Introduction

Lithium-ion batteries (LIBs) are not only attractive practical renewable energy storage devices for high-energy systems such as electrical vehicles, smart grids, etc., (Lu et al. 2013; Peterson et al. 2010; Thackeray et al. 2012) but they also fulfil the need of low energy gadgets such as PC memory, medical implants etc. (Armand and Tarascon 2008; Hu et al. 2010; Kyeremateng 2014). Anode materials of LIBs have a significant effect on the overall performance and efficiency of LIBs (Shehzad et al. 2016; Sagar et al. 2016). However, anode materials for LIBs currently suffer from disadvantages such as, slow ionic diffusion, weak electron transportation, and high interface resistance, which consequently limits the performance of LIBs (Thackeray et al. 2012).

Titanium dioxide (TiO2) is one of the attractive anode materials for LIBs due to its cost-effectiveness, chemical stability, and non-toxic nature. Various kinds of low dimensional TiO2 such as nanotubes (Armstrong et al. 2006; Qiu et al. 2010), nanowires (Cao et al. 2010), nanorods (Liu et al. 2012), and nanoparticles (Ren et al. 2012) etc., have been employed as anode for LIBs. Amongst these nano-structures of TiO2 (Armstrong et al. 2006; Qiu et al. 2010; Cao et al. 2010; Liu et al. 2012; Ren et al. 2012; Wang et al. 2011), titanium dioxide nanotube arrays (TNAs) (Guo et al. 2012; Anwar et al. 2015; Tauseef Anwar et al. 2016) are more advantageous owe to higher specific surface area, porosity, and vertical alignment. The TNAs not only provide the short lithium ion diffusion path but also accommodate the volume expansion, as well as easy preparation in large scale and self-standing structure facilitates film fabrication (Wu et al. 2012). However, TNAs have lower areal specific capacity (Anwar et al. 2015) which can be improved by adopting different strategies such as, metal or non-metal element doping (Liu et al. 2008, 2009, 2014), annealing in different atmosphere (Lu et al. 2012), conductive coating (Wang et al. 2015), and by compositing with the higher capacity materials (Anwar et al. 2016).

Molybdenum disulfide (MoS2) is an attractive material for the practical applications including hydrogen storage, (Chen et al. 2001; Ye et al. 2006) as catalysts, (Hinnemann et al. 2005; Lukowski et al. 2013) lubricants, (Chhowalla and Amaratunga 2000; Savan et al. 2000) double-layer capacitor (Cao et al. 2013; Soon and Loh 2007) as well as lithiun-ion batteries (Hwang et al. 2011; Stephenson et al. 2014; Feng et al. 2009; Li et al. 2009; Dominko et al. 2002). As an anode for lithium insertion/deinsertion, the volume of MoS2 has no significant expansion due to its unique layered structure and weak inter-layer interaction (Sun et al. 2016). Moreover, voids/dislocations in disordered MoS2 results in a significant increase in lithium capacity (~670 mAh g−1) as well as overall performance of the LIBs (Shehzad et al. 2016; Hwang et al. 2011; Liu et al. 2014; Zhu et al. 2014; Hu et al. 2016; Cui et al. 2015) Different composite of MoS2 with conductive materials (graphene, carbon nanotubes, etc.) have been synthesised for the use of anodes in LIBs. (Zhao et al. 2016; Cao et al. 2013; Hwang et al. 2014) Since, the assembly of layered materials into variety of morphologies such as, nanoarrays is still in infacny, hence, the making composite of MoS2 with arrays of TiO2 may prove an interesting choice.

In this article, MoS2 grafted TNAs have been prepared as a new hybrid anode material in order to improve LIBs performance. The MoS2/TNAs composites were fabricated via hydrothermal method and a high lithium stroage capacity of 430 µAh cm−2 has been observed. The magnitude of areal capacity of MoS2/TNAs is ~33 and ~55% higher as compared to the anatase TNAs and MoS2, respectively. Not only the specific capacity is enhanced but also a new morphology of MoS2 also helped to achieve a higher capacity retension. Better electrochemical performace of MoS2/TNAs indicates its utility arising from its novel hybrid structure.

