Method for synthesising a material for a lithium-ion battery consisting of nanoporous lithium iron phosphate particles

Abstract

The invention relates to a method for synthesising a material for a lithium-ion battery consisting of nanoporous lithium iron phosphate particles (1), the method comprising the following steps: (E1) forming a precipitation solution by mixing a lithium source (4), an iron(II) source (5), a phosphorus source (6), a reducing agent (8) and carbon nano-objects (7) in a solvent, so as to co-precipitate the lithium, the iron and the phosphorus around the carbon nano-objects in the form of particles, referred to as LFP / C particles (9), lithium iron phosphate incorporating the carbon nano-objects; (E2) separating the LFP / C particles (9) from the precipitation solution; (E3) drying the LFP / C particles (9); (E4) calcining the LFP / C particles (9) so as to decompose the carbon nano-objects (7) which are incorporated into said particles, the decomposition of the nano-objects (7) generating nanopores (3) within the lithium iron phosphate particles.

Description

[0001] DESCRIPTION

[0002] TITLE: Process for synthesizing a lithium-ion battery material consisting of nanoporous lithium iron phosphate particles

[0003] TECHNICAL FIELD

[0004] The invention relates to a method for synthesizing a lithium-ion battery material consisting of nanoporous lithium iron phosphate particles.

[0005] STATE OF THE ART

[0006] Rechargeable lithium-ion (Li-ion) batteries are one of the most important energy storage solutions today. Due to their high energy density and long service life, they are widely used in mobile phones, computers, household appliances, electric and hybrid vehicles, and renewable energy storage stations. The application areas and performance of these batteries have been constantly evolving for several years.

[0007] A Li-ion battery has three main components:

[0008] - a cathode comprising a lithium-containing material, a binder such as polyvinylidene fluoride, and an electrically conductive material such as carbon black, the lithium-containing material, the binder and the electrically conductive material being mixed randomly so as to form an organic-inorganic composite material, the composite material comprising, by mass, of the order of 85% lithium-containing material, 10% electrically conductive material and 5% binder,

[0009] - a liquid electrolyte containing a lithium salt which wets a thin plastic or polymeric sheet, called a separator,

[0010] - an anode made of a carbon-based material, for example, graphite.

[0011] The battery charge / discharge cycles take place through oxidation-reduction reactions which are accompanied by a reversible phenomenon of lithium insertion / deinsertion at the two electrodes. The electrical conductivity of the two electrodes as well as their structural stability during the charge / discharge cycles are therefore essential conditions for the proper functioning of the battery.

[0012] Transition metal oxides, such as LiCoO2 (LCO), LiNiC>2 (LNO), LiM^C (LMO) are widely used cathode materials, but they have several drawbacks. For example, cobalt present in LCO is a toxic and expensive metal. In addition, the main cobalt resources (nearly 70%) are concentrated in the Democratic Republic of Congo (DRC) and Zambia, two politically unstable countries. As for the use of a pure LNO cathode, it poses a major safety problem related to the instability of the nickel oxide structure after Li deinsertion and the risk of exothermic reaction of this nickel oxide with the electrolyte. LMO is also thermally and electrochemically unstable during charge / discharge cycles.

[0013] Lithium metal phosphate materials are alternative cathode materials to transition metal oxides. For example, lithium iron phosphate LiFePO4 (LFP), in the form of particles with a god-like crystal structure, has a high operating voltage of about 3.4 V (vs. Li7Li) and a high theoretical capacity of about 170 mAh / g. In addition, LFP exhibits excellent chemical and thermal stability and does not use toxic and / or expensive metals. Such characteristics make this material particularly advantageous for applications in which safety issues are decisive, for example in electric vehicles.

[0014] However, this material suffers from poor diffusion kinetics of Li ions +within it and has poor electronic conductivity, which leads to a significant loss of capacity, especially at high charge / discharge rates. Several strategies have been developed to improve these properties, such as reducing the size of LFP particles, coating the surface of the particles with a carbon layer or manufacturing LFP / C composites and doping with transition metals. Coating the surface of LFP particles with a carbon layer or manufacturing LFP and carbon composites, denoted LFP / C, makes it possible to increase the electronic conductivity of LFP particles.

[0015] The size and morphology of LFP particles play a crucial role in the electrochemical performance of the cathode. Nano-sized LFP particles allow for very good power density. Indeed, the diffusion paths of lithium ions are short and the exchange surface between the electrolyte and the cathode is large, which facilitates the insertion / deinsertion processes of lithium ions during charge / discharge cycles. On the other hand, nano-sized LFP particles lead to a low volumetric energy density due to a large specific surface area and a high amount of binders that tend to adsorb on the surface of the particles, thus giving rise to undesirable reactions and poor electrochemical stability during cycling.

[0016] One solution to the particle size problem is to prepare micro-nanostructured LFP, which comprises micrometric particles with nanometric pores. The preparation of micro-nanostructured LFP significantly improves the electrochemical performance of the LFP / C cathode. Indeed, micro-nanostructured LFP combines, during cycling, a high charge / discharge rate provided by the nanometric structure, with a high volumetric energy density and good electrochemical stability provided by the micrometric dimension. In addition, the larger the nanopores, the more they allow to increase the exchange surface with the electrolyte, and therefore the more they allow to improve the Li ion transport properties. + between the two electrodes. A battery that includes such a material has high charge and discharge rates, especially at high current flow.

[0017] Controlling particle size and morphology to improve material properties requires precise control of synthesis parameters during the various manufacturing steps. Synthesis methods based on solid-state reactions, which are widely used on an industrial scale, have major drawbacks. Indeed, it is difficult to obtain a homogeneous mixture of precursors in the solid state and to control the morphology and particle size of the final material. To overcome these difficulties, solid-state synthesis methods involve several steps of mixing, mechanical grinding, and high-temperature heat treatment, making the manufacturing process time-consuming and energy-intensive.

