A positive electrode material, a preparation method thereof, and a sodium ion battery

Abstract

This application provides a cathode material and its preparation method, as well as a sodium-ion battery, which enables the cathode material to possess air stability while also improving its specific capacity. The cathode material includes: an inner layer material and a coating layer covering the inner layer material; the molecular formula of the inner layer material is: Na. m Ni u Fe v Mn w O2, the coating layer is Na n Mn 2‑n‑x‑y Ru x Sn y O2; where 0.7<m≤1, 0.11≤u≤0.44, 0.11≤w≤0.44, 0.11≤v≤0.66, u+v+w=1, 1.10≤n≤1.20, x≥0, y≥0, 0.05≤x+y≤0.2.

Description

Technical Field

[0001] This application relates to the field of sodium-ion battery technology, and in particular to a cathode material and its preparation method, and a sodium-ion battery. Background Technology

[0002] With the deepening of environmental awareness and the improvement of people's living standards, current lithium-ion batteries can no longer meet people's needs. At the same time, lithium suffers from the problem of low and uneven distribution. Therefore, sodium, which is in the same group as lithium in the periodic table, has come into focus due to its similar physicochemical properties and abundant reserves; among these, sodium-ion cathode materials are one of the key research objects of sodium-ion batteries.

[0003] Currently, cathode materials for sodium-ion batteries include polyanionic materials, transition metal oxides, Prussian white materials, and organic materials. Among these, transition metal oxides are characterized by ease of preparation, non-toxicity, harmlessness, and low cost. However, these materials suffer from poor air stability; they readily react with water (H2O) and carbon dioxide (CO2) in the air to form non-electrochemically active sodium carbonate (Na2CO3) and sodium hydroxide (NaOH), leading to a decrease in the capacity of the sodium-ion cathode material and making it difficult to coat during battery fabrication. Although existing technologies improve the air stability of the aforementioned sodium-ion cathode materials by using metal oxides (e.g., zirconium oxide, magnesium oxide), phosphates, or carbon materials as coating layers, the lack of electrochemical activity in these coating layers still reduces the capacity of the sodium-ion cathode material. Currently, there is still a lack of sodium-ion cathode materials with both good air stability and high specific capacity. Summary of the Invention

[0004] This application provides a cathode material and its preparation method, as well as a sodium-ion battery, which enables the cathode material to have air stability while improving its specific capacity, thereby increasing the capacity of the sodium-ion battery when applied to it.

[0005] In a first aspect, embodiments of this application provide a cathode material, comprising:

[0006] Inner layer material and a coating layer covering the inner layer material; the molecular formula of the inner layer material is: Na m Ni u Fe v Mn w O2, the coating layer is Na n Mn 2-n-x-y Ru x Sn yO2; where 0.7<m≤1, 0.11≤u≤0.44, 0.11≤w≤0.44, 0.11≤v≤0.66, u+v+w=1, 1.10≤n≤1.20, x≥0, y≥0, 0.05≤x+y≤0.2.

[0007] The aforementioned cathode material, due to the presence of the coating layer, prevents its inner layer from contacting water and / or carbon dioxide in the air, thus enhancing the cathode material's air stability. Simultaneously, the coating layer is a sodium-rich material, allowing for the provision of more active material (Na₂O₃) during the charge-discharge process. + This is used for extraction and insertion, thereby increasing the capacity of the cathode material. Furthermore, in the cathode material coating layer provided in this application, the presence of transition metal atoms tin (Sn) and ruthenium (Ru) can regulate the local coordination of oxygen (O), which is beneficial for more oxygen atoms to participate in the redox reaction during the charging-discharging process of the cathode material, resulting in a higher proportion (i.e., the ratio of extracted / inserted atoms to those not involved in extraction / insertion) of Na. + Extraction and insertion significantly improve the specific capacity of the cathode material.

[0008] In one possible implementation, the mass of the covering layer is 0.3%-0.8% of the mass of the inner layer material.

