A composite positive electrode material and its preparation method and application

Abstract

The present invention provides a composite cathode material and its preparation method and application. The composite cathode material comprises a core and a carbon coating layer covering at least a portion of the surface of the core and / or embedded in the core; the core comprises NaFePO4 and a compound shown in Formula 1; Na 4+x Fe 3‑y (PO4) 2+z P2O7 has the formula 1; in the formula 1, -0.15≤x≤0.8, 0≤y≤0.5, -0.2≤z≤0.2; and the particle size of NaFePO4 is ≤100nm. When used in a battery, this composite positive electrode material can improve the battery's capacity and rate performance.

Description

Technical Field

[0001] The invention relates to a composite positive electrode material and a preparation method and application thereof, belonging to the technical field of secondary batteries. Background Art

[0002] Since sodium resources are abundant and low-cost compared to lithium resources, the industry is increasingly inclined to research sodium-ion batteries. Sodium iron pyrophosphate has the advantages of low cost and good structural stability, and is considered to be one of the most promising sodium-ion battery positive electrode materials. However, the electronic conductivity of sodium iron pyrophosphate is low. In order to improve the electronic conductivity of sodium iron pyrophosphate, the existing technology mainly uses carbon coating on sodium iron pyrophosphate. However, the composite positive electrode materials of carbon-coated sodium iron pyrophosphate still have the problems of low capacity and poor rate performance. Summary of the Invention

[0003] The present invention provides a composite positive electrode material, which can improve the capacity and rate performance of a battery when used in the battery.

[0004] The present invention provides a method for preparing a composite positive electrode material. The method can prepare the composite positive electrode material, has a simple preparation process, and is suitable for wide promotion and application.

[0005] The present invention provides a battery, which comprises the composite positive electrode material and thus has excellent rate performance, cycle performance and discharge specific capacity.

[0006] The present invention provides a composite positive electrode material, comprising a core and a carbon coating layer covering at least a portion of the surface of the core and / or embedded in the core;

[0007] The core includes NaFePO4 and a compound shown in Formula 1;

[0008] Na 4+x Fe 3-y (PO4) 2+z P2O7 formula 1;

[0009] In formula 1, -0.15≤x≤0.8, 0≤y≤0.5, -0.2≤z≤0.2;

[0010] The particle size of NaFePO4 is ≤100nm.

[0011] The composite cathode material as described above, wherein the atomic ratio of Fe to P in the composite cathode material gradually decreases from (0.9-1.1):1 from the outside to the inside, and then stabilizes at (0.65-0.85):1; and / or,

[0012] The atomic ratio of Fe to Na in the composite positive electrode material gradually decreases from (0.9-1.1):1 from the outside to the inside, and then stabilizes at (0.65-0.85):1.

[0013] The composite positive electrode material as described above, wherein the mass percentage of the carbon coating layer is 0.5-5% based on the total mass of the composite positive electrode material.

[0014] As described above, the composite positive electrode material, wherein, in the composite positive electrode material, PO4 3- and P2O7 4- The molar ratio is (1.5-3):1.

[0015] The composite positive electrode material as described above, wherein the particle size of the core is 100-900 nm.

[0016] The composite positive electrode material as described above, wherein the specific surface area of ​​the composite positive electrode material is 5-20m 2 g -1 .

[0017] The composite cathode material as described above, wherein the compaction density of the composite cathode material is 1.9-2.4 gcm -3 .

[0018] The present invention provides a method for preparing the composite positive electrode material as described above, comprising:

[0019] The raw material system including a sodium source, an iron source, a phosphorus source and a carbon source is sequentially subjected to sand milling and spray drying to obtain a powder;

[0020] performing a first calcination treatment, a second calcination treatment, a third calcination treatment, and a fourth calcination treatment on the powder in sequence to obtain the composite positive electrode material;

[0021] Among them, in the raw material system, the molar ratio a of the sodium element, the molar ratio b of the iron element, and the molar ratio c of the phosphorus element satisfy: 3.85≤a≤4.8; 2.5≤b≤3; 3.8≤c≤4.2;

[0022] In the first calcination treatment, the temperature is 255-260° C. and the time is 1-2 hours;

[0023] In the second calcination treatment, the temperature is 300-320°C and the time is 3-5h;

[0024] In the third calcination treatment, the temperature is 370-390° C. and the time is 1-2 hours;

[0025] In the fourth calcination treatment, the temperature is 470-500° C. and the time is 8-10 hours.

[0026] In the preparation method as described above, the sodium source and the iron source are both sodium ferric ethylenediaminetetraacetate.

[0027] The preparation method as described above, wherein the sanding process includes a first sanding process and a second sanding process;

[0028] The particle size of the sanding agent in the second sanding process is smaller than the particle size of the sanding agent in the first sanding process.

