Polyimide-driven innovations as “inert” components in high-performance lithium-ion batteries

Abstract

The rapid proliferation of lithium-ion batteries (LIBs) across portable electronics and electrified transportation systems has propelled unprecedented requirements for high energy density, prolonged cycle life, and improved safety protocols. Polyimides (PIs), attributed to the excellent thermal stability, mechanical robustness, chemical stability, and flame retardant properties, have been widely researched as “inert” materials to address critical challenges in advancing LIBs. Herein, this review provides design principles for employing PIs’ inherent characteristics to develop next-generation high-performance LIBs with balanced energy density, rate capability, and operational reliability. PI-based “inert” components, including PI-based separators, solid-state electrolytes, protective layers, and binders, overcome the limitations of conventional materials by enhancing the safety of liquid batteries, reinforcing the mechanical properties, stabilizing the electrolyte/electrode interface, and maintaining the electrode integrity. Key challenges and optimization pathways for practical implementation are discussed and proposed. Finally, prospective research directions of PIs in LIBs are also outlined to provide critical orientation for research fields.

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Wider impact

To meet escalating lithium-ion battery (LIB) performance requirements in multiple fields, the development of high-performance polymers is crucial to enhance electrochemical performance, safety, and durability. Polyimides (PIs) with outstanding stability and structural characteristics are hailed as the “trouble-shooting expert”. In the past few decades, PIs have been developed to address critical challenges in advancing LIBs and have demonstrated unprecedented potential in “inert” components (including separators, solid-state electrolytes, protective layers, and binders) of LIB technology. This review analyzes the past five years’ achievements and delineates foundational design principles across PI “inert” component domains, particularly highlighting their unique advantages in addressing thermal safety concerns and critical bottlenecks in ion transport, interfacial stability, and mechanical integrity. The future of PIs in LIBs is proposed to lie in overcoming challenges such as cost-effective synthesis, balancing low conductivity and mechanical strength, and optimizing interfaces through advanced molecular engineering. Key challenges and optimization pathways for practical implementation are discussed. The elucidated structure–property relationships establish a theoretical framework for the rational design of precisely controlled PI architectures and provide critical orientation for research fields.

1. Introduction

To get rid of the dependence on fossil fuels and meet the continuously growing energy demands, the development of renewable and clean energy technologies is imperative.^1–4 Rechargeable secondary batteries have been considered as the main candidates for electrochemical energy storage.^2,5 Among the multifarious energy storage devices, lithium-ion batteries (LIBs) hold the top position due to their high energy density and superior long-term stability.^6–9 Since Sony introduced LIBs in 1991, they have undergone decades of development and have expanded their applications to more areas, including electric vehicles, aerospace equipment, and large-scale energy storage systems.^5,8

In parallel, polymers have also experienced rapid development, with various methods for synthesizing polymers of different components.^10,11 Polymer materials have been extensively applied in all areas of the national economy, such as industrial and agricultural production, national defense, healthcare, and aerospace.^12–14 Nowadays, polymer materials have begun to play different roles in battery applications. With the increasing demand for LIB performance in various fields, the design and realization of functional polymers are crucial for enhancing the electrochemical performance, safety, and durability of LIBs, providing new avenues to rapidly address the urgent needs in the field of energy storage.^15–17

Polyimide (PI) is an aromatic heterocyclic polymer compound with imide groups (–C(O)–N–C(O)–) in its molecular structure. Fig. 1 presents the repeating unit geometries of the industrially dominant PI. It is one of the best heat-resistant varieties among engineering plastics, capable of withstanding temperatures up to 400 °C and can be used continuously in the temperature range of −200 to 300 °C. PIs have excellent resistance to high and low temperatures, radiation, corrosion, mechanical properties and dielectric constant.¹⁸ Known as the top material in the polymer pyramid, it has huge application prospects both as a structural material and a functional material due to its outstanding stability and structural characteristics, which is even hailed as the “trouble-shooting expert”.¹⁹

Fig. 1 Schematic diagram of the polyimide-based “inert” components in LIBs, which include separators, solid-state electrolytes, protective layers, and binders.

According to the difference of the main chain, PIs can be classified into aromatic, semi-aromatic, and aliphatic PIs. The unique aromatic heterocyclic conjugated structure of PIs exerts a strong conjugation effect on the molecular chain. The rigid aromatic structure endows the polymer chain with elevated rigidity and glass transition temperature (T_g), which strongly limits the localized motion of the segments.²⁰ Therefore, aromatic PIs have excellent high-strength, high-toughness, high-temperature resistance and other comprehensive properties.^21,22 Meanwhile, the aliphatic segments enhance the flexibility and localized mobility of the chain segments, thus improving interfacial stability. By virtue of exceptional physical and chemical characteristics of different types of PIs, they play an incrementally significant role in lithium battery technology, including separators,^23–25 solid-state electrolytes,^26–28 protective coatings,^29–32 binders,^33–35 and active storage materials,^36,37 not only enhancing the performance of the batteries but also contributing to their safety and reliability.^38,39

Herein, we systematically review the progress of PI-based materials as “inert” components of LIBs, incorporating cutting-edge advancements in this field. This review focuses on the design, fabrication, and multifunctional roles of PI-based “inert” components in advancing LIB performance. Fig. 1 schematically outlines the multidimensional focus on: (1) rationally designed PI separators with exceptional thermal/chemical stability innovations to improve the safety of LIBs, (2) PI-based solid-state electrolytes for enhancing mechanical robustness, (3) PI-derived protective layers for stabilizing electrode–electrolyte interfaces, and (4) PI-based multifunctional binders with enhanced adhesion and mechanical properties to improve the cyclability of structurally integral electrodes. We analyze the past five years’ achievements in PI applications across these non-electrode domains, particularly highlighting their unique advantages in addressing thermal safety concerns and critical bottlenecks in ion transport, interfacial stability, and mechanical integrity. Remaining challenges are identified regarding the optimization of PI synthesis routes for cost-effective manufacturing, advanced composite PI structure designs incorporating functional additives, balance between mechanical strength and ion conductivity, multifunctional integration, and sustainable manufacturing studies. This review establishes fundamental structure–property correlations to guide the reasonable design of PI materials for high-performance LIBs.

2. Design and fabrication of PIs

2.1 The role of PI properties in ensuring battery safety

PIs, with their unparalleled combination of properties in their unique molecular structure, have occupied a pre-eminent position in advanced polymer materials and have often been referred to as the “trouble-shooting expert”.¹⁹ The exceptional performance of PIs benefits from their rigid aromatic backbone, high-density interchain interactions, and tunable chemical functionalities, which synergistically confer the following structural advantages.⁴⁰

2.1.1 Mechanical strength. PIs exhibit high tensile strength and modulus, maintaining their structural integrity under various mechanical stresses, which is crucial for applications requiring robust materials. The PI backbone typically comprises alternating aromatic diamine and dianhydride units, forming a structure with extensive π–π conjugation and interchain charge-transfer complexes. This rigid aromatic structure, exemplified by biphenyl PI (e.g., Upilex S), features coplanar biphenyl groups that enhance the molecular alignment, allowing tensile strengths exceeding 400 MPa.⁴¹ The high modulus (up to 500 GPa) originating from restricted chain mobility and covalent bonding within the imide rings (–CO–NR–CO–) can resist deformation under stress.^42,43 Meanwhile the thermoplastic PIs (TPIs) achieve exceptional toughness (impact strength ∼260 kJ m⁻²) through controlled chain flexibility via ether (–O–) or ketone (–CO–) linkage, enabling energy dissipation via segmental motion while maintaining backbone integrity.

2.1.2 Temperature resistance. The superior thermal stability of PIs (initial decomposition of aromatic PIs > 500 °C) originates from their fully aromatic conjugated systems, where delocalized π-electrons dissipate thermal energy.^44,45 In biphenyl-type PIs, the para-linked benzene rings create a crystalline-like packing arrangement, increasing the decomposition temperature to 600 °C. For a short period, they can withstand temperatures as high as 555 °C and maintain the physical properties. The long-term use temperature is 333 °C, making it one of the most heat-resistant polymers currently available.⁴⁶ The resonance stabilization of imide rings and strong dipole–dipole interactions between carbonyl groups (–C=O) further inhibit chain breakage. Even under cryogenic conditions (−269 °C), PIs retain flexibility due to the absence of rotational barriers in their all-aromatic structure, preventing brittle fracture.

2.1.3 Chemical stability. PIs are highly resistant to chemical attack, making them suitable for applications involving exposure to corrosive chemicals or solvents. The electron-deficient imide rings and electron-rich aromatic groups form charge-transfer complexes that shield the polymer from chemical attack. This electron delocalization, combined with hydrophobic aromatic stacking, minimizes solvent penetration and oxidative degradation. Dimensional stability is ensured by covalent crosslinking in thermoset PIs and restricted chain mobility in semi-crystalline variants (e.g., Kapton®), which suppresses creep even under prolonged thermal–mechanical stress.⁴³ This property enables extending the service life of products under chemically harsh conditions.

2.1.4 Dimensional stability. PIs exhibit an exceptionally low coefficient of thermal expansion (CTE), typically ranging from 2 × 10⁻⁵ to 3 × 10⁻⁵ K⁻¹. Biphenyl-based PIs demonstrate even greater dimensional stability, with CTE values reaching as low as 10⁻⁶ K⁻¹ to 10⁻⁷ K⁻¹, which are close to the CTE values of metals.⁴⁰ The ultra-low CTE endows PI materials with exceptional dimensional stability under varying thermal and environmental conditions, which indicates significant potential for diverse applications in advanced battery separators.

2.1.5 Structural designability. The structural designability of PIs enables precise property optimization through controlled synthesis and processing strategies. By tailoring monomer selection and implementing solution casting or thermal imidization techniques, molecular structures of PIs with targeted characteristics can be engineered to achieve desired properties, such as increased flexibility via incorporating flexible ether units, improved adhesion by surface-functionalization, and enhanced electrical conductivity via carbon nanotube hybridization.

2.1.6 Electrical insulation properties. PIs have excellent electrical insulation and exhibit minor dielectric losses over a wide temperature range. In addition, the dielectric constant of PIs is generally about 3.4, which can be reduced to about 2.5 after introducing fluorine into the molecular chain or dispersing nanometer-sized air in PIs.⁴⁷ The dielectric loss of PIs is only 0.004–0.007, and the dielectric strength is 100–300 kV mm⁻¹.⁴⁸ The excellent electrical insulation properties make them suitable for use in electronic and electrical applications where dielectric strength and electrical resistance are critical, yet need to be overcome in the battery field.

2.1.7 Flame retardant properties. The limiting oxygen index of PI fibers is 35–38%, which is higher than those of most of the organic fibers, and they belong to non-combustible fibers.⁴³ The PI fibers do not burn in the air and are self-extinguishing polymers. Their smoke rate is low, they have good flame retardant properties, and they can meet the requirements of most areas of flame retardants.⁴⁹

These attributes collectively position PIs as a premier choice in the field of advanced materials, capable of addressing complex engineering challenges across a variety of industries.

2.2 Synthesis of PIs

2.2.1 One-step method. The one-step method is a common approach for synthesizing alicyclic polyimides, particularly those with good solubility.⁴¹ This method is primarily conducted by uniformly mixing diamines and dianhydrides in a high boiling point solvent (such as mesophenol, N-methyl pyrrolidone, etc.), and then directly dehydrated and condensed at a higher temperature (180–200 °C) to form PI, that is, directly synthesizing high molecular weight PI without generating a polyamic acid (PAA) intermediate (Fig. 2).⁵⁰ In the one-step method, polymerization and cyclization occur simultaneously. The one-step synthesis method employs lower reaction temperatures than conventional two-step processes, effectively minimizing side reactions (e.g., thermal degradation and crosslinking) while maintaining high product yields. Therefore, the one-step method is more suitable for preparing PI powders and transparent PI films.

Fig. 2 Synthesis of PIs using the one-step method.

2.2.2 Two-step method. The two-step method is the most widely used approach for synthesizing PIs, applicable to virtually all fully aromatic polyimides and some semi-aromatic polyimides. This process primarily involves synthesizing an intermediate polyamic acid in a polar solvent from diamines and dianhydrides and then obtaining polyimides through chemical imidization or thermal imidization processes. Chemical imidization involves relatively mild conditions but a more complex process, requiring the addition of catalysts (such as pyridine, triethylamine, etc.) and dehydrating agents (acetic anhydride) to the PAA to complete the imidization reaction.²¹ The two-step method is suitable for the large-scale preparation of PI films and is also applicable to the preparation of TPI and thermosetting PI, capable of preparing PIs with very high purity. The reaction process is shown in Fig. 3.

