Role of Anion Flexibility on Graphite Electrode Reactions in Bis(fluorosulfonyl)amide-Based Ionic Liquid Electrolytes for Lithium-Ion Batteries
Saki Sawayama, Tsubasa Kawaguchi, Kenta Fujii
Abstract
We subjected bis(fluorosulfonyl)amide (FSA)-based ionic liquid (IL) electrolytes for lithium (Li)-ion batteries to structural and electrochemical studies to elucidate the criticality of “FSA-anion flexibility” on Li-ion solvation/desolvation and electrode-reaction properties in the batteries. Raman spectroscopy, high-energy X-ray total scattering, and theoretical calculations were used to comprehensively conduct the structural investigation to reveal the occurrence of specific Li-ion solvation in the LiFSA/IL solutions exhibiting low and extremely high LiFSA concentrations ( c Li ). Li ions in the low- c Li solutions (<2.0 M) formed a mononuclear [Li(FSA) 3 ] 2– complex exhibiting monodentate- and bidentate-type coordination. The structure of this complex differed significantly from that formed in the high- c Li solutions (∼3.4 M in this case) and yielded ionic aggregates based on the FSA-linked ordered Li ions. Further, the calculation of the potential energy surface of the internal conformational change in FSA revealed its flexible conformation, which was changeable even when confined within Li–FSA complexes. This behavior differed significantly from that observed for the analogous anion (bis(trifluoromethanesulfonyl)amide), indicating the ease of changing the conformation of coordinated FSA from bidentate to monodentate, thus facilitating FSA desolvation from the Li-ion complexes via the weaker Li-ion interactions with monodentate FSA compared to its interactions with chelating-effect-exerting bidentate FSA. This feature was fully reflected in the reaction of the graphite negative electrode, where the favorable FSA desolvation in the low- c Li solutions contributed to lowering the activation energy ( E a ) during charge transfer, thus resulting in kinetically improved Li + insertion into the graphite layers. Notably, the obtained E a value was 10 kJ mol –1 higher in the high- c Li solution ( c Li = 3.4 M) than in the low- c Li solutions probably because of the highly ordered Li + structures, which exhibited higher structural-relaxation energy.