Limiting factors for low-temperature performance of electrolytes in LiFePO4/Li and graphite/Li half cells

Low-temperature performance of LiBF4 and LiPF6-based electrolytes in LiFePO4/Li and graphite/Li half cells was investigated. In the temperature range from 0 °C to −40 °C, electrochemical impedance spectroscopy (EIS) results show that the charge-transfer resistance (Rct) of graphite/Li cell decreases, the Rct of LiFePO4/Li cell increases, and sum resistance of LiFePO4/Li and graphite/Li cell decreases when replacing LiPF6 with LiBF4. In the temperature range from 25 °C to −40 °C, energy barrier (W) for Li-ion jump at the solid electrolyte interface (SEI) alters slightly from 16.04 kJ/mol to 13.60 kJ/mol in LiFePO4/Li cells, but declines greatly from 46.47 kJ/mol to 19.81 kJ/mol in graphite/Li cells when using LiBF4 instead of LiPF6, meanwhile, activation energy (ΔG) of electrode reaction is approximately the same (∼60 kJ/mol). The above results indicate that the ionic conductivity is the main limiting factor for low-temperature performance of electrolytes in LiFePO4/Li cell, while factors related with electrolyte-interface are more crucial in graphite/Li cell than in LiFePO4/Li cell.