Research progress on manganese-based Prussian blue as a cathode for sodium/potassium-ion batteries

Given the natural abundance and low cost of raw materials, sodium-ion batteries (SIBs) and potassium-ion batteries (PIBs) have emerged as highly promising alternatives to lithium-ion batteries for large-scale energy storage. However, the commercialization of these technologies is critically hindered by the lack of suitable cathode materials that simultaneously deliver high specific capacity, high operating voltage, and long-term cycling stability. Among various candidates, Prussian blue analogues (PBAs), which are a class of open-framework materials formed by transition metal ions connected via cyanide ligands, have garnered intense research interest due to their rigid three-dimensional diffusion channels, tunable chemical composition, and economical synthesis. In particular, manganese-based hexacyanoferrate (MnHCF) stands out as a leading cathode material for SIBs and PIBs, leveraging advantages such as simple synthesis, a high working voltage stemming from its multi-electron redox chemistry, and a compelling theoretical capacity. Despite these merits, the practical application of MnHCF is challenged by several intrinsic issues: the Jahn–Teller distortion associated with Mn3+ induces severe lattice strain and framework collapse; the inevitable presence of coordinated water and [Fe(CN)6] vacancies during synthesis blocks alkali metal sites, impairing reversible capacity and ion transport; and the dissolution of Mn2+ during cycling leads to active material loss and electrolyte degradation. To overcome these obstacles, concerted research efforts have been devoted to developing effective optimization strategies, including precise synthesis control, elemental doping, surface engineering, and composite architecture design.This review provides a systematic and dedicated overview of MnHCF as a cathode material for SIBs and PIBs, distinctly focused on its synthesis, mechanisms, challenges, and performance-enhancement pathways. Unlike broader reviews on PBAs, our work offers an in-depth, comparative analysis of MnHCF across different alkali-metal systems. The first section lays the foundation by detailing various synthesis methods and the fundamental material characteristics of MnHCF, which are crucial for achieving controlled fabrication and structural understanding. The second section delves into the charge storage mechanisms and systematically analyzes the key challenges hindering practical performance, such as limited intrinsic electronic/ionic conductivity, substantial volume changes during ion insertion/extraction, and insufficient cycling stability. In response, the third section comprehensively summarizes state-of-the-art modification strategies, encompassing composition regulation, microstructure design, surface coating, defect control, and the construction of advanced composites with conductive matrices. A unique and pivotal aspect of this review is presented in the fourth section, which conducts a systematic comparative analysis of the electrochemical performance of MnHCF in SIBs versus PIBs. This comparison critically evaluates key metrics like specific capacity, voltage profile, and cycle life, and delves into the underlying mechanisms for performance divergence. The analysis is framed through three fundamental dimensions: the thermodynamic properties of the host-guest interplay, the kinetic differences in Na+ versus K+ diffusion within the framework, and the structural stability influenced by the distinct ionic radii and interaction strengths of the two ions. Finally, the review concludes with a summary and a forward-looking perspective, highlighting emerging research trends and pinpointing unresolved scientific and technical challenges. By integrating fundamental insights with applied strategies and a distinctive comparative focus, this work aims to provide a comprehensive reference and to stimulate further innovation, ultimately accelerating the development of high-performance MnHCF cathodes toward practical applications in sustainable energy storage systems.