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

Abstract

The invention provides a lithium iron phosphate positive electrode material, a preparation method thereof, a positive electrode plate and a lithium ion battery. The preparation method comprises the following steps: preparing a ferrous source, a phosphorus source, an oxidant and a first pH regulator into first slurry, and carrying out first reaction on the first slurry to obtain second slurry; adding a second pH regulator into the second slurry, and carrying out a second reaction to obtain a third slurry; aging and filtering 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 with a lithium source to obtain a fourth precursor; mixing the fourth precursor with oxalic acid, and performing secondary 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. According to the invention, the preparation process of the precursor is optimized, and meanwhile, the whole preparation process of the lithium iron phosphate positive electrode material is optimized.

Description

Technical Field

[0001] The present invention relates to the field of lithium ion batteries, and in particular to a lithium iron phosphate positive electrode material, a preparation method thereof, a positive electrode sheet and a lithium ion battery. Background Art

[0002] Since the 1980s, lithium (Li) ion batteries have gradually entered the human vision and achieved commercialization in the 1990s. After the collaborative efforts of several generations of researchers and industrial practitioners, the performance of lithium ion batteries has been greatly improved. At present, lithium ion batteries have been widely used in small consumer electronic products, power tools, large power supplies, (plug-in) hybrid electric vehicles and power grid systems.

[0003] Lithium iron phosphate is an important cathode material for lithium-ion batteries, with many significant features and advantages. From the perspective of chemical structure, lithium iron phosphate has a stable olivine structure, which gives it excellent safety and cycle stability. During the charge and discharge process, its structural changes are relatively small, and it can withstand multiple charge and discharge cycles, thereby extending the service life of the battery. In terms of performance, lithium iron phosphate has a higher discharge platform, usually around 3.2V. Its theoretical specific capacity is about 170mAh / g, and its actual specific capacity can reach 130mAh / g~160mAh / g. At the same time, lithium iron phosphate also has good high-temperature performance, and its performance attenuation is relatively small under high temperature conditions. Safety is another outstanding advantage of lithium iron phosphate. It is not easy to have dangerous situations such as thermal runaway and combustion and explosion in the case of overcharge, over-discharge, short circuit, etc., which makes the equipment using lithium iron phosphate batteries have higher safety guarantees. In terms of cost, the raw materials of lithium iron phosphate are widely available and relatively low in price. For example, elements such as iron and phosphorus are abundant in the earth's crust, which helps to reduce the production cost of batteries.

[0004] However, lithium iron phosphate also has some shortcomings. For example, its electronic conductivity and ionic conductivity are relatively low, which limits its electrical performance to a certain extent. In order to improve this problem, it is usually necessary to perform modification treatments such as nano-sizing and carbon coating. However, simply taking this method to improve the electrical performance will significantly reduce the corresponding compaction density, resulting in a decrease in energy density. On the contrary, it is not advisable to simply improve the compaction and ignore the adverse effects on the electrical performance. Therefore, the research that takes into account both the electrical performance and the compaction performance of lithium iron phosphate is a focus of research in the industry.

[0005] Based on this, how to start from its preparation method and rationally design its doping elements, and then optimize the elemental composition and morphology and particle size of olivine-type lithium iron phosphate so that it exhibits better electrochemical properties, is one of the technical problems that need to be solved in this field. Summary of the invention

[0006] The main purpose of the present invention is to provide a lithium iron phosphate positive electrode material, a preparation method thereof, a positive electrode plate and a lithium ion battery, so as to solve the problem of poor electrochemical performance of olivine-type lithium iron phosphate positive electrode materials in the prior art.

[0007] In order to achieve the above-mentioned object, the first aspect of the present invention provides a method for preparing a lithium iron phosphate positive electrode material, comprising: step S1, preparing a ferrous source, a phosphorus source, an oxidant and a first pH regulator into a first slurry, and subjecting the first slurry to a first reaction to obtain a second slurry; step S2, adding a second pH regulator to the second slurry, and subjecting the second slurry to a second reaction to obtain a third slurry; step S3, taking a portion of the third slurry as slurry A, and another portion of the third slurry as slurry B; slurry A is subjected to static aging and filtration to obtain a first precursor, and slurry B is subjected to stirring aging and filtration to obtain a first precursor. After filtering, a second precursor is obtained; step S4, the first precursor, the second precursor and the metal doping source are formulated into a fourth slurry, and the fourth slurry is successively subjected to a first drying and a first calcination to obtain a third precursor; step S5, the third precursor and the lithium source are formulated into a fifth slurry, and the fifth slurry is subjected to a second drying to obtain a fourth precursor; step S6, the fourth precursor is mixed with oxalic acid, and a first product is obtained after a second calcination; step S7, the first product is formulated into a sixth slurry with a carbon source and a non-metallic doping source, and the sixth slurry is successively subjected to a third drying and a third calcination to obtain a lithium iron phosphate positive electrode material.

[0008] The present invention achieves the improvement of its compaction density and electrochemical performance by accurately designing the preparation process of the positive electrode material. Wherein in step S3, two kinds of iron phosphate dihydrate precursors are prepared by different aging conditions (standing and stirring aging), which leads to two different particle size distributions and morphologies. And the two kinds of iron phosphate dihydrate under different aging conditions are mixed to form a particle size grading effect, which can improve the compaction density of the final lithium iron phosphate material. Afterwards in step S4, by adding a metal doping source before dehydration, the uniform doping of the iron phosphate precursor and the metal doping element is promoted, and the electrical properties of lithium iron phosphate are improved. Afterwards in step S6, the synthesis of preliminary lithium iron phosphate uses oxalic acid as an auxiliary agent, which decomposes during the subsequent calcination process, and the reducing gas produced can reduce the trivalent iron in the precursor to divalent iron, which is conducive to the phase formation of lithium iron phosphate and improves the crystallinity and conductivity of the material. At the same time, there is basically no residual carbon after the decomposition of oxalic acid, which avoids the introduction of additional carbon sources, reduces the generation of magnetic foreign matter ferrophosphorus compounds, thereby improving the purity of the material. At the same time, a two-step sintering method is adopted, that is, the reducing gas decomposed by oxalic acid is first used to improve the crystallinity and structural integrity of lithium iron phosphate in the first sintering, and then the surface carbon coating is performed in the second sintering. At the same time, the non-metallic doping source is added in the process. On the one hand, it plays a good role in assisting sintering. The degree of graphitization of the carbon layer can be improved by calcining at a lower temperature. On the other hand, after the non-metallic doping source is calcined, the non-metallic elements are doped in the carbon source coating layer to form defects in part of the carbon layer, increase the hole carriers, and not only improve the electrical properties of the material, but also control the amount of carbon added, avoiding the adverse effect of excessive carbon on the compaction density. The lithium iron phosphate positive electrode material obtained by the above preparation method has good compaction and electrochemical properties.

