CN119208531B

CN119208531B - Negative electrode plate and battery

Info

Publication number: CN119208531B
Application number: CN202411621879.1A
Authority: CN
Inventors: 韩定宏; 江柯成; 张�浩; 徐晓晴; 章文; 於洪将
Original assignee: Jiangsu Zenio New Energy Battery Technologies Co Ltd
Current assignee: Jiangsu Zenio New Energy Battery Technologies Co Ltd
Priority date: 2024-11-14
Filing date: 2024-11-14
Publication date: 2025-02-25
Anticipated expiration: 2044-11-14
Also published as: CN119208531A

Abstract

The application relates to the technical field of batteries, in particular to a negative electrode plate and a battery. The negative electrode plate comprises a current collector and a coating layer arranged on at least one side surface of the current collector in the thickness direction, wherein the coating layer comprises an active substance, the active substance comprises graphite and a silicon-based material, the volume expansion coefficient of the silicon-based material is A, the hardness coefficient of the graphite is B, the unit of the B is g/cm ³, and the A and the B meet the conditions that A is more than or equal to 16 and B is more than or equal to 0.15 and less than or equal to 21. A battery comprises the negative electrode plate. According to the application, the silicon-based material with larger expansion force can be matched with harder graphite, the silicon-based material with relatively smaller expansion force can be matched with relatively softer graphite, and the problem of cycle degradation of the graphite and the silicon-based material when used together can be improved by a relatively simple method on the premise of not losing energy density and gram capacity.

Description

Negative electrode plate and battery

Technical Field

The invention relates to the technical field of batteries, in particular to a negative electrode plate and a battery.

Background

The conversion of fossil energy into new energy has become a trend, and under this trend, new energy automobiles using clean energy such as lithium power or hydrogen energy as a power source have been developed. Because lithium batteries are easier to commercialize than hydrogen energy sources, new energy automobiles are mainly lithium batteries at present. However, the installed duty ratio of new energy vehicles is rising year by year, but the duty ratio with respect to the oil vehicles is still low. The method is characterized in that the performance of the new energy automobile has a certain pain point, such as the user worry about the endurance mileage, the charging speed, the safety, the low-temperature performance and the like of the new energy automobile.

In order to solve these problems, improvement in the performance of the power source, i.e., the battery itself, is required. In terms of endurance mileage, it is required to increase the energy density of the battery, i.e., to exert higher energy at the same volume ratio. This requires an improvement from the overall structural design of the battery, and in terms of the battery anode material, an improvement in the gram capacity of the anode, such as the addition of some silicon to the graphite anode tab. Although the theoretical gram capacity of silicon is about ten times that of graphite, silicon itself has problems of volume expansion, insufficient conductivity, and the like. In the related art, in the use process of the graphite silicon negative electrode, the graphite negative electrode material and the silicon-based material are combined, so that the energy density is improved, the cycle stability is reduced to a certain extent, and the design requirement of a battery cannot be completely met.

Disclosure of Invention

In view of the above, the present invention aims to solve at least one of the technical problems in the related art to some extent. Therefore, the invention provides the negative electrode plate and the battery, which can keep good cycling stability of the battery under the condition of basically not losing energy density and gram capacity.

In order to solve the technical problems, the application is realized as follows:

According to one aspect of the application, the embodiment provides a negative electrode tab, which comprises a current collector and a coating layer arranged on at least one side surface of the current collector along the thickness direction;

the coating comprises an active substance;

The active substance comprises graphite and a silicon-based material, wherein the volume expansion coefficient of the silicon-based material is A, the hardness coefficient of the graphite is B, and the unit of B is g/cm ³;

The A and B satisfy the following conditions:

16≤A*B/0.15≤21。

In addition, the negative electrode plate according to the application can also have the following additional technical features:

in some embodiments, the a is a full charge expansion ratio of the negative electrode sheet when a silicon-based material is used as an active material, and the range of the a is 1.5-2.0.

In some embodiments, the B is an average value of compaction densities of the graphite under different pressures, and the range of the B is 1.55-1.70, and the unit is g/cm ³;

The pressure is in the range of 1-5T.

In some embodiments thereof, the silicon-based material comprises at least one of ground silicon, gas phase silicon carbon, silicon oxide, pre-lithiated silicon oxide, or pre-magnesia silicon oxide.

In some embodiments, the graphite comprises at least one of synthetic graphite, natural graphite, modified graphite, fast-charge graphite, or isostatic graphite.

In some embodiments, the mass ratio of the silicon-based material to the graphite is (25-40): 60-75.

In some of these embodiments, the coating further comprises a conductive agent and a binder.

In some embodiments thereof, the binder comprises at least two of polyacrylic acid, polyacrylonitrile, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, styrene butadiene rubber, or modified styrene butadiene polymer.

In some embodiments thereof, the conductive agent comprises at least two of single-walled carbon nanotubes, oligowalled carbon nanotubes, multiwalled carbon nanotubes, graphene, graphite alkyne, acetylene black, or ketjen black.

In some embodiments, the mass ratio of the active material to the binder to the conductive agent is (93-97): 2-5): 0.5-2.

In some embodiments, the current collector comprises at least one of copper foil, porous copper foil, nickel foam foil, copper foam foil, or carbon coated copper foil.

According to another aspect of the application, an embodiment of the application provides a battery, which includes the negative electrode tab.

