Positive plate, preparation method of positive plate and secondary battery

Abstract

The invention discloses a positive plate which comprises a current collector and an active material layer attached to at least one surface of the current collector, and the active material layer comprises an additive A. In the pulping process of the positive plate, the diphosphoric acid compound is added, so that the stability of the positive electrode material under high voltage is improved, and the cycle performance of a battery cell is further improved.

Description

Technical Field

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

[0002] Lithium-ion secondary rechargeable batteries have the advantages of high specific energy, long cycle life and low self-discharge rate, and are widely used in the field of consumer electronic products (such as mobile communications, tablet computers, smart wearables and medical fields, etc.). In recent years, the rapid iteration of electronic products has given consumer electronics more abundant product functions, and at the same time, it also requires the energy density of the secondary batteries used to be continuously improved to ensure sufficient use time and standby time.

[0003] The most significant way to increase the energy density of lithium-ion batteries is to increase the charging voltage of the battery. For example, in the commonly used lithium cobalt oxide / graphite system, increasing the upper charging voltage from 4.35V to 4.4V will increase the energy by about 6%. Further increasing it to 4.45V will increase the energy by another 4.5%. At present, there is still a trend and demand to increase the charging voltage of consumer lithium-ion batteries. However, as the charging voltage increases, the battery operates at a high voltage, the structure of the positive electrode material will be destroyed, and the internal electrolyte will oxidize and decompose on the exposed positive electrode surface. These phenomena significantly deteriorate the cycle life of the battery.

[0004] To address the above problems, the stability of positive electrode materials under high voltage can be improved by doping or coating the positive electrode materials in the bulk phase, or by combining the two methods, and this has been commercially applied. However, in order to obtain uniform and stable modified products, the manufacturing process is often complicated and costly. Summary of the invention

[0005] The purpose of the present invention is to provide a positive electrode sheet, a method for preparing the positive electrode sheet and a secondary battery, by adding diphosphoric acid compounds during the slurrying process of the positive electrode sheet to improve the stability of the positive electrode material under high voltage, thereby improving the cycle performance of the battery cell.

[0006] To achieve the above objectives, the following technical solutions are adopted:

[0007] A positive electrode sheet comprises 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 general formula of the additive A is

[0008]

[0009] Among them, R1 and R2 are selected from any one or more of hydrogen atom, fluorine atom, hydroxyl group, saturated hydrocarbon group, unsaturated hydrocarbon group, aromatic hydrocarbon group, fluorinated hydrocarbon group, amino group, carboxyl group, sulfonic acid group, sulfate group, phosphoric acid group, and amide group, and n=1-4.

[0010] Furthermore, the R1 is selected from a hydroxyl group or a fluorine atom; and the R2 is selected from an amino group or an amide group.

[0011] Furthermore, the active material layer at least includes a positive electrode active material, a conductive agent and a binder.

[0012] Furthermore, the content of the additive A is 0.01% to 1% by weight of the positive electrode active material.

[0013] Furthermore, the positive electrode active material includes a lithium-containing positive electrode material or a sodium-containing positive electrode material.

[0014] Furthermore, the positive electrode active material includes any one or more of the following materials: lithium cobalt composite oxide and its modified products, lithium manganese composite oxide and its modified products, lithium nickel composite oxide and its modified products, lithium iron phosphate composite oxide and its modified products, lithium manganese phosphate composite oxide and its modified products, lithium vanadium phosphate composite oxide and its modified products, multi-transition metal lithium oxide and its modified products, and lithium manganese-rich multi-transition metal lithium oxide and its modified products.

[0015] Furthermore, the sodium-containing positive electrode active material includes any one or more of the following materials: layered oxide positive electrode material, polyanion positive electrode material, Prussian white positive electrode material, Prussian blue positive electrode material.

[0016] A method for preparing a positive electrode sheet is also provided, which is at least used to prepare the above-mentioned positive electrode sheet, comprising the following steps:

[0017] S1: Mix the positive electrode active material, the conductive agent, the binder and the additive A, add the solvent and stir evenly to obtain the electrode slurry;

[0018] S2: After adjusting the viscosity of the electrode slurry, apply it on the current collector and dry it to obtain the positive electrode sheet of the battery.