[0009] Further, 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 0.8M-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°C-70°C, and the time of the first reaction is 0.5h-1h. In the process of forming the second slurry, by optimizing the molar ratio and concentration of the reactants, it is possible to promote the formation of iron phosphate with a moderate grain size and uniform distribution. At the same time, the pH value and reaction conditions of the reaction system are optimized, which can optimize the morphology of the obtained dihydrate iron phosphate, improve its structural integrity and surface morphology, and thus provide a higher quality precursor for the subsequent conversion into anhydrous iron phosphate and lithium iron phosphate.

[0010] Further, step S2 includes: adding a second pH regulator to the second slurry, adjusting the pH value of the second slurry to 1.5-2.0, so that the second slurry undergoes a second reaction for 1h-2h to obtain a third slurry. The pH range and precipitation reaction time during the formation of the third slurry are preferably such that the purity and consistency of the final cathode material can be improved, while 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 temperature of static aging is 85°C-95°C, and the time is 1h-2h; and / or, the temperature of stirring aging is 85°C-95°C, the time is 3h-4h, and the stirring frequency of stirring aging is 10Hz-20Hz. In the process of forming the first precursor and the second precursor, by optimizing the aging conditions, including the temperature, time and stirring frequency of static aging and stirring aging, the morphology, particle size and performance of the two obtained dihydrate iron phosphate precursors can be significantly improved, and ultimately the particle size distribution, compaction density and electrochemical performance of the obtained positive electrode material can be more effectively improved.

[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 doping 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 time of the first drying is 3h~4h; and / or, the temperature of the first calcination is 550℃~650℃, and the time is 4h~8h; and / or, the metal doping source is selected from one or more of a titanium source, a magnesium source, an aluminum source, a vanadium source and a strontium source, and the titanium source, the magnesium source, the aluminum source, the vanadium source and the strontium source are each independently added in the form of one or more of oxides, hydroxides, nitrates, phosphates, sulfates and acetates; preferably, the metal doping 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 doped with metal elements, by optimizing the above conditions, the doping of the metal doping elements in the third precursor can be made more uniform, thereby improving the compaction density and electrochemical performance of the material. In addition, the optimized conditions of drying and calcination also improve the purity and crystallinity of the resulting positive electrode material, giving it higher comprehensive electrochemical performance. Among the above-mentioned metal doping elements, titanium can not only improve the electrochemical performance of the resulting positive electrode material, but also inhibit the excessive growth of crystal particles during the calcination process, optimize the particle size distribution and structural stability of the material, and thus more effectively improve the electrochemical performance of the final positive electrode 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 performed at 150°C-200°C. Through the above-mentioned preferred lithium source molar ratio, the solid content of the fifth slurry and the temperature setting of the second drying, the preparation of high-performance lithium iron phosphate positive electrode material can be more effectively achieved, so that its compaction density, specific capacity and cycle stability can reach a better state.

[0014] Further, in step S6, the weight ratio of the fourth precursor to oxalic acid is (4-6): 1; and / or, the temperature of the second calcination is 700°C-800°C, 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, by 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, it has a significant effect on improving the electrochemical properties, compaction density, and high efficiency and environmental protection of the production process of the first product and the final cathode material. By optimizing the above conditions, the purity and activity of the cathode material during the conversion process can be promoted, thereby obtaining a lithium iron phosphate cathode material with superior electrochemical performance.

[0015] Further, in step S7, the amount of the carbon source is 8wt% to 10wt% based on the total weight of the first product as 100%, and the amount of the non-metallic doping source is 0.3wt% to 0.6wt% based on the total weight of the first product as 100%; and / or, the non-metallic doping 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 4h to 8h; 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 finally calcining and forming the positive electrode material, by preferably selecting the ratio of the first product to the carbon source, the ratio to the non-metallic doping source, the specific type and addition form of the non-metallic doping source, the solid content of the sixth slurry, and the conditions of the third calcination as above, a lithium iron phosphate positive electrode material with higher electronic conductivity, better compaction density, and more stable structure can be obtained.

[0016] Further, the ferrous source is selected from one or more of ferrous sulfate, ammonium ferrous 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 water 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. The above raw materials can be better matched, and finally a lithium iron phosphate positive electrode material with better electrochemical performance is prepared.

[0017] The second aspect of the present invention provides a lithium iron phosphate positive electrode material, which is prepared by the preparation method of the above-mentioned lithium iron phosphate positive electrode material, and the compaction density of the lithium iron phosphate positive electrode material at 350MPa is 2.65g / cc to 2.75g / cc. The prepared lithium iron phosphate positive electrode material has a higher compaction density and a more superior particle size distribution, so it can significantly improve the rate performance of the lithium ion battery in which it is located. Preferably, the lithium iron phosphate positive electrode material includes metal doping elements and non-metallic doping elements, and the total weight of the lithium iron phosphate positive electrode material is 100%, the doping amount of the metal doping element is 0.05% to 0.30%, and the doping amount of the non-metallic doping element is 0.05% to 0.10%. The obtained positive electrode material is doped with an appropriate amount of metal elements and non-metallic elements, so that the electrochemical performance of the lithium iron phosphate positive electrode material can be further improved.

[0018] The third aspect of the present invention provides a positive electrode sheet, which includes the above-mentioned lithium iron phosphate positive electrode material. Since the obtained lithium iron phosphate positive electrode material has high electrochemical performance and excellent particle size distribution, the positive electrode sheet in which it is located exhibits high energy density and good electrical performance.

[0019] The fourth aspect of the present invention provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet is the positive electrode sheet described above. The lithium-ion battery prepared by the positive electrode sheet having excellent electrical properties has higher rate performance and more stable electrical performance.

