Lithium iron phosphate material, preparation method and application thereof

Abstract

The application discloses a lithium iron phosphate material and a preparation method and application thereof. The preparation method of the lithium iron phosphate material comprises the following steps: S1, performing zoned sintering on a lithium iron phosphate precursor to obtain sintered material; wherein the preparation of the lithium iron phosphate precursor comprises: grinding and drying a mixture C containing product A and product B; the product A is prepared by reacting a mixture A containing an iron source and phosphoric acid; the product B is prepared by reacting a mixture B containing an organic acid, a lithium source and a carbon source; and S2, crushing the sintered material. The pole piece prepared from the lithium iron phosphate material prepared by the preparation method has high pole piece compactness density, is beneficial to the preparation of a small-size battery, and can also ensure excellent electrochemical performance of the obtained battery.

Description

Technical Field

[0001] The invention relates to a lithium iron phosphate material and a preparation method and application thereof. Background Art

[0002] Lithium iron phosphate (LiFePO4) cathode materials have gained widespread application in new energy power batteries and energy storage due to their comprehensive advantages, including excellent safety and cycling stability, low manufacturing costs, and environmental compatibility. In particular, increasing energy density has become a core requirement for industrial development in new energy vehicle power battery systems. Existing technologies primarily optimize the powder compaction density of lithium iron phosphate cathode materials to increase the electrode compaction density, thereby achieving breakthroughs in battery volumetric energy density. However, actual research has shown that, due to the synergistic effects of the material system's multi-level structural characteristics (including parameters such as particle grading, primary particle size distribution, and micromorphology), simply increasing the powder compaction density does not effectively and simultaneously increase the electrode compaction density. This limitation has become a key technical bottleneck in further improving the system's energy density.

[0003] Investigating its mechanism, the compaction process of lithium iron phosphate electrode plates is essentially subject to two structural constraints: first, the contact stress distribution between material particles is subject to the particle grading and morphological characteristics; second, the effective construction of the conductive network depends on the uniformity and continuity of the carbon coating layer. In the existing technology, the coordinated control of the carbon coating process and the sintering process is insufficient, resulting in a non-uniform distribution of the carbon source during the sintering process. Specifically, the local carbon content is too high to form an electronic conduction "island", resulting in an increase in ineffective porosity; at the same time, the carbon distribution gradient causes differences in contact impedance between particles, limiting the overall densification process of the electrode material. The above problems have seriously restricted the efficiency of the technology conversion of lithium iron phosphate materials from the laboratory to industrialization.

[0004] Therefore, there is an urgent need to develop a lithium iron phosphate material that can effectively improve the compaction density of the electrode. Summary of the Invention

[0005] To overcome the drawback of existing lithium iron phosphate material processes, which make it difficult to effectively increase the electrode sheet compaction density by increasing the powder compaction density, the present invention provides a lithium iron phosphate material, its preparation method, and its application. This lithium iron phosphate material can be used to produce electrodes with high electrode sheet compaction density, facilitating the transition from large batteries to smaller ones to meet higher application requirements, while also ensuring the excellent electrochemical performance of the resulting battery.

[0006] In order to achieve the above technical effects, the present invention provides the following technical solutions.

[0007] The present invention provides a method for preparing a lithium iron phosphate material, which comprises the following steps:

[0008] S1. Sintering the lithium iron phosphate precursor in different zones to obtain sintered material; wherein,

[0009] The preparation of the lithium iron phosphate precursor comprises: grinding and drying a mixture C containing product A and product B; the product A is prepared by reacting a mixture A containing an iron source and phosphoric acid; the product B is prepared by reacting a mixture B containing an organic acid, a lithium source and a carbon source;

[0010] S2. Crushing the sintered material.

[0011] In the present invention, the partitioned sintering means that the lithium iron phosphate precursor to be sintered is divided into at least two parts, and the parts are placed in different areas of a material holding device in a sintering device for sintering.

[0012] In the present invention, zoned sintering is used to achieve uniform heating of the lithium iron phosphate precursor during the sintering process. "Uniform heating" means that during the sintering process, the temperature difference between the lithium iron phosphate precursor in different zones is small, typically less than 10°C.

[0013] In the present invention, Product A, obtained by reacting a mixture A containing an iron source with phosphoric acid, is distinguished from commercially available iron phosphate in that it is a mixture containing, among other substances, amorphous ferrous phosphate and ferrous dihydrogen phosphate (which can be considered an iron phosphate precursor) and has a relatively small particle size. Therefore, the present invention's method for preparing a lithium iron phosphate precursor differs from prior art processes for directly preparing a lithium iron phosphate precursor from iron phosphate.

[0014] In the present invention, product A will generate crystals during the mixed grinding with product B and gradually grow. Subsequently, during the partitioned sintering process, the crystals continue to grow while promoting uniform carbon coating and lithium diffusion, and simultaneously regulating the micromorphology and gradation of the resulting sintered material, thereby reducing the differences in the micromorphology, particle size distribution, etc. of the sintered material. Finally, the sintered material is dissociated by crushing to obtain an excellent lithium iron phosphate material. The use of the lithium iron phosphate material obtained by the present invention to prepare pole pieces can effectively improve the pole piece compaction density of the obtained pole piece without adding too much binder; in practical applications, it has great practical significance for reducing the volume of the battery and has high efficiency. Compared with the traditional process of directly preparing the precursor with iron phosphate and sintering the obtained material in a conventional manner, it is significantly better.

[0015] In the present invention, the iron source does not include iron phosphate.

[0016] In some embodiments, in step S1, the iron source is selected from one or more of iron powder, ferrous oxide, and ferrosoferric oxide.

[0017] The purity of the iron powder is preferably 95 wt % or more, more preferably 99 wt % or more, and even more preferably 99.5 wt % or more, for example 99.7 wt %. The iron powder may be one or more of primary reduced iron powder, secondary reduced iron powder, carbonyl reduced iron powder, and electrolytic iron powder.

[0018] The purity of the ferric oxide is preferably 95 wt % or more, more preferably 99 wt % or more, and even more preferably 99.5 wt % or more.

[0019] The purity of the ferrosoferric oxide is preferably 95 wt % or more, more preferably 99 wt % or more, and even more preferably 99.5 wt % or more.

