A high-performance sodium iron sulfate polyanion cathode material and its preparation method

Abstract

The present invention provides a high-performance sodium iron sulfate polyanion cathode material and a preparation method thereof. By using spray drying technology, a linear carbon source treated with a hydrophilic anionic dispersant is prepared into porous carbon sphere particles with a porous structure and a three-dimensional network distribution. Then, such porous carbon sphere particles are added to a mixed solution containing sodium ions, iron ions and sulfate ions. The porous structure of the porous carbon sphere particles is used to adsorb the mixed solution inside the carbon spheres. Through drying and sintering processes, a high-performance sodium iron sulfate polyanion cathode material with sodium iron sulfate in-situ grown on the porous carbon sphere particles is finally obtained. This material has a uniformly distributed conductive layer, good electrochemical performance and stability, effectively alleviates the disadvantages in the prior art that the sodium iron sulfate material has poor conductivity and low sintering temperature, which is not conducive to in-situ sintering and coating of the carbon source, and has a significant promoting effect on the application of the sodium iron sulfate material in batteries.

Description

Technical Field

[0001] The present invention belongs to the technical field of battery materials, and particularly relates to a high-performance sodium iron sulfate polyanion cathode material and a preparation method thereof. Background Art

[0002] In cathode materials, the carbon source, as a conductive agent, has a decisive influence on the full play of material properties. Due to the poor inherent conductivity of sodium iron sulfate materials and the low decomposition temperature of sulfate radicals, the sintering temperature is low, and in-situ carbon source coating cannot be achieved. Currently, the preparation method of sodium iron sulfate cathode materials usually adopts dry mixing of sodium salts, iron salts, and carbon nanotubes followed by sintering. However, in subsequent characterization, it is found that the dispersion effect of carbon nanotubes in the material is poor, and there is an obvious carbon nanotube aggregation phenomenon.

[0003] However, in the traditional dry mixing process, although attempts can be made to disperse nanocarbon materials through long-term ball milling, a large amount of aggregation will inevitably still occur, which leads to a decrease in battery capacity.

[0004] In addition, when preparing NFS by spray drying method, due to the mixing of multiple materials, the viscosity of the slurry is high. To ensure the smooth progress of the spray process, the solid content is usually low (about 25%), and the content of carbon nanotubes in the aqueous solution is generally about 1%. This results in low production efficiency, increased energy consumption, and thus increased costs. At the same time, the particle size distribution of the spray product is uneven, with small particle sizes reaching several micrometers and large particle sizes exceeding 50 micrometers, which will lead to poor product consistency. Patent CN117720132A proposes to use H 3 BO 3 、B 2 O 3 or other boron-containing low-melting glass powders as sintering aids to achieve low-temperature solid-phase sintering of the material. At the same time, water-soluble organic substances such as ascorbic acid, citric acid, glucose, etc. are used as carbon sources. Due to the low sintering temperature of sodium iron sulfate, the organic carbon source cannot be fully carbonized, resulting in poor conductivity of the prepared material. Adding boron-containing glass powder will introduce boron impurities and affect the purity of the material. The preparation method of the modified sodium iron sulfate cathode material proposed in CN118117067A first disperses the carbon source in water to obtain a carbon source dispersion liquid; then mixes it with ascorbic acid, ferrous source, sodium source, sulfur source, and doped alkali metal source to obtain a suspension; then performs spray drying granulation to obtain a precursor powder. This method fails to effectively disperse the carbon source and only adds the carbon source to water for ultrasonic dispersion, unable to achieve effective dispersion of the carbon source.

[0005] Therefore, how to effectively disperse the carbon source and prepare a sodium iron sulfate cathode material with a continuous and uniform carbon conductive layer is of research significance for improving the conductivity and consistency of the material. Summary of the Invention

[0006] In view of the problems in the prior art, the present invention discloses a high-performance sodium iron sulfate polyanion cathode material and a preparation method thereof. A linear carbon source is selected and the spray drying technology is utilized to prepare porous carbon sphere particles with a porous structure and a three-dimensional network distribution; subsequently, a mixed solution containing sodium ions, iron ions and sulfate ions is adsorbed inside the carbon spheres through their porous structure; the prepared high-performance sodium iron sulfate polyanion cathode material has a composite porous structure with a uniformly distributed conductive network containing sodium iron sulfate active material, and has good electrochemical performance and stability.

[0007] The present invention is realized through the following technical solutions:

[0008] In the first aspect, the present invention provides a high-performance sodium iron sulfate polyanion cathode material; the material has a porous spherical structure; the particle size of the material is 10-160 nm; the pore volume is 0.010-0.040 cm 3 / g; the capacity retention rate after 100 cycles of 1C charge and discharge is 96-99.8%.

[0009] As a further scheme, the material satisfies (47.0 - 0.21*AD)*(100 ± 8.66)% = PV*10^ 3 ; where AD represents the particle size of the high-performance sodium iron sulfate polyanion cathode material, and PV represents the pore volume of the high-performance sodium iron sulfate polyanion cathode material.

[0010] The material of the present invention uses porous carbon spheres composed of linear carbon as the skeleton, and high-performance sodium iron sulfate polyanion cathode material is prepared by in-situ growth of sodium iron sulfate on its skeleton.

[0011] In the second aspect, the present invention provides a preparation method of the high-performance sodium iron sulfate polyanion cathode material described in the first aspect; the preparation method includes:

[0012] S1: Mix a linear carbon source, water and a hydrophilic anionic dispersant to obtain a carbon source slurry;

[0013] S2: Spray-dry the carbon source slurry described in S1 to prepare porous carbon sphere particles;

[0014] S3: Add the porous carbon sphere particles described in S2 into a mixed solution A containing sodium ions, iron ions and sulfate ions to obtain a mixed solution B; make the sodium ions, iron ions and sulfate ions adsorb on the porous carbon sphere particles;

[0015] S4: Under an inert atmosphere, subject the mixed solution B to heat-up drying treatment and pulverize to obtain a sodium iron sulfate polyanion precursor material;

[0016] S5: Under an inert atmosphere, sinter the sodium iron sulfate polyanion precursor material described in S4 to obtain a high-performance sodium iron sulfate polyanion cathode material with sodium iron sulfate in-situ grown on porous carbon sphere particles.

