Lamellar iron phosphate having high iron-to-phosphorus ratio, preparation method therefor, positive electrode material and positive electrode plate

Abstract

The present application relates to a lamellar iron phosphate having a high iron-to-phosphorus ratio, a preparation method therefor, a positive electrode material, and a positive electrode plate. Said lamellar iron phosphate has a lamellar structure and a Fe / P ratio greater than 0.99, and satisfies: the ratio of the length, width and thickness of the lamellar structure being (105-130):(90-100):(10-12), the specific surface area being not less than 3.5m2 / g and not more than 6.5m2 / g, and the particle size being less than 35 μm.

Description

Flake-shaped high-phosphorus iron phosphate and its preparation method, positive electrode material and positive electrode sheet

[0001] Related applications

[0002] This application claims priority to Chinese patent application filed on June 18, 2024, application number 202410790348.9, entitled "Flake-shaped high-iron-phosphorus ratio iron phosphate and its preparation method, positive electrode material and positive electrode sheet", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of battery materials technology, specifically to a sheet-like high iron-phosphorus ratio iron phosphate and its preparation method, a positive electrode material, and a positive electrode sheet. Background Technology

[0004] There are three main existing processes for producing ferric phosphate: ① ammonium method; ② sodium method; ③ iron method. Among them, the iron method involves reacting iron powder with phosphoric acid, followed by oxidation with hydrogen peroxide to produce ferric phosphate. Compared to the ammonium and sodium methods, the iron method has advantages such as a shorter process flow, no by-products, and minimal equipment and environmental investment for the same production capacity. Currently, the iron method for producing ferric phosphate is attracting increasing attention within the industry.

[0005] The main problems with the mainstream iron-based process for preparing iron phosphate in the market are: ① The iron-to-phosphorus ratio (i.e., the molar ratio of iron to phosphorus) is low, about 0.96 to 0.98; ② The morphology is difficult to control, or the size is inconsistent.

[0006] Therefore, there is an urgent need to provide a new method for preparing iron phosphate to solve the above-mentioned technical problems.

[0007] Summary of the Invention

[0008] Based on this, and according to various embodiments of this application, a sheet-like high-iron-phosphorus iron phosphate and its preparation method, a positive electrode material, and a positive electrode sheet are provided. The technical solution is as follows:

[0009] In a first aspect, embodiments of this application provide a sheet-like high-iron-phosphorus ratio iron phosphate, wherein the sheet-like high-iron-phosphorus ratio iron phosphate has a sheet-like structure with an Fe / P ratio greater than 0.99, and satisfies the following: the ratio of the length, width, and thickness of the sheet-like structure is (105-130):(90-100):(10-12), and 3.5m 2 / g≤Specific surface area≤6.5m² 2 / g, particle size <35μm.

[0010] Secondly, embodiments of this application provide a method for preparing flake-shaped high-phosphorus-ratio iron phosphate, comprising the following steps:

[0011] Provide a first ferrous dihydrogen phosphate solution and a second ferrous dihydrogen phosphate solution;

[0012] After heating the first ferrous dihydrogen phosphate solution, the oxidant is gradually added dropwise over the first dropping time. After the addition is completed, the temperature is maintained to obtain the seed slurry.

[0013] The seed slurry was added to the second ferrous dihydrogen phosphate solution and the temperature was raised. Then, the oxidant was gradually added dropwise over the second dropping time. After the dropping was completed, the temperature was maintained to obtain ferric phosphate dihydrate slurry.

[0014] The ferric phosphate dihydrate slurry was filtered, washed, dried, and sintered to obtain flaky high ferric phosphate (i.e., anhydrous ferric phosphate).

[0015] The first drop was added in a shorter time than the second drop.

[0016] In some embodiments, the concentrations of the first ferrous dihydrogen phosphate solution and the second ferrous dihydrogen phosphate solution are both 0.7 mol / L to 1.1 mol / L; the specific preparation steps are as follows: reacting dilute phosphoric acid with iron powder, filtering the slurry after the reaction is complete to obtain the first ferrous dihydrogen phosphate solution or the second ferrous dihydrogen phosphate solution; wherein, the mass fraction of dilute phosphoric acid is 20% to 35%, and the molar ratio of iron powder to phosphoric acid is (0.35 to 0.5):1.

[0017] In some embodiments, the preparation steps of the seed slurry are as follows: the first ferrous dihydrogen phosphate solution is heated to 70°C to 90°C, and then an oxidant is added dropwise evenly. The first dropwise addition time is 5 min to 15 min. After the dropwise addition is completed, the temperature is maintained for 60 min to 120 min to obtain the seed slurry.

[0018] In some embodiments, the preparation steps of ferric phosphate dihydrate slurry are as follows: seed slurry is added to the second ferrous dihydrogen phosphate solution and the temperature is raised to 70°C to 90°C. Then, oxidant is added dropwise evenly. The second dropwise addition time is 30 min to 90 min. After the dropwise addition is completed, the temperature is maintained for 60 min to 120 min to obtain ferric phosphate dihydrate slurry.

[0019] In some embodiments, in the preparation steps of the seed slurry and the preparation steps of the ferric phosphate dihydrate slurry, the molar ratio of the oxidant to divalent iron is (0.6-0.9):1; the oxidant includes at least one of hydrogen peroxide, ammonium persulfate, and sodium persulfate.

[0020] In some embodiments, the washing method is countercurrent washing, and the washing is carried out until the conductivity of the deionized water is ≤200μS / cm; the drying temperature is 90℃~110℃, and the drying time is 8h~24h; the sintering temperature is 600℃~700℃, and the sintering time is 2h~4h.

[0021] Thirdly, embodiments of this application provide a lithium iron phosphate cathode material, which is prepared by uniformly mixing sheet-like high iron-phosphorus ratio iron phosphate provided in the first aspect of this application with a lithium source and then by a high-temperature solid-state method.

[0022] Fourthly, embodiments of this application provide a positive electrode sheet, including the lithium iron phosphate positive electrode material provided in the third aspect of this application.