The implementation of the technical scheme of the invention has at least the following beneficial effects:

in the embodiment of the application, the provided negative electrode plate can be used as a negative electrode in batteries such as lithium ion batteries. The application solves the problem of poor cycling stability of the lithium ion battery containing more silicon-based materials (> 25%) by limiting specific parameters of the silicon-based materials and graphite in the negative electrode plate.

Considering from the pole piece level, when more silicon-based materials exist in the negative pole piece, larger expansion force can be generated in the lithium intercalation process, the expansion force can extrude surrounding graphite particles, and in the continuous charge and discharge process, the expansion pressure can repeatedly extrude graphite, so that the graphite particles are peeled off and can not reversibly deintercalate lithium, and the degradation of the cycle stability is brought.

According to the characteristic that different silicon-based materials have different expansion forces and the hardness of graphite is large or small, the expansion proportion of the silicon-based materials and the hardness of the graphite are measured, and the volume expansion coefficient and the hardness coefficient are quantized, so that the silicon-based materials meet a certain functional relation, namely, the volume expansion coefficient A of the silicon-based materials and the hardness coefficient B of the graphite meet that the volume expansion coefficient A is more than or equal to 16 and less than or equal to A, B/0.15 and less than or equal to 21. Therefore, the silicon-based material with larger expansion force can be matched with harder graphite, the silicon-based material with relatively smaller expansion force can be matched with relatively softer graphite, and the problem of cycle degradation caused by the common use of the graphite and the silicon-based material can be improved by a simpler method on the premise of not losing the energy density and gram capacity.

Additional aspects and advantages of the application will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the application.

Detailed Description

The application will be further illustrated with reference to specific examples. It is to be understood that these examples of the present application are merely illustrative of the present application and are not intended to limit the scope of the present application.

The endpoints and any values of the ranges disclosed herein are not limited to the precise range or value, and are understood to encompass values approaching those ranges or values. For numerical ranges, one or more new numerical ranges may be found between the endpoints of each range, or between the individual points, and are considered to be specifically disclosed herein.

All embodiments of the application and alternative embodiments may be combined with each other to form new solutions, unless otherwise specified.

All technical features and optional technical features of the application may be combined with each other to form new technical solutions, unless specified otherwise.

All the steps of the present application may be performed sequentially or randomly, preferably sequentially, unless otherwise specified. For example, the method comprises steps (a) and (b), meaning that the method may comprise steps (a) and (b) performed sequentially, or may comprise steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), may include steps (a), (c) and (b), may include steps (c), (a) and (b), and the like.

The terms "comprising" and "including" as used herein mean open ended or closed ended, unless otherwise noted. For example, the terms "comprising" and "comprises" may mean that other components not listed may be included or included, or that only listed components may be included or included.

In the related art, in order to improve the cycle stability of the lithium battery, a mode of modifying a silicon-based material or graphite in a silicon-carbon negative electrode material is mostly adopted, so that the structures of the silicon-based material and the graphite are changed, the silicon-based material is firmly and uniformly distributed on the surface of the graphite material, the agglomeration of the silicon-based material and the stripping of the silicon-based material and the graphite are avoided, and the lithium battery has high cycle stability. However, the existing modification method is too complicated in process, and the preparation cost of the lithium battery is further increased.

In view of the above, the present application provides a negative electrode sheet capable of improving the problem of cycle degradation occurring when graphite and a silicon-based material are used together by a relatively simple method without substantially losing energy density and gram capacity. The specific technical scheme is described below.

In some embodiments, the application provides a negative electrode tab comprising a current collector and a coating layer disposed on at least one side surface of the current collector in a thickness direction;

the coating comprises an active substance;

The A and B satisfy the following conditions:

16≤A*B/0.15≤21。

In the embodiment of the present application, the negative electrode sheet includes a current collector and a coating layer (may also be referred to as an active material layer) disposed on at least one side surface of the current collector in the thickness direction, and the "coating layer disposed on at least one side surface of the current collector in the thickness direction" means that the coating layer may be disposed on one surface of the current collector in the thickness direction thereof or may be disposed on both surfaces of the current collector in the thickness direction thereof. The "surface" herein may be the entire area of the current collector or may be a partial area of the current collector, and as in the present embodiment, the surface may be a partial area of the current collector, and the remaining area of the current collector may be used for connecting the tab, which is not particularly limited in the present application as long as the object of the present application can be achieved.

As an example, the current collector has two surfaces opposite in the thickness direction thereof, and the coating is provided on the two opposite surfaces of the current collector. It will be appreciated that in other embodiments, the coating may also be layered on either of the two surfaces of the current collector.

The active substances in the coating of the negative electrode plate comprise graphite and silicon-based materials, and the relation between the volume expansion coefficient A of the silicon-based materials and the hardness coefficient B of the graphite is limited, so that the active substances can be applied to batteries such as lithium ion batteries and are used for solving the problems of poor circulation stability or low capacity and the like of graphite silicon negative electrodes in the conventional lithium ion batteries.

More particularly, in the embodiment of the application, the A and the B satisfy that the A is less than or equal to 16 and less than or equal to A and the B/0.15 and less than or equal to 21, and further, the A and the B satisfy that the A is less than or equal to 17 and less than or equal to A and less than or equal to 0.15 and less than or equal to 20. For example, the value of a x B/0.15 may be any one of the point values 16, 17, 18, 19, 20, 21 or a range between any two.

