Method for preparing graphene-coated nitrogen-carbon-doped silicon negative electrode and application of graphene-coated nitrogen-carbon-doped silicon negative electrode

Abstract

According to the method for preparing the graphene-coated nitrogen-carbon-doped silicon negative electrode and the application of the graphene-coated nitrogen-carbon-doped silicon negative electrode, a high-energy ball milling method and a catalytic pyrolysis method are adopted, preparation of nitrogen-doped graphene can be completed in one step, silicon nanosheets and graphene can be effectively compounded, and the method is simpler, more convenient, more efficient and lower in cost; and meanwhile, the prepared material is relatively uniform in structure and has the advantage of easiness in large-scale production. The nitrogen-doped graphene is introduced into the silicon negative electrode material, the conductivity and the stability are remarkably improved, more active sites can be provided through nitrogen doping, the problem of volume expansion of silicon in the charging and discharging process of the silicon is solved, and therefore the electrochemical performance is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of negative electrode materials for lithium-ion batteries, and more specifically, to a method for preparing a graphene-coated nitrogen-carbon-doped silicon negative electrode and an application thereof. Background Art

[0002] At present, the negative electrode materials of lithium-ion batteries mainly use graphite, silicon and their composite materials. Among them, silicon negative electrode materials have become the focus of research due to their ultra-high specific capacity, but the volume expansion problem of silicon during the charging and discharging process has limited its application in batteries. Therefore, many researchers have tried to improve the cycle stability and conductivity of silicon negative electrodes by compounding silicon with conductive materials (such as graphene, carbon nanotubes, etc.). The role of nitrogen doping in carbon materials has been widely studied. Nitrogen doping can adjust the electronic structure of the material, enhance its conductivity, and improve the interaction with lithium ions. In silicon-carbon composite materials, nitrogen doping can effectively improve the electrochemical properties of silicon and alleviate the volume expansion problem by changing the surface structure of the material. In the prior art, nitrogen-doped silicon-carbon composite materials are mainly used in the development of high energy density batteries, with the goal of improving the capacity and cycle stability of the battery. Nitrogen doping is usually achieved by nitrogen treatment, chemical vapor deposition (CVD) or solution method. Through these methods, nitrogen elements can be evenly distributed on the surface of silicon or in the carbon layer, thereby improving the stability and conductivity of the composite material.

[0003] Among the existing methods for preparing silicon-carbon negative electrodes, the sol-gel method is to mix a silicon precursor (such as silanol) with a carbon source (such as glucose, polypyrrole, etc.), and carry out a chemical reaction in the solution to form a silicon and carbon composite. After heat treatment, a silicon-carbon composite material with a stable structure is formed. During the sintering process of the silicon-carbon composite material prepared by this method, the agglomeration of silicon particles may cause the specific surface area of ​​the material to decrease, affecting its electrochemical properties, and the carbon coating is incomplete. Although the sol-gel method can form a carbon coating layer on the silicon surface, the thickness and uniformity of the layer are difficult to control, which may lead to unstable performance of the composite material.

[0004] Another method is chemical vapor deposition (CVD) to prepare silicon-carbon composite negative electrodes. During the CVD process, silicon source and carbon source react in the gas phase, and carbon layers or carbon materials such as graphene are deposited on the surface of silicon particles through chemical reactions. This silicon-carbon composite negative electrode material is widely used in electric vehicle batteries and high-performance energy storage systems. However, the equipment of the CVD method is complex and the cost is high, which limits its application in large-scale production. Although this method can accurately control the quality of the carbon layer, it is difficult to achieve in large-scale production due to the complex process.

[0005] Therefore, it is necessary to propose a method for preparing graphene-coated nitrogen-carbon doped silicon negative electrode and its application to solve the problems of complex process for preparing silicon-carbon negative electrode and low conductivity and poor stability of the formed silicon negative electrode material, which is of great significance to improving the performance of lithium-ion batteries. Summary of the invention

[0006] The present invention provides a method for preparing a graphene-coated nitrogen-carbon-doped silicon negative electrode and application thereof, so as to solve the problems that the existing process for preparing a silicon-carbon negative electrode is complicated and the formed silicon negative electrode material has low conductivity and poor stability.

