How do plastic batteries differ fundamentally from conventional lithium-ion systems in terms of structure and function?
Esser: Lithium-ion batteries have two electrode materials that alternately absorb lithium ions from the electrolyte along with the electrons. The lithium ions move back and forth between the positive and negative electrodes between charging and discharging, which is where the battery gets its name. There are various possibilities for batteries in which a plastic replaces one or both of the electrode materials: After absorbing electrons, the plastic can also absorb a metal ion (e.g. a lithium ion) from the electrolyte, in which case it would be a so-called n-type material. Or the plastic can release an electron (thereby being oxidised) and absorb an anion from the electrolyte to compensate for the charge. This would be the case with a p-type material. This second class of material is very common in organic electrode materials but is rarely found in inorganic materials. One example from inorganic materials is graphite. In the development of a battery cell, both p-type and n-type organic electrode materials in the form of plastics can now be used in principle to obtain different configurations with regard to the movement of the ions in the battery.
With plastics as electrode materials, it is also possible to replace the lithium ions with other metal ions that have a more favourable geographical availability. Here we are working intensively on multivalent metal batteries based on aluminium or magnesium instead of lithium, among other things in the POLiS Cluster of Excellence at the University of Ulm.
What criteria must polymer materials fulfil in order to be suitable for batteries?
Esser: The most important criterion is redox activity, i.e., the material must be able to reversibly accept (n-type material) or release (p-type material) electrons. This requires redox-active groups in the material, e.g., conjugated quinones or aromatic heterocycles. Some of these are units that also occur in many natural substances.
A second important criterion is that the material is insoluble in the battery's liquid electrolyte. Battery electrolytes are very polar solvents that must dissolve the conductive salt (e.g., a lithium salt) required in the battery. However, they must not dissolve the organic electrode material, otherwise the battery would short-circuit. For this reason, the redox-active groups mentioned above are incorporated into a polymer, which is often also cross-linked. This results in a linked network of the redox polymer, which remains immobilised in the electrode. This type of cross-linking is also used in conventional plastics.
Other criteria are a redox potential that leads to an attractive cell voltage depending on the configuration of the battery. The desired cell voltage depends on the application of the battery. Organic electrode materials with a high redox potential are often desirable if, for example, a metal electrode is used as the opposite pole.
How do you manage to specifically adapt the properties of plastics?
Esser: Organic synthesis chemistry offers a wide range of possibilities here. Once a redox-active group has been identified that can be reversibly reduced or oxidised, the redox potential can be adjusted by chemical modification, e.g., the addition of substituents. This group can also be incorporated into a polymer structure by chemical modification in order to achieve insolubility in the battery electrolyte. Sometimes it is also the case that decomposition processes of the electrode material during its use in the battery are analysed in detail and provide information about sensitive positions in the organic molecular structure. These positions can then be modified by changing the molecular structure, and this can lead to a more stable, cyclisable electrode material.
Where do you currently see the biggest areas of application for plastic batteries?
Esser: On the one hand, there is the area of stationary energy storage, where a large increase in demand is expected in the coming decades. The criteria here include low costs, no toxicity, high safety, and longevity, and I see a lot of potential for organic electrode materials, e.g. also in combination with ‘post-lithium’ metals. On the other hand, plastics offer unique properties in terms of their processability and mechanical flexibility. They are therefore also excellent candidates for thin, bendable and printable batteries for applications with low-capacity requirements but strict specifications for the cell design of the battery.
