CN115088100A - Negative electrode composite material and application thereof - Google Patents
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- CN115088100A CN115088100A CN202080095939.4A CN202080095939A CN115088100A CN 115088100 A CN115088100 A CN 115088100A CN 202080095939 A CN202080095939 A CN 202080095939A CN 115088100 A CN115088100 A CN 115088100A
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Images
Classifications
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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
-
- 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
-
- 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
Abstract
A negative electrode composite material comprises a silicon-based material, graphene and graphite, wherein the graphene accounts for 1% -20% of the mass of the negative electrode composite material; the silicon-based material accounts for 10-100% of the total mass of the graphite and the silicon-based material; the Dv50 of the silicon-based material is 3.0-10 μm; the Dv50 of the graphite is 8.0-20 μm; the Dv50 of the negative electrode composite material is 9.5-40 μm. By adopting the cathode composite material provided by the application, the graphite can relieve the expansion of the silicon-based material, and the graphene can increase the conductivity of the composite material; further, the multilayer structure and the slip characteristic of the graphene can release the expansion stress of the silicon-based material in the lithium desorption and insertion process, so that pulverization of silicon-based particles caused by expansion is eliminated, and the cycle performance of the cathode material is improved.
Description
The application relates to the technical field of lithium ion batteries, in particular to a negative electrode composite material and application thereof.
The silicon material has high theoretical gram capacity (4200mAh/g), and has wide application prospect in lithium ion batteries. However, in the process of charge and discharge cycle, the silicon material can generate 120-300% volume change along with the insertion and extraction of lithium ions, so that the silicon material is pulverized and separated from a current collector, the conductivity of a negative electrode is poor, and the cycle performance of a lithium ion battery is reduced.
Disclosure of Invention
The application aims to provide a negative electrode composite material to at least solve the problems of large volume change and poor conductivity of a silicon-based negative electrode material.
The first aspect of the application provides a negative electrode composite material, which comprises a silicon-based material, graphene and graphite, wherein the graphene accounts for 1% -20% of the mass of the negative electrode composite material; the silicon-based material accounts for 10-100% of the total mass of the graphite and the silicon-based material;
the Dv50 of the silicon-based material is 3.0-10 μm; the Dv50 of the graphite is 8.0-20 μm; the Dv50 of the negative electrode composite material is 9.5-40 μm.
In some embodiments of the first aspect of the present application, the number of layers of graphene is 3 to 10.
In some embodiments of the first aspect of the present application, the graphite comprises at least one of natural graphite, artificial graphite, or mesocarbon microbeads.
In some embodiments of the first aspect of the present application, the silicon-based material comprises at least one of silicon, silicon oxide, or silicon carbon material.
In certain embodiments of the first aspect of the present application, carbon is present on at least a portion of a surface of the silicon-based material.
In some embodiments of the first aspect of the present application, the negative electrode composite material has an electrical conductivity of 2.0 to 30S/cm.
The second aspect of the present application provides a negative electrode sheet, which includes a current collector and a mixture layer coated on the current collector, where the mixture layer includes the negative electrode composite material provided in the first aspect of the present application.
The third aspect of the application provides a battery, which comprises the negative pole piece provided by the second aspect of the application.
In some embodiments of the third aspect of the present application, the battery has a swelling rate of 6.5 to 10%.
A fourth aspect of the present application provides an electronic device comprising the battery provided by the third aspect of the present application.
By using the cathode composite material provided by the application, a silicon-based material is compounded with graphene and graphite, wherein the graphite can relieve the expansion of the silicon-based material, and the graphene can increase the conductivity of the composite material;
further, the multilayer structure and the slip characteristic of the graphene can release the expansion stress of the silicon-based material in the lithium desorption and insertion process, so that pulverization of silicon-based particles caused by expansion is eliminated, and the cycle performance of the cathode material is improved.
In addition, negative pole piece, battery that this application provided have good cycle performance.
Herein, the term "Dv 50" denotes the particle size with a cumulative particle distribution of 50%; i.e. the volume content of particles smaller than this size is 50% of the total particles. The particle size is measured with a laser particle sizer.
