An iron phosphide negative electrode material, a preparation method thereof, and a lithium battery

Abstract

The present invention provides an iron phosphide negative electrode material and a preparation method thereof and a lithium battery, relating to the technical field of lithium ion batteries. Specifically, the preparation method comprises the following steps: (1) mixing an iron source, a nitrogen source and a solvent, and preparing a nano-sized N-FeP2 precursor by a solvent thermal reaction; (2) subjecting the N-FeP2 precursor and sodium hypophosphite to a gas-cured phosphating reaction to prepare nano-sized N-FeP2; (3) drying a solution containing a carbon source and the N-FeP2, heating the dried product in a protective gas atmosphere for pre-carbonization and carbonization reaction, and cooling to obtain the iron phosphide negative electrode material N-C@FeP2. The method of the present invention improves the volume expansion and fragmentation of the material through material nano-sizing, nitrogen doping and surface carbon coating, reduces resistance, and effectively improves the electrochemical performance of the product.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium-ion batteries, and in particular to an iron phosphide negative electrode material, a preparation method thereof, and a lithium battery. Background Art

[0002] In the existing commercial lithium-ion batteries, most of the negative electrodes are made of graphite, which is the mainstream choice of negative electrode materials for lithium-ion batteries. However, the theoretical specific capacity of graphite is only 372 mAh g -1 , which cannot meet the actual demand for high endurance of electric vehicles. Among the existing lithium energy storage negative electrode materials, transition metal phosphide materials have the advantages of high mass specific capacity and low price, and are currently popular candidates for lithium-ion battery negative electrode materials. Although FeP2 materials have high mass specific capacity, their uncontrollable volume expansion during the charge and discharge process will cause the SEI film to rupture and reorganize multiple times, resulting in active lithium loss and gas generation. In addition, the electronic conductivity of transition metal phosphides themselves is low, which seriously affects the application of transition metal phosphides in lithium-ion batteries.

[0003] In view of this, the present invention is proposed. Summary of the Invention

[0004] The first object of the present invention is to provide a method for preparing an iron phosphide negative electrode material. The method prepares a nanoparticle structure through a solvent thermal method, uses a nitrogen source to adjust the surface state of the product, increases the active sites to adjust the porosity, reduces the resistance, improves the conductivity of the material through carbon coating, and inhibits the volume expansion of the material during the charge and discharge process. After the material is nano-sized, nitrogen-doped and surface-coated with a carbon shell, the volume expansion and fragmentation of the material are improved, the resistance is reduced, the rate performance is effectively improved, and the electrochemical cycle stability is improved.

[0005] The second object of the present invention is to provide an iron phosphide negative electrode material, which is prepared using the above-mentioned method for preparing the iron phosphide negative electrode material.

[0006] A third object of the present invention is to provide a lithium battery comprising the iron phosphide negative electrode material as described above.

[0007] In order to achieve the above-mentioned purpose of the present invention, the following technical solutions are adopted:

[0008] A first aspect of the present invention provides a method for preparing an iron phosphide negative electrode material, comprising the following steps:

[0009] (1) mixing an iron source, a nitrogen source, and a solvent to perform a solvothermal reaction, and washing, drying, and grinding to obtain an N-FeP2 precursor after the reaction is completed;

[0010] (2) placing the N-FeP2 precursor and sodium hypophosphite in two reactors respectively, placing the sodium hypophosphite upstream of the incoming gas flow direction, and placing the N-FeP2 precursor downstream of the incoming gas flow direction, heating under a protective gas atmosphere for phosphating reaction, and then cooling, and grinding the cooled product under a protective gas atmosphere to obtain nano-sized N-FeP2;

[0011] (3) drying the solution containing the carbon source and the N-FeP2, heating the dried product in a protective gas atmosphere for pre-carbonization and carbonization reaction, and obtaining the iron phosphide negative electrode material NC@FeP2 after cooling.

[0012] Optionally, the mass ratio of the iron source to the nitrogen source is 4:1-1:2, and the usage ratio of the solvent to the iron source is 25-50 mL / g.

[0013] Optionally, the iron source includes at least one of FeCl2, Fe(NO3)2, FeSO4 and organic ferrous salts.

[0014] Optionally, the nitrogen source includes at least one of urea, melamine and ammonium nitrate.

[0015] More preferably, the nitrogen source is urea.

[0016] Optionally, the solvent includes water, a first organic solvent and a second organic solvent, wherein the first organic solvent includes NMP and the second organic solvent includes one of ethylene glycol and DMF.

