Synthesis and in Situ XAFS Investigation of MoO2 nano-Particles As Li-Ion Battery Anodes
Authors: Beaver Nathaniel M. , Aryal Shankar , Ding Yujia , Katsoudas John P. , Li Yue , Pelliccione Christopher J. , Segre Carlo U , Timofeeva Elena V. 
Published: February 2020

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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