(5d) Cycle Life Prediction for Ncm-Composite/Graphite Lithium Ion Battery Cells

Secondary lithium-ion batteries used in electric vehicles must last 10 years.  Predicting the capacity and power loss of the batteries over this span requires accelerated testing methods such as storing and cycling the individual cells at elevated temperatures.  Physics-based models may be able to improve capacity fade prediction using a smaller test matrix, greatly reducing the testing burden. In this paper, we study capacity fade in NCM-composite batteries (1.5 Ah) containing a composite Li(Ni0.33Mn0.33Co0.33)O2 + LiMn2O4 positive electrode and a graphite negative electrode.  The batteries were cycled at a wide range of conditions, including temperature (10°C to 46°C), discharge current (0.5C to 6.5C) and depth of discharge (10% to 90%).  The measured capacity loss is described by a single particle model which calculates lithium loss from two related mechanisms: (1) mechanical degradation at the graphite negative electrode particle surface caused by diffusion-induced stresses; (2) chemical degradation caused by lithium loss to the solid-electrolyte interphase (SEI).  Fatigue crack growth is modeled based upon the cyclic diffusion-induced stresses that are generated by repeated lithium insertion and de-insertion in the graphite particle.  This simplified model is able to reproduce the dominant capacity loss features observed for the NCM/graphite cells. In particular, the increased capacity fade observed for  cells at low temperature (10°C) and high discharge rates (5C–6.5C) is linked to crack propagation on graphite anode particles and subsequent formation of a Li-consuming SEI layer on newly exposed crack surfaces.  Fading of rate and pulse power capability during cycling is also modeled.