Potato-shaped graphite particles with low impurity rate at the surface, method for preparing same

Abstract

The invention concerns modified graphite particles obtained from graphite or based on graphite, said particles having impurities in their internal structure and having, at the surface, a low, even nil, rate of an impurity or several impurities. Said particles further comprise at least one of the following characteristics: a tab density ranging between 0.3 and 1.5 g/cc; the shape of a potato; and a size distribution such that the D90/D10 ratio varies between 2 and 5 and the particles have a size ranging between 1 and 50 μm. Said particles are useful for fuel cells, electrochemical generators or moisture sensor and/or oxygen sensor and they have interesting electrochemical properties. The resulting electrochemical cells and batteries are stable and safe.

1 . Modified graphite particles obtained from graphite (preferably from synthetic graphite), the structural parameters of the said particles corresponding to at least one of the equations fl e =[Y+1]/[(Y+1)+(B/2T)(Y−1)] and f2e=[Y+1]/[(Y+1)+(B/T)(Y−1)], where Y represents a whole number greater than or equal to 1, B represents the length of the particle in μm, T represents the thickness of the particle in μm, said particles having a potatolike shape and having at least one of the following two characteristics: 
 a tab density between 0.3 and 1.5, preferably between 0.5 and 1.4, most preferably between 1 and 1.3 g/cc; and  
 a granulometric dispersion such that the D90/D10 distribution ratio varies between 2 and 5 and the particles have a size between 1 μm and 50 μm, preferably such that the D90/D10 distribution ratio varies between 2.2 and 4.2 and the particles have a size between 2 μm and 30 μm.  
 
     
     
         2 . Modified graphite particles obtained from graphite, said particles having a potatolike shape, having impurities in their internal structure and having on their surface an amount of one or more impurities that preferably varies between 2% and 4%, and the said particles also having at least one of the following three characteristics: 
 a tab density measured according to the previously identified method between 0.3 and 1.5, preferably between 0.5 and 1.4, most preferably between 1 and 1.3 g/cc;    a granulometric dispersion measured according to the previously identified method, such that the D90/D10 ratio varies between 2.2 and 4.2 and the particles have a size between 2 and 30 μm; and    they have, attached to their surface, particles (preferably potatolike shaped, most preferably spherical) of NaCl and/or of NH 4 F; preferably the mass of these particles of NaCl and/or of NH 4 F represents 1 to 10%, preferably 1 to 4%, of the total mass of the modified graphite particles.    
     
     
         3 . Modified graphite particles according to  claim 2 , characterized in that TGA analysis values give an initial temperature value associated with the loss of weight between 560 and 660 degrees Celsius.  
     
     
         4 . Modified graphite particles according to any one of  claims 1  to  3 , in which the impurities present are at least one chemical element selected from the group comprising Fe, Mo, Sb, As, V, Cr, Cu, Ni, Pb, Co, Ca, Al, Ge, Si, Ba, Be, Cd, Ce, Co, Ciu, Dy, Eu, La, Li, Mo, Nd, Ni, Pb and Pr.  
     
     
         5 . Modified graphite particles according to  claim 4 , in which the percentage of impurities by weight present in the said particles, expressed with respect to the total weight of modified graphite particles and measured according to the ash method, is between 1 and 10%, and preferably between 2 and 4%.  
     
     
         6 . Modified graphite particles according to  claim 5 , substantially devoid of surface impurities, preferably devoid of surface impurities.  
     
     
         7 . Modified graphite particles obtained from graphite, said particles having a potatolike shape and containing from 5 to 20% of at least one of the following compounds SiO 2 , MgO, ceramic compounds or a mixture thereof, said compounds preferably being attached to the modified graphite particles by physical forces and having at least one of the following three characteristics: 
 a tab density measured according to the previously described method between 0.3 and 1.5, preferably between 0.4 and 1.4, most preferably between 1 and 1.3 g/cc;    a granulometric dispersion measured according to the previously defined method, such that the D90/D10 ratio varies between −2 and 5 for a particle size between 1 and 50 μm, preferably such that the D90/D10 ratio varies between 2.2 and 4.2 for a particle size between 2 and 30 μm; and    they have, attached to their surface, particles (preferably of . . . shape) of NaCl and/or of NH 4 F; preferably the mass of these particles of NaCl and/or of NH 4 F represents 1 to 4% of the total weight of the modified graphite particles.    
     
