DESCRIPTION OF THE INVENTION

[0030] A first object of the present invention consists of the synthesis of compounds of the formula 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 chosen 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,

[0031] by bringing into equilibrium, in the required proportions, a mixture containing at least:

[0032] a) a source of M, at least one part of the said transition metal or metals making up M being in an oxidation state greater than that of the metal in the final compound Li _x M _1−y M′ _y (XO ₄ ) _n ;

[0033] b) a source of an element M′;

[0034] c) a compound that is a source of lithium; and

[0035] d) possibly a compound that is a source of X,

[0036] the sources of the elements M, M′, Li and X being introduced or not, in whole or in part, in the form of compounds having more than one source element, and

[0037] the synthesis being carried out by reaction and bringing into equilibrium, in the proportions required, the mixture (preferably intimate and/or homogeneous mixture) of the precursors a) to d) and by reduction in such a way as to bring the transition metal or metals to the desired degree of valency.

[0038] The reduction may be carried out in different ways, by the techniques used in the production of ceramics and by different reducing agents, based on the chemistry of carbon derivatives, among them CO, hydrocarbons and various organic compounds, hydrogen and ammonia.

[0039] According to a preferred embodiment of the present invention, the reduction of the mixture of precursors a) to d) is carried out with a reducing gaseous atmosphere.

[0040] The source of M can also be the source of X and/or the source of M′ can also be the source of X and/or the source of lithium can also be the source of X and/or the source of X can also be the source of lithium.

[0041] According to a preferred embodiment of the invention, bringing the mixture of precursors a) to d) into equilibrium is carried out in the form of an intimate and/or homogeneous mixture. In the meaning of the present invention, intimate mixture is preferably understood as a mixture that does not contain aggregates of particles of individual components of the mixture and that has particle sizes preferably less than 10 micrometers, preferably less than 5 micrometers. The methods that make this implementation possible are known to the person skilled in the art, such as co-grinding in the presence of balls of hard material (such as steel, tungsten carbide, aluminum, zirconium), dry or in the presence of a liquid, i.e. with equipment of the cyclone type where the particles are ground by striking against the walls of the equipment or against itself, ultrasonic energy, or even spray drying of solutions or suspensions.

[0042] Homogeneous mixture is understood as a mixture in which no local variation in concentration of one of the components of the mixture is evident from which the fraction of the mixture chosen for sampling would be extracted. Mechanical agitation by the slanting of blades or of magnetic stirring bars for suspensions, planetary mixers with cycloid movement are examples of the methods used; by definition the solutions respond to this criterion.

[0043] In the scope of the present invention, the transition metal or metals is (are) advantageously chosen at least partially in the group constituted by iron, manganese, cobalt and nickel, the remainder of the transition metals preferably being chosen in the group constituted by vanadium, titanium, chromium and copper.

[0044] Advantageously, the compound that is the source of M is in an oxidation state that can vary from 3 to 7.

[0045] According to a preferred embodiment of the invention, the source compound of M is iron (III) oxide or magnetite, manganese dioxide, di-vanadium pentoxide, trivalent iron phosphate, trivalent iron nitrate, trivalent iron sulfate, iron hydroxyplosphate and lithium hydroxyphosphate or trivalent iron sulfate or nitrate or a mixture of the latter.

[0046] According to another preferred embodiment of the procedure, 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; still more preferably, the compound that is the source of lithium is lithium carbonate of the formula Li ₂ CO ₃ .

[0047] According to another advantageous method, the source of X is selected in 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 iron phosphate, manganese and ammonium phosphate (NH ₄ MnPO ₄ ), silica, lithiumn silicates, alkoxysilanes and their partial hydrolysis products and mixtures of the latter. Still more advantageously, the compound that is the precursor of X is iron phosphate, preferably iron (III) phosphate, anhydrous or hydrated.

[0048] The procedure according to the invention works particularly well for the preparation of one or more of the following compounds, in which at least one of the lithium derivatives obtained has 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, 0≦u≦2.5. The method according to the invention yields improved results when compounds of the formula Li _x M _1−y M′ _y (XO ₄ ) _n are obtained that have an olivine or Nasicon structure, including the monoclinic form.

[0049] The reduction is obtained by the action of a reducing atmosphere chosen in such a way as to reduce the oxidation state of the metallic ion M to the level required for the composition of the compound without, however, reducing it to the neutral metallic state. This reducing atmosphere preferably 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 nitroqen or argon).

