Degradation of surface film on LiCoO2 electrode at moderately elevated temperature and its effect on cell properties, and the countermeasures

Abstract

Lithium-ion batteries (LIBs) are one of the most popular energy conversion and storage devices, currently, used in electronic devices and electric vehicles. Because they can be heavy-duty used or abused, LIBs are frequently exposed to moderately elevated temperature (ca. 60–100 °C). LIBs are often degraded at the temperature. The cells life is shortened or safety problems are caused. Therefore, to unravel the failure mechanisms of LIBs at moderately elevated temperature, and to find the appropriate countermeasures are very important.<br/> Surface films on electrodes generated by reduction/oxidation of electrolytes are electrically insulating, so further electrochemical decompositions of electrolyte are prevented. It is the passivating role of the surface films. It is known that surface films on the negative electrode are damaged, and lose its passivation ability. Electrolyte is electrochemically decomposed on the damaged and naked electrode surface, leaving new films on the electrode. This process leads to capacity fading and shortened cell life. Compared to surface films of negative electrodes, these studies are insufficient for surface films of positive electrodes. <br/> Based on the discussion above, the objectives of this study are to reveal the degradation of surface film on LiCoO2 electrode at moderately elevated temperature and its effect on cell properties, as one of the failure mechanism of LiCoO2 electrode, and to suggest an appropriate countermeasure. (1) In the first part, it is attempted to reveal whether the surface film on the LiCoO2 positive electrode is degraded upon exposure to moderately elevated temperature, if yes, what is the cause, and how the film degradation affects the cell performances. (2) In the second part, when the cell is exposed to elevated temperature at higher State-Of-Charge (SOC), how surface films behave, and how it affects cell performances are studied. (3) Finally, it is tried to suppress degradation of surface film at moderately elevated temperature to mitigate degradation of the cell. To reduce HF in the cell, which is the cause of surface film degradation at the temperature, CuO is added into LiCoO2 electrode as a HF scavenger.<br/> To simulate the high-temperature exposure, Li/LiCoO2 cells were fabricated and cycled to deposit surface films on the LiCoO2 surface, and then stored at moderately elevated temperatures (60 or 70 °C). To investigate surface film degradation, the cells were stored at fully discharged state. To investigate the effect of SOC of the cell, they were stored at somewhat charged state. After the storage, the cells were cycled again at 25 °C to check for signs of cell degradation. CuO-added LiCoO2 electrode and Li/LiCoO2 (CuO-added) cells were fabricated and stored at same conditions with above. Postmortem analyses were performed on the damaged LiCoO2 electrodes. <br/> As a result, (1) in the first part, it was revealed that the surface films on the LiCoO2 positive electrode is degraded at 70 oC, and it is caused by HF attack from LiPF6 salt. After storage, electrolyte oxidation occurs on damaged LiCoO2 surface and charge capacity of Li/LiCoO2 cell increases and Coulombic efficiency decreases. (2) In the second part, it was revealed that when the cell is exposed to elevated temperature at higher SOC, surface films are repaired due to electrolyte oxidation on the degraded LiCoO2 surface. After storage, electrolyte oxidation is suppressed on the repaired surface films due to its passivating ability, while it suffers from self-discharged capacity during high temperature exposure, due to surface film repairing. Repaired surface films are similar to that of before storage. (3) Finally, CuO, a HF scavenger, suppresses degradation of surface films of LCO electrode. Charge capacity after storage, and capacity loss during storage are mitigated. <br/>