P2-type transition metal oxides for high performance Na-ion battery cathodes

Abstract

The particle pulverization induced by volume change and the disproportionation reaction of Mn³⁺ to Mn⁴⁺ and electrolyte-soluble Mn²⁺ are two major challenges for Na_0.67MnO₂ cathodes in Na-ion batteries. Herein, Ni and/or Fe doped Na_0.67MnO₂ was synthesized to suppress the particle pulverization and disproportionation reaction. The replacement of 33% Mn ions by Ni in Na_0.67MnO₂ can effectively reduce the particle pulverization and disproportionation of Mn³⁺, resulting in improved cycling stability at the cost of reduced capacity. To develop a high capacity and long cycle life cathode material, Ni in Na_0.67Ni_0.33Mn_0.67O₂ is further partially substituted by Fe to generate Na_0.67Fe_0.20Ni_0.15Mn_0.65O₂, which retains ∼70% of its initial capacity after 900 cycles, corresponding to a very low capacity decay rate of 0.033% per cycle. To the best of our knowledge, the Na_0.67Fe_0.20Ni_0.15Mn_0.65O₂ synthesized by ultrasonic spray pyrolysis (USP) represents one of the best cathode materials for Na-ion batteries to date. In addition, a thin layer (5 nm) of Al₂O₃ is deposited on the Na_0.67MnO₂ electrode by atomic layer deposition (ALD) to further reduce the dissolution of Mn ions and accommodate the volume change, which further extend the cycling stability of Na_0.67MnO₂ electrodes.