Solvent dependent relaxation dynamics in lithium ion battery electrolytes: Coupling to medium friction

Abstract

Solution heterogeneity and dynamics of the industrially relevant electrolyte system, [9.795{xEC + (1 − x)PC} + 0.869LiClO4] with x representing mole fraction, were explored at different loadings of ethylene carbonate (EC) via temperature dependent measurements of time-resolved spectral response and physico-chemical properties. This system was chosen because polarity differed considerably on replacing propylene carbonate (PC) by EC but the solution viscosity remained nearly the same, allowing to investigate the polarity impact on solution inhomogeneity and solute dynamics. Coumarin 153 (C153) and trans‑2‑[4‑(dimethylamino)styryl]‑benzothiazole (trans-DMASBT) were used as fluorescent probes which differ widely in their excited state lifetimes. Driven by the potential for applications of these electrolyte systems in the energy sector, we have investigated them at an optimal composition with (PC + EC)/LiClO4 mole ratio of ~11.3, where direct current (DC) conductivity reaches at the maximum. Signatures of spatio-temporal heterogeneity were detected in steady state fluorescence and time-resolved anisotropy measurements with DMASBT (lifetime ~100–200 ps) but not with C153, suggesting a timescale over which the inhomogeneity density distributions persist in these electrolyte systems. Subsequent Stokes shift dynamics measurements using C153 captured a dynamic shift of ~700–900 cm−1 with an estimated missing portion of ~50%, and biphasic solvation response characterized by sub-100 ps and nanosecond components. Time-resolved fluorescence anisotropy measurements reflect partial viscosity decoupling for the short lifetime probe and bimodal frictional response in these electrolyte systems.