Nitrogen-doped silicon-carbon composite negative electrode material and preparation method thereof

Abstract

The application discloses a kind of nitrogen-doped silicon-carbon composite negative materials and preparation method thereof.Silicon-carbon negative material inside uses nitrogen-doped needle-shaped array graphite sheet, needle-shaped array graphite sheet can effectively improve not only nano-silicon particle load, and nano-silicon particle is evenly dispersed on needle-shaped array graphite sheet surface, reduce reunion.At the same time, silicon-carbon composite material is mixed by needle-shaped array graphite sheet, nitrogen-doped one-dimensional carbon nanomaterial and nano-silicon, builds core structure, forms composite particle.And by amorphous carbon on the surface of composite particle, a layer of dense amorphous carbon is coated, finally form novel stable structure, excellent performance silicon-carbon negative material.The material can effectively alleviate the reunion of nano-silicon particle, and provide certain buffer space for silicon volume expansion, finally improve the initial efficiency and cycle stability of silicon-carbon negative material.

Description

TECHNICAL FIELD

[0001] The application relates to a nitrogen-doped silicon-carbon composite negative electrode material and a preparation method thereof, and belongs to the technical field of lithium battery negative electrode materials. BACKGROUND

[0002] At present, graphite-based negative electrode materials occupy a large market share due to low prices, and the theoretical specific capacity of commonly used graphite-based negative electrode materials is 372 mA.h / g. However, the reversible specific capacity of the graphite-based negative electrode materials on the market is close to the theoretical specific capacity, and the space for improvement is very limited. Silicon-based negative electrode materials are expected to replace the current commercial graphite negative electrode materials due to high capacity (the highest specific capacity is 4200 mAh / g), low lithium extraction voltage, low reactivity with electrolyte, environmental friendliness and other advantages. However, in practical applications, due to the low conductivity and large volume effect of the silicon-based negative electrode materials, the structure of the materials collapses and the materials fall off from the current collector during the lithium extraction process, resulting in rapid decrease of the cycle stability.

[0003] In recent years, inhibiting the volume expansion of silicon materials and improving the structural stability of the materials have become the main research direction of silicon-based composite materials. At present, the volume expansion of silicon is improved mainly through nanocrystallization of silicon, alloying of silicon and metals and silicon-carbon composite. The silicon-carbon composite has quite high application prospect.

[0004] Patent document CN103682287A discloses a lithium ion battery silicon-based composite negative electrode material, a preparation method and a battery, which is a mixture of hollow graphite and silicon nanomaterials. Since the graphite is subjected to hollow treatment, the specific surface area is greatly improved to 20-180 m2 / g. Although the silicon particles are also aggregated in the cavities of the graphite, the volume of the cavities is small and the specific surface area is large, the distribution of the silicon nanomaterials is improved, and the expansion of the silicon is reduced to a certain extent. However, the silicon nanomaterials still have a large amount of aggregation, the cycle performance is reduced, and the silicon-carbon composite belongs to the second generation of silicon-carbon composite. SUMMARY

[0005] The present application needs to design a negative electrode material capable of inhibiting the agglomeration of silicon, relieving the volume expansion of silicon and improving the lithium ion charge mobility in a long-time charge and discharge process. The main purpose of the present application is: on the one hand, to provide a nitrogen-doped needle-shaped array graphite sheet material and a preparation method thereof. The preparation method is simple in process, and the nitrogen-doped needle-shaped array graphite sheet material prepared has a needle-shaped surface on the graphite sheet, which can enhance the compounding of the graphite sheet with other materials, increase the migration channel of lithium ions, change the migration path of lithium ions, accelerate the migration rate of lithium ions by nitrogen doping, and improve the rate performance of the material; on the other hand, the present application also provides a nitrogen-doped silicon-carbon composite negative electrode material containing the above needle-shaped array graphite and a preparation method thereof. The needle-shaped array graphite can effectively prevent the agglomeration of nano-silicon material, so that the nano-silicon is uniformly loaded on the surface of the needle-shaped graphite sheet, which can effectively relieve the agglomeration of nano-silicon particles and provide a certain buffer space for the volume expansion of silicon; and by nitrogen doping, the lithium ion migration rate of the composite material is improved, and finally the initial efficiency and cycle stability of the silicon-carbon negative electrode material are improved.

[0006] A nitrogen-doped silicon-carbon composite negative electrode material has a core-shell structure, the inner core contains graphite sheets and nano-silicon, and the surface of the graphite sheet is left with a dot-shaped nitrogen-doped carbon material, and the outer shell is a carbon coating layer.