[0007] According to one aspect of the present invention, there is provided a method for preparing a graphene-coated nitrogen-carbon-doped silicon negative electrode, comprising the following steps:

[0008] Step 1, cleaning silicon with HF to remove the surface oxide layer to obtain silicon particles;

[0009] Step 2, transferring the silicon particles obtained in step 1 to a ball milling jar lined with stainless steel, and ball milling to obtain nano silicon nSi;

[0010] Step 3, adding the nSi, glucose and melamine obtained in step 2 into a ball mill and ball milling, and carbonizing the obtained product in a tube furnace under Ar conditions to obtain the product nSi@NC;

[0011] Step 4: Add the nSi@NC and graphene obtained in step 3 into a ball mill and perform ball milling to obtain nSi@NC / GN.

[0012] Preferably, based on the above scheme, the particle size of silicon in step 1 is 300um, and the HF concentration in step 1 is 2%; in step 2, the volume of the ball mill is 100ml, the ball-to-material ratio is 20:1, the rotation speed is 800rpm, and the time is 10h.

[0013] Preferably, in step 3, the mass ratio of nSi, glucose and melamine is 1:2:1, the volume of the ball mill is 100 ml, the ball-to-material ratio is 20:1, the rotation speed is 450 rpm, the ball milling time is 6 h, the carbonization temperature is 800 ° C, the carbonization time is 2 h, and the heating rate is 10 ° C / min;

[0014] In the step 4, the mass ratio of nSi@NC to graphene is 4:1, the volume of the ball mill is 50 ml, the ball-to-material ratio is 20:1, the rotation speed is 400 rpm, and the ball milling time is 2 h.

[0015] The present invention also provides an application of a graphene-coated nitrogen-carbon-doped silicon negative electrode for testing the lithium storage performance of a lithium-ion battery negative electrode.

[0016] Preferably, based on the above scheme, it comprises the following steps:

[0017] Step A1, grinding nSi@NC / GN, conductive carbon and PVDF to form a conductive slurry, during the grinding process, NMP is used as a grinding aid,

[0018] Step A2, coating the slurry on the surface of the copper foil current collector, drying it in a vacuum oven and then cutting it into discs as the negative electrode of the lithium ion battery;

[0019] Step A3, assembling into button cells in a glove box filled with argon;

[0020] Step A4, performing cyclic voltammetry test and AC impedance test on the assembled battery in an electrochemical workstation, and testing the cycle performance, rate performance and kinetic performance in a battery testing system.

[0021] Preferably, based on the above scheme, in step A1, the mass ratio of the nSi@NC / GN, conductive carbon and PVDF added is 7:2:1, and the specification of the PVDF is 99%.

[0022] Preferably, based on the above scheme, in step A2, the slurry coating thickness is 0.9 mm, and the vacuum drying conditions are 80° C. for 10 h, the diameter of the cut disc is 10 mm, and the conditions in the glove box are filled with argon and the water oxygen content is less than 0.1 ppm.

[0023] Compared with the prior art, the method for preparing a graphene-coated nitrogen-carbon-doped silicon negative electrode of the present invention has the following advantages:

[0024] 1. High-energy ball milling and catalytic pyrolysis methods are used, which can not only complete the preparation of nitrogen-doped graphene in one step, but also effectively composite silicon nanosheets with graphene. This method is simpler, more efficient, and less costly. At the same time, the prepared material has a more uniform structure and is easy to scale up for production.

[0025] 2. The present invention introduces nitrogen-doped graphene into the silicon negative electrode material, which significantly improves the conductivity and stability. The present invention can provide more active sites through nitrogen doping and alleviate the volume expansion problem of silicon during the charging and discharging process of silicon itself, thereby significantly improving the electrochemical performance.

[0026] 3. The present invention combines nitrogen-doped graphene with silicon nanosheets to give full play to the high specific capacity of silicon, improve the conductivity of the material, effectively increase the specific surface area and conductivity of the material, thereby improving the capacity and cycle stability. Graphene not only provides a better electron conduction channel, but also reduces the expansion problem of silicon through structural stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the following briefly introduces the drawings required for describing the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work. In the drawings:

[0028] Figure 1 It is a schematic diagram of the synthesis of preparing the graphene-coated nitrogen-carbon-doped silicon negative electrode of the present invention;

[0029] Figure 2a is the XRD pattern of nSi@NC / GN of the present invention;

[0030] Figure 2b This is the Raman spectrum of nSi@NC / GN of the present invention;

[0031] Figure 2c is a morphology diagram of nSi@NC / GN of the present invention;

[0032] Figure 3a is the XRD pattern of nSi@NC of the present invention;

