Communication pubs.acs.org/cm
Facile Synthesis of Chevrel Phase Nanocubes and Their Applications for Multivalent Energy Storage Yingwen Cheng,† Lucas R. Parent,‡ Yuyan Shao,† Chongmin Wang,‡ Vincent L. Sprenkle,† Guosheng Li,*,† and Jun Liu*,† †
Energy Processes & Materials Division and ‡Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99354, United States S Supporting Information *
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he Chevrel phase compounds are molybdenum chalcogenides with the general formula Mo6T8 or MxMo6T8 (M = metal, T = S, Se, or Te).1 They are important inorganic materials and have unique structural and physical properties. Most of them are superconducting with high critical temperatures and high critical magnetic fields.2,3 They are attractive for several other applications such as in magnetic, catalysis, and thermoelectrics areas.4−6 The general crystal structure of these compounds can be viewed as a three-dimensional array of Mo6T8 units that form tridirectional channels containing the metal (M), and each Mo6T8 unit consists a distorted Mo6 octahedron surrounded by a T8 cube.7 Experimental study of these compounds is currently limited by difficulties in preparing materials with controlled purity, composition, and structural characteristics. There are several synthetic methods developed by far, and the two most widely used methods are the high temperature (1100−1200 °C) solid-state-reaction method with elemental mixtures sealed in a quartz ampule and the molten salt method conducted at ∼850 °C.8,9 A self-propagating high temperature synthesis method with significantly reduced reaction time has also been developed recently.10 However, particles synthesized using these methods were mostly in micrometer sizes with very rough surface characteristics. Further development in material synthesis is still highly desired. Developing methods to synthesize nanometer Chevrel phase compounds is inherently attractive and should be pursued because this could bring new opportunities for studying these unique materials and lead to discoveries of novel properties and new applications, but little progress has been achieved. So far, only Aurbach et al. reported a method for synthesis of the nanoparticles, but the particles obtained had surface characteristics similar to those of microparticles and had a rather wide particle size distribution.11 In this paper, we report a facile method to synthesize Chevrel phase nanocubes through a graphene-assisted approach. This method yields pure materials with well-defined cubic shape. We further studied these nanocubes as electroactive materials for multivalent energy storage (Mg2+) and found they have substantially improved intercalation kinetics and higher reversible capacity. Our synthesis consists two steps: solution processing and precipitation of the precursors with graphene and high temperature solid-state reaction (see Supporting Information for details). This method was designed based on previous studies on synthesizing the Chevrel phase using soluble precursors12 but with some modifications in order to yield particles with desired nanometer sizes. In a typical synthesis, © 2014 American Chemical Society
500 mg of (NH4)2MoS4 (Aldrich, 99.97%), 86 mg of anhydrous CuCl2 (Aldrich, 99.995%), and 40 mg of graphene (high surface area reduced graphene oxide, Graphene Laboratories Inc.) were dissolved/dispersed in 10 mL of anhydrous dimethylformamide (Alfa Aesar, 99.9%). The mixture was heated at 95 °C for 4 h under Ar and was cooled afterward. A total of 50 mL of diethyl ether (Aldrich, 99.9%) was added to precipitate the precursors (they have low solubility in ether), and the black solids were collected by centrifuge and were vacuum-dried. The obtained precursor powders did not have good crystallinity from X-ray diffraction (XRD) measurement (Supporting Information Figure S1). Interestingly, it was found that most of the powders were coated on the surface of graphene sheets, and therefore it is evident that graphene provided templates for the precipitation (see transmission electron microscope (TEM) images, Supporting Information Figure S1). The powders were then heated at 1000 °C under flow of forming gas (100 sccm, 96% Ar and 4% H2) for solidstate synthesis of the Chevrel phase. The mechanism behind Chevrel phase synthesis has been known to be very complex,10 and here we attempted to study our process through characterizations of samples collected periodically using XRD and TEM (Figure 1). It can be seen that MoS2 was the reaction intermediate and was formed initially (within 30 min). The percentage of the Chevrel phase increased with the decrease of MoS2 for longer reaction, and pure Chevrel phase (Cu2Mo6S8) was obtained after 7 h of reaction (Figures 1a and 2a). Unlike previous studies that observed clear Cu diffraction peaks from intermediate products,13 we did not observe any Cu peaks during all stages of reaction, and this might indicate Cu was finely and homogeneously distributed in the mixture. Overall, the current reaction seems to proceed through a “MoS2 + Cu” intermediate state that agrees well with previous studies.13 The observation of MoS2 at early stages of reaction might indicate its formation is kinetically preferred. TEM analysis of the reaction process agrees well with XRD results as shown in Figure 1. The structural evolution can be clearly resolved from TEM images as MoS2 and the Chevrel phase have distinctive morphologies (sheets vs cubes).14 Overall, the synthesis successfully produced the Chevrel phase Cu2Mo6S8 nanocubes with an average size of ∼100 nm (see Figure 2a). It should be Received: June 25, 2014 Revised: July 30, 2014 Published: August 1, 2014 4904