[0017] As a further solution, the linear carbon source in S1 includes one or more of carbon fibers and carbon nanotubes.

[0018] As a further solution, the diameter of the linear carbon source in S1 is 5 - 100 nanometers; the aspect ratio is greater than 5.

[0019] Further, the diameter of the linear carbon source in S1 is preferably selected from 10 - 20 nanometers.

[0020] As a further solution, the types of hydrophilic anionic dispersants in S1 are selected from one or more of sulfate anionic dispersants and sulfonate anionic dispersants;

[0021] Further, the sulfate anionic dispersants are selected from one or more of sodium dodecyl sulfate and sodium laureth sulfate;

[0022] Further, the sulfonate anionic dispersants are selected from one or more of sodium dodecyl sulfonate and sodium styrene sulfonate.

[0023] As a further solution, the solid content in the carbon source slurry is 2 - 5%; the mass of the hydrophilic anionic dispersant accounts for the range of 0.5 - 5% of the mass of the linear carbon source.

[0024] As a further solution, the sources of sodium ions, iron ions, and sulfate ions in the mixed solution A of S3 are selected from the combination of sodium sulfate sources and iron sulfate sources.

[0025] Further, the sodium sulfate sources are selected from one or more of sodium sulfate, anhydrous sodium sulfate, sodium thiosulfate, and sodium sulfate heptahydrate.

[0026] Further, the iron sulfate sources are selected from one or more of ferric sulfate, anhydrous ferrous sulfate, ferrous sulfate monohydrate, and ferrous sulfate heptahydrate.

[0027] As a further solution, the molar concentration ratio of Na2:Fe in the mixed solution A of S3 is selected from 0.5 - 1; the particle size range of the porous carbon sphere particles in S2 is 5 - 10 μm; the proportion of the porous carbon sphere particles in the total mass of the mixed solution B in S3 is 2 - 5%.

[0028] As a further solution, the outlet temperature of spray drying in S2 is selected from 90 - 160 degrees; the inlet temperature of spray drying is selected from 200 - 300 degrees.

[0029] As a further embodiment, in the heating and drying treatment of S4, the heating rate is selected from 10-20 o °C / min; the drying temperature is 100-130 o °C; the drying time is selected from 0.5-1 h.

[0030] As a further embodiment, the particle size range of the sodium iron sulfate polyanion precursor material in S4 is 5-20 μm.

[0031] As a further embodiment, the sintering treatment temperature in S5 is 350-400 o °C; the sintering treatment time is selected from 10-16 h.

[0032] As a further embodiment, the mixing method in S1 is selected from ultrasonic dispersion; the ultrasonic dispersion power is selected from 600-2000 W; the ultrasonic dispersion time is selected from 30-120 min.

[0033] As a further embodiment, after the dispersion treatment in S1, a shearing treatment is further performed; the shearing power is 1500-2500 W, and the shearing treatment time is 15-60 min.

[0034] As a further embodiment, the mixed solution B in S3 is also subjected to ultrasonic dispersion treatment; the ultrasonic dispersion power is selected from 600-2000 W; the ultrasonic dispersion time is selected from 30-120 min.

[0035] As a further embodiment, the inert atmosphere in S4 and S5 includes one or more of a nitrogen atmosphere and a helium atmosphere in a compound form.

[0036] In a third aspect, the present invention provides a positive electrode plate; the positive electrode plate includes the high-performance sodium iron sulfate polyanion positive electrode material described in the first aspect or the high-performance sodium iron sulfate polyanion positive electrode material obtained by the preparation method described in the second aspect.

[0037] The present invention does not particularly limit the type of the conductive agent, as long as it can enhance the conductivity of the positive electrode and does not adversely affect the performance of the high-performance sodium iron sulfate polyanion positive electrode material. Those skilled in the art can select the conductive agents commonly used in the art according to actual needs. As some specific examples, the conductive agents used for preparing the positive electrode plate can be selected from carbon blacks such as acetylene black and conductive carbon black, carbon nanotubes, graphene, graphite, conductive polymers such as polyaniline and polythiophene, metal oxide conductive agents such as tin oxide and zinc oxide, and carbon nanofiber conductive agents.

[0038] The present invention does not particularly limit the type of binder, as long as it can enhance the adhesion between the positive electrode active material particles and does not adversely affect the performance of the high-performance sodium iron sulfate polyanion positive electrode material. Those skilled in the art can make selections according to actual needs. As some specific examples, the binders used to prepare the positive electrode sheet include polyvinylidene fluoride, polyvinylidene difluoride, polytetrafluoroethylene, polyvinyl alcohol, polyimide, polystyrene sulfonic acid, polyacrylonitrile, polycarbonate, or polyetherimide ketone, etc.

[0039] Fourthly, the present invention provides an electrode assembly, a battery cell, and an energy storage device including the high-performance sodium iron sulfate polyanion positive electrode material described in the first aspect or the high-performance sodium iron sulfate polyanion positive electrode material obtained by the preparation method described in the second aspect.

[0040] The characteristics and beneficial effects of the present invention are as follows:

[0041] (1) By adopting the spray drying technology, the present invention prepares porous carbon sphere particles with a porous structure and a three-dimensional network distribution from a linear carbon source dispersed by a hydrophilic anionic dispersant; then adds such porous carbon sphere particles to a mixed solution containing sodium ions, iron ions, and sulfate ions; utilizes the porous structure of the porous carbon sphere particles to adsorb the mixed solution inside the carbon spheres; and finally obtains a high-performance sodium iron sulfate polyanion positive electrode material with sodium iron sulfate in-situ grown on the porous carbon sphere particles through drying and sintering processes; this material has a uniformly distributed conductive layer; and has good electrochemical performance and stability; it provides a good reference value for the application and development of sodium iron sulfate materials.

