Pure-phase phosphate-based positive electrode material, preparation method and application thereof

Abstract

The application belongs to the technical field of battery materials, and particularly relates to a pure-phase phosphate-based positive electrode material and a preparation method and application thereof. a M b A c (PO4) d The stoichiometric ratio of elements in the active material is determined, a mixed solution is prepared from a lithium source, an M metal source and an A metal source, a complexing agent is added for complexation and coordination reaction, a polymerization agent is added for polymerization reaction, a phosphorus source is added for gelation reaction, and a dry gel precursor is subjected to sintering treatment to obtain a pure-phase positive electrode material. The gel network structure formed by the complexing agent and the polymerization agent can not only serve as a matrix of the three-dimensional porous carbon network structure, but also directly serve as a carbon-coated material to coat the active particles formed in situ, so that an additional carbon source is not needed. The network structure formed by the sol-gel method enables the active particles generated in situ to be uniformly distributed in the three-dimensional porous carbon network structure, so that the obtained positive electrode material has good structural stability, high energy density and excellent conductivity.

Description

Technical Field

[0001] This application belongs to the field of battery materials technology, and in particular relates to a pure-phase phosphate-based cathode material, its preparation method and application. Background Technology

[0002] Olivine LiMPO4 (M = Fe, Ni, Mn, etc.) phosphate-based cathode materials have been widely developed as cathode materials for lithium-ion batteries due to their good structural properties, high potential, and high specific capacity. However, the poor electronic and ionic conductivity of these cathode materials hinders their application in electrochemical devices. Therefore, the research and development of olivine cathodes is constantly increasing in line with their advanced applications. Although LiMPO4 can provide high charge and discharge capacity, its poor electronic and ionic conductivity due to Fe... 2+ / Fe 3+ The redox potential plateau is low, resulting in a relatively low energy density.

[0003] Therefore, improving the charge-discharge potential plateau and energy density of olivine LiMPO4 phosphate-based cathode materials is a pressing technical problem that needs to be solved. The preparation method of LiMPO4 cathode materials affects both the electrochemical performance and industrial application of the finished materials, and the preparation of pure-phase LiMPO4 cathode materials is currently difficult. Summary of the Invention

[0004] The purpose of this application is to provide a pure-phase phosphate-based cathode material, its preparation method, and its application, aiming to solve to some extent the problems of low charge-discharge potential plateau and low energy density of current phosphate-based cathode materials.

[0005] To achieve the above-mentioned objectives, the technical solution adopted in this application is as follows:

[0006] In a first aspect, this application provides a method for preparing a pure-phase phosphate-based cathode material, comprising the following steps:

[0007] According to Li a M b A c (PO4) d The stoichiometric ratio of elements in the active material is determined by preparing a mixed solution of lithium source, M metal source and A metal source, adding a complexing agent to carry out a complexation and coordination reaction with the metal elements, adding a polymerization agent to carry out a polymerization reaction to form a three-dimensional network structure, and obtaining a sol solution.

[0008] After adding a phosphorus source and mixing with the sol solution, a gelation reaction is carried out to form a wet gel, which is then dried to obtain a dry gel precursor.

[0009] The dry gel precursor was sintered to obtain Li a M b A c(PO4) d Pure phase phosphate-based positive electrode material; wherein, 0.95≤a≤1.05, 0.1≤b≤0.9, 0.1≤c≤0.9, 0.95≤d≤1.05, M and A are selected from different transition metal elements.

[0010] The application provides a preparation method of the pure phase phosphate-based positive electrode material. a M b A c (PO4) d After the raw material components such as the lithium source, the M metal source and the A metal source in the active material are made into a mixed solution, a complexing agent is added to perform a complex coordination reaction on the metal ions in the mixed solution, so that the metal ions are chelated into the complexing agent uniformly and stably, and then a polymerizing agent is added to perform a polymerization reaction with the complexing agent to form a three-dimensional network structure. Then, a phosphorus source is added, and the metal ions anchored in the three-dimensional network structure are used to coordinate and anchor the phosphate ions, so that the phosphate ions are uniformly dispersed into the three-dimensional network structure. After gelation, a wet gel is formed, and the solvent is removed by drying and volatilization, and the anion radicals are decomposed to generate gas, so that a dry gel precursor material with a loose and porous three-dimensional network structure is obtained, and each metal element is uniformly distributed in the three-dimensional porous structure. Through sintering treatment, the three-dimensional porous gel network structure is carbonized into a carbon material in situ, the three-dimensional porous network structure of the gel is retained, and a three-dimensional porous conductive carbon network structure is obtained. At the same time, each metal source and the phosphorus source in the three-dimensional porous conductive carbon network structure react in situ at high temperature to generate the pure phase Li a M b A c (PO4) d Active material, and the three-dimensional porous conductive carbon network structure can inhibit the agglomeration and growth of Li a M b A c (PO4) d Active material particles, so that the generated Li a M b A c (PO4) d Active material is uniformly distributed in the three-dimensional porous carbon network structure. The process is simple and easy to operate, the complexing agent and the polymerizing agent can be directly used as the carbon coating material to form a coating on Li a M b A c (PO4) d Active material in situ, and the process does not need to add an additional carbon source, and subsequent carbon coating treatment and carbonization sintering treatment are also not needed, so that the introduction of impurity phases is reduced, the purity of the product is improved, the pure phase phosphate-based positive electrode material is obtained, the structural stability is good, the energy density is high, the conductive performance is excellent, the material is environmentally friendly and pollution-free, and the safety is high.

[0011] In a second aspect, the application provides a pure-phase phosphate-based positive electrode material, comprising a three-dimensional porous carbon network structure and positive electrode active particles in-situ loaded inside the three-dimensional porous carbon network structure, wherein the positive electrode active particles comprise Li a M b A c (PO4) d , wherein 0.95≤a≤1.05, 0.1≤b≤0.9, 0.1≤c≤0.9, 0.95≤d≤1.05, M and A are respectively selected from at least one different transition metal element in Fe, Ni, Mn and Co.

[0012] The pure-phase phosphate-based positive electrode material of the application comprises a three-dimensional porous carbon network structure and Li a M b A c (PO4) d active particles in-situ loaded inside the three-dimensional porous carbon network structure. The active material particles are connected together through the loose porous carbon network structure, and the structural connectivity between the particles is strong, so that the pure-phase phosphate-based positive electrode material has good structural stability. Moreover, the three-dimensional porous carbon network structure has excellent electronic conductivity, which enhances the electronic conductivity of the pure-phase phosphate-based positive electrode material; at the same time, the three-dimensional porous carbon network structure has a super large specific surface area, which can make the electrolyte and the electrode material more fully contact, improve the ion migration efficiency of the pure-phase phosphate-based positive electrode material, and improve the energy density and the conductivity of the positive electrode material. Thus, the pure-phase phosphate-based positive electrode material has good structural stability, high energy density, excellent conductivity and other characteristics.

[0013] In a third aspect, the application provides a positive electrode sheet, comprising a current collector and an active layer formed on the surface of the current collector, wherein the active layer comprises the above pure-phase phosphate-based positive electrode material.

[0014] The positive electrode sheet of the application uses the above pure-phase phosphate-based positive electrode material in the active layer, and the pure-phase phosphate-based positive electrode material has good structural stability, high capacity and excellent conductivity, etc., so that the stability, energy density, rate performance, cycle performance and other electrochemical properties of the positive electrode sheet are improved.

[0015] In a fourth aspect, the application provides a secondary battery, wherein the secondary battery comprises the above positive electrode sheet.

