How could more renewable energies be used that also stabilise the grid? To answer this question, researchers at the MEET Battery Research Centre at Münster University developed a new energy storage system based on dual-ion battery technology. Its long service life is promising for stationary applications.
For more than 25 years, expensive, metal-containing active materials such as cobalt, nickel and manganese have been required to make lithium-ion batteries. However, the investment costs of these materials are very high, making the storage technology’s application relatively unattractive for large-scale, stationary energy storage systems. Lithium-cobalt-manganese oxides are typically used for the cathodes that, overall, account for a very high portion of the costs of the lithium-ion cell.
The processing of the cathode materials into electrodes, typically involving organic, toxic solvents, is very cost-intensive since the solvents must be collected and recycled due to their high toxicity. In addition, there are safety concerns with regard to the toxicity of the sometimes used components of cobalt and nickel as well as the use of fluorinated binders to produce the electrodes. Experts have long been discussing the long-term availability of nickel and, in particular, cobalt.
Cost-effective battery systems for stationary applications need to be developed. This was a focus of researchers at the MEET Battery Research Centre at Münster University during the INSIDER project. “We have taken up the basic idea of an energy storage system known since the 1990s, the so-called dual carbon battery, and modified its system structure on the basis of optimised electrolyte and other material variations,” explains Dr. Tobias Placke, division head for materials research at the MEET Battery Research Centre.
Within the INSIDER research project, the innovative concept of dual-ion technology as stationary energy accumulator was successfully established for the first time and gradually developed further and refined according to the intended applications. In contrast to the lithium-ion technology, no transition metal oxides (cobalt, nickel, etc.) are used with this storage technology. Both the anode and the cathode of this accumulator can consist of graphitic carbons and form what is referred to as the dual graphite cell. These carbons can be produced from renewable raw materials, for example, by thermal treatment of biological materials or carbon-containing waste materials. In addition, electrode processing can dispense with the use of toxic organic solvents and fluorinated binders.
Instead, water and biological, e.g. cellulose-based, binders that can be found in yoghurt, for example, are used. In combination with the ionic liquids used as electrolytes, a high degree of safety and long-term stability of the cycle and a long service life of this storage technology can also be ensured. Consequently, this system can make a valuable contribution towards the protection of important resources, particularly in view of renewable energies. The excellent electro-chemical performance and high cycle stability of the system has already been disclosed in two patent applications and a number of publications.
The significant further development of this dual-ion battery technology is the optimisation of the electrolyte components, particularly the electrolyte salt anion, which results in considerably higher stability and safety and thus ensures a longer service life many times over. For this reason, the technology is particularly attractive for stationary energy storage for major industries.
Fundamental difference in charging and discharging
Development of the dual-ion battery system was the heart of the joint project. In contrast to lithium ion batteries, not only lithium ions are intercalated into the anode, but additionally, anodes of the electrolyte are intercalated into the cathode. During charging, the lithium ions are embedded into the negative electrode and the electrolyte anions into the positive graphite electrode. Upon discharge of the cell, both ion types are returned to the electrolyte.
The crucial difference between a lithium ion battery and this technology is the function of the electrolyte, the chemist continues to explain, “In the lithium cell, it only functions as transport medium for lithium ions between the two electrolytes. In the dual-ion cell, it functions as active material.”
The high cycle stability is another advantage of this new battery system. At laboratory scale, several thousands of charging and discharging cycles of the dual-ion batteries could be performed at high capacity maintenance. However, the rate of self-discharge is slightly higher than in lithium ion batteries but significantly improved over lead batteries. The scientists have measured satisfactory load values for self-discharge, however, they were still inferior to lithium ion batteries.
Within the scope of the project, the scientists have examined current collectors materials, electrolytes, active materials and functions layers for their suitability for dual-ion technology. They also assessed modification and functionalisation processes and evaluated the entire process chain of electrode manufacturing. Within the scope of the INSIDER project, various issues of mechanistic understanding of this relatively new technology could be clarified. In potential follow-up projects, integral evaluation of the technology on pilot scale can be aimed at to achieve market availability of dual-ion batteries in the near future. What is more, various industrial companies have expressed their interest in the technology. Placke admits that the electrolytes are special chemicals and thus still rather on the expensive side, however, further electrolyte optimisation in coordination with the cell chemistry will significantly reduce the prices.
The dual-ion energy storage system has a specific gravimetric energy density of ≈40 to 70 Wh/kg that can be practically reached and is particularly attractive for stationary storage applications that are mandatory for efficient intermediate storage of renewable energies. “In addition, we were able to apply for innovative patents for economic use of the dual-ion batteries,” Placke proudly reports.