[0009] In one possible implementation, the specific surface area of ​​the positive electrode material is 0.75-0.85 g / m². 2 .

[0010] In one possible implementation, the loose packing density of the positive electrode material is 0.8-0.95 g / cm³. 3 The tap density of the positive electrode material is 1.65-1.80 g / cm³. 3 .

[0011] In one possible implementation, the cathode material has a specific capacity of 120-170 mAh / g within a voltage window of 2-4.2V.

[0012] In one possible implementation, the capacity retention rate of the positive electrode material is 85%-95%; wherein the capacity retention rate is the ratio between the capacity of the full cell containing the positive electrode material during the 20th charge-discharge cycle at 2-4.2V, 1C conditions and the capacity during the 1st cycle.

[0013] Secondly, embodiments of this application provide a method for preparing the cathode material as described in the first aspect and any possible implementation, comprising:

[0014] A first sintering treatment is performed on a first mixture including a tin source and / or a ruthenium source, and a sodium source and a manganese source to obtain a first sintered product corresponding to a coating layer of the cathode material; and a second sintering treatment is performed on a second mixture including a sodium source, a nickel source, an iron source and a manganese source, or a third mixture including a sodium source and a precursor, to obtain a second sintered product corresponding to an inner layer material of the cathode material; wherein the first sintering temperature of the first sintering treatment and the second sintering temperature of the second sintering treatment are each independently performed at a temperature of 900-1000°C;

[0015] The mixture of the first sintering product and the second sintering product is subjected to a third sintering treatment at a temperature of 450-750℃ to obtain the cathode material.

[0016] In one possible implementation, the sintering temperature of the third sintering treatment can be 500°C.

[0017] In one possible implementation, in the mixture of the first sintered product and the second sintered product, the mass of the first sintered product is 0.3%-0.8% of the mass of the second sintered product.

[0018] In one possible implementation, the first sintering time of the first sintering treatment and the second sintering time of the second sintering treatment are each independently selected from 10-15 hours, and the third sintering time of the third sintering treatment is 5-10 hours.

[0019] In one possible implementation, the first sintering time and the second sintering time are both 12 hours, and the third sintering time is 7 hours.

[0020] In one possible implementation, the first sintering temperature and the second sintering temperature are 930°C.

[0021] In one possible implementation, the first heating rate of the first sintering treatment and the second heating rate of the second sintering treatment are each carried out independently at a temperature of 1-5°C / min, and the third heating rate of the third sintering treatment is 1-5°C / min.

[0022] In one possible implementation, the first heating rate, the second heating rate, and the third heating rate are all 2°C / min.

[0023] Thirdly, embodiments of this application provide a sodium-ion battery, comprising:

[0024] The cathode material as described in the first aspect and any possible embodiment, or the cathode material prepared by the method described in the second aspect and any possible embodiment. Attached Figure Description

[0025] Figure 1 Electron micrograph of synthetic embodiment 1 provided for the present application;

[0026] Figure 2 Electron micrograph of synthetic embodiment 2 provided for the present application;

[0027] Figure 3 The electron microscope scan image of the synthetic comparative example 1 provided in the embodiments of this application;

[0028] Figure 4 Comparison of X-ray diffraction patterns of synthetic example 1 and synthetic comparative example 1 provided for embodiments of this application;

[0029] Figure 5 Capacity test data diagrams of Device Example 1 and Device Comparative Example 1 provided for embodiments of this application;

[0030] Figure 6 Cyclic performance test diagrams of device embodiments 2-3 and device comparative example 2 provided in this application. Detailed Implementation

[0031] To address the lack of a sodium-ion cathode material with good air stability and low specific capacity in existing technologies, this application provides a cathode material comprising an inner layer material (Na... m Ni u Fe v Mn w O2) and the coating layer (Na) covering the inner layer material n Mn 2-n-x-y Ru x Sn y O2). The coating layer of this cathode material is a sodium-rich material of the O3 (crystalline) phase. This O3 phase structure promotes the extraction and insertion of more sodium ions during the charge-discharge process, thereby increasing the capacity of the cathode material. Simultaneously, compared to other transition metal atoms (e.g., Ni, Fe, Mn), the Ru and Sn atoms in the coating layer can make oxygen participate more "actively" in the redox reaction during the charge-discharge process, thus achieving sodium ion (Na2) extraction. + The extraction and insertion of ) effectively improve the capacity of the cathode material.

