Molecular dynamics insights into Ca-based salts in organic electrolyte

Calcium-sulfur (Ca-S) batteries are gaining attention as a promising energy storage solution due to calcium's natural abundance and the system's high theoretical energy density. A critical factor influencing the performance and stability of Ca-S batteries is the behavior of calcium ions during solvation in the electrolyte and desolvation at the anode-electrolyte interface. This study aims to elucidate these processes to inform the development of more efficient and stable Ca-S battery systems. We conducted molecular dynamics (MD) simulations to characterize the solvation dynamics of calcium salts in various electrolytes. We focus on contact ion pairs (CIP) and solvent-shared ion pairs (SIP), which are prevalent in these systems and amenable to ab initio approaches due to their relatively smaller sizes while leveraging machine learning approaches to extend the MD trajectories. Our simulations provide insights into the solvation shells of calcium ions, including solvent molecule clustering, shell stability, and the extraction energies required for ion removal. We find that increased temperatures significantly alter solvation shell structures, and the solvation structure can significantly affect ion diffusion coefficients, affecting ion mobility and reaction kinetics. At the anode-electrolyte interface, MD simulations were utilized to model the desolvation of calcium ions as they approach the calcium anode. The study identifies distinct stages in the desolvation process and quantifies the associated energy barriers. By elucidating the factors that influence ion mobility and interfacial reactions, we contribute to developing strategies to enhance battery efficiency and stability, thereby advancing the practical application of calcium-based energy storage systems.