High-Entropy Engineering of Phosphate Based Polyanion Cathode Materials for Sodium-Ion Batteries: Advances and Perspectives.

Sodium-ion batteries (SIBs) are promising alternatives to lithium-ion batteries for large-scale energy storage due to their cost-effectiveness and resource abundance. However, conventional phosphate-based polyanionic cathodes suffer from low energy density, poor ion/electron conductivity, sluggish Na+ diffusion and cycling-induced structural degradation, limiting their industrial application. High-entropy engineering, which incorporates multiple cations into the same crystallographic site to maximize configurational entropy, is an effective strategy to address these issues, though its design principles and synergistic mechanisms remain unclear. This review systematically summarizes advances in high-entropy phosphate cathodes, focusing on their core functional mechanisms: activating multiple redox reactions for higher capacity, optimizing coordination environment to enhance conductivity and Na+ transport, and mitigating lattice distortion to improve structural stability. It classifies high-entropy applications in polyanionic cathodes, details synthesis methods for single-phase high-entropy solid solutions, and overviews multi-scale characterization techniques, theoretical calculations and machine-learning-assisted design. Finally, a comprehensive full-chain design framework is provided, offering guidance for unlocking the potential of high-entropy polyanionic cathodes in next-generation industrial SIBs.