Effects of depth of discharge on the electrochemical performance of LiFePO4-graphite pouch cells

Lithium iron phosphate-graphite (LFP-C) batteries are widely used in energy storage and electric vehicles due to their high safety and good cycling stability. However, there is still a lack of in-depth research to investigate the impact of depth of discharge (DOD) on LFP-C pouch cells. In this work, we systematically investigate the influence of DOD (2.5 V and 1.5 V) on the cycling performance of LFP-C pouch cells, and the evolution of the cathode–electrolyte interphase (CEI) and solid electrolyte interphase (SEI) layers. In comparison to a DOD of 2.5 V, the cell with a DOD of 1.5 V exhibits a rapid capacity degradation after 50 cycles. Combined with comprehensive characterizations, the mechanism of battery decay has been revealed. A deeper discharge to 1.5 V results in an increase in the disorder of graphite. Additionally, the organic components in the SEI layer decreases, while the Li₂CO₃, LiF, and Li₃PO₄ inorganic products enrich due to the continued decomposition of electrolyte. Meanwhile, the pouch cells generate a considerable quantity of H₂ and minor CH₄ gases. This study will pave the way to understand the effects of overdischarge on the electrochemical performance of commercial pouch cells and the evolution mechanism of CEI/SEI layers.