Nitrogen-doped iron phosphide and preparation method thereof, negative electrode material, secondary battery and electric device

Abstract

The application discloses nitrogen-doped iron phosphide and a preparation method thereof, a negative electrode material, a secondary battery and an electric equipment, and the preparation method comprises the following steps: S1, dissolving iron salt and urea in an organic solvent to obtain a mixed solution; then, performing a solvothermal reaction on the mixed solution to obtain an iron phosphide precursor; and S2, performing phosphorization treatment on the iron phosphide precursor by using a hypophosphite under a protective atmosphere to obtain the nitrogen-doped iron phosphide. The nitrogen-doped iron phosphide prepared by the method overcomes the problems of large irreversible capacity and low electronic conductivity of the iron phosphide negative electrode material.

Description

Technical Field

[0001] This invention relates to the field of secondary battery technology, and in particular to a nitrogen-doped iron phosphide and its preparation method, a negative electrode material, a secondary battery, and electrical equipment. Background Technology

[0002] Among existing lithium energy storage anode materials, iron phosphide (FeP2), a transition metal phosphide, stands out due to its high specific capacity (926 mAh·g). -1 Iron phosphide, with its advantages of low cost and high efficiency, has become a potential choice for anode materials in lithium-ion batteries. However, iron phosphide undergoes irreversible volume expansion during charge and discharge, leading to repeated rupture and recombination of the SEI film, resulting in the loss of active lithium and the generation of gas. In addition, iron phosphide itself has poor electronic conductivity, which severely limits its commercial application.

[0003] Therefore, it is necessary to modify iron phosphide to overcome its defects of poor conductivity and large volume change during charging and discharging. Summary of the Invention

[0004] This invention provides a method for preparing nitrogen-doped iron phosphide, which solves the problems of large irreversible capacity and low electronic conductivity of iron phosphide anode materials.

[0005] In a first aspect, the present invention provides a method for preparing nitrogen-doped iron phosphide, comprising the following steps:

[0006] S1. Iron salt and urea are dissolved in an organic solvent to obtain a mixture; then the mixture is subjected to a solvothermal reaction to obtain an iron phosphide precursor;

[0007] S2. Under a protective atmosphere, the iron phosphide precursor is phosphated with hypophosphite to obtain the nitrogen-doped iron phosphide.

[0008] Further, in step S1: the iron salt includes at least one of ferrous chloride, ferric chloride, ferrous sulfate, ferric sulfate, ferrous nitrate, ferric nitrate, ferrous acetate, and ferric acetate; and / or,

[0009] The mass ratio of the iron salt to the urea is 1-2:0.1-2.

[0010] Further, in step S1: the organic solvent includes at least one of ethylene glycol, N,N-dimethylformamide, and N-methylpyrrolidone; and / or,

[0011] The organic solvent includes ethylene glycol and N,N-dimethylformamide, and the volume ratio of ethylene glycol to N,N-dimethylformamide is 10-30:10-30.

[0012] Further, in step S1: first, iron salt and urea are dissolved in water to obtain an aqueous solution, then the aqueous solution is mixed with the organic solvent to obtain the mixture; and / or,

[0013] In the mixture, the concentration of the iron salt is 0.02–0.04 g / mL, and the concentration of the urea is 0.002–0.04 g / mL.

[0014] Further, in step S1: the temperature of the solvothermal reaction is 120–180°C, and the time is 5–8 hours; and / or,

[0015] After the solvothermal reaction, the process also includes collecting the product, washing and drying it, and then grinding it.

[0016] Further, in step S2, the protective atmosphere includes at least one of nitrogen and argon; and / or,

[0017] The hypophosphite includes at least one of sodium hypophosphite and potassium hypophosphite; and / or,

[0018] The mass ratio of the iron phosphide precursor to the hypophosphite is 1:8–20; and / or,

[0019] The phosphating treatment is carried out at a temperature of 320–380°C for 1–3 hours.

[0020] Secondly, the present invention provides nitrogen-doped iron phosphide, wherein the nitrogen-doped iron phosphide has a particle size D50 of 2-3 μm, a D90 of 6-8 μm, and a specific surface area of ​​15-25 m². 2 / g.

[0021] Thirdly, the present invention provides a negative electrode material comprising nitrogen-doped iron phosphide prepared by the aforementioned method, or comprising the aforementioned nitrogen-doped iron phosphide.

[0022] Fourthly, the present invention provides a secondary battery comprising a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode comprises the aforementioned negative electrode material.

[0023] Fifthly, the present invention provides an electrical device including the aforementioned secondary battery.

