CN110148708B - Negative plate and lithium ion battery - Google Patents
Negative plate and lithium ion battery Download PDFInfo
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- CN110148708B CN110148708B CN201910464690.9A CN201910464690A CN110148708B CN 110148708 B CN110148708 B CN 110148708B CN 201910464690 A CN201910464690 A CN 201910464690A CN 110148708 B CN110148708 B CN 110148708B
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M10/00—Secondary cells; Manufacture thereof
- H01M10/05—Accumulators with non-aqueous electrolyte
- H01M10/052—Li-accumulators
- H01M10/0525—Rocking-chair batteries, i.e. batteries with lithium insertion or intercalation in both electrodes; Lithium-ion batteries
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- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/133—Electrodes based on carbonaceous material, e.g. graphite-intercalation compounds or CFx
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- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/134—Electrodes based on metals, Si or alloys
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- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/362—Composites
- H01M4/366—Composites as layered products
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- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/38—Selection of substances as active materials, active masses, active liquids of elements or alloys
- H01M4/386—Silicon or alloys based on silicon
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M4/36—Selection of substances as active materials, active masses, active liquids
- H01M4/58—Selection of substances as active materials, active masses, active liquids of inorganic compounds other than oxides or hydroxides, e.g. sulfides, selenides, tellurides, halogenides or LiCoFy; of polyanionic structures, e.g. phosphates, silicates or borates
- H01M4/583—Carbonaceous material, e.g. graphite-intercalation compounds or CFx
- H01M4/587—Carbonaceous material, e.g. graphite-intercalation compounds or CFx for inserting or intercalating light metals
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- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
- H01M4/00—Electrodes
- H01M4/02—Electrodes composed of, or comprising, active material
- H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
- H01M2004/027—Negative electrodes
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/10—Energy storage using batteries
Abstract
A negative plate and a lithium ion battery belong to the technical field of lithium ion batteries. The negative plate comprises a negative current collector and a film layer coated on the negative current collector; the film layer is divided into two layers, and the first coating film layer is a bottom graphite coating layer close to the negative current collector; the second coating layer is a top silicon-containing coating layer far away from the copper foil; the preparation method is simple and convenient in process, the prepared lithium ion battery has the advantages of high energy density and good cycle performance, and can avoid lithium precipitation on the surface of the negative pole piece under the condition of rapid charging with larger multiplying power, thereby meeting the industrial application requirements.
Description
Technical Field
The invention belongs to the technical field of lithium ion batteries, and particularly relates to a negative plate and a lithium ion battery.
Background
In recent years, with the continuous expansion of the industrial scale of the lithium ion battery and the continuous development of the related technology, the lithium ion battery has become an energy storage device of mainstream electronic products, and the application of the lithium ion battery is greatly expanded. Meanwhile, the performance requirements of people on the lithium ion battery are further improved, and the lithium ion battery is required to have high energy density and long cycle life.
One method for improving the energy density of the negative electrode at present is to adopt technologies such as large and small particle matching and the like to improve the compaction of the negative electrode, but the high compaction can cause low porosity of a pole piece and low liquid retention of the battery, thereby causing the problem of water jumping at the later stage of the battery cycle; the other is to increase the gram capacity of the cathode, and the silicon has the theoretical capacity as high as 4200mAh/g and is abundant in nature, so the silicon can be used as one of the materials with great potential for replacing graphite cathode materials. However, the silicon-based negative electrode material has great volume change in the process of lithium ion intercalation and deintercalation, and the electrode material is likely to be broken due to internal stress generated by volume effect, so that the electric contact with a current collector is lost, and the problem of poor cycle performance is caused.
Disclosure of Invention
The invention aims to solve the problem of poor cycle performance caused by the volume effect of a silicon-based negative electrode, and provides a negative electrode plate and a lithium ion battery.