Fig. 3 Synthesis of PIs using the two-step method.

2.2.3 Three-step method. The preliminary steps of the three-step method are the same as the two-step method, i.e., the precursor PAA is obtained by the condensation polymerization of monomer dianhydride and diamine, and then PAA is dehydrated by partial chemical imidation to prepare polyimide. It is finally isomerized into PI by acid–base catalysis or high temperature heat treatment at 100–250 °C. This method does not produce small molecules such as water, and with this great advantage, the three-step method is also becoming more and more popular.²¹ The reaction process is shown in Fig. 4.

Fig. 4 Synthesis of PIs using the three-step method.

2.3 Fabrication technology of PI films

PIs are high-performance polymers renowned for their exceptional thermal and oxidative stability, chemical resistance and solvent tolerance. They have found extensive applications in advanced industries such as aerospace (e.g., spacecraft thermal shields), optoelectronics (e.g., flexible displays), renewable energy (e.g., solar cells), and microelectronics. These properties also position PIs as ideal base materials for membrane technologies, including osmotic evaporation, gas separation, fuel cells, and LIBs. The product forms of PIs are diverse, including films (or membranes), fibers, and foams, processed through distinct manufacturing processes. PI fibers are manufactured using wet or dry spinning of PAA solutions, followed by imidization under tension to align polymer chains, enhancing tensile strength. PI foams are synthesized via chemical or physical foaming of PAA precursors, often incorporating sacrificial templates to create porous structures with low density and high thermal insulation. PI membranes are fabricated using various methods depending on the intended application. For instance, solution casting is widely used to produce dense PI films for gas separation.^51,52 Chemical vapor deposition (CVD) allows the fabrication of ultrathin, defect-free PI coatings for high-precision microelectronic devices. Due to their relevance to battery technologies, the fabrication techniques for porous PI membranes are introduced in detail and summarized systematically as follows.

2.3.1 Nonsolvent-induced phase separation (NIPS) method. Immersion precipitation is the process of scraping a PAA precursor solution or soluble PI solution onto a carrier (e.g., glass) and immersing it in a non-solvent, utilizing the polymer to phase-separate in its solvent/non-solvent mixture. When the solvent is removed, the space occupied by the non-solvent forms a pore. The pore structure of porous membranes can be easily and effectively regulated by changing the formulation of the casting solution and the process conditions. Fig. 5a shows the preparation process of the porous PI separator via the NIPS method.⁵³ The PI membrane prepared by the NIPS method offered homogeneous and nanosized pores. Synergistically, the infiltrated PIL had a high affinity for the electrolyte, which enhances the electrolyte uptake to facilitate the ionic conductivity. Compared with a commercial polypropylene (PP) separator, the porous PI separator prepared by NIPS exhibited high porosity (76%), excellent electrolyte wettability (a contact angle of 11.0° with design (a pore size of 21 μm) using polylactide-b-polyimide-b-electrolyte), and prominent thermal resilience (stable at 140 °C).⁵⁶ However, while NIPS enables facile pore structure modulation via solvent/non-solvent ratio adjustments, electrolyte affinity and transportation of Li-ions (Li⁺) need to be further improved. Subsequent studies attempted to address these concerns through functional hybridization. Wang et al.⁵⁷ prepared a composite separator by blending a zeolite as an organic filler with PI. The addition of the zeolite effectively enhanced the separator porosity from 44% to 59%, allowed Lewis acid/base interactions with the electrolyte, and provided additional ionic transport channels to enhance the wettability of the liquid electrolyte. Although this hybrid design improved electrolyte wettability, inhomogeneous filler dispersion, a recurring issue in composite membranes, would compromise mechanical integrity during long-term cycling. Wu et al.⁵⁸ prepared a three-dimensional porous PI film with glycerol as a pore-forming agent. To reinforce ionic pathways, inorganic ionic conductor LAGP nanoparticles were incorporated to obtain a PI-LAGP separator. However, the high cost of LAGP and its interfacial incompatibility with PIs undermine practical feasibility. Besides, an in situ artificial layer with a low nucleation barrier obtained via spraying MoS₂ on the PI porous membrane was employed to reduce Li⁺ nucleation barriers, achieving enhanced wettability to the electrolyte (Fig. 5b).⁵⁴ Meanwhile, the thermal safety test conducted at the charged state revealed that a cell with PI@coating can tolerate 170 °C.

Fig. 5 (a) Schematic illustration of PI/PIL separator fabrication.⁵³ Copyright 2025 Elsevier. (b) Cross-sectional SEM image (i) of the PI film. Thermostability experiment conducted at 200 °C for 10 min (ii) and wettability examination (iii) of the PP separator, PI membrane and PI@coating membrane with the common electrolyte.⁵⁴ Copyright 2022 Elsevier. (c) Schematic structure (i) and the cross-sectional and top-view SEM images (ii) of the Zr-PIA@PP separator, respectively.⁵⁵ Copyright 2024 Wiley.

While material hybridization strategies demonstrate promise in enhancing ionic transport and thermal safety, challenges such as interfacial incompatibility and high costs require alternative approaches. To address these limitations, recent studies have shifted toward structural optimization of PI membranes through hierarchical pore engineering and chemical crosslinking. For instance, aerogel-based structures employ the intrinsic thermal resilience of PIs while introducing tunable porosity and lightweight frameworks, offering a pathway to simultaneously improve ion kinetics and mechanical stability without relying on expensive additives. By the substantial enhancements in heat resistance, electrolyte affinity, and uptake, Liu et al.⁵⁵ developed a polyolefin separator coated with a single-atom zirconium coordination polyimide aerogel (Zr-PIA@PP, Fig. 5c) to further selectively promote the Li⁺ transfer kinetics. The aerogel-coated layer on the Zr-PIA@PP separator has a highly interconnected nanopore structure that strengthens the separator–electrolyte interface and acts as a barrier to minimise electrolyte loss. The affinity of the interface with the liquid electrolyte significantly impacts the nature of ion transport in the LMB. The presence of connected pores in the PIA coating significantly increases the electrolyte uptake of the Zr-PIA@PP separator to 322%. Unlike the conventional thermal imidization process, Cheng and co-authors introduced a novel PI aerogel via the sol–gel method followed by chemical cross-linking.⁵⁹ The PI aerogel separator features a uniform porous structure with a porosity of 78.35%, exceptional thermal stability with a decomposition temperature of 500 °C, and superior electrochemical performance with the LiFePO₄|Li batteries stably cycled for over 300 times at 90 °C.

PI membranes prepared by the phase conversion method have excellent thermal stability and good electrolyte wettability. However, the low diffusion coefficient between the solvent and the non-solvent leads to the formation of some blind pores, which disrupt the connectivity of the internal pore structure of the separator and limit the free migration of Li⁺.⁶⁰

2.3.2 Sacrificial template method. The sacrificial template method is an intra-membrane pore creation technique widely used in the field of polymer materials. It involves adding a substance that is soluble in the synthesis solution during the synthesis process and then removing or disintegrating the substance once polymerization is complete. Specifically, PAA solution is first coated on the template by casting, spin-coating, or dip-coating, and then the precursor-coated template is chemically dissolved or thermally decomposed to remove the template and retain the porous film.⁶¹ The film is then thermally imidized to transform the PAA into a PI membrane.

For instance, a three-dimensionally ordered macro-porous PI separator with uniform pore size, high porosity, and good affinity for a liquid electrolyte was prepared by using the colloidal crystal template method with 300 nm monodispersed silica beads as a template.⁶² However, separators with macropores provide poor inhibition of Li dendrites and easily allow them to traverse (Fig. 6a), leading to internal short circuits and other undesirable consequences. This structural defect is a widespread concern in LMB research, where the pore size is directly related to dendrite mitigation effects. Based on these considerations, the subsequent work by Liu et al.⁶¹ addressed this limitation through an innovative mesoporous polylactide triblock copolymer template, followed by thermal cracking to remove the template of polylactide to obtain a mesoporous PI separator. Their strategy achieved a remarkable modulus of 1.8 GPa, representing an improvement over conventional macroporous separators, while maintaining sufficient porosity for ionic conduction. The mesoporous engineering and the high mechanical properties of PI synergistically assist in suppressing Li dendrites and improving the battery safety.

Fig. 6 (a) Effects of the pore size and modulus of separators on dendrite suppression (i) and the chemical structure of PLA-b-PAA-b-PLA and the preparation process of the mesoporous PI separator (ii).⁶² Copyright 2020 American Chemical Society. (b) Schematic illustration (i), cross-sectional SEM image (ii), and infrared thermal images (iii) of the PI/hBN separator.⁶³ Copyright 2022 American Chemical Society. (c) (i) Schematic illustration of the homogeneous deposition of Li and possible Li⁺ transport with the PIC/PE separator. (ii) SEM image of the PI and PIC microsphere. (iii) The breaking temperatures of various separators.⁶⁴ Copyright 2024 American Chemical Society.

The sacrificial template method allows fine control of the structure and size of the micropores by adjusting the particle size of the pore-inducer. However, it is difficult to remove the pore-inducer completely, which may adversely affect the subsequent imidization process, resulting in poor mechanical properties of the separator that do not meet the standards for industrial applications.

2.3.3 Track-etching method. Trace etching membranes are special membrane materials with pores or channels prepared by bombarding polymer films with heavy ions to form latent traces, which are then etched with specific chemical reagents. Liu et al.⁶³ prepared a porous PI membrane by ion track etching and coated it with hexagonal boron nitride to obtain a PI/hBN separator, as shown in Fig. 6b. The hBN coating layer improved the in-plane thermal conductivity of the separator while PI enabled extremely high heat resistance as high as 500 °C. With the synergistic effect of PI and hBN, the local heat accumulation in the battery was reduced, and the interfacial compatibility and conduction of Li⁺ were promoted. While the hBN layer effectively mitigates localized heat accumulation in batteries and improves Li⁺ ion transport, this approach inherits intrinsic limitations of the track-etching method, including the anisotropic pore orientation and scalability bottlenecks due to the reliance on high-energy ion bombardment equipment. In addition to the preparation of porous membranes, the etching method can also be utilized to modify the separators. Bifunctional PE-based separators incorporating acid-etched PI–COOH microspheres were engineered to address the high-temperature LMBs' safety and SEI instability.⁶⁴ The 3D carboxyl-enriched PI microspheres (Fig. 6c-ii) were coated on PE to obtain a composite separator (denoted as PIC/PE), which elevated thermal resilience (breaking temperatures were close to 150 °C, Fig. 6c-iii) and enabled homogeneous Li⁺ flux via electrostatic interactions, concurrently stabilizing the SEI layer (Fig. 6c-i).

Although significant progress has been made in porous PI films, separators prepared by NIPS, sacrificial templates, and track-etching methods still suffer from poor mechanical properties, difficulties in thinning, and limited preparation processes.

2.3.4 Electrospinning method. In recent years, electrostatic spinning has become an important method for the preparation of PI separators due to its simplicity, efficiency, and wide range of applications. The core mechanism of electrostatic spinning, as an advanced fiber preparation technology, is the use of a high-voltage electrostatic field to induce an electric charge on the polymer solution or melt, and the formation of a Taylor cone droplet at the end of the nozzle. The polymer solution ejected at high speed is stretched, deformed, split, and volatilized, and the polymer solution jet is solidified and finally deposited on the receiver to form a nanofiber film, as shown in Fig. 7a. The nanofiber film is deposited on the receiver.¹⁸ Electrostatic spinning has become one of the effective ways to prepare PI separators because of the advantages of a simple setup, a wide range of applicable substances, and macroscopic preparation. Electrostatic spinning can be used to produce nanofiber membranes with a three-dimensional network structure and high porosity, providing numerous channels for the rapid migration of Li⁺.⁶⁵

Fig. 7 (a) (i) Schematic illustration of the q-PBI@PI membrane. (ii) Synthetic routes of the q-PBI. (iii) Binding energies for the q-PBI with Li⁺.²⁵ Copyright 2025 Springer Nature. (b) (i) Schematic illustration of the 3D multiscale MOF networks enabling continuous and high-efficiency Li⁺ transport. (ii) Surface morphologies of PI@ZIF-8 nanofiber separators.²³ Copyright 2025 Royal Society of Chemistry.

