Supercapacitive properties of amorphous MoS₃ and crystalline MoS₂ nanosheets in an organic electrolyte

Abstract

Molybdenum sulfide materials receive high attention as high-performance electrodes for electrochemical energy storage devices. In this study, we investigate the electrochemical energy storage properties of amorphous MoS₃ and crystalline MoS₂ materials (prepared via thermal decomposition of ammonium tetrathiomolybdate) using an organic liquid electrolyte. Physicochemical characterization using X-ray diffraction pattern and laser Raman analysis confirms the formation of amorphous MoS₃ and crystalline MoS₂, respectively. The energy storage properties of MoS₃ and MoS₂ based symmetric supercapacitor devices were comparatively studied using cyclic voltammetry, electrochemical impedance spectroscopy, and galvanostatic charge–discharge analysis. The cyclic voltammetry analysis reveals the mechanism of charge storage in MoS₃ and MoS₂ is due to the ion-intercalation/de-intercalation pseudocapacitance. Electrochemical impedance spectroscopy studies reveal the better capacitance and charge-transfer nature of the crystalline MoS₂ symmetric supercapacitor compared to that of the amorphous MoS₃ symmetric supercapacitor. The charge–discharge analysis suggests that the MoS₂ symmetric supercapacitor device possesses better electrochemical energy storage properties with a high specific capacity of 20.81 mA h g⁻¹ (24.98 F g⁻¹) and energy density of about 20.69 W h kg⁻¹ with the excellent cyclic stability of about 2000 cycles. The experimental results suggest that the crystalline MoS₂ sheets might be a better choice than amorphous MoS₃ as an electrode material for supercapacitors using an organic liquid electrolyte.

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1. Introduction

In recent years, two-dimensional (2D) materials beyond graphene sheets have received much attention as electrode materials for electrochemical energy storage systems such as supercapacitors and batteries.^1–3 A lot of (2D) materials including transition metal chalcogenides (TMCs), M'Xenes (transition metal carbides/nitrides/carbonitrides), graphene derivatives such as graphdiyne, halogenated graphenes, and layered transition metal oxides/hydroxides are examined for electrochemical energy storage applications during this decade.^4–9 Among these emerging 2D materials, molybdenum sulfide sheets attracted much consideration due to their layered structure (analogous to graphene) and electrical conductivity (higher than those of molybdenum based oxides), with the advantage of rich electrochemistry due to the oxidation state of molybdenum.^10,11 The electrochemical properties of molybdenum disulfide (MoS₂) with different phases (such as 2H-hexagonal, 1-T trigonal and 3R-rhombohedral) and different morphologies (micro-/nanoflowers, spheres, particles, sheets, hierarchical structures) are explored during this decade.^12–16 A careful literature review revealed that exfoliated MoS₂ sheets showed superior energy storage properties compared to chemically synthesized MoS₂, which is mainly due to the poor crystallinity of the latter.^17,18 Additionally, the energy storage properties of the MoS₂ sheets can be tuned via engineering the thickness or layer number and the lateral size of the sheets.^19,20 A recent study showed that the energy storage properties of sonochemically exfoliated MoS₂ sheets are nearly 20 times higher than that of the bulk MoS₂ in an acidic electrolyte.²¹ In our earlier study, we demonstrated that mechanically milled few layered MoS₂ showed a nearly five-fold increase in specific capacitance compared to the bulk MoS₂ sheets in a basic electrolyte.²² Further, binder-free electrodes based on MoS₂ and their hybrids, and both symmetric and asymmetric devices based on MoS₂ are extensively studied using aqueous electrolytes by different research groups.^23–25 Hitherto, the energy storage properties of amorphous molybdenum tri-sulfide (MoS₃) are not explored well compared to the crystalline molybdenum disulfide (MoS₂) sheets.

