Iron phosphate as well as preparation method and application thereof

Abstract

The invention provides iron phosphate and a preparation method and application thereof.The preparation method of the iron phosphate comprises the steps that an iron source, a phosphorus source, an oxidizing agent and a pH regulator are mixed and then subjected to an oxidation precipitation reaction and first purification, and a reaction filter cake is obtained; pulping the reaction filter cake, adding an acid solution and a solution containing a metal ion doping agent, aging, purifying for the second time, and drying to obtain an aged filter cake containing iron phosphate dihydrate; and sintering the aged filter cake containing the iron phosphate dihydrate to obtain the iron phosphate. According to the preparation method of the iron phosphate, in the aging crystallization process of the iron phosphate precursor, the solution containing the metal ion doping agent is introduced, metal ions in the metal ion doping agent compete for phosphate radicals with ferric ions through a competitive precipitation mechanism to form a complex, the pH of the aging end is controlled, the complex is left in the system, and therefore the iron phosphate is obtained. Therefore, the content of phosphorus in a final product is effectively reduced, and the iron-phosphorus ratio of iron phosphate is remarkably increased.

Description

Technical Field

[0001] This application relates to the field of battery materials technology, and in particular to an iron phosphate, its preparation method and application. Background Technology

[0002] Iron phosphate (FePO4) is generally used as a precursor in the synthesis of lithium iron phosphate (LiFePO4), the positive electrode active material for lithium-ion batteries. Its morphology, particle size, specific surface area, and iron-to-phosphorus ratio (the molar ratio of iron to phosphorus) directly determine the electrochemical characteristics of the lithium iron phosphate positive electrode active material. For example, an excessively high specific surface area leads to increased side reactions; an excessively low specific surface area results in incomplete lithiation, directly affecting the battery's initial coulombic efficiency and safety stability. An imbalanced iron-to-phosphorus ratio leads to problems such as gas generation and voltage plateau fluctuations. Particle size affects the uniformity of the physicochemical reactions of lithium iron phosphate, thus impacting rate performance.

[0003] Therefore, obtaining high-quality iron phosphate is crucial for obtaining high-performance lithium iron phosphate and thus improving battery performance.

[0004] In related technologies, some methods for preparing iron phosphate involve reheating the aged filter cake for secondary aging to obtain high iron-to-phosphorus ratio iron phosphate. However, this method has a relatively complex process route, a long aging time, and the effect of improving the iron-to-phosphorus ratio is not obvious. Other methods for preparing iron phosphate add ammonium metavanadate during the aging stage. By controlling the amount of ammonium metavanadate added, high iron-to-phosphorus ratio iron phosphate is finally obtained. However, this method produces iron phosphate particles with small primary particles, which is not conducive to the preparation of high-pressure lithium iron phosphate. Summary of the Invention

[0005] This application aims to at least partially address one of the technical problems in the related art.

[0006] Therefore, one objective of this application is to provide an iron phosphate material with a high iron-to-phosphorus ratio and a wide range of primary particle sizes, which is more suitable for preparing high-performance lithium iron phosphate cathode materials.

[0007] Another objective of this application is to provide a method for preparing ferric phosphate.

[0008] Another object of this application is to provide a lithium iron phosphate.

[0009] Another objective of this application is to provide a positive electrode sheet.

[0010] Another object of this application is to provide a secondary battery.

[0011] To achieve the above objectives, a first aspect of this application provides an iron phosphate, wherein the iron phosphate is a secondary aggregate formed by primary particle agglomeration, and the iron phosphate includes at least one of the following characteristics: (1) The molar ratio of iron to phosphorus is 0.970-1.1; (2) The specific surface area of ​​BET is 5-12 m² 2 / g; (3) The particle size range of the primary particles is 200-500 nm.

[0012] The iron phosphate described in this application can bring at least the following beneficial effects: It features a high iron-to-phosphorus ratio and a wide range of primary particle sizes, making it more suitable for preparing high-performance lithium iron phosphate cathode materials.

[0013] The second aspect of this application discloses a method for preparing ferric phosphate, comprising: Iron source, phosphorus source, oxidant and pH adjuster are mixed and subjected to oxidation precipitation reaction and first purification to obtain reaction filter cake; The reaction filter cake was pulped and then added to an acid solution and a solution containing metal ion dopant for aging, second purification, and drying to obtain an aged filter cake containing ferric phosphate dihydrate. The aged filter cake containing ferric phosphate dihydrate is sintered to obtain the ferric phosphate.

[0014] The method for preparing ferric phosphate described in this application can bring at least the following beneficial effects: During the aging and crystallization process of the iron phosphate precursor, a solution containing metal ion dopants is introduced. The metal ions in these dopants compete with ferric ions for phosphate groups through a "competitive precipitation" mechanism to form complexes. By controlling the pH at the aging stage, these dopants remain in the system and are removed during subsequent purification, effectively reducing the phosphorus content in the final product and significantly increasing the iron-to-phosphorus ratio of iron phosphate. Simultaneously, due to the increased iron-to-phosphorus ratio, the iron phosphate crystal structure is more complete, making it easier to obtain iron phosphate with larger primary particles. This makes the prepared iron phosphate more suitable for preparing high-performance lithium iron phosphate cathode materials.

[0015] In some embodiments, the metal ions in the solution containing the metal ion dopant include Fe. 2+ Fe 3 + Al 3+ Cr 2+ Cr 3+ Mn 2+ Mn 3+ At least one of them.

[0016] Optionally, in the solution containing the metal ion dopant, the metal ion includes Fe. 2+ Fe 3+ Al 3+ Cr 3+ At least one of them.

[0017] Optionally, the metal ion dopant includes at least one of ferrous sulfate, ferric chloride, aluminum sulfate, chromium chloride, and manganese chloride.

[0018] In some embodiments, the molar amount of the metal ion in the solution containing the metal ion dopant is 0.1-10% of the molar amount of iron in the iron source, and optionally 0.1-0.5%.

[0019] In some embodiments, the reaction filter cake is pulped and then aged in an acid solution or a solution containing metal ion dopant, including: The reaction filter cake is pulped and then an acid solution is added to obtain the first slurry; The first slurry is heated to a preset temperature while the solution containing the metal ion dopant is added to obtain a second slurry; the preset temperature is the aging temperature. The second slurry is aged.

[0020] In some embodiments, the aging temperature is 70-98°C.

[0021] In some embodiments, the method for preparing ferric phosphate further includes: During the aging process, after the slurry Dv50 of the pulp, which has been mixed with acid solution and solution containing metal ion dopant, decreases from 10-20µm to 1-5µm, the first heat preservation is carried out.

[0022] In some embodiments, the temperature of the first heat preservation is 80-98°C, and the time of the first heat preservation is 2-4 hours.

[0023] In some embodiments, the acid solution includes at least one of phosphoric acid solution, sulfuric acid solution, and hydrochloric acid solution.

[0024] In some embodiments, the molar amount of additives in the acid solution is 10-15% of the molar amount of iron in the iron source.

[0025] In some embodiments, after adding the acid solution, the pH of the slurry is 1.5-2.5, optionally 1.8-2.2.

[0026] In some embodiments, the second purification includes filtration and washing, and the second purification is stopped when the washing agent used in the second purification has a conductivity of 200-400 μS / cm after washing.

[0027] In some embodiments, pulping the reaction filter cake includes pulping the reaction filter cake with water.

[0028] In some embodiments, during the pulping process, the total solids content of the reaction filter cake and water is 10-18%.

[0029] In some embodiments, the pulping temperature is room temperature, and the pulping time is 0.5-1 hour.

[0030] In some embodiments, the molar ratio of iron in the iron source to phosphorus in the phosphorus source is (1-1.2):1.

[0031] In some embodiments, the molar ratio of iron in the iron source to the oxidant is 1:(1-1.3), and can be optionally 1:1.15.

[0032] In some embodiments, the molar ratio of iron in the iron source to the pH adjuster is 1:(0.8-1.2).

[0033] In some embodiments, the iron source includes ferrous sulfate heptahydrate, the phosphorus source includes monoammonium phosphate, the oxidant includes hydrogen peroxide, and the pH adjuster includes ammonia.

