Negative electrode sheet, lithium ion battery, and device

Abstract

A negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer on at least one surface of the negative electrode current collector. The negative electrode current collector is a copper foil with a thickness of 1-4 µm. The negative electrode active material layer includes a first negative electrode active material layer and a second negative electrode active material layer. The first negative electrode active material layer includes a first negative electrode active material, and the OI value of the first negative electrode active material layer is 13-20. The second negative electrode active material layer includes a second negative electrode active material, and the OI value of the second negative electrode active material layer is 5-10. The application also provides a lithium ion battery including the negative electrode sheet and a device including the lithium ion battery. The copper foil of the negative electrode sheet of the application has a thin thickness and poor ductility. By designing a double-layer structure of the negative electrode active material layer, the negative electrode sheet has a small expansion, a high energy density, and no deformation problem under the conditions of a high tap density and a high coating weight.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a negative electrode sheet, a lithium ion battery comprising the negative electrode sheet and a device comprising the lithium ion battery. BACKGROUND

[0002] At present, with the increasing requirement of energy density of lithium ion batteries, the negative electrode sheet tends to be designed with high compaction density and high coating weight. However, such limit design will cause the ductility of the negative electrode sheet to be poor, resulting in the problem of battery deformation. Therefore, under the condition of high compaction density and high coating weight of the negative electrode sheet, how to avoid the problem of battery deformation is an urgent problem to be solved. SUMMARY

[0003] In view of this, the first aspect of the present application provides a negative electrode sheet, comprising:

[0004] A negative current collector, the negative current collector is a copper foil, the thickness of the copper foil is 1 μm to 4 μm;

[0005] A negative active material layer is located on at least one surface of the negative current collector, the negative active material layer comprises:

[0006] A first negative active material layer is located on the surface of the negative current collector, the first negative active material layer comprises a first negative active material, and the OI value of the first negative active material layer is 13 to 20; and

[0007] A second negative active material layer is located on the surface of the first negative active material layer away from the negative current collector, the second negative active material layer comprises a second negative active material, the OI value of the second negative active material layer is 5 to 10, and the weight of the second negative active material layer is greater than the weight of the first negative active material layer;

[0008] The OI value of the first negative active material layer is the peak area ratio of the 004 characteristic diffraction peak to the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the first negative active material layer;

[0009] The OI value of the second negative active material layer is the peak area ratio of the 004 characteristic diffraction peak to the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the first negative active material layer.

[0010] The thickness of the copper foil of the negative electrode sheet is relatively thin, and the ductility is poor. By designing the first negative electrode active material layer in the lower layer to have a relatively large OI value and a relatively small weight, the ductility of the first negative electrode active material layer in the XY direction is small, and the first negative electrode active material layer is directly coated on the thin copper foil to ensure that the overall ductility is small, thereby enhancing the ability to resist deformation. At the same time, the second negative electrode active material layer in the upper layer has a relatively small OI value and a relatively large weight, so as to ensure that the depth of lithium intercalation in the thickness direction is low, the expansion rate is small, and the energy density is high. This design can ensure that the negative electrode sheet has small expansion and high energy density under the conditions of high compaction density and high coating weight, and has no deformation problem.

[0011] In some embodiments, the weight ratio of the first negative electrode active material layer to the second negative electrode active material layer is 0.11 to 0.35.

[0012] In some embodiments, the weight ratio of the first negative electrode active material layer to the second negative electrode active material layer is 0.20 to 0.32.

[0013] The OI value of the first negative electrode active material layer in the lower layer is relatively large, so a relatively small weight needs to be coated. Therefore, the ductility of the first negative electrode active material layer in the XY direction is small. The OI value of the second negative electrode active material layer in the upper layer is relatively small, so a relatively large weight needs to be coated, so as to ensure that the depth of lithium intercalation in the thickness direction is low.

[0014] In some embodiments, the thickness of the negative electrode active material layer is 90 μm to 130 μm.

[0015] The thickness of the second negative electrode active material layer accounts for 70% to 90% of the thickness of the negative electrode active material layer, and the thickness of the first negative electrode active material layer accounts for 10% to 30% of the thickness of the negative electrode active material layer.

