Positive electrode sheet, battery, energy storage device, and electric device

Abstract

The application provides a positive electrode sheet, a battery, an energy storage device and an electric equipment. The positive electrode sheet comprises a positive electrode current collector and a positive electrode material layer arranged on at least one surface of the positive electrode current collector. The positive electrode material layer comprises a positive electrode material. The positive electrode material comprises secondary carbon-coated sodium iron pyrophosphate phosphate particles. The secondary carbon-coated sodium iron pyrophosphate phosphate particles are composed of a primary carbon-coated sodium iron pyrophosphate phosphate core and a carbon layer coated on the surface of the primary carbon-coated sodium iron pyrophosphate phosphate core. The surface contact angle of the positive electrode material layer is A, and 80°≤A≤100°.

Description

Technical Field

[0001] This application relates to the field of energy storage technology, and in particular to a positive electrode, a battery, an energy storage device, and an electrical device. Background Technology

[0002] Sodium-ion batteries, with their advantages of low cost, abundant sodium resources, and relatively high energy density, are expected to replace traditional lithium-ion batteries in the field of energy storage.

[0003] Sodium iron pyrophosphate (Na4Fe3(PO4)2(P2O7), abbreviated as NFPP, has become a research hotspot for cathode materials in sodium-ion energy storage batteries due to its advantages such as high structural stability, good thermal stability, long cycle life, and excellent safety performance. However, the capacity of NFPP cathode materials gradually decreases with increasing air exposure time, exhibiting poor air stability, which limits its actual energy density. Summary of the Invention

[0004] To address the aforementioned technical problems, this application discloses a positive electrode sheet, a battery, an energy storage device, and an electrical device to improve the air stability of NFPP positive electrode materials, thereby enhancing the energy density of NFPP positive electrode materials.

[0005] In a first aspect, this application provides a positive electrode sheet, comprising a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, wherein the positive electrode material layer comprises a positive electrode material, wherein: The cathode material includes secondary carbon-coated sodium iron pyrophosphate particles, which are composed of a primary carbon-coated sodium iron pyrophosphate core and a carbon layer coating the surface of the primary carbon-coated sodium iron pyrophosphate core. The surface contact angle of the positive electrode material layer is A, where 80°≤A≤100°.

[0006] In some embodiments of this application, the secondary carbon-coated sodium iron pyrophosphate particles are present in any predetermined observation area of ​​the scanning electron microscope image of the positive electrode material layer after being exposed to air with a relative humidity of 20%~30% for 48 hours. The surface of the secondary carbon-coated sodium iron pyrophosphate particles is coated with sodium compound particles, and the number of sodium compound particles in each secondary carbon-coated sodium iron pyrophosphate particle is N, where 0≤N≤30.

[0007] In some embodiments of this application, the average size of the sodium compound particles is 10 nm to 500 nm.

[0008] In some embodiments of this application, the sodium compound particles melt and volatilize after being scanned for a preset time at an accelerating voltage of 10 kV.

[0009] In some embodiments of this application, the carbon layer completely covers the surface of the primary carbon-coated sodium iron pyrophosphate core.

[0010] In some embodiments of this application, the resistivity of the positive electrode material is R, where 100 Ω·cm ≤ R ≤ 5000 Ω·cm.

[0011] In some embodiments of this application, the secondary carbon-coated sodium iron pyrophosphate particles have a spherical and / or near-spherical structure.

[0012] Secondly, this application provides a method for preparing a cathode material, comprising the following steps: Sodium source, iron source, phosphorus source, first carbon source and solvent are mixed to obtain slurry, and then spray dried to obtain sodium iron pyrophosphate precursor powder. The sodium iron pyrophosphate precursor powder is subjected to a first sintering process and a second sintering process to obtain a primary carbon-coated sodium iron pyrophosphate cathode material. The second carbon source is dissolved in a solvent to form a carbon source solution. Then, the prepared primary carbon-coated sodium iron pyrophosphate cathode material is uniformly mixed with the carbon source solution and dried to obtain the sodium iron pyrophosphate composite precursor. A third sintering process was carried out on the sodium iron pyrophosphate composite precursor to obtain a secondary carbon-coated sodium iron pyrophosphate cathode material.

[0013] In some embodiments of this application, the first sintering temperature is 300℃~330℃, and the first sintering time is 3h~8h; The second sintering temperature is 500℃~600℃, and the second sintering time is 10h~16h.

[0014] Thirdly, this application provides a sodium-ion battery, including the positive electrode sheet described in the first aspect.

[0015] Fourthly, this application provides an energy storage device, including a housing and at least one sodium-ion battery as described in the third aspect, the sodium-ion battery being housed in the housing.

[0016] Fifthly, this application provides an electrical device including the energy storage device described in the fourth aspect, wherein the energy storage device supplies power to the electrical device.

[0017] Compared with the prior art, this application has at least the following beneficial effects: This application provides a positive electrode sheet, a battery, an energy storage device, and an electrical device. The positive electrode material in the positive electrode sheet includes secondary carbon-coated sodium iron pyrophosphate particles. The secondary carbon-coated sodium iron pyrophosphate particles are composed of a primary carbon-coated sodium iron pyrophosphate core and a carbon layer coating the surface of the primary carbon-coated sodium iron pyrophosphate core. The surface contact angle of the positive electrode material layer is A, 80°≤A≤100°. The surface contact angle of the positive electrode material layer within the above range indicates that the positive electrode material has high hydrophobicity, which can reduce the capacity loss of NFPP positive electrode material due to air exposure during material processing and secondary battery manufacturing, and improve the actual energy density of the positive electrode material in secondary battery cell applications. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the structure of an energy storage system according to one embodiment of this application; Figure 2 This is a schematic diagram of the energy storage system according to another embodiment of this application; Figure 3 This is a schematic diagram of the energy storage system according to another embodiment of this application; Figure 4 This is a SEM image of the surface of the NFPP particles with secondary carbon coating after the positive electrode sheet prepared in Example 1 was exposed to air. Figure 5 This is a further enlarged SEM image of the surface of NFPP particles after air exposure, as shown in Example 1. Figure 6 SEM image of the surface of ordinary NFPP particles after the positive electrode sheet prepared in Comparative Example 3 was exposed to air; Figure 7 This is a further magnified SEM image of the surface of ordinary NFPP particles after air exposure, as shown in Comparative Example 3.

