Equilibrium Particle Shape and Surface Chemistry of Disordered Li-Excess, Mn-Rich Li-ion Cathodes through First-Principles Modeling

A novel methodology for calculating the surface energy of a disordered material was developed and is described here. The method was used to calculate the range of surface energies for {100}, {110}, {111}, and {112} type facets of the disordered rock salt (DRX) cathode material Li₂MnO₂F, as a function of surface cation and anion decoration. Boltzmann averaging was used to determine average surface energies for each facet which were then used to calculate the equilibrium particle shape. It was found that Li₂MnO₂F displays predominantly {100} type lithium/fluorine-rich facets favoring a cubic particle shape. The density of states along with electronic structure-based bonding analyses are calculated to rationalize differences observed in surface energy. Importantly, it is found that surface lithium and fluorine lower the surface energy of the majority facets, suggesting that surfaces of Li₂MnO₂F are likely enriched in lithium and fluorine and display less oxygen and manganese, which has implications for capacity and rate retention.