[0029] The preparation method as described above, wherein, in the first sand grinding treatment, the rotation speed is 900-1200 rpm and the time is 2-3 hours; and / or,

[0030] In the second sand grinding process, the rotation speed is 400-2000 rpm and the time is 1-2 hours.

[0031] The preparation method as described above, wherein the carbon source includes an organic carbon source.

[0032] The preparation method as described above, wherein the carbon source further comprises an inorganic carbon source;

[0033] The mass ratio of the organic carbon source to the inorganic carbon source is (12.5-30):1.

[0034] The preparation method as described above, wherein the raw material system further comprises a coupling agent;

[0035] Based on the total mass of the raw material system, the mass percentage of the coupling agent is 1-3%;

[0036] Preferably, the coupling agent is a titanate coupling agent.

[0037] The present invention provides a battery, comprising the composite positive electrode material as described above.

[0038] The composite positive electrode material of the present invention includes a core formed by the compound shown in Formula 1 and NaFePO4 of a specific particle size, and a carbon coating layer coated on at least a portion of the surface of the core. The composite positive electrode material of the present invention has excellent discharge capacity, cycle performance and rate performance.

[0039] The preparation method of the composite positive electrode material of the present invention can prepare the composite positive electrode material mentioned above, and the preparation method is simple to operate and suitable for wide promotion and application.

[0040] The battery of the present invention, because it includes the composite positive electrode material, has excellent cycle performance, discharge specific capacity and rate performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0041] To more clearly illustrate the technical solutions in the embodiments of the present invention or related technologies, the following briefly introduces the drawings required for use in the embodiments of the present invention or related technologies. Obviously, the drawings described below are only some embodiments of the present invention. Those skilled in the art can also derive other drawings based on these drawings without inventive effort.

[0042] Figure 1 This is the XRD pattern of the composite positive electrode material in Example 1 of the present invention;

[0043] Figure 2 This is the XRD pattern of the composite positive electrode material in Example 2 of the present invention;

[0044] Figure 3 This is the XRD pattern of the composite positive electrode material in Example 3 of the present invention;

[0045] Figure 4 This is the XRD pattern of the composite positive electrode material in Comparative Example 1 of the present invention;

[0046] Figure 5 This is a SEM image of the composite cathode material in Example 1 of the present invention;

[0047] Figure 6 This is a SEM image of the composite positive electrode material in Example 2 of the present invention;

[0048] Figure 7 This is a SEM image of the composite cathode material in Example 3 of the present invention;

[0049] Figure 8 This is a SEM image of the composite positive electrode material in Comparative Example 1 of the present invention;

[0050] Figure 9 is the charge and discharge curve of the battery in Example 1 of the present invention;

[0051] Figure 10 is the charge and discharge curve of the battery in Example 2 of the present invention;

[0052] Figure 11 is the charge and discharge curve of the battery in Example 3 of the present invention;

[0053] Figure 12 This is the charge and discharge curve of the battery in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0054] To make the objectives, technical solutions, and advantages of the present invention more clear, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts shall fall within the scope of protection of the present invention.

[0055] A first aspect of the present invention provides a composite cathode material, comprising a core and a carbon coating layer covering at least a portion of the surface of the core and / or embedded in the core;

[0056] The core includes NaFePO4 and a compound shown in Formula 1;

[0057] Na 4+x Fe 3-y (PO4) 2+z P2O7 formula 1;

[0058] In formula 1, -0.15≤x≤0.8, 0≤y≤0.5, -0.2≤z≤0.2;

[0059] The particle size of NaFePO4 is ≤100nm.

[0060] It can be understood that the carbon coating layer of the present invention can be coated on the entire surface of the core, or can be coated on a portion of the surface of the core, and the carbon coating layer can also be embedded in the core.

[0061] The composite positive electrode material of the present invention comprises a core and a carbon coating layer from the inside out.

[0062] The core includes NaFePO4 with a particle size of ≤100nm and a compound represented by Formula 1, wherein the particle size of NaFePO4 refers to the average particle size of NaFePO4. During the charge and discharge process of the battery, NaFePO4 with a particle size of ≤100nm will be converted into amorphous FePO4, thereby effectively exerting the capacity of NaFePO4 and improving the capacity of the composite positive electrode material; the compound represented by Formula 1 can be Na4Fe3(PO4)2P2O7, which has excellent stability and helps to improve the cycle performance of the composite positive electrode material. The carbon coating layer can improve the electron transport ability of the composite positive electrode material and greatly improve the conductivity of the composite positive electrode material. Therefore, the composite positive electrode material of the present invention has excellent capacity, cycle performance and rate performance.