[0020] In some embodiments, in step S1, the iron source is in powder form, and the mesh size of the iron source is preferably 200-1000 mesh, more preferably 200-500 mesh, for example, 250 mesh or 300 mesh.

[0021] In some embodiments, in step S1, the molar ratio of the iron source to the phosphoric acid is (0.94-1.05):1, preferably (0.96-1.0):1, for example 0.98:1, calculated based on phosphorus and iron.

[0022] In some embodiments, in step S1, the phosphoric acid may be conventional phosphoric acid in the art, such as industrial-grade phosphoric acid, food-grade phosphoric acid, appliance-grade phosphoric acid, or electronic-grade phosphoric acid.

[0023] In some embodiments, in step S1, the mixture A further comprises water and / or a protonic acid catalyst. The protonic acid catalyst refers to a compound that can dissociate to produce hydrogen ions, and those skilled in the art will know its specific meaning.

[0024] In some preferred embodiments, in step S1, the protonic acid catalyst satisfies one or more of the following conditions af:

[0025] a. The protonic acid catalyst includes one or more of carboxyl, hydroxyl, carbonyl, amino, nitro, cyano, sulfhydryl, sulfonic acid, ester and ether bonds;

[0026] b. The protonic acid catalyst includes heteroatoms, wherein the heteroatoms are selected from one or more of O, N, S, P, F, Cl, Br and I;

[0027] c. The protonic acid catalyst comprises one or more of a linear structure, a branched structure, a cyclic structure and a cage structure;

[0028] d. The protonic acid catalyst includes saturated bonds and / or unsaturated bonds;

[0029] e. The protonic acid catalyst is a polymer compound;

[0030] f. The protonic acid catalyst includes a skeleton structure constituting a biomolecule.

[0031] The cyclic structure preferably includes one or more of an aliphatic ring, an aromatic ring, and a heterocyclic ring. The heterocyclic ring refers to a ring composed of carbon atoms and other non-carbon atoms (such as nitrogen, oxygen, and sulfur), and those skilled in the art will know its specific meaning.

[0032] Wherein, the unsaturated bonds preferably include double bonds and / or triple bonds.

[0033] The biomolecules preferably include one or more of amino acids, fatty acids, and organic acid metabolites. Organic acid metabolites refer to acidic organic compounds produced by the decomposition of nutrients such as carbohydrates, fats, and proteins during human metabolism, and those skilled in the art will know their specific meanings.

[0034] In some more preferred embodiments, in step S1, the protonic acid catalyst is a carboxylic acid compound and / or ascorbic acid.

[0035] The carboxylic acid compound is preferably one or more of formic acid, acetic acid, propionic acid, sorbic acid, oxalic acid, salicylic acid, citric acid, tartaric acid and malic acid.

[0036] In some specific embodiments, in step S1, the protonic acid catalyst is one or more of formic acid, propionic acid, citric acid and malic acid.

[0037] The addition amount of the protonic acid catalyst is preferably 3%-11%, such as 3.4% or 10.5%, where the percentage is the percentage of the mass of the protonic acid catalyst to the sum of the masses of the water, the protonic acid catalyst and the phosphoric acid.

[0038] In some embodiments, in step S1, the preparation of the mixture A comprises: adding the iron source to the phosphoric acid under stirring.

[0039] In some preferred embodiments, in step S1, the preparation of the mixture A comprises: first mixing water, phosphoric acid and a protonic acid catalyst to obtain a mixed solution, and then adding the iron source under stirring.

[0040] In some preferred embodiments, in step S1, the viscosity of the product A is 2000-10000 cps, preferably 2000-6000 cps, for example 3000 cps or 4500 cps.

[0041] In some embodiments, in step S1, the reaction temperature of the mixture A is 20-95°C, preferably 30-90°C, for example 35°C, 45°C or 55°C.

[0042] In some embodiments, in step S1, after the reaction of the mixture A, a grinding step is further included; the grinding operation can be a conventional grinding operation in the art, such as ball milling or sand milling.

[0043] The sand milling device is preferably a sand mill, which is preferably a vertical sand mill, a horizontal sand mill (such as a nano-scale horizontal sand mill), a basket sand mill or a double-cone rod sand mill.

[0044] The grinding media used in the sand grinding process are preferably zirconium oxide grinding beads; the particle size of the zirconium oxide grinding beads is preferably 0.1-3.3 mm, such as 0.3 or 0.4 mm.

[0045] In some embodiments, in step S1, the organic acid is a carboxylic acid compound and / or ascorbic acid.

[0046] The carboxylic acid compound is preferably one or more of formic acid, acetic acid, propionic acid, sorbic acid, oxalic acid, salicylic acid, citric acid, tartaric acid and malic acid, such as one or more of oxalic acid, citric acid, tartaric acid and malic acid.

[0047] In some embodiments, in step S1, the amount of the organic acid added is 30%-150%, for example 68.25%; the percentage is the mass of the organic acid relative to the mass of the phosphoric acid in the mixture A.

[0048] In some embodiments, in step S1, the lithium source is one or more of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium nitrate, lithium phosphate and lithium acetate, preferably one or more of lithium hydroxide monohydrate, lithium carbonate and lithium acetate; the lithium carbonate is preferably industrial grade lithium carbonate or battery grade lithium carbonate.

[0049] In some embodiments, in step S1, the molar ratio of the lithium source to the phosphoric acid in the mixture A, calculated as phosphorus element and lithium element, is (0.98-1.05):1, for example, 1.02:1, 1.03:1 or 1.04:1.

[0050] In some embodiments, in step S1, the carbon source is one or more of glucose, sucrose, polyethylene glycol, and polyethylene glycol derivatives, preferably one or more of glucose, polyethylene glycol, and polyethylene glycol derivatives. The polyethylene glycol derivatives can be conventional in the art, generally referring to compounds obtained by replacing the hydroxyl groups at both ends of polyethylene glycol with different active groups, and those skilled in the art will know their specific meanings.

[0051] In some embodiments, in step S1, the amount of the carbon source added is 1%-80%, preferably 5%-50%, more preferably 10%-40%, for example 11%; the percentage is the mass of the carbon source as a percentage of the mass of the iron source in the mixture A.

