Positive electrode sheet, electrochemical device, and method for manufacturing positive electrode sheet

Abstract

The application provides a positive electrode sheet, an electrochemical device and a preparation method of the positive electrode sheet, and belongs to the technical field of secondary batteries.The negative electrode sheet comprises a positive current collector and a positive active material layer.The positive active material layer is arranged on the positive current collector, and the positive active material layer comprises a positive active material, a solid-state electrolyte and a binder.The binder comprises polyvinyl butyral, the molecular weight of the binder is 40000-53000, and the viscosity of the binder is 80-180 mPa S.The positive electrode sheet introduces polyvinyl butyral (PVB) as the binder in the positive active material layer, so that high coverage bonding of the contact interface between the positive active material and the solid-state electrolyte is achieved, stress concentration caused by volume change of the positive electrode in the charge-discharge cycle process can be relieved, stable contact of the interface between the positive active material and the solid-state electrolyte is maintained, and the rate performance and the cycle life of the solid-state battery are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of secondary batteries, in particular to a positive electrode sheet, an electrochemical device and a preparation method of the positive electrode sheet. BACKGROUND

[0002] With the rapid development of new energy vehicles and large-scale energy storage technology, the demand for safety, energy density, cycle life and cost of lithium ion secondary batteries is also increasing. The energy density of existing organic liquid lithium ion batteries has reached the theoretical limit; at the same time, the flammable organic liquid electrolyte also faces the risk of leakage and battery thermal runaway. All-solid-state batteries use non-flammable solid electrolytes instead of organic liquid electrolytes, which can simplify battery design, improve battery safety performance and energy density, and expand the battery temperature range.

[0003] The lithium ion transmission of the solid-state battery in the charging and discharging process depends on the interface contact between the solid-state electrolyte and the electrode active material particles. Whether the solid-solid contact between the two is good determines the reversible capacity and capacity retention rate of the all-solid-state battery. Because the solid-state electrolyte and the electrode active material particles cannot naturally maintain close contact, a composite positive electrode sheet usually introduces a binder to bond the solid components, so that the solid-solid interface between the solid-state electrolyte and the positive electrode active material is in close contact, thereby realizing high reversible capacity and capacity retention rate of the solid-state battery.

[0004] During the charging and discharging cycle of the solid-state battery, the volume change of the positive electrode active material caused by the deintercalation of lithium will deteriorate the interface contact with the solid-state electrolyte. The currently used binder is difficult to adapt to the volume change of the particles inside the electrode sheet, and cannot maintain effective contact between the positive electrode active material and the solid-state electrolyte interface, making it difficult to inhibit the deterioration of battery impedance and cycle life caused by interface contact failure and even active material shedding.

[0005] Therefore, it is necessary to design a positive electrode sheet, an electrochemical device and a preparation method of the positive electrode sheet to improve the above problems. SUMMARY

[0006] The present application provides a positive electrode sheet, an electrochemical device and a preparation method of the positive electrode sheet to improve the technical problem that the existing binder is difficult to maintain close contact between the positive electrode active material and the solid-state electrolyte during the charging and discharging process of the solid-state battery, thereby causing the cycle performance of the battery to decay.

[0007] In a first aspect, the present application provides a positive electrode sheet, which comprises a positive electrode current collector and a positive electrode active material layer.

[0008] The positive electrode active material layer is arranged on the positive electrode current collector, and includes a positive electrode active material, a solid-state electrolyte, and a binder.

[0009] In an example of the present application, the coverage of the binder on the surface of the positive electrode active material layer is greater than or equal to 90%.

[0010] In an example of the present application, at least part of the binder is coated on the surface of the solid-state electrolyte particles.

[0011] In an example of the present application, the thickness of the binder between the solid-state electrolyte and the positive electrode active material is 100-200 nm.

[0012] In an example of the present application, the mass content of the binder in the positive electrode active material layer is 1-3 wt%.

[0013] In an example of the present application, the average particle size of the solid-state electrolyte and the mass content of the binder in the positive electrode active material layer satisfy the following formula (1):

[0014] -0.1≤0.5x-y≤0.1 (1)

[0015] wherein the mass content of the binder in the positive electrode active material layer is x wt%, and the average particle size of the solid-state electrolyte is y μm.

[0016] In an example of the present application, the positive electrode active material includes at least one of lithium nickel cobalt manganese oxide and lithium nickel manganese oxide, the molar content of nickel in the lithium nickel cobalt manganese oxide is 60-95%, the mass content of the positive electrode active material in the positive electrode active material layer is 65-90 wt%, and the mass content of the positive electrode active material and the mass content of the binder satisfy the following formula (2):

[0017] -0.1≤0.06(z-40)-x≤0.1 (2)

[0018] wherein the mass content of the binder in the positive electrode active material layer is x wt%, and the mass content of the positive electrode active material in the positive electrode active material layer is z wt%.

[0019] In an example of the present application, the hydroxyl content of the binder is 21-36 mol%, and the butyral degree of the binder is 58-74 mol%.

[0020] In an example of the present application, the glass transition temperature of the binder is 60-67℃.

[0021] In an example of the present application, the Young's modulus of the solid-state electrolyte is 15-30 GPa, and the Young's modulus of the binder is 2-5 GPa.

[0022] In an example of the present application, the solid-state electrolyte comprises a sulfide electrolyte.

[0023] In a second aspect, the present application further provides a preparation method of a positive electrode tab, which comprises:

[0024] mixing the binder and the solid-state electrolyte in an organic solvent to obtain a first mixed slurry;

[0025] mixing the positive electrode active material and the first mixed slurry to obtain a second mixed slurry;

[0026] coating the second mixed slurry on a positive electrode current collector, and obtaining the positive electrode tab after drying.

[0027] In an example of the present application, the preparation process of the binder comprises:

[0028] mixing n-butyl aldehyde and polyvinyl alcohol in a solvent to form a precursor solution; the molar ratio of the n-butyl aldehyde to the polyvinyl alcohol is 0.6-1.2;

[0029] adding an acid catalyst to the precursor solution, and heating the precursor solution at a temperature of 45-65℃ to obtain the binder; during the reaction, the pH value of the precursor solution is controlled to be 1.5-2.5, and the reaction time is 4-8 hours.

[0030] In a third aspect, the present application further provides an electrochemical device, which comprises a negative electrode tab, a solid-state electrolyte film, and the positive electrode tab of any one of the examples described above, or the positive electrode tab prepared by the preparation method of any one of the examples described above.

[0031] The positive electrode tab provided by the present application introduces polyvinyl butyral (PVB) with a suitable molecular weight and a higher viscosity as a binder in the positive electrode active material layer, to achieve high coverage bonding of the contact interface between the positive electrode active material and the solid-state electrolyte, and effectively alleviate the stress concentration caused by the volume change of the positive electrode during the charge and discharge cycle, thereby maintaining the stable contact between the positive electrode active material and the solid-state electrolyte, ensuring the stability of the positive electrode structure during the charge and discharge process, and further improving the rate performance and cycle life of the high positive electrode active material content solid-state battery. BRIEF DESCRIPTION OF DRAWINGS

[0032] The accompanying drawings, which are incorporated herein and constitute part of the specification, illustrate embodiments consistent with the application and, together with the description, further serve to explain the principles of the application. It is to be understood that the drawings are only illustrations of some embodiments of the application and that, upon considering this description, other embodiments of the application will be apparent to those of ordinary skill in the art.

