PNNL

Abstract

Li-excess Mn-based disordered rock salt (DRX) oxides are promising Li-ion cathode materials, primarily owing to the cost-effectiveness of Mn and their high theoretical capacities. It has recently been shown that Mn-rich DRX Li1+xMnyM1-x-yO2 (y>0.5, M are hypervalent d0 ions such as Ti4+, Nb5+, and Mo6+) exhibit a gradual capacity increase during the first few charge-discharge cycles, which coincides with the emergence of local domains with a spinel-like cation arrangement within the long-range disordered structure coined as “d phase”. Here, we systematically study the structural evolution upon heating of Mn-based DRX compounds at different levels of delithiation to gain insight into the structural rearrangements taking place during battery cycling and the mechanism behind d phase formation. We found in all cases that the original DRX structure relaxes to a d phase, and these structural rearrangements in turn lead to capacity enhancement. Synchrotron X-ray and neutron diffraction were employed to examine the structure of the d phase, revealing that selective migration of Li and Mn/Ti cations to different crystallographic sites within the rock salt structure leads to the observed structural rearrangements. In addition, we show that both Mn-rich (y = 0.6) and Mn-poor (y