A positive electrode sheet and preparation method thereof, and secondary battery

Abstract

The present invention provides a positive electrode sheet and a preparation method thereof, and a secondary battery. The positive electrode sheet includes a current collector and an active material layer attached to at least one side of the current collector. The active material layer includes an additive A, and the additive A has the following general structural formula: The present invention introduces a specific additive A into the positive electrode slurry. Because its molecular structure contains an acidic group, it can neutralize the alkaline substance on the surface of the positive electrode active material. On the one hand, the generated alkali metal salt covers the surface of the positive electrode to form a protective layer with good ionic conductivity, thereby reducing the oxidative decomposition of the electrolyte; on the other hand, the additive interacts with the conductive agent and forms adsorption on its surface, which is conducive to the dispersion of the conductive agent in the slurry; on the other hand, one end of the additive molecule is anchored to the positive electrode particles through complexation, and the other end is adsorbed to the conductive agent, which is conducive to the uniform distribution of the conductive agent on the surface of the positive electrode particles, significantly improving the electronic conductivity of the electrode sheet and improving the rate performance of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a positive electrode sheet and a preparation method thereof, and a secondary battery. Background Art

[0002] Lithium-ion secondary batteries have the advantages of high specific energy, long cycle life and low self-discharge rate, and are widely used in the fields of consumer electronics, power batteries for vehicles such as automobiles, and energy storage industries. With the increasing requirements for the range of new energy vehicles and the continuous miniaturization of digital consumer electronic products, high energy density has become the main development trend of lithium-ion secondary batteries. Increasing the operating voltage of lithium-ion secondary batteries is one of the effective ways to increase the energy density of volume batteries.

[0003] However, as the charging voltage increases, the battery operates at a high voltage, the structure of the positive electrode material will be damaged, and the internal electrolyte will be oxidized and decomposed on the exposed positive electrode surface. These phenomena significantly deteriorate the cycle life of the battery. In addition, the dispersion of the conductive agent used in the positive electrode will also affect the electron transmission and current distribution of the positive electrode, thereby affecting the potential distribution on the surface of the positive electrode material, further accelerating the decomposition of the electrolyte and accelerating the decay of the battery cycle life.

[0004] Therefore, the cathode material system for high voltage applications still needs further optimization and improvement. Summary of the invention

[0005] In view of the above-mentioned shortcomings of the prior art, the purpose of the present invention is to provide a positive electrode sheet, which improves the stability of the positive electrode material at high voltage and improves the distribution of the conductive agent on the positive electrode surface by adding a specific additive A during the positive electrode slurrying process, thereby improving the cycle performance and rate performance of the battery; at the same time, the present invention will also provide a method for preparing the positive electrode sheet; in addition, the present invention will also provide a secondary battery using the positive electrode sheet.

[0006] In order to achieve the above-mentioned object and other related objects, the present invention provides the following technical solutions:

[0007] In a first aspect of the present invention, there is provided a positive electrode sheet, comprising a current collector and an active material layer attached to at least one side of the current collector, wherein the active material layer comprises an additive A, and the additive A has the following general structural formula:

[0008]

[0009] In the formula, R 1 independently represent at least one of a hydrogen atom, a fluorine atom, a hydroxyl group, a saturated hydrocarbon group, an unsaturated hydrocarbon group, an aromatic hydrocarbon group, a fluorinated hydrocarbon group, and a phenyl group;

[0010] R2 independently represent at least one of a carboxyl group, a sulfonic acid group, and a phosphonic acid group;

[0011] m and n represent the average number of two polymerized monomers, m=1-50, n=1-100.

[0012] Furthermore, m=10-30, n=10-50; m is preferably 10, 20, 30; n is preferably 10, 20, 30, 40, 50.

[0013] Furthermore, the R 1 Preferably, it is a hydrogen atom, an alkane group or a phenyl group; 2 Preferred is a phosphonic acid group.

[0014] Furthermore, the additive A is preferably a polystyrene phosphonic acid compound.

