The Configurational Space of Rocksalt-Type Oxides for High-Capacity Lithium Battery Electrodes

A unifying theory is presented to explain the lithium exchange capacity of rocksalt-like structures with any degree of cation ordering, and how lithium percolation properties can be used as a guideline for the development of novel high-capacity electrode materials is demonstrated. The lithium percolation properties of the three most common lithium metal oxide phases, the layered -NaFeO2 structure, the spinel-like LT-LiCoO2 structure, and the -LiFeO2 structure, are demonstrated and a strong dependence of the percolation thresholds on the cation ordering and the lithium content is observed. The poor performance of -LiFeO2-type structures is explained by their lack of percolation of good Li migration channels. The spinel-like structure exhibits excellent percolation properties that are robust with respect to off-stoichiometry and some amount of cation disorder. The layered structure is unique, as it possesses two different types of lithium diffusion channels, one of which is, however, strongly dependent on the lattice parameters, and therefore very sensitive to disorder. In general it is found that a critical Li-excess concentration exists at which Li percolation occurs, although the amount of Li excess needed depends on the partial cation ordering. In fully cation-disordered materials, macroscopic lithium diffusion is enabled by approximate to 10% excess lithium.