[0023] Fifthly, embodiments of this application provide a secondary battery, including the positive electrode sheet provided in the fourth aspect of this application.

[0024] Details of one or more embodiments of this application are set forth in the following description, and other features, objects, and advantages of this application will become apparent from the specification and its claims. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of this application or the conventional technology, the drawings used in the description of the embodiments or the conventional technology will be briefly introduced below. Obviously, the drawings described below are only embodiments of this application. For those skilled in the art, other drawings can be obtained based on the disclosed drawings without creative effort.

[0026] Figure 1 is a process flow diagram of one embodiment of the preparation method of flaky high-phosphorus ratio iron phosphate provided in this application;

[0027] Figure 2 is a SEM image of the flake-shaped high-phosphorus-ratio iron phosphate obtained in Example 1 of this application;

[0028] Figure 3 is a SEM image of the flake-shaped high ferric phosphorus ratio iron phosphate obtained in Example 2 of this application;

[0029] Figure 4 is a SEM image of the flake-shaped high ferric phosphorus ratio iron phosphate obtained in Example 3 of this application;

[0030] Figure 5 is a SEM image of the flake-shaped high ferric phosphorus ratio iron phosphate obtained in Example 4 of this application;

[0031] Figure 6 is a SEM image of the flake-shaped high ferric phosphorus ratio iron phosphate obtained in Example 5 of this application;

[0032] Figure 7 is a SEM image of the iron phosphate obtained in Comparative Example 1 of this application;

[0033] Figure 8 is a SEM image of the iron phosphate obtained in Comparative Example 2 of this application;

[0034] Figure 9 is a SEM image of the iron phosphate obtained in Comparative Example 3 of this application. Detailed Implementation

[0035] 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 some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0036] 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 belongs. The terminology used herein in the specification is for the purpose of describing particular embodiments only and is not intended to be limiting of 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.

[0037] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0038] 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.

[0039] 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, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0040] 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).

[0041] The main problems in the production of lithium iron phosphate using the iron-phosphorus process are: ① The iron-phosphorus ratio is low, about 0.96 to 0.98. The iron-phosphorus ratio mainly reflects the purity of lithium iron phosphate. The higher the purity, the fewer side reactions during the preparation of lithium iron phosphate; ② The morphology is difficult to control. The main reason is that the oxidation rate is difficult to control, which easily leads to the formation of a disordered plate-like structure (this structure has a large specific surface area, which is not conducive to controlling the grinding particle size during the preparation of lithium iron phosphate, and easily causes uneven particle size distribution. It also leads to poor processing performance problems such as powder shedding during the coating process of the prepared lithium iron phosphate) or a large octahedral structure (this structure has a small specific surface area, which is not conducive to the insertion and extraction of lithium ions).

[0042] To address the technical problems of low iron-to-phosphorus ratio, inconsistent morphology and size, and excessively large or small specific surface area in the preparation of iron phosphate using the iron-based process in existing technologies, this application provides a sheet-like high iron-to-phosphorus ratio iron phosphate, its preparation method, cathode material, and cathode electrode sheet. In the preparation method of the sheet-like high iron-to-phosphorus ratio iron phosphate provided in this application, primary iron phosphate particles with sheet-like structures but disordered morphology are first obtained through rapid oxidation as seed crystals. A small amount of these seed crystals is then added beforehand during the iron phosphate synthesis process. Simultaneously, the iron phosphate formation rate in the system is controlled by slow oxidation, allowing the iron phosphate generated in the system to grow and improve on the sheet-like structure of the seed crystals. This results in iron phosphate with regular morphology, uniform size, and more complete crystal growth, and the obtained iron phosphate has a higher iron-to-phosphorus ratio (closer to the theoretical value of 1.0) and a moderate specific surface area. The cathode material prepared from this sheet-like high iron-to-phosphorus ratio iron phosphate has advantages such as excellent processing performance and high compaction density, thereby improving the processing performance and energy density performance of the cathode electrode sheet and secondary battery.

[0043] In a first aspect, embodiments of this application provide a sheet-like high-iron-phosphorus ratio iron phosphate, wherein the sheet-like high-iron-phosphorus ratio iron phosphate has a sheet-like structure with an Fe / P ratio greater than 0.99, and satisfies the following: the ratio of the length, width, and thickness of the sheet-like structure is (105-130):(90-100):(10-12), and 3.5m 2 / g≤Specific surface area≤6.5m² 2 / g, particle size <35μm.

[0044] In this application, the Fe / P ratio of the flaky ferric phosphorus to ferric phosphate is further preferably 0.9924 to 1.0000, including but not limited to: 0.9924, 0.9925, 0.9930, 0.9935, 0.9940, 0.9945, 0.9950, 0.9955, 0.9960, 0.9965, 0.9970, 0.9975, 0.9980, 0.9985, 0.9990, or 1.0000.

[0045] In this application, the length of the sheet structure is further preferably 1050nm to 1300nm, including but not limited to: 1050nm, 1080nm, 1100nm, 1120nm, 1150nm, 1180nm, 1200nm, 1220nm, 1250nm, 1280nm or 1300nm.

[0046] In this application, the width of the sheet structure is further preferably 900nm to 1000nm, including but not limited to: 900nm, 910nm, 920nm, 930nm, 940nm, 950nm, 960nm, 970nm, 980nm, 990nm or 1000nm.

[0047] In this application, the thickness of the sheet structure is further preferably 100nm to 111nm, including but not limited to: 100nm, 101nm, 102nm, 103nm, 104nm, 105nm, 106nm, 107nm, 108nm, 109nm, 110nm or 111nm.

[0048] In this application, the specific surface area of ​​the flaky ferric phosphorus is further preferably 3.97 m² / g. 2 / g~6.22m 2 / g, including but not limited to: 3.97m 2 / g, 4.00m 2 / g、4.20m 2 / g, 4.40m 2 / g, 4.60m 2 / g, 4.80m 2 / g, 5.00m 2 / g, 5.20m 2 / g, 5.40m 2 / g, 5.60m 2 / g, 5.80m 2 / g, 6.00m 2 / g、6.20m 2 / g or 6.22m 2 / g.

