CN109119599B - Secondary battery and preparation method thereof - Google Patents
Secondary battery and preparation method thereof Download PDFInfo
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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
- H01M4/362—Composites
- H01M4/364—Composites as mixtures
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- 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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- 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
- H01M10/058—Construction or manufacture
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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
- H01M4/13—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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- 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
- H01M4/13—Electrodes for accumulators with non-aqueous electrolyte, e.g. for lithium-accumulators; Processes of manufacture thereof
- H01M4/139—Processes of manufacture
- H01M4/1393—Processes of manufacture of electrodes based on carbonaceous material, e.g. graphite-intercalation compounds or CFx
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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
- H01M4/62—Selection of inactive substances as ingredients for active masses, e.g. binders, fillers
- H01M4/628—Inhibitors, e.g. gassing inhibitors, corrosion inhibitors
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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
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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
- Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
- Y02P70/50—Manufacturing or production processes characterised by the final manufactured product
Abstract
The application relates to a secondary battery, including positive pole piece, negative pole piece, barrier film and electrolyte, positive pole piece includes positive active material layer, and positive active material layer contains high nickel cathode material, and negative pole piece includes negative active material layer, and negative active material layer contains the polymer, contains the structural unit that formula I is shown in the polymer. For the secondary battery with the positive electrode active material being the high-nickel positive electrode material, the polymer containing the structural unit shown in the formula I is used in the negative electrode active material layer, so that the storage capacity of the secondary battery under high temperature and high SOC can be obviously improved, and the gas generation of a battery core is reduced.
Description
Technical Field
The present invention relates to the field of secondary batteries, and more particularly, to a secondary battery and a method for manufacturing the same.
Background
In the face of increasingly severe environmental pollution problems, green and environment-friendly electric automobiles are more and more concerned and advocated by people.The popularization and promotion of electric vehicles are closely related to the rise and development of secondary batteries, especially lithium ion power batteries. Compared with the traditional consumer lithium ion battery (mobile phone, notebook computer battery and the like) taking the lithium cobalt material as the anode, the lithium ion power battery needs to adopt the technical route with better safety performance, higher energy density and lower cost, and in the face of the requirements, the layered ternary anode material NCM (Li [ Ni ] Ni) is adoptedxMnyCoz]O2Where x + y + z is 1) should be generated. Compared with an LCO anode material, Mn and Ni elements are introduced into the NCM material, wherein the Mn element has no chemical activity, but can improve the safety and stability of the material and simultaneously reduce the material cost. The Ni element can also reduce the material cost and improve the gram capacity of the material, and the higher the Ni content is, the better the gram capacity is. The chemical formula of a common high-nickel cathode material is Li [ Ni ]xCoyBz]O2Wherein B is Mn or Al, x + y + z is 1 and x is not less than 0.5. However, the Ni atoms are highly active and are prone to Li atoms mixed and discharged, which easily causes the material to continuously fade during the use process, especially when the Ni content in the high-nickel cathode material is high, the fading of the material is very obvious, and finally a series of problems such as gas generation in the battery cell storage, capacity attenuation and direct current Resistance (DC Resistance, DCR) increase are caused. The stored gas is used as an important index for safety performance and service life evaluation of the lithium ion battery, and is always a severe test faced by the high nickel material battery core.
The lithium ion battery contains positive and negative electrode active materials, a current collector, a separation film, electrolyte and the like, and is a very complex active system. The cells inevitably undergo chemical reactions when left for a long time, and these reactions generate gases. The state of charge (SOC) of the power battery pack is one of important parameters for representing the state of the lithium battery pack, and accurate estimation of the SOC is the guarantee for safety and optimal control of charge and discharge energy of the lithium battery pack. Under low SOC conditions, particularly at low storage temperatures, these chemical reactions proceed more slowly and gas evolution from the cell is essentially negligible. However, at high SOC, the high nickel anode material has strong oxidizability, and if high temperature catalysis is added, the electrolyte is contained in the high nickel anodeThe positive electrode of the material is rapidly oxidized and largely produced with CO2A predominantly gaseous species. The gases are filled in the battery cell to cause the battery cell to swell, and after the gases are accumulated to a certain degree, the hard-shell battery cell explosion-proof valve is opened, so that the battery cell is failed; in extreme cases, the risk of smoke generation, combustion and explosion of the battery cell due to deformation short circuit or combustible electrolyte leakage can be caused. Therefore, the problem of gas generation of the high-nickel material battery cell at high temperature and high SOC is not a little, and an effective method is needed to be adopted to improve the problem, so that the problems of shortened service life of the battery cell, potential safety hazard and the like caused by gas generation are avoided.