[0017] More preferably, the volume ratio of the water, the first organic solvent and the second organic solvent is 1:1-3:1-3.

[0018] Optionally, the temperature of the solvent thermal reaction is 120-180° C., and the reaction time is 5-8 h.

[0019] Optionally, the washing includes washing with anhydrous ethanol and / or washing with deionized water.

[0020] Optionally, in step (1), the grinding is ball milling, and the rotation speed of the ball mill is 400-600 rpm. More preferably, the number of ball milling is 3-5 times, and the time of each ball milling is 15-30 minutes.

[0021] Optionally, the mass ratio of the N-FeP2 precursor to the sodium hypophosphite is 1:10-1:5.

[0022] Optionally, in step (2), the heating rate is 3-5°C / min, the temperature of the phosphating reaction is 320-350°C, and the holding time is 1-3h.

[0023] Optionally, the cooling rate in step (2) is 5-10°C / min, and heating is stopped when the temperature drops to 180-150°C, and the mixture is allowed to cool naturally.

[0024] Optionally, in step (2), the grinding is ball milling, the rotation speed of the ball milling is 100-200 rpm, and the time of the ball milling is 0.5-2 h.

[0025] Optionally, in step (3), the concentration of the carbon source in the solution is 1-30 wt%, and the mass ratio of the carbon source to the N-FeP2 is 2:1-1:20.

[0026] Optionally, the carbon source includes at least one of glucose, asphalt, polydopamine, resorcinol+formaldehyde, starch, sucrose, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyvinyl pyrrolidone, tannic acid, polyvinyl alcohol, polypyrrole, vitamin C, polyethylene glycol, anthracene, and aniline.

[0027] Optionally, the heating rate of the pre-carbonization is 3-5° C. / min, the temperature of the pre-carbonization is 300-350° C., and the time of the pre-carbonization is 60-120 min.

[0028] Optionally, the heating rate of the carbonization reaction is 3-5° C. / min, the temperature of the carbonization reaction is 700-900° C., and the time of the carbonization reaction is 60-180 min.

[0029] Optionally, in step (2) and step (3), the protective gas is argon or nitrogen.

[0030] A second aspect of the present invention provides an iron phosphide negative electrode material, which is prepared using the above-mentioned method for preparing the iron phosphide negative electrode material.

[0031] A third aspect of the present invention provides a lithium battery comprising the iron phosphide negative electrode material described above.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] The present invention adopts a simple solvent thermal method, a low-temperature phosphating method and a carbon coating method to prepare a carbon-coated nitrogen-doped iron phosphide negative electrode material. The growth state of the crystal is controlled by the solvent thermal method to prepare an N-FeP2 precursor with a nano-particle structure. At the same time, a nitrogen source is used to adjust the state of the product surface, increase the active sites and adjust the porosity. The introduction of nitrogen helps the material to construct an internal pore structure, which is beneficial to the adsorption and embedding and extraction of Li by the porous material. +The introduction of nitrogen also reduces the material resistance and improves the rate performance. The surface carbon coating is used to improve the material conductivity, inhibit the volume expansion of the material during the charging and discharging process, and increase the pseudocapacitive performance of the material. The present invention suppresses its volume expansion and shortens the lithium ion transmission distance through material nano-material and surface carbon coating. At the same time, the bulk phase modification is carried out by introducing nitrogen and core-shell carbon structure to solve the problems of large irreversible capacity and low electronic conductivity of FeP2 negative electrode material. The rate performance of the iron phosphide negative electrode material prepared by the method of the present invention is significantly improved. Compared with pure phase FeP2, the electrochemical performance of the material is significantly improved after the introduction of nitrogen and surface coating with carbon shell. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0035] Figure 1 This is an SEM image of pure FeP2 prepared in Comparative Example 1 of the present invention;

[0036] Figure 2 SEM image of N-FeP2 provided for Comparative Example 3 of the present invention;

[0037] Figure 3 This is a SEM image of NC@FeP2 provided in Example 1 of the present invention;

[0038] Figure 4 This is a BET comparison diagram of N-FeP2 before and after carbon coating in Example 1 of the present invention;

[0039] Figure 5 This is a comparison chart of the long cycle performance of the materials obtained in Example 1, Comparative Example 1 and Comparative Example 3 of the present invention;

[0040] Figure 6 The AC impedance spectra of the materials obtained in Example 1, Comparative Example 1 and Comparative Example 3 of the present invention are shown; DETAILED DESCRIPTION

[0041] The technical scheme of the present invention will be clearly and completely described below in conjunction with specific embodiments, but it will be understood by those skilled in the art that the following described embodiments are part of embodiments of the present invention, rather than all embodiments, and are only used to illustrate the present invention, and should not be considered as limiting the scope of the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without making creative work premise belong to the scope of protection of the present invention. Those who do not specify specific conditions in the embodiments are carried out according to normal conditions or the conditions recommended by the manufacturer. Those whose reagents or instruments are not specified by the manufacturer are conventional products that can be purchased commercially.

