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

Abstract

The application provides a lithium iron phosphate positive electrode material, a preparation method thereof and a battery, and relates to the field of lithium ion battery positive electrode materials. The lithium iron phosphate positive electrode material comprises an inner core and a shell layer located on at least part of the surface of the inner core, the material of the inner core comprises lithium iron phosphate, and the material of the shell layer comprises a biomass carbon material. The number of mesopores in the lithium iron phosphate positive electrode material is 78 / μm 2 ~102 / μm 2 , the number of micropores in the lithium iron phosphate positive electrode material is 151 / μm 2 ~202 / μm 2 , and the biomass carbon material is doped with Cu and N. When the lithium iron phosphate positive electrode material is applied to a battery, the battery has good low-temperature conductivity, high-rate performance and cycle stability.

Description

Technical Field

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

[0002] Lithium iron phosphate (LFP) cathode materials are widely used in the fabrication of lithium-ion batteries due to their stable structure, high safety, and long cycle life. However, traditional LFP cathode materials suffer from the following problems: (1) insufficient conductivity at low temperatures; (2) low capacity utilization and poor cycle performance at high rates; and (3) high cost and the generation of large amounts of wastewater under large-scale production conditions. Based on this, although some research teams have coated LFP with a carbon layer, which can improve the low-temperature conductivity and high-rate performance of LFP cathode materials to some extent, the cycle stability of LFP cathode materials remains poor due to the weak bond between the coating layer and LFP. Summary of the Invention

[0003] In view of the technical problems existing in the background art, this application provides a lithium iron phosphate cathode material, its preparation method and battery, aiming to solve the problems of insufficient low-temperature conductivity, poor high-rate performance, poor cycle stability, high production cost and large amount of wastewater of lithium iron phosphate cathode materials.

[0004] In a first aspect, embodiments of this application provide a lithium iron phosphate cathode material, comprising a core and a shell layer located on at least a portion of the surface of the core. The core material comprises lithium iron phosphate, and the shell layer material comprises biomass carbon material, the biomass carbon material having mesopores and micropores; in the lithium iron phosphate cathode material, the number of mesopores per unit area is 78 per μm. 2 ~102 cells / μm 2 The number of micropores per unit area is 151 / μm. 2 ~202 cells / μm 2 Biomass carbon materials are doped with copper (Cu) and nitrogen (N).

[0005] In the technical solutions of this application embodiment, when the number of mesopores and micropores are within the aforementioned range, the biomass carbon material possesses a good hierarchical carbon framework structure, which helps to tightly bond the shell and core, while reducing interfacial impedance and shortening the lithium-ion diffusion path, thereby improving the low-temperature conductivity, high-rate performance, and cycle stability of the lithium iron phosphate cathode material. Furthermore, doping the biomass carbon material with Cu and N helps to lower the lithium-ion diffusion barrier, which can further improve the low-temperature conductivity and high-rate performance of the lithium iron phosphate cathode material.

[0006] In some embodiments, the shell includes a copper-nitrogen coordination structure and a copper-nitrogen-carbon coordination structure formed by copper, nitrogen and carbon elements.

[0007] In this embodiment, the copper-nitrogen coordination structure and the copper-nitrogen-carbon coordination structure help to further improve the conductivity of the shell, thereby improving the electrochemical performance of the lithium iron phosphate cathode material.

[0008] In some embodiments, the number of mesopores per unit area in the lithium iron phosphate cathode material is 94 per μm. 2 ~102 cells / μm 2 .

[0009] In this embodiment, when the number of mesopores is within the above range, it is beneficial to further improve the hierarchical carbon skeleton structure, thereby enhancing the rate performance and cycle stability of the lithium iron phosphate cathode material.

[0010] In some embodiments, the number of micropores per unit area in the lithium iron phosphate cathode material is 190 pores / μm. 2 ~202 cells / μm 2 .

[0011] In this embodiment, when the number of micropores is within the above range, it is beneficial to further improve the hierarchical carbon skeleton structure, thereby enhancing the rate performance and cycle stability of the lithium iron phosphate cathode material.

[0012] Furthermore, in some embodiments, the mass ratio of copper to nitrogen in the lithium iron phosphate cathode material is (0.216~1.179):1.

[0013] In the technical solution of this application embodiment, the mass ratio of copper to nitrogen is within the above range, which helps to form a good hierarchical carbon skeleton structure and also helps to introduce highly conductive active sites, thereby improving the rate performance and cycle stability of lithium iron phosphate cathode material.

[0014] In some embodiments, the mass fraction of copper in the lithium iron phosphate cathode material is 0.005% to 0.071%.

[0015] In this embodiment, the mass fraction of copper within the above range helps to form efficient electron transport channels on the surface and inside of the biomass carbon material, thereby improving the conductivity and rate performance of the lithium iron phosphate cathode material.

[0016] In some embodiments, the mass fraction of nitrogen in the lithium iron phosphate cathode material is 0.014% to 0.068%. In this embodiment, the mass fraction of nitrogen within the above range helps to fully introduce active sites on the surface and inside the biomass carbon material, thereby improving the rate performance of the lithium iron phosphate cathode material.

[0017] In some embodiments, the mass fraction of carbon in the lithium iron phosphate cathode material is 1.16% to 1.62%.

[0018] In this embodiment, the mass fraction of carbon element is within the above range, which helps the biomass carbon material to fully encapsulate lithium iron phosphate, effectively improving the cycle stability and rate performance of the lithium iron phosphate cathode material.

[0019] In some embodiments, the porosity of the lithium iron phosphate cathode material is 17.0% to 21.2%.

[0020] In this embodiment, a porosity within the aforementioned range helps to promote electrolyte penetration and reduce interfacial impedance, thereby improving the rate performance and cycle stability of the lithium iron phosphate cathode material.

[0021] In some embodiments, the specific surface area of ​​the lithium iron phosphate cathode material is 19.3 m². 2 / g~23.6m 2 / g.

[0022] In this embodiment, a specific surface area within the above-mentioned range helps to shorten the diffusion path of lithium ions and reduce interfacial impedance, thereby improving the high-rate performance and low-temperature conductivity of lithium iron phosphate cathode materials.

[0023] In some embodiments, the lithium iron phosphate cathode material I D / I G The value is 1.0 to 1.3.

[0024] In this embodiment, the lithium iron phosphate cathode material I D / I G When the value is within the above range, it helps to significantly improve the structural stability and electronic conductivity of the coating layer, thereby improving the high-rate performance, low-temperature conductivity and cycle stability of lithium iron phosphate cathode materials.

[0025] In some embodiments, the biomass carbon material is a lignin-based carbon material.

[0026] In this embodiment, lignin-based carbon material refers to carbon material prepared using lignin as a biomass carbon source. Using lignin as a biomass carbon source helps form a biomass carbon material with abundant porosity and a high degree of graphitization during subsequent sintering, thereby improving the conductivity, rate performance, and cycle stability of the lithium iron phosphate cathode material. Simultaneously, lignin, as a carbon source derived from agricultural waste, is not only low-cost but also achieves the resource utilization of waste.

[0027] Secondly, embodiments of this application provide a method for preparing the lithium iron phosphate cathode material described in the first aspect, comprising the following steps:

[0028] Preparation of copper-nitrogen complex solutions;

[0029] The biomass carbon source and copper-nitrogen complex solution are subjected to a first mixing, a first drying and a first pulverizing process to obtain the first material;

[0030] The first material, phosphorus iron source, lithium source and first solvent are subjected to a second mixing and a second pulverizing process to obtain a slurry;

[0031] The slurry undergoes a second drying and sintering process to obtain lithium iron phosphate cathode material.

[0032] In the technical solution of this application embodiment, biomass carbon source is carbonized during sintering to form biomass carbon material. During this process, Cu promotes carbon rearrangement, and N helps increase in-plane defects in carbon. Together, they promote the formation of a hierarchical carbon framework structure. Simultaneously, Cu and N participate in the sintering process in a coordinated manner, helping to increase the number of mesopores and micropores in the lithium iron phosphate cathode material, while also promoting the uniform distribution of mesopores and micropores, thus giving the biomass carbon material a good hierarchical carbon framework structure. This carbon framework structure has a tighter bond with lithium iron phosphate, resulting in a more stable coating structure, which helps improve the structural stability and cycle stability of the lithium iron phosphate cathode material. Furthermore, this carbon framework structure facilitates electrolyte wetting, thereby helping to improve the conductivity and rate performance of the lithium iron phosphate cathode material. In addition, the solid-phase method used in this application generates no wastewater, has low environmental pressure, and is suitable for large-scale production.

