Advances in anion-intercalated layered double hydroxides for supercapacitors: study of chemical modifications and classifications

Abstract

Hybrid material-based electrochemical supercapacitors (SCs) possessing improved energy density (ED), enhanced stability, high porosity, and a large accessible surface area have attracted attention as promising energy storage devices. SCs also demonstrate excellent specific capacitance (C_s) across various current densities, increased capacitance, and high cell voltages, all contributing to improved ED. Layered double hydroxides (LDHs), with their anionic exchange capabilities and laminar structures, offer significant potential for boosting charge transfer in SCs. This review provides a comprehensive overview of the recent advances in anion-based LDHs, discussing their storage mechanisms, chemical modifications, and classification based on interlayer anions. The roles of different anions, including monovalent, divalent, and polyoxometalates, in enhancing storage properties are examined. In addition, the challenges, future research directions, and practical perspectives of anion-storing LDHs are presented. Hence, this review provides a concise overview of anion-based LDHs for SCs, highlighting their potential significance in energy storage applications.

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Wider approach

To maximize the benefits for the readers and the broader research community, this review provides a detailed classification of layered double hydroxides (LDHs) based on intercalated anions and offers a novel resource for systematically understanding how different anionic species influence LDH properties and performance. Highlighting the role of structural modifications such as interlayer tuning and exfoliation ensures that researchers can replicate or build on these methods to enhance the energy storage performance of LDH-based supercapacitors. The focus on hybrid supercapacitors (HSCs) aligns the research with practical applications, ensuring direct relevance to current technological challenges. Thoughtful perspectives on unresolved challenges and emerging research directions can inspire future studies and collaborations. By including diverse ionic species and detailed discussions of their impacts on electrochemical behavior, this review serves as an essential guide for researchers aiming to design next-generation LDHs for energy storage. These aspects make this work invaluable for advancing sustainable energy solutions, equipping readers with foundational knowledge and actionable insights.

Introduction

Modern energy storage demands cost-effective devices capable of fast charging and possessing advanced properties.¹ SCs are emerging as the preferred energy storage solutions, showing superiority over capacitors, fuel cells, and batteries.^2–4 Their performance hinges on selecting the appropriate electrode–electrolyte pair.⁵ Electrode materials that exhibit a high surface area, stability, porosity, redox activity, and efficient ion diffusion are optimal for high-performance supercapacitors (SCs).⁶ In this regard, researchers have investigated strategies such as nanostructuring,⁷ surface modification,⁸ doping,^9,10 use of composite materials,¹¹ template-assisted synthesis,¹² hybridization,¹³ and controlled growth.¹⁴ These methods enhance surface chemistry, conductivity, wettability, ion accessibility, charge storage, and electrochemical stability.^15,16

However, LDHs are promising electrode materials for SCs owing to their unique layered structure, high surface area, remarkable specific capacitance (C_s), favorable anion exchange, numerous active sites, and stable ion diffusion channels.^17,18 Scientific communities worldwide are advancing SC technology through innovative material synthesis and engineering strategies,¹⁹ driving the commercialization of high-performance SCs.²⁰ The development of next-generation SCs,²¹ including electrochromic,^22,23 hybrid,^24–28 self-healing,²⁹ and piezoelectric types,³⁰ focuses on use of materials such as carbon-based compounds,³¹ metal oxides,³² conducting polymers,³³ MXenes,³⁴ and metal–organic frameworks.^35,36 Generally, three key electrode material classes are widely recognized: carbon-based materials, conductive polymers, and metal hydroxides/oxides/sulfides/phosphides.^37–40 However, existing materials have limitations, prompting research into advanced LDHs with superior properties.

Owing to their layered metal hydroxide structure with interlayer hydrated anions, LDHs exhibit higher C_s compared with MOFs, transition metal oxides, and carbon materials.⁴¹ Their large surface area enhances electrode–electrolyte interactions, accelerating charge storage and release, thus improving electrochemical performance.⁴² LDHs also offer high specific capacity, competent rate performance, and tunable interlayer spacing, further optimizing their electrochemical properties.^43,44 As ionic materials, LDHs consist of positively charged brucite-like layers balanced by interlayer anions,⁴⁵ making them ideal for hybrid nanostructures with high electrical conductivity, metal cation/anion tunability, and multiple oxidation states.^46,47 Their layered structure provides redox-active centers and synergistic effects in di/tri-valent metal-based hybrids, forming flexible 2D nanostructures.^48,49 The choice of interlayer anions, such as monovalent, divalent, organic, halo complexes, and metalate anions significantly influences SC performance by affecting ion intercalation, electrolyte accessibility, and structural stability.^50–52 By optimizing these parameters, researchers make efforts to enhance the electrochemical performance and achieve next-generation SC devices with superior energy storage capabilities.^53–55 Accordingly, a comprehensive analysis of LDHs is essential owing to their profound importance in energy storage applications, particularly in SCs. However, this review aims to provide a well-organized and in-depth overview of LDH materials, beginning with insights into the structural and electrochemical advantages of LDHs in SC applications. It then explores the chemical modifications of anion-based LDHs, specifically tailored to enhance the energy storage performance. This review further categorizes LDHs based on their interlayer anions: monovalent, divalent, and polyoxometalates, shedding light on how each type contributes to improved electrochemical properties. Concluding with future directions, this review emphasizes the potential in advancing LDHs for next-generation energy storage solutions.

Layered double hydroxides for electrochemical study

In recent years, LDHs have become increasingly important due to their distinctive 2D structure with laid-back ion-exchange capabilities, chemical tunability, and high active area.⁵⁶ This can be understood by observing an increasing number of publications related to LDH-based SCs over time. In this concern, Fig. 1(a) describes the number of publications for the same in each year from 2005. In addition, increased interest in LDH-based studies can be observed in Fig. 1(b), which shows the continued growth as well as recent trends.

Fig. 1 (a) Year-wise number of LDH-based research studies published 2005 onwards. Data based on the number of articles published on the Science Direct platform using the keyword “layered double hydroxides for electrochemical applications”. (b) Timeline template for the evolution of LDH-based studies.^44,57–70

The positively charged layers of LDHs are balanced via interchangeable anions, and electroneutrality is achieved.^71,72 The anions with water molecules help in layer stacking, resulting in a poorly ordered interlayer domain in the LDH structure.⁷³ Moreover, these layers are held together by hydrogen bonding. It is also believed that the interlayer domain of LDHs is a quasi-liquid state, giving the interlayer anions remarkable mobility.^74,75 LDHs have been extensively premeditated as an electrode active material for high-performance SC electrodes that exhibit properties such as multiple redox-active sites, tunable chemical composition, high anion exchange, intercalation capabilities, and interlayer distance in LDHs, leading to high-rate SCs.^76,77 Chemically tunable properties of LDHs can significantly enhance charge transfer during the charging and discharging processes. Consequently, electrochemical performance is influenced by factors such as the type of interlayer anions and the ratios of metal cations (M²⁺/M³⁺). Research also shows that LDHs with well-ordered crystalline structures promote faradaic redox charge transfer, exhibiting battery-like behavior and impressive energy storage capabilities.⁷⁸

LDHs consist of positively charged metal oxide/hydroxide sheets interspersed with water molecules and intercalated anions.^39,79 These materials are 2D, with a general formula shown in eqn (1):^80,81

[M_1−x²⁺ + M_x³⁺(OH)₂]^x+ + [Aⁿ⁻_x/n]^x−·mH₂O (1)

where M²⁺ and M³⁺ are divalent and trivalent metal cations, respectively, n is the valency of Aⁿ⁻ anion entered by utilizing the positive charge, x is the ratio of M³⁺/(M²⁺ + M³⁺)and 0.33 > x > 0.25.^82,83 Aⁿ⁻ are anions such as CO₃²⁻, NO₃⁻, SO₄²⁻, CH₃COO⁻, OH⁻, Cl⁻, and [V₁₀O₂₈]⁶⁻.⁸⁴ Zhang et al. reported the optimized structure of LDHs, also with different M²⁺/M³⁺ molar ratios, as shown in Fig. 2(a).^85–87 LDH is made up of the positively charged [M²⁺_1−x + M³⁺_x(OH)₂]^x+ layer, and the negatively charged [Aⁿ⁻_x/n]^x− interlayer region contains anions and water.⁸⁸Fig. 2(b) represents the electron and ion transport mechanism of LDHs.⁸⁹ Moreover, Fig. 2(c) displays the mechanisms of hybrid LDH structures on a Ni foam, designed for enhanced readability. The electrochemical performance of the NiMn-LDH@Ni₃S₂ hybrid array-based electrode at varying current densities, as depicted in Fig. 2(d), is analyzed through its GCD curves.⁹⁰

Fig. 2 (a) Schematic representation of LDHs based on positively charged metal cations and negatively charged anions. Reproduced with permission.⁸⁶ Copyright 2023, Elsevier, (b) Diagrammatic representation of the LDH/Ni-MOF/S electrode's ion and electron transport. Reproduced with permission.⁸⁹ Copyright 2022, Batteries. (c) Mechanism of the LDH structure formation on a nickel foam to increase legibility. (d) Hybrid array-electrode GCD waveforms at different CDs. Reproduced with permission.⁹⁰ (e) CV curves of the rGO (black) and NiCr LDH-POW (red) nanohybrid electrodes at a scan rate of 10 mV s⁻¹. (f) CV curves are displayed at different sweep speeds ranging from 5 to 100 mV s⁻¹ in the electrochemical analysis of the NCW-2//rGO AHSC device. (g) Relationship between current density (CD) and C_s. Reproduced with permission.⁵⁴ Copyright 2022, Elsevier.