In practical situations, in order to reduce the gas generation of the high-nickel battery cell at high temperature and high SOC, a method of coating the positive electrode or adding a positive electrode film-forming additive is usually adopted to avoid the direct contact between the positive electrode material and the electrolyte, so as to achieve the purpose of reducing the gas generation of the battery cell. However, the positive electrode coating requires special treatment of the positive electrode material, which greatly increases the raw material cost of the secondary battery, particularly the lithium ion battery, and these coating layers are slowly dissolved out and fail over time. The addition of the film additive can increase the impedance of the battery cell, and particularly can cause serious influence on the dynamic performance of the battery cell, such as multiplying power, high and low temperature performance and the like. Therefore, there is a need to effectively reduce the storage gas generation of secondary batteries containing high-nickel anode materials, especially lithium ion batteries, under the conditions of high temperature and high SOC, without increasing the production cost and affecting the cell performance.
In view of this, the present application is specifically made.
Disclosure of Invention
A primary object of the present application is to provide a secondary battery.
A second object of the present application is to provide a method of manufacturing the secondary battery.
In order to achieve the purpose, the technical scheme of the application is as follows:
the application provides a secondary battery, which comprises a positive pole piece, a negative pole piece, a separation film and electrolyte, wherein the positive pole piece comprises a positive active material layer, and the positive active material layer contains a high-nickel positive material; the chemical formula of the high-nickel anode material isLiaNixCoy MzO2Wherein M is selected from at least one of Mn, Al, Zr, Ti, V, Mg, Fe and Mo, a is more than or equal to 0.95 and less than or equal to 1.2, x is more than or equal to 0.5, y is more than or equal to 0, z is more than or equal to 0, and x + y + z is 1;
the negative electrode pole piece comprises a negative electrode active material layer, wherein the negative electrode active material layer contains a polymer, and the polymer contains a structural unit shown in a formula I;
wherein the content of the first and second substances,
R1selected from hydrogen, halogen, substituted or unsubstituted alkyl;
m is selected from one of metal ions, amino, substituted or unsubstituted alkyl, substituted or unsubstituted amine and substituted or unsubstituted alkylcarbonyl;
the substituents are selected from halogens.
Preferably, the surface of the high-nickel cathode material is provided with a coating layer, and the element of the coating layer is selected from at least one of Zr, Ti, Ce, V, Mg, Al, Fe, Cr, Mo, Zn, B and Si.
Preferably, the polymer contains at least one of structural units shown in formula II, formula III, formula IV, formula V and formula VI:
wherein:
M1selected from Na or K;
R11and R14Each independently selected from C which is substituted or unsubstituted1～C12An alkyl group;
R12and R13Each independently selected from hydrogen or C1～C12An alkyl group;
the substituent is halogen.
Preferably, the polymer is selected from at least one of homopolymers, copolymers or blends, preferably the monomer of the polymer is selected from methyl acrylate, ethyl acrylate, propyl acrylate, sodium acrylate, potassium acrylate, acrylamide.
Preferably, the number average molecular weight of the polymer is 2000 to 20,000,000.
Preferably, the mass percentage content of the polymer in the negative electrode active material layer is 0.5% to 10%, preferably 2% to 4%.
Preferably, the negative electrode active material layer further comprises a negative electrode active material, a binder and a thickening agent, wherein the negative electrode active material accounts for 85-98% by mass, the binder accounts for 0-5% by mass, and the thickening agent accounts for 0.5-1.5% by mass.
Preferably, the charge cutoff voltage of the secondary battery is 4.1V or more.
The application also provides a preparation method of the secondary battery, which at least comprises the following steps:
coating positive electrode slurry comprising the high-nickel positive electrode material, a conductive agent and a binder on the surface of a positive electrode current collector, and drying to form a positive electrode film layer to obtain a positive electrode piece;
coating the negative electrode slurry comprising the polymer, the negative electrode active material, the binder and the thickening agent on the surface of a negative electrode current collector, and drying to form a negative electrode film layer to obtain the negative electrode piece;
and step three, sequentially stacking the positive pole piece, the isolating membrane and the negative pole piece, then winding or pressing to obtain a bare cell, then injecting the electrolyte, and packaging to obtain the secondary battery.
