Lithium iron phosphate positive electrode active material, preparation method thereof, positive electrode sheet, battery, battery pack, and electric device

Abstract

The application provides a lithium iron phosphate positive electrode active material, a preparation method of the lithium iron phosphate positive electrode active material, a positive electrode sheet, a battery, a battery pack and an electric device. 16 spins / g~2.0000×10 16 spins / g. The lithium iron phosphate positive electrode active material provided by the application has a high compaction density and has normal temperature rate discharge and low temperature discharge performance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a lithium iron phosphate cathode active material and its preparation method, cathode sheet, battery, battery pack, and electrical equipment. Background Technology

[0002] Lithium iron phosphate (LiFePO4), as the cathode material for lithium-ion batteries, is widely used in new energy vehicle power batteries, energy storage systems, power tools, and rail transportation, mainly due to its good cycle stability, high thermal safety, and low cost. With the continuous advancement of the new energy industry, the market has placed higher demands on the performance of power batteries: not only do they need to further improve energy density and power density to support longer driving range and fast charging for electric vehicles, but they also need to ensure the stability of material performance in extreme environments (such as low-temperature conditions) to guarantee the reliability of batteries in cold regions or low-temperature operating conditions. How to achieve high energy density while simultaneously ensuring high power output and excellent low-temperature performance has become a pressing technical challenge for the industry. Summary of the Invention

[0003] This invention provides a lithium iron phosphate cathode active material and its preparation method, as well as a cathode sheet, a battery, a battery pack, and electrical equipment. This lithium iron phosphate cathode active material has a specific number of crystal defects, maintaining good power performance at both room temperature and low temperature while taking into account compaction characteristics.

[0004] In a first aspect, embodiments of this application provide a lithium iron phosphate cathode active material, wherein the number of crystal defects in the lithium iron phosphate cathode active material is 1.4000 × 10⁻⁶. 16 spins / g ~2.0000×10 16 spins / g.

[0005] In some embodiments, the lithium iron phosphate cathode active material includes a first particle and a second particle, and the Malvern particle size distribution curve of the lithium iron phosphate cathode active material includes a first peak corresponding to the first particle and a second peak corresponding to the second particle.

[0006] Wherein, the termination particle size of the first peak is the starting particle size of the second peak;

[0007] Based on the sum of the peak areas of the first peak and the second peak, the peak area of ​​the first peak accounts for 50% to 70%;

[0008] And / or, the peak area of ​​the second peak accounts for 30% to 50%.

[0009] In some embodiments, the peak size of the first peak is 0.25 μm to 0.65 μm;

[0010] And / or, the peak size of the second peak is 1.50 μm to 2.96 μm;

[0011] And / or, the initial particle size of the first peak is 0.189 μm to 0.244 μm;

[0012] And / or, the termination particle size of the first peak is 0.60 μm to 1.56 μm;

[0013] And / or, the initial particle size of the second peak is 0.60 μm to 1.56 μm;

[0014] And / or, the termination particle size of the second peak is 6.24 μm to 9.5 μm.

[0015] In some embodiments, the Dv5 of the lithium iron phosphate cathode active material is 0.30 μm - 0.37 μm, the Dv10 is 0.312 μm - 0.480 μm, the Dv50 is 0.784 μm - 1.600 μm, the Dv90 is 2.5 μm - 4.1 μm, and the Dv99 is 4.95 μm - 6.01 μm.

[0016] In some embodiments, the lithium iron phosphate cathode active material includes Fe 3+ Based on the lithium iron phosphate cathode active material, the Fe 3+ The mass percentage content is 0.19%~0.21%.

[0017] In some embodiments, the sphericity of the lithium iron phosphate cathode active material is 0.85~0.9;

[0018] And / or, the tap density of the lithium iron phosphate cathode active material is 1.3 g / cm³. 3 ~1.4g / cm 3 ;

[0019] And / or, the specific surface area of ​​the lithium iron phosphate cathode active material is 15 m². 2 / g~16m 2 / g.

[0020] In some embodiments, the lithium iron phosphate cathode active material includes the lithium iron phosphate matrix and a carbon coating layer covering at least a portion of the surface of the lithium iron phosphate matrix.

[0021] In some embodiments, the thickness of the carbon coating layer is 2 nm to 3 nm; and / or, based on the lithium iron phosphate cathode active material, the mass percentage content of the carbon coating layer is 1.35% to 1.4%.

[0022] Secondly, the present invention provides a method for preparing the above-mentioned lithium iron phosphate cathode active material, comprising the following steps: providing a mixed system of an iron source, a lithium source, and a phosphorus source; preparing the lithium iron phosphate cathode active material, wherein the lithium iron phosphate cathode active material comprises the lithium iron phosphate matrix, and the number of crystal defects in the lithium iron phosphate matrix is ​​1.4000 × 10⁻⁶. 16 spins / g ~2.0000×10 16 spins / g.

[0023] In some embodiments, the mixed system is processed to obtain an intermediate, and the intermediate is sintered to obtain the lithium iron phosphate cathode active material, wherein the preparation method satisfies at least one of the following conditions:

[0024] (1) The method of processing the mixed system to obtain the intermediate includes grinding the mixed system to obtain the intermediate, preferably, the grinding method includes sand milling;

[0025] (2) The sintering process includes a first sintering process and a second sintering process in sequence. Preferably, the highest temperature of the first sintering process is 760℃-790℃.

[0026] Preferably, the holding time of the first sintering treatment at the highest temperature is 8h-12h;

[0027] Preferably, the heating rate of the first sintering treatment is 2.5℃ / min ~ 5℃ / min;

[0028] Preferably, the temperature of the second sintering treatment is 700℃~750℃;

[0029] Preferably, the holding time for the second sintering treatment is 5h~6h;

[0030] Preferably, the heating rate of the second sintering treatment is 2.5℃ / min ~ 5℃ / min;

[0031] Preferably, a cooling process is further included between the first sintering process and the second sintering process;

[0032] Preferably, a cooling process is further included between the first sintering process and the second sintering process, wherein the target temperature for the cooling process is 90℃-100℃.

[0033] Preferably, a cooling process is further included between the first sintering process and the second sintering process, wherein the atmosphere for the cooling process is an oxygen-containing atmosphere;

[0034] Preferably, a cooling process is further included between the first sintering process and the second sintering process, wherein the atmosphere for the cooling process is an oxygen-containing atmosphere, and the flow rate of the oxygen-containing atmosphere for the cooling process is 0.6 L / min ~ 1.0 L / min;

[0035] Preferably, a cooling process is further included between the first sintering process and the second sintering process, wherein the cooling process is held at the target temperature for 40 min to 80 min.

[0036] (5) The method further includes drying the intermediate before the sintering process, preferably, the drying process includes spraying.

[0037] (6) The Dv50 of the intermediate is 370nm~390nm.

[0038] In some embodiments, both the iron source and the phosphorus source are iron phosphate, and the molar ratio of Fe to P in the iron phosphate is (0.965-0.972):1.

[0039] Thirdly, embodiments of this application provide a positive electrode sheet, comprising the lithium iron phosphate positive electrode active material described above;

[0040] The lithium iron phosphate positive electrode active material can be prepared by the preparation method provided above.

[0041] Fourthly, embodiments of this application provide a battery including the aforementioned positive electrode sheet.

[0042] Fifthly, embodiments of this application provide a battery pack comprising at least two of the aforementioned batteries.

[0043] Sixthly, embodiments of this application provide an electrical device including the aforementioned battery or battery pack.

[0044] By controlling the number of crystal defects in the lithium iron phosphate cathode active material to 1.4000 × 10⁻⁶ 16 spins / g ~2.0000×10 16Within the range of spins / g, it is possible to improve the compaction density of lithium iron phosphate cathode active materials while maintaining lithium-ion transport and electrochemical reaction performance, thereby improving the material's room temperature and low temperature discharge performance. On the one hand, it helps to relieve stress during the high compaction process and prevent particle breakage, thus maintaining the structural integrity of the particles while achieving high compaction density. On the other hand, these controlled defects (such as lithium vacancies or doping-induced defects) can serve as additional lithium-ion migration channels and active sites, effectively improving lithium-ion transport rate and electrochemical reaction kinetics. This optimization enables the material to have excellent high power output capability at room temperature, while at low temperatures, the enhanced ion conduction and reactivity significantly reduce charge transfer impedance, thus comprehensively improving the material's room temperature and low temperature discharge performance. Attached Figure Description

[0045] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with the invention and, together with the description, serve to explain the principles of the invention.

[0046] Figure 1 The above are the EPR curves of the lithium iron phosphate cathode active materials provided in Example 1 and Comparative Example 1 of this invention.

[0047] Figure 2 Malvern particle size distribution curve of the lithium iron phosphate cathode active material provided in Example 1 of this invention;

[0048] Figure 3 This is a TEM image of the lithium iron phosphate cathode active material provided in Example 1 of the present invention;

[0049] Figure 4 The first charge-discharge curve at 25°C for a coin cell made from the lithium iron phosphate cathode active material provided in Example 1 of this invention;

[0050] Figure 5 The discharge curve of a coin cell made from the lithium iron phosphate positive electrode active material provided in Example 1 of this invention at -20°C.

[0051] The accompanying drawings have illustrated specific embodiments of the invention, which will be described in more detail below. These drawings and descriptions are not intended to limit the scope of the invention in any way, but rather to illustrate the concept of the invention to those skilled in the art through reference to particular embodiments. Detailed Implementation

[0052] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions in the embodiments of this invention will be clearly and completely described below in conjunction with the embodiments of this invention. Obviously, the described embodiments are only some embodiments of this invention, not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this invention.

[0053] The inventors believe that the microstructure of lithium iron phosphate cathode active material can be controlled based on particle size distribution to shorten the ion transport path, thereby taking into account power performance under both room temperature and low temperature conditions.

[0054] Based on this, this application provides a lithium iron phosphate cathode active material, which includes a lithium iron phosphate matrix, wherein the number of crystal defects in the lithium iron phosphate matrix is ​​1.4000 × 10⁻⁶. 16 spins / g ~2.0000×10 16 spins / g.

[0055] This embodiment achieves a synergistic balance between microscopic lattice order, ion transport channels, and particle packing characteristics by quantitatively controlling the number of defects in the crystalline state of the material.

[0056] In this invention, the number of crystal defects in the lithium iron phosphate matrix refers to the paramagnetic defects with unpaired electrons in the lithium iron phosphate cathode active material, and the "number" is defined as the number of such paramagnetic centers per gram of sample.

