Phase Stability of Li-Mn-O Oxides As Cathode Materials for Li Ion Batteries

The over-lithiated-oxides (OLOs) are a composite material of layered structures of Li₂MnO₃ and LiMO₂ (M = Mn, Fe, Co, Ni). Their main advantage over current layered oxides is that they show much higher storage capacity as cathode material for Li-ion batteries, due to the presence of Li₂MnO₃ phase. However, experimental results indicate that Li₂MnO₃ is an almost inert material from an electrochemical point of view, and that it partially transforms into layered LiMnO₂ after the first charge/discharge cycle, thus forming a phase that is a mixture of both Li₂MnO₃ and LiMnO₂. During the subsequent charging cycles, OLO capacity is known to reduce gradually in connection with Mn spinel phase formation. To improve the OLO cathode material performance, it is desirable to suppress such spinel phase formation. In order to address the stability of Mn layered oxides, we have examined the phase diagram of the Li-Mn-O system on both physical and chemical (as a function of two chemical potentials, μ(Li) and μ(O)) potential space, using ab initio density-functional theory (DFT) simulations. These DFT findings will provide conceptual guidance in the experimental search for the mechanisms driving the structural transformation into the spinel phase, in an attempt to solve such structural instability problem and, thus, improving the performance of the OLO cathode materials.

This work was supported by Samsung GRO project.