[0020] By applying the technical solution of the present invention, the overall preparation process of lithium iron phosphate positive electrode materials is optimized by optimizing the precursor preparation process, introducing metal elements and non-metallic elements for doping during the preparation process, and utilizing the advantages that there is basically no residual carbon after oxalic acid decomposition and the reducing gas generated reduces trivalent iron to facilitate the formation of lithium iron phosphate phase. The obtained lithium iron phosphate positive electrode material has good compaction and electrochemical properties, thereby being able to significantly improve the rate performance of the lithium ion battery in which it is located. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] The drawings constituting a part of the present application are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute an improper limitation of the present invention. In the drawings:

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

[0023] Figure 2 This is a SEM photo of the lithium iron phosphate positive electrode material obtained in Example 1 of the present invention;

[0024] Figure 3 This is a SEM photo of the third precursor obtained in Comparative Example 1 of the present invention;

[0025] Figure 4 This is a SEM photo of the lithium iron phosphate positive electrode material obtained in Comparative Example 1 of the present invention;

[0026] Figure 5 This is a SEM photo of the third precursor obtained in Comparative Example 2 of the present invention;

[0027] Figure 6 This is a SEM photograph of the lithium iron phosphate positive electrode material obtained in Comparative Example 2 of the present invention. DETAILED DESCRIPTION

[0028] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The present invention will be described in detail below in conjunction with the embodiments.

[0029] As described in the background technology, the olivine-type lithium iron phosphate positive electrode material in the prior art has the problem of poor electrochemical performance. In order to solve the above technical problems, the first aspect of the present invention provides a method for preparing a lithium iron phosphate positive electrode material, including: step S1, preparing a ferrous source, a phosphorus source, an oxidant and a first pH regulator into a first slurry, and the first slurry undergoes a first reaction to obtain a second slurry; step S2, adding a second pH regulator to the second slurry, and obtaining a third slurry after a second reaction; step S3, taking a part of the third slurry as slurry A, and another part of the third slurry as slurry B; slurry A is sequentially subjected to static aging and filtration to obtain a first precursor, and slurry B is sequentially subjected to stirring aging and After filtration, a second precursor is obtained; step S4, the first precursor, the second precursor and the metal doping source are formulated into a fourth slurry, and the fourth slurry is successively subjected to a first drying and a first calcination to obtain a third precursor; step S5, the third precursor and the lithium source are formulated into a fifth slurry, and the fifth slurry is subjected to a second drying to obtain a fourth precursor; step S6, the fourth precursor is mixed with oxalic acid, and a first product is obtained after a second calcination; step S7, the first product is formulated into a sixth slurry with a carbon source and a non-metallic doping source, and the sixth slurry is successively subjected to a third drying and a third calcination to obtain a lithium iron phosphate positive electrode material.

[0030] The present invention optimizes the overall preparation process of lithium iron phosphate positive electrode materials by optimizing the precursor preparation process, introducing metal elements and non-metallic elements for doping during the preparation process, and utilizing the advantages that there is basically no residual carbon after oxalic acid decomposition, and the reducing gas generated reduces trivalent iron, which is beneficial to the formation of lithium iron phosphate. Specifically,

[0031] First, a ferrous source and a phosphorus source are mixed, and an oxidant is added for oxidation, so that the ferrous ions are oxidized to ferric ions, and at the same time, a first pH adjuster is used to control the pH so that the oxidation reaction is easy to occur, thereby obtaining a uniform second slurry. Then, the pH value of the second slurry is adjusted again to promote the iron ions to form iron phosphate and precipitate, thereby obtaining a third slurry containing iron phosphate precipitation.

[0032] The third slurry is divided into slurry A and slurry B, and two different ferric phosphate dihydrate precipitates are formed through different aging conditions. That is, slurry A is statically aged, tending to form larger, more regular crystals, while slurry B is stirred and aged, tending to form smaller, more uniform crystals. By mixing the two in subsequent steps, the effect of particle size grading can be achieved, that is, a composite material with both large and small particles is formed. Different particle sizes can fill each other, reduce the gaps between materials, and maintain good electron and ion transmission paths, thereby obtaining a positive electrode material with better electrochemical performance.

[0033] Afterwards, the first precursor and the second precursor with different particle size characteristics are formed into a uniform slurry with a metal doping source and then dried and calcined, that is, the doping metal element is introduced at the precursor stage to give full play to its advantages including promoting uniform mixing and improving the electrochemical properties of the material, and significantly improve the electrochemical properties of the obtained positive electrode material. Afterwards, the anhydrous iron phosphate doped with metal elements is mixed with a lithium source to enable lithium ions to be uniformly embedded in the iron phosphate lattice, forming a lithium iron phosphate positive electrode material with excellent electrical properties.

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

[0035] It should be noted that in the preparation process of the above-mentioned entire lithium iron phosphate positive electrode material, the valence state of the iron element undergoes a change from divalent to trivalent and then back to divalent. In step S1 and step S2, a ferrous source is first used, in which the valence state of the iron element is +2. Under the action of the first pH regulator, the presence of the oxidant triggers an oxidation reaction, which oxidizes part of the divalent iron ions into trivalent iron ions. In subsequent steps, especially due to the high-temperature decomposition of oxalic acid, the reducing gas produced, such as carbon monoxide (CO), reduces the trivalent iron ions to divalent iron ions again, thereby maintaining a stable Fe in the final formed lithium iron phosphate material. 2+ The reason why ferrous iron is chosen as the initial raw material is that Fe 2+ Compared with 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, which is beneficial to control the morphology and particle size of the material, and ultimately form a lithium iron phosphate positive electrode material with higher compaction density, better morphology and better electrochemical performance.

[0036] In summary, the above method provided by the present invention achieves the optimization of material structure and performance by controlling the morphology and particle size of the precursor, doping metal and non-metallic elements, using oxalic acid for reduction, and introducing a carbon source for carbon coating. The obtained lithium iron phosphate positive electrode material has excellent compaction density and good electrochemical properties, thereby significantly improving the rate performance of the lithium ion battery in which it is located.

[0037] In several typical embodiments, in step S1, the molar ratio of the ferrous source, the phosphorus source and the oxidant is 1: (1.1-1.2): 1. Optimizing the molar ratio of the ferrous source and the phosphorus source to the oxidant can promote the balance of the chemical reaction, facilitate the occurrence of the oxidation reaction, and thus improve the purity of the product. Preferably, in the first slurry, the concentration of the ferrous source is 0.8M-1.0M, so as to accelerate the reaction process, and also facilitate the formation of a positive electrode material with a higher compaction density.

[0038] In the above process, the pH value of the first slurry is preferably 1.0-1.5 to form a strongly acidic reaction system, which is conducive to the oxidation reaction. The ferrous ions can be oxidized more effectively, which promotes the formation of a ferric phosphate precursor with a better structure, and finally obtains a positive electrode material with higher structure, morphology and electrochemical performance. It is further preferred that the first reaction is carried out at 50°C-70°C, and the time of the first reaction is 0.5h-1h, so as to control the reaction rate and reduce the occurrence of side reactions, and also promote the uniform combination of ferrous ions and phosphate ions, forming a precursor with a tighter structure, thereby increasing the compaction density of the positive electrode material.

