Positive electrode active material and preparation method and application thereof

Abstract

The invention provides a positive electrode active material and a preparation method and application thereof, and the positive electrode active material at least comprises an inner core which comprises at least one ferric sodium pyrophosphate particle; the coating layer at least covers part of the surface of the inner core, and the material of the coating layer comprises boron carbide and carbon; wherein the content of the inner core in the positive electrode active material is 90wt%-98wt%, the content of the boron carbide in the positive electrode active material is 1wt%-4wt%, and the content of the carbon in the positive electrode active material is 1wt%-6wt%. According to the positive electrode active material as well as the preparation method and the application thereof, the positive electrode active material can be ensured to fully play a role in improving the rate capability and the cycling stability of the battery.

Description

Technical Field

[0001] This invention relates to the field of power battery technology, specifically to a positive electrode active material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries, compared to lithium-ion batteries, have advantages such as lower cost and abundant resources, and are rapidly emerging as a focal point in the energy storage market. Among the cathode materials for sodium-ion batteries, sodium iron pyrophosphate (SOP) is a typical polyanionic compound system, attracting significant attention due to its high theoretical capacity, voltage plateau, and stable structure. However, SOP itself has low electrochemical activity and insufficient conductivity, limiting battery capacity and rate performance. Therefore, a common strategy is to reduce the nanoscale degree of SOP and simultaneously coat its surface with carbon materials to improve the electrochemical activity and conductivity of the cathode active material, thus optimizing material performance. However, in actual optimization, the low bonding strength between the carbon material and the SOP particle surface results in a low tap density of the cathode active material, affecting battery design and failing to achieve the goal of improving material performance. Summary of the Invention

[0003] This invention proposes a positive electrode active material, its preparation method, and its application, which can improve the bonding strength between the coating layer and sodium iron pyrophosphate particles, avoid the coating layer from causing a decrease in the tap density of the positive electrode active material, and thus ensure that the positive electrode active material can fully play its role in improving the rate performance and cycle stability of the battery.

[0004] To solve the above-mentioned technical problems, the present invention provides a positive electrode active material, comprising at least:

[0005] The core comprises at least one sodium iron pyrophosphate particle; and

[0006] A coating layer, at least covering a portion of the surface of the core, wherein the material of the coating layer includes boron carbide and carbon;

[0007] The core content in the positive electrode active material is 90wt%-98wt%, the boron carbide content in the positive electrode active material is 1wt%-4wt%, and the carbon content in the positive electrode active material is 1wt%-6wt%.

[0008] In one embodiment of the present invention, the boron carbide is uniformly distributed in the coating layer, and the distribution difference of boron content in the coating layer is ≤10%.

[0009] In one embodiment of the present invention, the median particle size of the kernel is 0.5 μm-2 μm.

[0010] In one embodiment of the present invention, the thickness of the coating layer is 50nm-150nm.

[0011] In one embodiment of the present invention, the boron carbide has a particle size of 10 nm to 100 nm.

[0012] The present invention also proposes a method for preparing the above-mentioned positive electrode active material, comprising at least the following steps:

[0013] Sodium, iron and phosphorus sources are mixed to obtain a solid mixture, which is then dispersed in a solvent and followed by wet ball milling to obtain a slurry.

[0014] After drying the slurry, a precursor is obtained, and a carbon source and boron carbide are added for dry ball milling to obtain an intermediate material; and

[0015] The intermediate material is sintered to obtain the positive electrode active material.

[0016] In one embodiment of the present invention, at least one of the following features is included:

[0017] The mass ratio of the solid mixture to the solvent is 1:(3-6);

[0018] The rotational speed of the wet ball mill is 300 r / min-500 r / min;

[0019] The wet ball milling time is 6-10 hours;

[0020] The temperature for drying the slurry is 100℃-150℃;

[0021] The drying time for the slurry is 16-24 hours.

[0022] In one embodiment of the present invention, when sintering the intermediate material, the intermediate material is placed in an inert gas atmosphere, heated to a preset temperature at a preset rate, and held at the temperature for a preset time to obtain the positive electrode active material.

[0023] In one embodiment of the present invention, at least one of the following features is included:

[0024] The preset rate is 1℃ / min-5℃ / min;

[0025] The preset temperature is 450℃-550℃;

[0026] The preset time is 8h-10h.

[0027] The present invention also provides a battery comprising at least:

[0028] A positive electrode sheet includes at least a positive electrode material, wherein the positive electrode material includes the positive electrode active material described above, or a positive electrode active material obtained according to the preparation method described above;

[0029] Negative electrode plate;

[0030] A diaphragm is disposed between the positive electrode and the negative electrode; and

[0031] An electrolyte is filled between the positive electrode, the negative electrode, and the separator.

[0032] In summary, this invention proposes a positive electrode active material, its preparation method, and its application. By forming a coating layer on the surface of sodium iron pyrophosphate particles, the electrochemical activity, conductivity, and corrosion resistance of the positive electrode active material can be improved, thereby enhancing the rate performance and cycle stability of the battery. Furthermore, the coating layer can isolate the sodium iron pyrophosphate particles from direct contact with the electrolyte, thus suppressing side reactions and further improving the cycle performance of the positive electrode active material. In addition, the high-hardness boron carbide in the coating layer can reduce the damage to the carbon material in the coating layer during crushing and classification, improving the bonding strength between the core and the coating layer, thereby preventing a decrease in the tap density of the positive electrode active material and ensuring that the positive electrode active material can fully exert its role in improving the rate performance and cycle stability of the battery. Attached Figure Description

[0033] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a schematic diagram of the structure of the positive electrode active material in one embodiment of the present invention.