[0024] Through one or more embodiments of the above embodiments of the present invention, at least the following technical effects can be achieved:

[0025] This invention uses urea as a nitrogen source to prepare nitrogen-doped iron phosphide through solvothermal reaction and low-temperature phosphating reaction. The introduction of nitrogen helps to build a porous structure inside the material, which improves the volume expansion and fragmentation of iron phosphide during charge and discharge, and solves the defect of large irreversible capacity. At the same time, the introduction of nitrogen increases the electronic conductivity of iron phosphide, reduces the material resistance, effectively improves the rate performance and cycle performance. Attached Figure Description

[0026] The technical solution and other beneficial effects of the present invention will become apparent from the following detailed description of specific embodiments of the invention, in conjunction with the accompanying drawings.

[0027] Figure 1 Scanning electron microscope (SEM) images of FeP2 prepared in Comparative Example 3 at different magnifications;

[0028] Figure 2 Scanning electron microscope (SEM) images of N-FeP2 prepared in Example 2 at different magnifications;

[0029] Figure 3 Scanning electron microscope images of N-FeP2 prepared in Comparative Example 4 at different magnifications;

[0030] Figure 4 X-ray powder diffraction (XRD) pattern of N-FeP2 prepared in Example 4;

[0031] Figure 5 The specific capacity and coulombic efficiency of coin cells assembled using N-FeP2 in Example 2 and FeP2 in Comparative Example 3 as negative electrode materials are plotted at different current densities for 1-60 cycles; where the numbers on the upper horizontal axis represent the current density.

[0032] Figure 6 Using N-FeP2 as the negative electrode material in Examples 1-3, the assembled coin cells achieved a speed of 1000 mA·g -1 Specific capacity and coulombic efficiency diagrams for 1 to 1000 cycles at current densities;

[0033] Figure 7 This is a comparison of the rate performance of N-FeP2 prepared using different nitrogen sources in Example 1 and Comparative Examples 1-2;

[0034] Figure 8 The graph shows a comparison of the rate performance of N-FeP2 prepared under different solvent combinations in Examples 1, 4 and Comparative Example 4. Detailed Implementation

[0035] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0036] In the description of this invention, it should be noted that, unless otherwise explicitly specified and limited, the term "and / or" in this document is merely a description of the relationship between related objects, indicating that three relationships can exist. For example, A and / or B can represent: A existing alone, A and B existing simultaneously, and B existing alone. Furthermore, the character " / " in this document, unless otherwise specified, generally indicates that the preceding and following related objects have an "or" relationship.

[0037] As described in the background section, among existing lithium energy storage anode materials, transition metal phosphides have advantages such as high specific capacity and low price, making them popular candidates for lithium-ion battery anode materials. However, they suffer from low electronic conductivity and irreversible volume expansion during charging and discharging, which seriously affects the application of transition metal phosphides in lithium-ion batteries.

[0038] To address the aforementioned problems with iron phosphide, this application provides a modification method that involves doping iron phosphide with nitrogen, which not only improves the low electronic conductivity of iron phosphide but also reduces its volume expansion during charging and discharging.

[0039] Specifically, the preparation method of nitrogen-doped iron phosphide provided in this application includes the following steps:

[0040] S1. Iron salt and urea are dissolved in an organic solvent to obtain a mixture; then the mixture is subjected to a solvothermal reaction to obtain an iron phosphide precursor;

[0041] S2. Under a protective atmosphere, the iron phosphide precursor is phosphated with hypophosphite to obtain the nitrogen-doped iron phosphide.

[0042] In this application, the precursor of iron phosphide, iron hydroxyl oxide, is first prepared by solvothermal reaction. Urea is added as a nitrogen source during the solvothermal reaction, so that nitrogen element is doped into the bulk phase of iron hydroxyl oxide. Then, nitrogen-doped iron phosphide can be obtained by low-temperature phosphating reaction.

[0043] In this application, the iron salt can be a divalent iron salt and / or a trivalent iron salt. There is no limitation on the specific type of iron salt, as long as it can be dissolved in an organic solvent or dissolved in an organic solvent by some auxiliary means. Examples of iron salts include ferrous chloride, ferric chloride, ferrous sulfate, ferric sulfate, ferrous nitrate, ferric nitrate, ferrous acetate, and ferric acetate. Of course, two or more of the above iron salts can also be mixed.

[0044] The reason for using an organic solvent instead of water in this application is that the hydrothermal reaction is too fast and difficult to control, which can lead to insufficient density of the precursor crystals, easy cracking, and difficulty in controlling the uniformity of the crystals. The choice of organic solvent is primarily based on its ability to dissolve the aforementioned iron salts while ensuring a gentle solvothermal reaction, allowing the reaction product, iron hydroxyacid oxide, to nucleate and grow uniformly.

