Synergistic effects of transition metal substitution in conversion electrodes for lithium-ion batteries

Different solid solutions in the Fe(x)Co(1-x)C(2)O(4)center dot 2H(2)O system have been prepared in the form of nanoribbons by a reverse micelles method. The orthorhombic crystallographic structure differs from the monoclinic massive products FeC(2)O(4)center dot 2H(2)O and CoC(2)O(4)center dot 2H(2)O. The dehydration process is studied by thermal analysis to prepare the anhydrous solid solution oxysalts, in which the nanoribbon-shaped particles are preserved and a porous system is developed. Anhydrous mixed oxalates are used for the first time as high-capacity lithium storage materials with improved rate performance, and display synergistic effects as compared with the end members. Fe(0.75)Co(0.25)C(2)O(4) displays a reversible capacity ca. 600 mA h g(-1) at 5C rate with a very good capacity retention after 75 cycles by a hybrid mechanism. The solids display faradaic capacities due to a novel conversion reaction that produces nanodispersed transition metals, which is responsible for the high energy density, and a capacitive response that leads to high power densities in Li-ion batteries. The low temperature synthesis of these materials makes them an inexpensive option for this purpose.