Rechargeable batteries - Redox-flow" accumulators

Like all electrochemical generators, a redox-flow accumulator is the site of an oxidation reaction and a reduction reaction at each electrode. Its specificity lies in the fact that the reactants and reaction products are in solution in an ad hoc electrolyte, both at the anode and cathode. A redox-flow accumulator therefore comprises two half-cells, each containing one of these electrolytes. These electrolytes are circulated. It is because of this electrolyte circulation that the batteries studied in this document are called "redox-flow".

The electrolytes are stored in tanks and circulated to the half-cells, where the reaction processes take place. The continuity of the electrical circuit requires that the two half-cells be separated by a semi-permeable wall allowing the passage of an ion common to both electrolytes. A redox-flow accumulator comprises an electrochemical reactor, made up of half-cells, an electrolyte circulation device and storage compartments (reservoirs).

The capacity of a redox-flow accumulator is linked to the size of the tanks, while its power is linked to that of the reactor. Decoupling these two parameters is an advantage. The ability to control capacity means that this type of accumulator can be used for massive storage in energy networks. To do this, it is important to have sufficiently large reservoirs, while independence from power is conditioned by the intended use.

In a rechargeable battery, ageing or degradation processes generally affect the solid parts. These can be electrodes (e.g., shedding and softening in lead-acid batteries) or structural components (e.g., cracks in the β-alumina tube of high-temperature batteries). The reagents and products of the redox-flow accumulator, in the liquid phase, are immune to these problems, which potentially gives this accumulator a long service life.