Lithium iron phosphate positive electrode material and preparation method therefor, and electrochemical device

Abstract

A lithium iron phosphate positive electrode material and a preparation method therefor, and an electrochemical device. The preparation method of the present application comprises: mixing a phosphorus source, an iron source, a lithium source, a first carbon source, lithium fluoride, a first additive and a solvent to form a primary mixed material, and performing primary sintering to obtain a matrix material, wherein the mass percentage M of the lithium fluoride relative to the total mass of the iron source and the phosphorus source is 0-0.1%; mixing the matrix material, a second carbon source, lithium fluoride, a second additive and a solvent to form a secondary mixed material, wherein the mass percentage N of the lithium fluoride added in the step relative to the total mass of the iron source and the phosphorus source is 0-0.3%, and N and M are not both 0; and grinding the secondary mixed material to obtain a secondary mixed material A and a secondary mixed material B having different particle sizes, and then performing mixing and secondary sintering to obtain a lithium iron phosphate positive electrode material having good compaction density, rate capability and low-temperature performance.

Description

Lithium iron phosphate cathode material, its preparation method and electrochemical device

[0001] Cross-references to related applications

[0002] This application claims priority to Chinese Patent No. 202510112117.7, filed on January 23, 2025, entitled "Lithium iron phosphate cathode material, preparation method thereof and electrochemical device", the entire contents of which are incorporated herein by reference. Technical Field

[0003] This application relates to the field of new energy materials technology, specifically to a lithium iron phosphate cathode material, its preparation method, and an electrochemical device. Background Technology

[0004] Lithium-ion batteries are increasingly widely used in various fields due to their superior high energy density. The cathode materials for lithium-ion batteries generally include lithium cobalt oxide, ternary materials, lithium manganese oxide, and lithium iron phosphate. Among these, lithium iron phosphate cathode materials have become one of the most popular cathode materials due to their advantages such as high safety performance, low cost, stable chemical structure, and long cycle life.

[0005] However, in practical applications, lithium iron phosphate cathode materials have poor low-temperature performance and low energy density, which limits their large-scale application. Summary of the Invention

[0006] In view of this, in order to solve at least one of the above technical problems, this application provides a method for preparing lithium iron phosphate cathode material.

[0007] This application also provides a lithium iron phosphate cathode material prepared by the aforementioned method, and an electrochemical device using the lithium iron phosphate cathode material.

[0008] In a first aspect, embodiments of this application provide a method for preparing a lithium iron phosphate cathode material, comprising:

[0009] A primary mixture is formed by mixing a phosphorus source, an iron source, a lithium source, a first carbon source, lithium fluoride, a first additive, and a first solvent. The primary mixture is then ground and sintered once to obtain a matrix material. The mass percentage M of the lithium fluoride in the total mass of the iron source and the phosphorus source is 0 to 0.1%.

[0010] The matrix material, second carbon source, lithium fluoride, second additive, and second solvent are mixed to form a secondary mixture, wherein the lithium fluoride added in this step accounts for 0-0.3% of the mass percentage N of the matrix material, and N and M are not both 0; and

[0011] The secondary mixture is ground to obtain secondary mixture A with a particle size D50 of 0.6μm to 1.0μm and secondary mixture B with a particle size D50 of 0.2μm to 0.4μm. Secondary mixture A and secondary mixture B are mixed and subjected to secondary sintering to obtain the lithium iron phosphate cathode material.

[0012] In some possible embodiments, the secondary mixture A and the secondary mixture B are mixed in a mass ratio of (20-80):(80-20).

[0013] In some possible embodiments, the step of grinding the secondary mixture further includes:

[0014] The secondary mixture is subjected to a first grinding process to obtain secondary mixture A. The diameter of the grinding beads used in the first grinding process is 0.6 mm to 1.0 mm.

[0015] A portion of the secondary mixture A is subjected to a second grinding to obtain the secondary mixture B. The diameter of the grinding beads used in the second grinding is 0.2 mm to 0.35 mm.

[0016] In some possible embodiments, the step of grinding the primary mixture further includes:

[0017] The mixture is first ground until the particle size D50 is 1.5μm to 3.0μm, and then ground a second time until the particle size D50 is 0.8μm to 1.3μm. The diameter of the grinding beads used in the first grinding is 0.6mm to 1.0mm, and the diameter of the grinding beads used in the second grinding is 0.2mm to 0.35mm.

[0018] In some possible embodiments, the temperature of the first sintering is 450°C to 750°C, and the time of the first sintering is 3 hours to 15 hours; and / or

[0019] The secondary sintering temperature is 600℃~780℃, and the secondary sintering time is 3h~15h.

[0020] In some possible embodiments, in the primary mixture, the molar ratio P:Fe:Li of phosphorus in the phosphorus source, iron in the iron source, and lithium in the lithium source is (1.015-1.036):1:(1.0-1.04).

[0021] In some possible embodiments, the first additive and the second additive are each independently selected from at least one of titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide, and niobium pentoxide.

