Lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The application belongs to the technical field of positive electrode materials, and discloses a lithium iron phosphate positive electrode material and a preparation method thereof, which comprises the following steps: uniformly dissolving an organic lithium source, an organic iron source, an organic phosphorus source, an organic metal cation doping source and an organic anion doping source in an organic solvent in proportion to obtain slurry A; spraying and pyrolyzing the slurry A at 500-600 DEG C to obtain material B; sintering the material B in an oxidizing atmosphere at 400-600 DEG C first, and then sintering the material B in an inert atmosphere at 600-800 DEG C, and the lithium iron phosphate positive electrode material is obtained. The high-temperature spray pyrolysis is used to replace the spray drying, the organic matter is carbonized instantaneously at high temperature, and then the high-temperature oxidation and high-temperature reduction are carried out to realize the uniform coating of carbon. Meanwhile, the particle is subjected to three heat treatments, i.e., high-temperature pyrolysis, high-temperature oxidation and high-temperature reduction, so that the small particles are reduced, the particle roundness is increased, the uniformity is increased, and the compaction density is further improved.

Description

Technical Field

[0001] This invention relates to the field of lithium battery technology, and in particular to a lithium iron phosphate cathode material and its preparation method. Background Technology

[0002] The statements herein provide only background information in relation to this invention and do not necessarily constitute prior art.

[0003] Lithium iron phosphate cathode materials are widely used in power batteries and energy storage due to their safety and cycle performance advantages. However, with the increasing market demand for long driving range and high energy density, the materials face the challenge of balancing high compaction and high electrical performance: increasing particle size can improve compaction density, but it will worsen its inherently poor electronic and ionic conductivity, resulting in a decrease in rate capability and low-temperature performance.

[0004] Previous studies have explored using all-organic materials as raw materials for preparing lithium iron phosphate cathode materials, combined with organic metal cation doping sources and organic boron sources. The organic raw materials are dissolved in organic solvents, and the resulting slurry is then dried before sintering. This method allows for simultaneous doping of anions and cations, and to some extent improves the compaction density and specific capacity of the lithium iron phosphate cathode material.

[0005] However, the inventors discovered that when using the above method, during the sintering process, the organic matter slowly decomposes, and the components are prone to segregation, resulting in uneven carbon distribution. Furthermore, the coating adhesion of the carbon layer is weak, which easily leads to an incomplete, uneven coating layer or easy detachment from the particle surface. This prevents electrons from effectively transporting between and within the particles, resulting in a significant increase in the overall internal resistance of the electrode and a deterioration in the rate performance of the battery. That is, during high-current charging or discharging, the voltage drops rapidly, and the usable capacity decreases sharply. The detached carbon layer exposes the lithium iron phosphate particles, causing them to lose electronic connections with the conductive agent and current collector, resulting in a direct decrease in the reversible capacity of the battery. A strong carbon layer can protect the surface of lithium iron phosphate and reduce its direct contact with the electrolyte, thereby suppressing side reactions. A carbon layer with weak adhesion loses or weakens this protective effect, leading to an aggravation of side reactions and easy gas generation.

[0006] In addition, the lithium iron phosphate cathode material prepared by the above method has a high proportion of small particles, which has limited effect on improving the compaction density of the cathode material; moreover, the prepared cathode material particles have weak roundness and have sharp points and defects, which can easily lead to tip discharge and side reactions. Summary of the Invention

[0007] To address the shortcomings of existing technologies, the purpose of this invention is to provide a lithium iron phosphate cathode material and its preparation method.

[0008] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0009] In a first aspect, the present invention provides a method for preparing a lithium iron phosphate cathode material, comprising the following steps: uniformly dissolving an organic lithium source, an organic iron source, an organic phosphorus source, an organic metal cation doping source, and an organic anion doping source in an organic solvent in a certain proportion to obtain slurry A;

[0010] Slurry A is spray-pyrolyzed at 500-600℃ to obtain material B;

[0011] Material B is first sintered in an oxidizing atmosphere at 400-600℃; then sintered in an inert atmosphere at 600-800℃ to obtain the final product.