However, electrospun PI nanofiber membranes have shown great potential as ideal separators for safe LIBs due to their outstanding thermal stability.⁶⁶ Nevertheless, electrospun PI nanofiber membranes often suffer from inadequate mechanical robustness due to inter-fiber slippage, presenting a critical limitation for their application in LIB separators.⁶⁷ Therefore, numerous improvement strategies have been employed to enhance the performance of PI nanofiber membranes. For example, an interfacial reinforcement strategy was employed through using lithium polyacrylate (PAALi) as a polymeric binder, which significantly enhanced inter-fiber adhesion through the formation of crosslinked networks between adjacent nanofibers.²⁴ Modified membranes demonstrated a tensile strength of 16.1 MPa, representing a 222% enhancement compared to pristine counterparts (5.0 MPa), while maintaining essential porosity (78.4 ± 2.1%) for ionic transport. To further enhance the mechanical and ionic migration properties, the latest research has been conducted by in situ wrapping or growing functional nanolayers on electrospinning PI nanofiber membranes. A novel quaternized polybenzimidazolium (q-PBI)-armored PI nanofiber separator (q-PBI@PI) was developed (Fig. 7a-i), achieving an ultrahigh mechanical strength of 82.8 MPa and suppressing Li dendrites through TFSI⁻-mediated Li⁺ migration control (Fig. 7a-ii), which was demonstrated by density functional theory (DFT, Fig. 7a-iii).²⁵

Metal–organic frameworks (MOFs), with their high surface areas, ordered pores, and hybrid structures, have been considered as promising modifiers for the LIB separators to facilitate Li⁺ transport and improve ionic conductivity. A PI/MOF composite separator obtained via in situ polymerization and electrospinning exhibited excellent tensile strength and flame retardancy, especially superior in high lithium-ion transference numbers (t_Li⁺ = 0.79).⁶⁸ Different from simple co-blended electrospinning, a 3D hierarchical MOF nanofiber separator was engineered via electrospinning-assisted in situ self-assembly, integrating ZIF-8 units on PI fibers to establish continuous Li⁺ pathways and sub-nano ion-sieving channels (Fig. 7b). ZIF-8 exhibited selective anion restriction and Li-solvated cluster desolvation, enabling uniform Li⁺ intercalation and enhanced transport kinetics.²³

The development of porous PI membranes via techniques such as NIPS, sacrificial templating, and track etching has advanced significantly; yet, critical challenges persist in balancing mechanical robustness, thickness scalability, and process adaptability. To meet the diverse requirements of PIs in high-performance battery applications, advanced modification strategies have been proposed based on the above preparation processes. These innovative approaches, containing molecular-scale functionalization, hybrid composite architectures, and multi-process synergistic manufacturing, target the optimization of PI membranes for specific application scenarios, including high-safety, solid-state configuration, and high-stability LIBs.

3. How PIs power high-performance LIBs

3.1 Improving safety of liquid LIBs as separators

The separator of LIBs is between the cathode and the anode as an electrically insulating porous membrane to prevent direct contact between the two electrodes. The separator is non-electrochemically active but provides pathways for the conduction of Li⁺ and serves to avoid internal short circuits, which can significantly affect the performance and durability of the battery. It is also effective in preventing the battery from catching fire or exploding under harsh conditions.⁶⁹

Currently, commercially available separators are mainly based on polyolefin materials, including polyethylene (PE) and PP. There are still some issues with polyolefin separators: (1) the low melting point of polyolefin materials results in poor thermal dimensional stability, which poses a potential risk to the safety of batteries at high temperatures.⁷⁰ (2) The polarity of polyolefin materials poses a potential risk to the safety of batteries at high temperatures.⁴² (3) Polyolefin materials are mainly used in the dry or wet process, and the porosity of the prepared separators is low (about 40%), which may cause a certain degree of resistance to the trans-membrane transfer of Li⁺.⁷¹ Therefore, it is of great significance to develop high-temperature-resistant battery separators.

PIs, renowned for their exceptional thermal and chemical stability, have found extensive application as separator materials in LIBs. The remarkable thermal stability ensures that they maintain structural integrity even under high-temperature conditions, thereby significantly enhancing the safety of the battery. The PI-based separator exhibits excellent heat resistance and chemical corrosion resistance, effectively withstanding the electrochemical corrosion within the battery.⁷² This not only prevents thermal runaway reactions but also bolsters the overall safety of the battery. Furthermore, PI separators possess high mechanical strength, which allows them to maintain their structural stability throughout the battery assembly and during the charging/discharging processes. This effectively guards against mechanical damage and performance degradation of the battery separator. The preparation methods of PI separators are diverse, mainly including the NIPS method, template method, and electrostatic spinning method, which have been specifically explained in the previous section. While these fabrication methods provide foundational approaches for PI separator production, recent advancements have focused on the relationship between the structure and cell performance, illustrating how the novel structural design of PI-based materials address key battery challenges like dendrite growth, interfacial instability, and transport barriers.

3.1.1 PI/inorganic composite separator. The integration of functional inorganic nanoparticles into the PI membrane has emerged as a promising strategy to enhance separator performance, including thermal stability, mechanical strength, and dimensional stability. A hybrid architecture offers strong processing flexibility in terms of structural design. Yang et al.⁷⁴ dispersed SiO₂ nanoparticles in PAA solution and prepared SiO₂–PI nanofiber separators to obtain excellent battery performance. The interfacial stress transfer can be optimized through modulation of morphology and distribution of inorganic fillers (e.g. core–shell structures or nanofibre composites). For instance, a core–shell structure PI/zirconia (PI/ZrO₂) compound separator was developed via the in situ polar adsorption and hydrolysis technique, as shown in Fig. 8a.⁷³ The PI/ZrO₂ compound separator provided superior mechanical properties of 29.1 MPa, a thermal shrinkage of 0% even at 300 °C, a higher ionic conductivity of 1.32 mS cm⁻¹, and, more remarkably, battery safety without short circuit at 140 °C. However, the inherent incompatibility between the PI and inorganic fillers remains a challenge. Physical mixing generally leads to filler agglomeration, forming weak interfacial points that may delaminate after long-term cycling.

Fig. 8 (a) Schematic diagram of the PI/ZrO₂ separator (i) and the voltage–time curves of batteries with PP and PI/ZrO₂ separators at 140 °C (ii).⁷³ Copyright 2022 Elsevier. (b) Systematic illustration of the PI/TS-POSS@TiO₂ separator (i), the electrolyte uptake of separators (ii), and contact angle tests of PP and PI/TS-POSS@TiO₂ separators (iii).⁶⁵ Copyright 2024 American Chemical Society.

Despite these advancements, there is an organic–inorganic interface, which is poorly compatible, and thus needs to conduct surface functionalization strategies. Introducing bridging molecules or chemical groups to modify the surface of inorganic fillers can significantly improve the interfacial bond strength. Chemical bonding (e.g., covalent bonding and hydrogen bonding) replaces physical adsorption and enhances interfacial adhesion. Moreover, additional functionality can be imparted to the separator (e.g., polar groups to enhance ionic conductivity) without significantly increasing the thickness or density of the separator. In this regard, Song et al.⁷⁵ addressed interfacial compatibility challenges via immobilizing TiO₂ on the surface of PI nanofibers by self-assembly using polyhedral oligomeric silsesquioxane (POSS) as a bridge to obtain a PI/trisilanollsobutyl (TS)-POSS@TiO₂ nanofiber separator. As seen in Fig. 8b, benefiting from the excellent porosity and strong polarity of TiO₂ of the fiber membrane, the electrolyte uptake of the PI/TSPOSS@TiO₂ separators was 637%, higher than those of PP, PI, and PI@TiO₂. And the contact angle was only 9.7° (PP with 46.4°), indicating better electrolyte wettability of POSS@ TiO₂ separators. While this approach alleviated interfacial issues, the multi-step synthesis involving POSS functionalization raised concerns about scalability and cost-effectiveness.

Incorporating inorganic nanoparticles (e.g., SiO₂, ZrO₂, and TiO₂) into PI matrices has established a foundational strategy for enhancing separator performance through material hybridization. While this composite strategy successfully addresses thermal and mechanical limitations, their inherent interfacial incompatibility and increased separator density will not only increase the interface impedance, but also reduce the battery energy density, which should be compensated by structural design optimization. Although surface functionalization has demonstrated excellent interfacial properties at the laboratory scale, considering the scale-up challenges and performance enhancement trade-offs, the optimization direction should center on a synergistic strategy, with hybridization as the primary focus and functionalization as the secondary focus.

3.1.2 Interconnected PI fibers. Some drawbacks of the PI/inorganic composite separator can be overcome to some extent through enhancing the PI at a basic level via optimizing the structure. In addition to incorporating various inorganic filler materials into PI nanofibers to mitigate the risk, researchers propose that interconnected PI fibers bolster their mechanical strength, thermal stability, and flexibility. The development of three-dimensionally crosslinked PI networks represents a shift from nanoparticle enhancement to topological control.

Recent advances in hydrogen-bonded PI composite separators highlight the pivotal role of molecular-level crosslinking strategies in balancing mechanical robustness and electrochemical performance. For instance, cellulose/PI–COOH hybrid separators employed hydrogen bonding between cellulose hydroxyl groups and PI carboxyl/imide moieties to achieve a 5 time increase in tensile strength (34.2 MPa vs. 6.8 MPa for pure PI) and improved capacity retention (90% vs. 86%) in LiFePO₄ cells (Fig. 9a).⁷⁶ Similarly, PI/PVDF-HFP hybrids utilize NH–CF hydrogen bonds to enhance thermal stability (withstanding 140 °C) and t_Li⁺ of 0.7, enabling high Coulombic efficiency (94.5%) in aggressive thermal cycling (Fig. 9b).⁷¹ Further innovations include triple crosslinking via pre-calendering, solvent dissolution, and chemical imidization, obtaining separators with ultrahigh tensile strength (95.5 MPa, Fig. 9c).⁷⁷

Fig. 9 (a) Systematic illustration of the preparation process of the cellulose/PI–COOH separator.⁷⁶ Copyright 2022 Elsevier. (b) Schematic representation of the PI/PV separator.⁷¹ Copyright 2023 Elsevier. (c) Schematic diagram of the triple cross-linking strategy (i) and the tensile stress of the triple cross-linking PI separator.⁷⁷ Copyright 2024 American Chemical Society.

However, even optimized monolayer structures manage to simultaneously satisfy conflicting requirements for ionic selectivity and dendrite suppression. This trade-off drives innovation toward multidimensional configurations, epitomized by functionally graded sandwich structures.

3.1.3 Sandwich structure of the PI separator. The emergence of an asymmetric PI tri-layer structure demonstrates how hierarchical organization can resolve performance trade-offs. Pore sizes play a crucial role in influencing the performance of separators. Uneven micropores (∼1 μm) of PI nanofibers would potentially trigger Li dendrite issues. Therefore, the narrower pore size fabrication and multi-layer structure design are essential to enable fast Li⁺ transport and simultaneously inhibit the Li dendrite. For instance, Yim et al.⁷⁸ regulated the porous structure of the PI separator via coating lithium p-toluenesulfinate (PTSL) particles onto the PI surface to reduce the risk of internal short circuits (Fig. 10a). PTSL-coated PI separators achieved reduced pore heterogeneity through sulfonic group integration, enhancing ionic conductivity (1.25 × 10⁻³ S cm⁻¹) and the Li⁺ transference number (0.33). Similarly, a sandwiched separator (PIFA) was developed with PI nanofibers coated with two sides of the PI aerogel (Fig. 10b), in which the Si–(OCH₂CH₃)₃ groups of the PI coating were grafted onto the PI nanofibers.⁷⁹ The PIFA demonstrated exceptional thermal stability (450 °C) and mechanical strength (40.7 MPa), enabling 97.2% capacity retention in LiFePO₄ cells after 1500 cycles at 10C. Further innovations include a self-extinguishing PI sandwiched separator (PIFAP) with ammonium polyphosphate coatings, which suppress thermal runaway by oxygen deprivation during combustion as shown in Fig. 10c.⁸⁰ Fire resistance tri-layer separators (PBEI) integrated a structural support with polybenzimidazole-sheath@PI-core nanofibers and an interlayer with polyether imide nonwoven PI, offering shutdown functionality at 235 °C and stable 120 °C operation, as shown in Fig. 10d.⁸¹ Nevertheless, environmental concerns remain with these synthetic PI separator materials, whose energy-intensive manufacturing process (400–500 °C thermal imide) contradicts sustainable development goals. This necessity for sustainability has catalyzed a shift towards bio-derived alternatives, where composites inspired by nature offer superior solutions without sacrificing performance metrics.