[0034] In some embodiments, mixing the iron source, phosphorus source, oxidant, and pH adjuster includes: The phosphorus source, oxidant, and pH adjuster are mixed to obtain a mixture; The mixture is slowly added to the iron source while being stirred.

[0035] In some embodiments, the mixture is slowly added to the iron source using a peristaltic pump at a pumping speed of 1-10 rpm.

[0036] In some embodiments, the temperature of the oxidation precipitation reaction is 40-80°C.

[0037] In some embodiments, the oxidation precipitation reaction takes 0.5-1 hour.

[0038] In some embodiments, the pH of the oxidation precipitation reaction is 1.5-3.0.

[0039] In some embodiments, the first purification includes filtration and washing, and the first purification is stopped when the conductivity of the detergent used for washing in the first purification is 600-1000 μS / cm after washing.

[0040] In some embodiments, the method for preparing ferric phosphate further includes a second heat preservation step after the oxidation precipitation reaction and before the first purification.

[0041] In some embodiments, the second heat preservation time is 10-20 minutes.

[0042] In some embodiments, the ferric phosphate dihydrate is a spherical structure with lamellar intercalation.

[0043] In some embodiments, the molar ratio of iron to phosphorus in the iron phosphate dihydrate is 0.965-1.2.

[0044] In some embodiments, the sintering is carried out in an oxygen or oxygen-containing gas atmosphere.

[0045] In some embodiments, the sintering heating rate is 1-10°C / min, optionally 3-5°C / min.

[0046] In some embodiments, the sintering temperature is 550-700°C, and optionally 550-600°C.

[0047] In some embodiments, the sintering time is 1-5 hours.

[0048] The third aspect of this application discloses a lithium iron phosphate, which is prepared using iron phosphate as a raw material as described in the first aspect of this application, or using iron phosphate prepared by the method for preparing iron phosphate as described in the second aspect of this application.

[0049] The fourth aspect of this application discloses a positive electrode sheet comprising a positive electrode material, said positive electrode material comprising lithium iron phosphate as described in the third aspect of this application.

[0050] The fifth aspect of this application discloses a secondary battery, comprising a positive electrode, a separator, and a negative electrode, wherein the positive electrode is the same as the positive electrode described in the fourth aspect of this application.

[0051] The lithium iron phosphate, positive electrode sheet, and secondary battery described in this application all have at least the beneficial effects of the iron phosphate and the preparation method of iron phosphate described in this application.

[0052] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0053] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the following description of the embodiments taken in conjunction with the accompanying drawings. in: Figure 1 This is a flowchart illustrating a method for preparing ferric phosphate, which is an exemplary embodiment of this application.

[0054] Figure 2 The image shows the primary particle size and morphology of the iron phosphate prepared in Example 1.

[0055] Figure 3The image shows the primary particle size and morphology of the iron phosphate prepared in Example 2.

[0056] Figure 4 The image shows the primary particle size and morphology of the iron phosphate prepared in Example 3.

[0057] Figure 5 The primary particle size and morphology of the iron phosphate prepared for Comparative Example 1 are shown. Detailed Implementation

[0058] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of the embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0059] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of a particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is expected that ranges of 60-110 and 80-120 are also included. Furthermore, if minimum range values ​​of 1 and 2 are listed, and if maximum range values ​​of 3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0060] Unless otherwise specified, all raw materials and equipment involved in this application are self-made through commercial means or known methods; and all methods involved are conventional methods unless otherwise specified.

[0061] In this application, the iron-phosphorus ratio refers to the molar ratio of iron to phosphorus in ferric phosphate, abbreviated as Fe / P molar ratio.

[0062] In this application, BET specific surface area refers to the total area per unit mass of iron phosphate material.

[0063] In this application, room temperature refers to 20-30℃.

[0064] Ferric phosphate The ferric phosphate in this embodiment is a secondary agglomerate formed from primary particle agglomeration. The ferric phosphate includes at least one of the following characteristics: (1) The molar ratio of iron to phosphorus (i.e., the iron-to-phosphorus ratio, Fe / P) is 0.970-1.1; (2) The specific surface area of ​​BET is 5-12 m² 2 / g; (3) The particle size range of the primary particles is 200-500 nm.

[0065] In the embodiments of this application, controlling the iron-to-phosphorus ratio of lithium iron phosphate to 0.970-1.1 can improve the capacity utilization of lithium iron phosphate; controlling the BET specific surface area to 5-12 m² 2 By controlling the particle size of primary particles to a range of 200-500 nm, the compaction density of lithium iron phosphate can be improved.

[0066] For example, the iron-to-phosphorus ratio of the iron phosphate may include, but is not limited to, 0.975, 0.980, 0.985, 0.990, 0.995, 1.0, 1.05, or 1.1.

[0067] For example, the BET specific surface area of ​​the iron phosphate includes, but is not limited to, 6 m². 2 / g、7m 2 / g、8m 2 / g、9m 2 / g, 10m 2 / g or 11m 2 / g etc.

[0068] For example, the particle size range of the primary particles includes, but is not limited to, 250-500nm, 300-500nm, 350-500nm, 400-500nm, 450-500nm, 200-300nm, 200-400nm, or 300-400nm.

[0069] The iron phosphate in the embodiments of this application can bring at least the following beneficial effects: It features a high iron-to-phosphorus ratio and a wide range of primary particle sizes, making it more suitable for preparing high-performance lithium iron phosphate cathode materials.

[0070] <Preparation method of ferric phosphate> The method for preparing ferric phosphate according to the embodiments of this application can be used to prepare the ferric phosphate of the embodiments of this application, or in other words, the product obtained is the ferric phosphate of the embodiments of this application. The method includes three steps: reaction, aging, and sintering.

[0071] A method for preparing ferric phosphate according to an embodiment of this application is described below with reference to the accompanying drawings.

[0072] Figure 1 This is a flowchart illustrating a method for preparing ferric phosphate, which is an exemplary embodiment of this application.

[0073] like Figure 1 As shown, the preparation method includes the following steps: S101. After mixing the iron source, phosphorus source, oxidant and pH adjuster, an oxidation precipitation reaction and first purification are carried out to obtain a reaction filter cake.

[0074] In the embodiments of this application, there are no restrictions on the specific substances of phosphorus source, iron source, oxidant and pH adjuster. They can be any phosphorus source, iron source, oxidant and pH adjuster known in the art that can be used to prepare iron phosphate.

[0075] For example, the iron source includes, but is not limited to, at least one of ferrous sulfate heptahydrate, ferrous chloride, and ferrous nitrate, and may be selected as ferrous sulfate heptahydrate.

[0076] For example, the phosphorus source includes, but is not limited to, at least one of monoammonium phosphate (NH4H2PO4), diammonium hydrogen phosphate, and ammonium phosphate, and may be selected as monoammonium phosphate.

[0077] For example, the oxidant includes, but is not limited to, at least one of hydrogen peroxide, potassium permanganate, and sodium dichromate, and may be selected as hydrogen peroxide, and may be further selected as hydrogen peroxide with a molar concentration of 8-30%, such as hydrogen peroxide with a molar concentration of 10%, 15%, 20%, or 25%.

[0078] For example, the pH adjuster includes, but is not limited to, at least one of ammonia, sodium bicarbonate, and sodium hydroxide, and may be selected as ammonia, and may be further selected as ammonia with a molar concentration of 8-30%, such as ammonia with a molar concentration of 15%, ammonia with a molar concentration of 20%, or ammonia with a molar concentration of 25%.

[0079] It should be noted that in some cases, the iron source and phosphorus source are prepared into aqueous solutions of their respective selected substances to participate in the oxidation precipitation reaction.

[0080] In some embodiments, mixing the iron source, phosphorus source, oxidant, and pH adjuster includes the following steps: a) Mix the phosphorus source, oxidant, and pH adjuster to obtain a mixed solution; b) Under stirring conditions, the mixture is slowly added to the iron source.

[0081] Compared to directly mixing the iron source, phosphorus source, oxidant, and pH adjuster, the above method allows for a more uniform and complete reaction by using the iron source as a base and adding the phosphorus source, pH adjuster mixture, and oxidant separately.

