Lithium iron phosphate material and preparation method therefor, and positive electrode sheet and secondary battery

Abstract

The present application belongs to the field of battery materials, and provides a lithium iron phosphate material and a preparation method therefor, a positive electrode sheet, and a secondary battery. The lithium iron phosphate material comprises an inner core with a molecular formula LiaFe(PO4)b(BO3)c, where 1.02≤a≤1.05, 0.98≤b≤1.08, and 0.01≤c≤0.02, and the percentage of the number of particles having a primary particle size of 100 nm to 600 nm is ≥93.5%. The lithium iron phosphate material provided in the present application has a concentrated primary particle size range, with the number of particles having a primary particle size in the range of 100 nm to 600 nm being greater than or equal to 93.5%, which enables the lithium iron phosphate material to have good consistency during a discharge process. This avoids the problems of a low energy efficiency caused by large particles and a poor cycling performance caused by small particles, thus ensuring that the battery has the advantages of both a high energy efficiency and a good cycling performance.

Description

Lithium iron phosphate materials and their preparation methods, positive electrode sheets and secondary batteries Technical Field

[0001] This application relates to the field of battery materials technology, specifically to a lithium iron phosphate material and its preparation method, a positive electrode sheet, and a secondary battery. Background Technology

[0002] Lithium iron phosphate (LFP), with the molecular formula LiFePO4, is a water-insoluble, grayish-white solid, often appearing black due to its carbon coating. Because of its widely available and relatively inexpensive raw materials, its environmental friendliness, and the absence of harmful heavy metals, coupled with the difficulty of oxygen release from its olivine structure, LFP exhibits excellent stability and is a safe cathode material for lithium-ion batteries. Therefore, it has attracted extensive research and rapid development.

[0003] Lithium iron phosphate (LFP) is used as the cathode material in batteries (such as those used in new energy vehicles), and the uniformity of its particle size is crucial. However, the primary particle size distribution of LFP materials prepared by traditional solid-state methods is extremely uneven, leading to inconsistencies in its performance during discharge, resulting in lower battery efficiency and poorer cycle performance.

[0004] Summary of the Invention

[0005] In view of the technical problems existing in the background art, this application provides a lithium iron phosphate material, its preparation method and application, aiming to solve the technical problem of uneven primary particle size distribution of lithium iron phosphate material prepared by solid phase method in the traditional technology.

[0006] In a first aspect, embodiments of this application provide a lithium iron phosphate material, the lithium iron phosphate material comprising a core, the core having the molecular formula Li. a Fe(PO4) b (BO3) c Among them, 1.02≤a≤1.05, 0.98≤b≤1.08, 0.01≤c≤0.02, and the proportion of particles with a primary diameter of 100nm~600nm is ≥93.5%.

[0007] In the technical solution of this application embodiment, the provided lithium iron phosphate material has a concentrated primary particle size range, with the number of particles in the primary particle size range of 100nm to 600nm being greater than or equal to 93.5%, which makes it have good consistency during discharge, avoiding the problems of low energy efficiency caused by large particles and poor cycle performance caused by small particles, thereby ensuring that the battery has the advantages of both high energy efficiency and good cycle performance.

[0008] In some embodiments, the primary particle size of the lithium iron phosphate material is 0.5 to 2.5 mm, and the D50 particle size of the primary particle size is 140 nm to 320 nm.

[0009] The compacted density of the lithium iron phosphate material is 2.20 g / mL to 2.60 g / mL, and the BET specific surface area of ​​the lithium iron phosphate material is 8.0 m². 2 / g~15.0m 2 / g, the resistivity of the lithium iron phosphate material powder is 5.00Ω·cm~22.00Ω·cm.

[0010] In this embodiment, the primary particle size of the lithium iron phosphate material is not only uniformly distributed, but also relatively small, which makes the internal arrangement of the lithium iron phosphate material more uniform, which helps to form a balanced voltage platform and keep the battery stable during operation.

[0011] Meanwhile, by controlling the compaction density, BET specific surface area, and powder resistivity of lithium iron phosphate materials within the aforementioned ranges, it is not only beneficial for lithium iron phosphate materials to have good electronic conductivity and high energy density, but also beneficial for controlling the contact area between lithium iron phosphate materials and electrolytes, reducing the occurrence of side reactions between lithium iron phosphate materials and electrolytes, and improving the specific capacity, discharge efficiency, and cycle performance of batteries.

[0012] In some embodiments, the lithium iron phosphate material further includes a coating layer that coats the surface of the core; wherein the coating layer is made of carbon material, and the carbon material accounts for 1.10% to 2.00% of the mass of the lithium iron phosphate material.

[0013] In this embodiment, controlling the mass fraction of carbon material in the coating layer within the above-mentioned range effectively improves the electronic conductivity of the lithium iron phosphate material.

[0014] Secondly, embodiments of this application provide a method for preparing lithium iron phosphate material, comprising the following steps:

[0015] A mixture of ferrous ion source, oxalate ion source, borate ion source and first regulator is obtained by first reaction treatment in a base solution.

[0016] The mixture is subjected to solid-liquid separation and a first pulping process to obtain a first slurry;

[0017] After the first slurry is mixed with the first lithium source, the first phosphorus source and the second regulator, it is subjected to the first grinding process and the first drying process to obtain the first dried material;

[0018] After mixing the trivalent iron source, the second phosphorus source, the second lithium source and the carbon source, the mixture undergoes a second pulping treatment, a second grinding treatment and a second drying treatment to obtain the second dried material.

[0019] The first dried material and the second dried material are mixed and then calcined to obtain the lithium iron phosphate material.

[0020] In the technical solution of this application embodiment, firstly, raw materials such as ferrous iron source, oxalate ion source, borate ion source, first lithium source and second phosphorus source are used. After stirring reaction, solid-liquid separation, first slurrying, first grinding and first drying, a first dry material with a lower reaction temperature required to form lithium iron phosphate is obtained. At the same time, raw materials such as trivalent iron source, second phosphorus source, second lithium source and carbon source are used. After second slurrying, second grinding and second drying, a second dry material with a higher reaction temperature required to form lithium iron phosphate is obtained. The first dry material and the second dry material with different reaction temperatures are mixed and calcined. The first dry material first undergoes a crystallization reaction to form crystal nuclei. The second dry material preferentially nucleates and continues to grow on the crystal nuclei formed by the first dry material. This achieves stepwise nucleation and growth, avoiding the problem of extremely uneven primary particle size distribution caused by disordered growth and instantaneous rapid nucleation of lithium iron phosphate crystal nuclei. As a result, a lithium iron phosphate material with uniform primary particle size distribution and high primary particle size concentration is obtained.

[0021] In some embodiments, the step of mixing the ferrous source, the oxalate ion source, the borate ion source, and the first regulator in the base solution and obtaining the mixture through a first reaction treatment includes:

[0022] The ferrous source, the oxalate ion source, the borate ion source, and the first regulator are mixed in a base solution and subjected to a first treatment time at a first temperature to obtain the mixture.

[0023] The pH value of the base solution is 4 to 5.5, the first temperature is 35°C to 55°C, and the first treatment time is 60 min to 120 min.

[0024] In this embodiment, by controlling the pH, temperature, and time during the first reaction process, the formation of ferrous oxalate can be promoted, avoiding the formation of ferric iron products. Ferrous oxalate has a low decomposition temperature, and its iron content is ferrous (Fe2+), requiring no high-temperature reduction by carbon. Therefore, using ferrous oxalate as a raw material can lower the reaction temperature for lithium iron phosphate formation, allowing crystal nuclei to form at a lower calcination temperature. This facilitates the subsequent heterogeneous nucleation and growth of the second drying material on the already formed nuclei, achieving a stepwise process of nucleation and growth. This avoids the problem of extremely uneven primary particle size distribution caused by disordered growth and rapid, instantaneous nucleation of lithium iron phosphate nuclei, resulting in a lithium iron phosphate material with uniform primary particle size distribution and high primary particle size concentration.

[0025] In some embodiments, the molar ratio of iron, oxalate, and borate in the mixture is 1:(0.8-1):(0.1-0.2).

[0026] The molar ratio of iron in the first slurry, lithium in the first lithium source, phosphorus in the first phosphorus source, and the second regulator is 1:(1.04~1.1):(1.04~1.1):(0.15~0.2);

[0027] The molar ratio of iron in the trivalent iron source, phosphorus in the second phosphorus source, lithium in the second lithium source, and carbon in the carbon source is 1:1:(1.02~1.04):(1.2~1.8);

[0028] The molar ratio of lithium element in the first lithium source to lithium element in the second lithium source is 1:(8-9).

[0029] In this embodiment, by controlling the molar ratio of the raw materials used in each step within the above-mentioned range, the uniformity of the primary particle size distribution of lithium iron phosphate material can be further improved. While increasing the compaction density and tap density, the powder resistivity is significantly reduced, thereby exhibiting the advantages of high specific capacity, good rate performance, high discharge efficiency and excellent cycle performance.

[0030] In some embodiments, the ferrous source is selected from at least one of ferrous sulfate, ferrous chloride, and ferrous acetate; the oxalate ion source includes at least one of oxalic acid, ammonium oxalate, and sodium oxalate; the borate ion source includes at least one of boric acid, ammonium borate, and sodium borate; and the first regulator includes at least one of ammonium bicarbonate and sodium bicarbonate.

[0031] The first phosphorus source is selected from at least one of phosphoric acid, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate; the first lithium source is selected from at least one of lithium oxalate and lithium acetate; and the second regulator includes at least one of polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, and polycarboxylic acid compounds.

[0032] The trivalent iron source includes iron phosphate, the second phosphorus source includes iron phosphate, the second lithium source includes lithium carbonate, and the carbon source is selected from at least one of glucose, sucrose, starch, and cellulose.

[0033] In this embodiment, selecting suitable ferrous ion sources, oxalate ion sources, borate ion sources, a first phosphorus source, and a first lithium source ensures that the prepared first dried material has a low reaction temperature and enables anion doping of the lithium iron phosphate material, improving its electrochemical performance. Selecting suitable trivalent iron sources, a second phosphorus source, a second lithium source, and a carbon source ensures that the prepared second dried material has a high reaction temperature and enables carbon coating treatment of the lithium iron phosphate material, enhancing its electronic conductivity.

