CN112310363B - Silicon-carbon composite material, preparation method thereof and lithium ion battery - Google Patents

Silicon-carbon composite material, preparation method thereof and lithium ion battery Download PDF

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CN112310363B
CN112310363B CN201910699789.7A CN201910699789A CN112310363B CN 112310363 B CN112310363 B CN 112310363B CN 201910699789 A CN201910699789 A CN 201910699789A CN 112310363 B CN112310363 B CN 112310363B
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silicon
composite material
carbon composite
graphite
carbon
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CN112310363A (en
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苏航
李阳兴
谢封超
王平华
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Huawei Technologies Co Ltd
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Priority to CN202211004146.4A priority patent/CN115295784A/en
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
    • H01M4/366—Composites as layered products
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M10/00—Secondary cells; Manufacture thereof
    • H01M10/05—Accumulators with non-aqueous electrolyte
    • H01M10/052—Li-accumulators
    • H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/362—Composites
    • H01M4/364—Composites as mixtures
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/36—Selection of substances as active materials, active masses, active liquids
    • H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
    • H01M4/386—Silicon or alloys based on silicon
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/624—Electric conductive fillers
    • H01M4/625—Carbon or graphite
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
    • H01M4/628—Inhibitors, e.g. gassing inhibitors, corrosion inhibitors
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/021—Physical characteristics, e.g. porosity, surface area
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
    • H01M4/00—Electrodes
    • H01M4/02—Electrodes composed of, or comprising, active material
    • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
    • H01M2004/027—Negative electrodes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
    • Y02E60/10—Energy storage using batteries

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  • General Chemical & Material Sciences (AREA)
  • Composite Materials (AREA)
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  • Manufacturing & Machinery (AREA)
  • Battery Electrode And Active Subsutance (AREA)
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Abstract

The embodiment of the invention provides a silicon-carbon composite material, which comprises an inner core and a carbon layer coated on the surface of the inner core, wherein the inner core comprises a graphite skeleton, amorphous carbon filled in the graphite skeleton structure and silicon material uniformly distributed in the amorphous carbon, and the silicon-carbon composite material only has a pore structure with the pore diameter less than or equal to 50nm and does not have a pore structure with the pore diameter more than 50 nm. The silicon-carbon composite material has small internal pore size, can effectively reduce the contact area of the silicon material and electrolyte, reduce the occurrence of side reaction and prolong the service life of the battery; meanwhile, the silicon material is uniformly dispersed around the graphite framework without agglomeration, so that the graphite framework can effectively relieve the volume expansion and contraction of the silicon material, and the structural stability and the energy density of the composite material are improved. The embodiment of the invention also provides a preparation method of the silicon-carbon composite material and a lithium ion battery containing the silicon-carbon composite material.

Description

Silicon-carbon composite material, preparation method thereof and lithium ion battery
Technical Field
The embodiment of the invention relates to the technical field of lithium ion batteries, in particular to a silicon-carbon composite material, a preparation method thereof and a lithium ion battery.
Background
Since the early 90 s of the last century, graphite has been the mainstream negative electrode material of lithium ion batteries. However, the specific capacity of commercial high-quality graphite is 360mAh/g, which is close to the theoretical value 372mAh/g, and the energy density in a full battery reaches the ceiling, so that the requirement of various current consumer electronic devices, especially energy storage devices and electric vehicles on the energy density cannot be met. Therefore, it is urgently required to find a high energy density negative electrode material which can replace graphite.
The silicon-based material is one of the most studied negative electrode materials capable of replacing graphite at present. Depending on the reaction depth, silicon and lithium can form different products, such as Li 7 Si 3 、Li 13 Si 4 、Li 22 Si 5 、Li 12 Si 17 And so on. Li in which lithium is intercalated into silicon 4.4 The theoretical specific capacity of the Si alloy is 4200mAh/g, and the Si alloy is the anode material with the highest theoretical capacity. However, the silicon-based material undergoes severe volume expansion (0-300%) and shrinkage during the lithium-intercalation reaction, which leads to structural damage and pulverization of the electrode material, and a new SEI film (solid electrolyte interface film) is continuously generated on the silicon surface with the electrolyte, which leads to depletion of the electrolyte and rapid degradation of the battery capacity.
To combine the properties of both graphite and silicon materials, silicon carbon composites have been developed. The silicon-carbon composite material which is recognized to be practical at present is secondary particles formed by granulating nano silicon, graphite and carbon. However, as the nano-silicon and the graphite have a difference of 2 orders of magnitude in particle size and the nano-silicon has a high surface energy and is easy to agglomerate, the nano-silicon and the graphite are difficult to be uniformly dispersed, the nano-silicon often agglomerates on the graphite surface or concentrates on a certain position, the local volume expansion and shrinkage of particles is high, the graphite substrate cannot absorb and relieve the expansion of the silicon well, and finally the composite material is damaged in structure and performance is degraded. In addition, the inside of the secondary particles of the existing silicon-carbon composite material has a large number of macroporous structures with the size larger than 50nm, so that the structural stability of the composite material is not high, the contact area of the silicon material and the electrolyte is large, the side reaction is serious, and the performance of the material is finally rapidly declined.
