Motivation: Metal Sulfides as Materials in Lithium Ion Batteries

After transition metal sulfides were initially successfully used as lithium primary cell materials, Whittingham was the first to demonstrate fast, reversible Li insertion into TiS₂ over the solid-solution range 0 ≤ x ≤ 1 of Li_xTiS₂.^[1] On the other hand the technical feasibility of the electrochemical reaction of alkali metals with sulfur is also well-known and high temperature sodium-sulfur batteries are currently in use for certain stationary storage applications. In turn a battery based on lithium/elemental sulfur redox couples has a theoretical specific capacity of 1675 mAh g^–1 of active material and a theoretical specific energy of 2500 Wh kg^–1 assuming complete reaction to form Li₂S. Unfortunately the formation of a variety of lithium polysulfide intermediates which are partially soluble in the electrolyte result in severe inefficiencies. Nonetheless, in view of the reactivity of sulfur with lithium, it is not surprising that several transition metal sulfides have been reported to react with lithium following a conversion reaction to produce Li₂S and metal nanoparticles. These sulfides therefore share characteristics and issues as electrodes with elemental sulfur.

State of the Art: Copper Sulfides as Materials in Lithium Ion Batteries

Despite the reversibility of the reaction in the first cycle, severe capacity losses are observed due to irreversible sulfur dissolution in the electrolyte upon cycling.

Own Results: The electrochemical behaviour of nanostructured lithiated copper(I)sulfide phases^[5]

Two different approaches were used to synthesize lithiated copper(I)sulfide with the formal composition Li₂Cu₄S₃. In the first approach thiolato bridged copper cluster complexes Li (dme)₃][Cu₄(SPh)₆]    and [Li(diglyme)₂][Cu₄(SPh)₆] were decomposed/thermolized under vacuum conditions. In another approach solid state reactions of Li₂S and two Cu₂S were performed in quartz ampoules sealed under vacuum at 400 °C.

The as synthesized powders reveal complex room temperature XRD powder patterns. It is obvious that the patterns do not consist of simple mixtures of the binary sulfides Li₂S and Cu₂S. Interestingly structures of any `LiCuS´ phases have not been reported in literature to date.

SEM measurements show that the `Li₂Cu₄S₃´ material derived from the cluster molecules is highly porous while that obtained from solid state reactions of Li₂S and Cu₂S is rather compact.

The electrochemical behaviour of the materials investigated in solid state in Swagelock cells shows a reversible exchange of 2 Li⁺ ions between 1.9 and 2.7V.

Further discharge below 1.9 V leads to a fast degradation of the material. The two materials however show significant differences in their cycling stability for the reversible exchange of the two Li+ ions (Fig. 1). While for the porous material a higher initial capacity (140 Ah/kg) than for the compact material (120 Ah/kg) was observed the former material suffers from much faster degradation during cycling.

Future investigations aim on the one hand on the synthesis of nanostructured carbon stabilized ‘LiCuS’ materials in order to improve the cycling stability and on the other hand on the synthesis of lithium rich copper sulfides e. g. `Li_xCu₂S_1+0.5x´ ( x = 4, 6) and their electrochemical characterization.

References

List of Publications 2006-2011 as PDF

Subproject Report 2006-2010 as PDF