CN109119599A - Secondary battery and preparation method thereof - Google Patents

Abstract

The present application relates to a secondary battery, comprising a positive electrode piece, a negative electrode piece, a separator and an electrolyte, the positive electrode piece includes a positive electrode active material layer, the positive electrode active material layer contains a high-nickel positive electrode material, and the negative electrode electrode piece includes a negative electrode active material layer, the negative electrode active material layer contains a polymer, and the polymer contains a structural unit represented by formula I. For a secondary battery in which the positive electrode active material is a high-nickel positive electrode material, the present application uses a polymer containing a structural unit shown in formula I in the negative electrode active material layer, which can significantly improve the storage capacity of the secondary battery under high temperature and high SOC, Reduce cell gas production.

Description

A kind of secondary battery and preparation method thereof

technical field

The present application relates to the field of secondary batteries, and in particular, to a secondary battery and a method for preparing the secondary battery.

Background technique

Faced with the increasingly severe environmental pollution problem, green and environmentally friendly electric vehicles have attracted more and more people's attention and respect. The popularity and promotion of electric vehicles are closely related to the rise and development of secondary batteries, especially lithium-ion power batteries. Compared with consumer lithium-ion batteries (mobile phone and notebook batteries, etc.) with traditional lithium-cobalt materials as the positive electrode, lithium-ion power batteries need to adopt technical routes with better safety performance, higher energy density and lower cost. , the layered ternary cathode material NCM (Li[Ni _x Mn _y Co _z ]O ₂ , where x+y+z=1) came into being. Compared with the LCO cathode material, Mn and Ni elements are introduced into the NCM material. The Mn element has no chemical activity, but it can improve the safety and stability of the material, and can also reduce the material cost. Ni element can also reduce the material cost and increase the gram capacity of the material. From the perspective of increasing the gram capacity, the higher the Ni content, the better. The chemical formula of a common high-nickel cathode material is Li[Ni _x Co _y B _z ]O ₂ , wherein B is Mn or Al, x+y+z=1 and x≥0.5. However, Ni atoms themselves are highly active and easy to mix and arrange Li atoms, which easily leads to the continuous fading of the material itself during use, especially when the Ni content in the high-nickel positive electrode material is very high, the fading of the material is very obvious and will eventually lead to A series of problems such as gas production, capacity decay and DC resistance (DC Resistance, DCR) increase in electric core storage. Among them, gas production, as an important indicator for the safety performance and life evaluation of lithium-ion batteries, has always been a severe test for high-nickel battery cells.

The lithium-ion battery is a very complex active system, including positive and negative active materials, current collectors, separators, and electrolytes. When batteries are placed for a long time, chemical reactions will inevitably occur, and these reactions will generate gas. The state of charge (SOC) of the power battery pack is one of the important parameters to characterize the state of the lithium battery pack. Accurate estimation of the SOC is the guarantee for the safety of the lithium battery pack and the optimal control of the charge and discharge energy. Under low SOC conditions, especially when the storage temperature is low, these chemical reactions proceed relatively slowly, and the gas production of the cells can be basically ignored. However, at high SOC, the high-nickel cathode material has strong oxidizing properties. If coupled with high-temperature catalysis, the electrolyte will be rapidly oxidized by the cathode containing the high-nickel cathode material and produce a large amount of _CO2 -based gas. These gases fill the inside of the cell, causing the cell to swell. After accumulating to a certain extent, the explosion-proof valve of the hard-shell cell will be opened, which will cause the cell to fail; in extreme cases, the cell will be deformed and short-circuited or the flammable electrolyte will leak. Risk of smoking, burning and explosion. Therefore, the gas production problem of high-nickel material cells under high temperature and high SOC cannot be underestimated. Effective methods must be used to improve them to avoid problems such as shortened cell life and safety hazards caused by gas production.

In practical situations, in order to reduce the gas production of high-nickel material cells at high temperature and high SOC, the method of positive electrode coating or adding positive electrode film-forming additives is usually used to avoid the direct contact between the positive electrode material and the electrolyte, so as to reduce the The purpose of producing gas from the cell. However, the positive electrode coating requires special treatment of the positive electrode material, which will greatly increase the raw material cost of secondary batteries, especially lithium-ion batteries, and these coating layers will slowly dissolve and fail over time. The addition of film-forming additives tends to increase the impedance of the cell, especially on the dynamic properties of the cell, such as rate, high and low temperature performance, etc., which has a very serious impact. Therefore, there is an urgent need to effectively reduce the storage gas production of secondary batteries containing high nickel cathode materials, especially lithium ion batteries under high temperature and high SOC conditions, without increasing production costs and without affecting cell performance.