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days).17 Figure 2b−f shows a set TEM/scanning TEM image of Mo6S8 particles. The particles have faceted and cubic morphologies that resemble the starting Cu2Mo6S8 nanoparticles, evidencing that the acid leaching process did not change the particle morphology obviously. Most particles were single crystalline as revealed by high resolution STEM (Figure 2b,c) and TEM images (Figure 2d−f) and corresponding fast Fourier transformations. The crystal structure observed in these nanoparticles (Figure 2c−f) corresponds well with previous studies of the bulk Mo6S8 structure, where Mo−Mo intracluster spacing of 2.7 Å and a rhombahedral lattice parameter of 6.43 Å have been reported.11 The Mo6S8 nanocubes were assembled into electrodes using a standard approach (mix with 10 wt % of conductive carbon and 10 wt % of poly(vinylidene fluoride and using n-methyl-2pyrrolidone as dispersant). The batteries were prepared as coin cells (2032) using Mg metal as the anode and 0.4 M all-phenylcomplex (APC) dissolved in tetrahydorfuran (THF) as the electrolyte and were tested using cyclic voltammetry (CV) and galvanostatic charge−discharge techniques at room temperature (∼20 °C). The behavior of the microparticles prepared without graphene was also studied for comparison. It should be noted that the electrochemical behavior of Mo6S8 depends significantly on the electrolyte chemistry and temperature, as can be seen from previous studies focused on electrolyte development.18,19 Since the scope of this study was on material properties, we only used the APC electrolyte and the conclusion should be general. Figure 3a compares typical first and stabilized CV curves (acquired at 50 μV/s) for cells made with nanoparticles and
Figure 1. (a) XRD and (b, c) TEM characterization results of powders obtained at different reaction stages. The scale bars shown in (b) and (c) are 200 nm.
Figure 2. SEM and (S)TEM images of the Chevrel phase nanoparticles. (a) A representative SEM image of Cu 2 Mo 6 S 8 nanocubes. (b/c) High-angle annular dark field Z-contrast STEM images and (d) bright field TEM image of single crystalline, faceted Mo6S8 nanoparticles. High intensity “spots” in image c represent the locations Mo6 clusters, with a spacing of ∼6.5 Å. (e/f) High-resolution TEM image of the lattice structure of a crystal aligned on a zone axis where the intracluster Mo−Mo spacing and rhombohedral lattice spacing (blue and red markers, respectively) can be resolved. Fast Fourier transformation of the respective image is displayed in the inset.
noted that the incorporation of graphene was critical for obtaining nanoparticles, and particles synthesized using the same approach but without graphene were much larger and in micrometric scale (Supporting Information Figure S2). Graphene was important because it could provide heterogeneous nucleation sites for the nucleation of a particle. This property of graphene (and other materials such as carbon nanotubes) has been widely explored for synthesizing hybrid materials.15,16 We then removed Cu ions from the Cu2Mo6S8 nanocubes using an acid leaching method in order to prepare electroactive Mo6S8 for multivalent energy storage. To do this, the assynthesized nanocubes were stirred in 1.0 M HCl at room temperature for 12 h (Supporting Information Figure S3 for XRD of Mo6S8). We found that the nanocubes had remarkably better leaching kinetics than microparticles prepared by the molten salt method, as the microparticles require much concentrated acid for much longer time (8.0 M HCl for >7
Figure 3. Electrochemical characterization of batteries made with particles of different sizes: (a) comparison of their first and stabilized CV curves; (b) galvanostatic charge−discharge profiles at different Crate for nanoparticles; (c) comparison of the specific capacity of particles with different sizes as a function of C-rate; and (d) cyclic stability and Coulombic efficiency of nanocubes for 150 cycles.
microparticles. The particle size had strong influences on the electrochemical behavior of Mo6S8 and nanoparticles had better performance on the basis of the following three observations: (1) Both materials required some overpotential to initiate the first Mg2+ intercalation, but for nanoparticles the overpotential was noticeably lower. The observation of overpotential is in good agreement with previous works and indicates that Mg2+ insertion into “virgin” Mo6S8 has some thermodynamic barrier 4905
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and is intrinsically slow.20 Once some Mg2+ is inserted, the intercalation can proceed much faster. Such an understanding can also explain the reduction of overpotentials for subsequent CV cycles as some Mg ions got trapped during the first cycle. (2) Nanoparticles had a stronger anodic peak at ∼1.7 V. This peak can be assigned to extraction of Mg2+, and a stronger peak intensity essentially means more Mg2+ ions were extracted and the trapping effect for nanoparticles was reduced. (3) Nanoparticles exhibited narrower separation between the main anodic and cathodic peaks, indicating that the kinetics of the stabilized electrochemical reaction for nanoparticles was better. The behavior of nanoparticles and microparticles was further compared using the charge−discharge test conducted at different C-rates. The C-rates were determined using the theoretical capacity of Mo6S8 (128.8 mAh/g). Figure 3b,c shows typical charge−discharge profiles and the corresponding specific capacities (see Supporting Information Figure S4 for profiles of microparticles). Both materials had close to the theoretical capacities for the first discharge cycle, but their capacities dropped for the subsequent cycles due to partial trapping of Mg2+.21 A typical capacity drop of >30% was observed for the microparticles. In contrast, the drop for the nanoparticles was much less and was generally
