WO2024216590A1 - Positive electrode active material, positive electrode sheet, sodium-ion secondary battery, and electronic apparatus - Google Patents

Abstract

The present application provides a positive electrode active material, a positive electrode sheet, a sodium-ion secondary battery, and an electronic apparatus. The positive electrode active material comprises a layered oxide. In an infrared spectrum obtained by using 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≤890 cm^-1. The wave number range k₂ of the second peak is 1040 cm^-1≤k₂≤1100 cm^-1. 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％. The positive electrode active material of the present application has a low residual alkali content after synthesis and has excellent structural stability. When the positive electrode active material is applied to a sodium-ion secondary battery, the sodium-ion secondary battery has a high gram capacity.

Description

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

The present application relates to the field of electrochemical technology, and in particular to a positive electrode active material, a positive electrode sheet, a sodium ion secondary battery and an electronic device.

Background Art

Sodium-ion batteries received attention in the late 1970s and early 1980s, but they were not widely studied due to the excellent electrochemical performance of lithium-ion batteries. With the advent of the era of electric vehicles and smart grids, the shortage of lithium resources has become an important factor restricting their development.

As we all know, the reserves of metallic sodium in the earth's crust are relatively abundant (the sodium content in the earth's crust is about 2.75%, while the lithium content is about 0.065‰), and the distribution area is wide (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 extraction/insertion mechanisms. Therefore, the research and development of sodium-ion batteries is expected to alleviate the problem of limited development of energy storage batteries caused by the shortage of lithium resources to a certain extent. In addition to the advantages of abundant resources, low cost and wide distribution, sodium-ion batteries have greater 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 easily passivated and oxidized, and are not easy to produce flammability, which is the main disadvantage of lithium-ion batteries.

The positive electrode active materials of sodium ion batteries mainly include layered oxides, polyanion compounds, and Prussian blue analogues. Among them, the energy density of layered oxides is the highest among the three and the preparation process is mature. However, the positive electrode active materials synthesized by the existing technology have a high residual alkali content, which will affect the stability of the positive electrode active materials in the air, thereby resulting in a decrease in the gram capacity of the sodium ion battery.

Summary of the invention

The present application provides a positive electrode active material, a positive electrode sheet, a sodium ion secondary battery and an electronic device, which are used to reduce the residual alkali content of the positive electrode active material, improve the stability of the positive electrode active material in the air, and thus improve the gram capacity of the sodium ion secondary battery. The specific technical solution is as follows:

In a first aspect, the present application provides a positive electrode active material, which includes 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 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≤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 %, 0%＜ _h3≤15 %. When the positive electrode active material of the present application is applied to a sodium ion secondary battery, the surface of the positive electrode active material has a low residual alkali content. In this way, the sodium ions (Na ⁺ ) in the positive electrode active material are less precipitated and the content is higher, which can reduce the loss of irreversible capacity of the sodium ion secondary battery. During the charge and discharge process of sodium ion secondary batteries, more Na ⁺ participates in the deintercalation, and the gram capacity of sodium ion secondary batteries is improved. In addition, the processing performance and safety performance of sodium ion secondary batteries can be improved.

In some embodiments of the present application, 5 g of positive electrode active material is spread on aluminum foil to an area of 100±5 ^cm2 , and after being flattened with a glass dish, it is placed at a temperature of 25°C and a humidity of 50% for 3 days, and 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 thermogravimetric analysis, the positive electrode active material has a weight loss percentage of W _T4 %, W _T4≤3 in the temperature range of 35°C to 400°C, and a weight loss percentage of W _T3 %, W _T3 ＜1 in the temperature range of 35°C to 120°C; (3) the residual alkali content of the positive electrode active material is W _N2 %, W _N2≤5 . This indicates that the positive electrode active material of the present application has a high structural stability, so that Na ⁺ in the positive electrode active material is not easy to precipitate in a humid environment, thereby having a lower content of residual alkali Na ₂ CO ₃ , which can improve the stability of the positive electrode active material in the air, thus improving the gram capacity of the sodium ion secondary battery.

In some embodiments of the present application, the specific surface area of the positive electrode active material is B, ^{0.2m2/ g≤B≤0.8m2} /g; or, the residual alkali content of the positive electrode active material is _WN1 %, _WN1≤1.5 . Regulating the specific surface area of the positive electrode active material within the above range is conducive to improving the stability of the positive electrode active material in the air, reducing the residual alkali content of the positive electrode active material, and increasing the gram capacity of the sodium ion secondary battery.

In some embodiments of the present application, the molecular formula of the layered _oxide is _{NaxNiaFebMncMdO2} , 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<a≤0.5, 0.1<b≤0.5, 0.1<c≤0.7, 0≤d<0.2, 0.7≤x≤1.0. The selection of the above-mentioned layered oxide is conducive to reducing the residual alkali content of the positive electrode active material and improving the gram capacity of the sodium ion secondary battery.

In some embodiments of the present application, the surface of the layered oxide has an inorganic oxide, and the inorganic oxide includes at least one of TiO ₂ , Al ₂ O ₃ , MgO, V ₂ O ₅ , ZnO, ZrO ₂ , RuO ₂ , La ₂ O ₃ , CeO ₂ , B ₂ O ₃ or Nb ₂ O _5. The inorganic oxides of the above types are selected to be present on at least part of the outer surface of the layered oxide, which is beneficial to reduce the residual alkali content of the positive electrode active material and improve the stability of the positive electrode material in the air, thereby increasing the gram capacity of the sodium ion secondary battery.