[0052] In some embodiments, in step S1, the preparation of the mixture B comprises: adding the lithium source and the carbon source to the organic acid under stirring.

[0053] In some embodiments, in step S1, the mixture B further comprises water. The preparation of the mixture B preferably comprises: first mixing the water and the organic acid to obtain an organic acid solution, and then adding the lithium source and the carbon source while stirring. The mass percentage concentration of the organic acid in the organic acid solution is preferably 5% to 98%, for example, 55%, 62%, or 72%.

[0054] In some preferred embodiments, in step S1, the mixture B further comprises a metal oxide. The metal oxide can serve as a dopant to improve the ionic and electronic conductivity of the obtained lithium iron phosphate, thereby improving the electrical performance.

[0055] The metal oxide is preferably one or more of titanium oxide, vanadium oxide and niobium oxide, such as titanium dioxide.

[0056] Wherein, based on phosphorus element and metal element, the molar ratio of the metal oxide to the phosphoric acid is preferably (0.005-0.025):1, for example 0.01:1.

[0057] In some preferred embodiments, in step S1, the preparation of the mixture B comprises: first mixing the water and the organic acid to obtain an organic acid solution, and then adding the lithium source, carbon source and metal oxide under stirring.

[0058] In some embodiments, in step S1, the reaction temperature of the mixture B is 20-95°C, more preferably 30-90°C, for example 35°C, 40°C or 45°C.

[0059] In some embodiments, in step S1 , the particle size D10 of the solid particles in the mixture C is less than 0.45 μm; the particle size D50 is 1.4-2.3 μm, for example, 1.6 μm; and the particle size D90 is greater than 9.5 μm.

[0060] In some embodiments, in step S1 , the solid content of the mixture C is 40%-50%, for example 45%, where the percentage refers to the percentage of the mass of the solid in the mixture C to the total mass of the mixture C.

[0061] In the present invention, in step S1, the drying and grinding operations can be conventional in the art.

[0062] In some embodiments, in step S1, the drying method is spray drying.

[0063] Wherein, the spray drying device is preferably a spray dryer.

[0064] The air inlet temperature of the spray drying is preferably 260-290°C, for example 280°C.

[0065] The outlet temperature of the spray drying is preferably 100-130°C, for example 110°C.

[0066] In some embodiments, in step S1, the grinding method is ball milling or sand milling.

[0067] The sand milling device is preferably a sand mill, which is preferably a vertical sand mill, a horizontal sand mill (such as a nano-scale horizontal sand mill), a basket sand mill or a double-cone rod sand mill.

[0068] The grinding media used in the sand grinding process are preferably zirconium oxide grinding beads; the particle size of the zirconium oxide grinding beads is preferably 0.1-3.3 mm, such as 0.3 or 0.4 mm.

[0069] In some embodiments, in step S1, the zoned sintering includes 2-9 zones, for example, 4 zones.

[0070] In some embodiments, in step S1, the partitioned sintering is equal-amount partitioned sintering or gradient partitioned sintering.

[0071] Wherein, in the gradient zoned sintering, the mass of the lithium iron phosphate precursor in each zone preferably increases gradually from the center to the periphery.

[0072] In some embodiments, in step S1, the partitioned sintering is performed in an inert atmosphere, such as nitrogen.

[0073] In some embodiments, in step S1, the partitioned sintering includes the steps of first heating and then keeping the temperature.

[0074] The heating rate of the heating is preferably 1-5°C / min, for example 3°C / min.

[0075] The insulation temperature is preferably 680-820°C, such as 730°C, 740°C, 760°C or 780°C.

[0076] The insulation time is preferably 10-20 hours, for example 15 hours.

[0077] In some embodiments, in step S1, the partitioned sintering is performed using a partitioned sagger, the partitioned sagger includes at least two containing areas divided by a partition, and the lithium iron phosphate precursor is placed in each containing area of ​​the partitioned sagger.

[0078] In some preferred embodiments, in step S1, the partitioned sagger further comprises a cover plate, which is disposed above the partition plate and covers the containing area.

[0079] In some preferred embodiments, in step S1, the partitioned sagger includes 2-9 containing areas, for example, 4.

[0080] In some preferred embodiments, in step S1, in the partitioned sagger, the volumes of the various containing areas are equal, or the volumes of the various containing areas are distributed in a gradient manner.

[0081] The volume gradient distribution of each accommodation area is preferably such that the volume of each accommodation area gradually increases from the center to the periphery.

[0082] In some preferred embodiments, in step S1, in the partition sagger, the number of the partitions is 1-4, for example, 2; wherein the plurality of partitions are preferably arranged in the accommodating cavity in a vertically crossed, non-vertically crossed, circular, parallel or non-parallel manner, for example, in a vertically crossed manner.

[0083] In some preferred embodiments, in step S1, the partitioned sagger is made of one of graphite, alumina, or silicon carbide, preferably graphite.

[0084] In some embodiments, in step S2, the crushing method is mechanical crushing and / or air flow crushing; the air flow crushing device is, for example, an air flow mill.

[0085] The present invention also provides a lithium iron phosphate material, which is prepared by the above-mentioned preparation method of the lithium iron phosphate material.

[0086] In some embodiments, the particle size D10 of the lithium iron phosphate material is greater than 0.35 μm, D50 is 1.0-2.0 μm, and D99 is less than 9.5 μm.

[0087] In some embodiments, the carbon content of the lithium iron phosphate material is 1%-2%, for example, 1.15%, 1.16%, 1.17%, 1.19%, 1.21%, 1.25% or 1.27%.

[0088] The present invention also provides a use of the lithium iron phosphate material as described above in a lithium ion battery.

[0089] On the basis of conforming to the common sense in this field, the above-mentioned preferred conditions can be arbitrarily combined to obtain the preferred embodiments of the present invention.

[0090] The reagents and raw materials used in the present invention are commercially available.

[0091] The positive progress effect of the present invention is:

[0092] The lithium iron phosphate material preparation method of the present invention improves the lithium iron phosphate precursor preparation and sintering process, so that the obtained lithium iron phosphate material has high performance consistency, and effectively reduces the differences in micromorphology, particle distribution, etc. in different regions of the material caused by traditional preparation processes.

