Direct Observation of the Anisotropic Transport Behavior of Li⁺ in Graphite Anodes and Thermal Runaway Induced by the Interlayer Polarization

Graphite is one of the major anode materials for commercial lithium-ion batteries. Li + transport in a single graphite granule along intra and interlayer modes is a crucial factor for the battery performance. However, direct evidence and visualized details of the Li + transports are hardly provided. Here, we report the direct observation of the anisotropic transport behavior of Li + and investigate the electro-chemo-structure evolution during the lithiation of graphite through both the intra and interlayer pathways via in situ transmission electron microscopy. The in situ experiments of nano batteries give two extreme conditions, in which thermal runaway induced by polarization only occurs along the interlayer, not along the intralayer. The high diffusion energy barrier induced large polarization when the interlayer Li + transport became dominant. The energy of the polarization electric field would be instantaneously released like a short electric pulse, which generated a substantial amount of joule heat and created an extremely high temperature, causing the melting of the tungsten tip. We provide another possible fundamental mechanism of thermal failure in graphite-based Li-ion batteries and hope this insightful work would help the safety management of graphite-based lithium-ion batteries.