[0012] Secondly, the present invention provides a lithium iron phosphate cathode material, which is prepared by the aforementioned preparation method.

[0013] The beneficial effects achieved by one or more embodiments of the present invention described above are as follows:

[0014] This method enables simultaneous ionic and atomic-level dispersion and doping of anions and cations into lithium iron phosphate, significantly improving the doping effect and electrochemical performance of the product. This allows the prepared lithium iron phosphate cathode material to simultaneously achieve high compaction and high specific capacity.

[0015] The method uses soluble organic compounds as raw materials and basic carbon sources, lithium-containing organic compounds as lithium sources, iron-containing organic compounds as iron sources, phosphorus-containing organic compounds as phosphorus sources, the carbon in the organic compounds themselves as carbon sources, and metal-organic compounds as doping sources. The process involves mixing the ingredients, high-temperature pyrolysis, sintering in an oxidizing atmosphere, sintering in a nitrogen atmosphere, and gas crushing to obtain lithium iron phosphate. It has no precursor selectivity requirements and is highly adaptable to various raw materials.

[0016] During spray pyrolysis, organic matter decomposes instantly, avoiding the component segregation caused by slow pyrolysis during sintering. Carbon is evenly distributed, and lithium iron phosphate nucleation and growth are closely dependent on the carbon source, improving the coating and adhesion of the carbon layer. At the same time, due to the three high-temperature treatments of "spray pyrolysis-oxidation sintering-reduction sintering", the integrity and roundness of the particles are improved, reducing defects and tip discharge on the particle surface. Particle side reactions are reduced, resulting in better cycle performance.

[0017] The organic system uses high-temperature spray pyrolysis instead of spray drying to achieve instantaneous high-temperature carbonization of organic matter, followed by high-temperature oxidation and high-temperature reduction to achieve uniform carbon coating. Simultaneously, the particles undergo three heat treatments: high-temperature pyrolysis, high-temperature oxidation, and high-temperature reduction, resulting in a reduction in small particles, increased particle roundness and uniformity, and further improved compaction density. Attached Figure Description

[0018] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0019] Figure 1 This is an overall process flow diagram of an embodiment of the present invention;

[0020] Figure 2 This is an SEM image of the large-particle lithium iron phosphate cathode material provided in Embodiment 2 of the present invention;

[0021] Figure 3 This is a SEM image of the lithium iron phosphate cathode material provided in Comparative Example 1 of this invention.

[0022] Figure 4 This is a comparison chart of the number of cycles in Embodiments 1-2 and Comparative Examples 1-2 of the present invention. Detailed Implementation

[0023] It should be noted that the following detailed description is illustrative and intended to provide further explanation of the invention. Unless otherwise specified, all technical and scientific terms used in this invention have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0024] As described in the background section, in the existing preparation process of lithium iron phosphate cathode materials, all raw materials are organic and uniformly dissolved in organic solvents. After the obtained slurry is dried, it is sintered. During the sintering process, the organic matter slowly decomposes, and the components are prone to segregation, resulting in uneven carbon distribution. Moreover, the coating and adhesion of the carbon layer are weak, which easily leads to incomplete, uneven, or easily detached coating layers from the particle surface. This prevents electrons from being effectively transported between and within the particles, resulting in a significant increase in the overall internal resistance of the electrode and a deterioration in the rate performance of the battery. That is, during high-current charging or discharging, the voltage drops rapidly, and the usable capacity decreases sharply. The detached carbon layer exposes the lithium iron phosphate particles, causing them to lose electronic connections with the conductive agent and current collector, resulting in a direct decrease in the reversible capacity of the battery. A strong carbon layer can protect the surface of lithium iron phosphate and reduce its direct contact with the electrolyte, thereby inhibiting side reactions. A carbon layer with weak adhesion loses or weakens this protective effect, leading to an aggravation of side reactions and easy gas generation.