[0057] The number of crystal defects in lithium iron phosphate matrix can be detected using common detection methods in the field, such as electron paramagnetic resonance (EPR) testing, with the unit being spins / g, where spins / g represents the number of crystal defects per unit mass of sample.

[0058] In some embodiments of the present invention, the number of crystal defects in the lithium iron phosphate matrix of the lithium iron phosphate cathode active material is detected by a detection method including the following steps:

[0059] The sample to be tested is placed in an EPR spectrometer and tested under specific frequency (e.g., X-band, approximately 9.5 GHz) and low temperature (e.g., 77 K or lower). By analyzing the intensity of characteristic resonance signals generated by paramagnetic defect centers in the material (e.g., iron vacancies, lithium vacancies, or oxygen vacancies), and by calibrating with standard paramagnetic substances of known concentration, the number of unpaired electron spins per unit mass of the sample can be quantitatively calculated, thereby obtaining the number of crystal defects, which is expressed in units of spins / g.

[0060] For batteries prepared using the lithium iron phosphate cathode active material provided by this invention, the battery can be disassembled to obtain the lithium iron phosphate cathode active material. The number of crystal defects in the lithium iron phosphate matrix within the cathode active material can then be tested using the method described above. It should be noted that the results of directly testing the lithium iron phosphate cathode active material or testing the lithium iron phosphate cathode active material obtained from battery disassembly are similar and within the allowable error range.

[0061] The lithium iron phosphate cathode active material may include a coating layer, but the above detection method can directly detect the number of crystal defects in the lithium iron phosphate matrix without removing the coating layer from the lithium iron phosphate cathode active material.

[0062] In some embodiments, the battery disassembly process includes the following steps: first, the battery is disassembled to obtain a positive electrode sheet; the positive electrode sheet is immersed in water to obtain a positive electrode active material layer; the positive electrode active material layer is ground and pulverized and then immersed in an NMP solution, and heated and stirred at 80°C for 2 hours; subsequently, the mixed solution is filtered and separated, and then vacuum baked at 120°C for 12 hours to obtain lithium iron phosphate positive electrode active material.

[0063] Specifically, the number of crystal defects includes, but is not limited to, 1.4000 × 10⁻⁶. 16 spins / g, 1.5000×10 16 spins / g, 1.6000×10 16 spins / g, 1.7000×10 16 spins / g, 1.8000×10 16 spins / g, 1.9000×10 16 spins / g, 2.0000×10 16 spins / g or a range consisting of any two of them.

[0064] When this value is controlled at 1.4000×10 16 spins / g up to 2.00000 × 10⁻⁶ 16 When the spins / g range is within the range, it indicates that there are appropriate, adjustable, and not excessive lattice defects within the lithium iron phosphate matrix. These defects may include, but are not limited to, Fe. 3+ Related defects, Li + Local coordination imbalances and their combinations around vacancies, Fe sites, or O sites can effectively enhance lithium-ion transport rates and electrochemical reaction kinetics by providing additional lithium-ion migration channels and active sites. At low temperatures, the presence of defects can reduce the lithium-ion diffusion barrier, thereby alleviating charge transfer impedance and improving the low-temperature discharge performance of lithium iron phosphate cathode active materials.

[0065] In some embodiments, the lithium iron phosphate cathode active material includes a first particle and a second particle. The Malvern particle size distribution curve of the lithium iron phosphate cathode active material is obtained by detection with a Malvern particle size analyzer. The Malvern particle size distribution curve includes a first peak corresponding to the first particle and a second peak corresponding to the second particle, wherein the termination particle size of the first peak is the starting particle size of the second peak.

[0066] In the Malvern particle size distribution curve, the peak area refers to the area enclosed by the first perpendicular line drawn from the starting particle size of the peak to the horizontal axis, and the second perpendicular line drawn from the ending particle size of the peak to the horizontal axis; the area enclosed by the first perpendicular line, the horizontal axis, the second perpendicular line, and the peak curve.

[0067] Based on the Malvern particle size distribution curve, the peak area of ​​the first peak accounts for 50% to 70%;

[0068] Specifically, the Malvern particle size distribution curve is obtained using a Malvern particle size analyzer, and the peak area is calculated by fitting each peak using image processing software.

[0069] In some embodiments, the Malvern particle size distribution curve is obtained by a detection method including the following steps:

[0070] First, the sample was dispersed using a high-purity, low-conductivity organic dispersant, with the concentration controlled within the instrument's recommended shading range. Moderate depolymerization was achieved using low-frequency ultrasound and trace amounts of surfactant. Then, in the Malvern Mastersizer 3000 wet process module, a dual red and blue light source was used. The refractive index of lithium iron phosphate was set to 1.7-1.8, and the absorptivity to 0.1-0.3. The dispersant refractive index was accurately input, and both background and sample measurement times were no less than 10 seconds. A general analysis model was selected, and the high-resolution option was enabled. Finally, each sample was measured at least three times independently, and the average value was taken. The volumetric particle size distribution curve, peak diameter, and peak area percentage were obtained through peak analysis.

[0071] For batteries prepared using the lithium iron phosphate cathode active material provided by this invention, the batteries can be disassembled according to the method described above to obtain the lithium iron phosphate cathode active material, which can then be tested using the same method. It should be noted that the results of directly testing the lithium iron phosphate cathode active material or testing the lithium iron phosphate cathode active material obtained from battery disassembly are similar and within the allowable error range.

[0072] Malvern particle size distribution curves are used to characterize the overall gradation state of material particles. The first peak represents a finer particle group, and the second peak represents a coarser particle group. The first and second peaks together reflect the coordinated distribution of small and large particles.

[0073] In detail, the peak area percentage of the first peak includes, but is not limited to, 50%, 55%, 60%, 65%, 70%, or any combination thereof.

[0074] When the peak area ratio of the first peak is within the aforementioned range, it means that the first particle occupies a suitable proportion and can effectively fill the gaps between the second particles, achieving a tight packing of the lithium iron phosphate cathode active material. This significantly improves the compaction density and tap density of the lithium iron phosphate cathode sheet, thereby increasing the volumetric energy density of the battery. Simultaneously, a higher proportion of the first particle helps shorten the solid-phase diffusion path of lithium ions, enabling faster ion transport even at low temperatures, thus improving the battery's rate performance and low-temperature performance. In some embodiments, the peak area ratio of the second peak is 30% to 50%.

[0075] In detail, the peak area percentage of the second peak includes, but is not limited to, 30%, 35%, 40%, 45%, 50%, or any combination thereof.

[0076] The peak area ratio of the second peak means that the volume ratio of the second particle with a larger particle size in the lithium iron phosphate cathode active material is 30%~50%.

[0077] When the peak area ratio of the second peak is within the above range, while achieving close packing of particles and increasing compaction density, a sufficient proportion of second particles can construct a stable conductive framework and reduce the internal resistance of the electrode. In addition, an appropriate proportion of second particles helps to improve the tap density of the active material and the mechanical strength of the electrode, and improves the stability of coating and rolling processes, thereby further optimizing the cycle life and safety performance of the battery while ensuring high energy density.

[0078] In some embodiments of the present invention, the peak size of the first peak is 0.25~0.65μm.

[0079] The peak size of the first peak refers to the particle size at the highest point corresponding to the first peak in the Malvern particle size distribution curve. After the particle size distribution curve is obtained by fitting it with a Malvern particle size analyzer, a perpendicular line is drawn from the highest point of the peak corresponding to the first particle to the horizontal axis; the corresponding abscissa is the peak size. Specifically, the peak size of the first peak includes, but is not limited to, 0.25 μm, 0.30 μm, 0.35 μm, 0.40 μm, 0.45 μm, 0.50 μm, 0.55 μm, 0.60 μm, 0.65 μm, or any combination thereof.

[0080] By controlling the peak value of the first particle within the range of 0.25 to 0.65 μm, the first particle can effectively fill the gaps between coarse particles, significantly improving the packing density. At the same time, the first particle within this particle size range has a suitable specific surface area, which can maintain stable interfacial chemical activity.

[0081] In some embodiments of the present invention, the peak size of the second peak is 1.50~2.96μm.

[0082] The peak size of the second peak refers to the particle size at the highest point of the second peak in the Malvern particle size distribution curve. After obtaining the particle size distribution curve by fitting it with a Malvern particle size analyzer, a perpendicular line is drawn from the highest point of the second peak to the horizontal axis; the corresponding horizontal axis represents the peak size.

[0083] In detail, the peak particle size of the second peak includes, but is not limited to, a range of 1.50 μm, 1.60 μm, 1.80 μm, 2.00 μm, 2.20 μm, 2.40 μm, 2.50 μm, 2.60 μm, 2.70 μm, 2.80 μm, 2.90 μm, 2.96 μm, or any combination thereof.

[0084] By controlling the peak size of the second peak within the range of 1.50–2.96 μm, the second particles can provide a robust skeletal support and maintain a shorter lithium-ion diffusion distance, thereby achieving a lower polarization level. Through bimodal gradation, a tightly packed structure is formed, with the first particle filling the gaps and the second particle forming the framework, effectively reducing electrode porosity and increasing compaction density.

[0085] In some embodiments of the present invention, the initial particle size of the first peak is 0.189 μm to 0.244 μm.

[0086] The initial particle size of the first peak refers to the particle size corresponding to the point on the Malvern particle size distribution curve closest to the vertical axis.

[0087] The initial particle size of the first peak can be determined by obtaining the Malvern particle size distribution curve through the Malvern particle size analysis described above, and then drawing a perpendicular line from the point on the curve closest to the vertical axis to the horizontal axis.

[0088] In detail, the starting particle size of the first peak includes, but is not limited to, a range of 0.189 μm, 0.190 μm, 0.200 μm, 0.210 μm, 0.220 μm, 0.230 μm, 0.240 μm, 0.244 μm, or any two of these.

[0089] When the initial particle size of the first peak is within the above range, it is beneficial to achieve close packing between particles, significantly improve the compaction density of the electrode, and at the same time ensure that an appropriate amount of porosity is retained between particles to maintain electrolyte wetting and rapid ion transport. Thus, while maintaining high energy density, it effectively reduces the internal resistance of the electrode and improves rate performance and cycle stability.

[0090] In some embodiments of the present invention, the termination particle size of the first peak is 0.60~1.56 μm.