[0016] In some embodiments, the first negative electrode active material is graphite, and the OI value of the graphite powder is 3 to 8. The OI value of the graphite powder is the peak area ratio of the 004 characteristic diffraction peak to the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the graphite powder.

[0017] By controlling the OI value of the graphite powder of the first negative electrode active material to be 3 to 8, the OI value of the first negative electrode active material layer is 13 to 20.

[0018] In some embodiments, the second negative electrode active material is graphite, and the OI value of the graphite powder is 1 to 5. The OI value of the graphite powder is the peak area ratio of the 004 characteristic diffraction peak to the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the graphite powder.

[0019] The OI value of the second negative electrode active material layer is 5 to 10 by controlling the OI value of the graphite powder of the second negative electrode active material to be 1 to 5.

[0020] In some embodiments, the coating weight of the second negative electrode active material layer is 100 mg / 1540.25 mm 2 to 160 mg / 1540.25 mm 2 .

[0021] In some embodiments, the coating weight of the first negative electrode active material layer is 20 mg / 1540.25 mm 2 to 50 mg / 1540.25 mm 2 .

[0022] In some embodiments, the compaction density of the negative electrode active material layer is 1.47 g / cm 3 to 1.60 g / cm 3 .

[0023] The second aspect of the present application provides a lithium ion battery, which comprises a positive electrode sheet, a negative electrode sheet and an electrolyte, wherein the negative electrode sheet is the negative electrode sheet according to the first aspect of the present application.

[0024] The third aspect of the present application provides a device comprising the lithium ion battery according to the second aspect. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 FIG. 1 is a schematic view of the cross section of the negative electrode sheet according to an embodiment of the present application.

[0026] Explanation of main element symbols:

[0027] Negative electrode sheet 10

[0028] Negative electrode current collector 11

[0029] Negative electrode active material layer 13

[0030] First negative electrode active material layer 131

[0031] Second negative electrode active material layer 132 DETAILED DESCRIPTION

[0032] The data range values described in the present application should include the end values unless otherwise specified.

[0033] The present application provides a negative electrode sheet and a lithium ion battery using the same, which comprises a negative electrode sheet, a positive electrode sheet, an electrolyte and a separator arranged between the positive electrode sheet and the negative electrode sheet. The embodiments of the present application mainly improve the negative electrode sheet, which takes into account both high energy density and low expansion rate, thereby effectively reducing the risk of battery expansion and deformation.

[0034] Referring to Figure 1 The negative electrode sheet 10 provided by the embodiments of the present application includes a negative electrode current collector 11 and a negative electrode active material layer 13 located on at least one surface of the negative electrode current collector 11. The negative electrode current collector 11 is a copper foil, and the thickness of the copper foil is thin, i.e., 1 μm to 4 μm. The thin copper foil can correspondingly increase the thickness of the negative electrode active material layer 13, thereby increasing the energy density while maintaining the same overall negative electrode sheet. However, the thin copper foil has poor ductility and is prone to cause deformation of the battery.

[0035] In the embodiments of the present application, the negative electrode active material layer 13 is provided in a double-layer structure, including a first negative electrode active material layer 131 and a second negative electrode active material layer 132 stacked together. The first negative electrode active material layer 131 is located on the surface of the negative electrode current collector 11, and the second negative electrode active material layer 132 is located on the surface of the first negative electrode active material layer 131 away from the negative electrode current collector 11. That is, the first negative electrode active material layer 131 is located between the second negative electrode active material layer 132 and the negative electrode current collector 11.

[0036] The first negative electrode active material layer 131 includes a first negative electrode active material, and the OI value of the first negative electrode active material layer 131 is 13 to 20. The second negative electrode active material layer 132 includes a second negative electrode active material, and the OI value of the second negative electrode active material layer 132 is 5 to 10. The OI value of the first negative electrode active material layer 131 is the peak area ratio of the 004 characteristic diffraction peak to the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the first negative electrode active material layer 131, and the OI value of the second negative electrode active material layer 132 is the peak area ratio of the 004 characteristic diffraction peak to the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the first negative electrode active material layer 131.

