Niobium-doped reduced graphene oxide coated sodium vanadium phosphate positive electrode material and preparation method thereof

Abstract

The invention relates to the technical field of positive electrode materials, in particular to a niobium-doped reduced graphene oxide coated sodium vanadium phosphate positive electrode material and a preparation method thereof.The niobium-doped reduced graphene oxide coated sodium vanadium phosphate positive electrode material is prepared from sodium vanadium phosphate, niobium pentoxide, reduced graphene oxide, polyvinylpyrrolidone, lithium carbonate and carbon black; the particle size of the sodium vanadium phosphate is 0.8-1.5 [mu] m; niobium pentoxide is used as a metal doping source, Nb < 5 + > in niobium pentoxide can be embedded into sodium vanadium phosphate lattices through a pre-sintering process, on one hand, the ionic radius of Nb < 5 + > is close to V < 3 + > / V < 4 + > in the sodium vanadium phosphate lattices, lattice parameters can be effectively regulated and controlled, lattice distortion in the lithium ion deintercalation process is reduced, and material volume expansion is inhibited; on the other hand, due to introduction of Nb < 5 + >, an impurity energy level can be formed in a sodium vanadium phosphate forbidden band, an electron transition energy barrier is reduced, the intrinsic electron conductivity of the material is remarkably improved, and the problem that the conductivity of traditional sodium vanadium phosphate is extremely low is solved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of positive electrode materials, in particular to a niobium-doped reduced graphene oxide-coated sodium vanadium phosphate positive electrode material and a preparation method thereof. BACKGROUND

[0002] Lithium ion batteries have been widely used in new energy vehicles, energy storage systems and other fields due to their high energy density and long cycle life. As a core component of lithium ion batteries, the performance of the positive electrode material directly determines the overall electrochemical performance of the battery. Sodium vanadium phosphate (Na3V2(PO4)3) has become one of the most promising positive electrode materials due to its high theoretical specific capacity (117 mAh / g), stable working voltage, low cost and abundant sodium resources.

[0003] However, sodium vanadium phosphate has some significant technical shortcomings. Firstly, it is a typical insulator with extremely low electronic conductivity, which hinders the transmission of lithium ions within the material, resulting in low charge transfer efficiency during charging and discharging, and significantly limiting the rate performance and actual specific capacity of the battery. Secondly, during long-term cycling, sodium vanadium phosphate particles are prone to volume expansion and agglomeration, which not only damages the structural integrity of the electrode, but also causes the active material to separate from the conductive network, leading to rapid capacity decay and making it difficult to meet the stringent requirements of power batteries for cycle stability.

[0004] To solve the above problems, existing technologies often use modification methods such as carbon coating and metal ion doping. For example, by coating carbon black, carbon nanotubes and other carbon materials to construct a conductive network, the electronic conductivity can be improved to some extent, but the interface bonding strength between traditional carbon materials and sodium vanadium phosphate is weak, and the carbon materials are prone to fall off during cycling, which cannot maintain the conductive path for a long time. Metal ion doping can optimize the lattice structure stability, but single doping cannot simultaneously improve the electronic conductivity and inhibit the volume expansion. SUMMARY

[0005] In view of the problems in the prior art, the application provides a niobium-doped reduced graphene oxide-coated sodium vanadium phosphate positive electrode material and a preparation method thereof.

[0006] To achieve the above-mentioned purpose, the application provides the following technical scheme:

[0007] A niobium-doped reduced graphene oxide-coated sodium vanadium phosphate positive electrode material is composed of the following components by weight: 72-78 parts of sodium vanadium phosphate, 0.8-1.5 parts of niobium pentoxide, 3-5 parts of reduced graphene oxide, 1.8-2.5 parts of polyvinylpyrrolidone, 0.5-0.8 parts of lithium carbonate, and 2-3 parts of carbon black.

[0008] The particle size of the sodium vanadium phosphate is 0.8-1.5 microns.

[0009] The BET specific surface area of the positive electrode material is 15-20 m² / g, and the pore volume is 0.05-0.08 cm³ / g.

[0010] As a further technical solution, the sheet thickness of the reduced graphene oxide is 1-2 nm, and the sheet diameter is 0.8-1.5 μm.

[0011] As a further technical solution, the preparation step of the reduced graphene oxide is:

[0012] S1 graphene oxide pretreatment: disperse graphene oxide in deionized water to prepare a dispersion liquid with a concentration of 0.8-1.2 mg / mL, ultrasonic for 15-20 min until no obvious aggregation, and obtain a graphene oxide dispersion liquid;

[0013] The ultrasonic power is 200-250 W;

[0014] S2 plasma activation: place the graphene oxide dispersion liquid in a low-temperature plasma reaction chamber, introduce a mixed gas atmosphere of argon-hydrogen with a volume ratio of 9:1, control the vacuum degree to 10-15 Pa and the plasma power to 50-80 W, and treat for 10-12 min;

[0015] S3 in-situ reduction of tea polyphenol: add a tea polyphenol aqueous solution with a concentration of 5-8 mg / mL to the activated graphene oxide dispersion liquid, the mass ratio of tea polyphenol to graphene oxide is 1:2-3, and under nitrogen protection, constant temperature stirring at 60-80℃ for 3-4 h;

[0016] S4 purification and drying: centrifuge the mixed liquid after reduction at 9000-11000 rpm for 12-18 min, collect the precipitate, wash the precipitate with deionized water to pH 6.5-7, and place it in a vacuum drying oven at 50-60℃, and dry it in a vacuum degree ≤5 Pa for 6-8 h to obtain.

