Revisiting the overdischarge process as a novel accelerated aging method for LiFePO4/Graphite batteries through the unveiling of SEI evolution mechanism

The exceptional cycling stability of lithium-ion batteries in electric vehicles and large-scale grid energy storage applications necessitates the use of accelerated aging tests for rapid assessment. Overdischarge stress is an effective approach to accelerate battery aging, whereas its impact on solid electrolyte interphase (SEI) and battery aging performance remains elusive. Herein, the whole picture of SEI evolution under different overdischarge levels was quantitatively illustrated by combining the electrochemical analysis and spectrochemical techniques. Overdischarge leads to the decomposition of the organic components within SEI, such as ROCO2Li and CH3Li, while the damaged SEI is repaired during the subsequent charging process with its composition and structure reconstructed. Under overdischarge conditions, the SEI undergoes continuous cycles of destruction and repair, which suppresses its growth and evolution to inorganic components, resulting in a thinner and more uneven morphology with higher organic components and a lower Young's modulus. The unique SEI evolution mechanism of overdischarge effectively accelerates the loss of active lithium and exhibits similar thermodynamic degradation modes to normal aging, making overdischarge a potential accelerated aging method. This study provides a deeper understanding of the mechanisms behind accelerated aging in batteries and offers new insights into the evaluation and enhancement of battery performance.