Modified phosphate positive electrode material and preparation method thereof, positive plate and secondary battery

Abstract

The invention belongs to the technical field of batteries, and particularly relates to a modified phosphate positive electrode material, a preparation method thereof, a positive plate and a secondary battery. The modified phosphate positive electrode material comprises a phosphate active core and a carbon coating layer, wherein the carbon coating layer is doped with pyrrole nitrogen, and the crystallinity of the phosphate active core is not lower than 90%. In the modified phosphate positive electrode material, the crystal nucleus with high crystallinity can reduce grain boundary / defect obstruction by providing a regular diffusion channel, so that the diffusion rate and the electrochemical performance of lithium ions are remarkably improved. Meanwhile, pyrrole nitrogen doping is introduced into the carbon layer, the electronic conductivity of the carbon coating layer is enhanced, defects and active sites are introduced, the interface characteristic is improved, the carbon layer structure is regulated and controlled, and the lithium ion diffusion rate is increased in a multi-dimensional mode, so that the modified phosphate positive electrode material has high rate performance.

Description

Technical Field

[0001] This application belongs to the technical field of batteries, and particularly relates to a modified phosphate-based cathode material and a preparation method thereof, as well as a cathode sheet and a secondary battery. Background Art

[0002] As an important component of lithium-ion batteries, the cathode material of lithium-ion batteries plays an important role in the performance of lithium batteries. Currently, the most studied cathode materials are LiCoO2, LiNiO2, LiMn2O4, and LiFePO4 (lithium iron phosphate). Compared with other cathode materials, LiFePO4 electrode materials have many advantages, such as: relatively high theoretical specific capacity, stable working voltage, stable structure, good cycle performance, low raw material cost, and environmental friendliness. Therefore, lithium iron phosphate material is an ideal cathode material and is selected as one of the main cathode materials for power batteries.

[0003] However, due to the unique position of LiO6 in the LFP octahedral crystal unit, it forms a special one-dimensional channel in the

[010] direction (the width of this channel is mainly represented by the size of the b-axis), resulting in slow lithium-ion diffusion and restricting its further application under fast charging conditions. Therefore, it is particularly important to improve the rate performance of lithium iron phosphate materials. Summary of the Invention

[0004] The purpose of this application is to provide a modified phosphate-based cathode material and a preparation method thereof, as well as a cathode sheet and a secondary battery, aiming to solve the problem of low rate performance of existing lithium iron phosphate materials to a certain extent.

[0005] To achieve the above application purpose, the technical solutions adopted in this application are as follows:

[0006] In a first aspect, this application provides a modified phosphate-based cathode material, including a phosphate-based active core and a carbon coating layer; wherein, pyrrole nitrogen is doped in the carbon coating layer, and the crystallinity of the phosphate-based active core is not less than 90%.

[0007] In some possible implementation manners, pyridine nitrogen and / or graphitic nitrogen are also doped in the carbon coating layer.

[0008] In some possible implementation manners, at least one transition metal among titanium, manganese, cobalt, vanadium, niobium, magnesium, and zirconium is doped in the phosphate-based active core.

[0009] In some possible implementation manners, the particle size D50 of the modified phosphate-based cathode material is 0.2 μm to 1.4 μm.

[0010] In some possible implementation manners, the thickness of the carbon coating layer is 3 nm to 10 nm.

[0011] In some possible implementation manners, in the modified phosphate-based cathode material, the doping concentration of the transition metal is 1000 ppm to 6000 ppm.

[0012] In some possible implementation manners, in the carbon coating layer, based on the total content of doped nitrogen atoms being 100%, the nitrogen atom content of pyrrole nitrogen is 10% to 20%.

[0013] In some possible implementation manners, when the modified phosphate-based cathode material is lithium iron phosphate, the length of the b-axis of the unit cell parameter is 6.00 to 6.02.

[0014] In some possible implementation manners, when the modified phosphate-based cathode material is lithium iron manganese phosphate, the length of the b-axis of the unit cell parameter is 6.02 to 6.08.

[0015] In a second aspect, the present application provides a method for preparing a modified phosphate-based cathode material, including the following steps:

[0016] Mix raw materials including at least an iron source, a phosphorus source, a lithium source, a carbon source, and a nitrogen source, and prepare a precursor material;

[0017] After reaching a first sintering temperature, in a protective atmosphere, place the precursor material at the first sintering temperature for a first sintering treatment, and obtain an intermediate product after cooling;

[0018] After reaching a second sintering temperature, in a sintering atmosphere, place the intermediate product at the second sintering temperature for a second sintering treatment, and obtain a modified phosphate-based cathode material after cooling; the modified phosphate-based cathode material includes a phosphate-based active core and a carbon coating layer, wherein pyrrole nitrogen is doped in the carbon coating layer, and the crystallinity of the phosphate-based active core is not less than 90%.

[0019] In some possible implementation manners, the first sintering temperature is 550°C to 850°C, and the duration of the first sintering treatment is 8 min to 25 min.

[0020] In some possible implementation manners, the second sintering temperature is 750°C to 850°C, and the duration of the second sintering treatment is 2 min to 6 min.

[0021] In some possible implementation manners, the addition duration of the precursor material is 1 s to 3 s.

[0022] In some possible implementation manners, the addition duration of the intermediate product is 1 s to 3 s.

[0023] In some possible implementation manners, the temperature condition for cooling the intermediate product is in the range of 20°C to 40°C.

[0024] In some possible implementation manners, the temperature condition for cooling the modified phosphate-based cathode material is in the range of 20°C to 40°C.

[0025] In some possible implementation manners, the raw material further includes at least one transition metal doping source among a titanium source, a manganese source, a cobalt source, a vanadium source, a niobium source, a magnesium source, and a zirconium source.

[0026] In some possible implementation manners, the preparation method of the precursor material includes a solid-phase method, a liquid-phase method, or a gel method;

[0027] In some possible implementation manners, the carbon coating layer is further doped with pyridine nitrogen and / or graphitic nitrogen.

[0028] In some possible implementation manners, the mass ratio of the carbon source to the nitrogen source is (5 to 15):1.

[0029] In some possible implementation manners, the sintering atmosphere includes argon and ammonia with a volume ratio of (2 to 4):1.

[0030] In some possible implementation manners, the addition amount of the carbon source is 1% to 20% of the theoretical yield of the modified phosphate-based cathode material.

[0031] In some possible implementation manners, in the modified phosphate-based cathode material, the concentration of the transition metal doping source is 1000 ppm to 6000 ppm.

[0032] In some possible implementation manners, the transition metal doping source includes at least one titanium source among titanium dioxide, titanium nitrate, tetrabutyl titanate, tetraethyl titanate, tetrapropyl titanate, tetraisopropyl titanate, or titanium tetrachloride.

[0033] In some possible implementation manners, the nitrogen source includes at least one of polyvinylpyrrolidone, urea, melamine, ammonium carbonate, polypyrrole, chitosan, and polyaniline.

[0034] In a third aspect, the present application provides a positive electrode sheet, including a current collector and a positive electrode active layer formed on at least one surface of the current collector, where the positive electrode active layer contains the above-mentioned modified phosphate-based cathode material, or the modified phosphate-based cathode material prepared by the above method.

[0035] In a fourth aspect, the present application provides a secondary battery, and the secondary battery contains the above-mentioned positive electrode sheet.

