A positive electrode sheet, a lithium ion battery, and an electric device

Abstract

The application provides a positive plate, a lithium ion battery and an electric device, the positive plate comprising a current collector and a positive material layer; the positive material layer comprises a conductive agent, and the conductive agent comprises carbon black; the mass percentage content a of the carbon black in the positive material layer, the numerical value b of the surface resistance of the positive plate and the numerical value c of the adhesion between the positive material layer and the current collector satisfy the following relationship: 8*10 ‑6 ≤a / (b*c)≤0.073. The positive plate of the application controls the proportion of the conductive agent carbon black in the positive material layer, improves the electron transmission in the positive plate, adjusts the surface resistance of the positive plate, improves the conductivity of the positive plate and the fast charging performance of the battery, controls the adhesion between the positive material layer and the current collector, improves the stability of the positive material layer and guarantees the cycle life of the battery. Through the synergistic regulation of the three, the cycle life of the battery is guaranteed and the fast charging performance of the battery is improved.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery technology, specifically relating to a positive electrode, a lithium-ion battery, and an electrical device. Background Technology

[0002] With the continuous upgrading of the new energy vehicle industry and energy storage technology, the market has placed more stringent demands on the overall performance of lithium-ion batteries. Among these, fast charging performance, as a core indicator for improving ease of use and expanding application scenarios, is becoming increasingly urgent.

[0003] Lithium iron phosphate (LFP) materials have become the mainstream choice for cathode materials in lithium-ion batteries due to their outstanding comprehensive performance. LFP materials possess a stable olivine-type structure, making them less prone to oxygen evolution decomposition under overcharge, short-circuit, or thermal runaway conditions, thus exhibiting excellent thermal stability. Simultaneously, they also possess a long cycle life and resource-friendly and cost-effective advantages due to their absence of precious metals. Currently, LFP materials are widely used in various fields such as new energy vehicles and energy storage equipment.

[0004] However, the structure of lithium iron phosphate materials severely restricts the improvement of fast-charging performance of lithium-ion batteries, making it difficult to meet the high-frequency fast-charging requirements of current new energy devices. Therefore, improving its fast-charging performance has become an urgent technical challenge to be solved. Summary of the Invention

[0005] The purpose of this invention is to provide a positive electrode, a lithium-ion battery, and an electrical device that improves the fast-charging performance of the battery while ensuring its cycle life.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a positive electrode sheet, the positive electrode sheet comprising a current collector and a positive electrode material layer coated on at least one side of the current collector; the positive electrode material layer comprising a positive electrode active material and a conductive agent, the positive electrode active material comprising lithium iron phosphate, and the conductive agent comprising carbon black; the mass percentage a of carbon black in the positive electrode material layer, the sheet resistivity b of the positive electrode sheet, and the adhesion force c between the positive electrode material layer and the current collector satisfy the following relationship: 8 × 10 -6 ≤a / (b×c)≤0.073; where the unit of surface resistance is Ω, and the unit of adhesion between the positive electrode material layer and the current collector is N / m.

[0008] In a second aspect, the present invention provides a lithium-ion battery, the lithium-ion battery comprising the positive electrode sheet described in the first aspect.

[0009] Thirdly, the present invention provides an electrical device, the electrical device comprising the lithium-ion battery described in the first aspect.

[0010] Compared with the prior art, the present invention has the following beneficial effects:

[0011] The positive electrode of this invention controls the proportion of conductive carbon black in the positive electrode material layer to improve electron transport in the electrode. By adjusting the sheet resistance of the positive electrode, the conductivity of the electrode is improved, thereby enhancing the fast-charging performance of the battery. Furthermore, by controlling the adhesion between the positive electrode material layer and the current collector, the stability of the positive electrode material layer is improved, thus ensuring the cycle life of the battery. Through the synergistic regulation of these three factors, the fast-charging performance of the battery is improved while ensuring its cycle life. Detailed Implementation

[0012] The present invention provides a positive electrode sheet, the positive electrode sheet comprising a current collector and a positive electrode material layer coated on at least one side of the current collector; the positive electrode material layer comprises a positive electrode active material and a conductive agent, the positive electrode active material comprising lithium iron phosphate, and the conductive agent comprising carbon black;

[0013] The following relationship exists between the mass percentage of carbon black in the positive electrode material layer (a), the surface resistivity of the positive electrode sheet (b), and the adhesion force between the positive electrode material layer and the current collector (c):

[0014] 8×10 -6 ≤a / (b×c)≤0.073.

[0015] In this invention, the range of a / (b×c) is 8×10. -6 -0.073, for example, could be 8 × 10 -6 1×10 -5 5×10 -5 1×10 -4 5×10 -4 1×10 -3 5×10 -3 0.01, 0.05, or 0.073, but not limited to the listed values, other unlisted values ​​within the range also apply.

[0016] The unit of the sheet resistance of the positive electrode is Ω, and the unit of the bonding force between the positive electrode material layer and the current collector is N / m.

