Abstract
In recent years, LiFePO4 (LFP) has gained significant market acceptance in the area of electrical vehicles and grid energy storage due to the absence of Ni and Co, thermal stability, low toxicity, and long cycle life. However, the long-term performance of LFP may be degraded in mild overcharge scenarios. When placed in a battery pack, overcharge of an individual cell can occur due to charge imbalance between cells. Hence, it is important to understand the effect of overcharge on the cell materials.
The results of overcharge on pouch cells with a 10 Ah capacity utilizing LFP as the cathode and graphite as the anode will be presented. Post overcharge electrochemical and material characterization are used to elucidate the effects of overcharge on performance and materials degradation. Electrochemical experiments included in-operando electrochemical impedance spectroscopy (EIS) and differential capacity as a function of voltage scans. EIS revealed changes in internal resistances as a function of state of overcharge. Differential capacity scans determined the point during overcharge at which damage to the cell begins, loss of active material, and loss of Li inventory. Materials characterization included X-ray diffraction (XRD), scanning electron microscopy (SEM), and inductively coupled plasma (ICP) spectroscopy. XRD exposed changes to the crystal structure, composition, and contaminants. SEM highlighted changes in the morphology of the cathode due to damage from overcharging. ICP was used to detect the presence of cathode material in the anode and vice versa, which is detrimental to future cell performance. Results show cell degradation occurring at low overcharge percentages, indicating the sensitivity of LFP to overcharge scenarios. As a result, the inclusion of an LFP cathode in battery packs may require more significant SOC buffers and/or a more sophisticated battery management system that can monitor overcharge at the individual cell level.