[0082] In some embodiments, the mixture is slowly added to the iron source using a peristaltic pump at a pumping speed of 1-10 rpm (e.g., 3 rpm, 5 rpm, or 7 rpm).

[0083] In some embodiments, the molar ratio of iron in the iron source to phosphorus in the phosphorus source is (1-1.2):1, including but not limited to 1.02:1, 1.04:1, 1.06:1, 1.08:1, 1.1:1, 1.12:1, 1.14:1, 1.16:1 or 1.18:1, etc.

[0084] In some embodiments, the molar ratio of iron in the iron source to the oxidant is 1:(1-1.3), including but not limited to 1:1.12, 1:1.15, 1:1.18, 1:1.2, 1:1.22, 1:1.25 or 1:1.28, etc.

[0085] As an optional example, the molar ratio of iron to oxidant in the iron source is 1:1.15.

[0086] In some embodiments, the molar ratio of iron in the iron source to the pH adjuster is 1:(0.8-1.2), including but not limited to 1:0.85, 1:0.9, 1:0.95, 1:1, 1:1.05, 1:1.1 or 1:1.15, etc.

[0087] In some embodiments, the temperature of the oxidation precipitation reaction is 40-80°C, including but not limited to 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, or 75°C. In some embodiments, the reaction pressure of the oxidation precipitation reaction is atmospheric pressure.

[0088] In some embodiments, the oxidation precipitation reaction takes 0.5-1 hour, including but not limited to 0.5 hours, 0.6 hours, 0.7 hours, 0.8 hours, or 1 hour.

[0089] In some embodiments, the pH of the oxidation precipitation reaction is 1.5-3.0, including but not limited to 1.7, 2, 2.2, 2.5 or 2.8.

[0090] In some embodiments, the method for preparing ferric phosphate further includes a second heat preservation step after the oxidation precipitation reaction and before the first purification. In the embodiments of this application, the purpose of the second heat preservation is to promote the release of phosphorus atoms into the system.

[0091] In some embodiments, the second heat preservation time is 10-20 minutes, including but not limited to 12 minutes, 15 minutes or 18 minutes, and can be selected as 15 minutes.

[0092] In some embodiments, the temperature of the second insulation is 70-100°C, including but not limited to 72°C, 82°C, 92°C, 94°C or 96°C.

[0093] In some embodiments, the first purification includes filtration and washing. Exemplary examples include using pure water, distilled water, etc., as a washing agent in the first purification.

[0094] In some embodiments, the first purification is stopped when the conductivity of the detergent used in the first purification process reaches 600-1000 μS / cm (e.g., 700 μS / cm, 800 μS / cm, or 900 μS / cm, etc.) after washing. Conductivity is a physical indicator that measures the ability of a detergent such as pure water to conduct electricity. The more dissolved ions in the detergent such as pure water, the higher the conductivity value. Controlling the conductivity can detect whether the filter cake has been washed clean.

[0095] As an optional example, step S101 includes: Iron source solution, phosphorus source solution, hydrogen peroxide and ammonia solution are mixed in a certain molar ratio, the pH of the reaction system is controlled at 1.5-3.0, and an oxidation precipitation reaction is carried out at a certain temperature. After the reaction is completed, the mixture is kept at the temperature for 15 minutes to obtain a yellow reaction slurry, which is then filtered and washed.

[0096] S102. The reaction filter cake obtained in step S101 is pulped and then added to an acid solution and a solution containing metal ion dopant for aging, second purification, and drying to obtain an aged filter cake containing ferric phosphate dihydrate.

[0097] In the embodiments of this application, the purpose of adding acid solution is to dissolve the iron phosphide in the reaction filter cake to facilitate its subsequent recrystallization, and to adjust the pH of the slurry after beating. Therefore, the specific choice of acid solution is not limited, and it can be any acid solution known in the art that can achieve the above objectives.

[0098] For example, the acid solution includes, but is not limited to, at least one of phosphoric acid solution, sulfuric acid solution, hydrochloric acid solution, etc., and may be selected as phosphoric acid solution, and may be further selected as phosphoric acid solution with a mass fraction of 80-90%, such as phosphoric acid solution with a mass fraction of 85%.

[0099] In some embodiments, the molar amount of additives in the acid solution is 10-15% of the molar amount of iron in the iron source, including but not limited to 11%, 11.5%, 12%, 12.5%, 13%, 13.5%, 14% or 14.5%.

[0100] In some embodiments, after adding the acid solution, the pH of the slurry is 1.5-2.5, including but not limited to 1.7, 1.9, 2.1 or 2.3, and optionally 1.8-2.2.

[0101] In the embodiments of this application, a solution containing a metal ion dopant is introduced, wherein the metal ions in the metal ion dopant compete with ferric ions for phosphate ions to form a complex through a "competitive precipitation" mechanism. By controlling the pH at the aging stage (the pH of the slurry after pulping is 1.5-2.5 by adding an acid solution as described above), the metal ions remain in the system and are removed in the subsequent second purification (e.g., the washing process mentioned later), thereby effectively reducing the phosphorus content in the final product and significantly improving the iron-phosphorus ratio of ferric phosphate.

[0102] In some embodiments, the metal ion in the solution containing the metal ion dopant can be at least one of the metal ions of different valence states corresponding to iron (Fe), aluminum (Al), chromium (Cr), and manganese (Mn). Among these metal ions, iron is preferred over other metal ions, as some iron can enter the iron phosphate lattice, further increasing the iron-to-phosphorus ratio of iron phosphate.

[0103] Furthermore, in the solution containing metal ion dopant, the metal ion can be at least one of the following metal ions in different valence states: iron (Fe), aluminum (Al), chromium (Cr), and manganese (Mn). The purpose of this selection is to form competitive precipitation.

[0104] Therefore, as an optional example, in the solution containing the metal ion dopant, the metal ion includes Fe. 2+ Fe 3+ Al 3+ Cr 2+ Cr 3+ Mn 2+ Mn 3++ At least one of them, optionally Fe 2+ Fe 3+ Al 3+ Cr 2+ Cr 3+ At least one of them, further optionally Fe 2+ Fe 3+ Al 3+ Cr 3+ At least one of them.

[0105] For example, the metal ion dopant includes, but is not limited to, at least one of ferrous sulfate, ferric chloride, aluminum sulfate, chromium chloride, and manganese chloride.

[0106] In some embodiments, the molar amount of the metal ion in the solution containing the metal ion dopant is 0.1-10% of the molar amount of iron in the iron source, including but not limited to 0.1%, 0.2%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, 6.5%, 7%, 7.5%, 8%, 8.5%, 9%, or 9.5%. If too few metal ions are added, the iron-to-phosphorus ratio of iron phosphate will not be significantly improved; if too many are added, there will be more residual impurity ions in the iron phosphate.

[0107] As an optional example, in the solution containing the metal ion dopant, the molar amount of the metal ion is 0.1-0.5% of the molar amount of iron in the iron source.

[0108] In some embodiments, pulping the reaction filter cake includes pulping the reaction filter cake with water.

[0109] In some embodiments, during the pulping process, the total solids content of the reaction filter cake and water is 10-18%, including but not limited to 11%, 12%, 13%, 14%, 15%, 16% or 17%.

[0110] It should be noted that the reaction filter cake also contains water, which is water carried over after purification. Therefore, the total solids content of the reaction filter cake and water refers to the proportion of the mass of solids in the reaction filter cake to the total mass of the reaction filter cake and water.

[0111] In some embodiments, the pulping temperature is room temperature.

[0112] In some embodiments, the pulping time is 0.5-1 hour, including but not limited to 0.5 hours, 0.6 hours, 0.7 hours, 0.8 hours, or 1 hour.

[0113] In some embodiments, the aging temperature is 70-98°C, including but not limited to 72°C, 75°C, 78°C, 80°C, 83°C, 85°C, 88°C, 90°C, 92°C, or 95°C.

[0114] In some embodiments, the method for preparing ferric phosphate further includes: During the aging process, after the slurry Dv50 of the pulp, which has been mixed with acid solution and solution containing metal ion dopant, decreases from 10-20µm to 1-5µm, the first heat preservation is carried out.

