A study of the effect of current density on the internal short circuit of Cu contaminants in lithium-. An investigation of the effect of current density on the internal shorting of Cu contaminants in the lithium-ion battery using in-situ optical. In this research, we studied the internal short circuit when there was Cu contamination on the surface of the cathode.
The self-discharge phenomenon in the presence of a Cu impurity on the positive electrode was confirmed. Full cell voltage profile depending on whether Cu contamination is present or not in the cathode during constant current charging. situ image of whole cell when Cu contamination is present in the cathode during constant current charging. Image diagram of Cu redissolving during constant voltage charging. situ image of whole cell when Cu contamination is present in the cathode during constant charging.
Full cell voltage profile depending on whether or not Cu contamination is present in the cathode during constant voltage charging. Voltage profile of NCM622 cathode under 0.3 mA/cm2 current density charging as if Cu contamination is present or not. Voltage profile of NCM622 cathode under 0.6 mA/cm2 current density charging as if Cu contamination is present or not.
Voltage profile of NCM622 cathode during charging at a current density of 1.5 mA/cm2, as .. iv whether Cu contamination is present or not.
Introduction
Lithium-ion batteries
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Internal short in lithium-ion batteries
- Over-discharge
- Cu contamination
The lithium-ion cell is connected in series to meet the high voltage requirements for electric vehicles. Since the redox potential of Cu metal is more than 3 V compared to the redox potential of lithium, no oxidation takes place during the normal charging and discharging process. However, under excessive discharge, the anode potential increases abnormally to 5 V, as shown in Figure 7.
The potential of anode is higher than redox potential of Cu, so the Cu current collector is dissolved. Difference of electrode between fresh cell and overdischarged cell can be shown in figure 8. However, in the case of overdischarged cell, the surface of the anode was cracked and Cu current collector was thinner than that of fresh cell.
This caused the mechanical stability of the anode to decrease and the charge transfer resistance of the cell to increase. Although the Al current collector was not cracked, the surface of the cathode electrode was covered with Cu. With the higher discharge rate during overdischarge, the internal short circuit rate increases.
The manufacturing process of lithium-ion batteries is exposed to metal contaminants such as iron, copper, copper, zinc, aluminum. This metal contamination was dissolved during the charging process, making the internal short in the cell. Degrees of dissolution of metal pollution are different according to types of metal due to difference in redox potential of metal, as shown in figure 10.
This makes the degree of internal short metal contamination effect change according to the types of metal contamination, Figure 11 shows the degree of internal short effect following the redox potential of each metal contamination. Junju Kuwabara studied the formation of internal short-range contamination from Cu contamination. 2 Cu contamination dissipated after 2 min after declaring full cell charging. At the beginning of charging, the cell voltage below 3 V, however, the Cu impurity was dissolved that the redox potential of Cu is 3.3 V compared to the redox potential of Li.
The local low SOC region was charged during the constant voltage charging process, as shown in Figure 14. After charging the local low SOC region, Cu was redissolved because the potential of this region is greater than the redox potential of Cu.
Experimental Method
- Electrode manufacturing
- LSV experiment
- In-situ optical microscopy
- Half-cell manufacturing
- GITT
To identify the difference in the behavior of Cu contamination on the cathode with the current density, the cathode half-cell was charged to 4.3 V in difference of constant current density and 1/50 C constant voltage charging after cell production and 10 hours of rest. Due to overpotential due to difference in current density, because higher current density was applied, because higher potential was maintained at similar state of charge during constant current charging. Since higher current density was applied in constant current charging, it takes much longer during constant voltage charging.
The final capacity of cells as current density during the first cycle is all similar after charging with constant current and constant voltage. The potential of the cathode during charging was sufficiently high to resolve Cu contamination because the potential of the cathode material is more than 3.5 V after starting charging at any current density. Any current density for charging the NCM622 half cell, the holding time above 3.9V is higher than a few minutes, all the Cu contamination will be dissolved.
Figure 28, 29 shows the difference in effects due to the internal shortage of Cu impurity as current density. At low current density, the effect of the internal short circuit due to Cu contamination was indicated under constant current charging. At low current density, the leakage current due to the internal short circuit of Cu contamination affected a much more constant charging process compared to high current density.
Leakage current at any case of current density under constant current charging is below 1.5 mA/cm2. The current density of 0.3 and 0.6 mA/cm2 in the constant current charging was not large compared to leakage current, they are affected by the internal short circuit. However, at 1.5 mA/cm2 the leakage current is relatively small compared to the charge current, the effect of Cu contamination was less than the effect at low current density.
The image of the anode in front of the separator shows that the Cu impurity at the cathode is sufficiently dissolved and deposited on the anode regardless of the current density. As higher current density was applied, the lower the internal short rate with the Cu image of the cathode facing the spacer. In this work, the effect of current density on the potential dropout problems of Cu contamination was observed.
Results and Discussion s
Copper redox potential and shape of deposition
- The redox potential of Cu metal in the electrolyte containing Li salt
- Observation of copper deposition
LSV data shows the redox potential of copper (vs. Li/Li+) in the organic electrolyte and reaction current density at the charge potential of the cathode material. To confirm the voltage drop caused by the internal short circuit after charging, the experiment was carried out as shown in Figure 25a). After state of charge of 100 mAh/g, the effect of the internal short increases, then the difference in state of charge has grown as there is Cu contamination or not.
Since the cell was charged by the externally applied current and the internal one by the Cu contamination caused the cell to self-discharge continuously. For 24 hours of rest, the voltage of the half-cell without Cu contamination dropped to only 4.26 V after charging 4.3 V, the cathode potential with Cu contamination dropped to 3.81 V. Figure 25 Experimental process for confirming the voltage drop problem a), the image of the Cu surface - implanted cathode b). Guo, R.; Lu, L.; Ouyang, M.; Feng, X., Mechanism of the whole process of overdischarge and overdischarge-induced internal short circuit in lithium-ion batteries.
Ouyang, M., Internal short-circuit triggering method for lithium-ion battery based on shape memory alloy. Xu, J.; Wu, Y.; Yin, S., Investigation of effects of design parameters on the internal short circuit in cylindrical lithium ion batteries.
Voltage drop by the internal short induced by copper contamination
- Difference of state of charge as a current density
- Voltage drop by the internal short from Cu contamination after charging
- Difference of voltage drop as current density
Conclusion
Cu contamination on the cathode surface was dissolved during the charging process and deposited on the anode surface. The leakage current affects the constant current charging process at low current rate charging because the difference between the leakage current and the charging current is small. The low-potential area from the internal short is small at high current rates because the time the current spent by the internal short is short.
Low potential area is charged during constant voltage charging, contacted Cu at cathode surface will be dissolved again, the internal short circuit will be reduced. High rate charging is more effective than low rate charging for reducing internal short induced Cu contaminations dissolution and deposition.
Acknowledgement
