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

The particle pulverization induced by volume change and the disproportionation reaction of Mn 3+ to Mn 4+ and electrolyte-soluble Mn 2+ are two major challenges for Na 0.67 MnO 2 cathodes in Na-ion batteries. Herein, Ni and/or Fe doped Na 0.67 MnO 2 was synthesized to suppress the particle pulverization and disproportionation reaction. The replacement of 33% Mn ions by Ni in Na 0.67 MnO 2 can effectively reduce the particle pulverization and disproportionation of Mn 3+ , 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 0.67 Ni 0.33 Mn 0.67 O 2 is further partially substituted by Fe to generate Na 0.67 Fe 0.20 Ni 0.15 Mn 0.65 O 2 , 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 0.67 Fe 0.20 Ni 0.15 Mn 0.65 O 2 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 Al 2 O 3 is deposited on the Na 0.67 MnO 2 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 0.67 MnO 2 electrodes.