Multicomponent Phase Separation in Ternary Mixture Ionic Liquid Electrolytes

Abstract

We investigate the phase behavior of ternary mixtures of ionic liquid, organic solvent, and lithium salt by molecular dynamics simulations. We find that at room temperature, the electrolyte separates into distinct phases with specific compositions; an ion-rich domain that contains a fraction of solvent molecules and a second domain of pure solvent. The phase separation is shown to be entropy-driven and is independent of lithium salt concentration. Phase separation is only observed at microsecond time scales and greatly affects the transport properties of the electrolyte.

Phase Separation and Ion Diffusion in Ionic Liquid, Organic Solvent, and Lithium Salt Electrolyte Mixtures

“…Upon phase separation, the electrolyte mixture separates into two domains, one domain containing the organic solvent G4 and another domain containing a mixture of G4 and ionic species (termed the ionic domain). This is consistent with our previously reported results of a ternary mixture of [EMIM][BF 4 ] IL, G4, and [NO 3 ] salt. , The composition of the ionic domain varies as a function of temperature. By increasing temperature, the ionic domain becomes more condensed with ions by expelling G4 molecules.…”

“…Table S1 shows the number of molecules in each system, and a list of the simulations is summarized in Table S2. Our previous studies show that for systems of comparable size, 300 ns is enough to observe initial stages of phase separation. , This is consistent with other studies in which it is reported that structures like ion pairs and ion clusters take a few hundred nanoseconds to form. – Figure shows a snapshot of all the simulated ternary electrolyte mixtures and their corresponding temperature and duration of the simulation. In all but two of the systems, we observed a phase transition between the initial homogeneous system and a demixed system with two coexisting domains.…”

“…At each frame, the ionic domain is defined as the volume occupied by ions or by G4 molecules that are surrounded by ions. A two-step approach was used to differentiate between G4 molecules trapped within the ionic domain and those located on the surface, as described previously …”

“…In our recent study of a Li–O 2 battery, computational and experimental studies indicated that the ternary electrolyte mixture of tetraglyme (G4) and [EMIM][BF 4 ] IL (with a volumetric ratio of 75/25 for G4/IL) with LiI and LiNO 3 salts separated into the ion-rich and solvent-only phases . Later, we showed that this phase separation is entropy-driven and a function of composition of the solvent and IL in the mixture . Such phase separation is likely to happen when there is a strong attraction between two of the constituent components of the mixture. , In addition, as we showed in our previous study, at higher ratios of IL/solvent (40:60 and 50:50), the enthalpic part of the free energy is more dominant than the entropic part, favoring a homogeneous mixture at lower temperatures .…”

Smart interfaces in Li-ion batteries: Near-future key challenges

“…While in an aprotic IL the cation is preferentially coordinated by the anion, in a protic IL proton competition for TFSIcoordination can leave the Li + cations more free, resulting in higher conductivity and lower charge transfer resistance, implying the possibility to operate at higher discharge rate [10,36,37]. Changing the anion [44,55],…”