Positive electrode active material, positive electrode sheet, sodium-ion secondary battery, and electronic device

Abstract

The application provides a positive electrode active material, a positive electrode sheet, a sodium-ion secondary battery and an electronic device, wherein the positive electrode active material comprises a layered oxide; in an infrared spectrum obtained by Fourier transform infrared spectroscopy, the positive electrode active material has a first peak, a second peak and a third peak, a wave number range k1 of the first peak is 670 cm ‑1 ≤ k1 ≤ 890 cm ‑1 , a wave number range k2 of the second peak is 1040 cm ‑1 ≤ k2 ≤ 1100 cm ‑1 , and a wave number range k3 of the third peak is 1320 cm ‑1 ≤ k3 ≤ 1530 cm ‑1 ; a peak height h1 of the first peak, a peak height h2 of the second peak and a peak height h3 of the third peak satisfy: 0% < h1 ≤ 4%, 0% < h2 ≤ 2% and 0% < h3 ≤ 15%; the positive electrode active material has a low self residual alkali content after synthesis and has good structural stability, and when applied to a sodium-ion secondary battery, the sodium-ion secondary battery has a high gram capacity.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of electrochemistry, in particular to a positive electrode active material, a positive electrode sheet, a sodium-ion secondary battery and an electronic device. BACKGROUND

[0002] Sodium-ion batteries have been concerned since the late 1970s and early 1980s, but have not been widely studied due to the excellent electrochemical performance of lithium-ion batteries. With the advent of the era of electric vehicles and smart grids, lithium resource shortage has become an important factor restricting its development.

[0003] It is well known that the element of metallic sodium is relatively abundant in the earth's crust (the content of sodium in the earth's crust is about 2.75%, and the content of lithium is about 0.065‰), and is widely distributed (sodium is distributed all over the world, while about 70% of lithium is concentrated in South America). At the same time, sodium and lithium have similar physical and chemical properties and similar de- / intercalation mechanisms, so the research and development of sodium-ion batteries are expected to alleviate the problem of limited development of energy storage batteries caused by lithium resource shortage to some extent. In addition to the advantages of abundant resources, low cost and wide distribution, sodium-ion batteries have more safety advantages than lithium-ion batteries. The thermal runaway temperature of sodium-ion batteries is higher than that of lithium-ion batteries, and sodium-ion batteries are more likely to be passivated and oxidized, and are less likely to cause flammable phenomena, which is the main disadvantage of lithium-ion batteries.

[0004] The positive electrode active material of the sodium-ion battery mainly includes layered oxides, polyanion compounds and prussian blue analogues. Among them, the energy density of the layered oxide is the highest among the three, and the preparation process is mature. However, the positive electrode active material synthesized by the prior art has a high residual alkali content, which will affect the stability of the positive electrode active material in air, thereby reducing the gram capacity of the sodium-ion battery. SUMMARY

[0005] The present application provides a positive electrode active material, a positive electrode sheet, a sodium-ion secondary battery and an electronic device to reduce the residual alkali content of the positive electrode active material, improve the stability of the positive electrode active material in air, and thereby improve the gram capacity of the sodium-ion secondary battery. The specific technical solutions are as follows:

[0006] The first aspect of the present application provides a positive electrode active material, which comprises a layered oxide; in the infrared spectrum obtained by Fourier transform infrared spectroscopy, the positive electrode active material has a first peak, a second peak and a third peak, the wave number range k1 of the first peak is 670cm -1 ≤k1≤890cm -1 , the wave number range k2 of the second peak is 1040cm -1 ≤k2≤1100cm -1 , and the wave number range k3 of the third peak is 1320cm -1≤ k3≤ 1530 cm -1 ; the peak height h1 of the first peak, the peak height h2 of the second peak and the peak height h3 of the third peak satisfy: 0% < h1≤ 4%, 0% < h2≤ 2%, 0% < h3≤ 15%. When the positive electrode active material of the present application is applied to a sodium ion secondary battery, the positive electrode active material surface has a lower residual alkali content. In this way, the sodium ion (Na + ) in the positive electrode active material is less likely to be released, and the content is higher, which can reduce the loss of irreversible capacity of the sodium ion secondary battery. During the charging and discharging process of the sodium ion secondary battery, more Na + participates in deintercalation, so that the gram capacity of the sodium ion secondary battery is improved. Moreover, the processing performance and safety performance of the sodium ion secondary battery can be improved.

[0007] In some embodiments of the present application, 5 g of the positive electrode active material is laid flat on an aluminum foil with an area of 100±5 cm 2 , and after being flattened with a glass dish, it is placed in a humidity environment with a temperature of 25°C and a humidity of 50% for 3 days. The positive electrode active material satisfies at least one of the following characteristics: (1) the peak height h3 of the third peak satisfies: 0% < h3≤ 45%; (2) using thermal gravimetric analysis, the weight loss percentage of the positive electrode active material in the temperature range of 35°C to 400°C is W T4 %, W T4 ≤ 3, and the weight loss percentage of the positive electrode active material in the temperature range of 35°C to 120°C is W T3 %, W T3 < 1; (3) the residual alkali content of the positive electrode active material is W N2 %, W N2 ≤ 5. It indicates that the positive electrode active material of the present application has high structural stability, so that the Na + in the positive electrode active material is less likely to be released in a humidity environment, thereby having a lower content of residual alkali Na2CO3, which can improve the stability of the positive electrode active material in air. In this way, the gram capacity of the sodium ion secondary battery is improved.

[0008] In some embodiments of the present application, the specific surface area of the positive electrode active material is B, 0.2 m 2 / g≤ B≤ 0.8 m 2 / g; or, the residual alkali content of the positive electrode active material is W N1 %, W N1 ≤ 1.5. Controlling the specific surface area of the positive electrode active material within the above range is beneficial to improving the stability of the positive electrode active material in air, reducing the residual alkali content of the positive electrode active material, and improving the gram capacity of the sodium ion secondary battery.

[0009] In some embodiments of the present application, the molecular formula of the layered oxide is Na x Ni aFe b Mn c M d O2, M comprises at least one of Co, Mg, Ca, B, Al, Zr, Ti, W, Mo, Cr, Sr, Y, Cd, Sn, Sb, Ce, Li, K, Zn, La, F, Si or P; wherein 0.1

[0010] In some embodiments of the present application, the surface of the layered oxide has an inorganic oxide, and the inorganic oxide comprises at least one of TiO2, Al2O3, MgO, V2O5, ZnO, ZrO2, RuO2, La2O3, CeO2, B2O3 or Nb2O5. The presence of the inorganic oxide of the above type on at least part of the outer surface of the layered oxide is conducive to reducing the residual alkali content of the positive electrode active material, improving the stability of the positive electrode material in air, and thus improving the gram capacity of the sodium-ion secondary battery.

[0011] In some embodiments of the present application, the mass percentage content of the inorganic oxide is 0.1% to 0.5% based on the mass of the positive electrode active material. The mass percentage content of the inorganic oxide is controlled within the above range mainly because the inorganic oxide itself cannot play the role of active material, and a too high content will lead to a decrease in the gram capacity of the active material per unit mass, and will also affect the normal deintercalation of Na + . A too low content cannot achieve the purpose of isolating the moisture in the air from contacting the material.