[0018] Wet synthesis methods address homogeneity issues as lithium, iron, and phosphorus precursors are mixed in the atomic or molecular state in an organic solvent or water, resulting in a high chemical purity / crystalline quality LFP cathode at lower temperatures compared to solid-state synthesis methods. These so-called "soft" methods also allow for easier control of LFP morphology and particle size. However, they have drawbacks that make their transfer to industrial scale complex or economically unprofitable.

[0019] Sol-gel synthesis, for example, uses expensive and flammable organic solvents and organic precursors, such as acetates, which are relatively expensive. Drying, grinding, and heat treatment of the intermediate product (xerogel) are also essential. The sol-gel process is therefore long, expensive, and restrictive from an industrial and environmental perspective.

[0020] US2014 / 0342231 A1 discloses a method for hydrothermally synthesizing LFP / C particles from a lithium ion source, an iron source, a phosphorus source, and a first carbon source. These precursors are dissolved in water and mixed with a second carbon source based on carbon nanofibers before being transferred to the autoclave. The precipitate is then calcined to obtain composite LFP / C particles covered with a carbon coating incorporating the second carbon source. The carbon coating is intended to improve the electronic conductivity of the LFP particles.

[0021] Hydrothermal synthesis involves pressurized reactors, which pose safety concerns and require relatively high capital costs. Other variants of this method, such as solvothermal synthesis, allow for lower pressure in the reactors but use high-boiling organic solvents, such as polyethylene glycol (PEG), which are relatively expensive.

[0022] Document US2011 / 0027651 A1 discloses a method for synthesizing micro-nanostructured LFP / C particles (microparticles with nanopores) by co-precipitation in two synthesis steps. The first synthesis step consists of obtaining FePC particles by reaction between an iron(III) ion precursor and a phosphorus source. The particles obtained are then calcined. In a second synthesis step, the calcined FePC particles are mixed with a carbon source in a solvent. After evaporation of the solvent, a lithium precursor is added and all the precursors are calcined. The two-step co-precipitation process makes it possible to obtain micrometric particles covered with a carbon coating and having nanometric porosity.

[0023] However, this coprecipitation method still has drawbacks. For example, the process is based on a two-step synthesis and several mechanical mixing and grinding steps. The process is therefore long, complex, and energy-intensive. In addition, the process uses toxic and flammable organic solvents, particularly to disperse the carbon source. Finally, the process uses an iron(III) source that requires the use of a reducing gas to reduce the iron(III) ions to the iron(II) ions needed to obtain the FePC phase during the first calcination step. However, handling and storing a reducing gas such as dihydrogen on an industrial scale is a risk that should be avoided.

[0024] BRIEF DESCRIPTION OF THE INVENTION

[0025] An object of the invention is to design a method for preparing micrometric LFP / C particles having nanometric porosity, for use as a cathode material for a lithium-ion battery, which makes it possible to control the porosity of the particles while being easy to implement and having a reduced cost. To this end, the invention provides a method for synthesizing a lithium-ion battery material consisting of nanoporous lithium iron phosphate particles, the method comprising the following steps:

[0026] (E1) formation of a basic precipitation solution by mixing a lithium source (4), an iron(ll) source, a phosphorus source, a reducing agent and carbon nano-objects in a solvent, so as to co-precipitate the lithium, iron and phosphorus around the carbon nano-objects in the form of particles, called LFP / C particles, of iron and lithium phosphate incorporating the carbon nano-objects,

[0027] (E2) separation of LFP / C particles from the precipitation solution,

[0028] (E3) drying of LFP / C particles,

[0029] (E4) calcining the LFP / C particles so as to decompose the carbon nano-objects incorporated in said particles, the decomposition of said nano-objects generating nanopores within the lithium iron phosphate particles.

[0030] The process involves a single synthesis step by coprecipitation, which further minimizes the number of mechanical mixing, grinding, and heat treatment steps. This improves the cost-effectiveness of the process for preparing micro-nanostructured LFP / C particles by coprecipitation.

[0031] In addition, the process advantageously allows the morphology, grain size and porosity of the micro-nanostructured LFP / C particles obtained at the end of this single synthesis step to be controlled.

[0032] Finally, during the single step of synthesis of LFP / C particles by co-precipitation, the process allows the use of only water as a solvent, to avoid the use of a reducing gas such as dihydrogen and to operate in temperature and pressure conditions close to ambient. Thus, the process is easily transposable to industrial scale and less dangerous.

[0033] In this text, the term "micrometric" means an object with at least one dimension less than 1 mm and the term "nanometric" or "nano-" means an object with at least one dimension less than 1 pm.

[0034] According to advantageous but optional characteristics of the invention, possibly combined when technically possible:

[0035] - step (E4) comprises forming a coating layer around the particles by calcining a carbon source; - the reducing agent is carbon-based and the carbon source comprises said reducing agent;

[0036] - the carbon source is added to the precipitation solution in step (E1);

[0037] - the carbon source is added to the LFP / C particles during step (E4);

[0038] - the carbon nano-objects are chosen so as to obtain, at the end of the calcination step (E4), nanopores of the same size as said carbon nano-objects;

[0039] - the solvent is an aqueous solution and the carbon nano-objects are water-soluble;

[0040] - carbon nano-objects include carbon quantum dots;

[0041] - carbon nano-objects are chosen to decompose at a temperature between 400 and 700°C;

[0042] - carbon nano-objects are carbon nanoparticles obtained from sugars or sugar derivatives dissolved in water, at a concentration of between 0.1 M and 2 M, the solution being brought to a temperature above 100°C, preferably to a temperature of between 150°C and 200°C for a period of between 2 h and 4 h;

[0043] - step (E1) is carried out at atmospheric pressure in an open reactor at a temperature preferably between 50°C and 90°C;

[0044] - the reducing agent used in step (E1) is carbon-based and is preferably chosen from sugars and sugar derivatives, organic acids and glycols;

[0045] - the average size of nanopores within iron and lithium phosphate particles is between 1 nm and 500 nm;

[0046] - each nanoporous lithium iron phosphate particle is composed of primary particles, the average diameter of the primary particles preferably being between 50 nm and 500 nm and the average diameter of the nanoporous lithium iron phosphate particles preferably being between 1 pm and 50 pm;

[0047] - the mixing step (E1) comprises the addition of a base so as to control the pH of the solution during the co-precipitation, said base being chosen from the group comprising NH4OH, NH4HCO3, NaOH, KOH, Na2COs, Na2C2O4 or a water-soluble organic base;

[0048] - the formation of the precipitation solution in step (E1) comprises the mixing of the phosphorus source, the carbon nano-objects, the base and the reducing agent prior to a gradual addition of the lithium source and the iron(ll) source, the initial pH of the precipitation solution at the start of step (E1) before the introduction of the lithium source being between 1 and 3, preferably between 1.5 and 2.5;

[0049] - the final pH of the precipitation solution at the end of step (E1) is between 3.5 and 7.5, preferably between 4.5 and 7.

