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

摘要

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 Delta 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 = A Delta K (I) (n) . 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 Delta K to Delta K (eff).