In this work, three ionic liquids with two different cations and two different anions: trimethyl propyl phosphonium bis-fluorosulfonyl imide (P1113FSI), trimethyl isobutyl phosphonium bis-fluorosulfonyl imide (P111i4FSI) and trimethyl isobutyl phosphonium bistrifluoromethylsulfonyl imide (P111i4TFSI) have been characterized and evaluated as electrolytes in lithium ion half-cells. It is found that ionic liquids with FSI- anion have superior properties over their TFSI- counterparts and those with the smaller cation, P-1113, have better conductivity and viscosity. The two ionic liquids with FSI anion, P1113FSI and P111i4FSI, are liquid at room temperature and show high conductivities and low viscosities, reaching 10.0 mS/cm and 30 cP at room temperature for P1113FSI. They also exhibit electrochemical windows higher than 5 V and thermal stability exceeding 300 degrees C. Mixing the ionic liquids with 0.5 M LiPF6 increases viscosities, lowers conductivities but improves electrochemical cathodic stability. The electrolyte mixtures have been evaluated in graphite/Li half cells, Li/LiFePO4 and Li/LiMn1.5Ni0.5O4 at C/12 for 100 cycles and at different rates: C/6, C/3, C and 2C for rate capabilities. Battery testing shows that unlike their TFSI- counterparts both ionic liquids with FSI-anion perform well with graphite anode and LiFePO4 cathode but fail with the higher voltage LiMn1.5Ni0.5O4 cathode. (C) The Author(s) 2017. Published by ECS. All rights reserved.