[0042] (2) The present invention modifies the linear carbon source with a hydrophilic anionic dispersant, which can better inhibit the agglomeration phenomenon of the linear carbon source during the preparation process; the multiple hydrophilic groups and anionic characteristics of this dispersant enable it to maintain better dispersing ability in an acidic environment and play a good dispersing role in the preparation process of the high-performance sodium iron sulfate polyanion positive electrode material. Description of the Drawings

[0043] In order to more clearly illustrate the technical solutions in the embodiments of the present invention, the following will briefly introduce the drawings required for use in the embodiments or the description of the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0044] Figure 1 It is the XRD pattern of the products prepared in Example 1, Example 2, and Comparative Example 1 of the present invention; among them, PDF#21-1360 is the standard curve of sodium iron sulfate (NFS).

[0045] Figure 2 It is the surface SEM image of the product prepared in Example 1 of the present invention;

[0046] Figure 3 It is the cross-sectional SEM image of the product prepared in Example 1 of the present invention;

[0047] Figure 4 It is the surface SEM image of the product prepared in Comparative Example 1 of the present invention;

[0048] Figure 5 It is the surface SEM image of the product prepared in Comparative Example 2 of the present invention;

[0049] Figure 6 It is the pore volume-average particle size comparison chart of the original carbon nanotubes and porous carbon sphere particles prepared in Example 1 of the present invention;

[0050] Figure 7 It is the pore volume-average particle size comparison chart of the products prepared in Example 1 and Comparative Example 2 of the present invention. Detailed implementation manners

[0051] To facilitate the understanding of the present invention, the present invention will be described more comprehensively below. Embodiments of the present invention are given, but the scope of the present invention is not limited thereby.

[0052] In the first aspect, the present invention provides a preparation method of a high-performance sodium iron sulfate polyanion cathode material; the preparation method includes:

[0053] S1: Mix a linear carbon source, water, and a hydrophilic anionic dispersant to obtain a carbon source slurry;

[0054] S2: Spray-dry the carbon source slurry obtained in S1 to prepare porous carbon sphere particles;

[0055] S3: Add the porous carbon sphere particles obtained in S2 to a mixed solution A containing sodium ions, iron ions, and sulfate ions to obtain a mixed solution B; adsorb the sodium ions, iron ions, and sulfate ions on the porous carbon sphere particles;

[0056] S4: Under an inert atmosphere, subject the mixed solution B to a temperature-raising drying treatment and pulverize it to obtain a sodium iron sulfate polyanion precursor material;

[0057] S5: Under an inert atmosphere, sinter the sodium iron sulfate polyanion precursor material obtained in S4 to obtain a high-performance sodium iron sulfate polyanion cathode material with sodium iron sulfate in-situ grown on the porous carbon sphere particles.

[0058] In view of the disadvantages of the sodium iron sulfate material in the prior art, such as poor electrical conductivity and low sintering temperature, which are not conducive to in-situ sintering of the coated carbon source, the present invention has redesigned the process route. A hydrophilic anionic dispersant is selected to disperse the linear carbon source, and then the linear carbon source is prepared into porous carbon sphere particles with a porous structure and a three-dimensional network distribution by spray drying. Then, it is added to the mixed solution A containing sodium ions, iron ions, and sulfate ions. The porous structure of the porous carbon sphere particles is used to adsorb the mixed solution A inside the carbon spheres. After drying and sintering, a high-performance sodium iron sulfate polyanionic cathode material with sodium iron sulfate growing in-situ on the porous carbon sphere particles is prepared.

[0059] The present invention selects to use linear carbon to improve the electrical conductivity of the sodium iron sulfate material. This is because linear carbon has a higher specific surface area, high electrical conductivity, and good stability compared to other types of carbon sources, and is more inclined to form a stable porous structure and cross-link and lap to form a three-dimensional network distribution during the preparation process. This is to fully adsorb and mix with the mixed solution A containing sodium ions, iron ions, and sulfate ions subsequently to prepare a high-performance sodium iron sulfate polyanionic cathode material with a uniform conductive layer.

[0060] In addition, in order to ensure good dispersibility of the carbon source, the present invention modifies the linear carbon source with a hydrophilic anionic dispersant. On the one hand, the hydrophilic anionic dispersant has more hydrophilic groups, which helps to disperse the linear carbon source. When preparing the porous carbon sphere particles, the droplets generated by spray drying are quickly dried, and the linear carbon modified by the hydrophilic anionic dispersant can still remain dispersed during the drying process, avoiding agglomeration, and cross-linking and lapping to form a uniform three-dimensional network distribution. On the other hand, its anions help to interact with hydrogen ions in an acidic environment to generate electrostatic repulsion, which helps the dispersant molecules to remain stable in the solution. The hydrophilic groups interact with water molecules, which can further improve the solubility and stability of the dispersant in the acidic solution. Therefore, the modified porous carbon sphere particles can stably play their dispersing role in the acidic environment of the mixed solution A containing sodium ions, iron ions, and sulfate ions, prevent agglomeration between the porous carbon sphere particles, and help to adsorb the mixed solution A to form a uniform and continuous conductive layer. Moreover, when preparing the mixed solution A, the present invention also selects to use the mixed solution A containing sodium ions, iron ions, and sulfate ions. The selection of this type of mixed solution A helps to avoid introducing impurity ions and ensure the purity of the prepared sodium iron sulfate polyanionic cathode material.

[0061] The preparation method of the present invention can not only ensure the uniform dispersion of linear carbon, but also promote the in-situ growth of sodium iron sulfate on the porous carbon sphere particles to prepare a high-performance sodium iron sulfate polyanionic cathode material by adsorbing the mixed solution A inside the porous carbon sphere particles and then drying and sintering.

[0062] As a further solution, the linear carbon source in S1 includes one or more of carbon fiber and carbon nanotube.

[0063] As a further solution, the diameter of the linear carbon source in S1 is 5 - 100 nanometers; the aspect ratio is greater than 5.

[0064] Furthermore, the diameter of the linear carbon source in S1 is preferably selected from 10 - 20 nanometers.

