Na⁺ orientates Jahn-Teller effect to tune Li⁺ diffusion pathway and kinetics for Single-Crystal Ni-rich LiNi_xCo_yMn_1-x-yO₂ cathode materials

Single-crystallization is an effective strategy for enhancing both capacity and stability of Ni-rich LiNi_1-x-yCo_xMn_yO₂ (NCM) cathode materials, especially at high cut-off voltages. However, the kinetics limitation of solid-phase Li⁺ diffusion is a major concern because of the long diffusion path in large single-crystal particles. To address this issue, we synthesize a Na-doped single-crystal LiNi_0.82Co_0.125Mn_0.055O₂ (NCM-Na) cathode material by a facile mixed-molten-salt sintering process. Na⁺ is revealed to be uniformly doped at the Li⁺ lattice sites within the entire single-crystal particles. This Na⁺ doping effectively enhances the dynamics of Li⁺ transport in the layered oxide phases. The NCM-Na material with 2 at.% Na doping shows a Li⁺ diffusion coefficient up to more than 8 times higher than pristine NCM. In-situ X-ray diffraction and finite element analysis demonstrate significantly facilitated H1-H2-H3 phase transition in NCM-Na materials, compared with the severe phase separation phenomenon in NCM counterpart, hoisting their rate capacity and structure stability. Thus, the NCM-Na material achieves a superior reversible capacity of 177.7 mAh/g at 5C, and a capacity retention of 94.4 % after 100 cycles at 0.5C at a high cut-off voltage of 4.5 V. By density function theory calculations, we reveal that Na⁺ doping can selectively stabilize the surrounding Li⁺ at the second farthest hexagonal vertexes by tuning the orientation of the Jahn-Teller effect of Ni³⁺. These Li⁺ ions frame a high-speed pathway for preferential Li⁺ diffusion, which promotes the Li⁺ diffusion kinetics even in highly delithiated states. Our findings provide insights into the Na⁺ doping mechanism and present a low-cost, highly efficient, and scalable method to enhance the performance of single-crystal Ni-rich NCM materials.