Method for synthesizing nanoporous lithium iron phosphate particles for lithium-ion battery materials

Abstract

The present invention relates to a method for synthesizing a lithium-ion battery material comprising nanoporous lithium iron phosphate particles (1), the method comprising the steps of: (E1) 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 to form a precipitation solution, thereby coprecipitating lithium, iron, and phosphorus around the carbon nano-objects as LFP / C particles (9), which are lithium iron phosphate particles encapsulating the carbon nano-objects; (E2) separating the LFP / C particles (9) from the precipitation solution; (E3) drying the LFP / C particles (9); and (E4) calcining the LFP / C particles (9) to decompose the encapsulated carbon nano-objects (7), thereby generating nanopores (3) within the lithium iron phosphate particles.

Description

[Technical Field]

[0001] The present invention relates to a method for synthesizing lithium-ion battery materials consisting of nanoporous lithium iron phosphate particles. [Background technology]

[0002] Rechargeable lithium-ion (Li-ion) batteries are currently one of the main energy storage methods. Due to their high energy density and long life, they are widely used in mobile phones, computers, consumer electronics, electric and hybrid vehicles, renewable energy storage stations, etc. The application fields and performance of these batteries have been constantly evolving for several years.

[0003] A lithium-ion battery has three main components: A cathode consisting of a lithium-containing material, a binder such as polyvinylidene fluoride, and a conductive material such as carbon black, optionally mixed to form an organic / inorganic composite material having a mass ratio of approximately 85% lithium-containing material, 10% conductive material, and 5% binder. An electrolyte containing lithium salts that wets a thin plastic or polymer sheet called a separator. · Anode made of carbon-based material such as graphite.

[0004] The charge-discharge cycle of a battery is driven by a redox reaction involving the reversible phenomenon of lithium intercalation and deintercalation at both electrodes, so electrical conductivity and structural stability of both electrodes during the charge-discharge cycle are essential for the battery to function properly.

[0005] Transition metal oxides such as LiCoO2 (LCO), LiNiO2 (LNO), and LiMn2O4 (LMO) are widely used cathode materials, but they suffer from a number of drawbacks. For example, the cobalt contained in LCO is a toxic and expensive metal. Furthermore, the main cobalt sources (nearly 70%) are concentrated in the politically unstable Democratic Republic of the Congo (DRC) and Zambia. The use of pure LNO cathodes poses significant safety issues related to the instability of the nickel oxide structure after lithium desorption and the risk of exothermic reactions between the nickel oxide and the electrolyte. Furthermore, LMO is thermally and electrochemically unstable during charge-discharge cycling.

[0006] Lithium metal phosphate materials are cathode materials that can replace transition metal oxides. For example, particulate lithium iron phosphate LiFePO4 (LFP) with an olivine crystal structure has a voltage of about 3.4 V (vs. Li + / Li) and a high theoretical capacity of approximately 170 mAh / g. LFP also has excellent chemical and thermal stability and does not use toxic and expensive metals. These properties make the material particularly advantageous for applications where safety issues are crucial, such as electric vehicles.

[0007] However, this material is Li + The weak ion diffusion rate and low electronic conductivity result in a significant loss of capacity, especially at high charge / discharge rates. To improve these properties, several strategies have been considered, such as reducing the size of LFP particles, surface-coating the particles with a carbon layer, or fabricating an LFP / C composite and doping it with a transition metal. Surface-coating LFP particles with a carbon layer or fabricating an LFP / carbon composite (LFP / C) improves the electronic conductivity of LFP particles.

[0008] The size and morphology of LFP particles play an important role in the electrochemical performance of the cathode. Nano-sized LFP particles enable extremely high power density to be achieved. In fact, the short diffusion path of lithium ions and the large exchange surface area between the electrolyte and the cathode facilitate the lithium ion insertion / extraction process during charge / discharge cycling. However, nano-sized LFP particles have a large specific surface area and a large amount of binder that easily adsorbs onto the particle surface, resulting in a low volumetric energy density, undesirable reactions during cycling, and poor electrochemical stability.

[0009] One solution to the particle size problem is to prepare micro-nanostructured LFPs consisting of micrometer-sized particles with nanopores. The preparation of micro-nanostructured LFPs significantly improves the electrochemical performance of LFP / C cathodes. During cycling, micro-nanostructured LFPs combine high charge / discharge rates brought about by their nanometer structure with high volumetric energy density and good electrochemical stability brought about by their micrometer dimensions. Furthermore, larger nanopores increase the surface area for exchange with the electrolyte, thereby facilitating the transport of Li between the two electrodes. + The ion transport properties are improved, and batteries made from such materials exhibit high charge and discharge rates, especially at high currents.

[0010] Controlling particle size and morphology to improve material properties requires precise control of synthesis parameters during various manufacturing processes. Solid-state reaction-based synthesis methods, which are widely used on an industrial scale, have significant drawbacks: obtaining a uniform mixture of precursors in the solid state and controlling the morphology and particle size of the final material are difficult. To overcome these issues, solid-state synthesis methods involve multiple steps, such as mixing, mechanical grinding, and high-temperature heat treatment, making the manufacturing process time-consuming and energy-intensive.

[0011] Wet synthesis methods involve mixing lithium, iron, and phosphorus precursors in atomic or molecular form in organic solvents or water, which overcomes the homogeneity problem and allows for the production of LFP cathodes with higher chemical purity and crystalline quality at lower temperatures than solid-state synthesis methods. These methods are called "soft" and also allow for easier control of the morphology and size of LFP particles. However, they have the disadvantage of being complicated to scale up to industrial scale and economically unfavorable.