[0045] In some embodiments, the organic solvent is preferably a mixed solvent composed of two or more solvents. Compared with a single solvent, the advantage of using a mixed solvent is that it can slow down the growth rate of crystal nuclei during the precursor generation process, allowing for more uniform ion deposition, and resulting in a denser and crack-free precursor, iron hydroxyl oxide.

[0046] In some embodiments, the organic solvent is a mixture of two solvents, including a first solvent and a second solvent. The first solvent may be ethylene glycol (EG), and the second solvent may be N-methylpyrrolidone (NMP) or N,N-dimethylformamide (DMF).

[0047] In some embodiments, the volume ratio of the first solvent to the second solvent can be 10-30:10-30, for example, any value or a range between any two values ​​from 1:1, 1:2, 1:3, 2:1, 2:3, 3:1, 3:2.

[0048] Preferably, the first solvent is ethylene glycol and the second solvent is N,N-dimethylformamide.

[0049] When using organic solvents, it is preferable to combine the process with ultrasonic treatment during stirring to shorten the dissolution time and ensure that the iron salts are fully dissolved to form a homogeneous mixture. The ultrasonic treatment time is not limited, and can be, for example, 5 min, 10 min, 20 min, 30 min, 40 min, 50 min, etc.

[0050] In some embodiments, the iron salt can be first dissolved in water to form an aqueous solution, and then the aqueous solution can be mixed evenly with an organic solvent. This utilizes the high solubility of iron salt in water, which not only greatly shortens the dissolution time but also improves the homogeneity of the resulting solution. In this case, a water-miscible organic solvent, such as ethylene glycol, N,N-dimethylformamide, or N-methylpyrrolidone, is preferably selected.

[0051] It's understandable that when dissolving iron salts in water to form an aqueous solution, the amount of water should only be enough to fully dissolve the iron salts, not too much. Excessive water will cause the solvothermal reaction to proceed too quickly, affecting the quality of the precursor.

[0052] In some embodiments, the volume ratio of water to organic solvent is less than or equal to 1:4, for example, it can be any value or a range between any two values ​​from 1:10, 1:9, 1:8, 1:7, 1:6, 1:5, 1:4.

[0053] In this application, the concentration of iron salt in the mixture can be 0.02 to 0.04 g / mL, for example, any value or a range between any two of 0.02, 0.025, 0.03, 0.035, and 0.04 g / mL.

[0054] Compared to other nitrogen sources, this application selects urea as the nitrogen source. Urea exists in the form of small molecules, so it is easier to dop into the iron hydroxyl oxide precursor during the growth process, and it exists in the precursor lattice in the form of atomic doping.

[0055] In addition, urea, as a nitrogen source, not only plays a role in regulating the porosity of materials in solvothermal reactions, which helps to construct the internal pore structure of materials, but also plays a key role in the size regulation of precursor materials, which can obtain precursors with more uniform micro and nano structures.

[0056] In this application, if the amount of urea added is too small, it will not only affect the size and pore structure of the precursor, but also result in an excessively low nitrogen content in the final product, affecting the electrochemical performance of iron phosphide. If the amount of urea added is too large, it will also have a negative impact on the growth of the precursor crystals. In some embodiments, the mass ratio of iron salt to urea can be controlled as 1-2:0.1-2, for example, it can be any value or a range between any two values ​​from 20:1, 15:1, 10:1, 5:1, 2:1, 1:1, 2:3, 1:2.

[0057] In this application, the concentration of urea in the mixture can be 0.002 to 0.04 g / mL, for example, any value or a range between any two values ​​from 0.002, 0.005, 0.01, 0.02, 0.03, 0.04 g / mL.

[0058] In this application, the prepared mixture can be transferred to a polytetrafluoroethylene (PTFE) reactor for a solvothermal reaction. The temperature of the solvothermal reaction can be 120–180°C, for example, any value or any two of 120, 130, 140, 150, 160, 170, and 180°C; the reaction time can be 5–8 hours, for example, any value or any two of 5, 6, 7, and 8 hours. Within the above temperature and time range, sufficient nucleation and growth of the precursor crystals can be ensured, resulting in a precursor with a dense lattice and no cracks, while the crystal size is relatively uniform.

[0059] After the solvothermal reaction is complete and the temperature drops to room temperature, the product in the reactor is collected, washed, and dried. Centrifugation and / or filtration can be used to collect the product. Centrifugation is preferred for washing, as it effectively removes residual iron ions, organic solvents, and other substances from the surface of the precursor particles. The washing process can be repeated multiple times, using solvents such as water or ethanol.