[0042] A first aspect of the present invention provides a method for preparing an iron phosphide negative electrode material, comprising the following steps:

[0043] (1) mixing an iron source, a nitrogen source, and a solvent to perform a solvothermal reaction, and washing, drying, and grinding to obtain a nano-sized N-FeP2 precursor after the reaction is completed;

[0044] (2) placing the N-FeP2 precursor and sodium hypophosphite in two reactors respectively. For example, the reactor is a porcelain boat. The sodium hypophosphite is placed upstream in the flow direction of the incoming gas, and the N-FeP2 precursor is placed downstream in the flow direction of the incoming gas. After heating under a protective gas atmosphere for phosphating reaction, the cooled product is ground in a protective gas atmosphere to obtain nano-sized N-FeP2.

[0045] (3) drying the solution containing the carbon source and the N-FeP2, heating the dried product in a protective gas atmosphere for pre-carbonization and carbonization reaction, and obtaining the iron phosphide negative electrode material NC@FeP2 after cooling.

[0046] The present invention controls the growth state of the crystal by a solvent thermal method to prepare a nano-particle structure of N-FeP2 precursor, and uses a nitrogen source to adjust the state of the product surface, increase the active sites and adjust the porosity. The introduction of nitrogen helps the material to build an internal pore structure, which is beneficial to the adsorption and embedding and extraction of Li in the porous material. + The introduction of nitrogen also reduces the material resistance and improves the rate performance. In addition, the gas-curing phosphating reaction makes the product consistent with the outer contour morphology of the precursor, and obtains nano-sized nitrogen-doped iron phosphide N-FeP2. The specific surface area of the material is increased by nano-sizing to increase the active sites of the reaction and shorten the Li + The migration path of the material is used to improve the conductive performance; the surface carbon coating is used to improve the conductivity of the material, inhibit the volume expansion of the material during the charging and discharging process, and increase the pseudocapacitive performance of the material.

[0047] In some specific embodiments of the present invention, in step (1), the mass ratio of the iron source to the nitrogen source is 4:1-1:2, and the usage ratio of the solvent to the iron source is 25-50 mL / g; different ion concentrations and solvent types have a huge impact on the crystal nucleation reaction in the solvothermal reaction, and reasonable control of the raw material ratio to obtain an appropriate ion concentration can control the morphology and size of the precursor.

[0048] For example, in different embodiments, the mass ratio of the iron source to the nitrogen source can be any point value among 4:1, 3:1, 2:1, 1:1, and 1:2, or a range consisting of any two point values; the usage ratio of the solvent to the iron source can be any point value among 25 mL / g, 30 mL / g, 35 mL / g, 40 mL / g, 45 mL / g, and 50 mL / g, or a range consisting of any two point values.

[0049] In some specific embodiments of the present invention, in step (1), the iron source includes at least one of FeCl2, Fe(NO3)2, FeSO4 and an organic ferrous salt.

[0050] In some specific embodiments of the present invention, in step (1), the nitrogen source includes at least one of urea, melamine and ammonium nitrate. The nitrogen source not only plays a role in regulating the internal porosity of the material, but also plays a key role in regulating the size of the nanomaterial.

[0051] In some specific embodiments of the present invention, the nitrogen source is urea. Urea, as a transition substance between organic and inorganic substances, is easily decomposed and has high doping efficiency.

[0052] In some specific embodiments of the present invention, in step (1), the solvent includes water, a first organic solvent, and a second organic solvent, wherein the first organic solvent includes NMP and the second organic solvent includes one of ethylene glycol and DMF; to avoid impurities, the water is deionized water as an example. In a solvothermal reaction, the type of solvent affects the nucleation reaction of the crystals. The use of the above-mentioned mixed solvent is conducive to the synthesis of micro-nanostructures with uniform size. In addition, the addition of water can accelerate the dissolution of inorganic ferrous salts and urea.

[0053] In some specific embodiments of the present invention, the volume ratio of the water, the first organic solvent and the second organic solvent is 1:1-3:1-3.

[0054] For example, in different embodiments, the volume ratio of the water, the first organic solvent and the second organic solvent can be any point value among 1:1:1, 1:1:2, 1:1:3, 1:2:1, 1:2:2, 1:2:3, 1:3:1, 1:3:2, 1:3:3, or a range consisting of any two points.

