Method for synthesis of carbon-coated redox materials with controlled size

Abstract

A method for the synthesis of compounds of the formula C—Li x M 1−y M′ y (XO 4 ) n , where C represents carbon cross-linked with the compound Li x M 1−y M′ y (XO 4 ) n , in which x, y and n are numbers such as 0≦x≦2, 0≦y≦0.6, and 1≦n≦1.5, M is a transition metal or a mixture of transition metals from the first period of the periodic table, M′ is an element with fixed valency selected among Mg 2+ , Ca 2+ , Al 3+ , Zn 2+ or a combination of these same elements and X is chosen among S, P and Si, by bringing into equilibrium, in the required proportions, the mixture of precursors, with a gaseous atmosphere, the synthesis taking place by reaction and bringing into equilibrium, in the required proportions, the mixture of the precursors, the procedure including at least one pyrolysis step of the carbon source compound.

1 . A method for the synthesis of compounds of formula C—Li x M 1−y M′ y (XO 4 ) n , wherein C represents carbon cross-linked with the compound Li x M 1−y M′ y (XO 4 ) n  in which x, y and n are numbers such as 0≦x≦2, 0≦y≦0.6, and 1≦n≦1.5, M is a transition metal or a mixture of transition metals from the first line of the periodic table, M′ is an element with fixed valency selected among Mg 2+ , Ca 2+ , Al 3+ , Zn 2+  or a combination of these same elements and X is chosen from among S, P and Si,
 by bringing into equilibrium, in the required proportions, a mixture comprising at least:   a) a source of M;   b) a source of an element M′;   c) a compound that is a source of lithium; and   d) possibly a compound that is a source of X,   e) a source of carbon, called carbon conductor
 wherein
 the sources of the elements M, M′, Li and X may be introduced or not, in whole or in part, in at least one step, in the form of compounds having more than one source element, and 
 the synthesis is carried out by thermodynamic or kinetic reaction and bringing into equilibrium, in the required proportions, the mixture of the source compounds (also called precursors) a) to d), with a gaseous atmosphere, in such a way as to cause an oxidation state of the transition metal to the desired valency for the forming of Li x M 1−y M′ y (XO 4 ) n , by controlling the composition of the said gaseous atmosphere, the temperature of the synthesis reaction step, and the amount of the source compound c) relative to the other source compounds a), b) and d); 
 said method comprises at least one pyrolysis step of the source compound e) such as to obtain a compound whose electronic conductivity, measured on a sample of powder compressed at a pressure greater than or equal to 3000 is greater than 10 −8  S·cm −1 , and 
 the mixture of the sources is prepared by spray drying. 
 
   
   
   
       2 . A method according to  claim 1 , in which the synthesis reaction between the source compounds a) to d) is carried out simultaneously with the pyrolysis reaction of the source compound e). 
   
   
       3 . A method according to  claim 1 , in which the pyrolysis reaction is carried out in a second step, consecutive to the synthesis reaction between the source compounds a) to d) and in reducing or neutral gas atmosphere. 
   
   
       4 . A method according to  claim 1 , in which the amount of carbon-source compound is chosen in such a way as to coat at least a part of the surface of the particles of the compound of formula Li x M 1−y M′ y (XO 4 ) n  with carbon. 
   
   
       5 . A method according to  claim 1 , in which the amount of carbon conductor source compound in the reaction medium is chosen in such a way as to bond the particles of compound Li x M 1−y M′ y (XO 4 ) n  with each other and to constitute agglomerates with sizes comprised between 1 and 20 microns. 
   
   
       6 . A method according to  claim 1 , in which an organic substance that is the source of the carbon conductor is selected from the group constituted by polymers and oligomers containing a carbon skeleton, simple carbohydrates or polymers and the aromatic hydrocarbons. 
   
   
       7 . A method of synthesis according to  claim 1 , in which the carbon conductor source contains, in the same compound or in the mixture that constitutes this source, oxygen and hydrogen that are bound chemically and from which pyrolysis locally releases carbon monoxide and/or carbon dioxide and/or hydrogen and water vapor that contributes, in addition to depositing carbon, to creating locally the reducing atmosphere required for synthesis of the material Li x M 1−y M′ y (XO 4 ) n . 
   
   
       8 . A method according to  claim 1 , in which the carbon conductor source compound is mainly constituted by a block copolymer comprising at least one carbon source segment that can be pyrolyzed and a segment that is soluble in water and organic solvents in such a way as to allow its distribution throughout the compound Li x M 1−y M′ y (XO 4 ) n  or its precursors. 
   
   
       9 . A method of synthesis according to  claim 8 , in which an organic substance that is the carbon conductor source substance is at least one of the compounds of the group made up of polyethylene, polypropylene, glucose, fructose, sucrose, xylose, sorbose, starch, cellulose and its esters, block polymers of ethylene and ethylene oxide and polymers of furfuryl alcohol. 
   
