Facile synthesis of a novel Al-based composite as an anode for lithium-ion batteries

Abstract

A facile ball-milling method is developed to synthesize an Al/MoS₂/C composite, which can be used for scalable industrial mass production. The composite is characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), galvanostatic cycling and cyclic voltammetry. The electrochemical measurements demonstrate that the Al/MoS₂/C composite has a greatly improved electrochemical performance in comparison with pure Al. After 40 cycles, the capacity retentions of Al-40 wt%, Al-50 wt% and Al-60 wt% are 451.3 mA h g⁻¹, 419.6 mA h g⁻¹ and 378.2 mA h g⁻¹, respectively. This improved electrochemical performance may be attributed to the layer-structure MoS₂/C composite which can not only buffer the volume change but also provide capacity stability for the composite during the charge and discharge process. This suggests that the Al/MoS₂/C composite has a potential possibility to be developed as an anode material for LIBs.

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1. Introduction

Lithium ion batteries (LIBs) have been regarded as one of the most promising power sources for electric vehicles or hybrid electric vehicles because of their higher power density and longer cycle life.¹ However, graphite, widely used as the anode material for commercial LIBs, has a relatively low theoretical capacity and poor safety and cannot meet the demand for LIBs in the future.² In order to increase the energy density and power density for LIBs, it is necessary to develop a high-performance anode material. Recently, transition metal oxide anode materials and Sn-based oxide have shown significant potential for LIBs because they possess remarkably higher capacities than those of the current commercial anode materials. For example, Fe₃O₄,^3–5 Fe₂O₃,^6,7 Co₃O₄,⁸ and SnO₂^9–11 can all achieve a higher capacity than graphite, however, they suffer from a severe volume change during the lithium ion insertion/extraction, which can result in a poor cycle life.

Semimetal or metal based anodes are also believed to be the most promising replacement candidates due to their high capacities and desirable working potentials, such as Si,^12–15 Sn,^16–18 Sb,¹⁹ Al,^20–22 etc. The Li–Al alloy was used in the first lithium rechargeable batteries in the 1970s.²³ Al can form three kinds of alloys with lithium: AlLi, Al₂Li₃ and Al₄Li₉, all of which have a theoretical capacity much higher than that of graphite. In addition, Al also has flat and wide plateaus during the lithium ion insertion/extraction process, which is a very important factor for high-performance anode materials. Now, the poor cycle life of Al as well as the known huge volume change as a result of the cycling has hindered its replacement of graphite. To overcome these problems, enormous efforts have been made to develop two typical approaches; one way is to synthesize nanostructured Al materials with various morphologies, such as Al thin films²⁴ and Al nanorods.²⁵ The other promising strategy is to construct hybrid electrode materials. A series of Al-based composite materials such as the Al–Si–graphite composite,²⁶ Al–Fe–C composite,²⁷ and Co₃O₄ and SnO coated Al composite,^28,29 or nanostructured Al_1−xCu_x alloys³⁰ have been extensively reported. These composite materials can improve the cycling performance of Al-based composite materials to some degree, but they also do not meet the high performance of lithium ion batteries.

Recently, MoS₂, the typical layered transition metal sulfide with a single-layer or few-layer structure, has attracted great interest because of its excellent mechanical and electrical properties.³¹ In addition, such a layered structure of MoS₂ facilitates reversible Li⁺ intercalation/extraction, which enables MoS₂ nanosheets to have a high capacity up to 1000 mA h g⁻¹ for LIBs.³² Hence, metal oxide–MoS₂ composites using MoS₂ as the substrate have been developed, such as SnO₂/MoS₂ composites³³ and Fe₃O₄/MoS₂ composites;³⁴ in these composites MoS₂ can not only effectively buffer the volume change of the metal oxide but can also provide capacity stability for the composites, so this can be an effective method to prepare high-performance anode materials for LIBs. On the other hand, if the MoS₂ nanosheets are uniformly dispersed in carbon materials, this leads to enhanced electrochemical properties.³¹

In this paper, the two-step synthesis of a novel Al/MoS₂/C composite is achieved by ball milling Al and the MoS₂/C composite. Due to MoS₂ having excellent mechanical properties, the force offered by the ball milling could anchor the nanocrystalline Al particles to the MoS₂/C composite, forming a tight and stable structure. On one hand, the MoS₂/C matrix with a better conductivity offers paths for the fast electron transportation, increasing the reversible capacity of composites; on the other hand, the MoS₂/C matrix buffers the volume change of Al during the repeated charge–discharge cycles. Therefore, the capacity and cycling performance of the Al/MoS₂/C composite are greatly improved.

