Positive pole piece, secondary battery and electric device

Abstract

The invention relates to the technical field of batteries, and relates to a positive pole piece, a secondary battery and a power utilization device, the secondary battery comprises the positive pole piece, the positive pole piece comprises a water absorption material, and the water absorption material comprises at least one of polyacrylate, polyacrylamide and polyvinyl alcohol; according to the technical scheme provided by the invention, the long-term cycle performance of the secondary battery can be improved.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a positive electrode, a secondary battery, and an electrical device. Background Technology

[0002] Secondary batteries have attracted much attention due to their long lifespan, high energy density, and low maintenance costs, and are currently widely used in electric vehicles, portable electronic devices, energy storage systems, and other fields. However, in some related technologies, the structure of the positive electrode in a secondary battery is damaged, thus affecting its long-term cycle performance. Summary of the Invention

[0003] The main objective of this invention is to provide a positive electrode, a secondary battery, and an electrical device, which aims to improve the long-term cycle performance of the secondary battery.

[0004] To achieve the above objectives, the first aspect of the present invention provides a secondary battery, the secondary battery comprising a positive electrode sheet, the positive electrode sheet comprising a water-absorbing material, the water-absorbing material comprising at least one of polyacrylate, polyacrylamide and polyvinyl alcohol.

[0005] In the secondary battery provided by the present invention, the positive electrode sheet includes at least one water-absorbing material selected from polyacrylate, polyacrylamide and polyvinyl alcohol. Compared with conventional water-absorbing materials (such as silicon nitrogen compounds and isocyanate compounds), the above-mentioned water-absorbing materials absorb water faster, have higher efficiency, are non-toxic, and are less likely to form solid precipitates. They can effectively reduce the water content of the positive electrode sheet, thereby effectively reducing the damage to the structure of the positive electrode sheet and improving the long-term cycle performance of the secondary battery.

[0006] In one embodiment, the positive electrode sheet includes a positive active material layer, and the positive active material layer includes a water-absorbing material.

[0007] This invention incorporates a water-absorbing material into the positive electrode active material layer. Specifically, the water-absorbing material is added to the positive electrode slurry and then the positive electrode active material layer is fabricated. As a result, the water-absorbing material not only exhibits good water absorption but also demonstrates good long-term stability.

[0008] In one embodiment, the water-absorbing material accounts for 0.5%-2% of the total mass of the positive electrode active material layer.

[0009] The present invention selects an appropriate amount of water-absorbing material, which can ensure that the positive electrode has strong water absorption, thereby significantly reducing the water content of the positive electrode and thus more effectively improving the long-term cycle performance of the secondary battery.

[0010] In one embodiment, the positive electrode active material layer further includes a positive electrode active material with a particle size of 2 μm-20 μm; and / or, the specific surface area of ​​the positive electrode active material is 5 m².2 / g-20m 2 / g; and / or, the compaction density of the positive electrode active material is 2.3 g / cm³. 3 -2.8g / cm 3 .

[0011] The present invention selects positive electrode active materials with suitable particle size, suitable specific surface area, and suitable compaction density, which can ensure that the appropriate amount of water-absorbing material is used in the positive electrode sheet.

[0012] In one embodiment, the positive electrode active material includes lithium iron phosphate materials.

[0013] The secondary battery of the present invention is a lithium iron phosphate battery, wherein lithium iron phosphate is more sensitive to water, and the positive electrode includes a water-absorbing material, which significantly improves the long-term cycle performance of the lithium iron phosphate battery.

[0014] In one embodiment, the positive electrode includes a water-absorbing layer, which includes a water-absorbing material.

[0015] The water-absorbing layer of this invention can be disposed between the positive electrode current collector and the positive electrode active material layer (i.e., on the side of the positive electrode active material facing the positive electrode current collector), or on the side of the positive electrode active material facing away from the positive electrode current collector, or simultaneously on both sides of the positive electrode active material layer. The water-absorbing layer is made of water-absorbing material through coating or other reasonable methods, which can effectively reduce the water content of the positive electrode sheet, improve the long-term cycle performance of the secondary battery, and is relatively simple to manufacture.

[0016] In one embodiment, the content of the absorbent material in the absorbent layer is 80wt%-95wt%.

[0017] The water-absorbing layer of this invention uses an appropriate amount of water-absorbing material, which can fully exert its water absorption effect, ensure that the positive electrode has strong water absorption, significantly reduce the water content of the positive electrode, and thus more effectively improve the long-term cycle performance of the secondary battery; at the same time, it is easy to manufacture the water-absorbing layer.

[0018] In one embodiment, the thickness of the absorbent layer is 1 μm-10 μm.

[0019] The present invention selects an absorbent layer of appropriate thickness, which can give full play to its water absorption effect, while avoiding the problem of large internal resistance and reduced cycle life of secondary batteries due to excessive thickness.

[0020] In one embodiment, the bonding force of the positive electrode sheet is 5 N / m-15 N / m.

[0021] The present invention selects a positive electrode sheet with suitable adhesion, which can ensure the stability and reliability of the positive electrode sheet including the water-absorbing material, and also facilitates the processing and manufacturing of the positive electrode sheet.

[0022] A second aspect of the present invention also provides a positive electrode sheet, wherein the positive electrode sheet is the same as the positive electrode sheet included in the secondary battery provided in the first aspect of the present invention.

[0023] A third aspect of the present invention also provides an electrical device, the electrical device comprising the secondary battery provided in the first aspect of the present invention.

