100 nm Thin‐Film Solid‐Composite Electrolyte for Lithium‐Ion Batteries

A thin‐film composite electrolyte (CE) with thickness of only 100 nm is fabricated for the first time. The conductivity of the solid electrolyte is 2 × 10⁻⁷ S cm⁻¹ which corresponds to a cell resistance of only 50 Ω cm² at this thickness. The electrolyte is composed of a nanoporous thin film, a lithium salt, and a polymer, and is fabricated by a two‐step approach. First, a porous SiO_xC_yH_z film is deposited on the electrode substrate, then the porous structure is filled with a polyethylene glycol (PEG)‐LiX mixture by spin‐coating. The filling ratio of the mesoporous SiO_xC_yH_z is as high as 90% with less than 1% remaining porosity, as determined from ellipsometric porosimetry. The effect of the anion, X⁻, of the LiX salt as well as the molecular weight of the PEG on the Li⁺ ion conductivity of the CE is investigated. The CE with smaller anion and with lower molecular weight PEG yields the highest conductivity. A functional Li/CE/TiO₂ battery is built for demonstration purposes.