[0091] The terminus particle size of the first peak refers to the particle size at which the vertical line perpendicular to the horizontal axis is drawn from the end of the first peak in the Malvern particle size distribution curve. This can also be understood as the particle size at the inflection point where the volume density of the curve changes from decreasing to increasing.

[0092] In detail, the termination particle size of the first peak includes, but is not limited to, a range of 0.60 μm, 0.70 μm, 0.80 μm, 0.90 μm, 1.00 μm, 1.10 μm, 1.20 μm, 1.30 μm, 1.40 μm, 1.50 μm, 1.56 μm, or any two of these ranges.

[0093] When the termination particle size of the first peak is within the above range, the first particle can effectively shorten the lithium-ion diffusion path and improve the battery rate performance; at the same time, as the small particle size part in the bimodal distribution, it can tightly fill the gap between the second particles, optimize the powder packing density, and thus improve the compaction density and energy density of the electrode.

[0094] In some embodiments of the present invention, the initial particle size of the second peak is 0.60~1.56 μm.

[0095] In this invention, the starting particle size of the second peak is the same as the ending particle size of the first peak. Therefore, the starting particle size of the second peak can be determined using the method described above for determining the ending particle size of the first peak.

[0096] In detail, the starting particle size of the second peak includes, but is not limited to, a range of 0.60 μm, 0.70 μm, 0.80 μm, 0.90 μm, 1.00 μm, 1.10 μm, 1.20 μm, 1.30 μm, 1.40 μm, 1.50 μm, 1.56 μm, or any two of these.

[0097] In some embodiments of the present invention, the termination particle size of the second peak is 6.24 μm to 9.50 μm.

[0098] In detail, the termination particle size of the second peak includes, but is not limited to, a range of 6.24, 6.50 μm, 7.00 μm, 7.50 μm, 8.00 μm, 8.50 μm, 9.00 μm, 9.50 μm, or any combination thereof.

[0099] The termination particle size of the second peak is within the above range, indicating that the second particle has a suitable maximum particle size. The second particle within this particle size range serves as the skeleton structure in the bimodal distribution, ensuring the mechanical stability of the electrode and effectively improving the overall packing density of the powder by closely cooperating with the first particle, thus optimizing the compaction effect of the electrode and significantly improving the volumetric energy density and cycle life of the battery. In addition, the appropriate size of the second particle can also balance the long-distance transport of lithium ions and structural stability, ensuring the performance of the battery under high-rate charge and discharge.

[0100] In some embodiments of the present invention, the particle size distribution parameters of the lithium iron phosphate cathode active material are as follows: Dv5 is 0.30 μm to 0.37 μm, Dv10 is 0.312 μm to 0.480 μm, Dv50 is 0.784 μm to 1.600 μm, Dv90 is 2.50 μm to 4.10 μm, and Dv99 is 4.95 μm to 6.01 μm. These particle size distribution parameters are used to quantitatively characterize the overall cumulative particle size distribution of the lithium iron phosphate cathode active material, where Dv5, Dv10, Dv50, Dv90, and Dv99 correspond to the particle sizes at which the cumulative volume distribution reaches 5%, 10%, 50%, 90%, and 99%, respectively.

[0101] In this invention, the particle size distribution of the lithium iron phosphate cathode active material is detected by a method including the following steps:

[0102] First, the sample was dispersed using a high-purity, low-conductivity organic dispersant, with the concentration controlled within the instrument's recommended shading range. Moderate depolymerization was achieved using low-frequency ultrasound and trace amounts of surfactant. Then, in the Malvern Mastersizer 3000 wet process module, a dual red and blue light source was used. The refractive index of lithium iron phosphate was set to 1.7-1.8, and the absorption to 0.1-0.3. The dispersant refractive index was accurately input, and both background and sample measurement times were no less than 10 seconds. A general analysis model was selected, and the high-resolution option was enabled. Finally, each sample was measured independently at least three times, and the average value was taken. The volumetric particle size distribution curve was obtained through peak analysis, and Dv5, Dv10, Dv50, Dv90, and Dv99 were read.

[0103] The particle size distribution of lithium iron phosphate cathode active materials can be detected using common detection methods in the field, such as the Malvern laser particle size analyzer.

[0104] For batteries prepared using the lithium iron phosphate cathode active material provided by this invention, the batteries can be disassembled according to the method described above to obtain the lithium iron phosphate cathode active material, which can then be tested using the same method. It should be noted that the results of directly testing the lithium iron phosphate cathode active material or testing the lithium iron phosphate cathode active material obtained from battery disassembly are similar and within the allowable error range.

[0105] Specifically, the lithium iron phosphate cathode active material Dv5 includes, but is not limited to, a range of 0.30μm, 0.31μm, 0.32μm, 0.33μm, 0.34μm, 0.35μm, 0.36μm, 0.37μm, or any combination thereof; Dv10 includes, but is not limited to, a range of 0.312μm, 0.320μm, 0.340μm, 0.360μm, 0.380μm, 0.400μm, 0.420μm, 0.440μm, 0.460μm, 0.480μm, or any combination thereof; Dv50 includes, but is not limited to, 0.784μm, 0.800μm, 0.900μm, 1.000μm, 1.100μm, 1.200μm, ... 1.300μm, 1.400μm, 1.500μm, 1.600μm or any two of these ranges; Dv90 includes, but is not limited to, 2.50μm, 2.60μm, 2.80μm, 3.00μm, 3.20μm, 3.40μm, 3.60μm, 3.80μm, 4.00μm, 4.10μm or any two of these ranges; Dv99 includes, but is not limited to, 4.95μm, 5.00μm, 5.10μm, 5.20μm, 5.30μm, 5.40μm, 5.50μm, 5.60μm, 5.70μm, 5.80μm, 5.90μm, 6.00μm, 6.01μm or any two of these ranges.

[0106] By controlling Dv50 within the aforementioned range and constraining the proportion of lithium iron phosphate cathode active material with Dv5 and Dv10, the dense packing of particles can be improved without significantly increasing the specific surface area. This avoids the increase in electrode porosity and side reactions caused by an excessive number of first particles. At the same time, limiting Dv90 and Dv99 can shorten the lithium-ion diffusion path. The more concentrated the particle size distribution and the clearer the bimodal boundary, the more conducive it is for the first particles to fill the second gap and form a denser packing structure, thereby improving the tap density and compaction density, and making the electron and ion transport inside the electrode more balanced.

[0107] In some embodiments, the lithium iron phosphate cathode active material includes Fe 3+ Based on lithium iron phosphate cathode active material, Fe 3+ The mass percentage content is 0.19%~0.21%.

[0108] Common detection methods in this field can be used to detect the valence state of Fe and Fe in lithium iron phosphate cathode active materials. 3+ The mass percentage content, for example, X-ray photoelectron diffraction.

[0109] In some embodiments of the present invention, detection is performed by a detection method including the following steps:

[0110] First, a thin film suitable for testing was prepared from the lithium iron phosphate cathode active material sample. The sample surface was scanned using an X-ray photoelectron diffractometer, and the valence state of iron was determined by analyzing the position and shape of the characteristic energy spectrum peaks. Then, a certain mass of the sample to be tested was accurately weighed, dissolved and diluted with a strong acid, and an appropriate amount of sample solution was taken. The content of ferrous iron in the system was first determined by titration with potassium dichromate standard solution. Then, an equal amount of sample was taken, potassium iodide was added, and the reaction was carried out in an acidic dark place. After ferric iron oxidized iodide ions to elemental iodine, the content of ferric iron was determined by titration with sodium thiosulfate standard solution. Finally, the mass percentage of ferric iron in the lithium iron phosphate cathode active material was calculated based on the total mass of the sample.

[0111] For batteries prepared using the lithium iron phosphate cathode active material provided by this invention, the batteries can be disassembled according to the method described above to obtain the lithium iron phosphate cathode active material, which can then be tested using the same method. It should be noted that the results of directly testing the lithium iron phosphate cathode active material or testing the lithium iron phosphate cathode active material obtained from battery disassembly are similar and within the allowable error range.

[0112] In detail, Fe 3+ The mass percentage content includes, but is not limited to, 0.19%, 0.195%, 0.20%, 0.205%, 0.21%, or any combination thereof.

[0113] This invention does not limit Fe 3+ The control method can be achieved by adjusting the sintering atmosphere and holding time during the preparation process to regulate Fe. 3+ The percentage of mass content.

[0114] By Fe 3+ The Fe content is controlled within a narrow range of 0.19% to 0.21% by mass. 3+ The introduction of Fe maintains a local reversible defect environment, promoting the synergy between electron conduction and lithium-ion diffusion. During charge and discharge, Fe... 3+ By adjusting the valence state balance and charge compensation behavior inside the material, a balanced charge transfer path is formed, thereby improving the discharge efficiency under both room temperature and low temperature conditions.

[0115] In some embodiments of the present invention, the sphericity of the lithium iron phosphate cathode active material is 0.85~0.90.

[0116] In some embodiments, the sphericity testing method includes the following steps: acquiring precise three-dimensional morphology data of lithium iron phosphate particles using high-resolution three-dimensional imaging technology. Then, reconstructing the particle surface model using three-dimensional image processing software and calculating its minimum circumscribed sphere radius R. Next, analyzing the particle surface curvature, automatically identifying the sharpest region (i.e., the acuteest angle) with the largest curvature, fitting a local surface at that location, and calculating its radius of curvature ri. Finally, calculating the sphericity of a single particle according to the formula.

[0117] Sphericity = ri / R, where ri is the radius of curvature of the acute angle of the particle and R is the radius of the smallest circumscribed sphere.

[0118] Among them, the radius of curvature of the acuteest angle of a particle refers to the radius of curvature corresponding to the sharpest angle among the protruding edges on the particle's contour surface. The smallest circumscribed sphere refers to the smallest sphere that just encloses the actual contour of the object being measured.

[0119] For batteries prepared using the lithium iron phosphate cathode active material provided by this invention, the batteries can be disassembled according to the method described above to obtain the lithium iron phosphate cathode active material, which can then be tested using the same method. It should be noted that the results of directly testing the lithium iron phosphate cathode active material or testing the lithium iron phosphate cathode active material obtained from battery disassembly are similar and within the allowable error range.

[0120] In detail, the sphericity of lithium iron phosphate cathode active materials includes, but is not limited to, a range of 0.85, 0.86, 0.87, 0.88, 0.89, 0.90, or any two of these ranges.