Fig. 10 Systematic illustration of the preparation process of PTSL-coated PI (a)⁷⁸ Copyright 2024 Elsevier, PIFA (b),⁷⁹ Copyright 2023 WIELY, and PIFAP (c) separators.⁸⁰ Copyright 2024 Elsevier. (d) Schematic diagram of PBEI (i), digital photographs of thermal shrinkages for the PBEI and Celgard membranes (ii), and the thermal cycling performance of batteries with PBEI and Celgard separators.⁸¹ Copyright 2019 Elsevier.

3.1.4 PI/biomass composite separator. Sustainable and renewable biomass materials have become the most promising green materials,⁸² and their application in Li battery devices is an important direction for the future development of energy technology. Therefore, researchers have developed some strategies to incorporate biomass materials into PI separators. Kong et al.⁸³ demonstrated a bio-based separator consisting of PI blended with lignin (PI–L), in which the polar groups of lignin provide electrolyte affinity to enlarge the liquid absorption to 592%. The PI–L separator with high ion conductivity (1.78 × 10⁻³ S cm⁻¹) and Li mobility (0.787) enables the assembled battery to exhibit excellent cycle performance (a capacity retention of 95.1% after 100 cycles at 1C) and rate performance. Afterwards, their groups combined oxidized lignin (OL) and halloysite nanotubes (HNTs) with PI to prepare a bio-based nanofiber separator (PI–OL@HNTs).⁸⁴ Compared to lignin, OL had more polar groups, such as carboxyl groups and phenol hydroxyl groups, exhibited higher thermal stability, and improved electrolyte affinity and wettability. Moreover, OL would enhance the dispersion of HNTs in the PI nanofiber matrix. HNTs had hydroxyl groups on their surface and a positive charge on the inner surface, which endows not only excellent dispersion ability, but also absorbs the anion in the electrolyte, thereby adjusting the coordination state of anions and Li⁺. Consequently, LiFePO₄/Li with a PI–OL@HNTs separator presented exceptional cycling performance with a capacity retention rate of 92.1% after 300 cycles at 1C, higher than those of Celgard (86.8%) and PI@OL (89.6%). While OL and HNTs improve short-term performance, lignin's susceptibility to oxidative degradation under high-voltage cycling (>4.3 V) may compromise long-term durability, as observed in other bio-derived materials.

The above-mentioned innovations highlight the versatility of PI in safety-centric separator design but emphasize the need for scalable manufacturing, interfacial optimization, and cycling life assessments to bridge laboratory achievements with commercial viability. Future efforts should prioritize hybrid architecture (with simultaneous surface functionalization) and green synthesis routes to harmonize performance, cost, and sustainability.

3.2 Enhancing the mechanical robustness of solid electrolytes

As discussed above, the overall safety of liquid LIBs can be effectively improved by PI-based separators. However, significant drawbacks of liquid LIBs, such as leakage risks, degradation, and high corrosion of electrolytes, have also limited their performance and application.^12,85,86 The development of solid-state electrolytes (SSEs) has become a focus in the field of energy storage, particularly for LIBs, due to their enhanced safety, higher energy density, improved stability, and reinforced mechanical stability than those of liquid electrolytes.^87,88

Polymer electrolytes have garnered significant attention due to their superior interfacial compatibility compared to inorganic electrolytes.^89,90 This advantage derives from their flexible and adaptive characteristics, which allow for better contact with electrodes, reducing interfacial resistance and enhancing overall battery performance.^10,91 Additionally, volume changes during charge–discharge cycles can be accommodated via polymer electrolytes, mitigating issues such as electrode cracking or delamination, which are commonly observed with rigid inorganic electrolytes.^92–94 This unique combination of properties makes polymer electrolytes a promising candidate for next-generation solid-state batteries.^95–98 Nevertheless, due to the inherent softness of polymer electrolytes, which often leads to issues such as short circuits during operation, most reported studies on PI-based solid electrolytes utilize PI as a reinforcing framework to enhance mechanical strength and ensure structural stability.^99–101

Variations in PI backbone rigidity, especially the difference between aromatic and aliphatic structures, directly and significantly affect the ionic conductivity and interfacial stability of solid-state electrolytes. Ion transport in polymer electrolyte relies on either the motion of polymer chain segments (the coupling mechanism) or hopping through the transient free volumes created by this motion.¹⁰² Aromatic PIs are characterized by rigid aromatic rings and exhibit high rigidity and an elevated T_g.¹⁰³ This significantly restricts chain segment mobility and free volume, hindering Li⁺ migration and resulting in low ionic conductivity, particularly below the T_g. Furthermore, their high modulus and hardness make them difficult to deform. Therefore, researchers commonly utilize aromatic PIs as the solid electrolyte support. During Li deposition/dissolution cycles, they struggle to accommodate electrode volume changes. This leads to interfacial stress concentrations, voids, physical peeling, increased impedance and reduced long-term cycling stability. Conversely, aliphatic/semi-aromatic PIs incorporated with flexible segments reduce backbone rigidity, lower the T_g and enhance the chain segment mobility and free volume.²⁰ This facilitates Li⁺ transport via coupling or hopping mechanisms, directly boosting ionic conductivity. Additionally, their lower modulus and higher flexibility and ductility enable them to better conform to volume changes through elastic and plastic deformation. This maintains tighter physical contact at the electrode interface, minimising stress accumulation and void formation. Consequently, interfacial stability is enhanced, impedance growth is slowed, and battery longevity is improved. There is a trade-off between the rigid aromatic rings/flexible aliphatic segment and their effect on the performance of the battery.

In fact, PI-based polymer electrolytes can not only significantly enhance battery safety but also substantially improve the mechanical strength of solid-state polymer electrolytes. Their inherent thermal stability and non-flammability mitigate risks such as thermal runaway and electrolyte decomposition, while their robust mechanical properties, including high tensile strength and flexibility, ensure structural integrity under mechanical stress during battery operation. This dual advantage of safety and mechanical durability positions PI-based electrolytes as highly promising materials for advancing the performance and reliability of solid-state batteries. An overview of the last five years of work around PI-based solid-state electrolytes is presented below, and key data are summarized in Table 1.

Table 1 Overview of PI-based solid-state electrolytes applied in various LIBs

Electrolyte types	Thickness (μm)	Tensile strength (MPa)	Ionic conductivity (S cm^−1)	tLi^+	Cell types	Cycle life (RT)	Ref.
PEO/LiTFSI/PI CSPE	134	3.1	0.68 × 10^−4 (30 °C)	0.27	Li/LFP	89.5% for 418 cycles at 0.3C	100
PI/PVDF/LiTFSI	—	6.1	4.1 × 10^−4 (RT)	0.46	Li/LFP	86% for 500 cycles at 0.15 mA cm^−2	101
PI@PEO/DBDPE/LiTFSI	10–25	—	—	—	Li/LFP	∼98% for 300 cycles at 0.5C (60 °C)	104
PI–PEO–TMP CPE	—	5.5	4.70 × 10^−5 (30 °C)	0.64	Li/LFP	79.4% for 80 cycles at 0.5C (140 °C)	105
LiNO3–PPT@PI	—	21.8	7.57 × 10^−4 (30 °C)	0.62	Li/LFP	81.8% for 650 cycles at 1.0C	106
					Li/NMC811	80.4% for 150 cycles at 0.5C
PI@PEO/LiClO4/LiBOB	47	4.0	2.1 × 10^−4 (60 °C)	0.70	Li/LFP	88.3% for 500 cycles at 0.5C (60 °C)	107
PEI–PEG/PI-mod	—	41 ± 2	2.33 × 10^−3 (RT)	0.54	Graphene/LCO	∼90% for 100 cycles at 0.5C	109
PI–PEO/SN@LiTFSI	17.5	13.9	2.25 × 10^−4 (RT)		Li/LFP	85.9% for 1000 cycles at 1C	111
WPI-based PEO/SN/LiTFSI	—	12.2	2.20 × 10^−4 (80 °C)	0.51	Li/LFP	94.6% for 100 cycles at 0.3C	112
PI@PDOL (practiced with DME)	45	31.0	2.9 × 10^−3 (RT)	0.61	Li/LFP (18.7 mg cm^−2)	91.8% for 200 cycles	28
PI@ PETE-15	19	135	2.24 × 10^−4 (60 °C)	0.31	Li/LFP	99.0% for 250 cycles at 0.5C	114
PI–PEO–SN CPEs	84.3	4.52	1.03 × 10^−4 (30 °C)	0.38	Li/LFP	91.7% for 150 cycles at 0.2C	109
PI–LiCPSI/PVDF–HFP	—	30.0	1.4 × 10^−4 (30 °C)	0.97	Li/LFP	90.0% for 330 cycles at 0.2C	115
PVDF–HFP/F–PI-based GPEs	84	—	1.04 × 10^−3 (RT)	0.70	Cu/NMC811 pouch cell	70% for 100 cycles	26
Double bond PI–SIGPE (practiced with EC/DMC)	201	—	2.7 × 10^−4 (30 °C)	0.87	Li/LFP	97.1% for 250 cycles at 0.2C	116
PI–PVDF/LLZTO CSE	20	11.5	1.23 × 10^−4 (RT)	0.51	Li/NCM532 pouch cell	94.9% for 80 cycles at 0.1C	117
PI–PEO–LATP	—	—	1.24 × 10^−4 (30 °C)	0.48	Li/NMC811	95% for 100 cycles at 0.2C	118
PI/LLZTO/poly(PEGDA)	—	—	0.95 × 10^−3 (RT)	0.81	Li/NMC811	96.4% for 180 cycles at 0.5C	119
PLL10–PEO/PI–PLL40	26.8	12.8	1.1 × 10^−4 (30 °C)	0.58	Li/LFP	∼88.0% for 600 cycles at 1.0C	120
					Li/NMC811	∼77.0% for 200 cycles at 0.5C
PI@TiO2/PA(PMA)	8 (PI)	—	2.0 × 10^−5 (40 °C)	0.5–0.7	Li/LFP	94.0% for 100 cycles at 1.0C (70 °C)	121
PI–LLZTO/PVDF@[Pyr1,3FSI/LiTFSI]	25	—	3.7 × 10^−4 (RT)	0.39	Li/NCM532	88% for 100 cycles at 0.1C	122
PI@PVDF/PEO/LLZTO/LiTFSI	20	—	1.44 × 10^−4 (RT)	0.47	Li/LFP	63% for 300 cycles at 0.5C	123
					Li/NMC622	∼66% for 100 cycles at 0.1C
PI@PEO/CMC–Li	—	6.5	3.15 × 10^−4 (60 °C)	—	Li/LFP	∼82.0% for 150 cycles at 0.5C (60 °C)	124
PI@CAP(EC/DEC/LiPF6)	—	7.1	2.09 × 10^−3 (RT)	0.89	Li/LFP	95.0% for 300 cycles at 1.0C	125
PI@MOF network-PVDF	—	22.0	4.08 × 10^−4 (30 °C)	0.64	Li/LFP	96.0% for 500 cycles at 0.5C	27
PI@MOF/(EC-EMC-DMC)	28	12.6	6.7 × 10^−4 (30 °C)	0.88	Li/LFP	98.4% for 5000 cycles at 2.0C	126
					Li/NMC811	91.5% for 800 cycles at 0.5C
PINF@PEGMEM–SiO2	40	—	1.14 × 10^−4 (25 °C)	—	Li/LFP	87.2% for 400 cycles at 0.2C	127
					Li/NMC811	96.0% for 120 cycles at 0.2C

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3.2.1 PI frameworks in solid polymer electrolytes. Integrating PI frameworks into solid polymer electrolytes (SPEs) has progressed through diverse strategies to balance ionic conductivity, mechanical stability, and interfacial compatibility. For example, Cui and co-workers fabricated a poly(ethylene oxide)/decabromodiphenyl ethane/lithium bis(trifluoromethanesulfonyl)imide (PEO/DBDPE/LiTFSI) SPE combined with PI frameworks, which played a crucial role in providing thermal stability, nonflammability, and mechanical robustness to the SSE. The SPE effectively prevented short-circuiting in Li–Li symmetrical cells over extensive cycling periods (exceeding 300 h).¹⁰⁴ Han et al.¹⁰⁵ reinforced PEO-based SPEs with 3D PI fiber networks and enhanced thermal stability by incorporating trimethyl phosphate (TMP) as a plasticizer. Similarly, a novel rigid-flexible synergistic polymer electrolyte (LiNO₃–PPT@PI) with a high tensile strength of 21.8 MPa was developed. Combined with the robust PI fiber skeleton, a flexible poly pentaerythritol tetraacrylate (PPT) matrix was plasticized by LiNO₃, triethyl phosphate (TEP), and fluoroethylene carbonate (FEC) (Fig. 11b).¹⁰⁶ TEP enhanced the ionic conductivity of the electrolyte to 7.57 × 10⁻⁴ S cm⁻¹ at 30 °C and improved flame retardancy, while LiNO₃ promoted the formation of Li₃N- and LiF-rich interphases, stabilizing the electrode–electrolyte interfaces.

