CN113013394A

CN113013394A - Negative electrode material and preparation method and application thereof

Info

Publication number: CN113013394A
Application number: CN202110327502.5A
Authority: CN
Inventors: 郭明奎; 苏树发; 高飞
Original assignee: Svolt Energy Technology Co Ltd
Current assignee: Svolt Energy Technology Co Ltd
Priority date: 2021-03-26
Filing date: 2021-03-26
Publication date: 2021-06-22

Abstract

The invention provides a negative electrode material and a preparation method and application thereof. The negative electrode material comprises an inner core containing a silicon-based material and a buffer layer coated on the surface of the inner core, wherein the buffer layer comprises a low-expansion material, a conductive agent and a binder, and the low-expansion material comprises any one or a combination of at least two of hard carbon, soft carbon, lithium titanate, natural graphite and artificial graphite. According to the negative electrode material provided by the invention, under the condition of cyclic charge and discharge, lithium ions are preferentially extracted and inserted from the surface layer low-expansion material, so that excessive expansion and excessive volume change caused by excessive lithium extraction and lithium insertion of silicon-containing material system material particles are prevented, and long-term performance deterioration caused by excessive expansion and excessive volume change is prevented.

Description

Negative electrode material and preparation method and application thereof

Technical Field

The invention belongs to the technical field of lithium ion batteries, and relates to a negative electrode material, and a preparation method and application thereof.

Background

With the development of the electric vehicle, the requirement on the energy density (at present, the normal level is 180 plus 230wh/kg) of the power battery is higher and higher, the energy density of the power battery is used as a core component of the electric vehicle, the energy density of the power battery influences the design of the whole vehicle and comprises the performance and cost control of the whole vehicle, the high-energy-density power battery can effectively control the weight of the whole vehicle and the design of other parts, in addition, the cost of the power battery in the electric vehicle is nearly 50%, the cost of non-energy units such as mechanical parts can be reduced by improving the energy density of the power battery, and the cost of the whole vehicle.

The current methods for improving the energy density of the power battery comprise the following three methods:

(1) in the aspect of a positive electrode: the high Ni system improves the capacity exertion of the unit weight of the anode by improving the Ni content in the anode material, and the current battery core level technical state and level are 240-280 wh/kg; the high-voltage system improves the capacity exertion of the unit weight of the positive electrode by improving the upper limit voltage in the charging process of the positive electrode material, the current technical state and level of the battery core level is 230-260wh/kg, but the improvement of the Ni content reduces the potential of oxygen evolution of the positive electrode, thereby bringing high gas production risk of the battery core, which will deteriorate the long-term reliability of the battery core, such as the cycle life, the storage life and the battery core expansion control, and simultaneously, the higher Ni content reduces the temperature threshold value of the positive electrode material for thermal runaway, which will deteriorate the safety of the battery core under the relevant application, such as high temperature, overcharge, extrusion and the like; in a high-voltage system, the oxidation of the anode at the charging end is improved by increasing the upper limit service voltage of the anode, and the oxidation of the anode on the electrolyte and a diaphragm is accelerated, so that gas generation is deteriorated, and adverse effects are brought to long-term reliability; (2) negative electrode side: doping of silicon-based materials by addition of Si/SiO₂The silicon-based material is used for improving the capacity exertion of the unit weight of the negative electrode, and the current technical state and level of the battery cell level is 240-350 wh/kg; however, the addition of Si/SiO₂The lithium ion battery cell has very large shrinkage and expansion in the charging and discharging processes, and the negative electrode has the phenomena of demoulding and powder falling under a high shrinkage and expansion ratio along with the circulation, and meanwhile, the stability and the integrity of an SEI (solid electrolyte interphase) film on the surface of the negative electrode are influenced, so that the long-term service life and the capacity maintenance of the battery cell are greatly deteriorated; (3) aspects of the process: the effective capacity exertion of unit weight is improved by increasing the coating weight of the cathode and the anode and reducing the thickness of a base material (aluminum foil copper foil) and a diaphragm, and the current technical state and level of the battery cell level is 230-270 wh/kg; meanwhile, the method has some disadvantages, such as the increase of the coating weight of the cathode and the anode deteriorates the power of the battery core, the charging window and the long-term cycle life, and brings great challenges to the process and the equipment; the reduction of the base materials (aluminum foil and copper foil) mainly affects the process manufacturing, the thin base materials are easy to break in the manufacturing process of the battery cell, and the excellent rate of the battery cell manufacturing process is seriously affected, so that the production cost of the battery cell is increased; the diaphragm is used as a part for separating the direct contact short circuit of the positive electrode and the negative electrode, the thickness of the diaphragm is important for controlling the safety of the short circuit in the battery cell, and the safety risk of the short circuit in the battery cell caused by the thickness reduction is reduced.

