Toward Safer and Sustainable Lithium Metal Batteries: Fluorine‐Free Solid Polymer Electrolytes

Fluorinated compounds have long played a key role in solid‐state lithium metal polymer batteries (SSLMPBs). Yet, growing environmental and safety concerns associated with fluorine have intensified the search for safer and more sustainable alternatives. Cyano‐substituted imidazoles have recently emerged as promising candidates to replace conventional sulfonyl imide‐based anions, offering efficient charge delocalization and the potential to form fluorine‐free anions. In this work, we introduce lithium 2,4,5‐tricyanoimidazolide (LiTIM), a fluorine‐free lithium salt, as a component of a solid polymer electrolyte (SPE) based on poly(ethylene oxide) (PEO) and as a catholyte in LiFePO 4 (LFP)‐based cathodes. To assess the impact of fluorine, LiTIM is compared to its fluorinated analogues: lithium 4,5‐dicyano‐2‐(trifluoromethyl)imidazole (LiTDI) and lithium 4,5‐dicyano‐2‐(pentafluoroethyl)imidazole (LiPDI). In‐depth physicochemical and electrochemical characterization reveals that LiTIM exhibits competitive lithium diffusion coefficient ( D Li+ , at 40 °C, D Li+ (LiTIM) = 5.2 × 10 −9 cm 2 s −1 > D Li+ (LiPDI/PEO) = 1.7 × 10 −9 cm 2 s −1 > D Li + (LiTDI/PEO) = 1.4 × 10 −9 cm 2 s −1 ) and full cell performance (capacity retention at C/5, after 200 cycles (LiTIM/PEO = 92% > LiTDI/PEO = 88% > LiPDI/PEO = 82%)) comparable to that of its fluorinated counterparts. This is primarily attributed to its restricted anion mobility ( D anion (LiTIM/PEO) = 1.1 × 10 −8 cm 2 s −1 < D anion (LiPDI/PEO) = 1.6 × 10 −8 cm 2 s −1 ≈ D anion (LiTDI/PEO) = 1.7 × 10 −8 cm 2 s −1 ) and competitive LiCN‐based solid electrolyte interphase (SEI) layer formation. These findings highlight the viability of LiTIM as a nonfluorinated salt for SSLMPBs, offering a pathway toward more sustainable, environmentally friendly, and safer battery technologies.