Ball-milling in liquid media: Applications to the preparation of anodic materials for lithium-ion batteries

Abstract

Due to the intense development of portable electronic devices and the aim of the electric vehicle, the preparation of batteries with higher and higher electrochemical performances is of great interest. The best anodic material for lithium batteries should be the metal itself, but due to the formation of lithium dendrites provoking short-circuits while the battery is cycled, one uses graphite, since the lithium intercalation presents a high reversibility and occurs at very low potentials versus Li +/Li 0. In order to improve the anode performance, there are two possible ways, starting from either: • small particles of graphite obtained by grinding. In this case, the charge–discharge rate can be improved, as well as the amount of intercalated lithium, due to the lower particles size and to the presence of dangling carbon bonds, respectively. Unfortunately, ball-milling leads to disordered or amorphous carbons exhibiting a high hysteresis when electrochemically cycled, • graphite intercalation compounds with a high lithium content such as LiC 2 prepared at 300 °C under 50 kbars. This synthesis requires a heavy apparatus and the amount of prepared powders at once is quite small: several hundreds milligrams. Moreover, the compound is not stable and decomposes (into a mixture of LiC 6 and free lithium) as the pressure is released and the LiC 2 compound is not formed back by electrochemical reaction. By modeling and some experimental measurements, it was shown that the pressure and temperature temporarily induced by the shocks occurring during the ball-milling can reach values close to those used to prepare the LiC 2 compound. Therefore, the grinding of graphite–lithium mixtures was studied in order to obtain superdense graphite–lithium compounds stable under ambient conditions. Due to its high ductility, lithium agglomerates readily on the milling tools and, thus, the obtained powder, roughly LiC 6 in spite of a large excess of metal (the starting mixture is Li + 2C), is partly amorphous. This is the reason why we have added small quantities of a liquid, inert towards lithium: the n-dodecane (C 12H 26). The presence of this liquid allows a good lubrication and dispersion of lithium, which does not paste on the tools. With appropriate milling conditions, the formation of a superdense compound with a LiC 3 stoichiometry is possible. This compound is stable at room temperature under normal pressure. The electrochemical properties of the LiC 3 compound were investigated as anode in lithium-ion batteries. The primary capacity is very high (close to 1 A h/g) and the voltage profile is low (all the lithium removal occurs below 300 mV). Unfortunately, the superdense LiC 3 compound is not formed back by electrochemical mean. The reversible capacity corresponds to the formation of the LiC 6 compound and consequently is around 370 mA h/g. This compound constitutes a good candidate as anodic material since its first deintercalation corresponds to a high capacity and is partly reversible with the same behavior as graphite. In the same manner, graphite powders were milled together with dodecane. The powder obtained after separation from the liquid (by simple evaporation) is made of well crystallized thin particles of graphite (typically 20 nm thick, with a geometrical anisotropy of about 100). This graphite leads to a smaller consumption of lithium (used for the so-called solid electrolyte interface formation) than with the starting graphite powder, in spite of a much larger specific surface area. The previous syntheses were done with an inert liquid. In the case of graphite milled within water in a stainless steel vial, one observes the formation of graphite particles covered by small crystals of maghemite (γ Fe 2O 3), these last coming from the oxidation of the vial by water. This reactive ball-milling was used in two different directions: • easy preparation of pure nanosized maghemite crystals. The in situ production of hydrogen during the milling is relevant, because it favors a spinel type structure. For a milling of 48 h, the average diameter of the particles is 15 nm. The technique is remarkable due to the narrow size distribution of the obtained particles, which is very interesting for magnetic applications, • preparation of anode materials by depositing maghemite nanograins at the surface of graphite particles ground together with water. In this manner, the respective properties of graphite and transition metal oxide are added: it was shown recently that such oxides may present higher reversible capacities than graphite. The electrochemical performances of these graphite–maghemite composites were tested. The nanometric size of the maghemite grains, and consequently their great reactivity, allows a partial reversibility of the reaction between iron and maghemite. The corresponding capacity reaches 440 mA h/g and makes such composites good candidates for anodes in lithium-ion batteries. However, the reversible capacity decreases during the electrochemical cycling due to the coalescence of the maghemite nanoparticles.