[0050] Another object of the invention relates to a rechargeable Li-ion battery containing the material consisting of nanoporous lithium iron phosphate particles obtained by the method as described above. BRIEF DESCRIPTION OF THE FIGURES

[0051] Other characteristics and advantages of the invention will emerge from the detailed description which follows, with reference to the appended drawings, in which:

[0052] - figure 1 represents, in the form of a block diagram, the different stages of the process for manufacturing LFP / C particles according to the invention,

[0053] - Figure 2 represents an embodiment of the step of forming a precipitation solution by mixing a lithium source, an iron(II) source, a phosphorus source and carbon nano-objects in a solvent, so as to co-precipitate the LFP / C particles incorporating the carbon nano-objects,

[0054] - figure 3 represents the diagram of an example of experimental setup for carrying out the step of forming the precipitation solution according to the embodiment of figure 2,

[0055] - Figure 4 represents, in the form of a diagram, the process of formation of nano-porous LFP / C particles during the stages of formation of the precipitation and calcination solution of the process according to the invention,

[0056] - Figure 5 represents the result of X-ray diffraction analysis carried out on the LFP / C particles obtained in Example 3,

[0057] - Figure 6 represents an image obtained by scanning electron microscopy of the LFP / C particles obtained in Example 3,

[0058] - Figure 7 represents the curve of the first charge / discharge cycle of a battery whose manufacture from the LFP / C particles of example 3 is described in example 4,

[0059] - Figure 8 represents an image obtained by scanning electron microscopy of the carbon nanoparticles obtained in Example 5,

[0060] - Figure 9 represents the result of X-ray diffraction analysis carried out on the nano-porous LFP / C particles obtained in Example 7,

[0061] - Figure 10 represents an image obtained by scanning electron microscopy carried out on the nano-porous LFP / C particles obtained in Example 7,

[0062] - Figure 11 represents an image obtained by high magnification scanning electron microscopy of a nano-porous LFP / C particle obtained in Example 7,

[0063] - Figure 12 represents an image obtained by scanning electron microscopy of the polished section of a nano-porous LFP / C particle obtained in Example 7,

[0064] - Figure 13 represents the curve of the first charge / discharge cycle of a battery whose manufacture from the LFP / C particles of Example 7 is described in Example 8.

[0065] For readability reasons, the drawings are not necessarily drawn to scale.

[0066] Identical reference signs from one figure to another designate the same elements. DETAILED DESCRIPTION OF EMBODIMENTS

[0067] In the remainder of the description, the expressions "about" or "approximately" mean "within 10%". Furthermore, by the term "average diameter" is meant according to the present invention the diameter of the particles which is greater than the diameter of 50% of the particles and less than the diameter of 50% of the particles. The average diameter can be measured, for example, from a scanning electron microscopy (SEM) image.

[0068] The invention relates to an advantageous and economical process which can be used on an industrial scale to manufacture high performance lithium metal phosphates for use as cathode material in rechargeable lithium-ion batteries.

[0069] In particular, the invention relates to a method for synthesizing a lithium-ion battery material consisting of nanoporous lithium iron phosphate particles comprising the steps shown in block diagram form in Figure 1. In particular, the method comprises forming a precipitation solution so as to co-precipitate lithium, iron and phosphorus around carbon nano-objects in the form of lithium iron phosphate particles incorporating said carbon nano-objects, and calcining said particles so as to decompose the incorporated nano-objects. The decomposition of the nano-objects generates the nanopores within the lithium iron phosphate particles called LFP / C, so as to generate nanoporous LFP / C particles.

[0070] The inventors have noticed that the nanopores in the nanoporous iron and lithium particles obtained after calcination are approximately the same size as the carbon nano-objects introduced into the precipitation solution. In the method according to the invention, the carbon nano-objects introduced into the precipitation solution can therefore be chosen so as to obtain, after said calcination step, nanopores of the desired size to obtain the envisaged properties. Indeed, the larger the nanopores, the more they allow the exchange surface with the electrolyte to be increased and therefore the more they allow the Li ion transport properties to be improved. + between the two electrodes. The battery incorporating such a material thus has a high charge and discharge speed, particularly at high current flow.

[0071] The method according to the invention concerns the preparation of nanoporous LFP / C particles whose average diameter is between 1 pm and 50 pm, preferably between 5 pm and 10 pm. The nanoporous LFP / C particles are themselves composed of primary particles whose average diameter is preferably between 50 nm and 500 nm.

[0072] The size of the nanopores of the LFP / C particles according to the invention can be between 1 nm and 500 nm. Among these particles, a distinction is conventionally made (according to the IUCP (International Union of Pure and Applied Chemistry)) between microporous particles, whose pore size is less than 2 nm, mesoporous particles, whose pore size is between 2 nm and 50 nm, and macroporous particles, whose pore size is between 50 nm and 500 nm. As indicated above, the pore size is advantageously controlled by the size of the carbon nano-objects used in the co-precipitation step. A person skilled in the art can thus choose the type of porosity of the LFP / C particles.