In order to achieve the purpose, the technical scheme adopted by the invention is as follows:
a negative plate comprises a negative current collector and a film coated on the current collector, wherein the film is divided into two layers, and the first film is a bottom graphite coating close to the current collector; the second film layer is a top silicon-containing coating layer far away from the current collector; wherein the proportion of silicon material in the top layer silicon-containing coating is 5-30 wt.%; the silicon material in the negative plate accounts for 5-20% of the total weight of the active substances of the negative plate; after coating and drying, the thickness of the bottom graphite coating is 44-85 um, and the thickness of the top silicon-containing coating is 20-48 um.
Further, a double-die coating head extrusion type coating machine is adopted to simultaneously coat 2 kinds of slurry on a negative current collector, and after a double-layer paste coating structure is formed on the same side of the negative current collector, a double-layer coating negative plate with a silicon-containing coating layer on the top layer and a silicon-free pure graphite layer on the bottom layer is prepared; and after one surface of the current collector is coated, repeatedly coating the other surface according to the above mode.
The lithium ion battery containing the negative plate comprises a positive plate, a negative plate, a diaphragm and electrolyte.
Compared with the prior art, the invention has the beneficial effects that:
(1) the negative pole piece prepared by the invention utilizes the larger volume expansion of the silicon material in the silicon-containing coating at the top layer in the lithium embedding process, so that the porosity of the negative pole piece is improved, the pole piece can be ensured to have higher liquid retention capacity under a higher compaction state, and better ionic conductivity can be obtained; because the added silicon material is subjected to surface special treatment and the mixing amount is optimized, the bottom layer coating and the top layer silicon-containing coating are simultaneously coated and rolled, the coating-to-mortise work effect is stronger than that of a separately coated and dried double-layer pole piece, and the separation phenomenon between coatings can not occur, so that the integral electronic conductivity of the pole piece material can be ensured. The first reversible specific capacity of the prepared negative plate is 380-596 mAh/g, and the negative plate has high compaction and high capacity performance, so that the energy density of the negative electrode is improved.
(2) The lithium ion battery has better energy density and cycle performance, and can meet the requirements of industrial application.
(3) After the negative pole piece is used for the lithium ion battery, lithium precipitation on the surface of the negative pole piece can be avoided under the condition of high-rate quick charge, and the safety and the applicability are better.
Detailed Description
In order that the manner in which the above recited and other objects, features and advantages of the present invention are obtained will be readily apparent, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention. The reagents, materials and instruments used in the following description are all conventional reagents, conventional materials and conventional instruments, which are commercially available, and the reagents may be synthesized by a conventional synthesis method, if not specifically described.
The first embodiment is as follows: the embodiment describes a negative plate, which comprises a negative current collector and a film layer coated on the current collector, wherein the film layer is divided into two layers, the first film layer is a bottom graphite coating layer close to the current collector, and the coating has the performances of large compaction, small expansion of the coating layer after charging and poor liquid storage capacity; the second film layer is a top layer silicon-containing coating far away from the current collector, and the coating has the advantages of high gram volume, large expansion of the charged coating and strong liquid storage capacity; wherein the proportion of silicon material in the top layer silicon-containing coating is 5-30 wt.%; the silicon material in the negative plate accounts for 5-20% of the total weight of the active substances of the negative plate; after coating and drying, the thickness of the bottom graphite coating is 44-85 um, and the thickness of the top silicon-containing coating is 20-48 um.
The second embodiment is as follows: in the negative electrode sheet of the first embodiment of the present invention, a dual-die coating head extrusion coater is adopted to simultaneously coat 2 kinds of slurry on a negative electrode current collector, and after a double-layer paste coating structure is formed on the same side of the negative electrode current collector, a double-layer coated negative electrode sheet is prepared, in which a silicon-containing coating layer is on the top layer and a silicon-free pure graphite layer is on the bottom layer; and after one surface of the current collector is coated, repeatedly coating the other surface according to the above mode.
The third concrete implementation mode: in the negative electrode sheet of the first embodiment, the negative electrode current collector is one of a homogeneous copper foil, a porous copper foil and a copper foil with a carbon coating.