[0017] The preparation method of the niobium-doped reduced graphene oxide coated sodium vanadium phosphate positive electrode material comprises the following steps:

[0018] (1) raw material pretreatment: mix sodium vanadium phosphate, niobium pentoxide and carbon black according to weight parts, add deionized water filtered by a 0.22 μm filter and with an electrical resistivity ≥ , stir for 12-14 min;

[0019] (2) dispersion and doping precursor preparation: add polyvinylpyrrolidone to the mixed liquid obtained in step (1), and mill for 4.5-5.5 h at a speed of 320-380 rpm using agate balls with a diameter of 5-8 mm and dried at 120℃ for 2 h, and control the slurry temperature to 25-30℃ during the milling process to obtain a uniform slurry;

[0020] The ball-to-material ratio is 4-5:1;

[0021] (3) pre-sintering: the slurry obtained in step (2) is subjected to programmed temperature reduction freeze drying to obtain dry powder; the dry powder is placed in argon, and is subjected to staged temperature increase pre-sintering: temperature is increased to 630-640 DEG C at a rate of 6-7 DEG C / min, and is kept for 1.5-1.8 h; then, temperature is continuously increased to 690-700 DEG C at the same rate, and is kept for 1.5-1.8 h to promote lattice reconstitution, and then is decreased to below 300 DEG C at a rate of 3 DEG C / min to obtain niobium-doped sodium vanadium phosphate powder;

[0022] (4) coating: the pre-sintered powder obtained in step (3) and reduced graphene oxide are added into deionized water according to weight parts to prepare a mixed solution with a solid content of 15-20 wt%, and intermittent ultrasonic treatment is carried out at a power of 320-380 W for 22-28 min to form a coating slurry;

[0023] (5) secondary sintering: the coating slurry obtained in step (4) is subjected to spray drying to obtain composite powder; the composite powder is placed in argon, and is subjected to secondary sintering at 780-820 DEG C for 6-7 h, and then is naturally cooled to below 200 DEG C at a rate of 2 DEG C / min to obtain the target positive electrode material.

[0024] As a further technical solution, the amount of deionized water added in step (1) satisfies a solid-liquid ratio of 1:6-1:7.

[0025] As a further technical solution, the purity of the agate ball in step (2) is greater than or equal to 99.9%, and the ball milling tank is made of polytetrafluoroethylene.

[0026] As a further technical solution, the programmed temperature reduction freeze drying in step (3) is first reduced to -10 DEG C at a rate of 5 DEG C / min, kept for 1 h, then reduced to -45~-42 DEG C at a rate of 2 DEG C / min, and kept for 13-15 h under a vacuum degree of 5-8 Pa.

[0027] As a further technical solution, the intermittent ultrasonic treatment in step (4) is stopped for 1 min every 5 min of ultrasonic treatment.

[0028] As a further technical solution, the coating slurry in step (5) is subjected to spray drying at an air inlet temperature of 185-195 DEG C and an air outlet temperature of 85-88 DEG C.

[0029] As a further technical solution, after the secondary sintering in step (5), the target positive electrode material has a tap density of 1.2-1.4 g / cm3 and a compacted density of 2.8-3.0 g / cm3.

[0030] Compared with the prior art, the present application has the following beneficial effects:

[0031] In the present application, niobium pentoxide is used as a metal doping source, and Nb 5+Sodium vanadium phosphate lattice can be embedded through a pre-sintering process; on the one hand, Nb 5+ The ionic radius and V in the sodium vanadium phosphate lattice 3+ / V 4+ Similarly, it can effectively control lattice parameters, reduce lattice distortion during lithium-ion insertion / extraction, and suppress material volume expansion; on the other hand, Nb 5+ The introduction of [a specific substance] can form impurity energy levels in the band gap of sodium vanadium phosphate, reduce the electronic transition energy barrier, significantly improve the intrinsic electronic conductivity of the material, and solve the problem of extremely low conductivity of traditional sodium vanadium phosphate.

[0032] The reduced graphene oxide prepared by this invention possesses a single-atom-layer structure and excellent conductivity. It can be uniformly coated onto the surface of sodium vanadium phosphate particles using an intermittent ultrasonic process, forming a continuous conductive network. This further reduces the charge transfer resistance within the electrode and improves rate performance. Simultaneously, the sheet-like structure of the reduced graphene oxide acts as a buffer layer, absorbing the volume expansion stress of the sodium vanadium phosphate particles during cycling, preventing particle agglomeration and electrode structure damage, and extending cycle life. Furthermore, this invention prepares reduced graphene oxide through plasma activation and in-situ reduction of tea polyphenols. Plasma activation introduces defect sites on the graphene oxide surface, enhancing its interaction with tea polyphenols, preventing sheet agglomeration, and ensuring uniform coating. Tea polyphenols, as a green reducing agent, not only replace traditional toxic chemical reducing agents, but their hydroxyl and phenolic hydroxyl groups can also form hydrogen bonds with the hydroxyl groups on the sodium vanadium phosphate surface, improving interfacial bonding and preventing the reduced graphene oxide from detaching.

