Synthesis and in Situ XAFS Investigation of MoO 2 nano-Particles As Li-Ion Battery Anodes

Molybdenum dioxide (MoO 2 ) 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 MoO 2 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 MoO 2 structure accommodates 4 lithium atoms for every Mo atom, whereas graphite can only accommodate 1 lithium atom per 6 carbon atoms. MoO 2 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 MoO 2 , but also making it susceptible to hazardous lithium metal deposition. [6] Because MoO 2 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 MoO 2 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 MoO 2 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 MoO 2 materials, providing insight into the lithium intercalation mechanism. As a comparison to MoO 2 , MoO 3 was also investigated. [13] It shares morphology dependence and slow kinetics with MoO 2 , 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 MoO 2 half-cells vs. Li metal. This helps elucidate the charge and discharge mechanisms a