Lithium iron phosphate positive electrode material, preparation method thereof, positive electrode sheet and lithium ion battery

Abstract

The application provides a lithium iron phosphate positive electrode material, a preparation method thereof, a positive electrode sheet and a lithium ion battery. The preparation method comprises the following steps: preparing a first slurry by mixing a ferrous source, a phosphorus source, an oxidizing agent and a first pH regulator, performing a first reaction on the first slurry to obtain a second slurry; adding a second pH regulator to the second slurry, performing a second reaction to obtain a third slurry; performing aging and filtering on the third slurry to obtain a first precursor and a second precursor; mixing and calcining the first precursor, the second precursor and a metal doping source to obtain a third precursor; mixing the third precursor and a lithium source to obtain a fourth precursor; mixing the fourth precursor and oxalic acid, and performing a second calcination to obtain a first product; and reacting the first product with a carbon source and a non-metal doping source to obtain the lithium iron phosphate positive electrode material. The preparation process of the precursor is optimized, and the overall process for preparing the lithium iron phosphate positive electrode material is optimized.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion batteries, and more specifically, to a lithium iron phosphate cathode material, its preparation method, cathode sheet, and lithium-ion battery. Background Technology

[0002] Since the 1980s, lithium (Li)-ion batteries have gradually come into human view, and were commercialized in the 1990s. Through the collaborative efforts of several generations of researchers and industrial practitioners, the performance of lithium-ion batteries has been greatly improved. Currently, lithium-ion batteries are widely used in small consumer electronics, power tools, large power supplies, (plug-in) hybrid electric vehicles, and power grid systems.

[0003] Lithium iron phosphate (LFP) is an important cathode material for lithium-ion batteries, possessing numerous significant characteristics and advantages. Chemically, LFP exhibits a stable olivine-type structure, endowing it with excellent safety and cycle stability. Its structure undergoes relatively small changes during charge and discharge, allowing it to withstand multiple charge-discharge cycles, thus extending battery life. In terms of performance, LFP has a high discharge plateau, typically around 3.2V. Its theoretical specific capacity is approximately 170 mAh / g, with actual specific capacities reaching 130 mAh / g to 160 mAh / g. Furthermore, LFP demonstrates good high-temperature performance, exhibiting relatively small performance degradation at high temperatures. Safety is another major advantage of LFP; it is less prone to thermal runaway, combustion, and explosion under conditions of overcharging, over-discharging, and short circuits, providing enhanced safety for devices using LFP batteries. Regarding cost, the raw materials for LFP are widely available and relatively inexpensive; elements such as iron and phosphorus are abundant in the Earth's crust, contributing to lower battery production costs.

[0004] However, lithium iron phosphate also has some drawbacks. For example, its electronic and ionic conductivity are relatively low, which limits its electrical performance to some extent. To improve this, modification treatments such as nano-sizing and carbon coating are usually required. However, simply adopting this method to improve electrical performance will significantly reduce the corresponding compaction density, leading to a decrease in energy density. Conversely, simply improving compaction while ignoring the adverse effects on electrical performance is also not advisable. Therefore, research that balances the electrical and compaction performance of lithium iron phosphate is currently a key focus of the industry.

[0005] Therefore, one of the technical problems to be solved in this field is how to optimize the elemental composition, morphology and particle size of olivine-type lithium iron phosphate by starting from its preparation method and rationally designing its doping elements, so as to make it exhibit superior electrochemical performance. Summary of the Invention

[0006] The main objective of this invention is to provide a lithium iron phosphate cathode material, its preparation method, cathode sheet, and lithium-ion battery, so as to solve the problem of poor electrochemical performance of olivine-type lithium iron phosphate cathode materials in the prior art.

[0007] To achieve the above objectives, a first aspect of the present invention provides a method for preparing a lithium iron phosphate cathode material, comprising: step S1, preparing a first slurry by mixing an ferrous source, a phosphorus source, an oxidant, and a first pH adjuster; the first slurry undergoing a first reaction to obtain a second slurry; step S2, adding a second pH adjuster to the second slurry; and after a second reaction, obtaining a third slurry; step S3, taking a portion of the third slurry as slurry A and another portion as slurry B; slurry A being subjected to static aging and filtration sequentially to obtain a first precursor; and slurry B being subjected to stirring aging and filtration sequentially. After filtration, a second precursor is obtained; in step S4, the first precursor, the second precursor, and a metal doping source are formulated into a fourth slurry, and the fourth slurry is subjected to a first drying and a first calcination to obtain a third precursor; in step S5, the third precursor and a lithium source are formulated into a fifth slurry, and the fifth slurry is subjected to a second drying to obtain a fourth precursor; in step S6, the fourth precursor is mixed with oxalic acid and subjected to a second calcination to obtain a first product; in step S7, the first product, a carbon source, and a non-metal doping source are formulated into a sixth slurry, and the sixth slurry is subjected to a third drying and a third calcination to obtain a lithium iron phosphate cathode material.

[0008] This invention improves the compaction density and electrochemical performance of the cathode material by precisely designing the preparation process. In step S3, two iron phosphate dihydrate precursors are prepared under different aging conditions (static and stirred aging), resulting in two different particle size distributions and morphologies. Mixing the two iron phosphate dihydrates under different aging conditions creates a particle size distribution effect, which improves the compaction density of the final lithium iron phosphate material. In step S4, adding a metal dopant source before dehydration promotes uniform doping of the iron phosphate precursor and the metal dopant element, improving the electrical performance of lithium iron phosphate. In step S6, oxalic acid is used as an auxiliary agent in the initial synthesis of lithium iron phosphate. During subsequent calcination, oxalic acid decomposes, and the resulting reducing gas reduces the ferric iron in the precursor to ferrous iron, which is beneficial for the phase formation of lithium iron phosphate and improves the crystallinity and conductivity of the material. Simultaneously, the decomposition of oxalic acid leaves virtually no residual carbon, avoiding the introduction of additional carbon sources and reducing the generation of magnetic foreign iron phosphate compounds, thereby improving the purity of the material. A two-step sintering method was employed. First, the reducing gas from oxalic acid decomposition improved the crystallinity and structural integrity of lithium iron phosphate during a primary sintering process. Then, a secondary sintering process was used for surface carbon coating. During this process, a non-metallic dopant source was added. This served two purposes: firstly, it acted as a sintering aid, increasing the graphitization degree of the carbon layer at a lower calcination temperature; secondly, after calcination, the non-metallic elements were incorporated into the carbon coating layer, creating defects in the carbon layer and increasing hole carriers. This not only improved the material's electrical performance but also controlled the amount of carbon added, avoiding the adverse effects of excessive carbon on compaction density. The lithium iron phosphate cathode material prepared using this method exhibited good compaction and electrochemical performance.

[0009] Further, in step S1, the molar ratio of ferrous source, phosphorus source, and oxidant is 1:(1.1~1.2):1; and / or, the concentration of ferrous source in the first slurry is 0.8M~1.0M; and / or, the pH value of the first slurry is 1.0~1.5; and / or, the first reaction is carried out at 50℃~70℃, and the first reaction time is 0.5h~1h. During the formation of the second slurry, by optimizing the molar ratio and concentration of the reactants, it is possible to promote the formation of iron phosphate with moderate and uniform grain size. Simultaneously, optimizing the pH value and reaction conditions of the reaction system can optimize the morphology of the obtained iron phosphate dihydrate, improve its structural integrity and surface morphology, thereby providing a higher quality precursor for subsequent conversion into anhydrous iron phosphate and lithium iron phosphate.

[0010] Further, step S2 includes: adding a second pH adjuster to the second slurry to adjust the pH value of the second slurry to 1.5-2.0, so that the second slurry undergoes a second reaction lasting 1-2 hours to obtain a third slurry. Preferably, the pH range and precipitation reaction time during the formation of the third slurry can improve the purity and consistency of the final cathode material, while also giving it higher electrochemical performance and compaction density.