2. Experimental

2.1 Synthesis of the MoS₂/C composite

The MoS₂/C composite was synthesised according to the literature.³¹ In a typical synthesis, 3 g of Na₂MoO₄·2H₂O and 4 g NH₂CSNH₂ were dissolved in 400 ml deionized water, and then 10 g of glucose was added into the solution. After stirring for a few minutes, the obtained clear solution was transferred into a 500 ml Teflon-lined stainless steel autoclave and sealed tightly, then heated at 240 °C for 24 h. After cooling naturally, the black precipitates were collected by filtration, washed with deionized water, and dried in an oven at 80 °C for 12 h. The MoS₂/C composite was annealed in a conventional tube furnace at 800 °C for 2 h under Ar.

2.2 Preparation of the Al/MoS₂/C composite

Al and the MoS₂/C composite were used as received. The Al/MoS₂/C composite was prepared by ball milling; Al and the MoS₂/C composite were mixed in a 4 : 6 weight ratio, the weight ratio of the milling balls to the powder materials was maintained as 10 to 1. The mixture was milled at a rotation speed of 350 rpm for 20 h. To prevent metal oxidation, the material handling was performed in a dry glove box with a purified Ar atmosphere. Three samples, with different formal Al : MoS₂/C weight ratios were prepared. Different ratios of Al to MoS₂/C were used in order to optimize the composite for electrochemical performance. Three samples, with different formal Al : MoS₂/C weight ratios, were prepared, which are listed in Table 1.

Table 1 List of the samples with different formal Al : MoS₂/C weight ratios

Sample	Al	MoS2/C
40 wt% Al	40%	60%
50 wt% Al	50%	50%
60 wt% Al	60%	40%

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2.3 Characterization

The phase identification of the Al and Al/MoS₂/C composite was conducted by an X-ray diffractometer (XRD, Rigaku D/max 2500) using Cu Kα radiation. The microstructure of the composite and particle size distribution were investigated by scanning electron microscopy (SEM: Philips, FEI Quanta 200 FEG) and transmission electron microscopy (TEM, JEOL 2011).

2.4 Electrochemical measurements

The discharge/charge cycling performance of the samples was investigated using cell test systems (LAND BT2013A, Wuhan, China) with CR2032 coin-type cells assembled in an argon-filled glove box. The working electrodes consisted of 80 wt% of the active materials, 10 wt% conductivity agent (Super-P), and 10 wt% binder (polyvinyldifluoride), and the active material per electrode was about 0.8 mg cm⁻². Lithium foil was used both as a counter electrode and as a reference electrode in the half cells. The electrolyte was LiPF₆ (1 mol l⁻¹) in a mixture of ethylene carbonate (EC)–diethyl carbonate (DEC)–ethyl methyl carbonate (EMC) with a volume ratio EC–DEC–EMC = 1 : 1 : 1. Galvanostatic charge and discharge at a voltage interval of 0.05–3.0 V at a constant current of 100 mA g⁻¹ was carried out. Cyclic voltammetric measurements were also carried out on an electrochemical workstation (IM6) in a voltage range of 0.05–3.0 V at a scan rate of 0.1 mV s⁻¹.

3. Results and discussion

Fig. 1 displays the X-ray diffraction (XRD) patterns of pure Al (Fig. 1a), the MoS₂/C composite (Fig. 1b) and Al/MoS₂/C composite (Fig. 1c) with different Al content. The XRD patterns show no significant changes with increasing Al content. Four strong diffraction peaks at 2θ values of 38.57°, 44.81°, 65.21° and 78.33° are found, which are attributed to the (111), (200), (220) and (311) peaks of metallic aluminum. Weak peaks of MoS₂ at 2θ = 14.2°, 33.0° and 58.9° are attributed to (002), (100) and (110). However, no obvious carbon peaks appear in Fig. 1c due to the annealing temperature of 800 °C, which is much lower than the graphitization temperature of 3000 °C; the carbon in the Al/MoS₂/C composites should be amorphous. In the Al/MoS₂/C composite, diffraction lines of Al and MoS₂ were seen and no other XRD signals were detected, indicating that MoS₂/C acts only as a dispersing medium and does not alloy with the Al element to form any new phases after ball milling.

