The increasing demand for high-energy-density batteries has led to renewed interest in lithium (Li) metal anodes due to their low electrochemical redox potential and high theoretical specific capacity. However, standard non-aqueous electrolyte solutions in Li-ion batteries are unstable in high-voltage Li metal batteries (LMBs), exhibiting poor stability against both Li metal anodes and high-voltage cathodes. These instabilities (dendritic deposition and electrolyte decomposition) are exacerbated under fast-charging and low-temperature conditions. The ideal electrolyte for such batteries needs high electrode stability, high ionic conductivity across a wide temperature range, low density, and a high boiling point. Current strategies, including high-concentration electrolytes, localized high-concentration electrolytes, additive-regulated electrolytes, and fluorinated electrolytes, aim to address these challenges. Solvent fluorination enhances oxidative stability by decreasing the highest occupied molecular orbital (HOMO) level. While fluorinated carbonate-based electrolytes have been explored, their carbonyl groups are easily reduced by Li metal. Fluorinated ether-based electrolytes offer better compatibility with Li metal but often suffer from reduced ionic conductivity after fluorination, hindering high-rate and low-temperature performance. This is because difluoro and trifluoro groups reduce electron cloud density, weaken dissociation, and show weak coordination with Li cations, leading to ion aggregation and slow transport. This work focuses on designing a fluorinated electrolyte with high ionic conductivity, Li metal cyclability, and oxidative stability for fast-charging, low-temperature LMBs.

Several studies have investigated strategies to improve the performance of lithium metal batteries, particularly at high voltage and low temperatures. High-concentration electrolytes, localized high-concentration electrolytes, and additive-regulated electrolytes have shown promise. The use of fluorinated solvents has also been explored extensively due to their enhanced oxidative stability, offering improved compatibility with high-voltage cathodes such as LiNi0.8Co0.1Mn0.1O2 (NCM811). However, a key challenge remains balancing the improved oxidation stability with the need for high ionic conductivity, especially at low temperatures and high charge/discharge rates. Previous work with trifluoro- and difluoro-substituted ether-based electrolytes demonstrated improved stability with high-voltage cathodes, but a significant reduction in ionic conductivity was observed, ultimately limiting battery performance at high rates and low temperatures. This highlights the need for a novel approach to electrolyte design that can overcome this trade-off between stability and conductivity.

This study designed and synthesized monofluoride bis(2-fluoroethyl) ethers (BFE) as electrolyte solvents. The monofluoro substituent (-CH2F) was chosen to maximize ion conductivity while retaining high oxidation stability compared to difluoro (-CHF2) and trifluoro (-CF3) counterparts. The synthesis involved a one-step nucleophilic substitution reaction under mild conditions. The physicochemical properties of BFE, including density, boiling point, and viscosity, were characterized. Ionic conductivity and Li transference numbers were measured across a wide temperature range (-60 °C to +70 °C) using a conductivity meter. Electrochemical stability was assessed using linear sweep voltammetry (LSV) on Li||Al and Li||Pt cells. The coordination interaction between Li cations and the BFE solvent was investigated using 1H and 19F NMR, 17O NMR, and Raman spectroscopy. Density Functional Theory (DFT) calculations were performed to investigate the electrostatic potentials of fluorinated solvent molecules with different fluorination degrees and the solvation energies. Molecular dynamics (MD) simulations were used to analyze the solvation structure and Li+-solvate distribution. Electrochemical performance was evaluated using Li||Cu and Li||Li symmetric cells to assess Li metal compatibility. Li||NCM811 coin cells and pouch cells were assembled and tested under various conditions (areal capacity, current density, temperature, N/P ratio, and electrolyte amount) to evaluate the performance of the BFE electrolyte in practical scenarios. Ex situ characterization techniques, including scanning electron microscopy (SEM), cryo-transmission electron microscopy (Cryo-TEM), and X-ray photoelectron spectroscopy (XPS), were employed to analyze the morphology and composition of the solid electrolyte interphase (SEI) on the Li metal anode and the cathode electrolyte interphase (CEI) on the NCM811 cathode after cycling.

