Are Weakly Coordinating Anions Really the Holy Grail of Ternary Solid Polymer Electrolytes Plasticized by Ionic Liquids? Coordinating Anions to the Rescue of the Lithium Ion Mobility

TL;DR

This study challenges the idea that weakly coordinating anions are ideal in polymer electrolytes, demonstrating that more coordinating anions like TFSAM significantly enhance lithium ion transport and battery performance.

Contribution

It introduces the use of highly coordinating anions such as TFSAM to improve Li$^+$ mobility in polymer electrolytes, a novel approach compared to traditional weakly coordinating anions.

Findings

TFSAM increases Li$^+$ transference number by 600%

Li$^+$ conductivity is improved by 200-300% with TFSAM

Enhanced battery capacity retention of 86% after 300 cycles

Abstract

Lithium salts with low coordinating anions like bis(trifluoromethanesulfonyl)imide (TFSI) have been the state-of-the-art for PEO-based 'dry' polymer electrolytes for three decades. Plasticizing PEO with TFSI-based ionic liquids (ILs) to form ternary solid polymer electrolytes (TSPEs) increases conductivity and Li$^+$ diffusivity. However, the Li$^+$ transport mechanism is unaffected compared to their 'dry' counterpart and essentially coupled to the dynamics of the polymer host matrix, which limits Li$^+$ transport improvement. Thus, a paradigm shift is hereby suggested: The utilization of more coordinating anions such as trifluoromethanesulfonyl-N-cyanoamide (TFSAM), able to compete with PEO for Li$^+$ solvation to accelerate the Li$^+$ transport and reach higher Li$^+$ transference number. The Li-TFSAM interaction in binary and ternary TFSAM-based electrolytes was probed by experimental methods and discussed in the context of recent computational results. In PEO-based TSPEs, TFSAM drastically accelerates the Li$^+$ transport (increased Li$^+$ transference number by 600$\%$ and Li$^+$ conductivity by 200-300$\%$) and computer simulations reveal that lithium dynamics are effectively re-coupled from polymer to anion dynamics. Finally, this concept of coordinating anions in TSPEs was successfully applied in LFP$||$Li metal cells leading to enhanced capacity retention (86$\%$ after 300 cycles) and an improved rate performance at 2C.