Solid electrolytes for solid-state-supercapacitors and their interface chemistry: gel polymers, ionic liquids, ceramics, and beyond

Abstract

Conventional supercapacitors are limited by liquid electrolytes, which pose safety risks, have narrow electrochemical stability windows, are prone to leakage, and offer limited mechanical flexibility. Solid-state and quasi-solid-state electrolytes have appeared as promising alternatives, particularly for flexible and wearable energy storage devices. This review comprehensively summarizes recent developments in polymer electrolytes, inorganic ceramics, ionic gels, and organic–inorganic composites, highlighting their roles in ionic conductivity, electrochemical stability, operating voltage, and device performance. Polymer electrolytes, which combine polymer flexibility with thermal stability, mechanical strength, and enhanced ion transport facilitated by the incorporation of inorganic fillers, receive special attention due to their interface compatibility with solid electrodes. Key strategies such as filler incorporation, polymer crystallinity suppression, in situ polymerization and interfacial engineering to improve electrode–electrolyte contact are critically discussed. Furthermore, advanced in situ and operando characterization techniques are highlighted for their ability to probe buried solid–solid interfaces, track interfacial degradation, and monitor ion transport dynamics in real time. Finally, the review outlines the current challenges and future directions for designing high-performance, safe, and flexible solid-state supercapacitors, providing valuable insights for the development of next-generation energy storage technologies.

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1. Introduction

Modern technological advancements have significantly increased energy demand, leading to the exploitation of available energy sources. A balanced relationship between energy supply and demand needs to be maintained, and therefore, it is essential to avoid energy waste and ensure a stable energy grid.¹ To address this global issue, energy storage devices such as supercapacitors, batteries, and fuel cells have been intensively researched to meet future energy demand.^2–4 Although batteries play a crucial role in energy storage devices for electric and electronic applications, their limited power density and poor cycle life pose challenges to commercial deployment. In contrast, supercapacitors have emerged as a promising alternative, offering high power density, ultrafast charging and discharging characteristics, and excellent operational durability without compromising energy storage efficiency.^5–7 Supercapacitors primarily provide regenerative power and peak assist in industrial and automotive applications, while in consumer electronics, their miniaturized versions act as alternative power sources to batteries.⁸ Considering the charge storage mechanism, a supercapacitor is generally divided into electrochemical double-layer capacitors (EDLCs) and pseudocapacitors.⁹ EDLCs store energy through electrostatic charge separation at the electrode–electrolyte interface, whereas pseudocapacitors rely on Faradaic electrolyte interface, whereas pseudocapacitors rely on Faradaic charge transfer between the electrode and electrolyte.⁹ Structurally, supercapacitors are either symmetric, employing electrodes with identical capacitance, or asymmetric, with electrodes of different capacitances.¹⁰ The asymmetric type typically combines a double-layer electrode with a pseudocapacitive one, or two distinct pseudocapacitive electrodes.⁹

Historically, research on supercapacitors has primarily concentrated on developing various rigid, bulky electrode materials and liquid electrolytes to enhance energy and power densities. One of the most crucial components of supercapacitors is the electrolyte, which serves as an ionic conductor between the electrodes and strongly influences the voltage window, rate performance, and cyclic stability of the device.¹¹ Electrolytes are primarily characterized by their ionic conductivity, working voltage window, and operating temperature.¹¹ The energy density of a supercapacitor is governed by two primary factors: charge storage (Q) and operating voltage (V). Since energy density scales quadratically with V, a wider voltage window is highly beneficial. While Q is determined by the electrode used, surface area, pore size, and electrolyte ion size, optimizing the electrolyte would be an effective strategy because its potential window directly influences the operating voltage V. Electrolytes with high ionic conductivity and low equivalent series resistance (ESR) further improve power and energy density by reducing internal resistance and expanding the voltage window.¹² Traditional liquid electrolytes used offer excellent ionic conductivity and ability to effectively wet electrode surfaces; however, despite these advantages, systems such as aqueous (e.g., H₂SO₄ and KOH) and organic (e.g., ethylene carbonate, acetonitrile, or propylene carbonate-based) electrolytes exhibit several drawbacks in supercapacitors.¹² Aqueous electrolytes, while highly conductive, offer a narrow voltage window (∼1 V) and can corrode metal electrodes. While organic electrolytes provide a wider voltage range (∼2.5 to 3 V), they suffer from high volatility, flammability, and toxicity.¹³ Additionally, liquid electrolytes face challenges such as leakage, low mechanical stability, the need for hermetic sealing, and dendrite formation on metals (e.g., Li, Na, and Zn), limiting their use in flexible and portable energy storage devices.¹⁴ Consequently, the rising demand for flexible and portable electronics-including wearable multimedia devices, foldable smartphones, rollable displays, smart textiles, biomedical devices, and skin-inspired sensors-has driven research toward solid-state electrolyte configurations that could offer bendable, foldable, twistable, lightweight, ultrathin, wearable characteristics.¹⁵

In this context, developing structurally sound, secure, and high-performing energy storage systems depends on the design of innovative solid electrolytes, with high ionic conduction, flexibility, and stable electrochemical performance in solid and flexible supercapacitors. Considerable effort has been directed toward designing and optimizing a wide range of solid-state electrolytes, which are generally classified into organic polymer electrolytes (PEs) (e.g., poly(ethylene oxide) (PEO),¹⁶ polyvinylidene fluoride (PVDF),¹⁷ polyvinyl chloride,^18,19 poly(methyl methacrylate), etc.),²⁰ inorganic ceramic electrolytes (CEs) (e.g., aluminium oxide (Al₂O₃), silicon dioxide (SiO₂),^21,22 Li₇La₃Zr₂O₁₂ (LLZO),²³ and organic/inorganic composite polymer electrolytes (CPEs)).^23,24 Inorganic electrolytes are solid-state ion conductors, typically comprising conductive ceramics or glasses. CPEs, combining organic and inorganic components along with ion-conducting active or inactive fillers, offer the advantages of both phases, enhancing electrochemical, mechanical, and interfacial performance.²⁵

Fig. 1 presents a schematic illustration of different types of solid-state electrolytes, including solid polymer electrolytes, gel polymer electrolytes, composite polymer electrolytes containing inorganic fillers, and inorganic solid electrolytes, along with a comparative evaluation of their key performance characteristics. The left panel schematically depicts the structural differences among these electrolyte systems, highlighting the polymer matrix framework, mobile ionic species, and the presence of inorganic fillers or solid ceramic phases. As depicted in the radar plots, solid polymer electrolytes exhibit excellent mechanical flexibility but relatively low ionic conductivity, while gel polymer electrolytes demonstrate improved ionic transport due to the presence of liquid components.^14,26 Composite polymer electrolytes incorporate inorganic fillers to simultaneously enhance mechanical strength, ionic conductivity, and interfacial compatibility. In contrast, inorganic solid electrolytes typically exhibit high thermal stability and ionic conductivity but suffer from limited flexibility and interfacial contact issues. To get the advantages of individual components, new hybrid composite electrolytes have been developed, consisting of mixed properties such as high ionic conductivity, flexibility, and improved interfacial contact.¹⁴ Overall, the figure illustrates the inherent performance trade-offs among different electrolyte systems and underscores the potential of composite and hybrid electrolytes to achieve balanced and enhanced electrochemical performance.

Fig. 1 Schematic diagrams and performance of solid polymer electrolytes, gel polymer electrolytes, composite polymer electrolytes, and inorganic solid electrolytes. Reproduced with permission from ref. 25, Copyright 2022 American Chemical Society.

As recognized, the performance of flexible and portable energy devices mainly depends on the properties of semi-solid and solid electrolytes, their composition, and the electrode–electrolyte interfaces, which affect the voltage, charge–discharge rates, lifespan, energy and power density, and temperature range.²⁵ Recent research has focused on fine-tuning and improving these electrolytes and interfaces, but further innovations are still needed for practical use and commercial applications. Moreover, in situ characterization techniques have evolved as powerful techniques to understand interfacial mechanisms, offering real-time inside mechanisms for ion transport and electrolyte behaviour during charge–discharge processes in solid-state supercapacitors, which is essential for guiding the design of efficient and durable solid-state electrolytes. For example, in situ nuclear magnetic resonance (NMR) spectroscopy has been employed to track ion adsorption and desolvation within porous carbon electrodes, providing insights into how ion rearrangements affect capacitance during electrochemical cycling.²⁷ Likewise, in situ X-ray transmission studies have shown that ion-exchange processes predominate during the initial stages of charging, followed by slower ion equilibration, which impacts overall energy storage performance.²⁸ These advanced characterization methods allow direct monitoring of structural and chemical changes at the electrode–electrolyte interface, offering valuable guidance for designing more efficient and long-lasting solid-state electrolytes.

In recent years, several studies have focused on polymer electrolytes, inorganic solid electrolytes, and composite systems for supercapacitors. However, a comprehensive overview integrating different types of solid-state electrolytes, while emphasizing interface design, ionic transport, and in situ characterization, remains lacking. This review provides a holistic understanding of solid-state supercapacitors, highlighting how optimized electrode–electrolyte interfaces govern ion mobility, charge transfer, and device stability. We discuss the development of various solid electrolytes, including polymers, inorganic ceramics, ionic gels, and composites, and their roles in ionic conductivity, electrochemical stability, and overall performance. Strategies such as interfacial engineering, composite formation, and surface modification are analyzed for their impact on device efficiency. Finally, we emphasize in situ and operando techniques for monitoring buried interfaces and dynamic ion behaviour and offer perspectives on future directions for designing high-performance, safe, and flexible solid-state supercapacitors, guiding the development of next-generation energy storage devices.

2. Historical progress in solid-state electrolytes

The need for solid-state electrolytes for supercapacitors has evolved over time, driven by the goal of generating safer, leak-free, and flexible energy storage. The evolution of solid-state electrolytes for supercapacitors reflects a transition from conventional liquid systems to gel polymers, to hybrid composite electrolytes comprising gel and inorganic fillers, and, subsequently, to inorganic/ceramic oxide frameworks, as shown in Fig. 2.^14,25 In brief, during the 1970s–1980s, conventional supercapacitors predominantly employed aqueous, alkaline, neutral or organic-based electrolytes such as H₂SO₄, KOH, and organic salts with a very high ionic conductivity, but posed several other challenges like leakage, corrosion, limited mechanical robustness, reduced safety, and narrow electrochemical stability windows.¹²

Fig. 2 A historical overview of electrolyte development for supercapacitors.

The concept of solid-state ionics emerged in the mid-1970s, inspired by solid-state physics and aimed at understanding ion transport in solids.²⁹ A significant milestone occurred in 1914, when Tubandt et al. investigated AgI-type solid electrolytes, revealing that Ag⁺ ions in α-AgI could migrate with a “liquid-like” behaviour in the rigid anionic sublattice.^25,30,31 Later, in the 1960s, the discovery of fast sodium-ion conduction in β-alumina (Na₂O·11Al₂O₃) represented a significant advance, leading to the development of high-temperature Na–S batteries and inspiring decades of research on fast ion conductors.³² These findings established the foundation for modern solid and semi-solid electrolytes, which now play a vital role in flexible and portable electrochemical energy devices. By the 1970s and 1990s, the field of solid-state ionics expanded beyond inorganic materials with the emergence of quasi-solid or polymer gel electrolytes like poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA), marking a new phase in electrolyte design for energy devices. During the 2000s, attention shifted to fully solid polymer–salt complexes such as PEO–LiClO₄ and PMMA–NaClO₄, which provided truly solid, leak-free systems suitable for thin and portable configurations.³³ Wright et al. discovered PEO-based polymer–salt complexes, which demonstrated ionic conduction through polymer chains.^34,35 These gels combined good ionic transport with mechanical flexibility and safety, leading to the first generation of flexible solid-state devices. This breakthrough opened pathways to developing flexible, lightweight solid electrolytes, which were later adapted for supercapacitors and other solid-state energy storage systems. Following this breakthrough, numerous conductive polymer materials, such as poly(acrylonitrile) (PAN),³⁶ poly(methyl methacrylate) (PMMA)³⁷ and PVDF,³⁸ were extensively investigated for their potential in polymer-based energy storage systems.

In 1982, Weston and coworkers used Al₂O₃ as an inert filler in the PEO and Li salt polymer electrolyte to design a composite polymer electrolyte. The fabricated composite electrolyte improved the mechanical strength and interfacial stability.³⁹ Croce et al. advanced polymer electrolytes by incorporating micro- and nanosized inorganic fillers in the 1990s, promoting the advancement of CPEs.^40–42 However, their low ionic conductivity led to the introduction of gel polymer electrolytes (GPEs) by Feuillade et al. in 1975, incorporating propylene carbonate (PC) as a plasticizer,⁴³ and later the addition of ionic liquids (ILs) by Watanabe et al. in 1995.⁴⁴ In addition, other ceramics, such as TiO₂, SiO₂, and Al₂O_3, have been employed in polymer matrices to improve mechanical strength, thermal stability, high-voltage performance, and ion-transport properties.^45–48 These innovations laid the foundation for modern semi-solid and solid electrolytes in flexible energy storage devices. However, their relatively low ionic conductivity at ambient temperature and electrode/electrolyte interfacial resistance remained major challenges, which prompted the exploration of hybrid and ionic-liquid-based electrolytes. During the 2010s, hybrid systems combining polymer matrices with ILs or inorganic fillers offered broader electrochemical stability windows (up to 3 to 4 V), enhanced thermal and chemical stability, and compatibility with flexible substrates.⁴⁹ In addition to developing sustainable, environmentally friendly solid–gel electrolytes, research focused on biopolymer-based gels such as cellulose, chitosan, and agarose.⁴⁹

Ceramic oxides (e.g., β-alumina, NASICON, and perovskite-type oxides) have been known for their high ionic conductivity (10⁻³ to 10⁻⁵ S cm⁻¹) since the early (1960s–1980s) developments in solid-state ionics.^50–52 However, their use as solid-state electrolytes in supercapacitors remained largely limited for decades due to issues such as brittleness, high sintering temperature, and poor electrode–electrolyte interface.⁵³ In recent years (2020s), research on these materials has intensified, particularly with their use as inorganic fillers in polymer composite electrolytes, where they significantly improve thermal resistance, mechanical robustness, and ion transport properties.^22,54 Solid-state Na⁺ ion-based supercapacitors using a PEO polymer–NaCF₃SO₃ salt with a Na₃Zr₂Si₂PO₁₂ (NZSP) NASCICON dispersed filler has been reported. To enhance supercapacitor performance, a small amount of acetonitrile organic solvent is added to the electrode–electrolyte interface via a “solvent layer” technique. The device showed a specific capacitance of 260 F g⁻¹ and a specific power of 4780 W kg⁻¹ at 3 V/5 mA. The device remained stable for up to 10000 galvanostatic charge–discharge cycles, with ∼99% coulombic efficiency and ∼90% capacitance retention.⁵⁵ Modern solid-state supercapacitors increasingly rely on engineered composite electrolytes that integrate the advantages of polymeric and ceramic phases, enabling high energy density, wide voltage operation, and reliable performance for flexible and wearable electronic applications.

Very recently, ceramic oxides such as NASICON, garnet-type (LLZO), and perovskite-type (LLTO) structures have been designed as primary electrolyte frameworks in solid-state configurations, where they serve as the main ion-conducting media.^56,57 These materials exhibit high ionic conductivity, wide electrochemical stability windows, and superior thermal stability, making them promising options for future-generation solid-state supercapacitors and hybrid energy storage devices. For example, very recently Kaur et al. used LALZO (Li_6.75Al_0.25La₃Zr₂O₁₂) with ∼6 wt% of ILs IL-EMIM BF₄, a composite electrolyte for the development of temperature-tolerant solid-state supercapacitors, which displayed a high ionic conductivity ≥10⁻⁴ Ω⁻¹ cm⁻¹ with a high operating temperature range from 0 to 100 °C.⁵⁸ Despite challenges arising from brittleness, high interfacial resistance, and limited flexibility in solid electrolytes, these challenges still need to be overcome, which has recently drawn renewed research attention.