[0012] The second aspect of the present application provides a positive electrode sheet, which comprises a positive electrode current collector and a positive electrode active material layer arranged on at least one surface of the positive electrode current collector, and the positive electrode active material layer comprises the positive electrode active material provided in the first aspect of the present application. Therefore, the positive electrode sheet has a lower content of residual alkali and higher structural stability, and is applied to a sodium-ion secondary battery, which is conducive to improving the gram capacity of the sodium-ion secondary battery.

[0013] In some embodiments of the present application, the positive electrode active material layer has a compacted density of 2.8 g / cm 3 to 3.6 g / cm 3 . The compacted density of the positive electrode active material layer is controlled within the above range, so that the positive electrode active material layer can provide higher energy density, better cycle performance and rate performance on the basis of having a higher compacted density.

[0014] The third aspect of the present application provides a sodium-ion secondary battery, which comprises the positive electrode sheet provided by the second aspect of the present application. Therefore, the sodium-ion secondary battery has a higher gram capacity.

[0015] The fourth aspect of the present application provides an electronic device, which comprises the sodium-ion secondary battery provided by the third aspect of the present application. Therefore, the electronic device has a higher gram capacity.

[0016] The present application provides a positive electrode active material, a positive electrode sheet, a sodium-ion secondary battery and an electronic device, wherein the positive electrode active material comprises a layered oxide; in an infrared spectrum obtained by Fourier transform infrared spectroscopy, the positive electrode active material has a first peak, a second peak and a third peak, the wave number range k1 of the first peak is 670 cm- 1 ≤k1≤890cm- 1 , the wave number range k2 of the second peak is 1040 cm- 1 ≤k2≤1100cm- 1 , and the wave number range k3 of the third peak is 1320 cm- 1 ≤k3≤1530cm- 1 ; the peak height h1 of the first peak, the peak height h2 of the second peak and the peak height h3 of the third peak satisfy: 0% < h1 ≤ 4%, 0% < h2 ≤ 2%, and 0% < h3 ≤ 15%; the positive electrode active material of the present application has a lower self residual alkali content after synthesis, and has good structural stability, and when applied to a sodium-ion secondary battery, the sodium-ion secondary battery has a higher gram capacity. BRIEF DESCRIPTION OF DRAWINGS

[0017] The accompanying drawings, which are included to provide a further understanding of the present application, form a part of the present application and illustrate the illustrative embodiments of the present application and together with the description, serve to explain the present application. The accompanying drawings do not constitute an undue limitation on the scope of the present application.

[0018] Figure 1 The Fourier transform infrared spectroscopy (FTIR) test graph of the positive electrode active material in Example 2-2 and Comparative Example 3;

[0019] Figure 2 The thermogravimetric analysis (TG) test graph of the positive electrode active material in Example 2-2 and Comparative Example 3. DETAILED DESCRIPTION

[0020] To make the purpose, technical solutions and advantages of the present application more clear, the present application is further described in detail below with reference to the accompanying drawings and examples. Obviously, the described examples are only a part of the examples of the present application, but not all the examples. Based on the examples in the present application, all other examples obtained by those skilled in the art belong to the scope of protection of the present application.

[0021] The prior art synthesized positive electrode active material has a high residual alkali content. The positive electrode active material with a high residual alkali content will bring many negative effects to the sodium-ion secondary battery. First, after the positive electrode active material is prepared into a positive electrode slurry, a large amount of basic oxides (such as residual alkali Na2CO3 in the following) on the surface of the positive electrode active material will absorb water to cause the positive electrode slurry to form a jelly-like shape, thereby affecting the coating. Second, the basic oxides on the surface of the positive electrode active material will increase the irreversible capacity loss of the sodium-ion secondary battery and deteriorate the cycle performance of the sodium-ion secondary battery. In addition, the residual alkali Na2CO3 on the surface of the positive electrode active material will react with the electrolyte to generate CO2, which will cause the sodium-ion secondary battery to swell, which is not conducive to the cycle performance and safety and reliability of the sodium-ion secondary battery.

[0022] The first aspect of the present application provides a positive electrode active material, which comprises a layered oxide; the positive electrode active material has a first peak, a second peak and a third peak in the infrared spectrum obtained by Fourier transform infrared spectroscopy, the wave number range k1 of the first peak is 670 cm -1 ≤ k1 ≤ 890 cm -1 , the wave number range k2 of the second peak is 1040 cm -1 ≤ k2 ≤ 1100 cm -1 , and the wave number range k3 of the third peak is 1320 cm -1 ≤ k3 ≤ 1530 cm -1 ; the peak height h1 of the first peak, the peak height h2 of the second peak and the peak height h3 of the third peak satisfy: 0% < h1 ≤ 4%, 0% < h2 ≤ 2%, and 0% < h3 ≤ 15%. For example, h1 is 0.1%, 1%, 2%, 3%, 4% or any value within any two of the above numerical ranges. h2 is 0.1%, 0.5%, 1%, 1.5%, 2% or any value within any two of the above numerical ranges. h3 is 0.1%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15% or any value within any two of the above numerical ranges. Among them, the first peak and the second peak represent the fingerprint peaks of CO3 2- , and the third peak represents the anti-symmetrical stretching vibration peak of CO3 2- . The positive electrode active material has a first peak, a second peak and a third peak in the wave number ranges k1, k2 and k3, respectively, indicating that the positive electrode active material has residual alkali Na2CO3 on the surface. The peak height h3 of the anti-symmetrical stretching vibration peak of CO3 2- (the third peak) is within the above range of the present application, indicating that the positive electrode active material of the present application has a low content of residual alkali Na2CO3 on the surface. When the positive electrode active material of the present application is applied to a sodium-ion secondary battery, the positive electrode active material has a low residual alkali content on the surface. In this way, the sodium ions (Na +Less precipitation occurs, and the positive electrode active material exhibits high stability in air, which can reduce the irreversible capacity loss of sodium-ion secondary batteries. During the charging and discharging process of sodium-ion secondary batteries, a significant amount of Na₂O₂ is released. + By participating in intercalation and deintercalation, the specific capacity of sodium-ion secondary batteries can be increased. Furthermore, it can improve the cycle performance, processing performance, and safety and reliability of sodium-ion secondary batteries.

[0023] In this application, the peak height h1 of the first peak is the transmittance difference between the apex of the first peak and the baseline, the peak height h2 of the second peak is the transmittance difference between the apex of the second peak and the baseline, and the peak height h3 of the third peak is the transmittance difference between the apex of the third peak and the baseline.

