WO2021189211A1

WO2021189211A1 - Negative electrode composite material and application thereof

Info

Publication number: WO2021189211A1
Application number: PCT/CN2020/080745
Authority: WO
Inventors: 易婷; 陈志焕; 崔航; 谢远森
Original assignee: 宁德新能源科技有限公司
Priority date: 2020-03-23
Filing date: 2020-03-23
Publication date: 2021-09-30
Also published as: CN115088100B; CN115088100A

Abstract

A negative electrode composite material, comprising a silicon-based material, graphene, and graphite. The graphene accounts for 1-20% by mass of the negative electrode composite material; the silicon-based material accounts for 10-100% by total mass of the graphite and the silicon-based material; the silicon-based material has a Dv50 of 3.0-10 μm; the graphite has a Dv50 of 8.0-20 μm; the negative electrode composite material has a Dv50 of 9.5-40 μm. By adoption of the negative electrode composite material provided by the present application, the graphite can alleviate the expansion of the silicon-based material, and the graphene can increase the conductivity of the composite material; further, a multilayer structure and a sliding characteristic of the graphene can release the expansion stress of the silicon-based material in the lithium intercalation/deintercalation process, thereby eliminating powdering of silicon-based particles caused by expansion, and improving the cycle performance of the negative electrode material.

Description

A negative electrode composite material and its application

Technical field

This application relates to the technical field of lithium-ion batteries, in particular to a negative electrode composite material and its application.

Background technique

Silicon material has a high theoretical gram capacity (4200mAh/g), and its application in lithium-ion batteries has broad prospects. However, during the charge-discharge cycle of the silicon material, with the insertion and extraction of lithium ions, a volume change of 120% to 300% will occur, causing the silicon-based material to be powdered and separated from the current collector, resulting in poor conductivity of the negative electrode. Reduce the cycle performance of lithium-ion batteries.

Summary of the invention

The purpose of this application is to provide a negative electrode composite material to at least improve the problem of large volume changes and poor conductivity of silicon-based negative electrode materials.

The first aspect of the present application provides a negative electrode composite material, including silicon-based materials, graphene, and graphite, wherein the graphene accounts for 1% to 20% of the mass of the negative electrode composite material; the silicon-based material accounts for graphite and silicon 10-100% of the total mass of the base 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 graphene layers is 3-10 layers.

In some embodiments of the first aspect of the present application, the graphite includes at least one of natural graphite, artificial graphite, or mesocarbon microspheres.

In some embodiments of the first aspect of the present application, the silicon-based material includes 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 part of the surface of the silicon-based material.

In some embodiments of the first aspect of the present application, the electrical conductivity of the negative electrode composite material is 2.0-30 S/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, and the mixture layer comprises the negative electrode composite material provided in the first aspect of the present application.

The third aspect of the present application provides a battery, including the negative pole piece provided in the second aspect of the present application.

In some embodiments of the third aspect of the present application, the expansion rate of the battery is 6.5-10%.

The fourth aspect of the present application provides an electronic device, including the battery provided in the third aspect of the present application.

Using the negative electrode composite material provided in this application, the silicon-based material is composited with graphene and graphite, wherein graphite can relieve the expansion of the silicon-based material, and graphene can increase the conductivity of the composite material;

Further, the multilayer structure and slip characteristics of graphene can release the expansion stress of the silicon-based material in the process of deintercalating lithium, thereby eliminating the pulverization of silicon-based particles caused by expansion, and improving the cycle performance of the negative electrode material.

In addition, the negative pole piece and battery provided by this application have good cycle performance.

In this context, the term "Dv50" means the particle size at which the cumulative distribution of particles is 50%; that is, the volume content of particles smaller than this size accounts for 50% of all particles. The particle size is measured with a laser particle size analyzer.

Description of the drawings

In order to explain the embodiments of the present invention and the technical solutions of the prior art more clearly, the following briefly introduces the drawings that need to be used in the embodiments and the prior art. Obviously, the drawings in the following description are merely the present invention. For some of the embodiments of the invention, those of ordinary skill in the art can obtain other drawings based on these drawings without creative work.