[0032] The cathode material provided in this application embodiment has an inner layer and a coating layer that are both layered sodium ion materials. For ease of understanding, the crystal phase structure and related principles of sodium ion materials are explained below.

[0033] First, in the embodiments of this application, both the inner layer material and the coating layer are layered oxide materials. Specifically, the layered oxide is composed of octahedral sheets of MO6 (M is one or more transition metal elements or other doping elements; O indicates oxygen, and O6 represents 6 oxygen atoms). Further, based on the oxygen packing sequence number and Na... + The coordination environment of the layered oxide divides it into P2, P3, O2, and O3 crystal phase structures. In the aforementioned crystal phase structures, the letters represent Na. + The chemical environment in question is indicated by the number representing the number of repeating oxygen stacks within the unit cell. Specifically, P refers to prisms, and O refers to octahedrons.

[0034] Secondly, in the embodiments of this application, the coating layer is an O3 phase. O3 phase sodium-ion cathode materials are characterized by a high sodium ion content. According to the general formula for the theoretical capacity of sodium-ion cathode materials, the higher the sodium atom content, the higher the (gram) capacity of the corresponding cathode material. The above general formula is:

[0035]

[0036] Furthermore, in the aforementioned cathode material, the mass of the coating layer is 0.3%-0.8% of the mass of the inner layer material.

[0037] Furthermore, the specific surface area of ​​the above-mentioned cathode material is 0.75-0.85 g / m². 2 The loose bulk density is 0.8-0.95 g / cm³. 3 The tap density is 1.65-1.80 g / cm³. 3 .

[0038] Furthermore, the specific capacity of the above-mentioned cathode material is 120-170 mAh / g in a voltage window of 2-4.2V.

[0039] Furthermore, the capacity retention rate of the above-mentioned cathode material is 85%-95%; wherein, the capacity retention rate includes the ratio between the capacity of the full cell containing the cathode material after the 20th charge-discharge cycle and the capacity after the 1st cycle under the conditions of 2-4.2V, 1C.

[0040] It is worth noting that when sodium-rich materials are used as positive electrode materials in electrochemical reactions, due to the increased number of anions (O2), 2- Sodium-rich materials participate in redox reactions, resulting in lattice oxygen release, which leads to poor stability and severe gas generation during full-charge reactions. Therefore, sodium-rich materials are generally not used as cathode materials. However, in the embodiments of this application, the sodium-rich material coating layer has a low content and exists only as a coating layer on the surface of the inner layer material. Therefore, it will not cause excessive release of lattice oxygen in the cathode material, resulting in a decrease in the capacity retention rate of the cathode material or severe gas generation problems.

[0041] Therefore, in this embodiment, sodium-rich material is used as the cathode material for the coating layer. This not only avoids the impact of sodium-rich material on the capacity retention rate or gas production in the full cell, but also improves the specific capacity of the cathode material through sodium-rich material. Thus, the cathode material provided in this embodiment also has the effect of overcoming technical bias.

[0042] Furthermore, embodiments of this application also provide a method for preparing the cathode material as described above, the method comprising the following steps:

[0043] Step 101: Perform a first sintering treatment on a first mixture including a tin source and / or a ruthenium source, and a sodium source and a manganese source to obtain a first sintered product corresponding to the coating layer in the cathode material. Perform a second sintering treatment on a second mixture including a sodium source, a nickel source, an iron source and a manganese source, or on a third mixture including a sodium source and a precursor, to obtain a second sintered product corresponding to the inner layer material in the cathode material.

