A salient effect of density on the dynamics of nonaqueous electrolytes

Abstract

The mobility and solvation of lithium ions in electrolytes are crucial for the performance and safety of lithium ion batteries. It has been known that a single type of solvent cannot satisfy the requirements of both mobility and solvation simultaneously for electrolytes. Therefore, complex solvent mixtures have been used to optimize both properties. Here we present the effects of density on the dynamics and solvation of organic liquid electrolytes via extensive molecular dynamics simulations. Our study finds that a small variation in density can induce a significant effect on the mobility of electrolytes but does not influence the solvation structure of a lithium ion. It turns out that an adjustment of the density of electrolytes could provide a more effective way to enhance mobility than a control of the solvent mixture ratio of electrolytes. Our study reveals that the density change of electrolytes mainly affects the residence time of solvents in the first solvation shell of a lithium ion rather than the structural change of the solvation sheath. Finally, our results suggest an intriguing point for understanding and designing electrolytes of lithium ion batteries for better performance and safety.

Anionic effects on the structure and dynamics of water in superconcentrated aqueous electrolytes

“…Since the ion-water interactions are of great importance to the properties of water in the superconcentrated limit, it is natural to investigate how water molecules interact with ions by considering the structure and dynamics of hydration regarding the cationic and anionic hydration shells. For the structural properties of the cationic hydration shell, we rst calculated the lithium solvation number N C of water, which is dened as the number at the rst plateau in the cumulative coordination number, 24,51 nðrÞ ¼ 4pr…”

“…14,52,53 The residence time of water molecules within the hydration shell, also known to represent the rigidity of the hydration shell, plays an important role in the transport of ions. 14,51 We examine the fast and slow kinetics of the exchange dynamics of water molecules in the hydration shell, R(t) and C(t), which are dened in the same ways as in eqn (1) and 2, respectively. The mechanisms of the two exchange dynamics are different.…”

“…Lastly, we investigate how the existence of different anions would affect the translational and rotational dynamics of water molecules for increasing salt concentrations. First, we calculate the translational mean square displacement (TMSD), dened as 24,43,44,51,54,55 Dr…”

“…Using the results of the TMSD and RMSD, we examine the behavior of the translational diffusion constant D T and the rotational diffusion constant D R of water molecules for the different types of anion. We calculate D T from the TMSD via the Einstein relation, 24,43,44,51,54…”

Dynamic features of water molecules in superconcentrated aqueous electrolytes

“…For lithium ion batteries, there have been several important issues concerning safe, environmentally friendly and low cost batteries, which are mostly raised by nonaqueous electrolytes. Thus, finding suitable electrolytes for lithium ion batteries has been a big challenge for a long time 18 – 21 . Water has been one of strong candidates for electrolytes satisfied with most requirements.…”