Comparing the Insertion Properties of Mg²⁺ into V₃O₇ H₂O in Organic and Aqueous Electrolytes

Batteries are indispensable in our society due to their ability to store renewable energy efficiently and provide it on demand. [1] In this context, lithium-ion batteries offer one of the highest energy densities and are already used in numerous mobile applications. However, lithium reserves and their accessibility are limited, which motivates researcher to find alternatives with similar electrochemical properties, i.e. low redox potentials and high volumetric/gravimetric specific capacities as counter electrode in batteries compared to lithium.<br/><br/>In this context, magnesium is attractive because it is more abundant, has a low redox potential vs. SHE and a small ionic radius, comparable to lithium. [2] The double charge of magnesium results in advantages and disadvantages; on the one hand it offers the possibility to store a higher amount of charge, while on the other hand the stronger polarization of Mg²⁺ compared to Li⁺ leads to challenges concerning the intercalation process. Therefore, cathode materials, which allow the reversible intercalation of Mg²⁺, offer high theoretical capacities and are stable in various chemical environments <i>e.g</i>. in organic and aqueous electrolytes, are of great interest in current magnesium-ion battery research. One promising cathode material, which can fulfil these requirements, is hydrated vanadium oxide V₃O₇H₂O. [3]<br/><br/>In this context, we show via <i>operando</i> XRD how the structural evolution of V₃O₇H₂O during ion insertion is affected by adding small quantities of water to the organic electrolyte. In addition, we study Mg²⁺ insertion into V₃O₇ H₂O in a completely aqueous electrolyte for the first time. Then we compare the structural properties of hydrated vanadium oxide as cathode material with respect to Mg²⁺ insertion in organic and aqueous electrolytes.<br/>In the next step we investigate to which extent the chemical environment influences the electrochemical properties. Hereby, V₃O₇H₂O shows promising initial specific capacities of ~300 mAh g^-1 in a pure aqueous electrolyte at practical current densities of 100 mA g^-1, which demonstrates the versability of V₃O₇H₂O as cathode material for Mg²⁺ insertion.<br/><br/>Literature:<br/>[1] P. Wang <i>et al.</i>, Energy Storage Materials, <b>2022</b>, <i>45</i>, 142–181<br/>[2] P. Cunyuan <i>et al.,</i> <i>Small</i>, <b>2021</b>, <i>17</i>, 2004108<br/>[3] D. Söllinger et al., <i>Electrochimica Acta</i>, <b>2022</b>, <i>434</i>, 141294