Abstract:

Multiphasic interfaces can effectively inhibit the complex structural phase transition in layered transition metal oxide cathode materials containing sodium,thereby increasing cycle stability. Nevertheless, the design and manipulation of the multiphasic interface are intrinsically linked to the synthesis and calcination process of layered oxides. The O3/P2 multiphasic interface of the O3/P2-Na _x Ni_1/3Co_1/3Mn_1/3O₂ multiphasic cathode material can be controlled, and its electrochemical sodium storage performance can be regulated by adjusting the cooling process. It is found that natural cooling promotes the return of sodium ions diffused to the surface of the material during the high-temperature calcination period back to the bulk phase, forming a stable O3/P2 phase interface. Rapid cooling process, such as liquid nitrogen quenching, will prevent sodium ions from returning to the bulk phase, reducing the capacity of the multiphasic material. It is also not beneficial to forming a stable O3/P2 phase interface, which reduces the cycling performance. According to electrochemical impedance and cyclic voltammetry tests, liquid nitrogen quenching would increase the interface impedance of O3/P2-Na _x Ni_1/3Co_1/3Mn_1/3O₂, limit the diffusion kinetics of sodium ions, and reduce the diffusion coefficient of sodium ions. We attempted to establish a sodium ion diffusion equilibrium mechanism to describe the Na⁺ diffusion behavior and explain the impact of the cooling process on the O3/P2 multiphasic interface and bulk phase sodium content. Reasonable regulation of the cooling process during the cathode calcination process of layered oxide is crucial for constructing a stable multiphasic structure, inhibiting structural phase transition, and improving the electrochemical stability of layered oxide materials.

Key words: sodium-ion batteries, multiphasic structure, phase transition, cooling process, diffusion of sodium ions