Interestingly, amorphous MoS₃ is a material of emerging interest and proved to be a highly active electrocatalyst for the water splitting reaction compared with crystalline MoS₂.^26,27 Moreover, the band gap of amorphous MoS₃ is about 1.5 eV, which is lower than the band gap of MoS₂ sheets (1.8 eV).²⁸ The advantage of MoS₃ over MoS₂ is its higher electronic conductivity and additional sulfur species on the surface, which can provide enhanced electrochemical properties.²⁹ In general, amorphous materials are not suitable for electronics applications but highly recognized for applications in photo-/electro-catalysis and energy storage devices.^30–32 Earlier studies suggested that amorphous hydrous materials possess higher specific capacitance over their corresponding crystalline materials (such as α-RuO₂, α-MnO₂, graphene oxide, α-Ni(OH)₂).^33–36 A recent study showed the effectiveness of amorphous MoS₃ sheets as a negative electrode for supercapacitors using aqueous electrolytes.³⁷ Additionally, research also explored that MoS₃ possesses a superior electrochemical performance over crystalline MoS₂ sheets in aqueous electrolytes. The energy density (E = 0.5CV²) of a supercapacitor can be improved by increasing either the specific capacitance/capacity (C) or operating voltage window (V) of the device.^38,39 It is well known that organic liquid-based electrolytes can function over a wide voltage window (3.0 to 4.0 V). Therefore, supercapacitors utilizing organic liquid can provide a much higher energy density compared to aqueous electrolytes.⁴⁰

To date, only limited reports are available in the literature on the electrochemical energy storage properties of MoS₂ sheets using an organic liquid electrolyte. Recently, Acerce et al. explored the high-performance supercapacitive properties of metallic 1T MoS₂ sheets using organic electrolytes which can operate over a voltage window of 3.0 V and deliver a high specific capacitance of about 199 and 250 F cm⁻³ in TEABF₄/MeCN and EMIMBF₄/MeCN electrolytes.²¹ Pandey et al. studied the electrochemical properties of chemically prepared MoS₂ sheets using aqueous and non-aqueous (BMIMPF₆) electrolytes using a three-electrode configuration.⁴¹ On the other hand, there is no report available on the energy storage properties of MoS₃ sheets using non-aqueous electrolytes, which is a study of immense interest owing to the distinct electrical properties and electrochemical reactivity of MoS₃ over MoS₂. Therefore, we focused on examining the energy storage properties of both amorphous MoS₃ and crystalline MoS₂ sheets using an organic liquid (TEABF₄) electrolyte and understanding the mechanism of charge-storage properties in detail.

2. Results and discussion

In this study, (NH₄)₂MoS₄ was prepared via the precipitation followed by crystallization route and was thermally decomposed at 200 and 600 °C to obtain amorphous MoS₃ and crystalline MoS₂ as shown in Fig. 1(A). Fig. 1(B) shows the XRD pattern of the ammonium tetrathiomolybdate, MoS₃, and MoS₂ powders. The diffraction peaks observed at 2θ = 9.73°, 12.43° and 13.73° closely matched well with the standard diffraction pattern of ammonium tetrathiomolybdate (JCPDS card no. 38-0075), which indicated the formation of high-quality ammonium tetrathiomolybdate.³⁷ After heat treatment at 200 °C, the diffraction pattern showed the presence of one broad peak observed at 14.05° (corresponding to the (002) reflection plane of the layered structure), indicating the formation of MoS₃ and that the results are in agreement with the previous studies on the XRD pattern of MoS₃.³⁷ With an increase in decomposition temperature of about 600 °C, significant changes were observed in the XRD pattern, showing the presence of diffraction peaks which can be matched well with the standard diffraction pattern of 2H-MoS₂ (JCPDS card no. 37-1492).^18,42 Further, the (002) reflection plane was much stronger in 2H-MoS₂ compared to MoS₃, suggesting the highly crystalline nature of MoS₂ over MoS₃ powders. Laser Raman spectra of the (NH₄)₂MoS₄, MoS₃, and MoS₂ powders were measured to understand their crystallinity and bonding nature. Fig. 1(C) represents the laser Raman spectra of the (NH₄)₂MoS₄, MoS₃ and MoS₂ powders in which the former two represent almost identical Raman bands with little changes in their peak positions, whereas the latter (MoS₂) showed significant differences in the Raman spectrum. The presence of broad vibrational bands at 350 and 422 cm⁻¹ matched well with the Raman spectrum of amorphous MoS₃ reported by Chang et al.⁴³ After heat treatment at 600 °C, much stronger vibrational bands were observed in the spectrum of MoS₂ at 378.44 and 406.29 cm⁻¹, which arise from the E¹_2g mode (vibration of S–Mo–S trilayers against the adjacent ones) and A_1g mode (out of plane vibrations of the sulfur atoms in the opposite direction).^22,23 The peak position of the A_1g mode was red-shifted compared to the bulk MoS₂, which indicates the formation of few-layered MoS₂.²³ The Raman analysis confirmed the formation of amorphous MoS₃ and crystalline MoS₂via thermal decomposition of (NH₄)₂MoS₄.

