Investigation of the Cathode Matrix for High Energy Lithium Sulfur Systems

Abstract

Lithium-sulfur batteries are among the most promising systems for the future generation of rechargeable batteries. Main advantages are the high theoretical capacity (1672 Ah/kg), high gravimetric energy density (2455 Wh/kg) and natural abundance. These facts promise a three-to five-fold increased specific energy and greatly reduced costs compared to lithium-ion technology. Sulfur is available in almost unlimited quantities and is thus cheaper. However, these high energy densities are not yet achievable for the system since most of the sulfur cathodes proposed in the literature have an inherently lower active material loading than commercial cathodes of lithium-ion batteries.

In this work sulfur-cathodes were prepared with all commercially available raw materials. The standard battery set-up is realized using a lithium metal anode, a polymeric separator, a liquid aprotic electrolyte and a facile slurry-processed cathode (S/CB). The performance of such batteries is compared by its electrolyte:sulfur-ratio as well as its sulfur content in the cathode layer and the applicated active material loading in the range of 0.5 to 2.0 mg/cm². Furthermore the low-surface area standard carbon black is replaced by high-surface area structured carbon black. Within this attempt highly structured cathodes are produced showing an increase in sulfur utilization of sixty percentage, even at a sulfur content in the cathode of 70wt.-%. Additionally a viable slurry formulation, incorporating metall oxides into the cathode matrix in a polyvinylidene fluoride based system without the use of N-methyl-2-pyrrolidone is shown. The idea of using metall oxides comprising of a positive surface loading to trap the evolving polysulfides is realized only in N-methyl-2-pyrrolidone or N,N-dimethylmethanamid so far, which are both classified as Substances of Very High Concern [1,2,3]. The homogeneous incorporation into a polyvinylidene fluoride based system solved in dimethyl sulfoxide is realized using alumina oxide with hydrophobic surface treatment (S/CB/Al2O3). Starting at a similar capacity of 1000 mAh/g the /CB/Al2O3 cathode could maintain 920 mAh/g after 100 Cycles at 1C, while the standard S/CB cathode looses 15% of its initial capacity, means 840 mAh/g left.