CN112687843A - Composite conductive agent suitable for silicon-based negative electrode, preparation method of silicon-based negative electrode and lithium ion battery - Google Patents
Composite conductive agent suitable for silicon-based negative electrode, preparation method of silicon-based negative electrode and lithium ion battery Download PDFInfo
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Abstract
The invention discloses a composite conductive agent suitable for a silicon-based negative electrode, the silicon-based negative electrode, a preparation method of the silicon-based negative electrode and a lithium ion battery. The silicon-based negative electrode comprises a negative current collector and a silicon-based negative active layer positioned on the surface of the negative current collector, and is characterized in that the composite conductive agent comprises conductive carbon black, a carboxylated conductive carbon nanotube, conductive carbon fibers and a carboxylated Mxene; the mass percentage of the carboxylated Mxene is more than or equal to 0.1 percent based on 100 percent of the total dry-basis mass of the raw materials for preparing the silicon-based negative active layer. The silicon-based negative electrode prepared by the composite conductive agent has the advantages of good conductivity, small volume expansion, high theoretical gram capacity and high energy density.
Description
Technical Field
The invention relates to the technical field of new energy, and relates to a composite conductive agent suitable for a silicon-based negative electrode, the silicon-based negative electrode, a preparation method of the silicon-based negative electrode and a lithium ion battery.
Background
At present, the cathode material of the commercial lithium ion battery is mainly a graphite carbon cathode material, the theoretical specific capacity of the cathode material is only 372mAh/g (1 lithium ion is inserted into every 6 carbon atoms to form an LiC6 structure), and the gram capacity of the high-end graphite reaches 360-365mAh/g and is close to the theoretical capacity, so that the cathode material with higher energy density needs to be searched. At present, silicon-based materials become a research hotspot in negative electrode materials, the theoretical specific capacity is up to 4200mAh/g, and the silicon-based materials are considered to be substitute products of carbon negative electrode materials due to low lithium intercalation potential, low atomic mass and high energy density, and can effectively improve the energy density of lithium ion batteries, so the silicon-based materials are always paid extensive attention. However, there is a 300% expansion in volume of silicon during lithium deintercalation, resulting in pulverization during part of the silicon particles recycling without providing capacity, and destruction of physical connection with the conductive agent, which affects the battery cycle life.
How to select a conductive agent suitable for the silicon negative electrode is important, and the primary function of the conductive agent is to improve electron conductivity. The conductive agent plays a role in collecting micro-current between active substances and between the active substances and a current collector so as to reduce the contact resistance of the electrode, improve the migration rate of electrons in the lithium battery and reduce the polarization of the battery. The conductive agents commonly used for silicon-based materials at present are mainly conductive carbon black, conductive carbon fiber (VGCF), Carbon Nanotube (CNT), graphene, and the like. The conductive carbon black is a point-shaped conductive agent, also called as a zero-dimensional conductive agent, the conductivity is improved mainly through point contact among particles, the high specific surface area of carbon black particles is tightly stacked, so that the particles are tightly contacted together to form a conductive network in an electrode, but compared with other one-dimensional or two-dimensional conductive agents, the conductive carbon black has more addition amount, so that the energy density of a lithium battery is not favorably improved, and in addition, the conductive agent and a silicon cathode lose contact and a conductive network system is damaged due to repeated expansion and contraction in the circulation process of the silicon cathode. The conductive carbon fiber is a one-dimensional conductive agent, is in point-point contact with an active substance, can reduce the consumption of the conductive agent and improve the battery capacity compared with the point-point contact form of conductive carbon black and conductive graphite, but is difficult to disperse in VGCF (carbon black carbon), so that the conductivity of the conductive carbon fiber is limited to be exerted. The carbon nano tube has a one-dimensional structure and good electronic conductivity, and is easy to agglomerate under the action of van der waals force due to small tube diameter and large length-diameter ratio, so that the conductive effect is influenced. The graphene is a two-dimensional conductive agent, is in point-surface contact with an active substance, can reduce the consumption of the conductive agent greatly compared with zero-dimensional and one-dimensional materials, but the lamellar structure of the graphene can block the diffusion of lithium ions, thereby reducing the ionic conductivity of the pole piece. CN107978759A improves product performance by growing graphene as a conductive agent using a binder as a solid carbon source. However, the conductive agent of the above method is not suitable for a silicon-based material negative electrode with high expansion and low conductivity.
