Synthesis of Spherical Graphite Particles and Their Application as Cathode Material in Dual-Ion Cells

Graphitic carbon provides a redox-amphoteric character, which leads to the capability to intercalate a broad range of different cations and anions between its planar graphene sheets, resulting in so-called graphite intercalation compounds (GICs). Graphite intercalation compounds, formed by cations are called donor-type GICs and those intercalated by anions are called acceptor type GICs [1-4]. A well-known application for a donor-type GIC is lithium-intercalated graphite (LiC₆) as the anode material in lithium-ion batteries.

In recent publications, we reported an electrochemical energy storage system using graphite as both anion and cation intercalation host. During charge, cations and anions, stemming from the electrolyte, are intercalated into the graphitic lattice structure of negative and positive electrode, respectively. During discharge, they are released back into the electrolyte. Hence, the electrolyte can be considered as active material, as it does not only act as charge carrier for lithium ions [4, 5]. Since a high oxidative stability of the electrolyte is mandatory, it is most likely composed of an ionic liquid and a suitable conductive salt, possessing the same anion as the ionic liquid. This system has been proposed as dual-graphite cell [6]. The electrolyte of the dual-graphite system used in this work is composed of the ionic liquid N-butyl-N-methyl-pyrrolidinium bis(trifluoromethanesul-fonyl) imide (Pyr₁₄TFSI) using lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) as conductive and electro-active salt.

This work deals with the synthesis of spherical graphite particles, their physical characterization and the application as active material in dual-graphite cells. First, carbon spheres, possessing different sizes, are synthesized by a hydrothermal bottom-up synthesis from glucose as precursor. Afterwards, a heat treatment at 1200 °C under argon atmosphere was performed in order to decompose the glucose structure to a soft carbon structure. Finally, the resulting soft carbon spheres were heat treated at different temperatures above 2000 °C in order to investigate the graphitization degree as well as the influence of the carbonaceous material structure to the electrochemical performance for application in a dual-graphite cell. The structure of the novel materials was characterized by X-ray diffraction, Raman spectroscopy, specific surface area investigations, scanning electron microscopy and electrochemical measurements.

References

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[4] T. Placke, O. Fromm, S.F. Lux, P. Bieker, S. Rothermel, H.W. Meyer, S. Passerini, M. Winter, Journal of the Electrochemical Society, 159 (2012) A1755-A1765.

[5] T. Placke, P. Bieker, S.F. Lux, O. Fromm, H.W. Meyer, S. Passerini, M. Winter, Zeitschrift für Physikalische Chemie, 226 (2012) 391-407.

[6] S. Rothermel, P. Meister, G. Schmuelling, O. Fromm, H.-W. Meyer, S. Nowak, M. Winter, T. Placke,  Energy & Environmental Science, 2014, 7(10), 3412-3423.