Preparation of MoS₂/Ti₃C₂T_x composite as anode material with enhanced sodium/lithium storage performance

Abstract

MoS₂/Ti₃C₂T_x composite was synthesized by a facile one-step hydrothermal method without further annealing process. When tested as anode material for sodium-ion batteries, it exhibited a high reversible capacity of 331 mA h g⁻¹ at 100 mA g⁻¹ after 70 cycles with only 0.058% decay per cycle. When tested as anode material for lithium-ion batteries, it exhibited a reversible capacity of 614.4 mA h g⁻¹ at 100 mA g⁻¹ after 70 cycles with only 0.05% decay per cycle. Moreover, compared with pristine MoS₂ and pure Ti₃C₂T_x, the composite had better rate performance and faster ion diffusion kinetics, which might be caused by the as-prepared composite material having relatively rough surface, more active sites and more convenient diffusion paths.

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Introduction

Developing environmentally friendly, economical, secure and sustainable energy source is the greatest societal and scientific challenge in the 21^st century.^1–3 To meet these challenges, it is an urgent mission to develop renewable and sustainable energy sources, such as water, solar and tidal power.^4,5 However, these types of energy sources have inevitable shortcomings, such as instability and intermittency, and thus cannot be put into application directly. Therefore, energy storage and conversion devices have great significance for smooth integration of renewable energy sources into power grids. Specifically, lithium-ion batteries (LIBs) are playing an important role in powering many electronic devices in our daily lives from laptops to electric vehicles. LIBs have many merits, such as long cycle life, no memory effect, minor self-discharge and environment friendliness. However, the supply of natural lithium sources is limited. In recent years, taking sodium's cost and abundance into consideration, sodium-ion batteries (SIBs) have been regarded as a promising alternative to LIBs. Though many breakthroughs have been achieved, SIBs still have been relatively slowly developed and pales in comparison with that of LIBs.⁵ Therefore, in the foreseeable future, SIBs will draw more research attention while LIBs will still be the center of the commercial market. As a consequence, electrode materials for LIBs/SIBs with merits such as high reversible capacity, long lifespan and good electronic conductivity are promising.⁶

In terms of anode materials, graphene has excellent electrochemical performance due to its two-dimensional structure, which facilitates the insertion/extraction of ions between layers.^7–9 As a new type of two-dimensional material, MXene has received great attention since it was first reported in 2011 by Naguib et al.¹⁰ MXene is obtained by selectively etching the atom layers (such as Al or Si) from MAX phase with hydrofluoric acid. M represents early transition metal elements, X represents carbon and/or nitrogen and T_x represents the surface hydrophilic groups in the formula M_n+1X_nT_x (n = 1, 2, 3) of MXene.¹⁰ The applications of MXene are wide, including energy storage¹¹ and catalysis.¹² MXene possesses a surface with abundant functional groups, and it has merits such as high electronic conductivity and low ion diffusion resistance in LIBs/SIBs.^13–15 Nevertheless, pure MXene suffers from low capacity when used as LIBs/SIBs anode material, which is an obstacle for its application. Fortunately, it has surface hydrophilic groups (such as −OH, −F, =O) and a layered structure; hence, appropriate strategies can be devised to synthesize MXene composites with improved capacity. Many studies with the purpose of improving the reversible capacity of MXene composites have been reported,^16,17e.g., preparation of SnS/Ti₃C₂T_x composites.¹⁸ Therefore, combining sulfide with Ti₃C₂T_x is a worthy idea to improve the capacity of MXene.