[0060] In some embodiments, the collected product is first washed three times by centrifugation with anhydrous ethanol at a speed of 4000 rpm·min. -1 Then wash three times with deionized water and centrifuge at 6000 rpm·min. -1 .

[0061] The washed and dried precursor particles can be further ball-milled at low speed to improve particle size uniformity and consistency. This ball-milling process can be repeated multiple times at speeds of 400–600 rpm. -1 For example, 400, 450, 500, 550, 600 rpm·min -1 The value in the range or any two values ​​can be used; the time for each ball milling session can be 5 to 30 minutes, for example, any value or any two values ​​in the range of 5, 10, 15, 20, 25, and 30 minutes.

[0062] The precursor after ball milling has a smaller size and more uniform particle size, which is beneficial for thorough phosphating in subsequent processing. For phosphating, commonly used phosphorus sources in the art can be used, including but not limited to phosphorus-containing substances such as phosphorus trichloride, phosphorus pentachloride, phosphates, and hypophosphites; phosphating methods commonly used in the art can be used, including but not limited to impregnation, spraying, and heating methods.

[0063] In this application, hypophosphite is preferably used as the phosphating agent, and the precursor particles are phosphated by heating. Hypophosphite includes substances such as hypophosphite, sodium hypophosphite, and potassium hypophosphite, which decompose and release phosphine gas during heating. Phosphine has strong reducing properties and can react with metal oxides or metal salts to generate the corresponding phosphides.

[0064] In some embodiments, phosphating may be performed according to the following steps:

[0065] In a sealed container environment, precursor particles and sodium hypophosphite are placed in two separate ceramic boats, with the sodium hypophosphite upstream and the precursor particles downstream. A protective gas is then introduced into the container. The container is then heated to initiate a phosphating reaction. During this process, the upstream sodium hypophosphite decomposes, releasing phosphine gas, while the downstream precursor particles undergo a complete phosphating reaction in the phosphine gas atmosphere to form iron phosphide.

[0066] In the above reaction process, the protective gas can be nitrogen, an inert gas, or a mixture thereof, preferably argon.

[0067] In the above reaction process, the mass ratio of precursor particles to sodium hypophosphite can be 1:8 to 20, for example, any value or any range between two values ​​from 1:8, 1:10, 1:12, 1:15, 1:16, 1:18, to 1:20. If the mass ratio is less than 1:8, the amount of sodium hypophosphite will be insufficient, making it difficult to ensure that the precursor particles are fully phosphated.

[0068] During the above reaction process, the heating rate can be controlled within 1–10 °C·min. -1 For example, 1, 2, 3, 5, 6, 8, 10℃·min -1 Any value in the range or any two values.

[0069] In the above reaction process, the phosphating temperature can be 320–380℃, for example, any value or any two values ​​between 320, 330, 340, 350, 360, and 380℃. The holding time can be 1–3 hours, for example, any value or any two values ​​between 1, 1.5, 2, 2.5, and 3 hours.

[0070] After the phosphating reaction is complete, heating is stopped at 150°C, and the product is removed after further cooling to room temperature. In some embodiments, the obtained product is further subjected to ball milling to further improve the uniformity of particle size. This ball milling step needs to be carried out under a protective atmosphere, such as nitrogen. The ball milling speed can be 100–200 rpm·min. -1 Such as 100, 120, 150, 180, 200 rpm·min -1The value in the range can be any value or any two values. The ball milling time can be 10 to 120 minutes, such as any value or any two values ​​in the range of 10, 20, 30, 60, 90, 100, and 120 minutes.

[0071] Through the above steps, nitrogen-doped iron phosphide (N-FeP2) with uniform size and micro / nano structure can be prepared, which effectively improves the defect of low electronic conductivity of iron phosphide and also reduces the volume expansion of iron phosphide during charging and discharging.

[0072] In some embodiments, the nitrogen-doped iron phosphide has a D50 particle size of 2–3 μm, a D90 particle size of 6–8 μm, and a specific surface area of ​​15–25 m². 2 / g.

[0073] Furthermore, this application provides an anode material comprising the aforementioned nitrogen-doped iron phosphide. It is understood that the anode material can use the aforementioned nitrogen-doped iron phosphide alone as the active material, or it can be composited with existing anode materials (graphite, silicon, silicon suboxide, silicon-carbon composite materials, lithium titanate, tin-based anode materials) as the active material. The composite ratio can be determined according to the performance requirements of the anode, and this application does not limit this.