The performance of different classes of solid electrolytes was systematically compared based on insights from multiple literature reports. Key electrochemical and physicochemical parameters, such as ionic conductivity, electrochemical stability, cell voltage window, mechanical strength, operating temperature range, and cyclic stability, were carefully evaluated and the comparative results are summarized in Table 1.^59–65 Furthermore, broader trends are illustrated through graphical representations to provide a clearer visualization of the relationships between electrolyte properties and overall supercapacitor performance.^57,66–71 As illustrated in Fig. 3, the ionic conductivity of different solid electrolytes varies several orders of magnitude depending on their structural characteristics and ion transport mechanisms. Among them, hydrogel-based electrolytes typically exhibit the highest ionic conductivity, often reaching values in the range of 10⁻²–10⁻¹ S cm⁻¹ due to the presence of a water-rich polymer network that facilitates rapid ion diffusion.⁶⁶ In addition to their superior conductivity, these hydrogels also demonstrate excellent mechanical flexibility, making them suitable for wearable and flexible supercapacitor applications.⁶⁷ However, hydrogel electrolytes often suffer from a narrow electrochemical stability window and limited thermal stability, which can restrict their use in high-voltage and high-temperature environments. In contrast, gel polymer electrolytes (GPEs) typically exhibit ionic conductivities on the order of 10⁻³ to 10⁻² S cm⁻¹ and provide improved mechanical stability compared to hydrogels while maintaining reasonably efficient ion transport.⁶⁹ Their semi-solid structure effectively minimizes electrolyte leakage and enhances overall device reliability. Ionic liquid-based electrolytes, on the other hand, provide a significantly wider electrochemical stability window (often exceeding 3 to 4 V), thereby enabling the development of high-voltage supercapacitors. Meanwhile, solid polymer electrolytes (SPEs) and ceramic-based solid electrolytes typically exhibit lower ionic conductivities in the range of 10⁻⁶ to 10⁻⁴ S cm⁻¹ compared to hydrogel and gel systems. Despite these limitations, they offer distinct advantages in terms of superior mechanical strength, enhanced chemical stability, and improved safety, making them attractive for robust and long-term energy storage applications.^49,64 Hence, these electrolyte properties make them promising candidates for high-temperature and high-safety energy storage systems. However, their relatively high interfacial resistance and brittle nature still pose significant challenges for practical device integration. The ion transport mechanisms, interfacial challenges, and key physicochemical properties of each class of solid electrolytes are discussed in detail in the following sections.

Table 1 A performance comparison based on different solid electrolytes for supercapacitor applications

Solid electrolyte type	Solid electrolyte	Ionic conductivity (S cm^−1)	Electrode material	Voltage window (V)	Mechanical flexibility	Resistance (Ω cm^−2)	Temperature stability	Capacity	Cyclic stability	Ref.
Hydrogel electrolyte	CH3COONa·3H2O + PVA	81.27 × 10^−3	Activated carbon	0–2	Soft and flexible	1.92 and 2.23	RT	32.7 F g^−1 (1 A g^−1)	5000 cycles (110%)	59
Gel polymer electrolyte	PVA–LiClO4	—	V2O5/MWCNTs	0–2	Bending and flexible (0–180°)	3.2	RT	—	4000 cycles (93%)	60
Gel/hybrid polymer electrolyte	(C3(Br)DMAEMA) + (PEGMA) + Li2SO4	66.8 × 10^−3	Activated carbon	0–1.2	Bendable	2–3	RT	64.92 F g^−1 (1 A g^−1)	10 000 cycles (94.63%)	61
Iono gel electrolyte	PEGDA/[EMIM][TFSI]	9.4 × 10^−3	MWCNTs	0–2	Mechanical and bending stress	100	RT	5.3 F cm^−3 (0.01 V s^−1)	30 000 cycles (80%)	62
Solid polymer electrolyte	PAEK–PEG	2.6 × 10^−4	Activated carbon	1.5	Flexible bending (0–120°)	∼10	30–120 °C	103.17 F g^−1 (0.1 A g^−1)	2000 cycles (110%)	63
Ceramic/Inorganic solid electrolyte	Li1.4Al0.4Ti1.6(PO4)3	2.6 × 10^−4	MWCNTs	0–0.5	Rigid and brittle	3.9 × 10^3	RT	353.570 mF g^−1	—	64
Ceramic/polymer composite electrolyte	Li1.4Al0.4Ti1.6(PO4)3–PEO	10^−4	Graphite/AC	0–2	Moderate	10–60	40 °C	100 F g^−1 (1 A g^−1)	2500 cycles	65

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Fig. 3 Graphical comparison of various solid electrolyte systems based on key parameters, including ionic conductivity, electrochemical stability window, operating temperature, mechanical strength, and cyclic stability.^57,66–71

3. Interface challenges and engineering strategies in solid-state systems

While the substitution with conducting solid electrolytes is thought to be capable of aligning with these ideal electrodes and enhancing the safety of integrated batteries or supercapacitors to some degree, the resulting electrochemical performance of assembled solid-state devices remains considerably unsatisfactory. A major limitation arises from the poor interfacial contact between solid electrodes and solid electrolytes, which leads to large interfacial resistance, and continues to be a critical, unresolved scientific challenge.⁷² In liquid-state electrolytes, ions can readily move between the electrode and the electrolyte because of the electrodes' finely infiltrated surface in a liquid system. However, in solid-state energy storage devices, the sequential stacking of the anode, electrolyte, and cathode produces numerous interfaces.¹⁵ The interfaces between solid materials hinder the smooth infiltration of balanced redox processes involving ions into gaps and voids, resulting in a notable increase in interfacial resistance and continued degradation of electrochemical performance, thereby affecting the stability and lifespan of the entire cell.¹⁵ Hence, a comprehensive understanding of the characteristics and various aspects of the interfaces between solid-state electrolytes (SSEs) and electrode materials is crucial for the rational design of high-performance all-solid-state batteries (SSBs). This is because complex interfacial phenomena, including physical contact limitations and chemical or electrochemical instability, can arise at SSE–electrode interfaces, adversely affecting the electrochemical performance of SSBs.

3.1 Root causes of interface instability

The interfacial instability in solid-state energy storage systems originates from several fundamental root causes, including mechanical mismatch, poor physical contact, electrochemical potential differences, and chemical incompatibility (Fig. 4a).^73–75 These factors can lead to various interfacial problems such as crack formation, interfacial voids, space-charge layer formation, dendrite penetration, and poor wettability, as shown in Fig. 4b.^53,76,77 Furthermore, continuous interfacial reactions can expedite the irreversible capacity decay in assembled supercapacitors, thereby introducing additional challenges that must be addressed. These include issues such as contact degradation, interfacial deterioration, and dendrite formation during prolonged cycling.^76,78–80 Moreover, exceeding the limited thermodynamic stability windows of solid electrolytes results in the formation of additional interphases via solid-state electrolyte decomposition.⁸¹ This impedes the transport of metal ions between the solid-state electrolyte and the active materials. At the Li–SSE interface, side reactions and dendritic development are the main obstacles. When the solid-state electrolyte comes into contact with Li metal, it reduces to a lower valence compound, forming an unstable interface.⁸¹ Undesirable side reactions may occur at the interface between the cathode and solid-state electrolytes. The cathode/SSE interface can also experience significant resistance due to space charge effects and mutual diffusion effects, which limit the power density of solid-state batteries.

Fig. 4 Schematic illustration of major solid–solid interface challenges in solid-state energy storage systems and the corresponding engineering strategies used to mitigate them. (a) Root causes of interface issues, (b) interfacial challenges, and (c) engineering strategies for stabilizing interfaces in all-solid-state supercapacitors. Reproduced with permission from ref. 77 and 79, Copyright 2022 Science Advances 2020 Joule.

3.2 Mapping interface problems to engineering strategies

To address interfacial challenges and minimise resistance at the electrode–electrolyte interface in solid-state systems (both cathode and anode), an interfacial coating is the most effective strategy.⁸² Various interface engineering strategies have been developed, including surface coatings, soft interlayers, interface doping, incorporation of ceramic nanoparticle fillers, and in situ polymerization (Fig. 4c).^83–85 These approaches have been extensively implemented across different solid-state energy storage systems, employing polymer, ceramic, and composite solid electrolytes. The use of conformal and chemically inert coatings has proven effective in preventing undesirable interfacial reactions between electrode materials and solid electrolytes.⁸⁶ Such coatings mitigate the formation of space charge layers and significantly diminish interfacial resistance. Certainly, the incorporation of a LiNbO₃ layer onto the nickel layer cathode significantly enhances coulombic efficiency and discharge capacity.⁸⁷ These improvements are evident not only in the reduction of cathodic charge transfer impedance but also in maintaining stable impedance characteristics over extended periods.

Interfacial coating materials must exhibit high ionic conductivity, good electrochemical stability, and appropriate structural characteristics.⁸⁷ High ionic conductivity is essential to ensure a smooth pathway for ions across the interface, facilitating the desired rate of electrochemical reactions. A variety of techniques, including pulsed laser deposition, sol–gel, atomic layer deposition, and molecular layer deposition, have been employed to create an artificial interface.⁸⁸ The deposition of interfacial coating materials should occur at moderately low temperatures to preserve the electrode structure.⁸⁸ Materials containing Li are appealing choices for interfacial coatings due to their ability to stabilize the interface between solid-state electrolytes and electrodes.⁸⁹ This characteristic is anticipated to suppress parasitic reactions and reduce interfacial resistance. To provide a systematic understanding of the previous discussion, Table 2 summarizes the relationships among the root causes of interfacial instability, the resulting interfacial issues, the corresponding engineering strategies, representative electrode materials, and the resulting electrochemical performance improvements.^{40,68,90–95} This mapping helps clarify the design logic for stabilizing interfaces in solid-state supercapacitors. Based on the above discussion of the root causes of interfacial instability and the importance of corresponding engineering strategies, the focus in the preceding sections will be on different classes of solid electrolytes. In particular, emphasis is placed on their ion-transport mechanisms, key interfacial challenges, and the development of artificial interfacial-layer engineering strategies for each electrolyte system.

Table 2 Correlation between interface issues and engineering strategies in solid-state electrolytes

Interface problem	Root cause	Electrolyte type	Engineering strategy	Representative system	Effect achieved	Ref.
Poor interfacial wettability	High surface energy mismatch between Li and electrolyte	SPE (PEO-based)	Surface modification/soft interlayer	PEO–LiTFSI/Li metal	Reduced interfacial resistance	68
Mechanical mismatch	Rigid ceramic vs. soft electrode	Garnet-type ceramic	Polymer buffer interlayer	LLZO/Li	Suppressed interfacial cracking	90
Dendrite penetration	Non-uniform Li^+ flux	SPE/composite	Ceramic nanoparticle fillers	Al2O3–PEO	Uniform Li deposition	40
Chemical instability at the cathode interface	Interfacial redox reactions	Sulfide electrolyte	Cathode surface coating	LiNbO3-coated LNO	Enhanced cycling stability	91
Space charge layer formation	Li^+ concentration gradient at the ceramic interface	Oxide ceramic	Interface doping/surface treatment	LLZO surface modified	Reduced impedance	92
Poor ionic conductivity in the polymer	High crystallinity	Gel polymer electrolyte	Plasticizer/gel formation	PVDF-HFP gel electrolyte	Increased ionic conductivity	93
Interfacial side reactions	Electrolyte decomposition	Composite polymer electrolyte	In situ polymerization	In situ formed polymer/LLZO composite	Improved stability & contact	94
Interfacial void formation during cycling	Volume change of Li metal & insufficient stack pressure	Ceramic/composite SSE	Application of external pressure/elastic interlayer/3D current collector	LLZO/Li metal	Maintained interfacial contact & stable cycling	95

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4. Organic solid electrolytes

Polymer-based electrolytes, which address electrolyte leakage and enable easy fabrication of supercapacitors, influence both quasi-solid and solid electrolytes.¹² In addition to providing ionic conduction, they also serve as electrode separators, enabling the creation of lightweight, flexible, and wearable supercapacitors. Two main types exist: solid polymer electrolytes and GPEs, with GPEs being more extensively studied.¹²

4.1 Solid polymer electrolytes

Solid polymer electrolytes (SPEs), also referred to as dry polymer electrolytes, are solvent-free electrolyte systems that are described as polymer matrices swollen with ion-conducting salts, where the mobility of dissolved salt ions through the polymer matrix contributes to the ionic conductivity.⁹⁶ The impedance, cycle stability, and rate capability of solid-state batteries are all significantly influenced by the ionic conductivity of SPEs. Recognizing the process of ion transport in polymer electrolytes is therefore essential for developing high-conductivity materials suited for advanced solid-state battery performance. Several ion-conduction models, such as the polymer segmental migration model, the Vogel–Tamman–Fulcher (VTF) model, the ion-hopping model, the Arrhenius model, and the free-volume model, describe ion transport behaviour in polymer-based systems.⁹⁷ According to the polymer segmental migration model in SPEs, ion transport typically occurs through the amorphous regions of the polymer matrix via segmental motion of polymer chains, with solvated ions hopping between coordination sites associated with polar functional groups (e.g., ether or carbonyl oxygen atoms).^97,98

In brief, SPEs have two distinct phases-the crystalline and the amorphous regions. Within these polymer matrices, the lithium salt dissociates and Li⁺ ions coordinate with polymer chains, creating an ionically conductive phase. Ion transport in the amorphous phase is described by the polymer segment migration model.⁹⁷ For example, in PEO-based electrolytes, ether oxygen atoms coordinate with Li⁺ ions, enabling ion transport through segmental motion in the amorphous regions.⁹⁹ Under an electric field, as shown in Fig. 5a, repeated coordination–decoordination processes allow Li⁺ to migrate along and between polymer chains, making both free Li⁺ concentration and polymer segment mobility key factors governing ionic conductivity in SPEs. However there are some contrasting reports as well claiming the ionic conduction to be in the crystalline regions of the polymer.

Fig. 5 (a) Movement of Li⁺ ions through a single polymer chain (intrachain) and between different polymer chains (interchain) in PEO. Reproduced with permission from ref. 100 Copyright 2021 Springer Nature. (b) Li⁺ hopping through spiral channels in a static, ordered crystalline framework. Reproduced with permission from ref. 101, Copyright 2001 Nature.

In the crystalline regions of PEO-based electrolytes, polymer chains fold and stack to form tubular structures that enable Li⁺ transport via vacancy diffusion, similar to ion hopping in inorganic electrolytes. For example, Bruce et al. proposed that at an [EO] : [Li⁺] ratio of 6 : 1, two PEO chains form a double-helical channel providing a cylindrical channel through which Li⁺ ions migrate, while anions remain outside, as depicted in Fig. 5b.^101,102 This ordered helical structure enhances ionic conductivity by promoting Li⁺ hopping independent of polymer segmental motion. They also claimed that electrolytes with an ordered crystal structure show higher ionic conductivity. Although Li⁺ transport within ordered polymer crystalline structures has been observed, the underlying conduction mechanism remains insufficiently understood and requires further experimental validation. Hence, the ionic conductivity of SPEs depends significantly on polymer flexibility, degree of crystallinity, glass transition temperature (T₉), and salt–polymer interactions.