[0007] The thickness of the graphite sheet is selected to be 5-500nm, preferably 5-100nm, and the specific surface area is 10-270m 2 / g, preferably 200-270m 2 / g.

[0008] The inner core also contains one-dimensional conductive materials.

[0009] The one-dimensional conductive material is a one-dimensional carbon nanomaterial, the tube diameter of the one-dimensional carbon nanomaterial is 1-100nm, preferably 5-20nm, and the length is 1-50μm, preferably 5-20μm; the one-dimensional carbon nanomaterial can be one of carbon nanofibers, single-walled carbon nanotubes and multi-walled carbon nanotubes or a combination of at least two.

[0010] The one-dimensional carbon nanomaterial is doped with nitrogen elements.

[0011] The median particle size of the nano-silicon particles is 10-500nm, preferably 50-100nm.

[0012] The thickness of the carbon coating layer is 5-500nm, preferably 10-100nm.

[0013] The carbon material in the carbon coating layer is amorphous.

[0014] The preparation method of the nitrogen-doped silicon-carbon composite negative electrode material comprises the following steps:

[0015] Step 1, oxidizing the graphite sheet;

[0016] Step 2, placing the graphite sheet obtained in step 1 in a solution containing aniline to perform a polymerization reaction, growing a dot-matrix polyaniline on the surface, and performing a calcination treatment;

[0017] Step 3, uniformly mixing the graphite sheet obtained in step 2, nano-silicon, and a dispersing agent in an organic solvent, and then performing a drying treatment to obtain a first precursor;

[0018] Step 4, performing mechanical fusion on the first precursor obtained in step 3, and then performing carbon source coating to obtain a second precursor, and performing calcination to obtain a nitrogen-doped silicon-carbon composite negative electrode material.

[0019] In step 1, the oxidation treatment is to immerse the graphite sheet in a strong oxidizing agent solution to react.

[0020] The immersion time is 2-24 h, preferably 8-12 h, and more preferably 9-10 h.

[0021] The strong oxidizing agent can be selected from one or a combination of at least two of nitric acid, hydrogen peroxide solution, potassium persulfate solution, sulfuric acid, and ferric chloride solution.

[0022] After oxidation, the graphite sheet is dried by one or a combination of two of centrifugation, suction filtration, freeze-drying, vacuum air-drying, and spray drying.

[0023] In step 2, the solution containing aniline is a solution containing aniline, ammonium persulfate, and a protonic acid.

[0024] The concentration of aniline is 1-10 mmol, preferably 3-7 mmol, and more preferably 4-6 mmol; preferably, the concentration of ammonium persulfate is 0.2-1 mmol, preferably 0.5-0.8 mmol; preferably, the protonic acid solution can be selected from one or a combination of at least two of hydrochloric acid solution, sulfuric acid solution, p-aminobenzenesulfonic acid solution, dodecylbenzenesulfonic acid solution, sulfosalicylic acid solution, and camphorsulfonic acid solution; preferably, the concentration of the protonic acid solution is 0.2-5 mmol, preferably 0.5-0.8 mmol.

[0025] In step 2, the polymerization temperature is 0-20℃, preferably -3- -15℃, and more preferably -5- -10℃; the polymerization time is 6-48 h, preferably 8-36 h, and more preferably 12-24 h.

[0026] In step 2, the calcination is twice, the first calcination is at a temperature of 0-500℃, preferably 200-400℃, more preferably 250-350℃; the calcination time is 0-6h, preferably 0.5-4h, more preferably 1-2h; the heating rate is 1℃ / min; the second calcination is at a temperature of 700-1200℃, preferably 800-1100℃, more preferably 900-1000℃; the calcination time is 2-12h, preferably 2-8h, more preferably 3-7h; the heating rate is 5℃ / min; the protective gas for calcination is one or a combination of at least two of nitrogen, argon, helium, neon, krypton and xenon.

[0027] In step 3, the weight ratio of silicon nanoparticles, nitrogen-doped needle-shaped array graphite, dispersant and organic solvent is (1-50):(1-50):(0.1-10):(100-1000).

[0028] In step 1, one-dimensional carbon nanomaterials are also added during mixing; the weight ratio of one-dimensional carbon nanomaterials to silicon nanoparticles is (1-20):(1-50).

[0029] In step 3, the dispersant is one or a combination of at least two of sodium hexametaphosphate, sodium dodecyl sulfate, hexaalkyltrimethylammonium bromide, polyetherimide or polyacrylamide; the mixing method is one or a combination of both of low-speed stirring and ultrasonic, for a time of 1-10h; the drying is one or a combination of two of spray drying, reduced pressure distillation and freeze drying.