Fig. 1 (A) Digital photographs representing the thermal decomposition of ammonium tetrathiomolybdate into MoS₃ and MoS₂ powders, (B) X-ray diffraction pattern of precursor ammonium tetrathiomolybdate, MoS₃ and MoS₂ powder, and (C) laser Raman spectrum of precursor ammonium tetrathiomolybdate, prepared MoS₃ and MoS₂ powder.

The surface morphology of the prepared MoS₃ and MoS₂ powders was examined using field emission-scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HR-TEM) analysis, respectively. Fig. S1 and S2 (ESI†) show the FE-SEM micrographs of MoS₃ and MoS₂ powders measured under different magnifications. The figures show that the size of the MoS₃ and MoS₂ powders is in the range of 200 to 500 nm without any significant changes in the morphology. Moreover, they also show the presence of aggregation of individual sheets, which might be due to the drop-casting method of sample preparation. Fig. 2(A–C) shows the HR-TEM micrograph of MoS₃ with different levels of magnifications, which shows the presence of sheet-like morphologies of MoS₃ rather than the theoretically predicted linear chains or nanoclusters obtained by Vrubel et al. via acidification of MoO₃ using sodium sulfide.²⁶ The observed sheet-like morphology of MoS₃ is in agreement with the previous study of Zhang et al.³⁷ The selected area electron diffraction (SAED) patterns (Fig. 2(D)) showed the diffused ring pattern, confirming the amorphous nature of MoS₃.⁴⁴Fig. 2(E–G) shows the HR-TEM micrograph of MoS₂, revealing the sheet-like morphology of MoS₂ analogous to the structure of graphene.³⁷ The high magnification micrograph shown in Fig. 2(G) confirms the presence of few-layered sheets with crumpled or wrinkled regions at the edges, with an interplanar spacing of 0.62 nm.⁴⁴ The SAED pattern shown in Fig. 2(H) confirms the presence of crystalline diffraction spots, which corresponds to the hexagonal crystal structure of the MoS₂ nanosheets.^26,44Further, X-ray photoelectron spectroscopy was used to identify the chemical and surface states of MoS₃ and MoS₂ prepared via thermal decomposition of (NH₄)₂MoS₄. Fig. S3 (ESI†) shows the comparative survey spectrum of MoS₃ and MoS₂, indicating the presence of Mo 3d and S 2p states in addition to the C 1s components. Fig. 3(A and B) shows the core level de-convoluted spectrum Mo 3d and S 2p states in MoS₃ and MoS₂. The Mo 3d states of MoS₃ (Fig. 3(A)) showed the presence of two major peaks at the binding energies 228.80 and 231.77 eV corresponding to the Mo 3d_5/2 and Mo 3d_3/2 states, respectively. In addition to these peaks, small peaks corresponding to the S 2s and Mo 3d_3/2 states were also observed at binding energies 224.34 and 234.35 eV in the Mo 3d core-level spectrum of MoS₃ sheets. The Mo 3d state of MoS₂ sheets revealed the presence of S 2s, Mo 3d_5/2 and Mo 3d_3/2 states at the binding energies 225.42, 228.45, and 231.72 eV, respectively. Further, the peaks are sharpened in MoS₂ compared to the relatively broad peaks of MoS₃, suggesting the increase in crystallinity in the prepared MoS₂ sheets. Fig. 3(B) shows the comparative S 2p states of MoS₃ and MoS₂ sheets. The core-level spectrum of S 2p states in MoS₃ consisted of two doublets, viz (i) peaks observed in the range from 161.3 to 162.4 eV, highlighting the presence of bridging S₂²⁻ and/or apical S²⁻ ligands, and (ii) peaks observed at 163.28 to 164.33 eV for the terminal S₂²⁻ and/or S²⁻. The observed finding is in good agreement with the reported values in the literature.⁴⁵ On the other hand, the core-level spectrum of S 2p states in MoS₂ revealed the presence of two major peaks around 161.9 and 163.4 eV due to S 2p_3/2 and S 2p_1/2 states, respectively. The observed XPS analysis of MoS₃ and MoS₂ is in good agreement with the previous studies.^23,37,45 Surface area and pore-size distribution of an electrode material are important parameters which determine the electrochemical properties of the electrode and the device. Fig. S4† shows the N₂ adsorption/desorption isotherm and pore size distribution of (NH₄)₂MoS₄, MoS₃, and MoS₂ sheets, respectively. It is clearly evident from Fig. S4† that all the samples exhibited a type IV like isotherm according to the IUPAC standard. The surface area of (NH₄)₂MoS₄, MoS₃, and MoS₂ was found to be 10.34, 17.73 and 23.11 m² g⁻¹, respectively. The pore size distribution of MoS₃ and MoS₂ was found in the range of 30–40 and 25–50 nm, respectively.