[0022] In some possible embodiments, the lithium iron phosphate cathode material has a compaction density ≥2.5 g / cm³ at a pressure of 150 MPa. 3 ; and / or

[0023] The resistivity of the lithium iron phosphate cathode material at a pressure of 150 MPa is ≤45 Ω·cm.

[0024] Secondly, embodiments of this application provide a lithium iron phosphate cathode material prepared using the aforementioned method for preparing lithium iron phosphate cathode materials. The lithium iron phosphate cathode material includes a matrix material and a coating layer, wherein at least one of the matrix material and the coating layer contains lithium fluoride.

[0025] Thirdly, embodiments of this application provide an electrochemical device, the electrochemical device including a positive electrode sheet, the positive electrode sheet including a positive electrode active material, the positive electrode active material being the aforementioned lithium iron phosphate positive electrode material.

[0026] Compared to existing technologies, the lithium iron phosphate cathode material provided in this application incorporates a certain amount of lithium fluoride in at least one of the matrix material and the coating layer. This involves bulk doping of lithium iron phosphate with lithium fluoride, surface coating, or a combination of doping and coating. Since lithium fluoride has good electronic and ionic conductivity, it can improve the electronic conductivity and ion transport rate of the lithium iron phosphate cathode material, reduce the internal resistance of the lithium iron phosphate cathode material at low temperatures and high rates, thereby improving the rate performance and low-temperature performance of the lithium iron phosphate cathode material.

[0027] Secondly, by grinding the secondary mixture, lithium fluoride can be more evenly distributed on the surface or between the crystal lattices of lithium iron phosphate, thereby further improving the conductivity of lithium iron phosphate cathode material. During the grinding process, particles of different sizes are obtained to achieve particle size distribution, thereby improving the compaction density of lithium iron phosphate cathode material and improving the problem of compaction density reduction caused by the introduction of lithium fluoride.

[0028] In addition, the preparation process using two sintering stages involves first performing a solid-state reaction between raw materials through a single sintering stage, utilizing carbothermal reduction to form a matrix material with a lithium iron phosphate crystalline phase. Then, a second sintering stage is used to coat the surface of the matrix material with carbon, reducing the free carbon generated during the first sintering stage due to the fusion of lithium salts and improving the carbon coating effect. Lithium fluoride can work together with the carbon coating layer to construct a conductive network.

[0029] The embodiments of this application, through the synergistic effect of lithium fluoride doping and / or coating and grinding processes and two sintering processes, can effectively improve the conductivity of the cathode material, improve the particle size distribution, construct a conductive network and improve the structural stability, thereby achieving a joint improvement in the rate performance, low-temperature performance and compaction density of the lithium iron phosphate cathode material. Attached Figure Description

[0030] Figure 1 is a process flow diagram of a method for preparing lithium iron phosphate cathode material according to an embodiment of this application.

[0031] Figure 2 is a scanning electron microscope image of the lithium iron phosphate cathode material in Example 1 of this application.

[0032] Figure 3 is a scanning electron microscope image of the lithium iron phosphate cathode material in Example 2 of this application.

[0033] Figure 4 is a scanning electron microscope image of the lithium iron phosphate cathode material in Comparative Example 1 of this application.

[0034] Figure 5 is a scanning electron microscope image of the lithium iron phosphate cathode material in Comparative Example 3 of this application. Detailed Implementation

[0035] The embodiments of this application are described in detail below. The embodiments described below with reference to the accompanying drawings are exemplary and are only used to explain this application, and should not be construed as limiting this application; it should be noted that, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; where there is no conflict, the implementation methods and features of the implementation methods of this application can be combined with each other; many specific details are set forth in the following description to provide a full understanding of this application, and the described implementation methods are only a part of the implementation methods of this application, and not all of the implementation methods.

[0036] Please refer to Figure 1. This application provides a method for preparing lithium iron phosphate cathode material, which specifically includes the following steps:

[0037] Step S1: The phosphorus source, iron source, lithium source, first carbon source, lithium fluoride, first additive and first solvent are mixed to form a primary mixture. The primary mixture is ground and sintered once to obtain the matrix material. The mass percentage M of lithium fluoride in the total mass of iron source and phosphorus source is 0 to 0.1%.

[0038] In this step, a primary mixture is formed by mixing raw materials such as a phosphorus source, an iron source, a lithium source, a first carbon source, lithium fluoride, a first additive, and a first solvent in a certain proportion. For example, the first solvent can be water. The primary mixture is then ground to ensure uniform mixing and fine particle size. The ground primary mixture is then sintered to perform a solid-phase reaction between the raw materials, resulting in carbothermic reduction and the formation of a matrix material with a lithium iron phosphate crystalline phase. Lithium fluoride is uniformly doped and distributed within this matrix material. The doping of lithium fluoride in this step helps improve the electronic and ionic conductivity of the lithium iron phosphate cathode material, enhancing its rate performance and low-temperature performance.

[0039] In step S1, the mass percentage M of lithium fluoride added relative to the total mass of the iron and phosphorus sources is 0-0.1%. By controlling the amount of lithium fluoride doped in the lithium iron phosphate cathode material within the aforementioned range, the decrease in the compaction density of the lithium iron phosphate cathode material due to poor particle size distribution caused by excessive lithium fluoride doping (M greater than 0.1%) can be reduced. Alternatively, lithium fluoride may not be doped in this step. M can, for example, be 0, 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, or any value within the range of any two of the above values. M can further be 0.02% to 0.1%.

