Molecular insights into the ion-solvation and dynamics in a diglyme-based sodium-ion battery electrolyte Ardhra Shylendran, Prabhat Prakash, Rabin. Siva Dev, Arun Venkatnathan Indian Institute of Science Education and Research (IISER), Pune, India Glyme-based sodium salt solutions exhibit excellent electrochemical properties as battery electrolytes since they could enable the co-intercalation of sodium into graphite [1] . The electrolytic solutions of NaPF 6 in glyme have shown exceptional chemical and thermal stability than alternative carbonate-based electrolytes. The oxygen atoms present in the glyme molecules coordinate with the Na + ion in an octahedral manner, forming solvated ionic liquids {[Na(glyme)] + ---anion - } with an electrochemical window of 4V and ionic conductivity of 1mS/cm [2] . Understanding the solvation behavior of these electrolytes requires molecular-level exploration. Computer simulations based on classical molecular dynamics and plane-wave density functional theory were performed to explore atomic interactions and ionic mobilities. [3] We have studied the structure and ion dynamics over a range of concentrations of NaPF 6 in diglyme. The nature of the atomic interactions and the strength of these interactions were studied using radial distribution functions and uninterrupted lifetime analysis, indicating that Na + ions interact more prominently with the solvent molecules than the anions. The formation ion clusters were elucidated using cluster analysis, suggesting that the Na + ions mostly exist as solvated ions (SolI) that coordinate with solvent molecules (free ions or solvent-separated ion pairs). Also, some fraction exists as contact ion pairs (CIP) and very few as aggregated ion pairs (AGXIP), those increasing slightly with temperature and more with ion concentration. The mobilities of different species in the system were explained using the self-diffusion coefficients (calculated from the respective mean-squared displacements). The ionic conductivities at different concentrations were evaluated using Nernst-Einstein’s and corrected Nernst-Einstein’s relations, including ion-ion correlations, and compared with experiments. The insights from this work that studies the concentration and temperature effects and the impact of molecular cluster formation in the electrolytic solutions can provide a deep understanding of developing alternatives to conventional battery electrolytes.
Figure 1: A schematic representation of the type of ion clusters and their probabilities of occurrence (in percentile) at various concentrations at T = 298 K. References 1. Jache, B.; Adelhelm, P. Use of Graphite as a Highly Reversible Electrode with Superior Cycle Life for Sodium-Ion Batteries by Making Use of Co-Intercalation Phenomena. Angew. Chem. Int. Ed . 2014 , 53, 10169 –10173 2. Westman, K.; Dugas, R.; Jankowski, P.; Wieczorek, W.; Gachot, G.; Morcrette, M.; Irisarri, E.; Ponrouch, A.; Palacín, M. R.; Tarascon, J. M.; et al. Diglyme-Based Electrolytes for Sodium-Ion Batteries. ACS Appl. Energy Mater. 2018 , 1 (6), 2671– 2680.
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