[0074] Furthermore, this application provides a secondary battery, including a positive electrode, a negative electrode, a separator, and an electrolyte, wherein the negative electrode includes the aforementioned negative electrode material. This application does not limit the type of this secondary battery; it can be either a lithium-ion battery or a sodium-ion battery.

[0075] Furthermore, this application provides an electrical device including the aforementioned secondary battery. This electrical device can be a vehicle, mobile phone, portable device, laptop, ship, spacecraft, electric toy, and power tool, etc. Vehicles can be new energy vehicles, including pure electric vehicles, hybrid electric vehicles, or range-extended electric vehicles; 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. This application does not impose any special limitations on the aforementioned electrical device.

[0076] The present application will be further described below with reference to embodiments and comparative examples. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present application.

[0077] Example 1

[0078] (1) Place 1g FeCl2 and 0.5g urea in a 100mL beaker, add 10mL deionized water, 20mL ethylene glycol, and 20mL N-methylpyrrolidone. Stir magnetically for 30min, then sonicate for 30min to ensure complete dissolution of the solutes. Next, pour the solution into a 100mL polytetrafluoroethylene reactor and react in an oven at 160℃ for 5h. After the temperature drops to room temperature, first react with anhydrous ethanol at 4000rpm·min. -1 Centrifuge and wash three times at a rate of 6000 rpm, then rinse with deionized water at 6000 rpm·min. -1 The precursor material was centrifuged and washed three times at a certain speed, then dried in an oven for later use. The dried precursor material was then milled in a ball mill at 500 rpm·min. -1 The FeP2 precursor powder was obtained by ball milling three times at a high speed for 15 minutes each time, and then set aside for later use.

[0079] (2) Take 0.2g of precursor powder and 2g of sodium hypophosphite, and place them in two porcelain boats respectively, with the sodium hypophosphite placed upstream and the precursor powder downstream. In an argon atmosphere, incubate at 5℃·min. -1 The temperature was increased to 350℃ at a rate of [missing information], held for 2 hours, and then increased at a rate of 10℃·min [missing information]. -1 After cooling to 150℃, heating was stopped. Once the temperature had dropped to room temperature, the sample was removed and placed in a nitrogen-filled balloon mill beaker. The sample was then milled at 200 rpm·min. -1 After grinding at a certain rate for 1 hour, the product obtained is N-FeP2.

[0080] Example 2

[0081] (1) Place 1g FeCl2 and 1.5g urea in a 100mL beaker, add 10mL deionized water, 20mL ethylene glycol, and 20mL N-methylpyrrolidone. Stir magnetically for 30min, then sonicate for 30min to ensure complete dissolution of the solutes. Next, pour the solution into a 100mL polytetrafluoroethylene reactor and react in an oven at 160℃ for 5h. After the temperature drops to room temperature, first react with anhydrous ethanol at 4000rpm·min. -1 Centrifuge and wash three times at a rate of 6000 rpm, then rinse with deionized water at 6000 rpm·min. -1 The precursor material was centrifuged and washed three times at a certain speed, then dried in an oven for later use. The dried precursor material was then milled in a ball mill at 500 rpm·min. -1 The FeP2 precursor powder was obtained by ball milling three times at a high speed for 15 minutes each time, and then set aside for later use.

[0082] (2) Take 0.2g of precursor powder and 2g of sodium hypophosphite, and place them in two porcelain boats respectively, with the sodium hypophosphite placed upstream and the precursor powder downstream. In an argon atmosphere, incubate at 5℃·min.-1 The temperature was increased to 350℃ at a rate of [missing information], held for 2 hours, and then increased at a rate of 10℃·min [missing information]. -1 After cooling to 150℃, heating was stopped. Once the temperature had dropped to room temperature, the sample was removed and placed in a nitrogen-filled balloon mill beaker. The sample was then milled at 200 rpm·min. -1 After grinding at a certain rate for 1 hour, the product obtained is N-FeP2.

[0083] Example 3

[0084] (1) Place 1g FeCl2 and 2g urea in a 100mL beaker, add 10mL deionized water, 20mL ethylene glycol, and 20mL N-methylpyrrolidone. Stir magnetically for 30min, then sonicate for 30min to ensure complete dissolution of the solutes. Next, pour the solution into a 100mL polytetrafluoroethylene reactor and react in an oven at 160℃ for 5h. After the temperature drops to room temperature, first react with anhydrous ethanol at 4000rpm·min. -1 Centrifuge and wash three times at a rate of 6000 rpm, then rinse with deionized water at 6000 rpm·min. -1 The precursor material was centrifuged and washed three times at a certain speed, then dried in an oven for later use. The dried precursor material was then milled in a ball mill at 500 rpm·min. -1 The FeP2 precursor powder was obtained by ball milling three times at a high speed for 15 minutes each time, and then set aside for later use.

