Fluorine-Free Diluents for High-Performance Lithium Metal Batteries

With the increasing demand in electric vehicles and grid-scale energy storage, further increasing the energy density and improving the sustainability of batteries are needed. Compared to lithium-ion batteries, lithium metal batteries (LMBs) using lithium metal anodes (LMAs) are more promising due to their high theoretical specific capacity (3,860 mAh g -1 ) and high specific energy (>500 Wh kg -1 ) [1]. However, the high reactivity of LMAs causes the uncontrollable side reactions with electrolytes,which results in unstable solid electrolyte interphase (SEI) formation, low Coulombic efficiency and poor cycling stability.Thus, electrolyte design is crucial to enable reversible lithium plating and stripping, form a robust anion-rich SEI, and promote stable cycling of LMBs [2]. Among different electrolyte design strategies, localized high concentration electrolytes (LHCEs) have unique advantages. The diluents further promote the anion-rich Li + solvation structure formed in HCEs since they have weak Li + solvating ability [3] . However, the diluents are generally hydrofluoroethers, leading to the concerns of the environmental and human health risks [4] . Therefore, developing fluorine-free diluents are important for next-generation electrolyte design, paving a path toward the fluorine-free electrolyte goals. In this presentation, we will discuss fluorine-free diluents for LHCEs, which enable high Coulombic efficiency for LMBs. Conventional diluents can reduce the electron density around the oxygen in ether solvents by the electron-withdrawing effect of fluorine to weaken the Li + -solvent coordination, which promotes the formation of contact ion pairs (CIPs) and aggregates (AGGs) [5] . We will illustrate how the diluents without electron-withdrawing group can work to promote the formation of highly aggregated Li + solvation structure by conducing Raman and Nuclear Magnetic Resonance (NMR). We analyzed the SEI using X-ray Photoelectron Spectroscopy (XPS) and found it to be anion-rich, which is consistent with the aggregated solvation structure. The robust SEI formed by these electrolytes facilitates uniform and compact lithium deposition, confirming by top-view and cross-section Scanning Electron Microscope (SEM). We observed enhanced cycling stability for Li||LFP full cells using these electrolytes, showing their excellent application potential. Ultimately, this study provides new insights into the fluorine-free diluent design for LMB electrolytes. This will guide the fluorine-free electrolyte design for achieving high Coulombic efficiency and promoting practical application for LMBs. References: [1] Zhao, Yan, et al. "Fluorinated ether electrolyte with controlled solvation structure for high voltage lithium metal batteries." Nature Communications 13.1 (2022): 2575. [2] Wan, Hongli, Jijian Xu, and Chunsheng Wang. "Designing electrolytes and interphases for high-energy lithium batteries." Nature Reviews Chemistry 8.1 (2024): 30-44. [3] Cao, Xia, et al. "Localized high-concentration electrolytes for lithium batteries." Journal of The Electrochemical Society 168.1 (2021): 010522. [4] Ramesh, Sapna L., and Jeffrey Lopez. "Fluorine-Free Electrolytes for Lithium Metal Batteries: Challenges and Opportunities in Solvation Structure and Interphase Design." ACS Energy Letters 10 (2025): 1671-1679. [5] Wang, Hansen, et al. "Liquid electrolyte: The nexus of practical lithium metal batteries." Joule 6.3 (2022): 588-616.