State of charge dependent ordered and disordered phases in a Li[Ni1/3Co1/3Mn1/3]O2 cathode material

Abstract

We systematically investigated the structural phase transition of a Li[Ni_1/3Co_1/3Mn_1/3]O₂ (NCM) cathode material depending on the state of charge (SOC) using cluster expansion Monte Carlo simulation (CE-MCS) combined with density functional theory (DFT). Considering the charging/discharging process involving lithium intercalation/deintercalation, the oxidation state of transition metal (TM) cations varies with the SOC, resulting in a TM arrangement shift to a thermodynamically favorable NCM structure. Our results demonstrate that the phase transition from disordered to ordered happens at a low SOC with a high TM oxidation state, and that the phase transition is initiated by TM pop-up from the TM layer to the Li layer. Ni migration plays an especially fundamental role in the phase transition with a diffusion energy barrier comparable to that of Li ions. Furthermore, based on a thorough understanding of the structural phase transition, we propose cation dopants (Zr, Ti and V) which inhibit the Ni pop-up as an initiating step of the phase transition by enhancing the chemical bonding of Ni ions in the NCM structure, thereby preventing the phase transition from causing undesirable structural degradation and severe capacity fading in NCM. Our theoretical investigations will provide insights into the structural phase transition mechanism and the design of new cathode materials for lithium ion batteries (LIBs).