[0015] Furthermore, the active material layer at least includes a positive electrode active material, a conductive agent and a binder.

[0016] Furthermore, the content of the additive A is 0.01% to 1% by weight of the positive electrode active material.

[0017] Furthermore, the positive electrode active material can be selected from lithium-containing positive electrode materials, specifically including at least one of lithium cobalt composite oxides and their modifications, lithium manganese composite oxides and their modifications, lithium nickel composite oxides and their modifications, lithium iron phosphate composite oxides and their modifications, lithium manganese phosphate composite oxides and their modifications, lithium vanadium phosphate composite oxides and their modifications, multi-conductor transition metal lithium oxides and their modifications, and lithium-rich manganese-based multi-conductor transition metal lithium oxides and their modifications.

[0018] Furthermore, the positive electrode active material may also be selected from sodium-containing positive electrode materials, specifically including at least one of layered oxide positive electrode materials, polyanion positive electrode materials, Prussian white positive electrode materials, and Prussian blue positive electrode materials.

[0019] Furthermore, the present invention has no particular limitation on the conductive agent, which can be a conventional positive electrode conductive agent in the art. The conductive agent includes at least one of acetylene black, conductive carbon black, conductive graphite, carbon fiber, carbon nanotube, and graphene.

[0020] Furthermore, the present invention has no particular limitation on the binder, and it can be a binder known to those skilled in the art. The binder includes at least one of fluorine-containing resin, polyolefin compound, cellulose-type binder, rubber-type binder, polyacrylate-type binder, and polyimide; the polyolefin compound includes polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), etc., and the rubber-type binder includes styrene-butadiene rubber (SBR), etc.

[0021] Furthermore, the present invention has no particular limitation on the current collector, and the current collector may be any positive electrode current collector known to those skilled in the art, such as aluminum foil, carbon-coated aluminum foil, composite aluminum foil, and the like.

[0022] The second aspect of the present invention provides a method for preparing a positive electrode sheet, which is used to prepare the above-mentioned positive electrode sheet, comprising the following steps: mixing a positive electrode active material, a conductive agent, a binder and an additive A, adding a solvent and stirring evenly to obtain a positive electrode slurry, adjusting the viscosity of the positive electrode slurry and then applying it on a current collector, and drying to obtain the positive electrode sheet.

[0023] Furthermore, the additive A accounts for 0.01-1% of the mass of the positive electrode active material in the positive electrode slurry. When the amount of additive A is less than 0.01%, the coating effect is not obvious and a good protective layer cannot be formed; when the amount of additive A is higher than 1%, it is easy to increase the electrode impedance, affect the power output, and also reduce the battery energy density.

[0024] According to a third aspect of the present invention, a secondary battery is provided, comprising a negative electrode sheet, a separator, an electrolyte and a battery casing, and also comprising the above-mentioned positive electrode sheet.

[0025] As described above, a positive electrode sheet and a preparation method thereof, and a secondary battery of the present invention have the following beneficial effects:

[0026] The present invention introduces a specific polystyrene additive A into the positive electrode slurry. Because its molecular structure contains acidic groups, it can neutralize the alkaline substances (such as carbonates, alkali metal hydroxides, etc.) on the surface of the positive electrode active material. On the one hand, the generated alkali metal salt will cover the surface of the positive electrode to form a protective layer with good ionic conductivity, reduce the oxidative decomposition of the electrolyte, and thus improve the stability of the positive electrode material;

[0027] On the other hand, the additive molecules contain aromatic benzene rings and hydrophobic alkane groups, which interact with the conductive agent (such as carbon black, graphite, etc.) such as π-π and CH-π, forming adsorption on its surface, which helps to disperse the conductive agent in the slurry and prevent the conductive agent from agglomerating;

[0028] On the other hand, one end of the additive molecule is anchored to the positive electrode particles through complexation, and the other end is adsorbed to the conductive agent, which is beneficial to the uniform distribution of the conductive agent on the surface of the positive electrode particles, thereby improving the uneven distribution of the conductive agent on the surface of the particles, significantly improving the electronic conduction of the electrode sheet, and improving the rate performance of the battery; at the same time, it also improves the current density distribution on the positive electrode surface, improves the uniformity of the reaction on the surface of the positive electrode particles, and is beneficial to improving the structural stability of the positive electrode material and the electrode sheet, thereby further improving the battery cycle performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1The cycle curves of the lithium ion secondary batteries prepared in Example 1 and Comparative Example 1 of the present invention are shown.

