Bilayer surface modification of biphasic O3/P2 layered cathode for high voltage sodium ion battery

Biphasic composite materials have been demonstrated to be effective in enhancing the structural stability and electrochemical performance of layered cathodes for sodium-ion batteries. Herein, based on the cationic potential methodology and co-doping strategy, we design a biphasic O3/P2 composite layered cathode (85NFMLCC, Na 0.85 Ca 0.02 Ni 0.2 Li 0.1 Fe 0.15 Co 0.05 Mn 0.5 O 2 ) and further integrate surface modification to achieve superior cycling performance, which is prepared by a simple one-pot solid state sintering method. Comprehensive structural characterizations reveal that the surface modification is realized by an inner Co-enriched layer and an outmost Ca-coating layer, which enables much improved rate capability and cycling stability simultaneously. In situ XRD confirms that biphasic design effectively suppresses the harmful phase transition during high voltage cycling, and the bilayer surface modification further enhances the cathode interfacial kinetics and robustness. The electrochemical tests demonstrated that 85NFMLCC exhibits a high specific capacity (136.7 mAhg −1 at 0.1C), superior cycling stability and rate capability at 2.0–4.3 V, 97.8 % capacity retention after 100 cycles at 0.1C and 84.5 % capacity retention after 800 cycles at 1C 2.0–4.0 V. This work demonstrates that biphasic composite with proper surface modification is a very promising design route toward high performance high voltage cathode material for sodium-ion batteries.