[0024] In the secondary battery provided by the present invention, the positive electrode sheet includes at least one water-absorbing material of polyacrylate, polyacrylamide and polyvinyl alcohol. Compared with conventional water-absorbing materials (such as silicon nitrogen compounds and isocyanate compounds), the above-mentioned water-absorbing materials absorb water faster, have higher efficiency, are non-toxic, and are not prone to forming solid precipitates. They can effectively reduce the water content of the positive electrode sheet, thereby effectively reducing the damage to the structure of the positive electrode sheet and improving the long-term cycle performance of the secondary battery. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram comparing the storage capacity retention rates of the lithium-ion batteries obtained in Example 1 and Comparative Example 1 of the present invention.

[0027] Figure 2 This is a schematic diagram of a secondary battery according to an embodiment of the present invention;

[0028] Figure 3 yes Figure 2 An exploded view of a secondary battery according to an embodiment of the present invention is shown.

[0029] Explanation of icon numbers:

[0030] 5. Secondary battery; 51. Housing; 52. Electrode assembly; 53. Cover plate.

[0031] The realization of the objective, functional features and advantages of the present invention will be further explained in conjunction with the embodiments and with reference to the accompanying drawings. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0033] The positive electrode, secondary battery, and power supply device of the present invention are hereby specifically disclosed in detail with appropriate reference to the accompanying drawings. However, unnecessary detailed descriptions may be omitted. For example, detailed descriptions of well-known matters and repetitive descriptions of practically identical structures may be omitted. This is to avoid unnecessarily lengthy descriptions and to facilitate understanding by those skilled in the art. Furthermore, the accompanying drawings and the following description are provided to enable those skilled in the art to fully understand the invention and are not intended to limit the subject matter of the claims.

[0034] The "range" disclosed in this invention is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the particular range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a specific parameter, it is understood that ranges of 60-110 and 80-120 are also expected. Furthermore, if minimum range values ​​1 and 2 are listed, and if maximum range values ​​3, 4, and 5 are listed, then the following ranges are all expected: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In this invention, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers between a and b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed in this article; "0-5" is simply a shortened representation of these numerical combinations. Furthermore, when a parameter is stated as an integer ≥2, it is equivalent to disclosing that the parameter is, for example, an integer such as 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0035] Unless otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form new technical solutions.

[0036] Unless otherwise specified, all technical features and optional technical features of this invention can be combined to form new technical solutions.

[0037] Unless otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0038] Typically, a secondary battery includes a positive electrode, a negative electrode, a separator, and an electrolyte. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing active ions to pass through. The electrolyte, also positioned between the positive and negative electrodes, mainly conducts the active ions.

[0039] For secondary batteries, if the water content of the positive electrode is too high, it will react chemically with hexafluorophosphate ions in the electrolyte to produce hydrofluoric acid and hydrogen gas, which will damage the structure of the positive electrode and affect the long-term cycle performance of the secondary battery.

[0040] As one example, in a lithium-ion battery, excessive water content in the positive electrode will react chemically with lithium hexafluorophosphate in the electrolyte, producing hydrofluoric acid and hydrogen gas. This damages the positive electrode structure and affects the long-term cycle performance of the lithium-ion battery. Similarly, in a sodium-ion battery, excessive water content in the positive electrode will react chemically with sodium hexafluorophosphate in the electrolyte, producing hydrofluoric acid and hydrogen gas. This also damages the positive electrode structure and affects the long-term cycle performance of the sodium-ion battery.

[0041] In view of this, the first aspect of the present invention provides a secondary battery, which aims to reduce the water content of the positive electrode sheet of the secondary battery, thereby reducing the damage to the structure of the positive electrode sheet and improving the long-term cycle performance of the secondary battery.

[0042] The secondary battery provided by the present invention includes a positive electrode sheet, which includes a water-absorbing material, and the water-absorbing material includes at least one of polyacrylate, polyacrylamide and polyvinyl alcohol.

[0043] Typically, a secondary battery consists of a positive electrode, a negative electrode, an electrolyte, and a separator. During charging and discharging, active ions move back and forth between the positive and negative electrodes, inserting and releasing. The electrolyte acts as a conductor between the positive and negative electrodes. The separator, positioned between the positive and negative electrodes, primarily prevents short circuits while allowing ions to pass through.

[0044] The positive electrode includes a positive current collector and a positive active material layer disposed on at least one surface of the positive current collector, wherein the positive active material layer includes a positive electrode material.

[0045] As an example, the positive current collector has two surfaces opposite each other in its own thickness direction, and the positive active material layer is disposed on either or both of the two opposite surfaces of the positive current collector.

[0046] In some embodiments, the positive current collector may be a metal foil or a composite current collector. For example, aluminum foil may be used as the metal foil. The composite current collector may include a polymer substrate and a metal layer formed on at least one surface of the polymer substrate. The composite current collector may be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0047] The positive electrode sheet includes a water-absorbing material, which can be disposed within the positive electrode active material layer; alternatively, the water-absorbing material can be disposed outside the positive electrode active material layer, for example, the positive electrode sheet includes a water-absorbing layer, and the water-absorbing layer includes the water-absorbing material, i.e., the water-absorbing material is disposed separately. The water-absorbing layer can be disposed between the positive electrode current collector and the positive electrode active material layer, or it can be disposed on the side surface of the positive electrode active material layer facing away from the current collector. When the positive electrode current collector is a composite current collector, the composite current collector includes a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. In this case, the water-absorbing material can be disposed within the polymer material base layer, or it can be disposed separately as a water-absorbing layer on one side surface of the base layer. Of course, the water-absorbing material can also be disposed simultaneously in the positive electrode active material layer, the water-absorbing layer located on both sides of the positive electrode active material layer, and the polymer material layer of the composite current collector. Any positive electrode sheet that includes water-absorbing material is within the protection scope of this invention.

