Volte­ri­on and Fraun­hofer UMSICHT: Step towards mass mar­ket for redox flow batteries

Sun and wind are not ori­ent­ed to our elec­tric­i­ty needs — regen­er­a­tive ener­gy must there­fore be stored until it is con­sumed, for exam­ple in redox flow bat­ter­ies. These are both cycle-sta­ble — their capac­i­ty does not decrease notice­ably even after thou­sands of cycles — and non-flam­ma­ble, and their per­for­mance and capac­i­ty can be tai­lored to require­ments. In addi­tion, they do not require crit­i­cal mate­ri­als and their elec­trolytes can be ful­ly recov­ered. But: Until now, they were sim­ply too expen­sive for the mass market.

Cost effec­tive, light­weight and compact

Researchers at Fraun­hofer UMSICHT have now been able to find a sus­tain­able solu­tion to this prob­lem: They have rein­vent­ed the man­u­fac­tur­ing method of the cen­tral elec­tri­cal­ly con­duc­tive plas­tic so that it remains flex­i­ble and can be weld­ed. This new process has sig­nif­i­cant impli­ca­tions for redox flow bat­ter­ies. “The devel­oped stack, the heart of every redox flow bat­tery, is 40 per­cent cheap­er in terms of mate­r­i­al costs, and pro­duc­tion costs have also been sig­nif­i­cant­ly reduced. The stack weighs 80 per­cent less than a con­ven­tion­al stack and is only about half the size,” sum­ma­rizes Prof. Chris­t­ian Doetsch. The stack is mar­ket­ed by the spin-off Volte­ri­on. For this devel­op­ment, Chris­t­ian Doetsch and Lukas Kopi­etz from Fraun­hofer UMSICHT as well as Dr. Thorsten Seipp from Volte­ri­on GmbH & Co. KG receive the Joseph von Fraun­hofer Prize. The jury jus­ti­fied its deci­sion among oth­er things with “the spin-off and the suc­cess­ful exit of Fraun­hofer, which pro­to­typ­i­cal­ly show the way of mar­ket­ing new man­u­fac­tur­ing technologies”.

Typ­i­cal­ly, the stacks con­sist of 160 stacked com­po­nents held togeth­er with a vari­ety of screws and sol­id met­al plates and sealed with numer­ous gas­kets. Some of these com­po­nents are injec­tion mold­ed and are as brit­tle as pen­cil lead due to the high pres­sures and tem­per­a­tures required for injec­tion mold­ing. To get around this, the research team used sim­i­lar start­ing mate­ri­als, i.e. graphites and car­bon blacks, but approached the process in a dif­fer­ent way: pel­let-shaped plas­tic is cooled to as low as minus 80 degrees, ground into pow­der and mixed with 80 per­cent graphite by weight. The research team sends the result­ing pow­der through a sys­tem of sev­er­al rollers at dif­fer­ent tem­per­a­tures and speeds. Between the rollers, the pow­der is briefly melt­ed at mod­er­ate tem­per­a­tures and low pres­sures, knead­ed, rolled into an “end­less sheet” and final­ly rolled up. “The new mate­r­i­al acquires ther­mo­plas­tic prop­er­ties in the process, so it is pli­able and weld­able, even though the plas­tic only accounts for 20 per­cent of the mate­r­i­al,” explains Lukas Kopi­etz. This means that the stack does not require a sin­gle seal, and screws are also super­flu­ous — the cells are sim­ply weld­ed togeth­er. Anoth­er advan­tage is that this method not only allows bipo­lar plates to be pro­duced much faster and thus more cost-effec­tive­ly, but there are also no longer any size lim­its. Bipo­lar plates with up to sev­er­al square meters are pos­si­ble with­out any problems.

Imple­ment­ed in Volte­ri­on GmbH & Co KG up to the battery

The sec­ond deci­sive step, because it reduces costs, was the devel­op­ment of a con­tin­u­ous pro­duc­tion process: the pow­der-to-roll process, in which the bipo­lar plates can be pro­duced as a con­tin­u­ous roll. Very thin sheets can be pro­duced in this way. Where­as the sheet thick­ness in injec­tion mould­ing is lim­it­ed to sev­er­al mil­lime­tres for pro­duc­tion rea­sons, it can be between 0.1 and 0.4 mil­lime­tres thin in the pow­der-to-roll process. This means that sig­nif­i­cant­ly less mate­r­i­al is required, which in turn low­ers the price and enables lighter, more com­pact stacks. “All of this pro­vides com­plete­ly new pos­si­bil­i­ties in design, which we have imple­ment­ed at Volte­ri­on GmbH & Co. KG, right up to the entire bat­tery,” says Thorsten Seipp. In the mean­time, Volte­ri­on has already built and sold over a thou­sand stacks.