[0121] When the sphericity is controlled at 0.85~0.9, it means that the shape of the lithium iron phosphate cathode active material is closer to a regular sphere, the contact between particles is more uniform, and it is easier to rearrange and fill the gaps during the compaction or pressing process, thereby improving the compaction density of the lithium iron phosphate cathode active material.

[0122] In some embodiments, the tap density of the lithium iron phosphate cathode active material is 1.3 g / cm³. 3 ~1.4g / cm 3 .

[0123] In this invention, the tap density is determined in accordance with the method specified in the national standard GB / T 5162-2021 "Determination of Tap Density of Metal Powders".

[0124] For batteries prepared using the lithium iron phosphate cathode active material provided by this invention, the batteries can be disassembled according to the method described above to obtain the lithium iron phosphate cathode active material, which can then be tested using the same method. It should be noted that the results of directly testing the lithium iron phosphate cathode active material or testing the lithium iron phosphate cathode active material obtained from battery disassembly are similar and within the allowable error range.

[0125] Specifically, the tap density of lithium iron phosphate cathode active materials includes, but is not limited to, 1.30 g / cm³. 3 1.32g / cm 3 1.34 g / cm 3 1.36 g / cm 3 1.38g / cm 3 1.40g / cm 3 Or a range consisting of any two of them.

[0126] When the tap density is controlled within the above range, it indicates that the lithium iron phosphate cathode active material can form a relatively compact stacked structure under the action of external force, which is beneficial to increase the amount of active material loaded per unit volume of the electrode and reduce the non-uniformity of electrode pore distribution.

[0127] In some embodiments, the specific surface area of ​​the lithium iron phosphate cathode active material is 15 m². 2 / g ~16m 2 / g.

[0128] The specific surface area of ​​lithium iron phosphate cathode active materials can be detected using common detection methods in this field, such as according to GB / T19587-2017 "Determination of specific surface area of ​​solid materials by gas adsorption BET method".

[0129] For batteries prepared using the lithium iron phosphate cathode active material provided by this invention, the batteries can be disassembled according to the method described above to obtain the lithium iron phosphate cathode active material, which can then be tested using the same method. It should be noted that the results of directly testing the lithium iron phosphate cathode active material or testing the lithium iron phosphate cathode active material obtained from battery disassembly are similar and within the allowable error range.

[0130] Specifically, the specific surface area of ​​the lithium iron phosphate cathode active material includes, but is not limited to, 15.0 m². 2 / g, 15.2m 2 / g, 15.4m 2 / g, 15.6m 2 / g, 15.8m 2 / g, 16.0m 2 / g or a range consisting of any two of them.

[0131] When the specific surface area is controlled within the above range, it is possible to maintain a sufficient electrolyte wetting area and lithium-ion exchange interface while avoiding an increase in side reactions and an increase in interface impedance caused by an excessively large specific surface area.

[0132] Lithium iron phosphate cathode active materials can individually meet the above-mentioned sphericity, tap density, and specific surface area, or they can simultaneously meet the above-mentioned sphericity, tap density, and specific surface area.

[0133] In some embodiments, the lithium iron phosphate cathode active material simultaneously satisfies the aforementioned sphericity, tap density, and specific surface area. In this case, the low-friction stacking advantage brought by sphericity and the rapid wetting advantage brought by the moderate specific surface area work together to shorten the migration and intercalation / deintercalation path of lithium ions on the particle surface, reduce polarization, and improve rate performance. Meanwhile, the higher tap density and moderate specific surface area help form a dense and uniform porous structure, promoting sufficient electrolyte wetting and interface stability, thereby improving discharge consistency and cycle stability under both room temperature and low temperature conditions.

[0134] In some embodiments, it includes a lithium iron phosphate substrate and a carbon coating layer covering at least a portion of the surface of the lithium iron phosphate substrate.

[0135] The lithium iron phosphate matrix provides the host crystalline phase for lithium-ion intercalation and deintercalation, while the carbon coating layer disposed on at least part of its surface constitutes a continuous or semi-continuous conductive outer layer to improve interparticle electron transport and reduce interfacial contact resistance.

[0136] When a carbon coating covers all or part of the substrate surface, it can compensate for the low intrinsic electronic conductivity of lithium iron phosphate without significantly sacrificing the material's compaction density, thereby improving rate discharge capability and low-temperature output capability. On the one hand, the carbon coating forms a high-speed electron channel on the particle surface, shortening the charge transfer path; on the other hand, it provides physical isolation and protection for the substrate, reducing the side reaction erosion of the surface by the electrolyte and inhibiting the propagation of microcracks and pulverization of particles during cycling.

[0137] The present invention does not limit the source of the carbon coating layer. For example, the carbon coating layer can be formed by introducing carbon-containing precursors such as glucose and PEG into the precursor and carbonizing them in situ during the calcination process, or it can be prepared by post-coating, chemical vapor deposition, liquid phase deposition or heat treatment after spraying carbon source.

[0138] When the lithium iron phosphate matrix works synergistically with at least part of the surface carbon coating layer, the lithium iron phosphate cathode active material can not only maintain a high content of effective active material and good compaction performance, but also reduce polarization resistance, improve charge transfer efficiency, and alleviate the problem of limited ion and electron transport at low temperatures with the help of the carbon coating layer. As a result, the material can obtain better rate performance, low temperature discharge performance and cycle stability in scenarios such as power batteries, energy storage batteries and power tool batteries.

[0139] In some embodiments, the thickness of the carbon coating layer is 2nm-3nm.

[0140] The thickness of the carbon coating can be detected using methods commonly used in the field, such as transmission electron microscopy. For example, a cross-section of the sample particles can be placed on a TEM stage, and the thickness distribution of the carbon coating can be observed through high-resolution imaging, followed by statistical averaging using image analysis software.

[0141] In some embodiments, the thickness of the carbon coating layer is determined by a method including the following steps:

[0142] The carbon layer thickness was measured using high-resolution transmission electron microscopy (HRTEM). Lithium iron phosphate cathode active material powder samples were ultrasonically dispersed in ethanol, then dropped onto an ultrathin carbon support film copper grid. After drying, the samples were observed in HRTEM. Under a suitable accelerating voltage (e.g., 200 kV), thin regions with uniform particle dispersion, clear particle edges, visible lattice fringes, and intact coating layers were selected for high-magnification imaging. The width of the light-dark contrast transition region between the carbon layer and the lithium iron phosphate cathode active material matrix lattice at the edges of more than 200 random particles in 10 regions was measured. Precise calibration and measurement were performed using the instrument's attached scale or digital analysis software, and the average carbon coating layer thickness was obtained.

[0143] For batteries prepared using the lithium iron phosphate cathode active material provided by this invention, the batteries can be disassembled according to the method described above to obtain the lithium iron phosphate cathode active material, which can then be tested using the same method. It should be noted that the results of directly testing the lithium iron phosphate cathode active material or testing the lithium iron phosphate cathode active material obtained from battery disassembly are similar and within the allowable error range.

[0144] In detail, the thickness of the carbon coating includes, but is not limited to, a range of 2 nm, 2.2 nm, 2.4 nm, 2.6 nm, 2.8 nm, 3.0 nm, or any two of these.

[0145] By controlling the thickness of the carbon coating layer to 2-3 nm, a continuous and uniform conductive film can be formed, which can cover the particle surface and improve electron conduction, while also ensuring the proportion of active material and lithium ion diffusion, thus balancing compaction density and rate performance.

[0146] In some embodiments, based on lithium iron phosphate cathode active material, the carbon coating layer has a mass percentage content of 1.35% to 1.40%.

[0147] The mass percentage of the carbon coating can be determined using common detection methods in this field, such as thermogravimetric analysis.

[0148] In some embodiments of the present invention, the mass percentage of the carbon coating is detected by a method comprising the following steps: A suitable amount (e.g., 10-20 mg) of dried lithium iron phosphate cathode active material sample is weighed using a precision balance and placed in an alumina crucible. The sample is then heated from room temperature to a high temperature (e.g., 800 °C) at a constant heating rate (e.g., 10 °C / min) using high-purity air or oxygen (typically at a flow rate of 50 mL / min) as the atmosphere in a simultaneous thermal analyzer. During this process, the surface carbon coating is completely oxidized to CO2, while the LFP matrix remains stable in air.

[0149] For batteries prepared using the lithium iron phosphate cathode active material provided by this invention, the batteries can be disassembled according to the method described above to obtain the lithium iron phosphate cathode active material, which can then be tested using the same method. It should be noted that the results of directly testing the lithium iron phosphate cathode active material or testing the lithium iron phosphate cathode active material obtained from battery disassembly are similar and within the allowable error range.

[0150] In detail, based on lithium iron phosphate cathode active material, the mass percentage content of the carbon coating layer includes, but is not limited to, 1.35%, 1.36%, 1.37%, 1.38%, 1.39%, 1.40%, or any combination thereof.

[0151] This mass percentage corresponds to a thinner and denser surface conductive layer, which can maintain sufficient electronic connectivity with less introduction of inactive carbon, ensuring the material's volumetric energy density. An appropriate amount of carbon coating forms a stable conductive network on the surface of the lithium iron phosphate substrate, reducing the contact resistance and polarization resistance of the lithium iron phosphate cathode active material, allowing the electrode to maintain good electrochemical activity under high-rate charge-discharge and low-temperature conditions.

[0152] During the preparation process, the thickness and mass ratio of the carbon coating layer can be controlled by adjusting the amount of carbon source added, calcination temperature, holding time, atmosphere flow rate, or post-treatment conditions, so that it falls within the above range.

[0153] Secondly, the present invention provides a method for preparing the above-mentioned lithium iron phosphate cathode active material, comprising the following steps:

[0154] A mixed system of iron, lithium, and phosphorus sources was provided to prepare a lithium iron phosphate cathode active material. The lithium iron phosphate cathode active material includes a lithium iron phosphate matrix, and the number of crystal defects in the lithium iron phosphate matrix is ​​1.4000 × 10⁻⁶. 16 spins / g ~2.0000×10 16 spins / g.

[0155] This invention does not limit the method for preparing lithium iron phosphate cathode active materials that meet the above characteristics. Common methods in the field can be used to prepare lithium iron phosphate cathode active materials using a mixed system.

[0156] For example, in some embodiments, a high-temperature solid-state method can be used to grind and sinter a mixed system including an iron source, a lithium source, and a phosphorus source. By controlling the process parameters such as grinding and sintering, a lithium iron phosphate matrix that meets the above-mentioned number of crystal defects can be prepared, thereby obtaining a lithium iron phosphate cathode active material.