[0012] For example, sol-gel synthesis requires expensive, flammable organic solvents and relatively expensive organic precursors such as acetates. Furthermore, the intermediate product (xerogel) must be dried, crushed, and heat-treated. Therefore, the sol-gel method is time-consuming, costly, and restrictive from both an industrial and environmental perspective.

[0013] U.S. Patent Application Publication No. 2014 / 0342231 discloses a method for hydrothermal synthesis of 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, then transferred to an autoclave. The precipitate is then calcined to form LFP / C composite particles coated with a carbon coating that encompasses the second carbon source. This carbon coating is intended to improve the electronic conductivity of the LFP particles.

[0014] Hydrothermal synthesis uses pressurized reactors, which have safety issues and require relatively high investment costs. Other variations of this method, such as solvothermal synthesis, allow for lower pressure in the reactor but use relatively expensive high-boiling organic solvents such as polyethylene glycol (PEG).

[0015] US Patent Application Publication No. 2011 / 0027651 discloses a two-step co-precipitation method for synthesizing micro-nanostructured LFP / C particles (microparticles with nanopores). In the first step, FePO4 particles are obtained by reacting an iron(III) ion precursor with a phosphorus source. The resulting particles are then calcined. In the second step, the calcined FePO4 particles are mixed with a carbon source in a solvent. After the solvent is evaporated, a lithium precursor is added, and all the precursors are calcined. This two-step co-precipitation process produces micrometer-sized particles with nanometer-sized porosity and a carbon coating.

[0016] However, this coprecipitation method still has drawbacks. It is based on a two-step synthesis and several mechanical mixing and grinding steps. Therefore, it is time-consuming, complex, and energy-intensive. It also requires the use of toxic and flammable organic solvents, especially for dispersing the carbon source. Furthermore, because this method uses an iron(III) source, it is necessary to use a reducing gas to reduce iron(III) ions to iron(II) ions in the first calcination step to obtain the FePO4 phase. However, handling and storage of reducing gases such as dihydrogen on an industrial scale presents risks that should be avoided. Summary of the Invention

[0017] The present invention aims to design a method for preparing micrometer-sized LFP / C particles with nanometer-sized porosity for use as cathode materials in lithium-ion batteries, which is easy to implement, reduces costs, and allows for control of particle porosity.

[0018] To this end, the present invention provides a method for the synthesis of lithium-ion battery materials consisting of nanoporous lithium iron phosphate particles, comprising the steps of: (E1) mixing a lithium source, an iron(II) source, a phosphorus source, a reducing agent, and carbon nano-objects in a solvent to form a precipitation solution, thereby coprecipitating lithium, iron, and phosphorus around the carbon nano-objects as lithium iron phosphate particles, LFP / C particles, that encapsulate the carbon nano-objects; (E2) Separating the LFP / C particles from the precipitation solution; (E3) drying the LFP / C particles; (E4) calcining the LFP / C particles to decompose the carbon nano-objects contained in the particles, and generating nano-pores within the lithium iron phosphate particles due to the decomposition of the carbon nano-objects.

[0019] This method consists of a single co-precipitation synthesis step, and the number of mechanical mixing, grinding, and heat treatment steps can also be minimized, improving the cost-effectiveness of the preparation method for micro-nanostructured LFP / C particles by co-precipitation.

[0020] This method also advantageously allows for control over the morphology, particle size, and porosity of the micro-nanostructured LFP / C particles obtained at the end of this single synthesis step.

[0021] Furthermore, this method uses only water as a solvent in a single step to synthesize LFP / C particles by coprecipitation, avoids the use of reducing gases such as dihydrogen, and can be operated under temperature and pressure conditions close to ambient conditions, making it easy to scale up to industrial scale and less hazardous.

[0022] As used herein, the term "micrometric" refers to an object having at least one dimension less than 1 mm, and the terms "nanometric" or "nano-" refer to an object having at least one dimension less than 1 μm.

[0023] According to advantageous but optional features of the invention, the following features can be combined in any combination technically possible:

[0024] Step (E4) involves forming a coating layer around the particles by calcining the carbon source.

[0025] The reducing agent is carbon-based and the carbon source comprises the reducing agent.

[0026] In step (E1), a carbon source is added to the precipitation solution.

[0027] In step (E1), a carbon source is added to the LFP / C particles.

[0028] The carbon nano-objects are selected so as to obtain, at the end of the calcination step (E4), nanopores of the same size as the carbon nano-objects.

[0029] The solvent is an aqueous solution, and the carbon nano objects are water-soluble.

[0030] Carbon nano-objects include carbon quantum dots.

[0031] The carbon nano-objects are selected to decompose at temperatures between 400 and 700°C.

[0032] Carbon nanoparticles are carbon nanoparticles obtained from sugars or sugar derivatives by heating a solution of the sugars or sugar derivatives dissolved in water at a concentration of 0.1M to 2M to a temperature above 100°C, preferably 150°C to 200°C, for 2 to 4 hours.

[0033] Step (E1) is preferably carried out at a temperature between 50° C. and 90° C. in an open reactor at atmospheric pressure.

[0034] The reducing agent used in step (E1) is carbon-based and is preferably selected from sugars, sugar derivatives, organic acids, and glycols.

[0035] The average size of the nanopores in the lithium iron phosphate particles is 1 nm to 500 nm.

[0036] Each nanoporous lithium iron phosphate particle is composed of primary particles, the average diameter of which is preferably 50 nm to 500 nm, and the average diameter of the nanoporous lithium iron phosphate particles is preferably 1 μm to 50 μm.

