Positive electrode sheet, its preparation method and battery

Abstract

The present invention relates to the field of battery technology, and more specifically to a positive electrode sheet, a method for preparing the same, and a battery. The positive electrode sheet comprises: a current collector; a first positive electrode active material layer disposed on at least one surface of the current collector along the thickness direction, the first positive electrode active material layer comprising a first positive electrode active material; a second positive electrode active material layer disposed on a surface of the first positive electrode active material layer away from the current collector, the second positive electrode active material layer comprising a second positive electrode active material; and a safety layer disposed on a surface of the second positive electrode active material layer away from the first positive electrode active material layer, the safety layer comprising cordierite. This positive electrode sheet can improve the safety, structural stability, and electrochemical performance of the battery.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a positive electrode plate, a preparation method thereof, and a battery. Background Art

[0002] To achieve high energy density in lithium-ion batteries, the positive electrode sheet typically requires a high compaction density. Commonly used positive electrode active materials, such as ternary positive electrode materials, are composed of secondary particles formed by the agglomeration of primary particles. The bonding between primary particles within the secondary particles is weak, so the secondary particles are easily crushed under pressure during the cold pressing process of the positive electrode sheet. This is especially true where the positive electrode active material particles contact the cold pressing rollers, which ultimately leads to worse gassing in lithium-ion batteries at high temperatures.

[0003] Alumina ceramic coatings are often used as a safety layer, applied to the active layer surface, to address the problem of secondary particles being easily crushed under pressure during the cold pressing process of the positive electrode. However, conventional alumina ceramic coatings suffer from powder shedding and mold release issues.

[0004] Therefore, it is necessary to take effective measures to improve the safety, structural stability and electrochemical performance of batteries. Summary of the Invention

[0005] In view of this, the present invention aims to solve at least one of the technical problems in the related art to a certain extent. To this end, the present invention provides a positive electrode sheet and a preparation method thereof and a battery, which can improve the safety, structural stability and electrochemical performance of the battery.

[0006] In order to solve the above technical problems, this application is implemented as follows:

[0007] According to one aspect of the present application, an embodiment of the present application provides a positive electrode plate, the positive electrode plate comprising:

[0008] current collector;

[0009] a first positive electrode active material layer, disposed on at least one surface of the current collector in a thickness direction, the first positive electrode active material layer comprising a first positive electrode active material;

[0010] a second positive electrode active material layer, disposed on a surface of the first positive electrode active material layer away from the current collector, the second positive electrode active material layer comprising a second positive electrode active material; and

[0011] The safety layer is provided on a surface of the second positive electrode active material layer away from the first positive electrode active material layer, and the safety layer comprises cordierite.

[0012] In some embodiments, the cordierite satisfies at least one of the following characteristics (1) to (4):

[0013] (1) The cordierite has a porous structure, and the pores of the cordierite contain lithium salts;

[0014] Preferably, the mass content of lithium salt in the cordierite is 30% to 40%;

[0015] (2) The Dn90 of the cordierite is 50 nm to 500 nm;

[0016] (3) The specific surface area of the cordierite is 1.5 m 2 / g~3.5 m 2 / g;

[0017] (4) The porosity of the cordierite is 65% to 70%.

[0018] In some embodiments, the safety layer further includes a third adhesive, a fourth adhesive, and a third conductive agent.

[0019] In some embodiments, the mass ratio of the cordierite, the third binder, the fourth binder and the third conductive agent is (70-80): (0.3-1): (10-20): (1-5).

[0020] In some embodiments, the third binder includes at least one of polyvinylidene fluoride, styrene-butadiene rubber, nitrile rubber, or sodium polyacrylate.

[0021] In some embodiments, the fourth adhesive comprises a silicone resin adhesive.

[0022] In some embodiments, the security layer has a porous structure.

[0023] In some embodiments, the pore structure is honeycomb-shaped.

[0024] In some embodiments, the porosity is 50% to 60%.

[0025] In some embodiments, the thickness of the security layer is 2 μm to 3 μm.

[0026] In some embodiments, the first positive electrode active material and the second positive electrode active material both include primary particles and secondary particles, and the primary particle content in the first positive electrode active material is different from the primary particle content in the second positive electrode active material, and the secondary particle content in the first positive electrode active material is different from the secondary particle content in the second positive electrode active material.

[0027] In some embodiments, the primary particle content of the first positive electrode active material is greater than the primary particle content of the second positive electrode active material; and / or the secondary particle content of the first positive electrode active material is less than the secondary particle content of the second positive electrode active material.

[0028] In some embodiments, in the first positive electrode active material, the mass ratio of primary particles to secondary particles is (50-70): (30-50).

[0029] In some embodiments, the mass ratio of primary particles to secondary particles in the second positive electrode active material is (10-30): (70-90).

[0030] In some embodiments, the first positive electrode active material comprises LiNi x Co y M 1-x-y O2, where 0.7≤x≤0.9, 0.1≤y≤0.3.

[0031] In some embodiments, the M includes at least one of Mn or Al.

[0032] In some embodiments, the Dn50 of the primary particles is 1 μm to 5 μm, and the Dn50 of the secondary particles is 10 μm to 20 μm.

[0033] In some embodiments, the Dn50 of the primary particles is 1 μm to 2 μm, and the Dn50 of the secondary particles is 10 μm to 13 μm.

[0034] In some embodiments, the first positive electrode active material layer further includes a first conductive agent and a first binder.

[0035] In some embodiments, the mass ratio of the first positive electrode active material, the first conductive agent, and the first binder is (95-99): (1-3): (0.5-1.5).

[0036] In some embodiments, the thickness of the first positive electrode active material layer is 60 μm to 70 μm.

[0037] In some embodiments, the second positive electrode active material comprises LiNi x Co y M 1-x-y O2, where 0.7≤x≤0.9, 0.1≤y≤0.3.

[0038] In some embodiments, the M includes at least one of Mn or Al.

[0039] In some embodiments, the Dn50 of the primary particles is 4 μm to 10 μm, and the Dn50 of the secondary particles is 10 μm to 20 μm.

[0040] In some embodiments, the Dn50 of the primary particles is 4 μm to 6 μm, and the Dn50 of the secondary particles is 13 μm to 16 μm.

[0041] In some embodiments, the second positive electrode active material layer further includes a second conductive agent and a second binder.