[0036] The modified phosphate-based cathode material provided in the first aspect of the present application includes a phosphate-based active core and a carbon coating layer; wherein, pyrrole nitrogen is doped in the carbon coating layer, and the crystallinity of the phosphate-based active core is not less than 90%. By improving the crystallinity of the phosphate-based active core, the diffusion path of ions (one-dimensional channel along the

[010] direction) becomes more continuous, reducing the scattering effect of lattice defects (such as dislocations and vacancies) on the migration of lithium ions. And when the crystallinity is high, the arrangement of PO4 tetrahedrons and FeO6 octahedrons is more regular, the transition energy barrier of lithium ions in the channel decreases, and the diffusion activation energy decreases. By improving the structural order, the regularity of the diffusion channel is enhanced, thereby increasing the diffusion rate and improving the rate performance of the material. In addition, the phosphate-based active core with high crystallinity is usually composed of large grains, and the grain boundary density decreases, which can reduce the hindrance of grain boundaries and surface defects, significantly improving the long-range diffusion efficiency and thus the rate performance of the material. On the other hand, pyrrole nitrogen is doped in the carbon coating layer, which has a higher electronegativity. After doping, the electron cloud distribution in the carbon layer changes, forming local polarization regions, which can become "active centers" for electron transition, improving the intrinsic conductivity of the carbon layer. The improvement of electron conductivity can accelerate the migration of electrons during charge and discharge, indirectly promoting the diffusion of lithium ions at the interface between the phosphate-based active core and the coating layer. And doping pyrrole nitrogen will introduce topological defects and edge sites in the carbon layer, which can serve as "anchor points" for lithium ion adsorption, reducing the diffusion energy barrier. Therefore, the carbon coating layer can not only improve the conductivity of the coating layer, but also increase the defects of the coating layer, broaden the width of the lithium ion transmission channel, and optimize the interfacial reaction kinetics between the electrode material and the electrolyte. By regulating the electronic structure of the carbon layer, promoting the rapid insertion / extraction of lithium ions, the ion diffusion rate of the material can be further increased, and the rate performance can be improved.

[0037] The preparation method of the modified phosphate-based cathode material in the second aspect of the present application is to prepare a precursor material. After the temperature reaches the first sintering temperature, the precursor material is added again and sintered once under a protective atmosphere, and then cooled after sintering is completed. Then, after reaching the second sintering temperature, the intermediate product is added again and sintered twice in a sintering atmosphere, and then cooled after sintering is completed to obtain the modified phosphate-based cathode material. In the whole sintering process of the present application, the rapid non-equilibrium pyrolysis method can be used to obtain a high-crystallinity crystal nucleus, which significantly improves the diffusion rate of lithium ions and the electrochemical performance by providing a regular diffusion channel and reducing grain boundary / defect hindrance. At the same time, through dynamic atmosphere regulation, pyrrole nitrogen doping is introduced into the carbon layer, enhancing the electron conductivity of the carbon coating layer, introducing defects and active sites, improving the interfacial characteristics and regulating the carbon layer structure, and multi-dimensionally increasing the lithium ion diffusion rate. Thus, the prepared modified phosphate-based cathode material has high rate performance.

[0038] In the third aspect of the present application, for the positive electrode sheet, since the above-mentioned modified phosphate-based positive electrode material is used in the positive electrode active layer, the modified phosphate-based positive electrode material includes a phosphate-based active core and a carbon coating layer, wherein pyrrole nitrogen is doped in the carbon coating layer, and the crystallinity of the phosphate-based active core is not less than 90%, significantly improving the ionic conductivity, electronic conductivity, structural stability and other characteristics of the modified phosphate-based positive electrode material, and improving the rate performance of the positive electrode sheet.

[0039] In the fourth aspect of the present application, for the secondary battery provided, since it includes the above-mentioned positive electrode sheet with good rate performance, the fast charging efficiency of the secondary battery is improved. Description of the Drawings

[0040] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0041] Figure 1 It is a schematic flow chart of the preparation method of the modified phosphate-based positive electrode material provided by the embodiment of the present application;

[0042] Figure 2 It is the 0.1C electrochemical performance of the modified phosphate-based positive electrode materials provided by Embodiments 1-5 and Comparative Example 1 of the present application;

[0043] Figure 3 It is the 1C electrochemical performance of the modified phosphate-based positive electrode materials provided by Embodiments 1-5 and Comparative Example 1 of the present application. Detailed Embodiments

[0044] In order to make the technical problems to be solved, technical solutions and beneficial effects of the present application more clearly understood, the following further details the present application in conjunction with embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and are not used to limit the present application.

[0045] In the present application, the term "and / or" describes the association relationship of associated objects, indicating that three relationships can exist. For example, A and / or B can represent: A exists alone, A and B exist simultaneously, and B exists alone. Wherein A and B can be singular or plural. The character " / " generally represents an "or" relationship between the associated objects before and after.

[0046] In this application, "at least one" means one or more, and "a plurality" means two or more. "At least one of the following" or a similar expression means any combination of these items, including any combination of single items or plural items. For example, "at least one of a, b, or c", or "at least one of a, b, and c" can both represent: a, b, c, a - b (i.e., a and b), a - c, b - c, or a - b - c, where a, b, and c can be single or multiple respectively.

[0047] It should be understood that in various embodiments of this application, the magnitudes of the serial numbers of the above - mentioned processes do not imply the order of execution. Some or all of the steps can be executed in parallel or sequentially. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation to the implementation process of the embodiments of this application.

[0048] The terms used in the embodiments of this application are only for the purpose of describing specific embodiments and are not intended to limit this application. The singular forms "a" and "the" used in the embodiments of this application and the appended claims are also intended to include the plural forms unless the context clearly indicates otherwise.

[0049] The weight of the relevant components mentioned in the embodiments of the specification of this application not only can refer to the specific content of each component, but also can represent the proportional relationship of the weights between each component. Therefore, as long as the content of the relevant components in the embodiments of the specification of this application is scaled up or down proportionally, it is within the scope disclosed in the embodiments of the specification of this application. Specifically, the mass described in the embodiments of the specification of this application can be mass units well - known in the chemical industry such as μg, mg, g, kg, etc.

[0050] The terms "first" and "second" are only used for descriptive purposes to distinguish objects such as substances from each other, and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. For example, without departing from the scope of the embodiments of this application, the first XX can also be called the second XX. Similarly, the second XX can also be called the first XX. Thus, the features defined with "first" and "second" can explicitly or implicitly include one or more of such features.

[0051] In a first aspect of the embodiments of this application, a modified phosphate - based cathode material is provided, which includes a phosphate - based active core and a carbon coating layer; wherein, pyrrole nitrogen is doped in the carbon coating layer, and the crystallinity of the phosphate - based active core is not less than 90%.

