Electrochemical grinding-induced metallic assembly exploiting a facile conversion reaction route of metal oxides toward Li ions

Conversion-type electrodes generally feature unstable cycling stability as an anode for Li ion batteries. In this work, however, we suggest that the weird capacity increase after initial decay during cycling is likely a common feature for the electrode materials undergoing high-rate conversion reaction. The cycling with α-Fe₂O₃ anodes at the current density of 500 mA g⁻¹ or more could effectively trigger the typical capacity increase with cycle number after its initial degradation. Its morphological and structural evolutions during cycling are comprehensively characterized here by synchrotron X-ray scattering, X-ray absorption and photoelectron spectroscopies, and electron microscopy. The investigation demonstrates that enough fast conversion reaction tends to reduce the crystallite size of α-Fe₂O₃ significantly as a result of the electrochemical grinding and the continuous cycling can generate the Fe-Fe-Fe medium-range ordering terminated by long Fe-O bonds, which exploits a facile conversion reaction route catalyzing the dissociation of Li₂O toward lower redox potential and faster kinetics. In details, higher heterogeneity, larger stress/strain and thinner solid-electrolyte interphase layer accompanied by fast conversion reaction in α-Fe₂O₃ can make its pristine crystallites smaller, finally extending the proportion of Fe-Fe-Fe ordering and thereby facilitating the conversion reaction for higher capacity during the following charge/discharge. Most importantly, the new structure enables excellent performance for the full cell, in contrast to the catastrophic failure of the pristine oxides. The structural evolution addressed in the study updates the understanding of conversion reactions and sheds light on novel types of conversion-type electrodes towards the full-cell application of conversion-reaction electrodes.