[0011] Further, in step S3, the volume ratio of slurry A to slurry B is (0.8–1.2):1; preferably, the static aging temperature is 85°C–95°C, and the time is 1–2 hours; and / or, the stirring aging temperature is 85°C–95°C, and the time is 3–4 hours, with a stirring frequency of 10 Hz–20 Hz. In the process of forming the first and second precursors, by optimizing the aging conditions as described above, including the temperature, time, and stirring frequency of static and stirring aging, the morphology, particle size, and performance of the two obtained iron phosphate dihydrate precursors can be significantly improved, ultimately more effectively enhancing the particle size distribution, compaction density, and electrochemical performance of the obtained cathode material.

[0012] Further, in step S4, the ratio of the total molar amount of the first precursor and the second precursor to the molar amount of the metal dopant source is 1:(0.005~0.008); and / or, in the fourth slurry, the total solid content of the first precursor and the second precursor is 20%~25%; and / or, the first drying is carried out at 110℃~130℃, and the first drying time is 3h~4h; and / or, the first calcination temperature is 550℃~650℃, and the time is 4h~8h; and / or, the metal dopant source is selected from one or more of titanium source, magnesium source, aluminum source, vanadium source and strontium source, and the titanium source, magnesium source, aluminum source, vanadium source and strontium source are each independently added in one or more forms of oxide, hydroxide, nitrate, phosphate, sulfate and acetate; preferably, the metal dopant source is a titanium source, and the titanium source is added in the form of titanium dioxide and / or tetrabutyl titanate. In the process of forming a third precursor containing metal dopants, the optimization of the above conditions allows for more uniform doping of the metal dopant in the third precursor, thereby improving the compaction density and electrochemical performance of the material. Furthermore, optimized drying and calcination conditions also enhance the purity and crystallinity of the resulting cathode material, resulting in higher overall electrochemical performance. Among the aforementioned metal dopants, titanium not only improves the electrochemical performance of the resulting cathode material but also inhibits excessive crystal growth during calcination, optimizing the particle size distribution and structural stability of the material, thus more effectively improving the electrochemical performance of the final cathode material.

[0013] Further, in step S5, the molar ratio of the third precursor to the lithium source is 1:(0.4-0.6), preferably 1:(0.49-0.51); and / or, the solid content of the fifth slurry is 30%-40%; and / or, the second drying is carried out at 150°C-200°C. By setting the above-mentioned preferred lithium source molar ratio, the solid content of the fifth slurry, and the temperature of the second drying, the preparation of high-performance lithium iron phosphate cathode materials can be achieved more effectively, resulting in better compaction density, specific capacity, and cycle stability.

[0014] Further, in step S6, the weight ratio of the fourth precursor to oxalic acid is (4-6):1; and / or, the second calcination temperature is 700℃-800℃, and the time is 10h-15h; preferably, the second calcination is carried out under a protective atmosphere; more preferably, the protective atmosphere is nitrogen and / or argon. In the process of forming the first product, optimizing the weight ratio of the fourth precursor to oxalic acid, the temperature and time of the second calcination, and selecting a suitable protective atmosphere significantly improves the electrochemical performance, compaction density, and efficiency and environmental friendliness of the first product and the final cathode material. Optimizing these conditions promotes the purity and activity of the cathode material during the conversion process, thereby obtaining a lithium iron phosphate cathode material with superior electrochemical performance.

[0015] Further, in step S7, based on the total weight of the first product (100%), the amount of carbon source is 8 wt% to 10 wt%, and based on the total weight of the first product, the amount of non-metallic dopant source is 0.3 wt% to 0.6 wt%; and / or, the non-metallic dopant source is a boron source, and the boron source is added in the form of boric acid; and / or, the solid content of the sixth slurry is 30% to 40%; and / or, the temperature of the third calcination is 650°C to 750°C, and the time is 4 h to 8 h; preferably, the third calcination is carried out under a protective atmosphere; more preferably, the protective atmosphere is nitrogen and / or argon. In the process of final calcination and formation of the cathode material, by optimizing the ratio of the first product to the carbon source, the ratio to the non-metallic dopant source, the specific type and addition form of the non-metallic dopant source, the solid content of the sixth slurry, and the conditions of the third calcination as described above, a lithium iron phosphate cathode material with higher electronic conductivity, better compaction density, and a more stable structure can be obtained.

[0016] Furthermore, the ferrous source is selected from one or more of ferrous sulfate, ferrous ammonium sulfate, ferrous chloride, and ferrous nitrate; and / or, the phosphorus source is selected from one or more of ammonium dihydrogen phosphate, phosphoric acid, and diammonium hydrogen phosphate; and / or, the oxidant is hydrogen peroxide; and / or, the first pH adjuster is selected from one or more of sulfuric acid, phosphoric acid, nitric acid, and hydrochloric acid; and / or, the second pH adjuster is ammonia and / or sodium hydroxide; and / or, the lithium source is lithium carbonate; and / or, the carbon source is selected from one or more of glucose, sucrose, and polyvinyl alcohol. These raw materials can be better combined, ultimately resulting in a lithium iron phosphate cathode material with superior electrochemical performance.

[0017] A second aspect of the present invention provides a lithium iron phosphate cathode material, which is prepared by the above-described method for preparing lithium iron phosphate cathode materials, and the compaction density of the lithium iron phosphate cathode material at 350 MPa is 2.65 g / cc to 2.75 g / cc. The prepared lithium iron phosphate cathode material has a higher compaction density and a more superior particle size distribution, thus significantly improving the rate performance of the lithium-ion battery in which it is used. Preferably, the lithium iron phosphate cathode material includes metal dopant elements and non-metal dopant elements, and based on 100% of the total weight of the lithium iron phosphate cathode material, the doping amount of the metal dopant element is 0.05% to 0.30%, and the doping amount of the non-metal dopant element is 0.05% to 0.10%. The resulting cathode material is doped with appropriate amounts of metal elements and non-metal elements, thereby further improving the electrochemical performance of the lithium iron phosphate cathode material.

[0018] A third aspect of the present invention provides a positive electrode comprising the aforementioned lithium iron phosphate positive electrode material. Because the obtained lithium iron phosphate positive electrode material possesses high electrochemical performance and excellent particle size distribution, the positive electrode in which it is located exhibits high energy density and good electrical performance.

[0019] A fourth aspect of the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode is the aforementioned positive electrode. Using the aforementioned positive electrode with excellent electrical properties, the prepared lithium-ion battery exhibits higher rate performance and more stable electrical performance.

[0020] By applying the technical solution of this invention, the preparation process of the precursor is optimized, and metal and non-metal elements are introduced for doping during the preparation process. The advantages of oxalic acid decomposition leaving almost no carbon residue and generating reducing gas to reduce ferric iron are also utilized, which is beneficial to the phase formation of lithium iron phosphate. The overall preparation process of lithium iron phosphate cathode material is optimized, and the resulting lithium iron phosphate cathode material has good compaction and electrochemical performance, thereby significantly improving the rate performance of the lithium-ion battery in which it is located. Attached Figure Description

[0021] The accompanying drawings, which form part of this application, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings:

[0022] Figure 1 This is a SEM image of the third precursor obtained in Embodiment 1 of the present invention;

[0023] Figure 2 Here is a SEM image of the lithium iron phosphate cathode material obtained in Example 1 of this invention;

[0024] Figure 3 Here is a SEM image of the third precursor obtained in Comparative Example 1 of this invention;

[0025] Figure 4 Here is a SEM image of the lithium iron phosphate cathode material obtained in Comparative Example 1 of this invention;

[0026] Figure 5 Here is a SEM image of the third precursor obtained in Comparative Example 2 of this invention;

[0027] Figure 6 This is a SEM image of the lithium iron phosphate cathode material obtained in Comparative Example 2 of this invention. Detailed Implementation

[0028] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. The present invention will now be described in detail with reference to the embodiments.