[0024] In some embodiments of this application, 5g of positive electrode active material is spread evenly on an aluminum foil to form an area of ​​100±5cm². 2 After being flattened in a glass dish and placed at 25°C and 50% humidity for 3 days, the peak height h3 of the third peak satisfies the condition: 0% < h3 ≤ 45%. For example, h3 can be 0.1%, 2%, 4%, 6%, 8%, 10%, 12%, 14%, 16%, 18%, 20%, 25%, 30%, 35%, 40%, 45%, or any value between any two of the above ranges. After the positive electrode active material undergoes humidity treatment under the above conditions, the peak height h3 of the third peak falls within the range specified in this application, indicating that the positive electrode active material of this application has high structural stability, allowing the Na in the positive electrode active material to... + The material is less prone to precipitation in humid environments, resulting in a lower residual Na₂CO₃ content after storage in such environments. This leads to good stability of the positive electrode active material in air. When the positive electrode active material of this application is used in sodium-ion secondary batteries, it can reduce the irreversible capacity loss of the sodium-ion secondary battery, and minimize the amount of Na₂CO₃ released during the charging and discharging process. + By participating in intercalation and deintercalation, the specific capacity of sodium-ion secondary batteries can be increased. Furthermore, it can improve the cycle performance, processing performance, and safety and reliability of sodium-ion secondary batteries.

[0025] In some embodiments of this application, 5g of positive electrode active material is spread evenly on an aluminum foil to form an area of ​​100±5cm². 2 After being flattened in a glass dish and placed at 25°C and 50% humidity for 3 days, thermogravimetric analysis was performed. The percentage weight loss of the positive electrode active material within the temperature range of 35°C to 400°C was W. T4 %, W T4 ≤3, the percentage weight loss of the positive electrode active material in the temperature range of 35℃ to 120℃ is W. T3 %, W T3 <1. For example, W T4W is 0.1, 0.5, 1, 1.5, 2, 2.5, 3, or any value between any two of the above-mentioned numerical ranges. T3 W is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.99, or any value between any two of the above-mentioned numerical ranges. The percentage weight loss of the layered oxide under different conditions after the above humidity treatment T3 W is 0.1, 0.5, 1, 1.5, 2, 2.5, 3, or any value between any two of the above-mentioned numerical ranges. The percentage weight loss of the layered oxide under different conditions after the above humidity treatment T4 The fact that W is in the above range is because the positive electrode active material absorbs less water in the humidity environment, and the degree of degradation of the positive electrode active material due to the reaction between the positive electrode active material and water is reduced, thereby reducing the percentage weight loss at 35°C to 400°C or 35°C to 120°C, indicating that the positive electrode active material of the present application has high structural stability. In this way, Na in the layered compound + is less likely to be precipitated in a humidity environment, thereby having a lower content of residual alkali Na2CO3. Moreover, the positive electrode active material with high structural stability has a lower possibility of causing the alkali metal ion (such as Na + ) to be released and H + / H2O to be inserted, thereby reducing the risk of capacity decline and impaired ion transport. As a result, the content of Na + in the positive electrode active material is higher, and when the positive electrode active material of the present application is applied to a sodium-ion secondary battery, the loss of irreversible capacity of the sodium-ion secondary battery can be reduced, and more Na + participates in deintercalation during the charging and discharging process of the sodium-ion secondary battery, thereby improving the specific capacity of the sodium-ion secondary battery. Moreover, the cycle performance, processing performance, and safety and reliability of the sodium-ion secondary battery can be improved.

[0026] In some embodiments of the present application, 5 g of the positive electrode active material is laid flat on an aluminum foil to have an area of 100±5 cm 2 , and after being flattened with a glass dish, the positive electrode active material is placed in a humidity environment at a temperature of 25°C and a humidity of 50% for 3 days, and the residual alkali content of the positive electrode active material is W N2 %, W N2 ≤5. For example, W N2 is 0.1, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, or any value between any two of the above-mentioned numerical ranges. This indicates that the positive electrode active material after humidity treatment after preparation also has a lower content of residual alkali, which can indicate that the positive electrode active material has high structural stability, because the amount of alkali metal ion released and the area of the surface of the positive electrode active material that reacts are small when the positive electrode active material is exposed to a humidity environment, thereby making the positive electrode active material have a lower content of residual alkali. When the positive electrode active material is applied to a sodium-ion secondary battery, the surface of the positive electrode active material has a lower content of residual alkali. In this way, Na +precipitation, the content is higher, which can reduce the loss of irreversible capacity of sodium ion secondary battery, more Na + participate in deintercalation, the gram capacity of sodium ion secondary battery is improved. Moreover, the cycle performance, processing performance and safety reliability of sodium ion secondary battery can be improved.

[0027] In some embodiments of the present application, the specific surface area of the positive electrode active material is B, 0.2m 2 / g≤B≤0.8m 2 / g. For example, the value of B is 0.2m 2 / g, 0.3m 2 / g, 0.4m 2 / g, 0.5m 2 / g, 0.6m 2 / g, 0.7m 2 / g, 0.8m 2 / g or any value range between any two of the above values. By controlling the specific surface area of the positive electrode active material within the above range, the surface of the positive electrode active material in contact with air is smaller. In this way, the reaction surface can be reduced, thereby reducing the possibility of degradation of the positive electrode active material. As a result, the surface residual alkali content of the positive electrode active material can be reduced, and when the positive electrode active material of the present application is applied to a sodium ion secondary battery, the reaction of the positive electrode active material with moisture and carbon dioxide in the air can be reduced, and the gram capacity of the sodium ion secondary battery can be improved.

[0028] In some embodiments of the present application, the molecular formula of the layered oxide is Na x Ni a Fe b Mn c M d O2, M includes at least one of Co, Mg, Ca, B, Al, Zr, Ti, W, Mo, Cr, Sr, Y, Cd, Sn, Sb, Ce, Li, K, Zn, La, F, Si or P; wherein 0.1 + content is higher, more Na + participate in deintercalation, the gram capacity of sodium ion secondary battery is improved. Moreover, the cycle performance, processing performance and safety reliability of sodium ion secondary battery can be improved.

[0029] In some embodiments of the present application, the layered oxide is at least one of a single-crystal layered oxide or a polycrystalline layered oxide.

[0030] In some embodiments of the present application, at least part of the outer surface of the layered oxide is present with an inorganic oxide, and the inorganic oxide comprises at least one of TiO2, Al2O3, MgO, V2O5, ZnO, ZrO2, RuO2, La2O3, CeO2, B2O3, or Nb2O5. It can be understood that, in some embodiments of the present application, part of the outer surface of the layered oxide is present with the inorganic oxide; in other embodiments of the present application, the entire outer surface of the layered oxide is present with the inorganic oxide, i.e., the inorganic oxide completely coats the layered oxide. The presence of the inorganic oxide of the above-mentioned type on at least part of the outer surface of the layered oxide can reduce the area of the surface of the layered oxide in contact with air, reduce the release of Na+in the positive electrode active material; and the lattice defects in the positive electrode active material can be reduced to improve the structural stability of the positive electrode active material and reduce the release of Na+in the positive electrode active material in a humid environment. In this way, the residual alkali content on the surface of the positive electrode active material is further reduced, and the Na+content in the positive electrode active material is higher. When the positive electrode active material of the present application is applied to a sodium-ion secondary battery, the loss of irreversible capacity of the sodium-ion secondary battery can be further reduced, more Na+participates in deintercalation during the charging and discharging process of the sodium-ion secondary battery, and the gram capacity of the sodium-ion secondary battery is further improved. Moreover, the cycle performance, processing performance, and safety and reliability of the sodium-ion secondary battery can be improved. + + + +

[0031] In some embodiments of the present application, the mass percentage content of the inorganic oxide is 0.1% to 0.5% based on the mass of the positive electrode active material. For example, the mass percentage content of the inorganic oxide is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, or any value within the range between any two of the above-mentioned values. Controlling the mass percentage content of the inorganic oxide within the above-mentioned range can reduce the residual alkali content of the positive electrode active material without affecting the gram capacity of the positive electrode active material, improve the stability of the positive electrode material in air, and thus improve the gram capacity of the sodium-ion secondary battery.