[0040] The first carbon source can account for 5% to 7% of the total mass of the iron and phosphorus sources, which is beneficial to the solid-phase reaction and reduces the generation of free carbon. Specifically, the first carbon source may include at least one of glucose, sucrose, starch, and polyethylene glycol.

[0041] Lithium, iron, and phosphorus sources can be added according to the molar ratio of the elements, Li:Fe:P = (1.0~1.04):1:(1.015~1.036).

[0042] In some embodiments, the phosphorus source includes at least one selected from iron phosphate, phosphoric acid, lithium dihydrogen phosphate, monoammonium phosphate, and ammonium dihydrogen phosphate. The phosphorus source may further be iron phosphate.

[0043] In some embodiments, the iron source includes at least one selected from iron phosphate, iron oxide, iron(II,III) oxide, and iron hydroxide. The iron source may further be iron phosphate.

[0044] In some embodiments, the lithium source may include at least one of lithium dihydrogen phosphate, lithium carbonate, and lithium hydroxide.

[0045] In some embodiments, the first additive may include at least one of titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide, and niobium pentoxide. Doping with the above-mentioned first additive can effectively improve the crystal structure, electronic conductivity, ionic conductivity, thermal stability, and density of the matrix material, further improving the rate performance and low-temperature performance of the lithium iron phosphate cathode material.

[0046] In some embodiments, the mixing method may include mixing with a high-speed mixer, stirring, or grinding. It is understood that the mixing methods include, but are not limited to, the methods described above.

[0047] In some embodiments, grinding the primary mixture may include: first grinding (i.e., coarse grinding) with grinding beads of 0.6 mm to 1.0 mm in diameter until the particle size D50 is 1.5 μm to 3.0 μm, rapidly reducing the particle size and improving the mixing uniformity between raw materials; then, first grinding (i.e., fine grinding) with grinding beads of 0.2 mm to 0.35 mm in diameter until the particle size D50 is 0.8 μm to 1.3 μm, further refining the particle size and making the particle size distribution of the primary mixture more uniform. Adopting a step-by-step grinding method, that is, a combination of coarse and fine grinding, is beneficial for improving grinding efficiency and particle size distribution.

[0048] In some embodiments, the primary sintering temperature is 450°C to 750°C, and a primary sintering temperature within this range is conducive to the formation of the lithium iron phosphate crystalline phase. The primary sintering temperature can, exemplarily, be 450°C, 500°C, 550°C, 600°C, 650°C, 700°C, 750°C, or any value within the range of any two of the above values. Further, the primary sintering temperature can be 500°C to 750°C.

[0049] In some embodiments, the sintering time can be 3h to 15h, which is beneficial for the complete reaction of the raw materials. The sintering time can be exemplary, such as 3h, 5h, 5h, 10h, 12h, 14h, 15h, or any value within the range of any two of the above values. The sintering time can further be 7h to 10h.

[0050] In some embodiments, the heating rate of a single sintering step can be 2°C / min to 5°C / min, which is beneficial for the uniformity of the solid-phase reaction and reduces thermal stress and structural defects caused by rapid heating.

[0051] In some embodiments, the atmosphere for primary sintering can be an inert atmosphere to avoid oxidation of metal elements in the primary mixture and improve the integrity of the crystal structure and the stability of the chemical composition of the matrix material.

[0052] In some embodiments, the sintering equipment for a single sintering process may include one of a pusher kiln, a roller kiln, and a rotary kiln.

[0053] In some embodiments, before primary sintering, the primary mixture can be spray-dried to help quickly convert the liquid primary mixture into a uniform powder, which is beneficial for the uniform reaction of the primary mixture during primary sintering. The inlet air temperature for spray drying can be 200℃~270℃, and the outlet air temperature can be 85℃~100℃.

[0054] Step S2 involves mixing the matrix material, the second carbon source, lithium fluoride, the second additive, and the second solvent to form a secondary mixture. In this step, the lithium fluoride added accounts for 0 to 0.3% of the mass of the matrix material, N, and N and M are not both 0.

[0055] In this step, the matrix material, second carbon source, lithium fluoride, second additive, and second solvent are mixed to form a secondary mixture. For example, the second solvent can be water. Lithium fluoride is added again in this step. The lithium fluoride and second carbon source coat the surface of the matrix material, which helps improve the electronic and ionic conductivity of the lithium iron phosphate cathode material, enhancing its rate performance and low-temperature performance. The lithium fluoride coating also effectively reduces the erosion of the lithium iron phosphate cathode material by the electrolyte, reduces metal ion dissolution, and the lithium fluoride in the coating layer helps promote the formation of a stable and dense cathode-electrolyte interface (CEI), which is beneficial for improving the stability of the lithium iron phosphate cathode material and electrolyte interface, thereby improving the electrochemical performance of the lithium iron phosphate cathode material.

