Heat Generation Behavior of Lithium-Ion Battery Operated at Low Temperature
Authors: Okagaki Jun , Ueda Suguru , Nishijima Shun , Kawahara Yohei , Shibata Tsuyoshi , Okada Ken , Saito Yoshiyasu 
Published: December 2024

The demand for lithium-ion batteries (LIBs) used in electric vehicles (EVs) is rapidly increasing. LIBs face a serious problem of lithium deposition at the anode when the battery is rapidly charged, especially under low temperature conditions. The lithium deposition can cause battery to short-circuit. Self-heating at elevated temperature is also one of the factors of thermal runaway of LIBs. However, there have been few reports on heat generation behavior of LIBs with lithium deposition. In this study, the thermal analysis of battery was conducted to understand heat generation behavior at elevated temperature of LIBs after cycling test under harsh conditions for real applications.

The commercial 18650-type LIBs (US18650VTC4, Murata Manufacturing Co.) with a nominal capacity of 2100 mAh were used for thermal analysis. The battery consists of lithium nickel cobalt manganese oxides cathode, graphite anode, and mixed solvent of ethylene carbonate and dimethyl carbonate containing lithium hexafluorophosphate as electrolyte. The capacity of battery was obtained by charge and discharge with 1 C at 25 °C. The degradation test was carried out by cycling at 0 °C with 3 C charge, and 1 C discharge. Here, the ratio of discharge capacity after degradation to initial value before test is referred to as SOHQ. Thermal analysis of battery was carried out using a calvet calorimeter (C80-22, Setaram). The C80-22 is a customized machine based on C80 to measure whole 18650 battery without disassembly. The battery was charged to SOC 100% with 1 C at 25 °C and then enclosed in a stainless steel sample holder under air atmosphere. In the thermal analysis, the temperature was increased from 30 to 130 °C at a rate of 0.025 °C min-1. The pressure in sample holder was also measured during thermal analysis to detect gas evolution from the battery.

The capacity of battery decreased during charge-discharge cycle at 0 °C. The degraded batteries with SOHQ 70%, 85%, and 92% were used for thermal analysis. In the thermal analysis of a fresh battery, corresponding to SOHQ 100%, the heat was gradually generated with increase in temperature. Similar trends were observed in the degraded batteries, and the heat flow becomes greater as the battery degrades. In addition, the abrupt pressure increases in the sample holder occurred during measurement, suggesting the opening of the safety vent in the batteries. The heat generation can be attributed to chemical reaction at lithiated graphite anode [1] and degradation due to operation at low temperature. To evaluate heat generation of lithiated graphite, the thermal analysis was performed on a fresh battery with SOC 0%. The difference between the fresh battery with SOC 100% and 0% corresponds to heat flow attributed to lithiated graphite in anode. Based on the assumption that the capacity loss of degraded battery is dominated by graphite anode, the heat generation of lithiated graphite can be evaluated by multiplying SOHQ by the heat flow obtained above for the fresh batteries. The remaining heat flow by subtracting heat flow of lithiated graphite is related to product materials by degradation reaction at low temperature. The result is shown in Figure 1. The heat flows of SOHQ 85% and 92% increase monotonically with increasing temperature. In the result of SOHQ 70%, an additional heat generation is remarkably observed at 70 °C and above. Furthermore, lithium deposition on graphite anode was confirmed by disassembling battery with SOHQ 70%. Therefore, the rapid increase in heat flow after 70 °C with SOHQ 70% may be related to the deposited lithium. The abrupt pressure rise mentioned above corresponds to gas evolution from the battery, which may result from evaporation of electrolyte solvents and decomposition of SEI and electrolyte components. We found that the onset temperature decreases with decreasing SOHQ, as shown in Figure 1. This indicates that thick and unstable SEI was formed in highly degraded battery, resulting in evolution of large amount of gases through SEI decomposition. Thermal analysis was also carried out on batteries degraded by cycling at 50 °C with 1.5 C charge and 1 C discharge as shown in Figure 1. For the battery with SOHQ 80%, the heat flow is similar to that of a fresh battery up to 110 °C. In addition, for the battery with SOHQ 73%, the additional heat generation behavior was not observed at around 70 °C. These results suggest that the heat generation behavior of degraded LIBs at elevated temperature is not simply related to SOHQ, but is largely influenced by the temperature history.

References

[1] Y. Zheng et al., J. Energy Chem., 69 (2022) 593.

Figure 1