[0073] As illustrated in Figure 1, in a step E1 of the method, a precipitation solution is formed by mixing a lithium source, an iron(ll) source, a reducing agent, a phosphorus source and carbon nano-objects in a solvent, so as to co-precipitate the lithium, iron(ll) and phosphorus around the carbon nano-objects in the form of lithium iron phosphate particles incorporating the carbon nano-objects, in a single co-precipitation synthesis step. These particles are denoted LFP / C.

[0074] Figure 4 schematically represents the mechanism of the coprecipitation reaction implemented during step E1. First, primary nanoparticles 2 of iron and lithium phosphate are formed (CD in Figure 4). Then, these nanoparticles 2 agglomerate around the carbon nano-objects 7 ((2) in Figure 4). Finally, a maturation and Ostwald ripening phase gives rise to the formation of secondary particles 9 of iron and lithium phosphate by agglomeration of the primary nanoparticles 2 and incorporation of the carbon nano-objects 7 (® in Figure 4).

[0075] The co-precipitation reaction is preferably carried out at atmospheric pressure in an open reactor at a temperature between 50°C and 90°C, even more preferably at a temperature between 60°C and 80°C for a duration between 1 h and 20 h, preferably between 2 h and 15 h.

[0076] Such near-ambient pressure and temperature conditions and the fact that no gas such as dihydrogen is used in the single synthesis step make the process advantageously transposable to industrial scale and less hazardous. The reaction time is controlled to obtain secondary LFP / C particles of the desired morphology and size. The longer the synthesis time, the larger the size of the secondary particles at the end of the reaction. A person skilled in the art is able to adjust the synthesis time according to the desired particle size.

[0077] The source of iron(ll) (designated by the reference 5 in Figure 4) is an iron(ll) salt, for example FeSC . FW or Fe(NOs)2.

[0078] The lithium source (designated by reference 4 in Figure 4) can be chosen from the following precursors: UOH.I H2O, U2CO3, LiNCh, U2SO4.H2O and UH2PO4. Lithium hydroxide (UOH.I H2O) has the advantage of being basic, which contributes favorably to the pH of the co-precipitation medium.

[0079] In some embodiments, particularly in the case where the lithium source is not UOH.I H2O (which is basic), the precipitation solution may also comprise a base. Said base makes it possible to control the pH of the precipitation solution and the growth of the LFP particles. The base may be an inorganic type base such as, for example, ammonium hydroxide (NH4OH), sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium bicarbonate (Na2COs), ammonium hydrogen carbonate (NH4HCO3), sodium oxalate (Na2C2 <D4). Alternativement, on peut utiliser tout type de base organique qui soit soluble dans l’eau.

[0080] The phosphorus source (designated by reference 6 in Figure 4) may include for example: H3PO4, (NH4)3PO4, (NH4)2HPO4 and / or (NH4)H2PO4.

[0081] Carbon nano-objects may include, for example, carbon nano-spheres, carbon nano-rods, carbon nano-particles, carbon-based quantum dots and any form of carbon having at least one of its dimensions in the sub-micrometer range (less than 1 μm). The carbon nano-objects are chosen so that they decompose during the calcination step.

[0082] It will be apparent to a person skilled in the art that this list of precursors is not exhaustive and may extend to include several sources of lithium, iron(ll), phosphorus and different types of water-soluble carbon nano-objects. Therefore, all possible precursors of lithium, iron(ll), phosphorus and water-soluble carbon nano-objects are covered by the scope of this invention.

[0083] The precipitation solution according to the invention further comprises a reducing agent (designated by reference 8 in Figure 4). Said reducing agent makes it possible to maintain a reducing medium during the precipitation step in order to avoid the oxidation of Fe 2+ in Fe 3+ and the formation of parasitic phases (other phases than the LFP phase), which are undesirable.

[0084] In a particularly advantageous embodiment, the reducing agent may be carbon-based.

[0085] When carbon-based, the reducing agent may have an additional effect to that of maintaining the aforementioned reducing medium. Indeed, during the calcination step, the carbon-based reducing agent decomposes to generate a carbon coating layer 24 of the nanoporous LFP / C particles 1, thus improving the electrical conductivity of said particles.

[0086] Furthermore, the fact that the reducing agent is carbon-based, which is the constituent material of nano-objects, makes it possible to avoid contaminating the particles with a foreign material.

[0087] The carbon-based reducing agent may comprise a sugar and / or a sugar derivative, for example, glucose, lactose, fructose and / or dextrose. Alternatively or additionally, the carbon-based reducing agent comprises one or more organic acids selected from ascorbic acid, citric acid, lauric acid, malonic acid, acrylic acid and / or polyacrylic acid. Alternatively or additionally, the carbon-based reducing agent may comprise one or more glycols such as polyethylene glycol (PEG), tetraethylene glycol (TEG) and ethylene glycol (EG).

[0088] In other embodiments, the reducing agent may also be selected from a group comprising inorganic reducing agents, such as: potassium iodide (Kl), sodium sulfite (Na2SOs), sodium thiosulfate (Na2S20s), sodium dithionite (Na2S2O4), sodium tetrahydruroborate (NaBFL).

[0089] Figures 2 and 3 represent a particular embodiment of step E1 of forming the precipitation solution. In particular, Figure 2 represents, in the form of a block diagram, the different sub-steps that step E1 comprises according to this embodiment and Figure 3 represents an example of an experimental device making it possible to implement such an embodiment of step E1.

[0090] In step E1.1, the phosphorus source and the carbon nano-objects are mixed in a double-walled precipitation reactor 10, in which the values ​​of temperature, pH and redox potential are controlled. In addition, a base is added to adjust the pH of this initial solution to a value between 1 and 3, preferably between 1.5 and 2.5 and a carbon-based reducing agent to adjust the initial redox potential to a value between 200 mV and 350 mV relative to the potential of a normal hydrogen electrode. The temperature of the solution is between 10 and 90°C.

[0091] Controlling the initial pH using base helps to avoid the precipitation of undesirable crystalline phases, e.g., Fes(PO4)2 or Fe2P2O?, which may form at pH values ​​below 1.