[0019] Furthermore, the additive A is 0.01-1% by weight of the positive electrode active material in the electrode slurry.

[0020] A secondary battery is also provided, comprising a negative electrode sheet, a separator, an electrolyte and a battery casing, and also comprising the positive electrode sheet mentioned above.

[0021] By adopting the above scheme, the beneficial effects of the present invention are:

[0022] 1) The present invention adds a diphosphoric acid compound to the ingredients of the positive electrode material. Since the diphosphoric acid compound has two phosphate groups in its molecular structure, it can neutralize the alkaline substances on the surface of the positive electrode material, such as carbonates, alkali metal hydroxides, etc., and cover the surface of the positive electrode to form a protective layer with good ionic conductivity, thereby reducing the oxidative decomposition of the electrolyte, thereby improving the stability of the positive electrode material;

[0023] 2) Additive A contains a controllable solvation chain, which improves its compatibility with the electrolyte, facilitates the release of solvated lithium ions at the interface, reduces the charge transfer impedance of the reaction, and thus improves the rate performance of the battery;

[0024] 3) The addition of additive A reduces the alkalinity of the electrode slurry, improves the fluidity and stability of the electrode slurry, improves the uniformity of coating, and thus improves the cycle performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] Figure 1 The graphs are the cycle curves of the lithium ion secondary batteries of Example 1 and Comparative Example 1 of the present invention;

[0026] Figure 2 The sodium ion secondary battery cycle curve diagram of Example 2 of the present invention and Comparative Example 2;

[0027] Figure 3 1 is a cycle curve diagram of the lithium ion secondary battery of Example 3 of the present invention and Comparative Example 3. DETAILED DESCRIPTION

[0028] The present invention is described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0029] Reference Figures 1 to 3 As shown, in one embodiment, the present invention provides a positive electrode sheet, including a current collector, and an active material layer attached to at least one side of the current collector, wherein the active material layer includes an additive A, and the general formula of the additive A is

[0030]

[0031] Among them, R1 and R2 are selected from any one or more of hydrogen atom, fluorine atom, hydroxyl group, saturated hydrocarbon group, unsaturated hydrocarbon group, aromatic hydrocarbon group, fluorinated hydrocarbon group, amino group, carboxyl group, sulfonic acid group, sulfate group, phosphoric acid group, and amide group, and n=1-4.

[0032] Preferably, the R1 is selected from a hydroxyl group or a fluorine atom; and the R2 is selected from an amino group or an amide group.

[0033] Meanwhile, the active material layer at least includes a positive electrode active material, a conductive agent and a binder, wherein the content of the additive A is 0.01% to 1% of the weight of the positive electrode active material.

[0034] The current collector is not particularly limited and can 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.

[0035] The positive electrode active material includes a lithium-containing positive electrode material or a sodium-containing positive electrode material. When a lithium-containing positive electrode material is used, it can be any one or more of the following materials: lithium cobalt composite oxide and its modified products, lithium manganese composite oxide and its modified products, lithium nickel composite oxide and its modified products, lithium iron phosphate composite oxide and its modified products, lithium manganese phosphate composite oxide and its modified products, lithium vanadium phosphate composite oxide and its modified products, multi-transition metal lithium oxide and its modified products, and lithium manganese-rich multi-transition metal lithium oxide and its modified products.

[0036] When the positive electrode active material adopts a sodium-containing positive electrode material, it can be any one or more of the following materials: layered oxide positive electrode material, polyanion positive electrode material, Prussian white positive electrode material, Prussian blue positive electrode material.

[0037] In addition, there is no particular limitation on the conductive agent, which may be a conventional positive electrode conductive agent in the art, such as one or more of acetylene black, conductive carbon black, conductive graphite, carbon fiber, carbon nanotube and graphene.

[0038] There is no particular limitation on the binder, which may be any binder known to those skilled in the art, such as fluorine-containing resins and polyolefin compounds, such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), cellulose-based binders, rubber-based binders, such as styrene-butadiene rubber (SBR), polyacrylate-based binders, and one or more of polyimide.