[0017] The positive electrode of this invention controls the proportion of conductive carbon black in the positive electrode material layer. The conductive carbon black distributed in the positive electrode material layer can form a chain-like conductive structure, bonding with the positive electrode active material particles, improving electron transport in the electrode, and thus enhancing the battery's fast-charging performance. However, excessive carbon black reduces the bonding stability of the positive electrode material layer, easily causing peeling of the positive electrode material during charging and discharging, thereby exacerbating interfacial side reactions between the electrode and the electrolyte, consuming active lithium, and impairing the battery's cycle life. By adjusting the sheet resistance of the positive electrode, the ion transport performance of the electrode is improved. Simultaneously, the bonding force between the positive electrode material layer and the current collector is enhanced, improving the stability of the positive electrode material layer, thereby ensuring the battery's cycle life. Therefore, this invention ensures both fast-charging capability and cycle life by limiting the range of the a / (b×c) ratio. At the same time, it avoids situations where the ratio is too small, resulting in insufficient electrode conductivity and decreased fast-charging performance; or where the ratio is too large, resulting in excessive carbon black content, insufficient bonding strength of the positive electrode material layer, causing peeling of the positive electrode material, and reducing the battery's cycle life.

[0018] Preferably, the mass percentage of carbon black 'a' in the positive electrode material layer, the surface resistivity 'b' of the positive electrode sheet, and the adhesion force 'c' between the positive electrode material layer and the current collector satisfy the following relationship: 6 × 10 -5 ≤a / (b×c)≤1×10 -3 For example, it could be 6×10 -5 8×10 -5 1×10 -4 3×10 -4 5×10 -4 8×10 -4 Or 1×10 -3 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0019] Preferably, the mass percentage 'a' of carbon black in the positive electrode material layer is 0.02%-5.0%, for example, it can be 0.02%, 0.05%, 0.1%, 0.5%, 1.0%, 2.0%, 3.0%, 4.0% or 5.0%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, and it is further preferred to be 0.1%-0.2%.

[0020] Preferably, the sheet resistance of the positive electrode is b Ω, where b is 0.2-1.0, for example, it can be 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 or 1.0, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, and it is more preferably 0.3-0.8.

[0021] Preferably, the bonding force between the positive electrode material layer and the current collector is c N / m, where c is 3-40, for example, it can be 3, 5, 10, 15, 20, 25, 30, 35 or 40, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, and it is further preferred to be 6-25.

[0022] Preferably, the conductive agent further includes carbon nanotubes and / or graphene.

[0023] Carbon nanotubes have a high aspect ratio, which can bridge the positive electrode active material and the conductive agent carbon black to form a conductive network, thereby improving the ion transport of the positive electrode and increasing the conductivity of the positive electrode.

[0024] Preferably, the carbon nanotubes in the cathode material layer have a mass percentage content of 0.01%-1.0%, for example, 0.01%, 0.05%, 0.1%, 0.3%, 0.5%, 0.8% or 1.0%, but are not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0025] By incorporating carbon nanotubes, the surface resistance of the cathode can be controlled. Graphene, with its layered structure, large specific surface area, and good mechanical flexibility, can enhance the structural strength and flexibility of the cathode material layer.

[0026] Preferably, the graphene content in the positive electrode material layer is 0.01%-1.5% by mass, for example, it can be 0.01%, 0.05%, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.3% or 1.5%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0027] By adding graphene, the adhesion between the cathode material layer and the current collector can be controlled.

[0028] Preferably, the aspect ratio of the carbon nanotube is 100-2000, for example, it can be 100, 500, 1000, 1500 or 2000, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0029] By controlling the aspect ratio of carbon nanotubes within an appropriate range, a good conductive network can be formed in the cathode material layer, bridging the cathode active material and carbon black, improving the ion conduction of the cathode sheet, enhancing the fast charging performance of the battery, and at the same time, preventing entanglement and agglomeration that would block the pores of the electrode sheet.

[0030] Preferably, the thickness of the graphene sheets is 1-50 nm, for example, it can be 1 nm, 5 nm, 10 nm, 20 nm, 30 nm, 40 nm or 50 nm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0031] Controlling the thickness of graphene layers within an appropriate range ensures a good specific surface area, improves the mechanical strength and flexibility of the cathode material layer, enhances electrode stability, and increases battery cycle life. On the other hand, it avoids layer stacking, which hinders ion transport and ensures the battery's fast-charging performance.

[0032] Preferably, the positive electrode material layer further includes a binder.

[0033] The bonding force between the positive electrode material layer and the current collector can be adjusted by controlling the amount of binder used in the positive electrode material layer.

[0034] Preferably, the binder in the positive electrode material layer has a mass percentage content of 1.0%-3.0%, for example, it can be 1.0%, 1.3%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.8% or 3.0%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0035] Preferably, the compaction density of the positive electrode material layer is 2.3-2.8 g / cm³. 3 For example, it could be 2.3 g / cm³ 3 2.4g / cm 3 2.5g / cm 3 2.6g / cm 3 2.7g / cm 3 Or 2.8g / cm 3 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0036] Controlling the compaction density of the cathode material layer within a certain range ensures uniform coating of the cathode material layer, promotes rapid lithium-ion transport, and improves the fast-charging performance of the battery. On the other hand, it avoids excessive gaps between active materials, reduces interfacial side reactions between the electrolyte and the electrode, and improves the cycle life of the battery.

[0037] Preferably, the areal density of the positive electrode material layer is 200-600 g / m³. 2 For example, it could be 200g / m 2 300g / m 2 400g / m 2 500g / m 2 Or 600g / m 2 However, this does not limit the listed values; other unlisted values ​​within the range are also applicable.

[0038] Controlling the areal density of the cathode material layer can shorten the ion conduction path and improve the electrode conductivity. By adjusting the areal density of the cathode material layer, the sheet resistance of the cathode can be controlled.

