Local Lithium-Ion Transport of a Ternary Sulfolane-Lithium Bis(trifluoromethanesulfonyl)amide-Carbonate Electrolyte: Experimental and First-Principles Molecular Dynamics Analysis toward Quasi-Solid-State Lithium-Ion Battery

Abstract

The ion coordination and local ion transport of the ternary electrolyte containing sulfolane (SL), lithium bis(trifluoromethanesulfonyl)amide (LiTFSA), and a carbonate solvent were investigated by combining the electrochemical measurement and first-principles molecular dynamics (FPMD) simulation. Li(SL)3TFSA solution with the low-viscosity carbonate solvents, propylene carbonate (PC) and butylene carbonate (BC) was quasi-solidified by nanosized SiO2 and PTFE binder. Electrochemical impedance spectroscopy revealed that the quasi-solidified electrolyte sheets exhibited the Li+ transport number of 0.5, which was higher than that of Li(G4)TFSA containing PC and the conventional organic liquid electrolyte. The higher Li+ transport number contributed to an enhanced discharge capacity of the LIB at a high rate, and the LIB with Li(SL)3TFSA-PC and Li(SL)3TFSA-BC based electrolyte exhibited higher capacity retention in cycle test compared with Li(G4)TFSA-PC. The FPMD analysis revealed that a bridging coordination by SL and TFSA, where a single SL or TFSA coordinates two Li+, was observed even under coexistence with the carbonate solvent. It was also confirmed that some Li+ tend to transfer independently to the other molecules/anions via the bridging coordination, leading to the high Li+ transport number. In conclusion, the composition design of the electrolyte to keep the bridging coordination is desirable for a high Li+ transport number and superior LIB performances.