Sodium ion positive electrode material, preparation method thereof and sodium ion battery

Abstract

The invention relates to the technical field of battery materials, and particularly discloses a sodium ion positive electrode material, a preparation method thereof and a sodium ion battery, the sodium ion positive electrode material comprises a substrate particle, and a first coating layer and a second coating layer which sequentially coat the surface of the substrate particle; the substrate particles are O3 type layered oxides; the first coating layer is a P2 type layered oxide and a first carbon material; the second coating layer is a polyanion material and a second carbon material, and the polyanion material is a sodium ferric sulfate material. The sodium ion positive electrode material prepared by the invention is high in gram volume, excellent in cycle performance and relatively good in conductivity.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and particularly to a sodium-ion cathode material, a preparation method thereof, and a sodium-ion battery. Background Art

[0002] With the increasing global demand for clean energy, sodium-ion batteries, as a new type of battery technology, have received extensive attention due to their advantages such as rich resources, low cost, and high safety. Among them, the sodium-ion cathode material is the core raw material of sodium-ion batteries, and its performance directly affects the energy density, electrical performance, and cycle life of the battery.

[0003] Currently, the sodium-ion cathode materials that have been studied more include layered oxides, polyanion-based cathode materials, etc. Among them, polyanion-type cathode materials have the advantage of long cycle life, but their specific capacity is low. In the prior art, the performance is mainly improved by changing the structure or composition of the sodium-ion cathode material. For example, the electrochemical performance of the material is improved by bulk doping. However, the above methods still cannot provide a sodium-ion cathode material with a high specific capacity, good cycle performance, and good conductivity. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: aiming at the deficiencies existing in the prior art, to provide a sodium-ion cathode material, a preparation method thereof, and a sodium-ion battery. The sodium-ion cathode material prepared by the present invention has a high specific capacity, good cycle performance, and good conductivity.

[0005] To solve the above technical problem, the technical solution of the present invention is:

[0006] A sodium-ion cathode material includes substrate particles and a first coating layer and a second coating layer sequentially coated on the surface of the substrate particles; the substrate particles are O3-type layered oxides; the first coating layer is a P2-type layered oxide and a first carbon material; the second coating layer is a polyanion material and a second carbon material; the polyanion material is a sodium iron sulfate material.

[0007] Preferably, the structural formula of the O3-type layered oxide is Na x Ni a Fe b Mn c N d O2, where 0.95 ≤ x < 1.05, 0 ≤ a < 1, 0 ≤ b < 1, 0 ≤ c < 1, 0 ≤ d < 1, and N is selected from at least one of Co, Cu, Zn, and Ti metal elements.

[0008] Preferably, the structural formula of the P2-type layered oxide is Na x Ni 1 / (3-y) Mn 2 / (3-z) M(y+z) O2, where 0.95 ≤ x < 1.05, 0 ≤ y + z < 0.1, and M is selected from at least one of the metal elements Ni, Co, Mn, Fe, Cu, Zn, and Ti.

[0009] Preferably, the mass ratio of the O3-type layered oxide, P2-type layered oxide to the sodium iron sulfate material is (0.05 - 0.45):(0.05 - 0.45):(0.1 - 0.9).

[0010] To better solve the above technical problems, the present invention also provides the following technical solutions:

[0011] A preparation method of a sodium ion cathode material, comprising the following steps:

[0012] (1) Ball-mill the O3-type layered oxide to obtain substrate particles;

[0013] (2) Mix the substrate particles with the P2-type layered oxide and a first carbon source, and perform a first low-temperature sintering treatment. The P2-type layered oxide and the first carbon source form a first coating layer, and the first coating layer coats the substrate particles to obtain a first cathode material;

[0014] (3) Mix the first cathode material with a precursor material, and perform a second low-temperature sintering treatment. The precursor material forms a second coating layer, and the second coating layer wraps the first cathode material to obtain a sodium ion cathode material; the precursor material includes a sodium source, an iron source, and a second carbon source.

[0015] Preferably, in step (1), the average particle size of the substrate particles is 0.5 - 2 μm, the ball-milling time during the ball-milling treatment is 4 - 6 hours, and the ball-milling speed is 500 rpm.

[0016] Preferably, the carbon content in the first coating layer is less than or equal to the carbon content in the second coating layer.

[0017] Preferably, in step (2), the mass ratio of the substrate particles, P2-type layered oxide, and first carbon source is (0.05 - 0.45):(0.05 - 0.45):(0.0015 - 0.0135).

[0018] Preferably, in step (2), the conditions for the first low-temperature sintering treatment are: the sintering temperature is 280 - 380 °C, and the sintering time is 5 - 8 h.

[0019] Preferably, before the first low-temperature sintering treatment in step (2), it further includes: ball-milling the substrate particles with the P2-type layered oxide and the first carbon source, the ball-milling speed of the ball-milling treatment is 500 rpm, and the ball-milling time is 3 - 5 h.

[0020] Preferably, in step (3), the mass ratio of the first positive electrode material, the sodium source, the iron source, and the second carbon source is (0.11 - 9):(0.25 - 0.5):(0.2 - 0.48):(0.02 - 0.25).

[0021] Preferably, the sodium source is sodium sulfate; the iron source is at least one of ferrous sulfate, ferrous chloride, ammonium ferrous sulfate, ferrous lactate, ferric citrate, ferrocene, and ferrous glycinate;

[0022] or the sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium citrate, and sodium acetate, and the iron source is at least one of ferrous sulfate and ammonium ferrous sulfate.