Fig. 11 (a) Schematic illustration of the PI nanofiber membrane.¹²⁸ Copyright 2023 American Chemical Society. (b) Schematic illustration (i) and stress–strain curve (ii) of LiNO₃–PPT@PI.¹⁰⁶ Copyright 2024 Elsevier. (c) Synthesis route of PI–LiCPSI.¹¹⁵ Copyright 2023 Royal Society of Chemistry. (d) DFT investigations of chemical interactions.²⁶ Copyright 2024 Elsevier.

Beyond structure optimization, electrolyte formulation strategies, particularly dual-/multi-salt systems, have emerged to further refine interfacial dynamics and ion transport. Dual-salt/multi-salt systems (e.g., lithium perchlorate (LiClO₄) and lithium bis(oxalate)borate (LiBOB) or LiTFSI, lithium bis-(fluorosulfonyl)imide (LiFSI), and lithium bis(pentafluoroethanesulfonyl)imide (LiPFSI)) leverage synergistic anion effects to stabilize solid–electrolyte interphases (SEI), improving cycling retention by 5–10% compared to a single-salt system.^107,108 However, incompatibility between salts may induce parasitic reactions, accelerating capacity fade under high-voltage conditions.

Succinonitrile (SN), an effective solid-state non-ionic plasticizer with a structured molecular lattice, is incorporated into PEO-based electrolytes to suppress PEO crystallization and enhance the dissolution of Li-salts, thereby boosting the local ionic conductivity of SPEs. For instance, a PEO-based 3D network electrolyte (PEO/SN/LiTFSI) enhanced by interwoven high-modulus PI nanofibers with functional SN plasticizers has been proposed to synergistically improve both ionic conductivity and mechanical robustness.¹⁰⁹ In addition, the thickness of ultrathin electrolytes (e.g., 10–30 μm) directly lowers ionic migration resistance, as described by the relationship G = σA/l, where thinner electrolytes yield higher ionic conductance (G).¹¹⁰ For instance, a PI-PEO/SN@LiTFSI electrolyte with a thickness of 17.5 μm was sufficiently thin to have an ionic conductivity of 2.25 × 10⁻⁴ S cm⁻¹ at 25 °C, superior to thicker electrolytes.¹¹¹ To further obtain more stable PI nanofibers, an optimized design has been proposed. A water-immersed polyimide membrane (WPI) framework for the PEO/SN/LiTFSI electrolyte was reported to enhance the mechanical performance with a tensile strength of 12.2 MPa, despite a high porosity level of 85.6%.¹¹²

Compared to the solution-casting method, in situ polymerization has garnered significant attention. Benefiting from its high mechanical strength and porosity, PI nanofiber membranes are considered ideal scaffold materials for in situ polymerization, offering enhanced structural integrity and ion transport efficiency in solid-state electrolytes.¹¹³ In this regard, Yan and co-authors synthesized quasi-solid electrolytes by initiating the ring-opening reaction of DOL by LiPF₆ in the PI framework, while the LiTFSI promoted the self-polymerization, and 1,2-dimethoxyethane (DME) was used as the plasticizer.²⁸ Similarly, Meng et al. prepared an ultrathin (19 μm) SPE membrane via in situ thiol–ene click polymerization and reinforced structurally by the electrospinning PI substrate.¹¹⁴ In contrast to the simple compounding of PIs with polymers described above, a chemical grafting strategy was proposed.¹²⁸ Fig. 11a shows a heat-resistant PI nanofiber grafted by poly(ethylene glycol) (PEG) and cross-linked with amino-rich polyethyleneimine (PEI) to form a gel-like polymer-based SPE. The PEG-grafted PI membrane exhibited a significantly enhanced ionic conductivity of 2.33 mS cm⁻¹ at room temperature and an electrolyte uptake of 168%, attributed to its distinctive gel-like surface.

While safety improvements are crucial, simultaneous optimization of ionic selectivity remains essential for high-performance applications.

3.2.2 PI-based single ion electrolytes. The above-mentioned dual-ionic polymer electrolytes not only cause changes in the salt concentration on the surface and inside of the SPE, resulting in concentration polarization that produces high internal resistance, voltage loss, phase transition, or salt precipitation, but also exhibit a low t_Li⁺ (<0.5), which deteriorates their electrochemical performance. Therefore, concentration polarization can be reduced and t_Li⁺ increased by limiting the free transfer of anions so that the Li⁺ contribution to ionic conductivity dominates. A robust PI single-ion polymer electrolyte (PI-SIPE) was synthesized by utilizing hydroxyl-containing PI (PI–OH) as a thermally stable backbone and grafting it with lithium 3-chloropropanesulfonyl-(trifluoromethanesulfonyl)imide (LiCPSI) combined with PVDF–HFP, as shown in Fig. 11c.¹¹⁵ The resulting PI-SIPE exhibits exceptional thermal stability with minimal shrinkage at 200 °C and impressive mechanical properties, including a tensile strength of 30 MPa and a Young's modulus of 1.72 GPa. The PI-SIGPE, plasticized with EC/DMC, demonstrated a high t_Li⁺ (0.97), outstanding ionic conductivity (1.4 × 10⁻⁴ S cm⁻¹ at 30 °C), and a wide electrochemical window (5.2 V), and delivered over 90% capacity retention after 330 cycles in Li/PI-SIGPE/LFP batteries.

Unlike simple blending, the introduction of non-covalent interactions, hydrogen bonding, enables improved compatibility by enhancing interfacial bonding between different components. Hwang et al. composited PVDF and fluorinated polyimide (F-PI) via thermal treatment to obtain an anion-trapping composite gel electrolyte, in which H-bonding interaction existed between N–H of F-PI and –F of PVDF–HFP. The amide bond (N–H part) of F-PI exhibited anion (FSI⁻) trapping ability through strong hydrogen bonding interactions, which was confirmed by the density functional theory (DFT) investigation as shown in Fig. 11d.²⁶ The FSI⁻ strongly attached to F-PI via a hydrogen bond with a bond length of 1.93 Å.

Compared to physical blending, chemical bonding between polymer chains and single-ion functional monomers significantly improves compatibility and avoids phase separation. Based on this, Hu et al.¹¹⁶ synthesized a PI-SIPE via UV-induced in situ cross-linking of the double-bond PI, 4-styrene sulfonyl (benzenesulfonyl) imide, and pentaerythritol tetra(2-thiol acetate), incorporating plasticizers (EC/DMC) to enhance performance. By anchoring anions within the polymer network, the PI-SIPE achieves a high t_Li⁺ (0.87) and ionic conductivity (2.7 × 10⁻⁴ S cm⁻¹ at 30 °C). The electrolyte demonstrates exceptional thermal stability (no shrinkage at 200 °C for 0.5 h) and a wide electrochemical window (4.6 V), and promotes uniform Li deposition, as evidenced by stable Li stripping/plating over 900 hours. These properties, combined with their robust cross-linked structure, addressed the challenges of anion-induced polarization and improved the efficiency and safety of LIBs.

Although PI-based single-ion electrolytes greatly enhance the electrochemical performance of batteries, the cost of both single-ion monomers and PIs is high, and the preparation methods can significantly elevate the cost of materials, limiting their commercial applications.

3.2.3 PI/inorganic composite solid electrolytes. Composite solid electrolytes (CSEs) synergistically combine a polymeric matrix with inorganic fillers to overcome the “trade-off” between ionic conductivity and mechanical strength. This design allows for efficient optimization of electrochemical and thermo-mechanical properties through phase distribution. A mechanically robust CSE was developed with a porous PI film as a host structure, integrating with Li₇La₃Zr_2−xTa_xO₁₂ (LLZTO) nanoparticles and a PVDF polymer matrix (Fig. 12a).¹¹⁷ The PI network ensured structural robustness (a tensile strength of 11.5 MPa, Fig. 12a-ii) and uniform LLZTO dispersion, enabling continuous Li⁺ pathways that suppress dendrite growth. This design achieved stable cycling in symmetric cells (>1000 hours) and a high-capacity retention of 94.9% after 80 cycles in Li/LiNi_xCo_yMn_1−x−yO₂ (NCM) 532 cells. The CSE also demonstrated exceptional safety, resisting mechanical abuse (folding, cutting, and nail penetration), and highlighting its potential for safe solid-state batteries. Similarly, the PI–PEO–Li_1.4Al_0.4Ti_1.6(PO₄)₃ (LATP) CSE was developed via the solution-casting method, as shown in Fig. 12b.¹¹⁸ The crystallinity of PEO was reduced, and the electrochemical performance was enhanced. Benefiting from the high limit oxygen index (66%) and mechanical stability of the biphenyl tetracarboxylic dianhydride-4,4′-oxydianiline (BPDA-ODA) type PI, the PI–PEO–LATP solid electrolyte exhibited superior self-extinguishing properties (maintaining structural integrity after self-extinguishing test) than pure PEO (Fig. 12b-ii). Zhao et al.¹¹⁹ presented a novel CSE (PLSE) via in situ thermal curing of a poly(ethylene glycol) diacrylate-based polymer electrolyte onto the LLZTO-rich skeleton (well-dispersed LLZTO particles into the robust PI scaffold), followed by a roll-compression process, as shown in Fig. 12c-i. Compared to the simple blending described above, this design facilitates rapid Li⁺ transport as attributed to the high proportion of ionically conductive LLZTO nanoparticles and the abundant interfacial percolating channels (Fig. 12c-ii), yielding a high ionic conductivity (0.95 mS cm⁻¹) and t_Li⁺ of 0.81. Moreover, ultrathin electrolytes conform tightly to electrode surfaces, minimizing voids and interfacial impedance. For instance, an ultrathin asymmetric composite electrolyte reinforced with the PI fiber membrane (26.8 μm) was fabricated using a two-sided tape casting technique, achieving superior interfacial properties and high-voltage compatibility.¹²⁰ The double constraints of PI and Li₇La₃Zr₂O₁₂ extended Li symmetric battery cycling to 4000 h, while integrated interfacial layers enhanced the cycling stability. The resulting NCM811 pouch cell demonstrated exceptional safety and energy density (333.1 Wh kg⁻¹/713.2 Wh L⁻¹), offering a scalable solution for solid-state lithium metal battery (LMB) interface challenges.

Fig. 12 (a) Schematic illustration of lithium plating/stripping processes (i) and stress–strain curves of electrolytes (ii).¹¹⁷ Copyright 2020 Elsevier. (b) Morphology and EDS linear scanning profile of the Ti element (i) and the flame test of PI–PEO–LATP CSE (ii).¹¹⁸ Copyright 2022, American Chemical Society. (c) Schematic illustration of solid batteries with the LLZTO-rich skeleton (i) and Li⁺ transport in LLZTO/PISE (ii).¹¹⁹ Copyright 2024 Elsevier. (d) Schematic and physical characterization of the asymmetric ultrathin composite solid electrolyte.¹²³ Copyright 2022 Wiley.

While PI-based ultrathin electrolytes show promise, challenges remain. On the one hand, balancing the thickness with tensile strength requires advanced composites, such as PI-ceramic hybrids.¹²¹ An ultrathin (25 μm) and flexible composite SSE was developed using a PI film as a 3D network host, combined with a ceramic filler (LLZTO), a polymer binder (PVDF), and ionic liquid solution.¹²² The electrolyte exhibits a high ionic conductivity of 3.7 × 10⁻⁴ S cm⁻¹ at 25 °C and low interfacial impedance, enabling stability for 100 cycles in Li/NCM₅₃₂ batteries with 88% capacity retention at 0.1C. This design ensures both high safety and mechanical robustness for practical applications. Moreover, for large-scale production, scalable fabrication techniques need to be optimized. In this regard, Fan et al.¹²³ developed an asymmetric ultrathin composite solid electrolyte with a thickness of ∼20 μm reinforced by PI via roll-to-roll processing, featuring a ceramic-rich layer to inhibit Li dendrite growth and a polymer-rich layer to enhance cathode compatibility (Fig. 12d). The electrolyte exhibited an ionic conductivity of 1.44 × 10⁻⁴ S cm⁻¹ at 35 °C, enabling stable cycling in symmetric cells for over 1200 hours and superior performance in Li/NCM and Li/LFP solid-state batteries. This design offers a promising solution to interface challenges in high-energy-density solid-state LMBs.

Among composite strategies, PI-based systems have emerged as particularly promising ways due to their inherent thermal stability and structural versatility, paving the way for specialized functional enhancements.