[0093] At the same time, the use of the lithium iron phosphate material of the present invention to prepare pole pieces can significantly improve the pole piece compaction density, thereby realizing the conversion of large batteries to small-volume batteries to meet higher application requirements, and can further ensure the excellent electrochemical performance of the resulting battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0094] Figure 1 Schematic diagram of the structure of the sagger body of the partitioned sagger used in an embodiment of the present invention.

[0095] Figure 2 Schematic diagram of the structure of the sagger cover plate of the partitioned sagger used in an embodiment of the present invention.

[0096] Figure 3 This is a morphology diagram of the upper layer material obtained in the sintering step in Comparative Example 1 of the present invention.

[0097] Figure 4 This is a morphology diagram of the intermediate material obtained in the sintering step in Comparative Example 1 of the present invention.

[0098] Figure 5 This is a morphology diagram of the wall layer material obtained in the sintering step in Comparative Example 1 of the present invention.

[0099] Figure 6 This is a morphology diagram of the upper layer material obtained in the sintering step in Comparative Example 2 of the present invention.

[0100] Figure 7 This is a morphology diagram of the intermediate material obtained in the sintering step in Comparative Example 2 of the present invention.

[0101] Figure 8 This is a morphology diagram of the wall layer material obtained in the sintering step in Comparative Example 2 of the present invention.

[0102] Figure 9 This is a morphology diagram of the upper layer material obtained in the sintering step in Example 1 of the present invention.

[0103] Figure 10 This is a morphology diagram of the intermediate material obtained in the sintering step in Example 1 of the present invention.

[0104] Figure 11 This is a morphology diagram of the wall layer material obtained in the sintering step in Example 1 of the present invention.

[0105] Figure 12 This is a morphology diagram of the upper layer material obtained in the sintering step in Example 2 of the present invention.

[0106] Figure 13 This is a morphology diagram of the intermediate material obtained in the sintering step in Example 2 of the present invention.

[0107] Figure 14 This is a morphology diagram of the wall layer material obtained in the sintering step in Example 2 of the present invention.

[0108] Figure 15 This is a particle size distribution diagram of the mixed material obtained in the sintering step in Example 1 of the present invention.

[0109] Figure 16 This is a particle size distribution diagram of the mixed material obtained in the sintering step in Comparative Example 1 of the present invention.

[0110] Description of reference numerals:

[0111] The sagger body 100 , the side plate 110 , the groove 111 , the accommodating cavity 120 , the partition plate 130 , the cover plate 200 , the protrusion 210 , and the air vent 220 . DETAILED DESCRIPTION

[0112] The present invention is further illustrated by way of examples below, but the present invention is not limited to the scope of the examples. Experimental methods in the following examples where specific conditions are not specified were performed according to conventional methods and conditions, or selected according to the product specifications.

[0113] In the following embodiments and comparative examples, a partitioned sagger comprising a cover plate and four partitions is taken as an example, and its overall structure is described as follows:

[0114] like Figure 1 and Figure 2As shown, the partitioned sagger includes a sagger body 100, which includes a bottom plate and a side plate 110 fixedly connected to the bottom plate. The bottom plate and the side plate 110 form a accommodating chamber 120 for holding materials. The sintering sagger also includes a partition 130 and a cover plate 200. The partition 130 divides the accommodating chamber 120 into at least two independent accommodating areas. The top surface of the partition 130 is lower than the top surface of the side plate 110. The cover plate 200 is covered on the partition 130 and is located in the accommodating chamber 120; a first limiting structure is provided on the outer peripheral wall of the cover plate 200, and a second limiting structure matching the first limiting structure is provided on the inner wall surface of the side plate 110; and 8 air holes 220 are provided on the cover plate 200.

[0115] The four partitions 130 are arranged perpendicularly within the accommodating chamber 120, dividing the accommodating chamber 120 of the sagger body 100 into nine compartments. The first limiting structure is a protrusion 210, and the second limiting structure is a groove 111. The depth of the groove 111 from the inner wall of the side plate 110 to the outer wall is less than the length of the protrusion 210, and the width of the groove 111 is greater than the width of the protrusion 210. The distance that the groove 111 extends downward from the top surface of the side plate 110 is no greater than the distance between the top surface of the side plate 110 and the top surface of the partitions 130. A gap exists between the cover plate 200 and the inner sidewall of the accommodating chamber 120.

[0116] In the sagger, the sagger body 100 is a square structure; the material of the sagger body, partition and cover plate are all graphite; the thickness of the side plate 110 and the bottom plate is 15 mm, the thickness of the partition 130 is 5 mm, and the thickness of the cover plate 200 is 10 mm; the length, width and height of the accommodating cavity 120 are 300 mm*300 mm*165 mm, the height of the partition 130 is 115 mm, and the side length of the cover plate 200 is 295 mm*295 mm.

[0117] Example 1

[0118] The lithium iron phosphate material of this embodiment is prepared according to the following steps:

[0119] (1) According to the molar ratio of iron to phosphorus of 0.96:1, 6.895 kg (60 mol) of 85% industrial-grade phosphoric acid and 1 kg of formic acid were added to 5 L of deionized water and stirred and diluted. Under stirring, 3.25 kg of 200-mesh, 99% purity secondary reduced iron powder (58 mol) was slowly added to obtain mixture A. The mixture was then reacted at 45°C. During the reaction, some gas was generated and the color of the reactant gradually changed from gray-black to yellow-white. When no gas was generated during the reaction, the material was placed in a sand mill for sand grinding. The grinding beads in the sand mill were 0.3 mm zirconium oxide beads. During the sand grinding process, the viscosity gradually increased to 4500 cps and the color gradually changed to yellow-white, obtaining product A.

[0120] (2) According to the molar ratio of lithium to phosphorus being 1.02:1, 4 kg of tartaric acid was dissolved in 3.25 kg of deionized water to prepare a solution. 2.3 kg of battery-grade lithium carbonate (molar dosage of 62 mol, calculated as lithium element), 0.35 kg of polyethylene glycol and 0.05 kg of titanium dioxide (molar dosage of 0.6 mol, calculated as titanium element) were gradually added to the solution under stirring to obtain mixture B. The mixture was then reacted at 40°C, during which a large amount of gas was generated. The reaction was continued until no gas was generated to obtain product B.