Modifying the affinity of LDHs for specific anions is crucial for optimizing the driving force for anion intercalation, which, in turn, influences their electrochemical properties. The anion affinity for intercalation highly depends on the cation species present within the LDH layers. For instance, in LDHs, anions with higher charges tend to exhibit stronger binding energies, with the following general sequence of anions observed: PO₄³⁻ > CO₃²⁻ > SO₄²⁻ > OH⁻ > F⁻ > Cl⁻ > Br⁻ > NO₃⁻ > I⁻.⁹¹ In contrast, in CaAl LDHs, the anion affinity follows a different order: NO₃⁻ > NO₂⁻ > Cl⁻ > CO₃²⁻ > SO₄²⁻ > OH⁻.⁹² Moreover, Dillenburger et al. presented a lyotropic series highlighting the OH⁻ ion conductivities in LDH structures intercalated with a range of anions, including ClO₄⁻ ≥ NO₃⁻> Cl⁻ > SO₄²⁻ > CO₃²⁻.⁹³ These sequences are governed by the relative binding energies of the anions and the Gibbs free energy changes during the intercalation and exchange processes.⁹⁴ Thus, LDHs demonstrate exceptional anion exchange capabilities, a crucial feature in energy storage systems. Their extensive surface area, affordability, adjustable structure, and robust catalytic properties position them as promising materials for electrochemical applications. Their adaptability in composition, structural flexibility, ease of forming composites, and tunable morphology further enhance their potential for various applications.^95–99 Padalkar et al. concluded that the electrochemical performance of pristine LDH can be enhanced by the rational intercalation of polyoxotungstate (POW) anions in NiCr-LDH (NCW). To demonstrate the real-time applicability of NCW nanohybrids, an asymmetric hybrid SC (AHSC) was fabricated using rGO as the anode and NCW nanohybrid as the cathode in 2 M KOH, denoted as NCW-2//rGO AHSC (Fig. 2e), (inset). Based on the potential windows of rGO and NCW-2, the device operates up to 1.6 V without electrolyte polarization. Fig. 2(f) shows the scan rate effect on the cyclic voltammetry (CV) shape and current response. C_s, energy density (ED), and power density (PD) were evaluated using galvanostatic charge–discharge (GCD) curves (Fig. 2g), with the NCW-2//rGO AHSC achieving 120 F g⁻¹ at 2 A g⁻¹, a maximum ED of 43 W h kg⁻¹, and a PD of 1.3 kW kg⁻¹.⁷² Zhao et al. enhanced the specific surface area and interlayer spacing of LDH nanosheets by fine-tuning the Ni concentration, improving electron/ion transfer kinetics. The optimized NiCo-LDH-210 electrode achieved a high C_s of 2203.6 F g⁻¹ at 2 A g⁻¹, excellent rate capability, and stable cycling performance due to its vertically aligned nanosheets and cavity-induced ion transport. The assembled HSC device delivered a superior ED of 57.3 W h kg⁻¹ with remarkable cycling stability. This MOF-derived complex design holds great promise for energy storage applications.¹⁰⁰

The interlamellar domains of LDHs include various neutral or charged organic and inorganic species, anions, and water molecules.¹⁰¹ A distinguishing feature of LDHs is the weak interaction between these guest species and the host lamellae, which allows for their strategic placement either during the formation of the 3D structure or later through anionic exchange.¹⁰² The most widely employed method for synthesizing organo-LDHs is anion exchange, which takes advantage of the well-documented anion-exchange properties of simple inorganic anions in LDHs.¹⁰³ The anion-exchange approach has proven to be highly versatile, allowing for the incorporation of a diverse range of organic compounds into the LDH interlayers. Mallakpour et al. have further expanded this concept by developing various organo-LDHs, each featuring distinct arrangements of matrix cations and organic anions within the interlayer structure. Moreover, anion exchange has facilitated the exploration of intercalation batteries by combining two-layered hydroxides with the insertion of anionic or cationic materials.¹⁰⁴ The interlayer gallery of LDHs is capable of hosting various inorganic and organic anions, with the intercalation process being influenced by factors such as hydration state, hydrogen bonding, Coulombic forces, and the size of the anions.^105,106 This delicate balance determines the interlayer separation in LDHs, allowing for the accommodation of guest species. Moreover, their stable structure is maintained through a hydrogen bond network among water molecules, anions, and hydroxyl layers, along with electrostatic interactions between anions and hydroxyl layers.^50,99 The structure of LDHs is typically composed of hexagonal nanosheets, with the electrostatic forces between the exchangeable anions and brucite sheets playing a key role in maintaining the integrity of the material.^52,88 The thickness of the interlayer is particularly significant, as it is governed by the orientation, charge-balancing anions, and the size and strength of the brucite-like layers, along with the interactions between anions and hydroxyl groups. These factors collectively modify the basal spacing, influencing the nanosheet orientation and reflecting the unique properties of hydrotalcite-like materials.¹⁰⁷

The multi-layered nature of LDHs imparts a high intercalation capacity, owing to the larger interlayer spacing. This structural characteristic provides a theoretical framework for exploring the relationship between spatial effects and electrochemical activity. Despite several attempts to enhance the interlayer spacing through in situ anion-exchange processes, the slow diffusion of anionic guest species within the confined interlayer space has posed a significant challenge. The kinetic limitations associated with guest diffusion hinder the efficient expansion of the interlayer space, thereby restricting the potential for optimizing the LDH performance in some applications. However, the interlayer spacing plays a pivotal role in electrochemical activity, particularly in systems where the intercalation of electrolyte ions is crucial.¹⁰⁸ During charge–discharge cycles, the gallery space facilitates the movement of electrolyte ions between brucite layers, contributing to enhanced electrochemical performance.¹⁰⁹ This ion transport capability within the interlayer space highlights the remarkable electrochemical potential of LDHs, positioning them as promising candidates for energy storage applications.

Chemical modification of anion-based LDHs

LDHs have customizable metal–anion compositions that enable diverse applications, while tunable active sites, ion transport channels, and interlayer architecture make them promising electrode materials.¹¹⁰ With flexible metal cation combinations and high anion exchange capacity, LDHs offer versatility, though conventional bimetallic systems face limitations in conductivity and redox activity compared to ternary counterparts.¹¹¹ The mechanistic details of the chemistry and the characterization of intercalation compounds remain a significant challenge for our battery of modern spectroscopic techniques.¹¹² Intercalation often improves LDH performance, as incorporating consistent catalysts into the LDH interlayer can improve catalyst duration and thermal stability and make separation and purification processes easier.^113,114 Thus, LDHs may benefit from intercalation in energy conversion and storage.¹¹⁵ The d-spacing of LDHs and their layered structure enable electrolyte ions to interact with a large number of exposed electrochemically active spots. Owing to their excellent ion exchange and transport capabilities, LDHs have been investigated for EES.¹¹⁶ The indirect anion exchange method is based on the observation that anion diffusion and exchange are extremely susceptible to the lamellar structure of LDHs.¹¹⁷ By employing an anionic exchange processes, these characteristics have been used to create novel LDH phases, which can be explained by eqn (2):¹⁰²

[M²⁺ − M³⁺ − A] + B → [M²⁺ − M³⁺ − B] + A (2)

The electrostatic interaction and free energy of LDHs are influenced by the equilibrium constant of the ion exchange reactions, where ions with higher charge densities are typically preferred.¹⁰² The capacitive activities of LDHs are primarily governed by the modification of metal ions and interlayer anions. For example, OH⁻ ions in the electrolyte play a crucial role in redox reactions, facilitating charge accommodation and enabling valence changes of transition metal ions.¹¹⁸ Due to their tunable chemical composition, which includes various redox states, anion-intercalated layered structures, and ion-exchange properties, LDHs show significant promise as battery-type electrodes in SCs.¹¹⁹

Moreover, the pH of the electrolyte affects LDH-based electrodes by inducing discharge rates, charge storage, and stability. Thus, in the case of alkaline electrolytes enhanced redox reactions help to improve charge storage, while acidic conditions cause degradation. Controlled calcination optimizes surface area and conductivity, while generally, LDH-carbon composites improve thermal stability. However, polymers and metal-oxide coatings avoid degradation and assist in maintaining layered structures over different pH conditions. Therefore, maintaining a suitable pH and material modifications helps to enhance the long-term stability and capacitance in SCs.^120–122 Given these characteristics, several methods for the chemical modification of LDHs have been explored, including modifications to interlayer spacing, exfoliation techniques, and tuning of active metal ions. These approaches are pivotal for enhancing the electrochemical performance and functionality of LDHs in energy storage applications, as discussed below.