[0035] Figure 2 The 0.33C charge-discharge curves of the batteries in Examples 1-3 and Comparative Examples 1-4 are shown. Detailed Implementation

[0036] The following specific examples illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and various details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention.

[0037] It should be understood that the invention can be embodied in various forms and should not be construed as being limited to the embodiments set forth herein. Rather, providing these embodiments will make the disclosure thorough and complete, and will fully convey the scope of the invention to those skilled in the art.

[0038] The technical solution of the present invention will be further described in detail below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] Please see Figure 1 As shown, this invention proposes a positive electrode active material, comprising at least a core and a coating layer. The core includes at least one sodium iron pyrophosphate particle, and the coating layer covers at least a portion of the core's surface. The coating layer is composed of boron carbide and carbon. The core comprises 90 wt%-98 wt% of the positive electrode active material, boron carbide comprises 1 wt%-4 wt% of the positive electrode active material, and carbon comprises 1 wt%-6 wt% of the positive electrode active material. In the positive electrode active material provided by this invention, by forming a coating layer on the surface of the sodium iron pyrophosphate particles, the electrochemical activity, conductivity, and corrosion resistance of the positive electrode active material can be improved, thereby improving the rate performance and cycle stability of the battery. Moreover, the coating layer can isolate the sodium iron pyrophosphate particles from direct contact with the electrolyte, thereby suppressing the occurrence of side reactions and further improving the cycle performance of the positive electrode active material. In addition, the high-hardness boron carbide in the coating layer can reduce the damage to the carbon material in the coating layer during crushing and grading, and improve the bonding strength between the core and the coating layer. This avoids the coating layer causing a decrease in the tap density of the positive electrode active material, and ensures that the positive electrode active material can fully play its role in improving the rate performance and cycle stability of the battery.

[0040] Please see Figure 1As shown, in one embodiment of the present invention, the coating material in the positive electrode active material includes, for example, boron carbide and carbon. The boron carbide exists in the coating layer, for example, in the form of boron carbide particles, the shape of which includes, for example, spherical shapes, and the particle size of the boron carbide particles is, for example, 10 nm-100 nm. The carbon exists in the coating layer, for example, in the form of sheet-like carbon material. By mixing boron carbide and carbon materials as a coating layer to coat the core, the coating layer has high electrical conductivity and corrosion resistance. On the one hand, it can improve the electrochemical activity and conductivity of the positive electrode active material, thereby improving the rate performance and cycle stability of the battery. On the other hand, the coating layer can isolate the core from direct contact with the electrolyte, thereby suppressing the occurrence of side reactions and further improving the cycle performance of the positive electrode active material. Furthermore, boron carbide itself has high hardness, which can act as a milling medium during the ball milling process for preparing positive electrode active materials. This helps to further reduce the particle size of the core to the target particle size. Moreover, if the particle size of the core meets the target particle size after ball milling, there is no need for crushing and classification. This reduces the physical damage to the carbon material in the coating layer caused by the crushing and classification process, prevents the carbon material from detaching from the sodium iron pyrophosphate particles, and improves the bonding strength between the core and the coating layer. This avoids the coating layer causing a decrease in the tap density of the positive electrode active material, thereby ensuring that the coating layer can fully play its role in improving the rate performance and cycle stability of the battery.

[0041] Please see Figure 1 As shown, in one embodiment of the present invention, boron carbide particles are uniformly distributed in the coating layer. Specifically, the distribution difference of boron content in the coating layer is ≤10%. To calculate the distribution difference of boron content, the surface of the core is first evenly divided into multiple sites according to its perimeter. Then, the coating layer at each site is sampled and analyzed. The boron content per unit area at different sites is measured, and the average boron content C and the maximum boron content Cmax at all sites are calculated. max and minimum content C min Then according to (C) max -C minThe distribution difference of boron content in the coating layer is calculated using the ) / C method. The site shape is, for example, rectangular or circular, and the number of sites is, for example, 12-20. In this embodiment, the site shape is, for example, rectangular, the number of sites is, for example, 16, and the size of the rectangular sites is, for example, 200nm*150nm, 200nm*100nm, 200nm*50nm, 100nm*100nm, 100nm*50nm, 300nm*150nm, 300nm*100nm, or 300nm*50nm. Because boron carbide has good conductivity, introducing boron carbide with good conductivity into the coating layer can improve the ionic / electronic conductivity of the positive electrode active material and enhance its conductivity. Furthermore, if boron carbide is unevenly distributed in the coating layer, the conductivity of the enriched and depleted points of boron carbide in the coating layer will be inconsistent. This will not only lead to an increase in the overall polarization and impedance of the positive electrode active material, thereby reducing the battery's discharge capacity, but also cause local high impedance in the positive electrode active material, resulting in local high temperatures during battery cycling and further deteriorating the battery's cycle performance. Therefore, in this application, by making boron carbide uniformly distributed in the coating layer, the consistency of conductivity in the positive electrode active material can be improved, the polarization and impedance of the positive electrode active material can be reduced, and the ionic / electronic conductivity of the positive electrode active material can be further improved. This will improve the specific discharge capacity of the battery at 0.33C, the discharge capacity ratio at 2C, and the capacity retention rate after 500 cycles at 45°C, that is, ensure that the battery's discharge capacity is fully released and improve the battery's cycle performance.

[0042] Please see Figure 1 As shown, in one embodiment of the present invention, a coating layer is coated on the surface of the core in the positive electrode active material. The thickness of the coating layer is, for example, 50 nm to 150 nm. If the thickness of the coating layer is less than 50 nm, it is too thin to completely cover the core; if the thickness of the coating layer is greater than 150 nm, it will increase the mass ratio of the coating layer in the positive electrode active material, thereby affecting the specific capacity of the positive electrode active material, and consequently affecting the energy density and cycle life of the battery.

