Moisture-stable O3-type low-Ni layered oxides with improved electrochemical performance by Mg/Cu co-doping-enabled structural fine-tuning

As one of the most promising sodium-ion battery cathodes, O3-type transition metal (TM) layered oxides (Na x TMO 2 ) with a high specific capacity and operating voltage have received a wide attention. However, they still face the problems of slow Na + transfer kinetics and undesirable phase transitions, and suffer from severe structural degradation when stored in a humid atmosphere. In this work, we propose a Mg/Cu co-doping strategy to synthesize NaLi 0.04 (Mg 0.02 Cu 0.04 Ni 0.19 Fe 0.33 Mn 0.42 ) 0.96 O 2 (LMCNFM) cathode. In this material, Mg serves to stabilize the layer structure and improve reversibility of the phase transitions, while Cu with Cu 2+ /Cu 3+ redox reaction contributes to capacity and enhances moisture stability. Mg/Cu co-doping can expand the Na + transfer channels, leading to improved rate performance. LMCNFM can deliver a specific capacity of 109.1 mAh g −1 even at 10 C rate. Mg/Cu co-doping can stabilize the TM layer and reduce the surface side reactions with electrolyte. LMCNFM can maintain 92.7 % of its initial capacity after 200 cycles at 5 C rate. Furthermore, spontaneous extraction of Na + when exposed to water is suppressed via Mg/Cu co-doping. After being soaked in water for 6 h, LMCNFM shows a minor structural degradation with 88.7 % capacity retained after 200 cycles at 1 C.