Salt-Concentrated Siloxane-Based Electrolytes for Lithium Metal Batteries: Physical Properties, Electrochemical Properties, and Cell Performance

Abstract

Herein, salt systems were prepared using oligo(methyl(2-(tris(2-H methoxyethoxy)silyl)ethyl)siloxane)) grafted with Si-tripodant centers (390EO) as the solvent and amide salt (LiF(SO 2 CF 3 ) 2 (LiTFSI)) as the lithium (Li) ion source with varying ethylene oxide (EO)/Li + ratios. The LiTFSI concentration increased as the EO/ Li + ratio decreased from 10/1 to 5/1 and from 2.5/1 to 1.25/1. The molar concentrations of systems with EO/Li + ratios of 10/1, 5/1, 2.5/1, and 1.25/1 were 0.76, 1.38, 2.30, and >4 M, respectively, at 25 °C. In addition to the characterization of the electrochemical and physical properties, the battery performance of LiFePO 4 /Li with the 390EO/LiTFSI system with EO/Li + ratios of 10/1, 5/1, 2.5/1, and 1.25/1 was examined. Dynamic viscosities of the samples/systems were found to increase with increasing LiTFSI molar concentration; however, liquid electrolyte conductivity changed only minimally at 60 °C. Walden plot analysis revealed simultaneous strong ion dissociation and decoupling properties. The anion and cation diffusion coefficients clearly demonstrated that the high concentration systems contain more free Li ions, and the interactions between Si 3 at the tripodant centers and anions play a significant role in the increased amount of free Li. 390EO suppresses TFSI-induced anodic corrosion of the Al current collector and provides high electrolyte stability up to 5 V, even at 60 °C. This afforded Coulombic efficiencies exceeding 95% for the cell performance of all samples. The excellent thermal and electrochemical stability, as well as chemical inertness, adhesion strength, highly flexible backbone, and high segmental mobility of the siloxane-based polymers compared to their organic counterparts, afforded high cycling stability and electrochemical stability toward Li metal when the 5/1, 2/5, and 1/25 systems were used as electrolytes. Therefore, this study clearly demonstrates that siloxane polymers, particularly those grafted with Si-tripodant centers, are suitable solvents in high salt-concentrated electrolyte systems for Li metal batteries.