[0039] Preferably, the positive electrode material layer includes a positive electrode active material, and the positive electrode active material includes lithium iron phosphate.

[0040] Preferably, the Dv50 particle size of the positive electrode material layer is 0.5-2μm, for example, it can be 0.5μm, 1μm, 1.5μm or 2μm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0041] By controlling the particle size of the cathode material layer within a certain range, on the one hand, lithium-ion transport is ensured, improving the fast-charging performance of the battery; on the other hand, controlling the appropriate specific surface area of ​​the cathode material particles avoids excessive interfacial contact with the electrolyte, leading to side reactions and ensuring the cycle life of the battery.

[0042] Preferably, the lithium iron phosphate has a carbon coating layer on its surface.

[0043] Preferably, the thickness of the carbon coating layer is 2-15 nm, for example, it can be 2 nm, 5 nm, 8 nm, 10 nm, 12 nm or 15 nm, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0044] By controlling the appropriate thickness of the carbon coating layer, on the one hand, the interfacial side reactions caused by the contact between the positive electrode active material and the electrolyte are reduced, thus ensuring the cycle life of the battery; on the other hand, the lithium ion transport path is avoided to ensure the fast charging performance of the battery.

[0045] Preferably, the lithium iron phosphate includes doped lithium iron phosphate.

[0046] Preferably, the doped element includes at least one of Zr, Ti, Si, Mo, La, Ce, Te, Nb, Ta, Si, Al, W, Sr, V, Y, Mg, Co, F, Cl, or S.

[0047] By doping elements into lithium iron phosphate, on the one hand, the crystal structure stability of lithium iron phosphate materials is improved, thereby increasing the cycle life of the battery; on the other hand, it avoids hindering lithium-ion transport, ensuring the fast-charging performance of the battery.

[0048] Preferably, the content of the doped element in the doped lithium iron phosphate is 100-10000ppm, for example, it can be 100ppm, 500ppm, 1000ppm, 3000ppm, 5000ppm, 8000ppm or 10000ppm, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0049] The present invention also provides a lithium-ion battery.

[0050] Preferably, the lithium-ion battery includes a negative electrode sheet, and the negative electrode sheet includes a negative electrode material layer.

[0051] Preferably, the porosity of the negative electrode material layer is 10%-50%, for example, it can be 10%, 20%, 30%, 40% or 50%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0052] By controlling the porosity of the negative electrode material layer within a certain range, effective contact between the electrolyte and the negative electrode active material is ensured, lithium ion transport is guaranteed, and the fast charging performance of the battery is improved. On the other hand, excessive side reactions are avoided, thus ensuring the cycle life of the battery.

[0053] Preferably, the negative electrode material layer comprises a negative electrode active material.

[0054] Preferably, the Dv50 particle size of the negative electrode active material is 5-50 μm, for example, it can be 5 μm, 10 μm, 20 μm, 30 μm, 40 μm or 50 μm, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0055] Controlling the particle size of the negative electrode active material ensures rapid lithium-ion transport and reduces side reactions between the electrolyte and the negative electrode active material, thereby guaranteeing the fast-charging performance and cycle life of the lithium-ion battery. Preferably, the negative electrode active material comprises silicon-carbon and / or graphite.

[0056] Preferably, the mass percentage of silicon in the silicon-carbon anode material layer is 0.1%-30%, for example, it can be 0.1%, 0.5%, 1%, 5%, 10%, 15%, 20%, 25% or 30%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0057] Preferably, the graphitization degree of the graphite is 80%-99%, for example, it can be 80%, 85%, 90%, 95% or 99%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0058] Controlling the degree of graphitization of graphite serves two purposes: firstly, it reduces defects in graphite sheets and side reactions with the electrolyte, ensuring the cycle life of the battery; secondly, it prevents the graphite layers from being too tightly packed, which would hinder lithium-ion transport and ensure the battery's fast-charging performance.

[0059] Preferably, the lithium-ion battery further includes an electrolyte.

[0060] Preferably, the viscosity of the electrolyte is 1-50 mPa·s, for example, it can be 1 mPa·s, 5 mPa·s, 10 mPa·s, 20 mPa·s, 30 mPa·s, 40 mPa·s or 50 mPa·s, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0061] By controlling the viscosity of the electrolyte, the conductivity of the electrolyte is increased, thereby improving the lithium-ion transport rate and ensuring the fast-charging performance of the battery. On the other hand, it avoids the gas generation problem that is prone to occur in solvents with too low viscosity, which increases the side reactions at the electrode interface, thus ensuring the battery's cycle life.

[0062] Preferably, the electrolyte includes additives.

[0063] Preferably, the additive includes at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), or ethylene sulfate (DTD), and typical but non-limiting combinations include combinations of vinylene carbonate and fluoroethylene carbonate, combinations of fluoroethylene carbonate and ethylene sulfate, combinations of vinylene carbonate and ethylene sulfate, or combinations of vinylene carbonate, fluoroethylene carbonate, and ethylene sulfate.

[0064] Adding vinylene carbonate (VC) to the electrolyte allows it to preferentially decompose on the negative electrode surface to form an organic layer. Adding fluoroethylene carbonate (FEC) induces the formation of an inorganic layer containing LiF, which can form a dense and stable SEI film. Ethylene sulfate (DTD) can participate in the formation of an organic layer containing S at the negative electrode, forming an organic-inorganic composite solid electrolyte interphase (SEI) film, which can improve the mechanical strength and stability of the negative electrode material layer.