[0042] In some embodiments, the mass ratio of the second positive electrode active material, the second conductive agent, and the second binder is (95-99): (1-3): (0.5-1.5).

[0043] In some embodiments, the second positive electrode active material layer has a thickness of 60 μm to 70 μm.

[0044] According to another aspect of the present application, an embodiment of the present application provides a method for preparing a positive electrode sheet, comprising the following steps:

[0045] Coating a first positive electrode slurry on at least one side of the current collector to obtain a first positive electrode active material layer;

[0046] coating a second positive electrode slurry on the surface of the first positive electrode active material layer to obtain a second positive electrode active material layer;

[0047] coating a safety layer slurry on the surface of the second positive electrode active material layer to obtain a safety layer;

[0048] The first positive electrode active material layer includes a first positive electrode active material; the second positive electrode active material layer includes a second positive electrode active material; and the safety layer includes cordierite.

[0049] In some embodiments, the preparation of the first positive electrode slurry includes: uniformly mixing a first positive electrode active material, a first conductive agent, and a first binder in a solvent to obtain a first active material slurry.

[0050] In some embodiments, the preparation of the second positive electrode slurry includes: uniformly mixing a second positive electrode active material, a second conductive agent, and a second binder in a solvent to obtain a second active material slurry.

[0051] In some embodiments, the preparation of the safety layer slurry includes: uniformly mixing cordierite, a third binder, a fourth binder, and a third conductive agent in a solvent to obtain the safety layer slurry.

[0052] In some embodiments, the solvent includes at least one of water, ethanol, or N-methylpyrrolidone.

[0053] In some embodiments, the first positive electrode slurry is coated on the surface of the current collector, and after drying, a first positive electrode active material layer is formed on the surface of the current collector; the second positive electrode slurry is coated on the first positive electrode active material layer, and after drying, a second positive electrode active material layer is formed on the surface of the first positive electrode active material layer; the safety layer slurry is coated on the second positive electrode active material layer and dried to obtain the positive electrode sheet.

[0054] In some embodiments, the method for preparing cordierite includes:

[0055] An aluminum source, a magnesium source, a silicon source and an organic solvent are mixed to obtain a sol, and then a lithium salt is added and mixed, and the pH value is adjusted to obtain a gel;

[0056] The gel is allowed to stand, dried, calcined and ground to obtain cordierite.

[0057] In some embodiments, the molar ratio of the aluminum source, the magnesium source, and the silicon source is (2-6): (1-3): (2-8).

[0058] In some embodiments, the volume ratio of the silicon source to the organic solvent is 1:(3-5).

[0059] In some embodiments, based on the total mass of the sol, the lithium salt accounts for 30% to 40% of the total mass of the sol.

[0060] In some embodiments, the lithium salt includes at least one of lithium bis(trifluorosulfonyl)imide, lithium bis(fluorosulfonyl)amide, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium bis(oxalatoborate), lithium bis(oxalatoborate), lithium tetrafluoroborate, lithium phosphate, or lithium difluorophosphate.

[0061] In some embodiments, the pH value is adjusted to 5-7.

[0062] In some embodiments, the gel is allowed to stand for 6 to 10 hours.

[0063] In some embodiments, the calcination temperature is 500° C. to 700° C., and the calcination time is 1 h to 3 h.

[0064] According to another aspect of the present application, an embodiment of the present application provides a battery, including a positive electrode plate, wherein the positive electrode plate includes the aforementioned positive electrode plate, or includes a positive electrode plate prepared according to the aforementioned preparation method.

[0065] The implementation of the technical solution of the present invention has at least the following beneficial effects:

[0066] In the embodiments of the present application, a safety layer is provided in the positive electrode plate of the present application, which utilizes the electronic insulation properties of cordierite to prevent particles from piercing the diaphragm, causing a short circuit between the positive and negative electrodes, and replaces the uneven distribution and powder loss of ceramic powder on the diaphragm; the inert metal inorganic salt on the surface of the cordierite can replace the CEI film to improve the first effect and reversible discharge capacity; relying on the high mechanical strength and porous structure of the cordierite itself, the harmful structural evolution caused by the volume change during the deintercalation and insertion of lithium in the positive electrode is suppressed, and part of the gas generated after the secondary particles are broken is stored, reducing the impact of the gas on the long-term and safety performance of the battery cell. That is, the safety coating prepared using cordierite has high mechanical strength and honeycomb pores, which suppresses the crushing of secondary particles, increases the gas storage space, and improves the gas production problem caused by the easy breakage of secondary particles.

[0067] In some preferred embodiments of the present application, the active material layer (second active material layer) close to the electrolyte has a high proportion of secondary particles of active materials. Since the secondary particles have better kinetic properties, the fast charging capability of the battery can be improved; the active material layer (first active material layer) close to the current collector side has a high proportion of primary particles, which can provide better thermodynamic properties for the electrode and improve the pressure resistance and cycle performance of the electrode.

[0068] Additional aspects and advantages of the present application will be given in part in the following description and in part will become obvious from the following description or will be learned through practice of the invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0069] Figure 1 Shown is a schematic diagram of the positive electrode plate structure provided in Example 1 of the present invention.

[0070] Figure 2 This is a scanning electron microscope image of a positive electrode safety layer provided in Example 1 of the present invention.

[0071] Figure 3 This is a scanning electron microscope image of a positive electrode safety layer provided in Example 5 of the present invention.

[0072] 1-current collector;

[0073] 2-first positive electrode active material layer;

[0074] 3-second positive electrode active material layer;

[0075] 4- Security layer. DETAILED DESCRIPTION

[0076] The present application will be further described below with reference to specific examples. It should be understood that these examples are only used to illustrate the present application and are not intended to limit the scope of the present application.

[0077] The endpoints of the ranges and any values disclosed herein are not limited to the precise ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range or the individual point values, and the individual point values can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein.

[0078] Unless otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0079] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0080] Unless otherwise specified, all steps of the present application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), indicating that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0081] Unless otherwise specified, the terms "include" and "comprising" used in this application may be open-ended or closed-ended. For example, "include" and "comprising" may mean that other components not listed may also be included or that only the listed components are included.

[0082] [Positive electrode]

[0083] Alumina ceramic coatings are often used as safety layers, applied to the active layer surface, to address the problem of secondary particles being easily crushed under pressure during the cold pressing process of the positive electrode. However, conventional alumina ceramic coatings suffer from powder shedding and mold release, which can affect the battery's lifespan and safety performance.