[0033] During ball milling, polyvinylpyrrolidone can act as a dispersant and binder. On the one hand, it prevents the agglomeration of sodium vanadium phosphate and niobium pentoxide particles through steric hindrance, ensuring uniform mixing of all components and laying the foundation for subsequent uniform doping. On the other hand, its binding effect can enhance the stability of the slurry after ball milling, prevent particle sedimentation during drying, and ensure the uniformity of dry powder composition.

[0034] Segmented pre-sintering allows for slow doping and step-by-step lattice reconstruction, first promoting Nb at a lower temperature. 5+ Slow embedding into the sodium vanadium phosphate lattice avoids lattice defects caused by rapid heating; then, high-temperature holding promotes lattice ordering, reduces lattice distortion, and improves structural stability. Secondary sintering enhances the interfacial bonding strength between reduced graphene oxide and sodium vanadium phosphate, making the coating layer tighter, while further removing residual impurities and pores inside the material, increasing tap density and compaction density, and ensuring that the electrode has a high volumetric energy density.

[0035] Compared with the traditional hot air drying, the programmed cooling freeze drying can directly sublimate the water in the slurry in a low-temperature vacuum environment, avoids the particle agglomeration and component segregation caused by water evaporation in the drying process, ensures that the dry powder has a loose porous structure, facilitates the gas escape in the subsequent pre-sintering process, and provides more channels for lithium ion transmission, thereby improving the ion diffusion rate.

[0036] The doped niobium pentoxide and the reduced graphene oxide coating are synergistic, solving the problems of poor conductivity and insufficient cycle stability of sodium vanadium phosphate: the doped niobium pentoxide improves the intrinsic conductivity of the material through lattice doping, and the reduced graphene oxide further reduces the charge transfer resistance by constructing a continuous conductive network, so that the volume conductivity of the positive electrode material is improved to 1.5*10 -2 S / cm or more, far exceeding the traditional unmodified sodium vanadium phosphate; at the same time, the doped niobium pentoxide inhibits lattice distortion and the reduced graphene oxide buffers volume expansion, solving the technical problem of rapid cycle capacity decay of the traditional material. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 It is a flow chart of a preparation method of the niobium-doped reduced graphene oxide coated sodium vanadium phosphate positive electrode material. DETAILED DESCRIPTION

[0038] The technical solutions in the embodiments of the present application will be described below in a clear and complete manner. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0039] The present application provides a niobium-doped reduced graphene oxide coated sodium vanadium phosphate positive electrode material and a preparation method thereof. The positive electrode material is composed of the following components by weight: 72-78 parts of sodium vanadium phosphate, 0.8-1.5 parts of niobium pentoxide, 3-5 parts of reduced graphene oxide, 1.8-2.5 parts of polyvinylpyrrolidone, 0.5-0.8 parts of lithium carbonate, and 2-3 parts of carbon black. The particle size of the sodium vanadium phosphate is 0.8-1.5 μm. The BET specific surface area of the positive electrode material is 15-20 m² / g, and the pore volume is 0.05-0.08 cm³ / g.

[0040] In the present application, the sheet thickness of the reduced graphene oxide is 1-2 nm, and the sheet diameter is 0.8-1.5 μm. The preparation steps are as follows:

[0041] S1 graphene oxide pretreatment: disperse the graphene oxide in deionized water to prepare a dispersion liquid with a concentration of 0.8-1.2 mg / mL, and ultrasonic for 15-20 min until no obvious agglomeration occurs, to obtain a graphene oxide dispersion liquid; the ultrasonic power is 200-250 W;

[0042] S2 plasma activation: the graphene oxide dispersion liquid is placed in a low temperature plasma reaction cavity, a mixed gas atmosphere of argon-hydrogen with a volume ratio of 9:1 is introduced, the vacuum degree is controlled to be 10-15 Pa, the plasma power is controlled to be 50-80 W, and the treatment is performed for 10-12 min; S3 tea polyphenol in-situ reduction: a tea polyphenol aqueous solution with a concentration of 5-8 mg / mL is added to the activated graphene oxide dispersion liquid, the mass ratio of tea polyphenol to graphene oxide is 1:2-3, under nitrogen protection, constant temperature stirring is performed at 60-80 ℃ for 3-4 h;

[0043] S4 purification and drying: the mixed liquid after reduction is centrifuged at 9000-11000 rpm for 12-18 min, and the precipitate is collected; the precipitate is washed with deionized water until the pH is 6.5-7, and then the precipitate is placed in a vacuum drying box at 50-60 ℃, and dried in a vacuum degree ≤ 5 Pa for 6-8 h to obtain.

[0044] The preparation method of the positive electrode material comprises the following steps:

[0045] (1) raw material pretreatment: sodium vanadium phosphate, niobium pentoxide and carbon black are mixed according to weight parts, deionized water filtered by a 0.22 μm filter membrane and with an electrical resistivity ≥ is added, the amount of deionized water added satisfies the solid-liquid ratio of 1:6-1:7, and stirring is performed for 12-14 min;

[0046] (2) dispersion and doped precursor preparation: polyvinylpyrrolidone is added to the mixed liquid obtained in step (1), a diameter of 5-8 mm and a purity of ≥ 99.9% is adopted, the maroon ball is dried at 120 ℃ for 2 h, the maroon ball is used in the polytetrafluoroethylene material ball milling tank at a speed of 320-380 rpm, and the maroon ball is ball milled for 4.5-5.5 h, the ball-to-material ratio is 4-5:1, the slurry temperature is controlled to be 25-30 ℃ during the ball milling process, and a uniform slurry is obtained;