The fourth concrete implementation mode: in the negative electrode sheet according to the first embodiment, both the two film layers are made of a composite material containing a negative electrode active material, a conductive agent, a binder and a thickener; if the coating is a bottom graphite coating, the weight ratio of dry powder containing the following materials is that of the negative active material: conductive agent: adhesive: thickener = 95-98%: 0-2%: 1-1.5%: 1-1.5%, and the negative active material is graphite; if the coating layer contains silicon on the top layer, the ratio of the materials contained in the coating layer is that of the anode active material: conductive agent: adhesive: the thickening agent is 92.5-97%, 1-3%, 1.5-3%, 0.5-1.5%, and the negative active material is graphite and silicon material. The graphite of the bottom layer and the graphite of the top layer of the negative active material are both high-compaction graphite containing secondary particles, and the compaction density is 1.78g/cm2. The secondary particles are particles obtained by adopting small-particle-size natural graphite, petroleum coke, needle coke, asphalt coke and the like as base materials, adding a coating material and an additive, and performing secondary granulation on the small-particle-size base materials by controlling the material proportion, the temperature rise curve and the stirring speed under the high-temperature stirring condition. Compared with the product with the same granularity, the product of secondary granulation can effectively improve the liquid retention performance of the material and reduce the expansion rate of the materialShrinkage holes exist between the small particles), and the diffusion path of lithium ions is shortened (the lithium ions are shorter in the extraction/insertion path from the small particles), so that the lithium ions can be smoothly extracted/inserted from the negative electrode material under high voltage, and meanwhile, the low-temperature performance and the cycle life of the material can be improved. Graphite is made by the materials manufacturer and contains secondary particles and small, single particles.
The fifth concrete implementation mode: in the negative electrode sheet according to the fourth embodiment, the silicon material is a silicon alloy containing a special high-molecular conductive polymer on the surface, a composite material of silicon and carbon, and SiOxWherein x is more than 0.5 and less than 2, and the high molecular conductive polymer is one or a mixture of more of polyaniline, polythiophene and polypyrrole.
The sixth specific implementation mode: in the negative electrode sheet of the fourth embodiment, the conductive agent is one or a mixture of more of conductive carbon black Super P, vapor grown carbon fiber VGCF, graphene, or carbon nanotube CNTs.
The seventh embodiment: the negative electrode sheet of the fourth embodiment, wherein the binder is an aqueous binder, and the aqueous binder is one or a mixture of Styrene Butadiene Rubber (SBR), nitrile butadiene rubber, modified styrene butadiene rubber, sodium polyacrylate (PAA-Na), aqueous polyacrylonitrile copolymer and polyacrylate; the thickening agent is sodium carboxymethyl cellulose CMC and/or polyether modified organic silicon polymer with thickening effect.
The specific implementation mode is eight: a lithium ion battery comprising the negative electrode sheet of any one of the first to seventh embodiments, wherein the lithium ion battery comprises a positive electrode sheet, a negative electrode sheet, a separator and an electrolyte.
Example 1
(1) Preparing a positive plate:
LiCoO as positive electrode active material2Dissolving PVDF (polyvinylidene fluoride) as a binder and conductive black (Super P) in NMP (N-methyl pyrrolidone) according to a mass ratio of 97:1.5:1.5, uniformly stirring to prepare slurry, uniformly coating the slurry on an aluminum foil of a positive current collector, drying, rolling to prepare the positive electrode of the lithium ion batteryPole pieces; the positive electrode active material can also be a commercially available positive electrode material such as an NCM ternary material (532/622/811 and the like) or a lithium iron phosphate material; the drying and rolling are conventional steps in the art. The concentration of the positive electrode slurry and the coating thickness may be determined by using conventional parameters known in the art.