[0044] The first sintering temperature, first heating rate, and first sintering time of the first sintering treatment can be the same as, or different from, the second sintering temperature, second heating rate, and second sintering time of the second sintering treatment.

[0045] When the first sintering temperature is the same as the second sintering temperature, the first heating rate is the same as the second heating rate, and the first sintering time is the same as the second sintering time, the first sintering treatment and the second sintering treatment can be carried out in the same sintering equipment.

[0046] Specifically, the first sintering temperature and the second sintering temperature are each independently selected from 900-1000℃, for example, 930℃. The first heating rate and the second heating rate are each independently selected from 1-5℃ / min, for example, 2℃ / min. The first sintering time and the second sintering time are 10-15h, for example, 12h.

[0047] Furthermore, in the first mixture, the tin source can be tin dioxide; the ruthenium source can be ruthenium dioxide; the sodium source can be one or more of sodium carbonate, sodium hydroxide, and sodium nitrate; and the manganese source can be one or more of manganese carbonate, manganese acetate, manganese trioxide, and manganese tetroxide.

[0048] In the second mixture, the sodium source can be one or more of sodium carbonate, sodium hydroxide, and sodium nitrate; the nickel source can be one or more of nickel oxide, nickel nitrate, and nickel acetate; the iron source can be one or more of manganese tetroxide and ferric nitrate; and the manganese source can be one or more of manganese carbonate, manganese acetate, manganese trioxide, and manganese tetroxide.

[0049] In the third mixture, the sodium source can still be one or more of sodium carbonate, sodium hydroxide, and sodium nitrate; the precursor can be Ni. u Fe v Mn w (OH)2; where 0.11≤u≤0.44, 0.11≤w≤0.44, 0.11≤v≤0.66, u+v+w=1.

[0050] Step 102: The mixture of the first sintering product and the second sintering product is subjected to a third sintering treatment at a temperature of 450-750℃ to obtain the cathode material.

[0051] Specifically, in order to improve the capacity of the cathode material through the first sintering product (sodium-rich material) while avoiding its impact on the stability of the cathode material (e.g., gas generation performance, capacity retention rate), in one embodiment of this application, the mass of the first sintering product is 0.3%-0.8% of the mass of the second sintering product.

[0052] Furthermore, the temperature of the third sintering treatment can be 500℃. The third sintering time is 5-10 hours; for example, 7 hours. The third heating rate is 1-5℃ / min; for example, 2℃ / min.

[0053] Furthermore, in this embodiment, the first sintering product is a sodium-rich material corresponding to the coating layer. The grain size of this material is larger than the grain size of the second sintering product corresponding to the inner layer material. Therefore, it can be ensured that the first sintering product does not significantly enter the first sintering product (inner layer material) due to the third sintering process. Thus, it combines with the second sintering product in a coating form at 450-750°C to obtain the aforementioned cathode material. The inner layer material of this cathode material is: Na. m Ni u Fe v Mn w O2, with a coating of Na n Mn 2-n-x-y Ru x Sn y O2; where 0.7<m≤1, 0.11≤u≤0.44, 0.11≤w≤0.44, 0.11≤v≤0.66, u+v+w=1, 1.10≤n≤1.20, x≥0, y≥0, 0.05≤x+y≤0.2.

[0054] The following provides a detailed description through synthesis examples 1-3, synthesis comparative examples 1-4, device examples 1-3, and device comparative examples 1-4.

[0055] Synthesis Example 1

[0056] S1. Sodium carbonate, manganese dioxide, and tin dioxide are mixed according to Na... 1.2 Mn 0.7 Sn 0.1 O2 is mixed in a high-speed mixer with metal atoms in a specific molar ratio to obtain a first mixture; sodium carbonate, nickel oxide, manganese dioxide, and ferric oxide are then mixed according to the NaNi ratio. 0.22 Fe 0.12 Mn 0.66 The metal atoms in O2 are fed into a high-speed mixer and mixed to obtain a second mixture.