FIG. 1 is an SEM photograph of the negative electrode composite material prepared in Example 3 of the application.

Figure 2 shows the capacity decay curves of Example 5, Example 16, Example 19 and Comparative Example 4.

Detailed ways

In order to make the objectives, technical solutions, and advantages of the present invention clearer and more comprehensible, the following further describes the present invention in detail with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative work shall fall within the protection scope of the present invention.

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 particles is 9.5-40 μm, and the specific surface area BET is 1.0-15 m ² /g.

High-energy density batteries require a negative electrode material with a gram capacity of> 500mAh/g. The inventor found in the research that when the negative electrode composite material contains more than 10% of the total mass of graphite and silicon-based materials, it can meet high-energy requirements. Density requirements.

The inventor also discovered in the research that it is not limited to any theory. When the content of graphene in the negative electrode composite material of the present application is greater than 20%, there is too much graphene around silicon and graphite, which affects the insertion and extraction of lithium ions, thereby As a result, the rate performance of the full battery deteriorates; when the content of graphene is less than 1%, it cannot play the role of alleviating the expansion of the negative electrode composite material.

In the negative electrode composite material, the particle size of the silicon-based material, graphite and the negative electrode composite material has an important impact on the performance of the battery; not limited to any theory, the inventor found in the research that the silicon-based material Dv50<3μm, the composite The material Dv50<9.5μm, the material has a large specific surface area and a large contact area with the electrolyte, so the irreversible lithium loss is greater and the cycle capacity retention rate is reduced. When graphite Dv50>20μm, composite material Dv50>40μm, it will cause local expansion of the battery during cycling. Excessive expansion will result in poor electrical contact between the materials in the pole pieces and accelerate the decrease in battery capacity; in some of the preferred applications of this application In the embodiment, 8μm<graphite Dv50+silicon-based material Dv50<15um.

The inventor found in research that the addition of graphene can significantly improve the conductivity of the composite material. However, when the number of graphene layers is too large, such as more than 10 layers, or the number of layers is too small, such as single-layer graphene, Compared with the use of multi-layer graphene with less than 10 layers and more than 3 layers, the conductivity of the composite material is significantly reduced. It is not limited to any theory. The inventor believes that this may be because there are more exposed end faces when the number of graphene layers is too large. Defects will increase, resulting in a decrease in conductivity; while single-layer graphene is prone to wrinkles, resulting in increased resistance and decreased conductivity. The inventor found in the research that graphene has a sliding effect, can relieve the expansion stress, and can return to the original state after cyclic expansion. However, when the number of graphene layers is less than 3 layers, the negative electrode material intercalates lithium and expands to make the graphene slippery. Too much shift causes the graphene to fail to recover, thereby increasing the expansion and accelerating the cycle capacity attenuation.

In some embodiments of the first aspect of the present application, carbon is present on at least a part of the surface of the silicon-based material. It can be understood that at least part of the surface of the silicon-based material is coated with carbon, which may be partially coated or completely coated.

Carbon coating can increase the conductivity of silicon-based materials and improve their electrical properties. Carbon-coated silicon-based materials, such as carbon-coated silicon oxide, are materials known in the art; they can be prepared according to the prior art or obtained through commercial channels.

The preparation method of the negative electrode composite material provided in this application is not particularly limited, for example, it can be prepared by the following method:

1) Mix and stir powdered silicon-based materials with graphite;

2) Add the graphene slurry to the mixture of silicon-based material and graphite, and continue to stir;

3) Add water, adjust the solid content of the slurry to 30% to 60%, and continue to stir to obtain a mixed slurry;

4) Transfer the mixed slurry to the spray drying granulator for granulation;

5) The granulated material is fired in an inert atmosphere to obtain the negative electrode active material.

In research, the inventor found that the spray-drying granulation method does not contain a binder in the resulting negative electrode composite material, which is beneficial to improve the rate performance of the battery.

