Polysiloxane-based Electrolytes: Influence of Salt Type and Polymer Chain Length on the Physical and Electrochemical Properties

An oligo/poly(methyl(2-(tris(2-H methoxyethoxy)silyl)ethyl)siloxane)), 390EO, and 2550EO, were synthesized. Dilute electrolyte solutions of 390EO and 2550EO were prepared using LiTFSI, LiFSI, and LiPF6. The influence of the length of the siloxane polymer chain, salt type, and Si-tripodand centers at the side chain on ionic conductivity, t(Li)(+), and physical properties were examined. Both electrolyte systems showed high values of t(Li)(+) (0.35 for 2550EO/LiTFSI and 0.64 for 390EO/LiTFSI). Alternatively 390EO/LiPF6 and 2550EO/LiPF6 displayed high t(Li)(+) values of 0.61 and 0.44, respectively, while 390EO/LiFSI displayed the smallest t(Li+) (0.25). To clarify the role played by the Li+ environment in Li+ transport, the solvation states of electrolytes were examined. It was observed that anion solvation can be achieved using siloxane-based solvent in all systems. Walden plot analysis demonstrates that ionic diffusion was not controlled by either macroviscosity/microviscosity in the siloxane-based polymer electrolytes. Ions instead move along a relatively smooth ion-pathway without complete full segmental reorientation in 2550EO as a result of decoupling and high ion solvation behavior. Conversely, in 390EO, ions might move to available sites by a jumping after decoupling with low ion solvation behavior. Consequently, a high t (+)(Li) was achieved, and the oxidative stability of the salt was ensured.