Silicon-rich battery cells passed 1,000 full cycles at Fraunhofer ISE

What they found

In the StrOboBatt project (“Structure- and Surface-Optimized Silicon Anodes and High-Energy Cathodes for High-Energy-Density Lithium-Ion Batteries”), Fraunhofer ISE in Freiburg built 40-layer, 25 Ah pouch cells on a semi-automatic line in the cleanroom of its Center for Electrical Energy Storage. They used commercially available electrode foils: a silicon-carbon/graphite anode and a nickel-rich NMC (nickel-manganese-cobalt oxide) cathode. The silicon-carbon composite made up 51% of the anode active material; the anodes reached more than 1,000 mAh/g and more than 5 mAh/cm² [1].

Silicon has a theoretical specific capacity ten times that of graphite, but silicon-rich cells have been held back by short cycle life. The industry target is more than 1,000 cycles with at least 80% capacity retention. These cells passed more than 1,000 full cycles at 1C and kept over 83% of their original capacity at 280 Wh/kg in Fraunhofer's PrecISE research cell format; about 330 Wh/kg is expected in an industrially optimised pouch format [1].

Here's the twist: the team tested two axial contact pressures, 300 and 1,000 kPa (1,000 kPa is about ten times normal atmospheric pressure of roughly 101 kPa). At 1,000 kPa the cells kept over 83% after more than 1,000 cycles; at 300 kPa the usable capacity fell significantly more, especially at 1C. They also found a growing gap between the capacity measured in slow check-up cycles and the capacity actually available at 1C, so real-world operating conditions matter when judging silicon cells [1].

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About this page

The cell photos in the Short are official Fraunhofer ISE press images, shown unaltered (cropping/scaling only) and credited © Fraunhofer ISE; other photos are illustrative, from Wikimedia Commons. The “2.7 years” comparison is my own arithmetic (1,000 daily charges), and the “ten times air pressure” comparison is 1,000 kPa versus about 101 kPa.