[0016] The secondary battery of the application comprises the above positive electrode sheet with good stability, high energy density, good rate performance, good cycle stability and excellent electrochemical properties, so that the energy density and the cycle stability and other electrochemical properties of the secondary battery are improved. BRIEF DESCRIPTION OF DRAWINGS

[0017] 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. Obviously, the drawings in the following description only constitute some embodiments of the present application, and for those skilled in the art, other drawings can be obtained without creative effort based on these drawings.

[0018] Figure 1 is a flowchart of a preparation method of a pure-phase phosphate-based positive electrode material provided by the embodiments of the present application;

[0019] Figure 2 is a pure-phase LiFePO4 / C positive electrode material provided by Embodiment 1 of the present application; 0.8 Ni 0.2 is a scanning electron microscope image of the LiFePO4 / C positive electrode material;

[0020] Figure 3 is a pure-phase LiFePO4 / C positive electrode material provided by Embodiment 5 of the present application; 0.8 Ni 0.2 is a scanning electron microscope image of the LiFePO4 / C positive electrode material;

[0021] Figure 4 is an XRD spectrum of the positive electrode material provided by Embodiments 1-3 and Comparative Example 1 of the present application;

[0022] Figure 5 is a pure-phase LiFePO4 / C positive electrode material provided by Embodiment 6 of the present application; 0.8 Ni 0.2 is a scanning electron microscope image of the LiFePO4 / C positive electrode material. DETAILED DESCRIPTION

[0023] In order to make the technical problems, technical solutions and beneficial effects to be solved by the present application more clearly understood, the present application will be further described in detail below with reference to the 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.

[0024] 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, which can represent the following three cases: 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 an "or" relationship between the associated objects before and after it.

[0025] In this application, "at least one" means one or more, and "multiple" means two or more. "At least one of the following (one)" or the like means any combination of the items, including a single item (one) 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.

[0026] 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.

[0027] 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.

[0028] 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 proportional relationship between the weights of 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.

[0029] 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.

[0030] The first aspect of the embodiments of the present application provides a preparation method of a pure-phase phosphate-based positive electrode material, as shown in the accompanying drawings, comprising the following steps: Figure 1 As shown in the accompanying drawings, comprising the following steps:

[0031] S10. Li a M b A c (PO4) dThe stoichiometric ratio of elements in the active material, after the lithium source, the M metal source, and the A metal source are made into a mixed solution, a complexing agent is added to perform a complexation reaction with the metal elements, a polymerization agent is added to perform a polymerization reaction to form a three-dimensional network structure, and a sol solution is obtained;

[0032] S20. After the phosphorus source is added and mixed with the sol solution, a gelation reaction is performed to form a wet gel, and a dry gel precursor is obtained after drying;

[0033] S30. The dry gel precursor is subjected to a sintering treatment to obtain Li a M b A c (PO4) d a pure-phase phosphate-based positive electrode material; wherein 0.95≤a≤1.05, 0.1≤b≤0.9, 0.1≤c≤0.9, 0.95≤d≤1.05, M and A are selected from different transition metal elements.

[0034] The preparation method of the pure-phase phosphate-based positive electrode material provided by the first aspect of the embodiments of the present application has at least the following beneficial effects:

[0035] Firstly, the Li a M b A c (PO4) d After the raw material components such as the lithium source, the M metal source, and the A metal source are made into a mixed solution, a complexing agent is added to perform a complexation reaction on the metal ions in the mixed solution, so that the metal ions are uniformly and stably chelated into the complexing agent, and then a polymerization agent is added to perform a polymerization reaction with the complexing agent to form a three-dimensional network structure. Then, the phosphorus source is added, and the metal ions anchored in the three-dimensional network structure are coordinated with the phosphate ions to anchor the phosphate ions, so that the phosphate ions are uniformly dispersed in the three-dimensional network structure. After the three-dimensional network structure is generated, the phosphorus source is added, which can avoid affecting the chelation process of the complexing agent on the metal ions due to the premature combination of the phosphate ions with the metal ions, and reduce the influence on the polymerization reaction between the complexing agent and the polymerization agent, so as to ensure that each metal ion and phosphate ion is uniformly anchored in the three-dimensional network structure.

[0036] Secondly, after the gelation, a wet gel is formed, the solvent is removed by drying and volatilization, and the anion radicals are decomposed to generate gas, so that a dry gel precursor material with a loose and porous three-dimensional network structure is obtained, and each metal element is uniformly distributed in the three-dimensional porous structure. Through sintering treatment, the three-dimensional porous gel network structure is carbonized in situ into a carbon material, the three-dimensional porous network structure of the gel is retained, and a three-dimensional porous conductive carbon network structure is obtained. At the same time, each metal source and phosphorus source in the three-dimensional porous conductive carbon network structure reacts in situ at high temperature to generate Li a M b A c (PO4) dactive material, and the three-dimensional porous conductive carbon network structure can inhibit Li a M b A c (PO4) d active material particles grow in agglomerates, so that the generated Li a M b A c (PO4) d active material is uniformly distributed in the three-dimensional porous carbon network structure in small particles.

[0037] Thirdly, the prepared Li a M b A c (PO4) d active material particles are connected together through the loose porous carbon network structure, the structural connectivity between the particles is strong, so that the pure-phase phosphate-based positive electrode material has good structural stability. Moreover, the three-dimensional porous carbon network structure has excellent electronic conductivity, which enhances the electronic conductivity of the pure-phase phosphate-based positive electrode material; at the same time, the three-dimensional porous carbon network structure has a super large specific surface area, which can make the electrolyte and the electrode material more fully contact, improve the ion migration efficiency of the pure-phase phosphate-based positive electrode material, and improve the energy density and the conductivity of the positive electrode material.

[0038] Fourthly, the preparation method of the embodiment of the application has simple process and is easy to operate. The complexing agent and the polymerization agent can be directly used as carbon coating materials to form in-situ coating Li a M b A c (PO4) d active material, without additional carbon source, subsequent carbon coating treatment and carbonization sintering treatment, reducing the introduction of impurity phases, improving the purity of the product, obtaining pure-phase phosphate-based positive electrode material, and being environmentally friendly, pollution-free and safe.

[0039] In the above step S10, the stoichiometric ratio of the elements in the active material is Li a M b A c (PO4) d active material, wherein the stoichiometric ratio can be a molar ratio of raw material components, or a mass ratio calculated according to the molar ratio. The Li a M b A c (PO4) d active material contains transition metal elements at M and A sites, which can combine the performance of various materials and improve the electrochemical performance of the positive electrode material. a M b A c (PO4) d positive electrode material.

[0040] In some possible implementation manners, the molar ratio of the lithium source, the M metal source, the A metal source and the phosphorus source is (0.95-1.05):(0.1-0.9):(0.9-0.1):(0.95-1.05); in this ratio, the ratio of each raw material component is ensured to meet Li a M b A c (PO4) d stoichiometric ratio of the active material, avoiding a large amount of impurity phases generated by mismatched ratio, and ensuring Li a M b A c (PO4) d electrochemical performance of the active material.