In view of this, this application is hereby made.

SUMMARY OF THE INVENTION

The primary object of the present application is to provide a secondary battery.

A second object of the present application is to provide a method for preparing the secondary battery.

To achieve the above object, the technical scheme of the application is as follows:

The application provides a secondary battery, comprising a positive electrode piece, a negative electrode piece, a separator and an electrolyte, the positive electrode piece includes a positive electrode active material layer, and the positive electrode active material layer contains a high-nickel positive electrode material; the The chemical formula of the high-nickel cathode material is Li _a Ni _x Co _y M _z O ₂ , wherein M is selected from at least one of Mn, Al, Zr, Ti, V, Mg, Fe, and Mo, 0.95≤a≤1.2, x ≥0.5, y≥0, z≥0, and x+y+z=1;

The negative electrode pole piece includes a negative electrode active material layer, the negative electrode active material layer contains a polymer, and the polymer contains a structural unit shown in formula I;

in,

R ₁ is selected from hydrogen, halogen, substituted or unsubstituted alkyl;

M is selected from a metal ion, an amino group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted amine group, and a substituted or unsubstituted alkylcarbonyl group;

Substituents are selected from halogen.

Preferably, the surface of the high-nickel positive electrode material has a coating layer, and the elements of the coating layer are selected from Zr, Ti, Ce, V, Mg, Al, Fe, Cr, Mo, Zn, B, Si. at least one.

Preferably, the polymer contains at least one of the structural units represented by formula II, formula III, formula IV, formula V and formula VI:

in:

M ₁ is selected from Na or K;

R ₁₁ and R ₁₄ are each independently selected from substituted or unsubstituted C ₁ -C ₁₂ alkyl;

R ₁₂ and R ₁₃ are each independently selected from hydrogen or C ₁ -C ₁₂ alkyl;

Substituents are halogen.

Preferably, the polymer is selected from at least one of homopolymer, copolymer or blend, preferably the monomer of the polymer is selected from methyl acrylate, ethyl acrylate, propyl acrylate, sodium acrylate, Potassium acrylate, acrylamide.

Preferably, the polymer has a number average molecular weight of 2000-20,000,000.

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 thickener, preferably the negative electrode active material has a mass percentage of 85-98%, and the binder has a mass percentage content of 85-98%. is 0-5%, and the mass percentage content of the thickener is 0.5%-1.5%.

Preferably, the charge cut-off voltage of the secondary battery is greater than or equal to 4.1V.

The present application also provides a method for preparing the secondary battery, comprising at least the following steps:

Step 1: Coat the positive electrode slurry including the high-nickel positive electrode material, the conductive agent and the binder on the surface of the positive electrode current collector, form a positive electrode film layer after drying, and obtain the positive electrode electrode sheet;

Step 2, coating the negative electrode slurry including the polymer, the negative electrode active material, the binder and the thickener on the surface of the negative electrode current collector, and drying to form a negative electrode film layer to obtain the negative electrode pole piece;

Step 3: The positive electrode, separator and negative electrode are stacked in sequence and then rolled or pressed to obtain a bare cell, then injected into the electrolyte, and packaged to obtain the secondary battery.

Preferably, in step 2, the polymer is first dissolved in a part of the solvent for preparing the negative electrode slurry to obtain mixture A; then the negative electrode active material, binder and thickener are added to the remaining In the solvent, mixture B is obtained; finally, mixture A and mixture B are mixed to obtain the negative electrode slurry.

The technical solution of the present application has at least the following beneficial effects:

For the secondary battery in which the positive electrode active material is a high-nickel positive electrode material, the application uses a polymer containing the structural unit shown in formula I in the negative electrode active material layer, which can significantly reduce the gas production of the battery core and improve the secondary battery in high temperature and high temperature. Storage capacity under SOC.

