Understanding the Fast Charging Effect on Anode and Electrolyte in Li Ion Battery

In recent years, fast charging is a key enabler of mainstream adoption of battery-based electric vehicles into the transportation sector. Charging at high rates has been shown to accelerate degradation, causing the capacity, life and safety of batteries to deteriorate. Currently, the issues of fast charge residing inside the battery include cathode particle cracking, loss of active material, electrolyte-electrode side reactions, Li plating at the anode and electrolyte decomposition. In spite of all available information, there are no systematic studies in the open literature regarding the above issues caused by fast charging. Herein, we investigated the effects of charge rate in coin cells containing NMC532/1506-T graphite. The cells were cycled at room temperature using high charge rates of 1-, 2-, 4-, and 8-C and discharged at the 1-C rate for 50 cycles. Li plating and SEI formation on the anodes as a function of charging rates was examined by SEM and XPS. The amount of lithium plating and SEI composition were found to be correlated with charging rates. The concentrations of the transition metal dissolution on charge rate was studied by ICP-MS. The results show dependence of the concentrations of the TMs on charge rate is complex. The degraded electrolytes in the cells were collected and analyzed by HPLC-MS. Three decomposition products were identified in the organic electrolyte, regardless of the charge rates. The relative concentrations of these organics did not display sensitivity to charge rate. These findings in this work provide new understanding of the failure mechanisms of batteries during fast charging.