Structural Evolution in Disordered Rock Salt Cathodes

Li-excess Mn-based disordered rock salt oxides (DRX) are promising Li-ion cathode materials owing to their cost-effectiveness and high theoretical capacities. It has recently been shown that Mn-rich DRX Li1+xMnyM1-x-yO2 (y >= 0.5, M are hypervalent ions such as Ti4+ and Nb5+) exhibit a gradual capacity increase during the first few charge-discharge cycles, which coincides with the emergence of spinel-like domains within the long-range DRX structure coined as "delta phase". Here, we systematically study the structural evolution upon heating of Mn-based DRX at different levels of delithiation to gain insight into the structural rearrangements occurring during battery cycling and the mechanism behind delta phase formation. We find in all cases that the original DRX structure relaxes to a delta phase, which in turn leads to capacity enhancement. Synchrotron X-ray and neutron diffraction were employed to examine the structure of the delta phase, revealing that selective migration of Li and Mn/Ti cations to different crystallographic sites within the DRX structure leads to the observed structural rearrangements. Additionally, we show that both Mn-rich (y >= 0.5) and Mn-poor (y < 0.5) DRX can thermally relax into a delta phase after delithiation, but the relaxation processes in these distinct compositions lead to different domain structures. Thermochemical studies and in situ heating XRD experiments further indicate that the structural relaxation has a larger thermodynamic driving force and a lower activation energy for Mn-rich DRX, as compared to Mn-poor systems, which underpins why this structural evolution is only observed for Mn-rich systems during battery cycling.