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 degrees 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 degrees C while the LiBOB electrolyte is higher above -10 degrees C. For cell performance, LiBF4 cell has a better low temperature performance and a higher power capability, but it cannot survive above 60 degrees C. In contrast, the LiBOB cell performs very well at high temperature even up to 90 degrees C, but it fails to perform below -40 degrees 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 degrees C, for example, a Li/LiFePO4 cell using 90:10 (in mole) LiBF4-LiBOB salt mixture could provide up to similar to 30% of capacity at similar to 3.0 V and it still could be cycled at 90 degrees 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 degrees C discharges at a higher plateau voltage than the LiBOB cell, whereas the LiBF4 electrolyte has a lower ionic conductivity. Published by Elsevier B.V.