[0033] In the drawings:

[0034] Figure 1 A flowchart of a method for preparing a positive electrode sheet according to an embodiment of the application;

[0035] Figure 2 A flowchart of a method for preparing a binder PVB according to an embodiment of the application. DETAILED DESCRIPTION

[0036] The advantages and effects of the application disclosed in the specification can be easily understood by those skilled in the art through the following specific examples. The application can also be implemented or applied in other different specific embodiments, and the details in the specification can be modified or changed in various ways based on different views and applications without departing from the spirit of the application. It should be noted that the following embodiments and features in the embodiments can be combined with each other without conflict. It should also be understood that the terms used in the embodiments of the application are for describing specific specific embodiments, not for limiting the protection scope of the application. The test methods in the following embodiments are not specified, and are usually performed under conventional conditions or under conditions recommended by the manufacturers.

[0037] It should be noted that the terms such as "up", "down", "left", "right", "middle" and "one" used in the specification are only for the convenience of clear description, not for limiting the scope of the application, and the change or adjustment of the relative relationship without substantial change of the technical content is also regarded as the scope of the application.

[0038] In the specification, the average particle size (D50) can be defined as the equivalent diameter of the particles corresponding to the cumulative volume ratio of 50% in the particle size distribution curve of the particles. The average particle size (D50) can be measured by, for example, a laser diffraction method. The laser diffraction method can generally measure the particle size from the submicron range to several millimeters, so that a result with high reproducibility and high resolution can be obtained.

[0039] In the present specification, the Young's modulus represents the degree of change in the length of the particles in a unit cross section after the powder is subjected to a certain tensile stress, and the Young's modulus is defined as σ / ε, wherein σ represents the stress in a unit area of the material, and ε represents the strain in a unit length. The measurement method of the Young's modulus is referred to GB / T 34186-2017 test.

[0040] During the charge-discharge cycle of the solid-state battery, the positive electrode active material produces a certain amount of volume change due to the deintercalation of lithium, so that the contact interface between the positive electrode active material and the solid-state electrolyte is separated and even falls off under the stress of the volume change, resulting in a significant attenuation of the cycle performance of the solid-state battery during use.

[0041] The applicant found that the currently used binder (such as PVDF, HNBR, SBR, etc.) in the solid-state battery has the defects of too large molecular weight and insufficient adhesion, which cannot effectively disperse and cover the contact interface between the positive electrode active material and the solid-state electrolyte in the positive electrode sheet, and is difficult to adapt to the volume change of the particles inside the electrode sheet. Therefore, the binder is difficult to maintain the close contact of the interface between the positive electrode active material and the solid-state electrolyte during the battery charge-discharge process, and cannot effectively slow down the impedance deterioration and cycle life attenuation of the solid-state battery during use.

[0042] To solve the above problems, the present application provides a positive electrode sheet, which needs to use a binder with good dispersibility and adhesion to effectively improve the defect that the interface of the positive electrode sheet is easily separated and cracked during the charge-discharge process. The applicant found that the polyvinyl butyral (PVB) material particles also have good viscosity at low molecular weight, so the applicant selected PVB material with appropriate molecular weight and high viscosity as the binder of the composite positive electrode. In this way, the good dispersibility of the PVB material can be used to complete the high coverage adhesion of the contact interface between the positive electrode active material and the solid-state electrolyte, and the strong viscosity of the PVB material can be used to maintain the stable contact of the contact interface between the positive electrode active material and the solid-state electrolyte during the charge-discharge process, thereby effectively slowing down the impedance deterioration and cycle life attenuation of the solid-state battery during use.

[0043] In a first aspect, the present application provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode active material layer. The positive electrode current collector can be made of a foil material with good electrical conductivity and mechanical strength, such as aluminum, nickel, titanium, gold, silver, chromium, molybdenum, copper, stainless steel, or carbon, etc. In addition to the foil material, the positive electrode current collector can also be in any one or a combination of multiple forms such as film, mesh, porous, foam, or non-woven fabric, etc. The positive electrode current collector has two opposite surfaces in the thickness direction of itself, and the positive electrode active material layer is arranged on at least one surface of the positive electrode current collector, specifically, the positive electrode active material layer is arranged on any one or both of the two surfaces of the positive electrode current collector.

[0044] The positive electrode active material layer comprises a positive electrode active material, a solid-state electrolyte and a binder. The binder comprises polyvinyl butyral (PVB), the molecular weight of the binder is any value in the range of 40000-53000, for example, the molecular weight of the binder can be 40000, 42000, 45000, 48000, 50000 or 53000; the viscosity of the binder is any value in the range of 80-180 mPa·S, for example, the viscosity of the binder can be 80 mPa·S, 100 mPa·S, 120 mPa·S, 140 mPa·S, 150 mPa·S, 160 mPa·S or 180 mPa·S. Among them, the viscosity test condition is to dissolve the PVB binder in ethanol solvent to form a glue liquid containing 10wt% PVB, and test its viscosity at 25°C.

[0045] When the molecular weight and viscosity of the binder are in the above appropriate range, good dispersibility and strong adhesion can be achieved. The PVB material with moderate degree of polymerization has good dispersibility, and the binder can be uniformly attached to the contact interface between the positive electrode active material and the solid-state electrolyte particles to achieve high coverage adhesion of the positive electrode active material and the solid-state electrolyte. At the same time, the binder uniformly dispersed in the positive electrode active material layer has high viscosity, and the binder can better anchor the particles in the positive electrode active material layer by using its strong adhesion, which can resist the deformation of the positive electrode active material during charging and discharging, and maintain the close contact of the contact interface between the positive electrode active material and the solid-state electrolyte, ensuring the structural integrity of the positive electrode sheet, thereby slowing down the impedance deterioration and life attenuation of the solid-state battery during the cycle process.

[0046] Among them, considering that the positive electrode active material in the positive electrode sheet has a certain volume change during charging and discharging, the PVB binder used in the positive electrode sheet in the present application can maintain stable contact of the contact interface between the positive electrode active material and the solid-state electrolyte while uniformly covering the contact interface of the positive electrode particles. If a PVB material with low degree of polymerization is used, the binder can uniformly cover the interface between the positive electrode particles based on the low molecular weight, but the binder cannot effectively maintain the interface stability of the positive electrode during the cycle process of the battery due to the low viscosity; if a PVB material with high degree of polymerization is used, the dispersibility of the binder will be poor due to the high molecular weight, so that the binder cannot uniformly adhere to the contact interface of the positive electrode particles with high coverage, resulting in that the positive electrode particles not effectively adhered and fixed are easy to crack and fall off during the cycle process of the battery.

[0047] In some embodiments, the coverage of the binder on the surface of the positive active material layer is higher than 90%, by increasing the coverage of the binder in the positive active material layer, to reduce the interface of the insufficiently bonded positive active material and the solid-state electrolyte particles, the stability of the combination of the positive active material and the solid-state electrolyte can be further maintained in the charging and discharging process, and the impedance deterioration and life attenuation of the solid-state battery in the cycle process can be slowed down.

[0048] In some embodiments, at least part of the binder in the positive active material layer is coated on the surface of the solid-state electrolyte particles, and the binder coated on the surface of the solid-state electrolyte particles can achieve high coverage bonding of both the solid-state electrolyte and the positive active material at the contact interface.