[0025] The lithium iron phosphate cathode material prepared using the above methods has a high proportion of small particles, which limits the improvement in the compaction density of the cathode material. Furthermore, the prepared cathode material particles have poor roundness, with sharp points and defects. The sharp tips of the cathode material particles have a small radius of curvature, leading to a high concentration of electric field distribution at these points. During charging and discharging, lithium ions tend to preferentially insert and extract in these high electric field regions, resulting in excessively high local current density. This uneven lithium ion flow exacerbates the volume expansion and contraction in this region, making the particles more prone to microcracks or even breakage at the tips, thus causing an irreversible decrease in the overall battery capacity and a significant reduction in cycle life.

[0026] After the particles break down, on the one hand, the lithium-ion transport path is increased, and on the other hand, the original conductive network is destroyed, which significantly increases the internal resistance of the battery. This results in a larger polarization voltage and a faster drop in battery voltage during high-current charging and discharging, thus reducing the discharged capacity and deteriorating high-rate performance.

[0027] Due to the tip effect causing uneven local current density, lithium ions will also deposit unevenly on the negative electrode side. Under harsh conditions such as high-current charging or low-temperature charging, lithium metal is more likely to precipitate in areas with dense current, rather than being normally embedded in the graphite negative electrode. Lithium deposition not only consumes valuable lithium sources and leads to capacity reduction, but more seriously, dendrite lithium may puncture the separator, causing internal short circuits, leading to thermal runaway and posing a significant safety hazard.

[0028] The contact area between the broken active material particles and the electrolyte increases dramatically. The fresh fracture surface is very active and more likely to undergo side reactions with the electrolyte, releasing heat and gas, reducing the overall thermal stability of the battery, and increasing safety risks.

[0029] In addition, particles with sharp corners have poor flowability and are more likely to agglomerate when stirring to prepare slurry, making it difficult to disperse evenly. This leads to unstable slurry viscosity and problems such as scratches, uneven thickness, and particle agglomeration during coating, affecting the consistency of electrode quality.

[0030] The sharp tips of the particles can easily scratch or even puncture the high-quality polymer separator during coating and subsequent rolling processes, causing microscopic short circuits, reducing production yield, and posing a potential safety hazard for the long-term use of the battery. Moreover, during rolling, the pressure is concentrated at the sharp corners, making the particles more prone to breakage.

[0031] Irregularly shaped particles with sharp corners have a larger porosity when packed together, making it impossible to achieve the densest packing and affecting the improvement of compaction density.

[0032] To address the above problems, in a first aspect, the present invention provides a method for preparing lithium iron phosphate cathode material, comprising the following steps: uniformly dissolving an organic lithium source, an organic iron source, an organic phosphorus source, an organic metal cation dopant source, and an organic anion dopant source in an organic solvent in a certain proportion to obtain slurry A;

[0033] Slurry A is spray-pyrolyzed at 500-600℃ to obtain material B;

[0034] Material B is first sintered in an oxidizing atmosphere at 400-600℃; then sintered in an inert atmosphere at 600-800℃ to obtain the final product.

[0035] In traditional slow pyrolysis, after the raw materials are mixed, they need to undergo a long period of gradual heating and drying from low temperature to high temperature, and pre-decomposition. Due to the differences in decomposition temperature and diffusion rate of different components (such as lithium source, iron source, phosphorus source, doped ion source, etc.), component migration in the macroscopic area is likely to occur, resulting in overall component segregation and uneven carbon distribution.

[0036] Spray pyrolysis disperses slurry A into uniform droplets of micron size through an atomizer. Before atomization, slurry A has already been mixed at the molecular level with an organic solvent. The droplet size is small (large specific surface area), which ensures that the distribution of each element in a single droplet is uniform and there is no basis for macroscopic segregation.