[0092] For example, an aqueous solution of phosphoric acid is used as the source of phosphorus, and an aqueous solution of ammonium hydroxide as the base. The aqueous solutions of phosphoric acid and ammonium hydroxide are previously prepared by dissolving their precursors in water. The concentration of the aqueous solution of phosphoric acid is preferably between 1 mol / L and 3 mol / L. The concentration of the aqueous solution of ammonium hydroxide is between 0.1 mol / L and 3 mol / L, preferably between 0.4 mol / L and 2 mol / L. Similarly, the reducing agent can be dissolved in water at a concentration of between 1 mol / L and 3 mol / L before introducing this solution into the reactor.

[0093] As another example, a solution of carbon nanospheres with an average diameter between 1 nm and 1 pm, preferably between 1 nm and 500 nm, is used as a source of carbon nanoobjects.

[0094] Carbon nanospheres can be prepared in advance by different synthesis methods. For example, they can be obtained via a hydrothermal reaction from a carbon source, for example, sugar or its derivatives, in water, at a concentration between 0.1 mol / L and 2 mol / L, the solution being brought to a temperature above 100 °C, preferably at a temperature between 150 °C and 200 °C for a period of between 2 h and 6 h. For example, the synthesis of carbon nanospheres can be carried out according to the protocol described in the document "RSC Adv., 2015, 5, 59491-59494”. The average diameter of the carbon nanospheres can be adjusted by controlling the synthesis parameters such as, for example, the temperature, the reaction time and the concentration of the precursors.

[0095] Such a synthesis protocol allows obtaining carbon nanospheres with an average diameter between 1 nm and 500 nm. The diameter of the carbon particles can be measured experimentally by transmission electron microscopy (TEM) or high-resolution scanning electron microscopy (HRSEM). In addition, the carbon nanospheres obtained by such a synthesis protocol decompose advantageously at a temperature between 400 °C and 700 °C. Other synthesis methods based on a bottom-up approach can also be used to manufacture carbon nanoobjects. Another example of a bottom-up method is microwave-assisted pyrolysis synthesis from a water-soluble carbon precursor such as sugar or its derivatives. The advantage of these methods lies in the fact that they are easy to implement and can be scaled up.

[0096] The double-walled precipitation reactor 10 used has, for example, a capacity of 4 L and is further equipped with a mechanical stirrer 11, a collection valve 12 intended to collect the suspension at the end of synthesis to proceed to step E2, a redox potential measuring sensor 13, a pH measuring sensor 14, a temperature measuring sensor 15 and a thermostat 16 for regulating the temperature inside the reactor. The reactor can be heated, for example, by circulating hot water or water vapor in the double wall of the reactor or via a coil immersed in the initial solution contained in the reactor.

[0097] In step E1.2, the pH of the solution prepared in step E1.1 is adjusted between 1 and 3, preferably between 1.5 and 2.5, using a base (LiOH, NH4OH or other).

[0098] In step E1.3, the lithium source and the iron(ll) source are gradually added to the solution from step E1.2. The addition is preferably carried out so as to maintain the pH between 3.5 and 7.5, preferably between 5 and 7. Maintaining the pH at values ​​below 7 makes it possible to avoid the formation of undesirable phases, such as, for example, Fe(OH)s or U3PO4. Indeed, since the LiFePC compound is not yet stable at this stage of the process, it could redissolve in the medium at acidic pHs (below 3.5).

[0099] For example, a basic lithium source can be chosen to regulate the pH, for example an aqueous solution of lithium hydroxide at a concentration between 1 mol / L and 3 mol / L previously prepared by dissolving the lithium hydroxide salt and stored in a tank 17.

[0100] The source of iron(ll) is for example an aqueous solution of iron(ll) sulfate whose concentration is between 1 mol / L and 3 mol / L previously prepared in a tank 18 by dissolving solid iron(ll) sulfate in water.

[0101] In order to automatically ensure the pH regulation and maintain it in the targeted range, the sources of lithium and iron can be introduced into the reactor 10 using metering pumps 19 and 20 equipped with flow regulators 21 and 22 to control the rate of addition of these reagents 4,5 into the reactor 10. The reactor can also be equipped with a pH controller 23 connected to the regulator 21 of the metering pump 19 which supplies the reactor 10 with lithium hydroxide and which will regulate the rate of injection of the lithium hydroxide according to the pH value measured by the pH sensor 14.

[0102] During step E1.2, the aqueous lithium hydroxide (LiOH) solution is added to the mixture containing the phosphorus source, the reducing agent and, if applicable, a growth controller, to adjust the pH between 1 and 3, preferably between 1.5 and 2.5. Then, the rest of the LiOH solution and the iron sulfate are, for example, introduced into the reactor with a fixed flow rate of between 0.1 L / h and 2.5 L / h until the maximum tolerated pH value is reached. Then, during step E1.3, the lithium hydroxide solution or another base is introduced with a variable flow rate according to the pH setpoint set by the pH controller 23, so that the pH remains within the tolerated range of values, until the LiOH solution is exhausted.

[0103] In step E1.4, the precipitation solution thus formed is left stirring until particles of the desired size and morphology are obtained. Step E1.4 is therefore a maturation step.

[0104] According to an alternative embodiment of step E1, the sources of lithium, iron(ll), phosphorus, carbon nano-objects and the reducing agent are introduced directly into the precipitation reactor. Then, the base, for example an ammonium hydroxide solution, is introduced gradually, at a controlled flow rate, until the target pH value is reached.

[0105] Step E1 can be implemented with other orders of introduction of the reactants.

[0106] In the embodiments of step E1 previously described, step E1 is carried out in a stirred reactor 1, for example at a speed of between 800 rpm (revolutions per minute) and 1200 rpm. Alternatively, the precipitation reactor used in step E1 makes it possible to produce the LFP / C particles in continuous mode.

[0107] Preferably, the final oxidation-reduction potential (Ef) of the reaction mixture at the end of step E1 is between 0 mV and 100 mV, even more preferably between 10 mV and 50 mV.

