Exploring the electrochemical properties of lithium-ion battery electrodes composed of vacancy-defective zinc-substituted magnetite nanocrystallites

Abstract

Single phase Zn2+-substituted magnetite nanocrystallites are synthesized via co-precipitation method and used as lithium-ion battery electrodes. The structural properties and vacancy distribution of nanoparticles are evaluated by X-ray diffraction, X-ray photoelectron spectroscopy (XPS), and positron annihilation lifetime spectroscopy (PALS). The Rietveld refinement of the X-ray diffraction patterns shows that the lattice constant increases with Zn2+ content from 8.392 Å for magnetite to 8.448 Å for Zn0.2Fe2.8O4. The XPS measurements show that cation vacancies increase with Zn content on the surface of nanoparticles. Positron annihilation lifetime spectra suggest more concentration of monovacancies and larger size of vacancy clusters in the Zn0.1Fe2.9O4 sample compared with the other two samples, Fe3O4 and Zn0.2Fe2.8O4. Electrochemical measurements show better reversibility, higher initial discharge capacity, and lower electrochemical impedance for the Zn0.1Fe2.9O4 electrode, which are attributed to the higher concentration of vacancy defects in its nanocrystallites. The electrochemical impedance spectroscopy (EIS) shows that the Li+ diffusion coefficient (DLi+) increases with Zn2+ doping from 1.51 × 10–15 cm2s–1 for Fe3O4 to 3.25 × 10–13 cm2s–1 for Zn0.1Fe2.9O4, and 1.43 × 10–13 cm2s–1 for Zn0.2Fe2.8O4. The initial discharge capacities of the samples found to be about 1345.04 mAh g−1 for Fe3O4, 1435.82 mAh g−1 for Zn0.1Fe2.9O4, and 1063.72 mAh g−1 for Zn0.2Fe2.8O4 electrodes, respectively. During first few cycles, the discharge capacity decline faster as Zn content increases. The discharge capacity of the Zn0.2Fe2.8O4 electrode is stable at 410 mAhg−1 from the 60th to the 200th cycle, which is higher than the Fe3O4 electrode (150 mAhg−1) and the Zn0.1Fe2.9O4 electrode (270 mAhg−1). This variance is explained by the cation vacancy concentration and the lattice constant of the samples. The impedance of the electrodes is also affected by the vacancy defects formed during the sample preparation.