An ideal SPE should exhibit a strong coordination with cations and appropriate spacing between coordination sites. Effective salt dissolution requires low salt lattice energy and polymers containing Lewis bases and acid groups (e.g., carbonyl C=O, ether –O–, –P–, –N–, –S, etc.) to coordinate with lithium ions.^103,104 To improve ionic conductivity, various strategies, such as plasticizer addition, copolymerization, and incorporation of inorganic fillers or ILs have been explored to enhance ion mobility.¹⁰⁵ Several approaches for achieving highly ionic-conducting SPEs are schematically illustrated in Fig. 6. These improvements have expanded the potential of SPEs for solid-state supercapacitors and other flexible electrochemical devices that require high mechanical integrity and safety. However, their conductivity is generally lower (10⁻⁸ to 10⁻⁷ S cm⁻¹) than that of gel or liquid electrolytes at room temperature.

Fig. 6 Strategies for achieving SPEs with high ionic conductivity. Reproduced with permission from ref. 98, Copyright 2023 Wiley.

4.1.1 Interface engineering for SPEs. A persistent challenge with SPEs used alongside porous electrodes lies in creating a stable and sufficiently extensive electrode–electrolyte interface. Since the capacitance and energy storage capacity of a supercapacitor are directly related to the effective interfacial area, achieving maximum surface exposure and utilization of the electrode is crucial. In systems with liquid electrolytes, this is relatively easy to accomplish, as proper wetting ensures efficient contact and optimal use of the electrode surface.⁴⁹ Smooth electrodes with improved contact are required for SPEs to enable effective light pressing. Gentle pushing is ineffective when dealing with porous electrodes, because electrolyte penetration is extremely challenging. The electrode's porosity may potentially be destroyed by high pressure.⁴⁹

However, there have been reports of attempts to use merely mechanical force, such as laminating the polymer electrolyte with the nickel foam-based activated carbon electrodes using a roller press, which could maintain high stability and capacity retention.⁴⁹ Electrolytes can be created by UV light polymerization of the reactant solution soaked into the electrode¹⁰⁶ or electrodeposited directly on the electrode, resulting in excellent interfacial contact.¹⁰⁷ Another strategy to address interfacial challenges is the direct deposition of electrode materials onto SPE films.^108,109 For instance, Senokos et al. directly deposited a carbon nanotube network onto a polymer electrolyte membrane from a floating CNT aerogel to create transparent, flexible supercapacitors.¹⁰⁹ The resulting devices delivered a high power density (1370 kW kg⁻¹), 94% capacitance retention after 20 000 cycles, and excellent mechanical flexibility under repeated 180° bending. Li et al. proposed an alternative method to enhance interfacial contact by forming an ionogel in situ during device fabrication.¹¹⁰ By incorporating small ions into ILs and tuning their solvation structures, they improved ion transport kinetics within nanopores, thereby enhancing the capacitance and rate capability of the EDLCs. The resulting buffer layer demonstrated sufficient thermal stability to function with a PEEK-based solid electrolyte, enabling the devices to achieve 79% capacitance retention after 5000 cycles at 120 °C, along with a maximum energy density of 46.9 Wh kg⁻¹ and a power density of 926.9 W kg⁻¹. Huang et al. proposed another strategy involving the insertion of a thin (≈450 nm) porous TiO₂ layer sprayed onto the SPEs.¹¹¹ Introducing this interfacial TiO₂ layer among a 4.5 µm carbon/TiO₂ nanoparticle electrode and the polymer electrolyte membrane significantly enhanced the solid-state supercapacitor's electrochemical performance. Further impedance and charge–discharge analyses revealed that the additional layer lowered charge-transfer resistance, facilitated ion transport, and accelerated charge–discharge kinetics, resulting in an increase in areal capacitance from 45.3 to 111.1 mF cm⁻² at a current rate of 0.4 mA cm⁻².

Sharma et al. showed that adding ∼5 wt% of the same PEO–salt polymer into the activated carbon electrode markedly improved conformal contact with the SPE membrane, reduced ESR and improved capacitance (∼102 F g⁻¹) and efficiency.⁷¹ Liu et al. developed a composite SPE (PLI(70)@PP) by infusing a PEO-based SPE into a microporous polypropylene separator.¹¹² The softer SPE surface increased contact area with electrodes, lowering interface resistance and achieving ∼158 F g⁻¹ at 0.1 A g⁻¹. SPEs continue to encounter difficulties, including low ionic conductivity, high interfacial resistance, and insufficient electrode–electrolyte contact, despite substantial efforts documented in the literature. These persistent issues continue to hinder efficient ion transport and full utilization of the porous electrode surface, emphasizing the need for GPEs that offer improved flexibility, ionic mobility, and interfacial compatibility.

4.2 Gel polymer electrolytes and interface engineering

GPEs have become the front-runner for solid-state electrolytes for supercapacitor applications because they possess significant ion migration affinity, low corrosion, good mechanical stability, and ease of handling. GPEs are easy to handle and exhibit low corrosion, strong ion-migration affinity, and excellent mechanical stability. They are the frontrunners for solid-state electrolytes due to these characteristics.¹¹³ The fundamental constituents of aqueous GPEs include a polymer host matrix, an electrolytic salt, and a plasticizer. Based on the phase and ionic medium, these are classified as hydrogels, organogels, redox-active gels, and iono GPEs.¹¹³ Multiple factors influence the electrochemical performance of supercapacitors. While single-component electrolytes often have inherent limitations, combining different electrolyte components can mitigate these drawbacks, yielding a wider voltage window, lower viscosity, enhanced ionic conductivity, improved stability, and smaller effective ion size.

4.2.1 Hydrogel polymer system. Aqueous GPEs, commonly referred to as hydrogel electrolytes, are among the most promising solid-state electrolytes for developing flexible and ultrathin supercapacitors.^24,114 Their widespread use arises from their low cost, high ionic conductivity, and environmental compatibility. However, their narrow electrochemical stability window limits energy density, and the high-water content in conventional hydrogels leads to performance degradation at sub-zero and elevated temperatures due to water freezing or evaporation. To address these drawbacks, various functional fillers and additives, such as graphene oxide (GO), montmorillonite (MMT), PAA, PVA, etc., have been incorporated to improve thermal stability and ionic mobility.^114,115 Additionally, co-solvents such as dimethyl sulfoxide (DMSO), ethylene glycol, and sulfuric acid (H₂SO₄) are used to regulate hydrogen bonding and lower the freezing point. Typical hydrogel systems consist of a polymer host such as PVA, PAA, or polyethylene oxide (PEO), water as the plasticizer and an electrolyte salt that may be neutral (LiCl and Na₂SO₄), acidic (H₂SO₄ and H₃PO₄), or alkaline (KOH).^114,116,117 The interconnected pores of elastic, crosslinked hydrogels can retain a large volume of water, with some polymers absorbing up to 2000 times their own weight. The water content depends on the type of polymer and the solvents used, and it plays a crucial role in determining the physicochemical properties of the hydrogel. Higher water content not only affects the mechanical and structural characteristics but also enhances the ionic conductivity of hydrogel-based electrolytes. The poor adhesion/contact between the hydrogel electrolyte and the electrode surface is often a major bottleneck, leading to high interfacial resistance and poor ion transfer. Various interfacial engineering strategies have been developed to enhance adhesion, reduce resistance, and improve ion transport.⁶⁶ Surface modification with hydrophilic or zwitterionic coatings promotes intimate contact and lowers charge-transfer resistance.⁶⁷ The incorporation of functional groups such as catechol or sulfonate into the hydrogel matrix strengthens chemical bonding and facilitates ionic conduction at the interface.¹¹⁸ Constructing gradient interfacial layers helps balance ion flux and suppress interfacial side reactions.¹¹⁹ Regulating the coordination environment of water and ions further improves interfacial stability, while in situ and operando analyses provide critical insight into interfacial evolution and guide rational electrolyte design.

Using a PVA/H₂SO₄ hydrogel electrolyte and a graphene film electrode as a model system, the interfacial compatibility between the electrode and hydrogel electrolyte was improved by pre-adsorbing a very hydrophilic polyzwitterion layer, poly(propylsulfonate dimethylammonium propylmethacrylamide) (PPDP), onto the electrode surface.⁶⁷ Electrochemical analysis shows that this modification significantly reduces the interfacial charge-transfer resistance, leading to a threefold increase in areal capacitance compared to unmodified electrodes. Measurements using an electrochemical quartz crystal microbalance with dissipation monitoring indicate that the modified interface enables more reversible ion transfer during charge–discharge cycles, suggesting an increase in the accessible electrode surface area. While a hydrophilic PVA coating exhibits a similar effect, its efficiency is considerably lower. To improve the adhesion between hydrogel electrolytes and electrodes, Du et al. designed a polyacrylic acid Fe³⁺–chitosan hydrogel featuring a double-crosslinked network.¹²⁰ Strong interfacial binding arises from the Hofmeister effect, where chitosan macromolecular micelles within the polyacrylic acid network undergo precipitation and folding upon exposure to a ZnCl₂/NH₄Cl electrolyte rich in hydrated Cl⁻ and NH₄⁺ ions. Using this electrolyte, a flexible Zn-ion redox capacitor was constructed with a Zn-plated anode and an MXene/Ag-nanowire–bacterial cellulose composite cathode. The device exhibits high areal energy density (278.6 µWh cm⁻²), stable power output (46.1% energy retention from a battery-like plateau), and remarkable mechanical resilience, retaining over 89% capacity after 1000 bending cycles and resisting severe physical deformation.

Nan et al. prepared a hydrogel electrolyte by polymerizing a soybean protein isolate–calcium sulfoaluminate (SPI–CSA) solution with an acrylamide (AAm) solution, allowing the polymer network to chemically anchor to the Zn/CNT electrode via in situ polymerization.¹²¹ They proposed that strong adhesion between the hydrogel electrolyte and the electrode can be achieved through the combined effects of a mechanically robust polymer matrix and chemical interactions at the interface, as depicted in Fig. 7a. This chemical bonding creates a well-adhered, seamless interface, whereas conventional ex situ adhesion without chemical anchoring results in gaps and weak contact between the hydrogel and electrode (Fig. 7b). Electrochemical testing shows that the hybrid capacitor maintains symmetric charge–discharge curves across different temperatures, indicating effective ion transport and stable capacity output (Fig. 7c). Impedance analysis reveals that, at low temperatures (down to −60 °C), the internal resistance (R_s) increases slightly due to reduced ion mobility. In contrast, the charge-transfer resistance (R_ct) increases more markedly, indicating slower interfacial kinetics (Fig. 7d). Despite these effects, the device demonstrates excellent low-temperature performance, cycling for over 10 000 cycles with 98.4% coulombic efficiency and 98.7% capacity retention (Fig. 7e), confirming that chemical interfacial anchoring effectively enhances adhesion, interfacial stability, and electrochemical durability. As illustrated in Fig. 7f, the hybrid capacitor withstands diverse mechanical distortions, including bending, twisting, rolling, and compression, without detachment or cracking in a temperature range between 25 °C and −60 °C. These findings highlight that strategically engineered interfaces between hydrogel electrolytes and electrodes can effectively boost adhesion, facilitate ion transport, and maintain mechanical and electrochemical stability, even at extreme temperatures and under repeated mechanical stress.

Fig. 7 (a) Schematic illustration of the hydrogel electrolyte structure and adhesive interface. (b) Cross-sectional SEM images of hydrogel electrolyte–electrode interfaces with in situ and ex situ adhesion. (c) Charge–discharge profiles of the Zn‖CNTs hybrid capacitor over a temperature region of 25 to −80 °C. (d) Temperature vs. Nyquist plots of the hybrid capacitor. (e) Cycling performance showing capacity and coulombic efficiency at 25 and −60 °C at a current rate of 200 mA g⁻¹. (f) Photographs of the Zn‖CNTs hybrid capacitor under different mechanical distortions, including bending, twisting, rolling, and compression, from 25 to −60 °C. Reproduced with permission from ref. 122 Copyright 2023 Springer Nature.

4.2.2 Organic polymer gel system. Aqueous GPEs are typically limited because of their narrow electrochemical window, which hinders the achievable cell voltage and, consequently, the energy density of supercapacitors. To overcome this limitation, organic GPEs are often employed, allowing cell voltages to reach approximately 2.5 to 3 V.¹¹⁴ These electrolytes are generally prepared by physically blending high-molecular-weight polymers, such as PVDF-HFP or PMMA, with conducting salts in non-aqueous solvent systems, followed by gelation. Commonly used organic solvents-such as propylene carbonate (PC), ethylene carbonate (EC), or dimethylformamide (DMF), and their combinations, act as plasticizers to enhance ion mobility and broaden the electrochemical stability window. The enhancement in the voltage window would directly affect the energy storage capacity of supercapacitors. While both aqueous and organic GPEs face interfacial challenges, the nature of these issues differs. In aqueous gels, high water content promotes wetting but can cause swelling, freezing, or delamination, and the narrow electrochemical window limits voltage stability.¹²³ In contrast, organic gels operate at higher voltages and use less polar, viscous solvents, which often result in poor wetting, slower ion transport, and mechanical or thermal mismatch at the interface.¹²⁴ Additionally, organic gels are more prone to interfacial chemical decomposition.¹²⁵ Consequently, interface engineering strategies differ: aqueous systems rely on hydrophilic coatings and chemical anchoring, whereas organic systems require surface functionalization, in situ polymerization, and solvent-compatible interlayers.^126–128

Zhu et al. introduced a coherent integration strategy for organic GPEs, demonstrating that a PVDF-HFP-based GPE can simultaneously serve as an electrolyte, a binder, and a thin separator within porous electrodes.¹²⁹ This permeating distribution enhances ion-transport pathways and increases the effective electrode–electrolyte interfacial area, as shown in Fig. 8a, leading to marked improvements in capacitance and rate capability. Their study also revealed an activation process during early cycling, lowering charge-transfer resistance and diffusional impedance (Fig. 8b). For newly assembled GPE-based cells, the charge-transfer resistance (R_ct) increases as the PVDF-HFP gel layer becomes thicker. The sequence of R_ct values follows this trend: 5 µm PVDF-HFP (lowest) < cellulose separator (170 µm, 25 µm pores) < 12 µm PVDF-HFP < 36 µm PVDF-HFP < Celgard separator (25 µm) (highest). In contrast, the charge-transfer resistance of the GPE-based cells dropped significantly after activation. With an optimized ∼12 µm GPE layer and a hybrid AC/TEAPMo12 nanocomposite electrode, the resulting symmetric supercapacitor exhibited ambipolar redox behavior and a ∼40% increase in volumetric capacitance, as shown in Fig. 8c. Overall, the synergistic coupling between the infiltrated GPE and the redox-active hybrid electrode architecture enabled substantially higher energy density without sacrificing cycling stability, demonstrating the potential of coherent GPE–electrode integration for compact high-performance supercapacitors.

Fig. 8 (a) FESEM cross-section of the electrode/PVDF-HFP film/electrode configuration, (b) Nyquist plots of cells with PVDF-HFP GPEs of different thicknesses (5, 12, and 36 µm) compared with cellulose and Celgard separators, and (c) volumetric capacitance of AC/TEAPMo₁₂-GPE symmetric cells. Reproduced with permission from ref. 129 Copyright 2022 MDPI. (d) Schematic of the solution-casting synthesis of GPE-3, and (e) its polymerization via the reaction between DEGBA and Jeffamine, (f) strain behavior of GPE-3, (g) SEM images of the EDLC cross-section and GPE/electrode interface, (h) GCD curves at different current densities, (i) specific capacitance and Ragone plot, (j) electrochemical performance after rolling, and (k) demonstration of LED illumination powered by the rolled cell, reproduced with permission from ref. 130 Copyright 2020 Royal Society of Chemistry.