[0084] In view of this, an embodiment of the present application provides a positive electrode plate, which includes:

[0085] current collector 1;

[0086] A first positive electrode active material layer 2 is provided on at least one side of the current collector 1 in the thickness direction, and the first positive electrode active material layer 2 includes a first positive electrode active material;

[0087] A second positive electrode active material layer 3 is provided on a surface of the first positive electrode active material layer 2 away from the current collector 1 , and the second positive electrode active material layer 3 includes a second positive electrode active material; and

[0088] The safety layer 4 is provided on a surface of the second positive electrode active material layer 3 away from the first positive electrode active material layer 2 , and the safety layer 4 includes cordierite.

[0089] The provided positive electrode sheet has a multilayer structure, comprising a current collector 1, a first positive electrode active material layer 2, a second positive electrode active material layer 3, and a safety layer 4 stacked in sequence. Specifically, the first positive electrode active material layer 2 is disposed on at least one surface of the current collector 1, the second positive electrode active material layer 3 is disposed on the surface of the first positive electrode active material layer 2, and the safety layer 4 is disposed on the surface of the second positive electrode active material layer 3. The first positive electrode active material layer 2 can serve as a layer with a high content of primary particles of active material, the second positive electrode active material layer 3 can serve as a layer with a high content of secondary particles of active material, and the safety layer 4 can serve as a functional layer.

[0090] The phrase "the first positive electrode active material layer 2 is disposed on at least one surface of the current collector 1 along the thickness direction" means that the first positive electrode active material layer 2 can be disposed on one surface of the current collector 1 along its thickness direction, or on both surfaces of the current collector 1 along its thickness direction. The "surface" here can be the entire area of the current collector 1 or a portion of the current collector 1. For example, in this embodiment, the surface can be the entire area of the current collector 1. This application does not specifically limit this, as long as the purpose of this application can be achieved.

[0091] As an example, the current collector 1 has two surfaces facing each other in its thickness direction, and the first positive electrode active material layer 2 is disposed on the two facing surfaces of the current collector 1. Furthermore, a second positive electrode active material layer 3 is formed on the surfaces of the first positive electrode active material layer 2 on both sides. Furthermore, a safety layer 4 is formed on the surfaces of the second positive electrode active material layer 3 on both sides. It is understood that in other embodiments, the first positive electrode active material layer 2 can also be laminated on either of the two surfaces of the current collector 1.

[0092] In the present application, in the positive electrode sheet, the material of the current collector 1 is not specifically limited.

[0093] Therefore, a safety layer 4 is provided in the positive electrode sheet of the present application, which utilizes the electronic insulation properties of cordierite to prevent particles from piercing the diaphragm, causing a short circuit between the positive and negative electrodes, and replaces the uneven distribution and powder loss of ceramic powder on the diaphragm; the inert metal inorganic salt on the surface of the cordierite can replace the CEI membrane to improve the first effect and reversible discharge capacity; relying on the high mechanical strength and porous structure of the cordierite itself, it suppresses the harmful structural evolution caused by the volume change during the deintercalation of lithium in the positive electrode, stores part of the gas generated after the secondary particles are broken, and reduces the impact of the gas on the long-term and safety performance of the battery cell. That is, the safety coating prepared using cordierite has high mechanical strength and honeycomb pores, which suppresses the crushing of secondary particles, increases the gas storage space, and improves the gas production problem caused by the easy breakage of secondary particles.

[0094] In a preferred embodiment of the present application, the active material layer (second active material layer) close to the electrolyte has a high proportion of secondary particles of active materials. Since the secondary particles have better kinetic properties, the fast charging capability of the battery can be improved; the active material layer (first active material layer) close to the side of the current collector 1 has a high proportion of primary particles, which can provide better thermodynamic properties for the electrode and improve the pressure resistance and cycle performance of the electrode.

[0095] In some embodiments, the cordierite has a porous structure, and the pores of the cordierite contain lithium salts. The chemical formula of the cordierite is Mg2Al4Si5O 18 .

[0096] In some embodiments, the mass content of the lithium salt in the cordierite is 30% to 40%. As an example, the mass content of the lithium salt in the cordierite can be 30%, 35%, 40%, etc., or a value within the above range, and is not specifically limited here. If the lithium salt content is too high, it will lead to an increase in side reactions during the battery's charge and discharge process, and the safety coating will no longer be safe. If the lithium salt content is too low, it will lead to an increase in battery impedance and a sharp decrease in ionic conductivity, affecting the battery's charge and discharge rate.

[0097] In some embodiments, the Dn90 of cordierite is 50nm to 500nm. As an example, the Dn90 of cordierite can be 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, etc., and of course it can also be a point value within the above range, which is not specifically limited here. If the Dn90 of cordierite is too small, it will reduce the space for thermal expansion within the molecule, thereby increasing the thermal expansion coefficient of cordierite and reducing the thermal stability of cordierite. If the Dn90 of cordierite is too large, it will increase the risk of powder loss of the safety coating, which is not conducive to the conduction of lithium ions in the electrolyte and increases the risk of lithium plating.

[0098] In some embodiments, the specific surface area of cordierite is 1.5 m 2 / g~3.5 m 2 / g. As an example, the specific surface area of cordierite can be 1.5m 2 / g, 2m 2 / g, 2.5m 2 / g、3m 2 / g, 3.5m 2 / g, etc., of course, it can also be a point value within the above range, and is not specifically limited here. If the cordierite specific surface area is too large, it often indicates an increase in porosity or a decrease in cordierite wall thickness, increasing the brittleness of the safety coating and the risk of puncturing the separator. If the cordierite specific surface area is too small, the contact area with the electrolyte or lithium ions is reduced, making lithium ion transmission difficult and the kinetics worse.

[0099] In some embodiments, the porosity of cordierite is 65% to 70%. As an example, the porosity of cordierite can be 65%, 66%, 67%, 68%, 69%, 70%, etc., and of course it can also be a certain point value within the above range, which is not specifically limited here. If the porosity of cordierite is too large, it will affect the mechanical strength of the cordierite, and it will be difficult to suppress the harmful structural evolution caused by the volume change during the process of lithium insertion and removal of the positive electrode; if the porosity of cordierite is too small, the gas storage space is insufficient, and it is difficult to effectively store the gas generated after the secondary particles are crushed, and the long-term and safety performance of the battery cell will not be significantly improved.

