Degradation of lithium ion batteries employing graphite negatives and nickel–cobalt–manganese oxide + spinel manganese oxide positives: Part 2, chemical–mechanical degradation model

•A physics-based model is established to explain capacity fade in Li-ion cells.•The model is consistent with the capacity fade trends observed in NCM + LMO/graphite cells.•Fatigue crack growth in the graphite may be an important factor in the loss of usable lithium.

Capacity fade is reported for 1.5 Ah Li-ion batteries containing a mixture of Li–Ni–Co–Mn oxide (NCM) + Li–Mn oxide spinel (LMO) as positive electrode material and a graphite negative electrode. The batteries were cycled at a wide range of temperatures (10 °C–46 °C) and discharge currents (0.5C–6.5C). The measured capacity losses were fit to a simple physics-based model which calculates lithium inventory loss from two related mechanisms: (1) mechanical degradation at the graphite anode particle surface caused by diffusion-induced stresses (DIS) and (2) chemical degradation caused by lithium loss to continued growth of the solid-electrolyte interphase (SEI). These two mechanisms are coupled because lithium is consumed through SEI formation on newly exposed crack surfaces. The growth of crack surface area is modeled as a fatigue phenomenon due to the cyclic stresses generated by repeated lithium insertion and de-insertion of graphite particles. This coupled chemical–mechanical degradation model is consistent with the observed capacity loss features for the NCM + LMO/graphite cells.