[0115] In the embodiments of this application, the slurry Dv50 is reduced from 10-20µm to 1-5µm while the slurry changes from yellow to pinkish-white.

[0116] In some embodiments, the temperature of the first insulation is 80-98°C, including but not limited to 83°C, 85°C, 88°C, 90°C, 92°C or 95°C.

[0117] In some embodiments, the first heat preservation time is 2-4 hours, including but not limited to 2.5 hours, 3 hours or 3.5 hours.

[0118] In some embodiments, the reaction filter cake is pulped and then aged in an acid solution or a solution containing metal ion dopant, including the following steps: (1) The reaction filter cake is pulped and then an acid solution is added to obtain the first slurry; (2) The first slurry is heated to a preset temperature while the solution containing the metal ion dopant is added to obtain the second slurry; the preset temperature is the aging temperature; (3) The second slurry is aged.

[0119] In steps (1) to (3) above, an acid solution is added first, and then a solution containing metal ion dopant is added while the temperature is raised to the aging temperature. Compared with directly mixing the pulping product, acid solution and solution containing metal ion dopant, the pulping product and acid solution mixture can be used as a base, and the metal ion dopant can be added slowly.

[0120] It is understandable that, in some cases, the first heat preservation is carried out after the color of the second slurry changes from yellow to pinkish-white during the aging process of the second slurry in step (3).

[0121] In some embodiments, the second purification includes filtration and washing. Filtration can be performed using methods such as vacuum filtration or centrifugation, with vacuum filtration being a preferred option. Washing involves rinsing the ferric phosphate dihydrate slurry with pure water.

[0122] For example, in the second purification process, the impurities removed by washing include, but are not limited to, metal ions (Fe) in the solution containing metal ion dopants. 2+ Fe 3+ Except for), Al 3+ Cr 6+ At least one of the following.

[0123] For example, in the second purification process, the washing agent used includes at least one of pure water, distilled water, deionized water, ultrapure water, etc., and pure water can be selected.

[0124] In some embodiments, the second purification is stopped when the conductivity of the detergent used in the second purification process reaches 200-400 μS / cm (e.g., 250 μS / cm, 300 μS / cm, or 350 μS / cm, etc.). Conductivity is a physical indicator that measures the conductivity of the detergent used in the second purification process after washing. The more dissolved ions in the detergent after the second purification, the higher the conductivity value. Controlling the conductivity can detect whether the filter cake has been cleaned, that is, monitor whether the metal ions in the solution containing the metal ion dopant introduced in this application have been effectively removed.

[0125] In the embodiments of this application, after a second purification, especially washing in the second purification, the residual amount of impurities in the final ferric phosphate product can be reduced to 100-200 ppm.

[0126] It should be noted that the method of drying after the second purification is not limited in the embodiments of this application, and can be any drying method known in the art, such as baking.

[0127] In some embodiments, the drying temperature is 90-120°C, including but not limited to 95°C, 100°C, 105°C, 110°C, or 115°C.

[0128] In some embodiments, the drying time is 5-10 hours, including but not limited to 7 hours, 8 hours, 9 hours or 10 hours.

[0129] In some embodiments, the ferric phosphate dihydrate has a plate-like intercalated, near-spherical structure. This structure has the advantage of larger particle size compared to the ferric phosphate dihydrate formed during conventional ferric phosphate preparation processes.

[0130] In the embodiments of this application, a quasi-spherical structure refers to the degree to which the shape of a particle resembles a sphere, and is usually used to describe the morphological characteristics of a particle in three-dimensional space. The degree of quasi-sphericity reflects how close the geometry of the particle is to a sphere; particles with equal triaxiality have the highest quasi-sphericity, while plate-like or columnar particles have the lowest.

[0131] In some embodiments, the molar ratio of iron to phosphorus in the iron phosphate dihydrate is 0.965-1.2, including but not limited to 0.970, 0.975, 0.980, 0.985, 0.990, 0.995, 1, 1.05, 1.1 or 1.15.

[0132] As an optional example, step S102 includes: A certain amount of reaction filter cake and water are added according to the solid content and pulped at room temperature. After thorough mixing, a phosphoric acid solution with a certain molar ratio is added to the reaction system, and the reaction temperature is raised to 70-98℃. During the heating process, a dopant solution containing metal ion M is added. After the addition is complete, the mixture is aged until the slurry color changes from yellow to pinkish-white, and then kept at this temperature for 2-4 hours. After the aging process, the mixture is filtered and washed, and then dried at 90-120℃. M is a metal cation with a different valence state, specifically Fe. 2 + Al 3+ Cr 3+ One or more of them.

[0133] S103. The aged filter cake containing ferric phosphate dihydrate obtained in step S102 is sintered to obtain the ferric phosphate.

[0134] In some embodiments, the sintering is carried out in an oxygen or oxygen-containing gas atmosphere.

[0135] For example, the oxygen-containing gas includes at least one of air, a mixture of oxygen and an inert gas. Optionally, the volume percentage of oxygen in the mixture of oxygen and an inert gas is 20-30%, including but not limited to at least one of 20%, 20.9%, 22%, 25%, or 30%.

[0136] As an alternative example, the sintering is carried out in an air atmosphere.

[0137] In some embodiments, the heating rate of the sintering is 1-10℃ / min, including but not limited to 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min or 9℃ / min, and can be selected as 3-5℃ / min.

[0138] In some embodiments, the sintering temperature is 550-700℃, including but not limited to 560℃, 570℃, 580℃, 590℃, 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, 660℃, 670℃, 680℃ or 690℃, and can be selected as 550-600℃.

[0139] In some embodiments, the sintering time is 1-5 hours, including but not limited to 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours or 4.5 hours.

[0140] In some embodiments, the method for preparing ferric phosphate further includes a step of pulverizing the sintered product after sintering.

[0141] As an optional example, the above-mentioned method for preparing ferric phosphate includes the following steps: (1) Prepare an iron salt solution of a certain concentration (e.g., 220 g / L).

[0142] (2) Prepare a phosphate solution of a certain concentration (e.g., 150 g / L, etc.), add hydrogen peroxide and ammonia water in the required proportion to obtain a mixed solution.

[0143] (3) Preparation of solution containing metal ion dopant: Dissolve a certain amount of metal ion dopant in pure water to obtain a solution containing metal ion dopant with a concentration of 0.02-1 mol / L.

[0144] (4) Take the above iron salt solution and phosphate salt solution according to the required iron and phosphorus element feeding ratio. Use any one of the iron salt solution and the mixed solution as the base liquid, and slowly add the other solution at a fixed rate. Control the reaction temperature at 40-60℃. After the reaction is completed, slowly raise the temperature to 40-80℃, keep it at 15min, and stir at 250rpm to obtain a yellow reaction slurry.

[0145] (5) Filter the yellow reaction slurry and wash it multiple times to obtain a yellow reaction filter cake. Stop washing when the conductivity of the detergent used for washing is 600-1000 μS / cm (e.g., 800 μS / cm).

[0146] (6) Add a certain amount of water to the reaction filter cake and slurry it. The total solid content of the reaction filter cake and water is controlled at 10-18%. Add 10-15% of the required iron molar amount in the iron salt solution with phosphoric acid. Raise the system temperature to 80-98℃. During the heating process, add a solution containing metal ion dopant. The amount of solution containing metal ion dopant added should be based on the molar amount of metal ions being 0.2%-10% of the required iron molar amount in the iron salt solution. After the addition is completed, age the slurry. The slurry changes from yellow to pinkish white (during this process, Dv50 decreases from 10-20µm to 1-5µm). Then keep it warm for 2 hours.

[0147] (7) Filter the powdery white slurry to obtain a filter cake. Then wash the filter cake several times until the washing agent used for washing has a conductivity of 200-400 μS / cm (e.g., 300 μS / cm, etc.) after washing, and stop washing to obtain an aged filter cake containing ferric phosphate dihydrate.

[0148] (8) Dry the aged filter cake containing ferric phosphate dihydrate at 90-120℃ for 6 hours, sinter at 500-750℃ for 3 hours, and then crush it to obtain the finished ferric phosphate.

