Growing demand of Li-ion batteries initiates the on-going researches on new electrode materials with improved performance.  Conversion-type anode materials have been a topic of interest ever since the beginning of the 21st century due to their intrinsically high capacity, which can enhance significantly the energy density of future lithium-ion batteries [1]. For metal oxides, operating via conversion, the initial discharge involves the reduction of the transition metals to a composite material consisting of metallic nanoparticles (2-8 nm) dispersed in an amorphous Li2O matrix. Upon following charge, the metal particles are oxidized, however, the structure and composition of thus formed nanostructured metal oxide may be different comparing to the pristine oxide. Nanocrystalline transition metal Iron oxides MFe₂O₄ (M = Fe, Co, Ni and Cu) 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 electrochemical eevolution mechanism is investigated using in situ diffraction and pair distribution function analysis.

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. Due to low crystallite size and the formation of the amorphous (poorly diffracting) phases, laboratory and synchrotron conventional X-ray diffraction methods can provide only limited insight into the processes occurring upon cycling conversion-type materials. Pair Distribution Function (PDF) analysis can be used for probing local as well as the long-range structure of the material is more informative for such systems containing both crystalline and amorphous components [2]. The initial electrochemical lithiation of Fe3O4 anode operating via conversion mechanism is investigated by PDF analysis. The ex-situ PDF measurements have been performed on the samples with different lithium content, prepared by discharging in the electrochemical cell voltage range of 3.0 to 0.1 V vs. Li+/Li (See figure 1). The details of the electrochemical mechanism of transition metal ferrites as negative electrode material for Li-ion batteries will be discussed.

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 P02.1 beamline, PETRA, Hamburg.

[1] J. Cabana, L. Monconduit, D. Larcher, M. R. Palacín, Advanced Energy Materials, 22, E170-E192, 2010.

[2] T. Proffen and H. Kim, Journal of Materials Chemistry, 19, 5078, 2009.