[0033] In some embodiments, the copper-nitrogen coordination solution contains a copper-nitrogen coordination compound, and the steps for preparing the copper-nitrogen coordination solution include: mixing a soluble copper source, a soluble nitrogen source, and a second solvent in a third step to obtain the copper-nitrogen coordination solution.

[0034] In this embodiment, the pre-mixing of the third mixture helps to form a stable Cu-N coordination compound. The Cu-N coordination compound can promote the formation of a good hierarchical carbon framework structure in the subsequent sintering process. It also helps to dope Cu and N in biomass carbon materials and form copper-nitrogen coordination structures and copper-nitrogen-carbon coordination structures in the shell, thereby improving the rate performance and cycle stability of lithium iron phosphate cathode materials.

[0035] In some embodiments, the soluble copper source includes at least one of copper nitrate, copper acetate, and copper chloride.

[0036] In this embodiment, the soluble copper source can form a stable Cu-N coordination compound with the nitrogen source through a coordination reaction, thereby effectively improving the conductivity, rate performance and cycle stability of the lithium iron phosphate cathode material.

[0037] In some embodiments, the soluble nitrogen source includes at least one of melamine, dicyandiamine, aminocyanide, and urea.

[0038] In this embodiment, the soluble nitrogen source can form a stable Cu-N coordination compound with the copper source through a coordination reaction. It can also combine with the carbon source to form a pyrrole structure or a pyridine structure, thereby increasing the number of active sites at the interface between the biomass carbon material and lithium iron phosphate, reducing the diffusion barrier of lithium ions, and thus improving the conductivity, rate performance and cycle stability of the lithium iron phosphate cathode material.

[0039] In some embodiments, the mass ratio of the soluble copper source to the soluble nitrogen source is (3~10):6.

[0040] In this embodiment, the mass ratio of soluble copper source to soluble nitrogen source is within the above range, which helps to form a stable Cu-N coordination compound, thereby improving the conductivity, rate performance and cycle stability of the lithium iron phosphate cathode material.

[0041] In some embodiments, the mass ratio of the biomass carbon source to the copper element in the copper-nitrogen coordination solution is 100:(6~40).

[0042] In this embodiment, the mass ratio of copper in the biomass carbon source and the copper-nitrogen coordination solution is within the above range, which helps to ensure that copper is uniformly and fully loaded in the biomass carbon material, thereby improving the conductivity, rate performance and cycle stability of the lithium iron phosphate cathode material.

[0043] In some embodiments, the ratio of the total mass of the soluble copper source and the soluble nitrogen source to the mass of the second solvent is (7~13):50.

[0044] In this embodiment, the ratio of the total mass of the soluble copper source and the soluble nitrogen source to the mass of the second solvent is within the above range, which helps to promote the efficient reaction of the soluble copper source and the soluble nitrogen source and form a stable Cu-N coordination compound, while ensuring that the Cu-N coordination compound and the carbon source are uniformly mixed, thereby improving the conductivity, rate performance and cycle stability of the lithium iron phosphate cathode material.

[0045] In some embodiments, the second solvent includes at least one of deionized water, anhydrous ethanol, and ethylene glycol.

[0046] In this embodiment, the second solvent has good solubility and dispersibility, which helps to provide a stable solvent environment for the formation of Cu-N coordination compounds, while promoting uniform mixing of Cu-N coordination compounds and carbon sources, thereby improving the conductivity, rate performance and cycle stability of lithium iron phosphate cathode materials.

[0047] In some embodiments, the ferric phosphorus source includes at least one of anhydrous ferric phosphate, ferrous phosphate, ferrous dihydrogen phosphate, and ferrous pyrophosphate.

[0048] In this embodiment, the aforementioned iron-phosphorus source has good reactivity, which helps to form lithium iron phosphate efficiently and selectively, thereby improving the rate performance and cycle stability of the lithium iron phosphate cathode material.

[0049] In some embodiments, the lithium source includes at least one of lithium carbonate, lithium bicarbonate, and lithium hydroxide.

[0050] In this embodiment, the lithium source has high reactivity, which helps to improve the purity of the lithium iron phosphate obtained from the reaction, thereby improving the rate performance and cycle stability of the lithium iron phosphate cathode material.

[0051] In some embodiments, the molar ratio of lithium to iron in the slurry is 1:(1.04~1.06).

[0052] In this embodiment, limiting the molar ratio of lithium to iron within the above range helps to improve the crystallinity and structural stability of lithium iron phosphate, thereby improving the rate performance and cycle stability of lithium iron phosphate cathode materials.

[0053] In some embodiments, the first solvent includes at least one of deionized water, anhydrous ethanol, and ethylene glycol.

[0054] In this embodiment, the first solvent helps to uniformly disperse the first material, carbon source, iron phosphate source and lithium source, thereby promoting the uniform and sufficient coating of lithium iron phosphate by biomass carbon material and improving the rate performance and cycle stability of lithium iron phosphate cathode material.

[0055] In some embodiments, the solid content of the slurry is 10% to 30%.

[0056] In this embodiment, the slurry has good rheological properties when the solid content is within the above range, which is beneficial to the uniform and full coating of lithium iron phosphate by biomass carbon materials, thereby improving the rate performance and cycle stability of lithium iron phosphate cathode materials.

[0057] In some embodiments, the sintering temperature is 600°C to 800°C.

[0058] In this embodiment, the sintering temperature within the above range helps to fully carbonize the biomass carbon source. At the same time, under the action of Cu-N coordination compounds, it helps to form a good hierarchical carbon skeleton structure, thereby improving the high-rate performance, low-temperature conductivity and cycle stability of the lithium iron phosphate cathode material.

[0059] In some embodiments, the holding time for sintering is 6h to 8h.

[0060] In this embodiment, the holding time of the sintering process is within the above range, which helps to enable the biomass carbon material to have both a high degree of graphitization and a good hierarchical carbon skeleton structure, thereby improving the high rate performance, low temperature conductivity and cycle stability of the lithium iron phosphate cathode material.

[0061] In some embodiments, the temperature for the first drying is 120°C to 180°C.

[0062] In this embodiment, the temperature of the first drying process is within the above range, which can promote the uniform distribution of Cu and N in the shell, thereby improving the high-rate performance, low-temperature conductivity and cycle stability of the lithium iron phosphate cathode material.

[0063] In some embodiments, the temperature for the second drying is 90°C to 120°C.

[0064] In this embodiment, the temperature of the second drying is within the above range, which helps to stably remove the solvent in the slurry and promotes the uniform coating of biomass carbon materials, thereby improving the high-rate performance, low-temperature conductivity and cycle stability of the lithium iron phosphate cathode material.

[0065] In some embodiments, the median particle size (D50) of the biomass carbon source is 35 μm to 45 μm.

[0066] In this embodiment, the median particle size of the biomass carbon source is within the above range, which helps to mix it uniformly with Cu-N coordination compounds and promotes the formation of a good hierarchical carbon skeleton structure, thereby improving the high-rate performance, low-temperature conductivity and cycle stability of the lithium iron phosphate cathode material.

[0067] In some embodiments, the median particle size of the first material is 35 μm to 45 μm.

[0068] In this embodiment, the median particle size of the first material is within the above-mentioned range, which helps to form a continuous and uniformly thick carbon coating layer on the surface of lithium iron phosphate, thereby improving the high-rate performance, low-temperature conductivity and cycle stability of the lithium iron phosphate cathode material.

[0069] In some embodiments, the median particle size of the solid particles in the slurry is 0.35 μm to 0.45 μm.

[0070] In this embodiment, the median particle size of the solid particles in the slurry is within the above range, which helps to form a lithium iron phosphate cathode material with a moderate particle size and a moderate specific surface area, thereby improving the high-rate performance, low-temperature conductivity and cycle stability of the lithium iron phosphate cathode material.