CN111509208A discloses a lithium ion battery cathode material and a preparation method and a device thereof. The preparation method comprises the following steps: mixing micron-sized silicon and silicon dioxide, and compacting into blocks to obtain Si/SiO₂Mixing the materials; under vacuum, the mixed material and the metal magnesium are respectively heated to different preset temperatures at different heating rates to respectively obtain gas-phase SiO_xWith magnesium in the gas phase; SiO in gas phase_xDepositing the magnesium and the gas-phase magnesium by cooling mixing; and crushing, grading, demagnetizing and carbon coating to obtain the lithium ion battery cathode material. Si/SiO in this document₂The battery has very large shrinkage and expansion in the charging and discharging process, and along with the circulation, the negative electrode has the phenomena of demoulding and powder falling under a high shrinkage and expansion ratio, and meanwhile, the stability and the integrity of an SEI film on the surface of the negative electrode are influenced, so that the long-term service life and the capacity maintenance of the battery cell are greatly deteriorated.

Therefore, it is an urgent technical problem to control the expansion of the negative electrode active material containing the silicon-based material in the negative electrode, to prevent the long-term performance deterioration caused by the excessive expansion of the electrode, and to improve the energy density of the battery.

Disclosure of Invention

The invention aims to provide a negative electrode material and a preparation method and application thereof. According to the invention, the low-expansion material and the negative active material particles containing the silicon-based material are premixed through the linear conductive agent and the binder to form the core-shell-like design with the silicon-based material system material particles inside and the low-expansion material particles outside, and under the condition of cyclic charge and discharge, lithium ions are preferentially extracted and inserted from the surface low-expansion material, so that excessive expansion and excessive volume change caused by excessive lithium extraction and lithium insertion of the silicon-based material system material particles are prevented, and long-term performance deterioration caused by the excessive expansion and the excessive volume change is prevented.

In order to achieve the purpose, the invention adopts the following technical scheme:

in a first aspect, the present invention provides an anode material, which includes an inner core containing a silicon-based material and a buffer layer coated on a surface of the inner core, where the buffer layer includes a low expansion material, a conductive agent and a binder, and the low expansion material includes any one or a combination of at least two of hard carbon, soft carbon, lithium titanate, natural graphite and artificial graphite.

In the present invention, the silicon-based material includes SiO or SiO₂And the like.

Currently, in a negative electrode material, the volume expansion of material particles caused by lithium intercalation is greatly different from that of a graphite system or a silicon-containing base material system, wherein the volume expansion of the material particles is greatly related to the lithium intercalation amount, the volume expansion of the material particles is larger when the lithium intercalation amount is higher, and the expansion of a low-expansion material is far lower than that of the silicon-containing base material system, so that the expansion of the negative electrode can be controlled by controlling the lithium intercalation amount of the silicon-containing base material system.

The invention provides a core-shell structure-like cathode material, a silicon-containing material system material with higher expansion coefficient is used as an inner core, the surface of the core is coated with a buffer layer, and a low-expansion material in the buffer layer can ensure that lithium ions are preferentially embedded into the low-expansion material on the surface of the core, and the self expansion of the silicon-containing material system material in the core caused by lithium embedding is reduced, so that the expansion of the cathode material is effectively improved, the outer low-expansion material particles can be well wound together by a conductive agent on the surface of the inner core and uniformly coated and fixed on the surface of large particles of the inner cathode material, a binder plays a role in strengthening and fixing, the falling-off of the outer low-expansion material particles in the charge and discharge process is prevented, so that the expansion of a battery core is greatly improved under the condition of ensuring less energy density loss, the long-term reliability performance is improved, lithium ions will be preferentially extracted and inserted in the surface layer low-expansion graphite material, thereby preventing the negative electrode from being expanded too much due to the excessive lithium extraction and lithium insertion of the silicon-containing base material system material particles, and the long-term performance deterioration caused thereby.