Preferably, in the second step, the polymer is firstly dissolved in a part of the solvent for preparing the negative electrode slurry to obtain a mixture a; then adding the negative electrode active material, a binder and a thickener to the remaining solvent to obtain a mixture B; and finally, mixing the mixture A with the mixture B to obtain the negative electrode slurry.
The technical scheme of the application has at least the following beneficial effects:
for the secondary battery with the positive electrode active material being the high-nickel positive electrode material, the polymer containing the structural unit shown in the formula I is used in the negative electrode active material layer, so that the gas generation of the battery core can be obviously reduced, and the storage capacity of the secondary battery under high temperature and high SOC (state of charge) is improved.
Detailed Description
The present application is further illustrated with reference to specific examples. It should be understood that these examples are for illustrative purposes only and are not intended to limit the scope of the present application.
The application relates to a secondary battery, which comprises a positive pole piece, a negative pole piece, a separation film and electrolyte, wherein the positive pole piece comprises a positive active material layer, and the positive active material layer contains a high-nickel positive material; the negative electrode pole piece comprises a negative electrode active material layer, wherein the negative electrode active material layer contains a polymer, and the polymer contains a structural unit shown in a formula I.
In the present application, a high nickel positive electrode material refers to a nickel-containing positive electrode material having a nickel content (mole fraction of nickel in the positive electrode active material) of greater than or equal to 50%. Specifically, the chemical formula of the high-nickel cathode material is LiaNixCo y MzO2Wherein M is at least one of Mn, Al, Zr, Ti, V, Mg, Fe and Mo, a is more than or equal to 0.95 and less than or equal to 1.2, x is more than or equal to 0.5, y is more than or equal to 0, z is more than or equal to 0, and x + y + z is 1.
In the structural unit of the formula I, R1Selected from hydrogen, halogen, substituted or unsubstituted alkyl; m is selected from one of metal ions, amino, substituted or unsubstituted alkyl, substituted or unsubstituted amine and substituted or unsubstituted alkylcarbonyl; the substituents are selected from halogens.
The applicant researches and discovers that when the positive electrode active material of the secondary battery is a high-nickel positive electrode material and the negative electrode of the secondary battery is made of a polymer containing the structural unit shown in the formula I, the problem of storing and generating gas under high temperature and high SOC can be solved on the premise of not increasing the manufacturing cost of the battery and not damaging the performance of the battery, so that the shortening of the service life of a battery core and the potential safety hazard caused by gas generation are avoided.
Further mechanism research shows that the positive pole piece containing the high-nickel positive pole material has very strong oxidizability under the conditions of high temperature and high SOC, and can oxidize the electrolyte and generate a large amount of CO2Resulting in rapid gas generation from the cell. If the polymer containing the structural unit shown in the formula I is added into the negative electrode, the polymer can be slightly dissolved out by ester group-containing compounds such as cyclic ester, linear ester and carboxylic ester in the electrolyte to form electrolyte leachate containing the polymer, the leachate has certain corrosion effect on an SEI film of the negative electrode, so that the SEI film is damaged and fresh graphite is exposed, and CO generated by a positive electrode plate at the moment2Can react on the exposed graphite surface on the negative pole piece to produce a new SEI film, thereby leading to CO generated by the positive pole2Is consumed in a large amount, and the gas production of the secondary battery containing the high-nickel anode material under high temperature and high SOC is greatly reduced.
The secondary battery in the present application is preferably a lithium ion battery, which may be a wound or stacked lithium ion battery.
Further, in the chemical formula of the high nickel cathode material, M is Mn or Al, a is 1, x ≧ 0.5, y > 0, z > 0, and x + y + z is 1. Namely, the high nickel cathode material is selected from at least one of NCM and NCA.
In embodiments of the present application, the high nickel positive electrode material may be selected from Li [ Ni ]0.6Co0.2Mn0.2]O2、Li[Ni0.8Co0.1Mn0.1]O2、Li[Ni0.8Co0.1Al0.1]O2At least one of (1).
The surface of the high-nickel cathode material can be coated, and the coating element is selected from at least one of Zr, Ti, Ce, V, Mg, Al, Fe, Cr, Mo, Zn, B and Si.