[0149] The method for preparing ferric phosphate according to the embodiments of this application can bring at least the following beneficial effects: During the aging and crystallization process of the iron phosphate precursor, a solution containing metal ion dopants is introduced. The metal ions in these dopants compete with ferric ions for phosphate groups through a "competitive precipitation" mechanism to form complexes. By controlling the pH at the aging stage, these dopants remain in the system and are removed during subsequent purification, effectively reducing the phosphorus content in the final product and significantly increasing the iron-to-phosphorus ratio of iron phosphate. Simultaneously, due to the increased iron-to-phosphorus ratio, the iron phosphate crystal structure is more complete, making it easier to obtain iron phosphate with larger primary particles. This makes the prepared iron phosphate more suitable for preparing high-performance lithium iron phosphate cathode materials.

[0150] Lithium iron phosphate The lithium iron phosphate in this application embodiment is prepared using iron phosphate from this application embodiment as raw material, or using iron phosphate prepared by the method of preparing iron phosphate from this application embodiment as raw material.

[0151] It should be noted that the preparation method of lithium iron phosphate in the embodiments of this application is not limited, and can be any method well known in the art for preparing lithium iron phosphate using iron phosphate as a precursor.

[0152] As an optional example, the method for preparing lithium iron phosphate according to embodiments of this application includes the following steps: Lithium iron phosphate is obtained by mixing raw materials such as iron phosphate, lithium carbonate, and glucose in a certain proportion and then grinding, spraying, sintering, crushing, and sieving.

[0153] The lithium iron phosphate described in this application has at least the beneficial effects of the iron phosphate and the preparation method of iron phosphate in the embodiments of this application.

[0154] <Positive Electrode> The positive electrode sheet of this application embodiment includes a positive electrode material, wherein the positive electrode material includes lithium iron phosphate of this application embodiment.

[0155] It is understood that the role of lithium iron phosphate in the positive electrode active material in the embodiments of this application is that of a positive electrode active material.

[0156] In some embodiments, the positive electrode sheet further includes a positive current collector and a positive electrode film layer disposed on at least one surface of the positive current collector, the positive electrode film layer comprising the positive electrode material.

[0157] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive electrode film layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0158] In addition to lithium iron phosphate in this embodiment, the cathode material also includes other cathode active materials. These other cathode active materials can be selected from materials capable of absorbing and releasing lithium.

[0159] The specific types of other cathode active materials are not specifically limited and can be selected according to requirements. As examples, the cathode active materials can include, but are not limited to, lithium manganese phosphate (LiMnPO4), lithium cobalt phosphate (LiCoPO4), lithium iron pyrophosphate (Li2FeP2O7), lithium cobalt oxide (LiCoO2), spinel lithium manganese oxide (LiMn2O4), spinel lithium nickel manganese oxide (LiNi 0.5 Mn 1.5 O4), layered lithium manganese oxide (LiMnO2), lithium nickel oxide (LiNiO2), lithium niobate (LiNbO2), lithium ferrite (LiFeO2), lithium manganate (LiMgO2), lithium calcium oxide (LiCaO2), lithium copper oxide (LiCuO2), lithium zinc oxide (LiZnO2), lithium molybdate (LiMoO2), lithium tantalate (LiTaO2), lithium tungstate (LiWO2), lithium nickel cobalt aluminum oxide (LiNi x Co y Al 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1, for example LiNi 0.8 Co 0.15 Al 0.05 O2), lithium nickel cobalt manganese oxide (LiNi x Co y Mn 1-x-y O2, 0 < x < 1, 0 < y < 1, 0 < x + y < 1, for example LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 O2, LiNi 0.5 Co 0.2 Mn 0.3 O2, LiNi 0.6 Co 0.2 Mn 0.2 O2, LiNi 0.8 Co 0.1 Mn 0.1 O2, etc.), lithium-rich materials (such as lithium-rich nickel cobalt manganese oxide), manganese dioxide (MnO2), vanadium oxides, sulfur oxides, silicate oxides, and at least one of their respective modified compounds. These materials can be used alone or in combination of two or more.

[0160] The modified compounds of the above-mentioned other cathode active materials can be doping modification, surface coating modification, or simultaneous doping and coating modification of the above-mentioned other cathode active materials.

[0161] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0162] In some embodiments, the positive electrode material may optionally include a binder. As an example, the binder may include at least one selected from polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0163] In some embodiments, the cathode material may optionally include a conductive agent. As an example, the conductive agent may include at least one selected from superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0164] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as all positive active materials, conductive agents, binders and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto a positive current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0165] Secondary batteries The secondary battery of this application embodiment includes a positive electrode, a separator, and a negative electrode, wherein the positive electrode is the positive electrode of this application embodiment.

[0166] In some embodiments, the secondary battery includes lithium-ion batteries, etc.

[0167] In some embodiments, the secondary battery also includes an electrolyte.

[0168] [Negative electrode plate] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0169] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0170] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0171] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. The silicon-based material may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. The tin-based material may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, this application is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0172] In some embodiments, the negative electrode film layer may optionally include a binder. The binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0173] In some embodiments, the negative electrode film may optionally include a conductive agent. The conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0174] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0175] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0176] [Electrolytes] The electrolyte acts as a conductor of ions between the positive and negative electrodes. This application does not impose specific restrictions on the type of electrolyte; it can be selected according to requirements. For example, the electrolyte can be liquid, gel, or entirely solid.

[0177] In some embodiments, the electrolyte is an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0178] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0179] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0180] In some embodiments, the electrolyte may optionally include additives. For example, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0181] [Isolation membrane] This application does not impose any particular restrictions on the type of separator membrane; any known porous separator membrane with good chemical and mechanical stability can be selected.

[0182] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0183] The positive electrode and the secondary battery of the present application embodiments have at least the beneficial effects of the iron phosphate and the preparation method of iron phosphate of the present application embodiments.

[0184] The following non-limiting embodiments further illustrate certain features of the present technology.

[0185] I. Examples and Comparative Examples The sources of some of the raw materials involved in the following embodiments and comparative examples are as follows: The iron source ferrous sulfate solution was prepared in-house. The preparation method was as follows: 83.4g of ferrous sulfate heptahydrate was dissolved in 380mL of deionized water to obtain a ferrous phosphate solution with a concentration of 220g / L (i.e., 0.8mol / L).

[0186] The monoammonium phosphate solution was prepared in-house. The preparation method was as follows: 34.86g of monoammonium phosphate was dissolved in 232mL of deionized water to obtain a monoammonium phosphate solution with a concentration of 150g / L (i.e., 1.3mol / L).

[0187] Solution 1 containing metal ion dopant is a self-made ferrous sulfate solution, prepared by dissolving 0.166g of ferrous sulfate heptahydrate in 500g of pure water to obtain a ferrous sulfate solution with a concentration of 0.3g / L.

[0188] Solution 2 containing metal ion dopant is a self-made ferric chloride solution, prepared by dissolving 0.097g of ferric chloride in 500g of pure water to obtain a ferric chloride solution with a concentration of 0.19g / L.

[0189] Solution 3 containing metal ion dopant is a self-made aluminum sulfate solution, prepared by dissolving 0.205g of aluminum sulfate in 500g of pure water to obtain an aluminum sulfate solution with a concentration of 0.41g / L.

[0190] Solution 4 containing metal ion dopant is a self-made chromium chloride solution, prepared by dissolving 0.160g CrCl3·6H2O in 500g pure water to obtain a chromium chloride solution with a concentration of 0.32g / L.

[0191] Solution 5 containing metal ion dopant is a self-made manganese chloride solution, prepared by dissolving 0.075g of manganese chloride (MnCl2) in 500g of pure water to obtain a manganese chloride solution with a concentration of 0.15g / L.

[0192] In the following examples and comparative examples, room temperature refers to 25±1℃.

[0193] Example 1 The method for preparing ferric phosphate in this embodiment includes the following steps: (1) Prepare a mixed solution by mixing the phosphorus source monoammonium phosphate solution, 0.69 mol / L hydrogen peroxide and 0.48 mol / L ammonia solution, and prepare the iron source ferrous sulfate solution.

