Solid acrylonitrile‐based copolymer electrolytes and their potential application in solid state battery

Abstract Two solid acrylonitrile‐based polymer electrolytes (SPE1‐21 and SPE2‐31) were investigated. SPE1‐21 comprises the acrylonitrile/2‐ethylhexyl acrylate copolymer and 33 wt% lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). In contrast, SPE2‐31 comprises the AN/vinyl acetate (VAC) copolymer and 25 wt% LiTFSI. SPEP1‐21 shows inferior ionic conductivity and transference number to SPE2‐31. However, SPE1‐21 has higher oxidation potential and better interfacial stability against Li metal than SPE2‐31. These SPEs were used as the separator in solid‐state batteries with LiFePO 4 as the cathode and Li foil as the anode. For SPE1‐21 cell, the charge/discharge cycling remained relatively stable up to 223rd cycle, at which point the coulombic efficiency (CE) drops to 95.0% and the corresponding capacity retention is 98.1%. On the other hand, SPE2‐31 cell exhibited very poor cycling performance; the CE rapidly dropped to 55.5% at 10th cycle. This indicates that undesired side reaction occurs during the cycling process, which is attributed to the oxidation reaction of acetate group of the VAC unit in the AN‐VAC copolymer.