Improvement in Cycle Performance of a High-Voltage Hybrid Electrochemical Capacitor

Abstract

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...

Electrolytes for electrochemical energy storage

“…111 It was also reported that capacity loss of the LICs during charge-discharge cycling was obviously reduced by addition of Li metal into the cell. 112,113 The highest potential of the AC electrode should be lower than 4.5 V vs Li + /Li, while the lowest potential of the HC/stabilized Li metal powder (SLMP) electrode should be greater than 0.1 V vs. Li + /Li. Moreover, the capacitance degradation of this LIC was less than 1 % after 1300 cycles in a voltage range of 2.0 to 4.1 V. 114 Please do not adjust margins Please do not adjust margins Additionally, it was proposed to use quaternary alkyl ammonium and PC based organic electrolytes in LICs.…”

Cycle performance of lithium-ion capacitors using graphite negative electrodes at different pre-lithiation levels

“…The coulombic efficiency at a pre-lithiation of 7.5% was not high ($95%) and showed a moderate decrease. The absence of pre-lithiation resulted in the lowest coulombic efficiency (85% in cycles 1-5 and 20% in cycles [20][21][22][23][24][25].…”

“…This excellent property can alleviate electrolyte decomposition resulting from the increased redox potential (>4.5 V vs. Li/Li + ) at the PE [18,19]. The process of pre-lithiation or Li pre-doping is essential to provide graphite with a low redox potential, thus elevating the working voltage of the cell, which is typically 2.0-4.0 V. The potential swing at the graphite NE during Li intercalationdeintercalation can affect the capacitance and the cycle stability of the cells [19,20]. Zhang et al have studied the effect of the degree of pre-lithiation of graphite-type NEs (mesocarbon microbeads, MCMBs) on the electrochemical performance of LICs [21].…”

Electrical Double-Layer Capacitors and Pseudocapacitors

“…The common approach which has been recently introduced for realizing asymmetric capacitors using any of these two materials, as negative electrode combined with an activated carbon, consists in extending the working potential window of the positive electrode [113][114][115][116]. In this sense, Figure 8.32 shows a scheme of the potential profi les during charging two different asymmetric confi gurations (Figure 8.32a) and a symmetric double-layer capacitor (Figure 8.32b) [114].…”