Composite positive electrode material, and preparation method therefor and use thereof

Abstract

The invention relates to the technical field of secondary battery materials. Disclosed are a composite positive electrode material, and a preparation method therefor and the use thereof. The composite positive electrode material comprises a composite substrate and a carbon material layer, which coats at least part of the surface of the composite substrate, wherein the composite substrate comprises lithium iron phosphate secondary particles and sodium iron sulfate, which is wrapped around at least part of the surfaces of the lithium iron phosphate secondary particles. The composite positive electrode material has good electrochemical performance at a low temperature.

Description

A composite positive electrode material and its preparation method and application

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to Chinese patent application number 2023107880340 filed with the Patent Office of China on June 29, 2023, entitled “A composite positive electrode material, its preparation method and application”, the entire contents of which are incorporated by reference into this application. Technical Field

[0003] The present disclosure relates to the technical field of secondary battery materials, and in particular to a composite positive electrode material and a preparation method and application thereof. Background Art

[0004] LiFePO4 cathode materials have long occupied a prominent position in the lithium battery cathode material field due to their high safety, excellent cycle stability, superior high-temperature performance, and low production cost. However, due to the low electronic conductivity and lithium-ion diffusion coefficient of LiFePO4 at room temperature, it is difficult to use in rate-sensitive lithium-ion batteries. Researchers have made significant breakthroughs by improving the conductivity and ion mobility of cathode materials through modifications such as carbon coating, doping, and particle nano-sizing. However, in winter, at high altitudes, and in northern regions, batteries must be used in cold conditions with ambient temperatures below zero degrees Celsius. Cold temperatures reduce the migration rate of active lithium ions in the electrolyte, resulting in slow battery reaction dynamics, a significant reduction in battery capacity, a decrease in voltage, and reduced output power. Therefore, it is necessary to improve the battery's low-temperature kinetics to ensure the normal operation of lithium iron phosphate at low temperatures.

[0005] Current methods for improving the low-temperature performance of lithium iron phosphate materials primarily include adding additives to the electrolyte, reducing electrolyte concentration to improve the electrolyte's kinetic parameters at low temperatures, and reducing the migration rate of active lithium ions in the electrolyte. However, no breakthrough solutions have been achieved from a material perspective.

[0006] Summary of the Invention

[0007] The present disclosure provides a composite positive electrode material, comprising a composite matrix and a carbon material layer coated on at least a portion of the surface of the composite matrix; the composite matrix comprises lithium iron phosphate secondary particles and sodium ferric sulfate coated on at least a portion of the surface of the lithium iron phosphate secondary particles.

[0008] In one embodiment, the mass of the carbon material layer accounts for 0.5% to 1.5% of the mass of the composite matrix; the molar ratio of the sodium ferric sulfate to the lithium iron phosphate is (0.5-3): (7-9.5). The particle size of the sodium ferric sulfate is 180-300 nm; and the particle size of the lithium iron phosphate secondary particles is 5-10 μm.

[0009] The present disclosure provides a method for preparing a composite cathode material, comprising the following steps:

[0010] A mixed system of sodium ferric sulfate preparation raw materials, lithium iron phosphate, a carbon source, a surfactant and water is subjected to a hydrothermal reaction to obtain a precursor material, which is then calcined to obtain a composite positive electrode material.

[0011] In one embodiment, the raw materials for preparing the sodium ferric sulfate include ammonium ferric sulfate and sodium carbonate, and the molar ratio of the ammonium ferric sulfate to the sodium carbonate is 2:1.

[0012] In one embodiment, the molar ratio of the sodium iron sulfate to the lithium iron phosphate is (0.5-3): (7-9.5).

[0013] In one embodiment, the method for preparing lithium iron phosphate includes: mixing iron phosphate, lithium carbonate, a reducing agent and water, and then performing ball milling, spray drying and heat treatment.

[0014] In one embodiment, in the method for preparing lithium iron phosphate, the molar ratio of iron phosphate, lithium carbonate and reducing agent is (1.9-2.2):(0.9-1.1):(0.05-0.08).

[0015] In one embodiment, in the preparation method of lithium iron phosphate, the ball milling speed is 750-850 r / min, and the ball-to-material ratio is 1:(4-6); the heat treatment temperature is 700-800° C., and the heat treatment time is 4-6 hours.

