Intermediate storage of renewables
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.
Fundamental difference between charging and discharging
The development of the dual-ion battery system was the focus of the joint project. In contrast to lithium-ion batteries, not only are lithium ions intercalated into the anode but also anions of the electrolyte into the cathode. During the charging process, lithium ions are embedded in the negative electrode and electrolyte anions in the positive graphite electrode. When the cell is discharged, both ion types are released back into the electrolyte.
The key difference to lithium-ion batteries is the purpose of the electrolyte, and the chemist explains further that, “In the lithium cell, it serves only as a transport medium for lithium ions between both electrolytes. In the dual-ion cell, however, it is an active material.”
Its high cycle durability is another advantage of the new battery system. On a laboratory scale, several thousand charge and discharge cycles with the dual-ion battery have already been achieved with a high capacity retention. On the other hand, self-discharge is slightly higher than that of lithium-ion batteries but much better than lead-acid batteries. The researchers measured good load values during rapid discharge, but the dual-ion battery is still inferior to the lithium-ion battery.
Market maturity and patent application
As part of the project, the researchers investigated materials for accumulators, electrolytes, active materials and functional layers as to their suitability for the dual-ion technology. They also assessed modification and functionalisation processes and evaluated the entire process chain of electrode manufacture. As part of the Insider project, various mechanistic comprehension questions relating to this relatively new technology were addressed. In potential follow-up projects, the technology could be evaluated integrally on a technical centre and pilot plant scale in order to realise a rapid market availability for the dual-ion battery. Several industrial companies have already shown interest. Although the employed electrolytes are still relatively expensive as special chemicals, as Dr. Tobias Placke admits, the price could be significantly reduced by means of further electrolyte optimisations under consideration of the cell chemistry.
The dual-ion energy storage system has a practically achievable specific gravimetric energy density of ≈ 40 to 70 Wh/kg and is particularly attractive for stationary storage system applications, which are essential for the effective intermediate storage of renewable energies. “We have successfully applied for innovative patents for the economical use of dual-ion batteries,” Dr. Tobias Placke proudly states.
Further information on the INSIDER project is available on this website.