Since the energy shortage, climate and environment issues aggravate with each passing day, a demand for energy storage devices with superior electrochemical performance has been becoming increasingly urgent. Supercapacitors, as good supplements for Li-ion batteries, have drawn great attentions these years,, , ,  which help to the development of carbon-based electrochemical double layer capacitor,  MXene-based pseudocapacitor,  metallic compound-based battery-type supercapacitor. , , , ,  Among the varieties of materials, layered double hydroxides (LDHs) deliver a promising application potential, owning to their high specific capacitance (higher than 2000 F g-1), easy preparation, cost-effective, controllable composition and structure, environmental friendliness, and so forth. ,  They are a class of lamellar intercalation materials that comprise positively charged brucite-like host layers and intercalated anions and water. The composition formula of LDHs is [M2+1-xN3+x(OH)2](An-)x/n·mH2O, where M and N refer to metal elements (Co, Ni, Al, Fe, et al.), and An- refers to intercalated anions (CO32−, NO3−, SO42−, et al.). When using as battery-type electrode materials in hybrid supercapacitor, the transition metal ions in LDHs’ host layer could occur fast reversible redox reaction and provide high specific capacitance, while the interlayer anions function as the compensator for local charge equilibrium. Considering the significant influence of metal ions in the electrochemical performance of LDHs, a great deal of research has reported LDHs with different metal ions, such as binary NiCo, CoZn, CoGa-LDHs and ternary NiCoAl, NiCoZn, NiCoCe, NiCoMg-LDHs. , , , , ,  Compared to the extensive works on metal ions control, the effects of intercalated anions were relatively less studied and unclear.
2D lamellar structure with intercalated anions is one of the most typical characteristics of LDHs. The structure makes LDHs’ interlayer space adjustable. It has been reported that the interlayer distance of LDHs could be expanded by intercalating anions with large radius. Lan et al prepared sodium dodecyl benzene sulfonate intercalated NiCo-LDH, founding that the interlayer space increased from 0.79 nm to 1.52 nm, as the specific capacitance increased from 450 C g-1 to 748 C g-1.  Pan et al. used acetate anions to intercalate NiCo-LDH, which expanded the interlayer spacing from 0.8 to 0.94 nm, and increased the specific capacity from 690 to 1200 C g-1.  Ma et al. prepared 1,4-benzenedicarboxylic acid-intercalated NiCo-LDH, delivering a significant improvement in capacitance performance, which was ascribed to the increasing interlayer spacing and electrochemical active sites.  Zhao et al. synthesized a bunch of LDHs with gradient interlayer distance, which unveiled that increasing the interlayer distance could decrease the equivalent series resistance so as to improve the rate capability of LDH.  Kim et al. prepared Cl−, Br−, I− and NO3− intercalated CoAl-LDH, which also found that the electrochemical performance of LDHs was positively related to the interlayer spacing. With the combination of DFT, the phenomenon was attributed to the weakened interlayer interactions with low-charge-density ions, which further improved charge-transfer kinetics and electrochemical active surface area. Similar results were also found in dodecyl sulfate anion, acetate anion and benzoate anion intercalated LDHs. , , , , , ,  In summary, it is believed that larger interlayer distance could provide more electrochemical active sites and facilitate electrolyte ions transfer, leading to the improvements on capacitance and rate capability. However, the effect of intercalated anion on the cycling performance of LDH has been rarely discussed.
The poor cycling stability is one of the biggest weaknesses of LDH-based electrodes.  The Jahn-Teller distortion of Ni3+ is detrimental to the structural stability of Ni-based LDHs.  The phase change and distortion of host layer during charge-discharge cycles could lead to the collapse of structure and volume change of LDHs. Doping inactive metal ions to the host layer has been proved to be an effective way, ,  since it not only functions as the structural stabilizing unit in the host layer, but also break the long-term orderly arrangement of crystal structure and prevent the phase transition during charge and discharge. As a comparison, the effect of the intercalated anion on cycling performance is not the same and even contradictory in different works. Liu et al. prepared the SO42−, Cl− and NO3− intercalated CoMn-LDH, while the CoMn-LDH-SO4 with the largest interlayer spacing delivered the worst cycling stability.  In contrast, Xiao et al. compared the dodecyl sulfate (DS−), SO42− and NO3− intercalated CoAl-LDH, which found that CoAl-LDH-DS show the best cycling stability than the others.  Similar phenomenon is also reported in the glucose intercalated NiMn-LDH.  Nevertheless, neither of them has discussed the issue that how the intercalated anion influences the cycling performance.
