A method for preparing lithium iron phosphate

Abstract

The application provides a preparation method of lithium iron phosphate, which comprises the following steps: under an inert atmosphere, raw materials including ferrous carbonate, a phosphorus source, a lithium source and a carbon source are subjected to kneading reaction and calcination treatment in sequence to obtain lithium iron phosphate; and the solid content of the ferrous carbonate is 50-60%. The preparation method of the lithium iron phosphate can reduce the preparation cost of the lithium iron phosphate and improve the yield of the lithium iron phosphate.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a method for preparing lithium iron phosphate. Background Technology

[0002] Lithium iron phosphate (LiFePO4) is widely used as a cathode material for lithium-ion batteries in energy storage systems (such as home energy storage and grid peak shaving), electric vehicles (EVs), and power tools. In energy storage scenarios, its high cycle stability (cycle life of over 2000 cycles) and excellent safety (high thermal stability and low susceptibility to thermal runaway) make lithium iron phosphate the preferred material for large-scale energy storage systems.

[0003] However, with the rapid growth in market demand for lithium-ion batteries, the high cost of cathode materials (approximately 40%) has become a key factor restricting the large-scale application of lithium iron phosphate. Existing lithium iron phosphate preparation processes generally suffer from incomplete raw material reaction and conversion, large amounts of by-products, and low finished product output efficiency, making it difficult to meet the industry's needs for continuous and large-scale mass production, thus hindering the further popularization and application of lithium iron phosphate in the new energy field.

[0004] Therefore, developing a low-cost preparation method that can improve the yield of lithium iron phosphate products is of great significance for promoting the large-scale application of lithium iron phosphate. Summary of the Invention

[0005] This invention provides a method for preparing lithium iron phosphate, which can reduce the preparation cost of lithium iron phosphate while increasing the yield of lithium iron phosphate.

[0006] This invention provides a method for preparing lithium iron phosphate, comprising:

[0007] Under an inert atmosphere, raw materials including ferrous carbonate, phosphorus source, lithium source and carbon source are kneaded and roasted in sequence to obtain lithium iron phosphate.

[0008] The solid content of the ferrous carbonate is 50-60%.

[0009] In the preparation method described above, the ferrous carbonate is obtained by aging a ferrous salt and a carbonate.

[0010] The preparation method described above further includes determining the solid content of ferrous carbonate in the aging reaction product based on the iron ion content in the aging reaction product.

[0011] In the preparation method described above, the kneading reaction is carried out at a temperature of 50-90°C for 4-8 hours, a rotation speed of 400-800 rpm, and a flow rate of 100-200 m³ / h for the inert atmosphere. 3 / h.

[0012] In the preparation method described above, the calcination treatment includes a first calcination treatment and a second calcination treatment, wherein the temperature of the first calcination treatment is lower than the temperature of the second calcination treatment.

[0013] In the preparation method described above, the first calcination treatment is carried out at a temperature of 300-500°C for 2-5 hours, and the flow rate of the inert atmosphere is 3-20 m³ / h. 3 / h; and / or,

[0014] In the second calcination process, the temperature is 720~800℃, the time is 8~12h, and the flow rate of the inert atmosphere is 3~20m³. 3 / h.

[0015] In the preparation method described above, the aging reaction includes: adding a carbonate solution to a ferrous salt solution, wherein the flow rate of the carbonate solution is 2.5~3.5 m / s. 3 / h.

[0016] In the preparation method described above, the aging reaction is carried out at a temperature of 25-35°C for 10-24 hours.

[0017] In the preparation method described above, the raw materials further include additives, and the additives include metallic elements;

[0018] The metallic element includes at least one selected from Al, Mg, Ni, Co, Ti, Cu, Ca, Nb, Cr, Zn, La, Sb, Te, Sr, W, In, Y, and V.

[0019] In the preparation method described above, the molar ratio of lithium, iron, phosphorus, and the metal element in the additives in the raw materials is (1.02~1.08):(0.96~0.99):1:(0.002~0.01); and / or,

[0020] The mass of the carbon source is 10-15% of the mass of the ferrous carbonate.

[0021] This invention improves the uniformity of raw material mixing and the stability of the reaction process by sequentially kneading and calcining raw materials including ferrous carbonate, phosphorus source, lithium source and carbon source under an inert atmosphere, and controlling the solid content of ferrous carbonate to 50-60%, thereby promoting the formation of lithium iron phosphate phase and increasing the yield of lithium iron phosphate while reducing the preparation cost of lithium iron phosphate. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 Here are SEM images of lithium-containing precursors in some embodiments of the present invention;

[0024] Figure 2 The images shown are SEM images of lithium iron phosphate in some embodiments of the present invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. In the absence of conflict, the following embodiments and features can be combined with each other.