[0037] The weight of the second negative electrode active material layer 132 is greater than the weight of the first negative electrode active material layer 131. In some embodiments, the weight ratio of the first negative electrode active material layer 131 to the second negative electrode active material layer 132 is 0.11 to 0.35. In some embodiments, the weight ratio of the first negative electrode active material layer 131 to the second negative electrode active material layer 132 is 0.20 to 0.32. In addition, the weight ratio of the first negative electrode active material layer 131 to the second negative electrode active material layer 132 can also be equivalent to the thickness ratio of the first negative electrode active material layer 131 to the second negative electrode active material layer 132.

[0038] In some embodiments, the weight (or thickness) of the second negative electrode active material layer 132 accounts for 70% to 90% of the weight (or thickness) of the negative electrode active material layer 13, and the weight (or thickness) of the first negative electrode active material layer 131 accounts for 10% to 30% of the weight (or thickness) of the negative electrode active material layer 13.

[0039] The thickness of the copper foil of the negative electrode sheet 10 is thin, and the ductility is small. By designing the first negative electrode active material layer 131 in the lower layer to have a large OI value and a small coating weight, the ductility of the first negative electrode active material layer 131 in the XY direction is small, and the first negative electrode active material layer 131 is directly coated on the thin copper foil to ensure that the overall ductility is small, thereby enhancing the ability to resist deformation. At the same time, the second negative electrode active material layer 132 in the upper layer has a small OI value and a large coating weight, which ensures that the depth of lithium intercalation in the thickness direction is low, the expansion rate is small, and the energy density is high. This design can ensure that the negative electrode sheet 10 has small expansion, high energy density, and can effectively prevent deformation problems under the conditions of high compaction density and high coating weight.

[0040] The first negative electrode active material is graphite, and the OI value of the graphite powder is 3 to 8. The OI value of the graphite powder is the peak area ratio of the 004 characteristic diffraction peak to the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the graphite powder.

[0041] The second negative electrode active material is graphite, and the OI value of the graphite powder is 1 to 5. The OI value of the graphite powder is the peak area ratio of the 004 characteristic diffraction peak to the 110 characteristic diffraction peak in the X-ray diffraction spectrum of the graphite powder.

[0042] By controlling the OI value of the graphite powder of the first negative electrode active material to be 3 to 8, the OI value of the first negative electrode active material layer is controlled to be 13 to 20. By controlling the OI value of the graphite powder of the second negative electrode active material to be 1 to 5, the OI value of the second negative electrode active material layer is controlled to be 5 to 10.

[0043] The coating weight of the first negative electrode active material layer 131 is 20 mg / 1540.25 mm 2 to 50 mg / 1540.25 mm 2 . The coating weight of the second negative electrode active material layer 132 is 100 mg / 1540.25 mm 2 to 160 mg / 1540.25 mm 2 .

[0044] In some embodiments, the thickness of the negative electrode active material layer 13 is 90 μm to 130 μm. The compaction density of the negative electrode active material layer 13 is 1.47 g / cm 3 to 1.60 g / cm 3 . The thickness of the second negative electrode active material layer 132 accounts for 70% to 90% of the thickness of the negative electrode active material layer 13, and the thickness of the first negative electrode active material layer 131 accounts for 10% to 30% of the thickness of the negative electrode active material layer 13.

[0045] The first negative active material layer 131 and the second negative active material layer 132 are mainly composed of a negative active material, and contain some conventional components.

[0046] In some embodiments, the first negative active material layer 131 and the second negative active material layer 132 further contain a conductive agent to improve the conductivity of the electrode. Any electrically conductive material can be used as the conductive agent, as long as it does not undergo chemical changes during production and use. Examples of the conductive agent include, but are not limited to, carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, and the like; metal-based materials such as metal powder or metal fibers of copper, nickel, aluminum, silver, and the like; conductive polymers such as polyphenylene derivatives and the like; or mixtures thereof.

[0047] In some embodiments, the first negative active material layer 131 and the second negative active material layer 132 further contain a binder. The binder can be various binder polymers such as polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, butadiene-styrene rubber, acrylated butadiene-styrene rubber, epoxy resin, nylon, polyacrylic acid, poly(styrene-acrylate), and the like.

