Positive electrode sheet, method for manufacturing the same, energy storage device, and electric device

Abstract

The application discloses a positive electrode sheet, a preparation method thereof, an energy storage device and an electric device. The positive electrode sheet comprises a positive electrode active material layer, wherein the positive electrode active material layer comprises a positive electrode active material and a capacity compensation agent, the capacity compensation agent is dispersed in the positive electrode active material layer, and in a scanning electron microscope image of the positive electrode active material layer, an average particle size of the capacity compensation agent is not greater than an average particle size of the positive electrode active material.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of electrochemistry, and in particular, relates to a positive electrode sheet, a preparation method thereof, an energy storage device and an electric device. BACKGROUND

[0002] A secondary battery, also known as a rechargeable battery or a storage battery, refers to a battery that can be activated by charging after discharging to continue to be used. The recyclable nature of the secondary battery makes it gradually become the main power source of electric devices. With the gradual increase in demand for secondary batteries, people's requirements for their performance in all aspects are also getting higher and higher, for example, in terms of the energy density per unit volume of the battery, it is expected to obtain a battery with high energy density. SUMMARY

[0003] The present application aims to at least partially solve one of the technical problems in the related art. To this end, one object of the present application is to propose a positive electrode sheet, a preparation method thereof, an energy storage device and an electric device. The positive electrode sheet not only helps to develop specific capacity, but also helps to balance its electrical conductivity, which can improve the energy density of the battery and improve the electrochemical performance.

[0004] In a first aspect of the present application, a positive electrode sheet is proposed. The positive electrode sheet comprises: a positive electrode active material layer, the positive electrode active material layer comprising a positive electrode active material and a capacity compensation agent, the capacity compensation agent being dispersed in the positive electrode active material layer, and in a scanning electron microscope image of the positive electrode active material layer, the average particle size of the capacity compensation agent is not greater than the average particle size of the positive electrode active material.

[0005] The positive electrode sheet according to the above embodiments of the present application has at least the following beneficial effects: the introduction of the capacity compensation agent in the positive electrode sheet can make up for the active ions (such as Li + or Na + ) consumed by the formation of SEI film by the negative electrode sheet, improve the energy density, storage performance and cycle performance of the battery cell. At the same time, by controlling the average particle size of the capacity compensation agent in the positive electrode active material layer to be not greater than the average particle size of the positive electrode active material, the pore formed on the positive electrode active material layer after the decomposition of the capacity compensation agent can also be small, which can effectively solve the problem of the decrease in the electrical conductivity of the electrode sheet caused by the pore formed after the decomposition of the capacity compensation agent being too large. Therefore, the positive electrode sheet not only helps to develop specific capacity, but also helps to balance its electrical conductivity, which can improve the energy density of the battery and improve the electrochemical performance.

[0006] In addition, the positive electrode sheet according to the above embodiments of the present application can also have the following additional technical features:

[0007] In some embodiments of this application, the average particle size of the capacity compensator is not greater than 3 / 4 of the average particle size of the positive electrode active material.

[0008] In some embodiments of this application, the average particle size of the capacity compensator is ≤20μm.

[0009] In some embodiments of this application, based on the area of ​​the scanning electron microscope image of the positive electrode active material layer, the area ratio of the capacity compensator with an average particle size ≥ 5 μm in the scanning electron microscope image is ≤ 20%.

[0010] In some embodiments of this application, the capacity compensator includes inorganic acid salts and / or organic acid salts.

[0011] In some embodiments of this application, the capacity compensator includes one or more of Na2CO3, Na2C4O4, Na2C3O3, Na2C5O5, Na2C6O6, CH3COONa, Na2C2O4, NaNO3, NaOH, and NaHCO3.

[0012] In some embodiments of this application, the positive electrode active material includes Na. x M y (X a O b ) z Z w Where X is one or more of P, S and W, Z is one or more of F, OH and Cl, 1≤x≤6, 1≤y≤5, 1≤a≤6, 4≤b≤12, 2≤z≤6, and 0≤w≤6.

[0013] In some embodiments of this application, the Na x M y (X a O b ) z Z w In this context, M represents one or more of Fe, Mn, and Co.

[0014] In some embodiments of this application, the capacity compensator accounts for ≤5 wt% of the mass of the positive electrode active material layer.

[0015] In some embodiments of this application, the capacity compensator accounts for 1 wt% to 5 wt% of the mass of the positive electrode active material layer.

[0016] In some embodiments of this application, after the positive electrode sheet is formed into a battery, the average pore size of the pores on the surface of the positive electrode active material layer is not greater than the average particle size of the positive electrode active material.

[0017] In some embodiments of this application, the porosity of the positive electrode sheet changes by 1-5% before and after battery formation.

[0018] In a second aspect of this application, a method for preparing a positive electrode sheet is provided, comprising:

[0019] The positive electrode active material, capacity compensator, conductive agent and binder are mixed with solvent to obtain positive electrode active slurry;

[0020] The positive electrode active slurry is coated onto at least one side of the positive electrode current collector to form a positive electrode active material layer.

[0021] The capacity compensator comprises inorganic acid salts and / or organic acid salts, the solvent comprises at least one of water, acetone and N,N-dimethylformamide, and the water content in the positive electrode active slurry is not greater than 0.5 wt%. In the scanning electron microscope image of the positive electrode active material layer, the average particle size of the capacity compensator is not greater than the average particle size of the positive electrode active material.

