Facilitating the high voltage stability of NFM via transition metal slabs high-entropy configuration strategy

Abstract

Sodium ion batteries, which is viewed as the potential candidate of Li-ion batteries, are now at the edge of wildly application. The layered oxide cathode materials, represented by NaNi1/3Fe1/3Mn1/3O2 (NFM), inherits the experience of success from LIBs are now promising of commercialization. Yet, the relative low operation voltage of NFM causing by the existence of Fe migration above 4.2 V hinder its further application nowadays. Herein, high-entropy O3-type NaMg0.08Cu0.12Ni0.2Fe0.2Mn0.2Ti0.2O2(HNFM) was proposed addressing working voltage at 4.3 V with the inhibition of large amount of Fe migration. Electrochemical test showed a specific capacity of 131 mAh g−1 was achieved within the voltage range from 2.0 to 4.3 V at current density of 0.2 C which boosted an energy density of up to 425 Wh kg−1. After 200 cycles at a 1 C current density, the capacity retention remains at 84 %. In-situ XRD analysis revealed that HNFM alleviated phase transition at high voltage compared to NFM thus it can withstand a wide operating voltage range of 2–4.3 V. DFT calculations demonstrated that transition metal slabs with high-entropy configurations could effectively suppress the migration of Fe ions from the transition metal slabs to the Na layer, thereby further enhancing the material's stability during cycling. These research findings shed light on the high-voltage operating for the layered oxide materials which provide the novel insight of the material design and inspire the commercialization of sodium ion batteries.