[0048] In some embodiments, the method for preparing the negative electrode sheet 10 is a method for preparing a negative electrode for a lithium ion battery known in the art. For example, the negative electrode sheet 10 can be obtained by: (1) mixing the first negative active material, the conductive agent, the binder, and a solvent, and adding a thickening agent as needed to prepare a first negative active material slurry; (2) mixing the second negative active material, the conductive agent, the binder, and a solvent, and adding a thickening agent as needed to prepare a second negative active material slurry; and (3) sequentially coating the first negative active material slurry and the second negative active material slurry on the current collector, drying, and cold-pressing to form the negative active material layer. In some embodiments, suitable solvents include, but are not limited to, water and N-methylpyrrolidone.

[0049] The present application also provides a device comprising the lithium ion battery described above.

[0050] The use of the device of the present application is not particularly limited, and it can be used for any electronic device known in the art. For example, the electronic device includes, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile, a portable copier, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio player, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, an electric tool, a flashlight, a camera, a home-use large storage battery, a lithium ion capacitor, and the like. In addition, the lithium ion battery of the present application is applicable not only to the above-mentioned electronic devices, but also to energy storage power stations, marine vehicles, and air vehicles. The air vehicles include air vehicles within the atmosphere and air vehicles outside the atmosphere.

[0051] Positive electrode tab

[0052] The positive electrode tab includes a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector. It can be understood that the positive electrode active material layer mainly includes a positive electrode active material. The specific type of the positive electrode active material is not particularly limited, and can be selected according to the needs.

[0053] In some embodiments, the positive electrode active material includes a compound that reversibly intercalates and deintercalates lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive electrode active material can include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive electrode active material is selected from at least one of lithium cobaltate (LiCoO2), lithium nickel manganese cobalt ternary material (NCM), lithium manganate (LiMn2O4), lithium nickel manganate (LiNi 0.5 Mn 1.5 O4), lithium iron phosphate (LiFePO4), lithium nickel cobalt manganate, or lithium-rich manganese material.

[0054] In some embodiments, the positive electrode active material layer further includes a binder, and optionally, a conductive material. The binder can improve the binding between the positive electrode active material particles and each other, and can improve the binding between the positive electrode active material and the positive electrode current collector. In some embodiments, the binder includes, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymer, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, butadiene-styrene rubber, acrylated butadiene-styrene rubber, epoxy resin, and nylon, and the like.

[0055] In some embodiments, the positive electrode active material layer further includes a conductive material, thereby imparting electrical conductivity to the electrode. The conductive material can include any conductive material, so long as it does not cause chemical changes. Non-limiting examples of the conductive material include carbon-based materials (e.g., natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, etc.), metal-based materials (e.g., metal powders, metal fibers, etc., including, for example, copper, nickel, aluminum, silver, etc.), conductive polymers (e.g., polyphenylene derivatives), and mixtures thereof.

[0056] In some embodiments, the positive electrode current collector is a metal, for example including but not limited to aluminum foil.

[0057] In some embodiments, the structure of the positive electrode is a structure known in the art that can be used for a positive electrode of a lithium ion battery.

[0058] In some embodiments, the method of preparing the positive electrode is a method known in the art that can be used for preparing a positive electrode of a lithium ion battery. For example, the positive electrode can be obtained by mixing the active material, the conductive material, the binder, and the above-described positive electrode additive in a solvent to prepare a positive electrode active material slurry, and coating the positive electrode active material slurry on a current collector, drying, and cold-pressing to form a positive electrode active material layer. In some embodiments, the solvent can include water, N-methylpyrrolidone, etc., but is not limited thereto.

[0059] Electrolyte

[0060] The electrolyte used in the embodiments of the present application can be an electrolyte known in the art. The electrolyte can be classified into an aqueous electrolyte and a non-aqueous electrolyte, wherein a lithium ion battery using a non-aqueous electrolyte can operate at a wider voltage window compared to an aqueous electrolyte, thereby achieving a higher energy density. In some embodiments, the non-aqueous electrolyte includes an organic solvent, an electrolyte, and an additive.