[0049] In some embodiments, the thickness of the binder between the solid-state electrolyte and the positive active material is any value in the range of 100-200 nm, for example, it can be 100 nm, 120 nm, 140 nm, 150 nm, 160 nm, 180 nm or 200 nm. When the coverage of the binder in the positive active material layer is higher than 90% and the thickness of the binder is in the above range, the positive electrode sheet can have high adhesion and high toughness, thereby having good structural stability.

[0050] In some embodiments, the content of hydroxyl groups in the binder is 21-36 mol%, for example, it can be 21 mol%, 22 mol%, 24 mol%, 26 mol%, 28 mol%, 30 mol%, 32 mol%, 34 mol% or 36 mol%; the degree of butyralization of the binder is 58-74 mol%, for example, it can be 58 mol%, 61 mol%, 64 mol%, 66 mol%, 68 mol%, 70 mol%, 73 mol% or 74 mol%. The inventors have found that PVB material not only has hydroxyl groups with mutual attraction properties, but also has acetal groups with mutual non-attraction properties. The hydroxyl groups make the PVB material molecules have binding properties, and the acetal groups make the PVB material have softness. By adjusting the content of hydroxyl groups and the degree of butyralization of the PVB material to the above range, the binder can have strong hardness and toughness. This makes the binder can effectively inhibit and adapt to the volume change of the positive active material inside the positive electrode sheet, and can maintain effective bonding to the particle interface.

[0051] If the hydroxyl content of the PVB material is too high and the butyral content is too low, the binder molecules are too tightly bonded, the binder has strong hardness and bonding strength but low toughness, at this time the binder is not easy to deform but easy to be broken under stress, which makes the positive electrode sheet fragile and low in impact resistance; if the acetal group content of the PVB material is too high and the hydroxyl content is too low, the mutual attraction between the binder molecules is weak, the binder is too soft and easy to be torn and damaged under external action, and difficult to maintain the effective bonding of the particle interface under the deformation of the positive active material particles.

[0052] In some embodiments, the glass transition temperature of the binder is 60-67℃, for example, the glass transition temperature of the binder can be 60℃, 61℃, 62℃, 63℃, 64℃, 65℃, 66℃ or 67℃. When the glass transition temperature of the binder is within the above range, the binder can have strong thermal stability and toughness. When the glass transition temperature of the binder is too low, the binder has poor heat resistance, the binder is easy to soften and deform at high temperature, and is easy to be torn and damaged under external force; when the glass transition temperature of the binder is too high, the polymer molecular chains in the binder are easy to be frozen at room temperature and low temperature environment, resulting in insufficient toughness of the binder and too brittle texture, and easy to be broken under external force. Both of the above two cases will result in insufficient adhesion of the binder to the particles of each component in the positive electrode during the battery cycle, so limiting the glass transition temperature of the binder to the appropriate range of the present embodiment helps to improve the adhesion effect of the binder on each component of the positive electrode.

[0053] In some embodiments, the mass content of the binder in the positive active material layer is any value within the range of 1-3wt%, for example, the mass content of the binder can be 1wt%, 1.5wt%, 2wt%, 2.5wt% or 3wt%; the mass content of the solid-state electrolyte in the positive active material layer is any value within the range of 5-32wt%, for example, it can be 5wt%, 7wt%, 10wt%, 13wt%, 15wt%, 17wt%, 20wt%, 23wt%, 25wt%, 27wt%, 30wt% or 32wt%.

[0054] In some embodiments, the average particle size of the solid-state electrolyte and the mass content of the binder in the positive active material layer satisfy the relationship of formula (1), and formula (1) is as follows:

[0055] -0.1≤0.5x-y≤0.1(1)

[0056] In formula (1), the mass content of the binder in the positive electrode active material layer is x wt%, and the average particle size of the solid-state electrolyte is y μm. When the average particle size of the solid-state electrolyte and the mass content of the binder in the positive electrode active material layer satisfy the relationship of formula (1), high coverage of the binder on the particle interface of each component in the positive electrode active material layer can be achieved (for example, the coverage of the binder in the positive electrode active material layer is higher than 90%), and the thickness of the binder at the contact interface between the positive electrode active material and the solid-state electrolyte is in a suitable thickness range (for example, the thickness of the binder at the contact interface is 100-200 nm).

[0057] It should be noted that in the present application, the type of solid-state electrolyte used in the positive electrode sheet can not be limited, and the solid-state electrolyte can be selected from at least one of sulfide electrolyte, halide electrolyte, oxide electrolyte and polymer electrolyte. As an example, the sulfide electrolyte includes at least one of lithium phosphorus sulfur chloride (LPSCl), lithium germanium phosphorus sulfur (LGPS) and derivatives thereof; as an example, the halide electrolyte includes at least one of Li3YCl6, Li3InCl6 and derivatives thereof; as an example, the oxide electrolyte includes at least one of lithium lanthanum zirconium oxide (LLZO), lithium aluminum titanium phosphate (LATP), lithium lanthanum titanate (LLTO) and derivatives thereof.

[0058] In the positive electrode sheet, the Young's modulus of the binder is any value in the range of 2-5 GPa, for example, the Young's modulus of the binder can be 2 GPa, 3 GPa, 4 GPa or 5 GPa. Therefore, in some embodiments, the Young's modulus of the solid-state electrolyte used is any value in the range of 15-30 GPa, for example, the Young's modulus of the solid-state electrolyte can be 15 GPa, 17 GPa, 20 GPa, 23 GPa, 25 GPa, 27 GPa or 30 GPa. When the Young's modulus of the binder and the solid-state electrolyte is in the above range, the reasonable matching range of the flexible binder and the rigid electrolyte can be met, the uniform coverage of the binder on the solid-state electrolyte interface can be achieved, and when the volume of the positive electrode active material changes, the stress concentration of the solid-state electrolyte and the positive electrode active material at the bonding contact interface can be avoided, and the risk of the positive electrode sheet cracking due to insufficient internal adhesion can be reduced. If the Young's modulus of the solid-state electrolyte is too high, the binder will be difficult to uniformly cover the surface of the solid-state electrolyte, and the rigid contact interface between the solid-state electrolyte and the positive electrode active material will easily cause stress concentration during charging and discharging.

[0059] In addition, the Young's modulus of the oxide electrolyte is too different from that of the PVB material (generally more than 10 times), and although the Young's modulus of the halide electrolyte can be in the range of 15-30 GPa, the chemical stability of the halide electrolyte in the solvent is poor. Therefore, in some embodiments, the solid-state electrolyte is selected to be a sulfide electrolyte with a Young's modulus in the range of the above embodiments and more stable chemical properties.

[0060] In addition, it should be noted that in the present application, the positive active material used in the positive electrode sheet can be any conventional positive active material, and the type of positive active material can not be limited, for example, the positive active material is selected from one or more of lithium iron phosphate (LFP), lithium manganese iron phosphate (LMFP), lithium cobalt oxide (LCO), lithium nickel oxide (LNO), lithium manganese oxide (LMO), lithium nickel manganese oxide (LNMO), lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), and lithium-rich manganese-based oxide (LRMO).

[0061] In some embodiments, the positive active material can be selected from high specific capacity positive active materials containing nickel elements, for example, the positive active material can be selected from at least one of lithium nickel cobalt manganese oxide (NCM) and lithium nickel manganese oxide (LNMO), wherein the molar content of nickel in the lithium nickel cobalt manganese oxide is in the range of 60% to 95%, for example, it can be 60%, 70%, 80%, 85%, 90% or 95%.