Therefore, the volume expansion coefficient A of the silicon-based material and the hardness coefficient B of the graphite meet the functional relation, so that the silicon-based material with larger expansion force is matched with harder graphite, and the silicon-based material with relatively smaller expansion force is matched with relatively softer graphite. In the continuous charge and discharge process, the situation that graphite particles are peeled off can be weakened, and the problem of cycle degradation when graphite and a silicon-based material are used together can be improved by a simpler method on the premise of not losing energy density and gram capacity.

In the embodiment of the application, the volume expansion coefficient A of the silicon-based material is the full charge expansion proportion of the negative electrode plate when the silicon-based material is only used as an active substance.

In the embodiment of the application, the range of A is 1.5-2.0, and further, the range of A can be 1.6-1.9. For example, the value of a may be any one point value or a range value between any two of 1.5, 1.6, 1.8, 1.9, 2.0. Thus, the value of A can meet the requirement of the battery on energy density at the same time in the range, and the battery can also maintain high cycle stability. If the value of A is too small, gram capacity is relatively low, the requirement on energy density in practical application is difficult to meet, and if the value of A is too large, the material expansion is too large, the cycle performance is poor, and the practical application cannot be realized.

In the embodiment of the application, the volume expansion coefficient A of the silicon-based material is obtained by testing the full charge expansion proportion of the negative electrode pieces with different gradient silicon-based material content ratios to obtain a fitting relation of the silicon-based material content and the expansion ratio, and then the full charge expansion proportion of the pure silicon-based material negative electrode is obtained by extrapolation, namely the volume expansion coefficient A of the silicon-based material.

Illustratively, in the process of testing the volume expansion coefficient A of the silicon-based material, the mass percentage of the silicon-based material in the anode electrode active material is 40% -75%, the gradient difference is 1% -10%, and further, the mass percentage of the silicon-based material in the anode electrode active material is 45% -60%, and the gradient difference is 2% -5%. For example, the mass percentage of silicon-based material in different anode active materials may be 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75%, and the gradient difference may be 1%, 2%, 4%, 5%, 6%, 8%, 10%.

Alternatively, in some embodiments, the volume expansion coefficient A of the silicon-based material can be obtained by preparing a negative electrode sheet containing a gradient proportion silicon-based material (the number of the negative electrode sheets is more than or equal to 5, for example, 5, 6, 7,8, 9 and 10) by using graphite and the silicon-based material as active substances of the negative electrode sheet, sequentially stacking and winding the negative electrode sheet, the isolation film and the positive electrode sheet to obtain a bare cell and an ultrasonic welding tab, putting the bare cell into a battery shell, drying to remove moisture, injecting electrolyte into the battery shell, packaging, and forming the component to obtain the secondary battery. A series of gradient values are designed in the design of full-electric negative electrode gram capacity, such as 400,450,500,550,600,650,700,750, and the like, after the battery is fully charged after capacity division, the thickness of a negative electrode sheet is disassembled and measured after the full charge, and the ratio of the sheet after cold pressing is made to obtain a series of expansion ratios, then a relational expression (the relational expression is a linear relational expression under the general condition) of the content of a silicon-based material and the expansion ratio is fitted, and then the full-electric expansion ratio of the sheet is extrapolated when the silicon-based material is only used as a negative electrode sheet active substance, namely the volume expansion coefficient A. The structure of the positive electrode sheet and the structure of the isolating film can be a positive electrode sheet structure and an isolating film structure which are known in the art and can be used for the secondary battery, and are not limited herein, and are not described herein.

In addition, in some cases, the volume expansion coefficient a of the silicon-based material may be obtained according to practical experience and practical conditions.

In the embodiment of the application, the B is the average value of the compaction density of graphite under different pressures. It will be appreciated that characterizing the hardness factor of graphite by using the average value of the compaction densities of graphite at different pressures is a more common characterization in the art, which is more consistent with the fact that the pole piece is actually being cold pressed, because, relatively speaking, larger powder compacts are easier to compress when the pole piece is rolled, and smaller powder compacts are harder to compress when the pole piece is rolled.

The range of B is 1.55-1.70, and further, the range of B can be 1.58-1.66. For example, the value of B may be any one point value or a range value between any two of 1.55, 1.57, 1.59, 1.60, 1.63, 1.65, 1.66, 1.68, 1.70. Thus, the battery can also maintain high cycle stability by enabling the value of B to meet the energy density requirements of the battery at the same time within the above range. The applicant found that if the value of B is too low, e.g. below 1.55, this means that the mosaic of the graphite starting material is relatively high, harder and the gram volume is lower, if the value of B is too high, e.g. above 1.70, this means that the fibrous component of the graphite starting material is relatively high, the graphitization degree is relatively high, softer and the gram volume is high, but on the other hand there is an upper limit to the value of B because the graphite and silicon-based materials are not too soft to effectively inhibit the volume expansion and contraction of silicon when they are mated.

In the embodiment of the application, the B is the average value of the compaction density of the graphite under different pressures, the unit is g/cm ³, and preferably, the number of values is more than or equal to 5 when the average value of the density is calculated, for example, the values can be 5, 6, 7, 8, 9, 10 and the like.

