Enhanced specific energy for Li-S-cells through new cathode concept

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Lithium-Sulfur technology offers an outstanding specific energy due to the high specific capacity of sulfur as a conversion material. However, the weight of prototype cells is still dominated by its inactive components. In particular cathode current collectors (typically 12 – 18 µm thick Aluminum foils) add about 10-15 % to the cell mass.

 

            

 

Within the ECLIPSE project Fraunhofer IWS introduced an innovative, new approach to reduce the weight of the current collector through laser perforation. Circle shaped holes of up to 10 mm in diameter lead to weight reduction of up to 73 %.

Improved production process

This concept has been combined with freestanding electrode films produced by an environmental friendly solvent-free process. Those electrode films consist of sulfur/carbon nanocomposites and a binder. Through a dry mixing process the binder forms nanofibrils enclosing the nanocomposite particles. In a roll-pressing step this material is transferred into freestanding electrode films and subsequently laminated onto (perforated) current collectors.

 

            

 

This dry film process offers several advantages over state-of-the-art wet electrode coating procedures.

  1. Reduced costs through abandonment of solvent and drying steps
  2. High active material loadings and utilization 
  3. Flexibility in current collector design

Contacting the freestanding film with a perforated current collector requires a high electrical conductivity of the electrode layer which is achieved by the nanostructured carbons. The cathode concept has been evaluated on pouch cell level. First results reveal high sulfur utilization (> 60 %) for an electrode with 2,3 mg S / cm2 and a perforated aluminum foil with 73 % weight reduction (about 11 % weight reduction on stack level). No difference in rate capability is observed when compared to non-perforated Al current collectors.

The dry film concept has been successfully demonstrated also for electrode processing in other cell chemistries and potentially offers significant cost reduction in up-scaled cell production. The prototype equipment at Fraunhofer IWS allows small scale processing of electrode sheets for feasibility studies up to continuous powder-to-roll process development.