[0034] In some embodiments, the step of mixing the first dried material and the second dried material, followed by calcination to obtain the lithium iron phosphate material includes:

[0035] The first dried material and the second dried material are mixed and then pulverized to obtain pulverized material. The pulverized material is subjected to a first protective atmosphere, a second temperature for a second treatment time, and then a third temperature for a third treatment time to obtain the lithium iron phosphate material.

[0036] Wherein, the D50 particle size of the first dried material is 2μm to 5μm, the D50 particle size of the second dried material is 5μm to 15μm, the D50 particle size of the pulverized material is 0.5μm to 1μm, the second temperature is 350℃ to 450℃, the second processing time is 2h to 3h, the third temperature is 600℃ to 700℃, and the third processing time is 3h to 6h.

[0037] In this embodiment, a stepwise calcination method is used to induce nucleation, achieving a stepwise process of nucleation and growth. Specifically, the first dried material requires a lower reaction temperature to form lithium iron phosphate; calcination at 350℃ to 450℃ is sufficient for sufficient crystallization and nucleation. The second dried material requires a higher reaction temperature to form lithium iron phosphate, reacting at 600℃ to 700℃, and preferentially undergoes heterogeneous nucleation and further growth on the nuclei formed by the first dried material. Simultaneously, controlling the particle size of the first, second, and pulverized materials within the aforementioned range allows for regulation of the lithium iron phosphate nucleus growth rate, improving the uniformity of the primary particle size of the lithium iron phosphate material. Therefore, the stepwise calcination method can control nucleus growth, avoiding the problem of uneven particle distribution caused by rapid, instantaneous nucleation, resulting in lithium iron phosphate material with a uniform primary particle size distribution and more concentrated primary particle size.

[0038] Thirdly, embodiments of this application provide a positive electrode sheet, including a current collector and a positive electrode material disposed on at least one surface of the current collector along its thickness direction. The positive electrode material includes lithium iron phosphate material as described above or lithium iron phosphate material prepared by the preparation method described above.

[0039] In this embodiment, the positive electrode sheet and the positive electrode material contain the aforementioned lithium iron phosphate material, thus having the advantage of improving battery energy efficiency and cycle performance in the battery.

[0040] Fourthly, embodiments of this application provide a secondary battery, including a positive electrode, a negative electrode, and a separator, wherein the separator is disposed between the positive electrode and the negative electrode, and the positive electrode is the aforementioned positive electrode.

[0041] In this embodiment, the secondary battery includes the aforementioned positive electrode sheet, thus possessing the advantages of excellent cycle performance and high energy efficiency.

[0042] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0043] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0044] Figure 1 is a schematic flowchart of the preparation method of lithium iron phosphate material provided in the embodiments of this application;

[0045] Figure 2 shows the first morphology result of the lithium iron phosphate material prepared in Example 1 of this application;

[0046] Figure 3 shows the second morphology result of the lithium iron phosphate material prepared in Example 1 of this application;

[0047] Figure 4 shows the morphology of the lithium iron phosphate material prepared in Comparative Example 1 of this application. Detailed Implementation

[0048] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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).

[0054] In the description of the embodiments of this application, unless otherwise specified, the solvent or water in the "solution" or "base liquid" is at least one of distilled water, deionized water, pure water, and ultrapure water.

[0055] the term

[0056] Unless otherwise stated or in case of conflict, the terms or phrases used in this application shall have the following meanings:

[0057] Particle size: For spherical particles, particle size refers to the diameter of the spherical particle. For non-spherical particles, particle size usually refers to the equivalent particle size (generally referred to as particle size), which can be obtained by scanning electron microscopy (SEM) or laser particle size analyzer. The equivalent particle size means that when a particle has a physical property that is the same as or similar to that of a homogeneous spherical particle, the diameter of the spherical particle is used to represent the diameter of the actual particle. Unless otherwise stated or contradictory, the particle size in this application refers to the equivalent particle size.

[0058] Primary particle size: also known as original particle size, refers to the particle size of a single particle, reflecting the intrinsic size of particles when they do not interact in the early stages of formation.

[0059] Secondary particle size refers to the particle size of aggregates formed by interactions such as agglomeration and cohesion between particles. Secondary particle size is usually larger than primary particle size and has a wider distribution range.

[0060] Particle size distribution parameter: In the particle size distribution curve, the particle size corresponding to the cumulative particle size distribution percentage reaching N% is called the DN particle size. This indicates that particles smaller than this size account for N% of all particles, where N = 0–100. When N = 100, the D100 particle size represents the particle size corresponding to the cumulative particle size distribution percentage reaching 100%. When N = 50, the D50 particle size is the particle size corresponding to the cumulative particle size distribution percentage reaching 50%, representing the median particle size, indicating that particles smaller and larger than this size each account for 50%. For example, a D50 particle size of 1 mm means that particles smaller than 1 mm and particles larger than 1 mm each account for 50% of all particles. The DN particle size can be measured using a laser particle size analyzer.

[0061] Span: Also known as diameter, it is calculated as follows: Span = (D90 - D10) ÷ D50. The smaller the span, the more concentrated the particle size distribution; the larger the span, the greater the difference in particle size and the more dispersed the distribution.

[0062] In traditional technologies, when preparing lithium iron phosphate materials via solid-state methods, the lack of heterogeneous nucleation conditions during calcination results in all lithium iron phosphate crystals crystallizing simultaneously, generating a large number of lithium iron phosphate nuclei within the same time period. The growth rate of these nuclei is uncontrollable, leading to extremely uneven primary particle size distribution in the lithium iron phosphate material.

[0063] The non-uniformity of the primary particle size in lithium iron phosphate materials leads to inconsistencies in their discharge process. Specifically, large particles discharge slowly, especially those with a primary particle size exceeding 1 μm. These large particles exhibit high material polarization, resulting in a very rapid voltage drop during discharge and consequently lower energy efficiency. Small particles discharge quickly, but their excessively small size creates a large specific surface area, making them more susceptible to side reactions with trace amounts of acid or other substances released from the electrolyte. This damages and corrodes the material structure, leading to rapid degradation in the early stages of cycling and a significant deterioration in cycle performance.

[0064] For energy storage lithium iron phosphate materials, the required cycle performance after application in batteries is 10,000 or even 12,000 cycles or more, while the energy efficiency is required to reach 94% or even 95% or more under constant power discharge of 0.5P. However, the primary particle size distribution of lithium iron phosphate materials prepared by traditional solid-state synthesis methods is extremely uneven, resulting in poor consistency during discharge, leading to lower battery energy efficiency and deteriorated cycle performance.

[0065] To address the problem of extremely uneven primary particle size distribution in lithium iron phosphate materials prepared by solid-state methods in traditional technologies, this application provides a lithium iron phosphate material, its preparation method, a positive electrode, and a secondary battery. The lithium iron phosphate material provided in this application exhibits good uniformity and high concentration of primary particle size distribution, demonstrating excellent consistency during discharge. This lithium iron phosphate material is prepared using a solid-state synthesis method, and through induced nucleation technology, the difference in reaction temperatures between the first and second dried materials is utilized to achieve stepwise nucleation and growth, effectively controlling the growth rate of crystal nuclei and resulting in a more concentrated primary particle size distribution. The energy efficiency and cycle performance of the positive electrode and secondary battery incorporating this lithium iron phosphate material are also effectively improved.

[0066] In a first aspect, embodiments of this application provide a lithium iron phosphate material, which includes a core having the molecular formula Li. a Fe(PO4) b (BO3) c Among them, 1.02≤a≤1.05, 0.98≤b≤1.08, 0.01≤c≤0.02, and the proportion of particles with a primary diameter of 100nm~600nm is ≥93.5%.

[0067] In the technical solution of this application embodiment, the provided lithium iron phosphate material has a concentrated primary particle size range, with the number of particles in the primary particle size range of 100nm to 600nm being greater than or equal to 93.5%, which makes it have good consistency during discharge, avoiding the problems of low energy efficiency caused by large particles and poor cycle performance caused by small particles, thereby ensuring that the battery has the advantages of both high energy efficiency and good cycle performance.

[0068] In this embodiment, in the molecular formula of the core, a, b, and c all represent stoichiometric ratios. Values ​​of a include, but are not limited to, 1.02, 1.025, 1.03, 1.035, 1.04, 1.045, and 1.05; values ​​of b include, but are not limited to, 0.98, 0.99, 1.00, 1.01, 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, and 1.08; and values ​​of c include, but are not limited to, 0.01, 0.012, 0.014, 0.016, 0.018, and 0.02.

[0069] In this embodiment, the proportion of primary particles with a diameter of 100 nm to 600 nm in the lithium iron phosphate material is ≥93.5%, including but not limited to 93.5%, 94%, 94.5%, 95%, 95.5%, 96%, 96.5%, 97%, 97.5%, 98%, 98.5%, 99%, 99.5%, and 100%. Further, the proportion of primary particles with a diameter of 100 nm to 600 nm in the lithium iron phosphate material is preferably 93.5% to 97.3%, resulting in higher uniformity of primary particle size and ensuring better consistency of the lithium iron phosphate material during discharge. Even further, the proportion of primary particles with a diameter of 100 nm to 600 nm in the lithium iron phosphate material is preferably 95.5% to 97.3%, further improving the uniformity of primary particle size and resulting in very high consistency of the material during discharge.

[0070] In some embodiments, the primary particle size of the lithium iron phosphate material is 0.5 to 2.5 mm.

[0071] In some embodiments, the primary particle size (D50) of the lithium iron phosphate material is 140 nm to 320 nm.

[0072] When the lithium iron phosphate material meets the above-mentioned primary particle size range and / or the above-mentioned primary particle size D50 range, the primary particle size of the lithium iron phosphate material is not only uniformly distributed, but also small, which makes the internal arrangement of the lithium iron phosphate material more uniform, which helps to form a balanced voltage platform and keep the battery stable during operation.