[0056] In step S2, the lithium fluoride added accounts for 0-0.3% of the mass of the matrix material (N). By controlling the amount of lithium fluoride coating within the aforementioned range, it is beneficial to reduce the decrease in compaction density of the lithium iron phosphate cathode material caused by excessive lithium fluoride coating (N greater than 0.3%), which leads to poor particle size distribution. Alternatively, lithium fluoride coating may not be used in this step. N can exemplarily be 0, 0.001%, 0.005%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, or any value within the range of any two of the above values. N can further be 0.01% to 0.3%.

[0057] It is understandable that M and N are not both 0. Lithium fluoride can be added only in step S1 to dope the bulk phase of the matrix material; lithium fluoride can be added only in step S2 to coat the surface of the matrix material; or lithium fluoride can be added in both steps S1 and S2 to combine bulk doping and surface coating. All three of these methods can improve the low-temperature performance and rate performance of lithium iron phosphate cathode materials.

[0058] In some embodiments, the second carbon source may include at least one of glucose, sucrose, starch, and polyethylene glycol. Further, the second carbon source may be a mixture of glucose and polyethylene glycol.

[0059] In some embodiments, the second carbon source may comprise 3% to 14% of the matrix material by mass, which is beneficial for moderate carbon coating on the surface of the matrix material. Exemplarily, the second carbon source is a mixture of glucose and polyethylene glycol, wherein glucose comprises 2.0% to 7.0% of the matrix material by mass, and polyethylene glycol comprises 1.0% to 7.0% of the matrix material by mass.

[0060] In some embodiments, the second additive may include at least one of titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide, and niobium pentoxide. Coating with the above-mentioned second additive can further improve the rate performance and low-temperature performance of the lithium iron phosphate cathode material.

[0061] Step S3: Grind the secondary mixture to obtain secondary mixture A with a particle size D50 of 0.6μm to 1.0μm and secondary mixture B with a particle size D50 of 0.2μm to 0.4μm. Mix secondary mixture A and secondary mixture B and perform secondary sintering to obtain lithium iron phosphate cathode material.

[0062] In steps S1 and S2 of this application, lithium fluoride was added to improve the low-temperature performance and rate performance of the lithium iron phosphate cathode material. However, during the experiment, the inventors of this application found that the addition of lithium fluoride led to a decrease in the compaction density of the lithium iron phosphate cathode material. This may be related to the lower density of lithium fluoride, gas release during sintering, the compatibility difference between lithium fluoride and lithium iron phosphate, and its influence on the particle morphology of the lithium iron phosphate cathode material. Therefore, this application combines a special grinding process to improve the lithium iron phosphate cathode material, achieving a graded distribution of large and small particles. This allows smaller particles to fill the gaps between larger particles, thereby increasing the compaction density of the lithium iron phosphate cathode material and achieving a comprehensive improvement in the energy density, low-temperature performance, and rate performance of the lithium iron phosphate cathode material.

[0063] Specifically, the grinding process includes: first grinding (i.e., coarse grinding) of the secondary mixture using grinding beads with a diameter of 0.6 mm to 1.0 mm to obtain secondary mixture A with a particle size D50 of 0.6 μm to 1.0 μm; and then second grinding (i.e., fine grinding) of a portion of secondary mixture A using grinding beads with a diameter of 0.2 mm to 0.35 mm to obtain secondary mixture B with a particle size D50 of 0.2 μm to 0.4 μm.

[0064] After grinding, secondary mixture A and secondary mixture B, with different particle sizes, are mixed to achieve particle size distribution. This allows smaller particles to fill the gaps between larger particles, thereby increasing the compaction density of the lithium iron phosphate cathode material. Mixing methods can include high-speed mixer mixing, stirring, or grinding. It is understood that mixing methods include, but are not limited to, the methods described above. The mixing time can be 0.5 h to 3 h to ensure thorough mixing.

[0065] In this process, secondary mixture A and secondary mixture B can be mixed at a mass ratio of (20–80):(80–20). This mixing ratio affects the distribution of large and small particles in the primary particles of the lithium iron phosphate cathode material, thus influencing the compaction density and electrochemical performance of the lithium iron phosphate cathode material. When there are too many large particles (secondary mixture A), there is a lack of small particles to fill the gaps, resulting in insufficient improvement in the compaction density of the lithium iron phosphate cathode material. Furthermore, due to the excessive number of large particles, the electron migration path is lengthened, leading to a decrease in rate performance. Conversely, when there are too many small particles (secondary mixture B), the number of large particles is too small, reducing the number of gaps and also lowering the compaction density. Moreover, due to the excessive number of small particles, the specific surface area of ​​the lithium iron phosphate cathode material is too large, causing an increase in internal resistance and surface side reactions. Therefore, controlling the mass ratio of secondary mixture A and secondary mixture B at (20–80):(80–20) is beneficial for simultaneously improving the compaction density and electrochemical performance of the lithium iron phosphate cathode material. The mass ratio of secondary mixture A to secondary mixture B can be further (60-40):(40-60).