Interlayer spacing and ion exchange

The interlayer spacing of LDH was enhanced using a two-phase method, followed by a short reflux process and a mild solvothermal reaction. During the electrochemical reaction, the increased interlayer spacing in the ultrathin LDH nanosheets facilitates faster electron transport.¹²³ Moreover, Intercalation assists as a governing tool for functionalizing LDHs and increasing their applications, as diverse LDH-intercalant combinations influence performance. Its usefulness lies in allowing high doping levels, ensuring reversible property variations, consenting precise control, encouraging structural modifications, and accompanying other modification techniques. However, Ionic diffusion aids from expanded layer spacing, while anion size and hydration impact stability and mobility. Charge transfer resistance declines with conductive anions, whereas bulky or immobile anions subsidize higher ionic resistance. Furthermore, the choice of anion plays a critical role in augmenting electrochemical performance by improving ionic diffusion, enhancing charge transfer, and increasing capacitance with redox-active anions, electrolyte penetration is enhanced by optimized layer spacing, and structural stability supports long-term performance and eventually advancing efficiency in energy storage systems such as SCs.^114,124

Accordingly, high-ED SCs to meet the growing demand for electric vehicles and portable electronics, enhancing the mass loading of electrode materials while maintaining efficient electron and ion conductivity is a crucial strategy.¹²⁵ High amounts of intercalated hydrated anions boost electrostatic interactions concerning LDH layers and anions as shown in Fig. 3(a).¹²⁶ Also, the intercalated anions, interaction, anion exchange, basic sites, and structure of LDHs were studied with DFT.⁹¹ This makes LDHs-based materials a potent candidate for electrochemical SCs electrodes.⁹⁶ Also, in the gallery area water and anions are randomly distributed and are unrestricted in their motion as a result more space is taken up in the interlayer. Anionic clay known as LDHs has drawn a lot of interest as a potential electrode material because of its high C_s, inexpensive manufacture, and adjustable composition. Their low electron transfer and mass diffusion rates, however, prevent high-rate charging and discharging. In response, scientists have created sophisticated LDH-based nanoarchitectures such as hollow spheres, core–shell nanosheet arrays, and metal oxide hybrids improves the pseudocapacitive performance.¹²⁷ In LDHs, interlayer anions improve the anion exchange capacity through substitution while enhancing cation adsorption.¹²⁸ Jie Zhao et al. studied NiCo-LDHs, which offer high energy storage capacity but suffer from poor charge transport. To improve the performance, they intercalated multi-carboxylic anions to fine-tune the interlayer spacing, optimizing ion and electron transport. This revealed an “inverted-volcano” trend in equivalent series resistance (R_ESR), with 1,4-benzenedicarboxylic anion achieving the best balance. The material exhibited a high capacitance of 2115 F g⁻¹ at 1 A g⁻¹ and excellent rate capability. A hybrid SC (HSC) using this LDH achieved 11.2 W h kg⁻¹ at 30.7 kW kg⁻¹, providing key insights for high-rate LDH electrode development.⁷⁷ Abebaw Eshetie Kidie et al. synthesized a 3D NiMn-LDH structure with optimized interlayer spacing for enhanced ion diffusion and energy storage by optimizing the Ni/Mn molar ratio and reaction temperature. The electrode achieved 612 C g⁻¹ capacity and 67% rate capability at 20 A g⁻¹.¹²⁹

Fig. 3 (a) Representative structure of LDH materials. Reproduced with permission.¹²⁶ Copyright 2022, Elsevier. (b) Schematic representation of CoCr-LDH and CoCr-LDH-POV nanohybrids. Reproduced with permission.⁵³ Copyright 2022, Elsevier. (c) Schematic of di/tri-metal LDHs (MgAl, MgMAl). Reproduced with permission.¹³⁰ Copyright 2019, Royal Society of Chemistry.

For instance, Lin et al. modified NiCo-LDH nanosheets using sodium dodecylbenzene sulfonate (SDBS), which effectively expanded the interlayer spacing, resulting in a capacitance of 1094 F g⁻¹ at 5 A g⁻¹.¹³¹ Furthermore, the chemical modification of LDH-based SCs through surface redox reactions and ion intercalation facilitates a dual charge storage mechanism.^85,132

Exfoliation

Exfoliated LDH ultrathin nanosheets, which are only a few thick atomic layers, provide several new opportunities for basic and applied research via certain solvents, as interchangeable anions neutralize the positively charged host layers, making the fabrication of monolayer blocks easier.¹³³ LDH, a multimetallic clay material with diverse interlayer anions, features octahedral brucite-like layers that can be exfoliated into positively charged 2D single layers. Its tunable composition, high specific capacity, and layered structure make LDH a promising candidate for battery-type SCs.⁶⁷ However, Adachi-Pagano et al. proposed that LDHs with long-chain organic anions tend to delaminate in nonaqueous solvents. Such delamination-based LDHs have been recently studied, so there are not many published reports. LDHs were found to delaminate instantly and spontaneously in formamide without requiring heat or reflux.¹³⁴ It is considered a relatively straightforward and efficient method for delaminating LDHs, including various anions. Sadavar et al. utilized CoCr-LDH-POV nanohybrids for the exfoliation–restacking process, as shown in Fig. 3(b).⁵³ LDHs have excellent exfoliation capabilities, a large interlayer spacing, and a weak interlayer bonding force.¹³⁵ The interlayer distance can typically be changed by exchanging the intercalated anions for LDHs.¹³⁶ Exfoliating the LDHs and then condensing the layers while a guest is present would be an alternative method of inserting visitors.¹³³ The electrochemical exfoliation of 2D capacitive materials, coupled with electrochemical deposition and incorporation of faradaic materials, offers a promising approach to narrowing the performance gap between SCs and batteries.¹¹⁶

Investigations are now being conducted to determine the effects of delamination on the degree of hydration, anion composition, and layer charge,¹³⁷ as high-affinity anions are most dominant in LDH galleries. This causes a shortage of gallery space and could negatively impact electrolyte diffusion. As a result, various attempts have intensified to adjust the LDH gallery area through the use of lattice engineering.¹³⁸ Certain layered compounds have contact forces between their layers, making them exfoliate with the right solvents easily. After the solvent has evaporated, the exfoliated layers can be stacked once more and the polymer can then adsorb onto them. The polymer intercalates, producing an ordered multilayer structure.¹³⁹ Zhao et al. precisely prepared a hybrid structure of CoNi-LDH/PEDOT:PSS via electrostatic interactions, by the charge matching hypothesis in which CoNi-LDH monolayers are used as positively charged and PEDOT:PSS negatively charged components.¹⁴⁰ Liang et al. studied that LDH materials contain brucite-like M^II(OH)₂ layers, where M^II ions are partially substituted by M^III ions, creating positively charged layers with charge-matching anions in between.¹⁴¹

Xu et al. prepared NiAl-LDH nanosheets (NiAl-LDH-NSs) for ethanol oxidation by exfoliating bulk NiAl-LDHs.¹⁴² The 2D structure renders more active sites, with part Ni(II) converting into Ni(III). It has been possible to attain significantly higher activity and turnover frequency. This study is the first to exfoliate ultrathin LDH nanosheets with the goal of ethanol electrooxidation. This opens up new possibilities for the investigation of high-performance catalysts and broadens the range of applications for LDH-based materials.¹⁴² The most commonly employed LDH exfoliation techniques, namely liquid phase exfoliation,¹⁴² solvent-free exfoliation,¹⁴³ one-step synthesis,¹⁴⁴ exhibit their efficacy and versatility in producing high-quality LDH nanosheets.⁶⁹

Tuning the active metal ions

The main benefits of LDHs are their metal-anion pairings and adaptable compositions, which allow for a variety of uses. In addition, injected metal ions improve the structural stability, decrease mechanical stress during charge–discharge cycles, and widen the interlayer space.¹⁴⁵ The anion replacement is one of the key advantages of LDHs.¹⁴⁶ In this case, LDH constructed with several charge-balancing anions is achieved by adjusting the M²⁺/M³⁺.¹⁴⁷ The positive charge is generated by isomorphically replacing divalent cations with the trivalent cations in an LDH, resulting in a positive charge on the layers.^148,149 Naseem et al. reported that the isomorphous replacement of di/tri valent metal cations, such as Mg²⁺ with Co, Ni, Cu, and Zn and Al³⁺ with Fe, alters the interlayer.¹³⁰ Thus, substitution into the LDH structure with the transition metals is schematically represented in Fig. 3(c).¹³⁰ During charge and discharge, extensive space is required for complex redox reactions to undergo phase transformation, which can cause structural deformation and capacitance degradation. Multi-cation LDHs have gained increasing attention due to their hierarchical structure and large surface area, providing numerous active sites for chemical reactions.¹⁵⁰

Boumeriame et al. stated that LDHs have a brucite-like structure with edge-involvement Mg(OH)₆ octahedral units and metal cations arranged in the centers. Six OH⁻ ions toward the corners surround each cation to form endless sheets.¹²⁶ LDHs resemble brucite-like structures that are positively charged, where anions are added between the layers to counteract the excess positive charge for maintaining charge neutrality,¹⁵¹ where the cations are arbitrarily filled between the octahedral gaps created by the OH⁻ ions, leading to compact packing.¹⁵² Sandhiya et al. synthesized compositionally tuned NiCo-LDHs and NiCo-LDH@rGO (NC@RG10) composites via a simple one-pot solvothermal method. The NC@RG10 composites exhibited increased basal length, with charge storage kinetics influenced by the (Ni²⁺/Co³⁺) ratio and rGO content. The NC@RG10 electrode achieved a specific capacity of 963 C g⁻¹ at 1.5 A g⁻¹, while the hybrid device delivered 166 W h kg⁻¹ at 638 W kg⁻¹. A solid-state flexible capacitor maintained 28 W h kg⁻¹ at 553 W kg⁻¹, even at a 180° bent angle.¹⁵³

Chemical modification of LDHs has been explored, including modifications to interlayer spacing, exfoliation techniques, and tuning of active metal ions. These approaches are pivotal for enhancing the electrochemical performance and functionality of LDHs in energy storage applications; among them, we emphasize the role of interlayer spacing, which involves various charge-balancing anions, as discussed briefly below.

