Lithium-Sulfur Battery Cathodes with in Situ Polymerized Conductive Polymer Coating

The development of alternative strategies for rechargeable batteries is essential if energy technologies that can displace fossil fuels are to be successful. Among various types of rechargeable batteries, the lithium-sulfur battery is a striking candidate owing to its remarkable theoretical capacity of 1672 mA h/g and theoretical specific energy of 2600 W h/kg. In addition, elemental sulfur is environmentally friendly, inexpensive and abundant.

However, lithium-sulfur battery systems containing organic electrolytes show low cycle life due to the fact that sulfur is electronically and ionically insulating. This causes poor electrochemical accessibility and low utilization of sulfur in the electrode. In addition, generation of soluble intermediate lithium polysulfides during discharge causes an irreversible loss of sulfur-active materials during cycling via the shuttle mechanism. Such problems significantly reduce the cycle life of current lithium sulfur batteries.

We describe in this work the performance of novel cathode materials for lithium-sulfur batteries that utilize coatings of the electronically and ionically conductive polymer polythiophene.  Composite cathodes are synthesized by in situ chemical oxidative polymerization using the well-known Sugimoto–Yoshino method [1]. The conductive polythiophene functions as a conducting additive and as a porous adsorbing agent in the cathode. The PT-coated composites are found to be electrochemically stable within the range of 1.0 - 3.0V versus Li⁺/Li⁰,the operating voltage range of lithium-sulfur batteries.

A cyclic voltammogram of a cell using the PT-coated composite as the active cathode material, measured at scan rate of 1 mV s^-1, is shown in Figure 1. The reduction peak around 2.4 V corresponds to the transformation of sulfur to high-order lithium polysulfides. Reduction of high-order polysulfides to low-order polysulfides and eventually Li₂S shows another peak about 1.9 V. The oxidation peak near 2.6 V corresponds to the transformation to lithium polysulfides (Li₂S_n, n>2). The function of the PT-coated composite cathodes, their cyclic durability, and their rate capabilities will be compared with conventional cathode materials.

Acknowledgement:This work is supported as part of the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.

Figure 1: Cyclic Voltammogram of a polymer coated carbon-sulfur active cathode material. Voltammogram is obtained at 1.0 mV s⁻¹. Inset shows the cathode powder with C/S ratio of 1/2  before (Left) and after (right) polythiophene coating.

Reference 1) Yoshino, K.; Hayashi, S.; Sugimoto, R. Jpn. J. Appl. Phys. 1984, 23, L89.