Alkali metal lithium doping promotes the high stability and ionic kinetics of cathode materials for sodium-ion batteries†

Abstract

Layered transition metal oxides for sodium-ion batteries are regarded as the most promising cathode materials for commercialization owing to their high theoretical specific capacity, high rate performance, and low cost. However, their drawbacks, such as unfavorable phase transitions, Na⁺/vacancy disorder, and slow dynamics, seriously hinder their further practical applications. In this work, we prepared a P2-Na_0.67Ni_0.1Co_0.1Mn_0.8O₂ cathode with a heteroatom-substitution doped at the alkali metal position. The unfavorable phase transition was suppressed to a certain extent, and the order of Na⁺/vacancy was optimized. The initial discharge-specific capacity of the prepared [Na_0.57Li_0.1]Ni_0.1Co_0.1Mn_0.8O₂ at 0.1C (1C = 150 mA h g⁻¹) was 151.3 mA h g⁻¹. Doping with the alkali metal Li enhanced the stability of the layered structure, resulting in an improvement in cycling performance, and the capacity retention rate reached 87.9% after 100 cycles. In addition, the material had a stable structure and excellent Na⁺ diffusion coefficient at a high current density of 10C. It also exhibited an excellent rate capacity of 88.2 mA h g⁻¹ in an Na half-cell system. Kinetic analysis showed that the increase in Na⁺ diffusion rate was due to the increase in the Na⁺/vacancy disorder and the rise in Na interlayer spacing.