With the aim of developing secondary batteries for energy storage, electrode materials with long life span and low cost are favorable. MoS₂ is a type of layered transition metal dichalcogenide. It is an ideal electrode material for LIBs/SIBs because it has much higher capacity than traditional commercial materials. Particularly, Mo atoms have many different oxidation states, and MoS₂ has favorable charge storage capability.¹⁹

However, in the process of ion insertion/extraction, pristine MoS₂ suffers drastic capacity fading caused by large volume change owing to its large surface energy, leading to the stacking and aggregation of MoS₂ layers. Furthermore, with only a few accessible active sites and low electronic conductivity, MoS₂ has limited rate performance in LIBs/SIBs. In contrast, Ti₃C₂T_x has excellent electronic conductivity but lower capacity. Hence, combining Ti₃C₂T_x with MoS₂ may combine the merits of these two materials by synergistic effect.^16,20–22

In this work, MoS₂/Ti₃C₂T_x composite was synthesized by a facile one-step hydrothermal method. MoS₂ nanosheets anchored on Ti₃C₂T_x layers uniformly to create a hierarchical structure, which enhanced the capacity of Ti₃C₂T_x and the cycling stability of the MoS₂ nanosheets due to the synergistic effect. Furthermore, MoS₂/Ti₃C₂T_x composite, with the enlarged interlayer space of Ti₃C₂T_x and a rough surface, was conducive to ion diffusion and expanded the electrode–electrolyte contact surface area. Therefore, when used as anode material for SIBs and LIBs, impressive cycling stability, excellent rate performance and faster ion diffusion kinetics were achieved owing to the helpful synergistic effect.

Results and discussion

The MoS₂/Ti₃C₂T_x composite was synthesized by a facile one-step hydrothermal method without further annealing process. A schematic of the synthesis is shown in Fig. 1a. First, during the selective etching process, Al atom layers were removed from the MAX (Ti₃AlC₂) phase by HF solution and a loosely accordion-like MXene (Ti₃C₂T_x) phase was obtained. Then, the as-prepared Ti₃C₂T_x powder, Na₂MoO₄·2H₂O and CH₃CSNH₂ were dispersed into deionized water, due to which the functional groups (−F, −OH, =O) on the surface of Ti₃C₂T_x could firmly attach to the Mo⁶⁺ derived from Na₂MoO₄·2H₂O. Subsequently, CH₃CSNH₂ acted as a reductant and sulfur source; then, MoS₂ nanosheets were in situ deposited on the Ti₃C₂T_x substrate. This method has the following two merits: it required no further annealing process, and MoS₂ nanosheets could firmly and uniformly be decorated onto the Ti₃C₂T_x substrate.

Fig. 1 (a) Schematic of the synthesis processes of MoS₂/Ti₃C₂T_x composite. (b, c) FESEM and TEM images, respectively, of pure Ti₃C₂T_x. (d, e) FESEM images of MoS₂/Ti₃C₂T_x composite. (f, g) TEM images of MoS₂/Ti₃C₂T_x composite.

Fig. 1b and Fig. S1† are the FESEM images of pure Ti₃C₂T_x after the etching process. The structure appears accordion-like, where each layer of Ti₃C₂T_x is similar to graphene's two-dimensional structure and forms a hierarchical structure. The TEM image (Fig. 1c) further confirms the hierarchical structure of pure Ti₃C₂T_x.

The FESEM images of the MoS₂/Ti₃C₂T_x composite are shown in Fig. 1d and e. They demonstrate that MoS₂/Ti₃C₂T_x composite has a structure with MoS₂ nanosheets uniformly distributed on the Ti₃C₂T_x substrate. The energy dispersive X-ray mapping of the MoS₂/Ti₃C₂T_x composite is shown in Fig. S2,† which illustrates the uniform distribution of Ti (green), Mo (red) and S (yellow) on the surface and interlayers of Ti₃C₂T_x substrates, and further confirms the presence of MoS₂. Moreover, it can be clearly seen in Fig. 1e that a large number of MoS₂ nanosheets were deposited in Ti₃C₂T_x interlayers, providing many sites for ion intercalation and shortening the paths for ion diffusion. The FESEM image of pristine MoS₂ is shown in Fig. S3,† in which the MoS₂ nanosheets stacked closely with each other and few active sites were accessible. Further observation of MoS₂/Ti₃C₂T_x composite by TEM is shown in Fig. 1f, which confirmed that MoS₂ nanosheets were attached on the Ti₃C₂T_x layers. HRTEM images, as shown in Fig. 1g and inset, reveal spacings of the lattice fringe as 0.31 nm and 1.0 nm, corresponding to the (004) plane of MoS₂ and the (002) plane of Ti₃C₂T_x. Nevertheless, it is evident that MoS₂ nanosheets have a hierarchical structure with a few (∼3–5) monolayers. The coupling of few-monolayered MoS₂ nanosheets and highly conductive substrates such as graphene not only accelerates the electron transfer but also improves capacity and cycling stability.¹⁹ Therefore, it is a worthy idea to combine MoS₂ nanosheets with a highly conductive substrate such as Ti₃C₂T_x in order to achieve electrode materials with higher capacity, longer lifespan, lower electronic diffusion resistance and faster ion diffusion kinetics.