Disclosure of Invention
In view of the above problems in the prior art, an object of the present invention is to provide a composite conductive agent suitable for a silicon-based negative electrode, a preparation method thereof, and a lithium ion battery.
In order to achieve the purpose, the invention adopts the following technical scheme:
in a first aspect, the invention provides a composite conductive agent suitable for a silicon-based negative electrode, wherein the silicon-based negative electrode comprises a negative electrode current collector and a silicon-based negative electrode active layer positioned on the surface of the negative electrode current collector, and the composite conductive agent comprises conductive carbon black, carboxylated conductive carbon nanotubes, conductive carbon fibers and carboxylated Mxene;
the mass percentage content of the carboxylated Mxene is more than or equal to 0.1 percent, such as 0.1 percent, 0.3 percent, 0.5 percent, 0.8 percent, 1 percent, 1.5 percent, 2 percent, 3 percent, 3.5 percent, 4 percent or 5 percent and the like, based on the total dry-basis mass of the raw materials for preparing the silicon-based negative active layer.
The composite conductive agent is suitable for a silicon-based cathode, and the silicon-based cathode is prepared by adopting the composite conductive agent and silicon-based active substance ingredients, so that the silicon-based cathode has good conductivity, small volume expansion, high theoretical gram capacity and high energy density.
The technical principle is as follows: MXene has very high conductivity, a unique two-dimensional nano structure and good elasticity, a network structure built by the MXene, carbon nano tubes and carbon fibers has rich pores and good elasticity and can be used as a buffer structure of a silicon-based active substance, wherein small granular conductive carbon black is dispersed to form a strong conductive network system, and carboxyl groups in the conductive network system can form chemical bonds with hydroxyl groups on the surface of the silicon-based active substance due to carboxylation of the carbon nano tubes and Mxene, so that a conductive agent and the silicon-based active substance are firmly connected together and cannot lose contact with the conductive substance due to expansion of the silicon-based active substance. The factors are combined with the regulation and control of the Mxene content, so that compared with a conventional point-line-surface three-dimensional network, the conductive network system has better conductivity, better expansibility inhibition, ion transmission property and binding property with a carbon-based active substance, the defect of poor conductivity of silicon-based active substances such as silicon oxide and the like can be better changed, the volume expansion of the silicon-based active substances in the charge and discharge process is reduced, and the electrochemical properties such as high-rate charge and discharge performance, cycle performance and the like of a silicon-based cathode are improved.
Compared with the traditional conductive agent, the composite conductive agent disclosed by the invention can greatly reduce the usage amount, so that the content of active substances is increased, and the energy density of a battery is improved.
The following is a preferred technical solution of the present invention, but not a limitation to the technical solution provided by the present invention, and the technical objects and advantageous effects of the present invention can be better achieved and achieved by the following preferred technical solution.
Preferably, the carboxylated Mxene is selected from Ti3C2、Ti2C、Ta4C3、TiNbC、(V0.5Cr0.5)3C2、V2C、Nb2C and Nb4C3At least one of (1).
Preferably, the mass percentage of the carboxylated Mxene in the silicon-based anode material is 0.2-3%, such as 0.2%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 2.7%, 3%, etc., based on 100% of the total dry-based mass of the raw materials for preparing the silicon-based anode active layer.
Preferably, the conductive carbon black includes at least one of ketjen black, acetylene black, and supp.
Preferably, the conductive carbon black is contained in an amount of 0.5 to 10% by mass, for example, 0.5%, 0.8%, 1%, 1.2%, 2%, 2.5%, 3%, 4%, 5%, 6%, 7%, 8%, 10%, or the like, preferably 0.5 to 2% by mass, based on 100% by mass of the total dry basis of the raw materials used for preparing the silicon-based negative electrode active layer.
