Morphological control synthesis of layered double hydroxides for energy applications

In recent years, the growing demand for sustainable and efficient energy solutions has intensified research on electrochemical energy storage and conversion systems, such as batteries, supercapacitors, and water-splitting technologies [1, 2]. Layered double hydroxides (LDHs), a class of anionic clays, have garnered significant attention in these fields due to their unique layered structure, tunable composition, and exceptional chemical stability [3,4,5,6,7,8]. The specific configuration of LDHs, along with their adjustable metal cation ratios, expandable interlayer spaces, and surface modifiability, significantly influence their properties and performance in specific applications. For instance, the ability to tune metal cation ratios enhances electrochemical activity, while expandable interlayer spaces facilitate ion transport, a key factor in energy storage. Furthermore, surface modifiability enables functionalization, improving stability and efficiency for practical applications.

Number of publications from 2005 to 2025 related to four major topics of energy applications of LDHs: electrocatalysis, battery, capacitor, and fuel cell. Data source: Scopus. Access data: 1, Sept. 2025

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The synthesis and application of LDHs have been thoroughly investigated in the past two decades, and their rapidly expanding applications have made them a prominent research area. As shown in Fig. 1, number of publications reporting the energy applications of LDHs has been significantly growing over the last ten years, particularly in the sector of electrocatalysis and battery. However, there are just a limited number of papers among them that focus on the morphological management of LDHs. For instances, Sherryna et al. summarized the morphological design and dimensional approaches for photocatalytic applications [9]. Bai et al. reviewed the controllable synthesis from macroscopic morphology to microscopic coordination at the atomic level [10]. Notably, the morphological requirements for LDHs differ across energy-related applications. In catalysis, LDHs with specific morphologies are crucial for enhancing catalytic activity and efficiency by Maximizing surface area and exposing active sites. Thin nanosheets, with a thickness of less than 10nm, are widely studied for their ability to increase the number of active sites. For example, NiFe-LDH nanosheets exhibit enhanced catalytic performance in the oxygen evolution reaction (OER) by maximizing the exposure of specific crystal planes [11]. Flower-like LDHs, which incorporate macropores and mesopores, further facilitate reactant and product diffusion, thereby improving catalytic efficiency [12]. Additionally, heterostructured morphologies, such as core-shell structures combining LDHs with metal nanoparticles, optimize interfacial electron transfer and enhance catalytic activity [13]. For batteries, LDHs require morphologies that can buffer volume changes during charge-discharge cycles, optimize ion and electron transport, and Maintain structural stability. Hollow nanoparticles with particle sizes ranging from 50 to 200 nm are particularly effective. For instance, NiFe-LDH hollow spheres can accommodate volume changes during cycling, enabling a capacity retention rate of 724 mAh cm⁻² after 200 cycles [14]. Similarly, one-dimensional nanowires or nanorods, particularly those aligned along the c-axis, shorten Li⁺ diffusion paths and improve ion diffusion coefficients [15]. In supercapacitors, LDHs benefit from morphologies that promote short ion diffusion distances, high electrical conductivity, and large electrolyte contact areas. Vertically aligned nanosheet arrays, such as NiCo-LDH nanosheet arrays grown on carbon cloth substrates, have demonstrated excellent cycling stability, retaining performance after 10,000 cycles [16]. Mesoporous-layered composite structures accelerate ion adsorption and desorption, while LDH nanosheets coated with thin graphene layers enhance electrical conductivity [17, 18]. Thus, controlling structure of LDHs is highly significant for identifying the potential application.

The structural characteristics of LDHs, including morphology, size, layer thickness, crystallinity, and interlayer spacing, are strongly dictated by the choice of synthesis methods and reaction conditions. For examples, the exfoliation method, used for creating single-layer nanosheets, enhances the utility of each layer due to its large specific surface area [19]. While co-precipitation methods result in LDHs with a smaller size and low crystallinity [20]. Furthermore, parameters such as pH, temperature, and reaction sequence can significantly alter the resulting LDH structure, thereby affecting its functional performance. Up to date, although various synthesis techniques have been developed, including co-precipitation, hydrothermal synthesis, sol-gel processes, electrochemical deposition, and calcination-recovery, there remains a limited understanding of the systematic relationships between these methods, LDH structures, and their resulting properties [21].

This review provides a comprehensive summary of the intricate relationships between synthesis methods and the resulting structural properties of LDHs. We first outline three major synthetic strategies: (1) chemical methods, including co-precipitation, hydrothermal synthesis, and sol-gel processes; (2) electrochemical deposition; and (3) calcination-recovery, emphasizing their respective advantages and limitations (Fig. 2). We then analyze the influence of critical reaction parameters—such as pH, temperature, aging time, and additives—on the morphology, size, and thickness of LDHs. In addition, we review key characterization techniques, including X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM/TEM), atomic force microscopy (AFM), X-ray absorption spectroscopy (XAS), Inductively coupled plasma (ICP), and thermogravimetric analysis (TGA), which are indispensable for elucidating the structures of LDHs. Finally, we discuss the current challenges and future opportunities in optimizing LDH synthesis to enable precise morphological control for advanced energy-related applications.

Overview of LDH synthesis methods, morphologies, and energy applications. Synthesis techniques (hydrothermal, co-precipitation, electrodeposition) enable diverse morphologies (sheets, flower-like, nanoparticles, core-shell), tailored for batteries, supercapacitors, and catalysis

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2.1 LDH structure

LDHs, also known as anionic clays, are collectively referred to as hydrotalcite compounds (HT) and hydrotalcite-like compounds (HTLCs). LDHs are composed of positively charged metallic layers, interlayer anions, and water molecules. Their general structural formula is \(\:{\left[{\text{M}}_{\text{1-x}}^{\text{2+}}{\text{M}}_{\text{x}}^{\text{3+}}{\left(\text{OH}\right)}_{\text{2}}\right]}^{\text{x+}}{\left({\text{A}}^{\text{m-}}\right)}_{\raisebox{1ex}{$\text{x}$}\!\left/\:\!\raisebox{-1ex}{$\text{m}$}\right.}\cdot{\text{nH}}_{\text{2}}\text{O}\), where M²⁺ and M³⁺ represent divalent and trivalent metal cations, respectively, and Aⁿ⁻ corresponds to negatively charged interlayer anions. The molar ratio of M³⁺ to the total metal cations (M²⁺ + M³⁺), denoted as X, typically ranges from 0.22 to 0.33, depending on the intended application [22]. During synthesis, water molecules are readily intercalated into the interlayer region, where n represents the number of crystalline water molecules present in the structure. Figure 3 provides a schematic representation of the fundamental structure of LDHs, illustrating the arrangement of the metal hydroxide layers and the interlayer region that accommodates anions and water molecules. Within the structural unit of LDHs, metal cations are centrally positioned in octahedral coordination, surrounded by six hydroxide ions. These metal-hydroxide octahedra are edge-shared with neighboring units, forming a continuous, positively charged brucite-like layer. This brucite-like layer serves as the structural backbone of LDHs, providing a stable framework that defines the material’s overall stability and reactivity.

