Solid-liquid equilibria and thermo-physical properties of liquid electrolyte systems for lithium ion batteries

Abstract

Alkyl carbonate and $\text{γ}$-butyrolactone (GBL) are attractive organic electrolyte materials that are stable over a wide range of operating voltages and ethylene sulfite (ES) is used as a supplementary film-forming electrolyte additive for lithium ion batteries (LIBs). This paper reports the solid-liquid phase equilibrium (SLE) data and the thermo-physical mixture properties such as the density, refractive index, excess and deviation properties for organic liquid electrolyte solutions of carbonate-based or GBL electrolytes containing ES. The SLE data were correlated with two activity coefficient models: NRTL and UNIQUAC. In addition, the extent to which the excess volume ($V^E)$ and molar refraction deviation ($\Delta R$) of each of the binary systems correlated with the values calculated using the Redlich-Kister polynomial equations was determined.

Introduction

The high electropositivity of lithium and the fact that it is the lightest metal among the solid elements have, together with its high energy density and high specific current capacity, led to the consideration of several combinations of organic solvents and lithium salts as the most promising liquid electrolyte materials for rechargeable batteries. Consequently, research and development of lithium ion batteries (LIBs), which have found use in various applications such as a power source for electric vehicles and as storage medium for electric energy generated by solar and wind sources, has been underway all over the world [1,2]. Research pertaining to LIBs focuses on the three basic components of these batteries: the anode, cathode, and electrolyte. In reality, even though in LIBs the electrodes are responsible for energy storage, the electrodes function in collaboration with a liquid or solid electrolyte capable of conducting lithium. Additionally, the electrolyte (or combination of electrolyte materials) largely contributes to characterizing LIBs in terms of their specific power, safety, life time, and performance at both low and high temperatures [[3], [4], [5], [6]]. Therefore, the subsequent intensification of research into electrolytes and their additives, as one of the core elements of LIB technology, has been a major driving force behind the technological progress of LIBs [7].

A liquid electrolyte generally consists of a lithium salt, such as LiPF₆ or LiN(CF₃SO₂)₂ combined with linear and alkyl carbonates, because several lithium salts are highly soluble in these carbonates and the resulting conductivities are adequate for batteries. In addition, small amounts of other components, known as electrolyte additives, are incorporated in the electrolyte to improve its properties. For instance, ethylene sulfite (ES) or vinyl ethylene sulfite (VES), a film-forming electrolyte additive, are included in electrolyte formulations to increase the dielectric strength and enhance the electrode stability by facilitating the formation of a solid electrolyte interface (SEI) layer [[8], [9], [10]]. The selection of solvents for use in electrolyte formulations is therefore crucial to enhance the performance of LIBs. In practice, these solvent mixtures mainly contain ethylene carbonate (EC) or propylene carbonate (PC), which are used in combination with at least one of the following organic carbonates as co-solvents: dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. The permittivity of these electrolyte mixtures is sufficiently high to allow lithium salts to dissociate in the mixture; furthermore, the low melting point of these electrolytes make them suitable for low-temperature applications. Apart from the aforementioned linear carbonates, $\text{γ}$-butyrolactone (GBL) is another preferred electrolyte solvent for LIBs because of its similar electrochemical performance. However, its boiling point is much higher compared to that of linear carbonates such as DMC, DEC, and EMC and it would therefore be expected to enhance battery safety.

Many studies have used theoretical or empirical approaches to examine the properties of different electrolyte solutions. The results of these studies laid the foundation for subsequent advances in equilibrium and solution thermodynamics. Determining the dependence of the phase equilibrium and properties of solutions on the composition and temperature of these solutions remains an ongoing challenge for researchers in the chemical and physical sciences. This is because knowledge of these properties is of great importance to study the separation and interaction between the components of solutions [11]. Therefore, a good understanding of the phase equilibrium and thermodynamic properties of electrolyte mixtures for LIBs remains of interest and is helpful toward their utilization as electrolytes. In the present work, we determined the solid-liquid equilibrium (SLE) density ($\rho$), refractive index ($n_D$), excess volume ($V^E$), and molar refraction deviation ($\Delta R$) at atmospheric pressure and different temperatures for the binary systems: DMC + ES, EC + ES, EMC + ES, GBL + ES. To the best of our knowledge, the experimental SLE and thermo-physical properties of the systems considered in this work have not yet been reported.

The determined SLE data were correlated with the data calculated by the activity coefficient models NRTL [12] and UNIQUAC [13]. In addition, the calculated excess and deviation properties were modeled by known polynomial equations, namely the Redlich-Kister equations for binary fractions [14].