Fig. 1 XRD patterns of pure Al and the MoS₂/C and Al/MoS₂/C composites: (a) pure Al, (b) the MoS₂/C composite and (c) the Al/MoS₂/C composite.

Fig. 2 illustrates the SEM photograph of the pure aluminum powder and MoS₂/C composite. It was obvious that the pure aluminum powders present semi-spherical particles with an average width of about 10 μm, typical of gas-atomized metals (Fig. 2a and b). As shown in Fig 2c and d, the morphologies of the MoS₂/C composite prepared by a hydrothermal process and annealed exhibit a sphere-like or three-dimensional architecture with a rough surface. Fig. 3 shows the SEM images of the Al/MoS₂/C composite; the morphology of the composite consisted of irregularities at the nanoscale and sub-micron level. We cannot see obvious Al particles in the Al/MoS₂/C composite after ball milling because of the force induced by the milling balls during the ball milling process, and because the Al powder is broken down with a reduced particle size and uniformly dispersed in the MoS₂/C composite, so the Al powder (with a size of about 10 μm) disappears into the composite. On the other hand, it was obvious that the morphology and particle size of the Al/MoS₂/C composite differed from those of the pure Al and MoS₂/C composite after milling, indicating that metallic Al and the MoS₂/C composite have an interaction during ball milling.

Fig. 2 SEM images of (a and b) pure Al and (c and d) the MoS₂/C composite.

Fig. 3 SEM images of the Al/MoS₂/C composite: (a and b) 40 wt% Al, (c and d) 50 wt% Al and (e and f) 60 wt% Al.

The EDS elemental mapping in Fig. 4b–e depicts the homogeneous distribution of C, Al, Mo and S atoms in the Al/MoS₂/C composites. In addition to the four elements, the EDS also indicates the presence of trace amounts of oxygen in the composites, which could be from the introduction of some Al₂O₃ into the products during the ball milling or preparation process of the Al/MoS₂/C composite, due to the very easy oxidation of Al in air. However, the Al₂O₃ is not detected in the XRD, which could be because of the presence of trace amounts of Al₂O₃. So the elemental mapping images clearly revealed that the Al was homogeneously dispersed in the MoS₂/C matrix.

Fig. 4 EDS mappings of the Al/MoS₂/C composite: (a) the SEM image of the area selected, (b) C, (c) Al, (d) S, (e) Mo, (f) O.

To further observe the microstructure, the MoS₂/C composite and the Al/MoS₂/C composite were characterized by TEM. Fig. 5 shows that the MoS₂ nanoclusters comprised single-layer MoS₂ uniformly dispersed in amorphous carbon. Moreover, Fig. 6 shows the TEM images of the Al/MoS₂/C composite. In the original MoS₂/C composite, we can clearly see that the MoS₂ dispersed amorphously, but after the ball milling process, we can see some dark phase attached or embedded into the MoS₂/C matrix. According to the EDS mapping, the original Al particles were reduced and homogeneously dispersed in the MoS₂/C matrix after ball milling, so we think that the dark phase is Al. Therefore, large amounts of Al were significantly reduced to nanoscale particles and some aggregate clusters of Al were embedded into the MoS₂/C composite during ball milling, and the bright phase in Fig. 6c and d is MoS₂ and carbon.

Fig. 5 TEM (a) and HRTEM (b) images of the MoS₂/C composite.

Fig. 6 TEM (a and b) and the magnification (c and d) of the Al-40 wt% Al/MoS₂/C composite.

Fig. 7a shows the first charge and discharge curves of the three Al/MoS₂/C composite samples. All the electrodes exhibited similar charge and discharge profiles, giving two charging plateaus at ∼0.5 V and at ∼2.2 V; the plateau at about 0.5 V is indicative of the de-alloying reactions of the Li–Al alloy,²⁷ while the plateau at about 2.2 V is because of the high crystallinity of the annealed MoS₂. In addition, given the three discharging plateaus at ∼0.25 V, ∼1.9 V and ∼1.2 V, the plateau at about 0.25 V is indicative of the alloying reactions of Li with Al, and the plateaus at about 1.9 V and about 1.2 V are because of the formation of Li_xMoS₂ and Li₂S.^35,36

Fig. 7 (a) The first charge and discharge curves of the three Al/MoS₂/C composites at a constant current of 100 mA g⁻¹. (b) Cyclic voltammograms for the Al/MoS₂/C composite at the scan rate of 0.1 mV s⁻¹.