[0157] In other embodiments, a liquid-phase method can be used to co-precipitate a mixed system including an iron source, a lithium source, and a phosphorus source. By controlling parameters such as pH value, feeding rate, and stirring intensity during the co-precipitation process, a lithium iron phosphate matrix that meets the above-mentioned number of crystal defects can be prepared, thereby obtaining a lithium iron phosphate cathode active material.

[0158] Among these, iron, lithium, and phosphorus sources commonly used in this field can be selected according to the actual situation. For example, the iron source may include at least one of ferric phosphate, ferrous oxalate, ferric nitrate, and ferrous sulfate; the lithium source may include at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium nitrate, and lithium acetate; and the phosphorus source may include at least one of ferric phosphate, phosphoric acid, ammonium dihydrogen phosphate, and lithium dihydrogen phosphate.

[0159] In some embodiments, the mixed system further includes a carbon source, such as at least one selected from glucose, polyethylene glycol, sucrose, citric acid, and carbon nanotubes. The carbon source can act as a reducing agent, participating in the reaction and ensuring that various elements in the lithium iron phosphate cathode active material exist in the target valence state. On the other hand, it can serve as a source of the carbon coating layer in the lithium iron phosphate cathode active material, improving the electronic conductivity of the lithium iron phosphate cathode active material.

[0160] The preparation method provided by this invention can prepare a lithium iron phosphate matrix with a specific range of crystal defects, thereby obtaining a lithium iron phosphate cathode active material. This lithium iron phosphate cathode active material has a specific number of crystal defects, and while taking into account compaction characteristics, it can still maintain good power performance at both room temperature and low temperature.

[0161] In some embodiments of the present invention, the mixed system is processed to obtain an intermediate, and the intermediate is sintered to obtain a lithium iron phosphate cathode active material, wherein the preparation method satisfies at least one of the following conditions:

[0162] (1) The method of processing the mixed system to obtain the intermediate includes grinding the mixed system to obtain the intermediate, preferably, the grinding method includes sand milling;

[0163] (2) The sintering process includes a first sintering process and a second sintering process in sequence. Preferably, the highest temperature of the first sintering process is 760℃-790℃.

[0164] Preferably, the holding time at the highest temperature during the first sintering treatment is 8-12 hours.

[0165] Preferably, the heating rate of the first sintering treatment is 2.5℃ / min ~ 5℃ / min;

[0166] Preferably, the temperature of the second sintering treatment is 700℃~750℃;

[0167] Preferably, the holding time for the second sintering treatment is 5-6 hours;

[0168] Preferably, the heating rate of the second sintering treatment is 2.5℃ / min ~ 5℃ / min;

[0169] Preferably, a cooling process is further included between the first sintering process and the second sintering process;

[0170] Preferably, a cooling process is further included between the first sintering process and the second sintering process, and the target temperature of the cooling process is 90℃-100℃; preferably, a cooling process is further included between the first sintering process and the second sintering process, and the atmosphere of the cooling process is an oxygen-containing atmosphere.

[0171] Preferably, a cooling process is further included between the first sintering process and the second sintering process. The atmosphere for the cooling process is an oxygen-containing atmosphere, and the flow rate of the oxygen-containing atmosphere for the cooling process is 0.6 L / min ~ 1.0 L / min.

[0172] Preferably, a cooling process is included between the first sintering process and the second sintering process, and the holding time for the cooling process is 40 min to 80 min;

[0173] (3) The method before sintering further includes: drying the intermediate, preferably by spraying.

[0174] (4) Preferably, the Dv50 of the intermediate is 370nm~390nm.

[0175] In detail, the mixed system is processed to obtain an intermediate, and then the intermediate is sintered to obtain the lithium iron phosphate cathode active material.

[0176] First, a stoichiometric mixture of iron, lithium, and phosphorus sources is prepared. Additional sources such as carbon and dopant can be added as needed. The mixture is then ground and stirred with a solvent to obtain a homogeneous slurry or solution. The mixture is then processed to obtain an intermediate. Finally, the intermediate is sintered to obtain the lithium iron phosphate cathode active material.

[0177] Intermediates can be obtained using common processing methods in this field, such as hydrothermal reaction, magnetic field-induced directional treatment, pH-controlled precipitation, and stepwise grinding and granulation.

[0178] To avoid impurities from adversely affecting the sintering process, the sintering atmosphere in this invention is an inert atmosphere, such as nitrogen or argon.

[0179] For condition (1), the method of processing the mixed system to obtain the intermediate includes grinding the mixed system to obtain the intermediate. Preferably, the grinding method includes sand milling.

[0180] It is understandable that appropriate grinding parameters can be selected according to actual needs, such as the particle size of the grinding media and the grinding time.

[0181] Compared to traditional ball milling and other grinding methods, sand milling can refine the particle size of mixed raw materials to submicron or even nanometer scale in a shorter time. It can also precisely control the particle size range and make the product particle size distribution narrower. At the same time, it can achieve uniform mixing of raw materials at the molecular level through physical shearing, creating a larger contact area and a shorter atomic diffusion path for subsequent sintering reactions. This significantly improves the efficiency and uniformity of solid-phase reactions, and enhances the batch stability, compaction density and electrochemical performance of lithium iron phosphate cathode active materials.

[0182] For condition (2), the sintering process includes a first sintering process and a second sintering process in sequence. Preferably, the highest temperature of the first sintering process is 760℃-790℃.

[0183] The temperature of the first sintering treatment is 760-790℃. For example, the highest temperature of the first sintering treatment includes, but is not limited to, 760℃, 765℃, 770℃, 775℃, 780℃, 785℃, 790℃, or any combination thereof.

[0184] When the highest temperature of the first sintering treatment is within the above range, it can provide sufficient driving force for solid-phase diffusion and crystal phase transformation, making the raw material reaction more complete, reducing the content of residual impurity phases, and establishing a stable main lattice basis for subsequent defect regulation.

[0185] Preferably, the holding time of the first sintering treatment at the highest temperature is 8h-12h.

[0186] In detail, the holding time for the first sintering treatment includes, but is not limited to, a range of 8h, 9h, 10h, 11h, 12h, or any two of these.

[0187] The above time range ensures that the first sintering process is carried out sufficiently.

[0188] Preferably, the heating rate of the first sintering treatment is 2.5℃ / min to 5℃ / min.

[0189] In detail, the heating rate of the first sintering treatment includes, but is not limited to, a range of 2.5℃ / min, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, or any combination thereof.

[0190] When the heating rate of the first sintering process is within the above range, various raw materials can be heated evenly and gently in the early stage of sintering, thereby ensuring the integrity of the product structure, improving the density and mechanical properties of the sintered body, and laying a good microstructure foundation for subsequent process steps.

[0191] Preferably, the temperature of the second sintering treatment is 700℃~750℃.

[0192] In detail, the temperature of the second sintering treatment includes, but is not limited to, 700°C, 710°C, 720°C, 730°C, 740°C, 750°C, or any combination thereof.

[0193] The second sintering process, while ensuring further structural stability, suppresses over-sintering and excessive grain coarsening, making the edges of the lithium iron phosphate cathode active material more rounded, and forming a certain degree of overlapping connection between particles rather than a completely isolated and dispersed state. This is beneficial for reducing ineffective porosity and increasing compaction density during stacking.

[0194] Preferably, the holding time for the second sintering treatment is 5-6 hours.

[0195] In detail, the holding time for the second sintering treatment includes, but is not limited to, a range of 5h, 5.2h, 5.4h, 5.6h, 5.8h, 6h, or any two of these.

[0196] When the holding time of the second sintering process is within the above range, it is sufficient to allow atoms to diffuse fully, promote the full growth of lithium iron phosphate crystals, improve the crystallinity of the grains, optimize the stability of the crystal structure, and effectively improve the purity of the material, thus helping to improve the compaction density and cycle stability of the finished lithium iron phosphate cathode active material.

[0197] Preferably, the heating rate of the second sintering treatment is 2.5℃ / min to 5℃ / min.

[0198] In detail, the heating rate of the first sintering treatment includes, but is not limited to, a range of 2.5℃ / min, 3℃ / min, 3.5℃ / min, 4℃ / min, 4.5℃ / min, 5℃ / min, or any combination thereof.

[0199] Using a heating rate of 2.5℃ / min to 5℃ / min allows the particles to be heated evenly, enabling the atoms to diffuse and grow steadily. This ensures the crystallinity of lithium iron phosphate, optimizes the stability of the crystal structure, and controls the grain growth rate to obtain a suitable particle size distribution.

[0200] Preferably, a cooling process is further included between the first sintering process and the second sintering process.

[0201] During the first sintering process, volatile gases such as carbon monoxide, carbon dioxide, and water vapor generated from the decomposition of raw materials in the mixed system are discharged. Cooling after the first sintering process stabilizes the primary lithium iron phosphate grain structure generated by pre-sintering. Simultaneously, the cooled material is easier to crush and disperse, resulting in more uniform powder particles. This provides a foundation for the subsequent second sintering process, ultimately improving the compaction density, rate performance, and cycle stability of lithium iron phosphate.

[0202] Preferably, a cooling process is further included between the first sintering process and the second sintering process, and the target temperature of the cooling process is 90℃-100℃;

[0203] For example, the target temperature for the cooling process includes, but is not limited to, 90°C, 92°C, 94°C, 96°C, 98°C, 100°C, or any combination thereof.

[0204] By cooling the crystal under the conditions described above, controllable lithium vacancies and iron antisite defects can be introduced to optimize the crystal defect distribution.

[0205] Preferably, a cooling process is further included between the first sintering process and the second sintering process, and the atmosphere for the cooling process is an oxygen-containing atmosphere.

[0206] In this invention, "oxygen-containing atmosphere" refers to an atmosphere that includes oxygen, such as an oxygen atmosphere, an air atmosphere, or other oxygen-containing atmospheres.

[0207] Oxygen vacancies generated by oxygen-containing materials in an oxygen-containing atmosphere can stabilize the crystal structure and further control the number of crystal defects.

[0208] Preferably, a cooling process is further included between the first sintering process and the second sintering process. The atmosphere for the cooling process is an oxygen-containing atmosphere, and the flow rate of the oxygen-containing atmosphere for the cooling process is 0.6 L / min ~ 1.0 L / min.