[0062] In the above embodiments, the mass content of the positive active material in the positive active material layer is 65 to 90 wt%, for example, it can be 65 wt%, 70 wt%, 75 wt%, 80 wt%, 85 wt% or 90 wt%. In the positive active material layer, the mass content of the positive active material and the mass content of the binder satisfy the relationship of formula (2), and the formula (2) is as follows:

[0063] -0.1≤0.06(z-40)-x≤0.1(2)

[0064] wherein the mass content of the binder in the positive active material layer is x wt%, and the mass content of the positive active material in the positive active material layer is z wt%. When the mass content of the binder and the positive active material in the positive active material layer satisfies the relationship of formula (2), the binder can increase in appropriate content to match the additional volume change increase caused by the increase of nickel content in the positive active material. On the one hand, the increase of nickel content in the positive electrode sheet will bring additional volume change during charging and discharging, so by increasing the mass content of the binder and the mass content of the positive active material, it can help the positive electrode sheet to effectively adapt to the increase of volume expansion caused by high content of nickel element, and effectively maintain the close contact between the solid electrolyte and the positive active material interface during the charging and discharging process; on the other hand, under the relationship of formula (2), even if the positive electrode sheet with high nickel content is adapted, the mass content of the binder is still within the appropriate range (such as 4 wt% or less), which effectively avoids the introduction of too much binder to adapt to the increase of nickel content in the positive electrode sheet, thereby causing the decrease of battery loading capacity and the increase of impedance.

[0065] In some embodiments, the positive electrode active material layer further comprises a positive electrode conductive agent, and the mass content of the positive electrode conductive agent in the positive electrode active material layer is any value within the range of 1-5 wt%, for example, the mass content of the positive electrode conductive agent can be 1 wt%, 2 wt%, 3 wt%, 4 wt% or 5 wt%. In an example, the mass ratio of the positive electrode active material, the solid-state electrolyte, the positive electrode conductive agent and the binder is (65-90):(5-32):(1-5):(1-3).

[0066] It should be noted that the type of positive electrode conductive agent is not limited, and the positive electrode conductive agent can be selected from at least one of graphite, graphene, carbon black, carbon fiber and carbon nanotube. In some embodiments, the positive electrode conductive agent comprises carbon black and carbon fiber, and the mass ratio of the carbon black and the carbon fiber is 1:(0.4-0.8), for example, the mass ratio of the carbon black and the carbon fiber can be 1:0.4, 1:0.5, 1:0.6, 1:0.7 or 1:0.8; alternatively, the mass ratio of the carbon black and the carbon fiber is 1:0.6. When the mass content of the carbon black and the carbon fiber is within the above range, a positive electrode tab with excellent rate performance can be obtained. This is because the positive electrode tab has smaller positive electrode active material particle size, and more zero-dimensional conductive agents are needed to connect the active material that cannot be contacted by one-dimensional conductive agents with high aspect ratio, and the combination of zero-dimensional and one-dimensional conductive agents can form a conductive network with optimal coverage around the positive electrode active material particles at the above content ratio.

[0067] In a second aspect, the present application also provides a method for preparing a positive electrode tab, as shown in Figure 1 The method for preparing the positive electrode tab comprises the following steps:

[0068] S1, mixing the binder and the solid-state electrolyte in an organic solvent to obtain a first mixed slurry;

[0069] S2, mixing the positive electrode active material with the first mixed slurry to obtain a second mixed slurry;

[0070] S3, coating the second mixed slurry on a positive electrode current collector, and obtaining the positive electrode tab after drying.

[0071] In some embodiments, in step S1, the binder is first provided, and the binder is a PVB material powder with a molecular weight of 40,000-53,000 and a viscosity of 80-180 mPa·S; then, the binder is dissolved in an organic solvent to obtain a binder glue solution, and the concentration of the binder glue solution is 1-30 wt%; finally, a predetermined amount of the solid-state electrolyte is uniformly mixed with the binder glue solution in advance to obtain the first mixed slurry, and the solid content of the first mixed slurry is 30-80%.

[0072] In step S1, the solid-state electrolyte is pre-mixed with the adhesive glue solution. The shearing force during stirring can be used to preferentially adsorb the adhesive molecules on the surface of the solid-state electrolyte particles, thereby forming a coating layer on the surface of the solid-state electrolyte particles. This coating layer can effectively isolate and avoid the occurrence of side reactions between the solid-state electrolyte and the positive active material. At the same time, the polar groups (such as hydroxyl groups) in the PVB material of the adhesive can coordinate with the surface of the solid-state electrolyte (especially the sulfide electrolyte) through hydrogen bonding, forming a directional lithium ion transport path, which effectively improves the ionic conductivity of the prepared positive electrode sheet.

[0073] In some embodiments, in step S2, an organic solvent is added to the first mixed slurry, and a pre-set amount of positive active material and positive conductive agent is continuously added to the first mixed slurry, and after mixing uniformly, a second mixed slurry is prepared, and the solid content of the second mixed slurry is 30-80%.

[0074] In steps S1 and S2, the slurry can be stirred and mixed by a stirrer, the stirring speed is 500-2000 rpm, and the mixing time is 1-30 min each time.

[0075] In some embodiments, in steps S1 and S2, the mass ratio of the mixed positive active material, solid-state electrolyte, positive conductive agent, and adhesive is (65-90):(5-32):(1-5):(1-3).

[0076] In some embodiments, in steps S1 and S2, the organic solvent used can be selected from at least one of toluene, isobutyl isobutyrate, butyl butyrate, hexyl butyrate, diisobutyl ketone, and anisole.

[0077] In some embodiments, in step S3, the second mixed slurry is coated on the positive current collector (such as a carbon aluminum foil), and the positive current collector is dried at a temperature of 80-150°C for 12-24 hours to obtain a positive electrode sheet. The surface capacity of the prepared positive electrode sheet is 1-10 mAh·cm -2 .

[0078] In addition, as Figure 2 shown, in some embodiments, the preparation method of the PVB material used in step S1 includes the following steps:

[0079] S11, mixing n-butyraldehyde and polyvinyl alcohol into a solvent to form a precursor solution.

[0080] In step S11, polyvinyl alcohol with a molecular weight of 20000-150000 is provided, the polyvinyl alcohol (PVA) is dissolved in deionized water with a temperature of 80-95℃ to form a polyvinyl alcohol solution, the concentration of the polyvinyl alcohol solution is 5-15wt%; then n-butyl aldehyde is added to the polyvinyl alcohol solution to prepare a precursor solution. The molar ratio of n-butyl aldehyde to polyvinyl alcohol is 0.6-1.2, for example, it can be 0.6, 0.8, 1 or 1.2.

[0081] In step S12, an acid catalyst is added to the precursor solution, and the precursor solution is heated to make the precursor raw materials react to form the binder.

[0082] In step S12, an acid catalyst is added to the precursor solution, and the precursor solution is heated to make the precursor raw materials react to form the binder.

