The Interplay Between Electron Localization, Magnetic Order, and Jahn-Teller Distortion that Dictates LiMnO2 Phase Stability

The development of manganese (Mn)-rich cathodes for Li-ion batteries promises to alleviate potential supply chain bottlenecks in battery manufacturing. Fundamental challenges in Mn-rich cathodes arise from phenomena such as structural changes due to cooperative Jahn-Teller (JT) distortions of Mn3+ in octahedral environments, Mn migration, and phase transformations to spinel-like order, all of which affect the electrochemical performance. These physically complex phenomena motivate an ab initio re-examination of the Li-Mn-O rock-salt space, with a focus on the thermodynamics of the prototypical LiMnO2 polymorphs. It is found that the generalized gradient approximation (GGA – PBEsol) and meta-GGA (r2SCAN) density functionals with empirically fitted on-site Hubbard U corrections yield spurious stable phases for LiMnO2, such as predicting a phase with γ-LiFeO2-like order (γ-LiMnO2) to be the ground state instead of the orthorhombic (Pmmn) phase, which is the experimentally known ground state. Accounting for antiferromagnetic order in each structure is shown to have a substantial effect on the total energies and resulting phase stability. By using hybrid-GGA (HSE06) and GGA with extended Hubbard parameters (on-site U and inter-site V ) that are self-consistently determined, the experimentally observed LiMnO2 phase stability trends are recovered. The calculated on-site U between Mn-3d states in the experimentally observed orthorhombic, layered, and spinel phases (∼ 5.8 eV) are significantly smaller than U in γ-LiMnO2 and disordered layered structures (∼ 6.4 eV). The smaller values of U are shown to be correlated with a collinear ordering of JT distortions, in which all eg orbitals are oriented in the same direction. This cooperative JT effect can lead to greater electron delocalization from Mn along the eg states due to increased Mn-O covalency, which contributes to the greater electronic stability compared to the phases with noncollinear JT arrangements. The structures with collinear ordering of JT distortions also generate greater vibrational entropy, which helps stabilize these phases at high temperature. These phases are shown to be strongly insulating with large calculated band gaps > 3 eV, which are computed using HSE06 and G0W0.