[0043] Please see Figure 1 As shown, in one embodiment of the present invention, the core of the positive electrode active material includes at least one sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7, NFPP) particle. When there are multiple NFPP particles, the multiple NFPP particles aggregate to form the core. The median particle size of the core is, for example, 0.5 μm-2 μm.

[0044] Please see Figures 1 to 2 As shown, based on the above-mentioned positive electrode active material, the present invention also proposes a method for preparing a positive electrode active material, which includes at least steps S11-S13.

[0045] Step S11: Mix sodium source, iron source and phosphorus source to obtain a solid mixture, disperse it in a solvent, and then perform wet ball milling to obtain a slurry.

[0046] Step S12: After drying the slurry, a precursor is obtained, and a carbon source and boron carbide are added for dry ball milling to obtain an intermediate material.

[0047] Step S13: Sinter the intermediate material to obtain the positive electrode active material.

[0048] In one embodiment of the present invention, in step S11, the sodium source, iron source, and phosphorus source are mixed uniformly to obtain a solid mixture. The sodium source includes, for example, at least one of sodium carbonate, sodium bicarbonate, sodium sulfate, sodium nitrate, and sodium acetate; the iron source includes, for example, at least one of ferrous oxalate, ferrous phosphate, ferrous chloride, and ferrous nitrate; and the phosphorus source includes, for example, at least one of sodium dihydrogen phosphate and sodium pyrophosphate.

[0049] In one embodiment of the present invention, after obtaining the solid mixture, in step S11, a solvent is added to the solid mixture, and then wet ball milling is performed to obtain a slurry. The solvent includes, for example, at least one of deionized water, ethanol, and ethylene glycol; the mass ratio of the solid mixture to the solvent is, for example, 1:(3-6); the rotation speed of the wet ball mill is, for example, 300 r / min-500 r / min; and the wet ball milling time is, for example, 6 h-10 h.

[0050] In one embodiment of the present invention, after obtaining the slurry, in step S12, the slurry is dried to obtain a precursor. Then, the precursor is placed in an inert gas atmosphere and heated to a first preset temperature at a first preset rate, and held at that temperature for a first preset time to obtain NFPP particles. The drying temperature of the slurry is, for example, 100℃-150℃, the drying time is, for example, 16h-24h, the first preset rate is, for example, 1℃ / min-5℃ / min, the first preset temperature is, for example, 450℃-550℃, and the first preset time is, for example, 8h-10h.

[0051] In one embodiment of the present invention, after obtaining NFPP particles, in step S12, for example, the NFPP particles are placed in an air jet mill to pulverize the NFPP particles to obtain NFPP particles with smaller particle sizes. The pulverization time is, for example, 12-84 hours, and the air pressure in the air jet mill is, for example, 5-12 bar.

[0052] In one embodiment of the present invention, after pulverizing NFPP particles, in step S12, a carbon source and boron carbide are added to the NFPP particles for dry ball milling to obtain an intermediate material. The carbon source includes, for example, at least one of citric acid, glucose, and oxalic acid; the boron carbide is, for example, nano-boron carbide; the mass ratio of sodium source, iron source, phosphorus source, carbon source, and boron carbide is, for example, 8.48:20.88:4.6:15.36:1; and the dry ball milling time is, for example, 1-12 hours.

[0053] In one embodiment of the present invention, after obtaining the intermediate material, in step S13, the intermediate material is placed in an inert gas atmosphere, heated to a second preset temperature at a second preset rate, and held at that temperature for a second preset time to obtain the positive electrode active material. The second preset rate is, for example, 1℃ / min-5℃ / min, the second preset temperature is, for example, 450℃-550℃, and the second preset time is, for example, 8h-10h.

[0054] Based on the above-described positive electrode active material and its preparation method, this invention also provides a battery, such as a sodium-ion battery, which can be a primary or secondary battery. A secondary battery can be a pouch battery, a prismatic hard-shell battery, or a cylindrical battery, etc. This invention does not specifically limit the type or category of sodium-ion batteries. The sodium-ion battery includes a positive electrode, a separator, a negative electrode, and an electrolyte.

[0055] In one embodiment of the present invention, the positive electrode sheet includes a positive current collector and a positive electrode material layer coated on at least one surface of the positive current collector. The positive current collector is, for example, a foil formed by surface treatment of materials such as nickel, titanium, aluminum, silver, stainless steel, or carbon. Besides foil, the positive current collector can also be used in any one or more combinations of various forms such as film, mesh, porous, foam, or nonwoven fabric.

[0056] In one embodiment of the present invention, the positive electrode material layer includes a positive electrode active material, a positive electrode conductive agent, a positive electrode binder, and a positive electrode dispersant, etc. The positive electrode active material is the positive electrode active material mentioned above or the positive electrode active material obtained according to the preparation method mentioned above, which will not be elaborated here. The mass ratio of the positive electrode active material, the positive electrode conductive agent, the positive electrode binder, and the positive electrode dispersant can be selected according to actual needs, for example (90-97.3):(1-3.5):(1.5-4):(0.01-1). The positive electrode conductive agent is selected from at least one of conductive carbon black (Super P), acetylene black, nano-metal powder, carbon nanotubes or graphene, the positive electrode binder is selected from at least one of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene, polytetrafluoroethylene (PTFE), sodium carboxymethyl cellulose or styrene-butadiene rubber (SBR), and the positive electrode dispersant is selected from at least one of sodium citrate, potassium sodium tartrate or tributyl phosphate.