[0052] The modified phosphate-based cathode material provided in the first aspect of the embodiments of the present application includes a phosphate-based active core and a carbon coating layer; wherein, pyrrole nitrogen is doped in the carbon coating layer, and the crystallinity of the phosphate-based active core is not less than 90%. By improving the crystallinity of the phosphate-based active core, the diffusion path of ions (the one-dimensional channel along the

[010] direction) becomes more continuous, reducing the scattering effect of lattice defects (such as dislocations and vacancies) on the migration of lithium ions. And when the crystallinity is high, the arrangement of PO4 tetrahedrons and FeO6 octahedrons is more regular, the transition energy barrier of lithium ions in the channel decreases, and the diffusion activation energy decreases. By enhancing the structural orderliness, the regularity of the diffusion channel is improved, thereby enhancing the diffusion rate and improving the rate performance of the material. In addition, the phosphate-based active core with high crystallinity is usually composed of large grains, and the grain boundary density decreases, which can reduce the hindrance of grain boundaries and surface defects, significantly enhancing the long-range diffusion efficiency, thereby improving the rate performance of the material. On the other hand, pyrrole nitrogen is doped in the carbon coating layer, which has a higher electronegativity. After doping, the electron cloud distribution in the carbon layer changes, forming local polarization regions, which can become "active centers" for electron transition, improving the intrinsic conductivity of the carbon layer. The improvement of electron conductivity can accelerate the migration of electrons during charge and discharge, indirectly promoting the diffusion of lithium ions at the interface between the phosphate-based active core and the coating layer. And, pyrrole nitrogen doping introduces topological defects and edge sites in the carbon layer, which can serve as "anchor points" for lithium ion adsorption, reducing the diffusion energy barrier. Therefore, the carbon coating layer can not only improve the conductivity of the coating layer, but also increase the defects of the coating layer, broaden the width of the lithium ion transmission channel, and optimize the interfacial reaction kinetics between the electrode material and the electrolyte. By regulating the electronic structure of the carbon layer, promoting the rapid insertion / extraction of lithium ions, the ion diffusion rate of the material can be further enhanced, and the rate performance can be improved.

[0053] In some possible implementation manners, pyridine nitrogen and / or graphitic nitrogen are also doped in the carbon coating layer. In this case, among pyrrole nitrogen, pyridine nitrogen and graphitic nitrogen, how far the nitrogen atoms are embedded in the polycarbon ring, contributing lone electron pairs to the conjugated π system, enhancing the conductivity of the material in an electron-rich environment, and optimizing the interfacial reaction kinetics between the electrode and the electrolyte; by regulating the electronic structure of the carbon layer, promoting the rapid insertion / extraction of lithium ions. And, pyrrole nitrogen, as well as pyridine nitrogen and graphitic nitrogen, can provide lone pairs of electrons to form coordination bonds with lithium ions, enhancing the interfacial lithium ion adsorption ability.

[0054] In some possible implementation manners, in the carbon coating layer, based on the total content of doped nitrogen atoms being 100%, the nitrogen atom content of pyrrolic nitrogen is 10% to 20%. It can also be expressed as the pyrrolic nitrogen / total nitrogen amount being 10% to 20%. Exemplarily, the nitrogen atom content of pyrrolic nitrogen can be any typical but non-limiting point value such as 10%, 12%, 14%, 15%, 16%, 18%, 20%, etc., or an interval value between any two point values. In this case, too low a pyrrolic nitrogen content will result in insufficient conductivity of the carbon layer, and at the same time, insufficient adsorption of lithium ions by the carbon layer, slow interfacial kinetics, both of which will lead to an inability to effectively improve the rate performance. Too high a pyrrolic nitrogen content will lead to disordering of the carbon layer structure, hindering electron conduction. In addition, the polar functional groups of pyrrolic nitrogen are prone to side reactions (such as oxidation decomposition) with the electrolyte at high potentials, generating an interfacial impedance layer, which affects the improvement of the rate performance.

[0055] In some possible implementation manners, at least one transition metal among titanium, manganese, cobalt, vanadium, niobium, magnesium, and zirconium is doped in the phosphate-based active core. In this case, by doping these transition metal elements in the phosphate-based active core, the particle size can be optimized and the rate performance of the material can be improved. Specifically, the redox couple of cobalt (Co) and vanadium (V) has higher electron conductivity, which can form a continuous electron conduction network and reduce the contact resistance between the phosphate-based active material particles. The doping of niobium (Nb) can introduce impurity energy levels, narrow the band gap of the phosphate-based active material, and enhance the intrinsic electron conductivity. The ionic radii of titanium (Ti) and magnesium (Mg) are close to that of lithium, which can expand the

[010] diffusion channel of the olivine structure and reduce the lithium ion migration energy barrier. After zirconium (Zr) replaces iron, lattice distortion will generate a local stress field, promoting the transition of lithium ions in the lattice.

[0056] In some possible implementation manners, in the modified phosphate-based cathode material, the doping concentration of the transition metal is 1000 ppm to 6000 ppm. Exemplarily, the doping concentration of the transition metal can be any typical but non-limiting point value such as 1000, 2000 ppm, 3000 ppm, 4000 ppm, 5000 ppm, 6000 ppm, etc., or an interval value between any two point values. In this case, the doping content of the transition metal can not only effectively improve the conductivity and ion migration performance of the cathode material, but also avoid lattice collapse and side reactions.

[0057] In some possible implementation manners, the particle size D50 of the modified phosphate-based cathode material is 0.6 μm to 1.4 μm. Exemplarily, the particle size D50 of the modified phosphate-based cathode material can be any typical but non-limiting point value such as 0.6 μm, 0.8 μm, 1.0 μm, 1.2 μm, 1.4 μm, etc., or an interval value between any two point values. In this case, the diffusion path of ions is shorter, which is beneficial to improving the lithium ion diffusion rate.

[0058] In some possible implementation manners, the thickness of the carbon coating layer is 3 nm to 10 nm. Exemplarily, the thickness of the carbon coating layer can be any typical but non-limiting point value such as 3 nm, 5 nm, 6 nm, 8 nm, 10 nm or an interval value between any two point values. In this case, the modified phosphate-based cathode material has better electrochemical performances such as rate performance.

[0059] In some embodiments, the phosphate-based active core includes lithium iron phosphate, lithium manganese iron phosphate, lithium vanadium phosphate, lithium titanium iron phosphate, etc.

[0060] In some embodiments, when the modified phosphate-based cathode material is lithium iron phosphate, the length of the b-axis of the unit cell parameter is greater than 6.00. Exemplarily, the length of the b-axis of the unit cell parameter is 6.00 to 6.02. The larger the b-axis length is, the wider the transmission channel is, and the smaller the resistance is during the charge and discharge process, which is beneficial to improving the rate performance.

[0061] In some embodiments, when the modified phosphate-based cathode material is lithium manganese iron phosphate, the length of the b-axis of the unit cell parameter is greater than 6.02. Exemplarily, the length of the b-axis of the unit cell parameter is 6.02 to 6.08. The larger the b-axis length is, the wider the transmission channel is, and the smaller the resistance is during the charge and discharge process, which is beneficial to improving the rate performance.

[0062] The modified phosphate-based cathode material in the above embodiments of the present application can be obtained by the following example methods.

[0063] In a second aspect, an embodiment of the present application provides a preparation method of a modified phosphate-based cathode material, as shown in the appendix Figure 1 and includes the following steps:

[0064] S10. Mix raw materials including at least an iron source, a phosphorus source, a lithium source, a carbon source and a nitrogen source, and prepare a precursor material;

[0065] S20. After reaching the first sintering temperature, in a protective atmosphere, place the precursor material at the first sintering temperature for the first sintering treatment, and obtain an intermediate product after cooling;

[0066] S30. After reaching the second sintering temperature, in a sintering atmosphere, place the intermediate product at the second sintering temperature for the second sintering treatment, and obtain a modified phosphate-based cathode material after cooling; the modified phosphate-based cathode material includes a phosphate-based active core and a carbon coating layer, wherein, pyrrole nitrogen is doped in the carbon coating layer, and the crystallinity of the phosphate-based active core is not less than 90%.

