Towards a development of iron phosphate-based materials as positive and negative electrodes for Li-ion batteries

The design, synthesis and electrochemical characterization of new electrode materials hold the key for fundamental advances in energy conversion and storage technologies. Polyanionic compounds have been heavily investigated as possible electrode materials in lithium- and sodium-ion cells. This thesis focused on the design, synthesis, size and morphology tailoring of iron phosphate based electrode materials in order to enhance their electrochemical properties. Four iron phosphate-based electrode materials classified in three groups: Na2Mn1.5Fe1.5(PO4)3, NaxMxFe(3-X)(PO4)3 (X=1.25 when M=Ni and x=1.5 for M= Mn) and Fe1.19(PO4)(OH)0.57(H2O)0.43, were investigated in this work. All these compounds were obtained through wet chemical (hydrothermal or solvothermal) synthesis routes. The preparation protocols and characterization techniques to study the structural, particle size, morphological and electrochemical properties of the above materials have been discussed in the following chapters.
Sodium manganese iron phosphate alluaudite structure is one of the studied compounds group. Na2Mn1.5Fe1.5(PO4)3 is obtained via one step hydrothermal synthesis reaction and it was electrochemically studied without any other further heat treatment.
NaxMxFe(3-X)(PO4)3 (X=1.25 for M=Ni and x=1.5 for M= Mn) compounds were obtained through solvothermal method in ethylene glycol. Na1.25Ni1.25Fe1.75(PO4)3 is a new material. Both materials have been electrochemically characterized for the first time in this work.
Fe1.19(PO4)(OH)0.57(H2O)0.43 was obtained by modifying conventional hydrothermal synthesis of this materials by the addition of conducting carbon (carbon black and carbon nanotubes) in the precursors solution during synthesis that enhance its electrochemical properties as described in this thesis. The Li-ion intercalation reaction mechanisms in Fe1.19(PO4)(OH)0.57(H2O)0.43 cathode material were also investigated by using operando XRD and Mössbauer spectroscopy techniques.
The above studies of iron phosphate-based polyanionics as electrode materials in alkali metal-ion batteries show that this group may be the right key in replacing current commercial unsafe electrode materials. The possibility of improving alternative soft chemical synthesis methods to design new materials or improving electrochemical performance of existing electrode materials was also explored here. The roadmap for our current and future work has been proposed, the materials of our future interest have been chosen basing on their promising rich crystal structure and electrochemical properties by comparing them with the studied materials.