[0016] In one embodiment, the carbon source comprises at least one of sucrose, glucose, fructose, starch and cellulose.

[0017] In one embodiment, the mass of the carbon source is 0.5% to 1.5% of the total mass of lithium iron phosphate and sodium iron sulfate.

[0018] In one embodiment, the surfactant includes at least one of sodium dialkylbenzene sulfonate, polyvinyl pyrrolidone, and cetyltrimethylammonium bromide.

[0019] In one embodiment, the mass of the surfactant is 0.5% to 1.5% of the total mass of the lithium iron phosphate and sodium iron sulfate.

[0020] In one embodiment, the ratio of the total mass of the raw materials for the preparation of sodium ferric sulfate, lithium iron phosphate, carbon source and surfactant to water is (0.8-1.5) g: (90-150) mL.

[0021] In one embodiment, the temperature of the hydrothermal reaction is 150-170° C., and the time of the hydrothermal reaction is 8-16 hours.

[0022] In one embodiment, after the hydrothermal reaction, a cooling treatment is further included.

[0023] In one embodiment, before the calcination process, the precursor material is further subjected to washing and drying processes.

[0024] In one embodiment, the drying temperature of the precursor material is 80-120° C., and the drying time is 2-5 hours.

[0025] In one embodiment, the calcination temperature is 300-400° C., and the calcination time is 12-18 hours.

[0026] The present disclosure provides an electrode sheet, comprising the composite positive electrode material or the composite positive electrode material prepared by the method for preparing the composite positive electrode material.

[0027] The present disclosure provides a battery comprising the electrode sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In order to more clearly illustrate the technical solutions of the embodiments of the present disclosure, the following briefly introduces the drawings required for use in the embodiments. It should be understood that the following drawings only illustrate certain embodiments of the present disclosure and therefore should not be regarded as limiting the scope.

[0029] FIG1 shows the cycle capacity retention rate of a battery prepared with the composite positive electrode material of Example 1 of the present disclosure at -10°C. DETAILED DESCRIPTION

[0030] The advantages of the embodiments in the application content will be explained in the embodiment section of the specification below, and some of them are obvious from the specification, or can be obtained through some embodiments of the embodiments of the present disclosure.

[0031] The technical solution of the present disclosure will be further described below with reference to the accompanying drawings and through some implementation methods.

[0032] In order to make the purpose, technical solutions and advantages of the present disclosure more clear, the present disclosure is further described in detail below with reference to the accompanying drawings and examples. It should be understood that the embodiments described herein are only used to explain the present disclosure and are not intended to limit the present disclosure. In addition, the technical features involved in the various embodiments of the present disclosure described below can be combined with each other as long as they do not conflict with each other. Without departing from the principles of the embodiments of the present disclosure, several improvements and modifications can be made, and these improvements and modifications are also considered to be within the scope of protection of the embodiments of the present disclosure.

[0033] The present disclosure relates to a composite positive electrode material, comprising a composite matrix and a carbon material layer coated on at least a portion of the surface of the composite matrix; the composite matrix comprises lithium iron phosphate secondary particles and sodium iron sulfate coated on at least a portion of the surface of the lithium iron phosphate secondary particles.

[0034] The composite positive electrode material obtained by the present invention through the combination of lithium iron phosphate, sodium iron sulfate and a carbon material layer has excellent electrochemical performance at low temperatures.

[0035] In one embodiment, the mass of the carbon material layer accounts for 0.5% to 1.5% of the mass of the composite matrix; and the molar ratio of the sodium ferric sulfate to the lithium iron phosphate is (0.5 to 3): (7 to 9.5).

[0036] In one embodiment, the sodium iron sulfate has a particle size of 180 to 300 nm, such as 180 nm, 190 nm, 200 nm, 220 nm, 250 nm, 270 nm, or 300 nm. The lithium iron phosphate secondary particles have a particle size of 5 to 10 μm, such as 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm. In one embodiment, the lithium iron phosphate secondary particles are formed by agglomeration of lithium iron phosphate primary particles.