[0030] In step 4, the mechanical fusion process, the gap between the tool and the cavity wall is 10-1000mm, preferably 10-100mm; the rotation speed is 500-1800rpm, preferably 1200-1800rpm; the fusion time is 0.2-10h, preferably 0.5-3h.

[0031] In the process of coating with a carbon source, the first precursor and the organic carbon source are mixed by VC mixing, the mixing rotation speed is 500-1500rpm, preferably 1000-1500rpm; the mixing time is 0.5-10h, preferably 3-5h.

[0032] The carbon source is one or a combination of at least two of coal tar, coal pitch, petroleum pitch, epoxy resin, phenolic resin, acrylic resin, furfural resin, polyvinyl chloride, polyacrylonitrile and polyvinylidene fluoride; preferably, the median particle size of the carbon source is 0.01-100 microns, preferably 0.1-20 microns; preferably, the ratio of the precursor and the carbon source is 1:1-20:1, preferably 3:1-5:1; during the coating process, the rotation speed of the main shaft is 100-1000 rpm, preferably 150-500 rpm; the coating temperature is 100-1100 DEG C, preferably 300-850 DEG C; the heating rate is 1-20 DEG C / min, preferably 3-10 DEG C / min; the coating time is 0.5-30 h, preferably 8-20 h.

[0033] In step 4, the calcination temperature is 500-1250 DEG C, preferably 800-1200 DEG C; the heating rate is 1-20 DEG C / min, preferably 5-15 DEG C / min; the sintering time is 1-20 h, preferably 5-15 h; preferably, the protective gas is one or a combination of at least two of nitrogen, argon, helium, neon, krypton and xenon, and the nitrogen-doped silicon-carbon composite negative electrode material is obtained after high-temperature sintering.

[0034] A lithium ion battery containing the above-mentioned nitrogen-doped silicon-carbon composite negative electrode material.

[0035] The application of the above-mentioned nitrogen-doped silicon-carbon composite negative electrode material in the manufacture of lithium ion batteries.

[0036] Advantages

[0037] The nitrogen-doped needle-shaped array graphite sheet prepared by the application has the following advantages: the graphite sheet is soaked in a strong oxidizing agent solution, which will make the graphite sheet surface carry negative charge groups, which are one or a combination of at least two of hydroxyl, carboxyl and carbonyl, so that the graphite sheet surface is more easily combined with aniline monomers. The aniline monomers are deposited on the surface of the pre-oxidized graphite sheet through π-π bond conjugation and hydrogen bond effect, and gradually grow into an array. At the same time, the polyaniline still maintains an array form after carbonization, increases the migration channel of lithium ions, changes the migration path of lithium ions, and accelerates the transmission rate of lithium ions. The needle-shaped array structure can uniformly disperse the nanosilicon, which can effectively alleviate the huge volume expansion of the nanosilicon material caused by agglomeration, provide an effective attachment site for the nanosilicon, and provide a buffer space for the silicon material after calcination. BRIEF DESCRIPTION OF DRAWINGS

[0038] Figure 1 It is a flowchart of the preparation of the nitrogen-doped silicon-carbon composite negative electrode material in the examples.

[0039] Figure 2SEM image of the surface aciculate nitrogen-doped graphite sheet prepared. DETAILED DESCRIPTION Example 1

[0040] 1.2 kg of graphite sheets with a thickness of 100-300 nm were immersed in a 30% wt hydrogen peroxide solution for 4 h. The pre-oxidized graphite sheets were then placed in a 3 mmol of 60 ml aniline solution, followed by 0.4 mmol of ammonium persulfate added to 40 ml of 10 wt% dilute hydrochloric acid solution and poured into the above aniline solution, and polymerization was carried out at -5°C for 12 h. After polymerization, the graphite sheets were placed in a freeze dryer for drying. A polyaniline array / graphite sheet composite material was obtained. The polyaniline array / graphite sheet composite material was placed in a muffle furnace for the first calcination, the calcination temperature was 300°C, the heating rate was 1°C / min, and the holding time was 2 h. Subsequently, the second calcination was carried out at a calcination temperature of 900°C, a heating rate of 5°C / min, and a holding time of 4 h, and the aciculate array graphite sheet was obtained after the furnace was cooled. 1.1 kg of nano-silicon powder, 1 kg of nitrogen-doped aciculate array graphite sheet, 1 kg of one-dimensional nitrogen-doped single-walled carbon nanotube, and 18 kg of ethanol (ratio of 1.1:1:1:18) were dispersed in a ball milling device for 6 h to disperse the agglomerated silicon particles and mix them with the nitrogen-doped aciculate array graphite sheet. The mixed slurry was dried under vacuum blast drying under high-purity nitrogen protection (purity 99.999%), and a dry silicon, one-dimensional nitrogen-doped single-walled carbon nanotube, and nitrogen-doped aciculate array graphite sheet mixed powder was obtained. Then, after mechanical fusion of the dried mixed powder for 1 h (knife-to-cavity wall gap of 50 mm; rotation speed of 1500 rpm), 1.8 kg of the mixed powder and 0.384 kg of pitch were placed in a high-temperature coating device, and under high-purity nitrogen protection, the materials were mixed by stirring at 500 rpm for 3 h, then the temperature was increased to 150°C at a rate of 5°C / min and held for 5 h, then the temperature was increased to 500°C at the same rate, and held for 2 h, and finally the temperature was increased to 900°C and held for 3 h, and then naturally cooled to room temperature. Example 2