[0065] As a further solution, the types of hydrophilic anionic dispersants in S1 are selected from one or more of sulfate - type anionic dispersants and sulfonate - type anionic dispersants;

[0066] The hydrophilic anionic dispersant of the present invention contains hydrophilic groups, which can improve the dispersibility of the linear carbon source. When the spray - drying technology is used to prepare porous carbon sphere particles, the modified linear carbon can maintain a dispersed state, avoid particle agglomeration, and ensure that the particles form a uniform three - dimensional network structure. The anions in the dispersant interact with hydrogen ions in an acidic environment to generate electrostatic repulsion, maintaining the stability of the dispersant molecules in the solution. The hydrophilic groups interact with water molecules, enhancing the solubility and stability of the dispersant in the acidic solution. Therefore, this dispersant helps the dispersion of the linear carbon source, enables the porous carbon sphere particles to uniformly adsorb the mixed solution A, forms a uniform conductive layer, and prepares a sodium iron sulfate polyanion cathode material with excellent electrochemical performance.

[0067] Furthermore, the sulfate - type anionic dispersant is selected from one or more of sodium dodecyl sulfate and sodium lauryl polyoxyethylene ether sulfate;

[0068] Furthermore, the sulfonate - type anionic dispersant is selected from one or more of sodium dodecyl sulfonate and sodium styrene sulfonate.

[0069] As a further solution, the solid content in the carbon source slurry is 2-5%; the mass of the hydrophilic anionic dispersant accounts for the range of 0.5-5% of the mass of the linear carbon source. The appropriate solid content in the carbon source slurry helps to form uniform porous carbon sphere particles during the spray drying process, avoiding incomplete particles caused by too little solid content or nozzle blockage caused by too much solid content. At the same time, when the mass ratio of the hydrophilic anionic dispersant to the linear carbon source is within an appropriate range, the dispersibility and stability of the porous carbon sphere particles are ensured. Preventing too little content of the hydrophilic anionic dispersant from resulting in poor dispersion effect of the linear carbon source and causing agglomeration, and too much content affecting the viscosity of the slurry, resulting in discontinuous and uneven formation of porous carbon sphere particles during the spray drying process, which is not conducive to cross-linking and forming a stable three-dimensional network structure. By precisely controlling these parameters, it can be ensured that the carbon source particles maintain a good dispersion state during the spray drying process, avoiding agglomeration phenomena, so as to prepare porous carbon sphere particles with a uniform pore structure and appropriate particle size; at the same time, it is convenient to uniformly adsorb sodium ions, iron ions, and sulfate ions subsequently, ensuring the high-performance sodium iron sulfate polyanionic cathode material in which sodium iron sulfate grows in-situ on the porous carbon sphere particles.

[0070] As a further solution, the sources of sodium ions, iron ions, and sulfate ions in the S3 mixed solution A are selected from the combination of sodium sulfate sources and iron sulfate sources.

[0071] Furthermore, the sodium sulfate sources are selected from one or more of sodium sulfate, anhydrous sodium sulfate, sodium thiosulfate, and sodium sulfate heptahydrate.

[0072] Furthermore, the iron sulfate sources are selected from one or more of iron sulfate, anhydrous ferrous sulfate, ferrous sulfate monohydrate, and ferrous sulfate heptahydrate.

[0073] As a further solution, the molar concentration ratio of Na2:Fe in the S3 mixed solution A is selected from 0.5-1; the particle size range of the porous carbon sphere particles in the S2 is 5-10 μm; the proportion of the mass of the porous carbon sphere particles in the S3 in the total mass of the mixed solution B is 2-5%.

[0074] Porous carbon sphere particles with a suitable particle size range obtained by spray drying help to form a uniform internal pore structure, thus facilitating the smooth adsorption of appropriate concentrations of sodium atoms, iron atoms, and sulfate ions into the internal pores of the porous carbon sphere particles; in addition, we also adjusted the mass ratio of the porous carbon sphere particles in the mixed solution B; the porous carbon sphere particles with a suitable mass ratio and particle size range help sodium iron sulfate to uniformly adhere to the porous carbon sphere particles and grow, thus forming a uniform conductive layer. In addition, by controlling the mass ratio of the porous carbon sphere particles, it is possible to effectively avoid the problem of an overly thick conductive layer caused by too many porous carbon sphere particles or the inability to form a continuous conduction path due to too few porous carbon sphere particles. This uniform conductive layer has an advantageous effect on improving the overall electrochemical performance of the sodium iron sulfate material.

[0075] As a further solution, in step S2, the outlet temperature of the spray drying is selected from 90 - 160 degrees Celsius; the inlet temperature of the spray drying is selected from 200 - 300 degrees Celsius. By selecting a suitable range of inlet and outlet temperatures for spray drying, it helps to promote the drying rate and form good and stable porous carbon sphere particles; when the inlet and outlet temperatures are set too high, it is easy to damage the structure of the dispersant and the linear carbon material, which is not conducive to the subsequent preparation of a stable and uniform pore structure; while when the inlet and outlet temperatures are set too low, the carbon source slurry will cause caking and block the spray drying outlet during the spray drying process, resulting in an unstable carbon conductive layer during the adsorption and sintering process with the mixed solution A, and the sodium iron sulfate solution is not conducive to the stable adsorption and growth of the porous carbon sphere particles, resulting in a non-uniform conductive layer, thus unable to effectively improve the poor electrical conductivity of the sodium iron sulfate material.

[0076] As a further solution, in the heating and drying treatment of step S4, the heating rate is selected from 10 - 20 o °C / min; the drying temperature is 100 - 130 o °C; the drying time is selected from 0.5 - 1 h. First, drying the porous carbon sphere particles adsorbed with the mixed solution A at a suitable temperature helps to reduce the thermal shock caused by the rapid evaporation of water during the subsequent high-temperature sintering process. This thermal shock may cause the sodium iron sulfate solution to not grow smoothly along the porous carbon sphere particles; resulting in damage or non-uniformity of the material structure; therefore, by selecting a suitable drying temperature, it helps to ensure that the material has a relatively uniform physical state before entering the high-temperature sintering stage, forming a uniform conductive layer during the subsequent sintering process and improving the overall performance of the material.

[0077] As a further solution, the particle size range of the sodium iron sulfate polyanion precursor material in step S4 is 5 - 20 μm.

[0078] As a further solution, the sintering temperature in step S5 is 350 - 400 oC; The sintering treatment time is selected from 10 - 16 h. By means of a reasonable sintering temperature and time, it can be ensured that the sodium iron sulfate material has a complete and uniform carbon conductive layer; it is avoided that too high a sintering temperature causes the generation of defects in the crystal structure of sodium iron sulfate, damaging the adsorption stability with the carbon structure; it is not conducive to the full combination with the carbon material, and a high-performance sodium iron sulfate polyanion cathode material is prepared by sintering; and it has a complete and continuous carbon conductive layer coating.