(2) Preparation of conductive polymer coated silicon material:
dispersing 10g of silicon material into a lauryl sodium sulfate solution with the concentration of 2000mL and 0.15g/L, and carrying out ultrasonic treatment for 10 min; then 100mmol of aniline monomer is dripped, then ammonium persulfate 22.82g is added as an initiator, the mixture is stirred for 3 hours in an ice water mixed bath, deionized water and ethanol are used for washing, the obtained product is placed in a vacuum drying oven, and vacuum drying is carried out for 24 hours at the temperature of 60 ℃, so as to obtain the high molecular conductive polymer polyaniline-coated silicon material; the silicon material can be silicon alloy, silicon/carbon composite material and SiOxWherein 0.5 < x < 2. Besides polyaniline, the conductive polymer can also be one or a mixture of more of polythiophene, polypyrrole and the like.
(3) Preparing a double-layer film negative plate:
(a) preparation of primer coating slurry
Mixing a negative electrode active material 1, a conductive agent, a thickening agent and a binder according to a ratio of 97: 0.5: 1.5:1, the conductive agent of the embodiment is conductive carbon black (Super P), the thickening agent is sodium carboxymethylcellulose (CMC), the binder is water emulsion type Styrene Butadiene Rubber (SBR), the negative active material 1 is artificial graphite, and composite particle graphite with secondary particles and single particles matched is selected as a negative active material, wherein the weight percentage ratio of the single particles in the composite particle graphite is 20%, the average particle diameter D50 of the single particles is 7-10 um, the average particle diameter D50 of the secondary particles is 14-17 um, and the average particle diameter D50 of the composite particle graphite is 13-16 um; the composite particle graphite is compacted to 1.78g/cm2；
Adding deionized water into a double-planet stirrer, adding a conductive agent in the stirring process, dispersing for 20min, adding 0.4% sodium carboxymethylcellulose, and stirring for 30 min; after uniform mixing, adding active material 1 artificial graphite, performing thick stirring for 2 hours to complete the dispersion process of the conductive carbon black and the active material 1 artificial graphite, adding the rest sodium carboxymethylcellulose, stirring for 1 hour, adjusting the viscosity, adding the SBR emulsion, stirring for 30 minutes, finally vacuumizing to remove bubbles, and filtering by using a 150-mesh stainless steel screen mesh to obtain the bottom coating slurry with the solid content of 43%.
(b) Preparation of Top coating slip
Mixing a negative electrode active material 2, a conductive agent, a thickening agent and a binder according to a ratio of 95: 2: 2.2: the negative electrode slurry was prepared at a mass ratio of 0.8, which is a dry material mass ratio, and the negative electrode active material 2 of this example was a mixture of artificial graphite and a silicon material in a mass percentage of 90%: 10 percent, the conductive agent is a mixture of conductive carbon black (Super P) and Carbon Nanotubes (CNTs), and the mass ratio of the conductive carbon black to the carbon nanotubes is 2: 1, thickening agent is sodium carboxymethylcellulose (CMC), and binder is aqueous polyacrylonitrile copolymer; the silicon material of the embodiment is the polyaniline-coated silicon oxide prepared in the step (2), and the artificial graphite selects graphite with a single-particle component removed from the composite-particle graphite, because the general particle diameter ratio of the silicon material is smaller, and the loss of energy density due to the reduction of top layer compaction can be relieved by removing the single particles from the composite-particle graphite;
adding deionized water into a double-planet stirrer, adding a conductive agent in the stirring process, dispersing for 20min, adding 0.4% sodium carboxymethylcellulose, and stirring for 30 min; after uniform mixing, adding a mixture of the artificial graphite and the silicon material of the active material 2, performing thick stirring for 2 hours to complete the dispersion process of the conductive carbon black and the active material 2, adding the rest sodium carboxymethylcellulose, stirring for 1 hour, adjusting the viscosity, adding the polybenzonitrile binder, stirring for 30 minutes, finally vacuumizing to remove bubbles, and filtering by using a 150-mesh stainless steel screen to obtain the top coating slurry with the solid content of 41%.