Classifications of LDHs based on intercalated anions

The arrangement of hydrotalcite-like nanosheets in LDHs, which depends on the position and size of the interlayer anions, can reduce reflection intensities and enhance the material's overall properties.^{48,107,154,155} This alignment is crucial for optimizing the interaction between the LDH structure and OH⁻ ions, often achieved by introducing additional anions through straightforward methods.¹³² To overcome limitations imposed by particle clustering and dense layers, research has focused on increasing the surface area and interlayer spacing by incorporating larger guest anions.⁵⁴ The LDHs that are reported to date with their SC performance are summarized in Table 1.

Table 1 Interlayer anions in the layered structure of LDHs

Material	Anion	Electrolyte	Potential window [V]	Specific capacitance	Current density	Retention [%]/cycles	Ref.
NiCoAl-LDH	OH^−, CO3^2−	3 M KOH	0.0–0.45	1413.0 F g^−1	1.0 Ag^−1	77.0/5000	156
NiCo-LDH	OH^−, CO3^2−, NO3^−	2 M KOH	0.0–0.5	2762.7 F g^−1	1.0 A g^−1	61.0/—	157
CoAl-LDH/PG	OH^−, CO3^2−	30 wt% KOH	0.15–0.25	864.0 F g^−1	1.0 Ag^−1	75.0/5000	158
NiAl-LDH	OH^−, CO3^2−	6 M KOH	0.0–0.6	1250.7 C g^−1	2.0 A g^−1	76.7/5000	159
NiCo-LDHs	OH^−, CO3^2−, NO^3−	6 M KOH	0.0–0.6	2228.0 F g^−1	1.0 A g^−1	85.0/500	160
s-NiCo-LDH	OH^−, CO3^2−	2 M KOH	0.0–0.6	1321.0 F g^−1	5.0 mA cm^−2	97.0/6000	161
CC@NiCo2Al-LDH	OH^−, CO3^2−	1 M KOH	0.0–0.6	1137.0 F g^−1	0.5 A g^−1	—/12 000	162
NiCo-LDH	NO3^−	3 M KOH	0.0–0.6	4392.0 F g^−1	0.44 Ag^−1	90.6/—	107
NiCo-LDH	NO3^−	6 M KOH	0.0–0.8	802.0 C g^−1	1.0 A g^−1	86.7/—	163
NiAl-LDH/NNDG	OH^−, NO3^−	6 M KOH	0.0–0.8	975.0 F g^−1	1.0 A g^−1	75.0/—	67
NiAl-LDH-NO3	OH^−, NO3^−	6 M KOH	0.0–0.8	1616.0 F g^−1	1.0 A g^−1	82.0/2000	164
NiCr-LDH	NO3^−, CO3^2−	2 M KOH	0.2–0.6	815.0 C g^−1	1.0 A g^−1	84.0/5000	138
NiCo-LDH	NO3^−	1 M KOH	0.0–0.64	1050.0 F g^−1	1.0 A g^−1	—	155
CuCr-LDH	NO3^−, CO3^2−	2 M KOH	0.0–0.5	843.0 F g^−1	—	85.0/1500	51
NiCo-LDH	OH^−, NO3^−	1 M KOH	−1.0 to 0.7	2189.8 F g^−1	1.0 A g^−1	70.3/20 000	80
NiCo-LDH	Ac^−, OH^−	6 M KOH	0.0–0.55	1032.2 F g^−1	1.0 A g^−1	89.3/3000	165
A-NiCo-LDH	Ac^−, H2O, OH^−	2 M KOH	0.0–0.6	2445.0 F g^−1	0.5 A g^−1	93/10 000	93
NiCoAl-LDH	CO3^2−	2 M KOH	0.0–0.5	2369.4 F g^−1	1.0 Ag^−1	85.7/5000	166
CoGa-LDH	CO3^2−	6 M KOH	0.0–0.5	1431.4 C g^−1	1.0 A g^−1	83.4/8000	167
NiFe-LDHs	CO3^2−	2 M KOH	0.0–0.4	1061.0 F g^−1	1.0 A g^−1	80.0/1000	168
NiCo-LDH-CO3	CO3^2−, OH^−	2 M KOH	0.0–0.50	2391.0 F g^−1	2.0 mA cm^−2	—	169
G-NiCo-LDH	CO3^2−, OH^−	2 M KOH	0.0–0.6	1497.0 F g^−1	5.0 mA cm^−2	87.0/6000	170
NiMn-LDH	CO3^2−	2 M KOH	0.0–0.5	1635.0 F g^−1	1.0 A g^−1	74.1/2000	171
CoNi-LDHs/SCNTF	SO4^2−, CO3^2−	3 M KOH	0.0–0.5	1190.0 F g^−1	1.0 A g^−1	81.0/5000	172
NiCo hydroxides/CNTs	SO4^2−, CO3^2−	2 M KOH	0.0–0.4	1151.0 F g^−1	1.0 A g^−1	77.0/10 000	173
NiCoS@SBA-C	S^2−, CO3^2−, OH^−	6 M KOH	0.0–0.4	1732.5 F g^−1	—	59.26/5000	174
NiMn-LDH/Ni3S2	S^2−, DS^−	1 M KOH	−0.2 to 1.0	682.3 mA h g^−1	5.0 mA cm^−2	44.0/2500	175
Ni50Co50-LDH	NO3^−, CO3^2−	6 mol L^−1 KOH	0.0–0.5	1537.0 F g^−1	0.5 A g^−1	80.3/1000	176
NiCo-LDH	OH^−, H2O	2 M KOH	0.0–0.8	2156.0 F g^−1	1.0 A g^−1	86.8/6000	177
NiCo-LDH/S-Ni MOF	S^2−	1 M KOH	−0.2 to 0.5	1200.0 F g^−1	1.0 A g^−1	86/4000	89
CNFs/CoNiFe-SDS-LDH	SDS, CO3^2−, OH^−	6 M KOH	0.0–0.5	203.3 mA h g^−1	1.0 A g^−1	56.2/10 000	178
NiCo-SDBS-LDH	CO3^2−, OH^−	1 M KOH	0.0–0.7	1094.0 F g^−1	5.0 A g^−1	81/3000	179
NiCo hydroxide	CO3^2−	6 M KOH	0.0–0.5	2408.0 F g^−1	1.0 A g^−1	86.2/10 000	180
NiCo-LDH@NiCo-HOS-5	S^2−, CO3^2−, OH^−	1 M KOH	−1.2 to 0.2	1521.0 F g^−1	1.0 A g^−1	83/10 000	181
NiAl-LDHs-S	CO3^2−, OH^−	6 M KOH	0.0–0.6	1680.0 F g^−1	1.0 A g^−1	60/—	182

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Regarding the capacitive activity of LDHs, a pivotal role is being played by interlayer anions. These anions not only expand the interlayer spacing but also prevent the agglomeration of the nanosheets through interactions with hydrogen bonds. This expansion increases the number of redox-active sites, improves charge–discharge cycling performance, and enhances the diffusion coefficient due to the intercalation of anions.¹¹⁴ The reduction in the interplanar distance between cationic layers facilitates the diffusion of electrolyte ions into the active material, thus lowering internal diffusion resistance and boosting electrochemical performance.¹⁵⁶ Additionally, intercalated anions enhance ion diffusion, expose more active sites, and accelerate charge storage kinetics by further widening the interlayer gap.¹⁸³

Additionally, LDHs pose impressive potential as SC electrodes owing to their tunable composition and ion-exchange properties. However, their inherent characteristics of agglomeration and poor electron conductivity hinder stability, which significantly affects the overall electrochemical performance. Charge storage effectiveness depends on ionic and electrical conductivity, ion interchange, and volume alterations through charge–discharge cycles. However, the evolution of ion conductivities in intercalated LDHs with various anions such as ClO₄⁻ ≥ NO₃⁻ > Cl⁻ > SO₄²⁻ > CO₃²⁻ is shown in Fig. 4(a) and (b).⁹³

Fig. 4 (a) PXRD pattern of MgAl-CO₃/SO₄/Cl/NO₃/ClO₄. (b) Different anions were intercalated in the Hofmeister series of LDH compositions. Reproduced with permission.⁹³ Copyright 2023, ACS. (c) Schematic representation of the NiCr-LDH-POV (NCV) nanohybrid model. (d) Plot of specific capacity versus applied CD for the NiCr-LDH and NCV-2 nanohybrid electrodes. Reproduced with permission.⁸⁴ Copyright 2022, Wiley. (e) Cross-sectional HR-TEM image representing the crystallite thickness of the CoCr-LDH-POV nanohybrid. Reproduced with permission.⁵³ Copyright 2022, Elsevier. (f) Schematic illustration of the NCW nanohybrid. (g) Rate capability plot. Reproduced with permission.⁵⁴ Copyright 2022, Elsevier.