[0049] In this application, the D100 particle size of the flake-shaped ferric phosphate is further preferably 26.94 μm to 34.95 μm, including but not limited to: 26.94 μm, 27.00 μm, 27.50 μm, 28.00 μm, 28.50 μm, 29.00 μm, 29.50 μm, 30.00 μm, 30.50 μm, 31.00 μm, 31.50 μm, 32.00 μm, 32.50 μm, 33.00 μm, 33.50 μm, 34.00 μm, 34.50 μm or 34.95 μm.

[0050] The flake-shaped high-iron-phosphorus iron phosphate provided in this application has the advantages of regular morphology, uniform size, more complete crystal growth, higher iron-phosphorus ratio (closer to the theoretical value of 1.0), moderate specific surface area, and small particle size. The flake-shaped microstructure shortens the lithium-ion migration path, which is beneficial for lithium-ion extraction and insertion, and further enhances the electrochemical performance of the lithium iron phosphate cathode material. The small particle size also helps to shorten the ball milling time in the lithium iron phosphate preparation process, improving efficiency. The cathode material prepared from this precursor has the advantages of excellent processing performance and high compaction density.

[0051] Referring to Figure 1, in a second aspect, this application provides a method for preparing flake-shaped high-iron-phosphorus iron phosphate, comprising the following steps:

[0052] S1. Provide a first ferrous dihydrogen phosphate solution and a second ferrous dihydrogen phosphate solution;

[0053] S2. After heating the first ferrous dihydrogen phosphate solution, the oxidant is gradually added dropwise over the first dropping time. After the addition is completed, the solution is kept at the temperature to obtain the seed slurry.

[0054] S3. Add the seed slurry to the second ferrous dihydrogen phosphate solution and heat. Then, gradually add the oxidant over the second adding time. After the addition is complete, maintain the temperature to obtain ferric phosphate dihydrate slurry.

[0055] S4. The ferric phosphate dihydrate slurry is filtered, washed, dried and sintered to obtain flaky high iron-to-phosphorus ratio ferric phosphate (i.e. anhydrous ferric phosphate).

[0056] The first drop was added in a shorter time than the second drop.

[0057] This application addresses the problems of low iron-to-phosphorus ratio, inconsistent morphology and size, and excessively large or small specific surface area in existing iron-based processes. First, rapid oxidation is used to obtain primary ferric phosphate particles with a scattered, plate-like morphology, which serve as seed crystals. Then, a small amount of seed crystals is added beforehand during the ferric phosphate synthesis process to reduce the nucleation potential energy of the plate-like ferric phosphate in the reaction system, ensuring that the generated ferric phosphate has a plate-like structure instead of an octahedral structure with a similar nucleation potential energy. Simultaneously, slow oxidation is used to control the ferric phosphate formation rate, allowing the generated ferric phosphate to grow and mature on the plate-like structure of the seed crystals. This results in plate-like ferric phosphate with a regular morphology, uniform size, and more complete crystal growth, leading to a higher iron-to-phosphorus ratio (closer to the theoretical value of 1.0) and a moderate specific surface area. This solves the problems of low iron-to-phosphorus ratio, inconsistent morphology and size, and excessively large or small specific surface area in ferric phosphate prepared by existing iron-based processes.

[0058] In the technical solution of this application embodiment, the dropping time of the oxidant in steps S2 and S3 has a significant impact on the microstructure and crystal structure of the final obtained flaky high-iron-phosphorus iron phosphate. Specifically, under seedless conditions, if the oxidation rate is too slow, there are insufficient crystal nuclei in the system. The precipitate generated during the oxidation process continues to grow on the original crystal nuclei, growing from irregular flaky to octahedral structure (the specific process is irregular flaky-regular flaky-octahedral structure, but the conditions for generating regular flaky structure are harsh and difficult to control under seedless conditions. Extending the oxidation time generally results in a direct transformation from irregular flaky structure to octahedral structure). Under conditions with an appropriate amount of crystal nuclei, the system has a certain number of crystal nuclei, and the precipitate generated during the oxidation process grows on the crystal nuclei, perfecting the structure of the crystal nuclei and obtaining a flaky structure with a regular morphology. Therefore, in step S2, a shorter dropping time is selected, resulting in a faster oxidation rate and a large number of nuclei forming in the system. Since the crystal nuclei do not have time to grow, a large number of irregular plate-like structures are generated. In step S3, a longer dropping time is selected so that hydrogen peroxide is added slowly to oxidize and generate precipitate, allowing the generated precipitate to continue to grow on the seed crystal, thereby obtaining a regular plate-like structure. If the dropping time is too short, too much precipitate will be generated in a short time, causing agglomeration and affecting the microstructure, resulting in an irregular plate-like structure and a large specific surface area of ​​the obtained iron phosphate.

[0059] It is understood that in this application, the first ferrous dihydrogen phosphate solution and the second ferrous dihydrogen phosphate solution can be the same or different. This application does not limit this, and those skilled in the art can choose according to the actual situation.

[0060] Furthermore, in some embodiments, in step S1, the concentrations of the first ferrous dihydrogen phosphate solution and the second ferrous dihydrogen phosphate solution are both 0.7 to 1.1 mol / L, including but not limited to 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1.0 mol / L, 1.1 mol / L, etc., and this application does not impose any restrictions on them.

[0061] In the technical solution of this application embodiment, by controlling the concentrations of the first ferrous dihydrogen phosphate solution and the second ferrous dihydrogen phosphate solution within the above-mentioned range, it can be ensured that the first ferrous dihydrogen phosphate solution and the second ferrous dihydrogen phosphate solution in the system are not prone to crystallization, which is beneficial to process control. At the same time, the amount of mother liquor is small. If the concentration is too high, the ferrous solution will have a large viscosity and be easy to crystallize. If the concentration is too low, a large amount of reaction mother liquor needs to be treated, which is not conducive to industrial scale-up.

