Preparation method of core-shell structure positive electrode material

Abstract

The application provides a preparation method of a core-shell structure positive electrode material, and relates to the technical field of batteries. The method comprises the following steps: providing an inner core precursor for generating a layered oxide and a coating layer precursor for forming an inorganic coating layer; the coating layer precursor is a micellar precursor prepared by a sol-gel method; mixing the inner core precursor and the micellar precursor, drying and dispersing to obtain a composite material precursor; calcining the composite material precursor and annealing to obtain the core-shell structure positive electrode material. The preparation method provided by the application has high consistency in the quality of the positive electrode material and uniform coating.

Description

Technical Field

[0001] This application relates to the field of battery technology, and in particular to a method for preparing a core-shell structured cathode material. Background Technology

[0002] The cathode material in sodium-ion batteries accounts for more than one-third of the total cost and is also one of the key components affecting the battery's electrical performance, safety performance, and lifespan. Therefore, improving the production cost, efficiency, and quality of cathode materials for sodium-ion batteries is crucial for reducing overall cost and improving performance.

[0003] In the existing technology, the main preparation method for forming a core-shell composite cathode material by coating the cathode material with inorganic materials is the segmented sintering preparation method. Specifically, it includes first preparing and sintering a fast ion conductor to form the core, and then coating the surface of the fast ion conductor with a shell layer and sintering to obtain a core-shell composite cathode material. Summary of the Invention

[0004] The purpose of this application is to provide a method for preparing a core-shell structured cathode material, which produces cathode materials with high quality consistency and relatively uniform coating.

[0005] In a first aspect, to address the aforementioned problems, this application provides a method for preparing a core-shell structured cathode material, comprising the following steps:

[0006] A core precursor for generating layered oxides and a coating layer precursor for forming an inorganic coating layer are provided; the coating layer precursor is a micellar precursor prepared by the sol-gel method.

[0007] The core precursor and the micelle precursor are mixed, dried, and dispersed to obtain the composite material precursor.

[0008] The composite material precursor is calcined and annealed to obtain a core-shell structured cathode material.

[0009] The calcination process includes a first calcination section and a second calcination section. The sintering atmosphere of the first calcination section is a first sintering atmosphere. The first sintering atmosphere includes a first protective gas and oxygen, such as nitrogen and oxygen, with the oxygen accounting for 5%-8.5% of the mass in the first sintering atmosphere. The sintering atmosphere of the second calcination section is a second sintering atmosphere. The second sintering atmosphere includes a second protective gas, carbon dioxide, and gaseous water, with the carbon dioxide accounting for 3.2%-5.5% of the mass in the second sintering atmosphere and the gaseous water accounting for 1.0%~2.2 wt% of the mass in the second sintering atmosphere.

[0010] Furthermore, in some embodiments of this application, the sintering temperature of the first calcination section is higher than that of the second calcination section, and the difference between the sintering temperatures of the first calcination section and the second calcination section is not higher than 200°C.

[0011] Furthermore, in some embodiments of this application, the sintering temperature of the first calcination section is 850-900℃; the sintering temperature of the second calcination section is 700-750℃; and / or

[0012] The sintering time of the first calcination section is 3-6 hours; the sintering time of the second calcination section is 4-8 hours.

[0013] Furthermore, in some embodiments of this application, the sum of the sintering time of the first calcination section and the sintering time of the second calcination section does not exceed 10 hours.

[0014] Furthermore, in some embodiments of this application, the size of the micelle particles in the micelle precursor is 10-20 nm; and / or

[0015] The water content in the micelle precursor is 20-50 wt%.

[0016] Furthermore, in some embodiments of this application, the micelle precursor and the core precursor are mixed by ultrasonication; the solid content in the mixed system formed after mixing is 45~55 wt%.

[0017] Furthermore, in some embodiments of this application, a concentration step is included between the step of mixing the kernel precursor and the micelle precursor and the drying step;

[0018] The concentration process includes: concentrating the mixture at 80-85°C until the solid content in the system is 60-65%.

[0019] Furthermore, in some embodiments of this application, the annealing temperature is 300~350℃, the annealing time is 2~4h, and the annealing atmosphere is Ar and / or nitrogen.

[0020] Furthermore, in some embodiments of this application, the kernel precursor is provided by a method comprising the following steps:

[0021] Provide a first sodium source, a first metal source, a first solvent, and a dispersant; mix the sodium source, metal source, solvent, and dispersant, grind, and spray dry to obtain a core precursor; and / or

[0022] The micelle precursor is provided by a method comprising the following steps:

[0023] A second sodium source, an iron source, a second solvent, and a complexing agent are provided. The second sodium source, iron source, solvent, and complexing agent are mixed to obtain a coating layer precursor micelle.

[0024] Furthermore, in some embodiments of this application, the first sodium source is selected from at least one of sodium carbonate, sodium hydroxide, and sodium dihydrogen phosphate;

[0025] The metal source is selected from at least one of the following: iron source, nickel source, manganese source, copper source, cobalt source, titanium source, magnesium source, aluminum source, zinc source, and calcium source.