The 2D structure of LDHs, with its expansive interlayer spacing, serves as an “ion-buffering reservoir,” which helps manage the volume changes that occur during redox reactions.¹³² Intercalated anions, which can be either organic or inorganic, are used to tailor the physicochemical properties of the LDH materials.¹⁰⁵ These charge-balancing anions reside within the gallery spaces of metal LDHs, contributing significantly to their superior electrochemical energy storage capabilities.¹³⁶ This is achieved through a combination of high specific surface area, low internal resistance due to direct contact with the current collector, and enhanced ionic diffusion driven by the anions within the expanded interlayer space.¹⁸⁴

Moreover, charge storage in LDHs strikes via faradaic redox reactions of metal cations and interlayer anion interchange, while the ion migration is run by interlayer spacing, ion hydration energy, and electrolyte pH. Although LDHs have high ionic conductivity, intrinsically electronic conductivity is modest. Thus, to improve charge transport approaches such as compositing with carbon-based materials (e.g. carbon nanotubes and graphene) or doping with transition metals (e.g. Mn, Co, and Ni), conductive polymers are used. In addition, optimization is attained by modifying the morphology and composition via templating methods, material amalgamation, and oriented growth on conductive substrates.³⁶ For instance, NiFe-LDH/graphene nanohybrids improve the surface area, conductivity, and charge transportation by revealing more active metal sites, with performance induced by graphene dimensions and characteristics.¹⁸⁵ Besides, the hierarchical porous structure in LDHs, involving macropores, mesopores, and micropores, considerably boosts electrochemical performance by advancing ion transport and charge storage. For example, Co-based LDH nanoarrays (Co(OH)₂@CoAl LDH) were produced via a two-step hydrothermal process that holds mesoporous structures. Co(OH)₂@PLDH shows both areal and C_s while maintaining remarkable cycling stability over 5000 cycles, which emphasizes the status of structural tuning in supercapacitive performance. Thus, by augmenting interlayer spacing and hierarchical porosity, LDHs can accomplish admirable rate capability, high capacitance, and long-term durability, which makes them promising materials for next-generation SCs.¹⁸⁵ For instance, Zhang et al. highlighted the efficacy of anion intercalation in enhancing the electrochemical performance of NiCo-LDH, achieving an impressive C_s of 1032.2 F g⁻¹ at a CD of 1 A g⁻¹ in SCs.¹⁶⁵

Moreover, the electrode thickness considerably affects the performance, capacitance retention and ion transport of LDH-based SCs. The film thickness is much lesser than consequences in shortened redox reactions, whereas those moderately thicker inhibit ion diffusion and lift resistance. Maintaining the most favorable thickness ensures balanced capacitance and structural stability with ion accessibility, and improved CD advances hydration and capacitance. Besides, thicker deposited layers form porous surfaces that increase areal capacitance exclusive of changing C_s. However, controlling thickness, specifying preparation conditions, and normalising capacitance data are essential for accurate performance evaluations. A thicker preliminary film is linked with a high CD to achieve optimal performance.¹⁸⁶ However, various charge-balancing anions¹⁸⁷ such as polyoxometalates (POM), sodium dodecyl sulfate (SDS), acetate (Ac⁻), monovalent anions (OH⁻ and NO₃⁻), divalent anions such as SO₄²⁻ and CO₃²⁻, as well as oxyanions, halides, silicates, and complex organic anions (e.g., dodecyl sulfate (DS) and dodecylbenzene sulfonate (DBS)) are commonly used in LDHs. Due to their larger ionic radii, these anions increase the interlayer height, promoting ionic diffusion and reducing ohmic resistance. This results in improved electrochemical behavior such as higher C_s and enhanced rate capability.^48,54 Several of these anions and their contributions to LDH performance are discussed in further detail in this review (Table 1).

Polyoxometalates

POMs are nanoscale metal oxide clusters with a tunable structure, diverse electronic properties, and multi-electron redox capabilities, making them valuable for catalysis and energy storage. Keggin-type POMs, known as “electron sponges,” offer high storage capacity. Recent studies show that dispersing POMs within host materials enhances performance in transparent conductors and energy storage. However, research on anchoring highly dispersed POMs for micro-SC applications remains limited.²⁸ POMs serve as exceptional inorganic templates owing to their extensive range of chemical compositions, diverse structural topologies, tunable morphologies, and significant negative charge densities. These characteristics facilitate their utilization as anionic templates for the synthesis of cationic coordination polymer hosts.¹⁸⁸ The incorporation of various heteropoly anions further enhances the potential for developing POM-LDH intercalates. By judiciously modulating the degree of reduction of the heteropoly-blue anion, researchers can effectively control the density of intercalated pillars, thus optimizing the structural properties of the materials. Moreover, POMs exhibit rapid and reversible multi-electron transfer capabilities during charge and discharge cycles, maintaining structural integrity throughout these processes. This property positions them as ideal candidates for electrode materials in SCs.¹⁸⁹ POMs are generally categorized into two main classes: heteropolyacids, which contain heteroatoms such as phosphorus (P) or silicon (Si), and isopolyacids, which do not incorporate heteroatoms. To date, most POM-based hybrids developed for SC applications have been derived from heteropolyacids, with only a single study reported on an isopolyacid variant.¹⁹⁰ Notably, POMs, characterized by transition metal oxide clusters, are recognized for their rapid, reversible multi-electron redox reactions, which are crucial for effective charge storage in batteries and capacitors. Despite their promising properties, the exploration of POM materials specifically for SC applications remains limited. For instance, a pioneering study by Chen et al. introduced a polyoxovanadate (POV) Na₆V₁₀O₂₈ electrode material, demonstrating an outstanding capacitance of achieving 354 F g⁻¹ for SCs.¹⁹¹ These highlight the untapped potential and the need for further investigation into POM-based materials in energy storage technologies. Under pH control, the hydrolysis of a vanadium salt solution creates up to 1 nm POV anion and the LDH crystal's exfoliation, resulting in positively charged LDH monolayer nanosheets.⁷⁶ Next, an anion exchange process was used to create the appropriate POV-containing LDHs at various pH levels.⁸⁴ A distinct POV species is generated between the layers, according to the spectra of a sample prepared at pH levels between 8.5 and 10.5.¹³² Additionally, conducting polymers with flexible microstructures have demonstrated excellent conductivity, ensuring stability during repeated cycling.¹⁵⁰

Expanded interplanar spacing in NiCr-LDH-POV (NCV) nanohybrids, as observed by Padalkar et al., indicates [V₄O₁₂]⁴⁻ anion intercalation (Fig. 4(c)). Additionally, the specific capacity of NiCr-LDH and NCV-2 nanohybrids declines with increasing charge–discharge rates (1.0–3.0 mA cm⁻²), Fig. 4(d). The NCV nanohybrid electrode demonstrates 294.5 mA h g⁻¹ at 1 mA cm⁻², exceeding the 98.9 mA h g⁻¹ of NiCr-LDH, and 82% after 5000 cycles. NCV-2 outperforms other NCV hybrids, emphasizing the importance of optimal POV anion content for better electrode performance. Enhanced ion diffusion boosts specific capacities, making NCV-2 the best composition for electrochemical performance.⁸⁴ Sadavar et al. developed mesoporous CCV nanohybrids of CoCr-LDH nanosheets and POV anions, attaining 732 C g⁻¹ at 1 A g⁻¹ due to optimal gallery height. Fig. 4(e) focuses on the intercalative stabilization and layer-by-layer stacking of CoCr–(OH)₂ and POV anions, improving electrochemical performance. The expanded interlayer height from POV intercalation enhances ion diffusion and electrolyte percolation.⁵³ Padalkar et al. synthesized NiCr-LDH-POW (NCW) nanoclusters, employing them as a cathode with rGO as the anode in aqueous (AHSC) and solid-state (SSHSC) systems. Fig. 4(f) displays POW anion intercalation in the Ni-Cr-LDH. The NCW-2//rGO AHSC demonstrates 120 F g⁻¹ at 2 A g⁻¹, with an ED of 43 W h kg⁻¹ and a PD of 1.3 kW kg⁻¹. The potential windows of rGO and NCW-2 suggest an operating voltage of up to 1.6 V. Even at high CDs, the NCW-2//rGO AHSC exhibited stable capacity with minimal fading (Fig. 4(g)), demonstrating the superior electrochemical performance of NCW nanohybrids as cathodes for HSCs.⁵⁴ Sadavar et al. obtained CoCr-LDH-POW (CCW) nanohybrids. The HR-TEM analysis shows the CCW nanohybrid layered crystal structure and expanded basal spacing. The prepared CCW-nanohybrids show 1303 C g⁻¹ capacity and 85.43% retention over 5000 cycles.¹⁹²

Sodium dodecyl sulfate (SDS)

SDS is considered one of the eco-friendly anionic surfactants and is employed in several applications, such as eliminating metal cations from wastewater,^193,194 preventing magnesium alloys from corrosion by merging SDS with LDH films,^195,196 as a high-performance dye,¹⁹⁷ near-infrared and mid-infrared investigations,¹⁹⁸ applied for its photocatalytic activity and dielectric properties,¹⁹⁹ sorption of methyl orange dye,²⁰⁰ fabrication of organic modified composites for pollutants removal,²⁰¹ and ion exchange.¹⁵⁰ Wang et al. formed CoAl-layered metal oxide nanosheets intercalated with SDS, increasing the interlayer spacing from 0.76 to 1.33 nm. This expansion boosts ion diffusion and increases the employment of electroactive sites in the anode. Schematic illustrations of the ion exchange and electro-sorption mechanism of CoAl-LDH-SDS are discussed in Fig. 5(a).²⁰² Zhang et al. formed CoNi-SDS-LDH nanosheets via SDS ion intercalation, improving electrochemical performance. The SDS modulation roughens nanosheet edges, increasing active sites, advancing nanowire growth, lowering agglomeration, and improving performance. The specific capacity attains 509.2 C g⁻¹ at 1 A g⁻¹.²⁰³ Kumar et al. produced an SDS-functionalized binary Co/Ni hydroxide, achieving a C_s of 2765 F g⁻¹ at 5 A g⁻¹ with 88% capacity retention over 5000 cycles. Schematic representation of guest anions between the layers can change the layered structure (Fig. 5(b)).²⁰⁴

Fig. 5 (a) Ion exchange process and electro-sorption mechanism of CoAl-LDH-SDS. Reproduced with permission.²⁰² Copyright 2023, Wiley. (b) Schematic representation of guest anions between the layered structure. Reproduced with permission.²⁰⁴ Copyright 2021, ScienceDirect.