   
       10 . A method according to  claim 1 , in which the method is carried out continuously in a reactor that promotes the equilibrium of solid powders, agglomerated or not, with the gaseous phase, that allow control of the composition and the circulation of the gaseous atmosphere. 
   
   
       11 . A method according to  claim 1 , in which the method is carried out continuously in a reactor selected from the group consisting of rotary kilns, fluidized beds, and belt-driven kilns. 
   
   
       12 . A method according to  claim 10 , in which the solid feed is greater than 1 kg/h, the temperatures are between 650° C. and 800° C., the dwell time is less than 5 hours. 
   
   
       13 . Method of synthesis according to  claim 1 , in which the reduction is obtained by the action of a reducing atmosphere chosen in such a way as to be able to reduce the oxidation state of the metallic ion M to the level required for the composition of the compound without reducing it to the neutral metallic state. 
   
   
       14 . Method of synthesis according to  claim 13 , in which the reducing atmosphere contains hydrogen or a gas that is capable of generating hydrogen under the synthesis conditions, ammonia or a substance capable of generating ammonia under the synthesis conditions or carbon monoxide, these gases being used in their pure state or in mixtures and it also being possible to use them in the presence of water vapor and/or in the presence of carbon dioxide and/or in the presence of a neutral gas (such as nitrogen or argon). 
   
   
       15 . Method of synthesis according to  claim 13 , in which the reducing atmosphere is made of a mixture of CO/CO 2  or H 2 /H 2 O, NH 3 /H 2 O or a mixture of them, generating an oxygen equilibrium pressure less than or equal to that determined by the transition metal at the state of oxidation corresponding to the precursors introduced to form the compound Li x M 1−y M′ y (XO 4 ) n , but greater than that corresponding to the reduction of any one of the transition elements present in the metallic state, ensuring the thermodynamic stability of Li x M 1−y M′ y (XO 4 ) n  in the reaction mixture, independently of the synthesis reaction time. 
   
   
       16 . Method of synthesis according to  claim 13 , in which the gaseous atmosphere is made of a mixture of CO/CO 2  or H 2 /H 2 O, NH 3 /H 2 O or a mixture of them, generating an oxygen equilibrium pressure greater than or equal to that determined by at least the transition elements, when the precursor is introduced in the metallic form, to form the compound Li x M 1−y M′ y (XO 4 ) n , but greater than that corresponding to a superoxidation of the transition elements beyond their assigned valence in Li x M 1−y M′ y (XO 4 ) n , insuring the thermodynamic stability of Li x M 1−y M′ y (XO 4 ) n  in the reaction mixture, independently of the synthesis reaction time. 
   
   
       17 . Method of synthesis according to  claim 13 , in which the reducing atmosphere is made up of a mixture of CO/CO 2 , H 2 /H 2 O, NH 3 /H 2 O or a mixture of them, generating an oxygen equilibrium pressure less than or equal to that determined by one of the transition metals present in Li x M 1−y M′ y (XO 4 ) n , possibly being able to lead to the reduction of at least this transition element to the metallic state, the compound Li x M 1−y M′ y (XO 4 ) n  being obtained by controlling the temperature and the contact time with the gaseous phase or the proportion of the precursor c) in the reaction mixture; the synthesis temperature being comprised between 200 and 1200° C. and the time of contact between the reaction mixture and the gaseous phase being comprised between 2 minutes and 5 hours. 
   
   
       18 . Method according to  claim 13 , in which the gaseous reducing atmosphere is obtained by decomposition, in a vacuum or in an inert atmosphere, of an organic compound or of a mixture of organic compounds containing at least hydrogen and oxygen, bound chemically, and of which the pyrolysis generates carbon monoxide and/or a mixture of carbon dioxide and monoxide, of hydrogen and/or a mixture of hydrogen and water vapor that is able to carry out the reduction that leads to the formation of the compound Li x M 1−y M′ y (XO 4 ) n . 
   
   
       19 . A method of synthesis according to  claim 13 , in which the gaseous reducing atmosphere is obtained by partial oxidation by oxygen or by air, of a hydrocarbon and/or of carbon, possibly in the presence of water vapor, at an elevated temperature comprised between 400 and 1200° C., making possible the formation of carbon monoxide or hydrogen or of a mixture of carbon monoxide and hydrogen. 
   
   
       20 . A method according to  claim 1 , in which the gaseous phase is made up of a gas that is reformed in-situ or ex-situ. 
   
   
       21 . A method according to  claim 1 , in which the thermal processing (which includes the formation reaction of Li x M 1−y M′ y (XO 4 ) n  and the reduction and pyrolysis and possibly dehydration of one or several of sources a) to d)) is carried out by heating from normal temperature to a temperature between 500 and 1100° C. 
   