Disclosure of Invention
In view of this, the embodiment of the present invention provides a silicon-carbon composite material, in which graphite is used as a framework, a silicon material is uniformly dispersed among gaps of the graphite framework through amorphous carbon, and no pore with a size greater than 50nm is controlled in the material, so as to solve the problems of low structural stability and easy occurrence of side reactions of the composite material due to non-uniform dispersion and large internal pore of nano silicon in the existing silicon-carbon composite material to a certain extent.
Specifically, in a first aspect, the embodiment of the present invention provides a silicon-carbon composite material, which is used as a battery negative electrode material, and includes an inner core and a carbon layer coated on a surface of the inner core, where the inner core includes a graphite skeleton, an amorphous carbon filled in the graphite skeleton structure, and a silicon material uniformly distributed in the amorphous carbon, and the silicon-carbon composite material has only a pore structure with a pore diameter smaller than or equal to 50nm inside, and does not have a pore structure with a pore diameter larger than 50 nm.
In the embodiment of the invention, the area percentage of the silicon material is 20-50% in the range of 5 micrometers multiplied by 5 micrometers of any section of the silicon-carbon composite material.
In the embodiment of the invention, the grain size of the silicon material is 50nm-150 nm.
In the embodiment of the invention, in the silicon-carbon composite material, the mass percentage of the silicon material is 10-40%.
In the embodiment of the invention, in the silicon-carbon composite material, the mass ratio of the graphite skeleton is 50-70%.
In the embodiment of the invention, the mass ratio of the amorphous carbon in the silicon-carbon composite material is 10-30%.
In the embodiment of the invention, the particle size D10 of the silicon-carbon composite material is 4-7 μm, D50 is 8-18 μm, D90 is 25-35 μm, and D99 is 40-60 μm.
In an embodiment of the present invention, the graphite skeleton is composed of a graphite material having a particle size of 5 μm to 15 μm, and the graphite material includes at least one of artificial graphite and natural graphite.
In the embodiment of the invention, the graphite material is flake graphite with the length-diameter ratio of 1-3.
In an embodiment of the present invention, the carbon layer has a thickness of 5nm to 20 nm.
In the embodiment of the invention, the tap density of the silicon-carbon composite material is 0.8-1.0g/cm 3 The specific surface area is 1.5-3.0m 2 /g。
According to the silicon-carbon composite material provided by the first aspect of the embodiment of the invention, the graphite material is connected through the amorphous carbon, the silicon material is uniformly dispersed in the amorphous carbon, and the size of the internal pores is small, so that the contact area between the silicon material and the electrolyte can be reduced, the occurrence of side reactions is reduced, and the service life of the battery is prolonged; meanwhile, the silicon material is uniformly distributed around the graphite material through the amorphous carbon, so that the excessive local expansion shrinkage rate in the material can be effectively avoided, the volume expansion and shrinkage of the silicon material can be effectively relieved by the graphite material, and the structural stability and the energy density of the composite material are improved.
In a second aspect, an embodiment of the present invention further provides a method for preparing a silicon-carbon composite material, including:
adding a graphite material into a sodium carboxymethylcellulose aqueous solution, and uniformly stirring and dispersing to obtain a dispersion liquid A;
wetting a silicon material by using C1-C4 alcohol, and uniformly dispersing the silicon material and an amorphous carbon precursor in water together to obtain a dispersion liquid B;
adding water into the dispersion liquid A and the dispersion liquid B for mixing, and stirring and dispersing uniformly to obtain a mixed dispersion liquid;
drying the mixed dispersion liquid to obtain a primary precursor;
sequentially carrying out primary heat treatment, mould pressing treatment, cold isostatic pressing treatment and secondary heat treatment on the primary precursor, and crushing and grading to obtain a secondary precursor;
and coating the secondary precursor with carbon, and performing three-stage heat treatment to obtain the silicon-carbon composite material, wherein the silicon-carbon composite material comprises an inner core and a carbon layer coated on the surface of the inner core, the inner core comprises a graphite skeleton, amorphous carbon filled in the graphite skeleton structure and silicon material uniformly distributed in the amorphous carbon, and the silicon-carbon composite material only has a pore structure with the pore diameter smaller than or equal to 50nm and does not have a pore structure with the pore diameter larger than 50 nm.
In an embodiment of the present invention, the amorphous carbon precursor includes pitch or phenolic resin.
In the embodiment of the invention, the pressure in the mould pressing treatment process is 3-5Mpa, and the forming temperature is 300-400 ℃.
In the embodiment of the invention, the pressure of the cold isostatic pressing treatment is 90-120Mpa, and the time is 0.5-2 hours.
In an embodiment of the invention, the drying treatment comprises spray drying or rotary evaporation drying.
In the embodiment of the invention, the temperature of the primary heat treatment is 180-200 ℃, and the treatment time is 2-3 hours.
In an embodiment of the present invention, the secondary heat treatment specifically comprises: under the protection of nitrogen, firstly heating to 300-350 ℃ for 1-2 hours, then heating to 450-550 ℃ for 1-2 hours, finally heating to 600-700 ℃ for 2-4 hours, and then naturally cooling to room temperature.
In an embodiment of the present invention, the carbon coating specifically comprises: mixing the secondary precursor with the carbon layer precursor, heating to 200-250 ℃ for 60-90 minutes, slowly heating to 300-400 ℃ for 80-120 minutes; then heating to 450-500 deg.C, maintaining for 100-150 min; finally, the temperature is raised to 650-700 ℃ and maintained for 100-150 minutes to obtain the carbon layer.