[0073] In some embodiments, the primary particle size distribution of the lithium iron phosphate material includes, but is not limited to, 0.5, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2, 2.2, 2.4, and 2.5 mm. Further, the primary particle size distribution of the lithium iron phosphate material is preferably 1.2–2.5 mm, resulting in a more concentrated primary particle size distribution. This leads to better uniformity of the lithium iron phosphate material during discharge, which is beneficial for improving battery energy efficiency and cycle performance. Even further, the primary particle size distribution of the lithium iron phosphate material is preferably 1.2–1.8 mm, further increasing the concentration of the primary particle size distribution, resulting in better discharge uniformity and superior electrical performance.

[0074] In some embodiments, the D50 particle size of the primary particle size of the lithium iron phosphate material includes, but is not limited to, 140 nm, 160 nm, 180 nm, 200 nm, 220 nm, 240 nm, 260 nm, 280 nm, 300 nm, and 320 nm. Further, the D50 particle size of the primary particle size of the lithium iron phosphate material is preferably 140 nm to 305 nm, which has a smaller primary particle size, enabling close contact and uniform arrangement between the lithium iron phosphate material particles, thereby ensuring stable electrical performance. Even further, the D50 particle size of the primary particle size of the lithium iron phosphate material is preferably 160 nm to 235 nm, with a smaller primary particle size and more concentrated distribution, which is beneficial for further improving the electrical performance of the material.

[0075] In some embodiments, the compaction density of the lithium iron phosphate material is 2.20 g / mL to 2.60 g / mL.

[0076] In some embodiments, the tap density of the lithium iron phosphate material is 1.00 g / mL to 1.50 g / mL.

[0077] In some embodiments, the BET specific surface area of ​​the lithium iron phosphate material is 8.0 m². 2 / g~15.0m 2 / g.

[0078] In some embodiments, the resistivity of the lithium iron phosphate material powder is 5.00 Ω·cm to 22.00 Ω·cm.

[0079] When lithium iron phosphate materials meet the above-mentioned compaction density range, and / or, the above-mentioned tapped density range, and / or, the above-mentioned BET specific surface area range, and / or, the above-mentioned powder resistivity range, lithium iron phosphate materials not only have good electronic conductivity and good consistency during discharge, but also have good compactness between particles of different sizes in lithium iron phosphate materials. This improves the electronic conductivity of lithium iron phosphate materials and is beneficial to improving the specific capacity, discharge efficiency and cycle performance of batteries.

[0080] In some embodiments, the compaction density of the lithium iron phosphate material includes, but is not limited to, 2.20 g / mL, 2.25 g / mL, 2.30 g / mL, 2.35 g / mL, 2.40 g / mL, 2.45 g / mL, 2.50 g / mL, 2.55 g / mL, and 2.60 g / mL. Further, the compaction density of the lithium iron phosphate material is preferably between 2.25 g / mL and 2.55 g / mL, resulting in a higher degree of particle density, which can improve the battery's charge / discharge specific capacity, rate performance, and energy density, and is beneficial for optimizing the battery's cycle life. Even further, the compaction density of the lithium iron phosphate material is preferably between 2.40 g / mL and 2.50 g / mL, resulting in even better charge / discharge specific capacity, rate performance, and cycle performance of the battery.

[0081] In some embodiments, the tap density of the lithium iron phosphate material is 1.00 g / mL to 1.50 g / mL, including but not limited to 1.00 g / mL, 1.05 g / mL, 1.10 g / mL, 1.15 g / mL, 1.20 g / mL, 1.25 g / mL, 1.30 g / mL, 1.35 g / mL, 1.40 g / mL, 1.45 g / mL, and 1.50 g / mL. Further, the tap density of the lithium iron phosphate material is preferably 1.05 g / mL to 1.34 g / mL, resulting in closer particle contact, which is beneficial for increasing energy density, improving rate performance, and enhancing energy efficiency. Even further, the tap density of the lithium iron phosphate material is preferably 1.20 g / mL to 1.32 g / mL, resulting in superior charge / discharge specific capacity, rate performance, and cycle performance of the battery.

[0082] In some embodiments, the BET specific surface area of ​​the lithium iron phosphate material is 8.0 m². 2 / g~15.0m 2 / g, including but not limited to 8.0m 2 / g、9m 2 / g, 10m 2 / g、11m 2 / g、12m 2 / g、13m 2 / g、14m 2 / g, 15.0m 2 / g, etc. Furthermore, the preferred BET specific surface area of ​​the lithium iron phosphate material is 8.01 m². 2 / g~14.1m 2The lithium iron phosphate material has a relatively moderate specific surface area, which is neither too large, leading to excessive gaps between particles and hindering the improvement of compaction density, nor too small, resulting in excessively large particle size and affecting lithium ion insertion and extraction. Furthermore, the preferred BET specific surface area of ​​the lithium iron phosphate material is 8.01 m² / g. 2 / g~11.8m 2 / g, the material has a moderate specific surface area, which can further improve the compaction density while promoting the insertion and extraction of lithium ions.

[0083] In some embodiments, the powder resistivity of the lithium iron phosphate material is 5.00 Ω·cm to 22.00 Ω·cm, including but not limited to 5.00 Ω·cm, 6 Ω·cm, 8 Ω·cm, 10 Ω·cm, 12 Ω·cm, 14 Ω·cm, 16 Ω·cm, 18 Ω·cm, 20 Ω·cm, and 22.00 Ω·cm. Further, the powder resistivity of the lithium iron phosphate material is preferably 6.16 Ω·cm to 20.40 Ω·cm. A lower powder resistivity can accelerate electron transport speed, improve battery charge and discharge efficiency, meet the requirements of fast charging, and also enhance rate performance, improve cycle life, and improve battery safety. Even further, the powder resistivity of the lithium iron phosphate material is preferably 6.16 Ω·cm to 9.12 Ω·cm, which significantly improves the conductivity of the material and is beneficial for achieving superior electrical performance.

[0084] In some embodiments, the lithium iron phosphate material further includes a coating layer that coats the surface of the core; wherein the coating layer is made of carbon material, and the carbon material accounts for 1.10% to 2.00% of the mass of the lithium iron phosphate material.

[0085] In this embodiment, the lithium iron phosphate material obtained by carbon coating the core can improve the electrical contact between lithium iron phosphate materials, thereby improving the electronic conductivity of the lithium iron phosphate material.

[0086] In some embodiments, the mass percentage of carbon material in the lithium iron phosphate material includes, but is not limited to, 1.10%, 1.20%, 1.30%, 1.40%, 1.50%, 1.60%, 1.70%, 1.80%, 1.90%, and 2.00%. Further, the mass percentage of carbon material in the lithium iron phosphate material is preferably 1.45% to 1.98%. An appropriate amount of carbon material can form a complete conductive network on the surface of the lithium iron phosphate material, significantly improving the electronic conductivity of the material and facilitating the adjustment of the particle morphology and particle size distribution, thereby improving the charge / discharge specific capacity, rate performance, and energy efficiency of the battery.

[0087] Please refer to Figure 1. Secondly, according to embodiments of this application, a method for preparing lithium iron phosphate material is provided, comprising the following steps:

[0088] S1: A mixture of ferrous source, oxalate ion source, borate ion source and first regulator is obtained in a base solution and subjected to a first reaction treatment.

[0089] S2: After solid-liquid separation and first pulping treatment, the mixture yields the first slurry;

[0090] S3: After the first slurry is mixed with the first lithium source, the first phosphorus source and the second regulator, it is subjected to the first grinding treatment and the first drying treatment to obtain the first dried material;

[0091] S4: After mixing the trivalent iron source, the second phosphorus source, the second lithium source and the carbon source, the mixture undergoes a second pulping treatment, a second grinding treatment and a second drying treatment to obtain the second dried material;

[0092] S5: After the first and second dried materials are mixed, they are calcined to obtain lithium iron phosphate material.

[0093] In the technical solution of this application embodiment, firstly, using raw materials such as ferrous iron source, oxalate ion source, borate ion source, first lithium source and second phosphorus source, a first dry material with a lower reaction temperature is obtained through stirring reaction, solid-liquid separation, first slurrying, first grinding and first drying. Simultaneously, using raw materials such as trivalent iron source, second phosphorus source, second lithium source and carbon source, a second slurrying, second grinding and second drying are obtained to obtain a second dry material with a higher reaction temperature. The first and second dry materials with different reaction temperatures are mixed and calcined. The first dry material first undergoes a crystallization reaction to form crystal nuclei, while the second dry material preferentially undergoes heterogeneous nucleation and continues to grow on the crystal nuclei formed by the first dry material. This achieves stepwise nucleation and growth, avoiding the problem of extremely uneven primary particle size distribution caused by the disordered growth and rapid instantaneous nucleation of lithium iron phosphate crystal nuclei, thus obtaining a lithium iron phosphate material with uniform primary particle size distribution and high concentration.

[0094] The preparation method of lithium iron phosphate materials is described in detail below using a step-by-step approach.

[0095] S1: A mixture of ferrous ion source, oxalate ion source, borate ion source and first regulator is obtained in a base solution and subjected to a first reaction treatment.

[0096] In some embodiments, the step of mixing a ferrous source, an oxalate ion source, a borate ion source, and a first regulator in a base solution and obtaining a mixture through a first reaction treatment includes:

[0097] A mixture of ferrous ion source, oxalate ion source, borate ion source and a first regulator is obtained in a base solution and subjected to a first treatment time at a first temperature.

[0098] The pH value of the base solution is 4 to 5.5, the first temperature is 35℃ to 55℃, and the first treatment time is 60 min to 120 min.

[0099] In this embodiment, by controlling the pH, temperature, and time during the first reaction process, the formation of ferrous oxalate material can be promoted, avoiding the formation of ferric iron products. Ferrous oxalate material has a low decomposition temperature, and its iron content is ferrous (Fe2+), requiring no high-temperature, incandescent carbon reduction. Therefore, using ferrous oxalate as a raw material can lower the reaction temperature for lithium iron phosphate formation, allowing for sufficient crystallization and nucleation at a lower calcination temperature, which is beneficial for subsequent induced nucleation.

[0100] Understandably, the first processing time includes the first feeding time and the first stirring time. The first feeding time refers to the time when the ferrous source, oxalate ion source, borate ion source, and the first regulator are added to the base solution, and the first stirring time refers to the time during which the reaction continues to be stirred after the feeding is completed.

