The depletion of fossil-fuel resources and the amplification of the greenhouse effect caused by population growth and urbanization have posed unprecedented threats to the sustainable evolution of human society. In an attempt to overcome these issues, the exploration of environment-friendly, low-cost, and high-performance energy storage devices has garnered considerable research attention in recent decades^[1,2]. However, the safety concerns associated with nuclear energy and the unpredictable and intermittent nature of the resources of wind energy, tidal energy, and solar power production have limited the development of energy storage systems^[3,4]. Owing to their advantages of high energy density, high efficiency, rapid charging and discharging, and flexible combination, portable electrochemical energy storage devices have been widely investigated in recent decades^[5].

Supercapacitors have advantages such as high storage capacity, high power density, and safe operation, while alkali metal (Li, Na, K) ion batteries (AIBs) exhibit high specific energy density, high operating voltage, and long-cycle life. Consequently, they have both become the focus of research in electrochemical energy storage^[6,7]. These properties are essential for the development of electronic gadgets and electric vehicles^[8]. However, the low energy density of supercapacitors is the largest obstacle to their practical development. In particular, the frequent twisting of flexible supercapacitors causes irreversible capacity degradation. The development and application of AIBs are also hindered by their limited power density^[9,10]. The electrochemical performance of both of these systems depends essentially on the chemical composition and microstructure of the electrode material^[11,12]. Consequently, it is crucial to develop electrode materials with large surface areas, rapid reactivity, high cycling stability, and high electrical conductivity.

LDHs have the advantages of facile synthesis, abundant raw materials, high composition flexibility, and adjustable biocompatibility. In recent years, LDHs have been widely used in electrocatalysts^[13-15], sensors^[16,17], flame retardants^[18,19], drug delivery systems^[20,21], and energy storage and conversion devices^[22,23]. LDHs are a class of ionic layered compounds that are composed of positively charged brucite-like hydroxide layers known as anionic clays^[23,24]. The hydrogen bond network among water molecules, anion, and hydroxyl layers and the electrostatic force between the anion and hydroxyl layers have established its stable structure, as shown inFig. 1^[25]. The general formula of LDHs is [M²⁺_1−XM³⁺_X(OH)₂]^X+[Aⁿ⁻]_X/n·mH₂O, where M²⁺ and M³⁺ represent the divalent and trivalent metal cations, respectively. The molar ratio of M²⁺/(M²⁺+M³⁺), typically between 0.2 and 0.33, is denoted as X, and Aⁿ⁻¹ represents the interlayer anion. LDHs with diverse physicochemical features can be created by altering the molar ratio of M²⁺/M³⁺ to produce metal cations with different characteristics and interlayer anions of various types^[26-28].

Many studies have examined the preparation process of LDHs and their application in electrocatalysis. However, few reviews have focused on the applications of LDHs in supercapacitors (particularly flexible supercapacitors) and AIBs. To bridge the knowledge gap, this review will begin with a description of the design concepts and reaction mechanism It will then summarize the current research ideas and performance of LDHs as (flexible) supercapacitors and AIB electrode materials, as well as the possibilities and problems that they confront.

Owing to their high power density, extended cycle life, and exceptional temperature stability, supercapacitors stand out among numerous energy storage devices, and thus they have attracted considerable commercial interest^[29-31]. Compared with secondary batteries, supercapacitors have higher power densities and are widely used in various high-power equipment, electric vehicles, and energy supply/harvesting devices^[32]. However, their low energy density limits their development and application. In addition, with the rapid development of wearable electronic technology, it is difficult to find high-energy-density flexible energy storage materials that provide a stable power output during deformation in flexible supercapacitors to meet the needs of personalized electronic products^[33,34].

Flexible supercapacitors can be divided into electric double-layer capacitors (EDLCs) and pseudocapacitors (PDCs). In general, the specific capacitance and energy density of PDCs are higher than the former. Therefore, various nanostructured pseudocapacitive electrode materials have been widely designed, such as transition metal oxides, hydroxides, sulfides, phosphides, and selenides^[35-37]. Compared with other pseudocapacitive electrode materials, owing to their high redox activity, superior interlayer separation, and highly interconnected architecture, LDHs facilitate fast ion transport and reversible redox processes can be used as high-performance electrode materials for supercapacitors with significant growth potential^[38,39]. It is worth mentioning that the factors that affect the pseudocapacitive behavior of different LDHs are the difference in the type of transition metal elements in LDHs, the contact resistance between transition metal elements and OH^-, the electron transfer rate and the interlayer spacing^[40].