In the embodiment of the invention, the three-stage heat treatment is carried out in a tubular furnace, and is kept at 1000-1200 ℃ for 2-4 hours under the protection of nitrogen.
According to the preparation method of the silicon-carbon composite material provided by the second aspect of the embodiment of the invention, the raw materials are uniformly dispersed by adopting a three-step dispersion method to obtain a uniform dispersion system, so that the silicon material in the composite material can be uniformly distributed with the graphite material and the amorphous carbon, the capability of the composite material for relieving the volume expansion and contraction of silicon is improved, and the mould pressing and isostatic pressing treatment are further combined, so that the size of pores inside the composite material can be effectively reduced, the contact of the silicon material and electrolyte is reduced, and the service life of a battery cell is prolonged.
The embodiment of the invention also provides a lithium ion battery, which comprises a positive pole piece, a negative pole piece, a diaphragm and electrolyte, wherein the negative pole piece comprises a negative active material, and the negative active material comprises the silicon-carbon composite material of the first aspect of the invention.
In addition, the embodiment of the invention also provides a terminal, which comprises a shell, and a circuit board and a battery which are positioned in the shell, wherein the battery comprises the lithium ion battery provided by the embodiment of the invention, and the lithium ion battery is used for supplying power to the terminal. The terminal can be a mobile phone, or an electronic product such as a tablet personal computer, a notebook computer, a portable machine, an intelligent wearable product and the like.
Drawings
Fig. 1 is a schematic structural diagram of a silicon-carbon composite material provided in an embodiment of the present invention;
fig. 2 is a schematic structural diagram of a terminal according to an embodiment of the present invention;
FIG. 3 is a flow chart of a method for preparing a silicon carbon composite material according to example 1 of the present invention;
FIG. 4 is a Scanning Electron Microscope (SEM) picture of a precursor A obtained after spray drying in example 1 of the present invention;
FIG. 5 is a photograph of a precursor C obtained after molding and isostatic pressing in example 1 of the present invention;
FIG. 6 is a Scanning Electron Microscope (SEM) picture of the silicon carbon composite material of example 1 of the present invention;
FIG. 7 is a cross-sectional Scanning Electron Microscope (SEM) picture of a silicon carbon composite of a comparative example of the present invention;
fig. 8 is a graph showing the cycle profiles of the batteries fabricated using the silicon carbon composite material of example 1 of the present invention and the silicon carbon material of the comparative example.
Detailed Description
The embodiments of the present invention will be described below with reference to the drawings.
Referring to fig. 1, an embodiment of the present invention provides a silicon-carbon composite material 100 for a battery negative electrode, where the silicon-carbon composite material 100 is a secondary particle, and includes a core 10 and a carbon layer 20 coated on a surface of the core 10, the core 10 is composed of a graphite skeleton 101, amorphous carbon 102 filled in gaps of the graphite skeleton 101, and silicon material 103 uniformly distributed in the amorphous carbon 102, and the silicon-carbon composite material 100 only has a pore structure with a pore diameter less than or equal to 50nm, and does not have a pore structure with a pore diameter greater than 50 nm.
In the embodiment of the invention, amorphous carbon 102 is used as a connecting medium in the silicon-carbon composite material 100 to well connect the graphite framework materials 101 together, so that no pore structure with the size of more than 50nm exists between the graphite frameworks; in addition, the amorphous carbon 102 has uniformly dispersed silicon material 103 distributed therein, and the amorphous carbon 102 does not have a pore structure having a size of more than 50 nm. Because the inside of the whole secondary particle of the composite material does not have a macroporous structure with the size larger than 50nm, when electrolyte permeates into the inside of the composite material, the contact area of the silicon material and the electrolyte can be effectively reduced, the occurrence of side reaction is reduced, and the cycle life of the battery core is prolonged.
In the embodiment of the present invention, the silicon material 103 is uniformly dispersed in the amorphous carbon 102, that is, the silicon material 103 is uniformly dispersed between the gaps of the graphite skeleton 101, specifically, the highly uniform dispersion of the silicon material 103 is expressed in that the area percentage of the silicon material 103 is 20% to 50%, and further optionally, the area percentage of the silicon material 103 is 20% to 30%, within a range of 5 μm × 5 μm of any cross section of the silicon-carbon composite material 100. The silicon material 103 is uniformly dispersed in the gaps of the graphite framework 101 without agglomeration, so that the problem of overlarge local expansion shrinkage rate in the material can be effectively avoided, the expansion shrinkage of the silicon material in the material is more uniformly distributed, the graphite framework can effectively relieve the volume expansion and shrinkage of the silicon material, and the structural stability and energy density of the composite material are improved.
In the embodiment of the present invention, the grain size of the silicon material 103 may be 50nm to 150nm, and further, the grain size of the silicon material 103 may be 50nm to 100nm, 60nm to 80 nm. The nano silicon material can effectively reduce absolute expansion. The silicon material 103 may be spherical, spheroidal or acicular particles. The silicon material 103 is specifically elemental silicon.
In the embodiment of the invention, in the silicon-carbon composite material, the mass ratio of the graphite skeleton 101 may be 50% to 70%, and further, the mass ratio of the graphite skeleton 101 may be 55% to 65%. The graphite skeleton is the basic structure of composite material, can guarantee the structural stability of secondary particle, and graphite texture is softer, can effectively alleviate the volume expansion and the shrink of silicon material, finally makes the volume expansion of secondary particle reduce, and composite material's energy density obtains effectively improving.