(c) Preparation of double-layer film negative plate
The coating machine with the double-mold coating head device purchased from an equipment manufacturer is used, two discharge ports of the double-mold coating head point to the coating roller of the coating machine, a feeding channel respectively communicated with the two discharge ports is arranged in the double-mold coating head, two different sizing agents are simultaneously extruded by a high-precision screw pump, and double-layer simultaneous coating of pole pieces can be realized.
The 2 prepared slurries were applied simultaneously to the same side of a copper foil, the slurries fed a silicon-containing coating on the top layer and a silicon-free pure graphite layer on the bottom layer, with the centerline of the coated areal density of the bottom layer being 0.00512g/cm2The center line of the top coating surface density is 0.00394g/cm2The coating tolerance is controlled according to +/-1.5 wt%, the coating speed is 5m/min, drying is carried out by 5 sections of ovens after coating, the temperature of each section of oven is 60 ℃, 80 ℃, 110 ℃ and 100 ℃, the thickness of the top coating on one side of the copper foil of the pole piece after drying is 44 +/-3 um, and the thickness of the bottom coating is 56 +/-3 um; repeatedly coating to complete the double-layer film layer on the other side of the copper foil; the pressure treatment is carried out by a roller press, so that the compaction density of the negative plate is 1.75g/cm2Thus completing the preparation of the double-layer film negative plate; the silicon monoxide in the present example accounts for 5% of the total active material weight of the negative plate. The negative copper foil can be one of homogeneous copper foil, porous copper foil or copper foil with a carbon coating. The conductive agent is one or a mixture of more of conductive carbon black Super P, vapor-phase-generated carbon fiber VGCF, graphene or carbon nanotube CNTs. The binder is water-based binder, and specifically is one or a mixture of Styrene Butadiene Rubber (SBR), nitrile rubber, butadiene rubber, modified styrene butadiene rubber, sodium polyacrylate (PAA-Na), water-based polyacrylonitrile copolymer or polyacrylate; the thickening agent is one or a mixture of more of sodium carboxymethyl cellulose (CMC), polyether modified organic silicon polymer and the like with thickening effect.
(3) Preparing a lithium ion battery:
selecting a base material, single-sided ceramic and double-sided glued composite diaphragm for the diaphragms between the prepared positive plate and the prepared negative plate, and then winding by using a winding machine to prepare a winding core of a winding structure with the positive plate wrapped externally, packaging by adopting an aluminum-plastic film, baking for 48 hours in a vacuum state to remove moisture, injecting electrolyte, and forming and sorting the battery to obtain the square soft package lithium ion battery, wherein the label is C1. The electrolyte is prepared by adopting a conventional electrolyte formula: LiPF6+ solvent (EC + FEC + PC + DEC + SN + PS).
Example 2
This example differs from example 1 in that: the proportion of the silicon oxide in the negative electrode active material 2 was 20%; the coating surface density of the bottom layer of the double-layer coating is 0.00768 g/cm2The top coating surface density is 0.00158g/cm2The solid content of the top layer slurry is 38%, the thickness of the top layer coating on one side of the copper foil of the dried pole piece is 24 +/-3 um, and the thickness of the bottom layer coating is 74 +/-3 um; a lithium ion battery was obtained and recorded as C2.
Example 3
This example differs from example 1 in that: the proportion of the silicon monoxide in the negative electrode active material 2 was 30%; the coating surface density of the bottom layer of the double-layer coating is 0.00852 g/cm2The top coating surface density is 0.00089g/cm2The solid content of the top layer slurry is 38%, the thickness of the top layer coating on one side of the copper foil of the dried pole piece is 20 +/-3 um, and the thickness of the bottom layer coating is 82 +/-3 um; a lithium ion battery was obtained and recorded as C3.
