Solid-liquid equilibria and thermo-physical properties of liquid electrolyte systems for lithium ion batteries
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, -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 (), refractive index (), excess volume (), and molar refraction deviation () 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].
Section snippets
Materials
DMC and EMC were purchased from Sigma Aldrich (USA, >99%) and GBL was provided by Junsei (Japan, >99%), whereas EC and ES were supplied by Acros Organics (USA, EC: >99%, ES: >98%). All of these chemicals were dried with 0.3 nm molecular sieves and used without any further purification. Subsequently these chemical compounds were analyzed by gas chromatography (GC) and their final purities were determined to exceed 99.9 mass %. The water contents of the chemicals were also checked using
Results and discussion
Solid-liquid equilibria for eutectic systems can be calculated by knowledge of the real behavior in the liquid phase and the pure component properties using Eq. (1)
If a solid-solid phase transition (like as one crystal phase can transform into another under the same condition) does not occur, enthalpy of transition of the last term in Eq. (1) can be neglected and simplified to Eq. (2) as follows [23]:where , is the
Conclusions
The SLEs for the respective mixtures DMC + ES, EMC + ES, EC + ES, and GBL + ES exhibit a single eutectic point. The eutectic points regressed by the least-squares method are = 0.3460/ = 232.03 K, = 0.7028/ = 209.61 K, = 0.2300/ = 240.74 K, = 0.6010/ = 209.42 K for the systems DMC + ES, EMC + ES, EC + ES, and GBL + ES, respectively. They are in good agreement with the values obtained with the common models, NRTL and UNIQUAC, within 0.7 K of RMSD. The values of
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