- Sep 19, 2019
- 36° N 115° W
This study summed it up nicely:
With the complex material system used in LIBs, the performance degradation at low temperatures can be attributed to several different sources. First, the low temperature will affect the property of electrolyte. With the decrease of temperature, the viscosity of the electrolyte will increase, which will reduce the ionic conductivity. The internal resistance will subsequently rise due to the increase in the impedance of the directional migration of chemical ions. To counter such effect, electrolytes with low freezing point were explored, and different electrolyte additives were studied , , , , . Bugga et al.  presented a guideline for developing formulations of low temperature electrolytes during their research of LIBs for space applications. The improved electrolytes can be used in an extended operating temperature range (Fig. 2A) and were proved to be effective in aerospace batteries. Li et al.  also reported an optimized electrolyte formulation of 1.0 M LiPF6 in ethylene carbonate(EC)−propylene carbonate(PC)−ethyl methyl carbonate (EMC) (1:1:8 by wt) with 0.05 M CsPF6. Such formulation enabled a capacity retention of 68% for the batteries tested at −40 °C, while the ones with conventional formulation only showed a capacity retention of 20% (Fig. 2B). Specific electrolyte additives, such as lithium difluorophosphate (LiPO2F2), were also proved to be effective in improving the performance of LIBs at low temperature (Fig. 2C) .
The increase of charge-transfer resistance in LIBs is also an important factor that contributes to the performance degradation at low temperatures. The charge-transfer resistance of LiFePO4-based cathodes at −20 °C was reported to be three times higher than that at room temperature . Such high charge-transfer resistance largely affects the kinetics in batteries. The study of LIB performance at low temperatures by Zhang et al.  demonstrated that the charge-transfer resistance significantly increased when the temperature decreased. The charge-transfer resistance of a discharged battery normally is much higher than that of a charged one. Charging a battery at low temperatures is thus more difficult than discharging it. Additionally, performance degradation at low temperatures is also associated with the slow diffusion of lithium ions within electrodes. Such slow down can be countered by altering the electrode materials with low activation energy. For example, Li3V2(PO4)3 (LVP), which has an activation energy of 6.57 kJ mol–1, showed a 200x improvement of apparent chemical diffusion coefficient of lithium ions over LiFePO4 (LFP) with an activation energy of 47.48 kJ mol–1 at −20 °C .
Another typical effect that occurs at low temperatures is lithium plating , , . The cold condition will trigger the polarization of anodes and lead to the approach of the potential of graphite and other carbon based anodes to that of lithium metal, which would slow down the lithium-ion intercalation into the anodes during charging process . The aggregated lithium ions are thus deposited on the surface of the electrodes, which causes the reduction of the battery capacities. Furthermore, the lithium plating exists in the form of dendrite, which may penetrate the separators, and result in the internal short-circuit .