Local structural changes of nano-crystalline ZnFe <inf>2</inf> O <inf>4</inf> during lithiation and de-lithiation studied by X-ray absorption spectroscopy

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Journal Article
Electrochimica Acta, 2017, 246 pp. 699 - 706
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© 2017 Elsevier Ltd X-ray absorption spectroscopy was carried out to investigate local structural changes around Fe and Zn atoms of the nano-crystalline spinel ferrite ZnFe 2 O 4 anode material at various states-of-charge during the 1st and 2nd lithiation/de-lithiation. From the X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), we propose a possible structure evolution process of the ZnFe 2 O 4 electrode during the 1st discharge and charge cycle. A mixture of metallic iron, ZnO, metallic zinc, LiZn and Li 2 O phases seem to be formed as the cell is firstly discharged to 0.02 V. Instead of the original ZnFe 2 O 4 spinel phase, the metallic iron and zinc particles are re-oxidized to Fe 2 O 3 and ZnO phases during the subsequent de-lithiation. A reversible redox reaction between Fe 2 O 3 , ZnO and lithium ions is found in the 2nd cycle. The formation of SEI layer in the initial cycles plays a major role in the irreversible capacity of the electrode. The inactive disordered ZnO formed due to the conversion reaction of ZnFe 2 O 4 during the 1st lithiation is probably the main reason for the poor electrochemical behavior of the nano-crystalline ZnFe 2 O 4 electrode.
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