Abstract
Lithium-ion batteries (LIBs) capable of fast-charging are essential in the popularization of electric vehicles. Typical fast-charge requirements are charging to 80% state of charge within 10 mins. However, such high rates cause active material degradation and undesired lithium plating, leading to capacity fading and safety hazards. These issues are impacted by charging conditions, e.g. charging protocol, temperature, external pressure, etc. Here, we focus on the pressure-dependence of fast-charge batteries using an in-house designed gas bladder cell configuration. Through a systematic study, we revealed correlations between externally applied electrode stack pressure and capacity fade. Additionally, we propose possible capacity fade mechanisms during fast charge and a way to mitigate it.
We utilized single-layer pouch cell batteries with a graphite anode, LiNi0.5Mn0.3Co0.2O2 (NMC532) cathode, and organic liquid electrolyte solution. Electrode stack pressure is achieved by opposing pairs of external flexible gas bladders. These gas bladders are formed by airtight sealing of a Kapton dome against each side of the flexible battery pouch. (Fig. (a)). By controlling the gas pressure within the bladders, the external pressure is precisely set and the flexible bladders locally conform to provide uniform pressure across the electrode stack. This combination bypasses issues with rigid plates where electrode thickness variations and internal electrical connections prevent uniform pressure. Additionally, the pressure is stable during battery cycling process regardless of material expansion/contraction or gassing. Furthermore, the X-ray transparent Kapton domes allow operando X-ray characterization.
Using external pressures of 10, 50, and 125 psi, and on a control group with pressure applied by rigid plates, we performed a series of electrochemical experiments. All batteries were charged at 6C using CC-CV protocol and discharged at C/2 at room temperature for 140 cycles with a voltage range of 3 – 4.1 V. We found that the capacity fade decreases with increasing pressure (Fig. (b)); the capacity fade of rigid-plate cells is larger than gas-bladder cells. Additionally, the fast charge capacity increases with increasing pressure, indicating that the depth of charge/discharge in the entire cell is greater with higher external pressure (Fig. (c)). In contrast, the difference between the open circuit voltages at the end of charge and discharge is smaller at higher pressure. These two observations imply that the depth of reaction within the active material decreases with increasing pressure (Fig. (d)). Combining these two findings, we infer that there is more active material loss with lower external pressure, which is the main reason for capacity fading. The active material loss may come from the loss of electrical contact when particles crack or pulverize during fast charge, while higher external pressure can help preserve the electrical contact.
Our study sheds new light on the influence of external pressure on fast-charge performance of LIBs. The use of gas-bladder cells helps to eliminate non-reproducibility in non-uniform cell thickness and material expansion/contraction. We plan to conduct operando X-ray diffraction experiments using the X-ray transparent AP cell to monitor the evolution of anode and cathode materials and Li plating during fast charge to investigate the underlying mechanism of the pressure-dependent behaviors.
Figure 1