Secondary doped polymer-coated silicon-carbon composite material and method of making same

Abstract

The application relates to a secondary doped polymer-coated silicon-carbon composite material and a preparation method thereof and a lithium ion battery. The preparation method comprises the following specific steps: S1, providing a silicon-carbon base material, wherein the silicon-carbon base material is obtained by mixing a porous carbon precursor and a silicon source and then sintering at 400-700 DEG C; S2, providing an acid-doped polymer, forming an acid-doped polymer coating layer on the surface of the silicon-carbon base material, and obtaining a polymer-coated silicon-carbon composite material; S3, adding an alkaline solution to the polymer-coated silicon-carbon composite material for dedoping; and S4, adding a lithium salt for secondary doping, so as to obtain a required silicon-carbon composite material. The preparation method of the secondary doped polymer-coated silicon-carbon composite material provided by the application greatly improves the cycle and rate performance of the material, greatly reduces the specific surface area, is low in preparation cost, simple in operation, and suitable for industrialized production.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion battery materials, in particular to a secondary doped polymer coated silicon-carbon composite material, a preparation method thereof and a lithium ion battery. BACKGROUND

[0002] Lithium ion batteries, also known as secondary batteries, are one of the most popular energy storage devices and are widely used in electric vehicles, energy storage devices, portable electronic products and other fields. The rapid development of lithium ion batteries is stimulated by the surge in energy demand. Silicon has become the most promising anode material due to its high energy density, low discharge platform and abundant natural reserves. However, the electronic conductivity of silicon anodes is poor, and the volume expansion (~300%) during lithium ion intercalation and deintercalation causes material particle fragmentation and even loss of electrical contact with the current collector. The constantly updated solid electrolyte interface layer also greatly increases the irreversible capacity, reducing the cycle life of the battery. Therefore, the use of silicon in lithium ion batteries needs to overcome the shortcomings of poor silicon conductivity and significant volume expansion.

[0003] Prior art 1 (patent application number CN202310848485.9) discloses a preparation method of carbon-coated porous silicon anode material for lithium ion batteries. First, magnesium silicide undergoes a hydrothermal reaction at high temperature, and then the product is acid washed to remove magnesium salt and impurities, obtaining a light yellow powder. Then, a phenolic resin polymer is grown in situ in an alkaline aqueous solution, and then a dense carbon shell is formed by high temperature pyrolysis, which is uniformly coated on the porous silicon particles. This can effectively alleviate the volume change of silicon during the cycling process, improve the cycle stability and the electrical conductivity of the composite material. At the same time, the porous structure provides a large number of active sites for lithium ions, which helps to improve the electrochemical stability. The prepared carbon-coated porous silicon as an anode material applied to lithium ion batteries has significant effects on improving the specific capacity, cycle performance and rate performance of lithium ion batteries, and the process is simple, environmentally friendly and low in cost.

[0004] However, this method forms a carbon layer on the surface of porous silicon by high temperature pyrolysis of phenolic resin polymer. During the cycling process, the rigid carbon layer is damaged by the stress generated by the volume expansion of silicon and cannot be repaired, making it difficult to maintain the integrity of the electrode structure, leading to a decrease in performance. The capacity retention rate is less than 40% after 120 cycles at a current density of 0.5C. In addition, the use of high-temperature hydrothermal method and acid washing to prepare porous silicon makes it difficult to be applied on a large scale.

[0005] Prior art 2 (patent application number CN201810379550.7) discloses: a preparation method of a conductive polymer coated silicon-carbon composite negative electrode material. The present application discloses a preparation method of a conductive polymer coated silicon-carbon composite negative electrode material, which comprises the following steps: (1) in the presence of a surface dispersing agent, the conductive polymer is dissolved in a solvent to obtain a solution; (2) a certain proportion of nano-silicon and flake graphite is added to the above solution and uniformly dispersed; (3) the dispersion obtained in step (2) is evaporated at a certain temperature to obtain a solid precursor; (4) the obtained precursor is dried in a blast drying oven at 60-80℃; (5) the precursor is ground into powder, sieved and then pyrolyzed in a tube furnace under nitrogen protection, the temperature is raised to 600-800℃ at a rate of 3-7℃ / min, and kept constant for 3-5h, then the furnace is cooled to below 100℃, to obtain a silicon-carbon composite material. By using the method of the present application, the coating of conductive polymer not only improves the conductivity of the material, but also forms a three-dimensional network structure with certain strength to overcome the volume expansion problem of silicon, meeting the needs of use.