[0039] Furthermore, step S2 includes: adding a second pH adjusting agent to the second slurry, adjusting the pH value of the second slurry to 1.5-2.0, so that the second slurry undergoes a second reaction for 1h-2h to obtain a third slurry. In this process, the above pH range can promote the effective combination of iron ions and phosphate ions to form the required iron phosphate precipitation, while reducing the generation of other by-products and improving the purity and consistency of the final cathode material. The time of the above precipitation reaction can optimize the crystallinity and morphology of the precursor, thereby affecting the electrochemical properties 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 preferably using the volume ratio of slurry A to slurry B as above, it is possible to more effectively provide a stable skeleton structure for the dihydrated iron phosphate (first precursor) with a larger particle size, and the dihydrated iron phosphate (second precursor) with a smaller particle size fills the gaps between the large particles, thereby more effectively enhancing the stability of the overall structure, reducing internal stress, and contributing to the formation of a more complete structure and a more stable performance cathode material in the subsequent sintering process. In some more typical embodiments, the temperature of static aging is 85°C to 95°C, and the time is 1h to 2h; and / or, the temperature of stirring aging is 85°C to 95°C, and the time is 3h to 4h, and the stirring frequency of stirring aging is 10Hz to 20Hz. The above-mentioned more refined conditions are conducive to the formation of a first precursor with a larger particle size and a more stable structure, and a second precursor with a smaller particle size and a more uniform distribution. At the same time, they can also effectively accelerate the reaction process, reduce material structure damage or by-product generation, and ultimately successfully achieve the particle size grading of the resulting positive electrode material, thereby improving its compaction performance and electrochemical performance.

[0041] In the doping process of the metal doping 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 doping source in step S4 is 1: (0.005-0.008). This molar ratio relationship can promote the uniform doping of metal elements in lithium iron phosphate, thereby improving the electronic conductivity of the obtained positive electrode material. And, in several typical embodiments, in order to more significantly improve the electrochemical performance of the obtained positive electrode material, the metal doping source is preferably selected from one or more of a titanium source, a magnesium source, an aluminum source, a vanadium source and a strontium source, and the titanium source, the magnesium source, the aluminum source, the vanadium source and the strontium source are each independently added in the form of one or more of oxides, hydroxides, nitrates, phosphates, sulfates and acetates.

[0042] Furthermore, after a large number of experiments, the inventors have found that the metal doping source is preferably a titanium source from the above metal doping source types, and the titanium source is added in the form of titanium dioxide and / or tetrabutyl titanate. Compared with other metal elements with conventional doping effects, titanium can not only improve the electrochemical properties of the obtained positive electrode material, but also inhibit the excessive growth of crystal particles during the calcination process, optimize the particle size distribution and structural stability of the material, and thus more effectively improve the electrochemical properties of the final positive electrode material.

[0043] In order to make the materials more evenly dispersed and effectively contacted during the mixed slurry process, and thus promote the full mixing of the metal doping elements and the iron phosphate precursor, it is preferred that in the fourth slurry, the total solid content of the first precursor and the second precursor is 20% to 25%. And, in order to more effectively remove moisture without destroying the structure of the material, while also reducing the possibility of surface oxidation of the obtained third precursor, and improving its purity and activity, it is preferred that the first drying is carried out at 110°C to 130°C, and the first drying time is 3h to 4h. After a large number of experiments, the inventors preferably select the temperature of the first calcination to be 550°C to 650°C and the time to be 4h to 8h. This is because the optimization and setting of this temperature range and time helps to promote the effective formation of the third precursor, that is, anhydrous iron phosphate, and promotes the uniform doping and distribution of the above-mentioned metal doping elements therein. At the same time, the selection of the above-mentioned calcination conditions suppresses the destruction of the material structure caused by excessively high temperature and the incomplete dehydration caused by excessively low temperature, improves the crystallinity and structural integrity of the obtained third precursor and the final obtained positive electrode material, and makes it show more 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 is conducive to uniformly embedding lithium ions in the third precursor, while reducing the defects of the crystal structure caused by the irregular distribution of lithium ions in the material structure, so that the generated lithium iron phosphate positive electrode material has an ideal olivine structure and improves its electrochemical performance. Preferably, the solid content of the fifth slurry is 30% to 40%, which can improve the dispersion uniformity of the obtained fifth slurry during the mixed pulping process, thereby finally preparing a positive electrode material with uniform particle size distribution and stable structure. It is preferred that the second drying is carried out at 150°C to 200°C, which can effectively remove the residual moisture in the obtained fourth precursor and effectively promote 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 a reducing agent and a decomposing agent, and the above preferred range of its weight ratio is crucial for reducing trivalent iron ions to divalent iron ions that are more conducive to the formation of lithium iron phosphate phases. Under the above weight ratio relationship, oxalic acid can more effectively decompose and release reducing gases, such as carbon monoxide, to promote the reduction process. At the same time, the above dosage can also well inhibit the generation of carbon impurities or excessive gases that may be caused by excessive oxalic acid residues, thereby more significantly improving the purity and electrochemical properties of lithium iron phosphate materials. On this basis, the temperature of the second calcination is preferably 700°C to 800°C, and the time is 10h to 15h. This temperature condition can not only promote the crystallization of the first product, improve its structural integrity and electrochemical activity, but also promote the complete decomposition of oxalic acid. At the same time, the above calcination conditions are conducive to the optimization of the internal structure of the first product and the reduction of 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 properties and higher stability, the second calcination is preferably performed 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 doped with non-metallic elements on the surface of lithium iron phosphate. At the same time, it also suppresses the reduction of compaction density, energy density and deterioration of electrochemical performance caused by excessive carbon source. In addition, in practical applications, since the carbon source will undergo a certain amount of burnout after subsequent calcination treatment, the carbon content in the final lithium iron phosphate positive electrode material is 2wt% to 2.5wt%.