[0033] Figure 3b is the Raman spectrum of nSi@NC of the present invention;

[0034] Figure 3c is a morphology diagram of nSi@NC of the present invention;

[0035] Figure 4a is the XRD pattern of nSi of the present invention;

[0036] Figure 4b is the Raman spectrum of nSi of the present invention;

[0037] Figure 4c is a morphology diagram of nSi of the present invention;

[0038] Figure 5a Cyclic voltammetry curves of nSi, nSi@NC and nSi@NC / GN of the present invention;

[0039] Figure 5b It is the AC impedance fitting curve diagram of nSi, nSi@NC and nSi@NC / GN of the present invention;

[0040] Figure 6a The nSi@NC, nSi@NC / GN and nSi of the present invention are 0.5Ag -1 Cycling performance at current density of

[0041] Figure 6bThe rate performance diagram of nSi@NC, nSi@NC / GN and nSi of the present invention;

[0042] Figure 6c The nSi@NC, nSi@NC / GN and nSi in 1Ag -1 Cycling performance at current density of

[0043] Figure 7 The morphology comparison diagram of nSi@NC, nSi@NC / GN, nSi and original micron Si of the present invention;

[0044] Figure 8 These are the AC impedance diagrams of nSi@NC, nSi@NC / GN, nSi and original micron Si of the present invention. DETAILED DESCRIPTION

[0045] The specific implementation of the present invention is further described in detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present invention, but are not intended to limit the scope of the present invention.

[0046] It should be understood that when used in this specification and the appended claims, the term "comprising" indicates the presence of described features, integers, steps, operations, elements and / or components, but does not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or collections.

[0047] In order to simplify the drawings, only the parts related to the present invention are schematically shown in each figure, and they do not represent the actual structure of the product. In addition, in order to simplify the drawings and facilitate understanding, in some figures, only one of the parts with the same structure or function is schematically drawn or marked. In this article, "one" not only means "only one", but also means "more than one".

[0048] It should be further understood that the term “and / or” used in the specification and appended claims refers to any combination and all possible combinations of one or more of the associated listed items, and includes these combinations.

[0049] In the embodiments shown in the drawings, the indications of directions (such as up, down, left, right, front and back) used to explain the structure and movement of the various components of the present invention are not absolute but relative. These descriptions are appropriate when these components are in the positions shown in the drawings. If the descriptions of the positions of these components change, the indications of these directions also change accordingly.

[0050] In addition, in the description of the present application, the terms "first", "second", etc. are only used to distinguish the description and cannot be understood as indicating or implying relative importance.

[0051] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the specific implementation methods of the present invention will be described below with reference to the accompanying drawings. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings and other implementation methods can be obtained based on these drawings without creative work.

[0052] See also Figure 1 As shown, a method for preparing a graphene-coated nitrogen-carbon-doped silicon negative electrode of the present invention comprises the following steps:

[0053] Step 1, cleaning the silicon with HF, wherein the HF concentration is 2%, washing away the surface oxide layer to obtain silicon particles with a particle size of 300 um;

[0054] Step 2, transferring the silicon particles obtained in step 1 to a stainless steel-lined ball mill, and ball milling to obtain nano silicon nSi, wherein the ball mill volume is 100 ml, the ball-to-material ratio is 20:1, the rotation speed is 800 rpm, and the time is 10 hours;

[0055] Step 3, adding the nSi, glucose and melamine obtained in step 2 into a ball mill in a mass ratio of 1:2:1, the ball-to-material ratio is 20:1, the rotation speed is 450 rpm, the ball milling time is 6 hours, and the obtained product is carbonized in a tubular furnace under Ar conditions to obtain the product nSi@NC, wherein the carbonization temperature is 800°C, the carbonization time is 2 hours, and the heating rate is 10°C / min;

[0056] Step 4: Add the nSi@NC and graphene obtained in step 3 into a ball mill at a mass ratio of 4:1 for ball milling. The ball-to-material ratio in the ball mill is 20:1, the rotation speed is 400 rpm, and the ball milling time is 2 h to obtain nSi@NC / GN.