Detailed ways

The present application will be further described below with reference to specific embodiments. It should be understood that these examples are only used to illustrate the present application and not to limit the scope of the present application.

The application relates to a secondary battery, comprising a positive electrode piece, a negative electrode piece, a separator and an electrolyte, the positive electrode piece includes a positive electrode active material layer, and the positive electrode active material layer contains a high-nickel positive electrode material; the negative electrode electrode piece includes a negative electrode active material layer, the negative electrode active material layer contains a polymer, and the polymer contains a structural unit represented by formula I.

In the present application, the high-nickel cathode material refers to a nickel-containing cathode material with a nickel content (the mole fraction of nickel in the cathode active material) greater than or equal to 50%. Specifically, the chemical formula of the high-nickel cathode material is Li _a Ni _x Co _y M _z O ₂ , wherein M is selected from at least one of Mn, Al, Zr, Ti, V, Mg, Fe, and Mo, and 0.95≤a≤ 1.2, x≥0.5, y≥0, z≥0, and x+y+z=1.

In the structural unit shown in formula I, R ₁ is selected from hydrogen, halogen, substituted or unsubstituted alkyl; M is selected from metal ion, amino, substituted or unsubstituted alkyl, substituted or unsubstituted amino and One of substituted or unsubstituted alkanecarbonyl; the substituent is selected from halogen.

The applicant's research has found that when the positive electrode active material of the secondary battery is a high-nickel positive electrode material, and a polymer containing a structural unit represented by formula I is used in the negative electrode, it can be achieved without increasing the battery manufacturing cost and without damaging the battery performance. The problem of gas production in storage at high temperature and high SOC is improved, so as to avoid the shortened battery life and safety hazards caused by gas production.

Further mechanism studies show that the cathode electrode containing high nickel cathode material has a very strong oxidizing property under the condition of high temperature and high SOC, which can oxidize the electrolyte and generate a large amount of CO ₂ , resulting in the rapid gas production of the cell. If a polymer containing the structural unit shown in formula I is added to the negative electrode, the polymer can be dissolved in a small amount by compounds containing ester groups such as cyclic ester, linear ester and carboxylate in the electrolyte to form a polymer containing the polymer. Electrolyte leaching solution, which has a certain corrosive effect on the SEI film of the negative electrode, resulting in the destruction of the SEI film and the exposure of fresh graphite. At this time, the _CO2 generated by the positive electrode plate will react on the exposed graphite surface of the negative electrode plate. A new SEI film is produced, which results in a large amount of _CO2 produced by the cathode being consumed, which greatly reduces the gas production of secondary batteries containing high-nickel cathode materials at high temperature and high SOC.

The secondary battery in the present application is preferably a lithium ion battery, and the lithium ion battery may be a wound or stacked lithium ion battery.

Further, in the above chemical formula of the high nickel cathode material, M is Mn or Al, a=1, x≥0.5, y>0, z>0, and x+y+z=1. That is, the high-nickel positive electrode material is selected from at least one of NCM and NCA.

In the embodiments of the present application, the high-nickel cathode material may be selected from Li[Ni _0.6 Co _0.2 Mn _0.2 ]O ₂ , Li[Ni _0.8 Co _0.1 Mn _0.1 ]O ₂ , Li[Ni _0.8 Co _0.1 Al _0.1 ]O ₂ at least one of them.

In the present application, the surface of the high-nickel positive electrode material can also 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 the structural units shown in formula II, formula III, formula IV, formula V and formula VI:

Wherein, M ₁ is selected from Na or K;

R ₁₁ and R ₁₄ are each independently selected from substituted or unsubstituted C ₁ -C ₁₂ alkyl;

R ₁₂ and R ₁₃ are each independently selected from hydrogen or C ₁ -C ₁₂ alkyl;

Substituents are halogen.

Further, the polymer containing the structural unit represented by formula I, which is added to the conventional graphite negative electrode formulation involved in this application, is selected from at least one of homopolymers, copolymers or blends. The monomers of the polymer are selected from methyl acrylate, ethyl acrylate, propyl acrylate, sodium acrylate, potassium acrylate, acrylamide.