The positive charge of the brucite-like layer is neutralized by the negatively charged interlayer anions and water molecules that occupy the interlayer region. The precise arrangement of metal cations and hydroxide ions within the layer plays a pivotal role in determining the layer charge density, which directly affects the interlayer spacing. This, in turn, governs the interactions between the layers and the intercalated species, ultimately influencing the unique properties and performance of LDHs. By fine-tuning the composition and arrangement of cations and anions, LDHs can achieve tailored functionality for specific applications. For example, manipulating the layer charge density and interlayer spacing can optimize ion-exchange capacity, thermal stability, and surface reactivity, making LDHs versatile materials for catalysis, adsorption, and energy storage.

Schematic representation of the layered structure of LDHs. The positively charged layers consist of metal cations (red spheres) coordinated with hydroxide ions (pink spheres). The interlayer region accommodates water molecules (blue spheres) and negatively charged anions (yellow spheres), which balance the positive charge of the layers. The inset highlights the octahedral coordination of metal cations with hydroxide ions, forming the structural backbone of the LDHs

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2.2 Key structural parameters

The structure of LDHs, encompassing morphology, size, and thickness, is influenced by a complex interplay of factors. A comprehensive understanding of these parameters is essential for tailoring LDHs to meet the specific needs of various fields.

Morphology

Morphology plays a decisive role in dictating the structural and functional properties of LDHs. Beyond the commonly observed hexagonal platelets, [23] LDHs can be engineered into a wide range of morphologies, including spherical, [24] nanoparticles, [25] flower-like, [12] hollow, [26] and core-shell structures [27]. Such morphological diversity profoundly influences their physicochemical characteristics, such as surface area, pore distribution, ion diffusion pathways, and the accessibility of active sites. For instance, two-dimensional nanosheets and flower-like structures expose abundant reactive sites and facilitate interfacial reactions, while hollow and hierarchical architectures enhance mass transfer efficiency and accommodate structural stability during repeated use. Owing to these tunable morphologies, LDHs demonstrate remarkable adaptability across diverse applications, ranging from catalysis and energy storage to environmental remediation and biomedicine, where the optimization of morphology directly translates into enhanced performance.

Size

In addition to morphology, the size of LDHs represents another fundamental parameter that critically influences their physicochemical properties and practical applications. The thickness of the lamellar plates typically ranges from 0.4 to 0.8 nm, [28] while the interlayer spacing can be tuned from 0.3 to 2.0 nm depending on the nature of intercalated anions or guest species [29]. Beyond these intrinsic structural dimensions, the lateral size of LDH platelets can vary from several nanometers to a few micrometers, and the overall particle size spans from the nanoscale to the microscale. Differences in particle size exert a significant influence on the dispersibility of LDHs and determine their suitability for specific applications. Reducing the size, particularly into the nanoscale regime, markedly increases the specific surface area and exposes a greater number of active sites, thereby enhancing their performance in catalysis, adsorption, and energy storage. For example, Wang et al. demonstrated that a sandwich-like MXene@Ni-Mn LDH nano-microstructure in supercapacitors exhibited excellent energy and power density [30]. Nevertheless, ultrafine LDHs, while highly active, often suffer from structural instability and are prone to aggregation or dissolution. In contrast, larger particles generally exhibit improved crystallinity and stability, albeit at the expense of restricted diffusion and reduced accessibility of active sites. The precise control of LDH size can be achieved through employing specific synthetic methods, thereby enabling a fine balance between activity and stability.

Thickness

The thickness of LDHs constitutes a distinct structural dimension that profoundly affects their physicochemical properties. The thickness can vary widely, from monolayers with sub-nanometer dimensions to few-layer assemblies and even micrometer-scale multilayer structures. Monolayer LDHs, with thicknesses typically around 0.48 nm, are of particular interest due to their exceptionally high surface area and nearly complete exposure of active sites. However, their synthesis is nontrivial because of the strong hydrogen bonds and van der Waals forces that stabilize the stacked layers. Advanced strategies such as plasma-assisted exfoliation, surfactant- or solvent-mediated delamination, and confined microemulsion growth have been developed to obtain such ultrathin structures. For examples, O’Hare et al. employed a reverse microemulsion method to control the growth of LDHs, resulting in ultrathin monolayers [31]. Similarly, Sun et al. introduced formamide during synthesis, enabling the one-step production of monolayer LDHs [32]. In contrast, multilayer LDHs provide enhanced crystallinity and structural robustness, with their overall thickness strongly influenced by the type of interlayer anions [33]. Larger anions tend to increase interlayer spacing, while smaller anions result in more compact structures. For example, Zhang et al. found that oxalate ion intercalation into NiFe-LDH increased the interlayer spacing of NiOOH from 0.205 nm to 0.210 nm, thereby enhancing structural stability and electrochemical performance [34].

Overall, the structural characteristics of LDHs, including morphology, size, and thickness, collectively dictate their physicochemical behavior and functional performance. Morphological diversity determines the external architecture and accessibility of active sites, size governs dispersibility, surface area, and mass/charge transport efficiency, while thickness reflects the degree of layer stacking and interlayer interactions, balancing activity with structural stability. These parameters are not intrinsic constants but can be deliberately tuned through interlayer ion selection, synthesis methods, and growth conditions. By controlling these parameters, it is possible to design LDH materials with tailored properties to meet the demands of diverse application fields such as catalysis, energy storage, environmental remediation, and biomedicine.

2.3 Challenges of morphological control synthesis of LDHs

The intrinsic structural features of LDHs endow them with distinct advantages LDHs in morphology synthesis. On one hand, the compositional tunability offers a significant advantage, allowing for the straightforward modulation of morphological characteristics, including particle size, thickness, and stacking order, through modifications to the M²⁺/M³⁺ ratio or the selection of suitable interlayer anions. On the other hand, a variety of synthesis methods, including co-precipitation, hydrothermal/solvothermal methods, and templating, leverage the structural stability of LDHs to generate morphologies range from zero-dimensional (0D) nanoparticles to three-dimensional (3D) hierarchical structures. Additionally, the exchangeability of interlayer anions, which allows post-synthetic functionalization or structural adjustment without compromising the layered framework, thereby expanding the scope of LDH-based materials for targeted applications.

However, the morphological synthesis of LDHs also faces inherent challenges. Their structure is extremely sensitive to synthesis parameters, minor changes in parameters such as pH and temperature can disrupt the precise arrangement of ions within the hydroxide layers, leading to phase impurities or irregular morphologies. Reaction parameters will be discussed in 4.2. In addition, the exfoliation of nanosheets requires overcoming strong interlayer electrostatic forces and hydrogen bonds. Even after weakening these interactions via anion exchange, harsh conditions such as sonication are often necessary, which easily damage sheet integrity and yield only Limited amounts of high-quality nanosheets. Meanwhile, the brucite-like layers also exhibit relatively low thermal stability. Temperatures exceeding 300°C cause dehydroxylation and layer collapse, which restricts the applicability of LDHs under harsh thermal conditions unless further stabilization strategies are employed. Beyond these intrinsic limitations, issues such as particle aggregation, batch-to-batch reproducibility, and scale-up synthesis also remain obstacles to the practical exploitation of LDH morphological tunability.

