Mitigating kinetic hindrance of single-crystal Ni-rich cathodes through morphology modulation, nickel reduction, and lithium vacancy generation achieved by terbium doping

Abstract

Single crystallization has proven to be effective in enhancing the capacity and stability of Ni-rich LiNi1−x−yCoxMnyO2 (SNCM) cathode materials, particularly at high cut-off voltages. Nevertheless, the synthesis of high-quality single-crystal particles remains challenging because of severe particle agglomeration and irregular morphologies. Moreover, the limited kinetics of solid-phase Li+ diffusion pose a significant concern because of the extended diffusion path in large single-crystal particles. To address these challenges, we developed a Tb-doped single-crystal LiNi0.83Co0.11Mn0.06O2 (SNCM-Tb) cathode material using a straightforward mixed molten salt sintering process. The Tb-doped Ni-rich single crystals presented a quasi-spherical morphology, which is markedly different from those reported in previous studies. Tb4+ doping significantly enhanced the dynamic transport of Li+ ions in the layered oxide phase by reducing the Ni valence state and creating Li vacancies. A SNCM-Tb material with 1 at% Tb doping shows a Li+ diffusion coefficient up to more than 9 times higher than pristine SNCM in the non-diluted state. In situ X-ray diffraction analysis demonstrated a significantly facilitated H1-H2-H3 phase transition in the SNCM-Tb materials, thereby enhancing their rate capacity and structural stability. SNCM-Tb exhibited a reversible capacity of 186.9 mA h g−1 at 5 C, retaining 94.6% capacity after 100 cycles at 0.5 C under a 4.5 V cut-off. Our study elucidates the Tb4+ doping mechanisms and proposes a scalable method for enhancing the performance of single-crystal Ni-rich NCM materials.