[0108] Finally, the method according to the invention advantageously comprises a single step E1 of synthesis in a liquid medium, preferably in an aqueous medium, of the LFP / C particles. As a result, the method does not require mechanical mixing, grinding and heat treatment steps to obtain said LFP / C particles. The profitability of the method for preparing micro-nanostructured LFP / C particles by co-precipitation is improved. In addition, it is possible to control the morphology, grain size and porosity of the nanoporous LFP / C particles which will be obtained at the end of steps E2, E3 and E4 as described below simply by choosing the conditions for implementing this single synthesis step E1.

[0109] In a step E2, the LFP / C particles are separated from the precipitation solution. According to one embodiment, the solid-liquid separation is carried out by filtration, for example, using a filter press. Other mechanical separation methods, such as centrifugation, can be used. A wet LFP / C precipitate is thus obtained.

[0110] Step E2 may further comprise washing the wet LFP / C precipitate. According to one embodiment, the washing is carried out with water. The washing consists of removing the impurities adsorbed on the surface of the LFP / C precipitate, for example, salts and metal ions which are soluble in water, as well as the residual acid from step E1. The washing may be monitored by measuring the physicochemical properties of the washing liquid, for example, its pH or its ionic conductivity (in pS / cm). For example, the washing must be continued until the pH of the washing solution is between 6.5 and 7.5.

[0111] In step E3, the wet LFP / C particles are dried. For example, drying can be carried out under vacuum at a temperature between 60°C and 100°C. Alternatively, drying can be carried out under an inert atmosphere, for example under a nitrogen atmosphere.

[0112] Alternatively, drying can be achieved by the atomization method by spraying the suspension of LFP / C particles into a hot air flow reactor, using for example the Buchi B-290 mini-atomizer.

[0113] In one embodiment, particularly when the reducing agent is not carbon-based or has a low carbon content, a carbon source is added to the suspension of LFP / C particles before or after the drying step so that the particles are coated with a carbon film after the calcination step E4. Such a carbon source may optionally be added as early as step E1.

[0114] When the reducing agent is carbon-based, it may itself constitute the carbon source. For this purpose, the reducing agent is advantageously chosen to have good adsorption properties on the particles, in order to be present on the particles after step E2. If the residual carbon content at the end of step E2 is too low, the reducing agent may optionally be combined with an additional carbon source introduced in step E1 and / or E4.

[0115] In a step E4, calcination of the LFP / C particles dried in step E3 is carried out. As shown in step @ of FIG. 4, calcination E4 leads to the thermal decomposition of the carbon nano-objects 7 previously incorporated within the secondary LFP / C particles 9 to form pores 3 and to the thermal decomposition of the carbon source to form a carbon coating layer 24. At the end of calcination E4, nanoporous LFP / C micro-parties 1 containing pores 3 and a carbon layer 24 are therefore obtained.

[0116] The porosity of LFP / C microparticles is generated by the partial or complete decomposition of carbon nano-objects within the secondary LFP / C particles. The size and shape of the pores can thus be controlled by varying the shape and size of the carbon nano-objects.

[0117] Calcination can be carried out under an inert atmosphere, for example under a nitrogen atmosphere, at a temperature between 600°C and 800°C. E4 calcination is advantageously carried out in a rotary kiln which allows better homogenization and homogeneous heat diffusion within the material during heat treatment. The residence time of the LFP / C particles in the kiln is between 2 h and 15 h, preferably between 5 h and 10 h.

[0118] Alternatively, calcination step E4 may be carried out under an atmosphere comprising a mixture of an inert gas and a gaseous carbon source, for example a mixture of nitrogen and propylene C3H6 or a mixture of nitrogen and ethylene. The gas mixture may, for example, contain 1% to 5% of the gaseous carbon source. The gaseous carbon source present in the mixture makes it possible to deposit a layer of carbon in the vapor phase on the surface of the nanoporous LFP / C particles in order to improve the electrical conductivity of the final material.

[0119] The crystal structure of the nanoporous LFP / C particles from calcination step E4 can then be identified by X-ray diffraction (XRD) using, for example, a PHILIPS X'Pert apparatus with a copper anticathode of wavelength λcuKa=1.541 Å. Examples of X-ray diffractograms obtained on products from the embodiments covered by this invention are shown in Figures 5 and 9. The XRD results shown in Figures 5 and 9 show that the product obtained in step E4 is composed of LFP / C particles of divine structure which is electrochemically active.

[0120] Finally, the average diameter of nanoporous LFP / C particles, their constituent primary particles, and nanopores can be measured by high-resolution scanning electron microscopy (HRSEM), for example, using a ZEISS high-resolution scanning electron microscope.

[0121] Some examples of implementation of the method according to the invention by the inventors are described below (example 1 to 8). In these examples, embodiments of the method giving rise to the manufacture of pure nanoporous lithium iron phosphate particles LFP / C are described. However, it is obvious to a person skilled in the art that other chemical compositions based on phosphates can be obtained within the scope of the present invention. For example, a substitution of iron (Fe) by other metals can be carried out to obtain particles of formula LiFe(i. X )M xPO4, where M is a metal which can be chosen, for example, from the following elements: Ni, Mn, Co, Ti, V, Nd, Mg, Zn, Y, Al, and W and 0 < x < 1. In addition, other core / shell type formulations known by the English acronym "core / shell" having a transition metal concentration gradient between the center and the periphery of the particles can also be obtained within the scope of this invention.

