V2O5-Based Composite As Mg-Ion Battery Electrode Material Studied By Electrochemical SPM at the Nanoscale

The science and technology of rechargeable Li-ion batteries have dominated the field of advanced power sources and replaced many other batteries in the market, particularly in the areas of communication, computers, electronics, and in very high power devices such as power tools and electric vehicles.¹ The critical challenges for rechargeable Li-ion batteries are cost, safety, and energy density.² Since an innovative Mg-ion battery was demonstrated successfully by the Aurbach et al.,³ it has received much attention as a potential rechargeable battery to replace the Li-ion battery because of safety, abundance and inexpensiveness of Mg. Moreover, Mg anodes, being divalent and elemental, theoretically provide a higher volumetric capacity of up to ~3800 mAh cm^-3 compared to ~2050 mAh cm^-3 of Li.² Despite many promising aspects, there is few research work associated with Mg-ion batteries so far. Indeed, the development of rechargeable Mg-ion batteries has been impeded from various intrinsic limitations relating to the use of Mg metal anodes and Mg-ion intercalation cathodes.⁴ Passivation of Mg metal in the conventional non-aqueous electrolyte solution is a serious challenge, and Mg-ion intercalation is very slow or even impossible for most of the transition metal oxides serving as Li-ion insertion electrodes. In spite of these difficulties, it has been shown that Mg-ion may insert into transition metal oxides even in conventional non-aqueous Mg salt electrolyte solutions with a certain degree of reversibility.⁴ Specifically, nanostructured V₂O₅-based composites are known as high power and high energy density materials that might insert Mg ions reversibly, and they have already been studied intensively as anode and cathode materials for Li-ion batteries demonstrating a high reversible capacity, excellent cycling performance, and good rate capability.⁵

However, it is important to note that the research efforts dedicated to Mg-ion batteries are still in infancy. Recently, Gershinsky et al.⁶ explored that highly reversible Mg-ion insertion/extraction was possible with highly pure V₂O₅ thin-film electrodes using various electrochemical and spectroscopic analytical tools. A growing practical experience related to nanostructured materials suitable for Mg-ion battery applications requires a fundamental insight at nanoscale. In virtue of the Electrochemical Scanning Probe Microscopy (ECSPM) technique, especially Electrochemical Scanning Tunneling Microscopy (ECSTM), our approach to study magnesium intercalation is to use single-crystalline V₂O₅ to obtain an insight and accurate prediction of reaction mechanisms for the electrode of rechargeable Mg-ion batteries. In order to analyze the intercalation mechanisms of Mg²⁺, topographical changes of V₂O₅ will be analyzed under stepwise polarization to different potentials by ECSTM. The results will be described in terms of the combination among physicochemical phenomena, electrochemical metal-ion intercalation into single-crystalline host structures with different electrolyte systems, and electrochemical performance in rechargeable Mg-ion batteries. These studies will be complemented by three electrode electrochemical studies at nanostructured VO_xcomposites, and compared to results obtained for Li-ion insertion. 

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