[0032] Further, the layered oxide present on the outer surface is an O3-type layered oxide. The O3-type layered oxide has a higher Na+content, which is conducive to more Na+participating in deintercalation during the charging and discharging process of the sodium-ion secondary battery, and the gram capacity of the sodium-ion secondary battery is further improved. + +

[0033] The present application does not particularly limit the average particle size Dv50 of the inorganic oxide, as long as the purpose of the present application can be achieved. For example, the average particle size Dv50 of the inorganic oxide is 10 nm to 500 nm. ​​​​​​

[0034] In some embodiments of the present application, the weight loss percentage of the positive electrode active material is W T2 %, W T2 ≤ 0.2, the weight loss percentage of the positive electrode active material is W T1 %, W T1 < 0.1.

[0035] In some embodiments of the present application, the residual alkali content of the positive electrode active material is W N1 %, W N1 ≤ 1.5. For example, W N1 is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5 or any value range between any two of the above values. It is shown that the positive electrode active material after preparation without humidity treatment has a lower residual alkali content. When the positive electrode active material is applied to a sodium-ion secondary battery, the surface of the positive electrode active material has a lower residual alkali content. In this way, the Na + is less released and has a higher content in the positive electrode active material, which can reduce the loss of irreversible capacity of the sodium-ion secondary battery, and more Na + participates in deintercalation during the charging and discharging process of the sodium-ion secondary battery, so that the specific capacity of the sodium-ion secondary battery is improved. Moreover, the cycle performance, processing performance and safety reliability of the sodium-ion secondary battery are improved.

[0036] The preparation method of the positive electrode active material is not particularly limited in the present application, as long as the purpose of the present application can be achieved. For example, the preparation method of the positive electrode active material includes but is not limited to the following steps: preparing raw materials according to the stoichiometric ratio of the desired layered oxide formula Na x Ni a Fe b Mn c M d O2, grinding the raw materials Fe b Mn c M d (OH)2, Na2CO3 uniformly, calcining at 800-1000°C for 10-14h, crushing after natural cooling, to obtain the layered oxide Na x Ni a Fe b Mn c M d O2, denoted as Fresh state. Alternatively, the preparation method of the positive electrode active material can include but is not limited to the following steps: (1) preparing raw materials according to the stoichiometric ratio of the desired layered oxide formula Na x Ni a Feb Mn c M d O2 stoichiometric ratio of raw material Fe b Mn c M d (OH)2, Na2CO3, the raw material is ground uniformly, calcined at 800-1000°C for 10-14h, broken after natural cooling, to obtain layered oxide Na x Ni a Fe b Mn c M d O2; (2) mixing the layered oxide Na x Ni a Fe b Mn c M d O2 with inorganic oxide in a mass percentage ratio of (99.5-99.9):(0.1-0.5), uniformly mixed, sintered at 500-700°C for 5-7h, broken, to obtain layered oxide with at least part of the outer surface existing inorganic oxide, denoted as Fresh state.

[0037] The kind of raw material in the present application is not particularly limited, as long as it can prepare the positive electrode active material of the present application and achieve the purpose of the present application. For example, the raw material includes but is not limited to Na2CO3, and at least one of NaHCO3 or NaOH.

[0038] The present application does not particularly limit the kind of raw material Fe b Mn c M d The average particle size Dv50 of the raw material (OH)2 is not particularly limited, as long as it can achieve the purpose of the present application. For example, the average particle size Dv50 of the raw material is 2-15μm.

[0039] In the present application, the residual alkali content and the structural stability of the positive electrode active material can be regulated by controlling the ratio of Na / TM in the positive electrode active material, the kind of TM, the calcination temperature and time, and the sintering temperature and time. After the structural stability of the positive electrode active material is improved, Na + In a humid environment, it is not easy to precipitate, so the residual alkali content of the positive electrode active material after humidity treatment is reduced. And after the structural stability of the positive electrode active material is improved, the percentage of weight loss of the positive electrode active material is also reduced.

[0040] In the present application, TM includes at least one of metal elements Ni, Fe, Mn, Co, Mg, Ca, B, Al, Zr, Ti, W, Mo, Cr, Sr, Y, Cd, Sn, Sb, Ce, Li, K, Zn, La, F, Si or P.

[0041] The method for adjusting the specific surface area of the positive electrode active material is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the specific surface area of the positive electrode active material can be adjusted by adjusting the average particle size Dv50 of the raw material used for preparing the positive electrode active material.

[0042] In the present application, Dv50 represents the particle size at which 50% of the particles by volume reach from the small particle size side in the particle size distribution on a volume basis. The method for testing Dv50 is not particularly limited in the present application, and can be tested by a method known in the art, for example, by using a laser particle size analyzer.

[0043] The second aspect of the present application provides a positive electrode tab, which comprises a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode active material provided in the first aspect of the present application. Since the positive electrode active material provided in the first aspect of the present application has a lower residual alkali content, the positive electrode active material has a higher content of Na + When the positive electrode tab is applied to a sodium-ion secondary battery, there are more Na + participate in deintercalation, so that the specific capacity of the sodium-ion secondary battery is improved. Moreover, the sodium-ion secondary battery has good cycle performance, processing performance and safety reliability.

[0044] The "positive electrode active material layer disposed on at least one surface of the positive electrode current collector" refers to a positive electrode active material layer disposed on one surface of the positive electrode current collector, or a positive electrode active material layer disposed on two surfaces of the positive electrode current collector. The "surface" can be part of the surface or the entire surface of the positive electrode current collector.

[0045] In some embodiments of the present application, the positive electrode active material layer has a compaction density of 2.8 g / cm 3 to 3.6 g / cm 3 . For example, the positive electrode active material layer has a compaction density of 2.8 g / cm 3 , 2.9 g / cm 3 , 3.0 g / cm 3 , 3.1 g / cm 3 , 3.2 g / cm, 3.3 g / cm 3 , 3.4 g / cm 3 , 3.5 g / cm 3 , 3.6 g / cm 3or any value range between any two of the above-mentioned values. The compaction density of the positive electrode active material layer is regulated within the above-mentioned range, so that the positive electrode active material layer can provide a higher energy density, better cycle performance and rate performance on the basis of having a higher strength. Thus, the sodium-ion secondary battery has a higher energy density and has good processing performance, safety performance, cycle performance and rate performance.