[0037] The mixing step (E1) includes adding a base to control the pH of the solution during co-precipitation, the base being selected from the group consisting of NH4OH, NH4HCO3, NaOH, KOH, Na2CO3, Na2C2O4, or a water-soluble organic base.

[0038] Forming the precipitation solution in step (E1) involves mixing a phosphorus source, carbon nano objects, a base, and a reducing agent before gradually adding a lithium source and an iron(II) source, and the initial pH of the precipitation solution at the start of step (E1) before the introduction of the lithium source is between 1 and 3, preferably between 1.5 and 2.5.

[0039] The final pH of the precipitation solution at the end of step (E1) is 3.5-7.5, preferably 4.5-7.

[0040] Another object of the invention relates to a lithium-ion secondary battery comprising a material made of nanoporous lithium iron phosphate particles obtained by the method described above.

[0041] Other features and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings. [Brief explanation of the drawings]

[0042] [Figure 1] 1 is a block diagram illustrating the different steps of a method for producing LFP / C particles according to the present invention. [Figure 2] FIG. 1 illustrates one embodiment of a step of co-precipitating LFP / C particles containing carbon nano objects by mixing a lithium source, an iron(II) source, a phosphorus source, and carbon nano objects in a solvent to form a precipitation solution. [Figure 3] FIG. 3 shows an example of an experimental setup for forming a precipitation solution according to the embodiment of FIG. 2. [Figure 4] FIG. 1 shows the formation process of nanoporous LFP / C particles in the precipitation solution formation step and the calcination step of the method according to the present invention. [Figure 5] FIG. 1 shows the results of X-ray diffraction analysis performed on the LFP / C particles obtained in Example 3. [Figure 6] 1 is a scanning electron microscope image of the LFP / C particles obtained in Example 3. [Figure 7] 1 is a first charge-discharge cycle curve of a battery fabricated from the LFP / C particles of Example 3 as described in Example 4. [Figure 8] 1 is a scanning electron microscope image of the carbon nanoparticles obtained in Example 5. [Figure 9] FIG. 1 shows the results of X-ray diffraction analysis performed on the nanoporous LFP / C particles obtained in Example 7. [Figure 10] 1 is a scanning electron microscope image of nanoporous LFP / C particles obtained in Example 7. [Figure 11] 1 is a high-magnification scanning electron microscope image of nanoporous LFP / C particles obtained in Example 7. [Figure 12] 1 is an image obtained by observing the polished cross section of the nanoporous LFP / C particles obtained in Example 7 with a scanning electron microscope. [Figure 13] 1 is a first charge-discharge cycle curve of a battery fabricated from the LFP / C particles of Example 7 as described in Example 8.

[0043] For readability, the drawings are not necessarily to scale. Like reference numbers between the drawings indicate like elements. DETAILED DESCRIPTION OF THE INVENTION

[0044] Hereinafter, the terms "about" or "approximately" mean within 10%. The term "average diameter" is understood to mean a particle diameter that is larger than the diameter of 50% of the particles and smaller than the diameter of 50% of the particles according to the present invention. The average diameter can be measured, for example, from a scanning electron microscope (SEM) image.

[0045] The present invention relates to an advantageous and economical method that can be used to produce, on an industrial scale, high performance lithium metal phosphates for use as cathode materials in lithium ion secondary batteries.

[0046] In particular, the present invention relates to a method for synthesizing lithium-ion battery materials consisting of nanoporous lithium iron phosphate particles, which comprises the steps shown as a block diagram in Figure 1. In particular, the method includes forming a precipitation solution to co-precipitate lithium, iron, and phosphorus around the carbon nano-objects as lithium iron phosphate particles encapsulating the carbon nano-objects, and calcining the particles to decompose the encapsulated nano-objects. Decomposition of the nano-objects creates nanopores within the so-called LFP / C lithium iron phosphate particles, producing nanoporous LFP / C particles.

[0047] The inventors have observed that the nanopores of the nanoporous lithium-iron particles obtained after calcination are approximately the same size as the carbon nanoobjects introduced into the precipitation solution. Therefore, in the method according to the invention, the carbon nanoobjects introduced into the precipitation solution can be selected so that, after the calcination step, the nanopores have the desired size to obtain the envisaged properties. In fact, the larger the nanopores, the greater the surface area for exchange with the electrolyte, and therefore the greater the mobility of the Li-Fe particles between the two electrodes. + The ion transport properties are improved, and therefore batteries incorporating such materials exhibit high charge and discharge rates, especially at high currents.

[0048] The method according to the invention relates to the preparation of nanoporous LFP / C particles with an average diameter of 1 μm to 50 μm, preferably 5 μm to 10 μm. The nanoporous LFP / C particles are composed of primary particles with an average diameter preferably of 50 nm to 500 nm.

[0049] The nanopore size of the LFP / C particles according to the present invention can be between 1 nm and 500 nm. Among these particles, microporous particles with pore sizes less than 2 nm, mesoporous particles with pore sizes between 2 nm and 50 nm, and macroporous particles with pore sizes between 50 nm and 500 nm are conventionally distinguished according to the International Union of Pure and Applied Chemistry (IUPAC). As mentioned above, the pore size is advantageously controlled by the size of the carbon nano-objects used in the coprecipitation step. Therefore, those skilled in the art can select the type of porosity of the LFP / C particles.