[0029] As described in the background section, existing olivine-type lithium iron phosphate cathode materials suffer from poor electrochemical performance. To address this problem, a first aspect of the present invention provides a method for preparing a lithium iron phosphate cathode material, comprising: step S1, preparing a first slurry by mixing an ferrous source, a phosphorus source, an oxidant, and a first pH adjuster; subjecting the first slurry to a first reaction to obtain a second slurry; step S2, adding a second pH adjuster to the second slurry; subjecting the second reaction to a third slurry; step S3, taking a portion of the third slurry as slurry A and another portion as slurry B; subjecting slurry A to a static aging and filtration process to obtain a first precursor; and subjecting slurry B to a stirring aging and filtration process to obtain a second precursor. After filtration, a second precursor is obtained; in step S4, the first precursor, the second precursor, and a metal doping source are formulated into a fourth slurry, and the fourth slurry is subjected to a first drying and a first calcination to obtain a third precursor; in step S5, the third precursor and a lithium source are formulated into a fifth slurry, and the fifth slurry is subjected to a second drying to obtain a fourth precursor; in step S6, the fourth precursor is mixed with oxalic acid and subjected to a second calcination to obtain a first product; in step S7, the first product, a carbon source, and a non-metal doping source are formulated into a sixth slurry, and the sixth slurry is subjected to a third drying and a third calcination to obtain a lithium iron phosphate cathode material.

[0030] This invention optimizes the precursor preparation process, introduces both metallic and non-metallic elements for doping during preparation, and leverages the advantages of oxalic acid decomposition (which leaves virtually no carbon residue and generates a reducing gas that facilitates the reduction of ferric iron, thus promoting lithium iron phosphate phase formation). Specifically,

[0031] First, ferrous and phosphorus sources are mixed, and an oxidant is added for oxidation, converting ferrous ions into ferric ions. Simultaneously, a first pH adjuster is used to control the pH, facilitating the oxidation reaction and resulting in a homogeneous second slurry. Then, the pH of the second slurry is adjusted again to promote the formation and precipitation of ferric phosphate, thus yielding a third slurry containing ferric phosphate precipitate.

[0032] The third slurry is divided into slurry A and slurry B, and two different aging conditions are applied to form two different precipitates of ferric phosphate dihydrate. Specifically, static aging of slurry A tends to form larger, more regular crystals, while stirring aging of slurry B tends to form smaller, more uniform crystals. Subsequent mixing of the two achieves a particle size distribution, forming a composite material with both large and small particles. Different particle sizes can fill each other, reducing voids between materials while maintaining good electron and ion transport pathways, thus resulting in a cathode material with superior electrochemical performance.

[0033] Subsequently, the first and second precursors with different particle size characteristics were mixed with a metal doping source to form a homogeneous slurry, which was then dried and calcined. This process introduces the doping metal element at the precursor stage, leveraging its advantages, including promoting uniform mixing and improving the electrochemical performance of the material, significantly enhancing the electrochemical performance of the resulting cathode material. Next, the metal-doped anhydrous iron phosphate was mixed with a lithium source, promoting the uniform embedding of lithium ions into the iron phosphate lattice, thus forming a lithium iron phosphate cathode material with excellent electrical performance.

[0034] Subsequently, the obtained fourth precursor was mixed with oxalic acid and calcined at high temperature. Oxalic acid, acting as a reducing agent and sintering aid, decomposes at high temperature to produce reducing gas (CO), which helps reduce some of the iron ions in the fourth precursor from trivalent to divalent, while also avoiding the formation of residual carbon or the introduction of impurities, thus promoting the formation of a pure lithium iron phosphate phase. Through this process, the initially formed lithium iron phosphate material, i.e., the first product, possesses higher crystallinity and better electrochemical performance, laying the foundation for further processing in subsequent steps and the preparation of the final product. Finally, the stability and electrochemical performance of the obtained lithium iron phosphate cathode material are further improved through carbon source coating and the calcination and doping of non-metallic dopants.

[0035] It should be further explained that during the entire preparation process of the lithium iron phosphate cathode material, the valence state of iron underwent a transformation from divalent to trivalent and back to divalent. In steps S1 and S2, a ferrous source was initially used, where the iron valence state was +2. Under the action of the first pH adjuster, and with the presence of an oxidant, an oxidation reaction was initiated, oxidizing some divalent ferrous ions to trivalent ferric ions. In subsequent steps, especially due to the high-temperature decomposition of oxalic acid, the resulting reducing gases, such as carbon monoxide (CO), reduced the trivalent ferric ions back to divalent ferric ions, thus maintaining stable Fe in the final lithium iron phosphate material. 2+ Valence state. The choice of ferrous iron as the initial raw material is due to Fe... 2+ Compared to Fe 3+ The reaction activity is higher during the reaction synthesis process, and it is easier to form the required iron phosphate precursor during the reaction process. This is beneficial to control the morphology and particle size of the material, and ultimately form a lithium iron phosphate cathode material with higher compaction density, better morphology and better electrochemical performance.

[0036] In summary, the method provided by this invention optimizes the material structure and performance by controlling the morphology and particle size of the precursor, doping with metals and non-metals, using oxalic acid for reduction, and introducing a carbon source for carbon coating. The resulting lithium iron phosphate cathode material exhibits excellent compaction density and good electrochemical performance, thereby significantly improving the rate performance of the lithium-ion battery in which it is located.

[0037] In several typical implementations, in step S1, the molar ratio of ferrous source, phosphorus source, and oxidant is 1:(1.1~1.2):1. Optimizing the molar ratio of the two reactants, ferrous source and phosphorus source, to the oxidant as described above can promote the equilibrium of the chemical reaction, facilitate the oxidation reaction, and thus improve the purity of the product. Preferably, the concentration of ferrous source in the first slurry is 0.8M~1.0M to accelerate the reaction process and also facilitate the formation of a cathode material with a higher compaction density.

[0038] In the above process, the pH value of the first slurry is preferably 1.0 to 1.5 to form a strongly acidic reaction system, which is conducive to the oxidation reaction. Ferrous ions can thus be oxidized more effectively, promoting the formation of a better-structured iron phosphate precursor, and ultimately yielding a cathode material with superior structure, morphology, and electrochemical performance. Furthermore, the first reaction is preferably carried out at 50°C to 70°C for 0.5 to 1 hour to control the reaction rate, reduce side reactions, and promote the uniform binding of ferrous and phosphate ions, forming a more compact precursor and thus increasing the compaction density of the cathode material.

[0039] Further, step S2 includes: adding a second pH adjuster to the second slurry to adjust its pH value to 1.5–2.0, so that the second slurry undergoes a second reaction lasting 1–2 hours to obtain a third slurry. During this process, the aforementioned pH range promotes the effective combination of iron ions and phosphate ions, forming the desired iron phosphate precipitate, while reducing the formation of other byproducts and improving the purity and consistency of the final cathode material. The precipitation reaction time optimizes the crystallinity and morphology of the precursor, thereby affecting the electrochemical performance and compaction density of the final material.

[0040] In some typical embodiments, in step S3, the volume ratio of slurry A to slurry B is (0.8–1.2):1. By optimizing the volume ratio of slurry A to slurry B as described above, the larger-particle-size ferric phosphate dihydrate (first precursor) can more effectively provide a stable skeletal structure, while the smaller-particle-size ferric phosphate dihydrate (second precursor) fills the gaps between the large particles, thereby more effectively enhancing the overall structural stability, reducing internal stress, and contributing to the formation of a more complete and stable cathode material during subsequent sintering. In some more typical embodiments, the static aging temperature is 85°C–95°C for 1–2 hours; and / or, the stirring aging temperature is 85°C–95°C for 3–4 hours, and the stirring frequency is 10 Hz–20 Hz. The aforementioned refined conditions help to form a first precursor with a larger particle size and a more stable structure, as well as a second precursor with a smaller particle size and a more uniform distribution. At the same time, they can effectively accelerate the reaction process, reduce material structure damage or by-product generation, and ultimately achieve the particle size distribution of the obtained cathode material, thereby improving its compaction performance and electrochemical performance.