[0020] Explanation of reference numerals in the attached drawings: 400-Energy storage system, 410-First power conversion device, 420-First user load, 430-Second user load, 440-Energy storage device, 450-High voltage cable, 460-Second power conversion device, 470-Vehicle, 480-Photovoltaic-energy storage-charging station. Detailed Implementation

[0021] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0022] In this application, the terms "upper," "lower," "left," "right," "front," "rear," "top," "bottom," "inner," "outer," "vertical," "horizontal," "lateral," and "longitudinal" indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. These terms are primarily for the purpose of better describing this application and its embodiments, and are not intended to limit the indicated device, element, or component to having a specific orientation, or to be constructed and operated in a specific orientation.

[0023] Furthermore, in addition to indicating location or positional relationship, some of the aforementioned terms may also have other meanings. For example, the term "above" may also be used in some cases to indicate a certain dependency or connection relationship. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0024] Furthermore, the terms "installation," "setup," "equipped with," "connection," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral structure; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium, or an internal connection between two devices, components, or parts. Those skilled in the art can understand the specific meaning of these terms in this application based on the specific circumstances.

[0025] Furthermore, the terms "first," "second," etc., are primarily used to distinguish different devices, components, or parts (which may be the same or different in specific type and construction), and are not intended to indicate or imply the relative importance or quantity of the indicated devices, components, or parts. Unless otherwise stated, "a plurality of" means two or more.

[0026] It should be noted that this application uses sodium-ion batteries as an example of secondary batteries to explain the application, but the secondary batteries in this application are not limited to sodium-ion batteries.

[0027] In view of this, this application provides a positive electrode sheet, which includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector. The positive electrode material layer includes a positive electrode material, wherein: the positive electrode material includes carbon-coated sodium iron pyrophosphate (NFPP) particles, and the secondary carbon-coated sodium iron pyrophosphate particles are composed of a primary carbon-coated sodium iron pyrophosphate core and a carbon layer coating the surface of the primary carbon-coated sodium iron pyrophosphate core. The surface contact angle of the positive electrode material layer is A, 80°≤A≤100°. For example, A is 80°, 85°, 90°, 95° or 100°. When the surface contact angle of the cathode material layer is too small (e.g., less than 80°), the NFPP cathode material will absorb moisture from the air and subsequently undergo a sodium removal reaction, leading to a capacity decay of the NFPP cathode material. When the surface contact angle of the cathode material layer is too large (e.g., greater than 100°), although the hydrophobicity of the NFPP cathode material surface is enhanced, the water content after the NFPP cathode material is exposed to air is reduced, and the sodium removal reaction is also reduced accordingly, thus enhancing the air stability of the NFPP cathode material, at the same time, a higher contact angle also means an excessive proportion of additives. On the one hand, this increases the coating thickness, which actually reduces the ionic conductivity of the NFPP cathode material, affecting the actual electrical performance of the sodium-ion battery. On the other hand, an excessive carbon layer means a decrease in the actual proportion of active material in the cathode material, which actually reduces the total capacity provided by a unit of NFPP. At the cell level, the cell thickness required for the same capacity increases, resulting in an overall energy density loss in the sodium-ion battery. In the cathode material layer of this application, the conductive carbon in the NFPP core coated with primary carbon can improve the conductivity of the NFPP core. At the same time, the carbon layer on the surface of the NFPP core coated with primary carbon can improve both the electronic conductivity and air stability of the NFPP core. Furthermore, when the surface contact angle of the cathode material layer is within the aforementioned range, the hydrophobicity of the NFPP cathode material surface is enhanced, the water content after air exposure is reduced, and the resulting sodium removal reaction is also reduced. The enhanced air stability of the NFPP cathode material makes the cathode material highly hydrophobic, which can reduce the capacity loss caused by air exposure during material processing and sodium-ion battery manufacturing, thereby improving the actual energy density of the cathode material in sodium-ion battery cell applications.

[0028] In some embodiments of this application, secondary carbon-coated sodium iron pyrophosphate particles are present in any predetermined observation area of ​​the cathode material layer after exposure to air at 20%–30% relative humidity for 48 hours. The surface of each secondary carbon-coated sodium iron pyrophosphate particle is coated with sodium compound microparticles, and the number of sodium compound microparticles in each particle is N, where 0 ≤ N ≤ 30. For example, N can be 0, 1, 2, 3, 5, 8, 10, 15, 20, 25, or 30. The predetermined observation area in this application can be a rectangular region of 4000 nm × 4000 nm in the SEM image. The aforementioned sodium compound microparticles contain Na, P, O, and C elements, and may contain substances such as NaOH, Na3PO4, and Na2CO3. These microparticles are produced by the NFPP cathode material absorbing water and undergoing a desodium removal reaction, which affects the actual capacity utilization of the NFPP cathode material. The NFPP cathode material of this application produces a small number of sodium compound particles even when exposed to air, or even no sodium compound particles, exhibiting good air stability, which improves the energy density of the cathode material.

[0029] In some embodiments of this application, the average size of the sodium compound particles is 10 nm to 500 nm; in other embodiments, the average size of the sodium compound particles is 5 nm to 30 nm. The average size of the sodium compound particles within the above range distinguishes them from other particles on the surface of the NFPP cathode material particles, such as conductive carbon particles or single-crystal NFPP particles, in SEM images.

[0030] In this application, the average size of sodium compound particles refers to the diameter of sodium compound particles under SEM. For spherical white particles, the size is based on the longest length of the particle.

[0031] In some embodiments of this application, the sodium compound particles melt after scanning for a preset time at an accelerating voltage of 10 kV and volatilize from the surface of the secondary carbon-coated sodium iron pyrophosphate particles. This is because the sodium compound particles melt and volatilize under the energy of the accelerating voltage, indicating that the sodium compound particles are sodium compounds, rather than conductive carbon particles or single-crystal NFPP particles that cannot melt and disappear. This can be confirmed by the difference in SEM images before and after high accelerating voltage irradiation.

[0032] In this application, the preset time can be 1 min to 3 min.

[0033] In some embodiments of this application, the carbon layer fully coats the surface of the primary carbon-coated sodium iron pyrophosphate core. Compared to the partially coated NFPP cathode material formed when the carbon source is added only once, the fully coated NFPP cathode material of this application has a secondary carbon coating layer with uniform coating characteristics, which can further prevent external air from entering the NFPP core, thereby further improving the air stability of the NFPP cathode material.