[0194] in: The molar ratio of iron in the ferrous sulfate solution and phosphorus in the ammonium hydrogen phosphate solution in the mixed solution is 1.02:1. The molar ratio of iron in the ferrous sulfate solution to hydrogen peroxide in the mixed solution is 1:1.15. The molar ratio of iron in the ferrous sulfate solution to ammonia in the mixed solution is 1:0.8.

[0195] (2) While stirring the iron source ferrous sulfate solution at 250 rpm, add the mixture using a peristaltic pump at a pump speed of 6 rpm. Keep the temperature at 55°C and control the pH of the system at 2. After the mixture is added, continue to keep it at 55°C for 15 min to obtain slurry A.

[0196] (3) Slurry A is filtered and the filter cake obtained by filtration is washed with pure water until the conductivity of the detergent pure water is less than 800 μS / cm after multiple washings, and a light yellow reaction filter cake is obtained.

[0197] The method for washing the filter cake obtained after vacuum filtration with pure water is vacuum filtration.

[0198] (4) The reaction filter cake and pure water were pulped at room temperature for 1 hour, and then 85 wt% phosphoric acid solution was added to obtain slurry B.

[0199] The total solid content of the reaction filter cake and pure water is 12.5%, the molar amount of phosphoric acid solution added is 12.5% ​​of the molar amount of iron in the ferrous sulfate solution in step (1), and the pH of slurry B is 1.8-2.2.

[0200] (5) Heat slurry B to 92°C and add a solution containing metal ion dopant during the heating process to obtain slurry C.

[0201] Among them, the solution containing metal ion dopant is solution 1 containing metal ion dopant, and the amount of metal ion dopant added is to ensure its Fe 2+ The molar amount of Fe should preferably be 0.2% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1) (that is, the amount of Fe introduced into slurry C by the solution containing metal ion dopant). 2+ The molar amount is 0.2% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1).

[0202] (6) The slurry C is aged at 92°C. After the color of the slurry changes from yellow to pinkish-white, it is kept at 92°C for 2 hours. Then the slurry is filtered and washed multiple times with pure water until the conductivity of the washing agent pure water after multiple washes is less than 300 μS / cm, and a pinkish-white aged filter cake is obtained.

[0203] The method for washing the filter cake obtained after vacuum filtration with pure water is vacuum filtration.

[0204] (7) The aged filter cake was dried at 90°C for 6 hours, and then sintered in air atmosphere at a heating rate of 3°C / min and a sintering temperature of 550°C for 3 hours. After sintering, it was mechanically crushed to obtain the finished iron phosphate.

[0205] Example 2 This embodiment is basically the same as Embodiment 1, except that: In step (5), the solution containing metal ion dopant is solution 3 containing metal ion dopant, and the amount of solution containing metal ion dopant added is to ensure its Al 3+ The molar amount of iron should preferably be 0.2% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1) (that is, the amount of Al introduced into slurry C by the solution containing metal ion dopant). 3+ The molar amount is 0.2% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1).

[0206] Example 3 This embodiment is basically the same as Embodiment 1, except that: In step (5), the solution containing metal ion dopant is solution 4 containing metal ion dopant, and the amount of solution containing metal ion dopant added is to ensure that its Cr 3+ The molar amount of Cr introduced from the solution containing metal ion dopant in slurry C should preferably be 0.2% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1). 3+ The molar amount is 0.2% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1).

[0207] Example 4 This embodiment is basically the same as Embodiment 1, except that: In step (5), the solution containing metal ion dopant is solution 2 containing metal ion dopant, and the amount of metal ion dopant added is to ensure that its Fe 3+ The molar amount of Fe should preferably be 0.2% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1) (that is, the amount of Fe introduced into slurry C by the solution containing metal ion dopant). 3+ The molar amount is 0.2% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1).

[0208] Example 5 This embodiment is basically the same as Embodiment 1, except that: In step (5), the solution containing metal ion dopant is solution 5 containing metal ion dopant, and the amount of solution containing metal ion dopant added is to ensure that its Mn 2+The molar amount of Mn introduced from the solution containing metal ion dopant in slurry C should preferably be 0.2% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1). 2+ The molar amount is 0.2% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1).

[0209] Example 6 This embodiment is basically the same as Embodiment 1, except that: In step (5), the solution containing metal ion dopant is a mixture of solution 1, solution 3, and solution 4 containing metal ion dopant, and the mixture contains Fe 2+ Al 3+ Cr 3+ The molar ratio of the three is 1:1:1; the amount of metal ion dopant added to the solution is sufficient to ensure its Fe 2+ Al 3+ Cr 3+ The total molar amount should preferably be 0.2% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1) (that is, the amount of Fe introduced into slurry C by the solution containing metal ion dopant). 2+ Al 3+ Cr 3+ The total molar amount is 0.2% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1).

[0210] Example 7 This embodiment is basically the same as Embodiment 1, except that: In step (5), the amount of solution containing metal ion dopant added is determined to ensure its Fe... 2+ The molar amount of Fe should preferably be 10% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1) (that is, the amount of Fe introduced into slurry C by the solution containing metal ion dopant). 2+ The molar amount is 10% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1).

[0211] Example 8 This embodiment is basically the same as Embodiment 1, except that: In step (5), the amount of solution containing metal ion dopant added is determined to ensure its Fe... 2+ The molar amount of Fe should preferably be 5% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1) (that is, the amount of Fe introduced into slurry C by the solution containing metal ion dopant). 2+ The molar amount is 5% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1).

[0212] Example 9 This embodiment is basically the same as embodiment 7, except that: In step (5), the solution containing metal ion dopant is a mixture of solution 1, solution 3, and solution 4 containing metal ion dopant, and the mixture contains Fe 2+ Al 3+ Cr 3+ The molar ratio of the three is 1:1:1; the amount of metal ion dopant added to the solution is sufficient to ensure its Fe 2+ Al 3+ Cr 3+ The total molar amount should preferably be 0.3% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1) (that is, the amount of Fe introduced into slurry C by the solution containing metal ion dopant). 2+ Al 3+ Cr 3+ The total molar amount is 0.3% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1).

[0213] Example 10 This embodiment is basically the same as embodiment 7, except that: In step (5), the solution containing metal ion dopant is a mixture of solution 1, solution 3, and solution 4 containing metal ion dopant, and the mixture contains Fe 2+ Al 3+ Cr 3+ The molar ratio of the three is 1:1:1; the amount of metal ion dopant added to the solution is sufficient to ensure its Fe 2+ Al 3+ Cr 3+ The total molar amount should preferably be 9.9% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1) (that is, the amount of Fe introduced into slurry C by the solution containing metal ion dopant). 2+ Al 3+ Cr 3+ The total molar amount is 9.9% of the molar amount of iron in the ferrous sulfate solution of the iron source in step (1).

[0214] Example 11 This embodiment is basically the same as Embodiment 1, except that: In step (5), the slurry B is heated to 70°C.

[0215] In step (6), the slurry C is aged at 70°C, and after the color of the slurry changes from yellow to pinkish-white, it is kept at 98°C for 3 hours.

[0216] Example 12 This embodiment is basically the same as Embodiment 1, except that: In step (5), the slurry B is heated to 98°C.

[0217] In step (6), the slurry C is aged at 98°C, and after the color of the slurry changes from yellow to pinkish-white, it is kept at 80°C for 4 hours.

[0218] Example 13 This embodiment is basically the same as Embodiment 1, except that: In step (7), the dried aged filter cake is then sintered in an air atmosphere at a heating rate of 5℃ / min and a sintering temperature of 700℃ for 3 hours.

[0219] Example 14 This embodiment is basically the same as Embodiment 1, except that: In step (7), the dried aged filter cake is then sintered in an air atmosphere at a heating rate of 4℃ / min and a sintering temperature of 600℃ for 3 hours.

[0220] Example 15 This embodiment is basically the same as Embodiment 1, except that: In step (1): The molar ratio of iron in the ferrous sulfate solution and phosphorus in the ammonium hydrogen phosphate solution in the mixed solution is 1.2:1. The molar ratio of iron in the ferrous sulfate solution to hydrogen peroxide in the mixed solution is 1:1.3. The molar ratio of iron in the ferrous sulfate solution to ammonia in the mixed solution is 1:1.

