Development of metal-air batteries and gas diffusion electrodes

Metal-air batteries have a high energy density and constitute a potential low-cost energy storage technology. They are already commercially available as primary batteries. However, rechargeability is a major challenge and is currently the subject of research. Fraunhofer IFAM is developing rechargeable metal-air batteries. The focus is on the development of gas diffusion electrodes (GDE) with new (carbon) carrier materials and catalysts as well as novel designs with adapted porosity and wetting properties.

At Fraunhofer IFAM, various manufacturing technologies are used for metal-air batteries, such as doctor blade or roller coating of porous substrates, in-situ fabrication of mesoporous carbons, spray coating, or printing. The cell design requirements are also multi-layered, as these are "open" systems in which gaseous oxygen is the active component. Fraunhofer IFAM is developing special and also hybrid metal-air cell designs. Understanding the interaction of electrolyte and gas diffusion electrode is another main focus of our work with corresponding (in-situ) special analysis.

Alternative and high-energy energy storage

For future electromobile, stationary, and other industry-relevant applications, new battery materials and technologies are needed that represent real alternatives to the established lithium-ion battery in terms of sustainability and resource independence. Metal-air batteries have been part of energy storage research for decades and have been in development at Fraunhofer IFAM since 2009.

Metal-air technology is currently of particular interest across all industries, especially as a rechargeable variant. In addition to the potentially low-cost zinc-air system, the theoretically high-energy lithium-air system has received a great deal of attention in recent years, originally driven by the desire of the automotive industry to develop a new generation of high-energy storage systems for electromobility. Likewise, other metal-air alternatives such as calcium-air or sodium-air batteries are coming into the focus of research efforts.

Similarities and differences of the metal-air variants

There is a wide range of metal-air battery variants. The classic design of a metal-air battery consists of a metal anode, an electrolyte (solid, aqueous, organic), and a gas diffusion electrode (GDE) that provides the supply of the active oxygen component. Representative of aqueous, alkaline and organic, aprotic systems are often called the zinc-oxygen (Zn/O2) and lithium-oxygen (Li/O2) systems. Newer systems such as the Ca-oxygen (Ca/O2) system require a hybrid design, as the cathode and anode side electrolyte must be designed differently for potential stable operation. In this case, the anode compartment and cathode compartment are separated by an ion-conducting membrane (or solid-state electrolyte), which requires additional development in terms of components and cell design.

Common to all of them is the need for a gas diffusion electrode to introduce oxygen, the active component, into the cell. Mechanistically, there is a major difference in that with aqueous alkaline electrolytes the discharge products are dissolved or formed on the anode side, and in the aprotic electrolyte solid discharge products are formed which are deposited on the GDE. In the charging reaction, the conversion to the metal ions and O2 therefore takes place either electrocatalytically or homogeneously catalytically. Different heterogeneous or homogeneous catalysts or redox mediators are required to achieve efficient rechargeability. 

Challenges of metal-air batteries

The known system-side challenges of rechargeable metal/air batteries are:

• dendrite-free redeposition of metal ions or cycle-stable anode
• efficient bifunctional GDE (catalysts, heterogeneous or homogeneous) for the oxygen reduction and evolution reaction (ORR/OER) during discharging and charging
• electrolyte stability towards reactive oxygen species or parasitic CO2 from the air
• cyclability
• power handling and rate capability
• open cell operation

The performance of metal-air batteries is fundamentally highly dependent on ORR and OER. Despite clearly distinguishable mechanisms in the alkaline and aprotic systems, the compatibility remains the need for catalysts to present current carrying capacity and to make the ORR and OER more efficient. This is where Fraunhofer IFAM comes in: special GDE designs and cell constructions are developed here.

Production of gas diffusion electrodes

A standard material used for gas diffusion electrodes in the active layer is carbon powder. The carbon material is applied to the gas diffuser/conductor substrate (GDL: e.g.: carbon paper, carbon fleece/textile, metal mesh) via a coating technique (doctor blade, rolling, spraying, printing). For this purpose, it is mixed with a binder, such as Teflon (PTFE) or polyvinylidene fluoride (PVdF). Doughs, pastes or inks are produced with the aid of organic solvents (e.g. isopropanol or NMP). The manufacturing process and the porosity of the carbon powder and the compactness of the coating define the porosity of the gas diffusion electrode in the active layer.

Requirements for functional gas diffusion electrodes

For the design of porous GDE, the pore structure must ensure long-term oxygen and ion transport to the electrode surface. Parasitic intermediates or species react with the carbon and/or fluorinated binder from which the GDE are made. Accordingly, to realize a cycle-stable metal-air battery, the GDE must be made of chemically modified or inert and porous material. Corrosion-resistant, doped carbon materials or titanium carbide-based materials (TiC) as an alternative starting material are the subject of current research at Fraunhofer IFAM to improve electrochemical stability.