[0037] When the droplets enter the high-temperature pyrolysis furnace at 500-600℃, the solvent evaporates instantly, the organic matter decomposes simultaneously, and the inorganic components rapidly nucleate in the micro-region, avoiding cross-regional diffusion of elements caused by prolonged heating and fundamentally suppressing component segregation.

[0038] Organic matter inside the droplet undergoes rapid dehydrogenation and carbonization at 500-600℃, generating amorphous carbon. The carbon and inorganic phases are in close contact at the nanoscale, forming a uniform composite structure of carbon and inorganic phase.

[0039] The slurry concentration in each droplet is the same, and the compositional differences between droplets are very small. As a result, the composition of a single particle in the final material is basically the same as the overall composition of the material, which effectively avoids uneven carbon distribution.

[0040] The reasons for the improved particle roundness are analyzed as follows:

[0041] In traditional slow pyrolysis processes, particle morphology is affected by drying methods (such as oven drying which easily leads to agglomeration) and decomposition processes (such as solid-phase diffusion which leads to angular growth), easily forming irregular, multi-angular, or agglomerated particles.

[0042] During atomization, the droplets shrink into spherical shapes due to surface tension. During pyrolysis, the solvent evaporates rapidly, and the solute inside the droplets solidifies in the form of a spherical shell, avoiding morphological distortion caused by gravity or local contraction.

[0043] The atomized droplets are well dispersed in the airflow, with a low probability of collision between particles. The hard shell formed by rapid solidification further hinders particle adhesion, and the final material is a well-dispersed spherical particle.

[0044] Material B obtained from spray pyrolysis is a spherical precursor particle, but the particle surface may contain residual organic impurities (such as fragments of incompletely decomposed organic ligands), micropores or burrs (originating from local shrinkage during rapid evaporation of droplets), and the internal crystal structure is amorphous or low crystallinity. These defects will lead to insufficient particle roundness.

[0045] In an oxidizing atmosphere (air / O2), they are oxidized to CO2 / CO and escape, preventing these impurities from forming surface bulges or cracks due to local pyrolysis expansion during subsequent high-temperature sintering; the temperature of 400-600℃ can activate the surface diffusion ability of the atoms on the particle surface: atoms at the surface protrusions / edges have higher surface energy and tend to migrate to the depressions, filling the micropores or burrs left by spray pyrolysis, reducing the surface roughness of the particles and initially improving the sphericity;

[0046] After pretreatment in an oxidizing atmosphere, the surface of the particles has been initially smoothed, but the interior is still an amorphous / microcrystalline structure with low crystallinity and disordered surface atomic arrangement. High-temperature sintering is required to achieve crystal perfection and surface reconstruction, ultimately improving the roundness.

[0047] In some embodiments, the spray pyrolysis parameters are controlled as follows: pyrolysis temperature 500-600℃, particle size D50 10-40μm.

[0048] If the pyrolysis temperature is too low, the organic system will not be completely pyrolyzed, and further pyrolysis will occur during the oxidation stage, resulting in the segregation of pyrolysis components during oxidation and sintering.

[0049] Excessive pyrolysis temperature leads to energy waste; particles are prone to melting and forming dense spheres, affecting carbon emissions in the subsequent oxidation sintering process; and the carbon content inside and outside the particles is uneven during spray pyrolysis.

[0050] When the particle size is too small during pyrolysis, the loose density is low. During sintering, the material accumulates, the gaps between particles are large, the heat transfer efficiency is reduced, and the particles near the heat source of the furnace body are prone to forming dense spheres during subsequent sintering. Meanwhile, the temperature of the material in the center is low, the reaction is delayed, the reaction consistency is poor, and the performance difference is large.

[0051] When the particle size is too large during pyrolysis, the heat transfer difficulty of the spray pyrolysis particles increases, the temperature gradient is large and the internal temperature is lower than the external temperature, the sintering reaction temperature is inconsistent, which can easily cause surface overburning or complete reaction inside and outside, poor reaction consistency and large performance differences.