In some embodiments of the present application, the mass percentage of the inorganic oxide is 0.1% to 0.5% based on the mass of the positive electrode active material. The mass percentage of the inorganic oxide is regulated within the above range mainly because the inorganic oxide itself cannot play the role of the active material. If the content is too high, the gram capacity of the active material per unit mass will be reduced, and the normal deintercalation of Na ⁺ will be affected. If the content is too low, the purpose of isolating the moisture in the air from contacting the material cannot be achieved.

The second aspect of the present application provides a positive electrode plate, which includes 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 includes the positive electrode active material provided in the first aspect of the present application. Therefore, the positive electrode plate has a lower content of residual alkali and a higher structural stability, and is applied to a sodium ion secondary battery, which is beneficial to improving the gram capacity of the sodium ion secondary battery.

In some embodiments of the present application, the compaction density of the positive electrode active material layer is 2.8 g/cm ³ to 3.6 g/cm ^3. The compaction density of the positive electrode active material layer is adjusted 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 higher compaction density.

The third aspect of the present application provides a sodium ion secondary battery, which includes the positive electrode sheet provided in the second aspect of the present application. Therefore, the sodium ion secondary battery has a higher gram capacity.

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

The present application provides a positive electrode active material, a positive electrode plate, 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 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≤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 %, 0%＜ _h3 ≤15%; the positive electrode active material of the present application has a low residual alkali content after synthesis and has good structural stability. When applied to a sodium ion secondary battery, the sodium ion secondary battery has a higher gram capacity.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings described herein are used to provide further understanding of the present application and constitute a part of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute improper limitations on the present application.

FIG1 is a Fourier transform infrared spectroscopy (FTIR) test diagram of the positive electrode active materials in Example 2-2 and Comparative Example 3;

FIG. 2 is a thermogravimetric analysis (TG) test diagram of the positive electrode active materials in Example 2-2 and Comparative Example 3.

DETAILED DESCRIPTION

In order to make the purpose, technical scheme, and advantages of the present application more clearly understood, the present application is further described in detail with reference to the accompanying drawings and examples. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all of the embodiments. Based on the embodiments in the present application, all other embodiments obtained by ordinary technicians in the field belong to the scope of protection of the present application.

The positive electrode active materials synthesized by the prior art have a high residual alkali content. The high residual alkali content of the positive electrode active materials 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, in the process of coating the positive electrode slurry on the surface of the positive electrode current collector, a large amount of alkaline oxides on the surface of the positive electrode active material (such as the residual alkali Na ₂ CO ₃ mentioned below) will absorb water and cause the positive electrode slurry to form a jelly-like state, thereby affecting the coating. Secondly, the alkaline oxides on the surface of the positive electrode active material will increase the loss of the irreversible capacity of the sodium ion secondary battery and deteriorate the cycle performance of the sodium ion secondary battery. In addition, the residual alkali Na ₂ CO ₃ on the surface of the positive electrode active material will react with the electrolyte to generate CO ₂ , which will cause the sodium ion secondary battery to The secondary battery will swell, which is not conducive to the cycle performance and safety and reliability of the sodium ion secondary battery.

In a first aspect, the present application provides a positive electrode active material, which includes 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 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≤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 %, 0%＜ _h3≤15 %. For example, _h1 is 0.1%, 1%, 2%, 3%, 4% or any value between any two of the above numerical ranges. _h2 is 0.1%, 0.5%, 1%, 1.5%, 2% or any value between 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 between any two of the above numerical ranges. Wherein, the first peak and the second peak represent the fingerprint peaks of _CO32- ^, and the third peak represents the antisymmetric stretching vibration peak of _CO32- . The positive electrode active material has the first peak, the second peak and the ^third peak in the wave number ranges _k1 , _k2 and _k3, respectively, indicating that there is residual alkali _Na2CO3 on the surface of the positive electrode active material. The peak height ^h3 of the antisymmetric stretching vibration peak of _CO32- (i.e., _the third peak) is within the above range of the present application _, indicating that the surface of the positive electrode active material of the present application has a lower content of residual _{alkali Na2CO3} . When the positive electrode active material of the present application 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, less sodium ions (Na ⁺ ) are precipitated in the positive electrode active material, the positive electrode active material has high stability in the air, and the loss of irreversible capacity of the sodium ion secondary battery can be reduced. More Na ⁺ participates in the deintercalation during the charge and discharge process of the sodium ion secondary battery, and the gram capacity of the sodium ion secondary battery is improved. In addition, the cycle performance, processing performance and safety reliability of the sodium ion secondary battery can be improved.

In the present application, the peak height _h1 of the first peak is the transmittance difference between the apex and the baseline of the first peak, the peak height _h2 of the second peak is the transmittance difference between the apex and the baseline of the second peak, and the peak height _h3 of the third peak is the transmittance difference between the apex and the baseline of the third peak.

In some embodiments of the present application, 5 g of the positive electrode active material is spread on aluminum foil to an area of 100 ± 5 cm ² , and after being flattened with a glass dish, it is placed under the conditions of a temperature of 25°C and a humidity of 50% for 3 days, and the peak height h ₃ of the third peak satisfies: 0% < h ₃ ≤ 45%. For example, h ₃ is 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 numerical ranges. After the positive electrode active material is subjected to humidity treatment under the above conditions, the peak height h ₃ of 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 higher structural stability, so that Na ⁺ in the positive electrode active material is not easy to precipitate in a humid environment, so that the material placed in a humid environment has a lower content of residual alkali Na ₂ CO ₃ . In this way, the positive electrode active material has good stability in the air. 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, more Na ⁺ participates in the deintercalation during the charge and discharge process of the sodium ion secondary battery, and the gram capacity of the sodium ion secondary battery is improved. In addition, the cycle performance, processing performance and safety reliability of the sodium ion secondary battery can be improved.