3.2.4 PI/biomass composite electrolytes. The pursuit of sustainable alternatives has driven the integration of biological components with the synthetic advantages of PI, creating novel eco-hybrid systems. Sustainable and renewable biomass materials have emerged as the most promising green materials, and researchers have developed several strategies for incorporating biomass materials into solid-state electrolytes for LIBs. For instance, Min et al.¹²⁴ proposed an all-solid-state (ASS) Li battery with a hybrid polymer electrolyte membrane in which the PI non-woven fabric is used as a reinforcing framework. To enhance and facilitate the transport of Li⁺, they introduced modified N-carboxymethyl chitosan (CMC–Li) into the PEO matrix. The incorporation of CMC–Li significantly improved ionic conductivity, achieving values of 3.16 × 10⁻⁵ S cm⁻¹ at 30 °C and 3.15 × 10⁻⁴ S cm⁻¹ at 60 °C. While the PI framework enhanced mechanical stability with the tensile strength of PEO/CMC–Li@PI 40 times higher than that of PEO. This design leverages the synergistic effects of CMC–Li and PI, offers a promising approach for advancing solid polymer electrolytes in ASSBs, and highlights the importance of combining ionic conductivity enhancement with mechanical reinforcement for high-performance energy storage systems. Kong et al.¹²⁵ used PI and cellulose acetate propionate (CAP) to obtain electrospinning nanofibers for building a GPE. The incorporation of CAP enhanced electrolyte wettability through the introduction of polar functional groups while simultaneously improving the thermal and mechanical properties by forming hydrogen bonds with the rigid PI chains. The synergistic integration of biomass-derived components (e.g., CMC–Li and CAP) with PI frameworks demonstrates a transformative pathway toward sustainable, high-performance solid-state electrolytes, balancing ionic conductivity, mechanical robustness, and eco-compatibility while underscoring the need for further optimization in achieving long-term interfacial stability and scalable green synthesis.

3.2.5 PI substrates for nanoparticle growth. The development of PI/biomass composite electrolytes highlights the versatility of PI in integrating sustainable materials for enhanced ionic conductivity and interfacial stability. Building on this multifunctionality, PI substrates further demonstrate their structural adaptability by serving as tailored platforms for nanoparticle growth. PI's tunable surface chemistry and mechanical resilience are able to anchor and stabilize nanostructures for advanced catalytic or energy storage applications.

To further enhance the interfacial properties, the electrolyte microstructure can be precisely controlled by templated nanoparticle growth. The excellent 3D structure and inherent high mechanical strength of PI nanofiber membranes make them highly suitable for constructing robust metal–organic frameworks and 3D ceramic network structures. Their interconnected porous framework not only provides structural stability but also facilitates uniform dispersion and integration of nanoparticle fillers, enhancing the overall mechanical and electrochemical performance of composite solid electrolytes. This unique combination of properties positions PI nanofiber membranes as promising foundational materials for developing advanced SSEs with improved ionic conductivity, thermal stability, and resistance to mechanical stress. Mai et al.²⁷ designed composite solid electrolytes via a hierarchically self-assembled MOF network along PI fibers to create continuous Li⁺ pathways at the micrometer scale (Fig. 13a). The surface-etched carboxylated PI provided homogeneous nucleation sites for MOF growth. The sub-nanopores and Lewis acid sites of the MOF selectively confined the anions and enhanced Li⁺ transport. Consequently, the resulting MOF network-PVDF composite electrolyte possessed a high ionic conductivity of 4.08 × 10⁻⁴ S cm⁻¹ at 30 °C and 22.02 MPa (5 times higher than that of the pure PVDF solid electrolyte). Li symmetric batteries containing this electrolyte exhibited stable cycling at a current density of 0.5 mA cm⁻², superior to that of MOF powder-PVDF and pure PVDF electrolytes, which suffered from dendrite-induced short circuits (Fig. 13a-ii). Similarly, Xu et al.¹²⁶ utilized a PI membrane as a substrate for the growth of a MOF. However, different from Mai's work, Xu proposed a leaf-inspired MOF-based QSE, which is continuous and defect-free. In this design, a robust PI network serves as a “vein-like skeleton,” providing mechanical support, while the in situ grown MOF crystals behave as “mesophyll cells,” self-filling the PI framework to form a defect-free MOF membrane (Fig. 13b). The liquid electrolyte, restrained within the nano/sub-nano pores of the MOF crystals, functions as “blood,” ensuring efficient ionic transport and overall structural integrity. The results also showed that the Li symmetric batteries can be operated stably for more than 5000 h without short-circuiting (Fig. 13b-ii).

Fig. 13 (a) Schematic illustration of a MOF network (i), cycling performance of Li symmetric cells (ii), and a schematic illustration of Li dendrite behaviors in different solid electrolytes.²⁷ Copyright 2022 Wiley. (b) Leaf-inspired robust MOF-QSEs (i) and cycling performance of Li symmetric cells (ii).¹²⁶ Copyright 2023 Elsevier. (c) Design (i) and morphologies (ii) of PINF@PEGMEM-SiO₂, and ionic conductivity plots of different SPEs (iii).¹²⁷ Copyright 2024 American Chemical Society.

To address the limited Li⁺ conductivity of polyether-based polymer electrolytes at room temperature, Yang et al.¹²⁷ introduced a polyethylene glycol methyl ether methacrylate (PEGMEMA)-modified silica-coated PI fiber scaffold (PINF@PEGMEM-SiO₂) into poly(ethylene glycol) dimethyl ether (PEGDME), as shown in Fig. 13c. This innovative design enhanced Li⁺ transport at room temperature with an ionic conductivity of 1.14 × 10⁻⁴ S cm⁻¹ and a t_Li⁺ of 0.41, while improving the mechanical robustness of the ether-based electrolyte. The optimization derived from the carbonyl groups (C=O) on the PEGMEM attached to the SiO₂ promoted the dissociation of the Li salts and facilitated the transport of the free Li⁺ ions at the interface. The presence of identical –C–C–O– units in both PEGMEM and PEGDME confirmed excellent compatibility between the interfacial and the bulk. As a result, the NCM811 cells demonstrate excellent electrochemical stability for over 300 cycles at 1C.

Overall, these studies highlight the capabilities of nanoparticle structures based on PI substrates for versatility in advancing solid-state electrolytes. However, scalability, long-term interfacial stability, and cost-effective synthesis remain key barriers to industrial translation.

3.3 Stabilizing the electrode/electrolyte interfaces as protective layers

3.3.1 Stabilizing the interface of the cathode. Based on the fact that the gravimetric energy density (E) is proportional to the voltage U of the cathode according to the mathematical equation (Q and m are the capacity and mass of the battery, respectively).¹²⁹ The U value is determined by the difference in electrode potential between the cathode and the anode. Consequently, cathode materials with high reversible capacities and operating voltage will be key to obtaining high energy density LIBs. Therefore, elevating the working voltage seems to be an effective pathway to increase the specific capacity.

The layered transition metal oxide, Li_xMO₂ (M = transition metals, 0 < x ≤ 2), is the most promising for increasing the operation voltage and specific capacity for practical applications.^130,131 The metal-layered lithium cobalt oxide (LiCoO₂, LCO), as the first commercialized high voltage cathode, could deliver more capacity with increased charge cut-off voltage.^132,133 Nevertheless, the cobalt resources and cost restrict its further development. This accelerates the development of ternary layered oxide materials such as NCM, improving the cycling stability.^134,135 Recently, a Li-rich layered oxide represented by xLi₂MnO₃·(1 − x)LiMO₂ (M = Mn, Ni, Co, etc.) has been reported that could increase the operation voltage to >4.5 V because of its high capacity as much as or higher than 250 mAh g⁻¹.^136,137 However, the side reaction between the electrolyte and cathode materials would be further aggravated under high voltage. To suppress the side reactions and improve capacity retention, coating of a dense film on the cathode seems to be required when preparing and for application as a high-voltage cathode. Inorganic coatings on the cathode were widely investigated due to their facile preparation, and the surface modifications were conducted for high voltage cathodes with inorganic coatings such as Al₂O₃,^138,139 ZrO₂,¹⁴⁰ ZnO,¹⁴¹ AlF₃,¹⁴² and Li₃PO₄.¹⁴³ Although improvement in capacity retention and stability is obtained with the coating of inorganic materials, poor ionic or electronic conductivity of both metal oxides and fluorides still hinders its large-scale application for cathode electrode improvement. Importantly, building a uniform and ultrathin coating is an important part of research in cathode electrodes to prevent interfacial side reactions. There are new methods, such as atomic layer deposition (ALD)¹⁴² and CVD,¹⁴⁴ that can accomplish a continued and conformal coating on the cathode particles. However, the relatively costly equipment and complex preparation processes limit its large-scale production. Therefore, selecting polymers with designable molecular structures and high adhesion as coatings is an effective strategy.

The chemical resistance and long-term stability of PI ensure that it is insoluble in electrolytes and has good electrolyte wettability, making it an excellent choice for cathode coating. Meanwhile, the PI coating is easily applied as thin, lightweight films, which is particularly advantageous in cathode particle coating. The thermal imine method makes it easier to apply the coating than other methods. The PI protective coating was synthesized by thermally curing 4-component (pyromellitic dianhydride/biphenyl dianhydride/phenylenediamine/-oxydianiline) polyamic acid onto LiCoO₂.¹⁴⁵ The PI layer with high coverage and facile ion transport improved the high voltage performance and thermal stability of LiCoO₂. Fig. 14a shows a schematic of the further improved method reported by Lee et al.¹⁴⁶ The PAA-coated LiCoO₂ powder was thermally cured via a stepwise imidization process to form a highly continuous structure with a nanometer thickness (∼5 nm). The advantageous effects of PI-coated increased the charge cut-off voltage from 4.4 V to 4.6 V, which prevents the electrochemical decomposition of the liquid electrolyte. The cycle performance and thermal stability of a high voltage LiNi_1/3Co_1/3Mn_1/3O₂ cathode were also improved by the surface modification by an ultra-thin PI coating (∼10 nm).¹⁴⁷ Encouraged by the successful application of PI-coated LiCoO₂ and NCM, Li et al. controlled the thickness of the coating on the Li_1.2Ni_0.13Mn_0.54Co_0.13O₂ (LNMCO) to ∼3 nm, confirmed by high-resolution transmission electron microscopy (HR-TEM).¹⁴⁸ Electrochemical performances of LNMCO, including cycling stability and rate capability, were improved by coating a PI layer. Compared with the NCM electrode, a Co-free and Li-rich cathode, LiNi_0.5Mn_1.5O₄ (LNMO), offered high-power and economic advantages, and its practical adoption hinges on overcoming structural instability, kinetic limitations, and interfacial degradation. Lee et al. successfully deposited a PI coating on the LNMO surface via thermal imidization of a 4-component (pyromellitic dianhydride/biphenyl dianhydride/phenylenediamine/oxydianiline) PAA.¹⁴⁹ The manganese (Mn) dissolution was suppressed, and the high-voltage cell performance was improved. As a result, the PI-based protective coating layer could significantly improve the stability of the electrode at high voltage and reduce the liquid electrolyte decomposition on the positive side.

Fig. 14 (a) (i) TEM photograph of PI-coated LiCoO₂. (ii)Discharge capacities for cells assembled with pristine LiCoO₂ or PI-coated LiCoO₂ in a voltage range between 3.0 and 4.6 V.¹⁴⁶ Copyright 2012 Elsevier. (b) Schematic diagram showing the role of the PI coating and cycle performance of SnO₂ with different amounts of PI content.¹⁵⁰ Copyright 2017 Elsevier. (c) Schematic of battery and the SEM image of the sandwiched PI-TPP-Cu film.¹⁵⁴ Copyright 2020 Springer Nature. (d) Schematic illustration of 3D HT-Cu@PI enhancing Li deposition and battery performance.¹⁵⁵ Copyright 2024 American Chemical Society.

3.3.2 Stabilizing the interface of the anode. The anode is a critical component in LIBs, directly influencing the performance, safety, and cycle life. Each of the anode electrodes currently used in LIBs, such as alloy-based electrodes, silicon (Si) electrodes, carbon electrodes, Li–metal anode electrodes, and even in anode-free LIBs, still face different challenges that need to be overcome.

