Polyoctahedral Silsesquioxane-Nanoparticle Electrolytes for Lithium Batteries: POSS-Lithium Salts and POSS-PEGs

Nanocomposite electrolytes have been prepared from mixtures of two polyoctahedral silsesquioxanes (POSS) nanomaterials, each with a SiO_1.5 core and eight side groups. POSS-PEG₈ has eight polyethylene glycol side chains that have low glass transition (T_g) and melt (T_m) temperatures and POSS-benzyl₇(BF₃Li)₃ is a Janus-like POSS with hydrophobic phenyl groups and −Si–O–BF₃Li ionic groups clustered on one side of the SiO_1.5 cube. The electron-withdrawing POSS cage and BF₃ groups enable easy dissociation of the Li⁺. In the presence of polar POSS-PEG₈, the hydrophobic phenyl rings of POSS-benzyl₇(BF₃Li)₃ aggregate and crystallize, forming a biphasic morphology, in which the phenyl rings form the structural phase and the POSS-PEG₈ forms the conductive phase. The −Si–O–BF₃– Li⁺ groups of POSS-benzyl₇(BF₃Li)₃ are oriented toward the polar POSS-PEG₈ phase and dissociate so that the Li⁺ cations are solvated by the POSS-PEG₈. The nonvolatile nanocomposite electrolytes are viscous liquids that do not flow under their own weight. POSS-PEG₈/POSS-benzyl₇(BF₃Li)₃ at O/Li = 16/1 has a conductivity of σ = 2.5 × 10^–4 S/cm at 30 °C, which is 17 times greater than that of POSS-PEG₈/LiBF₄, and a low activation energy (E_a ∼ 3–4 kJ/mol); σ = 1.6 × 10^–3 S/cm at 90 °C and 1.5 × 10^–5 S/cm at 10 °C. The lithium ion transference number was t_Li⁺ = 0.50 ± 0.01, as a result of the reduced mobility of the large, bulky anion, and the system exhibited low interfacial resistance that stabilized after 3 days (both at 80 °C).