The search for high-energy density batteries especially stimulates the design of electrode materials with enhanced electrochemical storage properties. Downsizing the material to extend its electrochemically active surface as well as creating vacancies to create more available insertion sites are common approaches to improve the performance of electrode materials (1,3). In this work, our strategy is to maximize the cationic vacancies into nano-sized spinel Fe₂O₃ through substituting iron by molybdenum, with the final objective of extending the electrochemical insertion domain and accessing higher specific capacities. Our electrode materials were prepared by a simple solvothermal route which allows a carefully tuning of the cationic precursors.[2] The stabilization of molybdenum cations inside the spinel structure, and consequently the creation of cationic vacancies, were characterized by a wide range of complementary techniques, including pair distribution function analysis, X-ray absorption spectroscopy and ⁵⁷Fe Mössbauer spectroscopy. Interestingly, it is possible to tune both size and crystallinity of such nanomaterials by modifying the iron precursors and the synthesis conditions.

The positive influence of this nanoscale engineering was firstly verified by evaluating the synthesized materials as positive electrodes in lithium batteries, with a significant enhancement of the initial specific capacity (from 40 to 100 mAh/g) for Li insertion. As magnesium-ion batteries are emerging electrochemical storage systems that are still facing lack of positive electrode materials, we are also currently evaluating the magnesium insertion inside the Mo-substituted nanosized Fe₂O₃.

References
[1] B. P. Hahn, J. W. Long, and D. R. Rolison, “Something from nothing: Enhancing electrochemical charge storage with cation vacancies,” Acc. Chem. Res., vol. 46, no. 5, pp. 1181–1191, 2013.
[2] B. P. Hahn, J. W. Long, A. N. Mansour, K. A. Pettigrew, M. S. Osofsky, and D. R. Rolison, “Electrochemical Li-ion storage in defect spinel iron oxides: The critical role of cation vacancies,” Energy Environ. Sci., vol. 4, no. 4, pp. 1495–1502, 2011.