Preparation of 1T MoS2 by the solvated-ion-intercalated strategy and the traditional hydrothermal strategy. Ammonium molybdate ((NH4)6Mo7O24·4H2O) and thioacetamide (CH3CSNH2) were obtained from Shanghai Macklin Biochemical LTD. Lithium sulfate (Li2SO4) was purchased from Shanghai Aladdin Biochemical Technology LTD. Urea (CH4N2O) was purchased from Sinopharm Chemical Reagent LTD. All other reagents, including deionized water, were purchased commercially and utilized without any special pre-treatment.
To prepare the pristine 1T MoS2 samples by the traditional hydrothermal strategy, 25 mg ammonium molybdate, 30 mg thioacetamide and 100 mg urea were mixed in 25 mL deionized water followed by 2 h magnetic stirring at 600 rpm. Then, the mixed solution was transferred into a Teflon-lined stainless autoclave and kept in a furnace at 180 °C for 18 h. Subsequently, the autoclave was cooled down to room temperature rapidly by continuous water flow, and then further stabilized at 4 °C for 2 h. The as-prepared samples were collected after centrifugation and washing with deionized water and ethanol for several times, which remove the relatively large nanoparticles and retained the few-layer nanosheets in the dispersion. After 30 min ultrasonic treatment, the 1T MoS2 samples were stably dispersed in deionized water and kept at 4 °C environment for long-term storage. To prepare Li+-intercalated 1T MoS2 samples, 0.275 g, 0.550 g and 0.825 g of lithium sulfate were added into the well-mixed precursor solution mentioned above, which was continuously to stirred at 600 rpm for another 0.5 h. Therefore, the concentration of Li+ ion in the mixed precursor is 0.5 M, 1.0 M and 1.5 M.
Material characterization. The microstructures of samples were characterized by scanning electron microscope (SEM) (Hitachi SU-70) and transmission electron microscope (TEM) (JEOL JEM-2100). Raman spectra were measured by Raman spectrometer (LabRAM HR Evolution) with a 532 nm excitation wavelength. X-ray photoelectron spectroscopy (XPS) (Escalab Mark II, VG) with a monochromatic Mg Ka X-ray source (1253.6 eV) was used to obtain XPS data. Before Raman and XPS characterization, the 1T MoS2 dispersion was drop casted on a quartz plate and dried naturally at room temperature.
Density functional theory calculations. The relative stability of 1T and 2H MoS2 is obtained from density functional theory simulations, performed using Vienna ab initio simulation package (VASP)48,49 with generalized gradient approximation36 based on Perdew-Burke-Ernzerhof37 functions. The energy and force convergence criteria are set to 1 × 10-5 eV and 1 × 10-2 eV/Å, respectively, with an energy cutoff of 400 eV. A k-point mesh of 5 × 5 × 1 is applied to the Brillouin zone within Monkhorst−Pack grids. The relative stability of 1T MoS2 compared to 2H MoS2 (Es(c)) is calculated using:
Es(c) = Et(c) – EH(c), (1)
where ET(c) and EH(c) represent the total energy of 1T and 2H MoS2 with lithium adsorbed, and c is the surface charge density.
Preparation of MoS2 electrodes and asymmetric supercapacitor. To prepare thin-film electrodes, 1T MoS2 dispersion was filtered over membranes (25 nm-diameter pore size) and peeled off after drying in vacuum at 50 °C. The film thickness varied from ~1.45 to 9.30 μm depending on the amount of filtered dispersion, with the mass loading ranging from ~0.96 to 5.58 mg cm-2. To prepare submillimeter-thick film electrodes, the MoS2 dispersion was freeze-dried to obtain 1T MoS2 powder. It is worth noting that when the freeze-dried ice just completes the sublimation, the remaining powder is collected immediately to avoid the loss of water molecules between the MoS2 nanosheets. After manually stirring the mixture of 1T MoS2 powder and polytetrafluoroethylene (PTFE) with a ratio of 9 : 1 for over 3 hours, the well-mixed slurry was rolled, freeze-dried and pressed into film electrodes with the high mass loading of 36.7 mg cm-2 and thickness of 94.2 μm. The high packing density is mainly due to the high molecular weight of MoS2 and the compact packing of few-layer nanosheets. The compact packing morphology caused by rolling and pressing processes forms sufficient pores for the penetration of electrolyte. For the preparation of asymmetric MoS2||activated carbon supercapacitor, single-walled carbon nanotube (SWCNT) was used as the conducting agent to promote rapid ion transport, due to its high conductivity and stable nanochannel structure. 1T MoS2 dispersion was first mixed and stirred with SWCNT dispersion with an active substance mass ratio of 8 : 2. Then the well-mixed dispersion was freeze-dried to obtain mixture powder. Through adding a small amount of water and PTFE (10% of the mixture powder mass), the MoS2 electrode was prepared by rolling, freeze-drying and pressing the mixture powder into ~100 μm film with a high compaction density. To prepare the counter-electrode, YP-50 activated carbon, carbon black, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed with a ratio of 85 : 9 : 2 : 4. The mixture was stirred in vacuum and evenly coated and pressed on a 22-μm-thick aluminum foil, then dried at 80 °C to obtain the counter electrode. The asymmetric supercapacitor was assembled by stacking and pressing 22-μm-thick aluminum foil (collectors), MoS2 electrode (cathode), activated carbon electrode (anode), and glass fiber separator into a button supercapacitor cell. The mass loading of the cathode and anode are around 14.8 mg cm-2 and 10.2 mg cm-2, respectively, which are adjusted based on the comprehensive matching of charge balance and electrode thickness.
Electrochemical measurements. Electrochemical measurements were carried out in a three-electrode configuration using an electrochemical workstation (PGSTAT302N, Metrohm Autolab B.V.). Ag/AgCl electrode and activated carbon were used as counter and reference electrodes, respectively. In order to make the comparison more convincing, we normalized the thickness and quality of the tested electrodes. During the three-electrode tests, CV (Cyclic Voltammetry) data were obtained at the voltage windows between -1 and 0.2 V vs. Ag/AgCl with the scan rates ranging from 5 to 1000 mV s-1. For the asymmetric supercapacitor, the effective working voltage was chosen at 0 ~ 1.5 V with the scan rates of 1 ~ 100 mV s-1.
The specific gravimetric capacitance (Cg, F g-1) and the volumetric capacitance (Cv, F cm-3) were calculated by the following formula, respectively:
Cg = It / V, (2)
Cv = Cg × ρ, (3)
where I represents the mass normalized current (A g-1), t is the discharge time (s) obtained in galvanostatic charge/discharge (GCD) measurements, V is the voltage window (V), and ρ is the density of tested electrode.
The energy and power densities of the supercapacitor were calculated by the following formula:
E (Wh g-1) = 0.5CcellVcell2 / 3600, (4)
Ev (Wh cm-3) = E × ρ, (5)
P (W g-1) = E × 3600 / tcell, (6)
Pv (W cm-3) = P × ρ, (7)
where Ccell (F g-1) is the total capacitance of asymmetric electrode cell, Vcell (V) is the effective working voltage of the discharging process, tcell (s) is the discharging time, ρ (g cm-3) is the normalized density of the two-electrode cell, E and P are specific energy and power density, respectively.