Enhanced electrochemical performance of MoS₂/graphene nanosheet nanocomposites

Abstract

Molybdenum disulfide (MoS₂) is attractive as an anode material for next-generation batteries, because of its layered structure being favorable for the insertion/deinsertion of Li⁺ ions, and its fairly high theoretical capacity. However, since the MoS₂ anode material has exhibited disadvantages, such as low electrical conductivity and poor cycling stability, to improve the electrochemical performance of MoS₂ in this study, a nanocomposite structure consisting of MoS₂ and GNS (MoS₂/GNS) as an anode for LIBs was prepared, by controlling the weight ratios of MoS₂/GNS. The X-ray diffraction patterns and electron microscopic analysis showed that the nanocomposite electrode structure consisted of well-formed MoS₂ nanoparticles and GNS. Compared to MoS₂-only, the MoS₂/GNS composites exhibited high retention and improved capacity at high current densities. In particular, among these nanocomposite samples, MoS₂/GNS(8 : 2) with an appropriate portion of GNS exhibited the best LIB performance, due to the lowest interfacial resistance and highest Li-ion diffusivity.

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1 Introduction

Lithium-ion batteries (LIBs) are energy devices that convert chemical energy to electrical energy using electrochemical reactions, i.e., reduction and oxidation between Li⁺ ions and active electrode materials.^1–3 The electrochemical reduction and oxidation reactions spontaneously occur at the cathode and anode, respectively. In particular, the anode materials require typical characteristics, such as relatively low standard electrode potential, stable structure during the reactions with Li⁺ ions, large diffusion coefficient, high electrical conductivity, and excellent reversibility of the reaction with Li⁺ ions.⁴ Typically, the anodes for LIBs exhibit one of three reaction mechanisms (insertion, conversion, and alloying), during electrochemical reaction between Li⁺ ions and active anode material.⁵ According to the insertion reaction, Li⁺ ions are stored in the structure of the active material (M) with relatively low capacity, such as Li₄Ti₅O₁₂, TiO₂, and MoO₂, as follows: Li⁺ + M_yX_z → Li_xM_yX_z. The alloying materials, such as Si, Ge, and Sn, with high theoretical capacities and serious volumetric variation during cycling exhibit the alloying reaction of Li⁺ + M → Li_xM. Finally, the active materials, such as CoO, NiO, and MoO₃, with relatively high capacities, low efficiencies, and low cell voltages exhibit charging/discharging during the conversion reaction, such as Li⁺ + M_yX_z → yM + Li_xX_z.⁶

Molybdenum disulfide (MoS₂) has been attractive as an anode material for next-generation batteries, because of its layered structure being favorable for the insertion/deinsertion of Li⁺ ion, and a fairly high theoretical capacity of 670 mA h g⁻¹.^7–12 However, MoS₂ anode material has exhibited disadvantages, such as low electrical conductivity, and poor cycling stability, because of the low electric conductivity and the solid electrolyte interphase (SEI) layer during the conversion reaction.^13–15 Recently, to compensate the electrochemical properties of the MoS₂ anode for LIBs, carbon-based materials have been used for composite electrode structures, because of their excellent electrical conductivity, high specific surface area, and stability.^16–18 In particular, the composite electrodes consisting of MoS₂ and carbon can suppress the shuttling phenomenon of lithium polysulfides as by-products of the electrochemical reactions of MoS₂ with Li⁺ ions, resulting in the improved cycling performance of the composite electrodes.^19–21 Mengmeng et al. reported a hybrid electrode consisting of vertically aligned MoS₂ and nitrogen-doped carbon with improved cycling properties for 300 cycles and a high capacity of 995 mA h g⁻¹ at 0.2 A g⁻¹.²² Wang et al. confirmed that the MoS₂ anode structure with three-dimensional fibrous nitrogen-doped carbon sphere exhibited a high capacity of 700 mA h g⁻¹ at 1.2 A g⁻¹ for 400 cycles.²³ The MoS₂-based nanostructured electrode synthesized with nano-Si and carbon maintained a fairly high capacity of 767 mA h g⁻¹ at 100 mA g⁻¹ for 400 cycles.²⁴ In particular, graphene nanosheet (GNS) exhibits a 2-dimensional layered graphitic nanostructure with a weak van-der Waals force between layers, and has offers, such as high electrical conductivity, and high specific surface area.^25–27 Thus, in this study, to improve the electrochemical performance of MoS₂, a nanocomposite structure consisting of MoS₂ and GNS (MoS₂/GNS) as an anode for LIBs was prepared by controlling the weight ratios of MoS₂/GNS. The electrochemical properties of the MoS₂/GNS nanocomposite samples were characterized and evaluated using coin-type cells.

