Ion Transport Mechanisms in Pectin-containing EC-LiTFSI Electrolytes

TL;DR

This study uses molecular dynamics simulations to explore ion transport in pectin-based solid electrolytes, revealing how pectin influences ion coordination, aggregate size, and transport properties, with implications for electrolyte stability and conductivity.

Contribution

It introduces a new class of biodegradable, mechanically stable pectin-containing electrolytes and characterizes their ion transport mechanisms through detailed simulations.

Findings

Pectin reduces lithium ion coordination numbers and promotes smaller ionic aggregates.

Increasing pectin content raises viscosity and relaxation times, decreasing ion diffusion and conductivity.

Lithium and TFSI$^-$ diffusivities correlate differently with ion-pair relaxation timescales.

Abstract

Using all-atom molecular dynamics simulations, we report the structure and ion transport characteristics of a new class of solid polymer electrolytes that contain biodegradable and mechanically stable biopolymer pectin. We simulate a highly conducting ethylene carbonate (EC) as a solvent for lithium-trifluoromethanesulfonimide (LiTFSI) salt containing different weight percentages of pectin. Our simulations reveal that the pectin chains reduce the coordination numbers of lithium ions around the counterions (and vice-versa) because of stronger lithium-pectin interactions compared to lithium-TFSI interactions. Further, the pectin is found to promote smaller ionic aggregates over larger ones, contrary to results typically reported for liquid and polymer electrolytes. We observe that the loading of pectin in EC-LiTFSI electrolytes increases the viscosity ($\eta$) and relaxation timescales ($\tau_c$), indicating higher mechanical stability and, consequently, the mean squared displacements, diffusion coefficients (D), and the Nernst-Einstein conductivity ($\sigma_{NE}$) decrease. Interestingly, while the lithium diffusivities are related to the ion-pair relaxation timescales as $D_+\sim \tau_c^{-3.1}$, the TFSI$^-$ diffusivities exhibit excellent correlations with ion-pair relaxation timescales as $D_-\sim \tau_c^{-0.95}$. On the other hand, the NE conductivities are dictated by distinct transport mechanisms and scales with ion-pair relaxation timescale as $\sigma_{NE}\sim \tau_c^{-1.85}$.