[0079] As a further solution, the mixing method in S1 is selected from ultrasonic dispersion; the ultrasonic dispersion power is selected from 600 - 2000 W; the ultrasonic dispersion time is selected from 30 - 120 min. Ultrasonic dispersion in S1 helps to fully mix the linear carbon source and water, ensuring the uniformity of the carbon source slurry. The high-frequency vibration of ultrasonic waves can break the agglomeration between carbon source particles, making the carbon source particles more uniformly dispersed in water, so as to form more uniform porous carbon sphere particles in the subsequent spray drying process. In addition, the reasonable selection of the power and time of ultrasonic dispersion can further optimize the particle size distribution of the porous carbon sphere particles, making it more in line with the preparation requirements of the high-performance sodium iron sulfate polyanion cathode material.

[0080] As a further solution, after the dispersion treatment in S1, there is also a shearing treatment; the shearing power is 1500 - 2500 W, and the shearing treatment time is 15 - 60 min.

[0081] The shearing treatment helps the uniform dispersion of linear carbon in the solution, thereby improving the uniformity and consistency of the porous carbon sphere particles after spray drying; at the same time, the shearing treatment is beneficial to form a richer pore structure after spray drying; improving the adsorption strength of the subsequent mixed solution A containing sodium ions, iron ions, and sulfate ions; and further preparing a high-performance sodium iron sulfate polyanion cathode material with a uniform conductive layer.

[0082] As a further solution, the mixed solution B in S3 also undergoes ultrasonic dispersion treatment; the ultrasonic dispersion power is selected from 600 - 2000 W; the ultrasonic dispersion time is selected from 30 - 120 min. By using the ultrasonic dispersion method in S3, the Na ions, Fe ions, and sulfate ions in the solution are uniformly dispersed, which helps to adsorb into the pores of the porous carbon sphere particles for sintering to prepare a uniform and stable conductive layer, thereby preparing a high-performance sodium iron sulfate polyanion cathode material.

[0083] As a further solution, the inert atmosphere in S4 and S5 includes one or more of a nitrogen atmosphere and a helium atmosphere in a compound form.

[0084] Second aspect, the present invention provides a high-performance sodium iron sulfate polyanion cathode material obtained by the preparation method described in the first aspect; the material is a porous spherical structure; the particle size of the material is 10 - 160 nm; the pore volume is 0.010 - 0.040 cm 3 / g; the capacity retention rate after 100 cycles of 1C charge and discharge is 96 - 99.8%.

[0085] As a further scheme, the material satisfies (47.0 - 0.21*AD)*(100 ± 8.66)% = PV*10^ 3 ; where AD represents the average particle size of the high-performance sodium iron sulfate polyanion cathode material, and PV represents the pore volume of the high-performance sodium iron sulfate polyanion cathode material.

[0086] The material of the present invention uses a porous carbon sphere composed of linear carbon as the skeleton, and a high-performance sodium iron sulfate polyanion cathode material is prepared by in-situ growth of sodium iron sulfate on its skeleton.

[0087] Third aspect, the present invention provides a positive electrode sheet; the positive electrode sheet includes the high-performance sodium iron sulfate polyanion cathode material described in the second aspect or the high-performance sodium iron sulfate polyanion cathode material obtained by the preparation method described in the first aspect.

[0088] The present invention does not particularly limit the type of conductive agent as long as it can enhance the conductivity of the positive electrode and does not adversely affect the performance of the high-performance sodium iron sulfate polyanion cathode material. Those skilled in the art can select the conductive agents commonly used in the art according to actual needs. As some specific examples, the conductive agents used to prepare the positive electrode sheet can be selected from carbon blacks such as acetylene black and conductive carbon black, carbon nanotubes, graphene, graphite, conductive polymers such as polyaniline and polythiophene, metal oxide conductive agents such as tin oxide and zinc oxide, and carbon nanofiber conductive agents, etc.

[0089] The present invention does not particularly limit the type of binder as long as it can enhance the adhesion between the positive electrode active material particles and does not adversely affect the performance of the high-performance sodium iron sulfate polyanion cathode material. Those skilled in the art can select according to actual needs. As some specific examples, the binders used to prepare the positive electrode sheet include polyvinylidene fluoride, polyvinylidene difluoride, polytetrafluoroethylene, polyvinyl alcohol, polyimide, polystyrene sulfonic acid, polyacrylonitrile, polycarbonate, or polyetherimide ketone, etc.

[0090] Fourth aspect, the present invention provides an electrode assembly, a battery cell, and an energy storage device including the high-performance sodium iron sulfate polyanion cathode material described in the second aspect or the high-performance sodium iron sulfate polyanion cathode material obtained by the preparation method described in the first aspect.

[0091] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present application without creative efforts belong to the scope of protection of the present application.

[0092] The chemical raw materials involved in the following examples and comparative examples are all prior art and obtained through commercial purchase. The experimental devices, testing devices, etc. involved in the following examples and comparative examples are all conventional devices in the art, without special requirements and limitations.

[0093] Example 1

[0094] S1: Add 4 grams of carbon nanotubes with a diameter of 15 ± 2 nm and an aspect ratio of 7 ± 2 nm, 79.8 grams of water, and 0.2 grams of sodium dodecyl sulfate into a container; perform ultrasonic dispersion, set the ultrasonic dispersion power to 800 W, and the ultrasonic dispersion time to 60 min; then use an emulsifier to perform shearing treatment at a shearing power of 2000 W and a shearing time of 15 min; stir evenly to obtain a carbon source slurry; wherein, the solid content in the carbon source slurry is adjusted to 5%, and the mass ratio of sodium dodecyl sulfate to carbon nanotubes is 5%.

[0095] S2: Spray-dry the carbon source slurry obtained in S1 to prepare porous carbon sphere particles with a fan of 90 CFM. Set the outlet temperature of the spray drying to 120 °C and the inlet temperature to 250 °C. Control the particle size range of the obtained porous carbon sphere particles within 7 μm.