Han et al. developed an epoxy-based organic GPE for flexible EDLCs that addresses interfacial adhesion challenges between the electrode and electrolyte under mechanical deformation.¹³⁰ The epoxy-based polymer matrix of the GPE is formed through a simple cross-linking reaction between epoxide rings and amine groups (curing process), as shown in Fig. 8d and e. By polymerizing epoxy with an IL, the GPE achieved strong chemical bonding with the electrode, high flexibility (up to 509%) (Fig. 8f), and high ionic conductivity (∼10⁻³ S cm⁻¹), while remaining stable at elevated voltage and temperature. The adhesive nature of GPE firmly binds the two electrodes, keeping the cell intact even under twisting. Cross-sectional SEM (Fig. 8g) shows good contact between the electrode and GPE-3, forming effective capacitive interfaces. EDLCs incorporating this GPE exhibited enhanced electrode utilization, maintaining electrochemical performance with a higher specific capacitance (99 F g⁻¹) from GCD curves (Fig. 8h), energy density (113 Wh kg⁻¹), and power density (4.5 kW kg⁻¹) (Fig. 8i), and even under mechanical deformation the performance remained unchanged (Fig. 8j and k), which demonstrates the importance of interface engineering in organic GPE-based supercapacitors for wearable and flexible electronics.

UV-assisted in situ polymerization has emerged as an effective strategy for forming GPEs directly on electrode surfaces, thereby improving interfacial contact, mimicking liquid-like electrolyte behavior, and enhancing ionic transport and device performance.¹³¹ For instance, a non-aqueous GPE based on poly(2-hydroxy-3-phenoxy propyl acrylate) has been synthesized via this method, entrapping LiClO₄/propylene carbonate to form a polymer network with both covalent crosslinks and reversible interactions between Li⁺ ions and the polymer–solvent system, thereby improving interfacial contact (Fig. 9a and b).⁶⁹ The electrolyte showed a high ionic conductivity (∼4.7 × 10⁻³ S cm⁻¹), which is analogous to that of liquid electrolytes, and excellent mechanical stability (Fig. 9c). The formation of GPE directly on the electrode surfaces facilitates a 2.0 V supercapacitor with high mass loading (3.8 mg cm⁻² of YP-80F carbon), exhibiting low equivalent series resistance (2.2 Ω) and high capacitance retention (113 F g⁻¹ at 2 mA cm⁻², 81% retention at 20 mA cm⁻²). They successfully scaled the method to large-area flexible devices (16 cm², 4 mg cm⁻²) operating at 2.5 V, retaining high capacitance (111 F g⁻¹) and low ESR (Fig. 9d). This study highlights the importance of in situ polymerization for mimicking liquid-like electrode–electrolyte interfaces, thereby enhancing ion transport and overall device performance in flexible supercapacitors.

Fig. 9 (a) Schematic illustration and photographs of the all-solid-state supercapacitor fabricated via the in situ polymer gel electrolyte formation approach, (b) schematic comparison of the electrode–electrolyte interfaces in in situ generated GPEs versus conventional GPE systems, (c) mechanical robustness and reversibility of the H-P-L-3M-80% GPE, and (d) CV curves of H-P-L-3M-S-4.0 taken at 50 mV s⁻¹ at different bending states, as depicted in the inset images, reproduced with permission from ref. 69 Copyright 2017 Royal Society of Chemistry.

4.2.3 IONO gel system. Another system, ionic liquid-based GPEs (IL-GPEs), also known as ionogels, is designed by confining ILs within a polymer network, combining solid-like mechanical stability with liquid-like ion transport properties and has been promisingly studied for solid-state supercapacitors.¹³² These electrolytes exhibit features of both aqueous and organic gel systems, including high ionic conductivity, an extensive electrochemical stability window of up to 3.5 V, and high thermal stability. Additional advantages of IL-GPEs include broad operational temperature ranges and self-healing capabilities, making them particularly attractive for high-performance and multifunctional flexible supercapacitors.¹³² These are classified into three main categories according to their cation–anion combinations: aprotic, protic, and zwitterionic. These types are commonly applied in lithium-ion batteries and supercapacitors, fuel cells, and membrane technologies, respectively. Due to their excellent stability and non-corrosive nature, ILs are particularly promising for flexible and stretchable supercapacitors and being well-suited for printed supercapacitor devices where nozzle corrosion must be avoided.¹¹ Moreover, devices utilizing IL-GPEs often do not require a separate separator between electrodes, which can reduce manufacturing complexity and cost. The chemical and physical characteristics of IL-GPEs are readily tunable, providing benefits such as leak-proof behavior, enhanced safety due to non-flammability and low vapor pressure, and a more compact device design.¹³² Based on the nature of their host network, ionogels can be classified as polymer-based, silica-based, or hybrid organic–inorganic systems, with polymer-based IL-GPEs being the most commonly applied in flexible supercapacitor applications.¹³²

Liew et al. reported that incorporating the ionic liquid 1-butyl-3-methylimidazolium chloride (BmImCl) into PVA-based polymer electrolytes significantly lowers their glass transition temperature (T_g) to below ambient conditions. When the IL serves as the solvent, selecting or designing a suitable host polymer is critical to achieve a high electrochemical activity in IL-based GPEs.¹³³ To date, various polymers have been explored as hosts for ionogel electrolytes, including PVA, PMMA, poly(ethylene glycol) diacrylate, PEO, and PVDF-HFP.^9,133 Tamilarasan and Ramaprabhu developed a transparent, highly elastic ionogel by embedding the ionic liquid [BMIM][TFSI] into a PMMA polymer matrix, achieving a material capable of stretching up to four times its original length. When used in a supercapacitor with graphene electrodes, this PMMA/[BMIM][TFSI] system delivered a specific capacitance of 83 F g⁻¹, an energy density of 26.1 Wh kg⁻¹, and a power density of 5 kW kg⁻¹, demonstrating excellent electrochemical performance.⁷⁰ In addition, Yamagata et al. reported that using chitosan as a host polymer can enhance the activity of ionogel-based EDLCs, underlining the significant impact of polymer selection on the overall electrochemical performance and flexibility of the device.¹³⁴

Ionogels, despite offering superior thermal stability, wide electrochemical windows, and non-volatility, present unique interfacial challenges not typically seen in aqueous or organic gel systems. Unlike hydrogels, where the polymer network is swollen by water and maintains high ion mobility due to a large mesh size that accommodates hydrated ions, ionogels use viscous ILs whose mobility is intrinsically lower and whose interaction with the polymer host often leads to increased crosslinking or confinement.^135,136 This confinement at the electrode–electrolyte interface contributes to higher interfacial resistance and sluggish ion transport.¹³⁷ Moreover, while aqueous hydrogels can form strong adhesion to electrodes through hydrogen bonding or in situ polymerization to minimize contact resistance¹²² ionogels frequently suffer from poor wetting or mechanical mismatch at the interface, particularly under deformation or cycling, thereby exacerbating interfacial polarization and degrading long-term performance.^138,139 These factors make the interfacial design in ionogel-based supercapacitors more complex and critical than in conventional aqueous or organic gel systems. For example, Li et al. addressed the limited ability of these electrolytes to infiltrate the electrode interface, which often deteriorates device performance, by implementing a hierarchical design strategy. They created an in situ ionogel buffer layer that enabled uninterrupted ionic pathways throughout the multi-level porous structure of the 3D electrode.¹¹⁰ The schematic illustration of the assembling process for the device using in situ buffer layer gelation is shown in Fig. 10a. The performance of the cell was evaluated at various temperatures, as shown in the CV and GCD (Fig. 10b and c). The current and discharge times notably increased with temperature, with no deterioration in the curves. The capacitance increases from 70 F g⁻¹ at 0 °C to 220 F g⁻¹ at 120 °C as shown in Fig. 10d.

Fig. 10 (a) Schematic diagram depicting the fabrication of a solid-state EDLC incorporating a buffer layer on the electrode surface through an in situ gelation process. The enlarged view highlights the interfacial structure, showing effective infiltration of the buffer layer into the electrode pores. High-temperature electrochemical performance. (b) CV curves, (c) GCD profiles, and (d) capacity vs. current density at different temperatures. Reproduced with permission from ref. 110 Copyright 2021, Elsevier. (e) Schematic of an all-solid-state supercapacitor using a viscoelastic polymer binder for electrode adhesion and covalent bonding with the ionogel electrolyte, (f) Nyquist plots comparing cells with different binders, (g) lithium-ion diffusion coefficients during the charging process for all-solid-state supercapacitors employing various binders, (h) photographs of a flexible capacitor in the flat (top) and bent (bottom) states, (i) CV and (j) GCD curves of a flexible all-solid-state supercapacitor consuming an ionogel electrolyte, (k) lap shear testing, and (l) cyclic stability at different bending cycles, reproduced with permission from ref. 140 Copyright 2025 American Chemical Society.

Ion transport typically accelerates with temperature, as indicated by the Stokes–Einstein relationship of the diffusion coefficient with temperature and viscosity, i.e., D = KT/6πrη, when the morphological factors are not considered (D is the diffusion coefficient, K is a constant, T is the Kelvin temperature, r is the ionic radius and η is the viscosity).¹¹⁰ In essence, higher temperatures lower the effective resistance to ionic movement, allowing ions to reach electroactive sites more efficiently and thereby boosting capacitance. This temperature-driven improvement in charge transport is also reflected in interfacial resistance and cyclic stability, with 79% capacity retention and 100% coulombic efficiency after 5000 cycles. The use of polymeric binders in supercapacitors resulted in limited ionic mobility and they do not adhere strongly to electrode surfaces, which ultimately constrains device performance and structural integrity.

To overcome these drawbacks, Park et al. proposed an all-solid-state supercapacitor design that employs a specially engineered binder capable of forming covalent linkages with the electrolyte phase.¹⁴⁰ A cross-linked polymeric binder composed of poly(ethyl acrylate) with allyl methacrylate (ALMA) as a cross-linker was used as the binder. Their formulation based on poly(ethyl acrylate-co-allyl methacrylate) enables strong van der Waals interactions with conductive materials through conformal contact, while its inherent flexibility accommodates electrode structural changes and enhances the electrode–electrolyte interface as shown in Fig. 10e, while the accompanying ionogel, constructed from lithium perchlorate and the ionic liquid EMIM–TFSI dispersed within a PEGDA network, delivers high ionic conductivity, mechanical softness, and stability under ambient conditions. This covalent integration facilitates more efficient ion migration, resulting in a notable rise in specific capacitance (55.8 F g⁻¹ at 0.2 A g⁻¹, approximately 79% higher than the non-covalent system).

As shown in Fig. 10f, EIS of electrodes with various polymeric binders displayed a constant solution resistance; however, the charge-transfer resistance at the electrode–electrolyte interface was significantly lowered for the poly(X₆EA) binder. Specifically, R_ct decreased to ∼78% of the control without covalent bonding, highlighting enhanced interfacial charge transfer and improved electrode stability and GITT measurements further supported faster ion diffusion (Fig. 10g). To check the feasibility of the device for flexible supercapacitors PDMS was used as a substrate replacing aluminium foil and copper foil was used as a current collector which provides good flexibility to the overall device as shown in Fig. 10h. Hence, further electrochemical tests were carried out in bending conditions. The device demonstrated strong durability, preserving about 96% of its capacitance subsequently after 90 days of operation in air and remaining working after 500 bending cycles (Fig. 10i–l). Hence, chemical bonding at the electrode–electrolyte interface can be an effective route toward enhancing the electrochemical activity and consistency of flexible solid-state energy-storage systems.

Park et al. introduced a novel phase-transitional ionogel electrolyte based on an ionic liquid ([EMIM]⁺[NO₃]⁻) embedded in a crosslinked acrylamide (PAAm)/PEGDA network, which undergoes a reversible crystallization-melting transition at around ∼44 °C.¹⁴¹ When polymerized directly on the porous carbon electrode, it forms a strong, bonded interface that effectively acts as an interlayer, suppressing interfacial delamination. This phase transition drastically reduces resistivity from ∼2318 kΩ cm to ∼43 Ω cm and increases capacitance from ∼0.02 F g⁻¹ to ∼37.35 F g⁻¹, due to the activation of ionic mobility in the molten state. The device also achieves an energy density of ∼7.77 Wh kg⁻¹ at a power density of 4000 W kg⁻¹ in the operating mode, while retaining ∼87.5% of its capacitance after 3000 cycles, demonstrating both high performance and interfacial stability. Moreover, when cooled to the storage mode, the crystallized state significantly suppresses self-discharge: about 89.5% of the stored charge is retained after 24 h, highlighting how the phase-transitional interface acts as a controllable “gate” for ion transport.

Moreover, Liu et al. used directional freezing combined with solvent exchange to create an aligned nanocomposite ionogel, which reduces tortuosity at the electrode interface, achieving an ionic conductivity of 22.1 mS cm⁻¹ and a capacitance up to 176 F g⁻¹.¹⁴² Neoh et al. used irradiation-induced phase separation to create ionogels with IL-rich nanodomains, enhancing ionic transport and retaining mechanical stability.¹⁴³ The optimized ionogel exhibited an ionic conductivity of 5.51 mS cm⁻¹ and enabled the fabrication of a flexible supercapacitor with ∼103 F g⁻¹ at 0.5 A g⁻¹, an energy density of 122 Wh kg⁻¹, and outstanding cyclic stability (93% capacitance retention after 30 000 cycles). This phase-separated structure acts like a quasi-interlayer, improving interfacial ion transport while preserving flexibility. Wei et al. incorporated carbon nanotubes into a redox-active ionogel, forming an interpenetrating conductive network that promotes both ionic and electronic transport at the interface.¹⁴⁴ This structure improves capacitance and rate capability, acting like an artificial interlayer to reduce interfacial resistance. Finally, Moura and coworkers demonstrated that a flexible ionogel layer coating on electrodes can act as a soft interphase, strongly influencing the electrochemical performance.¹⁴⁵ Together, these studies highlight how structural alignment, phase separation, conductive additives, and interfacial coatings can be used to optimize ionogel/electrode interfaces for improved supercapacitor performance.

4.3 Opportunities, challenges and future perspectives of organic solid electrolytes

Based on the above discussion, organic solid electrolytes-including solid polymer electrolytes (SPEs), gel polymer electrolytes (GPEs), hydrogels, organogels, and ionogels-have attracted significant attention for next-generation solid-state supercapacitors due to their high flexibility, tunable ionic conductivity, and improved electrode–electrolyte interfacial compatibility.¹⁴⁶ Compared with conventional liquid electrolytes, these materials offer enhanced safety by eliminating highly flammable organic solvents, such as linear and cyclic carbonates. As a result, challenges commonly associated with liquid electrolytes, including electrolyte leakage, volatility, and short-circuit risks, can be significantly reduced in organic solid electrolyte systems.¹⁴⁷

Polymer-based gel electrolytes have demonstrated significant improvement in electrolyte stability and electrochemical performance. For example, Chiou et al. reported that incorporating a PVDF-HFP gel polymer into LiPF₆-based organic electrolytes effectively suppressed electrolyte decomposition and improved the cycling stability of lithium-ion capacitors, particularly at higher voltage windows of 2.5 to 4.0 V, achieving nearly 20% higher capacity retention after 10 000 cycles compared with conventional liquid electrolytes.¹⁴⁸ Similarly, Na et al. developed a ladder-structured poly(poly(ethylene oxide)-co-methacryloxypropyl)silsesquioxane (PEO-SQ) hybrid polymer gelator to fabricate high-performance ionogel electrolytes.¹⁴⁹ The synergistic integration of a thermally stable polysilsesquioxane backbone and ion-conducting PEO chains resulted in a hybrid ionogel exhibiting excellent thermal stability (∼400 °C), improved ionic conductivity, and enhanced cycling performance compared with conventional ionic liquid electrolytes. Collectively, these studies highlight the importance of polymer architecture and hybrid electrolyte design in improving the electrochemical performance and long-term stability of organic solid electrolytes.