[0121] (3) Product B was added to product A, and the mixture was stirred to obtain mixture C. The viscosity of the system was rapidly reduced to 500 cps-1000 cps, and the solid content was 45%.

[0122] (4) Grinding the obtained mixture C until the particle size D50 is about 1600 nm, spray drying (inlet temperature is 280°C, outlet temperature is 110°C) and sintering are performed to obtain a sintered material;

[0123] The sintering conditions are as follows: a partitioned sagger with a specification of 330*330*180mm (the sagger capacity is 8kg, and the other parameters are as recorded above) is used, and two partitions (vertically cross-placed, corresponding to four equal-volume containing areas) and a cover are provided inside the sagger. In a nitrogen atmosphere with a purity of 99.999%, the temperature is gradually increased from room temperature to 760°C at a heating rate of 5°C / min, and the temperature is kept at 760°C for 10 hours, and the temperature is then lowered to obtain a sintered product.

[0124] (5) The sintered material is crushed by a jet crusher to obtain lithium iron phosphate material with a particle size D10 of 0.35 μm or more, D50 of 1.0-2.0 μm, and D99 of 9.5 μm or less.

[0125] Example 2

[0126] Compared with Example 1, the only difference is that in step (4), four partitions (vertically cross-placed, corresponding to nine equal-volume containing areas) and a cover are provided inside the partition sagger.

[0127] Example 3

[0128] Compared with Example 1, the only difference is that in step (4), the sintering holding temperature is 780°C.

[0129] Example 4

[0130] Compared with Example 1, the only difference is that in step (4), the sintering holding temperature is 740°C.

[0131] Example 5

[0132] Compared with Example 1, the only difference is that in step (1), the amount of organic acid used in product A is 0.42 kg.

[0133] Example 6

[0134] Compared with Example 1, the only difference is that in step (1), the amount of organic acid used in product A is 1.39 kg.

[0135] Example 7

[0136] Compared with Example 1, the only difference is that in step (1), the organic acid in product A is malic acid.

[0137] Example 8

[0138] Compared with Example 1, the only difference is that in step (2), the amount of metal oxide in product B is 0.025 kg, and the sintering holding temperature is 730 ° C.

[0139] Example 9

[0140] Compared with Example 1, the only difference is that in step (2), the amount of metal oxide in product B is 0.125 kg, the sintering holding temperature is 780° C., and the holding time is 15 h.

[0141] Comparative Example 1

[0142] Compared with Example 1, the only difference is that in step (4), the sagger used has no partitions and cover plates inside, and the sagger loading capacity is 6 kg.

[0143] Comparative Example 2

[0144] Compared with Example 1, the only difference is that in step (4), the sagger used has no partition inside, but is provided with a cover, and the amount of the sagger is 6 kg.

[0145] Comparative Example 3

[0146] Compared with Example 1, the only difference is that in step (4), there is no partition plate and cover plate inside the sagger used.

[0147] Comparative Example 4

[0148] Compared with Example 1, the only difference is that in step (4), the sagger used has no partition plate inside, but is provided with a cover plate.

[0149] Effect Example 1 Physical and Chemical Parameters Test of Sintered Materials

[0150] For the sintered materials at different positions in Examples 1-9 and Comparative Examples 1-4 (the materials were stacked into a square in each receiving cavity during sintering), relevant detailed parameters were tested, specifically the upper layer material, the wall layer material, the middle material, and the mixed material; wherein:

[0151] The upper layer of material refers to the surface sintered material near the sagger cover in each holding area (about 30mm thick). During testing, the upper layer of material in different holding areas must be tested separately, and then the average value is calculated.

[0152] The wall material refers to the surface sintered material close to the inner wall of the sagger in each holding area (thickness about 30mm). During the test, the wall material of each holding area should be tested separately and the average value should be taken.

[0153] The middle material refers to the sintered material in the center of the entire sagger. For example, when there are 4 partitions corresponding to 9 equal-volume containing cavities, the material is the central sintered material of the middle containing cavity.

[0154] Mixed material refers to the sintered material obtained by mixing sintered materials at different positions.

[0155] 1. Micromorphology test

[0156] Electron microscope scanning was performed on the upper layer material, the middle material and the wall layer material in Examples 1 and 2 and Comparative Examples 1 and 2, respectively, to characterize the corresponding microscopic morphologies.

[0157] The results are as follows Figure 3-Figure 14 As shown in Table 1, Figure 3-Figure 5 These are the morphologies of the upper layer material, the middle layer material, and the wall layer material obtained in the sintering step in Comparative Example 1; Figure 6-Figure 8 These are the morphology images of the upper layer material, the middle layer material and the wall layer material obtained in the sintering step in Comparative Example 2; Figures 9-11 These are the morphology images of the upper layer material, the middle layer material and the wall layer material obtained in the sintering step in Example 1; Figure 12-14 These are the morphology pictures of the upper layer material, the middle layer material and the wall layer material obtained in the sintering step in Example 2 respectively.

[0158] 2. Test of particle size distribution of sintered materials

[0159] The particle size distribution of the mixed materials in Example 1 and Comparative Example 1 was tested respectively using a laser particle size analyzer (model: TopSizer), and the refractive index was 1.33%.

[0160] The results are as follows Figure 15 (Example 1) and Figure 16 (Comparative Example 1), specifically as follows:

[0161] Example 1: D10 = 0.447 μm, D25 = 0.677 μm, D50 = 1.471 μm, D75 = 2.617 μm, D90 = 4.406 μm;

[0162] Comparative Example 1: D1=0.306μm, D5=0.349μm, D25=0.495μm, D50=0.835μm, D99=4.295μm.

[0163] 3. Tests of resistivity, carbon content, and powder compaction density

[0164] The following performance tests were performed on the sintered materials obtained in Examples 1-9 and Comparative Examples 1-4, respectively:

[0165] (1) Resistivity test

[0166] The resistivity of the sintered materials in Examples 1-9 and Comparative Examples 1-4 was tested using a powder resistivity tester (model: ST2742B) using a four-probe test at a pressure of 8 MPa.