In the embodiment of the application, the pressure range is 1-5T, and further, the pressure range can be 2-4T. For example, the pressure may be 1T, 2T, 3T, 4T, 5T.

That is, in embodiments of the present application, the hardness factor of graphite is indirectly characterized by the powder compaction density of the graphite, and generally, the harder graphite compacts less and the softer graphite compacts more easily. For example, the hardness coefficient B of the graphite is obtained by testing the compaction density of the graphite under different pressures, and then taking an average value, wherein the value range of B is more than or equal to 1.55 and less than or equal to 1.7, and the different pressures can be 1T,2T,3T,4T and 5T. In addition, according to practical situations, other specific pressure values, such as 1t,1.5t,2t,2.5t,3t,4t, etc., may be used, which is not limited.

Alternatively, in some embodiments, the compaction density (ρ=m/v=m/pi r ² h) may be obtained by weighing a mass of powder, loading the graphite powder into a mold (e.g., 13mm diameter of the cylinder of loaded powder or other determined diameter value), monitoring the height and pressure values of the graphite column by two sensors, respectively, calculating the powder compaction density by the height h of the powder column (depressurization stage), and the procedure is as follows:

1) Initial control mode-displacement control (10 mm/min) final control mode-force control (10000N);

2) Initial control mode-force load (30 s);

3) Initial control mode-displacement control (30 mm/min) final control mode-force control (20N);

4) Initial control mode-force load (10 s);

when testing the compaction density under other pressures, the pressure value of step 1) is modified. As an example of the presence of a metal such as, the powder compaction densities of 2t,3t,4t,5t were repeated.

It should be appreciated that the specific testing methods and programming principles of compaction density described above are known to those skilled in the art and will not be described in detail with respect to the specific operation thereof.

In an embodiment of the present application, the silicon-based material includes, but is not limited to, at least one of ground silicon, gas phase silicon carbon, silicon oxide, pre-lithiated silicon oxide, or pre-magnesia silicon oxide.

The silicon-based material may be a micrometer-sized silicon material (micrometer silicon), or may be a nanometer-sized silicon material (nanometer silicon).

The above-mentioned silica is a conventional silica such as a conventional silica containing no other metal. The pre-lithiated silica is pre-lithiated silica and the pre-magnesia silica is pre-magnesia silica.

Optionally, in the embodiment of the present application, the ground silicon is nano silicon obtained by grinding, and a particle size range of the ground silicon (nano silicon) may be 50nm to 100nm.

In an embodiment of the present application, the graphite includes, but is not limited to, at least one of artificial graphite, natural graphite, modified graphite, quick charge graphite, or isostatic graphite.

The present invention is not limited to the shape of the graphite and silicon-based materials, and as an example, the shape of the graphite particles and silicon-based materials is selected from one or more of a block, a sphere-like shape, and a sphere shape, and the shape of the particles affects the compacted density.

Further, the shapes of the graphite and the silicon-based material can be spherical or spheroidic, so that the purpose of protecting the silicon particles can be better realized.

In the embodiment of the application, the mass ratio of the silicon-based material to the graphite is (25-40): (60-75), and as an example, the mass ratio of the silicon-based material to the graphite can be 25:75, 30:70, 32:68, 35:65, 38:62, 40:60 and the like. Therefore, in the process of volume expansion of the negative electrode, stress can be well released, the possibility of cracking of silicon particles is reduced, and the cycle performance of the battery is improved.

In an embodiment of the present application, the coating further includes a conductive agent. As non-limiting examples, the conductive agent includes, but is not limited to, at least two of single-walled carbon nanotubes, oligowalled carbon nanotubes, multiwalled carbon nanotubes, graphene, graphite alkyne, acetylene black, or ketjen black. Among them, the conductive carbon black may include acetylene black, ketjen black, and the like. As an example, the conductive agent includes at least one of single-walled carbon nanotubes, oligowalled carbon nanotubes, multiwall carbon nanotubes, and at least one of graphene, graphite alkyne, acetylene black, or ketjen black.

Among the above-mentioned several conductive agents, a dot-shaped conductive agent and a long Cheng Daodian agent are included, and preferably, the conductive agent includes both the dot-shaped conductive agent and the long Cheng Daodian agent. Therefore, the dot-shaped conductive agent can be better coated on the surface of the active material, and can be used cooperatively with the long Cheng Daodian agent of the carbon tube with better conductivity, so that the overall conductivity of the negative electrode plate can be better improved.

In an embodiment of the present application, the coating further includes a binder. As non-limiting examples, the binder includes at least two of polyacrylic acid, polyacrylonitrile, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, styrene-butadiene rubber, or modified styrene-butadiene polymer.

The adhesive can play a better role in bonding by combining two or more types of adhesives. For example, polyacrylic acid has better dynamic performance of the adhesive, but has insufficient flexibility, and the dynamic performance, flexibility and other performances of the adhesive can be better improved by cooperatively using the polyacrylic acid and the softer adhesive of styrene-butadiene rubber.