[0100] In some embodiments, the security layer 4 further includes a third adhesive, a fourth adhesive, and a third conductive agent.

[0101] In some embodiments, the mass ratio of cordierite, the third binder, the fourth binder, and the third conductive agent is (70-80):(0.3-1):(10-20):(1-5). For example, the mass ratio of cordierite, the third binder, the fourth binder, and the third conductive agent can be 70:0.3:10:1, 75:0.6:15:3, 80:1:20:5, etc. Of course, any ratio within the above range is also possible and is not specifically limited here.

[0102] In some embodiments, the third binder includes but is not limited to at least one of polyvinylidene fluoride, styrene butadiene rubber, nitrile rubber, or sodium polyacrylate. As an example, the third binder can be polyvinylidene fluoride, styrene butadiene rubber, or a mixture of the two.

[0103] In some embodiments, the fourth binder comprises a silicone resin adhesive. The safety layer 4 also contains a single-component silicone resin adhesive, which can secure the cordierite powder and prevent the safety coating from shedding. However, excessive silicone resin adhesive should not be added. First, it has poor ionic conductivity and provides a good seal. Excessive addition can lead to a sharp increase in impedance or even insulation. It can also complicate the slurry preparation process, making stirring and coating difficult.

[0104] In some embodiments, the security layer 4 has a porous structure.

[0105] In some embodiments, the pore structure of the safety layer 4 is honeycomb-shaped, see the attached specification. Figure 2 or attached to the instruction manual Figure 3 This can provide the safety layer 4 with stronger mechanical properties and a larger gas storage space. It can also inhibit the breakage of secondary particles, increase the gas storage space, and improve the gas production problem caused by the easy breakage of secondary particles.

[0106] In some embodiments, the porosity of the security layer 4 is 50% to 60%. As an example, the porosity of the security layer 4 can be 50%, 55%, 60%, etc., or any point within the above range, which is not specifically limited here.

[0107] In some embodiments, the thickness of the security layer 4 is 2 μm to 3 μm. As an example, the thickness of the security layer 4 can be 2 μm, 2.5 μm, 3 μm, etc., or any value within the above range, which is not specifically limited here.

[0108] In some embodiments, the first positive electrode active material and the second positive electrode active material both include primary particles and secondary particles, and the primary particle content in the first positive electrode active material is different from the primary particle content in the second positive electrode active material, and the secondary particle content in the first positive electrode active material is different from the secondary particle content in the second positive electrode active material.

[0109] In some embodiments, the content of primary particles in the first positive electrode active material is greater than the content of primary particles in the second positive electrode active material. Since the secondary particles have better kinetic properties, the fast charging capability of the battery can be improved; and / or, the content of secondary particles in the first positive electrode active material is less than the content of secondary particles in the second positive electrode active material, which can provide better thermodynamic properties for the electrode and improve the compressive resistance and cycle performance of the electrode.

[0110] In some embodiments, the mass ratio of primary particles to secondary particles in the first positive electrode active material is (50-70):(30-50). For example, the mass ratio of primary particles to secondary particles in the first positive electrode active material can be 50:50, 40:60, 30:70, etc., and can also be any ratio within the above range, without specific limitation.

[0111] In some embodiments, the mass ratio of primary particles to secondary particles in the second positive electrode active material is (10-30):(70-90). For example, the mass ratio of primary particles to secondary particles in the second positive electrode active material can be 10:90, 20:80, 30:70, etc., and can also be any ratio within the above range, without specific limitation herein.

[0112] In some embodiments, the first positive electrode active material includes LiNi x Co y M 1-x-y O2, where 0.7≤x≤0.9, 0.1≤y≤0.3.

[0113] In some embodiments, M includes but is not limited to at least one of Mn and Al. For example, M may be Mn or Al.

[0114] In some embodiments, the Dn50 of the primary particles in the first positive electrode active material is 1μm to 5μm, and the Dn50 of the secondary particles is 10μm to 20μm. Preferably, the Dn50 of the primary particles is 1μm to 2μm, and the Dn50 of the secondary particles is 10μm to 13μm. As an example, the Dn50 of the primary particles in the first positive electrode active material can be 1μm, 2μm, 3μm, 4μm, 5μm, etc., and of course it can also be a value within the above range, and no specific limitation is made here. The Dn50 of the secondary particles in the first positive electrode active material can be 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, etc., and of course it can also be a value within the above range, and no specific limitation is made here. The relatively small particle size of the primary and secondary particles in the first positive electrode active material can prevent stress concentration caused by larger particles, prevent them from separating from the current collector 1, causing electrochemical disconnection and loss of active material.

[0115] In some embodiments, the first positive active material layer 2 further includes a first conductive agent and a first binder.

[0116] In some embodiments, the mass ratio of the first positive electrode active material, the first conductive agent, and the first binder is (95-99):(1-3):(0.5-1.5). As an example, the mass ratio of the first positive electrode active material, the first conductive agent, and the first binder can be 95:1:0.5, 97:2:1, 99:3:1.5, etc. Of course, it can also be any ratio within the above range and is not specifically limited here.

[0117] In some embodiments, the thickness of the first positive electrode active material layer 2 is 60 μm to 70 μm. As an example, the thickness of the first positive electrode active material layer 2 can be 60 μm, 65 μm, 70 μm, etc., or any value within the above range, which is not specifically limited here.

[0118] In some embodiments, the second positive electrode active material includes LiNi x Co y M 1-x-y O2, where 0.7≤x≤0.9, 0.1≤y≤0.3.

[0119] In some embodiments, M includes but is not limited to at least one of Mn and Al. For example, M may be Mn or Al.

