Lithium Iron Methylenediphosphonate: a Model Material for New Organic-Inorganic Hybrid Positive Electrode Materials for Li Ion Batteries.

To increase the energy density of lithium ion batteries, new electrode materials with superior performance are ceaselessly being searched for. To combine the advantages of organic and inorganic materials, the creation of organic inorganic hybrid materials is a promising option for the future. In this work, we introduce hydrothermally synthesized lithium iron methylenediphosphonate (Li1.4Fe6.8[CH2(PO3)(2)](3) [CH2(PO3)(PO3H)]center dot 4H(2)O) as a model material for a new class of organic inorganic hybrid materials for Li ion battery positive electrodes. Hereby, we validate the concept of using diphosphonate ligands for hybrid Li ion battery materials. Structure determination based on neutron and X-ray powder diffraction data indicates a monoclinic phase containing three different Fe positions, which is confirmed by Mossbauer spectroscopy. The material achieves a specific charge of 128 (mA h)/g upon galvanostatic cycling after 200 cycles at a theoretical value of 168 (mA h)/g. Operando X-ray absorption near-edge spectroscopy results confirm the reversible cycling of Fe ions between Fe(II) and Fe(III), and ex situ superconducting quantum interference device measurements indicate an exchange of 0.6 electron per Fe atom in the first cycle, in agreement with the specific charge obtained for the first reduction. This proves the applicability of transition-metal diphosphonates as positive electrode materials for Li ion batteries.