New battery strategies with a polymer/Al2O3 separator

•Al2O3/PEO composite film was fabricated as a battery electrolyte membrane.•The membrane is thin, flexible, and mechanically robust.•The membrane can block Li/Na dendrites and enable assembly of charged-state batteries.•A hybrid redox-flow battery could be achieved by using the membrane together with a PEDOT:PSS-coated polypropylene separator.•Osmosis issues could be minimized by balancing the solute concentrations of catholyte and anolyte.

A low-cost, thin, flexible, and mechanically robust alkali-ion electrolyte separator is shown to allow fabrication of a safe rechargeable alkali-ion battery with alternative cathode strategies. A Na-ion battery with an insertion host as cathode and a Li-ion battery with a redox flow-through cathode are demonstrated to cycle without significant fade. The separator membrane is a composite of Al2O3 particles and cross-linked ethylene-oxide chains; it can be fabricated at low cost into a large-area thin membrane that blocks dendrites from an alkali-metal anode. To block a soluble ferrocene redox molecule from crossing from the cathode side to the anode in a Li-ion battery with a redox-flow cathode, a thin mixed Li+/electronic-conducting film has been added to the cathode side of the composite separator. An osmosis issue was minimized by balancing concentrations of solutes on the two sides of the separator where the cathode side contains a soluble redox molecule.

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