[0120] In some embodiments, the Dn50 of the primary particles in the second positive electrode active material is 4μm to 10μm, and the Dn50 of the secondary particles is 10μm to 20μm. Preferably, the Dn50 of the primary particles is 4μm to 6μm, and the Dn50 of the secondary particles is 13μm to 16μm. As an example, the Dn50 of the primary particles in the second positive electrode active material can be 4μm, 6μm, 8μm, 10μm, etc., and of course it can also be a point value within the above range, and no specific limitation is made here. The Dn50 of the primary particles in the second positive electrode active material can be 4μm, 6μm, 8μm, 10μm, etc., and of course it can also be a point value within the above range, and no specific limitation is made here. The Dn50 of the secondary particles in the second positive electrode active material can be 10μm, 12μm, 14μm, 16μm, 18μm, 20μm, etc., and of course it can also be a point value within the above range, and no specific limitation is made here. The particle sizes of the primary particles and the secondary particles in the second positive electrode active material are relatively large, which facilitates the penetration of the cordierite slurry, makes the adhesion of the safety coating stronger, and prevents it from falling off.

[0121] In some embodiments, the second positive active material layer 3 further includes a second conductive agent and a second binder.

[0122] In some embodiments, the mass ratio of the second positive electrode active material, the second conductive agent, and the second binder is (95-99):(1-3):(0.5-1.5). As an example, the mass ratio of the second positive electrode active material, the second conductive agent, and the second binder can be 95:1:0.5, 97:2:1, 99:3:1.5, etc., and of course, it can also be any ratio within the above range, and is not specifically limited here.

[0123] In some embodiments, the thickness of the second positive electrode active material layer 3 is 60 μm to 70 μm. As an example, the thickness of the second positive electrode active material layer 3 can be 60 μm, 65 μm, 70 μm, etc., or any value within the above range, which is not specifically limited here.

[0124] It should be noted that the first conductive agent, the second conductive agent, and the third conductive agent each independently include, but are not limited to, at least one of conductive graphite, conductive carbon black, conductive carbon fiber, carbon nanotubes, or graphene. The conductive carbon black includes acetylene black, Ketjen black, and the like. The conductive carbon fiber includes vapor-grown carbon fiber.

[0125] It should also be noted that the first binder and the second binder independently include but are not limited to at least one of polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate or polyacrylic acid.

[0126] It is understood that the mass ratios of the first positive electrode active material, the first conductive agent, and the first binder, the mass ratios of the second positive electrode active material, the second conductive agent, and the second binder, and the mass ratios of cordierite, the third binder, the fourth binder, and the third conductive agent are related to the electrochemical performance of the corresponding battery and further affect the structural stability of the positive electrode sheet. By controlling the ratios of the various substances in the first positive electrode active material layer 2, the second positive electrode active material layer 3, and the safety layer 4 within the above ranges, the interactions between the various materials are fully utilized, effectively improving the cycling performance, fast charging performance, and structural stability of the positive electrode sheet.

[0127] It is also understandable that the surface thickness of the coating will affect the battery's electrical performance (such as energy density, cycle performance, and rate performance), preparation cost, and safety performance. If the thickness of the first positive electrode active material layer 2, the second positive electrode active material layer 3, and the safety layer 4 is too large, the electron transmission distance will increase, the electronic resistance will increase, and the rate performance will decrease, which will have an adverse effect on the battery's electrical performance and further increase the difficulty of thermal management of the battery. However, if the thickness of the first positive electrode active material layer 2, the second positive electrode active material layer 3, and the safety layer 4 is too low, the structural stability of the positive electrode sheet will not be significantly improved, which will have an adverse effect on the safety and long-term cycle stability of the battery.

[0128] In some embodiments, the current collector 1 includes, but is not limited to, aluminum foil and stainless steel. As an example, the current collector 1 can be made of aluminum foil.

[0129] Therefore, based on the above scheme, in the positive electrode plate of the present application, the active material layer (second active material layer) close to the electrolyte has a high proportion of secondary particles of active materials. Since the secondary particles have better kinetic properties, the fast charging capability of the battery can be improved; the active material layer (first active material layer) close to the current collector 1 has a high proportion of primary particles, which can provide better thermodynamic properties for the plate and improve the pressure resistance and cycle performance of the plate. In addition, the particle size of the primary and secondary particles in the first positive electrode active material is relatively small, which can prevent stress concentration caused by large particles and prevent them from detaching from the current collector 1 and causing electrochemical disconnection and loss of active materials. The particle size of the primary and secondary particles in the second positive electrode active material is relatively large, which facilitates the penetration of cordierite slurry, makes the adhesion of the safety coating stronger, and prevents it from falling off.

[0130] In addition, the positive electrode plate of the present application is also provided with a safety layer 4, which utilizes the electronic insulation properties of cordierite to prevent particles from piercing the diaphragm, causing a short circuit between the positive and negative electrodes, and replaces the uneven distribution and powder loss of ceramic powder on the diaphragm; the inert metal inorganic salt on the surface of the cordierite can replace the CEI film to improve the first effect and reversible discharge capacity; relying on the high mechanical strength and porous structure of the cordierite itself, it suppresses the harmful structural evolution caused by the volume change during the deintercalation of lithium in the positive electrode, stores part of the gas generated after the secondary particles are broken, and reduces the impact of the gas on the long-term and safety performance of the battery cell. That is, the safety coating prepared using cordierite has high mechanical strength and honeycomb pores, which inhibits the crushing of secondary particles, increases the gas storage space, and improves the gas production problem caused by the easy breakage of secondary particles.

[0131] [Preparation of positive electrode sheet]

[0132] According to another aspect of the present application, an embodiment of the present application provides a method for preparing a positive electrode sheet, comprising the following steps:

[0133] Coating a first positive electrode slurry on at least one side of the current collector 1 to obtain a first positive electrode active material layer 2;

[0134] Coating a second positive electrode slurry on the surface of the first positive electrode active material layer 2 to obtain a second positive electrode active material layer 3;

[0135] Coating a safety layer 4 slurry on the surface of the second positive electrode active material layer 3 to obtain a safety layer 4;

[0136] The first positive electrode active material layer 2 includes a first positive electrode active material; the second positive electrode active material layer 3 includes a second positive electrode active material; and the safety layer 4 includes cordierite.

[0137] It should be understood that all the features and advantages described above for the “positive electrode sheet” are also applicable to the “method for preparing the positive electrode sheet” and will not be described in detail here.

[0138] In some embodiments, the preparation of the first positive electrode slurry includes: uniformly mixing the first positive electrode active material, the first conductive agent, and the first binder in a solvent to obtain a first active material slurry.

[0139] In some embodiments, the preparation of the second positive electrode slurry includes: uniformly mixing the second positive electrode active material, the second conductive agent, and the second binder in a solvent to obtain a second active material slurry.