     
         8 . Modified graphite particles according to'any one of  claims 1  to  7 , in which the interplane distance d 002  varies from 33 to 3.4 angstroms and/or the BET varies between 0.5 g/m 2  and 50 g/m 2 .  
     
     
         9 . Modified graphite particles according to any one of  claims 1  to  8 , having a cycling stability greater than 500 cycles.  
     
     
         10 . Process for preparing graphite particles (preferably from natural graphite) according to any one of  claims 6  to  9 , by using at least one physical means that permits a reduction of at least 50% of the basal function (fb) and an increase of at least 50% of the edge function (fe) of the graphite particles, such physical means preferably being attrition, a jet mill, ball mill, hammer mill, atomizer mill, in the presence of at least one chemical compound chosen from the group consisting of compounds of the formula MF z , in which M represents an alkaline or alkaline-earth metal and z represents 1 or 2 (preferably MFz represents CaF 2 , BaF 2 ,LiF), NaC 1  and NH 4 F or a mixture thereof, said compound or compounds preferably being added in solid form, preferably at the beginning of the step using the physical means that permit a reduction of the basal function.  
     
     
         11 . Process according to  claim 10 , in which the reduction of the basal function and the increase of the edge function, preferably the rounding of the particles is carried out preferably using a means of attrition, the said means preferably being made up of balls such as steel balls, ceramic balls or a mixture of steel and ceramic balls.  
     
     
         12 . Process for preparing modified graphite particles according to  claim 1  or  2 , preferably from natural graphite, comprising at least the following two steps: 
 i) modification of the shape of the graphite particles by using at least one physical means permitting a reduction of at least 0.50% of the basal function (fb) and an increase of at least 50% of the edge function (fe) of synthetic graphite particles (preferably of natural graphite), such physical means preferably being attrition (preferably a jet mill, ball mill, hammer mill, an atomizer mill) in the presence of at least one chemical compound selected from the group comprising compounds of the formula MF z , in which M represents an alkaline or alkaline-earth metal and z represents 1 or 2, MF z  preferably represents CaF 2 , BaF 2 , LiF or a mixture of these, the compound or compounds preferably being added in solid form, preferably at ambient temperature, preferably at the beginning of the step for use of the means that permit the reduction of the basal function and the increase of the edge function; and  
 ii) reduction in the amount of surface impurities, preferably by purification, preferably by chemical purification of the graphite particles obtained in the preceding step i).  
 
     
     
         13 . Process according to any one of  claims 10  to  12 , in which the graphite particles used at the beginning of the process have a size between 1 and 450 μm, preferably between 2 and 350 μm.  
     
     
         14 . Process according to any one of  claims 10  to  13 , in which the attrition process is carried out in the presence of an additive, preferably an additive of the metallic oxide type such as SiO 2 , TiO 2 , ZrO 2 , and preferably in the presence of steel balls, ceramic balls or in the presence of a mixture of steel and ceramic balls.  
     
     
         15 . Process according to any one of  claims 10  to  14 , in which at least one of the two steps is carried out in a controlled atmosphere or in air, the controlled atmosphere preferably being based on nitrogen, argon, helium or a mixture of these gases.  
     
     
         16 . Process according to any one of  claims 9  to  15 , in which step i) is a hybrid step using jet milling and attrition, the attrition preferably being carried out after jet milling is used.  
     
     
         17 . Process according to any one of  claims 10  to  15 , in which step i) is carried out using jet milling.  
     
     
         18 . Use of the modified graphite particles according to any one of  claims 1  to  9  for fuel cells, electrochemical generators, as moisture sensors and/or as oxygen sensors.  
     
     
         19 . Negative electrode, preferably for a rechargeable electrochemical generator prepared with a bonding agent, preferably a bonding agent of the PVDF or PTFE type, and with graphite particles according to any one of  claims 1  to  8 .  
     
