Rapid prediction of vapour pressure for organic solvents and electrolytes using the Two-Phase Thermodynamic Model

Abstract

The development of electrochemical energy storage systems (ESSs) is essential if we are to transition away from fossil fuels and alleviate global warming. Lithium-ion batteries (LIBs), one of the ESSs, are now widely used in electrical devices. However, with the rapidly developing market of Electric vehicles (EV) and Hybrid electric vehicles (HEV), lithium supply is expected to meet a shortage in the near future. Sodium-ion batteries (SIBs) are one of the most promising and sustainable technologies alternatives to LIBs due to their lower cost and larger global reserves of sodium. The electrolyte is one of the most critical components of a battery. The safe utilization of electrolytes is a prerequisite for building a safer battery technology. Since the vapour pressure indicates the volatility and safety of the electrolytes in batteries, the emphasis of this research will be placed on the development of an efficient model for screening the vapour pressure of the organic solvent based SIB electrolytes quickly. The vapour pressure is closely linked to the solvation free energy which is defined as the work associated with moving a molecule from a fixed position in an ideal gas phase to another fixed position in a liquid phase at constant temperature and pressure in a thought experiment. The screening of the solvation free energy and vapour pressure can be achieved by combining the ideal gas model and the Two-Phase Thermodynamic (2PT) model. The 2PT model was originally developed and used to study the noble gas Argon. Further simulations on water were then performed using this method. In this thesis, we have extended the 2PT model so that it can be used to study the vapour pressure of the mixtures of complex molecules that make up the electrolytes of ESSs. We first applied the 2PT model to compute the vapour pressure for the pure organic solvents including typical dimethyl carbonate (DMC) and ethylene carbonate (EC). Then we extended and refined the 2PT model to calculate the vapour pressure for DMC-EC mixtures. Finally, we utilized the 2PT model to calculate the vapour pressure of electrolytes by adding 1 mol/kg Na salts in the above mixtures. At the same time, we also showed that the transport properties including the self-diffusion coefficient and the ionic conductivity can be readily obtained while using the 2PT scheme. We thus showed that the 2PT not only provides a way to calculate the thermodynamic properties. It can also be used to calculate important kinetic properties in electrolytes and is thus ideally suited as a fast screening tool for choosing suitable electrolytes for a safer battery design. To validate the applicability of the 2PT model, we compared the solvation free energy and vapour pressure results from the 2PT model with those obtained by thermodynamic integration (TI). The results for typical pure organic solvent and solvent mixtures at any molar ratio from 311K to 391K were in good quantitative agreement. We found that the 2PT model cannot be simply extended to the non-ideal mixtures in electrolytes, and there are differences between the TI and 2PT results for such systems. The 2PT model must be further improved in order to treat the non-ideal electrolytes more precisely.