[0037] The present disclosure also relates to a method for preparing a composite positive electrode material, comprising the following steps:

[0038] A mixed system of sodium ferric sulfate preparation raw materials, lithium iron phosphate, a carbon source, a surfactant and water is subjected to a hydrothermal reaction to obtain a precursor material, which is then calcined to obtain a composite positive electrode material.

[0039] The present invention discloses a composite cathode material prepared by a hydrothermal method. The cathode material is coated with a carbon layer, which improves the ionic conductivity of the cathode material and is beneficial to the active ion Li + 、Na + Rapid transmission between the electrolyte and the electrode material; the composite positive electrode material enables the battery to have excellent electrochemical performance at a low temperature of -10°C. Not only that, while improving the battery rate performance, the blended sodium ion battery material will also reduce the battery cost to a certain extent.

[0040] In one embodiment, the raw materials for preparing the sodium ferric sulfate include ammonium ferric sulfate and sodium carbonate, and the molar ratio of the ammonium ferric sulfate to the sodium carbonate is 2:1.

[0041] In one embodiment, based on the theoretical sodium ferric sulfate obtained by the reaction of ammonium ferric sulfate and sodium carbonate, the molar ratio of the sodium ferric sulfate to the lithium iron phosphate is (0.5-3):(7-9.5), for example, 1:9, 2:8, 3:7, etc.

[0042] In one embodiment, lithium iron phosphate is synthesized by a method of spray drying combined with a high-temperature solid phase. In one embodiment, the method for preparing the lithium iron phosphate comprises: mixing iron phosphate, lithium carbonate, a reducing agent and water, and then ball milling, spray drying and heat treating. In one embodiment, in the method for preparing the lithium iron phosphate, the molar ratio of iron phosphate, lithium carbonate and reducing agent is (1.9-2.2): (0.9-1.1): (0.05-0.08), for example, 1.98: 1: 0.07, etc. In one embodiment, the rotation speed of the ball milling process is 750-850r / min, for example, 750r / min, 760r / min, 780r / min, 800r / min, 820r / min, 850r / min, etc. The pellet-to-material ratio is 1:(4-6), such as 1:5; the heat treatment temperature is 700-800°C, such as 720°C, 750°C, 770°C, 780°C, 790°C, etc., and the heat treatment time is 4-6 hours, such as 4.5 hours, 5 hours, or 5.5 hours. The above method is used to prepare lithium iron phosphate with a specific microstructure, which is advantageous for compounding with sodium iron sulfate and carbon materials to obtain a composite positive electrode material with excellent low-temperature performance.

[0043] In one embodiment, the carbon source comprises any one of sucrose, glucose, fructose, starch, and cellulose, or a combination of at least two thereof. In one embodiment, when multiple carbon sources are used in combination, the amount of each carbon source is the same.

[0044] In one embodiment, the added mass of the carbon source is 0.5% to 1.5% of the total mass of lithium iron phosphate and sodium iron sulfate, such as 0.8%, 1%, 1.9%, 2%, 2.2%, 2.5%, etc. By adding an appropriate amount of carbon source, a carbon layer with an appropriate coverage rate is formed on the surface of the composite positive electrode material, which is beneficial to improving the ionic conductivity of the positive electrode material and is beneficial to the active ion Li + 、Na + Rapid transport between electrolyte and electrode materials.

[0045] In one embodiment, the surfactant comprises at least one of sodium dialkylbenzene sulfonate, polyvinyl pyrrolidone, and cetyltrimethylammonium bromide. In one embodiment, the mass of the surfactant is 0.5% to 1.5% of the total mass of the lithium iron phosphate and sodium iron sulfate, for example, 0.8%, 1%, 1.9%, 2%, 2.2%, 2.5%, etc. The addition of an appropriate amount of surfactant in the present disclosure facilitates uniform dispersion of the various materials, thereby ensuring that the resulting composite positive electrode material has excellent electrochemical performance.

[0046] In one embodiment, the total mass ratio of the raw materials for preparing sodium ferric sulfate, lithium iron phosphate, carbon source, and surfactant to water is (0.8-1.5) g: (90-150) mL, for example, 0.8 g: 90 mL, 1 g: 100 mL, or 1.2 g: 130 mL.