Preferably, the length of the carboxylated conductive carbon nanotube is 1 μm to 10 μm, such as 1 μm, 2 μm, 3 μm, 3.5 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 10 μm, or the like; a diameter of 1nm to 70nm, such as 1nm, 3nm, 5nm, 8nm, 10nm, 15nm, 20nm, 25nm, 30nm, 35nm, 40nm, 50nm, 60nm or 70nm, and an aspect ratio of 100 to 800, such as 100, 200, 300, 400, 500, 600, 700 or 800.
Preferably, the specific surface area of the carboxylated conductive carbon nanotube is 150-1300m2G, e.g. 150m2/g、200m2/g、300m2/g、400m2/g、450m2/g、500m2/g、600m2/g、800m2/g、1000m2(iv)/g or 1200m2And/g, etc.
Preferably, the carboxylated conductive carbon nanotubes are contained in an amount of 0.04 to 0.5% by mass, such as 0.04%, 0.06%, 0.08%, 0.1%, 0.13%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.5% or the like, preferably 0.05 to 0.3% by mass, based on 100% by mass of the total dry basis of the raw materials used for preparing the silicon-based negative active layer.
Preferably, the mass percentage of the conductive carbon fiber is 0.5-3%, such as 0.5%, 1%, 1.3%, 1.5%, 2%, 2.5% or 3%, and preferably 0.5-1%, based on 100% of the total dry mass of the raw materials used for preparing the silicon-based negative active layer.
Preferably, the mass ratio of the total mass of the carboxylated conductive carbon nanotubes and the conductive carbon fibers to the mass of the carboxylated Mxene is 1.2-2.5, such as 1.2, 1.3, 1.5, 1.7, 1.8, 2 or 2.5, and the like, and in this range, the effects of obtaining better buffer volume expansion, increasing conductivity and increasing ion conduction under the condition of low addition amount are favorably achieved.
In a second aspect, the present invention provides a method for preparing a silicon-based negative electrode using the composite conductive agent of the first aspect, the method comprising the steps of:
(1) preparing a first conductive glue solution containing carboxylated Mxene;
(2) mixing conductive carbon black, carboxylated conductive carbon nanotubes and conductive carbon fibers with the first conductive glue solution in the step (1), and uniformly dispersing to obtain a second conductive glue solution;
(3) adding a silicon-based active substance and a binder into the second conductive glue solution in the step (2), and uniformly stirring to obtain a negative electrode slurry;
(4) and (4) coating the negative electrode slurry obtained in the step (3) on the surface of a negative electrode current collector, and drying to form a silicon-based negative electrode active layer on the surface of the negative electrode current collector, namely obtaining the silicon-based negative electrode.
In the method, the carboxylated Mxene and the carboxylated conductive carbon nano tube have good hydrophilicity, can be well dispersed in the negative electrode slurry in the preparation process of the silicon-based negative electrode, and the problem that the single carbon fiber or the carbon nano tube is difficult to disperse is solved through the matching of the components, so that the preparation method of the whole silicon-based negative electrode is simple, and the large-scale production is easy to realize. By adopting a plurality of specific conductive agents for compounding, the conductivity of the silicon-based negative electrode can be improved, the volume expansion of the silicon-based active material can be inhibited, and the rate capability and the cycle performance of the silicon-based negative electrode are improved.
The preparation method of the carboxylated Mxene is not specifically limited in the present invention, and the preparation method of Mxene is simpler than that of graphene materials, and can be performed by the following methods, for example and without limitation, by referring to the methods disclosed in the prior art:
600mg of Mxene material and 500ml of cold deionized water were placed in a beaker and mechanically stirred at 2 ℃ for 60min, then 10g of ClCH was added2COOH was slowly added to the solution and stirred for 4h, then 0.4mol sodium hydroxide solution was added to the solution and stirred for 5h at 58 ℃ to obtain a carboxylated MXene sample. The carboxylated Mxene may have a thickness of, for example, 3 to 50nm and a number of layers of 1 to 13.
