Diffusion of Lithium Cations in Concentrated Sulfolane-Based Liquid Electrolytes
Adilson DE FREITAS1, Karina SHIMIZU1, José CANONGIA LOPES1
1Centro de Química Estrutural, Institute of Molecular Sciences, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
Since its inception 30 years ago, lithium-ion batteries (LIB) have become ubiquitous energy storage devices because of its advantageous qualities over its predecessors. However, volatile, and flammable carbonate-based electrolytes are commonly employed in LIBs, raising concerns on thermal stability and toxicity. The use of nonaqueous polyelectrolyte solutions, aqueous Li+-ion batteries, eutectic mixtures, superconcentrated or highly concentrated electrolyte (HCE) solutions have received much attention recently as strategies to overcome the safety and environmental issues about conventional LIBs.
In highly concentrated aqueous ionic solutions and supercooled liquids, the time correlation function transients are usually described by a non-exponential function rather than the exponential decays usually seen in diluted solutions. In this work we employed molecular dynamics simulations to explore the solvation behaviour of sulfolane-LiBF4 mixtures going to [LiBF4] up to ca. 6 mol.L-1, a well-characterized system from the experimental point of view (Figure 1). We surveyed some structural aspects of the mixtures to understand how the solvation environment around Li+ ions changes as a function of the molar fraction of sulfolane. Then, we focused on the dynamical aspects of the sulfolane-LiBF4 mixtures, namely the first steps of the relaxation process of the cation-anion and cation-sulfolane pairs.
The MD results suggest that the relaxation dynamics of the contact ion pair and Li-sulfolane pair at room or even higher temperatures is analogous to that seen in glass-forming materials at much lower temperatures. In addition, the Li+ ion dynamics is influenced to such an extent that the hopping diffusion of Li+ ions starts to take place in highly concentrated electrolyte solutions. Also, the extension of the departure from the exponential behaviour seems to reflect the aggregation of the ion pair in solution. The correlations with the Stokes-Einstein (SE) equation indicated that the largest contributor to the SE breakdown is the cation-anion pair dynamics.