[0048] The positive electrode of this invention includes a water-absorbing material. The water-absorbing material's molecular structure contains a large number of anions, enabling it to exchange ions with cations in water, forming an ion adsorption layer. This causes water molecules to aggregate and align, increasing the surface tension of the water and making it easier to absorb. Alternatively, its molecules have strong hydrophilicity, forming hydrogen bonds and electrostatic interactions with water molecules, allowing the water-absorbing material molecules to combine with water molecules to form a stable substance that does not react with the electrolyte. In other words, the water-absorbing material is a substance that can react with water or can absorb water without reacting with the electrolyte. Compared to conventional positive electrode sheets, using a positive electrode sheet including a water-absorbing material significantly reduces the water content, thereby improving the long-term cycle performance of the secondary battery. Simultaneously, because water undergoes electrochemical decomposition during cycling to generate hydrogen and oxygen, the reduced water content in the positive electrode sheet also reduces its gas production during cycling.

[0049] The polyacrylates of the present invention include, but are not limited to, sodium polyacrylate and potassium polyacrylate.

[0050] In some embodiments, the absorbent material includes sodium polyacrylate. Sodium polyacrylate is a highly absorbent substance with a large number of anions in its molecular structure, which can exchange ions with cations in water to form an ion adsorption layer. This causes water molecules to aggregate and arrange, increasing the surface tension of the water and making it easier to absorb. Polyacrylic acid molecules have strong hydrophilicity and can form hydrogen bonds and electrostatic forces with water molecules, so that polyacrylic acid molecules and water molecules combine to form a stable white flocculent substance. This substance does not react with the electrolyte and can still remain stable and lock in moisture at temperatures below 80°C.

[0051] This embodiment utilizes the strong water absorption property of sodium polyacrylate to add sodium polyacrylate powder to the bottom coating to prepare a positive electrode sheet. When water enters the positive electrode sheet, the sodium polyacrylate will absorb this part of the water and form a white flocculent. The flocculent formed by the combination of polyacrylic acid molecules and water molecules will not react with the electrolyte. At the same time, it is a soft and elastic flocculent and will not produce particles that puncture the diaphragm and cause safety problems. The water in the compound will not decompose at a high temperature of 120°C, so it can be used in high-temperature conditions.

[0052] It should be noted that if potassium polyacrylate is used as the absorbent material, the resulting flocculent material after absorbing water will be relatively yellowish.

[0053] In some embodiments, the absorbent material is polyacrylamide. Polyacrylamide absorbs water molecules and hydrates with the groups of the cross-linked polymer, releasing a large number of cations. The cations move relatively freely, while the released polymer ions are in a relatively stationary state. There is a strong electrostatic repulsion between the two, which can cause the polymer chains to extend and eventually lead to the spatial expansion of the entire network structure. Water molecules hydrate with the ions released in the network structure. After hydration, an environment different from free water is formed, with a strong osmotic pressure inside and outside. Under the action of this osmotic pressure, surrounding water molecules can continuously enter the cross-linked polymer network, giving polyacrylamide a strong water absorption capacity.

[0054] In some embodiments, the absorbent material is polyvinyl alcohol (PVA), which is a strong absorbent material. Before absorbing water, the PVA crosslinks are in a tight solid state, with their long polymer chains intertwined and coiled, forming a three-dimensional network-like crosslinked structure. When exposed to water, some groups in the crosslinks are released from the network, while the remaining charged groups repel each other, fully expanding the polymer chains, much like opening a large net bag, expanding the three-dimensional network many times over, which is then filled with absorbed water.

[0055] In the secondary battery provided by the present invention, the positive electrode sheet includes at least one water-absorbing material of polyacrylate, polyacrylamide and polyvinyl alcohol. Compared with conventional water-absorbing materials (such as silicon nitrogen compounds and isocyanate compounds), the above-mentioned water-absorbing materials absorb water faster, have higher efficiency, are non-toxic, and are not prone to forming solid precipitates. They can effectively reduce the water content of the positive electrode sheet, thereby effectively reducing the damage to the structure of the positive electrode sheet and improving the long-term cycle performance of the secondary battery.

[0056] In some embodiments, the positive electrode sheet includes a positive active material layer, which includes a water-absorbing material. This invention incorporates the water-absorbing material within the positive active material layer, i.e., the water-absorbing material is added to the positive electrode slurry before forming the positive active material layer. This allows the water-absorbing material to exhibit both good water absorption and good long-term stability.

[0057] In some embodiments, the mass of the water-absorbing material accounts for 0.5%-2% of the total mass of the positive electrode active material layer (e.g., 0.5%, 1%, 1.5%, 2%, and any range between the two endpoints). By selecting an appropriate amount of water-absorbing material, this invention ensures that the positive electrode sheet has strong water absorption, resulting in a more significant reduction in the water content of the positive electrode sheet, thereby more effectively improving the long-term cycle performance of the secondary battery.

