Solvent and Anion Controlled Ionic Clustering in Halide Containing Electrolytes for use in Rechargeable Magnesium Batteries

The higher charge density, cheaper cost and relative stability over lithium-ion batteries have driven the development of Rechargeable Magnesium batteries (RMBs) [1,2]. However, at electrode/electrolyte interfaces (called Solid-Electrolyte Interfaces, SEIs), the high charge<br/>density of Mg²⁺ ions trigger the formation of ion impermeable deposits. Electrolytes need to remain stable against reductive reactions at the electrode/electrolyte interface to overcome the stability and reversibility issues caused by SEIs [3,4]. Ethereal solvents containing Magnesium<br/>halide salts provide an exciting prospect towards development of stable electrolytes for RMBs.The primary charge carriers in solution are a variety of electroactive species (EAS) formed due to complex equilibria of the Mg-halide system balanced with a counter-ion such as AlCl₄^-. The<br/>existence of larger ionic architectures formed by the anion bridged ionic agglomerations have been previously shown [5].<br/><br/>Here we investigated ion agglomeration and transport of several such EAS in MgCl₂ salts dissolved in five ethereal solvents and in the presence of two anions under both equilibrium and operating conditions using large scale atomistic simulations. We find that clusters were discovered irrespective of the anion used, the rate of ionic clustering is affected by the nature of the anion present. Highly interacting anions such as Cl^- tend to enhance clustering rates while weakly interacting anions lower clustering rates in all solvent environments. Solvents<br/>and anions perform complementary roles in enhancing the clustering of the EAS, with clustering in heavier solvents being significantly affected by the solvents. We further illustrate that ionic transport in the electrolytes are affected by the weight of the solvent used with heavier solvents<br/>showing significantly lowered diffusivities. We highlight the role of the solvent in determining cluster morphology and ion mobility by controlling ion diffusion pathways and ion coverage. Our findings of the solvent and anion controls on cluster formation and kinetics can provide useful insight into the electrochemical reactions at the anode/electrolyte interface in RMBs.<br/><br/><b>References</b><br/>1. C. B. Bucur, T. Gregory, A. G. Oliver and J. Muldoon, The Journal of Physical Chemistry Letters, 2015, 6, 3578-3591.<br/>2. M. John, B. C. B. and G. Thomas, Angewandte Chemie International Edition, 2017, 56, 12064-12084.<br/>3. P. Baofei, H. Jinhua, H. Meinan, B. S. M., V. J. T., Z. Lu, B. A. K., Z. Zhengcheng and L. Chen, ChemSusChem, 2016, 9, 595-599.<br/>4. K. A. See, K. W. Chapman, L. Zhu, K. M. Wiaderek, O. J. Borkiewicz, C. J. Barile, P. J. Chupas and A. A. Gewirth, Journal of the American Chemical Society, 2016, 138, 328-337.<br/>5. V. Vasudevan, M. Wang, J. A. Yuwono, J. Jasieniak, N. Birbilis and N. V. Medhekar, The Journal of Physical Chemistry Letters, 2019, 10, 7856-7862.