2 Experimental

2.1. Synthesis of MoS₂ and GNS

To prepare the MoS₂/GNS nanocomposites, the precursor for MoS₂ nanoparticle was firstly synthesized. Ammonium molybdate (0.88 g, (NH₄)₆Mo₇O₂·4H₂O, Sigma-Aldrich) and sodium sulfide nonahydrate (2.64 g, Na₂S·9H₂O, Sigma-Aldrich) were mixed in 0.8 M HCl solution (100 mL) at 80 °C with stirring for 30 min. Hydroxylamine hydrochloride (0.7 g, NH₂OH·HCl, Sigma-Aldrich) was added to the precursor solution at 80 °C, and then mixed with continuous stirring for 90 min. After the complete reaction, the dark brown precipitate was washed with de-ionized (DI) water and ethanol and dried in a 50 °C oven for 24 h. The GNS was obtained by mixing graphene (Graphene Supermarket) in a solution of H₂SO₄ : HNO₃ = 3 : 1 at 120 °C for 2 h, washing with de-ionized (DI) water, and drying using a freeze-dryer.

2.2. Synthesis of MoS₂/GNS

The MoS₂ precursor and GNS with different weight ratios of 9 : 1, 8 : 2, and 7 : 3 were added to 50 mL N-methyl-2-pyrrolidone (NMP) solution and then mixed with ultrasonication for 4 h (denoted respectively) (Fig. 1). The resulting precipitate was washed using ethanol and DI water and then dried in a 50 °C oven for 12 h. Finally, the dried sample was heated in an N₂ atmosphere at 500 °C for 3 h.

Fig. 1 Schematic illustration of the synthetic process of the MoS₂/GNS nanocomposite.

2.3. Materials characterization

The crystal structure of the as-prepared samples was characterized by an X-ray diffractometry (XRD, Bruker, D2 Phase system) operating at 40 kV and 100 mA with an X-ray source of Cu K_α (λ = 0.15418 nm) and a Ni filter. The morphology of the samples was confirmed using field-emission scanning electron spectroscopy (FE-SEM, ZEISS, Germini 300). The morphology and crystal structure of the samples were characterized by Cs-corrected transmission electron microscopy (Cs-TEM, JEM-ARM200F, JEOL Ltd, Japan). Thermogravimetric analysis (TGA) were conducted using a thermal analyzer (SDTA851, Mettler Toledo) in the range of 25 to 700 °C under an air flow of 50 cm³ min⁻¹. Raman spectroscopy analysis was performed using a DM2700 Raman spectroscope (RENISHAW, 2GTE70) with the excitation wavelength of 532 nm of Nd:YAG laser.

2.4. Cell assembly and electrochemical characterization

To characterize the electrochemical properties of the samples, coin-type half-cells with the samples as anodes were prepared in an Ar-filled glove box. The slurry was prepared using a paste mixer with the as-prepared powder samples as active materials (80 wt%), Ketjen black as a conducting agent (10 wt%), and polyvinylidene fluoride (PVDF, 10 wt%) as a binder. The as-prepared slurry was coated on a Cu foil using the doctor blade method, and then dried in a 110 °C convection oven for 24 h. The half-cell was assembled with an active material-coated electrode, 1.1 M LiPF₆ as an electrolyte, polyethylene as a separator, and lithium metal as a counter electrode. Charge–discharge curves of the samples were obtained by a multichannel battery tester (WBCS3000L, Wonatech Co.) in the potential range of 0–3 V vs. Li/Li⁺ with various current densities of 100, 200, 500, 1000, and 2000 mA g⁻¹. Cyclic voltammograms (CVs) of the samples were measured at a scan rate of 0.2 mV s⁻¹ in the potential range of 0–3 V vs. Li/Li⁺. The interface resistances of the cells were measured by electrochemical impedance spectroscopy (EIS). To measure Li-ion conductivity for the samples, galvanostatic intermittent titration technique (GITT) measurement was performed at a current density of 100 mA g⁻¹ for 10 min intervals, with rest periods of 10 min, in the potential range of 0–2 V Li/Li⁺. The electrical conductivity of the samples was obtained by measuring I–V curves using a conductivity kit.