Alternatively, in the embodiment of the present application, the conductive agent may be used in any combination, and the binder may be used in any combination, without particular limitation. As an example, single-walled carbon nanotubes and oligowall carbon nanotubes are mixed in an arbitrary ratio as a conductive agent, polyacrylic acid and polyacrylonitrile are mixed in an arbitrary ratio as a binder, single-walled carbon nanotubes, oligowall carbon nanotubes and graphene are mixed in an arbitrary ratio as a conductive agent, polyacrylic acid and lithium carboxymethyl cellulose are mixed in an arbitrary ratio as a binder, and multiwall carbon nanotubes, graphene, graphite alkyne and acetylene black are mixed in an arbitrary ratio as a conductive agent, and lithium carboxymethyl cellulose, styrene-butadiene rubber and a modified styrene-butadiene polymer are mixed in an arbitrary ratio as a binder.

In the embodiment of the application, the mass ratio of the active substance to the conductive agent to the binder is (93-97): 0.5-2): 2-5, and the mass ratio of the active substance to the conductive agent to the binder is 93:2:5, 95:1:4, 93:1:5, 96:1:3, 97:0.5:2.5, and the like as an example.

In an embodiment of the present invention, the current collector includes, but is not limited to, at least one of copper foil, porous copper foil, nickel foam foil, copper foam foil, or carbon coated copper foil.

Therefore, based on the arrangement, the application improves the problem of poor cycling stability of the lithium ion battery containing more silicon cathodes (> 25%) by designing the cathode plate structure. When more silicon-based materials exist in the negative electrode plate, larger expansion force can be generated in the lithium intercalation process, surrounding graphite particles can be extruded by the expansion force, and graphite can be repeatedly extruded by the expansion pressure in the continuous charge and discharge process, so that the graphite particles are peeled off and can not reversibly deintercalate lithium, and the deterioration of the circulation stability is brought. According to the application, according to the characteristics that different silicon-based materials have different expansion forces and the hardness of graphite has large and small, the expansion proportion of the silicon-based materials and the hardness of the graphite are measured, and the expansion coefficient and the hardness coefficient of the volume are quantized, so that the silicon-based materials meet a certain functional relation. The silicon-based material with larger expansion force is matched with harder graphite, so that the harder graphite can bear larger expansion force and generate smaller deformation, and delamination among graphite particles is avoided. The silicon-based material with relatively smaller expansion force is matched with the relatively softer graphite, the expansion force of the softer graphite is relatively smaller, and delamination among graphite particles can be avoided.

In the embodiment of the application, the preparation method of the negative electrode plate is a common method in the field, and the specific operation mode is not limited.

As an example, the present application provides a method for preparing a negative electrode tab, the method comprising the steps of:

Mixing an active material, a binder, a conductive agent and water to obtain a negative electrode slurry;

And (3) coating the surface of the current collector with negative electrode slurry to obtain a negative electrode plate.

It should be understood that the preparation method of the negative electrode plate and the foregoing negative electrode plate are based on the same inventive concept, and all features and advantages described in the foregoing for the negative electrode plate are equally applicable to the preparation method of the composite negative electrode plate, and are not described in detail herein.

In the embodiment of the application, the solid content of the anode slurry is 47-55%, and as an example, the solid content of the anode slurry can be 47%, 48%, 50%, 52%, 55%, and the like.

In the embodiment of the present invention, the coating is performed on at least one side surface of the current collector in the thickness direction, and as an example, the same amount of coating is performed on both side surfaces of the current collector in the thickness direction.

In the embodiment of the invention, the single-sided coating weight of the negative electrode plate is 80-150 mg, and the single-sided coating weight can be 80mg, 90mg, 100mg, 120mg, 150mg and the like as an example.

In the embodiment of the application, the compaction density of the negative electrode plate is 1.5-1.7 g/cm ³, and as an example, the compaction density of the negative electrode plate can be 1.5g/cm ³、1.55g/cm³、1.60g/cm³、1.65g/cm³、1.70g/cm³ and the like. In this range, the prepared negative electrode sheet has good porosity, and after being used for preparing a battery, the battery has excellent cycle performance and rate discharge performance.

In some embodiments, a battery is provided that includes a positive electrode tab, a negative electrode tab, an electrolyte, and a separator. Alternatively, the battery may be a secondary battery, which may be a lithium ion battery.

The battery is mainly characterized by comprising any negative electrode plate. Due to the use of the negative electrode plate, the problem of cyclic degradation of graphite and silicon-based materials when used together can be solved by a simpler method on the premise of not losing energy density and gram capacity, so that the cycle service life of the battery is prolonged.

In the embodiment of the application, the structure of the positive electrode plate and the structure of the isolating film can be a positive electrode plate structure and an isolating film structure which are known in the art and can be used for a secondary battery, and the structure is not limited and is not repeated herein.

Examples

Hereinafter, embodiments of the present application will be described. The embodiments described below are exemplary only for explaining the present application and are not to be construed as limiting the present application. The embodiments are not to be construed as limited to the particular techniques or conditions disclosed in the literature or as per the specifications of the product. The reagents, materials or apparatus used were conventional products commercially available without the manufacturer's knowledge.