Despite these promising developments, organic solid electrolytes still face several critical challenges that limit their large-scale practical implementation in solid-state energy storage systems. One major limitation is the relatively low ionic conductivity at room temperature, particularly for SPE systems, which strongly depends on polymer chain mobility and the underlying ion transport mechanisms. In addition, the limited electrochemical stability at high voltages restricts the compatibility of polymer electrolytes with widely used high-energy cathode materials such as LiCoO₂ (LCO), LiNiCoAlO₂ (NCA), LiNiMnCoO₂ (NMC), and LiNi_0.5Mn_1.5O₄ (LNMO). Currently, LFP remains the most commonly used cathode material in SPE-based systems due to its relatively lower operating voltage (∼3.8 V vs. Li⁺/Li). However, to fully exploit high-energy cathode materials such as NCA (∼4.3 V), NMC622 (∼4.5 V), and LiNi_0.5Mn_1.5O₄ (LNMO, ∼4.9 V), polymer electrolytes must exhibit significantly improved oxidative stability at higher voltage cut-offs.¹⁵⁰

Continuous electrolyte oxidation and impedance buildup at high-voltage cathodes remain significant issues. Furthermore, the inherent mechanical instability of polymer matrices and high interfacial resistance between electrodes and solid electrolytes can significantly hinder ion transport and reduce battery performance, particularly in high mass loading electrode configurations required for practical energy storage devices.¹⁵⁰ From an application perspective, organic solid electrolytes remain promising candidates for solid-state lithium batteries (SSLBs) due to their intrinsic advantages in safety, flexibility, and processability. Their ability to form intimate contact with electrode surfaces further enhances their suitability for flexible batteries, lithium-metal batteries, and next-generation energy storage systems. However, translating these electrolytes into practical battery configurations, such as high-mass-loading cathodes and pouch cell architectures, remains challenging. In such systems, maintaining continuous ion transport pathways and stable electrode–electrolyte interfaces becomes difficult, often leading to increased interfacial resistance and reduced electrochemical performance.

Therefore, evaluating newly developed organic solid electrolytes in practical pouch cell configurations is a crucial step toward demonstrating their real-world applicability. For instance, prototyping facilities such as the Battery Technology Research and Innovation Hub (BatTRI-Hub) at Deakin University enable the fabrication of multilayer Li metal pouch cells with an active area of up to ≈14 cm² using automated stacking systems (Fig. 11a).¹⁵⁰ Initial demonstrations of single-layer SSLB pouch cells (cathode|anode|cathode configuration) have achieved stable discharge capacities of approximately 7 mAh (Fig. 11b–e). However, further challenges remain in fabricating composite electrodes with high mass loading (≥1.5 mAh cm⁻²) while maintaining adequate film cohesion, strong adhesion to current collectors, and efficient utilization of active materials.

Fig. 11 (a) Robotic stacking unit used for pouch cell fabrication at the BatTRI-Hub, Deakin University. (b) PDADMA-TFSI : LiFSI : [C3mpyr][FSI] composite electrolyte. (c) Assembled Li metal pouch cell. (d and e) Charge–discharge profiles and cycling performance of a Li∣NMC111 pouch cell using the PDADMA-TFSI : LiFSI : [C3mpyr][FSI] electrolyte (0.18 : 0.59 : 0.23). The cell was cycled at C/20, 4 V cutoff, 50 °C, with an NMC111 loading of 9.5 mg cm⁻², reproduced with permission from ref. 150 Copyright 2020 Wiley.

Addressing these issues through improved electrode formulation, optimized coatings, and advanced materials design will be critical for translating organic solid electrolyte systems into high-energy practical battery devices.¹⁵¹ With this focus SPEs as the first solid-state electrolytes have been commercialized for electric vehicle applications, notably in the Bolloré Bluecar. Among them, polyethylene oxide (PEO)-based electrolytes have been widely utilized in electric vehicles, although their practical use is often limited by a relatively narrow electrochemical stability window (≤4 V) and the requirement for elevated operating temperatures.¹⁵² However, with continued research and improved material design, PEO-based systems have shown potential to operate at higher voltages, with studies indicating stable charging up to approximately 4.3 V.¹⁵³

In this context, future research should focus on the rational design of advanced polymer architectures with enhanced ion transport properties, improved oxidative stability for compatibility with high-voltage cathodes, and optimized electrode–electrolyte interfaces. Strategies such as molecular engineering of polymer backbones, incorporation of inorganic or hybrid frameworks, advanced interface engineering, and computational-guided electrolyte design are expected to play key roles in overcoming current limitations.¹⁵³ Furthermore, translating these materials into high-energy-density solid-state battery prototypes with practical electrode loading and scalable fabrication processes will be essential for the commercialization of organic solid electrolyte-based energy storage technologies.¹⁵³

5. Inorganic solid electrolytes

Inorganic solid-state electrolytes have recently been considered for supercapacitor applications; however, they are less explored compared to polymer-based solid electrolytes. Solid-state ionic conductors contain mobile ions embedded within frameworks made of metal and non-metal ions, which generally assemble into ligand-coordinated polyhedra forming the material's structural backbone.¹⁵⁴ To date, more than half of the periodic table's elements have been utilized in designing such solid-state ion-conducting materials. Many different charged species are capable of migrating through solid materials, ranging from monovalent ions, like H⁺, Li⁺, Na⁺, K⁺, Cu⁺, and Ag⁺, to divalent cations like Mg²⁺ and anions including F⁻, Cl⁻, and O²⁻.¹⁵⁵ To build the structural framework that supports the ion movement, inorganic solid electrolytes commonly rely on metal cations that are electronically insulating, such as Ti⁴⁺, Zr⁴⁺, Nb⁵⁺, and Ta⁵⁺-elements, as they lack d electrons in their electronic configuration and therefore minimize electronic conductivity. Elements from groups 13 (Al³⁺, Ga³⁺), 14 (Si⁴⁺, Ge⁴⁺), and 15 (P⁵⁺) also play a major role in constructing the host lattice.¹⁵⁵ These ions form coordination units-tetrahedral, octahedral, cubic, or higher-coordination polyhedra that link together in various patterns to establish the crystal's backbone.¹⁵⁵ At present, the most extensively researched inorganic solid electrolytes fall into two main categories: oxide and sulfide solid electrolytes. Oxide electrolytes include NASICON-type, perovskite-type, and garnet-type materials. Sulfide electrolytes, on the other hand, can be classified as glassy sulfides, glass-ceramic sulfides, and crystalline sulfides.⁷²

Fig. 12a illustrates how the ion-conduction mechanisms in solid-state conductors differ greatly from those in liquid electrolytes and gel electrolytes. In aprotic electrolytes, lithium ions experience a nearly uniform environment due to rapid exchange between the solvating molecules and the bulk solvent.¹⁵⁶ As a result, the potential energy landscape for these mobile ions is relatively flat (Fig. 12a). By contrast, in crystalline solids, the movement of ions must occur through a series of bottleneck points (Fig. 12b).^157,158 These bottlenecks create an energy barrier that separates the crystallographic sites for lithium. This barrier, commonly referred to as the migration energy (E_m) or motional energy, is a key factor controlling ionic transport: lower migration energies correspond to higher ionic mobility and increased conductivity. In addition, ionic conductivity in crystalline solids is influenced by lattice defects such as interstitials, vacancies, and partially occupied sites. In stoichiometric ion conductors, the concentration of these defects is dictated by the ionic energy gap or the defect formation energy. Additionally, defects can be introduced intentionally by substituting aliovalent cations, with their stability and formation determined by the trapping energy, which defines the extrinsic regime.¹⁵⁸

Fig. 12 Ion-conduction mechanism in (a) solid electrolyte and (b) liquid electrolyte. Reproduced with permission from ref. 155 Copyright 2016 American Chemical Society.

Typically, these materials are rigid and lack flexibility, limiting their use in bendable or wearable devices. However, they exhibit excellent mechanical strength, chemical stability, elevated intrinsic ionic conductivity, and extreme thermal tolerance, making them strong contenders for high-performance, safe solid-state supercapacitors. With proper interface engineering, they can deliver stable charge–discharge performance over extended cycles.¹⁵⁹ Several inorganic solid electrolytes, including Li₂S–P₂S₅,¹⁶⁰ LATP,⁶⁴ LLTO,^161,162 LiClO₄Al₂O₃,¹⁶³ LLZO,¹⁶⁴ Li_9.6P₃S₁₂, and LGPS,¹⁶⁵ have been studied for solid-state supercapacitor applications. Francisco et al. reported a Li₂S–S–PS₅ inorganic solid electrolyte functioning as both an ion conductor and a separator. This material exhibited high Li-ion transport, and devices incorporating nanostructured electrodes delivered a specific capacitance of 7.75 F g⁻¹.¹⁶⁰ Ulihin et al. introduced a composite electrolyte composed of 0.4LiClO₄–0.6Al₂O₃, which was suitable for both symmetric and asymmetric supercapacitor configurations.¹⁶³ Using oxide electrodes such as LiMn_1.5Ni_0.45Mg_0.05O₄, Mn₂O₃, and MnO₂ the device achieved a specific capacitance of 29 F g⁻¹ at 150 °C. In another study, Inguama et al. prepared a tetragonal Li_0.5La_0.5TiO₃ (LLTO) electrolyte via a solid-state reaction process. The Li_2/3−xLa_3xTiO₃ (LLTO) ceramic exhibited ionic conductivities in the range of 10⁻⁵ to 10⁻³ S cm⁻¹ at room temperature.¹⁶² Additionally, Hu et al. developed a three-dimensional layered carbon framework (porous/dense/porous) combined with a Li_1.3Al_0.3Ti_1.7P₃O₁₂ ceramic electrolyte incorporating LiMnPO₄ in solid-state supercapacitors. The device delivered a maximum capacitance of 0.13 F cm⁻¹ at a low scan rate of 2 mV s⁻¹.¹⁶⁶ A NASICON-type LATP electrolyte, specifically, has been used as solid electrolyte Li_1.4Al_0.4Ti_1.6(PO₄)₃.⁶⁴ By addition of carbon nanotubes to LATP, electrodes are prepared and fabricated for EDLC devices using LATP as the separator. These devices achieved specific capacitances of 0.52 mF cm⁻³ and 11.59 mF cm⁻³ once the CNT content was raised to 7.5%.

5.1 Interface engineering for inorganic solid electrolytes

As discussed earlier, ceramic ionic conductors typically suffer from low ionic conductivity, limiting their direct use in electrochemical devices. Recent efforts to make ceramic solid-state electrolytes more compatible with flexible supercapacitors have focused on interlayer strategies. The point-to-point contact of these electrolytes can be converted to enclosed surface contact by incorporating polymer electrolytes or ILs as interlayers on both sides of inorganic solid electrolytes, providing a soft feature at the interface and significantly enhancing the ionic conductivity. For instance, in a recent report, a NASICON-type ceramic electrolyte is used in which an ultrathin, gel-like solvent layer is introduced at the ceramic/electrode interface. This interfacial layer greatly enhances contact, reduces charge-transfer resistance, and boosts capacitance and long-term cycling stability in Na⁺-ion supercapacitors.¹⁶⁷ Additionally, research inspired by solid-state battery technology has shown that incorporating soft polymer phases alongside ceramic particles can improve both mechanical compliance and interface adhesion without severely compromising conductivity.¹⁶⁸ These hybrid designs bridge the gap between the rigid yet high-conductivity ceramic framework and the soft, deformable needs of flexible devices, thereby enabling improved interfacial ion transport and resilience under mechanical strain.

Recent studies have demonstrated the effectiveness of IL–ceramic composite electrolytes (having IL content ≤13 wt%) in supercapacitor applications.⁵⁷ For instance, Li⁺-conducting fast ionic ceramics such as LiTi₂(PO₄)₃ (LTP) and Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) dispersed with ILs have been employed as electrolytes, paired with activated charcoal-coated electrodes in 2032 coin cells as shown in the cell assembly photographs in Fig. 13a. Device performance strongly depends on the IL choice (ion size) and the composition of the ceramic electrolyte; 13 wt% EMIM BF exhibited the highest conductivity, as shown in Fig. 13b. A supercapacitor using LATP with 13 wt% EMIM BF₄ at ∼35 °C shows a high specific capacitance of ∼181 F g⁻¹ (Fig. 13c), energy density of ∼6.1 Wh kg⁻¹, and power density of ∼140 W kg⁻¹ (Fig. 13d) at 0.56 A g⁻¹ and 1 V. The composite exhibits remarkable cycling stability, maintaining ∼99% coulombic efficiency over ∼13 000 charge–discharge cycles at 0.56 A g⁻¹ (Fig. 13e), and shows an enhanced capacitance of up to ∼600 F g⁻¹ at 100 °C. At elevated currents (≥0.34 A g⁻¹), a typical EDLC mechanism is observed. A practical demonstration shows that a two-cell stack can successfully power a 3 V white LED for ∼30 minutes (Fig. 13f). These results highlight the potential of IL–ceramic composites for high-performance, thermally robust supercapacitors.

Fig. 13 (a) Schematic of the fabrication stages for the SSC using activated carbon-coated copper foil electrodes and IL-dispersed Li⁺-NASICONs, (b) ionic conductivity of LATP containing 13 wt% different ILs measured at three temperatures, (c) charge–discharge profiles of the SSC with LATP-13 EMIM BF₄ electrolyte recorded over various cycles at ∼35 °C, (d) evolution of specific capacitance and ESR with the cycle number at ∼35 °C for the SSC using LATP-13 EMIM BF₄, with the inset showing coulombic efficiency up to 13 000 cycles, and (e) photographs of an LED illuminated using two coin cells connected in series-one using EMIM BF₄ electrolyte (5 min) and the other using LATP-13 EMIM BF₄ electrolyte (25 min), reproduced with permission from ref. 57 Copyright 2022 Elsevier.

Sharma et al. highlighted the importance of interface engineering in all-solid-state supercapacitors by incorporating a PEO–LiClO₄ polymer phase into activated carbon electrodes to improve contact with a NASICON-type solid ceramic electrolyte (Li_1.3Al_0.3Ti_1.7(PO₄)₃).⁷¹ This modification led to a smoother and more uniform interface, reducing interfacial resistance and enhancing ion transport. SEM characterization confirmed enhanced wetting of the electrolyte on the electrode surface with a significant decrease in charge-transfer resistance. Electrochemical testing confirmed a specific capacitance of ∼102 F g⁻¹ at 1.5 A g⁻¹ (2 V), along with stable performance over repeated cycling, indicating effective interface stabilization.

5.2 Opportunities, challenges, and future perspectives of inorganic solid electrolytes

When examining the hierarchy of solid electrolyte systems, inorganic solid electrolytes compared with solid polymer electrolytes have outperformed due to their wider electrochemical stability window, superior thermal stability, and higher ionic conductivity.^169,170 These characteristics make them promising candidates for high-energy-density solid-state battery systems. However, despite these advantages, inorganic solid electrolytes still face several critical challenges that have obstructed their practical implementation.

