High-energy-density energy storage systems are increasingly in demand, driving research into high-voltage (>4.0 V vs. Li+/Li) lithium metal batteries (LMBs). Lithium metal is an ideal anode material due to its high theoretical specific capacity (3860 mAh g⁻¹) and low reduction potential. When paired with high-voltage Ni-rich cathodes, LMBs can achieve nearly double the energy density (400-500 Wh/kg) of conventional graphite-based Li-ion batteries. However, LMBs face challenges due to the thermodynamic instability of lithium metal. Uncontrolled side reactions between the lithium metal anode and the electrolyte lead to the formation of a fragile solid electrolyte interphase (SEI) and its mechanical failure during cycling, resulting in electrolyte and lithium consumption, lithium dendrite growth, "dead" lithium formation, and poor cycle life. Therefore, optimizing electrolyte chemistry to control reactivity, electrochemical stability, ion transport, and solvation ability is crucial for stabilizing the SEI and extending cycle life. Commercial carbonate electrolytes, while effective in graphite anodes, suffer from severe side reactions with lithium metal anodes, leading to dendrite growth and short cycle life. Ether-based electrolytes form larger, flatter lithium grains, improving Coulombic efficiency, but their oxidative stability is limited at typical salt concentrations, restricting their use in high-voltage LMBs. High-concentration electrolytes (HCEs) offer compatibility with both high-voltage cathodes and lithium metal anodes by altering the solvation structure, making anions dominate the solvation sheaths. This reduces HOMO-LUMO gaps, facilitating anion decomposition and the formation of an anion-derived inorganic SEI. However, the high cost and viscosity of HCEs limit their practicality. Hydrofluoroethers (HFEs), while having high anodic stability, poorly dissolve salts and lack ionic conductivity. Their use often requires pairing with other solvents, leading to parasitic reactions. Research on fluorinated ethers as effective lithium-ion solvents remains limited. This study focuses on designing a high-voltage fluorinated ether that combines the redox stability of HFEs with the solvation ability and ionic conductivity of ether-based electrolytes, maintaining a solvent-in-salt solvation structure at standard salt concentrations. The covalent attachment of fluorinated and ether segments is a promising approach, and the spatial arrangement of functional groups is crucial for optimal ionic conductivity, solvation, and redox stability.

Extensive research has explored various electrolyte chemistries to address the challenges of high-voltage lithium metal batteries. Carbonate-based electrolytes are widely used in conventional lithium-ion batteries due to their ability to form a stable SEI layer on the graphite anode. However, their use with lithium metal anodes leads to significant side reactions, dendrite formation, and poor cycle life. Ether-based electrolytes have shown promise in mitigating dendrite formation and enhancing Coulombic efficiency, but their limited oxidative stability restricts their application in high-voltage systems. High-concentration electrolytes (HCEs) have emerged as a potential solution by modifying the solvation structure and promoting the formation of an anion-derived inorganic SEI layer, leading to improved stability. However, HCEs suffer from high viscosity and cost. The use of hydrofluoroethers (HFEs) as diluents has been investigated to enhance the oxidative stability of electrolytes. However, HFEs alone have poor salt solubility and ionic conductivity, and their use often requires additional solvents, potentially leading to new limitations. Previous studies have explored modifications to ether structures to enhance both their oxidative stability and lithium-ion solvation ability. For example, work by Bao et al. demonstrated improved oxidative stability by lengthening alkyl chains and fluorination between oxygen atoms. This highlights the importance of a balanced molecular design that considers both factors simultaneously.

This study introduces a novel fluorinated ether solvent, 2,2-dimethoxy-4-(trifluoromethyl)-1,3-dioxolane (DTDL), synthesized via a one-pot acid-catalyzed condensation of tetramethyl orthocarbonate (TMOC) and 1,1,1-trifluoro-2,3-propanediol (TFPD). The synthesis procedures for TMOC and TFPD are detailed, along with the final synthesis of DTDL, including NMR and mass spectrometry characterization. The electrochemical properties of DTDL-based electrolytes (1 M and 2 M LiFSI-DTDL) were characterized using various techniques. Linear sweep voltammetry (LSV) was employed in Li||Al half-cells to assess oxidation stability. Electrochemical impedance spectroscopy (EIS) was used to determine ionic conductivity and lithium-ion transference numbers (t⁺Li) in Li||Li symmetric cells. The wettability of the electrolytes was evaluated by contact angle measurements on copper foil and separators. The morphology of lithium plating on copper foil was examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The composition and thickness of the solid electrolyte interphase (SEI) layer were analyzed using X-ray photoelectron spectroscopy (XPS). Raman and NMR spectroscopy were utilized to investigate the solvation structure of the electrolytes. Cyclic voltammetry (CV) was performed on Li||Al and Li stainless steel half-cells. Electrochemical characterization of lithium-copper half-cells was performed to determine Coulombic efficiency (CE) over extended cycling periods. Finally, Li|NCM811 full cells were assembled and tested with varying electrolytes to assess cell performance, including capacity retention and voltage profiles. The flammability of the electrolyte was assessed.

