Synthesis and characterization of novel manganese and vanadium oxides as cathodes for lithium rechargeable batteries

The development of advanced rechargeable lithium batteries depends on cathodes that can reversibly intercalate lithium ions. Intercalation chemistry plays a key role in the electrochemical reduction and oxidation by lithium of many solid electrodes including transition metal compounds. Intercalation compounds such as Li_xTiS₂ and LiCoO₂ exhibit some of the ideal characteristics expected of a cathode for advanced batteries. The former shows high rate capability, excellent electronic conductivity and almost perfect reversibility; the latter shows a high voltage suitable for carbon based anodes and good reversibility but an energy density no higher than that of Li_xTiS₂, a prohibitively high cost and an environmental hazard. The spinel LiMn₂O₄ removes the cost issue but not the others. Thus, much effort has been directed at synthesizing new structures that exhibit enhanced electrochemical activity.; Soft chemistry approaches have been applied for this purpose. One such approach is mild hydrothermal reactions which lead to the formation of new metastable transition metal oxides, not accessible by conventional high temperature methods. The nature of the reactants, the pH of the reaction medium, heating temperature and heating duration have dramatic effects on the crystal structure of the phase formed.; The mild hydrothermal decomposition of aqueous permanganate solutions has been found to lead to new layered manganese oxides. In the case of alkali permanganates AMnO₄, layered birnessite-type compounds are formed with the general formula A_xMnO₂.nH ₂O (A = Li, Na, K). These compounds have R m rhombohedral structures analogous to the layered disulfides. The water is reversibly lost on heating, and the compounds readily react with lithium through an intercalation mechanism. The capacity for lithium is a function of the alkali ion present, and the larger potassium ion maintains the capacity best. For the lithium compound, there is a tendency to convert to the spinel structure which leads to loss of capacity. In the case of hydrothermal decomposition of tetramethylammonium permanganate in the presence of nickel, a new structure compound Ni_1−xMn_1−yO ₃ is formed that has a different structure from that of the known ilmenite form. The structure contains empty tunnels into which lithium ions can be intercalated. Reaction of this nickel manganese oxide with n-butyl lithium showed the uptake of 0.91 Li per formula unit. It converts into the known ilmenite form of NiMnO₃ at around 400°C. This compound also has interesting magnetic properties.; A wide variety of vanadium oxides can be prepared using hydrothermal methods. Organic templates play an important role in directing the structures formed. Although tetramethyl ammonium hydroxide is a widely used organic template, other amines and long chain amine surfactants also yield interesting new structures many of which are layered phases. The hydrothermal reaction of vanadium pentoxide with methylamine leads to a series of new layered vanadium oxides, which differ in structure from the corresponding ones prepared in the presence of the tetramethylammonium ion because of the existence of hydrogen bonding. Methylamine is the first organic to form a double sheet vanadium oxide, (CH₃NH₃) _0.75V₄O₁₀.0.67H₂O, with δ-Ag_xV₂O₅ structure. (CH ₃NH₃)V₃O₇ shows significant buckling of the vanadium oxide layers compared with N(CH₃)₄V ₃O₇. Both of these two compounds are monoclinic. (CH ₃NH₂)₂V₈O₁₇ has a tetrahed...