[0050] According to an advantageous embodiment, 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 to the metallic state, insuring the thermodynamic stability of Li _x M _1−y M′ _y (XO ₄ ) _n in the reaction mixture, independent of the synthesis reaction time.

[0051] According to another advantageous embodiment of the invention, the reducing atmosphere is made 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; the synthesis temperature preferably being between 200 and 1200° C., still more preferably between 500 and 800° C. and the time of contact between the reaction mixture and the gaseous phase preferably being between 2 minutes and 5 hours and still more preferably between 10 and 60 minutes. This control is implemented more easily at the time of reduction by a gaseous phase due to the rapid diffusion of the gas molecules around the grains. In addition, the nucleation of the metallic phase is slow and thus can be more easily avoided due to the rapid reduction by the gaseous phase.

[0052] The gaseous reducing atmosphere is preferably 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 pyrolysis of which 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 .

[0053] The reducing gas atmosphere is preferably obtained by partial oxidation by oxygen or by air, of a hydrocarbon and/or carbon called sacrificial carbon.

[0054] Preferably, sacrificial carbon is understood as carbon that is intended to carry out a chemical reaction, in particular with the gaseous phase, which in fact serves as a vector for reduction of precursors of the material, the carbon thus being intended to be consumed.

[0055] In the scope of the present invention) the amount of water vapor preferably corresponds to between 0.1 and 10 molecules, inclusively, of H ₂ O per atom of hydrocarbon at an elevated temperature (preferably comprised between 400 and 1200° C.) that makes possible the formation of carbon monoxide or hydrogen or a mixture of carbon monoxide and hydrogen.

[0056] According to an advantageous method, the sacrificial carbon is chosen from the group constituted by natural or artificial graphite carbon black or acetylene black and coke (preferably from petroleum), the sacrificial carbon preferably being in the form of particles with a size that is preferably less than 15 micrometers, and still more preferably less than 2 micrometers.

[0057] The quantity of sacrificial carbon is preferably less than or essentially equal to the quantity required to reduce the reaction mixture without allowing residual sacrificial carbon; this quantity is preferably a carbon atom that is able to combine with an oxygen atom at temperatures greater than 750° C. and it is preferably an atom that can combine with 2 oxygen atoms at temperatures less than 750° C.

[0058] According to another advantageous embodiment, the synthesis is carried out with a quantity of sacrificial carbon that is (essentially) equal, mol for mol, to half the quantity of oxygen needed to be removed from the mixture of the precursor compounds to obtain the material Li _x M _1−y M′ _y (XO ₄ ) _n by reduction, when the reaction is carried out below 710° C. and equal, mol for mol, to this quantity when the reaction is carried out below this temperature.

[0059] A second object of the present invention is a method of synthesis for a material comprising a core and a coating and/or being connected to each other by cross-linking, the said core comprising at least one compound of the formula Li _x M _1−y M′ _y (XO ₄ ) _n , in which x, y and n arc 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 chosen from among Mg ²⁺ , Ca ²⁺ , Al ³⁺ , Zn ²⁺ and X is chosen from among S, P and Si, and

[0060] the said coating is made of a layer of carbon,

[0061] the said cross-linking is made of carbon (connecting at least two particles to each other),

[0062] the said method consists of bringing into equilibrium (preferably intimate and/or homogeneous), in the proportions required, a mixture containing at least

[0063] a) a source of M, at least one part of the said transition metal or metals making up M being in an oxidation state greater than that of the metal in the final compound Li _x M _1−y M′ _y (XO ₄ ) _n ;

[0064] b) a source of an element M′;

[0065] c) a compound that is a source of lithium, and

[0066] d) possibly a compound that is a source of X;

[0067] the sources of the elements M, M′, Li and X being introduced or not, in whole or in part, in the form of compounds having more than one source element, and

[0068] the synthesis being carried out by reaction and bringing into equilibrium, in the proportions required, the mixture of the precursors a) to d) with a reducing gaseous atmosphere, in such a way as to bring the transition metal or metals to the desired degree of valency,

[0069] the synthesis being carried out in the presence of a carbon source called carbon conductor,

[0070] the synthesis thus leading to the said material, by a pyrolysis step for the carbon source compound after, or preferably simultaneously with, the steps of preparation of the mixture (preferably intimate and/or homogeneous) of the precursors and reduction of the mixture obtained.

[0071] The carbon present in the material, in the form of coating and cross-linking, adheres intimately to the material and lends to the latter an electronic conductivity that is greater than that of the material constituted by the corresponding non-coated particles.