     
         20 . Process for preparing an electrode based on graphite particles as defined in any one of  claims 1  to  9  or based on graphite particles such as those obtained by a process according to any one of  claims 1  to  17  for a rechargeable generator comprising at least the following steps: 
 a—solubilization of at least one bonding agent (preferably selected from the group comprising PVDF, PTFE) in a solvent (preferably in a strong solvent selected from the group comprising NMP (N-methylpyrrolidone), cyclopentanone at the highest possible concentration (preferably greater than 1 g/cc)) to obtain a viscous solution (A);  
 b—coating the viscous solution obtained in the preceding step, which is a powder-bonding agent-composition mixture (B), on a device of the collector type, preferably on a collector of the metallic type and/or of the perforated metal collector type, said collector thus treated making up an electrode; and  
 c—drying the electrode prepared in step b the drying is preferably carried out using an infrared lamp or using a heating element.  
 
     
     
         21 . Process for preparing an electrode based on graphite for a rechargeable generator according to  claim 20 , the method comprising all the steps cited in  claim 18  with the exception of step (c) where two means of drying the electrode are used in parallel, drying with an infrared lamp and a heating element.  
     
     
         22 . Modified graphite-based particles made up of prismatic particles of graphite covered with a metallic deposit and/or a carbonic deposit, the structural parameters of the said particles corresponding to the equations fe 1 =[Y+l]/[(Y+1)+(B/2T)(Y−1)] and fe 2 =[Y+1]/(Y+1)+(B/T) (Y−1)], in which: Y represents a real number greater than or equal to 1, B represents the length of the particle, T represents the thickness of the particle, said particles having the a potatolike shape and having at least one of the following two characteristics: 
 a tab density measured according to the previously defined method, preferably between 0.3 and 1.5, more preferably between 0.5 and 1.4, and most preferably between 1 and 1.3 g/cc;  
 a granulometric dispersion measured according to the previously defined method, such that the D90/D10 ratio varies between 2 and 5 and the particles have a size between 1 and 50 μm, preferably such that the D90/D10 ratio varies between 2.2 and 4.2 and the particles have a size between 2 and 30 μm.  
 
     
     
         23 . Graphite-based particles according to  claim 22  having a size between 1 and 50 μm.  
     
     
         24 . Graphite-based particles according to  claim 22  or  23 , having a sphericity of 80% or more.  
     
     
         25 . Graphite-based particles according to any one of  claims 22  to  24 , in which the average thickness of the metallic and/or carbonated coating is between 50 nm and 2 μm.  
     
     
         26 . Graphite-based particles made up of a coated graphite core, said core making up at least 90% by weight of the total weight of the graphite-based particle, the remaining 10% preferably being made up of at least one metal selected from the group comprising Ag, Si, Al and Cu and/or of carbon and/or of carbonated polymer, preferably in prismatic or fiber form.  
     
     
         27 . Process for preparing graphite-based particles according to any one of  claims 22  to  26 , using prismatic-shaped particles, by coating the particles while keeping the basal function (fb) and the edge function (fe) constant while wrapping the graphite surface with a metallic or carbonic deposit in such a way as to obtain a sphericity of 80% or more.  
     
     
         28 . Use of the modified graphite-based particles such as defined in any one of  claims 22  to  26  or such as obtained by one of the processes defined in any one of  claims 13  to  27  for fuel cells, electrochemical generators or as moisture absorbers and/or oxygen absorbers.  
     
     
         29 . Negative electrode, preferably for a rechargeable electrochemical generator prepared with a bonding agent, preferably a bonding agent of the PVDF, PTFE type and with graphite particles as defined in any one of  claims 22  to  26  or such as obtained by the process defined in  claim 27 .  
     
     
         30 . Process for in situ purification of the surface of graphite particles by coating the particles, in the presence of their impurities, with carbon.  
     
     
         31 . Use in an electrochemical cell of the particles according to  claim 1 , with a control of the basal function (fb) that permits their use in the presence of an electrolyte based on polyethylene carbonate (PC), the concentration of PC in the electrolyte then being less than 50% by volume of the electrolytic mixture.  
     
     
         32 . Safe battery resulting from the use described in  claim 31 .  
     
     
         33 . Use of the graphite-based particles according to  claim 22 , with a constant basal function (fb) which permits their use in the presence of an electrolyte based on polyethylene carbonate (PC), up to a concentration of PC in the electrolyte that is then less than 100% by volume of the electrolytic mixture.  
     
     
         34 . Safe battery resulting from the use described in  claim 33.