In order to more clearly illustrate the embodiments of the present invention and the technical solutions of the prior art, the following briefly introduces the drawings required for the embodiments and the prior art, and obviously, the drawings in the following description are only some embodiments of the present invention, and other drawings can be obtained by those skilled in the art without creative efforts.
Fig. 1 is an SEM photograph of the negative electrode composite prepared in example 3 of the present application.
FIG. 2 is a graph of the capacity fade curves for example 5, example 16, example 19 and comparative example 4.
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail below with reference to the accompanying drawings and examples. It is to be understood that the described embodiments are merely exemplary of the invention, and not restrictive of the full scope of the invention. All other embodiments, which can be obtained by a person skilled in the art without making any creative effort based on the embodiments in the present invention, belong to the protection scope of the present invention.
The first aspect of the application provides a negative electrode composite material, which comprises a silicon-based material, graphene and graphite, wherein the graphene accounts for 1% -20% of the mass of the negative electrode composite material; the silicon-based material accounts for 10-100% of the total mass of the graphite and the silicon-based material;
the Dv50 of the silicon-based material is 3.0-10 μm; the Dv50 of the graphite is 8.0-20 μm; the Dv50 of the negative electrode composite material particle is 9.5-40 mu m, and the BET of the specific surface area is 1.0-15 m 2 /g。
The high energy density battery requires the cathode material with gram capacity more than 500mAh/g, and the inventor finds in research that the high energy density battery can meet the requirement when the silicon-based material in the cathode composite material accounts for more than 10% of the total mass of the graphite and the silicon-based material.
The inventors also found in research, without being limited to any theory, that when the content of graphene in the negative electrode composite material of the present application is greater than 20%, too much graphene around silicon and graphite affects intercalation and deintercalation of lithium ions, thereby causing deterioration of the full battery rate performance; when the content of the graphene is lower than 1%, the effect of relieving the expansion of the negative electrode composite material cannot be achieved.
In the negative electrode composite material, the particle sizes of the silicon-based material, the graphite and the negative electrode composite material particles have important influence on the performance of the battery; without being limited to any theory, the inventor finds that when the Dv50 of the silicon-based material is less than 3 μm, the Dv50 of the composite material is less than 9.5 μm, the specific surface area of the material is large, the contact area with the electrolyte is large, so that the irreversible lithium loss is more, and the cycle capacity retention rate is reduced. When the graphite Dv50 is more than 20 mu m, the composite material Dv50 is more than 40 mu m, the local expansion of the battery in the circulating process can be caused, the electric contact between materials in the pole piece can be deteriorated due to the excessive expansion, and the reduction and acceleration of the battery capacity can be realized; in some preferred embodiments of the present application, 8 μm < graphite Dv50+ silicon-based material Dv50 < 15 um.
In some embodiments of the first aspect of the present application, the number of layers of graphene is 3 to 10.
The inventor finds in research that the addition of graphene can significantly improve the conductivity of the composite material, however, when the number of layers of graphene is too large, such as more than 10 layers, or too small, such as when single-layer graphene is used, the conductivity of the composite material is significantly reduced, not limited to any theory, compared with when multi-layer graphene with less than 10 layers and more than 3 layers is used, and the inventor believes that this may be because when the number of graphene layers is too large, the exposed end faces are more, the defects are increased, and the conductivity is reduced; single-layer graphene is prone to wrinkling, resulting in increased resistance and reduced conductivity. The inventor finds that graphene has a slipping effect, can relieve expansion stress, and can restore to an original state after cyclic expansion, but when the number of layers of graphene is less than 3, lithium-embedded expansion of a negative electrode material enables the graphene to slip too much, so that the graphene cannot restore, expansion is increased, and cyclic capacity attenuation is accelerated.
In certain embodiments of the first aspect of the present application, the graphite comprises at least one of natural graphite, artificial graphite, or mesocarbon microbeads.
In some embodiments of the first aspect of the present application, the silicon-based material comprises at least one of silicon, silicon oxide, or silicon carbon material.