[0085] (2) Take 0.2g of precursor powder and 2g of sodium hypophosphite, and place them in two porcelain boats respectively, with the sodium hypophosphite placed upstream and the precursor powder downstream. In an argon atmosphere, incubate at 5℃·min. -1 The temperature was increased to 350℃ at a rate of [missing information], held for 2 hours, and then increased at a rate of 10℃·min [missing information]. -1 After cooling to 150℃, heating was stopped. Once the temperature had dropped to room temperature, the sample was removed and placed in a nitrogen-filled balloon mill beaker. The sample was then milled at 200 rpm·min. -1 After grinding at a certain rate for 1 hour, the product obtained is N-FeP2.

[0086] Example 4

[0087] (1) Place 1g FeCl2 and 0.5g urea in a 100mL beaker, add 10mL deionized water, 20mL ethylene glycol, and 20mL N,N-dimethylformamide. Stir magnetically for 30min, then sonicate for 30min to ensure complete dissolution of the solutes. Next, pour the solution into a 100mL polytetrafluoroethylene reactor and react in an oven at 160℃ for 5h. After the temperature drops to room temperature, first react with anhydrous ethanol at 4000rpm·min. -1 Centrifuge and wash three times at a rate of 6000 rpm, then rinse with deionized water at 6000 rpm·min. -1The precursor material was centrifuged and washed three times at a certain speed, then dried in an oven for later use. The dried precursor material was then milled in a ball mill at 500 rpm·min. -1 The FeP2 precursor powder was obtained by ball milling three times at a high speed for 15 minutes each time, and then set aside for later use.

[0088] (2) Take 0.2g of precursor powder and 2g of sodium hypophosphite, and place them in two porcelain boats respectively, with the sodium hypophosphite placed upstream and the precursor powder downstream. In an argon atmosphere, incubate at 5℃·min. -1 The temperature was increased to 350℃ at a rate of [missing information], held for 2 hours, and then increased at a rate of 10℃·min [missing information]. -1 After cooling to 150℃, heating was stopped. Once the temperature had dropped to room temperature, the sample was removed and placed in a nitrogen-filled balloon mill beaker. The sample was then milled at 200 rpm·min. -1 After grinding at a certain rate for 1 hour, the product obtained is N-FeP2.

[0089] Example 5

[0090] (1) Place 1g FeCl2 and 0.5g urea in a 100mL beaker, add 20mL ethylene glycol and 20mL N-methylpyrrolidone, stir magnetically for 30min, and then sonicate for 30min to ensure complete dissolution of the solute. Next, pour the solution into a 100mL polytetrafluoroethylene reactor and react in an oven at 160℃ for 5h. After the temperature drops to room temperature, first react with anhydrous ethanol at 4000rpm·min. -1 Centrifuge and wash three times at a rate of 6000 rpm, then rinse with deionized water at 6000 rpm·min. -1 The precursor material was centrifuged and washed three times at a certain speed, then dried in an oven for later use. The dried precursor material was then milled in a ball mill at 500 rpm·min. -1 The FeP2 precursor powder was obtained by ball milling three times at a high speed for 15 minutes each time, and then set aside for later use.

[0091] (2) Take 0.2g of precursor powder and 2g of sodium hypophosphite, and place them in two porcelain boats respectively, with the sodium hypophosphite placed upstream and the precursor powder downstream. In an argon atmosphere, incubate at 5℃·min. -1 The temperature was increased to 350℃ at a rate of [missing information], held for 2 hours, and then increased at a rate of 10℃·min [missing information]. -1 After cooling to 150℃, heating was stopped. Once the temperature had dropped to room temperature, the sample was removed and placed in a nitrogen-filled balloon mill beaker. The sample was then milled at 200 rpm·min. -1 After grinding at a certain rate for 1 hour, the product obtained is N-FeP2.

[0092] Comparative Example 1

[0093] The difference between Comparative Example 1 and Example 1 is that ethylenediamine was used instead of urea.

[0094] Comparative Example 2

[0095] The difference between Comparative Example 2 and Example 1 is that melamine was used instead of urea.

[0096] Comparative Example 3

[0097] The difference between Comparative Example 3 and Example 1 is that no urea was added.

[0098] Comparative Example 4

[0099] The difference between Comparative Example 4 and Example 1 is that 50 mL of water was added as a solvent, and ethylene glycol and N-methylpyrrolidone were not added.