The method for preparing the polymer of the present application is conventional. For example, the above-mentioned monomers are added to the reactor in proportion, an initiator and a molecular weight regulator are added, and water is used as a solvent, and the reaction is carried out at 60 to 80 ° C for 4 to 8 hours. When the number average molecular weight is 2,000 to 20,000,000, the reaction is terminated, and the polymer of the present application is obtained.

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, it is easy to be dissolved by the electrolyte, and the improvement of the gas production of the cell is not obvious. When the number average molecular weight of the polymer is greater than 20,000,000, the processability of the slurry becomes poor, and it is difficult to form a uniform negative electrode active material layer on the negative electrode surface. 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 polymers are composed of homologous mixtures with the same chemical composition but different degrees of polymerization, that is, a mixture of high polymers with different molecular chain lengths, the average molecular weight is usually used to characterize the size of the molecules. According to the number of molecules, the average is called the number-average molecular weight, and the symbol is MN (Number-average Molecular Weight).

As an improvement to the secondary battery of the present application, the mass percentage content of the polymer in the negative electrode active material layer is 0.5% to 10%, that is, 0.5 to 10% by weight of the compound shown in formula I is added to the conventional graphite negative electrode formulation. Structural units of polymers. When the content of the polymer is less than 1%, the improvement effect on the gas production in the high SOC storage of the cell cannot achieve the desired effect, and when the content is higher than 10%, the energy density of the cell is damaged. Further preferably, the lower limit of the mass percentage is selected from 0.5%, 1%, 2%, and 3%, and the upper limit is selected from 6%, 8%, 9%, and 10%, most preferably 2% to 4%.

Further, the negative electrode active material layer further includes a negative electrode 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, silicon oxide compound, silicon carbon composite, lithium titanate, and metals capable of forming alloys with lithium. Here, the silicon oxide compound is SiO _x , and 0.5<x<2. The silicon-carbon composite is selected from graphite-hard carbon mixed materials, graphite-silicon material composite materials, and graphite-hard carbon-silicon composite materials.

Specifically, the binder is selected from at least one of 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% to 98%, the mass percentage of the binder is 0 to 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 selected from polyvinyl alcohol, polytetrafluoroethylene, polyvinylidene fluoride, sodium carboxymethyl cellulose, water-based acrylic resin, ethylene-vinyl acetate copolymer, styrene-butadiene rubber, fluorinated rubber and polyurethane at least one of them.

Specifically, the conductive agent is a carbon material, at least one selected from graphite, carbon black, graphene, and carbon nanotube conductive fibers. Commonly used conductive agents include Ketjen black (ultrafine conductive carbon black, particle size is 30-40nm), SP (Super P, small particle conductive carbon black, particle size is 30-40μm), S-O (ultrafine graphite powder, particle size). diameter is 3-4 μm), KS-6 (large particle graphite powder, particle size is 6.5 μm), acetylene black, VGCF (vapor grown carbon fiber, particle size is 3-20 μm). Optional conductive agents also include metal powders, conductive whiskers, conductive metal compounds, conductive polymers, and the like.

Specifically, in the positive electrode active material layer, the mass percentage content of the high nickel positive electrode material is 80-98%, the mass percentage content of the binder is 1-10%, and the mass percentage content of the conductive agent is 1-10% %.

Further, the material of the separator is not particularly limited in this application, and it is a polymer separator, which can be selected from 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 carboxylate. Specifically, it can be selected from the following organic solvents without limitation: ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, dipropyl carbonate, ethyl methyl carbonate, methyl formate, ethyl 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 at least one selected from lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroarsenate (LiAsF ₆ ), and lithium perchlorate (LiClO ₄ ). The organic lithium salt is selected from lithium bis-oxalate borate (LiB(C ₂ O ₄ ) ₂ , abbreviated as LiBOB), lithium bisfluorosulfonimide (LiFSI), and lithium bistrifluoromethanesulfonimide (LiTFSI). at least one.

Specifically, the additive is selected from one or more of fluorine-containing, sulfur-containing and unsaturated double bond-containing compounds. Specifically, it can be selected from the following substances without limitation: fluoroethylene carbonate, vinyl sulfite, propane sultone, N-methylpyrrolidone, N-methylformamide, N-methylacetamide, acetonitrile, Acrylonitrile, γ-butyrolactone, methyl sulfide.