Experimental section

Synthesis of MoS2/TNAs

MoS2 grafted TNAs were synthesized according to the previous literature by using hydrothermal method (Fig. 1) (Anwar et al. 2015, 2016). The Ti-foil with grown TNAs was placed with top surface downward in the Teflon liner wall. The 30 mL solution of (NH4)6Mo7O24·4H2O (1 mmol) and thiourea (H2NCSNH2 ~ 0.484 g) was poured into the autoclaves which was Teflon lined. This sealed autoclave placed in oven for 3 h at the temperature of 180 °C. The autoclave was cooled down to room temperature. After cleaning samples were dehydrated at 80 °C for 30 min in vacuum oven. Annealing of MoS2 deposited TNAs (MoS2/TNAs) sample was performed at 400 °C for 2 h in argon atmosphere.

Fig. 1
figure 1

The schematic diagram for MoS2/TNAs composite fabrication: I Ti foil, II growth of TNAs at Ti foil, III MoS2/TNAs fabrication

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Characterization

The morphology of TNAs and MoS2/TNAs was characterized by using field emission scanning electron microscopy (FE-SEM LEO 1530). The confirmation of TNAs, MoS2 and MoS2/TNAs phases was performed by using Cu Kα radiation (λ = 0.15 nm) of X-ray powder diffraction (Rigaku D/max). Raman spectroscopy of the anatase TNAs, MoS2, MoS2/TNAs was recorded on a HR800 micro-Raman spectrometer (Horiba Jobin–Yvon) using a 633 nm He–Ne laser.

Electrochemical characterization

The electrochemical properties of MoS2/TNAs composite were assessed by using Li| MoS2/TNAs half-cells. The coin cell (2032) was assembled in a glove box filled with argon. MoS2/TNAs was used as cathode without additives, while lithium foil was used as the anode. The cell was ready for the measurement after inserting a separator of celgard 2300 between anode and cathode. The electrolyte of 1 M LiPF6 was dissolved in 1:1 volumetric ratio mixture of dimethyl carbonate (DMC) and ethylene carbonate (EC). The galvanostatically discharge and charge were conducted between 0.005 and 3 V (vs. Li/Li+) at Land battery test system at room temperature. The electrodes used for comparison (TNAs and MoS2) were prepared and characterized at same parameters. The cyclic voltammetry (CV) measurement was recorded using electrochemical workstation (CHI660C, CH Instruments, Shanghai, PRC).

Results and discussion

The morphology of anatase TNAs and MoS2/TNAs under SEM indicate the smooth, uniform, and vertically aligned TNAs. The top surface and lateral surface do not have any unwanted nanostructures (Fig. 2a–d). The length and average pore diameter of the TNAs was ~3–5 µm and ~80 nm, respectively. The MoS2 was grafted at TNAs by hydrothermal reaction for different time durations (i.e. 0–10 h) and found excellent performance of the device at 3 h as the excessive coating hindered the ions and electrons movement (Anwar et al. 2016). The partially covered top surface of TNAs with MoS2 can facilitate Li ions and electrons movement through the inner of the nanotubes (Fig. 2c). The MoS2 is also grafted at the lateral sides of TNAs (Fig. 2d), suggesting the filling of TNAs with MoS2. The MoS2 at bare titanium plate by similar method for 10 h is also shown in Fig. S1. The MoS2 nanoflakes at titanium substrate are 500 nm long and 10 nm thick. The dimension of MoS2 increases with the increase in the time.