[0047] As for the non-metallic doping elements, it is preferred that the total weight of the first product is 100%, and the amount of the non-metallic doping source is 0.3wt% to 0.6wt%. Doping the non-metallic elements according to this addition amount can more significantly improve the electrochemical properties of the final positive electrode material. In particular, when the non-metallic doping source is preferably a boron source, and the boron source is added in the form of boric acid, the boron content in the carbon layer of the obtained positive electrode material is about 0.05wt% to 0.10wt%, and the boron-doped carbon coating layer with this content can play a good sintering role on the one hand, and the graphitization degree of carbon can be improved by calcining at a lower temperature; on the other hand, after calcining with boric acid, boron atoms are doped in the carbon source coating layer to form defects in part of the carbon layer, increase hole carriers, and further improve the electronic conductivity of the material and improve its electrochemical properties. And, in step S7, the solid content of the sixth slurry is preferably 30% to 40%, which helps to promote the uniform mixing and dispersion of the first product with the carbon source and the non-metallic doping source during the mixed slurry process, and helps to form a more ideal particle structure and distribution in the subsequent drying and third calcination process, and finally improve the compaction density and electrochemical activity of the obtained positive electrode material. And, after a large number of experiments, the inventors preferably have a third calcination temperature of 650°C to 750°C and a time of 4h to 8h. Calcination within this temperature range can effectively convert the carbon source into a carbon coating layer, promote the uniform distribution of the non-metallic doping source therein, and reduce the material structure damage or by-product generation that may occur at high temperatures, and finally obtain a lithium iron phosphate positive electrode material with better electrochemical performance. In order to reduce the introduction of impurities and the high-temperature oxidation of the obtained positive electrode material, and improve its purity and structural integrity, it is preferred that the third calcination is carried out under a protective atmosphere, and more preferably the protective atmosphere is nitrogen and / or argon.

[0048] In some typical embodiments, the ferrous source is selected from one or more of ferrous sulfate, ammonium ferrous 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 water 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, the above raw materials can all be selected from the types commonly used in the art. The inventors screened out the above raw material types through a large number of experiments because they can achieve better coordination between them, thereby more effectively realizing the above preparation process flow, and finally obtaining a lithium iron phosphate positive electrode material with better electrochemical performance.

[0049] The second aspect of the present invention provides a lithium iron phosphate positive electrode material, which is prepared by the preparation method of the above-mentioned lithium iron phosphate positive electrode material, and the compaction density of the lithium iron phosphate positive electrode material at 350MPa is 2.65g / cc to 2.75g / cc. It should be noted that due to the particularity of the material field and the limitations of existing test characterization methods, it is difficult to conduct a comprehensive quantitative characterization of the complex microstructure of the above-mentioned lithium iron phosphate positive electrode material, but the performance test results show that the positive electrode material obtained in this application has better electrochemical properties and can significantly improve the various performances of the lithium ion battery in which it is located. The prepared lithium iron phosphate positive electrode material has a higher compaction density and a more superior particle size distribution, so it can significantly improve the rate performance of the lithium ion battery in which it is located.

[0050] In several preferred embodiments, the lithium iron phosphate positive electrode material includes a metal doping element and a non-metal doping element, and based on the total weight of the lithium iron phosphate positive electrode material being 100%, the doping amount of the metal doping element is 0.05% to 0.30%, and the doping amount of the non-metal doping element is 0.05% to 0.10%. The obtained positive electrode material is doped with the above-mentioned appropriate amount of metal elements and non-metal elements, thereby further improving the electrochemical performance of the lithium iron phosphate positive electrode material.

[0051] The third aspect of the present invention provides a positive electrode sheet, which includes the above-mentioned lithium iron phosphate positive electrode material. Since the obtained lithium iron phosphate positive electrode material has high electrochemical performance and excellent particle size distribution, the positive electrode sheet in which it is located exhibits high energy density and good electrical performance.

[0052] The fourth aspect of the present invention provides a lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the positive electrode sheet is the positive electrode sheet described above. The lithium-ion battery prepared by the positive electrode sheet having excellent electrical properties has higher rate performance and more stable electrical performance.

[0053] The present application is further described in detail below in conjunction with specific embodiments. These embodiments should not be construed as limiting the scope of protection claimed in the present application.

[0054] Unless otherwise defined, all professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0055] Example 1

[0056] A method for preparing a lithium iron phosphate positive electrode material:

[0057] (1) 1 mol of ferrous sulfate, 1.1 mol of ammonium dihydrogen phosphate, and 1 mol of hydrogen peroxide were mixed in deionized water (i.e., the molar ratio of the ferrous source, the phosphorus source, and the oxidant was 1:1.1:1), and sulfuric acid was added to adjust the pH to obtain a first slurry with a pH value of 1, wherein the ferrous sulfate concentration was 1 M. The first slurry was heated to 60° C. to perform a first reaction, and a second slurry was obtained after the reaction for 0.5 h.

[0058] (2) Add 25 wt % of ammonia water to the second slurry and adjust its pH value to 1.5 to cause a precipitation reaction (i.e., the second reaction) for 1 h. Meanwhile, stir at a stirring frequency of 15 Hz throughout the second reaction process to obtain a third slurry.

[0059] (3) The third slurry obtained after precipitation is divided into two parts, one half of which, slurry A, is heated to 90°C, aged for 1 hour without stirring, and filtered to obtain dihydrated iron phosphate A, i.e., the first precursor; the other half of slurry B is heated to 90°C, stirred and aged at a stirring frequency of 15 Hz for 4 hours, and filtered to obtain dihydrated iron phosphate B, i.e., the second precursor.

[0060] (4) Iron phosphate dihydrate A and iron phosphate dihydrate B were mixed with deionized water at a target solid content of 20%, and titanium dioxide (i.e., a metal doping source) was added at a ratio of 0.008:1 to the total molar amount of the two, and then mixed at a ball mill speed of 400 rpm for 1 hour to obtain a fourth slurry. The fourth slurry was baked in an oven at a temperature of 120° C. for 4 hours, and then placed in a box furnace for a first calcination at a temperature of 600° C. for 5 hours, and dehydrated to obtain anhydrous iron phosphate, i.e., the third precursor.

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

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

[0063] (7) Taking the total weight of the first product obtained above as 100%, 9wt% of glucose and 0.5wt% of boric acid (i.e., non-metallic doping source) were added and mixed, and the two were mixed with deionized water at a total solid content of 35%, and the ball-to-material ratio was 4:1, the rotation speed was 400rpm, and the ball milling was performed for 5h. After the ball milling was completed, the sixth slurry was obtained. The sixth slurry was spray dried to remove water; then the third calcination was performed in a box furnace at 700°C in a nitrogen atmosphere for 6h, and then the slurry was crushed by a jet mill to obtain a lithium iron phosphate positive electrode material.

[0064] The SEM photo of the obtained anhydrous iron phosphate, i.e. the third precursor, is shown in Figure 1 The SEM image of the obtained lithium iron phosphate cathode material is shown in Figure 2 .

[0065] Example 2

[0066] A method for preparing a lithium iron phosphate positive electrode material:

[0067] (1) 1 mol of ferrous sulfate, 1.2 mol of ammonium dihydrogen phosphate, and 1 mol of hydrogen peroxide were mixed in deionized water (i.e., the molar ratio of the ferrous source, the phosphorus source, and the oxidant was 1:1.2:1), and sulfuric acid was added to adjust the pH to obtain a first slurry with a pH value of 1, wherein the concentration of ferrous sulfate was 1 M. The first slurry was heated to 50° C. to perform a first reaction, and a second slurry was obtained after the reaction for 1 hour.

