Surface preparation of natural graphite and the effect of impurities on grinding and the particle distribution

Abstract

The present invention relates to the physical or chemical specific purification of natural mineral graphite. This purification is preferably applied to the surface of natural graphite in order to allow the formation of a passivation film during the first electrical discharge or the insertion of lithium in the graphite when the latter is used in a lithium-ion cell. The grinding to a small size before purification allows the optimization of the distribution of the particles, resulting in a more uniform electrode. This grinding is carried out in the presence of the natural impurities of the graphite that play the role of a micro-abrasive and result in a hardness of the graphite that increases its mechanical properties.

1 . A process for the preparation of graphite particles which are surfaced-purified, said process comprising at least the following consecutive steps: 
 a) a grinding of the graphite particles, carried out until said particles attain a size between 1 and 50 μm;    b) A purification of the graphite particles obtained in a) performed for less than 16 hours, by chemical, mechanical or thermal means, at a temperature lower than 2,000° C. or by combining these means, until impurities, as well as sites of corrosion are substantially eliminated.    
     
     
         2 . A process for the purification of the surface of particles of a natural graphite, said process comprising at least the following steps: 
 a) a grinding of the natural graphite carried out until said natural graphite attains a size between 1 and 50 μm; and    b) a purification of the particles obtained in a) carried out until said natural graphite, as well as sites or corrosion, are substantially eliminated.    
     
     
         3 . A process according to  claim 2 , wherein the surface of the particles is purified by chemical or mechanical means for less than 16 hours at a temperature lower than 2,000° C., or by a combination of these means.  
     
     
         4 . A process according to  claim 2 , characterized in that the purification of particles as carried out by mechanical means. the crystallographic parameters L c  and L a  are controlled.  
     
     
         5 . A process according to  claim 1  or  2 , characterized in that the crystallographic parameters L c  and L a  are controlled.  
     
     
         6 . A process according to  claim 1  or  2 , characterized in that the graphite is of the StratminGraphite® type.  
     
     
         7 . A process according to  claim 1  or  2 , characterized in that the graphite is of the Chinese type.  
     
     
         8 . A process according to  claim 1  or  2 , characterized in that the graphite is of the Lac Knife type.  
     
     
         9 . A process according to  claim 1  or  2 , characterized in that the graphite is of the Brazilian type.  
     
     
         10 . A process according to any one of  claims 1  to  9 , characterized in that steps a) and b) are performed under conditions, which are adapted to obtain graphite particles having the following additional properties: 
 an interplanar distance d002 measured by X-rays, varying between 3.35 Å to 3.38 Å;    a specific surface varying from 0.4 to 55 m 2 /g; and    a level of purity varying from 98.5% to 99.99%.    
     
     
         11 . A process according to  claim 1  or  2 , characterized in that the purification is carried out by chemical means in an acid or in a basic medium.  
     
     
         12 . A process according to  claim 11 , characterized in that the acid medium comprises H 2 SO 4 —HF, H 2 SO 4 —NH 4 F, H 2 SO 4 , HNO 3 , HCl or their mixtures thereof and the basic medium comprises a fluoridated derivative.  
     
     
         13 . A process according the  claim 11 , characterized in that the acid medium comprises HF or a fluoridated derivative generating HF in said medium.  
     
     
         14 . A process according to  claim 13 , characterized in that the fluoridated derivative comprises NH 4 F, NH 5 F 2 , XF wherein X is an alkaline or a rare earth metal or their mixtures thereof.  
     
     
         15 . A process according to  claim 12 , wherein said acid medium comprises H 2 SO 4  or HCl, as well as a fluoridated derivative, and additionally sufficient HNO 3  to exfoliate graphite while inserting itself between the layers of said graphite.  
     
     
         16 . A process according to  claim 1  or  4 , characterized in that the mechanical purification is carried out by grinding until the particles reach a size from 1 μm to 50 μm, followed by the separation of the impurities by flotation.  
     
     
         17 . A process according to  claim 11 , characterized in that the purification is carried out by an acid in the presence of at least two salts generating two acids including HF.  
     
     
         18 . A process according to  claim 1  or  2 , characterized in that said graphite particles are conditioned in the form of a carbon anode for a rechargeable electrochemical generator comprising an alkaline or rare earth metal.  
     
     
         19 . A process according to  claim 18 , characterized in that the metal is lithium.  
     
     
         20 . A process according to  claim 18 , characterized in that the anode is prepared by mixing the graphite particles with a binder and a solvent, and the mixture thereby obtained is then coated onto a metallic collector.  
     
     
         21 . A process according to  claim 1 , characterized in that the purification is performed in such a way to eliminate said impurities and sites of corrosion only on the surface.  
     
     
         22 . Graphite particles with a size between about 1 and 50 μm and where impurities and sites of corrosion have been partially or totally eliminated from the surface of the particles.  
     
     
         23 . A carbon anode based on graphite particles according to  claim 22 .  
     
     
         24 . An anode according to  claim 23 , characterized in that the natural graphite particles which are ground and purified have the following additional properties: 
 an interplanar distance d002 measure by X-rays varying between 3.35 Å and 3.38 Å;    a specific surface varying from 0.4 to 55 m 2 /g; and    a level of purity varying from 98.5% to 99.99%.    
     
     
         25 . Electrochemical battery comprising an anode according to  claim 23 .  
     
     
         26 . A battery according to  claim 25 , characterized in that it is a battery of a lithium-ion type.