A promising approach is a graded pore structure in the GDE consisting of an appropriate ratio of meso- and macropores. Macropores allow for good oxygenation and mesopores provide a larger reaction surface and thus a greater power density. Since sustained and reliable oxygen transport is essential for the reaction, wetting of the pores is also critical to ensure good diffusion. Here, the pore surface should remain completely wetted, while rapid oxygen transport can take place through gas channels in the macropores.

Cell testing and performance of metal-air batteries

The greatest challenge for stable cell operation is the containment of undesirable side reactions. These currently still lead to significantly lower practical energy densities as well as extremely low cycle stabilities. The combination of GDE and selective membrane can ensure operation in air here. The optimal wetting of the GDE, the change in the pore structure and the stability of the metal anode with respect to the electrolyte are important influencing factors here, in addition to the stability of the electrolyte itself.

Our employees develop new material and electrode designs and have a special metal-air test rig to test them under a wide range of conditions. In situ analysis is also used here if required. In addition, we are involved in the conceptual design of metal-air cell stacks. 

Various research projects on metal-air are funded in the electrical energy storage department:

• BMBF projects
  • Akuzil (development of materials and components for zinc-air secondary elements)
  • AMaLiS (alternative materials and components for aprotic lithium-oxygen batteries: ionic liquids and titanium carbide-based gas diffusion electrodes in combination with protected lithium anode)
  • Melubatt (A fresh breeze for metal-oxygen batteries - what can be learned from lithium-ion batteries)
• EU-project: ZABAT (ZABAT – Next generation rechargeable and sustainable Zinc-Air batteries

Publications

D. Fenske, I. Bardenhagen, J. Schwenzel; The Role of Gas Diffusion Electrodes in the Zinc-Air and Lithium-Air Battery: Chem. Ing. Tech., 91, 707-719 (2019)

M. Augustin, D. Fenske, J. Parisi, Study on Electrolyte Stability and Oxygen Reduction Reaction Mechanisms in the Presence of Manganese Oxide Catalysts for Aprotic Lithium–Oxygen Batteries, Energy Technol., 4, 1 – 13 (2016)

H. Bülter, P. Schwager, D. Fenske, G. Wittstock; Observation of Dynamic Interfacial Layers in Li-Ion and Li-O2 Batteries by Scanning Electrochemical Microscopy. Electrochim. Acta, 199, 366-379 (2016)

P. Schwager, S. Dongmo, D. Fenske, G. Wittstock; Reactive oxygen species formed in organic lithium-oxygen batteries. Phys. Chem. Chem. Phys., 18, 10774-10780 (2016)

P. Schwager, D. Fenske, G. Wittstock; Scanning electrochemical microscopy of oxygen permeation through air-cathodes in lithium-air batteries, J. Electroanal. Chem., 740, 82-87 (2015)

I. Bardenhagen, O. Yezerska, M. Augustin, D. Fenske, A. Wittstock, M. Bäumer, In situ investigation of pore clogging during discharge of a Li/O2 battery by electrochemical impedance spectroscopy. J. Power Sources, 278, 255-264 (2015)

M. Augustin, D. Fenske, I. Bardenhagen, A. Westphal, M. Knipper, T. Plaggenborg, J. Kolny-Olesiak, J. Parisi, Manganese oxide phases and morphologies: A study on calcination temperature and atmosphere dependence. Beilstein J. Nanotechn., 6, 47-59 (2015)

I. Bardenhagen, M. Fenske, D. Fenske, A. Wittstock, M. Bäumer, Distribution of discharge products inside of the lithium/oxygen battery cathode, J. Power Sources, 299, 162–169 (2015)

M. Augustin, O. Yezerska, D. Fenske, I. Bardenhagen, A. Westphal, M. Knipper, T. Plaggenborg, J. Kolny-Olesiak, J. Parisi, Mechanistic study on the activity of manganese oxide catalysts for oxygen reduction reaction in an aprotic electrolyte. Electrochim. Acta, 158, 383-389 (2015)

T. Dabrowski, A. Struck, D. Fenske, P. Maaß, L. Colombi Ciacchi, Optimization of catalytically active sites positioning in porous cathodes of lithium/air batteries filled with different electrolytes, J. Electrochem. Soc., 162, 2796-2804 (2015)

I. Bardenhagen, W. Dreher, A. Wittstock, M. Bäumer, Fluid Distribution and Pore Wettability of Monolithic Carbon Xerogels measured by 1H NMR Relaxation, Carbon, 68, 542-552 (2014)

We will be happy to provide further information on projects and publications upon request.