Fraunhofer’s silicon battery result puts the pack under pressure
Image: © Fraunhofer ISE
Fraunhofer ISE has reported more than 1,000 full cycles from silicon-rich 25 Ah pouch cells, followed by a residual-capacity measurement above 83%. The cells achieved 280 Wh/kg. They were also operated under 1 MPa of axial pressure.
That last condition brings a substantial mechanical assembly into the picture. Applied uniformly over an illustrative 100 cm² cell face, 1 MPa requires 10 kN of force. That is a calculation to show the scale, not a published dimension of Fraunhofer’s cell. Even so, it makes clear why the compression arrangement belongs in the discussion of battery performance.
Fraunhofer compared 300 and 1,000 kPa during cycling. The cells were charged and discharged at 1C, with periodic capacity checks at lower current. At the lower pressure, usable capacity declined more severely, particularly during regular 1C operation.
Retained capacity and available capacity are diverging
The institute observed a growing difference between capacity measured during those gentler check-ups and the capacity delivered at 1C. That is the most revealing qualification in the result. A cell can retain electrochemical capacity while becoming less able to deliver it at the rate the application demands.
Quoting 83% retention without the measurement conditions would conceal that distinction. The pressure result is valuable precisely because it exposes how strongly the usable performance depends on the assembled cell’s operating state.
“Mechanical cell pressure is therefore an important parameter for the stable operation of silicon-rich pouch cells,” said Marc Kissling, Fraunhofer ISE’s group leader for battery cell production and technology evaluation.
There is no universal pressure setting to extract from this work. Earlier research in Electrochimica Acta, using pure-silicon anodes and an LTO positive electrode, found that greater pressure could increase ionic pore resistance. That chemistry differs from Fraunhofer’s, but the finding illustrates why simply applying more compression is an incomplete answer.
The StrOboBatt programme extends from active materials through electrode processing to pouch-cell manufacture. Its objectives include reducing dependence on graphite and developing processes that can move along an industrial production chain. Testing multilayer cells brings some of the mechanical complications of that scale into the experiment.
The remaining question is how much of the cell’s energy-density advantage survives the pack. Compression plates, restraints and enclosure mass all enter that calculation, while the structure must maintain useful pressure as cell dimensions and temperature change. A strong cell result can therefore coexist with a demanding packaging problem. Fraunhofer’s data gives that problem a measurable starting point.
