A Wide Operating Temperature Range Electrolyte Containing Lithium Salts Mixture and a Co-Solvent for the LiFePO4 Cathode

A series electrolytes for the operation of a LiFePO4 cathode over a wide temperature range were designed by employing a lithium tetrafluoroborate (LiBF4) and lithium bis(oxalato) borate (LiBOB) salt mixture and an ester co-solvent methyl butanoate (MB) in a solvent mixture of 1:1:3 (v/v) propylene carbonate (PC) /ethylene carbonate (EC) /ethylmethyl carbonate (EMC). And prepared electrolytes were compared to the reference mixture EC/PC/3EMC (LiPF6, 1 M). Cells temperature performance was systematically investigated by ionic conductivity test and various electrochemical tests, such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge-discharge test. It was found that the temperature performance of Li/LiFePO4 cells could be optimized by using a LiBF4-LiBOB salt mixture and MB co-solvent. At -40 degrees C, for example, a Li/LiFePO4 cell using 1 M LiBF4:LiBOB(8: 2) PC: EC: EMC: MB(1:1:1:2 v/v) electrolyte could provide up to 60% of capacity, while still had a voltage platform of 3 V. And it still retained 80% of capacity after 100 cycles at 65 degrees C, 1 C. (C) 2014 The Electrochemical Society.