3 Results and discussion

Fig. 2 shows XRD patterns of MoS₂/GNS 9 : 1, MoS₂/GNS 8 : 2, MoS₂/GNS 7 : 3, and MoS₂-only prepared in the absence of GNS. All of the samples contained XRD peaks at 14.4°, 32.7°, 39.6°, 49.8°, and 58.3°, corresponding to the (002), (100), (103), (105), and (110) planes, with the MoS₂ crystal structure (PDF-65-1951) with lattice parameters of a = 3.16 Å, c = 12.30 Å, without diffraction peaks related to impurities or molybdenum oxides. In particular, MoS₂/GNS 9 : 1, MoS₂/GNS 8 : 2, and MoS₂/GNS 7 : 3 exhibited a distinct peak at 26.5° associated with the (002) plane of graphene crystal structure as well as the characteristic peaks corresponding to the MoS₂ structure.^28,29 The average particle sizes of MoS₂/GNS 9 : 1, MoS₂/GNS 8 : 2, MoS₂/GNS 7 : 3, and MoS₂-only were determined to be 3.97, 3.47, 4.47, and 4.48 nm, respectively, using the Scherrer equation. The order of the particle size in the samples is as follows: MoS₂/GNS 7 : 3 ∼ MoS₂/GNS 8 : 2 < MoS₂/GNS 9 : 1 ∼ MoS₂-only. According to the present synthesis, the MoS₂ precursor and GNS with different weight ratios were heated in an N2 atmosphere at 500 °C for 3 h. Thus, as the content of GNS increased in the synthesis, the particle size of MoS₂ structure decreased. This demonstrates the GNS could hinder the growth of MoS₂ nanoparticles. In the Raman spectra of the samples, the E2g1 and A_1g peaks at ∼380 and ∼408 cm⁻¹, correspond to Mo–S and S vibrations in MoS₂ structure (Fig. S1(a)†). The D- and G-bands at ∼1350 and ∼1584 cm⁻¹, correspond to the disorder and crystal structure of carbon, respectively, with a low I_D/I_G, demonstrating the high crystallinity of the carbon structure (Fig. S1(b)†).^30–32 Thus, it can be inferred that a nanocomposite electrode structure consisting of MoS₂ nanoparticles and GNS could be formed as an anode for LIBs.

Fig. 2 Wide-range XRD patterns of the as-prepared MoS₂/GNS samples as anodes.

The morphology and particle size of the samples were observed through SEM analysis (Fig. 3). Compared to MoS₂-only, for the MoS₂/GNS samples, MoS₂ nanoparticles were homogeneously mixed with 2-dimensional GNS structure. As the content of GNS increased in the synthesis, the portion of GNS increased as the particle size of MoS₂ decreased. The structure of the samples was characterized using TEM analysis (Fig. 4). The interplanar spacing of the samples was determined from the HR-TEM images to be ∼0.620 nm corresponding to that of the (002) plane of MoS₂. The average particle sizes of MoS₂/GNS 9 : 1, MoS₂/GNS 8 : 2, MoS₂/GNS 7 : 3, and MoS₂-only were determined to be 3.9, 3.6, 3.7, and 4.4 nm, respectively, which were in good agreement with those from the XRD analysis (Fig. S2†). To investigate the contents in the samples, TGA was performed with the samples in the temperature range of 25–950 °C under an air atmosphere (Fig. S3†). The weight losses at 25–200 °C and ∼300 °C might result from an evaporation of water molecules in the samples and the transition of MoS₂ to MoO₃, respectively. Furthermore, the weight loss at >400 °C implies the content of carbon, resulting from the carbon oxidation. The Thus, the contents of carbon in MoS₂/GNS 9 : 1, MoS₂/GNS 8 : 2, and MoS₂/GNS 7 : 3 were 9.5, 32.7, and 45.6 wt%, respectively.^33,34 The electrical conductivity values of MoS₂-only, MoS₂/GNS 9 : 1, MoS₂/GNS 8 : 2, and MoS₂/GNS 7 : 3 were 2.1 × 10⁻³, 2.693, 3.545, and 1.420 S cm⁻¹, respectively, demonstrating an excellent electronic motion of the composite structure.