In some embodiments of the present application, 5 g of positive electrode active material is spread on aluminum foil to an area of 100 ± 5 cm ² , and after being flattened with a glass dish, it is placed at a temperature of 25°C and a humidity of 50% for 3 days. Thermogravimetric analysis is performed, and the positive electrode active material has a weight loss percentage of W _T4 %, W _T4 ≤3 in the temperature range of 35°C to 400°C, and a weight loss percentage of W _T3 %, W _T3 <1 in the temperature range of 35°C to 120°C. For example, W _T4 is 0.1, 0.5, 1, 1.5, 2, 2.5, 3, or any value between any two of the above numerical ranges. W _T3 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 numerical ranges. After the layered oxide is subjected to the above humidity treatment, the weight loss percentages W _T3 % and W _T4 % under different conditions are within the above ranges. This is because the positive electrode active material absorbs less water in a humid environment, and the degree of degradation of the positive electrode active material caused by the reaction between the positive electrode active material and water is reduced, which is further reflected in the reduction of the weight loss percentage from 35°C to 400°C or from 35°C to 120°C, indicating that the positive electrode active material of the present application has a higher structural stability. In this way, Na ⁺ in the layered compound is not easy to precipitate in a humid environment, thereby having a lower content of residual alkali Na ₂ CO _3. Moreover, the positive electrode active material with higher structural stability has a lower possibility of alkali metal ions (such as Na ^+, etc.) being released and H ⁺ /H ₂ O being embedded when it is exposed to air, thereby reducing the risk of capacity decline and impaired ion transport. Therefore, the Na ⁺ content in the positive electrode active material is relatively high. When the positive electrode active material of the present application is applied to a sodium ion secondary battery, the irreversible capacity loss of the sodium ion secondary battery can be reduced, more Na ⁺ participates in the deintercalation during the charge and discharge process of the sodium ion secondary battery, and the gram capacity of the sodium ion secondary battery is increased. In addition, the cycle performance, processing performance and safety reliability of the sodium ion secondary battery can be improved.

In some embodiments of the present application, 5g of positive electrode active material is spread on aluminum foil to an area of 100± ^5cm2 , and after being flattened with a glass dish, it is placed 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 _WN2 %, _WN2≤5 . For example, _WN2 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 numerical ranges. It shows that the positive electrode active material after humidity treatment after preparation also has a lower content of residual alkali, which can explain that the positive electrode active material has a higher structural stability. This is because the positive electrode active material is exposed to a humid environment, the amount of alkali metal ions released is small and the area of reaction on its surface is small, so that the positive electrode active material has 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 residual alkali content. In this way, less Na ⁺ is precipitated in the positive electrode active material and the content is higher, which can reduce the loss of irreversible capacity of the sodium ion secondary battery. More Na ⁺ participates in the deintercalation during the charge and discharge process of the sodium ion secondary battery, and the gram capacity of the sodium ion secondary battery is improved. In addition, the cycle performance, processing performance and safety reliability of the sodium ion secondary battery can be improved.

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 between any two of the above numerical ranges. When the specific surface area of the positive electrode active material is controlled within the above range, the surface of the positive electrode active material in contact with the air is small. In this way, the reaction surface can be reduced, thereby reducing the possibility of degradation of the positive electrode active material. Thus, the residual alkali content on the surface of the positive electrode active material can be reduced. When the positive electrode active material of the present application is applied to a sodium ion secondary battery, the reaction between the positive electrode active material and moisture and carbon dioxide in the air can be reduced, and the gram capacity of the sodium ion secondary battery can be increased.

In some embodiments of the present application, the molecular formula of _the layered oxide is _{NaxNiaFebMncMdO2} , _where 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<a≤0.5, 0.1<b≤0.5, 0.1<c≤0.7, 0≤d<0.2, 0.7≤x≤1.0. The layered oxide of the above type has high structural stability, low residual alkali content on its surface, high Na ⁺ content, more Na ⁺ participates in the deintercalation during the charge and discharge process of the sodium ion secondary battery, and the gram capacity of the sodium ion secondary battery is improved. In addition, the cycle performance, processing performance and safety reliability of the sodium ion secondary battery can be improved.

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.

In some embodiments of the present application, inorganic oxides exist on at least part of the outer surface of the layered oxide, and the inorganic oxides include at least one of TiO ₂ , Al ₂ O ₃ , MgO, V ₂ O ₅ , ZnO, ZrO ₂ , RuO ₂ , La ₂ O ₃ , CeO ₂ , B ₂ O ₃ or Nb ₂ O _5. It is understood that in some embodiments of the present application, inorganic oxides exist on part of the outer surface of the layered oxide; in other embodiments of the present application, inorganic oxides exist on the entire outer surface of the layered oxide, that is, the inorganic oxides completely cover the layered oxide. The selection of the above-mentioned types of inorganic oxides to exist on at least part of the outer surface of the layered oxide can reduce the area of contact between the layered oxide surface and the air, and reduce the precipitation 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 precipitation 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 relatively high. 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, and more Na ⁺ participates in the deintercalation during the charge and discharge process of the sodium ion secondary battery, and the gram capacity of the sodium ion secondary battery is further improved. In addition, the cycle performance, processing performance, and safety and reliability of the sodium ion secondary battery can be improved.

In some embodiments of the present application, the mass percentage 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 of the inorganic oxide is 0.1%, 0.2%, 0.3%, 0.4%, 0.5% or any value between any two of the above numerical ranges. By regulating the mass percentage of the inorganic oxide within the above range, the residual alkali content of the positive electrode active material can be reduced without affecting the gram capacity of the positive electrode active material, and the stability of the positive electrode material in the air can be improved, thereby increasing the gram capacity of the sodium ion secondary battery.