[0071] Thirdly, embodiments of this application provide a battery comprising a positive electrode material, a negative electrode material, and an electrolyte, wherein the positive electrode material is the lithium iron phosphate positive electrode material provided in the first aspect, or the lithium iron phosphate positive electrode material prepared by the preparation method provided in the second aspect.

[0072] In this embodiment, the battery contains the aforementioned lithium iron phosphate cathode material, thus exhibiting excellent low-temperature conductivity, rate performance, and cycle performance.

[0073] The above description is only an overview of the technical solution of this application. In order to better understand the technical means of this application and to implement it in accordance with the contents of the specification, and to make the above and other objects, features and advantages of this application more obvious and understandable, the following are specific embodiments of this application. Attached Figure Description

[0074] To more clearly illustrate the technical solutions of this application, the accompanying drawings used in this application will be briefly described below. Obviously, the drawings described below are merely some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without any creative effort.

[0075] Figure 1 The image shown is a SEM image of the lithium iron phosphate cathode material provided in Example 1 of this application, magnified 20,000 times.

[0076] Figure 2 This is a SEM image of the lithium iron phosphate cathode material provided in Example 1 of this application, magnified at 50,000 times. Detailed Implementation

[0077] The embodiments of the technical solution of this application will now be described in detail with reference to the accompanying drawings. These embodiments are only used to more clearly illustrate the technical solution of this application and are therefore merely examples, and should not be used to limit the scope of protection of this application.

[0078] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having”, and any variations thereof, in the specification, claims, and foregoing description of the drawings are intended to cover non-exclusive inclusion.

[0079] In the description of the embodiments of this application, technical terms such as "first" and "second" are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly specifying the number, specific order, or primary and secondary relationship of the indicated technical features. In the description of the embodiments of this application, "multiple" means two or more, unless otherwise explicitly defined.

[0080] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0081] In the description of the embodiments in this application, the term "and / or" is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Additionally, the character " / " in this document generally indicates that the preceding and following related objects have an "or" relationship.

[0082] Traditional lithium iron phosphate cathode materials have the following problems: (1) insufficient conductivity at low temperatures; (2) low capacity utilization and poor cycle performance at high rates; (3) high cost under large-scale production conditions and easy generation of large amounts of wastewater. Coating the surface of lithium iron phosphate with a carbon layer can improve the low-temperature conductivity and high-rate performance of lithium iron phosphate cathode materials to a certain extent, but there is a problem that the coating layer is not tightly bonded to lithium iron phosphate, resulting in poor cycle stability of lithium iron phosphate cathode materials.

[0083] To address at least one of the aforementioned technical problems, this application provides a lithium iron phosphate cathode material, its preparation method, and a battery. By limiting the number of mesopores and micropores per unit area in the lithium iron phosphate cathode material, the biomass carbon material can be ensured to have a good hierarchical carbon framework structure. At the same time, the doping of Cu and N can introduce highly conductive active sites, thereby improving the battery's low-temperature conductivity, high-rate performance, and cycle stability.

[0084] In a first aspect, embodiments of this application provide a lithium iron phosphate cathode material, comprising a core and a shell layer located on at least a portion of the surface of the core. The core material comprises lithium iron phosphate, and the shell layer material comprises biomass carbon material, the biomass carbon material having mesopores and micropores; in the lithium iron phosphate cathode material, the number of mesopores per unit area is 78 per μm. 2 ~102 cells / μm 2 The number of micropores per unit area is 151 / μm. 2 ~202 cells / μm2 The biomass carbon material is doped with Cu and N.

[0085] The mesopores have a pore size of 2 nm to 50 nm, while the micropores have a pore size of less than 2 nm. In this invention, when the number of mesopores and micropores is within the above range, the biomass carbon material possesses a good hierarchical carbon framework structure. This carbon framework structure helps to tightly bond the shell and core, improving the structural and cycle stability of the lithium iron phosphate cathode material. Furthermore, it helps to reduce interfacial impedance and shorten the lithium-ion diffusion path, thereby improving the low-temperature conductivity and high-rate performance of the lithium iron phosphate cathode material. In addition, doping the biomass carbon material with Cu and N helps to lower the lithium-ion diffusion barrier, which can further improve the low-temperature conductivity and high-rate performance of the lithium iron phosphate cathode material.

[0086] Specifically, a hierarchical carbon framework structure refers to a carbon framework structure that simultaneously possesses a large number of mesopores and a large number of micropores. More specifically, the ratio of mesopores to micropores per unit area is (0.469~0.555):1. Preferably, the ratio of mesopores to micropores per unit area is (0.495~0.505):1.

[0087] Typical, but not limiting, lithium iron phosphate cathode materials have a mesopore count of 78 per μm. 2 80 cells / μm 2 85 cells / μm 2 90 cells / μm 2 95 cells / μm 2 100 cells / μm 2 102 cells / μm 2 Or a range of values ​​consisting of any two of its values.

[0088] Typical, but not limiting, lithium iron phosphate cathode materials have a micropore count of 151 per μm. 2 160 cells / μm 2 170 cells / μm 2 180 cells / μm 2 190 cells / μm 2 202 cells / μm 2 Or a range of values ​​consisting of any two of its values.

[0089] Furthermore, in some embodiments, the shell includes copper-nitrogen coordination structures and copper-nitrogen-carbon coordination structures formed by copper, nitrogen, and carbon elements.

[0090] In the technical solution of this application embodiment, the above-mentioned coordination structure helps to further improve the electronic conductivity of the shell, and at the same time introduces active sites at the interface between the shell and the core, thereby reducing the diffusion barrier of lithium ions and simultaneously improving the electrochemical performance of lithium iron phosphate cathode material.

[0091] Furthermore, in some embodiments, the number of mesopores per unit area in the lithium iron phosphate cathode material is 94 per μm. 2 ~102 cells / μm 2 .

[0092] In the technical solutions of this application embodiment, when the number of mesopores is within the above range, it is beneficial to further improve the hierarchical carbon skeleton structure, promote electrolyte wetting and efficient penetration, thereby improving the rate performance and cycle stability of lithium iron phosphate cathode material.

[0093] Furthermore, in some embodiments, the lithium iron phosphate cathode material has a micropore count of 190 per μm per unit area. 2 ~202 cells / μm 2 .

[0094] In the technical solutions of this application embodiment, when the number of micropores is within the above range, it is beneficial to further improve the hierarchical carbon skeleton structure, promote the tight bonding between the shell and the core, and effectively shorten the diffusion path of lithium ions, thereby improving the rate performance and cycle stability of lithium iron phosphate cathode materials.

[0095] Furthermore, in some embodiments, the mass ratio of copper to nitrogen in the lithium iron phosphate cathode material is (0.216~1.179):1. Preferably, the mass ratio of copper to nitrogen in the lithium iron phosphate cathode material is (0.333~1.039):1.

[0096] In the technical solution of this application embodiment, the mass ratio of copper to nitrogen is within the above range, which helps to further improve the hierarchical carbon skeleton structure. At the same time, it fully introduces highly conductive active sites into the lithium iron phosphate cathode material, reduces the lithium-ion diffusion barrier, and thus improves the rate performance and cycle stability of the lithium iron phosphate cathode material.

[0097] Furthermore, in some embodiments, the mass fraction of copper in the lithium iron phosphate cathode material is 0.005% to 0.071%. Preferably, the mass fraction of copper in the lithium iron phosphate cathode material is 0.011% to 0.053%.

[0098] In the technical solutions of this application embodiment, the mass fraction of copper within the aforementioned range helps to form efficient electron transport channels on the surface and inside the biomass carbon material, improving the conductivity and rate performance of the lithium iron phosphate cathode material. Furthermore, a copper content within the aforementioned range also helps to promote the formation of a graded carbon framework, thereby improving the rate performance and cycle stability of the lithium iron phosphate cathode material.

[0099] Typical, but not limiting, copper content in lithium iron phosphate cathode materials is 0.005%, 0.010%, 0.020%, 0.030%, 0.040%, 0.050%, 0.060%, 0.071%, or any two of these values.