[0057] S2. The first mixture and the second mixture are sintered in air and then cooled to obtain the corresponding first sintered product Na. 1.2 Mn 0.7 Sn 0.1 O2 and the second sintering product NaNi 0.22 Fe 0.12 Mn 0.66 O2.

[0058] The sintering parameters for the first and second mixtures in this step are the same. The sintering temperature is 930℃, the sintering time is 12h, and the heating rate is 3℃ / min.

[0059] S3. The first sintered product and the second sintered product are mixed in a high-speed mixer at a mass ratio of 0.5:100 to obtain a third mixture; the third mixture is then sintered in air at 500°C for 10 hours at a heating rate of 3°C / min, and cooled to obtain the cathode material. The coating layer of this cathode material is Na. 1.2 Mn 0.7 Sn 0.1 O2, inner layer material is NaNi 0.22 Fe 0.12 Mn 0.66 O2. Please refer to the scanning electron microscope (SEM) image of this cathode material. Figure 1 .

[0060] Synthesis Example 2

[0061] S1, Sodium carbonate, manganese carbonate, ruthenium dioxide, and tin dioxide are mixed according to Na... 1.2 Mn 0.6 Sn 0.18 Ru 0.02 The elements in O2 are added in a specific molar ratio and mixed in a high-speed mixer to obtain the first mixture; sodium carbonate and the precursor Ni are then added separately. 1 / 3 Fe 1 / 3 Mn 1 / 3 (OH)2 is fed into a high-speed mixer at a molar ratio of 0.5:1 to obtain a second mixture.

[0062] S2. The first mixture and the second mixture are sintered in air and then cooled to obtain the corresponding first sintered product Na. 1.2 Mn 0.6 Sn 0.18 Ru 0.02 O2 and the second sintering product NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2.

[0063] The sintering parameters for the first and second mixtures in this step are the same. The sintering temperature is 950℃, the sintering time is 10h, and the heating rate is 2℃ / min.

[0064] S3. The first sintered product and the second sintered product are mixed in a high-speed mixer at a mass ratio of 0.75:100 to obtain a third mixture. The third mixture is then sintered in air at 600°C for 7 hours at a heating rate of 2°C / min. After cooling, the cathode material is obtained. The coating layer of this cathode material is Na. 1.2 Mn 0.6 Sn 0.18 Ru 0.02 O2, inner layer material is NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3O2. Please refer to the scanning electron microscope (SEM) image of this cathode material. Figure 2 .

[0065] Synthesis Example 3

[0066] S1, Sodium carbonate, manganese carbonate, ruthenium dioxide, and tin dioxide are mixed according to Na... 1.2 Mn 0.6 Sn 0.18 Ru 0.02 The elements in O2 are added in a specific molar ratio and mixed in a high-speed mixer to obtain the first mixture; sodium carbonate and the precursor Ni are then added separately. 1 / 3 Fe 1 / 3 Mn 1 / 3 (OH)2 is fed into a high-speed mixer at a molar ratio of 0.5:1 to obtain a second mixture.

[0067] S2. The first mixture and the second mixture are sintered in air and then cooled to obtain the corresponding first sintered product Na. 1.2 Mn 0.6 Sn 0.18 Ru 0.02 O2 and the second sintering product NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2.

[0068] The sintering parameters for the first and second mixtures in this step are the same. The sintering temperature is 950℃, the sintering time is 10h, and the heating rate is 2℃ / min.

[0069] S3. The first sintered product and the second sintered product are mixed in a high-speed mixer at a mass ratio of 1.5:100 to obtain a third mixture. The third mixture is then sintered in air at 600°C for 7 hours at a heating rate of 2°C / min. After cooling, the cathode material is obtained. The coating layer of this cathode material is Na. 1.2 Mn 0.6 Sn 0.18 Ru 0.02 O2, inner layer material is NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3O2.