[0063] XPS testing of the composite cathode material of the present invention reveals that the signal of carbon atoms gradually decreases from the outside to the inside of the composite cathode material of the present invention, indicating that the carbon coating layer of the present invention is located on the outer surface of the composite cathode material.

[0064] In some embodiments of the present invention, the atomic ratio of Fe to P in the composite cathode material decreases from (0.9-1.1):1 from the outside to the inside, and then stabilizes at (0.65-0.85):1; and / or,

[0065] From the outside to the inside of the composite positive electrode material, the atomic ratio of Fe to Na gradually decreases from (0.9~1.1):1, and then stabilizes at (0.65-0.85):1, indicating that the NaFePO4 in the inner core is closer to the outer surface of the composite positive electrode material.

[0066] In the present invention, the atomic ratio of Fe to P or the atomic ratio of Fe to Na in the composite positive electrode material can be detected using a detection method commonly used in the art. For example, XPS or SEM-EDS can be used to detect and obtain the atomic ratio of Fe to P or the atomic ratio of Fe to Na in the composite positive electrode material.

[0067] In the present invention, the mass percentage of the carbon coating layer in the composite positive electrode material can be specifically selected to further improve the rate performance of the composite positive electrode material while ensuring the capacity and cycle performance of the composite positive electrode material. In some embodiments of the present invention, the mass percentage of the carbon coating layer is 0.5-5% based on the total mass of the composite positive electrode material.

[0068] In some embodiments of the present invention, in the composite cathode material, PO4 3- and P2O7 4- The content ratio is (1.5-3):1.

[0069] It can be understood that the present invention, P2O7 3- From the compound shown in formula 1, PO4 3- One part comes from the compound shown in formula 1, and the other part comes from NaFePO4. 3- and P2O7 4- When the content ratio is (1.5-3):1, the advantages of the compound shown in Formula 1 and NaFePO4 can be better utilized, further improving the cycle performance and capacity of the composite positive electrode material.

[0070] In the present invention, a spectrophotometer can be used to detect the PO4 in the composite cathode material. 3- and P2O7 4- content.

[0071] In some embodiments of the present invention, the particle size of the core is 100-900 nm.

[0072] The primary particles of the composite cathode material of the present invention are spherical or round, and HRTEM can be used to observe that any two composite cathode material particles are connected by the carbon coating layer.

[0073] In the present invention, the particle size of the core refers to the average particle size of the core. In the composite positive electrode material of the present invention, the cores with a particle size of 100-900nm are connected by a carbon coating layer, and electrons and ions are transferred through the carbon coating layer, which helps to improve the specific capacity and rate performance of the composite positive electrode material. When the particle size of the core is 100-900nm, the composite positive electrode material can have a lower specific surface area. During the preparation of the positive electrode sheet, the slurry can be prevented from forming jelly, the coating effect of the positive electrode sheet can be improved, and the industrial application prospects of the composite positive electrode material are further enhanced.

[0074] In some embodiments, the surface contact angle of the composite cathode material of the present invention is 35-60°.

[0075] In some embodiments, the surface of the core has no obvious porous structure, which helps to further reduce the specific surface area of ​​the composite positive electrode material. In some embodiments of the present invention, when the specific surface area of ​​the composite positive electrode material is 5-20m 2 g -1 When the composite positive electrode material is used in the process of preparing the positive electrode sheet, the slurry can be further prevented from forming jelly, thereby improving the coating effect of the positive electrode sheet.

[0076] In some embodiments of the present invention, when the compaction density of the composite cathode material is 1.9-2.4 g cm -3 In some embodiments, the composite positive electrode material of the present invention can be used to prepare a positive electrode with a compaction density of 2.2-2.6 g cm -3 The positive electrode sheet is provided, and a battery including the positive electrode sheet has excellent volume specific energy.

[0077] A second aspect of the present invention provides a method for preparing the composite positive electrode material as described above, comprising:

[0078] The raw material system including a sodium source, an iron source, a phosphorus source and a carbon source is sequentially subjected to sand milling and spray drying to obtain a powder;

[0079] The powder is subjected to a first calcination treatment, a second calcination treatment, a third calcination treatment, and a fourth calcination treatment in sequence to obtain a composite positive electrode material;

[0080] Among them, in the raw material system, the molar ratio a of the sodium element, the molar ratio b of the iron element, and the molar ratio c of the phosphorus element satisfy: 3.85≤a≤4.8; 2.5≤b≤3; 3.8≤c≤4.2;

[0081] In the first calcination treatment, the temperature is 255-260°C and the time is 1-2 hours;

[0082] In the second calcination treatment, the temperature is 300-320°C and the time is 3-5h;

[0083] In the third calcination treatment, the temperature is 370-390°C and the time is 1-2 hours;

[0084] In the fourth calcination treatment, the temperature is 470-500° C. and the time is 8-10 hours.