Further, the polymer contains at least one of structural units shown in formula II, formula III, formula IV, formula V and formula VI:
wherein M is1Selected from Na or K;
R11and R14Each independently selected from C which is substituted or unsubstituted1～C12An alkyl group;
R12and R13Each independently selected from hydrogen or C1～C12An alkyl group;
the substituent is halogen.
Further, the polymer containing the structural unit shown in the formula I, which is added into the conventional graphite negative electrode formula, is selected from at least one of homopolymer, copolymer or blend. The monomer of the polymer is selected from methyl acrylate, ethyl acrylate, propyl acrylate, sodium acrylate, potassium acrylate and acrylamide.
The method for preparing the polymer is conventional, for example, the monomers are added into a reactor in proportion, an initiator and a molecular weight regulator are added, water is used as a solvent, the reaction is carried out for 4-8 hours at the temperature of 60-80 ℃, and the reaction is finished when the number average molecular weight is 2000-20,000,000, so as to obtain the polymer.
Wherein the initiator is selected from ammonium persulfate, and the molecular weight regulator is dodecyl mercaptan.
Further, the number average molecular weight of the polymer is 2000 to 20,000,000. When the number average molecular weight of the polymer is less than 2000, the polymer is easy to dissolve by electrolyte, and the improvement of gas generation of the battery cell is not obvious. When the number average molecular weight of the polymer is more than 20,000,000, the processability of the slurry becomes poor, and it is difficult to form a uniform anode active material layer on the surface of the anode. The lower limit of the number average molecular weight is selected from 2000, 5000, 10,000, 15,000, and the upper limit is selected from 20,000,000, 10,000,000, 1,000,000, 100,000.
Since the polymer is composed of a mixture of homologous polymers having the same chemical composition but different degrees of polymerization, i.e., a mixture of polymers having different molecular chain lengths, the average molecular weight is usually used to characterize the size of the molecule. The statistical average by Number of molecules is referred to as the Number average Molecular Weight and is denoted by MN (Number-average Molecular Weight).
As an improvement of the secondary battery, the mass percentage of the polymer in the negative electrode active material layer is 0.5-10%, namely 0.5-10 wt% of the polymer containing the structural unit shown in the formula I is added into the conventional graphite negative electrode formula. When the content of the polymer is less than 1%, the improvement effect on the high SOC storage gas production of the battery cell cannot achieve an ideal effect, and when the content of the polymer is more than 10%, the energy density of the battery cell is damaged. More preferably, the lower limit of the mass percentage is selected from 0.5%, 1%, 2%, 3%, and the upper limit is selected from 6%, 8%, 9%, 10%, and most preferably 2% to 4%.
Further, the anode active material layer further includes an anode active material, a binder, and a thickener.
Specifically, the negative electrode active material is selected from at least one of soft carbon, hard carbon, artificial graphite, natural graphite, silicon-oxygen compound, silicon-carbon composite, lithium titanate, and a metal capable of forming an alloy with lithium. Wherein the silicon oxide is SiOx，0.5<x<2. The silicon-carbon composite is selected from graphite-hard carbon mixed material, graphite-silicon material composite material and graphite-hard carbon-silicon material composite material.
Specifically, the binder is at least one selected from polyvinyl alcohol, polytetrafluoroethylene, polyvinylidene fluoride, water-based acrylic resin, ethylene-vinyl acetate copolymer, styrene-butadiene rubber, fluorinated rubber and polyurethane.
Specifically, the thickener is a surfactant, such as sodium carboxymethylcellulose (CMC).
Specifically, in the negative electrode active material layer, the mass percentage of the negative electrode active material is 85-98%, the mass percentage of the binder is 0-5%, and the mass percentage of the thickener is 0.5-1.5%.
Further, the positive electrode active material layer of the present application further includes a binder and a conductive agent.
Specifically, the binder is at least one selected from polyvinyl alcohol, polytetrafluoroethylene, polyvinylidene fluoride, sodium carboxymethylcellulose, water-based acrylic resin, ethylene-vinyl acetate copolymer, styrene-butadiene rubber, fluorinated rubber and polyurethane.