[0072] According to a preferred method, the addition of carbon conductor is carried out after the synthesis of the Li _x M _1−y M′ _y (XO ₄ ) _n .

[0073] According to another advantageous embodiment of the invention, the addition of carbon conductor is carried out simultaneously With the synthesis of the Li _x M _1−y M′ _y (XO ₄ ) _n .

[0074] The reaction parameters, in particular the kinetics of the reduction by the gaseous phase, are chosen in such a way that the carbon conductor does not participate in a significant way in the reduction process.

[0075] According to another important variation, the reaction parameters of the synthesis, such as flow and composition of the gaseous phase, temperature and contact time, are chosen in such a way that the carbon conductor does not participate in a significant way in the reduction process, i.e. the reduction process is due to the gaseous phase, and in particular in such a way that the reaction temperature is preferably less than 900° C. and the reaction time less than 5 hours, in a manner that is even more advantageous if the reaction temperature is below 800° C. and/or for a time less than 1 hour.

[0076] According to another variation of the synthesis, the value of x in Li _x M _1−y M′ _y (XO ₄ ) _n is chosen in such a way as to insure thermodynamic control and/or rapid kinetics of the reduction by making it possible to select reducing gaseous atmospheres that are easily accessible by simple mixture of eases or by reforming simple organic molecules.

[0077] The organic substance that is the source of carbon conductor is selected in such a way that the particles of material obtained after the pyrolysis step essentially have the form and granulometric distribution of the precursors of the synthesis reaction.

[0078] Thus, the organic substance that is the source of the carbon conductor is advantageously selected from the group constituted by polymers and oligomers containing a carbon skeleton, simple carbohydrates or polymers and the aromatic hydrocarbons.

[0079] The organic substance that is the source of carbon conductor is chosen in such a way as to leave a deposit of carbon conductor on the surface (coating) of the solid particles that are made up of the material and/or between these solid particles making up the carbon bridges (cross-linking) at the time of pyrolysis.

[0080] According to another variation, 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 .

[0081] Thus, the organic substance that is the source of carbon conductor source is at least one of the compounds of the group constituted by polyethylene, polypropylene, glucose, fructose, sucrose, xylose, sorbose, starch, cellulose and its esters, block polymers of ethylene and ethylene oxide and polymers of furfuryl alcohol.

[0082] The source of carbon conductor is preferably added at the start of, or in the course of, the mixing step of the reaction precursors a) to d).

[0083] According to a preferred method, 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 mixture is preferably comprised between 0.1 and 25%, inclusively, and still more preferably comprised between 0.3 and 1.5%, inclusively, of the total mass of the reaction mixture.

[0084] Advantageously, in the method according to the first object of the invention, as well as in the method according to the second object of the present intention, the thermal processing (which includes the formation reaction of Li _x M _1−y M′ _y (XO ₄ ) _n and the reduction and pyrolysis) is carried out by heating from normal temperature to a temperature comprised between 500 and 1100° C. in the presence of a reducing atmosphere such as defined above. This maximum temperature reached is even more advantageously comprised between 500 and 800° C.

[0085] According to another advantageous embodiment, 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.

[0086] According to another advantageous embodiment of the procedure according to the second object of the present invention, the heart of the core of the particles of the core synthesized is at least 95% an Li _x M _1−y M′ _y (XO ₄ ) _n compound (preferably the compound synthesized has the formula LiMPO ₄ ), the remainder may be an oxide of one or [more] of the metals of the precursors, functioning as an insertion or inert compound, carbon, carbonate or lithium phosphate and the amount of carbon conductor after pyrolysis is comprised between 0.1 and 10% by mass in comparison to the mass of the compound LiMPO ₄ .

[0087] The compound that is the source of carbon conductor is advantageously chosen such that it is easily dispersible at the time of mixture with the precursors. The intimate and/or homogeneous mixture of precursors a) to d) is advantageously produced by agitation and/or by mechanical grinding and/or by ultrasonic homogenizing, in the presence, or not, of a liquid or by spray-drying of a solution of one or more precursors and/or of a suspension and/or of an emulsion.

[0088] According to a particularly advantageous embodiment the synthesis according to the present invention comprise the two steps:

[0089] i) intimate grinding dry or in a solvent, of the source compounds including carbon, and drying if necessary; and

[0090] ii) thermal processing with scavenging by a controlled reducing atmosphere.

[0091] The procedures according to the invention make it possible to carry out the synthesis of materials having a conductivity that is greater than 10 ⁻⁸ Scm ⁻¹ , measured on a sample of powder compacted at a pressure greater than or equal to 3000, preferably 3750 Kg.cm ⁻² .