[0039] In another embodiment, the present invention further provides a method for preparing a positive electrode sheet, which is at least used to prepare the above-mentioned positive electrode sheet, comprising the following steps:

[0040] S1: Mix the positive electrode active material, conductive agent, binder and additive A, add solvent and stir evenly to obtain electrode slurry, wherein the present invention has no particular limitation on the slurry mixing preparation method, and methods known in the industry can be used, including wet homogenization, semi-wet homogenization, etc.

[0041] S2: After adjusting the viscosity of the electrode slurry, apply it on the current collector and dry it to obtain the positive electrode sheet of the battery.

[0042] Wherein, the additive A is 0.01-1% of the weight of the positive electrode active material in the electrode slurry.

[0043] In addition, in another embodiment, a secondary battery is provided, including a negative electrode sheet, a separator, an electrolyte and a battery casing, and the positive electrode sheet mentioned above.

[0044] Among them, the secondary battery can be prepared by existing processes, such as lithium-ion battery preparation processes or sodium-ion battery preparation processes. For example, the above-mentioned positive electrode plates, separators and negative electrode plates are stacked in sequence to form a pole core or made into a pole core by winding, and then loaded into a battery casing, and the electrolyte is injected into the battery casing. After sealing, formation, capacity division, and standing, it is made into a secondary battery. Other components required for the secondary battery, such as battery casings, separators, electrolytes, negative electrodes, etc. are all commonly used devices in the field, and the present invention is not limited to this.

[0045] In the present invention, by adding a diphosphate compound to the ingredients of the positive electrode material, because its molecular structure contains two phosphate groups, it will neutralize the alkaline substances on the surface of the positive electrode material, such as carbonates, alkali metal hydroxides, etc., and will cover the surface of the positive electrode to form a protective layer with good ionic conductivity to reduce the oxidative decomposition of the electrolyte, thereby improving the stability of the positive electrode material; at the same time, the additive A contains a controllable solvation chain, which improves its compatibility with the electrolyte, is conducive to the release of solvated lithium ions at the interface, reduces the charge migration impedance of the reaction, and thus improves the rate performance of the battery; in addition, the addition of additive A reduces the alkalinity of the electrode slurry, improves the fluidity and stability of the electrode slurry, improves the uniformity of the coating, and thus improves the cycle performance of the battery.

[0046] In addition, in order to better understand the technical solutions provided by the present invention, the following will be described in conjunction with specific examples:

[0047] Embodiment 1:

[0048] 1) Preparation of positive electrode

[0049] Commercial positive electrode material lithium cobalt oxide LiCoO2, conductive carbon black and polyvinylidene fluoride, and additive A are dispersed in N-methylpyrrolidone, and then subjected to wetting, kneading and dispersion treatment to obtain positive electrode slurry, wherein in the structural formula of additive A, R1=hydroxyl OH, n=1, and R2=amino NH2.

[0050] The weight ratio of LCO: conductive carbon black: polyvinylidene fluoride is 98:1:1, and the weight of additive A is 0.05% of the weight of the positive electrode material LCO.

[0051] Subsequently, the positive electrode active slurry is coated on both surfaces of the aluminum foil, and then dried, rolled and cut to obtain the required positive electrode sheet.

[0052] 2) Preparation of negative electrode

[0053] Dissolve the binder CMC (sodium carboxymethyl cellulose) in water, then add the negative electrode material artificial graphite, mix and stir thoroughly, then add SBR styrene butadiene rubber emulsion, continue to mix and stir; then evenly coat the negative electrode slurry on both sides of 10μm copper foil, bake and dry at 100℃, and roll to obtain the negative electrode sheet. The weight ratio of the negative electrode material composition is: graphite: CMC: SBR = 96.5:1.5:2.