[0100] Furthermore, in some embodiments, the mass fraction of nitrogen in the lithium iron phosphate cathode material is 0.014% to 0.068%. Preferably, the mass fraction of nitrogen in the lithium iron phosphate cathode material is 0.033% to 0.051%.

[0101] In the technical solution of this application embodiment, the mass fraction of nitrogen element within the above range helps to fully introduce active sites on the surface and inside of biomass carbon materials, significantly improving ion transport performance. At the same time, it also helps to promote the formation of hierarchical carbon skeleton, thereby improving the rate performance and cycle stability of lithium iron phosphate cathode materials.

[0102] Typical, but not limiting, nitrogen content in lithium iron phosphate cathode materials is 0.014%, 0.020%, 0.030%, 0.040%, 0.050%, 0.060%, 0.068%, or any two of these values.

[0103] Furthermore, in some embodiments, the mass fraction of carbon in the lithium iron phosphate cathode material is 1.16% to 1.62%. Preferably, the mass fraction of carbon in the lithium iron phosphate cathode material is 1.18% to 1.62%.

[0104] In the technical solutions of this application embodiment, the carbon mass fraction within the aforementioned range helps the biomass carbon material to fully encapsulate lithium iron phosphate, effectively improving the cycle stability and rate performance of the lithium iron phosphate cathode material. Furthermore, a carbon mass fraction within the aforementioned range also helps to form a hierarchical carbon framework structure with better mechanical properties, thereby improving the cycle stability of the lithium iron phosphate cathode material.

[0105] Typical, but not limiting, carbon content in lithium iron phosphate cathode materials is 1.16%, 1.20%, 1.30%, 1.40%, 1.50%, 1.60%, 1.62%, or any two of these values.

[0106] Furthermore, in some embodiments, the porosity of the lithium iron phosphate cathode material is 17.0% to 21.2%. Preferably, the porosity of the lithium iron phosphate cathode material is 19.8% to 21.2%.

[0107] In the technical solutions of this application embodiment, the porosity within the above range helps to promote electrolyte penetration and reduce interfacial impedance, thereby improving the rate performance and cycle stability of lithium iron phosphate cathode materials.

[0108] Typical, but not limiting, porosity of lithium iron phosphate cathode materials is 17%, 18%, 19%, 20%, 21%, 21.2%, or any two of these values.

[0109] Furthermore, in some embodiments, the specific surface area of ​​the lithium iron phosphate cathode material is 19.3 m². 2 / g~23.6m 2 / g. Preferably, the specific surface area of ​​the lithium iron phosphate cathode material is 22.1m². 2 / g~23.6m 2 / g.

[0110] In the technical solution of this application embodiment, when the specific surface area is within the above range, the lithium iron phosphate cathode material has more exposed active sites on its surface and is more easily wetted by the electrolyte, which helps to shorten the diffusion path of lithium ions and reduce the interface impedance, thereby improving the high rate performance and low temperature conductivity of the lithium iron phosphate cathode material.

[0111] A typical, but not limiting, specific surface area of ​​lithium iron phosphate cathode materials is 19.3 m². 2 / g、20m 2 / g、21m 2 / g、22m 2 / g、23m 2 / g, 23.6m 2 / g or a range of values ​​consisting of any two of its values.

[0112] Furthermore, in some embodiments, the lithium iron phosphate cathode material I D / I G The value is 1.0~1.3. Preferably, the I of the lithium iron phosphate cathode material is... D / I G The value is 1.2~1.3.

[0113] I D / I G The value represents the ratio of the intensity of peak D to the intensity of peak G in the Raman spectrum, where peak D is located at 1350 cm⁻¹. -1 ~1360cm -1Within the range, peak G is located at 1580 cm. -1 ~1600cm -1 Within the scope. In the technical solution of this application embodiment, the lithium iron phosphate cathode material I D / I G When the value is within the above range, it indicates that the graphitization degree of the biomass carbon material is moderate, which helps to significantly improve the structural stability and electronic conductivity of the coating layer, thereby improving the high-rate performance, low-temperature conductivity and cycle stability of the lithium iron phosphate cathode material.

[0114] Typical, but not limiting, lithium iron phosphate cathode materials I D / I G The value is a range of 1.0, 1.1, 1.2, 1.3 or any two of them.

[0115] Furthermore, in some embodiments, the median particle size of the lithium iron phosphate cathode material is 35.6 μm to 45.2 μm. Preferably, the median particle size of the lithium iron phosphate cathode material is 39.8 μm to 41.5 μm.

[0116] In the technical solutions of this application embodiment, when the median particle size of the lithium iron phosphate cathode material is within the above range, it has a suitable specific surface area and mechanical properties, which helps to improve the electrochemical performance of the lithium iron phosphate cathode material.

[0117] Furthermore, in some embodiments, the biomass carbon material is a lignin-based carbon material.

[0118] In the technical solutions of this application embodiment, lignin-based carbon materials refer to carbon materials prepared using lignin as a biomass carbon source. Using lignin as a biomass carbon source helps form biomass carbon materials with rich porosity and a high degree of graphitization during subsequent sintering, thereby improving the conductivity, rate performance, and cycle stability of lithium iron phosphate cathode materials. Simultaneously, lignin, as a carbon source derived from agricultural waste, is not only low-cost but also achieves the resource utilization of waste.

[0119] Secondly, embodiments of this application provide a method for preparing the lithium iron phosphate cathode material described in the first aspect, comprising the following steps:

[0120] Preparation of copper-nitrogen complex solutions;

[0121] The biomass carbon source and copper-nitrogen complex solution are subjected to a first mixing, a first drying and a first pulverizing process to obtain the first material;

[0122] The first material, phosphorus iron source, lithium source and first solvent are subjected to a second mixing and a second pulverizing process to obtain a slurry;

[0123] The slurry undergoes a second drying and sintering process to obtain lithium iron phosphate cathode material.

[0124] In the above preparation method, a biomass carbon source and a Cu-N coordination solution are first premixed. Then, the premixed material is mixed with an iron phosphate source and a lithium source. Next, a carbon coating layer is formed in situ on the surface of the lithium iron phosphate through sintering. The carbon coating layer is doped with Cu and N. During this process, the biomass carbon source carbonizes during sintering to form biomass carbon material. Cu promotes carbon rearrangement, and N helps increase in-plane defects in the carbon, thereby promoting the formation of a hierarchical carbon framework structure. Simultaneously, Cu and N participate in the sintering process in a coordinated manner, helping to increase the number of mesopores and micropores in the lithium iron phosphate cathode material, and promoting the uniform distribution of mesopores and micropores. Therefore, the biomass carbon material has a good hierarchical carbon framework structure. The hierarchical carbon framework structure binds more tightly to the lithium iron phosphate, resulting in a more stable coating structure, which helps improve the structural stability and cycle stability of the lithium iron phosphate cathode material. Furthermore, the hierarchical carbon framework structure facilitates electrolyte wetting, thus helping to improve the conductivity and rate performance of the lithium iron phosphate cathode material. In low-temperature environments, when the electrolyte viscosity increases and it becomes difficult to wet the cathode material, the hierarchical carbon framework structure helps to promote the wetting effect of the electrolyte, thereby improving the low-temperature conductivity of the lithium iron phosphate cathode material. Furthermore, the solid-phase method used in this application generates no wastewater, has low environmental impact, and is suitable for large-scale production.

[0125] Furthermore, in some embodiments, the copper-nitrogen coordination solution contains a copper-nitrogen coordination compound, and the steps for preparing the copper-nitrogen coordination solution include: mixing a soluble copper source, a soluble nitrogen source, and a second solvent in a third step to obtain the copper-nitrogen coordination solution.

[0126] In the technical solution of this application embodiment, the third mixing is performed in advance, which helps to form a stable Cu-N coordination compound. The Cu-N coordination compound can promote the formation of a good hierarchical carbon skeleton structure in the subsequent sintering process. At the same time, it helps to dope Cu and N in biomass carbon materials and form copper-nitrogen coordination structure and copper-nitrogen-carbon coordination structure in the shell, thereby improving the rate performance and cycle stability of lithium iron phosphate cathode material.