[0066] In some embodiments, the secondary sintering temperature is 600°C to 780°C. A secondary sintering temperature within this range is beneficial for the coating of lithium fluoride, the second carbon source, and the second additive, and reduces the poor carbon coating effect caused by free carbon generated during the primary sintering process due to the fused lithium salt. The secondary sintering temperature can, exemplarily, be 600°C, 650°C, 700°C, 750°C, 780°C, or any value within the range of any two of the above values. Further, the secondary sintering temperature can be 700°C to 780°C. The secondary sintering time can be 3 hours to 15 hours, which is beneficial for thorough sintering. The secondary sintering time can, exemplarily, be 3 hours, 5 hours, 5 hours, 10 hours, 12 hours, 14 hours, 15 hours, or any value within the range of any two of the above values. The secondary sintering time can further be 7 hours to 10 hours.

[0067] In some embodiments, the heating rate of the secondary sintering can be 2°C / min to 5°C / min, which helps to reduce thermal stress and structural defects caused by rapid heating.

[0068] In some embodiments, the atmosphere for secondary sintering can be an inert atmosphere, which helps to reduce the oxidation of metal elements in lithium iron phosphate cathode materials and improve the crystal structure integrity and chemical composition stability of lithium iron phosphate cathode materials.

[0069] In some embodiments, the sintering equipment for secondary sintering may include one of a pusher kiln, a roller kiln, and a rotary kiln.

[0070] In some embodiments, before secondary sintering, secondary mixture A and secondary mixture B can be spray-dried, which is beneficial for uniform secondary sintering. The inlet air temperature for spray drying can be 200℃~270℃, and the outlet air temperature can be 85℃~100℃.

[0071] In some embodiments, the powder resistivity of the lithium iron phosphate cathode material at 150 MPa pressure is ≤45 Ω·cm. Lithium fluoride improves the electronic and ionic conductivity of the lithium iron phosphate cathode material, effectively reducing its internal resistance. The powder resistivity of the lithium iron phosphate cathode material at 150 MPa pressure can further be 20 Ω·cm to 35 Ω·cm.

[0072] In some embodiments, the compaction density of the lithium iron phosphate cathode material at a pressure of 150 MPa is ≥2.5 g / cm³. 3 The grinding of the secondary mixture achieved particle size distribution, effectively improving the compaction density of the lithium iron phosphate cathode material. The compaction density of the lithium iron phosphate cathode material under 150 MPa pressure can be further increased to 2.5 g / cm³. 3 ~2.6g / cm 3 .

[0073] Compared with the prior art, the method for preparing lithium iron phosphate cathode material provided in this application has the following beneficial effects:

[0074] 1. By adding a certain amount of lithium fluoride to lithium iron phosphate in bulk, or coating it on the surface, or combining doping and coating, the electronic conductivity and ion transport rate of lithium iron phosphate cathode materials can be effectively improved, thereby enhancing the rate performance and low-temperature performance of lithium iron phosphate cathode materials.

[0075] 2. By combining grinding processes, the secondary mixture is ground to achieve particle size distribution, reduce the decrease in compaction density caused by lithium fluoride, and simultaneously improve the energy density, rate performance and low-temperature performance of lithium iron phosphate cathode materials.

[0076] 3. This preparation method is simple, efficient, and environmentally friendly, which is conducive to the large-scale production of lithium iron phosphate cathode materials and has excellent commercial prospects.

[0077] This application also provides a lithium iron phosphate cathode material prepared using the aforementioned method for preparing lithium iron phosphate cathode materials. The lithium iron phosphate cathode material includes a matrix material and a coating layer, wherein at least one of the matrix material and the coating layer contains lithium fluoride.

[0078] Compared to existing technologies, the lithium iron phosphate cathode material exhibits improved electronic conductivity and ion transport rate, reduced internal resistance under high rate and low temperature conditions, and increased compaction density due to the introduction of a certain amount of lithium fluoride into at least one of the matrix material and the coating layer, coupled with a unique grinding process. Therefore, the lithium iron phosphate cathode material provided in this application possesses excellent energy density, rate performance, and low-temperature performance.

[0079] This application also provides an electrochemical device (e.g., a battery) including a positive electrode, wherein the positive electrode includes a positive active material, which is the lithium iron phosphate positive electrode material as described above.

[0080] The electrochemical device prepared using the aforementioned lithium iron phosphate cathode material has advantages such as high capacity, high rate performance and good low-temperature performance, and can be used under low-temperature conditions.

[0081] The following specific examples further illustrate the aforementioned lithium iron phosphate cathode material, its preparation method, and electrochemical device.

[0082] Example 1

[0083] Step S1: Mix 6 kg of glucose, 525.5 g of titanium dioxide, 25 kg of lithium carbonate, 100 kg of ferric phosphate, 44 g of lithium fluoride, and 142 kg of water until homogeneous. The molar ratio of Li:Fe:P is 1.039:1:1.032, resulting in a primary mixture. The added lithium fluoride accounts for 0.044% of the mass of ferric phosphate (phosphorus and iron sources). The primary mixture is then fed into a sand mill for a first grinding (coarse grinding, zirconium bead diameter 0.6 mm) to a particle size D50 = 1.86 μm. It is then fed into the sand mill for a second grinding (fine grinding, zirconium bead diameter 0.3 mm) to a particle size D50 = 1.0 μm. Finally, it is spray-dried at an inlet air temperature of 220℃ and an outlet air temperature of 90℃. The dried primary mixture was heated to 650℃ at a rate of 2.5℃ / min under an inert atmosphere and held for 7.5h for primary sintering. After cooling, the matrix material was obtained.

