Cyclic strain-induced crack growth in graphite during electrochemical testing in propylene carbonate-based Li-ion battery electrolytes

Lithiation/de-lithiation cycles induced cracks in isostatically pressed graphite samples subjected to constant-load bending tests while simultaneously conducting cyclic voltammetry (CV) experiments in propylene carbonate-based Li-ion battery electrolyte solutions. A large cyclic strain of Δ#epsilon# = 0.95% was generated by lithiation/de-lithiation cycles as determined by micro-Raman measurements carried out concurrently with CV experiments. In-situ optical microscopy measurements of crack lengths, a, showed that the crack-growth rate, da/dt, depended on the stress intensity factor at the crack tip and could be expressed as da/dt = AΔK_Iⁿ. A two-stage crack-growth behaviour was determined with n = 51.3 in the first stage. However, lower crack propagation rates observed in the second stage (n = 9.9) were affected by crack closure due to (1) a rough fracture surface morphology of the graphite cracks, and (2) the deposition of solid electrolyte reduction products on facetted crack surfaces, where both the factors likely contributed to reducing ΔK to ΔK_eff.