The researchers in the PoSiBat project developed a method for fabricating porous silicon anode materials in layers. Replacing current graphite by a silicon anode promises an almost tenfold increase in the specific charging capacity of the anode. However, the charging and discharging process leads to an enormous change in volume of the silicon and therefore to a rapid mechanical and electrochemical destruction of the material compound and thus to cell failure.
"We have developed a process in which silicon and zinc are simultaneously deposited on metal substrates," said Dr Stefan Saager from Fraunhofer FEP. "By applying a subsequent heat treatment, the zinc re-evaporates from the layer and pores are generated at the locations of former zinc grains. The porous structure in the silicon provides adequate space for its expansion during charging process and thus capacity fade is minimized. The porous structure can be manipulated and optimized to the specific battery requirements by adapting process parameters. The zinc can be collected and reused in the conceived process."
The porous silicon layers show an initial charging capacity of more than 3,000 mAh/gSi and a comparably good cycle stability.
The expertise of the Fraunhofer FEP lies in the coating of metal substrates and foils with zinc and silicon, which is possible with very high coating rates in conventional non-toxic vacuum processes. These processes enable high throughput and low manufacturing costs. At the Fraunhofer Institute for Material and Beam Technology IWS, the electrochemical properties of the porous coatings were characterized.
The development is aimed in particular at applications in vehicles where high energy densities and high currents are required of the battery. The project also developed a concept to scale up the new technologies to a production process. The scientists at the Fraunhofer FEP are now looking to collaborate with battery manufacturers to transfere the results into high-performance products.