Preparation of carbon nanoparticles from electrolysis of molten carbonates and use as anode materials in lithium-ion batteries

The electrochemical reduction of molten Li-Na-K carbonates at 450 degrees C provides quasi-spherical carbon nanoparticles with size comprised between 40 and 80 nm (deduced from AFM measurements). XRD analyses performed after washing and heat-treatment at various temperatures have revealed the presence of graphitised and amorphous phases. The d(002) values were close to the ideal one obtained for pure graphite. Raman spectroscopy has pointed out surface disordering which increases with increasing temperature of the heat-treatment. The presence of Na and Li on the surface of the carbon powder has been evidenced by SIMS. The maximum Na and Li contents were observed for carbon samples heat-treated at 400 degrees C. Their electrochemical performances vs. the insertion/deinsertion of lithium cations were studied in I M LiPF6-EC:DEC:DMC (2:1:2). The first charge-discharge cycle is characterised by a high irreversible capacity as in the case of hard-disordered carbon materials. However, the potential profile in galvanostatic mode is intermediate between that usually observed for graphite and amorphous carbon: rather continuous charge-discharge curves sloping between 1.5 and 0.3V vs. Li/Li+, and successive phase transformations between 0.3 and 0.02V vs. Li/Li+. The best electrochemical performances were obtained with carbon powders heat-treated at 400 degrees C which exhibits a reversible capacity value of 1080mAh g(-1) (composition of Li2.9C6). This sample has also both the lowest surface disordering (deduced from Raman spectroscopy), and the highest Na and Li surface contents (deduced from SIMS). (c) 2006 Elsevier B.V. All rights reserved.