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

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journal contribution
posted on 13.12.2011, 00:00 authored by Parameswara Rao Chinnam, Stephanie L. Wunder
Nanocomposite electrolytes have been prepared from mixtures of two polyoctahedral silsesquioxanes (POSS) nanomaterials, each with a SiO1.5 core and eight side groups. POSS-PEG8 has eight polyethylene glycol side chains that have low glass transition (Tg) and melt (Tm) 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 SiO1.5 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 (Ea ∼ 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 tLi+ = 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).