Quasi-solid-state polymer plastic crystal electrolyte for subzero lithium-ion batteries

Succinonitrile（SN）-based polymer plastic crystal electrolytes（PPCEs） have attracted considerable attention as solid-state electrolytes owing to their high ionic conductivities similar to those of liquid electrolytes, excellent contacts with the electrodes, and good mechanic properties. As a crucial property of a solid-state electrolyte, the ionic conductivity of the PPCE directly depends on the interactions between the constituent parts including the polymer, lithium salt, and SN. A few studies have focused on the effects of polymer–lithium–salt and polymer–SN interactions on the PPCE ionic conductivity. Nevertheless, the impact of the lithium–salt–SN combination on the PPCE ionic conductivity has not been analyzed. In particular, tuning of the lithium-salt–SN interaction to fabricate a subzero PPCE with a high low-temperature ionic conductivity has not been reported. In this study, we design and fabricate five PPCE membranes with different weight ratios of Li N（SO2 CF3）2（Li TFSI） and SN to investigate the effect of the Li TFSI–SN interaction on the PPCE ionic conductivity. The ionic conductivities of the five PPCEs are investigated in the temperature range of –20 to 60 °C by electro-chemical impedance spectroscopy.The interaction is analyzed by Fourier-transform infrared spectroscopy, Raman spectroscopy, and differential scanning calorimetry. The Li TFSI–SN interaction significantly influences the melting point of the PPCE, dissociation of the Li TFSI salt, and thus the PPCE ionic conductivity. By tuning the Li TFSI–SN interaction, a subzero workable PPCE membrane having an excellent low-temperature ionic conductivity(6 × 10-4 S cm–1 at 0 °C) is obtained. The electro-chemical performance of the optimal PPCE is evaluated by using a Li Co O2/PPCE/Li4 Ti5 O12 cell, which confirms the application feasibility of the proposed quasisolid-state electrolyte in subzero workable lithium-ion batteries.