[0058] In some embodiments, the secondary battery is a lithium-ion battery, and the positive electrode active material can be a positive electrode active material known in the art for lithium-ion batteries. As an example, the positive electrode active material may include at least one of the following materials: lithium phosphates with an olivine structure, lithium transition metal oxides, and their respective modified compounds. However, the present invention is not limited to these materials, and other conventional materials that can be used as battery positive electrode active materials may also be used. These positive electrode active materials may be used alone or in combination of two or more. Examples of lithium transition metal oxides include, but are not limited to, lithium cobalt oxides (such as LiCoO2), lithium nickel oxides (such as LiNiO2), lithium manganese oxides (such as LiMnO2, LiMn2O4), lithium nickel cobalt oxides, lithium manganese cobalt oxides, lithium nickel manganese oxides, and lithium nickel cobalt manganese oxides (such as LiNi). 1 / 3 Co 1 / 3 Mn 1 / 3 O2 (also known as NCM) 333 LiNi 0.5 Co 0.2 Mn 0.3 O2 (also known as NCM) 523 LiNi 0.5 Co 0.25 Mn 0.25 O2 (also known as NCM) 211 LiNi 0.6 Co 0.2 Mn 0.2 O2 (also known as NCM) 622 LiNi 0.8 Co 0.1 Mn 0.1 O2 (also known as NCM) 811 ), lithium nickel cobalt aluminum oxide (such as LiNi) 0.85 Co 0.15 Al 0.05 At least one of O2 and its modified compounds. Examples of lithium phosphates with an olivine structure include, but are not limited to, lithium iron phosphate (such as LiFePO4 (also referred to as LFP)), lithium iron phosphate and carbon composites, lithium manganese phosphate (such as LiMnPO4), lithium manganese phosphate and carbon composites, lithium manganese iron phosphate, and lithium manganese iron phosphate and carbon composites.

[0059] In some embodiments, the secondary battery is a sodium-ion battery, and the positive electrode active material may be a positive electrode active material known in the art for sodium-ion batteries.

[0060] In an optional embodiment of the present invention, the transition metal in the sodium transition metal oxide can be at least one selected from Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce, and the sodium transition metal oxide is, for example, Na. x MO2, where M is one or more of Ti, V, Mn, Co, Ni, Fe, Cr and Cu, and 0 < x ≤ 1.

[0061] In an optional embodiment of the present invention, the polyanionic compound may be a compound containing sodium ions, transition metal ions, or a tetrahedral (YO4) structure. n- A class of compounds with anionic units. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si; n represents (YO4). n- The price state.

[0062] Polyanionic compounds can also contain sodium ions, transition metal ions, or tetrahedral (YO4) ions. n- A class of compounds containing anionic units and halide anions. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; Y can be at least one of P, S, and Si; n represents (YO4). n- The valence state; the halogen can be at least one of F, Cl and Br.

[0063] Polyanionic compounds can also be sodium-containing tetrahedral (YO4) compounds. n- Anionic unit, tetrahedral unit (ZO) y ) m+ And a class of compounds with optional halide anions, where Y can be at least one of P, S, and Si; n represents (YO4). n- The valence state; Z represents a transition metal, which can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce; m represents (ZO) y ) m+ The valence state; the halogen can be at least one of F, Cl and Br.

[0064] Prussian blue compounds can contain sodium ions, transition metal ions, and cyanide ions (CN). - A class of compounds. The transition metal can be at least one of Mn, Fe, Ni, Co, Cr, Cu, Ti, Zn, V, Zr, and Ce. Prussian blue compounds are, for example, Na. a Me b Me' c(CN)6, wherein Me and Me' are each independently at least one of Ni, Cu, Fe, Mn, Co and Zn, 0 < a ≤ 2, 0 < b < 1, 0 < c < 1.

[0065] In some embodiments, the adhesive may include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), PVDF-tetrafluoroethylene-propylene terpolymer, PVDF-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, and fluorinated acrylate resin.

[0066] In some embodiments, the conductive agent may include at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0067] In some embodiments, the solvent includes at least one of N-methylpyrrolidone and dimethyl sulfoxide.

[0068] In some embodiments, the mass ratio of positive electrode active material, conductive agent, binder, and water-absorbing material in the positive electrode active material layer is (90-97):(1-4):(1-4):(0.5-2), for example, mass ratios of 90:1:1:0.5, 94:1:1:0.5, 97:1:1:0.5, 90:2:2:1, 90:4:2:1, 90:4:4:2, 94:2:3:1, 97:1:1:1, and any range between the two endpoints. While ensuring a high content of positive electrode active material in the positive electrode active material layer, this invention also ensures that the positive electrode sheet has strong water absorption, resulting in a more significant reduction in the water content of the positive electrode sheet, thereby more effectively improving the long-term cycle performance of the secondary battery.

[0069] In some embodiments, the particle size of the positive electrode active material is 2μm-20μm (e.g., 2μm, 5μm, 10μm, 15μm, 20μm, and any range between two endpoints). 。

[0070] The particle size of the positive electrode active material affects its specific surface area, which in turn affects its water content, and consequently, the amount of absorbent material required. Smaller particle sizes result in larger specific surface areas and higher water content, thus requiring a greater amount of absorbent material. Therefore, to ensure an appropriate amount of absorbent material is used, the positive electrode active material must be selected with a suitable particle size.

[0071] In some embodiments, the specific surface area of ​​the positive electrode active material is 5m². 2 / g-20m 2 / g(5m 2 / g, 10m 2 / g, 15m 2 / g、20m2 / g and the range between any two endpoints). This invention selects a positive electrode active material with a suitable specific surface area, ensuring an appropriate amount of water-absorbing material in the positive electrode sheet. As an example, the specific surface area of ​​the positive electrode active material is 15m². 2 / g.

[0072] In some embodiments, the compaction density of the positive electrode active material is 2.3 g / cm³. 3 -2.8g / cm 3 (e.g., 2.3g / cm) 3 2.4g / cm 3 2.5g / cm 3 2.6g / cm 3 2.7g / cm 3 2.8g / cm 3 (And the range between any two endpoints). Compacted density refers to the mass of the positive electrode active material per unit volume; the higher the compacted density, the lower the water content. This invention selects positive electrode active materials with appropriate compacted densities to ensure a suitable amount of water-absorbing material in the positive electrode sheet, while simultaneously ensuring good energy density and cycle life of the secondary battery. If the compacted density is too high, it will cause the positive electrode active material to pulverize, reducing the cycle life of the secondary battery; if the compacted density is too low, it will reduce the battery's energy density.

