Amorphous Cathodes for Lithium Batteries

Abstract

Interest in ambient temperature secondary lithium batteries has focused recently on the use of crystalline layered compounds as cathode materials (1). In particular the dichalcogenide, TiS₂, has been found to possess many of the characteristics desired of a secondary electrode, such as a continuous non-stoichiometric phase, Li_xTiS₂ for o<χ<l, high electron conductivity, and high free energy of reaction. Many of the other dichalcogenides show similar but generally inferior electrochemical characteristics. However, amongst the group VIB dichalcogenides, crystalline molybdenum disulfide only intercalates about 0.1 Li/Mo and the tungsten compounds react directly to the lithium chalcogenide, Li₂S or Li₂Sε. This behavior is just that expected from thermodynamic calculations. Ideally to achieve optimum energy storage capacity, either volumetric or gravimetric, reaction with more than one alkali metal per transition metal is desired without breaks in the discharge curve as observed with VSe₂. Although some crystalline trichalcogenides such as TiS₃, MoO₃ and NbSe₃ have been studied only one, NbSe₃, shows significant reversibility under deep discharge conditions and it contains heavy, expensive and toxic components; moreover, its gravimetric energy density under load is less than that of TiS_2. This paper discusses some initial work on a new class of cathode materials, amorphous transition metal chalcogenides. As examples, the properties of MoS₂, MoS₃ and V₂S₅ will be described (2).