[0101] In some embodiments, the first feeding time and the first stirring time are each independently 30 min to 60 min, including but not limited to 30 min to 60 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, etc.

[0102] In some embodiments, the substrate is an oxalic acid solution with a pH value of 4 to 5.5, including but not limited to 4, 4.2, 4.4, 4.6, 4.8, 5, 5.2, 5.4, and 5.5.

[0103] In some embodiments, during the feeding process and during the stirring reaction process, the pH value of the first reaction treatment is maintained at 4 to 5.5, including but not limited to 4, 4.2, 4.4, 4.6, 4.8, 5, 5.2, 5.4, 5.5, etc.; and the temperature of the first reaction treatment is maintained at 35°C to 55°C, including but not limited to 35°C, 37°C, 39°C, 41°C, 43°C, 45°C, 47°C, 49°C, 51°C, 53°C, 55°C, etc.

[0104] In some embodiments, the molar ratio of iron in the ferrous source, oxalate in the oxalate ion source, and borate in the borate ion source is 1:(0.8-1):(0.1-0.2).

[0105] In this embodiment, the molar ratio of iron in the ferrous ion source to oxalate in the oxalate ion source is 1:(0.8-1), including but not limited to 1:0.8, 1:0.85, 1:0.9, 1:0.95, 1:1, etc.; the molar ratio of iron in the ferrous ion source to borate in the borate ion source is 1:(0.1-0.2), including but not limited to 1:0.1, 1:0.12, 1:0.14, 1:0.16, 1:0.18, 1:0.2, etc.; the molar ratio of oxalate in the oxalate ion source to borate in the borate ion source is (0.8-1):(0.1-0.2), including but not limited to 0.8:0.1, 0.8:0.2, 0.9:0.1, 0.9:0.2, 1:0.1, 1:0.2, etc.

[0106] In some embodiments, the ferrous source is selected from at least one of ferrous sulfate, ferrous chloride, and ferrous acetate. It can be any one of ferrous sulfate, ferrous chloride, and ferrous acetate, or a combination of at least two of them, such as a combination of ferrous sulfate and ferrous chloride, a combination of ferrous sulfate and ferrous acetate, a combination of ferrous chloride and ferrous acetate, or a combination of ferrous sulfate, ferrous chloride, and ferrous acetate.

[0107] In some embodiments, the oxalate ion source includes at least one of oxalic acid, ammonium oxalate, and sodium oxalate. It can be any one of oxalic acid, ammonium oxalate, and sodium oxalate, or a combination of at least two of them, such as a combination of oxalic acid and ammonium oxalate, a combination of oxalic acid and sodium oxalate, a combination of ammonium oxalate and sodium oxalate, or a combination of oxalic acid, ammonium oxalate, and sodium oxalate.

[0108] In some embodiments, the borate ion source includes at least one of boric acid, ammonium borate, and sodium borate. It can be any one of boric acid, ammonium borate, and sodium borate, or a combination of at least two of them, such as a combination of boric acid and ammonium borate, a combination of boric acid and sodium borate, a combination of ammonium borate and sodium borate, or a combination of boric acid, ammonium borate, and sodium borate.

[0109] In this embodiment, the borate ion source anionizes lithium iron phosphate to improve the electrochemical performance of the lithium iron phosphate material, while the borate ion source acts as a flux to further reduce the reaction temperature when the first dried material forms lithium iron phosphate.

[0110] In some embodiments, the first regulator includes at least one of ammonium bicarbonate and sodium bicarbonate. Understandably, the first regulator, as an acid-base regulator, is used to adjust the pH value of the reaction system during the feeding process, and it can be ammonium bicarbonate, sodium bicarbonate, or a combination of ammonium bicarbonate and sodium bicarbonate.

[0111] S2: After solid-liquid separation and first pulping treatment, the mixture yields the first slurry.

[0112] In some embodiments, the solid-liquid separation process in step S2 includes one or more of filtration, vacuum filtration, pressure filtration and centrifugation, and this application does not make any particular limitation on this.

[0113] In some embodiments, after the solid-liquid separation process in step S2, the method further includes the following step: washing the solid material obtained from the solid-liquid separation process until the conductivity of the washing water is ≤200μS / cm to obtain the washing material.

[0114] In some embodiments, the first pulping process in step S2 includes the following steps: after mixing the washing material and water, the mixture is pulped to obtain a first pulp.

[0115] In this embodiment, the water can be distilled water, reverse osmosis water, deionized water, pure water, or ultrapure water, and is more preferably pure water. The solid content of the first slurry is 30% to 40%, including but not limited to 30%, 32%, 34%, 36%, 38%, and 40%, etc., and this application does not impose any particular limitation on this. The solid content of the first slurry refers to the mass fraction of solids in the first slurry.

[0116] S3: After the first slurry is mixed with the first lithium source, the first phosphorus source and the second regulator, it is subjected to the first grinding treatment and the first drying treatment to obtain the first dried material.

[0117] In some embodiments, the molar ratio of iron in the first slurry, lithium in the first lithium source, phosphorus in the first phosphorus source, and the second regulator is 1:(1.04~1.1):(1.04~1.1):(0.15~0.2);

[0118] In this embodiment, the molar ratio of iron in the first slurry to lithium in the first lithium source is 1:(1.04~1.1), including but not limited to 1:1.04, 1:1.05, 1:1.06, 1:1.07, 1:1.08, 1:1.09, 1:1.1, etc.; the molar ratio of iron in the first slurry to phosphorus in the first phosphorus source is 1:(1.04~1.1), including but not limited to 1:1.04, 1:1.05, 1:1.06, 1:1.07, 1:1.08, 1:1.09, 1:1.1, etc.; the molar ratio of iron in the first slurry to the second regulator is 1:(0.15~0.2), including but not limited to 1:0.15, 1:0.16, 1:0.17, 1:0.18, 1:0.19, 1:0.2, etc.

[0119] In some embodiments, the first lithium source is selected from at least one of lithium oxalate and lithium acetate, and may be lithium oxalate, lithium acetate, or a combination of lithium oxalate and lithium acetate.

[0120] In this embodiment, the first lithium source is lithium oxalate and lithium acetate, which have low melting points and low decomposition temperatures. When combined with iron oxalate, which has a low decomposition temperature and does not require carbothermal reduction reaction, lithium iron phosphate crystal nuclei can be formed at a lower reaction temperature, which is beneficial for subsequent induced nucleation.

[0121] In some embodiments, the first phosphorus source is selected from at least one of phosphoric acid, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate. It may be selected from any one of phosphoric acid, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate, or from a combination of at least two of them, such as a combination of phosphoric acid and diammonium hydrogen phosphate, a combination of phosphoric acid and ammonium dihydrogen phosphate, a combination of diammonium hydrogen phosphate and ammonium dihydrogen phosphate, or a combination of phosphoric acid, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate.

[0122] In some embodiments, the second modifier includes at least one selected from polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, and polycarboxylic acid compounds. The molecular weight of the polyethylene glycol can be from 200 to 10,000, including but not limited to PEG200, PEG400, PEG600, PEG1000, PEG2000, PEG4000, PEG6000, PEG8000, PEG10000, etc., and more preferably PEG6000.

[0123] In this embodiment, the first regulator can act as a dispersant to improve the dispersion uniformity of each component in the first slurry, thereby ensuring that each component in the first dried material is fully mixed. This facilitates the full contact and reaction of each component when the first dried material forms lithium iron phosphate under low temperature conditions, thereby improving the product quality of the final lithium iron phosphate material and thus improving the energy efficiency and cycle performance of batteries using lithium iron phosphate material.

[0124] In some embodiments, the first grinding process in step S3 includes the following steps: after grinding the first slurry, a first grinding material with a particle size of 0.1 mm to 0.2 mm is obtained.

[0125] In this embodiment, the D50 particle size of the first abrasive is 0.1mm to 0.2mm, including but not limited to 0.1mm, 0.11mm, 0.12mm, 0.13mm, 0.14mm, 0.15mm, 0.16mm, 0.17mm, 0.18mm, 0.19mm, and 0.2mm. The first grinding process can be carried out using a ball mill or a sand mill. The inner wall of the grinding equipment is made of ceramic material, and the grinding media are ceramic balls with a diameter of 0.2mm ± 0.03mm.

[0126] In some embodiments, the first drying process in step S3 includes the following steps: spray drying the first abrasive to obtain the first dried material.

[0127] In some embodiments, the D50 particle size of the first dried material is 2μm to 5μm, including but not limited to 2μm, 2.5μm, 3μm, 3.5μm, 4μm, 4.5μm, 5μm, etc.

[0128] The first dried material obtained through steps S1 to S3 contains raw materials such as lithium oxalate and ferrous oxalate with low melting point and low decomposition temperature, and contains borate as a flux to lower the melting point, which is conducive to the reaction at a lower reaction temperature to form lithium iron phosphate crystal nuclei, and serves as seed crystals for subsequent heterogeneous nucleation.

[0129] S4: The trivalent iron source, the second phosphorus source, the second lithium source and the carbon source are mixed and then subjected to the second pulping treatment, the second grinding treatment and the second drying treatment to obtain the second dried material.

[0130] In some embodiments, the molar ratio of iron in the trivalent iron source, phosphorus in the second phosphorus source, lithium in the second lithium source, and carbon source is 1:1:(1.02-1.04):(0.2-0.3).

[0131] In this embodiment, the molar ratio of iron in the trivalent iron source to phosphorus in the second phosphorus source is 1:1; the molar ratio of iron in the trivalent iron source to lithium in the second lithium source is 1:(1.02~1.04), including but not limited to 1:1.02, 1:1.025, 1:1.03, 1:1.035, 1.04, etc.; the molar ratio of iron in the trivalent iron source to carbon source is 1:(0.2~0.3), including but not limited to 1:0.2, 1:0.21, 1:0.22, 1:0.23, 1:0.24, 1:0.25, 1:0.26, 1:0.27, 1:0.28, 1:0.29, 1:0.3, etc.

[0132] In some embodiments, the first phosphorus source is selected from at least one of phosphoric acid and ammonium dihydrogen phosphate, the first lithium source is selected from at least one of lithium oxalate and lithium acetate, and the second regulator includes polyethylene glycol.

