Effect of Ni/S Alloy Coating on the Composite Electroplated 3D-Sulfur Cathodes in Lithium/Sulfur Cells

Abstract

The demand for high energy density batteries is increasing as they are required especially in the area of mobile applications. Lithium-sulfur batteries have attracted increasing attention due to their high theoretical energy density (2500 Wh/kg) and theoretical capacity (1675 mAh/g), low cost and environmental friendliness. However these batteries suffer from poor observed capacity and cycle life which is mainly due the low electrical conductivity of sulfur (5×10−30 S/cm at 25 ◦C), electrochemical irreversibility (i.e. the dissolved polysulfides) and morphology/volume change of the cathode material upon cycling. Improvements of the capacity and cycle life of the batteries have already been achieved by addressing some of the above mentioned problems [1]. Most of the research concentrates on reducing polysulfide shuttle mechanism and/or improving conductivity of sulfur by sulfur-conducting carbon composite [2].The state-of-art sulfur electrodes consist of a multicomponent mixture which is applied on a current collector by casting. The multicomponent mixture contains, in addition to the sulfur active material, electrically conducting carbon particles to improve conductivity and binder material to stabilize the cathode integrity. The inclusion of binder and electrically conducting particles increases the costs of production and also limits the energy density of the cell. Additionally, mechanical integrity of the multicomponent mixture and adhesion of the multicomponent mixture to the current collector is not stable enough so that it diminishes upon battery cycling [3]. In this work, for the first time, polythiophene encapsulated sulfur particles, which are homogeneously composite electroplated throughout a 3D-nickel foam, are further coated with NiS alloy galvanostatically. Composite electroplating provides production of battery electrodes with a high active material loading in the absence of a binder and conducting carbon [4, 5]. In this way, the energy density of the battery, adhesion of sulfur to the current collector, mechanical integrity of the cathode during cycling, sulfur utilization and accordingly battery capacity will be improved. Additionally, sulfur loading will be enhanced by using 3D-substrate with high surface area. The purpose of the NiS coating is to further improve sulfur utilization and kinetics of the electrochemical reactions on the cathode. The cross-section SEM micrographs show that the polythiophene encapsulated sulfur particles are homogeneously coated with NiS alloy and uniformly and densely distributed throughout the 3D-nickel foam. The developed cathodes were tested in Li/S batteries and a battery capacity of about 1000 mAh/g at a rate of 1 mA/cm2 is achieved. [1] I. Bauer, S. Thieme, J. Brückner, H. Althues, S. Kaskel, J. Power Sources 251 (2014) 417–422[2] X. Ji, L.F. Nazar, J. Mater. Chem. 20 (2010) 9821-9826[3] B.H. Jeon, J.H. Yeon, K.M. Kim, I.J. Chung, J. Power Sources 109 (2002) 89–97[4] T.Sörgel, S. Meinhard, S. Sörgel, Verbundmaterial, DE 102014003128.4, applied [5] C. Erhardt, S. Sörgel, S. Meinhard, T. Sörgel, J. power Sources, 296 (2015) 70-77