Further, since the secondary battery can increase the capacity when the charge cut-off voltage is increased, for the secondary battery using high-nickel cathode materials, the gas generation problem becomes more prominent after the charge cut-off voltage is increased, and it has to be charged at a lower Use at cut-off voltage. However, with the solution in the present application, the charge cut-off voltage of the secondary battery using the high-nickel positive electrode material can reach 4.1V and above.

The present application also relates to a method for preparing a secondary battery, comprising at least the following steps:

Step 1, coating the positive electrode slurry including high nickel positive electrode material, conductive agent and binder on the surface of the positive electrode current collector, and drying to form a positive electrode active material layer to obtain a positive electrode pole piece;

Step 2: Coating the negative electrode slurry including the polymer, the negative electrode active material, the binder and the thickener on the surface of the negative electrode current collector, and drying to form a negative electrode active material layer to obtain a negative electrode pole piece.

Step 3: The positive pole piece, the separator and the negative pole piece are stacked in sequence and then rolled or pressed to obtain a bare cell, and then the electrolyte is injected, and the secondary battery is obtained after encapsulation.

As an improvement to this method, in step 2, in order to ensure that the polymer can be uniformly distributed in the negative electrode active material layer, it is generally pre-dissolved in a part of the solvent used to prepare the negative electrode slurry to obtain mixture A; The negative electrode active material, the binder and the thickening agent are added to the remaining solvent to obtain the mixture B; finally, the mixture A and the mixture B are mixed to obtain the negative electrode slurry. In particular, for the water-based negative electrode slurry, the polymer is generally first dissolved in water, and its mass percentage in the aqueous solution is 4% to 40%. Then, the aqueous solution containing the polymer is added while stirring the negative electrode slurry, so that the mass percentage of the polymer in the solid content of the entire negative electrode slurry is 0.5% to 10%.

The present application will be described in detail below with reference to the examples, but the present application is not limited to these examples.

In the embodiment, the high nickel ternary materials NCM622 (Li[Ni _0.6 Co _0.2 Mn _0.2 ]O ₂ ), NCM811 (Li[Ni _0.8 Co _0.1 Mn _0.1 ]O ₂ ), NCA811 (Li[Ni _0.8 Co _0.1 Al _0.1 ] O ₂ ) and Zr-coated NCM811 are commercially available.

Example 1

Preparation of positive electrode

The high-nickel positive electrode material, the conductive agent acetylene black, and the binder polyvinylidene fluoride (PVDF) are mixed, and the weight ratio of the three is 95:3:2. The solvent N-methylpyrrolidone is added, and the positive electrode slurry is obtained after mixing and stirring uniformly. The positive electrode slurry was uniformly coated on the positive current collector aluminum foil, then dried at 85°C, cold-pressed, trimmed, cut into pieces, and slit, and then dried under vacuum at 85°C for 4 hours to obtain a positive electrode piece. . The specific types of high-nickel cathode materials used therein are shown in Table 1.

Preparation of negative pole piece

Mix the negative active material artificial graphite, conductive agent acetylene black, binder styrene-butadiene rubber, and thickener CMC (sodium carboxymethyl cellulose) in a weight ratio of 95:2:2:1, add solvent deionized water, stir After mixing uniformly, add the aqueous solution of the polymer containing the structural unit represented by formula I, and stir again to obtain negative electrode slurry. The negative electrode slurry is uniformly coated on the negative electrode current collector copper foil. After coating, it is dried at 80-90 °C, cold pressed, edge trimmed, cut into pieces, and slit, and then dried under vacuum at 110 °C for 4 hours. , get the negative pole piece. Table 1 shows the specific types of polymers containing structural units represented by formula I, and their mass percentages in the solid content of the negative electrode slurry.

Lithium-ion battery preparation

A 12 μm thick polyethylene film was used as the separator. The concentration of lithium hexafluorophosphate in the electrolyte is 1mol/L, and the organic solvent in the electrolyte is composed of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and ethylene carbonate (EC), and the mass ratio of the three is 5:2 :3.

