Electrochemical Conversion Reaction of NiFe2O4 Electrode As an Anode Material for Li-Ion Battery

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Transition metal oxides (TMOs) are recently emerged for the negative electrode of advanced lithium-ion batteries [1,2]. Two transition metal oxides NiM2O4 (where M was Fe or Mn), were synthesized by a simple and easily scalable sol-gel method and evaluated as anode materials for Li-ion batteries. Cyclic voltammetry and Galvano static charge/discharge investigations in lithium half-cells revealed a difference between the first cycle and the following charge-discharge cycles for both samples, which is characteristic for conversion-type electrode systems [3,4]. NiFe2O4 combines high abundant elements and significant lithium storage properties. Battery cycling showed that NiFe2O4 exhibited excellent discharge capacity and rate capability, nonetheless of capacity fading with cycling. From the electrochemical tests performed, the specific capacity of NiFe2O4 electrode at current rates of 10 and 20 Ag-1 were found to be 365 and 150 mAh g-1, respectively. To the best of our knowledge, reports on such outstanding capacities of NiFe2O4 nanoparticles as an anode at these high current rates are quite rare. Investigation by using Ex-situ XRD technique on the discharged and charged electrode were performed, and it was found that the material completely lost its crystallinity after first electrochemical discharge process. This fact restraint the reliability of conventional diffraction technique for the reaction mechanisms study [5]. Thus, different spectroscopic techniques are required to apply for studying the reduced/oxidized electrode vs Li [6]. X-ray absorption near-edge spectroscopy (XANES) and Extended X-ray absorption fine structure (EXAFS) spectroscopy measurements were used to study the environment of iron and nickel ions during cycling in lithium test cells. Ni K-edge and Fe K-edge XANES results give evidence of the successive steps in the reduction/oxidation mechanism of the oxide during the cell dis/charge. In a first step NiFe2O4 reacts with lithium and the reduction of both Ni2+ and Fe3+ to the zero oxidation state and following re-oxidation in a second step were shown by a peak shifting related to energy values. In the subsequent (second) discharging process, nickel oxide was found to undergo complete reduction. In contrast, more or less amount of iron oxide remained in the oxidized state at the end of the discharge (end of third cycle). This incomplete reduction of iron oxide in the applied voltage range could be the main reason behind reversible capacity fading over the cycling often reported for this conversion electrode system.