[0006] In this method, nano-silicon is high in cost and easy to agglomerate, and it is difficult to uniformly disperse with flake graphite. Direct coating with polymer is difficult to ensure the uniformity of the coating layer, and it is not enough to support the volume expansion of silicon during long-term cycling.

[0007] Therefore, the prior art needs to be improved. SUMMARY

[0008] In the method for solving the expansion effect of silicon in lithium ion batteries, the method of forming a carbon layer on the surface of porous silicon by high temperature pyrolysis of phenolic resin polymer may cause the rigid carbon layer to be damaged by the stress generated by the volume expansion of silicon and cannot be repaired; and the direct coating with polymer is difficult to ensure the uniformity of the coating layer, and it is not enough to support the volume expansion of silicon during long-term cycling. Therefore, the present application provides a secondary doped polymer coated silicon-carbon composite material and a preparation method thereof, and a lithium ion battery for solving the expansion effect of silicon in lithium ion batteries.

[0009] To achieve the above-mentioned purpose, in a first aspect, the present application provides a preparation method of a secondary doped polymer coated silicon-carbon composite material, which comprises the following specific steps:

[0010] S1, providing a silicon-carbon substrate, which is obtained by mixing a porous carbon precursor and a silicon source and then sintering at 400-700℃;

[0011] S2, providing an acid doped polymer, forming an acid doped polymer coating layer on the surface of the silicon-carbon substrate to obtain a polymer coated silicon-carbon composite material;

[0012] S3, adding a basic solution to the polymer-coated silicon-carbon composite material for dedoping;

[0013] S4, adding a lithium salt for secondary doping to obtain the desired silicon-carbon composite material.

[0014] In an implementation manner, in S1, the silicon-carbon substrate has a porous structure and contains silicon nanoparticles in the pores, the size of the pores is 0.1-100 nm, and the size of the silicon nanoparticles is 0.1-50 nm.

[0015] In an implementation manner, in S2, specifically comprising: adding the silicon-carbon substrate into ethanol, adding a silane coupling agent after stirring to reflux, the reflux temperature is 0-100 ℃, the reflux time is 0.5-3 h, adding an acid and a polymer monomer to stir uniformly to obtain a solution A; slowly adding an ammonium persulfate solution into the solution A, reacting for 2-8 h, after the reaction is completed, the obtained mixed solution is washed with ethanol / distilled water alternately for three times, and then is dried completely in a vacuum environment at 80 ℃ to obtain the polymer-coated silicon-carbon composite material.

[0016] In an implementation manner, in S2, the acid includes any one of hydrochloric acid, sulfuric acid, phosphoric acid, perchloric acid, nitric acid, hydrobromic acid, dodecylbenzenesulfonic acid, salicylic acid, benzoic acid, citric acid and oxalic acid.

[0017] In an implementation manner, in S2, the polymer monomer can be any one of aniline, pyrrole and thiophene.

[0018] In an implementation manner, in S2, the silane coupling agent includes any one of 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-(2-aminoethylamino)propyltrimethoxysilane, aminoethylaminopropyltrimethoxysilane, N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane and vinyltris(2-methoxyethoxy)silane.

[0019] In an implementation manner, in S3, the basic solution includes any one of ammonia water, sodium hydroxide solution, potassium hydroxide solution, sodium carbonate solution, potassium carbonate solution and hydrazine hydrate.

[0020] In an implementation manner, in S4, the lithium salt includes any one of lithium metaborate, lithium bistrifluoromethanesulfonimide, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bis(oxalato)borate and lithium chloride.

[0021] In a second aspect, the application further provides a secondary-doped polymer-coated silicon-carbon composite material, which is prepared by the method for preparing the secondary-doped polymer-coated silicon-carbon composite material.

[0022] In a third aspect, the present application also provides a lithium ion battery comprising the secondary doped polymer-coated silicon-carbon composite material provided by the present application.