[0209] In detail, the flow rate of the oxygen-containing atmosphere includes, but is not limited to, 0.6 L / min, 0.65 L / min, 0.7 L / min, 0.75 L / min, 0.8 L / min, 0.85 L / min, 0.9 L / min, 0.95 L / min, 1.0 L / min, or any combination thereof.

[0210] By controlling the flow rate of the oxygen-containing atmosphere during the cooling process within the range of 0.6 L / min to 1.0 L / min, sufficient oxygen is ensured to compensate for potential oxygen vacancies in the material and stabilize the crystal structure. This guarantees the uniformity and controllability of the cooling process, which is beneficial for obtaining lithium iron phosphate cathode active materials with consistent structure and stable performance.

[0211] Preferably, a cooling process is included between the first sintering process and the second sintering process, and the cooling process is held at the target temperature for 40 min to 80 min.

[0212] In detail, the holding time at the target temperature during the cooling process includes, but is not limited to, a range of 40 min, 45 min, 50 min, 55 min, 60 min, 65 min, 70 min, 75 min, 80 min, or any combination thereof.

[0213] When the holding time for the cooling process is within the above range, it can ensure that the temperature of the intermediate material at different locations drops uniformly to the target temperature after the first sintering process, and also allow the crystal structure of the intermediate to fully relax and stabilize.

[0214] In response to condition (3), the method further includes drying the intermediate before sintering, preferably by spraying.

[0215] In this process, spray treatment, by controlling the feed rate and drying temperature, causes the intermediate particles to form a rounded shape during the droplet evaporation process, thus providing a basis for dense packing in subsequent sintering. Appropriate spray treatment conditions, such as inlet temperature and outlet temperature, can be selected according to actual needs.

[0216] For condition (4), the Dv50 of the intermediate is 370nm~390nm.

[0217] It is understandable that the Dv50 of the intermediate can be made to be 370nm~390nm by controlling the parameters of the sand milling process, such as the sand milling time and the diameter of the zirconium balls.

[0218] In detail, the Dv50 of the intermediate includes, but is not limited to, a range of 370nm, 375nm, 380nm, 385nm, 390nm, or any two of these.

[0219] When the Dv50 of the intermediate is within the above range, it first ensures that the intermediate slurry has excellent uniformity and stability; moreover, the intermediate particles within this specific size range can achieve more controllable crystal growth kinetics through surface energy drive during the subsequent sintering process, ultimately resulting in the lithium iron phosphate cathode active material having a suitable number of crystal defects and particle size distribution.

[0220] In some embodiments, the molar ratio of Fe to P in ferric phosphate is (0.965-0.972):1.

[0221] In detail, the molar ratio of Fe to P in ferric phosphate includes, but is not limited to, 0.965:1, 0.966:1, 0.967:1, 0.968:1, 0.969:1, 0.970:1, 0.971:1, 0.972:1, or any combination thereof.

[0222] A suitable iron-to-phosphorus ratio can promote the full growth of lithium iron phosphate particles, enabling them to reach the target compaction density; at the same time, this ratio can also ensure that the material obtains excellent electrochemical performance and meets design requirements.

[0223] In another aspect, the present invention provides a positive electrode sheet comprising the above-mentioned lithium iron phosphate positive electrode active material, or the lithium iron phosphate positive electrode active material prepared by the above-mentioned preparation method.

[0224] By configuring the aforementioned lithium iron phosphate cathode active material into the cathode system of lithium-ion batteries, the material's controlled crystal defect count structure can be directly inherited. This allows the active particles in the cathode to maintain good lithium-ion diffusion and electron transport synergy even under high compaction conditions, thereby improving the active material loading level per unit volume. Simultaneously, the moderately controlled crystal defects help balance structural stability and transport efficiency, enabling the cathode to maintain good discharge response under both room temperature and low temperature conditions. This improves rate performance and low-temperature capacity retention, thus better meeting the comprehensive requirements of power batteries, energy storage batteries, and high-power devices for energy density, safety, and cycle stability.

[0225] Specifically, the positive electrode sheet provided by the present invention includes a positive current collector and a positive active layer disposed on at least one side of the positive current collector, wherein the positive active layer includes lithium iron phosphate positive active material.

[0226] In the specific preparation of the positive electrode sheet, for example, the lithium iron phosphate positive electrode active material of the present invention can be dispersed with a conductive agent and a binder in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and thoroughly stirred and mixed to form a uniform positive electrode slurry; the positive electrode slurry is uniformly coated on the positive electrode current collector, and after drying, rolling and cutting, the positive electrode sheet is obtained.

[0227] In one specific embodiment, the positive electrode active layer comprises, by weight percentage, 70-99 wt% lithium iron phosphate positive electrode active material, 0.5-15 wt% conductive agent, and 0.5-15 wt% binder; more specifically, it comprises 80-98 wt% lithium iron phosphate positive electrode active material, 1-10 wt% conductive agent, and 1-10 wt% binder.

[0228] The positive current collector can be made of at least one of aluminum foil or nickel foil; the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber; and the binder can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0229] In another aspect, the present invention provides a battery comprising the above-described positive electrode sheet.

[0230] When this battery uses the aforementioned positive electrode sheet, the positive electrode active material contained within it can maintain good particle packing and ion transport channel matching under high compaction conditions. This results in the battery possessing both high electrode packing efficiency and stable electrochemical reaction activity. Because the positive electrode sheet itself can balance compaction density, rate performance, and low-temperature discharge performance, the assembled battery can maintain good capacity output and power response in scenarios such as power batteries, energy storage batteries, and power tool batteries. This allows the battery to maintain stable discharge performance under high load and low-temperature conditions, thereby improving cycle stability and application reliability.

[0231] It is conceivable that, in addition to the aforementioned positive electrode, the battery of the present invention also includes a negative electrode, an electrolyte, and a separator.

[0232] This invention does not strictly limit the negative electrode active material in the negative electrode sheet. It can be at least one of the negative electrode active materials commonly used in batteries, such as graphite, hard carbon, soft carbon, mesophase carbon microspheres, silicon-based negative electrode materials (mainly including silicon suboxide and silicon-carbon negative electrode), and tin-based negative electrode materials (mainly including tin and tin alloy).

[0233] This invention does not strictly limit the choice of electrolyte, and may include one or more solvents commonly used in current battery electrolytes, as well as lithium salts commonly used in current electrolytes. For example, the solvent may be ethylene carbonate, propylene carbonate, butene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, γ-butyrolactone, etc.; the lithium salt may be one or more of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), and lithium bis(trifluoromethanesulfonyl)imide (LiTFSI).

[0234] This invention does not strictly limit the choice of separator material. It can be one of the separator materials commonly used in batteries, such as polypropylene separator (PP), polyethylene separator (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun separator (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven separator, and separator with ceramic coating.

[0235] In battery manufacturing, the positive electrode, separator, and negative electrode are wound or stacked to obtain a bare cell, which is then packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery is dried at 85°C, the electrolyte is injected into the dried battery. The battery undergoes resting, formation, and secondary sealing to complete battery manufacturing.

[0236] In another aspect, the present invention provides a battery pack comprising at least two of the aforementioned batteries.

[0237] Because the battery has high compaction density, good rate performance and low temperature discharge performance, the battery pack can maintain stable power output and good low temperature adaptability while meeting the energy integration requirements. Therefore, it is suitable for new energy vehicles, energy storage systems and power consumption scenarios with high requirements for safety and cycle life. Thus, it helps to improve the overall energy utilization efficiency, operational stability and application reliability of the battery pack.

[0238] Generally, a battery pack includes multiple batteries as individual cells, which are connected to form the battery pack. These batteries can be electrically connected using methods conventional in the art, such as series connection, parallel connection, or a combination of these connection methods, without any particular limitation.

[0239] The present invention also provides an electrical device, including the battery or battery pack described above.

[0240] Because the number of crystal defects in the positive electrode active material is controlled at 1×10 16spins / g up to 2×10 16 Within the spins / g range, structural stability and lithium-ion migration and electronic conduction performance can be balanced, so that the battery can maintain good rate discharge capability and low-temperature discharge performance at high compaction density, thereby improving the energy density, power output and low-temperature performance of electrical equipment.

[0241] The electrical equipment used in the embodiments of the present invention can be conventional electrical equipment in the art, such as power equipment (e.g., electric vehicles, electric cars), electronic equipment (e.g., mobile phones, tablets, laptops, digital cameras, etc.), wearable devices (e.g., watches, bracelets, VR glasses, etc.), energy storage power stations, etc., and there are no particular limitations on this.

[0242] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0243] Example 1

[0244] The method for preparing lithium iron phosphate cathode active material provided in this embodiment includes the following steps:

[0245] (1) Iron phosphate (Fe / P = 0.970:1), lithium carbonate, glucose and PEG were mixed in a mass ratio of 4:1:0.123:0.369 to obtain a mixed system. The mixed system was then subjected to sand milling to obtain an intermediate with a particle size Dv50 of 380 nm.

[0246] (2) The intermediate was sprayed to obtain a light yellow uniform powder. The powder was sintered at 770°C for 10 h under nitrogen protection in a tube furnace at a heating rate of 3.5°C / min. Then, it was cooled to a target temperature of 100°C and kept warm with air for 1 h at a flow rate of 0.8 L / min. Finally, under nitrogen protection, the temperature was raised to 730°C for a second sintering treatment for 5.5 h at a heating rate of 3.5°C / min to obtain lithium iron phosphate cathode active material.

[0247] Example 2

[0248] The method for preparing lithium iron phosphate cathode active material provided in this embodiment includes the following steps:

[0249] (1) Iron phosphate (Fe / P = 0.965:1), lithium carbonate, glucose and PEG were mixed in a mass ratio of 4:1:0.123:0.369 to obtain a mixed system. The mixed system was then subjected to sand milling to obtain an intermediate with a particle size Dv50 of 370 nm.

[0250] (2) The intermediate was sprayed to obtain a light yellow uniform powder. The powder was sintered at 760°C for 12 h under nitrogen protection in a tube furnace at a heating rate of 2.5°C / min. Then, it was cooled to a target temperature of 95°C and held at 0.6 L / min with air. Finally, it was heated to 700°C for 6 h under nitrogen protection at a heating rate of 2.5°C / min to obtain lithium iron phosphate cathode active material.

