Role of Structural H₂O in Intercalation Electrodes: The Case of Mg in Nanocrystalline Xerogel‑V₂O₅

Cointercalation is a potential approach to influence the voltage and mobility with which cations insert in electrodes for energy storage devices. Combining a robust thermodynamic model with first-principles calculations, we present a detailed investigation revealing the important role of H₂O during ion intercalation in nanomaterials. We examine the scenario of Mg²⁺ and H₂O cointercalation in nanocrystalline Xerogel-V₂O₅, a potential cathode material to achieve energy density greater than Li-ion batteries. Water cointercalation in cathode materials could broadly impact an electrochemical system by influencing its voltages or causing passivation at the anode. The analysis of the stable phases of Mg-Xerogel V₂O₅ and voltages at different electrolytic conditions reveals a range of concentrations for Mg in the Xerogel and H₂O in the electrolyte where there is no thermodynamic driving force for H₂O to shuttle with Mg during electrochemical cycling. Also, we demonstrate that H₂O shuttling with the Mg²⁺ ions in wet electrolytes yields higher voltages than in dry electrolytes. The thermodynamic framework used to study water and Mg²⁺ cointercalation in this work opens the door for studying the general phenomenon of solvent cointercalation observed in other complex solvent–electrode pairs used in the Li- and Na-ion chemical spaces

Materials Design Rules for Multivalent Ion Mobility in Intercalation Structures

The diffusion of ions in solid materials plays an important role in many aspects of materials science such as the geological evolution of minerals, materials synthesis, and in device performance across several technologies. For example, the realization of multivalent (MV) batteries, which offer a realistic route to superseding the electrochemical performance of Li-ion batteries, hinges on the discovery of host materials that possess adequate mobility of the MV intercalant to support reasonable charge and discharge times. This has proven especially challenging, motivating the current investigation of ion mobility (Li⁺, Mg²⁺, Zn²⁺, Ca²⁺, and Al³⁺) in spinel Mn₂O₄, olivine FePO₄, layered NiO₂, and orthorhombic δ-V₂O₅. In this study, we not only quantitatively assess these structures as candidate cathode materials, but also isolate the chemical and structural descriptors that govern MV diffusion. Our finding that matching the intercalant site preference to the diffusion path topology of the host structure controls mobility more than any other factor leads to practical and implementable guidelines to find fast-diffusing MV ion conductors