In summary, LDHs are composed of positively charged metal hydroxide layers with intercalated anions and water molecules, represent a highly versatile class of materials with tunable structural and functional properties. Key parameters, including morphology, size, and thickness, can be precisely controlled through selection of synthesis methods, metal compositions, and interlayer anions, enabling the design of LDHs with tailored performance. This tunability confers notable advantages, such as the ability to generate diverse morphologies, optimize surface area and active site exposure, and achieve post-synthetic functionalization. However, these materials also present inherent challenges, including high sensitivity to reaction conditions, difficulty in achieving uniform nanosheets, and limited thermal stability, which can constrain their practical implementation. Despite these limitations, the controlled manipulation of structural parameters allows LDHs to be engineered for a wide range of applications, including catalysis, energy storage, environmental remediation, and biomedicine, highlighting their potential as adaptable and high-performance functional materials.

To tailor the structural parameters of LDHs, this section highlights three primary synthesis approaches: chemical synthesis, electrochemical deposition, and calcination recovery, as well as emerging techniques. Each subsection examines the key features, benefits, and limitations of these methods, offering a comprehensive understanding of their roles in LDHs synthesis.

3.1 Chemical synthesis

Chemical synthesis methods are among the most widely utilized approaches for preparing LDHs, valued for their flexibility and simplicity. This category includes techniques such as co-precipitation, hydrothermal, and sol-gel methods. A comparison of these three chemical synthesis methods is presented in Table 1.

3.1.1 Co-precipitation method

The co-precipitation method remains one of the most widely used techniques for synthesizing LDHs, which involves the dissolved inorganic salt in an alkaline medium [35]. In this approach, the growth of LDHs can be divided into two strategies: (i) low supersaturation and (ii) high supersaturation. The low supersaturation approach is based on the addition of a solution comprising M²⁺ and M³⁺ cations at the desired molar ratio, which serve as precursor salts, to an alkaline aqueous solution containing a salt of the interlayer anions, with the pH being Maintained at the selected value. Generally, the pH is kept above 8 throughout the titration to ensure rapid coprecipitation. The precipitate then undergoes aging at room temperature for at least 24 h, followed by centrifugation, a wash with deionized water, and vacuum drying to create the desired product. This method has been employed to achieve specific morphologies, such as the flower-like NiCo-LDH nanoparticles synthesized by Yang et al. [36] In contrast, the high supersaturation technique involves introducing a titrant into the cationic solution along with the alkaline aqueous solution that holds the anion to be intercalated, with the pH being adjusted until the target level is attained. For example, an alkaline aqueous mixture, consisting of NaOH and Na₂CO₃, is introduced into a cationic solution containing Co²⁺ and Fe³⁺, with the pH being adjusted from 2 to 7, in order to synthesize CoFe LDHs [37]. Starting at pH 2, elevated pH levels cause Fe³⁺ to precipitate as ferrihydrite, forming the primary reaction intermediate. Once pH exceeds 2, the adsorption of Co²⁺ onto ferrihydrite facilitates a redox reaction involving ferrihydrite’s Fe³⁺ and the adsorbed Co²⁺ to form Co³⁺. With an increase in pH, the crystallinity of the weakly crystallized ferrihydrite steadily declines. At pH 7, the interplay of this phenomenon and the hydrolysis of both sorbed Co³⁺ and available Co²⁺ produces novel hydroxylated Fe²⁺/Co³⁺ LDHs. Besides, This technique is widely used to synthesize LDHs with carbonate (CO₃²⁻) as the interlayer anion, such as NiFe-LDHs, [38] ZnAl-LDHs, [39] and MgAl-LDHs [40].

A significant limitation of the co-precipitation method is the poor crystallinity of the synthesized LDHs. To overcome this drawback, a thermal aging process is often performed after initial precipitation. During this process, the precipitated LDHs are heated in an autoclave or oven at elevated temperatures (typically 60–120 °C) for several hours. This promotes crystal growth and rearrangement, thereby improving crystallinity. For instance, Guo et al. synthesized Co_xCu_y-LDHs with thermal aging at 65 °C for 24 h, resulting in enhanced crystallinity [41].

Meanwhile, two key factors, pH and the type of alkaline solution, significantly influence the structure of LDHs. The pH controls the nucleation and growth of LDH crystals, while the choice of alkaline solutions affects the morphology and composition of the resulting LDHs. These parameters are critical for optimizing the performance and properties of LDHs in various applications.

3.1.2 Hydrothermal method

The hydrothermal method has emerged as a highly effective technique for synthesizing stable LDHs [42]. In this process, metal salt and base solutions are combined in a sealed autoclave, which is then Heated to a specific temperature, typically in the range of 30–300 °C, under autogenous pressure for a set duration, ranging from several hours to a few days. The elevated temperature and pressure within the autoclave create an optimal environment for the growth of well-ordered crystals, resulting in LDHs with large, uniform particle sizes and high crystallinity. Most commonly, metal oxides and hydroxides are used as reactants. For instance, Xu et al. reported that the mixed MgO and Al₂O₃ oxides were used as metal source to generate MgAl LDHs by controlling the pH of reaction solution [43]. Initially, MgO and Al₂O₃ were understood to hydrate, forming Mg(OH)₂ and Al(OH)₃. Subsequently, in a neutral environment, Mg(OH)₂ rapidly dissociated into Mg²⁺ and OH^- ions, which then deposited onto the Al(OH)₃/Al₂O₃ surface to create a MgAl pre-LDH material. By contrast, an alkaline reaction solution caused Al(OH)₃ to ionize into Al(OH)₄^- and precipitate onto the Mg(OH)₂/MgO surface, yielding a similar MgAl pre-LDH material. Afterward, continuous heating in the hydrothermal process led to the pre-LDH material becoming well crystallized. Notably, the increased pressure is not necessary for the synthesis of LDHs. Sun’s group reported a rapid hydrothermal method for producing MgAl-LDH single-layer nanosheets at 80 °C, involving 23 vol% formamide. [44]They observed that formamide’s presence is crucial for single-layer nanosheet development, as its molecules attach to the LDH sheet surface, thereby preventing the interlayer growth. Beyond its direct application as a synthesis technique, mild hydrothermal treatment (at temperatures up to 200 °C) also helps to improve the crystallinity and crystallite size of LDH precursors that have been prepared using other methods, including coprecipitation and the sol-gel method, etc [45].