Example 4
This example differs from example 1 in that: the mass fraction of the silicon monoxide in the total active substances of the negative plate is 10 percent; the proportion of the silicon oxide in the negative electrode active material 2 was 20%; the coating surface density of the bottom layer of the double-layer coating is 0.00512g/cm2The top coating surface density is 0.00316g/cm2The solid content of the top layer slurry is 40%, the thickness of the top layer coating on one side of the dried pole piece copper foil is 37 +/-3 um, and the thickness of the bottom layer coating is 60 +/-3 um; rolling compaction of 1.70 g/cm2A lithium ion battery was obtained, denoted C4.
Example 5
This example differs from example 1 in that: the mass fraction of the silicon monoxide in the total active substances of the negative plate is 20 percent; the proportion of the silicon monoxide in the negative electrode active material 2 was 30%; the coating surface density of the bottom layer of the double-layer coating is 0.00342g/cm2The top coating surface density is 0.00351g/cm2The solid content of the bottom layer slurry is 41%, the thickness of the top layer coating on one side of the copper foil of the dried pole piece is 48 +/-3 um, and the thickness of the bottom layer coating is 37 +/-3 um; rolling compaction of 1.60 g/cm2A lithium ion battery was obtained, denoted C5.
Comparative example 1
This example differs from example 1 in that: the coating is a single layer, and the mass fraction of the silicon monoxide in the total active substances of the negative plate is 5%; the active material 2 is slurry according to the ratio of 0.00899 g/cm2And (5) coating at the surface density, wherein the thickness of the coating on one side of the copper foil of the pole piece after drying is 95 +/-3 um, and thus obtaining the lithium ion battery which is marked as C6.
Comparative example 2
This example differs from example 1 in that: the coating is a single layer, and is active substance 1 slurry with the concentration of 0.01024g/cm2And (5) coating at the surface density, wherein the thickness of the coating on one side of the copper foil of the pole piece after drying is 102 +/-3 um, and thus obtaining the lithium ion battery which is marked as C7.
Comparative example 3
This example differs from example 1 in that: the double-layer coating is characterized in that first bottom layer slurry is coated on one surface of a copper foil, the copper foil is dried, then the bottom layer slurry is coated on the other surface of the copper foil, then the top layer slurry is coated on the bottom layer coating in the same steps, the thickness of the top layer coating on one side of the copper foil of the pole piece after drying is 44 +/-3 microns, and the thickness of the bottom layer coating is 58 +/-3 microns, so that the lithium ion battery is obtained and is marked as C8.
Table 1 shows the main relevant parameter tables of examples 1 to 5 and comparative examples 1 to 3
Table 2 shows the performance test results of the lithium ion batteries C1-C5 obtained in examples 1-6 and the lithium ion batteries C6-C8 obtained in comparative examples 1-3. Wherein the content of the first and second substances,
negative electrode compacted density: and the density of the negative pole piece after cold pressing for 1 h.
Cathode gram capacity: the first discharge capacity of the battery and the mass ratio of the negative active material.
Energy density: charging to 4.45V at 25 ℃ and 0.2C by constant current, and then charging to 0.025C by constant voltage; then discharging to 2.75V at 0.2C, and measuring the energy density at the moment;
battery liquid retention amount: the battery liquid retention amount = the weight of the battery after two seals + the weight of the air bag-the original weight of the package, and the unit is g;
full-electric first efficiency: and (3) charging to the upper limit voltage by adopting a 0.1C constant current, then charging to a 0.02C current at a constant voltage, stopping, and then discharging by adopting a 0.1C constant current, wherein the discharge capacity is compared with the first charging capacity to obtain the first efficiency of the full battery.
And (3) testing the cycle life of the battery:
the lithium ion batteries C1-C5 obtained in examples 1-5 and the lithium ion batteries C6-C8 obtained in comparative examples are charged to 4.45V at a constant current of 1C rate at 25 ℃, then charged at a constant voltage of 4.45V, the cut-off current is 0.025C, and then discharged at a constant current of 0.5C rate, the cut-off voltage is 2.75V, which is a charge-discharge cycle process, and the charge-discharge cycle process is repeated until the capacity retention rate of the batteries is lower than 80%.