[0133] In some embodiments, the trivalent iron source includes iron phosphate, the second phosphorus source includes iron phosphate, the second lithium source includes lithium carbonate, and the carbon source is selected from at least one of glucose, sucrose, starch, and cellulose. The carbon source can be selected from any one of glucose, sucrose, starch, and cellulose, or from a combination of at least two of them, such as: a combination of glucose and sucrose, a combination of glucose and starch, a combination of glucose and cellulose, a combination of sucrose and starch, a combination of sucrose and cellulose, a combination of starch and cellulose, a combination of glucose, sucrose, and starch, or a combination of glucose, sucrose, starch, and cellulose.

[0134] In this embodiment, iron phosphate serves as both a trivalent iron source and a phosphorus source, requiring carbon to undergo a reduction reaction at high temperatures. Simultaneously, lithium carbonate, with its high melting point and high decomposition temperature, is selected as the lithium source. This results in a higher reaction temperature required for the second dried material to form lithium iron phosphate. This facilitates the preferential formation of heterogeneous nucleation of the second dried material on the surface of the crystal nuclei obtained from the first dried material, avoiding disordered growth and rapid instantaneous nucleation. Consequently, the primary particle size distribution of the resulting lithium iron phosphate material is more uniform and concentrated.

[0135] In some embodiments, the molar ratio of lithium element in the first lithium source to lithium element in the second lithium source is 1:(8-9), including but not limited to 1:8, 1:8.1, 1:8.2, 1:8.3, 1:8.4, 1:8.5, 1:8.6, 1:8.7, 1:8.8, 1:8.9 or 1:9.

[0136] In this embodiment, by adjusting the molar ratio of the two lithium sources, the mass ratio of the first dried material and the second dried material can be controlled, thereby precisely controlling the mass ratio of lithium iron phosphate generated at different reaction temperatures. This is beneficial for obtaining lithium iron phosphate materials with a more concentrated primary particle size distribution. Specifically, if the molar ratio of lithium elements in the first lithium source and the second lithium source is less than 1:9, the amount of the first lithium source used is relatively small, resulting in a smaller mass of the first dried material. This leads to a smaller number of crystal nuclei formed during calcination, which cannot provide sufficient phase interface for subsequent heterogeneous nucleation. This results in disordered growth of crystal nuclei and rapid, instantaneous nucleation, ultimately causing uneven primary particle size distribution. If the molar ratio of lithium elements in the first lithium source and the second lithium source is greater than 1:8, the amount of the first lithium source used is relatively large, resulting in an excessive number of crystal nuclei formed during pre-crystallization. This means that some crystal nuclei do not participate in subsequent heterogeneous nucleation, also ultimately causing uneven primary particle size distribution in the lithium iron phosphate material.

[0137] In some embodiments, the second pulping process in step S4 includes the following steps: mixing a trivalent iron source, a second phosphorus source, a second lithium source, and a carbon source, then adding water to pulp the mixture to obtain a second slurry.

[0138] In this embodiment, the water can be distilled water, reverse osmosis water, deionized water, pure water, or ultrapure water, and is more preferably pure water. The solid content of the second slurry is 30% to 40%, including but not limited to 30%, 32%, 34%, 36%, 38%, and 40%, etc., and this application does not impose any particular limitation on this. The solid content of the second slurry is the mass fraction of solids in the first slurry.

[0139] In some embodiments, the second grinding process in step S4 includes the following steps: after the second slurry is ground, a second grinding material with a particle size of 0.2 μm to 0.5 μm is obtained.

[0140] In this embodiment, the D50 particle size of the second abrasive is 0.2μm to 0.5μm, including but not limited to 0.2μm, 0.25μm, 0.3μm, 0.35μm, 0.4μm, 0.45μm, and 0.5μm. The second grinding process can be carried out using a ball mill or a sand mill, with the inner wall of the grinding equipment made of ceramic material and the grinding media being ceramic balls with a diameter of 0.3mm ± 0.03mm.

[0141] In some embodiments, the second drying process in step S4 includes the following steps: spray drying the second abrasive to obtain a second dried material.

[0142] In some embodiments, the D50 particle size of the second dried material is 5μm to 15μm, including but not limited to 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, etc.

[0143] S5: After the first and second dried materials are mixed, they are calcined to obtain lithium iron phosphate material.

[0144] In some embodiments, the step of mixing the first dried material and the second dried material, followed by calcination treatment to obtain lithium iron phosphate material includes:

[0145] The first dried material and the second dried material are mixed and then crushed to obtain crushed material;

[0146] The pulverized material is placed in a first protective atmosphere, subjected to a second treatment time at a second temperature, and then subjected to a third treatment time at a third temperature to obtain lithium iron phosphate material;

[0147] The D50 particle size of the pulverized material is 0.5μm~1μm, the second temperature is 350℃~450℃, the second treatment time is 2h~3h, the third temperature is 600℃~700℃, and the third treatment time is 3h~6h.

[0148] In this embodiment, nucleation is induced through a stepwise calcination method, achieving a stepwise process of nucleation and growth. Specifically, the first dried material has a lower chemical reaction temperature, and calcination at 350℃ to 450℃ allows it to fully crystallize and form crystal nuclei. The second dried material has a higher reaction temperature, reacting at 600℃ to 700℃ to form lithium iron phosphate, which preferentially undergoes heterogeneous nucleation and continues to grow on the crystal nuclei formed by the first dried material. Therefore, the stepwise calcination method can control the growth rate of the crystal nuclei, thereby avoiding the problem of uneven particle distribution caused by rapid instantaneous nucleation, resulting in lithium iron phosphate material with a more concentrated primary particle size distribution.

[0149] In this embodiment, the D50 particle size of the pulverized material is 0.5μm to 1μm, including but not limited to 0.5μm, 0.6μm, 0.7μm, 0.8μm, 0.9μm, and 1μm. The pulverization of the mixed first and second dried materials can be carried out using a mechanical pulverizer or an air jet mill.

[0150] In this embodiment, the second temperature is 350℃~450℃, including but not limited to 350℃, 360℃, 370℃, 380℃, 390℃, 400℃, 410℃, 420℃, 430℃, 440℃, 450℃, etc.; the second processing time is 2h~3h, including but not limited to 2h, 2.1h, 2.2h, 2.3h, 2.4h, 2.5h, 2.6h, 2.7h, 2.8h, 2.9h, 3h, etc.; the third temperature is 600℃~700℃, including but not limited to 600℃, 610℃, 620℃, 630℃, 640℃, 650℃, 660℃, 670℃, 680℃, 690℃, 700℃, etc.; the third processing time is 3h~6h, including but not limited to 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, 6h.

[0151] In some embodiments, the rate of temperature increase from room temperature to the second temperature is 100℃ / h to 150℃ / h, including but not limited to 100℃ / h, 110℃ / h, 120℃ / h, 130℃ / h, 140℃ / h, 150℃ / h, etc.; the rate of temperature increase from the second temperature to the third temperature is 30℃ / h to 50℃ / h, including but not limited to 30℃ / h, 35℃ / h, 40℃ / h, 45℃ / h, 50℃ / h, etc.

[0152] In some embodiments, the first protective atmosphere satisfies the following conditions: oxygen content is less than 1 ppm and humidity is less than 1%. The oxygen content being less than 1 ppm is achieved by introducing a protective gas, which includes, but is not limited to, nitrogen, helium, neon, argon, xenon, etc., and is more preferably nitrogen. During the calcination process, an induced draft fan is activated in the heating section to ensure that the humidity in the heat preservation section is less than 1%.

[0153] In some embodiments, after calcination, the process further includes the following steps: crushing, sieving, iron removal, and packaging to obtain lithium iron phosphate material.

[0154] In some embodiments, the pulverization process after calcination is carried out using a fluidized bed air jet mill under a protective atmosphere. Specifically, the pulverization chamber of the fluidized bed air jet mill uses Laval nozzles to achieve an airflow velocity of Mach 2 to Mach 3, thereby disturbing the material and pulverizing it. The protective atmosphere includes, but is not limited to, nitrogen, helium, neon, argon, xenon, etc., and is more preferably nitrogen; the pressure of the protective atmosphere is 0.4 MPa to 0.8 MPa, including but not limited to 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa, 0.8 MPa, etc.; the temperature of the protective atmosphere is 100℃ to 150℃, including but not limited to 100℃, 110℃, 120℃, 130℃, 140℃, 150℃, etc.

[0155] In some embodiments, the pulverized material is graded in the grading chamber using a grading wheel, and the graded material is screened using an ultrasonic vibrating screen. Screening, iron removal, and packaging are all carried out in a constant temperature and humidity room.

[0156] Thirdly, embodiments of this application provide a positive electrode sheet, including a current collector and a positive electrode material disposed on at least one surface of the current collector along its thickness direction. The positive electrode material includes lithium iron phosphate material as described above or lithium iron phosphate material prepared by the preparation method described above.

[0157] In this embodiment, the positive electrode sheet and the positive electrode material contain the aforementioned lithium iron phosphate material, thus having the advantage of good discharge performance.

[0158] Fourthly, embodiments of this application provide a secondary battery, including a positive electrode, a negative electrode, and a separator, wherein the separator is disposed between the positive electrode and the negative electrode, and the positive electrode is the aforementioned positive electrode.

[0159] In this embodiment, the secondary battery includes the aforementioned positive electrode sheet, thus possessing the advantages of excellent cycle performance and high energy efficiency.

[0160] 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.

[0161] I. Preparation Method

[0162] Example 1

[0163] The method for preparing lithium iron phosphate material provided in this embodiment includes the following steps:

[0164] (1) Add the ferrous source, oxalate ion source, borate ion source and the first regulator to the bottom liquid. During the feeding process, maintain the pH value at 4.5, the temperature at 45℃, and the feeding time at 50 min. Then continue stirring and reacting for 40 min to obtain a mixture.

[0165] The ferrous source is ferrous sulfate solution, the oxalate ion source is oxalic acid solution, the borate ion source is boric acid solution, the first regulator is ammonium bicarbonate, and the base solution is oxalic acid base solution with a pH of 4.5; the molar ratio of iron, oxalate and borate in the mixture is 1:0.9:0.1.