[0061] Electrolytes that can be used in the electrolyte of embodiments of the application include, but are not limited to, inorganic lithium salts such as LiCIO4, LiAsF6, LiPF6, LiBF4, LiSbF6, LiSO3F, LiN(FSO2)2, and the like; fluorine-containing organic lithium salts such as LiCF3SO3, LiN(FSO2)(CF3SO2), LiN(CF3SO2)2, LiN(C2F5SO2)2, cyclic 1,3-hexafluoropropanedisulfonimide lithium, cyclic 1,2-tetrafluoroethane disulfonimide lithium, LiPF4(CF3)2, LiN(CF3SO2)(C4F9SO2), LiC(CF3SO2)3, LiPF4(CF3SO2)2, LiPF4(C2F5)2, LiPF4(C2F5SO2)2, LiBF2(CF3)2, LiBF2(C2F5)2, LiBF2(CF3SO2)2, LiBF2(C2F5SO2)2; dicarboxylic acid complex-containing lithium salts such as lithium bis(oxalato)borate, lithium difluoro(oxalato)borate, lithium tris(oxalato)phosphate, lithium difluorobis(oxalato)phosphate, lithium tetrafluoro(oxalato)phosphate, and the like. Additionally, the above electrolytes can be used singly, or two or more kinds can be used simultaneously. For example, in some embodiments, the electrolyte includes a combination of LiPF6and LiBF4. In some embodiments, the electrolyte includes LiPF6. In some embodiments, the mass percentage of the electrolyte is in the range of 8% to 15% based on the mass of the electrolyte.

[0062] The additive that can be used in the electrolyte of the present application can be any additive known in the art that can be used to improve the electrochemical performance of the battery. In some embodiments, the additive includes, but is not limited to, at least one of a polynitrile compound, a sulfur-containing additive, fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), 1,4 butane sultone.

[0063] The organic solvent that can be used in the electrolyte of the present application can be any organic solvent known in the art. In some embodiments, the organic solvent includes, but is not limited to, a carbonate compound, an ester-based compound, an ether-based compound, a ketone-based compound, an alcohol-based compound, an aprotic solvent, or a combination thereof. Among them, examples of the carbonate compound include, but are not limited to, a chain carbonate compound, a cyclic carbonate compound, a fluorinated carbonate compound, or a combination thereof.

[0064] In some embodiments, the organic solvent includes at least one of ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, methyl acetate, or ethyl propionate.

[0065] The preparation method of the electrolyte of the embodiments of the present application is not limited, and can be prepared according to the conventional electrolyte method. In some embodiments, the electrolyte of the present application can be prepared by mixing the components.

[0066] Separation film

[0067] A separation film is provided between the positive electrode and the negative electrode to prevent short circuit. The material and shape of the separation film are not particularly limited, and it can be any technology disclosed in the prior art. In some embodiments, the separation film comprises a polymer or inorganic substance formed of a material stable to the electrolyte of the present application, etc.

[0068] For example, in some embodiments, the separation film comprises a substrate layer. The substrate layer is a non-woven fabric, a film or a composite film having a porous structure. The material of the substrate layer can be selected from at least one of polyethylene, polypropylene, polyethylene terephthalate and polyimide. Specifically, the material of the substrate layer can be selected from polypropylene porous film, polyethylene porous film, polypropylene non-woven fabric, polyethylene non-woven fabric or polypropylene-polyethylene-polypropylene porous composite film.

[0069] A surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer can be a polymer layer, an inorganic layer or a layer formed by mixing polymer and inorganic substance. Specifically, the inorganic layer comprises inorganic particles and a binder. The inorganic particles can be selected from one or a combination of several of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The binder can be selected from one or a combination of several of polyvinylidene fluoride, a polymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, polymethyl methacrylate, polytetrafluoroethylene and polyhexafluoropropylene.

[0070] The technical solutions of the embodiments of the present application are further described below through specific examples.

[0071] Comparative Example 1

[0072] A negative electrode slurry was prepared by adding artificial graphite (powder OI value 5) as a negative electrode active material, conductive carbon black (Super P) as a conductive agent, sodium carboxymethyl cellulose (Daxion 2200) as a thickening agent, and the above-mentioned negative electrode binder as a negative electrode binder in a weight ratio of 97.3%:0.5%:1.2%:1.0% to water as a solvent, using a ROSS double planetary mixer. An extrusion coater was used for coating, the coating speed was 18 m / min, and the coating weight was set to 160 mg / 1540.25 cm 2The above negative electrode slurry was coated onto the surface of a 4 μm copper foil as a current collector. Then, baking was performed in an oven until the electrode sheet was dried, and cold-pressing was performed using a cold press to a density of 1.55 g / cm 3 , to produce a negative electrode sheet. The OI value of the coated layer was 15, and the thickness was 120 μm.