[0050] 1, in step E1 of the method, a lithium source, an iron(II) source, a reducing agent, a phosphorus source, and carbon nano-objects are mixed in a solvent to form a precipitation solution, and in a single co-precipitation step, the lithium, iron(II), and phosphorus are co-precipitated around the carbon nano-objects as lithium iron phosphate particles that encase the carbon nano-objects. These particles are referred to as LFP / C.

[0051] Figure 4 shows a schematic diagram of the mechanism of the co-precipitation reaction carried out in step E1. First, primary lithium iron phosphate nanoparticles 2 are formed (step 1 in Figure 4). Next, these nanoparticles 2 aggregate around carbon nanoobjects 7 (step 2 in Figure 4). Further maturation and Ostwald ripening steps result in the formation of secondary lithium iron phosphate particles 9 by aggregation of the primary nanoparticles 2 and incorporation of the carbon nanoobjects 7 (step 3 in Figure 4).

[0052] The coprecipitation reaction is preferably carried out in an open reactor at atmospheric pressure at a temperature of 50°C to 90°C, more preferably 60°C to 80°C, for 1 to 20 hours, preferably 2 to 15 hours.

[0053] The near-ambient pressure and temperature conditions in this single synthesis step, and the fact that gases such as dihydrogen are not used, make this method advantageously scalable 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. Those skilled in the art can adjust the synthesis time depending on the desired particle size.

[0054] The iron(II) source (designated 5 in FIG. 4) is an iron(II) salt, such as FeSO4·7H2O or Fe(NO3)2.

[0055] The lithium source (designated 4 in Figure 4) can be selected from the precursors LiOH·1H2O, Li2CO3, LiNO3, Li2SO4·H2O, and LiH2PO4. Lithium hydroxide (LiOH·1H2O) has the advantage of being basic, which contributes favorably to the pH of the coprecipitation medium.

[0056] In some embodiments, the precipitation solution may contain a base, especially if the lithium source is not (basic) LiOH·1H2O. The base is used to control the pH of the precipitation solution and the growth of the LFP particles. The base may be an inorganic base, such as ammonium hydroxide (NH4OH), sodium hydroxide (NaOH), potassium hydroxide (KOH), sodium carbonate (Na2CO3), ammonium bicarbonate (NH4HCO3), and sodium oxalate (Na2CO4). Alternatively, any organic base that is soluble in water can be used.

[0057] The phosphorus source (designated 6 in FIG. 4) includes, for example, H3PO4, (NH4)3PO4, (NH4)2HPO4, and / or (NH4)H2PO4.

[0058] Carbon nano-objects include, for example, carbon nanospheres, carbon nanorods, carbon nanoparticles, carbon-based quantum dots, and any form of carbon having at least one of its dimensions submicrometer in size (less than 1 μm). The carbon nano-objects are selected to decompose during the calcination step.

[0059] It will be apparent to those skilled in the art that the listed precursors are not exhaustive and can be expanded to include sources of lithium, iron(II), phosphorus, and various types of water-soluble carbon nano objects, respectively. Thus, all possible precursors of lithium, iron(II), phosphorus, and water-soluble carbon nano objects are within the scope of the present invention.

[0060] The precipitation solution according to the present invention also contains a reducing agent (designated 8 in FIG. 4). 2+ Fe 3+ To avoid oxidation to HCl and the formation of unwanted parasitic phases (phases other than the LFP phase), a reducing medium can be maintained during the precipitation step.

[0061] In a particularly advantageous embodiment, the reducing agent is carbon-based.

[0062] If carbon-based, the reducing agent can have an additional effect in addition to the aforementioned effect of maintaining the reducing medium: indeed, during the calcination step, the carbon-based reducing agent decomposes to produce a carbon coating layer 24 on the nanoporous LFP / C particles 1, thereby improving the electrical conductivity of the particles.

[0063] Furthermore, because the reducing agent is based on carbon, the material that nano-objects are made of, foreign particle contamination can be avoided.

[0064] The carbon-based reducing agent may include a sugar and / or a sugar derivative, such as glucose, lactose, fructose, and / or dextrose. Alternatively or additionally, the carbon-based reducing agent may include 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 include one or more glycols, such as polyethylene glycol (PEG), tetraethylene glycol (TEG), and ethylene glycol (EG).

[0065] In other embodiments, the reducing agent may be selected from the group consisting of inorganic reducing agents such as potassium iodide (KI), sodium sulfite (NaSO), sodium thiosulfate (NaS0), sodium dithionite (NaS0), sodium tetrahydridoborate (NaBH).

[0066] Figures 2 and 3 show a particular embodiment of step E1 of forming a precipitation solution. In particular, Figure 2 shows, in block diagram form, the various substeps that make up step E1 according to this embodiment, and Figure 3 shows an example of an experimental setup for carrying out such an embodiment of step E1.

[0067] In step E1.1, the phosphorus source and carbon nano-objects are mixed in a double-walled precipitation reactor 10, where the temperature, pH, and redox potential are controlled. A base is added to adjust the pH of the initial solution to 1-3, preferably 1.5-2.5, and a carbon-based reducing agent is added to adjust the initial redox potential to 200-350 mV relative to the potential of a normal hydrogen electrode. The temperature of the solution is 10-90°C.

[0068] Controlling the initial pH with a base prevents the precipitation of undesirable crystalline phases, such as Fe3(PO4)2 and Fe2P2O7, which can form at pH values ​​below 1.