[0140] In some embodiments, the preparation of the safety layer 4 slurry includes: uniformly mixing cordierite, a third binder, a fourth binder, and a third conductive agent in a solvent to obtain the safety layer 4 slurry.

[0141] In some embodiments, the solvent includes but is not limited to at least one of water, ethanol, or N-methylpyrrolidone. As an example, the solvent can be water or ethanol.

[0142] In some embodiments, a first positive electrode slurry is coated on the surface of a current collector 1, and after drying, a first positive electrode active material layer 2 is formed on the surface of the current collector 1; a second positive electrode slurry is coated on the first positive electrode active material layer 2, and after drying, a second positive electrode active material layer 3 is formed on the surface of the first positive electrode active material layer 2; a safety layer 4 slurry is coated on the second positive electrode active material layer 3 and dried to obtain a positive electrode sheet.

[0143] [Preparation of Cordierite]

[0144] In some embodiments, the method for preparing cordierite includes:

[0145] An aluminum source, a magnesium source, a silicon source and an organic solvent are mixed to obtain a sol, and then a lithium salt is added and mixed, and the pH value is adjusted to obtain a gel;

[0146] The gel is allowed to stand, dried, calcined and ground to obtain cordierite.

[0147] In some embodiments, the molar ratio of the aluminum source, the magnesium source, and the silicon source is (2-6):(1-3):(2-8). As an example, the molar ratio of the aluminum source, the magnesium source, and the silicon source can be 2:1:2, 4:1.5:5, 6:3:8, etc. Of course, it can also be any ratio within the above range and is not specifically limited here.

[0148] In some embodiments, the volume ratio of the silicon source to the organic solvent is 1:(3-5). As an example, the volume ratio of the silicon source to the organic solvent can be 1:3, 1:4, 1:5, etc., and of course, it can also be any ratio within the above range, and is not specifically limited here.

[0149] In some embodiments, the silicon source includes, but is not limited to, at least one of ethyl orthosilicate, methyl orthosilicate, or alkoxysilane.

[0150] In some embodiments, the magnesium source includes, but is not limited to, at least one of magnesium nitrate, magnesium carbonate, or magnesium oxide.

[0151] In some embodiments, the aluminum source includes, but is not limited to, at least one of aluminum nitrate, aluminum chloride, or aluminum hydroxide.

[0152] In some embodiments, the lithium salt accounts for 30% to 40% of the total mass of the sol, based on the total mass of the sol. For example, based on the total mass of the sol, the lithium salt may account for 30%, 35%, 40%, etc., or any value within the above range, and is not specifically limited here.

[0153] In some embodiments, the lithium salt includes, but is not limited to, at least one of lithium bis(trifluorosulfonyl)imide, lithium bis(fluorosulfonyl)amide, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium bis(oxalatoborate), lithium bis(oxalatoborate), lithium tetrafluoroborate, lithium phosphate, or lithium difluorophosphate. For example, the lithium salt may be lithium bis(trifluorosulfonyl)imide, lithium bis(fluorosulfonyl)amide, or a mixture thereof.

[0154] In some embodiments, sufficient stirring after adding the lithium salt can reduce the residual lithium content.

[0155] In some embodiments, the pH value is adjusted to 5-7. As an example, the pH value can be adjusted to 5, 6, 7, etc., or it can be any point within the above range, which is not specifically limited here. The pH value has a significant impact on the particle size and porosity of the cordierite powder. A neutral environment is more likely to produce small particles and densified cordierite powder. However, the smaller the powder, the better. Excessively small particles may reduce the intramolecular thermal expansion space, thereby increasing the thermal expansion coefficient of the cordierite and reducing the thermal stability of the cordierite.

[0156] In some embodiments, the gel is allowed to stand for 6 to 10 hours. As an example, the gel can be allowed to stand for 6 hours, 8 hours, 10 hours, etc., or any point within the above range, which is not specifically limited here.

[0157] In some embodiments, the calcination temperature is 500°C to 700°C, and the calcination time is 1 hour to 3 hours. As an example, the calcination temperature can be 500°C, 600°C, 700°C, etc., and of course it can also be a value within the above range, and is not specifically limited here. The calcination time can be 1 hour, 2 hours, 3 hours, etc., and of course it can also be a value within the above range, and is not specifically limited here.

[0158] In some embodiments, the grinding method is not limited, as long as the particle size of the cordierite reaches a predetermined range. As an example, the grinding can be performed by ball milling.

[0159] In some embodiments, the preparation of the silicone resin adhesive includes: copolymerizing 80 parts of vinyl diisopropoxysilyl-terminated dimethylsiloxane, 20 parts of bis(p-glycidylphenyl)propane, and 70 parts of styrene at 160° C. for 6 hours to obtain a polymer mixture, and then mixing the obtained 80 parts of the polymer mixture with 2 parts of a silane coupling agent (γ-aminopropyltriethoxysilane), 60 parts of butyl acrylate, and 0.02 parts of a catalyst (dibutyltin dimethoxide) to obtain a heat-cured silicone resin adhesive.

[0160] Therefore, based on the above scheme, in the positive electrode plate of the present application, the active material layer (second active material layer) close to the electrolyte has a high proportion of secondary particles of active materials. Since the secondary particles have better kinetic properties, the fast charging capability of the battery can be improved; the active material layer (first active material layer) close to the current collector 1 has a high proportion of primary particles, which can provide better thermodynamic properties for the plate and improve the pressure resistance and cycle performance of the plate. In addition, the particle size of the primary and secondary particles in the first positive electrode active material is relatively small, which can prevent stress concentration caused by large particles and prevent them from detaching from the current collector 1 and causing electrochemical disconnection and loss of active materials. The particle size of the primary and secondary particles in the second positive electrode active material is relatively large, which facilitates the penetration of cordierite slurry, makes the adhesion of the safety coating stronger, and prevents it from falling off.

