Analyzing the internal interface in localized high-concentration electrolytes

We present molecular dynamics simulations on localized high-concentration electrolytes (LHCE) based on the conducting salt lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis(trifluoromethanesulfonyl)imide dissolved in the solvent 1,2-dimethoxyethane and diluted to two different degrees with the diluent 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether. Due to the immiscibility of the conducting salt phase (salt + solvent) and the diluent phase, LHCEs feature a complex microstructure of two phases forming an internal interface. In this study, we not only investigate the lithium coordination structure in the conducting salt phase but also the size and composition of its interface to the diluent phase by Voronoi tessellations. Furthermore, we investigate the influence on the ion transport by evaluating Onsager coefficients. We show that an LHCE containing the surface-active anion TFSI- creates an anion-rich internal interface, leading to enhanced ion dissociation and anticorrelated ion movement. On the other hand, the smaller FSI- anion with a more localized charge distribution and less amphiphilic character shows no enrichment at the internal interface, but rather a depletion. By increasing LiFSI concentration, we even observe a solvent-rich internal interface due to a large and branched Li-anion network. Furthermore, the less diffuse interface and enlarged Li-anion network lead to lower ion-ion anticorrelations and a stronger convective flux of the conducting salt phase, which is compensated by a flux of the diluent phase, especially in the higher concentrated LiFSI based LHCE.