[0068] (2) 25% wt of aqueous ammonia was added to the second slurry, and the pH value was adjusted to 2.0 to cause a precipitation reaction (i.e., the second reaction) to take place for 1 h. Meanwhile, a stirring frequency of 15 Hz was used throughout the second reaction process to obtain a third slurry.

[0069] (3) The third slurry obtained after precipitation is divided into two parts, wherein half of the slurry A is heated to 85°C, aged for 2 hours without stirring, and filtered to obtain dihydrated iron phosphate A, i.e., the first precursor; the other half of the slurry B is heated to 90°C, stirred and aged for 3 hours at a stirring frequency of 15 Hz, and filtered to obtain dihydrated iron phosphate B, i.e., the second precursor.

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

[0071] (5) The third precursor obtained above is mixed with lithium carbonate in a molar ratio of the third precursor: lithium carbonate = 1:0.49, and the mixed two are mixed with deionized water with a total solid content of 30%, and the ball-to-material ratio is 4:1, the rotation speed is 400 rpm, and the ball milling is performed 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 a fourth precursor.

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

[0073] (7) Taking the total weight of the first product obtained above as 100%, 8wt% of glucose and 0.3wt% of boric acid (i.e., non-metallic doping source) were added and mixed, and the two were mixed with deionized water at a total solid content of 40%, and the ball-to-material ratio was 4:1, the rotation speed was 400rpm, and the ball milling was performed for 5h. After the ball milling was completed, the sixth slurry was obtained. The sixth slurry was spray dried to remove water; then the third calcination was performed in a box furnace at 750°C in a nitrogen atmosphere for 4h, and then the slurry was crushed by a jet mill to obtain a lithium iron phosphate positive electrode material.

[0074] Example 3

[0075] A method for preparing a lithium iron phosphate positive electrode material:

[0076] (1) 1 mol of ferrous sulfate, 1.1 mol of ammonium dihydrogen phosphate, and 1 mol of hydrogen peroxide were mixed in deionized water (i.e., the molar ratio of the ferrous source, the phosphorus source, and the oxidant was 1:1.1:1), and sulfuric acid was added to adjust the pH to obtain a first slurry with a pH value of 1.5, wherein the concentration of ferrous sulfate was 0.8 M. The first slurry was heated to 70° C. to perform a first reaction, and the second slurry was obtained after the reaction for 0.5 h.

[0077] (2) 25% wt of aqueous ammonia was added to the second slurry, and the pH value was adjusted to 1.5 to allow a precipitation reaction (i.e., the second reaction) to occur for 2 h. Meanwhile, a stirring frequency of 15 Hz was used throughout the second reaction process to obtain a third slurry.

[0078] (3) The third slurry obtained after precipitation is divided into two parts, wherein half of the slurry A is heated to 90°C, aged for 1 hour without stirring, and filtered to obtain dihydrated iron phosphate A, i.e., the first precursor; the other half of the slurry B is heated to 95°C, stirred and aged for 4 hours at a stirring frequency of 20 Hz, and filtered to obtain dihydrated iron phosphate B, i.e., the second precursor.

[0079] (4) Iron phosphate dihydrate A and iron phosphate dihydrate B were mixed with deionized water at a target solid content of 20%, and titanium dioxide (i.e., a metal doping source) was added at a ratio of 0.005:1 to the total molar amount of the two, and then mixed at a ball mill speed of 400 rpm for 1 hour to obtain a fourth slurry. The fourth slurry was baked in an oven at a temperature of 130° C. for 4 hours, and then placed in a box furnace for a first calcination at a temperature of 550° C. for 8 hours, and dehydrated to obtain anhydrous iron phosphate, i.e., the third precursor.

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

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

[0082] (7) Taking the total weight of the first product obtained above as 100%, 10wt% of glucose and 0.6wt% of boric acid (i.e., non-metallic doping source) were added and mixed, and the two were mixed with deionized water at a total solid content of 30%, and the ball-to-material ratio was 4:1, the rotation speed was 400rpm, and the ball milling was performed for 5h. After the ball milling was completed, the sixth slurry was obtained. The sixth slurry was spray dried to remove water; then the third calcination was performed in a box furnace at 650°C in a nitrogen atmosphere for 8h, and then the slurry was crushed by a jet mill to obtain a lithium iron phosphate positive electrode material.

[0083] Example 4

[0084] A method for preparing a lithium iron phosphate positive electrode 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 a lithium iron phosphate positive electrode 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 10 h.

[0089] Example 6

[0090] A method for preparing a lithium iron phosphate positive electrode material:

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

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

[0093] Example 7

[0094] A method for preparing a lithium iron phosphate positive electrode material:

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

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

[0097] Example 8

[0098] A method for preparing a lithium iron phosphate positive electrode 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 according to a volume ratio of 1:3 to obtain slurry A and slurry B, and the volume ratio of slurry A:slurry B=1:3.

[0100] Example 9

[0101] A method for preparing a lithium iron phosphate positive electrode 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 according to a volume ratio of 1:3 to obtain slurry A and slurry B, and the volume ratio of slurry A:slurry B=3:1.

[0103] Example 10

[0104] A method for preparing a lithium iron phosphate positive electrode 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 ratio of the molar amount of titanium dioxide to the total molar amount of iron phosphate dihydrate A and iron phosphate dihydrate B is 0.002:1; and at the same time in step (7), the amount of boric acid added is changed so that the amount of boric acid added accounts for 0.05wt% of the first product.

[0106] Embodiment 11

[0107] A method for preparing a lithium iron phosphate positive electrode 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 ratio of the molar amount of titanium dioxide to the total molar amount of iron phosphate dihydrate A and iron phosphate dihydrate B is 0.010:1; and at the same time in step (7), the amount of boric acid added is changed so that the amount of boric acid added accounts for 0.80wt% of the first product.

[0109] Example 12

[0110] A method for preparing a lithium iron phosphate positive electrode 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 10 h.

[0112] Example 13

[0113] A method for preparing a lithium iron phosphate positive electrode 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] Embodiment 14

[0116] A method for preparing a lithium iron phosphate positive electrode 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] Embodiment 15

[0119] A method for preparing a lithium iron phosphate positive electrode 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 18 h.