The negative pole piece, the separator and the positive pole piece are stacked in sequence, the separator is in the middle of the positive pole piece and the negative pole piece, and then rolled into a square bare cell with a thickness of 8mm, a width of 60mm and a length of 130mm. Put the bare cell into an aluminum foil packaging bag, bake it in a vacuum at 75°C for 10h, inject a non-aqueous electrolyte, seal it in a vacuum, let it stand for 24h, and then charge it to 4.2V with a constant current of 0.1C (160mA), and then use 4.2V constant voltage charge until the current drops to 0.05C (80mA), then discharge to 3.0V at a constant current of 0.1C (160mA), repeat the charge and discharge twice, and finally charge to 3.8V at a constant current of 0.1C (160mA) , that is, the preparation of the lithium ion secondary battery is completed. Batteries 1 to 9 were obtained in the above-described manner.

The preparation process of the battery 10 is similar to that of the battery 1, the difference is that when preparing the negative pole piece, the polymer containing the structural unit shown in formula I is directly mixed with the negative electrode active material, the conductive agent, the binder and the thickener, and then water is added. , and stir to obtain a negative electrode slurry.

Table 1

Comparative Example 1

The preparation process of batteries 1# to 5# is shown in Table 2:

Table 2

test case

High temperature storage test

Take 3 batteries from each group in Example 1 and Comparative Example 1, charge them with a constant current of 0.5C rate at room temperature until the voltage is higher than 4.2V, and further charge them under a constant voltage of 4.2V until the current is lower than 0.025C , so that it is fully charged at 4.2V. The volume of the fully charged battery before storage is measured and recorded as V ₀ . The fully charged battery was then placed in an 85°C oven, stored for 2D, and taken out. After cooling the battery cell for 1 hour, the volume of the battery cell after storage was measured and recorded as V _n .

According to the formula: ε=(V _n -V ₀ )/V ₀ , the volume change rate of the battery before and after storage is calculated. The average volume change rates of the obtained batteries after storage are shown in Table 3.

Cyclic performance test

Three batteries were taken from each group in Example 1 and Comparative Example 1, and the batteries were repeatedly charged and discharged through the following steps, and the discharge capacity retention rate of the batteries was calculated.

First, in an environment of 25°C, perform the first charge and discharge, and then perform constant current charging at a charging current of 0.5C (that is, the current value that fully discharges the theoretical capacity within 2 hours), and then perform constant voltage charging until The upper limit voltage was 4.2 V, followed by constant current discharge at a discharge current of 0.5 C until the final voltage was 2.8 V, and the discharge capacity of the first cycle was recorded. Then 200 charge and discharge cycles were performed, and the discharge capacity at the 200th cycle was recorded.

According to the formula: cycle capacity retention ratio=(discharge capacity of the 200th cycle/discharge capacity of the first cycle)×100%, the capacity retention ratio before and after the battery cycle was calculated. The average capacity retention rates of the obtained batteries after cycling are shown in Table 3.

table 3

From the test results of batteries 1# to 4# and batteries 1 to 6, it can be seen that in the NCM 622 system cells, by adding a certain amount of polymer containing the structural unit represented by formula I to the negative pole piece, the battery cell can be improved. The gas production problem can be improved, and the capacity retention rate of the battery can be improved.

In battery 2, when the amount of polymer added is 0.2 wt %, the gas production and cycle capacity retention rate of the battery cell are only slightly improved, and the expected effect is not achieved.

In battery 3, when the amount of polymer added is 0.5wt%, compared with battery 1#, the gas production of the 4.2V fully charged cell stored at 85°C for 2D can be reduced by 14.1%, and the gas production after 200 cycles is reduced by 14.1%. The retention rate of capacity can be improved by 5.8%.

In battery 1, when the amount of polymer added is increased to 2 wt%, a good effect of improving gas production can be achieved. Compared with battery 1#, the gas production of 4.2V fully charged cells can be reduced by 29.4% in 2D storage at 85°C, and the capacity retention rate after 200 cycles can be increased by 7.9%.

In battery 4, when the amount of polymer added was further increased to 4 wt %, compared with the added amount of 2 wt %, although the gas production and capacity retention rate were further improved, the improvement was not large. Considering that adding an excessive amount of polymer will adversely affect the energy density, it is not necessary to add more than 4 wt %. Especially in battery 5 and battery 6, when the amount of polymer added is increased to 10 or 15 wt%, although the gas generation of the battery cell is further improved, the cycle capacity retention rate of the battery cell is greatly damaged.