[0041] In some possible implementation manners, the M metal source and the A metal source are each independently selected from at least one of an iron source, a nickel source, a manganese source and a cobalt source. In this case, Li a M b A c (PO4) d The addition of iron elements in the active material can improve the electrochemical performance of the material, enhance the stability and safety of the material, enhance the conductivity of the positive electrode material, improve the safety performance, so that the battery has higher energy density and longer cycle life. The addition of nickel elements can improve the crystal structure, redox performance and ion conduction performance of lithium iron phosphate. At the same time, nickel can also effectively reduce the structural change of the positive electrode material, improve the cycle performance and safety of the battery. The addition of manganese elements to lithium iron phosphate can improve the voltage platform. Manganese ions and iron ions have similar ionic radii and can be miscible in any proportion. By adding manganese elements to lithium iron phosphate and adjusting the ratio of the number of atoms of manganese to iron (manganese-iron ratio), high-voltage platforms can be achieved while avoiding the inherent defects of lithium manganese phosphate. The crystal of lithium manganese iron phosphate has an olivine structure, and the biggest advantage of this structure is high stability, even if all lithium ions are completely removed during charging, the structure will not collapse, so the safety performance is good. The addition of cobalt can improve the energy density of lithium iron phosphate battery, thereby improving the capacity of the battery. In addition, cobalt can also improve the discharge platform of the battery, so that the cycle life of the battery is more stable. Cobalt can also improve the ion conductivity of lithium iron phosphate battery, thereby increasing the rate of battery charging and discharging. Cobalt can also optimize the crystal structure of the battery, thereby improving the cycle life of the battery.

[0042] In some possible implementation manners, LiNiPO4 has similar theoretical capacity to LiFePO4, and the Ni 2+ / Ni 3+The redox pair exhibits a higher voltage plateau (5.1V), thus LiNiPO4 possesses a higher energy density. However, the electronic conductivity and lithium-ion diffusion coefficient of LiNiPO4 are significantly lower than those of LiFePO4 (by several orders of magnitude). This poor electronic conductivity and lithium-ion diffusion coefficient are attributed to the separation of NiO6 octahedra and PO4 tetrahedra by oxygen atoms, as well as the smaller unit cell volume. Consequently, LiNiPO4 actually exhibits a lower charge-discharge specific capacity (approximately 60–150 mAh g⁻¹). -1 By combining LiFePO4 and LiNiPO4 to form LiFeNiPO4, the problems of low energy density of LiFePO4 and poor electronic conductivity and lithium-ion diffusion coefficient of LiNiPO4 can be overcome. Thus, the electrochemical performance of pure-phase phosphate cathode materials can be improved through the complexation of multiple metal elements.

[0043] In some possible implementations, a mixed solution of lithium source, M metal source, and A metal source is prepared, and a complexing agent is added to conduct a complexation coordination reaction with the metal element. The ratio of the total molar amount of M metal source and A metal source to the molar amount of the complexing agent is 1:(1-3). Under this ratio, the complexing agent can fully coordinate with the metal ions at the M and A sites, chelating the metal ions fully and uniformly onto the complexing agent. For example, the ratio of the total molar amount of lithium source, M metal source, and A metal source to the molar amount of the complexing agent can be 1:1, 1:2, 1:3, or any value between 1:(1-3).

[0044] In some specific embodiments, deionized water is used as the solvent in the mixed solution, which is environmentally friendly, pollution-free, and highly safe.

[0045] In some possible implementations, the complexing agent includes at least one of citric acid, a metal citrate salt, tartaric acid, a metal tartaric acid salt, a metal gluconate salt, and a metal alginate salt. These complexing agents can coordinate with metal ions such as lithium ions, iron ions, M, and A metal ions, stably chelating lithium ions into the complexing agent. In some possible implementations, the metal citrate salt includes sodium citrate, potassium citrate, etc., the metal tartaric acid salt includes sodium citrate, the metal gluconate salt includes sodium gluconate, and the metal alginate salt includes sodium alginate, etc. That is, the complexing agent includes at least one of citric acid, sodium citrate, potassium citrate, tartaric acid, sodium tartrate, sodium gluconate, and sodium alginate. In some specific embodiments, the complexing agent is selected from citric acid, which is a commonly used transition metal ion chelating agent and has moderate to strong acidity, which can meet the pH requirements for chelating most metal ions, and has a strong complexing effect.

[0046] In some possible implementation manners, the temperature condition of the complexation reaction is 60-90°C, the rotating speed is 300-600 rpm, and the reaction time is 1-3 hours. Under the reaction condition, the complexing agent can be fully complexed with the metal ions in the mixed solution, and the metal ions can be stably chelated in the complexing agent, which is beneficial to the subsequent uniform distribution in the three-dimensional network structure. For example, the temperature of the complexation reaction can be 60-70°C, 70-80°C, 80-90°C, etc., the rotating speed can be 300 rpm, 400 rpm, 500 rpm, 600 rpm, etc., and the reaction time can be 1 hour, 2 hours, 3 hours, etc.

[0047] In some possible implementation manners, the polymerization agent is added to react with the complexing agent to form a three-dimensional network structure, and the polymerization agent is selected from organic polymers containing hydroxyl groups. In this case, the hydroxyl groups in the polymerization agent can be polymerized with the carboxyl groups in the complexing agent to form a network molecular structure, thereby forming a three-dimensional network structure.

[0048] In some possible implementation manners, the polymerization agent includes at least one of polyethylene glycol with a number average molecular weight of 200-2000 and polyvinyl alcohol. The polyethylene glycol includes, but is not limited to, polyethylene glycol-200 with a number average molecular weight, polyethylene glycol-400 with a number average molecular weight, polyethylene glycol-2000 with a number average molecular weight, etc. These polymerization agents have good polymerization effects with the complexing agent, and can form a porous three-dimensional network polymer structure.

[0049] In some possible implementation manners, the molar ratio of the polymerization agent to the complexing agent is (0.5-1):1. Under the ratio condition, the polymerization effect between the polymerization agent and the complexing agent is fully ensured, a stable three-dimensional network polymer structure is formed, and the generation of by-products is reduced. For example, the molar ratio of the polymerization agent to the complexing agent can be 0.5:1, 0.6:1, 0.7:1, 0.8:1, 0.9:1, 1:1, etc.

[0050] In some possible implementation manners, the temperature condition of the polymerization reaction is 60-90°C, the rotating speed is 300-600 rpm, and the reaction time is 3-6 hours. Under the reaction condition, the cross-linking polymerization between the complexing agent and the polymerization agent is promoted, the polymerization efficiency is improved, and the full polymerization to form a porous three-dimensional network structure is ensured. For example, the temperature of the polymerization reaction can be 60-70°C, 70-80°C, 80-90°C, etc., the rotating speed can be 300 rpm, 400 rpm, 500 rpm, 600 rpm, etc., and the reaction time can be 3 hours, 4 hours, 5 hours, 6 hours, etc.

[0051] In some possible implementation manners, the lithium source includes at least one of lithium chloride, lithium nitrate, lithium oxalate, lithium acetate, and lithium phosphate.

[0052] In some possible implementation manners, the iron source includes at least one of ferric chloride, ferric sulfate, ferric nitrate, ferric acetate, ferric phosphate, ferrous sulfate, ferrous phosphate, ferrous oxalate.

[0053] In some possible implementation manners, the nickel source includes at least one of nickel chloride, nickel sulfate, nickel nitrate, nickel acetate, nickel oxalate.

[0054] In some possible implementation manners, the manganese source includes at least one of manganese chloride, manganese sulfate, manganese nitrate, manganese acetate, manganese oxalate.

[0055] In some possible implementation manners, the cobalt source includes at least one of cobalt chloride, cobalt sulfate, cobalt nitrate, cobalt acetate, cobalt oxalate.

[0056] The raw material components used in the above embodiments of the present application have good solubility, which is beneficial to the subsequent preparation of pure-phase phosphate-based positive electrode materials.