   
       22 . A method of synthesis according to  claim 21 , in which the maximum temperature reached is comprised between 500 and 800° C. 
   
   
       23 . A method of synthesis according to  claim 1 , in which the dwell time of the reagents in the thermal processing step is less than 5 hours. 
   
   
       24 . A method of synthesis according to  claim 1 , in which the source of M is also the source of X and/or the source of M′ is also the source of X and/or the source of lithium is also the source of X and/or the source of X is also the source of lithium. 
   
   
       25 . A method of synthesis according to  claim 1 , in which the transition metal or metals is (are) chosen at least partially from the group constituted by iron, manganese, cobalt and nickel, the complement for the transition metals being chosen from the group constituted by vanadium, titanium, chromium and copper. 
   
   
       26 . A method of synthesis according to  claim 1 , in which the compound that is the source of M is in an oxidation state that can vary from 3 to 7. 
   
   
       27 . A method of synthesis according to  claim 26 , in which the compound that is the source of M is iron (III) oxide or magnetite, manganese dioxide, di-vanadium pentoxide, trivalent ferric phosphate, ferric hydroxyphosphate and lithium or trivalent ferric nitrate or a mixture of the latter. 
   
   
       28 . A method of synthesis according to  claim 1 , in which the compound that is the source of lithium is chosen from the group constituted by lithium oxide or lithium hydroxide, lithium carbonate, the neutral phosphate Li 3 PO 4 , the acid phosphate LiH 2 PO 4 , the orthosilicates, the metasilicates or the polysilicates of lithium, lithium sulfate, lithium oxalate and lithium acetate or a mixture of the latter. 
   
   
       29 . A method of synthesis according to  claim 1 , in which the source of X is chosen from the group constituted by sulfuric acid, lithium sulfate, phosphoric acid and its esters, the neutral phosphate Li 3 PO 4  or the acid phosphate LiH 2 PO 4 , the monoammonium or diammonium phosphates, trivalent ferric phosphate, manganese and ammonium phosphate (NH 4 MnPO 4 ), silica, lithium silicates, alkoxysilanes and their partial hydrolysis products and mixtures of the latter. 
   
   
       30 . A method according to  claim 1 , in which at least one of the lithium derivatives obtained is of the formula LiFePO 4 , LiFe 1−s Mn s PO 4  wherein 0≦s≦0.9, LiFe 1−y Mg y PO 4  and LiFe 1−y Ca y PO 4  wherein 0≦y≦0.3, LiFe 1−s−y Mn s Mg y PO 4  wherein 0≦s≦1 and 0≦y≦0.2, Li 1+x FeP 1−x Si x O 4  wherein 0≦x≦0.9, Li 1+x Fe 1−s Mn s P 1−x Si x O wherein 0≦s≦1, Li 1+z Fe 1−s−z Mn s P 1−z S z O 4  wherein 0≦s≦1, 0≦z≦0.2, Li 1+2q Fe 1−s−q Mn s PO 4  wherein 0≦s≦1, and 0≦q≦0.3, Li 1+r Fe 1−s Mn s (S 1−r P r O 4 ) 1.5  wherein 0≦r≦1, 0≦s, t≦1 or Li 0.5+u Fe 1−t Ti t (PO 4 ) 1.5  wherein 0≦t≦1 and wherein 0≦u≦1.5. 
   
   
       31 . A method of synthesis according to  claim 1 , in which the reaction parameters, and in particular the kinetics of the reduction by gaseous phase, are chosen in such a way that the carbon conductor is not consumed in the course of the reduction process. 
   
   
       32 . A method of synthesis according to  claim 31 , in which the amount of substance that is the carbon conductor source, present in the reaction medium subjected to reduction, is chosen such that the amount of carbon conductor in the reaction medium will be between 0.1 and 15% of the total mass of the reaction mixture. 
   
   
       33 . A method of synthesis according to  claim 1 , in which the temperature and duration of the synthesis are chosen as a function of the nature of the transition metal, i.e. above a minimum temperature at which the reactive atmosphere is capable of reducing the transition element or elements to their oxidation state required in the compound Li x M 1−y M′ y (XO 4 ) n  and below a temperature or a time leading to a reduction of the transition element or elements to the metallic state or an oxidation of the carbon resulting from pyrolysis of the organic substance. 
   
   
       34 . A method of synthesis according to  claim 1 , in which the compound that is the source of carbon is chosen in such a way that it is easily dispersible at the time of the processing used to insure an intimate mixture with precursors a) to d) by solubilization, by agitation and/or by mechanical grinding and/or by ultrasound homogenization in the presence, or not, of a liquid or by spray-drying of a solution of one or several precursors and/or of a suspension and/or of an emulsion.