Phosphate positive electrode material and preparation method thereof, positive plate and secondary battery

Abstract

The invention belongs to the technical field of battery materials, and particularly relates to a phosphate positive electrode material, a preparation method thereof, a positive plate and a secondary battery. The phosphate positive electrode material comprises a phosphate active core and a first carbon layer coating the outer surface of the core, the first carbon layer is modified with an element S and / or an element X, and the element X comprises at least one of boron, silicon, selenium and tellurium. The surface of the phosphate active core is coated with the first carbon layer modified with the S element and / or the X element, so that the electronic conductivity and the ion diffusion rate of the phosphate positive electrode material can be effectively improved, and meanwhile, the improvement of the capacity and the improvement of the cycle performance are facilitated.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery materials, and particularly relates to a phosphate-based positive electrode material, a preparation method thereof, a positive electrode sheet and a secondary battery. BACKGROUND

[0002] Lithium ion batteries have been widely used in portable electronic devices, electric vehicles and large-scale energy storage systems due to their high energy density, long cycle life and environmental friendliness. The positive electrode material is one of the key factors determining the performance of lithium ion batteries. Phosphate-based positive electrode materials such as lithium iron phosphate and lithium manganese iron phosphate are considered to be one of the most promising positive electrode materials due to their abundant raw material sources, high thermal stability, excellent cycle performance and safety and non-toxicity. However, the intrinsic electronic conductivity and ionic conductivity of the phosphate-based positive electrode material are relatively low.

[0003] Therefore, it is necessary to modify the phosphate-based positive electrode material to improve the electronic conductivity and ion diffusion rate while ensuring the processing performance and improving the electrochemical performance. SUMMARY

[0004] The application aims to provide a phosphate-based positive electrode material and a preparation method thereof, and a positive electrode sheet and a secondary battery, and aims to solve the problem of low intrinsic electronic conductivity and ionic conductivity of the existing phosphate-based positive electrode material to some extent.

[0005] To achieve the above application purposes, the technical solutions adopted by the application are as follows: In a first aspect, the application provides a phosphate-based positive electrode material, comprising a phosphate-based active inner core and a first carbon layer coated on the outer surface of the inner core, wherein the first carbon layer is modified with S elements and / or X elements, and the X elements include at least one of boron, silicon, selenium and tellurium.

[0006] In some possible implementation manners, the first carbon layer is modified with the S elements and the X elements, and the mass ratio of carbon elements to the S elements and the X elements is (3-9):(1-7):(1-7).

[0007] In some possible implementation manners, the first carbon layer is modified with the S elements and the X elements, and carbon elements and the S elements and the X elements form a C-S-X three-dimensional network structure.

[0008] In some possible implementation manners, the first carbon layer contains ketone functional groups.

[0009] In some possible implementation manners, a second carbon layer is further included between the inner core and the first carbon layer.

[0010] In some possible implementations, the phosphate-based positive electrode material has a particle size D50 of 0.5 μm to 1.1 μm.

[0011] In some possible implementations, the second carbon layer has a thickness of 1 nm to 2 nm.

[0012] In some possible implementations, the first carbon layer has a thickness of 2 nm to 3 nm.

[0013] In some possible implementations, the second carbon layer has a mass percentage of 0.4% to 0.6% based on the total mass of the phosphate-based positive electrode material; and / or, the first carbon layer has a mass percentage of 0.5% to 0.7%.

[0014] In some possible implementations, the phosphate-based active core includes at least one of lithium iron phosphate and lithium manganese iron phosphate.

[0015] In some possible implementations, when the phosphate-based active core is lithium iron phosphate, the phosphate-based positive electrode material has a particle size D50 of 0.9 μm to 1.1 μm.

[0016] In some possible implementations, when the phosphate-based active core is lithium manganese iron phosphate, the phosphate-based positive electrode material has a particle size D50 of 0.5 μm to 0.9 μm.

[0017] In a second aspect, the present application provides a preparation method of a phosphate-based positive electrode material, including the following steps: Preparation of an active particle, the active particle including a phosphate-based active material; Preparation of a first carbon layer on the outer surface of the active particle to obtain a phosphate-based positive electrode material; wherein the first carbon layer is modified with S elements and / or X elements, and the X elements include at least one of boron, silicon, selenium and tellurium.

[0018] In some possible implementations, the active particle includes a phosphate-based active core and a second carbon layer coated on the outer surface of the core, and the first carbon layer is coated on the outer surface of the second carbon layer.

[0019] In some possible implementations, the step of preparing the active particle includes: preparing raw material components including a lithium source, a phosphorus source, an iron source and a carbon source I into a phosphate-based active precursor, and performing a first sintering treatment under an inert atmosphere to form the active particle in which the second carbon layer coats the core.

[0020] In some possible implementations, the step of preparing the first carbon layer comprises: mixing the active particles with a carbon source II, a sulfur source and an X source, and then performing a second sintering treatment under an inert atmosphere to form the first carbon layer on the surface of the second carbon layer, thereby obtaining the phosphate-based positive electrode material.

[0021] In some possible implementations, the raw material components further comprise a manganese source.

[0022] In some possible implementations, the carbon source I comprises at least one of glucose, fructose, sucrose, lactose, starch, and cellulose.

[0023] In some possible implementations, the carbon source II is selected from a carbon source with a ketone functional group.

[0024] In some possible implementations, the sulfur source comprises at least one of thiourea, thioacetamide, sulfamic acid, 2-thiophene carboxylic acid, ferrous sulfide, sodium sulfide, and sulfur powder.

[0025] In some possible implementations, the X source comprises at least one of a boron source, a silicon source, a selenium source, and a tellurium source.

[0026] In some possible implementations, the conditions of the first sintering treatment comprise: heating to 400-600°C at a heating rate of 3-10°C / min, and then holding for 8-12 h; and heating to 700-800°C and holding for 4-6 h.

[0027] In some possible implementations, the conditions of the second sintering treatment comprise: heating to 400-550°C at a heating rate of 3-10°C / min, and then holding for 3-6 h.

[0028] In some possible implementations, the carbon source II comprises at least one of nonanone, hexanedione, phenylhydrazine ketone, ginger ketone, benzophenone, hexanedial, furfural, p-nitrobenzaldehyde, salicylic acid, tartaric acid, malic acid, oxalic acid, lauric acid, clomazone, dimethyl oxalate, acrylate, ethyl acetate.

[0029] In some possible implementations, the boron source comprises at least one of boric acid, boron oxide, boron nitride, titanium diboride, sodium borate pentahydrate, sodium borohydride, trimethoxyborane, sodium tetraphenylborate, and decaborane. In some possible implementations, the silicon source comprises at least one of triphenylsilane, hexamethyldisilane, γ-aminopropyl triethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, and ureidopropyl triethoxysilane.

[0030] In some possible implementations, the selenium source includes at least one of selenium powder, sodium selenate, sodium selenite, selenium dioxide, selenious acid, potassium selenocyanate, bismuth selenide, cadmium selenide, and zinc selenide.

[0031] In some possible implementations, the tellurium source includes at least one of tellurium powder, tellurium dioxide, cadmium telluride, bismuth telluride, tin telluride, sodium tellurate, and telluric acid.

[0032] In some possible implementations, the mass ratio of the carbon source II, the sulfur source, and the X source is (3-9):(1-7):(1-7).

[0033] In some possible implementations, in the phosphate-based active precursor, the mass percentage of the carbon source I is 3%-10%.

[0034] In some possible implementations, the total mass of the carbon source II, the sulfur source, and the X source is 0.5%-3% based on the mass of the active particles.

[0035] In a third aspect, the present application provides a positive electrode sheet, which includes a current collector and a positive electrode active layer formed on at least one surface of the current collector, and the positive electrode active layer includes the above-mentioned phosphate-based positive electrode material and / or the phosphate-based positive electrode material prepared by the above-mentioned method.

[0036] In a fourth aspect, the present application provides a secondary battery containing the above-mentioned positive electrode sheet.