Ding et al. synthesized CNFs/CoNiFe-SDS-LDH, demonstrating outstanding electrochemical performance with a C_s of 203.3 mA h g⁻¹ at 1 A g¹ and a retention of 71.3% from 1 to 30 A g⁻¹. The incorporation of SDS expanded layer spacing and promoted the formation of porous, flower-like LDH nanosheets, increasing electrochemically active sites and enhancing charge/electrolyte ion transfer. Additionally, the introduction of carbon materials improved electrical conductivity, further enhancing the overall performance.¹⁷⁸ Yinyin Lin et al. synthesized NiCo-SDBS-LDH nanosheets using the intrinsic pillar effect of SDBS. Electrochemical analysis revealed simultaneous surface redox and intercalation behaviors. Despite a modest surface area (15.28 m² g⁻¹), the electrode achieved a high C_s of 1094 F g⁻¹ at 5 A g⁻¹ and retained 81% capacitance over 3000 cycles. A dual charge storage mechanism was proposed, attributing performance gains to SDBS stabilization and expanded interlayer spacing. These findings highlight the potential of functional pillaring to optimize TM-LDH-based materials for energy storage.¹⁷⁹ The LDH is a promising pseudocapacitive electrode material with an adjustable interlayer spacing, excellent electrochemical properties, and ion exchangeability. By leveraging LDH's structural features and the high conductivity of sulfides, a large-spacing LDH can be grown on a conductive substrate as a precursor. Partial sulfidation then combines LDH's sheet morphology with sulfides’ conductivity, significantly enhancing the electrochemical performance.¹⁷⁵ Bing Li et al. synthesized, via TA etching and partial sulfurization, hollow nanoflower-like structures, with S atoms modifying ZIF-67 to increase its pore size and surface activity, thus promoting NiCo-LDH nanosheet growth. The optimized electrocatalyst (S-NCCO) achieved 3744.4 F g⁻¹ (1684.98 C g⁻¹) at 1 A g⁻¹, while the S-NCCO//AC ACS delivered 92.3 W h kg⁻¹ at 750 W kg⁻¹, offering insights for the high-performance SC design.²⁰⁵ The conductive substrate gives the electrodes more electrical conductivity, which improves electrochemical performance, while sulfidation increases the electrical conductivity, which strengthens the interfacial interaction.¹⁷⁵

Hydroxides (OH⁻)

Su et al. integrated LDHs to refine the hierarchical structure, enhancing the electrochemical performance. The highly active hydroxide in LDH promotes ion and electron diffusion, accelerating charge transfer and boosting pseudocapacitance. Additionally, LDH strengthens the structure, preventing collapse and improving the buffer layer's effectiveness, leading to greater electrode stability. This optimized design advances energy storage technology by significantly enhancing electrochemical efficiency.²⁰⁶ Hu et al. demonstrated that evenly coordinated LDH flake layers efficiently sieve target ions, with hydrogen bonds between interlayer anions, hydroxyl groups, and water content facilitating instant hydroxide ion transport.¹⁷⁷ Constructing advanced architectures is crucial for high-performance HSCs. FeCoNi-LDH, with porous spindle-like nanoplates, achieves 1960 F g⁻¹ at 1 A g⁻¹, benefiting from metal synergy and oxygen vacancies. Density functional theory (DFT) confirms that hydroxide strength lowers oxygen vacancy formation energy, enhancing ion transport and electrolyte adsorption. The FeCoNi-LDH//AC HSC delivers 53.2 W h kg⁻¹ at 800 W kg⁻¹, surpassing similar devices. This study highlights LDH's potential for electrochemical energy storage and its role in oxygen vacancy generation, warranting further research for SC performance enhancement.²⁰⁷

The LDH model in Fig. 6(a) demonstrates that water molecules and OH⁻ ions exist in the interlayer gallery. Fig. 6(b) delivers pictures from advanced ab initio molecular dynamics simulations, showcasing OH⁻ ion and proton transportation within nano-confined LDH layers. Remarkably, the interplay between Grotthuss and Vehicular mechanisms improve surface –OH groups (e.g., –OH_f1 and –OH_f2) in Fig. 6(c), substantially upholding OH⁻ ion carrying, mainly via the Grotthuss mechanism. The initial position of the H_w1 proton at the O_w2 atom, while the OH⁻ ion was located at O_w1H⁻ (Fig. 6(b)), was used to calculate the mean square displacement. The migration of O_w1H⁻ to O_w2H⁻ shortened the O_w2–H_f1 distance, leading to the formation of a hydrogen bond and highlighting the crucial role of surface –OH groups (–OH_f1) in facilitating H_w1 proton transport (Fig. 6(d)). Likewise, at ∼8.84 ps, extra surface –OH group (–OH_f2) supported H_w2 moving from O_w3 to O_w2 (Fig. 6(e)), assisting the development of O_w2H⁻ to O_w3H⁻ (Fig. 6(b)). These findings demonstrate that surface –OH groups within the LDH layers significantly enhance proton dissociation and transport, resulting in high OH⁻ conductivity. Fig. 6(f) shows the rate performance of the alkaline zinc–iron flow battery (AZIFB) with LDH-M and substrate. As the CD increased from 80 to 200 mA cm⁻², the AZIFB with LDH-M and substrate showed increased coulombic efficiency (CE). In contrast, voltage and energy efficiency slightly declined due to higher Ohmic and electrochemical polarization.²⁰⁸ Wang et al. prepared NiCo-LDH/CFC with organized 3D porous nanosheets, achieving a C_s of 2762.7 F g⁻¹ (1243.2 C g⁻¹) at 1 A g⁻¹.¹⁵⁷ Similarly, Zhang et al. reported that a NiAl-LDH (3D flower-like) on a nickel foam achieves a specific capacity of 1250 C g⁻¹ at 2 A g⁻¹ with 76.7% retention over 5000 cycles at 50 A g⁻¹ following 5000 activation cycles at 20 A g⁻¹. The stability tests on NiAl LDH, NiAl LDH-NF, and NiAl LDH-NF-H electrodes included 5000 cycles at 20.0 A g⁻¹ (Fig. 6(g)). Notably, the specific capacity of the NiAl LDH-NF electrode increased by 66% after 1300 cycles, reaching 1250.7 C g⁻¹ at 2.0 A g⁻¹ (Fig. 6(h)). This is due to the excellent electrochemical performance of the NiAl LDH, activated through continuous cycling.¹⁶⁴

Fig. 6 (a)–(e) Structure, channel, and hydroxide ion conduction of the LDH-based membrane. (f) Rate performance as a function of CD ranging from 80 to 200 mA cm⁻². Reproduced with permission.²⁰⁸ Copyright 2021, Nature. (g) Cycling performance. (h) Cycling test showing the specific capacity of the LDH-NF electrode. Reproduced with permission.¹⁵⁹ Copyright 2016, Elsevier.

Acetates (Ac⁻)

Zhang et al. prepared a NiCo-LDH intercalated with acetate ions by a one-step hydrothermal method (Fig. 7(a)). The spherical structure size varied with acetate ion content, initially decreasing and then increasing. Additionally, Ac⁻ significantly influenced the surface morphology associated with LDH composite materials, showing a C_s of 1032.2 F g⁻¹ at 1 A g⁻¹.¹⁶⁵ The enhancement in the concentration of Ac⁻ reveals the gradual impoverishment in the specific surface area of the sample. Here, the number of micropores decreased initially, followed by the subsequent improvement. The particle size distribution (PSD) curve, as displayed in Fig. 7(b), possesses a hierarchical porous structure with mesopores being the predominant pore type.¹⁶⁵ Zha et al. studied acetate anion-intercalated A-NiCo-LDHs and the NF approach has a C_s of 2445 F g⁻¹ at 0.5 A g⁻¹. The cathodic peak current densities (I_ap) for both A-Ni₅Co₅-LDH and A-Ni₅Co₅-LDH/NF display a linear correlation with the square root of the scan rate (ν^1/2), as shown in Fig. 7(c). Remarkably, the slope of the I_ap − ν^1/2 plot for A-Ni₅Co₅-LDH/NF is ∼3.57 times higher than that of A-Ni₅Co₅-LDH, emphasizing enriched ion diffusion and charge transfer supported by its conductive porous structure. In addition, Fig. 7(d) describes the Nyquist plots from electrochemical impedance spectroscopy, illustrating small variations in semicircle diameter at high frequencies and slope at low frequencies after 10 000 cycles, representing steady electrochemical performance.²⁰⁹

Fig. 7 (a) Schematic of a promising strategy for enhancing NiCo-LDH-based energy storage systems. (b) Particle size distribution curves for pristine NiCo-LDH and NiCo-LDH intercalated with Ac⁻ anions. Reproduced with permission.¹⁶⁵ Copyright 2024, ScienceDirect. (c) Anodic peak current densities for A-Ni₅Co₅-LDH and A-Ni₅Co₅-LDH/NF as a function of the scan rates. (d) Nyquist plots of A-Ni₅Co₅-LDH and A-Ni₅Co₅-LDH/NF before cycling and after 10 000 cycles. Reproduced with permission.²⁰⁹ Copyright 2017, ScienceDirect. (e) Nitrogen adsorption–desorption isotherms of NiAl-LDH/NNDG. (f) Relationship curves of logarithms of peak currents on scan rates, (g) Nyquist plots of the NiAl-LDH/NNDG hybrid composites, and Bode plots (inset). Reproduced with permission.⁶⁷ Copyright 2018, Chemical Engineering. (h) Schematic representation and formation mechanism of NiAl LDH-NO₃. Reproduced with permission.¹⁶⁴ Copyright 2019, Springer.