[0026] The preparation technology of lithium iron phosphate (LFP) cathode materials belongs to the field of lithium-ion battery material manufacturing and is widely used in power batteries for new energy vehicles, energy storage batteries, and electrochemical energy storage scenarios with high requirements for safety and cycle life. In actual production, this type of material usually requires first obtaining a uniform iron-containing precursor, which is then combined with phosphorus, lithium, and carbon sources in a subsequent heat treatment process to finally form LFP powder that can be used for electrode preparation. Since the particle morphology, density, impurity content, and conductivity of the material directly affect the battery's compaction density, low-temperature discharge performance, and batch consistency, the preparation process focusing on precursor state control, raw material mixing uniformity, and calcination atmosphere management has become a key link in the large-scale production of LFP.

[0027] Existing lithium iron phosphate (LFP) preparation methods mostly employ the lithium iron phosphate salt method or the lithium iron oxalate salt method. This involves first mixing an iron source with a phosphorus source, a lithium source, and a carbon source, then calcining the mixture at a high temperature to induce a solid-state reaction and generate LFP crystals. While this process can produce the target product, it typically relies on numerous pretreatment steps and is highly sensitive to mixing uniformity, temperature rise, and atmospheric conditions. In actual production, insufficient dispersion of raw materials or inconsistent local reaction rates can easily lead to microscopic distribution deviations in lithium, iron, and phosphorus, resulting in problems such as impurity phase residues, uneven particle size, and crystal agglomeration, thus reducing the yield of LFP.

[0028] Therefore, improving the yield of lithium iron phosphate is an urgent technical problem to be solved. The inventors discovered that by sequentially kneading and calcining raw materials including ferrous carbonate, phosphorus source, lithium source, and carbon source under an inert atmosphere, lithium iron phosphate can be obtained. By controlling the solid content of ferrous carbonate, the aforementioned problem can be solved.

[0029] This invention provides a method for preparing lithium iron phosphate, comprising:

[0030] Under an inert atmosphere, raw materials including ferrous carbonate, phosphorus source, lithium source and carbon source are kneaded and roasted in sequence to obtain lithium iron phosphate.

[0031] The solid content of ferrous carbonate is 50-60%.

[0032] Specifically, in the kneading reaction, ferrous carbonate contacts and reacts with phosphorus and lithium sources to generate an amorphous precursor of lithium iron phosphate. A carbon source coats the surface of the ferrous carbonate and the forming amorphous precursor, forming a carbon source coating layer, thus yielding a lithium-containing precursor. In the calcination process, the lithium-containing precursor gradually dehydrates and undergoes a crystal phase change, forming lithium iron phosphate crystals. The carbon source pyrolyzes to form amorphous carbon, which coats the surface of the lithium iron phosphate crystals, ultimately yielding lithium iron phosphate. Since both the kneading reaction and the calcination process are carried out in an inert atmosphere, ferrous iron can remain stable during the preparation of lithium iron phosphate due to the absence of oxygen.

[0033] In the preparation method of this invention, the solid content of ferrous carbonate is 50-60%, and the remaining liquid phase is water. An appropriate amount of water can form a continuous water film on the surface of ferrous carbonate, isolating oxygen in the air and reducing the rate at which ferrous ions are oxidized to ferric ions, thus ensuring the purity of the reaction raw materials. At the same time, in the kneading reaction, water can provide a wetting contact interface for the phosphorus source and lithium source, promoting full diffusion of ions and achieving uniform dispersion of the phosphorus source, lithium source, and carbon source. This allows ferrous ions, lithium source, and phosphorus source to be converted into lithium iron phosphate to the maximum extent, reducing the generation of by-products. Meanwhile, a small amount of ferrous carbonate in the lithium-containing precursor can decompose into carbon monoxide and carbon dioxide during the calcination process. Carbon monoxide has reducing properties and can protect divalent iron from oxidation, thus improving the yield of lithium iron phosphate.

[0034] Based on this, relying on the inherent antioxidant effect of water in ferrous carbonate, no additional reducing agents, antioxidants, or other auxiliaries need to be added to the reaction system, nor is it necessary to configure complex anti-oxidation equipment and protection systems, thus reducing the cost of raw material input and equipment investment. Furthermore, the preparation method of this invention also omits traditional high-energy-consuming processes such as high-temperature drying, deep dehydration, and multi-stage grinding, shortening the production time of lithium iron phosphate and reducing energy consumption costs. Therefore, the preparation method of this invention also has excellent economic efficiency, is suitable for the continuous production process of lithium iron phosphate, can be compatible with existing industrial equipment, and realizes the large-scale production of lithium iron phosphate.

[0035] Furthermore, during the roasting stage, the moisture in the lithium-containing precursor gradually escapes, making the generated lithium iron phosphate particles more regular in shape. The moisture also allows the lithium source, phosphorus source, and carbon source to be more evenly dispersed before roasting, making it less likely for the particles to agglomerate. After roasting, lithium iron phosphate with uniform particle size, dense packing, and few internal defects is formed, thereby increasing the compaction density of lithium iron phosphate.