[0055] In some specific embodiments of the present invention, in step (1), the iron source, the nitrogen source and the solvent are mixed and then stirred and ultrasonically treated to ensure that the solute is fully dissolved.

[0056] In some specific embodiments of the present invention, the stirring speed is 300-600 rpm, the stirring time is 30-60 min, the ultrasonic power is 500-1200 W, and the ultrasonic treatment time is 30-60 min.

[0057] For example, in different embodiments, the rotation speed of the stirring and dissolving can be any value among 300rpm, 350rpm, 400rpm, 450rpm, 500rpm, 550rpm, and 600rpm, or a range consisting of any two value values; the time of the stirring and dissolving can be any value among 30min, 35min, 40min, 45min, 50min, 55min, and 60min, or a range consisting of any two value values; the power of the ultrasonic dissolution can be any value among 500W, 600W, 700W, 800W, 900W, 1000W, 1100W, and 1200W, or a range consisting of any two value values; the time of the ultrasonic dissolution can be any value among 30min, 35min, 40min, 45min, 50min, 55min, and 60min, or a range consisting of any two value values.

[0058] In some specific embodiments of the present invention, the stirring is magnetic stirring.

[0059] In some specific embodiments of the present invention, in step (1), the temperature of the solvent thermal reaction is 120-180° C., and the reaction time is 5-8 h.

[0060] For example, in different embodiments, the temperature of the solvent thermal reaction can be any point value among 120°C, 130°C, 140°C, 150°C, 160°C, 170°C, and 180°C, or a range consisting of any two point values; the time of the solvent thermal reaction can be any point value among 5h, 5.5h, 6h, 6.5h, 7h, 7.5h, and 8h, or a range consisting of any two point values.

[0061] In some specific embodiments of the present invention, in step (1), the washing includes washing with anhydrous ethanol and / or washing with deionized water.

[0062] In some specific embodiments of the present invention, the anhydrous ethanol washing is centrifugal washing, the rotation speed is 4000-5000 rpm, the time of each washing is 10-20 minutes, and the number of washing times is 2-4 times.

[0063] For example, in different embodiments, the centrifugal speed during the anhydrous ethanol washing can be any value among 4000 rpm, 4200 rpm, 4400 rpm, 4600 rpm, 4800 rpm, and 5000 rpm, or a range consisting of any two value points; the time for each washing can be any value among 10 min, 12 min, 14 min, 15 min, 16 min, 18 min, and 20 min, or a range consisting of any two value points; the number of washing times can be 2 times, 3 times, or 4 times.

[0064] In some specific embodiments of the present invention, the deionized water washing is centrifugal washing with a rotation speed of 6000-7000 rpm, a washing time of 10-20 minutes per washing, and 2-4 washing times.

[0065] For example, in different embodiments, the rotation speed during deionized water washing can be any point value among 6000 rpm, 6200 rpm, 6400 rpm, 6600 rpm, 6800 rpm, 7000 rpm, or a range consisting of any two point values; the time for each washing can be any point value among 10 min, 12 min, 14 min, 15 min, 16 min, 18 min, 20 min, or a range consisting of any two point values, and the number of washing times can be 2 times, 3 times, or 4 times.

[0066] In some specific embodiments of the present invention, in step (1), the grinding is ball milling, the rotation speed of the ball mill is 400-600 rpm, the number of ball milling is 3-5 times, and the time of each ball milling is 15-30 minutes.

[0067] For example, in different embodiments, the rotation speed of the ball mill in step (1) can be any value among 400 rpm, 450 rpm, 500 rpm, 550 rpm, 600 rpm, or a range consisting of any two values; the time for each ball milling can be any value among 15 min, 20 min, 25 min, 30 min, or a range consisting of any two values; the number of ball millings can be 3 times, 4 times, or 5 times.

[0068] In some specific embodiments of the present invention, the mass ratio of the N-FeP2 precursor to the sodium hypophosphite is 1:10-1:5.

[0069] For example, in different embodiments, in step (2), the mass ratio of the N-FeP2 precursor to the sodium hypophosphite can be any value among 1:10, 1:9, 1:8, 1:7, 1:6, 1:5, or a range consisting of any two values.

[0070] In some specific embodiments of the present invention, in step (2), the heating rate is 3-5°C / min, the temperature of the phosphating reaction is 320-350°C, and the holding time is 1-3h.

