New Insights in the Development of 3D Metal-Air Microbatteries
Authors: Freixas Jérémy , Brousse Thierry , Lethien Christophe 
Published: February 2020

An increasingly number of studies is dedicated to the development of miniaturized sensors (“smartdust”), either for industrial purposes or for health monitoring. The requested power to get autonomous such miniaturized sensors led researchers to design efficient power sources. Microbatteries and microsupercapacitors have to be combined to fulfill such requirements. Our approach aims at miniaturizing metal-air batteries which can drastically boost the energy density available for smartdust sensors. In the frame of this study, the common thread is to design and to produce a full 3D microdevice in which energy is stored in the same way as in a metal-air battery. To address this task, the fabrication and characterization of miniaturized 3D metal anode and 3D macro-porous air cathode are proposed, each of those electrodes having a footprint area close to several mm2.
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 [1]. 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 [2]. 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 [3]. 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 [4]. 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.

References

[1] Chamran, F. et al. (2007), Zinc-Air Microbattery with Electrode Array of Zinc Microposts, MEMS 2007, Kobe, Japan, 21-25 January 2007
[2] 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
[3] Xiaoming, G. et al. (2015), Oxygen Reduction in Alkaline Media: From Mechanisms to Recent Advances of Catalysts, ACS Catal., 5, 4643-4667
[4] Létiche, M et al. (2016), Atomic layer deposition of functional layers for on Chip 3D Li-ion all solid state microbattery, Adv. Energy Mater. 1601402

Figure 1