Investigations on the improvement of the cycling behavior of lithium metal electrodes in combination with slurry-based sulfdic electrolyte layers

Sulfide solid electrolytes, such as Lithium Phosphorus Sulfide _-Li₃PS₄
(LPS), show promising results in all solid-state battery research due to higher lithium-ion conductivity compared to solid ceramic electrolytes. Current research is trying to cope with different challenges with sulfidic electrolytes, among them:

• The use of a lithium (Li) metal anode to increase energy density. Inhomogeneous deposition of the lithium (dendrite formation), strong side reactions, and contact loss between electrolyte layer and the electrode occur which severely limit the lifetime of the cells with lithium metal anodes and sulfide-based electrolytes.

• Slurry-based electrolyte layers for later commercial production. Oh et al. show that binders interrupt the Li⁺-ionic contacts between the solid electrolyte particles, which lower the ionic conductivity of the electrolyte layer.

There are different approaches on how to improve the cyclability on the side of the lithium electrode. In this poster, approaches that are suitable for commercial production are compared experimentally.

The experiments are carried out as follows:

The production of the electrolyte layers is slurry-based using Li3PS4 as electrolyte, hydrogenated nitrile-butadiene rubber (HNBR) as binder and p-xylene as solvent.

The slurry is coated on different metal foils as current collectors (stainless steel, nickel, aluminum).

For improvement of the Li+ contacts between the solid electrolyte particles, different solvate ionic liquids (SIL) are added to the slurry before coating on a stainless-steel current collector.

Further processing results in the following half-cell structure: Lithium – solid electrolyte layer – current collector. With this cell assembly platting and stripping tests are performed: Lithium is plated on the current collector, in the next step it is tried to deposit the same amount of charge back again (stripping). The different half cells are compared with regard to Coulombic efficiency (CE) and overpotentials.

The variation of the current collector shows the best results for stainless steel. It shows a stable course of the CE with an average efficiency of about 98%. Likewise, low overpotentials can be observed.

The stainless-steel current collector is combined with electrolyte layer containing different SILs. Tributylmethylphosphonium bis(fluorosulfonyl)imide has the best cycling results among the SILs used. A stable course of the CE with an average efficiency of about 95% and relatively low overpotentials can be observed.

However, in these investigations, there is no improvement by using SILs. A stainless-steel current collector shows the best cycling results for insitu lithium deposition during plating and stripping tests in half cells with a slurry based LPS electrolyte layer.