[0166] (2) The mixture is filtered to obtain solid material; the solid material is washed with pure water until the conductivity of the washing water is ≤200μS / cm to obtain washing material; the washing material is added to pure water for the first slurry treatment to obtain the first slurry with a solid content of 35wt%.

[0167] (3) Add the first lithium source, the first phosphorus source and the second regulator to the first slurry, stir and dissolve, and then perform the first grinding treatment using a ball mill. The inner wall of the ball mill is made of ceramic material, and the grinding medium is ceramic balls with a particle size of 0.2 mm ± 0.03 mm to obtain the first grinding material with a particle size of 0.15 mm. The first grinding material is spray-dried to obtain the first dry material with a particle size of 3.5 μm.

[0168] The first lithium source is lithium oxalate; the first phosphorus source is a mixture containing phosphoric acid and ammonium dihydrogen phosphate, with a pH of 2.9; the second regulator is PEG6000; the molar ratio of iron in the first slurry, lithium in the first lithium source, phosphorus in the first phosphorus source, and the second regulator is 1:1.06:1.08:0.18.

[0169] (4) After mixing the trivalent iron source, the second phosphorus source, the second lithium source and the carbon source, water is added for the second slurry treatment to obtain a second slurry with a solid content of 35%; the second slurry is subjected to a second grinding treatment using a ball mill with the inner wall of the ball mill made of ceramic material and the grinding media being ceramic balls with a particle size of 0.3mm±0.03mm to obtain a second grinding material with a particle size of 0.34μm; the second grinding material is subjected to spray drying treatment to obtain a second dry material with a particle size of 12μm.

[0170] Among them, the trivalent iron source and the second phosphorus source are both iron phosphate, the second lithium source is lithium carbonate, and the carbon source is glucose; the molar ratio of iron or phosphorus in iron phosphate, lithium in lithium carbonate, and glucose is 1:1.03:0.25; the molar ratio of lithium in the first lithium source and lithium in the second lithium source is 1:8.5.

[0171] (5) After mixing the first dry material and the second dry material, the mixture is pulverized to a particle size of 0.8 μm using an air jet mill to obtain pulverized material. The pulverized material is then calcined in a roller furnace. Nitrogen gas is introduced to maintain the oxygen content in the roller furnace below 1 ppm. At the same time, the induced draft fan is turned on in the heating section to keep the humidity in the heat preservation section below 1%. The temperature is first raised to 400℃ at a heating rate of 120℃ / h and kept at that temperature for 2.5h. Then, the temperature is raised to 600℃ at a heating rate of 40℃ / h and kept at that temperature for 5h. The material is then cooled down to a temperature of ≤100℃ before being removed from the furnace.

[0172] (6) After the calcined material is discharged from the furnace, it is pulverized by a fluidized bed air jet mill. The pulverization process is carried out in a nitrogen atmosphere with a pressure of 0.6 MPa and a temperature of 135°C. Laval nozzles are used in the pulverization chamber to make the airflow speed reach Mach 2.5, disturbing the material and pulverizing it by collision. The pulverized material is classified in the classification chamber by a classifying wheel. The classified material is screened by an ultrasonic vibrating screen. Screening, iron removal and packaging are all carried out in a constant temperature and humidity room to obtain lithium iron phosphate material.

[0173] Example 2

[0174] The method for preparing lithium iron phosphate material provided in this embodiment includes the following steps:

[0175] (1) Add the ferrous source, oxalate ion source, borate ion source and the first regulator to the bottom liquid. During the feeding process, maintain the pH value at 4, the temperature at 35℃, and the feeding time at 30 min. Then continue stirring and reacting for 30 min to obtain a mixture.

[0176] The ferrous source is ferrous acetate, the oxalate ion source is oxalic acid, the borate ion source is boric acid, the first regulator is sodium bicarbonate, and the bottom solution is an oxalic acid bottom solution with a pH of 4; the molar ratio of iron, oxalate, and borate in the mixture is 1:0.8:0.1.

[0177] (2) The mixture is filtered to obtain a solid material; the solid material is washed with pure water until the conductivity of the washing water is ≤200μS / cm to obtain a washed material; the washed material is added to pure water for a first slurry treatment to obtain a first slurry with a solid content of 35%.

[0178] (3) Add the first lithium source, the first phosphorus source and the second regulator to the first slurry, stir and dissolve, and then perform the first grinding treatment using a sand mill. The inner wall of the sand mill is made of ceramic material, and the grinding medium is ceramic balls with a particle size of 0.2mm±0.03mm to obtain the first grinding material with a particle size of 0.1mm. The first grinding material is spray-dried to obtain the first dry material with a particle size of 2μm.

[0179] The first lithium source is lithium oxalate; the first phosphorus source is a mixture containing phosphoric acid and ammonium dihydrogen phosphate with a pH of 2.5; the second regulator is PEG6000; the molar ratio of iron in the first slurry, lithium in the first lithium source, phosphorus in the first phosphorus source, and the second regulator is 1:1.04:1.04:0.15.

[0180] (4) After mixing the trivalent iron source, the second phosphorus source, the second lithium source and the carbon source, water is added for the second slurry treatment to obtain a second slurry with a solid content of 35%; the second slurry is subjected to a second grinding treatment using a sand mill. The inner wall of the sand mill is made of ceramic material and the grinding media is ceramic balls with a particle size of 0.3mm±0.03mm to obtain a second abrasive with a particle size of 0.2μm; the second abrasive is subjected to spray drying treatment to obtain a second dry material with a particle size of 5μm.

[0181] Among them, the trivalent iron source and the second phosphorus source are both iron phosphate, the second lithium source is lithium carbonate, and the carbon source is glucose; the molar ratio of iron or phosphorus in iron phosphate, lithium in lithium carbonate, and glucose is 1:1.02:0.2; the molar ratio of lithium in the first lithium source and lithium in the second lithium source is 1:8.

[0182] (5) After mixing the first dry material and the second dry material, the mixture is pulverized to a particle size of 0.5 μm using an air jet mill to obtain pulverized material. The pulverized material is then calcined in a roller furnace. Nitrogen gas is introduced to maintain the oxygen content in the roller furnace below 1 ppm. At the same time, the induced draft fan is turned on in the heating section to keep the humidity in the heat preservation section below 1%. The temperature is first raised to 350℃ at a heating rate of 100℃ / h and kept at that temperature for 2 hours. Then, the temperature is raised to 600℃ at a heating rate of 30℃ / h and kept at that temperature for 3 hours. The material is then cooled down to a temperature of ≤100℃ before being removed from the furnace.

[0183] (6) After the calcined material is discharged from the furnace, it is pulverized by a fluidized bed air jet mill. The pulverization process is carried out in a nitrogen atmosphere with a pressure of 0.4 MPa and a temperature of 100°C. Laval nozzles are used in the pulverization chamber to make the airflow speed reach Mach 2, disturbing the material and pulverizing it. The pulverized material is classified in the classification chamber by a classifying wheel. The classified material is screened by an ultrasonic vibrating screen. Screening, iron removal and packaging are all carried out in a constant temperature and humidity room to obtain lithium iron phosphate material.

[0184] Example 3

[0185] The method for preparing lithium iron phosphate material provided in this embodiment includes the following steps:

[0186] (1) Add the ferrous source, oxalate ion source, borate ion source and the first regulator to the bottom liquid. During the feeding process, maintain the pH value at 5.5, the temperature at 55℃, and the feeding time at 60 min. Then continue stirring and reacting for 60 min to obtain a mixture.

[0187] The ferrous source is ferrous sulfate solution, the oxalate ion source is oxalic acid solution, the borate ion source is boric acid solution, the first regulator is ammonium bicarbonate, and the base solution is oxalic acid base solution with a pH of 5.5; the molar ratio of iron, oxalate and borate in the mixture is 1:1:0.2.

[0188] (2) The mixture is filtered to obtain a solid material; the solid material is washed with pure water until the conductivity of the washing water is ≤200μS / cm to obtain a washed material; the washed material is added to pure water for a first slurry treatment to obtain a first slurry with a solid content of 40%.

[0189] (3) Add the first lithium source, the first phosphorus source and the second regulator to the first slurry, stir and dissolve, and then perform the first grinding treatment using a ball mill. The inner wall of the ball mill is made of ceramic material, and the grinding medium is ceramic balls with a particle size of 0.2 mm ± 0.03 mm to obtain the first grinding material with a particle size of 0.2 mm. The first grinding material is spray-dried to obtain the first dry material with a particle size of 5 μm.

[0190] The first lithium source is lithium oxalate; the first phosphorus source is a mixture containing phosphoric acid and ammonium dihydrogen phosphate with a pH of 2.9; the second regulator is PEG6000; the molar ratio of iron in the first slurry, lithium in the first lithium source, phosphorus in the first phosphorus source, and the second regulator is 1:1.1:1.1:0.2.

[0191] (4) After mixing the trivalent iron source, the second phosphorus source, the second lithium source and the carbon source, water is added for the second slurry treatment to obtain a second slurry with a solid content of 40wt%; the second slurry is then subjected to a second grinding treatment using a ball mill with the inner wall of the ball mill made of ceramic material and the grinding media being ceramic balls of 0.3mm±0.03mm to obtain a second grinding material with a particle size of 0.5μm; the second grinding material is then subjected to spray drying treatment to obtain a second dry material with a particle size of 15μm.

[0192] Among them, the trivalent iron source and the second phosphorus source are both iron phosphate, the second lithium source is lithium carbonate, and the carbon source is glucose; the molar ratio of iron or phosphorus in iron phosphate, lithium in lithium carbonate, and glucose is 1:1.03:0.25; the molar ratio of lithium in the first lithium source and lithium in the second lithium source is 1:9.

[0193] (5) After mixing the first dry material and the second dry material, the mixture is pulverized to a particle size of 1μm using an air jet mill to obtain pulverized material. The pulverized material is then calcined in a roller furnace. Nitrogen gas is introduced to maintain the oxygen content in the roller furnace below 1ppm. At the same time, the induced draft fan is turned on in the heating section to keep the humidity in the heat preservation section below 1%. The temperature is first raised to 450℃ at a heating rate of 150℃ / h and kept at that temperature for 3h. Then, the temperature is raised to 700℃ at a heating rate of 50℃ / h and kept at that temperature for 6h. The material is then cooled down to a temperature of ≤100℃ before being removed from the furnace.