[0069] As an example, an aqueous solution of phosphoric acid is used as the phosphorus source, and an aqueous solution of ammonium hydroxide is used as the base. The aqueous solution of phosphoric acid and the aqueous solution of ammonium hydroxide are prepared by dissolving their precursors in water in advance. The concentration of the aqueous solution of phosphoric acid is preferably 1 mol / L to 3 mol / L. The concentration of the aqueous solution of ammonium hydroxide is 0.1 mol / L to 3 mol / L, preferably 0.4 mol / L to 2 mol / L. Similarly, the reducing agent can be dissolved in water at a concentration of 1 mol / L to 3 mol / L before introducing the solution into the reactor.

[0070] As yet another example, a solution of carbon nanospheres with an average diameter between 1 nm and 1 μm, preferably between 1 nm and 500 nm, is used as a source of carbon nano objects.

[0071] Carbon nanospheres can be prepared in advance by various synthesis methods. For example, they can be obtained from a carbon source via a hydrothermal reaction, for example, by heating a solution of sugar or its derivatives dissolved in water at a concentration of 0.1 mol / L to 2 mol / L to a temperature above 100°C, preferably 150°C to 200°C, for 2 to 6 hours. For example, carbon nanospheres can be synthesized according to the protocol described in the literature "RSC Adv. 2015, 5, 59491-59494." The average diameter of the carbon nanospheres can be adjusted by controlling synthesis parameters such as temperature, reaction time, and precursor concentration.

[0072] This synthesis protocol can yield carbon nanospheres with average diameters between 1 nm and 500 nm. The diameter of the carbon particles can be experimentally measured using transmission electron microscopy (TEM) or high-resolution scanning electron microscopy (HRSEM). Furthermore, the carbon nanospheres obtained by this synthesis protocol decompose favorably at temperatures between 400 °C and 700 °C. Other synthesis methods based on bottom-up approaches can also be used to produce carbon nanoobjects. Another example of a bottom-up method is microwave-assisted pyrolysis synthesis from water-soluble carbon precursors, such as sugars and their derivatives. These methods are advantageous in that they are easy to implement and scalable.

[0073] The double-walled precipitation reactor 10 used has a capacity of, for example, 4 L and is equipped with a mechanical agitator 11, a recovery valve 12 intended to recover the suspension at the end of the synthesis in order 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 steam inside the double wall of the reactor or by means of a coil immersed in the initial solution contained in the reactor.

[0074] In step E1.2, the pH of the solution prepared in step E1.1 is adjusted to 1 to 3, preferably 1.5 to 2.5, using a base (LiOH, NH4OH, etc.).

[0075] In step E1.3, the lithium and iron(II) sources are gradually added to the solution obtained in step E1.2. The addition is preferably carried out so as to maintain a pH between 3.5 and 7.5, preferably between 5 and 7. Maintaining a pH below 7 avoids the formation of undesired phases such as Fe(OH)3 and Li3PO4. In fact, LiFePO4 is not yet stable at this stage and may redissolve in the solvent at acidic pH (below 3.5).

[0076] For example, a basic lithium source can be selected to adjust the pH, and for example, an aqueous lithium hydroxide solution with a concentration of 1 mol / L to 3 mol / L prepared in advance by dissolving a lithium hydroxide salt may be stored in tank 17.

[0077] As the iron (II) source, for example, an aqueous solution of iron (II) sulfate with a concentration of 1 mol / L to 3 mol / L, which is prepared in advance in the tank 18 by dissolving solid iron (II) sulfate in water, is used.

[0078] Metering pumps 19, 20 equipped with flow regulators 21, 22 can be used to introduce the lithium source and the iron source into reactor 10 and control the rate at which these reagents 4, 5 are added to reactor 10 in order to automatically adjust and maintain the pH within a target range. The reactor may also be equipped with a pH controller 23 connected to regulator 21 of metering pump 19, which supplies lithium hydroxide to reactor 10 and adjusts the injection rate of lithium hydroxide depending on the pH value measured by pH sensor 14.

[0079] In step E1.2, aqueous lithium hydroxide (LiOH) solution is added to the mixture containing the phosphorus source, reducing agent, and optionally growth regulator to adjust the pH to 1-3, preferably 1.5-2.5. The remaining LiOH solution and iron sulfate are then introduced into the reactor at a fixed flow rate of, for example, 0.1 L / h to 2.5 L / h until the maximum allowable pH value is reached. Next, in step E1.3, lithium hydroxide solution or another base is introduced at a variable flow rate according to the pH setpoint set by the pH controller 23, ensuring that the pH remains within the allowable range until the LiOH solution is exhausted.

[0080] In step E1.4, the precipitation solution thus formed is kept under stirring until particles of the desired size and morphology are obtained, i.e., step E1.4 is an aging step.

[0081] According to another embodiment of step E1, the lithium source, the iron(II) source, the phosphorus source, the carbon nano-objects, and the reducing agent are directly introduced into the precipitation reactor, and then a base, for example, an ammonium hydroxide solution, is gradually introduced at a controlled flow rate until the target pH value is reached.

[0082] Step E1 can also be performed by changing the order in which the reagents are introduced.

[0083] In the embodiment of step E1 described above, step E1 is carried out in a reactor 1 that is stirred at a speed of, for example, 800 to 1200 revolutions per minute (rpm). Alternatively, the precipitation reactor used in step E1 produces LFP / C particles in a continuous mode.

[0084] Preferably, the final redox potential (Ef) of the reaction mixture at the end of step E1 is 0 mV to 100 mV, more preferably 10 mV to 50 mV.