[0221] In step (2), the temperature is kept at 40°C, and after the mixture is added, it is kept at 40°C for 20 minutes.

[0222] Example 16 This embodiment is basically the same as Embodiment 1, except that: Replace the iron source ferrous sulfate solution with a commercially available 0.8 mol / L ferrous chloride solution.

[0223] Step (1) is as follows: Prepare a mixed solution by mixing a commercially available 1.3 mol / L monoammonium phosphate solution as the phosphorus source, a commercially available 0.69 mol / L hydrogen peroxide solution as the oxidant, and a 0.48 mol / L ammonia solution as the pH adjuster, and prepare a commercially available 0.8 mol / L ferrous chloride solution.

[0224] The molar ratio of iron in ferrous chloride solution to phosphorus in a commercially available 1.3 mol / L monoammonium phosphate solution is 1.1:1. The molar ratio of iron in the ferrous chloride solution to the commercially available 0.69 mol / L hydrogen peroxide solution in the mixed solution is 1:1. The molar ratio of iron in the ferrous chloride solution to commercially available 0.48 mol / L ammonia solution, a pH adjuster, is 1:2.

[0225] Example 17 This embodiment is basically the same as Embodiment 1, except that: The raw materials that participate in the oxidation precipitation reaction are mixed at once.

[0226] Steps (1) and (2) are modified as follows: Ferrous sulfate solution (iron source), monoammonium phosphate solution (phosphorus source), 0.69 mol / L hydrogen peroxide, and 0.48 mol / L ammonia solution were mixed and then kept at 55°C for 15 min while stirring at 250 rpm to obtain slurry A.

[0227] in, The molar ratio of iron in the ferrous sulfate solution (iron source) to phosphorus in the ammonium hydrogen phosphate solution (phosphorus source) is 1.02:1. The molar ratio of iron to hydrogen peroxide in the ferrous sulfate solution is 1:1.15. The molar ratio of iron to ammonia in the ferrous sulfate solution is 1:0.8.

[0228] Comparative Example 1 This comparative example is basically the same as Example 1, except that: In step (1), the molar ratio of iron in the ferrous sulfate solution and phosphorus in the ammonium hydrogen phosphate solution in the mixed solution is 1:1.

[0229] In step (4), the total solids content of the reaction filter cake and pure water is 15%.

[0230] Step (5) does not include the step of "adding a solution containing metal ion dopant during the heating process".

[0231] That is: Steps (5) and (6) are combined as follows: slurry B is heated to 92°C for aging. After the slurry color changes from yellow to pinkish-white, it is kept at 92°C for 2 hours. Then, the slurry is filtered and washed with pure water until the conductivity of the filtrate is less than 300 μS / cm, resulting in a pinkish-white aged filter cake. The method of washing the filter cake obtained after filtration with pure water is the same as step (6) in Example 1.

[0232] II. Material Characterization and Performance Testing 1. Material Characterization (1) Iron-to-phosphorus ratio (Fe / P molar ratio) The molar content of iron and phosphorus in the iron phosphate products of each embodiment or comparative example was tested by titration. The specific test method was as follows: the mass content of Fe was tested by potassium dichromate titration, and the mass content of P was tested by quinomolybdate gravimetric method. Then, the molar content of iron and phosphorus was obtained by chemical calculation, and the ratio of the molar content of iron to the molar content of phosphorus was used as the iron-phosphorus ratio (Fe / P molar ratio).

[0233] The test results are shown in Table 1.

[0234] (2) BET specific surface area The specific surface area of ​​the iron phosphate products in each embodiment or comparative example was tested using a nitrogen adsorption test instrument manufactured by RuboLab in Germany, model Infrasorb. The specific test method was the BET method, which is based on the multilayer gas adsorption theory. The sample to be tested was placed in a liquid nitrogen environment (-196℃), and the amount of nitrogen adsorbed under different pressures was measured. The volume of single-layer adsorption was calculated by fitting the data, and the specific surface area was finally derived.

[0235] The test results are shown in Table 1.

[0236] (3) Particle morphology and size The microstructure and primary particle size range of ferric phosphate dihydrate, finished ferric phosphate, and the comparative ferric phosphate were imaged using a field emission electron microscope (FET) manufactured by Nippon Electronics Corporation. The specific testing method was to achieve nanoscale surface imaging through secondary electron signals, which can clearly show the size, shape, surface roughness, and aggregation state of the particles.

[0237] Among them, the test results of Examples 1-3 and Comparative Example 1 Figure 2-5 As shown in Table 1. Compare. Figures 2 to 5 It can be seen that, compared with the iron phosphate prepared in Comparative Example 1, the iron phosphate prepared in Examples 1-3 has a significantly larger particle size.

[0238] (4) Impurity content The impurity content of ferric phosphate in each example or comparative example was tested using a Shimadzu ICPMS-2040 LF / 2050 LF inductively coupled plasma mass spectrometer. The specific testing method was as follows: the sample was pretreated, typically by dissolving or grinding a solid sample into a homogeneous powder. The ICP instrument was calibrated using a standard solution of known concentration. The instrument's operating conditions were set, including gas flow rate and power supply. The element content was determined by measuring the intensity of light emitted by the element at a specific wavelength in the sample. The impurity was Al. 3+ Cr 2+ Cr 3+ Mn2+ Mn 3+ .

[0239] The test results are shown in Table 1.

[0240] Table 1. Test Results of Materials

[0241] As can be seen from Table 1, under the premise that the impurity content is basically the same, the finished iron phosphate prepared in each embodiment of this application has a higher BET specific surface area, iron-phosphorus ratio and a larger primary particle size range compared with the comparative example.

[0242] 2. Electrochemical performance, etc. The finished iron phosphate products from each example or comparative example were used to prepare lithium iron phosphate. The preparation method of lithium iron phosphate was as follows: 2 kg of deionized water was weighed and added to a sand mill, and stirred at 500 rpm. Then, 250 g of lithium carbonate was weighed and added to the sand mill, and stirred for 5 min to disperse evenly. Then, 80 g of glucose and 30 g of PEG1500 were weighed and added to the above dispersion, and stirred for 5 min to disperse evenly. Then, 4 g of titanium dioxide was weighed and added to the above dispersion, and stirred for 5 min to disperse evenly. Finally, 1 kg of anhydrous iron phosphate prepared in the examples and comparative examples was weighed and added to the above dispersion. The mixture was stirred for 5 minutes to disperse it evenly, and then homogenized by a sand mill for 50 minutes to obtain a slurry with a particle size of 0.5 μm to 0.6 μm. The slurry was then spray-dried at a frequency of 47 Hz, an inlet temperature of 250 ℃, and an outlet temperature of 90 ℃ to obtain 1 kg of yellow or brown powder with a particle size of 30 μm to 40 μm. The powder obtained by spraying was then sintered at 790 ℃ under an inert atmosphere at a heating rate of 3 ℃ / min for 10 h, and then pulverized by airflow at a pulverizing frequency of 135 Hz to obtain lithium iron phosphate with a particle size of 1 μm to 2 μm.

[0243] Lithium iron phosphate (LiFePO4) prepared from the finished iron phosphate products of each embodiment or comparative example was used to prepare positive electrode sheets, and the prepared positive electrode sheets were used to prepare lithium-ion batteries. Subsequently, the compaction density of LiFePO4 and the electrochemical performance of each lithium-ion battery were tested. Wherein: The preparation method of the positive electrode sheet is as follows: take the lithium iron phosphate prepared in each example and comparative example, and mix it with conductive agent carbon black and binder PVDF in a weight ratio of 90:5:5 to form a slurry. The slurry parameters are 2000 rpm and 15 min. The solvent is NMP and the solid content of the slurry is 50 wt%. Coat the slurry prepared above onto carbon-coated aluminum foil with a coating thickness of 200 μm. Transfer it to a vacuum dryer for drying to obtain the positive electrode sheet. The drying conditions are 120 °C and 3 h.

[0244] The method for preparing a lithium-ion battery includes the following steps: (1) Preparation of negative electrode sheet: Lithium metal sheet is used as negative electrode sheet.

