Insight View of Lithium Doped Imidazolium-Based Ionic Liquids in Presence of Organic Additive

Lithium ion batteries are extensively used as the power source for the consumer nomad electronics due to their high energy density (1). To increase their safety and stability, the development of electrolytes based on ionic liquids instead of flammable, organic carbonates gained a lot of attention in recent years. However, ILs show a high viscosity implying low cycling and power delivery (2). Addition of organic carbonates (e.g. vinylene carbonate, VC) with graphite (Cgr) electrode is essential for improving their performance. In the literature, the role of VC is associated to the creation of interfacial compatibility between the Cgr electrode and the ionic liquid (3). Simulations of the lithium doped ILs showed that the transport of the [Li⁺] cations, coordinated by four different [NTf₂^-] anions, is realized through [NTf₂^-] exchange in the first coordination shell. Nevertheless, it is reported that the carbonate additive destroys the existing ion pairs between [Li⁺] and [NTf₂^-] (4). Note that in carbonate electrolytes; [Li⁺] is solvated by carbonyl of carbonate (5). In an IL//VC media, could VC contribute not only in the SEI formation but also have a role in the coordination shell of [Li⁺]? The aim of this work is to determine by NMR experiments the effects of additive VC on coordination shell of [Li⁺] and on its transport properties.

The study has been carried out on in function of the temperature on the four samples C₁C₆ImNTf₂, C₁C₆ImNTf₂//VC (5%vol.) C₁C₆ImNTf₂//LiNTf₂ (1mol.L^-1) and C₁C₆ImNTf₂//VC(5% vol.)//LiNTf₂ (1mol.L^-1), by NMR using {¹H-⁷Li}, {¹H-¹⁹F} NOE correlations (HOESY), and pulsed field gradient spin-echo (PGSE) NMR. In neat IL and in IL//VC media, the diffusion coefficients follow the general trend reported in the literature (6), with a the slowest diffusion of [Li⁺] ions explained by the formation of a Li-anion aggregates, [Li(NTf₂)_n]^(n-1)-(7). On contrary, the diffusion coefficient of VC is remarkably decreasing in the presence of [Li⁺] ions. Moreover, the ratio of diffusion coefficients (D₁/D₂) was 4 time more the ratio of the viscosities (ŋ₂:ŋ₁), of the electrolyte C₁C₆ImNTf₂//LiNTf₂ (ŋ₂,D₂) and the neat IL (ŋ₁,D₁), suggesting a strong interaction/coordination of VC with [Li⁺] ions. Furthermore, 2D {¹H-⁷Li} NOE correlations (HOESY) prove the vicinity of Li and VC. Both sets of experiments could be explained by the presence of VC in the coordination sphere of [Li⁺]. This potential formation of a VC-[Li⁺] interaction/coordination could have a major influence on the observed electrochemical behaviour of theses batteries. Since i) the D[Li⁺] is expected to increase because of the larger diffusivity of VC compared to [NTf₂⁻] anion , and ii) also the activation energy for [Li⁺] ions diffusion is likely to increase, due to a somewhat more complicated diffusion mechanisms based on Borodin’ work,  passing through Li-[NTf₂⁻] complex diffusion, disruption, Li-VCcomplex formation etc. as depicted in Scheme 1.

Acknowledgements:

E.B. thanks to the financial support by COST Action Number CM1206 EXIL - Exchange on Ionic Liquids for STSM STSM-CM1206-14833 grant.

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