[0062] Furthermore, in some embodiments, step S1, the step of preparing the first ferrous dihydrogen phosphate solution or the second ferrous dihydrogen phosphate solution, includes: reacting dilute phosphoric acid with iron powder, and filtering the slurry after the reaction is completed to obtain the first ferrous dihydrogen phosphate solution or the second ferrous dihydrogen phosphate solution.

[0063] In the technical solution of this application embodiment, unreacted iron can be removed by filtration.

[0064] Furthermore, in some embodiments, the mass fraction of dilute phosphoric acid is 20% to 35%, including but not limited to 20%, 25%, 30%, 35%, etc., and this application does not limit it.

[0065] Furthermore, in some specific embodiments, dilute phosphoric acid is obtained by diluting refined phosphoric acid with pure water at a mass fraction of ≥75%.

[0066] Furthermore, in some embodiments, the molar ratio of iron powder to phosphoric acid is (0.35 to 0.5):1, including but not limited to 0.35:1, 0.4:1, 0.45:1, 0.5:1, etc., and this application does not limit it.

[0067] In the technical solution of this application embodiment, by controlling the molar ratio of iron powder to phosphoric acid within the above-mentioned range, ferrous iron can be made more stable under acidic conditions. If the amount of iron powder is excessive, the pH value of the obtained ferrous dihydrogen phosphate solution will be too high, and ferrous iron will be easily oxidized.

[0068] In this application, it should be noted that the complete reaction system of iron powder and phosphoric acid refers to the reaction system in which iron powder and phosphoric acid react completely to produce ferrous dihydrogen phosphate. The specific reaction equation is as follows:

[0069] Fe + 2H3PO4 → Fe(H2PO4)2 + H2.

[0070] Furthermore, in some embodiments, the reaction temperature during the reaction of dilute phosphoric acid and iron powder is 55°C to 70°C, including but not limited to 55°C, 60°C, 65°C, 70°C, etc., and this application does not limit it.

[0071] Furthermore, in some embodiments, the endpoint of the reaction between dilute phosphoric acid and iron powder is: system pH > 1.45, ferrous iron content > 1.3 mol / L.

[0072] Furthermore, in some embodiments, the step of preparing the first or second ferrous dihydrogen phosphate solution further includes diluting the filtrate to the target concentration.

[0073] In the technical solution of this application embodiment, by diluting the filtrate, the concentration of ferrous dihydrogen phosphate solution can be controlled within the target range.

[0074] Further, in some embodiments, step S1, the step of providing the first ferrous dihydrogen phosphate solution or the second ferrous dihydrogen phosphate solution includes: heating dilute phosphoric acid to the reaction temperature, then adding iron powder to the dilute phosphoric acid, filtering the slurry after the reaction is completed, and then diluting the filtrate to obtain the first ferrous dihydrogen phosphate solution or the second ferrous dihydrogen phosphate solution.

[0075] Furthermore, in some embodiments, in step S2, the molar ratio of oxidant to ferrous iron is (0.6 to 0.9):1, including but not limited to 0.6:1, 0.7:1, 0.8:1, 0.9:1, etc., and this application does not limit it.

[0076] In the technical solution of this application embodiment, by controlling the amount of oxidant added within the above-mentioned range, Fe can be made 2+ It is fully oxidized, and the waste of oxidant is avoided.

[0077] Furthermore, in some embodiments, in step S2, the oxidant includes at least one of hydrogen peroxide, ammonium persulfate, and sodium persulfate.

[0078] Furthermore, in some preferred embodiments, the oxidant in step S2 is hydrogen peroxide. This application does not limit the mass fraction of hydrogen peroxide used, and those skilled in the art can select it according to actual conditions. In some specific embodiments, the mass fraction of hydrogen peroxide is 20% to 30%, including but not limited to 20%, 22%, 24%, 26%, 28%, 30%, etc., and this application does not limit this.

[0079] Further, in some embodiments, in step S2, the first ferrous dihydrogen phosphate solution is heated to 70°C to 90°C, including but not limited to 70°C, 75°C, 80°C, 85°C, 90°C, etc., which is not limited in this application; then the oxidant is added dropwise uniformly, and the first dropwise addition time is 5 min to 15 min, including but not limited to 5 min, 7 min, 9 min, 11 min, 13 min, 15 min, etc., which is not limited in this application; after the dropwise addition is completed, the temperature is maintained for 60 min to 120 min, including but not limited to 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, 120 min, etc., to obtain the seed slurry.

[0080] In the technical solution of this application embodiment, by controlling the temperature of step S2 within a certain range, crystalline ferric phosphate dihydrate can be generated in the system in one step. If the temperature is too high, the slurry boils during the addition of the oxidant, making the addition operation difficult and causing severe decomposition of the oxidant. If the temperature is too low, the slurry cannot undergo complete crystal transformation or the process time is too long. Simultaneously, by controlling the addition time within a certain range, irregularly shaped ferric phosphate dihydrate seed crystals can be obtained. If the addition time is too short, the oxidant in the reaction solution becomes too concentrated locally, the local temperature rise is too high, and hydrogen peroxide decomposes violently. If the addition time is too long, blocky ferric phosphate is generated, and the desired shaped ferric phosphate seed crystals cannot be obtained. Furthermore, by controlling the holding time within a certain range, the reaction can be completed without excessive time. If the holding time is too short, the system will not react completely; if the holding time is too long, it will affect the production cycle.

[0081] Furthermore, in some embodiments, in step S3, the amount of seed slurry added is 5% to 15% of the mass of the second ferrous dihydrogen phosphate solution, including but not limited to 5%, 7%, 9%, 11%, 13%, 15%, etc., and this application does not limit this.

[0082] In the technical solution of this application embodiment, by controlling the amount of seed slurry added within this range, a specified plate-like crystal can be obtained, and the formation of other crystal forms can be avoided. If the amount of seed slurry added is too high, the crystallinity will be reduced, resulting in irregular plate-like crystals; if the amount of seed slurry added is too low, other crystal forms (orthorhombic structure, with an octahedral microstructure) will be generated.