[0084] Step S2: Mix 100kg of matrix material, 3.5kg of glucose, 6.0kg of polyethylene glycol (molecular weight 6000), 166.8g of titanium dioxide and 136kg of water until homogeneous to obtain a secondary mixture.

[0085] Step S3: The secondary mixture is fed into a sand mill for the first grinding (coarse grinding, zirconium bead diameter 0.6 mm) to obtain secondary mixture A with a particle size D50 = 0.78 μm. A portion of secondary mixture A is then ground a second time (fine grinding, zirconium bead diameter 0.3 mm) to obtain secondary mixture B with a particle size D50 = 0.36 μm. Secondary mixture A and secondary mixture B are mixed at a mass ratio of 50:50. The mixture is then spray-dried at an inlet air temperature of 220℃ and an outlet air temperature of 90℃. The dried mixture is then heated to 750℃ at a rate of 2.5℃ / min under an inert atmosphere and held for 7.5 h for secondary sintering. After cooling, air jet milling, sieving, and iron removal, lithium iron phosphate cathode material is obtained.

[0086] Example 2

[0087] The difference from Example 1 is that lithium fluoride is not added in step S1, i.e., M is 0, and lithium fluoride is added to the secondary mixture in step S2, with the added lithium fluoride accounting for 0.1% of the mass percentage N of the matrix material. The other steps are basically the same as in Example 1, please refer to Example 1.

[0088] Example 3

[0089] The difference from Example 2 is that in step S2, the mass percentage N of lithium fluoride added to the matrix material is 0.3%. The other steps are basically the same as in Example 2; please refer to Example 2.

[0090] Example 4

[0091] The difference from Example 1 is that in step S1, the mass percentage M of lithium fluoride in iron phosphate (phosphorus source and iron source) is 0.02%. The other steps are basically the same as in Example 1, please refer to Example 1.

[0092] Example 5

[0093] The difference from Example 1 is that in step S1, the mass percentage M of lithium fluoride in iron phosphate (phosphorus source and iron source) is 0.1%. The other steps are basically the same as in Example 1, please refer to Example 1.

[0094] Example 6

[0095] The difference from Example 1 is that lithium fluoride is added to the secondary mixture in step S2, and the mass percentage N of the added lithium fluoride is 0.3% of the matrix material. The other steps are basically the same as in Example 1, please refer to Example 1.

[0096] Comparative Example 1

[0097] Step S1: Mix 8.5 kg glucose, 2.6 kg polyethylene glycol (molecular weight 6000), 525.5 g titanium dioxide, 25 kg lithium carbonate, 100 kg iron phosphate, and 142 kg water until homogeneous. The molar ratio of Li:Fe:P is 1.039:1:1.032. The resulting mixture is coarsely ground in a sand mill (zirconium bead diameter 0.6 mm) to a particle size D50 = 1.86 μm, then finely ground in the same mill (zirconium bead diameter 0.3 mm) to a particle size D50 = 0.46 μm. Spray drying is then performed at an inlet air temperature of 220℃ and an outlet air temperature of 90℃. The dried mixture is then sintered under an inert atmosphere at a temperature of 780℃ at a rate of 2.5℃ / min for 7.5 h. After cooling, air jet milling, sieving, and iron removal, lithium iron phosphate cathode material is obtained.

[0098] Comparative Example 2

[0099] The difference from Comparative Example 1 is that lithium fluoride is added to the mixture in step S1, and the mass percentage of lithium fluoride added is 0.044% of iron phosphate (phosphorus source and iron source). The other steps are basically the same as those in Comparative Example 1. Please refer to Comparative Example 1.

[0100] Comparative Example 3

[0101] The difference from Example 1 is that in step S1, the mass percentage M of lithium fluoride in iron phosphate (phosphorus source and iron source) is 0. The other steps are basically the same as in Example 1; please refer to Example 1.

[0102] Comparative Example 4

[0103] The difference from Example 1 is that in step S1, the mass percentage M of lithium fluoride in iron phosphate (phosphorus source and iron source) is 0.15%. The other steps are basically the same as in Example 1, please refer to Example 1.

[0104] Comparative Example 5

[0105] The difference from Example 1 is that: in step S1, lithium fluoride is not added, i.e., M is 0; in step S2, the mass percentage N of lithium fluoride in iron phosphate (phosphorus source and iron source) is 0.5%. The other steps are basically the same as in Example 1, please refer to Example 1.

[0106] The lithium iron phosphate cathode materials obtained in Examples 1-6 and Comparative Examples 1-5 were tested accordingly.