In this work, we prepare a series of NiCo-LDHs with different intercalated anions, including NO3−, Cl−, SO42−, MoO42− and WO42−, and try to figure out the effects of anions on electrochemical performance of LDH. The intercalated anions are all inorganic ions with gradient increasing radius, resulting in the expansion of interlayer spacing from 0.73 to 1.06 nm. Interestingly, the correlation between interlayer spacing and cycling performance exhibits a “volcano” plot. The MoO42− intercalated NiCo-LDH with a 0.96 nm interlayer spacing delivers a high specific capacitance of 795 C g-1 and the best cycling stability with 80% capacitance retention after 20000 cycles. In addition, the asymmetric supercapacitor also exhibits an excellent cycling stability with ∼100% capacity retention after 10000 cycles, and a high energy density of 45.7 Wh kg-1 at a power density of 400 W kg-1.
Carbon cloth (CC, W0S1002) was supplied by Taiwan CeTech Co., Ltd. Cobalt sulfate heptahydrate (CoSO4·7H2O), cobalt nitrate heptahydrate (Co(NO3)2·6H2O), cobalt chloride heptahydrate (CoCl2·6H2O), nickel sulfate hexahydrate (NiSO4·6H2O), nickel nitrate hexahydrate (Ni(NO3)2·6H2O), nickel chloride hexahydrate (NiCl2·6H2O), ammonium molybdate ((NH4)2MoO4) and sodium tungstate (Na2WO4) were purchased from Aladdin reagent (Shanghai, China). Urea and potassium hydroxide (KOH) were supplied from
Preparation of NiCo-LDH-MoO4, NiCo-LDH-WO4:
The precursor solution was prepared by dissolving 1.2 mmol Ni(NO3)2, 1.2 mmol Co(NO3)2, 12 mmol urea into 50 mL deionized water to form a homogeneous solution. Then, 0.5 mmol anion-intercalated additives, (NH4)2MoO4 and NaWO4, were added into the solution to get the different precursor solutions, respectively. The solutions were then transferred into a 100 mL stainless Teflon-lined autoclave with pristine carbon cloth emerged in and undergone a heating process at 105 °C for 8 h before cooled to
Physical Characterization of NiCo-LDH-An electrodes
In the preparation process, a one-pot hydrothermal method with different salts containing target anions used as precursors is employed to obtain NiCo-LDH-An. For the NiCo-LDH-NO3, NiCo-LDH-Cl and NiCo-LDH-SO4, nitrate, chloride and sulfate of nickel and cobalt are directly used as reactants. While preparing NiCo-LDH-MoO4 and NiCo-LDH-WO4, nickel and cobalt nitrate are used as the resources of Ni2+ and Co2+. Though urea could slowly produce carbon dioxide and form carbonate during the
In this study, we employed intercalation engineering method to prepare NiCo-LDHs with different intercalated anions and interlayer spacing so as to elucidate the effects of anions on the electrochemical performance, especially cycling stability of the electrodes. Although expanding the interlayer spacing could increase the specific capacity by providing more electrochemical active sites, it could only facilitate the diffusion process but not guarantee the improvement in rate capability. A
Conflicts of interest
The authors declare no conflict of interest.
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Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
This research received funding from the National Natural Science Foundation of China (Grant Nos. 51973183, 52173024, 51873180) and Shanxi-Zheda Institute of Advanced Materials and Chemical Engineering (No. 2022SZ-TD008). The authors also thank Shiyanjia Lab (www.shiyanjia.com) for its support in HRTEM.
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