Abstract
Hybrid capacitors should ideally exhibit high volumetric energy density, favorable low-temperature performance and safe operation. Here we describe a negative electrode comprising an intercalated metal–organic framework, 4,4′-biphenyl dicarboxylate dilithium [4,4′-Bph(COOLi)2], which forms a repeating organic–inorganic layered structure of π-stacked biphenyl and tetrahedral LiO4 units. The electrode shows a stepwise two-electron transfer and has a capacity of 190 mAh g−1 at 0.7 V vs. Li/Li+, which can suppress the lithium metal deposition reaction occurring an internal short circuit. A hybrid capacitor containing 4,4′-Bph(COOLi)2 negative and activated carbon positive electrodes possesses high volumetric energy density of approximately 60 Wh L−1 and good high-rate performance, particularly at the low temperature of 0 °C, because of low charge-transfer resistance along with low activation energy. Hopping mobility calculations suggest the observed low resistance properties are the result of high electron mobility arising from two electron-hopping pathways between adjacent molecules in the π-stacked biphenyl packing layer by lithium intercalation.
Highlights
Hybrid capacitors should ideally exhibit high volumetric energy density, favorable lowtemperature performance and safe operation
The framework was maintained during lithium intercalation with a remarkably small change in volume (0.33%); we named the series of electrochemically active metal–organic framework (MOF) as “intercalated MOFs”
To design an intercalated MOFs (iMOFs) electrode that provides low resistance and favorable lowtemperature characteristics, we selected a dicarboxylate lithium salt containing a biphenyl (Bph) framework[10,11] based on the following characteristics: (i) stepwise formation of a stable complex with one and two lithium atoms per unit (Li− Bph− and Li2−Bph2−, respectively)[24]; (ii) further stabilization of lithium through coordination with oxygen[24,25]; and (iii) expectation of a lower reduction potential of the biphenyl dicarboxylate lithium salt than that of the naphthalene because biphenyl exhibits a lower reduction potential than naphthalene (0.45 V vs. Li/Li+ for biphenyl compared to 0.51 V vs. Li/Li+ for naphthalene)[26]
Summary
Hybrid capacitors should ideally exhibit high volumetric energy density, favorable lowtemperature performance and safe operation. In existing LICs that use graphite as the negative electrode operating at a potential of 0.05 V vs Li/Li+, the deposition of metallic Li dendrites can cause internal short circuits between the positive and negative electrodes at high charging rates and/or low temperatures[8,9]. To address this safety issue, we proposed to control the negative electrode potential using aromatic dicarboxylate Li salts, which operate at potentials ranging from 0.5 to 1.0 V10,11. The mechanism of electronic conduction, which greatly contributes to the low resistance of this material, was not detailed in the report
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