[0070] Synthetic Comparative Example 1

[0071] S1, Sodium carbonate is reacted with the precursor Ni 0.22 F e0.12 Mn 0.66 (OH)₂ was added to a high-speed mixer at a molar ratio of 0.5:1 and mixed. The mixture was then sintered in air at 930°C for 12 hours to obtain NaNi. 0.22 Fe 0.12 Mn 0.66 O2.

[0072] S2, NaNi 0.22 Fe 0.12 Mn 0.66 After O2 was pulverized, it was sintered in air at 500℃ for 10 hours with a heating rate of 2℃ / min to obtain the cathode material NaNi. 0.22 Fe 0.12 Mn 0.66 O2. Please refer to the scanning electron microscope (SEM) image of this cathode material. Figure 3 .

[0073] Figure 4 The X-ray diffraction patterns are for Synthesis Example 1 and Synthesis Comparative Example 1. Figure 3 Therefore, although Synthesis Example 1 added a coating and sintering step compared to Synthesis Comparative Example 1, it did not change the crystal phase structure of the cathode material. Thus, the cathode materials in both Synthesis Example 1 and Synthesis Comparative Example 1 have an O3 phase structure.

[0074] Synthetic Comparative Example 2

[0075] S1, Mix nickel oxide, manganese tetroxide, ferric oxide, and sodium carbonate according to NaNi 1 / 3 Fe 1 / 3 Mn1 / 3 The metal atoms in O2 were mixed in a high-speed mixer; the resulting mixture was then sintered in air at 900°C at a heating rate of 5°C / min for 10 hours, followed by cooling to obtain NaNi. 1 / 3 Fe 1 / 3 Mn 1 / 3 O2.

[0076] S2, ZrO2 and NaNi 1 / 3 Fe 1 / 3 Mn 1 / 3 O2 was added to a high-speed mixer at a mass ratio of 0.5:100 and mixed. The resulting mixture was then sintered in air at 500°C for 7 hours at a heating rate of 3°C / min. After cooling, the positive electrode material was obtained. The coating layer of this positive electrode material was ZrO, and the inner layer material was NaNi. 1 / 3 Fe 1 / 3 Mn 1 / 3 O2.

[0077] Synthetic Comparative Example 3

[0078] S1. Sodium carbonate and manganese carbonate are mixed according to Na 0.6 The elements in MnO2 are added in the following molar ratios and mixed in a high-speed mixer to obtain the first mixture; separately, sodium carbonate, manganese dioxide, nickel oxide, and ferric oxide are mixed according to the NaNi ratio. 0.28 Fe 0.36 Mn 0.36 The metal atoms in O2 are fed into a high-speed mixer and mixed to obtain a second mixture.

[0079] S2. The first mixture and the second mixture are sintered in air and then cooled to obtain the corresponding first sintered product Na. 0.6 MnO2 and the second sintering product NaNi 0.28 Fe 0.36 Mn 0.36 O2.

[0080] The sintering parameters for the first and second mixtures in this step are the same. The sintering temperature is 950℃, the sintering time is 10h, and the heating rate is 5℃ / min.

[0081] S3. The first sintered product and the second sintered product are mixed in a high-speed mixer at a mass ratio of 0.5:100 to obtain a third mixture; the third mixture is then sintered in air at 550°C for 7 hours at a heating rate of 5°C / min, and cooled to obtain the cathode material. The coating layer of this cathode material is Na. 0.6 MnO2, with NaNi as the inner layer material. 0.28 Fe 0.36 Mn 0.36 O2.

