(Keynote) Understanding Performance Degradation of Li-Ion Batteries Under Fast Charge Conditions

Fast charge is a critical challenge for the electric vehicles in competing with rapid gasoline refueling of the traditional internal combustion engine vehicles. Fast charge not only causes accelerated performance degradation, but also increases safety risk of the Li-ion batteries, which are attributed to a number of physical and chemical processes, such as overheating by Joule heat, Li plating on the graphite anode, structural deterioration of the layered cathode material, and electrolyte depletion. The performance degradation behaves as a decline in the capacity and operating voltage, and it is often accompanied by impedance growth and volumetric swelling of the battery. Owing to the high reactivity of Li metal with electrolyte solvents and its dendritic electrodeposition nature, Li plating has long been considered as the key issue resulting in the accelerated performance degradation and potential safety risk.1, 2 However, our recent work3, 4 shows that in health power-optimized Li-ion cells, Li plating does not occur even when the charging rate is increased to 10C, while the accelerated performance degradation is still present. The above results suggest that there must be other factors triggering the accelerated performance degradation, and that the safety risk by Li plating may be overestimated. In this work, we made a comprehensive survey on the accelerated performance degradation by analyzing the literature and our own results. It is concluded that the accelerated capacity degradation under the fast charge conditions can be ultimately attributed to the loss of cyclable Li+ ions from the limited capacity of the cathode, namely a decrease in the cathode-to-anode (C/A) capacity ratio (or an increase in the negative-to-positive (N/P) capacity ratio, as called elsewhere), and that such a process is always accompanied by the depletion of electrolyte and the resultant impedance growth and volumetric swelling of the battery, as summarized in Fig. 1, though degradation mechanisms in the graphite anode and layered transition metal oxide cathode are fundamentally different. In this presentation, the effect of various factors on the performance degradation of the Li-ion batteries and their correlation with the depletion of electrolyte will be discussed, and several strategies towards enhancing the fast-charging capability of the Li-ion batteries will be presented. References A. Tomaszewska, Z. Chu, X. Feng, S. O’Kane, X. Liu, J. Chen, C. Ji, E. Endler, R. Li, L. Liu, Y. Li, S. Zheng, S. Vetterlein, M. Gao, J. Du, M. Parkes, M. Ouyang, M. Marinescu, G. Offer, and B. Wu, eTransportation, 1, 100011 (2019). H. Zhao, L. Wang, Z. Chen, and X. He, Energies, 12 (20) (2019). S. S. Zhang, InfoMat, 2, 1606823 (2020). https://doi.org/10.1002/inf2.12058. S. S. Zhang, ChemElectroChem, 7, 555-560 (2020). Fig. 1. A summary of the factors leading to accelerated performance degradation of Li-ion batteries under fast charge conditions. Figure 1