[0047] In one embodiment, the temperature of the hydrothermal reaction is 150-170° C., for example, 150° C., 155° C., 160° C., 165° C., 170° C., etc.; the time of the hydrothermal reaction is 8-16 hours, for example, 8 hours, 9 hours, 10 hours, 11 hours, 12 hours, 15 hours, 16 hours, etc. The present disclosure ensures that a composite cathode material with a specific microstructure is obtained by adopting an appropriate hydrothermal reaction temperature and time, thereby ensuring that the composite cathode material has excellent electrochemical performance at low temperatures.

[0048] In one embodiment, after the hydrothermal reaction, the method further comprises: cooling treatment; cooling to room temperature.

[0049] In one embodiment, before the calcination process, the precursor material is further subjected to washing and drying processes.

[0050] In one embodiment, the washing treatment uses deionized water for 2 to 5 times to remove impurities and ensure the purity of the obtained composite negative electrode material.

[0051] In one embodiment, the drying temperature of the precursor material is 80-120°C, such as 90°C, 100°C, 110°C, etc., and the drying time is 2-5h, such as 2h, 2.5h, 3h, 4h, 4.5h, 5h, etc.

[0052] In one embodiment, the calcination temperature is 300-400°C, for example, 300°C, 320°C, 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, etc. The calcination time is 12-18 hours, for example, 12 hours, 13 hours, 14 hours, 15 hours, 16 hours, 17 hours, 18 hours, etc. The present disclosure can change the microstructure of the positive electrode material through appropriate calcination time and temperature, making the material performance more stable and more durable, thereby increasing the capacity of the battery prepared with the positive electrode material, making charging and discharging more efficient, safer, and more durable.

[0053] The present disclosure also relates to an electrode sheet comprising the composite positive electrode material or the composite positive electrode material prepared by the method for preparing the composite positive electrode material. In one embodiment, the composite positive electrode material is mixed with a conductive agent, a binder, and a solvent to prepare a positive electrode slurry, which is then applied to at least one side of a positive electrode current collector and dried to obtain a positive electrode sheet.

[0054] The present disclosure also relates to a battery, comprising the electrode sheet.

[0055] The battery disclosed herein includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte.

[0056] The battery of the present disclosure has excellent electrochemical performance at low temperatures.

[0057] The following are typical but non-limiting embodiments of the present disclosure:

[0058] Example 1

[0059] The preparation method of the composite positive electrode material comprises the following steps:

[0060] (a) First, the lithium iron phosphate material was synthesized by spray drying combined with high temperature solid phase method: iron phosphate (FePO4), lithium carbonate (Li2CO3) and glucose (C6H 12 O6) were placed in a ball mill and mixed evenly with deionized water as the medium. The molar ratio of the three was 0.98:0.5:0.035. The speed of the ball mill was 800 r / min and the ball milling time was 4 h. Glucose was used as a reducing agent to prevent Fe 3+ Reduction to Fe 2+ The ball-to-material ratio is 1:5. After ball milling, the mixture is dried using a spray dryer, placed in a porcelain boat, and calcined at 750°C in an argon atmosphere in a muffle furnace for 5 hours. The mixture is then cooled to room temperature to obtain lithium iron phosphate.

[0061] (b) The raw materials of sodium ferric sulfate are ammonium ferric sulfate [NH4Fe(SO4)2] and sodium carbonate (Na2CO3), and the molar ratio of the two is 2:1; according to the theoretical molar ratio of sodium ferric sulfate to lithium iron phosphate of 1:9, the corresponding masses of lithium iron phosphate, ammonium ferric sulfate, and sodium carbonate are weighed and placed in a hydrothermal reactor; 1% sucrose of the theoretically obtained lithium iron phosphate and sodium ferric sulfate and the surfactant sodium dodecylbenzene sulfonate (SDBS) are also added; deionized water is added to mix evenly and dissolve fully, and the hydrothermal reaction is carried out at 160°C for 15 hours. After the reaction is completed, it is naturally cooled to room temperature to obtain a precursor material, which is washed with deionized water 3 times and placed in a vacuum oven to dry for 4 hours. After drying, it is transferred to a porcelain boat and placed in a muffle furnace for calcination at 350°C for 15 hours to obtain the final lithium iron phosphate-sodium ferric sulfate-carbon composite positive electrode material.