Among them, the inlet temperature of the spray drying granulator is 240-280°C, preferably 260°C; the outlet temperature is 100-110°C, preferably 105°C.

In research, the inventor also found that baking the granulated material in an inert atmosphere can effectively remove impurities such as moisture and organic matter in the material; the baking temperature of the negative electrode material has a greater impact on the cycle expansion and capacity retention of the battery. Limited to any theory, too high a processing temperature, such as higher than 900°C, will cause material to agglomerate, resulting in a significant increase in Dv50, resulting in poorer contact between small particles after cyclic expansion, resulting in accelerated cycle capacity attenuation, and increased battery expansion . When the treatment temperature is too low, such as lower than 600°C, the residual dispersant in the graphene preparation process will not be completely decomposed, the composite material has more surface active groups, and more solid electrolyte interfaces (SEI) are generated, and the cyclic expansion increases. , The capacity maintenance rate is reduced.

In some embodiments of the present application, the firing conditions are: the firing temperature is 600-900°C, the holding time is 1-5 hours, preferably 2 hours; during firing, the heating rate can be selected from 3-8°C/min, preferably 5°C/min. min.

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, and the mixture layer includes the negative electrode composite material provided in the first aspect of the present application.

The mixture layer can be coated on one or both surfaces of the current collector, and those skilled in the art can make specific selections 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 can 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, as the negative electrode current collector, copper foil or copper alloy foil is particularly preferred. The above-mentioned materials may be used singly or in combination of two or more in any ratio.

In some embodiments of the present application, the mixture layer may further include an adhesive. The adhesive is not particularly limited, and can be any adhesive or combination known to those skilled in the art. For example, polyacrylate, polyimide, polyamide, polyamideimide, polyvinylidene fluoride, butylene Benzene rubber, sodium alginate, polyvinyl alcohol, polytetrafluoroethylene, polyacrylonitrile, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, sodium carboxymethyl cellulose, potassium carboxymethyl cellulose, etc. These binders may be used alone or in combination of two or more in any 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 known to those skilled in the art or a combination thereof. 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 (Vapour Grown Carbon Fiber) or graphene. The amount of the conductive agent is not particularly limited, and can be selected according to common knowledge in the art. The above-mentioned conductive agent may be used alone or in combination of two or more in any ratio.

The battery in this application includes but is 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 secondary battery. The battery, such as a lithium ion battery, generally includes a negative pole piece, a positive pole piece, a separator, and an electrolyte.

In the battery provided in this application, the negative pole piece of this application adopts the negative pole piece provided in this application; and other components, including the positive pole piece, separator, electrolyte, etc., are not particularly limited. Exemplarily, the positive electrode material contained in the positive pole piece 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, glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or a combination thereof. The electrolyte generally includes organic solvents, lithium salts and additives. Organic solvents may include, but are not limited to, carbon ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate , At least one of 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 ), and double trifluorophosphate (LiPF6). Lithium fluoromethanesulfonimide LiN(CF ₃ SO ₂ ) ₂ (LiTFSI), Lithium bis(fluorosulfonyl) imide Li(N(SO ₂ F) ₂ )(LiFSI), Lithium bisoxalate borate LiB(C ₂ At least one of O ₄ ) ₂ (LiBOB), lithium difluorooxalate borate LiBF ₂ (C ₂ O ₄ ) (LiDFOB)

The preparation process of the battery is familiar to those skilled in the art, and this application is not particularly limited. For example, a secondary battery can be manufactured by the following process: overlap the positive electrode and the negative electrode via spacers, and place them in the battery container after winding, folding and other operations as needed, and inject the electrolyte into the battery container and seal it. The negative electrode used is The above-mentioned negative pole piece provided in this application. In addition, an overcurrent prevention element, a guide plate, etc. can also be placed in the battery container as needed, so as to prevent the internal pressure of the battery from rising and overcharging and discharging.

The application also provides an electronic device, which includes the battery provided in the application.

Hereinafter, the present application will be specifically described based on examples, but the present application is not limited to these examples.