Specifically, the conductive agent is a carbon material selected from at least one of graphite, carbon black, graphene, and carbon nanotube conductive fibers. Commonly used conductive agents include Ketjen black (ultra fine conductive carbon black, particle size 30-40nm), SP (Super P, small particle conductive carbon black, particle size 30-40 μm), S-O (ultra fine graphite powder, particle size 3-4 μm), KS-6 (large particle graphite powder, particle size 6.5 μm), acetylene black, VGCF (vapor grown carbon fiber, particle size 3-20 μm). The optional conductive agent also includes metal powder, conductive whisker, conductive metal compound, conductive polymer, etc.
Specifically, in the positive electrode active material layer, the mass percentage of the high-nickel positive electrode material is 80-98%, the mass percentage of the binder is 1-10%, and the mass percentage of the conductive agent is 1-10%.
Further, the material of the release film is not particularly limited in the present application, and is a polymer release film, and may be selected from one of polyethylene, polypropylene, and ethylene-propylene copolymer.
Further, the electrolyte includes an organic solvent, a lithium salt, and an additive.
Specifically, the organic solvent is selected from one or more of conventional organic solvents such as cyclic carbonate, linear carbonate and carboxylic ester. The organic solvent which can be specifically selected from the following is not limited thereto: ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl formate, ethyl propionate, propyl propionate, methyl butyrate, ethyl acetate.
Specifically, the lithium salt is selected from at least one of inorganic lithium salts and organic lithium salts. The inorganic lithium salt is selected from lithium hexafluorophosphate (LiPF)6) Lithium tetrafluoroborate (LiBF)4) Lithium hexafluoroarsenate (LiAsF)6) Lithium perchlorate (LiClO)4) At least one of (1). The organic lithium salt is selected from lithium bis (oxalato) borate (LiB (C)2O4)2Abbreviated as LiBOB), lithium bis (fluorosulfonyl) imide (LiFSI), and lithium bis (trifluoromethanesulfonyl) imide (LiTFSI).
Specifically, the additive is selected from one or more of fluorine-containing compounds, sulfur-containing compounds and unsaturated double bond-containing compounds. The following substances can be selected in particular and are not limited thereto: fluoroethylene carbonate, ethylene sulfite, propane sultone, N-methyl pyrrolidone, N-methyl formamide, N-methyl acetamide, acetonitrile, acrylonitrile, gamma-butyrolactone and methyl sulfide.
Further, although the capacity of the secondary battery can be increased by increasing the charge cut-off voltage, the problem of gas generation is more pronounced in the secondary battery using a high nickel positive electrode material by increasing the charge cut-off voltage, and the secondary battery has to be used at a lower charge cut-off voltage. By adopting the scheme in the application, the charge cut-off voltage of the secondary battery using the high-nickel anode material can reach 4.1V or more.
The present application also relates to a method for manufacturing a secondary battery, comprising at least the steps of:
coating positive electrode slurry comprising a high-nickel positive electrode material, a conductive agent and a binder on the surface of a positive electrode current collector, and drying to form a positive electrode active substance layer to obtain a positive electrode piece;
and step two, coating the negative electrode slurry comprising the polymer, the negative electrode active material, the binder and the thickening agent on the surface of the negative electrode current collector, and drying to form a negative electrode active material layer to obtain the negative electrode plate.
And step three, sequentially stacking the positive pole piece, the isolating membrane and the negative pole piece, then winding or pressing to obtain a bare cell, then injecting electrolyte, and packaging to obtain the secondary battery.
As an improvement to this method, in step two, in order to ensure that the polymer can be uniformly distributed in the negative electrode active material layer, it is generally dissolved in advance in a part of the solvent for preparing the negative electrode slurry to obtain a mixture a; then adding the negative electrode active material, a binder and a thickener to the remaining solvent to obtain a mixture B; and finally, mixing the mixture A with the mixture B to obtain the negative electrode slurry. In particular, for water-based negative electrode slurry, the polymer is generally dissolved in water, and the mass percentage of the polymer in the water solution is 4-40%. Then adding the aqueous solution containing the polymer while stirring the negative electrode slurry to ensure that the mass percentage of the polymer in the solid content of the whole negative electrode slurry is 0.5-10%.
The present application will be described in detail with reference to examples, but the present application is not limited to these examples.
In the examples, the high nickel ternary material NCM622(Li [ Ni ]0.6Co0.2Mn0.2]O2)、NCM811(Li[Ni0.8Co0.1Mn0.1]O2)、NCA811(Li[Ni0.8Co0.1Al0.1]O2) NCM811 coated with Zr is commercially available.
