We report a new approach toward formulating an electrolyte for low temperature operation of Li-ion batteries. The core of this new approach is to use LiBF4 salt instead of LiPF6, which is the chosen solute in the state-of-the-art Li-ion electrolytes. We found that although LiBF4-based electrolyte has lower ionic conductivity than the LiPF6 analogue, it provides improved low temperature performance. In particular, at −30 °C, a Li-ion cell with 1 m (mol/kg solvent) LiBF4 dissolved in 1:1:3 (wt.) propylene carbonate (PC)/ethylene carbonate (EC)/ethylmethyl carbonate (EMC) mixed solvent delivers as high as 86% of capacity, in comparison to that obtained at 20 °C. Whereas the counterpart one, using LiPF6, only retains 72%. Furthermore, the cell with LiBF4-based electrolyte shows lower polarization at −30 °C. The above results suggest that the ionic conductivity of the electrolyte is not the only limitation to the low temperature operation of Li-ion batteries. Analysis of cell impedance reveals that the improved low temperature performance by LiBF4 arises from a reduced charge-transfer resistance.
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