A novel fluorine-free lithium salt derived from malononitrile for electrolyte applications in liquid and solid-state batteries

Abstract

The transition to clean energy heavily relies on lithium-ion batteries, known for their high energy density and long lifespan. However, the use of fluorinated salts such as LiPF₆ and LiTFSI in commercial cells raises environmental concerns, leading to increased efforts to develop fluorine-free alternatives. In this study, we present the synthesis of lithium (1-naphthalenesulfonyl)-dicyanomethide (LiNPDM), a novel fluorine-free salt containing a dicyanomethide moiety and a naphthalene group, obtained in three steps from naphthalene sulfonate. For comparison, its trifluoromethanesulfonimide analogue (LiNPTFSI) was also prepared. The electrochemical performance of both salts were studied in conventional liquid electrolyte solvents (EC:DMC) and polycarbonate-based solid polymer electrolytes (SPEs). Although LiNPDM shows a narrower electrochemical stability window, attributed to cyano group oxidation at 4.0 V vs Li⁺/Li, its conductivity performance matched those of its fluorinated analogue LiNPTFSI, both in conventional liquid electrolytes (3.5 mS∙cm^‑1) and SPEs (∼ 1 × 10^‑6 S∙cm^‑1 at 80 °C). These results suggest that the dicyanomethide functional group is a promising fluorine-free alternative to the conventional trifluoromethylsulfonimide group.