[0041] The difference from Example 1 is that the polymerization temperature of the pre-oxidized graphite sheet in the aniline mixed solution is 0°C. Example 3

[0042] The difference from Example 1 is that the second calcination temperature of the aciculate array graphite sheet is 1000°C. Example 4

[0043] The difference from Example 1 is that the polymerization time of the graphite sheet in the aniline solution is 24 h. Example 5

[0044] The difference from Example 1 is that the strong oxidant solution for pre-oxidation of the graphite sheet is a mixed solution of 98.3wt% concentrated sulfuric acid and 42.24wt% concentrated nitric acid.

[0045] Comparative Example 1

[0046] The difference from Example 1 is that the graphite sheet is not pre-oxidized.

[0047] Comparative Example 2

[0048] The difference from Example 1 is that the surface of the graphite sheet is not coated with polyaniline, but is directly mixed and coated with silicon nanoparticles, nitrogen-doped single-walled carbon nanotubes, etc.

[0049] Battery preparation and test procedure

[0050] The electrochemical performance of the nitrogen-doped silicon-carbon composite negative electrode material was evaluated by assembling it into a CR2032-coin half-cell. The coin cell was prepared as follows: the mass ratio of active material (Si / C), acetylene black, CMC, and SBR was 80:10:4:6, and the CMC was a 1% aqueous solution. The slurry was dispersed with a high-speed shear mixer (Fluko FA25, Germany) at 10,000 rpm for 30 min. The well-mixed slurry was then uniformly coated on a 15-μm-thick copper foil. After natural air-drying, the copper foil was dried in a vacuum drying oven at 80°C for 10 h, and the dried copper foil was compacted with a roller press; the electrode sheet was punched and cut into a 13-mm-diameter disc. The half-cell was assembled in a glove box under high-purity argon protection, using a lithium metal foil as the counter electrode and a polypropylene porous membrane as the separator, and 1 M LiPF6 and 5wt% fluoroethylene carbonate were added to a mixed solution of ethylene carbonate / dimethyl carbonate / methyl ethyl carbonate (volume ratio 1:1:1) as the electrolyte. The battery charge and discharge tests were performed in a multi-channel battery, and the test voltage range of the silicon-carbon material was 0.01-1.5 V (vs. Li + / Li).

[0051] The main electrochemical test results of the electrode materials prepared in the above examples and comparative examples after being made into batteries are summarized as follows:

[0052]

[0053] From the above table, it can be seen that the first coulombic efficiency and cycle capacity retention in the comparative example are far lower than those in Example 1, which can well prove that the needle-like array structure is beneficial to disperse the nano-silicon particles and can provide good buffer space for the nano-silicon, so that the cycle capacity retention of the battery can be greatly improved. From the comparison of Example 1 and Examples 2 and 3, it can also be clearly seen that the different polymerization temperatures and polymerization times of polyaniline have an effect on the battery performance, because the different polymerization temperatures and polymerization times have a great influence on the array formation, thereby changing the dispersibility of the nano-silicon particles. It can be seen from the comparison of Comparative Example 1 and Example 1 that the pre-oxidation treatment on the surface of the graphite sheet can make the surface generate negative charged groups, improve the combination amount of aniline, and make the various performances of the battery have a significant improvement; it can be seen from the comparison of Comparative Example 2 and Example 1 that when the graphite sheet surface does not generate polyaniline with a needle-like shape, the battery performance is significantly decreased, which shows that the formation of polyaniline modification points on the surface of the graphite sheet can effectively improve the performance of the electrode material.