[0046] The regulation manner of the compaction density of the positive electrode active material layer in the present application is not particularly limited, as long as the purpose of the present application can be achieved. For example, it can be achieved by regulating the pressure during the cold pressing process of the positive electrode sheet, adjusting the type or average particle size of the positive electrode active material, etc. The present application does not have a particular limitation on the size of the above-mentioned pressure, as long as the purpose of the present application can be achieved. For example, the pressure is 40 t to 80 t.

[0047] The present application does not have a particular limitation on the type of the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the positive electrode current collector can include an aluminum foil, an aluminum alloy foil, etc.

[0048] In the present application, the thickness of the positive electrode current collector and the positive electrode active material layer is not particularly limited, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5 μm to 20 μm, preferably 6 μm to 18 μm. The thickness of the positive electrode active material layer is 30 μm to 120 μm. Optionally, the positive electrode active material layer can also include a positive electrode conductive agent and a positive electrode binder. The present application does not have a particular limitation on the type of the positive electrode conductive agent and the positive electrode binder in the positive electrode active material layer, as long as the purpose of the present application can be achieved. The present application does not have a particular limitation on the mass ratio of the positive electrode active material, the positive electrode conductive agent and the positive electrode binder in the positive electrode active material layer, and a person skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved. For example, the mass ratio of the positive electrode active material, the positive electrode conductive agent and the positive electrode binder in the positive electrode active material layer is (90-98):(0.5-5):(0.5-5).

[0049] The third aspect of the present application provides a sodium-ion secondary battery, which includes the positive electrode sheet provided by the second aspect of the present application. Thus, the sodium-ion secondary battery has a higher specific capacity.

[0050] In some embodiments of the present application, the sodium-ion secondary battery of the present application further includes a negative electrode sheet and a separator, and the separator is arranged between the positive electrode sheet and the negative electrode sheet to separate the positive electrode sheet and the negative electrode sheet, prevent internal short circuit of the electrochemical device, allow the electrolyte ions to pass freely, and not affect the electrochemical charging and discharging process.

[0051] The negative electrode sheet is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector. The negative electrode current collector is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the negative electrode current collector can include an aluminum foil, an aluminum alloy foil, a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, or a copper foam, etc. The negative electrode active material layer of the present application includes a negative electrode active material, and the type of the negative electrode active material is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the negative electrode active material can include, but is not limited to, at least one of hard carbon, soft carbon, sodium metal, or bismuth metal. In the present application, the thickness of the negative electrode current collector and the negative electrode active material layer is not particularly limited, as long as the object of the present application can be achieved. For example, the thickness of the negative electrode current collector is 6 to 10 pm, and the thickness of the negative electrode active material layer is 30 to 130 pm. Optionally, the negative electrode active material layer can further include at least one of a negative electrode conductive agent or a thickening agent, and the type of the negative electrode conductive agent and the thickening agent is not particularly limited in the present application, as long as the object of the present application can be achieved. The mass ratio of the negative electrode active material, the negative electrode conductive agent, the thickening agent, and the negative electrode binder in the negative electrode active material layer is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the mass ratio of the negative electrode active material, the negative electrode conductive agent, the thickening agent, and the negative electrode binder in the negative electrode active material layer is (90-98):(0-5):(0.5-1.5):(0.5-5).

[0052] The separator is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the material of the separator can include, but is not limited to, at least one of polyethylene (PE), polyolefin (PO) based on polypropylene (PP), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex, or aramid. The type of the separator can include at least one of a woven film, a non-woven film, a microporous film, a composite film, a calendered film, or a spunlaid film.

[0053] The sodium-ion battery of the present application further comprises a packaging bag and an electrolyte, and the electrolyte, the positive electrode sheet, the separator and the negative electrode sheet are contained in the packaging bag. The packaging bag and the electrolyte of the present application are not particularly limited, and can be the packaging bag and the electrolyte known in the art, as long as the purpose of the present application can be achieved. For example, the packaging bag can be an aluminum plastic film or a stainless steel, aluminum shell, steel shell. The electrolyte can be an electrolyte comprising a sodium salt and a non-aqueous solvent. In some embodiments of the present application, the sodium salt can include, but is not limited to, at least one of NaPF6, NaClO4, NaBF4, NaNO3, NaSCN, NaCN, NaAsF6, NaCF3CO2, NaSbF6, NaC6H5CO2, Na(CH3)C6H4SO3, NaHSO4or NaB(C6H5)4. The non-aqueous solvent can be at least one of a carbonate compound, a carboxylic acid ester compound, an ether compound or other organic solvents. The above-mentioned carbonate compound can include, but is not limited to, at least one of a chain carbonate compound, a cyclic carbonate compound or a fluorinated carbonate compound. The above-mentioned chain carbonate compound can include, but is not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC) or methyl ethyl carbonate (MEC). The cyclic carbonate compound can include, but is not limited to, at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinyl ethylene carbonate (VEC). The fluorinated carbonate compound can include, but is not limited to, at least one of fluoroethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methyl ethylene carbonate, 1-fluoro-1-methyl ethylene carbonate, 1,2-difluoro-1-methyl ethylene carbonate, 1,1,2-trifluoro-2-methyl ethylene carbonate or trifluoromethyl ethylene carbonate. The above-mentioned carboxylic acid ester compound can include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valeronolactone, methylvaleronolactone or caprolactone. The above-mentioned ether compound can include, but is not limited to, at least one of dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, ethoxymethoxyethane, 2-methyltetrahydrofuran or tetrahydrofuran. The above-mentioned other organic solvents can include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate or phosphoric acid ester.

[0054] The preparation steps of the sodium-ion battery of the present application can also include, but are not limited to, the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and winding, folding, etc. according to the needs to obtain an electrode assembly with a winding structure, placing the electrode assembly into a packaging bag, injecting electrolyte into the packaging bag and sealing to obtain a sodium-ion battery; or stacking the positive electrode sheet, the separator and the negative electrode sheet in order, then fixing the four corners of the entire stack structure with adhesive tape to obtain an electrode assembly with a stack structure, placing the electrode assembly into a packaging bag, injecting electrolyte into the packaging bag and sealing to obtain a sodium-ion battery.

[0055] The fourth aspect of the present application provides an electronic device comprising the sodium-ion secondary battery provided by the third aspect of the present application. Therefore, the electronic device has a higher gram capacity.

[0056] The electronic device of the present application is not particularly limited, and it can be any electronic device known in the prior art. For example, the electronic device can include, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a head-mounted stereo headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a household large storage battery and a sodium-ion capacitor.

[0057] Embodiment

[0058] Hereinafter, examples and comparative examples are given to more specifically explain the embodiments of the present application. Various tests and evaluations are carried out according to the following methods.

[0059] Test methods and equipment:

[0060] Fourier transform infrared spectroscopy (FTIR) test:

[0061] The reference standard is GB / T 21186-2007 Fourier Transform Infrared Spectrometer National Standard. Each positive electrode active material (Fresh state) of the examples and comparative examples is irradiated with light waves with a wavelength of 2.5 μm to 25 μm and a frequency of 4000 cm -1 to 400 cm -1 to produce an absorption spectrum caused by intramolecular vibration and rotational energy level transition.