Compared to the co-precipitation method, the hydrothermal method offers multiple advantages. The controlled growth conditions in the autoclave enable precise regulation of the crystallization process, leading to the formation of more homogeneous LDHs. Additionally, this method is versatile, allowing for the synthesis of LDHs with complex compositions and structures [46]. By incorporating various metal salts and additives into the reaction mixture, LDHs with tailored structures can be achieved. For example, Zhang et al. demonstrated that adding surfactants during hydrothermal synthesis could alter the morphology of ZnAl-LDHs, resulting in unique structural formations (Fig. 4) [47]. Recently, Huang et al. employed a facile hydrothermal method to synthesize Ru single atoms and sulfur anions dual-doped LDHs. This approach yielded nanosheets with a diameter of 100nm and a thickness of 2.5 nm, demonstrating the method’s ability to achieve precise control over nanoscale dimensions [48]. Yet, the hydrothermal method also has certain drawbacks. Generally, increasing the preparation temperature and extending the hydrothermal treatment time leads to the observation of a higher hydrotalcite content [49]. The high energy consumption and prolonged reaction periods (from hours to days) are significant disadvantages for the industrial application of hydrothermal treatment. Besides, specialized equipment is also necessary, such as autoclaves, which increases both the cost and complexity of the synthesis process.

SEM images of (a, b) ZnAl(CO₃²⁻)-LDH. (c, d) ZnAl(SDS)-LDH. And (e, f) ZnAl(SDBS)-LDH. SDS: sodium dodecyl sulfate; SDBS: sodium dodecyl benzene sulfonate. Figure adapted and reproduced with permission from Ref. [47]. Copyright 2013 Springer Nature

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3.1.3 Sol-gel method

The sol-gel method is highly regarded for its low cost and minimal energy demands, facilitating the production of high-purity materials. This method allows for the precise regulation of the final materials’ structural characteristics through simple modifications to the chemical composition of the precursors and the aging duration, along with the selective addition or removal of reactant types. In this process, LDH synthesis involves forming a sol, which is a colloidal suspension of solid particles in a liquid, using metal alkoxides or metal salts as precursors [50]. Initially, the metal precursors are dissolved in a suitable solvent, followed by a series of chemical reactions, including hydrolysis and condensation, to create a gel-like network. The gel is then dried and calcined, taking multiple hours or days, at temperatures from ambient to a Maximum of 100°C with slight heating, yielding the ultimate LDH product. A wide variety of materials can be synthesized by modifying the metal cation concentration, which in turn adjusts the M²⁺ and M³⁺ cation substitution ratio.

One of the key advantages of the sol-gel method is its ability to mix precursors at an atomic level, offering superior control over particle size and morphology [22]. By fine-tuning parameters such as the type of solvent, the concentration of metal precursors, and reaction conditions, LDHs with varied shapes and sizes can be synthesized. For example, Smalenskaite et al. compared the synthesis of MgAl-LDHs via co-precipitation and sol-gel methods. Their findings revealed that the sol-gel method resulted in products with enhanced crystallinity and larger particle sizes compared to those produced by the co-precipitation method (Fig. 5) [51]. The sol-gel method thus provides a valuable approach for synthesizing LDHs with high crystallinity and precise morphological control, making it suitable for advanced material applications.

SEM images of (a) Mg/Al/Ce 1 mol% LDH synthesized by the co-precipitation method, (b) reconstructed Mg/Al/Ce 1 mol% LDH, (c) Mg/Al/Ce 1 mol% LDH, and (d) Mg/Al/Ce 10 mol% LDH synthesized by the sol-gel method. Figure adapted and reproduced with permission from Ref. [51]. Copyright 2017 Elsevier B.V

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However, the sol-gel method has several limitations that must be considered. The use of organic solvents and the involvement of complex chemical reactions raise environmental concerns and increase the risk of product contamination. Furthermore, maintaining the long-term stability of the sol and effectively controlling the gel-to-solid conversion process can be challenging, potentially compromising the reproducibility and consistency of the synthesis. For low-cost mass production, the co-precipitation method remains the most practical option due to its straightforward operation and low cost, making it particularly suitable for energy-related applications. In contrast, the hydrothermal and sol-gel methods offer superior structural control, allowing for the synthesis of materials with precise morphologies and enhanced crystallinity. However, these advantages come at the expense of higher equipment and energy costs, which restrict their viability for cost-sensitive, large-scale production.

3.1.4 Calcination-recovery

The calcination-recovery method utilizes the distinct “memory effect” of LDHs, which contains a versatile two-step process for synthesizing and modifying LDHs [52]. In the first step, the synthesized LDHs are calcined at high temperatures, typically between 300 and 600 °C, in an inert atmosphere. During calcination, the interlayer anions and water molecules are removed, causing the LDH structure to collapse into a mixed metal oxide known as a calcined-LDH or layered double oxide (LDO). These calcined products exhibit high surface areas and enhanced reactivity, making them suitable for a wide range of applications. In the second step, the LDO is rehydrated or reduced in the presence of water and an appropriate anion, thereby restoring the LDH structure. This rehydration process is typically carried out by immersing the LDO in an aqueous solution containing the desired interlayer anion, which facilitates an anion exchange reaction and reconstructs the LDH. For example, Carja et al. utilized the memory effect to reconstruct CuZnAl LDHs, producing abundant Cu/Zn interfaces that enhanced catalytic activity [53].

The calcination-recovery method has significant advantages. One of its key benefits is the ability to tailor the interlayer composition and properties of LDHs. By selecting different anions during the rehydration process, LDHs with customized structures can be synthesized. For instance, Lu et al. demonstrated the intercalation of LDHs with organic corrosion inhibitors to absorb chloride ions (Cl⁻), enhancing their performance as anti-corrosion materials [54]. Additionally, the calcined LDOs generated in the first step possess enhanced chemical reactivity and high surface areas, which are especially beneficial for catalytic, adsorption, and other functional applications.

Despite its versatility, this method also has notable limitations. The high-temperature calcination process can lead to particle sintering, which reduces the surface area and pore volume of the LDHs, potentially diminishing their effectiveness. Furthermore, the rehydration process does not always guarantee complete restoration of the original LDH structure. Factors such as the type of anion, rehydration temperature, and rehydration time can significantly influence the quality and properties of the rehydrated LDHs.

3.2 Electrochemical deposition

Electrochemical deposition is a method based on the cathodic reduction of anions in a solution containing both divalent and trivalent metal ions. This process generates a basic environment, enabling the precipitation of LDHs [55]. In this technique, the working electrode, counter electrode, and reference electrode are immersed in an electrolyte solution containing metal salts. When an appropriate potential or current is applied between the working and counter electrodes, metal ions are reduced at the surface of the working electrode. Simultaneously, hydroxide ions are generated due to the reduction of water or anions. The presence of both metal ions and hydroxide ions at the electrode surface facilitates the formation of LDHs directly on the electrode.

Electrochemical deposition offers several advantages. One of its key benefits is the precise control it provides over the thickness and composition of the deposited LDH films. By adjusting parameters such as the applied potential, current, deposition time, and metal salt concentration, LDH films with tailored properties can be fabricated. This method is particularly effective for synthesizing monolayer or thin LDHs. For example, Guo et al. demonstrated the direct synthesis of monolayer Ni(OH)₂ films on electrodes via in situ electrochemical conversion, as illustrated in Fig. 6. [56] Similarly, Zhao et al. achieved precise growth control of NiFe-LDH nanosheets through electrodeposition, enabling the formation of stable β-NiOOH films [57].