Example 1
Preparation of positive pole piece
Mixing a high-nickel positive electrode material, a conductive agent acetylene black and a binder polyvinylidene fluoride (PVDF), wherein the mixing weight ratio of the high-nickel positive electrode material to the conductive agent acetylene black is 95: 3: 2. adding solvent N-methyl pyrrolidone, and mixing and stirring uniformly to obtain the anode slurry. And uniformly coating the positive electrode slurry on a positive electrode current collector aluminum foil, drying at 85 ℃, then carrying out cold pressing, trimming, cutting into pieces and slitting, and drying for 4 hours at 85 ℃ under a vacuum condition to obtain a positive electrode piece. Specific kinds of the high nickel positive electrode material used therein are shown in table 1.
Preparation of negative pole piece
Preparing negative active material artificial graphite, conductive agent acetylene black, binder styrene butadiene rubber and thickening agent CMC (sodium carboxymethylcellulose) according to a weight ratio of 95: 2: 2: 1, adding solvent deionized water, stirring and mixing uniformly, adding an aqueous solution of a polymer containing a structural unit shown in formula I, and stirring uniformly again to obtain the cathode slurry. And uniformly coating the negative electrode slurry on a copper foil of a negative electrode current collector, drying at 80-90 ℃ after coating, carrying out cold pressing, trimming, cutting into pieces and slitting, and then drying for 4h under the vacuum condition of 110 ℃ to obtain a negative electrode pole piece. The specific types of the polymers containing the structural unit shown in the formula I and the mass percentage of the polymers in the solid content of the negative electrode slurry are shown in the table 1.
Lithium ion battery preparation
A polyethylene film 12 μm thick was used as the separator. The concentration of lithium hexafluorophosphate in the electrolyte is 1mol/L, and the organic solvent in the electrolyte consists of dimethyl carbonate (DMC), Ethyl Methyl Carbonate (EMC) and Ethylene Carbonate (EC) in a mass ratio of 5:2: 3.
And the negative pole piece, the isolation film and the positive pole piece are sequentially stacked, the isolation film is positioned between the positive pole piece and the negative pole piece, and then the positive pole piece and the negative pole piece are wound into a square bare cell with the thickness of 8mm, the width of 60mm and the length of 130 mm. And (2) filling the bare cell into an aluminum foil packaging bag, baking for 10h at 75 ℃, injecting a non-aqueous electrolyte, carrying out vacuum packaging, standing for 24h, charging to 4.2V by using a constant current of 0.1C (160mA), then charging to 0.05C (80mA) by using a constant voltage of 4.2V until the current is reduced to 0.05V, then discharging to 3.0V by using a constant current of 0.1C (160mA), repeating the charging and discharging for 2 times, and finally charging to 3.8V by using a constant current of 0.1C (160mA), thus completing the preparation of the lithium ion secondary battery. The batteries 1-9 are obtained by adopting the mode.
The battery 10 is prepared in a similar manner to the battery 1, except that when preparing the negative electrode plate, the polymer containing the structural unit shown in formula I is directly mixed with the negative active material, the conductive agent, the binder and the thickener, and then water is added and uniformly stirred to obtain the negative electrode slurry.
TABLE 1
Comparative example 1
The preparation process of batteries 1# to 5# is shown in table 2:
TABLE 2
Test example
High temperature storage test
Example 1 and the pair3 batteries in the proportion 1 are respectively taken from each group, and are charged at the constant current of 0.5C multiplying power to the voltage higher than 4.2V at normal temperature, and further charged at the constant voltage of 4.2V to the current lower than 0.025C, so that the batteries are in a full charge state of 4.2V. The fully charged cell volume before storage was tested and recorded as V0. Then the battery in the full charge state is placed in an oven at 85 ℃, is stored for 2D and then is taken out, the volume of the battery after the storage is tested and recorded as V after the battery is cooled for 1hn。
According to the formula: epsilon ═ Vn-V0)/V0And calculating the volume change rate before and after the storage of the battery. The average volume change rate after storage of each battery pack is shown in table 3.
Cycle performance test
The batteries in example 1 and comparative example 1 were each taken out of 3 pieces, and the charging and discharging of the batteries were repeated by the following procedure, and the discharge capacity retention rate of the batteries was calculated.