In some embodiments of the first aspect of the present application, carbon is present on at least a portion of a surface of the silicon-based material. It is understood that at least a portion of the surface of the silicon-based material is coated with carbon, either partially or fully.
The carbon coating can improve the conductivity of the silicon-based material and improve the electrical property of the silicon-based material. Carbon-coated silicon-based materials, such as carbon-coated silicon oxides, are known in the art; can be prepared according to the prior art or obtained commercially.
In some embodiments of the first aspect of the present application, the negative electrode composite material has an electrical conductivity of 2.0 to 30S/cm.
The preparation method of the anode composite material provided by the present application is not particularly limited, and may be prepared, for example, by the following method:
1) mixing and stirring a powdery silicon-based material and graphite;
2) adding the graphene slurry into a mixture of a silicon-based material and graphite, and continuously stirring;
3) adding water, adjusting the solid content of the slurry to 30-60%, and continuously stirring to obtain mixed slurry;
4) transferring the mixed slurry to a spray drying granulator for granulation;
5) and roasting the granulated material in an inert atmosphere to obtain the negative active material.
The inventor finds that the generated negative electrode composite material does not contain a binder by adopting a spray drying granulation method, and is beneficial to improving the rate capability of the battery.
Wherein the inlet temperature of the spray drying granulator is 240 ℃ and 280 ℃, preferably 260 ℃; the outlet temperature is 100 ℃ and 110 ℃, preferably 105 ℃.
The inventor also finds that the granulated material can effectively remove impurities such as water, organic matters and the like in the material by roasting in an inert atmosphere; while the firing temperature of the negative electrode material has a large influence on the cycle expansion and capacity retention of the battery, it is not limited to any theory that too high a processing temperature, for example, above 900 ℃, may cause agglomeration of the material, resulting in a significant increase in Dv50, poor contact between small particles after cycle expansion, resulting in accelerated cycle capacity fade, and increased battery expansion. When the treatment temperature is too low, for example, below 600 ℃, the decomposition of the residual dispersant in the graphene preparation process is incomplete, the number of surface active groups of the composite material is large, more Solid Electrolyte Interface (SEI) is generated, the cyclic expansion is increased, and the capacity retention rate is reduced.
In some embodiments of the present application, the firing conditions are: the roasting temperature is 600-900 ℃, and the heat preservation time is 1-5 hours, preferably 2 hours; during roasting, the heating rate can be 3-8 ℃/min, and is preferably 5 ℃/min.
In a second aspect, the present application provides a negative electrode sheet comprising a current collector and a mixture layer coated on the current collector, wherein the mixture layer comprises the negative electrode composite material provided in the first aspect of the present application.
The mixture layer may be coated on one or both surfaces of the current collector, and may be specifically selected by those skilled in the art according to actual needs, and the application is not limited herein.
The current collector is not particularly limited, and any current collector known to those skilled in the art may be used. Specifically, for example, a current collector formed of iron, copper, aluminum, nickel, stainless steel, titanium, tantalum, gold, platinum, or the like can be used. Among them, copper foil or copper alloy foil is particularly preferable as the negative electrode current collector. One of the above materials may be used alone, or two or more of them may be used in combination in any ratio.
In some embodiments of the present application, the mixture layer may further include a binder. The binder is not particularly limited, and may be any binder or combination thereof known to those skilled in the art, and for example, polyacrylate, polyimide, polyamide, polyamideimide, polyvinylidene fluoride, styrene-butadiene rubber, sodium alginate, polyvinyl alcohol, polytetrafluoroethylene, polyacrylonitrile, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, sodium hydroxymethyl cellulose, potassium hydroxymethyl cellulose, and the like may be used. These binders may be used alone, or two or more thereof may be used in combination at an arbitrary ratio.
