Enhancing Cation Diffusion and Suppressing Anion Diffusion via Lewis-Acidic Polymer Electrolytes

TL;DR

This paper demonstrates that Lewis-acidic polymer electrolytes can significantly enhance lithium-ion diffusion and suppress anion diffusion, offering a promising route to improve solid polymer electrolytes for lithium batteries.

Contribution

The study introduces Lewis-acidic polyboranes as a new class of polymer electrolytes that outperform traditional PEO in Li-ion diffusivity and ion selectivity, based on molecular dynamics simulations.

Findings

Lewis-acidic polymers increase Li-ion diffusivity up to tenfold.

They significantly decrease anion diffusivity compared to PEO.

The results reveal a general principle for designing better polymer electrolytes.

Abstract

Solid polymer electrolytes (SPE) have the potential to increase both the energy density and stability of lithium-based batteries, but low Li-ion conductivity remains a barrier to technological viability. SPEs are designed to maximize Li-ion diffusivity relative to the anion, while maintaining sufficient salt solubility. It is thus remarkable that polyethylene oxide (PEO), the most widely used SPE, exhibits Li-ion diffusivity that is an order of magnitude smaller than that of typical counter-ions, such as TFSI, at moderate salt concentrations. Here, we show that Lewis-basic polymers like PEO intrinsically favor slow cation and rapid anion diffusion while this relationship can be reversed in Lewis-acidic polymers. Using molecular dynamics (MD) simulations, Lewis-acidic polyboranes are identified that achieve up to a ten-fold increase in Li-ion diffusivity and a significant decrease in anion diffusivity, relative to PEO. The results for this new class of Lewis-acidic SPEs illustrate a general principle for increasing Li-ion diffusivity and transference number with polymer chemistries that exhibit weaker cation and stronger anion coordination.