Ionic Liquid Based Electrolytes for High Temperature Lithium-Ion Batteries

Lithium-ion batteries (LIBs) have primarily been developed for application in portable devices, but during the past decade much attention has been paid on making progress also for electric vehicles (EVs). With this aim, elevated/high temperature (HT) operation and life-length is a critical issue, but HT is also an opportunity for simplified cooling systems (working at ca. 80-110ºC) and faster charge/discharge possible. Conventional organic electrolytes such as LiPF₆ in EC:DEC is not a safe option due to high volatility and flammability, ultimately leading to the ease of catching fire, dangerous fumes, and possibly also explosions. Ionic liquid (IL) based electrolytes, on the other hand, are an interesting alternative for HT-LIBs owing to negligible vapor pressures, non-flammability, and high ionic conductivities.

In this work, we demonstrate that the IL Pyr₁₃TFSI with an addition of LiTFSI is a potential electrolyte family for HT-LIBs. These electrolytes possess high thermal stability, up to 350°C, wide electrochemical stability windows, 0-5 V vs Li⁺/Li^°, and ionic conductivities of the order of 10^-3 S cm^-1 at RT and 10^-2 S cm^-1 at HT (above 80°C). The LiTFSI_0.2Pyr₁₃TFSI_0.8 electrolyte shows promising electrochemical stability in a LFP half-cell at 80°C delivering an average discharge capacity of 145 mAh g^-1 at 1C (Fig.1). Additional doping of the IL based electrolytes with cyclic carbonates such as propylene carbonate (PC) and/or vinylene carbonate (VC), creating hybrid electrolytes, further improves the cycling performance in terms of capacity, stability and Coulombic efficiency. This is attributed mainly to a decrease in viscosity, leading to higher ionic conductivities and a better wetting of the electrode, and VC has the additional advantage of forming a stable SEI.

High Temperature Lithium-ion Batteries is a project funded by the Swedish Energy Agency and the collaboration between Chalmers University of Technology, Lund University, Uppsala University and Scania CV AB was created within the Swedish Hybrid Vehicle Centre.

Fig. 1 Specific capacity of LFP half-cells using IL based electrolytes cycled between 2.5 to 4.2 V vs Li⁺/Li^° at 80°C and 1C rate. The open and closed circles represent the charge and discharge capacities, respectively.