Ball-milling: an alternative way for the preparation of anodes for lithium-ion batteries
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Ball-milling : an alternative way for the preparation of anodes for lithium-ion batteries Daniel Guérard and Raphaël Janot Laboratoire de Chimie du Solide Minéral, UMR 7555 CNRS, UHP Nancy, BP 239, 54506 Vandoeuvre lès Nancy Cedex, France ABSTRACT The preparation of new anodic materials for lithium-ion batteries is possible by ball-milling within liquid media. Those powders can be as diverse as : highly anisometric graphite particles (geometrical anisotropy around 100), graphite-maghemite (γ Fe2O3) composites and graphiteintercalation compounds with high lithium contents (e.g. LiC3 is obtained by ball-milling of Li + 2C powders). These phases are the first superdense graphite-lithium compounds stable under ambient pressure and lead to a capacity of 1 Ah/g as primary battery and 370 mAh/g as secondary one. The synthesis and characterizations (chemical analysis, XRD, pycnometry, TEM, SEM, 7Li NMR, magnetic measurements) are presented for each materials as well as the electrochemical behavior. INTRODUCTION Due to the important development of portable electronic devices, the needs for lithium-ion batteries is still increasing and the improvement of the electrochemical performances is, therefore, of a large interest. Most of the researches concerning the anodic materials are oriented towards disoriented carbon matrices, since these powders present a larger capacity than graphite. However, this increase is generally linked to a larger irreversible lithium loss, due to the formation of an important passivating layer, and to a large hysteresis of the charge-discharge process. On the other hand, nanosized oxides like CoO are good candidates as anodic materials with very high capacity. Nevertheless, these powders require to be supported on graphite in order to obtain a good electric conduction between the grains and to avoid the agglomeration of the nanoparticles during cycling [1, 2]. Our research on the preparation of new materials by ballmilling covers those two types of anodes for lithium-ion batteries. EXPERIMENTAL Graphite is generally used as anodes in commercial batteries. Its capacity, corresponding to the reversible lithium intercalation into graphite leading to LiC6 (372 mAh/g), is quite high and the charge/discharge process occurs mainly below 0.3 V vs. Li. By using low-temperature carbon materials (as cokes), the capacity is higher (up to 800 mAh/g), but the charge-discharge potentials are around 1.0 V limiting the practical use for such disorganized carbons. On the other hand, the use of small and thin graphite particles can increase the rate of the intercalation /deintercalation process. If one mills graphite, the resulting powder is made of carbon particles less anisotrope than those of starting graphite and presenting a high defects content : as a result, the electrochemical behavior of these ball-milled carbons is quite similar to that of low temperature carbons [3]. In order to limit the formation of defects during the mechanical milling, we have chosen to mill the graphite in the presence of a liq
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