In this review, we explore the composition of LDHs and discuss the most recent research advancements related to the application of LDHs in supercapacitors (particularly flexible supercapacitors) and the synthesis strategies of electrode materials (Table 1).

Ni-Co LDH is frequently employed as an electroactive material for energy storage owing to its straightforward manufacturing process and the highly reversible reaction capability of the binary combination^[41]. The reaction mechanism follows Eqs. (1)–(3)^[42,43]:

1Co(OH)2+OH−↔CoOOH+H2O+e−,

2CoOOH+OH−↔CoO2+H2O+e−,

3Ni(OH)2+OH−↔NiOOH+H2O+e−.

Based on ZIF-67, in which some Co ions were replaced by Ni, Xuan et al.^[44] developed a hollow-structured Ni–Co LDH on flexible acidified carbon cloth by an in situ growth method (Figs. 2(a)–2(c)), designing a hollow-structured Ni–Co LDH@acidified carbon cloth (H-NiCo LDH@ACC) flexible free-standing supercapacitor active material. The sample with a transfer resistance of 0.15 Ω exhibited a capacity of 1377 mC/cm (3060 mF/cm²) at 1 mA/cm², which is significantly higher than that of H-NiCo LDH@ACF (Fig. 2(d)). Additionally, a 99% coulombic efficiency retention rate and 70% capacity retention rate were still observed after 10 000 cycles at 80 mA/cm² (Fig. 2(f)). The PDMS-sealed solid-state H-NiCo LDH@ACC//AC devices assembled under the conditions of 0.0708 mWh/cm² energy density and 0.7 mW/cm² power density showed no obvious signs of capacity decay after 0°–180° bending (Fig. 2(e)). This is due to the large specific surface area of hollow LDHs, which facilitates charge transfer and ion diffusion and the higher conductivity of the acidified carbon fiber cloth than that of 2D LDHs^[44].

Graphene has become a popular topic over the past decade owing to its excellent physical and chemical properties. Because of its high carrier mobility, electrical conductivity, transparency, and mechanical strength, graphene is widely used in flexible supercapacitors^[45,46]. Using a 3D dendritic-cell-nanostructured Ni–Co LDH@rGO as the cathode and wrinkled reduced graphene oxide sheets as the anode, Kiran et al.^[47] reported a solid-state asymmetric hybrid supercapacitor system (Ni-Co LDH@rGO//crumpled rGO) (Figs. 3(a)–3(b)). The system achieved an energy density of 58.4 W·h/kg and a power density of 3.732 kW/kg at a current density of 0.5 A/g.Figs. 3(c)–3(e) displays the specific performance indicators. The radial alignment of Ni–Co LDHs on rGO improves ion and charge transport, while the dendritic-cell-like morphology offers a broad contact surface for electrolyte entry (Figs. 3(f)–3(h)). Moreover, the large surface area and high electrical conductivity of the crumpled rGO sheets enable the cathode and anode to operate synergistically in order to improve the electrochemical performance of the supercapacitor^[47].

Among the various wearable energy storage devices, carbon nanotube yarns are considered to be good candidates for high-performance flexible energy storage devices owing to their inherent mechanical flexibility, high toughness, and good electrical properties^[48-50]. In a study by Le et al.^[51], the pseudocapacitive Ni–Co LDH was wrapped on the surface of a ZnO nanorod forest grown on a carbon nanotube yarn system (Fig. 4(a)). The obtained NiCo-OH/ZnO-NR/CNT-yarn exhibited high electrochemical performance in a three-electrode test system. Furthermore, the symmetric supercapacitor device composed of two NiCo-OH/ZnO-NR/CNT-yarn electrodes exhibited almost no fluctuation in the CV curve when bent to 150°, and the capacitance retention was as high as 75% after 1000 cycles (Figs. 4(b)–4(d)). The excellent performance, strong mechanical flexibility, and reliable cycling characteristics confirmed the feasibility of using flexible fiber-shaped composite yarn materials in wearable energy storage devices^[51].

