Protective PVDF-HFP-based membranes for air de-hydration at the cathode of the rechargeable Li–air cell

Non-aqueous electrolyte Li-air batteries with an O2-selective membrane at the cathode are developed for operation in ambient air at constant relative humidity (RH). The membrane allows oxygen permeation and acts as a moisture barrier. Our group prepared highly hydrophobic films of poly(vinylidene fluoride co-hexafluoropropylene) (PVDF-HFP) via non-solvent induced phase separation. The addition of sacrificial silica nanoparticles (SiO2 NPs) to the precursor polymer solution increased the surface roughness of the casted membrane. Subsequent SiO2 NPs removal created a homogeneous porous network inside the membrane in which the silicone oil was trapped. The silicone oil accomplished the role of O2-selective medium. The Li-air cell with the oxygen selective membrane forwarded capacities of 640 mA h g-1, at the discharge current density of 0.05 mA cm-2, in air (17 % RH). In the same experimental conditions, but without the membrane at the cathode, the Li-air cell delivered a capacity of 220 mA h g-1. We demonstrate that the main cause of the unsatisfying behaviour of the unprotected cell is the corrosion of the Li anode in the presence of water, although the cathode is also sensitive to CO2. The galvanostatic discharge-charge tests, in the capacity-controlled mode, confirmed the effectiveness of the membrane in protecting the Li-air cell in ambient air, assuring a cycle-life comparable to that in dry oxygen atmosphere.