[0194] (6) After the calcined material is discharged from the furnace, it is pulverized by a fluidized bed air jet mill. The pulverization process is carried out in a nitrogen atmosphere with a pressure of 0.8 MPa and a temperature of 150°C. Laval nozzles are used in the pulverization chamber to make the airflow speed reach Mach 3, disturbing the material and pulverizing it. The pulverized material is classified in the classification chamber by a classifying wheel. The classified material is screened by an ultrasonic vibrating screen. Screening, iron removal and packaging are all carried out in a constant temperature and humidity room to obtain lithium iron phosphate material.

[0195] Example 4

[0196] The preparation method of lithium iron phosphate material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0197] In step (4), the molar ratio of iron or phosphorus in ferric phosphate, lithium in lithium carbonate, and glucose is 1:1.04:0.3.

[0198] Example 5

[0199] The preparation method of lithium iron phosphate material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0200] In step (1), the ferrous source is replaced with ferrous chloride solution, the oxalate ion source is ammonium oxalate solution, and the borate ion source is ammonium borate solution.

[0201] Example 6

[0202] The preparation method of lithium iron phosphate material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0203] In step (1), the ferrous source is replaced with an equimolar amount of ferrous acetate solution, the oxalate ion source is sodium oxalate solution, and the borate ion source is sodium borate solution.

[0204] In step (3), the first lithium source is lithium acetate.

[0205] Example 7

[0206] The preparation method of lithium iron phosphate material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0207] In step (1), the molar ratio of iron, oxalate and borate in the mixture is 1:0.6:0.05;

[0208] In step (3), the molar ratio of iron in the first slurry, lithium in the first lithium source, phosphorus in the first phosphorus source, and the second regulator is 1:1:1.02:0.12.

[0209] Example 8

[0210] The preparation method of lithium iron phosphate material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0211] In step (1), the molar ratio of iron, oxalate and borate in the mixture is 1:1.2:0.3;

[0212] In step (3), the molar ratio of iron in the first slurry, lithium in the first lithium source, phosphorus in the first phosphorus source, and the second regulator is 1:1.2:1.2:0.3.

[0213] Example 9

[0214] The preparation method of lithium iron phosphate material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0215] In step (4), the molar ratio of iron or phosphorus in ferric phosphate, lithium in lithium carbonate, and glucose is 1:0.96:0.18.

[0216] Example 10

[0217] The preparation method of lithium iron phosphate material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0218] In step (4), the molar ratio of iron or phosphorus in ferric phosphate, lithium in lithium carbonate, and glucose is 1:1.10:0.35.

[0219] Example 11

[0220] The preparation method of lithium iron phosphate material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0221] In step (4), the molar ratio of lithium element in the first lithium source to lithium element in the second lithium source is 1:7.

[0222] Example 12

[0223] The preparation method of lithium iron phosphate material provided in this embodiment is the same as or similar to that in Example 1, except that:

[0224] In step (4), the molar ratio of lithium element in the first lithium source to lithium element in the second lithium source is 1:10.

[0225] Comparative Example 1

[0226] Compared with Example 1, the process of preparing the first dried material is omitted. The second dried material obtained in step (4) is directly calcined according to step (5), and then crushed, sieved, de-ironized and packaged according to step (6) to obtain lithium iron phosphate material.

[0227] Comparative Example 2

[0228] Compared with Example 1, there is no process for preparing the second dry material. After preparing the first dry material according to steps (1) to (3), the first dry material is calcined according to step (5), and then crushed, sieved, de-ironized and packaged according to step (6) to obtain lithium iron phosphate material.

[0229] II. Testing Methods

[0230] (I) Property Testing of Cathode Materials

[0231] 1. SEM Testing: The morphology of the lithium iron phosphate material was characterized using a Zeiss MERLIN Compact Quanta 200FEG field emission scanning electron microscope (SEM). The results are shown in Figures 2 and 3. Figure 2 shows the first morphological result of the lithium iron phosphate material prepared in Example 1 of this application; Figure 3 shows the second morphological result of the lithium iron phosphate material prepared in Example 1 of this application; and Figure 4 shows the morphological result of the lithium iron phosphate material prepared in Comparative Example 1 of this application.

[0232] 2. Element content: The content of each element in the lithium iron phosphate material was tested using an inductively coupled plasma optical emission spectrometer (ICP-OES).

[0233] 3. BET specific surface area: determined by gas adsorption BET method.

[0234] 4. Powder resistivity: Tested using the four-probe method at a pressure of 10 MPa.

[0235] 5. Compacted density: Tested using a compaction density meter with a test pressure of 3T and a compaction time of 30S.

[0236] 6. Tap density: Tested using a tap density meter with 5000 vibrations.

[0237] 7. pH value: Refer to GB / T 9724-2007 General Rules for pH Value Determination of Chemical Reagents.

[0238] 8. Primary particle size distribution: The D10, D50, and D90 particle sizes of the primary particles are measured using a laser particle size analyzer, and the primary particle size distribution is calculated based on (D90-D10)÷D50.

[0239] 9. D50 particle size of primary particles: observed using SEM.

[0240] 10. Free Li: Measured by automatic potentiometric titration.

[0241] 11. Iron leaching: Add 10g of the test sample to 100mL of 0.1mol / L hydrogen fluoride-ethanol solution, stir and dissolve at 45℃ for 30min, then filter. The iron content in the filtrate is measured by ICP-OES, which is the amount of iron leached.

[0242] (II) Properties Testing of Secondary Batteries

[0243] The lithium iron phosphate materials prepared in each embodiment and comparative example were mixed with ultrafine carbon powder (Super P, SP) and polyvinylidene fluoride (PVDF) in a mass ratio of 92:4:4, respectively. Then, N-methylpyrrolidone (NMP) was added to form a slurry, which was then coated onto aluminum foil, dried, and pressed into sheets. Using the lithium sheet as the negative electrode, lithium hexafluorophosphate (LiPF6) as the lithium salt, and ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) in a volume ratio of 1:1:1 as organic solvents, the lithium salt was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1M. Button batteries were then prepared, and their charge specific capacity and discharge specific capacity were measured at 0.1C and 1C rates. Their discharge efficiency was tested at a discharge rate of 0.5P. The test temperature was 25°C, and the voltage range was 2.0-3.75V.

[0244] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0245] The results obtained according to the above test methods are shown in Tables 1 to 3.

[0246] Table 1

[0247] Table 2

[0248] Table 3

[0249] Table 1 shows that the mass fractions of each element in the lithium iron phosphate materials prepared in Examples 1-12 are as follows: Li 4.76%-5.98%, Fe 24.15%-35.78%, P 18.05%-20.89%, B 367ppm-1258ppm, and C 1.45%-1.98%. Therefore, it can be deduced that the lithium iron phosphate material includes a core and a coating layer covering the surface of the core, wherein the molecular formula of the core is Li... a Fe(PO4) b (BO3) c 1.02≤a≤1.05, 0.98≤b≤1.08, 0.01≤c≤0.02, the coating layer includes carbon material, and the mass percentage of carbon material in lithium iron phosphate material is 1.45% to 1.98%.

[0250] As shown in Table 1 and Figures 2-4, the primary particle size (D50) of the lithium iron phosphate material prepared by the traditional solid-state method in Comparative Example 1 is 325 nm, the primary particle spacing is 4.8, and the proportion of particles with a primary particle size of 100 nm to 600 nm is only 91.5%, indicating that its primary particle size distribution is uneven. Comparative Example 2, which uses a first-stage dried material prepared at a lower reaction temperature and undergoes calcination, crushing, and sieving, produces lithium iron phosphate material with a very small primary particle size (D50), but the primary particle spacing reaches 6.1, and the proportion of particles with a primary particle size of 100 nm to 600 nm is only 92.1%, indicating that its primary particle size distribution is also uneven. The lithium iron phosphate materials prepared in Examples 1-12 have a primary particle size of D50 ranging from 142 nm to 305 nm, a primary particle spacing of 1.2 to 2.5 nm, and a primary particle size of 100 nm to 600 nm accounting for 93.5% to 97.3%. Compared with the lithium iron phosphate materials obtained in Comparative Examples 1-2, the primary particle size distribution is more uniform and the primary particle D50 size is smaller. This indicates that Examples 1-12, through the synergistic effect of the first and second drying materials with different reaction temperatures, effectively regulated the growth rate of lithium iron phosphate crystal nuclei, achieving stepwise nucleation and growth, thereby producing lithium iron phosphate materials with uniform primary particle size distribution and smaller primary particle size. Meanwhile, the lithium iron phosphate materials prepared in Examples 1-6 have a smaller particle spacing and a higher proportion of primary particles with a primary particle size of 100 nm to 600 nm, indicating that precise control of the molar ratio of each raw material can improve the uniformity of primary particle size distribution.

[0251] As shown in Table 2, the compacted density of the lithium iron phosphate materials prepared in Examples 1-12 was 2.25 g / mL to 2.55 g / mL, the tapped density was 1.05 g / mL to 1.34 g / mL, and the BET specific surface area was 8.1 m². 2 / g~14.1m 2 The powder resistivity ranges from 6.16 Ω·cm to 20.4 Ω·cm, the free Li content ranges from 213 ppm to 986 ppm, the iron leaching amount ranges from 14.7 ppm to 125.8 ppm, and the pH value ranges from 8.38 to 10.89. Furthermore, the lithium iron phosphate materials in Examples 1-6 have a compacted density of ≥2.41 g / mL, a tapped density of ≥1.21 g / mL, and a BET specific surface area of ​​≥11.8 m². 2 The following parameters were observed: powder resistivity below 9.12 Ω·cm, free Li content below 548 ppm, iron leaching below 17.5 ppm, and pH value between 8.57 and 9.13. This indicates that controlling the molar ratio of each raw material within a suitable range is beneficial for obtaining higher compaction density and tapped density, more suitable BET specific surface area, lower powder resistivity, lower free Li content, lower iron leaching content, and a suitable pH value.