One of the major limitations is the large interfacial resistance between the solid electrolyte and electrode materials. Since both the electrolyte and the electrode are in the solid phase, their interface involves solid–solid contact, which often leads to poor physical contact and sluggish ion-transport kinetics.¹⁷⁰ This weak interfacial interaction significantly restricts the overall electrochemical performance of solid-state batteries, as discussed in previous sections. In addition, air sensitivity is another critical drawback of many inorganic solid electrolytes, particularly sulfide- and oxide-based systems, which complicates their handling, processing, and large-scale application.¹⁵³ Although inorganic solid electrolytes exhibit high ionic conductivity at room temperature, comparable to that of conventional liquid electrolytes, their chemical instability in humid environments remains a major concern. For example, argyrodite-type Li₆PS₅Cl exhibits an ionic conductivity of approximately 4.6 × 10⁻⁴ S cm⁻¹, while thio-LISICON-type Li₁₀GeP₂S₁₂ (LGPS) can achieve ionic conductivities as high as 1.2 × 10⁻² S cm⁻¹, approaching those of liquid electrolytes used in conventional lithium-ion batteries. However, the presence of sulfur-containing species makes these materials highly reactive toward moisture. Upon exposure to humid air, sulfur ions undergo hydrolysis due to the weak Li–S bonding, resulting in the formation of toxic H₂S gas (Fig. 14a).¹⁷¹

Fig. 14 Air sensitivity of solid-state electrolytes (SSEs): (a) schematic illustration of the reaction between Li₂S and H₂O. (b) Formation of Li₂CO₃ upon air exposure and its role in increasing the interfacial resistance between LLZO and Li metal, reproduced with permission from ref. 171 Copyright 2027 American Chemical Society. (c) C K-edge XAS spectra of Li₂CO₃, air-exposed LLZO, and Ar-treated LLZO recorded in TEY and TFY modes. Reproduced with permission from ref. 172 Copyright 2024 Wiley. (d) EIS profile of the Li|LALZOBr_0.15-600|Li cell. (e) Cross-sectional SEM images of Li|LALZOBr_0.15-600, reproduced with permission from ref. 173 Copyright 2023 Elsevier. (f) Schematic depiction of enhanced interfacial contact between garnet electrolyte and Li metal via surface modification with a thin Ge layer. Reproduced with permission from ref. 174 Copyright 2017 Wiley. (g) EIS comparison of Li–LLZO interfaces with and without Al coating, and (h) cross-sectional SEM image demonstrating uniform bonding of Li metal with LLZO facilitated by ALD-deposited Al₂O₃, reproduced with permission from ref. 175 Copyright 2017 Nature Materials.

This instability can also be explained by the structural composition of many sulfide electrolytes, which can be considered as combinations of Li₂S and metal sulfides (M_xS_γ). Since Li₂S is highly sensitive to moisture, exposure to water leads to rapid decomposition and H₂S gas evolution.^176,177 In addition to sulfide electrolytes, oxide-based solid electrolytes are also affected by air exposure. The underlying mechanism involves H⁺/Li⁺ exchange within the crystal lattice, which leads to the formation of surface contaminants such as Li₂CO₃ and LiOH. These species significantly increase interfacial resistance and shorten battery lifetime. Garnet-type Li₇La₃Zr₂O₁₂ (LLZO), initially considered air-stable, was later found to undergo surface reactions under ambient conditions. Studies by Sakamoto and co-workers demonstrated that the formation of Li₂CO₃ and LiOH on the LLZO surface reduces the wettability between LLZO and the lithium metal anode, thereby increasing interfacial resistance. Removal of these surface layers through wet polishing followed by heat treatment significantly improved lithium wetting and reduced interfacial impedance (Fig. 14b).¹⁷¹

Similarly, a simple physical polishing method can be done to eliminate the Li₂CO₃ surface layer and achieve a compact LLZO/Li interface (Fig. 14c).¹⁷² Electrochemical impedance spectroscopy measurements of Li‖garnet symmetric cells confirmed that these surface carbonate layers are primarily responsible for poor interfacial contact between garnet and lithium metal (Fig. 14d).¹⁷³ Ma et al. reported improved interfacial contact observed in Li|LALZOBr_0.15 systems by polishing, where a close and gap-free interface was observed (Fig. 14e).¹⁷³ Furthermore, long-term exposure to air can lead to the formation of additional impurity phases such as La(OH)₃ and Li(OH)·H₂O, which further increase interfacial resistance and reduce ionic conductivity due to reactions occurring at grain boundaries.¹⁵³

Beyond sulfide and oxide systems, thin-film solid electrolytes such as lithium phosphorus oxynitride (LiPON) also exhibit sensitivity to ambient conditions. Freshly deposited LiPON films have been observed to gradually darken during prolonged air exposure, indicating chemical reactions with atmospheric components, most likely moisture and carbon dioxide, which ultimately lead to electrolyte degradation.¹⁵³ To address these challenges, several materials engineering strategies have been proposed. One important approach involves defect engineering through aliovalent and isovalent doping, which can regulate defect concentrations and enhance ion transport in crystalline electrolytes such as Na₃PS₄, LLZO, and LISICON-type systems (Fig. 14f–h).^174,175 In aliovalent doping, vacancies are introduced by substituting ions with different valence states to maintain charge neutrality. For instance, in Na₃PS₄, vacancies can be generated by replacing Na⁺ with higher-valent cations such as Ca²⁺, or by substituting S²⁻ with lower-valent anions such as Cl⁻. When Ca²⁺ occupies the Na site in cubic Na₃PS₄, the resulting Na⁺ vacancies facilitate inter-site ion hopping and reduce the migration energy barrier from 0.21 eV to 0.19 eV, thereby enhancing ionic conductivity.^174,178,179 In addition to aliovalent substitution, isovalent doping is another effective strategy to enhance ion mobility.¹⁷⁹ This approach typically involves replacing framework atoms such as P or S with larger ions, thereby expanding the lattice and widening ion diffusion pathways. Such structural modifications not only improve Na⁺ transport but may also enhance other properties, including electrochemical stability and air tolerance. Therefore, controlled doping strategies represent a promising route for optimizing the ionic conductivity and stability of inorganic solid electrolytes.

Interface engineering has emerged as a critical strategy for improving the performance of solid-state electrolytes, particularly through advanced surface modification techniques such as plasma-enhanced chemical vapor deposition (PECVD), thermal evaporation, and atomic layer deposition (ALD) (Fig. 14g).¹⁷⁹ For example, the surface properties of LLZO can be effectively tailored to transform its behaviour from super-lithiophobicity to super-lithiophilicity through the application of thin semiconductor coatings. Another key factor influencing ion transport in inorganic solid electrolytes is the presence of grain boundaries. In chalcogenide electrolytes such as Na₃PS₄, the blocking effect of grain boundaries is generally less pronounced than in oxide electrolytes due to the relatively soft lattice and favourable intergranular ion conduction pathways.^178,180 Nevertheless, grain boundaries can still influence overall ionic conductivity. The accumulation of point defects, such as Na⁺ vacancies, near grain boundaries can disrupt continuous ion migration, particularly in systems where ion transport is predominantly defect-mediated.¹⁷⁸ In addition, structural variations at the grain boundary core may act as trapping sites for Na⁺ ions, further limiting ion mobility. Consequently, the experimentally measured ionic conductivity is often lower than theoretical predictions. To address these limitations, several strategies have been proposed, such as increasing grain size, enhancing amorphous phase fraction, and implementing grain-boundary engineering to improve ionic conductivity of Na₃PS₄.^181,182 Furthermore, crystallite nanosizing and disorder induction through mechanochemical ball-milling have emerged as promising methods for enhancing ion transport, particularly in borohydride-based solid electrolytes.¹⁷⁸ Continuous mechanical impacts during ball-milling reduce crystallite size and induce structural disorder, enabling the formation of conductive disordered phases that are typically stable only at elevated temperatures. Stabilizing these metastable phases at room temperature significantly improves ionic conductivity and overall electrolyte performance.¹⁷⁸

Overall, inorganic solid electrolytes offer significant opportunities for next-generation solid-state batteries due to their high ionic conductivity, wide electrochemical stability window, and excellent thermal stability. Despite these advantages, several critical challenges remain, including interfacial resistance, air instability, grain-boundary effects, and processing limitations, which provide opportunities to modify and design interface electrolytes, and microstructural optimization to develop new solid electrolytes, which will be essential for obtaining high-performance solid-state batteries. In particular, translating laboratory-scale materials into practical battery configurations with high mass loading, thin electrolyte layers, and scalable manufacturing processes remains a key step toward commercialization.¹⁷⁸ Furthermore, translating these materials into thin electrolyte membranes and high-energy-density solid-state battery configurations with scalable manufacturing techniques will also be crucial for their commercialization.

6. Composite polymer electrolytes

To bridge the gap between purely inorganic and purely polymeric organic electrolytes, composite solid electrolytes (CSEs) have emerged as promising candidates for supercapacitors. Dispersing inorganic fillers within polymer matrices combines the mechanical robustness and electrochemical stability of ceramics with the flexibility and processability of polymers.⁴⁰ This synergistic design not only mitigates the limitations of each component but also enables significant enhancements in ionic transport and structural stability.^40,98 The simplest method for producing composite electrolytes involves mechanically blending the polymer with the inorganic filler in a suitable solvent. These inorganic fillers are generally categorized into two types: (i) inert or passive fillers, such as TiO₂,⁴⁷ SiO₂,¹⁸³ Al₂O₃,¹⁸⁴ ZnO,,¹⁸⁵ ZrO₂,¹⁸⁶ palygorskite,¹⁸⁷ and nanoclay¹⁸⁸ and (ii) active fillers Li₁₀GeP₂S₁₂ (LGPS),¹⁸⁹ LLZO,¹⁹⁰ LLTO,¹⁹¹ Li_1.4Al_0.4Ti_1.6(PO₄)₃ (LATP),¹⁹² and Li-ion conducting MOFs.¹⁹³

Recent studies demonstrated that the strategic incorporation of these fillers into the polymer matrix can markedly improve the physicochemical and electrochemical properties of the resulting CSEs. Inert fillers, such as Al₂O₃, SiO_2, or TiO₂, which are non-Li-ion conductors, are often added to polymer–salt matrices to disrupt polymer crystallinity, increase free volume and segmental mobility, and thus enhance salt dissociation and ionic conduction.¹⁹⁴ By reducing polymer crystallinity and facilitating Lewis-acid–base interactions between filler surface groups and polymer/salt species, these passive fillers can increase the concentration and mobility of free ions in the polymer matrix.¹⁹⁵ On the other hand, active fillers ceramics that themselves conduct Li⁺, such as LLZO, LiLaTiO₄ (LLTO), Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP), Li₁₀GeP₂S₁₂ (LGPS), and related NASICON-type or garnet-type ceramics, have been extensively researched.¹⁹⁴ When dispersed within polymer matrices, these active fillers may not only impart mechanical strength and stability, but also potentially contribute additional continuous ion-conduction pathways beyond the polymer phase and the ceramic solid phase.

Recent studies have shown that optimizing filler concentration, particle size, and interfacial compatibility plays a decisive part in influencing the performance of CSEs.⁹⁸ Numerous experimental studies illustrate the benefits of CSEs. For instance, a composite polymer electrolyte based on LLZO nanofibers dispersed in a PVDF-HFP matrix achieved ∼9.5 × 10⁻⁴ S cm⁻¹ at room temperature when 10 wt% LLZO nanofibers were used, attributed to continuous 1D Li⁺ conduction paths through the nanofiber network.¹⁹⁶ In a recent study incorporating LLZTO (a variant of LLZO with doping) into PVDF-HFP/LiTFSI polymer electrolyte, ionic conductivity reached ∼8.2 × 10⁻⁴ S cm⁻¹ at 60 °C, along with good interface stability with the lithium metal anode.¹⁹⁷ Experimental ionic conductivity has also reached 10⁻⁴–10⁻³ S cm⁻¹ by incorporating ceramic filler content below the percolation threshold, which suggests that filler-induced polymer structure modification and interfacial conduction pathways may contribute significantly.

A recent study reported a PVDF/PEO-based ceramic CPE incorporating various loadings of LLTO (10 to 40 wt%) to enhance ionic transport for hybrid supercapacitor applications.¹⁹¹ Structural FTIR, microscopy, thermal TGA, and electrochemical impedance spectroscopy analyses revealed that increasing LLTO content progressively disrupted the PVDF/PEO crystalline domains, increased amorphous characteristics, and improved polymer-segmental mobility. The optimized composition with 40 wt% LLTO exhibited a markedly high electrolyte acceptance of ∼275% and an enhanced ionic conductivity of 3.94 mS cm⁻¹. Hybrid supercapacitor coin cells fabricated with this electrolyte delivered a high specific capacitance of 182 F g⁻¹ and demonstrated stable cycling for up to 10 000 cycles, outperforming lower-filler compositions. These results highlight the effectiveness of LLTO-based ceramic reinforcement in PVDF/PEO matrices for achieving high-performance solid-state hybrid supercapacitors.

6.1 Transport mechanism in composite polymer electrolytes

Ionic conductivity is the key metric for assessing CSEs, making it essential to closely examine the underlying ion-transport mechanisms. These fillers were introduced first to reduce polymer crystallinity and thereby increase the amorphous phase fraction-where ionic transport is more favourable. However, later studies have shown that filler incorporation not only suppresses crystallization but also induces beneficial interfacial interactions and surface effects that further enhance ion mobility.¹⁹⁸ As illustrated in Fig. 15a, Li⁺ transport in PEO–inert-filler composites can occur through two main pathways: (path 1) along the PEO chains via their segmental motion and (path 2 Fig. 15b) through the interfacial regions formed between PEO and the filler particles. Because the presence of fillers significantly boosts ionic conductivity, the interfacial pathway is considered the dominant contributor to this improvement.¹⁰⁰ Polymer–ceramic interfaces are essential for improving Li⁺ transport in these electrolytes. They limit the recrystallization of PEO, increasing the amorphous fraction and thus boosting ionic conductivity at room temperature (Fig. 15c). Moreover, these interfaces help release additional free Li⁺ and create fast ion-migration pathways. This occurs through coordinated interactions among Lewis acid sites on the ceramic surface, Lewis base oxygen atoms on the polymer chains, and the anions from the lithium salt (Fig. 15d), while in the case of PEO–active-filler composites, there is an additional fast Li⁺ transport pathway compared with PEO–passive-filler systems. As illustrated in Fig. 15b, Li⁺ ions can migrate via the PEO matrix (path 1), along the polymer–ceramic interfaces (path 2), and directly through the active ceramic fillers themselves (path 3).

Fig. 15 (a) Two proposed Li⁺ transport pathways in PEO composites containing passive fillers, (b) three Li⁺ transport pathways in PEO composites with active fillers, (c) the role of passive fillers in disrupting the ordered structure of PEO, thereby suppressing recrystallization, and (d) Lewis acid–base interactions at the PEO–ceramic interface, involving Al–X⁻, Al–O, O–Li⁺, and Li⁺–X⁻ species, reproduced with permission from ref. 100 Copyright 2021 Springer Nature.

