Understanding the Graphite Anode Electrode Failure Mode in Cycled Commercial Lithium-Ion Batteries

Abstract

The structure degradation of commercial Lithium-ion battery (LIB) graphite anodes was investigated and reported here. The first was to investigate the electrode degradation by examining the PVDF binder degradation after extended cycling. We concluded that the PVDF binder is extremely stable and has no structure degradation after 800 cycles. The graphite materials with different cycling numbers and charge rates was investigated by focused ion beam (FIB) and scanning electron microscopy (SEM). The cross-section image of graphite anode by FIB milling shows that cracks, resulted in the volume expansion of graphite electrode during long-term cycling, were formed in parallel with the current collector. The crack occurs in the bulk of graphite particles near the lithium insertion surface, which might derive from the stress induced during lithiation and de-lithiation cycles. Subsequently, crack takes place along grain boundaries of the polycrystalline graphite, but only in the direction parallel with the current collector. Furthermore, fast charge graphite electrodes are more prone to form cracks since the tensile strength of graphite is more likely to be surpassed at higher charge rates. Therefore, for LIBs long-term or high charge rate applications, the tensile strength of graphite anode should be considered.