[0073] Example 1

[0074] A powder having an OI value of 3.5, artificial graphite, was used as a negative electrode active material, conductive carbon black (Super P) was used as a conductive agent, sodium carboxymethyl cellulose (Dailu 2200) was used as a thickening agent, and SBR / phenylpropyl emulsion was used as a negative electrode binder at a weight ratio of 97.3%:0.5%:1.2%:1.0% in water as a solvent, and a slurry for a second negative electrode active material layer (upper layer) was prepared using a ROSS double planetary mixer. A powder having an OI value of 5.3, artificial graphite, was used as a negative electrode active material, conductive carbon black (Super P) was used as a conductive agent, sodium carboxymethyl cellulose (Dailu 2200) was used as a thickening agent, and SBR / phenylpropyl emulsion was used as a negative electrode binder at a weight ratio of 97.3%:0.5%:1.2%:1.0% in water as a solvent, and a slurry for a first negative electrode active material layer (lower layer) was prepared using a ROSS double planetary mixer. Coating was performed using an extrusion double-layer coater, the coating speed was 18 m / min, the coating weight of the second negative electrode active material layer was set to 140 mg / 1540.25 cm 2 , and the coating weight of the first negative electrode active material layer was set to 20 mg / 1540.25 cm 2 The above two slurries were coated onto the surface of a 4 μm copper foil as a current collector, and the slurry for the first negative electrode active material layer (lower layer) was directly coated on the surface of the copper foil. Then, baking was performed in an oven until the electrode sheet was dried, and cold-pressing was performed using a cold press to a density of 1.55 g / cm 3 , to produce a negative electrode sheet. The OI value of the first negative electrode active material layer prepared after cold-pressing was 20, and the OI value of the second negative electrode active material layer was 10. The thickness of the first negative electrode active material layer was 15 μm, and the thickness of the second negative electrode active material layer was 105 μm.

[0075] A positive electrode sheet was prepared, and the material for the positive electrode active material layer included lithium cobaltate, conductive carbon black, and polyvinylidene fluoride. Lithium cobaltate, acetylene black, and polyvinylidene fluoride and an NMP solvent were stirred to prepare a positive electrode slurry, the positive electrode slurry was coated on the surface of a 6 μm aluminum foil as a positive electrode current collector and dried, and cold-pressing was performed using a cold press to a density of 4.15 g / cm 3 , to produce a positive electrode sheet.

[0076] A single-side ceramic coating + double-side water-based vinylidene fluoride-hexafluoropropylene copolymer coating was used as a separator.

[0077] The electrolyte was prepared using a conventional electrolyte formula: 1 mol / L lithium hexafluorophosphate + (ethylene carbonate + propylene carbonate + diethyl carbonate + ethyl propionate + fluoroethylene carbonate + 1,3-propane sultone) solvent.

[0078] After welding the above positive electrode tab and negative electrode tab and the separator into a battery cell, the battery cell was packaged with an aluminum plastic film, baked for 24 hours in a vacuum state to remove moisture, and then injected with the above electrolyte. The battery was formed and sorted at high temperature to obtain a square soft-pack lithium ion polymer battery with a thickness / width / height of 3.8 mm, 64 mm, and 82 mm, respectively.

[0079] Comparative Examples 2-5 and Example 2-8 refer to the preparation of lithium ion polymer batteries according to Example 1.

[0080] In Table 1, the differences between Comparative Examples 1-5 and Examples 1-7 are shown in Table 1 below, and other preparation process parameters such as negative electrode tab, positive electrode tab, separator, electrolyte, and battery are basically the same. For example, the second negative electrode active material layer has a coating weight of 140 mg / 1540.25 cm 2 , the first negative electrode active material layer has a coating weight of 20 mg / 1540.25 cm 2 , the thickness of the first negative electrode active material layer is 15 μm, the thickness of the second negative electrode active material layer is 105 μm, the thickness of the first negative electrode active material layer is 15 μm, the thickness of the second negative electrode active material layer is 105 μm, and the compaction density of the negative electrode tab is 1.55 g / cm 3 .

[0081] Table 1

[0082]

[0083] The measurement methods of the performance parameters of the examples and comparative examples are as follows.