[0083] Furthermore, in some embodiments, in step S3, the molar ratio of oxidant to ferrous iron is (0.6 to 0.9):1, including but not limited to 0.6:1, 0.7:1, 0.8:1, 0.9:1, etc., and this application does not limit it.

[0084] In the technical solution of this application embodiment, by controlling the amount of oxidant added within the above-mentioned range, Fe can be made 2+ It is fully oxidized, and the waste of oxidant is avoided.

[0085] Furthermore, in some embodiments, in step S3, the oxidant includes at least one of hydrogen peroxide, ammonium persulfate, and sodium persulfate.

[0086] Furthermore, in some preferred embodiments, the oxidant in step S3 is hydrogen peroxide. This application does not limit the mass fraction of hydrogen peroxide used, and those skilled in the art can select it according to actual conditions. In some specific embodiments, the mass fraction of hydrogen peroxide is 20% to 30%, including but not limited to 20%, 22%, 24%, 26%, 28%, 30%, etc., and this application does not impose any limitation on this.

[0087] Further, in some embodiments, in step S3, the seed slurry is added to the second ferrous dihydrogen phosphate solution and the temperature is raised to 70°C to 90°C, including but not limited to 70°C, 75°C, 80°C, 85°C, 90°C, etc., which are not limited in this application; then the oxidant is added uniformly, and the second addition time is 30 min to 90 min, including but not limited to 30 min, 40 min, 50 min, 60 min, 70 min, 80 min, 90 min, etc., which are not limited in this application; after the addition is completed, the temperature is maintained for 60 min to 120 min, including but not limited to 60 min, 70 min, 80 min, 90 min, 100 min, 110 min, 120 min, etc., to obtain ferric phosphate dihydrate slurry.

[0088] In this embodiment, by controlling the temperature within this range, crystalline ferric phosphate dihydrate can be generated in one step within the system. If the temperature is too high, the solution boils during the oxidant addition process, making the addition difficult and causing severe decomposition of the oxidant; if the temperature is too low, the slurry cannot undergo complete crystal transformation or the process time is too long. By controlling the addition time within this range, the generated ferric phosphate precipitate has sufficient growth time on the seed crystal, resulting in a regular morphology of the prepared ferric phosphate. If the addition time is too short, the precipitation rate is too fast, preventing complete growth on the seed crystal and resulting in an irregular morphology; if the addition time is too long, the crystals grow abnormally, leading to a decrease in specific surface area. By controlling the holding time within this range, the reaction can be completed without excessive time. If the holding time is too short, the system will not react completely; if the holding time is too long, it will affect the production cycle.

[0089] Furthermore, in some embodiments, in step S4, the washing method is countercurrent washing.

[0090] Furthermore, in some embodiments, in step S4, the washing continues until the conductivity of the deionized water is ≤200 μS / cm.

[0091] In the technical solution of this application embodiment, by washing until the conductivity of the deionized water is ≤200μS / cm, impurity ions (mainly phosphate ions) present in the slurry can be better removed.

[0092] Furthermore, in some embodiments, in step S4, the drying temperature is 90℃~110℃, including but not limited to 90℃, 95℃, 100℃, 105℃, 110℃, etc., and this application does not limit it; the drying time is 8h~24h, including but not limited to 8h, 12h, 16h, 20h, 24h, etc., and this application does not limit it.

[0093] Furthermore, in some embodiments, in step S4, the sintering temperature is 600℃~700℃, including but not limited to 600℃, 620℃, 640℃, 660℃, 680℃, 700℃, etc., and this application does not limit it; the sintering time is 2h~4h, including but not limited to 2h, 2.5h, 3h, 3.5h, 4h, etc., and this application does not limit it.

[0094] In the technical solution of this application embodiment, by controlling the sintering temperature and sintering time within this range, ferric phosphate dihydrate can be transformed into flake-shaped high-iron-phosphorus ratio ferric phosphate through high-temperature sintering. If the sintering temperature is below 600°C, pure-phase anhydrous ferric phosphate cannot be obtained; if the sintering temperature is above 700°C, a quartz-type structure with low chemical activity will be generated.

[0095] Thirdly, embodiments of this application provide a lithium iron phosphate cathode material, which is prepared by uniformly mixing sheet-like high iron-phosphorus ratio iron phosphate provided in the first aspect of this application with a lithium source and then by a high-temperature solid-state method.

[0096] The lithium iron phosphate cathode material of this application is prepared from the above-mentioned sheet-like high iron-phosphorus ratio iron phosphate, and therefore has the advantages of excellent processing performance and high energy density.

[0097] Fourthly, embodiments of this application provide a positive electrode sheet, including the lithium iron phosphate positive electrode material provided in the third aspect of this application.

[0098] The positive electrode of this application includes the above-mentioned lithium iron phosphate positive electrode material, and therefore has the advantages of excellent processing performance and high energy density.

[0099] Fifthly, embodiments of this application provide a secondary battery, including the positive electrode sheet provided in the fourth aspect of this application.

[0100] The secondary battery of this application includes the above-mentioned positive electrode sheet, and therefore has the advantages of excellent processing performance and high energy density.

[0101] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0102] I. Preparation Method

[0103] Example 1

[0104] ① Preparation of ferrous dihydrogen phosphate: Take refined phosphoric acid (mass fraction 85%), add pure water to prepare a 30% (mass fraction) dilute phosphoric acid solution for later use. Heat the dilute phosphoric acid solution to 65℃, and after reaching the temperature, add an appropriate amount of high-purity iron powder (iron powder purity 99.2%) to the dilute phosphoric acid solution. Fe / n H3PO4 =0.4:1.0), after the reaction is complete, the slurry is filtered to obtain a ferrous dihydrogen phosphate solution;

[0105] ② Preparation of seed crystals: Take the ferrous dihydrogen phosphate solution from step ①, add pure water to prepare a ferrous dihydrogen phosphate solution with a molar concentration of 1.0 mol / L. Heat the prepared ferrous dihydrogen phosphate solution to 80℃. After heating, uniformly add hydrogen peroxide (25% by mass) dropwise to the ferrous dihydrogen phosphate solution. H2O2 / n Fe2+ =0.75:1.0), control the dropping time to 10 min, and keep warm for 90 min after dropping to obtain seed slurry;

