Frameworks for the future: PBA-templated nanocomposites for next-generation alkali-ion batteries

Alkali-ion batteries (AIBs), including lithium-ion batteries (LIBs), sodium-ion batteries (SIBs), and potassium-ion batteries (PIBs), are essential for efficient energy storage systems. However, challenges such as lithium scarcity, high costs, and sluggish ion diffusion—particularly for the larger Na⁺ and K⁺ ions—have hindered their widespread adoption. To overcome these limitations, researchers are developing advanced electrode materials, among which Prussian blue analogues (PBAs) have emerged as promising candidates due to their open frameworks, abundant redox-active sites, tunable compositions, and ease of synthesis. A research team from Queensland University of Technology, led by Dr. Jiaye Ye, has systematically explored the synthesis methods, ion storage mechanisms, electrochemical performance, and future development strategies of PBA-templated materials.

PBAs are a class of metal-organic frameworks composed of transition metal ions coordinated by cyanide ligands, forming a highly ordered, porous, and rigid structure. While pristine PBAs have been investigated as electrode materials, they often suffer from structural degradation, limited electronic conductivity, and irreversible phase transitions during charge-discharge cycles. To address these shortcomings, PBAs are more commonly used as self-sacrificial templates to produce nanocomposites with enhanced physicochemical and electrochemical properties. These derived materials—such as metal oxides, metal chalcogenides, metal phosphides, and carbon composites—retain the advantageous features of the parent PBAs while overcoming their intrinsic limitations.

The synthesis of PBA-templated nanocomposites typically begins with a coprecipitation process. This straightforward, cost-effective, and environmentally friendly method enables precise control over the composition and morphology of the resulting PBA crystals. Further structural refinement can be achieved through post-treatment techniques such as electrospinning, surface coating, and ion-exchange reactions. These methods introduce additional complexity, allowing for the creation of hierarchical, hollow, or core-shell architectures that improve mechanical stability and surface accessibility.

Thermal conversion is a critical step in transforming PBAs into functional nanomaterials. When PBAs are subjected to high-temperature treatment under specific atmospheres—oxidative, inert, or reductive—the metal centers and cyano ligands undergo decomposition and rearrangement, forming metal-containing phases embedded in carbon matrices. Depending on the conditions, this process yields a wide variety of nanocomposites, including metal oxides, metal nitrides, carbides, sulphides, and phosphides. Importantly, PBAs themselves act as sources of carbon and nitrogen, enabling in situ doping that enhances conductivity, promotes ion transport, and stabilizes electrode structures.

The resulting nanocomposites can be classified based on their ion storage mechanisms: intercalation, alloying, and conversion reactions. Each mechanism plays a distinct role in the electrochemical performance of electrodes and determines their suitability for LIBs, SIBs, and PIBs. Intercalation-type materials, such as PBA-derived LiCoO₂ and LiFePO₄, facilitate reversible insertion and extraction of alkali ions without significant structural change, offering long cycle life and stability. Alloying-type materials, including Zn-based compounds, undergo reactions that form metal-ion alloys (e.g., Li_xZn or Na_xZn), delivering high theoretical capacities but facing challenges related to volume expansion. Conversion-type materials, such as Co₃O₄, FeS₂, and various metal phosphides, provide high capacities through redox reactions that involve complete phase transformation, although they often suffer from poor reversibility and rate performance.

In LIBs, PBA-templated materials exhibit excellent capacity and cycling stability. Metal oxide composites, such as FeMnO₃/Mn₂O₃ and ZnO/Co₃O₄, demonstrate initial capacities exceeding 1000 mAh·g^-1. By incorporating carbon coatings or doping with nitrogen, these materials show enhanced electronic conductivity and mechanical robustness, which mitigate the effects of volume change during cycling. For example, oxygen-deficient MnO/Co nanoparticles embedded in a carbon network derived from MnCo-PBA exhibit long-term cycling stability and fast lithium-ion diffusion, thanks to built-in electric fields around defect sites.

SIBs benefit from similar strategies, though sodium’s larger ionic radius requires materials with expanded interlayer spacing or porous architectures to facilitate ion transport. PBA-derived heterostructures such as ZnSe/FeSe and NiCoP in carbon matrices have shown synergistic electrochemical behavior, where multiple active centers contribute to sodium storage through a combination of alloying and conversion mechanisms. In situ and ex situ characterizations confirm that these materials undergo reversible structural transformations, enabling stable capacity retention over hundreds of cycles.

For PIBs, the challenges are even more pronounced due to the size of the K⁺ ion. However, PBA-templated nitrogen-doped layered oxides and porous carbons have proven effective in accommodating large ions. Materials such as K_0.5Mn_0.67Fe_0.33O_1.95N_0.05 (KMFON), synthesized via thermal conversion of PBAs in the presence of potassium sources, exhibit stable intercalation behavior and high voltage plateaus suitable for cathode applications. Meanwhile, composites like Ni₃S₂-Co₉S₈ heterostructures show reversible conversion behavior and benefit from the synergistic effect of dual metal centers, enabling high capacity and rate performance.

A key advantage of PBA-templated materials is their strong pseudocapacitive behavior, which contributes significantly to overall capacity, particularly at high rates. Due to their high surface area, interconnected pore structure, and abundant active sites, these materials allow for fast ion adsorption and surface redox reactions. This enhances power density without compromising energy density, a critical advantage in real-world battery applications. Cyclic voltammetry and other electrochemical analyses confirm that PBA-derived electrodes often exhibit a mixed charge storage mechanism, combining diffusion-controlled and surface-controlled processes for optimal performance.

Despite these promising results, challenges remain. The coprecipitation method, while simple and scalable, may result in structural defects or limited morphological diversity. Post-treatment steps, though effective in tailoring material properties, can introduce impurities or increase production costs. Moreover, the thermal conversion process is energy-intensive and may generate toxic byproducts, which raises concerns regarding environmental sustainability and commercial feasibility.

To bridge the gap between laboratory research and industrial application, future efforts must focus on optimizing synthesis routes, improving scalability, and reducing environmental impact. Innovations such as low-temperature conversion, solvent-free processing, and the use of renewable precursors may help address these concerns. Additionally, greater integration of PBA-templated materials into full-cell configurations is necessary to evaluate their practical performance in real-world battery systems.

In conclusion, the review provides a comprehensive and insightful overview of PBA-templated nanocomposites as advanced electrode materials for alkali-ion batteries. By leveraging the structural advantages of PBAs and combining them with strategic post-synthetic modifications, researchers have developed a versatile platform for addressing the performance limitations of current AIB technologies. With continued innovation in material design and process engineering, PBA-templated nanocomposites are poised to play a central role in the development of next-generation, high-performance, and sustainable energy storage systems.