Preferably, the inner core further comprises a carbon material.

When the inner core also contains a carbon material, the anode material provided by the invention can greatly improve the energy density on the premise of ensuring the long-term reliability of the battery.

Preferably, the carbon material has a median particle diameter of 5 to 20 μm, for example 5 μm, 10 μm, 15 μm or 20 μm.

Preferably, the volume ratio of the carbon material in the inner core containing the carbon material is 2 to 50%, for example, 2%, 5%, 10%, 20%, 30%, 40%, or 50%.

Preferably, the carbon material comprises any one of natural graphite, artificial graphite, soft carbon or hard carbon or a combination of at least two thereof.

Preferably, the silicon-based material has a median particle diameter of 8 to 20 μm, such as 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, or 20 μm, and the like.

Preferably, the low expansion material in the buffer layer has a median particle size of 0.5 to 5 μm, such as 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, or 5 μm.

Preferably, the conductive agent is selected from fibrous, tubular, linear or sheet conductive agents, preferably including any one or a combination of at least two of carbon fibers, carbon nanotubes or graphene.

Preferably, the diameter of the fibrous, tubular, linear conductive agent is 50 to 500nm, such as 50nm, 100nm, 200nm, 300nm, 400nm, or 500 nm.

Preferably, the aspect ratio of the fibrous, tubular, and linear conductive agent is greater than or equal to 10, such as 10, 11, 12, or 13.

Preferably, the aspect ratio of the flake-like conductive agent is not less than 10, such as 10, 11, 12, 13, 14 or 15.

According to the invention, the conductive agent is selected, so that the outer-layer low-expansion material particles can be well wound together and uniformly coated and fixed on the surface of the inner negative electrode material large particles.

Preferably, the binder comprises any one of polyvinylidene fluoride, polytetrafluoroethylene, polypropylene, styrene butadiene rubber, styrene butadiene latex, polybutylene, polyacrylate or polybutylene ester or a combination of at least two of them.

Preferably, in the negative electrode material, the mass ratio of the low expansion material is 3 to 10%, for example, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, or the like.

According to the invention, the mass ratio of the low-expansion graphite material to the active substance of the silicon-containing base material system in the composite core-shell structure particles can be controlled by regulating the mass ratio of the low-expansion material, so that the electrode expansion improvement degree is low due to too low temperature, the improvement degree of long-term reliability is directly influenced, and the energy density is improved to a small extent due to too high temperature.

Preferably, in the negative electrode material, the mass ratio of the conductive agent is 0.5-5%, for example, 0.5%, 1%, 2%, 3%, 4%, or 5%.

Preferably, in the negative electrode material, the mass ratio of the binder is 1.2-3%, for example, 1.2%, 1.5%, 2%, 2.5%, or 3%.

In a second aspect, the present invention provides a method for producing the anode material according to the first aspect, the method comprising:

and mixing the silicon-based material, the low-expansion material, the conductive agent, the binder and the solvent to obtain mixed slurry, and drying to obtain the cathode material.

According to the invention, through premixing, the low-expansion material, the conductive agent and the binder can form a buffer layer on the surface of the silicon-based material, so that the cell expansion can be greatly improved and the long-term reliability performance can be improved under the condition of ensuring that the energy density loss is small, and lithium ions are preferentially extracted and inserted from the surface layer low-expansion graphite material under the condition of cyclic charge and discharge, so that the excessive expansion of a negative electrode caused by the excessive lithium extraction and lithium insertion of silicon-based material system material particles and the long-term performance deterioration caused by the excessive lithium extraction and lithium insertion are prevented. The preparation method provided by the invention is simple to operate and suitable for large-scale production.

Preferably, the method of mixing comprises stirring.

Preferably, the stirring linear speed of the stirring is 60-80 m/min, such as 60m/min, 65m/min, 70m/min, 75m/min or 80 m/min.

Preferably, the stirring time is 30-50 min, such as 30min, 35min, 40min, 45min or 50 min.