[0023] Beneficial effects: In the preparation method of the secondary doped polymer-coated silicon-carbon composite material provided by the present application, the nanosilicon combined with the porous carbon material composite is provided as a substrate, wherein the porous carbon skeleton can enhance the conductivity of the material while limiting the volume effect of silicon, and the nanoscale silicon particles can achieve long cycle stability; further, the polymer is in-situ polymerized on the surface of the substrate, the high-elasticity polymer coating layer makes up for the defect of the rigid carbon coating layer, reduces the direct contact between the electrolyte and the silicon nanoparticles, reduces the side reaction, helps to form a stable solid electrolyte interface layer, and prevents further decomposition of the electrolyte; then, the de-doping and lithium salt secondary doping are performed, the secondary doping promotes the re-growth of the polymer molecular chain, the steric hindrance effect makes the surface polymer layer more easily doped, thereby forming a continuous conductive path, improving the electrical conductivity, providing more ion transmission channels, improving the diffusion rate of lithium ions in the electrode material, and improving the rate performance. The material prepared by the preparation method of the secondary doped polymer-coated silicon-carbon composite material provided by the present application has greatly improved cycle and rate performance, the specific surface area is greatly reduced, the preparation cost is low and the operation is simple, and it is suitable for industrial production. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 is a step flow chart of the preparation method of the secondary doped polymer-coated silicon-carbon composite material provided by the present application;

[0025] Figure 2 is a 1C cycle performance graph of the secondary doped polymer-coated silicon-carbon composite material provided by the present application;

[0026] Figure 3 is a rate performance graph of the secondary doped polymer-coated silicon-carbon composite material provided by example 1 and example 3 of the present application;

[0027] Figure 4 is an electrochemical impedance graph of the secondary doped polymer-coated silicon-carbon composite material provided by example 3 and comparative example 1 of the present application;

[0028] Figure 5 is a GITT graph of the secondary doped polymer-coated silicon-carbon composite material provided by example 3 and comparative example 1 of the present application;

[0029] Figure 6 is a graph of the electrical conductivity of the secondary doped polymer-coated silicon-carbon composite material provided by the present application varying with pressure;

[0030] Figure 7 is an FT-IR graph of the secondary doped polymer-coated silicon-carbon composite material provided by the present application;

[0031] Figure 8 is the XPS chart of the secondary doped polymer coated silicon-carbon composite material provided by the embodiment of the present application.

[0032] The object, functional characteristics and advantages of the present application will be further described with reference to the accompanying drawings and embodiments. DETAILED DESCRIPTION

[0033] In order to make the object, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not to limit the present application. In addition, the description of the terms "one embodiment", "some embodiments", "an example", "a specific example", or "some examples" described below means that the specific features, structures, materials or characteristics described in conjunction with the embodiment or example are included in at least one embodiment or example of the present application. In the present specification, the illustrative description of the above terms is not necessarily directed to the same embodiment or example. Moreover, the technical features involved in each embodiment of the present application can be combined with each other as long as they do not conflict with each other.

[0034] In detail, refer to Figure 1 , Figure 1 is the step flow chart of the preparation method of the secondary doped polymer coated silicon-carbon composite material provided by the present application. The present application provides a preparation method of a secondary doped polymer coated silicon-carbon composite material, which comprises the following specific steps:

[0035] S1, providing a silicon-carbon substrate, which is obtained by mixing a porous carbon precursor and a silicon source and then sintering at 400-700℃;

[0036] S2, providing an acid doped polymer, forming an acid doped polymer coating layer on the surface of the silicon-carbon substrate to obtain a polymer coated silicon-carbon composite material;

[0037] S3, adding an alkaline solution to de-dope the polymer coated silicon-carbon composite material;

[0038] S4, adding lithium salt for secondary doping to obtain the required silicon-carbon composite material.

[0039] Specifically, in S1, the silicon-carbon substrate has a porous structure and contains silicon nanoparticles in the pores, the size of the pores is 0.1-100nm, and the size of the silicon nanoparticles is 0.1-50nm. In this step, the sintering temperature is 400-700℃, preferably, the sintering temperature is 400℃, 450℃, 500℃, 550℃, 600℃, 650℃ or 700℃.

[0040] In S1, the preparation process of the silicon-carbon substrate specifically comprises: adding a carbon source into a chemical vapor deposition furnace (CVD furnace), introducing an inert gas, and keeping for 60 min. Then, adjusting the temperature in the furnace for sintering, introducing a silicon source for a period of time, introducing a carbon source gas for a period of time, and obtaining the silicon-carbon substrate.

[0041] The inert gas includes any one of nitrogen, argon and hydrogen, and the flow rate of the introduced inert gas is 1-20 L / min. Preferably, the flow rate of the introduced inert gas is 1 L / min, 2 L / min, 4 L / min, 8 L / min, 10 L / min, 14 L / min, 18 L / min or 20 L / min.

