Dominant Solid‐Solution Reactions in a Medium‐Entropy NASICON Cathode for High‐Rate Sodium‐Ion Batteries

Abstract Manganese‐based NASICON cathodes enable high‐energy‐density sodium‐ion batteries via multi‐electron Mn 2+/3+/4+ redox and fast Na + diffusion. However, they suffer from Jahn‐Teller distortion (Mn 3+ ), causing structural degradation and compromised rate/cycling stability. Herein, a medium‐entropy engineered Na 3 Mn 2/3 Ti 2/3 V 2/3 (PO 4 ) 3 cathode material, synthesized via a scalable ball‐milling method is reported, which integrates the Ti 3+/4+ , V 3+/4+ , and Mn 2+/3+ redox couples. The NMTVP‐V 2/3 achieves a reversible capacity of 178.8 mAh g −1 at 20 mA g −1 and an energy density of 509.6 Wh kg −1 . After cycling 1200, its capacity retains 82.24% at 200 mA g −1 . In‐situ XRD reveals that near‐equimolar Ti/Mn/V mixing creates a medium‐entropy structure, promoting the mechanism shifted from a mixed solid‐solution and two‐phase reaction to dominant solid‐solution reactions. Integrated with electrochemical characterization and DFT, the results indicate that the medium‐entropy optimized NMTVP‐V 2/3 enhances ionic transport and mitigates phase degradation. Furthermore, the NMTVP‐V 2/3 ||hard carbon full cell exhibits a high capacity of 162.9 mAh g −1 at 50 mA g −1 with an outstanding rate performance. This strategy can be generalized to other polyanionic systems, presenting a novel approach for developing SIB cathodes.