In the embodiment of the invention, in the silicon-carbon composite material, the mass ratio of the silicon material 103 may be 10% to 40%, and further, the mass ratio of the silicon material 103 may be 15% to 35% and 20% to 30%. The silicon material with proper amount can not only improve the energy density of the composite material, but also ensure that the composite material can be uniformly dispersed in the whole structure, maintain the structural stability and ensure that the composite material cannot be damaged due to the expansion of the silicon material.
In the embodiment of the present invention, the mass ratio of the amorphous carbon 102 in the silicon-carbon composite material may be 10% to 30%, and further, the mass ratio of the amorphous carbon 102 may be 15% to 25%. Amorphous carbon is filled in the gaps of the graphite framework, so that on one hand, the size of internal pores can be reduced, and the contact area of the silicon material and electrolyte is effectively reduced; on the other hand, an integral conductive network can be formed in the material, the silicon material and graphite can be effectively linked, the conductivity of secondary particles is improved, the defect of poor conductivity of the silicon is overcome, and the structural stability of the material can be improved.
In the embodiment of the present invention, the graphite skeleton 101 is made of a graphite material having a particle size of 5 μm to 15 μm, and the graphite material may be at least one of artificial graphite and natural graphite. Optionally, the graphitic material is flake graphite having an aspect ratio of 1-3. The flake graphite with the proper length-diameter ratio is selected to form a graphite framework structure, so that the structural stability of the framework is enhanced.
In the embodiment of the present invention, the thickness of the carbon layer 20 may be 5nm to 20nm, and further, the thickness of the carbon layer 20 may be 10nm to 15 nm. The carbon layer 20 may specifically be an amorphous carbon layer. The outermost layer of the secondary particles is coated by the carbon layer, so that the inner core of the secondary particles can be further protected, the specific surface area is reduced, the occurrence of side reactions is reduced, and the cycle performance of the composite material is improved.
In the embodiment of the invention, the particle size D10 of the silicon-carbon composite material 100 is 4-7 μm, D50 is 8-18 μm, D90 is 25-35 μm, and D99 is 40-60 μm.
In the embodiment of the present invention, the silicon carbon composite material 100 may be spherical or spheroidal particles having a tap density of 0.8 to 1.0g/cm 3 Specific surface area of 1.5-3.0m 2 (ii) in terms of/g. The silicon-carbon composite material disclosed by the embodiment of the invention has higher tap density, and is beneficial to improving the processing performance of the material for later-stage battery core manufacturing; meanwhile, the material has a lower specific surface area, so that the side reaction between the material and the electrolyte can be effectively reduced, and the cycle life is prolonged. The silicon-carbon composite material provided by the embodiment of the invention has higher first coulombic efficiency, and can effectively improve the energy density of the full cell.
According to the silicon-carbon composite material provided by the embodiment of the invention, the graphite material in the silicon-carbon composite material is connected through the amorphous carbon, the silicon material is uniformly dispersed in the amorphous carbon, and the size of internal pores is small, so that the contact area of the silicon material and an electrolyte can be reduced, the occurrence of side reactions is reduced, and the service life of a battery is prolonged; meanwhile, the silicon material is uniformly distributed around the graphite material through the amorphous carbon, so that the excessive local expansion shrinkage rate in the material can be effectively avoided, the volume expansion and shrinkage of the silicon material can be effectively relieved by the graphite material, and the structural stability and the energy density of the composite material are improved.
Correspondingly, the embodiment of the invention also provides a preparation method of the silicon-carbon composite material, which comprises the following steps:
s101, adding a graphite material into a sodium carboxymethylcellulose aqueous solution, and uniformly stirring and dispersing to obtain a dispersion liquid A;
s102, wetting a silicon material by using C1-C4 alcohol, and uniformly dispersing the silicon material and an amorphous carbon precursor in water to obtain a dispersion liquid B;
s103, adding water into the dispersion liquid A and the dispersion liquid B for mixing, and stirring and dispersing uniformly to obtain a mixed dispersion liquid;
s104, drying the mixed dispersion liquid to obtain a primary precursor;
s105, sequentially carrying out primary heat treatment, mould pressing treatment, cold isostatic pressing treatment and secondary heat treatment on the primary precursor, and crushing and grading to obtain a secondary precursor;
and S106, coating the secondary precursor with carbon, and performing three-stage heat treatment to obtain the silicon-carbon composite material, wherein the silicon-carbon composite material comprises an inner core and a carbon layer coated on the surface of the inner core, the inner core comprises a graphite framework, amorphous carbon filled in the graphite framework structure and silicon material uniformly distributed in the amorphous carbon, and the silicon-carbon composite material only has a pore structure with the pore diameter of less than or equal to 50nm and does not have a pore structure with the pore diameter of more than 50 nm.
In step S101, the sodium carboxymethyl cellulose may be used as a surfactant to modify the surface of the graphite material, and may adjust the surface charge of the graphite, thereby facilitating affinity with silicon and forming secondary particles. The graphite material can be dispersed in the sodium carboxymethylcellulose aqueous solution by adopting a high-speed stirrer, the stirring speed can be 2000-3000 r/min, and the stirring time can be 1-2 hours. Of course, in other embodiments of the present invention, other available methods can be used to uniformly disperse the graphite material in the sodium carboxymethyl cellulose aqueous solution.