[0161] In addition, the positive electrode plate of the present application is also provided with a safety layer 4, which utilizes the electronic insulation properties of cordierite to prevent particles from piercing the diaphragm, causing a short circuit between the positive and negative electrodes, and replaces the uneven distribution and powder loss of ceramic powder on the diaphragm; the inert metal inorganic salt on the surface of the cordierite can at least partially replace the CEI film, improving the first efficiency and reversible discharge capacity; relying on the high mechanical strength and porous structure of the cordierite itself, it suppresses the harmful structural evolution caused by the volume change during the deintercalation and insertion of lithium in the positive electrode, stores part of the gas generated after the secondary particles are broken, and reduces the impact of the gas on the long-term and safety performance of the battery cell. That is, the safety coating prepared using cordierite has high mechanical strength and honeycomb pores, which inhibits the crushing of secondary particles, increases the gas storage space, and improves the gas production problem caused by the easy breakage of secondary particles.

[0162] A preparation method for cordierite is also provided. Compared with cordierite prepared by traditional solid-phase sintering methods, the cordierite prepared in this application has a smaller particle size and a larger specific surface area. When used to prepare a safety coating, it is not easy to puncture the separator during hot pressing, thereby improving the safety performance of the battery. The cordierite in this application has excellent chemical stability and can prevent HF acid from attacking the positive electrode material, reducing the consumption of lithium on the CEI membrane during the first charge and improving the initial efficiency.

[0163] According to another aspect of the present application, an embodiment of the present application provides a battery, including a positive electrode plate, wherein the positive electrode plate includes the aforementioned positive electrode plate, or includes a positive electrode plate prepared according to the aforementioned preparation method.

[0164] Since the battery includes the positive electrode plate provided in the embodiment of the present application, it has relatively excellent safety, structural stability, and electrochemical performance.

[0165] In some embodiments, the battery may be a lithium-ion battery. The stacking type of the battery is, for example, a wound type or a laminated type battery, and the structural type is, for example, a square shell (aluminum shell, steel shell, etc.) battery, a soft-pack battery, or a cylindrical battery, etc., which is not specifically limited. The battery has excellent safety and structural stability.

[0166] In some embodiments, the above battery further includes a negative electrode sheet, an electrolyte, and a separator. That is, the battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator.

[0167] In this embodiment, for the negative electrode sheet, materials and structures such as the negative electrode current collector, conductive agent, binder, etc. in the negative electrode active material layer are not limited, and the negative electrode sheet structure and composition that can be used in secondary batteries well-known to those skilled in the art can be selected.

[0168] In this embodiment, the specific material or type of the separator is not limited, and the separator that can be used in secondary batteries well-known to those skilled in the art can be selected.

[0169] It should also be noted that for the electrolyte of the battery of the present application, the specific material or type, etc. are not limited, and the components and types that can be used in secondary batteries well-known to those skilled in the art can be selected, as long as the purpose of the present application can be achieved.

[0170] Since the battery provided by the embodiment of the present invention adopts all the technical solutions of the above embodiment, it has at least all the beneficial effects brought by the technical solutions of the above embodiment, which will not be elaborated here one by one.

[0171] Hereinafter, the implementation manners of the present application will be described. The implementation manners described below are exemplary and are only used to explain the present application and should not be construed as a limitation to the present application. For those without specific technologies or conditions noted in the implementation manners, they shall be carried out according to the technologies or conditions described in the literature in the art or according to the product specifications. For reagents, materials or instruments without the producer noted, they are all conventional products that can be obtained through commercial purchase.

[0172] Example 1

[0173] S1: Mix LiNi 0.8 Co 0.1 Mn 0.1 O2 positive electrode material (the proportion of secondary particle material is 40 wt%), acetylene black, and polyvinylidene fluoride (PVDF) in a mass ratio of 97:2:1 in a solvent to prepare a first positive electrode slurry;

[0174] Among them, the particle sizes of the primary particles and the secondary particles are 1 μm < Dn50 < 4 μm and 10 μm < Dn50 < 13 μm respectively, the discharge viscosity ≤ 3000 mPa·s, and the solid content is 71%.

[0175] S2: Mix LiNi0.8 Co 0.1 Mn 0.1 The O2 cathode material (the proportion of secondary particle material is 80 wt%) and acetylene black, PVDF are mixed in a solvent according to a mass ratio of 97:2:1 to prepare a second cathode slurry;

[0176] Among them, the particle sizes of the primary particles and the secondary particles are 4μm < Dn50 < 8μm and 13μm < Dn50 < 16μm respectively, the discharge viscosity ≤ 3000 mPa·s, and the solid content is 70%.

[0177] S3: Mix cordierite powder and acetylene black in a ratio of 1:0.05, dry stir in a stirring kettle for 30 min, then add 10 wt% of the solvent and stir for another 30 min. During this period, PVDF and silicone resin adhesive are dispersed in the solvent in a ratio of 1:0.6 to form a glue solution with a solid content of 15 wt%. After the cordierite slurry is stirred, add the glue solution and continue to stir for 2 h before discharging;

[0178] Among them, the particle size of the cordierite powder is within the range of 50nm ≤ Dn90 ≤ 100nm, the specific surface area is within the range of 2 - 3 m 2 / g, the porosity of cordierite is 70%, and a safety layer slurry with a solid content of 40% is obtained;

[0179] The mass ratio of cordierite powder, PVDF, silicone resin adhesive and acetylene black is 75:15:0.6:3;

[0180] The preparation of cordierite includes: using tetraethyl orthosilicate (TEOS) as the silicon source, aluminum nitrate (Al(NO3)3) as the aluminum source, magnesium nitrate (Mg(NO3)2) as the magnesium source, and the molar ratio of tetraethyl orthosilicate, aluminum nitrate, and magnesium nitrate is 5:4:2; adding (Al(NO3)3) and Mg(NO3)2 to anhydrous ethanol and standing for a period of time, heating under reflux to dissolve all the powder. After the mixture cools to room temperature, add TEOS and ethanol solution (the volume ratio of TEOS and ethanol is 1:4), stir evenly to obtain a sol; based on the mass of the sol, add 40 wt% of lithium phosphate and stir well. Then, while stirring, add a mixed solution of ammonia water or nitric acid and ethanol until the solution pH is 6 to form a three-dimensional network cordierite ceramic gel; let the gel stand at room temperature for 8 h, dry it at 80℃, then remove the organic matter and water at 200℃. Finally, calcine it at 600℃ for 2 h and put the obtained powder into a ball mill for full grinding to obtain nano-scale cordierite powder.