[0036] Furthermore, since the lithium iron phosphate of the present invention has excellent compaction density, when the lithium iron phosphate of the present invention is used in lithium-ion batteries, the stacking between lithium iron phosphate particles is more compact, the actual migration path of lithium ions in the electrolyte is greatly shortened, the resistance to lithium ion migration at low temperatures is reduced, the intercalation and deintercalation of lithium ions during charging and discharging is more stable, and thus the cycle stability of lithium-ion batteries at low temperatures is improved.

[0037] The present invention does not specifically limit the inert gas, and can be any inert gas commonly used in the art. For example, the inert gas can be nitrogen, argon or a mixture thereof.

[0038] The present invention does not impose any particular limitation on the carbon source, and can use any carbon source commonly used in the art. For example, the carbon source can be at least one of glucose, sucrose, dextrin, citric acid, fructose, polyethylene glycol, polyvinyl alcohol, and polypropylene glycol.

[0039] The present invention does not impose any particular limitation on the lithium source, and can be any lithium source commonly used in the art. For example, the lithium source can be at least one of lithium carbonate, lithium hydroxide, lithium oxalate, trilithium phosphate, lithium acetate, and lithium dihydrogen phosphate.

[0040] The present invention does not impose any particular limitation on the phosphorus source, and can be any phosphorus source commonly used in the art. For example, the phosphorus source can be at least one of lithium dihydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

[0041] It should be understood that the above examples are merely illustrative and not limiting. All equivalent substitutions or variations made based on the technical concept of this invention should fall within the protection scope of this invention.

[0042] In some embodiments of the present invention, ferrous carbonate is obtained by aging ferrous salt and carbonate.

[0043] Specifically, ferrous salts (such as ferrous sulfate) and carbonates (such as ammonium bicarbonate) undergo an aging reaction to generate ferrous carbonate nuclei. Through this aging reaction, the ferrous carbonate nuclei further grow and rearrange, forming a wet precipitate with a uniform structure and low agglomeration tendency. This invention uses an aging reaction to allow the initially generated ferrous carbonate nuclei to grow, rearrange, and stabilize, thereby improving particle uniformity, reducing agglomeration tendency, and minimizing impurities. This provides a foundation for subsequent uniform contact and solid-phase reaction with phosphorus, lithium, and carbon sources, improving the conversion rate of raw materials and the yield of lithium iron phosphate.

[0044] In one possible implementation, the ferrous salt may be at least one of ferrous sulfate, ferrous chloride, or ferrous nitrate; the carbonate may be at least one of ammonium bicarbonate, sodium carbonate, or ammonium carbonate; further, the ferrous salt is ferrous sulfate and the carbonate is ammonium bicarbonate, so as to facilitate the formation of ferrous carbonate precipitate under milder conditions.

[0045] In another possible implementation, the ferrous salt is ferrous sulfate and the carbonate is sodium carbonate to improve the controllability of the precipitation reaction; a composite salt system that can release carbonate ions can also be used to adapt to different concentrations and equipment conditions.

[0046] In some embodiments of the present invention, the solid content of ferrous carbonate in the aging reaction product is determined based on the iron ion content in the aging reaction product.

[0047] Specifically, after the aging reaction is completed, the residual iron ions, free iron ions, or total iron ions in the reaction system are quantitatively detected, and the degree of ferrous carbonate formation and the proportion of wet solids in the aged product are inferred in reverse based on the correspondence between iron ions and the generation and consumption of ferrous carbonate.

[0048] The lower the iron ion content, the more complete the reaction between the ferrous salt and the carbonate, and the higher the solid content of the ferrous carbonate. This technique transforms traditional empirical judgment into quantifiable and feedback-based process parameters, ensuring the stability of the solid content of ferrous carbonate and the rheological properties between raw materials in the subsequent kneading reaction, thereby improving the uniformity of raw material mixing and the conversion rate of the kneading reaction.

[0049] In one possible embodiment, the iron ion content can be detected using one or more combinations of titration analysis, spectrophotometry, atomic absorption spectrometry, or inductively coupled plasma analysis. Titration is suitable for rapidly determining the concentration of residual iron ions in the solution, spectrophotometry is suitable for quantitatively tracking low concentrations of iron ions, and atomic absorption and inductively coupled plasma methods are suitable for improving detection accuracy and batch consistency. In another possible embodiment, an online iron ion electrode, process analyzer, or automatic sampling and detection module can be arranged on the discharge pipeline, circulation pipeline, or filtrate pipeline of the aging reactor to achieve near real-time monitoring. In yet another exemplary embodiment, the wet precipitate sample can be digested first, and then the total iron content in the digestion solution can be indirectly determined, thereby converting the ferrous carbonate solid content in the aging product. Generally, the lower the iron ion content in the aging reaction product, the more complete the reaction between the iron source and carbonate ions, and the higher the proportion of ferrous carbonate solids formed.