[0084] (1) Current collector thickness test

[0085] Using a micrometer, align the current collector, and rotate the micrometer to test the reading h2.

[0086] (2) Tab thickness test

[0087] Using a micrometer, align the tab, and rotate the micrometer to test the reading h1.

[0088] (3) Tab compaction test

[0089] A tab with an area of 1540.25 mm 2 was punched, and the mass was weighed with a balance to be m1; a tab with an area of 1540.25 mm 2The current collector is weighed with a balance to have a mass m2, and the thickness h1 and h2 are used to calculate the compaction PD = (m1-m2) / (h1-h2).

[0090] (4) Graphite powder OI value test

[0091] The graphite powder is taken, and the diffraction peak is obtained by XRD test, and the C004 / C110 peak intensity ratio is calculated, that is, the OI value of the graphite powder.

[0092] (5) Pole piece OI value test

[0093] A pole piece is taken, and the diffraction peak is obtained by XRD test, and the C004 / C110 peak intensity ratio is calculated, that is, the OI value of the pole piece.

[0094] (6) Pole piece adhesion

[0095] The double-sided adhesive tape is pasted on the steel plate, the cold-pressed pole piece is cut into a 20mm long strip and pasted on the double-sided adhesive tape, the pole piece sample is rolled back and forth on the roller for 4 times, one end of the pole piece sample is clamped in the tensile machine clamp, and the tensile machine is started to test, and the adhesion value is obtained after the test is completed.

[0096] (7) Pole piece cohesion

[0097] The double-sided adhesive tape is pasted on the steel plate, the cold-pressed pole piece is cut into a 20mm long strip and pasted on the double-sided adhesive tape, the anode special cohesive green tape with a width of 20mm and a length of 80mm is pasted in the middle of the pole piece, the white paper strip with a width of 20mm and a length of 60mm is cut and inserted into the gap between the pole piece and the green tape, the overlapping length is about 15mm, the pole piece sample is rolled back and forth on the roller for 4 times, the white paper is clamped in the tensile machine clamp, and the tensile machine is started to test, and the cohesion value is obtained after the test is completed.

[0098] (8) Full charge XY elongation rate

[0099] The battery is disassembled, and the negative pole piece is taken out. The empty foil area of the negative pole piece is placed on the X-ray, and the size reading of the empty foil area is tested from the width direction of the pole piece, which is a1. The pole piece with active material layer coated on both sides after full charging is placed on the X-ray, and the size reading of the current collector is tested from the width direction of the pole piece, which is a2. The full charge elongation rate is calculated as (a2-a1) / a*100%.

[0100] The initial battery is charged and discharged for 1cls, and the initial thickness of the battery is measured by using a micrometer as H1. The battery is continuously charged and discharged for 500cls, and the thickness of the battery is measured by using a micrometer as H2. The thickness MMC retention rate of the battery after 25℃ cycle for 500cls is (H1-H2) / H1*100%.

[0101] The initial battery is charged and discharged for 1 cls, and the initial thickness of the battery is measured as H1 using a flat plate thickness gauge. The battery is continuously charged and discharged for 500 cls, and the thickness of the battery is measured as H2 using a flat plate thickness gauge. The thickness retention rate of 500 cls at 25°C is (H1-H2) / H1*100%.

[0102] As can be seen from Table 1, compared with Comparative Examples 1 to 4, the negative electrode sheet of Examples 1 to 7 has significantly lower battery MMC expansion rate and battery PPG expansion rate while ensuring the adhesion and cohesion of the electrode sheet, indicating that the battery has smaller expansion and higher energy density. Furthermore, the difference between the battery MMC expansion rate and the battery PPG expansion rate is smaller, i.e. 0.2% or 0.3%, indicating that the battery has smaller deformation. Note: The closer the battery MMC expansion rate and the battery PPG expansion rate (the smaller the difference), the smaller the deformation of the battery, and vice versa. (The deformation of the battery refers to surface unevenness, such as bulging of the battery surface, warping of both ends, convex points on the surface, etc.)

[0103] In Table 2, in each of Examples 9 to 18, the OI value of the graphite powder in the first negative electrode active material layer is 4.1, the OI value of the graphite powder in the second negative electrode active material layer is 3, the OI value of the first negative electrode active material layer is 15, the OI value of the first negative electrode active material layer is 9, and the thickness of the copper foil is 3 μm; the differences are shown in Table 2, and the ratio of the functional components in the two negative electrode slurries, the preparation process parameters of the positive electrode sheet, the separator, the electrolyte, the battery, etc. are all the same.