[0047] (3) pre-sintering: the slurry obtained in step (2) is subjected to programmed cooling freeze-drying, first reduced to -10 ℃ at a speed of 5 ℃ / min and kept for 1 h, then reduced to -45~-42 ℃ at a speed of 2 ℃ / min, and kept for 13-15 h under a vacuum degree of 5-8 Pa, to obtain dry powder; the dry powder is placed in argon, and is subjected to segmented temperature rising pre-sintering: the temperature is raised to 630-640 ℃ at a speed of 6-7 ℃ / min, kept for 1.5-1.8 h; then the temperature is continuously raised to 690-700 ℃ at the same speed, kept for 1.5-1.8 h to promote lattice reconstruction, and then reduced to below 300 ℃ at a speed of 3 ℃ / min and naturally cooled, to obtain niobium-doped sodium vanadium phosphate powder;

[0048] (4) coating: the pre-sintered powder obtained in step (3) and reduced graphene oxide are added to deionized water according to weight parts to prepare a mixed solution with a solid content of 15-20 wt%, and an intermittent ultrasonic wave of 320-380 W power, 5 min on and 1 min off, is used for 22-28 min to form a coating slurry;

[0049] (5) secondary sintering: the coating slurry obtained in step (4) is spray dried at an inlet temperature of 185-195 DEG C and an outlet temperature of 85-88 DEG C to obtain a composite powder; the composite powder is placed in argon, and secondary sintering is carried out at 780-820 DEG C for 6-7 h, and then cooled to below 200 DEG C at a rate of 2 DEG C / min to obtain the target positive electrode material, and the tap density of the target positive electrode material is 1.2-1.4 g / cm3, and the compacted density is 2.8-3.0 g / cm3.

[0050] The positive electrode material and the preparation method provided by the application have the synergistic effect of specific component proportioning and preparation process, effectively improve the conductivity and structural stability of the positive electrode material while ensuring that the positive electrode material has appropriate BET specific surface area and pore volume, the process parameters are accurately controlled during preparation, the material is uniformly doped and completely coated, the problems of poor conductivity and insufficient cycle stability of the traditional sodium vanadium phosphate positive electrode material are solved, the preparation process is easy to scale up, and the production cost is reduced and the product competitiveness is improved.

[0051] In order to further illustrate the application, the following examples, comparative examples and tests are described in detail. The particle size of sodium vanadium phosphate used in the following examples of the application is 0.8-1.5 μm, the niobium pentoxide and lithium carbonate are analytically pure, the reduced graphene oxide is prepared according to the preparation method of the application (the thickness of the sheet is 1-2 nm, and the sheet diameter is 0.8-1.5 μm), the polyvinylpyrrolidone is chemically pure, the carbon black is conductive carbon black, the deionized water is filtered through a 0.22 μm filter membrane and has a resistivity of ≥ 18.2 MΩ·cm, the agate ball has a purity of ≥ 99.9%, and the ball milling tank is made of polytetrafluoroethylene.

[0052] The following are specific examples:

[0053] Example 1: preparation of reduced graphene oxide

[0054] S1: pretreatment of graphene oxide: graphene oxide is dispersed in deionized water to prepare a dispersion liquid with a concentration of 1.0 mg / mL, and ultrasonic wave with a power of 220 W is used for 18 min until no obvious agglomeration occurs to obtain a graphene oxide dispersion liquid;

[0055] S2: plasma activation: the graphene oxide dispersion liquid is placed in a low-temperature plasma reaction cavity, a mixed gas atmosphere of argon-hydrogen with a volume ratio of 9:1 is introduced, the vacuum degree is controlled at 12 Pa, the plasma power is 65 W, and the treatment time is 11 min; ​

[0056] S3 In-situ reduction of tea polyphenol: 6.5 mg / mL tea polyphenol aqueous solution was added into the activated graphene oxide dispersion, the mass ratio of tea polyphenol to graphene oxide was 1:2.5, under nitrogen protection, constant temperature stirring at 70℃ for 3.5h;

[0057] S4 Purification and drying: the mixed solution after reduction was centrifuged at 10000 rpm for 15 min, and the precipitate was collected; the precipitate was washed with deionized water until pH 6.8, and then placed in a vacuum drying oven at 55℃, dried in a vacuum degree of 3Pa for 7h, to obtain reduced graphene oxide.

[0058] Preparation of positive electrode material:

[0059] (1) Raw material pretreatment: 75 parts of sodium vanadate, 1.2 parts of niobium pentoxide and 2.5 parts of carbon black were mixed according to weight, deionized water was added (solid-liquid ratio 1:6.5), and stirred for 13 min;

[0060] (2) Preparation of dispersion and doped precursor: 2.2 parts of polyvinylpyrrolidone was added to the above mixture, and the mixture was ball milled in a polytetrafluoroethylene ball mill tank with a ball-to-material ratio of 4.5:1 and a ball diameter of 6.5mm dried at 120℃ for 2h, at a rotation speed of 350rpm, for 5h, and the slurry temperature was controlled at 28℃ during ball milling, to obtain a uniform slurry;