[0067] In the preparation method of the modified phosphate-based cathode material according to the embodiment of the present application, a precursor material is prepared. After the temperature reaches the first sintering temperature, the precursor material is added again and sintered once under a protective atmosphere, and then cooled after sintering is completed. Then, after reaching the second sintering temperature, the intermediate product is added again and sintered twice in a sintering atmosphere, and then cooled after sintering is completed to obtain the modified phosphate-based cathode material. In the entire sintering process of the embodiment of the present application, the rapid non-equilibrium pyrolysis method can be used to obtain highly crystalline crystal nuclei. By providing regular diffusion channels and reducing grain boundary / defect hindrance, the diffusion rate of lithium ions and the electrochemical performance are significantly improved. At the same time, through dynamic atmosphere regulation, pyrrole nitrogen is doped in the carbon layer, enhancing the electronic conductivity of the carbon coating layer, introducing defects and active sites, improving the interfacial characteristics and regulating the carbon layer structure, and multi-dimensionally enhancing the lithium ion diffusion rate. Therefore, the prepared modified phosphate-based cathode material has high rate performance.

[0068] In some embodiments, the preparation method of the modified phosphate-based cathode material according to the embodiment of the present application undergoes two sintering treatments. The protective atmosphere is an inert atmosphere, and the sintering atmosphere is a mixed gas containing ammonia. Among them, the role of rapid sintering under the protective atmosphere is to generate lithium iron phosphate under an inert gas and ensure that the carbon layer structure is relatively complete, which is beneficial to protecting the internal lithium iron phosphate from reacting with the electrolyte as little as possible. The role of rapid sintering in the sintering atmosphere is to facilitate the formation of pyrrole nitrogen. A carbon layer structure with a relatively complete internal carbon layer and pyrrole nitrogen defects on the outer layer is prepared by combining the two sintering treatments.

[0069] In some embodiments, after the first sintering treatment, the sintered product is cooled under the condition of 20°C to 40°C, and the gas source is replaced and heated to the second sintering temperature for secondary sintering, which is beneficial to a rapid heating process after replacing the sintering atmosphere and is beneficial to the formation of pyrrole nitrogen.

[0070] In some embodiments, in both the first sintering and the second sintering of the embodiment of the present application, the material is added again after reaching the sintering temperature to achieve rapid heating and sintering of the material, which mainly includes the following functions: ① Generation of non-equilibrium carbon skeleton: Rapid heating shortens the pyrolysis time of the precursor, inhibits the ordering of the carbon layer, and forms a large number of edge defects, vacancies and microporous structures; ② Topological stress-induced defects: Rapid sintering causes uneven local shrinkage of the carbon layer, resulting in lattice distortion (such as five-membered rings / seven-membered rings), providing an anchoring site for pyrrole nitrogen. ③ In a sintering atmosphere of a mixed gas containing ammonia, the active nitrogen atoms (·N) generated by the high-temperature cracking of the gaseous nitrogen source are preferentially adsorbed on the defect sites to form five-membered ring pyrrole nitrogen.

[0071] In some embodiments, the temperature condition for cooling the intermediate product is in the range of 20°C to 40°C. In some embodiments, the temperature condition for cooling the modified phosphate-based cathode material is in the range of 20°C to 40°C. After the first sintering and the second sintering in the embodiments of the present application are completed, the sintered products are taken out and rapidly cooled at room temperature of 20°C to 40°C, rather than slowly cooled in the sintering system. The main functions include the following: ① Suppressing annealing repair: Rapid cooling prevents the annealing healing of defects by instantaneously reducing the temperature (>100°C / s), maintaining a high density of active sites; ② Retaining the microporous structure: The mesopores / micropores in the carbon layer are fixed due to rapid cooling, shortening the diffusion path of lithium ions (Li+); ③ Blocking the conversion of nitrogen species: Pyrrolic nitrogen is prone to convert to graphitic nitrogen at high temperatures, and pyrrolic nitrogen is retained. Through the synergy of rapid heating and rapid cooling, an optimized electron-ion dual channel can be constructed. The doped nitrogen acts as an additional carrier, enhancing the electronic conductivity of the carbon layer. The defects in the carbon layer make the conjugated π-electron distribution more uniform, reducing electrode polarization. In addition, the edge defects and micropores in the carbon layer form a rapid diffusion channel for Li ions. The lone pair electrons introduced by the doped nitrogen can have a weak coordination interaction with Li ions, reducing the desolvation energy barrier. Through the defect-nitrogen synergistic sites, "dual active centers" are formed, simultaneously promoting charge transfer and Li ion adsorption and reducing the interfacial impedance.

[0072] In the above step S10:

[0073] In some possible implementation manners, the preparation method of the precursor material includes a solid-phase method, a liquid-phase method or a gel method. Among them, the solid-phase method directly generates the precursor material by mechanically mixing solid raw materials such as an iron source, a phosphorus source, a lithium source, a carbon source and a nitrogen source, and triggering a solid-solid diffusion reaction by high-temperature calcination. The liquid-phase method is to dissolve raw materials such as a soluble iron source, a phosphorus source, a lithium source, a carbon source and a nitrogen source in a solution, and generate a precursor through coprecipitation, hydrothermal or sol-gel reaction. The gel method is to form a three-dimensional sol network through the hydrolysis-condensation reaction of metal alkoxides or inorganic salts, and obtain the precursor material after drying and calcination.

[0074] In some possible implementation manners, the raw materials further include at least one transition metal doping source among a titanium source, a manganese source, a cobalt source, a vanadium source, a niobium source, a magnesium source and a zirconium source. By doping these transition metal elements in the phosphate-based active core, the particle size can be optimized and the rate performance of the material can be improved.

[0075] In some possible implementation manners, in the modified phosphate-based cathode material, the concentration of the transition metal doping source is 1000 ppm to 6000 ppm. Exemplarily, the concentration of the transition metal doping source can be typical but non-limiting arbitrary point values such as 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm, 5000 ppm, 6000 ppm or interval values between any two point values. In this case, the doping content of the transition metal can not only effectively improve the electrical conductivity and ion migration performance of the cathode material, but also avoid lattice collapse and side reactions.

[0076] In some possible implementation manners, the molar ratio of the phosphorus source to the iron source is (0.95 to 1.05):1. Exemplarily, the molar ratio of the phosphorus source to the iron source can be typical but non-limiting arbitrary point values such as 0.95:1, 0.96:1, 0.97:1, 0.98:1, 0.99:1, 1.0:1, 1.02:1, 1.04:1, 1.05:1 or interval values between any two point values. In this case, the multi-objective optimization of stoichiometry, product purity, particle morphology and process cost is balanced. This ratio ensures high purity, uniform crystallization and excellent electrochemical performance of the product.

[0077] In some possible implementation manners, the molar ratio of the lithium source to the iron source is (1.0 to 1.09):1. Exemplarily, the molar ratio of the lithium source to the iron source can be typical but non-limiting arbitrary point values such as 1:1, 1.02:1, 1.04:1, 1.05:1, 1.06:1, 1.08:1, 1.09:1 or interval values between any two point values. In this case, the lithium source is slightly in excess, compensating for the high-temperature volatilization loss, ensuring stoichiometric balance, optimizing the particle morphology, and at the same time balancing the process tolerance and economy.