[0252] Compared with Example 1, the molar ratio of lithium element in the first lithium source to lithium element in the second lithium source in Examples 11-12 was not controlled within the range of 1:(8-9). This resulted in a significant decrease in the compaction density and tap density of the lithium iron phosphate material prepared in Example 11, a higher BET specific surface area, and a significant increase in powder resistivity. Although the lithium iron phosphate material prepared in Example 12 had higher compaction density and tap density, its powder resistivity was more than twice that of Example 1. This means that the conductivity of the lithium iron phosphate material prepared in Example 12 was significantly worse, thus affecting the charge / discharge specific capacity, rate performance, energy efficiency, and cycle performance. This indicates that controlling the feeding ratio of the first and second lithium sources is beneficial to significantly reduce the powder resistivity of the lithium iron phosphate material while increasing the compaction density and tap density, thereby greatly improving its conductivity and achieving better electrical performance.

[0253] Compared with Comparative Examples 1 and 2, the resistivity of the lithium iron phosphate powder obtained in Example 1 was significantly reduced. This demonstrates that uniformity of primary particle size distribution can significantly reduce the resistivity of lithium iron phosphate powder, improve its conductivity, and ensure good consistency during discharge. Simultaneously, compared with Comparative Example 1, the specific surface area of ​​the lithium iron phosphate material obtained in Example 1 was significantly increased, meaning a larger contact area between the lithium iron phosphate material and the electrolyte, increasing the channels for ion transport and electron conduction, which is beneficial for reducing internal resistance, increasing specific capacity, and improving rate performance. Compared with Comparative Example 2, the lithium iron phosphate material obtained in Example 1 had higher compaction density and tap density, a more suitable BET specific surface area, and lower free Li content, iron dissolution, and pH value. This demonstrates that by utilizing the synergistic effect of the first and second drying materials, a more suitable specific surface area can be obtained for lithium iron phosphate material while ensuring uniformity of primary particle size distribution, thus improving various properties such as compaction density and tap density.

[0254] As shown in Table 3, the lithium iron phosphate materials prepared in Examples 1-12 have the following specific capacities: 0.1C initial charge specific capacity: 160.5 mAh / g to 163.5 mAh / g; 0.1C initial discharge specific capacity: 157.6 mAh / g to 159.9 mAh / g; 1C initial charge specific capacity: 143.7 mAh / g to 148.9 mAh / g; 1C initial discharge specific capacity: 144.8 mAh / g to 147.9 mAh / g; and 0.5P discharge efficiency: 94.6%. The specific capacity, rate performance, discharge efficiency, and cycle performance of the lithium iron phosphate material prepared by the conventional solid-state method in Comparative Example 1 are significantly better than those of the lithium iron phosphate material prepared by the conventional solid-state method. This demonstrates that the uniformity of the primary particle size distribution can effectively ensure the consistency of the lithium iron phosphate material during discharge, thereby ensuring that the battery has higher specific capacity, better rate performance, higher discharge efficiency, and better cycle performance. Compared with the lithium iron phosphate material obtained by calcining the first dry material at a lower reaction temperature in Comparative Example 2, the lithium iron phosphate material prepared in Example 1 also shows a significant increase in specific capacity, rate performance, discharge efficiency, and cycle performance. This is because the uniformity of the primary particle size distribution can obtain lower powder resistivity, higher compaction density and tap density, and a more suitable BET specific surface area, thereby ensuring that the specific capacity, rate performance, discharge efficiency, and cycle performance of the material are comprehensively improved. Meanwhile, the lithium iron phosphate materials prepared in Examples 1-6 exhibited a 0.1C initial charge specific capacity ≥162.9 mAh / g, a 0.1C initial discharge specific capacity ≥159.2 mAh / g, a 1C initial charge specific capacity ≥148.3 mAh / g, a 1C initial discharge specific capacity ≥147.1 mAh / g, a 0.5P discharge efficiency ≥95.30%, and a room temperature cycling capacity retention rate ≥98.2% after 1000 cycles at 1C rate. The charge / discharge specific capacity, discharge efficiency, and cycling performance were significantly better than those in Examples 7-12, indicating that controlling the molar ratio of each raw material within a suitable range can further improve the uniformity of the primary particle size distribution of the lithium iron phosphate material, increase the compaction density and tap density, and reduce the powder resistivity, thus exhibiting superior electrical performance.

[0255] In summary, this application uses raw materials with low melting point and low decomposition temperature to prepare a first dry material with a low chemical reaction temperature, and uses raw materials with high melting point and high decomposition temperature to prepare a second dry material with a high chemical reaction temperature. The difference in chemical reaction temperature between the two dry materials is used to achieve stepwise nucleation and growth, avoiding the problem of extremely uneven primary particle size distribution caused by the disordered growth and instantaneous rapid nucleation of lithium iron phosphate crystal nuclei. This results in a lithium iron phosphate material with uniform primary particle size distribution and high concentration, thus exhibiting advantages such as high specific capacity, good rate performance, high discharge efficiency, and excellent cycle performance.

[0256] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

A lithium iron phosphate material, characterized in that, The lithium iron phosphate material includes a core, the molecular formula of which is Li. a Fe(PO4) b (BO3) c Among them, 1.02≤a≤1.05, 0.98≤b≤1.08, 0.01≤c≤0.02, and the proportion of particles with a primary diameter of 100nm~600nm is ≥93.5%. The lithium iron phosphate material according to claim 1 is characterized in that, The primary particle size of the lithium iron phosphate material is 0.5 to 2.5 mm, and the D50 particle size of the primary particle is 140 nm to 320 nm. The compacted density of the lithium iron phosphate material is 2.20 g / mL to 2.60 g / mL, and the BET specific surface area of ​​the lithium iron phosphate material is 8.0 m². 2 / g~15.0m 2 / g, the resistivity of the lithium iron phosphate material powder is 5.00Ω·cm~22.00Ω·cm. The lithium iron phosphate material according to claim 1 or 2 is characterized in that, The lithium iron phosphate material further includes a coating layer, which coats the surface of the core; The coating layer is made of carbon material, and the carbon material accounts for 1.10% to 2.00% of the mass of the lithium iron phosphate material. A method for preparing lithium iron phosphate material, characterized in that, Includes the following steps: A mixture of ferrous ion source, oxalate ion source, borate ion source and first regulator is obtained by first reaction treatment in a base solution. The mixture is subjected to solid-liquid separation and a first pulping process to obtain a first slurry; After the first slurry is mixed with the first lithium source, the first phosphorus source and the second regulator, it is subjected to the first grinding process and the first drying process to obtain the first dried material; After mixing the trivalent iron source, the second phosphorus source, the second lithium source and the carbon source, the mixture undergoes a second pulping treatment, a second grinding treatment and a second drying treatment to obtain the second dried material. The first dried material and the second dried material are mixed and then calcined to obtain the lithium iron phosphate material. The method for preparing lithium iron phosphate material as described in claim 4, characterized in that, The step of mixing the ferrous source, the oxalate ion source, the borate ion source, and the first regulator in the base solution and obtaining the mixture through a first reaction treatment includes: The ferrous source, the oxalate ion source, the borate ion source, and the first regulator are mixed in a base solution and subjected to a first treatment time at a first temperature to obtain the mixture. The pH value of the base solution is 4 to 5.5, the first temperature is 35°C to 55°C, and the first treatment time is 60 min to 120 min. The method for preparing lithium iron phosphate material as described in claim 4 or 5, characterized in that, The molar ratio of iron, oxalate, and borate in the mixture is 1:(0.8-1):(0.1-0.2). The molar ratio of iron in the first slurry, lithium in the first lithium source, phosphorus in the first phosphorus source, and the second regulator is 1:(1.04~1.1):(1.04~1.1):(0.15~0.2); The molar ratio of iron in the trivalent iron source, phosphorus in the second phosphorus source, lithium in the second lithium source, and carbon in the carbon source is 1:1:(1.02~1.04):(1.2~1.8); The molar ratio of lithium element in the first lithium source to lithium element in the second lithium source is 1:(8-9). The method for preparing lithium iron phosphate material as described in claim 6, characterized in that, The ferrous source is selected from at least one of ferrous sulfate, ferrous chloride and ferrous acetate; the oxalate ion source includes at least one of oxalic acid, ammonium oxalate and sodium oxalate; the borate ion source includes at least one of boric acid, ammonium borate and sodium borate; and the first regulator includes at least one of ammonium bicarbonate and sodium bicarbonate. The first phosphorus source is selected from at least one of phosphoric acid, diammonium hydrogen phosphate, and ammonium dihydrogen phosphate; the first lithium source is selected from at least one of lithium oxalate and lithium acetate; and the second regulator includes at least one of polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, and polycarboxylic acid compounds. The trivalent iron source includes iron phosphate, the second phosphorus source includes iron phosphate, the second lithium source includes lithium carbonate, and the carbon source is selected from at least one of glucose, sucrose, starch, and cellulose. The method for preparing lithium iron phosphate material according to any one of claims 4 to 5 and 7, characterized in that, The step of mixing the first dried material and the second dried material, followed by calcination to obtain the lithium iron phosphate material, includes: The first dried material and the second dried material are mixed and then pulverized to obtain pulverized material; The pulverized material is subjected to a first protective atmosphere, a second temperature for a second treatment time, and then a third temperature for a third treatment time to obtain the lithium iron phosphate material. Wherein, the D50 particle size of the first dried material is 2μm to 5μm, the D50 particle size of the second dried material is 5μm to 15μm, the D50 particle size of the pulverized material is 0.5μm to 1μm, the second temperature is 350℃ to 450℃, the second processing time is 2h to 3h, the third temperature is 600℃ to 700℃, and the third processing time is 3h to 6h. A positive electrode sheet, characterized in that, The present invention includes a current collector and a positive electrode material disposed on at least one surface of the current collector along its thickness direction, wherein the positive electrode material includes lithium iron phosphate material as described in any one of claims 1 to 3 or lithium iron phosphate material prepared by the preparation method as described in any one of claims 4 to 8. A secondary battery, characterized in that, It includes a positive electrode, a negative electrode, and a separator, wherein the separator is disposed between the positive electrode and the negative electrode, and the positive electrode is the positive electrode as described in claim 9.

Images