The specific type of the silicon-based active material is not limited in the invention, and the silicon-based active material can be nano silicon, a silicon-carbon material or a silicon-oxygen material, and is preferably a silicon-carbon material. The silicon carbon material is a composite or mixture of at least one of SiO and Si and a carbon material, such as at least one of artificial graphite and natural graphite.
The type of the negative electrode current collector in the present invention is not particularly limited, and may be, for example, a copper foil.
As a preferred technical scheme of the method of the invention, the step (1) comprises the following steps: dispersing a dispersing agent into water, uniformly stirring to obtain a glue solution with a solid content of 1.2-3% (such as 1.2%, 1.5%, 1.8%, 2%, 2.2%, 2.5% or 3%, and the like), then adding a carboxylated Mxene into the glue solution, and performing dispersion treatment to obtain a first conductive glue solution.
Preferably, the dispersant is sodium carboxymethyl cellulose (CMC).
Preferably, the temperature of the dispersion treatment is 25 to 30 ℃, for example, 25 ℃, 26 ℃, 27 ℃ or 30 ℃.
Preferably, step (2) comprises: and (2) sequentially adding conductive carbon fibers, carboxylated conductive carbon nanotubes and conductive carbon black into the first conductive adhesive in the step (1), adding one conductive agent each time, and stirring for 1-3 hours (for example, 1 hour, 1.5 hours, 1.7 hours, 2 hours, 2.5 hours or 3 hours and the like, wherein the stirring time for each time can be the same or different), so that the second conductive adhesive solution is uniformly dispersed, and thus the second conductive adhesive solution is obtained. The preferable technical scheme can better ensure the good dispersibility of each substance.
Preferably, step (3) comprises: adding the silicon-based active material into the second conductive glue solution in the step (2), uniformly stirring, adding a solvent to adjust the viscosity of the slurry to be 4500-7500 mPa.s (such as 4500mPa.s, 5000mPa.s, 5500mPa.s, 6000mPa.s, 6500mPa.s, 7000mPa.s or 7500 mPa.s), then adding a binder, uniformly stirring, and filtering to obtain the negative electrode slurry.
Preferably, the mass percentage of the silicon-based active material is 80-96%, for example, 80%, 83%, 85%, 88%, 90%, 92%, 93%, 94%, or 96%, and preferably 92-96%, based on 100% of the total dry-based mass in the negative electrode slurry in the step (3).
Preferably, the mass percentage of the dispersant is 0.5-5%, such as 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, or 5%, etc., based on 100% of the total dry mass in the negative electrode slurry in step (3).
Preferably, the binder of step (3) includes at least one of styrene-butadiene rubber, polyvinylidene fluoride, and polyacrylic acid.
Preferably, the mass percentage of the binder is 1-5%, for example, 1%, 2%, 3%, 3.5%, 4%, or 5%, etc., based on 100% of the total dry mass in the negative electrode slurry in step (3).
As a further preferred technical solution of the method of the present invention, the method comprises the steps of:
(1) dispersing a dispersing agent into water, uniformly stirring to obtain a glue solution with the solid content of 1.2-3%, then adding carboxylated Mxene into the glue solution, and performing dispersion treatment to obtain a first conductive glue solution;
(2) sequentially adding conductive carbon fibers, carboxylated conductive carbon nanotubes and conductive carbon black into the first conductive adhesive in the step (1), adding one conductive agent each time, and stirring for 1-3 hours to uniformly disperse to obtain a second conductive adhesive solution;
(3) adding a silicon-based active substance into the second conductive glue solution in the step (2), uniformly stirring, adding a binder, then adding a solvent to adjust the viscosity of the slurry to be 4500-7500 mPa.s, uniformly stirring, and filtering to obtain a negative electrode slurry;
(4) and (4) coating the negative electrode slurry obtained in the step (3) on the surface of a negative electrode current collector, and drying to form a silicon-based negative electrode active layer on the surface of the negative electrode current collector, namely obtaining the silicon-based negative electrode.
In a third aspect, the invention provides a silicon-based anode prepared according to the method of the second aspect.
In a fourth aspect, the present invention provides a lithium ion battery, which includes the silicon-based negative electrode of the third aspect.