[0167] (2) Carbon content test

[0168] The sintered materials in Examples 1-9 and Comparative Examples 1-4 were tested respectively, and the test was carried out in accordance with the national standard: "GB / T223.86-2009 Determination of total carbon content in steel and alloys - Infrared absorption method after induction furnace combustion".

[0169] (3) Powder compaction density test

[0170] The sintered materials involved in Examples 1-9 and Comparative Examples 1-4 were tested respectively using an electronic pressure testing machine (model UTM7305Z09) with a pressure parameter of 30 kN.

[0171] The results are shown in Table 1.

[0172] Effect Example 2: Test of Pole Piece Compaction Density

[0173] For the lithium iron phosphate materials obtained in Examples 1-9 and Comparative Examples 1-4, pole pieces were prepared according to the following steps, and then the pole piece compaction density was tested:

[0174] The lithium iron phosphate material, conductive carbon black and polyvinylidene fluoride obtained in Examples 1-9 and Comparative Examples 1-4 were respectively dissolved in NMP solution in a mass ratio of 97:1.2:1.8, and stirred in a vacuum mixer for 3 hours, and the solid content of the slurry was controlled to be 60%, and the viscosity was 6000-8000 mPa.s to prepare a positive electrode slurry; the positive electrode slurry was evenly coated on aluminum foil, and then placed in a vacuum drying oven at 60°C for 4 hours. After drying, it was punched into square positive electrode sheets with a width of 5 cm and a length of 10 cm, and the surface density of the electrode sheet was controlled to be 40 mg / cm 2 The electrode is rolled on a roller press. The electrode density is then tested under the condition of an elongation of 0.7%, with the electrode not peeling off when folded forward and backward, light transmission and breakage as the standards.

[0175] The results are shown in Table 1.

[0176] Effect Example 3 Battery Performance Test

[0177] For the lithium iron phosphate materials obtained in Examples 1-9 and Comparative Examples 1-4, button batteries were prepared according to the following steps, and then the relevant electrical properties of the obtained batteries were tested:

[0178] (1) Positive electrode production

[0179] The lithium iron phosphate material, conductive carbon black, and polyvinylidene fluoride obtained in Examples 1-9 and Comparative Examples 1-4 were dissolved in an NMP solution at a mass ratio of 90:5:5, and stirred in a vacuum mixer for 3 hours. The solid content of the slurry was controlled to 50% to prepare a positive electrode slurry. The positive electrode slurry was evenly coated on aluminum foil and then dried in a vacuum drying oven at 120°C for 12 hours. After drying, the slurry was punched into 12 mm diameter discs to serve as positive electrode sheets.

[0180] (2) Production of button batteries

[0181] A lithium metal sheet was used as the negative electrode, a Celgard 2400 microporous membrane as the separator, and a 1.0 mol / L LiPF6 solution as the electrolyte. The solvent was a mixture of ethylene carbonate (EC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC) in a 1:1:1 volume ratio. The positive electrode sheet, negative electrode sheet, separator, and electrolyte were assembled into a CR2016 button cell in an argon-filled glove box.

[0182] (3) The electrical performance of the button battery prepared using the above method was tested.

[0183] During the test, the cut-off voltage of the charging process was 3.75V, and the cut-off voltage of the discharging process was 2.0V.

[0184] Charge at a constant current of 0.1C to a voltage of 3.75V, then switch to constant voltage charging to a current of 0.02C, and discharge at a constant current of 0.1C to a voltage of 2.0V. Take the 0.1C charge and discharge specific capacity as the test result.

[0185] Charge at a constant current of 1C to a voltage of 3.75V, then switch to constant voltage charging to a current of 0.02C, and discharge at a constant current of 1C to a voltage of 2.0V. Take the 1C discharge specific capacity as the test result.

[0186] The results are shown in Table 2.

[0187] Table 1

[0188]

[0189] In Table 1, the calculation methods of the corresponding parameters are as follows:

[0190] (1) Overall carbon content difference:

[0191] Overall carbon content difference (%) = (carbon content of upper material - carbon content of middle material) / carbon content of middle material × 100%;

[0192] (2) Compaction density improvement rate:

[0193] Compaction density improvement rate (%) = (pole compaction density - powder compaction density) / powder compaction density × 100%.

[0194] In Table 1, the carbon content of the mixed material obtained by sintering remains almost unchanged before and after crushing, and the powder compaction density and resistivity are slightly different; that is, the carbon content of the mixed material in the table is the carbon content of the final lithium iron phosphate material.

[0195] Table 2

[0196]

[0197] From Table 1 and Table 2, Figures 9-15 The data obtained in the embodiment of the present invention can simultaneously achieve the following excellent effects:

[0198] (1) During the sintering step, the carbon content of the sintered materials at different locations is relatively uniform, with the overall carbon content difference being as low as 8.87%-32.56%. The present invention primarily reflects the overall carbon content difference of the sintered materials at different locations by using the difference in carbon content of the upper material and the intermediate material in the longitudinal direction, which have a relatively large theoretical carbon content difference, and takes into account the influence of multiple factors such as the thermal field, gravity, and thermal buoyancy. Furthermore, in actual operation, the mixing of different sintered materials also involves a crushing step, which ultimately results in a low overall carbon content in the resulting mixed material.

[0199] (2) During the sintering step, the overall morphology of the sintered materials obtained at different positions is relatively small, and the particle size distribution is also relatively uniform ( Figures 9-14 The obtained mixture has a moderate proportion of large and small particles ( Figure 15 ), which can maximize the effect of electrode compaction density.

[0200] (3) During the sintering step, the obtained mixed materials can ensure a high powder compaction density, which can be as high as 2.523g / cm 3 After the lithium iron phosphate material is prepared, the carbon content can be as high as 2.58g / cm2 by adding less than 2% binder. 3 The electrode compaction density is above 1.45%; and based on the powder compaction density, the electrode compaction density is increased by at least 1.45%, and can be increased by up to 4.67%.

[0201] (4) After actual use in batteries, it can simultaneously ensure that: the 0.1C charge specific capacity is not less than 159.4mAh / g, the 0.1C discharge specific capacity is not less than 157.6mAh / g, and the 1C discharge specific capacity is not less than 141.4mAh / g.