The BFE solvent exhibited a lower density (0.98 g cm⁻³) and higher boiling point (128 °C) than conventional carbonate and fluorinated solvents. The 2 M LiFSI/BFE electrolyte demonstrated a high bulk ionic conductivity (8 mS cm⁻¹ at 30 °C) and maintained high conductivity across a wide temperature range (-60 °C to +70 °C). 19F NMR analysis revealed strong coordination interactions between the monofluoride groups and Li⁺ cations, leading to an upfield shift of 0.55 ppm after dissolving 1 M LiFSI. DFT calculations and Li NMR confirmed the strong solvation capability of BFE and the formation of a stable five-member-ring molecular structure after coordinating with Li⁺ ions. Molecular dynamics (MD) simulations showed that BFE electrolytes formed small and evenly distributed solvation clusters at both 30 °C and -30 °C, unlike DEE electrolytes which formed large and unevenly distributed clusters. Radical distribution functions (RDFs) showed a higher intensity of Li-OBFE peak compared to Li-OFSI, indicating that most LiFSI are fully dissociated by BFE molecules. The Li||Cu cells using BFE electrolyte exhibited a high average Coulombic efficiency (CE) of 99.75%. Li||Li symmetric cells showed low overpotential and negligible fluctuation during repeated plating/stripping processes. Li||NCM811 coin cells showed high specific capacities at various current densities (up to 17.5 mA cm⁻²) and excellent cycling stability with >80% capacity retention after 300 cycles at 3.5 and 7 mA cm⁻². High-voltage Li||NCM811 coin cells (areal capacity: 3.5 mAh cm⁻², low N/P ratio of 2.8) demonstrated good cycling stability with >90% capacity retention after 200 cycles. SEM images showed smooth and compact Li metal deposits in BFE electrolyte, unlike porous deposits in DME and DEE electrolytes. Cryo-TEM and XPS analyses revealed a dense and uniform SEI layer rich in LiF on the Li metal anode. The NCM811 cathode in the BFE electrolyte showed a uniform CEI layer with a thickness of <1 nm and a LiF-rich composition. Li||NCM811 coin cells performed well at low temperatures (-60 °C to 60 °C), even retaining >45% capacity at -60 °C compared to the DEE electrolyte. A 320 mAh Li||NCM811 pouch cell achieved an initial specific energy of 426 Wh kg⁻¹ and 80% capacity retention after 200 cycles.

The findings demonstrate that the monofluoride ether-based electrolyte significantly improves the performance of lithium metal batteries, particularly at high rates and low temperatures. The high ionic conductivity, enhanced Li metal compatibility, and wide electrochemical stability window are attributed to the unique tridentate coordination chemistry of the BFE solvent, which facilitates the formation of small and well-distributed solvation clusters, promoting fast ion transport. The formation of a stable and uniform SEI layer on the Li metal anode and a thin LiF-rich CEI layer on the NCM811 cathode contributes to improved cycling stability and suppresses side reactions. The successful demonstration of a high-energy-density pouch cell under practical conditions highlights the potential of this electrolyte for next-generation LMBs. The superior performance compared to conventional electrolytes with DME and DEE underscores the importance of the monofluoride substitution strategy in overcoming the trade-off between oxidation stability and ionic conductivity in fluorinated ether electrolytes.

This work successfully designed and synthesized a monofluoride ether-based electrolyte that exhibits high ionic conductivity, exceptional Li metal compatibility, and wide electrochemical stability. The electrolyte enables high-performance lithium metal batteries capable of fast charging and low-temperature operation. The results presented demonstrate the effectiveness of this approach and pave the way for the development of high-energy-density lithium metal batteries for practical applications. Future studies could focus on exploring other monofluorinated ether derivatives with further optimized properties and evaluating their performance in different battery configurations and with various cathode materials.

While this study demonstrates significant advancements in electrolyte design for lithium metal batteries, some limitations exist. The synthesis of the BFE solvent might require optimization for large-scale production. The long-term stability of the electrolyte under extreme conditions (e.g., extended cycling at very high rates or very low temperatures) requires further investigation. The study primarily focuses on NCM811 cathodes; further research is needed to evaluate compatibility with other cathode materials.