[0245] (2) Preparation of electrolyte: 1 mol / L LiPF6 / EC+DMC (the volume ratio of EC and DMC is 1:1) was used as the electrolyte.

[0246] (3) Screening of the separator: The separator model BOPP produced by Shanghai Enjie New Material Technology Co., Ltd. was used.

[0247] (4) Assembly: Transfer the positive electrode, separator and negative electrode to an inert atmosphere glove box. Place the negative electrode shell flat on an insulated table. Place the lithium metal sheet in the center of the negative electrode shell and flatten the lithium metal sheet with a pressing mold. Then place the separator flat on the lithium metal sheet. Use a pipette to add an appropriate amount of electrolyte drop to the surface of the separator. Use insulated tweezers to place the test electrode, gasket, spring sheet and positive electrode shell on the separator in sequence. Then use insulated tweezers to place the button cell with the negative side facing up on the button cell sealing machine mold. You can use a paper towel to pad the top of the battery to absorb the overflowing electrolyte. Adjust the pressure (generally 800 Pa) and press for 5 seconds to complete the assembly of the button cell. Take it out with insulated tweezers, observe whether the appearance is complete and wipe it clean with a paper towel.

[0248] The powder compaction density of lithium iron phosphate was tested, and the electrochemical performance of lithium-ion batteries, such as 1P discharge capacity and energy efficiency, was tested.

[0249] The test method for the compaction density of lithium iron phosphate is as follows: according to GB / T 41232.2, the test is conducted using a powder compaction tester (pressure 3t) of model UTM7305 generated by Sansi Instruments.

[0250] The testing methods for the electrochemical performance of lithium-ion batteries are as follows: At 25℃, the battery is charged to 4.3V at a constant power of 1P, and then discharged to 2.0V at a constant rate of 1P to obtain a discharge capacity of 1P. Energy efficiency (%) = (total energy released at voltages of 3.2V and above during the discharge process ÷ total energy released during the discharge process) × 100%, that is, the amount of electricity released at voltages of 3.2V and above during the discharge curve divided by the total amount of electricity released.

[0251] The results of compaction density and electrochemical performance tests are shown in Table 2.

[0252] Table 2. Test results of compaction density and electrochemical performance

[0253] As can be seen from Table 2, the finished iron phosphate prepared in each embodiment of this application, after being converted into lithium iron phosphate, has a higher discharge capacity (second column) and higher energy efficiency (third column) compared to the comparative example with the same compaction density. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion. In the description of the embodiments of this application, technical terms such as "first", "second", "I", "II" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features.

[0254] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0255] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, or B existing alone. Additionally, the character " / " in this document, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0256] In the description of the embodiments of this application, the term "multiple" refers to two or more (including two), similarly, "multiple sets" refers to two or more (including two sets), and "multiple pieces" refers to two or more (including two pieces), unless otherwise explicitly specified.

Claims

1. A type of iron phosphate, characterized in that, The ferric phosphate is a secondary aggregate formed by primary particle agglomeration, and the ferric phosphate includes at least one of the following characteristics: (1) The molar ratio of iron to phosphorus is 0.970-1.1; (2) The specific surface area of ​​BET is 5-12 m² 2 / g; (3) The particle size range of the primary particles is 200-500 nm.

2. A method for preparing ferric phosphate, characterized in that, include: Iron source, phosphorus source, oxidant and pH adjuster are mixed and subjected to oxidation precipitation reaction and first purification to obtain reaction filter cake; The reaction filter cake was pulped and then added to an acid solution and a solution containing metal ion dopant for aging, second purification, and drying to obtain an aged filter cake containing ferric phosphate dihydrate. The aged filter cake containing ferric phosphate dihydrate is sintered to obtain the ferric phosphate.

3. The preparation method according to claim 2, characterized in that, In the solution containing the metal ion dopant, the metal ions include Fe. 2+ Fe 3+ Al 3+ Cr 2+ Cr 3+ Mn 2+ Mn 3+ At least one of them; And / or, in the solution containing the metal ion dopant, the molar amount of the metal ion is 0.1-10% of the molar amount of iron in the iron source; And / or, the second purification includes filtration and washing, and the second purification is stopped when the washing agent used in the second purification has a conductivity of 200-400 μS / cm after washing.

4. The preparation method according to claim 3, characterized in that, In the solution containing the metal ion dopant, the metal ions include Fe. 2+ Fe 3+ Al 3+ Cr 3+ At least one of them; And / or, the metal ion dopant includes at least one of ferrous sulfate, ferric chloride, aluminum sulfate, chromium chloride, and manganese chloride; And / or, in the solution containing the metal ion dopant, the molar amount of the metal ion is 0.1-0.5% of the molar amount of iron in the iron source.

5. The preparation method according to claim 2, characterized in that, After adding the acid solution, the pH of the slurry is 1.5-2.5, preferably 1.8-2.2; And / or, after pulping the reaction filter cake, it is added to an acid solution or a solution containing metal ion dopant for aging, including: The reaction filter cake is pulped and then an acid solution is added to obtain the first slurry; The first slurry is heated to a preset temperature while the solution containing the metal ion dopant is added to obtain a second slurry; the preset temperature is the aging temperature. The second slurry is aged; And / or, the aging temperature is 70-98°C; And / or, the method for preparing ferric phosphate further includes: During the aging process, after the Dv50 of the slurry containing acid solution and metal ion dopant solution added after pulping decreases from 10-20µm to 1-5µm, the first heat preservation is carried out. Optionally, the temperature of the first insulation is 80-98℃, and the duration of the first insulation is 2-4 hours.

6. The preparation method according to claim 2, characterized in that, The acid solution includes at least one of phosphoric acid solution, sulfuric acid solution, and hydrochloric acid solution; And / or, the molar amount of the acid solution added is 10-15% of the molar amount of iron in the iron source; And / or, pulping the reaction filter cake, including: The reaction filter cake and water are then pulped together. And / or, the molar ratio of iron in the iron source to phosphorus in the phosphorus source is (1-1.2):1; And / or, the molar ratio of iron element in the iron source to the oxidant is 1:(1-1.3), optionally 1:1.15; And / or, the molar ratio of iron in the iron source to the pH adjuster is 1:(0.8-1.2). And / or, the iron source includes ferrous sulfate heptahydrate, the phosphorus source includes monoammonium phosphate, the oxidant includes hydrogen peroxide, and the pH adjuster includes ammonia. And / or, the mixing of the iron source, phosphorus source, oxidant, and pH adjuster includes: The phosphorus source, oxidant, and pH adjuster are mixed to obtain a mixture; Under stirring conditions, the mixture is slowly added to the iron source; Optionally, the mixture can be slowly added to the iron source using a peristaltic pump at a pump speed of 1-10 rpm.

7. The preparation method according to claim 2, characterized in that, The temperature of the oxidation precipitation reaction is 40-80℃; And / or, the oxidation precipitation reaction takes 0.5-1 h; And / or, the pH of the oxidation precipitation reaction is 1.5-3.0; And / or, the first purification includes filtration and washing, and the first purification is stopped when the conductivity of the washing agent used in the first purification is 600-1000 μS / cm after washing; And / or, the method for preparing ferric phosphate further includes a second heat preservation step after the oxidation precipitation reaction and before the first purification; optionally, the second heat preservation time is 10-20 min; And / or, the iron phosphate dihydrate is a spherical structure with lamellar intercalation; And / or, the molar ratio of iron to phosphorus in the iron phosphate dihydrate is 0.965-1.2; And / or, the sintering is carried out in an oxygen or oxygen-containing gas atmosphere; And / or, the sintering heating rate is 1-10℃ / min, optionally 3-5℃ / min; And / or, the sintering temperature is 550-700℃, optionally 550-600℃; And / or, the sintering time is 1-5 hours.

8. A lithium iron phosphate, characterized in that, Prepared using ferric phosphate as a raw material according to claim 1, or ferric phosphate prepared by any one of claims 2 to 7.

9. A positive electrode sheet, characterized in that, It includes a cathode material, said cathode material including lithium iron phosphate as described in claim 8.

10. A secondary battery, comprising a positive electrode, a separator, and a negative electrode, characterized in that, The positive electrode is the positive electrode as described in claim 9.

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