[0057] The present invention also provides an application of a graphene-coated nitrogen-carbon-doped silicon negative electrode for testing the lithium storage performance of a lithium-ion battery negative electrode, which comprises the following steps:

[0058] Step A1, grinding nSi@NC / GN, conductive carbon and PVDF in a mass ratio of 7:2:1 to form a conductive slurry, during the grinding process, NMP is used as a grinding aid, the conductive carbon is analytical grade and purchased from Sinopharm Group, and the PVDF specification is 99% and purchased from Solvay Co., Ltd., USA;

[0059] Step A2, coating the slurry on the surface of the copper foil current collector, wherein the coating thickness is 0.9 mm, and drying in a vacuum oven at 80° C. for 10 h, and cutting into discs as the negative electrode of the lithium ion battery after drying, wherein the diameter of the cut discs is 10 mm;

[0060] Step A3, assembling a button cell in a glove box filled with argon, the glove box is filled with argon and the water oxygen content is less than 0.1 ppm, the assembled button cell model is CR2302, the counter electrode used is a metal lithium sheet, the diaphragm used is a polypropylene diaphragm (Celgard 2500), and the electrolyte used is 1M lithium hexafluorophosphate (LiPF6) dissolved in a mixed solvent of ethyl carbonate (EC), dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC) (1:1:1, v / v / v);

[0061] Step A4, performing cyclic voltammetry test and AC impedance test on the assembled battery in an electrochemical workstation, and testing the cycle performance, rate performance and kinetic performance in a battery testing system.

[0062] The cyclic voltammetry test was carried out on a Shanghai Chenhua CHI660E electrochemical workstation, with the test voltage range set to 0.01-1.5 V and the scan rate set to 0.2 mV s -1 , 0.4mV s -1 , 0.6mV s -1 , 0.8mV s -1 and 1.0mV s -1 ;

[0063] The AC impedance test was carried out on a Shanghai Chenhua CHI660E electrochemical workstation with a test frequency of 0.01-100000 Hz;

[0064] The cycle performance test was carried out on the LAND CT2001A test system with a current density of 0.5Ag -1 ;

[0065] The rate performance was tested on the LAND CT2001A test system with a current density of 0.1Ag -1 , 0.2Ag -1 , 0.3Ag -1 , 0.4Ag -1 , 0.5Ag -1 , 1.0Ag -1 ;

[0066] The kinetic performance GITT was carried out in a LAND CT2001A test system with a current density of 0.1A. -1 , set the interval time to 30 minutes and the recording time to 10 seconds.

[0067] Compared with the prior art, the method for preparing a graphene-coated nitrogen-carbon-doped silicon negative electrode of the present invention has the following advantages:

[0068] High-energy ball milling and catalytic pyrolysis methods were used, which not only completes the preparation of nitrogen-doped graphene in one step, but also effectively compounds silicon nanosheets with graphene. This method is simpler, more efficient, and less costly. At the same time, the prepared material has a more uniform structure and is easy to scale up for production.

[0069] The present invention introduces nitrogen-doped graphene into the silicon negative electrode material, which significantly improves the conductivity and stability. The present invention can provide more active sites through nitrogen doping and alleviate the volume expansion problem of silicon during the charging and discharging process of silicon itself, thereby significantly improving the electrochemical performance.

[0070] The present invention combines nitrogen-doped graphene with silicon nanosheets to give full play to the high specific capacity of silicon, improve the conductivity of the material, effectively increase the specific surface area and conductivity of the material, thereby improving the capacity and cycle stability. Graphene not only provides a better electron conduction channel, but also reduces the expansion problem of silicon through structural stability.

[0071] In order to further illustrate the technical effects of the present invention in detail, the following will be described in detail with specific embodiments:

[0072] Example 1 Preparation of nSi@NC / GN and its application as negative electrode in lithium-ion batteries

[0073] (1) 5 g of silicon powder with a particle size of 300 μm was poured into a beaker containing 20 ml of 2% HF, stirred for 60 min on a magnetic stirrer, and then vacuum filtered. The mixture was vacuum filtered with deionized water for 3 times, and the product was collected and dried in a vacuum oven at 120° C. for 10 h to obtain 4.86 g of black solid powder.

[0074] (2) The black powder obtained in (1) was placed in a stainless steel lined 100 ml solvent ball mill, 100 g of a 5 mm radius stainless steel ball was added, and the mixture was ball milled at 800 rpm for 10 h. The resulting product was a gray-black powder of 3.78 g, i.e., nSi.

