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-sc ale sodium-ion batteries(SIBs).As promising high-capacity cathode materials for SIBs,O_(3)-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 O_(3)-Na_(1-2x)Ca_xNi_(0.25)Fe_(0.5)Mn_(0.25)O_(2)(x=0.0.02 and 0.05)that is proposed.With proper Ca content,O_(3)-type Na_(0.96)Ca_(0.02)-Ni_(0.25)Fe_(0.5)Mn_(0.25)O_(2)can deliver a reversible capacity of122.1 mAh·g^(-1)at 10 mA·g^(-1),with capacity retention of83.4%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 O_(3)-P_(3)-O_(3)phase transitions of such cathode during the whole electrochemical reaction.Such a simple and effective strategy may reveal a new avenue for high-stable O_(3)-type cathodes and promote the practical applications of SIBs.