[0065] Preferably, the total mass percentage of the additive in the electrolyte is 0.1%-5%, for example, it can be 0.1%, 0.5%, 1%, 2%, 3%, 4% or 5%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0066] Preferably, the electrolyte further includes lithium salt.

[0067] Preferably, the lithium salt comprises at least one of lithium hexafluorophosphate (LiPF6), lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), lithium di(oxo)borate (LiBOB), lithium tetrafluoroborate (LiBF4), or lithium hexafluoroarsenate (LiAsF6).

[0068] Preferably, the concentration of lithium salt in the electrolyte is 0.8-1.5 mol / L, for example, it can be 0.8 mol / L, 1 mol / L, 1.2 mol / L, 1.4 mol / L or 1.5 mol / L, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0069] Preferably, the lithium-ion battery further includes a separator.

[0070] Preferably, the porosity of the diaphragm is 20%-80%, for example, it can be 20%, 30%, 40%, 50%, 60%, 70% or 80%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0071] By controlling the porosity of the separator, lithium-ion transport is ensured, thereby guaranteeing the battery's fast-charging performance. On the other hand, lithium dendrite growth and penetration are prevented, ensuring the battery's cycle life.

[0072] Preferably, the thickness of the diaphragm is 2-30 μm, for example, it can be 2 μm, 5 μm, 10 μm, 20 μm or 30 μm, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0073] In this invention, the lithium-ion battery is prepared using the following method:

[0074] (1) Preparation of positive electrode sheet: According to the mass percentage of the positive electrode material layer composition, 90%-99% of positive electrode active material, 0.01%-8% of conductive agent, 1%-5% of binder and 0-0.5% of dispersant are mixed evenly, solvent is added and stirred to disperse evenly to obtain positive electrode slurry. The positive electrode slurry is coated on at least one surface of the positive electrode current collector, dried, rolled and cut to obtain positive electrode sheet. The areal density of the positive electrode material layer is 200-600 g / m². 2 The compaction density of the positive electrode material layer is 2.3-2.8 g / cm³. 3 .

[0075] In the aforementioned positive electrode sheet, the positive electrode active material includes lithium iron phosphate.

[0076] In some embodiments, lithium iron phosphate can be carbon-coated doped lithium iron phosphate, and the general chemical formula of doped lithium iron phosphate can be represented as: LiFe 1-x M x PO y Q zWhere x≤0.1, 3.85≤y≤4, 0≤z≤0.05; M is a doping element, including at least one of Zr, Ti, Si, Mo, La, Ce, Te, Nb, Ta, Si, Al, W, Sr, V, Y, Mg, Co, F, Cl or S, and the content of M in the doped lithium iron phosphate is 100-10000ppm.

[0077] In some embodiments, the preparation process of the positive electrode active material includes: using lithium dihydrogen phosphate (LiH2PO4), trivalent iron source (Fe2O3 or FePO4·2H2O), carbon source (sucrose, glucose or acetylene black) and compounds of dopant elements as raw materials, weighing them according to the stoichiometric ratio (Li:Fe:P=1:1:1), adding them to a ball mill, using anhydrous ethanol as the dispersion medium, ball milling to obtain a uniform slurry, drying the slurry to obtain powder, placing the powder in an atmosphere furnace, heating it under argon or nitrogen protection, first pre-calcining at 300~400℃ for 2~4h, then heating to 700~850℃, holding at that temperature for 8~12h, naturally cooling to room temperature, pulverizing the sintered product, passing it through a 300-mesh sieve, to obtain the positive electrode active material powder.

[0078] In the above positive electrode, the conductive agent is selected from conductive carbon black, or a combination of carbon black and other conductive agents, including carbon nanotubes and / or graphene.

[0079] In the above-mentioned positive electrode sheet, the binder is selected from at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene-propylene terpolymer, ethylene-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, or fluorinated acrylate resin.

[0080] In the above-mentioned positive electrode sheet, the dispersant is selected from at least one of acrylic acid, acrylate, polyether ester, phosphate ester, small molecule alkanolamine, polyurethane, modified styrene / maleic anhydride, hydrogenated nitrile butadiene rubber (HNBR), polyvinylpyrrolidone (PVP) or polyethylene glycol (PEG).

[0081] In the above-mentioned positive electrode, the positive current collector can be a metal foil or a composite current collector.

[0082] In some embodiments, the metal foil may be aluminum or an aluminum alloy.

[0083] In some embodiments, the composite current collector includes an intermediate high-molecular layer and metal layers disposed on both sides of the polymer layer; wherein the polymer layer includes polymeric materials, including at least one of polyamide (PA), polyterephthalate, polyimide (PI), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), aramid, acrylonitrile-butadiene-styrene copolymer (ABS), polybutylene terephthalate (PET), poly(p-phenylene terephthalamide) (PPTA), polypropylene (PPE), polyoxymethylene (POM), epoxy resin, phenolic resin, polytetrafluoroethylene (PTEE), polyvinylidene fluoride (PVDF), silicone rubber, polycarbonate (PC), polyvinyl alcohol (PVA), polyethylene glycol (PEG), cellulose, starch, protein, its derivatives, its crosslinks, or copolymers thereof; the metal layer includes at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver alloy.