[0202] In comparison:

[0203] like Figure 3-Figure 5 and Figure 16 As shown, in Comparative Example 1, when the amount of sagger is relatively small and the sagger is without inner cover and partition, the overall carbon content of the sintered material obtained is as high as 60.47%, and the proportion of small particles in the mixed material obtained is relatively high ( Figure 16 ), there is a better grading effect of small and large particles, and the powder compaction density is correspondingly higher. However, a comparison found that, precisely because the small particles account for a larger proportion and the large particles account for a smaller proportion in the material obtained in this comparative example, it is not conducive to the performance of its electrode compaction density; specifically, after further manufacturing the electrode, under the condition of the same amount of binder (both 1.8%), the electrode compaction density of this comparative example is lower than that of Example 1, and the electrode compaction density of this comparative example is only increased by 1.1% on the basis of its powder compaction density. In addition, due to the large specific surface area corresponding to small particles, more binder (about 2%) is required to maintain the electrode compaction density in Table 1; however, research has found that in actual application, when the amount of PVDF binder is controlled at about 2%, it cannot meet the use requirements; if the binder is increased, the electrical performance will be seriously attenuated.

[0204] like Figure 6-Figure 8As shown, in Comparative Example 2, when the amount of filling is relatively small and the sagger used is without a partition, the overall carbon content difference of the obtained sintered material is still as high as 39.84%, the proportion of small particles in the obtained mixed material is still relatively high, and there is still a certain grading effect of small particles and large particles; when the powder compaction density is higher than that of Example 1, the electrode compaction density is also lower, and the electrode compaction density of this comparative example is only increased by 2.3% based on its powder compaction density.

[0205] Compared with Example 1, in Comparative Example 3, when the same filling amount is used and the sagger used has no inner cover and partition, the overall carbon content difference of the obtained sintered material is as high as 139%, and when the powder compaction density is higher, the obtained electrode compaction density is significantly lower than that of Example 1, and the improvement is only 0.23%; compared with Example 1, in Comparative Example 4, when the same filling amount is used and the sagger used has no partition, the overall carbon content difference of the obtained sintered material is still as high as 113%, and when the powder compaction density is higher, the obtained electrode compaction density is still lower than that of Example 1, and the improvement is only 0.54%.

[0206] The above results show that the present invention effectively controls the carbon content distribution and uniformity of the microstructure of the obtained material through improvements in the precursor preparation process and the sintering process. Ultimately, without increasing the powder compaction density and using a lower amount of binder, a pole piece with a higher pole piece compaction density is obtained, which can better meet the preparation requirements of small-volume batteries and effectively improve the efficiency of the pole piece compaction density. Moreover, after the lithium iron phosphate material obtained in the embodiment of the present invention is used to make a battery, comparable electrochemical performance can also be guaranteed.

Claims

1. A method for preparing a lithium iron phosphate material, characterized in that: The preparation method of the lithium iron phosphate material comprises the following steps: S1. Sintering the lithium iron phosphate precursor in different zones to obtain sintered material; wherein, The preparation of the lithium iron phosphate precursor comprises: grinding and drying a mixture C containing product A and product B; the product A is prepared by reacting a mixture A containing an iron source, phosphoric acid, water, and a protonic acid catalyst; the amount of the protonic acid catalyst added is 1%-50%, where the percentage is the percentage of the mass of the protonic acid catalyst to the sum of the mass of water, the protonic acid catalyst, and the phosphoric acid; the product B is prepared by reacting a mixture B containing an organic acid, a lithium source, and a carbon source; The zoned sintering includes 4-9 zones; The partitioned sintering includes the steps of heating up first and then keeping the temperature; the keeping temperature is 680-820°C; The partitioned sintering is carried out using a partitioned sagger, which includes 4-9 containing areas divided by partitions, and the lithium iron phosphate precursor is placed in each containing area of ​​the partitioned sagger; the partitioned sagger also includes a cover plate, which is arranged above the partition and covers the containing area; S2, crushing the sintered material.

2. The method for preparing the lithium iron phosphate material according to claim 1, wherein: In step S1, the partitioned sintering satisfies one or more of the following conditions: (1) The partitioned sintering is equal-amount partitioned sintering or gradient partitioned sintering; (2) The partitioned sintering is carried out in an inert atmosphere.

3. The method for preparing the lithium iron phosphate material according to claim 2, wherein: In step S1, the partitioned sintering satisfies one or more of the following conditions: (1) During the gradient zone sintering, the mass of the lithium iron phosphate precursor in each zone gradually increases from the center to the periphery; (2) The heating rate of the heating is 1-5°C / min; (3) The insulation time is 10-20 hours; (4) In the partitioned sagger, the volumes of the various containing areas are equal, or the volumes of the various containing areas are distributed in a gradient manner.

4. The method for preparing the lithium iron phosphate material according to claim 3, wherein: In step S1, the partitioned sintering satisfies one or more of the following conditions: (1) The heating rate of the heating is 3°C / min; (2) The insulation temperature is 730°C, 740°C, 760°C or 780°C; (3) The insulation time is 15 hours (4) The volume gradient distribution of each accommodation area is such that the volume of each accommodation area gradually increases from the center to the periphery.

5. The method for preparing the lithium iron phosphate material according to claim 1, wherein: In step S1, the preparation method of the lithium iron phosphate precursor meets one or more of the following conditions: (1) The iron source is selected from one or more of iron powder, ferric oxide and ferrosoferric oxide; (2) The iron source is in powder form; (3) Calculated based on phosphorus and iron, the molar ratio of the iron source to the phosphoric acid is (0.94-1.05):1; (4) The lithium source is one or more of lithium hydroxide monohydrate, lithium carbonate, lithium dihydrogen phosphate, lithium nitrate, lithium phosphate and lithium acetate; (5) Calculated based on phosphorus and lithium, the molar ratio of the lithium source to the phosphoric acid is (0.98-1.05):1; (6) The carbon source is one or more of glucose, sucrose, polyethylene glycol, and polyethylene glycol derivatives; (7) The amount of the carbon source added is 1%-80%, where the percentage is the mass of the carbon source relative to the mass of the iron source; (8) The organic acid is a carboxylic acid compound and / or ascorbic acid; (9) The amount of the organic acid added is 30%-150%, where the percentage is the mass of the organic acid relative to the mass of the phosphoric acid.