[0034] In some embodiments of this application, the resistivity of the cathode material powder is R, where 100 Ω·cm ≤ R ≤ 5000 Ω·cm. For example, R is 100 Ω·cm, 500 Ω·cm, 1000 Ω·cm, 3000 Ω·cm, or 5000 Ω·cm. A resistivity within the above range indicates that the cathode material has a low resistivity characteristic.

[0035] In some embodiments of this application, the secondary carbon-coated sodium iron pyrophosphate particles have a spherical and / or near-spherical structure. This structure of NFPP cathode material is synthesized by sand milling and spraying, which is beneficial for obtaining high-energy-density NFPP cathode material with high compaction density and high capacity.

[0036] In some embodiments of this application, the preparation method of the NFPP cathode material includes: Step A: Sodium source, iron source, phosphorus source, first carbon source and solvent are mixed to obtain slurry, and then spray dried to obtain NFPP precursor powder; Step B: The NFPP precursor powder is subjected to a first sintering process and a second sintering process to obtain a carbon-coated NFPP cathode material. Step C: Dissolve the second carbon source in a solvent to form a carbon source solution, then uniformly mix the prepared primary carbon-coated NFPP cathode material with the carbon source solution, and dry to obtain the NFPP composite precursor. Step D: The NFPP composite precursor is subjected to a third sintering process to obtain a secondary carbon-coated NFPP cathode material.

[0037] In step A, the sodium source includes, but is not limited to, sodium pyrophosphate; the iron source includes, but is not limited to, ferrous oxalate dihydrate and ferric nitrate nonahydrate; the phosphorus source includes, but is not limited to, ammonium dihydrogen phosphate; the first carbon source includes, but is not limited to, at least one of glucose, citric acid, sucrose, starch, ascorbic acid, and polyvinyl alcohol; and the solvent includes, but is not limited to, water. This application does not impose any particular restrictions on the addition ratio of the sodium, iron, and phosphorus sources; technicians can adjust them according to actual needs, as long as an NFPP cathode material can be formed.

[0038] In step B, the first sintering temperature is 300℃~330℃, and the first sintering time is 3h~8h; the second sintering temperature is 500℃~600℃, and the second sintering time is 10h~16h. The first and second sintering processes can be performed without interval, with the temperature of the second sintering process reached through a single heating process. Through these two sintering processes, high-capacity spherical NFPP cathode materials can be obtained. The gas atmosphere in both the first and second sintering processes is either nitrogen or argon.

[0039] In step C, the second carbon source corresponds to the solvent, ensuring that the second carbon source is fully dissolved in the solvent. For example, when the solvent is water, the second carbon source includes, but is not limited to, at least one of glucose, citric acid, and polyvinyl alcohol. The atmosphere of the corresponding solution is nitrogen, and nitrogen must be passed through first to remove oxygen from the deionized water to avoid volume loss. When the solvent is ethanol, the second carbon source includes, but is not limited to, at least one of furfuryl alcohol resin, phenolic resin, polyvinyl alcohol, polyvinylpyrrolidone, and polyacryl alcohol. Drying can be either spray drying or vacuum drying, with a drying temperature of 80℃~130℃. In step D, the temperature of the third sintering process is 500℃~600℃, and the sintering time is 1h~15h; the gas in the gas atmosphere of the third sintering process is either nitrogen or argon. A temperature within the above range for the third sintering process is beneficial for achieving a dense carbon-coated NFPP cathode material with good electronic conductivity, while also preventing the growth of impurity phases in the NFPP core.

[0040] The inventors found that the main sources of air exposure capacity loss in existing NFPP cathode materials are as follows: (1) Low effective carbon coating carbon ratio: In industrial applications, glucose is usually used as the coating carbon source. The mainstream method for synthesizing NFPP cathode materials is the sand milling-spray method. However, in the existing method, glucose is added as a carbon source at the raw material sand milling stage. After dissolving in water, it is evenly dispersed on the surface of the raw material particles. After carbonization, the carbonized glucose is more distributed between the finished NFPP particles, rather than on the surface of the NFPP particles. Therefore, the effective carbon ratio is small, resulting in the active sites in the NFPP cathode material being exposed to the air; (2) Low carbonization rate: The actual carbonization rate of glucose at 510°C in a nitrogen atmosphere is only 20%. This means that even if glucose is coated on the surface of the NFPP cathode material in the sand milling-spraying stage, only about 20% carbon will be formed after high-temperature sintering. The excess O and H will break down and volatilize, exposing a large number of NFPP sites. The method for preparing NFPP cathode material provided in this application introduces a second carbon source, which fills the pores generated during the carbonization and decomposition of the first carbon source, reducing the number of active sites of the NFPP cathode material exposed to air, thereby improving the air stability of the NFPP cathode material. Furthermore, the second carbon source uses a high carbonization rate carbon source, with a carbonization rate greater than or equal to that of the first carbon source, which can also reduce the exposure of active sites caused by carbonization. Moreover, the carbon layer formed after the second carbon source is carbonized can increase the electronic conductivity of the surface layer of the NFPP cathode material, thereby reducing the polarization of the NFPP cathode material and improving the electrochemical performance of the NFPP cathode material.

[0041] The positive electrode active material layer of this application can be disposed on one or both surfaces of the positive electrode current collector in the thickness direction. In this application, the positive electrode active material layer is disposed on the surface of the positive electrode current collector; that is, the positive electrode active material layer can be disposed on a portion of one surface of the positive electrode current collector, or it can be disposed on the entire surface of one surface of the positive electrode current collector. This application does not have any particular limitation on the positive electrode current collector, as long as it can achieve the purpose of this application; for example, it can be, but is not limited to, aluminum foil, aluminum alloy foil, or composite current collectors. In this application, there is no particular limitation on the thickness of the positive electrode current collector, as long as it can achieve the purpose of this application; for example, a thickness of 8μm to 13μm. The single-sided thickness of the positive electrode active material layer in this application can be 150μm to 400μm.

[0042] In this application, the positive electrode active material layer may further include a positive electrode conductive agent. This application does not impose any particular limitation on the positive electrode conductive agent, as long as it achieves the purpose of this application. For example, it may include, but is not limited to, at least one of conductive carbon black (SuperP), carbon nanotubes (CNT), Ketjen black (KB), graphene, graphene oxide, and acetylene black. The mass percentage of the conductive agent in the positive electrode active material layer is 10% to 40%. In this application, the positive electrode active material layer may further include a positive electrode binder. This application does not impose any particular limitation on the positive electrode binder, as long as it achieves the purpose of this application. For example, it may include, but is not limited to, at least one of fluorinated resin, polypropylene resin, fiber-type binder, rubber-type binder, polyimide-type binder, and polyvinylidene fluoride (PVDF).