[0050] In some embodiments, the mass percentage of iron ions in the aging reaction product is <0.1%.

[0051] In some embodiments, when ferrous sulfate is used to carry out an aging reaction with carbonates, the mass percentage of sulfate ions in the aging reaction product is <0.05%.

[0052] In some implementations, the content of iron ions in the aging reaction can be confirmed by potassium dichromate titration.

[0053] The present invention does not specifically limit the carbonate solution, and can be any carbonate solution commonly used in the art. For example, the carbonate solution can be an ammonium bicarbonate solution or a sodium carbonate solution.

[0054] The present invention does not specifically limit the ferrous salt solution, and can be any ferrous salt solution commonly used in the art. For example, the ferrous salt solution can be at least one of ferrous sulfate solution, ferrous chloride solution, and ferrous nitrate solution.

[0055] In some embodiments of the present invention, the kneading reaction is carried out at a temperature of 50-90°C, for a time of 4-8 hours, at a rotation speed of 400-800 rpm, and with an inert atmosphere flow rate of 100-200 m³ / h. 3 At a speed of / h, the oxidation of ferrous ions by oxygen can be suppressed by mild reaction conditions. At the same time, water can be used to promote uniform contact between phosphorus source, lithium source and ferrous carbonate, accelerate ion diffusion, and make the raw materials react more fully to generate lithium-containing precursors. Furthermore, the control of rotation speed and gas flow rate can optimize the uniformity of material mixing, reduce the generation of by-products caused by excessive local reaction, and further improve the yield of lithium iron phosphate.

[0056] For example, in the kneading reaction, the temperature can be any one or a combination of two of 50°C, 60°C, 70°C, 80°C, and 90°C; the time can be any one or a combination of two of 4h, 5h, 6h, 7h, and 8h; the rotational speed can be any one or a combination of two of 400rpm, 500rpm, 600rpm, 700rpm, and 800rpm; and the inert atmosphere flow rate can be 100m³. 3 / h, 120m 3 / h, 140m 3 / h, 160m 3 / h, 180m 3 / h、200m 3 A range consisting of any one of / h or any two of them.

[0057] In some embodiments of the present invention, the calcination process includes a first calcination process and a second calcination process, wherein the temperature of the first calcination process is lower than the temperature of the second calcination process.

[0058] The first roasting process is used for low-temperature pretreatment of the raw materials after the kneading reaction, while the second roasting process is used to complete the main crystallization reaction and final phase formation at a higher temperature. The two processes are connected by a temperature gradient, forming a progressively increasing thermal process. The first roasting process can be understood as a pre-decomposition roasting stage, which aims to slowly release or rearrange residual moisture, some low-boiling-point volatile components, and intermediate structures formed during the kneading process in the lithium-containing precursor. This reduces the risk of agglomeration, cracking, or abnormal expansion of internal pores in the lithium-containing precursor due to excessively rapid gas escape in the subsequent high-temperature stage. The second roasting process can be understood as a phase-forming roasting stage, which aims to promote further solid-phase diffusion reactions between the phosphorus source, lithium source, carbon source, and iron-containing components to form lithium iron phosphate with complete crystals and uniform composition.

[0059] This invention employs a two-stage roasting method, first at a low temperature and then at a high temperature, which allows the lithium-containing precursor to react gradually under heat, reducing raw material loss and impurity generation, improving the overall reaction conversion degree of the raw materials, and further increasing the yield and overall productivity of lithium iron phosphate products.

[0060] In some embodiments of the present invention, in the first calcination treatment, the temperature is 300~500℃, the time is 2~5h, and the flow rate of the inert atmosphere is 3~20m³. 3 At a flow rate of / h, adsorbed water, crystal water, and some low-molecular-weight volatile components in the lithium precursor can be smoothly released; at the same time, a suitable inert atmosphere flow rate can smoothly guide the gas generated by the reaction of the lithium precursor, so that the internal pores of lithium iron phosphate grains can be uniformly released during the growth process, the particle morphology develops regularly, and the stacking arrangement is more compact, thereby improving the compaction density of lithium iron phosphate.

[0061] For example, in the first calcination process, the temperature can be any one of 300°C, 350°C, 400°C, 450°C, 500°C, or any combination thereof; the time can be any one of 2h, 3h, 4h, 5h, or any combination thereof; and the flow rate of the inert atmosphere can be 3m³ / h. 3 / h、6m 3 / h、9m 3 / h、12m 3 / h, 15m 3 / h、18m 3 / h, 20m 3 A range consisting of any one of / h or any two of them.