[0042] Specifically, the silicon source gas includes any one of monosilane, disilane and silane derivatives. The flow rate of the introduced silicon source gas is 0.5-10 L / min, and preferably, the flow rate of the introduced silicon source gas is 0.5 L / min, 1 L / min, 2 L / min, 3 L / min, 4 L / min, 5 L / min, 6 L / min, 7 L / min, 8 L / min, 9 L / min or 10 L / min, etc. The time for introducing the silicon source gas is 100-1000 min, and preferably, the time for introducing the silicon source gas is 100 min, 200 min, 300 min, 400 min, 500 min, 600 min, 700 min or 1000 min, etc.

[0043] Specifically, the carbon source gas includes any one of acetylene, ethylene and methane. The flow rate of the introduced carbon source gas is 1-10 L / min, and preferably, the flow rate of the introduced carbon source gas is 1 L / min, 2 L / min, 3 L / min, 4 L / min, 5 L / min, 6 L / min, 7 L / min, 8 L / min, 9 L / min or 10 L / min, etc. Further, the carbon coating temperature when introducing the carbon source is 500-800℃, and preferably, the temperature in the furnace can be increased to 500℃, 550℃, 600℃, 650℃, 700℃, 750℃ or 800℃, etc. Further, the time for introducing the carbon source gas is 50-500 min, and preferably, the time for introducing the carbon source gas is 50 min, 100 min, 150 min, 200 min, 250 min, 300 min, 350 min, 400 min, 450 min or 500 min, etc.

[0044] In S2, specifically comprising: adding the silicon-carbon substrate into ethanol, adding silane coupling agent after stirring, refluxing at a temperature of 0℃ to 100℃, refluxing for 0.5 to 3 hours, adding acid and polymer monomer to stir uniformly to obtain solution A; slowly adding ammonium persulfate solution into solution A, reacting for 2 to 8 hours, after the reaction, the obtained mixed solution is washed with ethanol / distilled water alternately for three times, and then is dried completely in a vacuum environment at 80℃ to obtain a polymer-coated silicon-carbon composite material.

[0045] Specifically, the acid comprises any one of hydrochloric acid, sulfuric acid, phosphoric acid, perchloric acid, nitric acid, hydrobromic acid, dodecylbenzenesulfonic acid, salicylic acid, benzoic acid, citric acid and oxalic acid. The polymer monomer can be any one of aniline, pyrrole and thiophene. Further, the thickness of the polymer coating layer is 1 to 20 nm. The silane coupling agent comprises any one of 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-(2-aminoethylamino)propyltrimethoxysilane, aminoethylaminopropyltrimethoxysilane, N-β-(aminoethyl)-γ-aminopropyltrimethoxysilane and vinyltris(2-methoxyethoxy)silane.

[0046] In S3, specifically comprising: placing the composite material obtained in S2 into an alkaline solution, stirring for 2 to 6 hours, washing with ethanol / distilled water alternately for three times, and then drying completely in a vacuum environment at 80℃ to obtain a de-doped polymer-coated silicon-carbon composite material.

[0047] Specifically, the alkaline solution comprises any one of ammonia, sodium hydroxide solution, potassium hydroxide solution, sodium carbonate solution, potassium carbonate solution and hydrazine hydrate. The amount of the de-doped alkaline solution is 0.1% to 1% of the volume of the polymer-coated silicon-carbon composite material. For example, the amount of ammonia is 0.1%, 0.25%, 0.5% or 1%.

[0048] In S4, specifically comprising: placing the composite material obtained in S3 into water, stirring and dispersing, then adding lithium salt, stirring for 2 to 8 hours, washing with ethanol / distilled water alternately for three times, and then drying completely in a vacuum environment at 80℃ to obtain the required silicon-carbon composite material.

[0049] Specifically, the lithium salt comprises any one of lithium metaborate, lithium bistrifluoromethanesulfonimide, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bisoxalate borate and lithium chloride.

[0050] The technical solutions of the present application are further illustrated by a plurality of embodiments and comparative examples.

[0051] Embodiment 1:

[0052] S1, 3000g of porous carbon was added to the CVD furnace, nitrogen was passed at a flow rate of 14L / min for 60min. The temperature in the CVD furnace was raised to 520℃, silane was passed at a flow rate of 3L / min for 500min, acetylene gas was passed at a flow rate of 3L / min for 200min to obtain a silicon-carbon base material.

