Structural innovation and electrochemical progress in phosphate-based polyanionic oxide cathodes for high-performance sodium-ion batteries

The growing need for sustainable and high-performance energy storage technologies has positioned sodium-ion batteries (SIBs) as a viable alternative to lithium-ion batteries (LIBs), largely owing to the abundance, widespread geographic distribution, and economic benefits of sodium resources. Phosphate- and pyrophosphate-based polyanionic oxide materials are prominent cathode candidates due to their strong structural integrity, excellent thermal stability, and high operational safety. These materials demonstrate elevated working voltages and consistent sodium storage characteristics, rendering them especially suitable for extensive applications. However, their extensive use is constrained by inherent limitations, such as inadequate electronic conductivity and moderate specific capacities, which limit their rate capability and energy density. Despite the increasing amount of research in polyanionic cathodes, there is currently no comprehensive analysis that systematically outlines optimization methodologies from a structural evolution perspective or creates a coherent framework linking crystal chemistry and sodium storage behavior. Addressing this gap is critical for directing rational design and hastening technological adoption. This review examines recent developments in phosphate-based polyanion cathode materials, including simple phosphates, pyrophosphates, and mixed polyanionic frameworks. Modification strategies are emphasized, including elemental doping, surface carbon coating, morphology control, and advanced electrode design, all intended to enhance electrochemical performance and address conductivity limitations. The review critically assesses the relationship among crystal structure, synthesis methodology, and sodium-ion diffusion kinetics. Future research directions are outlined, focusing on scalable synthesis, enhanced rate performance, and commercial viability, to provide a roadmap for advancing next-generation phosphate-based polyanionic cathodes for SIBs.