Physical and Electrochemical Properties of Some Phosphonium-Based Ionic Liquids and the Performance of Their Electrolytes in Lithium-Ion Batteries

In this work, three ionic liquids with two different cations and two different anions: trimethyl propyl phosphonium bis-fluorosulfonyl imide (P 1113 FSI), trimethyl isobutyl phosphonium bis-fluorosulfonyl imide (P 111 i 4 FSI) and trimethyl isobutyl phosphonium bis-trifluoromethylsulfonyl imide (P 111 i 4 TFSI) 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, P 1113 FSI and P 111 i 4 FSI, are liquid at room temperature and show high conductivities and low viscosities, reaching 10.0 mS/cm and 30 cP at room temperature for P 1113 FSI. They also exhibit electrochemical windows higher than 5 V and thermal stability exceeding 300°C. Mixing the ionic liquids with 0.5 M LiPF 6 increases viscosities, lowers conductivities but improves electrochemical cathodic stability. The electrolyte mixtures have been evaluated in graphite/Li half cells, Li/LiFePO 4 and Li/LiMn 1.5 Ni 0.5 O 4 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 LiFePO 4 cathode but fail with the higher voltage LiMn 1.5 Ni 0.5 O 4 cathode.