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

Abstract

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 bis-trifluoromethylsulfonyl 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, P1113, 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°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.