(a) Schematic illustration of the whole monolayer fabrication process. (b) TEM image of monolayer Ni(acac)₂. (c) The corresponding fast Fourier transform pattern. (d) monolayer Ni(acac)₂. (e) Height profile. (f) Monolayer Ni(acac)₂ nanosheets. (g) Repeated LSV cycles of the exfoliated Ni(acac)₂. (h) In situ Raman spectra demonstrating the structural changes along with the electrochemical decomposition that occur during the different stages of the fabrication process. (i) height profile. (j) monolayer Ni(OH)₂ nanosheets. acac: acetylacetonate. Figure adapted and reproduced with permission from Ref. ⁵⁶. Copyright 2021 Springer Nature

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Although electrochemical deposition offers significant advantages, it is not without limitations. One major drawback is that LDHs synthesized through this method often exhibit lower crystallinity and uniformity compared to those produced via chemical synthesis methods. Furthermore, the process demands precise control over critical parameters such as potential and current density. Failure to maintain these parameters can lead to uneven coating quality, reduced structural integrity, and degraded performance of the resulting LDH films. These challenges, in turn, make the method less suitable for the large-scale production of LDHs, where high consistency and uniformity are essential.

However, these limitations can be partially mitigated by optimizing process parameters and incorporating advanced techniques. For example, the introduction of magneto-electrochemical deposition has shown promise in overcoming some of these issues [58]. Zhu et al. demonstrated the synthesis of a three-dimensional cross-linked NiCo-LDH with ultrahigh capacity by applying magneto-electrodeposition [59]. This approach highlights the potential of magnetic fields in improving the quality and performance of LDHs produced via electrochemical deposition.

3.3 Other synthesis methods

3.3.1 Exfoliation

The exfoliation method is a widely used approach for preparing layered double hydroxides (LDHs) with individual layers that are well-separated [60, 61]. This approach requires choosing a suitable method to increase the interlayer spacing and diminish the interlayer interactions, thereby separating the LDH layers from the main substance, usually via intercalation and delamination. In the first step, an appropriate intercalating agent is introduced into the interlayer region of the LDHs. This agent expands the interlayer spacing, weakening the interlayer interactions and facilitating the separation of the layers. Once the intercalated LDHs are prepared, the delamination process is performed using techniques such as sonication, centrifugation, or mechanical shearing in a suitable solvent [62]. According to the factors involved in the synthesis process, the exfoliation approach can be divided into three methods: (i) liquid phase exfoliation with pre-treatment; (ii) direct liquid phase exfoliation; (iii) solid phase exfoliation. Liquid phase exfoliation with pre-treatment is considered an efficient approach to lessen the intense interactions between host layers and anions, accomplished by substituting the original anions with other preferred organic/inorganic molecules or anionic species, such as dodecyl sulfate (DDS), NO₃⁻, graphdiyne, laurate, etc. For example, Li et al. reported that large MgAl-LDH crystals were exfoliated by vigorously shaking the bulk materials in formamide at room temperature [63]. It is important to note that a preliminary step, involving the treatment of MgAl–CO₃²⁻-LDH in an aqueous solution with specific quantities of NaNO₃ and HNO₃, is necessary for effective exfoliation. The CO₃²⁻ can be substituted by NO₃⁻ species and thus weaken the interaction between CO₃²⁻ and the host layers, resulting in improved exfoliation. In addition to pre-treatment, the exfoliation of bulk LDHs can also be realized directly in liquid or solid media by applying external forces such as sonication, [64] mechanical shearing, [65] or plasma treatment [66]. The exfoliated LDH nanosheets produced through these methods possess large surface areas and ultrathin layers, which enhance reactivity and improve dispersion in various media [67]. For example, Yu et al. utilized silicon tetrachloride (SiCl₄) as an intercalating agent to chemically exfoliate Si-ZnAl-LDH nanosheets [35]. Similarly, Hu et al. reported an orthogonal liquid-phase exfoliation approach that improved the catalytic activity of LDHs without altering their composition or structure [68]. As illustrated in Fig. 7, the exfoliated single-layer nanosheets exhibited significantly higher oxygen evolution activity in alkaline conditions compared to bulk LDH materials. The exfoliation process created additional active sites, enhancing the electronic conductivity of the nanosheets and thereby improving their performance.

Despite numerous investigations into LDH synthesis through exfoliation, liquid phase exfoliation persists as the predominantly employed method, possessing certain disadvantages. It is theoretically feasible to produce ultrathin LDH structures through liquid phase exfoliation, owing to their layered arrangement bound by one-dimensional van der Waals forces. However, the method still presents several disadvantages, such as ineffective exfoliation, challenges in regulating layer count and lateral dimensions, the requirement for organic solvents, and the risk of contamination from adsorbed organic molecules. Selecting a suitable intercalating agent and optimizing the exfoliation process are critical to obtaining high-quality LDH nanosheets. Furthermore, the exfoliated nanosheets often have a tendency to restack during storage or processing, which can reduce their effectiveness and limit their performance in practical applications. For instance, Yu et al. observed that synthetic monolayer MgAl-LDH nanosheets tended to re-stack after their dispersed sample was cast and dried on a silicon wafer [32]. The re-stacking or clumping phenomenon could arise from physical interlinking, attractive electrostatic interactions, or the significant surface energy inherent to the ultrathin nanosheets.

(a) Optical images of colloidal solutions of exfoliated nanosheets. (b) TEM images of bulk NiCo LDH nanoparticles. Inset is the corresponding selected area electron diffraction (SAED) pattern. Scale bar, 400 nm. (c) TEM images of exfoliated single layer nanosheet of NiCo LDH. Scale bar, 100 nm. (d) AFM image of exfoliated single-layer nanosheets of NiCo LDH. (e) Height profile of the exfoliated monolayer of NiCo LDH. Figure adapted and reproduced with permission from Ref. [68]. Copyright 2014 the Authors

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3.3.2 Microwave-assisted synthesis

Microwave-assisted synthesis is an innovative and rapidly emerging technique that has gained significant attention in recent years for the efficient synthesis of LDHs [69]. Unlike conventional heating methods, where heat is transferred indirectly through conduction and convection, microwave irradiation directly couples with the reaction medium, leading to uniform and volumetric heating. In this method, the reaction mixture is irradiated with microwaves, thereby accelerating nucleation and growth processes while significantly reducing reaction times, making it a time-efficient alternative to traditional synthesis methods.

One of the primary advantages of microwave-assisted synthesis is the significant reduction in reaction time due to the uniform and rapid heating provided by microwaves. This promotes faster crystal growth and the efficient formation of LDHs. Moreover, this method enables the synthesis of LDHs with high purity and excellent crystallinity. The rapid and homogeneous temperature distribution minimizes the formation of impurities and facilitates the growth of well-ordered crystals. For example, Li et al. successfully designed and synthesized an array core-shell Heterostructure graphene nanoscrolls composite, where petal-like NiCo-LDH nanoflakes were vertically anchored on a 3D interconnected graphene nanoscrolls skeleton using a highly facile microwave-assisted method [70]. The resulting morphology, as shown in Fig. 8, highlighted the capability of this technique to produce advanced LDH-based composites with precise structural control.