The XRD patterns of MoS₂/Ti₃C₂T_x composite, pure Ti₃C₂T_x and pristine MoS₂ are shown in Fig. 2a. The XRD pattern of pure Ti₃C₂T_x clearly exhibits the planes of (002), (008), (0010), (0012) and (110), which are in agreement with the previously reported data.²³ In the XRD pattern of MoS₂/Ti₃C₂T_x composite, the MoS₂ diffraction peaks are clearly visible. Diffraction peaks at 2θ = 33.8°, 39.4° and 57.1° correspond to (100), (103) and (110) planes of MoS₂ (JCPDS No. 37-1492), which are consistent with a previous report.²⁴ It should be mentioned that the peak intensity of (002) signifies a structure of stacked layers.²⁵ However, the (002) peak (2θ = 14.378°) is not observed in this work, illustrating that the obtained MoS₂ is a very thin layered structure, which is consistent with the HRTEM image (Fig. S4†). The characteristic diffusion peaks of MoS₂ and Ti₃C₂T_x were well retained in the MoS₂/Ti₃C₂T_x composite, indicating successful synthesis of the MoS₂/Ti₃C₂T_x composite. On comparing the XRD patterns of Ti₃C₂T_x and MoS₂/Ti₃C₂T_x composite materials, we can see that the diffusion peaks of MoS₂/Ti₃C₂T_x have lower intensity; this illustrates that Ti₃C₂T_x flakes have been effectively modified by MoS₂ nanosheets. Moreover, the XRD pattern of the MoS₂/Ti₃C₂T_x composite illustrates that the (002) peak of Ti₃C₂T_x shifts to a lower angle, indicating that the Ti₃C₂T_x interplanar spacing increased, which may facilitate the diffusion of ions. It is noticeable that there is a TiO₂ diffraction peak at 61°, which may have evolved due to exposing the composite to air.

Fig. 2 (a) XRD patterns of MoS₂/Ti₃C₂T_x composite, pure Ti₃C₂T_x and pristine MoS₂. (b) Raman spectra of MoS₂/Ti₃C₂T_x composite and pure Ti₃C₂T_x. (c) Pore size distribution and nitrogen sorption isotherms of MoS₂/Ti₃C₂T_x composite and pure Ti₃C₂T_x. (d) XPS survey spectrum of MoS₂/Ti₃C₂T_x composite. (e, f) XPS spectra of Mo 3d and S 2p in MoS₂/Ti₃C₂T_x composite, respectively.

The Raman spectra of pure Ti₃C₂T_x and MoS₂/Ti₃C₂T_x composite are shown in Fig. 2b. For pure Ti₃C₂T_x, the Raman modes located at 410 cm⁻¹ and 638 cm⁻¹ represent the vibrations from Ti–O and Ti–C bonds, respectively. It is clear that there are two typical peaks in the Raman spectrum of the MoS₂/Ti₃C₂T_x composite: the E¹_2g peak at 364.7 cm⁻¹, which originate from the vibration of Mo–S in-plane mode and the A_1g peak at around 389.6 cm⁻¹ from out-of-plane vibrations, which are typical first-order Raman active modes E¹_2g and A_1g due to in-plane vibrational modes within the S–Mo–S layer.²⁵ Notably, there is a shift in the peak of Ti–C to lower energy, implying the appearance of strong interactions at the interfaces between the MoS₂ nanosheets and Ti₃C₂T_x layers.

