Bild: Chalmers University of Technology

Big break­through for “mass­less” ener­gy stor­age systems

Researchers at Chalmers Uni­ver­si­ty of Tech­nol­o­gy have pro­duced a struc­tur­al bat­tery that works ten times bet­ter than any pre­vi­ous ver­sion. It con­tains car­bon fibers that serve simul­ta­ne­ous­ly as an elec­trode, con­duc­tor and load-bear­ing mate­r­i­al. Their lat­est research break­through paves the way for essen­tial­ly “mass­less” ener­gy stor­age in vehi­cles and oth­er technologies.

The bat­ter­ies in today’s elec­tric cars make up a large part of the vehi­cle’s weight with­out per­form­ing a load-bear­ing func­tion. A struc­tur­al bat­tery, on the oth­er hand, is a bat­tery that works both as a pow­er source and as part of the struc­ture — for exam­ple, in a car body. This is called “mass­less” ener­gy stor­age because the weight of the bat­tery essen­tial­ly dis­ap­pears when it becomes part of the sup­port­ing struc­ture. Cal­cu­la­tions show that this type of mul­ti­func­tion­al bat­tery could great­ly reduce the weight of an elec­tric vehicle.

The devel­op­ment of struc­tur­al bat­ter­ies at Chalmers Uni­ver­si­ty of Tech­nol­o­gy is based on years of research, includ­ing ear­li­er dis­cov­er­ies with cer­tain types of car­bon fibres. These are not only stiff and strong, but also have a good abil­i­ty to chem­i­cal­ly store elec­tri­cal ener­gy. This work was named one of the top ten sci­en­tif­ic break­throughs of 2018 by Physics World.

The first attempt to make a struc­tur­al bat­tery was made back in 2007, but it has so far proved dif­fi­cult to pro­duce bat­ter­ies with both good elec­tri­cal and mechan­i­cal properties.

But now devel­op­ment has tak­en a real step for­ward: researchers at Chalmers, in col­lab­o­ra­tion with the Roy­al Insti­tute of Tech­nol­o­gy in Stock­holm (KTH), have unveiled a struc­tur­al bat­tery whose prop­er­ties in terms of elec­tri­cal ener­gy stor­age, stiff­ness and strength far exceed any­thing that has gone before. Their mul­ti­func­tion­al per­for­mance is ten times high­er than that of pre­vi­ous struc­tur­al bat­tery prototypes.

The bat­tery has an ener­gy den­si­ty of 24 Wh/kg, which is about 20 per­cent capac­i­ty com­pared to sim­i­lar lithi­um-ion bat­ter­ies cur­rent­ly avail­able. How­ev­er, because the weight can be great­ly reduced, less ener­gy is need­ed to pow­er an elec­tric car, for exam­ple, and the low­er ener­gy den­si­ty also leads to greater safe­ty. And with a stiff­ness of 25 GPa, the struc­tur­al bat­tery can actu­al­ly com­pete with many oth­er com­mon build­ing materials.

“Pre­vi­ous attempts to make struc­tur­al bat­ter­ies have result­ed in cells that have either good mechan­i­cal prop­er­ties or good elec­tri­cal prop­er­ties. But here we have suc­ceed­ed in using car­bon fibres to design a struc­tur­al bat­tery that has both com­pet­i­tive ener­gy stor­age capac­i­ty and stiff­ness,” explains Leif Asp, pro­fes­sor at Chalmers and leader of the project.

Superlight elec­tric bikes and con­sumer elec­tron­ics could soon be a reality

The new bat­tery has a neg­a­tive elec­trode made of car­bon fiber and a pos­i­tive elec­trode made of alu­minum foil coat­ed with lithi­um iron phos­phate. They are sep­a­rat­ed by a glass fibre fab­ric in an elec­trolyte matrix. Although the researchers have man­aged to cre­ate a struc­tur­al bat­tery that is ten times bet­ter than any pre­vi­ous, they did not choose the mate­ri­als to break records — rather, they want­ed to study and under­stand the effects of the mate­r­i­al archi­tec­ture and the thick­ness of the separator.

A new project is now under­way, fund­ed by the Swedish Space Agency, to increase the per­for­mance of the struc­tur­al bat­tery even fur­ther. The alu­minum foil is replaced by car­bon fiber as the sup­port­ing mate­r­i­al in the pos­i­tive elec­trode, increas­ing both stiff­ness and ener­gy den­si­ty. The glass-fibre sep­a­ra­tor is replaced by an ultra-thin vari­ant that enables a sig­nif­i­cant­ly greater effect — and also faster charg­ing cycles. The new project is expect­ed to be com­plet­ed with­in two years.

Leif Asp, who is also lead­ing this project, esti­mates that such a bat­tery could achieve an ener­gy den­si­ty of 75 Wh/kg and a stiff­ness of 75 GPa. This would make the bat­tery about as strong as alu­minum, but com­par­a­tive­ly much lighter in weight.

“The next-gen­er­a­tion struc­tur­al bat­tery has fan­tas­tic poten­tial. If you look at con­sumer tech­nol­o­gy, it could well be pos­si­ble with­in a few years to pro­duce smart­phones, lap­tops or elec­tric bicy­cles that weigh half as much as they do today and are much more com­pact,” says Leif Asp.

And in the longer term, it is quite con­ceiv­able that elec­tric cars, elec­tric air­craft and satel­lites will be designed and pow­ered by struc­tur­al batteries.

“We’re real­ly only lim­it­ed by our imag­i­na­tions here. We have received a lot of atten­tion from many dif­fer­ent com­pa­nies in con­nec­tion with the pub­li­ca­tion of our sci­en­tif­ic arti­cles in this field. There is under­stand­ably a lot of inter­est in these light­weight, mul­ti­func­tion­al mate­ri­als,” says Leif Asp.

Read the arti­cle in the jour­nal Advanced Ener­gy & Sus­tain­abil­i­ty Research: A Struc­tur­al Bat­tery and its Mul­ti­func­tion­al Performance