[0084] (2) Preparation of negative electrode sheet: According to the mass percentage of the negative electrode material layer, 90%-98% of negative electrode active material, 0.05%-5% of conductive agent, 1.5%-5% of binder and 0.3%-1.5% of dispersant are mixed evenly, and solvent is added to obtain negative electrode slurry. The negative electrode slurry is coated on at least one surface of the negative electrode current collector, dried, rolled and cut to obtain negative electrode sheet.

[0085] In the above-mentioned negative electrode sheet, the negative electrode active material is selected from carbon materials and / or silicon-based materials.

[0086] In some embodiments, the carbon material is selected from at least one of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, or soft carbon.

[0087] In some embodiments, the silicon-based material is selected from at least one of elemental silicon, silicon oxides, silicon carbide compounds, or silicon alloys.

[0088] In the aforementioned negative electrode sheet, the conductive agent is selected from at least one of superconducting carbon (SP), conductive graphite, acetylene black, carbon black, Ketjen black, carbon dots, carbon nanotubes, graphene, or carbon nanofibers.

[0089] In the aforementioned negative electrode sheet, the binder is selected from at least one of styrene-butadiene rubber (SBR), water-soluble unsaturated resin SR-1B, waterborne acrylic resin, polyvinyl alcohol (PVA), sodium alginate (SA), or sodium carboxymethyl methacrylate (CMC). The waterborne acrylic resin may be at least one of polyacrylic acid (PAA), polymethacrylic acid (PMAA), sodium polyacrylate (PAAS), or polyacrylamide (PAM).

[0090] In the aforementioned negative electrode sheet, the negative electrode current collector is selected from metal foil or composite current collector.

[0091] In some embodiments, the metal foil may be copper or a copper alloy.

[0092] In some embodiments, the composite current collector includes an intermediate high-molecular layer and metal layers disposed on both sides of the polymer layer; wherein the polymer layer includes polymeric materials, including at least one of polyamide (PA), polyterephthalate, polyimide (PI), polyethylene (PE), polypropylene (PP), polystyrene (PS), polyvinyl chloride (PVC), aramid, acrylonitrile-butadiene-styrene copolymer (ABS), polybutylene terephthalate (PET), poly(p-phenylene terephthalamide) (PPTA), polypropylene (PPE), polyoxymethylene (POM), epoxy resin, phenolic resin, polytetrafluoroethylene (PTEE), polyvinylidene fluoride (PVDF), silicone rubber, polycarbonate (PC), polyvinyl alcohol (PVA), polyethylene glycol (PEG), cellulose, starch, protein, its derivatives, its crosslinks, or copolymers thereof; the metal layer includes at least one of aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, or silver alloy.

[0093] (3) Electrolyte preparation: Mix organic solvents, then dissolve the dried lithium salt in the mixed organic solvents, add additives, and obtain the electrolyte.

[0094] (4) Separator: The separator is placed between the positive electrode and the negative electrode to separate the positive electrode and the negative electrode and prevent the positive electrode and the negative electrode from short-circuiting.

[0095] The material of the diaphragm is selected from at least one of glass fiber, nonwoven fabric, polyethylene (PE), polypropylene (PP) and / or polyvinylidene fluoride.

[0096] In some embodiments, the membrane surface may also be coated, which may be an inorganic coating and / or an organic coating, wherein the inorganic coating material includes at least one of alumina, silicon oxide, titanium oxide, magnesium oxide, zirconium oxide or boehmite; and the organic coating includes at least one of aramid coating or polyvinylidene fluoride (PVDF) coating.

[0097] (5) The positive electrode, separator and negative electrode are stacked in sequence so that the separator is between the positive and negative electrode sheets to play a role in isolation. Then the bare cell is wound up and placed in the outer packaging shell. After drying, the electrolyte is injected. After vacuum sealing, standing, formation and shaping, a lithium-ion battery is obtained.

[0098] Steps (1) to (4) are not in any particular order.

[0099] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0100] Example 1

[0101] (1) Preparation of positive electrode sheet: According to the mass percentage composition of the positive electrode material layer, 96.77% lithium iron phosphate, 2.0% binder PVDF, 0.13% conductive agent carbon black, 0.5% conductive agent carbon nanotubes, 0.3% conductive agent graphene, and 0.3% dispersant polyvinylpyrrolidone are mixed evenly. The Dv50 particle size of lithium iron phosphate is 1 μm, the average aspect ratio of carbon nanotubes is 500, and the layer thickness of graphene is 30 nm. The mixed material is dispersed in the solvent N-methylpyrrolidone to form a positive electrode slurry. The obtained positive electrode slurry is coated on aluminum foil to obtain a double-sided coated positive electrode sheet. Then, the double-sided coated positive electrode sheet is rolled and cut to obtain a positive electrode sheet. The areal density of the positive electrode material layer is 480 g / m². 2 The compaction density of the positive electrode material layer is 2.6 g / cm³. 3 .

[0102] (2) Preparation of negative electrode sheet: According to the mass percentage composition of the negative electrode material layer, 95.85% Dv50 graphite with a particle size of 30μm, 0.6% conductive agent SP, 0.5% binder CMC, 1% binder SBR, 1.05% binder PAA and 1% dispersant polyvinylpyrrolidone are mixed evenly and dispersed in deionized water to obtain a negative electrode slurry. The negative electrode slurry is coated on copper foil to obtain a double-sided coated electrode sheet, which is then rolled and cut to obtain a negative electrode sheet. The areal density of the negative electrode material layer is 220g / m². 2 The compaction density of the negative electrode material layer is 1.6 g / cm³. 3 .