[0127] It should also be noted that the first mixing helps to uniformly mix Cu-N coordination compounds with biomass carbon materials, promotes the uniform distribution of mesopores and micropores in lithium iron phosphate cathode materials, further improves the hierarchical carbon framework structure, and thus enhances the rate performance and cycle stability of lithium iron phosphate cathode materials.

[0128] Specifically, the third mixing is carried out under stirring conditions.

[0129] Furthermore, in some embodiments, the soluble copper source includes at least one of copper nitrate, copper acetate, and copper chloride.

[0130] In the technical solution of this application embodiment, the above-mentioned soluble copper source can form a stable Cu-N coordination compound with the nitrogen source through a coordination reaction, thereby effectively improving the conductivity, rate performance and cycle stability of the lithium iron phosphate cathode material.

[0131] Furthermore, in some embodiments, the soluble nitrogen source includes at least one of melamine, dicyandiamine, aminocyanide, and urea.

[0132] In the technical solution of this application embodiment, the above-mentioned soluble nitrogen source can form a stable Cu-N coordination compound with the copper source through a coordination reaction. At the same time, it can also combine with the carbon source to form a pyrrole structure or a pyridine structure, which increases the number of active sites at the interface between the biomass carbon material and lithium iron phosphate, reduces the diffusion barrier of lithium ions, and thus improves the conductivity, rate performance and cycle stability of the lithium iron phosphate cathode material.

[0133] Furthermore, in some embodiments, the mass ratio of the soluble copper source to the soluble nitrogen source is (3~10):6. In the technical solutions of this application embodiments, the mass ratio of the soluble copper source to the soluble nitrogen source within the above range helps to form a stable Cu-N coordination compound, thereby improving the conductivity, rate performance, and cycle stability of the lithium iron phosphate cathode material.

[0134] Furthermore, in some embodiments, the mass ratio of the biomass carbon source to the copper element in the copper-nitrogen coordination solution is 100:(6~40). Preferably, the mass ratio of the biomass carbon source to the copper element in the copper-nitrogen coordination solution is 100:(6~15).

[0135] In the technical solution of this application embodiment, the mass ratio of copper element in the biomass carbon source and copper-nitrogen coordination solution is within the above range, which helps to ensure that copper element is uniformly and fully loaded in the biomass carbon material, thereby improving the conductivity, rate performance and cycle stability of the lithium iron phosphate cathode material.

[0136] Furthermore, in some embodiments, the mass ratio of the total mass of the soluble copper source and the soluble nitrogen source to the mass of the second solvent is (7~13):50. Preferably, the mass ratio of the total mass of the soluble copper source and the soluble nitrogen source to the mass of the second solvent is (8~10):50.

[0137] In the technical solution of this application embodiment, the ratio of the total mass of the soluble copper source and the soluble nitrogen source to the mass of the second solvent is within the above range, which helps to promote the efficient reaction of the soluble copper source and the soluble nitrogen source and form a stable Cu-N coordination compound, while ensuring that the Cu-N coordination compound and the carbon source are uniformly mixed, thereby improving the conductivity, rate performance and cycle stability of the lithium iron phosphate cathode material.

[0138] Furthermore, in some embodiments, the second solvent includes at least one of deionized water, anhydrous ethanol, and ethylene glycol.

[0139] In the technical solution of this application embodiment, the second solvent has good solubility and dispersibility, which helps to provide a stable solvent environment for the formation of Cu-N coordination compounds, and at the same time promotes the uniform mixing of Cu-N coordination compounds and carbon source, thereby improving the conductivity, rate performance and cycle stability of lithium iron phosphate cathode material.

[0140] Furthermore, in some embodiments, the ferric phosphorus source includes at least one of anhydrous ferric phosphate, ferrous phosphate, ferrous dihydrogen phosphate, and ferrous pyrophosphate.

[0141] In the technical solution of this application embodiment, the above-mentioned iron phosphate source has good reactivity, which helps to form lithium iron phosphate efficiently and selectively, thereby improving the rate performance and cycle stability of lithium iron phosphate cathode material.

[0142] Furthermore, in some embodiments, the lithium source includes at least one of lithium carbonate, lithium bicarbonate, and lithium hydroxide.

[0143] In the technical solution of this application embodiment, the lithium source has high reactivity, which helps to improve the purity of the lithium iron phosphate obtained by the reaction, thereby improving the rate performance and cycle stability of the lithium iron phosphate cathode material.

[0144] Furthermore, in some embodiments, the molar ratio of lithium to iron in the slurry is 1:(1.04~1.06).

[0145] In the technical solutions of this application embodiment, limiting the molar ratio of lithium to iron to the above range helps to improve the crystallinity and structural stability of lithium iron phosphate, thereby improving the rate performance and cycle stability of lithium iron phosphate cathode material.

[0146] Furthermore, in some embodiments, the first solvent includes at least one of deionized water, anhydrous ethanol, and ethylene glycol.

[0147] In the technical solution of this application embodiment, the first solvent helps to uniformly disperse the first material, carbon source, iron phosphate source and lithium source, thereby promoting the uniform and sufficient coating of lithium iron phosphate by biomass carbon material and improving the rate performance and cycle stability of lithium iron phosphate cathode material.

[0148] Furthermore, in some embodiments, the solid content of the slurry is 10% to 30%.

[0149] In the technical solution of this application embodiment, the slurry has better rheological properties when the solid content is within the above range, which is conducive to the uniform and full coating of lithium iron phosphate by biomass carbon materials, optimizes the carbon layer structure, and improves the rate performance and cycle stability of lithium iron phosphate cathode materials.

[0150] Furthermore, in some embodiments, the sintering temperature is 600°C to 800°C.

[0151] In the technical solution of this application embodiment, the sintering temperature within the above range helps to fully carbonize the biomass carbon source. At the same time, under the action of Cu-N coordination compounds, it helps to form a good hierarchical carbon skeleton structure, thereby improving the high rate performance, low temperature conductivity and cycle stability of lithium iron phosphate cathode material.

[0152] Preferably, the sintering temperature is 740℃~760℃. This temperature range is conducive to the formation of smaller lithium iron phosphate particles, thereby increasing the specific surface area of ​​the carbon layer, improving the conductivity, and enhancing the high-rate performance and low-temperature conductivity of the lithium iron phosphate cathode material.

[0153] Furthermore, in some embodiments, the holding time for sintering is 6h to 8h.

[0154] In the technical solution of this application embodiment, the holding time of the sintering process is within the above range, which helps to enable the biomass carbon material to have both a high degree of graphitization and a good hierarchical carbon skeleton structure, thereby improving the high rate performance, low temperature conductivity and cycle stability of the lithium iron phosphate cathode material.

[0155] Furthermore, in some embodiments, the sintering process is carried out in a nitrogen or inert atmosphere; the inert atmosphere includes at least one of argon and helium.

[0156] In the technical solution of this application embodiment, the sintering process is carried out in the above atmosphere, which helps to provide an oxygen-free environment for the carbonization of biomass carbon source, promotes the transformation of biomass carbon material into biomass carbon material with both high graphitization degree and good pore structure, thereby improving the high rate performance, low temperature conductivity and cycle stability of lithium iron phosphate cathode material.

[0157] Furthermore, in some embodiments, the temperature for the first drying is 120°C to 180°C.

[0158] In the technical solution of this application embodiment, the temperature of the first drying is within the above range, which can ensure that the biomass carbon source and Cu and N are uniformly mixed while fully removing the solvent, thereby promoting the uniform distribution of Cu and N in the shell layer, and thus improving the high rate performance, low temperature conductivity and cycle stability of the lithium iron phosphate cathode material.

[0159] Furthermore, in some embodiments, the temperature for the second drying is 90°C to 120°C.

[0160] In the technical solution of this application embodiment, the temperature of the second drying is within the above range, which helps to stably remove the solvent in the slurry and promotes the uniform coating of biomass carbon materials, thereby improving the high-rate performance, low-temperature conductivity and cycle stability of lithium iron phosphate cathode materials.

[0161] Specifically, in the second drying process, spray drying granulation is used to process the slurry.

[0162] Furthermore, in some embodiments, the median particle size of the biomass carbon source is 35 μm to 45 μm.