[0057] In one embodiment of the present invention, the positive electrode current collector is, for example, aluminum foil; the positive electrode conductive agent is, for example, acetylene black; the positive electrode binder is, for example, PTFE; and the positive electrode dispersant is, for example, sodium citrate. Specifically, the positive electrode active material, acetylene black, PTFE, and sodium citrate are added to a mixing tank in a mass ratio of, for example, 93:2:4:1 for mixing. Then, an organic solvent is added at a solid content of 45wt%-70wt%, and the mixture is stirred evenly to obtain a positive electrode slurry. Then, 0.2g / 1540.25mm... 2 -0.4g / 1540.25mm 2 The areal density is determined by uniformly coating the positive electrode slurry onto aluminum foil, followed by coating, drying, rolling, and slitting processes to obtain the positive electrode sheet. The organic solvent used is, for example, N-methylpyrrolidone. The thickness of the positive electrode material layer on the current collector side after rolling is, for example, 100μm-360μm, and the compaction density of the positive electrode sheet is, for example, 1.5g / cm³. 3 -2.5g / cm 3 .

[0058] In one embodiment of the present invention, the negative electrode sheet is, for example, an indium sheet, a lithium sheet, an aluminum sheet, or an alloy sheet composed of at least two of the above metals. In other embodiments of the present invention, the negative electrode sheet further includes, for example, a negative current collector and a negative active layer coated on at least one surface of the negative current collector. The negative current collector is, for example, a copper foil current collector, a composite copper foil current collector, a carbon current collector, a foamed copper current collector, or a stainless steel current collector, etc., and the negative active layer includes a negative active material, a negative conductive agent, a negative binder, and a negative dispersant, etc. The mass ratio of the negative active material, the negative conductive agent, the negative binder, and the negative dispersant can be selected according to actual needs, specifically, for example, (93-98):(0.2-1.5):(1-3):(0.01-1.5). In this embodiment, the negative electrode active material is selected from at least one of graphite, silicon, or hard carbon, the negative electrode conductive agent is selected from at least one of Super P, acetylene black, nano silver powder, carbon nanotubes, or graphene, the negative electrode binder is selected from at least one of PVDF, polyvinylidene fluoride-hexafluoropropylene, PTFE, or SBR, and the negative electrode dispersant includes, for example, sodium carboxymethyl cellulose (CMC-Na).

[0059] In one embodiment of the present invention, the negative electrode current collector is, for example, copper foil; the negative electrode active material is, for example, hard carbon; the negative electrode conductive agent is, for example, Super P; the negative electrode binder is, for example, SBR; and the negative electrode dispersant is, for example, CMC-Na. Specifically, the negative electrode active material, negative electrode conductive agent, negative electrode binder, and negative electrode dispersant are mixed in a mass ratio of 97:1:1:1, and deionized water solvent is added at a solid content of 45wt%-70wt%. The mixture is then thoroughly stirred and mixed uniformly under vacuum to obtain a negative electrode slurry. Then, a 0.13g / 1540.25mm... 2 -0.22g / 1540.25mm 2 The areal density is determined by coating the negative electrode slurry onto copper foil, followed by coating, drying, rolling, and slitting processes to obtain the negative electrode sheet. The thickness of the negative electrode active layer on the current collector side after rolling is, for example, 100μm-260μm, and the compaction density of the negative electrode sheet is, for example, 0.8g / cm³. 3 -1.2g / cm 3 .

[0060] In one embodiment of the present invention, a separator is positioned between the positive and negative electrode plates to prevent short circuits between them, allowing lithium ions to pass through. The separator can be, for example, a conventional separator, a ceramic separator, a polymer separator, a non-woven fabric separator, or an inorganic-organic composite separator. Specifically, the separator can be, for example, a single-layer polypropylene (PP) membrane, a single-layer polyethylene (PE) membrane, a double-layer PP / PE membrane, a double-layer PP / PP membrane, or a triple-layer PP / PE / PP membrane. In this embodiment, a single-layer PP membrane is selected as the separator.

[0061] In one embodiment of the present invention, an electrolyte is filled between the positive electrode, the separator, and the negative electrode to conduct ions. The electrolyte comprises at least a solvent and a sodium salt. The solvent includes, for example, at least one selected from diethyl carbonate, fluoroethylene carbonate, difluoroethyl acetate, dimethyl carbonate, ethylene carbonate, propylene carbonate, or methyl ethyl carbonate. The sodium salt includes at least one selected from sodium hexafluorophosphate (NaPF6), sodium perchlorate (NaClO4), sodium bis(fluorosulfonyl)imide (NaFSI), or sodium bis(trifluoromethanesulfonyl)imide (NaTFSI), and the sodium salt content in the electrolyte is, for example, 10 wt% to 18 wt%.

[0062] In one embodiment of the present invention, the solvent in the electrolyte includes, for example, diethyl carbonate, fluoroethylene carbonate, difluoroethyl acetate, and dimethyl carbonate, and the sodium salt is, for example, NaPF6. Specifically, diethyl carbonate, fluoroethylene carbonate, difluoroethyl acetate, dimethyl carbonate, and NaPF6 are mixed uniformly in a mass ratio of 60:10:10:6:14 to obtain the electrolyte.

[0063] In one embodiment of the present invention, the above-mentioned positive electrode, separator and negative electrode are placed in sequence, with the positive electrode material layer and the negative electrode active layer facing the separator, so that the separator is in the middle of the positive electrode and the negative electrode to play a role in isolation, and is put into an aluminum-plastic film. After being fully baked so that the water content is below 450ppm, the electrolyte injection, formation, sealing and inspection processes are completed to obtain a sodium-ion battery.

[0064] The present invention will be explained in more detail below by referring to embodiments, which should not be construed as limiting. Appropriate modifications can be made within the scope of the present invention, and all such modifications fall within the technical scope of the present invention.