[0062] In the second calcination process, the temperature is 720~800℃, the time is 8~12h, and the flow rate of the inert atmosphere is 3~20m³. 3 At a rate of / h, it can promote the full diffusion reaction between the lithium source, phosphorus source and iron-containing intermediate and complete the stable formation of the lithium iron phosphate crystal phase; and the residual ferrous carbonate in the lithium precursor can decompose to generate carbon monoxide and carbon dioxide. Among them, carbon monoxide can continuously create a reducing atmosphere, further preventing ferrous iron from being oxidized to ferric iron in the second roasting process, and further improving the raw material conversion rate and the yield of lithium iron phosphate.

[0063] For example, in the second calcination process, the temperature can be any one of 720°C, 740°C, 760°C, 780°C, 800°C, or any combination thereof; the time can be any one of 8h, 9h, 10h, 11h, 12h, or any combination thereof; and the flow rate of the inert atmosphere can be 3m³ / h. 3 / h、6m 3 / h、9m 3 / h、12m 3 / h, 15m 3 / h、18m 3 / h, 20m 3 A range consisting of any one of / h or any two of them.

[0064] In some embodiments of the present invention, the aging reaction includes: adding a carbonate solution to a ferrous salt solution, wherein the flow rate of the carbonate solution is 2.5~3.5 m / s. 3 / h.

[0065] Adding a carbonate solution to a ferrous salt solution for aging, utilizing the weakly acidic environment of the ferrous salt solution itself, preferentially promotes the combination of ferrous ions and carbonate ions to form ferrous carbonate precipitate, laying the foundation for improving the yield of the finished product; the flow rate of the carbonate solution is 2.5~3.5m. 3At a rate of / h, the carbonate ion concentration in the aging reaction system can be maintained within a stable range, allowing ferrous carbonate grains to nucleate and grow uniformly. The resulting grains are of uniform size and have a dense structure, which is beneficial for the full reaction and conversion of raw materials, improving the yield of lithium iron phosphate. It also makes the particles obtained from subsequent roasting more compact, further optimizing the compaction density of lithium iron phosphate.

[0066] For example, the flow rate of carbonates can be 2.5 m. 3 / h, 3.0m 3 / h, 3.5m 3 A range consisting of any one of / h or any two of them.

[0067] In some embodiments of the present invention, during the aging reaction, when the temperature is 25~35°C and the time is 10~24h, the carbonate and ferrous salt can react more fully under mild conditions, and the ferrous carbonate crystals are fully developed and more uniformly distributed, which further improves the conversion rate of the raw materials and thus improves the yield of lithium iron phosphate.

[0068] In some embodiments of the present invention, the raw materials also include additives, which include metallic elements;

[0069] Metallic elements include at least one of Al, Mg, Ni, Co, Ti, Cu, Ca, Nb, Cr, Zn, La, Sb, Te, Sr, W, In, Y, and V.

[0070] Specifically, additives refer to metal dopant components introduced into the system along with ferrous carbonate, phosphorus source, lithium source, and carbon source during the preparation of lithium iron phosphate. These additives participate in solid solution, substitution, site occupation, or surface modification during precursor formation, crystal phase transformation, and calcination crystallization stages, thereby regulating the crystal structure and interface state of the final lithium iron phosphate material. The role of additives is to adjust the coordination environment of Fe, Li, and P-related sites through trace modifications of the local crystal lattice by metal elements, inhibiting abnormal grain growth during sintering and reducing the probability of residual impurity phase formation. Simultaneously, some metal elements can enhance the surface structural stability of the material, weaken the tendency of interfacial side reactions during high-temperature calcination and subsequent cycling, thus helping to improve the rate performance, cycle life, and batch consistency of lithium-ion batteries, including lithium iron phosphate, at low temperatures.

[0071] For example, aluminum can optimize the crystal structure of lithium iron phosphate, improving the structural stability and cycle life of lithium-ion batteries; magnesium can enhance the ion conductivity in lithium-ion batteries, improving their electrochemical rate performance; titanium can refine lithium iron phosphate grains and reduce electrode polarization; rare earth and transition metal elements such as lanthanum and niobium can further regulate the microstructure of lithium iron phosphate crystals, improving their conductivity and stability during cycling.

[0072] In some embodiments of the present invention, when the molar ratio of lithium, iron, phosphorus, and metal elements in the additives is (1.02~1.08):(0.96~0.99):1:(0.002~0.01), the raw material components can achieve an optimal balance, and the materials can be synergistically matched. This ensures precise and controllable phase formation during the solid-phase reaction, promotes the formation of a complete crystal structure of lithium iron phosphate, and gives lithium iron phosphate a higher yield. At the same time, it optimizes the microstructure and particle packing state of lithium iron phosphate grains. Furthermore, the appropriate ratio of each element allows the metal elements to uniformly enter the lithium iron phosphate lattice, resulting in better reaction compatibility between materials, ensuring uniform and consistent distribution of system components, and improving the structural regularity of lithium iron phosphate.