[0043] The sodium-ion battery of this application also includes a negative electrode, a separator, and an electrolyte, wherein the separator is located between the positive electrode and the negative electrode and plays a role in isolation.

[0044] This application does not impose any particular limitation on the negative electrode sheet, as long as it achieves the purpose of this application. For example, the negative electrode sheet typically includes a negative current collector and a negative active material layer. The negative active material layer can be disposed on the surface of the negative current collector; that is, the negative active material layer can be disposed on a portion of the surface of the negative current collector, or it can be disposed on the entire surface of the negative current collector. This application does not impose any particular limitation on the negative current collector, as long as it achieves the purpose of this application. For example, it can include, but is not limited to, copper foil, copper alloy foil, nickel foil, or composite current collectors. In this application, there is no particular limitation on the thickness of the negative current collector, as long as it achieves the purpose of this application, for example, a thickness of 4μm to 12μm. The single-sided thickness of the negative material layer in this application can be 70μm to 200μm.

[0045] In this application, the negative electrode active material layer may also include a negative electrode binder. This application does not impose any particular limitation on the negative electrode binder, as long as it can achieve the purpose of this application. For example, it may include at least one of acrylate, polyamide, polyimide, polyamide-imide, polyvinylidene fluoride, styrene-butadiene rubber, sodium alginate, polyvinyl alcohol, polytetrafluoroethylene, and sodium carboxymethyl cellulose.

[0046] In some embodiments, the diaphragm material can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The diaphragm can be a single-layer film or a multi-layer composite film, without particular limitation. When the diaphragm is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0047] The sodium-ion battery of this application also includes an electrolyte. This application does not impose any particular limitation on the electrolyte; those skilled in the art can choose according to actual needs, as long as the purpose of this application is achieved. For example, at least one of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl propionate (EP), propyl propionate (PP), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), vinylene carbonate (VC), or fluoroethylene carbonate (FEC) can be mixed in a certain mass or volume ratio to obtain a non-aqueous organic solvent, and then a sodium salt can be added to dissolve and mix evenly. This application does not limit the type of sodium salt, as long as the purpose of this application is achieved. For example, the sodium salt may include at least one of sodium hexafluorophosphate, sodium perchlorate, sodium tetrafluoroborate, sodium trifluoromethanesulfonate, and sodium p-toluenesulfonate. This application does not impose any particular limitation on the concentration of the sodium salt in the electrolyte, as long as the purpose of this application is achieved. For example, the concentration of the sodium salt is 1.0 mol / L to 2.0 mol / L.

[0048] The sodium-ion battery of this application also includes a casing. This application does not impose any particular restrictions on the casing, and those skilled in the art can choose one according to actual needs, as long as it can achieve the purpose of this application. For example, the casing may include an aluminum-plastic film.

[0049] This application does not impose any particular limitation on the preparation method of sodium-ion batteries. Any preparation method known in the art can be used, as long as it can achieve the purpose of this application. For example, the preparation method of sodium-ion batteries includes, but is not limited to, the following steps: stacking the positive electrode, separator, and negative electrode in sequence, and winding and folding them as needed to obtain a bare cell with a wound structure; placing the bare cell in a packaging bag; injecting electrolyte into the packaging bag and sealing it to obtain a sodium-ion battery.

[0050] This application also provides an energy storage device, including a housing and at least one battery as described in any of the above embodiments, the battery being housed within the housing. The energy storage device with this battery exhibits excellent performance, which is beneficial for its use. Housing the battery within the housing increases its stability and protection, thereby extending the lifespan of the energy storage device. It is understood that the energy storage device may contain one or more batteries, and when the energy storage device contains multiple batteries, the multiple batteries can be connected in at least one manner, such as parallel or series connection.

[0051] This application also provides an electrical device including the energy storage device described in the above embodiments, which is beneficial for improving the product competitiveness and performance of the electrical device. In an optional embodiment, the electrical device includes an electrical device body, and the energy storage device is used to supply power to the electrical device body. In an optional embodiment, the electrical device body includes a positive terminal and a negative terminal, the positive electrode of the battery in the energy storage device is used to electrically connect to the positive terminal of the electrical device body, and the negative electrode of the battery in the energy storage device is used to electrically connect to the negative terminal of the electrical device body, so as to supply power to the electrical device.

[0052] The electrical equipment in this application may include, but is not limited to: prefabricated energy storage cabins, electric vehicles, electric cars, ships, spacecraft, electric toys, and power tools, etc. Among them, spacecraft include, for example, airplanes, rockets, space shuttles, and spacecraft, etc. Electric toys include, for example, stationary or mobile electric toys, specifically, electric car toys, electric ship toys, and electric airplane toys, etc. Power tools include, for example, metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, specifically, electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers.

[0053] Because the energy we need is highly time- and space-dependent, in order to utilize energy rationally and improve energy efficiency, it is necessary to store one form of energy in the same way or by converting it into another, and then release it in a specific energy form for future applications. Currently, the main way to generate green electricity is to develop green energy sources such as photovoltaics and wind power to replace fossil fuels. Currently, the generation of green electricity generally relies on solar, wind, and hydropower. However, wind and solar power are generally characterized by strong intermittency and large fluctuations, which can cause grid instability, insufficient power during peak demand periods, and excessive power during off-peak periods. Unstable voltage can also damage the power grid. Therefore, insufficient electricity demand or insufficient grid capacity may lead to the problem of "wind and solar curtailment." Solving these problems requires energy storage. This involves converting electrical energy into other forms of energy through physical or chemical means and storing it. When needed, this energy can be converted back into electrical energy and released. Simply put, energy storage is like a large "power bank," storing electrical energy when solar and wind power are abundant and releasing the stored electricity when needed.

[0054] Taking electrochemical energy storage as an example, this solution provides an energy storage device 440, which is applied to an energy storage system 400. The energy storage device 440 is equipped with a set of chemical batteries, which mainly use the chemical elements in the batteries as energy storage medium. The charging and discharging process is accompanied by the chemical reaction or change of the energy storage medium. Simply put, the electrical energy generated by wind and solar energy is stored in the chemical batteries. When the use of external electrical energy reaches its peak, the stored electrical energy is released for use, or transferred to places with a shortage of electricity for use.