[0065] Example 1

[0066] Preparation of the positive electrode active material: 212g sodium carbonate, 522g ferrous oxalate, and 115g sodium dihydrogen phosphate were mixed evenly to obtain a solid mixture. The solid mixture was then dispersed in deionized water at a mass ratio of 1:4 (solid mixture:water). The mixture was then wet-milled at 300 rpm for 6 hours to obtain a slurry. The slurry was then dried at 110℃ for 16 hours to obtain a precursor. This precursor was then sintered in a tube furnace under argon protection at a temperature increased to 500℃ at a rate of 2℃ / min for 8 hours to obtain NFPP. This NFPP was then treated in an air jet mill at a pressure of 5 bar for 24 hours to obtain NFPP with a smaller particle size. 384g citric acid and 25g nano-boron carbide were added to the pulverized NFPP. The mixture was dry-milled for 6 hours and then sintered in a tube furnace under argon protection at a temperature increased to 500℃ at a rate of 2℃ / min for 10 hours to obtain the positive electrode active material. The positive electrode active material has the molecular formula NFPP·C@0.04C4B, with an NFPP particle core and a coating layer consisting of boron carbide and carbon.

[0067] Preparation of the positive electrode sheet: The positive electrode active material, acetylene black, PTFE, and sodium citrate were added to a mixing tank in a mass ratio of 93:2:4:1 and mixed. Then, N-methylpyrrolidone was added at a solid content of 45 wt%, and the mixture was stirred until homogeneous to obtain the positive electrode slurry. Next, 0.2 g / 1540.25 mm... 2 The areal density is achieved by uniformly coating the positive electrode slurry onto one side of an aluminum foil. After coating, drying, rolling, and slitting processes, a positive electrode sheet is obtained, forming a positive electrode material layer on one side of the aluminum foil. The thickness of the positive electrode material layer after rolling is 100 μm, and the compaction density of the positive electrode sheet is 1.5 g / cm³. 3 .

[0068] Preparation of the negative electrode sheet: Hard carbon, Super P, SBR, and CMC-Na were mixed in a mass ratio of 97:1:1:1, and deionized water was added at a solid content of 45 wt%. The mixture was then thoroughly stirred and homogenized under vacuum to obtain a negative electrode slurry. Then, the slurry was prepared at a ratio of 0.13 g / 1540.25 mm. 2 The areal density is determined by coating the negative electrode slurry onto one side of a copper foil. After coating, drying, rolling, and slitting processes, a negative electrode sheet is obtained, forming a negative electrode active layer on one side of the copper foil. The thickness of the negative electrode active layer after rolling is 100 μm, and the compaction density of the negative electrode sheet is 0.8 g / cm³. 3 .

[0069] Preparation of electrolyte: 60g of diethyl carbonate, 10g of fluoroethylene carbonate, 10g of difluoroethyl acetate, 6g of dimethyl carbonate and 14g of NaPF6 were mixed evenly to obtain the electrolyte.

[0070] Preparation of the diaphragm: A single-layer PP membrane was selected as the diaphragm.

[0071] Battery fabrication: The positive electrode, separator, and negative electrode are placed sequentially, with the positive electrode material layer and the negative electrode active layer facing the separator. The separator is positioned between the positive and negative electrode to act as a separator. The battery is then encased in an aluminum-plastic film and thoroughly baked until the water content is below 450 ppm. Electrolyte injection, formation, sealing, and inspection are then completed to obtain the sodium-ion battery.

[0072] Particle size measurement of the core and boron carbide in the positive electrode active material: After charging the battery to a State of Health (SOH) ≥ 95%, charging was stopped. The battery was then disassembled, the positive electrode sheet was removed and cleaned, and cross-sectional composition analysis was performed on the positive electrode sheet using a scanning electron microscope (SEM). At least one of Na, Fe, and P was used as a characteristic element to identify the core, and the median particle size of the core was measured and statistically analyzed. Boron was used as a characteristic element to identify boron carbide particles, and the particle size of the boron carbide particles was measured. In this embodiment, the median particle size of the core was 3 μm, and the particle size of boron carbide was 120 nm.

[0073] Content testing of the core and coating layer in the positive electrode active material: After charging the battery to SOH ≥ 95%, charging was stopped. The battery was then disassembled, the positive electrode sheet was removed and cleaned, and at least 500g of the positive electrode material layer was scraped from the surface of the positive electrode sheet. Inductively Coupled Plasma (ICP) and Thermogravimetric Analysis (TGA) tests were performed to determine the content of the core, boron carbide, and carbon in the positive electrode active material. In this embodiment, the core content in the positive electrode active material is 90wt%, the boron carbide content is 4wt%, and the carbon content is 6wt%.

[0074] Measurement of the coating thickness in the positive electrode active material: After charging the battery to SOH ≥ 95%, charging was stopped. The battery was then disassembled, the positive electrode sheet was removed and cleaned, and at least 500g of the positive electrode material layer on the surface of the positive electrode sheet was scraped for sample preparation and testing. The thickness of the coating layer could be measured using a transmission electron microscope (TEM). In this embodiment, the coating thickness was 160nm.

[0075] Uniformity test of boron carbide distribution in the coating layer of the positive electrode active material: The battery was charged to SOH ≥ 95% and then charging was stopped. The battery was then disassembled, the positive electrode sheet was removed and cleaned. The cross-sectional composition of the positive electrode sheet was analyzed using a scanning electron microscope (SEM). At least one of Na, Fe and P was used as a characteristic element to identify the core, boron and carbon were used as characteristic elements to identify the coating layer, and boron carbide particles were used as a characteristic element. The surface of the core was evenly divided into 16 square sites with a perimeter of 100 nm * 100 nm. Each square site included part of the surface of the core and the coating layer on its surface. Then, the coating layer in each site was tested by energy dispersive X-ray spectroscopy (EDS) to measure the boron content per unit area on the surface of the core in different sites, and the average boron content C and maximum boron content C in all sites were calculated. max and minimum content C min Then according to (C) max -C min The boron content distribution in the coating layer at 16 sites on the surface of the core is calculated using the ) / C method. Furthermore, the positive electrode can be tested at least 10 times according to the above testing procedure to obtain the average value of the content distribution difference, thereby reducing testing errors. In this embodiment, the boron content distribution difference in the coating layer at the 16 sites on the surface of the core is 12%.