[0085] Finally, the method according to the present invention advantageously comprises a single synthesis step E1 of LFP / C particles in a liquid medium, preferably an aqueous medium. Therefore, the method does not require mechanical mixing, grinding, or heat treatment steps to obtain the LFP / C particles described above. This improves the cost-effectiveness of the method for preparing micro-nanostructured LFP / C particles by coprecipitation. Furthermore, simply by selecting the conditions for carrying out this single synthesis step E1, the morphology, particle size, and porosity of the nanoporous LFP / C particles obtained at the end of steps E2, E3, and E4, as described below, can be controlled.

[0086] In step E2, the LFP / C particles are separated from the precipitation solution. According to one embodiment, solid-liquid separation is performed by filtration, for example, using a filter press. Other mechanical separation methods, such as centrifugation, can also be used. This results in a wet LFP / C precipitate.

[0087] Step E2 may further include washing the wet LFP / C precipitate. According to one embodiment, washing is performed with water. Washing consists of removing impurities adsorbed on the surface of the LFP / C precipitate, such as water-soluble salts and metal ions, as well as residual acid from step E1. Washing can be monitored by measuring the physicochemical properties of the washing solution, such as pH and ionic conductivity (μS / cm). For example, washing is continued until the pH of the washing solution reaches 6.5-7.5.

[0088] In step E3, the wet LFP / C particles are dried. By way of 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, such as a nitrogen atmosphere.

[0089] Alternatively, drying can be carried out by atomization, for example using a Buchi B-290 Mini Spray Dryer, by spraying a suspension of LFP / C particles into a hot air current reactor.

[0090] In one embodiment, particularly if the reducing agent is non-carbon-based or has a low carbon content, a carbon source is added to the LFP / C particle suspension before or after the drying step so that the particles are coated with a carbon film after calcination step E4. Such a carbon source may optionally be added earlier in step E1.

[0091] If the reducing agent is carbon-based, it can itself be a carbon source. To this end, the reducing agent is advantageously selected to have good adsorption properties on the particles so that it is present on the particles after step E2. If the residual carbon content at the end of step E2 is too low, the reducing agent can optionally be combined with an additional carbon source introduced in steps E1 and / or E4.

[0092] In step E4, the LFP / C particles dried in step E3 are calcined. As shown in step 4 of Figure 4, calcination E4 pyrolyzes the carbon nanoobjects 7 previously encapsulated within the secondary LFP / C particles 9 to form pores 3, and pyrolyzes the carbon source to form a carbon coating layer 24. Thus, at the end of calcination E4, nanoporous LFP / C particles 1 containing pores 3 and a carbon layer 24 are obtained.

[0093] The porosity of LFP / C microparticles is generated by the partial or total decomposition of carbon nano-objects within the secondary LFP / C particles, and therefore the size and shape of the pores can be controlled by varying the shape and size of the carbon nano-objects.

[0094] Calcination can be carried out in an inert atmosphere, for example in a dinitrogen atmosphere, at temperatures between 600°C and 800°C. Calcination E4 is advantageously carried out in a rotary furnace, which allows for better homogenization and uniform diffusion of heat within the material during the heat treatment. The residence time of the LFP / C particles in the furnace is between 2 and 15 hours, preferably between 5 and 10 hours.

[0095] Alternatively, calcination step E4 can be carried out in an atmosphere of a mixture of an inert gas and a gaseous carbon source, such as a mixture of dinitrogen and propylene CH or a mixture of dinitrogen and ethylene. The mixture may contain, for example, 1% to 5% of the gaseous carbon source. The gaseous carbon source present in the mixture can deposit a carbon layer in the vapor phase on the surface of the nanoporous LFP / C particles to improve the electrical conductivity of the final material.

[0096] The crystalline structure of the nanoporous LFP / C particles obtained from the calcination step E4 is, for example, CuKαIt can be identified by X-ray diffraction (XRD) using a PHILIPS X’Pert device with a copper anticathode of 1.541 Å. Examples of X-ray diffraction patterns obtained for the products of the embodiments targeted by the present invention are shown in FIGS. 5 and 9. The XRD results shown in FIGS. 5 and 9 indicate that the product obtained in step E4 is composed of electrochemically active olivine-structured LFP / C particles.

[0097] Furthermore, the nanoporous LFP / C particles, the primary particles constituting them, and the average diameter of the nanopores can be measured using a high-resolution scanning electron microscope (HRSEM), for example, a high-resolution ZEISS scanning electron microscope.

[0098] Some examples of the method of the present invention by the present inventors are described below (Examples 1 to 8). In these examples, embodiments of a method for producing pure nanoporous lithium iron phosphate particles LFP / C are described. However, it will be apparent to those skilled in the art that other phosphate-based chemical compositions can be obtained within the scope of the present invention. For example, iron (Fe) can be replaced with other metals to obtain particles of the formula LiFe (1-x) M x PO4 (M is a metal selected from elements such as Ni, Mn, Co, Ti, V, Nd, Mg, Zn, Y, Al, W, and 0 < x < 1). Also, within the scope of the present invention, other core / shell type formulations having a transition metal concentration gradient between the central part and the peripheral part of the particles can be obtained.

[0099] (Comparative Example 1) In this example, coprecipitation step E1 was performed. First, 1 L of H3PO4 solution (1 M) was added to a 4 L reactor. Next, 60 mL of NH4OH solution (1 M) and 100 mL of glucose solution (1 M) were added to the H3PO4 solution while mechanically 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 pH 2 using the prepared 2 M LiOH solution. The initial potential of the mixture was 350 mV. Then, the remaining LiOH solution (2 M) and 1 L of FeSO4·7H2O solution (1 M) were added at flow rates of 0.4 L / h and 0.3 L / h, respectively. pH max The set value is fixed at 7, and the pH min The value was fixed at 6.7. max Once the set point is reached, the automatic pH control will activate and the pH min The addition of LiOH·1H2 solution will automatically stop until the pH value is reached. max The LiOH solution addition pump was started again until the value reached 0.05, and this was repeated until the LiOH solution was consumed. The reaction mixture was then stirred at 60°C for 10 hours.