Fig. 2 High-resolution transmission electron micrographs of MoS₃ (A–D) and MoS₂ (E–H) powders prepared via thermal decomposition of ammonium thiomolybdate at a temperature of 200 and 600 °C, respectively.

Fig. 3 X-ray photoelectron spectroscopy of MoS₃ and MoS₂ powders prepared via thermal decomposition of ammonium thiomolybdate. (A) Core-level spectrum of Mo 3d states present in MoS₃ and MoS₂ powders and (B) core-level spectrum of S 2p states present in MoS₃ and MoS₂ powders.

The energy storage properties of the prepared MoS₃ and MoS₂ were investigated by fabricating a symmetric coin cell (CR2032) configuration using 0.5 M TEABF₄ as the electrolyte. At first, the cyclic voltammogram (CV) of the fabricated symmetric supercapacitor (SSC) based on MoS₃ and MoS₂ was measured at different potential regimes to understand the polarization of the electrode materials and for the optimization of the ideal operating potential window (OPW) of the SSC. The CV profiles of the MoS₃ SSC device measured over different OPWs between −3.0 and +3.0 V shown in Fig. 4(A) indicated the presence of typical rectangle like curves within the range of ±1.5 V, resembling the ideal capacitive behavior. An increase in OPW to ±3.0 V (Fig. 4(B)) showed the presence of rectangular curves with a pair of redox peaks, indicating the mechanism of ion intercalation/de-intercalation assisted pseudocapacitance.⁴⁶Fig. 4(B) shows the CV profiles of the MoS₃ SSC device recorded over different OPWs from 0 to 3.0 V, which clearly shows that the ion-intercalation reaction at the electrode is diffusion controlled. Fig. 4(C) shows the effect of the scan rate on the CV profiles of the MoS₃ SSC device recorded over an OPW of 0.0 to 3.0 V. There is an increase in current values with an increase in the scan rate as an indication of ideal capacitive nature.⁴⁷ Further, the CV profiles show that the ion-intercalation/de-intercalation process is of irreversible nature, which is probably because the electrolyte ions stuck between the MoS₃ layers after the intercalation process, thus limiting the reversibility of the process.^48,49 However, the scenario is entirely different in the MoS₂ SSC device, as observed from the experimental results shown in Fig. 4(D–F). Fig. 4(D) shows the similar type of electrochemical reactions occurred at the MoS₂ electrode, as seen in the case of MoS₃ (given in Fig. 4(A)). However, the current range in the MoS₂ SSC was higher than that of the MoS₃ SSC, suggesting a higher number of charges are stored at the MoS₂ SSC. Further, the reversible redox peaks were observed in the CV profiles as an indication of the ion intercalation/de-intercalation process. Fig. 4(E) further highlights the presence of a reversible intercalation and de-intercalation reaction occurred at the MoS₂ electrodes when tested in the positive voltage regime. The CV profiles of the MoS₂ SSC device measured over 0.0 to 3.0 V with different scan rates shown in Fig. 4(F) showed the presence of quasi-rectangular shaped curves at high scan rates, indicating the ideal capacitive nature of MoS₂.

