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

• Published in: Journal of power sources 2006-09, Vol.159 (1), p.702-707
• Main Authors: ZHANG, S. S, XU, K, JOW, T. R
• Format: Article
• Language: English
• Subjects: Applied sciences; Direct energy conversion and energy accumulation; Electrical engineering. Electrical power engineering; Electrical power engineering; Electrochemical conversion: primary and secondary batteries, fuel cells; Exact sciences and technology
• ISSN: 0378-7753; 1873-2755
• DOI: 10.1016/j.jpowsour.2005.11.042

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