Hydrogen-bond-driven high ionic conductivity, Li+ transfer number, and lithium-interface stability of poly(vinylidene fluoride-hexafluoropropylene)-based solid-state electrolytes

Abstract

Poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP)-based solid electrolytes with wide electrochemical windows and high thermal stabilities are promising candidates for use in solid-state lithium batteries. However, the practical applications of these electrolytes are currently hindered by their low room-temperature ionic conductivity, limited Li+ transfer number, and the poor stability of the lithium/electrolyte interface. This study proposes a ‘one stone, three birds’ modification method, wherein a small amount of lithium carboxymethylcellulose (CMC-Li) containing rich hydroxyl groups is added to form abundant hydrogen bonds with numerous electronegative C–F groups in the PVDF-HFP molecular chain and lithium bistrifluoromethane sulfonimide (LiTFSI). The strong connections formed between CMC-Li and PVDF-HFP or TFSI− significantly reduced the crystallinity of PVDF-HFP, promoted the dissociation of LiTFSI, and anchored TFSI−, achieving a synchronous improvement in the ionic conductivity and Li+ transfer number. The hydrogen bonds drove the densification of the membrane surface, improving the lithium/electrolyte compatibility at the interface. The ionic conductivity of the modified electrolytes was 5.1 × 10−4 S cm−1 and the Li + transfer number was 0.72 at room temperature. A Li||Li symmetric cell demonstrated stable cycling over 1000 h at 0.2 mA cm−2, where the initial discharge capacity of an all-solid-state cell was enhanced to 165.1 mAh g−1, with steady cycling for 400 runs. Thus, this study provides a novel and effective method for promoting the practical application of solid-state electrolytes.