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 SiO1.5 core and eight side groups. POSS-PEG(8) has eight polyethylene glycol side chains that have low glass transition (T-g) and melt (T-m) temperatures and POSS-benzyl(7)(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-PEG(8), the hydrophobic phenyl rings of POSS-benzyl(7)(BF3Li)(3) aggregate and crystallize, forming a biphasic morphology, in which the phenyl rings form the structural phase and the POSS-PEG(8) forms the conductive phase. The -Si-O-BF3- Li+ groups of POSS-benzyl(7)(BF3Li)(3) are oriented toward the polar POSS-PEG(8) phase and dissociate so that the Li+ cations are solvated by the POSS-PEG(8). The nonvolatile nanocomposite electrolytes are viscous liquids that do not flow under their own weight. POSS-PEG(8)/POSS-benzyl(7)(BF3Li)(3) at O/Li = 16/1 has a conductivity of sigma = 2.5 X 10(-4) S/cm at 30 degrees C, which is 17 times greater than that of POSS-PEG(8)/LiBF4, and a low activation energy (E-a similar to 3-4 kJ/mol); sigma = 1.6 X 10(-3) S/cm at 90 degrees C and 1.5 X 10(-5) S/cm at 10 degrees 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 degrees C).