Today, the lithium-ion battery is the system of choice for many uses in consumer applications, electric vehicles and the stationary sector. Continuous improvements to the electrode materials and cell design have enabled the energy density and performance to be considerably increased over the past twenty years. However, although further improvements are possible, the lithium-ion technology does not allow for unlimited improvements, which is why new battery systems need to be considered.
At ZSW, we are pursuing various alternative approaches for storing electrochemical energy. The motivation behind this research include factors such as further increasing the energy density and performance, the behaviour at high and low temperatures, the size and safety of the system as well as issues regarding the availability and cost of the materials.
Our work focuses on metal-air cells, supercapacitors, hybrid supercapacitors, redox flow batteries and much more. ZSW can contribute over 20 years of interdisciplinary experience in the battery and fuel cell sector to the development of these new systems.
Reversible air electrodes present a particular challenge for the involved catalysts, additives and electrode structures. For many years, ZWS has been researching the production and characterisation of bifunctional oxygen electrodes in aqueous alkaline electrolytes. In this regard, both dry preparation methods and processes using pastes have been established.
In addition to electrodes for aqueous systems, electrodes for non-aqueous metal-oxygen cells (e.g. lithium or magnesium) are also being developed.
High-surface metal oxides based on cobalt, nickel or manganese have proved themselves in practice as bifunctional catalysts in metal-air accumulators. ZSW is equipped with suitable synthesis methods such as a high-temperature and sol gel route.
Metal-oxygen accumulators exhibit very high values for specific energy and energy density because when oxygen is the active material on the positive electrode it can, in principle, be extracted from the surrounding air. However, particular challenges are presented by the kinetics of the oxygen electrode and the cycle stability of the respective metal electrodes. Metal-air systems both with aqueous and non-aqueous electrolytes are processed at ZSW.
Zinc-air systems with aqueous electrolytes are very well known as primary systems, for example, for hearing aids. Even rechargeable systems have been researched for many years. However, they have not yet become established on the market due to the high voltage difference between charging and discharging, as well as the comparably low cycle stability.
Current research projects at ZSW are therefore concerned with increasing the cycle stability of the zinc electrode through microstructural adjustment and improving the electrolyte composition. In addition, new concepts for oxygen electrodes with stable cycles are also being researched.
Metal-oxygen systems with non-aqueous electrolytes allow higher cell voltages than aqueous systems when lithium or magnesium are used as active materials. However, they are still in a very early stage of development. New active materials and electrolytes for oxygen electrodes with cycle stability have been discovered at ZSW. Furthermore, it has been proven that the currently observed limitation in the cycle stability of lithium-oxygen cells is caused by the lithium metal-electrode that has been used until now.
Our current work focuses on the deeper understanding of the relevant processes in lithium- and magnesium-oxygen systems with the aim of further increasing the reaction kinetics on the oxygen electrode and improving the cycle stability of the metal electrode by using suitable materials for the electrolytes and active materials.
Lithium-oxygen batteries (Li-O2-systems) are a new kind of rechargeable battery with a maximum theoretical specific energy of 11,700 Wh/kg-1. This is particularly interesting because this value roughly matches the values for aqueous fuels. As a result, rechargeable Li-O2 batteries are most likely to be used for the drive unit in future generations of electric vehicles.
The research and development projects are aimed at enabling a fundamental understanding of the Li-O2 systems. One of the biggest challenges is to select a suitable electrolyte that enables multiple cycling of the Li-O2 battery.
Dr. Mario Marinaro
Telefon: +49 (0)731 95 30 406
As asymmetric double-layer capacitors, powercaps bridge the gap between batteries and symmetric double-layer capacitors (“supercapacitors, supercaps”). As a result, high performance densities up to 5 kW kg-1 with high energy densities in a range from 20 to 100 Wh kg-1 can be achieved. While the performance densities of the powercaps are comparable with those of double-layer capacitors, their energy densities are clearly higher than those of supercapacitors.
This astounding performance capacity is possible due to the combination of fast redox reactions, as seen, for example, on cathodes in nickel metal hydride (NiMH) cells, with the equally fast adsorption processes on activated carbon. The cell is completed by an aqueous electrolyte with very high performance capacities that also reduces the risk of fire. The powercaps’ high level of safety means that the cost of external electronic monitoring is also greatly reduced.
As well as investigating industrial electrodes as the benchmark, innovative positive and negative active materials for powercaps are also being developed. Particular attention is paid to the high performance capacity of the newly produced materials. In order to further increase the performance capacity of the powercaps, bipolar cells with very low internal resistance are being developed. External cell connections are not required in bipolar cells because there is an internal series connection with a large exchange surface. In the course of our research, prototypes for 12V, 24V and 48V voltage levels have been developed.
The cost and availability of raw materials are important driving forces in the development of new storage materials. They also explain the rapidly growing interest in the development of sodium-ion batteries. Sodium-ion batteries work according to the same principle as lithium-ion batteries, but the charge transport within the cells is performed by sodium ions instead of lithium ions.
Whereas lithium deposits are mined in geopolitically unstable regions and lithium represents a significant portion of the material cost, sodium is ubiquitous and available in virtually infinite quantities. In contrast to lithium, sodium does not alloy with aluminium. Therefore replacing the critical copper with aluminium should lead to significant cost savings at the cell level and to a reduction of the cell weight.
ZSW is currently already working on the first projects to develop cathode materials for sodium-ion batteries.
Dr. Peter Axmann
Telefon: +49 (0)731 95 30 404
Redox-flow batteries are a cost-effective option for balancing the power supply from renewable energy sources and the electricity demand. For many years, ZSW has been researching cell technology and the operation mode of redox flow accumulators. Our focus has been on the cell and battery design as well as on the assessment of new redox systems. In this regard, the combination of reversible hydrogen electrodes with redox electrodes in the aqueous phase is of particular interest because it presents particular challenges for the design of the electrode and the reactant distribution.