[0106] ③ Preparation of ferric phosphate dihydrate: Take the ferrous dihydrogen phosphate solution from step ①, add pure water to prepare a ferrous dihydrogen phosphate solution with a molar concentration of 1.0 mol / L. Simultaneously, add 10% (by mass) of the seed slurry from step ② to the ferrous dihydrogen phosphate solution. After preparation, heat the mixed slurry to 80℃. After heating, uniformly add hydrogen peroxide (25% by mass) dropwise to the ferrous dihydrogen phosphate solution. H2O2 / n Fe2+ =0.75:1.0), control the dropping time to 60 min, and keep warm for 90 min after dropping is completed to obtain ferric phosphate dihydrate slurry with complete reaction;

[0107] ④ Preparation of flake-shaped high-iron-phosphorus ratio ferric phosphate: The slurry from step ③ was filtered, washed countercurrently, dried at 95℃ for 8 hours, and sintered at 650℃ for 2 hours to obtain flake-shaped high-iron-phosphorus ratio ferric phosphate.

[0108] Example 2

[0109] ① Preparation of ferrous dihydrogen phosphate: Take refined phosphoric acid (mass fraction 85%), add pure water to prepare a 35% (mass fraction) dilute phosphoric acid solution for later use. Heat the dilute phosphoric acid solution to 70℃, and after reaching the temperature, add an appropriate amount of high-purity iron powder (iron powder purity 99.2%) to the dilute phosphoric acid solution. Fe / n H3PO4 =0.5:1.0), after the reaction is complete, the slurry is filtered to obtain ferrous dihydrogen phosphate solution;

[0110] ② Preparation of seed crystals: Take the ferrous dihydrogen phosphate solution from step ①, add pure water to prepare a ferrous dihydrogen phosphate solution with a molar concentration of 0.7 mol / L. Heat the prepared ferrous dihydrogen phosphate solution to 70℃. After heating, uniformly add hydrogen peroxide (20% by mass) dropwise to the ferrous dihydrogen phosphate solution. H2O2 / n Fe2+ =0.60:1.0), control the dropping time to 5 min, and keep warm for 60 min after dropping to obtain seed slurry;

[0111] ③ Preparation of ferric phosphate dihydrate: Take the ferrous dihydrogen phosphate solution from step ①, add pure water to prepare a ferrous dihydrogen phosphate solution with a molar concentration of 0.7 mol / L. Simultaneously, add 5% (by mass) of the seed slurry from step ② to the ferrous dihydrogen phosphate solution. After preparation, heat the mixed slurry to 70℃. After heating, uniformly add hydrogen peroxide (20% by mass, nH₂O₂ / nFe) dropwise to the ferrous dihydrogen phosphate solution. 2+ =0.60:1.0), control the dropping time to 30 min, and keep warm for 60 min after dropping to obtain ferric phosphate dihydrate slurry with complete reaction;

[0112] ④ Preparation of flake-shaped high-iron-phosphorus ratio iron phosphate: The slurry from step ③ was filtered, washed countercurrently, dried at 90℃ for 10h, and sintered at 600℃ for 3h to obtain flake-shaped high-iron-phosphorus ratio iron phosphate.

[0113] Example 3

[0114] ① Preparation of ferrous dihydrogen phosphate: Take refined phosphoric acid (mass fraction 85%), add pure water to prepare a 25% (mass fraction) dilute phosphoric acid solution for later use. Heat the dilute phosphoric acid solution to 55℃, and after reaching the temperature, add an appropriate amount of high-purity iron powder (iron powder purity 99.2%) to the dilute phosphoric acid solution. Fe / n H3PO4 =0.35:1.0), after the reaction is complete, the slurry is filtered to obtain a ferrous dihydrogen phosphate solution;

[0115] ② Preparation of seed crystals: Take the ferrous dihydrogen phosphate solution from step ①, add pure water to prepare a ferrous dihydrogen phosphate solution with a molar concentration of 1.1 mol / L. Heat the prepared ferrous dihydrogen phosphate solution to 90℃. After heating, uniformly add hydrogen peroxide (30% by mass) dropwise to the ferrous dihydrogen phosphate solution. H2O2 / n Fe2+ =0.90:1.0), control the dropping time to 15 min, and keep warm for 120 min after dropping to obtain seed slurry;

[0116] ③ Preparation of ferric phosphate dihydrate: Take the ferrous dihydrogen phosphate solution from step ①, add pure water to prepare a ferrous dihydrogen phosphate solution with a molar concentration of 1.1 mol / L. Simultaneously, add 15% (by mass) of the seed slurry from step ② to the ferrous dihydrogen phosphate solution. After preparation, heat the mixed slurry to 90℃. After heating, uniformly add hydrogen peroxide (30% by mass) dropwise to the ferrous dihydrogen phosphate solution. H2O2 / n Fe2+ =0.90:1.0), control the dropping time to 90 min, and keep warm for 120 min after dropping to obtain ferric phosphate dihydrate slurry with complete reaction;

[0117] ④ Preparation of flake-shaped high-iron-phosphorus ratio iron phosphate: The slurry from step ③ was filtered, washed countercurrently, dried at 110℃ for 12h, and sintered at 700℃ for 2h to obtain flake-shaped high-iron-phosphorus ratio iron phosphate.