[0073] For example, the molar ratio between lithium, iron, phosphorus and metal elements in the raw materials and additives can be any one of 1.02:0.96:1:0.002, 1.08:0.96:1:0.002, 1.08:0.99:1:0.01 or any two of them.

[0074] In some embodiments of the present invention, when the mass of the carbon source in the raw material is 10-15% of the mass of ferrous carbonate, a uniform and continuous carbon coating layer can be formed on the surface of lithium iron phosphate particles; at the same time, a sufficient reducing environment can be created during the roasting process, further reducing the possibility of oxidation of ferrous iron, ensuring the stable formation of lithium iron phosphate crystal phase, and improving the yield of lithium iron phosphate.

[0075] For example, the mass of the carbon source can be any one of 10%, 11%, 12%, 13%, 14%, or 15% of the mass of ferrous carbonate, or any combination thereof.

[0076] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0077] Example 1

[0078] The preparation of lithium iron phosphate in this embodiment includes the following steps:

[0079] (a) Under nitrogen protection, ammonium bicarbonate solution is added to ferrous sulfate solution to carry out an aging reaction, wherein the molar ratio of ferrous sulfate to ammonium bicarbonate is 1:1; and the flow rate of ammonium bicarbonate solution is 2.5 m. 3After the ammonium bicarbonate solution was added, the pH of the reaction system was adjusted to 8.5, and the reaction was carried out at 25°C for 14 hours to obtain a mixture. The mixture was then subjected to solid-liquid separation to obtain a precipitate and a supernatant. The mass percentage of iron ions in the supernatant was determined to be 0.03% by potassium dichromate titration, and the mass of ferrous carbonate in the precipitate was calculated. The precipitate was washed with deionized water to make the solid content of the precipitate 55%.

[0080] (b) Under nitrogen protection, 1 mol of lithium dihydrogen phosphate, glucose, and titanium dioxide were added sequentially to the precipitate, and a kneading reaction was carried out at 60°C for 6 hours with a nitrogen flow rate of 120 m³ / h. 3 At a speed of 600 rpm, a lithium-containing precursor was obtained.

[0081] (c) Under nitrogen protection, the lithium-containing precursor is placed in a rotary kiln for a first calcination treatment, and then transferred to a track kiln for a second calcination treatment under nitrogen atmosphere; wherein the temperature of the first calcination treatment is 300℃, the time is 4h, and the nitrogen flow rate is 15m³ / h. 3 / h; the second calcination treatment was carried out at a temperature of 750℃ for 10.5h, with a nitrogen flow rate of 15m³ / h. 3 / h, to obtain lithium iron phosphate.

[0082] In the above steps, the molar ratio of lithium, iron, phosphorus and titanium in the raw materials is 1.03:0.973:1:0.01, and the mass of glucose is 13% of the mass of ferrous carbonate.

[0083] Example 2

[0084] The preparation steps of lithium iron phosphate in this embodiment are basically the same as those in Example 1, except that:

[0085] (a) The flow rate of the ammonium bicarbonate solution is 3.0 m. 3 After the ammonium bicarbonate solution has been added, adjust the pH of the reaction system to 8.2.

[0086] Example 3

[0087] The preparation steps of lithium iron phosphate in this embodiment are basically the same as those in Example 1, except that:

[0088] (a) The aging reaction time is 10 hours;

[0089] The solid content of the precipitate was 58%.

[0090] Example 4

[0091] The preparation steps of lithium iron phosphate in this embodiment are basically the same as those in Example 1, except that:

[0092] (c) The temperature of the first calcination treatment was 450℃, the time was 5h, and the nitrogen flow rate was 20m³. 3 / h; the second calcination treatment was carried out at a temperature of 780℃ for 10 hours, with a nitrogen flow rate of 20m³ / h. 3 / h.

[0093] Example 5

[0094] The preparation steps of lithium iron phosphate in this embodiment include the following steps:

[0095] (a) Under nitrogen protection, ammonium bicarbonate solution is added to ferrous sulfate solution to carry out an aging reaction, wherein the molar ratio of ferrous sulfate to ammonium bicarbonate is 1:1; and the flow rate of ammonium bicarbonate solution is 3.5 m. 3 After the ammonium bicarbonate solution was added, the pH of the reaction system was adjusted to 8.3, and the reaction was carried out at 30°C for 12 hours to obtain a mixture. The mixture was then subjected to solid-liquid separation to obtain a precipitate and a supernatant. The mass percentage of iron ions in the supernatant was determined to be 0.05% by potassium dichromate titration, and the mass of ferrous carbonate in the precipitate was calculated. The precipitate was washed with deionized water to make the solid content of the precipitate 55%.