Anode materials undergo a large volume change during Li⁺ insertion and extraction. Yu et al. proposed a high-modulus PI coating to improve Si, tin dioxide (SnO₂), and antimony (Sb)-based anodes in recent years. SnO₂, as a high-capacity, large-volume-change material, undergoes both alloy and conversion reactions. As shown in Fig. 14b, the capacity retention of SnO₂ was improved by the PI coating from 80% to 100% after 80 cycles at 250 mA g⁻¹.¹⁵⁰ With this study, it is clear that an alloy-based material is very promising as an anode electrode material in LIBs. With appropriate operating potential (0.8 V vs. Li/Li⁺), Sb-based electrodes are considered promising alternatives as fast-charging anodes for LIBs. However, the multiple phase transitions during charge and discharge hinder the widespread application of the Sb electrode. Yu et al. proposed a simple method by encapsulating each Sb particle with a high modulus PI protective coating layer.¹⁵¹ The Sb particle cracking after lithiation was suppressed by the high modulus PI protective coating layer. Combined with CMC, the rate performance of the Sb electrode was further improved. To meet high-energy density demands, the Si electrode shows higher potential as next-generation anode materials for LIBs. However, the drastic volume expansion (400%) of Si particles during the lithiation/delithiation could induce cracks and pulverization.^152,153 Structural design is an effective method to improve the volume expansion of Si particles. The PI protective coating layer shows higher mechanical strength than other coating layers, which have been widely used in cathode coating and as separators. Zhang et al. used a facile mechanical stirring process to prepare Si nanoparticles coated with PI transformed from PAA.¹⁵⁶ The Si@PI composites showed lower Li⁺ diffusion resistances compared with pure Si particles. To solve the large volume change of the SiO particles during charge/discharge, smaller sizes could accommodate the stress, and voids could alleviate volume changes. However, the nanoparticles are difficult to produce on a large scale and are easily prone to side reactions between the liquid electrolyte and the electrode. Inspired by previous research, Yu et al. illustrated a self-assembled monolayer (SAM) to covalently bond the active material with a high-modulus PI protective coating layer.¹⁵⁷ The mechanical strength and chemical stability of PIs ensure them as a superior coating for improving the performance of the anode materials.

3.3.3 Modifying the Cu current collector. The Li metal has been considered one of the most promising anodes due to its superior specific capacity and low electrochemical potential. However, the practical application of the Li metal anode is hindered because the Li dendrites during the cycling cause low CE and continued cycling life fade. A series of safety hazards were posed, such as short circuits, thermal runaway, and combustion, affecting the commercialization of LMBs. Consequently, various strategies have been considered to stabilize Li metal anodes. Notably, many methods need excess Li during the assembly process of the battery. The 200% excess Li metal would reduce the theoretical volumetric capacity of LMBs from 2060 mAh L⁻¹ to 687 mAh L⁻¹, which significantly hampers the development of high energy density LMBs.¹⁵⁸ In recent years, the Cu current collectors pre-deposited with a finite amount of Li were used as the anode, and even the Cu current collectors can be directly used as the anode in the anode-free LMBs. However, the lithiophobic nature of Cu will significantly lead to the nucleation and growth of Li on the surface, resulting in Li dendrite growth and low Coulombic efficiency. Building a functional modification layer on the Cu surface is an effective strategy that improves the performance of LMBs. Furthermore, the wrinkles and fracture of Cu foil are still challenges in industrial production for battery anode current collectors. Due to its excellent mechanical strength, high thermal stability, and lighter weight than Cu, the PI protective coating layer has been widely used as the current collector coating. Combining the advantages of electroless plating techniques, Fu et al. utilized laser-etched porous arrays to obtain copper-plated polymer films (P@Cu). The P@Cu current collector exhibited superior bending performance and tensile strength compared to commercial Cu foil current collectors.¹⁵⁹ The weight of the metal current collector is a research priority in the modification of the current collector. The lightweight PI simultaneously minimizes the dead weight. Cui et al. prepared an ultralight polyimide-based current collector with a low specific mass of 1.54 mg cm⁻² by sandwiching a polyimide embedded between two super-thin Cu layers (Fig. 14c).¹⁵⁴ PI coatings significantly improve the strength and commercial application of Cu current collectors. Nevertheless, the issue of infinite volume change on a current collector during the Li plating and stripping still influences cycling stability. Constructing a 3D current collector could effectively alleviate the volume change of the electrode. Cui et al. used the electrospinning method and ALD to fabricate a PI–ZnO matrix. A porous metallic Li anode prepared by infusing molten Li into a core–shell PI–ZnO matrix exhibited long-term cycling stability even at a high current density of 5 mA cm⁻².¹⁶⁰ Similarly, Xu et al. proposed a novel polymeric-based current collector prepared via combined electrospinning of PI and Cu coating, which efficiently boosted the transport of Li⁺ and increased Li nucleation sites (Fig. 14d).¹⁵⁵

In summary, PIs have good mechanical strength, thermal stability, and superior wettability for liquid electrolytes. Furthermore, the favorable adhesive characteristics of PIs ensure good contact with electrodes under various conditions. These fascinating properties make PIs an excellent choice for electrode coating.

3.4 Improving the structural integrity of electrodes as binders

Binders play a crucial role in integrating active materials and conductive agents into a cohesive structure, thereby enhancing the electrochemical reaction interface and active surfaces of the electrode. Despite its minimal proportion in the electrode composition, the binder's contribution is indispensable.¹⁶¹ An ideal binder should exhibit the following characteristics: (1) strong adhesion and high tensile strength to maintain electrode structural integrity, (2) excellent flexibility to accommodate volume expansion of active materials without detachment, (3) superior chemical and electrochemical stabilities to minimize side reactions, and (4) favorable dispersibility, electrolyte insolubility, cost-effectiveness, and environmental compatibility.¹⁶²

The commercial LIB industry predominantly employs PVDF as the primary binder material. This linear homopolymer, characterized by its solubility in organic solvents, has maintained its dominant position owing to its superior adhesive characteristics and electrochemical stability.¹⁶³ However, conventional PVDF-based materials present several technical constraints, including limited thermal endurance and mechanical robustness, which may lead to premature aging and interfacial delamination, ultimately compromising battery performance and safety.¹⁶¹

In this context, PIs have emerged as promising alternative binder materials, offering distinct advantages over traditional PVDF systems. The thermal properties of PIs are particularly noteworthy, as they maintains structural integrity and binding efficacy at elevated operational temperatures, effectively mitigating thermal degradation risks and enhancing battery safety parameters. This thermal stability is especially crucial for high-energy-density batteries operating under harsh conditions, as it prevents binder decomposition at elevated temperatures.³³ From a chemical perspective, PIs demonstrate remarkable resistance to electrolyte-induced corrosion, effectively suppressing undesirable interfacial reactions between the electrode and electrolyte components, which contributes to extended cycle life. Furthermore, the mechanical characteristics of PIs, particularly their enhanced tensile modulus (up to 30 MPa) and interfacial adhesion strength, provide robust cohesion between active materials and current collectors. This structural reinforcement effectively accommodates the volumetric stresses associated with repeated charge–discharge cycles, thereby improving both the cyclic performance and operational safety of LIBs.

3.4.1 Anode binders. The exceptional mechanical properties of PIs are particularly beneficial in addressing the mechanical instability of electrodes, especially in Si-based anodes, where volume expansion during lithiation can exceed 300%.¹⁶⁴ The PI's unique combination of high tensile strength and flexibility enables it to withstand repeated stress without fracture.⁷⁰ For instance, Aurbach et al.³⁴ proposed a co-polymerized PI, denoted as P84 (3,3,4,4-benzophenone-tetracarboxylic dianhydride (BTDA)), as an innovative binder for composite Si anodes and investigated thermal treatment on the performance. By comparing the P84 binder with other common binders such as sodium alginate (SA) and lithium polyacrylate (LiPAA), the Si anodes employing the P84 binder exhibited superior performance and formed a passivated layer.

The mechanical failure of electrodes in LIBs is often attributed to the insufficient mechanical properties of polar binders. To address this issue, binders with both mechanical stability and electron-rich functional groups have been specifically designed for micro-silicon (micro-Si) anodes. For example, a PI-based binder functionalized with hydroxyl (–OH) and trifluoromethyl (–CF₃) groups (denoted as PI–CF₃) was developed for commercial micro-particulate Si anodes as shown in Fig. 15a.¹⁶⁵ The PI–CF₃ binder established a dynamic cross-linking network through extensive hydrogen bonding between its functional groups and –OH on the silicon particles' surface, significantly enhancing the adhesive strength within the anode, mitigating electrode fracture and improving structural integrity. As a result, the PI–CF₃-based micro-silicon anode demonstrated a reversible capacity of 1478 mAh g⁻¹ after 200 cycles at a current density of 0.6 A g⁻¹, highlighting its potential for high-performance LIB applications.

Fig. 15 (a) Schematic illustration of the action mechanism of PI–CF₃ binders with Si particles, and the PI–CF₃ with PI–CF₃.¹⁶⁵ Copyright 2024, Wiley. (b) Schematic diagram of the SiO_x/C anode during lithiation/delithiation states and the advantages of fatigue-resistant polyimide binders (i) and the cycling performance of the SiO_x/C electrodes with average areal mass loading of SiO_x/C at 2 mg cm⁻² (ii).¹⁶⁶ Copyright 2025, American Chemical Society. (c) Synthesis scheme of Si@PMTI-Ag.¹⁶⁷ Copyright 2025, Wiley.

To address the unclear impact of PI fatigue resistance on battery performance, a series of PIs were synthesized, and their strength retention was evaluated after 100 cycles of stress load-release at 30% strain stretching.¹⁶⁶ Electrochemical cycling tests of SiO_x/C anodes using these PIs as binders revealed that BPDA-ODA and BPDA-PDA, which exhibit superior fatigue resistance, maintained over 95% capacity retention after 100 cycles, delivering specific capacities of approximately 1400 mAh g⁻¹ and areal capacities of 2.8 mAh cm⁻² (Fig. 15b). Nevertheless, achieving superior electron transport and fostering a durable SEI while maintaining structural integrity during long cycling remains a critical challenge for the PI-based Si binder. Wu et al.¹⁶⁷ developed a 3D branched ion–electron dual-conductive PI/silver (PMTI/Ag, Fig. 15c) interface layer on nano-Si, integrating in situ-synthesized silver nanoparticles within a PMDA-ODA type PI matrix branched with 1,3,5-tris(4-aminophenoxy) benzene (TAPOB) and composed of flexible poly(dimethylsiloxane)etherimide (DMS). The structure combines rigid aromatic networks for mechanical robustness with flexible Si–O–Si chains to mitigate electrode expansion, enhance electrolyte compatibility, and accelerate Li⁺ transport, while the uniformly distributed Ag nanoparticles in PMTI create continuous electron pathways, improve charge-carrier mobility, and promote uniform Li⁺ diffusion with thin SEI layer formation, collectively maintaining structural integrity.

3.4.2 Cathode binders. NCM811 has garnered significant attention as a cathode material for LIBs due to its high theoretical capacity, extended cycle life, and lower production costs. However, challenges such as structural instability, surface degradation, and safety concerns must be addressed to enable its widespread commercial application. Rational structural engineering of functional binders can effectively enhance the structural stability of cathode materials for achieving prolonged electrochemical cycling life. Zhang et al.¹⁶⁸ recently reported novel sulfonyl-containing PI binders synthesized by a one-step method, leveraging their strong polarity to form a uniform protective layer on NCM811 particles, which minimized side reactions and electrolyte decomposition (Fig. 16a). The high dielectric constant of sulfonyl groups facilitated Li⁺ dissociation and transport, while experimental and computational analyses confirmed the stabilization of the cathode–electrolyte interface and suppression of transition metal dissolution. This resulted in exceptional cycling stability under high-voltage conditions.

Fig. 16 (a) A schematic diagram of the mechanism of PI binders on NCM811 particles (i) and the chemical structure of the synthesized PI binders (ii).¹⁶⁸ Copyright 2024 Elsevier. (b) The cross-sectional SEM image of the cathode sheet of the LIBs and its corresponding schematic diagram, wherein molecular fragments Ar1 and Ar2 are attributed to dicarboxylic acids and diamines, respectively.¹⁶⁹ Copyright 2022 Elsevier. (c) Chemical structure of cPIS (i) and cycling performance of NCM811/cPIS cathodes at various cutoff voltages during 100 cycles at 0.2C (ii).¹⁷⁰ Copyright 2023 Elsevier.