[0022] The method for preparing the positive electrode sheet according to the above embodiments of this application has at least the following beneficial effects: By introducing at least one of water, acetone, and N,N-dimethylformamide into the solvent, the solubility of inorganic acid salts and / or organic acid salts as capacity compensators in the positive electrode active slurry can be improved, thereby improving the dispersion uniformity of the capacity compensator in the positive electrode active slurry and the positive electrode active material layer. This effectively suppresses the formation of large-diameter particles in the positive electrode active material layer after the capacity compensator decomposes, resulting in a positive electrode active material layer with an average particle size of the capacity compensator not greater than the average particle size of the positive electrode active material, reducing the risk of excessive lithium / sodium supplementation in local areas. Simultaneously, by controlling the mass ratio of water in the positive electrode active slurry to be no more than 0.5 wt%, the adverse effects of the presence of water on the electrochemical performance of the positive electrode active material and / or the processability of the positive electrode sheet can also be taken into account. In summary, the positive electrode sheet prepared by this method can not only compensate for the active ions (such as Li) consumed by the negative electrode sheet in the formation of the SEI film (such as Li...), but also... + Or Na + This process improves the energy density, storage performance, and cycle performance of the battery cell. It also reduces the pores formed on the positive electrode active material layer after the capacity compensator decomposes, effectively solving the problem of decreased electrode conductivity caused by excessively large pores formed after the capacity compensator decomposes. As a result, the positive electrode produced not only benefits the specific capacity but also balances its conductivity, thereby improving the battery's energy density and electrochemical performance.

[0023] In some embodiments of this application, the capacity compensator includes one or more of the following: Na2CO3, Na2C4O4, Na2C3O3, Na2C5O5, Na2C6O6, CH3COONa, Na2C2O4, NaNO3, NaOH, and NaHCO3.

[0024] In some embodiments of this application, the positive electrode active material includes Na. x M y (X a O b ) z Z w Where M is selected from transition metal elements, X is one or more of P, S and W, Z is one or more of F, OH and Cl, 1≤x≤6, 1≤y≤5, 1≤a≤6, 4≤b≤12, 2≤z≤6, and 0≤w≤6.

[0025] In some embodiments of this application, the mass percentage of the capacity compensator is ≤5wt%, based on the solid content of the positive electrode active slurry.

[0026] In some embodiments of this application, the mass percentage of the capacity compensator is 1 wt% to 5 wt%, based on the solid content of the positive electrode active slurry.

[0027] In some embodiments of this application, the solvent includes N-methylpyrrolidone and water, wherein the mass ratio of N-methylpyrrolidone to water is 99.5:0.5 to 99.95:0.05.

[0028] In some embodiments of this application, the solvent includes N-methylpyrrolidone and acetone, wherein the mass ratio of N-methylpyrrolidone to acetone is 80:20 to 95:5.

[0029] In some embodiments of this application, the solvent includes N-methylpyrrolidone and N,N-dimethylformamide, wherein the mass ratio of N-methylpyrrolidone to N,N-dimethylformamide is 70:30 to 95:5.

[0030] In some embodiments of this application, the solvent contains N-methylpyrrolidone at a mass percentage of not less than 95 wt%.

[0031] In some embodiments of this application, the mixing temperature is 10–120°C.

[0032] In some embodiments of this application, the solubility of the capacity compensator in the solvent is not less than 0.1 wt% to 5 wt% of the solid content of the positive electrode active slurry.

[0033] In a third aspect, this application proposes an energy storage device. This energy storage device includes the aforementioned positive electrode sheet, and / or a positive electrode sheet prepared using the aforementioned method for preparing the positive electrode sheet. This energy storage device possesses all the features and effects of the aforementioned positive electrode sheet and the aforementioned method for preparing the positive electrode sheet, which will not be elaborated further here. In general, this energy storage device combines high energy density, storage performance, and good electrochemical performance.

[0034] In a fourth aspect, this application proposes an electrical device. According to an embodiment of this application, the electrical device includes the aforementioned energy storage device. This electrical device possesses all the features and effects of the aforementioned energy storage device, which will not be elaborated further here.

[0035] Additional aspects and advantages of this application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of this application. Attached Figure Description

[0036] The above and / or additional aspects and advantages of this application will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which:

[0037] Figure 1 This is a scanning electron microscope image of the positive electrode sheet prepared according to Comparative Example 2 of this application.

[0038] Figure 2 This is a scanning electron microscope image of the positive electrode sheet prepared according to Comparative Example 2 of this application after battery formation.

[0039] Figure 3 This is a scanning electron microscope image of the positive electrode sheet prepared according to Example 2 of this application.

[0040] Figure 4 This is a scanning electron microscope image of the positive electrode sheet prepared according to Example 2 of this application after battery formation.

[0041] Figure 5 This is a schematic diagram of an energy storage device according to an embodiment of this application. Detailed Implementation

[0042] The embodiments of this application are described in detail below. Examples of these embodiments are shown in the accompanying drawings, wherein the same or similar reference numerals denote the same or similar elements or elements having the same or similar functions throughout. The embodiments described below with reference to the accompanying drawings are exemplary and intended to explain this application, and should not be construed as limiting this application.

[0043] This application is primarily based on the following issues and findings: In alkali metal ion batteries, the formation and growth of the SEI film on the surface of the negative electrode consumes active metal ions, reducing the battery's capacity and energy density. Currently, to compensate for the active ions consumed by the formation of the SEI film on the negative electrode, capacity compensation agents (such as lithium or sodium supplements) are typically introduced into the positive or negative electrode to compensate for capacity loss, or the electrolyte is improved. Taking sodium-ion batteries as an example, sodium supplements can be introduced into the positive electrode to pre-sodium-encapsulate the positive electrode, compensating for the capacity loss caused by SEI film consumption. However, due to the limited solubility of sodium supplements in the positive electrode active slurry, a large amount of sodium supplements may exist directly in the positive electrode active slurry in particulate form, resulting in a large number of sodium supplement particles larger than the particle size of the positive electrode active material in the formed positive electrode active material layer (see reference). Figure 1 (Understanding), while the sodium supplement particles, after decomposition, will form a large porous structure in the positive electrode active material layer (refer to...). Figure 2 (Understanding) This leads to a decrease in the conductivity of the positive electrode.

