Superstructure and Correlated Na⁺ Hopping in a Layered Mg-Substituted Sodium Manganate Battery Cathode are Driven by Local Electroneutrality

Abstract

In this work, we present a variable-temperature 23Na NMR and variable-temperature and variable-frequency EPR analysis of the local structure of a layered P2 NIB cathode material, Na0.67[Mg0.28Mn0.72]O2 (NMMO). For the first time, we elucidate the superstructure in this material using synchrotron X-ray diffraction and total neutron scattering and show that this superstructure is consistent with NMR and EPR spectra. To complement our experimental data, we carry out ab initio calculations of the quadrupolar and hyperfine 23Na NMR shifts, the Na+ ion hopping energy barriers and the EPR g-tensors. We also describe an in-house simulation script for modelling the effects of ionic mobility on variable-temperature NMR spectra and use our simulations to interpret the experimental spectra, available upon request. We find long-zig-zag-type Na ordering with two different types of Na sites, one with high mobility and the other low mobility, and reconcile the tendency towards Na+/vacancy ordering to the preservation of local electroneutrality. The combined magnetic resonance methodology for studying local paramagnetic environments from the perspective of electron and nuclear spins will be useful for examining the local structures of materials for devices.