Direct In Situ Observation and Numerical Simulations of Non-Shrinking-Core Behavior in an MCMB Graphite Composite Electrode

Most models of battery charging assume a shrinking core model with a Lithium-rich shell and Li-poor core. In this study, we provide direct experimental evidence from in situ optical measurements and that suggests that the lithiation of (spherical) MCMB (meso carbon micro beads) particles do not follow the shriking-core model, even for particles which eventually become uniformly lithiated. We observe "hot spots" which expand until the particles are fully charged. The observations are explained using a microstructural model of MCMB particles that 1) accounts for their polycrystalline nature, 2) anisotropy in diffusion and 3) dependence of the intercalation rate at the surface on the orientation of the graphene planes. We show that our model reproduces the key features of the charging process observed in experiments. Implications of these results to Li-plating are discussed.