[0083] In the preparation process of the PVB material, the molecular weight of the PVB material prepared is controlled by adjusting the molecular weight of the PVA raw material and the reaction time in step S12. The greater the molecular weight of the PVA raw material and the longer the reaction time, the higher the molecular weight of the PVB material prepared. The hydroxyl content of the PVB material prepared is controlled by adjusting the acetalization reaction degree in step S12. For every 5℃ increase in reaction temperature, the hydroxyl content of the PVB material prepared decreases by 3-5mol%. The butyral degree of the PVB material prepared is controlled by adjusting the molar ratio of n-butyl aldehyde to PVA. For every 0.1 increase in the molar ratio of n-butyl aldehyde to PVA, the butyral degree of the PVB material increases by 8-10mol%. The glass transition temperature of the PVB material prepared is controlled by adjusting the hydroxyl / acetyl ratio in the raw materials n-butyl aldehyde and PVA. For every 5mol% increase in the hydroxyl content, the glass transition temperature of the PVB material prepared increases by 2-3℃.

[0084] In a third aspect, the present application also provides an electrochemical device, which can be a solid-state lithium ion secondary battery. The electrochemical device comprises a negative electrode sheet, a solid-state electrolyte film and the positive electrode sheet of any one of the above embodiments. The solid-state electrolyte film is arranged between the positive electrode sheet and the negative electrode sheet to separate the positive electrode sheet and the negative electrode sheet and serve as a lithium ion conductor between the positive electrode sheet and the negative electrode sheet.

[0085] The solid electrolyte film can be a solid electrolyte film of any type known in the art.For example, in some embodiments, the solid-state electrolyte film comprises a solid-state electrolyte and a film layer binder. The mass ratio of the solid-state electrolyte to the film layer binder in the solid-state electrolyte film is (90-100):(0.5-10), the solid-state electrolyte is selected from at least one of a sulfide electrolyte, a halide electrolyte, an oxide electrolyte, and a polymer electrolyte, and the film layer binder is selected from any one or a combination of several compositions mixed in any ratio of nitrile rubber (NBR), styrene-butadiene rubber (SBR), polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), and linear tri-embedded copolymer.

[0086] The negative electrode tab can be a negative electrode tab of any type known in the art.

[0087] In some embodiments, the negative electrode tab comprises a negative electrode current collector and a negative electrode active material layer coated on at least one side of the negative electrode current collector, and the negative electrode active material layer comprises a negative electrode active material, a solid-state electrolyte, a negative electrode binder, and a negative electrode conductive agent. The preparation process of such a negative electrode tab is as follows: the negative electrode active material, the solid-state electrolyte, the negative electrode conductive agent, and the negative electrode binder are mixed in a mass ratio of (70-90):(5-25):(1-5):(1-3), a solvent, deionized water, is added, and then the mixture is stirred sufficiently under vacuum to obtain a negative electrode slurry. The negative electrode slurry is uniformly coated on the negative electrode current collector, and then the negative electrode current collector is transferred to an oven for drying after being dried at room temperature, followed by cold pressing and slitting to obtain the negative electrode tab.

[0088] The negative electrode current collector can be a foil material with good electrical conductivity and mechanical strength, such as aluminum, nickel, titanium, gold, silver, chromium, molybdenum, copper, stainless steel, or carbon, etc. In addition to the foil material, the negative electrode current collector can also adopt any one or a combination of multiple forms such as a film, a mesh, a porous shape, a foam, or a non-woven fabric, etc.

[0089] The negative electrode active material is selected from one or more of tin, artificial graphite (single-crystal graphite, polycrystalline graphite, pyrolytic graphite, graphite fibers, etc.), natural graphite (block graphite, flake graphite, earthy graphite, etc.), soft carbon, hard carbon, pure silicon (crystalline silicon, amorphous silicon, or organic silicon), silicon oxide compounds, silicon carbon compounds, and nano metal oxides (Fe2O3, CuO, SnO2, Mn3O4 nanoparticles).

[0090] The solid-state electrolyte is selected from at least one of a sulfide electrolyte, a halide electrolyte, an oxide electrolyte, and a polymer electrolyte.

[0091] The negative electrode conductive agent is selected from one or a combination of two or more compositions mixed in any ratio of carbon black, acetylene black, graphene, carbon nanotubes, and carbon nanofibers.

[0092] The negative electrode binder is selected from any one of polyvinylidene fluoride (PVDF), fluoroethylene-hexafluoropropylene (PVDF-HFP), polyethylene oxide (PEO), polymethyl methacrylate (PMMA), nitrile rubber (NBR), hydrogenated nitrile rubber (HNBR), styrene-butadiene rubber (SBR), or a combination thereof mixed in any proportion.

[0093] In other embodiments, the negative electrode plate can be a metal lithium plate, a metal indium plate, or a lithium-containing alloy plate (such as a lithium-tin-indium alloy plate, a lithium-silicon alloy plate, a lithium-tin alloy plate, or a lithium-aluminum alloy plate). For example, in one example, the negative electrode plate is selected from a metal lithium plate.

[0094] An example of the assembly method of a solid-state battery is described as follows: a positive electrode sheet is integrated on one side of a solid electrolyte membrane, for example, the solid electrolyte membrane is placed on the positive active material layer of the positive electrode sheet, and the solid electrolyte membrane and the positive electrode sheet are pressed into one at a pressure of 200 MPa; and then a negative electrode sheet is integrated on the other side of the solid electrolyte membrane, for example, the negative electrode sheet is placed on the other side of the solid electrolyte membrane, so that the negative active material layer of the negative electrode sheet is in contact with the solid electrolyte membrane, and the negative electrode sheet, the solid electrolyte membrane and the positive electrode sheet are pressed into one at a pressure of 200 MPa; a solid-state lithium-ion battery can be obtained by sealing and packaging the pressed battery cell under vacuum or inert atmosphere.

[0095] The technical solutions of the present invention are described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art.

[0096] Example 1

[0097] This embodiment provides a positive electrode sheet, which includes a positive electrode current collector and a positive electrode active material layer coated on the positive electrode current collector. The positive electrode active material layer includes a positive electrode active material, a solid electrolyte, a positive electrode conductive agent and a binder. The mass ratio of the positive electrode active material, the solid electrolyte, the positive electrode conductive agent and the binder is 75:21:2:2. Among them, the positive electrode active material is LiNi 0.8 Co 0.1 Mn 0.1O2, from Xiamen Tungsten Co., Ltd., model M821A, average particle size D50 of 3.3 μm; solid-state electrolyte Li6PS5Cl, from Shandong Xingjieneng Lithium Battery Co., Ltd., model LPSCl, average particle size D50 of 1 μm; positive electrode conductive agent super p (Tianjin Youmeng Chemical Technology Co., Ltd., superp) and VGCF (Showa Denko K.K., VGCF-H) in a mass ratio of 1:1; binder PVB material, molecular weight of the binder 53000, hydroxyl content 22 mol%, butyral degree 73 mol%, viscosity 120 mPa·S, glass transition temperature 60 ℃, PVB material from Shokubai Chemical Industries Co., Ltd., model BM-S.

[0098] The preparation method of the positive electrode tab is as follows:

[0099] (1) 10 g of positive electrode active material, 2.67 g of solid-state electrolyte, 0.27 g of conductive agent, 0.4 g of binder, and 6 g of solvent dimethylbenzene were mixed in a glove box with a stirrer to form a positive electrode slurry with a solid content of 70 wt%; wherein the high-speed stirrer speed was 2000 rpm, and the mixing time was 30 min each time.