[0041] In the doping process of the metal dopant source, it is further preferred that the ratio of the total molar amount of the first precursor and the second precursor to the molar amount of the metal dopant source in step S4 is 1:(0.005~0.008). This molar ratio can promote the uniform doping of metal elements in lithium iron phosphate, thereby improving the electronic conductivity of the obtained cathode material. Furthermore, in several typical embodiments, for the purpose of more significantly improving the electrochemical performance of the obtained cathode material, it is preferred that the metal dopant source be selected from one or more of titanium, magnesium, aluminum, vanadium, and strontium sources, and that the titanium, magnesium, aluminum, vanadium, and strontium sources are each independently added in one or more forms selected from oxides, hydroxides, nitrates, phosphates, sulfates, and acetates.

[0042] Furthermore, through extensive experimentation, the inventors selected titanium as the preferred metal doping source from the aforementioned types of metal doping sources, and the titanium source was added in the form of titanium dioxide and / or tetrabutyl titanate. Compared to other metal elements with conventional doping effects, titanium not only improves the electrochemical performance of the resulting cathode material, but also inhibits excessive crystal growth during calcination, optimizes the particle size distribution and structural stability of the material, and thus more effectively improves the electrochemical performance of the final cathode material.

[0043] To ensure more uniform dispersion and effective contact of materials during the mixing and slurry preparation process, thereby promoting thorough mixing of the metal dopant elements and the iron phosphate precursor, the total solid content of the first and second precursors in the fourth slurry is preferably 20%–25%. Furthermore, to more effectively remove moisture without damaging the material structure, and to reduce the possibility of surface oxidation of the resulting third precursor, thereby improving its purity and activity, the first drying is preferably carried out at 110°C–130°C for 3–4 hours. Through extensive experimentation, the inventors have optimized the first calcination temperature to 550°C–650°C and the time to 4–8 hours. This optimized temperature range and time setting helps to effectively form the third precursor, i.e., anhydrous iron phosphate, while also promoting the uniform doping and distribution of the aforementioned metal dopant elements. Simultaneously, the selection of the above calcination conditions suppresses material structure damage caused by excessively high temperatures and incomplete dehydration caused by excessively low temperatures, improving the crystallinity and structural integrity of the resulting third precursor and the final cathode material, resulting in superior electrochemical performance.

[0044] Further, in step S5, the molar ratio of the third precursor to the lithium source is 1:(0.4-0.6), preferably 1:(0.49-0.51). Adding the lithium source according to this preferred molar ratio facilitates the uniform embedding of lithium ions in the third precursor, while reducing defects in the crystal structure caused by the irregular distribution of lithium ions in the material structure. This results in the generated lithium iron phosphate cathode material having an ideal olivine-type structure, improving its electrochemical performance. Preferably, the solid content of the fifth slurry is 30%-40%, which can improve the dispersion uniformity of the obtained fifth slurry during the mixing and slurry preparation process, thereby ultimately preparing a cathode material with uniform particle size distribution and stable structure. Preferably, the second drying is carried out at 150°C-200°C, which can effectively remove residual moisture in the obtained fourth precursor, while effectively promoting the chemical bonding between the lithium source and the anhydrous iron phosphate precursor.

[0045] In some typical embodiments, in step S6, the weight ratio of the fourth precursor to oxalic acid is (4-6):1. In step S6, oxalic acid is introduced into the reaction system as both a reducing agent and a decomposing agent. The aforementioned preferred range of its weight ratio is crucial for reducing ferric ions to a ferrous ion state more favorable for lithium iron phosphate phase formation. Under the aforementioned weight ratio, oxalic acid can more effectively decompose and release reducing gases, such as carbon monoxide, promoting the reduction process. Simultaneously, the aforementioned dosage can effectively suppress the generation of carbon impurities or excessive gases that may result from excessive oxalic acid residue, thereby significantly improving the purity and electrochemical performance of the lithium iron phosphate material. Based on this, the preferred second calcination temperature is 700℃-800℃, and the time is 10h-15h. This temperature condition not only promotes the crystallization of the first product, improving its structural integrity and electrochemical activity, but also promotes the complete decomposition of oxalic acid. Furthermore, the aforementioned calcination conditions are beneficial for optimizing the internal structure of the first product and reducing defects, thereby improving its compaction density and cycle stability. Furthermore, in order to reduce the introduction of impurities and make the final cathode material have better electrochemical performance and higher stability, it is preferable to carry out the second calcination under a protective atmosphere, and more preferably the protective atmosphere is nitrogen and / or argon.

[0046] Furthermore, in step S7, the amount of carbon source used is 8wt% to 10wt%, based on the total weight of the first product as 100%. Adding the carbon source in the above proportion can more effectively form a carbon coating layer with non-metallic element doping on the surface of lithium iron phosphate. It also suppresses the decrease in compaction density, energy density, and deterioration of electrochemical performance caused by excessive carbon source. In practical applications, because the carbon source will undergo some burn-off after subsequent calcination, the carbon content in the final lithium iron phosphate cathode material is 2wt% to 2.5wt%.

[0047] Regarding the non-metallic dopant elements, it is preferable that the amount of non-metallic dopant source is 0.3wt% to 0.6wt%, based on 100% of the total weight of the first product. Adding non-metallic elements at this amount significantly improves the electrochemical performance of the final cathode material. In particular, when the preferred non-metallic dopant source is a boron source, and the boron source is added in the form of boric acid, the boron content in the carbon layer of the resulting cathode material is approximately 0.05wt% to 0.10wt%. This boron-doped carbon coating layer benefits from boric acid's good calcination aid effect, increasing the graphitization degree of carbon at lower calcination temperatures. Furthermore, after calcination, boron atoms dope into the carbon source coating layer, forming defects in some carbon layers, increasing hole carriers, and further improving the electronic conductivity and electrochemical performance of the material. Furthermore, in step S7, the solid content of the sixth slurry is preferably 30%–40%, which helps to promote the uniform mixing and dispersion of the first product with the carbon source and non-metallic dopant source during the mixing and slurry preparation process. This also helps to form a more ideal particle structure and distribution during subsequent drying and third calcination, ultimately improving the compaction density and electrochemical activity of the obtained cathode material. Additionally, through extensive experimentation, the inventors have preferred a third calcination temperature of 650°C–750°C and a time of 4–8 hours. Calcination within this temperature range effectively converts the carbon source into a carbon coating layer, promotes the uniform distribution of non-metallic dopant sources within it, and reduces potential material structure damage or byproduct formation at high temperatures, ultimately resulting in a lithium iron phosphate cathode material with superior electrochemical performance. To reduce the introduction of impurities and high-temperature oxidation of the obtained cathode material, and to improve its purity and structural integrity, the third calcination is preferably carried out under a protective atmosphere, more preferably nitrogen and / or argon.

[0048] In some typical embodiments, the ferrous source is selected from one or more of ferrous sulfate, ferrous ammonium sulfate, ferrous chloride, and ferrous nitrate; and / or, the phosphorus source is selected from one or more of ammonium dihydrogen phosphate, phosphoric acid, and diammonium hydrogen phosphate; and / or, the oxidant is hydrogen peroxide; and / or, the first pH adjuster is selected from one or more of sulfuric acid, phosphoric acid, nitric acid, and hydrochloric acid; and / or, the second pH adjuster is ammonia and / or sodium hydroxide; and / or, the lithium source is lithium carbonate; and / or, the carbon source is selected from one or more of glucose, sucrose, and polyvinyl alcohol. Theoretically, all of the above raw materials can be of types commonly used in the art. However, the inventors selected the above raw material types through extensive experimentation because they can achieve better synergy, thereby more effectively realizing the above preparation process and ultimately obtaining lithium iron phosphate cathode materials with superior electrochemical performance.