Example 1

(1) Preparation of negative electrode slurry

Pre-stirring the active substances and the conductive carbon black according to the proportion, adding 1/2 part of polyacrylic acid and deionized water for stirring, and adding the single-wall carbon nano tube, the rest 1/2 part of polyacrylic acid, styrene-butadiene rubber and deionized water for stirring to ensure that the solid content of the obtained negative electrode slurry is about 51%;

wherein the mass ratio of the active substance to the styrene-butadiene rubber to the polyacrylic acid to the single-walled carbon nanotube to the conductive carbon black is 94.3:1.4:2.8:0.06:1.44;

The active material consists of pre-lithium silicon oxide and artificial graphite, and the pre-lithium silicon oxide and the artificial graphite are prepared into a composite material with the gram capacity of 650 mAh/g according to the respective capacity calculation (the mass ratio of the pre-lithium silicon oxide to the artificial graphite is 30:70);

The volume expansion coefficient A of the pre-lithium silicon oxide is 1.8, the hardness coefficient B of the artificial graphite is 1.58, the unit is g/cm ³ (the same applies below), and A is B/0.15=18.96.

(2) Preparation of negative electrode plate

And (3) coating the negative electrode slurry on a copper foil, wherein the single-sided coating weight is 102.7mg, performing back coating after preliminary baking in an oven, enabling the back coating weight to be consistent with the front surface, performing secondary baking, and performing cold pressing (compaction density is 1.6 g/cm ³), thereby obtaining the negative electrode plate.

Example 2

Example 2 differs from example 1 in that the active substance used consists of gaseous silicon carbon and artificial graphite, and the gaseous silicon carbon and artificial graphite are formulated as a composite material having a gram capacity of 650 mAh/g, calculated on the respective capacities;

Wherein the volume expansion coefficient A of the gas phase silicon carbon is 1.5, the hardness coefficient B of the artificial graphite is 1.67, and A is B/0.15=16.70.

The remainder was the same as in example 1.

Example 3

Example 3 differs from example 1 in that,

The volume expansion coefficient A of the pre-lithium silicon oxide is 1.65, the hardness coefficient B of the artificial graphite is 1.62, and A is B/0.15=17.82.

The remainder was the same as in example 1.

Example 4

Example 4 differs from example 1 in that,

The silicon-based material adopts gas-phase silicon carbon, the volume expansion coefficient A of the gas-phase silicon is 1.5, the hardness coefficient B of the artificial graphite is 1.6, and the ratio A of the artificial graphite to the artificial graphite is 0.15=16.00.

The remainder was the same as in example 1.

Example 5

Example 5 differs from example 1 in that,

The volume expansion coefficient A of the pre-lithium silicon oxide is 2.0, the hardness coefficient B of the artificial graphite is 1.58, and A is B/0.15=21.07.

The remainder was the same as in example 1.

Example 6

Example 6 differs from example 1 in that the active substance used consists of ground silicon and natural graphite, and the ground silicon and natural graphite are formulated as a composite material with a gram capacity of 650 mAh/g, calculated on the respective capacities;

Wherein the volume expansion coefficient A of the ground silicon is 1.8, the hardness coefficient B of the natural graphite is 1.58, and A is B/0.15=18.96.

Example 7

Example 7 differs from example 1 in that the active material used consists of silica and modified graphite, and that the silica and the modified graphite are formulated as a composite material having a gram capacity of 650 mAh/g, calculated on the respective capacities;

wherein the volume expansion coefficient A of the silicon oxide is 1.8, the hardness coefficient B of the modified graphite is 1.58, and A is B/0.15=18.96.

Example 8

Example 8 differs from example 1 in that,

The pre-lithium silica and the artificial graphite are prepared into a composite material with the gram capacity of 750mAh/g according to the respective capacity calculation (the mass ratio of the pre-lithium silica to the artificial graphite is 40:60), and the mass ratio of the active substance, the styrene-butadiene rubber, the polyacrylic acid, the single-walled carbon nano tube and the conductive carbon black is 93.9:1.4:3.2:0.06:1.44.

Example 9

Example 9 differs from example 1 in that,

The pre-lithium silica and the artificial graphite are prepared into a composite material with the gram capacity of 550mAh/g according to the respective capacity calculation (the mass ratio of the pre-lithium silica to the artificial graphite is 20.5:79.5), and the mass ratio of the active substance, the styrene-butadiene rubber, the polyacrylic acid, the single-walled carbon nano-silica and the conductive carbon black is 94.7:1.2:2.6:0.06:1.44.

Example 10

Example 10 differs from example 1 in that,

Pre-stirring an active substance and conductive carbon black, adding 1/2 part of sodium carboxymethyl cellulose and deionized water, stirring, and adding the multiwall carbon nanotube, the rest 1/2 part of sodium carboxymethyl cellulose, styrene-butadiene rubber and deionized water, stirring to obtain a negative electrode slurry with a solid content of about 51%;

Wherein the mass ratio of the active substance to the styrene-butadiene rubber to the sodium carboxymethyl cellulose to the multi-wall carbon nano tube to the conductive carbon black is 94.3:1.4:2.8:0.06:1.44.

The remainder was the same as in example 1.

Comparative example 1

Comparative example 1 differs from example 1 in that,

The volume expansion coefficient A of the pre-lithium silicon oxide is 1.9, the hardness coefficient B of the artificial graphite is 1.7, and A is B/0.15=21.53.

The remainder was the same as in example 1.

Comparative example 2

Comparative example 2 differs from example 2 in that,

The volume expansion coefficient A of the gas phase silicon carbon is 1.5, the hardness coefficient B of the artificial graphite is 1.55, and A is equal to or smaller than 0.15=15.50.

The remainder was the same as in example 2.