Compared with the prior art, the invention has the following beneficial effects:
the composite conductive agent adopts the cooperative matching of the conductive carbon black, the carboxylated conductive carbon nano tube, the conductive carbon fiber and the carboxylated Mxene with specific content to form a strong conductive network system, and compared with the conventional point-line-surface three-dimensional network, the composite conductive agent has better conductivity, better inhibition expansibility, ion transmission property and associativity with carbon-based active substances, can better change the defect of poor conductivity of silicon-based active substances such as silicon oxide and the like, reduce the volume expansion of the silicon-based active substances in the charging and discharging process, and improve the electrochemical properties such as high-rate charging and discharging performance, cycle performance and the like of a silicon-based cathode.
Detailed Description
The technical solution of the present invention is further explained by the following embodiments.
The silicon-based active substance adopted in the embodiment of the invention is a silicon-carbon material, specifically a mixture formed by Si and natural graphite, and the mass ratio of Si to natural graphite is 10: 85.
Example 1
The embodiment provides a preparation method for preparing a silicon-based negative electrode by adopting a composite conductive agent, which comprises the following steps:
(1) dispersing a dispersing agent CMC into water, stirring uniformly to obtain a glue solution with the solid content of 2%, then adding a carboxylated Mxene (the thickness is 20nm, the number of layers is 4) into the glue solution, and performing dispersion treatment to obtain a first conductive glue solution;
(2) adding conductive carbon fibers into the first conductive adhesive obtained in the step (1), stirring for 1h, then continuously adding carboxylated conductive carbon nanotubes (the length is 10 micrometers and the diameter is 50nm), stirring for 1h, then adding conductive carbon black (specifically acetylene black), and stirring for 3h to uniformly disperse to obtain a second conductive adhesive solution;
(3) adding a silicon-based active substance into the second conductive glue solution in the step (2), uniformly stirring, adding a binder styrene butadiene rubber, adding a solvent to adjust the viscosity of the slurry to 6500mPa.s, uniformly stirring, and filtering to obtain a negative electrode slurry;
(4) and (4) coating the negative electrode slurry obtained in the step (3) on the surface of a negative electrode current collector, and drying to form a silicon-based negative electrode active layer on the surface of the negative electrode current collector, namely obtaining the silicon-based negative electrode.
In this example, the mass percentage of each substance is 100% of the total mass of the dispersing agent, the conductive carbon black, the carboxylated conductive carbon nanotube, the conductive carbon fiber, the carboxylated Mxene, the silicon-based active material, and the binder used in the preparation process (i.e., the total mass of the raw materials used for preparing the silicon-based negative active layer on a dry basis).
In this embodiment, the composite conductive agent includes conductive carbon black, a carboxylated conductive carbon nanotube, conductive carbon fibers, and a carboxylated Mxene, and the composite conductive agent includes the following components:
in this embodiment, the dispersant CMC is 1% by mass, the silicon-based active substance is 95% by mass, and the binder styrene-butadiene rubber is 2% by mass.
Example 2
(1) Dispersing a dispersing agent CMC into water, stirring uniformly to obtain a glue solution with the solid content of 1.5%, then adding a carboxylated Mxene (the thickness is 20nm, the number of layers is 4) into the glue solution, and performing dispersion treatment to obtain a first conductive glue solution;
(2) adding conductive carbon fibers into the first conductive adhesive in the step (1), stirring for 1.5h, then continuously adding a carboxylated conductive carbon nanotube (with the length of 8 mu m and the diameter of 15nm), stirring for 1h, then adding conductive carbon black (particularly SP), and stirring for 3h to uniformly disperse to obtain a second conductive adhesive liquid carboxylated conductive carbon nanotube;
(3) adding a silicon-based active substance into the second conductive glue solution in the step (2), uniformly stirring, adding a binder polyacrylic acid, adding a solvent to adjust the viscosity of the slurry to 6000mPa.s, uniformly stirring, and filtering to obtain a negative electrode slurry;
(4) and (4) coating the negative electrode slurry obtained in the step (3) on the surface of a negative electrode current collector, and drying to form a silicon-based negative electrode active layer on the surface of the negative electrode current collector, namely obtaining the silicon-based negative electrode.