Furthermore, the layered oxide with inorganic oxide on the outer surface is an O3-type layered oxide. The O3-type layered oxide has a high Na ⁺ content, which is conducive to more Na ⁺ participating in the deintercalation during the charge and discharge process of the sodium ion secondary battery, and the gram capacity of the sodium ion secondary battery is further improved.

The present application has no particular limitation on 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.

In some embodiments of the present application, using thermogravimetric analysis, the positive electrode active material has a weight loss percentage of W _T2 %, W _T2 ≤0.2 in the temperature range of 35°C to 400°C, and a weight loss percentage of W _T1 %, W _T1 ＜0.1 in the temperature range of 35°C to 120°C.

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 between any two of the above numerical ranges. It shows that the positive electrode active material in the state without humidity treatment after preparation has 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 residual alkali content. In this way, less Na ⁺ is precipitated in the positive electrode active material and the content is higher, which can reduce the loss of irreversible capacity of the sodium ion secondary battery, and more Na ⁺ participates in the deintercalation during the charge and discharge process of the sodium ion secondary battery, and the gram capacity of the sodium ion secondary battery is improved. In addition, the cycle performance, processing performance and safety reliability of the sodium ion secondary battery can be improved.

The present application has no particular restrictions on the preparation method of the positive electrode active material, 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 _{layered oxide} molecular _{formula NaxNiaFebMncMdO2} to be prepared, _{grinding the raw materials FebMncMd ( OH} ) _{2 and Na2CO3} uniformly, calcining at 800 _℃ to 1000℃ for 10h to 14h, and crushing after natural _cooling to obtain the layered oxide _{NaxNiaFebMncMdO2 , which is} recorded as Fresh state. Alternatively, the preparation method of the positive electrode active material may include but is not limited to the following steps: (1) preparing raw materials Fe _b Mn _c M _d (OH) ₂ and Na ₂ CO ₃ according to the stoichiometric ratio of the layered oxide molecular formula Na _x Ni _a Fe _b Mn _c M _d O ₂ to be prepared, grinding the raw materials evenly, calcining them at 800° C. to 1000° C. for 10 h to 14 h, and crushing them after natural cooling to obtain the layered oxide Na _x Ni _a Fe _b Mn _c M _d O ₂ ; (2) mixing the layered oxide Na _x Ni _a Fe _b Mn _c M _d O ₂ prepared in step (1) with an inorganic oxide in a mass percentage ratio of (99.5% to 99.9%): (0.1% to 0.5%), sintering them at 500° C. to 700° C. for 5 h to 7 h, and crushing them to obtain a layered oxide having an inorganic oxide on at least a portion of its outer surface, which is recorded as a Fresh state.

The present application has no particular restrictions on the types of raw materials, as long as the positive electrode active material of the present application can be prepared and the purpose of the present application can be achieved. For example, the raw materials include but are not limited to Na ₂ CO ₃ , and at least one of NaHCO ₃ or NaOH.

The present application has no particular limitation on the average particle size Dv50 of the raw material Fe _b Mn _c M _d (OH) ₂ , as long as the purpose of the present application can be achieved. For example, the average particle size Dv50 of the raw material is 2 μm to 15 μm.

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

In the present application, TM includes at least one of the 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.

The present application has no particular restrictions on the method for regulating the specific surface area of the positive electrode active material, as long as the purpose of the present application can be achieved. For example, the specific surface area of the positive electrode active material can be regulated by regulating the average particle size Dv50 of the raw materials used to prepare the positive electrode active material.

In the present application, Dv50 means the particle size at which the volume accumulation reaches 50% from the small particle size side in the volume-based particle size distribution. The present application has no particular restrictions on the test method of Dv50, and it can be measured by a test method known in the art, for example, by a laser particle size analyzer.

The second aspect of the present application provides a positive electrode plate, which includes 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 includes the positive electrode active material provided in the first aspect of the present application. Since the surface of the positive electrode active material provided in the first aspect of the present application has a low residual alkali content, and the positive electrode active material has a high content of Na ⁺ , the positive electrode plate is applied to a sodium ion secondary battery, and more Na ⁺ participates in the intercalation and deintercalation process of the sodium ion secondary battery, so that the gram capacity of the sodium ion secondary battery is improved. In addition, the sodium ion secondary battery has good cycle performance, processing performance and safety and reliability.

The above-mentioned “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 both surfaces of the positive electrode current collector. The “surface” may be a partial surface or the entire surface of the negative electrode current collector.

In some embodiments of the present application, the compaction density of the positive electrode active material layer is 2.8 g/cm ³ to 3.6 g/cm ^3. For example, the compaction density of the positive electrode active material layer is 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 ³ or any value between any two of the above numerical ranges. The compaction density of the positive electrode active material layer is regulated within the above 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. Thereby, the sodium ion secondary battery has a higher energy density and good processing performance, safety performance, cycle performance and rate performance.

The present application has no particular restrictions on the method for regulating the compaction density of the positive electrode active material layer, 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 of the positive electrode sheet, adjusting the type or average particle size of the positive electrode active material, etc. The present application has no particular restrictions on the size of the above pressure, as long as the purpose of the present application can be achieved. For example, the pressure is 40t to 80t.

The present application has no particular restrictions on the type of positive electrode current collector, as long as the purpose of the present application can be achieved. The electrode current collector may include aluminum foil, aluminum alloy foil, or the like.