[0076] Example 2

[0077] The amount of boron carbide nanoparticles added was changed to 19g. The molecular formula of the positive electrode active material was: NFPP·C@0.03C4B. The content of boron carbide in the positive electrode active material was 3wt%. Other steps were the same as in Example 1.

[0078] Example 3

[0079] The amount of boron carbide nanoparticles added was changed to 6g. The molecular formula of the positive electrode active material was: NFPP·C@0.01C4B. The content of boron carbide in the positive electrode active material was 1wt%. Other steps were the same as in Example 1.

[0080] Example 4

[0081] The processing time of NFPP by air jet mill was extended to 36 hours, so that the median particle size of the kernel was 2 μm. Other steps were the same as in Example 1.

[0082] Example 5

[0083] The processing time of NFPP by air jet mill was extended to 72 hours, so that the median particle size of the kernel was 0.5 μm. Other steps were the same as in Example 1.

[0084] Example 6

[0085] The processing time of NFPP by air jet mill was extended to 72 hours, and the air pressure was increased to 12 bar to make the median particle size of the core 0.2 μm. Other steps were the same as in Example 1.

[0086] Example 7

[0087] Boron carbide with a particle size of 100 nm was selected, and the other steps were the same as in Example 1.

[0088] Example 8

[0089] Boron carbide with a particle size of 50 nm was selected, and the other steps were the same as in Example 1.

[0090] Example 9

[0091] Boron carbide with a particle size of 10 nm was selected, and the other steps were the same as in Example 1.

[0092] Example 10

[0093] Boron carbide with a particle size of 5 nm was selected, and the other steps were the same as in Example 1.

[0094] Example 11

[0095] The processing time of NFPP by air jet mill was extended to 36 hours to achieve a median particle size of 2 μm in the core, and boron carbide with a particle size of 50 nm was selected. Other steps were the same as in Example 1.

[0096] Example 12

[0097] The dry ball milling time was extended to 6.5 hours to make the coating thickness 150 nm. Other steps were the same as in Example 1.

[0098] Example 13

[0099] The dry ball milling time was extended to 9 hours to make the coating thickness 80 nm. Other steps were the same as in Example 1.

[0100] Example 14

[0101] The dry ball milling time was extended to 10 hours to make the coating thickness 50 nm. Other steps were the same as in Example 1.

[0102] Example 15

[0103] The dry ball milling time was extended to 12 hours to make the coating thickness 40 nm. Other steps were the same as in Example 1.

[0104] Example 16

[0105] The processing time of NFPP by air jet mill was extended to 36 hours to make the median particle size of the core 2 μm, and boron carbide with a particle size of 50 nm was selected. The dry ball milling time was extended to 9 hours to make the coating thickness 80 nm. Other steps were the same as in Example 1.

[0106] Example 17

[0107] Before adding citric acid and boron carbide, the citric acid and boron carbide were dry-mixed and ball-milled for 2 hours. Then, the pulverized NFPP particles were added in two batches: half of the NFPP was added first, and after ball milling for 2 hours, the remaining half of the NFPP was added and ball milling was continued for 2 hours. This resulted in a 10% difference in the distribution of boron content in the coating layer among the 16 sites on the surface of the core. The other steps were the same as in Example 1.

[0108] Example 18

[0109] Before adding citric acid and boron carbide, the citric acid and boron carbide were dry-mixed and ball-milled for 2 hours. Then, the pulverized NFPP particles were added in two batches: half of the NFPP was added first, and after ball milling for 3 hours, the remaining half of the NFPP was added, and ball milling was continued for 3 hours. This resulted in a 5% difference in the distribution of boron content in the coating layer among the 16 sites on the surface of the core. The other steps were the same as in Example 1.

[0110] Example 19

[0111] Before adding citric acid and boron carbide, the citric acid and boron carbide were dry-mixed and ball-milled for 2 hours. Then, the pulverized NFPP particles were added in two batches: half of the NFPP was added first, and after ball milling for 3 hours, the remaining half of the NFPP was added, and ball milling continued for 6 hours. This resulted in a 5% difference in the distribution of boron content in the coating layer among the 16 sites on the surface of the core. The other steps were the same as in Example 16.

[0112] Comparative Example 1

[0113] The amount of boron carbide nanoparticles added was changed to 3g. The molecular formula of the positive electrode active material was: NFPP·C@0.005C4B. The content of boron carbide in the positive electrode active material was 0.5wt%. Other steps were the same as in Example 1.

[0114] Comparative Example 2

[0115] The amount of boron carbide nanoparticles added was changed to 0, and the other steps were the same as in Example 1.

[0116] Comparative Example 3

[0117] The amount of boron carbide nanoparticles added was changed to 38g. The molecular formula of the positive electrode active material was: NFPP·C@0.06C4B. The content of boron carbide in the positive electrode active material was 6wt%. Other steps were the same as in Example 1.

[0118] Comparative Example 4

[0119] In the preparation of the positive electrode active material, boron carbide nanoparticles are not added. However, in the preparation of the positive electrode sheet, boron carbide nanoparticles, the positive electrode active material, acetylene black, PTFE, and sodium citrate are mixed together, and the other steps are the same as in Example 1. The amount of boron carbide nanoparticles added remains 25g, and the mass ratio of the positive electrode active material, acetylene black, PTFE, and sodium citrate remains 93:2:4:1.