[0092] The conductivity measurement is carried out on powders of the sample. This powder (from 100 mg to around 1 g) is placed in a hollow cylindrical mold, 1.3 cm in diameter, made of poly(oxymethylene) (Delrin®) and it is compacted between two stainless steel pistons with a laboratory press having a force of 5.10 ³ Kg, which corresponds to a pressure of 3750 Kg.cm ⁻² .

[0093] The conductivity measurement is carried out by using the pistons (plungers) as electrodes and using the complex impedance method known to the person skilled in the art. The conductivity is obtained from the resistance, using the formula ρ=RS/l where R is the measured resistance, S is the surface (1.33 cm ² for 1.3 cm diameter), l is the thickness of the sample and the resistivity is determined using the formula ρ=RS/l.

[0094] One of the compounds advantageously prepared by the procedure according to the first object of the present invention is the compound of formula LiFePO ₄ .

[0095] One of the materials advantageously obtained by the synthesis procedure according to the second object of the present invention is made up of particles having a core, a coating and/or a cross-linking. The core of the particles is essentially (preferably at least 95%) made up of a compound of formula LiFePO ₄ , the remainder being made up of other compounds, in particular other oxides having an activity or not, or lithium phosphate or ferric phosphate and in which the coating and/or cross-linking of the particles of the material by carbon preferably represents an amount of carbon conductor between 0.2 and 5%, preferably between 0.3 and 3%, in comparison to the total mass of the material obtained.

[0096] The compound that is the source of iron, in particular the compound that is the source of iron in the synthesis of the compound of formula LiFePO ₄ , is chosen at least partially from the group constituted by iron phosphates, iron oxyphosphates or hydroxyphosphates, iron oxides and lithium oxides, in which at least a part of the iron is in the oxidation state III, as well as mixtures of the latter.

[0097] The compound that is the source of lithium is advantageously lithium phosphate, lithium dihydrogenophosphate, lithium carbonate, lithium acetate or lithium hydroxide, as well as mixtures of the latter.

[0098] The compound that is the source of phosphorus is advantageously ammonium phosphate, orthophosphoric, metaphosphoric or pyrophosphoric acid or phosphorus pentoxide.

[0099] The synthesis procedure according to the invention can be carried out in the reactor that is used, or was used, for the preparation of the mixture of precursors or in a different reactor (preferably in a reformer).

[0100] A third object of the present invention is made up by particles of a compound of formula 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 chosen from among Mg ²⁺ , Ca ²⁺ , Al ³⁺ , Zn ²⁺ and X is chosen from among S, P and Si, the said compound having a conductivity greater than 10 ⁻⁸ S.cm ⁻¹ , measured on a sample of powder compressed at a pressure of 7350 Kg.cm ² , the particles having a size between 0.05 micrometers and 15 micrometers, preferably between 0.1 and 10 micrometers.

[0101] A fourth object of the present invention consists of a material that can be obtained by a procedure according to the second object of the present invention, comprising a core and a coating and/or a cross-linking, said material having a total carbon amount greater than 0.1%, preferably between 0.1 and 0.2%, of the total mass of the material.

[0102] According to an advantageous embodiment of this fourth object, the said core contains at least one compound of the formula 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 chosen from among Mg ²⁺ , Ca ²⁺ , Al ³⁺ , Zn ²⁺ and X is chosen from among S, P and Si, the said material having a conductivity greater than 10 ⁻⁸ S.cm ⁻¹ , measured on a sample of powder compacted at a pressure of 3750 Kg.cm ⁻² .

[0103] A fifth object of the present invention consists of a material having a core and a coating and/or a cross-linking, the said core comprising at least one compound of formula 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 chosen in the group constituted by Mg ²⁺ , Ca ²⁺ , Al ³⁺ , Zn ²⁺ and X is in the group constituted by S, P and Si, the said material having a conductivity greater than 10 ⁻⁸ S.cm ⁻¹ , measured on a sample of powder compacted at a pressure of 3750. The materials thus obtained have, according to the measurement method explained above, good conductivity that in some cases is greater than 10 ⁻⁸ Scm ^{31 1} on a sample of compacted powder and one of carbon greater than 0.1%, preferably between 0.1 and 0.2%, of the total mass of the material.

