Investigation of multi-step fast charging protocol and aging mechanism for commercial NMC/graphite lithium-ion batteries

Fast charging is considered a promising protocol for raising the charging efficiency of electric vehicles. However, high currents applied to Lithium-ion (Li-ion) batteries inevitably accelerate the degradation and shorten their lifetime. This work designs a multi-step fast-charging method to extend the lifetime of LiNi0.5Co0.2Mn0.3O2 (NMC)/graphite Li-ion batteries based on the studies of half cells and investigates the aging mechanisms for different charging methods. The degradation has been studied from both full cell behaviour and materials perspectives through a combination of non-destructive diagnostic methods and post-mortem analysis. In the proposed multi-step charging protocol, the state-of-charge (SOC) profile is subdivided into five ranges, and the charging current is set differently for different SOC ranges. One of the designed multi-step fast charging protocols is shown to allow for a 200 full equivalent cycles longer lifetime as compared to the standard charging method, while the charging time is reduced by 20%. From the incremental capacity analysis and electrical impedance spectroscopy, the loss of active materials and lithium inventory on the electrodes, as well as an increase in internal resistance for the designed multi-step constant-current-constant-voltage (MCCCV) protocol have been found to be significantly lower than for the standard charging method. Post-mortem analysis shows that cells aged by the designed MCCCV fast charging protocol exhibit less graphite exfoliation and crystallization damage, as well as a reduced solid electrolyte interphase (SEI) layer growth on the anode, leading to a lower Rsei resistance and extended lifetime.