[0120] In this invention, the specific capacity of the batteries in Examples 1-19 and Comparative Examples 1-4 was tested, and the test results are shown in Table 1. Specifically, the batteries were subjected to a constant current discharge of 0.33C (1C = 1 nominal capacity) within the range of 1.5V-3.5V at 25°C. The specific capacity at 0.33C was obtained by dividing the discharge capacity by the mass of the positive electrode active material.

[0121] In this invention, the batteries in Examples 1-16 and Comparative Examples 1-4 were subjected to discharge capacity tests, and the test results are shown in Table 1. Specifically, at 25°C, the batteries were sequentially charged at a constant current and constant voltage of 0.33C (1C = 1 nominal capacity) within a range of 1.5V-3.5V, and then discharged at different rates of 0.33C and 2C to obtain the discharge capacity at 0.33C and 2C respectively. Then, the discharge capacity at 0.33C was taken as 100% to obtain the percentage of 2C discharge capacity.

[0122] In one embodiment of the present invention, the batteries in Examples 1-19 and Comparative Examples 1-4 were subjected to cycle performance tests, and the test results are shown in Table 1. Specifically, at 45°C, the batteries were sequentially charged at a constant current and constant voltage of 1C (1C = 1 nominal capacity) within a range of 1.5V-3.5V, and then discharged at 1C, with the battery capacity C0 recorded. The above charge-discharge steps were repeated for 500 cycles, and the battery discharge capacity C1 after 500 cycles was recorded. The battery capacity retention rate was calculated according to the following formula:

[0123] Capacity retention rate = C1 / C0 * 100%.

[0124] Table 1. Performance test results of Examples 1-19 and Comparative Examples 1-4

[0125]

[0126] Please refer to Table 1. Comparing Examples 1-3 and Comparative Example 2, it can be seen that when the coating layer contains boron carbide, the 0.33C discharge specific capacity, the 2C discharge capacity ratio, and the capacity retention rate at 45°C significantly increase. This indicates that incorporating boron carbide into the coating layer can ensure sufficient release of the battery's discharge capacity and improve the battery's cycle performance. Moreover, comparing Examples 1-3 and Comparative Example 4, it can be seen that compared to preparing the cathode material by mechanically mixing nano-boron carbide and sodium iron pyrophosphate particles, this application, by coating sodium iron pyrophosphate particles with nano-boron carbide before preparing the cathode material, significantly increases the 0.33C discharge specific capacity, the 2C discharge capacity ratio, and the capacity retention rate at 45°C, indicating that the battery's discharge and cycle performance are improved.

[0127] Please refer to Table 1. Comparing Examples 1-3, Comparative Example 1, and Comparative Example 3, it can be seen that when the boron carbide content in the coating layer is less than 1 wt%, the 0.33C discharge capacity, the 2C discharge capacity percentage, and the 45°C capacity retention rate decrease significantly. Moreover, as the boron carbide content in the coating layer increases from 1 wt% to 4 wt%, the 0.33C discharge capacity, the 2C discharge capacity percentage, and the 45°C capacity retention rate gradually increase. However, as the boron carbide content in the coating layer continues to increase from 4 wt% to 6 wt%, the 0.33C discharge capacity and the 45°C capacity retention rate decrease. This indicates that by controlling the boron carbide content in the coating layer between 1 wt% and 4 wt%, the discharge performance and cycle performance of the battery can be improved.

[0128] Please refer to Table 1. Comparing Examples 1 and 4-6, it can be seen that when the median particle size of the core is less than 0.5 μm, although the proportion of 0.33C discharge capacity and 2C discharge capacity increases slightly, the capacity retention rate at 45°C decreases significantly. When the median particle size of the core is greater than 2 μm, the 0.33C discharge capacity, the proportion of 2C discharge capacity, and the capacity retention rate at 45°C decrease. This shows that by controlling the median particle size of the core between 0.5 μm and 2 μm, both the discharge performance and cycle performance of the battery can be balanced.

[0129] Please refer to Table 1. Comparing Examples 1 and 7-10, it can be seen that when the boron carbide particle size is less than 10 nm, the 0.33C discharge specific capacity, the 2C discharge capacity ratio, and the capacity retention rate at 45°C are significantly reduced; when the boron carbide particle size is greater than 100 nm, the 0.33C discharge specific capacity, the 2C discharge capacity ratio, and the capacity retention rate at 45°C are also reduced. This shows that by controlling the boron carbide particle size to 10 nm-100 nm, the discharge performance and cycle performance of the battery can be improved.

[0130] Please refer to Table 1. Comparing Examples 1 and 12-15, it can be seen that when the thickness of the coating layer increases from 40nm to 150nm, the specific capacity of 0.33C discharge, the proportion of 2C discharge capacity, and the capacity retention rate at 45°C gradually increase. However, when the thickness of the coating layer continues to increase from 150nm to 160nm, the specific capacity of 0.33C discharge, the proportion of 2C discharge capacity, and the capacity retention rate at 45°C decrease. This indicates that by controlling the thickness of the coating layer to 50nm-150nm, the discharge performance and cycle performance of the battery can be improved.

[0131] Please refer to Table 1. Comparing Examples 1 and 17-18, it can be seen that when the distribution difference of boron content in the coating layer is ≤10%, the 0.33C discharge capacity remains almost unchanged, while the 2C discharge capacity ratio and the capacity retention rate at 45°C increase significantly. This indicates that by controlling the distribution difference of boron content in the coating layer to ≤10%, the discharge performance and cycle performance of the battery can be improved.

