Researchers develop dual-ion battery
Scientists from the MEET battery research centre at the University of Münster are working on a new energy storage system which is based on dual-ion battery technology. This system is to be used in stationary plants for the intermediate storage of renewable energies and to stabilise the electricity grid.
Under the project name “INSIDER”, scientists are working on an alternative to the lithium-ion battery. INSIDER stands for “Anionen-Interkalation basierende Dual-Ionen-Energiespeicher” (dual-ion energy storage based on anion intercalation).The name gives a first indication of the storage principle: In contrast to the lithium-ion battery, in which only one type of ions, namely lithium ions, are involved in the storage process, in the dual-ion battery, the electrolyte anions also participate in the energy storage mechanism. While in the lithium-ion cell, the lithium ions between the anode and the cathode are pushed back and forth by the charging and discharging processes, there is a fundamental difference in the dual-ion cell’s charging and discharging mechanism. During the charging process, the lithium ions are deposited on the negative electrode, and the electrolyte anions, such as bis(trifluoromethanesulfonyl)imide (TFSI-) for example, are deposited on the positive graphite electrode. During the discharging process both ion types are returned to the electrolyte. The crucial difference between the battery types is the function of the electrolyte, which, in the case of the lithium cells, only acts as a transport medium for the lithium ions between both electrodes, whereas in the case of the dual-ion cells, it can be seen as an active material. If metallic lithium is used as an anode material, discharging currents of up to 5.3 volts are possible. By comparison: lithium-ion batteries have 3.6 volts.
The overall goal of the project is to find new, cost-efficient and eco-friendly materials for the dual-ion technology and, at the same time, to analyse processing opportunities in the cell manufacturing process. In this way, scientists want to ensure it is possible to quickly introduce the new technology and the new material concepts to industrial practice.
Properties of dual-ion technology
Dual-ion batteries have some advantages over lithium-ion batteries. They allow us to avoid using environmentally damaging, expensive and heavy transition metals, such as nickel, cobalt and manganese. Instead, the cathodes are made of graphites. However, scientists are still conducting basic research. The laboratory cells currently have approximately half the energy density of lithium-ion batteries. Dr Hinrich-Wilhelm Meyer, project manager of the MEET battery research centre is confident, “We are continuously testing alternative electrolyte formulations. One of the systems we are currently examining already achieves the energy density of lithium-ion systems.”
Dr Meyer sees stationary mass storage facilities for stabilising supply networks as primary areas of use, but the batteries also have interesting properties for electric vehicle applications. He answers the question as to whether batteries for devices (“power tools”) are possible, carefully, saying that these are indeed feasible, but researchers are not yet focussing on such applications.
Dr Meyer highlights one particular advantage of the technology: its high cycle durability, “Our dual-ion systems are the only batteries that still have 99 per cent of their original capacity after 500 charge/discharge cycles.” The storage capacity of batteries decreases with each charging cycle. In the mobile field, batteries are usually replaced when their storage capacity has fallen to 80 per cent of their original capacity. For commonly used systems, this is often the case after 500 charge cycles.
Self-discharge is slightly higher than in lithium-ion batteries, but significantly lower than in lead acid batteries. It is, however, negligible for its intended purpose of going through high numbers of cycles in a short time. Dual-ion batteries show good stress-test values for fast discharging, but they are inferior to lithium-ion batteries.
“Our goal is an inexpensive and, at the same time, eco-friendly system with a high degree of efficiency,” explains Dr Meyer. Expensive transition metals are to be replaced with comparatively inexpensive graphite. The electrolytes used are still expensive since they are special chemicals, but the price is falling to the usual level for mass production. Dr Meyer is confident that, overall, dual-ion batteries would be less expensive than lithium-ion batteries. Various industrial companies are already showing interest.
Scientists from the MEET battery research centre - Institute for Physical Chemistry and the Institute of Inorganic and Analytical Chemistry at the University of Münster will be working on the project until 2016. In addition, working groups from the FZ Jülich, the FAU Erlangen and the TU Braunschweig are also involved in the project. The INSIDER project receives 5.5 million euros from the German Federal Ministry of Education and Research as part of its funding initiative.