[0075] (3) 1 g of nSi obtained in step (2), 2 g of glucose, and 1 g of melamine were added to a ball mill, and 80 g of stainless steel balls were added. The ball mill was milled at 450 rpm for 6 h. The product was carbonized in a tube furnace at a heating rate of 10 °C / min at 800 °C under Ar conditions for 2 h to obtain 3.6 g of a black product, namely nSi@NC;

[0076] (4) 3.6 g of nSi@NC and 0.9 g of graphene obtained in step (3) were added to a ball mill, and 90 g of stainless steel balls were added, and ball milled at 400 rpm for 2 h to obtain 3 g of black solid powder, namely nSi@NC / GN;

[0077] (5) The structural morphology of nSi@NC / GN was characterized by XRD, Raman, SEM and STEM. The results are as follows Figure 2a-2c As shown;

[0078] (6) 35 mg of nSi@NC / GN, 10 mg of conductive carbon and 5 mg of PVDF were ground in an agate mortar for 20 min. 10 drops of NMP were added to obtain a black viscous conductive slurry, namely nSi@NC / GN slurry. The slurry was evenly coated on a copper foil conductive current collector with a thickness of 0.9 mm using an automatic coating machine and dried in a vacuum drying oven at 80 °C for 10 h.

[0079] (7) Take out the dried nSi@NC / GN electrode, use the slicer produced by Shenzhen Kejing Material Technology Co., Ltd. to cut the electrode into circular electrode pieces with a diameter of 10 mm, take out five electrode pieces and weigh them to obtain that the mass of active materials of the five nSi@NC / GN electrode pieces is 0.5 mg;

[0080] (8) Five weighed electrode pieces and a polypropylene diaphragm with a diameter of 16 mm were placed in the transition compartment of the glove box and evacuated three times before being brought into the glove box (the battery assembly, metal lithium sheet and electrolyte were originally in the glove box). The button lithium-ion battery was assembled in the order of negative electrode shell → spring → gasket → nSi@NC / GN electrode piece → two drops of electrolyte → polypropylene diaphragm → one drop of electrolyte → metal lithium sheet → positive electrode shell, and the battery was sealed and compacted using a battery sealing machine to obtain five prepared nSi@NC / GN button batteries;

[0081] (9) Cyclic voltammetry test was performed on the nSi@NC / GN-1 battery on a CHI660E electrochemical workstation in the voltage range of 0.01-1.5 V with a scan rate of 0.2 mV s -1 , 0.4mV s -1 , 0.6mV s -1 , 0.8mV s -1 and 1.0mV s -1 ; Conduct AC impedance test on nSi@NC / GN-2, the test frequency is 0.01-100000Hz; conduct charge and discharge cycle performance test on nSi@NC / GN-3, the current density is 0.5Ag -1 ; The charge and discharge cycle test of nSi@NC / GN-4 was carried out with a current density of 1.0A -1 ; The rate performance of nSi@NC / GN-5 was tested, and the current density was 0.1A -1 , 0.2Ag -1 , 0.3Ag -1 , 0.4Ag -1 , 0.5Ag-1 , 1.0Ag -1 .

[0082] Example 2 Preparation of nSi@NC and its application as negative electrode in lithium-ion batteries

[0083] (1) 5 g of silicon powder with a particle size of 300 μm was poured into a beaker containing 20 ml of 2% HF, stirred for 60 min on a magnetic stirrer, and then vacuum filtered. The mixture was vacuum filtered with deionized water for 3 times, and the product was collected and dried in a vacuum oven at 120° C. for 10 h to obtain 4.56 g of black solid powder.

[0084] (2) The black powder obtained in (1) was placed in a stainless steel lined 100 ml solvent ball mill, 100 g of a 5 mm radius stainless steel ball was added, and the mixture was ball milled at 800 rpm for 10 h. The resulting product was a gray-black powder of 3.6 g, i.e., nSi.

[0085] (3) 1 g of nSi obtained in step (2), 2 g of glucose, and 1 g of melamine were added to a ball mill, and 80 g of stainless steel balls were added. The ball mill was performed at 450 rpm for 6 h. The obtained product was carbonized in a tube furnace at a heating rate of 10 °C / min at 800 °C under Ar conditions for 2 h to obtain 3.47 g of a black product, namely nSi@NC;

[0086] (4) The structural morphology of nSi@NC / GN was characterized by XRD, Raman, SEM and STEM. The results are as follows Figure 3a-3c As shown;

[0087] (5) 35 mg of nSi@NC, 10 mg of conductive carbon and 5 mg of PVDF were ground in an agate mortar for 20 min. 10 drops of NMP were added to obtain a black viscous conductive slurry, namely nSi@NC slurry. The slurry was evenly coated on a copper foil conductive current collector with a thickness of 0.9 mm using an automatic coating machine and dried in a vacuum drying oven at 80 °C for 10 h.

