In the quest of high voltage electrolytes for lithium batteries, we developed a dinitrile based   electrolyte, using mononitrile as a co-solvent with 1 M LiTFSI salt, which possesses electrochemical stability above 5.5 V under ambient conditions. It offers ionic conductivity in the order of 10⁻³ Scm⁻¹ at 30 ^oC, analogous to the carbonate based electrolytes. However, the mononitriles, propionitrile and butyronitrile, decompose readily with lithium metal and form some dimers, trimer crystals and oligomers/polymers. These by-products are well characterized using NMR and single crystal XRD techniques, and a possible mechanism of the decomposition is demonstrated. To overcome the mononitrile decomposition, 5 wt % of vinylene carbonate is added to the glutronitrile/butyronitrile (6/4 ratio) electrolyte system, and several coin cells are fabricated with this electrolyte system using LiFePO₄ cathode and Li metal anode. With capacity retention of more than 95%, the cells performs more than 100 cycles at 0.5 C charge/discharge rate and also found capable to perform upto 3C rate. The developed electrolyte system may pave way to use nitriles as the lithium battery electrolytes.