[0073] In some embodiments, the positive electrode active material includes lithium iron phosphate materials, which include lithium iron phosphate and modified lithium iron phosphate doped with certain other elements. Specifically, the general chemical formula of lithium iron phosphate materials is LiFe. x M (1-x) PO4, where M is at least one of Mn and Co, and 0 < x ≤ 1.

[0074] In one specific embodiment, x = 1, and the general chemical formula of the lithium iron phosphate material is LiFePO4; in another specific embodiment, x = 0.8, M is Mn, and the general chemical formula of the lithium iron phosphate material is LiFe 0.8 Mn 0.2 PO4.

[0075] The secondary battery of the present invention is a lithium iron phosphate battery. Lithium iron phosphate is more sensitive to water. After adding water-absorbing materials to the positive electrode slurry, the long-term cycle performance of the lithium iron phosphate battery is improved more significantly.

[0076] Furthermore, lithium iron phosphate should also be selected with an appropriate particle size to ensure an appropriate amount of absorbent material is used. Selective lithium iron phosphate can be 2μm-20μm (e.g., 2μm, 5μm, 10μm, 15μm, 20μm and any range between the two endpoints).

[0077] In some embodiments, the positive electrode sheet can be prepared by dispersing the above-mentioned components for preparing the positive electrode sheet, such as positive active material, conductive agent, binder, water-absorbing material and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry; coating the positive electrode slurry onto the positive electrode current collector, and then obtaining the positive electrode sheet after drying, cold pressing and other processes.

[0078] In some embodiments, the solid content of the positive electrode slurry is 60wt%-75wt%, such as 60wt%, 62wt%, 65wt%, 68wt%, 70wt%, 72wt%, 75wt%, and any range between the two endpoints. Thus, the appropriate solid content makes the positive electrode slurry of the present invention less prone to particle agglomeration, layering, or precipitation, thereby extending the storage time of the positive electrode slurry, reducing the number of agglomeration points on the positive electrode sheet, reducing the resistivity difference of the positive electrode sheet, and thus reducing the DC impedance of the secondary battery and improving the long-term cycle performance of the secondary battery.

[0079] In some embodiments, the viscosity of the cathode slurry at room temperature (25°C) is 4000 mPa·s to 10000 mPa·s (e.g., 4000 mPa·s, 5000 mPa·s, 6000 mPa·s, 7000 mPa·s, 8000 mPa·s, 9000 mPa·s, 10000 mPa·s, and any range between these two endpoints). The cathode slurry of this invention has a suitable viscosity, preventing damage to dispersibility due to excessive viscosity and preventing stratification or precipitation due to excessively low viscosity. This maintains high dispersibility while improving the stability of the cathode slurry and extending its shelf life.

[0080] In some embodiments, the positive electrode slurry of the present invention can be prepared by the following steps:

[0081] (a) A mixture is obtained by mixing the positive electrode active material, conductive agent, binder and water-absorbing material;

[0082] (b) Add solvent to the mixture and stir to obtain the positive electrode slurry.

[0083] Specifically, according to the above mass ratio and solid content, weigh the positive electrode active material, conductive agent, binder, water-absorbing material and solvent. Then, first mix the positive electrode active material, conductive agent, binder and water-absorbing material, and then add the solvent. After stirring, the positive electrode slurry can be obtained. This preparation method is simple to operate and can obtain a uniformly dispersed positive electrode slurry.

[0084] Therefore, the positive electrode slurry prepared by the present invention contains water-absorbing material that can react with water or absorb water without reacting with the electrolyte. Compared with conventional positive electrode sheets, the positive electrode sheet of the present invention, which includes the above-mentioned positive electrode slurry, has a significantly reduced water content, thereby significantly improving the long-term cycle performance of the secondary battery.

[0085] In some embodiments, in the step of adding the solvent to the mixture and stirring to obtain the positive electrode slurry, the stirring temperature is 5-60°C (e.g., 5°C, 10°C, 20°C, 30°C, 40°C, 50°C, 60°C, and any range between two endpoints), the stirring speed is 600-1800 rpm (e.g., 600 rpm, 800 rpm, 1000 rpm, 1200 rpm, 1400 rpm, 1600 rpm, 1800 rpm, and any range between two endpoints), and the stirring time is 80-150 min (e.g., 80 min, 90 min, 100 min, 110 min, 120 min, 130 min, 140 min, 150 min, and any range between two endpoints). This results in a uniformly dispersed positive electrode slurry, which helps to extend the storage time of the positive electrode slurry, reduce the number of agglomeration points on the positive electrode, reduce the resistance difference of the positive electrode, thereby reducing the DC resistance of the secondary battery, improving the long-term cycle performance of the secondary battery, and also reducing its cycle gas production.

[0086] In some embodiments, the method for preparing the positive electrode sheet includes the following steps:

[0087] The positive electrode slurry is coated on at least one surface of the positive electrode current collector, and after drying, cold pressing and cutting, a positive electrode sheet is obtained.

[0088] Therefore, this invention utilizes the water absorption properties of water-absorbing materials to add them to the positive electrode slurry to prepare the positive electrode sheet. When water enters the positive electrode sheet, the water-absorbing material will absorb this part of the water and will not react with the electrolyte. Compared with conventional positive electrode sheets, the water content of the positive electrode sheet prepared by this invention is significantly reduced, thereby significantly improving the long-term cycle performance of the secondary battery, while also significantly reducing its cycle gas production.