Preferably, the solvent comprises water and/or N-methylpyrrolidone.

Preferably, the solid content of the mixed slurry is 45-55%, such as 45%, 48%, 50%, 52% or 55%, and the like.

As a preferable technical solution, the preparation method of the anode material includes:

and stirring the silicon-based material, the low-expansion material, the conductive agent, the binder and water at a stirring linear speed of 60-80 m/min for 30-50 min to obtain mixed slurry with a solid content of 45-55%, and drying to obtain the cathode material.

In a third aspect, the present invention also provides a lithium ion battery, which includes the negative electrode material according to the first aspect.

Compared with the prior art, the invention has the following beneficial effects:

according to the invention, the low-expansion material and the active substance particles of the silicon-containing material system material are premixed through the linear conductive agent and the binder to form the core-shell-like design with the active substance particles of the silicon-containing material system inside and the low-expansion material particles outside, so that lithium ions are preferentially extracted and inserted from the low-expansion material on the surface layer under the condition of cyclic charge and discharge, and thus lithium extraction and lithium insertion of the particles of the silicon-containing material system material are preventedWhen the core of the negative electrode system is a silicon-based material, the capacity can reach 133.3Ah and above, the energy density is 259Wh/kg and above, the BOL expansion rate of the electrode is reduced to 43.2 percent or even to 32.7 percent or below, the capacity is reduced to 80 percent or below after at least 1098 cycles, the expansion force is also 24.6N and below, the EOL expansion rate of the electrode is also reduced, and when the core of the negative electrode system is a SiO core₂When the electrode is used for preparing the battery, the capacity of the battery is not particularly obviously reduced, the capacity is only reduced to 112.8Ah, even 124Ah or more, the energy density is 219Wh/kg or more, the BOL expansion rate of the electrode is reduced to 32.3 percent or even 29.2 percent or less, the capacity is reduced to 80 percent or less after at least 1312 cycles, the expansion force is also 20.9N or less, and the EOL expansion rate of the electrode is also reduced, so that the cell expansion can be greatly improved under the condition of ensuring that the energy density loss is small or improved, the long-term reliability of the battery is improved, and the cycle performance and the storage performance of the battery are improved.

Drawings

Fig. 1 is a schematic structural view of an anode material provided in example 1.

1-core, 2-low expansion material, 3-conductive agent, 4-binder

Detailed Description

The technical solution of the present invention is further explained by the following embodiments. It should be understood by those skilled in the art that the examples are only for the understanding of the present invention and should not be construed as the specific limitations of the present invention.

Example 1

The present embodiment provides a negative electrode material, as shown in fig. 1, the negative electrode material includes a core 1 and a buffer layer coated on a surface of the core, where the core 1 is SiO with a median particle size of 10 μm, the buffer layer is a low-expansion material 2, a conductive agent 3 and a binder 4, the low-expansion material 2 is natural graphite with a median particle size of 2 μm, the conductive agent 3 is a carbon nanotube with a diameter of 200nm and an aspect ratio of 13, and the binder 4 is styrene butadiene rubber; the mass ratio of the natural graphite is 5%.

The preparation method of the negative electrode material comprises the following steps:

stirring SiO, natural graphite, carbon nano tubes and styrene butadiene rubber for 40min at a mass ratio of 93:5:1:1 and water at a stirring linear speed of 70m/min to obtain mixed slurry with the solid content of 50%, and drying at 60 ℃ for 8h to obtain the cathode material.

Example 2

This example provides a negative electrode material comprising SiO having a median particle diameter of 8 μm₂The artificial graphite composite material comprises an inner core of an artificial graphite composite material with a median particle size of 5 mu m and a buffer layer coated on the surface of the inner core, wherein the buffer layer comprises lithium titanate with a median particle size of 0.5 mu m, a carbon nano tube with a diameter of 50nm and an aspect ratio of 10 and polyvinylidene fluoride; the volume percentage of the artificial graphite is 30%, and the mass percentage of the lithium titanate is 10%.

mixing SiO₂And stirring the mixed material of the artificial stone mill, lithium titanate, carbon fiber and polyvinylidene fluoride for 50min at a mass ratio of 87:5:5:3 and water at a stirring linear speed of 60m/min to obtain mixed slurry with the solid content of 55%, and drying at 60 ℃ for 8h to obtain the cathode material.

