Kavli Affiliate: Kristin A. Persson
| First 5 Authors: Ronald L. Kam, Luca Binci, Aaron D. Kaplan, Kristin A. Persson, Nicola Marzari
| Summary:
The development of Mn-rich cathodes for Li-ion batteries promises to
alleviate supply chain bottlenecks in battery manufacturing. Challenges in
Mn-rich cathodes arise from Jahn-Teller (JT) distortions of Mn$^{3+}$, Mn
migration, and phase transformations to spinel-like order, which can affect the
electrochemical performance. These phenomena motivate an ab initio
re-examination of the thermodynamics of the LiMnO2 polymorphs. It is found that
the generalized gradient approximation (GGA – PBEsol) and meta-GGA (r2SCAN)
density functionals with empirical on-site Hubbard U corrections yield spurious
stable phases for LiMnO2, such as predicting a phase with gamma-LiFeO2-like
order (g-LiMnO2) to be the ground state instead of the orthorhombic (Pmmn)
phase, which is the experimentally known ground state. Accounting for the
antiferromagnetic (AFM) order in each structure has 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 Hubbard U in the experimentally
observed orthorhombic, layered, and spinel phases (around 5.8 eV) are
significantly smaller than U in g-LiMnO2 and disordered layered structures
(around 6.4 eV). The smaller values of U are correlated with a collinear
ordering of JT distortions, in which all $e_g$ orbitals are oriented in the
same direction. This cooperative JT effect leads to increased Mn-O covalency,
which contributes to the greater electronic stability compared to the phases
with noncollinear JT arrangements, and 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,
computed using HSE06 and $G_0W_0$.
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