Nitrates (NO₃⁻)

Guest molecule integration is facilitated by LDHs on the surface and in the interlayer galleries. Because NO₃⁻ can be easily displaced from the interlayer region, LDHs carrying NO₃⁻ anions are especially advantageous as precursors for anion replacement activities.²¹⁰ Since the system needs twice as many NO₃⁻ anions to offset the charge of CO₃²⁻, this preference results from the higher steric repulsion between NO₃⁻ onions compared to CO₃²⁻ anions.²¹¹ Recently reported NO₃⁻ anion-based LDHs with their SCs performance are collectively mentioned in Table 1. In concern with recent advances, Tian et al. synthesized a Ni–Al LDH/NNDG hybrid with a self-assembly process.⁶⁷ In Fig. 7(e), nitrogen sorption isotherms demonstrate that every sample has the usual mesoporous structure made up of parallel-plate building blocks. The layered structure of NiAl-LDH/NNDG hybrids improves electrolyte-reachable active sites, as demonstrated by their surface areas of 192.8, 204.7, and 236.3 m² g⁻¹ for NiAl-LDH-NO₃, NiAl-LDH/rGO, and NiAl-LDH/NNDG, respectively. The XPS spectra of NiAl-LDH-NO₃ reveal a prominent N 1s peak at 406.9 eV, corresponding to the spin–orbit characteristics of anions. The studied NiCo-LDHs were synthesized via a redox reaction between and C₂H₅OH, which contributed to a remarkable specific capacity.⁶⁷ The electrode materials demonstrate diffusion-controlled intercalation activities with diverse redox peaks, maintained by fitting-curve slopes of almost 0.5 in scan rate plots. NiAl-LDH/NNDG exhibits pointed fitting curves, indicating faster diffusion-controlled faradaic reactions because of its layered structure (Fig. 7(f)). Electrochemical impedance spectroscopy features the superior ion and electron transfer of NiAl-LDH/NNDG. Nyquist plots (Fig. 7(g)) reveal a high-frequency semicircle and a low-frequency straight line.⁶⁷ In addition, Bao et al. prepared a NiAl-LDH via the dual-anion intercalation method with hydroxyl and NO₃⁻ anions represented in Fig. 7(h). The NO₃⁻ anions act as compensatory charges, separating the LDH layers. Bao et al. demonstrated the role of anions in charge storage for SCs through electrochemical analysis, achieving a C_s of 1616 F g⁻¹ at a CD of 1 A g⁻¹. The diffusion-controlled intercalation behavior indicates that the LN-25 sample exhibits enhanced ion accessibility, attributed to its nanolayered structure. This underscores the importance of structural engineering in improving electrochemical performance.¹⁶⁴

Thereafter, Jeong et al. optimized NiCo-LDH materials by using an expanded interlayer gap and hollow structural arrangement to improve the electroactive sites, mass and charge transfer, and stability. The FTIR spectra show a band at 1380 cm⁻¹, linked to the asymmetric vibration of the NO₃⁻ group, with a peak strength increasing with the Co content, indicating enhanced NO₃⁻ intercalation. The higher capacitance with increased Co content is due to conductive CoOOH, which improves electrode conductivity and NO₃⁻ intercalation, thereby enlarging the interlayer spacing.¹⁵⁵ CaLiAl-NO₃ LDHs and their adsorption behavior towards chloride were created by Xiang He et al. for the scientific design of cement-based repair materials that are suited for marine conditions. This method is essential for improving the robustness and prolonging the useful life of damaged maritime concrete structures. The objective of this study is to investigate the characterization and adaptation of CaLiAl-LDHs in OPC-SAC-GGBFS repair materials.²⁰⁷

Carbonates (CO₃²⁻)

The layered structure containing CO₃²⁻ anions has reduced the interplanar distance,¹⁵⁶ thereby enhancing the rate performance, as shown in Table 1. CO₃²⁻ has the highest intercalation affinity due to its substantial charge-to-size ratio, whereas NO₃⁻ has the lowest affinity. Expanding interlayer spacing by altering the morphology and arrangement of active materials can increase ion accessibility and enhance the interface area related to the electrolyte and the electrode. This leads to a more efficient charge/discharge process.²¹² Zou et al. synthesized NiCo LDH nanosheet arrays by a hydrothermal method with CO₃²⁻ as the interlayer anion. Anion exchange between CO₃²⁻ and OH⁻ in an alkaline solution improved the material structure and capacitive performance. The NiCo–CO₃ LDH nanosheets form wall-like arrays attached to the NF (Fig. 8(a)). The electrode exhibited a capacitance of 1.78 F cm⁻² (684.0 F g⁻¹) at 2.0 mA cm⁻² within 0.0–0.5 V. The Bode graphs of the acquired electrodes are shown in Fig. 8(b). At low frequencies, the phase angles change between the electrodes. The NiCo LDH electrode and the Ni–Co–S sulfide electrode display phase angles of ∼−72° and −62° at 1 Hz, respectively, demonstrating a robust capacitive character and improved charge transfer performance.¹⁶⁹ To investigate the service life at 20 mA cm⁻², cyclic charge/discharge tests were conducted for the NiCo–OH LDH electrode following a 12-hour soaking period and for the NiCo–S electrode (Fig. 8(c)). Because of the extended soaking period in the KOH, the capacitance of the NiCo–OH LDH rises with the charge/discharge cycles. After 1500 cycles, the capacitance gets 6.48 F cm⁻², and over 3000 cycles, 107% capacitance is still present, which is better than the NiCo–S electrode retention (53% after 3000 cycles).¹⁶⁹

Fig. 8 (a) SEM image of NiCo-CO₃ LDH nanoarray electrodes. (b) Frequency dependence of the NiCo electrode phase angle. (c) Cycling behaviors of NiCo-OH LDH at 20 mA cm⁻² and NiCo-S electrodes after soaking for 12 h. Reproduced with permission.¹⁶⁹ Copyright 2018, Wiley. (d) Schematic representation and (e) Nyquist plots of the experimental impedance of CNTs-Ni-Co hydroxide nanoflakes, (f) C_s of the samples at a CD of 10 A g⁻¹. Reproduced with permission.¹⁷³ Copyright 2015, ScienceDirect. (g) SEM image of Co₁Ni₄LDHs/SCNTF. (h) Nyquist plots of Co(OH)₂/SCNTF, Ni(OH)₂/SCNTF and Co₁Ni₄LDHs/SCNTF electrodes. (i) Specific capacity of Co_xNi_5−xLDHs/SCNTF at current densities from 1 A g⁻¹ to 10 A g⁻¹. Reproduced with permission.¹⁷² Copyright 2021 International journal of current research.

Thereafter, Li et al. synthesized sandwich-like NiMn LDH/reduced graphene oxide (rGO) hybrids with a flower-like Ni–Mn LDH. XPS revealed that both CO₃²⁻ and rGO anions contribute to the C 1s spectrum, with a prominent peak for non-oxygenated carbon (C=C) at 284.8 eV. Carboxylate (O–C=O) and carbonyl peaks appear at 288.7 eV and 286.2 eV, likely from rGO and CO₃²⁻. A nitrocellulose separator demonstrated strong performance, with a maximum capacitance of 84.26 F g⁻¹ at 1.0 A g⁻¹ and an ED of 33.8 W h kg⁻¹ at 0.85 kW kg⁻¹. In practical tests, it powered a small lamp for 34 seconds after charging to 1.7 V, showcasing its potential for high-performance capacitors.¹⁷¹

Sulphates (SO₄²⁻)

The interlayer gallery of LDHs has been effectively intercalated with DS anions to increase the interlayer gap and maintain the structural stability.⁷⁷ Typically, the interlayer charge-compensating anion is CO₃²⁻, which has a strong affinity for LDH layers but can be replaced with other inorganic or organic anions such as Cl⁻, SO₄²⁻, acetate, lactate, and DS. Organically modified LDHs (organo-LDHs) are studied for their ability to remove organic contaminants from natural waters, with research focusing on phenols, aromatic sulfonates, and carboxylates. Long-chain anions such as DS and DBS, with a single negatively charged end, can replace typical charge-balancing anions (CO₃²⁻, NO₃⁻), significantly increasing the interlayer distance.⁷⁷ Panpan Li et al. designed a NiCo-LDH@NiCo-HOS core–shell heterostructure on a nickel foam via two-step electrodeposition to enhance energy storage. This architecture integrates interconnected NiCo-LDH nanosheets, conductive NiCo-HOS, and robust NF, providing ample redox-active sites and efficient electron transport. The electrode achieves 1521 F g⁻¹ at 1 A g⁻¹, with 65% retention at 20 A g⁻¹ and 83% capacity retention after 10 000 cycles at 30 A g⁻¹.¹⁸¹ Guiquan Liu et al. emphasized that ion and electron conductivity is a key factor limiting electrochemical advancement. They synthesized a nanoflower-like NiAl-LDHs-S electrode via a solvothermal reaction followed by vulcanization, where surface sulfide formation enhanced performance, achieving a high C_s of 1680 F g⁻¹ at 1 A g⁻¹ and a capacity retention of 60% at 20 A g⁻¹, surpassing conventional NiAl-LDH electrodes.¹⁸² Li et al. highlighted the scarcity of comprehensive studies on combined strategies for improving LDH electrode conductivity. This study synthesized self-supporting CoAl-LDH, followed by hydrothermal synthesis of NiCoAl-LDH in varying ratios and partial vulcanization to form NiCo sulfide by sulfidation. The resulting heterostructure enhanced conductivity and a hybrid SCs was assembled with the best-performing electrode to assess its practical performance.²¹³ Zhanjun Yu et al. synthesized a 3D flower-like NiCo₂O₄@NiCoMnS₄@NiCo-LDH multilayer nanostructured film on NF. Electrodeposition and heat treatment formed NiCo₂O₄ nanosheets, followed by hydrothermal sulfiding to create NiCo₂O₄@NiCoMnS₄. The electrode exhibited outstanding electrochemical performance, achieving a high C_s of 2956 F g⁻¹ at 1 A g⁻¹ and 95.75% retention after 5000 cycles.³⁸