[0026] The solvent is selected from at least one of deionized water, ethylene glycol, N-methylpyrrolidone, and ethanol;

[0027] The dispersant is selected from at least one of polyacrylic acid, ethylene glycol, and polyvinyl alcohol;

[0028] The second sodium source is selected from at least one of sodium carbonate, sodium hydroxide, and sodium dihydrogen phosphate;

[0029] The iron source is selected from at least one of ferric nitrate, ferric oxide, ferric phosphate, ferrous sulfate, ferrous oxalate dihydrate, and iron.

[0030] The second solvent is selected from at least one of deionized water, ethylene glycol, N-methylpyrrolidone, and ethanol;

[0031] The complexing agent is selected from at least one of citric acid, oxalic acid, glycine, and polyacrylic acid.

[0032] This application provides a method for preparing a core-shell structured cathode material. The method involves pre-forming precursor particles for the core and a gel-like coating layer precursor for the coating layer. The core-shell structured cathode material is then grown in a single, segmented, dynamic atmosphere during calcination. This method results in a clear core-shell structure and good coating uniformity. Furthermore, it eliminates the need for separate sintering of the core material and coating layer, reducing sintering time and energy consumption, thus lowering production costs. It also facilitates crystal growth of the core and coating layer, improving the charge / discharge capacity and its consistency. Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the specific embodiments of this application or the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0034] Figure 1This is a SEM image of the cathode material obtained in Example 1 of this application;

[0035] Figure 2 This is a SEM image of the cathode material obtained in Comparative Example 1 of this application. Detailed Implementation

[0036] The technical solutions of this application will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0037] In the description of this application, it should be understood that "multiple" means two or more, unless otherwise expressly and specifically limited.

[0038] The following disclosure provides many different embodiments or examples for implementing different structures of this application. To simplify the disclosure, specific examples of components and arrangements are described below. Of course, these are merely examples and are not intended to limit the scope of this application. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. In addition, various specific examples of processes and materials are provided in this application, but those skilled in the art will recognize the application of other processes and / or the use of other materials.

[0039] The preparation of coating layers for core-shell structured cathode materials in existing technologies includes single-sintering, double-sintering, and multiple-sintering methods. Currently, single-sintering is mainly used for carbon-coated cathode materials and organic-coated cathode materials. For carbon-coated cathode materials, a precursor is formed by mixing the carbon source for the coating layer and other raw materials for the core layer, followed by a single-sintering process to form a core-shell structure. For organic-coated cathode materials, the organic layer is coated after the core is sintered. Other non-carbon-coated inorganic coating layers are typically prepared using double-sintering or multiple-sintering methods, i.e., preparing the core and coating material separately (usually including a sintering step), and then coating the core with the coating material and sintering to form a core-shell structured cathode material. In the process of preparing NFPP (sodium iron pyrophosphate) coated O3-phase layered oxide cathode material, the applicant found that the NaM oxygen@NFPP coating prepared using the above process had poor coating uniformity, poor quality consistency, and high production costs. Based on this, this application provides a method for preparing a core-shell structured cathode material, which involves forming a spherical or near-spherical core precursor and a gel-like coating layer precursor, respectively, and then coating the core precursor onto the surface of the core precursor, followed by sintering to allow the core and coating layer to grow in situ to obtain a core-shell structured cathode material. During this process, the sintering atmosphere is controlled to promote the crystal growth of the core and the coating layer, resulting in a core-shell structured cathode material with uniform coating, good quality consistency, and lower cost compared to secondary sintering.

[0040] Specifically, the method for preparing the core-shell structured cathode material provided in this application includes the following steps:

[0041] A core precursor for generating layered oxides and a coating layer precursor for forming an inorganic coating layer are provided; the coating layer precursor is a micellar precursor prepared by the sol-gel method.

[0042] The core precursor and the micelle precursor are mixed, dried, and dispersed to obtain the composite material precursor.

[0043] The composite material precursor is calcined and annealed to obtain a core-shell structured cathode material.

[0044] The calcination process includes a first calcination section and a second calcination section. The sintering atmosphere of the first calcination section is a first sintering atmosphere, which includes a first protective gas and oxygen, with the oxygen accounting for 5%-8.5% of the total mass. The sintering atmosphere of the second calcination section is a second sintering atmosphere, which includes a second protective gas, carbon dioxide, and gaseous water, with the carbon dioxide accounting for 3.2%-5.5% of the total mass and the gaseous water accounting for 1.0%-2.2 wt%.

[0045] It should be noted that the core precursor described in this application refers to spherical or near-spherical precursor particles to form the general structure of the core. The core crystals are also grown in situ during the subsequent sintering process. The coating layer precursor is a micellar precursor with a certain degree of fluidity. During the mixing process of the core precursor and the coating layer precursor, its fluidity allows it to uniformly coat the surface of the core precursor. The coating layer is also grown in situ during the subsequent sintering process. That is, in the preparation method provided in this application, both the core and coating layer crystals are grown in situ during the sintering process.