In some embodiments of the present application, the mixture layer may further include a conductive agent. The conductive agent is not particularly limited, and may be any conductive agent or a combination thereof known to those skilled in the art, and for example, at least one of a zero-dimensional conductive agent, a one-dimensional conductive agent, and a two-dimensional conductive agent may be used. Preferably, the conductive agent may include at least one of carbon black, conductive graphite, carbon fiber, carbon nanotube, VGCF (vapor grown carbon fiber), or graphene. The amount of the conductive agent is not particularly limited and may be selected according to the common general knowledge in the art. The conductive agent may be used alone, or two or more of them may be used in combination at an arbitrary ratio.
The third aspect of the application provides a battery, which comprises the negative pole piece provided by the second aspect of the application.
In some embodiments of the third aspect of the present application, the battery has a swelling rate of 6.5 to 10%.
Batteries of the present application include, but are not limited to: all kinds of primary batteries, secondary batteries, fuel cells, solar cells or capacitors. A typical battery is a lithium ion battery, which is a type of secondary battery. The battery, such as a lithium ion battery, generally includes a negative electrode plate, a positive electrode plate, a separator, and an electrolyte.
According to the battery provided by the application, the negative pole piece adopts the negative pole piece provided by the application; the other components including the positive electrode sheet, the separator, the electrolyte, and the like are not particularly limited. Illustratively, the positive electrode material included in the positive electrode sheet may include, but is not limited to, lithium cobaltate, lithium manganate, lithium iron phosphate, and the like. The material of the diaphragm may include, but is not limited to, fiberglass, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or a combination thereof. The electrolyte generally includes an organic solvent, a lithium salt, and an additive. The organic solvent may include, but is not limited to, at least one of Ethylene Carbonate (EC), Propylene Carbonate (PC), diethyl carbonate (DEC), Ethyl Methyl Carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, and ethyl propionate. The lithium salt may include at least one of an organic lithium salt or an inorganic lithium salt; for example, the lithium salt may include lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF) 4 ) Lithium difluorophosphate (LiPO) 2 F 2 ) Lithium bis (trifluoromethanesulfonylimide) LiN (CF) 3 SO 2 ) 2 (LiTFSI), lithium bis (fluorosulfonyl) imide Li (N (SO) 2 F) 2 ) (LiFSI), lithium LiB (C) bis (oxalato) borate 2 O 4 ) 2 (LiBOB), lithium difluoro (oxalato) borate LiBF 2 (C 2 O 4 ) (LiDFOB)
The preparation process of the battery is well known to those skilled in the art, and the present application is not particularly limited. For example, the secondary battery may be manufactured by the following process: the positive electrode and the negative electrode are overlapped through a spacer, and are placed into a battery container after operations such as winding, folding and the like are performed according to needs, an electrolyte is injected into the battery container and the battery container is sealed, wherein the negative electrode used is the negative electrode plate provided by the application. In addition, an overcurrent prevention element, a guide plate, or the like may be placed in the battery container as necessary to prevent a pressure rise, overcharge, and discharge inside the battery.
The application also provides an electronic device which comprises the battery provided by the application.
The present application will be specifically described below with reference to examples, but the present application is not limited to these examples.
Testing the particle size of the composite material:
about 0.02g of each sample powder was added to a 50ml clean beaker, about 20ml of deionized water was added, several drops of 1% surfactant were added dropwise to completely disperse the powder in water, and the resulting mixture was ultrasonically cleaned in a 120W ultrasonic cleaner for 5 minutes, and the particle size distribution was measured using a MasterSizer 2000.
Testing the specific surface area of the composite material:
after measuring the adsorption amount of gas on the solid surface at constant temperature and low temperature under different relative pressures, the adsorption amount of the monomolecular layer of the sample is obtained based on the Broenson-Eltt-Taylor adsorption theory and the formula (BET formula) thereof, and the specific surface area of the solid is calculated.
Composite powder conductivity test:
taking 5g composite material powder sample, keeping constant pressure to 5000kg + -2 kg with electronic press, maintaining for 15-25s, and electrically testing the sample in resistivity tester (Suzhou lattice electron ST-2255A)Interelectrode, sample height h (cm), voltage U at two ends, current I and resistance R (K omega); area S after powder compression was 3.14cm 2 The electronic conductivity of the powder was calculated according to the formula δ ═ h/(S × R)/1000, with the unit of S/cm.
