High‐Entropy and Multiphase Cathode Materials for Sodium‐Ion Batteries

Abstract Cycling stability is the biggest challenge for cathodes of sodium‐ion batteries (SIBs), which is directly affected by the structure design. Herein, the combination of high‐entropy (HE) and multiphase structure is demonstrated to be helpful for maintaining the structure and improving the cycling stability. In the Ni/Mn/Cu/Ti/Sn five‐component HE multiphase cathode, the multiple elements at transition metal sites can enlarge the lattice and stabilize the structure simultaneously without causing an obvious capacity drop, achieving the synergistic effect of multi‐cations. In the HE cathodes consisting of P2 and O3 phases, the harmful phase transition in high‐voltage is suppressed and the cycling performance is improved. A capacity retention of 77.3 mAh g −1 after 300 cycles is delivered, and an improved rate performance of 88.7 mAh g −1 at 750 mA g −1 is observed, better than that of the low‐entropy multiphase cathode(P2 and O3) and the HE oxide single O3‐phase cathode. The weighted average ionic radius( WAIR ) of all transition metals is demonstrated critical for the formation of the phase composition in HE composites. Through comparing a series of HE and multiphase cathodes, an empirical range of WAIR is obtained, which shows guidance for the design of other cathode materials.