Mesoporous Iron Phosphonate Electrodes with Crystalline Frameworks for Lithium-Ion Batteries

A new family of mesoporous iron phosphonate (FeP) materials has been prepared through cooperative assembly of cetyltrimethylammonium bromide (CTAB), iron nitrate, and nitrilotris(methylene)triphosphonic acid (NMPA). CTAB is used as a structure directing agent, while the other two chemicals are used as precursors for the formation of pore walls. An extraction procedure is employed to remove the template without damaging the as-prepared ordered mesostructure. The obtained mesoporous FeP materials are well characterized by low angle X-ray diffraction (XRD), N-2 adsorption isotherms, and transmission electron microscopy. The mesostructural ordering of the obtained materials strongly depends on the synthetic conditions. The morphology and the crystallinity of the pore walls are investigated by scanning electron microscopy and wide-angle XRD measurements, respectively. It is revealed that the FeP framework is crystallized in the tetragonal crystal phase (I4(1)/amd), according to the Rietveld refinement of the XRD patterns through the MAUD program. The unit cell parameters of the obtained crystals are a = b = 5.1963 (3) angstrom, c = 12.9808 (1) angstrom (alpha = beta = gamma = 90 degrees). Also, the homogeneous distribution of both Fe species and organo-phosphonic acid groups in the mesoporous architectures is confirmed by Fourier transform infrared spectroscopy and elemental mapping. Mesoporous FeP materials with high surface area have great applicability as high performance electrode materials for lithium-ion (Li-ion) batteries, due to several advantages including a large contact area with the electrolyte, high structural stability, and short transport paths for Li+ ions. Mesoporous FeP electrodes exhibit high reversible specific capacity with very good cycling stability and excellent retention of capacity.