[0037] The phosphate-based positive electrode material provided in the first aspect of the present application has a first carbon layer modified with S elements and / or X elements (X includes at least one of boron, silicon, selenium, and tellurium) on the surface of the active core of the phosphate-based positive electrode material. First, the carbon material in the first carbon layer has high electronic conductivity, and the modified S elements and / or X elements can form a cross-linked network with carbon atoms, serving as a conductive coating to form a continuous electronic conduction network on the surface of the active core, thereby providing an electronic transmission path, and thus significantly improving the overall electronic conductivity. Second, the introduction of S elements and / or X elements (such as B, Si, Se, and Te) in the carbon layer can change the electronic structure of the carbon layer: S atoms have a larger atomic radius and higher electronegativity than carbon atoms, and after doping, defects can be introduced and the graphitization degree of the carbon layer can be increased, while S acts as an electron donor to provide additional free electrons, enhancing n-type conductivity. B atoms are electron acceptors, forming holes in the carbon layer to improve p-type conductivity and promote the sp 2Hybrid, optimize electron mobility. Si, Se, Te element doping, these elements can also introduce defect states or change the energy band structure of carbon, for example, Se and Te have similar electron donor properties, while Si doping can form a silicon carbide interface, further reducing the interface resistance. Multi-element co-modification can produce a synergistic effect, by adjusting the electron cloud distribution of the carbon layer, increasing the electron cloud density, and providing a more optimal conductive channel. In addition, the first carbon layer as an interface layer, the heteroatoms doped in the carbon layer can improve the density and chemical environment of the carbon layer, forming a more uniform coating layer, reducing the barrier to lithium ion diffusion; and the modification elements can create more surface defects or nanopores to provide additional diffusion channels for lithium ions, thereby improving the intrinsic ionic conductivity. The modification elements can also improve the adhesion of the carbon layer to the phosphate core, form a more stable interface, reduce the interface impedance, thereby promoting ion transmission at the phase boundary and improving the intrinsic ionic conductivity. Therefore, by coating the surface of the phosphate active core with a first carbon layer modified with S elements and / or X elements, the electronic conductivity and ion diffusion rate of the phosphate positive electrode material can be effectively improved, and the capacity and cycle performance can also be improved.

[0038] The preparation method of the phosphate positive electrode material provided by the present application realizes in-situ and uniform compounding of the functionalized carbon layer and the active particles through a simple and controllable process, thereby providing an efficient solution for fundamentally improving the interface conductivity of the phosphate positive electrode material. A first carbon layer is prepared on the outer surface of the active particles, and the first carbon layer is modified with S elements and / or X elements (X includes at least one of boron, silicon, selenium, and tellurium) to form a cross-linked network with carbon atoms, thereby forming a continuous electron conductive network on the surface of the active particles and improving the overall electronic conductivity. Moreover, the S elements and / or X elements introduced into the carbon layer as dopants can change the electronic structure of the carbon layer, adjust the electron cloud distribution of the carbon layer, increase the electron cloud density, and provide a more optimal conductive channel. At the same time, more surface defects or nanopores are created to provide additional diffusion channels for lithium ions, thereby improving the intrinsic ionic conductivity. Therefore, the electronic conductivity and ion diffusion rate of the prepared phosphate positive electrode material are improved, and the capacity and cycle performance are also improved.

[0039] The positive electrode sheet provided by the present application can directly convert the excellent electron / ion dual high-conductivity characteristics of the material level into outstanding performance at the electrode sheet level by using the aforementioned phosphate positive electrode material with a unique core-shell structure and an interface modified by a C-S-X three-dimensional network function. The positive electrode sheet exhibits extremely low internal impedance, uniform current distribution, and fast electrochemical reaction kinetics.

[0040] The secondary battery of the present application adopts the positive plate with low internal impedance, uniform current distribution and fast electrochemical reaction kinetics, so that the secondary battery assembled therefrom has high rate charge and discharge capability, high capacity output and long cycle life, realizing effective conversion of high-performance positive material to high-performance battery product. BRIEF DESCRIPTION OF DRAWINGS

[0041] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0042] Figure 1 is a structural schematic diagram of the phosphate-based positive material provided by the embodiments of the present application; Figure 2 is a flow schematic diagram of the preparation method of the phosphate-based positive material provided by the embodiments of the present application; Figure 3 is an SEM diagram of the lithium iron phosphate positive material provided by the embodiment 2 and the comparative example 1 of the present application; Figure 4 is a TEM diagram of the lithium iron phosphate positive material provided by the embodiment 4 of the present application; Figure 5 is an EIS diagram of the secondary batteries prepared by the comparative example 1 and the embodiments 2 and 4 of the present application; Figure 6 is a charge-discharge curve diagram of the secondary batteries prepared by the comparative example 1 and the embodiments 2 and 4 of the present application at 1C; Figure 7 is a cycle capacity retention rate diagram of the secondary batteries prepared by the comparative example 1 and the embodiments 2 and 4 of the present application at 0.5P. DETAILED DESCRIPTION

[0043] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clear and explicit, the present application will be further described in detail in combination 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.

[0044] In the present application, the term "and / or" describes the association relationship of the associated objects, which means that there can be three kinds of relationships, for example, A and / or B can mean that A exists alone, A and B exist together, and B exists alone. Wherein A and B can be singular or plural. The character " / " generally represents that the associated objects before and after it are in an "or" relationship.

[0045] In this application, "at least one" means one or more, "multiple" means two or more. "At least one of the following" or the like means any combination of these items, including a single item or any combination of multiple items. For example, "at least one of a, b or c", or "at least one of a, b and c", can 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.

[0046] It should be understood that the size of the sequence number of the above-mentioned processes in various embodiments of the present application does not mean the order of execution, and part or all of the steps can be executed in parallel or in sequence, and the execution order of the processes should be determined according to their functions and internal logic, and should not constitute any limitation on the implementation process of the embodiments of the present application.

[0047] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments, and are not intended to limit the present application. The singular forms "a" and "the" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0048] The weight of the related components mentioned in the embodiments of the present application can not only refer to the specific content of each component, but also represent the weight ratio relationship between each component, therefore, as long as the content of the related components in the embodiments of the present application is enlarged or reduced in proportion, it is within the scope disclosed in the embodiments of the present application. Specifically, the mass mentioned in the embodiments of the present application can be µg, mg, g, kg and other mass units commonly known in the chemical field.

[0049] The terms "first", "second" are only for the purpose of description, used to distinguish objects such as substances from each other, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of indicated technical features. For example, without departing from the scope of the embodiments of the present application, the first XX can also be called the second XX, and similarly, the second XX can also be called the first XX. Therefore, the features limited by "first" and "second" can explicitly or implicitly include one or more of the features.

[0050] The first aspect of the embodiments of the present application provides a phosphate-based positive electrode material, comprising a phosphate-based active core and a first carbon layer coated on the outer surface of the core, wherein the first carbon layer is modified with S element and / or X element, and the X element includes at least one of boron, silicon, selenium and tellurium.

[0051] The first carbon layer modified with S element and / or X element (X includes at least one of boron, silicon, selenium and tellurium) is coated on the surface of the active core of the phosphate-based positive electrode material. First, the carbon material in the first carbon layer has high electronic conductivity, and the modified S element and / or X element can form a cross-linked network with carbon atoms, serving as a conductive coating to form a continuous electron conduction network on the surface of the active core, providing an electron transport path, thereby significantly improving the overall electronic conductivity. Second, the introduction of S element and / or X element (such as B, Si, Se and Te) in the carbon layer can change the electronic structure of the carbon layer: S atoms have larger atomic radius and higher electronegativity than carbon atoms, and after doping, defects can be introduced and the graphitization degree of the carbon layer can be increased, and S as an electron donor can provide additional free electrons to enhance n-type conductivity. B atoms are electron acceptors, forming holes in the carbon layer to improve p-type conductivity and promote sp 2 hybridization of the carbon layer, optimizing electron mobility. Si, Se and Te elements can also introduce defect states or change the energy band structure of carbon, for example, Se and Te have similar electron donor properties, while Si doping can form a silicon carbide interface to further reduce interface resistance. Multi-element co-modification can produce a synergistic effect by adjusting the electron cloud distribution of the carbon layer, increasing the electron cloud density, and providing a more optimal conductive channel. In addition, the first carbon layer as an interface layer, the heteroatoms doped in the carbon layer can improve the density and chemical environment of the carbon layer, forming a more uniform coating layer and reducing the diffusion resistance of lithium ions; and the modified elements can create more surface defects or nanopores to provide additional diffusion channels for lithium ions, thereby improving the intrinsic ionic conductivity. The modified elements can also improve the adhesion of the carbon layer to the phosphate core, forming a more stable interface and reducing interface impedance, thereby promoting ion transport at the phase boundary and improving intrinsic ionic conductivity. Therefore, by coating the surface of the phosphate-based active core with the first carbon layer modified with S element and / or X element, the electronic conductivity and ion diffusion rate of the phosphate-based positive electrode material can be effectively improved, and the capacity and cycle performance can also be improved.

