3/23/2023 0 Comments Anode vs cathode![]() ![]() Until the voltage is less than 0.2 V, the framework structures of LiMn2O4 are deteriorated, and further overlithiation caused decomposition into Li2MnO2 and Li2O. As shown by X-ray diffraction and X-ray photoelectron spectroscopy with Ar-ion etching, the new phase, Li2Mn2O4, appears starting from 2.5 V. The study shows that LiMn2O4 and LiFePO4 still maintain good crystal morphology during the deep over-discharge process, showing better over-discharge resistance capability with different overlithiation mechanisms. The cells were dismantled to study the electrode structure, surface morphology, and compositional changes. In this study, both LiMn2O4 and LiFePO4 cathode materials at different over-discharge conditions were tested using half cell (vs. Li) application, the overlithiation mechanism of the different cathode materials is worthy to investigate. ![]() Positive and negative materials were attached on both sides of the separator, which reduced the effective area of ionic transportation. 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. 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. Regarding the cyclic aging of the 50.9% SOH battery, the fusion temperature of the separator decreased by 22.3 ☌, indicating a substantial degradation of the separator and thus reducing battery safety. However, severe thermal runaway occurred in the 79.6% and 50.9% SOH batteries. The battery without high-rate aging and over-discharge did not experience thermal runaway. 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 micro short-circuit was found by the discharge voltage curve and impedance spectrum. Additionally, the temperature sharply increased in the over-discharge stage. By setting the end-of-discharge of 1 V, the battery capacity rapidly decayed after 130 cycles. ![]() The aging behavior of cylindrical NCM811 batteries under high-rate aging and over-discharge was studied. Although deep over-discharge can be avoided by optimizing the battery control system, slight over-discharge still often occurs in the battery pack. Inconsistencies in a monomer battery pack can lead to the over-discharge of a single battery. ![]()
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