[0023] Preferably, in step (2), the first carbon source is selected from at least one of carbon nanotubes, graphene, acetylene black, conductive carbon black, glucose, sucrose, polyethylene glycol, citric acid, starch, and phenolic resin.

[0024] Preferably, in step (3), the second carbon source is selected from at least one of carbon nanotubes, graphene, acetylene black, conductive carbon black, glucose, sucrose, polyethylene glycol, citric acid, starch, and phenolic resin.

[0025] Preferably, in step (3), the conditions for the second low-temperature sintering treatment are: sintering temperature 300 - 400 °C, sintering time 10 - 20 h.

[0026] Preferably, in step (3), before the second low-temperature sintering treatment, it further includes sequentially performing ball milling treatment and vacuum drying treatment on the first positive electrode material and the precursor material; the ball milling speed of the ball milling treatment is 500 rpm, and the ball milling time is 4 - 6 h; and / or the temperature of the vacuum drying is 75 - 85 °C, and the vacuum drying time is 10 - 20 h.

[0027] In order to better solve the above problems, the present invention also provides the following technical solutions:

[0028] A sodium-ion battery, the positive electrode sheet of the sodium-ion battery includes the above-mentioned sodium-ion positive electrode material.

[0029] Preferably, in the positive electrode sheet, the mass percentage content of the sodium-ion positive electrode material is 1 - 99 wt%.

[0030] Due to the adoption of the above technical solutions, the beneficial effects of the present invention are:

[0031] 1. The sodium-ion cathode material of the present invention has a "shell-II core-I core" structure. In the present invention, an O3-type layered oxide is used as the "I core" in the core-shell structure, a P2-type layered oxide-first carbon material layer is used as the "II core" in the core-shell structure, and a polyanion material (sodium iron sulfate material)-second carbon material layer is used as the "shell" in the core-shell structure. The P2-type layered oxide-first carbon material layer and the polyanion material (sodium iron sulfate material)-second carbon material layer are sequentially coated on the surface of the O3-type layered oxide. The O3-type layered oxide is used as the innermost core, which has a relatively high specific capacity; compared with the O3-type layered oxide, the P2-type layered oxide has a larger sodium layer spacing, faster ion transport ability, and high structural stability. It is used as the first coating layer to provide support for the material and protect the innermost O3-type layered oxide. At the same time, it can serve as a bridge for ion transport between the innermost core (I core) and the second coating layer (shell); the polyanion material has high thermal stability and excellent cycling performance. Using it as the second coating layer can improve the thermal stability and cycling performance of the core-shell material; the "shell-II core-I core" structure setting of the present invention enables better contact between the materials through interfacial bonding, thereby improving the conductivity of the material. The multi-layer core-shell structure can provide a certain "support" to improve the overall cycling stability of the material. At the same time, the core-shell structure also helps to improve the migration channels of sodium ions, thereby enhancing the conductivity and capacity utilization of the material. The "shell-II core-I core" structure setting of the present invention obtains a sodium-ion cathode material with a relatively high specific capacity, good cycling stability, and good conductivity.

[0032] In addition, when the polyanion material is wrapped on the outer layer of the layered oxide material, the layered oxide material is sensitive to the production environment, which will increase the production control cost. The structural design of this application avoids the direct contact between the layered oxide material and the environment and reduces the production cost.

[0033] The preparation method of the present invention can be realized by low-temperature sintering. On the one hand, it avoids the damage to the structure of the core material caused by high-temperature sintering and ensures the final performance of the sodium-ion cathode material; on the other hand, it reduces the production energy consumption, which is beneficial to large-scale production, and solves the problems of high energy consumption in the preparation method in the prior art and being not conducive to large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the drawings in the following description are only the embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained according to the provided drawings.

[0035] Figure 1 It is a schematic cross-sectional view of the sodium-ion cathode material;

[0036] In the figure, 10 is the base particle; 11 is the first coating layer; 12 is the second coating layer. Detailed implementation manners

[0037] In order to more clearly understand the above objects, features, and advantages of the present invention, the solution of the present invention will be further described below. It should be noted that, without conflict, the embodiments of the present invention and the features in the embodiments can be combined with each other.

[0038] In the following description, many specific details are set forth in order to fully understand the present invention, but the present invention can also be implemented in other ways different from those described herein; obviously, the embodiments in the specification are only a part of the embodiments of the present invention, rather than all the embodiments.

[0039] In order to solve the problem of "the existing sodium ion cathode materials in the prior art cannot simultaneously meet the requirements of high specific capacity and long cycle life" pointed out in the background art, a new sodium ion cathode material is provided.

[0040] The specific solution is as follows:

[0041] In a first aspect, the present invention provides a sodium ion cathode material, as Figure 1 shown, comprising a base particle 10 and a first coating layer 11 and a second coating layer 12 sequentially coated on the surface of the base particle; the base particle 10 is an O3-type layered oxide; the first coating layer 11 is a P2-type layered oxide and a first carbon material; the second coating layer 12 is a polyanion material and a second carbon material; the polyanion material is a sodium iron sulfate material.