[0044] To address this problem, the inventors discovered that it can be solved by controlling the particle size of the capacity compensator in the positive electrode active material layer. Specifically, this can be achieved by controlling the average particle size of the capacity compensator in the positive electrode active material layer to be no larger than the average particle size of the positive electrode active material. Control methods may include, but are not limited to, increasing the solubility of the capacity compensator in the positive electrode active slurry, thereby improving the dispersion uniformity and particle size of the capacity compensator in the positive electrode active material layer, and suppressing the formation of large sodium-compensating agent particles in the positive electrode active material layer. Smaller sodium-compensating agent particles, after decomposition, form smaller pores, having less impact on the conductivity of the positive electrode sheet. Furthermore, the small-sized pores facilitate electrolyte penetration, accelerate sodium ion transport within the electrode sheet, and improve the battery's rate performance. Simultaneously, the smaller pore structure also helps alleviate the volume expansion of the electrode sheet during charge-discharge cycles, improving cycle stability.

[0045] In view of this, in a first aspect of this application, a positive electrode sheet is provided. The positive electrode sheet includes: a positive active material layer, the positive active material layer comprising a positive active material and a capacity compensator, the capacity compensator being dispersed in the positive active material layer, with reference to... Figure 3 It is understood that, in the scanning electron microscope image of the positive electrode active material layer, the average particle size of the capacity compensator is not greater than the average particle size of the positive electrode active material.

[0046] The positive electrode sheet according to the above embodiments of this application has at least the following beneficial effects: introducing a capacity compensator into the positive electrode sheet can compensate for the active ions (such as Li) consumed by the negative electrode sheet in the formation of the SEI film. + Or Na +This improves the energy density, storage performance, and cycle performance of the battery cell. Simultaneously, by controlling the average particle size of the capacity compensator in the positive electrode active material layer to be no larger than the average particle size of the positive electrode active material, the pores formed on the positive electrode active material layer after the capacity compensator decomposes can also be kept smaller (see reference). Figure 4 (Understanding) This design effectively solves the problem of decreased electrode conductivity caused by excessively large pores formed after the decomposition of the capacity compensator. Therefore, this positive electrode not only facilitates the utilization of specific capacity but also maintains good conductivity, thereby improving the battery's energy density and electrochemical performance.

[0047] It should be noted that in this application, the capacity compensator is used to provide metallic active particles to compensate for the initial irreversible capacity loss of the battery. The specific type of capacity compensator can be flexibly selected according to the application scenario of the positive electrode. For example, for lithium batteries, the capacity compensator in the positive electrode can be a lithium supplement; for sodium batteries, the capacity compensator in the positive electrode can be a sodium supplement. In addition, the average particle size of the capacity compensator and the average particle size of the positive electrode active material can be obtained by selecting a specific number (e.g., 10 to 50 capacity compensator nanoparticles in the range of 100 μm × 67 μm) of corresponding particles in the scanning electron microscope image of the positive electrode active material layer using image processing software or particle analysis software for labeling and statistical analysis.

[0048] In some embodiments of this application, the average particle size of the capacity compensator may not be greater than 3 / 4 of the average particle size of the positive electrode active material. For example, in a scanning electron microscope of the positive electrode active material layer, the average particle size of the capacity compensator may not be greater than 3 / 4, 1 / 2, 1 / 3, or 1 / 4 of the average particle size of the positive electrode active material. This further reduces the pores formed on the positive electrode active material layer after the capacity compensator decomposes, thereby further mitigating the problem of decreased electrode conductivity caused by excessively large pores formed after the capacity compensator decomposes. This allows the positive electrode to improve both the battery's energy density and its electrochemical performance.

[0049] In some embodiments of this application, the average particle size of the capacity compensator can be ≤20μm, for example, it can be 20μm, ≤15μm, ≤10μm, ≤5μm, ≤1μm, ≤0.5μm, etc. This further reduces the pores formed on the positive electrode active material layer after the capacity compensator decomposes, thereby further improving the problem of decreased electrode conductivity caused by excessively large pores formed after the capacity compensator decomposes. This allows the positive electrode to improve both the battery's energy density and its electrochemical performance.

[0050] In some embodiments of this application, based on the area of ​​the scanning electron microscope (SEM) image of the positive electrode active material layer, the area ratio of the capacity compensator with an average particle size ≥5 μm in the SEM image can be ≤20%, such as ≤18%, ≤15%, ≤12%, ≤10%, ≤8%, ≤5%, etc. In the positive electrode active material layer that meets the given conditions, the dispersibility of the capacity compensator is relatively better, and the proportion of the capacity compensator existing in the form of large particles is also smaller. This can further reduce the pores formed on the positive electrode active material layer after the capacity compensator decomposes, thereby further improving the problem of decreased electrode conductivity caused by excessively large pores formed after the decomposition of the capacity compensator. This allows the positive electrode to improve the battery's energy density while also improving the battery's electrochemical performance.

[0051] In some embodiments of this application, the capacity compensator may include inorganic acid salts and / or organic acid salts. The specific type of inorganic or organic acid salt can be flexibly selected according to actual needs. Taking sodium-ion batteries as an example, commonly used sodium compensators have some shortcomings. For example, Na2NiO2 and NaCrO2 have poor sodium compensating effects (low specific capacity) and are not easily decomposed, easily leading to residues after sodium compensating that affect battery performance; Na2NiO2, NaN3, Na3P, etc. are unstable in air, which increases the processing difficulty of the positive electrode sheet. At the same time, there are also problems such as high decomposition voltage or incomplete decomposition due to the generally large particle size, and poor dispersion uniformity due to uneven particle size distribution. Inorganic acids and organic acid salts generally have better stability in air and are easily fully decomposed, which helps to overcome the above problems to a certain extent.

