Lithium Salt Effects on the Liquid Structure of Choline Chloride–Urea Deep Eutectic Solvent

Abstract

Deep eutectic solvents (DESs) added with lithium salts are emerging as alternative electrolytes for lithium-ion batteries (LIBs). Yet, to design, optimize, and develop efficient DES-based electrolytes for LIBs, an in-depth understanding of the role played by the lithium cations in the intermolecular interactions between all species in the mixture is crucial. A joint approach of experimental NMR techniques and polarizable molecular dynamics (MD) simulations is used here to gather a comprehensive picture of the structure and dynamics of the prototypical system composed of the DES choline chloride:urea (ChCl:U, x ChCl = 0.33) and the lithium salt containing the same anion, LiCl. Strong coordination of lithium cations by chloride anions, resulting in the formation of LiCl 3 2− units, is revealed. Other species (especially, urea) are present in the second coordination shell of lithium, creating an extensive hydrogen-bond network. The effect of small quantities of water, typically absorbed by DES from air moisture, on the studied properties is discussed.

Insights into the Effect of Lithium Doping on the Deep Eutectic Solvent Choline Chloride:Urea

“…Lithium is hence the slowest species in the mixtures, due to the strong interactions involving this cation. Indeed, Li + interacts strongly with chloride anions, as already observed for the parent system with LiCl 0.4 mol/kg [ 14 ] and is widely known for similar systems like ionic liquid mixtures [ 20 , 21 ]. An additional contribution to the slow translational dynamics of Li + stems from its interactions with urea in the second coordination shell [ 14 ].…”

“…Indeed, Li + interacts strongly with chloride anions, as already observed for the parent system with LiCl 0.4 mol/kg [ 14 ] and is widely known for similar systems like ionic liquid mixtures [ 20 , 21 ]. An additional contribution to the slow translational dynamics of Li + stems from its interactions with urea in the second coordination shell [ 14 ]. The addition of LiCl 0.8 mol/kg roughly halves the diffusion of both U and Ch, which is further slowed down when more LiCl is added.…”

“…At the light of the results obtained on archetypal lithium-doped ChCl:U [ 14 ], here we propose a more systematic investigation on two samples containing a higher amount of metal salt (0.8 and 1.0 mol/kg), and compare the results with the pure DES. This allows to gather novel insights into the effect of Li-doping and Li-concentration on the dynamic behavior of the different species in the mixtures.…”

“…Note that ChCl and most type III DESs are highly hygroscopic, hence preventing the preparation of a water-free system when working under ambient conditions. We found previously that a small amount of water (3.9 wt%) substantially does not alter the intermolecular structure of the ChCl DES electrolyte, but has a remarkable effect on dynamics [ 14 ]. This is in line with the by now established concept that a controlled amount of water does not seriously break up the intermolecular network of neat DESs [ 18 ], but has a non-negligible effect in reducing viscosity.…”

“…It should be noted, however, that preparing those type IV DESs requires large amounts of expensive lithium salt, rising the production costs when compared to dissolving controlled amounts of the same salt in low-cost type III DESs, hence jeopardizing one of the merits of DESs. In a previous work [ 14 ], we studied a prototypical system composed of choline chloride:urea at 1:2 mole fraction (ChCl:U x ChCl = 0.33) with LiCl dissolved at a concentration of 0.4 mol/kg. A combination of multinuclear NMR measurements and molecular dynamics simulations pointed towards a strong coordination of Li + cations by chloride anions and a marked involvement of U in the H-bond network.…”