The High-Performance Asymmetric Capacitors with Lithium-Intercalated Metal-Organic Framework Electrodes

Abstract

Text: High-power energy storage devices can provide an improvement of drivability and a load-levelling system in an electric vehicle, a hybrid electric vehicle, and a fuel-cell vehicle.1 Asymmetric capacitors known as lithium-ion capacitors (LIC), in which a faradaic, battery-type electrode is combined with a non-faradaic, capacitor-type electrode, are up-and-coming devices because they show faster charge and discharge responses than lithium ion batteries and larger energy densities than electric double-layer capacitors (EDLC).1 Safety is also an important factor for the onboard devices. However, LICs using conventional graphite carbon negative electrodes operating at 0.1 V have the risk of a metallic Li deposition causing internal short circuits. Thus, we have studied lithium-intercalated metal-organic frameworks (iMOFs)2 and reported the novel negative electrode of 2,6-naphthalene dicarboxylate dilithium (2,6-Naph(COOLi)2), whose operating potential (0.8 V vs. Li/Li+) is expected to suppress a metallic Li deposition. In this report, we will introduce a novel iMOF negative electrode for asymmetric capacitors, a 4,4’-biphenyl dicarboxylate dilithium (4,4’-Bph(COOLi)2), which is fast responsible because of the remarkably low internal resistances. 4,4’-Bph(COOLi)2 can be prepared from biphenyl-4,4’-dicarboxilic acid by the deprotonation in LiOH under methanol solution. The powder X-ray diffraction of 4,4’-Bph(COOLi)2 and the corresponding Rietvelt refinement revealed the repeating structure of organic p-stacked biphenyl moieties and inorganic tetrahedral LiO4 units (Figure 1a). 4,4’-Bph(COOLi)2 has a larger lattice parameter of the a-axis direction than 2,6-Naph(COOLi)2because of the longer aromatic unit. The charge/discharge curve of a Li/4,4’-Bph(COOLi)2 cell indicates that the 4,4’-Bph(COOLi)2 electrode shows reversible two-electron transfer reaction at a potential of 0.7 V vs. Li/Li+, which is 0.1 V lower operating potential than that of 2,6-Naph(COOLi)2 (Figure 1b). Compared to the polarization of the 2,6-Naph(COOLi)2 electrode, 4,4’-Bph(COOLi)2 electrode exhibits small and large polarization in the deep and shallow depth of discharge (DOD), respectively (Figure 1b). For further study, the internal resistances of 2,6-Naph(COOLi)2 and 4,4’-Bph(COOLi)2 electrodes were examined by using the electrochemical impedance spectroscopy using symmetric cells. The symmetric cells were constructed of two iMOF electrodes lithiated up to DOD 50% under the same condition. The results showed the charge transfer resistances of 4,4’-Bph(COOLi)2 were lower than 2,6-Naph(COOLi)2. The activation energy of the charge transfer resistance obtained from Arrenius plots was slight lower in 4,4’-Bph(COOLi)2 than in 2,6-Naph(COOLi)2 or graphite. The low internal resistance with low activation energy of 4,4’-Bph(COOLi)2electrode seems to be suitable for the negative electrode of high-power devices, and can lead a high-rate performance even at a low temperature. The asymmetric capacitors composed of the positive electrodes of active carbon (AC), and the negative electrodes of 4,4’-Bph(COOLi)2 (4,4’-Bph(COOLi)2/AC cell) or 2,6-Naph(COOLi)2 (2,6-Naph(COOLi)2/AC cell) were prepared. The 4,4’-Bph(COOLi)2/AC cell showed 56 Wh L－ 1 energy density at 1C and 2.3 kW L－ 1 power density at 60C (Figure 1d). 4,4’-Bph(COOLi)2/AC cell showed higher energy density at 60C than conventional EDLC, LIC4, LTO/AC cell, and 2,6-Naph(COOLi)2/AC cell. Interestingly, 4,4’-Bph(COOLi)2/AC cell maintained a high-rate performance at a low temperature of 0°C compared with 2,6-Naph(COOLi)2/AC cell, which could derive from the low internal resistance with the low activation energy of 4,4’-Bph(COOLi)2. The capacity retention of 4,4’-Bph(COOLi)2/AC cell was 86 % after 1000 cycles at 10C. In summary, the asymmetric capacitor with 4,4’-Bph(COOLi)2electrode can be safer than the LIC using graphite, and showed high power capability and favorable low-temperature performance, therefore our proposed capacitors will be promising for the future capacitors.