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

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 Zn 0.2 Fe 2.8 O 4. 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 Zn 0.1 Fe 2.9 O 4 sample compared with the other two samples, Fe 3 O 4 and Zn 0.2 Fe 2.8 O 4. Electrochemical measurements show better reversibility, higher initial discharge capacity, and lower electrochemical impedance for the Zn 0.1 Fe 2.9 O 4 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 (D Li +) increases with Zn2+ doping from 1.51 × 10–15 cm2s–1 for Fe 3 O 4 to 3.25 × 10–13 cm2s–1 for Zn 0.1 Fe 2.9 O 4 , and 1.43 × 10–13 cm2s–1 for Zn 0.2 Fe 2.8 O 4. The initial discharge capacities of the samples found to be about 1345.04 mAh g−1 for Fe 3 O 4 , 1435.82 mAh g−1 for Zn 0.1 Fe 2.9 O 4 , and 1063.72 mAh g−1 for Zn 0.2 Fe 2.8 O 4 electrodes, respectively. During first few cycles, the discharge capacity decline faster as Zn content increases. The discharge capacity of the Zn 0.2 Fe 2.8 O 4 electrode is stable at 410 mAhg−1 from the 60th to the 200th cycle, which is higher than the Fe 3 O 4 electrode (150 mAhg−1) and the Zn 0.1 Fe 2.9 O 4 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. [Display omitted] • Single-phase Zn2+ substituted magnetite nanoparticles were synthesized via co-precipitation method at room temperature. • The vacancy distribution in nanoparticles due to substitution of Zn cation are measured by XPS and PALS respectively. • Discharge capacity of samples improved with Zn content which were explained based on lattice constant and cation vacancies. [ABSTRACT FROM AUTHOR]