And (3) testing the performance of the full battery:
and (3) cycle testing:
the test temperature was 25 ℃, and the voltage was charged to 4.45V at a constant current of 0.5C, to 0.025C at a constant voltage, and discharged to 3.0V at 0.5C after standing for 5 minutes. Taking the capacity obtained in the step as initial capacity, carrying out 0.5C charging/0.5C discharging for cycle test, and taking the ratio of the capacity of each step to the initial capacity to obtain a capacity fading curve; wherein, the capacity fade curves of examples 5, 16, 19 and comparative example 4 are shown in fig. 2; the capacity retention after 400 cycles of each example and comparative example is shown in tables 1 and 2.
Rate capability:
the test temperature was 25 ℃, and was charged to 4.45V at a constant current of 0.5C, to 0.025C at a constant voltage, and discharged to 3.0V at 0.2C after standing for 5 minutes. And taking the capacity obtained in the step as initial capacity, carrying out 0.5C charging and 2C discharging, wherein the ratio of the 2C discharging capacity to the 0.2C discharging capacity is the rate capability.
And (3) testing the full charge expansion rate of the lithium ion battery:
and testing the thickness of the lithium ion battery during initial half-charging by using a spiral micrometer. And (3) at 25 ℃, when the charging and discharging are cycled to 400 times, the lithium ion battery is in a full charge state, the thickness of the lithium ion battery at the moment is tested by using a spiral micrometer, and the thickness is compared with the thickness of the lithium ion battery at the initial half charge, so that the expansion rate of the full charge lithium ion battery at the moment can be obtained.
And (3) testing the first efficiency of the full battery: in the first charge and discharge process of the whole battery, the constant current is charged to 4.45V at 0.5C, the constant voltage is charged to 0.025C at 4.45V, the capacity obtained before is C0, and after standing for 5min, the 0.5C is discharged to 3.0V (the discharge capacity D0 is obtained). Full cell first efficiency D0/C0.
First reversible capacity test of half-cell: and cutting the negative pole pieces obtained in the embodiments and the comparative examples into round pieces with the diameter of 1.4cm by a punch in a dry environment, taking a metal lithium piece as a counter electrode, selecting a ceglard composite membrane as an isolating membrane, and adding an electrolyte to assemble the button cell. The first reversible capacity of the half cell was measured using a blue electricity (LAND) series cell test system, and the results are shown in tables 1 and 2.
Testing the first efficiency of the half cell: assembling a button cell by using a lithium sheet as a counter electrode, discharging to 5mV at 0.05C, then discharging to 5mV at 0.05mA, standing for 1 hour, and then discharging to 5mV at 0.01mA (the previously obtained lithium-embedded capacity C is obtained 0 ) Standing for 5min, charging to 2.0V at 0.05C (to obtain delithiation capacity D 0 ) First half-cell efficiency D 0 /C 0 。
Preparing a full battery:
preparation of a negative electrode mixture layer material:
1) respectively adding 2.8kg of the negative electrode composite material prepared in each embodiment and the negative electrode composite material prepared in each comparative example and 35g of conductive carbon black into an MSK-SFM-10 vacuum stirrer, and stirring for 40min at the revolution speed of 10-30 rpm;
2) adding 95g of polyvinylidene fluoride into the mixture stirred in the step 1), stirring for 60min to disperse uniformly, adding deionized water, stirring for 120min to disperse uniformly, and obtaining mixed slurry with the viscosity of 2000mPa.S and the solid content of 35%; the composite material, the conductive agent and the binder are in mass ratio: 95.6:1.2: 3.2;
3) filtering the mixed slurry obtained in the step 2) by using a 170-mesh double-layer screen to obtain the negative electrode mixture layer material.