Example 3

This example provides a negative electrode material comprising SiO having a median particle diameter of 20 μm₂The core is made of a natural graphite mixed material with a median particle size of 10 microns, and the buffer layer is coated on the surface of the core and comprises hard carbon with a median particle size of 5 microns and graphene and polyacrylate with an aspect ratio of 15; the volume of the natural graphite accounts for 50%, and the mass of the hard carbon accounts for 5%.

mixing SiO₂And stirring the mixed material of the artificial stone mill, hard carbon, graphene and polyacrylate for 30min at a mass ratio of 93.3:5:0.5:1.2 and a stirring linear speed of 80m/min of N-methyl pyrrolidone to obtain mixed slurry with a solid content of 45%, and drying for 8h at 60 ℃ to obtain the cathode material.

Example 4

This example provides a negative electrode material comprising SiO having a median particle diameter of 12 μm₂The buffer layer comprises artificial graphite with the median diameter of 1 mu m, carbon nano tubes with the median diameter of 250nm and styrene butadiene rubber; the mass percentage of the artificial graphite is 1%.

mixing SiO₂And stirring the artificial graphite, the carbon nano tube and the styrene butadiene rubber at a mass ratio of 98:1:1:1 and water at a stirring linear speed of 70m/min for 40min to obtain mixed slurry with the solid content of 50%, and drying at 60 ℃ for 8h to obtain the cathode material.

Example 5

This example provides a negative electrode material comprising SiO having a median particle diameter of 12 μm₂The buffer layer comprises artificial graphite with the median particle size of 1 mu m, carbon nano tubes with the median particle size of 250nm and styrene butadiene rubber; the volume percentage of the artificial graphite in the inner core is 2%, and the mass percentage of the artificial graphite in the buffer layer is 1%.

mixing SiO₂And stirring the mixed material of the artificial stone mill, the artificial graphite, the carbon nano tube and the styrene butadiene rubber for 50min at a mass ratio of 97:1:1:1 and water at a stirring linear speed of 60m/min to obtain mixed slurry with the solid content of 55%, and drying at 60 ℃ for 8h to obtain the cathode material.

Example 6

The present example is different from example 4 in that the artificial graphite in the present example has a mass ratio of 3%. In the preparation method, SiO₂The mass ratio of the artificial graphite to the carbon nano tube to the styrene butadiene rubber is 95:3:1: 1.

The remaining preparation methods and parameters were in accordance with example 4.

Example 7

The present example is different from example 5 in that the artificial graphite in the present example has a mass ratio of 3%. In the preparation method, SiO₂The mass ratio of the mixed material of the artificial stone mill to the artificial graphite to the carbon nano tube to the styrene butadiene rubber is 95:3:1: 1.

The remaining preparation methods and parameters were in accordance with example 5.

Example 8

The difference between this example and example 4 is that the mass ratio of the artificial graphite in this example is 5%. In the preparation method, SiO₂The mass ratio of the artificial graphite to the carbon nano tube to the styrene butadiene rubber is 93:5:1: 1.

Example 9

The present example is different from example 5 in that the artificial graphite in the present example has a mass ratio of 5%. In the preparation method, SiO₂The mass ratio of the mixed material of the artificial stone mill to the artificial graphite to the carbon nano tube to the styrene butadiene rubber is 93:5:1: 1.

Example 10

The difference between this example and example 4 is that the mass ratio of the artificial graphite in this example is 10%. In the preparation method, SiO₂The mass ratio of the artificial graphite to the carbon nano tube to the styrene butadiene rubber is 88:10:1: 1.

Example 11

The present example is different from example 5 in that the mass ratio of the artificial graphite in the present example is 10%. In the preparation method, SiO₂The mass ratio of the mixed material of the artificial stone mill to the artificial graphite to the carbon nano tube to the styrene butadiene rubber is 88:10:1: 1.

Example 12

The difference between the present example and example 4 is that the mass ratio of the artificial graphite in the present example is 15%. In the preparation method, SiO₂The mass ratio of the artificial graphite to the carbon nano tube to the styrene butadiene rubber is 83:15:1: 1.

