Electrochemical device having a solid alkaline ion-conducting electrolyte and an aqueous electrolyte

#The present application is filed pursuant to 35 U.S.C. §371 as a U.S. National Phase application of International Patent Application No. PCT/FR2010/052246, which was filed on Oct. 21, 2010, claiming the benefit of priority to French Patent Application No. FR 09 57528 filed on Oct. 27, 2009. The entire content of each of the aforementioned applications is incorporated herein by reference in its entirety.

#The present invention relates to an electrochemical device, in particular to a rechargeable battery, comprising both a solid electrolyte membrane that can conduct alkali cations and a saturated aqueous electrolyte, these two electrolytes being separated from each other by an organic polymer film.

#The energy density per unit mass (expressed in Wh/kg) of batteries is still the main factor limiting their use in mobile devices such as portable electronics or electric vehicles. The limited energy density of these batteries is mainly due to the performance of the materials from which they are made. The best negative-electrode materials currently available generally have a specific capacity of 300 to 350 Ah/kg. The specific capacity is only about 100 to 150 Ah/kg for positive-electrode materials.

#The present invention is illustrated below using the appended figures, in which:

#FIG. 1 shows the structure of a lithium-air battery according to the invention; and

#FIG. 2 shows how the voltage required to maintain a current of 2 mA, in the trials described in the example below, varied over time.

#In FIG. 1, the negative-electrode compartment comprises a negative electrode 1 made of lithium metal and connected to an electronic conductor 2. The positive-electrode compartment comprises a liquid electrolyte 3 consisting of a saturated LiOH solution, in which an air electrode 4 and an oxygen-emitting electrode 5 are immersed. An LiOH precipitate 6 accumulates at the bottom of the positive-electrode compartment. The two compartments are separated from each other by a ceramic membrane 7 covered, on the negative-electrode side, by an LiPON-based protective coating 8, and, on the positive-electrode side, with a layer of an organic cation-conductive polymer 9 that is water insoluble. The coating 8 serves to isolate the ceramic membrane from the lithium metal whereas the organic polymer layer 9 prevents a layer of crystallized LiOH from forming on the surface of the ceramic membrane 7.

#To demonstrate the effect of placing a thin layer of hydrophobic, cation-conductive, organic polymer on the surface of a ceramic electrolyte membrane an electrochemical device was prepared comprising two compartments separated from each other by a ceramic membrane that could conduct Li+ ions (LISICON membrane); the membrane was 300 μm thick and was sold by Ohara. The two compartments were filled with an aqueous solution of 5M LiOH. A platinum electrode was inserted into each compartment. Using a potentiostat, a 2 mA current was made to flow through the cell between the two platinum electrodes, thereby causing Li+ ions to pass from the anode compartment to the cathode compartment. The Li+ ion migration was accompanied by the formation of OH− ions in the positive-electrode compartment by water or oxygen reduction. The positive-electrode compartment of this device mimicked the operation of the positive-electrode compartment of a lithium-water or lithium-air battery at the interface between the LISICON ceramic and the LiOH-containing aqueous electrolyte. The 2 mA current flow was maintained until LiOH saturation and precipitation occurred, and the voltage required to maintain a 2 mA current flow was measured throughout the experiment.

#After about 45 hours of operation, at the moment when LiOH started to precipitate, a rapid and significant increase in the voltage required to maintain a current of 2 mA was observed (see Curve A in FIG. 2).