[0049] A second aspect of this invention provides a lithium iron phosphate cathode material, which is prepared by the aforementioned method for preparing lithium iron phosphate cathode materials, and the compaction density of the lithium iron phosphate cathode material at 350 MPa is 2.65 g / cc to 2.75 g / cc. It should be noted that, due to the specific nature of the materials field and limitations of existing testing and characterization methods, it is difficult to perform a comprehensive quantitative characterization of the complex microstructure of the aforementioned lithium iron phosphate cathode material. However, performance test results show that the cathode material obtained in this application possesses superior electrochemical performance and can significantly improve the various performance characteristics of the lithium-ion battery in which it is located. Furthermore, the prepared lithium iron phosphate cathode material has a higher compaction density and a more superior particle size distribution, thus significantly improving the rate performance of the lithium-ion battery in which it is located.

[0050] In several preferred embodiments, the lithium iron phosphate cathode material includes metal dopants and non-metal dopants, and based on 100% of the total weight of the lithium iron phosphate cathode material, the doping amount of the metal dopants is 0.05% to 0.30%, and the doping amount of the non-metal dopants is 0.05% to 0.10%. The resulting cathode material, doped with the aforementioned appropriate amounts of metal and non-metal elements, can further improve the electrochemical performance of the lithium iron phosphate cathode material.

[0051] A third aspect of the present invention provides a positive electrode comprising the aforementioned lithium iron phosphate positive electrode material. Because the obtained lithium iron phosphate positive electrode material possesses high electrochemical performance and excellent particle size distribution, the positive electrode in which it is located exhibits high energy density and good electrical performance.

[0052] A fourth aspect of the present invention provides a lithium-ion battery, comprising a positive electrode, a negative electrode, and an electrolyte, wherein the positive electrode is the aforementioned positive electrode. Using the aforementioned positive electrode with excellent electrical properties, the prepared lithium-ion battery exhibits higher rate performance and more stable electrical performance.

[0053] The present application will be further described in detail below with reference to specific embodiments, which should not be construed as limiting the scope of protection claimed in the present application.

[0054] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0055] Example 1

[0056] A method for preparing lithium iron phosphate cathode material:

[0057] (1) Mix 1 mol ferrous sulfate, 1.1 mol ammonium dihydrogen phosphate, and 1 mol hydrogen peroxide in deionized water (i.e., the molar ratio of ferrous source, phosphorus source, and oxidant is 1:1.1:1), and add sulfuric acid to adjust the pH to obtain a first slurry with a pH of 1, wherein the concentration of ferrous sulfate is 1M. Heat the first slurry to 60℃ to carry out the first reaction, and after reacting for 0.5 h, obtain the second slurry.

[0058] (2) Add 25wt% ammonia water to the second slurry and adjust its pH value to 1.5 so that it can undergo a precipitation reaction for 1 hour (i.e., the second reaction). At the same time, stir at a stirring frequency of 15Hz throughout the second reaction process to obtain the third slurry.

[0059] (3) The third slurry obtained after precipitation is divided into two parts. One half of the slurry A is heated to 90°C and aged without stirring for 1 hour. After filtration, ferric phosphate dihydrate A is obtained, which is the first precursor. The other half of the slurry B is heated to 90°C and aged with stirring at a stirring frequency of 15Hz for 4 hours. After filtration, ferric phosphate dihydrate B is obtained, which is the second precursor.

[0060] (4) Ferric phosphate dihydrate A and ferric phosphate dihydrate B were mixed with deionized water at a target solid content of 20%, and titanium dioxide (i.e., the metal dopant source) was added at a molar ratio of 0.008:1 to the total molar amount of the two. The mixture was then ball-milled at 400 rpm for 1 hour to obtain the fourth slurry. The fourth slurry was dried in an oven at 120°C for 4 hours, and then placed in a box furnace for the first calcination at 600°C for 5 hours. After dehydration, anhydrous ferric phosphate, i.e., the third precursor, was obtained.

[0061] (5) The third precursor obtained above is mixed with lithium carbonate at a molar ratio of third precursor: lithium carbonate = 1:0.5, and the mixture is mixed with deionized water with a total solid content of 35%. The ball-to-material ratio is 4:1, the speed is 400 rpm, and the ball milling is carried out for 8 hours. After the ball milling is completed, the fifth slurry is obtained. The fifth slurry is dried in an oven at 180°C to obtain the fourth precursor.

[0062] (6) The fourth precursor obtained above is mixed with oxalic acid at a weight ratio of fourth precursor: oxalic acid = 5:1, and then calcined in a box furnace at 750°C under a nitrogen atmosphere for 12 hours to obtain preliminary lithium iron phosphate, i.e. the first product.

[0063] (7) Based on the total weight of the first product obtained above as 100%, 9 wt% glucose and 0.5 wt% boric acid (i.e., non-metallic dopant source) were added and mixed. The mixture was then combined with deionized water with a total solid content of 35%. The mixture was ball-milled for 5 hours at a ball-to-material ratio of 4:1 and a rotation speed of 400 rpm. After ball milling, the sixth slurry was obtained. The sixth slurry was dehydrated by spray drying. Subsequently, it was calcined for 6 hours in a box furnace at 700°C under a nitrogen atmosphere. After that, it was pulverized by an air jet mill to obtain lithium iron phosphate cathode material.

[0064] SEM images of the obtained anhydrous ferric phosphate, i.e., the third precursor, are shown below. Figure 1 The SEM images of the obtained lithium iron phosphate cathode material are shown below. Figure 2 .

[0065] Example 2

[0066] A method for preparing lithium iron phosphate cathode material:

[0067] (1) Mix 1 mol of ferrous sulfate, 1.2 mol of ammonium dihydrogen phosphate, and 1 mol of hydrogen peroxide in deionized water (i.e., the molar ratio of ferrous source, phosphorus source, and oxidant is 1:1.2:1), and add sulfuric acid to adjust the pH to obtain a first slurry with a pH of 1, wherein the concentration of ferrous sulfate is 1M. Heat the first slurry to 50℃ to carry out the first reaction, and after reacting for 1 hour, obtain the second slurry.

[0068] (2) Add 25%wt ammonia water to the second slurry and adjust its pH value to 2.0 so that it can undergo a precipitation reaction for 1 hour (i.e., the second reaction). At the same time, stir at a stirring frequency of 15Hz throughout the second reaction process to obtain the third slurry.

[0069] (3) The third slurry obtained after precipitation is divided into two parts. One half of the slurry A is heated to 85°C and aged without stirring for 2 hours. After filtration, ferric phosphate dihydrate A is obtained, which is the first precursor. The other half of the slurry B is heated to 90°C and aged with stirring at a stirring frequency of 15Hz for 3 hours. After filtration, ferric phosphate dihydrate B is obtained, which is the second precursor.

[0070] (4) Ferric phosphate dihydrate A and ferric phosphate dihydrate B were mixed with deionized water at a target solid content of 20%, and titanium dioxide (i.e., the metal dopant source) was added at a molar ratio of 0.008:1 to the total molar amount of the two. The mixture was then ball-milled at 400 rpm for 1 hour to obtain the fourth slurry. The fourth slurry was dried in an oven at 110°C for 8 hours, and then placed in a box furnace for the first calcination at 600°C for 5 hours. After dehydration, anhydrous ferric phosphate, i.e., the third precursor, was obtained.

[0071] (5) The third precursor obtained above is mixed with lithium carbonate at a molar ratio of third precursor: lithium carbonate = 1:0.49, and the mixture is mixed with deionized water with a total solid content of 30%. The ball-to-material ratio is 4:1, the speed is 400 rpm, and the ball milling is carried out for 8 hours. After the ball milling is completed, the fifth slurry is obtained. The fifth slurry is dried in an oven at 150°C to obtain the fourth precursor.

[0072] (6) The fourth precursor obtained above is mixed with oxalic acid at a weight ratio of fourth precursor: oxalic acid = 4:1, and then calcined in a box furnace at 750°C under a nitrogen atmosphere for 12 hours to obtain preliminary lithium iron phosphate, i.e. the first product.

