A high-voltage hybrid electrochemical capacitor (HEC) which uses nongraphitizable carbon for a negative electrode and activated carbon for a positive electrode has been constructed with lithium salt for the electrolyte. Under high-voltage conditions, large capacity loss during charge-discharge cycling was observed when the positive electrode was highly polarized at a potential over
4V
vs
Li∕normalLi+
. This capacity loss was induced by the increase of the interfacial resistance at the negative electrode. A large amount of lithium fluoride (LiF) was observed on the surface of the negative electrode after charge-discharge cycling. It is considered that the LiF was generated by a reaction of hydrofluoric acid (HF), generated by the decomposition of hexafluorophosphate anion
(PnormalF6−)
, with lithium alkyl carbonate (semicarbonate, ROCOOLi) or lithium carbonate
(normalLi2CnormalO3)
on the surface of the negative electrode. The generation of LiF and the capacity loss were suppressed by providing lithium metal between the positive electrode and the negative electrode. It was thus confirmed that the suppression of LiF generation, caused by high polarization of activated carbon positive electrode, can realize a high-voltage HEC with good cycleability.
A technology for preventing capacity loss during charge-discharge cycling of a high-voltage hybrid electrochemical capacitor is presented. During high-voltage cycling of the capacitor, which combined the negative electrode of a lithium-ion battery with the positive electrode of an electric double-layer capacitor, extreme capacity loss occurred through a decline in the capacity of the negative electrode when the positive electrode was highly polarized ͑at over 4 V vs Li/Li + ͒. A method of preventing this capacity loss by adding a lithium metal to the cell is shown here.In recent years, several types of so-called hybrid electrochemical capacitors ͑HECs͒, which combine a faradaically rechargeable battery-type electrode and a nonfaradically rechargeable ͑polariz-able͒ electrode, have been proposed. 1-16 These HECs have higher energy density than electric double-layer capacitors ͑EDLCs͒ and higher power density than pure battery systems. In particular, HECs that combine the negative electrode of a lithium-ion battery ͑LIB͒ with the positive electrode of an EDLC are expected to be useful as power sources for hybrid electric vehicles ͑HEVs͒ and fuel-cell electric vehicles ͑FCEVs͒, because they have a higher energy density than EDLC due to the higher capacity of the Li-dopable carbon negative electrode and the higher cell voltage. [1][2][3][4][5][6][7] However, the specific capacity of the activated-carbon positive electrode in HECs is much lower than that of the negative electrode. Therefore, the positive electrode needs to have a large volume, about 10 times as large as that of the negative electrode, making the energy density of HECs only a few times higher than that of conventional EDLCs. To achieve an even higher energy density in HECs, the capacity of the positive electrode is required to improve. Extension of the potential range to a lower potential region has been proposed to achieve this end. 6,7 However, this type of HEC has a disadvantage in that the bottom cutoff voltage becomes lower than that of a conventional HEC unless the negative electrode is charged preliminarily by using a lithium-metal auxiliary electrode. In contrast, extension of the potential range to a higher potential region is more effective in improving the energy density of HECs. However, the upper potential limit of the positive electrode is usually controlled to below 4 V vs Li/Li + to obtain cycle durability, and no attempts to extend the potential range of the activated-carbon positive electrode to a higher potential region have been reported. In this paper, we focused on improving the cycle durability of a highvoltage HEC to improve its energy density.
ExperimentalPreparation of the positive electrode.-Commercially available activated carbon ͑steam activated, average particle size 2.9 m, 1500 m 2 g −1 ͒ was used as the material for the positive electrode. The activated carbon was mixed with polytetrafluoroethylene ͑PTFE͒ ͑D-210C, Daikin͒ and carbon black ͑Ketjenblack ECP, Lion Co., Ltd.͒ in a mass ratio of 85:10:5, stiffened by mixing i...
A waste heat recovery system is investigated basically. Original electro-thermodynamic cycle and novel system are expected to be viable in any heat sources with time dependent temperature changes instead of the spatial temperature gradient.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.