[0052] In some possible implementations, the first carbon layer is modified with S elements and X elements, and the carbon elements form a C-S-X three-dimensional network structure with the S elements and the X elements. In this case, the doped S elements and X elements are not randomly dispersed, but are combined with carbon (C) atoms through chemical bonds (such as C-S, C=S, C-X, and possibly existing S-X bonds) to jointly build a stable covalent network structure extending in three-dimensional space. The conductive framework of the C-S-X three-dimensional network structure can regulate the electron cloud distribution in the entire three-dimensional network through the synergistic effect of S atoms (providing lone pair electrons) and X atoms (accepting electrons), generate a stronger "charge synergistic effect", greatly promote the delocalization and migration of electrons, and equivalently build a highly interconnected electron conduction network and ion rapid channel on the surface of the active particles, significantly reduce the interface resistance, and improve the electronic conductivity and ion diffusion rate. Moreover, this network structure is more stable than a physically mixed or simply doped structure, is not easily corroded or structurally collapsed by the electrolyte during long-term cycling, is also conducive to capacity improvement and cycle performance improvement, ensures the persistence and reliability of the electron conduction path, and thus guarantees the long cycle life of the battery.

[0053] In some possible implementations, the first carbon layer is modified with S elements and X elements, and the mass ratio of the carbon elements to the S elements and the X elements is (3-9):(1-7):(1-7). In this case, the ratio range ensures the balance between the dominant framework role of the carbon matrix and the effective modification role of the S / X elements. Among them, the proportion of carbon elements is relatively high, ensuring a continuous conductive network, and the proportions of S and X elements are comparable, ensuring effective three-dimensional network construction and functional synergy, and effectively constructing an ion transmission channel and synergistically improving electron transmission.

[0054] For example, the first carbon layer is modified with S elements and X elements, and the mass ratio of the carbon elements to the S elements and the X elements can be 3:4:4, 4:3:3, 5:2:3, 6:2:2, 7:1:2, 8:1:1, or any interval value between any two point values.

[0055] In some possible implementations, the first carbon layer contains ketone functional groups. In this case, the ketone (C=O) functional groups in the first carbon layer are polar functional groups, which can form hydrogen bonds or dipole-dipole interactions with polar molecules in a liquid medium (such as an electrolyte). Thus, the first carbon layer containing these functional groups can improve the solvent affinity of the phosphate-based positive electrode material, reduce direct contact between positive electrode material particles, improve the surface wettability of the positive electrode material particles, and reduce the agglomeration tendency of the positive electrode material particles. When the positive electrode material is slurried, it can be better dispersed with the solvent (similar compatibility principle), the material particles are less agglomerated, the solvent is uniformly spread on the surface of the powder, the viscosity of the slurry is lower, the solid content is higher, and the processing performance of the positive electrode material is improved.

[0056] Among some possible implementations, as shown in the appendix Figure 1 As shown, a second carbon layer is also included between the core and the first carbon layer. That is, the inner layer is the second carbon layer, and the outer layer is the first carbon layer. In this case, by introducing a layer of pure carbon as a second carbon layer between the core and the first carbon layer of the CSX three-dimensional network structure modified with S / X elements, a "core-shell-shell" gradient coating structure is constructed. This structure utilizes the inner pure carbon layer to achieve dense coating and stable electron collection, while utilizing the outer CSX network to provide efficient electron conduction and fast lithium-ion transport channels. Through the synergistic effect of the two layers, while ensuring interface stability, the intrinsic electronic conductivity and interface ion diffusion rate of the material are significantly improved, thereby optimizing the high-rate performance and cycle stability of the phosphate cathode material as a whole.

[0057] In some possible implementations, the particle size D50 of the phosphate-based cathode material is 0.5 μm to 1.1 μm. Exemplary examples include any typical but non-limiting point value or a range between any two points, such as 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1.0 μm, and 1.1 μm. In this case, the smaller particle size of the phosphate-based cathode material is beneficial for improving the tap density.

[0058] In some possible implementations, the thickness of the second carbon layer is 1 nm to 2 nm. For example, it can be any typical but non-limiting value such as 1 nm, 1.5 nm, or 2 nm, or a range between any two values. In this case, by precisely controlling the thickness of the second carbon layer (pure carbon) to 1 nm to 2 nm, it is ensured that while achieving continuous and dense coating to provide stable electron channels and physical isolation, it does not significantly hinder lithium-ion diffusion.

[0059] In some possible implementations, the thickness of the first carbon layer is 2nm to 3nm. For example, it can be any typical but non-limiting value such as 2nm, 2.5nm, or 3nm, or a range between any two values. In this case, controlling the thickness of the first carbon layer (CSX functional layer) to 2nm to 3nm ensures that the S / X modifying elements have sufficient space to form an effective three-dimensional conductive network and high-density ionic active sites, thereby maximizing their synergistic function of improving electronic / ionic conductivity, while avoiding excessively thick carbon layers that would lead to excessively long overall ion migration paths. This nanoscale thickness matching allows the bilayer structure to achieve an optimal balance between electronic conduction and ion diffusion dynamics while ensuring interface stability, thus significantly improving the material's rate performance and cycle life.

[0060] In some possible implementations, with the total mass of the phosphate-based cathode material being 100%, the mass percentage of the second carbon layer is 0.4% to 0.6%; and / or, the mass percentage of the first carbon layer is 0.5% to 0.7%. In this case, the energy density of the battery is maximized while reducing the proportion of inactive materials; at the same time, a continuous and dense inner coating (second carbon layer) and a functionally rich surface modification layer (first carbon layer) are formed, thereby achieving a balance between high energy density and excellent electrochemical performance while improving both conductivity.

[0061] For example, taking the total mass of the phosphate-based cathode material as 100%, the mass percentage of the second carbon layer can be any typical but non-limiting point value or a range between any two points, such as 0.4%, 0.5%, or 0.6%; the mass percentage of the first carbon layer can be any typical but non-limiting point value or a range between any two points, such as 0.5%, 0.6%, or 0.7%.

[0062] In some possible implementations, the phosphate-based active core includes at least one of lithium iron phosphate (LFP) and lithium manganese iron phosphate (LFP). LFP is renowned for its excellent safety and cycle life, but its main performance bottlenecks are extremely low electronic conductivity and moderate ion mobility. The bilayer carbon coating structure of this application directly addresses these core shortcomings. The second carbon layer ensures that each active particle is fully connected to the conductive network, overcoming the problem of poor intrinsic conductivity; the CSX three-dimensional network of the first carbon layer further accelerates interfacial charge transfer and ion diffusion. This allows LFP materials to leverage their high capacity and stable plateau while achieving excellent high-rate performance. Lithium manganese iron phosphate increases the operating voltage (thus improving energy density) by introducing manganese, but also brings challenges such as manganese dissolution leading to cycle decay and still relatively low electronic conductivity. In this application, the dense second carbon layer (pure carbon) and the first carbon layer together constitute a robust physical and chemical barrier, effectively suppressing the dissolution of manganese ions during charging and discharging, greatly improving the material's cycle stability. The uniform carbon coating ensures the stability of the interface under high voltage, while the strong dual conductivity of the CSX network compensates for the inherent kinetic limitations of lithium iron manganese, enabling it to achieve rapid charging and discharging even at high voltage.

[0063] In some possible implementations, when the phosphate-based active core is lithium iron phosphate, the particle size D50 of the phosphate-based cathode material is 0.9 μm to 1.1 μm.

[0064] In some possible implementations, when the phosphate-based active core is lithium manganese iron phosphate, the particle size D50 of the phosphate-based cathode material is 0.5 μm to 0.9 μm.

[0065] The phosphate-based cathode materials described in the above embodiments of this application can be prepared by the methods described in the following embodiments.

[0066] Secondly, embodiments of this application provide a method for preparing a phosphate-based cathode material, as shown in the attached figure. Figure 2 As shown, it includes the following steps: S10. Prepare active particles, including phosphate-based active materials; S20. A first carbon layer is prepared on the outer surface of the active particles to obtain a phosphate-based cathode material; wherein the first carbon layer is modified with S element and / or X element, and X element includes at least one of boron, silicon, selenium and tellurium.

[0067] The method for preparing phosphate-based cathode materials provided in this application, through a simple and controllable process, achieves in-situ, uniform composite formation of functionalized carbon layers and active particles, offering an efficient solution for fundamentally improving the interfacial conductivity of phosphate cathode materials. A first carbon layer is prepared on the outer surface of the active particles. This first carbon layer is modified with sulfur (S) and / or x (X includes at least one of boron, silicon, selenium, and tellurium) elements, which can form a cross-linking network with carbon atoms, creating a continuous electronic conductive network on the surface of the active particles and improving the overall electronic conductivity. Furthermore, introducing sulfur and / or x elements as dopants into the carbon layer can alter the electronic structure of the carbon layer, adjust the electron cloud distribution, increase the electron cloud density, and provide better conductive channels. Simultaneously, it creates more surface defects or nanopores, providing additional diffusion channels for lithium ions and improving intrinsic ionic conductivity. Therefore, the electronic conductivity and ion diffusion rate of the prepared phosphate-based cathode material are improved, which also contributes to capacity enhancement and improved cycle performance.

