Study of model surfaces of molecularly functionalized vanadium oxide

Abstract

Global warming is one of the biggest threats to civilization, the greenhouse gases (GHGs) in the atmosphere increase the absorbed solar radiation, increasing the global temperature. In Chile, the transport industry has the highest GHG emissions. Therefore, it is necessary to transit from conventional fossil fuels to hybrid or electric vehicles. Additionally, the use of renewable energy sources to produce electricity can further reduce GHG emissions. However, the seasonality of these energy sources makes it necessary to store the energy produced during periods of high production to supply the periods of low production. As a result, research aimed at improving energy storage has been increasing. Particularly, chemical energy storage is widely used due to its presence in almost every electronic device in the form of a rechargeable battery. In the field of rechargeable batteries, lithium-ion batteries (LIBs) have attracted a lot of attention due to their high volumetric and gravimetric energy density, reaching up to 400 Wh/L and 180 Wh/g, respectively. LIBs are also design flexible and do not suffer memory problems of first-generation Ni-based batteries. However, the performance of LIBs is primarily limited by the cathode, which often exhibits issues such as low ionic and/or electronic conductivity, irreversible phase transitions, and dissolution in the electrolyte. As a result, various cathode options are being studied, with transition metal oxides such as LiCoO2, LiMn2O4, LiFePO4, LiNi0.5Mn1.5O4 and V2O5 being the most promising. Among these options, vanadium pentoxide stands out due to its high theoretical capacity (442 mAh/g for three Li+ intercalation and 294 mAh/g for two Li+ intercalation per formula), abundance, low cost, and ease of preparation. Although V2O5 has a high theoretical capacity, it also presents problems such as a low ionic diffusion coefficient and dissolution in the electrolyte. For this reason, research on more stable electrolyte-electrode interfaces has become crucial to improve the performance of V2O5. Solutions have emerged from the study of cathode-electrolyte interface (CEI) such as coating the surface of the cathode with an inert material to reduce the contact of the electrode with the electrolyte and regulating or promoting the formation of a stable solid electrolyte interface (SEI) to reduce the cathode dissolution. In this context, the formation of a self-assembled monolayer (SAM) on the surface of the electrode before contact with the electrolyte can be an efficient way to improve the performance of V2O5. This work studied the formation of self-assembled monolayers on the V2O5 surface and their influence on the cyclability as cathodes of lithium-ion batteries. Samples of vanadium oxides with different thicknesses were prepared on silicon substrates and characterized chemically and topographically. The most oxidized sample was then functionalized with 4- (amino)benzoic acid and characterized. A new set of V2O5 samples was prepared on stainless steel and functionalized with 4-(phenylazo)benzoic acid (PPBA). Both bare V2O5 and PPBAcapped V2O5 surfaces were characterized and tested as LIB cathodes, resulting in an increase in both the charge/discharge capacity and the energy efficiency. Furthermore, postmortem analysis indicates the formation of a protective CEI in the PPBA-capped sample.