[0042] The sodium-ion cathode material of the present invention has a "shell-II core-I core" structure. In the present invention, an O3-type layered oxide is used as the "I core" in the core-shell structure, a P2-type layered oxide-first carbon material layer is used as the "II core" in the core-shell structure, and a polyanion material (sodium iron sulfate material)-second carbon material layer is used as the "shell" in the core-shell structure. The P2-type layered oxide-first carbon material layer and the polyanion material (sodium iron sulfate material)-second carbon material layer are sequentially coated on the surface of the O3-type layered oxide. The O3-type layered oxide serves as the innermost core and has a relatively high specific capacity; compared with the O3-type layered oxide, the P2-type layered oxide has a larger sodium layer spacing, faster ion transport ability, and high structural stability. It is used as the first coating layer to provide support for the material and protect the innermost O3-type layered oxide. At the same time, it can act as a bridge for ion transport between the innermost core (I core) and the second coating layer (shell); the polyanion material has high thermal stability and excellent cycling performance. Using it as the second coating layer can improve the thermal stability and cycling performance of the core-shell material; the "shell-II core-I core" structure setting of the present invention enables better contact to be formed between the materials through interfacial bonding, thereby improving the conductivity of the material. The multi-layer core-shell structure can provide a certain "support" and improve the overall cycling stability of the material. At the same time, the core-shell structure also helps to improve the migration channels of sodium ions, thereby enhancing the conductivity and capacity utilization of the material. The "shell-II core-I core" structure setting of the present invention results in a sodium-ion cathode material with a relatively high specific capacity, good cycling stability, and good conductivity.

[0043] In addition, when the polyanion material is wrapped on the outer layer of the layered oxide material, the layered oxide material is sensitive to the production environment, which will increase the production control cost. The structural design of this application avoids the direct contact between the layered oxide material and the environment and reduces the production cost.

[0044] The specific capacity of the sodium-ion cathode material of this application is greater than 100 mAh / g. The theoretical specific capacity of sodium iron sulfate is less than 100 mAh / g, and in the prior art, the specific capacity of the sodium iron sulfate composite cathode material will not exceed 100 mAh / g even through doping modification. The structural design of this application can achieve a specific capacity of the sodium-ion cathode material greater than 100 mAh / g.

[0045] In some embodiments, the carbon content in the first coating layer is less than or equal to the carbon content in the second coating layer.

[0046] During coating, the present invention adds a first carbon material to the first coating layer and a second carbon material to the second coating layer. By adjusting the carbon content in the first coating layer to be less than or equal to that in the second coating layer, the conductivity of the first coating layer and the second coating layer is made similar, thereby avoiding the interlayer accumulation of sodium ions formed by a large difference in conductivity between the first coating layer and the second coating layer, improving the structural stability of the material, increasing the radial electronic conductivity of the material particles, further improving the electrical performance of the sodium-ion battery, enhancing the tap density of the sodium-ion cathode material, and further increasing the volumetric specific capacity of the sodium-ion battery.

[0047] In some embodiments, the structural formula of the O3-type layered oxide is Na x Ni a Fe b Mn c N d O2, where 0.95 ≤ x < 1.05, 0 ≤ a < 1, 0 ≤ b < 1, 0 ≤ c < 1, 0 ≤ d < 1, and N is selected from at least one of the metal elements Co, Cu, Zn, and Ti.

[0048] In some embodiments, the structural formula of the P2-type layered oxide is Na x Ni 1 / (3-y) Mn 2 / (3-z) M (y+z) O2, where 0.95 ≤ x < 1.05, 0 ≤ y + z < 0.1, and M is selected from at least one of the metal elements Ni, Co, Mn, Fe, Cu, Zn, and Ti.

[0049] In some embodiments, the mass ratio of the O3-type layered oxide, the P2-type layered oxide, and the sodium iron sulfate material is (0.05 - 0.45):(0.05 - 0.45):(0.1 - 0.9).

[0050] The present invention regulates the O3-type layered oxide as the "Ⅰ core" in the core-shell structure, the P2-type layered oxide - first carbon material layer as the "Ⅱ core" in the core-shell structure, and the polyanion material (sodium iron sulfate material) - second carbon material layer as the "shell" in the core-shell structure. By optimizing the ratio among the O3-type layered oxide, the P2-type layered oxide, and the sodium iron sulfate material in the sodium-ion cathode material, the advantages of each layer of the material are enhanced, thereby ensuring the overall capacity and stability of the material. After testing, the sodium-ion cathode material prepared by the present invention has a stable structure and less capacity decay during charge and discharge, improving the service life of the battery, and solving the problems of unstable structure, capacity decay, and affecting the service life of the battery that may occur in the sodium-ion cathode material in the prior art during charge and discharge.

[0051] In a second aspect, the present invention provides a method for preparing a sodium-ion cathode material, comprising the following steps:

[0052] (1) The O3-type layered oxide is ball-milled to obtain substrate particles;

[0053] (2) The substrate particles are mixed with P2-type layered oxide and a first carbon source, and a first low-temperature sintering treatment is carried out. The P2-type layered oxide and the first carbon source form a first coating layer, and the first coating layer coats the substrate particles to obtain a first positive electrode material;

[0054] (3) The above-mentioned first positive electrode material is mixed with a precursor material, and a second low-temperature sintering treatment is carried out. The precursor material forms a second coating layer, and the second coating layer wraps the first positive electrode material to obtain a sodium-ion positive electrode material; the precursor material is a mixture of a sodium source, an iron source, and a second carbon source; the second coating layer is a sodium iron sulfate material and a second carbon material.

[0055] In the above solution, the present invention coats the surface of the substrate particles with P2-type layered oxide and a first carbon material through the first low-temperature sintering treatment; through the second low-temperature sintering treatment, a polyanion material and a second carbon material are in-situ coated on the surface of the substrate particles coated with P2-type layered oxide and carbon material. This material structure design can effectively improve the specific capacity of the sodium-ion positive electrode material and improve the cycling performance of the material. The preparation method of the present invention can be realized by low-temperature sintering. On the one hand, it avoids the damage to the structure of the core material caused by high-temperature sintering, and ensures the final performance of the sodium-ion positive electrode material; on the other hand, it reduces the production energy consumption, is conducive to large-scale production, and solves the problem of high energy consumption in the preparation method in the prior art and is not conducive to large-scale production.

