Synthesis and in Situ XAFS Investigation of MoO₂ nano-Particles As Li-Ion Battery Anodes

Molybdenum dioxide (MoO2) has appealing properties as an alternative to graphite for Li-ion battery anodes. It has a low electrical resistivity (8.8⋅10⁻⁵ Ω⋅cm for bulk MoO2 at room temperature[1] vs. about 1⋅10⁻¹ Ω⋅cm for graphite powder electrodes[2]) and a high theoretical capacity (840 mAh/g[3] vs 372 mAh/g for graphite[4]). The MoO2 structure accommodates 4 lithium atoms for every Mo atom, whereas graphite can only accommodate 1 lithium atom per 6 carbon atoms. MoO2 has a 1.1 V chemical potential versus lithium ions,[5] suitable for an anode material. Graphite lithiates at about 0.1 V, yielding a higher cell voltage than MoO2, but also making it susceptible to hazardous lithium metal deposition.[6] Because MoO2 undergoes a large volume change while intercalating Li-ions (12% according to DFT calculations[5]), its bulk form has limited capacity and cycling stability. This can be significantly enhanced by changing its morphology, but its performance is strongly dependent on synthesis method.[7] Li-ion diffusion kinetics in MoO2 are relatively slow, so the material benefits from nano-scale particles that reduce the diffusion length.[1] Many studies have examined synthesis routes for nano-sized MoO2 oxides[8][9] or hybrid materials.[10][11][12] This work focuses on synthesizing particles of micron to nano-scale using a low-temperature, wet chemical synthesis, and characterizing the effect of particle size on the electrochemical performance of MoO2 materials, providing insight into the lithium intercalation mechanism. As a comparison to MoO2, MoO3 was also investigated.[13] It shares morphology dependence and slow kinetics with MoO2, but is an insulator.[14] Its higher chemical potential makes it more suitable as a cathode material.[5] The electrode materials were characterized using X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX), and X-ray absorption spectroscopy fine structure (XAFS). XAFS is an element-specific technique, probing the local electronic and atomic environment. As XAFS measurements do not require long-range crystalline order, they yield information for both crystalline and amorphous phases, making XAFS a valuable technique for battery material characterization. XAFS spectra were taken in situ during charge and discharge cycles of bulk and nanoscale MoO2 half-cells vs. Li metal. This helps elucidate the charge and discharge mechanisms and provides insight into the cycling behavior of the cell. [1] Shi, Y. et al. Ordered Mesoporous Metallic MoO2 Materials with Highly Reversible Lithium Storage Capacity. Nano Letters 9, 4215-4220 (2009). doi:10.1021/nl902423a. [2] Imoto, K., et al. High-performance carbon counter electrode for dye-sensitized solar cells. Solar Energy Materials and Solar Cells 79, no. 4, 459-469 (2003). doi:10.1016/S0927-0248(03)00021-7 [3] Wang, Z., et al. One-pot synthesis of uniform carbon-coated MoO2 nanospheres for high-rate reversible lithium storage. Chemical Communications 46, 6906 (2010). doi:10.1039/c0cc01174f. [4] Shu, Z. X., et al. Electrochemical intercalation of lithium into graphite. Journal of The Electrochemical Society 140, no. 4 (1993): 922-927. doi: 10.1149/1.2056228 [5] Dillon, A., et al. High Capacity MoO3 Nanoparticle Li-Ion Battery Anode. Vehicle Technologies Program AMR, Feb. 27, 2008. http://energy.gov/sites/prod/files/2014/03/f11/merit08_dillon.pdf [6] Jansen, A. N., et al. Development of a high-power lithium-ion battery. 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