[0118] Example 4

[0119] ① Preparation of ferrous dihydrogen phosphate: Take refined phosphoric acid (mass fraction 85%), add pure water to prepare a 30% (mass fraction) dilute phosphoric acid solution for later use. Heat the dilute phosphoric acid solution to 60℃, and after reaching the temperature, add an appropriate amount of high-purity iron powder (iron powder purity 99.2%) to the dilute phosphoric acid solution. Fe / n H3PO4 =0.45:1.0), after the reaction is complete, the slurry is filtered to obtain a ferrous dihydrogen phosphate solution;

[0120] ② Preparation of seed crystals: Take the ferrous dihydrogen phosphate solution from step ①, add pure water to prepare a ferrous dihydrogen phosphate solution with a molar concentration of 0.9 mol / L. Heat the prepared ferrous dihydrogen phosphate solution to 85℃. After heating, uniformly add hydrogen peroxide (25% by mass) dropwise to the ferrous dihydrogen phosphate solution. H2O2 / n Fe2+ =0.70:1.0), control the dropping time to 12 min, and keep warm for 75 min after dropping to obtain seed slurry;

[0121] ③ Preparation of ferric phosphate dihydrate: Take the ferrous dihydrogen phosphate solution from step ①, add pure water to prepare a ferrous dihydrogen phosphate solution with a molar concentration of 0.9 mol / L. Simultaneously, add 10% (by mass) of the seed slurry from step ② to the ferrous dihydrogen phosphate solution. After preparation, heat the mixed slurry to 85℃. After heating, uniformly add hydrogen peroxide (25% by mass) dropwise to the ferrous dihydrogen phosphate solution. H2O2 / n Fe2+ =0.70:1.0), control the dropping time to 60 min, and keep warm for 75 min after dropping is completed to obtain ferric phosphate dihydrate slurry with complete reaction;

[0122] ④ Preparation of flake-shaped high-iron-phosphorus ratio iron phosphate: The slurry from step ③ was filtered, washed countercurrently, dried at 95℃ for 16h, and sintered at 650℃ for 3h to obtain flake-shaped high-iron-phosphorus ratio iron phosphate.

[0123] Example 5

[0124] ① Preparation of ferrous dihydrogen phosphate: Take refined phosphoric acid (mass fraction 85%), add pure water to prepare a 30% (mass fraction) dilute phosphoric acid solution for later use. Heat the dilute phosphoric acid solution to 60℃, and after reaching the temperature, add an appropriate amount of high-purity iron powder (iron powder purity 99.2%) to the dilute phosphoric acid solution. Fe / n H3PO4 =0.45:1.0), after the reaction is complete, the slurry is filtered to obtain a ferrous dihydrogen phosphate solution;

[0125] ② Preparation of seed crystals: Take the ferrous dihydrogen phosphate solution from step ①, add pure water to prepare a ferrous dihydrogen phosphate solution with a molar concentration of 1.0 mol / L. Heat the prepared ferrous dihydrogen phosphate solution to 90℃. After heating, uniformly add hydrogen peroxide (25% by mass) dropwise to the ferrous dihydrogen phosphate solution. H2O2 / n Fe2+ =0.75:1.0), control the dropping time to 10 min, and keep warm for 60 min after dropping to obtain seed slurry;

[0126] ③ Preparation of ferric phosphate dihydrate: Take the ferrous dihydrogen phosphate solution from step ①, add pure water to prepare a ferrous dihydrogen phosphate solution with a molar concentration of 1.0 mol / L. Simultaneously, add 5% (by mass) of the seed slurry from step ② to the ferrous dihydrogen phosphate solution. After preparation, heat the mixed slurry to 90℃. After heating, uniformly add hydrogen peroxide (25% by mass) dropwise to the ferrous dihydrogen phosphate solution. H2O2 / n Fe2+ =0.75:1.0), control the dropping time to 90 min, and keep warm for 60 min after dropping is completed to obtain ferric phosphate dihydrate slurry with complete reaction;

[0127] ④ Preparation of flake-shaped high-iron-phosphorus ratio iron phosphate: The slurry from step ③ was filtered, washed countercurrently, dried at 95℃ for 16h, and sintered at 650℃ for 2.5h to obtain flake-shaped high-iron-phosphorus ratio iron phosphate.

[0128] Comparative Example 1

[0129] The seed slurry prepared in step ② is used as the reaction completion slurry for subsequent filtration, washing, drying and sintering processes. Other steps and parameter settings are the same as in Example 1.

[0130] Comparative Example 2

[0131] Step ③ Preparation of ferric phosphate dihydrate: Take the ferrous dihydrogen phosphate solution from step ①, add pure water to prepare a ferrous dihydrogen phosphate solution with a molar concentration of 1.0 mol / L, and uniformly add hydrogen peroxide (mass fraction of hydrogen peroxide is 25%) to the ferrous dihydrogen phosphate solution. H2O2 / n Fe2+ =0.75:1.0), and the dropping time was controlled at 60 min to completely oxidize the ferrous iron in the ferrous solution. After oxidation, 10% of the seed slurry from step ② was added to the solution. After preparation, the mixed slurry was heated to 80°C and held at that temperature for 90 min to obtain the ferric phosphate slurry after the reaction was completed. Other steps and parameter settings were the same as in Example 1.

[0132] Comparative Example 3

[0133] The hydrogen peroxide dropping time in step ② of the seed crystal preparation process was adjusted to 60 min. All other steps and parameter settings were the same as in Example 1.

[0134] II. Testing Methods

[0135] 1. SEM testing

[0136] The iron phosphate prepared in the examples and comparative examples was detected by SEM, and the test results are shown in Figures 2-9.

[0137] 2. Fe and P content test

[0138] Iron content was determined by potassium dichromate titration, and phosphorus content was determined by quinoline phosphomolybdate gravimetric method.

[0139] 3. Specific surface area test

[0140] The specific surface area was measured using a specific surface area meter via nitrogen adsorption method.

[0141] 4. Microscopic size testing

[0142] Microscopic morphology was tested using a scanning electron microscope.

[0143] 5. D100 test

[0144] Particle size was measured using a Malvern laser particle size analyzer.

[0145] III. Test Results and Analysis of Each Embodiment and Comparative Example

[0146] Table 1. Test Data for Examples and Comparative Examples

[0147] Table 2. Test Data for Examples and Comparative Examples

[0148] As can be seen from Tables 1-2 and Figures 2-6, the iron phosphate obtained in the embodiments of this application all have a high iron-to-phosphorus ratio and a moderate specific surface area, and all have a plate-like structure.

