Safe assembly of battery cells

The bat­tery is the core of every elec­tric car.  It should be com­pact and as pow­er­ful as pos­si­ble — and above all safe. This places great demands on pro­duc­tion. Researchers from Fraun­hofer IPA are demon­strat­ing what this could look like in the future at the Cen­ter for Dig­i­tized Bat­tery Cell Pro­duc­tion (ZDB).

A mod­ern bat­tery sys­tem cou­ples sev­er­al bat­tery mod­ules in which a large num­ber of bat­tery cells are installed. Depend­ing on the man­u­fac­tur­er, these cells have a dif­fer­ent for­mat. The pilot plant in the ZDB at the Fraun­hofer Insti­tute for Man­u­fac­tur­ing Engi­neer­ing and Automa­tion IPA in Stuttgart is designed for cylin­dri­cal cell formats.

The elec­trodes are locat­ed inside a bat­tery cell. They con­sist of wafer-thin coat­ed foils that are rolled up into a coil togeth­er with a sep­a­ra­tor. Even a small defect or a speck of dust that gets inside can sig­nif­i­cant­ly weak­en the per­for­mance or even lead to a short cir­cuit and thus to a fire. At Fraun­hofer IPA, there­fore, a lab­o­ra­to­ry with spe­cial clean and dry room con­di­tions was cre­at­ed, sup­port­ed by fund­ing from the state of Baden-Würt­tem­berg. It is equipped with plant tech­nol­o­gy that enables com­plete assem­bly of bat­tery cells.

What makes it spe­cial is the dig­i­ti­za­tion and net­work­ing of all process steps. This pro­vides the researchers with a pro­duc­tion line that is unique in Europe, which they can use to sup­port both poten­tial cell man­u­fac­tur­ers and machine and plant builders in the devel­op­ment and automa­tion of process­es and the opti­miza­tion of assem­bly in terms of reli­a­bil­i­ty and through­put. The spec­trum ranges from the analy­sis and inves­ti­ga­tion of crit­i­cal process steps to the appli­ca­tion of dig­i­tal tools and pro­to­type construction.

Process monitoring for an optimal solution

Around a dozen steps are required before a cell is ready for use, and each of these steps is cru­cial for its qual­i­ty and for that of the entire bat­tery sys­tem. The first step is the coat­ing of the pos­i­tive and neg­a­tive elec­trodes, which are then rolled up togeth­er with a sep­a­ra­tor to form a coil, the so-called “jel­ly roll”. This is fol­lowed by assem­bly. For this pur­pose, the jel­ly roll must be guid­ed with high pre­ci­sion and, if pos­si­ble, with­out touch­ing the cup wall. The roll is then weld­ed to the base of the buck­et by means of a rod elec­trode insert­ed through the cen­tral hole in the roll. To pre­vent the wrap from shift­ing or com­ing loose, a bead of a spe­cif­ic shape and depth, an annu­lar depres­sion, is incor­po­rat­ed at a defined point.

The next step, the fill­ing of the liq­uid elec­trolyte, is par­tic­u­lar­ly del­i­cate and requires an envi­ron­ment with­out oxy­gen and with as lit­tle humid­i­ty as pos­si­ble. The nec­es­sary equip­ment is there­fore locat­ed in a her­met­i­cal­ly sealed box, a so-called “glove box,” in which it is pos­si­ble to han­dle from the out­side with gloves worn inside.

“In the process, which takes place under an argon atmos­phere, a defined quan­ti­ty of the liq­uid elec­trolyte must be filled in with high pre­ci­sion and with­out over­flow­ing, because this has an influ­ence on the per­for­mance and ser­vice life of the cell.”

- Matthias Burgard

This is all the more dif­fi­cult because the liq­uid seeps in only slow­ly because the pore spaces are nar­row. Final­ly, a lid ele­ment with a defined join­ing force is insert­ed, which is fixed by reshap­ing the edge of the cup, thus seal­ing the cell. Due to the desired reli­a­bil­i­ty of the process­es, there should be no safe­ty-crit­i­cal elec­trolyte on the out­side, but the assem­bled cell is still cleaned before com­ple­tion. Final­ly, the cell is wrapped in a pro­tec­tive tube and labeled.

Knowledge from production data converging in the cloud — new knowledge is created

In order to min­i­mize waste and increase qual­i­ty, researchers led by Flo­ri­an Maier and Ozan Yesi­lyurt from Fraun­hofer IPA have dig­i­tized and net­worked the entire pro­duc­tion process. In a sense, pro­duc­tion is becom­ing trans­par­ent. To this end, numer­ous sen­sors on all devices col­lect data that con­verge in the cloud in real time. Trace­abil­i­ty tech­nolo­gies devel­oped at Fraun­hofer IPA enable the col­lect­ed data to be assigned to the bat­tery cells pro­duced. The high­light: Each indi­vid­ual bat­tery cell that is pro­duced is avail­able as a dig­i­tal twin for data analy­sis and train­ing of an arti­fi­cial intel­li­gence. This makes it pos­si­ble to trace the con­di­tions under which it was man­u­fac­tured and how it relates to the prod­uct qual­i­ty achieved. Researchers like Soumya Singh are orches­trat­ing this data and using it to devel­op ser­vices with mon­i­tor­ing, analy­sis and pre­dic­tion capa­bil­i­ties. This makes it pos­si­ble to con­tin­u­al­ly opti­mize the pro­duc­tion process and elim­i­nate sources of error more quick­ly than before.

In addi­tion, the data col­lect­ed in pro­duc­tion also helps to devel­op bet­ter pre­dic­tive mod­els for the aging behav­ior of bat­tery cells dur­ing use, to eval­u­ate fur­ther pos­si­ble uses for used bat­tery cells and to improve the effi­cien­cy of recy­cling process­es. The sci­en­tists involved are cur­rent­ly sum­ma­riz­ing fur­ther approach­es to dig­i­tiz­ing and data-dri­ven opti­miza­tion of bat­tery pro­duc­tion in the book “Hand­book on Smart Bat­tery Cell Man­u­fac­tur­ing”. It is due to be pub­lished lat­er this year.