Electrolyte formulations play a critical role in enabling stable lithium metal anodes, particularly in addressing challenges such as dendrite formation and low Coulombic efficiency. Lithium metal anodes offer high theoretical capacity and low electrochemical potential, making them attractive for next-generation batteries. However, uncontrolled dendrite growth and parasitic reactions with electrolytes have hindered their practical adoption. Advanced electrolyte systems, including high-concentration salts and ether-based solvents, have emerged as promising solutions to these challenges.

High-concentration electrolytes, often referred to as localized high-concentration electrolytes (LHCEs), have demonstrated significant improvements in stabilizing lithium metal anodes. These formulations typically involve a high molarity of lithium salts, such as lithium bis(fluorosulfonyl)imide (LiFSI), dissolved in ether-based solvents like 1,2-dimethoxyethane (DME) or tetrahydrofuran (THF). The high salt-to-solvent ratio alters the solvation structure, reducing free solvent molecules that would otherwise participate in side reactions with lithium metal. Research has shown that electrolytes with salt concentrations exceeding 3 M can form a robust solid-electrolyte interphase (SEI) layer, which is crucial for suppressing dendrite growth. The SEI layer acts as a physical barrier, preventing further electrolyte decomposition and promoting uniform lithium deposition.

Ether-based solvents are particularly effective in high-concentration electrolytes due to their compatibility with lithium metal. Unlike carbonate-based solvents, which tend to decompose aggressively on lithium surfaces, ethers exhibit higher reduction stability. This property minimizes gas evolution and SEI degradation, leading to improved Coulombic efficiency. For instance, electrolytes combining LiFSI and DME have achieved Coulombic efficiencies exceeding 98% over hundreds of cycles. The solvation structure in these systems also facilitates the formation of a lithium fluoride (LiF)-rich SEI, which is mechanically strong and ionically conductive. LiF-rich interfaces are known to enhance lithium ion transport while blocking electron transfer, further mitigating dendrite formation.

Another approach involves the use of fluorinated solvents or additives to enhance electrolyte stability. Fluorinated compounds, such as fluoroethylene carbonate (FEC) or hydrofluoroethers (HFEs), can improve the electrochemical window of the electrolyte and contribute to a more stable SEI. These additives reduce the reactivity of lithium metal with the electrolyte, leading to fewer side reactions and higher cycling efficiency. Studies have shown that electrolytes containing FEC can achieve Coulombic efficiencies of over 99% in symmetric lithium-lithium cells, indicating minimal parasitic losses.

Dendrite suppression is further addressed through the design of electrolytes with optimized viscosity and ionic conductivity. High-concentration electrolytes often exhibit higher viscosity, which can limit ion mobility. To counteract this, diluents such as bis(2,2,2-trifluoroethyl) ether (BTFE) are introduced to reduce viscosity without compromising the solvation structure. These diluted high-concentration electrolytes maintain the benefits of concentrated systems while improving rate capability. The balance between viscosity and ionic conductivity is critical for ensuring uniform lithium deposition at high current densities. For example, electrolytes with ionic conductivities above 5 mS/cm have demonstrated stable cycling at current densities exceeding 5 mA/cm².

The role of lithium salt anions in electrolyte formulations cannot be overlooked. Anions such as FSI⁻ or bis(trifluoromethanesulfonyl)imide (TFSI⁻) influence the SEI composition and interfacial stability. FSI⁻ anions tend to promote the formation of LiF, while TFSI⁻ anions contribute to organic-rich SEI components. The choice of anion affects the mechanical properties and ionic conductivity of the SEI, impacting long-term cycling performance. Electrolytes with FSI⁻ salts have shown superior dendrite suppression compared to those with TFSI⁻ salts, attributed to the more favorable SEI characteristics.

Coulombic efficiency, a key metric for lithium metal batteries, is heavily influenced by electrolyte design. High Coulombic efficiency indicates minimal lithium loss due to side reactions or dead lithium formation. Electrolytes that enable efficient lithium plating and stripping are essential for practical applications. Advanced formulations have achieved Coulombic efficiencies above 99.5% in laboratory settings, though scaling these results to commercial cells remains a challenge. The interplay between salt concentration, solvent selection, and additive chemistry determines the overall efficiency of the system.

Thermal stability is another consideration for electrolyte formulations targeting lithium metal anodes. Ether-based solvents generally exhibit lower boiling points compared to carbonates, raising concerns about safety under high-temperature conditions. However, the incorporation of flame-retardant additives or ionic liquids can enhance thermal stability without sacrificing electrochemical performance. For instance, phosphazene-based additives have been shown to improve the thermal robustness of ether electrolytes while maintaining high Coulombic efficiency.

Scaling these advanced electrolyte formulations for industrial production requires addressing cost and compatibility with existing manufacturing processes. High-concentration electrolytes often involve expensive lithium salts and fluorinated compounds, which can increase material costs. Research efforts are focused on optimizing formulations to reduce reliance on costly components while maintaining performance. Compatibility with electrode materials and separators is also critical, as inhomogeneous wetting or chemical degradation can compromise cell performance.

In summary, electrolyte formulations based on high-concentration salts and ether-based solvents offer a viable path toward stable lithium metal anodes. By tailoring solvation structures, SEI composition, and interfacial chemistry, these systems address dendrite growth and Coulombic efficiency challenges. Continued advancements in additive chemistry and diluent strategies will further enhance the practicality of lithium metal batteries. The development of cost-effective, scalable electrolytes remains a key focus for enabling next-generation energy storage technologies.