A transmission electron microscopy study of crack formation and propagation in electrochemically cyc
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 cointercalation
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.
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