The sodium carboxymethyl cellulose aqueous solution can be prepared by the following steps: adding sodium carboxymethylcellulose into water, and stirring with a high-speed stirrer to completely dissolve the sodium carboxymethylcellulose to obtain sodium carboxymethylcellulose aqueous solution.
The graphite material may be at least one of artificial graphite and natural graphite. Alternatively, the graphite material has a particle size of 5 μm to 15 μm, and specifically may be flake graphite having an aspect ratio of 1 to 3.
In the step S102, the silicon material is wetted by the alcohol of C1-C4, so that the dispersibility of the nano silicon can be effectively improved, and the agglomeration is avoided. Specifically, the C1-C4 alcohol can be absolute ethyl alcohol, methyl alcohol, propyl alcohol, and butyl alcohol.
The specific operation of wetting the silicon material with the alcohol of C1-C4 and dispersing the silicon material and the amorphous carbon precursor in water together can be as follows: adding C1-C4 alcohol into the silicon material, stirring at low speed to fully wet the silicon material, adding water after wetting, then adding an amorphous carbon precursor, stirring by a high-speed stirrer, and then adding into a sand mill for circular dispersion. Amorphous carbon precursors include, but are not limited to, pitch, phenolic resins. The stirring speed of the high-speed stirrer can be 2000-3000 rpm.
In the embodiment of the invention, the grain size of the silicon material can be 50nm-150nm, and further, the grain size of the silicon material can be 50nm-100nm and 60nm-80 nm. The silicon material may be spherical, spheroidal or acicular particles. The silicon material is specifically simple substance silicon.
In the embodiment of the present invention, the amorphous carbon precursor may be pitch, phenol resin, or the like.
In step S103, the stirring speed may be 2000-3000 rpm, and the stirring dispersion time may be 1-2 hours. By the special three-step dispersion method, graphite, silicon material and amorphous carbon precursors in the prepared mixed dispersion liquid are uniformly dispersed, so that the composite material with uniformly dispersed silicon material and graphite can be finally obtained.
In step S104, the drying process may be spray drying or rotary evaporation drying.
In step S105, the first-stage heat treatment can be performed in an electric heating type high-speed modification mixer, the heat treatment temperature can be 180-200 ℃, the treatment time can be 2-3 hours, and the stirring speed in the heat treatment process is 1200-1500 rpm. In the heat treatment process, when the temperature reaches more than 200 ℃, nitrogen can be introduced (the flow rate is 200 plus 300mL/min), and the volatile matter is controlled to be 1.5-2.5%. The first-order heat treatment can enable the amorphous carbon precursor to be crosslinked, so that light components in the system are volatilized.
In order to effectively reduce the size of pores inside the composite material, reduce the contact between a silicon material and an electrolyte in the charging and discharging process and prolong the cycle life of a battery cell, the embodiment of the invention respectively adopts a mould pressing mode and an isostatic pressing mode for processing after primary heat treatment. Wherein, compression molding (also called compression molding or compression molding) is a technology that powder, granular or fibrous powder is firstly put into a mold cavity at a molding temperature, and then the mold is closed to pressurize so as to mold and solidify the powder; isostatic pressing is an ultrahigh pressure hydraulic technique that utilizes a closed high pressure vessel to form a product under an isotropic ultrahigh pressure condition.
In the embodiment of the invention, the pressure in the mould pressing treatment process is 3Mpa-5Mpa, and the forming temperature is 300 ℃ to 400 ℃. Specifically, the pressure during the molding treatment may be 3MPa, 4MPa or 5MPa, and the molding temperature may be 300 deg.C, 350 deg.C or 400 deg.C.
In the embodiment of the invention, the cold isostatic pressing treatment is carried out at the normal temperature under the pressure of 90-120MPa for 0.5-2 hours. Specifically, the cold isostatic pressing treatment may be performed at a pressure of 90MPa, 100MPa, 110MPa, 120MPa for a period of 0.5 hour, 1 hour, 1.5 hours, or 2 hours.
By adopting mould pressing and cold isostatic pressing treatment and controlling proper operation parameters, the size of the internal pores of the composite material is greatly reduced.
In the embodiment of the invention, the secondary heat treatment can be carried out in a tubular furnace, under the protection of nitrogen, the temperature is firstly increased to 300-350 ℃ and kept for 1-2 hours, then increased to 450-550 ℃ and kept for 1-2 hours, and finally increased to more than 600-700 ℃ and kept for 2-4 hours, and then naturally reduced to the room temperature. In the secondary heat treatment process, the amorphous carbon precursor is subjected to pyrolysis crosslinking, so that light components are further removed, and the amorphous carbon precursor is fully and uniformly mixed with graphite and silicon materials.
In the embodiment of the present invention, the crushing treatment may be performed by crushing the block-shaped sample with a mortar and then crushing the sample with a crusher. The classification treatment can adopt jet classification, cyclone classification and other modes.
In step S106, specifically, a carbon layer may be obtained by coating with pitch or phenolic resin or depositing a carbon layer by chemical vapor deposition. In one embodiment of the present invention, the carbon coating is specifically performed by: mixing the secondary precursor and the carbon layer precursor, placing the mixture into an electric heating type high-speed modification mixer, heating to 200-250 ℃ firstly, keeping the temperature for 60-90min, then slowly heating to 300-400 ℃, and keeping the temperature for 80-120 min; then heating to 450-500 deg.C, maintaining for 100-150 min; finally, the temperature is raised to 650-700 ℃ and maintained for 100-150min, thus obtaining the carbon layer.