[0052] In some embodiments of this application, the capacity compensator may include one or more of Na2CO3, Na2C4O4, Na2C3O3, Na2C5O5, Na2C6O6, CH3COONa, Na2C2O4, NaNO3, NaOH, and NaHCO3. For sodium batteries, the sodium compensators within the given range not only exhibit good stability in air and are easily decomposed, but also provide excellent sodium compensation with minimal residue. Using them in the positive electrode not only improves processability but also further enhances the specific capacity of the positive electrode, improves the battery's energy density, and avoids or reduces the adverse effects of sodium residue on battery performance.

[0053] In some embodiments of this application, the positive electrode active material may include Na x M y (X a O b ) z Z wWhere M is selected from transition metal elements, X is one or more of P, S and W, Z is one or more of F, OH and Cl, 1≤x≤6, 1≤y≤5, 1≤a≤6, 4≤b≤12, 2≤z≤6, and 0≤w≤6. For example, M can be one or more of Fe, Mn, and Co; x can be 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, etc.; y can be 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, etc.; a can be 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, etc.; b can be 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.; z can be 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, etc.; w can be 0, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, etc. For sodium batteries, the selected polyanionic compound Na... x M y (X a O b ) z Z w It has advantages such as high operating voltage, good rate performance, and cycle stability. Using it in the positive electrode sheet is beneficial to further improve the energy density and electrochemical performance of the battery.

[0054] In some embodiments of this application, the Na x M y (X a O b ) z Z w In this context, M can be one or more of Fe, Mn, and Co. For sodium-ion batteries, sodium iron pyrophosphate further offers the advantage of high specific capacity, making its use in the positive electrode sheet beneficial for improving the battery's energy density and electrochemical performance. Furthermore, during the preparation of the positive electrode sheet, to reduce the particle size of the capacity compensator in the positive electrode active material layer, a trace amount of water can be added to the positive electrode active slurry to increase the solubility of the capacity compensator, thereby improving the dispersion uniformity and particle size of the capacity compensator in the positive electrode active material layer. Choosing sodium iron pyrophosphate as the positive electrode active material results in relatively low sensitivity to water, which helps to reduce the adverse effects of trace amounts of water in the positive electrode active slurry on the performance of the positive electrode active material.

[0055] In some embodiments of this application, the mass percentage of the capacity compensator in the positive electrode active material layer can be ≤5wt%, for example, the mass percentage of the capacity compensator in the positive electrode active material layer can be 0.5wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, etc. Introducing a capacity compensator into the positive electrode sheet can compensate for the active metal ions consumed by the negative electrode sheet in the formation of the SEI film, promote the specific capacity of the positive electrode sheet, and improve the energy density of the battery. However, the irreversible cycle capacity of the capacity compensator is generally low. By controlling the content of the capacity compensator in the positive electrode active material layer to meet the given range, it is beneficial to further enable the positive electrode sheet itself to have a higher reversible specific capacity, so that the battery can obtain a higher energy density. Furthermore, the mass percentage of the capacity compensator in the positive electrode active material layer can be 1wt% to 5wt%, thereby achieving both a good capacity compensation effect and enabling the positive electrode sheet itself to have a high reversible specific capacity, which is beneficial to further improving the energy density of the battery.

[0056] In some embodiments of this application, after battery formation, the average pore size of the pores on the surface of the positive electrode active material layer is not greater than the average particle size of the positive electrode active material. This further avoids the problem of decreased conductivity of the positive electrode due to the decomposition of the capacity compensator.

[0057] In some embodiments of this application, the porosity change of the positive electrode sheet before and after battery formation can be 1% to 5%, for example, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, etc. The positive electrode sheet can be formed by charging at a rate of 0.2C to the upper limit cutoff voltage (e.g., 4.2V). The porosity change of the positive electrode sheet before and after formation is mainly affected by the amount of capacity compensator, which can be achieved by adjusting the mass ratio of the capacity compensator in the positive electrode active material layer. The porosity of the positive electrode sheet after battery formation is obtained by testing the positive electrode sheet obtained after fully discharging the battery following formation and disassembling the discharged battery. Controlling the porosity change of the positive electrode sheet before and after battery formation to meet the given range is beneficial for achieving better capacity compensation and maintaining the structural stability of the positive electrode active material layer during charge-discharge cycles, thereby improving the battery's cycle life.

[0058] Based on the same inventive concept, in a second aspect of this application, a method for preparing a positive electrode sheet is proposed, comprising:

[0059] The positive electrode active material, capacity compensator, conductive agent and binder are mixed with solvent to obtain positive electrode active slurry;

[0060] The positive electrode active slurry is coated onto at least one side of the positive electrode current collector to form a positive electrode active material layer.

[0061] The capacity compensator comprises inorganic acid salts and / or organic acid salts, the solvent comprises at least one of water, acetone and N,N-dimethylformamide (DMF), and the mass percentage of water in the positive electrode active slurry is not greater than 0.5 wt%. In the scanning electron microscope image of the positive electrode active material layer, the average particle size of the capacity compensator is not greater than the average particle size of the positive electrode active material.