[0030] Figure 2 The cycle curve diagram of the sodium ion secondary battery prepared in Example 2 of the present invention and Comparative Example 2.

[0031] Figure 3 The cycle curve diagram of the lithium ion secondary battery prepared in Example 3 of the present invention and Comparative Example 3. DETAILED DESCRIPTION

[0032] The following is a description of the implementation of the present invention by means of specific embodiments. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification.

[0033] Example 1

[0034] 1) Preparation of positive electrode

[0035] The commercial cathode material LiCoO 2 , conductive carbon black, polyvinylidene fluoride, additive A(R 1 =H, R 2 =phosphonic acid group, m=n=20) is dispersed in N-methylpyrrolidone, and then subjected to wetting, kneading and dispersion treatment to obtain a positive electrode slurry;

[0036] Among them, LiCoO 2 : The weight ratio of conductive carbon black: polyvinylidene fluoride is 97.8:1.2:1, and the weight of additive A is the positive electrode material LiCoO 2 0.1% by weight;

[0037] Subsequently, the positive electrode slurry is coated on both surfaces of the aluminum foil, and then dried, rolled, and cut to obtain the desired positive electrode sheet.

[0038] 2) Preparation of negative electrode

[0039] The binder sodium carboxymethyl cellulose (CMC) is dissolved in water, and then the negative electrode material artificial graphite is added, and after sufficient mixing and stirring, styrene-butadiene rubber latex (SBR) is added, and the mixing and stirring are continued to obtain a negative electrode slurry; then the negative electrode slurry is evenly coated on both sides of a 10 μm copper foil, baked and dried at 100°C, and rolled to obtain a negative electrode sheet.

[0040] Among them, artificial graphite: CMC: SBR = 96.6: 1.4: 2.

[0041] 3) Preparation of lithium-ion secondary batteries

[0042] After the positive and negative electrodes and polyethylene separators prepared above are respectively wound, assembled, inserted into battery shells, and baked, an appropriate amount of the commonly used electrolyte is injected into the battery aluminum shell and sealed, and then a lithium-ion secondary battery is made after aging, formation, and capacity separation.

[0043] The mixed solvent ratio in the electrolyte is EC / EMC / DEC=3:5:2 (volume ratio), and the LiPF 6 The concentration is 1 mol / L, and the additives are 2.0 wt% of vinylene carbonate VC and 1 wt% of vinyl sulfate DTD.

[0044] Comparative Example 1:

[0045] Compared with Example 1, the only difference is that the positive electrode sheet does not contain additive A, and other materials and preparation processes are the same.

[0046] Example 2

[0047] 1) Preparation of positive electrode

[0048] The ternary positive electrode material is nickel iron manganese oxide (NFM111), conductive carbon black, polyvinylidene fluoride, additive A (R 1 =H, R 2 =phosphonic acid group, m=n=20) is dispersed in N-methylpyrrolidone, and then subjected to wetting, kneading and dispersion treatment to obtain a positive electrode slurry;

[0049] The weight ratio of NFM111: conductive carbon black: polyvinylidene fluoride is 97.5:1:1.5; the weight of additive A is 0.3% of the weight of the positive electrode material NFM111;

[0050] Subsequently, the positive electrode slurry is coated on both surfaces of the aluminum foil, and then dried, rolled, and cut to obtain the desired positive electrode sheet.