[0121] Example 16

[0122] A method for preparing a lithium iron phosphate positive electrode material:

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

[0124] Embodiment 17

[0125] A method for preparing a lithium iron phosphate positive electrode material:

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

[0127] Comparative Example 1

[0128] A method for preparing a lithium iron phosphate positive electrode material:

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

[0130] (3) The third slurry obtained after precipitation was heated to 90° C., allowed to stand without stirring for 1 hour, and filtered to obtain ferric phosphate dihydrate.

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

[0132] The SEM photo of the obtained anhydrous iron phosphate, i.e. the third precursor, is shown in Figure 3 The SEM image of the obtained lithium iron phosphate cathode material is shown in Figure 4 .

[0133] Comparative Example 2

[0134] A method for preparing a lithium iron phosphate positive electrode material:

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

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

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

[0138] The SEM photo of the obtained anhydrous iron phosphate, i.e. the third precursor, is shown in Figure 5 The SEM image of the obtained lithium iron phosphate cathode material is shown in Figure 6 .

[0139] Comparative Example 3

[0140] A method for preparing a lithium iron phosphate positive electrode material:

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

[0142] Comparative Example 4

[0143] A method for preparing a lithium iron phosphate positive electrode material:

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

[0145] Comparative Example 5

[0146] A method for preparing a lithium iron phosphate positive electrode material:

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

[0148] Test Method

[0149] Compacted density: Referring to GB / T 30835-2014 "Carbon composite lithium iron phosphate positive electrode material for lithium ion battery" standard, the powder compaction density method GB / T24533-2019, the compaction density of lithium iron phosphate material was measured, and the compaction density of the positive electrode material samples obtained in each embodiment and comparative example at 350MPa was obtained. The specific steps are as follows: Take 0.5g of sample and place it in the sample slot of the compaction density measuring instrument, set the measurement pressure range to 30-350MPa, stay for 10s at every 30MPa interval, measure the data once, and obtain the compaction curve and the compaction density at the end pressure of 350MPa.

[0150] Battery sample preparation and performance test: 1. Battery sample preparation. Weigh and mix the obtained positive electrode material, SP conductive agent, and PVDF binder in a mass ratio of 8:1:1, and make a CR2032 button-type half-cell in an argon protective atmosphere. In the button battery preparation process, except for the positive electrode material, all other auxiliary materials and auxiliary materials are any auxiliary materials or auxiliary materials that can be purchased on the market. 2. Button battery sample test: The prepared button-type half-cell samples are charged and discharged at room temperature, with a voltage range of 2.0V to 4.0V. Obtained: the first charge specific capacity at 0.2C; the first discharge specific capacity at 0.2C; the first efficiency % at 0.2C (first discharge capacity / first charge capacity×100%); 0.2C discharge specific capacity (the discharge specific capacity obtained by performing a charge and discharge test at 0.2C again after the first 0.2C charge and discharge test); 1C discharge specific capacity (the discharge specific capacity obtained by performing a charge and discharge test at 1C after the 0.2C charge and discharge test); 2C discharge specific capacity (the discharge specific capacity obtained by performing a charge and discharge test at 2C after the 1C charge and discharge test).

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

[0152] Table 1

[0153]

[0154]

[0155] From the above description, it can be seen that the above embodiments of the present invention achieve the preparation of lithium iron phosphate materials with excellent electrochemical properties. When the obtained materials are used as positive electrode materials for lithium ion batteries and prepared into lithium ion batteries, the corresponding batteries exhibit higher rate performance.

[0156] Specifically, compared with Example 1, in the crystallization process of step (3), the ferric phosphate dihydrate of Comparative Example 1 only adopts the process condition of aging for 1 h without stirring. Figure 3 , Figure 4 ) It can be seen that the anhydrous iron phosphate precursor and lithium iron phosphate particles obtained in Comparative Example 1 are relatively large, and the measured compaction density is also relatively high, but its rate performance is much different from that of Example 1. This is because the lithium iron phosphate particles are large, which is not conducive to sufficient infiltration with the electrolyte, resulting in a large capacity loss during large current charging and discharging, and poor performance.

[0157] Compared with Example 1, the anhydrous iron phosphate precursor and lithium iron phosphate particles obtained in Comparative Example 2 are uniform and small (see Figure 5 and Figure 6 ), has good electrical properties, but its compaction density is low. Because the particle size does not form a good particle size distribution, the gaps between the particles cannot be filled. Therefore, the energy density of the lithium-ion battery prepared subsequently is low, which is not conducive to practical application.

[0158] Compared with Example 1, Comparative Example 3 does not add titanium and boron elements, and its electrical properties are poor and the compaction density is average. This is because titanium, as a metal doping element, has a doping effect, which is beneficial to improving electrical properties, and has the effect of inhibiting excessive particle growth during high-temperature sintering. The lack of titanium may cause the final particles to be too large, fail to form a good grading, and have poor electrical properties.

[0159] Compared with Example 1, Comparative Example 4 changes the two-step sintering into one-step sintering, and does not add oxalic acid. However, a portion of the organic carbon source will be lost during sintering due to the reduction of trivalent iron ions, resulting in less final coated carbon. Although the compaction density is improved to a certain extent, the rate performance of the obtained lithium iron phosphate positive electrode material is adversely affected, and the corresponding lithium ion battery performance is poor.

[0160] Compared with Example 1, Comparative Example 5 replaces oxalic acid with citric acid. Citric acid can also reduce trivalent iron to divalent iron when decomposed at high temperature, thereby generating a lithium iron phosphate phase. However, the residual carbon produced by its decomposition inhibits the growth of particles during sintering, resulting in the inability to improve the compaction performance of the final material, and the corresponding full battery energy density is low and the performance is poor.