[0089] In some embodiments, the positive electrode sheet includes a water-absorbing layer, which includes a water-absorbing material.

[0090] The absorbent layer, as a separate layer, can selectively contain the positive electrode active material. For example, the absorbent layer can be placed between the positive electrode current collector and the positive electrode active material layer (i.e., on the surface of the positive electrode active material facing the positive electrode current collector), or on the surface of the positive electrode active material facing away from the positive electrode current collector. Alternatively, it can be placed on both surfaces of the positive electrode active material layer. The absorbent layer is made of absorbent material through coating or other suitable methods. This effectively reduces the water content of the positive electrode sheet, improves the long-term cycle performance of the secondary battery, and is relatively simple to manufacture.

[0091] In some embodiments, the content of the absorbent material in the absorbent layer is 80wt%-95wt% (e.g., 80wt%, 85wt%, 90wt%, 95wt%, and any range between two endpoints).

[0092] The water-absorbing layer of this invention uses an appropriate amount of water-absorbing material, which can fully exert its water absorption effect, ensure that the positive electrode has strong water absorption, significantly reduce the water content of the positive electrode, and thus effectively improve the long-term cycle performance of the secondary battery; at the same time, it is easy to manufacture the water-absorbing layer.

[0093] It should be noted that the absorbent layer may also include at least one of a conductive agent and a binder. For example, when the absorbent layer is disposed between the current collector and the positive electrode active material layer (i.e., the inner layer of the positive electrode sheet), the absorbent layer includes an absorbent material, a conductive carbon agent, and a binder, thereby ensuring both good water absorption and conductivity. When the absorbent layer is disposed on the outer surface of the positive electrode active material layer (i.e., the outer layer of the positive electrode sheet), the absorbent layer only needs to include an absorbent material and a binder. Since the absorbent materials used in this invention (including at least one of polyacrylate, polyacrylamide, and polyvinyl alcohol) themselves have good film-forming ability and good adhesion, the amount of binder used in making the absorbent layer is relatively small; and it is beneficial to produce a uniform, continuous, and stable absorbent layer, thereby improving the long-term water absorption performance and service life of the positive electrode sheet, while also being easy to process and manufacture.

[0094] In some embodiments, the thickness of the absorbent layer is 1μm-10μm (e.g., 1μm, 2μm, 5μm, 8μm, 10μm, and any range between two endpoints). This invention selects an absorbent layer of suitable thickness to fully utilize its absorbency without causing excessive internal resistance or reduced cycle life in the secondary battery due to excessive thickness.

[0095] As an example, a water-absorbing layer is provided on both sides of the positive electrode active material layer, and the thickness of the water-absorbing layer is 2μm.

[0096] In some embodiments, the bonding force of the positive electrode sheet is 5 N / m-15 N / m (e.g., 5 N / m, 10 N / m, 15 N / m, and any range between the two endpoints). The present invention selects a positive electrode sheet with suitable bonding force to ensure the stability and reliability of the positive electrode sheet including the absorbent material, while also facilitating the processing and fabrication of the positive electrode sheet. If the bonding force is lower than 5 N / m, the positive electrode sheet including the absorbent material will detach during the preparation process or when subjected to external impact or vibration, thereby affecting the performance and lifespan of the secondary battery; if the bonding force is higher than 15 N / m, the flexibility of the positive electrode sheet is relatively low, which may lead to cracks or breakage.

[0097] It should be noted that the absorbent material used in this invention not only has good water absorption properties, but also good adhesion properties, thereby improving the adhesion of the positive electrode sheet including the absorbent material.

[0098] The second aspect of the present invention also provides a positive electrode sheet. The specific structure and fabrication of the positive electrode sheet can be referred to the above embodiments, and will not be repeated here.

[0099] The negative electrode sheet includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0100] As an example, the negative electrode current collector has two surfaces opposite each other in its own thickness direction, and the negative electrode film layer is disposed on either or both of the two opposite surfaces of the negative electrode current collector.

[0101] In some embodiments, the negative electrode current collector may be a metal foil or a composite current collector. For example, copper foil may be used as the metal foil. The composite current collector may include a polymer material substrate and a metal layer formed on at least one surface of the polymer material substrate. The composite current collector may be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver and silver alloy, etc.) on a polymer material substrate (such as a substrate of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0102] In some embodiments, the negative electrode active material may be a negative electrode active material known in the art for use in batteries. As an example, the negative electrode active material may include at least one of the following materials: artificial graphite, natural graphite, soft carbon, hard carbon, silicon-based materials, tin-based materials, and lithium titanate, etc. Silicon-based materials may be selected from at least one of elemental silicon, silicon oxide compounds, silicon-carbon composites, silicon-nitrogen composites, and silicon alloys. Tin-based materials may be selected from at least one of elemental tin, tin oxide compounds, and tin alloys. However, the present invention is not limited to these materials, and other conventional materials that can be used as negative electrode active materials for batteries may also be used. These negative electrode active materials may be used alone or in combination of two or more.

[0103] In some embodiments, the negative electrode film layer may optionally include a binder. As an example, the binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0104] In some embodiments, the negative electrode film layer may optionally include a binder. As an example, the binder may be selected from at least one of styrene-butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0105] In some embodiments, the negative electrode film may optionally include a conductive agent. As an example, the conductive agent may be selected from at least one of superconducting carbon, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0106] In some embodiments, the negative electrode film may optionally include other additives, such as thickeners (e.g., sodium carboxymethyl cellulose (CMC-Na)).

