Researchers from Xi’an Jiaotong University in China have developed a new ionic liquid-based electrolyte (ILE) that exhibits high conductivity that is twice that of pure ILE and also constructs a LiF-rich solid electrolyte interphase (SEI) for electrochemical stability.

A paper on their work is published in Journal of Power Sources.

Commercial lithium-ion batteries commonly use organic carbonate electrolytes which offer advantages such as low viscosity, high ionic conductivity, and good oxidative stability. However, their volatility and flammability can easily lead to localized overheating of battery, which ultimately induces thermal runaway, explosions, combustion and other safety issues under abusive conditions.

Further, as battery developers seek to produce higher energy density batteries, the adoption of lithium-containing high-energy-density anode and high-voltage cathode is crucial. However, organic carbonate electrolyte always present poor compatibility with high-energy-density anode, which seriously hinders the development of high-energy-density batteries.

Ionic liquids (ILs) with a series of desirable properties such as non-flammability, negligible vapor pressure, wide electrochemical window and high electrochemical stability present the possibility of realizing lithium metal batteries (LMBs) with both high-energy-density and high safety. Nevertheless, their relatively high viscosity and low ionic conductivity limit the commercialization of ionic liquid-based electrolytes (ILEs) in LMBs.

… in this work, a new ILE with high ionic conductivity and high electrochemical stability was designed by employing ethyl EMC and FEC to facilitate the movement of Li⁺ and develop a stable SEI. The new ILE was prepared by dissolving 1M LiFSI in a mixture of N-methyl-N-propylpyrrolidinium bis(fluorosulfonyl)imide (Pyr₁₃FSI), EMC and FEC with a volume ratio of 3:4.9:2.1 (denoted as EF).

The addition of carbonate reagents remarkably reduces the viscosity, making the ionic conductivity of EF system be about twice as high as in pure IL system. Furthermore, unlike the solvating properties of other carbonate reagents, FEC with a certain fluorination partially solvates Li⁺, increasing the coordination between Li⁺ and FSI⁻ indirectly.

Meanwhile, the solvated FEC and FSI₋ decompose synergistically, producing a deeper decomposition, thereby generating a robust inorganic LiF-based SEI on lithium metal surface, and consequently achieving an excellent lithium stripping/plating Coulombic efficiency (CE) of 96.3 % at 250 cycles. Furthermore, the superior compatibility toward LiFePO₄ (LFP) cathodes in EF electrolyte results in a highly stable cycling of Li/LFP for up to 150 cycles with a capacity retention of 93.2 %.

—Xue et al.

Resources

• Sa Xue, Yongqi Liu, Kun Hou, Maogang He, Xiangyang Liu, “A new ionic liquid-based electrolyte with both high conductivity and electrochemical stability”, Journal of Power Sources, Volume 615, 2024, doi: 10.1016/j.jpowsour.2024.235097