In this example, the mass percentage of each substance is 100% of the total mass of the dispersing agent, the conductive carbon black, the carboxylated conductive carbon nanotube, the conductive carbon fiber, the carboxylated Mxene, the silicon-based active material, and the binder used in the preparation process (i.e., the total mass of the raw materials used for preparing the silicon-based negative active layer on a dry basis).
In this embodiment, the composite conductive agent includes conductive carbon black, a carboxylated conductive carbon nanotube, conductive carbon fibers, and a carboxylated Mxene, and the composite conductive agent includes the following components:
in this example, the dispersant CMC was 1% by mass, and the silicon-based active substance was 94.6% by mass. The mass percentage of the polyacrylic acid as the binder is 2%.
Example 3
(1) Dispersing a dispersing agent CMC into water, stirring uniformly to obtain a glue solution with the solid content of 2%, then adding a carboxylated Mxene (the thickness is 20nm, the number of layers is 4) into the glue solution, and performing dispersion treatment to obtain a first conductive glue solution;
(2) adding conductive carbon fibers into the first conductive adhesive in the step (1), stirring for 1.5h, then continuously adding carboxylated conductive carbon nanotubes (the length is 9 mu m, and the diameter is 20nm), stirring for 2h, then adding conductive carbon black (particularly SP), stirring for 1.5h, uniformly dispersing, and carboxylating the conductive carbon nanotubes to obtain a second conductive adhesive solution;
(3) adding a silicon-based active substance into the second conductive glue solution in the step (2), uniformly stirring, adding a binder polyacrylic acid, adding a solvent to adjust the viscosity of the slurry to 5500mPa.s, uniformly stirring, and filtering to obtain a negative electrode slurry;
(4) and (4) coating the negative electrode slurry obtained in the step (3) on the surface of a negative electrode current collector, and drying to form a silicon-based negative electrode active layer on the surface of the negative electrode current collector, namely obtaining the silicon-based negative electrode.
In this example, the mass percentage of each substance is 100% of the total mass of the dispersing agent, the conductive carbon black, the carboxylated conductive carbon nanotube, the conductive carbon fiber, the carboxylated Mxene, the silicon-based active material, and the binder used in the preparation process (i.e., the total mass of the raw materials used for preparing the silicon-based negative active layer on a dry basis).
In this embodiment, the composite conductive agent includes conductive carbon black, a carboxylated conductive carbon nanotube, conductive carbon fibers, and a carboxylated Mxene, and the composite conductive agent includes the following components:
in this example, the dispersant CMC was 1% by mass, and the silicon-based active substance was 92% by mass. The mass percentage of the polyacrylic acid as the binder is 2%.
Example 4
The difference from the embodiment 2 is that the composite conductive agent comprises the following components in percentage by weight:
in this example, the mass ratio of the total mass of the carboxylated conductive carbon nanotubes and conductive carbon fibers to the carboxylated Mxene was 0.5.
Example 5
The difference from the embodiment 2 is that the composite conductive agent comprises the following components in percentage by weight:
the mass percentage of the dispersant CMC is 1 percent, and the mass percentage of the silicon-based active substance is 86 percent. The mass percentage of the polyacrylic acid as the binder is 2%.
Example 6
The difference from example 2 is that the electrically conductive carbon nanotubes (length 12 μm, diameter 0.8nm) are carboxylated.
Comparative example 1
The difference from example 2 is that carboxylated Mxene was not added and the mass ratio between the substances in the composite conductive agent was maintained as in example 2.
Comparative example 2
The difference from example 2 is that the addition amount of carboxylated Mxene was 0.05%, and the mass ratio between the respective substances in the composite conductive agent was maintained as in example 2.
Comparative example 3
The difference from the example 2 is that the carboxylated conductive carbon nanotube is not added, and the mass ratio among the substances in the composite conductive agent is kept the same as the example 2.
Comparative example 4
The difference from example 2 is that the conductive carbon fiber is not added and the mass ratio between the substances in the composite conductive agent is maintained the same as in example 2.
