Overcharge study in Li₄Ti₅O₁₂ based lithium-ion pouch cell II. Experimental investigation of the degradation mechanism

An Li₄Ti₅O₁₂ || LiNi_1/3 Mn_1/3 Co_1/3 O₂ lithium-ion pouch cell has been subjected to an overcharge early in its cycle-life and kept cycling it up to 1500 cycles afterwards. We report on the non-invasive experimental verifications we conducted to corroborate our initial findings obtained via incremental capacity analysis. First, we used incremental capacity analysis on the negative electrode during overcharge to bring evidence of electrolyte reduction below 1 V vs. Li/Li⁺. Second, we used X-ray computerized tomography (CT scan) to demonstrate that gas bubbles were trapped between layers of positive and negative electrodes, thereby disabling the ionic conduction pathway between the two and causing capacity fade. Third, we administered a massage to the pouch cell to establish that the gas bubbles could be displaced, thereby recovering about 60% of the faded capacity. Finally, we proposed a new approach based on the study of the open-circuit voltage to describe quantitatively the gains of active materials that led to the partial capacity recovery.