[0163] In the technical solution of this application embodiment, the median particle size of the biomass carbon source is within the above range, which helps to uniformly mix it with Cu-N coordination compounds and promote the formation of a good hierarchical carbon skeleton structure, thereby improving the high rate performance, low temperature conductivity and cycle stability of lithium iron phosphate cathode material.

[0164] Furthermore, in some embodiments, the median particle size of the first material is 35 μm to 45 μm.

[0165] In the technical solution of this application embodiment, the median particle size of the first material is within the above range, which helps to form a continuous and uniformly thick carbon coating layer on the surface of lithium iron phosphate, thereby improving the high rate performance, low temperature conductivity and cycle stability of lithium iron phosphate cathode material.

[0166] Furthermore, in some embodiments, the median particle size of the solid particles in the slurry is 0.35 μm to 0.45 μm.

[0167] In the technical solution of this application embodiment, the median particle size of the solid particles in the slurry is within the above range, which helps to form a lithium iron phosphate cathode material with moderate particle size and specific surface area, thereby improving the high rate performance, low temperature conductivity and cycle stability of the lithium iron phosphate cathode material.

[0168] Thirdly, embodiments of this application provide a battery comprising a positive electrode material, a negative electrode material, and an electrolyte, wherein the positive electrode material is the lithium iron phosphate positive electrode material provided in the first aspect, or the lithium iron phosphate positive electrode material prepared by the preparation method provided in the second aspect.

[0169] In the technical solution of this application embodiment, the battery contains the above-mentioned lithium iron phosphate cathode material, and thus has good low-temperature conductivity, rate performance and cycle performance.

[0170] The following are some specific embodiments. It should be noted that the embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they shall be performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments whose manufacturers are not specified are all conventional products that can be obtained commercially.

[0171] I. Preparation Method

[0172] Example 1

[0173] This embodiment provides a lithium iron phosphate cathode material, comprising a core and a shell located on at least a portion of the surface of the core. The core is made of lithium iron phosphate, and the shell is made of lignin-based carbon material (biomass carbon material). The lignin-based carbon material has mesoporous and microporous structures, and is doped with copper and nitrogen. The shell includes copper-nitrogen coordination structures and copper-nitrogen-carbon coordination structures formed by copper, nitrogen, and biomass carbon material. The mass fractions of copper, nitrogen, and carbon in the lithium iron phosphate cathode material are shown in Table 1.

[0174] The preparation method of lithium iron phosphate cathode material includes the following steps:

[0175] In step S1, copper chloride, dicyandiamine, and deionized water are mixed in a mass ratio of 3:6:50 and stirred for 30 min to prepare a copper-nitrogen complex solution; straw is crushed to a median particle size of 40 μm to obtain a lignin-based carbon source.

[0176] Step S2: The lignin-based carbon source obtained in step S1 and the copper-nitrogen coordination solution obtained in step S1 are mixed, ultrasonically dispersed for 20 min, and then magnetically stirred for 2 h to obtain a first mixture. The first mixture is then placed in a vacuum environment and dried at 150°C for 12 h to obtain a first compound. The first compound is then pulverized to a median particle size of 40 μm to obtain the first material. The mass ratio of the lignin-based carbon source to copper chloride is 100:6.

[0177] Step S3: The first material obtained in step S2, anhydrous iron phosphate, lithium carbonate and deionized water are mixed and ground to obtain a slurry with a median particle size of 0.4 μm; wherein the solid content of the slurry is 20%; in the slurry, the molar ratio of lithium to iron is 1:1.05, and the mass ratio of lignin-based carbon source to lithium carbonate is 3:7.4.

[0178] Step S4: Spray dry and granulate the slurry at 105°C to obtain a second mixture. Place the second mixture in an argon atmosphere and sinter at 750°C for 7 hours to obtain lithium iron phosphate cathode material.

[0179] Example 2

[0180] This embodiment provides a lithium iron phosphate cathode material, comprising a core and a shell located on at least a portion of the surface of the core. The core is made of lithium iron phosphate, and the shell is made of lignin-based carbon material. The lignin-based carbon material has mesoporous and microporous structures and is doped with copper and nitrogen. The shell includes copper-nitrogen coordination structures and copper-nitrogen-carbon coordination structures formed by copper, nitrogen, and biomass carbon material. The mass fractions of copper, nitrogen, and carbon in the lithium iron phosphate cathode material are shown in Table 1.

[0181] The preparation method of lithium iron phosphate cathode material includes the following steps:

[0182] Step S1: Copper nitrate trihydrate, melamine and deionized water are mixed in a mass ratio of 5:3:50 and stirred for 20 min to prepare a copper-nitrogen complex solution; straw is crushed to a median particle size of 35 μm to obtain a lignin-based carbon source.

[0183] Step S2: The lignin-based carbon source obtained in step S1 and the copper-nitrogen coordination solution obtained in step S1 are mixed, ultrasonically dispersed for 15 min, and then magnetically stirred for 1.5 h to obtain a first mixture. The first mixture is then dried in a vacuum environment at 120°C for 14 h to obtain a first compound. The first compound is then pulverized to a median particle size of 35 μm to obtain the first material. The mass ratio of the lignin-based carbon source to copper nitrate trihydrate is 100:10.

[0184] Step S3: The first material obtained in step S2, ferrous phosphate, lithium bicarbonate and deionized water are mixed and ground to obtain a slurry with a median particle size of 0.35 μm; wherein the solid content of the slurry is 10%; in the slurry, the molar ratio of lithium to iron is 1:1.04, and the mass ratio of lignin-based carbon source to lithium carbonate is 3.5:7.4.

[0185] Step S4: Spray dry and granulate the slurry at 90°C to obtain a second mixture. Place the second mixture in a helium atmosphere and sinter at 740°C for 8 hours to obtain lithium iron phosphate cathode material.

[0186] Example 3

[0187] This embodiment provides a lithium iron phosphate cathode material, comprising a core and a shell located on at least a portion of the surface of the core. The core is made of lithium iron phosphate, and the shell is made of lignin-based carbon material. The lignin-based carbon material has mesoporous and microporous structures and is doped with copper and nitrogen. The shell includes copper-nitrogen coordination structures and copper-nitrogen-carbon coordination structures formed by copper, nitrogen, and biomass carbon material. The mass fractions of copper, nitrogen, and carbon in the lithium iron phosphate cathode material are shown in Table 1.

[0188] The preparation method of lithium iron phosphate cathode material includes the following steps:

[0189] In step S1, copper acetate, urea and deionized water are mixed in a mass ratio of 6:4:50 and stirred for 40 min to prepare a copper-nitrogen complex solution; straw is crushed to a median particle size of 45 μm to obtain a lignin-based carbon source.

[0190] Step S2: The lignin-based carbon source obtained in step S1 and the copper-nitrogen coordination solution obtained in step S1 are mixed, ultrasonically dispersed for 25 min, and then magnetically stirred for 2.5 h to obtain a first mixture. The first mixture is then dried in a vacuum environment at 180°C for 10 h to obtain a first compound. The first compound is then pulverized to a median particle size of 45 μm to obtain the first material. The mass ratio of the lignin-based carbon source to copper acetate is 100:15.

[0191] Step S3: The first material obtained in step S2, ferrous pyrophosphate, lithium hydroxide and deionized water are mixed and ground to obtain a slurry with a median particle size of 0.45 μm; wherein the solid content of the slurry is 30%; in the slurry, the molar ratio of lithium to iron is 1:1.06, and the mass ratio of lignin-based carbon source to lithium carbonate is 4:7.4.

[0192] Step S4: Spray dry and granulate the slurry at 120°C to obtain a second mixture. Place the second mixture in a nitrogen atmosphere and sinter at 760°C for 6 hours to obtain lithium iron phosphate cathode material.

[0193] Example 4

[0194] The difference between this embodiment and Embodiment 1 is that in step S1, copper chloride, dicyandiamine and deionized water are mixed in a mass ratio of 3:10:50, and the mass ratio of lignin-based carbon source to copper chloride is 100:6.

[0195] Example 5

[0196] The difference between this embodiment and Embodiment 1 is that in step S1, copper chloride, dicyandiamine and deionized water are mixed in a mass ratio of 4:3:50, and the mass ratio of lignin-based carbon source to copper chloride is 100:20.