[0062] The method for preparing the positive electrode sheet according to the above embodiments of this application has at least the following beneficial effects: By introducing at least one of water, acetone, and N,N-dimethylformamide into the solvent, the solubility of inorganic acid salts and / or organic acid salts as capacity compensators in the positive electrode active slurry can be improved, thereby improving the dispersion uniformity of the capacity compensator in the positive electrode active slurry and the positive electrode active material layer, effectively suppressing the formation of large-sized particles of the capacity compensator in the positive electrode active material layer, and obtaining a positive electrode active material layer with an average particle size of the capacity compensator not greater than the average particle size of the positive electrode active material, reducing the risk of excessive lithium / sodium supplementation in local areas. Simultaneously, by controlling the mass ratio of water in the positive electrode active slurry to be no more than 0.5 wt%, the adverse effects of the presence of water on the electrochemical performance of the positive electrode active material and / or the processability of the positive electrode sheet can also be taken into account. In summary, the positive electrode sheet prepared by this method can not only compensate for the active ions (such as Li) consumed by the negative electrode sheet in the formation of the SEI film (such as Li...), but also... + Or Na + This process improves the energy density, storage performance, and cycle performance of the battery cell. It also reduces the pores formed on the positive electrode active material layer after the capacity compensator decomposes, effectively solving the problem of decreased electrode conductivity caused by excessively large pores formed after the capacity compensator decomposes. As a result, the positive electrode produced not only benefits the specific capacity but also balances its conductivity, thereby improving the battery's energy density and electrochemical performance.

[0063] It should be noted that in this application, the combination of positive electrode active material and capacity compensator can be flexibly selected according to the application scenario of the positive electrode sheet when preparing the positive electrode active slurry. For example, for lithium batteries, lithium-containing positive electrode active material and lithium compensator can be selected; while for sodium batteries, sodium-containing positive electrode active material and sodium compensator can be selected. The specific types of lithium-containing positive electrode active material and lithium compensator, as well as sodium-containing positive electrode active material and sodium compensator, are not particularly limited, and those skilled in the art can flexibly select them according to actual needs; for example, they can all be conventional choices in the field. Furthermore, the selection of conductive agent, binder, and positive electrode current collector in this application is also not particularly limited and can be flexibly selected according to actual needs; for example, they can all be conventional choices in the field. Specifically, conductive agent can include, but is not limited to, conductive carbon black, graphene, carbon nanotubes, etc., binder can include, but is not limited to, polyvinylidene fluoride (PVDF), and positive electrode current collector can include, but is not limited to, aluminum foil.

[0064] In some embodiments of this application, the capacity compensator may include at least one of the following: Na2CO3, Na2C4O4, Na2C3O3, Na2C5O5, Na2C6O6, CH3COONa, Na2C2O4, NaNO3, NaOH, and NaHCO3. For sodium batteries, the sodium-compensating agents within the given range not only exhibit good stability in air and are easily decomposed, but also provide excellent sodium-compensating effects with minimal residue. Furthermore, they demonstrate relatively good solubility in water, acetone, and N,N-dimethylformamide. Dispersing these agents in the positive electrode active slurry to prepare the positive electrode sheet not only improves processability and facilitates further enhancement of the specific capacity of the positive electrode sheet, but also avoids or reduces the adverse effects of sodium residue on battery performance. It also promotes the full dissolution of the capacity compensator in the positive electrode active slurry, thereby improving the uniformity of the capacity compensator's dispersion in the positive electrode active material layer and reducing the particle size of the capacity compensator within the positive electrode active material layer. This helps to further mitigate the problem of decreased electrode conductivity caused by excessively large pores formed after the capacity compensator decomposes. Therefore, the resulting positive electrode sheet can improve both the battery's energy density and electrochemical performance.

[0065] In some embodiments of this application, the positive electrode active material may include Na x M y (X a O b ) z Z w Wherein, M is selected from transition metal elements, X is one or more of P, S, and W, Z is one or more of F, OH, and Cl, 1≤x≤6, 1≤y≤5, 1≤a≤6, 4≤b≤12, 2≤z≤6, and 0≤w≤6. Further, the Na... x M y(X a O b ) z Z w In this context, M can be one or more of Fe, Mn, and Co. Na is chosen. x M y (X a O b ) z Z w The beneficial effects of sodium iron pyrophosphate as a positive electrode active material have been explained in detail in the preceding sections and will not be repeated here.

[0066] In some embodiments of this application, based on the solid content of the positive electrode active slurry, the mass percentage of the capacity compensator can be ≤5 wt%, such as 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%, 2.5 wt%, 3 wt%, 3.5 wt%, 4 wt%, 4.5 wt%, 5 wt%, etc. Further, the mass percentage of the capacity compensator can be 1 wt% to 5 wt%. This allows the battery to achieve a higher energy density. It should be noted that the solid content of the positive electrode active slurry refers to the percentage of the total mass of the solvent-free components in the positive electrode active slurry. For example, for a positive electrode active slurry that only includes positive electrode active material, capacity compensator, conductive agent, binder, and solvent, the solid content of the positive electrode active slurry refers to the total mass of the positive electrode active material, capacity compensator, conductive agent, and binder in the slurry. In some specific examples, the solid content of the positive electrode active slurry can be 47 to 65 wt%, such as 47 wt%, 50 wt%, 55 wt%, 60 wt%, or 65 wt%, etc.