[0088] (6) Take out the dried nSi@NC electrode, use the slicer produced by Shenzhen Kejing Material Technology Co., Ltd. to cut the electrode into circular electrode pieces with a diameter of 10 mm, take out five electrode pieces and weigh them to obtain that the mass of active materials of the five nSi@NC electrode pieces is 0.48 mg;

[0089] (7) Five weighed electrode pieces and a polypropylene diaphragm with a diameter of 16 mm were placed in the transition compartment of the glove box and evacuated three times before being brought into the glove box (the battery assembly, metal lithium sheet and electrolyte were originally in the glove box). The button lithium-ion battery was assembled in the order of negative electrode shell → spring → gasket → nSi@NC electrode piece → two drops of electrolyte → polypropylene diaphragm → one drop of electrolyte → metal lithium sheet → positive electrode shell, and the battery was sealed and compacted using a battery sealing machine to obtain five prepared nSi@NC button batteries;

[0090] (8) For nSi@NC -1 The cyclic voltammetry test was carried out on a CHI660E electrochemical workstation in the voltage range of 0.01-1.5 V and the scan rate of 0.2 mV s -1 , 0.4mV s -1 , 0.6mV s -1 , 0.8mV s -1 and 1.0mV s -1 ; Conduct AC impedance test on nSi@NC-2, the test frequency is 0.01-100000Hz; conduct charge and discharge cycle performance test on nSi@NC-3, the current density is 0.5Ag -1 ; The charge and discharge cycle test of nSi@NC-4 was carried out with a current density of 1.0A -1 ; The rate performance of nSi@NC-5 was tested, and the current density was 0.1A -1 , 0.2Ag -1 , 0.3Ag -1 , 0.4Ag -1 , 0.5Ag -1 , 1.0Ag -1 .

[0091] Example 3nSi for lithium-ion battery negative electrode

[0092] (1) 5 g of silicon powder with a particle size of 300 μm was poured into a beaker containing 20 ml of 2% HF, stirred on a magnetic stirrer for 60 min, and then vacuum filtered. The mixture was vacuum filtered with deionized water for 3 times, and the product was collected and dried in a vacuum oven at 120° C. for 10 h to obtain 4.8 g of black solid powder.

[0093] (2) The black powder obtained in (1) was placed in a stainless steel lined 100 ml solvent ball mill, 100 g of a 5 mm radius stainless steel ball was added, and the mixture was ball milled at 800 rpm for 10 h. The resulting product was a gray-black powder of 4.1 g, i.e., nSi.

[0094] (3) The results of XRD, Raman, SEM and STEM characterization of nSi are as follows Figure 4a-4c As shown;

[0095] (4) 35 mg of nSi, 10 mg of conductive carbon and 5 mg of PVDF were ground in an agate mortar for 20 min. 10 drops of NMP were added to obtain a black viscous conductive slurry, namely nSi slurry. The slurry was evenly coated on a copper foil conductive current collector with a thickness of 0.9 mm using an automatic coating machine and dried in a vacuum drying oven at 80 °C for 10 h.

[0096] (5) Take out the dried nSi electrode pieces, use a slicer produced by Shenzhen Kejing Material Technology Co., Ltd. to cut the electrode pieces into circular electrode pieces with a diameter of 10 mm, take out five electrode pieces and weigh them to obtain that the mass of active materials of the five nSi electrode pieces is 0.52 mg;

[0097] (6) Five weighed electrode pieces and a polypropylene diaphragm with a diameter of 16 mm were placed in the transition compartment of the glove box and evacuated three times before being brought into the glove box (the battery assembly, metal lithium sheet and electrolyte were originally in the glove box). The button-type lithium-ion battery was assembled in the order of negative electrode shell → spring → gasket → nSi electrode piece → two drops of electrolyte → polypropylene diaphragm → one drop of electrolyte → metal lithium sheet → positive electrode shell, and the battery was sealed and compacted using a battery sealing machine to obtain five prepared nSi button-type batteries;

[0098] (7) Cyclic voltammetry test was performed on the nSi-1 battery on a CHI660E electrochemical workstation in the voltage range of 0.01-1.5 V with a scan rate of 0.2 mV s -1 , 0.4mV s -1 , 0.6mV s -1 , 0.8mV s -1 and 1.0mV s -1 ; Conduct AC impedance test on nSi-2, the test frequency is 0.01-100000Hz; conduct charge and discharge cycle performance test on nS-3, the current density is 0.5Ag -1 ; The charge and discharge cycle test of nSi-4 was carried out with a current density of 1.0A -1 ; The rate performance of nSi-5 was tested, and the current density was 0.1A -1 , 0.2Ag -1 , 0.3Ag -1 , 0.4Ag -1 , 0.5Ag -1 , 1.0Ag -1 .