[0062] Example 2

[0063] The preparation method of the composite positive electrode material comprises the following steps:

[0064] (a) Iron phosphate, lithium carbonate, and glucose were placed in a ball mill and uniformly mixed with deionized water. The molar ratio of the three was 0.98:0.5:0.035. The speed of the ball mill was 800 r / min and the ball milling time was 4 h. Glucose was used as a reducing agent to prevent Fe 3+ Reduction to Fe 2+ The ball-to-material ratio is 1:5. After ball milling, the mixture is dried using a spray dryer, placed in a porcelain boat, and calcined at 750°C in an argon atmosphere in a muffle furnace for 5 hours. The mixture is then cooled to room temperature to obtain lithium iron phosphate.

[0065] (b) The raw materials of sodium ferric sulfate are ammonium ferric sulfate and sodium carbonate, and the molar ratio of the two is 2:1; according to the theoretical molar ratio of sodium ferric sulfate to lithium iron phosphate of 1:9, the corresponding masses of lithium iron phosphate, ammonium ferric sulfate, and sodium carbonate are weighed and placed in a hydrothermal reactor; 1% of the theoretical mass of lithium iron phosphate and sodium ferric sulfate and the surfactant sodium dodecylbenzene sulfonate are also added; deionized water is added to mix evenly and dissolve fully, and a hydrothermal reaction is carried out at 150°C for 8 hours. After the reaction is completed, it is naturally cooled to room temperature to obtain a lithium iron phosphate-sodium ferric sulfate-carbon precursor, which is washed with deionized water 3 times and placed in a vacuum oven to dry for 4 hours. After drying, it is transferred to a porcelain boat and placed in a muffle furnace for calcination at 300°C for 12 hours to obtain the final lithium iron phosphate-sodium ferric sulfate-carbon composite positive electrode material.

[0066] Example 3

[0067] The preparation method of the composite positive electrode material comprises the following steps:

[0068] (a) Iron phosphate, lithium carbonate, and glucose were placed in a ball mill and uniformly mixed with deionized water. The molar ratio of the three was 0.98:0.5:0.035. The speed of the ball mill was 800 r / min and the ball milling time was 4 h. Glucose was used as a reducing agent to prevent Fe 3+ Reduction to Fe 2+ The ball-to-material ratio is 1:5. After ball milling, the product is dried using a spray dryer, placed in a porcelain boat, and calcined in an argon atmosphere at 750°C for 5 hours in a muffle furnace. The product is then cooled to room temperature to obtain lithium iron phosphate.

[0069] (b) The raw materials of sodium ferric sulfate are ammonium ferric sulfate and sodium carbonate, and the molar ratio of the two is 2:1; according to the theoretical molar ratio of sodium ferric sulfate to lithium iron phosphate of 1:9, the corresponding masses of lithium iron phosphate, ammonium ferric sulfate, and sodium carbonate are weighed and placed in a hydrothermal reactor; 1% sucrose of the theoretically obtained lithium iron phosphate and sodium ferric sulfate and the surfactant sodium dodecylbenzene sulfonate are also added; deionized water is added to mix evenly and dissolve fully, and the mixture is hydrothermally reacted at 170°C for 18 hours. After the reaction is completed, it is naturally cooled to room temperature to obtain a lithium iron phosphate-sodium ferric sulfate-carbon precursor, which is washed with deionized water 3 times and dried in a vacuum oven for 4 hours. After drying, it is transferred to a porcelain boat and placed in a muffle furnace for calcination at 400°C for 18 hours to obtain the final lithium iron phosphate-sodium ferric sulfate-carbon composite positive electrode material.

[0070] Example 4

[0071] The preparation method of the composite positive electrode material is the same as that of Example 1, except that the corresponding masses of lithium iron phosphate, ammonium iron sulfate, and sodium carbonate are weighed according to the theoretical molar ratio of sodium iron sulfate to lithium iron phosphate of 0.5:9.5. Other conditions are the same as those of Example 1.