[0104] Table 2

[0105]

[0106] As can be seen from Table 2, in Examples 8 to 13, adjusting the weight ratio of the first negative electrode active material layer to the second negative electrode active material layer within a reasonable range (0.11-0.33) does not substantially affect the battery MMC expansion rate and the battery PPG expansion rate, and compared with Comparative Examples 1 to 5, the battery MMC expansion rate and the battery PPG expansion rate of Examples 9 to 14 are substantially similar, indicating that the battery has smaller deformation.

[0107] As can be seen from Table 2, in Examples 14 to 17, under the condition that other parameters are all the same (the coating weight ratio of the first and second negative electrode active material layers is 1:7), adjusting the thickness of the negative electrode active material layer (the sum of the thicknesses of the first and second negative electrode active material layers) does not change the battery MMC expansion rate and the battery PPG expansion rate.

[0108] The above embodiments are only used to illustrate the technical solutions of the present application but not limit the present application, and although the present application is explained in detail with reference to the above preferred embodiments, it should be understood by those skilled in the art that the technical solutions of the present application can be modified or equivalent replaced without departing from the spirit and scope of the technical solutions of the present application.

Claims

1. A negative electrode sheet, characterized by, The negative electrode active material layer includes: The first negative electrode active material layer includes a first negative electrode active material, and an OI value of the first negative electrode active material layer is 13 to 20; and The second negative electrode active material layer includes a second negative electrode active material, and an OI value of the second negative electrode active material layer is 5 to 10, and a weight of the second negative electrode active material layer is greater than a weight of the first negative electrode active material layer. The OI value of the first negative electrode active material layer is a ratio of peak areas of a 004 characteristic diffraction peak and a 110 characteristic diffraction peak in an X-ray diffraction spectrum of the first negative electrode active material layer. The OI value of the second negative electrode active material layer is a ratio of peak areas of a 004 characteristic diffraction peak and a 110 characteristic diffraction peak in an X-ray diffraction spectrum of the first negative electrode active material layer. A ratio of the weight of the first negative electrode active material layer to the weight of the second negative electrode active material layer is 0.11 to 0.

35. The ratio of the weight of the first negative electrode active material layer to the weight of the second negative electrode active material layer is 0.20 to 0.

32.

2. The negative electrode sheet according to claim 1, characterized by, A thickness of the negative electrode active material layer is 90 μm to 130 μm.

3. The negative electrode sheet according to claim 1, characterized by The first negative electrode active material is graphite, and an OI value of a powder of the graphite is 3 to 8; 4. The negative electrode sheet according to claim 1, wherein The OI value of the powder of the graphite is a ratio of peak areas of a 004 characteristic diffraction peak and a 110 characteristic diffraction peak in an X-ray diffraction spectrum of the powder of the graphite.

5. The negative electrode sheet according to claim 1, wherein The second negative electrode active material is graphite, and an OI value of a powder of the graphite is 1 to 5; The OI value of the powder of the graphite is a ratio of peak areas of a 004 characteristic diffraction peak and a 110 characteristic diffraction peak in an X-ray diffraction spectrum of the powder of the graphite.

6. The negative electrode sheet according to claim 1, wherein The negative electrode sheet is the negative electrode sheet according to any one of claims 1 to 9. The lithium ion battery includes the lithium ion battery according to claim 10.

7. The negative electrode sheet according to claim 1, wherein The coating weight of the second negative electrode active material layer is 100 mg / 1540.25 mm 2 to 160 mg / 1540.25 mm 2 .

8. The negative electrode plate of claim 1, wherein, The coating weight of the first negative electrode active material layer is 20 mg / 1540.25 mm 2 to 50 mg / 1540.25 mm 2 .

9. The negative electrode plate of claim 1, wherein, The compacted density of the negative active material layer is 1.47 g / cm 3 to 1.60 g / cm 3 .

10. A lithium-ion battery comprising a positive electrode sheet, a negative electrode sheet, and an electrolyte, characterized by, ​ 11. An apparatus, comprising: ​

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