[0062] Specific surface area test of positive electrode active material:

[0063] The specific surface area test of the positive electrode active material (Fresh state) in the examples and the comparative examples was measured by nitrogen adsorption / desorption method using a specific surface area analyzer (Tristar II 3020M). Among them, the specific test was carried out in accordance with the national standard GB / T 19587-2017.

[0064] Thermogravimetric analysis (TG) test:

[0065] The change of the mass of the positive electrode active material with temperature was tested by using a thermogravimetric analyzer, and the test was carried out in accordance with JY_T0589.1-2020 / JY_T0589.4-2020 / JY_T0589.5-2020 "General methods for thermal analysis".

[0066] (1) Take 10 mg of the positive electrode active material in Fresh state in each example or the comparative example and place it in the thermogravimetric analyzer, set the test atmosphere in a nitrogen atmosphere, and test the temperature from 35°C to 400°C at a heating rate of 5°C / min.

[0067] During the test, the mass loss between 35°C and 120°C and between 35°C and 400°C was recorded, and the mass loss percentage W T1 % between 35°C and 120°C, and the mass loss percentage W T2 % between 35°C and 400°C.

[0068] (2) Except that the positive electrode active material in Fresh state is replaced by the positive electrode active material in A2 state, the rest is the same as the above (1).

[0069] The mass loss percentage W T3 % between 35°C and 120°C, and the mass loss percentage W T4 % between 35°C and 400°C.

[0070] Humidity treatment:

[0071] 5 g of the positive electrode active material in each example and the comparative example was laid flat on an aluminum foil with an area of 100±5 cm 2 2, and after being flattened with a glass dish, it was placed in a condition of a temperature of 25°C and a humidity of 50% for 3 days. The state of the positive electrode active material after the above humidity treatment is A2 state.

[0072] Test of residual alkali content:

[0073] By extracting Na2CO3 and NaOH in the positive electrode active material, filtering, and then testing the filtrate by potentiometric titration, the residual alkali content in the positive electrode active material can be obtained.

[0074] Na2CO3 + HC1 = NaCl + NaHCO3;

[0075] NaOH + HC1 = NaCl + H2O;

[0076] NaHCO3 + HC1 = NaCl + H2CO3.

[0077] Specific operation: 1 g of sample was weighed in a 100 mL conical flask, 20 mL of ethylene glycol (purity 99.999%) was added, the flask was sealed with sealing film after the magnetic stirrer was put in, and the filter was performed after the magnetic stirring for 30 min. The filtrate was diluted to 100 mL with pure water, and 50 mL of the filtrate was taken into a beaker for potential titration test.

[0078] The above "sample" is the positive electrode active material of each example or comparative example in the Fresh state and the A2 state, respectively.

[0079] Test of the compaction density of the positive electrode active material layer:

[0080] The compaction density Pc of the positive electrode active material layer is calculated by the formula: Pc = mc / Vc. In the formula, mc is the mass of the positive electrode active material layer, unit: g; Vc is the volume of the positive electrode active material layer, unit: cm 3 , wherein the volume Vc is the product of the area Sc of the positive electrode active material layer and the thickness of the positive electrode active material layer.

[0081] Test of the gram capacity:

[0082] The positive electrode active material powder of each example or comparative example in the Fresh state and the A2 state was mixed with the conductive agent conductive carbon black and the binder polyvinylidene fluoride (PVDF) according to the mass ratio of 8:1:1, N-methyl pyrrolidone (NMP) was added to grind into a slurry with a solid content of 72 wt%, which was coated on an aluminum foil with a coating thickness of 200 μm, dried in a vacuum oven, and then cut into pieces. A metal sodium sheet was used as the counter electrode, a solution obtained by dissolving NaPF6 in PC solvent was used as the electrolyte, and glass fiber was used as the separator to assemble a CR2032 button cell. The assembly of the button cell was carried out in an argon-filled glove box. The assembled button cell was tested on a blue electric test system with a current density of 10 mA / g and a charge / discharge voltage window of 2.0 V to 4.0 V to test the gram capacity of the Fresh state and the A2 state, respectively.

[0083] Example 1-1

[0084] <Preparation of the positive electrode active material>

[0085] According to Na 0.95 Ni0.33 Fe 0.33 Mn 0.33 Ti 0.01 O2 with stoichiometric ratio Ni:Fe:Mn:Ti = 0.33:0.33:0.33:0.01 0.33 Fe 0.33 Mn 0.33 Ti 0.01 (OH)2, Na2CO3, the precursor Ni 0.33 Fe 0.33 Mn 0.33 Ti 0.01 (OH)2, Na2CO3 were ground uniformly, calcined at 900°C for 12h, broken after natural cooling, to obtain layered oxide Na 0.95 Ni 0.33 Fe 0.33 Mn 0.33 Ti 0.01 O2, noted as Fresh state (hereinafter abbreviated as F state).

[0086] 5g of the layered oxide Na 0.95 Ni 0.33 Fe 0.33 Mn 0.33 Ti 0.01 O2 obtained above was subjected to the humidity treatment as described above to obtain layered oxide Na 0.95 Ni 0.33 Fe 0.33 Mn 0.33 Ti 0.01 O2 in A2 state.

[0087] <Preparation of positive electrode tab>

[0088] Preparation of positive electrode tab using layered oxide in F state (hereinafter abbreviated as positive electrode tab in F state):

[0089] The layered oxide Na 0.95 Ni 0.33 Fe 0.33 Mn 0.33 Ti 0.01O2as the positive active material, positive electrode conductive agent conductive carbon black (Super P), and positive electrode binder polyvinylidene fluoride (PVDF, molecular weight 1000000 to 110000) were mixed in a mass ratio of 80:10:10, N-methyl pyrrolidone (NMP) was added as a solvent, and the positive electrode slurry was obtained by fully stirring to have a solid content of 72 wt% and a uniform system. The positive electrode slurry was uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 10 μm, and was dried at 85°C to obtain a positive electrode tab with a single-sided coated positive active material layer. Then, the above steps were repeated on the other surface of the aluminum foil to obtain a positive electrode tab with a double-sided coated positive active material layer. After cold pressing, cutting, and slitting, the positive electrode tab with a size of 74 mm x 851 mm was dried at 85°C under vacuum for 4 h. The thickness of the positive electrode tab was 140 μm.

[0090] Preparation of a positive electrode tab using a layered oxide in A2 state (hereinafter referred to as a positive electrode tab in A2 state):

[0091] Except that the layered oxide Na 0.95 Ni 0.33 Fe 0.33 Mn 0.33 Ti 0.01 O2was replaced by the layered oxide Na 0.95 Ni 0.33 Fe 0.33 Mn 0.33 Ti 0.01 O2in A2 state, the rest was the same as “Preparation of a positive electrode tab using a layered oxide in F state”.

