An improved electrolyte for the LiFePO4 cathode working in a wide temperature range

A LiBF4–LiBOB (lithium bis(oxalato)borate) salt mixture was used to formulate an electrolyte for the operation of a LiFePO4 cathode over a wide temperature range (−50 to 80 °C) by employing a solvent mixture of 1:1:3 (wt.) propylene carbonate (PC)/ethylene carbonate (EC)/ethylmethyl carbonate (EMC). In comparison with the ionic conductivity of a single salt electrolyte, LiBF4 electrolyte has a higher conductivity below −10 °C while the LiBOB electrolyte is higher above −10 °C. For cell performance, LiBF4 cell has a better low temperature performance and a higher power capability, but it cannot survive above 60 °C. In contrast, the LiBOB cell performs very well at high temperature even up to 90 °C, but it fails to perform below −40 °C. We found that the temperature performance of Li/LiFePO4 cells could be optimized by using a LiBF4–LiBOB salt mixture. At 1C and at −50 °C, for example, a Li/LiFePO4 cell using 90:10 (in mole) LiBF4–LiBOB salt mixture could provide up to ∼30% of capacity at ∼3.0 V and it still could be cycled at 90 °C. In addition, we observed and explained an opposite correlation between the ionic conductivity of the electrolyte and the power capability of the cell. That is, the LiBF4 cell at 20 °C discharges at a higher plateau voltage than the LiBOB cell, whereas the LiBF4 electrolyte has a lower ionic conductivity.