[0056] In some embodiments, in step (1), the average particle size of the substrate particles is 0.5 - 2 μm, the ball-milling time during the ball-milling treatment is 4 - 6 hours, and the ball-milling speed is 500 rpm.

[0057] The O3-type layered oxide is refined by ball-milling treatment. The smaller the particle size of the substrate particles, the larger the specific surface area, thereby increasing the contact area between the electrolyte and the active material, which is beneficial to improving the charge-discharge efficiency and cycling stability. The present invention controls the particle size of the substrate particles to prevent large volume changes, cracks, and pulverization of the material during the charge-discharge process, thereby enhancing the stability of the material and improving the cycling performance.

[0058] In some embodiments, in step (2), the mass ratio of the substrate particles, P2-type layered oxide, and first carbon source is (0.05 - 0.45):(0.05 - 0.45):(0.0015 - 0.0135).

[0059] In the above solution, the theoretical specific capacity of the base particles is high. The P2-type layered oxide has a large sodium layer spacing, fast ion transport ability, and high structural stability. Using it as the first coating layer can provide support for the material, protect the core, and also serve as a bridge for ion transport between the core and the second coating layer. During specific coating, the mass ratio of the base particles, P2-type layered oxide, and the first carbon source is controlled to be (0.05 - 0.45):(0.05 - 0.45):(0.0015 - 0.0135), so as to form a first coating layer with an appropriate thickness on the surface of the base particles, ensuring the structural stability of the material without affecting the specific capacity of the material, preventing volume changes of the base particles during charge and discharge, reducing cracks and pulverization of the material, and improving the cycle life.

[0060] In some embodiments, in step (2), the conditions for the first low-temperature sintering treatment are: the sintering temperature is 280 - 380 °C, and the sintering time is 5 - 8 h.

[0061] In the above solution, in the preparation of the sodium-ion cathode material, when using P2-type layered oxide and the first carbon material to coat the O3-type layered oxide, the control of the sintering temperature and sintering time has a significant impact on the coating effect. Specifically, if the temperature is too low or the sintering time is too short, the P2-type layered oxide and the first carbon material cannot be fully combined with the O3-type layered oxide, resulting in a weak coating layer, which affects the conductivity and cycle stability of the material. If the temperature is too high, it may cause phase transformation or destruction of the crystal structure of the material, affecting the uniformity of the coating and the overall performance of the cathode material. If the sintering time is too long, it may lead to over-sintering or melting of the material, causing degradation of the coating layer and having a negative impact on the structure and performance of the material. Therefore, to ensure good coating, the present invention controls the first sintering temperature to be 280 - 380 °C and the sintering time to be 5 - 8 h.

[0062] In some embodiments, in step (2), before the first low-temperature sintering treatment, it further includes: ball-milling the base particles, P2-type layered oxide, and the first carbon source. The ball-milling speed of the ball-milling treatment is 500 rpm, and the ball-milling time is 3 - 5 h. In the present invention, before sintering, through reasonable ball-milling speed and ball-milling time, the base particles, P2-type layered oxide, and the first carbon source are fully mixed, improving the uniformity of the coating during sintering.

[0063] In some embodiments, in step (3), the mass ratio of the first cathode material, sodium source, iron source, and the second carbon source is (0.11 - 9):(0.25 - 0.5):(0.2 - 0.48):(0.02 - 0.25).

[0064] In some embodiments, the sodium source is sodium sulfate; the iron source is at least one of ferrous sulfate, ferrous chloride, ammonium ferrous sulfate, ferrous lactate, ferric citrate, ferrocene, and iron glycinate;

[0065] or the sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium citrate, and sodium acetate, and the iron source is at least one of ferrous sulfate and ammonium ferrous sulfate.

[0066] In the present invention, by regulating the O3-type layered oxide as the "core Ⅰ" in the core-shell structure, the P2-type layered oxide - first carbon material layer as the "core Ⅱ" in the core-shell structure, and the polyanion material (sodium iron sulfate material) - second carbon material layer as the "shell" in the core-shell structure, by optimizing the ratio between the mass ratios of the first cathode material to the sodium source, iron source, and second carbon source in the sodium-ion cathode material, the advantages of each layer of materials are enhanced, thereby ensuring the overall capacity and stability of the material. The sodium-ion cathode material prepared by the present invention has a stable structure, less capacity decay during charge and discharge, and improves the service life of the battery.

[0067] In some embodiments, in step (2), the first carbon source is selected from at least one of carbon nanotubes, graphene, acetylene black, conductive carbon black, glucose, sucrose, polyethylene glycol, citric acid, starch, and phenolic resin.

[0068] In some embodiments, in step (3), the second carbon source is selected from at least one of carbon nanotubes, graphene, acetylene black, conductive carbon black, glucose, sucrose, polyethylene glycol, citric acid, starch, and phenolic resin.

[0069] In some embodiments, in step (3), the conditions for the second low-temperature sintering treatment are: sintering temperature 300 - 400 °C, sintering time 10 - 20 h.