[0073] (7) Based on the total weight of the first product obtained above as 100%, 8 wt% glucose and 0.3 wt% boric acid (i.e., non-metallic dopant source) were added and mixed. The mixture was then combined with deionized water with a total solid content of 40%. The mixture was ball-milled for 5 hours at a ball-to-material ratio of 4:1 and a rotation speed of 400 rpm. After ball milling, the sixth slurry was obtained. The sixth slurry was dehydrated by spray drying. Subsequently, it was calcined for the third time in a box furnace at 750°C under a nitrogen atmosphere for 4 hours. After that, it was pulverized by an air jet mill to obtain lithium iron phosphate cathode material.

[0074] Example 3

[0075] A method for preparing lithium iron phosphate cathode material:

[0076] (1) Mix 1 mol of ferrous sulfate, 1.1 mol of ammonium dihydrogen phosphate, and 1 mol of hydrogen peroxide in deionized water (i.e., the molar ratio of ferrous source, phosphorus source, and oxidant is 1:1.1:1), and add sulfuric acid to adjust the pH to obtain a first slurry with a pH of 1.5, wherein the concentration of ferrous sulfate is 0.8M. Heat the first slurry to 70℃ to carry out the first reaction, and after reacting for 0.5 h, obtain the second slurry.

[0077] (2) Add 25%wt ammonia water to the second slurry and adjust its pH value to 1.5 so that it can undergo a precipitation reaction for 2 hours (i.e., the second reaction). At the same time, stir at a stirring frequency of 15Hz throughout the second reaction process to obtain the third slurry.

[0078] (3) The third slurry obtained after precipitation is divided into two parts. One half of the slurry A is heated to 90°C and aged without stirring for 1 hour. After filtration, ferric phosphate dihydrate A is obtained, which is the first precursor. The other half of the slurry B is heated to 95°C and aged with stirring at a stirring frequency of 20Hz for 4 hours. After filtration, ferric phosphate dihydrate B is obtained, which is the second precursor.

[0079] (4) Ferric phosphate dihydrate A and ferric phosphate dihydrate B were mixed with deionized water at a target solid content of 20%, and titanium dioxide (i.e., the metal dopant source) was added at a molar ratio of 0.005:1 to the total molar amount of the two. The mixture was then ball-milled at 400 rpm for 1 h to obtain the fourth slurry. The fourth slurry was dried in an oven at 130°C for 4 h, and then placed in a box furnace for the first calcination at 550°C for 8 h. After dehydration, anhydrous ferric phosphate, i.e., the third precursor, was obtained.

[0080] (5) The third precursor obtained above is mixed with lithium carbonate at a molar ratio of third precursor: lithium carbonate = 1:0.51, and the mixture is mixed with deionized water with a total solid content of 40%. The ball-to-material ratio is 4:1, the speed is 400 rpm, and the ball milling is carried out for 8 hours. After the ball milling is completed, the fifth slurry is obtained. The fifth slurry is dried in an oven at 200°C to obtain the fourth precursor.

[0081] (6) The fourth precursor obtained above is mixed with oxalic acid at a weight ratio of fourth precursor: oxalic acid = 6:1, and then calcined in a box furnace at 700°C under a nitrogen atmosphere for 15 hours to obtain preliminary lithium iron phosphate, i.e. the first product.

[0082] (7) Based on the total weight of the first product obtained above as 100%, 10 wt% glucose and 0.6 wt% boric acid (i.e., non-metallic dopant source) were added and mixed. The mixture was then combined with deionized water with a total solid content of 30%. The mixture was ball-milled for 5 hours at a ball-to-material ratio of 4:1 and a rotation speed of 400 rpm. After ball milling, the sixth slurry was obtained. The sixth slurry was dehydrated by spray drying. Subsequently, it was calcined for 8 hours in a box furnace at 650°C under a nitrogen atmosphere. After that, it was pulverized by an air jet mill to obtain lithium iron phosphate cathode material.

[0083] Example 4

[0084] A method for preparing lithium iron phosphate cathode material:

[0085] The only difference between this embodiment and Embodiment 1 is that in step (4), the temperature of the first calcination is changed to 650°C and the time is changed to 4 hours.

[0086] Example 5

[0087] A method for preparing lithium iron phosphate cathode material:

[0088] The only difference between this embodiment and Embodiment 1 is that in step (6), the temperature of the second calcination is changed to 800°C and the time is changed to 10h.

[0089] Example 6

[0090] A method for preparing lithium iron phosphate cathode material:

[0091] The only difference between this embodiment and Embodiment 1 is that, in step (1), the amount of ammonium dihydrogen phosphate added is changed to 0.8 mol.

[0092] At this point, the molar ratio of ferrous source, phosphorus source and oxidant is 1:0.8:1.

[0093] Example 7

[0094] A method for preparing lithium iron phosphate cathode material:

[0095] The only difference between this embodiment and Embodiment 1 is that, in step (1), the amount of ammonium dihydrogen phosphate added is changed to 1.5 mol.

[0096] At this point, the molar ratio of ferrous source, phosphorus source and oxidant is 1:1.5:1.

[0097] Example 8

[0098] A method for preparing lithium iron phosphate cathode material:

[0099] The only difference between this embodiment and embodiment 1 is that in step (3), the obtained third slurry is divided into two parts in a volume ratio of 1:3 to obtain slurry A and slurry B, and the volume ratio of slurry A to slurry B is 1:3.

[0100] Example 9

[0101] A method for preparing lithium iron phosphate cathode material:

[0102] The only difference between this embodiment and embodiment 1 is that in step (3), the obtained third slurry is divided into two parts in a volume ratio of 1:3 to obtain slurry A and slurry B, and the volume ratio of slurry A to slurry B is 3:1.

[0103] Example 10

[0104] A method for preparing lithium iron phosphate cathode material:

[0105] The only difference between this embodiment and Embodiment 1 is that, in step (4), the amount of titanium dioxide added is changed so that the molar ratio of titanium dioxide to the total molar ratio of ferric phosphate dihydrate A and ferric phosphate dihydrate B is 0.002:1; and in step (7), the amount of boric acid added is changed so that the amount of boric acid added accounts for 0.05 wt% of the first product.

[0106] Example 11

[0107] A method for preparing lithium iron phosphate cathode material:

[0108] The only difference between this embodiment and embodiment 1 is that, in step (4), the amount of titanium dioxide added is changed so that the molar ratio of titanium dioxide to the total molar ratio of ferric phosphate dihydrate A and ferric phosphate dihydrate B is 0.010:1; and in step (7), the amount of boric acid added is changed so that the amount of boric acid added accounts for 0.80 wt% of the first product.

[0109] Example 12

[0110] A method for preparing lithium iron phosphate cathode material:

[0111] The only difference between this embodiment and Embodiment 1 is that in step (4), the temperature of the first calcination is changed to 500°C and the time is changed to 10h.

[0112] Example 13

[0113] A method for preparing lithium iron phosphate cathode material:

[0114] The only difference between this embodiment and Embodiment 1 is that in step (6), the weight ratio of the fourth precursor to oxalic acid is changed to 2:1.

[0115] Example 14

[0116] A method for preparing lithium iron phosphate cathode material:

[0117] The only difference between this embodiment and Embodiment 1 is that in step (6), the weight ratio of the fourth precursor to oxalic acid is changed to 8:1.

[0118] Example 15

[0119] A method for preparing lithium iron phosphate cathode material:

[0120] The only difference between this embodiment and Embodiment 1 is that in step (6), the temperature of the second calcination is changed to 600°C and the time is changed to 18h.

[0121] Example 16

[0122] A method for preparing lithium iron phosphate cathode material:

[0123] The only difference between this embodiment and Embodiment 1 is that, in step (4), an equimolar amount of aluminum hydroxide is used instead of titanium dioxide.

