( Invited ) The Affect of Mono and Multivalent Metal Ions in Structural and Dynamical Behavior of Ternary Electrolytes (PE-PEO/Ionic-Liquids/Salts) Using MD Simulations

The development of advanced electrolytes with high ionic conductivity and robust electrochemical stability is critical for next-generation energy storage technologies. To this end, ternary systems combining polymers, ionic liquids (ILs), and metal salts present a promising strategy for designing tunable conductive media. This study focuses on employing all-atom molecular dynamics (MD) simulations to investigate ternary electrolytes comprising a short-chain PE-PEO block copolymer surfactant, the IL 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), and salts of the form M x+ (TFSI) x , comparing the behavior of monovalent (Li⁺, Na⁺) and multivalent (Mg²⁺, Ca²⁺) metallic cations. The coordination preferences and transport mechanisms of these metal ions are critically influenced by their interactions with the PEO block and TFSI ‒ . Simulations performed across four compositions of IL/salt reveal that while Li⁺ and Na⁺ maintain a stronger, persistent interaction with the polymer ether oxygens even at higher salt, Mg²⁺ and Ca²⁺ show a clear transition from PEO to TFSI ‒ dominated solvation shells. Detailed structural analysis demonstrates how solvation environments evolve with salt composition. The contribution in coordination number of alkali metal ions with PEO ether oxygens decreases significantly with increased salt in the system, shifting from ~80% of their primary coordination shell at low salt to ~40% at high salt. This indicates a progressive displacement by TFSI ‒ around the metal ions from polymer’s ether oxygen from the first coordination shell of metal ions. However, for Mg²⁺ and Ca²⁺, this shift is far more pronounced, moving from ~80% PEO coordination at low salt to only ~20% at high salt. This dramatic decrease, coupled with a correspondingly larger increase in anion coordination number, confirms that the multivalent cations are preferentially electrostatically bound to TFSI ‒ . These distinct solvation structures have direct consequences for ionic transport. The self-diffusion coefficients for all ionic species follow the trend D [EMIM]+ > D TFSI– > D MX+ at low salt compositions. However, the mobility of multivalent ions (Mg²⁺, Ca²⁺) is lower than that of their monovalent counterparts, a result of their higher charge density, stronger coordination shells, and increased effective ionic radius due to tightly bound anions. At high salt, diffusivities converge for all species, indicating a transition to a strongly correlated ion-hopping or aggregate-based transport regime. This convergence suggests the formation of persistent, neutral-like clusters, such as [EMIM]⁺/TFSI ‒ pairs and [M x+ (TFSI) x ]⁽ˣ⁻²⁾ complexes, which move in a coupled manner, thereby reducing overall ionic conductivity. Radial distribution function analysis further supports the existence of these stable complexes, particularly for the M x ⁺/(TFSI ‒ ) x pairs. This comparative analysis provides fundamental insights into how cation charge density, size, and solvation shell dictate the microstructure and dynamics in ternary electrolytes. The results suggest a critical trade off while multivalent ions offer higher energy density, their strong coulombic interactions with both polymer and anions can impede mobility and promote ion aggregation. These findings establish key design principles for optimizing the composition of IL and salt to selectively enhance transport for either mono- or multivalent charge carriers in advanced electrochemical devices.