[0070] In the above solution, the present invention selects appropriate sodium source, iron source, and second carbon source as coating raw materials to coat the base particles coated with the first coating layer again, and forms a polyanion material doped with the second carbon material as the second coating layer outside the first coating layer. The polyanion material has high thermal stability and excellent cycling performance. Using it as the second coating layer can improve the thermal stability and cycling performance of the core-shell material. At the same time, the polyanion material can be prepared by a low-temperature sintering process, which has no influence on the material properties of the inner core and the first coating layer, and can ensure the electrical performance of the material.

[0071] During specific sintering, the selection of sintering temperature and sintering time is also extremely crucial. For the sintering temperature, if the sintering temperature is too low, it will lead to insufficient reaction or poor bonding between materials, resulting in poor coating effect and limited electrochemical performance. While if the sintering temperature is too high, it may cause phase transformation or structural damage, leading to a decline in the quality of the coating layer and affecting the performance of the overall material. For the sintering time, if the sintering time is too short, the reaction will also be insufficient, resulting in an uneven coating layer. If the sintering time is too long, it may cause over-sintering, affecting the structural stability and electrochemical performance of the material, and even leading to thermal degradation.

[0072] In some embodiments, in step (3), before the second low-temperature sintering treatment, it further includes: successively performing ball milling treatment and vacuum drying treatment on the first cathode material, sodium source, iron source, and second carbon source. The ball milling speed of the ball milling treatment is 500 rpm, and the ball milling time is 4 - 6 h; the temperature of the vacuum drying is 75 - 85 °C, and the time of the vacuum drying is 10 - 20 h. In the present invention, before sintering, through ball milling treatment, the materials are mixed more uniformly, thereby improving the uniformity of the coating.

[0073] In some embodiments, the structural formula of the O3-type layered oxide is Na x Ni a Fe b Mn c N d O2, where 0.95 ≤ x < 1.05, 0 ≤ a < 1, 0 ≤ b < 1, 0 ≤ c < 1, 0 ≤ d < 1, and N is selected from at least one of the metal elements Co, Cu, Zn, and Ti.

[0074] In some embodiments, the O3-type layered oxide and the P2-type layered oxide can be prepared according to the prior art or purchased externally.

[0075] In some embodiments, the preparation method of the O3-type layered oxide is as follows:

[0076] Dissolve the transition metal salt and the sodium salt in deionized water, and obtain a sodium-containing precursor through spray drying. Press the sodium-containing precursor into tablets, and perform high-temperature solid-phase sintering in an air atmosphere. The sintering conditions are a temperature of 800 - 900 °C for 10 - 14 h, and a heating rate of 5 - 12 °C / min. After crushing and screening, an O3-type layered oxide sodium-ion cathode material is obtained.

[0077] In some embodiments, the transition metal salt can be at least one of sulfates, acetates, and chlorates of elements such as Ni, Fe, Mn, Co, Cu, Zn, and Ti;

[0078] In some embodiments, the sodium salt can be one of sodium bicarbonate, sodium carbonate, sodium hydroxide, and sodium sulfate;

[0079] In some embodiments, the structural formula of the P2-type layered oxide is Na x Ni 1 / (3-y) Mn 2 / (3-z) M (y+z) O2, where 0.95 ≤ x < 1.05, 0 ≤ y + z < 0.1, and M is selected from at least one of the metal elements Ni, Co, Mn, Fe, Cu, Zn, and Ti.

[0080] In some embodiments, the method for preparing the P2-type layered oxide is as follows:

[0081] Add transition metal salts and sodium salts to deionized water, and carry out ball milling and mixing. The ball milling conditions are a rotation speed of 200 - 350 rpm and a time of 2 - 4 h; obtain a preform through spray drying and pressing, subject the preform to high-temperature calcination. The calcination conditions are maintaining the temperature at 800 - 950 °C for 8 - 12 h, with a heating rate of 3 - 10 °C / min. After calcination, carry out crushing and screening to obtain the P2-type layered oxide sodium ion cathode material.

[0082] In some embodiments, the transition metal salts can be at least one of carbonates and oxides of elements such as Mn, Ni, Fe, Mg, Al, and Co;

[0083] In some embodiments, the sodium salt can be one of sodium carbonate, sodium hydroxide, and sodium oxide.

[0084] In a third aspect, the present invention provides a sodium ion battery, and the positive electrode sheet of the sodium ion battery includes the above-mentioned sodium ion cathode material. Further, in the positive electrode sheet, the mass percentage content of the sodium ion cathode material is 1 - 99 wt%.

[0085] To further understand the present invention, the following describes the preferred implementation embodiments of the present invention in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, rather than limiting the claims of the present invention.

[0086] Unless otherwise specified, the materials in the following examples and comparative examples are all commercially available.

[0087] Example 1

[0088] A method for preparing a sodium ion cathode material includes the following steps:

[0089] S1: Place the O3-type layered oxide with the chemical formula Na 0.7 Co 0.2 Mn 0.8 O2 into a ball mill, add ethanol as the ball milling medium, and ball mill at a ball milling speed of 500 rpm for 5 h, and obtain substrate particles with a particle size of 0.5 μm through spray drying;

[0090] S2: Mix the above base particles with a P2-type layered oxide with the chemical formula Na 0.6 Fe 0.4 Mn 0.6 O2 and glucose (the mass ratio of the base particles, P2-type layered oxide, and glucose is 1:0.5:0.03) and add them to a ball mill. Ball mill for 4 h at a ball milling speed of 500 rpm. Sinter the ball-milled solid powder for 6 h under an inert atmosphere at a sintering temperature of 280 °C. After the sintered powder is crushed and ball milled, base particles coated with a first coating layer are obtained;

[0091] S3: Mix the above base particles coated with a first coating layer with anhydrous ferrous sulfate, sodium sulfate, carbon nanofibers, and glucose (the mass ratio of the base particles coated with a first coating layer, anhydrous ferrous sulfate, sodium sulfate, carbon nanofibers, and glucose is 1:0.35:0.4:0.06:0.04), put them into a ball mill, ball mill for 5 h at a ball milling speed of 500 rpm, vacuum dry the ball-milled material at 80 °C for 12 h, then sinter for 15 h under an inert atmosphere at 300 °C. After the sintered material is crushed and sieved, a sodium ion cathode material is obtained.