[0046] The cathode material prepared by the method provided in this application exhibits good coating uniformity, which is beneficial to its performance improvement, and good quality consistency, with capacity fluctuations between different batches of products <±1.5%. Furthermore, the core-shell structured cathode material prepared by this method shows superior electrical performance compared to core-shell structured cathode materials prepared by secondary sintering. The reason for this may be:

[0047] The preparation method used in this application involves the in-situ growth of the core and coating layer during the same sintering process. This eliminates the need for post-construction dispersion processes that could damage or introduce uncontrollable issues to the core and coating materials, thus improving the quality consistency and electrical performance of the resulting cathode material. Furthermore, the core-shell structure cathode material provided in this application exhibits superior mechanical properties, particularly reducing the likelihood of cracking. This is because the coating layer is formed before sintering the precursors for the core and coating layer, allowing for the in-situ generation of the core-shell structure. This creates a transition zone at the core-shell interface during core and shell growth, enhancing the bonding between the coating layer and the core. Consequently, the coating layer's ability to mitigate defects such as core volume expansion is more pronounced.

[0048] In some embodiments, the first protective gas is selected from at least one inert gas such as nitrogen, helium, argon, and neon; the second protective gas is selected from at least one inert gas such as helium, argon, and neon. To reduce costs, nitrogen can be selected as the first protective gas.

[0049] This application employs a specific sintering atmosphere of (91.5%~95%) nitrogen and (5%-8.5%) oxygen in the first calcination stage, which is beneficial to Ni 2+ It is almost completely oxidized to Ni 3+ To avoid incomplete oxidation or unstable crystal structure due to lattice distortion, an atmosphere is used in the second calcination stage. This atmosphere consists of an inert gas mixture containing 3.2%-5.5% carbon dioxide and 1.0%-2.2 wt% gaseous water. The carbon dioxide helps to regulate the local pH value to approximately 7, while simultaneously suppressing iron... 2+ Hydrolysis, in which gaseous water molecules can promote solid-phase reactions and accelerate iron production. 2+ Diffusion is beneficial for forming a uniform and stable core-shell structure.

[0050] The calcination process provided in this application uses the aforementioned specific sintering atmosphere. When calcining to generate core-shell structured cathode materials, there is no need to add hydrogen fluoride, which is required in traditional cathode material preparation processes. This means that the preparation method provided in this application does not require the cathode material to be treated with hydrogen fluoride, reducing the use of this toxic solution, making it environmentally friendly, and avoiding the toxicity of hydrogen fluoride and its corrosive effects on equipment.

[0051] In some embodiments of this application, the sintering temperature of the first calcination section is higher than that of the second calcination section, and the difference between the sintering temperatures of the first and second calcination sections is not higher than 200°C. Specifically, the sintering temperature of the first calcination section is 850-900°C; the sintering temperature of the second calcination section is 700-750°C. The sintering time of the first calcination section is 3-6 hours; the sintering time of the second calcination section is 4-8 hours. Preferably, the sintering temperature of the first calcination section is 840-850°C; the sintering temperature of the second calcination section is 720-730°C; the sintering time of the first calcination section is 2-3 hours; and the sintering time of the second calcination section is 4-6 hours.

[0052] Furthermore, in the preparation method provided in this application, the sum of the sintering times of the first calcination stage and the second calcination stage can be controlled within a sintering time range of 6-9 hours to obtain cathode materials with excellent quality and good consistency. Compared with other methods for preparing cathode materials with a core-shell structure formed by stepwise calcination, the preparation method provided in this application requires a shorter sintering time, which can improve preparation efficiency and reduce preparation costs. Specifically, the sum of the sintering times of the first calcination stage and the second calcination stage does not exceed 9 hours, preferably not exceeding 8 hours.

[0053] In some embodiments, the micelle particles in the micelle precursor have a size of 10-20 nm, so that they can uniformly coat the surface of the core precursor particles to form a precursor coating layer, which is beneficial to the in-situ growth and uniform coating of the coating layer in the subsequent calcination process.

[0054] In some embodiments, the water content in the micelle precursor is 20-50 wt%, providing sufficient fluidity to facilitate the dispersion of the core precursor particles and the uniform coating of the coating layer precursor. In this application, the viscosity of the micelle precursor can be controlled between 100-500 mPa·s, preferably 100-500 mPa·s, which is beneficial for dispersing the core precursor particles and ensuring the stability of the mixed system, preventing agglomeration and sedimentation during process operations.

[0055] In some embodiments, the micelle precursor and the core precursor are mixed by ultrasonication; the solid content in the resulting mixture is 45-55 wt%, preferably 50-52%. The solid content in the mixture should not be too high or too low. Too high a solid content can lead to uneven dispersion of the core precursor, longer dispersion time, aggregation of the micelle precursor, and destruction of the micelles during ultrasonication; while too low a solid content can lead to micelle aggregation during the concentration process, thereby affecting the uniformity of the coating thickness.

[0056] It should be noted that the solid content in the hybrid system in this application is the mass ratio of the sum of the masses of the micelle precursor and the core precursor in the hybrid system.

[0057] In some embodiments, a concentration step is included between the step of mixing the core precursor and the micelle precursor and the drying step; the concentration step includes: concentrating the mixed system at 80-85°C to a solid content of 60-65% in the system, so that the water content in the mixed system is removed to below 40%, which is beneficial for the micelles to uniformly coat the core, avoiding the problem of damage to the micelle structure and poor coating uniformity caused by directly drying the mixed system, which leads to increased impedance. In addition, the concentration step can also reduce drying energy consumption and reduce costs.

[0058] In some embodiments, the annealing temperature is 300~350℃, the annealing time is 2~4h, and the annealing atmosphere is argon and / or nitrogen.

