Molecular anchoring of free solvents for high-voltage and high-safety lithium metal batteries

Lithium metal is an ideal anode material for batteries due to its high specific capacity and low electrochemical potential. However, the development of high-voltage lithium metal batteries (>4.0 V vs Li+/Li) is hindered by the lack of suitable electrolytes. Conventional carbonate electrolytes are highly reactive with the Li anode, leading to continuous Li consumption and dendrite growth. Ether electrolytes offer superior reduction stability, lower viscosity, and faster Li+ transport, but their oxidation stability is limited (<4.0 V), restricting their use with high-voltage cathodes like nickel-rich cathodes. Recent efforts to address the compatibility issue between ether-based electrolytes and high-voltage cathodes include the use of artificial cathode-electrolyte interphases (CEIs) and high-concentration electrolytes (HCEs) or localized high-concentration electrolytes (LHCEs). HCEs and LHCEs minimize the population of free ether molecules through Li+ coordination, but they are still limited by factors such as limited anodic stability, low ion transport kinetics, high salt cost, and poor low-temperature performance. Furthermore, the safety of Li metal batteries using concentrated electrolytes with reactive anions is a concern. Exothermic reactions between the electrolyte and electrode materials can trigger thermal runaway. Therefore, there's a need for ether-based electrolytes with high Li metal Coulombic efficiency (CE), high-voltage stability, and suppressed exothermic reactions to improve safety.

The literature extensively explores strategies to improve the compatibility of ether-based electrolytes with high-voltage cathodes. High-concentration electrolytes (HCEs) and localized high-concentration electrolytes (LHCEs) have been investigated, focusing on minimizing free solvent molecules by maximizing Li+ coordination. The formation of inorganic-rich electrode-electrolyte interphases (particularly LiF) is highlighted as a crucial aspect of this approach, acting as a kinetic barrier against side reactions. However, HCEs and LHCEs suffer from limitations such as reduced anodic stability, slower ion transport, high cost, and poor low-temperature performance. Research also highlights the importance of considering the safety implications of concentrated electrolytes, especially the exothermic reactions that can lead to thermal runaway, even with flame-retardant solvents. The literature emphasizes the need for electrolytes that balance high performance with enhanced safety.

This study introduces a molecular anchoring diluent electrolyte (MADE) to address the limitations of existing approaches. The MADE design utilizes a high molar ratio of an oxidation-resistant hydrofluoroether (TTE) to an ether solvent (DME) with a low concentration of Li bis(fluorosulfonyl)imide (LiFSI) salt. The researchers investigated the thermodynamic interaction between DME and TTE using isothermal titration calorimetry (ITC), nuclear magnetic resonance (NMR), and Fourier-transform infrared spectroscopy (FT-IR). Density functional theory (DFT) calculations were used to understand the interaction between molecules and their oxidation potentials. Electrochemical tests, including linear sweep voltammetry (LSV), cyclic voltammetry (CV), and galvanostatic cycling, were performed using Li||NMC811 full cells to evaluate the oxidation stability and cycling performance of the MADEs at various salt concentrations (MADE-1, MADE-2, MADE-3). The effects of different salts, solvents, and anchoring agents were investigated. Characterization techniques such as scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HR-TEM), X-ray photoelectron spectroscopy (XPS), Raman mapping, and X-ray absorption spectroscopy (XAS) were used to analyze the cathode/electrolyte interface (CEI) and elucidate the mechanism of enhanced stability. Differential scanning calorimetry (DSC) and accelerating rate calorimetry (ARC) were used to assess the thermal stability of the electrolytes and full cells under thermal abuse conditions. Molecular dynamics (MD) simulations were employed to investigate the solvation structures of the MADEs and LHCE, focusing on the coordination of Li+ ions with solvent and anion molecules. DFT calculations were used to determine the oxidation potentials of various complexes identified from MD simulations.

The study found that the MADE design, with its hydrogen-bonding interactions between the anchoring solvent (TTE) and the ether solvent (DME), significantly enhances the oxidation stability of ether electrolytes. The oxidation onset potential for MADE-1 was >4.7 V, considerably higher than that of LHCE (~4.6 V). This enhanced stability was observed across various solvents and salts. The MADEs exhibited improved high-voltage stability in Li||NMC811 full cells, showing highly reversible charge-discharge cycling at 4.7 V cut-off voltage, unlike electrolytes with higher salt concentrations. Characterization of cycled cathodes revealed that MADEs resulted in thinner and more uniform CEI layers compared to LHCE. XPS analysis showed a lower accumulation of N-Ox/S-Ox species in MADEs, indicating less anion decomposition. XAS measurements supported the formation of a thin CEI layer in MADEs and showed evidence of partial oxidation of LiFSI in LHCE. Ex-situ NMR confirmed anion decomposition in LHCE during cycling. DFT calculations indicated that the DME-TTE complex had a much higher oxidation potential (5.739 V) than the DME-FSI complex (5.002 V), explaining the enhanced stability in MADEs. The improved stability stems from the shift in oxidation potential due to hydrogen bonding, rather than solely from the protective CEI layer. Li||Cu half-cell tests showed that MADEs achieved high Coulombic efficiencies (>99%) for Li plating/stripping, much higher than the DE (87.8%). XPS analysis of the SEI layer formed on Li metal in MADEs showed a higher F/S ratio and more organic C species, leading to a more flexible SEI. AFM measurements revealed that MADEs yielded a less rigid SEI than LHCE. DSC and ARC tests showed that MADEs significantly suppressed exothermic reactions between the electrolyte and both Li metal and the charged cathode under thermal abuse conditions, raising the thermal runaway temperature from 141 °C for LHCE to 209 °C for MADE-1.

The findings demonstrate the effectiveness of the molecular anchoring approach in improving the electrochemical stability and safety of ether-based electrolytes for high-voltage lithium metal batteries. The hydrogen-bonding interactions between the anchoring solvent and the ether solvent fundamentally alter the electrolyte's electrochemical properties, overcoming the voltage limitations of dilute ether-based electrolytes. Unlike HCEs and LHCEs, which rely on anion decomposition for interface passivation, MADEs achieve stability through thermodynamic stabilization of the solvent-anchor complex. The improved performance is attributed to the suppressed reactivity of both the solvent and the anion at the electrode interfaces, resulting in thinner and more uniform CEI and SEI layers. The less rigid SEI layer is crucial for accommodating volume changes during Li plating/stripping, leading to higher Coulombic efficiency. The significantly increased thermal runaway temperature underlines the enhanced safety profile of MADEs.

This study introduces a novel molecular anchoring approach for designing high-voltage and high-safety electrolytes for lithium metal batteries. The MADEs, characterized by hydrogen bonding interactions between a hydrofluoroether and an ether solvent, exhibit significantly enhanced oxidation stability, high Coulombic efficiency for lithium plating/stripping, and increased thermal stability. This approach offers a new perspective for developing next-generation high-energy-density and high-safety lithium metal batteries. Further optimization of the MADE design is warranted to address challenges related to long-term cycling performance and practical battery applications.

While the study demonstrates the significant advantages of the MADE design, some limitations should be noted. The extremely low ion concentration in MADE-1 might limit its rate capability. Further work is needed to optimize the electrolyte composition to balance high ionic conductivity with the benefits of the molecular anchoring approach. The long-term cycling stability of MADEs under practical operating conditions also requires further investigation.