First, in an environment of 25 ℃, first charging and discharging were performed, constant current charging was performed at a charging current of 0.5C (i.e., a current value at which the theoretical capacity was completely discharged within 2 hours), then constant voltage charging was performed until the upper limit voltage was 4.2V, constant current discharging was performed at a discharging current of 0.5C until the final voltage was 2.8V, and the discharge capacity of the first cycle was recorded. Then, 200 cycles of charge and discharge were performed, and the discharge capacity at the 200 th cycle was recorded.
According to the formula: the cycle capacity retention rate (discharge capacity at 200 th cycle/discharge capacity at first cycle) × 100%, and the capacity retention rates before and after the battery cycle were calculated. The average capacity retention after cycling of each battery group is shown in table 3.
TABLE 3
According to the test results of batteries 1# to 4# and batteries 1 to 6, in the battery cell of the NCM622 system, a polymer containing a structural unit shown in formula I in a certain content is added into the negative electrode plate, so that the gas generation problem of the battery cell can be improved, and the capacity retention rate of the battery can be improved.
In the battery 2, when the polymer addition amount is 0.2 wt%, the storage gas generation and the cycle capacity retention rate of the battery cell are only slightly improved, and the expected effect is not achieved.
In battery 3, when the polymer addition amount is 0.5 wt%, the gas production rate of the 4.2V fully charged cell stored at 85 ℃ in 2D can be reduced by 14.1% and the capacity retention rate after 200 cycles can be improved by 5.8% compared with battery 1 #.
In the battery 1, when the addition amount of the polymer is increased to 2 wt%, a good gas generation improvement effect can be achieved. Compared with the battery 1#, the gas production rate of the 4.2V fully-charged battery core stored for 2D at 85 ℃ can be reduced by 29.4%, and the capacity retention rate after 200 cycles can be improved by 7.9%.
In the battery 4, when the amount of the polymer added was further increased to 4 wt%, the gas generation and capacity retention rate were further improved as compared with the amount added of 2 wt%, but the improvement was not so great. It is not necessary to add a content exceeding 4 wt% in view of the adverse effect on energy density caused by the addition of an excessive amount of the polymer. Particularly in battery 5 and battery 6, when the amount of the polymer added was increased to 10 or 15 wt%, although the gas evolution from the cell was further improved, the deterioration of the cell cycle capacity retention rate was large.
Comparing battery # 1 with batteries # 2 to # 3, it can be seen that NCM811 and NCA 811 have a larger cell capacity than NCM622, but the gas evolution from storage is more severe and the cycle capacity retention is worse. Therefore, the higher the Ni content of the battery cell is, the more serious the stored gas generation is, and the faster the cycle capacity is attenuated, so the improvement on the gas generation of NCM811 and NCA 811 is more urgent.
By comparing battery # 2 with battery # 4, it can be seen that the surface of NCM811 modified with Zr has improved gas storage and evolution at high SOC and cycle capacity fading to a greater extent, but still does not reach the ideal state.
By comparing battery 2# and battery 7, it can be seen that the test results are similar to the NCM622 system after the polymer containing the structural unit shown in formula I is added to the negative electrode sheet of the NCM811 system. Namely, the addition of 2 wt% of the polymer can greatly improve the gas generation condition of the battery cell under high-SOC high-temperature storage on the premise of not reducing the capacity of the battery cell, and can improve the retention rate of the circulating capacity. This conclusion applies equally to the NCA 811 system (battery 3# and battery 8).
By comparing battery 4# and battery 9, it can be seen that NCM811 is Zr modified, and that 2 wt% polymer is added to the negative electrode tab, which can increase the cell capacity, and the cell storage gassing and cyclic capacity fade can be kept in a better state.
Comparing battery 5# and battery 1# battery 1, it can be seen that the addition of the polymer containing the structural unit shown in formula I to the positive electrode tab can improve the gas generation of the cell, and also partially improve the cycle performance of the cell, but the improvement effect is not as significant as the use of the polymer in the negative electrode tab, and therefore the use is not recommended.
By comparing battery 10 with battery 1, it can be seen that the cell capacities of the two are substantially equivalent to the improvement of the gas generation of the cell, but the cycle performance of the cell is reduced. Since the polymer is directly mixed and stirred with the raw material of the anode slurry, which may result in poor dispersibility in the anode slurry, it is preferable to form the anode slurry by dissolving the polymer in water, adding the anode active material, and the like, and stirring.