[0059] The core precursor provided in this application is a core precursor for forming layered oxides, and the raw materials required can be adjusted according to the structural formula of the desired layered oxide. The micelle precursor provided in this application for forming a coating layer can also have its raw materials adjusted according to the structural formula of the desired coating layer.

[0060] In some embodiments, the kernel precursor is provided by a method comprising the following steps:

[0061] A first sodium source, a first metal source, a first solvent, and a dispersant are provided; the sodium source, metal source, solvent, and dispersant are mixed, ground, and spray-dried to obtain a core precursor.

[0062] The micelle precursor is provided by a method comprising the following steps:

[0063] A second sodium source, an iron source, a second solvent, and a complexing agent are provided. The second sodium source, iron source, solvent, and complexing agent are mixed to obtain a coating layer precursor micelle.

[0064] In some embodiments, the first sodium source is selected from at least one of sodium carbonate, sodium hydroxide, and sodium dihydrogen phosphate;

[0065] The metal source is selected from at least one of the following: iron source, nickel source, manganese source, copper source, cobalt source, titanium source, magnesium source, aluminum source, zinc source, and calcium source.

[0066] The solvent is selected from at least one of deionized water, ethylene glycol, N-methylpyrrolidone, and ethanol;

[0067] The dispersant is selected from at least one of polyacrylic acid, ethylene glycol, and polyvinyl alcohol;

[0068] The second sodium source is selected from at least one of sodium carbonate, sodium hydroxide, and sodium dihydrogen phosphate;

[0069] The iron source is selected from at least one of ferric nitrate, ferric oxide, ferric phosphate, ferrous sulfate, ferrous oxalate dihydrate, and elemental iron.

[0070] The second solvent is selected from at least one of deionized water, ethylene glycol, N-methylpyrrolidone, and ethanol;

[0071] The complexing agent is selected from at least one of citric acid, oxalic acid, glycine, and polyacrylic acid.

[0072] For example, the preparation method provided in this application specifically includes the following steps:

[0073] Preparation of layered oxide precursors

[0074] Sodium, iron, nickel, and manganese sources, as well as dopants (added if present, not required if absent), are provided in a molar ratio of 0.8-1:0.2-0.4:0.2-0.4:0.2-0.4. The core raw materials are mixed with the first solvent and dispersion, and the Zeta potential is adjusted to above +35mV by adding the dispersion. The mixture is then ground and spray-dried to obtain layered oxide precursor particles.

[0075] (2) Preparation of sodium iron pyrophosphate precursor

[0076] The outer shell material is provided in a molar ratio of sodium source, iron source and phosphorus source of 3.5-4:2.5-3:3.8-4.2. The outer shell material and the second solvent are stirred in a water bath at 40-90℃ for 4-24 hours to obtain a transparent sol-like sodium iron pyrophosphate precursor.

[0077] (3) The sodium iron pyrophosphate precursor and the layered oxide precursor particles are mixed at a mass ratio of 1:10~50 and ultrasonically dispersed for 30~180 min. The mixture is then concentrated at 80~85℃ until the sodium iron pyrophosphate precursor increases in weight by 5-8%, so that the sodium iron pyrophosphate precursor is uniformly attached to the surface of the layered oxide precursor particles to obtain the composite material.

[0078] (4) Sintering

[0079] The composite material is vacuum dried for 4-24 hours, heated to 850-900℃ under a first protective gas and 5%-8.5% oxygen, and sintered at that temperature for 3-6 hours. The sintering atmosphere is then adjusted to a mixed atmosphere of a second protective gas, 3.2%-5.5% carbon dioxide, and 1.0%-2.2wt% gaseous water, and sintered at 700-750℃ for 4-8 hours, followed by annealing at 200-300℃ for 2-8 hours to obtain the core-shell structure cathode material of this application.

[0080] Preferably, the sodium source in step (1) can be the same as that in step (2) to reduce the introduction of more impurities.

[0081] To facilitate a better understanding of the innovative aspects of this application by those skilled in the art, the technical solutions of this application are further described in detail below with reference to embodiments. The embodiments of this application described in detail below are exemplary and are only used to explain this application, and should not be construed as limiting this application.

[0082] Example 1

[0083] This embodiment provides a method for preparing a core-shell structured cathode material, including the following steps:

[0084] (1) Preparation of layered oxide precursors

[0085] Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese trioxide, and copper oxide were provided in a molar ratio of Na:Ni:Fe:Mn:Cu = 1:0.4:0.3:0.2:0.1. These components, along with an ethanol solution, were added to a planetary ball mill. Ammonium polyacrylate dispersant was added, and the Zeta potential was adjusted to above +40mV. The mill was then ground at 400 rpm for 24 hours to obtain wet abrasive. The wet abrasive was then spray-dried under the following conditions: inlet air temperature: 220-250℃; outlet air temperature: 80-100℃; atomization pressure: 0.3-0.5MPa; nozzle diameter: 0.5mm; and peristaltic pump-controlled feed rate: 10mL / min. This yielded spherical core precursor particles with an average particle size of D50: 8μm.