Example 13

The difference between the present example and example 5 is that the mass ratio of the artificial graphite in the present example is 15%. In the preparation method, SiO₂The mass ratio of the mixed material of the artificial stone mill to the artificial graphite to the carbon nano tubes to the styrene butadiene rubber is 83:15:1: 1.

Comparative example 1

In this comparative example, artificial graphite was used as the negative electrode material.

Comparative example 2

In this comparative example, SiO is used₂As the anode material.

Comparative example 3

This comparative example uses SiO₂And artificial graphite, wherein the volume percentage of the artificial graphite is 2%.

The negative electrode materials provided in examples 1 to 13 and comparative examples 1 to 3 were used as negative electrode active materials, and were homogenized with SBR (styrene butadiene rubber), CMC (sodium carboxymethylcellulose) and SP (conductive carbon black) at a weight ratio of 95:2.5:1.5:1, wherein water was added to control the solid content to 50% and the viscosity to 3000mpa s, and after completion of stirring, the negative electrode slurry was uniformly coated on the surface of a copper foil base material of 8 μm with a double-side coating weight of 210g/m²And then drying, rolling, die cutting and punching to obtain the negative pole piece.

Taking positive active material LiNi_0.5Co_0.2Mn_0.3O₂Homogenizing according to weight ratio of NCM (ternary material), PVDF (polyvinylidene fluoride) and SP (conductive carbon black) of 95:3:2, adding NMP (N-methyl-2 pyrrolidone) to control solid content to be 70% and viscosity to be 8000mpa s, stirring, uniformly coating positive electrode slurry on surface of aluminum foil substrate with 12 mu m, wherein coating weight of both surfaces is 450g/m²And then drying, rolling, die cutting and punching to obtain the positive pole piece.

The membrane is 16 μmThe polyethylene PE diaphragm and the electrolyte adopt LiPF containing lithium salt₆A conventional electrolyte having a concentration of 1.12mol/l and a volume ratio of DEC/EC/EMC (dimethyl carbonate/ethylene carbonate/ethyl methyl carbonate) in the solvent of 1:1: 1.

Taking a punched positive pole piece and the negative pole pieces provided by the comparative examples and the comparative examples 1 to 13 and the comparative examples 1 to 3, stacking the pole pieces layer by layer in the order of the negative pole of the positive diaphragm and the negative diaphragm of the negative diaphragm to manufacture a naked cell, controlling the thickness of the comparative examples 1 to 3 to be consistent with that of the naked cells of the examples 1 to 13 by controlling the number of the stacked positive and negative poles, then putting the naked cells into a shell, baking, injecting liquid, forming, and sealing to manufacture the cell.

The cells of comparative examples 1 to 3 and examples 1 to 13 were taken at room temperature, and three cells of each example (examples 1 to 1, 1 to 2, and 1 to 3, and the like were similarly expressed as parallel experiments) were charged to 4.2V at a constant current and a constant voltage at a charge of 0.33C using a charge-discharge test cabinet, and discharged to 2.5V at a discharge of 0.33C after leaving for 10min, and the discharge capacity was recorded.

At room temperature, the battery cores provided in comparative examples 1 to 3 and examples 1 to 13 were charged to 4.2V at a constant current and a constant voltage of 0.33C using a charge-discharge test cabinet, and then the negative electrode sheets of comparative examples 1 to 3 and examples 1 to 13 were disassembled and tested for thickness, which is recorded as BOL (cycle initial state) negative electrode thickness, and BOL negative electrode expansion rate ═ BOL negative electrode thickness-negative electrode roll thickness/negative electrode roll thickness.

The cell impedances of comparative examples 1-3 and examples 1-13 were measured using a resistance tester and the values recorded.

The results of the above tests are shown in table 1.

TABLE 1

As can be seen from the data results of examples 4, 6, 8, 10 and 12 and comparative example 2, with respect to SiO₂The capacity and energy density of the negative electrode system are reduced to a certain extent, but the reduction range is lower, but the expansion rate is obviously reduced.

