Ionic liquid (IL) and hybrid IL/organic electrolytes with pyrrolidinium cation based ILs have been investigated for application in high temperature lithium-ion batteries (HT-LIBs). The IL based electrolytes show high thermal stabilities, up to 340°C, ionic conductivities of >5×10−3 Scm−1 at 80°C, and broad electrochemical stability windows: 0–5V vs. Li+/Li°. The performance of LiFePO4 based half-cells at 80°C is promising, delivering ca. 160mAhg−1 at 1C, with a rate capability up to 4C and ca. 98% coulombic efficiency. The creation of hybrid IL/organic electrolytes by adding different organic cyclic carbonate solvents reduces viscosity of the electrolytes by 28% at 80°C, thereby improving the ion transport, and further improves the electrochemical performance; higher stability, better rate capability, and ≥99% coulombic efficiency. Overall, the electrolytes proposed have a potential to be applied in HT-LIBs, a concept with large advantages at the vehicle system level.