By comparing battery 11, battery 1 and battery 1#, it can be seen that the cell capacities of the three are basically equivalent, but battery 11 is equivalent to battery 1# with respect to improving the gas generation condition of the cell and the cycle performance of the cell. The reason is that the molecular weight of the polymer is too small, the polymer is easily dissolved by electrolyte, and the improvement effect on the gas generation and the cycle performance of the battery cell is not obvious.
By comparing battery 12, battery 1 and battery 1#, it can be seen that battery 12 is equivalent to battery 1 in terms of improving the gas generation of the cell, but the cycle performance of the cell is poor, even lower than battery 1 #. The reason may be that the polymer has an excessively large molecular weight, the slurry has poor processability, and it is difficult to form a uniform negative electrode active material layer on the surface of the negative electrode, and the cycle performance is rapidly lowered although gas generation from the cell can be improved.
The preferred embodiments disclosed above are not intended to limit the scope of the claims. A number of possible variations and modifications can be made by anyone skilled in the art without departing from the concept of the present application, and the scope of protection of the present application shall therefore be subject to the ambit defined by the claims.
Claims (9)
1. A secondary battery comprises a positive pole piece, a negative pole piece, a separation film and electrolyte, and is characterized in that,
the positive pole piece comprises a positive active material layer, and the positive active material layer contains a high-nickel positive material; the chemical formula of the high-nickel cathode material is LiaNixCoyMzO2Wherein M is selected from at least one of Mn, Al, Zr, Ti, V, Mg, Fe and Mo, a is more than or equal to 0.95 and less than or equal to 1.2, x is more than or equal to 0.5, y is more than or equal to 0, z is more than or equal to 0, and x + y + z is 1;
the negative electrode pole piece comprises a negative electrode active material layer, wherein the negative electrode active material layer contains a polymer;
the polymer contains at least one of structural units shown in formulas II, III, V and VI:
wherein:
M1selected from Na or K;
R11and R14Each independently selected from C which is substituted or unsubstituted1～C12An alkyl group;
R12and R13Each independently selected from hydrogen or C1～C12An alkyl group;
the substituent is halogen;
the polymer is selected from at least one of copolymer or blend;
the number average molecular weight of the polymer is 2000-20,000,000;
the mass percentage content of the polymer in the negative electrode active material layer is 2-10%.
2. The secondary battery according to claim 1, wherein a surface of the high-nickel positive electrode material has a coating layer, and an element of the coating layer is at least one element selected from Zr, Ti, Ce, V, Mg, Al, Fe, Cr, Mo, Zn, B, and Si.
3. The secondary battery according to claim 1, wherein the monomer of the polymer is selected from the group consisting of methyl acrylate, ethyl acrylate, propyl acrylate, sodium acrylate, potassium acrylate, and acrylamide.
4. The secondary battery according to claim 1, wherein the polymer is contained in the negative electrode active material layer in an amount of 2 to 4% by mass.
5. The secondary battery according to claim 1, wherein the anode active material layer further contains an anode active material, a binder, and a thickener.
6. The secondary battery according to claim 5, wherein the negative electrode active material is 85 to 98% by mass, the binder is 0 to 5% by mass, and the thickener is 0.5 to 1.5% by mass.
7. The secondary battery according to claim 1, wherein a charge cut-off voltage of the secondary battery is 4.1V or more.
8. A method for manufacturing a secondary battery according to any one of claims 1 to 7, characterized by comprising at least the steps of:
coating positive electrode slurry comprising the high-nickel positive electrode material, a conductive agent and a binder on the surface of a positive electrode current collector, and drying to form a positive electrode film layer to obtain a positive electrode piece;
coating the negative electrode slurry comprising the polymer, the negative electrode active material, the binder and the thickening agent on the surface of a negative electrode current collector, and drying to form a negative electrode film layer to obtain the negative electrode piece;
and step three, sequentially stacking the positive pole piece, the isolating membrane and the negative pole piece, then winding or pressing to obtain a bare cell, then injecting the electrolyte, and packaging to obtain the secondary battery.
9. The method according to claim 8, wherein in step two, the polymer is dissolved in a part of the solvent for preparing the negative electrode slurry to obtain a mixture A; then adding the negative electrode active material, a binder and a thickener to the remaining solvent to obtain a mixture B; and finally, mixing the mixture A with the mixture B to obtain the negative electrode slurry.
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