[0124] Example 17

[0125] A method for preparing lithium iron phosphate cathode material:

[0126] The only difference between this embodiment and Embodiment 1 is that, in step (4), strontium nitrate is used instead of titanium dioxide in an equal molar amount.

[0127] Comparative Example 1

[0128] A method for preparing lithium iron phosphate cathode material:

[0129] The only difference between this comparative example and Example 1 is step (3), as follows.

[0130] (3) The third slurry obtained after precipitation is heated to 90°C and aged without stirring for 1 hour. After filtration, ferric phosphate dihydrate is obtained.

[0131] That is, the comparative example did not divide the third slurry into slurry A and slurry B and age them separately, but directly aged the third slurry as a whole to obtain the iron phosphate dihydrate precursor.

[0132] SEM images of the obtained anhydrous ferric phosphate, i.e., the third precursor, are shown below. Figure 3 The SEM images of the obtained lithium iron phosphate cathode material are shown below. Figure 4 .

[0133] Comparative Example 2

[0134] A method for preparing lithium iron phosphate cathode material:

[0135] The only difference between this comparative example and Example 1 is step (3), as follows.

[0136] (3) The third slurry obtained after precipitation is heated to 90°C and then stirred and aged at a stirring frequency of 15Hz for 4 hours. After filtration, ferric phosphate dihydrate is obtained.

[0137] That is, the comparative example did not divide the third slurry into slurry A and slurry B and age them separately, but directly stirred and aged the third slurry as a whole to obtain the iron phosphate dihydrate precursor.

[0138] SEM images of the obtained anhydrous ferric phosphate, i.e., the third precursor, are shown below. Figure 5 The SEM images of the obtained lithium iron phosphate cathode material are shown below. Figure 6 .

[0139] Comparative Example 3

[0140] A method for preparing lithium iron phosphate cathode material:

[0141] The only difference between this comparative example and Example 1 is that titanium dioxide and boric acid were not added in step (4).

[0142] Comparative Example 4

[0143] A method for preparing lithium iron phosphate cathode material:

[0144] The only difference between this comparative example and Example 1 is that step (6) was not performed, but the fourth precursor obtained in step (5) was used as the first product and step (7) was performed.

[0145] Comparative Example 5

[0146] A method for preparing lithium iron phosphate cathode material:

[0147] The only difference between this comparative example and Example 1 is that, in step (6), oxalic acid is replaced with an equal weight of citric acid.

[0148] Test methods

[0149] Compacted density: Referring to GB / T 30835-2014 "Carbon Composite Lithium Iron Phosphate Cathode Material for Lithium-ion Batteries" standard, the method for powder compaction density (GB / T24533-2019) was used to measure the compaction density of lithium iron phosphate materials, obtaining the compaction density of the cathode material samples obtained in each example and comparative example at 350 MPa. The specific steps are as follows: 0.5 g of sample was placed in the sample cell of the compaction density measuring instrument, the measurement pressure range was set to 30-350 MPa, and the pressure was measured for 10 seconds at 30 MPa intervals. The compaction curve and the compaction density at the final pressure of 350 MPa were obtained.

[0150] Battery Sample Preparation and Performance Testing: 1. Battery Sample Preparation. The obtained positive electrode material, SP conductive agent, and PVDF binder were weighed and mixed in a mass ratio of 8:1:1, and CR2032 coin cell half-cells were prepared under an argon protective atmosphere. During the coin cell preparation process, all auxiliary materials except the positive electrode material were any commercially available auxiliary materials. 2. Coin Cell Sample Testing: Each prepared coin cell half-cell sample was subjected to charge-discharge tests at room temperature, with a voltage range of 2.0V to 4.0V. The following values ​​were obtained: first charge specific capacity under 0.2C conditions; first discharge specific capacity under 0.2C conditions; first-time efficiency % under 0.2C conditions (first discharge capacity / first charge capacity × 100%); 0.2C discharge specific capacity (discharge specific capacity obtained by performing a charge-discharge test under 0.2C conditions after the first charge-discharge test at 0.2C); 1C discharge specific capacity (discharge specific capacity obtained by performing a charge-discharge test under 1C conditions after the 0.2C charge-discharge test); 2C discharge specific capacity (discharge specific capacity obtained by performing a charge-discharge test under 2C conditions after the 1C charge-discharge test).

[0151] The test results are shown in Table 1. The amount of metal doping and non-metal doping in the cathode material samples obtained in each embodiment and comparative example are also shown in Table 1 (metal doping amount = (mass of metal element / total mass of cathode material) × 100%, and the calculation of non-metal doping amount is the same).

[0152] Table 1

[0153]

[0154]

[0155] As can be seen from the above description, the embodiments of the present invention have achieved the preparation of lithium iron phosphate materials with excellent electrochemical performance. When the obtained material is used as a cathode material for lithium-ion batteries and lithium-ion batteries are prepared, the corresponding batteries exhibit higher rate performance.

[0156] Specifically, compared to Example 1, in the crystallization process of ferric phosphate dihydrate in Comparative Example 1, only the process condition of aging without stirring for 1 hour was used, as shown by its SEM image (i.e. Figure 3 , Figure 4 As can be seen, the anhydrous iron phosphate precursor and lithium iron phosphate particles obtained in Comparative Example 1 are both relatively large, and the measured compaction density is also relatively high. However, their rate performance is much worse than that of Example 1. This is because the large size of the lithium iron phosphate particles is not conducive to sufficient wetting with the electrolyte, resulting in greater capacity loss during high current charging and discharging and poor performance.

[0157] Compared to Example 1, the anhydrous iron phosphate precursor and lithium iron phosphate particles obtained in Comparative Example 2 are more uniform and smaller (see Example 2). Figure 5 and Figure 6 While it possesses good electrical properties, its compaction density is low because the particle size distribution is not well formed, and the gaps between the particles cannot be filled. Consequently, the energy density of the lithium-ion batteries prepared subsequently is low, which is not conducive to practical applications.

[0158] Compared to Example 1, Comparative Example 3 did not contain titanium and boron, resulting in poorer electrical properties and lower compaction density. This is because titanium, as a metal dopant, has a doping effect that can improve electrical properties and inhibit excessive particle growth during high-temperature sintering. The absence of titanium may lead to excessively large particles, poor gradation, and poor electrical performance.

[0159] Compared to Example 1, Comparative Example 4 changed the two-step sintering to one-step sintering and did not add oxalic acid. However, the organic carbon source will lose some of its carbon during sintering due to the reduction of ferric ions, resulting in less carbon coating in the end. Although the compaction density is improved to some extent, the rate performance of the resulting lithium iron phosphate cathode material is adversely affected, and its corresponding lithium-ion battery performance is poor.

[0160] Compared to Example 1, Comparative Example 5 replaced oxalic acid with citric acid. Citric acid can also reduce ferric iron to ferrous iron when it decomposes at high temperature, thereby generating lithium iron phosphate phase. However, the residual carbon produced by its decomposition inhibits the growth of particles during sintering, resulting in the compaction performance of the final material not being improved. Consequently, the energy density of the full cell is low and the performance is poor.