[0068] In step S10 above: In some possible implementations, the active particles comprise a phosphate-based active core and a second carbon layer coating the outer surface of the core, with a first carbon layer covering the outer surface of the second carbon layer. In this case, a unique "core-shell-shell" layered structure is constructed by introducing a pure carbon second carbon layer between the core and the first carbon layer of the CSX three-dimensional network structure modified with S / X elements. This design itself achieves effective physicochemical isolation, preventing side reactions between the outer modified elements and the core, ensuring the intrinsic stability of the material, and achieving dense coating and stable electron aggregation. Through the synergistic effect of the two layers, the intrinsic electronic conductivity and interfacial ion diffusion rate of the material are better improved while ensuring interfacial stability.

[0069] In some possible implementations, the steps for preparing active particles include: preparing a phosphate-based active precursor from raw material components including a lithium source, phosphorus source, iron source, and carbon source I; performing a first sintering treatment under an inert atmosphere to form an active particle with a second carbon layer coating the core. In this case, by preparing a precursor together with raw materials such as carbon source I, lithium source, phosphorus source, and iron source, and performing a first sintering treatment, the synthesis of active materials and carbon coating are completed in one step in situ. This process utilizes carbon source I at high temperatures to both act as a reducing agent to ensure the pure phase formation of phosphate crystals (such as lithium iron phosphate) and to pyrolyze to generate a second carbon layer that directly and uniformly coats the surface of the newly formed active core. This ensures close contact between the carbon layer and the active material.

[0070] In some possible implementations, the raw material components also include a manganese source; in this case, the resulting active core is lithium manganese iron phosphate.

[0071] In some possible implementations, the total molar amount of iron and manganese sources, relative to the molar ratios of phosphorus and lithium sources, range from (0.93 to 0.98), (0.98 to 1.00), to (1.01 to 1.06). This approach helps ensure high capacity and long cycle life for the active core.

[0072] In some possible implementations, the lithium source can be one or more materials such as lithium oxide, lithium carbonate, lithium hydroxide, lithium acetate, lithium phosphate, and lithium citrate. These lithium sources all have good solubility, which is beneficial for the preparation of phosphate-based cathode materials.

[0073] In some possible implementations, the iron source can be one or more of ferric nitrate, ferrous sulfate, ferric citrate, ferrous oxalate, ferric oxide, and ferrous phosphate. These iron sources all have good solubility, which is beneficial for the preparation of phosphate-based cathode materials.

[0074] In some possible implementations, the phosphorus source can be one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium dihydrogen phosphate, and iron phosphate. These phosphorus sources all have good solubility, which is beneficial for the preparation of phosphate-based cathode materials.

[0075] In some possible implementations, the manganese source can be at least one of manganese carbonate, manganese oxalate, manganese acetate, manganese dioxide, manganese trioxide, and manganese tetroxide. These manganese sources all have good solubility, which is beneficial for the preparation of phosphate-based cathode materials.

[0076] In some possible implementations, carbon source I includes at least one of glucose, fructose, sucrose, lactose, starch, and cellulose. These carbon sources I are all water-soluble or well-dispersible organic compounds in the aqueous phase, allowing for molecular-level homogeneous mixing with metal ions (lithium, iron, manganese) during the precursor preparation stage. During subsequent sintering, the second carbon layer formed by the carbonization of these carbon source I particles can continuously and in situ coat the surface of the phosphate active core. Simultaneously, these carbon sources I generate reducing gases (such as CO and H2) during high-temperature pyrolysis, creating and maintaining a strongly reducing environment for phosphate synthesis. This effectively prevents the oxidation of active metal ions such as iron and manganese, ensuring that the final product is a pure-phase, highly electrochemically active olivine structure, thus guaranteeing the material's capacity from the source.

[0077] In some possible implementations, the mass percentage of carbon source I in the phosphate-based active precursor is 3% to 10%. This balances the material's conductivity, the content of the active substance, and the feasibility of the synthesis process. This content range ensures that a continuous and uniform conductive carbon layer is formed after the carbon source pyrolysis, effectively improving electronic conductivity, while its role as a reducing agent guarantees the formation of a pure-phase phosphate.

[0078] In some possible implementations, the conditions for the first sintering treatment include: heating to 400℃~600℃ at a heating rate of 3℃ / min~10℃ / min, holding for 8h~12h, then heating to 700℃~800℃ and holding for 4h~6h. In this case, the first stage of holding allows the lithium source, iron source and / or manganese source, and phosphorus source to nucleate, i.e., form lithium iron phosphate or lithium manganese iron phosphate crystal nuclei, and the second stage of holding allows the crystal nuclei to grow more completely. The first sintering treatment, through staged heating and sintering, ensures effective carbon coating and modification, while maximally suppressing the secondary growth of lithium iron phosphate or lithium manganese iron phosphate grains and the formation of impurity phases, maintaining the primary particle morphology and structural stability of the material.

[0079] For example, in the first sintering process, the heating rate can be any typical but non-limiting point value or a range between any two points, such as 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min; the holding temperature in the first stage can be any typical but non-limiting point value or a range between any two points, such as 400℃, 450℃, 500℃, 550℃, 600℃; and the holding time can be any typical but non-limiting point value or a range between any two points, such as 8h, 9h, 10h, 11h, 12h. The holding temperature in the second stage can be 700℃, 720℃, 750℃, 780℃, 800℃; and the holding time can be any typical but non-limiting point value or a range between any two points, such as 4h, 5h, 6h.

[0080] In some embodiments, the inert atmosphere includes at least one of nitrogen, argon, and helium.

[0081] In some embodiments, the step of preparing active particles includes: mixing a lithium source, an iron source and / or a manganese source, a phosphorus source, carbon source I, and deionized water uniformly, and then drying the mixed solution by heat treatment to obtain a solid-phase phosphate-based active precursor. The precursor is crushed and placed in a tube furnace for a first sintering under an inert gas atmosphere. The sintering is divided into two stages: heating at a rate of 3°C / min to 10°C / min to 400°C to 600°C and holding for 8 to 12 hours, followed by further heating to 700°C to 800°C and holding for 4 to 6 hours. After the material cools to room temperature, it is removed and pulverized to obtain the first sintered product, which is a second carbon layer-coated active particle of lithium iron phosphate / lithium manganese iron phosphate.

[0082] In step S20 above: In some possible implementations, the steps for preparing the first carbon layer include: mixing active particles with carbon source II, sulfur source, and X source, followed by a second sintering treatment under an inert atmosphere to form the first carbon layer on the surface of the second carbon layer, thus obtaining a phosphate-based cathode material. By mixing the active particles coated with the second carbon layer with carbon source II, sulfur source, and X source and performing a second sintering treatment, a "one-step" process is used to simultaneously achieve carbon layer deposition and in-situ doping of S / X elements. During the heat treatment process, carbon source II is pyrolyzed to form a carbon matrix, while simultaneously promoting the bonding reaction between sulfur and X elements and newly formed carbon atoms (S atoms from the decomposition of the sulfur source and X atoms from the decomposition of the X source react with defect sites in the carbon layer to form CS bonds, C=S bonds, CX bonds, and CSX bonds). This not only simplifies the process flow but also efficiently constructs a functionally integrated composite interface layer, namely the first carbon layer. It also enhances the stability, continuity, and density of the coated carbon layer, thereby more effectively suppressing particle agglomeration. More importantly, it ensures that the functional elements (S, X) are uniformly dispersed and stably dissolved at the molecular level in the carbon layer, forming a three-dimensional CSX network structure, which changes the electron cloud distribution of the carbon layer, increases the electron cloud density, provides better conductive channels, improves electronic conductivity, and optimizes interfacial ion transport dynamics.

[0083] In some possible implementations, carbon source II is selected from carbon sources with ketone functional groups. Carbon source II containing ketone C=O functional groups has high reactivity at lower temperatures and can decompose under low-temperature conditions of secondary sintering to form a large number of well-organized graphitized carbons, which have better electronic conductivity.

[0084] In some possible implementations, carbon source II includes at least one of nonanone, hexanedione, phenanthrene, gingerone, benzophenone, adipical, furfural, p-nitrobenzaldehyde, salicylic acid, tartaric acid, malic acid, oxalic acid, lauric acid, clodinafop-propargyl, dimethyl oxalate, acrylate, and ethyl acetate. These carbon source IIs all contain small molecules or polymer precursors with specific ketone functional groups (such as carbonyl, carboxyl, and ester groups), which can decompose and crosslink under relatively mild heat treatment conditions. Utilizing the high reactivity of the specific organic molecules in carbon source II, a high-performance CSX three-dimensional network functional layer can be reliably constructed by guiding the efficient and uniform co-doping carbonization process of S / X elements in the carbon matrix through functional group chemistry.