[0096] S3: Add 17 grams of anhydrous sodium sulfate and 30 grams of ferrous sulfate into 50 grams of water, and stir at a speed of 500 rpm / min at 50 °C for 60 min to obtain a mixed solution A. The molar concentration ratio of Na2:Fe in the mixed solution A is 0.6. The pH value of the mixed solution A is about 4.

[0097] S3: Add 2 grams of the porous carbon sphere particles obtained in S2 into the mixed solution A in S3, and perform dispersion treatment by ultrasonic stirring at a power of 2000 W for 120 min to obtain a mixed solution B. Among them, the mass ratio of the porous carbon sphere particles to the total mass of the mixed solution B is 2%.

[0098] S4: Under a nitrogen atmosphere, the temperature curve of the tubular furnace is heated to 120 o °C at a rate of 15 o °C / min, hold for 1 h, and perform heating and drying treatment on the mixed solution B. Then perform mechanical crushing; the crushing particle size is 5 μm; to obtain a sodium iron sulfate polyanion precursor material.

[0099] S5: Continuing under a nitrogen atmosphere, place the sodium iron sulfate polyanion precursor material obtained in S4 into a crucible, heat it to 370 °C at a rate of 10 °C / min, and perform pressure sintering for 12 hours to finally obtain a high-performance sodium iron sulfate polyanion cathode material.

[0100] Energy consumption calculation:

[0101] To obtain 1 ton of sodium iron sulfate - CNT composite material, 40 kg of carbon nanotubes, 10 kg of dispersant, and 760 kg of water are required for spray drying. 760 kg of water needs to be evaporated to obtain 40 kg of porous carbon sphere particles. After mixing 40 kg of CNT spheres with 1920 kg of an aqueous solution containing 50% NFS salt and then drying, only 800 * 95% + 1920 * 50% = 1.72 tons of water needs to be evaporated.

[0102] Example 2

[0103] The specific steps and preparation method are the same as those in Example 1, except that: sodium dodecyl sulfate dispersant is used.

[0104] Example 3

[0105] The specific steps and preparation method are the same as those in Example 1, except that: the solid content in the carbon source slurry is adjusted to 4%, and the mass ratio of sodium dodecyl sulfate to carbon nanotubes is 1%.

[0106] Example 4

[0107] The specific steps and preparation method are the same as those in Example 1, except that: the proportion of the mass of the porous carbon sphere particles in the total mass of the mixed solution B is 3%.

[0108] Example 5

[0109] The specific steps and preparation method are the same as those in Example 1, except that: the proportion of the mass of the porous carbon sphere particles in the total mass of the mixed solution B is 5%.

[0110] Example 6

[0111] The specific steps and preparation method are the same as those in Example 1, except that: in S1, the ultrasonic dispersion power is set to 600 W and the ultrasonic dispersion time is set to 40 min.

[0112] Example 7

[0113] The specific steps and preparation method are the same as those in Example 1, except that: the outlet temperature of spray drying is set to 90 °C and the inlet temperature is set to 200 °C. The particle size range of the obtained porous carbon sphere particles is controlled within 10 μm.

[0114] Example 8

[0115] The specific steps and preparation method are the same as those in Example 1, except that: in S4, under a nitrogen atmosphere, the temperature curve of the tubular furnace is increased to 100 o °C at a rate of 10 o °C / min, held for 1 h, and the mixed solution B is subjected to heating and drying treatment.

[0116] Example 9

[0117] The specific steps and preparation method are the same as those in Example 1, except that: in S5, under a nitrogen atmosphere, the sodium iron sulfate polyanion precursor material obtained in S4 is placed in a crucible, heated to 400

[0118] Comparative Example 1

[0119] The specific steps and preparation method are the same as those in Example 1, except that: a polyvinylpyrrolidone dispersant is used.

[0120] Comparative Example 2

[0121] S1: Add 4 g of carbon nanotubes with a diameter of 15 ± 2 nm and an aspect ratio of 6 ± 2 nm, 79.8 g of water, 0.2 g of sodium dodecyl sulfate, 17 g of anhydrous sodium sulfate, and 30 g of ferrous sulfate into a container; perform ultrasonic dispersion, set the ultrasonic dispersion power to 800 W, and set the ultrasonic dispersion time to 60 min; obtain a mixed slurry;

[0122] S2: Spray-dry the mixed slurry obtained in S1, and use a blower of 90 CFM to prepare porous carbon sphere particles. Set the outlet temperature of the spray drying to 120 °C and the inlet temperature to 250 °C. Obtain particles with a particle size of 5 μm;

[0123] S3: Directly increase the temperature of the particles described in S2 to 370 o °C at a rate of 15 °C / min, and perform pressure sintering for 12 h to obtain a sodium iron sulfate polyanion cathode material.

[0124] Energy consumption calculation:

[0125] In order to obtain 1 ton of sodium iron sulfate-CNT composite material, a slurry of 1% CNT, 24% NFS salt, and 75% water is spray-dried. It is necessary to mix 40 kg of carbon nanotubes, 10 kg of dispersant, 1920 kg of NFS salt, and 3 tons of water, and 3 tons of water needs to be evaporated during spray drying.

[0126] Comparative Example 3

[0127] The specific steps and preparation method are the same as those in Example 1, except that: in S3, the mass ratio of the porous carbon sphere particles to the total mass of the mixed solution B is 10%.

[0128] Comparative Example 4

[0129] The specific steps and preparation method are the same as those in Example 1, except that in S3, the mass ratio of the porous carbon sphere particles to the total mass of the mixed solution B is 1%.

[0130] Comparative Example 5

[0131] The specific steps and preparation method are the same as those in Example 1, except that the outlet temperature of spray drying is set at 60 °C and the inlet temperature is set at 100 °C. The particle size range of the obtained porous carbon sphere particles is 20 ± 5 μm.

[0132] Comparative Example 6

[0133] The specific steps of S1 - S3 and the preparation method are the same as those in Example 1, except that the mixed solution B is directly heated to 370 °C at a rate of 15 o C / min and pressure sintered for 12 hours to obtain a sodium iron sulfate polyanion cathode material.