[0085] The present invention performs a sand milling process on a raw material system including a sodium source, an iron source, a phosphorus source, and a carbon source in specific contents to uniformly mix the sodium source, the iron source, the phosphorus source, and the carbon source to obtain a uniform suspension; then the suspension is spray dried to evaporate water in the suspension to obtain a powder;

[0086] The powder is subjected to a first calcination treatment at 255-260°C for 1-2 hours; the temperature is increased to 300-320°C for a second calcination treatment for 3-5 hours; the temperature is increased to 370-390°C for a third calcination treatment for 1-2 hours; the temperature is increased to 470-500°C for a fourth calcination treatment for 8-10 hours, thereby forming the composite positive electrode material with a core-shell structure in the present invention.

[0087] It is understood that the raw material system of the present invention also includes water, and the present invention does not limit the water content in the raw material system. In some embodiments, the mass ratio of the total mass (solid phase) of the sodium source, iron source, phosphorus source, and carbon source to the water (liquid phase) is (0.2-0.4):1, and the Dv50 of the solid phase in the raw material system is 100-1000 nm, which helps to improve the yield of the powder. In specific embodiments, the water can be ultrapure water.

[0088] It is understood that during sand milling, a portion of the solid phase dissolves in the water, while another portion forms a suspended solid in the water. The inventors have discovered that increasing the suspended solids content in the suspension improves powder yield, reduces raw material loss during the spray drying process, and reduces raw material costs. In some embodiments, when the mass percentage of suspended solids in the sand milled suspension is 35-75%, the powder yield can be improved.

[0089] The present invention does not impose any particular limitation on the spray drying process, as long as the suspension can be dried to form a powder. In some embodiments, a spray dryer can be used for the spray drying process, and the fan temperature of the spray dryer can be 180-240°C, and the outlet temperature can be 80-100°C.

[0090] The yield of the powder obtained by the preparation method of the present invention can be 70-97%, which helps to improve the yield of the composite positive electrode material.

[0091] In some embodiments, the powder may be crushed and sieved in sequence, wherein the mesh size of the sieve may be 250-350 meshes, in order to further improve the effect of the subsequent calcination treatment, thereby obtaining a composite positive electrode material with excellent performance.

[0092] The present invention does not particularly limit the specific methods of the first calcination treatment, the second calcination treatment, the third calcination treatment and the fourth calcination treatment, as long as the temperature and time are within the above ranges. In some embodiments, the first calcination treatment, the second calcination treatment, the third calcination treatment and the fourth calcination treatment can be carried out in a calcination atmosphere, and the calcination atmosphere can be a nitrogen atmosphere or a nitrogen-hydrogen mixed atmosphere. When the calcination atmosphere is a nitrogen-hydrogen mixed atmosphere, the volume ratio of nitrogen to hydrogen can be 95:5. In some embodiments, the flow rate of the calcination atmosphere can be 0.3-0.5 L / min to further improve the effect of the calcination treatment and obtain a composite positive electrode material with excellent performance.

[0093] In the present invention, the heating rate of the first calcination treatment, the second calcination treatment, the third calcination treatment and the fourth calcination treatment can be set to 3°C / min to ensure that each raw material can fully react, improve the graphitization degree of the carbon material, and thus improve the comprehensive performance of the composite positive electrode material.

[0094] The preparation method of the composite positive electrode material of the present invention can prepare the composite positive electrode material mentioned above, and the preparation method is simple to operate and suitable for wide promotion and application.

[0095] The present invention does not impose any particular limitation on the sanding process, as long as the raw material system can be mixed uniformly. In some embodiments of the present invention, the sanding process includes a first sanding process and a second sanding process;

[0096] The particle size of the sanding agent in the second sanding process is smaller than the particle size of the sanding agent in the first sanding process.

[0097] The present invention first uses a large-particle sanding agent for the first sanding treatment and then uses a small-particle sanding agent for the second sanding treatment, which helps to improve the effect of the sanding treatment, obtain a uniformly mixed suspension, and further helps to improve the comprehensive performance of the composite positive electrode material.

[0098] The sanding treatment of the present invention can be performed in a sand mill. Furthermore, the present invention can further define the specific processes of the first sanding treatment and the second sanding treatment to further improve the comprehensive performance of the composite positive electrode material. In some embodiments of the present invention, the speed of the first sanding treatment is 900-1200 rpm and the time is 2-3 hours; and / or,

[0099] In the second sanding treatment, the rotation speed is 400-2000 rpm and the time is 1-2 hours.