[0052] In some embodiments, the molar ratio of the organolithium source, organoiron source, organophosphorus source, organometallic cation dopant source, and organoanion dopant source is 1-1.05:1:1-1:05:0.1%-0.5%:0.5%-2.0%.

[0053] Preferably, in the slurry A, the molar concentration of all solutes is 1-5 mol / L.

[0054] In some embodiments, the organic lithium source is lithium formate, lithium acetate, or lithium methyl; the organic iron source is ferric citrate, ferrous lactate, or ferrous succinate; and the organic phosphorus source is urea phosphate or tributyl phosphate.

[0055] In some embodiments, the organometallic cation dopant source is niobium acetate or niobium formate; the organic anion dopant source is trimethyl borate, 4-methylphenylboronic acid, or phenylboronic acid.

[0056] In some embodiments, the organic solvent is methanol, ethanol, acetic acid, or formic acid.

[0057] In some embodiments, the sintering time in an oxidizing atmosphere is 2-12 h; the sintering time in an inert atmosphere is 4-20 h.

[0058] Preferably, the oxygen partial pressure in the oxidizing atmosphere is 0.05-0.4 MPa. If the oxygen partial pressure is too low, the oxygen cannot effectively penetrate into the interior of the pyrolysis sphere to oxidize the internal carbon, resulting in uniform oxidation and consumption of carbon inside and outside the sphere; if the oxygen partial pressure is too high, the carbon oxidized inside the sphere exists in the form of carbon monoxide and carbon dioxide, the exhaust from the sphere is not smooth, the oxidation efficiency is reduced, and the uniformity of carbon oxidation inside and outside the sphere is also affected.

[0059] Secondly, the present invention provides a lithium iron phosphate cathode material, which is prepared by the aforementioned preparation method.

[0060] The present invention will be further described below with reference to the embodiments.

[0061] Example 1

[0062] Tributyl phosphate was used as the phosphorus source, ferrous succinate as the iron source, lithium methyl phosphate as the lithium source, niobium formate as the niobium source, and trimethyl borate as the boron source.

[0063] Weigh 1.0 mol reagent-grade tributyl phosphate, 1.0 mol ferrous succinate, 1.0 mol lithium methyl ester, 0.5% mol niobium formate, and 2.0% mol trimethyl borate, add them to 1 L of methanol, and stir thoroughly to dissolve and obtain mixed slurry A;

[0064] Slurry A was spray-cracked at a temperature of 500℃, and the particle size D50 was 10.2μm to obtain material B.

[0065] Material B was placed in a sintering furnace and heated to 400°C in an oxygen atmosphere, and kept at that temperature for 12 hours.

[0066] Then nitrogen gas was introduced to replace the inert atmosphere, and the temperature was raised to 800°C and held at that temperature for 6 hours. After cooling and gas fragmentation, lithium iron phosphate cathode material was obtained.

[0067] Example 2

[0068] Urea phosphate was used as the phosphorus source, ferric citrate as the iron source, lithium formate as the lithium source, niobium acetate as the niobium source, and trimethyl borate as the boron source.

[0069] Weigh 1.05 mol reagent-grade urea phosphate, 1.0 mol ferric citrate, 1.05 mol lithium formate, 0.15% mol niobium acetate, and 2.0% mol trimethyl borate, add them to 10 L of ethanol, and stir thoroughly to dissolve to obtain slurry A;

[0070] Slurry A was spray-cracked at a temperature of 400℃, and the particle size D50 was 25.4μm to obtain material B;

[0071] Material B was placed in a sintering furnace and heated to 600°C in an oxygen atmosphere, and held at that temperature for 2 hours. Then nitrogen was introduced to replace the inert atmosphere, and the temperature was raised to 750°C and held at that temperature for 4 hours. After cooling and gas crushing, lithium iron phosphate cathode material was obtained.