[0251] Example 3

[0252] The method for preparing lithium iron phosphate cathode active material provided in this embodiment includes the following steps:

[0253] (1) Iron phosphate (Fe / P = 0.972:1), lithium carbonate, glucose and PEG were mixed in a mass ratio of 4:1:0.123:0.369 to obtain a mixed system. The mixed system was then subjected to sand milling to obtain an intermediate with a particle size Dv50 of 390 nm.

[0254] (2) The intermediate was sprayed to obtain a light yellow uniform powder. The powder was sintered at 790°C for 8 hours under nitrogen protection in a tube furnace at a heating rate of 5°C / min. Then, it was cooled to a target temperature of 90°C and held at 1.0 L / min for 40 minutes. Finally, it was heated to 750°C for 5 hours under nitrogen protection to obtain lithium iron phosphate cathode active material.

[0255] Example 4

[0256] The preparation method of the lithium iron phosphate cathode active material provided in this embodiment is basically the same as that in Example 1, except that:

[0257] In step (2), the first sintering temperature is 765℃.

[0258] Example 5

[0259] The preparation method of the lithium iron phosphate cathode active material provided in this embodiment is basically the same as that in Example 1, except that:

[0260] In step (2), the first sintering temperature is 775℃.

[0261] Example 6

[0262] The preparation method of the lithium iron phosphate cathode active material provided in this embodiment is basically the same as that in Example 1, except that:

[0263] In step (1), the Dv50 of the intermediate is 350nm.

[0264] Example 7

[0265] The preparation method of the lithium iron phosphate cathode active material provided in this embodiment is basically the same as that in Example 1, except that:

[0266] In step (2), the heating rate of the first sintering treatment is 2℃ / min.

[0267] Example 8

[0268] The preparation method of the lithium iron phosphate cathode active material provided in this embodiment is basically the same as that in Example 1, except that:

[0269] In step (2), the air flow rate during the cooling process is 0.5 L / min.

[0270] Example 9

[0271] The preparation method of the lithium iron phosphate cathode active material provided in this embodiment is basically the same as that in Example 1, except that:

[0272] In step (2), the heating rate of the second sintering treatment is 5.5℃ / min.

[0273] Example 10

[0274] The preparation method of the lithium iron phosphate cathode active material provided in this embodiment is basically the same as that in Example 1, except that:

[0275] In step (1), the Fe / P ratio in ferric phosphate is 0.982:1.

[0276] Comparative Example 1

[0277] The preparation method of the lithium iron phosphate cathode active material provided in this comparative example includes the following steps:

[0278] (1) Iron phosphate (Fe / P = 0.970), lithium carbonate, glucose and PEG were mixed in a mass ratio of 4:1:0.123:0.369 to obtain a mixed system. The mixed system was then subjected to sand milling to obtain an intermediate with a particle size Dv50 of 380 nm.

[0279] (2) The intermediate was sprayed to obtain a light yellow uniform powder. The powder was sintered at 770°C for 8 h in a tube furnace under nitrogen protection at a heating rate of 2.5°C / min to obtain lithium iron phosphate positive electrode active material.

[0280] Comparative Example 2

[0281] The preparation method of the lithium iron phosphate cathode active material provided in this comparative example is basically the same as that in Example 1, except that:

[0282] In step (1), ferric phosphate is replaced with ferrous oxalate and lithium carbonate is replaced with lithium hydroxide.

[0283] Comparative Example 3

[0284] The preparation method of the lithium iron phosphate cathode active material provided in this comparative example is basically the same as that in Example 1, except that:

[0285] In step (2), the temperature of the first sintering treatment is 750℃ and the time is 15h.

[0286] Comparative Example 4

[0287] The preparation method of the lithium iron phosphate cathode active material provided in this comparative example is basically the same as that in Example 1, except that:

[0288] In step (2), the cooling process is carried out under a nitrogen atmosphere, with a target temperature of 250°C.

[0289] Comparative Example 5

[0290] The preparation method of the lithium iron phosphate cathode active material provided in this comparative example is basically the same as that in Example 1, except that:

[0291] In step (2), the temperature of the second sintering treatment is 760℃ and the time is 10h.

[0292] Test case

[0293] (a) Physical property testing

[0294] 1. Crystal Defect Count Detection

[0295] The number of crystal defects in the lithium iron phosphate cathode active materials provided in the examples and comparative examples was determined sequentially using electron paramagnetic resonance (EPR), including the following steps:

[0296] The sample to be tested is placed in an EPR spectrometer and tested under specific frequency (e.g., X-band, approximately 9.5 GHz) and low temperature (e.g., 77 K or lower). By analyzing the intensity of characteristic resonance signals generated by paramagnetic defect centers in the material (e.g., iron vacancies, lithium vacancies, or oxygen vacancies), and calibrating with a standard paramagnetic substance of known concentration (e.g., DPPH), the number of unpaired electron spins per unit mass of the sample can be quantitatively calculated, thus obtaining the number of crystal defects, with the unit being spins / g.

[0297] The test results are shown in Table 1.

[0298] in Figure 1The above are the EPR curves of the lithium iron phosphate cathode active materials provided in Example 1 and Comparative Example 1 of this invention. Figure 1 The number of crystal defects can be calculated using fitting; in Example 1, the number of crystal defects is 1.4268 * 10^6. 16 The number of spins / g is 3.6195*10, while the number of crystal defects in the lithium iron phosphate cathode active material provided in Comparative Example 1 is 3.6195*10. 16 As can be seen from the spins / g, by adjusting the sintering process in Example 1, the number of defects in the resulting lithium iron phosphate cathode material is significantly reduced, the crystal is more complete, and the atomic arrangement is more orderly, which is beneficial to the transport of lithium ions during the insertion and extraction process.

[0299] 2. Malvern particle size distribution detection

[0300] The lithium iron phosphate cathode active materials provided in the examples and comparative examples were tested sequentially using a Malvern particle size analyzer, including the following steps:

[0301] 1. Disperse the sample using a high-purity, low-conductivity organic dispersant. Precisely control the sample concentration within the instrument's recommended shading range. Employ a combination of physical and chemical dispersion methods to ensure proper depolymerization and stable suspension.

[0302] 2. The test must be performed in the "Wet Method" module of a laser particle size analyzer (such as the Malvern Mastersizer 3000). Select a red light (λ=632.8 nm) main light source and a blue light (λ=470 nm) auxiliary light source suitable for submicron to micron level measurements to enhance the detection sensitivity for small particles. Based on the characteristics of lithium iron phosphate materials, set the particle refractive index (RI) to approximately 1.7-1.8, the absorptivity to 0.1-0.3, and the dispersant refractive index (e.g., 1.36 for ethanol) to be accurately entered. Both the background measurement time and the sample measurement time should be no less than 10 seconds to ensure signal stability.

[0303] 3. Each sample must be measured at least three times independently, and the results should be averaged to ensure repeatability. In the obtained volumetric particle size distribution curve, use the "peak analysis" function of the instrument software (such as Mastersizer software) to identify and confirm two distinct distribution peaks. The software will automatically calculate the particle size distribution based on volume and provide the corresponding particle sizes Dv0, Dv5, Dv10, Dv50, Dv90, Dv100, peak apex size, and peak area percentage for each peak.

[0304] The test results are shown in Tables 1 and 2.

[0305] in, Figure 2 The Malvern particle size distribution curve of the lithium iron phosphate cathode active material provided in Example 1 of this invention is obtained through... Figure 2As can be seen, the lithium iron phosphate cathode active material provided in Example 1 has the following particle sizes: Dv5 = 0.316 μm, Dv10 = 0.367 μm, Dv50 = 1.07 μm, Dv90 = 3.17 μm, and Dv99 = 5.39 μm. The peak size of the first peak is 0.32 μm, and the peak size of the second peak is 2.51 μm. The peak area of ​​the first peak accounts for 63%, and the peak area of ​​the second peak accounts for 37%. The lithium iron phosphate provided in Example 1 has sufficiently small particles, ensuring the rate performance and low-temperature performance of the battery when lithium iron phosphate is used as a cathode material. At the same time, it contains a suitable proportion and size of large particles, providing a high-compaction framework.

[0306] Table 1

[0307]

[0308] Table 2

[0309]

[0310] 3. Fe 3+ Content detection

[0311] The Fe content in lithium iron phosphate cathode active materials was determined by X-ray photoelectron spectroscopy combined with chemical titration. 3+ The percentage content is determined by the following steps:

[0312] First, a thin film suitable for testing was prepared from the lithium iron phosphate cathode active material sample. The sample surface was scanned using an X-ray photoelectron diffractometer, and the valence state of iron was determined by analyzing the position and shape of the characteristic energy spectrum peaks. Then, a certain mass of the sample to be tested was accurately weighed, dissolved and diluted with a strong acid, and an appropriate amount of sample solution was taken. The content of ferrous iron in the system was first determined by titration with potassium dichromate standard solution. Then, an equal amount of sample was taken, potassium iodide was added, and the reaction was carried out in an acidic dark place. After ferric iron oxidized iodide ions to elemental iodine, the content of ferric iron was determined by titration with sodium thiosulfate standard solution. Finally, the mass percentage of ferric iron in the lithium iron phosphate cathode active material was calculated based on the total mass of the sample.

[0313] The test results are shown in Table 3.

[0314] 4. Scanning electron microscopy (SEM) examination

[0315] The lithium iron phosphate cathode active materials provided in the examples and comparative examples were tested sequentially using a scanning electron microscope, including the following steps:

[0316] A cross-sectional sample was prepared by embedding and polishing lithium iron phosphate cathode active material powder, and high-resolution backscattered / secondary electron images were captured using FE-SEM in multiple random fields of view.

[0317] The image is imported into image analysis software, and the particle outline is identified through threshold segmentation, binarization and other processing. After calibrating the size ratio, the equivalent circle diameter or maximum Feret diameter of the particles is measured in batches.

[0318] Collect particle size data from at least 500 particles, plot a quantity-particle size distribution histogram, and analyze to obtain the main distribution range and characteristic particle size parameters.

[0319] 5. Sphericity test

[0320] The sphericity of the lithium iron phosphate cathode active materials provided in the examples and comparative examples was tested sequentially.

[0321] High-resolution 3D imaging technology was used to acquire precise 3D morphology data of lithium iron phosphate particles. Subsequently, the particle surface model was reconstructed using 3D image processing software, and its minimum circumscribed sphere radius R was calculated. Next, the surface curvature of the particles was analyzed to identify the sharpest regions of maximum curvature (i.e., the acuteest angles), and a local surface was fitted at these locations to calculate its radius of curvature ri. Finally, the sphericity of individual particles was calculated using the formula.

