Compatible composite electrolyte membrane Li7La3Zr2O12/SB-PVDF for solid-state lithium ion battery

Organic/inorganic composite solid electrolyte has been a hot topic due to combining the merits of both components. Herein, a series of organic/inorganic composite solid electrolytes were prepared by casting method, in which polyvinylidene fluoride (PVDF) was modified with amphoteric sulfonate betaine (SB) and combined with the different contents (0 %, 10 %, 20 % and 30 %) of Li7La3Zr2O12 (LLZO) and 20 % bis(trifluoromethane) sulphonyl (LiTFSI). The resultant samples were marked as 0%LLZO/SB-PVDF/LiTFSI, 10%LLZO/SB-PVDF/LiTFSI, 20%LLZO/SB-PVDF/LiTFSI and 30%LLZO/SB-PVDF/LiTFSI, respectively. As expected, composite solid electrolyte exhibited the desirable overall performances. Especially, 20%LLZO/SB-PVDF/LiTFSI membrane exhibited high ionic conductivity (1.73 × 10−4 S/cm), wide electrochemical window (5.22 V), high Li+ transference number (tLi+ = 0.67) and high mechanical property, probably due to the synergistic effect of LLZO and amphoteric SB modification. Specifically, the amphoteric SB promoted the dissociation of LiTFSI and endowed PVDF matrix with the ionic conductivity, while LLZO facilitated high ionic conductivity, tLi+ and mechanical property. Resultantly, Li//LiFePO4 cell with 20%LLZO/SB-PVDF/LiTFSI membrane delivered 129.5 mAh/g at 1.0C for 200 cycles and 94.0 % capacity retention, demonstrating high application feasibility in the solid-state lithium ion battery. Furthermore, density functional theory (DFT) calculations showed that the binding energy of the amphoteric SB-Li+ (C11H21NO5S-Li+) system was 80.93 kcal/mol, higher than 75.55 kcal/mol of the LiTFSI system, indicating that the amphoteric SB could indeed promote the dissociation of LiTFSI.