[0122] Comparison Example 1

[0123] In this example, a co-precipitation step E1 was carried out. First, 1 L of a 1 M H3PO4 solution was introduced into a 4 L reactor. Then, 60 mL of a 1 M NH4OH solution and 100 mL of a 1 M glucose solution were added to the H3PO4 solution under mechanical stirring at 1000 rpm. The reactor was then closed and heated at 60 °C for 30 min. The initial pH of the mixture was adjusted to 2 using the prepared 2 M LiOH solution. The initial potential of the mixture was 350 mV. The remaining 2 M LiOH solution and 1 L of a 1 M FeSO4.7H2O solution were then added at a flow rate of 0.4 L / h and 0.3 L / h, respectively. The pH setpoint max was set to 7 while the pHmin value is set to 6.7. When the pH setpoint maxis reached, the automatic pH control is activated, the addition of the UOH.I H2O solution stops automatically until the pHmin value is reached, in which case, the LiOH solution addition pump restarts again to reach the pH value max and so on until the LiOH solution is used up. The reaction mixture is then left stirring at 60°C for 10 h. Comparison example 2

[0124] In this example, the intermediate LFP suspension obtained in Example 1 is used to carry out the E2 solid-liquid separation and washing steps. In this test, approximately 3 L of the LFP suspension was filtered. The resulting solid is then washed several times with water. The washing efficiency is monitored by measuring the conductivity of the filtrate (washing liquid) after each filtration with a conductivity meter. Table 1 shows the evolution of the ionic conductivity of the filtrate as a function of the number of washes.

[0125] [Table 1]

[0126] Table 1 shows that washing allows the conductivity of the washing water to be lowered to 60 pS / cm, a value close to that of the water used in this test. Then, a step E3 of vacuum drying of the washed solid is carried out in a vacuum oven at 90 °C for 12 h.

[0127] Comparison Example 3

[0128] In this example, the washed and dried LFP precipitate from Example 2 is used to perform an E4 calcination step. In this test, approximately 25 g of the product from Example 2 was mixed with 2 g of anhydrous glucose (VWR Chemicals). The mixture is then dispersed in approximately 10 mL of ultrapure water and the mixture is placed in an ultrasonic bath for approximately 15 min. After evaporation of water and drying of the mixture under vacuum, the product is transferred to an alumina crucible and then placed in a tube furnace for 3 h at 150 °C and then 10 h at 700 °C under an argon flow. Anhydrous glucose is a carbon source to form a carbon film on the surface of the particles during the calcination step.

[0129] Figure 5 shows the X-ray diffraction analysis result of the product obtained in Example 3. All the diffraction lines observed correspond to the positions of the LiFePC reference (ASTM-JCPDS sheet number 40-1499) with an orthorhombic divine structure. No other lines are observed, indicating the absence of secondary phases in this product. Figure 6 shows a scanning electron microscopy (SEM) image of the product obtained in Example 3. The image shows the formation of LFP / C particles of relatively homogeneous size and shape with an average diameter of approximately 5 μm. A high-magnification SEM image shows that the particles are formed by agglomeration of primary particles with an average diameter of approximately 400 nm.

[0130] The LFC / C particles obtained in Example 3 were used to fabricate a positive electrode to evaluate their electrochemical properties in CR2025 button cells. An LFP / C formulation was first prepared by mixing 8.5 g of the LFP / C powder from Example 3 with 1 g of a conductive material (acetylene black) and 0.5 g of binder (PVDF) in the solvent N-methyl pyrolidone (NMP). The electrode was prepared by depositing the LFP / C formulation on the surface of an aluminum foil using the doctor blade method. Metallic lithium was used as the anode material. The cathodes were dried under vacuum at 85 °C for 24 h. The electrolyte used is a 1 M solution of lithium hexafluorophosphate (LiPFe) in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) solvents prepared at a volume ratio of 1:1 (Supplied by Merck).The cells were then assembled in an airtight glove box and tested using a Biologie MPG-200 cycler under a C / 10 charge and discharge regime.

[0131] Figure 7 shows the curve of the first charge and discharge cycle of a cell using the LFP / C particles of Example 3 as the cathode material formulated as described in Example 4. This cell shows a low polarization (approximately 95 mV at 60 mA / g) and a relatively reversible charge / discharge process. The cell delivers a charge capacity of approximately 150 mAh / g.

[0132] Example 5

[0133] In this example, a synthesis of carbon nanoparticles was carried out. First, a glucose solution was prepared by dissolving 15 g of glucose in 70 mL of distilled water. Then, 1 g of citric acid was added to the glucose solution. Second, the solution was transferred to an autoclave and heated at 150 °C for 6 h.

[0134] Figure 8 shows an image taken by scanning electron microscopy (SEM) in high-resolution mode on a sample of the carbon nanoparticle solution obtained in Example 5. The image shows the formation of carbon nanoparticles of homogeneous size and shape with an average diameter of approximately 30 nm.

[0135] The carbon nanoparticles obtained in Example 5 were used to manufacture nanoporous LFP / C particles. A precipitation step E1 was first carried out under the following conditions. First, 1 L of a H3PO4 solution (1 M) was introduced into a 4 L reactor. Then, 150 mL of a NH4OH solution (0.4 M) and 100 mL of a glucose solution (1 M) were added to the H3PO4 solution under mechanical stirring at 1000 rpm. The reactor was then closed and heated to 65 °C for 30 min. When the temperature had stabilized at 65 °C, 40 mL of the carbon nanoparticle suspension obtained in Example 5 was added. Then, the pH of the solution was adjusted to 2 using the prepared 2 M LiOH solution. The remaining LiOH solution (2 M) and 1 L of FeSG . FW solution (1 M) were then added with a flow rate of 0.4 L / h and 0.3 L / h, respectively.The pHmax setpoint was set to 7 while the pHmin value was set to 6.7. When the pHmax setpoint was reached, the automatic pH control was activated, the addition of the UOH.I H2O solution was automatically stopped until the pHmin value was reached, in which case, the LiOH solution addition pump was restarted again to reach the pHmax value and so on until the LiOH solution was exhausted. The reaction mixture was then left stirring at 65 °C for 12 h.

[0136] In this example, step E2 of solid-liquid separation and washing and step E3 of vacuum drying were also carried out as in example 2.

[0137] Example 7

[0138] In this example, the LFP / carbon nanoparticle precipitate obtained in Example 6 is used to perform a calcination step E4. In this test, approximately 12.5 g of the product of Example 6 was mixed with 1 g of anhydrous glucose (VWR Chemicals). The mixture is then dispersed in approximately 5 mL of ultrapure water and placed in an ultrasonic bath for approximately 20 min. After evaporation of water and drying of the mixture under vacuum, the product is transferred to an alumina crucible and then placed in a tube furnace for 3 h at 150 °C and then 10 h at 700 °C under an argon flow.