Negative pole lithium extraction test:
the lithium ion batteries C1-C5 obtained in examples 1-5 and the lithium ion batteries C6-C8 obtained in comparative examples were respectively charged to 4.45V at a constant current of 1.5C rate at 25 ℃, then charged at a constant voltage of 4.45V with a cut-off current of 0.025C, and then discharged at a constant current of 1C rate with a cut-off voltage of 2.75V, which is a charge-discharge cycle, and the charge-discharge cycle was repeated 10 times. And (4) fully charging the battery after the operation is finished, disassembling the battery cell in a drying room environment, and observing the lithium precipitation condition on the surface of the negative electrode. The results of the tests for analyzing lithium levels as no lithium analysis, slight lithium analysis, moderate lithium analysis and severe lithium analysis are shown in Table 1. Slight lithium deposition means that the lithium deposition region on the surface of the negative electrode is 1/10 or less of the entire region, and severe lithium deposition means that the lithium deposition region on the surface of the negative electrode exceeds 1/3 of the entire region.
TABLE 2 Performance test results of examples 1 to 5 and comparative examples 1 to 3
From the results in table 2, it can be seen that in examples 1 to 5, the liquid retention and energy density of the battery prepared by the method of the present invention are improved and the cycle life of the battery is prolonged as compared with comparative examples 1 and 2. In example 5, the cycle life is deteriorated due to excessive silicon content in the negative electrode sheet, because the electronic conductivity of the whole material is damaged in the repeated expansion process of the silicon material, and the exposed new negative electrode interface is repeatedly formed into a film, so that the capacity is quickly attenuated due to the interaction of 2 factors; compared among examples 1, 2 and 3, it can be seen that increasing the proportion of the top layer coated with silica reduces the top layer coating areal density, reduces the top layer coating thickness, and does not deteriorate the ultimate compaction of the pole piece, but too much top layer content deteriorates the first efficiency of the pole piece; compared with examples 2, 4 and 5, the content of the top silicon material is 20-30%, if the silicon accounts for the total amount of the negative electrode plate and is increased, the thickness of the top coating is increased, and the first efficiency of the full battery and the limit compaction of the negative electrode are greatly reduced; compared with the comparative example 2, the full battery prepared by the method has better first-time efficiency performance, higher energy density and better cycle performance; the reason why the embodiment 1 of the invention is better than the comparison 3 is that the embodiment 1 adopts the bottom layer coating and the top layer silicon-containing coating to be simultaneously coated and rolled, the mortise and tenon cooperation between the coatings of the double-layer pole piece which is separately dried in the separate coating step is stronger, and the separation phenomenon between the coatings can not occur, so that the electronic conductivity of the whole material can be ensured, and the process of the invention is more energy-saving, simpler and more capable of meeting the industrial requirements. As can be seen from Table 2, the lithium battery obtained by using the formulation of the material in example 2 has the best overall performance.
The side reaction products of the battery can be deposited in the pores of the negative electrode, the porosity of the negative electrode material can be reduced, the closer to the diaphragm, the more serious the porosity of the negative electrode material is reduced, the larger the concentration polarization of the negative electrode is, the slower the lithium embedding speed is, and the lithium precipitation phenomenon can be generated when the charging current is too large; the lithium ion batteries C1-C5 of the negative plate can improve the charging performance because the negative plate utilizes the charge expansion characteristic of a silicon material to improve the porosity close to the diaphragm side, so that the liquid retention of the batteries is increased; in example 5, the reason why lithium is separated out is that the content of silicon in the negative electrode sheet is too high, the electronic conductivity of the whole material is damaged in the repeated expansion process of the silicon material, and the exposed new negative electrode interface is repeatedly formed into a film, and the polarization of the battery is increased due to the interaction of 2 factors, so that the negative electrode reaches the lithium separation potential in advance.