As shown in Fig. 2c is the comparison of N₂ adsorption/desorption curves. The specific surface areas of Ti₃C₂T_x and MoS₂/Ti₃C₂T_x composite are 4.0 m² g⁻¹ and 12.91 m² g⁻¹, respectively. This enhancement can be described as the intercalation of MoS₂ nanosheets into Ti₃C₂T_x interlayers and anchoring on the surface. Pore size distribution (inset of Fig. 2c) data demonstrated that the pores of MoS₂/Ti₃C₂T_x composite were mainly mesoporous. The increased specific surface area and pore size distribution of MoS₂/Ti₃C₂T_x composite were beneficial to expand the electrode–electrolyte contact area and contributed more active sites to improve the electrochemical performance.

To detect the valences and chemical components of the MoS₂/Ti₃C₂T_x composite, XPS analysis was performed. From the survey spectrum of MoS₂/Ti₃C₂T_x composite in Fig. 2d, the peaks of elements sulfur, molybdenum, carbon, titanium, oxygen and fluoride can be clearly seen. The Mo 3d region is displayed in Fig. 2e, in which the two peaks centered at 228.78 eV and 232.08 eV correspond to Mo 3d_5/2 (Mo–S bond) and Mo 3d_3/2 (Mo–S bond), respectively.²⁶ The peak at 224.98 eV corresponds to S 2s of MoS₂. The other two peaks at 227.68 eV and 230.98 eV are ascribed to Mo 3d_5/2 (Mo–C bond) and Mo 3d_3/2 (Mo–C bond), respectively. Mo 3d_5/2 and Mo 3d_3/2 are the characteristic Mo⁴⁺ oxidation states of MoS₂, while the peak at 235.1 eV corresponds to MoO₃ and MoO₄²⁻, indicating that the MoS₂/Ti₃C₂T_x composite was partly oxidized due to preparation and exposure to air. Fig. 2f shows the S 2p spectrum. The two main peaks at 160.5 eV and 161.8 eV are ascribed to the characteristic S²⁻ species in MoS₂, while the peak at 163.6 eV corresponds to C–S–C bonding, which illustrates the firm binding between MoS₂ nanosheets and Ti₃C₂T_x substrate. The other two peaks at 167.9 eV and 168.9 eV coincide with C–So_x–C bonding, which may occur due to the exposure of the composite to air.²⁷ All the XPS analysis results confirm that the MoS₂/Ti₃C₂T_x composite was successfully synthesized and the MoS₂ nanosheets had firm interfacial contact with the Ti₃C₂T_x layers, which could maintain the stable structure and ensure steady cycling performance.

In order to determine the advantages of the MoS₂/Ti₃C₂T_x composite, it was used as the anode material for sodium-ion batteries. Fig. 3a shows the CV curves of the first three cycles. In the first cathodic process, an irreversible peak at around 0.06 V could be ascribed to the decomposition of the electrolyte and formation of the solid electrolyte interface (SEI). In the second and third cathodic process, a distinct peak at around 1.32 V can be associated with Na⁺ insertion into the MoS₂ lattice. Moreover, the peaks overlap very well, which demonstrates a highly reversible reaction between MoS₂ and Na⁺. In contrast, there is one oxidation peak at around 1.83 V in the first cycle, which could be ascribed to the reversible reaction of Na₂S to polysulfide,²⁸ and the oxidation peak shifting to a higher voltage in the following two cycles should be ascribed to the polarization of the electrode.

