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

Abstract

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 LiFePO₄ (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 cm² s⁻¹ > D_Li+ (LiPDI/PEO) = 1.7 × 10⁻⁹ cm² s⁻¹ > D_Li⁺ (LiTDI/PEO) = 1.4 × 10⁻⁹ cm² 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 cm² s⁻¹ < D_anion (LiPDI/PEO) = 1.6 × 10⁻⁸ cm² s⁻¹ ≈ D_anion (LiTDI/PEO) = 1.7 × 10⁻⁸ cm² 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.

Keywords

fluorine-free lithium salt / lithium metal batteries / solid polymer electrolytes / sustainability

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