electronic insulator with abundant oxygen-containing groups (epoxy, hydroxyl, and carboxyl groups), which can be easily functionalized to improve its ionic conductivity. [12,16] Owing to the wide range of oxygen functional groups on its basal planes and edges, GO is also readily exfoliated to yield well-dispersed solution of individual GO sheets in both water and organic solvents, and then flow-directed assembled into a free-standing paper-like material by vacuum filtration. [12,17] However, despite all the progress of the development of a free-standing GO electrolyte membrane, its advantages are diminished with the difficulty to form a mechanically robust membrane. [17,18] Consequently, that makes the free-standing GO membranes less attractive for being used as practical solid-state electrolytes in energy storage systems. To enhance the mechanical strength, several fillers can be incorporated into GO matrix. Among the multifarious materials, cellulose-the most abundant renewable material-is recognized as a good binder and surface modifier thanks to its low cost, high hygroscopicity, flexibility, ad porous substrate that allows strong binding of other materials. Particularly, considerable interest has been directed to nanocellulose fiber because of its low thermal expansion, high surface area, high surface area-to-volume ratio, strengthening effect, good mechanical and optical properties which may find many applications in nanocomposites. [19] Meanwhile, due to its highly reactive area rich in hydroxyl groups, cellulose can be easily refashioned through a surface-functionalization to conduct ions and be utilized in advanced batteries. [7] Herein, for the first time, we report on a laminate-structured nanocellulose/GO membrane functionalized with highly hydroxide-conductive quaternary ammonium (QA) groups to be applied as a robust solid-state electrolyte in flexible, rechargeable zinc-air batteries. The QA-functionalized nanocellulose/GO (QAFCGO) membrane is fabricated through chemical functionalization, layer-by-layer filtration, cross-linking, and ion-exchange processes (Figure 1a). For the functionalization process, dimethyloctadecyl [3-(trimethoxysilyl)-propyl] ammonium chloride (DMAOP) has been selected as the functional precursor containing QA moieties. The hydroxide conductivity and the alkaline stability in the highly surface-active DMAOP are provided by quaternary ammonium groups which are already attached to this organic compound. Thus, the hydroxide conducting characteristic of the electrolyte membrane can be achieved directly through the precursor DMAOP without complex organic synthesis. First, the trimethoxy groups of trimethoxysilyl are hydrolyzed to form the corresponding silanols, and the hydrolyzed silanols undergo self-condensation to yield silanol oligomers intermediates ( Figure S1, step 1, Supporting Information). Then, these intermediates are adsorbed onto nanocellulose/GO surface rich in oxygen-containing groups through hydrogen bonding ( Figure S1,
