1. Introduction – MoS₂ is promising as alternative to traditional graphite anodes. MoS₂ possesses a low redox potential relative to lithium and approximately 3x the theoretical capacity of graphite, making it an ideal candidate for lithium ion battery anodes [1, 2]. To achieve high storage capacities and stable capacities over many charge/discharge cycles, high electrochemically active surface areas and access to layer edge intercalation sites is required. Controlled growth of MoS₂ microspheres, whose robust morphology should provide a stable structure and abundant access to intercalation sites also allows for the control of particle surface area to volume ratios, a key component to determining electrochemical activity of the anode particles. In this work, experimental parameters such as precursor materials, temperature and pressure are varied for the hydrothermal synthesis of MoS₂ microparticles, giving control over particle morphology and size. The influence of surface area to volume ratios are then examined with respect to material stability and capacity. Fabrication procedures and preliminary results are shown in the following sections.
2. Fabrication procedures – Synthesis of MoS₂ microspheres is accomplished by a hydrothermal procedure under high pressure. In a typical procedure hexammonium heptamolybdate tetrahydrate and thiourea are vigorously in a solution of deionized water. The solution is then transferred to a Teflon lined autoclave reactor vessel and heated to 220 °C for an extended period of time. The resulting black MoS₂ precipitate is collected by centrifugation and washed with ethanol and deionized water several times to remove impurities. Lingering solvent is then removed by drying the material at low temperatures in atmosphere overnight.
3. Preliminary results – X-ray diffraction spectra of MoS₂ microspheres taken at different reaction times are shown in Figure 1(a). Material crystallinity is observed to be dependent on reaction time. An SEM image typical of the fabricated MoS₂ microspheres is shown in Figure 1(b), showing that the microspheres are composed of conglomerations of MoS₂ sheets. Figure 1(c) shows particle size distributions of the three reaction times. All resulted in two size distributions per reaction time, with an increase in reaction time yielding an increase in both particle size distributions. Figure 1(d) shows surface area to volume ratios (S.V.R.) of the measured particle size distributions plotted against the S.V.R. function for a sphere.
4. References –

[1] T. Stephenson, Z. Li, B. Olsen, D. Mitlin, Energy & Environmental Science, 7 (2014) 209-231.

[2] N. Yabuuchi, K. Kubota, Y. Aoki, S. Komaba, The Journal of Physical Chemistry C, 120.