[0161] Compared to Examples 1 to 5, Example 6 reduced the amount of phosphorus source, resulting in iron phosphate with a higher iron-to-phosphorus ratio. While the lithium iron phosphate produced from the high iron-to-phosphorus ratio exhibited good electrical performance, its compaction performance was relatively average, leading to a lower energy density in the final full cell. Example 7 increased the phosphorus source, lowering the iron-to-phosphorus ratio of the iron phosphate and promoting lithium iron phosphate particle growth, resulting in relatively better compaction, but slightly worse electrical performance. Example 8 effectively increased the proportion of small-particle iron phosphate, resulting in relatively better electrical performance for the corresponding lithium iron phosphate, but a slight decrease in compaction performance. Example 9 increased the proportion of large-particle iron phosphate, resulting in slightly better compaction performance, but the electrical performance of the half-cell sample was lower than that of Example 1. Example 10 reduced the amount of titanium and boron added, resulting in a slight decrease in electrical performance, but slightly better compaction. Example 11 increased the amount of titanium and boron added, significantly improving electrical performance, but simultaneously worsening compaction performance. Therefore, in practical applications, the energy density of the full cell product produced in Example 1 was relatively lower than that of Example 1. In Example 12, the dehydration temperature of iron phosphate dihydrate was lowered, affecting its crystallinity and resulting in a slight decrease in the performance of the prepared lithium iron phosphate battery. In Example 13, reducing the amount of oxalic acid slightly decreased the electrical performance. In Example 14, increasing the amount of oxalic acid also led to a decline in the electrical performance of the half-cell sample. In Example 15, lowering the secondary sintering temperature affected the growth of large particles, resulting in a slight decrease in compaction, which was not conducive to the high energy density of the subsequent full-cell product. In Examples 16 and 17, the type of metal dopant source was changed, making it difficult to utilize the size control effect of titanium on particles during calcination. Therefore, the optimal particle size distribution could not be formed, and although the electrical performance of the final battery sample was improved due to the doping effect, it was still slightly worse than that of Example 1.

[0162] It should be noted that the terms "first," "second," etc., used in the specification and claims of this application are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such terms can be used interchangeably where appropriate so that the embodiments of this application described herein can be implemented, for example, in a sequence other than those described herein.

[0163] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, include: Step S1: The ferrous source, phosphorus source, oxidant and first pH adjuster are mixed to prepare a first slurry with a pH value of 1.0~1.

5. The first slurry undergoes a first reaction to obtain a second slurry. Step S2: Add a second pH adjuster to the second slurry to adjust the pH value of the second slurry to 1.5~2.

0. After the second reaction, a third slurry is obtained. Step S3: Take a portion of the third slurry as slurry A and another portion of the third slurry as slurry B; after slurry A is subjected to static aging and filtration in sequence, a first precursor is obtained; after slurry B is subjected to stirring aging and filtration in sequence, a second precursor is obtained. The volume ratio of slurry A to slurry B is (0.8~1.2):1; Step S4: The first precursor, the second precursor, and the metal doping source are prepared into a fourth slurry, and the fourth slurry is subjected to a first drying and a first calcination to obtain a third precursor. The ratio of the total molar amount of the first precursor and the second precursor to the molar amount of the metal dopant source is 1:(0.005~0.008). The metal doping source is selected from one or more of titanium, magnesium, aluminum, vanadium and strontium sources; Step S5: The third precursor and lithium source are mixed to form a fifth slurry, and the fifth slurry is dried in a second process to obtain a fourth precursor. Step S6: The fourth precursor is mixed with oxalic acid and then calcined a second time to obtain the first product; The weight ratio of the fourth precursor to the oxalic acid is (4~6):1; Step S7: The first product is prepared into a sixth slurry by combining the first product with a carbon source and a non-metallic doping source. The sixth slurry is then subjected to a third drying and a third calcination to obtain the lithium iron phosphate cathode material. Based on the total weight of the first product being 100%, the amount of the non-metallic dopant source is 0.3wt%~0.6wt%; The non-metallic dopant source is a boron source, and the boron source is added in the form of boric acid.

2. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, In step S1 The molar ratio of the ferrous source, the phosphorus source, and the oxidant is 1:(1.1~1.2):1; and / or, In the first slurry, the concentration of the ferrous source is 0.8 M to 1.0 M; and / or, The first reaction is carried out at 50℃~70℃, and the reaction time is 0.5h~1h.

3. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, Step S2 includes: adding the second pH adjuster to the second slurry to adjust the pH value of the second slurry to 1.5~2.0, so that the second slurry undergoes the second reaction for 1h~2h to obtain the third slurry.

4. The method of producing a lithium iron phosphate cathode material according to any one of claims 1 to 3, characterized in that, In step S3, the temperature for static aging is 85℃~95℃ and the time is 1h~2h; and / or, the temperature for stirring aging is 85℃~95℃ and the time is 3h~4h, and the stirring frequency for stirring aging is 10Hz~20Hz.

5. The method of producing a lithium iron phosphate cathode material according to any one of claims 1 to 3, characterized in that, In step S4 In the fourth slurry, the total solid content of the first precursor and the second precursor is 20%~25%; and / or, The first drying is carried out at 110℃~130℃, and the first drying time is 3h~4h; and / or, the first calcination is performed at a temperature of 550-650°C for 4-8 hours; and / or, each of the titanium source, the magnesium source, the aluminum source, the vanadium source, and the strontium source is independently added in the form of one or more of an oxide, a hydroxide, a nitrate, a phosphate, a sulfate, and an acetate.

6. The method of claim 5, wherein the lithium iron phosphate cathode material is prepared by the steps of: the metal doping source is a titanium source, and the titanium source is added in the form of titanium dioxide and / or tetrabutyl titanate. ​ 7. The method of producing a lithium iron phosphate cathode material according to any one of claims 1 to 3, characterized in that, in the step S5, the molar ratio of the third precursor to the lithium source is 1:(0.4-0.6); and / or, the solid content of the fifth slurry is 30%-40%; and / or, the second drying is performed at 150-200°C.

8. The method of claim 7, wherein the lithium iron phosphate cathode material is prepared by the steps of: in the step S5, the molar ratio of the third precursor to the lithium source is 1:(0.49-0.51). ​ 9. The method of producing a lithium iron phosphate cathode material according to any one of claims 1 to 3, characterized in that, in the step S6, the second calcination is performed at a temperature of 700-800°C for 10-15 hours.

10. The method of claim 9, wherein the lithium iron phosphate cathode material is prepared by the steps of: in the step S6, the second calcination is performed under a protective atmosphere. ​ 11. The method of claim 10, wherein the lithium iron phosphate cathode material is prepared by the steps of: in the step S6, the protective atmosphere is nitrogen and / or argon. ​ 12. The method of producing a lithium iron phosphate cathode material according to any one of claims 1 to 3, characterized in that, in the step S7, the amount of the carbon source is 8-10 wt% based on the total weight of the first product; and / or, the solid content of the sixth slurry is 30%-40%; and / or, the third calcination is performed at a temperature of 650-750°C for 4-8 hours.

13. The method for preparing lithium iron phosphate cathode material according to claim 12, characterized in that, in the step S7, the third calcination is performed under a protective atmosphere.

14. The method of claim 13, wherein the lithium iron phosphate cathode material is prepared by the steps of: in the step S7, the protective atmosphere is nitrogen and / or argon. ​ 15. The method of claim 1-3, wherein, the ferrous source is selected from one or more of ferrous sulfate, ferrous ammonium sulfate, ferrous chloride, and ferrous nitrate; and / or, the phosphorus source is selected from one or more of ammonium dihydrogen phosphate, phosphoric acid, and ammonium dihydrogen phosphate; and / or, the oxidizing agent is hydrogen peroxide; and / or, the first pH adjuster is selected from one or more of sulfuric acid, phosphoric acid, nitric acid, and hydrochloric acid; and / or, the second pH adjuster is ammonia and / or sodium hydroxide; and / or, the lithium source is lithium carbonate; and / or, the carbon source is selected from one or more of glucose, sucrose, and polyvinyl alcohol.

16. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate positive electrode material is prepared by the method of any one of claims 1-15, and the compaction density of the lithium iron phosphate positive electrode material at 350 MPa is 2.65-2.75 g / cc.

17. The lithium iron phosphate cathode material of claim 16, wherein, The lithium iron phosphate positive electrode material comprises metal doping elements and non-metal doping elements, and the doping amount of the metal doping elements is 0.05%-0.30% and the doping amount of the non-metal doping elements is 0.05%-0.10% based on the total weight of the lithium iron phosphate positive electrode material.

18. A positive electrode sheet characterized by comprising: The positive electrode sheet comprises the lithium iron phosphate positive electrode material of claim 16 or 17.

19. A lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, characterized by, The positive electrode sheet is the positive electrode sheet of claim 18.

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