[0086] (2) Preparation of sodium iron pyrophosphate precursor

[0087] Ferrous oxalate dihydrate and sodium pyrophosphate were dissolved in 0.1 mol / L citric acid solution at a molar ratio of 3:1 and stirred in a water bath at 80°C for 4 hours to form a blue-green transparent sol, thus obtaining a nano-micelle precursor of sodium iron pyrophosphate; wherein the average particle size of the micelles was 10~20 nm, the water content was 35%, and the viscosity was 200 mPa·s.

[0088] (3) The core precursor particles were added to a transparent sol at a mass ratio of 10:1 of core precursor to sodium iron pyrophosphate precursor, ultrasonically dispersed for 30 minutes, and concentrated under reduced pressure at 80°C until the sol completely covered the particle surface (controlling the weight gain of 5-8%); so that the sodium iron pyrophosphate precursor was uniformly attached to the surface of the core precursor particles to obtain the composite material.

[0089] (4) Sintering

[0090] The composite material was vacuum dried at 100℃ for 12 hours. Nitrogen and oxygen were then introduced into the calcination equipment, with a nitrogen flow rate of 9.5 L / min and an oxygen flow rate of 0.5 L / min, creating a calcination atmosphere of 95% nitrogen + 5% oxygen, maintained at a pressure of 0.5~0.6 MPa. The temperature was then raised to 850℃ and sintered for 4 hours. Argon was then introduced into the calcination equipment to replace the sintering atmosphere, followed by the introduction of water-containing carbon dioxide gas, creating an atmosphere of 93% argon + 5% carbon dioxide + 2% water, at a pressure of 0.3~0.5 MPa. The temperature was raised to 750℃ and sintered for 6 hours, then cooled to 300℃ and annealed for 2 hours to obtain the cathode material of this application: NaNi. 0.4 Fe 0.3 Mn 0.3 Cu 0.1 O2@1 / 11Na4Fe3(PO4)2(P2O7), its morphology is as follows Figure 1 As shown.

[0091] Five batches of cathode materials were prepared according to the above preparation method, and samples S11~S15 were taken from each batch for later use.

[0092] Example 2

[0093] This embodiment provides a method for preparing a core-shell structured cathode material, including the following steps:

[0094] (1) Preparation of layered oxide precursors

[0095] Sodium carbonate, nickel hydroxide, ferrous oxalate dihydrate, manganese trioxide, and cerium trioxide were provided in a molar ratio of Na:Ni:Fe:Mn:Ce = 1:0.3:0.3:0.3:0.1. A solution of sodium carbonate, nickel hydroxide, ferrous oxalate dihydrate, manganese trioxide, cerium trioxide, and ethanol was added to a planetary ball mill. PVP dispersant was added, and the Zeta potential was adjusted to above +40mV. The mill was ground at 400 rpm for 24 hours to obtain wet abrasive. The wet abrasive was then spray-dried under the following conditions: inlet air temperature: 220-250℃, outlet air temperature: 80-100℃, atomization pressure: 0.3-0.5MPa, nozzle diameter: 0.5mm, and peristaltic pump-controlled feed rate: 10mL / min, to obtain spherical core precursor particles with an average particle size of D50: 10μm.

[0096] (2) Preparation of sodium iron pyrophosphate precursor

[0097] Ferrous oxalate dihydrate and sodium pyrophosphate were dissolved in 0.1 mol / L citric acid solution at a molar ratio of 3:1 and stirred in a water bath at 80°C for 4 hours to form a blue-green transparent sol, thus obtaining a nano-micelle precursor of sodium iron pyrophosphate; wherein the average particle size of the micelles was 10~20 nm, the water content was 35%, and the viscosity was 200 mPa·s.

[0098] (3) The core precursor particles are added to a transparent sol according to the mass ratio of core precursor to sodium iron pyrophosphate precursor of 10:1, ultrasonically dispersed for 30 minutes, and concentrated under reduced pressure at 80°C until the sol completely covers the particle surface (weight gain of 5-8%); so that the sodium iron pyrophosphate precursor is uniformly attached to the surface of the core precursor particles to obtain the composite material.

[0099] (4) Sintering

[0100] The composite material was vacuum dried at 100°C for 12 hours. Nitrogen and oxygen were then introduced into the calcination equipment at a flow rate of 9.5 L / min and 0.5 L / min, respectively, to create a calcination atmosphere of 95% nitrogen and 5% oxygen, with the pressure maintained between 0.5 and 0.6 MPa. The temperature was then raised to 850°C and sintered for 4 hours. Argon was then introduced into the calcination equipment to replace the sintering atmosphere, followed by the introduction of water-containing carbon dioxide gas to create an atmosphere of 80% argon, 18% carbon dioxide, and 2% water, with a pressure of 0.3 to 0.5 MPa. The temperature was then raised to 750°C and sintered for 6 hours, followed by annealing at 300°C for 2 hours to obtain the cathode material of this application.

[0101] NaNi 0.3 Fe 0.3 Mn 0.3 Ce 0.1 O2@1 / 11Na4Fe3(PO4)2(P2O7).

[0102] Five batches of cathode materials were prepared according to the above preparation method, and samples S21~S25 were taken from each batch for later use.

[0103] Example 3

[0104] This embodiment provides a method for preparing a core-shell structured cathode material, including the following steps: (1) preparing a layered oxide precursor.