[0061] (3) Pre-sintering: the slurry was freeze-dried by programmed cooling, first reduced to-10℃ at a rate of 5℃ / min and kept for 1h, then reduced to-43℃ at a rate of 2℃ / min, and kept for 14h under a vacuum degree of 6Pa, to obtain dry powder; the dry powder was heated to 635℃ at a rate of 6.5℃ / min in argon, kept for 1.6h; then continued to heat to 695℃ at the same rate, kept for 1.6h, and then naturally cooled to below 300℃ at a rate of 3℃ / min, to obtain niobium-doped sodium vanadate powder;

[0062] (4) Coating: the pre-sintered powder and 4 parts of reduced graphene oxide were added to deionized water to prepare a mixture with a solid content of 18wt%, and the mixture was intermittently ultrasonicated for 25min at a power of 350W, 5min on and 1min off, to form a coated slurry;

[0063] (5) Secondary sintering: the coated slurry was spray dried at an inlet temperature of 190℃ and an outlet temperature of 86℃, to obtain a composite powder; the composite powder was sintered at 800℃ for 6.5h in argon, and then naturally cooled to below 200℃ at a rate of 2℃ / min, to obtain the target positive electrode material.

[0064] Example 2: Preparation of reduced graphene oxide:

[0065] S1 graphene oxide pretreatment: graphene oxide was dispersed in deionized water to prepare a dispersion liquid with a concentration of 0.8 mg / mL, and ultrasonic treatment was performed at a power of 200 W for 15 min until no obvious aggregation was observed, thereby obtaining a graphene oxide dispersion liquid;

[0066] S2 plasma activation: the graphene oxide dispersion liquid was placed in a low-temperature plasma reaction chamber, and a mixed gas atmosphere of argon-hydrogen with a volume ratio of 9:1 was introduced, the vacuum degree was controlled at 10 Pa, and the plasma power was controlled at 50 W, and the treatment was performed for 10 min;

[0067] S3 in-situ reduction of tea polyphenol: 5 mg / mL tea polyphenol aqueous solution was added to the activated graphene oxide dispersion liquid, and the mass ratio of tea polyphenol to graphene oxide was 1:2, and the mixture was stirred at 60°C under nitrogen protection for 3 h;

[0068] S4 purification and drying: the mixture after reduction was centrifuged at 9000 rpm for 12 min, and the precipitate was collected; the precipitate was washed with deionized water until the pH was 6.5, and then placed in a vacuum drying oven at 50°C, and dried at a vacuum degree of 2 Pa for 6 h, thereby obtaining reduced graphene oxide.

[0069] Preparation of positive electrode material:

[0070] (1) raw material pretreatment: 72 parts of sodium vanadate, 0.8 parts of niobium pentoxide and 2 parts of carbon black were mixed, deionized water was added (solid-liquid ratio 1:6), and stirred for 12 min;

[0071] (2) dispersion and preparation of doped precursor: 1.8 parts of polyvinylpyrrolidone was added to the above mixture, and a diameter of 5 mm and 120°C was used for 2 h in a maroon ball, and the ball was milled in a polytetrafluoroethylene ball mill tank at a ball-to-material ratio of 4:1 at a speed of 320 rpm for 4.5 h, and the slurry temperature was controlled at 25°C during ball milling, thereby obtaining a uniform slurry;

[0072] (3) pre-sintering: the slurry was freeze-dried by programmed cooling, first reduced to-10°C at a rate of 5°C / min, then reduced to-45°C at a rate of 2°C / min, and the vacuum degree was 5 Pa, and the temperature was kept for 13 h, thereby obtaining a dry powder; the dry powder was placed in argon, and the temperature was increased to 630°C at a rate of 6°C / min, and the temperature was kept for 1.5 h; then the temperature was continuously increased to 690°C at the same rate, and the temperature was kept for 1.5 h, and then the temperature was decreased to below 300°C at a rate of 3°C / min, and the temperature was naturally cooled, thereby obtaining a niobium-doped sodium vanadate powder;

[0073] (4) coating: the pre-sintered powder and 3 parts of reduced graphene oxide were added to deionized water to prepare a mixture with a solid content of 15 wt%, and intermittent ultrasonic treatment was performed at a power of 320 W for 5 min and stopped for 1 min, and the treatment was performed for 22 min, thereby forming a coating slurry;

[0074] (5) Secondary sintering: spray drying the coating slurry at an inlet temperature of 185°C and an outlet temperature of 85°C to obtain a composite powder; placing the composite powder in argon, and secondary sintering at 780°C for 6h, and then naturally cooling to below 200°C at a rate of 2°C / min, to obtain the target positive electrode material.

[0075] Example 3: Preparation of reduced graphene oxide:

[0076] S1 Pretreatment of graphene oxide: dispersing graphene oxide in deionized water to prepare a dispersion liquid with a concentration of 1.2mg / mL, and ultrasonicating at a power of 250W for 20min until no obvious agglomeration is observed, to obtain a graphene oxide dispersion liquid;

[0077] S2 Plasma activation: placing the graphene oxide dispersion liquid in a low-temperature plasma reaction cavity, and introducing a mixed gas atmosphere of argon-hydrogen with a volume ratio of 9:1, controlling the vacuum degree to be 15Pa and the plasma power to be 80W, and processing for 12min;

[0078] S3 In-situ reduction of tea polyphenol: adding a tea polyphenol aqueous solution with a concentration of 8mg / mL to the activated graphene oxide dispersion liquid, and the mass ratio of tea polyphenol to graphene oxide is 1:3, and stirring at a constant temperature of 80°C for 4h under nitrogen protection;

[0079] S4 Purification and drying: centrifuging the mixed liquid after reduction at 11000rpm for 18min, and collecting the precipitate; washing the precipitate with deionized water until the pH is 7, and then placing it in a vacuum drying oven at 60°C, and drying in a vacuum degree of 5Pa for 8h, to obtain reduced graphene oxide.