[0096] (b) Phosphoric acid, lithium triphosphate, glucose, polyethylene glycol, and vanadium pentoxide were added sequentially to hydrated ferrous carbonate, and a kneading reaction was carried out at 70°C for 4 hours with a nitrogen flow rate of 100 m³ / h. 3 / h, to obtain a lithium-containing precursor;

[0097] (c) The first calcination treatment was carried out at a temperature of 350°C for 4 hours, with a nitrogen flow rate of 15 m³ / h. 3 / h; the second calcination treatment was carried out at a temperature of 780℃ for 8 hours, with a nitrogen flow rate of 15m³ / h. 3 / h, to obtain lithium iron phosphate.

[0098] In the above steps, the molar ratio of lithium, iron, phosphorus and vanadium in the raw materials is 1.05:0.98:1:0.005; the mass of glucose is 12.5% ​​of the mass of ferrous carbonate.

[0099] Comparative Example 1

[0100] The preparation steps of lithium iron phosphate in this comparative example include the following steps:

[0101] (a) Iron phosphate, lithium carbonate, titanium dioxide and glucose were weighed as raw materials, wherein the molar ratio of lithium, iron, phosphorus and titanium in the raw materials was 1.03:0.973:1:0.01, and the mass of glucose was 12.5% ​​of the mass of iron phosphate. The raw materials were mixed with deionized water and added to a ball mill jar. The solid content of the raw material system in the ball mill jar was 45%. Zirconia was used as the grinding beads and the ball-to-material mass ratio was 3:1. The mixture was ground in a planetary ball mill at 300 rpm for 8 hours to obtain a slurry.

[0102] (b) The slurry is spray-dried with an inlet air temperature of 180°C, an outlet air temperature of 90°C, and an atomizing wheel frequency of 23 Hz to obtain powder.

[0103] (c) Place the powder in a tube furnace and introduce nitrogen gas at a flow rate of 15 m³ / h. 3 Set the heating program to increase the temperature to 750℃ at a rate of 5℃ / min, and hold for 10.5h to obtain lithium iron phosphate.

[0104] Performance testing

[0105] The following performance tests were performed on the examples and comparative examples respectively:

[0106] (1) SEM test

[0107] The lithium-containing precursor and lithium iron phosphate in Example 1 were subjected to SEM tests using an S-4800 scanning electron microscope.

[0108] Figure 1 The image shows a SEM image of the lithium-containing precursor in Example 1. Figure 1 As can be seen from the data, the lithium-containing precursor in Example 1 exhibits a regular one-dimensional rod-shaped or needle-shaped morphology. The lithium-containing precursor has a clear growth orientation and uniform morphology, with no obvious agglomeration or fragmentation. The lithium-containing precursor is well dispersed, without large-area adhesion or agglomeration. This uniform and well-dispersed microstructure ensures that the lithium-containing precursor is fully converted into lithium iron phosphate grains during the subsequent roasting process, thereby improving the lithium iron phosphate yield. At the same time, the regular particle morphology provides a good foundation for the crystal growth of lithium iron phosphate during the subsequent roasting process, which is conducive to the formation of dense lithium iron phosphate particles.

[0109] Figure 2 The image shows a SEM image of lithium iron phosphate in Example 1. As can be seen from the image, the lithium iron phosphate particles are nearly spherical or ellipsoidal, with a regular overall morphology, smooth and dense surface, concentrated particle size distribution, and no obvious abnormally large particles, fragmented crystals, or excessive sintering and adhesion. The particles are well dispersed.

[0110] (3) Yield test of lithium iron phosphate

[0111] The yield of lithium iron phosphate in the examples and comparative examples was calculated respectively. The formula for calculating the yield is: Yield (%) = (actual mass of lithium iron phosphate product obtained ÷ theoretical mass of lithium iron phosphate that can be generated from all raw materials) × 100%.

[0112] The calculation results are shown in Table 1.

[0113] (2) Low-temperature cycling performance test

[0114] The lithium iron phosphate used in the examples and comparative examples was prepared into lithium-ion batteries, specifically as follows:

[0115] First, lithium iron phosphate, conductive carbon black Super P, and binder polyvinylidene fluoride (model 5130) were mixed in a planetary mixer at a mass ratio of 96.5:2:1.5 to obtain a positive electrode mixture. N-methylpyrrolidone (NMP) was then added to the positive electrode mixture to obtain a slurry. The solid content of the slurry was controlled at 50 wt%, the stirring speed was 1500 rpm, and the stirring time was 2 hours to obtain a uniform positive electrode slurry. Subsequently, the positive electrode slurry was uniformly coated onto a 12 μm thick aluminum foil current collector using a transfer coating machine, with the single-sided coating surface density controlled at 12 mg / cm². 2 After coating, the material is pre-dried in an oven at 80℃ for 30 minutes, and then dried in a vacuum oven at 120℃ for 12 hours to obtain the positive electrode sheet. The positive electrode sheet is then subjected to roll pressing to achieve a compaction density of 2.5 g / cm³. 3 The thickness of the positive electrode sheet after rolling is 100μm. Then, the CR2032 button cell is assembled in an argon glove box using lithium sheet as negative electrode, 1 mol / L lithium hexafluorophosphate ethylene carbonate solution (EC) and 1 mol / L lithium hexafluorophosphate dimethyl carbonate solution (DMC) as electrolyte (where the volume ratio of EC to DMC is 1:1) and polypropylene (PP) as separator.