[0107] Test method:

[0108] 1. Field emission scanning electron microscopy (SEM) test. Thermo Fisher Scientific's AxiaChemiSEMHiVac scanning electron microscope was used. This instrument has high-resolution imaging capabilities, which can clearly observe the microstructure and structural features of the cathode material. The accelerating voltage is 10.00 kV, the working distance is 10 mm, and the magnification is 10,000x.

[0109] 2. Carbon content determination: The carbon content of LFP cathode material is tested by infrared analysis. A carbon-sulfur analyzer is used. The sample is burned in a high-temperature oxygen-rich state. The carbon element is oxidized into carbon dioxide and enters the infrared detector with the carrier gas. The carbon content is quantitatively calculated by statistically analyzing the change in the intensity of the infrared absorption wavelength of the carbon dioxide signal.

[0110] 3. Compacted density determination: The test was conducted in accordance with the method specified in the standard "Determination of compacted density of lithium-ion battery cathode material powder" drafted by the National Technical Committee for Standardization of Nonferrous Metals.

[0111] 4. Resistivity Measurement: The test was conducted in accordance with the method specified in the standard "Determination of Resistivity of Powdered Cathode Material for Lithium-ion Batteries" drafted by the National Technical Committee for Standardization of Nonferrous Metals.

[0112] 5. Electrochemical Performance Testing: Electrochemical performance testing was conducted using coin cells. The above-mentioned positive electrode material, polyvinylidene fluoride (PVDF), and conductive agent (such as acetylene black or conductive carbon black) were mixed at a mass ratio of 93:3.5:3.5, and an appropriate amount of NMP was added to prepare a slurry. Next, the slurry was uniformly coated onto aluminum foil and vacuum dried at 115℃±5℃ / 8h, then compacted and cut into circular pieces. Simultaneously, lithium metal sheets were used as the negative electrode material. Finally, all materials were transferred to a glove box and assembled into CR2025 specification coin cells. Constant current charge-discharge testing was performed using the Xinwei Battery Testing System.

[0113] (I) Rate performance test: The test operating voltage range is 2.0V~3.75V, the temperature is 25℃, and the 0.1C capacity, initial efficiency and 1.0C capacity of the button cell are measured.

[0114] (II) Low temperature performance test: The test operating voltage range is 2.0V~3.75V, the temperature is 25℃, and the 1.0C charging capacity of the button cell is measured; then the battery is transferred to -15℃, discharged to 2.0V, and the 1.0C discharge capacity of the button cell is measured to obtain the capacity retention rate relative to the 1.0C charging capacity at 25℃.

[0115] The relevant process parameters for Examples 1-6 and Comparative Examples 1-5 are shown in Table 1, and the relevant test results are shown in Table 2.

[0116] Table 1

[0117] Table 2

[0118] The above results show that:

[0119] The scanning electron microscope (SEM) results of the lithium iron phosphate (LFP) cathode material prepared in Example 1 are shown in Figure 2, the results in Example 2 are shown in Figure 3, the results in Comparative Example 1 are shown in Figure 4, and the results in Comparative Example 3 are shown in Figure 5. Comparing Figures 2-5, it can be seen that, compared to the single-sintering process of Comparative Example 1, Example 1, using a two-sintering process, yields LFP cathode materials with small particle sizes ranging from 200 nm to 400 nm and large particle sizes ranging from 1.0 μm to 1.3 μm, achieving a well-balanced particle size distribution. In contrast, the LFP cathode material obtained in Comparative Example 1 has small particle sizes ranging from 300 nm to 500 nm and large particle sizes ranging from 0.8 μm to 1.0 μm, showing an indistinct particle size distribution. Compared to Comparative Example 3, Examples 1 and 2 also involved mixing slurries of different particle sizes during the secondary grinding stage to achieve particle size distribution and improve the compaction density of lithium iron phosphate cathode materials.

[0120] As shown in Tables 1 and 2, compared to Comparative Examples 1 and 2, Examples 1-6, by combining grinding and stepwise sintering processes, achieved particle size distribution and sufficient lithium fluoride doping and / or coating, resulting in lithium iron phosphate cathode materials with higher compaction density, better rate performance, and better low-temperature performance. Compared to Comparative Example 3, the lithium iron phosphate cathode materials in Examples 1-6, after lithium fluoride doping and / or coating, exhibited better rate performance and improved low-temperature performance.

[0121] Compared to the lithium fluoride doping in Examples 1-2, Example 3, by coating with lithium fluoride, can not only improve the rate performance and low-temperature performance of the lithium iron phosphate cathode material, but also effectively protect the cathode from electrolyte corrosion and reduce metal dissolution. In addition, the lithium fluoride in the coating layer is conducive to the formation of a stable CEI, which is beneficial to improving the stability of the cathode and electrolyte interface, thereby improving the electrochemical performance of the lithium iron phosphate cathode material.

[0122] Comparing Examples 1, 3, and 6, it was found that in Example 6, by adding lithium fluoride in both steps S1 and S2, i.e., simultaneously performing bulk doping and surface coating on the cathode material, the rate performance and low-temperature performance of the lithium iron phosphate cathode material were further improved. However, due to the increase in the lithium fluoride content in the lithium iron phosphate cathode material, the compaction density of the lithium iron phosphate cathode material decreased.