[0067] In some embodiments of this application, the solvent used in preparing the positive electrode active slurry may include N-methylpyrrolidone (NMP) and water. The mass ratio of N-methylpyrrolidone to water may be 99.5:0.5 to 99.95:0.05, for example, 99.5 / 0.5, 99.6 / 0.4, 99.7 / 0.3, 99.8 / 0.2, 99.9 / 0.1, 99.95 / 0.05, etc. Inorganic acid salts and organic acid salts have relatively high solubility in water. Increasing the water content in the solvent is beneficial to improving the solubility of the capacity compensator in the positive electrode active slurry. Controlling the mass percentage of water in the solvent to no more than 0.5 wt% is beneficial to reducing the adverse effects of the presence of water on the performance of the positive electrode active material. When preparing the positive electrode active slurry, by controlling the mass ratio of N-methylpyrrolidone and water in the solvent to meet the given range, the capacity compensator can be fully dissolved in the positive electrode active slurry without significantly changing the performance of the positive electrode active material. This can further improve the dispersion uniformity of the capacity compensator in the positive electrode active material layer and reduce the particle size of the capacity compensator in the positive electrode active material layer. This is beneficial to further improve the problem of decreased conductivity of the electrode due to excessively large pores formed after the decomposition of the capacity compensator. Thus, the prepared positive electrode can improve the battery's energy density and electrochemical performance at the same time.

[0068] In some embodiments of this application, when preparing the positive electrode active slurry, the solvent may include N-methylpyrrolidone and acetone, and the mass ratio of N-methylpyrrolidone to acetone may be 80:20 to 95:5, for example, 80 / 20, 85 / 15, 90 / 10, 95 / 5, etc. Introducing acetone into the solvent also helps improve the solubility of the capacity compensator in the positive electrode active slurry. The inventors found that as the acetone content in the solvent increases, the improvement effect on the solubility of the capacity compensator in the positive electrode active slurry first increases and then decreases. When preparing the positive electrode active slurry, by controlling the mass ratio of N-methylpyrrolidone and acetone in the solvent to meet the given range, the capacity compensator can have a high solubility in the positive electrode active slurry. This can further improve the dispersion uniformity of the capacity compensator in the positive electrode active material layer and reduce the particle size of the capacity compensator in the positive electrode active material layer. This helps to further improve the problem of decreased conductivity of the electrode due to excessively large pores formed after the decomposition of the capacity compensator. Thus, the prepared positive electrode can improve the battery's energy density and electrochemical performance at the same time.

[0069] In some embodiments of this application, when preparing the positive electrode active slurry, the solvent may include N-methylpyrrolidone and N,N-dimethylformamide (DMF), and the mass ratio of N-methylpyrrolidone to N,N-dimethylformamide may be 70:30 to 95:5, for example, 70 / 30, 75 / 25, 80 / 20, 85 / 15, 90 / 10, 95 / 5, etc. Introducing N,N-dimethylformamide into the solvent is also beneficial for improving the solubility of the capacity compensator in the positive electrode active slurry. The inventors discovered that as the N,N-dimethylformamide content in the solvent increases, the improvement in solubility of the capacity compensator in the positive electrode active slurry first increases and then decreases. When preparing the positive electrode active slurry, by controlling the mass ratio of N-methylpyrrolidone and N,N-dimethylformamide in the solvent to meet a given range, the capacity compensator can achieve high solubility in the positive electrode active slurry. This further improves the dispersion uniformity of the capacity compensator in the positive electrode active material layer and reduces the particle size of the capacity compensator in the positive electrode active material layer. This helps to further improve the problem of decreased electrode conductivity caused by excessively large pores formed after the decomposition of the capacity compensator. Therefore, the prepared positive electrode can improve both the battery energy density and the battery's electrochemical performance.

[0070] In some embodiments of this application, when preparing the positive electrode active slurry, the mass percentage of N-methylpyrrolidone in the solvent can be no less than 95 wt%. Meeting the given conditions is beneficial to further improve the coatability of the positive electrode active slurry and the adhesion strength between the positive electrode active material layer and the positive electrode current collector.

[0071] In some embodiments of this application, when preparing the positive electrode active slurry, the mixing temperature of the positive electrode active material, capacity compensator, conductive agent, and binder with the solvent can be 10℃ to 120℃, for example, 10℃, 20℃, 30℃, 50℃, 80℃, 100℃, 120℃, etc., and optionally 25℃ to 80℃. Appropriately increasing the mixing temperature is beneficial to improving the solubility of the capacity compensator in the positive electrode active slurry, thereby improving the dispersion uniformity of the capacity compensator in the positive electrode active material layer, reducing the particle size of the capacity compensator in the positive electrode active material layer, reducing the impact of capacity compensator decomposition on the decrease in the conductivity of the positive electrode sheet, and also taking into account the stability of the positive electrode active slurry. This is beneficial to avoid problems such as the dissolution of transition metal elements and the side reactions of residual alkali in the positive electrode active material that may occur due to high temperature.

[0072] In some embodiments of this application, the solubility of the capacity compensator in the solvent can be no less than 0.1 wt% to 5 wt% of the solid content of the positive electrode active slurry. For example, it can be 0.5 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt%, 5 wt%, 6 wt%, 8 wt%, 10 wt%, etc., of the solid content of the positive electrode active slurry. This allows the capacity compensator to have high solubility in the positive electrode active material layer.

[0073] It should be noted that the method for preparing the positive electrode sheet described above in this application is based on the same inventive concept as the positive electrode sheet described above. The features and effects described for the positive electrode sheet described above are also applicable to the method for preparing the positive electrode sheet, and will not be repeated here.

[0074] In a third aspect, this application proposes an energy storage device. This energy storage device includes the aforementioned positive electrode sheet, and / or a positive electrode sheet prepared using the aforementioned method for preparing the positive electrode sheet. This energy storage device possesses all the features and effects of the aforementioned positive electrode sheet and the aforementioned method for preparing the positive electrode sheet, which will not be elaborated further here. In general, this energy storage device combines high energy density, storage performance, and good electrochemical performance.