In the present application, there is no particular restriction on the thickness of the positive current collector and the positive active material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the positive current collector is 5μm to 20μm, preferably 6μm to 18μm. The thickness of the positive active material layer is 30μm to 120μm. Optionally, the positive active material layer may also include a positive electrode conductor and a positive electrode binder. The present application does not particularly limit the types of positive electrode conductors and positive electrode binders in the positive electrode active material layer, as long as the purpose of the present application can be achieved. The present application does not particularly limit the mass ratio of the positive active material, positive electrode conductor, and positive electrode binder in the positive electrode active material layer, and those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved. For example, the mass ratio of the positive active material, the positive electrode conductor, and the positive electrode binder in the positive electrode active material layer is (90-98): (0.5-5): (0.5-5).

The third aspect of the present application provides a sodium ion secondary battery, which includes the positive electrode sheet provided in the second aspect of the present application. Therefore, the sodium ion secondary battery has a higher gram capacity.

In some embodiments of the present application, the sodium ion secondary battery of the present application also includes a negative electrode plate and a diaphragm, and the diaphragm is arranged between the positive electrode plate and the negative electrode plate to separate the positive electrode plate and the negative electrode plate, prevent internal short circuit of the electrochemical device, allow electrolyte ions to pass freely, and do not affect the electrochemical charge and discharge process.

The present application has no special restrictions on the negative electrode sheet, as long as the purpose 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 present application has no special restrictions on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the negative electrode current collector may include aluminum foil, aluminum alloy foil, copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foam nickel or foam copper, etc. The negative electrode active material layer of the present application includes a negative electrode active material, and the present application has no special restrictions on the type of negative electrode active material, as long as the purpose of the present application can be achieved. For example, the negative electrode active material may include but is not limited to at least one of hard carbon, soft carbon, sodium metal or bismuth metal. In the present application, there is no special restriction on the thickness of the negative electrode current collector and the negative electrode active material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 6μm to 10μm, and the thickness of the negative electrode active material layer is 30μm to 130μm. Optionally, the negative electrode active material layer may also include at least one of a negative electrode conductive agent or a thickener. The present application does not particularly limit the types of the negative electrode conductive agent and the thickener, as long as the purpose of the present application can be achieved. The present application does not particularly limit the mass ratio of the negative electrode active material, the negative electrode conductive agent, the thickener, and the negative electrode binder in the negative electrode active material layer, as long as the purpose of the present application can be achieved. For example, the mass ratio of the negative electrode active material, the negative electrode conductive agent, the thickener, and the negative electrode binder in the negative electrode active material layer is (90-98): (0-5): (0.5-1.5): (0.5-5).

The present application has no particular restrictions on the diaphragm, as long as the purpose of the present application can be achieved. For example, the material of the diaphragm may include but is not limited to polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyester (e.g., polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid. The type of diaphragm may include woven membranes, nonwoven membranes, microporous membranes, composite membranes, rolled membranes or spun membranes. One less.

The sodium ion battery of the present application further includes 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 present application has no particular restrictions on the packaging bag and the electrolyte, and can be packaging bags and electrolytes 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 stainless steel, an aluminum shell, or a steel shell. The electrolyte can include 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 NaPF ₆ , NaClO ₄ , NaBF ₄ , NaNO ₃ , NaSCN, NaCN, NaAsF ₆ , NaCF ₃ CO ₂ , NaSbF ₆ , NaC ₆ HsCO ₂ , Na(CH ₃ )C ₆ H ₄ SO ₃ , NaHSO ₄ or NaB(C ₆ Hs) _4. The non-aqueous solvent can be at least one of a carbonate compound, a carboxylate compound, an ether compound or other organic solvents. The above-mentioned carbonate compound may include but is not limited to at least one of a linear carbonate compound, a cyclic carbonate compound or a fluorinated carbonate compound. The above-mentioned linear carbonate compound may 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 ethyl methyl carbonate (MEC). The cyclic carbonate compound may 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 may 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-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate, or trifluoromethylethylene carbonate. The above-mentioned carboxylate compound may include, but is not limited to, at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decalactone, valerolactone, mevalonolactone, or caprolactone. The above-mentioned ether compound may 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 may include but are not limited to at least one of dimethyl sulfoxide, 1,2-dioxolane, cyclopentane, methyl cyclopentane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, formamide, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, trioctyl phosphate or phosphate.

The preparation steps of the sodium ion battery of the present application may also include but are not limited to the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in sequence, and winding, folding and other operations as needed to obtain an electrode assembly with a wound structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a sodium ion battery; or, stacking the positive electrode sheet, the separator and the negative electrode sheet in sequence, and then fixing the four corners of the entire stacked structure with tape to obtain an electrode assembly with a stacked structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a sodium ion battery.

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

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, electronic devices may include, but are not limited to, laptop computers, pen-type computers, mobile computers, electronic book players, portable phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, LCD televisions, portable cleaners, portable CD players, mini-discs, transceivers, electronic notepads, calculators, memory cards, portable recorders, radios, backup power supplies, motors, cars, motorcycles, power-assisted bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household batteries, and sodium-ion capacitors.

Example

Hereinafter, the embodiments of the present application will be described in more detail with reference to Examples and Comparative Examples. Various tests and evaluations were performed according to the following methods.

Test methods and equipment:

Fourier transform infrared spectroscopy (FTIR) test:

The reference standard is GB/T 21186-2007 National Standard for Fourier Transform Infrared Spectrometer. The positive active materials (fresh state) of each embodiment and comparative example are 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 induce the absorption spectrum produced by the intramolecular vibration and rotation energy level transition.

Specific surface area test of positive electrode active material:

The specific surface area of the positive electrode active materials (fresh state) in the examples and comparative examples was tested by nitrogen adsorption/desorption method using a specific surface area analyzer (TristarⅡ3020M). The specific test was performed in accordance with the national standard GB/T19587-2017.

