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. Journal of power sources 81 (1999): 902-905. doi:10.1016/S0378-7753(99)00268-2
[7] 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.
[8] Ellefson, Caleb A., et al. Synthesis and applications of molybdenum (IV) oxide. Journal of Materials Science 47, no. 5 (2012): 2057-2071. doi:10.1007/s10853-011-5918-5
[9] Manthiram, A., A. Dananjay, and Y. T. Zhu. New route to reduced transition-metal oxides. Chemistry of materials 6, no. 10 (1994): 1601-1602. doi: 10.1021/cm00046a006
[10] Hirsch, Ofer, et al. Aliovalent Ni in MoO2 Lattice— Probing the Structure and Valence of Ni and Its Implication on the Electrochemical Performance. Chemistry of Materials 26, no. 15 (2014): 4505-4513. doi:10.1021/cm501698a
[11] Huang, Z. X., et al. 3D graphene supported MoO 2 for high performance binder-free lithium ion battery. Nanoscale 6, no. 16 (2014): 9839-9845. doi:10.1039/C4NR01744G
[12] Bhaskar, A., et al. MoO2/multiwalled carbon nanotubes (MWCNT) hybrid for use as a Li-ion battery anode. ACS applied materials & interfaces 5, no. 7 (2013): 2555-2566. doi:10.1021/am3031536
[13] Luigi, C., and Pistoia, G. MoO3: A New Electrode Material for Nonaqueous Secondary Battery Applications. Journal of The Electrochemical Society 118, no. 12 (1971): 1905-8. doi: 10.1149/1.2407864
[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