Comparative example 5
The difference from example 2 is that the conductive carbon black is not added, and the mass ratio between the respective substances in the composite conductive agent is maintained the same as in example 2.
Firstly, rate performance testing:
the negative pole piece, the ternary positive pole piece (the positive active material is NCM532) prepared by the traditional mature process and 1mol/L LiPF6The electrolyte of/EC + DMC + EMC (v/v is 1:1:1), Celgard2400 diaphragm and shell adopt the conventional production technology to assemble 18650 cylindrical single-cell battery.
The multiplying power performance and the expansion rate of the negative pole piece at full charge of the battery are tested under the cylindrical battery test condition, the charging and discharging voltage is limited to 2.0V-4.2V when the battery is tested on a LAND battery test system of Wuhanjinuo electronic Limited company at normal temperature (25 ℃), and the result is shown in Table 1.
II, testing cycle performance:
the negative pole piece is adopted to be assembled into a button cell in an argon atmosphere glove box, the diaphragm is a polypropylene microporous membrane, and the electrolyte is 1mol/L LiPF6(v/v ═ 1:1:1) of/EC + DMC + EMC, pairs usedThe electrode is a lithium metal sheet.
The button cell was subjected to a cycling test using a blue cell test system CT2001C at room temperature (25 ℃) and a current density of 1C for 50 weeks, as shown in table 1.
TABLE 1
And (3) analysis:
the comparison between the example 2 and the example 4 shows that the mass ratio of the total mass of the carboxylated conductive carbon nanotubes and the conductive carbon fibers to the carboxylated Mxene is in the range of 1.2-2.5, which is beneficial to obtaining high rate capability and cycle performance under the condition of low content of the composite conductive agent, and the electrochemical performance of the example 4 is inferior to that of the example 2.
It can be seen from the comparison between example 2 and example 6 that the aspect ratio of the carbon nanotube has an important influence on the performance of the silicon-based negative electrode of the present invention, and the too large aspect ratio of the carbon nanotube is not conducive to dispersion, resulting in the reduction of rate capability and cycle performance.
It can be seen from examples 1-6 that the method of the present invention can increase the amount of active material used under the condition of lower amount of conductive agent and increase the energy density of the battery by adjusting the ratio of each material in the composite conductive agent.
As can be seen from the comparison between the example 2 and the comparative examples 1 to 5, the four components of the conductive carbon black, the carboxylated conductive carbon nanotube, the conductive carbon fiber and the carboxylated Mxene in the composite conductive agent are all absent, and the multiple components synergistically improve the rate capability and the cycle performance of the material. Moreover, if the content of the carboxylated Mxene or the carboxylated carbon nanotube is too low, the full electrode plate expands greatly, the cycle performance is poor, and the silicon-based negative electrode expands greatly and the cycle performance is poor probably mainly because the carboxyl groups are few and are not combined with hydroxyl on the surface of the silicon negative electrode to form chemical bonds.
The applicant states that the present invention is illustrated in detail by the above examples, but the present invention is not limited to the above detailed methods, i.e. it is not meant that the present invention must rely on the above detailed methods for its implementation. It should be understood by those skilled in the art that any modification of the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific modes, etc., are within the scope and disclosure of the present invention.
Claims (10)
1. The composite conductive agent is suitable for a silicon-based negative electrode, and the silicon-based negative electrode comprises a negative electrode current collector and a silicon-based negative electrode active layer positioned on the surface of the negative electrode current collector, and is characterized in that the composite conductive agent comprises conductive carbon black, carboxylated conductive carbon nanotubes, conductive carbon fibers and carboxylated Mxene;
the mass percentage of the carboxylated Mxene is more than or equal to 0.1 percent based on 100 percent of the total dry-basis mass of the raw materials for preparing the silicon-based negative active layer.