[0197] Example 6

[0198] The difference between this embodiment and Embodiment 1 is that in step S4, the second mixture is sintered at 600°C for 7 hours in a nitrogen atmosphere to obtain lithium iron phosphate cathode material.

[0199] Example 7

[0200] The difference between this embodiment and Embodiment 1 is that in step S4, the second mixture is sintered at 800°C for 7 hours in a nitrogen atmosphere to obtain lithium iron phosphate cathode material.

[0201] Example 8

[0202] The difference between this embodiment and Embodiment 2 is that in step S1, copper chloride, dicyandiamine, and deionized water are mixed in a mass ratio of 5:3.5:50, and the mass ratio of lignin-based carbon source to copper chloride is 100:40.

[0203] Example 9

[0204] The difference between this embodiment and Embodiment 2 is that in step S1, copper chloride, dicyandiamine, and deionized water are mixed in a mass ratio of 3:6:50, and the mass ratio of lignin-based carbon source to copper chloride is 100:3.

[0205] Comparative Example 1

[0206] The difference between this comparative example and Example 1 is that the lignin-based carbon material is not doped with copper and nitrogen.

[0207] The preparation method of lithium iron phosphate cathode material includes the following steps:

[0208] Step S1: The straw is crushed to a median particle size of 40 μm to obtain a lignin-based carbon source.

[0209] In step S2, the lignin-based carbon source, anhydrous iron phosphate, lithium carbonate and deionized water obtained in step S1 are mixed and ground to obtain a slurry with a median particle size of 0.40 μm; wherein, the solid content of the slurry is 20%; in the slurry, the molar ratio of lithium to iron is 1:1.05, and the mass ratio of lignin-based carbon source to lithium carbonate is 3:7.4.

[0210] Step S3: Spray dry and granulate the slurry at 105°C to obtain a second mixture. Place the second mixture in an argon atmosphere and sinter at 750°C for 7 hours to obtain lithium iron phosphate cathode material.

[0211] Comparative Example 2

[0212] The difference between this comparative example and Example 1 is that the lignin-based carbon source is replaced with glucose (a non-biomass carbon material).

[0213] Comparative Example 3

[0214] The difference between this comparative example and Example 1 is that the lignin-based carbon source is replaced with glucose (a non-biomass carbon material), and the lignin-based carbon material is not doped with copper and nitrogen.

[0215] Comparative Example 4

[0216] The difference between this comparative example and Example 1 is that step S2 is not included. That is, the copper-nitrogen coordination solution and the lignin-based carbon source are not premixed. Instead, the copper-nitrogen coordination solution and the lignin-based carbon source obtained in step S1 are directly mixed with anhydrous iron phosphate, lithium carbonate and deionized water to obtain a slurry.

[0217] Comparative Example 5

[0218] The difference between this embodiment and Embodiment 1 is that a copper-nitrogen coordination solution is not prepared in step S1, and step S2 is omitted. Copper chloride, dicyandiamine, lignin-based carbon source are directly mixed with anhydrous iron phosphate, lithium carbonate and deionized water to obtain a slurry.

[0219] II. Testing Methods

[0220] 1. Morphology of lithium iron phosphate cathode material

[0221] The surface morphology of lithium iron phosphate cathode material was observed using scanning electron microscopy (SEM), and the number of mesopores, the number of micropores, and the median particle size of lithium iron phosphate cathode material particles per unit area were measured.

[0222] 2. Specific surface area of ​​lithium iron phosphate cathode material

[0223] The specific surface area was obtained by testing using the BET (Brunauer-Emmett-Teller) method.

[0224] 3. Porosity of lithium iron phosphate cathode material

[0225] The porosity of lithium iron phosphate cathode material was tested using the ethanol immersion method. The specific steps are as follows: Weigh the lithium iron phosphate cathode material dried to constant weight as the sample before adsorption, and record the mass of the sample before adsorption as m1; Under vacuum conditions, immerse the sample in anhydrous ethanol of volume V1 for 2 hours to allow the ethanol to fully fill the sample pores, and measure the total volume V2 of the anhydrous ethanol and the sample. Calculate the sample volume V3 according to formula (1); Then, remove the sample and gently wipe away the excess ethanol adsorbed on the sample surface with filter paper to obtain the sample after adsorption, and record the mass of the sample after adsorption as m2. In addition, the density of ethanol is recorded as ρ, and the pore volume of the sample is recorded as V4. The porosity P is calculated according to formulas (2) to (3).

[0226] V3 = V2 - V1 Formula (1);

[0227] V4 = (m2 - m1) / ρ (Formula 2);

[0228] P = V4 / (V3 + V4) Formula (3).

[0229] 4. I of lithium iron phosphate cathode material D / I G value

[0230] Raman spectroscopy was performed on the lithium iron phosphate cathode material to obtain the ID / IG value.

[0231] 5. Carbon content in lithium iron phosphate cathode materials

[0232] The carbon content of lithium iron phosphate cathode material was obtained by testing with a carbon-sulfur analyzer.

[0233] 6. Nitrogen content in lithium iron phosphate cathode materials

[0234] The nitrogen content of the lithium iron phosphate cathode material was obtained by testing with an organic elemental analyzer.

[0235] 7. Copper content in lithium iron phosphate cathode materials

[0236] The copper content of lithium iron phosphate cathode material was determined by inductively coupled plasma atomic emission spectrometry (ICP-AES).

[0237] 8. Performance testing of lithium iron phosphate cathode materials

[0238] Lithium iron phosphate cathode material, conductive agent SuPer P, polyvinylidene fluoride, and N-methylpyrrolidone were uniformly mixed and stirred at a mass ratio of 8:1:1:25 to obtain a cathode slurry. The cathode slurry was uniformly coated onto the surface of an aluminum foil current collector, and then dried, rolled, and die-cut to obtain a cathode sheet with a compaction density of 2.3 g / cm³. 3 The negative electrode is a lithium metal sheet, the separator is Celgard 2400, and the electrolyte is a mixture of lithium hexafluorophosphate (1 mol / L), ethylene carbonate, and dimethyl carbonate. V 碳酸乙烯酯 V 碳酸二甲酯 =1:1. CR2032 button cells were assembled in an argon-filled glove box using positive electrode, separator, electrolyte, and negative electrode for subsequent performance testing.

[0239] (1) At -20℃, the battery was first charged at a constant current of 0.1C to 3.75V, and then charged at a constant voltage until the current decayed to 0.05C. After standing for 30 minutes, the battery was discharged at a constant current of 0.1C to 2V. The first discharge capacity of the battery was recorded. Then, the battery was charged and discharged in accordance with the above-mentioned charging and discharging method, and the capacity retention rate after 500 cycles was recorded.

[0240] (2) At 25°C, the battery was first charged at a constant current of 1C to 3.75V, and then charged at a constant voltage until the current decayed to 0.05C. After standing for 30 minutes, the battery was discharged at a constant current of 1C to 2V. The first discharge capacity of the battery was recorded. Then, the battery was charged and discharged in accordance with the above-mentioned charging and discharging method, and the capacity retention rate after 500 cycles was recorded.

[0241] (3) At 25°C, the battery is first charged at a constant current of 10C to 3.75V, then charged at a constant voltage until the current decays to 0.05C. After standing for 30 minutes, the battery is discharged at a constant current of 10C to 2V. The first discharge capacity of the battery is recorded. Then, the battery is charged and discharged in accordance with the above-mentioned charging and discharging method. The capacity retention rate after 500 cycles is recorded.

[0242] III. Analysis of Test Results for Each Embodiment and Comparative Example

[0243] The SEM image of the lithium iron phosphate cathode material provided in Example 1 is shown below. Figures 1 to 2 As shown, by Figure 1 As can be seen, the biomass carbon material fully encapsulates the lithium iron phosphate, and the particle size distribution of the lithium iron phosphate cathode material is uniform. (From...) Figure 2 As can be seen, lithium iron phosphate cathode materials have a large number of mesopores and micropores, exhibiting a hierarchical carbon framework structure, and the distribution of mesopores and micropores is relatively uniform.