By comparing battery 1# and battery 2# to 3#, it can be seen that NCM 811 and NCA 811 have larger capacity than NCM 622, but their storage gas generation is more serious, and the cycle capacity retention rate is worse. This shows that the higher the Ni content in the cell, the more serious the storage gas production, and the faster the cycle capacity decay, so it is more urgent to improve the gas production of NCM811 and NCA 811.

By comparing battery 2# and battery 4#, it can be seen that after the surface of NCM 811 is modified with Zr, its high SOC storage gas production and cycle capacity decay have been greatly improved, but it has not yet reached the ideal state.

By comparing battery 2# and battery 7, it can be seen that after adding the polymer containing the structural unit shown in formula I to the negative pole piece of the NCM 811 system, the test results are similar to those of the NCM 622 system. That is, adding 2 wt % of the polymer can greatly improve the gas production of the cell under high SOC and high temperature storage without reducing the capacity of the cell, and can improve the cycle capacity retention rate. This conclusion also applies to the NCA 811 system (cell 3# and cell 8).

By comparing battery 4# and battery 9, it can be seen that NCM 811 is modified by Zr, and adding 2wt% of polymer to the negative pole piece can improve the cell capacity, and the storage gas production and cycle capacity decay of the cell can be maintained. better condition.

By comparing battery 5# and battery 1#, battery 1, it can be seen that adding a polymer containing a structural unit shown in formula I to the positive pole piece can improve the gas production of the cell, and also partially improve the cycle performance of the cell. However, the improvement effect is not as obvious as the use of polymers for negative pole pieces, so it is not recommended.

By comparing the battery 10 and the battery 1, it can be seen that the capacity of the battery cells and the gas production of the improved battery cells are basically the same, but the cycle performance of the battery cells will be reduced. The reason is that mixing and stirring the polymer directly with the raw materials of the negative electrode slurry will lead to poor dispersibility in the negative electrode slurry. Therefore, it is preferable to dissolve the polymer in water first, and then add the negative electrode active material and stir to form the negative electrode slurry. .

By comparing battery 11, battery 1, and battery 1#, it can be seen that the cell capacities of the three are basically the same, but battery 11 improves the gas production of the battery cells and the cycle performance of the battery cells is comparable to battery 1#. The reason is that the molecular weight of the polymer is too small, it is easily dissolved by the electrolyte, and the improvement effect on the gas production and cycle performance of the cell is not obvious.

By comparing battery 12, battery 1 and battery 1#, it can be seen that battery 12 is comparable to battery 1 in terms of improving the gas production of the cell, but the cycle performance of the battery cell is poor, even lower than that of battery 1#. The reason may be that the molecular weight of the polymer is too large, the processing performance of the slurry is deteriorated, and it is difficult to form a uniform negative electrode active material layer on the surface of the negative electrode. Although it can improve the gas production of the cell, it will lead to a rapid decline in the cycle performance.

Although the present application discloses the preferred embodiments as above, it is not intended to limit the claims. Any person skilled in the art can make several possible changes and modifications without departing from the concept of the present application. Therefore, the protection scope of the present application should be based on the scope defined by the claims.

Claims (10)

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 Li_aNi_xCo_yM_zO₂Wherein 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,

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.

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 polymer contains at least one of structural units represented by 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.

4. The secondary battery according to claim 1, wherein the polymer is selected from at least one of a homopolymer, a copolymer, or a blend,

preferably, the monomers of the polymer are selected from the group consisting of methyl acrylate, ethyl acrylate, propyl acrylate, sodium acrylate, potassium acrylate, acrylamide.

5. The secondary battery according to any one of claims 1 to 4, wherein the number average molecular weight of the polymer is 2000 to 20,000,000.

6. The secondary battery according to claim 1, wherein the polymer is contained in the negative electrode active material layer in an amount of 0.5 to 10% by mass, preferably 2 to 4% by mass.

7. The secondary battery according to claim 1, wherein the anode active material layer further contains an anode active material, a binder, and a thickener,

preferably, 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%.

8. The secondary battery according to claim 1, wherein a charge cut-off voltage of the secondary battery is 4.1V or more.

9. A method for manufacturing a secondary battery according to any one of claims 1 to 8, 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.

10. The method according to claim 9, 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.