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

Abstract

Different solid solutions in the Fe_xCo_1−xC₂O₄·2H₂O system have been prepared in the form of nanoribbons by a reverse micelles method. The orthorhombic crystallographic structure differs from the monoclinic massive products FeC₂O₄·2H₂O and CoC₂O₄·2H₂O. 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_0.75Co_0.25C₂O₄ 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.