Composite material particle size test:

Add about 0.02g of each sample powder to a 50ml clean beaker, add about 20ml of deionized water, and then add a few drops of 1% surfactant to completely disperse the powder in the water. Ultrasound in a 120W ultrasonic cleaning machine for 5 minutes, using MasterSizer 2000 Test the particle size distribution.

Composite material specific surface area test:

At a constant temperature and low temperature, after measuring the adsorption amount of gas on the solid surface at different relative pressures, the adsorption amount of the sample monolayer is calculated based on the Brownauer-Ett-Taylor adsorption theory and its formula (BET formula), thereby calculating The specific surface area of the solid.

Composite material powder conductivity test:

Take 5g composite material powder sample, use an electronic press to constant pressure to 5000kg±2kg, maintain for 15-25s, place the sample between the electrodes of the resistivity tester (Suzhou Jingge Electronics ST-2255A), the height of the sample is h(cm), The voltage U, the current I, the resistance R (KΩ) at both ends; the area after the powder is pressed is S=3.14cm ² , the electronic conductivity of the powder is calculated according to the formula δ=h/(S*R)/1000, the unit is S/ cm.

Full battery performance test:

Cycle test:

The test temperature is 25°C, and it is charged to 4.45V at a constant current of 0.5C, charged to 0.025C at a constant voltage, and discharged to 3.0V at 0.5C after standing for 5 minutes. The capacity obtained in this step is the initial capacity, and the 0.5C charge/0.5C discharge is carried out for a cycle test, and the capacity at each step is used as the ratio of the initial capacity to obtain the capacity attenuation curve; among them, Examples 5, 16, 19 and comparative examples The capacity decay curve of 4 is shown in Figure 2; the capacity retention rates of each example and comparative example after 400 cycles are shown in Table 1 and Table 2.

Rate performance:

The test temperature is 25°C, and it is charged to 4.45V at a constant current of 0.5C, and charged to 0.025C at a constant voltage. After standing for 5 minutes, it is discharged to 3.0V at 0.2C. The capacity obtained in this step is the initial capacity, charge at 0.5C, discharge at 2C, and the ratio of the 2C discharge capacity to the 0.2C discharge capacity is the rate performance.

Full charge expansion rate test of lithium ion battery:

Use a spiral micrometer to test the thickness of the lithium-ion battery at the initial half-charge. At 25℃, when the charge-discharge cycle reaches 400 times, the lithium-ion battery is fully charged, and the thickness of the lithium-ion battery at this time is measured with a spiral micrometer. This can be obtained by comparing the thickness of the lithium-ion battery with the initial half-charged lithium-ion battery. Expansion rate of Li-ion battery when fully charged.

The first efficiency test of the whole battery: During the first charge and discharge of the whole battery, charge to 4.45V at a constant current of 0.5C, then charge to 0.025C at a constant voltage of 4.45V, (the capacity obtained before is C0), after standing for 5 minutes, 0.5C Discharge to 3.0V (obtain discharge capacity D0). The first efficiency of the whole battery = D0/C0.

The first reversible capacity test of a half-cell: The negative pole pieces obtained in each example and comparative example were cut into discs with a diameter of 1.4cm with a punching machine in a dry environment. The metal lithium sheet was used as the counter electrode, and the isolation membrane was selected as ceglard composite. Membrane, adding electrolyte to assemble button cell. The LAND series battery test system was used to detect the first reversible capacity of the half-cell, and the results are shown in Table 1 and Table 2.

The first efficiency test of a half-cell: Use a lithium sheet as the counter electrode to assemble a button battery, discharge at 0.05C to 5mV, then discharge at 0.05mA to 5mv, after standing for 1 hour, discharge 5mv with 0.01mA, (previously get the lithium insertion capacity C ₀ ), stand for 5 min, charge at 0.05C to 2.0V (obtain delithiation capacity D ₀ ), the first half-cell efficiency = D ₀ /C ₀ .