[0092] Example 2

[0093] Compared with Example 1, the distinguishing feature is that in step S2, the mass ratio of the base particles, P2-type layered oxide, and glucose is 1:0.5:0.05, and other conditions are the same as those in Example 1.

[0094] Example 3

[0095] Compared with Example 1, the distinguishing feature is that in step S2, the mass ratio of the base particles, P2-type layered oxide, and glucose is 1:0.7:0.03, and other conditions are the same as those in Example 1.

[0096] Example 4

[0097] Compared with Example 1, the distinguishing feature is that in step S2, the sintering temperature is 350 °C, and other conditions are the same as those in Example 1.

[0098] Example 5

[0099] Compared with Example 1, the distinguishing feature is that in step S2, the sintering time is 8 h, and other conditions are the same as those in Example 1.

[0100] Example 6

[0101] Compared with Example 1, the distinguishing feature is that in step S3, the mass ratio of the substrate particles coated with the first coating layer to anhydrous ferrous sulfate, sodium sulfate, carbon nanofibers, and glucose is 1:0.35:0.4:0.08:0.06, and other conditions are the same as those in Example 1.

[0102] Example 7

[0103] Compared with Example 1, the distinguishing feature is that in step S3, the sintering temperature is 400 °C, and other conditions are the same as those in Example 1.

[0104] Example 8

[0105] Compared with Example 1, the distinguishing feature is that in step S3, the sintering time is 20 h, and other conditions are the same as those in Example 1.

[0106] In order to better verify that the sodium ion cathode material prepared by the present invention has excellent performance, multiple comparative examples are given below for detailed description.

[0107] Comparative Example 1

[0108] Compared with Example 1, the distinguishing feature is that in step S2, the mass ratio of the substrate particles, P2-type layered oxide, and glucose is 1:0.5:0.35, and other conditions are the same as those in Example 1.

[0109] Comparative Example 2

[0110] Compared with Example 1, the distinguishing feature is that in step S2, the mass ratio of the substrate particles, P2-type layered oxide, and glucose is 1:10:0.03, and other conditions are the same as those in Example 1.

[0111] Comparative Example 3

[0112] Compared with Example 1, the distinguishing feature is that in step S2, the sintering temperature is 400 °C, and other conditions are the same as those in Example 1.

[0113] Comparative Example 4

[0114] Compared with Example 1, the distinguishing feature is that in step S2, the sintering time is 10 h, and other conditions are the same as those in Example 1.

[0115] Comparative Example 5

[0116] Compared with Example 1, the distinguishing feature is that in step S3, the mass ratio of the substrate particles coated with the first coating layer to anhydrous ferrous sulfate, sodium sulfate, carbon nanofibers, and glucose is 1:0.5:0.5:0.06:0.04, and other conditions are the same as those in Example 1.

[0117] Comparative Example 6

[0118] Compared with Example 1, the distinguishing feature is that in step S3, the mass ratio of the substrate particles coated with the first coating layer to ferrous sulfate anhydrous, sodium sulfate, carbon nanofibers and glucose is 1:0.35:0.4:0.15:0.15, and other conditions are the same as those in Example 1.

[0119] Comparative Example 7

[0120] Compared with Example 1, the distinguishing feature is that in step S3, the sintering temperature is 420 °C, and other conditions are the same as those in Example 1.

[0121] Comparative Example 8

[0122] Compared with Example 1, the distinguishing feature is that in step S3, the sintering time is 22 h, and other conditions are the same as those in Example 1.

[0123] Comparative Example 9

[0124] Compared with Example 1, the distinguishing feature is that step S3 is not included, and other conditions are the same as those in Example 1.

[0125] Comparative Example 10

[0126] Compared with Example 1, the distinguishing feature is that step S2 is not included, and the polyanionic material and the carbon material are directly coated on the surface of the substrate particles. When coating, the mass ratio of the substrate particles to ferrous sulfate anhydrous, sodium sulfate, carbon nanofibers and glucose is 1:0.35:0.4:0.06:0.04, and other conditions are the same as those in Example 1.

[0127] Comparative Example 11

[0128] The sodium-ion cathode material does not have a core-shell structure and is a mixture of O3-type layered oxide, P2-type layered oxide, polyanionic material and carbon material, and the proportion of each component is the same as that of the sodium-ion cathode material in Example 1.

[0129] Comparative Example 12

[0130] The sodium-ion cathode material is an O3-type layered oxide.

[0131] Comparative Example 13

[0132] The sodium-ion cathode material is a polyanionic material. The specific preparation method is as follows: Mix ferrous sulfate anhydrous and sodium sulfate in a mass ratio of 0.35:0.4 and put them into a ball mill. Ball mill at a ball milling speed of 500 rpm for 5 h. Vacuum dry the ball-milled material at 80 °C for 12 h, and then sinter at 300 °C under an inert atmosphere for 15 h. The sintered material is crushed and sieved to obtain the sodium-ion cathode material.