[0051] 2) Preparation of negative electrode

[0052] The binder sodium carboxymethyl cellulose (CMC) is dissolved in water, and then conductive carbon black (super-p) and negative electrode material hard carbon are added, and after sufficient mixing and stirring, styrene-butadiene rubber emulsion (SBR) is added, and the mixing and stirring is continued to obtain a negative electrode slurry; then the negative electrode slurry is evenly coated on both sides of a 10μm copper foil, baked and dried at 100°C, and rolled to obtain a negative electrode sheet.

[0053] Among them, hard carbon:super-p:CMC:SBR=94.3:1.5:1.5:2.7.

[0054] 3) Preparation of sodium ion secondary batteries

[0055] After the positive and negative electrodes and polyethylene separators prepared above are respectively wound, assembled, inserted into battery shells, and baked, an appropriate amount of the commonly used electrolyte is injected into the battery aluminum shell and sealed, and then a sodium ion secondary battery is made after aging, formation, capacity separation and other processes.

[0056] The mixed solvent ratio in the electrolyte is EC / PC / DMC=0.4:0.5:0.1 (volume ratio), and the additive is 6 wt% fluoroethylene carbonate FEC, wherein NaPF 6 The concentration is 0.8mol / L.

[0057] Comparative Example 2:

[0058] Compared with Example 2, the only difference is that the positive electrode sheet does not contain additive A, and other materials and preparation processes are the same.

[0059] Example 3

[0060] 1) Preparation of positive electrode

[0061] The ternary cathode material LiNi 0.5 Co 0.2 Mn 0.3 O 2 (NCM523), conductive carbon black, polyvinylidene fluoride, additive A(R 1 =H, R 2 =phosphonic acid group, m=10, n=30) are dispersed in N-methylpyrrolidone, and then the positive electrode slurry is obtained through wetting, kneading and dispersion treatment;

[0062] Wherein, the weight ratio of NCM523: conductive carbon black: polyvinylidene fluoride is 98:1:1, and the weight of additive A is 0.2% of the weight of the ternary positive electrode material NCM523;

[0063] The positive electrode active slurry is then coated on both surfaces of the aluminum foil, and then dried, rolled and cut to obtain the required positive electrode sheet.

[0064] 2) Preparation of negative electrode

[0065] The binder sodium carboxymethyl cellulose (CMC) is dissolved in water, and then the negative electrode material artificial graphite is added, and after sufficient mixing and stirring, styrene-butadiene rubber latex (SBR) is added, and the mixing and stirring are continued to obtain a negative electrode slurry; then the negative electrode slurry is evenly coated on both sides of an 8 μm copper foil, baked and dried at 100°C, and rolled to obtain a negative electrode sheet.

[0066] Among them, artificial graphite: CMC: SBR = 96.6: 1.4: 2.

[0067] 3) Preparation of lithium-ion secondary batteries

[0068] After the positive and negative electrodes and polyethylene separators prepared above are respectively wound, assembled, inserted into battery shells, and baked, an appropriate amount of the commonly used electrolyte is injected into the battery aluminum shell and sealed, and then a lithium-ion secondary battery is made after aging, formation, and capacity separation.

[0069] The mixed solvent ratio in the electrolyte is EC / EMC / DEC=1:2:1 (volume ratio), and the LiPF 6 The concentration is 1 mol / L, and the additives are 2 wt% of ethylene carbonate VC, 1.5 wt% of vinyl sulfate DTD and 5 wt% of fluoroethylene carbonate FEC.

[0070] Comparative Example 3:

[0071] Compared with Example 3, the only difference is that the positive electrode sheet does not contain additive A, and other materials and preparation processes are the same.

[0072] Example 4

[0073] Compared with Example 1, the only difference is the structure of the additive A in the positive electrode sheet, wherein R 1 =CH 3 , R 2 =phosphonic acid group, m=n=20.

[0074] Example 5

[0075] Compared with Example 1, the only difference is the structure of the additive A in the positive electrode sheet, wherein R 1 =H, R 2 =sulfonic acid group, m=n=20.