Fig. 3 Electrochemical measurements of the MoS₂/Ti₃C₂T_x composite in sodium-ion batteries. (a) Cyclic voltammetry curves at a scan rate of 0.1 mV s⁻¹. (b) Charge/discharge curves at 100 mA g⁻¹. (c, d) Rate performance and corresponding charge/discharge curves. (e) Cycling performances of MoS₂/Ti₃C₂T_x composite, pristine MoS₂ and pure Ti₃C₂T_x. (f) Comparison of electrochemical impedance spectra of MoS₂/Ti₃C₂T_x composite and pristine MoS₂.

Fig. 3b shows the galvanostatic charge and discharge voltage profiles of the MoS₂/Ti₃C₂T_x composite at 100 mA g⁻¹. The charge and discharge capacities of the MoS₂/Ti₃C₂T_x composite in the first cycle were 339 mA h g⁻¹ and 543 mA h g⁻¹, respectively, with a coulombic efficiency of only 62.4%. Formation of SEI is the cause of the initial irreversible capacity. In the subsequent cycles, there is one charge plateau and one discharge plateau, which corresponds to the cyclic voltammetry analysis.

Fig. 3c and d show the rate performance and the corresponding charge and discharge curves of the MoS₂/Ti₃C₂T_x composite, which delivered high discharge capacities of 411 mA h g⁻¹, 356.2 mA h g⁻¹, 312.6 mA h g⁻¹, 259.6 mA h g⁻¹ and 186.2 mA h g⁻¹ at 50 mA g⁻¹, 100 mA g⁻¹, 200 mA g⁻¹, 400 mA g⁻¹ and 800 mA g⁻¹, respectively. It is worth mentioning that the electrode can still deliver a reversible capacity of 333.6 mA h g⁻¹ when the current density returned to 100 mA g⁻¹ almost without capacity decay. The rate performance and the corresponding charge–discharge curves of pristine MoS₂ and pure Ti₃C₂T_x are shown in Fig. S5.† It is clearly demonstrated that the pristine MoS₂ has a higher reversible discharge capacity but suffers fast capacity decay during cycling, while pure Ti₃C₂T_x is stable but its capacity is too low for its application. The excellent rate performance illustrates that the MoS₂/Ti₃C₂T_x composite electrode could maintain a stable structure even under a high current density. Furthermore, the performance could also be explained as the result of synergistic effect. On the one hand, the Ti₃C₂T_x substrate prevented the stacking of MoS₂ nanosheets during the cycling and contributed more active sites for the reversible actions. On the other hand, the MoS₂ nanosheets anchored in the Ti₃C₂T_x interlayers expanded the space of the interlayers to improve the electrode–electrolyte contact area and shorten the ion diffusion paths. As a consequence, an excellent rate performance was obtained.

The cycling performances of the MoS₂/Ti₃C₂T_x composite, pristine MoS₂ and pure Ti₃C₂T_x are shown in Fig. 3e. The MoS₂/Ti₃C₂T_x composite electrode still had a high reversible capacity of 331 mA h g⁻¹ even after 70 cycles at 100 mA g⁻¹, with a low capacity decay rate of only ∼0.058% per cycle. Pristine MoS₂ and pure Ti₃C₂T_x delivered reversible capacities of only 170.7 mA h g⁻¹ and 45.7 mA h g⁻¹, respectively. Additionally, coulombic efficiency of the MoS₂/Ti₃C₂T_x composite electrode remained at around 100% during cycling, proving that the MoS₂/Ti₃C₂T_x composite possessed stable structural integrity. Moreover, it is evident that the MoS₂/Ti₃C₂T_x composite electrode had a more stable cycling performance than pristine MoS₂ and a much higher charge/discharge capacity than pure Ti₃C₂T_x. Therefore, the MoS₂/Ti₃C₂T_x composite is an electrode material with great promise for sodium-ion batteries.