2. The composite conductive agent of claim 1, wherein the carboxylated Mxene is selected from the group consisting of Ti3C2、Ti2C、Ta4C3、TiNbC、(V0.5Cr0.5)3C2、V2C、Nb2C and Nb4C3At least one of;
preferably, the mass percentage of the carboxylated Mxene in the silicon-based negative electrode material is 0.2-3% by taking the total dry-basis mass of the raw materials for preparing the silicon-based negative electrode active layer as 100%.
3. The composite conductive agent according to claim 1 or 2, wherein the conductive carbon black includes at least one of ketjen black, acetylene black, and supp;
preferably, the mass percentage of the conductive carbon black is 0.5-10%, preferably 0.5-2%, calculated by 100% of the total dry-basis mass of the raw materials for preparing the silicon-based negative active layer;
preferably, the length of the carboxylated conductive carbon nanotube is 1-10 μm, the diameter is 1-70 nm, and the length-diameter ratio is 100-800;
preferably, the specific surface area of the carboxylated conductive carbon nanotube is 150-1300m2/g；
Preferably, the mass percentage of the carboxylated conductive carbon nanotube is 0.04-0.5%, and preferably 0.05-0.3%, based on 100% of the total dry-based mass of the raw materials for preparing the silicon-based negative active layer;
preferably, the mass percentage of the conductive carbon fiber is 0.5-3%, preferably 0.5-1%, calculated by 100% of the total dry basis mass of the raw materials for preparing the silicon-based negative electrode active layer;
preferably, the mass ratio of the total mass of the carboxylated conductive carbon nanotubes and the conductive carbon fibers to the carboxylated Mxene is 1.2 to 2.5.
4. A method for preparing a silicon-based negative electrode by using the composite conductive agent as defined in any one of claims 1 to 3, wherein the method comprises the steps of:
(1) preparing a first conductive glue solution containing carboxylated Mxene;
(2) mixing conductive carbon black, carboxylated conductive carbon nanotubes and conductive carbon fibers with the first conductive glue solution in the step (1), and uniformly dispersing to obtain a second conductive glue solution;
(3) adding a silicon-based active substance and a binder into the second conductive glue solution in the step (2), and uniformly stirring to obtain a negative electrode slurry;
(4) and (4) coating the negative electrode slurry obtained in the step (3) on the surface of a negative electrode current collector, and drying to form a silicon-based negative electrode active layer on the surface of the negative electrode current collector, namely obtaining the silicon-based negative electrode.
5. The method of claim 4, wherein step (1) comprises: dispersing a dispersing agent into water, uniformly stirring to obtain a glue solution with the solid content of 1.2-3%, then adding carboxylated Mxene into the glue solution, and performing dispersion treatment to obtain a first conductive glue solution;
preferably, the dispersant is sodium carboxymethyl cellulose;
preferably, the temperature of the dispersion treatment is 25-30 ℃.
6. The method of claim 4 or 5, wherein step (2) comprises: and (2) sequentially adding conductive carbon fibers, carboxylated conductive carbon nanotubes and conductive carbon black into the first conductive adhesive in the step (1), adding one conductive agent each time, and stirring for 1-3 hours to uniformly disperse to obtain a second conductive adhesive solution.
7. The method according to any one of claims 4-6, wherein step (3) comprises: adding the silicon-based active substance into the second conductive glue solution in the step (2), uniformly stirring, adding the binder, then adding the solvent to adjust the viscosity of the slurry to be 4500-7500 mPa.s, uniformly stirring, and filtering to obtain the negative electrode slurry.
8. The method according to any one of claims 4 to 7, wherein the mass percentage of the silicon-based active material is 80 to 96%, preferably 92 to 96%, based on 100% of the total dry mass in the anode slurry in the step (3);
preferably, the mass percentage of the dispersant is 0.5-5% based on 100% of the total dry mass of the negative electrode slurry in the step (3);
preferably, the binder of step (3) comprises at least one of styrene-butadiene rubber, polyvinylidene fluoride and polyacrylic acid;
preferably, the mass percentage of the binder is 1-5% based on 100% of the total dry mass of the negative electrode slurry in the step (3).
9. A silicon-based anode prepared according to the method of any one of claims 4 to 8.
10. A lithium ion battery comprising the silicon-based negative electrode of claim 9.
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