[0100] (2) The slurry was coated on the positive electrode current collector carbon-coated aluminum foil with a doctor blade, and the positive electrode current collector was quickly transferred to a vacuum environment and dried at 100 ℃ for 12 h to finally form a positive electrode tab with a surface capacity of 3 mAh·cm -2 .

[0101] Example 2

[0102] This example provides a positive electrode tab of the same system as Example 1, and the difference between this example and Example 1 is that the preparation method of the positive electrode tab is as follows:

[0103] (1) 5 g of PVB binder was dissolved in 45 g of solvent dimethylbenzene to obtain a first mixed slurry with a concentration of 10 wt%;

[0104] (2) 2.67 g of solid-state electrolyte and 4 g of binder glue were mixed in a glove box with a stirrer to form a second mixed slurry; 10 g of positive electrode active material, 0.27 g of conductive agent, and 2.11 g of solvent dimethylbenzene were added to the second mixed slurry and continued to be mixed with a stirrer to form a third mixed slurry with a solid content of 70 wt%; wherein the high-speed stirrer speed was 2000 rpm, and the mixing time was 30 min each time.

[0105] (3) The slurry is coated on the positive current collector carbon-coated aluminum foil with a doctor blade, and the positive current collector is quickly transferred to a vacuum environment and dried at 100 ℃ for 12 h to finally prepare a positive electrode sheet with a surface capacity of 3 mAh·cm -2 .

[0106] Example 3

[0107] This example provides a positive electrode sheet of the same system as Example 2, and the difference between this example and Example 2 is that the molecular weight of the binder PVB in the positive electrode sheet is 40000, the hydroxyl content is 34 mol%, the butyral degree is 63 mol%, the viscosity is 80 mPa·S, and the glass transition temperature is 67 ℃. The binder PVB is from Shikoku Chemicals Corporation, and the model number is BM-1.

[0108] Example 4

[0109] This example provides a positive electrode sheet of the same system as Example 2, and the difference between this example and Example 2 is that the molecular weight of the binder PVB in the positive electrode sheet is 52000, the hydroxyl content is 31 mol%, the butyral degree is 66 mol%, the viscosity is 130 mPa·S, and the glass transition temperature is 67 ℃. The binder PVB is from Shikoku Chemicals Corporation, and the model number is BM-2.

[0110] Example 5

[0111] This example provides a positive electrode sheet of the same system as Example 2, and the difference between this example and Example 2 is that the molecular weight of the binder PVB in the positive electrode sheet is 53000, the hydroxyl content is 34 mol%, the butyral degree is 63 mol%, the viscosity is 180 mPa·S, and the glass transition temperature is 67 ℃. The binder PVB is from Shikoku Chemicals Corporation, and the model number is BM-5.

[0112] Example 6

[0113] This example provides a positive electrode sheet of the same system as Example 2, and the difference between this example and Example 2 is that the solid-state electrolyte in the positive electrode sheet is Li 5.5 PS 4.5 Cl 1.5 , from Hunan Enjie, model number LPSC (lithium phosphorus sulfur chloride), average particle size D50 is 0.7 μm; in the positive electrode sheet, the mass ratio of the positive electrode active material, the solid-state electrolyte, the positive electrode conductive agent, and the binder is adjusted to 65:31.5:2:1.5.

[0114] Example 7

[0115] The embodiment provides the positive electrode sheet of the same system as that of embodiment 2, and the difference between the embodiment and embodiment 2 is that the solid-state electrolyte in the positive electrode sheet is Li 5.5 PS 4.5 Cl 1.5 , which is from Hunan Enjie, the model is LPSC (lithium phosphorus sulfur chloride), and the average particle size D50 is 0.9 μm.

[0116] Embodiment 8

[0117] The embodiment provides the positive electrode sheet of the same system as that of embodiment 2, and the difference between the embodiment and embodiment 2 is that the solid-state electrolyte in the positive electrode sheet is Li 5.5 PS 4.5 Cl 1.5 , which is from Hunan Enjie, the model is LPSC (lithium phosphorus sulfur chloride), and the average particle size D50 is 1.3 μm; in the positive electrode sheet, the mass ratio of the positive electrode active material, the solid-state electrolyte, the positive electrode conductive agent and the binder is adjusted to 85:10.4:2:2.6.

[0118] Embodiment 9

[0119] The embodiment provides the positive electrode sheet of the same system as that of embodiment 2, and the difference between the embodiment and embodiment 2 is that the solid-state electrolyte in the positive electrode sheet is Li 5.5 PS 4.5 Cl 1.5 , which is from Hunan Enjie, the model is LPSC (lithium phosphorus sulfur chloride), and the average particle size D50 is 1.5 μm; in the positive electrode sheet, the mass ratio of the positive electrode active material, the solid-state electrolyte, the positive electrode conductive agent and the binder is adjusted to 90:5:2:3.

[0120] Embodiment 10

[0121] The comparative example provides the positive electrode sheet of the same system as that of embodiment 2, and the difference between the comparative example and embodiment 2 is that in the positive electrode sheet, the mass ratio of the positive electrode active material, the solid-state electrolyte, the positive electrode conductive agent and the binder is adjusted to 75:19:2:4.

[0122] Embodiment 11

[0123] The embodiment provides the positive electrode sheet of the same system as that of embodiment 2, and the difference between the embodiment and embodiment 2 is that the average particle size D50 of the solid-state electrolyte is 4 μm.

[0124] Embodiment 12

[0125] The embodiment provides the positive electrode sheet of the same system as that of embodiment 2, and the difference between the embodiment and embodiment 2 is that the positive electrode active material is LiNi 0.6 Co 0.2 Mn 0.2O2, from Xiamen Tungsten Co., Ltd., model XW10, average particle size D50 of 3.5 μm.

[0126] Example 13

[0127] This example provides a positive electrode tab of the same system as Example 2, and the difference between this example and Example 2 is that the positive electrode active material is LiNi 0.9 Co 0.05 O 0.05 , from Hubei Maidaihe Chemical Co., Ltd., model NCM900505, average particle size D50 of 3.5 μm.

[0128] Example 14

[0129] This example provides a positive electrode tab of the same system as Example 2, and the difference between this example and Example 2 is that the positive electrode active material is LNMO instead of LiNi 0.8 Co 0.1 Mn 0.1 O2. LNMO is specifically single-crystal LiNi 0.5 Co 1.5 O4 coated with Li2ZrO3, from Xiamen Tungsten Co., Ltd., model XW46.

[0130] Example 15

[0131] This example provides a positive electrode tab of the same system as Example 2, and the difference between this example and Example 2 is that the positive electrode conductive agent is selected to be super p and VGCF in a mass ratio of 1:0.4.

[0132] Example 16

[0133] This example provides a positive electrode tab of the same system as Example 2, and the difference between this example and Example 2 is that the positive electrode conductive agent is selected to be super p and VGCF in a mass ratio of 1:0.8.

[0134] Example 17

[0135] This example provides a positive electrode tab of the same system as Example 2, and the difference between this example and Example 2 is that the positive electrode conductive agent is selected to be KB and CNT in a mass ratio of 1:0.6. The KB is from Foshan Pears Carbon Material Technology Co., Ltd., model EC600JD; and the CNT is from LG New Energy, model CP1002M.

[0136] Example 18

[0137] The embodiment provides the positive electrode sheet of the same system as that of the embodiment 2, and the difference between the embodiment and the embodiment 2 is that super p and VGCF are selected as the positive electrode conductive agent in a mass ratio of 1:2.