[0105] Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese trioxide, and copper oxide were provided in a molar ratio of Na:Ni:Fe:Mn:Cu = 1:0.4:0.3:0.2:0.1. These components, along with an ethanol solution, were added to a planetary ball mill. Ammonium polyacrylate dispersant was added, and the Zeta potential was adjusted to above +40mV. The mill was then ground at 400 rpm for 24 hours to obtain wet abrasive. The wet abrasive was then spray-dried under the following conditions: inlet air temperature: 220-250℃; outlet air temperature: 80-100℃; atomization pressure: 0.3-0.5MPa; nozzle diameter: 0.5mm; and peristaltic pump-controlled feed rate: 10mL / min. This yielded spherical core precursor particles with an average particle size of D50: 8μm.

[0106] (2) Preparation of sodium iron pyrophosphate precursor

[0107] Ferric nitrate and sodium pyrophosphate were dissolved in a 0.1 mol / L glycine solution at a molar ratio of 3:1 and stirred in a water bath at 60°C for 3 hours to form a transparent sol, thus obtaining a nano-micelle precursor of sodium ferric pyrophosphate; wherein the average particle size of the micelles was 20~30 nm, the water content was 30%, and the viscosity was 100 mPa·s.

[0108] (3) The core precursor particles were added to a transparent sol at a mass ratio of 10:1, ultrasonically dispersed for 30 minutes, and concentrated under reduced pressure at 80°C until the sol completely covered the particle surface (weight gain of 5-8%); so that the sodium iron pyrophosphate precursor was uniformly attached to the surface of the core precursor particles to obtain the composite material.

[0109] (4) Sintering

[0110] The composite material was vacuum dried at 100°C for 12 hours. Nitrogen and oxygen were then introduced into the calcination equipment at a flow rate of 9.2 L / min and 0.8 L / min, respectively, to create a calcination atmosphere of 92% nitrogen and 8% oxygen, with the pressure maintained between 0.5 and 0.6 MPa. The temperature was then raised to 850°C and sintered for 4 hours. Argon was then introduced into the calcination equipment to replace the sintering atmosphere, followed by the introduction of water-containing carbon dioxide gas to create an atmosphere of 93% argon, 5% carbon dioxide, and 2% water, with a pressure between 0.3 and 0.5 MPa. The temperature was then raised to 750°C and sintered for 6 hours, followed by annealing at 300°C for 2 hours to obtain the cathode material of this application.

[0111] NaNi 0.4 Fe 0.3 Mn 0.3 Cu 0.1 O2@1 / 11Na4Fe3(PO4)2(P2O7).

[0112] Five batches of cathode materials were prepared according to the above preparation method, and samples S31~S35 were taken from each batch for later use.

[0113] Comparative Example 1

[0114] Compared to Example 1, the sintering atmosphere in step (4) of this comparative example is 100% nitrogen for the first calcination and 100% argon for the second calcination. The remaining steps are the same as in Example 1, resulting in a core-shell structured cathode material D1, the morphology of which is as follows. Figure 2 As shown.

[0115] Comparative Example 2

[0116] Compared with Example 1, the sintering atmosphere in step (4) of this comparative example is 50% nitrogen-50% oxygen for the first calcination and 50% argon-25% carbon dioxide-25% water for the second calcination. The remaining steps are the same as in Example 1, and a core-shell structured cathode material D2 is obtained.

[0117] Comparative Example 3

[0118] This embodiment provides a method for preparing a core-shell structured cathode material, including the following steps:

[0119] (1) Preparation of layered oxide precursors

[0120] Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese trioxide, and copper oxide were provided in a molar ratio of Na:Ni:Fe:Mn:Cu = 1:0.4:0.3:0.2:0.1. These components, along with an ethanol solution, were added to a planetary ball mill. Ammonium polyacrylate dispersant was added, and the Zeta potential was adjusted to above +40mV. The mill was then ground at 400 rpm for 24 hours to obtain wet abrasive. The wet abrasive was then spray-dried under the following conditions: inlet air temperature: 220-250℃; outlet air temperature: 80-100℃; atomization pressure: 0.3-0.5MPa; nozzle diameter: 0.5mm; and peristaltic pump-controlled feed rate: 10mL / min. This yielded spherical core precursor particles with an average particle size of D50: 10μm.

[0121] (2) Calcination of kernel precursor particles

[0122] The above-mentioned core precursor particles were calcined at 850°C for 6 hours in an oxygen atmosphere to obtain layered oxide particles.

[0123] (3) Preparation of sodium iron pyrophosphate precursor

[0124] Ferrous oxalate dihydrate and sodium pyrophosphate were dissolved in 0.1 mol / L citric acid solution at a molar ratio of 3:1 and stirred in a water bath at 80°C for 4 hours to form a blue-green transparent sol, thus obtaining a nano-micelle precursor of sodium iron pyrophosphate; wherein the average particle size of the micelles was 10-20 nm, the water content was 40%, and the viscosity was 120 mPa·s.

[0125] (4) The layered oxide particles were added to a transparent sol at a mass ratio of 10:1, ultrasonically dispersed for 30 minutes, and concentrated under reduced pressure at 50°C until the sol completely covered the particle surface (weight gain of 5-8%); the sodium iron pyrophosphate precursor was uniformly attached to the surface of the layered oxide particles to obtain the composite material.

