Imidazolium-organic solvent mixtures as electrolytes for lithium batteries

γ-Butyrolactone (BL) has been mixed to the room temperature ionic liquid (RTIL) 1-butyl 3-methyl-imidazolium tetrafluoroborate (BMIBF4) (ratio: 3/2, v/v) in the presence of lithium tetrafluoroborate (LiBF4) for use as electrolyte in lithium-ion batteries. This mixture exhibits a larger thermal stability than the reference electrolyte EC/DEC/DMC (2/2/1) + LiPF6 (1 M) and can be considered as a new RTIL as no free BL molecules are present in the liquid phase. The cycling ability of this electrolyte has been investigated at a graphite, a titanate oxide (Li4Ti5O12) and a cobalt oxide (LixCoO2) electrodes. The ionic liquid is strongly reduced at the graphite electrode near 1 V and leads to the formation of a blocking film, which prevents any further cycling. The titanate oxide electrode can be cycled with a high capacity without any significant fading. Cycling of the positive cobalt oxide electrode was unsuccessfully owing to an oxidation reaction at the electrode surface, which prevents the intercalation or de-intercalation of Li ions in and from the host material. Less reactive cathode material than cobalt oxide must be employed with this RTIL.