Self-assembled Janus-like multi-ionic lithium salts form nano-structured solid polymer electrolytes with high ionic conductivity and Li+ ion transference number

Abstract

A solid polymer electrolyte with high ambient temperature conductivity, 4 × 10⁻⁴ S cm⁻¹, and transference number, t_Li⁺ = 0.6, is formed from blends of polyethylene oxide (PEO) and a multi-ionic polyoctahedral silsesquioxane lithium salt, POSS-phenyl₇(BF₃Li)₃, with Janus-like properties. A two-phase morphology is proposed in which the hydrophobic phenyl groups cluster and crystallize, and the three –BF₃⁻ form an anionic pocket, with the Li⁺ ions solvated by the PEO phase. The high ionic conductivity results from interfacial migration of Li⁺ ions loosely bonded to three –BF₃⁻ anions and the ether oxygens of PEO. Physical crosslinks formed between PEO/Li⁺ chains and the POSS clusters account for the solid structure of the amorphous PEO matrix. The solid polymer electrolyte has an electrochemical stability window of 4.6 V and excellent interfacial stability with lithium metal.