[0072] Example 3

[0073] Tributyl phosphate was used as the phosphorus source, ferrous lactate as the iron source, lithium acetate as the lithium source, niobium formate as the niobium source, and trimethyl borate as the boron source.

[0074] Weigh 1.0 mol reagent-grade tributyl phosphate, 1.0 mol ferrous lactate, 1.0 mol lithium acetate, 0.5% mol niobium formate, and 0.5% mol trimethyl borate, add them to 1 L of methanol, and stir thoroughly to dissolve and obtain slurry A;

[0075] Slurry A was spray-cracked at a temperature of 600℃, and the particle size D50 was 39.5μm to obtain material B.

[0076] Material B was placed in a sintering furnace and heated to 500°C in an oxygen atmosphere, and kept at that temperature for 6 hours.

[0077] Then nitrogen gas was introduced to replace the inert atmosphere, and the temperature was raised to 700°C and held at that temperature for 12 hours. After cooling and gas fragmentation, lithium iron phosphate cathode material was obtained.

[0078] Comparative Example 1

[0079] The difference from Example 2 is that the spray pyrolysis step in Example 2 is replaced with flash drying, while everything else is the same as in Example 2.

[0080] Specifically, urea phosphate is used as the phosphorus source, iron citrate as the iron source, lithium formate as the lithium source, niobium acetate as the niobium source, and trimethyl borate as the boron source.

[0081] Weigh 1.05 mol reagent-grade urea phosphate, 1.0 mol ferric citrate, 1.05 mol lithium formate, 0.15% mol niobium acetate, and 2.0% mol trimethyl borate, add them to 10 L of ethanol, and stir thoroughly to dissolve to obtain slurry A;

[0082] Slurry A was placed in a flash drying oven and dried at a temperature of 230℃ to obtain material B;

[0083] Material B was placed in a sintering furnace and heated to 600°C in an oxygen atmosphere, and held at that temperature for 2 hours. Then nitrogen was introduced to replace the inert atmosphere, and the temperature was raised to 750°C and held at that temperature for 4 hours.

[0084] After sintering, the material is cooled and gas-crushed to obtain lithium iron phosphate cathode material.

[0085] Depend on Figure 2 , Figure 3 It can be seen that, Example 2 ( Figure 2 ) and Comparative Example 1 ( Figure 3 Compared with lithium iron phosphate cathode materials prepared by other methods, lithium iron phosphate cathode materials have higher particle size uniformity and roundness, which is conducive to particle stacking and achieving higher compaction density.

[0086] Comparative Example 2

[0087] Ammonium phosphate was used as the phosphorus source, iron oxide as the iron source, lithium carbonate as the lithium source, sucrose as the carbon source, niobium pentoxide as the niobium source, and boric acid as the boron source. The molar ratio of the ingredients was: lithium source: iron source: phosphorus source: carbon source: niobium source: boron source = 1:1:1:0.11:0.005:0.005.

[0088] Weigh out 1 mol of reagent-grade ammonium phosphate, 0.5 mol of iron oxide, 0.5 mol of lithium carbonate, 0.11 mol of sucrose, 0.0025 mol of niobium pentoxide, and 0.005 mol of boric acid, and add them together to 1 L of deionized water. Grind, dry, and sinter to obtain lithium iron phosphate. The sintering temperature is 700℃, and the sintering time is 12 h. The sintered lithium iron phosphate is then pulverized to obtain the finished lithium iron phosphate product.