Unfortunately, microwave-assisted synthesis exhibits certain limitations. One major drawback lies in the requirement for specialized equipment, such as microwave reactors, can be expensive, making it less accessible for some research and industrial settings. In addition, the scale-up of microwave-assisted synthesis remains a significant challenge. Current reactor designs typically offer limited capacity, and when extended to larger volumes, issues such as non-uniform microwave penetration and localized overheating may occur. These effects can lead to heterogeneous nucleation, poor crystallinity, or variations in particle morphology, ultimately compromising the reproducibility and quality of the synthesized LDHs. Moreover, the high energy input associated with microwave irradiation raises concerns about energy efficiency and cost-effectiveness in large-scale production, which requires advances in reactor design, improved process control, and a deeper understanding of microwave–matter interactions.

SEM images of (a,b) pure GNSs. (c) pristine NiCo-LDH. (d) NiCo-LDH@GNSs. Figure adapted and reproduced with permission from Ref. [70] Copyright 2022 Elsevier B.V

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3.4 Feasibility analysis for energy applications

The synthesis methods of LDHs play a critical role in determining their morphology, performance, and commercial feasibility for energy applications (Table 2). Traditional chemical methods, such as co-precipitation, are well-suited for low-cost mass production of simple morphologies, such as nanoparticles. However, these methods often yield products with limited crystallinity, which may limit their performance in high-demand energy applications. In contrast, hydrothermal and sol-gel methods enable precise control over morphology, allowing the synthesis of uniform nanosheets or porous structures that are ideal for high-performance applications, such as electrocatalysts. Nevertheless, these methods are associated with higher production costs due to their energy-intensive processes and the need for specialized equipment.

Advanced techniques, including electrochemical deposition, exfoliation, and microwave-assisted synthesis, further expand the range of LDH morphologies for energy applications. Electrochemical deposition facilitates the fabrication of thin-film electrodes, which are particularly advantageous for applications like batteries and supercapacitors. Similarly, exfoliation methods produce high-surface-area, single-layer nanosheets that improve charge storage and distribution, making them suitable for supercapacitors. Additionally, microwave-assisted synthesis offers rapid fabrication of complex nanostructures, such as core-shell composites, which exhibit enhanced performance in energy storage and conversion systems. However, these advanced methods often involve elevated costs due to the need for specialized equipment, intricate procedures, or scalability challenges.

This section presents a review of the current advancements and improvements in synthesis methods. Moreover, each synthesis technique has been elaborated upon to reveal its advantages and drawbacks concerning structural and textural aspects, although each synthesis method offers unique advantages tailored to specific energy applications, the advancement of LDHs toward commercial deployment will ultimately depend on reducing production costs and optimizing synthesis processes. Developing cost-effective and scalable methods that maintain precise control over LDH morphologies is necessary to meet the growing demands of energy storage and conversion technologies.

4.1 Crystal growth kinetics

Understanding crystal growth kinetics and anisotropic regulation is essential for controlling the morphology of LDHs. The crystal growth of LDHs typically follows two primary mechanisms: dissolution–recrystallization and Ostwald ripening. These mechanisms govern the nucleation and growth processes, leading to anisotropic crystal development.

During synthesis, anisotropic growth of LDHs arises from differences in the hydrolysis rates of metal ions, nucleation density, and growth rates of crystal facets. Specifically, In terms of horizontal growth, metal ions within the layer plates form rigid structures through hydroxyl bonding, which restricts horizontal expansion and facilitates the formation of nanosheets or nanoparticles. Vertical growth, on the other hand, is strongly influenced by the type and concentration of interlayer anions (e.g., CO₃²⁻, NO₃⁻) which determine the interlayer spacing and regulate the rate of vertical growth. For instance, large organic anions (e.g., dodecylsulfonate) can expand the interlayer space, promoting growth perpendicular to the layer plates and resulting in expanded layered structures.

4.2 Reaction parameters

The morphology and size of LDHs are highly sensitive to reaction parameters during synthesis. Key factors include the metal salt ratio, pH, reaction temperature, and reaction duration, all of which directly affect the structure and performance of LDHs.

Metal Salt Ratio: This ratio affects the layer charge density and interlayer spacing, which in turn influence LDH morphology, especially for bimetallic LDHs. Cavani et al. suggested that an M²⁺:M³⁺ ratio in the range of 2 ~ 4 is optimal for obtaining pure LDHs, while a higher molar ratio can lead to the formation of undesired secondary phases [71]. An elevated M³⁺ content increases the positive charge density on the layer plates, requiring compensation by a higher concentration of interlayer anions, which promotes interlayer expansion and vertical growth. This process often results in thin-layer structures. Incorporating transition metals (e.g., Co, Ni) with variable valence states can modify the electronic structure of LDHs, induce defect formation, and control the exposure of specific crystal facets. For example, Liu et al. demonstrated that co-doping NiFe-LDHs regulated active sites via electronic effects, leading to porous nanosheets with enhanced electrocatalytic performance [72].

pH of the reaction solution

The pH significantly affects the precipitation and crystallization of LDHs. At low pH, incomplete or disordered precipitation of metal hydroxides often results in poor-quality LDHs. At high pH, byproducts such as metal oxides May form, reducing Material purity. Betelu et al. found that CoFe-LDHs synthesized at a pH of 9 exhibited excellent morphology and size (Fig. 9; Table 3) [73]. Similarly, Seron et al. reported that successful LDH synthesis occurs within a pH range of 10–13.2, with optimal crystallinity and composition [74].

Reaction Temperature and Duration: Temperature: Higher temperatures accelerate crystal growth by providing more energy for ion diffusion and hydroxide complex formation. Wu et al. observed that at 80°C, amorphous LDHs mixed with nanowires formed, while at 120 °C, nanoparticles and nanowires coexisted [75]. Duration: Prolonged reaction times allow for more complete crystallization and growth. Zhao et al. proposed a method involving separate nucleation and aging steps during MgAl-LDH synthesis, yielding uniform and small crystallites [76]. Longer reaction times enable larger, more crystalline particles to form as crystal layers stack and grow. At the same time, Table 4 indicates that regardless of whether LDHs grow along the a or c direction, the size of MgAl-LDHs gets bigger with the increasing temperature and duration.

SEM images for CoFe-LDHs synthesized at pH values 7, 8, 9, 11. Figure adapted and reproduced with permission from Ref. [73]. Copyright 2022 the Authors

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4.3 Role of additives and templates

Additives and templates are essential tools for tailoring the structure and morphology of LDHs, playing a critical role in optimizing their performance for energy-related applications such as catalysis, batteries, and supercapacitors. Additives, including surfactants, polymers, and inorganic salts, modify the surface properties of LDHs and influence their growth kinetics [78]. For instance, surfactants like sodium dodecyl sulfate (SDS), sodium lauryl sulfate (SLS), and cetyl trimethyl ammonium bromide (CTAB) can adsorb onto LDH particle surfaces, acting as barriers to control particle growth and prevent agglomeration. Zhang et al. demonstrated that SDS produced flower-like nanospheres with uniform size, SLS resulted in irregularly shaped particles with poor crystallinity, and CTAB led to hexagonal flakes with small grain sizes and uneven distribution (Fig. 10) [79]. The choice of surfactant is thus critical when specific morphologies are required. For example, CTAB is particularly effective for producing ultrathin nanosheets, which are desirable for supercapacitors as they enhance charge storage capacity.