[0103] (3) Electrolyte preparation: Ethyl carbonate (EC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC) and fluoroethylene carbonate (FEC) are mixed in a volume ratio of 3:3:4:1 to obtain an organic solvent. Then, fully dried lithium salts LiPF6 and LiFSI (in a mass ratio of 8:5) are dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L and a viscosity of 2.5 mPa·s.

[0104] (4) Stack the positive electrode, separator and negative electrode in sequence so that the separator is between the positive electrode and the negative electrode to play a role in isolation. The separator is a PP separator with a thickness of 20μm and a porosity of 50%. Then, the bare cell is wound up and placed in the outer packaging shell. After drying, the electrolyte is injected and vacuum sealed. After formation and shaping, a lithium-ion battery is obtained.

[0105] Example 2

[0106] The difference between this embodiment and Embodiment 1 lies in the following: In the preparation step of the positive electrode, carbon-coated doped lithium iron phosphate material is used, with a carbon coating layer thickness of 10 nm and Zr as the dopant element. The Zr content in the doped lithium iron phosphate is 500 ppm. By changing the amounts of carbon black, carbon nanotubes, graphene, and binder, the areal density of the positive electrode material layer is altered to obtain different positive electrode sheets. In the electrolyte preparation step, additives VC, FEC, and DTD are added to the electrolyte, with each additive added at a mass percentage of 0.5% of the electrolyte. The remaining preparation process remains consistent with Embodiment 1. By changing the amount of carbon nanotubes and the areal density of the positive electrode material layer, different sheet resistances are achieved in the positive electrode sheets. By changing the amounts of graphene and binder, different bonding forces between the positive electrode material layer and the current collector are achieved in the positive electrode sheets.

[0107] The preparation process of lithium iron phosphate material includes: using 1000g of iron phosphate dihydrate as the iron source, 260g of lithium carbonate as the lithium source, 4g of water-soluble zirconium nitrate as the dopant, and 100g of sucrose as the carbon source, adding deionized water and a dispersant to prepare a mixed slurry with a solid content of 45%. The slurry is then treated with zirconium oxide bead milling, and spray-dried to obtain spherical composite precursor powder. The precursor powder is then subjected to segmented sintering in a nitrogen inert atmosphere, with the temperature raised from room temperature to 350℃ and held for 2 hours, then raised to 720℃ and held for 8 hours. The cooled product is then subjected to air jet milling, sieving, and iron removal treatment to obtain the target lithium iron phosphate material.

[0108] Example 3-16

[0109] The difference between Examples 3-16 and Example 1 is that the amounts of carbon black, carbon nanotubes, graphene and binder are changed in the preparation steps of the positive electrode sheet, and the areal density of the positive electrode material layer is changed to obtain different positive electrode sheets. The rest of the preparation process is the same as that of Example 1.

[0110] Comparative Examples 1-4

[0111] The difference between Comparative Examples 1-4 and Example 1 is that the amounts of carbon black, carbon nanotubes, graphene, and binder are changed in the preparation steps of the positive electrode sheet, and the areal density of the positive electrode material layer is changed to obtain different positive electrode sheets. The rest of the preparation process is the same as that of Example 1.

[0112] In the examples and comparative examples, the amount of conductive agent and binder was changed by replacing them with an equal mass of lithium iron phosphate to ensure that the sum of the contents of each component in the cathode material layer meets 100%.

[0113] The variations in the amounts of carbon nanotubes, graphene, and binders in the examples and comparative examples are shown in Table 1.

[0114] The lithium-ion batteries provided in the examples and comparative examples were tested, including: the areal density of the positive electrode material layer, the mass percentage of carbon black in the positive electrode material layer, the sheet resistance of the positive electrode sheet, and the adhesion force between the positive electrode material layer and the current collector. The results are shown in Table 1. By changing the amount of carbon nanotubes and the areal density of the positive electrode material layer, different sheet resistances were achieved for the positive electrode sheet; by changing the amount of graphene and binder, different adhesion forces between the positive electrode material layer and the current collector were achieved for the positive electrode sheet.

[0115] The testing method is as follows:

[0116] (1) Areal density of the cathode material layer:

[0117] The battery was discharged at 0.33C to the lower limit voltage of 2.5V. The positive electrode was then disassembled. Using a standard circular punch die of Φ50mm, three samples were taken from a smooth and uniform location on the electrode. The three circular samples were weighed and the average value was calculated as 'a'. The positive electrode material layer was then scraped off, and the current collector was weighed and the average value was calculated as 'b'. The areal density was calculated as (ab) / A, where A is the sample area, and the calculation was A=π×(0.05 / 2). 2 ≈0.001963m².

[0118] (2) Mass percentage of carbon black in the cathode material layer:

[0119] ①Pretreatment: Discharge the battery at 0.33C to the lower limit voltage of 2.5V, disassemble the positive electrode sheet, scrape off the positive electrode material layer from the positive electrode sheet, weigh the mass of the positive electrode material layer as m0, wash it 2-3 times with NMP / ethanol solution, and vacuum dry it at 60℃ for 12h to remove residual binder and solvent.

[0120] ② Calcination: Place the pretreated sample in a crucible, introduce air, and heat to 800℃ at a rate of 10℃ / min. Record the TG curve and perform segmented integration at different temperatures. The weight loss of water at RT-200℃ is denoted as m1, and the weight loss of carbon black at 350-450℃ is denoted as m2. The mass percentage of carbon black is then calculated as m2 / m0×100%.

