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Inconsistencies in a monomer battery pack can lead to the over-discharge of a single battery. Although deep over-discharge can be avoided by optimizing the battery control system, slight over-discharge still often occurs in the battery pack. The aging behavior of cylindrical NCM811 batteries under high-rate aging and over-discharge was studied. By setting the end-of-discharge of 1 V, the battery capacity rapidly decayed after 130 cycles. Additionally, the temperature sharply increased in the over-discharge stage. The micro short-circuit was found by the discharge voltage curve and impedance spectrum. Batteries with 100%, 79.6% and 50.9% SOH (state of health = Q_now/Q_new × 100%) as a result of high-rate aging and over-discharging were subjected to thermal testing in an adiabatic environment. The battery without high-rate aging and over-discharge did not experience thermal runaway. However, severe thermal runaway occurred in the 79.6% and 50.9% SOH batteries. Regarding the cyclic aging of the 50.9% SOH battery, the fusion temperature of the separator decreased by 22.3 °C, indicating a substantial degradation of the separator and thus reducing battery safety. Moreover, the results of scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses revealed that the particles of the positive material were broken and detached, and that large-area cracks and delamination had formed on the negative material. Furthermore, Ni deposition and the uneven deposition of P and F on the negative surface were observed, which increased the risk of short-circuit in the battery. Positive and negative materials were attached on both sides of the separator, which reduced the effective area of ionic transportation.

Effect of High-Rate Cycle Aging and Over-Discharge on NCM811 (LiNi0.8Co0.1Mn0.1O2) Batteries