[0099] Comprehensive comparison Figure 2a , Figure 3a as well as Figure 4aThe XRD spectrum and PDF standard card show the successful synthesis of the material. The difference in crystallinity can be seen through the peak intensity comparison, indicating the change in lattice structure after ball milling and doping. Figure 2b The Raman spectrum shows that with the doping of nitrogen and carbon, the order of the composite material changes. According to previous studies, the larger the Id / Ig value, the greater the degree of material defects and the enhanced disorder. This structure is conducive to ion diffusion, proving that this solution example effectively improves the original silicon-carbon material from a structural aspect and the effect is significant.

[0100] To further verify this conclusion, the morphology of the materials was characterized and compared, such as Figure 2c , Figure 3c , Figure 4c as well as Figure 7 As shown in the figure, it can be seen that the material size is reduced from the original micron level to the nanometer level through the ball milling process. From the morphology point of view, the porosity of the material is increased and the specific surface area is expanded by doping nitrogen and carbon. After being compounded with graphene, the specific surface area is further increased due to the stacking of graphene and the attachment of surface nano-silicon, providing more ion transmission channels. In addition, due to nitrogen and carbon doping and graphene compounding, the volume expansion of silicon materials is effectively suppressed and the conductivity is improved. This conclusion can also be based on Figure 5b and Figure 8 From the AC impedance diagram, it can be seen that the interface resistance of the composite material is significantly reduced and the conductivity is enhanced, which is beneficial to ion transport.

[0101] In addition, through Figure 5a , Figure 6 shows a series of electrochemical performance tests, through Figure 5a The cyclic voltammetry test diagram shows the redox reaction during the charge and discharge process from the appearance of two pairs of redox peaks. From the comparison of cycle performance, it can be seen that at 0.5Ag -1 After 50 cycles at a current density of 1.34 Å, nSi, nSi@NC, and nSi@NC / GN have a current density of 552.37 mAh g -1 , 725.5mAhg -1 and 2284.53mAh g -1 The specific capacity at 1Ag -1 After 1000 cycles at a current density of 233.52 mAh g -1 , 787.3mAh g -1 and 1987.2mAh g -1 The specific capacity of the material is obviously improved after doping and compounding with graphene, and from the rate performance point of view, the rate performance is improved after doping and compounding with graphene, which is consistent with our conjecture.

[0102] In general, the successful synthesis of each example material can be proved from the structural test. The successful synthesis and structural morphology of each material can be seen from the morphology. The nitrogen doping of silicon and the compounding with graphene can show the reduction of material particle size and the increase of defect degree, which is conducive to the diffusion of lithium ions. Through the performance comparison, it is obvious that the original silicon has large volume expansion, poor cycle performance and rate performance. After nitrogen doping and compounding with graphene, the advantages of structural stability are fully utilized, and the intrinsic conductivity of the material is increased. It can be seen that the capacity of the material is significantly improved, and the stability is significantly enhanced. Specifically, at 0.5Ag -1 The current density can reach 2284.5 mAh g -1 The specific capacity at 1.0Ag -1 The current density can reach 1987.2 mAh g -1 At the same time, the performance can be seen to be significantly improved from the rate performance, indicating that the material combines the high theoretical specific capacity of silicon, and successfully alleviates volume expansion and enhances conductivity by nitrogen doping and compounding with graphene.

[0103] The present invention provides a method for preparing a graphene-coated nitrogen-carbon-doped silicon negative electrode and its application. Compared with traditional silicon-carbon composite materials, the newly added nitrogen-doped component can improve the electrochemical stability of the composite material. Nitrogen doping acts on the silicon surface, enhances the affinity between the material and the electrolyte, and promotes the embedding and de-embedding of lithium ions. The introduction of graphene effectively improves the conductivity of the composite material, so that during high-rate charge and discharge, the electrode material can provide a good electron conduction channel and improve the rate performance of the battery.