[0053] Example 2:

[0054] S1, 3000g of porous carbon was added to the CVD furnace, nitrogen was passed at a flow rate of 14L / min for 60min. The temperature in the CVD furnace was raised to 520℃, silane was passed at a flow rate of 3L / min for 500min, acetylene gas was passed at a flow rate of 3L / min for 200min to obtain a silicon-carbon base material.

[0055] S2, 20g of the obtained silicon-carbon base material was weighed and added to ethanol for magnetic stirring, silane coupling agent was added and refluxed for 3h, the reflux temperature was 80℃, a certain amount of 2M phosphoric acid solution and 2ml aniline solution were added in turn and stirred uniformly to obtain solution A; 1M ammonium persulfate solution was slowly added to solution A, and in-situ polymerization was carried out for 5h, after the reaction was completed, the obtained mixed solution was washed and filtered with ethanol and deionized water alternately for three times, and then dried completely in a vacuum environment at 80℃ to obtain an acid-doped polyaniline-coated silicon-carbon composite material.

[0056] S3, 10g of the above acid-doped composite material was weighed and placed in 100ml of 0.1M ammonia water, stirred for 4h, washed and filtered with ethanol / distilled water alternately for three times, and then dried completely in a vacuum environment at 80℃ to obtain a de-doped polyaniline-coated silicon-carbon composite material.

[0057] S4, 7g of the above de-doped composite material was weighed and placed in 70ml of deionized water, stirred and dispersed, 1% lithium metaborate was added, stirred for 5h, washed and filtered with ethanol / distilled water alternately for three times, and then dried completely in a vacuum environment at 80℃ to obtain a secondary doped polyaniline-coated silicon-carbon composite material.

[0058] Example 3:

[0059] Different from example 2, in this embodiment, in the step of de-doping the polymer-coated silicon-carbon composite material, 10g of the above acid-doped composite material was weighed and placed in 100ml of 0.25M ammonia water for de-doping. The other implementation steps are the same as in example 2, which will not be repeated here.

[0060] Example 4:

[0061] Different from example 2, in this embodiment, in the step of de-doping the polymer-coated silicon-carbon composite material, 10 g of the above acid-doped composite material was weighed and placed in 100 ml of 0.5 M ammonia water for de-doping. The other implementation steps were the same as in example 2, and are not repeated here.

[0062] Example 5:

[0063] Different from example 2, in this embodiment, in the step of de-doping the polymer-coated silicon-carbon composite material, 10 g of the above acid-doped composite material was weighed and placed in 100 ml of 1 M ammonia water for de-doping. The other implementation steps were the same as in example 2, and are not repeated here.

[0064] Comparative Example 1:

[0065] 3000 g of porous carbon was added to a CVD furnace, and nitrogen gas was passed in at a flow rate of 14 L / min for 60 min. The temperature in the CVD furnace was raised to 520°C, and silane was passed in at a flow rate of 3 L / min for 500 min. In this comparative example, no carbon source gas was passed in, and an uncoated silicon-carbon substrate was obtained.

[0066] Comparative Example 2:

[0067] The silicon-carbon substrate in example 1 was provided, 20 g of the silicon-carbon substrate was weighed and added to ethanol and ultrasonically treated, 4 ml of ammonia water was added dropwise and magnetically stirred, and 5 ml of a solution of isopropyl titanium in ethanol was added dropwise, and the reaction was carried out for 8 h. After centrifugation, the product was dried completely in a vacuum environment at 80°C, and calcined at 550°C under an Ar atmosphere for 3 h, to obtain a titanium dioxide-coated silicon-carbon material.

[0068] Comparative Example 3:

[0069] Nitrogen gas was passed into a CVD furnace at a flow rate of 14 L / min for 30 min. The temperature in the CVD furnace was raised to 520°C, and silane was passed in at a flow rate of 3 L / min, and acetylene gas was passed in at a flow rate of 3 L / min, and the gases were passed in continuously for 300 min, to obtain a nanosilicon-carbon composite material.

[0070] Comparative Example 4:

[0071] Different from example 2, in this embodiment, only the acid-doped polyaniline-coated silicon-carbon composite material in S2 was obtained, and the specific steps were the same as in example 2, and are not repeated here.