The introduction of heteroatoms into carbon-based materials in recent years has also proven to be an efficient and straightforward means of enhancing capacitance^[52-54]. In addition, Mn doping increases the electrode capacity and ion mobility and decreases the surface hydrogen desorption energy^[55-57]. In this context, Cao et al.^[58] deposited Mn ions into NiCo-LDH-nanosheet-coated polyaniline (PANI)-derived carbon (PAC) (Fig. 5(a)), whose specific capacity showed a fourfold increase (from 310.02 to 1282.06 C/g). Furthermore, the flexible all-solid-state asymmetric supercapacitor fabricated with MLDH@PAC as the positive electrode (nitrogen/oxygen self-doped PAC as the negative electrode) exhibited a high energy density of 78.9 W·h/kg under a 1.6-V voltage window and a power density of 1.55 kW/kg. When the curvature changed, and the capacitance retention rate was stable at 82.66% after 8000 cycles (Fig. 5(b)). The experimental results indicate that the introduction of heteroatoms is an effective means of improving the capacitive performance of flexible energy storage devices^[58].

Iron (Fe) and aluminum (Al) are also used in combination with nickel to form LDHs. Li et al. used a two-step electrodeposition method to wrap NiFe-LDHs on reduced graphene oxide (rGO)-modified nickel foam. The prepared Ni-Fe LDHs/rGO/NF composite exhibited a specific capacitance of 1462.5 F/g at a current density of 5 A/g. Moreover, the capacitance retention rate was ~64.7% after 2000 cycles. Additionally, the Ni-Fe-LDHs/rGO/NF-cathode-based flexible asymmetric supercapacitor (mesoporous carbon coated on nickel foam as the anode) displayed a power density of 348.49 W/kg and an energy density of 17.71 W·h/kg. The presence of Fe³⁺ in this system greatly reduced the manufacturing cost in relation to that of the Co-based system^[59].

Recently, a novel rGO/silver nanowire (Ag NW)@Ni–Al LDH composite thin-film electrode with a hierarchical core–shell structure has exhibited excellent electrochemical performance. Composites with core–shell structures were formed by growing ultrathin Ni–Al LDH sheets on Ag NWs using the hydrothermal method. Then Ag NW@Ni–Al LDH and rGO were then assembled into GAL hybrid membranes through vacuum filtration (Figs. 6(a)–6(c)). The symmetric all-solid-state flexible supercapacitor (ASFC) prepared with the GAL film as the electrode exhibited a specific capacitance of 127.2 F/g at a current density of 1 A/g, and the capacity retention rate was 83.2% after 10 000 cycles. In addition, the electrochemical performance of the AFSC did not change significantly in the mechanical flexibility test (Figs. 6(d) and6(e)). This excellent performance is attributed to the remarkable electrical conductivity and mechanical flexibility of the Ag NWs after intercalation in carbon materials, coupled with the pseudocapacitive properties of the high-capacity Al core–shell acting as spacers, which alleviates the aggregation of rGO nanosheets (Figs. 6(f) and6(g))^[10].

Cobalt-based materials have always been attractive electrode materials thanks to their high theoretical capacity and excellent electrochemical performance. However, their actual capacity is often unsatisfactory because of their slow reaction kinetics, short cycle life, and low power density^[60]. Recently, a hybrid product of Co–Mn LDH@MnO₂ was grown on nickel foam using a simple hydrothermal method (Fig. 7(a)). The electrode material exhibited an energy density of 59.73 W·h/kg at a power density of 1000.09 W/kg and a specific capacitance of 2325.01 F/g at a current density of 1 A/g in the assembled asymmetric supercapacitors (Fig. 7(b)). The high capacity and energy density benefit from the synergistic effect of the inherent high theoretical capacity of Co-based materials [such as Co₃O₄ (~3560 F/g)] and MnO₂ (theoretical specific capacitance of 1370 F/g)]^[61,62]. The excellent capacitance retention (95% capacity retention after 10 000 cycles at 30 mA/cm²) is attributed to the unique hierarchical structure of the Co-Mn LDH^[63]. In addition, these flexible energy storage devices based hybrid material might be as a Power supply platform for wearable system (Figs. 7(c)–7(f))^[63].