[0121] (3) Surface resistance of the positive electrode:

[0122] The battery was discharged at 0.33C to the lower limit voltage (2.5V for lithium iron phosphate), and the positive electrode was removed. The positive electrode was then immersed in dimethyl carbonate (DMC) solution for 4 hours and air-dried. A four-probe method was used for testing with the Chuanyuan Technology-film resistance testing system. During the test, the voltage was increased to 25MPa at a rate of 1MPa / s, held for 6 seconds, and then decreased at a rate of 5MPa / s. The test area was 154.025 mm². 2 The through resistance of the electrode is tested, and the surface resistance of the electrode is automatically read after the test is completed.

[0123] (4) Adhesion between the positive electrode material layer and the current collector:

[0124] ① Discharge the battery at 0.33C to the lower limit voltage (2.5V for lithium iron phosphate), disassemble the positive electrode, soak the positive electrode in dimethyl carbonate (DMC) solution for 4 hours, and then air dry;

[0125] ② Take a standard steel plate (50mm×125mm) as the rigidity test base plate. Wipe the surface of the steel plate clean with lint-free paper soaked in alcohol. Attach one side of the 50mm×125mm 3M double-sided tape to the steel plate, ensuring a smooth and wrinkle-free adhesion. Cut the positive electrode sheet into 50mm×200mm test samples. Adhere the electrode sheet to be tested to the other adhesive side of the double-sided tape, ensuring a smooth and wrinkle-free contact during the adhesion process. After rolling with a pressure roller, use a universal testing machine's tensile clamps to hold the steel plate at one end and the electrode sheet at the other. Perform a tensile test at a speed of 300mm / min. Set the tensile test machine's stroke to 100mm. Record the curve in the tensile test machine's software graph as flat and the displacement greater than 80mm. Stop the machine and read the average tensile force value of the flattened part of the curve, which is the adhesive force.

[0126] Table 1

[0127]

[0128] Performance testing

[0129] The relationship between the mass percentage of carbon black in the positive electrode material layer (a), the surface resistivity of the positive electrode sheet (b), and the adhesion force between the positive electrode material layer and the current collector (c) in a lithium-ion battery is calculated using the formula a / (b×c). The results are shown in Table 2.

[0130] The lithium-ion batteries of the examples and comparative examples were tested for fast charging performance and cycle life, and the results are shown in Table 2.

[0131] The fast charging performance testing method is as follows:

[0132] Using copper wire as a reference electrode, after the battery was charged to 0% SOC, lithium was plated onto the three-electrode copper wire at a rate of 0.01C for 10 hours on the positive side. After lithium plating, the following steps were performed for testing: the battery was charged at a constant current of 0.33C to the cutoff voltage (3.65V for lithium iron phosphate), and then charged at a constant voltage until the current was less than or equal to 0.05C; then discharged at 0.33C to the cutoff voltage of 2.5V. The above steps were repeated 3 times, and the capacity discharged in the third cycle was taken as the battery discharge capacity. After resting for 10 minutes, discharge at 1C to 2.5V, rest for 10 minutes, and charge at 0.33C to 10% SOC. Then charge at 4C to the upper limit voltage or auxiliary voltage of 0mV, and record the time t0. Then charge at 0.4C in descending order, and charge at 3.6C, 3.2C, 2.8C, 2.4C, 2.0C, 1.6C, 1.2C, 0.8C, and 0.4C. The cutoff condition for each charge is to charge to the upper limit voltage or auxiliary voltage of 0mV. Record the time t1. t1-t0 is the fast charging time.

[0133] The cycle life test method is as follows:

[0134] Place the battery in a 25°C chamber and charge it at 0.33C to the upper limit voltage of 3.65V, then charge it at a constant voltage until the cutoff current is 0.05C; let it rest for 30 minutes, then discharge it at 0.33C to the lower limit voltage of 2.5V; repeat the above operation 3 times, and use the discharge capacity of the third cycle as the battery's initial capacity. Perform the test according to the following steps: charge at a constant current of 1C to 3.65V, then charge at a constant voltage until the current drops to 0.05C; let it rest for 30 minutes; discharge at a 1C rate to 2.5V; let it rest for 30 minutes; repeat the above process until the lithium-ion battery capacity is less than 80% of the initial capacity, and record the number of cycles.

[0135] For ease of comparison, the mass percentage of carbon black in the positive electrode material layer (a), the sheet resistance of the positive electrode sheet (b), and the adhesion force between the positive electrode material layer and the current collector (c) in lithium-ion batteries are listed together in Table 2.

[0136] Table 2

[0137]

[0138] As can be seen from the test results in Table 2, referring to Examples 1-16, by coordinating and controlling the mass percentage of carbon black in the positive electrode material layer (a), the surface resistivity of the positive electrode sheet (b), and the adhesion force between the positive electrode material layer and the current collector (c), the ratios a and b×c are controlled within a certain range, ensuring the fast-charging performance and cycle life of the lithium-ion battery. However, in Comparative Examples 1-4, when the ratio of the three exceeds 8×10... -6When the value is in the range of -0.073, it can be seen that the fast charging performance and cycle life of the battery show a significant decrease. This is because by controlling the ratio of the three components, the ion conduction of the electrode can be improved and the stability of the positive electrode material layer can be enhanced, thereby ensuring the fast charging performance and cycle life of the battery.