The rigid-flexible structure of PI binders synergistically enhances electrochemical performance, with rigid aromatic segments providing mechanical support and Li⁺ affinity, while flexible segments ensure solubility and accommodate volume changes during cycling. For example, Wu et al.¹⁶⁹ designed aromatic PI binders integrated with soft functional segments (ether bond) and hard functional segments (biphenyl structure) by two steps (Fig. 16b). The cathode slurry with PAA precursor glue containing polar groups of –CONH and –COOH was first prepared and formed a lot of active sites in the cathode, contributing to the robust and stable cathode interface. Subsequently, the PAA glue was converted into the PI binder by thermal treatment. Their group further synthesized a gel poly(imide-siloxane) (cPIS, Fig. 16c) binder via in situ crosslinking of PI and aminopropyl-terminated polydimethylsiloxane (DMS) precursors.¹⁷⁰ The flexible dimethylsiloxane (DMS) segments served as a stress–strain buffer to accommodate volume changes in the cathode during prolonged cycling and facilitated faster, enhanced Li⁺ diffusion. The synergistic combination of rigid and flexible components formed a self-adaptive 3D network binder, which ensured the structural integrity of thecathode–electrolyte interface. The NCM811 cathode with the cPIS binder demonstrates reduced polarization and improved electrochemical kinetics and achieved a specific capacity of 149.7 mAh g⁻¹ at 5C within a voltage range of 2.5–4.3 V and cycling stability with capacity retentions of 100%, 87%, and 65% after 100 cycles at cut-off voltages of 4.3, 4.5, and 4.7 V, respectively (Fig. 16c-ii). This innovative polymer binder design offers significant potential for advancing high-energy LIBs operating under high-voltage conditions. Similarly, Chen et al.¹⁷¹ incorporated a polar and soft ether segment into the rigid PI chains using 3,5-diaminobenzoic acid (DABA) as a crosslinking agent to synthesize a rigid-flexible binder for the LFP cathode. The optimized PI binder demonstrated superior rate capability and cycling stability compared to commercial PVDF.

4. Limitations of PI applications in LIBs

4.1 High cost

4.1.1 High raw material costs. The commercialization of PI components faces significant cost barriers from raw materials through synthesis and processing, compared to conventional battery materials (e.g. polyolefin separators and PVDF binders). The high market price of PI's key monomers, such as dianhydride (e.g. PMDA) and diamine (e.g. ODA), is approximately 15 times higher than that of polyolefin resin, particularly for high-purity grades required in battery applications.^52,172 While the prices of polyolefins have maintained long-term stability as petrochemical commodities, PVDF resin costs less than half that of PI monomers. Moreover, PI synthesis also requires high-purity polar solvents (like NMP and DMAc), which significantly elevate raw materials costs. In comparison, polyolefin separators use a solvent-free melt extrusion process, and PVDF only needs conventional solvents (e.g., DMF), making them relatively low cost.

4.1.2 High synthesis costs. Additionally, the synthesis of PIs is energy-intensive, especially the thermal imidization demanding high-temperature (above 300 °C) treatment. In contrast, polyolefin is processed by melt processing, and PVDF is commonly used in solution coating with lower energy consumption than PI. Collectively, considering raw material costs, specialized solvents, and energy-intensive production processes, the PI-based component cost is significantly higher than the cost of conventional materials like polyolefin separators and PVDF binders.

4.1.3 High processing costs. While laboratory-scale studies demonstrate promising results, scaling up production faces a series of technical and process challenges in production efficiency, product quality stability, and cost control.^173,174 For instance, commercial PI separators require advanced techniques like NIPS, ion-track etching or electrospinning method, which demand specialized equipment and rigorous parameter control and are less scalable than melt extrusion used for polyolefins. However, the electrospinning method is highly effective in producing separators with ultra-high porosity (>80%) and fine fiber diameter (50–500 nm), enhancing electrolyte wettability and ionic conductivity. However, industrialization is hindered by the high processing energy costs even with multi-needle or needleless technologies. The recycling costs of solvents such as NMP and DMAc during processing also account for a large portion of the production costs. Conversely, the NIPS method forms porous structures via diffusion-induced phase separation, offering precise control over pore characteristics. It is more suited for high-speed continuous production and roll-to-roll manufacturing, with lower energy consumption than electrospinning. Nevertheless, it also faces the challenge of solvent recovery. These factors limit the adoption of PIs in cost-sensitive markets, such as consumer electronics, where polyolefin separators dominate due to their lower price.⁴²

4.2 Low conductivity and trade-offs

4.2.1 Low intrinsic ion conductivity. Although the PI's ion conductivity can be improved through certain methods, compared to some traditional LIB electrolyte materials, its ion conductivity still needs to be enhanced. Pure PIs exhibit low intrinsic ionic conductivity (∼10⁻¹⁰ S cm⁻¹), necessitating modifications such as blending with ionic liquids or incorporating sulfonic acid groups. While composite PI electrolytes exhibit conductivities up to 10⁻⁴ S cm⁻¹, they still fall behind those of liquid electrolytes (>10⁻³ S cm⁻¹).³⁹ This limitation restricts fast-charging capabilities and high-rate performance. For instance, PI-based solid-state electrolytes may suffer from polarization losses at high current densities, reducing battery efficiency. Lower ion conductivity may affect the charging and discharging rates and performance of the battery.

4.2.2 Trade-off between mechanical strength and ion conductivity. Although PIs are ideal candidates for solid-state electrolytes due to their excellent mechanical strength, thermal stability, and chemical inertness, their rigid backbone severely limits the ionic conductivity while enhancing the mechanical properties, creating an irreconcilable contradiction. First, the conduction mechanism of PIs is limited. The high rigidity of the chain segments (high T_g) inhibits the polymer chain movement and prevents the migration of Li⁺ through chain-coupled transport or free-volume hopping, especially at low temperatures, where the conductivity decreases dramatically. High tensile strength (>150 MPa) and modulus (>5 GPa) of PIs often come at the expense of flexibility.³⁹ There is a conflict between interfacial stability and mechanical properties. Although high-modulus aromatic PI can inhibit lithium dendrite penetration, the lack of elasticity makes it difficult to adapt to changes in electrode volume, leading to the accumulation of interfacial stress and contact failure. While copolymerization with flexible segments (e.g., ether or siloxane linkages) improves ion conductivity and toughness, it can compromise mechanical strength, thermal stability and ion transport efficiency. Moreover, existing studies have not yet clarified the optimal ratio of rigid/flexible chain segments. Excessive flexibility improves conductivity but sacrifices thermal stability (T_g reduction) and dendrite resistance. Therefore, how to improve the material's ion conductivity while maintaining sufficient mechanical strength to adapt to different battery structures and usage scenarios urgently needs to be uncovered.

4.3 Poor compatibility with electrodes

The adhesion of PI to the electrode can be hindered by its chemical inertness, and the compatibility issues need to be solved. When used as an electrode coating or binder, its surface may require special treatment or the addition of other additives to enhance the binding force and interfacial stability with electrode materials. PI binders may require surface functionalization (e.g., sulfonation) to enhance bonding with silicon anodes, which undergo significant volume expansion during cycling.¹⁷⁰ Otherwise, they may affect the performance and cycle life of the battery. Similarly, PI-coated electrodes often exhibit higher interfacial resistance compared to PVDF-based systems, needing additives like conductive carbon or metal nanoparticles. Poor interfacial contact exacerbates issues like lithium dendrite growth and capacity fading.

5. Summary and perspectives

5.1 Summary

In summary, this review comprehensively discussed the recent progress of PI-based materials in “inert” components of LIBs. Over the past five years, significant efforts have been made in employing PIs with exceptional thermal stability, mechanical strength, and chemical inertness to address critical challenges in LIB safety and performance. PI-based separators significantly enhance the thermal stability and improve the tolerance of liquid LIBs by suppressing dendrite penetration and electrolyte decomposition. However, there is a contradiction between regulating separator porosity and mechanical strength, which often results in the loss of their intrinsic advantages. In solid-state systems, PI reinforcement improves the mechanical robustness of brittle electrolytes, effectively inhibiting Li dendrite propagation under high current densities. Nevertheless, the solid-state electrolytes itself suffer from the problem of insufficient ionic conductivity, and so does the PI material, which is still around 10⁻⁴ S cm⁻¹ even with many modification strategies. Surface-engineered PI coatings on electrodes establish stable interfaces by mitigating side reactions and regulating Li⁺ flux uniformity. Furthermore, PI binders enable structurally integral electrodes through strong adhesion and volume change buffering, thereby improving cyclability in high-capacity systems. Similarly, the key limitation of PI-based coatings and binders is the ionic transport barrier due to their low intrinsic ionic conductivity. Most critically, the industrialisation of PI-based “inert” materials is facing a central obstacle. There are critical challenges in achieving cost-effective manufacturing and optimizing production scalability to enable widespread commercialization. One of the core conflicts faced in the industrialization of PI materials is achieving the balance between excellent thermal stability/mechanical properties vs. high cost and processing difficulty.

5.2 Perspectives

The design of molecular structures and functionalized composites of PIs is also a future development direction. Previous sections reviewed the application of PIs in each non-electrode component, and PIs may play multiple roles at the same time in achieving multi-functional integration. Here, some significant directions for future research and development from this perspective are proposed:

(1) Despite that the future development of PI-based materials in LIBs presents great potential, challenges persist in optimizing PI synthesis pathways for scalable production. Current PI synthesis relies on high-temperature imidization and toxic solvents, hindering large-scale applications. Future research should prioritize green chemistry approaches, such as water-soluble PI precursors or bio-derived monomers, to reduce the environmental impacts. Moreover, to achieve cost efficiency, future research efforts should consider low-cost synthesis routes that employ scalable polymerization techniques, thereby reducing reliance on expensive raw materials and energy-intensive processes.

(2) To address the issue of intrinsically low ionic conductivity of PIs, molecular structure design and the development of functionalized composites have emerged as promising future directions. Additionally, based on nanoscale engineering breakthroughs, dimensional optimization represents the next frontier in electrolyte development for compact energy storage systems.¹⁷⁵ Polyimide-based ultrathin electrolytes (<30 μm) have emerged as a transformative solution, addressing these limitations by reducing ionic diffusion distances and improving interfacial contact with electrodes.¹⁷⁶ By leveraging PI's inherent thermal and mechanical advantages, coupled with nanostructural innovation, these electrolytes pave the way for developing next-generation energy storage systems.

(3) Multifunctional integration represents a future development direction in advanced battery design. PI-based components can be used not only as a single component, but also they play dual or even tripartite roles. For instance, PI membranes could serve as both the electrolyte and the separator simultaneously. Also, PI-derived binders might integrate ion-conductive polymers and viscoelastic networks to concurrently buffer electrode volume expansion and facilitate Li⁺ transport, eliminating the need for separator coating and electrolyte additives. These multifunctional architectures exemplify the shift toward “all-in-one” components, addressing the growing demand for compact, high-performance energy storage systems.

(4) Smart PI composites integrated with stimuli-responsive additives (e.g., thermal/voltage-sensitive fillers, shape-memory materials, or self-healing agents) will enable dynamic adaptation to operational stresses of LIBs, enhancing safety and lifespan under fluctuating conditions.

(5) To break the shackles of the trade-off between mechanical strength and ionic conductivity, future research should focus on the non-equilibrium structural design. For example, the development of gradient PI architectures (e.g. surface rigid/internal flexible) to maintain high-modulus barrier dendrimers at the electrode interfaces while constructing high-migration channels within the bulk phase.

(6) Concurrently, advancing sustainable and scalable manufacturing is critical. Green synthesis protocols (e.g., solvent-free processing or recyclable templates) and closed-loop recycling frameworks must be developed to minimize environmental footprints.¹⁷⁷

Overall, considerable advances have been made in the PI-based “inert” components for the LIBs, and future development prospects are extremely promising. Prospective research should prioritize the development of cost-effective fabrication techniques, advanced composite designs incorporating functional additives, multifunctional integration, non-equilibrium structural design, and sustainable manufacturing studies. By addressing these gaps, PI-based materials will play a pivotal role in enabling next-generation LIBs with enhanced energy density, safety, and durability, thereby accelerating the transition toward sustainable energy storage technologies.

Author contributions

Yayue He, Zhenxi Li, and Shilun Gao: writing – original draft, review and editing; Yayue He, Huabin Yang, and Peng-Fei Cao: conceptualization and supervision; and Yayue He, Zhenxi Li, Shilun Gao, Yinkui He, Yurong Liang, Yan Zhai, Yuxuan Li, Huabin Yang, and Peng-Fei Cao: review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

No primary research results, software, or code have been included, and no new data were generated or analysed as part of this review.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (22379073 and 52373275), the Natural Science Foundation of Tianjin, China (18JCZDJC31400), the Taiyuan Institute of Technology Scientific Research Initial Funding (2024KJ035 and 2025LJ004), and the Basic Research Program of Shanxi Province, China (Grant No. 202203021211211).

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