[0123] Comparing Examples 1, 4, 5, and Comparative Example 4, it can be seen that as the amount of lithium fluoride added in step S1 increases, i.e., the amount of lithium fluoride doping increases, the rate performance and low-temperature discharge capacity of the lithium iron phosphate cathode material improve. When the addition amount M reaches 0.044% (Example 5), the 0.1C charging capacity of the lithium iron phosphate cathode material reaches as high as 162.0 mAh / g, but the compaction density decreases slightly. When the addition amount M is too high, reaching 0.15% (Comparative Example 4), the compaction density, rate performance, and low-temperature discharge capacity of the lithium iron phosphate cathode material all decrease.

[0124] Comparing Examples 2, 3, and 5, it can be seen that as the amount of lithium fluoride added in step S2 increases, i.e., the amount of lithium fluoride coating increases, the rate performance and low-temperature discharge capacity of the lithium iron phosphate cathode material improve. When the amount of N added reaches 0.3% (Example 3), the resistivity of the lithium iron phosphate cathode material is only 22 Ω·cm, the 1C discharge capacity increases to 150.2 mAh / g, the low-temperature discharge capacity also increases to 112.8 mAh / g, and the volume retention rate reaches 70.0%. When the amount of N added is too high, reaching 0.50% (Comparative Example 5), the rate performance and low-temperature discharge capacity of the lithium iron phosphate cathode material decrease, and the compaction density decreases significantly.

[0125] Therefore, it can be seen that by using a certain amount of lithium fluoride in bulk doping, surface coating, or a combination of doping and coating, and combining unique grinding and sintering processes, lithium iron phosphate cathode materials can exhibit ideal compaction density, energy density, rate performance, and low-temperature performance.

[0126] It is understood that the above embodiments are merely exemplary implementations used to illustrate the principles of the present invention, and the present invention is not limited thereto. For those skilled in the art, various modifications and improvements can be made without departing from the spirit and essence of the present invention, and these modifications and improvements are also considered to be 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: A primary mixture is formed by mixing a phosphorus source, an iron source, a lithium source, a first carbon source, lithium fluoride, a first additive, and a first solvent. The primary mixture is then ground and sintered once to obtain a matrix material. The mass percentage M of the lithium fluoride in the total mass of the iron source and the phosphorus source is 0 to 0.1%. The matrix material, second carbon source, lithium fluoride, second additive, and second solvent are mixed to form a secondary mixture, wherein the lithium fluoride added in this step accounts for 0-0.3% of the mass percentage N of the matrix material, and N and M are not both 0; and The secondary mixture is ground to obtain secondary mixture A with a particle size D50 of 0.6μm to 1.0μm and secondary mixture B with a particle size D50 of 0.2μm to 0.4μm. Secondary mixture A and secondary mixture B are mixed and subjected to secondary sintering to obtain the lithium iron phosphate cathode material.

2. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The secondary mixture A and the secondary mixture B are mixed in a mass ratio of (20-80):(80-20).

3. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The step of grinding the secondary mixture further includes: The secondary mixture is subjected to a first grinding to obtain secondary mixture A. The diameter of the grinding beads used in the first grinding is 0.6 mm to 1.0 mm. A portion of the secondary mixture A is subjected to a second grinding to obtain the secondary mixture B. The diameter of the grinding beads used in the second grinding is 0.2 mm to 0.35 mm.

4. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The step of grinding the primary mixture further includes: The mixture is first ground until the particle size D50 is 1.5μm to 3.0μm, and then ground a second time until the particle size D50 is 0.8μm to 1.3μm. The diameter of the grinding beads used in the first grinding is 0.6mm to 1.0mm, and the diameter of the grinding beads used in the second grinding is 0.2mm to 0.35mm.

5. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The temperature of the first sintering is 450℃~750℃, and the sintering time is 3h~15h; and / or The secondary sintering temperature is 600℃~780℃, and the secondary sintering time is 3h~15h.

6. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, In the primary mixture, the molar ratio of phosphorus in the phosphorus source, iron in the iron source, and lithium in the lithium source, P:Fe:Li, is (1.015~1.036):1:(1.0~1.04).

7. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, Both the first additive and the second additive are independently selected from at least one of titanium dioxide, tetrabutyl titanate, magnesium oxide, magnesium acetate, magnesium hydroxide, magnesium nitrate, zirconium nitrate, zirconium hydroxide, zirconium oxide, and niobium pentoxide.

8. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, The lithium iron phosphate positive electrode material has a compaction density ≥ 2.5 g / cm under a pressure of 150 MPa 3 ; and / or The resistivity of the lithium iron phosphate cathode material at a pressure of 150 MPa is ≤45 Ω·cm.

9. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is prepared using the method for preparing lithium iron phosphate cathode material as described in any one of claims 1 to 8. The lithium iron phosphate cathode material includes a matrix material and a coating layer, wherein at least one of the matrix material and the coating layer contains lithium fluoride.

10. An electrochemical device, characterized in that, The electrochemical device includes a positive electrode, wherein the positive electrode includes a positive active material, and the positive active material is the lithium iron phosphate positive electrode material as described in claim 9.

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