[0100] The present invention does not particularly limit the sodium source, as long as it can provide sodium element. In some embodiments, the sodium source can be at least one of sodium dihydrogen phosphate dihydrate, sodium bicarbonate, sodium carbonate, sodium pyrophosphate decahydrate, and sodium ferric ethylenediaminetetraacetate; the present invention does not particularly limit the iron source, as long as it can provide iron element. In some embodiments, the iron source can be at least one of ferrous oxalate dihydrate, ferric pyrophosphate, ferric phosphate, and sodium ferric ethylenediaminetetraacetate; the present invention does not particularly limit the phosphorus source, as long as it can provide phosphorus element. In some embodiments, the phosphorus source can be at least one of sodium dihydrogen phosphate dihydrate, ferric phosphate, ferric pyrophosphate, and sodium pyrophosphate decahydrate. It can be understood that when sodium ferric ethylenediaminetetraacetate is included in the raw material system, nitrogen, iron, and sodium elements can be introduced at the same time, which not only helps to simplify the preparation process, but also can complex iron atoms to achieve the confinement of the metal active center (limiting the dissolution of the variable valence metal).

[0101] The carbon source in the present invention can be an organic carbon source and / or an inorganic carbon source. In some embodiments of the present invention, when the carbon source includes an organic carbon source, during the calcination process, the organic molecules interact with each other to form a three-dimensional network for sodium ion transmission, achieving continuous transfer of ions or electrons, which can greatly improve the conductivity of the composite positive electrode material.

[0102] The organic carbon source of the present invention may be any commonly used organic carbon source in the art. For example, the organic carbon source may be at least one of polyethylene glycol, citric acid, glucose, sucrose, melamine, and polyacrylamide.

[0103] In the present invention, when the organic carbon source includes not only citric acid but also at least one of polyethylene glycol, glucose and sucrose, the carboxyl and hydroxyl groups in the citric acid can not only undergo an esterification reaction to form a cross-linked framework, but the citric acid can also be tightly combined with the hydroxyl groups of at least one of polyethylene glycol, glucose and sucrose to form a precursor polymer. During the subsequent calcination process, the precursor polymer will form a carbon skeleton, so that the core is evenly loaded in the carbon skeleton, forming a composite positive electrode material with excellent performance.

[0104] In the present invention, when the organic carbon source is selected from at least one of melamine and polyacrylamide, nitrogen atoms can also be introduced into the system to improve the specific capacity and rate performance of the composite positive electrode material.

[0105] In some embodiments of the present invention, the carbon source may also include an inorganic carbon source;

[0106] And the mass ratio of the organic carbon source to the inorganic carbon source can be (12.5-30):1.

[0107] In the present invention, when the carbon source also includes an inorganic carbon source, the inorganic carbon source will enter the inner core, and the organic carbon source will form a carbon coating layer. The combined action of the inorganic carbon source and the organic carbon source will achieve rapid transfer of electrons or ions.

[0108] The present invention does not particularly limit the inorganic carbon source, and it can be any inorganic carbon source commonly used in the art. For example, the inorganic carbon source can be Ketjen black and / or carbon nanotubes.

[0109] In some embodiments, when the carbon source includes an organic carbon source and an inorganic carbon source, a coupling agent may be added, with the weight percentage of the coupling agent being 1-3% based on the total weight of the raw material system. In some embodiments, the coupling agent may be a titanate coupling agent.

[0110] When the raw material system also includes a coupling agent, the coupling agent can promote the reaction of the organic carbon source and the inorganic carbon source, thereby better realizing the rapid transfer of electrons or ions and improving the electrochemical performance of the battery.

[0111] In the present invention, the theoretical mass of sodium ferric pyrophosphate can be calculated based on the added amounts of the sodium source, iron source, and phosphorus source and the reaction equation, and the content of the carbon source can be determined based on the theoretical mass of the sodium ferric pyrophosphate. In some embodiments, the added amount of the inorganic carbon source can be 0.5-2% of the theoretical mass of the sodium ferric pyrophosphate, and the added amount of the organic carbon source can be 8-25% of the theoretical mass of the sodium ferric pyrophosphate.

[0112] A third aspect of the present invention provides a battery, comprising the above-mentioned composite positive electrode material.

[0113] In the present invention, a conductive agent, a binder and the composite positive electrode material may be mixed to prepare a positive electrode slurry, which is then placed on at least one functional surface of a positive electrode current collector and dried to form a positive electrode sheet.

[0114] In a specific embodiment, the conductive agent can be acetylene black or conductive carbon black, the binder can be PVDF, and the mass ratio of the composite positive electrode material, the conductive agent and the binder is 8:1:1; the drying can be carried out in a vacuum oven, and the drying time can be 2 hours.

