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.
Highlights
As technologies and markets of portable electronic devices and electric vehicles are rapidly growing in recent years, rechargeable batteries such as lithium ion batteries have become one of the most active research fields and industrial markets[1,2,3,4,5]
We explore the effects of density on the dynamics of an electrolyte consisting of a lithium hexafluorophosphaste (LiPF6) salt in a binary solvent mixture of ethylene carbonate (EC) and Dimethyl carbonate (DMC) with a mixture ratio of EC:DMC = 50%:50%
Our starting point is the two electrolyte systems with densities of ρ = 1.3446 g/cm[3] for EC 50% and ρ = 1.2677 g/ cm[3] for EC 20%, and we investigate the dynamics for EC 50% as a function of ρ
Summary
As technologies and markets of portable electronic devices and electric vehicles are rapidly growing in recent years, rechargeable batteries such as lithium ion batteries have become one of the most active research fields and industrial markets[1,2,3,4,5]. Whereas larger diffusivity of ions obviously increases the ionic conductivity, forming a pair of a cation and an anion does not contribute the ionic conductivity due to its charge neutrality. The formation of pairs of cations and anions is closely related with a decrease in diffusivity due to an increasing size of ionic clusters in addition to a decrease in the number of ions contributing the ionic conductivity. A molecule with a large dielectric constant can fulfill a good solvent in the view of the ion pairing, but it is easy to fail to enhance the mobility of ions due to its large viscosity. A molecule with a small dielectric constant has lower viscosity in order to enhance mobility, but its fulfillment in the solvation process is not satisfied. A combination of cyclic and linear www.nature.com/scientificreports/
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