[0075] It should be noted that this application does not impose any particular restrictions on the specific type of energy storage device. Those skilled in the art can choose flexibly according to the actual situation, for example, referring to... Figure 5It is understood that the energy storage device may include a battery, which may include a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode may be the aforementioned positive electrode. The negative electrode may include a negative current collector and a negative active material layer disposed on at least one side of the negative current collector. The negative active material layer may include a negative active material, a conductive agent, a binder, and other optional conventional functional additives. The specific ratio and type of the negative active material, conductive agent, and binder in the negative electrode, as well as the selection of the negative current collector, can adopt conventional choices in the art. For example, the negative active material may include, but is not limited to, artificial graphite, soft carbon, hard carbon, silicon-carbon composite materials, etc.; the conductive agent may be conductive carbon black, graphene, carbon nanotubes, etc.; the binder may be styrene-butadiene rubber, sodium carboxymethyl cellulose, etc.; and the negative current collector may include, but is not limited to, copper foil, etc. Furthermore, this application does not impose any particular restrictions on the specific composition of the separator and electrolyte. Conventional choices in the art can be used. For example, the separator may include at least one of polypropylene (PP), polyethylene (PE), and ceramic separators; the electrolyte may include electrolyte salts and organic solvents, and the specific types and compositions of the electrolyte salts and organic solvents are not particularly limited, allowing those skilled in the art to select according to actual needs. In addition, this application does not impose any particular restrictions on the specific type of battery. Those skilled in the art can flexibly choose according to actual conditions. For example, the battery can be a prismatic battery or a cylindrical battery. Another example is that the battery can be a liquid battery or a semi-solid battery. Yet another example is that the battery can be a lithium battery or a sodium battery.

[0076] In a fourth aspect, this application proposes an electrical device. According to an embodiment of this application, the electrical device includes the aforementioned energy storage device. This electrical device possesses all the features and effects of the aforementioned energy storage device, which will not be repeated here. In general, this electrical device offers better performance. It should be noted that this application does not impose any particular limitation on the specific type of electrical device; those skilled in the art can flexibly choose according to actual circumstances. For example, the electrical device may include, but is not limited to, electronic devices, vehicles, aircraft, and household appliances.

[0077] The embodiments of this application are described in detail below. The embodiments described below are exemplary and are only used to explain this application, and should not be construed as limiting this application. Where specific techniques or conditions are not specified in the embodiments, they are performed in accordance with the techniques or conditions described in the literature in this field or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0078] Example 1

[0079] 1. Preparation of the positive electrode sheet:

[0080] The positive electrode active material NaFeP2O7, sodium supplement Na2C2O4, conductive agent Super-P (conductive carbon black), and binder PVDF (polyvinylidene fluoride) were mixed together in a mass ratio of 94%:2%:2%:2% and dispersed in a mixed solvent comprising 0.25 wt% water and 99.75 wt% N-methylpyrrolidone (NMP). After thorough stirring, a positive electrode active slurry with a solid content of 47–65 wt% was formed. This slurry was then uniformly coated on one side of the positive electrode current collector, which was made of aluminum foil. After drying, the slurry was rolled to a compaction density of 14 mg / cm³. 2 Then, it is punched to obtain the positive electrode sheet.

[0081] 2. Preparation of the negative electrode sheet:

[0082] Hard carbon, conductive carbon black, and water-based carboxymethyl cellulose were mixed together in a mass ratio of 80:10:10 and dispersed in water to obtain a uniformly dispersed negative electrode slurry. The negative electrode slurry was then uniformly coated onto the surface of a copper foil negative electrode current collector, and subsequently transferred to a vacuum drying oven for complete drying. The resulting electrode sheet was then rolled to a compaction density of 6 mg / cm³. 2 Then, it is punched to obtain the negative electrode sheet.

[0083] 3. Preparation of electrolyte:

[0084] Ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 1:1. Then, 5 wt% FEC (fluoroethylene carbonate) was added (based on the total mass of EC, DEC, and FEC). Next, fully dried lithium salt NaClO4 was dissolved in the mixed organic solvent at a concentration of 1 mol / L to prepare the electrolyte.

[0085] 4. Preparation of the separating membrane:

[0086] A 260-micron glass fiber film was selected.

[0087] 5. Battery assembly:

[0088] Stack the positive electrode, separator, and negative electrode in sequence, with the separator positioned between the positive and negative electrodes to provide isolation. Add the electrolyte and assemble into a button cell.

[0089] Examples 2-15 and Comparative Examples 1-2

[0090] The differences between Examples 2-15 and Comparative Examples 1-2 and Example 1 are detailed in Table 1. In Examples 1-9, the water content in the positive electrode active slurry is less than 0.5 wt%.

[0091] The coin cells prepared in the above-described embodiments and comparative examples were subjected to electrochemical performance tests, including cycle performance tests and initial charge specific capacity tests. The test results are shown in Table 1 and [Table data missing]. Figures 1-4 .in:

[0092] 1) Specific capacity during the first charge

[0093] At 25°C, the batteries prepared in the examples and comparative examples were charged to 4.0V at a rate of 0.1C, and then subjected to full discharge and full charge tests at a rate of 0.1C to obtain the initial charge specific capacity.

[0094] 2) Cyclic performance test

[0095] At 25°C, the batteries prepared in the examples and comparative examples were discharged to 2V at a rate of 0.2C and charged to 4.0V at a rate of 0.2C for 3 cycles. Then, a full charge-discharge cycle test was performed by discharging to 2V at a rate of 0.2C and charging to 4.0V at a rate of 0.2C. After 100 cycles, the test data were recorded and the capacity retention rate was calculated.

[0096] 3) Scanning electron microscopy test

[0097] The microstructure of the positive electrode sheet surface prepared in each embodiment and comparative example was observed using a scanning electron microscope. After the battery underwent its first full charge and discharge test, it was disassembled, the electrolyte on the surface was cleaned off, and the microstructure of the positive electrode sheet surface was observed again using a scanning electron microscope.

