Photo: ZSW

Go-ahead for light­weight and safe lithi­um-ion bat­ter­ies for elec­tric flying

Mobil­i­ty and thus also air traf­fic cause a large CO2 foot­print. In order to be able to reduce this sig­nif­i­cant­ly in the future, light­weight, safe and cost-effec­tive high-ener­gy bat­ter­ies are need­ed. And these should, as far as pos­si­ble, only con­tain mate­ri­als that con­serve resources and are envi­ron­men­tal­ly com­pat­i­ble. A research project of the Cen­ter for Solar Ener­gy and Hydro­gen Research Baden-Würt­tem­berg (ZSW) with three indus­tri­al com­pa­nies starts exact­ly here. The project will devel­op new active mate­ri­als with high spe­cif­ic ener­gy and safe­ty, as well as the process­es to turn them into bat­tery elec­trodes. Crit­i­cal and expen­sive mate­ri­als are to be sub­sti­tut­ed. Both anode and cath­ode are processed aque­ous instead of with the harm­ful sol­vent NMP. The knowl­edge gained is to be used for the pro­duc­tion of round cells suit­able for indus­tri­al use. The project start­ed at the end of 2021.

In addi­tion to ZSW, the mate­r­i­al man­u­fac­tur­er John­son Matthey Bat­tery Mate­ri­als GmbH, the mechan­i­cal engi­neer­ing com­pa­ny Cope­ri­on GmbH and the cell man­u­fac­tur­er VARTA AG are also involved. VARTA is coor­di­nat­ing the project, which is being fund­ed by the Ger­man Fed­er­al Min­istry of Edu­ca­tion and Research (BMBF) with 1.6 mil­lion euros. The research project will run for three years until Octo­ber 31, 2024.

Today, com­mer­cial high-ener­gy cells still con­tain sig­nif­i­cant amounts of the expen­sive and car­cino­genic met­al cobalt, which is also some­times mined under pre­car­i­ous con­di­tions. “An impor­tant project goal will be to use cobalt-free cath­ode mate­ri­als in future bat­ter­ies by devel­op­ing suit­able process con­di­tions,” says Prof. Dr. Markus Höl­zle, ZSW board mem­ber and head of the Elec­tro­chem­i­cal Ener­gy Tech­nolo­gies busi­ness unit in Ulm. “On the anode side, the use of sil­i­con oxide is expect­ed to sig­nif­i­cant­ly increase the ener­gy con­tent. Anoth­er new fea­ture is that the pro­duc­tion of both elec­trodes is water-based, i.e. with­out the use of the tox­ic sol­vents that are com­mon today.”

Envi­ron­men­tal­ly friend­ly mate­ri­als with more spe­cif­ic energy

The con­sor­tium aims to increase the spe­cif­ic ener­gy of the bat­tery cells by up to 20 per­cent — com­pared to an already avail­able sus­tain­able cell chem­istry. This would sig­nif­i­cant­ly reduce the weight of the bat­ter­ies for the same ener­gy con­tent. This is to be achieved through four mea­sures. First, the use of intrin­si­cal­ly safe lithi­um man­ganese iron phos­phate (LMFP) in place of the estab­lished lithi­um iron phos­phate (LFP) as the cath­ode mate­r­i­al. Both mate­ri­als are free of the crit­i­cal raw mate­ri­als nick­el and cobalt, but LMFP con­tains more ener­gy than LFP. The sec­ond mea­sure is derived from this: The part­ners want to increase the sur­face capac­i­ty by 40 per­cent com­pared to an LFP cathode.

Third, on the anode side, graphite, which is becom­ing increas­ing­ly scarce and expen­sive, is to be replaced by the abun­dant­ly avail­able sil­i­con oxide (SiOx). Because the ener­gy con­tent of SiOx is sig­nif­i­cant­ly high­er than that of graphite, this can save weight and vol­ume of the batteries.
The fourth mea­sure is to sub­sti­tute water for the haz­ardous sol­vent NMP, which has been used almost exclu­sive­ly to coat the electrodes.

Devel­op low-cost cell pro­duc­tion suit­able for indus­tri­al use

All improve­ments are to be trans­lat­ed into process­es suit­able for indus­tri­al use in order to be able to build high-per­for­mance and safe bat­ter­ies on a pilot scale. To do this, the process require­ments for very high capac­i­ty elec­trodes — anode and cath­ode — dur­ing mix­ing, coat­ing, dry­ing and cal­en­der­ing must be inves­ti­gat­ed and under­stood. To this end, the sci­en­tists and engi­neers are research­ing, among oth­er things, the process tech­nol­o­gy of extru­sion as an inno­v­a­tive, high­ly effi­cient way of con­tin­u­ous pro­cess­ing. Based on the find­ings, it should ulti­mate­ly be pos­si­ble to man­u­fac­ture thick water-based elec­trodes in an indus­try-rel­e­vant roll-to-roll process and wind them for use in round cells.

At the ZSW in Ulm, the process­es are being test­ed on a ded­i­cat­ed pilot line and val­i­dat­ed in small lab­o­ra­to­ry bat­ter­ies as a demon­stra­tor. In par­al­lel, VARTA is intro­duc­ing the results of the project into the pro­duc­tion of wound but­ton cells and 21700 round cells.

The bat­ter­ies devel­oped in this way can be used in numer­ous appli­ca­tions. An impor­tant appli­ca­tion in mobil­i­ty is avi­a­tion. Bat­tery-pow­ered air­craft are cur­rent­ly being devel­oped by a num­ber of com­pa­nies, and high ener­gy con­tent com­bined with max­i­mum safe­ty are key fac­tors in the suc­cess of the high-per­for­mance bat­ter­ies required for this purpose.