[0138] Comparative example 1

[0139] The comparative example provides the positive electrode sheet of the same system as that of the embodiment 1, and the difference between the comparative example and the embodiment 1 is that the PVDF material is used as the binder in the positive electrode sheet, and the PVDF material is from Arkema, and the model is Film 302 PGM TR.

[0140] Comparative example 2

[0141] The comparative example provides the positive electrode sheet of the same system as that of the embodiment 2, and the difference between the comparative example and the embodiment 2 is that the PVDF material is used as the binder in the positive electrode sheet, and butyl butyrate solvent (from Aldrich) is used to dissolve the PVDF material when the first mixed slurry is prepared. The PVDF material is from Arkema, and the model is Film 302 PGM TR.

[0142] Comparative example 3

[0143] The comparative example provides the positive electrode sheet of the same system as that of the embodiment 2, and the difference between the comparative example and the embodiment 2 is that the HNBR material is used as the binder in the positive electrode sheet. The HNBR material is from Japan Zeon, and the model is Zetpol HSN 1010.

[0144] Comparative example 4

[0145] The comparative example provides the positive electrode sheet of the same system as that of the embodiment 2, and the difference between the comparative example and the embodiment 2 is that the PVB material is used as the binder in the positive electrode sheet, the molecular weight of the PVB material is 19000, the hydroxyl content is 36 mol%, the butyral degree is 61 mol%, the viscosity is 20 mPa·S, and the glass transition temperature is 66 DEG C. The PVB binder is from Shikoku Chemicals Corporation, and the model is BL-1.

[0146] Comparative example 5

[0147] The comparative example provides the positive electrode sheet of the same system as that of the embodiment 2, and the difference between the comparative example and the embodiment 2 is that the PVB material is used as the binder in the positive electrode sheet, the molecular weight of the PVB material is 27000, the hydroxyl content is 36 mol%, the butyral degree is 61 mol%, the viscosity is 45 mPa·S, and the glass transition temperature is 68 DEG C. The PVB binder is from Shikoku Chemicals Corporation, and the model is BL-2.

[0148] Comparative example 6

[0149] The positive electrode sheet of the present comparative example is the same system as that of Example 2, and the difference between the present comparative example and Example 2 is that the molecular weight of the PVB material used in the binder in the positive electrode sheet is 110000, the hydroxyl content is 34 mol%, the butyral degree is 63 mol%, the viscosity is 800 mPa·S, and the glass transition temperature is 71℃. The PVB binder is from Shikoku Chemicals Corporation, and the model number is BH-3.

[0150] Comparative Example 7

[0151] The positive electrode sheet of the present comparative example is the same system as that of Example 2, and the difference between the present comparative example and Example 2 is that the molecular weight of the PVB material used in the binder in the positive electrode sheet is 115000, the hydroxyl content is 29 mol%, the butyral degree is 60 mol%, the viscosity is 500 mPa·S, and the glass transition temperature is 62℃. The PVB binder is from Shikoku Chemicals Corporation, and the model number is BH-A.

[0152] Comparative Example 8

[0153] The positive electrode sheet of the present comparative example is the same system as that of Example 2, and the difference between the present comparative example and Example 2 is that the molecular weight of the PVB material used in the binder in the positive electrode sheet is 200000, the hydroxyl content is 19 mol%, the butyral degree is 80 mol%, the viscosity is 240 mPa·S, and the glass transition temperature is 73℃. The PVB binder is from Eastman Chemical Company, and the model number is BUTVAR-B-72.

[0154] Comparative Example 9

[0155] The positive electrode sheet of the present comparative example is the same system as that of Example 2, and the difference between the present comparative example and Example 2 is that the molecular weight of the PVB material used in the binder in the positive electrode sheet is 90000, the hydroxyl content is 11 mol%, the butyral degree is 88 mol%, the viscosity is 23 mPa·S, and the glass transition temperature is 65℃. The PVB binder is from Eastman Chemical Company, and the model number is BUTVAR-B-76.

[0156] The positive electrode sheets prepared in Examples 1 to 18 and Comparative Examples 1 to 9 were subjected to peeling force test, and the positive electrode sheets prepared in Examples 1 to 18 and Comparative Examples 1 to 9 were assembled into lithium ion solid-state batteries, and the solid-state batteries assembled with Examples 1 to 18 and Comparative Examples 1 to 9 were subjected to cycle performance test to verify the improvement effect of the positive electrode sheet on the cycle performance of the battery, and the test results are shown in Table 1.

[0157] The process of the positive electrode sheet peeling force test is as follows:

[0158] The peeling strength of the positive electrode tab, i.e., the peeling force, was quantitatively determined by a 180° peeling method. The free end of the positive electrode tab prepared in Examples 1 to 18 and Comparative Examples 1 to 9 was folded by 180°, and the free end of the positive electrode tab and the test plate were clamped on the upper and lower clamps, respectively, and continuous peeling was performed in the same environment using a tensile testing machine until the positive current collector and the positive active material layer were completely separated, and the peeling strength value could be directly read to obtain the peeling force between the positive active material layer and the positive current collector.

[0159] The assembly process of the solid-state battery is as follows:

[0160] The solid-state electrolyte powder was placed in a mold sleeve with a diameter of φ10 mm, and was pressed at a pressure of 200 MPa for 5 minutes to obtain a solid-state electrolyte block with a diameter of φ10 mm; wherein the solid-state electrolyte was Li6PS5Cl. The lithium-indium alloy sheet was used as the negative electrode tab, and the positive electrode tab and the negative electrode tab prepared in Examples 1 to 18 and Comparative Examples 1 to 9 were cut into small round pieces with a diameter of φ10 mm. The prepared positive electrode tab and negative electrode tab were placed in the mold sleeve on both sides of the solid-state electrolyte block for assembly, and after assembly, the pressure was increased to 100 MPa and the nut at the top of the stand was tightened to maintain the pressure, thereby obtaining a full-solid-state lithium ion battery.

[0161] The cycle performance test process of the solid-state battery is as follows:

[0162] At 25°C, the solid-state battery prepared in Examples 1 to 18 and Comparative Examples 1 to 9 was subjected to constant volume at a current of 0.6 mA, and after constant volume, the battery was subjected to charge-discharge cycling at a current rate of 0.3C / 0.3C, and the first cycle discharge specific capacity and the cycle number when the SOH capacity was less than or equal to 80% of the initial capacity were recorded during the charge-discharge cycling. The cycle working voltage range of the battery for Examples 1 to 13, 15 to 18 and Comparative Examples 1 to 9 was 1.9-3.7 V; and the cycle working voltage range of the battery for Example 14 was 1.4-4.25 V.

[0163] Table 1: Parameters of the positive electrode tab prepared in Examples 1 to 18 and Comparative Examples 1 to 9 and performance test results of the assembled battery

[0164]

[0165] By comparing the test results of Examples 1 to 5 and Comparative Examples 1 to 3, it can be seen that, compared with the insulating polymer binders HNBR and PVDF commonly used in solid-state batteries, the PVB binder in the present disclosure has certain ionic conductivity due to the coordination structure of its own hydroxyl group with the surface of the solid-state electrolyte, which can promote the utilization rate of the positive electrode active material and further improve the reversible specific capacity of the battery; at the same time, the PVB binder also has good dispersibility and strong viscosity, which can effectively improve the structural stability of the positive electrode sheet during the charging and discharging process, and further improve the cycle life of the battery.

