Na layer pillar ion post-doping facilitates diffusion kinetics and structural stability in NaNi0.5Mn0.5O2†

Abstract

Elemental doping is an effective strategy to enhance the structural stability of O3-type layered cathodes, but few studies focus on the differences in influence of dopants on sodium-ion diffusion kinetics during material synthesis and charge–discharge processes. Herein, two Ca-incorporating materials, pre-doped and post-doped Na_1−2xCa_xNi_0.5Mn_0.5O₂ accompanied by Na vacancies, were successfully synthesized. Ca²⁺ pre-doping inhibits Na⁺ diffusion into the bulk during synthesis and consequently causes electrochemical performance degradation, while the post-doping strategy, by introducing Ca²⁺ after the synthesis of NNM, effectively circumvents these detrimental effects. The designed post-doping sample enlarges the Na interlayer spacing with fast Na⁺ diffusion behavior and reinforces a layered structure with the “pillar” effect of strong Ca²⁺–O²⁻ bonds, thus enhancing rate capability and cycling stability. Meanwhile, enhanced Na⁺ diffusion kinetics ensures uniform phase transitions from the surface to the bulk. Consequently, the post-doping approach provides inspiration for the design and synthesis of high-performance O3-type Na-layered oxides.