[0115] In some embodiments, the above-mentioned positive electrode sheet, separator, and negative electrode sheet are stacked to obtain an electrode assembly, the electrode assembly is placed in an outer package, and an electrolyte is injected into the outer package to obtain the battery of the present invention.

[0116] The battery of the present invention, because it includes the composite positive electrode material, has excellent cycle performance, discharge specific capacity and rate performance.

[0117] Hereinafter, the technical solution of the present invention will be further explained in conjunction with specific embodiments.

[0118] Example 1

[0119] The battery of this embodiment is prepared by a method comprising the following steps:

[0120] 1. Preparation of composite cathode materials

[0121] 1) Weighing 2 mol of sodium dihydrogen phosphate dihydrate and 1.5 mol of ferrous oxalate dihydrate, calculating the theoretical mass of sodium ferric pyrophosphate according to the reaction equation, then adding polyethylene glycol and melamine, dissolving them together in 2 L of deionized water, sequentially performing a first sand milling process and a second sand milling process using a sand mill to obtain a suspension, and spray drying the suspension using a spray dryer to obtain a powder;

[0122] The amount of polyethylene glycol and melamine added is 25% of the theoretical mass of sodium ferric pyrophosphate, and the mass ratio of polyethylene glycol to melamine is 1:1;

[0123] The particle size of the sand mill in the first sand grinding process is larger than the particle size of the sand mill in the second sand grinding process, and the time of the first sand grinding process is 3 hours and the speed of the sand mill is 1000 rpm min -1 In the second sand milling process, the time is 1.5h and the speed of the sand mill is 500rpm min -1 ;

[0124] During the spray drying process, the fan temperature of the spray dryer was 210°C and the outlet temperature was 85°C;

[0125] 2) placing the powder in a tubular furnace continuously filled with nitrogen and sequentially performing a first calcination treatment, a second calcination treatment, a third calcination treatment, and a fourth calcination treatment, and obtaining a composite positive electrode material after cooling;

[0126] The nitrogen ventilation flow rate is 0.3L min -1 ;

[0127] In the first calcination treatment, the temperature was 255°C and kept at this temperature for 2 h;

[0128] In the second calcination treatment, the temperature was 300°C and kept at this temperature for 5 h;

[0129] In the third calcination treatment, the temperature was 370°C and kept at this temperature for 2 hours;

[0130] In the fourth calcination treatment, the temperature was 470° C. and the temperature was maintained for 10 hours.

[0131] 2. Preparation of batteries

[0132] The composite cathode material, acetylene black, and binder PVDF were mixed in a mass ratio of 80:10:10 to form a cathode slurry. The cathode slurry was applied onto the two functional surfaces of the aluminum foil, dried, and baked in a vacuum oven for 2 hours to obtain a cathode sheet.

[0133] The positive electrode sheet, the separator, and the metal sodium negative electrode are stacked in a glove box with a water and oxygen content of less than 0.001 ppm to obtain an electrode assembly, the electrode assembly is placed in an outer package, and the electrolyte is injected into the outer package to obtain a CR2032 button battery;

[0134] Among them, the diaphragm is Celgard membrane,

[0135] The electrolyte includes NaClO4, EC, PC and FEC, the volume ratio of EC to PC is 1:1, the concentration of NaClO4 is 0.8 mol / L, and the mass percentage of FEC is 7.5%.

[0136] Example 2

[0137] The preparation method of the battery of this embodiment is basically the same as that of the battery of Example 1, except that:

[0138] In the preparation of composite cathode materials,

[0139] Weigh 0.6 mol of sodium pyrophosphate decahydrate, 0.6 mol of ferrous oxalate dihydrate, 1.47 mol of ferric phosphate, and 0.27 mol of sodium bicarbonate, calculate the theoretical mass of sodium ferric pyrophosphate according to the reaction equation, and then add polyethylene glycol and glucose;

[0140] The added amounts of polyethylene glycol and glucose are 11.1% of the theoretical mass of sodium ferric pyrophosphate, and the mass ratio of polyethylene glycol to glucose is 1:1.

[0141] Example 3

[0142] The preparation method of the battery of this embodiment is basically the same as that of the battery of Example 1, except that:

[0143] In the preparation of composite cathode materials,

[0144] Weigh 1 mol of sodium pyrophosphate decahydrate, 1 mol of ferrous oxalate dihydrate, and 2 mol of ferric phosphate, calculate the theoretical mass of sodium ferric pyrophosphate according to the reaction equation, and then add polyethylene glycol, Ketjen black, and titanate coupling agent;

[0145] The amount of polyethylene glycol added is 25% of the theoretical mass of sodium ferric pyrophosphate, the amount of Ketjen black added is 1% of the theoretical mass of sodium ferric pyrophosphate, and the amount of titanate coupling agent added is the same as that of Ketjen black.