[0055] Current energy storage applications are quite widespread, including generation-side energy storage, grid-side energy storage, and consumption-side energy storage. The corresponding types of energy storage devices include: (1) Large-scale energy storage power stations (composed of multiple prefabricated energy storage modules) applied to wind power and photovoltaic power stations can help renewable energy power generation meet grid connection requirements and improve the utilization rate of renewable energy. As a high-quality active / reactive power regulation power source on the power supply side, the energy storage power station realizes the load matching of power in time and space, enhances the renewable energy absorption capacity, reduces instantaneous power changes, reduces the impact on the power grid, improves the problem of new energy power generation absorption, and is of great significance in power grid system backup, alleviating peak load power supply pressure and peak regulation and frequency regulation. (2) The energy storage prefabricated cabin applied on the grid side mainly functions as peak regulation, frequency regulation and grid congestion relief. In terms of peak regulation, it can realize peak shaving and valley filling of electricity load, that is, charging the energy storage battery when the electricity load is low and releasing the stored electricity during the peak electricity load period, thereby achieving a balance between power production and consumption. (3) Small energy storage cabinets applied to the electricity consumption side mainly function as self-consumption of electricity, peak-valley price arbitrage, capacity cost management, and improvement of power supply reliability. Depending on the application scenario, electricity consumption side energy storage can be divided into industrial and commercial energy storage cabinets, household energy storage devices, energy storage charging piles, etc., which are generally used in conjunction with distributed photovoltaics. Industrial and commercial users can use energy storage for peak-valley price arbitrage and capacity cost management. In the electricity market implementing peak-valley pricing, by charging the energy storage system when the electricity price is low and discharging the energy storage system when the electricity price is high, peak-valley price arbitrage can be achieved, reducing electricity costs. In addition, industrial enterprises subject to two-part tariffs can use energy storage systems to store energy during off-peak hours and discharge during peak loads, thereby reducing peak power and the maximum demand declared, achieving the goal of reducing capacity costs. Household photovoltaics with energy storage can improve the level of self-consumption of electricity. Due to high electricity prices and poor power supply stability, the demand for household photovoltaic installations is driven. Given that photovoltaic power generation occurs during the day, while user load is generally higher at night, configuring energy storage can better utilize photovoltaic power, improve self-consumption levels, and reduce electricity costs. Furthermore, energy storage is needed in areas such as communication base stations and data centers for backup power.

[0056] In some embodiments, see Figure 1 , Figure 1 This is a schematic diagram of the structure of an energy storage system 400 according to an embodiment of this application. Figure 1 This application Figure 1 The embodiments are illustrated using a home energy storage scenario in user-side energy storage as an example. The energy storage device 440 of this application is not limited to the home energy storage scenario.

[0057] This application provides an energy storage system 400, which includes a first power conversion device 410 (photovoltaic panel), a first user load 420 (household lighting fixture), a second user load 430 (e.g., household appliances such as air conditioners), and an energy storage device 440. The energy storage device 440 is a small energy storage box that can be wall-mounted on an outdoor wall. However, the energy storage device 440 is not limited to wall mounting and can also be placed in a user's residence in other ways. Specifically, the photovoltaic panel can convert solar energy into electrical energy during periods of low electricity prices, and the energy storage device 440 stores this electrical energy and supplies it to lighting fixtures and household appliances during peak electricity prices, or provides power during power outages / power interruptions.

[0058] In some embodiments, see Figure 2 , Figure 2 This is a schematic diagram of the structure of an energy storage system 400 according to an embodiment of this application. Figure 1 And this application Figure 2 The embodiments are illustrated using a shared energy storage scenario on the generation / distribution side as an example. The energy storage device 440 of this application is not limited to the energy storage scenario on the generation / distribution side.

[0059] This application provides an energy storage system 400, which includes: a high-voltage cable 450, a first power conversion device 410, a second power conversion device 460, and an energy storage device 440 provided in this application. In some embodiments of the power generation scenario, the second power conversion device 460 can be a wind power conversion device. Since the electricity generated by wind power conversion is volatile, random, and intermittent, the unstable electricity output by the wind power conversion device can be stored in the energy storage device 440 through grid connection. The energy storage device 440 is connected to the high-voltage cable and outputs smooth electricity to the power consumption side of the distribution network, realizing peak shaving and frequency regulation, and stable grid operation; or, wind power... The conversion device is always connected to the high-voltage cable. Under normal power generation conditions, the power output of the wind power conversion device is supplied to the power consumption side of the distribution network through the high-voltage cable. When the current power load is low and the wind power conversion device generates excess power, the excess power is first stored in the energy storage device 440 to reduce wind and solar curtailment rates and improve the problem of new energy power generation consumption. When the power load is high, the power grid issues an instruction to transmit the power stored in the energy storage device 440 together with the high-voltage cable 450 in grid-connected mode to supply power to the power consumption side. This provides the power grid with various services such as peak shaving, frequency regulation, and backup, giving full play to the peak shaving role of the power grid, promoting peak shaving and valley filling, and alleviating the power supply pressure of the power grid.

[0060] In some embodiments on the distribution network side, the first power conversion device 410 can be a photovoltaic panel, and the energy storage device 440 is connected to the high-voltage cable 450 and installed downstream of the high-voltage cable 450 and between the user load. The electrical energy output by the photovoltaic power conversion device is stored in the energy storage device 440, which can respond in a timely manner to act as a backup power source when the power grid / distribution network fails; or, it can provide power supply support to alleviate line congestion when the high-voltage cable 450 transmission line is blocked, and to delay the economic pressure caused by the expansion of the power grid / distribution capacity when the power grid is planned to be expanded.

[0061] In some embodiments, see Figure 3 , Figure 3 This is a schematic diagram of the structure of an energy storage system 400 according to an embodiment of this application. Figure 3 And this application Figure 3 The embodiments are illustrated using an industrial and commercial energy storage scenario as an example. The energy storage device 440 of this application is not limited to industrial and commercial energy storage scenarios.