[0166] By comparing the test results of Example 1 and Examples 2 to 5, it can be seen that, compared with the positive electrode active material layer formed by single mixing of the binder with the positive electrode active material and the solid-state electrolyte in Example 1, the pre-mixing and coating of the binder with the solid-state electrolyte in Examples 2 to 5 can improve the coverage of the binder in the positive electrode active material layer, thereby further improving the interface stability of the positive electrode sheet during the charging and discharging process, and improving the charging and discharging capacity and cycle life of the solid-state battery.

[0167] By comparing the test results of Examples 2 to 5 and Comparative Examples 4 to 9, it can be seen that, when the molecular weight of the PVB binder is 40000~53000 and the viscosity is 80~180 mPa·S, the positive electrode sheet peeling force and the reversible charging and discharging capacity and normal temperature cycle number of the solid-state battery can be effectively improved. This is because the PVB binder with appropriate molecular weight and viscosity can better anchor the particles in the sheet while ensuring dispersibility, enhance the interface adhesion of the particles in the positive electrode sheet, and be beneficial to improving the reversible discharge capacity and cycle life of the solid-state battery.

[0168] By comparing the test results of Examples 2, 6 to 10, it can be seen that, in the positive electrode active material layer, when the mass content (x wt%) of the binder and the mass content (z wt%) of the positive electrode active material satisfy the formula (2) relationship -0.1≤0.06(z-40)-x≤0.1, the binder can increase the content appropriately to match the additional volume change increase caused by the increase of nickel content in the positive electrode active material, thereby effectively inhibiting and adapting the volume deformation of the positive electrode active material without introducing too much binder to occupy the space of the solid-state electrolyte, effectively maintaining the close contact between the solid-state electrolyte and the positive electrode active material interface during the charging and discharging process, further improving the structural stability of the positive electrode sheet, and improving the cycle performance of the solid-state battery.

[0169] By comparing the test results of Example 2, Examples 6 to 8 and Example 11, it can be seen that, in the positive electrode active material layer, when the mass content (x wt%) of the binder and the average particle size (y μm) of the solid-state electrolyte satisfy the relationship -0.2≤0.5x-y≤0.1, the binder can be bound at a suitable thickness on the interface between the particles of each component in the positive electrode active material layer to achieve high coverage, thereby effectively improving the structural stability of the positive electrode sheet and further improving the rate performance and cycle life of the solid-state battery.

[0170] By comparing the test results of Example 2, Examples 15 to 17 and Example 18, it can be seen that, by limiting the content ratio of carbon black and carbon fiber to 1:(0.4~0.8), a positive electrode sheet with excellent rate performance can be obtained, thereby further improving the reversible discharge capacity and cycle life of the solid-state battery. This is because the positive electrode active material particles in the positive electrode sheet are small in size, and more zero-dimensional conductive agents are needed to connect the active materials that cannot be contacted by the one-dimensional conductive agents with high aspect ratio. Under the above content ratio, the zero-dimensional and one-dimensional conductive agents can form a conductive network with optimal coverage around the positive electrode active material particles.

[0171] By comparing the test results of Examples 2 to 5, Examples 7, 8, 11 and 12, it can be seen that, when the mass ratio of the positive electrode active material, the solid-state electrolyte, the positive electrode conductive agent and the binder in the positive electrode sheet is (70~90):(5~25):(1~5):(1~3), the theoretical capacity, rate performance and structural stability of the positive electrode sheet can be considered, so that the cycle performance of the battery loaded with the positive electrode sheet is optimal.

[0172] The above examples are only illustrative of the principles and effects of the present application, and are not intended to limit the present application. Any person skilled in the art can modify or change the above examples without departing from the spirit and scope of the present application. Therefore, all equivalent modifications or changes made by those skilled in the art without departing from the spirit and technical ideas disclosed by the present application should be covered by the claims of the present application.

Claims

1. A positive electrode plate, characterized in that: include: positive electrode current collector; A positive electrode active material layer is provided on the positive electrode current collector, wherein the positive electrode active material layer comprises a positive electrode active material, a solid electrolyte and a binder; Wherein, the binder comprises polyvinyl butyral, the molecular weight of the binder is 40,000-53,000, and the viscosity of the binder is 80-180 mPa·s; The binder is coated on the surface of the solid electrolyte particles, the Young's modulus of the solid electrolyte is 15-30 GPa, and the Young's modulus of the binder is 2-5 GPa.

2. The positive electrode sheet according to claim 1, characterized in that: The coverage rate of the binder on the surface of the positive electrode active material layer is greater than or equal to 90%.

3. The positive electrode sheet according to claim 1 or 2, characterized in that: The thickness of the binder located between the solid electrolyte and the positive electrode active material is 100-200 nm.

4. The positive electrode sheet according to claim 1, characterized in that: The binder has a mass content of 1 to 3 wt % in the positive electrode active material layer.

5. The positive electrode sheet according to claim 1 or 4, characterized in that: The average particle size of the solid electrolyte and the mass content of the binder in the positive electrode active material layer satisfy the relationship of formula (1): -0.1≤0.5xy≤0.1(1) The binder has a mass content of x wt % in the positive electrode active material layer, and an average particle size of the solid electrolyte is y μm.

6. The positive electrode sheet according to claim 1 or 4, characterized in that: The positive electrode active material includes at least one of lithium nickel cobalt manganese oxide and lithium nickel manganese oxide, and the molar content of nickel in the lithium nickel cobalt manganese oxide is 60% to 95%. In the positive electrode active material layer, the mass content of the positive electrode active material is 65 to 90 wt%, and the mass content of the positive electrode active material and the mass content of the binder satisfy the relationship of formula (2): -0.1≤0.06(z-40)-x≤0.1(2) The binder has a mass content of x wt % in the positive electrode active material layer, and the positive electrode active material has a mass content of z wt % in the positive electrode active material layer.

7. The positive electrode sheet according to claim 1, characterized in that: The hydroxyl content of the binder is 21-36 mol %, the butyralization degree of the binder is 58-74 mol %; and / or the glass transition temperature of the binder is 60-67° C.

8. The positive electrode sheet according to claim 1, characterized in that: The solid-state electrolyte includes a sulfide electrolyte.

9. A method for preparing the positive electrode sheet according to any one of claims 1 to 8, characterized in that: include: Mixing a binder and a solid electrolyte in an organic solvent to prepare a first mixed slurry; Mixing the positive electrode active material with the first mixed slurry to prepare a second mixed slurry; The second mixed slurry is coated on the positive electrode current collector, and dried to obtain a positive electrode sheet.

10. The preparation method according to claim 9, characterized in that The preparation process of the binder includes: Mixing n-butyraldehyde and polyvinyl alcohol in a solvent to form a precursor solution; the molar ratio of the n-butyraldehyde to the polyvinyl alcohol is 0.6 to 1.2; An acid catalyst is added to the precursor solution, and the precursor solution is placed at a temperature of 45-65° C. for heating reaction to obtain a binder; during the reaction, the pH value of the precursor solution is controlled to be 1.5-2.5, and the reaction time is 4-8 hours.

11. An electrochemical device, characterized in that: The invention comprises the positive electrode sheet according to any one of claims 1 to 8, or the positive electrode sheet prepared by the preparation method according to claim 9 or 10.

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