Lithium (Li) transition metal (TM) oxides composed of earth-abundant TMs, such as manganese (Mn), are a promising class of Li battery cathode materials with high energy density that can alleviate potential TM supply chain bottlenecks in battery manufacturing [1, 2]. Fundamental challenges in Mn-rich cathodes arise from phenomena such as large structural changes due to collective Jahn-Teller distortion of Mn³⁺, Mn migration, and phase transformations to spinel-like order, all of which impact the electrochemical performance [1, 3]. These physically complex phenomena motivate our ab initio re-examination of the Li-Mn-O rock salt space.

In this presentation, we focus on the thermodynamic and structural properties of several low-energy LiMnO₂ polymorphs, which include the orthorhombic (Pmmn), layered (C2/m), lithiated spinel (I4/amd), and a previously unreported LiMnO₂ phase with 𝛾-LiFeO₂-like order [1, 3, 4]. DFT ionic relaxations are performed using an extensive range of density functionals spanning the generalized gradient approximation (GGA), meta-GGA, and hybrid-GGA, with or without Hubbard corrections (on-site and inter-site). Across all functional forms, we find that the anti-ferromagnetic (AFM) super-exchange interaction has a remarkably large impact on the total energies and resulting phase stability trends of these polymorphs, in agreement with previous studies [5]. We identify and analyze the substantial changes in electronic structure and bonding that arise from different spin configurations. The experimentally observed phase stability trends can be reproduced when accurately describing electron exchange and the magnetic ground-state. By comparing the electronic structure from DFT and many-body theory, we show that even the most advanced functionals do not fully capture the complex electronic correlations of this phase space. The phase stability at finite temperature is also evaluated with harmonic phonon calculations. Our first-principles analysis elucidates the rich relationship between the phase stability, electronic structure, magnetic order, and ionic configuration of the LiMnO₂ polymorphs. Our methodology and fundamental understanding of Li-Mn-O phases developed in this study can be further harnessed to investigate compositionally similar, more recently developed Li TM oxide cathodes that are derived from this phase space.