[0057] In the above step S20, after the phosphorus source is added and mixed with the sol solution, a gelation reaction is performed to form a wet gel, and drying is performed to obtain a dry gel precursor. After the complexation reaction and the polymerization reaction generate a three-dimensional network structure, the phosphorus source is added, which can avoid the influence of the premature combination of phosphate ions and metal ions on the chelation process of the metal ions by the complexing agent, reduce the influence on the polymerization reaction between the complexing agent and the polymerizing agent, and ensure that each metal ion and phosphate ion is uniformly anchored in the three-dimensional network structure. The added phosphorus source is anchored to the phosphate ions through the coordination of the metal ions already anchored in the three-dimensional network structure to the phosphate ions, so that the phosphate ions are uniformly dispersed in the three-dimensional network structure. After gelation, a wet gel is formed, and drying and volatilization are performed to remove the solvent and generate gas by decomposing anion radicals, so that a dry gel precursor material with a loose and porous three-dimensional network structure is obtained, and each element is uniformly distributed in the three-dimensional porous structure. Further, after the dry gel precursor is made into a powder, subsequent sintering treatment is performed, so that the sintering efficiency is improved.

[0058] In some possible implementation manners, the phosphorus source includes at least one of phosphoric acid, diammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, ferric phosphate, lithium phosphate, which can all provide sufficient phosphate ions and have good coordination effect with metal ions.

[0059] In some possible implementations, the temperature condition of the gelation reaction is 80-100°C, and the rotation speed is 300-600 rpm. During the gelation reaction, the solvent gradually volatilizes, and the flowability of the sol-like solution gradually deteriorates until the gelation is completed and the wet gel is formed, as the solvent volatilizes. For example, the temperature of the gelation reaction can be 80-90°C, 90-100°C, etc., and the rotation speed can be 300 rpm, 400 rpm, 500 rpm, 600 rpm, etc.

[0060] In some possible implementations, the drying treatment condition includes drying for 12-36 hours under vacuum condition at a temperature of 120-150°C. During the drying process, the solvent in the wet gel gradually volatilizes and is removed, and the loose and porous dry gel precursor is obtained. For example, the drying treatment temperature can be 120°C, 130°C, 140°C, 150°C, etc., under vacuum condition, and the drying time can be 12-18 hours, 18-24 hours, 24-30 hours, 30-36 hours, etc.

[0061] In step S30, the dry gel precursor is sintered to obtain Li a M b A c (PO4) d pure-phase phosphate-based positive electrode material. Through the sintering treatment, the three-dimensional porous gel network structure is in-situ carbonized into carbon material, and the three-dimensional porous carbon network structure is retained. Meanwhile, the three-dimensional porous carbon network structure in-situ generates Li a M b A c (PO4) d active material, and the three-dimensional porous carbon network structure can inhibit the agglomeration and growth of Li a M b A c (PO4) d active material particles, so that the generated Li a M b A c (PO4) d active material is uniformly distributed in the three-dimensional porous carbon network structure, and the Li a M b A c (PO4) d pure-phase phosphate-based positive electrode material is obtained.

[0062] In some possible implementations, the step of sintering treatment includes: heating at a rate of 2-10℃ / min to 300-500℃ under an inert atmosphere, holding for 1-6 hours, and then cooling; and heating at a rate of 2-10℃ / min to 600-900℃, holding for 2-10 hours. In this case, the xerogel precursor is subjected to two sintering treatments, the first sintering treatment is at a lower temperature, and mainly involves removal of crystal water, decomposition of carbon sources (complexing agents and polymerizing agents), metal source anions, and phosphorus sources. The second sintering treatment is at a higher temperature, and mainly involves reaction of lithium, iron, M, A metals, and phosphate to form Li a M b A c (PO4) d active material, while the three-dimensional porous network polymer structure is converted into a three-dimensional porous carbon network structure, and Li a M b A c (PO4) d active material is in-situ coated in the three-dimensional carbon network structure, to obtain a pure-phase phosphate-based positive electrode material.

[0063] In some embodiments, the heating rate of the first sintering stage of the sintering treatment can be 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, etc., the holding temperature can be 300-350℃, 350-400℃, 400-450℃, 450-500℃, etc., and the holding time can be 1-2 hours, 2-3 hours, 3-4 hours, 2-4 hours, 4-5 hours, 5-6 hours, etc. The heating rate of the second sintering stage can be 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min, 9℃ / min, 10℃ / min, etc., the holding temperature can be 600-650℃, 650-700℃, 700-750℃, 750-800℃, 800-850℃, 850-900℃, etc., and the holding time can be 2-3 hours, 4-8 hours, 3-5 hours, 5-7 hours, 7-9 hours, 9-10 hours, etc.

[0064] In a second aspect, the embodiments of the present application provide a pure-phase phosphate-based positive electrode material, which includes a three-dimensional porous carbon network structure and positive electrode active particles in-situ loaded inside the three-dimensional porous carbon network structure, and the positive electrode active particles include Li a M b A c (PO4) dWherein, 0.95≤a≤1.05, 0.1≤b≤0.9, 0.1≤c≤0.9, 0.95≤d≤1.05, M, A are respectively selected from at least one different transition metal element in Fe, Ni, Mn, Co.

[0065] The pure-phase phosphate-based positive electrode material of the embodiment of the present application comprises a three-dimensional porous carbon network structure and positive electrode active particles in-situ loaded inside the three-dimensional porous carbon network structure, the positive electrode active particles comprise Li a M b A c (PO4) d . Wherein, the three-dimensional porous carbon network structure not only provides a carrier for the uniform and stable loading and carbon coating of the active particles, but also enhances the electronic conductivity of the pure-phase phosphate-based positive electrode material due to its excellent electronic conductivity. Meanwhile, the three-dimensional porous carbon network structure has a super large specific surface area, which can make the electrolyte and the electrode material more fully contact, thereby improving the ion migration efficiency of the pure-phase phosphate-based positive electrode material. Thus, the pure-phase phosphate-based positive electrode material has the characteristics of good structural stability, high energy density, excellent conductivity, etc.

[0066] The pure-phase phosphate-based positive electrode material of the embodiment of the present application can be prepared by the method of the above embodiment.

[0067] In some possible implementations, the mass percentage content of the carbon material in the pure-phase phosphate-based positive electrode material is 10wt%-15wt%. In this content, it is ensured that the carbon material can form a stable three-dimensional porous carbon network structure, which not only provides a three-dimensional network carrier for the in-situ loading of the active material, but also improves the uniformity and stability of the distribution of the active particles and the specific surface area of the pure-phase phosphate-based positive electrode material, and can also provide in-situ carbon coating for the active material. a M b A c (PO4) d The in-situ loading of the active material provides a three-dimensional network carrier, improves the uniformity and stability of the distribution of the active particles, and improves the specific surface area of the pure-phase phosphate-based positive electrode material, and can also provide in-situ carbon coating for the active material. a M b A c (PO4) d The in-situ loading of the active material provides a three-dimensional network carrier, improves the uniformity and stability of the distribution of the active particles, and improves the specific surface area of the pure-phase phosphate-based positive electrode material, and can also provide in-situ carbon coating for the active material.

[0068] In some possible implementations, in the pure-phase phosphate-based positive electrode material, M includes Fe, and A includes Ni. In this case, in combination with Li a M b A c (PO4) d the performance of the multiple metal elements can improve the electrochemical performance of the Li a M b A c (PO4) d positive electrode material.

[0069] In some possible implementations, in the pure-phase phosphate-based positive electrode material, the D50 particle size of the positive electrode active particles is 300-500 nm. In this case, in the pure-phase phosphate-based positive electrode material, Li a M b A c (PO4) d active particles are loaded in the three-dimensional porous carbon network structure, and the inhibition of the agglomeration of the active material particles by the three-dimensional porous conductive carbon network structure makes the active material uniformly distributed in the three-dimensional porous carbon network structure in the form of small particles, the particle size of the positive electrode active particles is nanoscale, the specific surface area of the active material is high, and the ion insertion / extraction path is shortened, thereby facilitating the improvement of the rate performance of the applied battery. For example, in the pure-phase phosphate-based positive electrode material, the D50 particle size of the positive electrode active particles can be 300-350 nm, 350-400 nm, 400-450 nm, 450-500 nm, etc.