[0071] In some specific embodiments of the present invention, in step (2), the cooling rate is 5-10°C / min, heating is stopped when the temperature drops to 180-150°C, and the mixture is then naturally cooled to room temperature before being taken out.

[0072] Sodium hypophosphite decomposes at temperatures above 300°C to produce phosphine gas, which phosphine reacts with iron oxyhydroxide to produce FeP2. Temperature affects the phosphine generation rate, iron phosphide generation rate and phosphating degree. This process rationally controls synthesis parameters such as phosphating temperature, heating conditions and cooling conditions to obtain nano-sized N-FeP2 with stable and uniform phase morphology.

[0073] For example, in different embodiments, in step (2), the heating rate of the heating can be any value among 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min, and 5°C / min, or a range consisting of any two values; the temperature of the phosphating reaction can be any value among 320°C, 325°C, 330°C, 335°C, 340°C, 345°C, and 350°C, or a range consisting of any two values; the cooling rate of the cooling can be any value among 5°C / min, 6°C / min, 7°C / min, 8°C / min, 9°C / min, and 10°C / min, or a range consisting of any two values; the temperature when stopping heating can be any value among 150°C, 160°C, 170°C, and 180°C, or a range consisting of any two values.

[0074] In some specific embodiments of the present invention, in step (2), the grinding is ball milling, the rotation speed of the ball mill is 100-200 rpm, and the time of the ball milling is 0.5-2 h.

[0075] For example, in different embodiments, in step (2), the ball milling rotation speed can be any value among 100 rpm, 120 rpm, 140 rpm, 160 rpm, 180 rpm, 200 rpm, or a range consisting of any two values; the ball milling time can be any value among 0.5 h, 1 h, 1.5 h, 2 h, or a range consisting of any two values.

[0076] In some specific embodiments of the present invention, in step (3), the concentration of the carbon source in the solution is 1-30wt%, and the mass ratio of the carbon source to the N-FeP2 is 2:1-1:20. The presence of the carbon layer plays a significant role in improving the conductivity of the material and inhibiting its volume expansion. The amount of carbon source affects the thickness of the carbon layer. Reasonable control of the thickness of the carbon layer can improve the electrochemical properties of the material.

[0077] For example, in different embodiments, the concentration of the carbon source in the solution can be any one of 1wt%, 2.5wt%, 4wt%, 5wt%, 8wt%, 10wt%, 12wt%, 15wt%, 17wt%, 20wt%, 22wt%, 25wt%, 28wt%, 30wt%, or a range consisting of any two values; the mass ratio of the carbon source to the N-FeP2 can be any one of 2:1, 1.25:1, 1:1, 1:2, 1:3.2, 1:4, 1:6.4, 1:8, 1:10, 1:12, 1:14, 1:16, 1:18, or 1:20, or a range consisting of any two values.

[0078] In some specific embodiments of the present invention, the carbon source includes at least one of glucose, asphalt, polydopamine, resorcinol + formaldehyde, starch, sucrose, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyvinyl pyrrolidone, tannic acid, polyvinyl alcohol, polypyrrole, vitamin C, polyethylene glycol, anthracene, and aniline, wherein resorcinol + formaldehyde means that resorcinol and formaldehyde are used simultaneously as carbon sources.

[0079] In some specific embodiments of the present invention, in step (3), the solution is prepared by dispersing the carbon source and the N-FeP2 in deionized water, and the dispersion includes stirring dispersion and ultrasonic dispersion, and the solute is ensured to be uniformly dispersed by combining stirring dispersion and ultrasonic dispersion.

[0080] In some specific embodiments of the present invention, the stirring and dispersing speed is 300-600 rpm, the stirring and dispersing time is 30-60 min, the ultrasonic dispersing power is 500-1200 W, and the ultrasonic dispersing time is 30-60 min.

[0081] For example, in different embodiments, the stirring and dispersing speed can be any value among 300rpm, 350rpm, 400rpm, 450rpm, 500rpm, 550rpm, and 600rpm, or a range consisting of any two value points; the stirring and dispersing time can be any value among 30min, 35min, 40min, 45min, 50min, 55min, and 60min, or a range consisting of any two value points; the ultrasonic dispersion power can be any value among 500W, 600W, 700W, 800W, 900W, 1000W, 1100W, and 1200W, or a range consisting of any two value points; the ultrasonic dispersion time can be any value among 30min, 35min, 40min, 45min, 50min, 55min, and 60min, or a range consisting of any two value points.

[0082] In some specific embodiments of the present invention, the stirring and dispersing is magnetic stirring.

[0083] In some specific embodiments of the present invention, in step (3), the drying is freeze-drying.