Fig. 4 Electrochemical performance of the MoS₃ and MoS₂ SSC devices measured using 0.5 M TEABF₄ electrolyte. The cyclic voltammetric profiles of the MoS₃ SSC device measured under different operating potential regimes (A) from −3 to +3 V, (B) from 0 to +3 V, and (C) CV profiles of the MoS₃ SSC device in the potential regime from 0 to +3 V measured under different scan rates. The cyclic voltammetric profiles of the MoS₂ SSC device measured under different operating potential regimes from (D) −3 to +3 V, (E) 0 to +3 V, and (F) CV profiles of the MoS₂ SSC device in the potential regime from 0 to +3 V measured under different scan rates.

Fig. S5 and S6 (ESI†) show the plot of specific capacity of the MoS₃ and MoS₂ SSC devices versus the OPW. This evidences that the MoS₂ SSC device has higher energy storage properties than the MoS₃ SSC device. Fig. 5(A) shows the effect of the scan rate on the specific capacity of the MoS₃ and MoS₂ SSC devices obtained from the CV analysis. This indicates that the MoS₂ SSC device possesses a higher specific capacity (specific capacitance) of about 33.28 mA h g⁻¹ (39.93 F g⁻¹) compared to the MoS₃ SSC device (27.95 mA h g⁻¹/33.54 F g⁻¹) obtained at a scan rate of 5 mV s⁻¹. Fig. 5(A) also reveals that the amount of charge stored in both MoS₂ and MoS₃ SSC devices is high at low scan rates, which is in agreement with the previous finding of Soon et al.⁵⁰ Further, the MoS₂ SSC device retained a specific capacity of about 64.57% even with an increase in the scan rate up to 20 times, whereas the MoS₃ SSC held only 16.24%. Further, the MoS₂ SSC device retained a specific capacity of about 33.02% even with an increase in the scan rate up to 200 times, suggesting the rate capability of the MoS₂ SSC over MoS₃ SSC.

Fig. 5 Electrochemical performance of the MoS₃ and MoS₂ SSC devices measured using 0.5 M TEABF₄ electrolyte. (A) Effect of the scan rate on the specific capacity of the MoS₃ and MoS₂ SSC devices, (B) Nyquist plot of the MoS₃ and MoS₂ SSC devices, (C) Bode phase angle plots of the MoS₃ and MoS₂ SSC devices, and (D) plot of impedance versus applied frequency of the MoS₃ and MoS₂ SSC devices.