[0092] <Preparation of a negative electrode tab>

[0093] The negative active material hard carbon, the negative electrode binder styrene butadiene rubber (SBR, weight average molecular weight 200000 to 1000000), and the thickening agent carboxymethyl cellulose sodium (CMC Na) were mixed in a mass ratio of 97:2:1, and then deionized water was added as a solvent. The negative electrode slurry was obtained by fully stirring to have a solid content of 40 wt% and a uniform system. The negative electrode slurry was uniformly coated on one surface of a negative electrode current collector aluminum foil with a thickness of 10 μm, and was dried at 70°C to obtain a negative electrode tab with a single-sided coated negative active material layer. Then, the above steps were repeated on the other surface of the aluminum foil to obtain a negative electrode tab with a double-sided coated negative active material layer. After cold pressing, cutting, and slitting, the negative electrode tab with a size of 76 mm x 867 mm was dried at 120°C under vacuum for 12 h. The thickness of the negative electrode tab was 240 μm.

[0094] <Preparation of an electrolyte>

[0095] In a glove box under argon atmosphere, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a mass ratio of 50:50 and stirred well, then sodium salt sodium hexafluorophosphate (NaPF6) was added, and the electrolyte was obtained after mixing well. The concentration of the sodium salt in the electrolyte was 1 mol / L.

[0096] Preparation of a sodium-ion battery

[0097] Preparation of a sodium-ion battery using a positive electrode sheet in F state

[0098] The positive electrode sheet in F state, the separator, and the negative electrode sheet were sequentially stacked in order, with the separator between the positive electrode sheet in F state and the negative electrode sheet to play a role of isolation, and the electrode assembly was obtained by stacking. The electrode assembly was placed in an aluminum plastic film packaging bag, and after removing water at 150 DEG C, the above electrolyte was injected, and the sodium-ion battery was obtained after vacuum packaging, standing, formation, degassing, shaping, and capacity testing.

[0099] Preparation of a sodium-ion battery using a positive electrode sheet in A2 state

[0100] Except that the positive electrode sheet in F state was replaced by the positive electrode sheet in A2 state, the rest was the same as the preparation of a sodium-ion battery using a positive electrode sheet in F state.

[0101] Examples 1-2 to 1-5

[0102] Except that the corresponding preparation parameters were adjusted according to Table 1, the rest was the same as Example 1-1.

[0103] Example 2-1

[0104] Preparation of a positive electrode active material

[0105] (1) Na 0.95 Ni 0.31 Fe 0.33 Mn 0.33 Ti 0.03 O2 with a stoichiometric ratio of Ni:Fe:Mn:Ti = 0.31:0.33:0.33:0.03, and the raw material with a Dv50 = 6 μm was prepared: precursor Ni 0.31 Fe 0.33 Mn 0.33 Ti 0.013 (OH)2, Na2CO3, the precursor Ni 0.31 Fe 0.33 Mn 0.33 Ti 0.013 (OH)2, Na2CO3 was ground well, calcined at 900 DEG C for 12 h, and broken after natural cooling to obtain a layered oxide Na with a Dv50 = 6 μm.0.95 Ni 0.31 Fe 0.33 Mn 0.33 Ti 0.03 O2;

[0106] (2) The layered oxide Na 0.95 Ni 0.31 Fe 0.33 Mn 0.33 Ti 0.03 O2 prepared in step (1) is mixed with the inorganic oxide Al2O3 in a mass ratio of 99.7:0.3, and is sintered at 600°C for 6 h, and is crushed to obtain a positive electrode active material Na 0.95 Ni 0.31 Fe 0.33 Mn 0.33 Ti 0.03 O2·Al2O3, which is denoted as F state.

[0107] 10 g of the positive electrode active material Na 0.95 Ni 0.33 Fe 0.33 Mn 0.33 Ti 0.01 O2·Al2O3 prepared above is spread on an aluminum foil to have an area of 100±5 cm 2 2, and is pressed flat with a glass dish, and is left in a humidity chamber at a temperature of 25°C and a humidity (RH) of 50% for 3 days to obtain a humidity-treated positive electrode active material Na 0.95 Ni 0.31 Fe 0.33 Mn 0.33 Ti 0.03 O2·Al2O3, which is denoted as A2 state.

[0108] The rest is the same as in Example 1-4.

[0109] Examples 2-2 to 2-20

[0110] The rest is the same as in Example 2-1 except that the corresponding preparation parameters are adjusted according to Table 3.

[0111] Comparative Examples 1 and 2

[0112] The rest is the same as in Example 1-1 except that the relevant preparation parameters are adjusted according to Table 1.

[0113] Comparative Example 3

[0114] The rest is the same as in Example 1-1 except that the relevant preparation parameters are adjusted according to Table 3.

[0115] The preparation parameters and performance parameters of each example and comparative example are shown in Tables 1 to 4.

[0116] Table 1

[0117]

[0118] Table 2

[0119]

[0120] As can be seen from Examples 1-1 to 1-5 and Comparative Examples 1 to 2, when the sodium ion battery of the embodiments of the present application selects the layered oxide within the scope of the present application as the positive electrode active material, the peak height h3 of the third peak in the infrared spectrum of the layered oxide in F state is less than or equal to 15%, the residual alkali content of the layered oxide in F state is less, W N1 %≤1.5%, the weight loss percentage of the layered oxide in F state measured by thermogravimetric analysis is low, W T1 %<0.1%, W T2 %≤0.2%. The peak height h3 of the third peak in the infrared spectrum of the layered oxide in A2 state is less than or equal to 45%, the residual alkali content of the layered oxide in A2 state is less, W N2 %≤5%, the weight loss percentage of the layered oxide in A2 state measured by thermogravimetric analysis is low, W T3 %<1%, W T4 %≤3%, indicating that the positive electrode active material of the embodiments of the present application has a lower residual alkali content in F state and A2 state, and the positive electrode active material of the embodiments of the present application has a lower residual alkali content in A2 state, Na + is not easy to precipitate in a humid environment, and has high stability in air, so that the sodium ion secondary battery has a high gram capacity, indicating that the gram capacity of the sodium ion secondary battery is improved. The positive electrode active material in the comparative examples has a high residual alkali content in F state and A2 state, especially a higher residual alkali content in A2 state, indicating that the positive electrode active material in the comparative examples has poor stability in air, and the sodium ion secondary battery of the comparative examples has a lower gram capacity when the positive electrode active material in A2 state is used.

[0121] Table 3

[0122]

[0123]

[0124]

[0125] Note: “\” in Table 3 indicates no relevant preparation parameters.