[0078] In some possible implementation manners, the mass ratio of the carbon source to the nitrogen source is (5 to 15):1. Exemplarily, the mass ratio of the carbon source to the nitrogen source can be typical but non-limiting arbitrary point values such as 5:1, 8:1, 10:1, 12:1, 14:1, 15:1 or interval values between any two point values. In this case, if the nitrogen source content is too low, it is difficult to form sufficient active sites (such as pyridine nitrogen and pyrrole nitrogen), resulting in limited improvement in electrochemical performance. If the nitrogen source content is too high, excessive nitrogen doping introduces defects such as vacancies and grain boundaries, which may reduce the conductivity and accelerate the degradation of the material. And it is easy to form non-active nitrogen species (such as oxidized nitrogen N-O), which is not beneficial to the performance.

[0079] In some possible implementation manners, the addition amount of the carbon source is 1% to 20% of the theoretical yield of the modified phosphate-based cathode material. Exemplarily, the addition amount of the carbon source is 1%, 2%, 4%, 5%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20% and other typical but non-limiting arbitrary point values or interval values between any two point values of the theoretical yield of the modified phosphate-based cathode material. In this case, the addition amount of the carbon source fully ensures the improvement of the conductivity and stability of the phosphate-based active material, and also ensures that the modified phosphate-based cathode material has a relatively high capacity.

[0080] In some possible implementation manners, the transition metal doping source includes at least one titanium source of titanium dioxide, titanium nitrate, tetrabutyl titanate, tetraethyl titanate, tetrapropyl titanate, tetraisopropyl titanate or titanium tetrachloride. In this case, adding a titanium source to the phosphate-based active core is beneficial to the improvement of the rate performance.

[0081] In some possible implementation manners, the nitrogen source includes at least one of polyvinylpyrrolidone, urea, melamine, ammonium carbonate, polypyrrole, chitosan and polyaniline. These nitrogen sources can all generate nitrogen-containing substances such as pyrrole nitrogen during the sintering process and be doped into the carbon coating layer. Exemplarily, taking polyvinylpyrrolidone PVP as an example, the molecule contains a pyrrolidone five-membered ring (composed of 4 carbons and 1 nitrogen), and its cyclic structure can be directly embedded in the carbon layer during the pyrolysis process. The pyrolysis path includes: low temperature stage (300°C to 500°C): The side chain of PVP breaks, releasing small molecule gases (such as CO, H2O), but part of the pyrrolidone ring remains. Medium temperature stage (600°C to 850°C): The remaining pyrrolidone ring conjugates with the carbon skeleton, and the nitrogen atom is fixed at the edge or defect site of the carbon layer in the form of a five-membered ring, forming pyrrole nitrogen. In addition, pyridine nitrogen, graphitic nitrogen, gases (such as: CO, CO2, H2O, NH3 and HCN, etc.), amorphous carbon, etc. will also be generated.

[0082] In some possible implementation manners, the iron source includes at least one of ferrous oxalate, ferrous acetate, ferrous chloride, ferrous sulfate, iron oxide, iron phosphate, ferric nitrate, ferric chloride, ferric sulfate, iron hydroxide, ferric citrate and ferric acetate.

[0083] In some possible implementation manners, the phosphorus source includes at least one of ammonium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, iron phosphate, lithium phosphate and lithium dihydrogen phosphate.

[0084] In some possible implementation manners, the lithium source includes at least one of lithium hydroxide, lithium oxalate, lithium carbonate, lithium acetate, lithium phosphate, lithium dihydrogen phosphate, lithium hydrogen phosphate, lithium chloride and lithium nitrate.

[0085] In some possible implementation manners, the carbon source includes at least one of glucose, sucrose, starch, citric acid, amino acid, polyethylene glycol, and polyvinylidene fluoride.

[0086] In the above step S20: After reaching the first sintering temperature, under a protective atmosphere, the precursor material is placed at the first sintering temperature for the first sintering treatment, and the sintered product is cooled under the condition of 20°C to 40°C (which can also be room temperature condition) to obtain an intermediate product.

[0087] In some possible implementation manners, the addition duration of the precursor material is 1 s to 3 s; that is, the entire process of the material entering the first sintering temperature from room temperature only takes 1 - 3 s. In this case, the precursor material can be rapidly heated, a non-equilibrium carbon skeleton is generated, and compressive stress-induced defects are formed. Taking 850°C as the first sintering temperature and 25°C as the room temperature, the heating rate is between 275°C / s and 825°C / s.

[0088] In some possible implementation manners, after the first sintering treatment, the sintered product is rapidly cooled in the room temperature condition of 20°C to 40°C within 1 s to 3 s to inhibit annealing repair and retain the microporous structure.

[0089] In some possible implementation manners, the first sintering temperature is 550°C to 850°C, and the duration of the first sintering treatment is 8 min to 25 min. Exemplarily, the sintering temperature can be typical but non-limiting arbitrary point values such as 550°C, 600°C, 650°C, 700°C, 750°C, 800°C, 850°C, or interval values between any two point values; the sintering duration can be typical but non-limiting arbitrary point values such as 8 min, 10 min, 12 min, 15 min, 18 min, 20 min, 25 min, or interval values between any two point values. In this case, it can promote the complete growth of grains, improve the electrolyte wettability and electrochemical activity, increase the lithium ion diffusion path length, and improve the charge-discharge rate performance.

[0090] In some embodiments, the protective atmosphere includes inert atmospheres such as argon, nitrogen, and helium.

[0091] In the above step S30: After reaching the second sintering temperature, under the sintering atmosphere, the intermediate product is placed at the second sintering temperature for the second sintering treatment, and the sintered product is cooled under the condition of 20°C to 40°C (which can also be room temperature condition) to obtain a modified phosphate-based cathode material.

[0092] In some possible implementation manners, the sintering atmosphere includes argon and ammonia with a volume ratio of (2 - 4):1; in the argon-ammonia mixed gas, the active nitrogen atoms (·N) generated by the high-temperature cracking of NH3 are preferentially adsorbed on the defect sites to form five-membered ring pyrrole nitrogen.

[0093] In some possible implementation manners, the addition duration of the intermediate product is 1 s to 3 s; that is, the entire process of the intermediate product entering the second sintering temperature from room temperature only takes 1 - 3 s. In this case, the intermediate product can be rapidly heated, a non-equilibrium carbon skeleton is generated, and compressive stress induces defects. In the sintering atmosphere containing a nitrogen source, the active nitrogen atoms (·N) generated by the high-temperature cracking of the nitrogen source are preferentially adsorbed on the defect sites to form five-membered pyrrole nitrogen. Taking 850 °C as the second sintering temperature and 25 °C as the room temperature, the heating rate is 275 °C / s to 825 °C / s.

[0094] In some possible implementation manners, after the second sintering treatment, the sintered product is rapidly cooled at 20 °C to 40 °C room temperature conditions within 1 s to 3 s to inhibit annealing repair, retain the microporous structure, and block the conversion of nitrogen species.

[0095] In some possible implementation manners, the second sintering temperature is 750 °C to 850 °C, and the duration of the second sintering treatment is 2 min to 6 min. Exemplarily, the sintering temperature can be typical but non-limiting arbitrary point values such as 750 °C, 800 °C, 850 °C, etc. or interval values between any two point values; the sintering duration can be typical but non-limiting arbitrary point values such as 2 min, 3 min, 4 min, 5 min, 6 min, etc. or interval values between any two point values. In this case, it can promote the complete growth of grains, improve the electrolyte wettability and electrochemical activity, increase the lithium ion diffusion path length, and improve the charge-discharge rate performance.

