Physical and electrochemical properties of lithium bis(oxalate)borate—organic mixed electrolytes in Li-ion batteries

A lithium bis(oxalate)borate (LiBOB) salt in many of the common organic solvents (cyclic and linear carbonates, ionic liquid, sulfolane and gamma-butyrolactone) were tested as electrolytes for Li-ion batteries. These electrolytes were obtained by dissolution of LiBOB in sulfolane and in a mixture of propylene carbonate and ethylene carbonate (PC:EC (1:1 wt)); propylene carbonate and dimethyl carbonate (PC:DMC (1:1 wt)); N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulphonyl)imide and PC (MePrPyrNTf2:PC (2:1 wt)); gamma-butyrolactone and vinylene carbonate (GBL:VC (9:1 wt)). Such properties of the novel electrolyte systems as flammability, solubility, conductivity and electrochemical stability were investigated. The resulting electrolytes have good cathode and anode stability, high conductivity (almost 6 mS cm−1 for 0.3 M LiBOB in MePrPyrNTf2:PC (2:1 wt)) and some of them are practically non-flammable (the flash point above 164 °C and 189 °C for the ionic liquid with lithium salt and LiBOB in sulfolane, respectively). The spinel lithium titanate (Li4Ti5O12, LTO) anode working together with the electrolyte were tested with the use of cyclic voltammetry (CV), galvanostatic charge/discharge cycles and electrochemical impedance spectroscopy (EIS). The surface morphology of the electrode was studied using scanning electron microscopy (SEM). Among the range of tested electrolytes, the 0.3 M LiBOB in MePrPyrNTf2:PC (2:1 wt) and 1 M LiBOB in GBL:VC (9:1 wt) have the best battery performances. The discharge specific capacity after 50 cycles of LTO|1 M LiBOB in GBL:VC|Li was 137 mAh g−1. Charge/discharge tests of LTO in 0.3 M LiBOB in the MePrPyrNTf2:PC electrolyte at different C rates exhibit a good specific capacity of 126 mAh g−1 and 119 mAh g−1 at 0.5C and 1C, respectively.