[0084] In some specific embodiments of the present invention, the pre-carbonization heating rate is 3-5°C / min, the pre-carbonization temperature is 300-350°C, and the pre-carbonization time is 60-120min. The purpose of pre-carbonization is to allow the carbon source to slowly shrink on the material to form a dense and stable carbon layer.

[0085] For example, in different embodiments, the heating rate of the pre-carbonization can be any value among 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min, and 5°C / min, or a range consisting of any two values; the temperature of the pre-carbonization can be any value among 300°C, 310°C, 320°C, 330°C, 340°C, and 350°C, or a range consisting of any two values.

[0086] In some specific embodiments of the present invention, the heating rate of the carbonization reaction is 3-5°C / min, the temperature of the carbonization reaction is 700-900°C, and the time of the carbonization reaction is 60-180min. The time and temperature of the carbonization reaction have an important influence on the electrochemical properties of the material. Reasonable control of the temperature and time of the carbonization reaction can improve the electrochemical properties of the material.

[0087] For example, in different embodiments, the heating rate of the carbonization reaction can be any value among 3°C / min, 3.5°C / min, 4°C / min, 4.5°C / min, and 5°C / min, or a range consisting of any two values; the temperature of the carbonization reaction can be any value among 700°C, 750°C, 800°C, 850°C, and 900°C, or a range consisting of any two values; the time of the carbonization reaction can be any value among 60min, 90min, 120min, 150min, and 180min, or a range consisting of any two values.

[0088] In some specific embodiments of the present invention, in step (2) and step (3), the protective gas is argon or nitrogen.

[0089] A second aspect of the present invention provides an iron phosphide negative electrode material, which is prepared by the method described above.

[0090] A third aspect of the present invention provides a lithium battery comprising the iron phosphide negative electrode material described above.

[0091] Some embodiments of the present invention are described in detail below with reference to specific examples. Unless otherwise specified, the raw materials used in the examples can be purchased from the market.

[0092] Example 1

[0093] (1) Preparation of N-FeP2 precursor

[0094] Take 2g of FeCl2 and 1g of urea and place them in a 100ml beaker, add 10ml of deionized water, 20ml of ethylene glycol, and 20ml of nitrogen methyl pyrrolidone, stir magnetically at 400rpm for 45min, and then ultrasonically dissolve for 45min with an ultrasonic power of 800W to ensure that the solute is fully dissolved. Pour the solution into a 100ml polytetrafluoroethylene reactor and react in a 150℃ oven for 6.5h. After the temperature drops to room temperature, centrifuge and wash with anhydrous ethanol at 4000rpm for 3 times, each washing time is 15min, and then centrifuge and wash with deionized water at 6000rpm for 3 times, each washing time is 15min, and then put it in an oven to dry for use. The dried precursor material is ball milled in a ball mill at 500rpm for 5 times, each ball milling time is 15min.

[0095] (2) Preparation of N-FeP2

[0096] Take N-FeP2 precursor powder and sodium hypophosphite in a mass ratio of 1:8 and place them in two porcelain boats respectively. Place the sodium hypophosphite upstream of the incoming gas flow direction, and the N-FeP2 precursor powder downstream of the incoming gas flow direction. Under an argon atmosphere, heat to 330℃ at 3℃ / min and keep warm for 2h. Cool to 150℃ at 5℃ / min and stop heating. Take out after the temperature drops to room temperature, place the sample in a nitrogen-filled balloon mill beaker, grind at 200rpm for 1h, and take out. The product is N-FeP2.

[0097] (3) Preparation of NC@FeP2

[0098] 0.125 g of glucose was dissolved in 2 ml of deionized water to prepare a glucose solution, 0.4 g of N-FeP2 prepared in step (2) was added to the glucose solution, and the mixture was dispersed by magnetic stirring at a speed of 400 rpm for 60 min, ultrasonically dispersed for 60 min each, and then freeze-dried to prepare Pre-NC@FeP2, with an ultrasonic power of 800 W; the freeze-dried Pre-NC@FeP2 was placed in a corundum porcelain boat, heated to 320°C at 3°C / min under an argon atmosphere for pre-carbonization for 90 min, and then heated to 800°C at 3°C / min for reaction for 120 min. After cooling, the product was NC@FeP2.

[0099] Example 2-22

[0100] The differences between Examples 2-22 and Example 1 are shown in Table 1.

[0101] Table 1

[0102]

[0103]

[0104] Comparative Example 1

[0105] Preparation of FeP2

[0106] The difference from Example 1 is that the amount of nitrogen source added in step (1) is 0, and step (3) is not performed, that is, nitrogen doping and carbon coating are not performed, and the other conditions are the same as in Example 1.