[0096] In some possible implementation manners, in the prepared modified phosphate-based cathode material, the carbon coating layer is also doped with pyridine nitrogen and / or graphitic nitrogen. In this case, among pyrrole nitrogen, as well as pyridine nitrogen and graphitic nitrogen, nitrogen atoms are embedded in the polycyclic carbon ring, contributing lone electron pairs to the conjugated π system, enhancing the conductivity of the material in an electron-rich environment, and optimizing the interfacial reaction kinetics between the electrode and the electrolyte; by regulating the electronic structure of the carbon layer, the rapid insertion / extraction of lithium ions is promoted. Moreover, pyrrole nitrogen, as well as pyridine nitrogen and graphitic nitrogen, can provide lone pairs of electrons to form coordination bonds with lithium ions, enhancing the interfacial lithium ion adsorption capacity.

[0097] In some possible implementation manners, in the carbon coating layer, taking the total content of doped nitrogen atoms as 100%, the nitrogen atom content of pyrrole nitrogen is 10% to 20%.

[0098] In some possible implementation manners, the particle size D50 of the modified phosphate-based cathode material is 0.2 μm to 1.4 μm, and the thickness of the carbon coating layer is 3 nm to 10 nm.

[0099] The beneficial effects of the above embodiments of this application have been elaborated in detail in the foregoing text and will not be repeated here.

[0100] In a third aspect, an embodiment of the present application provides a positive electrode sheet, which includes a current collector and a positive electrode active layer formed on the surface of the current collector. The positive electrode active layer contains the above-mentioned modified phosphate-based positive electrode material.

[0101] In the positive electrode sheet of the embodiment of the present application, since the above-mentioned modified phosphate-based positive electrode material is adopted in the positive electrode active layer, the modified phosphate-based positive electrode material includes a phosphate-based active core and a carbon coating layer. Among them, pyrrole nitrogen is doped in the carbon coating layer, and the crystallinity of the phosphate-based active core is not less than 90%, which significantly improves the ionic conductivity, electronic conductivity, structural stability and other characteristics of the modified phosphate-based positive electrode material, and improves the rate performance of the positive electrode sheet.

[0102] In some possible implementation manners, the preparation of the positive electrode active layer includes the steps of: mixing the above-mentioned modified phosphate-based positive electrode material, a conductive agent and a binder to form an electrode paste, and then coating the electrode paste on the current collector, and preparing the positive electrode sheet through steps such as drying, rolling and die-cutting.

[0103] In some possible implementation manners, the mass percentage of the modified phosphate-based positive electrode material in the positive electrode active layer of the positive electrode sheet is 90% to 95%. Specifically, the mass percentage of the modified phosphate-based positive electrode material in the positive electrode active material layer can be 90%, 91%, 92%, 93%, 94%, 95%, etc.

[0104] In some possible implementation manners, the current collector of the positive electrode sheet includes, but is not limited to, any one of copper foil and aluminum foil.

[0105] In some possible implementation manners, the content of the binder in the positive electrode active material layer of the positive electrode sheet is 2 wt% to 5 wt%. In specific embodiments, the content of the binder can be typical but non-limiting contents such as 2 wt%, 3 wt%, 4 wt%, 5 wt%, etc.

[0106] In some possible implementation manners, the binder includes one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

[0107] In some possible implementation manners, the content of the conductive agent in the positive electrode active material layer of the positive electrode sheet is 1 wt% to 5 wt%. In specific embodiments, the content of the conductive agent can be typical but non-limiting contents such as 3 wt%, 4 wt%, 5 wt%, etc.

[0108] In some possible implementation manners, the conductive agent includes one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C 60 and carbon nanotubes.

[0109] Fourthly, an embodiment of the present application provides a secondary battery, which includes the above-mentioned positive electrode sheet.

[0110] Since the secondary battery provided by the embodiment of the present application includes the above-mentioned positive electrode sheet with good rate performance, the fast charging efficiency of the secondary battery is improved.

[0111] The present application does not specifically limit the negative electrode sheet, electrolyte, separator, etc. in the secondary battery of the embodiment, and it can be applied to any battery system.

[0112] In some possible implementation manners, the negative electrode active material of the secondary battery includes, but is not limited to, carbon materials such as graphite, soft carbon (such as coke, etc.), hard carbon, or nitrides, tin-based oxides, tin-based oxides, tin alloys, and nano negative electrode materials, etc. The current collector includes, but is not limited to, any one of copper foil and aluminum foil.

[0113] In some possible implementation manners, the steps of manufacturing the negative electrode sheet include: mixing the negative electrode active material with a conductive agent such as conductive carbon black, a binder such as carboxymethyl cellulose and styrene-butadiene rubber, and a solvent such as water in a mass ratio of (80-99):(1-5):(2-10):100 to form a positive electrode mixed slurry, then performing vacuum degassing, discharging, coating on a coater, and obtaining the negative electrode sheet after rolling, slitting, and die-cutting.

[0114] In some possible implementation manners, the separator can block the passage of electrons while allowing ions to pass through. Exemplarily, the separator includes, but is not limited to, at least one material among polypropylene fiber, polyacrylonitrile fiber, polyvinyl formal fiber, poly(ethylene glycol terephthalate), polyethylene terephthalate, polyamide fiber, and poly(p-phenylene terephthalamide).

[0115] In some possible implementation manners, the electrolyte includes at least one soluble metal salt. In some specific embodiments, the metal salt includes at least one of LiClO4, LiBF4, LiPF6, LiAsF6, LiCF3SO3, LiTDI, Li[(CF3SO2)2N], Li[(FSO2)2N], Li[(C m F 2m+1 SO2)(C n F 2n+1 SO2)N], where m and n are natural numbers. These electrolytic salts can all ensure high ionic conductivity of the electrolyte, and do not undergo harmful side reactions with electrode materials, electrolytes, separators, etc., and have good chemical stability.

[0116] In some possible implementation manners, the secondary battery includes at least one of a battery cell, a battery module, and a battery pack.

[0117] In some possible embodiments, the battery cell types include lithium-ion batteries, as well as new types of batteries such as lithium-air batteries and lithium-metal batteries.

[0118] In some possible embodiments, the battery cells of the present application can be assembled into a battery module. The number of battery cells included in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. The battery module can also include a housing having an accommodation space, and a plurality of battery cells are accommodated in the accommodation space.

[0119] In one possible embodiment, the battery cells and / or the battery module can also be assembled into a battery pack, and the number of battery cells or battery modules included in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0120] To enable those skilled in the art to clearly understand the above implementation details and operations of the present application, and to significantly reflect the improved performance of the modified phosphate-based cathode material, its preparation method, the cathode sheet and the secondary battery of the embodiments of the present application, the following technical solutions will be illustrated by multiple embodiments.

[0121] Example 1

[0122] A preparation method of modified lithium iron phosphate includes the following steps:

[0123] 1. Disperse iron phosphate and lithium carbonate in deionized water at a P / total iron molar ratio of 1.05 and a Li / Fe molar ratio of 1.03. Additionally, add 10% of the theoretical weight of glucose, 1% of the theoretical weight of urea, and 4000 ppm of titanium dioxide, and perform ball milling at 600 rpm for 6 h, then dry to obtain a lithium iron phosphate precursor;

[0124] 2. After heating to the first sintering temperature of 850 °C, place an appropriate amount of the lithium iron phosphate precursor into a graphite boat with a graphite long rod; under an argon atmosphere (protective atmosphere), quickly push the lithium iron phosphate precursor into the central heating zone at 850 °C and hold for the first sintering time (15 min), then quickly pull it out of the heating zone to obtain an intermediate product;

[0125] 3. Adjust the gas in the tubular furnace to an argon-ammonia mixture (volume ratio of argon: ammonia = 3:1, i.e., sintering atmosphere), blow for 30 min, heat up to the second sintering temperature of 850 °C, then quickly push the intermediate product into the central heating zone at 850 °C, hold for the second sintering time (3 min), then quickly pull it out of the heating zone and place it in the room temperature zone for cooling. After the sample cools to room temperature, obtain the modified lithium iron phosphate.