Fig. 2
figure 2

a TNAs c MoS2/TNAs and lateral side of b TNAs d MoS2/TNAs

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The XRD peaks given in Fig. 3a indexed well with standard JCPDS No. 44-1294 and JCPDS No. 21-1272 for TNAs and MoS2/TNAs, respectively. After MoS2 coating the intensities of TiO2 peaks decrease which indicates successful coating (Li et al. 2015). The peaks observed at 35°, 38.4°, 40.1°, 53°, 62.9°, 70.6°, 74.1°, 76.2° and 77.4° represents Ti foil planes while observed peaks at 14.38°, 29.03°, 33.51° and 62.81° can be attributed to MoS2 (002), (004), (101), and (101) planes, respectively. Anatase-TiO2 peaks were observed at 25.0° (101) and 47.9° (200). The MoS2 peaks could be observed only in MoS2/TNAs composite, and anatase TiO2 did not show any MoS2 peak. The XRD results suggest that anatase TNAs and MoS2/TNAs are polycrystalline in nature.

Fig. 3
figure 3

a XRD and b Raman of TNAs, MoS2 and MoS2/TNAs

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The comparison of Raman spectra of anatase TNAs, MoS2, and MoS2/TNAs is performed for the investigation of the changes in the electronic structure (Fig. 3b). The intense Raman band at 145 cm−1 in MoS2/TNAs corresponds to the main Eg vibration mode of anatase TiO2. Moreover, the peaks located at 392 (B1g), 513 (A1g), and 634 cm−1 (Eg) also confirm the presence of anatase TiO2. In MoS2 spectrum, two broad peaks centered at 397 (\({\text{E}}_{{2{\text{g}}}}^{1}\)) cm−1 and 406 (A1g) cm−1 corresponds to the modes of MoS2. The MoS2 peaks are also observed in the Raman spectrum of the MoS2/TNAs composites, confirming the successful coating of MoS2 species on the anatase TNAs. The blue shift and peak broadening is observed in the mode of Eg, E1g, A1g, and Eg in the MoS2/TNAs as compared to anatase TNAs. The blue shift might be attributed to the induced surface strain by the grafted MoS2 nanoflakes at anatase TNAs surface (Fig. 4).

Fig. 4
figure 4

CV curves for anatase TNAs (a), MoS2 (b) and MoS2/TNAs (c). d The 3rd galvanostatic discharge/charge curve of anatase TNAs, MoS2 and MoS2/TNAs at a potential of 0.005–3 V

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The cyclic voltammetry (CV) of all prepared electrodes were collected at scan rate of 5 mV/s in a potential window of 0.005–3 V vs. Li/Li+ (Fig. 4a–c). The CV curve of anatase TNAs showed cathodic peaks at 1.2 and 1.35 V for 1st and 2nd cycle, respectively (Fig. 4a). Moreover, anodic peaks at 2.5 and 2.65 V correspond to the 1st and 2nd cycle of anatase TNAs, respectively, indicating the Li ion intercalation and de-intercalation potentials of the anatase TiO2. The cathodic peak potentials are higher for 2nd cycle as compared to 1st cycle. The shoulder peak was observed at 0.2–1.0 V for both cycles with major peak, which depicted more lithium storage occurred at nanotubes surfaces and interfaces. The intensity of cathodic shoulder peak of the 2nd cycle is larger as compared to 1st cycle. Two cathodic/anodic peaks appeared at 0.6 V and 1.3 V (vs. Li/Li+), respectively in the both cycles of MoS2 anodes (Fig. 4b). The MoS2/TNAs electrode showed the similar peaks of anatase TNAs cathodic peak and MoS2 cycles behavior, indicating the contribution of TNAs and MoS2 in MoS2/TNAs anode (Fig. 4c). The whole Li+ intercalation and deintercalation reaction can be described as:(Li et al. 2015)

$${\text{TiO}}_{2} + x{\text{Li}}^{ + } + x{\text{e}}^{ - } \leftrightarrow {\text{Li}}_{x } {\text{TiO}}_{2}$$
(1)
$${\text{MoS}}_{2} + x{\text{Li}}^{ + } + x{\text{e}}^{ - } \to {\text{Li}}_{x} {\text{MoS}}_{2}$$
(2)
$${\text{Li}}_{x} {\text{MoS}}_{2} + (4 - x){\text{Li}}^{ + } + (4 - x){\text{e}}^{ - } \to {\text{Mo}} + 2{\text{Li}}_{2} {\text{S}}$$
(3)

