Suppressing the formation of OP4 phase in P2-Structured Na0.67Ni0.1Co0.1Mn0.8O2 by in-situ formed NiF2 layer

The P2-structured sodium transition metal oxides suffer from an irreversible phase transition to OP4, which results in the -1.3 Å shrinking of lattice c and locks the following electrochemical cycles between OP4 and P2’, seriously aggravating the cracking of cathode materials. Therefore, strategies to suppress the phase transition from P2 to OP4 are currently pursued to alleviate the destructively volumetric changes. Here, the DFT calculations reveal that the transition to OP4 phase is not driven by thermodynamical mechanism but is induced by structural defects. To verify this prediction, an in-situ formed NiF2 layer via electrolyte additives is coated on the P2-structured Na0.67Ni0.1Co0.1Mn0.8O2 during initial charging, which mitigates the generation of structural defects from the side reaction of electrolyte with active sites of cathode particles. The sample protected with NiF2 layer presents a continuous, moderate and reversible structural evolution, free from phase P2’. Therefore, the coated sample delivers 230.1 mAh/g at 0.1C and retains 83.4% capacity at 0.5C after 150 cycles, significantly higher than the values, 174.5 mAh/g and 63.4%, respectively, of the pristine materials. This work demonstrates the severe phase transition in P2-type cathode can be effectively alleviated by a simple surface modification.