A Comparative Investigation of Single Crystal and Polycrystalline Ni-Rich NCMs as Cathodes for Lithium-Ion Batteries

Nickel-rich LiNi_1−x−yCo_xMn_yO₂ (NCM, 1−x−y ≥ 0.6) is known as a promising cathode material for lithium-ion batteries since its superiority of high voltage and large capacity. However, polycrystalline Ni-rich NCMs suffer from poor cycle stability, limiting its further application. Herein, single crystal and polycrystalline LiNi_0.84Co_0.07Mn_0.09O₂ cathode materials are compared to figure out the relation of the morphology and the electrochemical storage performance. According to the Li⁺ diffusion coefficient, the lower capacity of single crystal samples is mainly ascribed to the limited Li⁺ diffusion in the large bulk. In situ XRD illustrates that the polycrystalline and single crystal NCMs show a virtually identical manner and magnitude in lattice contraction and expansion during cycling. Also, the electrochemically active surface area (ECSA) measurement is employed in lithium-ion battery study for the first time, and these two cathodes show huge discrepancy in the ECSA after the initial cycle. These results suggest that the single crystal sample exhibits reduced cracking, surface side reaction, and Ni/Li mixing but suffers the lower Li⁺ diffusion kinetics. This work offers a view of how the morphology of Ni-rich NCM effects the electrochemical performance, which is instructive for developing a promising strategy to achieve good rate performance and excellent cycling stability.