[0104] The uses of these materials are very important in the area of electrochemistry, as electrode material, used alone or in a mixture with other electrically active materials, in particular in cells serving as primary or secondary generators, possibly connected in batteries or accumulators; in super-capacities, systems capable of storing electrical energy with significant power (≧800 Wl ^{31 1} ), in electrochromic light modulation systems and antiglare mirrors for automobiles. In systems for separation or purification of metals, especially of lithium, water purification, in oxidation reactions or reduction reactions in organic synthesis; in the case of oxidation reaction it may be necessary to delithiate the material chemically or electrochemically to increase its oxidizing power.

[0105] A sixth object of the present invention consists of electrochemical cells containing at least two electrodes and at least one electrolyte, these cells being characterized in that at least one of their electrodes comprises at least one compound according to the third object of the invention.

[0106] A seventh object of the present invention consists of electrochemical cells containing at least two electrodes and at least one electrolyte, these cells being characterized in that at least one of their electrodes comprises at least one material according to the fourth object of the invention.

[0107] These cells are preferably designed in such a way that the electrolyte is a polymer, solvating or not, optionally plasticized or gelled by a polar liquid containing one or more metallic salts in solution, by way of example.

[0108] Advantageously, the electrolyte is a polar liquid immobilized in a microporous separator, containing one or more metallic salts in solution; by way of example at least one of these metallic salts is a lithium salt.

[0109] Preferably at least one of the negative electrodes is made of metallic lithium, a lithium alloy, especially with aluminum, antimony, zinc, tin, possibly in nanomolecular mixture with lithium oxide or a carbon insertion compound, especially graphite, a double nitride of lithium and iron, cobalt or manganese, a lithium titanate of the formula Li _x Ti _(5+3y)/4 O ₄ , wherein 1≦x≦(11−3y)/4 (or) wherein 0≦y≦1.

[0110] According to another embodiment of the cells according to the invention, at least one of the positive electrodes contains one of the products that can be obtained by a procedure according to the invention, used alone or in a mixture with a double oxide of cobalt and lithium or with a complex oxide of the formula Li _x Ni _{1−y−z−q−r} Co _y Mg _z Al _r O ₂ wherein 0.1≦x≦1, 0≦y, z and r≦0.3or with a complex oxide of the formula Li _x Mn _{1−y−z−q−r} Co _y Mg _z AlrO ₂ −qF _q wherein 0.05≦x≦1 and 0≦y, z, r, q≦0.3.

[0111] The polymer used to bond the electrodes or used as electrolytes is advantageously a polyether, a polyester, a polymer based on methyl methacrylate units, an acrylonitrile-based polymer and/or a vinylidene fluoride, or a mixture of the latter.

[0112] Preferably, the cell contains a solvent that is preferably a non-protogenic solvent that contains, e.g. ethylene or propylene carbonate, an alkyl carbonate having 1 to 4 carbon atoms, γ-butyrolactone, a tetraalkylsulfamide, an α-ω dialkyl ether of a mono-, di-, tri-, tetra- or oligo-ethylene glycol with molecular weight less than or equal to 5000, as well as mixtures of the above-named solvents.

[0113] The cells according to the invention preferably function as primary or secondary generator, as supercapacity or as light modulation system.

[0114] According to another preferred method, the electrochemical cells according to the present invention function as supercapacity, characterized in that the positive electrode material is a material according to the third, fourth or the fifth object of the present invention and the negative electrode is a carbon with a specific surface area greater than 50 m ² .g ⁻¹ in the form of powder, fiber or mesoporous composite of the carbon-carbon composite type.

[0115] According to another embodiment, the electrochemical cells function as a light modulation system and in that the optically inactive counter-electrode is a material according to the third, fourth and fifth object of the present invention, spread in a thin layer on a transparent conductor support of a glass or polymer type covered with doped tin oxide (SnO ₂ :Sb or SnO ₂ :F) or doped indium oxide (In ₂ O ₃ :Sn).

[0116] Preferred Methods

[0117] The proposed invention relates to a new method for simplified synthesis of Li _x MXO ₄ compounds with olivine structure obtained by reduction of a mixture in which at least a part of the transition metal M is in an oxidation state higher than that of the final compound LiMPO ₄ . Another surprising advantage of the present invention is to also be compatible with the synthesis described in CA-A-2,270,771, which leads to optimized performance. In this case, the organic compound that is the carbon source is added to the mixture of the initial reagents containing, at least partially, transition metal in a state of oxidation greater than that of the lithium compound LiMPO ₄ and the simplified synthesis leads directly to the material covered in carbon. The simplification involves, in particular, a reduction in the number of steps and above all, in the number of steps where control of the atmosphere is necessary. Reference can be made to “Modern Batteries”, by C. A. Vincent & B. Scrosati, Arnold publishers, London, Sydney, Auckland, (1997).

