A transmission electron microscopy study of crack formation and propagation in electrochemically cycled graphite electrode in lithium-ion cells

Subsurface defects and local compositional changes that occurred in graphite anodes subjected to cyclic voltammetry tests (vs. Li/Li⁺, using an electrolyte consisting of 1 M LiClO₄ in a 1:1 volumetric mixture of ethylene carbonate and 1,2-dimethoxy ethane) were investigated using high-resolution transmission electron microscopy (HR-TEM). Cross-sections of anodes prepared by focused ion beam (FIB) milling indicated that graphite layers adjacent to solid electrolyte (SEI)/graphite interface exhibited partial delamination due to the formation of interlayer cracks. The SEI layer formed on the graphite surface consisted of Li₂CO₃ that was identified by {1 1 0} and {0 0 2} crystallographic planes. Lithium compounds, LiC₆, Li₂CO₃ and Li₂O, were observed on the surfaces of separated graphite layers. Deposition of these co-intercalation compounds near the crack tip caused partial closure of propagating graphite cracks during electrochemical cycling, and possibly reduced the crack growth rate. Graphite fibres that were observed to bridge crack faces likely provided an additional mechanism for the retardation of crack propagation.