[0107] Comparative Example 2

[0108] Preparation of C@FeP2

[0109] The difference from Example 1 is that the amount of nitrogen source added in step (1) is 0, that is, no nitrogen doping is performed, only carbon coating is performed, and the other conditions are the same as Example 1.

[0110] Comparative Example 3

[0111] Preparation of N-FeP2

[0112] The difference from Example 1 is that step (3) is not performed, that is, only nitrogen doping is performed without carbon coating, and the other conditions are the same as Example 1.

[0113] Comparative Example 4

[0114] The difference from Example 1 is that in step (1), only 50 ml of deionized water is used as the solvent.

[0115] Comparative Example 5

[0116] The difference from Example 1 is that in step (1), only 50 ml of nitrogen-methyl pyrrolidone is used as the solvent.

[0117] Comparative Example 6

[0118] The difference from Example 1 is that in step (1), only 50 ml of ethylene glycol is used as the solvent.

[0119] Comparative Example 7

[0120] The difference from Example 1 is that no pre-carbonization was performed.

[0121] Experimental example

[0122] 1. Scanning electron microscope test

[0123] The products prepared in each embodiment and comparative example were subjected to scanning electron microscopy test, and the results showed that the particle size of the materials was all at the micro-nano level, and the uniformity of the particle size distribution of the materials prepared in Example 1 was significantly better than that of comparative examples 4-6; in addition, Figure 1 、 Figure 2 and Figure 3 As shown in the figure, the N-FeP2 adjusted with urea presents a block structure that is significantly different from the uneven block structure obtained without the participation of urea. The overall uniformity of the particle size of the material after N doping is better; SEM shows that the carbon layer wraps the FeP2 as a whole. This structure can greatly limit the volume expansion during the charge and discharge process and improve the conductivity of the material.

[0124] 2.BET Test

[0125] The N2 adsorption-desorption curves of N-FeP2 before and after carbon coating were tested. The results are as follows: Figure 4 As shown in the figure, BET analysis shows that the specific surface area of the material is significantly increased after carbon coating, and the coated carbon layer has a porous structure, which is composed of macropores (500nm-100μm), mesopores (10nm-500nm) and micropores (1nm-10nm), which significantly improves the adsorption performance after carbon coating.

[0126] 3. Electrochemical performance test

[0127] The electrochemical performance of lithium-ion batteries assembled with the negative electrode materials obtained in the Examples and Comparative Examples was tested. The negative electrode sheet was prepared by mixing FeP2 / N-FeP2 / NC@FeP2 material, CMC, SBR, SP, and H2O in a mass ratio of 95:2.5:1.5:1:150. A lithium sheet served as the positive electrode, and the electrolyte employed was LiPF6 / EC+DEC (LiPF6 as the electrolyte, a 1:1 volume ratio of EC and DEC as the solvent, with an electrolyte concentration of 1.3 mol / L). The separator employed a composite membrane of polyethylene (PE), polypropylene (PP), and polyethylene (PEP). The button-type batteries were assembled in an argon-filled glove box. Refer to GB / T 37201:2018 for the initial discharge specific capacity of button-type batteries at a current density of 100 mA / g. Refer to GB / T 37207:2018 for the capacity retention of button-type batteries after 1000 charge and discharge cycles at a current density of 200 mA / g.

[0128] The results are shown in Table 2.

[0129] Table 2

[0130]

[0131]

[0132] It can be seen from the data in Table 1 that compared with pure FeP2, the electrochemical performance of the composite material is significantly improved after the introduction of nitrogen elements and surface coating with a carbon shell, and the parameter changes in the preparation process affect the electrochemical performance of the product.

[0133] like Figure 5 As shown in the figure, at a current density of 200 mA / g, the capacity retention rate of pure FeP2 after more than 400 cycles is only 35%, the capacity retention rate of N-FeP2 after 1000 cycles is 69%, and the capacity retention rate of NC@FeP2 after 1000 cycles is 87%. After nitrogen doping and carbon coating, the electrochemical cycle stability of the material is significantly improved; Figure 6 As shown, the resistance of pure phase FeP2 is significantly greater than that of N-FeP2, and the resistance of N-FeP2 is significantly greater than that of NC@FeP2, indicating that nitrogen doping and carbon coating reduce the resistance of the material.

[0134] After N doping and surface carbon coating, the volume expansion and fragmentation of the material have been improved accordingly. At the same time, the introduction of nitrogen element reduces the material resistance and effectively improves the rate performance, which also contributes to the improvement of electrochemical cycle stability.