Preparing a negative pole piece:
coating the prepared negative electrode mixture layer material on two surfaces of a copper foil current collector with the thickness of 10 mu m, wherein the coating thickness is 100 mu m; drying the pole piece and then cold-pressing the pole piece, wherein the double-sided compaction density is 1.8g/cm 3 。
Preparing a positive pole piece:
active material LiCoO 2 The conductive carbon black and the adhesive polyvinylidene fluoride (PVDF) are mixed according to the weight ratio of 96.7: 1.7: 1.6 in N-methyl pyrrolidone solvent system, preparing into slurry with solid content of 0.75, and stirring uniformly. Uniformly coating the slurry on one surface of a positive electrode current collector aluminum foil with the thickness of 12 mu m, wherein the coating thickness is 115 mu m and the coating temperature is 90 DEG CDrying and cold pressing under the condition to obtain the positive pole piece.
Assembling the whole battery:
a PE porous polymer film having a thickness of 15 μm was used as a separator. And stacking the positive pole piece, the isolating film and the negative pole piece in sequence to enable the isolating film to be positioned between the cathode and the anode to play an isolating role, and winding to obtain the bare cell. Placing the naked cell in an external package, and injecting the prepared electrolyte (EC: DMC: DEC: 1:1:1 vol%, 10 wt% FEC, 1mol/L LiPF 6 ) And packaging, and carrying out technological processes of formation, degassing, edge cutting and the like to obtain the full cell.
Preparation of negative electrode composite material
Example 1
1.1 kg of silicon oxide (Dv50 of 5 μm) powder and 9kg of graphite (Dv50 of 8 μm) were added to water, and the mixture was stirred in an MSK-SFM-10 vacuum stirrer for 180 minutes at a revolution speed of 10 to 40rpm and mixed uniformly (silicon-based material content: 10 wt%).
2. 2.04kg of graphene (10-layer structure) water-based slurry with solid content of 10% is added into a stirrer and stirred for 120min to be uniformly dispersed. Adding 5kg of deionized water, and continuously stirring for 120min to obtain mixed slurry, wherein the revolution speed is 40rpm, the rotation speed is 1500rpm, and the content of graphene in the solid is 2%.
3. Transferring the slurry in the step 2) to a centrifugal turntable nozzle of a spray drying granulator, wherein the centrifugal rotating speed is 5000rpm, and forming tiny fog drops. The inlet temperature of the spray drying granulator is 260 ℃, the outlet temperature is 105 ℃, and the granules are cooled and collected.
4. Roasting the granulated material in the step 3) in an inert atmosphere (Ar), controlling the heating rate at 5 ℃/min, and keeping the temperature at 750 ℃ for 2 h. And (4) sieving the powder with a 400-mesh sieve to obtain the required cathode composite material.
Examples 2 to 4
Replacing the graphene in the embodiment 1 with graphene with 7-layer, 5-layer and 3-layer structures, and the rest is the same as the embodiment 1; wherein a scanning electron micrograph (SEM photograph) of the negative electrode composite material prepared in example 3 is shown in fig. 1.
Examples 5 to 8
The amounts of the graphene slurries used in example 3 were adjusted to 5.26kg, 11.11kg, 17.65kg and 25kg so that the contents of graphene were 5%, 10%, 15% and 20%, respectively, and the rest was the same as in example 3.
Example 9
The silicon-based material in example 5 was replaced with silicon (Si), and the rest was the same as in example 5.
Example 10
The silicon-based material in example 5 was replaced with a silicon carbon material (SiC), and the rest was the same as example 5.
Examples 11 and 12
The calcination temperatures in example 5 were adjusted to 600 ℃ and 900 ℃ respectively, and the same procedure as in example 5 was repeated.
Example 13
In example 5, 2kg of silicon oxide powder was mixed with 8kg of graphite so that the content of the silicon-based material was 20%, and the rest was the same as in example 5.
Example 14
In example 5, 5kg of silicon oxide powder was mixed with 5kg of graphite so that the content of the silicon-based material was 50%, and the rest was the same as in example 5.
Example 15
In example 5, 8kg of silicon oxide powder was mixed with 2kg of graphite so that the content of the silicon-based material was 80%, and the rest was the same as in example 5.