[0062] This application provides an energy storage system 400, which includes: an energy storage device 440, a high-voltage cable 450, a factory equipped with a first power conversion device 410, a photovoltaic-energy storage-charging station 480, and a vehicle 470. In some embodiments of industrial and commercial scenarios, the first power conversion device 410 can be a photovoltaic panel, which converts solar energy into electrical energy and stores it in the energy storage device 440 in the factory. In the event of a power grid failure, the energy storage device 440 provides power to ensure the safe and stable operation of the factory without interruption. Alternatively, when the factory's power load is high, the power grid issues an instruction to transmit the electricity stored in the energy storage device 440 in conjunction with the high-voltage cable 450 in a grid-connected mode to supply the factory with electricity, providing various services such as peak shaving / frequency regulation and backup for the power grid operation. In addition, the first power conversion device 410 can also convert solar energy into electrical energy and store it in the energy storage device 440 of the photovoltaic-energy storage-charging station 480, which can then directly charge the vehicle 470, making it fast and convenient.

[0063] Optionally, the first power conversion device 410 may include, but is not limited to, a photovoltaic panel, and the second power conversion device 460 may include, but is not limited to, a wind power conversion device. The first power conversion device 410 and the second power conversion device 460 can convert at least one of solar energy, light energy, wind energy, thermal energy, tidal energy, biomass energy, and mechanical energy into electrical energy.

[0064] Optionally, the energy storage device 440 may include, but is not limited to, energy storage applications such as energy storage power stations, hydropower / thermal / wind power generation systems, solar power generation systems, mobile power systems, smart home systems, or temporary power supply systems, and may also be applied in multiple fields such as data centers, military equipment, aerospace, charging piles, and electric vehicles.

[0065] Optionally, the energy storage device 440 may include, but is not limited to, individual batteries, or battery modules, battery packs, battery clusters, power banks, energy storage cabinets / prefabricated energy storage compartments, and other battery integrated systems composed of individual batteries. The actual application form of the energy storage device 440 provided in this application embodiment may be, but is not limited to, the listed products, and may also be other application forms. This application embodiment does not strictly limit the application form of the energy storage device 440.

[0066] Optionally, the individual cell can be, but is not limited to, at least one of cylindrical, prismatic, prismatic, or other shaped cells.

[0067] Optionally, the single cell may be a secondary battery, which refers to a single cell that can be reused by reactivating the active material through charging after being discharged. The single cell may be a lithium ion battery, a sodium ion battery, a sodium lithium ion battery, a lithium metal battery, a sodium metal battery, a lithium sulfur battery, a magnesium ion battery, a nickel hydrogen battery, a nickel cadmium battery, a lead storage battery, etc., which is not specifically limited in the present application.

[0068] Examples Hereinafter, examples and comparative examples are given to more specifically illustrate the embodiments of the present application. Various tests and evaluations are carried out according to the following methods.

[0069] Example 1 <Preparation of NFPP Cathode Material> <Preparation of NFPP Precursor> Sodium pyrophosphate (as sodium source and phosphorus source), ammonium dihydrogen phosphate (as phosphorus source), iron nitrate nonahydrate (as iron source), glucose (as first carbon source) and water (as solvent) are stirred uniformly to obtain a slurry with a solid content of 35%, which is then spray-dried to obtain NFPP precursor powder, wherein the mass ratio of sodium pyrophosphate, ammonium dihydrogen phosphate and ferrous oxalate dihydrate is 17.2:14.88:76.06, the addition amount of glucose is adjusted according to the designed carbon content in the primary carbon-coated NFPP cathode material, and the carbonization rate of the carbon source can be confirmed by a differential scanning calorimeter-thermogravimetric analyzer (DSC-TG).

[0070] <Preparation of Primary Carbon-coated NFPP Cathode Material> The NFPP precursor powder is subjected to a first sintering process and a second sintering process under a nitrogen atmosphere to obtain a primary carbon-coated NFPP cathode material, and the mass proportion of carbon formed after carbonization of the first carbon source in the primary carbon-coated NFPP cathode material is shown in Table 1. Wherein, the first sintering temperature is 310°C, and the first sintering time is 5h; the second sintering temperature is 550°C, and the second sintering time is 14h.

[0071] <Preparation of NFPP Composite Precursor> The second carbon source furfuryl alcohol resin is dissolved in solvent ethanol to form a carbon source solution, then the prepared primary carbon-coated NFPP cathode material is uniformly mixed with the carbon source solution, and dried to obtain an NFPP composite precursor; <Preparation of Secondary Carbon-coated NFPP Cathode Material> The NFPP composite precursor is subjected to a third sintering process to obtain a secondary carbon-coated NFPP cathode material. Wherein, the third sintering temperature is 550°C, the third sintering time is 5h, and the mass proportion of carbon formed after carbonization of the second carbon source in the secondary carbon-coated NFPP cathode material is shown in Table 1.

[0072] <Preparation of Cathode Pole Piece> The prepared secondary carbon-coated NFPP cathode material, conductive agent Super P, and binder PVDF are mixed at a mass ratio of 90:5:5, then NMP is added and stirred uniformly to obtain a cathode slurry with a solid content of 60%. The cathode slurry is then uniformly coated on an aluminum foil with a thickness of 10 μm, and the single-side coating thickness is 20 μm. After that, the coated foil is dried in vacuum at 110°C for 12 h, and then rolled to obtain a cathode electrode sheet. The obtained cathode electrode sheet is placed in an air atmosphere with a humidity of 20% to 30% and exposed to air for 48 h. The exposed electrode sheet is dried in vacuum and cut into round sheets with a diameter of 14 mm for later use.

[0073] <Preparation of Electrolyte> In an argon atmosphere glove box with a water content of ≤1 ppm, ethylene carbonate (EC) and diethyl carbonate (DEC) are mixed at a volume ratio of 1:1, then sodium salt NaClO4 is added and dissolved in the above solvent, and the mixture is uniformly mixed to obtain an electrolyte. Wherein, the molar concentration of NaClO4 in the electrolyte is 1 mol / L.

[0074] <Preparation of Separator> A glass fiber membrane with a thickness of 260 μm is selected as the separator.

[0075] <Assembly of Coin Cell> A circular sodium sheet with a diameter of 14 mm is used as the counter electrode. The circular cathode electrode sheet prepared above, the separator, and the circular sodium sheet are stacked in order, so that the separator is located between the circular cathode electrode sheet and the circular sodium sheet to play an insulating role. Then the prepared electrolyte is injected, and a coin cell is assembled.

[0076] Examples 2 to 4 Except that in <Preparation of NFPP Composite Precursor>, the type of the second carbon source and the corresponding solvent are adjusted according to Table 1, the rest are the same as in Example 1.

[0077] Examples 5 to 10 Except that in <Preparation of Secondary Carbon-Coated NFPP Cathode Material>, the temperature and time of the third sintering process are adjusted according to Table 1, the rest are the same as in Example 1.

