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

Tuesday, 26 May 2015: 09:00
Salon A-4 (Hilton Chicago)
N. M. Beaver, S. Aryal, Y. Ding, J. P. Katsoudas, Y. Li, C. J. Pelliccione, C. U. Segre (Illinois Institute of Technology), and E. V. Timofeeva (Energy Systems Division, Argonne National Laboratory)
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.

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[14] Sunu, S. S., et al. Electrical conductivity and gas sensing properties of MoO3. Sensors and Actuators B: Chemical 101, no. 1 (2004): 161-174. doi:10.1016/j.snb.2004.02.048