[0100] Performance testing

[0101] 1. Physical and chemical performance testing

[0102] The iron phosphide materials prepared in Example 2 and Comparative Examples 3-4 were subjected to SEM testing, and the test results are as follows: Figure 1-2 As shown.

[0103] In Comparative Example 3, since no nitrogen source, urea, was added, undoped iron phosphide was prepared. Figure 1 As can be seen, the particle size is uneven and the crystallinity is poor, presenting an uneven blocky structure.

[0104] In Example 2, urea was used as the nitrogen source, and a mixture of deionized water, ethylene glycol, and NMP (volume ratio 1:2:2) was used as the solvent to prepare nitrogen-doped iron phosphide. Figure 2 As can be seen, the particles are relatively uniform in size, with a particle size D50 of approximately 2 μm and a D90 of approximately 6 μm. They also exhibit good crystallinity, with the grains displaying a blocky structure. This is because when urea is used as the nitrogen source for N-doping of iron phosphide, urea acts as a nucleation regulator during the solvothermal reaction, promoting the nucleation and growth of hydroxyl iron phosphide crystals, ensuring complete crystallization, and playing a crucial role in controlling the grain size, resulting in uniform grain size.

[0105] In addition, the presence of numerous pores between the grains results in a material with high porosity and specific surface area, reaching up to 20 m². 2 / g. This is because the introduction of nitrogen helps to construct the internal pore structure of the material, increasing its porosity. This is beneficial for the adsorption and extraction of Li. + This reduces volume expansion during the process and improves the material's cycle performance.

[0106] In Comparative Example 4, urea was used as the nitrogen source and deionized water as the solvent to prepare nitrogen-doped iron phosphide. Figure 3As can be seen, the particle size is uneven, with a significant difference between large and small grains, and some large grains exceeding 10 μm in size; moreover, the crystal growth quality is poor, with numerous cracks appearing on the grain surface. This is because the hydrothermal reaction was too rapid when water was used as the solvent, resulting in incomplete crystallization and thus low crystallinity and uneven particle size.

[0107] The sample from Example 4 was subjected to XRD testing, and the results are as follows: Figure 4 As shown in the figure, compared with the PDF standard card of FeP2, the sample prepared in Example 4 showed corresponding characteristic peaks, indicating that the material prepared in Example 1 was FeP2. Furthermore, the characteristic peaks in the sample prepared in Example 4 had wider half-peak widths, indicating that its crystallinity was not high compared to the pure FeP2 phase.

[0108] 2. Button cell battery test

[0109] 2.1 Button assembly

[0110] Iron phosphide is used as the negative electrode material for lithium-ion batteries and assembled into coin cells. The specific preparation method of the negative electrode material is as follows:

[0111] A negative electrode was prepared by mixing FeP2 / N-FeP2 material, CMC, SBR, SP, and H2O in a mass ratio of 95:2.5:1.5:1:150. A lithium sheet was used as the positive electrode, and the electrolyte was LiPF6 / EC+DEC (LiPF6 as the electrolyte, a 1:1 volume ratio of EC and DEC as the solvent, and an electrolyte concentration of...).

[0112] (1.3 mol / L); the separator is a composite membrane of polyethylene (PE), polypropylene (PP), and polyethylene propylene (PEP). The button cell assembly is carried out in an argon-filled glove box.

[0113] 2.2 Button performance test

[0114] According to GB / T 37201-2018, the initial discharge specific capacity of the coin cell at 0.1C rate was tested; according to GB / T 37207-2018, the capacity retention rate of the coin cell after 50 charge-discharge cycles at 1C rate was determined.

[0115] 3. Effects of different urea addition amounts on battery performance

[0116] Figure 5The graphs show the specific capacity and coulombic efficiency of coin cells assembled using the iron phosphide materials from Examples 2 and 3 as anode materials, cycling at different current densities for 1-60 cycles. As can be seen from the graphs, the specific capacity of the coin cells assembled using the N-FeP2 material from Example 3 as the anode material is significantly higher than that of Comparative Example 3 during 1-60 cycles. This indicates that after doping, the volume expansion and fragmentation of the iron phosphide material are correspondingly improved. Simultaneously, the introduction of nitrogen reduces the material's resistance, increases electronic conductivity, and effectively improves the rate performance of the battery.

[0117] Figure 6 Using nitrogen-doped iron phosphide material from Examples 1-3 as the negative electrode material, the assembled coin cells achieved an efficiency of 1000 mA·g. -1 The specific capacity and coulombic efficiency at current densities of 1-1000 cycles are plotted. As can be seen from the figure, compared to Examples 1 and 3, Example 2, which doped with 1.5 g of urea, yielded the battery with the best cycle performance at 1000 mA·g. -1 At a current density of 978 mAh·g, the specific capacity is 978 mAh·g after 100 cycles. -1 Even after 1000 cycles, the specific capacity is still 1056 mAh·g. -1 .

