Role of structural H$_2$O in intercalation electrodes: the case of Mg in nano-crystalline Xerogel-V$_2$O$_5$ (1603.05342v1)

Abstract: Co-intercalation 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 H$_2$O during ion intercalation in nano-materials. We examine the scenario of Mg$^{2+}$ and H$_2$O co-intercalation in nano-crystalline Xerogel-V$_2$O$_5$, a potential cathode material to achieve energy density greater than Li-ion batteries. Water co-intercalation 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 V$_2$O$_5$ and voltages at different electrolytic conditions reveals a range of concentrations for Mg in the Xerogel and H$_2$O in the electrolyte where there is no thermodynamic driving force for H$_2$O to shuttle with Mg during electrochemical cycling. Also, we demonstrate that H$_2$O shuttling with the Mg$^{2+}$ ions in wet electrolytes yields higher voltages than in dry electrolytes. The thermodynamic framework used to study water and Mg$^{2+}$ co-intercalation in this work opens the door for studying the general phenomenon of solvent co-intercalation observed in other complex solvent-electrode pairs used in the Li- and Na-ion chemical spaces.

Role of Structural H$_2$O in Intercalation Electrodes: The Case of Mg in Nano-Crystalline Xerogel-V$_2$O$_5$

The paper presents a detailed investigation into the role of structural water co-intercalation in nano-crystalline Xerogel-V$_2$O$_5$, with a specific focus on magnesium (Mg$^{2+}$) ion intercalation. Through a robust thermodynamic model supported by first-principles calculations, the authors provide insights into how the presence of H$_2$O impacts Mg intercalation, exploring its influence on voltages and phase behavior across varying electrolytic conditions. The research emphasizes the significance of H$_2$O co-intercalation in enhancing voltages for energy storage applications potentially beyond current Li-ion battery capabilities.

Summary of Findings

1. Coe-Intercalation Dynamics: The paper delineates three distinct electrolytic regimes—wet, dry, and superdry—each influencing the structure's ability to retain or release water during Mg$^{2+}$ intercalation. In wet electrolytes, water fully co-intercalates with Mg, increasing the phase's stability and resulting in high Mg insertion voltages. Conversely, in superdry conditions, no water co-intercalation occurs, leading to different phase behaviors and a singular voltage plateau.
2. Voltage Characteristics: The research shows that the water activity in the electrolyte is positively correlated with the Mg insertion voltage. A wet electrolyte can increase the Mg insertion voltage by approximately 150 mV compared to dry conditions, aligning with experimental observations of higher voltages in aqueous versus dry electrolytes.
3. Practical Implications: The findings offer significant technological implications for Mg-ion batteries. Higher voltages and improved kinetic properties due to H$_2$O co-intercalation could enhance the intrinsic performance of V$_2$O$_5$-based batteries. However, the risk of H$_2$O causing anode passivation in Mg batteries underscores the importance of optimizing electrolytic environments.

Implications and Future Directions

The research provides a critical framework for understanding how solvent co-intercalation affects the electrochemical properties of energy storage systems. This thermodynamic approach could be extended to other complex solvent-electrode combinations in Li- and Na-ion chemical spaces. Future developments may explore:

• Alternative Electrolytes: Identify solvents that can leverage the co-intercalation effects without adverse side-effects, such as Mg anode passivation.
• Material Design: Integrate these insights into designing electrodes that can utilize co-intercalation to enhance capacity and cycle stability.
• Simulation and Modelling: Expand computational studies to include temperature and pressure effects, potentially uncovering novel phase behaviors or kinetics in different battery chemistries.

This work enhances the fundamental understanding of solvent co-intercalation phenomena in multi-valent ion batteries and offers pathways to innovate future rechargeable battery systems with better energy densities and stability.