[0082] Synthetic Comparative Example 4

[0083] S1. Sodium carbonate, manganese carbonate, and tin dioxide are mixed according to Na... 0.6 Mn 0.9 Sn 0.1 The elements in O2 are added in the same molar ratio and mixed in a high-speed mixer to obtain the first mixture; separately, sodium carbonate, manganese dioxide, nickel oxide, and ferric oxide are mixed according to the NaNi ratio. 0.28 Fe 0.36 Mn 0.36 The metal atoms in O2 are fed into a high-speed mixer and mixed to obtain a second mixture.

[0084] S2. The first mixture and the second mixture are sintered in air and then cooled to obtain the corresponding first sintered product Na. 0.6 Mn 0.9 Sn 0.1 O2 and the second sintering product NaNi 0.28 Fe 0.36 Mn 0.36 O2.

[0085] The sintering parameters for the first and second mixtures in this step are the same. The sintering temperature is 930℃, the sintering time is 12h, and the heating rate is 2℃ / min.

[0086] S3. The first sintered product and the second sintered product are mixed in a high-speed mixer at a mass ratio of 0.5:100 to obtain a third mixture; the third mixture is then sintered in air at 550°C for 7 hours at a heating rate of 2°C / min, and cooled to obtain the cathode material. The coating layer of this cathode material is Na. 0.6 Mn 0.9 Sn 0.1 O2, inner layer material is NaNi 0.28 Fe 0.36 Mn 0.36 O2.

[0087] For the cathode materials in Synthesis Examples 1-3 and Comparative Examples 1-4, the surface pH (measured by a Leici pHS-3C pH meter), tap density (measured by a Dandong Baite tap density meter), loose pack density (measured by a FS4-2 Scott loose pack density meter), and specific surface area (measured by a US McTriStar II 3020) were tested respectively. The Sn and Zr contents in the cathode materials of Synthesis Examples 1-3, Comparative Examples 2, and Comparative Examples 4 were also tested (measured by ICP-OES, Agileent 5110). Please refer to Table 1 for the test results.

[0088] Table 1

[0089]

[0090] Because the lower the surface pH of sodium ion cathode materials, the less likely they are to react chemically with water and / or carbon dioxide in the air to form strong alkalis (sodium carbonate and / or sodium hydroxide), meaning they have better air stability. Therefore, the air stability of sodium ion cathode materials can be determined by their pH value. In Table 1, the surface pH values ​​of synthesis examples 1-3 are lower than those of synthesis comparative examples 1-4, so the air stability of synthesis examples 1-3 is better than that of synthesis comparative examples 1-4. Furthermore, the loose packing density, tap density, and specific surface area of ​​synthesis examples 1-3 are also better than those of synthesis comparative examples 1-4. Additionally, the ratio between the coating layer and the inner layer material in the corresponding cathode materials can be determined from the Sn and Zr contents in Table 1 (consistent with the ratio of the first sintered product and the second sintered product). Device Examples 1-3, Device Comparative Examples 1-4

[0091] Device Examples 1-3 and Device Comparative Examples 1-4 are half-cells using Synthetic Examples 1-3 and Synthetic Comparative Examples 1-4 as positive electrode materials, respectively.

[0092] The following describes the fabrication of devices in Examples 1-3 and Comparative Examples 1-4:

[0093] The active material, Super-P, and PVDF were dissolved in NMP solution at a mass ratio of 0.8:0.15:0.5 to achieve a solid content of 30%. The solution was stirred at 50 rpm for 5 hours, and then the areal density was adjusted to 7.5-7.95 mg / cm³. 2 The mixture is evenly coated onto aluminum foil, dried in an oven at 80°C for 12 hours, punched into a disc with a diameter of 12 cm, and assembled into a half-cell using metallic sodium as the counter electrode.

[0094] Furthermore, the capacitance of devices 1-3 and 1-4 was tested under a voltage window of 2-4.2V and a charge-free condition of 0.1C. Please refer to the test data graphs for device 1 and 1-4. Figure 5 The capacity of device Example 1 is 167 mAh / g, and the capacity of device Comparative Example 1 is 163 mAh / g.