[0078] Examples 11 to 14 Except that in <Preparation of NFPP Precursor>, the type of the first carbon source is adjusted according to Table 1, and in <Preparation of NFPP Composite Precursor>, the type of the second carbon source and the corresponding solvent are adjusted according to Table 1, the rest are the same as in Example 1.

[0079] Examples 15 to 17 Except for adjusting the addition amount of the second carbon source to adjust the mass proportion of the carbonized second carbon source in the secondary carbon-coated NFPP cathode material according to Table 1, as described in <Preparation of NFPP composite precursor>, the remaining steps are the same as those in Example 1.

[0080] Comparative Example 1 Sodium pyrophosphate (sodium source and phosphorus source), ammonium dihydrogen phosphate (phosphorus source), iron nitrate nonahydrate (iron source), citric acid (carbon source) and water (solvent) are stirred uniformly to obtain a slurry with a solid content of 35%, and then spray-dried to obtain NFPP precursor powder, wherein the mass ratio of sodium pyrophosphate, ammonium dihydrogen phosphate and ferrous oxalate dihydrate is 17.2:14.88:76.06, and the addition amount of citric acid is adjusted according to the designed carbon content in the primary carbon-coated NFPP cathode material; the slurry is spray-dried at 105°C to obtain precursor powder; the precursor powder is sintered in a sintering furnace under nitrogen protection at 510°C for 12h, and cooled to obtain the NFPP cathode material.

[0081] Comparative Example 2 Except that the carbon source is replaced with glucose, and the mass proportion of the carbonized carbon source in the primary carbon-coated NFPP cathode material is adjusted according to Table 1, the remaining steps are the same as those in Comparative Example 1.

[0082] Comparative Example 3 Except that the carbon source is replaced with glucose, the remaining steps are the same as those in Comparative Example 1.

[0083] Table 1: Preparation parameters of each example and comparative example

[0084] Note: In Table 1, " / " indicates that the relevant preparation parameter does not exist.

[0085] Test methods and equipment: Surface contact angle test of cathode material layer: The surface contact angle test of the cathode material layer is carried out by a contact angle measuring instrument. Firstly, a 500 μL micro-syringe is used to drop one drop of deionized water onto the surface of the cathode material layer of the NFPP cathode sheet, the dropping height is 0.3 cm, the standing time is 20 min, and the surface contact angle of the cathode material layer after standing is recorded.

[0086] Measurement of the number of sodium compound particles: The NFPP positive electrode sheet was exposed to air with a relative humidity of 20%–30% for 48 hours, and then observed using a scanning electron microscope at a magnification of 10k–20k to obtain SEM images of the NFPP positive electrode material. Within the SEM images, five arbitrary 4000 nm × 4000 nm regions were selected, and ten random secondary carbon-coated sodium iron pyrophosphate particles were identified within each region. At least five of these particles were found to be unrolled NFPP particles. Furthermore, sodium compound particles (bright white particles) adhering to the surface of the secondary carbon-coated sodium iron pyrophosphate particles were identified and their number was recorded. The number of sodium compounds was statistically determined using ImageJ software. The SEM images of the NFPP positive electrode material were processed, and by setting area limits and other parameters, the total number of sodium compound particles (N), average size (Dmean), minimum particle size (Dmin), and maximum particle size (Dmax) were measured. The software requirements are not fixed; the software only needs to accurately measure the number and size of the sodium compounds. When conductive carbon particles interfere with the number of sodium compounds, the difference in the number of particles in the same SEM area before and after high-acceleration voltage irradiation is the number of white sodium particles.

[0087] Mass percentage test of carbon formed after carbonization of the first carbon source: The NFPP sample after secondary sintering was tested and analyzed using a high-frequency infrared carbon-sulfur analyzer. The results were measured 5 times, and the average carbon content was taken as C1 after outliers were excluded. C1 is the mass percentage of carbon formed after the first carbon source is carbonized in the NFPP cathode material.

[0088] Mass percentage test of carbon formed after carbonization of the second carbon source: The NFPP samples after three sinterings were tested and analyzed using a high-frequency infrared carbon-sulfur analyzer. The results were measured five times, and after excluding outliers, the average carbon content was taken as C2. C2-C1 is the mass percentage of carbon formed after the second carbon source carbonization in the NFPP cathode material.

[0089] Measurement of the average size of sodium compound particles: The SEM images of the NFPP cathode material were processed using ImageJ software. By setting upper limits for area and other parameters, the average size of the sodium compound particles, Dmean, was measured. The software is not required to meet specific conditions; it only needs to measure the number and average size of the sodium compounds. When conductive carbon particles interfere with the number of sodium compounds, the number of white sodium particles was determined by comparing the number of particles in the same SEM area before and after high-acceleration voltage irradiation.

[0090] Powder resistivity test of cathode material: The powder resistivity of the NFPP cathode powder is determined by standard testing using a powder resistivity tester at a pressure of 200 MPa.

[0091] First charge capacity and first discharge capacity test: The test temperature was 25℃. The coin cell battery was charged at a constant current of 0.1C to 3.5V, which is the charging stage. After resting for 10 minutes, it was discharged at a constant current of 0.1C to 2V, and then rested for 10 minutes, which is the discharging stage. The charging capacity of the first charging stage was recorded as the first-cycle charging capacity, in mAh / g; the discharging capacity of the first discharging stage was also recorded as the first-cycle discharging capacity, in mAh / g.

[0092] Cyclic performance test: The test temperature was 25℃. The coin cell battery was charged to 3.5V at a constant current of 0.1C, allowed to stand for 10 minutes, and then discharged to 2V at 0.1C. The capacity obtained in this step is the initial discharge capacity C. i Perform 200 cycles of 0.1C charge / 0.1C discharge, and record the discharge capacity on the 200th cycle. Cycle capacity retention = (Discharge capacity on the 200th cycle / Initial discharge capacity C) i )×100%.

[0093] Table 2: Performance data of each embodiment and comparative example

[0094] Note: In Table 2, " / " indicates that the relevant parameter does not exist.