Templates, on the other hand, are used to guide the growth of LDHs into specific morphologies. Hard templates, such as SiO₂ spheres and anodic aluminum oxide (AAO), enable the formation of ordered porous structures, while soft templates, such as surfactants like CTAB or block copolymers, self-assemble into frameworks that direct crystal growth. Chen et al. synthesized NiCo-LDH nanocages using a Cu₂O template etching method, demonstrating how templates can create hollow and well-defined structures (Fig. 11) [80]. Templates also find application-specific utility. In catalysis, templates are used to create nanosheets or porous nanospheres, Maximizing the surface area for active site exposure and improving reaction efficiency. For battery applications, morphologies Like hollow nanoparticles or 1D nanowires are preferred as they buffer volume changes and enhance ion transport, with carbon-based hard templates or self-template strategies derived from metal-organic frameworks (MOFs) often employed for these designs. In supercapacitors, ultrathin nanosheets or porous architectures are crucial for improving charge storage. Templates like CTAB and AAO are particularly effective in exfoliating LDH layers or forming ordered porous structures.

In summary, the choice of additives and templates is critical for achieving desired LDH morphologies tailored to specific applications. Additives such as surfactants are particularly useful for controlling particle size and preventing aggregation, while templates enable the formation of complex and application-specific structures. By integrating these strategies with the appropriate synthesis methods, LDH materials can be precisely engineered to optimize their performance in energy storage and conversion technologies.

Field Emission SEM (FESEM) images of the MgAl-LDH samples synthesized at 160 °C for 6 h with different surfactants as the soft-templates: (a) MgAl-SDS-LDH, (b) MgAl-SLS-LDH, (c) MgAl-CTAB-LDH, and (d) MgAl-CO3-LDH. SEM images of the MgAl-SDS-LDH samples synthesized at 160 °C for 6 h with different concentrations of SDS: (e) 0.005 mol L⁻¹, (f) 0.02 mol L⁻¹, (g) 0.04 mol L⁻¹, and (h) 0.06 mol L⁻¹. Figure adapted and reproduced with permission from Ref. [79]. Copyright 2015 Royal Society of Chemistry

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Preparation process of the NiCo-LDH nanocage. Figure adapted and reproduced with permission from Ref. [80]. Copyright 2023 Royal Society of Chemistry

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Accurate characterization of LDHs is essential for understanding their structural, compositional, and morphological properties. A suite of complementary techniques is typically employed to provide comprehensive insights into the composition, morphology, size, and thickness of LDHs. These insights facilitate the rational design and optimization of LDHs for various applications, particularly in the energy sector.

X-ray Photoelectron Spectroscopy (XPS) is a powerful tool for analyzing the elemental composition and chemical states of LDHs [81]. By irradiating the sample with X-rays, XPS ejects electrons from the atoms on the surface of the material. The kinetic energy of these ejected electrons is specific to the element and its chemical environment, allowing for the identification and quantification of elements present in LDHs. Additionally, XPS provides information on the valence states of the elements. By analyzing the electronic environment, XPS can help determine how elements contribute to the material’s functionality, such as catalytic activity or electronic conductivity. Furthermore, the application of near ambient pressure XPS can reveal quantitative insights into the water content in the interlayer region, and ultraviolet photoemission spectroscopy serves to analyze the particle’s outermost surface layer and distinguish the diverse valence band structures influenced by the intercalated anions, offering supplementary insights to XPS.

X-ray Diffraction (XRD) is another fundamental technique used to characterize the crystalline structure of LDHs. When X-rays interact with the ordered atomic lattice of LDHs, they are diffracted at specific angles, producing a unique diffraction pattern that corresponds to the material’s crystal structure. Analysis of these patterns provides critical insights into phase purity, crystallinity, and interlayer spacing of LDHs. By comparing experimental XRD patterns with standard reference patterns, researchers can identify the type of LDHs synthesized and assess their structural integrity. Additionally, XRD is particularly valuable for monitoring changes in interlayer spacing, which play a key role in processes such as ion intercalation and exfoliation. For instance, XRD analysis was used to determine the lattice parameters of Ru-S-NiFe LDH, revealing values of a = 12.116 Å, b = 6.317 Å, c = 23.227 Å and a volume of V = 1539.5 ų, compared to NiFe LDH values of a = 12.112 Å, b = 6.313 Å, c = 23.220 Å, V = 1537.6 ų.⁴⁸ Such subtle variations highlight the sensitivity of XRD in detecting structural modifications induced by elemental incorporation or lattice distortion.

Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) are widely utilized to investigate the morphology, size, and thickness of LDHs. SEM provides high-resolution images of the surface and external morphology of LDH particles, enabling detailed examination of particle shape, size distribution, and surface features. This technique is particularly useful for identifying overall morphology, such as whether LDHs form platelets, spheres, or more complex structures. TEM, on the other hand, offers higher-resolution imaging and provides information on the internal structure of LDHs. Through TEM, researchers can directly measure the thickness of individual layers, dispersion, and, in some cases, resolve atomic arrangements within the layered framework. It is especially useful for studying the morphology of nanosheets produced by exfoliation methods, offering direct evidence of their ultrathin morphology and structural integrity. Further understanding of crystal orientation and defects is gained by associating TEM with selected area electron diffraction (SAED) patterns. Elemental mapping is achieved through the application of energy dispersive X-ray spectroscopy (EDS) with SEM or TEM. Additionally, electron energy loss spectroscopy (EELS) has the capability to assess atomic composition, chemical bonding, valence, and other characteristics of surfaces.

Atomic Force Microscopy (AFM) is a valuable tool for analyzing the size and thickness of LDHs at the nanoscale [82]. AFM operates by scanning a sharp probe over the sample surface and measuring the forces between the probe and the sample to generate a topographical image. This technique can precisely measure the thickness of LDH layers and provide insights into surface roughness and mechanical properties. For instance, AFM analysis of Ru-S-NiFe LDH revealed a thickness of approximately 2.5 nm (Fig. 12) [48]. AFM has also been used to monitor the dynamic transformation of LDHs during reactions. Stefano et al. combined operando near-edge X-ray absorption fine structure (NEXAFS) spectroscopy with AFM to study the structural evolution of CoAl-LDH in real-time during oxygen evolution reaction (OER), as shown in Fig. 13 [83].

