Degradation Mechanism Study and Safety Hazard Analysis of Overdischarge on Commercialized Lithium-ion Batteries

Abstract

With the continuous improvement of the energy density of traction batteries for electric vehicles, the safety of batteries over their entire lifecycle has become the most critical issue in the development of electric vehicles. Abuse of electricity encountered in the application of batteries has a great impact on the safety of traction batteries. In this study, focused on the overdischarge phenomenon that is most likely to be encountered in the practical use of electric vehicles and grid storage, the impact of overdischarge on battery performance degradation is analyzed by neutron imaging technology and its safety hazards is systematically explored, combined with multimethods including electrochemical analysis and structural characterization. Results reveal the deterioration of the internal structure of traction batteries due to the overdischarge behavior and play a guiding role in the testing and evaluation of the safety of traction batteries.

Reviewing the Safe Shipping of Lithium-Ion and Sodium-Ion Cells: A Materials Chemistry Perspective

“…Similarly, in Figure 3(b), Ma et al reported that 2.5 Ah 18650-type NMC//graphite Li-ion cells, when overdischarged to 110% or 150% DOD (corresponding to full cell voltages of -1.12 V and approximately -0.3 V), showed significant metallic Cu deposition on the cathode and that the extent of metallic Cu deposition increased as the extent of overdischarge increased [34]. It might be worth mentioning that as overdischarging is allowed to continue, studies have reported that the voltage of Li-ion cells first decrease to a minimum (such as -1 or -2 V) and then increase, gradually tending towards 0 V-such an increase in voltage is believed to arise due to internal shorting in the cell caused by the growth of metallic dendritic Cu on the cathode, which can pierce through the separator and hence cause localised contacts between the cathode and the anode [28,30,[34][35][36]. Extreme overdischarging can have serious consequences as the internal microstructure of the battery is compromised: issues such as internal shorting due to growth of metallic dendritic Cu and decrease in the mechanical integrity of the separator, reduced porosities in the cathode and blockage of electrochemical active sites on the cathode can not only result in rapid capacity loss but also pose serious thermal hazards [32,34,35].…”

“…Under such conditions, Langner et al reported extensive metallic Cu plating on the cathode, along with white spots belonging to the breakdown of the polyolefinic Energy Material Advances separator typically used in commercial Li-ion batteries [28]. Similarly, in Figure 3(b), Ma et al reported that 2.5 Ah 18650-type NMC//graphite Li-ion cells, when overdischarged to 110% or 150% DOD (corresponding to full cell voltages of -1.12 V and approximately -0.3 V), showed significant metallic Cu deposition on the cathode and that the extent of metallic Cu deposition increased as the extent of overdischarge increased [34]. It might be worth mentioning that as overdischarging is allowed to continue, studies have reported that the voltage of Li-ion cells first decrease to a minimum (such as -1 or -2 V) and then increase, gradually tending towards 0 V-such an increase in voltage is believed to arise due to internal shorting in the cell caused by the growth of metallic dendritic Cu on the cathode, which can pierce through the separator and hence cause localised contacts between the cathode and the anode [28,30,[34][35][36].…”

“…It should be understood that the nature of full cell balancing as designed by a cell manufacturer would largely dictate how rapid this voltage slippage process would be and that this might vary between different cell manufacturers even for the same Li-ion chemistry. In these extreme overdischarged states, metallic Cu can easily be deposited on the cathode, as presented in Figures 3(a) and 3(b), with data from Langner et al [28] and Ma et al [34], respectively. Figure 3(a) displays metallic Cu plating on the cathode of a 20 Ah Li-ion pouch cell, which was made to discharge up to 31.5 Ah (the full cell voltage dropped as low as around -2 V).…”

“…It might be worth mentioning that as overdischarging is allowed to continue, studies have reported that the voltage of Li-ion cells first decrease to a minimum (such as -1 or -2 V) and then increase, gradually tending towards 0 V-such an increase in voltage is believed to arise due to internal shorting in the cell caused by the growth of metallic dendritic Cu on the cathode, which can pierce through the separator and hence cause localised contacts between the cathode and the anode [28,30,[34][35][36]. Extreme overdischarging can have serious consequences as the internal microstructure of the battery is compromised: issues such as internal shorting due to growth of metallic dendritic Cu and decrease in the mechanical integrity of the separator, reduced porosities in the cathode and blockage of electrochemical active sites on the cathode can not only result in rapid capacity loss but also pose serious thermal hazards [32,34,35]. For example, Fear et al showed that a Li-ion cell's temperature can rapidly increase when overdischarged at 1C cycling rate (discharge in 1 h)-as shown in Figure 3(c), a 3.35 Ah 18650-type Liion cell's temperature rose from 21 °C at the beginning of discharge to 65 °C at 0 V and reached a peak temperature of 79 °C shortly after reaching 0 V in the course of overdischarging the cell [35].…”

New developments in battery safety for large-scale systems

“…This can then hinder the intercalation of lithium ions in the anode during the charge process causing heat to be produced that can then result in thermal runaway. [8][9][10][11] In general, a reduced voltage range will help to avoid the side reactions that occur at the two ends of the voltage range and provides a higher margin for safety. Studies have also shown that reducing the voltage range can more than double the number of cycles obtained with Li-ion.…”