Thermogravimetric analysis (TG) test:

A thermogravimetric analyzer was used to test the change in mass of the positive electrode active material with temperature. The test was based on JY_T0589.1-2020/JY_T0589.4-2020/JY_T0589.5-2020 "General Rules for Thermal Analysis Methods".

(1) Take 10 mg of the fresh positive electrode active material in each embodiment or comparative example and place it in a thermogravimetric analyzer. The test atmosphere is set in a nitrogen atmosphere. The test temperature is 35° C. to 400° C. and the heating rate is 5° C./min.

During the test, the weight loss between 35°C and 120°C and 35°C and 400°C are recorded respectively, the weight loss percentage from 35°C to 120°C W _T1 %=the mass fraction corresponding to the temperature from 35°C to 120°C, and the weight loss percentage from 35°C to 400°C W _T2 %=the mass fraction corresponding to the temperature from 35°C to 400°C.

(2) The process is the same as the process (1) above, except that the positive electrode active material in the Fresh state is replaced by the positive electrode active material in the A2 state.

The weight loss percentage from 35°C to 120°C W _T3 %=the mass fraction corresponding to the temperature from 35°C to 120°C, and the weight loss percentage from 35°C to 400°C W _T4 %=the mass fraction corresponding to the temperature from 35°C to 400°C.

Humidity treatment:

5 g of the positive electrode active material of each embodiment and comparative example was spread on aluminum foil to an area of 100± ⁵ cm2, and flattened with a glass dish, and then placed at 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 state A2.

Test of residual alkali content:

The residual alkali content in the positive electrode active material can be obtained by extracting Na ₂ CO ₃ and NaOH from the positive electrode active material, filtering and testing the filtrate by potentiometric titration.

Na ₂ CO ₃ +HC1=NaCl+NaHCO ₃ ;

NaOH+HCl＝NaCl+H ₂ O;

NaHCO ₃ +HCl=NaCl+H ₂ CO ₃ .

Specific operation: Weigh 1g sample into a 100mL conical flask, add 20mL ethylene glycol (purity 99.999%), put in a magnetic rod and seal the bottle mouth with a sealing film, stir magnetically for 30 minutes and then filter, dilute the filtrate to 100mL with pure water, take 50mL of the filtrate and put it in a beaker for potentiometric titration test.

The above “samples” are the positive electrode active materials in the Fresh state and the A2 state of each embodiment or comparative example.

Compaction density test of positive electrode active material layer:

The compaction density Pc of the positive electrode active material layer is calculated by the formula: Pc = mc/Vc, where mc is the mass of the positive electrode active material layer, in g; Vc is the volume of the positive electrode active material layer, in cm ³ , where 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.

Test of gram capacity:

The positive active material powders of each embodiment or comparative example in the Fresh state and the A2 state are mixed with the conductive agent conductive carbon black and the binder polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1, and N-methylpyrrolidone (NMP) is added to grind into a slurry with a solid content of 72wt%, coated on aluminum foil, the coating thickness is 200μm, placed in a vacuum oven to dry, and then cut into pieces. Using a metal sodium sheet as the counter electrode, 1mol/L NaPF ₆ dissolved in a PC solvent to obtain a solution as an electrolyte, glass fiber as a diaphragm, assembled into a CR2032 button cell, and the assembly of the button cell is carried out in a glove box filled with argon. The assembled button cell is subjected to a constant current charge and discharge test on a blue electric test system, with a current density of 10mA/g and a charge and discharge voltage window of 2.0V to 4.0V, and the gram capacity of the Fresh state and the A2 state is obtained by testing respectively.

Example 1-1

<Preparation of positive electrode active material>

According to the stoichiometric ratio of Na _0.95 Ni _0.33 Fe _0.33 Mn _0.33 Ti _0.01 O _2, Ni:Fe:Mn:Ti=0.33:0.33:0.33:0.01, the raw materials with Dv50=6μm were prepared: precursor Ni _0.33 Fe _0.33 Mn _0.33 Ti _0.01 (OH) ₂ and Na ₂ CO _3. The precursor Ni _0.33 Fe _0.33 Mn _0.33 Ti _0.01 (OH) ₂ and Na ₂ CO ₃ were ground evenly, calcined at 900℃ for 12h, and crushed after natural cooling to obtain The layered oxide Na _0.95 Ni _0.33 Fe _0.33 Mn _0.33 Ti _0.01 O ₂ with Dv50=6 μm is referred to as a Fresh state (hereinafter abbreviated as F state).

5 g of the layered oxide Na _0.95 Ni _0.33 Fe _0.33 Mn _0.33 Ti _0.01 O ₂ prepared above was subjected to the aforementioned humidity treatment to obtain the layered oxide Na _0.95 Ni _0.33 Fe _0.33 Mn _0.33 Ti _0.01 O ₂ in the A2 state.

<Preparation of positive electrode sheet>

Preparation of positive electrode sheet using F-state layered oxide (hereinafter referred to as F-state positive electrode sheet):

The above-prepared F-state layered oxide Na _0.95 Ni _0.33 Fe _0.33 Mn _0.33 Ti _0.01 O ₂ is used as a positive electrode active material and mixed with a positive electrode conductive agent conductive carbon black (Super P) and a positive electrode binder polyvinylidene fluoride (PVDF, molecular weight 1000000 to 110000) in a mass ratio of 80:10:10, and N-methylpyrrolidone (NMP) is added as a solvent, and stirred thoroughly to obtain a positive electrode slurry with a solid content of 72wt% and a uniform system. The positive electrode slurry is evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 10μm, and dried at 85°C to obtain a positive electrode sheet coated with a positive electrode active material layer on one side. Afterwards, the above steps are repeated on the other surface of the aluminum foil to obtain a positive electrode sheet coated with a positive electrode active material layer on both sides. After cold pressing, cutting, slitting, and drying at 85°C under vacuum for 4 hours, a positive electrode sheet with a specification of 74mm×851mm was obtained for use. The thickness of the positive electrode sheet was 140μm.