[0133] The performance of the sodium-ion cathode materials prepared in the above examples and comparative examples was tested. The test methods and results are as follows:

[0134] 1. Specific capacity test of sodium-ion cathode material:

[0135] The test method is as follows:

[0136] Preparation of coin sodium-ion battery: The sodium-ion cathode materials, conductive agent (acetylene black), and binder (polyvinylidene fluoride) prepared in the above examples and comparative examples were dispersed in NMP at a ratio of 90 wt%: 5 wt%: 5 wt% to obtain a positive electrode slurry. The above slurry was coated on one side of aluminum foil using a squeegee, baked at 120 °C for 12 h, and then roll-pressed and cut to obtain a coin cell positive electrode sheet; the above positive electrode sheet, a 20-μm glass fiber separator, and a sodium metal sheet were dropped into a sodium-ion battery electrolyte to assemble a 2032-type coin sodium-ion battery;

[0137] Battery performance test: The above 2032-type coin sodium-ion battery after standing at room temperature was connected to a charge-discharge device, and the capacity was tested at a current density of 0.1C and a voltage range of 2.0 - 4.5V.

[0138] 2. Cycle performance test of the battery:

[0139] The test method is as follows:

[0140] Battery preparation: A battery stack was prepared by laminating a sodium-ion battery positive electrode sheet, a hard carbon negative electrode sheet, and a 16-μm wet separator using a lamination process, encapsulated with an aluminum-plastic film, dried, and then injected with a sodium-ion battery electrolyte. After activation, exhaust, and sealing, a 10-Ah soft-pack battery was obtained;

[0141] Battery cycle performance test: The above soft-pack battery was connected to a test device, and under the conditions of a room temperature of 0.5C rate and a voltage range of 2.6 - 4.2V, a charge-discharge cycle test was carried out; the cycle was 500 weeks, and the discharge capacity of the battery in the 500th week was recorded; the 500-week cycle capacity retention rate = 500-week cycle capacity / initial capacity * 100%.

[0142] 3. Compaction density:

[0143] A certain mass of the sample was placed in the inner hole groove of the mold, and the parameters were set: the test pressure was 5000 kg, the mold diameter was 13 mm, the constant pressure time was 30 s, and the test mass was 0.5577 g.

[0144] 4. Resistivity of the material powder:

[0145] The resistivity of the powder of the sample was tested using an SZT-D type semiconductor powder resistivity tester (90 MPa, pressure holding for 10 s).

[0146] The test results are shown in Table 1.

[0147] Table 1

[0148]

[0149]

[0150] It can be seen from the above test results that, compared with the comparative examples, the sodium-ion cathode material obtained by successively coating a P2 layered oxide - first carbon material layer and a polyanion material - second carbon material layer on the surface of the O3 layered oxide in the present invention has a multi-layer core-shell structure. This material structure design effectively improves the specific capacity of the sodium-ion cathode material, improves the cycling performance of the material, and the interfacial bonding between the layers in this material is good, effectively improving the tap density of the material.

[0151] According to Examples 1-3, Example 6, Comparative Examples 1-2, and Comparative Examples 5-6, it can be known that during coating, the amounts of the first coating layer and the second coating layer have a great influence on the performance of the material. Within a certain range, as the amounts of the first coating layer and the second coating layer increase, the specific capacity of the sodium-ion cathode material also increases, and the cycling performance of the material is significantly improved; however, if the amounts of the first coating layer and the second coating layer are too large, resulting in too thick a coating layer, the performance of the material will instead decline. This is mainly because too large a coating thickness will affect the proportion of the high-capacity layered oxide in the material.

[0152] According to Examples 4-5, Examples 7-8, Comparative Examples 3-4, and Comparative Examples 7-8, it can be known that during coating, within a certain range, as the sintering temperature increases and the sintering time prolongs, the coating effect is better, and the performance of the prepared sodium-ion cathode material is also more excellent. However, if the sintering temperature is too high and the sintering time is too long, the specific capacity and cycling performance of the material will instead decline. This is mainly because too high a sintering temperature will cause phase transformation or structural damage, resulting in a decline in the quality of the coating layer and affecting the performance of the overall material. And too long a sintering time not only increases energy consumption but also causes over-sintering, affecting the structural stability and electrochemical performance of the material, and even leading to thermal degradation.

[0153] According to Example 1 and Comparative Examples 9-10, in Comparative Example 9, a P2-type layered oxide - first carbon material is coated on the surface of the O3-type layered oxide, and in Comparative Example 10, a polyanion material - second carbon material is coated on the surface of the O3-type layered oxide. The specific capacity and cycling performance of the material both decline to a certain extent.

[0154] According to Example 1 and Comparative Examples 11-13, it can be seen that the sodium ion cathode materials in Comparative Example 12 and Comparative Example 13 are of a single type, and the comprehensive performance of the materials is significantly inferior to that of the material in Example 1. Compared with Example 1, the performance of the material in Comparative Example 11 is also significantly reduced. When preparing the sodium ion cathode material, directly mixing various materials will not improve the defect points of a single material, and there will be no synergistic effect between the materials. However, the material of the present invention has a multi-layer core-shell structure. Through coating, better contact is formed between various materials through interfacial bonding, thereby improving the conductivity of the material. The multi-layer core-shell structure can provide a certain "support" to improve the overall cycle stability of the material. In addition, the core-shell structure also helps to improve the migration channel of sodium ions, thereby enhancing the conductivity and capacity performance of the material.