Example 16
In example 5, 10kg of silicon oxide was used as it is so that the content of the silicon-based material was 100%, and the rest was the same as in example 5.
Examples 17 and 18
The particle size of the silicon oxide (Dv50) in example 5 was adjusted to 3.5 μm and 8 μm, respectively, and the procedure was otherwise the same as in example 5.
Examples 19 and 20
The particle size of graphite (Dv50) in example 5 was adjusted to 15 μm and 20 μm, respectively, and the procedure was repeated in the same manner as in example 5.
Comparative examples 1 and 2
The graphene in example 5 was replaced with graphene having a 15-layer structure and a single-layer structure, respectively, and the rest was the same as in example 5.
Comparative example 3
The amount of the graphene slurry used in example 5 was adjusted to 33.3kg so that the content of each graphene was 25%, and the rest was the same as in example 5.
Comparative example 4
In example 5, 5kg of deionized water was directly used instead of the graphene slurry, and the rest was the same as example 5 (i.e., no graphene was included).
Comparative examples 5 and 6
The calcination temperatures in example 5 were adjusted to 500 ℃ and 1000 ℃ respectively, and the rest was the same as in example 5.
Comparative example 7
In example 5, 10kg of graphite was used as is, i.e., without the silicon-based material, and the rest was the same as in example 5.
Comparative example 8
In example 5, 0.5kg of silicon oxide powder was mixed with 9.5kg of graphite so that the content of the silicon-based material was 5%, and the rest was the same as in example 5.
Comparative example 9
The particle size of the silicon oxide (Dv50) in example 5 was adjusted to 1.5. mu.m, and the procedure was repeated in the same manner as in example 5.
Comparative example 10
The particle size of graphite (Dv50) in example 5 was adjusted to 25 μm, and the procedure was repeated in the same manner as in example 5.
The parameters and performance test results of the negative electrode composite material in each example are shown in table 1; the parameters and performance test results of the negative electrode composite materials in each comparative example are shown in table 2.
TABLE 1
TABLE 2
Comparing examples 1-12 with comparative example 4, it can be seen that the conductivity of the negative electrode material can be significantly improved by mixing graphene granulation under the condition of the same silicon content, and the full battery rate is significantly improved.
By comparing examples 1, 2, 3, 4, when the number of graphene layers satisfies 10 "3 layers, the powder conductivity increases as the number of graphene layers decreases; and as the conductivity increases, lithium ions are more easily extracted and inserted in the negative electrode material, so the first reversible capacity increases. With the increase of the number of graphene layers, the specific surface area of the material is also increased, and particles are more easily agglomerated, so that the particle size of the composite material is increased; in addition, the increase of the specific surface area can cause the contact surface between the electrolyte and an active material to be larger, more SEI films are generated, the cyclic expansion of the full battery is increased, the capacity fading is accelerated, and the capacity retention rate is reduced more obviously.
Example 5 can be seen by comparing with comparative examples 1 and 2, when the number of graphene layers exceeds 10 or is a single layer, the conductivity is obviously reduced, and the inventor believes that, without being limited to any theory, when the number of graphene layers is too large, more end faces are exposed, defects are increased, and the conductivity is reduced; single-layer graphene is prone to wrinkle, resulting in increased resistance and thus reduced conductivity.
In addition, the graphene has a slipping effect, expansion stress can be relieved, and the graphene can be restored to an original state after cyclic expansion, but in the comparative example 2, when the number of layers is less than 3, lithium-embedded expansion of the negative electrode material can cause excessive slipping of the graphene, so that the graphene cannot be restored, expansion is increased, and cyclic capacity attenuation is accelerated.
As can be seen from comparison among examples 3, 5, 6, 7 and 8 and comparative example 3, the conductivity of the material can be obviously improved even if the content of graphene is increased, and meanwhile, the rate performance of the battery is improved along with the increase of the content of graphene; however, when the graphene content is > 20% (comparative example 3), the electric rate performance is rather lowered, and without being limited to any theory, the inventors believe that too high graphene content may cause too much graphene around silicon and graphite, affecting intercalation and deintercalation of lithium ions, so that the full battery rate performance is deteriorated.