[0070] In some possible implementations, the pore volume of the pure-phase phosphate-based positive electrode material is 0.05 cm 3 / g-0.06 cm 3 / g. In some possible implementations, the specific surface area of the pure-phase phosphate-based positive electrode material is 50 m 2 / g-80 m 2 / g. The pure-phase phosphate-based positive electrode material prepared in the embodiments has a three-dimensional porous carbon network structure carrier, and the active particles are in-situ loaded and carbon-coated in the three-dimensional porous carbon network structure, so that the pure-phase phosphate-based positive electrode material has a large porosity and specific surface area, the electrolyte and the electrode material can be more fully contacted, and the ion migration efficiency of the pure-phase phosphate-based positive electrode material is improved. For example, the pore volume of the pure-phase phosphate-based positive electrode material can be 0.05 cm 3 / g, 0.06 cm 3 / g, etc., and the specific surface area of the pure-phase phosphate-based positive electrode material can be 50-60 m 2 / g, 60-70 m 2 / g, 70-80 m 2 / g, etc.

[0071] In a third aspect, the embodiments of the present application provide a positive electrode sheet, comprising a current collector and an active layer formed on the surface of the current collector, wherein the active layer comprises the pure-phase phosphate-based positive electrode material described above.

[0072] The positive electrode sheet of the embodiments of the present application has the pure-phase phosphate-based positive electrode material in the active layer, which has the characteristics of good structural stability, high capacity, excellent conductivity and the like, thereby improving the stability, energy density, rate capability, cycle performance and other electrochemical properties of the positive electrode sheet.

[0073] In some possible implementation manners, the preparation of the active layer comprises the following steps: mixing the pure-phase phosphate-based positive electrode material, the conductive agent and the binder to prepare an electrode slurry, and then coating the electrode slurry on the current collector, and performing drying, rolling, die cutting and the like to obtain the positive electrode sheet.

[0074] In some possible implementation manners, the mass percentage of the pure-phase phosphate-based positive electrode material in the active layer of the positive electrode sheet is 90% to 95%. Specifically, the mass percentage of the coated modified positive electrode material in the positive electrode active material layer can be 90%, 91%, 92%, 93%, 94%, 95% and the like.

[0075] In some possible implementation manners, the current collector of the positive electrode sheet comprises any one of a copper foil and an aluminum foil.

[0076] In some possible implementation manners, the content of the binder in the active material layer of the positive electrode sheet is 2wt% to 5wt%. In specific embodiments, the content of the binder can be 2wt%, 3wt%, 4wt%, 5wt% and the like, which are typical but not limiting contents.

[0077] In some possible implementation manners, the binder comprises one or more of polyvinylidene chloride, soluble polytetrafluoroethylene, styrene butadiene rubber, hydroxypropyl methyl cellulose, methyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, acrylonitrile copolymer, sodium alginate, chitosan and chitosan derivatives.

[0078] In some possible implementation manners, the content of the conductive agent in the active material layer of the positive electrode sheet is 1wt% to 5wt%. In specific embodiments, the content of the conductive agent can be 3wt%, 4wt%, 5wt% and the like, which are typical but not limiting contents.

[0079] In some possible implementation manners, the conductive agent comprises one or more of graphite, carbon black, acetylene black, graphene, carbon fiber, C60 and carbon nanotube.

[0080] In a fourth aspect, the embodiments of the present application provide a secondary battery, wherein the secondary battery comprises the positive electrode sheet described above.

[0081] The secondary battery provided by the embodiments of the present application has high energy density and good cycle stability due to the positive electrode sheet with good stability, high energy density, good rate performance, and good cycle stability.

[0082] The negative electrode sheet, electrolyte, and separator in the secondary battery of the embodiments of the present application are not specifically limited and can be applied to any battery system.

[0083] In some possible implementations, the negative active material of the secondary battery includes, but is not limited to, carbon materials such as graphite, soft carbon (e.g., coke), hard carbon, nitrides, tin-based oxides, tin-based oxides, tin alloys, and nano negative materials.

[0084] In some possible implementations, the step of manufacturing the negative electrode sheet includes: mixing the negative active material, a conductive agent such as conductive carbon black, a binder such as carboxymethyl cellulose and butadiene rubber, and a solvent such as water in a mass ratio of (80-99):(1-5):(2-10):100 to form a positive electrode mixed slurry, vacuum degassing, discharging, coating on a coating machine, and rolling, slitting, and die cutting to obtain the negative electrode sheet.

[0085] In some possible implementations, the separator can block electrons and allow ions to pass. For example, the separator includes, but is not limited to, at least one material including polypropylene fibers, polyacrylonitrile fibers, polyvinyl formal fibers, poly(ethylene glycol terephthalate), polyethylene terephthalate, polyamide fibers, and poly(p-phenyleneterephthalamide).

[0086] In some possible implementations, the electrolyte includes at least one soluble metal salt. In some specific embodiments, the metal salt includes at least one of LiClO4, LiBF4, LiPF6, LiAsF6, LiCF3SO3, LiTDI, Li[(CF3SO2)2N], Li[(FSO2)2N], Li[(C m F 2m+1 SO2)(C n F 2n+1 SO2)N], wherein m and n are natural numbers. These electrolyte salts can ensure high ionic conductivity of the electrolyte and have good chemical stability without causing harmful side reactions with electrode materials, electrolytes, separators, and the like.

[0087] In some possible implementations, the secondary battery includes at least one of a battery cell, a battery module, and a battery pack.

[0088] In some possible implementations, the battery cell type includes a lithium ion battery, and new battery types such as a lithium-air battery and a lithium metal battery.

[0089] In some possible embodiments, the battery cell of the present application can be assembled into a battery module, and the number of battery cells contained in the battery module can be multiple, and the specific number can be adjusted according to the application and capacity of the battery module. Further, the battery module can further include a shell having an accommodation space, and the plurality of battery cells are accommodated in the accommodation space.

[0090] In one possible embodiment, the battery cell and / or the battery module 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.

[0091] In order to enable the above-mentioned implementation details and operations of the present application to be clearly understood by those skilled in the art, and the performance of the pure-phase phosphate-based positive electrode material and the preparation method thereof is significantly embodied, the following will be illustrated by multiple examples.

[0092] Example 1

[0093] A pure-phase LiFe 0.8 Ni 0.2 PO4 / C positive electrode material, the preparation thereof includes the following steps:

[0094] ①Iron nitrate, nickel nitrate and lithium nitrate are weighed according to the element molar ratio Fe:Ni:Li=0.8:0.2:1.05, respectively, and dissolved in deionized water; under the magnetic stirring at 80°C and 400 rpm, a certain amount of citric acid solution is added to ensure that the molar ratio of citric acid to the total molar amount of iron source and nickel source is 2:1, and is kept for 2 hours to make the chelation reaction of transition metal ions and citric acid; then a certain amount of polyethylene glycol solution is added to the above solution under the action of constant magnetic stirring to ensure that the molar ratio of citric acid to polyethylene glycol is 1:1, and is kept for 4 hours to obtain a sol solution containing metal ions.

[0095] ②A certain amount of ammonium dihydrogen phosphate with the same molar amount as the lithium source is dissolved in deionized water, and is added to the above sol solution under the action of constant magnetic stirring, and is kept for 1 hour, and then the solution is continuously stirred at 80°C until the water solvent is gradually evaporated, and the solution is gradually changed from sol to wet gel; then the wet gel is transferred to a vacuum drying oven, and is kept at 130°C for 24 hours, and the dry gel is made into a loose and porous network structure dry gel due to the decomposition of nitrate to produce gas, and the dry gel is ground and crushed to obtain a dry gel precursor powder.

