First, a strategy for designing a 3D metal anode has been drawn, using CMOS compatible process. 3D zinc electrode (Zn micropillars) has already been electroplated through a porous membrane . Despite an interesting technology, the 3D zinc scaffold is totally consumed after the first discharge (primary battery). The aim of our project is to produce a secondary metal-air microbattery by keeping safe the 3D template after the first discharge. A 3D metal electrode is then electroplated onto an inert 3D scaffold. To reach this goal, a 3D silicon template, consisting in an array of microtubes (Φ = 3 µm / depth = 50 µm) is fabricated. The lateral surface providing by the 3D scaffold allows the enhancement of the material mass loading while keeping constant the footprint area . Once the geometry has been optimized by pushing back the limits of the silicon etching, the 3D scaffold is coated with a conformal deposit of metals. Zinc electroplated thin films has been deposited from an aqueous bath, and a similar approach for the electrodeposition of aluminum thin films was carried out from ionic liquids.
Then, a micro-structured air cathode has been engineered using similar microelectronics processes. Air cathodes are composed of a conductive mesh, a catalyst and carbon material . The project aims at reproducing the behavior of such air cathodes but at the millimeter scale: silicon wafer is double side etched with a tuned porosity. The top side of this topology acts as gas diffusion layer allowing the diffusion of oxygen into the miniaturized device. The bottom side of this electrode is composed of a large cavity, allowing the contact between the aqueous electrolyte, the catalytic sites and the oxygen. Conformal deposition of a platinum thin film is achieved by atomic layer deposition (ALD) to add the conductive contribution to the porous silicon scaffold. This platinum layer acts as the current collector and as the seed layer for catalysts electrodeposition process.
These two electrodes have been separately characterized. Concerning the 3D metallic anode, various Zn-air prototypes have been assembled in KOH electrolyte using a homemade electrochemical cell with a commercially available air cathode. In the best configuration tested so far, this 3D Zn electrode can deliver a discharge capacity close to 1 mAh.cm-² with a flat discharge plateau at 1.2 V (fig. 1C). This value is three times higher than the one reported for state-of-art 3D Li-ion microbattery capacity using material thickness in the same order of magnitude . Similarly, various Zn-air prototypes have been assembled using a Zn foil and the silicon micromachined air cathode in KOH electrolyte. A discharge plateau around 1.3V has been observed (fig. 1D). Finally, both components have been assembled to demonstrate the feasibility of a 3D metal-air primary microbattery with improved performance which will be reported in this communication.
Acknowledgment: The RS2E and the DGA financially support this work. The French RENATECH network is greatly acknowledged for the microfabrication facilities.
 Chamran, F. et al. (2007), Zinc-Air Microbattery with Electrode Array of Zinc Microposts, MEMS 2007, Kobe, Japan, 21-25 January 2007
 Eustache, E. et al. (2014), Silicon-Microtube Scaffold Decorated with Anatase TiO2 as a Negative Electrode for a 3D Litium-Ion Microbattery, Adv. Energy Mater. 1301612
 Xiaoming, G. et al. (2015), Oxygen Reduction in Alkaline Media: From Mechanisms to Recent Advances of Catalysts, ACS Catal., 5, 4643-4667