[0072] Comparative Example 5:

[0073] Different from example 2, in this embodiment, only the de-doped polyaniline-coated silicon-carbon composite material in S3 was obtained, and the specific steps were the same as in example 2, and are not repeated here.

[0074] Comparative Example 6:

[0075] Different from Example 2, after obtaining the silicon-carbon substrate in S1, 20 g of the obtained silicon-carbon substrate was weighed into ethanol and magnetically stirred, 1 g of doped polyaniline was added, and the reaction was performed for 5 h. After the reaction, the obtained mixed solution was washed and filtered with ethanol and deionized water alternately for three times, and then dried completely in a vacuum environment at 80°C to obtain a silicon-carbon composite material coated with doped polyaniline. In this comparative example, no acid doping was performed on the coated polymer.

[0076] Comparative Example 7:

[0077] Different from Example 2, after obtaining the silicon-carbon substrate in S1, 20 g of the obtained silicon-carbon substrate was weighed into ethanol and magnetically stirred, 1 g of doped polyaniline was added, and the reaction was performed for 5 h. After the reaction, the obtained mixed solution was washed and filtered with ethanol and deionized water alternately for three times, and then dried completely in a vacuum environment at 80°C to obtain a silicon-carbon composite material coated with doped polyaniline. In this comparative example, no acid doping was performed on the coated polymer.

[0078] Some data of the above examples and comparative examples can be seen in Table 1 Figures 2-8 , wherein, Figure 2 is a 1C cycle performance graph of the secondary doped polymer coated silicon-carbon composite material provided by the application, Figure 3 is a rate performance graph of the secondary doped polymer coated silicon-carbon composite material provided by Example 1 and Example 3 of the application, Figure 4 is an electrochemical impedance graph of the secondary doped polymer coated silicon-carbon composite material provided by Example 3 and Comparative Example 1 of the application, Figure 5 is a GITT graph of the secondary doped polymer coated silicon-carbon composite material provided by Example 3 and Comparative Example 1 of the application, Figure 6 is a graph of the change of the conductivity of the secondary doped polymer coated silicon-carbon composite material with pressure provided by the application, Figure 7 is an FT-IR graph of the secondary doped polymer coated silicon-carbon composite material provided by the application, Figure 8 is an XPS graph of the secondary doped polymer coated silicon-carbon composite material provided by the application.

[0079] Electrochemical performance test:

[0080] Electrode preparation: the above-mentioned composite material, conductive carbon black and binder CMC were weighed according to the proportion, and deionized water was added to prepare a uniform slurry, wherein the proportion of composite material: conductive carbon black: binder CMC solid content was 95: 1.5: 3.5, and the sample was prepared by using a homogenizer at a speed of 2000 rpm for 20 min.

[0081] After the slurry was sieved, it was uniformly coated on a copper foil and placed in a 90°C vacuum drying oven for drying. The dried electrode sheet was rolled and cut into a certain size of circular electrode, and the electrode mass was recorded.

[0082] Button assembly: in an argon atmosphere glove box, a metal lithium sheet was used as a counter electrode, and the above-mentioned electrode, a separator, a gasket and the like were assembled into a button cell, and a silicon-carbon special electrolyte was used as the electrolyte.

[0083] Through the data results of Figure 2 , Table 1 was obtained.

[0084] Table 1: Figure 2 Performance test results of the examples and comparative examples are shown in Table 1.

[0085] .

[0086] From Table 1 and Figure 2 , it can be seen that the secondary doped polyaniline coated silicon-carbon composite material provided in Example 3 has a first cycle reversible capacity of 1752.1 mAh / g, a first cycle coulombic efficiency of 91.88%, a reversible capacity of 1008.1 mAh / g after 200 cycles, and a capacity retention rate of 71.12%, and has excellent cycle performance and first cycle coulombic efficiency.