[0118] The improvements also relate to the reproducibility of the synthesis, to the control of the size and distribution of the particles and to a reduction in the number and cost of the initial reagents and naturally of the final material. This synthesis, when combined with the teachings of CA-A-2,270,771, also makes it possible to control the amount of carbon in the final material.

[0119] We are reporting here, for the first time, the synthesis of a Li _x MXO ₄ compound of olivine type, in this case LiFePO ₄ , produced by reduction of an iron (III) salt. Since the initial salts are no longer sensitive to oxidation, the synthesis process is greatly simplified. In addition, the possible use of Fe ₂ O ₃ as a source of iron considerably reduces the cost of synthesizing LiFePO ₄ . This material would thus be preferable to other cathode materials for lithium batteries, such as cobalt or nickel oxides in the case of lithium-ion batteries, or vanadium oxides V ₂ O ₅ or analogs that are less inoffensive to the environment.

[0120] LiFePO ₄ can be prepared using an iron (III) salt that is stable in air, e.g. FePO ₄ .2H ₂ O or Fe ₂ O ₃ or any other source of iron (III). The lithium source would be e.g. Li ₂ CO ₃ in the first case, or LiOH, LiH ₂ PO ₄ or Li ₃ PO ₄ would be used as a source of both lithium and phosphorus in the second case. The stoichiometric mixtures, as well as the carbon precursor, are processed at 700° C. for 4 hours with scavenging by an excess of reducing atmosphere in such a way as to reduce the oxidation state of the iron. The choice of the synthesis atmosphere and temperature is very important in order to be able to reduce the iron (III) to iron (II) without the gaseous atmosphere or the carbon present being able to reduce the iron to the metallic state. The latter will preferably, but in a non-limiting manner, be made up e.g. of hydrogen, ammonia, of a Gaseous mixture capable of supplying hydrogen under the synthesis conditions, the hydrogen being able to be used pure or diluted in a dry or hydrated inert gas, carbon monoxide, possibly mixed with carbon dioxide and/or a dry or hydrated neutral gas. The maximum thermal processing temperature is chosen such that the carbon present will be thermodynamically stable with respect to the iron (II) and preferably with respect to the gaseous phase. In the case of iron, the limit temperature zone is between 500 and 800° C., preferably around 700° C. Beyond these temperatures, the carbon becomes sufficiently reducing to reduce the iron (II) to metallic iron. In the case of other transition metals., any person skilled in the art would be able to use Ellingham curves to adapt the temperature and the nature of the gaseous atmosphere in order to obtain an equivalent result.

[0121] An unexpected and surprising aspect of the invention that is advantageous is the relative chemical inertia of the carbon deposited on the surface of the material with respect to reactions that make it possible to reduce the degree of oxidation of the transition metal, in particular, of iron. From a thermodynamic point of view, the carbon formed by decomposition of the pyrolyzed organic substance has a reducing power that is adequate to oxidize into CO ₂ or CO and to reduce, even in an inert atmosphere, Iron (III) to Iron (II), which would make controlling the amount of carbon in the final product difficult. The inventors have noted that the reduction reaction was almost totally due to the action of the reducing gas atmosphere, of which the kinetics are faster than those due to the action of the carbon deposited on the surface, in spite of the intimate contact between the two solid phases (carbon and redox material). By using a reducing atmosphere, preferably based on hydrogen, ammonia or carbon monoxide, the reduction of the iron by the solid carbon is not promoted kinetically and the Iron (III) is reduced to Iron (II) mainly by reaction with the reducing atmosphere. The amount of carbon in the final product thus essentially corresponds to the decomposition yield of the organic substance, which makes it possible to control this amount.

[0122] The following examples are given to better illustrate the present invention, but they should not be interpreted as constituting a limitation to the scope of the present invention.

EXAMPLES

Example 1

Synthesis of LiFePO ₄ Using Iron Phosphate in Reducing Atmosphere

[0123] LiFePO ₄ was prepared by reaction of FePO ₄ .2H ₂ O and Li ₂ CO ₃ in the presence of hydrogen. In a first step, stoichiometric quantities of the two compounds are ground together in isopropanol, then heated progressively (6° C. per minute up to 700° C.) in a tube kiln under reducing gas scavenging (8% hydrogen in argon). This temperature is maintained for one hour. The sample is cooled for 40 minutes, which would be with a cooling speed of around 15° C. per minute.