[0149] Compared with Example 1, Comparative Example 1 directly used the iron phosphate slurry prepared in the seed crystal preparation process as the reaction completion slurry. The resulting iron phosphate had a lower iron-to-phosphorus ratio and a larger specific surface area. The reason is that the oxidation time of Comparative Example 1 was shorter, and a large number of crystal nuclei were formed in a short time. The surface energy of the crystal nuclei was large, and they were easy to form agglomerates (as shown in Figure 7). The rinsing process could not completely wash away the adsorbed and encapsulated phosphate ions, resulting in a lower iron-to-phosphorus ratio.

[0150] Compared to Example 1, in the iron phosphate synthesis steps of Comparative Example 2, hydrogen peroxide was added first, followed by the seed slurry. The resulting iron phosphate had a lower iron-to-phosphorus ratio. This is because in Comparative Example 2, the ferrous dihydrogen phosphate solution underwent pre-oxidation, resulting in an uncontrollable nucleation process and significant agglomeration of the prepared microstructure (as shown in Figure 8). The rinsing process was insufficient to completely remove the adsorbed and encapsulated phosphate ions, leading to a low iron-to-phosphorus ratio. Therefore, the order of addition of the seed slurry plays a crucial role in the synthesis of the plate-like high iron-to-phosphorus ratio iron phosphate of this application.

[0151] Compared with Example 1, Comparative Example 3 extended the oxidation time during the seed preparation process, resulting in iron phosphate with a lower iron-to-phosphorus ratio and a smaller specific surface area. This is because the oxidation time during the seed preparation process was extended, and the oxidation rate was too slow without seed crystals. As a result, there were insufficient crystal nuclei in the system, the precipitate generated during the oxidation process continued to grow on the original crystal nuclei, growing from irregular sheet-like structures to octahedral structures, ultimately yielding orthorhombic iron phosphate with an octahedral microstructure.

[0152] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0153] The embodiments described above are merely illustrative of several implementation methods of this application, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of this application, and these all fall within the protection scope of this application. Therefore, the protection scope of this patent application should be determined by the appended claims.

Claims

1. A flake-shaped high-iron-phosphorus iron phosphate, characterized in that, The flaky ferric phosphate has an Fe / P ratio greater than 0.99 and satisfies the following conditions: the length, width, and thickness ratio of the flaky structure is (105-130):(90-100):(10-12), and the thickness is 3.5m. 2 / g≤Specific surface area≤6.5m² 2 / g, particle size <35μm.

2. A method for preparing the flaky high-phosphorus-ratio iron phosphate as described in claim 1, characterized in that, Includes the following steps: Provide a first ferrous dihydrogen phosphate solution and a second ferrous dihydrogen phosphate solution; After heating the first ferrous dihydrogen phosphate solution, the oxidant is gradually added dropwise over the first dropping time. After the addition is completed, the solution is kept at the temperature to obtain the seed slurry. The seed slurry was added to the second ferrous dihydrogen phosphate solution and the temperature was raised. Then, the oxidant was gradually added dropwise over the second adding time. After the addition was completed, the temperature was maintained to obtain ferric phosphate dihydrate slurry. The ferric phosphate dihydrate slurry was filtered, washed, dried, and sintered to obtain flaky high ferric phosphate with a high ferric-to-phosphorus ratio. Wherein, the first dripping time is less than the second dripping time.

3. The method for preparing flake-shaped high-phosphorus-ratio iron phosphate according to claim 2, characterized in that, The concentrations of the first ferrous dihydrogen phosphate solution and the second ferrous dihydrogen phosphate solution are both 0.7 mol / L to 1.1 mol / L. The specific preparation steps for both are as follows: react dilute phosphoric acid with iron powder, and after the reaction is completed, filter the slurry to obtain the first ferrous dihydrogen phosphate solution or the second ferrous dihydrogen phosphate solution. The mass fraction of the dilute phosphoric acid is 20% to 35%, and the molar ratio of iron powder to phosphoric acid is (0.35 to 0.5):

1.

4. The method for preparing flake-shaped high-phosphorus-ratio iron phosphate according to claim 2, characterized in that, The specific steps for preparing the seed slurry are as follows: The first ferrous dihydrogen phosphate solution is heated to 70℃~90℃, and then an oxidant is added dropwise evenly. The first dropwise addition time is 5min~15min. After the dropwise addition is completed, the temperature is maintained for 60min~120min to obtain the seed slurry.

5. The method for preparing flake-shaped high-phosphorus-ratio iron phosphate according to claim 2, characterized in that, The specific steps for preparing the ferric phosphate dihydrate slurry are as follows: The seed slurry was added to the second ferrous dihydrogen phosphate solution and the temperature was raised to 70°C–90°C. Then, the oxidant was added dropwise uniformly over a period of 30–90 minutes. After the addition was completed, the temperature was maintained for 60–120 minutes to obtain the desired solution. Ferric phosphate dihydrate slurry.

6. The method for preparing flake-shaped high-phosphorus-ratio iron phosphate according to claim 2, characterized in that, In the preparation steps of the seed slurry and the preparation steps of the ferric phosphate dihydrate slurry, the molar ratio of the oxidant to divalent iron is (0.6-0.9):

1. The oxidant includes at least one of hydrogen peroxide, ammonium persulfate, and sodium persulfate.

7. The method for preparing flaky high-phosphorus iron phosphate according to claim 2, characterized in that, In the preparation steps of the flaky high-iron-phosphorus iron phosphate, the washing method is countercurrent washing, and the washing is carried out until the conductivity of the deionized water is ≤200μS / cm; the drying temperature is 90℃~110℃, and the drying time is 8h~24h; the sintering temperature is 600℃~700℃, and the sintering time is 2h~4h.

8. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared by uniformly mixing the sheet-like high iron-phosphorus ratio iron phosphate as described in claim 1 with a lithium source and then using a high-temperature solid-state method.

9. A positive electrode sheet, characterized in that, The positive electrode sheet includes the lithium iron phosphate positive electrode material as described in claim 8.

10. A secondary battery, characterized in that, The secondary battery includes the positive electrode sheet as described in claim 9.

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