Degradation Behavior of 21700 Cylindrical Lithium-Ion Cells during over-Discharge Cycling at Low Temperature

In case of electric vehicles (EVs) and energy storage systems (ESSs), multiple lithium-ion battery (LIB) cells are connected in series or parallel to supply the required capacity and power of appliances. However, there is inevitably a small difference in capacity among mass-produced cells. Furthermore, all the cells cannot degrade at the same rate in long-term usage. Thus, relatively more degraded cells in the module or pack can be over-charged or over-discharged, which can trigger the thermal runaway of LIBs. In this sense, many research groups have investigated the behaviors of over-charged cells with experiments and simulation works. However, the over-discharge was not dealt as much as the over-charge, even though the over-discharge can frequently occur and must be one of the electrical abuses. Especially, most of the over-discharging studies have been limited to room-temperature degradations like Cu dissolution, SEI decomposition, etc. However, considering the wider operating temperature of EVs and other applications, it is also necessary to investigate this over-discharging behavior at various temperatures. Thus, in this work, we reveal how the operating temperature (-20 ℃, 25 ℃) and over-discharge cycling influence the degradation behavior of 21700 LIB cells through both electrochemical analysis (EIS and DC-IR) and postmortem analysis (SEM, EDS, XPS, and XRD). Furthermore, we constructed a thermo-electrochemical model applicable to the over-discharge condition (0 V cut-off at -20 ℃ and 25 ℃). Thus, we can systematically study the abnormal thermal behavior of over-discharged cells at the low temperature. This result will be a good case study how the over-discharge affects the LIB cells at low temperatures.