A Theory of Li Percolation in Rocksalts and Demonstration as Capacity Cathode Materials with Substantial Cation Disorder

There is a clear need to develop dense, lithium intercalation oxides with > 200 mAh/g practical capacity.  Today, nearly all high energy density cathodes for rechargeable lithium batteries are well-ordered materials where lithium and other cations occupy distinct sites. Cation-disordered materials are generally disregarded because lithium diffusion tends to be limited in them. The recently demonstrated performance of Li_1.211Mo_0.467Cr_0.3O₂, achieving close to 300 mAh/g, shows that lithium diffusion can be facile in disordered materials [1] and made us revisit the question of how Li diffuses through rocksalt-like materials.  We have combined ab initio computations of local Li migration barriers with percolation modeling to develop a unified understanding of Li diffusion in close-packed oxides.   The theory explains the high capacity of layered and spinel-like materials, and the lack of reversible capacity in γ-LiFeO2. More surprisingly, the new percolation theory also clearly supports that Li-excess is needed to achieve high capacity in partially or fully disordered materials.  We can now give very specific guidelines for the amount of Li-excess needed in order to achieve a particular reversible capacity, and open up a new direction for finding very high capacity cathodes.

[1] J. Lee, A. Urban, X. Li, D. Su, G. Hautier, G. Ceder, Unlocking the Potential of Cation-Disordered Oxides for Rechargeable Lithium Batteries, Science, 343 (6170), 519-522 (2014)