Claims

1. A lithium-ion battery, characterized by, The nitrogen-doped silicon-carbon composite negative electrode material has a core-shell structure, the inner core contains graphite sheets and nano-silicon, and the surface of the graphite sheets is left with a dot array of nitrogen-doped carbon material, and the shell is a carbon coating layer. The preparation method of the nitrogen-doped silicon-carbon composite negative electrode material comprises the following steps: Step 1: oxidizing treatment of graphite sheets; Step 2: placing the graphite sheets obtained in step 1 in a solution containing aniline to perform a polymerization reaction, growing a dot array of polyaniline on the surface, and performing calcination treatment; Step 3: uniformly mixing the graphite sheets, nano-silicon and dispersing agent obtained in step 2 in an organic solvent, and then performing drying treatment to obtain a first precursor; Step 4: performing mechanical fusion on the first precursor obtained in step 3, then performing carbon source coating to obtain a second precursor, and performing calcination to obtain the nitrogen-doped silicon-carbon composite negative electrode material; In step 1, the oxidizing treatment is to immerse the graphite sheets in a strong oxidizing agent solution for reaction; the immersion time is 2-24 hours; the type of strong oxidizing agent is selected from one or a combination of at least two of concentrated nitric acid, hydrogen peroxide solution, potassium persulfate solution, concentrated sulfuric acid or ferric chloride solution; In step 2, the solution containing aniline is a solution containing aniline, ammonium persulfate and a proton acid; The concentration of aniline is 1-10 mmol; the concentration of ammonium persulfate is 0.2-1 mmol; the proton acid solution is selected from one or a combination of at least two of hydrochloric acid solution, sulfuric acid solution, p-aminobenzenesulfonic acid solution, dodecylbenzenesulfonic acid solution, sulfosalicylic acid solution and camphorsulfonic acid solution, and the concentration of the proton acid solution is 0.2-5 mmol; In step 2, the polymerization temperature is 0-20°C, and the polymerization time is 6-48 hours; In step 2, the calcination is twice, the first calcination temperature is 200-400°C, the calcination time is 0.5-4 hours, and the heating rate is 1°C / min; the second calcination temperature is 700-1200°C, the calcination time is 2-12 hours, and the heating rate is 5°C / min; the protective gas for calcination is nitrogen; In step 3, the weight ratio of silicon nanoparticles, nitrogen-doped needle-shaped array graphite, dispersing agent and organic solvent is (1-50):(1-50):(0.1-10):(100-1000).

2. The lithium-ion battery of claim 1, wherein, The graphite sheet has a thickness of 5-500 nm and a specific surface area of 10-270 m 2 / g.

3. The lithium-ion battery of claim 1, wherein, The inner core further contains one-dimensional conductive material; The one-dimensional conductive material is one-dimensional carbon nanomaterial, the tube diameter of the one-dimensional carbon nanomaterial is 1-100 nm, and the length is 1-50 μm; The one-dimensional carbon nanomaterial is one or a combination of at least two of carbon nanofiber, single-walled carbon nanotube and multi-walled carbon nanotube; The one-dimensional carbon nanomaterial is doped with nitrogen elements; in step 3, the one-dimensional carbon nanomaterial is also added in the mixing process; the weight ratio of the one-dimensional carbon nanomaterial to the silicon nanoparticles is (1-20):(1-50).

4. The lithium-ion battery of claim 1, wherein, The median particle size of the nano-silicon is 10-500 nm; The thickness of the carbon coating layer is 5-500 nm; The carbon material in the carbon coating layer has an amorphous structure.

5. The lithium-ion battery of claim 1, wherein, The graphite sheet is dried after oxidation, and the drying method is one or a combination of two of centrifugation, suction filtration, freeze drying, vacuum air drying and spray drying.

6. The lithium-ion battery of claim 1, wherein, In step 3, the dispersant is one or a combination of two of sodium hexametaphosphate, sodium dodecyl sulfate, hexaalkyltrimethylammonium bromide, polyetherimide or polyacrylamide; the mixing method is one or a combination of two of low-speed stirring and ultrasonic, and the time is 1-10 h; and the drying is one or a combination of two of spray drying, reduced pressure distillation and freeze drying.

7. The lithium-ion battery of claim 5, wherein, In step 4, the calcination temperature is 500-1250 DEG C, the heating rate is 1-20 DEG C / min, the sintering time is 1-20 h, the protective gas is nitrogen, and the nitrogen-doped silicon-carbon composite negative electrode material is obtained after high-temperature sintering.

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