Effect of a Weak Coordination Solvent on a Kinetically Favorable Electrode Reaction in Concentrated Lithium-Ion Battery Electrolytes

Abstract

We present highly concentrated electrolytes prepared with a fluorinated acetate solvent that has much weaker coordination power, allowing for a kinetically favorable electrode reaction in lithium (Li)-ion batteries (LIBs). Among the concentrated electrolytes using conventional organic solvents, the concentrated electrolyte containing a 2,2,2-trifluoroethyl acetate (TFEAc) solvent and 3.2 mol dm–3 LiFSA salt (FSA: bis(fluorosulfonyl)amide) had the lowest ionic conductivity but a wide electrochemical window (∼5 V), allowing for a LIB electrode reaction. Compared to an analogous electrolyte containing LiTFSA salt (TFSA: bis(trifluoromethanesulfonyl)amide), the concentrated LiFSA/TFEAc electrolyte demonstrated significantly improved charge–discharge behavior with a high C-rate performance on the graphite negative electrode. The activation energy (Ea) of the graphite electrode reaction (i.e., Li+-insertion reaction) was determined experimentally to be Ea = 19.9 kJ mol–1 in the concentrated LiFSA/TFEAc electrolyte, which was significantly lower than that reported previously for dilute carbonate-based electrolytes. Furthermore, a combination of high-energy X-ray total scattering experiments and all-atom molecular dynamics simulations revealed that Li ions are co-coordinated with TFEAc molecules and FSA anions to form intricate ion-ordered complexes in the bulk solution with no free solvent and counteranion. From a structural viewpoint, we discussed the characteristic Ea in the current concentrated electrolyte system and concluded that the weak interaction between Li-ion and TFEAc remaining in the activation state plays a key role in triggering energetically easier decoordination during the charge transfer process, achieving a kinetically unique graphite electrode reaction.