[0072] Example 5

[0073] The preparation method of the composite positive electrode material is the same as that of Example 1, except that the corresponding masses of lithium iron phosphate, ammonium iron sulfate, and sodium carbonate are weighed according to the theoretical molar ratio of sodium iron sulfate to lithium iron phosphate of 3:7.

[0074] Example 6

[0075] The preparation method of the composite positive electrode material is the same as that of Example 1 except that 0.5% of the theoretical mass of lithium iron phosphate and sodium iron sulfate is added to sucrose.

[0076] Example 7

[0077] The preparation method of the composite positive electrode material is the same as that of Example 1 except that 1.5% of the theoretical mass of lithium iron phosphate and sodium iron sulfate is added to sucrose.

[0078] Comparative Example 1

[0079] The positive electrode material was prepared by the method of step (a) of Example 1 to obtain lithium iron phosphate material.

[0080] Comparative Example 2

[0081] The method for preparing a positive electrode material comprises the following steps:

[0082] Ammonium ferric sulfate and sodium carbonate with a molar ratio of 2:1 are placed in a hydrothermal reactor; sucrose (1% of the theoretical mass of sodium ferric sulfate) and a surfactant, sodium dodecylbenzenesulfonate, are added; deionized water is added and mixed evenly and dissolved fully, and a hydrothermal reaction is carried out at 160°C for 15 hours. After the reaction is completed, it is naturally cooled to room temperature to obtain a sodium ferric sulfate-carbon precursor, which is washed with deionized water 3 times and dried in a vacuum oven for 4 hours. After drying, it is transferred to a porcelain boat and placed in a muffle furnace and calcined at 350°C for 15 hours to obtain the final sodium ferric sulfate positive electrode material.

[0083] Comparative Example 3

[0084] The preparation method of the positive electrode material is the same as that of Example 1 except that sucrose is not added.

[0085] Experimental example

[0086] The positive electrode materials of the embodiment and the comparative example are respectively prepared into button batteries, comprising the following steps:

[0087] The above-mentioned positive electrode material, conductive agent Super-P, and binder PVDF were added to a glass bottle in a ratio of 8:1:1. N-methylpyrrolidone (NMP) was used as a solvent to prepare a slurry. The slurry was coated on the surface of carbon-containing aluminum foil and dried in a positive air environment at 100°C for 10 hours to obtain a positive electrode sheet. In a glove box, the above-mentioned positive electrode sheet, polymer separator (Celgard 2400), and hard carbon negative electrode were assembled into a CR2032 button cell. Lithium-ion battery electrolyte (1M LiPF6 dissolved in a mixed solvent of EC and DEC, with a volume ratio of EC and DEC of 1:1) was added. The cell was sealed using a sealing machine to obtain the final lithium-ion battery. After standing at room temperature for 24 hours, the cell was transferred to a thermostat, the temperature was adjusted to -10°C, and the cell was allowed to stand for 3 hours to allow the cell to reach the same temperature as the environment. Rate charge and discharge tests were then carried out.

[0088] The discharge capacity of each battery was measured at rates of 0.5C and 5C, as well as the cycle retention rate at 0.5C, -10°C, and 120 cycles. The test results are shown in Table 1. The capacity retention rate of the battery corresponding to the positive electrode material of Example 1 at 0.5C and -10°C is shown in Figure 1.

[0089] Table 1 Performance test results

[0090] As shown in Table 1, the battery prepared with the cathode material obtained by the method of the present disclosure has a high capacity and excellent electrochemical performance at a low temperature of -10°C. The discharge capacity of the lithium iron phosphate material of Comparative Example 1 and the sodium iron sulfate cathode material of Comparative Example 2 are both inferior to that of the composite cathode material of the present application. Industrial Applicability

[0091] In summary, the present disclosure provides a composite positive electrode material, a preparation method thereof, and an application thereof. The composite positive electrode material has excellent electrochemical properties at low temperatures through the combination of lithium iron phosphate, sodium iron sulfate, and a carbon material layer. The preparation method disclosed herein is simple and easy to implement, and is environmentally friendly. The obtained composite positive electrode material enables the battery to have excellent electrochemical properties at a low temperature of -10°C. Not only that, while improving the battery rate performance, the addition of sodium ion battery materials also reduces the battery cost to a certain extent. The battery prepared from this composite material has excellent electrochemical properties at low temperatures.