[0087] In summary, the technical scheme provided by the application uses silicon-carbon material containing nano-silicon nanoparticles in the pores of the porous carbon as a substrate, the porous structure can limit the expansion of silicon and reduce the damage to the electrode structure caused by volume change; the acid-doped polymer is in-situ polymerized on the surface of the substrate, the modification of the substrate surface by silane coupling agent can improve the dispersibility of the substrate and use the surface groups as initiation sites to initiate polymerization, and a highly conductive polymer shell is synthesized to covalently connect the coating layer and the silicon-carbon substrate; the heteroatoms in the polymer can improve the conductivity of the composite material while defects around the heteroatoms can provide additional lithium storage capacity; the coating layer can greatly reduce the specific surface area of the material, and the low specific surface area can effectively reduce agglomeration and promote lithium ion diffusion; on the other hand, the highly elastic conductive polymer can accommodate the volume expansion of silicon, reduce the direct contact between the electrolyte and the silicon nanoparticles, thereby reducing side reactions, helping to form a stable solid electrolyte interface layer, and preventing further decomposition of the electrolyte; and then de-doping and secondary doping of lithium salt, the secondary doping can promote the re-growth of the polymer molecular chain, and the steric hindrance effect makes the surface polymer layer more easily doped, thereby forming a continuous conductive path, improving the electrical conductivity (1.25 times), providing more ion transport channels, improving the diffusion rate of lithium ions in the electrode material, and improving the rate performance.

[0088] The preparation method of the composite material provided by the application greatly improves the cycle and rate performance of the prepared material, greatly reduces the specific surface area, has low preparation cost and simple operation, and is suitable for industrial production.

[0089] The above description is only the preferred embodiments of the application, and does not limit the patent scope of the application, and any equivalent structure or equivalent process transformation obtained by using the content of the specification and drawings, or direct or indirect application in other related technical fields, is also included in the patent protection scope of the application.

Claims

1. A method for preparing a twice-doped polymer-coated silicon-carbon composite material, characterized by, The method comprises the following specific steps: S1, providing a silicon-carbon substrate, which is obtained by mixing a porous carbon precursor and a silicon source and then sintering at 400-700 DEG C; S2, providing an acid-doped polymer, forming an acid-doped polymer coating layer on the surface of the silicon-carbon substrate to obtain a polymer-coated silicon-carbon composite material; S3, adding an alkaline solution to the polymer-coated silicon-carbon composite material for dedoping; S4, adding a lithium salt for secondary doping to obtain a desired silicon-carbon composite material; In S2, specifically comprising: adding the silicon-carbon substrate into ethanol, stirring, then adding a silane coupling agent to reflux, the reflux temperature is 0-100 DEG C, the reflux time is 0.5-3 h, adding phosphoric acid and a polymer monomer to stir uniformly to obtain a solution A; slowly adding an ammonium persulfate solution into the solution A, reacting for 2-8 h, after the reaction is completed, the obtained mixed solution is washed with ethanol / distilled water alternately and then suction filtered three times, and then dried completely in a vacuum environment at 80 DEG C to obtain the polymer-coated silicon-carbon composite material; The polymer monomer comprises any one of aniline, pyrrole and thiophene; In S1, the silicon-carbon substrate has a porous structure and contains silicon nanoparticles in the pores, the size of the pores is 0.1-100 nm, and the size of the silicon nanoparticles is 0.1-50 nm; In S2, the thickness of the polymer coating layer is 1-20 nm; In S3, the amount of the alkaline solution for dedoping is 0.1%-1% of the volume of the polymer-coated silicon-carbon composite material.

2. The method of claim 1, wherein the secondary doped polymer-coated silicon- carbon composite is prepared by the steps of: In S2, the silane coupling agent comprises any one of 3-aminopropyltrimethoxysilane, 3-aminopropyltriethoxysilane, 3-(2-aminoethylamino)propyltrimethoxysilane, aminoethylaminopropyltrimethoxysilane, N-beta-(aminoethyl)-gamma-aminopropyltrimethoxysilane and vinyltris(2-methoxyethoxy)silane.

3. The method of claim 1, wherein the secondary doped polymer- coated silicon-carbon composite is prepared by the steps of: In S3, the alkaline solution comprises any one of ammonia water, a sodium hydroxide solution, a potassium hydroxide solution, a sodium carbonate solution, a potassium carbonate solution and hydrazine hydrate.

4. The method of claim 1, wherein the secondary doped polymer- coated silicon-carbon composite is prepared by the steps of: In S4, the lithium salt comprises any one of lithium metaborate, lithium bistrifluoromethanesulfonimide, lithium perchlorate, lithium tetrafluoroborate, lithium hexafluorophosphate, lithium bisoxalate borate and lithium chloride.

5. A twice-doped polymer-coated silicon-carbon composite material, characterized in that, The secondary-doped polymer-coated silicon-carbon composite material is prepared by the method of any one of claims 1-4.

6. A lithium-ion battery, characterized by The secondary-doped polymer-coated silicon-carbon composite material of claim 5.

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