In the case of active fillers, they provide additional functions that further boost ionic conductivity.¹⁹⁹ Unlike inert fillers, they can significantly restructure the polymer–filler interface. The resulting interfacial regions, which may extend to about twice the filler radius, create percolative pathways that dominate the enhancement of conductivity.^199,200 Li et al. proposed that active fillers generate space-charge layers at these interfaces, causing Li⁺ to accumulate on one side.²⁰¹ When these space-charge zones link across neighbouring nanoparticles, they form continuous, high-conductivity Li-ion pathways, greatly improving overall ionic transport. Another important polymer/filler interfacial mechanism is the space-charge effect, in which contact between an active ceramic filler and a polymer disturbs local electroneutrality, creating a nanometer-thick charged layer with altered ion concentration. This interfacial zone can host a significantly higher density of mobile Li⁺ than the bulk, thereby forming fast-ion pathways throughout the composite. For example, Guo et al. suggested that in PEO/LLZO systems, Li⁺ migrates from LLZO lattice sites toward the surface upon contact with the polymer, producing Li-rich space-charge layers that greatly enhance interfacial conduction.²⁰¹ When these charged regions connect across multiple LLZO nanoparticles or aligned ceramic structures, they form continuous high-conductivity networks (Fig. 16), contributing substantially to the overall ionic transport in polymer–ceramic composites.²⁰² Moreover, an optimal number of active fillers can create interconnected conductive pathways that facilitate Li⁺ diffusion through the fillers. However, exceeding this optimal amount can reduce ionic conductivity due to irregular agglomeration of the fillers.²⁰³

Fig. 16 Schematic illustration of Li⁺ migration from LLZO lattice sites to the polymer interface, leading to the formation of Li-rich space-charge layers. Reproduced with permission from ref. 202 Copyright 2017 American Chemical Society.

6.2 Interface engineering for composite solid electrolytes

Even though the ionic conductivity of composite electrolytes has significantly improved, interfacial challenges remain critical. Composite systems generally provide more stable interfaces than fully inorganic solid electrolytes and can help suppress lithium-related instability, yet key issues persist. In particular, the stability of the cathode interface, reactions at the anode–electrolyte interface, and the risk of dendrite penetration still pose obstacles. When the electrolyte components come into contact with lithium metal in a composite electrolyte, various organic and inorganic lithium compounds are generated, with the specific reactions depending on the polymer and lithium salt used. For example, AFM and FTIR studies showed that in a PEO–LiSO₃CF₃ system, CF₃ radicals form at the lithium surface.²⁰⁴ These radicals abstract hydrogen from the polymer backbone, after which lithium further breaks the C–O bonds, producing Li–O–R species, while in PVDF–LiX systems (X = FSI, TFSI, ClO₄), interfacial compounds include LiF, Li₂S, Li₂O, and Li₂CO₃/LiOH for X = FSI, and LiF, Li₂O, and Li₂CO₃/LiOH for X = TFSI, which are inorganic-rich SEI layers and are beneficial for suppressing lithium dendrite growth.²⁰⁵

Addressing the interfacial issues in the composite electrolyte system is therefore essential for further performance enhancement and enabling its practical application. Yet, despite their importance, only a limited number of recent studies have systematically addressed interfacial challenges in CSE-based supercapacitors. For example, Neha and Dalvi reported that activated carbon electrodes combined with a Na₃Zr₂Si₂PO₁₂ (NZSP) dispersed polymer electrolyte membrane show improved performance for supercapacitor applications.²⁰⁶ NASICON structured NZSP (Na₃Zr₂Si₂PO₁₂) is synthesized using the solid-state reaction route and the composite solid polymer electrolyte (CSPE) is synthesized by the solution casting route, as shown in Fig. 17a–i. Introducing a tiny volume of acetonitrile as a “solvent layer” at the interface, along with better-surface-area activated carbon, establishes smooth contact and also eliminates the residual pores or cavities on the surface, as shown in Fig. 17j and k. Raman spectroscopy illustrates the importance of acetonitrile addition towards the local structure of CSPEs (Fig. 17l). In the untreated PEO-based CSPE, characteristic Raman peaks indicate an ordered, crystalline polymer structure. Addition of NZSP and salt reduces crystallinity, broadening and decreasing the intensity of the peaks. When acetonitrile is introduced at the surface, Raman signals become substantially weaker, suggesting the formation of a localized gel-like layer where the polymer and solvent coexist. This gel layer improves interfacial contact with electrodes, facilitating enhanced ion transport between the electrode–electrolyte interface. The interface modified cell shows a specific capacitance of ∼260 F g⁻¹ and high specific power (∼4780 W kg⁻¹) at 3 V/5 mA, with ∼99% coulombic efficiency and ∼90% capacitance retention after 10 000 cycles. Multi-cell stacks successfully power an 8 V LED for over 30 minutes, confirming their suitability for practical applications. The solvent layer promotes the formation of a local gel-like structure, improving coupling between the electrode and SPEs and facilitating faster ion transport at the interface, which highlights interface engineering and solvent incorporation as effective strategies to enhance solid-state supercapacitor performance.

Fig. 17 Schematic of the CSPE membrane preparation and supercapacitor assembly: (a) preparation of the composite electrolyte slurry, (b) hot pressing to form the CSPE membrane, (c) flexible CSPE membrane, (d) acetonitrile treatment of the CSPE surface, (e and f) supercapacitor parts assembled, (g and h) diagram and a photograph of the sandwich-type supercapacitor, and (i) the final fabricated device. Cross-sectional SEM images illustrating interfacial effects, (j) without and (k) with acetonitrile treatment, and (l) Raman spectra of the polymer electrolyte surface before and after acetonitrile application, reproduced with permission from ref. 167 Copyright 2025 Royal Society of Chemistry.

Recent studies have shown that incorporating a small amount of polymer into activated carbon-based electrodes can also significantly improve the electrode–electrolyte interface in solvent-free all-solid-state supercapacitors. For example, Sharma et al. incorporated a PEO–LiClO₄ polymer into an AC electrode medium in combination with a Li_1.3Al_0.3Ti_1.7(PO₄)₃ (LATP) NASICON-based composite solid polymer electrolyte (CSP membrane), which provides smoother interfacial contact, thereby enhancing ionic transport.⁷¹

Fig. 18a and b show the photograph of the CSP membrane and the device fabricated with polymer-added electrodes. The membrane displays a consistent thickness throughout and possesses good mechanical flexibility. When impedance-derived real (C′) and imaginary (C″) capacitance components were evaluated from the frequency-dependent Z′ and Z″ values using standard relations, the C″–ω plot displayed a distinct relaxation peak corresponding to the characteristic relaxation time (τ = 2π/ω_max), associated with ion movement between the electrode–electrolyte interface and penetration into the activated-carbon pores. At this peak, C′ typically shows an inflection. Comparison between the activated carbon electrode deprived of polymer (ACE) and activated carbon electrode having the polymer (P-ACE) revealed that the P-ACE cell displayed a better relaxation frequency and a markedly shorter relaxation time (∼14 s vs. ∼33 s) as shown in Fig. 18c, indicating that polymer incorporation in the electrode facilitates faster interfacial ion dynamics. This strategy enables a specific capacitance of ∼102 F g⁻¹ at 1.5 A g⁻¹, with an operating voltage of 2 V near room temperature (Fig. 18d), along with improved cycling stability, coulombic efficiency, specific energy, and power compared to conventional activated carbon electrodes.

Fig. 18 (a) Photograph of the as-prepared free-standing CSPE membrane (40% LATP) used for device assembly, (b) all-solid-state supercapacitor (ASSC) fabricated with polymer-modified electrodes, (c) frequency-dependent real (C′) and imaginary (C″) components of capacitance for the ASSC employing P-ACE electrodes, and (d) corresponding GCD curves compared with those of ASSCs using conventional ACE electrodes, reproduced with permission from ref. 71 Copyright 2023 MDPI.

6.3 Opportunities, challenges and future perspectives of composite solid electrolytes

Despite significant progress in the development of composite solid electrolytes (CSEs) and their interfaces, their practical implementation in all-solid-state energy storage systems remains at an early stage.^207,208 One of the major challenges is the limited understanding of ion transport mechanisms in heterogeneous composite systems, particularly at the complex interfaces between polymer matrices, inorganic fillers, and electrodes.²⁰⁸ These interfacial regions often dominate ionic migration behavior but remain difficult to characterize. In addition, the ionic conductivities of most CSEs at room temperature remain in the range of 10⁻⁵–10⁻⁴ S cm⁻¹, which is still lower than that required for practical devices and comparable liquid electrolytes. Enhancing ionic conductivity through rational polymer design, filler engineering, and structural optimization, therefore, remains a key research direction.

Another major limitation arises from the complex processing requirements compared with conventional liquid electrolyte systems. For example, polymer-based and composite solid electrolytes, particularly those based on poly(ethylene oxide) (PEO), often exhibit poor wetting behavior toward electrode surfaces.²⁰⁸ As a result, achieving intimate interfacial contact between the electrolyte and electrode layers requires additional processing techniques such as hot pressing, calendaring, rolling, or hot isostatic pressing, which increase fabrication complexity.²⁰⁸ To address these challenges, several advanced processing routes have been developed for composite solid electrolytes. A common approach involves slurry mixing followed by tape casting, where the electrolyte components are dispersed in a solvent to form a homogeneous slurry that can be cast into thin films.²⁰⁹ After solvent evaporation, the resulting green film is further processed through lamination or low-temperature sintering to improve mechanical integrity and ionic transport pathways, as illustrated in Fig. 19a.²⁰⁹ In some cases, aerosol deposition techniques are employed to fabricate dense solid electrolyte layers directly onto electrode surfaces. This method enables the formation of high-density electrolyte films without requiring high-temperature co-sintering, thereby minimizing undesirable side reactions between the composite electrolyte and electrode materials while improving interfacial adhesion.

Fig. 19 Schematic of fabrication process sequences for (a) cathode-supported cells and (b) tri-layer cells. Reproduced with permission from ref. 209 Copyright 2019 Royal Society of Chemistry.

Another emerging strategy involves the fabrication of multilayer or tri-layer composite electrolyte structures through sequential tape casting (Fig. 19b).²⁰⁹ In this approach, electrolyte layers are either prepared as separate green sheets and subsequently laminated or directly cast layer-by-layer with intermediate solvent evaporation.²⁰⁸ Such architectures enable tailored ion-conduction pathways and improved mechanical stability, enhancing the structural integrity of energy storage devices during operation.

Although these processing techniques introduce additional manufacturing steps compared to conventional liquid electrolyte systems, they simultaneously provide significant opportunities for materials engineering and interface optimization. The ability to precisely control layer thickness, composition, and structural architecture enables the design of hierarchical composite structures with enhanced ionic conductivity, improved electrode–electrolyte interfacial contact, and better mechanical durability.²⁰⁸ Furthermore, these approaches open possibilities for integrating multiple solid electrolyte materials within a single architecture, thereby allowing the synergistic combination of their individual advantages.

Another critical challenge is the interfacial stability between CSEs and electrode materials, including chemical, electrochemical, thermal, and mechanical compatibility during long-term cycling.^210,211 Interfacial resistance, dendrite formation, and degradation under high current densities and high voltages and in wide temperature ranges can significantly affect device performance.²¹² Future studies should therefore focus on advanced interface engineering strategies, including protective interlayers, surface modifications, and multilayer architectures, to improve interfacial compatibility and extend the electrochemical stability window.

From a practical perspective, both economic and technological feasibility remain significant barriers to the commercialization of CSEs. The fabrication of CSEs often involves complex synthesis routes and high cost materials, which hinder large-scale production relative to conventional liquid electrolyte systems.²¹² Therefore, future research should emphasize the development of low-cost materials, scalable fabrication techniques, and a sustainable polymer framework, while maintaining an optimal balance among mechanical flexibility, ionic conductivity, and energy density. Overall, advancing the field of composite solid electrolytes will require integrated efforts in materials design, interface engineering, mechanistic understanding, and scalable manufacturing. Such developments will be essential for bridging the gap between laboratory research and practical deployment of composite solid electrolytes in next-generation solid-state energy storage technologies. Table 3 summarizes reports from different literature based on their electrolyte type with electrode composition, operating voltage window, and electrochemical performance, highlighting the critical role of interfacial engineering in improving device efficiency.^{57,71,111,167,213–217} It shows that inherent limitations such as poor electrode–electrolyte contact, high interfacial resistance, and mechanical mismatch can be effectively mitigated through targeted modifications. Crosslinking in polymer electrolytes enhances mechanical stability and reduces crystallinity, thereby improving ionic transport. The incorporation of inorganic fillers (e.g., Al₂O₃ and SiO₂) facilitates ion dissociation and creates continuous ion conduction pathways while simultaneously improving interfacial wettability and mechanical strength. Ionic liquids further enhance ionic conductivity and widen the electrochemical stability window. Meanwhile, polymer–ceramic composite electrolytes combine the advantages of both components, enabling improved electrode contact and stable ion transport. Overall, the table emphasizes that interface engineering plays a crucial role in enhancing ionic conductivity, reducing interfacial resistance, and improving cycling stability in solid-state supercapacitors.

Table 3 A performance comparison based on different interface modifications for various solid electrolytes for energy storage applications

Solid electrolyte type	Electrolyte composition	Electrode	Interface/electrolyte modification	Specific capacitance	Cyclic stability	Voltage window	Operating temperature	Key findings	Ref.
Hydrogel electrolyte	IL + PVA/H3PO4	Activated carbon	Cross-linked network + ionic liquid improves ion transport	∼271 F g^−1 @ 0.5 A g^−1	3000 cycles	∼1.0 V	25 °C	IL addition enhances capacitance and ion mobility	213
Hydrogel + filler	PVA/V4C3Tx MXene/H2SO4	V4C3Tx	MXene-enhanced interfacial ion transport	—	∼99.4% after 5500 cycles	∼1.0 V	RT	Enhanced conductivity and long stability	214
Iono gel electrolyte	PVDF-HFP + EMIM-BF4	Activated carbon	Ionic liquid confinement, polymer–IL interaction	∼161.8 F g^−1	∼86% after 10 000 cycles	∼3.91 V	Up to 320 °C	Wide voltage window; thermal stability	215
Silica-based ionogel	SiO2-IL ionogel	rGO film	Porous silica matrix improves electrode contact	—	93% after 10 000 cycles	∼3.0 V	30–120 °C	Excellent stability at elevated temperature	216
Solid polymer electrolyte	(SiWA)–H3PO4–poly(vinyl alcohol)	Stainless steel foil	A cross-linking effect of SiO2	23 µF cm^−2	100 000 cycles	∼0.9 V	25 °C	High-rate capability of 5000 V s^−1	217
Inorganic solid electrolyte	Li1.3Al0.3Ti1.7(PO4)3 + EMIM BF4	Activated charcoal/Cu	IL facilitates the Li^+ ion transport lowering the energy barriers at the grain boundaries	∼181 F g^−1 (35 °C)	13 000 cycles	1 and 1.8 V	30–100 °C	High capacity at elevated temperature	57
				∼600 F g^−1 (100 °C)
Composite solid polymer electrolyte	PEO–LiClO4 + Li1.3Al0.3 Ti1.7(PO4)3	PEO–LiClO4 added activated carbon	Interlayer avoids parasitic reactions and pseudo-capacitance at the interface	∼102 F g^−1 @ 1.5 A g^−1	∼96% after 500 cycles	2 V	RT	High specific energy and coulombic efficiency	71
Polymer electrolyte	Nafion membrane	TiO2/MWNTs	TiO2 interface layer reduced interfacial resistance and enhanced ion transfer	101.8 F g^−1 @ 0.36 A g^−1	∼96% after 10 000 cycles	1 V	—	Interlayer improved dynamic capacitive performance	111
Composite solid polymer electrolyte	Na3Zr2Si2PO12/NaCF3SO3/PEO	Activated carbon/graphite	Acetonitrile interlayer establishes smooth contact	260 F g^−1 @ 5 mA	10 000 cycles	3 V	50 °C	Powers an 8 V LED circuit; high stability	167

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7. In situ measurements

Over the past decade, the field of supercapacitor research has seen notable progress through advances in in situ characterization techniques that track dynamic processes during operation.^218–220 These strategies have facilitated a deeper understanding of charge storage mechanisms and interfacial behaviour in working devices. For example, Griffin et al. highlighted the power of in situ NMR to provide molecular-level insight into ion populations and adsorption–desorption dynamics in EDLCs during cycling.²²¹ In addition, Lin et al. demonstrated the utility of techniques such as XPS, NMR, and electrochemical quartz crystal microbalance (EQCM) for probing ion interactions within nanoporous electrodes under realistic conditions.²²² More recently, comprehensive summaries of synchrotron-based in situ studies have expanded understanding of structural evolution and electronic changes in pseudocapacitive systems during charge/discharge.²²³ However, buried solid–solid interfaces in solid electrolytes are inherently difficult to access. Unlike liquid electrolytes, where interfaces can often be probed using optical or spectroscopic techniques, solid–solid interfaces are hidden within the device architecture, severely limiting direct observation. This presents significant challenges for in situ and operando imaging or spectroscopy.

