Effects of salt fraction on the Na+ transport in salt-in-ionic liquid electrolytes

In the practical operations of the sodium ion (Na+) batteries (SIBs), the fast transport of Na+ is desired for the rate performance, because the other ions in an electrolyte are electrochemically inert. In this study, we use molecular dynamics simulations to investigate the partial conductivity of Na+ (σNa+) in the salt-in-ionic liquid electrolytes (SILEs) composed of 1-ethyl-3-methylimidazolium (EMIM+) and bis(fluorosulfonyl)imide (FSI−) with various molar fraction of NaFSI. The simulations show that while the ionic conductivity of the SILE decreases monotonically with the increase of salt fraction of NaFSI, σNa+ peaks in the SILE with 0.5 molar fraction of NaFSI. Detailed analyses indicate that with the increase of salt fraction, the coordination structure of FSI− around Na+ changed from bidentate manner to monodentate manner which weakens the binding of FSI− to Na+. The effects are two folds. On one hand, the increased monodentate coordinations cause a large aggregate that hinders the transport of Na+ within the aggregate; on the other hand, the large aggregate captures most FSI− to form percolating ion network, and thus leaves a small portion of Na+’s that are not in the large aggregate to be more “free” to transport in the SILE.