[0080] Preparation of positive electrode material:

[0081] (1) Pretreatment of raw materials: weighing 78 parts of sodium vanadate, 1.5 parts of niobium pentoxide and 3 parts of carbon black by weight, and adding deionized water (solid-liquid ratio 1:7), and stirring for 14min;

[0082] (2) Preparation of dispersion and doped precursor: adding 2.5 parts of polyvinylpyrrolidone to the above mixture, and using agate balls with a diameter of 8mm and dried at 120°C for 2h to ball mill the mixture in a polytetrafluoroethylene ball mill tank at a ball-to-material ratio of 5:1 and a rotation speed of 380rpm for 5.5h, and controlling the slurry temperature to be 30°C during ball milling, to obtain a uniform slurry;

[0083] (3) Pre-sintering: The slurry was subjected to programmed cooling freeze-drying, first 5 ℃ / min to -10 ℃ for 1 h, then 2 ℃ / min to -42 ℃, and vacuum degree 8 Pa for 15 h to obtain dry powder; the dry powder was placed in argon, and heated to 640 ℃ at a rate of 7 ℃ / min, and kept for 1.8 h; then continue to heat at the same rate to 700 ℃, and keep for 1.8 h, and then decrease to below 300 ℃ at a rate of 3 ℃ / min to obtain niobium-doped sodium vanadium phosphate powder;

[0084] (4) Coating: The pre-sintered powder and 5 parts of reduced graphene oxide were added to deionized water to prepare a mixed solution with a solid content of 20wt%, and intermittent ultrasonic treatment was performed at a power of 380W for 5min and then stopped for 1min for 28min to form a coating slurry;

[0085] (5) Second sintering: The coating slurry was spray dried at an inlet temperature of 195 ℃ and an outlet temperature of 88 ℃ to obtain a composite powder; the composite powder was placed in argon and secondarily sintered at 820 ℃ for 7 h, and then naturally cooled to below 200 ℃ at a rate of 2 ℃ / min to obtain the target positive electrode material.

[0086] Comparative Example 1: The difference between this comparative example and Example 1 is that no niobium pentoxide is added, and the rest of the preparation steps and parameters are consistent with Example 1.

[0087] Comparative Example 2: The difference between this comparative example and Example 1 is that no reduced graphene oxide is added, and the rest of the preparation steps and parameters are consistent with Example 1.

[0088] Comparative Example 3: The difference between this comparative example and Example 1 is that the reduced graphene oxide is prepared by a conventional chemical reduction method (without plasma activation treatment), and the rest of the preparation steps and parameters are consistent with Example 1. The conventional chemical reduction method is as follows: graphene oxide is dispersed in deionized water to prepare a dispersion liquid with a concentration of 1.0 mg / mL, hydrazine hydrate is added (mass ratio of graphene oxide to hydrazine hydrate 1:5), and stirred at 80 ℃ for 4 h, and the subsequent purification and drying steps are consistent with S4 in Example 1.

[0089] Test 1:

[0090] Physical property test of positive electrode material;

[0091] BET specific surface area and pore volume test: Referring to GB / T19587-2017, the BET specific surface area and pore volume of the positive electrode materials of Examples 1-3 and Comparative Examples 1-3 were tested by liquid nitrogen adsorption-desorption method at liquid nitrogen temperature (-196 ℃) with nitrogen as adsorbate.

[0092] Tap density test: refer to GB / T5162-2022 "Determination of Apparent and Tap Density of Metal Powders", use tap density tester, load the sample into the measuring cylinder, vibrate according to the specified frequency and times, then calculate the tap density.

[0093] Compacted density test: take an appropriate amount of positive material powder, press it into a round sheet on a tablet press with a pressure of 300 MPa, measure the mass, diameter and thickness of the round sheet, and calculate the compacted density according to the formula "compacted density = mass / (π x radius 2 x thickness)", the results are as follows:

[0094] Table 1

[0095] Sample BET specific surface area (m 2 / g) Pore volume (cm 3 / g) Tap density Example 1 18.2 0.068 1.32 Example 2 15.5 0.055 1.23 Example 3 19.8 0.078 1.38 Comparative Example 1 16.3 0.061 1.15 Comparative Example 2 12.1 0.042 1.08

[0096] As can be seen from Table 1, the BET specific surface area, pore volume, tap density and compacted density of the positive material of Examples 1-3 are excellent, which shows that the component ratio and preparation process of the application can effectively control the physical structure and packing performance of the material. Comparative Example 1 does not add niobium pentoxide, and niobium pentoxide can play a role in regulating the pore structure and improving the particle packing density in the material. The absence of it leads to a decrease in the tap density and compacted density of the material, and the BET specific surface area and pore volume also decrease slightly, affecting the ion transport channel and structural stability of the material; Comparative Example 2 does not add reduced graphene oxide, and the sheet structure of reduced graphene oxide can construct a loose porous network. The absence of it reduces the pore structure of the material, and the BET specific surface area and pore volume decrease significantly, and the adhesion of the particles disappears, resulting in a decrease in the tap density and compacted density; Comparative Example 3 uses the conventional chemical reduction method to prepare reduced graphene oxide, which is not subjected to plasma activation treatment, and the obtained reduced graphene oxide sheet layer has serious agglomeration phenomenon, which cannot effectively construct a porous network, resulting in a decrease in the BET specific surface area, pore volume and packing density of the material, and poor physical properties.

