(692b) A Comparison of Capacity Fade in Li-Ion Batteries Employing Lfp, NCA and Blended LMO+Ncm Cell Chemistries, with Insights from a Physics-Based Model of SEI Formation

Li-ion batteries in satellites and electric vehicles operate for at least 10 years, and predicting the capacity and power loss of Li-ion batteries over this span can be challenging.  Physics-based models can help to predict the capacity losses arising from different usage profiles, by identifying the key degradation mechanisms and extrapolating them forward in time.  However, a major obstacle is the fact that numerous Li-ion cell cathode chemistries are currently in wide use.  For example, layered Ni-Co-Al (NCA) and Ni-Co-Mn (NCM) oxides are favored for energy applications, while lithium iron phosphate (LFP) is attractive for high-power applications where safety is an issue.  For PHEV and EV batteries, where a  balance between power and capacity is desired, a physical mixture of high-power Li-Mn oxide spinel (LMO)  and high-capacity NCM or NCA is often employed for the positive electrode.  Each Li-ion cell chemistry has a distinct capacity fade mechanism, so it is unclear whether a single physics-based model can be employed.  However, a unifying theme is that nearly all commercial cells employ a negative electrode based on natural or artificial graphite.  Further, the formation of a solid-electrolyte-interphase (SEI) layer on the graphite surface through lithium consuming side reactions is widely considered to play a dominant role in the capacity fade in all Li-ion cells.  Our findings from nearly five years of cycling studies on LFP, NCA and LMO+NCM cells also point to the significant contribution of lithium-consuming SEI formation towards the overall capacity fade.  In this paper, we explore the feasibility of using a single physics-based model to predict capacity fade  in LFP, NCA and blended LMO+NCM cells under varied operating conditions.  The model includes the main formation and degradation mechanisms of the graphite SEI layer, and also allows us to assess the significance that other factors (such as the cathode) have on the capacity and power fade. The model is compared against the measured rates of capacity fade and resistance rise in commercial LFP, NCA and LMO+NCM cells cycled over a range of temperatures (10°C to 46°C), discharge rates (0.5C to 10C) and discharge depths (10% to 90%).