A Model for Lithium-Ion Battery Electrolyte Mixture Densities

Abstract

Lithium-ion battery (LIB) electrolytes are generally composed of an organic hydrocarbon and lithium salt (e.g., LiPF6). The effect of salt addition to the electrolyte solvent increases its density. Experimental density data for basic and realistic, multicomponent mixtures are scarce in the literature, and a predictive model for electrolyte solution density is not yet available. This work resolves to create a predictive method that can be used to estimate the density of these mixtures. A predictive method which accounts for intramolecular forces in the electrolyte mixture was developed and fit to the sparse density data available in the literature. The model exhibited high accuracy for single component electrolyte mixtures with most predictions falling within 1% of measured values and all predictions falling within 5%. The model was further extended to more realistic, multi-solvent electrolyte mixtures which exhibited similar accuracy, seen in Fig. 1. It was noted that the multi-component predictions were moderately worse than for the single component mixtures. The source of this disparity is not clear but may be derived from uncertainty in the data reported by electrolyte vendors and/or the presence of solvent-to-solvent interactions in the solution which are not accounted for in the current modeling framework. In addition to the density model, an accurate method for computing absolute molar and mass fractions of multi-solvent mixtures with specified volumetric concentrations (e.g., 1.2 M LiPF6 in 1:1:1 %vol. EC/ DEC/DMC) is also described for when that is the only known relation for a solution. Computed mass concentration values yielded an average error of 1.4% with a maximum value of 9.6%. The molar and mass approaches were combined to yield a modeling framework capable of computing molar concentrations and densities of all LIB electrolyte solutions based on LiPF6 loaded in any combination of ethylene carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), and propylene carbonate (PC). The largest fault with the final modeling system is that it suffers from a lack of sufficient, high-quality experimental data for validation of its accuracy over a wide range of electrolyte formulations and conditions. This model’s accuracy is dependent on the experimental data utilized for correlation. Accordingly, more data from differing sources will help increase the accuracy and confidence of the model developed herein and allow it for expansive use in LIB fields. Future work will be required to extend the current approach to any novel electrolyte formulations developed in the research and development sector. Figure 1