The effects of charge/discharge rate on capacity fade of lithium ion batteries

Abstract

In this work, various techniques were used to evaluate the impact of c-rate on capacity fade of lithium ion batteries. The c-rate is the rate of charge or discharge which correlates with the rate of lithation or delithiation of electrode material. Historically, a higher c-rate results in accelerated capacity fade due to mechanical-induced damage of active particles. However, there has been no comprehensive study showing direct evidence for this hypothesis. In this study, a variety of experiments were performed to generate a cohesive understanding of how c-rate affects capacity fade. Electrochemical techniques were used to evaluate capacity, resistance, and rate capability. Together, capacity and resistance measurements were used to segregate chemical-induced degradation (associated with resistance rise and capacity loss) from mechanical-induced degradation (primarily associated with capacity loss) for different c-rates throughout cycling. Rate capability measurements were used to show the impact of chemical degradation and physical damage on utilization of the electrodes at various c-rates. Raman spectroscopy was used to measure Li+ inventory losses caused by film growth on the anode surface and to evaluate how it contributes to the capacity loss that was measured electrochemically. Lastly, microscopy techniques were used to assess mechanical damage in the cathode material. The damage accumulates in the form of micro-cracks and dislocation defects which lower Li ion transport through Li ion trapping mechanisms in LiCoO₂ particles. This was demonstrated through diffusivity measurements using galvanostatic intermittent titration techniques.

The results from this work show that, for slow c-rate applications, formation of a stable solid electrolyte interphase (SEI) on the graphite anode is critical for long term battery performance. For high c-rate applications, both the formation of a stable SEI and optimization of LiCoO₂ particle morphology to minimize intercalation stresses are vital for improved long term performance.

These experiments show that chemical degradation accounts for the entirety of capacity loss at slow c-rates (C/10). At high c-rates, both chemical and mechanical degradation contribute to fade. Mechanical degradation plays an increasing role as c-rate is increased. Additionally, higher c-rates effectively increase the strain rate for lithiation of LiCoO₂ particles resulting in intercalation induced stresses that lead to micro-crack formation, defect generation, and eventual particle fracture. The accumulation of this damage reduces the rate capability of the cell.