[0100] Comparative Example 2 In this example, the intermediate LFP suspension obtained in Example 1 was used to carry out the solid-liquid separation and washing step E2. In this test, approximately 3 L of the LFP suspension was filtered. The resulting solid was then washed several times with water. The effectiveness of the washings was monitored by measuring the conductivity of the filtrate (washing liquid) after each filtration using a conductivity meter. Table 1 shows the change in the ionic conductivity of the filtrate as a function of the number of washes.

[0101] [Table 1] From Table 1, it can be seen that washing reduces the conductivity of the wash water to 60 μS / cm, which is close to the conductivity of the water used in this study. Next, step E3, in which the washed solid was dried under vacuum, was carried out in a vacuum oven at 90° C. for 12 hours.

[0102] Comparative Example 3 In this example, calcination step E4 was performed using the washed and dried LFP precipitate from Example 2. In this experiment, approximately 25 g of the product from Example 2 was mixed with 2 g of anhydrous glucose (VWR Chemicals). The mixture was then dispersed in approximately 10 mL of ultrapure water and placed in an ultrasonic bath for approximately 15 minutes. After the water was evaporated and the mixture was dried under vacuum, the product was transferred to an alumina crucible and placed in a tube furnace where it was heated at 150 °C for 3 hours and then at 700 °C for 10 hours under argon flow. The anhydrous glucose was the carbon source that formed the carbon film on the particle surface during the calcination step.

[0103] Figure 5 shows the results of the X-ray diffraction analysis of the product obtained in Example 3. All observed diffraction lines correspond to the positions of reference LiFePO4 (ASTM-JCPDS File No. 40-1499) with an orthorhombic olivine structure. No other lines were observed, indicating the absence of secondary phases in this product.

[0104] Figure 6 shows a scanning electron microscope (SEM) image of the product obtained in Example 3. This image shows the formation of LFP / C particles with a relatively uniform size and shape, with an average diameter of approximately 5 μm. High-magnification SEM images show that the particles are formed by aggregation of primary particles with an average diameter of approximately 400 nm.

[0105] Comparative Example 4 A positive electrode was fabricated using the LFP / C particles obtained in Example 3, and its electrochemical properties were evaluated in a CR2025 button battery. First, an LFP / C blend was prepared by mixing 8.5 g of the LFP / C powder from Example 3, 1 g of conductive material (acetylene black), and 0.5 g of binder (PVDF) in N-methylpyrrolidone (NMP) solvent. An electrode was fabricated by depositing the LFP / C blend on the surface of aluminum foil using a doctor blade method. Metallic lithium was used as the anode material. The cathode was dried under vacuum at 85°C for 24 hours. The electrolyte was a 1 M solution of lithium hexafluorophosphate (LiPF6) dissolved in a 1:1 volumetric mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (Merck). The battery was assembled in an air-protected glove box and tested under C / 10 charge / discharge conditions using a Biologic MPG-200 cycler.

[0106] Figure 7 shows the first charge-discharge cycle curves of a battery formulated as described in Example 4 using the LFP / C particles of Example 3 as the cathode material. The battery exhibits low polarization (about 95 mV at 60 mA / g) and a relatively reversible charge-discharge process. The charge capacity of the battery is about 150 mAh / g.

[0107] Example 5 In this example, carbon nanoparticles were synthesized. First, a glucose solution was prepared by dissolving 15 g of glucose in 70 mL of distilled water. Next, 1 g of citric acid was added to the glucose solution. This solution was then transferred to an autoclave and heated at 150 °C for 6 hours.

[0108] Figure 8 is an image of the carbon nanoparticle solution sample obtained in Example 5 taken with a scanning electron microscope (SEM) in high resolution mode. This image shows that carbon nanoparticles of uniform size and shape with an average diameter of about 30 nm have been formed.

[0109] Example 6 Nanoporous LFP / C particles were prepared using the carbon nanoparticles obtained in Example 5. Precipitation step E1 was performed under the following conditions: 1 L of H3PO4 solution (1 M) was placed in a 4 L reactor. Next, 150 mL of NH4OH solution (0.4 M) and 100 mL of glucose (1 M) solution were added to the H3PO4 solution while mechanically stirring at 1000 rpm. The reactor was then closed and heated to 65 °C for 30 minutes. Once the temperature stabilized at 65 °C, 40 mL of the carbon nanoparticle suspension obtained in Example 5 was added. Next, the pH of the solution was adjusted to 2 using a 2 M LiOH solution. The remaining LiOH solution (2 M) and 1 L of FeSO4·7H2O solution (1 M) were added at flow rates of 0.4 L / h and 0.3 L / h, respectively. pH max The set value is fixed at 7, and the pH min The value was fixed at 6.7. max Once the set point is reached, the automatic pH control will activate and the pH min The addition of LiOH·1H2 solution will automatically stop until the pH value is reached. max The LiOH solution addition pump was started again until the value reached 0.05, and this was repeated until the LiOH solution was consumed. The reaction mixture was then stirred at 65°C for 12 hours.

