The development of halogen-free electrolyte salts for electrochemical devices is desirable from the viewpoint of safety and cost. We have investigated the performance of electrical double-layer capacitors (EDLCs) composed of activated carbon electrodes as a positive and a negative electrode, tetraethylammonium bis(oxalato)borate (TEABOB) as an electrolyte salt, and propylene carbonate (PC) as a solvent. In spite of the larger anion size and lower ionic conductivity of the TEABOB, rate capability of a 2025-type coin cell utilizing a PC solution containing 1 mol dm−3 TEABOB is more excellent than that obtained for tetraethylammonium tetrafluoroborate (TEABF4).
The use of bis͑oxalato͒borate ion makes it possible to increase the solubility of the tetramethylammonium salt in propylene carbonate ͑PC͒. We have investigated the thermal and electrolytic properties of tetramethylammonium bis͑oxalato͒borate ͑TMA-BOB͒, tetraethylammonium bis͑oxalato͒borate ͑TEABOB͒, and tetraethylammonium bis͓salicylato͑2-͔͒borate and their application to electric double-layer capacitors ͑EDLCs͒. The anion sizes of the fluorine-free chelatoborates are larger than that of tetrafluoroborate, and the ionic conductivities and wettability of the PC solutions become lower. Nevertheless, the gravimetric capacitances of model cells using the TMABOB and the TEABOB are comparable to that obtained for tetraethylammonium tetrafluoroborate ͑TEABF 4 ͒.Electric double-layer capacitor ͑EDLC͒ is an energy storage device that accumulates electric charges in an interphase between an electrode and an electrolyte. 1 The EDLC is regarded as one of the most promising electrochemical devices for hybrid electric vehicles. Tetraethylammonium tetrafluoroborate ͑TEABF 4 ͒ is commonly used as an electrolyte salt for the EDLC. Much attention has recently been paid to the application of room-temperature ionic liquids such as 1-ethyl-3-methylimidazolium tetrafluoroborate ͑EMIBF 4 ͒. However, the electrolyte salts based on BF 4 − are not only expensive, but also they can be hydrolyzed to generate hydrogen fluoride ͑HF͒ in the presence of moisture. The development of fluorine-free electrolyte salts is desirable from the viewpoint of safety and cost.The same thing is true of electrolyte salts for lithium and lithiumion batteries. LiPF 6 is a typical electrolyte salt, though it is more hydrolytic. A potential replacement of the LiPF 6 by lithium bis͑ox-alato͒borate ͑LiBOB͒ has been tried by several researchers. 2-12 Chelate compounds are more stable than the corresponding nonchelated analogs because of the chelate effect. Sasaki and co-workers have reported the physical and electrochemical properties of lithium bis͓salicylato͑2-͔͒borate ͑LBSB͒ and its derivatives and their application to lithium batteries. [13][14][15][16] They found that the solutions containing the LBSB afford lithium cycling efficiency of more than 80% over the range of high cycle numbers. Surprisingly, only one attempt has so far been made at the application of tetraethylammonium bis͑oxalato͒borate ͑TEABOB͒ to an electrolyte salt for EDLCs. 17 The use of smaller quaternary ammonium ions such as tetramethylammonium ion ͑TMA + ͒ may increase the number of ions accumulated in pores of an activated carbon electrode and bring about high electrostatic capacitance. Regretably, the solubility of tetramethylammonium tetrafluoroborate ͑TEABF 4 ͒ in propylene carbonate ͑PC͒ is 0.1 mol dm −3 or less at 25°C. 18 The combination of the TMA + and the larger anion, BOB − , can improve the solubility in the PC. Ue et al. have investigated the electrochemical properties of several tetramethylammonium chelatoborates including the TMABOB in ␥-butyrolactone ͑␥-BL͒. 19 In this pape...
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