Tetraethylammonium difluoro(oxalato)borate as electrolyte salt for electrochemical double-layer capacitors

Abstract

Difluoro(oxalato)borate (ODFB−) is a less symmetric borate anion, which makes it possible to increase the solubility of tetraethylammonium (TEA+) salt in propylene carbonate (PC) and improve the capacitance of electrochemical double-layer capacitors (EDLCs). The use of TEAODFB with PC solvent in EDLCs was investigated in the paper. The results show that TEAODFB has good solubility in PC, and the conductivity is comparable to TEABF4/PC electrolyte. When the molar concentration of TEAODFB reaches to 1.6M, the TEAODFB/PC electrolyte has superior conductivity of 14.46mScm−1 and good capacitor characteristics. Despite the less accessible to the electrode and low energy density was achieved, the specific capacitance of 1.6M TEAODFB/PC electrolyte is 21.4Fg−1 at 1Ag−1, and the energy density and power density were comparable to 1M TEABF4/PC electrolyte at 1–5Ag−1. Temperature characteristic was also tested by 3.3F circular capacitors from −40 to 60°C, the result demonstrates that capacitors using 1.6M TEAODFB/PC electrolyte show much higher capacitance and energy density at the investigated temperatures, and the discharge capacitance of capacitors using 1.6M TEAODFB/PC electrolyte varies with the temperature less than that of 1M TEABF4/PC electrolyte.