[0085] In some possible implementations, the sulfur source includes at least one of thiourea, thioacetamide, aminosulfonic acid, 2-thiophenic acid, ferrous sulfide, sodium sulfide, and sulfur powder. These sulfur sources provide sufficient chemical feasibility for achieving efficient, uniform, and controllable in-situ doping reactions, thereby ensuring the construction of CSX composite carbon layers with optimized electrochemical functionality.

[0086] In some possible implementations, the X source includes at least one of boron, silicon, selenium, and tellurium sources. These X sources provide sufficient chemical feasibility for achieving efficient, uniform, and controllable in-situ doping reactions, thereby ensuring the construction of CSX composite carbon layers with optimized electrochemical functionality.

[0087] In some possible implementations, the boron source includes at least one of boric acid, boron oxide, boron nitride, titanium diboride, sodium borate pentahydrate, sodium borohydride, trimethoxyborane, sodium tetraphenylborate, and decaborane. These boron sources are highly reactive and readily form substitution dopants (B atoms replacing C atoms) in the carbon network, introducing charge carriers (holes), thereby effectively enhancing the p-type semiconductor properties of the carbon layer and significantly reducing resistance.

[0088] In some possible implementations, the silicon source includes at least one of triphenylsilane, hexamethyldisilane, γ-aminopropyltriethoxysilane (NH2(CH2)3Si(OC2H5)3), vinyltrimethoxysilane, vinyltriethoxysilane (CH2=CH-Si(OC2H5)3), and ureapropyltriethoxysilane. These silicon sources possess excellent interfacial bridging and structural enhancement capabilities, contributing to the formation of a robust Si-C bond network.

[0089] In some possible implementations, the selenium source includes at least one of selenium powder, sodium selenate, sodium selenite, selenium dioxide, selenite, potassium selenocyanate, bismuth selenide, cadmium selenide, and zinc selenide. In these selenium sources, selenium atoms have a good affinity for lithium ions, which may help lower the interfacial ion diffusion barrier.

[0090] In some possible implementations, the tellurium source includes at least one of tellurium powder, tellurium dioxide, cadmium telluride, bismuth telluride, tin telluride, sodium tellurite, and telluric acid. Among these tellurium sources, tellurium is the most metallic element in the chalcogen group, and its doping is expected to bring the highest improvement in electronic conductivity.

[0091] In some possible implementations, the mass ratio of carbon source II, sulfur source, and X source is (3~9):(1~7):(1~7). This ratio range provides a precise stoichiometric basis for achieving the optimal balance between efficient doping of functional elements (S, X) and maintaining the structural integrity of the carbon matrix. This ensures the dominant framework role of the carbon matrix in the coating layer to maintain excellent intrinsic conductivity, while providing a sufficient and controllable doping concentration window for sulfur and X elements. This effectively modulates the electronic structure of the carbon layer at the atomic scale, introduces ion migration active sites, and ultimately achieves a synergistic increase in both electronic conductivity and ion diffusion rate.

[0092] In some possible implementations, the total mass of carbon source II, sulfur source, and X source is 0.5% to 3% based on the mass of the active particles. This low addition range is sufficient to form a thin and effective CSX functional layer through subsequent sintering, significantly improving interfacial electronic conductivity and optimizing ion transport kinetics. At the same time, it minimizes the overall proportion of inactive materials (coating layer), avoiding dilution of the active material proportion due to excessive additives. Thus, while significantly improving the rate performance and cycle stability of the material, it effectively ensures the final energy density of the battery.

[0093] For example, based on the mass of the active particles, the total mass of carbon source II, sulfur source and X source can be any typical but non-limiting point value or an interval between any two point values, such as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%.

[0094] In some possible implementations, the conditions for the second sintering process include: heating to 400℃~550℃ at a rate of 3℃ / min~10℃ / min, and holding at that temperature for 3h~6h. In this case, a moderate heating rate facilitates the smooth decomposition and uniform coverage of the organic precursor, avoiding violent reactions that could lead to uneven coating layers. A lower sintering temperature ensures sufficient pyrolysis of carbon source II to form a conductive carbon matrix, while providing the activation energy required for the doping reactions of the sulfur and X sources, and preventing excessively high temperatures from causing elemental volatilization or damaging the underlying structure. The holding time ensures that the carbonization and doping reactions proceed fully and thoroughly, ultimately forming a chemically stable and functionally uniform composite coating layer.

[0095] For example, the heating rate of the second sintering treatment can be any typical but non-limiting point value or a range between any two points, such as 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min; the holding temperature can be any typical but non-limiting point value or a range between any two points, such as 400℃, 450℃, 500℃, 550℃; and the holding time can be any typical but non-limiting point value or a range between any two points, such as 3h, 4h, 5h, 6h.

[0096] In some embodiments, the inert atmosphere includes at least one of nitrogen, argon, and helium.

[0097] In some embodiments, the second sintering process, followed by cooling and air jet milling, yields the final phosphate-based cathode material product.

[0098] Thirdly, embodiments of this application provide a positive electrode sheet, which includes a current collector and a positive electrode active layer formed on at least one surface of the current collector. The positive electrode active layer includes the phosphate-based positive electrode material described above and / or the phosphate-based positive electrode material prepared by the above method.

[0099] The cathode provided in this application, by employing the aforementioned phosphate-based cathode material with a unique core-shell structure and interface functionalized by a CSX three-dimensional network, can directly translate the excellent electronic / ionic high conductivity characteristics at the material level into superior performance at the electrode level. This cathode exhibits extremely low internal impedance, uniform current distribution, and rapid electrochemical reaction kinetics.

[0100] In some possible implementations, the preparation of the positive electrode active layer includes the following steps: mixing the above-mentioned phosphate-based positive electrode material, conductive agent and binder to form an electrode slurry, coating the electrode slurry onto the current collector, and then preparing the positive electrode sheet through steps such as drying, rolling and die cutting.

[0101] In some possible implementations, the mass percentage of phosphate-based cathode material in the cathode active layer of the cathode sheet is 90% to 95%. Specifically, the mass percentage of phosphate-based cathode material in the cathode active material layer can be 90%, 91%, 92%, 93%, 94%, 95%, etc.

[0102] In some possible implementations, the current collector of the positive electrode includes, but is not limited to, any one of copper foil or aluminum foil.

[0103] In some possible implementations, the binder content in the positive electrode active material layer is 2wt% to 5wt%. In specific embodiments, the binder content can be typical but not limited to 2wt%, 3wt%, 4wt%, 5wt%, etc.

[0104] In some possible implementations, the binder includes one or more of the following: polyvinylidene chloride, soluble polytetrafluoroethylene, styrene-butadiene rubber, hydroxypropyl methylcellulose, methylcellulose, carboxymethylcellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan, and chitosan derivatives.

[0105] In some possible implementations, the conductive agent content in the positive electrode active material layer is 0.7wt% to 5wt%. In specific embodiments, the conductive agent content can be a typical but not limited content such as 3wt%, 4wt%, or 5wt%.

[0106] In some possible implementations, the conductive agent includes graphite, carbon black, acetylene black, graphene, carbon fiber, and C. 60 And one or more of carbon nanotubes.

[0107] Fourthly, embodiments of this application provide a secondary battery that includes the aforementioned positive electrode plate.

[0108] The secondary battery in this application uses the aforementioned positive electrode sheet with low internal impedance, uniform current distribution, and fast electrochemical reaction kinetics. Therefore, the secondary battery assembled from it has high-rate charge and discharge capability, high capacity output, and long cycle life, realizing the effective transformation of high-performance positive electrode materials into high-performance battery products.

[0109] This application does not specifically limit the negative electrode, electrolyte, separator, etc. in the secondary battery of the embodiments, and can be applied to any battery system.

[0110] In some possible implementations, the negative electrode active material of the secondary battery includes, but is not limited to, carbon materials such as graphite, soft carbon (e.g., coke), and hard carbon, or nitrides, tin-based oxides, tin alloys, and nano-anode materials. The current collector includes, but is not limited to, any one of copper foil and aluminum foil.

[0111] In some possible implementations, the steps for making the negative electrode sheet include: mixing the negative electrode active material with conductive agents such as conductive carbon black, binders such as carboxymethyl cellulose and styrene-butadiene rubber, and solvents such as water in a mass ratio of (80~99):(1~5):(2~10):100 to make a positive electrode mixed slurry, then degassing under vacuum, discharging the material, coating it on a coating machine, and obtaining the negative electrode sheet after rolling, slitting, and die-cutting.