6. The method for preparing the lithium iron phosphate material according to claim 5, wherein: In step S1, the preparation method of the lithium iron phosphate precursor meets one or more of the following conditions: (1) The mesh size of the iron source is 200-1000 mesh; (2) Calculated based on phosphorus and iron, the molar ratio of the iron source to the phosphoric acid is (0.96-1.0):1; (3) The lithium source is one or more of lithium hydroxide monohydrate, lithium carbonate and lithium acetate; (4) The carbon source is one or more of glucose, polyethylene glycol, and polyethylene glycol derivatives; (5) The amount of the carbon source added is 5%-50%, where the percentage is the percentage of the mass of the carbon source to the mass of the iron source; (6) The carboxylic acid compound is one or more of formic acid, acetic acid, propionic acid, sorbic acid, oxalic acid, salicylic acid, citric acid, tartaric acid and malic acid.

7. The method for preparing the lithium iron phosphate material according to claim 6, wherein: In step S1, the preparation method of the lithium iron phosphate precursor meets one or more of the following conditions: (1) The mesh size of the iron source is 200-500 mesh; (2) The amount of the carbon source added is 10%-40%, where the percentage is the percentage of the mass of the carbon source to the mass of the iron source; (3) The carboxylic acid compound is one or more of oxalic acid, citric acid, tartaric acid and malic acid.

8. The method for preparing the lithium iron phosphate material according to claim 1, wherein: In step S1, the preparation method of the lithium iron phosphate material meets one or more of the following conditions: (1) The preparation of the mixture A comprises: adding the iron source to the phosphoric acid under stirring; (2) The reaction temperature of the mixture A is 20-95°C; (3) After the reaction of the mixture A, a grinding step is further included; (4) The preparation of the mixture B comprises: adding the lithium source and the carbon source to the organic acid under stirring; (5) The mixture B further comprises water; (6) The mixture B further comprises a metal oxide; (7) The reaction temperature of the mixture B is 20-95°C.

9. The method for preparing the lithium iron phosphate material according to claim 8, wherein: In step S1, the preparation method of the lithium iron phosphate material meets one or more of the following conditions: (1) The protonic acid catalyst satisfies one or more of the following conditions a-f: a. The protonic acid catalyst includes one or more of carboxyl, hydroxyl, carbonyl, amino, nitro, cyano, sulfhydryl, sulfonic acid, ester and ether bonds; b. The protonic acid catalyst includes heteroatoms, wherein the heteroatoms are selected from one or more of O, N, S, P, F, Cl, Br and I; c. The protonic acid catalyst comprises one or more of a linear structure, a branched structure, a cyclic structure and a cage structure; d. The protonic acid catalyst includes saturated bonds and / or unsaturated bonds; e. The protonic acid catalyst is a polymer compound; f. The protonic acid catalyst includes a skeleton structure constituting a biomolecule; (2) The preparation of the mixture A comprises: first mixing water, phosphoric acid and a protonic acid catalyst to obtain a mixed solution, and then adding the iron source under stirring; (3) The reaction temperature of the mixture A is 30-90°C; (4) After the reaction of the mixture A, the grinding method is ball milling or sand milling; (5) In the mixture B, the metal oxide is one or more of titanium oxide, vanadium oxide and niobium oxide; (6) In the mixture B, the molar ratio of the metal oxide to the phosphoric acid in the mixture A is (0.005-0.025):1, calculated based on phosphorus element and metal element; (7) The mixture B is prepared by adding the lithium source, the carbon source and the metal oxide to the organic acid under stirring; Alternatively, the water and the organic acid are first mixed to obtain an organic acid solution, and then the lithium source and the carbon source are added under stirring; Alternatively, the water and the organic acid are first mixed to obtain an organic acid solution, and then the lithium source, the carbon source and the metal oxide are added under stirring; (8) The reaction temperature of the mixture B is 30-90°C.

10. The method for preparing the lithium iron phosphate material according to claim 9, wherein: In step S1, the protonic acid catalyst satisfies one or more of the following conditions: (1) The cyclic structure includes one or more of an aliphatic ring, an aromatic ring, and a heterocyclic ring; (2) The unsaturated bonds include double bonds and / or triple bonds; (3) The biomolecules include one or more of amino acids, fatty acids and organic acid metabolites.

11. The method for preparing the lithium iron phosphate material according to claim 10, wherein: In step S1, the protonic acid catalyst is a carboxylic acid compound and / or ascorbic acid; Wherein, the carboxylic acid compound is one or more of formic acid, acetic acid, propionic acid, sorbic acid, oxalic acid, salicylic acid, citric acid, tartaric acid and malic acid.

12. The method for preparing the lithium iron phosphate material according to claim 1, wherein: The preparation method of the lithium iron phosphate material meets one or more of the following conditions: (1) In step S1, the solid content of the mixture C is 40%-50%, where the percentage refers to the percentage of the mass of the solid in the mixture C to the total mass of the mixture C; (2) In step S1, the particle size D10 of the solid particles in the mixture C is less than 0.45 μm; the particle size D50 is 1.4-2.3 μm; and the particle size D90 is greater than 9.5 μm; (3) In step S1, the drying method is spray drying; (4) In step S1, the grinding method is ball milling or sand milling; (5) In step S2, the crushing method is air flow crushing.

13. The method for preparing the lithium iron phosphate material according to claim 12, wherein: The preparation method of the lithium iron phosphate material meets one or more of the following conditions: (1) In step S1, the air inlet temperature of the spray drying is 260-290°C; (2) In step S1, the outlet temperature of the spray drying is 100-130°C.

14. A lithium iron phosphate material, characterized in that: The lithium iron phosphate material is prepared by the preparation method of the lithium iron phosphate material according to any one of claims 1 to 13.

15. The lithium iron phosphate material according to claim 14, wherein: The lithium iron phosphate material meets one or more of the following conditions: (1) Particle size D10 is above 0.35 μm, D50 is 1.0-2.0 μm, and D99 is below 9.5 μm; (2) Carbon content is 1%-2%.

16. Use of the lithium iron phosphate material according to claim 14 or 15 in a lithium ion battery.

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