[0135] Although the present invention has been illustrated and described using specific embodiments, it should be appreciated that the above embodiments are merely intended to illustrate the technical solutions of the present invention rather than to limit them. Those skilled in the art should understand that the technical solutions described in the above embodiments may be modified, or some or all of the technical features thereof may be replaced by equivalents, without departing from the spirit and scope of the present invention. However, these modifications or replacements do not deviate from the essence of the corresponding technical solutions within the scope of the technical solutions of the embodiments of the present invention. Therefore, this means that all such replacements and modifications within the scope of the present invention are included in the appended claims.

Claims

1. A method for preparing an iron phosphide negative electrode material, characterized in that: The following steps are involved: (1) mixing an iron source, a nitrogen source, and a solvent, and performing a solvothermal reaction; after the reaction is completed, washing, drying, and grinding to obtain an N-FeP2 precursor; wherein the nitrogen source comprises at least one of urea, melamine, and ammonium nitrate; and the solvent comprises water, a first organic solvent, and a second organic solvent, wherein the first organic solvent comprises NMP, and the second organic solvent comprises one of ethylene glycol and DMF; (2) placing the N-FeP2 precursor and sodium hypophosphite in two reactors respectively, placing the sodium hypophosphite upstream of the incoming gas flow direction, and placing the N-FeP2 precursor downstream of the incoming gas flow direction, heating under a protective gas atmosphere for phosphating reaction, and then cooling, and grinding the cooled product under a protective gas atmosphere to obtain nano-sized N-FeP2; (3) Drying the solution containing the carbon source and the N-FeP2, heating the dried product in a protective gas atmosphere for pre-carbonization and carbonization reaction, and obtaining the iron phosphide negative electrode material NC@FeP2 after cooling.

2. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: The mass ratio of the iron source to the nitrogen source is 4:1-1:2, and the usage ratio of the solvent to the iron source is 25-50 mL / g.

3. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: The iron source includes at least one of FeCl2, Fe(NO3)2, FeSO4 and organic ferrous salts.

4. The method for preparing the iron phosphide negative electrode material according to claim 1, wherein: The volume ratio of the water, the first organic solvent and the second organic solvent is 1:1-3:1-3.

5. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: The temperature of the solvent thermal reaction is 120-180° C., and the reaction time is 5-8 hours.

6. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: The washing includes washing with anhydrous ethanol and / or washing with deionized water.

7. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: In step (1), the grinding is ball milling, and the rotation speed of the ball mill is 400-600 rpm.

8. The method for preparing the iron phosphide negative electrode material according to claim 7, characterized in that: The ball milling is performed 3-5 times, and the time for each ball milling is 15-30 minutes.

9. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: The mass ratio of the N-FeP2 precursor to the sodium hypophosphite is 1:10-1:

5.

10. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: In step (2), the heating rate is 3-5°C / min, the temperature of the phosphating reaction is 320-350°C, and the holding time is 1-3h.

11. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: In step (2), the cooling rate is 5-10°C / min, and heating is stopped when the temperature drops to 180-150°C, and the mixture is cooled naturally.

12. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: In step (2), the grinding is ball milling, the rotation speed of the ball mill is 100-200 rpm, and the ball milling time is 0.5-2 h.

13. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: In step (3), the concentration of the carbon source in the solution is 1-30 wt %, and the mass ratio of the carbon source to the N-FeP2 is 2:1-1:

20.

14. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: The carbon source comprises at least one of glucose, asphalt, polydopamine, resorcinol+formaldehyde, starch, sucrose, sodium carboxymethyl cellulose, polyvinylidene fluoride, polyvinyl pyrrolidone, tannic acid, polyvinyl alcohol, polypyrrole, vitamin C, polyethylene glycol, anthracene, and aniline.

15. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: The heating rate of the pre-carbonization is 3-5°C / min, the temperature of the pre-carbonization is 300-350°C, and the time of the pre-carbonization is 60-120min.

16. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: The heating rate of the carbonization reaction is 3-5°C / min, the temperature of the carbonization reaction is 700-900°C, and the time of the carbonization reaction is 60-180min.

17. The method for preparing the iron phosphide negative electrode material according to claim 1, characterized in that: In step (2) and step (3), the protective gas is argon or nitrogen.

18. The iron phosphide negative electrode material prepared by the method for preparing the iron phosphide negative electrode material according to any one of claims 1 to 17.

19. A lithium battery comprising the iron phosphide negative electrode material according to claim 18.

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