[0110] In this example, the solid-liquid separation and washing step E2 and the vacuum drying step E3 were carried out in the same manner as in Example 2.

[0111] Example 7 In this example, calcination step E4 was performed using the LFP / carbon nanoparticle precipitate obtained in Example 6. In this test, approximately 12.5 g of the product from Example 6 was mixed with 1 g of anhydrous glucose (VWR Chemicals). The mixture was then dispersed in approximately 10 mL of ultrapure water and placed in an ultrasonic bath for approximately 20 minutes. After the water was evaporated and the mixture was dried under vacuum, the product was transferred to an alumina crucible and heated in a tube furnace at 150°C for 3 hours, followed by 700°C for 5 hours under argon flow.

[0112] Figure 9 shows the results of X-ray diffraction analysis of the product obtained in Example 7. All observed diffraction lines correspond to those of LiFePO4 with an olivine structure (ASTM-JCPDS File No. 40-1499). No other crystalline phases are observed in this product.

[0113] Figure 10 shows a scanning electron microscope (SEM) image of the product obtained in Example 7. This image shows the formation of LFP / C particles of relatively uniform size and shape with an average diameter of approximately 6 μm. A higher magnification SEM image (Figure 11) shows that secondary particles are formed by aggregation of smaller particles (primary particles) with an average diameter of approximately 300 nm.

[0114] The product obtained in Example 7 was dispersed in resin, and a polished cross section was prepared. Figure 12 shows an SEM image of the cross section of a secondary particle from this product. This image reveals that pores of relatively uniform size are formed within the particle. The average diameter of these pores is approximately 30 nm, which corresponds to the average diameter of the carbon nanoparticles used in this example. Therefore, these pores are the result of thermal decomposition of the carbon nanoparticles during the calcination step.

[0115] Example 8 In this example, a positive electrode was fabricated using the nanoporous LFP / C powder obtained in Example 7, and its electrochemical properties were evaluated in a CR2025 button battery. The experimental conditions for electrode fabrication and electrochemical testing were the same as those described in Example 4. Figure 13 shows the first charge-discharge cycle curves of a battery using the nanoporous LFP / C powder of Example 7 as the cathode material. The battery using the nanoporous LFP / C powder exhibits significantly lower polarization (only 63 mV at 60 mAh / g) compared with the battery using the nonporous LFP / C powder of Example 4 (9 mV at 60 mAh / g). In this example, the battery has a reversible charge-discharge process and exhibits a charge capacity of approximately 156 mAh / g. This example suggests that the pores generated within the particles significantly improve the electrochemical performance of the battery.

Claims

1. A method for synthesizing a lithium-ion battery material consisting of nanoporous lithium iron phosphate particles (1), comprising: (E1) 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 to form a precipitation solution, and co-precipitating lithium, iron, and phosphorus around the carbon nano-objects as lithium iron phosphate particles (LFP / C particles) (9) containing 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) to decompose the carbon nano-objects (7) contained in the particles, and generating nano-pores (3) within the lithium iron phosphate particles by decomposition of the carbon nano-objects (7).

2. 2. The method of claim 1, wherein step (E4) includes 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 the reducing agent.

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

5. The method of claim 2, wherein in step (E1), the carbon source is added to the LFP / C particles.

6. The method according to any one of claims 1 to 5, wherein said carbon nano objects are selected so as to obtain nanopores of the same size as said carbon nano objects at the end of the calcination step (E4).

7. The method according to any one of claims 1 to 6, wherein the solvent is an aqueous solution and the carbon nano objects are water-soluble.

8. The method of any one of claims 1 to 7, wherein the carbon nano-objects comprise carbon quantum dots.

9. 9. The method of any one of claims 1 to 8, wherein the carbon nano-objects are selected to decompose at a temperature of 400 to 700°C.

10. The method according to any one of claims 1 to 9, wherein the carbon nano objects are carbon nanoparticles obtained by heating a solution of sugar or sugar derivative in water at a concentration of 0.1 M to 2 M to a temperature exceeding 100°C, preferably 150°C to 200°C, for 2 to 4 hours.

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

12. The method according to any one of claims 1 to 10, wherein the reducing agent used in step (E1) is carbon-based and preferably selected from sugars, sugar derivatives, organic acids, and glycols.

13. The method according to any one of claims 1 to 12, wherein the average size of the nanopores in the lithium iron phosphate particles (1) is between 1 nm and 500 nm.

14. 14. The method according to any one of claims 1 to 13, wherein each nanoporous lithium iron phosphate particle (1) is composed of primary particles (2), the average diameter of said primary particles being preferably between 50 nm and 500 nm, and the average diameter of said nanoporous lithium iron phosphate particles (1) being preferably between 1 μm and 50 μm.

15. The mixing step (E1) includes adding a base to control the pH of the solution during coprecipitation, the base being NH ４ OH, NH ４ HCO ３、 NaOH, KOH, Na ２ CO ３ , Na ２ C ２ O ４ or a water-soluble organic base.

16. 16. The method of any one of claims 1 to 15, wherein forming the precipitation solution in step (E1) comprises mixing the phosphorus source, the carbon nano objects, a base, and the reducing agent before gradually adding the lithium source and the iron(II) source, and wherein the initial pH of the precipitation solution at the start of step (E1) before introduction of the lithium source is between 1 and 3, preferably between 1.5 and 2.

5.

17. 17. The method according to any one of claims 1 to 16, wherein 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. A lithium-ion secondary battery comprising a nanoporous lithium iron phosphate particulate material (1) obtained by the method according to any one of claims 1 to 17.

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