Claims

1. A composite positive electrode material, characterized in that: It comprises a composite matrix and a carbon material layer coated on at least a part of the surface of the composite matrix; the composite matrix comprises lithium iron phosphate secondary particles and sodium iron sulfate coated on at least a part of the surface of the lithium iron phosphate secondary particles.

2. The composite positive electrode material according to claim 1, characterized in that The mass of the carbon material layer accounts for 0.5% to 1.5% of the mass of the composite matrix; the molar ratio of the sodium iron sulfate to the lithium iron phosphate is (0.5 to 3): (7 to 9.5).

3. The composite positive electrode material according to claim 1, characterized in that: The particle size of the sodium iron sulfate is 180-300 nm; the particle size of the lithium iron phosphate secondary particles is 5-10 μm.

4. A method for preparing a composite positive electrode material, characterized in that: The following steps are involved: A mixed system of sodium iron sulfate preparation raw materials, lithium iron phosphate, a carbon source, a surfactant and water is subjected to a hydrothermal reaction to obtain a precursor material, which is then subjected to a calcination treatment to obtain a composite positive electrode material.

5. The method for preparing a composite positive electrode material according to claim 4, characterized in that: The raw materials for preparing the sodium ferric sulfate include ammonium ferric sulfate and sodium carbonate, and the molar ratio of the ammonium ferric sulfate to the sodium carbonate is 2:

1.

6. The method for preparing a composite positive electrode material according to claim 4, characterized in that: The molar ratio of the sodium iron sulfate to the lithium iron phosphate is (0.5-3):(7-9.5).

7. The method for preparing a composite positive electrode material according to claim 4, characterized in that: The preparation method of lithium iron phosphate comprises: mixing iron phosphate, lithium carbonate, a reducing agent and water, and then performing ball milling, spray drying and heat treatment.

8. The method for preparing a composite positive electrode material according to claim 4, characterized in that: In the preparation method of lithium iron phosphate, the molar ratio of iron phosphate, lithium carbonate and reducing agent is (1.9-2.2):(0.9-1.1):(0.05-0.08).

9. The method for preparing a composite positive electrode material according to claim 4, characterized in that: In the preparation method of lithium iron phosphate, the rotation speed of ball milling is 750-850 r / min, and the ball-to-material ratio is 1:(4-6); the temperature of heat treatment is 700-800° C., and the time of heat treatment is 4-6 hours.

10. The method for preparing a composite positive electrode material according to claim 4, characterized in that: Contains at least one of the following features (1) to (5): (1) The carbon source comprises at least one of sucrose, glucose, fructose, starch and cellulose; (2) The mass of the carbon source is 0.5% to 1.5% of the total mass of lithium iron phosphate and sodium iron sulfate; (3) the surfactant comprises at least one of sodium dialkylbenzene sulfonate, polyvinyl pyrrolidone and hexadecyltrimethylammonium bromide; (4) The mass of the surfactant is 0.5% to 1.5% of the total mass of the lithium iron phosphate and sodium iron sulfate; (5) The total mass ratio of the raw materials for the preparation of sodium ferric sulfate, lithium iron phosphate, carbon source and surfactant to the amount of water is (0.8-1.5) g: (90-150) mL.

11. The method for preparing a composite positive electrode material according to claim 4, characterized in that: Contains at least one of the following features (1) to (3): (1) The temperature of the hydrothermal reaction is 150 to 170° C., and the time of the hydrothermal reaction is 8 to 16 hours; (2) Also includes: cooling treatment; (3) The method further includes: washing and drying the precursor material.

12. The method for preparing a composite positive electrode material according to claim 11, characterized in that: The drying temperature of the precursor material is 80-120° C., and the drying time is 2-5 hours.

13. The method for preparing a composite positive electrode material according to claim 4, characterized in that: The calcination temperature is 300-400° C., and the calcination time is 12-18 hours.

14. An electrode sheet, characterized in that: The invention comprises the composite positive electrode material according to any one of claims 1 to 3 or a composite positive electrode material prepared by the method for preparing the composite positive electrode material according to any one of claims 4 to 13.

15. A battery, characterized in that: Comprising the electrode sheet as claimed in claim 14.

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