Density Functional Investigation on Li2MnO3

Abstract

Li 2 MnO 3 component plays a key role in Li-rich Mn-based layered materials ( m Li 2 MnO 3 · n LiMO 2, M = Mn, Ni, Co, etc.) for achieving unusually high lithium storage capacity. However, detailed lithium storage mechanism in Li 2 MnO 3, such as structure evolution and charge compensation are still not very clear. In this work, the redox mechanism, the delithiation process, the kinetics of lithium diffusion, and the oxygen stability of Li 2 MnO 3 are investigated through density functional calculations. The ground-state Li/vacancy configurations of Li 2– x MnO 3 (0 ≤ x ≤ 1) at five Li concentrations are determined, from which the delithiation potential is calculated as ∼4.6 V vs Li + /Li, and the charge compensation during Li removal is contributed mainly by oxygen. According to the Li/vacancy configuration in each ground state, the sequence of lithium removal is suggested from an energetic view. Both the Li + in the lithium layer and in the transition-metal layer can be extracted. The first-principles molecular dynamics (FPMD) simulations indicate that the lithium layer is the main diffusion plane in this material, while the Li + in the transition-metal LiMn 2 layer can migrate into the lithium layer first, and then diffuse through the lithium plane or move back to the LiMn 2 layer. The energy barriers of such migrations are in the range of 0.51–0.84 eV, according to the calculations with the nudged elastic band method. The release of O 2 gas from Li 2– x MnO 3 (0 ≤ x ≤ 1) happens spontaneously if x ≥ 0.5, from the point of view of enthalpy change. Further understanding on the evolution of oxygen in Li 2– x MnO 3 with x ≥ 0.5 is needed to find a way to stabilize the structure during electrochemical cycles.