To date, most in situ interface studies for solid-state systems have been limited to the battery field particularly lithium- and sodium-metal solid-state batteries or to liquid-based supercapacitors. This focus largely reflects the more severe interfacial challenges in batteries, such as dendrite formation, solid electrolyte interphase (SEI) growth, and mechanical degradation, which have driven the development of advanced interface-characterization tools. As a result, the direct transfer of these techniques to solid-electrolyte supercapacitors remains nontrivial and is still in its early stages. Nevertheless, methodologies originally developed for battery systems, including in situ polymerization strategies,²²⁴ solid-state NMR,^225,226 interface modelling, and operando impedance spectroscopy,²²⁷ are increasingly being adapted and offer promising pathways for probing buried interfaces in solid-state supercapacitors. For example, Blanc et al. systematically demonstrated that in situ solid-state NMR spectroscopy is a powerful method for probing electrochemical processes in working supercapacitors at the atomic and molecular levels.²²⁶ The authors highlight the use of in situ NMR to directly monitor ion adsorption, desolvation, and redistribution within porous carbon electrodes during polarization, providing a real-time mechanism for the formation of the electric double layer. Notably, 11B, 19F, and 7Li NMR nuclei are shown to be highly sensitive probes for tracking electrolyte ions and their local chemical environments under applied potential. Moreover, the changes in chemical shift, linewidth, and signal intensity during charging–discharging can be correlated with ion confinement, interfacial charge accumulation, and restricted ion mobility key phenomena governing the performance of solid-state supercapacitors. Furthermore, operando NMR experiments captured both reversible and irreversible interfacial processes, enabling differentiation between purely electrostatic charge storage and pseudocapacitive contributions. Hence in situ characterization techniques can provide valuable insights into ion transport and charge storage mechanisms in electrochemical capacitors. Although such studies have primarily been conducted in liquid-electrolyte systems, they provide important mechanistic insights that are also relevant to solid-state supercapacitors.

7.1 In situ techniques to understand the interface in a solid-state supercapacitor

Applying in situ techniques directly to solid-state systems remains challenging due to the low ionic mobility and limited signal sensitivity of solid electrolytes. Future advances in in situ spectroscopy and microscopy are therefore expected to provide deeper insights into interfacial ion migration and degradation processes in solid-state supercapacitors. However, only a limited number of studies have explored these approaches in solid-state systems. Representative examples are discussed here; for instance, Qiao et al. combined electrochemical analysis with in situ spectroscopic monitoring to elucidate the role of Prussian Blue (PB) as a redox mediator in a solid-state supercapacitor.²²⁸ X-ray absorption near edge structure (XANES) and Fourier-transformed EXAFS data at different potentials (at open-circuit, +2 V, and −2 V) enable direct tracking of the reversible redox transitions of PB during charge and discharge, providing real-time evidence of its active participation in charge storage rather than acting solely as a passive additive. Variations in characteristic spectral features are correlated with applied potential, confirming the Fe²⁺/Fe³⁺ redox conversion of PB under operating conditions. Importantly, the in situ analysis reveals a clear synchronization between PB redox chemistry and the electrochemical response of the solid-state device, demonstrating that the mediator facilitates charge transfer and enhances capacitance through fast, reversible surface-confined reactions. Furthermore, the study shows that the spectroscopically observed redox processes remain stable during repeated cycling, highlighting the robustness of PB in a solid electrolyte environment. Such observations provide direct evidence of interfacial charge-transfer processes and highlight the importance of redox mediators in improving charge transfer kinetics and capacitance in solid-state supercapacitors.²²⁹

Generally, electrochemical cells containing liquid electrolytes are not suitable for in situ scanning electron microscopy (SEM) under vacuum conditions due to electrolyte evaporation and leakage issues. This limitation can be overcome by using solid-state electrolytes, which reduce the associated risks. The transfer chamber is often used to move the electrochemically cycled electrode to the SEM for analysis, preventing exposure of the electrode surface to environmental contamination. However, designing operando cells that maintain electrochemical functionality while allowing real-time structural observation remains challenging, which has limited the number of in situ studies in solid-state supercapacitors.²³⁰ Using in situ SEM analysis, various changes occurring at the electrode interface during electrochemical processes, such as interfacial structural evolution, morphological transformations, crack propagation, and volume expansion, within the electrode materials can be identified.^231–233 Therefore, in situ SEM has emerged as a powerful tool for investigating structural evolution and mechanical stability of electrode materials in solid-state devices. For instance, Xiong et al. investigated a hierarchical pseudocapacitive electrode composed of Ni–Co hydroxide nanopetals grown on a three-dimensional graphene petal foam using in situ SEM.²³⁴ The electrode was fabricated using a two-step synthesis process, in which graphene petal foam served as a conductive and mechanically robust scaffold for the growth of Ni–Co hydroxide nanostructures. In situ SEM analysis was employed to investigate the mechanical behaviour of the graphene foam during compression and bending. It was bent to an angle of 45°, and after the external force was removed, it returned to its initial shape, indicating its excellent elastic behaviour (Fig. 20a–d), revealing a high Young's modulus of approximately 3.42 GPa and excellent structural stability, with no observable cracking or damage at the bending joints. The 3D graphene framework provided a conductive scaffold and strong interfacial contact with Ni–Co hydroxide nanopetals, enabling efficient electron transport and electrolyte accessibility. As a result, the asymmetric solid-state supercapacitor exhibited a high volumetric capacitance of ∼765 F cm⁻³, an energy density of ∼10 mWh cm⁻³, and an outstanding cycling stability over 15 000 cycles, highlighting the importance of mechanically robust electrode-interface design for high-performance supercapacitors.

Fig. 20 (a) Free-standing graphene petal foam (GPF) after removal of the Ni foam template (inset: the optical image of the GPF), (b) hollow channel structures formed upon etching of Ni ligaments (inset: thickness of the channel walls), (c) in situ SEM characterization illustrating the bending behavior of GPF from 0° to 45° and back to 0° (inset: the pristine GPF sample before bending), and (d) variation of bending moment (M) of GPFs as a function of the bending angle (θ), reproduced with permission from ref. 234 Copyright 2016 Wiley.

Operando ambient-pressure X-ray photoelectron spectroscopy (APXPS) has recently emerged as a powerful technique for probing the surface electronic and chemical states of electrochemical systems under realistic operating conditions. In 2018, Camci et al. developed an in situ XPS-based electrochemical cell to directly investigate ion transport within porous graphene electrodes of an electric double-layer capacitor (EDLC).²³⁵ The experimental setup consisted of a multilayer graphene electrode in contact with an ionic liquid electrolyte, integrated with an XPS measurement chamber that enabled monitoring of the electrode surface while an external voltage was applied, as shown in Fig. 21a. By tracking binding energy shifts and intensity changes of ionic species in the electrolyte during polarization, the authors monitored the movement of cations and anions within the graphene micropores in real time. The operando XPS measurements, as depicted in Fig. 21b, revealed a voltage-driven asymmetric ion migration mechanism, where cations and anions exhibit different transport behaviours during charging and discharging.

Fig. 21 Operando XPS analysis of a graphene electrode: (a) schematic illustration of the experimental setup used for operando XPS measurements, (b) XPS spectra of the electrode surface recorded at applied potentials ranging from 0 to 4 V, and (c) corresponding C 1s, N 1s, and F 1s spectra with the variation in intensity ratios as a function of the applied voltage, reproduced with permission from ref. 235 Copyright 2018 American Chemical Society.

When a positive potential was applied to the graphene electrode, anions preferentially migrated and accumulated within the electrode pores, while cations were partially expelled from the interfacial region. Conversely, under negative polarization, cations were driven into the pores while anions moved away from the interface. Importantly, the XPS spectra showed that this ion redistribution leads to a potential drop across the electric double layer near the electrode–electrolyte interface, providing direct experimental evidence of charge compensation and ion rearrangement during device operation (Fig. 21c). This study provides direct spectroscopic evidence of ion redistribution within porous carbon electrodes during electrochemical polarization, offering important insight into the formation and evolution of the electric double layer. The operando XPS results reveal how ions migrate and rearrange within confined nanopores under applied voltage. Although the study was conducted in a liquid-electrolyte system, the mechanistic understanding of ion migration and interfacial potential gradients obtained from operando XPS measurements provides important guidance for investigating similar processes in solid-state supercapacitors.

In addition to these techniques, in situ nuclear magnetic resonance (NMR) spectroscopy has also been employed to probe ion transport and charge-storage mechanisms in electrochemical capacitors. For example, in situ NMR spectroscopy has been used to monitor ion adsorption and desorption within porous carbon electrodes during charging, revealing the dynamic evolution of ion populations inside micropores and their role in electric double-layer formation, primarily in liquid-electrolyte systems. For example, Céline et al. utilized operando NMR spectroscopy to quantitatively track ion populations inside nanoporous carbon electrodes, revealing the dynamic redistribution of ions within micropores and their crucial role in the formation of the electric double layer.²³⁶ Even though the investigations have mainly been conducted in liquid-electrolyte systems, the mechanistic insights obtained are highly relevant for understanding ion transport and interfacial processes in solid-state supercapacitors. However, the direct implementation of in situ techniques in solid-state systems remains challenging due to the low ionic mobility and reduced signal sensitivity of solid electrolytes. Future developments in high-resolution in situ spectroscopy and microscopy are therefore expected to provide deeper insights into ion migration, interfacial evolution, and degradation processes in solid-state supercapacitors.

8. Challenges and prospects

Despite significant progress in solid electrolytes for solid-state supercapacitors, several critical challenges continue to hinder their practical deployment. A primary limitation arises from the large interfacial resistance at the electrode–electrolyte interface, which originates from poor physical contact, limited ion transport pathways, and interfacial instability.^237,238 Unlike liquid electrolytes, solid electrolytes cannot effectively wet porous electrode structures and are frozen at the solid–solid interface, leading to incomplete interfacial contact and increased charge-transfer resistance.²³⁹ As highlighted by He et al., interfacial impedance often dominates the overall device resistance, thereby restricting rate capability and long-term cycling stability.²⁴⁰

In addition, the relatively low ionic conductivity of many solid electrolytes under ambient conditions, particularly in polymer and gel-based systems, remains a key bottleneck. This limitation becomes even more severe at low temperatures due to the suppressed segmental motion of polymer chains, which significantly impedes ion transport.^241,242 Furthermore, mechanical mismatch between rigid electrodes and comparatively softer electrolytes can induce interfacial stress, cracking, and delamination during repeated cycling, ultimately leading to performance degradation.²⁴³ The narrow electrochemical stability window of many solid electrolytes also constrains the achievable operating voltage and energy density.²⁴⁴ Another major challenge lies in the limited understanding of buried solid–solid interfaces. Conventional characterization techniques often fail to probe these interfaces in situ, thereby hindering mechanistic insights into interfacial degradation. Advanced operando and in situ techniques, such as solid-state NMR, Raman spectroscopy, synchrotron-based X-ray absorption spectroscopy, and emerging cryogenic electron microscopy, are therefore essential to unravel dynamic interfacial processes at multiple length scales.^245–247 From a practical perspective, although numerous high-performance systems have been demonstrated, most studies remain confined to laboratory-scale prototypes, and scalable, cost-effective fabrication strategies suitable for industrial production are still underdeveloped.²⁴⁸

To address these challenges, future research should prioritize the development of adaptive and ionically conductive interfacial layers capable of simultaneously minimizing interfacial resistance and accommodating mechanical stress. Promising approaches include polymer–ionic liquid hybrid interphases, dynamically crosslinked (self-healing) polymer networks, and artificial solid electrolyte interphases (ASEIs) that can stabilize electrode–electrolyte interactions during long-term cycling.^248–250 In addition, the design of advanced solid electrolytes with enhanced low-temperature ionic conductivity, such as ionic liquid-based gels, plastic-crystal electrolytes, and polymer–ceramic hybrid systems with percolated ion-conduction pathways, will be crucial for expanding the operational window.^251–253

The integration of multi-scale simulations with machine learning (ML)-assisted materials discovery offers a powerful strategy to accelerate the rational design of next-generation electrolytes.^254,255 Data-driven approaches can enable high-throughput screening of polymer architectures, ceramic dopants, and interface chemistries and predictive modelling of ion transport mechanisms and activation barriers.^256,257 Equally important is the translation of laboratory-scale innovations into scalable manufacturing technologies, including roll-to-roll processing, 3D printing of solid electrolytes, and printable/flexible device architectures.^258–261 The development of structurally integrated electrodes (e.g., 3D porous frameworks) compatible with solid electrolytes can further improve interfacial contact and device performance.^262,263 Establishing standardized testing protocols and reliability benchmarks will also be essential for bridging the gap between academic research and industrial implementation.²⁶⁴

Overall, the synergistic integration of interface engineering, advanced characterization techniques, scalable processing, and data-driven materials design is expected to overcome current limitations and enable the development of next-generation solid-state supercapacitors with high energy density, mechanical robustness, and practical applicability in wearable and portable electronics.

9. Conclusion

This review provides a comprehensive overview of solid-state electrolytes for supercapacitor applications, encompassing a broad spectrum of materials, including polymers, aqueous and nonaqueous gels, ionic gels, composites, and inorganic oxides. The impact of electrolyte properties-including ionic and electronic conductivity, electrochemical stability window, mechanical flexibility, and interfacial compatibility, on key device metrics such as energy density, power density, cycle life, and internal resistance has been critically examined. Despite significant progress, interface-related challenges, which remain the primary bottleneck in the development of high-performance solid-state supercapacitors, are discussed alongside emerging strategies to minimize interfacial resistance, enhance ion transport, and ensure long-term stability. Furthermore, the growing importance of in situ and operando characterization techniques is highlighted, as these methods provide crucial insights into dynamic interfacial processes and degradation mechanisms, guiding the rational design of high-performance solid-state supercapacitor systems. Looking forward, the advancement of solid-state supercapacitors will rely on the synergistic integration of electrolyte innovation, interface engineering, and advanced characterization methodologies. Accordingly, this review outlines a forward-looking roadmap that emphasizes the rational design of electrolytes and interfaces, supported by in situ diagnostic tools, to accelerate the development of next-generation solid-state supercapacitors with improved performance, reliability, and practical applicability.

Conflicts of interest

There are no conflicts to declare.

Data availability

This review article does not report any new experimental or computational data. Data sharing is not applicable to this article.

Acknowledgements

P. B. acknowledges IISc for the Fellowship (DST-National Postdoctoral Fellowship) in Nano Science and Technology. A. K. S. is grateful to ANRF (CRG/2022/006284), New Delhi, and Jain University (JU/MRP/CNMS/97/2025) for the research funding.

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