Lithium-ion batteries (LIBs) have primarily been developed for application in portable devices, but are now prospering also for hybrid and electric vehicles (xEVs). Safety and life-length at elevated/high temperature (HT) is a critical issue, but HT operation is also an opportunity for simplified commercial vehicle cooling systems (working at ca. 80-110ºC) and faster charge/discharge rates. Conventional organic electrolytes such as 1 M LiPF₆in EC:DEC are volatile and flammable, and thus a safety risk; fires, 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 also have high ionic conductivities.

In this work, we demonstrate that the IL N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (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 in the order of 10^-3 S cm^-1 at RT and 10^-2 S cm^-1 at HT (above 80°C) (Fig. 1a). LiTFSI_0.2Pyr13TFSI_0.8 i.e. an electrolyte with 0.2 mole fraction of LiTFSI shows the most promising electrochemical stability in a half-cell with LiFePO₄ at 80°C. The cell delivers an average discharge capacity of 160 mAh g^-1 at 1C and shows rate capability up to 4C with a ca.25% capacity drop (Fig. 1b).

Additional doping of the IL based electrolytes with organic cyclic carbonate solvents such as propylene carbonate (PC), vinylene carbonate (VC) and fluoroethylene carbonate (FEC) is made to create organic-IL hybrid electrolytes. Although these solvents do not significantly change the total ionic conductivity (Fig. 1a), they improve the cycling performance in terms of capacity, stability and rate capability (Fig. 1b). While phase transitions at ca. 10-15°C are clearly indicated in the conductivity data for both the pure IL and the IL based electrolyte – the doping mitigates this problematic feature. The improvement in cyclability is attributed to a decreased viscosity and a better wetting of the electrode. Both FEC and VC have the additional advantage of forming a stable SEI on the negative electrode, preventing further electrochemical degradation of the electrolytes. The FEC doped electrolyte shows the highest stability and rate capability with only a 5% capacity drop from 1C to 4C (12 and 13% for the VC and PC doped electrolytes, respectively). The capacity at 1C is recovered to 99% after cycling at 4C for all the hybrid electrolytes, but only to 90% for LiTFSI_0.2Pyr13TFSI_0.8. A slight capacity fading is observed at 4C for the PC and VC doped electrolytes, but still these perform better than LiTFSI_0.2Pyr13TFSI_0.8.

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

Fig. 1 (a) Ionic conductivities of the pure Pyr13TFSI IL and the IL and hybrid based electrolytes in the temperature range of -20 to 120°C. (b) Discharge capacity of half-cells of LiFePO₄ at 80°C using pure IL and hybrid electrolytes cycled between 2.5-4.2 V vs Li⁺/Li^° at rates of 1-4C.