[0139] Figure 9 shows the X-ray diffraction analysis result of the product obtained in Example 7. All observed diffraction lines correspond to the diffraction lines of LiFePC with olivine structure (ASTM-JCPDS sheet number 40-1499). No other crystalline phases are observed on this product.

[0140] Figure 10 shows a scanning electron microscopy (SEM) image of the product obtained in Example 7. The image shows the formation of LFP / C particles of relatively homogeneous size and shape with an average diameter of approximately 6 μm. A high-magnification SEM image (Figure 11) shows that the secondary particles are formed by agglomeration of smaller (so-called primary) particles with an average diameter of approximately 300 nm.

[0141] Furthermore, a polished section of the product obtained in Example 7 was made by dispersing this product in a resin. Figure 12 shows an SEM image made on the section of a secondary particle taken from this product. The figure clearly shows that pores of relatively homogeneous sizes have formed within the particle. The average diameter of these pores is about 30 nm, which corresponds to the average diameter of the carbon nanoparticles used in this example. These pores are therefore the result of the thermal decomposition of the carbon nanoparticles at the calcination stage.

[0142] In this example, the nanoporous LFP / C powder obtained in Example 7 was used to fabricate a positive electrode to evaluate its electrochemical properties in CR2025 button cell batteries. The experimental conditions for electrode preparation and electrochemical testing are similar to those described in Example 4. Figure 13 shows the curve of the first charge-discharge cycle of a cell using the nanoporous LFP / C powder of Example 7 as the cathode material. The cell using the nanoporous LFP / C powder clearly exhibits a lower polarization (only 63 mV at 60 mAh / g) compared to the cell of Example 4 (95 mV at 60 mAh / g) using non-porous LFP / C powder. In this example, the cell also exhibits a reversible charge / discharge process and delivers a charge capacity of approximately 156 mAh / g.This example indicates that the porosity generated within the particles significantly improves the electrochemical performance of the battery.

Claims

CLAIMS 1. A method of synthesizing a lithium-ion battery material consisting of nanoporous lithium iron phosphate particles (1), the method comprising the following steps: (E1) formation of a precipitation solution by mixing a lithium source (4), an iron(I) source (5), a phosphorus source (6), a reducing agent (8) and carbon nano-objects (7) in a solvent, so as to co-precipitate the lithium, iron and phosphorus around the carbon nano-objects in the form of particles, called LFP / C particles (9), of iron and lithium phosphate incorporating the carbon nano-objects, (E2) separation of LFP / C particles (9) from the precipitation solution, (E3) drying of LFP / C particles (9), (E4) calcination of the LFP / C particles (9) so as to decompose the carbon nano-objects (7) incorporated in said particles, the decomposition of said nano-objects (7) generating nanopores (3) within the lithium iron phosphate particles.

2. The method of claim 1, wherein step (E4) comprises forming a coating layer (24) around the lithium iron phosphate particles by calcining a carbon source.

3. The method of claim 2, wherein the reducing agent is carbon-based and the carbon source comprises said reducing agent.

4. The method of claim 2, wherein the carbon source is added to the precipitation solution in step (E1).

5. The method of claim 2, wherein the carbon source is added to the LFP / C particles during step (E4).

6. Method according to one of claims 1 to 5, in which the carbon nano-objects are chosen so as to obtain, at the end of the calcination step (E4), nanopores of the same size as said carbon nano-objects.

7. Method according to one of claims 1 to 6, in which the solvent is an aqueous solution and the carbon nano-objects are water-soluble.

8. Method according to one of claims 1 to 7, in which the carbon nano-objects comprise carbon quantum dots.

9. Method according to one of claims 1 to 8, in which the carbon nano-objects are chosen to decompose at a temperature between 400 and 700°C.

10. Method according to one of claims 1 to 9, in which the carbon nano-objects are carbon nanoparticles obtained from sugars or sugar derivatives dissolved in water, at a concentration of between 0.1 M and 2 M, the solution being brought to a temperature above 100°C, preferably to a temperature of between 150°C and 200°C for a period of between 2 h and 4 h.

11. Method according to one of claims 1 to 10, in which step (E1) is carried out at atmospheric pressure in an open reactor at a temperature preferably between 50°C and 90°C.

12. Method according to one of claims 1 to 10, in which the reducing agent used in step (E1) is carbon-based and is preferentially chosen from sugars and sugar derivatives, organic acids and glycols.

13. Method according to one of claims 1 to 12, in which the average size of the nanopores within the particles (1) of iron and lithium phosphate is between 1 nm and 500 nm.

14. Method according to one of claims 1 to 13, in which each nanoporous iron and lithium phosphate particle (1) is composed of primary particles (2), the average diameter of the primary particles being preferably between 50 nm and 500 nm and the average diameter of the nanoporous iron and lithium phosphate particles (1) being preferably between 1 pm and 50 pm.

15. Method according to one of claims 1 to 14, in which the mixing step (E1) comprises the addition of a base so as to control the pH of the solution during the coprecipitation, said base being chosen from the group comprising NH4OH, NH4HCO3, NaOH, KOH, NazCOs, NazCsC^ or a water-soluble organic base.

16. Method according to one of claims 1 to 15, in which the formation of the precipitation solution in step (E1) comprises the mixing of the phosphorus source, the carbon nano-objects, the base and the reducing agent prior to a gradual addition of the lithium source and the iron(ll) source, the initial pH of the precipitation solution at the start of step (E1) before the introduction of the lithium source being between 1 and 3, preferably between 1.5 and 2.

5.

17. Method according to one of claims 1 to 16, in which the final pH of the precipitation solution at the end of step (E1) is between 3.5 and 7.5, preferably between 4.5 and 7.

18. Rechargeable Li-ion battery containing the material consisting of nanoporous lithium iron phosphate particles (1) obtained by the method according to one of claims 1 to 17.

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