[0146] Comparative Example 1

[0147] The preparation method of the battery of this comparative example is basically the same as that of the battery of Example 1, except that:

[0148] In the preparation of the composite positive electrode material, polyethylene glycol and melamine are not added.

[0149] Comparative Example 2

[0150] The preparation method of the battery of this comparative example is basically the same as that of the battery of Example 1, except that:

[0151] In step 1), the amount of polyethylene glycol and melamine added is 18% of the theoretical mass of sodium ferric pyrophosphate;

[0152] In step 2), the powder is placed in a tubular furnace continuously filled with nitrogen and sequentially subjected to a first calcination treatment, a second calcination treatment, and a third calcination treatment, and then cooled to obtain a composite positive electrode material;

[0153] The nitrogen ventilation flow rate is 0.3L min -1 ;

[0154] In the first calcination treatment, the temperature was 255°C and kept at this temperature for 2 h;

[0155] In the second calcination treatment, the temperature was 390°C and kept at this temperature for 5 h;

[0156] In the third calcination treatment, the temperature was 530° C. and the temperature was maintained for 15 hours.

[0157] Performance Testing

[0158] 1. The following performance tests were performed on the composite cathode materials in the examples and comparative examples, and the test results are shown in Table 1.

[0159] 1) XRD test

[0160] The composite cathode materials in the examples and comparative examples were subjected to XRD tests, and the test results are shown in Figure 1-4 ;

[0161] Figure 1 This is the XRD pattern of the composite positive electrode material in Example 1 of the present invention; Figure 2 This is the XRD pattern of the composite positive electrode material in Example 2 of the present invention; Figure 3 This is the XRD pattern of the composite positive electrode material in Example 3 of the present invention; Figure 4 This is the XRD pattern of the composite positive electrode material in Comparative Example 1 of the present invention. Figure 1-4It can be seen that the samples of the composite cathode materials in the embodiments of the present invention and the comparative examples have good crystallinity, and there is no obvious 4+x Fe 3-y (PO4) 2+z Impurity peaks other than P2O7) and NFP (NaFePO4).

[0162] The spectrum was subjected to Rietveld refinement to obtain the ratio of NFPP and NFP in the composite cathode material. The grain size of the three independent peaks of NFPP and NFP was calculated using the Scherrer formula to obtain the grain size of different phases.

[0163] 2) SEM test

[0164] The composite cathode materials in the embodiment and the comparative example were subjected to SEM tests respectively, and the test results are shown in FIG. Figure 5-8 ;

[0165] Figure 5 This is a SEM image of the composite cathode material in Example 1 of the present invention; Figure 6 This is a SEM image of the composite positive electrode material in Example 2 of the present invention; Figure 7 This is the SEM image of the composite positive electrode material in Example 3 of the present invention. Figure 8 This is the SEM image of the composite positive electrode material in Comparative Example 1 of the present invention. Figure 5-8 It can be seen that the composite positive electrode materials in the embodiment of the present invention are connected via the carbon coating layer, while the composite positive electrode materials in the comparative example have larger grain sizes and do not have a carbon coating layer.

[0166] 3) BET Testing

[0167] The BET test of the composite cathode materials in the examples and comparative examples was performed using a N2 adsorption BET tester.

[0168] 4) Compaction density of composite cathode material

[0169] A 13 mm diameter cylindrical tool was used, 2 g of material was loaded into it, and a pressure of 3 T was applied. The thickness of the final sample was measured and the compacted density of the material was calculated.

[0170] 5) Atomic ratio of Fe to Na (Fe / Na) or atomic ratio of Fe to P (Fe / P)

[0171] The composite cathode material was tested using XPS to obtain the initial atomic ratio of Fe to Na (Fe / Na) or the atomic ratio of Fe to P (Fe / P) from the outside to the inside of the composite cathode material, as well as the final stable atomic ratio of Fe to Na (Fe / Na) or the atomic ratio of Fe to P (Fe / P).

[0172] 2. The following performance tests were performed on the batteries in the examples and comparative examples. The test results are shown in Table 2.

[0173] 1) Charge and discharge specific capacity

[0174] The battery's charge and discharge range is 1.7-4.1V, and the charge and discharge current density is 1C (1C = 129mAh g -1 ), the results are shown in Table 1 and Figure 9-12 ;

[0175] Figure 9 is the charge and discharge curve of the battery in Example 1 of the present invention; Figure 10 is the charge and discharge curve of the battery in Example 2 of the present invention; Figure 11 is the charge and discharge curve of the battery in Example 3 of the present invention; Figure 12 is the charge and discharge curve of the battery in comparative example 1 of the present invention. Figure 9-12 It can be seen that the battery in the embodiment of the present invention has excellent charge and discharge specific capacity.

