Ionic conductivity of bis(2-cyanoethyl) ether-lithium salt and poly(propylether imine)-lithium salt liquid electrolytes

Time-Temperature Scaling and Dielectric Modeling of Conductivity Spectra of Single-Ion Conducting Liquid Dendrimer Electrolytes

We discuss here the time-temperature scaling and dielectric modeling of the variation of single-ion conductivity with frequency of first generation (G₁) liquid dendrimer electrolyte, viz., Poly(propyl ether imine) (PETIM):Li-salt. The PETIM:Li-salt electrolyte exhibits a cation/anion transference number close to unity in the liquid state. On switching from an ester (G₁-COOR) to cyano (G₁-CN) peripheral group, keeping constant the linker (ether) and branching groups (amine), an interesting transformation from cationic ( t₊ ∼1) to anionic conductor ( t_- ∼1) takes place. The switch in the nature of the predominant charge carrier is directly related to the change in the magnitude of anion diffusion ( D_-), which increases by 1 order of magnitude from D_- = 1.1 × 10^-12 m² s^-1 (at 30 °C) in G₁-COOR to D_- = 1.3 × 10^-11 m² s^-1 (at 30 °C) in G₁-CN. This intriguing ion transport mechanism is probed comprehensively using ac-impedance spectroscopy. The frequency dependent ionic conductivity of G₁-CN/G₁-COOR, comprised of distinct frequency regimes, is analyzed using the time-temperature superposition scaling principle (TTSP) based on Summerfield and Baranovski scaling methods. To gain insight into the electrical polarization (EP) phenomenon, the relevant frequency regime is converted from conductivity to dielectric versus frequency. The dielectric versus frequency data is modeled using Macdonald and Coelho. The combined approach of TTSP and dielectric modeling provide explicitly the extent of the influence of ion-dendrimer, ion-ion interactions, and also the mobile charge carrier density on the effective ion transport in the homogeneous single-ion conducting dendrimer electrolytes. The combined analysis suggests that ion transport in PETIM-COOR is only due to enhanced ion mobility, whereas in PETIM-CN it is due to both mobile charge carrier concentration and ion mobility. To the best of our knowledge, the scaling and modeling approaches employed here constitute a rare example for validation of such concepts in the context of dendrimer electrolytes.

A single cation or anion dendrimer-based liquid electrolyte

We propose here a novel liquid dendrimer-based single ion conductor as a potential alternative to conventional molecular liquid solvent-salt solutions in rechargeable batteries, sensors and actuators. A specific change from ester (-COOR) to cyano (-CN) terminated peripheral groups in generation-one poly(propyl ether imine) (G1-PETIM)-lithium salt complexes results in a remarkable switchover from a high cation (tLi+ = 0.9 for -COOR) to a high anion (tPF6- = 0.8 for -CN) transference number. This observed switchover draws an interesting analogy with the concept of heterogeneous doping, applied successfully to account for similar changes in ionic conductivity arising out of dispersion of insulator particle inclusions in weak inorganic solid electrolytes. The change in peripheral group simultaneously affects the effective ionic conductivity, with the room temperature ionic conductivity of PETIM-CN (1.9 × 10-5 Ω-1 cm-1) being an order of magnitude higher than PETIM-COOR (1.9 × 10-6 Ω-1 cm-1). Notably, no significant changes are observed in the lithium mobility even following changes in viscosity due to the change in the peripheral group. Changes in the peripheral chemical functionality directly influence the anion mobility, being lower in PETIM-COOR than in PETIM-CN, which ultimately becomes the sole parameter controlling the effective transport and electrochemical properties of the dendrimer electrolytes.