[0244] Examples 1-9 and Comparative Examples 1-5 respectively provide lithium iron phosphate cathode materials with mesopore and micropore content, elemental content, porosity, specific surface area, and Ig. D / I G The median and maximum particle size are shown in Table 1, and the performance test results are shown in Table 2.

[0245] Table 1

[0246]

[0247] Table 2

[0248]

[0249] As shown in Tables 1-2, the lithium iron phosphate cathode materials provided in Examples 1-9 contain an appropriate amount of carbon, and have a good pore structure, moderate specific surface area, and I0 D / I G The moderate value indicates that its shell has a good carbon skeleton structure. In addition, the lithium iron phosphate cathode material also contains appropriate amounts of copper and nitrogen elements. Applying the aforementioned lithium iron phosphate cathode material to batteries results in good low-temperature performance, rate performance, and cycle performance.

[0250] Comparative Example 1 did not dope with copper or nitrogen, therefore the carbonization process of the biomass carbon material did not involve Cu and N. The resulting lithium iron phosphate cathode material had a poor carbon framework structure and fewer exposed active sites, leading to poorer low-temperature performance, rate performance, and cycle performance compared to Examples 1-9. Comparative Example 2 replaced the carbon source, resulting in a poorer carbon framework structure in the lithium iron phosphate cathode material, leading to poorer low-temperature performance, rate performance, and cycle performance compared to Examples 1-9. Comparative Example 3 replaced the carbon source and did not dope with copper or nitrogen. The resulting lithium iron phosphate cathode material had a poorer carbon framework structure and fewer exposed active sites, resulting in poorer performance than Comparative Examples 1-2.

[0251] In Comparative Examples 4 and 5, the mixing methods of copper, nitrogen, and carbon sources were changed. In Comparative Example 4, the copper-nitrogen coordination compound was not preloaded with a carbon source. Due to the lack of a carbon source as a carrier to pre-fix the copper-nitrogen coordination compound, copper elements tend to migrate and agglomerate during subsequent sintering, forming large copper particles or inactive copper phases. This fails to promote the graphitization and pore formation of the carbon source, resulting in low graphitization degree and poor pore structure of the final carbon skeleton, and even uneven carbon coating. In Comparative Example 5, a copper-nitrogen coordination solution was not prepared in advance. Since copper and nitrogen did not form a stable coordination environment beforehand, they were difficult to anchor synchronously in the carbon skeleton during subsequent sintering. This weakened the graphitization effect of the carbon material and failed to effectively construct the "micropore-mesopore" hierarchical pore structure. Therefore, the carbon skeleton structure of the lithium iron phosphate cathode materials provided by Comparative Examples 4 and 5 was poor, resulting in poorer low-temperature performance, rate performance, and cycle performance of the lithium iron phosphate cathode materials compared to Examples 1 to 9.

[0252] It should be noted that this application is not limited to the above-described embodiments. The above embodiments are merely examples, and any embodiments with the same structure and effect as the technical concept within the scope of this application are included in the technical scope of this application. Furthermore, various modifications that can be conceived by those skilled in the art to the embodiments, and other ways of constructing by combining some of the constituent elements of the embodiments, without departing from the spirit of this application, are also included in the scope of this application.

Claims

1. A lithium iron phosphate cathode material, characterized in that, The cathode material comprises a core and a shell located on at least a portion of the surface of the core. The core is made of lithium iron phosphate, and the shell is made of biomass carbon material having mesopores and micropores. The lithium iron phosphate cathode material has a mesopore count of 78 per μm. 2 ~102 cells / μm 2 The number of micropores per unit area is 151 / μm. 2 ~202 cells / μm 2 The biomass carbon material is doped with copper and nitrogen.

2. The lithium iron phosphate cathode material according to claim 1, characterized in that, The shell includes copper-nitrogen coordination structures and copper-nitrogen-carbon coordination structures formed by the copper, nitrogen, and carbon elements; and / or, In the lithium iron phosphate cathode material, the number of mesopores per unit area is 94 per μm. 2 ~102 cells / μm 2 ; and / or, In the lithium iron phosphate cathode material, the number of micropores per unit area is 190 per μm. 2 ~202 cells / μm 2 .

3. The lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, In the lithium iron phosphate cathode material, the mass ratio of copper to nitrogen is (0.216~1.179):1; and / or, In the lithium iron phosphate cathode material, the mass fraction of copper is 0.005%~0.071%; and / or, In the lithium iron phosphate cathode material, the mass fraction of nitrogen is 0.014%~0.068%; and / or, In the lithium iron phosphate cathode material, the mass fraction of carbon is 1.16%~1.62%; and / or, The porosity of the lithium iron phosphate cathode material is 17.0%~21.2%; and / or, The specific surface area of ​​the lithium iron phosphate cathode material is 19.3 m². 2 / g~23.6m 2 / g; and / or, The lithium iron phosphate cathode material I D / I G The value is 1.0 to 1.3; and / or, The biomass carbon material is a lignin-based carbon material.

4. A method for preparing a lithium iron phosphate cathode material according to any one of claims 1 to 3, characterized in that, Includes the following steps: Preparation of copper-nitrogen complex solutions; The biomass carbon source and the copper-nitrogen coordination solution are subjected to a first mixing, a first drying, and a first pulverizing process to obtain the first material; The first material, the phosphorus iron source, the lithium source, and the first solvent are subjected to a second mixing and a second pulverizing process to obtain a slurry. The slurry undergoes a second drying and sintering process to obtain the lithium iron phosphate cathode material.

5. The method for preparing the lithium iron phosphate cathode material according to claim 4, characterized in that, The copper-nitrogen coordination solution contains a copper-nitrogen coordination compound. The steps for preparing the copper-nitrogen coordination solution include: mixing a soluble copper source, a soluble nitrogen source, and a second solvent in a third step to obtain the copper-nitrogen coordination solution.

6. The method for preparing the lithium iron phosphate cathode material according to claim 5, characterized in that, At least one of the following conditions must be met: (1) The soluble copper source includes at least one of copper nitrate, copper acetate and copper chloride; (2) The soluble nitrogen source includes at least one of melamine, dicyandiamine, aminocyanide and urea; (3) The mass ratio of the soluble copper source to the soluble nitrogen source is (3~10):6; (4) The mass ratio of the biomass carbon source to the soluble copper source is 100:(6~40). (5) The ratio of the total mass of the soluble copper source and the soluble nitrogen source to the mass of the second solvent is (7~13):50; (6) The second solvent includes at least one of deionized water, anhydrous ethanol and ethylene glycol.

7. The method for preparing the lithium iron phosphate cathode material according to any one of claims 4 to 6, characterized in that, At least one of the following conditions must be met: (1) The phosphorus iron source includes at least one of anhydrous ferric phosphate, ferrous phosphate, ferrous dihydrogen phosphate and ferrous pyrophosphate; (2) The lithium source includes at least one of lithium carbonate, lithium bicarbonate and lithium hydroxide; (3) In the slurry, the molar ratio of lithium to iron is 1:(1.04~1.06). (4) The first solvent includes at least one of deionized water, anhydrous ethanol and ethylene glycol; (5) The solid content of the slurry is 10%~30%.

8. The method for preparing the lithium iron phosphate cathode material according to any one of claims 4 to 6, characterized in that, At least one of the following conditions must be met: (1) The sintering temperature is 600℃~800℃; (2) The holding time for the sintering treatment is 6h~8h; (3) The temperature of the first drying process is 120℃~180℃; (4) The temperature of the second drying is 90℃~120℃.

9. The method for preparing the lithium iron phosphate cathode material according to claim 8, characterized in that, At least one of the following conditions must be met: (1) The median particle size of the biomass carbon source is 35 μm to 45 μm; (2) The median particle size of the first material is 35 μm to 45 μm; (3) The median particle size of the solid particles in the slurry is 0.35μm~0.45μm.

10. A battery, characterized in that, The battery comprises a positive electrode material, a negative electrode material, and an electrolyte. The positive electrode material is a lithium iron phosphate positive electrode material according to any one of claims 1 to 3, or a lithium iron phosphate positive electrode material prepared by any one of claims 4 to 9.

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