High-Entropy Doped P'2 Mn-Based Layered Oxide with Superior Stability and High Capacity for Sodium-Ion Batteries.

Abstract

P'2-Na_xMnO₂ (NMO) features an ultra-high specific capacity in sodium-ion batteries, which, however, suffers from a fast capacity decay. To improve the stability, a high-entropy doped P'2-Na_0.59Mn_0.90Ti_0.02Cu_0.02Ni_0.02Co_0.02Fe_0.02O_1.95F_0.05 (NMHE_0.1OF) is developed to lessen the Jahn-Teller distortion and address the multiple phase transition issue. Physicochemical characterizations reveal that the NMHE_0.1OF yields a lower anisotropy in the Mn─O bond than does the undoped NMO. Theoretical calculations indicate that the cation doping enhances the coordination ability of oxygen and the F doping breaks the electronic symmetry of Mn. The in situ X-ray diffraction result reveals that the NMO experiences a more abrupt and irreversible OP4-P'2-P″2 tri-phase transition; and the NMHE_0.1OF features a mild and reversible OP4-P'2 bi-phase transition, which originates from the alleviation in the contraction/expansion of the transition metal slabs evidenced by ex situ extended X-ray absorption fine structure. The bi-phase transition favors the compatibility between the NMHE_0.1OF and the ether-based electrolyte at high voltages. As a result, the NMHE_0.1OF yields a superior cyclability (97.8% capacity retention after 100 cycles at 100 mA g^-1) with a notable specific capacity of 224 mAh g^-1 at 10 mA g^-1. This work provides an effective strategy for the rational design of cathode materials with high capacity and superior stability.