Fig. 3 SEM images of (a) MoS₂-only, (b) MoS₂/GNS 9 : 1, (c) MoS₂/GNS 8 : 2, and (d) MoS₂/GNS 7 : 3.

Fig. 4 TEM images of (a and e) MoS₂-only, (b and f) MoS₂/GNS 9 : 1, (c and g) MoS₂/GNS 8 : 2 and (d and h) MoS₂/GNS 7 : 3.

Fig. 5 shows the charge/discharge characteristic curves of the samples as anodes measured at a current density of 200 mA g⁻¹ in the potential range of 0–3 V vs. Li/Li⁺ for 100 cycles. After an initial activation cycling, the discharge capacities of MoS₂/GNS 9 : 1, MoS₂/GNS 8 : 2, MoS₂/GNS 7 : 3, and Mo₂S-only measured after the 3^rd cycle were 686, 662, 542, and 760 mA h g⁻¹, respectively. The MoS₂/GNS composite samples exhibited a lower initial capacity than that of the Mo₂S-only. Furthermore, the discharge capacities of MoS₂/GNS 9 : 1, MoS₂/GNS 8 : 2, MoS₂/GNS 7 : 3, and Mo₂S-only after 100 cycles were 278, 613, 213, and 159 mA h g⁻¹, respectively. The retentions of MoS₂/GNS 9 : 1, MoS₂/GNS 8 : 2, MoS₂/GNS 7 : 3, and Mo₂S-only were 40.5%, 92.6%, 39.2%, and 20.9%, respectively. After 100 cycles, compared to the Mo₂S-only, composite anodes showed relatively relieved aggregation effect between the electrochemically active particles during cycling (Fig. S4†). As compared to the MoS₂-based anode in the literatures, MoS₂/GNS 8 : 2 exhibited fairly improved electrochemical performance, i.e. a discharge capacity of 613 mA h g⁻¹ at 200 mA g⁻¹ and a retention of 92.6% after 100 cycles (Table S1†).^8,24,35–38 The MoS₂/GNS samples, despite relatively low capacities, exhibited significantly improved cycling performance, because of the enhanced stability of the nanocomposite structure with GNS during the conversion reaction. For the nanocomposite electrodes, the GNS with chemical and structural stability, excellent electrical properties, and high specific surface area can enhance the electrical conductivity, improving the cycling performance. To characterize the electrochemical properties of the samples, the CVs were measured at a scan rate of 0.2 mV s⁻¹ in the potential range of 0–3 V vs. Li/Li+ (Fig. 6). In the 1st cycle during a cathodic potential scanning, lithium intercalation and conversion reactions appeared at ∼0.9 and ∼0.5 V, respectively (eqn (1) and (2)), forming the solid electrolyte interphase (SEI) layer on electrode surfaces from the decomposition products of electrolytes. The decomposition of Li_xMoS₂ and oxidation of Li₂S occurred at ∼1.8 and ∼2.4 V, respectively, during an anodic scanning (eqn (3) and (4)). Followed by the 1st scan, the electrochemical reduction reactions appeared at 1.3 V, respectively, during cathodic scanning in the subsequent cycles (eqn (4) and (5)).^39–41 In particular, the CVs for MoS₂/GNS composite samples contained the oxidation characteristic peaks at 0.2 V associated with an intercalation of GNS as an anode, compared to Mo₂S-only without GNS.

MoS₂ + xLi⁺ + xe⁻ → Li_xMoS₂ (1)

Li_xMoS₂ + (4 − x)Li⁺ + (4 − x)e⁻ → Mo + 2Li₂S (2)

Li_xMoS₂ → MoS₂ + xLi⁺ + xe⁻ (3)

Li₂S → 2Li⁺ + S + 2e⁻ (4)

2Li⁺ + S + 2e⁻ → Li₂S (5)

Fig. 5 (a) Cycle performances of MoS₂/GNS electrodes at 200 mA g⁻¹. Charge–discharge profiles of (b) MoS₂-only, (c) MoS₂/GNS 9 : 1, (d) MoS₂/GNS 8 : 2, and (e) MoS₂/GNS 7 : 3 measured at a current density of 200 mA g⁻¹.

