Internal Short-Circuit Tendency of Overdischarged Li-Ion Batteries: Effect of Subsequent Charging Conditions
Authors: Lee Ga-Eun , Min Yu-Jeong , Shin Heon-Cheol 
Published: December 2024

Lithium-ion batteries (LIBs) are currently the most widely used energy sources for electric vehicles and energy storage systems due to their high-energy, long-life, and high-efficiency characteristics. To achieve high voltage and capacity, tens to hundreds of unit cells are connected in series and parallel, and their performance is managed by a battery management system (BMS). However, despite the use of BMS, battery abuse such as overcharge and overdischarge is still being reported. Among these, overdischarge is a situation when the battery is discharged below the lower cut-off voltage, and is roughly divided into slight(2.5-0V) and extreme (<0V) overdischarge depending on the depth of the discharge. The latter situation, caused by unintended accidents, BMS failure, extreme SoC variances between cells, etc., has a critical impact on the performance and safety of the battery. There have been many reports on the association of overdischarge with internal short circuits, which are directly related to cell safety. Examples include analyzing the tendency of internal short circuit as a function of the depth and number of overdischarges, and the recoverability of internal short circuit as a function of the depth of overdischarge (i.e., the critical overvoltage). However, these studies have only addressed the state of the cell immediately after overdischargeor occurrence of an internal short circuit, and have not focused on what happens inside the cell during a normal charge and discharge of the overdischarged cell. During overdischarge, the anode substrate (Cu) oxidizes, resulting in the dissolution of Cu2+ ions in the electrolyte. The ions are then deposited on the cathode. Subsequently, during charging, the Cu2+ ions oxidize again and are deposited back on the anode. Since the anode potential does not reach the Cu oxidation potential in the normal charge/discharge voltage range, the Cu precipitation formed on the anode remains intact. Therefore, Cu precipitation on the anode during the charging process immediately after the initial overdischarge can affect the cell performance, especially the cell self-discharge.
In this study, we investigate the internal short-circuit behavior of LIBs that has been overdischarged and subsequently charged. In particular, we examined the changes in the morphology of the Cu precipitate on the anode surface as a function of the charging current during subsequent charging and investigated how morphology of Cu is related to self-discharge. To do this, we first disassembled cells that were subsequently charged to observe changes of the Cu electrodeposited on the anode. The results showed that Cu deposited as coarse spherical particles at low charging currents, transitioned to a mixture of spherical and dendritic morphology as the charging current increased, and finally only dendritic morphology was observed above a specific current. To correlate Cu morphology on anode with the internal short circuit of the cell, the leakage current of the cell was measured to calculate the internal short resistance (ISR). When the overdischarged cells were charged at low current, the ISR was almost the same as that of the non-overdischarged cells, but as the charge current increased, the ISR decreased significantly. Taking these results together with the Cu morphology analysis, it can be inferred that the Cu morphology formed during subsequent charge after overdischarge has a significant impact on the internal short circuit of the cell. On the other hand, the findings suggest that internal short-circuit characteristics which were initially determined do not change when overdischarged cells are repeatedly charged and discharged under normal conditions. This is because Cu is electrochemically stable over the normal charge and discharge voltage range, so it remains at the anode and does not change shape. Moreover, it demonstrates that the Cu morphology dominates the internal short-circuit tendency of the cell. In this presentation, we will discuss in depth the correlation between Cu morphology and internal short circuits with the subsequent charge current in overdischarged cells. Based on this, we suggest subsequent charging conditions to minimize the self-discharge caused by overdischarge.