Preparation of positive electrode sheet using A2 state layered oxide (hereinafter referred to as positive electrode sheet in A2 state):

Except for replacing the F-state layered oxide Na _0.95 Ni _0.33 Fe _0.33 Mn _0.33 Ti _0.01 O ₂ with the A2-state layered oxide Na _0.95 Ni _0.33 Fe _0.33 Mn _0.33 Ti _0.01 O ₂ , the rest is the same as the “Preparation of positive electrode sheet using F-state layered oxide”.

<Preparation of negative electrode sheet>

The negative electrode active material hard carbon, the negative electrode binder styrene butadiene rubber (SBR, weight average molecular weight 200000 to 1000000), and the thickener sodium carboxymethyl cellulose (CMCNa) are mixed in a mass ratio of 97:2:1, and then deionized water is added as a solvent, and the mixture is stirred thoroughly to obtain a negative electrode slurry with a solid content of 40wt% and a uniform system. The negative electrode slurry is evenly coated on one surface of a negative electrode current collector aluminum foil with a thickness of 10μm, and dried at 70°C to obtain a negative electrode sheet with a single-sided coating of a negative electrode active material layer. After that, the above steps are repeated on the other surface of the aluminum foil to obtain a negative electrode sheet with a double-sided coating of a negative electrode active material layer. After cold pressing, cutting, and slitting, it is dried at 120°C under vacuum conditions for 12h to obtain a negative electrode sheet with a specification of 76mm×867mm for standby use. Among them, the thickness of the negative electrode sheet is 240μm.

<Preparation of Electrolyte>

In a dry (humidity <1%) argon atmosphere glove box, ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed at a mass ratio of 50:50 and stirred thoroughly, and then sodium hexafluorophosphate (NaPF ₆ ) was added and mixed evenly to obtain an electrolyte solution, wherein the concentration of the sodium salt in the electrolyte solution was 1 mol/L.

<Preparation of Sodium Ion Batteries>

Preparation of sodium ion battery using positive electrode sheet in F state:

The positive electrode sheet, separator and negative electrode sheet in F state are stacked in order, so that the separator is between the positive electrode sheet and the negative electrode sheet in F state to play an isolating role, and the sheets are stacked to obtain an electrode assembly. The electrode assembly is placed in an aluminum-plastic film packaging bag, and after dehydration at 150°C, the above electrolyte is injected, and a sodium ion battery is obtained through vacuum packaging, standing, formation, degassing, shaping, capacity testing and other process flows.

Preparation of sodium ion battery using positive electrode sheet in A2 state:

Except for replacing the positive electrode sheet in F state with the positive electrode sheet in A2 state, the rest is the same as "Preparation of sodium ion battery using positive electrode sheet in F state".

Example 1-2 to Example 1-5

Except for adjusting the corresponding preparation parameters according to Table 1, the rest is the same as Example 1-1.

Example 2-1

<Preparation of positive electrode active material>

(1) According to the stoichiometric ratio of Na _0.95 Ni _0.31 Fe _0.33 Mn _0.33 Ti _0.03 O ₂ of Ni:Fe:Mn:Ti=0.31:0.33:0.33:0.03, raw materials with Dv50=6 μm were prepared: precursors Ni _0.31 Fe _0.33 Mn _0.33 Ti _0.013 (OH) ₂ and Na ₂ CO ₃ ; the precursors Ni _0.31 Fe _0.33 Mn _0.33 Ti _0.013 (OH) ₂ and Na ₂ CO ₃ were ground uniformly, calcined at 900°C for 12 h, and crushed after natural cooling to obtain layered oxide Na _0.95 Ni _0.31 Fe _0.33 Mn _0.33 Ti _0.03 O ₂ with Dv50=6 μm;

(2) The layered oxide Na _0.95 Ni _0.31 Fe _0.33 Mn _0.33 Ti _0.03 O ₂ prepared in step (1) and the inorganic oxide Al ₂ O ₃ were uniformly mixed in a mass ratio of 99.7:0.3, sintered at 600°C for 6 h, and crushed to obtain the positive electrode active material Na _0.95 Ni _0.31 Fe _0.33 Mn _0.33 Ti _0.03 O ₂ ·Al ₂ O ₃ , which was recorded as F state.

Take _10g of the above-prepared positive electrode active _{material Na0.95Ni0.33Fe0.33Mn0.33Ti0.01O2} · _Al2O3 and spread it on aluminum foil to an area of 100 _± ^5cm2. _After pressing it flat with a glass dish _, it is placed in a humidity box at _{a temperature of 25°C and a humidity (RH) of 50% for 3 days to obtain the positive electrode active material Na0.95Ni0.31Fe0.33Mn0.33Ti0.03O2·Al2O3 after humidity treatment , which is recorded as} A2 state.

The rest is the same as Example 1-4.

Example 2-2 to Example 2-20

Except for adjusting the corresponding preparation parameters according to Table 3, the rest is the same as Example 2-1.

Comparative Example 1 to Comparative Example 2

Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

Comparative Example 3

Except for adjusting the relevant preparation parameters according to Table 3, the rest is the same as Example 1-1.