[0134] Specific test methods and conditions

[0135] Sodium iron sulfate, carbon black, and PVDF are mixed evenly in a mass ratio of 90:5:5, and then coated on aluminum foil using a 100 - um four - sided coater. Then, the electrode film is pre - dried in a forced - air drying oven at 80 °C for 30 min and transferred to a vacuum drying oven for drying at 110 °C for 10 hours. The electrode film is punched into a disc with a radius of 0.6 mm using a punching machine, with metallic sodium as the counter electrode, and 1M NaClO 4 EC + DEC (1:1 vol%) + 5% FEC as the electrolyte, using Whatman - D fiberglass diaphragm, and assembled into a CR2032 - type button cell in a glove box.

[0136] The above - mentioned button cell is subjected to constant - current charge - discharge tests. The current density is 0.1C (1C = 100 mAh / g), and 0.1C / 0.5C / 2C / 5C discharge specific capacity tests and 100 - cycle tests at a 1C rate are carried out within a voltage range of 2.0 - 4.3V.

[0137] Table 1

[0138]

[0139] It can be seen from the data in Table 1 that the examples have better electrochemical performance than the comparative examples; the discharge specific capacities of Examples 1 to 9 are all higher than those of Comparative Examples 1 to 6, and the rate performance and cycle performance are more stable. It shows that the preparation method of the present invention has obvious advantages in improving the electrochemical performance such as the conductivity of the sodium iron sulfate cathode material and the cycle stability.

[0140] Specifically, it can be seen from the examples and Comparative Example 1 that, compared with this example, when Comparative Example 1 uses polyvinylpyrrolidone non-hydrophilic anionic dispersant, the discharge specific capacity, cycle efficiency and other properties of the prepared cathode material are all poor; this is because the dispersant used in the present invention not only has good dispersing ability and high conductivity, but also has good stability in acidic solutions containing sulfate ions, which can effectively avoid the inactivation of the dispersant. During the sintering process of preparing the high-performance sodium iron sulfate polyanion cathode material, it can effectively ensure the dispersibility of the carbon source and prevent agglomeration to form an uneven and continuous conductive layer. The dispersant used in the example helps to prepare a uniformly continuous carbon conductive layer, which not only has high conductivity and charge-discharge specific capacity, but also can maintain the stability of the sodium iron sulfate structure and has better cycle efficiency.

[0141] It can be seen from the examples and Comparative Example 2 that the difference between Comparative Example 2 and this example is that during the process of carbon coating to prepare the high-performance sodium iron sulfate cathode material, Comparative Example 2 selects the operation steps of directly mixing the carbon source, dispersant, ferrous sulfate, and anhydrous sodium sulfate for spray drying and then sintering; this preparation method is likely to cause uneven carbon source coating and easy agglomeration of the carbon source. At the same time, this preparation method also increases the process difficulty of spray drying preparation, resulting in blockage and easy structural collapse during the high-temperature sintering process; it is not conducive to forming a high-performance sodium iron sulfate polyanion cathode material with a stable structure and a uniform and continuous carbon layer.

[0142] It can be seen from the examples and Comparative Example 3 and Comparative Example 4 that during the process of drying and sintering to prepare the high-performance sodium iron sulfate cathode material; the electrochemical performance and cycle stability of the examples are better than those of Comparative Example 3 and Comparative Example 4; this is because the examples select an appropriate range of the proportion of porous carbon sphere particles to mixed solution B; it can ensure that the sodium iron sulfate solution in the mixed solution B can uniformly grow on the surface of the porous carbon sphere particles, thus forming a uniform conductive layer; while the proportion of porous carbon sphere particles in Comparative Example 3 and Comparative Example 4 is too high or too low, which is likely to cause the conductive layer to be too thick or unable to form a continuous conduction path; thus, it is not conducive to improving the problem of poor conductivity of the sodium iron sulfate material.

[0143] From the comparative example 5 and the examples, it can be seen that in comparative example 5, different spray drying conditions were selected, and the inlet and outlet temperatures were lower than those in this example. The prepared high-performance sodium iron sulfate polyanion material had lower electrochemical performance during the charge and discharge cycles of the battery. This is because when the spray drying temperature is too low, it is easy for the carbon source slurry to agglomerate during the spray drying process, blocking the spray drying outlet, and the particle size of the prepared porous carbon spheres is uneven, which is not conducive to having a porous structure and a three-dimensional network structure of cross-linking and overlapping. At the same time, too much moisture affects its drying and sintering rates, and is not conducive to reducing the growth binding force between sodium iron sulfate and the carbon structure during the sintering preparation process. In example 1, a suitable range of inlet and outlet temperatures for spray drying was selected, which helped the carbon source slurry to prepare carbon source particles with a good porous structure and uniform particle size. Thus, a high-performance sodium iron sulfate polyanion cathode material was successfully prepared.

[0144] In comparative example 6, compared with the example, steps S4 and S5 were combined; the cathode material prepared by this method showed poorer electrochemical performance and cycle stability during the battery cycle compared with the example. This is because when directly sintering the porous carbon sphere particles adsorbed with the mixed solution A, thermal shock will occur during the sintering process due to the presence of moisture, resulting in the inability of sodium iron sulfate to grow smoothly along the porous carbon sphere particles; causing unstable binding with the carbon conductive layer and easily causing structural defects; thus affecting the structural stability during the cycle. At the same time, the structural defects will also cause uneven binding with the carbon conductive layer, which is not conducive to the improvement of electrochemical performance.

[0145] Through Figure 1 From the XRD performance diagrams of comparative example 1, example 1, and example 2, it can be seen that the peaks of example 1 and 2 are consistent with the standard card of NFS, and the purity of the obtained material is higher than that of comparative example 1. Obvious impurity peaks in comparative example 1 will affect the performance of the material.

[0146] Through Figure 2 、 Figure 3 From the SEM image of example 1, Figure 2 , 3 are the surface and cross-section SEM images of the carbon spheres after spraying respectively. Figure 3 It can be seen that there are uniformly distributed pores in the porous carbon sphere particles after spraying, which helps the NFS salt solution to be better and more uniformly filled and tightly combined with the carbon tubes as Figure 2 , enabling it to obtain better electrical conductivity.