[0054] 3) Preparation of lithium-ion secondary batteries

[0055] After the positive and negative electrodes and the polyethylene separator are wound, assembled, inserted into the battery shell, baked, etc., the commonly used electrolyte is injected into the battery aluminum shell in an appropriate amount and sealed, and then the lithium-ion secondary battery is made after aging, formation, volume separation, etc. The mixed solvent ratio in the electrolyte is specifically EC / EMC / DEC=3:5:2 (volume ratio), the concentration of LiPF6 is 1 mol / L, and the additives are 2.5wt% of vinylene carbonate VC and 1wt% of vinyl sulfate DTD.

[0056] Comparative Example 1:

[0057] Compared with Example 1, except that the positive electrode sheet does not contain additive A, other materials and preparation processes are the same.

[0058] Example 2:

[0059] 1) Preparation of positive electrode

[0060] The ternary positive electrode material sodium nickel iron manganese oxide (NFM111), conductive carbon black and polyvinylidene fluoride, and additive A are dispersed in N-methylpyrrolidone, and then the positive electrode slurry is obtained through wetting, kneading and dispersion treatment, wherein in the structural formula of additive A, R1=hydroxyl OH, n=1, and R2=amino NH2.

[0061] The weight ratio of NFM111: conductive carbon black: polyvinylidene fluoride is 97.5:1:1.5; the weight of additive A is 0.2% of the weight of the positive electrode material NFM111.

[0062] Subsequently, the positive electrode active slurry is coated on both surfaces of the aluminum foil, and then dried, rolled and cut to obtain the required positive electrode sheet.

[0063] 2) Preparation of negative electrode

[0064] Dissolve the binder CMC (sodium carboxymethyl cellulose) in water, then add conductive carbon black and negative electrode material hard carbon, mix and stir thoroughly, then add SBR styrene-butadiene rubber emulsion, continue mixing and stirring; then evenly coat the negative electrode slurry on both sides of 10μm copper foil, bake and dry at 100℃, and roll to obtain the negative electrode sheet. The weight ratio of the negative electrode material composition is: hard carbon: super-p: CMC: SBR = 94.3: 1.5: 1.5: 2.7.

[0065] 3) Preparation of sodium ion secondary batteries

[0066] After the positive and negative electrodes and the polyethylene separator are respectively wound, assembled, inserted into the battery shell, baked and other processes, the commonly used electrolyte is appropriately injected into the battery aluminum shell and sealed, and then the sodium ion secondary battery is made after aging, formation, volume separation and other processes. Among them, the mixed solvent ratio in the electrolyte is specifically EC / PC / DMC=0.45:0.45:0.1 (volume ratio), the additive is 5wt% of fluoroethylene carbonate FEC, and the concentration of NaPF6 is 0.8mol / L.

[0067] Comparative Example 2:

[0068] Compared with Example 2, except that the positive electrode sheet does not contain additive A, other materials and preparation processes are the same.

[0069] Embodiment 3:

[0070] 1) Preparation of positive electrode

[0071] The ternary positive electrode material LiNi0.5Co0.2Mn0.3O2 (NCM523), conductive carbon black and polyvinylidene fluoride, and additive A are dispersed in N-methylpyrrolidone, and then the positive electrode slurry is obtained through wetting, kneading and dispersion treatment, wherein in the structural formula of additive A, R1=hydroxyl OH, n=2, and R2=amino NH2.

[0072] Among them, 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 523.

[0073] 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.

[0074] 2) Preparation of negative electrode

[0075] Dissolve the binder CMC (sodium carboxymethyl cellulose) in water, then add the negative electrode material artificial graphite, mix and stir thoroughly, then add SBR styrene-butadiene rubber emulsion, continue to mix and stir; then evenly coat the negative electrode slurry on both sides of a 10μm copper foil, bake and dry at 100°C, and roll to obtain a negative electrode sheet. The weight ratio of the negative electrode material composition is: graphite: CMC: SBR = 96.6:1.4:2.

[0076] 3) Preparation of lithium-ion secondary batteries

[0077] After the positive and negative electrodes and the polyethylene separator are wound, assembled, inserted into the battery shell, baked, etc., the commonly used electrolyte is injected into the battery aluminum shell in an appropriate amount and sealed, and then the lithium-ion secondary battery is made after aging, formation, volume separation, etc. The mixed solvent ratio in the electrolyte is specifically EC / EMC / DEC=1:1:1 (volume ratio), the concentration of LiPF6 is 1 mol / L, and the additives are 2wt% of ethylene carbonate VC and 1wt% of ethylene sulfate DTD.

