This paper reports on the synthesis and characterization of novel dicationic ionic liquid electrolytes based on asymmetrical imidazolium-trialkylammonium ionic liquids. The results obtained indicate that the length of the alkyl chain attached to the nitrogen atom has a significant influence on the thermal and electrochemical properties of the electrolytes, as well as on parameters such as lithium ion coordination number and lithium ion transport number. All synthesized dicationic ionic liquids and the corresponding electrolytes are liquid at room temperature being thermally stable up to 300 °C; well above battery operation temperature. Room temperature ionic conductivities of the electrolytes are in the range of 0.8–7.2 × 10−5 S cm−1, with an electrochemical stability window of 1.5 V–5.0 V vs. Li/Li+. Synthesized electrolytes have been tested in Li half-cells using LiMn2O4 (LMO) spinel as cathode material. Cyclic voltammetry, rate capability and galvanostatic cycling studies at 60 °C have been performed. Again, the alkyl chain length attached to the nitrogen atom plays a significantly role in the electrochemical performance of the LMO cells. Among the cells studied, the one assembled with the electrolyte 1M LITFSI doped IMI1,6-TEA exhibits the best rate capability and the high cycling performance.