Fig. 6 CV curves of (a) MoS₂-only, (b) MoS₂/GNS 9 : 1, (c) MoS₂/GNS 8 : 2, (d) MoS₂/GNS 7 : 3 in the 1^st, 3^rd, 10^th scans at a scan rate of 0.2 mV s⁻¹ in the potential range of 0.0–3.0 V vs. Li/Li⁺.

Fig. 7 shows the rate cycling performance of the samples as cathodes measured at various current densities from 100 to 2000 mA g⁻¹ in the potential range of 0–3 V for five cycles each. The average capacities of MoS₂-only at 100, 200, 500, 1,000, 2,000, 100, and 200 mA g⁻¹ were 732, 442, 166, 64, 28, 264, and 119 mA h g⁻¹, respectively. The average capacities of MoS₂/GNS 9 : 1 at 100, 200, 500, 1,000, 2,000, 100, and 200 mA g⁻¹ were 817, 721, 545, 386, 261, 887, and 556 mA h g⁻¹, respectively. The average capacities of MoS₂/GNS 8 : 2 at 100, 200, 500, 1,000, 2,000, 100, and 200 mA g⁻¹ were 822, 764, 623, 480, 323, 879, and 736 mA h g⁻¹, respectively. The average capacities of MoS₂/GNS 7 : 3 at 100, 200, 500, 1,000, 2,000, 100, and 200 mA g⁻¹ were 489, 430, 279, 154, 74, 479, and 264 mA h g⁻¹, respectively. The MoS₂/GNS composite samples exhibited improved high-rate performance and recovery rate because of the enhancement of electronic motion caused by GNS as an excellent conductor, compared to MoS₂-only without GNS.

Fig. 7 High-rate performance of the MoS₂/GNS anodes measured at varying current densities for each 10 cycles.

Fig. 8 compares the interfacial resistance (R_ct) of the MoS₂/GNS composites and MoS₂-only after the 3^rd cycle between electrolyte and electrode. The values of R_ct for Mo₂S-only, MoS₂/GNS 9 : 1, MoS₂/GNS 8 : 2, and MoS₂/GNS 7 : 3 were 131, 20, 17, and 43 Ω, respectively. Compared to MoS₂-only, the MoS₂/GNS composites showed decreased resistance, resulting from the improved conductivity of the composite caused by GNS with excellent electrical conductivity. In particular, among these composites, MoS₂/GNS 8 : 2 with an appropriate portion of GNS has the lowest interfacial resistance.^42,43 Furthermore, in the initial states, MoS₂-only showed increased interfacial resistance whereas the MoS₂/GNS composites maintained the low resistance due to the improved electrical and electrochemical properties by GNS (Fig. S5†). To determine the Li-ion conductivity of the samples as anodes, the GITT analysis was performed after the 3^rd cycle.⁴⁴ Based on the diffusivity of Li⁺ ion according to the state of charge (SOC), compared to MoS₂-only, the MoS₂/GNS composites exhibited overall improved ionic motion (Fig. 9a). Also, when compared to the average diffusivity, the MoS₂/GNS composites exhibited higher values than MoS₂-only (Fig. 9b). In particular, MoS₂/GNS 8 : 2 showed the best Li-ion diffusivity. Thus, the improved LIB performance of MoS₂/GNS 8 : 2 with a proper portion of GNS results from the lowest interfacial resistance and highest Li-ion diffusivity.

Fig. 8 Nyquist plots of the samples after 3 cycles at a current of 200 mA g⁻¹.

Fig. 9 The GITT analysis after the 3^rd cycle. (a) Diffusivity of Li⁺ ion according to the state of charge (SOC) and (b) the average Li⁺ ion diffusion coefficients of the samples.

4 Conclusions

In summary, nanocomposite electrodes with MoS₂ and GNS were prepared to improve the electrochemical performance of MoS₂ as an anode for LIBs. The nanocomposite electrode structure was found to consist of MoS₂ nanoparticles and GNS. Compared to MoS₂-only, the MoS₂/GNS composites exhibited improved electrochemical performance, i.e. high retentions and improved capacities at high current densities. In particular, MoS₂/GNS 8 : 2 with a proper portion of GNS exhibited improved LIB performance due to the lowest interfacial resistance between electrode and electrolyte and highest Li-ion diffusivity. Thus, the nanocomposite electrode consisting of MoS₂ and GNS can be utilized as an anode for high-performance LIBs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No.2020R1A2C2010510).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0ra03539d

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