In the embodiment of the invention, the three-stage heat treatment is calcination treatment, and specifically, the three-stage heat treatment can be carried out in a tubular furnace, and is kept at 1000-1200 ℃ for 2-4h under the protection of protective atmosphere. The protective atmosphere may be nitrogen, argon, etc. The nitrogen flow may be 1L/min.
According to the preparation method of the silicon-carbon composite material provided by the embodiment of the invention, the uniform dispersion system is obtained by adopting the three-step dispersion method, so that the silicon material in the composite material can be uniformly distributed with the graphite material and the amorphous carbon, the problem that all components in the composite material are difficult to uniformly disperse in the prior art is effectively solved, and the capability of the composite material in relieving silicon volume expansion and shrinkage is improved; and further combining mould pressing and isostatic pressing treatment, the size of pores in the secondary particles of the material is reduced to a preset level, so that the stable composite material resistant to expansion and shrinkage changes is formed, the integral specific surface area of the material is reduced, and the cycle performance of the material is improved.
The embodiment of the invention also provides a lithium ion battery, which comprises a positive pole piece, a negative pole piece, a diaphragm and electrolyte, wherein the negative pole piece comprises a negative active material, and the negative active material comprises the silicon-carbon composite material disclosed by the embodiment of the invention.
As shown in fig. 2, an embodiment of the present invention further provides a terminal 200, where the terminal 200 may be a mobile phone, or an electronic product such as a tablet computer, a notebook computer, a portable device, or a smart wearable product, and the terminal 200 includes a housing 201 assembled outside the terminal, and a circuit board and a battery (not shown in the figure) located inside the housing 201, where the battery is the lithium ion battery provided in the embodiment of the present invention, the housing 201 may include a display screen assembled on a front side of the terminal and a rear cover assembled on a rear side, and the battery may be fixed inside the rear cover to supply power to the terminal 200.
The invention is further illustrated by the following specific examples.
Referring to fig. 3, the method for preparing a silicon-carbon composite material according to embodiment 1 of the present invention includes the following steps:
1. three-step dispersion of graphite/silicon material/asphalt:
1.1 preparation of graphite dispersion a (one-step dispersion): adding 3000g of deionized water into a 10L barrel, starting a high-speed stirrer, slowly adding 20.5g of sodium carboxymethylcellulose (CMC) under the low-speed stirring state, and after adding, adjusting the stirring speed to 900-. And adding 270g of graphite into the CMC solution, adjusting the stirring speed to 2000-2400 rpm, and stirring and dispersing for 60-120min to obtain the uniformly dispersed graphite slurry dispersion liquid A.
1.2 preparation of silicon material/asphalt dispersion liquid B: (two-step dispersion): adding 180g of simple substance silicon into a 5L plastic barrel, then adding 270g of absolute ethyl alcohol, starting a stirrer, stirring at a low speed to fully wet the simple substance silicon, adding 1750g of deionized water after wetting, then adding 270g of asphalt powder, stirring at a high speed, and then adding into a sand mill for circular dispersion to obtain uniformly dispersed silicon material/asphalt mixed slurry to obtain dispersion liquid B.
1.3 preparation of graphite/silicon material/asphalt mixed dispersion (three-step dispersion): adding the silicon material/asphalt mixed slurry prepared in the step 1.2 into the graphite slurry prepared in the step 1.1, adding 2000g of deionized water, adjusting the stirring speed of a stirrer to 3000 revolutions per minute, and stirring and blending for 60-120min to obtain uniformly dispersed graphite/silicon material/asphalt mixed slurry and obtain a mixed dispersion liquid. Through three-step dispersion, silicon materials, asphalt and graphite can be uniformly dispersed.
2. Spray drying and primary heat treatment:
2.1 starting a sprayer, carrying out spray drying on the mixed dispersion liquid, adjusting the inlet temperature to 200-230 ℃, starting the sprayer after the temperature reaches, adjusting the atomization rotating speed to be 30-45Hz, starting a peristaltic pump to adjust the feeding speed of a feeding pump to be 20-30Hz, and starting spraying to obtain a precursor A;
2.2 the material after spray drying is put into an electric heating type high-speed modification mixer (hot VC machine), stirring is started (the stirring speed is 1200-.
As shown in fig. 4, after the spray drying, the nano silicon material, the amorphous carbon precursor and the graphite are granulated into spherical secondary particles, and the nano silicon material is uniformly dispersed in and on the secondary particles.
3. Embossing and isostatic pressing
And (2) putting the precursor powder (graphite substrate/nano silicon material/asphalt) subjected to spray drying and primary heat treatment into a mould pressing mould, treating the mould with the pipe diameter of 53mm and the length of 105mm for 10 minutes at the oil pressure gauge pressure of 3-5MPa, then putting the mould pressing mould into a polyethylene bag, vacuumizing and sealing the polyethylene bag, putting the polyethylene bag into an isostatic press, increasing the pressure to 90-120MPa at the pressure of 8-10MPa/min, keeping the pressure for 0.5-2 hours, continuously relieving the pressure at the pressure of 8-10MPa/min, and discharging the product. After mold pressing and isostatic pressing, the material is formed into a block, and a precursor C is obtained, as shown in fig. 5.