Comparative example 3

Comparative example 3 differs from example 2 in that,

The silicon-based material adopts gas-phase silicon carbon, the volume expansion coefficient A of the gas-phase silicon carbon is 1.4, the hardness coefficient B of the artificial graphite is 1.5, and A is B/0.15=14.00.

The remainder was the same as in example 2.

Comparative example 4

Comparative example 1 differs from example 1 in that,

The volume expansion coefficient A of the pre-lithium silicon oxide is 2.1, the hardness coefficient B of the artificial graphite is 1.8, and A is B/0.15=25.20.

The remainder was the same as in example 1.

Performance testing

1. Preparation of a lithium battery:

(1) The preparation of the positive plate comprises the steps of homogenizing a mixture of eight-system nickel cobalt manganese material, PVDF, conductive carbon black and carbon nano tubes in a mass ratio of 0.978:0.009:0.008:0.005 to obtain positive electrode slurry, coating the positive electrode slurry on aluminum foil, coating the aluminum foil with the surface density of 0.3 g/m ², coating the aluminum foil on both sides, drying, rolling the aluminum foil to obtain the positive plate, and selecting the compacted density of 3.4 g/cm ³.

(2) Preparation of a release film A release film comprising a polypropylene-based film and a coating layer of aluminum oxide was used, wherein the thickness of the polypropylene-based film was 12 μm and the thickness of the coating layer was 3. Mu.m.

(3) The preparation of the electrolyte comprises the steps of mixing Ethylene Carbonate (EC), diethyl carbonate (DEC) and methyl ethyl carbonate (EMC) according to a volume ratio of 1:1:1, dissolving fully dried lithium hexafluorophosphate into a mixed organic solvent according to a ratio of 1.2 mol/L, and adding ethylene carbonate to prepare the electrolyte.

(4) And respectively stacking and winding the negative electrode pieces prepared in each example and the comparative example with the isolating film and the positive electrode piece in sequence to obtain a bare cell and an ultrasonic welding tab, putting the bare cell into a battery shell, drying to remove moisture, injecting electrolyte into the battery shell, packaging, and forming the components to obtain the lithium secondary battery.

2. Electrochemical performance testing of the cells:

(1) The first discharge capacity and the first charge capacity are that when the formation charge is carried out, the constant current is firstly carried out at 0.02C to 3.4V, then the constant current is carried out at 0.1C to 3.75V, the constant current is firstly carried out at 0.5C to 4.25V when the separation is carried out, then the constant voltage is carried out until the current is reduced to 0.05C, then the constant current is firstly carried out at 1C, then the constant current is carried out at 0.2C to 2.5V, and the charge-discharge gram capacity is respectively obtained according to the division of the charge-discharge capacity by the mass of active substances.

(2) First coulombic efficiency test first coulombic efficiency = first discharge capacity/first charge capacity 100%.

The test results are shown in Table 1.

Table 1 gram capacity and first coulombic efficiency for each of the examples and comparative examples

As can be seen from table 1, the first charge capacities of the examples and comparative examples are relatively close, with the main differences being the first discharge capacity and the first coulombic efficiency. Examples 1-3 exhibited near reversible capacity and first coulombic efficiency due to reasonable collocation of silicon material and graphite. The first effect of example 4 is slightly lower and the first effect of example 5 is slightly higher, mainly because the harder graphite is generally the lower the first effect, the larger the silica domains the higher the first effect. Example 7 has the lowest discharge capacity and initial efficiency because its silicon negative electrode material is silicon oxide without pre-lithium. Examples 1, 8 and 9 negative electrode gram capacities were designed to be 650mAh/g, 750mAh/g and 550mAh/g, respectively, and it can be seen from the data in table 1 that the higher the gram capacity, the lower the initial efficiency and the lower the reversible capacity. The reversible capacity and first coulombic efficiency of comparative example 1 are slightly higher than those of examples 1-3, mainly because the volume expansion coefficient of the matched pre-lithium silica is higher, the internal silicon domain is larger, the first effect is higher, the hardness coefficient of graphite is also higher, and less irreversible capacity consumption is brought. In contrast, comparative example 2 has lower discharge capacity and first coulombic efficiency because the matched silicon material is gas phase silicon carbon, lower volume expansion coefficient brings lower first effect, and the hardness coefficient of matched graphite is lower, the graphitization degree is lower, and the first coulombic efficiency is also reduced. Comparative example 3 has slightly lower initial and reversible capacity than comparative example 2, mainly because of its lower coefficient of expansion and harder graphite in the gas phase silicon carbon, which causes difficulty in rolling the pole pieces, crushing the particles, and thus losing gram capacity. Comparative example 4 is similar to comparative example 1 and comparative example 1 is similar to example 1.

(3) The cycle performance test is that under the normal temperature of 25 ℃, the initial and cut-off voltages are 2.8V, 4.25V,1C is charged to 4.25V, then 4.25V is charged at constant voltage until the current is reduced to 0.05C, 1C is discharged to 2.8V, the ratio of the discharge capacity and the initial discharge capacity of each circle is recorded, and the cycle number under different capacity retention rates (such as 90%, 85% and 80%) is recorded.

The test results are shown in Table 2.