[0089] Depend on Figure 4 It can be seen that the lithium iron phosphate prepared in Examples 1 and 2 has better cycle performance than the comparative examples. The main reason is that during spray pyrolysis, the organic matter is instantly pyrolyzed, which avoids the component segregation caused by slow pyrolysis during sintering. The carbon is evenly distributed, and the lithium iron phosphate nucleation and growth are closely attached to the carbon source, which improves the coating and adhesion of the carbon layer. At the same time, due to the three high-temperature treatments of "spray pyrolysis-oxidation sintering-reduction sintering", the integrity / roundness of the particles is improved, defects and tip discharge on the particle surface are reduced, and particle side reactions are reduced. The cycle performance of Examples 1-2 is significantly better than that of Comparative Examples 1-2.

[0090] Comparative Example 3

[0091] Iron phosphate was used as the iron and phosphorus source, lithium carbonate as the lithium source, sucrose as the carbon source, niobium pentoxide as the niobium source, and boric acid as the boron source.

[0092] Weigh out 1 mol of reagent iron phosphate, 0.525 mol of reagent lithium carbonate, 0.12 mol of sucrose, 0.005 mol of niobium pentoxide, and 0.005 mol of boric acid, and add them together to 1 L of deionized water. After grinding, drying, and sintering in a furnace, lithium iron phosphate is obtained. The sintering temperature is 750℃, and the sintering time is 6 hours. The sintered lithium iron phosphate is then pulverized to obtain the finished lithium iron phosphate product.

[0093] Comparative Example 4

[0094] The difference from Example 2 is that spray pyrolysis in Example 2 is replaced by spray drying, and the spray drying temperature is 230°C. Everything else is the same as in Example 2.

[0095] Comparative Example 5

[0096] The difference from Example 2 is that "heating to 600°C in an oxygen atmosphere and holding at that temperature for 2 hours" in Example 2 is replaced with "heating to 650°C in an oxygen atmosphere and holding at that temperature for 2 hours". Everything else is the same as in Example 2.

[0097] Comparative Example 6

[0098] The difference from Example 2 is that the spray pyrolysis particle size D50 in Example 2 is 5.6 μm, while all other aspects are the same as in Example 2.

[0099] Comparative Example 7

[0100] The difference from Example 2 is that the spray pyrolysis particle size D50 in Example 2 is 45.8 μm, while all other aspects are the same as in Example 2.

[0101] Button cell manufacturing:

[0102] (1) Dissolve polyvinylidene fluoride (PVDF) in methylpyrrolidone (NMP) to obtain PVDF solution, add conductive carbon black to PVDF solution, and then add lithium iron phosphate cathode material prepared in the example or comparative example. The mass ratio of PVDF, methylpyrrolidone, conductive carbon black and cathode material is 5:115:5:90. Stir evenly and coat the slurry evenly on copper foil on a coating machine to form an electrode sheet.

[0103] (2) Place the coated electrode in a vacuum drying oven at 120°C and dry it for 6 hours. Then take out the electrode and roll it on a roller press for later use.

[0104] The button cell assembly was carried out in a glove box under an argon atmosphere. The electrolyte was 1 M LiPF6+ EC:DEC:DMC = 1:1:1 (volume ratio), and the lithium metal sheet was used as the counter electrode.

[0105] Capacity testing was conducted on an Arbin BT2000 battery tester in the United States, with a charge / discharge voltage range of 2.0-3.65V and a charge / discharge rate of 0.1C.

[0106] Preparation of soft-pack lithium-ion battery: The lithium iron phosphate cathode material prepared in this example and the comparative example is dispersed in methylpyrrolidone with conductive agent Super P and binder polyvinylidene fluoride (PVDF) at a mass ratio of 96:2:2 and stirred evenly to obtain electrode slurry. The mass percentage of solute in the slurry is 60%.

[0107] The electrode paste is coated onto the surface of an aluminum foil with a coating density of 400 g / m². 2 Dry at 85°C to obtain the positive electrode sheet.

[0108] A pouch cell with a capacity of approximately 3 Ah was fabricated by combining a positive electrode sheet with a commercial graphite negative electrode, an electrolyte of 1 mol / L LiPF6 / EC+PC+DEC+EMC (volume ratio 1:0.3:1:1), and a PP / PE / PP three-layer separator with a thickness of 14 μm. Full cell performance was tested in the range of 2.5V-3.8V.

