Revealing the Intrinsic Li Mobility in the Li2MnO3 Lithium-Excess Material

One of the most promising avenues for future high energy Li-ion batteries originate from the family of Li-rich layered cathodes. However, while exhibiting excellent initial capacity, these materials also suffer from voltage fade, high impedance, and poor rate capability, particularly in the Mn-rich, high Li excess concentration regime. Though it is clear that the Li2MnO3 component contributes to the high capacity as well as the chemical and structural degradation of the material, the inherent ionic conductivity of the material has not been clarified. In this work, we investigate the delithiation mechanism, involving coherent Li migration from two layers by first-principles density functional theory. Surprisingly, and contrary to expectations from available experimental results, we find that the pristine material exhibits excellent Li mobility enabling facile Li extraction from both the transition metal layer and Li-layer. Generally, the Li-extractions are highly accelerated by di- and trivacancy clusters, which stabilize the saddle point tetrahedral sites. Hence, we deduce that the observed inferior rate behavior of this class of Li cathode materials is not due to intrinsic poor bulk ionic mobility, but more likely due to surface-passivation, structural deterioration, and/or particle–particle electrode-level transport limitations.