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.
Recently, fire and explosion accidents associated with lithium ion battery failure occurred frequently. Safety has become one of the main constraints on the wide application of lithium ion batteries in the field of electric vehicles (EVs). By using a simultaneous thermal analyzer (STA8000) and accelerating rate calorimetry (ARC), we studied the thermal stability of high nickel battery materials and the high temperature thermal runaway of the battery, combining the two experimental results to analyze the battery thermal runaway process. We studied the temperature difference between inside and outside during thermal runaway by arranging two temperature sensors inside and outside the battery. The chemical reactions of the battery at high temperature through the thermal performance of the anode, cathode, and separator are also revealed. In-depth exploration of the occurrence process and the trigger mechanism of thermal runaway of lithium batteries was made. The main findings of the study are as follows: The temperature at which the anode materials begin to decompose is 77.13 °C, caused by decomposition of the solid electrolyte interface and the temperature at which the cathode materials begin to decompose is 227.09 °C. The maximum surface temperature of the battery during thermal runaway is 641.41 °C; and the maximum inside temperature of the battery is 1117.80 °C. The time difference between the maximum temperatures inside and outside the battery is 40 s. The thermal runaway temperature of the battery T c is 228.47 °C, which is mainly contributed by the internal short circuit of the anode and cathode to release Joule heat and the cathode/electrolyte reaction. The maximum temperature of T m is 642.65 °C, which is mainly caused by the reaction between oxygen and electrolyte.
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