X-ray absorption spectroscopy (XAS) is a powerful tool for probing the local atomic and electronic structure of LDHs and their precursor materials [84, 85]. Extended X-ray absorption fine structure (EXAFS) analysis provides detailed information on the immediate atomic environment, including bond lengths, coordination numbers, and local vibrational dynamics, which are critical for understanding structural distortions, cation substitutions, and lattice disorder. X-ray absorption near edge structure (XANES), on the other hand, offers valuable insights into the oxidation state and coordination geometry of specific elements, enabling the assessment of redox behavior and electronic structure in LDHs. The recent development of ambient-pressure X-ray spectroscopies, such as AP-XPS and AP-XAS, further extends the applicability of these techniques by allowing surface-sensitive measurements under realistic reaction conditions. This advancement facilitates in situ studies that capture the dynamic structural and chemical evolution of LDHs during processes such as electrocatalysis, ion intercalation, or gas adsorption, providing a more comprehensive understanding of their functional behavior.

Inductively coupled plasma (ICP) and atomic absorption spectroscopy (AAS) are widely employed for determining the chemical composition of LDHs, particularly the metal cation ratios, after dissolution in an acidic medium [86]. In ICP, the sample is introduced into a high-temperature plasma torch, where it is fully atomized and excited; as the atoms return to their ground electronic states, they emit light at characteristic wavelengths, which is detected and quantified to determine elemental concentrations. In AAS, the sample is vaporized in a flame or graphite furnace to generate free atoms of the target element, which absorb light from an element-specific lamp, with the amount of absorption directly correlating to the element’s concentration. Both techniques provide precise and reliable measurements of LDH composition, enabling accurate assessment of stoichiometry and cation ratios.

Finally, thermogravimetric analysis (TGA) provides valuable information on the hydration degree of LDHs and the number of hydroxyl groups within the layers. Additionally, TGA can be coupled with gas chromatography (GC) to quantify CO₂ released from interlayer carbonate ions, as well as other volatile species present in the interlayer space. For more precise characterization, TGA can be combined with Fourier-transform infrared spectroscopy (TGA-FTIR) or GC coupled to mass spectrometry (TGA-GC-MS), enabling detailed analysis of the evolved gases [87]. Such integrated approaches offer deeper insights into the thermal behavior, interlayer composition, and decomposition mechanisms of LDHs, thereby enhancing understanding of their structural and functional properties.

Overall, a comprehensive understanding of LDH materials relies on the integration of multiple complementary characterization techniques. Surface-sensitive methods such as XPS and near-ambient pressure variants provide detailed insights into elemental composition, chemical states, and interlayer water content. XRD and electron microscopy techniques, including SEM, TEM, and AFM, reveal the crystalline structure, morphology, layer thickness, and nanoscale architecture, while advanced approaches such as SAED, EDS, and EELS offer additional information on crystal orientation, defects, and elemental distribution. Local atomic and electronic structures can be further probed using XAS, including EXAFS and XANES, with ambient-pressure setups enabling in situ monitoring of dynamic transformations during functional processes. Chemical composition and stoichiometry are accurately determined through ICP and AAS, and TGA, especially when coupled with GC, FTIR, or MS, provides critical information on hydration levels, interlayer species, and thermal stability. Collectively, these techniques provide a multifaceted toolkit for elucidating the structure–property relationships of LDHs, thereby guiding their rational design and optimization for diverse applications in catalysis, energy storage, and so on.

(a) TEM (b) AFM (c) High Resolution (HR)-TEM images and corresponding SAED (inset) of Ru-S-NiFe LDH. (d) Experimental XRD patterns and corresponding fitting patterns on Ru-S-NiFe LDH and NiFe LDH. (e) Raman spectra of Ru-S-NiFe LDH and NiFe LDH. (f) Aberration-corrected high-angle annular dark field-scanning TEM (AC HAADF-STEM) image of Ru-S-NiFe LDH, with the Ru single atoms marked with circles. (g) The enlarged areas in (f) and corresponding atomic intensity profiles along the dashed lines. (h) Corresponding elemental mappings of Ru-S-NiFe LDH. Figure adapted and reproduced with permission from Ref. [48]. Copyright 2025 Wiley

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Operando AFM images of a representative flake of CoAl-LDH deposited on the HOPG surface. (a,d,g) Images acquired at OCP (0.8 V). (b, e,h) Images acquired at 1.35 V. (c,f,i) Images acquired at OER (1.75 V). Panels (a,b,c) report the amplitude signal; panels (d,e,f) show a zoom-in of the topography at the center of the flake; panels (g,h,i) show the 3D topography in false colors. Figure adapted and reproduced with permission from Ref. [83]. Copyright 2025 Wiley

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LDHs have emerged as promising materials for addressing the growing need for efficient and sustainable energy storage and conversion technologies. Their unique structural characteristics, including tunable metal cation ratios, adjustable interlayer spacings, and high surface modifiability, allow for application-specific optimization across a wide range of energy systems, including supercapacitors, batteries, and electrocatalysts. Through precise control of synthesis methods and reaction parameters, LDHs can be engineered to achieve tailored morphologies, such as ultrathin nanosheets, porous structures, and core-shell architectures, which enhance their performance by improving ion transport, surface area, and structural stability.

Despite these advances, several challenges remain. Cost-effective and scalable synthesis techniques must be developed to meet the demands of industrial-scale production. Traditional methods, such as co-precipitation, offer simplicity and low cost but often yield products with poor crystallinity and limited structural control. In contrast, advanced methods like hydrothermal synthesis, sol-gel processes, and microwave-assisted synthesis enable precise morphology control but are accompanied by high energy costs and complex operational requirements. Addressing these trade-offs will require innovative approaches, such as hybrid synthesis techniques or the integration of automation and machine learning to optimize reaction conditions in real time.

Another critical challenge is achieving long-term structural stability and performance under operational conditions. For example, LDHs tend to re-stack during storage or processing, which can reduce their surface area and catalytic activity. Strategies such as incorporating additives, modifying interlayer anions, or designing composite structures with carbon-based materials (e.g., graphene) can mitigate these issues by enhancing structural integrity and conductivity. LDHs also become deactivated after long-term use due to structural damage or loss of active sites. The regeneration and recyclability of LDHs in energy applications has emerged as a prominent research focus, with the restoration of morphological architectures presenting a particularly formidable challenge. Nevertheless, from a material property standpoint, the inherent tunability of the unique layered structure of LDHs offers a fundamental basis for their recycling and regeneration, thereby underscoring substantial prospects for future advancement in this domain. At the same time, AI-assisted parameter design will make a difference. For example, a database of reaction conditions, morphologies and applications to predict optimal synthesis paths via AI algorithms will be promising.

Future research should also focus on expanding the functionality of LDHs by exploring new compositions and intercalation chemistries. The incorporation of multivalent cations, heteroatoms, or dual-anion systems could open new pathways for achieving superior electrochemical and catalytic performance. Furthermore, combining LDHs with emerging nanomaterials, such as metal-organic frameworks (MOFs) or MXenes, [88, 89] could lead to novel hybrid structures with enhanced multifunctionality.

Overall, LDHs hold immense potential for advancing energy technologies, but their practical implementation is currently limited by challenges associated with cost, scalability, and performance stability. By integrating advanced synthesis techniques, optimizing structural properties, and leveraging innovative material designs, LDHs can be strategically engineered to overcome these limitations. Such efforts position LDHs as promising candidates for enabling the transition toward sustainable and efficient energy solutions.