Full battery preparation:

Preparation of negative electrode mixture layer material:

1) In the MSK-SFM-10 vacuum stirrer, 2.8 kg and 35 g of conductive carbon black prepared in each embodiment and comparative example were added to the stirrer and stirred for 40 minutes at a revolution speed of 10-30 rpm;

2) Add 95g of polyvinylidene fluoride to the mixture stirred in 1), stir for 60min to disperse evenly, then add deionized water and stir for 120min to disperse evenly to obtain a mixed slurry with a viscosity of 2000mPa.S and a solid content of 35%; The mass ratio of composite material, conductive agent and binder is 95.6:1.2:3.2;

3) Filter the mixed slurry obtained in 2) with a 170-mesh double-layer screen to obtain a negative electrode mixture layer material.

Preparation of negative pole piece:

The prepared negative electrode mixture layer material was coated on both surfaces of a copper foil current collector with a thickness of 10 μm, and the coating thickness was 100 μm; the pole pieces were dried and cold pressed, and the double-sided compaction density was 1.8 g/cm ³ .

Preparation of positive pole piece:

The active material LiCoO ₂ , conductive carbon black, and binder polyvinylidene fluoride (PVDF) are formulated into a slurry with a solid content of 0.75 in a N-methylpyrrolidone solvent system at a weight ratio of 96.7:1.7:1.6, and Stir well. The slurry was uniformly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 12 μm, the coating thickness was 115 μm, dried at 90° C., and cold pressed to obtain a positive pole piece.

Full battery assembly:

A PE porous polymer film with a thickness of 15 μm is used as the separator. The positive pole piece, the isolation film, and the negative pole piece are stacked in order, so that the isolation film is in the middle of the cathode and anode for isolation, and the bare cell is obtained by winding. Place the bare cell in the outer package, inject the prepared electrolyte (EC:DMC:DEC=1:1:1vol%, 10wt% FEC, 1mol/L LiPF ₆ ) and package, after forming, degassing, cutting The full battery is obtained by waiting for the technological process.

Anode composite material preparation

Example 1

1. Add 1kg of silicon oxide (Dv50 is 5μm) powder and 9kg of graphite (Dv50 is 8um) into water, stir for 180min in MSK-SFM-10 vacuum stirrer, revolution speed 10-40rpm, mix evenly (silicon-based material Content 10wt%).

2. Add 2.04 kg of graphene (10-layer structure) water-based slurry with a solid content of 10% into the mixer, and stir for 120 minutes to disperse evenly. Add 5 kg of deionized water, continue stirring for 120 minutes to obtain a mixed slurry, the revolution speed is 40 rpm, the rotation speed is 1500 rpm, and the graphene content in the solid is 2%.

3. Transfer the slurry in 2) to the centrifugal turntable nozzle of the spray drying granulator, and the centrifugal speed is 5000 rpm to form tiny droplets. The spray drying granulator has an inlet temperature of 260°C and an outlet temperature of 105°C. The granulated material is collected by cooling.

4. The pellets in 3) are calcined in an inert atmosphere (Ar), the heating rate is controlled at 5°C/min, and the temperature is kept at 750°C for 2h. The powder is passed through a 400-mesh sieve to obtain the desired negative electrode composite material.

Example 2-4

The graphene in Example 1 was replaced with graphene with 7, 5, and 3 layers, and the rest were the same as Example 1. The scanning electron micrograph (SEM) of the negative electrode composite material prepared in Example 3 Photo) is shown in Figure 1.

Example 5-8

Adjust the amount of graphene slurry in Example 3 to 5.26 kg, 11.11 kg, 17.65 kg, and 25 kg, so that the content of graphene is 5%, 10%, 15%, and 20%, respectively, and the rest are the same as in Example 3.

Example 9

The silicon-based material in Example 5 is replaced with silicon (Si), and the rest is the same as in Example 5.

Example 10

The silicon-based material in Example 5 is replaced with silicon carbon material (SiC), and the rest is the same as in Example 5.

Examples 11, 12

The firing temperature in Example 5 was adjusted to 600°C and 900°C, respectively, and the rest were the same as in Example 5.