[0096] ③Put the dry gel precursor powder into a tube furnace, first increase the temperature to 350℃ at a rate of 5℃ / min under the protection of argon atmosphere, keep for 2 hours, and then naturally cool to room temperature. Then increase the temperature to 730℃ at a rate of 5℃ / min, keep for 6 hours, and then naturally cool to room temperature. The sample is recorded as LiFe 0.8 Ni 0.2 PO4 / C positive electrode material.

[0097] Example 2

[0098] A pure phase LiNi 0.5 Fe 0.5 PO4 / C positive electrode material, the preparation comprising the steps of:

[0099] ①Iron nitrate, nickel nitrate and lithium nitrate are weighed according to the element molar ratio Fe:Ni:Li=0.5:0.5:1.05, respectively, and dissolved in deionized water. Under the condition of 80℃ and 400rpm magnetic stirring, a certain amount of citric acid solution is added to ensure that the molar ratio of citric acid to the total molar amount of iron source and nickel source is 2:1, and kept for 2 hours to make the transition metal ions chelate with citric acid. Then a certain amount of polyethylene glycol solution is added to the above solution under the action of constant magnetic stirring to ensure that the molar ratio of citric acid to polyethylene glycol is 1:1, and kept for 4 hours to obtain a sol solution containing metal ions.

[0100] ②A certain amount of ammonium dihydrogen phosphate is dissolved in deionized water under constant magnetic stirring, and then added to the above sol solution and kept for 1 hour. Then the solution is continuously stirred at 80℃ until the water solvent is gradually evaporated, and the solution gradually changes from sol to viscous wet gel. Then the wet gel is transferred to a vacuum drying oven and kept at 130℃ for 24 hours. The dry gel has a loose and porous network structure due to the decomposition of nitrate to produce gas, and the dry gel is ground and crushed to obtain a dry gel precursor powder.

[0101] ③Put the dry gel precursor powder into a tube furnace, first increase the temperature to 350℃ at a rate of 5℃ / min under the protection of inert atmosphere, keep for 4 hours, and then naturally cool. Then increase the temperature to 720℃ at a rate of 5℃ / min, keep for 8 hours, and then naturally cool to room temperature. The sample is recorded as LiNi 0.5 Fe 0.5 PO4 / C positive electrode material.

[0102] Example 3

[0103] A pure phase LiNi 0.7 Fe 0.3 PO4 / C positive electrode material, the preparation comprising the steps of:

[0104] ①Iron nitrate, nickel nitrate and lithium nitrate were weighed according to the element molar ratio Fe:Ni:Li=0.3:0.7:1.05, and dissolved in deionized water; under magnetic stirring at 80°C and 400 rpm, a certain amount of citric acid solution was added to ensure that the molar ratio of citric acid to the total molar amount of iron source and nickel source was 2:1, and the chelation reaction between transition metal ions and citric acid was allowed to proceed for 2 hours; then a certain amount of polyethylene glycol solution was added to the above solution under continuous magnetic stirring to ensure that the molar ratio of citric acid to polyethylene glycol was 1:1, and the solution was kept for 4 hours to obtain a sol solution containing metal ions.

[0105] ②A certain amount of ammonium dihydrogen phosphate was dissolved in deionized water in an amount equal to that of the lithium source, and was added to the above sol solution under continuous magnetic stirring for 1 hour, and then the solution was continuously stirred at 80°C until the water solvent was gradually evaporated, and the solution gradually changed from sol to viscous wet gel; then the wet gel was transferred to a vacuum drying oven and kept at 130°C for 24 hours; the dry gel was ground into a powder to obtain a dry gel precursor powder.

[0106] ③The dry gel precursor powder was placed in a tube furnace, and first heated to 400°C at a rate of 5°C / min under inert atmosphere protection, and naturally cooled; then heated to 720°C at a rate of 5°C / min, and kept for 8 hours; naturally cooled to room temperature, and the sample was recorded as LiNi 0.7 Fe 0.3 PO4 / C positive electrode material.

[0107] Example 4

[0108] A pure-phase LiFe 0.8 Ni 0.2 PO4 / C positive electrode material, the preparation comprising the steps of:

[0109] ①Iron nitrate, nickel nitrate and lithium nitrate were weighed according to the element molar ratio Fe:Ni:Li=0.3:0.7:1.05, and dissolved in deionized water; under magnetic stirring at 80°C and 400 rpm, a certain amount of citric acid solution was added to ensure that the molar ratio of citric acid to the total molar amount of iron source and nickel source was 2:1, and the chelation reaction between transition metal ions and citric acid was allowed to proceed for 2 hours; then a certain amount of polyethylene glycol solution was added to the above solution under continuous magnetic stirring to ensure that the molar ratio of citric acid to polyethylene glycol was 1:1, and the solution was kept for 4 hours to obtain a sol solution containing metal ions.

[0110] ② Dissolve ammonium dihydrogen phosphate in deionized water in an equal molar amount to the lithium source. Add the solution to the above sol solution while continuously stirring with a magnetic force. Keep the solution for 1 hour. Then, stir the solution continuously at 80°C until the aqueous solvent gradually evaporates and the solution gradually changes from a sol state to a viscous wet gel. Then, transfer the wet gel to a vacuum drying oven and keep it at 130°C for 24 hours. Due to the decomposition of nitrate ions, the dry gel produces gas, which makes the wet gel into a loose and porous network structure dry gel. Grind the dry gel to obtain the dry gel precursor powder.

[0111] ③ The dry gel precursor powder was placed in a tube furnace and heated to 350°C at a rate of 5°C / min under an argon atmosphere. The temperature was held for 2 hours and then allowed to cool naturally to room temperature. Next, the temperature was increased to 730°C at a rate of 5°C / min and held for 6 hours. The sample was then allowed to cool naturally to room temperature. The sample was designated LiFe. 0.8 Ni 0.2 PO4 / C cathode material.

[0112] Example 5

[0113] A pure phase LiFe 0.8 Ni 0.2 The preparation of PO4 / C cathode material includes the following steps:

[0114] ① Weigh ferric nitrate, nickel nitrate, and lithium nitrate according to the elemental molar ratio Fe:Ni:Li = 0.8:0.2:1.05 and dissolve them in deionized water. Under magnetic stirring at 80℃ and 400rpm, add a certain amount of citric acid solution to ensure that the molar ratio of citric acid to the total molar ratio of iron and nickel sources is 3:1. Maintain this for 2 hours to allow the transition metal ions to undergo a chelation reaction with citric acid. Then, under continuous magnetic stirring, add a certain amount of polyethylene glycol solution to the above solution to ensure that the molar ratio of citric acid to polyethylene glycol is 1:1. Maintain this for 4 hours to obtain a sol solution containing metal ions.

[0115] ② Dissolve ammonium dihydrogen phosphate in deionized water in an equal molar amount to the lithium source. Add the solution to the above sol solution while continuously stirring with a magnetic force. Keep the solution for 1 hour. Then, stir the solution continuously at 80°C until the aqueous solvent gradually evaporates and the solution gradually changes from a sol state to a viscous wet gel. Then, transfer the wet gel to a vacuum drying oven and keep it at 130°C for 24 hours. Due to the decomposition of nitrate ions, the dry gel produces gas, which makes the wet gel into a loose and porous network structure dry gel. Grind the dry gel to obtain the dry gel precursor powder.