[0118] 4. The impact of different nitrogen source types on battery performance

[0119] Figure 7 The figure shows a comparison of the rate performance of N-FeP2 prepared using different nitrogen sources. As can be seen from the figure, N-FeP2 prepared using urea as the nitrogen source exhibits the highest rate performance compared to ethylenediamine and melamine. This is because the introduction of urea increases the electrochemical reaction active sites in the material and introduces surface functional groups, thereby increasing the adsorption capacity for lithium ions and enhancing the charge-discharge capability under high current, resulting in better rate performance.

[0120] 5. The impact of different solvent types on battery performance

[0121] Figure 8 A comparison chart of the rate performance of N-FeP2 prepared with different solvents is shown. As can be seen from the chart, N-FeP2 prepared with a mixed solvent of water and organic solvents exhibits higher specific capacity and cycle performance compared to using water alone. This is because using a mixed solvent to prepare the N-FeP2 precursor slows down the growth rate of crystal nuclei, allowing for more uniform ion deposition and resulting in a precursor with better crystallinity and uniformity. Therefore, the final N-FeP2 structure prepared has better stability, which is beneficial for improving the battery's specific capacity and cycle performance.

[0122] In summary, although the present invention has been disclosed above with reference to preferred embodiments, the above preferred embodiments are not intended to limit the present invention. Those skilled in the art can make various modifications and refinements without departing from the spirit and scope of the present invention. Therefore, the scope of protection of the present invention shall be determined by the scope defined in the claims.

Claims

1. A method for preparing nitrogen-doped iron phosphide, characterized in that, Includes the following steps: S1. Iron salt and urea are dissolved in water to obtain an aqueous solution, which is then mixed with an organic solvent to obtain a mixture. The organic solvent includes a first solvent and a second solvent. The first solvent is ethylene glycol, and the second solvent is N,N-dimethylformamide or N,N-dimethylformamide. The volume ratio of the first solvent to the second solvent is 10~30:10~30. The mixture is then subjected to a solvothermal reaction to obtain an iron phosphide precursor. The mass ratio of the iron salt to the urea is 1~2:0.1~2. S2. Under a protective atmosphere, the iron phosphide precursor is phosphated with hypophosphite to obtain nitrogen-doped iron phosphide. The particle size D50 of the nitrogen-doped iron phosphide is 2µm to 3µm. The nitrogen-doped iron phosphide is used in anode materials.

2. The method for preparing nitrogen-doped iron phosphide as described in claim 1, characterized in that, In step S1: the iron salt includes at least one of ferrous chloride, ferric chloride, ferrous sulfate, ferric sulfate, ferrous nitrate, ferric nitrate, ferrous acetate, and ferric acetate.

3. The method for preparing nitrogen-doped iron phosphide as described in claim 1, characterized in that, In step S1: the concentration of the iron salt in the mixture is 0.02~0.04 g / mL.

4. The method for preparing nitrogen-doped iron phosphide as described in claim 1, characterized in that, In step S1: The concentration of the urea is 0.002~0.04 g / mL.

5. The method for preparing nitrogen-doped iron phosphide as described in claim 1, characterized in that, In step S1: the temperature of the solvothermal reaction is 120~180℃, and the time is 5~8 h; and / or, After the solvothermal reaction, the process also includes collecting the product, washing and drying it, and then grinding it.

6. The method for preparing nitrogen-doped iron phosphide as described in claim 1, characterized in that, In step S2, the protective atmosphere includes at least one of nitrogen and argon; and / or, The hypophosphite includes at least one of sodium hypophosphite and potassium hypophosphite; and / or, The mass ratio of the iron phosphide precursor to the hypophosphite is 1:8~20; and / or, The phosphating treatment is performed at a temperature of 320~380℃ for 1~3 hours.

7. A nitrogen-doped iron phosphide, characterized in that, The nitrogen-doped iron phosphide has a particle size D50 of 2-3 µm, a D90 of 6-8 µm, and a specific surface area of ​​15-25 m². 2 / g.

8. A negative electrode material, characterized in that, It includes nitrogen-doped iron phosphide prepared by the method of any one of claims 1 to 6, or nitrogen-doped iron phosphide as described in claim 7.

9. A secondary battery, comprising a positive electrode, a negative electrode, a separator, and an electrolyte, characterized in that, The negative electrode sheet includes the negative electrode material as described in claim 8.

10. An electrical appliance, characterized in that, Includes the secondary battery as described in claim 9.

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