[0139] Furthermore, in Examples 1-16, the performance of Examples 1-4 is superior compared to other examples. Comparing the results of Examples 1-8, when the content of conductive carbon black in the positive electrode material layer is too high, it reduces the bonding stability of the positive electrode material layer, making it prone to peeling during long-term charge-discharge cycles and damaging the battery's cycle life. When it is too low, it is difficult to form a good conductive network in the positive electrode material layer, reducing the ion conduction of the electrode and decreasing the battery's fast-charging performance. When the sheet resistance of the positive electrode is too high, it reduces the ion conduction of the electrode and decreases the battery's fast-charging performance. When it is too low, it also reduces the stability of the positive electrode material layer and decreases the battery's cycle life. When the bonding force between the positive electrode material layer and the current collector is too high, the positive electrode active material is tightly coated, easily causing obstruction of ion transport on the electrode and reducing the battery's fast-charging performance. When it is too low, the positive electrode material layer is prone to peeling during long-term cycling, exacerbating the side reactions between the electrode and the electrolyte and damaging the battery's cycle life. Comparing the results of Examples 9-16, a higher ratio of a to b×c results in decreased bonding stability of the positive electrode material layer and a worse cycle life of the battery; conversely, a lower ratio of a to b×c hinders ion transport on the electrode and reduces the battery's fast-charging performance. This demonstrates that precisely coordinating the numerical relationships among these three factors can simultaneously ensure both fast-charging performance and cycle life of the battery.

[0140] In summary, the positive electrode of this invention improves electron transport by controlling the proportion of conductive carbon black in the positive electrode material layer, enhances the conductivity of the electrode by adjusting its sheet resistance, and improves the fast-charging performance of the battery. Furthermore, it improves the stability of the positive electrode material layer by controlling the adhesion between the positive electrode material layer and the current collector, thereby ensuring the cycle life of the battery. Through the synergistic regulation of these three factors, the fast-charging performance of the battery is improved while ensuring its cycle life.

[0141] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A positive electrode plate, characterized in that, The positive electrode includes a current collector and a positive electrode material layer coated on at least one side of the current collector; the positive electrode material layer includes a positive electrode active material and a conductive agent, the positive electrode active material includes lithium iron phosphate, and the conductive agent includes carbon black; the mass percentage a of carbon black in the positive electrode material layer, the sheet resistivity b of the positive electrode, and the adhesion force c between the positive electrode material layer and the current collector satisfy the following relationship: 8 × 10 -6 ≤a / (b×c)≤0.073; where the unit of surface resistance is Ω, and the unit of adhesion between the positive electrode material layer and the current collector is N / m.

2. The positive electrode sheet according to claim 1, characterized in that, The following relationship exists between the mass percentage of carbon black in the positive electrode material layer (a), the surface resistivity of the positive electrode sheet (b), and the adhesion force between the positive electrode material layer and the current collector (c): 6 × 10 -5 ≤a / (b×c)≤1×10 -3 .

3. The positive electrode sheet according to claim 1, characterized in that, The mass percentage (a) of carbon black in the cathode material layer is 0.02%-5.0%; And / or, the surface resistance of the positive electrode is b Ω, where b is 0.2-1.0; And / or, the adhesion force between the positive electrode material layer and the current collector is c N / m, where c is 3-40.

4. The positive electrode sheet according to claim 3, characterized in that, The mass percentage 'a' of carbon black in the cathode material layer is 0.1%-0.2%; And / or, the surface resistance of the positive electrode is b Ω, where b is 0.3-0.8; And / or, the adhesion force between the positive electrode material layer and the current collector is c N / m, where c is 6-25.

5. The positive electrode sheet according to claim 1, characterized in that, The conductive agent also includes carbon nanotubes and / or graphene materials.

6. The positive electrode sheet according to claim 5, characterized in that, The aspect ratio of the carbon nanotubes is 100-2000; And / or, the thickness of the graphene sheets is 1-50 nm.

7. The positive electrode sheet according to claim 1, characterized in that, The lithium iron phosphate has a carbon coating layer on its surface, and the thickness of the carbon coating layer is 2-15 nm.

8. The positive electrode sheet according to claim 1, characterized in that, The lithium iron phosphate includes doped lithium iron phosphate, wherein the doping element includes at least one selected from Zr, Ti, Si, Mo, La, Ce, Te, Nb, Ta, Si, Al, W, Sr, V, Y, Mg, Co, F, Cl or S, and the content of the doping element in the doped lithium iron phosphate is 100-10000 ppm.

9. A lithium-ion battery, characterized in that, The lithium-ion battery includes the positive electrode sheet as described in any one of claims 1-8.

10. The lithium-ion battery according to claim 9, characterized in that, The lithium-ion battery further includes a negative electrode sheet, which includes a negative electrode active material, including silicon carbon and / or graphite; the silicon element in the silicon carbon has a mass percentage content of 0.1%-30% in the negative electrode material layer; and the graphitization degree of the graphite is 80%-99%.

11. The lithium-ion battery according to claim 9, characterized in that, The lithium-ion battery further includes an electrolyte, which includes additives; the additives include at least one of vinylene carbonate, fluoroethylene carbonate, or ethylene sulfate. The total mass percentage of the additive in the electrolyte is 0.1%-5%.

12. An electrical appliance, characterized in that, The electrical device includes the lithium-ion battery as described in any one of claims 9-11.

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