Li et al. designed a core–shell structure comprising carbon nanotubes (CNTs) coated with Ni–Co hydroxide nanoflakes, where the hydroxide flakes with a turbostratic disorder conformally covered the CNT surface. As the Co/Ni ratio increases, the nanoflakes on the CNTs grow larger, suggesting that Co-rich grains require less energy for growth. Fig. 8(d) shows the relationship between C_s and discharge CDs, revealing that the C_s decreases as the discharge CD rises, consistent with electrochemical polarization. At higher CDs, charge storage is limited to the external active surface due to reduced electrolyte diffusion and reaction times. The samples exhibit a maximum C_s of 1151 F g⁻¹ at a low CD of 1 A g⁻¹. Among the samples, the capacitance decreases at the slowest rate with the increasing CD, indicating superior rate capability. This improvement is attributed to intercalated SO₄²⁻ ions, which enhance conductivity and increase interlayer spacing. EIS measurements, conducted over a frequency range of 10⁻² to 10⁵ Hz, further support these findings. Nyquist plots (Fig. 8(e)) were used to analyze the EIS data. Specifically, Ni_0.36Co_0.64(OH)₂ achieves a C_s of 1151 F g⁻¹ at 1 A g⁻¹ (Fig. 8f), with the intercalated SO₄²⁻ ions contributing to the improved conductivity and rate capability.¹⁷³

In addition, Akbar et al. described the intercalation of SO₄²⁻ to improve the interlayer spacing with improved LDH conductivity. This is associated with microscopic analysis, revealing the tubular morphology of Co₁Ni₄LDHs/SCNTF after carbonization. Herein, sulfonic acid present on the SCNTF surface is responsible for increasing the wettability and dispersibility of CNTs and serves as a nucleation site for CoNi LDH microspheres, ensuring uniform coating. By sighting Fig. 8(g), which reveals the SEM image of the composite materials, the spherical structure of Co₁Ni₄LDHs/SCNTF composites can be understood. The thin lamellar structure, flower-like morphology, and many porous structures on the surface of Co₁Ni₄LDHs/SCNTF microspheres are advantageous for fostering electrochemical reactions. Fig. 8(h) displays the Nyquist impedance spectrum. The diagonal at low frequencies and the semicircle at high frequencies make up the Nyquist curve. The intercept between the graph line and the real axis shows the amount of the equivalent series resistance in the high-frequency zone. The rate performance of Co₁Ni₄LDHs/SCNTF is shown in Fig. 8(i) at a CD ranging from 1 to 10 A g⁻¹. These results highlight the improved performance of the Co₁Ni₄LDHs/SCNTF material.¹⁷²

Sulfuretted Ni MOF nanosheets were strategically incorporated into the NiCo LDH nano-scaffold to enhance the ionic and electronic transport kinetics. Sulfide treatment facilitates rapid ion diffusion within the sheets while lowering the overall resistance. Additionally, the interlayer expansion induced by sulfur ions helps prevent structural degradation, ensuring superior stability. Notably, this approach of designing well-engineered interfaces and surface structures using MOF templates holds potential for broader applications in electrochemical fields.⁸⁹

In this context, the discussed chemical modifications and structural tuning of LDHs have established an effective approach for electrochemical studies. These modifications play a crucial role in enhancing the performance, ultimately providing improved capacitance, conductivity, stability, etc., for SC applications.

Conclusions and future perspectives

LDHs have drawn attention as prospective SC materials because of their enormous surface area, abundance, affordability, and compositional flexibility. Their great anion exchange capability, varied morphologies that expand active sites and diffusion, and composition manipulation that affects oxidation states are all advantages for energy storage. The techniques used to enhance the LDH performance have been studied, including nanostructure and interface engineering, composite design, and optimization. To enhance energy storage, heterostructured composite electrodes have also been fabricated by making use of metal–atom interactions. The intercalation of cations from aqueous solutions into LDHs presents significant challenges for developing new energy storage materials. These challenges stem from the occupation of anions in the interlayer gallery and charge repulsion between the positively charged LDH layers and the incoming cations. Similarly, the pH of the solution plays a crucial role in facilitating or inhibiting ion exchange, affecting the overall performance of LDHs. Large interlayer spacings in LDHs exhibit excellent electrochemical performance. Pillaring the brucite-like layers with long-chain anions such as multi-carboxylic anions with conjugated plane or straight-chain configurations can fine-tune the charge transport kinetics at a sub-nanometer scale. The anion length can be precisely adjusted to regulate interlayer distances, promoting the passage of OH⁻ ions between the brucite-like layers during charging/discharging, which enhances high-rate capability. This increase in basal spacing is accompanied by an initial rise in platelet thickness.

Interlayer anions play a critical role in maintaining the overall charge neutrality of LDHs, which contain mountains of hydroxide layers with positively charged mixed metals which have garnered significant attention due to their potential applications in various fields. The gallery height in LDHs depends on the size, dimensions, and orientation of the charge-balancing anion, which significantly impacts their capacitive behavior. Numerous studies have demonstrated that replacing the original interlayer anions can improve interlayer spacing, making ion intercalation a successful strategy for modifying LDH-based SCs.

Despite the extensive use of LDHs involving the intercalation or exchange of specific guests, the size of molecules that can be introduced into the LDH galleries is limited. Hybrid LDH materials have shown great potential in intercalating a wide range of organic molecules and macromolecules, providing an open environment for catalytic processes. Interlayer solvent exchange can facilitate molecular rearrangement, particularly in a slurry phase. Modest, high-symmetry, monoatomic anions with reasonably high charge densities intercalate more straightforwardly, whereas larger anions with low charge densities, typical of organic species, present more complex challenges. The 2D, sheet-like inorganic clay framework of both anionic clays and organo-clays is critical. The arrangement of the interlayer area's charge-balancing ions and related water molecules, as well as the various forces present, are essential considerations. Hydration results in various water contents, influencing the properties of the materials. Several factors are crucial for the potential use of these materials:

✓ The parameters of synthesis, including pH, temperature, and environment, are required to yield phase-pure products.

✓ The structures' thermal stability is determined by the charge-balancing anion and the type of octahedral substituents.

✓ The size and shape of the product crystals affect porosity and accessible surface area.

✓ It is possible to successfully modify the charge density, structural properties, and interlayer gaps of LDHs by altering the valence state, species, molar ratio of metal cations, and size. These changes affect LDHs’ anion-exchange performance, increasing their adaptability and usefulness in a range of applications.

✓ Enhancing anion intercalation, electron/ion transport, and interlayer engineering improves the electrochemical performance, charge transfer, and conductivity.

✓ Evolving durable LDH structures ensure long-term stability while executing cost-effective and scalable synthesis methods.

✓ Integrating LDHs with conductive materials and adapting compositions enlarge their applications in energy storage, catalysis, and beyond.

✓ Endorsing sustainability over green synthesis techniques and enlightening efficiency with real-time monitoring of structural and intercalation changes are beneficial.

The complexity and variety of forces within the interlayer, particularly water hydrogen bonding, play a significant role in determining the structure. Forces with a correlation length of at least three layers influence both layer expansion and the registry between layers. The optimization of hydrogen bonding is crucial for the placement of anions, rather than ensuring maximum coulombic attractions to the positively charged sheets. The host layer charge density and cation ratio significantly induce anion selection, light absorption, crystallinity, stability, and photocatalytic performance of LDHs. The lack of crosslinking between the cation layers allows the interlayer to accommodate a wide range of anions. The potential energy surface associated with the interlayer anion helps calculate changes in the anion-layer interaction's strength. The adsorption of both anions and cations creates capacitance at the electrode/electrolyte interface.

Future research should focus on optimizing the synthesis conditions, understanding the role of interlayer forces, and exploring hybrid materials' potential. Tailoring interlayer spacings and improving ion exchange capabilities will likely lead to significant advancements in electrochemical performance and broaden the range of applications. Continued investigation into the fundamental interactions within LDHs will provide deeper insights into their behavior, enabling the design of more efficient and versatile materials for energy storage and beyond.

Author contributions

S. P. S. and S. D. D. conceived the project. S. P. S., S. V. M., P. A. K., S. V. S., and S. D. D. participated in discussions and data analysis. S. P. S. and S. D. D. wrote the manuscript with contributions from all the authors. S. D. D. supervised this project.

Data availability

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

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

S. D. D. acknowledges Anusandhan National Research Foundation (ANRF) for funding under the ASEAN-India Collaborative R&D scheme under ASEAN-India S&T Development Fund (AISTDF) [no. CRD/2024/000886]. Also, S. P. S. gratefully acknowledges Chhatrapati Shahu Maharaj Research, Training and Human Development Institute (SARTHI), Pune, for providing financial assistance under CSMNRF-2023 (CSMNRF-2023/2024-25/1773).

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