The first two cycles were discharged/charged at 10 µA cm−2 to stabilize the electrochemical properties. The consecutive cycles (3rd–50th) were discharged/charged at current density of 50 µA cm−2 in the potential window of 0.005–3 V (Fig. 4d). In the 3rd discharge, the slope at 1.5–1.2 V corresponds to the TiO2 lithiation. The slope after 1.0 V corresponded to the phase transformation from MoS2 to Li x MoS2 and conversion into Mo and Li2S (Eq. 3), respectively. The discharge capacity of 3rd cycle is 180, 290, and 430 µAh cm−2 for MoS2, anatase TNAs and MoS2/TNAs, respectively. The areal specific capacity of MoS2/TNAs is higher as compared to MoS2 and anatase TNAs. The improved electrochemical performance may be attributed to the synergistic effect of high capacity containing MoS2 and vertically aligned nature of TNAs helps fast Li+ kinetics.

Fig. 5
figure 5

Cyclic performance and efficiency (a) and rate performance (b)

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To characterize stabilized electrochemical performance, first two discharged/charged were measured at the current density of 10 µA cm−2 (Fig. S2 (a) and (b)) while the remaining cycles were measured at 50 µA cm−2 (Fig. 5a). The cyclic stability of anatase TNAs reduces continuously from 3rd to 50th cycle with 1.47% capacity loss per cycle, while for MoS2 cyclic performance is very stable from 3rd to 50th cycle with 0.08% capacity loss per cycle.

In the case of MoS2/TNAs low capacity fading of TNAs is achieved and the capacity loss per cycle is just 0.21%. So it is an effective way to improve capacity retention. The composite electrode MoS2/TNAs showed higher discharge/charge capacity as compared to individual anatase TNAs and MoS2 electrodes. The MoS2/TNAs nanostructures electrode exhibits a discharge capacity for 3rd cycle was 430 µAh cm−2 and low capacity fading until 50th cycle (388 µAh cm−2) with capacity loss of 0.21% per cycle. While the discharge capacity of anatase TNAs and MoS2 electrodes was 84 and 242 µAh cm−2 for 50th cycle, respectively. The efficiency of all electrodes is 100% during galvanostatic discharge/charge and fluctuates between 101 and 103% (Fig. 5a).

The rate performance was conducted at current densities of 50, 100, 150, 200, 250 and again 50 mA cm−2 for all electrodes. MoS2/TNAs have high rate capability in comparison to anatase TNAs and MoS2. At highest current density when discharge/charge rate switched again at 50 mA cm−2, the areal capacity for MoS2/TNAs was still more and stable as compared to anatase TNAs and MoS2 (Fig. 5b). So the results depicted that TiO2 and MoS2 incorporate with each other and enhanced the lithium storage rate performance.

The MoS2/TNAs can be a practical anode material in LIBs due to the achievement of larger areal capacity as well as high capacity retention. At First, there are no reports about MoS2 grafted TNAs electrode for lithium ion battery and MoS2 grafted TNAs of few micron length with higher specific capacity of 430 µAh cm−2. It might be due to the fabrication process, nanostructures combination, and the synergetic effect of both nanostructured materials (i.e. MoS2 and TNAs). The synergestic effect of MoS2 and TNAs increased the lithium intercalation, which results into a higher capacity and better capacity retention. The hydrothermal fabrication helps to control MoS2 thickness which brings the exciting performance and controllable performance of LIBs.

Conclusion

The electrochemical properties of MoS2 nanoflakes grafted TNAs were studied for the first time to the best of our knowledge. The specific capacity (~430 µAh cm−2) of MoS2/TNAs is higher than anatase TNAs (~84 µAh cm−2) and MoS2 (~84 and 242 µAh cm−2). Anatase TNAs have high capacity fading (i.e. 1.47% capacity loss/cycle), which was significantly reduced to 0.21% capacity loss/cycle, owing to the MoS2 nanostructured coating over TNAs. Thus, MoS2/TNAs material can be a promising anode material for lithium ion batteries.