Lithium ion capacitors have been recognized as high energy density energy harvesting devices, owing to the synergetic combination of double-layer charge storage at the positive electrode and lithiated carbon negative electrode. The performance of such hybrid supercapacitors are mainly governed by the capacitive cathode, thus devices which employ higher capacitance capacitive electrodes are desired.
We have developed a new hybrid supercapacitor design based on water stable, protected Li anode technology that uses a water-stable solid electrolyte as a separator . Our preliminary cell configuration allowed the use of pseudocapacitive cathodes in aqueous electrolyte with a 4 V rated voltage [1,2]. Unfortunately, the high resistance of the protected Li anode allowed charge/discharge only at 60oC. Here we report are recent progress of room temperature performance of 4-V aqueous hybrid supercapacitors with energy density higher than lithium ion capacitors . A multi-layered lithium-doped carbon (LixC6) negative electrode was developed, which consisted of LixC6 anode, poly(ethylene oxide) (PEO) polymer or alginate gel electrolyte doped with lithium bis(trifluoromethansulfonyl)imide (LiTFSI) and N-methyl-N-propylpiperidinium bis(trifluoromethansulfonyl)imide (PP13TFSI) ionic liquid as the buffer layer and LISICON-type glass ceramic (LTAP). The protected LixC6 anode employing PEO-LiTFSI-PP13TFSI (Li | PEO-LiTFSI | LTAP) at 25oC exhibited comparable performance to a protected Li negative electrode without PP13TFSI addition at 60oC (Li | PEO-LiTFSI | LTAP), owing to a drastic increase in conductivity. Charge/discharge tests of an aqueous hybrid supercapacitor using the newly developed protected LixC6 anode with PEO-LiTFSI-PP13TFSI at 25oC showed good capacitive behavior and long-term capability. In addition, an aqueous hybrid supercapacitor employing a RuO2 nanosheet positive electrode with specific capacitance of 1000 F/g in acetic acid-lithium acetate catholyte [4,5] (LixC6 | PEO-LiTFSI-PP13TFSI | LTAP | AcOH-AcOLi | RuO2 nanosheet) showed excellent specific capacity of 196 mAh/(g-RuO2) and specific energy of 625 Wh/(kg-RuO2) at 25oC. Further decrease in cell resistance was realized by using alginate gel electrolyte in place of PEO polymer electrolyte.
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