Carbothermal reduction synthesis of LixMoyOz/C composite material as a cathode material for Li-ion batteries

Abstract

Li2MnO3 is extensively investigated as a cathode material for Li-ion batteries. However, oxygen loss (oxygen will oxidize prior to Mn4+ to Mn5+) resulting in irreversible structural changes decreases the electrochemical performance. The layered disordered NaFeO2 type structured cathode material Li2MoO3 has several advantages over Li2MnO3. This includes the accessibility of the Mo4+/Mo6+ redox couple, improved kinetics due to the higher electronic conductivity and lower oxygen evolution from the cathode’s active material oxygen sublattice. Most commonly, Li2MoO3 is prepared by reducing Li2MoO4 under Ar/H2 flow1 or N2/H2 flow 2. However, the use of H2 comprises severe safety issues. As an alternative, Li2MoO3/C has been obtained in literature via a solid state reaction between Li2CO3 and MoO2 with acetylene black as an additive under inert Argon atmosphere, excluding the use of H2.3 The major limitation of the proposed synthesis method is the extended anneal period of 24 hours at 900°C or 1000°C. Here, we report the synthesis of LixMoyOz/C composite materials via a carbothermal reduction method with organic ligands serving as the carbon source, requiring an anneal period of less or equal to 12 hours at 900°C or less. Molybdenum is present in a 6+ oxidation state in the aqueous precursor. The end products are characterized by Raman spectroscopy, XRD and SEM. The XRD patterns indicate Li2MoO3 and Li4Mo5O24 as major phases, hinting the ability to effectively reduce Mo6+ to Mo4+ by our method. The composite end product has been electrochemically characterized vs. Li metal as an anode in a coin cell configuration. The initial galvanostatic charging curve has at first glance a similar shape as compared to literature for Li2MoO3, including the prominent potential plateau around 3.5V. In order to gain understanding of the mechanisms behind initial charge/discharge of our LixMoyOz /C composite material, with focus on the contribution of the individual compounds of which the composite is composed of, in situ Raman spectroscopy has been applied.