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-PEG 8 has eight polyethylene glycol side chains that have low glass transition ( T g ) and melt ( T m ) temperatures and POSS-benzyl 7 (BF 3 Li) 3 is a Janus-like POSS with hydrophobic phenyl groups and −Si–O–BF 3 Li ionic groups clustered on one side of the SiO 1.5 cube. The electron-withdrawing POSS cage and BF 3 groups enable easy dissociation of the Li + . In the presence of polar POSS-PEG 8, the hydrophobic phenyl rings of POSS-benzyl 7 (BF 3 Li) 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–BF 3 – Li + groups of POSS-benzyl 7 (BF 3 Li) 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 (BF 3 Li) 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-PEG 8 /LiBF 4, 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).