Fig. 3f shows the comparison of the electrochemical impedance spectra (EIS) of MoS₂/Ti₃C₂T_x composite and pristine MoS₂ used as anode materials for sodium-ion batteries and Fig. S6† displays the dependence of reciprocal square root of angular frequency on the real impedance in the low frequencies of MoS₂/Ti₃C₂T_x composite and pristine MoS₂. According to the equation:

Z′ = R_s + R_ct + δ_wω^−1/2

in which the Warburg coefficient (δ_w) is equal to the slope of the fitted line (Fig. S6†) in the low frequencies of EIS spectrum. As calculated, the slope of MoS₂/Ti₃C₂T_x composite was k₁ = 23.48 and that of pristine MoS₂ was k₂ = 26.94. According to the equation:
under the same conditions, the diffusivity of Na⁺ (D_Na⁺) is inversely proportional to δ_w². δ_w² of the MoS₂/Ti₃C₂T_x composite was smaller than that of pristine MoS₂; therefore, the diffusivity of Na⁺ in the MoS₂/Ti₃C₂T_x composite is faster than that in pristine MoS₂. The faster diffusion kinetics of the MoS₂/Ti₃C₂T_x composite may be due to the unique structure of MoS₂ nanosheets uniformly decorated on Ti₃C₂T_x layers. The Ti₃C₂T_x substrate with expanded interlayer space favored the electrode–electrolyte contact, while the ultrathin MoS₂ nanosheets shortened the ion diffusion path; as a consequence, fast reaction kinetics is obtained.

The same electrochemical measurements were adopted in lithium-ion batteries to prove the advantages of the MoS₂/Ti₃C₂T_x composite.

Fig. 4a shows the first three CV curves of the MoS₂/Ti₃C₂T_x composite with a scan rate of 0.1 mV s⁻¹. In the first cycle, it is clear that there is an irreversible reduction peak at around 0.6 V that disappeared in the subsequent cycles, which could be ascribed to the lattice transformation from triangular prism into octahedral structure. The reason for the irreversibility is that the nearly amorphous MoS₂ only reformed after the first charge process.^24,29 The reduction peaks at around 1.4 V in the second and third cycles correspond to the reaction MoS₂ + 4Li + 4e⁻ → Mo + 2Li₂S. Moreover, a broad reduction peak is located at around 2.0–2.5 V. In the anodic scanning, the distinguishable peaks at around 1.8 V and 2.3 V can be ascribed to lithium extraction and oxidation of Mo to MoS₂.²⁸ It is noticeable that the CV curves of the second and the third cycles are looped very well, which illustrates that the action between the MoS₂/Ti₃C₂T_x composite and Li⁺ is highly reversible.

Fig. 4 Electrochemical measurements of the MoS₂/Ti₃C₂T_x composite in lithium-ion batteries. (a) Cyclic voltammetry curves at a scan rate of 0.1 mV s⁻¹. (b) Charge/discharge curves at 100 mA g⁻¹. (c) Rate performance of MoS₂/Ti₃C₂T_x composite. (d) Cycling performances of MoS₂/Ti₃C₂T_x composite, pristine MoS₂ and pure Ti₃C₂T_x. (e) Comparison of electrochemical impedance spectra of MoS₂/Ti₃C₂T_x composite and pristine MoS₂. (f) Schematic diagram of ion diffusion and electronic conduction.

Fig. 4b shows the galvanostatic charge and discharge voltage profiles at 100 mA g⁻¹. The charge and discharge capacities of the first cycle are 647.2 mA h g⁻¹ and 843.7 mA h g⁻¹, respectively, and the coulombic efficiency is 76.7%. The drastic initial irreversible capacity may be obtained because of the decomposition of electrolyte and formation of SEI. It is clear that the discharge profiles have two plateaus at around 2.0–2.5 V and 1.4 V. Moreover, the charge profiles have two plateaus at around 1.8 V and 2.3 V, which corresponds to the cyclic voltammetry analysis.

