Engineered nanocomposites as high-energy density positive electrode materials for rechargeable lithium batteries

This dissertation explores new routes towards the development of very high energy density cathodes for non-aqueous batteries. Chapter VII relates the sol-gel synthesis and the characterization of a vanadium oxide/propylene carbonate nanocomposite. The nanocomposite, consisting of mesopores filled with PC molecules and a network of hydrated vanadium oxide, is engineered for the insertion of polyvalent cations and exhibited a specific capacity of 465 mAh/(g of vanadium oxide) when Ca2+ was the guest cation. The critical role of the PC has been demonstrated and it has been shown that the nanocomposite exhibited better electrochemical performances with Ca 2+ than with Li+.; The majority of this dissertation focuses on macro and nanostructured bismuth fluoride and oxyfluorides. The room temperature synthesis and the physical characterization of BiOF and BiO0.5F2 are described in chapter IIX. Some ammonium bismuth fluorides and oxyfluorides, some of them previously unknown, were prepared from the same synthesis method and are also reported.; We demonstrated that a reversible conversion occurs in BiOxF 3-2x(x = 0, 0.5, 1, 1.5)/C nanocomposites prepared by high-energy milling. In BiOF and BiO0.5F2 the fluoride component reacts first at high voltage during lithiation to form Bi0, LiF and Bi 2O3 and Bi2O3 is then reduced into Li2O and Bi0 at lower voltage. During the delithiation Li2O reacts first at low voltage and then LiF. The electrochemical activity of BiF3 is significantly improved by oxygen substitution, suggesting that metal oxyfluorides could be an attractive alternative to metal fluorides, combining the high voltage of the fluorides with the high electrochemical activity of the oxides.; A more detailed investigation of the BiF3/C nanocomposite has been conducted as it is of particular interest due to its high voltage and large volumetric capacity. It has been shown that the nanocomposite exhibits a specific capacity of 230 mAh/(g of composite), which corresponds to 90% utilization of the active material, when discharged to 2V at rates ranging from C/30 to C. A model, based on the variation of the ionic and electronic transport mechanisms as a function of the degree of completion of the reactions, has been proposed to explain the different voltage plateaus occurring during cycling.