[0124] The reducing gas flow is maintained during the entire thermal processing time and also during the temperature drop. The total thermal processing time is around three and a half hours.

[0125] The structure of the sample was verified by X-ray diffraction and the rays correspond to those of pure triphylite LiFePO ₄ .

Example 1′

Preparation of LiFePO ₄ Coated with Carbon Synthesized Using the Sample Prepared in Example 1

[0126] The triphylite obtained in example 1 is impregnated with a solution of cellulose acetate (39.7% by weight of acetyl, average molecular weight M _w of 50,000) in acetone. The quantity of cellulose acetate added represents 5% of the weight of the processed triphylite. The use of a carbon precursor in solution makes possible a perfect distribution over the particles of triphylite. After drying, the mixture is placed in the kiln described above under scavenging by an argon atmosphere. The temperature is increased by 6° C. per minute up to 700° C. The latter temperature is maintained for one hour. The sample is then cooled progressively, still under argon scavenging. This sample contains 1% by weight of carbon, which corresponds to a carbonation yield of the cellulose acetate of 20%.

[0127] The material exhibits electronic surface conductivity. The latter was measured on a pastille of compacted powder. A force of 5 tons is applied at the time of measurement on a sample that is 1.3 cm in diameter. Under these conditions, the electronic conductivity measured is a 5.10 ⁻⁵ S.cm ⁻¹ .

Example 1″

Comparison of the Electrochemical Behavior of Materials Prepared in Examples 1 and 1′ in Electrochemical Cells

[0128] The materials prepared in example 1 and 1′ were tested in button-type CR 2032 cells of lithium polymer batteries at 80° C. The cathodes were prepared by mixing the powder of the active material with carbon black (Ketjenblack®) to insure electronic exchange with the current collector and poly(ethylene oxide) with mass 400,000 used as the binding agent on one hand, and ionic conductor on the other. The proportions by weight are 51:7:42. Acetonitrile is added to the mixture to dissolve the poly(ethylene oxide) in a quantity that is adequate to form a homogeneous suspension. This suspension is then dripped onto a 1 cm ² stainless steel disk. The cathode thus prepared is dried in a vacuum, then transferred in a glove box under helium atmosphere (<1 ppm H ₂ O, O ₂ ). A sheet of lithium (27 μm) laminated on a nickel substrate was used as the anode. The polymer electrolyte was made of poly(ethylene oxide) with mass 5,000,000 and a bistrifluorosulfonimide lithium salt Li[(CF ₃ SO ₂ ) ₂ N]) (hereinafter referred to as LiTFSI) in oxygen proportions of oxyethylene units/lithium ions of 20:1.

[0129] Electrochemical experiments were carried out at 80° C., the temperature at which the ionic conductivity of the electrolyte is adequate (2×10 ⁻³ Scm ⁻¹ ).

[0130] FIG. 1 shows the first cycle obtained by slow voltametry, a technique well known to the person skilled in the art (20 mV.h ⁻¹ ), controlled by a Macpile® type battery cycler (Biologic™, Claix, France), of the samples prepared in example 1 and 1′.

[0131] The non-carbonated compound in example 1 shows the oxidoreduction peaks characteristic of LiFePO ₄ . The capacity exchanged at the time of the reduction process represents 74% of the theoretical value. The reaction kinetics are slow and the discharge extends to 3 volts. These capacity and kinetic limitations of the reactions are currently observed for the samples of non-carbonated LiFePO ₄ . The carbonated compound from example 1′ shows well-defined oxidoreduction peaks and reaction kinetics that are much more rapid than those of the material resulting from the synthesis described in example 1. The capacity achieved in discharge is 87% of the theoretical value, which represents an improvement in the electrochemical generator capacity of 17% in comparison to that of the non-carbonated sample in example 1.

Example 2

Synthesis of Carbonated LiFePO ₄ Using Iron Phosphate in Reducing Atmosphere

[0132] Carbonated LiFePO ₄ was prepared by reducing reaction of FePO ₄ .2H ₂ O and Li ₂ CO ₃ in the presence of hydrogen. In a first step, the stoichiometric quantities of the two compounds, as well as the carbon source, (cellulose acetate, 39.7% by weight of acetyl, average molecular weight M _w of 50,000) in low proportion (5% by weight in comparison to the weight of FePO ₄ 2H ₂ O, i.e. 4.2% in comparison to the weight of the mixture of FePO ₄ .2H ₂ O and Li ₂ CO ₃ ) are ground together in isopropanol. The solvent is evaporated and the mixture subjected to the thermal processing described in examples 1 and 1′. Throughout the entire thermal processing and also at the time of the temperature drop, the reducing atmosphere is applied by a scavenging of a mixture of 8% hydrogen in argon.

