Currently, there are many research activities worldwide for developing new materials and alternative manufacturing processes in order to improve the performance of lithium-ion batteries, especially for mobile devices and electromobility. Important aspects for prospective technologies are resource conservation, environmental protection and safety. In addition, batteries should be produced sustainably and cost-effectively with large throughputs.
An anode material that is currently being investigated is porous silicon layers. Replacing the currently used graphite materials by silicon 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.
Dr. Stefan Saager from Fraunhofer FEP explains the innovation: "We have developed a process in which silicon and zinc are simultaneously deposited on metal substrates. 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 encouraging battery performance, which is demonstrated by 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 IWS, the electrochemical properties of the porous coatings were characterized.