[0078] Comparative Example 3:

[0079] Compared with Example 3, except that the positive electrode sheet does not contain additive A, other materials and preparation processes are the same.

[0080] 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 leaving it for 30 minutes, discharge it to the cut-off voltage at 0.5C, leave it for 30 minutes, repeat the charge and discharge steps, and stop when the test termination conditions are reached.

[0081] Among them, the charging range of Example 1 and Comparative Example 1 is 3.0-4.45V; 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 3.0-4.35V.

[0082] 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.

[0083] Among them, the charging range of Example 1 and Comparative Example 1 is 3.0-4.45V; 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 3.0-4.35V. The test results are shown in the following table:

[0084]

[0085] Table 1 Comparison of rate performance of batteries of Examples 1-3 and Comparative Examples 1-3

[0086] from Figures 1 to 3 It can be clearly seen that the lithium / sodium ion secondary battery made of the positive electrode sheet containing additive A has a significantly higher capacity residual rate than that of the comparative example, and the cycle performance is significantly improved.

[0087] Specifically, Figure 1 As shown, in the lithium cobalt oxide high-voltage system, after 500 cycles, the battery capacity remaining rate of Example 1 is 93.5%, and the capacity remaining rate of Comparative Example 1 is 88.2%, 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 obtained in the layered ternary sodium ion battery system and the ternary lithium ion battery system.

[0088] In addition, it can be seen from Table 1 that after adding 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 0.6%, 1.6%, and 3.2% 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 interfacial polarization resistance at the positive electrode of the battery and improves the battery discharge rate performance.

[0089] The above are only preferred embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the protection scope 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, the general formula of which is Among them, R1 and R2 are selected from any one or more of hydrogen atom, fluorine atom, hydroxyl group, saturated hydrocarbon group, unsaturated hydrocarbon group, aromatic hydrocarbon group, fluorinated hydrocarbon group, amino group, carboxyl group, sulfonic acid group, sulfate group, phosphoric acid group, and amide group, and n=1-4.

2. The positive electrode sheet according to claim 1, characterized in that: The R1 is selected from a hydroxyl group or a fluorine atom; and the R2 is selected from an amino group or an amide group.

3. The positive electrode sheet according to claim 1, characterized in that: The active material layer at least includes a positive electrode active material, a conductive agent and a binder.

4. The positive electrode sheet according to claim 3, characterized in that: The content of the additive A is 0.01% to 1% of the weight of the positive electrode active material.

5. The positive electrode sheet according to claim 3, characterized in that: The positive electrode active material includes a positive electrode material containing lithium or a positive electrode material containing sodium.

6. The positive electrode sheet according to claim 5, characterized in that: The positive electrode active material includes any one or more of the following materials: lithium cobalt composite oxide and its modified products, lithium manganese composite oxide and its modified products, lithium nickel composite oxide and its modified products, lithium iron phosphate composite oxide and its modified products, lithium manganese phosphate composite oxide and its modified products, lithium vanadium phosphate composite oxide and its modified products, multi-conductor transition metal lithium oxide and its modified products, and lithium manganese-rich multi-conductor transition metal lithium oxide and its modified products.

7. The positive electrode sheet according to claim 5, characterized in that: The positive electrode active material includes any one or more of the following materials: layered oxide positive electrode material, polyanion positive electrode material, Prussian white positive electrode material, Prussian blue positive electrode material.

8. A method for preparing a positive electrode sheet, at least for preparing the positive electrode sheet as claimed in any one of claims 3 to 7, characterized in that: The steps include: S1: Mix the positive electrode active material, the conductive agent, the binder and the additive A, add the solvent and stir evenly to obtain the electrode slurry; S2: After adjusting the viscosity of the electrode slurry, apply it on the current collector and dry it to obtain the positive electrode sheet of the battery.

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

10. 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 3 to 7.

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