Furthermore, the inventors have found that the first reversible capacity of the composite material increases significantly with increasing conductivity when the graphene content is < 5%, but that the gram capacity of the composite material decreases inversely with increasing conductivity when the content exceeds 5%, which may be caused by, without being limited to any theory, the increase in the graphene content leading to a decrease in the overall content of high-capacity silicon-based materials. On the other hand, as the graphene content increases, the full cell first efficiency decreases, and without being limited to any theory, the inventors believe that this may be due to the fact that the increase in the graphene content significantly increases the specific surface area of the composite, the contact area with the electrolyte is larger, more SEI films are generated, and thus the first efficiency is lower.
As can be seen from the comparison of examples 5, 11 and 12 with comparative examples 5 and 6, when the firing temperature is higher than 900 ℃, the composite material is liable to aggregate, and thus the particle size of the composite material increases; without being bound to any theory, the inventors believe that an increase in particle size results in poor contact between small particles after cyclic expansion, accelerated cyclic capacity fade, and increased cell expansion; when the roasting temperature is lower than 600 ℃, the decomposition of a dispersing agent in the graphene slurry is incomplete, a large number of surface active groups are generated, more SEIs are generated, the cyclic expansion is increased, and the capacity maintenance rate is reduced.
Examples 5, 13, 14, 15, 16 in comparison with comparative examples 7, 8 show that the higher the silicon-based active content, the higher the gram capacity of the material. But the swelling during cell cycling increases with increasing capacity, and the swelling increases significantly when the silicon base content exceeds 20%. When the silicon content is less than 10 percent, the requirement of high energy density cannot be met (the high energy density requirement is that the gram capacity of the anode is more than 500mAh/g)
Examples 5, 17 and 18 are compared with comparative example 9, and it is demonstrated that when Dv50 of the silicon-based material is less than 3 μm, the particle size Dv50 of the composite material is less than 9.5 μm, the specific surface area of the material is large, the contact area with the electrolyte is large, the irreversible lithium loss is more, and the cycle capacity retention rate is reduced. Examples 5, 19, 20 compare with comparative example 10 and it can be seen that when the graphite Dv50 is greater than 20 μm, the composite material Dv50 is greater than 40 μm, which causes local swelling during battery cycling, and that excessive swelling causes poor electrical contact between the materials in the pole pieces and accelerated battery capacity drop; the data are combined to give a preference for 8 μm < graphite Dv50+ silicon Dv50 < 15 um.
The above description is only exemplary of the present invention and should not be taken as limiting the invention, as any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
- A negative electrode composite material comprises a silicon-based material, graphene and graphite, wherein,the graphene accounts for 1-20% of the mass of the negative electrode composite material; the silicon-based material accounts for 10-100% of the total mass of the graphite and the silicon-based material;the Dv50 of the silicon-based material is 3.0-10 μm; the Dv50 of the graphite is 8.0-20 μm; the Dv50 of the negative electrode composite material is 9.5-40 μm.
- The negative electrode composite according to claim 1, wherein the number of graphene layers is 3 to 10.
- The negative electrode composite of claim 1, wherein the graphite comprises at least one of natural graphite, artificial graphite, or mesocarbon microbeads.
- The negative electrode composite of claim 1, wherein the silicon-based material comprises at least one of silicon, a silicon oxide, or a silicon carbon material.
- The negative electrode composite of any of claims 1-4, wherein carbon is present on at least a portion of the surface of the silicon-based material.
- The negative electrode composite according to any one of claims 1 to 4, wherein the negative electrode composite has an electrical conductivity of 2.0 to 30S/cm.
- A negative electrode sheet comprising a current collector and a mixture layer coated on the current collector, the mixture layer comprising the negative electrode composite material according to any one of claims 1 to 6.
- A battery comprising the negative electrode tab of claim 7.
- The battery according to claim 8, wherein the expansion rate of the battery is 6.5-10%.
- An electronic device comprising the battery of claim 8 or 9.
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