Table 2 electrical performance of each of the examples and comparative examples

As can be seen from table 2, the different silicon-based materials and graphite were greatly different in collocation, examples 1,3,5, 8, 9,10, 1,4 were pre-lithiated silica, 2,4, 2 and 3 were gas-phase silicon-carbon, and in addition, nano-silicon-carbon was used in example 6 and non-pre-lithiated silica was used in example 7. In contrast, from the point of view of cycle stability, the gas phase silicon carbon is larger overall than the non-pre-lithiated silicon oxide is larger than the nano-silicon material. In classification, the cycle stability of examples 1,3,5, 8, 9 and 10 is far better than that of comparative examples 1 and 4, which is mainly benefited by reasonable collocation of pre-lithiated silica and graphite, and the collocation of pre-lithiated silica with larger volume expansion coefficient and harder artificial graphite can well buffer the extrusion of silica on graphite particles in the continuous cycle process, so that graphite is not cracked and active capacity is not lost, and the cycle stability is remarkably improved. Of these, example 9 had the best cycle stability, with its lowest gram capacity, and example 8 had the worst cycle stability, with its highest gram capacity, and its gram capacity, with its 750mAh/g. The cycling stability of example 7 is greater than that of example 1, mainly because the silicon negative electrode material is non-pre-lithiated silica, and the electrochemical agglomeration of silicon domains during cycling is less, thus improving the cycling stability to a certain extent.

In addition, the cycle stability of example 2 is superior to example 4 and both are superior to comparative example 2, and the cycle stability of comparative example 2 is superior to comparative example 3, because it is consistent that when the volume expansion coefficient of the vapor phase silicon carbon material is too low, the compaction density using graphite is too low, meaning that both types of materials are hard, and are not well pressed when the pole pieces are rolled, the active substances therein are easily crushed, thereby causing a decrease in the cycle stability.

The invention is not described in detail in a manner known to those skilled in the art.

The basic principles of the present invention have been described above in connection with specific embodiments, but it should be noted that the advantages, benefits, effects, etc. mentioned in the present invention are merely examples and not intended to be limiting, and these advantages, benefits, effects, etc. are not to be construed as necessarily possessed by the various embodiments of the invention. Furthermore, the specific details disclosed herein are for purposes of illustration and understanding only, and are not intended to be limiting, as the invention is not necessarily limited to practice with the above described specific details.

It should be noted that the term "and/or"/"as used herein is merely an association relation describing the association object, and indicates that three kinds of relations may exist, for example, a and/or B, and that three kinds of cases where a exists alone, while a and B exist alone, exist alone. As used in this application and the appended claims, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if item A, B is listed, the phrase "at least one of A, B" means only A, only B, or A and B. In another example, if item A, B, C is listed, then the phrase "at least one of A, B, C" means all of A alone, or B alone, C alone, A and B (excluding C), A and C (excluding B), B and C (excluding A), or A, B and C. Item a may comprise a single element or multiple elements. Item B may comprise a single element or multiple elements. Item C may comprise a single element or multiple elements.

It should be noted that the above-mentioned embodiments are merely for illustrating the technical solution of the present invention, and not for limiting the same, and although the present invention has been described in detail with reference to the above-mentioned embodiments, it should be understood by those skilled in the art that the technical solution described in the above-mentioned embodiments may be modified or some technical features may be equivalently replaced, and these modifications or substitutions do not make the essence of the corresponding technical solution deviate from the spirit and scope of the technical solution of the embodiments of the present invention.

Claims

1. A negative electrode plate, characterized in that it comprises a current collector and a coating disposed on at least one side of the current collector along the thickness direction;

The coating includes an active substance;

The active material comprises graphite and silicon-based material, and the mass ratio of the silicon-based material to graphite is (25-40): (60-75);

The volume expansion coefficient of the silicon-based material is A, the hardness coefficient of the graphite is B, and the unit of B is g/cm ³ ;

A and B satisfy:

16≤A*B/0.15≤21;

A is the full charge expansion ratio of the negative electrode when a silicon-based material is used as the active material, and the range of A is 1.5 to 2.0;

B is the average value of the compaction density of graphite under different pressures, and the range of B is 1.55~1.70; wherein the range of the pressure is 1~5T.

2. The negative electrode plate according to claim 1 is characterized in that the silicon-based material includes at least one of ground silicon, gas-phase silicon carbon, silicon oxide, pre-lithium silicon oxide or pre-magnesium silicon oxide.

3 . The negative electrode plate according to claim 1 , wherein the graphite comprises at least one of artificial graphite or natural graphite.

4. The negative electrode sheet according to any one of claims 1 to 3, characterized in that the coating further comprises a conductive agent and a binder;

The binder comprises at least two of polyacrylic acid, polyacrylonitrile, sodium carboxymethyl cellulose, lithium carboxymethyl cellulose, styrene-butadiene rubber or modified styrene-butadiene rubber;

And/or, the conductive agent includes at least two of single-walled carbon nanotubes, few-walled carbon nanotubes, multi-walled carbon nanotubes, graphene, graphyne, acetylene black or Ketjen black.

5. The negative electrode sheet according to claim 4, characterized in that the mass ratio of the active material, the binder and the conductive agent is (93~97): (2~5): (0.5~2).

6. The negative electrode sheet according to any one of claims 1 to 3, characterized in that the current collector comprises at least one of copper foil, red copper foil or foam nickel foil.

7. A battery comprising a negative electrode plate, characterized in that the negative electrode plate comprises the negative electrode plate according to any one of claims 1 to 6.