[0095] Cyclic tests were performed on devices 1-3 and 1-4 under a voltage window of 2-4.2V and a capacitance retention rate at 1C, and the capacitance retention rate was calculated after 20 cycles. Please refer to the test data graphs for devices 2-3 and 2-4.2V. Figure 6 .Depend on Figure 6 As can be seen, the capacity retention rate of device embodiment 2 is 95.05%, the capacity retention rate of device embodiment 3 is 86.12%, and the capacity retention rate of device comparative example 2 is 90.04%. Please refer to Table 2 for detailed data.

[0096] Table 2

[0097]

[0098] As shown in Table 2, the capacities of devices in Examples 1-3 are all superior to those in Comparative Examples 1-4, and the capacity retention rates of devices in Examples 1-2 are all superior to those in Comparative Examples 1-4. Device Example 3 exhibits lower stability, which is due to the fact that the mass ratio of the (sodium-rich) coating layer to the inner layer material in Synthesis Example 3 is higher than 0.3%-0.8%.

[0099] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A positive electrode material, characterized in that, include: Inner layer material and a coating layer covering the inner layer material; the molecular formula of the inner layer material is: Na m Ni u Fe v Mn w O2, the coating layer is Na in the O3 phase n Mn 2-n-x-y Ru x Sn y O2; where 0.7<m≤1, 0.11≤u≤0.44, 0.11≤w≤0.44, 0.11≤v≤0.66, u+v+w=1, 1.10≤n≤1.20, x≥0, y≥0, 0.05≤x+y≤0.

2.

2. The cathode material as described in claim 1, characterized in that, The mass of the coating layer is 0.3%-0.8% of the mass of the inner layer material.

3. The positive electrode material as described in claim 1, characterized in that, The specific surface area of ​​the positive electrode material is 0.75-0.85 g / m². 2 .

4. The positive electrode material as described in claim 1, characterized in that, The loose packing density of the positive electrode material is 0.8-0.95 g / cm³. 3 The tap density of the positive electrode material is 1.65-1.80 g / cm³. 3 .

5. The cathode material according to any one of claims 1-4, characterized in that, The specific capacity of the cathode material is 120-170 mAh / g in a voltage window of 2-4.2V.

6. The cathode material according to any one of claims 1-4, characterized in that, The capacity retention rate of the positive electrode material is 85%-95%; wherein, the capacity retention rate is the ratio between the capacity of the full cell containing the positive electrode material after the 20th charge-discharge cycle under 2-4.2V, 1C conditions and the capacity after the 1st charge-discharge cycle.

7. A method for preparing the cathode material according to any one of claims 1-6, characterized in that, include: A first sintering treatment is performed on a first mixture including a tin source and / or a ruthenium source, and a sodium source and a manganese source to obtain a first sintered product corresponding to the coating layer in the cathode material; A second sintering treatment is performed on a second mixture including a sodium source, a nickel source, an iron source, and a manganese source, or on a third mixture including a sodium source and a precursor, to obtain a second sintered product corresponding to the inner layer material in the cathode material; wherein the first sintering temperature of the first sintering treatment and the second sintering temperature of the second sintering treatment are each independently performed at a temperature of 900-1000°C. The mixture of the first sintered product and the second sintered product is subjected to a third sintering treatment at a temperature of 450-750℃ to obtain the cathode material.

8. The method as described in claim 7, characterized in that, In the mixture of the first sintered product and the second sintered product, the mass of the first sintered product is 0.3%-0.8% of the mass of the second sintered product.

9. The method as described in claim 7, characterized in that, The first sintering time of the first sintering treatment and the second sintering time of the second sintering treatment are each independently selected from 10-15h, and the third sintering time of the third sintering treatment is 5-10h.

10. A sodium-ion battery, characterized in that, include: The cathode material as described in any one of claims 1-6, or the cathode material prepared by the method described in any one of claims 7-9.

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