[0126] (5) Sintering

[0127] The composite material was vacuum dried at 100°C for 12 hours. Nitrogen and oxygen were then introduced into the calcination equipment at a flow rate of 9.5 L / min for nitrogen and 0.5 L / min for oxygen, creating a calcination atmosphere of 95% nitrogen and 5% oxygen, with the pressure maintained between 0.5 and 0.6 MPa. The temperature was then raised to 850°C and sintered for 4 hours. Argon was then introduced into the calcination equipment to replace the sintering atmosphere. Water-containing carbon dioxide was then introduced to create an atmosphere of 80% argon, 18% carbon dioxide, and 2% water, with a pressure between 0.3 and 0.5 MPa. The temperature was raised to 750°C and sintered for 6 hours. The temperature was then lowered to 300°C and annealed for 2 hours to obtain the cathode material of this application.

[0128] Five batches of cathode materials were prepared according to the above preparation method, and samples D31~D35 were taken from each batch for later use.

[0129] Comparative Example 4

[0130] This embodiment provides a method for preparing a core-shell structured cathode material, including the following steps:

[0131] (1) Preparation of layered oxide precursors

[0132] Sodium carbonate, ferrous oxalate dihydrate, nickel hydroxide, manganese trioxide, and copper oxide were provided in a molar ratio of Na:Ni:Fe:Mn:Cu = 1:0.4:0.3:0.2:0.1. These components, along with an ethanol solution, were added to a planetary ball mill. Ammonium polyacrylate dispersant was added, and the Zeta potential was adjusted to above +40mV. The mill was then ground at 400 rpm for 24 hours to obtain wet abrasive. The wet abrasive was then spray-dried under the following conditions: inlet air temperature: 220-250℃; outlet air temperature: 80-100℃; atomization pressure: 0.3-0.5MPa; nozzle diameter: 0.5mm; and peristaltic pump-controlled feed rate: 10mL / min. This yielded spherical core precursor particles with an average particle size of D50: 10μm.

[0133] (2) Calcination of kernel precursor particles

[0134] The above-mentioned core precursor particles were calcined at 850°C for 6 hours in an oxygen atmosphere to obtain layered oxide particles.

[0135] (3) NFPP particles with an average particle size D50 of 6 μm purchased from Hunan Meite Company were wet-mixed in a batch mixer at a mass ratio of 10:1 with ethanol as the solvent to obtain a mixture O3@NFPP.

[0136] (4) Sintering

[0137] The mixture was vacuum dried at 100°C for 12 hours. Nitrogen and oxygen were then introduced into the calcination equipment. The nitrogen flow rate was 9.5 L / min and the oxygen flow rate was 0.5 L / min, creating a calcination atmosphere of 95% nitrogen and 5% oxygen, with the pressure maintained between 0.5 and 0.6 MPa. The temperature was then raised to 850°C and sintered for 4 hours. Argon was then introduced into the calcination equipment to replace the sintering atmosphere. Water-containing carbon dioxide gas was then introduced to create an atmosphere of 93% argon, 5% carbon dioxide, and 2% water, with a pressure between 0.3 and 0.5 MPa. The temperature was raised to 750°C and sintered for 6 hours. The temperature was then lowered to 300°C and annealed for 2 hours to obtain the cathode material of this application.

[0138] Five batches of cathode materials were prepared according to the above preparation method, and samples D41~D45 were taken from each batch for later use.

[0139] The applicant used the core-shell structured cathode materials obtained in Examples 1-3 and the cathode materials in Comparative Examples 1-4 as cathode active materials to prepare corresponding sodium-ion batteries. The specific preparation process is as follows:

[0140] (1) Preparation of the positive electrode: The positive electrode material, SP, PVDF and CNT were mixed and stirred in a mass ratio of 95:1:3:1. N-methylpyrrolidone solvent was added to adjust the slurry to a solid content of 60% and a viscosity of 6000 mPa·s. The positive electrode slurry was transferred and coated onto a 15 μm thick carbon-coated aluminum foil using a transfer coating method. The coated electrode was then rolled to a thickness of 3 mg / cm. 3 The compaction density is determined; the rolled electrode sheets are then die-cut into electrode sheets with a length of 48mm and a width of 38mm for later use.

[0141] (2) Preparation of negative electrode: Hard carbon material, SP, CMC and SBR are mixed and kneaded in a mass ratio of 92:3:2:3, deionized water is added, and the slurry is adjusted to a negative electrode slurry with a solid content of 45% and a viscosity of 5000 mPa·s; the negative electrode slurry is transferred and coated onto a 15 μm thick carbon-coated aluminum foil using a transfer coating method; the coated electrode is rolled to 1 mg / cm³. 2 The compaction density is determined; the rolled electrode sheets are then die-cut into 50mm long and 40mm wide sheets for later use.

[0142] (3) Fabrication of sodium-ion batteries: The slit positive and negative electrode sheets are stacked on a stacking machine. The separator is made of PP / PE / PP three-layer material to form a soft-pack cell. Electrolyte (components: carbonate solvent and 1M sodium hexafluorophosphate) is injected, packaged, dried, and subjected to capacity testing to obtain a sodium-ion battery.

[0143] The sodium-ion batteries prepared above were subjected to morphology characterization tests, cycle performance tests, charge-discharge capacity tests, and rate performance tests; the specific test methods are as follows.