[0112] In some possible implementations, the membrane is capable of blocking electrons while allowing ions to pass through. Exemplary membranes include, but are not limited to, at least one material selected from polypropylene fibers, polyacrylonitrile fibers, polyvinyl formal fibers, poly(ethylene glycol terephthalate), polyethylene terephthalate, polyamide fibers, and poly(p-phenylene terephthalamide).

[0113] In some possible implementations, the electrolyte comprises at least one soluble metal salt. In some specific embodiments, the metal salt includes LiClO4, LiBF4, LiPF6, LiAsF6, LiCF3SO3, LiTDI, Li[(CF3SO2)2N], Li[(FSO2)2N], Li[(C m F 2m+1 SO2)(C n F 2n+1 At least one of SO2(N)[m, n], where m and n are natural numbers. These electrolytic salts can ensure high ionic conductivity of the electrolyte and do not undergo harmful side reactions with electrode materials, electrolyte, diaphragm, etc., and have good chemical stability.

[0114] In some possible implementations, the secondary battery includes at least one of a battery cell, a battery module, and a battery pack.

[0115] In some possible implementations, the battery cell types include lithium-ion batteries, as well as novel batteries such as lithium metal batteries.

[0116] In some possible implementations, the battery cells of this application can be assembled into a battery module. The battery module can contain multiple battery cells, the specific number of which can be adjusted according to the application and capacity of the battery module. The battery module may also include a housing with a receiving space in which multiple battery cells are received.

[0117] In one possible implementation, battery cells and / or battery modules can also be assembled into a battery pack, and the number of battery cells or battery modules contained in the battery pack can be adjusted according to the application and capacity of the battery pack.

[0118] To enable those skilled in the art to clearly understand the above-described implementation details and operations, and to highlight the significant improvements in the performance of the phosphate-based cathode material, its preparation method, cathode sheet, and secondary battery in the embodiments of this application, the following examples illustrate the above technical solutions.

[0119] Example 1 A phosphate-based cathode material includes a lithium iron phosphate core, and a second carbon layer and a first carbon layer (containing ketone groups and sulfur elements, with a mass ratio of C to S of 9:1) sequentially coated on the outer surface of the core. Its preparation includes the following steps: 1. Ferric nitrate, ammonium dihydrogen phosphate, and lithium hydroxide were mixed in a ratio of 0.95:1.00:1.04, and 12% by weight of deionized water was added as a dispersant. At the same time, 5% glucose was added as carbon source I to prepare a mixed solution. After drying the solution, a solid-phase lithium iron phosphate precursor was obtained. 2. The solid-phase lithium iron phosphate precursor was placed in a tube furnace and kept at 500°C for 10 hours under the atmosphere of nitrogen and carbon source I, with a heating rate of 3°C / min. Then the temperature was increased to 700°C and held for 6 hours. After the material cooled to room temperature, it was taken out and crushed to obtain the first sintered product. 3. Add a second raw material component to the first sintered product. The second raw material component is a uniform mixture of carbon source II (phenidone) and sulfur source (aminosulfonic acid) in a mass ratio of 9:1. The amount of the second raw material component added is 1 wt% of the mass of the first sintered product. The mixture is heated to 400℃ at a heating rate of 3℃ / min under a nitrogen atmosphere, held at this temperature for 4 hours, cooled, and then pulverized by air jet milling to obtain the final lithium iron phosphate cathode material.

[0120] Example 2 A lithium iron phosphate cathode material includes a lithium iron phosphate core, and a second carbon layer and a first carbon layer (containing ketone groups and S and B elements, with the mass ratio of C element to S element and B element being 8:1:1) sequentially coated on the outer surface of the core. Its preparation includes the following steps: 1. Ferric nitrate, ammonium dihydrogen phosphate, and lithium hydroxide were mixed in a ratio of 0.95:1.00:1.04, and 12% by weight of deionized water was added as a dispersant. At the same time, 5% glucose was added as carbon source I to prepare a mixed solution. After drying the solution, a solid-phase lithium iron phosphate precursor was obtained. 2. The solid-phase lithium iron phosphate precursor was placed in a tube furnace and kept at 500°C for 10 hours under the atmosphere of nitrogen and carbon source I, with a heating rate of 3°C / min. Then the temperature was increased to 700°C and held for 6 hours. After the material cooled to room temperature, it was taken out and crushed to obtain the first sintered product. 3. A second raw material component is added to the first sintered product. The second raw material component consists of carbon source II (phenidone), sulfur source (sulfamic acid), and boron source (boron oxide) mixed evenly in a mass ratio of 8:1:1. The amount of the second raw material component added is 1 wt% of the mass of the first sintered product. The mixture is heated to 400°C at a heating rate of 3°C / min under a nitrogen atmosphere, held at that temperature for 4 hours, cooled, and then pulverized by an air jet mill to obtain the final lithium iron phosphate cathode material.

[0121] Example 3 A lithium iron phosphate cathode material includes a lithium iron phosphate core, and a second carbon layer and a first carbon layer (containing ketone groups and S and B elements, with the mass ratio of C to S and B elements being 5:3:2) sequentially coated on the outer surface of the core. Its preparation includes the following steps: 1. Ferric nitrate, ammonium dihydrogen phosphate, and lithium hydroxide were mixed in a ratio of 0.95:1.00:1.04, and 12% by weight of deionized water was added as a dispersant. At the same time, 5% glucose was added as carbon source I to prepare a mixed solution. After drying the solution, a solid-phase lithium iron phosphate precursor was obtained. 2. The solid-phase lithium iron phosphate precursor was placed in a tube furnace and kept at 500°C for 10 hours under the atmosphere of nitrogen and carbon source I, with a heating rate of 3°C / min. Then the temperature was increased to 700°C and held for 6 hours. After the material cooled to room temperature, it was taken out and crushed to obtain the first sintered product. 3. A second raw material component is added to the first sintered product. The second raw material component consists of carbon source II (phenidone), sulfur source (sulfamic acid), and boron source (boron oxide) mixed evenly in a mass ratio of 5:3:2. The amount of the second raw material component added is 1 wt% of the mass of the first sintered product. The mixture is heated to 400°C at a heating rate of 3°C / min under a nitrogen atmosphere, held at this temperature for 4 hours, cooled, and then pulverized by an air jet mill to obtain the final lithium iron phosphate cathode material.

[0122] Example 4 A lithium iron phosphate cathode material includes a lithium iron phosphate core, and a second carbon layer and a first carbon layer (containing ketone groups and S and Si elements, with the mass ratio of C to S and Si elements being 5:3:2) sequentially coated on the outer surface of the core. Its preparation includes the following steps: 1. Ferric nitrate, ammonium dihydrogen phosphate, and lithium hydroxide were mixed in a ratio of 0.95:1.00:1.04, and 12% by weight of deionized water was added as a dispersant. At the same time, 5% glucose was added as carbon source I to prepare a mixed solution. After drying the solution, a solid-phase lithium iron phosphate precursor was obtained. 2. The solid-phase lithium iron phosphate precursor was placed in a tube furnace and kept at 500°C for 10 hours under the atmosphere of nitrogen and carbon source I, with a heating rate of 3°C / min. Then the temperature was increased to 700°C and held for 6 hours. After the material cooled to room temperature, it was taken out and crushed to obtain the first sintered product. 3. A second raw material component is added to the first sintered product. The second raw material component consists of carbon source II (phenidone), sulfur source (sulfamic acid), and silicon source (triphenylsilane) mixed evenly in a mass ratio of 5:3:2. The amount of the second raw material component added is 1 wt% of the mass of the first sintered product. The mixture is heated to 400°C at a heating rate of 3°C / min under a nitrogen atmosphere, held at this temperature for 4 hours, cooled, and then pulverized by an air jet mill to obtain the final lithium iron phosphate cathode material.

[0123] Example 5 A lithium iron phosphate cathode material includes a lithium iron phosphate core, and a second carbon layer and a first carbon layer (containing ketone groups and S and Se elements, with the mass ratio of C to S and Se elements being 5:3:2) sequentially coated on the outer surface of the core. Its preparation includes the following steps: 1. Ferric nitrate, ammonium dihydrogen phosphate, and lithium hydroxide were mixed in a ratio of 0.95:1.00:1.04, and 12% by weight of deionized water was added as a dispersant. At the same time, 5% glucose was added as carbon source I to prepare a mixed solution. After drying the solution, a solid-phase lithium iron phosphate precursor was obtained. 2. The solid-phase lithium iron phosphate precursor was placed in a tube furnace and kept at 500°C for 10 hours under the atmosphere of nitrogen and carbon source I, with a heating rate of 3°C / min. Then the temperature was increased to 700°C and held for 6 hours. After the material cooled to room temperature, it was taken out and crushed to obtain the first sintered product. 3. A second raw material component is added to the first sintered product. The second raw material component consists of carbon source II (phenidone), sulfur source (sulfamic acid), and selenium source (selenium dioxide) mixed evenly in a mass ratio of 5:3:2. The amount of the second raw material component added is 1 wt% of the mass of the first sintered product. The mixture is heated to 400°C at a heating rate of 3°C / min under a nitrogen atmosphere, held at this temperature for 4 hours, cooled, and then pulverized by an air jet mill to obtain the final lithium iron phosphate cathode material.