Example 13

In Example 5, 2 kg of silicon oxide powder was mixed with 8 kg of graphite so that the content of the silicon-based material was 20%, and the rest were the same as in Example 5.

Example 14

In Example 5, 5 kg of silicon oxide powder and 5 kg of graphite were mixed so that the content of the silicon-based material was 50%, and the rest were the same as in Example 5.

Example 15

In Example 5, 8 kg of silicon oxide powder was mixed with 2 kg of graphite so that the content of the silicon-based material was 80%, and the rest were the same as in Example 5.

Example 16

In Example 5, 10 kg of silicon oxide was directly used to make the content of the silicon-based material 100%, and the rest were the same as in Example 5.

Examples 17, 18

The particle size (Dv50) of the silicon oxide in Example 5 is adjusted to 3.5 μm and 8 μm, respectively, and the rest is the same as in Example 5.

Examples 19, 20

The particle size (Dv50) of graphite in Example 5 was adjusted to 15 μm and 20 μm, and the rest was the same as in Example 5.

Comparative example 1, 2

The graphene in Example 5 was replaced with graphene having a 15-layer structure and a single-layer structure, and the rest were the same as in Example 5.

Comparative example 3

The dosage of the graphene slurry in Example 5 is adjusted to 33.3 kg, so that the content of graphene is 25%, and the rest are the same as in Example 5.

Comparative example 4

In Example 5, 5 kg of deionized water was directly used instead of the graphene slurry, and the rest were the same as in Example 5 (that is, without graphene).

Comparative Examples 5, 6

The firing temperature in Example 5 was adjusted to 500° C. and 1000° C., and the rest were the same as in Example 5.

Comparative example 7

In Example 5, 10 kg of graphite was directly used, that is, no silicon-based material was used, and the rest were the same as in Example 5.

Comparative example 8

In Example 5, 0.5 kg of silicon oxide powder was mixed with 9.5 kg of graphite so that the content of the silicon-based material was 5%, and the rest were the same as in Example 5.

Comparative example 9

The particle size (Dv50) of the silicon oxide in Example 5 was adjusted to 1.5 μm, and the rest were the same as in Example 5.

Comparative example 10

The particle size (Dv50) of graphite in Example 5 was adjusted to 25 μm, and the rest were the same as in Example 5.

The parameters and performance test results of the negative electrode composite material in each embodiment are shown in Table 1; the parameters and performance test results of the negative electrode composite material 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 under the same silicon content condition, mixed graphene granulation can significantly improve the conductivity of the negative electrode material, and the full battery rate is significantly improved.

Compared with Examples 1, 2, 3, and 4, when the number of graphene layers meets 10-3 layers, as the number of graphene layers decreases, the powder conductivity increases; and as the conductivity increases, lithium ions are more likely to be The negative electrode material is detached and embedded, so the reversible capacity is increased for the first time. As the number of graphene layers increases, the specific surface area of the material also increases, and the particles are more likely to agglomerate. Therefore, the particle size of the composite material increases. In addition, the increase in the specific surface area will result in a larger contact surface between the electrolyte and the active material, resulting in more generation. With SEI film, the cycle expansion of the full battery increases, the capacity attenuation accelerates, and the capacity retention rate decreases more obviously.

Comparing Example 5 with Comparative Examples 1 and 2, it can be seen that when the number of graphene layers exceeds 10 or is a single layer, the conductivity is significantly reduced. It is not limited to any theory. The inventor believes that because the number of graphene layers is too large, the exposed More end faces will increase the number of defects, which will lead to a decrease in conductivity; single-layer graphene is prone to wrinkles, which leads to an increase in resistance and a decrease in conductivity.

In addition, graphene has a sliding effect, can relieve the expansion stress, and can return to the original state after cyclic expansion. However, it can be seen from Comparative Example 2 that when the number of layers is less than 3 layers, the negative electrode material lithium intercalation will cause graphene to expand. Too much slip causes the graphene to be unable to recover, thereby increasing the expansion and accelerating the attenuation of the cycle capacity.

