Multi-functional Electrolyte for Li-Ion Batteries



The high thermal conduction resistances of lithium-ion batteries severely limits the effectiveness of conventional external thermal management systems. To remove heat from the insulated interior portions of the cell, a large temperature gradient is required across the cell, and the center of the electrode stack can exceed the thermal runaway onset temperature even under normal cycling conditions. One potential solution is to remove heat locally inside the cell by evaporating a volatile component of the electrolyte. In this system, a high vapor pressure co-solvent evaporates at low temperature prior to triggering thermal runaway. The vapor generated is transported to the skin of the cell, where it is condensed and transported back to the internal portion of the cell via surface tension forces. For this system to function, a co-solvent that has a boiling point below the thermal runaway onset temperature must also allow the cell to function under normal operating conditions. Low boiling point hydrofluoroethers (HFE) were first used by Arai to reduce LIB electrolyte flash points, and have been proven to be compatible with LIB chemistry. In the present study, HFE-7000 and ethyl methyl carbonate (EMC) are used to solvate 1.0 M LiTFSI to produce a candidate electrolyte for the proposed cooling system. Lithium titanate oxide (Li4Ti5O12), copper antimonide (Cu2Sb), and lithium iron phosphate (LiFePO4) are used in half and full cells with the candidate electrolyte for cycling and electrochemical impedance spectroscopy tests, and testing results show similar performance characteristics as compared with a conventional carbonate-only electrolyte (1.0 M LiPF6 in 3:7 ethylene carbonate/diethyl carbonate). The same battery active materials are evaluated in a custom electrolyte boiling facility to evaluate electrochemical performance, and test results show that full electrochemical cells operate similarly even when a portion of the more volatile HFE-7000 is continuously evaporated.