[0322] Sphericity = ri / R, where ri is the radius of curvature of the acute angle of the particle and R is the radius of the smallest circumscribed sphere.

[0323] The test calculation results are shown in Table 3.

[0324] 6. Tap density

[0325] The lithium iron phosphate cathode active materials provided in the embodiments and comparative examples of the present invention were tested in sequence according to the methods specified in GB / T 5162-2021 "Determination of tap density of metal powder".

[0326] The test results are shown in Table 3.

[0327] 7. Specific surface area

[0328] The lithium iron phosphate cathode active materials provided in the embodiments and comparative examples of the present invention were tested in accordance with GB / T19587-2017 "Determination of specific surface area of ​​solid materials by gas adsorption BET method".

[0329] The test results are shown in Table 3.

[0330] 8. Carbon coating detection

[0331] The lithium iron phosphate cathode active materials provided in the embodiments and comparative examples of the present invention were tested sequentially, including the following steps:

[0332] Cross-sections or edges of the sample particles were observed using a high-resolution transmission electron microscope. On HR-TEM images, the carbon coating appears as an amorphous or less graphitized layer with contrast differences surrounding a crystalline lithium iron phosphate matrix.

[0333] Using image analysis software, the width of the dark or gray contrast layer was measured at multiple representative locations, and the statistical average value was taken as the thickness of the carbon coating layer.

[0334] In air, the sample was burned to completely oxidize the carbon coating to CO2 and remove it, while the lithium iron phosphate matrix remained essentially stable. The carbon content was calculated based on the mass loss before and after burning.

[0335] The test calculation results are shown in Table 3.

[0336] in, Figure 3 This is a TEM image of the lithium iron phosphate cathode active material provided in Example 1 of the present invention. Figure 3 As can be seen, the surface of the lithium iron phosphate cathode active material provided in Example 1 is covered with a carbon coating layer, the thickness of which is approximately 2.58 nm.

[0337] Table 3

[0338]

[0339] (II) Electrochemical performance testing

[0340] 1. Compacted density

[0341] The compaction density of the lithium iron phosphate cathode active materials provided in the examples and comparative examples was tested according to GB / T 44330-2024 "Determination of compaction density of lithium-ion battery cathode material powder". The test results are shown in Table 4.

[0342] 2. Discharge specific capacity and discharge efficiency

[0343] The lithium iron phosphate cathode active materials provided in the examples and comparative examples are sequentially assembled into coin cell half-cells, including the following steps:

[0344] The battery slurry was prepared using N-methylpyrrolidone as a solvent, polyvinylidene fluoride (PVDF) as the binder, and acetylene black as the conductive agent. The mass ratio of the various components in the slurry was: lithium iron phosphate positive electrode active material / binder / conductive agent = 90 / 5 / 5. The battery slurry was uniformly coated on aluminum foil and vacuum dried. In a glove box, the positive electrode sheets obtained after cutting and pressing were combined with lithium negative electrodes, lithium-ion battery commercial separators, electrolytes, and spacer spring assemblies to form coin cells.

[0345] Electrochemical testing of coin cells was conducted using Xinwei's coin cell test channel. Tests were performed in air at 25°C and at -20°C. The capacity test conditions were as follows: test window 2.5-4.3V, charging current 0.05C, 4.3V constant voltage charging cutoff current 0.01C, and discharging current 0.05C. The charging and discharging capacities were recorded.

[0346] Where discharge specific capacity = discharge capacity / mass of lithium iron phosphate cathode active material; and discharge efficiency = discharge capacity / charge capacity × 100%.

[0347] The test results are shown in Table 4.

[0348] in, Figure 4 The battery prepared using the lithium iron phosphate cathode active material provided in Example 1 of this invention has its first charge-discharge curve at 25°C. Figure 4 It can be seen that the initial charging capacity at 25℃ is approximately 161.98 mAh / g, and the discharge capacity is approximately 159.63 mAh / g. Figure 5 The discharge curve of the battery prepared using the lithium iron phosphate positive electrode active material provided in Example 1 of this invention at -20°C is shown in the figure. Figure 5 It can be seen that the discharge capacity at -20℃ is 73.66mAh / g and the discharge efficiency is 47.14%.

[0349] Table 4

[0350]

[0351] As can be seen from Table 4, the lithium iron phosphate cathode active material provided in the embodiments of the present invention can simultaneously improve the battery's compaction performance, room temperature rate discharge performance, and low temperature discharge performance when used in a battery.

[0352] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A lithium iron phosphate cathode active material, characterized in that, The lithium iron phosphate cathode active material comprises a lithium iron phosphate matrix, wherein the number of crystal defects in the lithium iron phosphate matrix is ​​1.4000 × 10⁻⁶. 16 spins / g ~2.0000×10 16 spins / g.

2. The lithium iron phosphate cathode active material according to claim 1, characterized in that, The lithium iron phosphate cathode active material includes a first particle and a second particle. The Malvern particle size distribution curve of the lithium iron phosphate cathode active material includes a first peak corresponding to the first particle and a second peak corresponding to the second particle. Wherein, the termination particle size of the first peak is the starting particle size of the second peak; Based on the sum of the peak areas of the first peak and the second peak, the peak area of ​​the first peak accounts for 50% to 70%; And / or, the peak area of ​​the second peak accounts for 30% to 50%.

3. The lithium iron phosphate cathode active material according to claim 2, characterized in that, The peak size of the first peak is 0.25 μm to 0.65 μm; And / or, the peak size of the second peak is 1.50 μm to 2.96 μm; And / or, the initial particle size of the first peak is 0.189 μm to 0.244 μm; And / or, the termination particle size of the first peak is 0.60 μm to 1.56 μm; And / or, the initial particle size of the second peak is 0.60 μm to 1.56 μm; And / or, the termination particle size of the second peak is 6.24 μm to 9.50 μm.

4. The lithium iron phosphate cathode active material according to any one of claims 1-3, characterized in that, The lithium iron phosphate cathode active material has the following diameters: Dv5 is 0.30μm - 0.37μm, Dv10 is 0.312μm - 0.480μm, Dv50 is 0.784μm - 1.600μm, Dv90 is 2.50μm - 4.10μm, and Dv99 is 4.95μm - 6.01μm.

5. The lithium iron phosphate cathode active material according to any one of claims 1-4, characterized in that, The lithium iron phosphate cathode active material includes Fe 3+ Based on the lithium iron phosphate cathode active material, the Fe 3+ The mass percentage content is 0.19%~0.21%.

6. The lithium iron phosphate cathode active material according to any one of claims 1-5, characterized in that, The sphericity of the lithium iron phosphate cathode active material is 0.85~0.90; And / or, the tap density of the lithium iron phosphate cathode active material is 1.3 g / cm³. 3 ~1.4g / cm 3 ; And / or, the specific surface area of ​​the lithium iron phosphate cathode active material is 15 m². 2 / g~16m 2 / g.

7. The lithium iron phosphate cathode active material according to any one of claims 1-6, characterized in that, The lithium iron phosphate cathode active material includes the lithium iron phosphate matrix and a carbon coating layer covering at least a portion of the surface of the lithium iron phosphate matrix.

8. The lithium iron phosphate cathode active material according to claim 7, characterized in that, The thickness of the carbon coating layer is 2nm~3nm; And / or, based on the lithium iron phosphate cathode active material, the carbon coating layer has a mass percentage content of 1.35% to 1.40%.

9. A method for preparing the lithium iron phosphate cathode active material according to any one of claims 1-8, characterized in that, Includes the following steps: A mixed system of iron, lithium, and phosphorus sources is provided to prepare the lithium iron phosphate cathode active material. The lithium iron phosphate cathode active material includes the lithium iron phosphate matrix, and the number of crystal defects in the lithium iron phosphate matrix is ​​1.4000 × 10⁻⁶. 16 spins / g ~2.0000×10 16 spins / g.

10. The preparation method according to claim 9, characterized in that, The mixed system is processed to obtain an intermediate, and the intermediate is sintered to obtain the lithium iron phosphate cathode active material. The preparation method satisfies at least one of the following conditions: (1) The method of processing the mixed system to obtain the intermediate includes grinding the mixed system to obtain the intermediate, preferably, the grinding method includes sand milling; (2) The sintering process includes a first sintering process and a second sintering process in sequence. Preferably, the highest temperature of the first sintering process is 760℃-790℃. Preferably, the holding time of the first sintering treatment at the highest temperature is 8h-12h; Preferably, the heating rate of the first sintering treatment is 2.5℃ / min ~ 5℃ / min; Preferably, the temperature of the second sintering treatment is 700℃~750℃; Preferably, the holding time for the second sintering treatment is 5h~6h; Preferably, the heating rate of the second sintering treatment is 2.5℃ / min ~ 5℃ / min; Preferably, a cooling process is further included between the first sintering process and the second sintering process; Preferably, a cooling process is further included between the first sintering process and the second sintering process, wherein the target temperature of the cooling process is 90℃-100℃; Preferably, a cooling process is further included between the first sintering process and the second sintering process, wherein the atmosphere for the cooling process is an oxygen-containing atmosphere; Preferably, a cooling process is further included between the first sintering process and the second sintering process, wherein the atmosphere for the cooling process is an oxygen-containing atmosphere, and the flow rate of the oxygen-containing atmosphere for the cooling process is 0.6 L / min ~ 1.0 L / min; Preferably, a cooling process is further included between the first sintering process and the second sintering process, wherein the cooling process is held at the target temperature for 40 min to 80 min. (3) The method further includes drying the intermediate before the sintering process, preferably, the drying process includes spraying. (4) The Dv50 of the intermediate is 370nm~390nm.

11. The preparation method according to claim 9 or 10, characterized in that, Both the iron source and the phosphorus source are iron phosphate, and the molar ratio of Fe to P in the iron phosphate is (0.965-0.972):

1.

12. A positive electrode plate, characterized in that, Includes the lithium iron phosphate cathode active material according to any one of claims 1-8; Or the lithium iron phosphate cathode active material prepared by the preparation method according to any one of claims 9-11.

13. A battery, characterized in that, Includes the positive electrode sheet as described in claim 12.

14. A battery pack, characterized in that, It includes at least two batteries as described in claim 13.

15. An electrical appliance, characterized in that, Includes the battery of claim 13, or the battery pack of claim 14.

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