[0126] Table 4

[0127]

[0128]

[0129] As can be seen from Comparative Example 3, Examples 1-4, and Examples 2-1 to 2-20, when the layered oxide of the present application with at least part of the outer surface having inorganic oxide is selected as the positive electrode active material, the positive electrode active material in the F state and the positive electrode active material in the A2 state have a lower residual alkali content on the surface, and the positive electrode active material in the F state and the positive electrode active material in the A2 state have a lower weight loss percentage measured by thermogravimetric analysis, indicating that the positive electrode active material of the present application has a lower residual alkali content in the F state and the A2 state, and the positive electrode active material of the present application has a lower residual alkali content in the A2 state, and the Na content in the positive electrode active material is lower. + It is not easy to precipitate in a humid environment, and it has high stability in air, so that the sodium-ion secondary battery has a high gram capacity, indicating that the gram capacity of the sodium-ion secondary battery is improved. Among them, Figure 1 The infrared spectra of the positive electrode active materials in Examples 2-2 and Comparative Example 3 in the Fresh state and the A2 state are shown. Figure 2 The thermogravimetric analysis test graphs of the positive electrode active materials in Examples 2-2 and Comparative Example 3 in the Fresh state and the A2 state are shown.

[0130] As can be seen from Examples 2-2, 2-8 to 2-12, and Comparative Example 3, the positive electrode active material containing the layered oxide of the present application has a lower residual alkali content and a lower weight loss percentage in the F state and the A2 state, indicating that the positive electrode active material of the present application has a lower residual alkali content and a higher structural stability, and the sodium-ion secondary battery using the positive electrode active material of the present application has a higher gram capacity.

[0131] The type of inorganic oxide generally affects the gram capacity of the sodium-ion secondary battery. As can be seen from Examples 2-1 to 2-3, the positive electrode active material with the type of inorganic oxide within the scope of the present application has a lower residual alkali content and a lower weight loss percentage, and the sodium-ion secondary battery has a higher gram capacity.

[0132] The content of inorganic oxide generally affects the gram capacity of the sodium-ion secondary battery. As can be seen from Examples 2-2, 2-4 to 2-7, the positive electrode active material with the content of inorganic oxide within the scope of the present application has a lower residual alkali content and a lower weight loss percentage, and the sodium-ion secondary battery has a higher gram capacity.

[0133] The kind of layered oxide generally affects the gram capacity of the sodium-ion secondary battery. As can be seen from Example 2-2, Example 2-8 to Example 2-12, the kind of layered oxide in the positive electrode active material within the scope of the present application has a lower residual alkali content and a lower weight loss percentage, and the sodium-ion secondary battery has a higher gram capacity.

[0134] The specific surface area of the layered oxide generally affects the gram capacity of the sodium-ion secondary battery. As can be seen from Example 2-2, Example 2-13 to Example 2-16, the specific surface area of the layered oxide in the positive electrode active material within the scope of the present application has a lower residual alkali content and a lower weight loss percentage, and the sodium-ion secondary battery has a higher gram capacity. The specific surface area of the positive electrode active material is strongly related to the particle size, the larger the particle size, the smaller the specific surface area, and generally, the larger the Dv50, the smaller the specific surface area. The smaller the specific surface area, the smaller the contact area between the moisture in the environment and the positive electrode active material, so that the positive electrode active material is less likely to absorb water under the humidity treatment condition, and the Na + is less likely to be precipitated from the bulk phase, so that the degree of degradation of the positive electrode active material is lower, and the gram capacity is higher. However, the larger the particle size, the lower the gram capacity, mainly because the larger particle size will result in a longer migration path of Na + , which manifests as a decrease in gram capacity.

[0135] The compaction density of the positive electrode active material layer generally affects the gram capacity of the sodium-ion secondary battery. As can be seen from Example 2-2, Example 2-17 to Example 2-20, the sodium-ion secondary battery with the compaction density of the positive electrode active material layer within the scope of the present application has a higher gram capacity.

[0136] It should be noted that in this document, the terms "comprising", "containing", or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or apparatus that includes a series of elements not only includes those elements, but also includes other elements not explicitly listed, or inherent to such a process, method, article or apparatus.

[0137] Each of the embodiments in the specification is described in a related manner, and the same or similar parts between the embodiments can be referred to each other, and each embodiment focuses on the difference from other embodiments.

[0138] The above only describes the preferred embodiments of the present application, and does not limit the present application, and any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A positive electrode active material comprising layered oxides; In the infrared spectrum obtained by Fourier transform infrared spectroscopy, the positive electrode active material has a first peak, a second peak, and a third peak, with the wavenumber range k1 of the first peak being 670 cm⁻¹. -1 ≤k1≤890cm -1 The wavenumber range k2 of the second peak is 1040 cm⁻¹. -1 ≤k2≤1100cm -1 The wavenumber range k3 of the third peak is 1320 cm⁻¹. -1 ≤k3≤1530cm -1 ; The peak height h1 of the first peak, the peak height h2 of the second peak, and the peak height h3 of the third peak satisfy the following conditions: 0% < h1 ≤ 4%, 0% < h2 ≤ 2%, and 0% < h3 ≤ 15%. The positive electrode active material is used in sodium-ion batteries.

2. The positive electrode active material according to claim 1, wherein, The positive electrode active material is placed at a temperature of 25°C and a humidity of 50% for 3 days, and the positive electrode active material meets at least one of the following characteristics: (1) The peak height h3 of the third peak satisfies: 0% < h3 ≤ 45%; (2) Using thermogravimetric analysis, the percentage weight loss of the positive electrode active material in the temperature range of 35℃ to 400℃ is W. T4 %, W T4 ≤3, the percentage weight loss of the positive electrode active material is W in the temperature range of 35℃ to 120℃. T3 %, W T3 <1; (3) The residual alkali content of the positive electrode active material is W N2 %, W N2 ≤5.

3. The positive electrode active material according to claim 1, wherein, The specific surface area of ​​the positive electrode active material is B, 0.2m². 2 / g≤B≤0.8m 2 / g; or, The residual alkali content of the positive electrode active material is W. N1 %, W N1 ≤1.

5.

4. The positive electrode active material according to claim 1, wherein, The molecular formula of the layered oxide is Na. x Ni a Fe b Mn c M d O2, M includes at least one of Co, Mg, Ca, B, Al, Zr, Ti, W, Mo, Cr, Sr, Y, Cd, Sn, Sb, Ce, Li, K, Zn, La, F, Si or P; Where 0.1 < a ≤ 0.5, 0.1 < b ≤ 0.5, 0.1 < c ≤ 0.7, 0 ≤ d < 0.2, and 0.7 ≤ x ≤ 1.

0.

5. The positive electrode active material according to claim 1, wherein, The surface of the layered oxide has an inorganic oxide, which includes at least one of TiO2, Al2O3, MgO, V2O5, ZnO, ZrO2, RuO2, La2O3, CeO2, B2O3 or Nb2O5.

6. The positive electrode active material according to claim 5, wherein, Based on the mass of the positive electrode active material, the mass percentage of the inorganic oxide is 0.1% to 0.5%.

7. A positive electrode sheet, comprising a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, wherein the positive active material layer comprises the positive active material according to any one of claims 1 to 6.

8. The positive electrode sheet according to claim 7, wherein, The compaction density of the positive electrode active material layer is 2.8 g / cm³. 3 Up to 3.6 g / cm 3 .

9. A sodium-ion secondary battery comprising the positive electrode sheet as described in claim 7 or 8.

10. An electronic device comprising the sodium-ion secondary battery of claim 9.

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