[0126] Example 2

[0127] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that: the first sintering temperature is 750 °C, the second sintering temperature is 750 °C, and other conditions remain unchanged.

[0128] Example 3

[0129] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that: the first sintering temperature is 950 °C, the second sintering temperature is 950 °C, and other conditions remain unchanged.

[0130] Example 4

[0131] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that: the first sintering duration is 8 min, and other conditions remain unchanged.

[0132] Example 5

[0133] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that: the first sintering duration is 25 min, and other conditions remain unchanged.

[0134] Example 6

[0135] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that: the molar ratio of P / (Fe + Mn) is 1.05, and the molar ratio of Fe / Mn is 6:4, and other conditions remain unchanged.

[0136] Example 7

[0137] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that: the sintering atmosphere is changed to pure argon, and other conditions remain unchanged.

[0138] Example 8

[0139] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that: the second sintering duration is 1 min, and other conditions remain unchanged.

[0140] Example 9

[0141] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that: the second sintering duration is 8 min, and other conditions remain unchanged.

[0142] Example 10

[0143] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that: the first sintering temperature is 850 °C, the second sintering temperature is 800 °C, and other conditions remain unchanged.

[0144] Example 11

[0145] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that titanium dioxide is replaced with manganese nitrate, and other conditions remain unchanged.

[0146] Example 12

[0147] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that urea is replaced with melamine, and other conditions remain unchanged.

[0148] Comparative Example 1

[0149] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that during the first sintering process, the lithium iron phosphate precursor is heated from room temperature (20 - 40 °C) to 850 °C at a heating rate of 5 °C / min, the holding time is still 15 minutes, then wait for the tube furnace to cool slowly, take out the intermediate product; adjust the gas in the tube furnace to argon-ammonia mixture (volume ratio of argon to ammonia = 3:1, i.e., sintering atmosphere), blow for 30 min, quickly push the intermediate product into the tube furnace, heat from room temperature (20 - 40 °C) to 850 °C at a heating rate of 5 °C / min, hold for 3 min, then wait for the tube furnace to cool slowly, take out to obtain the sample of Comparative Example 1.

[0150] Comparative Example 2

[0151] A preparation method of modified lithium iron phosphate has the same raw material ratio as Example 1, the sintering conditions are the same as those of Comparative Example 1, and the sintering atmosphere is pure argon.

[0152] Comparative Example 3

[0153] A preparation method of modified lithium iron phosphate has the same raw material ratio as Example 6, and the sintering conditions and atmosphere are the same as those of Comparative Example 2.

[0154] Comparative Example 4

[0155] A preparation method of modified lithium iron phosphate has the same raw material ratio and sintering conditions as Comparative Example 3, and the atmosphere is pure argon.

[0156] Comparative Example 5

[0157] A preparation method of modified lithium iron phosphate is different from Example 1 in that only the first sintering treatment is carried out.

[0158] Comparative Example 6

[0159] A preparation method of modified lithium iron phosphate, which is different from Example 1 in that: only the second sintering treatment is carried out. After heating to the first sintering temperature of 850 °C, an appropriate amount of lithium iron phosphate precursor is placed in a graphite boat with a graphite long rod; in an atmosphere with a volume ratio of argon: ammonia = 3:1, the lithium iron phosphate precursor is quickly pushed into the central heating zone at a temperature of 850 °C and kept for the sintering time (15 min), and then quickly pulled out of the heating zone.

[0160] The material types and preparation conditions of the above examples and comparative examples are shown in Table 1 below:

[0161] Table 1

[0162]

[0163] In order to verify the progressiveness of the embodiments of the present application, the following performance tests are carried out on the above examples and comparative examples:

[0164] 1. Test the unit cell parameters (a-axis length, b-axis length, c-axis length, V unit cell volume) of the phosphate-based active core: obtained by refining the XRD data through GSAS software;

[0165] 2. Calculate the crystallinity of the phosphate-based active core: adopt the X-ray diffraction (XRD) integral intensity method, that is, by measuring the ratio of the integral intensity of the crystalline peak to the total integral intensity in the XRD pattern, and combining the diffraction data of the standard crystal to calculate the crystallinity. The formula is: crystallinity = (integral intensity of crystalline peak / total integral intensity) × 100%;

[0166] 3. Measure the nitrogen atom content of pyrrole nitrogen in the carbon coating layer (calculated based on the total nitrogen content of 100%): use the equipment-related parameters: Thermo Fisher Scientific ESCALAB Xi + , monochromatized Al Kα (1486.6 eV), power setting 150 W (voltage 15 kV, current 10 mA) for N1s high-resolution narrow scan, binding energy range set to 395 - 405 eV, pass energy 20 - 50 eV, step size 0.05 - 0.1 eV, cumulative scan times 5 - 10 times to improve the signal-to-noise ratio. Subsequently, use the spectral peak fitting technique combined with the standard binding energy parameters of chemical states to analyze the nitrogen species in different chemical environments in the nitrogen-doped carbon layer (such as pyridine-type, pyrrole-type, and graphite-type nitrogen), and calculate their relative contents through peak area integration;

[0167] 4. Apply the modified phosphate-based cathode materials of the above examples and comparative examples to lithium-ion batteries respectively, and conduct electrochemical performance tests respectively. The preparation process of the lithium-ion batteries is as follows:

[0168] ① Preparation of the positive electrode sheet: According to the mass ratio, in the examples or comparative examples, phosphate-based positive electrode material: SP: PVDF: NMP = 93.5:2.5:4:100. The ball mill mixer was stirred at a speed of 360 rpm for 1 h to mix evenly, obtaining the positive electrode paste. The prepared positive electrode paste was poured on the aluminum foil and evenly scraped flat with a scraper. The positive electrode sheet was dried in an oven at 120 °C for 20 min and then roll-pressed under a pressure of 10 MPa to obtain the positive electrode sheet.

[0169] ② The battery assembly process is as follows: The prepared positive electrode sheet was pasted on the positive electrode metal shell with conductive adhesive and then dried; a lithium metal sheet was used as the negative electrode, Celgard 2400 microporous membrane was used as the battery separator, and a mixed solution containing 1.0 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) (the volume ratio of EC / DEC was 1:1) was used as the electrolyte. The button battery was assembled in a glove box to obtain the lithium-ion button battery.

[0170] The specific discharge capacities (mAh / g) at 0.1 C at room temperature (25 °C), 1 C at room temperature (25 °C), and 1 C at low temperature (-20 °C) were respectively tested.