[0133] The structure of the sample was verified using X-ray diffraction and the rays correspond to those of pure triphylite LiFePO ₄ .

[0134] The prepared sample is constituted by very fine particles on the order of a micrometer ( FIG. 2 ). These particles are covered with a fine layer of carbon, of which the weight represents 1.2% of the total weight of the sample, measured by gravimetry after dissolving the core of LiFePO ₄ in 2M hydrochloric acid.

[0135] The material exhibits electronic surface conductivity. The latter was measured according to the procedure described in example 1′. Under these conditions, the electronic conductivity measured is 2.10 ⁻³ Scm ⁻¹ .

[0136] Taking into account the residual quantity of carbon in the sample, the carbonation yield of the cellulose acetate at the time of synthesis is 20%. It is important to note that this yield is identical to that obtained in example 1′, where the triphylite LiFePO ₄ is already formed and no reducing step is necessary.

[0137] Thus it is evident that the carbon that comes from decomposition of the cellulose acetate is not consumed and does not interfere in the reaction that reduces iron (III) to iron (II). Thus this reduction is carried out by means of the gaseous phase.

Example 2′

Comparison of the Electrochemical Behavior of the Carbonated Triphylite LiFePO ₄ Prepared in Example 1 to That of a Sample of Carbonated Triphylite Synthesized by Another Method

[0138] The material prepared in example 2 was tested in CR 2032 button cells described in example 1′. For comparison, we also are reporting several results obtained for the best carbonated sample synthesized using iron (II) (vivianite Fe ₃ (PO ₄ ) ₂ .8H ₂ O), the synthesis of which has already been described in CA-A-2,270,771.

[0139] FIG. 3 presents the 5 ^th cycle obtained by slow voltametry (20 mV.h ⁻¹ ) controlled by a battery cycler of the Macpiles® type with the sample resulting from classical synthesis (dotted lines) on one hand, to that obtained in example 2 (solid lines) on the other. The two syntheses lead to samples having the same electrochemical behavior on the level of oxidoreduction potentials and electrochemical kinetics.

[0140] The charging and discharging profiles of batteries assembled with the sample resulting from the synthesis described in example 2 are presented in FIG. 4 for two loads. These results are obtained in galvanostatic mode between 2.8 and 3.8 volts for two charging and discharging speeds C/8 and C/2 (the current applied (expressed in mA) at the time of charging or discharging corresponds to ⅛ (or ½ respectively) of the theoretical capacity of the battery expressed in mAh). We have reported the 20 ^th cycle and in the two cases, the discharge plateau is flat and the capacities involved correspond to 95% of the theoretical capacity.

[0141] The trend in capacities exchanged at the time of discharging is represented in FIG. 5 . In both cases, the initial capacity is around 80% of the theoretical capacity but, after around ten cycles, it is greater than 95%, ie. at 160 mAh.g ⁻¹ , and remains stable for the duration of the experiment. These results are comparable to those obtained with classical synthesis (reaction of divalent iron phosphate (vivianite) with lithium phosphate).

Example 3

Control of the Carbon Quantity

[0142] Samples of triphylite with different amounts of carbon were prepared by reaction of FePO ₄ .2H ₂ O and Li ₂ CO ₃ in the presence of a 1:1 mixture by volume of CO/CO ₂ . This atmosphere was chosen for its reducing power with respect to iron (III) while maintaining a stability of the iron (II), in particular at the end of the cycle for the rise to the synthesis temperature at 700° C. In a first step, the stoichiometric quantities of the two compounds, as well as the cellulose acetate, are ground together in isopropanol. The cellulose acetate quantities added represent 2.4 and 5%, respectively, of the mixture weight. After drying, these mixtures are heated progressively (6° C. per minute up to 700° C.) in a tube kiln with scavenging of the reducing gas (CO/CO ₂ : 1:1). This temperature is maintained for one hour. The sample is cooled for 40 minutes, which would be with a cooling speed of around 15° C. per minute. The reducing gas flow is maintained during the entire thermal processing time and also during the temperature drop. The total thermal processing time is around three and a half hours.

[0143] The structure of the samples was verified by X-ray diffraction and in all cases, the rays correspond to those of pure triphylite LiFePO ₄ .

[0144] The amounts of carbon were determined by elementary analysis. The results, as well as the electronic conductivities, of the samples are shown in Table 1 below. 1