[0097] Test 2: Electrochemical performance test of positive material;

[0098] First charge-discharge efficiency and specific capacity test: the positive electrode material, conductive carbon black and polyvinylidene fluoride (PVDF) were mixed in a mass ratio of 8:1:1, N-methyl pyrrolidone (NMP) was added to prepare a slurry, which was coated on an aluminum foil, vacuum dried at 80°C for 12h, and then cut into a round piece as a positive electrode sheet; with lithium metal as the negative electrode, Celgard2400 as the separator, 1 mol / L LiPF6 (EC / DMC / EMC volume ratio 1:1:1) as the electrolyte, CR2032 button cells were assembled in a glove box. According to GB / T33829-2017, the first charge-discharge test was carried out at room temperature at a rate of 0.1C (1C=117mAh / g) in the voltage range of 2.5-4.3V, and the first charge specific capacity, the first discharge specific capacity were recorded, and the first charge-discharge efficiency (first discharge specific capacity / first charge specific capacity x 100%) was calculated.

[0099] Cycle stability test: the above button cell was cycled at a rate of 1C in the voltage range of 2.5-4.3V for 100 times, the discharge specific capacity of each cycle was recorded, and the capacity retention rate after 100 cycles (100th discharge specific capacity / 1st discharge specific capacity x 100%) was calculated, and the results were as follows:

[0100] Table 2

[0101] Sample Initial charge specific capacity (mAh / g) Initial discharge specific capacity (mAh / g) First charge-discharge efficiency (%) 100-cycle capacity retention rate (%) Example 1 128.5 119.2 92.8 90.5 Example 2 125.3 115.8 92.4 89.2 Example 3 130.2 121.5 93.3 91.8 Comparative Example 1 118.6 105.2 88.7 78.3 Comparative Example 2 110.5 96.8 87.6 72.5

[0102] As can be seen from Table 2, the positive electrode materials of Examples 1-3 exhibit excellent electrochemical performance, which is due to the synergistic effect of the suitable physical structure and composition of the materials: the doped niobium pentoxide optimizes the lattice structure and improves the ion diffusion rate; the reduced graphene oxide constructs a conductive network, reduces the charge transfer resistance, and at the same time relieves the volume expansion during the cycle process. Comparative Example 1 does not add niobium pentoxide, the lattice defects increase, the ion diffusion is hindered, resulting in a decrease in the first charge-discharge specific capacity and efficiency, and the structure is easy to collapse during the cycle process, and the capacity retention rate is significantly reduced; Comparative Example 2 lacks reduced graphene oxide, the material has poor conductivity and low charge transfer efficiency, the first charge-discharge performance is poor, and there is no conductive network support, the active material falls off seriously during the cycle process, and the capacity decays quickly; the reduced graphene oxide of Comparative Example 3 has not been activated by plasma, the sheet layer agglomeration leads to discontinuous conductive network, the conductivity is limitedly improved, and at the same time the material has poor pore structure, the ion transmission is hindered.

[0103] Test 3: positive electrode material conductivity and structure stability test;

[0104] Conductivity test: according to GB / T36373-2018, the positive electrode material powder was pressed into a round piece with a diameter of 10mm and a thickness of 2-3mm on a tablet press with a pressure of 200MPa, and the volume conductivity of the round piece was measured by a four-probe tester.

[0105] Volume expansion rate test: After the assembled button cell was initially charged to 4.3V, the battery volume V1 was measured using Archimedes' displacement method; after 100 cycles, it was charged to 4.3V again, and the battery volume V2 was measured. The volume expansion rate ((V2-V1) / V1×100%) was calculated, and the results are as follows:

[0106] Table 3

[0107] Sample Bulk conductivity (S / cm) Bulk expansion rate after 100 cycles (%) Example 1 1.85 x 10 -2 ]]> 4.2 Example 2 1.52 x 10 -2 ]]> 4.8 Example 3 2.03 x 10 -2 ]] 3.8 Comparative Example 1 0.86 x 10 -2 ]] 7.5 Comparative Example 2 0.52 x 10 -2 ]] 9.3

[0108] As can be seen from Table 3, the volume conductivity of the cathode materials in Examples 1-3 is higher than that of the cathode materials in Examples 1-3. After 100 cycles, the volume expansion rate was less than 5%, indicating that the material possesses good conductivity and structural stability. This is because niobium pentoxide doping introduces electronic active sites, improving electronic conductivity; reduced graphene oxide constructs a continuous conductive network, further reducing resistance; simultaneously, the two work synergistically to suppress volume changes of the active material during cycling, reducing structural damage. Comparative Example 1, without niobium pentoxide, has insufficient electronic active sites, resulting in a significant decrease in volume conductivity and poor lattice structure stability, leading to significant volume expansion during cycling; Comparative Example 2, lacking the conductive network and structural support of reduced graphene oxide, has the lowest conductivity, and the active material is prone to aggregation and volume collapse during cycling, resulting in the highest volume expansion rate; Comparative Example 3, with its reduced graphene oxide, suffers from incomplete conductive network due to aggregation, limiting the improvement in conductivity, while also weakening the suppression of volume expansion and exhibiting poor structural stability.

