Electrolytes optimized for low-temperature operation are critical for ensuring battery performance in cold climates, electric vehicles, aerospace applications, and other environments where sub-zero conditions are prevalent. Conventional lithium-ion batteries suffer from reduced ionic conductivity, increased interfacial resistance, and electrolyte freezing at temperatures below −20°C. To address these challenges, specialized electrolyte formulations incorporate solvent blends, low-viscosity salts, and functional additives that maintain electrochemical stability and ion transport efficiency in extreme cold.
**Solvent Blends for Low-Temperature Performance**
The choice of solvents significantly influences electrolyte behavior in low-temperature conditions. Carbonate-based solvents like ethylene carbonate (EC) and dimethyl carbonate (DMC), commonly used in standard electrolytes, exhibit high viscosity and freezing points below −20°C, leading to poor ionic mobility. To mitigate this, low-freezing-point solvents such as esters (e.g., methyl acetate, ethyl acetate) and sulfones (e.g., sulfolane) are blended to reduce viscosity and extend the liquid range.
Esters demonstrate particularly favorable properties, with methyl acetate having a freezing point of −98°C and a viscosity of 0.37 cP at 25°C, compared to EC’s viscosity of 1.9 cP. Sulfolane, while more viscous, provides superior oxidative stability, making it useful in hybrid solvent systems. Optimal formulations often combine esters with small amounts of carbonates to balance low-temperature fluidity with sufficient SEI-forming capability. For instance, a ternary blend of ethyl acetate, methyl acetate, and fluoroethylene carbonate (FEC) has been shown to maintain conductivity above 2 mS/cm at −40°C.
**Low-Viscosity Lithium Salts**
The lithium salt selection is equally crucial. Lithium hexafluorophosphate (LiPF6), the industry standard, suffers from dissociation issues and increased resistance at low temperatures. Alternatives such as lithium bis(fluorosulfonyl)imide (LiFSI) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) offer lower viscosity and better dissociation in cold environments. LiFSI, in particular, has demonstrated superior ionic conductivity in ester-based electrolytes, with reported values of 1.8 mS/cm at −30°C in ethyl acetate-based solutions.
However, these salts present challenges, including aluminum current collector corrosion (LiTFSI) and higher cost. Recent studies explore dual-salt systems, such as LiFSI-LiPF6 mixtures, which balance cost, conductivity, and corrosion inhibition.
**Additives for Freezing Suppression and Interfacial Stability**
Additives play a key role in preventing freezing and reducing interfacial resistance. Film-forming additives like FEC and vinylene carbonate (VC) enhance SEI stability, preventing excessive charge transfer resistance buildup at low temperatures. FEC also lowers the freezing point of the electrolyte due to its low melting temperature (−20°C).
Cosolvents such as 1,3-dioxolane (DOL) are incorporated to further depress freezing while improving wetting properties. Additionally, small quantities of lithium nitrate (LiNO3) have been found to stabilize the lithium metal interface in sub-zero conditions, relevant for lithium-metal and anode-free batteries.
**Performance Metrics and Limitations**
Low-temperature electrolytes must meet several performance benchmarks:
- Ionic conductivity >1 mS/cm at −30°C
- Charge transfer resistance below 100 Ω·cm² at −20°C
- Capacity retention >80% after 100 cycles at −30°C
While optimized formulations achieve these metrics, trade-offs exist. Ester-based electrolytes often exhibit reduced oxidative stability, limiting their use in high-voltage cathodes (>4.3 V). Sulfolane-containing blends improve high-voltage compatibility but increase viscosity. Furthermore, SEI formation at low temperatures tends to be less uniform, leading to accelerated degradation in some cases.
**Conclusion**
Electrolytes designed for low-temperature operation rely on carefully balanced solvent blends, advanced lithium salts, and targeted additives to overcome the inherent limitations of conventional systems. While significant progress has been made, challenges remain in achieving wide electrochemical stability windows and long-term cycling performance. Future developments may focus on novel solvent-salt-additive combinations or solid-state hybrid systems to further push the boundaries of low-temperature battery operation.