The negative plate has high compaction and high capacity performance, the preparation method is simple and convenient in process, and the prepared lithium ion battery has high energy density and high battery liquid retention capacity, so that the cycle life of the battery is prolonged; the charging performance of the battery is improved, the lithium precipitation window of the battery is enlarged, and the battery is safer in practical application;
in the description, each part is described in a progressive manner, each part is emphasized to be different from other parts, and the same or similar parts among the parts are referred to each other. The combination of each component is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and the foregoing description of the disclosed embodiments is provided to enable any person skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.
Claims (8)
1. The utility model provides a negative plate, includes the negative pole current collector and coat in the rete on the current collector, its characterized in that: the film layer is divided into two layers, wherein the first layer of film layer is a bottom layer graphite coating layer close to the current collector; the second film layer is a top silicon-containing coating layer far away from the current collector; wherein the proportion of silicon material in the top layer silicon-containing coating is 5-30 wt.%; the silicon material in the negative plate accounts for 5-20% of the total weight of the active substances of the negative plate; after coating and drying, the thickness of the bottom graphite coating is 44-85 um, and the thickness of the top silicon-containing coating is 20-48 um; the two film layers are both composite materials containing negative active materials, conductive agents, binders and thickening agents; the graphite of the bottom layer and the graphite of the top layer of the negative active material both contain secondary particles, and the compacted density is 1.78g/cm2The high compacted graphite of (1); the secondary particles are prepared by using natural graphite, petroleum coke, needle coke and pitch coke with small particle size as base materials, adding a coating material and an additive, and controlling the material proportion and heating the mixture under the condition of high-temperature stirringLine and stirring speed, and carrying out secondary granulation on the base material with small granularity to obtain particles; the bottom coat and the top coat are applied simultaneously and rolled simultaneously.
2. A negative electrode sheet according to claim 1, wherein: a double-die coating head extrusion type coating machine is adopted to simultaneously coat 2 kinds of slurry on a negative current collector, and after a double-layer paste coating structure is formed on the same side of the negative current collector, a double-layer coated negative plate with a silicon-containing coating on the top layer and a silicon-free pure graphite layer on the bottom layer is prepared; and after one surface of the current collector is coated, repeatedly coating the other surface according to the above mode.
3. A negative electrode sheet according to claim 1, wherein: the negative current collector is one of a homogeneous copper foil, a porous copper foil and a copper foil with a carbon coating.
4. A negative electrode sheet according to claim 1, wherein: the bottom graphite coating contains the following substances in dry powder weight ratio: conductive agent: adhesive: thickener = 95-98%: 0-2%: 1-1.5%: 1-1.5%, and the negative active material is graphite; the top layer silicon-containing coating comprises the following materials in proportion of the anode active material: conductive agent: adhesive: the thickening agent is 92.5-97%, 1-3%, 1.5-3%, 0.5-1.5%, and the negative active material is graphite and silicon material.
5. A negative electrode sheet according to claim 4, wherein: the silicon material is a silicon alloy with a surface containing a high-molecular conductive polymer, a composite material of silicon and carbon and SiOxWherein x is more than 0.5 and less than 2, and the high molecular conductive polymer is one or a mixture of more of polyaniline, polythiophene and polypyrrole.
6. A negative electrode sheet according to claim 4, wherein: the conductive agent is one or a mixture of more of conductive carbon black Super P, vapor-phase-generated carbon fiber VGCF, graphene or carbon nanotube CNTs.
7. A negative electrode sheet according to claim 4, wherein: the binder is a water-based binder, and the water-based binder is one or a mixture of more of Styrene Butadiene Rubber (SBR), nitrile rubber, butadiene rubber, modified styrene butadiene rubber, sodium polyacrylate (PAA-Na), water-based polyacrylonitrile copolymer and polyacrylate; the thickening agent is sodium carboxymethyl cellulose CMC and/or polyether modified organic silicon polymer with thickening effect.
8. A lithium ion battery containing the negative electrode sheet according to any one of claims 1 to 7, characterized in that: the lithium ion battery comprises a positive plate, a negative plate, a diaphragm and electrolyte.
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