The rate performance and corresponding charge/discharge curves are shown in Fig. 4c and Fig. S7,† respectively. The discharge capacities at 100 mA g⁻¹, 200 mA g⁻¹, 400 mA g⁻¹ and 800 mA g⁻¹ are 653 mA h g⁻¹, 587 mA h g⁻¹, 544 mA h g⁻¹ and 488 mA h g⁻¹, respectively, with coulombic efficiency being almost 100%. Impressively, the MoS₂/Ti₃C₂T_x composite electrode maintained reversible capacity of 645 mA h g⁻¹, with almost no capacity decay, when the current density returned to 100 mA g⁻¹. It was speculated that the MoS₂/Ti₃C₂T_x composite electrode had a stable structure even under high current density. The rate performances of pristine MoS₂ and pure Ti₃C₂T_x and the corresponding charge–discharge curves are shown in Fig. S8.† The drastic decay of reversible discharge capacity of pristine MoS₂ when the current density was turned to 100 mA g⁻¹ illustrates that the structure of pristine MoS₂ is not stable. Pure Ti₃C₂T_x had a low capacity of 170.4 mA h g⁻¹, making it difficult for use in practical applications. Therefore, the MoS₂/Ti₃C₂T_x composite has the best rate performance of all the three electrode materials.

The cycling performance is exhibited in Fig. 4d. The MoS₂/Ti₃C₂T_x composite could maintain a reversible capacity of 614.4 mA h g⁻¹ at 100 mA g⁻¹ after 70 cycles with only a 0.05% decay per cycle. In contrast, pristine MoS₂ suffered a drastic capacity decay of 63.8% after only 30 cycles, while pure Ti₃C₂T_x delivered a very low capacity of 123.2 mA h g⁻¹. The MoS₂/Ti₃C₂ composite had the best cycling performance of all the three materials, which was attributed to its unique structure and synergistic effect. On the one hand, the MoS₂ nanosheets with ultrathin thickness shortened the diffusion paths of Li⁺ and promoted the intercalation and accommodation of Li⁺. On the other hand, the Ti₃C₂T_x substrate prevented the stacking of MoS₂ nanosheets and maintained the stability of the structure during the cycling process. Furthermore, the Ti₃C₂T_x substrate had lower resistance for electron conduction, which also promoted the cycling stability of the MoS₂/Ti₃C₂T_x composite.

The comparison of electrochemical impedance spectra (EIS) of MoS₂/Ti₃C₂T_x composite and pristine MoS₂ as anodes for lithium-ion batteries is shown in Fig. 4e. The semicircles at high and medium frequencies can be described as the resistance of charge-transfer and SEI film, while the sloping line at low-frequency corresponds to the diffusion of lithium ions. Similar to the MoS₂/Ti₃C₂T_x composite being used as the anode material in sodium-ion batteries, when used in lithium-ion batteries, the composite electrode exhibited a smaller semicircle and a steeper line than the pristine MoS₂ electrode, which implied that the MoS₂/Ti₃C₂T_x composite electrode had smaller electrochemical impedance and faster Li⁺ diffusion kinetics than pristine MoS₂. Overall, the superior electrochemical performance of MoS₂/Ti₃C₂T_x could be ascribed to the unique structure and synergistic effect, as shown in Fig. 4f, in which two-dimensional MoS₂ nanosheets are uniformly decorated on the Ti₃C₂T_x layers. First, the MoS₂ nanosheets shortened the ionic diffusion paths and contribute more active sites for ion intercalation. Second, the improved specific surface area facilitated electrode–electrolyte contact. Finally, Ti₃C₂T_x matrix promoted electronic conduction and prevented the stacking of MoS₂ nanosheets, which also enhanced the electrochemical performance.

