Metal/Sulfur Composite Electroplated Cathodes for the Use in Lithium-Sulfur Cells
Authors: Kesten Oliver , Erhardt Claudia , Sörgel Seniz , Meinhard Sandra , Bock Anne , Sörgel Timo 
Published: February 2020

The development of high energy density batteries is of extraordinary importance especially in the area of mobile applications. The lithium/sulfur (Li/S) battery has a high theoretical specific capacity of 1672 mAh/g and theoretical energy density of 2500 Wh/kg which is pronounceably higher than that of a commercial Li-ion-battery. Although sulfur is very abundant and cheap, the commercial use is not economically interesting as the theoretically possible capacities are not reached yet. The known problems are the low electrical conductivity of sulfur, electrochemical irreversibility (shuttle-mechanism) and morphology/volume change of the cathode upon battery cycling.

Due to the electrically isolating properties of sulfur, the cathode material of lithium-sulfur batteries usually consists of a multi-component mixture. For example, carbon particles are added to increase conductivity and binders are added to mechanically stabilize the cathode material [1]. The mixture is then applied to a current collector, whose task it is to conduct the electrons to an external circuit.

The inclusion of binding material and electrically conductive particles such as carbon nano tubes, is not only expensive but also limits the energy density of the cell, given that the volume of binder and conductive particles reduces the volume available for the active material. A further problem is the relatively high electrical resistance between the surfaces of the conductive particles in these kind of composite cathode materials. These cathode mixtures have a low overall conductivity and a limited mechanical stability. Furthermore, the integrity of these composite cathode materials fades as the cycle number increases, and the capacity as well as the coulombic and energy efficiency of the cell diminish.

In this work, for the first time, sulfur cathodes for lithium-sulfur batteries were produced by a newly developed single-step electroplating process which allows sulfur incorporation into an electrically conducting metal matrix (which may simultaneously serve as current collector) in the absence of binder and conducting carbon [2, 3].

We further developed this process by applying the electroplated composite layer onto the inner surface of a 3D-nickel foam, using nickel as matrix metal. The use of 3D-foams as current collectors immensely increases the specific surface area of the cathodes, which lowers the local current densities in the battery and thereby improves the energy efficiency. Furthermore, the foam ensures maximum conductivity with minimum weight. As the pores of the foam can be filled up with electrolyte, in contrast to 2D electrodes, additional cell volume, required for the electrolyte, is significantly reduced.

The new approach thereby combines the advantages of composite electroplating (no binder, no carbon black, high mechanical stability) and the usage of 3D substrates (high electrode conductivity, less need for additional electrolyte volume, improved electrode kinetics).

The achieved energy density and the specific energy of the batteries were compared with those of a ”benchmark” Li/S battery concept, as defined by Koller [4].

[1] X. Ji, L.F.Nazar, J. Mater.
Chem.
2010, 20, 9821-9826

[2] T. Sörgel, S. Meinhard, Ş. Sörgel, WO 2015/131977 A1, 2015

[3] C. Erhardt, Ş. Sörgel, S. Meinhard, T. Sörgel, J. Power Sources
2015, 296, 70-77

[4] S. Koller, Li-S vs. Li-Ion - Chances and Challenges, 3rd workshop "Li-S Batteries", Dresden, 2014.

Figure 1