[0124] Example 6 A lithium iron phosphate cathode material includes a lithium iron phosphate core, and a second carbon layer and a first carbon layer (containing ketone groups and S and Te elements, with the mass ratio of C to S and Te elements being 5:3:2) sequentially coated on the outer surface of the core. Its preparation includes the following steps: 1. Ferric nitrate, ammonium dihydrogen phosphate, and lithium hydroxide were mixed in a ratio of 0.95:1.00:1.04, and 12% by weight of deionized water was added as a dispersant. At the same time, 5% glucose was added as carbon source I to prepare a mixed solution. After drying the solution, a solid-phase lithium iron phosphate precursor was obtained. 2. The solid-phase lithium iron phosphate precursor was placed in a tube furnace and kept at 500°C for 10 hours under the atmosphere of nitrogen and carbon source I, with a heating rate of 3°C / min. Then the temperature was increased to 700°C and held for 6 hours. After the material cooled to room temperature, it was taken out and crushed to obtain the first sintered product. 3. A second raw material component is added to the first sintered product. The second raw material component consists of carbon source II (phenidone), sulfur source (sulfamic acid), and tellurium source (tellurium dioxide) mixed evenly in a mass ratio of 5:3:2. The amount of the second raw material component added is 1 wt% of the mass of the first sintered product. The mixture is heated to 400°C at a heating rate of 3°C / min under a nitrogen atmosphere, held at this temperature for 4 hours, cooled, and then pulverized by an air jet mill to obtain the final lithium iron phosphate cathode material.

[0125] Example 7 A phosphate-based cathode material includes a lithium iron phosphate core, and a second carbon layer and a first carbon layer (containing ketone groups and B elements, with a mass ratio of C elements to B elements of 9:1) sequentially coated on the outer surface of the core. Its preparation includes the following steps: 1. Ferric nitrate, ammonium dihydrogen phosphate, and lithium hydroxide were mixed in a ratio of 0.95:1.00:1.04, and 12% by weight of deionized water was added as a dispersant. At the same time, 5% glucose was added as carbon source I to prepare a mixed solution. After drying the solution, a solid-phase lithium iron phosphate precursor was obtained. 2. The solid-phase lithium iron phosphate precursor was placed in a tube furnace and kept at 500°C for 10 hours under the atmosphere of nitrogen and carbon source I, with a heating rate of 3°C / min. Then the temperature was increased to 700°C and held for 6 hours. After the material cooled to room temperature, it was taken out and crushed to obtain the first sintered product. 3. Add a second raw material component to the first sintered product. The second raw material component is a uniform mixture of carbon source II (phenidone) and boron source (boron oxide) in a mass ratio of 9:1. The amount of the second raw material component added is 1 wt% of the mass of the first sintered product. The mixture is heated to 400°C at a heating rate of 3°C / min under a nitrogen atmosphere, held at this temperature for 4 hours, cooled, and then pulverized by air jet milling to obtain the final lithium iron phosphate cathode material.

[0126] Example 8 A lithium iron phosphate cathode material includes a lithium iron phosphate core, and a second carbon layer and a first carbon layer sequentially coated on the outer surface of the core (carbon source elements I, S and B, with the mass ratio of C to S and B elements being 8:1:1). Its preparation includes the following steps: 1. Ferric nitrate, ammonium dihydrogen phosphate, and lithium hydroxide were mixed in a ratio of 0.95:1.00:1.04, and 12% by weight of deionized water was added as a dispersant. At the same time, 5% glucose was added as carbon source I to prepare a mixed solution. After drying the solution, a solid-phase lithium iron phosphate precursor was obtained. 2. The solid-phase lithium iron phosphate precursor was placed in a tube furnace and kept at 500°C for 10 hours under the atmosphere of nitrogen and carbon source I, with a heating rate of 3°C / min. Then the temperature was increased to 700°C and held for 6 hours. After the material cooled to room temperature, it was taken out and crushed to obtain the first sintered product. 3. A second raw material component is added to the first sintered product. The second raw material component consists of carbon source I, sulfur source (aminosulfonic acid), and boron source (boron oxide) mixed evenly in a mass ratio of 8:1:1. The amount of the second raw material component added is 1 wt% of the mass of the first sintered product. The mixture is heated to 400°C at a heating rate of 3°C / min under a nitrogen atmosphere, held at that temperature for 4 hours, cooled, and then pulverized by an air jet mill to obtain the final lithium iron phosphate cathode material.

[0127] Comparative Example 1 A lithium iron phosphate cathode material includes a lithium iron phosphate core and a carbon layer coated on the outer surface of the core. Its preparation includes the following steps: 1. Ferric nitrate, ammonium dihydrogen phosphate, and lithium hydroxide were mixed in a ratio of 0.95:1.00:1.04, and 12% by weight of deionized water was added as a dispersant. At the same time, 5% glucose was added as carbon source I to prepare a mixed solution. After drying the solution, a solid-phase lithium iron phosphate precursor was obtained. 2. The solid-phase lithium iron phosphate precursor was placed in a tube furnace and kept at 500°C for 10 hours under the atmosphere of nitrogen and carbon source I, with a heating rate of 3°C / min. Then the temperature was increased to 700°C and held for 6 hours. After the material cooled to room temperature, it was taken out and crushed to obtain the first sintered product. 3. The first sintered product is placed in a tube furnace and heated to 400°C at a rate of 3°C / min under the atmosphere of nitrogen and carbon source I. The temperature is maintained for 4 hours. The amount of carbon source I added is 1 wt% of the mass of the first sintered product. After cooling, the product is pulverized by air jet mill to obtain the final lithium iron phosphate cathode material.

[0128] Key process information for the comparative examples and embodiments above is summarized in Table 1 below:

[0129] To verify the progressiveness of the embodiments of this application, the lithium iron phosphate cathode materials prepared in the above embodiments and comparative examples were subjected to the following performance tests: 1. The morphology of the lithium iron phosphate cathode materials prepared in Example 4 and Comparative Example 1 was observed by scanning electron microscopy, as shown in the attached figure.Figure 3 As can be seen, the unique double-layer coating design prepared in Example 4 of this application endows the lithium iron phosphate cathode material with a more rounded, discrete, and uniform particle morphology, which is beneficial to structural stability and rapid lithium ion desorption, thereby significantly improving the electrochemical performance and cycle life of the material. The discrete particle state and the unique groups (ketone groups) of carbon source II endow the lithium iron phosphate cathode material powder with excellent dispersion and stability in solvents such as NMP (N-methylpyrrolidone), thereby reducing the viscosity of the slurry and increasing the solid content.

[0130] The lithium iron phosphate cathode material prepared in Example 4 was subjected to transmission electron microscopy (TEM) testing, and the results are shown in the attached figure. Figure 4 As shown in the TEM image, a double-layer coating is formed on the surface of lithium iron phosphate particles. The inner layer (second carbon layer) is a relatively loosely arranged carbon coating layer, while the outer layer (first carbon layer) is a more uniform and regularly arranged cross-linked network coating layer. This inner layer structure is beneficial to improving the diffusion rate of lithium ions in the coating layer, and this outer layer structure can improve electronic conductivity and reduce side reactions during cycling, thereby improving electrochemical and cycling performance.

[0131] 2. Application in secondary batteries for electrochemical performance testing. Specifically, the secondary battery is prepared as follows: Prepared lithium iron phosphate cathode material, polyvinylidene fluoride (PVDF), and conductive carbon black are mixed with a certain amount of N-methylpyrrolidone (NMP) at mass percentages of 96.5%, 2.2%, and 1.3%, respectively, and stirred thoroughly. The slurry is then dried on a coating machine at 120°C to form a film, followed by rolling to obtain the cathode sheet. The prepared cathode sheet, along with a graphene anode and separator, is then coated, sliced, rolled, slit, dried, tape-coated, wound into cells, and dried at 80°C for 48 hours. Finally, Zhuhai Saiwei 2001a electrolyte is used to fill and seal the lithium-ion battery, followed by 24 hours of resting, formation, first final sealing, aging, and second final sealing to prepare the lithium-ion battery.