[0107] In some embodiments, the negative electrode sheet can be prepared by dispersing the components used to prepare the negative electrode sheet, such as the negative electrode active material, conductive agent, binder and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry onto the negative electrode current collector, and then obtaining the negative electrode sheet after drying, cold pressing and other processes.

[0108] electrolytes

[0109] The electrolyte plays a role in conducting ions between the positive and negative electrode plates.

[0110] In some embodiments, the electrolyte is an electrolyte solution, which includes an electrolyte salt and a solvent.

[0111] In some embodiments, the electrolyte salt may be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bis(fluorosulfonyl)imide, lithium bis(trifluoromethanesulfonyl)imide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluorooxalate borate, lithium dioxalate borate, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0112] In some embodiments, the solvent may be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butyl carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, sulfolane, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0113] In some embodiments, the electrolyte may optionally include additives. As examples, additives may include negative electrode film-forming additives, positive electrode film-forming additives, and may also include additives that can improve certain battery performance, such as additives that improve battery overcharge performance, additives that improve battery high-temperature or low-temperature performance, etc.

[0114] Separating membrane

[0115] In some embodiments, the secondary battery also includes a separator. The present invention does not impose any particular limitation on the type of separator; any known porous separator with good chemical and mechanical stability can be selected.

[0116] In some embodiments, the material of the separator can be selected from at least one of glass fiber, nonwoven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single-layer film or a multi-layer composite film, without particular limitation. When the separator is a multi-layer composite film, the materials of each layer can be the same or different, without particular limitation.

[0117] In some embodiments, the above-mentioned positive electrode, negative electrode and separator can be fabricated into an electrode assembly by a winding process or a stacking process.

[0118] In some embodiments, the secondary battery may include an outer packaging. This outer packaging may be used to encapsulate the electrode assembly and electrolyte described above.

[0119] In some embodiments, the outer packaging of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, or a steel shell. The outer packaging of the secondary battery can also be a soft pack, such as a pouch. The material of the soft pack can be plastic; examples of plastics include polypropylene, polybutylene terephthalate, and polybutylene succinate.

[0120] This invention does not impose any particular limitation on the shape of the secondary battery; it can be cylindrical, square, or any other arbitrary shape. For example, Figure 2 This is an example of a square-structured secondary battery 5.

[0121] In some implementations, refer to Figure 3 The outer packaging may include a housing 51 and a cover 53. The housing 51 may include a base plate and side plates connected to the base plate, the base plate and side plates forming a receiving cavity. The housing 51 has an opening communicating with the receiving cavity, and the cover 53 can be placed over the opening to close the receiving cavity. The positive electrode, negative electrode, and separator may be formed into an electrode assembly 52 by a winding process or a stacking process. The electrode assembly 52 is encapsulated within the receiving cavity. Electrolyte is immersed in the electrode assembly 52. ​​The secondary battery 5 may contain one or more electrode assemblies 52, which can be selected by those skilled in the art according to specific practical needs.

[0122] Furthermore, a third aspect of the present invention provides an electrical device, which includes the secondary battery provided in the first aspect of the present invention. The secondary battery can be used as the power source of the electrical device or as the energy storage unit of the electrical device. The electrical device may include, but is not limited to, mobile devices (e.g., mobile phones, laptops, etc.), electric vehicles (e.g., pure electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, electric bicycles, electric scooters, electric golf carts, electric trucks, etc.), electric trains, ships and satellites, energy storage systems, etc.

[0123] Example

[0124] The following describes embodiments of the present invention. The embodiments described below are exemplary and are only used to explain the present invention, and should not be construed as limiting the present invention. Where specific techniques or conditions are not specified in the embodiments, they are performed according to the techniques or conditions described in the literature in the art or according to the product instructions. Reagents or instruments used, unless otherwise specified, are all conventional products that can be obtained commercially.

[0125] Example 1

[0126] Preparation of positive electrode sheet

[0127] Lithium iron phosphate, conductive carbon black, PVDF, and sodium polyacrylate were mixed in a mass ratio of 95:2:2:1. After mixing, N-methylpyrrolidone was added as a solvent, and the mixture was stirred at 1800 rpm for 80 min at 60℃ to obtain a positive electrode slurry. The lithium iron phosphate had a particle size of 10 μm, the solid content of the positive electrode slurry was 70 wt%, and the viscosity was 8000 mPa·s. The positive electrode slurry was coated on both surfaces of an aluminum foil, and after drying, cold pressing, and cutting, a positive electrode sheet was obtained.

[0128] Preparation of negative electrode sheet

[0129] Artificial graphite, conductive carbon black, carboxymethyl cellulose (CMC) binder, and water solvent are mixed uniformly in a weight ratio of 95:2:3:100. The mixture is then coated on both surfaces of a copper foil. After drying, cold pressing, and cutting, the negative electrode sheet is obtained.

[0130] diaphragm

[0131] A polyethylene film with a thickness of 15 μm was used as the separator.

[0132] Preparation of electrolyte

[0133] Ethylene carbonate, diethyl carbonate, and dimethyl carbonate were mixed in a volume ratio of 1:1:1, and LiPF6 was dissolved in the mixture to obtain an electrolyte. The concentration of LiPF6 in the electrolyte was 1 mol / L.

[0134] Assembly of lithium-ion batteries

[0135] The positive electrode, negative electrode, and two separators are arranged in the order of "separator-negative electrode-separator-positive electrode". One end of the positive electrode, negative electrode, and two separators is fixed to the discharge roller, and the other end is stacked together and fixed to the winding shaft. The winding shaft is rotated by a motor to wind the positive electrode, negative electrode, and two separators to obtain a wound battery.