[0076] Example 6

[0077] Compared with Example 1, the only difference is the structure of the additive A in the positive electrode sheet, wherein R 1 =H, R 2 =carboxyl, m=n=20.

[0078] Example 7

[0079] Compared with Example 3, the only difference is the structure of the additive A in the positive electrode sheet, wherein R 1 =phenyl, R 2 =phosphonic acid group, m=10, n=30.

[0080] Example 8

[0081] Compared with Example 3, the only difference is the structure of the additive A in the positive electrode sheet, wherein R 1 =H, R 2 =phosphonic acid group, m=35, n=65.

[0082] Performance Testing

[0083] The test method of the electrode resistivity is as follows: the electrode resistivity is tested by a four-probe tester, 10 data are evaluated in parallel, and the average value and standard deviation are taken. The test results are shown in Table 1.

[0084] Table 1. Resistivity of Examples 1 to 8 and Comparative Examples 1 to 3

[0085]

[0086] Cycle life test method: At room temperature, use 0.5C constant current and constant voltage to charge to the cut-off voltage, the cut-off current is 0.02C, after 20 minutes, discharge to the cut-off voltage at 0.5C, and leave for 20 minutes. Repeat the charge and discharge steps and stop when the test termination conditions are met. The test results are as follows Figures 1 to 3 shown.

[0087] Among them, the charging range of Example 1 and Comparative Example 1 is 3.0-4.5V; the charging range of Example 2 and Comparative Example 2 is 1.5-4.0V; the charging range of Example 3 and Comparative Example 3 is 3.0-4.4V.

[0088] Rate performance test method: At room temperature, use 0.5C constant current and constant voltage charging mode to the cut-off voltage, and the cut-off current is 0.02C; discharge at different discharge currents (0.5C, 1C, 2C, 3C) to different cut-off voltages, record the capacity at different discharge rates, and compare them with the capacity at 0.5C. The test results are shown in Table 2.

[0089] Table 2. Comparison of rate performance of batteries of Examples 1 to 3 and Comparative Examples 1 to 3

[0090]

[0091] It can be clearly seen from the test results in Table 1 that the resistivity and standard deviation of the lithium / sodium ion battery made of the positive electrode sheet containing additive A are smaller than those of the battery without additive A, indicating that the addition of additive A is conducive to the dispersion of the conductive agent, reduces the electronic resistance of the electrode sheet, and thus helps to improve the rate performance of the battery.

[0092] from Figures 1 to 3 It can be clearly seen from the cycle curve diagram that the capacity remaining rate of the lithium / sodium ion secondary battery made of the positive electrode sheet containing additive A is significantly higher than that of the secondary battery in the comparative example that does not contain additive A. It can be seen that the cycle performance of the secondary battery with the addition of additive A is significantly improved.

[0093] Specifically, Figure 1As shown, in the lithium cobalt oxide high-voltage system, after 500 cycles, the capacity remaining rate of the comparative example 1 is 89.2%, and the capacity remaining rate of the battery of Example 1 is 91.9%, and the capacity remaining rate is significantly improved, which shows that the introduction of additive A can significantly improve the cycle stability of the positive electrode, thereby extending the cycle life of the battery cell. Similarly, the same effect is also obtained in the layered ternary sodium ion battery system and the ternary lithium ion battery system.

[0094] In addition, it can be seen from Table 2 that after the addition of additive A, in the lithium cobalt oxide battery system of Example 1, under the discharge conditions of 1C, 2C, and 3C, the discharge capacity is increased by 1.8%, 2.8%, and 4.3% respectively compared with that of comparative example 1. Similarly, the same effect is also obtained in the layered ternary sodium ion battery system and the ternary lithium ion battery system. This shows that the addition of additive A significantly reduces the polarization resistance of the positive electrode of the battery, thereby improving the discharge rate performance of the battery.