[0104] In addition, surface modification of nitrogen-doped silicon helps inhibit the growth of lithium dendrites and improve battery safety. The chemically active sites formed by nitrogen doping on the modified silicon surface can enhance the structural stability of the material, thereby effectively alleviating the cracking and capacity decay caused by silicon expansion during the charge and discharge process.

[0105] The mechanism of action of the graphene-coated nitrogen-carbon-doped silicon negative electrode of the present invention is different from that of traditional silicon negative electrode materials. The expansion problem of traditional silicon electrodes is difficult to overcome, resulting in a short cycle life. The material of the present invention not only improves the stability of silicon, but also enhances its interfacial compatibility and conductivity with the electrolyte through the dual advantages of composite graphene and nitrogen doping. And the silicon-doped nitrogen and graphene composite material are prepared by high-energy ball milling, aiming to solve the problems of volume expansion and poor conductivity of traditional silicon negative electrodes in lithium-ion batteries by improving the electrical conductivity, stability and electrochemical properties of silicon. The advantages of this method are simplicity, high efficiency and low cost. It can prepare silicon-carbon composite negative electrode materials with excellent electrochemical properties and long cycle stability, and has broad application prospects.

[0106] Finally, the method of the present application is only a preferred implementation scheme and is not intended to limit the protection scope of the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection scope of the present invention.

Claims

1. A method for preparing a graphene-coated nitrogen-carbon-doped silicon negative electrode, characterized in that: The following steps are involved: Step 1, cleaning silicon with HF to remove the surface oxide layer to obtain silicon particles; Step 2, transferring the silicon particles obtained in step 1 to a ball milling jar lined with stainless steel, and ball milling to obtain nano silicon nSi; Step 3, adding the nSi, glucose and melamine obtained in step 2 into a ball mill and ball milling, and carbonizing the obtained product in a tube furnace under Ar conditions to obtain the product nSi@NC; Step 4: Add the nSi@NC and graphene obtained in step 3 into a ball mill and perform ball milling to obtain nSi@NC / GN.

2. A method for preparing a graphene-coated nitrogen-carbon-doped silicon negative electrode according to claim 1, characterized in that: In step 1, the particle size of silicon is 300um, and the HF concentration in step 1 is 2%; in step 2, the volume of the ball mill is 100ml, the ball-to-material ratio is 20:1, the rotation speed is 800rpm, and the time is 10h.

3. A method for preparing a graphene-coated nitrogen-carbon-doped silicon negative electrode according to claim 1, characterized in that: In step 3, the mass ratio of nSi, glucose and melamine is 1:2:1, the volume of the ball mill is 100 ml, the ball-to-material ratio is 20:1, the rotation speed is 450 rpm, the ball milling time is 6 h, the carbonization temperature is 800 ° C, the carbonization time is 2 h, and the heating rate is 10 ° C / min; In the step 4, the mass ratio of nSi@NC to graphene is 4:1, the volume of the ball mill is 50 ml, the ball-to-material ratio is 20:1, the rotation speed is 400 rpm, and the ball milling time is 2 h.

4. Application of a graphene-coated nitrogen-carbon-doped silicon negative electrode for testing the lithium storage performance of a lithium-ion battery negative electrode.

5. The use according to claim 4, characterized in that It includes the following steps: Step A1, grinding nSi@NC / GN, conductive carbon and PVDF to form a conductive slurry, during the grinding process, NMP is used as a grinding aid, Step A2, coating the slurry on the surface of the copper foil current collector, drying it in a vacuum oven and then cutting it into discs as the negative electrode of the lithium ion battery; Step A3, assembling into button cells in a glove box filled with argon; Step A4, performing cyclic voltammetry test and AC impedance test on the assembled battery in an electrochemical workstation, and testing the cycle performance, rate performance and kinetic performance in a battery testing system.

6. The use according to claim 4, characterized in that In step A1, the mass ratio of nSi@NC / GN, conductive carbon and PVDF is 7:2:1, and the specification of PVDF is 99%.

7. The use according to claim 4, characterized in that In step A2, the slurry coating thickness is 0.9 mm, and the vacuum drying condition is 80° C. for 10 h. The diameter of the cut disc is 10 mm. The glove box is filled with argon and the water oxygen content is less than 0.1 ppm.

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