[0161] Compared with Examples 1 to 5, Example 6 reduces the amount of phosphorus source added, and finally obtains iron phosphate with a relatively high iron-phosphorus ratio. Although the lithium iron phosphate obtained from the iron phosphate with a high iron-phosphorus ratio has good electrical properties, its compaction performance is relatively general, and the final full battery energy density is low. The increase of the phosphorus source in Example 7 reduces the iron-phosphorus ratio of iron phosphate and promotes the particle growth of lithium iron phosphate, so its compaction is relatively good, but its electrical performance is slightly poor. Example 8 is equivalent to increasing the proportion of small-particle iron phosphate, so the corresponding lithium iron phosphate has relatively good electrical performance, but the compaction performance is slightly reduced. Example 9 increases the proportion of large-particle iron phosphate, and the compaction performance is slightly better, but the electrical performance of the half-cell sample is not as good as that of Example 1. Example 10 reduces the addition of titanium and boron elements, and the electrical performance is slightly reduced, and the compaction is slightly better. Example 11 increases the addition of titanium and boron elements, and the electrical performance is significantly improved, but the compaction performance is also deteriorated. Therefore, in actual application, the energy density of the full battery product is relatively low compared to Example 1. In Example 12, the dehydration temperature of dihydrate iron phosphate is reduced, which affects its crystallinity, resulting in a slight decrease in the electrical performance of the obtained lithium iron phosphate. In Example 13, the amount of oxalic acid used is reduced, and the electrical performance is slightly reduced. In Example 14, the amount of oxalic acid used is increased, and the electrical performance of the half-cell sample also declines. In Example 15, the temperature of the secondary sintering is reduced, which affects the growth of large particles and slightly reduces the compaction, which is not conducive to the subsequent full-battery product to show high energy density. In Examples 16 and 17, the type of metal doping source is changed. At this time, it is difficult to play the role of titanium element in regulating the size of particles during the calcination process, so the optimal particle size distribution cannot be formed. Although the electrical performance of the final battery sample is improved due to the doping effect, it is still slightly worse than Example 1 as a whole.

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

[0163] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: include: Step S1, preparing a first slurry with a ferrous source, a phosphorus source, an oxidant and a first pH adjuster, and subjecting the first slurry to a first reaction to obtain a second slurry; Step S2, adding a second pH regulator to the second slurry, and obtaining a third slurry after a second reaction; Step S3, taking a portion of the third slurry as slurry A, and another portion of the third slurry as slurry B; the slurry A is sequentially subjected to static aging and filtration to obtain a first precursor, and the slurry B is sequentially subjected to stirring aging and filtration to obtain a second precursor; Step S4, preparing the first precursor, the second precursor and a metal doping source into a fourth slurry, and subjecting the fourth slurry to a first drying and a first calcination in sequence to obtain a third precursor; Step S5, preparing the third precursor and a lithium source into a fifth slurry, and obtaining a fourth precursor by second drying the fifth slurry; Step S6, mixing the fourth precursor with oxalic acid, and performing a second calcination to obtain a first product; Step S7, preparing the first product with a carbon source and a non-metallic doping source into a sixth slurry, and subjecting the sixth slurry to a third drying and a third calcination in sequence to obtain the lithium iron phosphate positive electrode material.

2. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: In the 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 0.8M-1.0M; and / or, The pH value of the first slurry is 1.0 to 1.5; and / or, The first reaction is carried out at 50° C. to 70° C., and the first reaction time is 0.5 h to 1 h.

3. The method for preparing the lithium iron phosphate positive electrode material according to claim 1 or 2, characterized in that: The 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 1 hour to 2 hours to obtain the third slurry.

4. The method for preparing a lithium iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: In the step S3, The volume ratio of the slurry A to the slurry B is (0.8-1.2):1; Preferably, the temperature of the static aging is 85°C to 95°C, and the time is 1h to 2h; and / or the temperature of the stirring aging is 85°C to 95°C, and the time is 3h to 4h, and the stirring frequency of the stirring aging is 10Hz to 20Hz.

5. The method for preparing a lithium iron phosphate positive electrode material according to any one of claims 1 to 4, characterized in that: In the step S4, The ratio of the total molar amount of the first precursor and the second precursor to the molar amount of the metal doping 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% to 25%; and / or, The first drying is performed at 110° C. to 130° C., and the first drying time is 3 h to 4 h; and / or, The first calcination temperature is 550°C to 650°C, and the time is 4h to 8h; and / or, The metal doping source is selected from one or more of a titanium source, a magnesium source, an aluminum source, a vanadium source and a strontium source, and the titanium source, the magnesium source, the aluminum source, the vanadium source and the strontium source are each independently added in the form of one or more of oxides, hydroxides, nitrates, phosphates, sulfates and acetates; Preferably, the metal doping source is a titanium source, and the titanium source is added in the form of titanium dioxide and / or tetrabutyl titanate.

6. The method for preparing a lithium iron phosphate positive electrode material according to any one of claims 1 to 5, characterized in that: In the 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% to 40%; and / or, The second drying is performed at 150°C to 200°C.

7. The method for preparing a lithium iron phosphate positive electrode material according to any one of claims 1 to 6, characterized in that: In step S6, The weight ratio of the fourth precursor to the oxalic acid is (4-6):1; and / or, The second calcination temperature is 700°C to 800°C, and the time is 10h to 15h; Preferably, the second calcination is performed under a protective atmosphere; more preferably, the protective atmosphere is nitrogen and / or argon.

8. The method for preparing a lithium iron phosphate positive electrode material according to any one of claims 1 to 7, characterized in that: In step S7, Based on the total weight of the first product as 100%, the amount of the carbon source is 8wt% to 10wt%; and / or, Based on the total weight of the first product being 100%, the amount of the non-metallic doping source is 0.3 wt% to 0.6 wt%; and / or, the non-metallic doping source is a boron source, and the boron source is added in the form of boric acid; The solid content of the sixth slurry is 30% to 40%; and / or, The third calcination is performed at a temperature of 650°C to 750°C and for a time of 4h to 8h; Preferably, the third calcination is performed under a protective atmosphere; more preferably, the protective atmosphere is nitrogen and / or argon.

9. The method for preparing a lithium iron phosphate positive electrode material according to any one of claims 1 to 8, characterized in that: The ferrous source is selected from one or more of ferrous sulfate, ammonium ferrous sulfate, ferrous chloride and ferrous nitrate; and / or, The phosphorus source is selected from one or more of diammonium phosphate, phosphoric acid and diammonium phosphate; and / or, The oxidant is hydrogen peroxide; and / or, The first pH regulator is selected from one or more of sulfuric acid, phosphoric acid, nitric acid and hydrochloric acid; and / or, the second pH regulator is ammonia water 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.

10. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material is prepared by the preparation method of the lithium iron phosphate positive electrode material according to any one of claims 1 to 9, and the compaction density of the lithium iron phosphate positive electrode material at 350 MPa is 2.65 g / cc to 2.75 g / cc; Preferably, the lithium iron phosphate positive electrode material includes metal doping elements and non-metal doping elements, and based on the total weight of the lithium iron phosphate positive electrode material as 100%, the doping amount of the metal doping element is 0.05% to 0.30%, and the doping amount of the non-metal doping element is 0.05% to 0.10%.

11. A positive electrode sheet, characterized in that: The positive electrode plate includes the lithium iron phosphate positive electrode material according to claim 10.

12. A lithium-ion battery, comprising a positive electrode sheet, a negative electrode sheet and an electrolyte, characterized in that: The positive electrode sheet is the positive electrode sheet according to claim 11.

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