LETTER pubs.acs.org/NanoLett
MoS2 Nanoplates Consisting of Disordered Graphene-like Layers for High Rate Lithium Battery Anode Materials Haesuk Hwang,† Hyejung Kim,† and Jaephil Cho* Interdisciplinary School of Green Energy, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, Korea
bS Supporting Information ABSTRACT:
MoS2 nanoplates, consisting of disordered graphene-like layers, with a thickness of ∼30 nm were prepared by a simple, scalable, onepot reaction using Mo(CO)6 and S in an autoclave. The product has a interlayer distance of 0.69 nm, which is much larger than its bulk counterpart (0.62 nm). This expanded interlater distance and disordered graphene-like morphology led to an excellent rate capability even at a 50C (53.1 A/g) rate, showing a reversible capacity of 700 mAh/g. In addition, a full cell (LiCoO2/MoS2) test result also demonstrates excellent capacity retention up to 60 cycles. KEYWORDS: Rate capability, MoS2, nanoplate, anode, lithium battery
L
ayered transition metal dichalcogenide compounds MX2 (M = Mo, Ti, V, and W, X = S or Se) can introduce or intercalate foreign atoms or alkali metals into their structure between the layers.1 3 Atoms within layers of these compounds are bound by strong covalent bonds, while the individual layers are bound by weak van der Waals interactions, forming a sandwich-like structure (graphene analogues). Because of this structural feature, they had been considered as host materials in the field of primary or rechargeable batteries. Among these compounds, MoS24 6 received great attention due to its high lithium ion capacity. Recently, there have been some reports on MoS2 about improving the capacity via structure modification. The most popular approach to increase the capacity is to enlarge the interlayer distance to relax the strain and lower the barrier for Li intercalation. For instance, Du et al. reported that elongated, restacked MoS2, along the c axis, prepared from commercially available MoS2, showed the first charge (Li+ extraction) capacity of 800 mAh/g and maintained a capacity of 750 mAh/g after 20 cycles at the current density of 50 mA/g.7 On the other hand, bulk MoS2 electrodes exhibited decreased charge capacity from over 800 to 226 mAh/g after 50 cycles. Xiao et al. and Chang et al. reported that nanocomposites formed by inserting the lithium ion coordination properties of PEO and amorphous carbon, respectively, into the interlayer spacing of MoS2, led to a reversible capacity of >900 mAh/g at a rate of 100 mA/g.5,6 On the other hand, 1D and 3D MoS2 nanotubes, nanoflakes, and flower-like structures have been demonstrated with the reversible capacity of >850 mAh/g at low current rates.8 10 However, these synthetic methods for obtaining MoS2 are quite complex. For instance, 3D- flowerlike MoS2 was prepared r 2011 American Chemical Society
from a hydrothermal synthesis, assisted with an ionic liquid (IL) 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), via two-step procedures (hydrothermal treatment and annealing at 240 and 800 °C, respectively).8 The thioacetamide (CH3CSNH2) and sodium molybdate (Na2MoO4 3 2H2O) were used as precursors.8 In order to use a MoS2 electrode as a possible candidate in a lithium battery, a synthetic method should be simple for an easy scale-up. In addition, a high rate performance needs to be satisfied up to 10 C (>10 A/g). In these regards, we report a simple, onepot synthesis of disordered graphene-like MoS2 nanoplates via a solvothermal method. The first discharge and charge capacities were 1062 and 917 mAh/g, respectively, showing an efficiency of 87% at a 1C rate (1C = 1.06 A/g). The MoS2 nanoplate electrode demonstrated excellent capacity retention at 50C (=53.1 A/g), delivering a reversible capacity of 700 mAh/g. Figure 1a shows a schematic for the preparation of disordered graphene-like MoS2 nanoplates via a simple one-pot reaction. Mo(CO)6 and sulfur powders are mixed in xylene and annealed at 250 °C in an autoclave for 24 h (typical batch size was 15 20 g). This simple method leads to disordered graphene-like MoS2 nanoplates. The XRD pattern (Figure 1b) of the as-prepared sample does not exhibit a (002) reflection peak that is typically observed in bulk analogue (Figure S1, Supporting Information), indicating that the material contains five or less layered graphenelike MoS 2. 10 12 Indicatively coupled plasma atomic emission Received: August 3, 2011 Revised: September 12, 2011 Published: September 29, 2011 4826
dx.doi.org/10.1021/nl202675f | Nano Lett. 2011, 11, 4826–4830
Nano Letters
LETTER
Figure 1. (a) Schematic of preparation of MoS2 nanoplates. (b) XRD pattern of as-prepared MoS2 nanoplates. (c) SEM image of MoS2 nanoplates. (d and e) TEM images of (c), and (f) digitized fast Fourier transformed (FFT) image of the red rectangular area in (e).
Figure 2. (a) AFM image of as-prepared MoS2 nanoplates and (b) plot of height of the plates at points A, B, and C in (a). (c) Schematic of MoS2 structure.
spectroscopy (ICP-AES) confirmed that the product was stoichiometric MoS2. Panels c and d of Figure 1 show the SEM and TEM images of the as-prepared MoS2 sample, consisting of stacked nanoplates with average particle size of ∼50 nm. MoS2 nanoplates consist of randomly oriented graphene-like layers with an interlayer separation of 0.69 nm, which is larger than the interlayer distance (0.62 nm) for the (002) plane of the bulk MoS2. Moreover, the digitalized fast Fourier transformed (FFT) image (Figure 1f) of the MoS2 nanoplates shows that d-spacing value of (002) plane along the [020] zone axis is 0.69 nm, while that of the bulk sample shows 0.62 nm (Figure S1, Supporting Information). Long-range
ordering of the stacked graphene-like layers with a longitudinal length of
