2022/08/05 by Hoffknecht, Jan-Philipp, Wettstein, Alina, Atik, Jaschar +7
#Chemical Physics (physics.chem-ph) #FOS: Physical sciences #Materials Science (cond-mat.mtrl-sci)
paper · doi:10.48550/arxiv.2208.03245
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.