4. Two-stage heat treatment and pulverization classification
Placing the precursor C in a ceramic crucible, putting the ceramic crucible into a tube furnace, and raising the temperature to 300 ℃ for 1-2 hours; raising the temperature to 450 ℃ again, and maintaining the temperature for 1 to 2 hours; and finally, raising the temperature to 600 ℃, maintaining for more than 2 hours, ending the process, and naturally cooling to the room temperature under the protection of atmosphere. The whole process is protected by nitrogen, and the flow is 1L/min. After this step, a precursor D was obtained.
Grinding the blocks by adopting a mortar, sieving by using a 80-mesh sieve, and adding into a grinder, wherein the frequency of a grinder host is 12-15Hz, the frequency of a grader is 25-30Hz, and the feeding speed is 5 Hz. After this step, precursor E was obtained.
5. Carbon coating treatment
Uniformly mixing the precursor E and the asphalt in a high-speed mixer according to the mass ratio of 100: 10-15, and then putting the mixture into an electric heating type high-speed modification mixer (thermal VC machine) for coating treatment, wherein the treatment conditions are as follows: heating to 200 deg.C, and maintaining for 60-90 min; slowly heating to 300 deg.C, and maintaining for 80-120 min; heating to 450 deg.C, and maintaining for 100-150 min; finally, the temperature is raised to 650 ℃, the temperature is maintained for 100-150min, and the process is ended to obtain a precursor F.
6. Three stage heat treatment
And (3) filling the precursor F of the carbon-coated sample into a crucible, putting the crucible into a tubular furnace, quickly heating to 1000-1100 ℃, maintaining for 2-3 hours, and carrying out the whole process with nitrogen at the flow rate of 1L/min to obtain the target material, namely the silicon-carbon composite material particles.
Comparative example
The difference from example 1 is that graphite, silicon material and asphalt are dispersed in one step, specifically, graphite, silicon material and asphalt are stirred and dispersed in water together to obtain a mixed dispersion, and the other steps are the same as example 1 without mould pressing and isostatic pressing treatment.
As shown in fig. 6 and 7, it can be seen from fig. 6 that the silicon-carbon composite material prepared by the inventive example 1 and the comparative example, which are respectively observed in cross section by using a Scanning Electron Microscope (SEM), has an internal silicon material (shown as white in the figure) and graphite (shown as black in the figure) uniformly distributed therein, has a small internal pore structure size, and does not have a macroporous structure with a size greater than 50nm, when the inventive example 1 adopts a three-step dispersion method, and is combined with the mold pressing and the isostatic pressing treatment. And fig. 7 shows that the silicon material and graphite are not uniformly distributed in the composite material prepared by the comparative example, and a large number of macroporous structures exist in the composite material.
The results of comparing the physical and electrochemical properties of the silicon carbon composites of inventive example 1 and comparative example are shown in table 1.
TABLE 1
Item Example 1 silicon carbon composite Comparative example silicon carbon composite
D10(μm) 6.83 5.33
D50(μm) 17.67 12.55
D90(μm) 34.95 23.63
Tap density (g/cm) 3 ) 0.98 0.8
Specific surface area (m) 2 /g) 1.85 2.17
Reversible capacity (mAh/g) 1083.4 1080.6
First coulombic efficiency (%) 86.7 84.8
Table 1 shows that the silicon-carbon composite material of example 1 of the present invention has a high tap density, which is beneficial to improving the processability of the material for later cell fabrication; secondly, the silicon-carbon composite material has a lower specific surface area, so that the side reaction of the material and the electrolyte can be effectively reduced, and the cycle life is prolonged; the silicon-carbon composite material disclosed by the embodiment 1 of the invention has higher first coulombic efficiency, and can effectively improve the energy density of a full battery.
The silicon-carbon composite material of the embodiment 1 and the silicon-carbon composite material of the comparative example are respectively used as negative electrodes, and are dispersed in deionized water with the conductive agent SP, the binder SBR and the CMC according to the mass ratio of 95:0.3:3.2:1.5, and the electrode slurry is obtained after uniform stirring. And coating the surface of the copper foil, and drying at 85 ℃ to obtain the negative electrode plate. The electrolyte is 1mol/L LiPF matched with a commercial lithium cobaltate positive plate 6 The material is characterized by comprising three layers of membranes, namely/EC + PC + DEC + EMC (volume ratio of 1:0.3:1:1) and PP/PE/PP, wherein the thickness of the three layers of membranes is 10 mu m, and the three layers of membranes are manufactured into a soft package battery with the thickness of about 3.7Ah and used for testing the full battery performance of the material. As shown in fig. 8, the battery using the silicon carbon composite material of example 1 of the present invention was excellent in cycle performance of the cell, and the capacity retention rate at 500 cycles was 80%, while the battery using the silicon carbon composite material of the comparative exampleThe capacity had decayed to 80% for 200 weeks of the cycle. The silicon-carbon composite material provided by the embodiment of the invention has the advantages that the graphite material in the silicon-carbon composite material is connected through the amorphous carbon, the silicon material is uniformly dispersed in the amorphous carbon, and the size of internal pores is small, so that the contact area between the silicon material and electrolyte can be reduced, the occurrence of side reactions is reduced, and the service life of a battery is prolonged; meanwhile, the silicon material is uniformly distributed around the graphite material through the amorphous carbon, so that the excessive local expansion shrinkage rate in the material can be effectively avoided, the volume expansion and shrinkage of the silicon material can be effectively relieved by the graphite material, the structural stability of the composite material is improved, and the service life of a battery is prolonged.

