Significant Enhancement in Conductivity in Ethylene Oxide-Based Branched Polymer Electrolytes in the Presence of Sodium Salt

In this paper, we conduct all-atom molecular dynamics simulations to explore and compare the performance of a sodium-salt-based (NaTFSI) solid polymer electrolyte (SPE), which is suitable for use in sodium-ion batteries, with the lithium-salt based (LiTFSI) SPE. The choice of polymer for both cases is poly(oligo oxyethylene methacrylate) or POEM which is a comb-architecture based polymer with poly(ethylene oxide) or PEO side chains. For a wide range of temperature values and concentration ratios (ratio of the concentration of Li⁺ or Na⁺ to ethylene oxide), NaTFSI-doped POEM shows a significantly larger ionic conductivity and more enhanced transport properties (e.g., mean square displacements) as compared to those of the LiTFSI-doped POEM. The improvement in the conductivity of the NaTFSI-doped POEM is especially evident at lower temperatures. A faster hopping of the Na⁺ ions from one ether oxygen (EO) to another (along POEM’s side chains), which is associated with a smaller distance between the solvation sites (as suggested by the corresponding enhanced solvation site connectivity value), explains this conductivity increase. A greater average mobility of the EOs of the NaTFSI-doped POEM SPE further contributes to the conductivity enhancement. Finally, through separate density functional theory calculations, we show that the binding energy of the Na⁺ ions to the PEO oxygen atoms is higher (or smaller negative) than that of the Li⁺ ions to the PEO oxygens, thereby confirming a weaker binding and faster hopping of the Na⁺ ions in NaTFSI-doped POEM SPE leading to a more enhanced conductivity of the NaTFSI-doped POEM SPE. We anticipate that these findings will inspire the exploration of sodium-salt-based SPEs to be used in sodium-ion batteries.