A Ca-substituted air-stable layered oxide cathode material with facilitated phase transitions for high-performance Na-ion batteries

Cost-efficient and high-performance cathodes play a crucial role in the advancement of grid-scale sodium-ion batteries (SIBs). As promising high-capacity cathode materials for SIBs, O₃-type Na-based layered transition metal oxides constantly experience inadequate air stability and complex phase transitions. Herein, a comprehensive investigation was conducted to explore the impact of Ca substitution on both the electrochemical performance and structural stability of O₃–Na_1−2xCa_xNi_0.25Fe_0.5Mn_0.25O₂ (x = 0, 0.02 and 0.05) that is proposed. With proper Ca content, O₃-type Na_0.96Ca_0.02Ni_0.25Fe_0.5Mn_0.25O₂ can deliver a reversible capacity of 122.1 mAh·g⁻¹ at 10 mA·g⁻¹, with capacity retention of 83.4% after 200 cycles at 50 mA·g⁻¹ and good rate capability. Its air sensitivity is investigated, and its potential as the Na host in full cells has been studied. In situ X-ray diffraction reveals the facilitated O₃–P₃–O₃ phase transitions of such cathode during the whole electrochemical reaction. Such a simple and effective strategy may reveal a new avenue for high-stable O₃-type cathodes and promote the practical applications of SIBs.