Impact of Solvent Co-Intercalation in Cathodes: A Study on Mg-Xerogel V₂O₅

The electrochemical performance of several known Li- and Na-ion cathode materials can be influenced by the presence of specific solvents, such as water, either in the electrolyte or the electrodes. Indeed co-intercalation of solvent molecules, along with the redox active ions, is an approach to potentially impact the voltage and mobility with which cation species intercalate into electrodes in energy storage systems. While Multi-valent (MV) ion intercalation batteries that replace the Li + with a MV cation, such as Mg 2+ , provide a pragmatic approach to improve upon the energy densities of current cutting-edge Li-ion batteries, several cathode hosts suffer from sluggish MV ion mobility within the cathode structure. One of the approaches that has been deployed to mitigate poor MV ion migration so far, specifically in layered oxide cathodes such as Xerogel-V 2 O 5 , is to introduce solvent molecules (water) in the structure that electrostatically shield the high positive charge density around MV ions and lower the local structural distortions. However, the role of water during the process of Mg 2+ (or MV) intercalation is not well known yet, specifically if the structural water in the cathode “shuttles” along with the Mg 2+ ion or stays in the structure during Mg-cycling. In this study, we use first-principles calculations combined with the framework of grand potential phase diagrams to gain insight into the thermodynamics of Mg-water co-intercalation into Xerogel-V 2 O 5 . Based on our calculations, we find that Mg-water shuttling depends heavily on the water concentration of the electrolyte, ranging from full co-intercalation in wet to none in superdry electrolytes. Also, the Mg insertion voltages depend on the activity of water in the electrolyte in regimes where water shuttles with the Mg 2+ , leading to important consequences in the design of electrodes and electrolytes. We believe that this study can be further extended to other complex solvent-electrode pairs to develop a general understanding of the phenomenon of solvent co-intercalation.