To exploit LIBs to their maximum applications, innovative electrode materials should be developed and existing materials should be improved. As negative electrode materials are needed in excess in comparison with positive electrode materials (in terms of specific capacity) in a lithium-ion battery, it is important to reduce their amounts so as to reduce the cell size and production cost^[1]. Moreover, innovative negative electrode material classes delivering high capacity and offering fast Li diffusion kinetics should be developed. Parallel to this, research in alternative low cost energy storage systems became more intensive. Due to the high abundance and cost effectiveness of the electrode materials used, Na-ion batteries have become one of the best alternative energy storage systems to LIBs. Many of the attractive Li-insertion electrode materials are also tested for Na-insertion as a part of developing this system further. As different from the wide variety of positive electrode materials, there are only few choices for negative electrode materials for Na-ion batteries^[2]. Na does not insert into graphite. Binary alloys of Na with Sn, Pb or other metals and conversion metal oxides are expected to have very high gravimetric capacity^[3]. However, the enormous volume changes are supposed to bring out poor cycling behavior of these materials.

SnO₂ is a promising negative electrode material for Li-ion batteries due to its high theoretical specific capacity of 1491 mAh g^-1[4]. The first step in the Li insertion of SnO₂ is a conversion reaction involving 4 Li, where metallic Sn particles are formed in a matrix of Li₂O. In the second step, metallic Sn alloys with Li and produce Li_4.4Sn. Hence in total 8.4 Li ions can be inserted per formula unit of SnO₂^[4]. Nevertheless, this material suffers from extreme volume changes which can lead to a drop in the electric contact and further to severe capacity fading during cycling. This can be overcome by preparing nanocrystalline materials, composite materials with carbon derivatives etc. In the present work, SnO₂ nanoparticles are synthesized via a hydrothermal process. The obtained SnO₂ material has a particle size of ~50 nm. The material was investigated as negative electrode candidate for both Li- and Na-ion batteries in corresponding half cells. The respective electrochemical mechanism was investigated by in situ and quasi in situ X-ray absorption spectroscopy. It was observed that at the end of discharge, SnO₂ was converted to metallic Sn nanoparticles in a Li-ion cell (see Figure 1). Meanwhile, no complete reduction of the SnO₂ was observed in the Na-ion cell at the end of discharge. The details of the electrochemical mechanism of SnO₂ as negative electrode material for Li-ion and Na-ion batteries will be discussed.

Acknowledgement:

Financial support from DFG within the Research Priority Program SPP 1473, “Materials with new design for improved Li ion batteries-WeNDeLIB” under grant no EH183/16-2 is gratefully acknowledged. This work has benefitted from beam time allocation by the core-level absorption and emission spectroscopies (CLAESS) beamline, ALBA, Barcelona.

References:

1.        N. Nitta and G. Yushin, Part. Part. Syst. Charact., 2014, 31, 317–336.

2.        A. Moretti, M. Secchiaroli, D. Buchholz, G. Giuli, R. Marassi, and S. Passerini, J. Electrochem. Soc., 2015, 162, A2723–A2728.

3.        N. Zhang, X. Han, Y. Liu, X. Hu, Q. Zhao, and J. Chen, Adv. Energy Mater., 2014, 1401123.

4.        X. Liu, J. Zhang, W. Si, L. Xi, S. Oswald, C. Yan, and O. G. Schmidt, Nanoscale, 2015, 7, 282–288.