Influence of Nickel-Content and Cycling Rate on the Phase Behavior of Layered Nickel-Rich Cathode Materials for Lithium-Ion Batteries

Nickel-rich layered cathode materials, such as LiNi_<i>x</i>Mn_<i>y</i>Co_1-x-yO₂ (NMC), are essential for high-energy-density lithium-ion batteries used in electric vehicles due to their higher specific capacities as compared to their lower nickel-content analogs. However, these materials suffer from structural instability, which becomes increasingly severe as the nickel content rises. Despite their significant importance, the intrinsic structural change mechanisms of nickel-rich cathodes, especially at practical cycling rates, remain unclear. This study investigates the influence of nickel content and cycling rate on the phase behavior and electrochemical performance of three representative nickel-rich cathode materials: LiNi_0.83Mn_0.05Co_0.12O₂ (Ni-83), LiNi_0.90Mn_0.05Co_0.05O₂ (Ni-90), and LiNiO₂ (Ni-100). Using synchrotron <i>operando</i> X-ray diffraction alongside electrochemical analysis, we have elucidated distinct structural transformation mechanisms: a solid-solution process for Ni-83, a quasi-two-phase mechanism for Ni-90, and classic H1-M-H2-H3 phase transitions at slow rates for Ni-100. Our findings highlight significant rate-dependent behaviors which affect these materials' electrochemical performance and stability under practical conditions. Notably, high cycling rates impede the H2-H3 transition in Ni-100 due to substantial lattice contraction, emphasizing the need for optimizing nickel content to enhance the stability and performance of high-nickel cathodes for next-generation lithium-ion batteries.