Inspired by these hybrid-design ideas, Wang et al. synthesized a 3D hybrid structure of CoAl-LDH@NiCo-LDH on surface-modified carbon cloth (CC) using a two-step hydrothermal method. Ni–Co LDH nanoneedles grew in situ on the periphery of the Co–Al LDH, and then CoAl-LDH@NiCo-LDH nucleates grew on the modified CC, preventing the undesirable aggregation of LDHs during cycling. Simultaneously, the open area on the surface of the material makes it easier for the electrolyte to penetrate the interior, which improves the ionic conduction efficiency. In this context, flexible asymmetric quasi-solid-state supercapacitors prepared with CoAl-LDH@NiCo-LDH/CC as the positive electrode and active carbon as the negative electrode exhibited energy densities of 57.8 and 38.0 W·h/kg at a power density of 0.81 and 16.09 kW/kg, respectively, under a wide potential window of 1.55 V. The innovation of cobalt-based electrode materials with hybrid architectures represents a new opportunity for future flexible energy storage systems to overcome their performance constraints^[64].

In addition to introducing two ions into LDHs, three metal cations have been added to the LDH layer to synthesize one material in recent years, which is also a novel method for improving the extremely flexible ion exchange and composition-tunable features of LDHs^[65,66]. Based on previously reported composites of the Ni-Co LDH with carbon materials^[67-69], metal oxides^[70], and MXenes^[71], Wang et al. introduced new concepts and created CC@NiCo₂Al_X-LDHs using a surface hydrothermal technique.Fig. 8(a) shows a schematic of the synthesis. Their results indicated that CC@NiCo₂Al-LDH exhibited an energy density of 44 W·h/kg at a power density of 462 W/kg and an ultrahigh capacity retention rate of 91.2% after 15 000 charge–discharge cycles. In addition, the capacitor remained stable during large-angle bending (Figs. 8(b)–8(d)). By resolving the bottleneck issue experienced by LDHs, this study offered a novel method for constructing ternary LDHs with strong structures and electrochemical activity (Fig. 8(e))^[72].

Kim et al. used a hydrothermal process to vertically confine ternary ZnAlMg-LDH (ZMA-LDH) nanosheets and hematite α-Fe₂O₃ nanorods on coaxial electrospun 3D hollow porous carbon nanofibers (3DHPCNF). It is worth noting that ZMA-LDH@Fe₂O₃/3DHPCNF with an appropriate dose of added Zn had a capacity of 3437 mF/cm² at a current density of 1 mA/cm², and it exhibited superior cycling stability in 40 000 charge–discharge tests (106.5% capacity retention rate). The synergistic impact of three-element or multi-element LDH electrode materials offers promise for overcoming the capacity and stability restrictions of existing flexible energy storage systems^[73].

Lithium, sodium, and potassium are members of the IA group and have an outermost electron in the S orbital; hence, their chemical characteristics exhibit obvious homologous behavior^[74]. The performance of AIBs is largely determined by the electrode material (Table 2). Scientists have used various methods to improve the performance of the anode material, such as construction of a solid nanoframe structure, elemental doping, and etching to increase the specific surface area and enhance the reaction properties, stability, and conductivity of such materials^[75-77]. LDHs with a 2D layered structure can possess one or more of the abovementioned properties after a series of chemical transformations; therefore, they are gradually being widely explored by scientists. In addition, the calcination of LDHs allows the formation of well-dispersed metal oxides with controllable sizes and large surface areas, which can effectively prevent the volume expansion, shrinkage, and aggregation of LIBs during charging and discharging.

Although research on composite materials composed of LDHs and lithium has been conducted for over a decade, the low coulombic efficiency, reversible capacity, and electrical conductivity of such materials limit their further development^[78]. Thus, researchers have been attempting to utilize the solid-state electrochemical modification techniques of other metal oxides to construct sophisticated hierarchical nanostructures with excellent structural morphology control and superior performance^[79].

As early as 2008, Liu et al. demonstrated the growth of ZnAl-LDH thin films on galvanized stainless-steel substrates at room temperature through the mature thin-film coating technology. Through this approach, ZnAl-LDH was converted to the oxide of the ZnO/ZnAl₂O₄ composite after calcination at high temperature (650 °C), which was used to synthesize a negative electrode material for lithium-ion batteries (LIBs) for the first time. The test results showed that the discharge capacity of the ZnO/ZnAl₂O₄ porous nanosheets reached 1275 mA·h/g in the first cycle and stabilized at 500 mA·h/g after 10 cycles. ZnO/ZnAl₂O₄ has better electrochemical properties than pure ZnO (Fig. 9(a)). This is attributed to the uniform distribution of ZnAl₂O₄ in the ZnO particle grid, which effectively mitigates the phenomenon of energy attenuation during charging–discharging. The research results provide unlimited possibilities for the development of LDH-based anode materials^[80].