[0155] In this article, specific examples are used to elaborate on the principles and implementation manners of the present invention. The descriptions of the above embodiments are only used to help understand the method and its core idea of the present invention, including the best mode, and also enable any person skilled in the art to practice the present invention, including manufacturing and using any device or system, and implementing any combined method. It should be noted that for those of ordinary skill in the art in the technical field of the present invention, without departing from the principle of the present invention, several improvements and modifications can be made to the present invention, and these improvements and modifications also fall within the protection scope of the claims of the present invention. The protection scope of the present invention is defined by the claims and may include other embodiments that can be conceived by those skilled in the art. If these other embodiments have structural elements similar to the literal description of the claims, or if they include equivalent structural elements that have no substantial difference from the literal description of the claims, then these other embodiments should also be included within the scope of the claims.

Claims

1. A sodium ion positive electrode material, characterized in that: It includes a base particle and a first coating layer and a second coating layer sequentially coated on the surface of the base particle; the base particle is an O3 type layered oxide; the first coating layer is a P2 type layered oxide and a first carbon material; the second coating layer is a polyanion material and a second carbon material; the polyanion material is a sodium iron sulfate material.

2. A sodium ion positive electrode material according to claim 1, characterized in that: The structural formula of the O3-type layered oxide is Na x Ni a Fe b Mn c N d O2, wherein 0.95≤x<1.05, 0≤a<1, 0≤b<1, 0≤c<1, 0≤d<1, and N is selected from at least one of Co, Cu, Zn, and Ti metal elements; And / or the structural formula of the P2 type layered oxide is Na x Ni 1 / (3-y) Mn 2 / (3-z) M (y+z) O2, wherein 0.95≤x<1.05, 0≤y+z<0.1, and M is selected from at least one of Ni, Co, Mn, Fe, Cu, Zn, and Ti metal elements; And / or the mass ratio of the O3 type layered oxide, the P2 type layered oxide and the sodium iron sulfate material is (0.05-0.45):(0.05-0.45):(0.1-0.9).

3. A method for preparing a sodium ion positive electrode material, characterized in that: The following steps are involved: (1) ball milling the O3-type layered oxide to obtain base particles; (2) mixing the base particles, the P2-type layered oxide and the first carbon source, and performing a first low-temperature sintering treatment, wherein the P2-type layered oxide and the first carbon source form a first coating layer, and the first coating layer coats the base particles, thereby obtaining a first positive electrode material; (3) The first positive electrode material is mixed with a precursor material and subjected to a second low-temperature sintering treatment, wherein the precursor material forms a second coating layer, and the second coating layer wraps the first positive electrode material to obtain a sodium ion positive electrode material; the precursor material includes a sodium source, an iron source and a second carbon source; and the second coating layer is a sodium iron sulfate material and a second carbon material.

4. The method for preparing a sodium ion positive electrode material according to claim 3, characterized in that: The first carbon source is selected from at least one of carbon nanotubes, graphene, acetylene black, conductive carbon black, glucose, sucrose, polyethylene glycol, citric acid, starch, and phenolic resin; and / or the second carbon source is selected from at least one of carbon nanotubes, graphene, acetylene black, conductive carbon black, glucose, sucrose, polyethylene glycol, citric acid, starch, and phenolic resin; And / or the carbon content in the first coating layer is less than or equal to the carbon content in the second coating layer.

5. The method for preparing a sodium ion positive electrode material according to claim 3, characterized in that: In step (2), the conditions of the first low-temperature sintering treatment are: the first sintering temperature is 280-380° C., and the first sintering time is 5-8 hours; And / or in step (3), the conditions of the second low-temperature sintering treatment are: a second sintering temperature of 300-400° C., and a second sintering time of 10-20 h.

6. The method for preparing a sodium ion positive electrode material according to claim 3, characterized in that: In step (2), before performing the first low-temperature sintering process, the method further comprises: ball milling the base particles, the P2-type layered oxide and the first carbon source; The ball milling speed of the ball milling treatment is 500 rpm, and the ball milling time is 3-5 h.

7. The method for preparing a sodium ion positive electrode material according to claim 3, characterized in that: In step (2), the mass ratio of the base particles, the P2-type layered oxide, and the first carbon source is (0.05-0.45): (0.05-0.45): (0.0015-0.0135); In step (3), the mass ratio of the first positive electrode material to the sodium source, the iron source and the second carbon source is (0.11-9): (0.25-0.5): (0.2-0.48): (0.02-0.25).

8. The method for preparing a sodium ion positive electrode material according to claim 3, characterized in that: The sodium source is sodium sulfate; the iron source is at least one of ferrous sulfate, ferrous chloride, ammonium ferrous sulfate, ferrous lactate, ferrous citrate, ferrocene, and glycine iron; or the sodium source is at least one of sodium carbonate, sodium bicarbonate, sodium hydroxide, sodium citrate, and sodium acetate, and the iron source is at least one of ferrous sulfate and ammonium ferrous sulfate.

9. The method for preparing a sodium ion positive electrode material according to claim 3, characterized in that: In step (3), before performing the second low-temperature sintering process, the method further includes: The first cathode material and the precursor material are sequentially subjected to ball milling treatment and vacuum drying treatment; the ball milling speed of the ball milling treatment is 500 rpm, and the ball milling time is 4-6 hours; the vacuum drying temperature is 75-85° C., and the vacuum drying time is 10-20 hours.

10. A sodium ion battery, characterized in that: The positive electrode sheet of the sodium ion battery comprises a sodium ion positive electrode material prepared by the method according to any one of claims 3 to 9; in the positive electrode sheet, the mass percentage of the sodium ion positive electrode material is 1-99wt%.

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