The cyclic voltammogram of the composite electrode is shown in Fig. 7b, two oxidation peaks appeared at ∼0.5 V and ∼2.3 V, and three reduction peaks appeared at ∼0.25 V, ∼0.6 V and ∼0.8 V during the first cycle, but the first peak at 0.6 V and 1.8 V appeared only in the initial scan and disappeared from the second and subsequent scans, implying that this reduction peak is caused by the electrochemical reduction of the electrolyte solvent for the formation of the SEI film on the composite anode. Obviously, the pair of redox peaks at 0.2 V and 0.5 V should be attributed to the reversible alloying and dealloying reactions of lithium with aluminum, the oxidation peak at 2.3 V is attributed to the formation of MoS₂ in the second and third cycle, and the reduction peak at 1.9 V is attributed to the formation of Li_xMoS₂,³⁶ which are in accordance with the charge and discharge profiles.

Fig. 8 shows the cycling behaviors of the Al/MoS₂/C composite and the pure Al electrode under a current density of 100 mA g⁻¹ and a potential range from 0.05–3.0 V. The pure Al electrode delivers a diminishing discharge capacity from over 1000 mA h g⁻¹ during initial cycles to 134 mA h g⁻¹ at the 20^th cycle with a capacity retention of 13.4%. In the mean time, the initial discharge capacities of the three composite electrodes were 1644.4 mA h g⁻¹, 1100.8 mA h g⁻¹ and 1206.0 mA h g⁻¹, respectively; the Al-60 wt% composite electrode reaches the maximum initial discharge capacity, and it is shown that a higher initial discharge capacity of the composites is reached when the Al content increases. However, though the composites have a higher capacity, the capacity retention of them is poor; with the addition of more Al, the capacity of the composites dropped dramatically. The composites of Al-40 wt%, Al-50 wt% and Al-60 wt% deliver reversible capacities of 451.3 mA h g⁻¹, 419.6 mA h g⁻¹ and 378.2 mA h g⁻¹ after 40 cycles, respectively. This is further proof that the Al/MoS₂/C composite exhibits a much improved cycle performance and initial discharge capacity in comparison to pure Al. In addition, the cycle performance of our Al/MoS₂/C composite is much better than previously reported Al-based composites for lithium ion batteries, such as Al–Fe–C composites,²⁷ Al–Si–graphite composites,²⁶ and SnO and Co₃O₄ coated Al particle composites.^28,29 Fig. 8b shows the rate performance of the Al/MoS₂/C composite. It is observed that the discharge capacities of the Al/MoS₂/C composites were 614 mA h g⁻¹, 497 mA h g⁻¹ and 413 mA h g⁻¹ at the 100 mA g⁻¹, 200 mA g⁻¹ and 500 mA g⁻¹ current densities, respectively. Furthermore, when increasing the current density to 1 A g⁻¹, a discharge capacity of 379 mA h g⁻¹ is still achieved. These improved electrochemical performances of the Al/MoS₂/C composites no doubt contributed to the cooperating action of MoS₂/C composites, which not only buffers the volume charge of Al, but can also provide a stability of capacity for the composite electrode during the charge and discharge process. As we all know, MoS₂ has an excellent mechanical property,³¹ and according to the EDS and TEM, the Al particles have been reduced to nanoparticles and attached or embedded into the MoS₂/C matrix, so that the MoS₂ can buffer the volume change of Al during the repeated charge–discharge cycles.

Fig. 8 (a) Cycle performance of the Al/MoS₂/C composite and pure Al electrode. (b) Rate performance of the Al-40 wt% Al/MoS₂/C composite.

4. Conclusions

This paper describes the structures and electrochemical performances of Al/MoS₂/C composites prepared by ball milling. The experimental results revealed that the Al/MoS₂/C composites have better electrochemical performances compared with pure Al and the previously reported Al-based anode materials. Particularly, the Al-40 wt% composite delivers a reversible capacity of 451.3 mA h g⁻¹ after 40 cycles under a current density of 100 mA g⁻¹, which showed a good cyclability. This suggests that the Al/MoS₂/C composite has a potential possibility to be developed as an anode material for lithium ion batteries.

Acknowledgements

This research was supported by National Science Foundation of China (U1401246, 51364004, 51474077 and 21473042),and the Guangxi Natural Science Foundation (2013GXNSFDA019027).

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