Subsequently, Zhang et al. fabricated nitrogen and sulfur co-doped porous carbon materials (NSPCs) using the 2D interlayer confinement effect of the LDHs. It is worth noting that the authors confined organic molecular precursors to the interlayer of LDHs, and at the same time replaced a part of the aluminum ions in Mg-Al LDHs with catalytically active iron ions. The yield of the desired product is increased; the degree of graphitization of the material is enhanced; and the construction of a porous structure is promoted. Finally, the addition of N and S introduces many edge defects into the material, and the micropores formed by these defects can be used as active sites for lithium-ion storage. It is precisely because of the existence of these micropores and mesopores that the specific surface area of the material is as high as 1493.2 m²/g. This structure maintains ultrahigh stability of 1175 mA·h/g after 120 cycles at 0.5 C current density, and exhibits satisfactory cycling stability in the rate performance test with large gradient current density changes (Fig. 9(b))^[81].

In addition to lamellar structures, LDHs can be synthesized into hollow nanocages. Lu et al. prepared a hollowed Ni-Co LDH polyhedron H-(Ni, Co)-LDHP) using a sacrificial template method. The hollow structure had a large specific surface area and good permeability, allowing the generation of numerous active sites and promoting charge transfer; its capacity reached 928.3 mA·h/g under 0.1 A/g. The experimental results also provide a new idea for the use of LDHs with a hollow polyhedral structure as an anode material^[82].

Since 2011, the emergence of the MXene family headed by Ti₃C₂ has promoted the selection of electrode materials for alkali metal ion batteries. MXene has many advantages such as a large specific surface area, short ion diffusion path, and extremely small volume change during charging and discharging; however, its use as an electrode material is limited by its low theoretical capacity^[83]. Zhao et al. combined a ZIF-67-polyhedron-derived negatively charged MXene framework with Co–Mo LDHs via a simple electrostatic self-assembly method (Fig. 10(a)). After thermal annealing, transition metal oxides (TMOs)@MXene (CoO/Co₂Mo₃O₈@MXene) composites were obtained (Fig. 10(b)), which were used as anodes of Li-ion batteries. The test results show that TMOs@MXene had a reversible capacity of 947.4 mA·h/g at a current density of 0.1 A/g, and it maintained a surprising specific capacity of 435.8 mA·h/g under 5 A/g (Fig. 10(c)). Even after 1200 cycles at 2 A/g, the capacity remained stable at 545 mA·h/g (Figs. 10(d) and10(e)). This happens because the Co–Mo LDH derivatives compensate for the low theoretical capacity of MXene, and the CoO/Co₂Mo₃O₈ nanosheets greatly alleviate the agglomeration phenomenon of MXene. Moreover, the three-dimensional lamellar structure of MXene provides mechanical stability to the entire electrode system, overcoming the short cycle life problem caused by the volume expansion of internal TMOs@MXene. The advantages of both MXene and Co-Mo LDHs have been fully explored, showing another successful combination of LDHs and MXene^[76].

Compared with LIBs, sodium-ion batteries (SIBs) have more abundant reserves and uniform distribution; however, both batteries exhibit similar electrochemical behaviors. SIBs have gradually become more effective alternatives to LIBs^[84]. However, the diameter of Na⁺ is larger than that of Li⁺, which makes it difficult to find suitable anode materials for Na⁺ to shuttle freely^[85,86]. Zhao et al. used this property to synthesize SIB anode materials for the first time by intercalating Co-Fe LDH layers with two-dimensional nitrate columns. The CoFe-NO₃^– LDH half-cell exhibited an initial discharge capacity of 498 mA·h/g at a 1 A/g current density after testing (Fig. 11(a)). A high reversible capacity of 209 mA·h/g was still maintained after 200 charging–discharging cycles (Fig. 11(b)). Unlike the conversion reaction of LIB metal oxides, the excellent sodium-ion storage of the CoFe-NO₃^– LDH is attributed to the support of nitrate pillars within the Co-Fe LDH layer, which results in a large interlayer spacing. It exhibits a weaker Coulomb force than that of the LDH, which is favorable for the intercalation/deintercalation reaction of sodium ions. In addition, the low Na⁺ diffusion barrier of 0.147 eV among the layers of the CoFe-NO₃^– LDH in the DFT calculation results indicates the applicability of LDHs as a superior and promising Na-ion conductor (Figs. 11(c)–11(e))^[87].

