Introduction

The commercial demand of Li-ion batteries for various applications necessitates the development of new electrode materials with improved energy density, cycle life, safety and low cost. The anode materials which operate through reversible conversion mechanism have been brought to interest in the beginning of 21^st century due to their intrinsically higher capacity comparing to the insertion-type electrodes [1]. Nanocrystalline Iron oxides are especially attractive because of low cost and environmental compatibility. In this work, a comparative study of different metal ferrites as conversion type model system for Li-ion batteries, to elucidate the influence of partial substitution of Fe in the spinel structure with different 3d-cations, are reported and also a comparative study of their performance with respect to particle size is investigated using in situ diffraction, transmission electron microscope and pair distribution function analysis. 

Discussion

The 3d-transition metal ferrites MFe₂O₄ (M = Fe, Co, Ni and Cu) were synthesized by a simple and environmental friendly co-precipitation route[2,3]. The final calcination was performed at different temperatures. The obtained powders were characterized by Scanning Electron Microscopy (SEM) and X-Ray Diffraction (XRD). The structural characterization by XRD confirmed the existence of phase pure compounds with spinel structure except for CuFe₂O₄ and Fe₃O₄ samples which contains a mixture of different oxides. The electrochemical performance is strongly influenced by the specific 3d-cation, final calcination temperature, electrode composition as well as particle size.

 The electrochemical performance of nano (20-100 nm) and micro (2-5 µm) sized Fe₃O₄ and CuFe₂O₄ is investigated to elucidate the remarkable influence of the specific surface area on the obtainable specific capacities as well as the cycling stability of the electrode materials. Nano sized materials show higher specific capacity in the beginning but drastic fading during cycling whereas micro sized materials show comparatively lower specific capacity which remains stable over 100 cycles. In order to investigate the electrochemical mechanism in detail, the structural evolution of the materials in the 1^st cycle was tracked using in situ synchrotron diffraction and ex situ PDF techniques. It can be observed that the electrochemical mechanism in the first discharge cycle could occur via two different processes. Either by the direct reduction of initial material into respective binary oxide or by a Li-intercalation process in the spinel structure during the initial discharge, which is further transformed to metal nanoparticles dispersed in Li₂O matrix. In our previous investigations on nano-sized CoFe₂O₄ and NiFe₂O₄ using in situ X-ray absorption studies we found that, at the end of first discharge metallic nanoparticles are formed and reoxidation of Ni and Co was found to be incomplete whereas Fe is completely reoxidized.

Conclusions and Outlook

3d-transition metal ferrites were synthesized and investigated as conversion-type model system in Li-ion half cells. The synthesis via co-precipitation enabled the production of phase pure nanocrystalline compounds. The electrochemical studies (cycling stability and cyclic voltammetry) showed that CoFe₂O₄ annealed at 800 ºC with an electrode composition of 60:20:20 exhibited the best electrochemical performance among all the metal ferrites. In situ investigations revealed the influence of particle size on the electrochemical reaction pathways.

Acknowledgement: The financial support from DFG within the Research Priority Program SPP 1473, “Materials with new Design for improved Li ion batteries-WeNDeLIB” is gratefully acknowledged. This work has benefited from the beamtime allocated by the MSPD beam line, ALBA, Barcelona and P02.1 beamline, PETRA, Hamburg. Dr. Alexander Schoekel, Murat Yavuz is gratefully acknowledged for his help with the diffraction measurements.

References:

[1] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nature 2000, 407, 496-499.

[2] X. Yang, X. Wang, Z. Zhang, J. Cryst. Growth 2005, 277, 467– 470.

[3] P. Reddy, Y. Raja, M. Ashok, Adv. Mater. Res. 2014, 895, 287– 290.