[0109] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not describe all details exhaustively, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification.

Claims

1. A niobium-doped reduced graphene oxide-coated sodium vanadium phosphate cathode material, characterized in that, It is composed of the following components in parts by weight: 72-78 parts sodium vanadium phosphate, 0.8-1.5 parts niobium pentoxide, 3-5 parts reduced graphene oxide, 1.8-2.5 parts polyvinylpyrrolidone, 0.5-0.8 parts lithium carbonate, and 2-3 parts carbon black; The sodium vanadium phosphate has a particle size of 0.8-1.5 μm; The positive electrode material has a BET specific surface area of ​​15-20 m² / g and a pore volume of 0.05-0.08 cm³ / g.

2. The cathode material according to claim 1, characterized in that, The reduced graphene oxide has a sheet thickness of 1-2 nm and a sheet diameter of 0.8-1.5 μm.

3. The cathode material according to claim 2, characterized in that, The preparation steps of the reduced graphene oxide are as follows: S1 Graphene oxide pretreatment: Graphene oxide is dispersed in deionized water to prepare a dispersion of 0.8-1.2 mg / mL, and sonicated for 15-20 min until there is no obvious agglomeration to obtain a graphene oxide dispersion. The ultrasonic power is 200-250W; S2 plasma activation: Place the graphene oxide dispersion in a low-temperature plasma reaction chamber, introduce an argon-hydrogen mixed atmosphere with a volume ratio of 9:1, control the vacuum degree to 10-15 Pa and the plasma power to 50-80 W, and process for 10-12 min. S3 Tea Polyphenol In-situ Reduction: Add a 5-8 mg / mL tea polyphenol aqueous solution to the activated graphene oxide dispersion. The mass ratio of tea polyphenol to graphene oxide is 1:2-3. Stir at a constant temperature of 60-80℃ for 3-4 hours under nitrogen protection. S4 Purification and Drying: Centrifuge the reduced mixture at 9000-11000 rpm for 12-18 min and collect the precipitate; wash the precipitate with deionized water until pH 6.5-7, and then place it in a vacuum drying oven at 50-60℃ and dry it in a vacuum degree ≤5Pa for 6-8 h to obtain the final product.

4. A method for preparing niobium-doped reduced graphene oxide coated sodium vanadium phosphate cathode material as described in any one of claims 1-3, characterized in that, Includes the following steps: (1) Raw material pretreatment: Sodium vanadium phosphate, niobium pentoxide, and carbon black are mixed in parts by weight and added to a filter that has been filtered through a 0.22 μm filter membrane with a resistivity ≥ Add deionized water and stir for 12-14 minutes. (2) Preparation of dispersion and doping precursor: Add polyvinylpyrrolidone to the mixture obtained in step (1), and ball mill for 4.5-5.5 hours at a speed of 320-380 rpm using agate balls with a diameter of 5-8 mm that have been dried at 120℃ for 2 hours. During the ball milling process, control the slurry temperature at 25-30℃ to obtain a uniform slurry. The ratio of balls to feed is 4-5:1; (3) Pre-sintering: The slurry obtained in step (2) is freeze-dried by programmed cooling to obtain dry powder; the dry powder is placed in argon gas and pre-sintered by segmented heating: the temperature is increased to 630-640℃ at a rate of 6-7℃ / min and held for 1.5-1.8h; then the temperature is increased to 690-700℃ at the same rate and held for 1.5-1.8h to promote lattice reconstruction, and then the temperature is reduced to below 300℃ at a rate of 3℃ / min and naturally cooled to obtain niobium-doped sodium vanadium phosphate powder; (4) Coating: The pre-sintered powder obtained in step (3) and reduced graphene oxide are added to deionized water in parts by weight to prepare a mixture with a solid content of 15-20wt%. The mixture is then subjected to intermittent ultrasonication at 320-380W power for 22-28 minutes to form a coating slurry. (5) Secondary sintering: The coating slurry obtained in step (4) is spray-dried to obtain composite powder; the composite powder is placed in argon gas and sintered at 780-820℃ for 6-7h, and then cooled naturally at 2℃ / min to below 200℃ to obtain the target cathode material.

5. The preparation method according to claim 4, characterized in that, The amount of deionized water added in step (1) satisfies the solid-liquid ratio of 1:6-1:

7.

6. The preparation method according to claim 4, characterized in that, The agate ball mentioned in step (2) has a purity of ≥99.9%, and the grinding jar is made of polytetrafluoroethylene.

7. The preparation method according to claim 4, characterized in that, The freeze-drying process described in step (3) involves first reducing the temperature at 5℃ / min to -10℃ and holding it for 1 hour, then reducing the temperature at 2℃ / min to -45~-42℃ and holding it under a vacuum of 5-8Pa for 13-15 hours.

8. The preparation method according to claim 4, characterized in that, The intermittent ultrasound in step (4) is performed by pausing for 1 minute after every 5 minutes of ultrasound.

9. The preparation method according to claim 4, characterized in that, The coating slurry described in step (5) is spray-dried at an inlet air temperature of 185-195℃ and an outlet air temperature of 85-88℃.

10. The preparation method according to claim 4, characterized in that, After the secondary sintering in step (5), the tap density of the target cathode material is 1.2-1.4 g / cm³, and the compaction density is 2.8-3.0 g / cm³.

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