Identification of ferrimagnetic orbitals preventing spinel degradation by charge ordering in Li$_x$Mn$_2$O$_4$

TL;DR

This study reveals that in Li$_x$Mn$_2$O$_4$, a ferrimagnetic metallic phase surrounds the anti-ferromagnetic insulator phase, and charge ordering suppression in this phase helps prevent spinel degradation in lithium-ion batteries.

Contribution

It combines experimental x-ray magnetic Compton scattering with first-principles calculations to identify ferrimagnetic orbitals that inhibit charge ordering and degradation.

Findings

Ferrimagnetic phase stabilizes with small Li variations.

Charge ordering and distortions are suppressed in the ferrimagnetic state.

Identification of specific orbitals involved in ferrimagnetism.

Abstract

Spinel Li$_x$Mn$_2$O$_4$ is a key cathode material that is used extensively in commercial Li-ion batteries. A challenge with this material has been that the capacity of the battery fades with cycling, an effect that can be traced to the presence of an anti-ferromagnetic insulator phase in the fully lithiated LiMn$_2$O$_4$ (LMO) and the associated charge disproportionation that drives distortions of the MnO$_6$ octahedra. Here, by combining x-ray magnetic Compton scattering experiments with parallel first-principles computations, we show that the anti-ferromagnetic phase of LMO is surrounded by a robust ferrimagnetic metallic phase, which becomes stable when even a small amount of Li is removed from or added to the charge-ordered LMO. In this surprising ferrimagnetic state, charge-ordering and octahedral distortions are found to be strongly suppressed. We identify the nature of the ferrimagnetic orbitals involved through theoretical and experimental analyses of the magnetic Compton scattering spectra.