[0095] As can be seen from Examples 1-17 and Comparative Examples 1-3, when the NFPP cathode material is a carbon-coated material obtained by adding citric acid as a carbon source and sintering it once (e.g., Comparative Example 1), the surface contact angle of the NFPP cathode material is small, the powder resistivity of the NFPP cathode material is high, the number of sodium compound particles is large, the first charge capacity and the first discharge capacity of the battery are low, and the capacity retention rate after 200 cycles is also low. When the NFPP cathode material is a carbon-coated material obtained by adding glucose as a carbon source and sintering it once (e.g., Comparative Example 2), the surface contact angle of the NFPP cathode material is also relatively small. Although the powder resistivity of the NFPP cathode material is reduced to a certain extent, it is still higher than that of the embodiments of this application, and the number of sodium compound particles is large. The battery exhibited low initial charge and discharge capacities, and low capacity retention after 200 cycles. When the amount of primary carbon source added was further increased (e.g., in Comparative Example 3), the surface contact angle of the NFPP cathode material was also relatively small. Although the powder resistivity of the NFPP cathode material decreased to some extent, it was still higher than that of the embodiments in this application, and the number of sodium compound particles was relatively large. The improvement in the battery's initial charge and discharge capacities and capacity retention after 200 cycles was not significant compared to Comparative Example 2. In contrast, the secondary carbon-coated NFPP cathode material of this application exhibited lower powder resistivity, and the number of sodium compound particles was small or even absent. The battery's initial charge and discharge capacities were improved, and the capacity retention after 200 cycles was also improved.

[0096] The type and amount of the first carbon source (the percentage of carbon formed by the first carbon source), the type and amount of the second carbon source (the percentage of carbon formed by the second carbon source), the third sintering temperature, and the third sintering time, among other preparation parameters, also have a certain impact on the performance of the NFPP cathode material. As can be seen from Examples 1 to 17, based on the secondary carbon coating characteristics of the NFPP cathode material, by adjusting the above preparation parameters within the scope of this application, it is beneficial to obtain an NFPP cathode material with good air stability, good capacity performance, and good cycle performance, thereby improving the actual energy density of the NFPP cathode material in sodium-ion battery cell applications.

[0097] Figure 4 This is a SEM image of the surface of the NFPP particles with secondary carbon coating after the positive electrode sheet prepared in Example 1 was exposed to air. Figure 5 This is a further enlarged SEM image of the surface of NFPP particles after air exposure, as shown in Example 1. From... Figure 4 and Figure 5 It can be seen that there are no dot-like white sodium compound particles on the surface of the secondary carbon-coated NFPP particles, indicating that the air stability of the secondary carbon-coated NFPP cathode material of this application has been significantly improved, and there is no obvious sodium desorption behavior.

[0098] Figure 6 SEM image of the surface of ordinary NFPP particles after the positive electrode sheet prepared in Comparative Example 3 was exposed to air; Figure 7 This is a further magnified SEM image of the surface of ordinary NFPP particles after air exposure, as shown in Comparative Example 3. From... Figure 4 and Figure 5 It can be seen that a large number of white particles (>100) grow on the surface of ordinary NFPP particles. These white particles melt and disappear after being irradiated with a 10kV accelerating voltage for a period of time. Upon further exposure to air, the white particles continue to grow, and EDS confirms that these white particles are sodium-containing products.

[0099] The above provides a detailed description of a positive electrode sheet, battery, energy storage device, and electrical equipment disclosed in this application. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of the above embodiments are only for the purpose of helping to understand the technical solutions and core inventive points of the embodiments of this application. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this application. Therefore, the content of this specification should not be construed as a limitation of this application.

Claims

1. A positive electrode plate, characterized in that, The device includes a positive current collector and a positive electrode material layer disposed on at least one surface of the positive current collector, wherein the positive electrode material layer includes a positive electrode material, wherein: The cathode material includes secondary carbon-coated sodium iron pyrophosphate particles, which are composed of a primary carbon-coated sodium iron pyrophosphate core and a carbon layer coating the surface of the primary carbon-coated sodium iron pyrophosphate core. The surface contact angle of the positive electrode material layer is A, where 80°≤A≤100°.

2. The positive electrode sheet according to claim 1, characterized in that, In any predetermined observation area of ​​the scanning electron microscope image of the cathode material layer after being exposed to air with a relative humidity of 20%~30% for 48 hours, there are secondary carbon-coated iron sodium pyrophosphate particles. The surface of the secondary carbon-coated iron sodium pyrophosphate particles is attached with sodium compound particles. The number of sodium compound particles in each secondary carbon-coated iron sodium pyrophosphate particle is N, where 0≤N≤30.

3. The positive electrode sheet according to claim 2, characterized in that, The average size of the sodium compound particles is 10 nm to 500 nm.

4. The positive electrode sheet according to claim 2, characterized in that, The sodium compound particles melt and volatilize after being scanned for a preset time at an accelerating voltage of 10 kV.

5. The positive electrode sheet according to claim 1, characterized in that, The carbon layer completely covers the surface of the primary carbon-coated sodium iron pyrophosphate core.

6. The positive electrode sheet according to claim 1, characterized in that, The resistivity of the positive electrode material powder is R, where 100 Ω·cm ≤ R ≤ 5000 Ω·cm.

7. The positive electrode sheet according to claim 1, characterized in that, The secondary carbon-coated iron pyrophosphate sodium particles have a spherical and / or near-spherical structure.

8. A method for preparing a positive electrode material, characterized in that, Includes the following steps: Sodium source, iron source, phosphorus source, first carbon source and solvent are mixed to obtain slurry, and then spray dried to obtain sodium iron pyrophosphate precursor powder. The sodium iron pyrophosphate precursor powder is subjected to a first sintering process and a second sintering process to obtain a primary carbon-coated sodium iron pyrophosphate cathode material. The second carbon source is dissolved in a solvent to form a carbon source solution. Then, the prepared primary carbon-coated sodium iron pyrophosphate cathode material is uniformly mixed with the carbon source solution and dried to obtain the sodium iron pyrophosphate composite precursor. A third sintering process was carried out on the sodium iron pyrophosphate composite precursor to obtain a secondary carbon-coated sodium iron pyrophosphate cathode material.

9. The preparation method according to claim 8, characterized in that, The first sintering temperature is 300℃~330℃, and the first sintering time is 3h~8h; The second sintering temperature is 500℃~600℃, and the second sintering time is 10h~16h.

10. A sodium-ion battery, characterized in that, Includes the positive electrode sheet as described in any one of claims 1 to 7.

11. An energy storage device, characterized in that, It includes a housing and at least one sodium-ion battery as described in claim 10, the sodium-ion battery being housed within the housing.

12. An electrical appliance, characterized in that, The device includes the energy storage device of claim 11, wherein the energy storage device supplies power to the electrical equipment.

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