[0176] 2) Rate performance

[0177] The battery's charge and discharge range is 1.7-4.1V, and the rate is 5C. The test results are shown in Table 1.

[0178] 3) Pole compaction density

[0179] The dried positive electrode sheet is rolled at a set pressure and then punched with a standard die. The punched electrode sheet is weighed and the thickness is measured. The thickness and weight of the aluminum foil are deducted and the density calculation formula is used to obtain the compaction density of the electrode sheet after rolling.

[0180] 4) Cycle performance

[0181] The battery was tested for 200 cycles at 5C rate at room temperature (25°C), and the cycle retention rate was calculated based on the initial 5C cycle capacity.

[0182] Table 1

[0183]

[0184] Table 2

[0185]

[0186] It can be seen from Table 1 and Table 2 that the composite positive electrode material of the embodiment of the present invention, when used in a battery, can improve the rate performance, cycle performance and discharge specific capacity of the battery.

[0187] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A composite positive electrode material, characterized in that The invention comprises a core and a carbon coating layer covering at least a portion of the surface of the core and embedded in the core; The core includes NaFePO4 and a compound shown in Formula 1; Na 4+x Fe 3-y (PO4) 2+z P2O7 formula 1; In formula 1, -0.15≤x≤0.8, 0≤y≤0.5, -0.2≤z≤0.2; The particle size of NaFePO4 is ≤50.23nm; The particle size of the core is 100-900 nm.

2. The composite cathode material according to claim 1, characterized in that The atomic ratio of Fe to P in the composite positive electrode material gradually decreases from (0.9-1.1):1 from the outside to the inside, and then stabilizes at (0.65-0.85):1; and / or, The atomic ratio of Fe to Na in the composite positive electrode material gradually decreases from (0.9-1.1):1 from the outside to the inside, and then stabilizes at (0.65-0.85):

1.

3. The composite cathode material according to claim 1, characterized in that Based on the total mass of the composite positive electrode material, the mass percentage of the carbon coating layer is 0.5-5%.

4. The composite cathode material according to claim 1, characterized in that In the composite positive electrode material, PO4 3- and P2O7 4- The molar ratio is (1.5-3):

1.

5. The composite cathode material according to any one of claims 1 to 4, characterized in that: The specific surface area of ​​the composite positive electrode material is 5-20m 2 g -1 and / or, The compaction density of the composite cathode material is 1.9-2.4 g cm -3 .

6. A method for preparing the composite cathode material according to any one of claims 1 to 5, characterized in that: include: The raw material system including a sodium source, an iron source, a phosphorus source and a carbon source is sequentially subjected to sand milling and spray drying to obtain a powder; performing a first calcination treatment, a second calcination treatment, a third calcination treatment, and a fourth calcination treatment on the powder in sequence to obtain the composite positive electrode material; In the first calcination treatment, the temperature is 255-260° C. and the time is 1-2 hours; In the second calcination treatment, the temperature is 300-320°C and the time is 3-5h; In the third calcination treatment, the temperature is 370-390° C. and the time is 1-2 hours; In the fourth calcination treatment, the temperature is 470-500° C. and the time is 8-10 hours.

7. The preparation method according to claim 6, characterized in that The sodium source and the iron source are both sodium ferric ethylenediaminetetraacetate.

8. The preparation method according to claim 6, characterized in that The sanding process includes a first sanding process and a second sanding process; The particle size of the sanding agent in the second sanding process is smaller than the particle size of the sanding agent in the first sanding process.

9. The preparation method according to claim 8, characterized in that In the first sanding treatment, the rotation speed is 900-1200 rpm and the time is 2-3 hours; and / or, In the second sand grinding process, the rotation speed is 400-2000 rpm and the time is 1-2 hours.

10. The preparation method according to any one of claims 6 to 9, characterized in that: The carbon source includes an organic carbon source.

11. The preparation method according to claim 10, characterized in that: The carbon source also includes an inorganic carbon source; The mass ratio of the organic carbon source to the inorganic carbon source is (12.5-30):

1.

12. The preparation method according to claim 11, characterized in that The raw material system also includes a coupling agent; Based on the total mass of the raw material system, the mass percentage of the coupling agent is 1-3%.

13. The preparation method according to claim 12, characterized in that The coupling agent is a titanate coupling agent.

14. A battery, characterized in that: The composite positive electrode material comprises the composite positive electrode material according to any one of claims 1 to 5.

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