[0136] Performance testing

[0137] Water content test

[0138] (1) Bottles and rubber stoppers were baked at 85℃ for more than 4 hours;

[0139] (2) Sample preparation in a drying room: Weigh an appropriate amount of positive electrode sample into a vial;

[0140] (3) Seal the bottle with sealing pliers and mark the sample mass (excluding the bottle) on the bottle body. Take an empty bottle and seal it under the same environment to conduct an empty sample comparison test: Put the vial containing the positive electrode sample into the automatic sample feeding system of the equipment. During the test, heat the vial containing the positive electrode sample and pass in dry gas. Purge the gas in the vial into the titration cup for water absorption titration and convert the result into the water content of the solid sample.

[0141] Test of circulating gas production

[0142] A gas-generating nail was welded onto the battery cell. The oil pipe and the barometer were connected to the gas-generating nail. The battery cell was then charged at 60°C with a constant current of 0.5C to 3.65V, and then charged with a constant voltage of 0.05C. After standing for 10 minutes, it was discharged with a current of 0.5C to 2.5V, and then discharged with a current of 0.1C to 2.0V. This cycle was repeated 1000 times, and the gas production was recorded.

[0143] Storage performance testing

[0144] Lithium-ion batteries were stored in a constant temperature chamber at 60°C for 365 days. The battery capacity was measured every 30 days. After storage, the batteries were charged at a constant current rate of 0.5C to 3.65V, then charged at a constant voltage rate to 0.05C, then discharged at a current rate of 0.5C to 2.5V, and finally discharged at a current rate of 0.1C to 2.0V. The ratio of the current capacity to the initial capacity was recorded, which is the storage capacity retention rate.

[0145] Cyclic performance testing

[0146] At 45℃, charge the battery to 3.65V using a constant current of 0.5C, then charge it to 0.05C using a constant voltage, let it stand for 10 minutes, then discharge it to 2.5V using a current of 0.5C, and then discharge it to 2.0V using a current of 0.1C, until the discharge capacity of the lithium-ion battery decays to 80% of the initial capacity. Record the number of cycles at this point.

[0147] Examples 2-8

[0148] Examples 2-8 are similar to the lithium battery preparation method in Example 1, but the type and amount of water-absorbing material in the positive electrode slurry and the particle size and amount of lithium iron phosphate are adjusted. The different parameters are detailed in Table 1.

[0149] Comparative Example 1

[0150] The difference between this comparative example and Example 1 is that no water-absorbing material was used in the positive electrode slurry; all other operations are the same as in Example 1.

[0151] Table 1. Parameters and performance test results of Examples 1-8 and Comparative Example 1

[0152]

[0153] As can be seen from the data in Table 1, compared with Comparative Example 1, the positive electrode slurry in Examples 1-8 of the present invention includes water-absorbing material. By reasonably adjusting the type and amount of water-absorbing material and the particle size and amount of positive electrode active material, the water content of the positive electrode sheet obtained is significantly reduced, by more than 28.57%; the cycle gas production of the battery obtained is significantly reduced, by more than 27.27%; the storage capacity retention rate of the battery obtained is significantly improved, by more than 0.8%; and the number of cycles of the battery obtained is significantly increased, that is, its long-term cycle performance is significantly improved.

[0154] Please see Figure 1 , Figure 1 This is a schematic diagram comparing the storage capacity retention rates of the lithium-ion batteries obtained in Example 1 and Comparative Example 1 of the present invention. Figure 1As can be seen, compared with Comparative Example 1, the lithium-ion battery obtained in Example 1 has a 0.8% improvement in storage capacity retention rate over 150 days and is expected to improve by 2.5% over 365 days.

[0155] The above description is merely an exemplary embodiment of the present invention and does not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention specification and drawings under the technical concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

Claims

1. A secondary battery, characterized in that, The secondary battery includes a positive electrode sheet, which includes a water-absorbing material, and the water-absorbing material includes at least one of polyacrylate, polyacrylamide, and polyvinyl alcohol.

2. The secondary battery as described in claim 1, characterized in that, The positive electrode sheet includes a positive active material layer, and the positive active material layer includes a water-absorbing material.

3. The secondary battery as described in claim 2, characterized in that, The water-absorbing material accounts for 0.5%-2% of the total mass of the positive electrode active material layer.

4. The secondary battery as described in claim 2, characterized in that, The positive electrode active material layer further includes a positive electrode active material, wherein the particle size of the positive electrode active material is 2μm-20μm; and / or, The specific surface area of ​​the positive electrode active material is 5m². 2 / g-20m 2 / g; and / or, The compaction density of the positive electrode active material is 2.3 g / cm³. 3 -2.8g / cm 3 .

5. The secondary battery as described in claim 2, characterized in that, The positive electrode active material layer further includes a positive electrode active material, which includes lithium iron phosphate materials.

6. The secondary battery as described in claim 1, characterized in that, The positive electrode sheet includes a water-absorbing layer, and the water-absorbing layer includes a water-absorbing material.

7. The secondary battery as described in claim 6, characterized in that, The absorbent material content in the absorbent layer is 80wt%-95wt%.

8. The secondary battery as described in claim 6, characterized in that, The thickness of the absorbent layer is 1μm-10μm.

9. The secondary battery as described in any one of claims 1 to 8, characterized in that, The bonding strength of the positive electrode sheet is 5N / m-15N / m.

10. A positive electrode plate, characterized in that, The positive electrode is the positive electrode included in any one of the secondary batteries described in any one of 1 to 9.

11. An electrical appliance, characterized in that, The electrical device includes a secondary battery as described in any one of claims 1 to 9.

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