[0132] The lithium-ion batteries prepared in the above embodiments and comparative examples were subjected to the following performance tests: 2-1 Processing performance test: The finished lithium iron phosphate cathode material, binder (PVDF) and conductive carbon material were mixed at a mass percentage of 96.5%, 2.2% and 1.3%, respectively. Then, a certain amount of solvent N-methylpyrrolidone (NMP) was added and mixed and stirred thoroughly. The solid content and viscosity were tested using a viscometer (Bolefeld viscometer, model: DV2TLVTJ0). The test results are shown in Table 2 below.

[0133] 2-2 Battery charge and discharge performance test: The battery was tested at 0.5C and 1C charge and discharge specific capacity data under the conditions of temperature 25℃±2℃ and humidity <2%RH, according to the operating procedure of the battery charge and discharge tester. The test results are shown in Table 2 below.

[0134] 2-3. Cycling Performance: Cycling was conducted in a 25℃ constant temperature chamber using a constant power of 0.5P for charge-discharge cycles. The charging and discharging voltage range was 2.5V~3.65V. The capacity retention rate was calculated for different numbers of cycles. The capacity retention rate (%) of the lithium-ion battery after N cycles = (discharge capacity of the Nth cycle / initial discharge capacity) × 100%, where N is the number of cycles of the lithium-ion battery. The test results are shown in Table 2 below.

[0135]

[0136] As shown in Table 2 above, the test results of this application, through the design of specific sintering conditions and raw material components in the first and / or second sintering processes, not only reduce the viscosity of the cathode material and improve its processing performance such as solid content, but also significantly improve the battery's capacity, cycle life, and other electrochemical performance. The test results also show that the lithium iron phosphate cathode materials prepared in Examples 1-9 of this application all exhibit higher solid content and lower viscosity when preparing cathode slurries, which is beneficial for improving processing performance. The prepared secondary batteries all exhibit higher charge-discharge performance and cycle stability.

[0137] The electrochemical impedance spectroscopy (EIS) spectra of Examples 2, 4, and Comparative Example 1 are attached. Figure 5 As shown, the ohmic impedance (4.3Ω) and interface transfer impedance (54.5Ω) of Example 2, and the ohmic impedance (2.6Ω) and interface transfer impedance (42.3Ω) of Example 4 are all less than the ohmic impedance (8.8Ω) and interface transfer impedance (75.3Ω) of Comparative Example 1, indicating an improvement in electronic conductivity and lithium-ion transport at the interface. This explains the improvement in electrochemical and cycling performance.

[0138] The charge-discharge curves of Examples 2, 4, and Comparative Example 1 at 1C are attached. Figure 6 As shown, the 1C discharge capacity of Examples 2 and 4 is higher than that of Example 1, with increases of 3.17 mAh / g and 5.55 mAh / g, respectively.

[0139] The cycle capacity retention diagrams for Examples 2, 4, and Comparative Example 1 at 0.5P are attached. Figure 7As shown, it can be seen that the capacity retention rates of Examples 2 and 4 after 200 cycles at 0.5P are higher than those of Example 1, with capacity retention rates increasing by 1.94% and 4.53%, respectively.

[0140] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A phosphate-based cathode material, characterized in that, It includes a phosphate-based active core and a first carbon layer covering the outer surface of the core, wherein the first carbon layer is modified with S and / or X elements, and the X element includes at least one of boron, silicon, selenium, and tellurium.

2. The phosphate-based cathode material as described in claim 1, characterized in that, The first carbon layer is modified with the S element and the X element, and the mass ratio of carbon element to the S element and the X element is (3~9):(1~7):(1~7). And / or, the first carbon layer is modified with the S element and the X element, and the carbon element, the S element and the X element form a CSX three-dimensional network structure; And / or, the first carbon layer contains ketone functional groups.

3. The phosphate-based cathode material as described in claim 1 or 2, characterized in that, A second carbon layer is also included between the core and the first carbon layer.

4. The phosphate-based cathode material as described in claim 3, characterized in that, The particle size D50 of the phosphate-based cathode material is 0.5 μm to 1.1 μm; And / or, the thickness of the second carbon layer is 1 nm to 2 nm; And / or, the thickness of the first carbon layer is 2nm~3nm; And / or, the phosphate-based active core includes at least one of lithium iron phosphate and lithium manganese iron phosphate.

5. The phosphate-based cathode material as described in claim 4, characterized in that, Based on the total mass of the phosphate-based cathode material as 100%, the mass percentage of the second carbon layer is 0.4% to 0.6%; and / or, the mass percentage of the first carbon layer is 0.5% to 0.7%. And / or, when the phosphate-based active core is lithium iron phosphate, the particle size D50 of the phosphate-based cathode material is 0.9 μm to 1.1 μm; And / or, when the phosphate-based active core is lithium manganese iron phosphate, the particle size D50 of the phosphate-based cathode material is 0.5 μm to 0.9 μm.

6. A method for preparing a phosphate-based cathode material, characterized in that, Includes the following steps: Prepare active particles, wherein the active particles comprise phosphate-based active materials; A first carbon layer is prepared on the outer surface of the active particles to obtain a phosphate-based cathode material; wherein the first carbon layer is modified with S element and / or X element, and the X element includes at least one of boron, silicon, selenium and tellurium.

7. The method for preparing the phosphate-based cathode material as described in claim 6, characterized in that, The active particles comprise a phosphate-based active core and a second carbon layer covering the outer surface of the core, wherein the first carbon layer covers the outer surface of the second carbon layer.

8. The method for preparing the phosphate-based cathode material as described in claim 7, characterized in that, The steps for preparing the active particles include: preparing a phosphate-based active precursor from raw material components including lithium source, phosphorus source, iron source and carbon source I, and performing a first sintering treatment under an inert atmosphere to form an active particle with the second carbon layer covering the core. And / or, the step of preparing the first carbon layer includes: mixing the active particles with carbon source II, sulfur source and X source, and then performing a second sintering treatment under an inert atmosphere to form the first carbon layer on the surface of the second carbon layer, thereby obtaining the phosphate-based cathode material.

9. The method for preparing the phosphate-based cathode material as described in claim 8, characterized in that, The raw material components also include a manganese source; And / or, the carbon source I includes at least one of glucose, fructose, sucrose, lactose, starch, and cellulose; And / or, the carbon source II is selected from carbon sources with ketone functional groups; And / or, the sulfur source includes at least one of thiourea, thioacetamide, aminosulfonic acid, 2-thiophenic acid, ferrous sulfide, sodium sulfide, and sulfur powder; And / or, the X source includes at least one of boron source, silicon source, selenium source, and tellurium source; And / or, the conditions for the first sintering treatment include: heating to 400℃~600℃ at a heating rate of 3℃ / min~10℃ / min and holding for 8h~12h; then heating to 700℃~800℃ and holding for 4h~6h. And / or, the conditions for the second sintering treatment include: heating to 400°C to 550°C at a heating rate of 3°C / min to 10°C / min, and holding at that temperature for 3h to 6h.

10. The method for preparing the phosphate-based cathode material as described in claim 9, characterized in that, The carbon source II includes at least one of nonanone, hexanedione, phenanthrene, gingerone, benzophenone, adipic aldehyde, furfural, p-nitrobenzaldehyde, salicylic acid, tartaric acid, malic acid, oxalic acid, lauric acid, clodinafop-propargyl, dimethyl oxalate, acrylate, and ethyl acetate. And / or, the boron source includes at least one of boric acid, boron oxide, boron nitride, titanium diboride, sodium borate pentahydrate, sodium borohydride, trimethoxyborane, sodium tetraphenylborate, and decaborane; And / or, the silicon source includes at least one of triphenylsilane, hexamethyldisilane, γ-aminopropyltriethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, and ureapropyltriethoxysilane; And / or, the selenium source includes at least one of selenium powder, sodium selenate, sodium selenite, selenium dioxide, selenite, potassium selenocyanate, bismuth selenide, cadmium selenide, and zinc selenide. And / or, the tellurium source includes at least one of tellurium powder, tellurium dioxide, cadmium telluride, bismuth telluride, tin telluride, sodium tellurite, and telluric acid; And / or, the mass ratio of the carbon source II, the sulfur source and the X source is (3~9):(1~7):(1~7); And / or, in the phosphate-based active precursor, the mass percentage of carbon source I is 3% to 10%; And / or, based on the mass of the active particles, the total mass of the carbon source II, the sulfur source and the X source is 0.5% to 3%.

11. A positive electrode plate, characterized in that, The positive electrode sheet includes 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 includes a phosphate-based positive electrode material as described in any one of claims 1 to 5 and / or a phosphate-based positive electrode material prepared by the method as described in any one of claims 6 to 10.

12. A secondary battery, characterized in that, The secondary battery includes the positive electrode as described in claim 11.

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