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

Table 1

Table 2

It can be seen from Examples 1-1 to 1-5 and Comparative Examples 1 to 2 that the sodium ions in the embodiments of the present application When the battery selects the layered oxide within the scope of this application as the positive electrode active material, in the infrared spectrum of the layered oxide in the F state, the peak height _h3 of the third peak is less than or equal to 15%, the residual alkali content of the layered oxide in the F state is relatively low, W _N1 % ≤ 1.5%, and the weight loss percentage of the layered oxide in the F state measured by thermogravimetric analysis is relatively low, W _T1 % < 0.1%, W _T2 % ≤ 0.2%. In the infrared spectrum of the layered oxide in the A2 state, the peak height _h3 of the third peak is less than or equal to 45%, the residual alkali content of the layered oxide in the A2 state is less, _WN2 %≤5%, and the weight loss percentage of the layered oxide in the A2 state measured by thermogravimetric analysis is low, _WT3 %＜1%, _WT4 %≤3%, indicating that the positive active material of the embodiment of the present application has a lower residual alkali content in both the F state and the A2 state, and the positive active material of the embodiment of the present application has an even lower residual alkali content in the A2 state, and the Na ⁺ in the positive active material is not easy to precipitate in a humid environment, and it has a higher stability in the air, so that the sodium ion secondary battery has a higher gram capacity, indicating that the gram capacity of the sodium ion secondary battery is improved. The positive active material in the comparative example has a higher residual alkali content in both the F state and the A2 state, especially the residual alkali content in the A2 state is higher, indicating that the positive active material of the comparative example has poor stability in the air, and the sodium ion secondary battery of the comparative example has a lower gram capacity when using the positive active material in the A2 state.

Table 3

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

Table 4

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

It can be seen from Example 2-2, Example 2-8 to Example 2-12 and Comparative Example 3 that 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 both the F state and the A2 state, indicating that the positive electrode active material of the embodiments 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.

The type of inorganic oxide usually affects the gram capacity of the sodium ion secondary battery. From Example 2-1 to Example 2-3, it can be seen that the positive electrode active material with the type of inorganic oxide within the scope of this 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 content of inorganic oxide usually affects the gram capacity of sodium ion secondary batteries. From Examples 2-2, 2-4 to 2-7, it can be seen that the positive electrode active material with an inorganic oxide content within the range of this application has a lower The residual alkali content and the lower weight loss percentage make the sodium ion secondary battery have a higher gram capacity.

The type of layered oxide usually affects the gram capacity of the sodium ion secondary battery. It can be seen from Examples 2-2, 2-8 to 2-12 that the positive electrode active material of the layered 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.

The specific surface area of the layered oxide usually affects the gram capacity of the sodium ion secondary battery. It can be seen from Example 2-2, Example 2-13 to Example 2-16 that the positive electrode active material with a specific surface area of the layered oxide within the scope of this 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 correlated with the particle size. The larger the particle, the smaller the specific surface area. In general, 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. In this way, the positive electrode active material is less likely to absorb water under humidity treatment conditions, and Na ⁺ is not easy to precipitate from the bulk phase, so that the degree of deterioration of the positive electrode active material is lower and the gram capacity is higher. However, the larger the particle, the lower the gram capacity, mainly because the larger the particle, the longer the migration path of Na ⁺ , which is manifested as a decrease in gram capacity.

The compaction density of the positive electrode active material layer usually affects the gram capacity of the sodium ion secondary battery. From Examples 2-2, 2-17 to 2-20, it can be seen that the sodium ion secondary battery with a compaction density of the positive electrode active material layer within the scope of the present application has a higher gram capacity.

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

Each embodiment in this specification is described in a related manner, and the same or similar parts between the embodiments can be referenced to each other, and each embodiment focuses on the differences from other embodiments.

The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims (10)

A positive electrode active material comprising 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≤1530cm} ^-1 ; The peak height h ₁ of the first peak, the peak height h ₂ of the second peak and the peak height h ₃ of the third peak satisfy: 0%<h ₁ ≤4%, 0%<h ₂ ≤2%, 0%<h ₃ ≤15%. The positive electrode active material according to claim 1, wherein the positive electrode active material is placed under conditions of a temperature of 25° C. and a humidity of 50% for 3 days, and 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 thermogravimetric analysis, the positive electrode active material has a weight loss percentage of W _T4 %, W _T4 ≤ 3 in the temperature range of 35°C to 400°C, and a weight loss percentage of W _T3 %, W _T3 <1 in the temperature range of 35°C to 120°C; (3) The residual alkali content of the positive electrode active material is W _N2 %, W _N2 ≤5. The positive electrode active material according to claim 1, wherein the specific surface area of the positive electrode active material is B, 0.2m 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. The positive electrode _active material according to claim ₁ , wherein the molecular formula of the layered oxide is _{NaxNiaFebMncMdO2} , and 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; Among them, 0.1＜a≤0.5, 0.1＜b≤0.5, 0.1＜c≤0.7, 0≤d＜0.2, 0.7≤x≤1.0. The positive _electrode active material according to claim 1, wherein the surface of the layered oxide has an _inorganic oxide, _{and the} inorganic oxide includes at least one of _TiO2 , _Al2O3 , MgO, _V2O5 , _ZnO , _{ZrO2 , RuO2} , _La2O3 , CeO2, _B2O3 or _Nb2O5 . The positive electrode active material according to claim 5, wherein the mass percentage of the inorganic oxide is 0.1% to 0.5% based on the mass of the positive electrode active material. A positive electrode sheet 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, wherein the positive electrode active material layer comprises the positive electrode active material according to any one of claims 1 to 6. The positive electrode sheet according to claim 7, wherein the compaction density of the positive electrode active material layer is 2.8 g/cm ³ to 3.6 g/cm ³ . A sodium ion secondary battery comprising the positive electrode sheet according to claim 7 or 8. An electronic device comprising the sodium ion secondary battery according to claim 9.