[0116] ③Put the dry gel precursor powder into a tube furnace, first increase the temperature to 350℃ at a rate of 5℃ / min under the protection of argon atmosphere, keep for 2 hours, and then naturally cool to room temperature. Then increase the temperature to 730℃ at a rate of 5℃ / min, keep for 6 hours; naturally cool to room temperature, and the sample is recorded as LiFePO4 / C. 0.8 Ni 0.2 PO4 / C positive electrode material.

[0117] Example 6

[0118] A pure phase LiFePO4 / C positive electrode material, the preparation comprising the steps of: 0.8 Ni 0.2 PO4 / C positive electrode material, the preparation comprising the steps of:

[0119] ①Iron nitrate, nickel nitrate and lithium nitrate are weighed according to the element molar ratio of Fe:Ni:Li=0.8:0.2:1.05, and dissolved in deionized water; under the condition of 80℃ and 400rpm magnetic stirring, a certain amount of citric acid solution is added to ensure that the molar ratio of citric acid to the total molar amount of iron source and nickel source is 2:1, and kept for 2 hours to make the transition metal ions chelate with citric acid; then a certain amount of polyethylene glycol solution is added to the above solution under the condition of continuous magnetic stirring to ensure that the molar ratio of citric acid to polyethylene glycol is 2:1, and kept for 4 hours to obtain a sol solution containing metal ions.

[0120] ②A certain amount of ammonium dihydrogen phosphate is dissolved in deionized water under the condition of continuous magnetic stirring, and then added to the above sol solution and kept for 1 hour; then the solution is continuously stirred at 80℃ until the water solvent is gradually evaporated, and the solution gradually changes from sol to viscous wet gel; then the wet gel is transferred to a vacuum drying oven and kept at 130℃ for 24 hours; the dry gel is loose and porous due to the decomposition of nitrate to produce gas, and the dry gel is ground and crushed to obtain a dry gel precursor powder.

[0121] ③Put the dry gel precursor powder into a tube furnace, first increase the temperature to 350℃ at a rate of 5℃ / min under the protection of argon atmosphere, keep for 2 hours, and then naturally cool to room temperature. Then increase the temperature to 730℃ at a rate of 5℃ / min, keep for 6 hours; naturally cool to room temperature, and the sample is recorded as LiFePO4 / C. 0.8 Ni 0.2 PO4 / C positive electrode material.

[0122] Comparative Example 1

[0123] A pure phase LiFePO4 / C positive electrode material, the preparation comprising the steps of:

[0124] ① Weigh ferric nitrate and lithium nitrate according to the elemental molar ratio Fe:Li = 1:1.05 and dissolve them in deionized water. Under magnetic stirring at 80℃ and 400rpm, add a certain amount of citric acid solution to ensure that the molar ratio of citric acid to lithium source is 2:1 and keep it for 2 hours to allow the transition metal ions to undergo a chelation reaction with citric acid. Then, under continuous magnetic stirring, add a certain amount of polyethylene glycol solution to the above solution to ensure that the molar ratio of citric acid to polyethylene glycol is 1:1 and keep it for 4 hours to obtain a sol solution containing metal ions.

[0125] ② Dissolve ammonium dihydrogen phosphate in deionized water in an equal molar amount to the lithium source. Add the solution to the above sol solution while continuously stirring with a magnetic force. Keep the solution for 1 hour. Then, stir the solution continuously at 80°C until the aqueous solvent gradually evaporates and the solution gradually changes from a sol state to a viscous wet gel. Then, transfer the wet gel to a vacuum drying oven and keep it at 130°C for 24 hours. Due to the decomposition of nitrate ions, the dry gel produces gas, which makes the wet gel into a loose and porous network structure dry gel. Grind the dry gel to obtain the dry gel precursor powder.

[0126] ③ The dry gel precursor powder was placed in a tube furnace and heated to 450°C at a rate of 5°C / min under an inert atmosphere. The temperature was then maintained for 4 hours. The temperature was then increased to 750°C at a rate of 5°C / min and maintained for 6 hours. The mixture was then allowed to cool naturally to room temperature to obtain a porous lithium iron phosphate cathode material, which was designated as LiFePO4 / C cathode material.

[0127] Comparative Example 2

[0128] Using Example 1 as a comparison, LiFe was provided. 0.8 Ni 0.2 Conventional preparation methods for PO4 cathode materials (i.e., preparation methods that do not involve forming a gel network structure through an aqueous sol-gel method followed by sintering):

[0129] ① Weigh ferric nitrate, nickel nitrate, lithium nitrate and ammonium dihydrogen phosphate according to the elemental molar ratio Fe:Ni:Li:P=0.8:0.2:1.05:1, place them in a ball mill jar, add ethanol as solvent, ball mill at 300 rpm for 4 hours, and then transfer to a 100℃ oven to dry for 24 hours.

[0130] ② The precursor powder was pulverized and then placed in a tube furnace. Under an inert atmosphere, the temperature was first increased to 450°C at a rate of 5°C / min and held for 4 hours. Then, the temperature was increased to 720°C at a rate of 5°C / min and held for 6 hours. After natural cooling to room temperature, lithium iron phosphate cathode material was obtained, and the sample was designated LiFe. 0.8 Ni 0.2 PO4 cathode material.

[0131] ③ Weigh the sample after the first sintering into a ball mill jar, add 10 wt% glucose as a carbon source, and ball mill at 300 rpm for 4 hours using ethanol as the medium. Transfer to a 100℃ oven to dry for 24 hours. Grind the dried powder into a fine powder, then place it in a tube furnace and heat it to 750℃ at a rate of 5℃ / min under an inert atmosphere, holding for 2 hours. Allow it to cool naturally to room temperature to obtain carbon-coated lithium iron phosphate cathode material, denoted as LiFe. 0.8 Ni 0.2 PO4 / C cathode material.

[0132] Furthermore, to verify the progressiveness of the embodiments of this application, the following performance tests were conducted on the above embodiments and comparative examples:

[0133] 1. Pure-phase LiFe prepared in Examples 1, 5, and 6 0.8 Ni 0.2 The morphology of the PO4 / C cathode material was observed using scanning electron microscopy, and the test results are attached. Figure 2 (Scanning electron microscope image of the cathode material in Example 1), Appendix Figure 3 (Scanning electron microscope image of the cathode material in Example 5) and attached Figure 5 (Scanning electron microscope image of the cathode material in Example 6) shows that the cathode material has a loose, porous three-dimensional network structure. The active cathode particles are wrapped in carbon layers and interconnected through these carbon layers. This structure facilitates electron transport, and the porous structure facilitates electrolyte wetting, increasing the contact area and thus improving the performance of pure-phase LiFe. 0.8 Ni 0.2 Lithium-ion diffusion coefficient of PO4 / C cathode material.

[0134] 2. X-ray powder diffraction analysis was performed on the cathode materials prepared in Examples 1-3 and Comparative Example 1, and the test results are shown in the appendix. Figure 4 As shown, the diffraction peaks of the LiFePO4 cathode material prepared in Comparative Example 1 are consistent with those of pure-phase olivine-structured LiFePO4 (PDF#81-1173), proving the formation of pure-phase lithium iron phosphate. In Examples 1, 2, and 3, as the nickel content gradually increases, the diffraction peaks of the cathode materials are between those of pure-phase LiFePO4 and pure-phase LiNiPO4 (PDF#81-1528), and are of uniform composition, proving the successful synthesis of pure-phase nickel-iron-phosphate solid solution.