From the comparison of Examples 3, 5, 6, 7, 8 and Comparative Example 3, it can be seen that the increase of graphene content can also significantly improve the conductivity of the material, and at the same time, the battery rate performance also improves with the increase of graphene content; but When the graphene content is greater than 20% (comparative example 3), the electrical performance will decrease instead. It is not limited to any theory. The inventor believes that too high graphene content may cause too much graphene around silicon and graphite, which affects lithium ions. The insertion and extraction of the battery, so the full battery rate performance deteriorates.

In addition, the inventor also found that when the graphene content is less than 5%, the first reversible capacity of the composite material increases significantly as the conductivity increases, but when the content exceeds 5%, as the conductivity increases, the composite material’s gram On the contrary, the capacity drops, not limited to any theory, this may be caused by the increase in graphene content leading to a decrease in the overall content of high-capacity silicon-based materials. On the other hand, as the content of graphene increases, the efficiency of the whole battery decreases for the first time. It is not limited to any theory. The inventor believes that this may be due to the increase in the content of graphene. Larger, more SEI film is generated, resulting in lower efficiency for the first time.

Comparing Examples 5, 11, and 12 with Comparative Examples 5 and 6, it can be seen that when the calcination temperature is higher than 900°C, the composite material is prone to agglomeration, so the particle size of the composite material increases; not limited to any theory, the inventor It is believed that the increase in particle size leads to poor contact between small particles after cyclic expansion, accelerated cycle capacity attenuation, and increased battery expansion; and when the calcination temperature is lower than 600°C, it will cause the dispersant in the graphene slurry to decompose Incomplete, more surface active groups, more SEI generated, increased cycle expansion, decreased capacity maintenance rate.

The comparison of Examples 5, 13, 14, 15, 16 and Comparative Examples 7 and 8 shows that the higher the content of the silicon-based active material, the higher the gram capacity of the material. However, with the increase of capacity, the expansion of the battery during cycling increases. When the silicon-based content exceeds 20%, the expansion increases significantly. When the silicon content is less than 10%, the demand for high energy density cannot be met (high energy density requires anode gram capacity> 500mAh/g)

The comparison of Examples 5, 17, 18 and Comparative Example 9 shows that when the silicon-based material Dv50<3μm, the composite material particle size Dv50<9.5μm, the material specific surface area is larger, the contact area with the electrolyte is larger, and the irreversible lithium loss is greater. The cycle capacity retention rate is reduced. Comparing Examples 5, 19, and 20 with Comparative Example 10, it can be seen that when the graphite Dv50>20μm, the composite material Dv50>40μm, will cause the local expansion of the battery during the cycle, and the excessive expansion will lead to the gap between the materials in the pole piece. The electrical contact becomes worse, and the battery capacity declines faster; comprehensive data considerations are preferably 8μm<graphite Dv50+silicon Dv50<15um.

The above are only the preferred embodiments of the present invention and are not intended to limit the present invention. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention shall be included in the protection of the present invention. Within the range.

Claims

A negative electrode composite material, including silicon-based materials, graphene and graphite, wherein:

The graphene accounts for 1% to 20% of the mass of the negative electrode composite material; the silicon-based material accounts for 10 to 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 material according to claim 1, wherein the number of graphene layers is 3-10 layers.
The negative electrode composite material according to claim 1, wherein the graphite includes at least one of natural graphite, artificial graphite, or mesocarbon microspheres.
The negative electrode composite material according to claim 1, wherein the silicon-based material includes at least one of silicon, silicon oxide, or silicon carbon material.
The negative electrode composite material according to any one of claims 1 to 4, wherein carbon is present on at least a part of the surface of the silicon-based material.
The negative electrode composite material according to any one of claims 1 to 4, wherein the electrical conductivity of the negative electrode composite material is 2.0-30 S/cm.
A negative pole piece, 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 1-6.
A battery comprising the negative pole piece according to 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 according to claim 8 or 9.