Understanding the Lithium Ion Transport in Concentrated Block-Copolymer Electrolytes on a Microscopic Level

TL;DR

This study uses all-atom MD simulations to uncover the microscopic mechanism behind enhanced Li$^+$ transport in concentrated lamellar block-copolymer electrolytes composed of PS and short PEO blocks doped with LiTFSI and THF. It identifies a central salt-rich layer that forms at high salt loadings, where LiTFSI-THF networks coordinate Li$^+$ and promote fast cation diffusion, decoupled from the polymer host. The coordination environment shifts from polymer-dominated to TFSI/THF-dominated as salt increases, with THF capable of breaking up clusters and further increasing mobility, thereby yielding high cation transference numbers in agreement with experiments. These results provide a microscopic rationale for the experimentally observed salt-rich phase and high conductivity, and suggest solvent engineering as a means to optimize transport in other microstructured electrolytes.

Abstract

Block-copolymer electrolytes with lamellar microstructure show promising results regarding the ion transport in experiments. Motivated by these observations we study block-copolymers consisting of a polystyrene (PS) block and a poly(ethylene oxide) (PEO) block which were assembled in a lamellar structure. The lamella was doped with various amounts of lithium-bis(trifluoromethane)sulfonimide (LiTFSI) until very high loadings with ratios of EO monomers to cations up to 1:1 were reached. We present insights into the structure and ion transport from extensive Molecular Dynamics simulations. For high salt concentrations most cations are not coordinated by PEO but rather by TFSI and THF. More specifically, LiTFSI partially separates from the PEO domain and forms a network-like structure in the middle of the lamella. This central salt-rich layer plays a decisive role to enable remarkably good cationic mobilities as well as high transport numbers in agreement with the experimental results.

Figures (8)

• Figure 1: Snapshot of the 2:2:1 system with bilayer height $L_0$: PS blue, PEO red, Li+ green, TFSI magenta, THF iceblue.
• Figure 2: Distribution of the mass density $\rho$ of the different residues along the normalized bilayer height $z/L_0$. Li+ correlates with TFSI and is not shown. Due to the symmetry of the bilayer only one half of the density profile is shown. The densities are averaged over the whole production run except the 3:2:1 system which is an average of the last 50ns of the equilibration run (see \ref{['sec:sim_details']}).
• Figure 3: Snapshots of the initial polymer structure and the polymer localization after 500ns of equilibration in the 3:2:1 system. LiTFSI and THF are not shown to highlight the formation of the salt layer in the middle. The accompanying volume change during the simulation is too small to be clearly visible in the snapshots.
• Figure 4: Average number $\bar{N}_{\ce{O}}$ of cation--coordinating oxygen atoms for the various ligand residues.
• Figure 5: Fraction $p$ of cations with a certain number of coordinating residues.