The strong effect of microstructure stability on the performance of anode materials is obvious. Park et al. used ZIF-67 nanocubes as precursors and etched them with nickel nitrate in ethanol to generate cubic ZIF-67/Ni-Co LDHs with a shell–core structure. Subsequently, under Ar/H₂ conditions, a box-in-box structure product, Co/(NiCo)Se₂, was obtained by the selenization of CoSe₂ in the core and (NiCo)Se₂ in the outer shell (Fig. 12(a)). Because of the unique configuration and the synergistic effect of multicomponent selenides, Co/(NiCo)Se₂ can serve as a good electrocatalyst for the hydrogen evolution reaction (HER), while acting as the anode for SIBs. The electrochemical test results show that the capacity remained at 497 mA·h/g after 80 cycles at a current density of 0.2 A/g, and the slope of its Tafel curve was only 39.8 mV/dec (Figs. 12(b) and12(c)). Moreover, the rate performance was stable at a current density of 0.2–5 A/g (Fig. 12(d)). This work exploits the high capacity and excellent electrocatalytic properties of LDH derivatives^[77].

In addition to the construction of a stable architecture, the combination of LDHs with carbon materials is a crucial approach for overcoming the significant capacity loss of batteries during cycling. In Chen's work, multiphase Mn-doped Ni sulfide (NMS) nanoparticles were prepared by the sulfidation method using the Ni–Mn LDH as the precursor and immobilized on the rGO surface as the final product of heterodoping. When used as an SIB anode in an ether-based electrolyte, it exhibits excellent rate capability (229.2 mA·h/g at 5.0 A/g) and cycling stability (206 mA·h/g after 2000 cycles at 0.5 A/g). The high-capacity properties of LDH nanomaterials further demonstrate the potential of metal sulfides as SIB anodes after being compounded with carbon materials^[75].

Similarly, using the carbon composite method, Diao et al. used polyvinylpyrrolidone to carbon-coat a Zn–Ti LDH through annealing. Next, the zinc element in Zn–Ti LDH@PVP was etched with hydrochloric acid to obtain ultrasmall TiO₂-nanoparticle-embedded carbon nanosheets (TCNS). The comparative experiments in the flow chart and SEM image demonstrate that the addition of PVP improves the preservation of the lamellar structure of the Zn–Ti LDH and prevents agglomeration (Fig. 13). Therefore, the composite exhibits great advantages for sodium storage, with ultrahigh stability and a specific capacity of 196.7 mA·h/g after 2000 cycles at 2 A/g. The research results show that the two-dimensional lamellar structure of LDHs is fragile, easy to agglomerate, and unstable in the absence of special protection. Upon overcoming these shortcomings, the application scope of such composites can be expanded^[88].

In the future, the depleting lithium resource deposits in the Earth's crust and the geopolitical concentrations of lithium may impede the expansion of lithium mining. However, the crustal contents of sodium and potassium are much higher than those of lithium, which suggests that rechargeable sodium and potassium batteries are attractive alternatives to LIBs^[89,90]. In the past decade, rechargeable potassium batteries (KIBs) have gained great attention. Compared with SIBs, KIBs have higher voltages (0.2–0.3 V) and a greater possibility of using graphite as an anode material^[91-93]. However, the development of rechargeable KIBs is still in its infancy; there are only a few reports on the application of LDHs as electrode materials for KIBs. Here, we briefly summarize the applicability of LDHs in KIBs as represented by two recent research results.