[0147] Through Figure 4 From the SEM image of the surface of NFS after spraying in comparative example 1 in

[0148] Through Figure 5It can be seen from the SEM image of Comparative Example 2 that there is obvious agglomeration of carbon nanotubes inside the particles, and NFS and carbon nanotubes fail to bind well, thus affecting the performance of the material.

[0149] Through Figure 6 and Figure 7 It can be seen that the product prepared in Example 1 can have a deeper pore volume under the same particle size compared with Comparative Example 2, can bind more tightly with the sodium iron sulfate material, and the average particle size and pore volume of the product prepared in Example 1 of the present invention also satisfy the formula (47.0 - 0.21*AD)*(100 ± 8.66)% = PV*10^ 3 .

[0150] It should be noted that the above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principle of the present invention shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a high-performance sodium iron sulfate polyanion positive electrode material; characterized in that: The preparation method comprises: S1: The linear carbon source, water and a hydrophilic anionic dispersant are subjected to ultrasonic dispersion and shearing treatment to mix and obtain a carbon source slurry; S2: spray drying the carbon source slurry described in S1 to prepare porous carbon sphere particles; S3: adding the porous carbon sphere particles described in S2 to a mixed solution A containing sodium ions, iron ions, and sulfate ions to obtain a mixed solution B; allowing the sodium ions, iron ions, and sulfate ions to be adsorbed on the porous carbon sphere particles; S4: In an inert atmosphere, the mixed solution B is subjected to a heating and drying treatment, and then crushed to obtain a sodium iron sulfate polyanion precursor material; S5: Under an inert atmosphere, the sodium iron sulfate polyanion precursor material described in S4 is sintered to obtain a high-performance sodium iron sulfate polyanion positive electrode material in which sodium iron sulfate is in situ grown on porous carbon sphere particles.

2. The method for preparing the high-performance sodium iron sulfate polyanion positive electrode material according to claim 1; characterized in that: The linear carbon source in S1 includes one or more of carbon fiber and carbon nanotube; the solid content in the carbon source slurry is 2-5%; the mass of the hydrophilic anionic dispersant accounts for 0.5-5% of the mass of the linear carbon source.

3. The method for preparing the high-performance sodium iron sulfate polyanion positive electrode material according to claim 2; characterized in that: The diameter of the linear carbon source in S1 is 5-100 nanometers; the aspect ratio is greater than 5.

4. The method for preparing the high-performance sodium iron sulfate polyanion positive electrode material according to claim 3; characterized in that: The diameter of the linear carbon source in S1 is 10-20 nanometers.

5. The method for preparing the high-performance sodium iron sulfate polyanion positive electrode material according to claim 1; characterized in that: The sources of sodium ions, iron ions and sulfate ions in the S3 mixed solution A are selected from a combination of sodium sulfate sources and iron sulfate sources.

6. The method for preparing the high-performance sodium iron sulfate polyanion positive electrode material according to claim 5; characterized in that: The sodium sulfate source is selected from one or more of sodium sulfate, anhydrous sodium sulfate, sodium thiosulfate, and sodium sulfate heptahydrate.

7. The method for preparing the high-performance sodium iron sulfate polyanion positive electrode material according to claim 5; characterized in that: The iron sulfate source is selected from one or more of ferrous sulfate, anhydrous ferrous sulfate, ferrous sulfate monohydrate, and ferrous sulfate heptahydrate.

8. The method for preparing the high-performance sodium iron sulfate polyanion positive electrode material according to claim 1; characterized in that: The molar concentration ratio of Na2:Fe in the S3 mixed solution A is selected from 0.5 to 1; the particle size range of the porous carbon sphere particles in the S2 is 5-10 μm; the mass of the porous carbon sphere particles in the S3 accounts for 2-5% of the total mass of the mixed solution B.

9. The method for preparing the high-performance sodium iron sulfate polyanion positive electrode material according to claim 1; characterized in that: The spray drying outlet temperature in S2 is selected from 90-160 degrees; the spray drying inlet temperature is selected from 200-300 degrees; the particle size range of the sodium iron sulfate polyanion precursor material in S4 is 5-20 μm.

10. The method for preparing the high-performance sodium iron sulfate polyanion positive electrode material according to claim 1; characterized in that: In the S4 heating and drying process, the heating rate is selected from 10-20 o C / min; drying temperature is 100-130 o C; drying time is selected from 0.5-1h.

11. The method for preparing the high-performance sodium iron sulfate polyanion positive electrode material according to claim 1; characterized in that: The sintering temperature in S5 is 350-400 o C; the sintering treatment time is selected from 10-16h.

12. The method for preparing the high-performance sodium iron sulfate polyanion positive electrode material according to claim 1; characterized in that: The inert atmosphere in S4 and S5 includes one or more combinations of nitrogen atmosphere and helium atmosphere.

13. The high-performance sodium iron sulfate polyanion positive electrode material obtained by the preparation method according to any one of claims 1 to 12, characterized in that: The material has a porous spherical structure; the particle size of the material is 10-160nm; the pore volume is 0.010-0.040cm 3 / g; the capacity retention rate after 100 cycles of 1C charge and discharge is 96-99.8%.

14. The high performance sodium iron sulfate polyanion positive electrode material according to claim 13, characterized in that: The material satisfies (47.0-0.21*AD)*(100±8.66)%=PV*10^ 3 ; Wherein, AD represents the particle size of the high-performance sodium iron sulfate polyanion positive electrode material, and PV represents the pore volume of the high-performance sodium iron sulfate polyanion positive electrode material.

15. A positive electrode plate; characterized in that: The positive electrode plate comprises the high-performance sodium ferric sulfate polyanion positive electrode material described in any one of claims 13-14 or the high-performance sodium ferric sulfate polyanion positive electrode material obtained by the preparation method described in any one of claims 1-12.

16. An electrode assembly, a battery cell or an energy storage device; characterized in that: The high-performance sodium ferric sulfate polyanion positive electrode material comprises the high-performance sodium ferric sulfate polyanion positive electrode material described in any one of claims 13-14 or the high-performance sodium ferric sulfate polyanion positive electrode material obtained by the preparation method described in any one of claims 1-12.

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