[0135] 3. The physicochemical properties of the cathode materials prepared in the above embodiments and comparative examples were tested respectively, and the test results are shown in Table 1 below:

[0136] Table 1

[0137]

[0138] 3. The positive electrode materials prepared in the examples and comparative examples were respectively subjected to electrochemical performance tests by being put into lithium ion batteries, which were prepared as follows:

[0139] ① Preparation of the positive electrode sheet: the positive electrode materials in the above examples or comparative examples, SP, PVDF and NMP were mixed uniformly in a ball mill stirrer at a stirring speed of 360 rpm for 1 h at a mass ratio of 93.5:2.5:4:100, to obtain a positive electrode slurry. The prepared positive electrode slurry was poured on an aluminum foil, and then uniformly scraped flat with a spatula. The positive electrode sheet was dried in an oven at 120°C for 20 min, and then rolled under a pressure of 10 MPa to obtain a rolled electrode sheet. A 12 mm diameter disc was cut from the middle area of the electrode sheet by a disc cutter, weighed and measured in thickness, to calculate the compacted density, and thus obtain the positive electrode sheet, which was ready for use.

[0140] ② The battery assembly process was as follows: the prepared positive electrode sheet was attached to the positive electrode metal shell with conductive glue, and then dried; a lithium metal sheet was used as the negative electrode, Celgard 2400 microporous membrane was used as the battery separator, and a mixed solution containing 1.0 M LiPF6 in ethylene carbonate (EC) and diethyl carbonate (DEC) was used as the electrolyte (the volume ratio of EC / DEC was 1:1). The assembly of the coin cell was completed in a glove box, to obtain a lithium ion coin cell.

[0141] The electrochemical performance of the above coin cell was tested by using a blue electric LAND electrochemical tester, and the charge termination voltage was 3.75 V and the discharge cutoff voltage was 2.0 V. The obtained lithium ion battery was subjected to 0.1C rate charge-discharge test, 1C rate charge-discharge test, and performance tests such as initial efficiency and 1C 100 cycle stability. The test results are shown in Table 2 below:

[0142] Table 2

[0143]

[0144] From the above test results, it can be seen that the positive electrode material prepared in the examples of the present application comprises a three-dimensional porous carbon network structure and Li a M b A c (PO4) dActive particles. Benefited from the multiple metal elements complexing and the excellent electronic conductivity and three-dimensional porous structure of the three-dimensional porous carbon network structure, the positive electrode material has an ultra-large specific surface area, significantly improves the wettability of the electrode material and the electrolyte, increases the electronic conductivity and lithium ion diffusion performance of the battery material, and improves the charge and discharge capacity, rate performance and capacity retention rate of the battery. Under the same iron-nickel element ratio, compared with Comparative Example 2, the pure-phase phosphate-based positive electrode materials of Example 1, Example 4, Example 5 and Example 6 with higher carbon content and three-dimensional porous network structure show higher 0.1C discharge capacity, initial efficiency and 1C rate discharge capacity. Under the same feeding ratio, different complexing agents and polymer addition amounts (as shown in Examples 2 and 4-6), which lead to slightly different particle morphologies and carbon contents, but the overall still maintains the three-dimensional porous network structure. In Examples 2, 4-6, Example 5 adds the most complexing agent and polymer, so the carbon content of the example is the highest, and too much carbon will affect the migration and diffusion of lithium ions of lithium iron phosphate, so Example 5 shows relatively low charge and discharge capacity, but due to the electronic conduction and protection effect of the carbon structure, it shows higher 1C charge and discharge efficiency and cycle capacity retention rate. Thus, the positive electrode material prepared in the examples of the present application has the advantages of excellent electronic conductivity, high charge and discharge capacity, high efficiency, good rate performance, and good structural stability.

[0145] The above only describes the preferred embodiments of the present application and is not intended to limit the present application. Any modifications, equivalent replacements and improvements made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for producing a pure-phase phosphate-based positive electrode material, characterized by, The method comprises the following steps: According to Li a M b A c (PO4) d The stoichiometric ratio of elements in the active material, after the lithium source, the M metal source and the A metal source are made into a mixed solution, a complexing agent is added to carry out a complexation reaction with the metal elements, a polymerization agent is added to carry out a polymerization reaction to form a three-dimensional network structure, and a sol solution is obtained. adding a phosphorus source to mix with the sol solution, and then performing a gelation reaction to form a wet gel, and drying to obtain a dry gel precursor; sintering treatment to the dry gel precursor to obtain Li a M b A c (PO4) d pure-phase phosphate-based positive electrode material; wherein 0.95≤a≤1.05, 0.1≤b≤0.9, 0.1≤c≤0.9, 0.95≤d≤1.05, M and A are selected from different transition metal elements.

2. The method for producing a pure-phase phosphate-based cathode material according to claim 1, characterized by, the ratio of the total mole amount of the M metal source and the A metal source to the mole amount of the complexing agent is 1:(1-3); and / or, the mole ratio of the polymerization agent to the complexing agent is (0.5-1):

1.

3. The method for producing a pure-phase phosphate-based cathode material according to claim 1 or 2, characterized by, The M metal source and the A metal source are each independently selected from at least one of an iron source, a nickel source, a manganese source, and a cobalt source; and / or, the complexing agent comprises at least one of citric acid, a metal citrate, tartaric acid, a metal tartrate, a metal gluconate, and a metal alginate; and / or, the polymerization agent is selected from an organic polymer containing a hydroxyl group.

4. The method for producing a pure-phase phosphate-based cathode material according to claim 3, characterized by, The complexing agent comprises at least one of citric acid, sodium citrate, potassium citrate, tartaric acid, sodium tartrate, sodium gluconate, and sodium alginate; and / or, the polymerization agent comprises at least one of a polyethylene glycol with a number average molecular weight of 200-2000 and a polyvinyl alcohol.

5. The method for producing a pure-phase phosphate-based cathode material according to any one of claims 1 to 2 or 4, characterized by, The temperature condition of the complexing reaction is 60-90°C, the rotation speed is 300-600 rpm, and the reaction time is 1-3 hours; and / or, the temperature condition of the polymerization reaction is 60-90°C, the rotation speed is 300-600 rpm, and the reaction time is 3-6 hours.

6. The method for producing a pure-phase phosphate-based cathode material according to claim 5, characterized by, The temperature condition of the gelation reaction is 80-100°C, and the rotation speed is 300-600 rpm; and / or, the drying treatment condition comprises drying at a temperature of 120-150°C under vacuum for 12-36 hours; and / or, the sintering treatment comprises: heating at a rate of 2-10°C / min to 300-500°C under an inert atmosphere, maintaining for 1-6 hours, and then cooling; and heating at a rate of 2-10°C / min to 600-900°C, and maintaining for 2-10 hours.

7. A pure phase phosphate-based cathode material, characterized by, comprising a three-dimensional porous carbon network structure and positive electrode active particles loaded in-situ inside the three-dimensional porous carbon network structure, the positive electrode active particles comprising Li a M b A c (PO4) d wherein 0.95≤a≤1.05, 0.1≤b≤0.9, 0.1≤c≤0.9, 0.95≤d≤1.05, M, A are respectively selected from at least one different transition metal element from Fe, Ni, Mn, Co.

8. The pure-phase phosphate-based cathode material of claim 7, wherein, In the pure-phase phosphate-based positive electrode material, the mass percentage of the carbon material is 10-15 wt%; and / or, in the pure-phase phosphate-based positive electrode material, M comprises Fe and A comprises Ni; and / or, in the pure-phase phosphate-based positive electrode material, the D50 particle size of the active particles is 300-500 nm; and / or the pore volume of the pure-phase phosphate-based positive electrode material is 0.05 cm 3 / g ~ 0.06 cm 3 / g; and / or the specific surface area of the pure-phase phosphate-based positive electrode material is 50 m 2 / g ~ 80 m 2 / g.

9. A positive electrode sheet characterized by comprising: The secondary battery comprises the positive electrode sheet according to claim 9.

10. A secondary battery characterized by comprising: The secondary battery comprises the positive electrode sheet according to claim 9.

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