Claims (21)

1. A preparation method of a silicon-carbon composite material is characterized by comprising the following steps:
adding a graphite material into a sodium carboxymethylcellulose aqueous solution, and uniformly stirring and dispersing to obtain a dispersion A;
wetting a silicon material by using C1-C4 alcohol, and uniformly dispersing the silicon material and an amorphous carbon precursor in water together to obtain a dispersion liquid B;
adding water into the dispersion liquid A and the dispersion liquid B for mixing, and stirring and dispersing uniformly to obtain a mixed dispersion liquid;
drying the mixed dispersion liquid to obtain a primary precursor;
sequentially carrying out primary heat treatment, mould pressing treatment, cold isostatic pressing treatment and secondary heat treatment on the primary precursor, and crushing and grading to obtain a secondary precursor;
and coating the secondary precursor with carbon, and performing three-stage heat treatment to obtain the silicon-carbon composite material, wherein the silicon-carbon composite material comprises an inner core and a carbon layer coated on the surface of the inner core, the inner core comprises a graphite skeleton, amorphous carbon filled in the graphite skeleton structure and silicon material uniformly distributed in the amorphous carbon, and the silicon-carbon composite material only has a pore structure with the pore diameter smaller than or equal to 50nm and does not have a pore structure with the pore diameter larger than 50 nm.
2. The method of preparing a silicon carbon composite material according to claim 1, wherein the amorphous carbon precursor comprises pitch or phenolic resin.
3. The method for preparing the silicon-carbon composite material according to claim 1, wherein the pressure in the molding process is 3Mpa to 5Mpa, and the molding temperature is 300 ℃ to 400 ℃.
4. The method of claim 1, wherein the cold isostatic pressing is performed at a pressure of 90Mpa to 120Mpa for a time of 0.5 to 2 hours.
5. The method of claim 1, wherein the drying process comprises spray drying or rotary evaporation drying.
6. The method of claim 1, wherein the primary heat treatment is performed at a temperature of 180 ℃ to 200 ℃ for a period of 2 to 3 hours.
7. The method for preparing the silicon-carbon composite material according to claim 1, wherein the secondary heat treatment is carried out by the following specific operations: under the protection of nitrogen, firstly heating to 300-350 ℃ for 1-2 hours, then heating to 450-550 ℃ for 1-2 hours, finally heating to 600-700 ℃ for 2-4 hours, and then naturally cooling to room temperature.
8. The method for preparing the silicon-carbon composite material according to claim 1, wherein the carbon coating is specifically performed by: mixing the secondary precursor with the carbon layer precursor, heating to 200-250 ℃ for 60-90 minutes, slowly heating to 300-400 ℃ for 80-120 minutes; then heating to 450-500 deg.C, maintaining for 100-150 min; finally, the temperature is raised to 650-700 ℃ and maintained for 100-150 minutes to obtain the carbon layer.
9. The method for preparing the silicon-carbon composite material according to claim 1, wherein the three-stage heat treatment is performed in a tube furnace and is maintained at 1000 ℃ to 1200 ℃ for 2 to 4 hours under the protection of nitrogen.
10. The method according to claim 1, wherein the silicon material has an area ratio of 20% to 50% in a range of 5 μm x 5 μm in any cross section of the silicon-carbon composite material.
11. The method of claim 1, wherein the silicon material has a particle size of 50nm to 150 nm.
12. The method for preparing the silicon-carbon composite material according to claim 1, wherein the silicon material is 10-40% by mass of the silicon-carbon composite material.
13. The method for preparing the silicon-carbon composite material according to claim 1, wherein the mass ratio of the graphite skeleton in the silicon-carbon composite material is 50-70%.
14. The method for preparing a silicon-carbon composite material according to claim 1, wherein the mass ratio of the amorphous carbon in the silicon-carbon composite material is 10-30%.
15. The method of claim 1, wherein the silicon-carbon composite material has a particle size of D10 ranging from 4 μm to 7 μm, D50 ranging from 8 μm to 18 μm, D90 ranging from 25 μm to 35 μm, and D99 ranging from 40 μm to 60 μm.
16. The method of preparing a silicon-carbon composite material according to claim 1, wherein the graphite skeleton is composed of a graphite material having a particle size of 5 μm to 15 μm, and the graphite material includes at least one of artificial graphite and natural graphite.
17. The method of claim 1, wherein the graphite material is flake graphite having an aspect ratio of 1 to 3.
18. The method of claim 1, wherein the carbon layer has a thickness of 5nm to 20 nm.
19. The method of claim 1, wherein the silicon-carbon composite has a tap density of 0.8 to 1.0g/cm 3 The specific surface area is 1.5-3.0m 2 /g。
20. A lithium ion battery is characterized by comprising a positive pole piece, a negative pole piece, a diaphragm and electrolyte, wherein the negative pole piece comprises a negative active material, and the negative active material comprises the silicon-carbon composite material prepared by the preparation method of any one of claims 1 to 19.
21. A terminal comprising a housing, and a circuit board and a battery located inside the housing, the battery comprising the lithium ion battery of claim 20.
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