The newly synthesized solvent, DTDL, demonstrates significantly improved electrochemical properties compared to conventional electrolytes and other fluorinated ethers. LSV measurements revealed a high oxidation stability of up to 5.5 V vs. Li/Li⁺ for 1 M LiFSI-DTDL, substantially higher than 1 M LiFSI-DME (4.0 V) and 1 M LiPF6-EC-DEC. The lithium-ion transference number (t⁺Li) for 1 M LiFSI-DTDL was determined to be 0.75, much larger than that of 1 M LiFSI-DME (0.39). SEM images showed that DTDL facilitated uniform and dendrite-free lithium plating on the copper foil, unlike DME which resulted in uneven plating and dendrite formation. The Coulombic efficiency (CE) in Li||Cu half-cells was exceptionally high and stable, reaching 99.2% after 500 cycles at 0.5 mA cm⁻² and 0.5 mAh cm⁻² for 1 M LiFSI-DTDL, in contrast to the fluctuating CE observed with 1 M LiFSI-DME. XPS analysis indicated that the SEI layer formed with DTDL was significantly richer in LiF and S-F components than that formed with DME, consistent with an anion-derived inorganic SEI. Li|NCM811 full cells with 2 M LiFSI-DTDL exhibited superior cycling performance, retaining 84% of their initial capacity after 200 cycles at 0.5 C, significantly outperforming cells with 1 M LiPF6-EC-DEC. Raman and NMR spectroscopy provided evidence of the formation of contact ion pairs (CIP) and aggregates (AGG) in the DTDL-based electrolytes, particularly at higher salt concentrations, suggesting a unique solvation structure that promotes the formation of a stable SEI layer. The flammability test shows that 2 M LiFSI-DTDL has better flame-retardant properties compared to 1 M LiPF6-EC-DEC.

The superior performance of DTDL-based electrolytes can be attributed to its unique molecular design and resulting solvation structure. The combination of a cyclic fluorinated ether and a linear ether segment in DTDL allows for both high oxidative stability and sufficient lithium-ion solvation ability. The formation of CIP and AGG clusters at relatively low salt concentrations is unusual and contributes to the formation of a stable, inorganic-rich SEI layer, which is crucial for preventing dendrite formation and ensuring long-term cycling stability. The high lithium-ion transference number further enhances the performance by facilitating efficient lithium-ion transport. The high voltage stability, excellent CE, and improved cycling performance of the Li|NCM811 full cells with 2 M LiFSI-DTDL demonstrate the potential of this new electrolyte for practical high-voltage LMB applications. The improved homogeneity and inorganic-rich nature of the SEI layer formed with DTDL, as evidenced by XPS, are consistent with the enhanced performance and stability. The weaker decomposition of the solvent at higher salt concentrations further suggests improved compatibility with the cathode material. The findings of this study contribute significantly to the ongoing efforts to develop advanced electrolytes for high-performance LMBs.

This research successfully demonstrated the synthesis and application of a novel fluorinated ether solvent, DTDL, for high-voltage lithium metal batteries. DTDL's unique molecular structure allows for a controlled solvation structure that promotes the formation of a stable, inorganic-rich SEI layer, leading to superior electrochemical performance, including high oxidative stability, high lithium-ion transference number, and excellent cycling stability in both half-cell and full-cell configurations. This approach offers a promising pathway for the development of advanced electrolytes for next-generation high-energy-density batteries. Future research could explore variations of the DTDL structure to further optimize its properties and investigate its compatibility with other high-voltage cathode materials.

While this study demonstrated excellent performance of the DTDL-based electrolytes, further research is needed to fully explore its long-term stability under various operating conditions, including different temperatures and charge/discharge rates. The scalability and cost-effectiveness of the synthesis process should also be evaluated for practical applications. Investigating the electrolyte’s performance with other high-nickel cathode materials and at higher areal capacities would further validate its applicability.