As can be seen from the data results of examples 5, 7, 9, 11 and 13 and comparative example 3, with respect to SiO₂Compared with the negative electrode system made of the artificial graphite mixed material, the capacity and the energy density are reduced to a certain extent, but the reduction range is controlled within a certain range, but the expansion rate is obviously reduced.

Although the capacity and energy density of the cathode of the silicon-based material are more outstanding, the problem that particles are deformed too much in the circulation process due to the overhigh expansion coefficient of the pure silicon-based material at present, the active substances are broken and fall off along with the circulation, side reactions are increased, and lithium is separated out due to overhigh expansion force, so that the defect of the circulation performance is overcome, the cathode is difficult to be applied to actual production and life.

SiO-containing materials can be seen by comparing the core capacity and energy density data of the comparative and the example₂Compared with a pure graphite system, the graphite mixed system has the advantages that the cell capacity and the energy density are obviously improved, and the content of the cell is increased along with the SiO₂The increase of the cell capacity and the energy density is mainly due to SiO₂The material has high specific capacity, and more active substances can be filled in the same space, so that the effective energy exertion can be improved. In the embodiment, the low-expansion material is uniformly coated on SiO by premixing₂Surface of system material particles, relative to SiO₂The capacity and energy density of the system are reduced to a certain extent, but the reduction range is lower when the system is controlled within a certain range, compared with the current graphite system, the reduction range still has great increase, and the capacity of the material per se of the current graphite system can play a role due to the fact that the capacity and energy density are too lowThe force is a bottleneck for restricting the development and application of the high-energy-density power battery in the future.

Through the data of the expansion rate of the negative electrode of the full-electricity battery cell, the SiO content can be seen₂The system and the mixed system with graphite have higher expansion rate in the full-charge state compared with the pure graphite system and are accompanied with SiO₂The content is increased, the full-electric expansion amplitude of the negative electrode is increased, mainly because of SiO₂The material has higher volume expansion after lithium is embedded. In the embodiment, the low-expansion material is uniformly coated on SiO by premixing₂Surface of system material particles, relative to SiO₂The expansion of the system can be well controlled, and the SiO after coating can be gradually increased along with the gradual increase of the coating amount of the low-expansion material₂The expansion of the system electrode gradually decreases.

The cells provided in comparative examples 1-3 and examples 1-13 were tested for cycle life and storage life:

at room temperature, the cells provided in comparative examples 1 to 3 and examples 1 to 13 were taken, 2 cells were charged to 4.2V at a constant current and a constant voltage of 0.33C for 5min, and then discharged to 2.5V at 0.33C for each example, and the discharge capacity was recorded, and the capacity retention rate was the corresponding cycle discharge capacity/initial discharge capacity. The process is repeated until the capacity retention rate is less than or equal to 80 percent, and the number of the recording cycles is recorded.

At room temperature, after the cycle provided by comparative examples 1 to 3 and examples 1 to 13 is finished, the battery core is charged to 4.2V at constant current and constant voltage of 0.33C by using a charging and discharging test cabinet, then the negative electrode plates of comparative examples 1 to 3 and examples 1 to 13 are disassembled, the thickness is tested and is recorded as EOL (end of cycle) negative electrode thickness, and the EOL negative electrode expansion rate is (EOL negative electrode thickness-negative electrode rolling thickness)/negative electrode rolling thickness.

And testing the expansion force change condition in the circulation process by adopting expansion force testing equipment.

At room temperature, 2 cells of the comparative example and the example were charged to 4.2V at a constant current and a constant voltage of 0.33C, and then the cells were stored in a high-temperature 45 ℃ incubator for 500 days, and were taken out every 30 days to test the capacity retention rate. The results are shown in Table 2.

TABLE 2

As can be seen from the data results of examples 4, 6, 8, 10 and 12 and comparative example 2, with respect to SiO₂In terms of a system, the cathode material provided by the application effectively controls the over expansion of the cathode, and the surface low-expansion graphite preferentially completes lithium intercalation and deintercalation, thereby controlling the SiO of the inner layer₂The lithium intercalation degree of the system, and SiO₂Swelling amount of system, and volume change degree, SiO inhibition₂The cycle deterioration of the system caused by excessive volume change is increased along with the increase of the coating content of the low-expansion graphite, so that the cycle performance and the storage performance of the battery are improved.