[0098] Table 1. Differences and test results between Examples 1-18 and Comparative Examples 1-2

[0099]

[0100]

[0101] Results and conclusions:

[0102] Based on Examples 1-9 and Comparative Example 2, and related test results, it can be seen that introducing a small amount of water into the positive electrode active slurry can reduce the particle size of the sodium supplement in the positive electrode active material layer and improve its dispersion uniformity. The reduction in the particle size of the sodium supplement in the positive electrode active slurry is beneficial for improving the cycle capacity retention rate while improving the battery's energy density. Taking Example 2 and Comparative Example 2 as examples, refer to... Figures 1-4As shown, in Example 2, the sodium-supplementing particles in the positive electrode active material layer were uniformly distributed before battery formation, and no obvious large-diameter sodium-supplementing particles were observed. After battery formation, no large pores were formed on the surface of the positive electrode. In contrast, in Comparative Example 2, larger sodium-supplementing particles were clearly visible dispersed in the positive electrode active material layer before battery formation, and many large pores were formed on the surface of the positive electrode after battery formation. Furthermore, combining Examples 1-3 and Comparative Examples 1-2, it can be seen that the improvement effect on battery energy density and cycle capacity retention rate first increases and then decreases with the increase of water content in the positive electrode active slurry. In addition, combining Examples 1 and Examples 9-12 and the corresponding test results, it can be seen that, based on Example 1, increasing the mixing temperature of the positive electrode active slurry has an improvement effect on battery energy density and cycle capacity retention rate that first increases and then decreases. In addition, based on Examples 13-18 and Comparative Example 2 and the corresponding scanning electron microscopy results, it can be seen that introducing acetone and DMF into the positive electrode active slurry is also beneficial to reducing the particle size of the sodium supplement in the positive electrode active material layer and improving its dispersion uniformity.

[0103] In the description of this specification, the references to terms such as "one embodiment," "some embodiments," "example," "specific example," or "some examples," etc., refer to specific features, structures, materials, or characteristics described in connection with that embodiment or example, which are included in at least one embodiment or example of this application. In this specification, the illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples. Moreover, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of different embodiments or examples.

[0104] Although embodiments of this application have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting this application. Those skilled in the art can make changes, modifications, substitutions and variations to the above embodiments within the scope of this application.

Claims

1. A method for preparing a positive electrode sheet, characterized in that, include: The positive electrode active material, capacity compensator, conductive agent and binder are mixed with solvent to obtain positive electrode active slurry; The positive electrode active slurry is coated onto at least one side of the positive electrode current collector to form a positive electrode active material layer. The capacity compensator includes inorganic acid salts and / or organic acid salts. The solvent comprises N-methylpyrrolidone and water, wherein the mass ratio of N-methylpyrrolidone to water is 99.5:0.5 to 99.95:0.05; or, the solvent comprises N-methylpyrrolidone and acetone, wherein the mass ratio of N-methylpyrrolidone to acetone is 80:20 to 95:5; or, the solvent comprises N-methylpyrrolidone and N,N-dimethylformamide, wherein the mass ratio of N-methylpyrrolidone to N,N-dimethylformamide is 70:30 to 95:

5. In the scanning electron microscope image of the positive electrode active material layer, the average particle size of the capacity compensator is not greater than the average particle size of the positive electrode active material.

2. The method according to claim 1, characterized in that, At least one of the following conditions must be met: The average particle size of the capacity compensator is no greater than 3 / 4 of the average particle size of the positive electrode active material; The average particle size of the capacity compensator is ≤20μm; Based on the area of ​​the scanning electron microscope (SEM) image of the positive electrode active material layer, the area of ​​the capacity compensator with an average particle size ≥ 5 μm in the SEM image is ≤ 20%; The capacity compensator includes inorganic acid salts and / or organic acid salts.

3. The method according to claim 1 or 2, characterized in that, The capacity compensator includes one or more of Na₂CO₃, Na₂C₄O₄, Na₂C₃O₃, Na₂C₅O₅, Na₂C₆O₆, CH₃COONa, Na₂C₂O₄, NaNO₃, NaOH, and NaHCO₃; and / or, The positive electrode active material includes Na. x M y (X a O b ) z Z w Where M is selected from transition metal elements, X is one or more of P, S and W, Z is one or more of F, OH and Cl, 1≤x≤6, 1≤y≤5, 1≤a≤6, 4≤b≤12, 2≤z≤6, and 0≤w≤6.

4. The method according to claim 3, characterized in that, At least one of the following two conditions must be met: The Na x M y (X a O b ) z Z w In this context, M represents one or more of Fe, Mn, and Co. The capacity compensator has a mass percentage of ≤5wt% in the positive electrode active material layer.

5. The method according to claim 4, characterized in that, The capacity compensator accounts for 1 wt% to 5 wt% of the mass of the positive electrode active material layer.

6. The method according to claim 1 or 4, characterized in that, After the battery is formed, the average pore size of the pores on the surface of the positive electrode active material layer is not greater than the average particle size of the positive electrode active material; and / or, The porosity of the positive electrode sheet changes by 1 to 5% before and after battery formation.

7. The method according to claim 1, characterized in that, The solvent contains N-methylpyrrolidone at a mass percentage of not less than 95 wt%.

8. The method according to claim 1 or 7, characterized in that, The mixing temperature is 10~120℃; and / or the solubility of the capacity compensator in the solvent is not less than 0.1wt%~5wt% of the solid content of the positive electrode active slurry.

9. An energy storage device, characterized in that, include: A positive electrode sheet prepared by any one of claims 1 to 8.

10. An electrical appliance, characterized in that, Includes the energy storage device as described in claim 9.

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