Room Temperature Molten Salts (Ionic Liquids) as Electrolytes in Rechargeable Lithium Batteries

Molten salts are ionic, nonflammable, nonvolatile liquids with high ionic conductivity, oxidation voltage greater than 5 V vs. Li and high thermal stability (>300°C). So far the application of molten salts in batteries has been limited to those operating at relatively high temperature (>150°C). Rechargeable lithium-ion and lithium batteries have attained wide acceptance in both commercial and military applications. However, most of the solvents used in these cells are volatile and flammable; hence, they represent a significant safety hazard, especially, when operated at higher temperatures. Therefore the application of molten salts as electrolytes in lithium and lithium-ion cells containing LiMn₂O₄ (cathode) was investigated. The preliminary results show that rechargeable lithium and lithium-ion cells can be constructed and operated using molten salts as electrolytes. Test cells were cycled at ambient temperature and at higher temperature (55°C). They were cathode limited and their capacities ranged from 0.6 to 2 mAh. The open circuit voltage of charged cell was 4.2 V. Charge/discharge efficiencies were high (80%). However, the cells showed an increase of cell resistance with cycling which decreased the charge capacity (voltage limited) and efficient utilization of the cell capacity.