Experimental

Preparation of the MoS₂/Ti₃C₂T_x composite

Ti₃C₂T_x powder was obtained as described in our previous study.¹⁸

The MoS₂/Ti₃C₂T_x composite was synthesized by a facile one-step hydrothermal method. In detail, 100 mg Ti₃C₂T_x powder was dispersed in 36 mL DI water with 192 mg Na₂MoO₄·2H₂O and 66 mg CH₃CSNH₂ under mechanical stirring at room temperature for 1 h. Then, the suspension was transferred into a Teflon-lined stainless-steel autoclave and kept at 180 °C for 12 h. After cooling, the product was washed with ethyl alcohol and DI water several times and collected by filtration, and then dried in a vacuum oven at 60 °C for 12 h. For comparison, pristine MoS₂ was synthesized by the same method without Ti₃C₂T_x.

Characterization of materials

The morphologies of the MoS₂/Ti₃C₂T_x composite were observed by field emission electron microscopy (FESEM, JSM-7800F, Japan) and transmission electron microscopy (TEM, JEM-2100, Japan). X-ray diffraction (XRD MAXima-X XRD-7000) patterns were recorded with Cu Kα radiation (λ = 1.5406 Å). Raman spectra were recorded by Invia Refl (Renishaw, UK) with a laser wavelength of 532 nm. X-ray photoelectron spectroscopy was conducted on an electron spectrometer (ESCALAB 250Xi). The specific surface area and distribution of pore size were determined by Quadrasorb evo 2QDS-MP-30 (Quantachrome Instruments, USA). The elemental analysis was performed on an energy dispersive X-ray spectrometer attached to a SEM instrument.

Electrochemical measurements

MoS₂/Ti₃C₂T_x composite, pure Ti₃C₂T_x and pristine MoS₂ were tested as anode materials in SIBs and LIBs by assembling CR2032 type coin cells in an argon filled glovebox with sodium and lithium metals as counter electrodes. Typically, the anode electrodes were prepared by a slurry coating procedure. In detail, the as-prepared material was mixed with polyvinylidene fluoride (PVDF) and acetylene black (AB) with a mass ratio of 8 : 1 : 1 in N-methyl-2-pyrrolidinone (NMP) to get a slurry. Following this, the slurry was uniformly coated on the copper foil current collector, and then dried under vacuum at 120 °C for 12 h. The area of each electrode is 1.13 cm².

Celgard 2400 was used as a separator and 1 M NaClO₄ dissolved in a mixed solution of diethyl carbonate (DEC) and ethylene carbonate (EC) (1 : 1, v/v) was used as the electrolyte. Galvanostatic charge and discharge cycling was tested between 0.01 V and 3.0 V vs. Na/Na⁺ and Li/Li⁺ on the Land battery test system. The rate performance was tested with a variety of current densities. Cyclic voltammetry (CV) was conducted between 0.01 V and 3.0 V with a scanning rate of 0.1 mV s⁻¹. Electrochemical impedance spectroscopy (EIS) was conducted on an electrochemical workstation (CHI 660C). All electrochemical measurements were conducted under normal environment.

Conclusions

In summary, the MoS₂/Ti₃C₂T_x composite was successfully synthesized by a facile one-step hydrothermal method. The MoS₂ nanosheets were tightly and uniformly anchored on the Ti₃C₂T_x layers. Benefiting from the rough surface, firm contact between MoS₂ nanosheets and Ti₃C₂T_x layers, improved number of active sites, and high electrical conductivity of Ti₃C₂T_x substrate, the MoS₂/Ti₃C₂T_x composite as anode material for sodium/lithium ion batteries delivered high reversible discharge capacity, better rate performance and lower electrochemical impedance than those of pristine MoS₂ and pure Ti₃C₂T_x. Therefore, we think that the MoS₂/Ti₃C₂T_x composite is an anode material for SIBs and LIBs with great promise.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was financially supported by grants from the National Natural Science Foundation of China (No. 21773188), Fundamental Research Funds for the Central Universities (XDJK2017A002, XDJK2017B048) and Program for Innovation Team Building at Institutions of Higher Education in Chongqing (CXTDX201601011).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8qi01081a

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