[0109] Table 1. Comparison of phase properties of lithium iron phosphate cathode materials prepared in the examples and comparative examples.

[0110]

[0111] As shown in Table 1, by using an organic raw material high-temperature spray pyrolysis system for synthesis, compared with Comparative Example 1, the lithium iron phosphate cathode materials prepared by Examples 1-3 with the same carbon coating content have lower powder resistivity than the sample of Comparative Example 1; compared with Comparative Example 1, under the same particle size conditions, Examples 1-3 have higher compaction density and a slight improvement in 1C discharge performance compared with Example 1.

[0112] Table 2 Comparison of Rate Discharge Performance of Lithium Iron Phosphate Prepared in Examples and Comparative Examples

[0113]

[0114] As shown in Table 2, since organic raw materials are mixed at the atomic level, the uniformity of each element is improved, which can quickly and effectively provide more Li ion insertion and extraction channels and improve the rate discharge performance. Secondly, the use of organic raw materials as dopants avoids the component segregation caused by the "solid-solid" mass transfer of nanoparticle additives, which further improves the lithium ion diffusion ability and improves the rate discharge performance.

[0115] Furthermore, since the raw materials themselves can decompose carbon to achieve in-situ dense coating, improve electronic conductivity and reduce electronic resistance, on the one hand, it can reduce charge transfer impedance and reduce heat generation; on the other hand, it can quickly transfer the generated heat to the surface of the cell through the current collector, avoid heat accumulation, effectively reduce the temperature rise of the cell, and improve safety performance.

[0116] Compared with Comparative Example 1, the high-temperature pyrolysis in Examples 1-3 enables rapid pyrolysis of organic carbon, avoids component segregation, and helps improve the uniformity of carbon distribution. After spray pyrolysis, high-temperature oxidation, and high-temperature reduction, the roundness and integrity of the particles are improved, and their rate performance is improved compared with the comparative example.

[0117] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that: From the following steps Composition: Organolithium source, organoiron source, organophosphorus source, organometallic cation dopant source and organoboron source are uniformly dissolved in an organic solvent in a certain proportion to obtain slurry A; the organoboron source is trimethyl borate, 4-methylphenylboronic acid or phenylboronic acid; Slurry A is spray-pyrolyzed at 500-600℃ to obtain material B; Material B is first sintered in an oxidizing atmosphere at 400-600℃; then sintered in an inert atmosphere at 600-800℃ to obtain the final product.

2. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that: The parameters for spray pyrolysis are controlled as follows: pyrolysis temperature 500-600℃, particle size D50 10-40μm.

3. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that: The molar ratio of organolithium source, organoiron source, organophosphorus source, organometallic cation dopant source and organoboron source is 1-1.05:1:1-1:05:0.001-0.005:0.005-0.

02.

4. The method for preparing the lithium iron phosphate cathode material according to claim 3, characterized in that: In the slurry A, the molar concentration of all solutes is 1-5 mol / L.

5. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that: The organic lithium source is lithium formate, lithium acetate, or methyl lithium; the organic iron source is ferric citrate, ferrous lactate, or ferrous succinate; and the organic phosphorus source is urea phosphate or tributyl phosphate.

6. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that: The organometallic cation doping source is niobium acetate or niobium formate.

7. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that: The organic solvent is methanol, ethanol, acetic acid, or formic acid.

8. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that: The sintering time in an oxidizing atmosphere is 2-12 hours; the sintering time in an inert atmosphere is 4-20 hours.

9. The method for preparing the lithium iron phosphate cathode material according to claim 8, characterized in that: In the oxidizing atmosphere, the partial pressure of oxygen is 0.05-0.4 MPa.

10. A lithium iron phosphate cathode material, characterized in that: It is prepared by any one of the preparation methods described in claims 1-9.

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