[0171] The above test results are shown in Tables 2 and 3 as follows:

[0172] Table 2

[0173]

[0174] Table 3

[0175]

[0176] It can be seen from the test results in Tables 2 and 3 above that in Examples 1-12 of the present application, through a dynamic atmosphere (argon + argon-ammonia mixed gas) and rapid sintering, the prepared modified lithium iron phosphate all showed better rate performance. In particular, for the modified lithium iron phosphate prepared in Example 1, the discharge capacities at 0.1 C and 1 C at room temperature were the highest, and the specific discharge capacity at 1 C at low temperature (-20 °C) was also the highest, indicating that the lithium iron phosphate sample prepared under this condition had the best rate performance and better kinetic performance. In Examples 2-12, by changing conditions such as material ratio, sintering duration, temperature, nitrogen source type, and doping metal type, the performance of the prepared modified lithium iron phosphate positive electrode material fluctuated to some extent, but the electrochemical performance was still higher than that of Comparative Examples 1-6.

[0177] By comparing Example 2 and Example 3, it can be seen that during rapid heating, at a low temperature (750 °C), the grain growth energy is small, resulting in smaller grains and a reduced unit cell volume; at a high temperature (950 °C), the grain growth energy is large and the unit cell volume increases. Among them, the b-axis mainly represents Li +During the transmission process, the width of the one-dimensional channel. It can be seen that the b-axis length in Embodiment 1 is the largest, the transmission channel is wider, and the resistance during the charge and discharge process is smaller, which is also a proof of its better rate performance.

[0178] In addition, compared with the samples with slow heating rate in Comparative Examples 1-4, the crystallinity of the modified lithium iron phosphate prepared in the examples of this application is significantly higher, indicating that under this condition, the lithium iron phosphate has the best crystallinity and can provide more excellent electrochemical performance. In addition, only the first sintering treatment is carried out in Comparative Examples 5 and 6, which is also not conducive to crystal growth.

[0179] In addition, the ratio of pyrrole nitrogen content / total nitrogen content in Embodiment 1 is higher than that in Comparative Example 1 with slow heating rate and Embodiment 7 sintered in argon atmosphere. Pyrrole nitrogen can enhance the conductivity of the material, optimize the interfacial reaction kinetics between the electrode and the electrolyte, enable rapid lithium ion deintercalation and insertion, and improve the rate performance.

[0180] The 0.1C and 1C electrochemical performance test graphs of Examples 1-5 and Comparative Example 1 are as shown in the Figure 2 and 3 figure. It can be seen that the lithium iron phosphate cathode material prepared by the rapid non-equilibrium method of the examples of this application has a higher 0.1C and 1C discharge capacity than Comparative Example 1 at room temperature, and the difference is even greater at 1C, indicating that the rate performance of the examples is more excellent than that of the comparative examples, proving that the method provided by the examples of this application has an obvious improvement effect on the rate performance.

[0181] The above are only the preferred embodiments of this application and are not intended to limit this application. Any modifications, equivalent replacements, and improvements made within the spirit and principle of this application shall be included in the protection scope of this application.

Claims

1. A modified phosphate-based positive electrode material, characterized in that: It comprises a phosphate-based active core and a carbon coating layer; wherein the carbon coating layer is doped with pyrrole nitrogen, and the crystallinity of the phosphate-based active core is not less than 90%.

2. The modified phosphate-based positive electrode material according to claim 1, characterized in that: The carbon coating layer is also doped with pyridinic nitrogen and / or graphitic nitrogen; and / or, the phosphate-based active core is doped with at least one transition metal selected from titanium, manganese, cobalt, vanadium, niobium, magnesium and zirconium; And / or, the particle size D50 of the modified phosphate-based positive electrode material is 0.6 μm to 1.4 μm; And / or, the carbon coating layer has a thickness of 3 nm to 10 nm.

3. The modified phosphate-based positive electrode material according to claim 2, characterized in that: In the modified phosphate-based positive electrode material, the doping concentration of the transition metal is 1000ppm to 6000ppm; And / or, in the carbon coating layer, based on the total content of doped nitrogen atoms being 100%, the nitrogen atom content of the pyrrolic nitrogen is 10% to 20%.

4. The modified phosphate-based positive electrode material according to any one of claims 1 to 3, characterized in that: When the modified phosphate-based cathode material is lithium iron phosphate, the unit cell parameter b axis length 6.00~6.02; Alternatively, when the modified phosphate-based cathode material is lithium manganese iron phosphate, the unit cell parameter b axis length It is 6.02~6.

08.

5. A method for preparing a modified phosphate-based positive electrode material, characterized in that: The following steps are involved: Mixing raw materials including at least an iron source, a phosphorus source, a lithium source, a carbon source, and a nitrogen source to prepare a precursor material; After reaching the first sintering temperature, placing the precursor material at the first sintering temperature in a protective atmosphere for a first sintering treatment, and obtaining an intermediate product after cooling; After reaching the second sintering temperature, the intermediate product is placed in the second sintering temperature for a second sintering treatment under a sintering atmosphere, and a modified phosphate-based positive electrode material is obtained after cooling; the modified phosphate-based positive electrode material includes a phosphate-based active core and a carbon coating layer, wherein the carbon coating layer is doped with pyrrole nitrogen, and the crystallinity of the phosphate-based active core is not less than 90%.

6. The method for preparing a modified phosphate-based cathode material according to claim 5, characterized in that: The first sintering temperature is 550°C to 850°C, and the duration of the first sintering treatment is 8min to 25min; And / or, the second sintering temperature is 750° C. to 850° C., and the duration of the second sintering treatment is 2 min to 6 min; and / or, the precursor material is placed at the first sintering temperature for a period of 1 second to 3 seconds; and / or, the intermediate product is placed at the second sintering temperature for 1 second to 3 seconds; And / or, the temperature condition for cooling the intermediate product is 20°C to 40°C; And / or, the temperature condition for cooling the modified phosphate-based positive electrode material is 20° C. to 40° C.

7. The method for preparing a modified phosphate-based positive electrode material according to claim 5 or 6, characterized in that: The raw material also contains at least one transition metal doping source selected from the group consisting of a titanium source, a manganese source, a cobalt source, a vanadium source, a niobium source, a magnesium source and a zirconium source; And / or, the preparation method of the precursor material includes a solid phase method, a liquid phase method or a gel method; And / or, the carbon coating layer is further doped with pyridinic nitrogen and / or graphitic nitrogen.

8. The method for preparing a modified phosphate-based cathode material according to claim 7, characterized in that: The amount of the carbon source added is 1% to 20% of the theoretical output of the modified phosphate-based positive electrode material; And / or, the mass ratio of the carbon source to the nitrogen source is (5-15):1; And / or, the sintering atmosphere comprises argon and ammonia in a volume ratio of (2-4):1; And / or, in the modified phosphate-based positive electrode material, the concentration of the transition metal doping source is 1000 ppm to 6000 ppm.

9. The method for preparing a modified phosphate-based positive electrode material according to claim 8, characterized in that: The transition metal doping source includes at least one titanium source selected from titanium dioxide, titanium nitrate, tetrabutyl titanate, tetraethyl titanate, tetrapropyl titanate, tetraisopropyl titanate or titanium tetrachloride; And / or, the nitrogen source includes at least one of polyvinyl pyrrolidone, urea, melamine, ammonium carbonate, polypyrrole, chitosan and polyaniline.

10. A positive electrode sheet, characterized in that: It comprises a current collector and a positive electrode active layer formed on at least one surface of the current collector, wherein the positive electrode active layer contains the modified phosphate positive electrode material as described in any one of claims 1 to 4, or the modified phosphate positive electrode material prepared by the method as described in any one of claims 5 to 9.

11. A secondary battery, characterized in that: The secondary battery comprises the positive electrode sheet as claimed in claim 10.

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