Trace Mg2+ doping enhances structural stability of single-crystal layered oxides for sodium-ion batteries

Abstract

Sodium-ion batteries (SIBs) have emerged as a promising alternative to lithium-ion batteries due to the abundance and low cost of sodium resources. However, the structural instability of P2-type layered oxides during cycling remains a significant challenge, limiting their practical application. Single-crystal materials, with their highly ordered atomic structure and superior electronic and mechanical properties, are pivotal for the development of high-performance SIBs. In this study, we optimize the electrochemical performance of P2-type single-crystal layered oxides by trace doping with Mg²⁺, carefully preserving the inherent single-crystal structure. Mg²⁺ doping not only stabilizes the P2 phase and mitigates detrimental phase transitions, but also enhances Na⁺ diffusion kinetics by subtly expanding the interplanar spacing without disrupting the single-crystal integrity. We demonstrate that trace Mg²⁺ doping improves the structural and electrochemical properties, resulting in superior cycling stability and rate capability. NMCMg_0.05 maintains 83.7 ​% of its initial capacity after 1000 cycles at 5 ​C, while full-cell tests with a hard carbon anode show 75.0 ​% capacity retention after 300 cycles and an energy density of 124.1 ​Wh·kg⁻¹. This work underscores the critical role of single-crystal engineering and trace doping in enhancing the stability and performance of SIB cathodes, offering a pathway for next-generation, high-performance sodium-ion batteries.