As can be seen from the data results of examples 5, 7, 9, 11 and 13 and comparative example 3, with respect to SiO₂Compared with the cathode system of the artificial graphite mixed material, the cathode material provided by the application can effectively inhibit the excessive expansion of the cathode, and improve the cycle performance and storage performance of the battery.

By combining the data of all the examples and comparative examples in table 2, the anode material provided by the application not only effectively reduces the expansion rate of the anode material under the silicon-based material system, but also improves the deterioration of the cycle performance of the battery caused by the expansion of the anode, and effectively improves the cycle performance and the storage life of the battery.

Combining tables 1 and 2, the cathode material provided by the present application has a capacity of 133.3Ah or more, an energy density of 259Wh/kg or more, and a BOL expansion rate of the electrode reduced to 43.2% or even as low as 32.7% or less, and up to 32.7% or less, when the core of the cathode system is a silicon-based materialAt least 1098 cycles, the capacity will be reduced to 80% or less, the expansion force is 24.6N or less, the EOL expansion rate of the electrode is reduced, when the core system in the negative electrode is SiO₂And graphite, the capacity of the material is not particularly obviously reduced, the capacity is only reduced to 112.8Ah, even 124Ah or more, the energy density is 219Wh/kg or more, the BOL expansion rate of the electrode is reduced to 32.3 percent or even to 29.2 percent or less, the capacity is reduced to 80 percent or less after at least 1312 cycles, the expansion force is also 20.9N or less, and the EOL expansion rate of the electrode is also reduced, so that the full-charge expansion rate of the battery is obviously reduced after initial cycles and multiple cycles under the condition that the capacity and the energy density of the battery are small and even are improved, the performance deterioration of the battery is further restrained, and the cycle performance and the storage life of the battery are improved.

The applicant declares that the above description is only a specific embodiment of the present invention, but the scope of the present invention is not limited thereto, and it should be understood by those skilled in the art that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope of the present invention are within the scope and disclosure of the present invention.

Claims

1. The negative electrode material is characterized by comprising a core containing a silicon-based material and a buffer layer coated on the surface of the core, wherein the buffer layer comprises a low-expansion material, a conductive agent and a binder, and the low-expansion material comprises any one or a combination of at least two of hard carbon, soft carbon, lithium titanate, natural graphite and artificial graphite.

2. The anode material of claim 1, wherein the inner core further comprises a carbon material;

preferably, the median particle size of the carbon material is 5-20 μm;

preferably, in the inner core containing the carbon material, the volume ratio of the carbon material is 2-50%;

3. The negative electrode material as claimed in claim 1, wherein the silicon-based material has a median particle diameter of 8 to 20 μm;

preferably, the median particle diameter of the low-expansion material in the buffer layer is 0.5-5 μm.

4. The negative electrode material of any one of claims 1 to 3, wherein the conductive agent is selected from fibrous, tubular, wire-like or sheet-like conductive agents, preferably comprising any one or a combination of at least two of carbon fibers, carbon nanotubes or graphene;

preferably, the diameter of the fibrous, tubular and linear conductive agent is 50-500 nm;

preferably, the length-diameter ratio of the fibrous, tubular and linear conductive agent is more than or equal to 10;

preferably, the aspect ratio of the flaky conductive agent is more than or equal to 10;

5. The negative electrode material according to any one of claims 1 to 4, wherein the low expansion material is present in an amount of 3 to 10% by mass.

6. The negative electrode material of any one of claims 1 to 5, wherein the conductive agent is present in an amount of 0.5 to 5% by mass;

preferably, in the negative electrode material, the mass ratio of the binder is 1.2-3%.

7. The method for producing the anode material according to any one of claims 1 to 6, characterized by comprising:

8. The method for producing the anode material according to claim 7, wherein the mixing method includes stirring;

preferably, the stirring linear speed of the stirring is 60-80 m/min;

preferably, the stirring time is 30-50 min;

preferably, the solvent comprises water and/or N-methylpyrrolidone;

preferably, the solid content of the mixed slurry is 45-55%.

9. The method for producing the anode material according to claim 7 or 8, characterized by comprising:

10. A lithium ion battery comprising the negative electrode material according to any one of claims 1 to 6.