Enabling fast and reversible kinetics via TM–O bond modulation in homogeneous solid-solution medium-entropy NASICON for sodium ion batteries

Na 4 MnCr(PO 4 ) 3 (NMCP) and Na 2 VTi(PO 4 ) 3 (NVTP) are promising binary electrode materials for sodium-ion batteries (SIBs) due to their high theoretical specific capacity and high voltage redox potential associated with V/Mn redox couples. However, the Jahn–Teller distortion induced by Mn 3+ in NMCP and inactive V 4+ /V 5+ in NVTP lead to inadequate rate capability and cyclic instability, rendering their utilization impractical. To overcome these intrinsic bottlenecks, we rationally designed and synthesized homogeneous solid-solution medium-entropy Na 3.0 V 0.5 Mn 0.5 Ti 0.5 Cr 0.5 (PO 4 ) 3 (NVMTCP) NASICON. The synergetic effect of medium-entropy modulation and charge redistribution around the TM–O 6 bond at the octahedral site enhances intrinsic lattice stability and electronic conductivity. Consequently, it stabilizes and activates reversible V 3+ /V 4+ /V 5+ and Mn 2+ /Mn 3+ /Mn 4+ redox couples. NVMTCP exhibits a high specific capacity of 175.3 mA h g −1 and excellent cyclic stability with 91% capacity retention after 5000 cycles at 2 A g −1 . Comprehensive X-ray analysis and theoretical calculations demonstrate shorter and stronger TM–O 6 bonding, which facilitates faster electronic kinetics, enabling a smooth phase transition and mitigating the biphasic reaction with a small volume change of 5.3% during sodiation–desodiation. The proposed strategy could be applied to design low-cost, ultra-stable, and high-performance NASICON electrode materials for the commercialization of SIBs.