Characteristics of molybdenum oxide and chromium oxide cathodes in primary and secondary organic electrolyte lithium batteries I. Morphology, structure and their changes during discharge and cycling

Abstract Rechargeable positive electrodes for ambient temperature Li-batteries are mainly based on the same general principle: reversible topotactic Li + -intercalation into transition metal chalcogenide host lattices, yielding ternary phases Li n MCh x . After an intercalation/deintercalation cycle the original host lattice may be retained practically unchanged if the structure of the host matrix is determined by strong covalent bonds and if the concentration of intercalated ions was small. On the other hand, high concentration of unsolvated Li + ions in fairly ionic oxide host lattices such as MoO 3 , Cr 2 O 5 or Cr 3 O 8 (CrO x ) cause significant irreversible structural and also morphological changes of the host matrix. Moreover, ternary oxides Li n MO x prepared at room temperature are non-equilibrium phases — their structural parameters and transport properties vary with conditions of preparation, ageing time etc., and elevated temperature finally causes a complete and irreversible breakdown of their original structure. Prolonged cycling of crystalline large particle size MoO 3 leaves quasi-amorphous powders which, however, are still usable for rechargeable cathodes. Failure of Li n MoO 3 and Li n CrO x electrodes is due to recrystallization via solution, forming e.g. Li 2 MoO 4 from Li n MoO 3 . To obtain a better understanding of the limitations of MoO 3 and CrO x cathodes under practical operation conditions changes in structure and morphology during electrochemical and chemical lithiation and after electrochemical cycling were followed by X-ray diffraction and SEM micrographs; supplementary results were obtained by electrochemical methods and by thermal analysis. Electronically conducting preparations of “hexagonal MoO 3 ” (which is a hydrated MoO 3 modification) were also characterized with respect to their discharge behaviour and reversibility in organic Li + -electrolytes − their “sloping” discharge characteristics disqualify them for practical applications.