[0116] The above-mentioned button cell batteries were subjected to low-temperature cycling performance testing. The test steps were as follows:

[0117] The assembled CR2032 coin cell was placed in a 25℃ environment and subjected to a 0.2C constant current and constant voltage charge-discharge cycle within a voltage window of 2.0~3.75V for two consecutive cycles. The specific capacity at room temperature during the second cycle was recorded. After completing the room temperature cycle, the CR2032 coin cell was fully charged at a 0.2C constant current and constant voltage. The fully charged CR2032 coin cell was then placed in a -20℃ environment for 4 hours, followed by a constant current discharge at -20℃ at 0.2C with a discharge cutoff voltage of 2.0V. The low-temperature discharge specific capacity was recorded, and the specific capacity at room temperature during the second cycle of the CR2032 coin cell was extracted according to the formula:

[0118] Capacity retention rate = Low-temperature discharge specific capacity / Room temperature discharge specific capacity in the second week × 100%, calculate the capacity retention rate of CR2032 coin cell at -20℃.

[0119] The test results are shown in Table 1.

[0120] Table 1

[0121]

[0122] As shown in Table 1, the yield of lithium iron phosphate in Examples 1-5 is higher than that in Comparative Example 1, and it has a better capacity retention rate at -20℃. The reason is that: the solid content of ferrous carbonate in the examples is controlled at 50~60%. At this solid content range, water can form a liquid phase coating layer on the surface of ferrous carbonate, which protects the ferrous carbonate particles and prevents them from oxidizing and deteriorating; in addition, the contact between ferrous carbonate and phosphorus source, lithium source and carbon source in the kneading reaction is more sufficient and uniform, the raw material conversion is more complete, and the raw material loss and defect rate caused by the formation of impurity phases due to the oxidation of iron source are reduced, thus improving the yield of lithium iron phosphate; at the same time, the lithium iron phosphate phase formed in the roasting treatment is pure and has a regular lattice structure, and the carbon source coating is more uniform and dense, which optimizes the intrinsic conductivity of lithium iron phosphate. In lithium-ion batteries, lithium ions have a faster ion transport channel, which reduces the charge transfer impedance at low temperature, thus enabling lithium-ion batteries to maintain a good capacity retention rate at -20℃.

[0123] Finally, it should be noted that other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein, and is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A method for preparing lithium iron phosphate, characterized in that, include: Under an inert atmosphere, raw materials including ferrous carbonate, phosphorus source, lithium source and carbon source are kneaded and roasted in sequence to obtain lithium iron phosphate. The solid content of the ferrous carbonate is 50-60%.

2. The preparation method according to claim 1, characterized in that, The ferrous carbonate is obtained by aging ferrous salt and carbonate.

3. The preparation method according to claim 2, characterized in that, It also includes determining the solid content of ferrous carbonate in the aging reaction products based on the iron ion content in the aging reaction products.

4. The preparation method according to claim 1, characterized in that, In the kneading reaction, the temperature is 50~90℃, the time is 4~8h, the rotation speed is 400~800rpm, and the flow rate of the inert atmosphere is 100~200m³. 3 / h.

5. The preparation method according to claim 1, characterized in that, The roasting process includes a first roasting process and a second roasting process, wherein the temperature of the first roasting process is lower than the temperature of the second roasting process.

6. The preparation method according to claim 5, characterized in that, In the first calcination treatment, the temperature is 300~500℃, the time is 2~5h, and the flow rate of the inert atmosphere is 3~20m³. 3 / h; and / or, In the second calcination process, the temperature is 720~800℃, the time is 8~12h, and the flow rate of the inert atmosphere is 3~20m³. 3 / h.

7. The preparation method according to claim 2, characterized in that, The aging reaction includes adding a carbonate solution to a ferrous salt solution, wherein the flow rate of the carbonate solution is 2.5~3.5 m / s. 3 / h.

8. The preparation method according to claim 2, characterized in that, The aging reaction is carried out at a temperature of 25-35℃ for 10-24 hours.

9. The preparation method according to claim 1, characterized in that, The raw materials also include additives, which include metallic elements; The metallic element includes at least one selected from Al, Mg, Ni, Co, Ti, Cu, Ca, Nb, Cr, Zn, La, Sb, Te, Sr, W, In, Y, and V.

10. The preparation method according to claim 9, characterized in that, In the raw materials, the molar ratio between lithium, iron, phosphorus, and the metal elements in the additives is (1.02~1.08):(0.96~0.99):1:(0.002~0.01); and / or, The mass of the carbon source is 10-15% of the mass of the ferrous carbonate.

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