In 2021, Yang et al. compared the performance of a Mg–Al LDH and Mg–Fe LDH as anodes for KIBs through comparative experiments. The test findings indicated that the specific capacity of the Mg–Al LDH without multivalent metal elements was only 195.2 mA·h/g at a current density of 0.1 A/g, but the initial coulombic efficiency was as high as 99.4%. In contrast, the Mg–Fe LDH with variable valence iron ions exhibited a potassium storage capacity of 371.6 mA·h/g at the same current density and cycle number, which is almost twice that of the Mg–Al LDH; the initial coulombic efficiency was 88.4%. This discrepancy is attributed to the difference in the potassium storage mechanism: The Mg–Al LDH/K battery utilizes the electric double-layer effect, while the Mg–Fe LDH/K battery mainly accomplishes potassium storage through the Faradaic reaction of conversion between Fe³⁺ and Fe²⁺. The battery capacity retention rates of Mg–Fe LDH/K at current densities of 1, 2, 5, and 10 A/g were 60.9%, 51.3%, 42.9%, and 39.5%, respectively, in the rate performance test. These unsatisfactory results are attributed to the intrinsically low electrical conductivity of LDHs and the high surface activation energy of up to 79.5 kJ/mol, making it difficult for potassium ions to diffuse within the anode. This comparative experiment confirmed the feasibility of LDHs containing variable valence metal elements as an anode for KIBs. However, their low rate performance must be overcome for the application of LDHs as electrode materials for KIBs^[94].

To alleviate this difficulty and to expand the application scope of LDHs, Jiang et al. developed Ni–Co LDH hollow nanocages utilizing ZIF-67 as a precursor in the same year. Subsequently, using an in situ sulfidation process, ultrafine metal sulfide nanoparticles were inserted into LDH nanocages to generate LDH-S (Figs. 14(a) and14(b)). Consequently, the surface of LDH-S was modified with carbon dots (CDs), originating the final composite product CDs@LDH-S; its specific capacity was 188 mA·h/g after 8000 cycles at 1.0 A/g (Figs. 14(c) and14(d)). Compared with the capacity retention of 70% for pure Ni–Co LDH and 88% for LDH-S, the capacity retention of CDs@LDH-S was as high as 96% at a current density of 0.1–5 A/g (Fig. 14(e)). The excellent performance can be attributed to the enhanced electrical conductivity owing to the interlayer confinement formed by ultrafine metal sulfides. The establishment of the three-dimensional cage-like structure alleviates the agglomeration phenomenon of the material during long-term cycling and compensates for the shortcomings of the traditional two-dimensional sheet structure of LDHs, which are fragile and easy to agglomerate. In addition, the performance comparison of LDH-S and CDs@LDH-S indicates that the addition of CDs can further improve the conductivity of the anode material, resulting in higher cycling stability at high current (Figs. 14(f) and14(g)). This work has confirmed the applicability of LDHs as KIB electrodes toward energy storage, similar to those the LIBs and SIBs; the combination of carbon materials and LDHs can be utilized to create exceptional high-efficiency electrode materials, compensate for the low conductivity of LDHs, and provide a solution to overcome the significant performance degradation when LDHs with variable valence metal elements are used as anode materials^[95].

This review of the research on LDHs toward the development of (flexible) supercapacitors and AIBs has placed emphasis on how their bi-component or even multicomponent composition might further increase the cycle capacity and life of electrode materials, which promotes their application in flexible wearable electronic devices. This review also expounded the common synthesis strategies of LDHs, including the utilization of the core–shell structure, layered structure, hollow polyhedron, and carbon composites, as well as analyzed the various performance indicators of as-prepared electrochemical energy storage devices. The exciting results obtained thus far will guide follow-up research and development of LDHs.

Although the research on LDHs has been extensively conducted, there are still some shortcomings that need to be addressed. It is difficult to maintain a flexible electrode structure when LDHs are used as the electrode material in flexible supercapacitors owing to frequent and long-term bending deformation. Moreover, the energy density of LDH-based supercapacitors needs to be further improved. While LDHs in AIBs have a high theoretical capacity, some instability phenomena occur during charging and discharging, such as development of a fragile structure and agglomeration, which leads to low cycling stability and impede the guarantee of reversible capacity. Adjusting the metal type and interlayer spacing in the LDH host layer is often necessary to overcome these challenges. Optimization is achieved by combining LDH bases with conductive support materials and improving the surface morphology and internal structure of the LDHs.

In summary, it is still difficult to precisely regulate the structure of LDH-based materials and identify their active sites throughout the synthesis process. Under such circumstances, it is very important to develop cutting-edge characterization techniques to obtain a deep understanding of the microscale active-site characteristics of LDH materials and the synergistic effects between various constituents. Moreover, from the standpoint of environmental resources, attempts to design low-cost, high-quality, and environment-friendly LDH-based materials are critical for sustainable development^[96,97].