The electrochemical impedance spectroscopy (EIS) technique is one of the versatile tools for understanding the electrochemical reactions, capacitive nature, and diffusion kinetics of an electrolyte/electrode interfacial system.^51–53 Herein, EIS analysis was used to examine the charge storage behavior in the MoS₂ and MoS₃ SSCs in a frequency dependent manner, and the results were analyzed using Nyquist and Bode plots. Fig. 5(B) shows the plot of the real component against the imaginary component of the impedance (Nyquist plot) of the MoS₂ and MoS₃ SSCs. In general, the Nyquist plot is comprised of three specific regions. viz (i) high-, (ii) mid-, and (iii) low-frequency regions as seen in Fig. 5(B).⁵¹ At the high-frequency region, the presence of small semi-circle was observed for the MoS₂ SSC, whereas the diameter of the semi-circle was higher for the MoS₃ SSC.^54,55 The equivalent series resistance (ESR) of the MoS₃ and MoS₂ devices was found to be 20.53 and 3.64 Ω, respectively, indicating that the MoS₂ SSC possesses good power capabilities than the MoS₃ SSC.⁵⁶ The MoS₂ SSC device showed a higher maximal areal power density (P_max = V²/4ESR) of about 332.32 mW cm⁻² compared to the MoS₃ SSC device (58.92 mW cm⁻²). The obtained maximal areal power density of the MoS₂ SSC device (332.32 mW cm⁻²) was quite higher compared to the reported values of the onion-like carbon-based SSC (240 mW cm⁻²) and silicon nanowire based SSC (182 mW cm⁻²).^56,57 At the low-frequency region, both devices showed the presence of a straight line/Warburg line, which is due to the frequency dependent ion-transfer kinetics.⁵⁸ The Warburg line of the MoS₂ SSC device was closer towards the imaginary component of the impedance compared to that of the MoS₃ SSC, suggesting the better capacitive nature of MoS₂ over MoS₃ in organic liquid electrolytes.⁵⁹Fig. 5(C) shows the Bode phase angle plot of the MoS₃ and MoS₂ SSC devices. It shows that the phase angle at the low-frequency region is about 54.18° and 72.26° for the MoS₃ and MoS₂ SSC, indicating that MoS₂ the SSC device possesses better capacitive nature.⁶⁰ The capacitor-response frequency (f_o) measured at the phase of −45° was found to be about 73.35 and 8.9 Hz for the MoS₃ and MoS₂ SSC devices. The relaxation time (τ) of the MoS₃ and MoS₂ SSC devices was calculated to be 13.6 and 112 ms using the relation (τ = 1/f_o).⁶¹ The plot of impedance against the frequency of the MoS₃ and MoS₂ SSCs is shown in Fig. 5(D), which shows the higher resistance of the MoS₃ SSC device than the MoS₂ SSC device in the low-frequency region (0.01 to 100 KHz).

The galvanostatic CD profiles of the MoS₃ SSC measured using different currents are provided in Fig. 6(A). The figure shows the presence of sloppy CD profiles indicating the irreversible charging and discharging process due to the pseudocapacitive nature of the MoS₃ electrodes. On the other hand, the CD profiles of the MoS₂ SSC obtained at different currents (as shown in Fig. 6(B)) displayed almost symmetric profiles at high current ranges due to the reversible ion intercalation and de-intercalation process. The CD profiles obtained at a low current of 0.5 mA showed sloppy discharge curves, which are due to the time constraints of electrolyte ions at the low current range. Fig. 6(C) shows the comparative CD profiles of the MoS₃ and MoS₂ SSC devices obtained using a current of 0.5 mA. It shows that the charging and discharging time is higher for the MoS₂ SSC compared to that of the MoS₃ SSC device, thus indicating the amount of charge stored at the MoS₂ SSC device is higher than at the MoS₃ SSC device. The comparative performance of the MoS₃ and MoS₂ SSC devices was studied using the plot of specific capacity against the current range, as shown in Fig. 6(D). The specific capacity of the MoS₂ SSC device is quite higher compared to that of the MoS₃ SSC device at all the tested current ranges. A maximum specific capacity (specific capacitance) of 5.25 mA h g⁻¹ (6.3 F g⁻¹) and 20.81 mA h g⁻¹ (24.98 F g⁻¹) was obtained for the MoS₃ and MoS₂ SSCs measured using a constant current of 0.5 mA, thus highlighting the superior energy-storage performance of the MoS₂ SSC. This study further evidences the better performance of MoS₂ over MoS₃ for electrochemical energy storage devices using the organic liquid electrolyte.

Fig. 6 Electrochemical performance of the MoS₃ and MoS₂ SSC devices measured using 0.5 M TEABF₄ electrolyte. The CD profiles of the (A) MoS₃ and (B) MoS₂ SSCs measured using different current densities, (C) comparative CD profiles of the MoS₃ and MoS₂ SSCs measured at a constant current of 0.5 mA, (D) effect of applied current ranges on the specific capacity of the MoS₃ and MoS₂ SSCs, (E) Ragone plot of the MoS₃ and MoS₂ SSC devices. Table S1† summarizes the references given in (E), and (F) the practical application of a fully charged MoS₂ SSC to glow ten LEDs.

