In Silico Based Rank Order Determination of Electrolytes for Sodium Ion Battery Applications

Abstract

The selection of an appropriate electrolyte is an important consideration in the design of next-generation batteries. Low-cost sodium ion based batteries have assumed the role of attractive alternatives to Lithium ion batteries for dispensing and storage of energy. Here, we perform in silico based molecular dynamics simulation (MD) study on a variety of electrolytes and NaClO4 salt. We perform potential of mean force (PMF) calculations to extract the free energy of solvation for these different electrolyte-salt combinations in bulk and rank them based on the free energy predictions. The energetics of ion solvation will be used to understand the surface preference of the intercalating ion for a given electrolyte. The PMF calculations will provide insights into the solubility of the salt in the various electrolytes and will serve as a metric for choosing the most suitable electrolyte-salt combination for a given electrode. In designing new electrolytes, it is generally preferred to have high solubility of the salt. We will also determine the heats of vaporization of the various organic electrolytes and this information will be used to identify the operational range and safety aspects of the electrolyte. Electrolytes with low heat of vaporization can be considered unsafe. Additionally, the MD simulation trajectories will also be used to calculate the diffusivity of the ions in the various electrolytes. Additional transport metrics such as activation barriers to diffusion, ionic conductivity are used to refine the rank order of the electrolytes, in addition to the thermodynamics and equilibrium considerations. Determination of micro-structure suggests varying degree of coordination of sodium ion with these electrolytes commensurate with the ion solvation energetics. Suitable comparisons of computational predictions are made with experimental resutls. Collectively, this would form an apriori, in silico study informing experiments and would be a useful tool and metric for screening variety of solvent and electrode candidates in terms of their ability to be used for battery applications.