[0144] (1) Cyclic performance test

[0145] The cycle performance of the battery was tested using the test method described in GB / T 31485-2015.

[0146] (2) Volume expansion rate test

[0147] The volume change of the battery before and after storage at 55°C for 28 days was tested using the water displacement method.

[0148] (3) Charge and discharge capacity test

[0149] The charge and discharge capacity of the battery was tested using the test method described in GB / T 31467.2-2015.

[0150] The test results are shown in Table 1.

[0151] Table 1

[0152]

[0153] As can be seen from Table 1, the cathode material preparation method provided in this application adopts segmented sintering during the sintering process, and uses a specific sintering atmosphere in each sintering segment to control the morphology of the obtained cathode material and control the directional growth of crystals in the core and outer shell layers. This is beneficial to the optimization of the cathode material's cycle performance, surface impedance, volume expansion rate, and other properties. At the same time, it can also improve the uniformity of the cathode material particles' coating, thereby improving the consistency of product quality.

[0154] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A method for preparing a core-shell structured cathode material, characterized in that, Includes the following steps: A core precursor for generating layered oxides and a coating layer precursor for forming an inorganic coating layer are provided; the coating layer precursor is a micellar precursor prepared by the sol-gel method. The core precursor and the micelle precursor are mixed, dried, and dispersed to obtain the composite material precursor. The composite material precursor is calcined and annealed to obtain a core-shell structured cathode material. The calcination process includes a first calcination section and a second calcination section. The sintering atmosphere of the first calcination section is a first sintering atmosphere, which includes a first protective gas and oxygen, with the oxygen accounting for 5%-8.5% of the total mass. The sintering atmosphere of the second calcination section is a second sintering atmosphere, which includes a second protective gas, carbon dioxide, and gaseous water, with the carbon dioxide accounting for 3.2%-5.5% of the total mass and the gaseous water accounting for 1.0%-2.2 wt%.

2. The method for preparing the core-shell structured cathode material according to claim 1, characterized in that, The sintering temperature of the first calcination section is higher than that of the second calcination section, and the difference between the sintering temperatures of the first calcination section and the second calcination section is not higher than 200℃.

3. The method for preparing the core-shell structured cathode material according to claim 2, characterized in that, The sintering temperature of the first calcination section is 850-900℃; the sintering temperature of the second calcination section is 700-750℃; and / or The sintering time of the first calcination section is 3-6 hours; the sintering time of the second calcination section is 4-8 hours.

4. The method for preparing the core-shell structured cathode material according to claim 3, characterized in that, The sum of the sintering time of the first calcination section and the sintering time of the second calcination section shall not exceed 10 hours.

5. The method for preparing the core-shell structured cathode material according to any one of claims 1 to 4, characterized in that, The size of the micelle particles in the micelle precursor is 10-20 nm; and / or The water content in the micelle precursor is 20-50 wt%.

6. The method for preparing the core-shell structured cathode material according to claim 5, characterized in that, The micelle precursor and the core precursor are mixed by ultrasonication; the solid content in the resulting mixture is 45-55 wt%.

7. The method for preparing the core-shell structured cathode material according to claim 6, characterized in that, Between the process of mixing the core precursor and the micelle precursor and the drying process, there is also a concentration process; The concentration process includes: concentrating the mixture at 80-85°C until the solid content in the system is 60-65%.

8. The method for preparing the core-shell structured cathode material according to claim 1, characterized in that, The annealing temperature is 300~350℃, the annealing time is 2~4h, and the annealing atmosphere is argon and / or nitrogen.

9. A method for preparing a core-shell structured cathode material according to any one of claims 1 to 4, 6, and 7, characterized in that, The kernel precursor is provided by a method comprising the following steps: Provide a first sodium source, a first metal source, a first solvent, and a dispersant; mix the sodium source, metal source, solvent, and dispersant, grind, and spray dry to obtain a core precursor; and / or The micelle precursor is provided by a method comprising the following steps: A second sodium source, an iron source, a second solvent, and a complexing agent are provided. The second sodium source, iron source, solvent, and complexing agent are mixed to obtain a coating layer precursor micelle.

10. The method for preparing the core-shell structured cathode material according to claim 9, characterized in that, The first sodium source is selected from at least one of sodium carbonate, sodium hydroxide, and sodium dihydrogen phosphate; The metal source is selected from at least one of the following: iron source, nickel source, manganese source, copper source, cobalt source, titanium source, magnesium source, aluminum source, zinc source, and calcium source. The solvent is selected from at least one of deionized water, ethylene glycol, N-methylpyrrolidone, and ethanol; The dispersant is selected from at least one of polyacrylic acid, ethylene glycol, and polyvinyl alcohol; The second sodium source is selected from at least one of sodium carbonate, sodium hydroxide, and sodium dihydrogen phosphate; The iron source is selected from at least one of ferric nitrate, ferric oxide, ferric phosphate, ferrous sulfate, ferrous oxalate dihydrate, and elemental iron. The second solvent is selected from at least one of deionized water, ethylene glycol, N-methylpyrrolidone, and ethanol; The complexing agent is selected from at least one of citric acid, oxalic acid, glycine, and polyacrylic acid.

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