Nonaqueous electrolytes for wide-temperature-range operation of Li-ion cells

Nonaqueous electrolytes play a key role in extending the operating temperature range of Li-ion batteries. In developing electrolytes for wide temperature operations, we adopted an approach of starting with thermally stable lithium tetrafluoroborate (LiBF4) and lithium bis(oxalato)borate (LiB(C2O4)2, or LiBOB) salts. We have demonstrated that the capacity of Li-ion cells fades much slower in electrolytes using LiBF4 or LiBOB than in electrolytes using LiPF6. For low temperatures applications, suitable solvent systems for LiBF4 and LiBOB were explored. We found that the charge transfer resistance (Rct) is smaller in Li-ion cells in electrolytes based on LiBF4 in selected solvent systems than that based on LiPF6 and results in better capacity utilization at low temperatures. We also found that the electrolytes based on LiBOB in PC-based solvent system would allow Li-ion cells with graphite anode to be cycled. By comparing the properties of LiBF4 and LiPF6 in the propylene carbonate and diethyl carbonate (PC–DEC) solvent system, we found that it is possible to formulate proper solvent mixtures for enhanced conductivity for LiBF4 and LiBOB salts at low temperatures. It is concluded that nonaqueous electrolytes for wide-temperature-range operations of Li-ion cells are achievable.