Lithium-metal batteries represent a significant advancement in energy storage technology due to their potential for high energy density, which is critical for applications such as electric vehicles and portable electronics. A key component influencing their performance and safety is the electrolyte formulation. Electrolytes in lithium-metal batteries must meet stringent requirements, including high ionic conductivity, wide electrochemical stability, and effective dendrite suppression. Three primary electrolyte systems are employed: liquid, gel, and hybrid electrolytes, each with distinct chemistries and trade-offs.

Liquid electrolytes remain the most widely used due to their high ionic conductivity and excellent electrode wetting properties. Conventional liquid electrolytes consist of lithium salts dissolved in organic carbonate solvents. However, these systems face challenges such as low oxidation stability and flammability. Advanced solvents like fluorinated carbonates have been developed to address these issues. Fluorinated carbonates, such as fluoroethylene carbonate (FEC) and difluoroethylene carbonate (DFEC), exhibit enhanced electrochemical stability and reduced flammability. These solvents form stable solid-electrolyte interphases (SEIs) on lithium-metal anodes, which are crucial for preventing dendrite growth and improving cycle life.

Lithium salts play a pivotal role in determining electrolyte performance. Lithium hexafluorophosphate (LiPF6) has been traditionally used, but its sensitivity to moisture and thermal instability limit its suitability for lithium-metal batteries. Alternatives such as lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) offer superior thermal stability and higher ionic conductivity. LiFSI, in particular, has demonstrated excellent compatibility with lithium-metal anodes, promoting uniform lithium deposition and reducing dendrite formation. However, these salts can be corrosive to aluminum current collectors at high voltages, necessitating careful formulation to mitigate side reactions.

Localized high-concentration electrolytes (LHCEs) represent a breakthrough in liquid electrolyte design. Unlike conventional electrolytes, LHCEs use a small amount of a diluent to reduce viscosity while maintaining a high local concentration of lithium salt around the electrodes. This approach combines the benefits of high-concentration electrolytes—such as improved SEI stability and suppressed dendrite growth—with the practical advantages of lower viscosity and cost. Common diluents include hydrofluoroethers (HFEs), which are non-flammable and electrochemically inert. LHCEs have shown remarkable performance in lithium-metal batteries, enabling high Coulombic efficiency and extended cycle life.

Gel electrolytes offer a middle ground between liquid and solid electrolytes, providing higher mechanical strength while retaining reasonable ionic conductivity. These systems typically consist of a liquid electrolyte immobilized within a polymer matrix, such as poly(ethylene oxide) (PEO) or poly(vinylidene fluoride) (PVDF). Gel electrolytes can suppress dendrite growth more effectively than liquid electrolytes due to their semi-solid nature, which physically hinders lithium dendrite penetration. However, their ionic conductivity is generally lower than that of liquid electrolytes, particularly at room temperature. To address this, researchers have incorporated plasticizers or ceramic fillers to enhance ion transport. For instance, adding nanosized alumina or silica particles can improve mechanical stability without significantly compromising conductivity.

Hybrid electrolyte systems combine multiple components to leverage the advantages of different materials. A common approach involves blending liquid electrolytes with polymer or ceramic phases to achieve synergistic effects. For example, a hybrid electrolyte might consist of a liquid carbonate-based electrolyte infused into a porous polymer membrane. This design enhances mechanical robustness while maintaining high ionic conductivity. Another strategy incorporates ionic liquids into gel electrolytes to improve thermal stability and reduce flammability. Ionic liquids, such as those based on pyrrolidinium or imidazolium cations, exhibit negligible vapor pressure and non-flammability, making them attractive for high-safety applications. However, their high viscosity and cost remain challenges for widespread adoption.

Flammability is a critical concern for lithium-metal batteries, as conventional organic solvents are highly combustible. To mitigate this risk, flame-retardant additives such as phosphates, phosphazenes, and ionic liquids are incorporated into electrolyte formulations. These additives can either chemically interfere with the combustion process or form a protective layer that prevents fire propagation. For instance, trimethyl phosphate (TMP) is an effective flame retardant but can degrade battery performance if used in excessive amounts. Balancing flame resistance with electrochemical performance requires precise optimization of additive concentrations.

Electrochemical stability is another essential criterion for lithium-metal battery electrolytes. The electrolyte must remain stable across a wide voltage range to accommodate high-voltage cathodes such as nickel-manganese-cobalt (NMC) or lithium-rich layered oxides. Decomposition of the electrolyte at high voltages leads to gas generation and capacity fade. To enhance stability, additives like vinylene carbonate (VC) and lithium nitrate (LiNO3) are introduced. These compounds participate in the formation of stable SEI and cathode-electrolyte interphase (CEI) layers, which protect the electrodes from degradation. LiNO3 is particularly effective in ether-based electrolytes, where it promotes uniform lithium deposition and reduces side reactions.

Dendrite suppression remains one of the most challenging aspects of lithium-metal battery development. Dendrites, which are needle-like lithium protrusions, can pierce the separator and cause internal short circuits. Advanced electrolyte formulations aim to promote homogeneous lithium deposition by modifying the SEI composition and properties. For example, electrolytes containing lithium bis(oxalato)borate (LiBOB) or lithium difluoro(oxalato)borate (LiDFOB) facilitate the formation of a robust and flexible SEI layer that adapts to volume changes during cycling. Additionally, solvents with high donor numbers, such as dimethyl sulfoxide (DMSO), can coordinate lithium ions more effectively, reducing ion depletion at the electrode surface and mitigating dendrite growth.

Trade-offs between performance and safety are inherent in electrolyte design. High-concentration electrolytes and LHCEs offer superior stability and dendrite suppression but may increase viscosity and cost. Gel and hybrid electrolytes improve safety through enhanced mechanical properties but often at the expense of ionic conductivity. Flame-retardant additives enhance safety but can introduce unwanted side reactions if not carefully optimized. The choice of electrolyte system depends on the specific application requirements, balancing energy density, cycle life, and safety.

In summary, electrolyte formulations for lithium-metal batteries have evolved significantly to address the challenges of high energy density and safety. Liquid electrolytes with advanced solvents and salts, gel electrolytes with improved mechanical properties, and hybrid systems combining multiple materials each offer unique advantages. Innovations such as LHCEs and flame-retardant additives demonstrate the potential for further improvements in performance and safety. Continued research into optimizing these formulations will be crucial for realizing the full potential of lithium-metal batteries in next-generation energy storage systems.