[0132] Please refer to Table 1. Comparing Examples 1, 4, 11, 16, and 19, it can be seen that, compared to individually controlling the median particle size of the core, the particle size of boron carbide, the thickness of the coating layer, or the distribution difference of boron content in the coating layer, simultaneously controlling the median particle size of the core to 2 μm, the particle size of boron carbide to 50 nm, the thickness of the coating layer to 80 nm, and the distribution difference of boron content in the coating layer ≤10%, the 0.33C discharge specific capacity, the 2C discharge capacity ratio, and the capacity retention rate at 45°C are the highest. This indicates that by simultaneously controlling the median particle size of the core to 0.5 μm-2 μm, the particle size of boron carbide to 10 nm-100 nm, the thickness of the coating layer to 50 nm-150 nm, and the distribution difference of boron content in the coating layer ≤10%, the discharge performance and cycle performance of the battery can be further improved.

[0133] This invention also provides an electronic device comprising at least one of the aforementioned batteries, the batteries being used to provide electrical energy. The electronic device can be a vehicle, mobile phone, portable device, laptop computer, ship, spacecraft, electric toy, or power tool, etc. In one embodiment of this invention, the vehicle is, for example, a new energy vehicle, which can be a pure electric vehicle, a hybrid electric vehicle, or a range-extended electric vehicle, etc. Spacecraft include airplanes, rockets, space shuttles, and spacecraft, etc. Electric toys include stationary or mobile electric toys, such as game consoles, electric car toys, electric ship toys, and electric airplane toys, etc. Power tools include metal cutting power tools, grinding power tools, assembly power tools, and railway power tools, such as electric drills, electric grinders, electric wrenches, electric screwdrivers, electric hammers, impact drills, concrete vibrators, and electric planers, etc. The electronic device includes the aforementioned battery, and therefore the advantages of including the aforementioned battery will not be elaborated upon here.

[0134] In summary, this invention proposes a positive electrode active material, its preparation method, and its application. A coating layer is formed on the surface of sodium iron pyrophosphate particles, which improves the electrochemical activity, conductivity, and corrosion resistance of the positive electrode active material, thereby enhancing the rate performance and cycle stability of the battery. Furthermore, the coating layer isolates the sodium iron pyrophosphate particles from direct contact with the electrolyte, suppressing side reactions and further improving the cycle performance of the positive electrode active material. In addition, the high-hardness boron carbide in the coating layer reduces damage to the carbon material in the coating layer during crushing and grading, increasing the bonding strength between the core and the coating layer. This prevents a decrease in the tap density of the positive electrode active material, ensuring that the positive electrode active material can fully exert its role in improving the rate performance and cycle stability of the battery.

[0135] The above description is merely a preferred embodiment of this application and an explanation of the technical principles employed. Those skilled in the art should understand that the scope of the invention involved in this application is not limited to the technical solutions formed by specific combinations of the above-described technical features. It should also cover other technical solutions formed by arbitrary combinations of the above-described technical features or their equivalents without departing from the inventive concept. For example, technical solutions formed by substituting the above-described features with (but not limited to) technical features with similar functions disclosed in this application. Except for the technical features described in the specification, the remaining technical features are known to those skilled in the art. To highlight the innovative features of this invention, the remaining technical features will not be described further here.

Claims

1. A positive electrode active material, characterized by, At least comprising: a core comprising at least one sodium ferric pyrophosphate particle; and a coating layer covering at least part of a surface of the core, and a material of the coating layer comprising boron carbide and carbon; wherein a content of the core in the positive electrode active material is 90wt%-98wt%, a content of the boron carbide in the positive electrode active material is 1wt%-4wt%, and a content of the carbon in the positive electrode active material is 1wt%-6wt%.

2. The positive electrode active material according to claim 1, characterized by The boron carbide is uniformly distributed in the coating layer, and a difference in distribution of a content of boron element in the coating layer is ≤10%.

3. The positive electrode active material according to claim 1, characterized by A median particle size of the core is 0.5μm-2μm.

4. The positive electrode active material according to claim 1, characterized by A thickness of the coating layer is 50nm-150nm.

5. The positive electrode active material according to claim 4, characterized by A particle size of the boron carbide is 10nm-100nm.

6. A method for producing the positive electrode active material according to any one of claims 1 to 5, characterized by, At least comprising the following steps: mixing a sodium source, an iron source and a phosphorus source, obtaining a solid mixture, dispersing the solid mixture in a solvent, then performing wet ball milling to obtain a slurry; after drying the slurry, obtaining a precursor, and adding a carbon source and boron carbide to perform dry ball milling to obtain an intermediate material; and sintering the intermediate material to obtain the positive electrode active material.

7. The preparation method according to claim 6, characterized in that, At least comprising one of the following features: a mass ratio of the solid mixture to the solvent is 1:(3-6); a rotation speed of the wet ball milling is 300r / min-500r / min; a time of the wet ball milling is 6h-10h; a temperature of drying the slurry is 100℃-150℃; a time of drying the slurry is 16h-24h.

8. The preparation method according to claim 6, characterized in that, When sintering the intermediate material, the intermediate material is placed in an inert gas atmosphere, heated to a preset temperature at a preset rate, and kept at the preset temperature for a preset time to obtain the positive electrode active material.

9. The production method according to claim 8, characterized by, At least comprising one of the following features: the preset rate is 1℃ / min-5℃ / min; the preset temperature is 450℃-550℃; the preset time is 8h-10h.

10. A battery, characterized by At least comprising: a positive electrode tab at least comprising a positive electrode material, the positive electrode material comprising the positive electrode active material according to any one of claims 1-5, or obtained by the preparation method according to any one of claims 6-9; a negative electrode tab; a separator arranged between the positive electrode tab and the negative electrode tab; and an electrolyte filled between the positive electrode tab, the negative electrode tab and the separator.

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