Enhanced Charge Transport through Ion Networks in Highly Concentrated LiSCN-Polyethylene Carbonate Solid Polymer Electrolytes.

Challenging the preference for bulky anions due to low binding energy with Li ion, the lithium thiocyanate-polyethylene carbonate (LiSCN-PEC) solid polymer electrolyte (SPE) demonstrates higher ionic conductivities (3.16 × 10 S cm) at polymer-in-salt concentration (100 mol%) compared to those with lithium bis(fluorosulfonyl)imide (LiFSI, 1.01 × 10 S cm) and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, 1.72 × 10 S cm). Through the careful selection of PEC and LiSCN as components of SPE, the carbonyl stretching of PEC and the SCN stretching band as vibrational reporters provide detailed structural insights into the Li ion transport channel. Spectroscopic investigations reveal that enhanced ion aggregation alters the solvation structure around the Li and diminishes the interaction between Li and polymer (PEC) with increasing LiSCN concentrations, promoting faster segmental motion as a major transport mechanism. However, the transition observed from subionic to superionic behavior in the Walden plot indicates the onset of segmental motion decoupled charge transport pathway. The SCN vibrational spectrum elucidates the evolution from a Li-SCN-Li type chain-like structure to a Li > SCN < Li type extended ion network with increasing LiSCN concentration, revealing that the ion network provides an alternative channel for Li ion transfer at higher concentrations, enhancing conductivity.