The energy density (E), power density (P), and cyclic stability are some of the important parameters which determine the practical applications of a supercapacitor device.⁶²Fig. 6(E) shows the comparative analysis of energy/power performance of the MoS₃ and MoS₂ SSC devices using the Ragone plot. The energy density of the MoS₂ SSC device ranges from 20.68 to 12.80 W h kg⁻¹ with the corresponding change in power density from 496.71 to 1858.64 W kg⁻¹, whereas the corresponding energy density of the MoS₃ SSC device ranges from 3.39 to 0.659 W h kg⁻¹ with the power density ranging from 323.29 to 790.98 W kg⁻¹. The energy/power rating of the MoS₂ SSC device is about 20.68 to 4.60 W h kg⁻¹ with a corresponding power density from 496.71 to 3380.06 W kg⁻¹ as obtained from the CD analysis recorded using different current ranges (0.5 to 5 mA). The energy density of the MoS₂ SSC device (20.68 W h kg⁻¹) is quite higher than many of the SSCs using organic electrolytes as evidenced from Fig. 6(E) and Table S1 (ESI†).^13,63–65 Fig. S7 (ESI†) shows the cyclic stability analysis of the MoS₃ and MoS₂ SSC devices over 2000 cycles obtained using a current range of 1 and 2.5 mA, respectively. It shows that the MoS₂ SSC device has a higher capacity retention of about 90% of its initial capacity, whereas the MoS₃ SSC device holds only 79.66% of its initial capacity after 2000 cycles. The better electrochemical performance of MoS₂ over MoS₃ in the organic liquid electrolyte can be described based on the structure and surface of these two materials as follows: (i) MoS₂ is a well crystalline layered material with a large interlayer spacing compared to that of amorphous MoS₃ with limited spacing, which can make the crystalline MoS₂ as a good candidate for the ion intercalation and de-intercalation process over MoS₃; (ii) the amorphous MoS₃ possesses additional sulphur ligands bonded with Mo (compared with MoS₂) which can bind with the electrolyte ions during the charging and discharge process, thereby increasing the possibility of ions being stuck between the layers and the surface of MoS₃ (as seen from Fig. 3(B)); (iii) the surface area and pore size distribution are higher in MoS₂ compared to MoS₃, suggesting that MoS₂ (as evident from Fig. S4†) can provide superior electrochemical properties over MoS₃. Fig. 6(F) shows the practical application of the fully charged MoS₂ SSC to light up 10 LEDs. Altogether, these studies revealed that crystalline MoS₂ possess superior energy storage properties in the organic liquid electrolyte with better power capabilities over the amorphous MoS₃.

3. Conclusions

In conclusions, we have demonstrated the cost-effective preparation of MoS₃ and MoS₂ sheets and investigated their energy storage properties in the organic liquid electrolyte. The significant findings of this study suggested that the crystalline MoS₂ holds promising electrochemical energy storage properties using an organic liquid electrolyte compared to the amorphous MoS₃. The MoS₂ SSC device operates over a potential window of 3.0 V, delivering a high specific capacity (specific capacitance) of 20.81 mA h g⁻¹ (24.98 F g⁻¹) with a high energy density of 20.68 W h kg⁻¹, which is almost six times higher than that of the MoS₃ SSC device. The better electrochemical properties of the crystalline MoS₂ sheets are mainly due to their better crystallinity and the high surface area, which provides better reactive sites for the ion intercalation/de-intercalation pathways compared to the amorphous MoS₃ sheets. The collective findings of this study can give new insight on molybdenum sulfide materials towards the development of next-generation energy storage devices.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018R1A4A1025998 and 2019R1A2C3009747).

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Footnotes

† Electronic supplementary information (ESI) available: The experimental section, additional results, and discussion. See DOI: 10.1039/c9qi00623k

‡ These authors contributed equally.

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