[0095] In summary, the present invention introduces a specific additive A into the positive electrode slurry. Because its molecular structure contains an acidic group, it will neutralize the alkaline substance on the surface of the positive electrode active material. On the one hand, the generated alkali metal salt covers the surface of the positive electrode to form a protective layer with good ionic conductivity, reducing the oxidative decomposition of the electrolyte; on the other hand, the additive molecule contains an aromatic benzene ring and a hydrophobic alkane group, which interacts with the conductive agent and forms adsorption on its surface, which helps the conductive agent to be dispersed in the slurry; on the other hand, one end of the additive molecule is anchored to the positive electrode particles through complexation, and the other end is adsorbed with the conductive agent, which is conducive to the uniform distribution of the conductive agent on the surface of the positive electrode particles, significantly improving the electronic conductivity of the electrode sheet and improving the rate performance of the battery. Therefore, the present invention effectively overcomes the various shortcomings of the prior art and has a high industrial utilization value.

[0096] The above embodiments are merely illustrative of the principles and effects of the present invention, and are not intended to limit the present invention. Anyone familiar with the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by a person of ordinary skill in the art without departing from the spirit and technical concept disclosed by the present invention shall still be covered by the claims of the present invention.

Claims

1. A positive electrode sheet, comprising a current collector and an active material layer attached to at least one side of the current collector, characterized in that: The active material layer includes an additive A, and the additive A has the following general structural formula: In the formula, R1 independently represents at least one of a hydrogen atom, a fluorine atom, a hydroxyl group, a saturated hydrocarbon group, an unsaturated hydrocarbon group, an aromatic hydrocarbon group, a fluorinated hydrocarbon group, and a phenyl group; R2 independently represents a phosphonic acid group; m and n represent the average number of two polymerized monomers, m = 10 to 30, n = 10 to 50; The active material layer further includes at least a positive electrode active material, a conductive agent and a binder, and the content of the additive A is 0.01% to 1% of the weight of the positive electrode active material.

2. The positive electrode sheet according to claim 1, characterized in that: The positive electrode active material is selected from lithium-containing positive electrode materials, and the positive electrode active material includes at least one of lithium cobalt composite oxides and their modifications, lithium manganese composite oxides and their modifications, lithium nickel composite oxides and their modifications, lithium iron phosphate composite oxides and their modifications, lithium manganese phosphate composite oxides and their modifications, lithium vanadium phosphate composite oxides and their modifications, multi-conductor transition metal lithium oxides and their modifications, and lithium manganese-rich multi-conductor transition metal lithium oxides and their modifications.

3. The positive electrode sheet according to claim 1, characterized in that: The positive electrode active material is selected from sodium-containing positive electrode materials, and the positive electrode active material includes at least one of layered oxide positive electrode materials, polyanion positive electrode materials, Prussian white positive electrode materials, and Prussian blue positive electrode materials.

4. The positive electrode sheet according to claim 1, characterized in that: The conductive agent includes at least one of acetylene black, conductive carbon black, conductive graphite, carbon fiber, carbon nanotube, and graphene.

5. The positive electrode sheet according to claim 1, characterized in that: The binder includes at least one of fluorine-containing resin, polyolefin compound, cellulose type binder, rubber type binder, polyacrylate type binder and polyimide.

6. The positive electrode sheet according to claim 1, characterized in that: The current collector is selected from aluminum foil, carbon-coated aluminum foil or composite aluminum foil.

7. A method for preparing a positive electrode sheet, for preparing the positive electrode sheet according to any one of claims 1 to 6, characterized in that: The method comprises the following steps: mixing positive electrode active material, conductive agent, binder and additive A, adding solvent and stirring evenly to obtain positive electrode slurry, adjusting the viscosity of the positive electrode slurry and coating it on the current collector, and drying to obtain the positive electrode sheet.

8. The method for preparing a positive electrode sheet according to claim 7, characterized in that: The additive A accounts for 0.01-1% of the mass of the positive electrode active material in the positive electrode slurry.

9. A secondary battery comprising a negative electrode sheet, a separator, an electrolyte and a battery casing, characterized in that: It also includes the positive electrode sheet as described in any one of claims 1 to 6.

Images