[0181] S4. Use a double-layer die head to evenly coat the first positive electrode slurry and the second positive electrode slurry on a 13 μm thick aluminum foil, and form the first active material layer and the second active material layer after drying; continue to coat the safety layer slurry at a coating speed of 5 m / min, and dry it in a 5-section oven after coating. The temperature of each section of the oven is 100°C, 120°C, 150°C, 120°C, and 100°C respectively; the thickness of the first positive electrode active material layer is 70 μm, the thickness of the second positive electrode active material layer is 70 μm, the thickness of the safety layer is 2.5 μm, and the porosity is 55%; repeat the coating to complete the coating of the second surface of the aluminum foil opposite to the first surface, and then use a roller press to pressurize it to obtain a compaction density of 3.4 g / cm 3 The structure of the positive electrode plate is shown in the appendix of the manual. Figure 1 .

[0182] Example 2

[0183] The difference between Example 2 and Example 1 is that the particle size Dn90 of the cordierite powder used to prepare the safety layer is less than 50 nm.

[0184] Example 3

[0185] The difference between Example 3 and Example 1 is that the particle size Dn90 of the cordierite powder used to prepare the safety layer is ≤300 nm.

[0186] Example 4

[0187] The difference between Example 4 and Example 1 is that the particle size of the cordierite powder used to prepare the safety layer is 120nm≤Dn90≤150nm.

[0188] Example 5

[0189] The difference between Example 5 and Example 1 is that the porosity of the cordierite powder used to prepare the safety layer is 65%, and the porosity of the safety layer is 50%.

[0190] Example 6

[0191] The difference between Example 6 and Example 1 is that the secondary particle material in the first positive electrode active material accounts for 50 wt %;

[0192] The secondary particulate material accounts for 90 wt % of the second positive electrode active material.

[0193] Example 7

[0194] The difference between Example 7 and Example 1 is that the thickness of the first positive electrode active material layer is 65 μm, the thickness of the second positive electrode active material layer is 65 μm, and the thickness of the safety layer is 2 μm.

[0195] Example 8

[0196] Example 8 is different from Example 1 in that the safety layer is prepared from cordierite powder, PVDF, silicone resin adhesive, and acetylene black in a ratio of 60:18:0.5:1.5.

[0197] Example 9

[0198] Example 9 is different from Example 1 in that the safety layer is prepared from cordierite powder, PVDF, silicone resin adhesive, and acetylene black in a ratio of 95:4:0.5:1.5.

[0199] Example 10

[0200] Example 10 is different from Example 1 in that the particle sizes of the primary and secondary particles of the first positive electrode active material are 10μm < Dn50 < 15μm and 25μm < Dn50 < 30μm, respectively;

[0201] The particle sizes of the primary and secondary particles of the second positive electrode active material are 15μm < Dn50 < 20μm and 30μm < Dn50 < 35μm, respectively.

[0202] Example 11

[0203] Example 11 is different from Example 1 in that the particle sizes of the primary and secondary particles of the first positive electrode active material are 0.01μm < Dn50 < 1μm and 5μm < Dn50 < 10μm, respectively;

[0204] The particle sizes of the primary and secondary particles of the second positive electrode active material are 0.01μm < Dn50 < 1μm and 5μm < Dn50 < 10μm, respectively.

[0205] Example 12

[0206] Example 12 is different from Example 1 in that the particle sizes of the primary and secondary particles of the first positive electrode active material are 4μm < Dn50 < 8μm and 13μm < Dn50 < 16μm, respectively;

[0207] The particle sizes of the primary and secondary particles of the second positive electrode active material are 1μm < Dn50 < 4μm and 10μm < Dn50 < 13μm, respectively.

[0208] Example 13

[0209] Example 13 is different from Example 1 in that the safety layer slurry does not contain silicone resin adhesive.

[0210] Comparative Example 1

[0211] Comparative Example 1 is different from Example 1 in that mullite ceramic (3Al2O3·2SiO2) is selected to replace cordierite.

[0212] Comparative Example 2

[0213] The difference between Comparative Example 2 and Example 1 is that spinel ceramic (MgO·Al2O3) is used to replace cordierite.

[0214] Comparative Example 3

[0215] The difference between Comparative Example 3 and Example 1 is that the safety layer is not included.

[0216] Performance Testing

[0217] 1. Battery preparation

[0218] Isolation film: composite film (base material (polypropylene) 9μm + double-sided adhesive 3μm) is selected.

[0219] Electrolyte: In a glove box filled with inert gas, ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate were mixed in a mass ratio of 1:1:1 to obtain an organic solvent. LiPF6 was then dissolved in the organic solvent to a concentration of 1.2 mol / L to obtain an electrolyte.

[0220] Negative electrode sheet: Graphite, conductive carbon black, styrene-butadiene rubber (SBR), and carboxymethyl cellulose (CMC) were dissolved in deionized water at a mass ratio of 97:0.5:1.5:1 and stirred to form a slurry. The slurry was evenly coated on both sides of the negative electrode current collector copper foil, baked at 100°C for 30 minutes, and then cold pressed and cut to form the negative electrode sheet of the lithium-ion battery. The compaction density was 1.65g / cm 3 .

[0221] The positive electrode sheets, negative electrode sheets and separators prepared in each embodiment and comparative example were wound using a winding machine to obtain a winding core with the positive electrode wrapped therein, which was then encapsulated with an aluminum-plastic film. After being baked under vacuum for 48 hours to remove moisture, an electrolyte was injected, and the battery was subjected to conventional formation and capacity separation to obtain a square soft-pack lithium-ion battery.

[0222] 2. Testing

[0223] Peel strength test: Fix the electrode to the upper jaw of the tensile testing machine, stick one end of the tape to the electrode, fold the other end 180° and fix it to the lower jaw of the tensile testing machine, pull it apart at a test rate of 50mm / min, and measure the peel strength (N / m) by the force required to continuously peel the tape from the electrode.

[0224] Initial efficiency test: During formation, charge the battery at 0.1C to 85% SOC, recording the charge capacity at this point as C1. After 5 minutes of rest, continue charging at a low current of 0.05C to 4.25V, recording the charge capacity at this point as C2. After 5 minutes of rest, discharge the battery at 1C constant current to 2.8V. After another 5 minutes of rest, discharge the battery at 0.2C constant current to 2.8V, recording the total discharge capacity of these two steps as C3. The charge and discharge capacities of the battery are calculated separately: Initial efficiency = ((C1 + C2) / C3) * 100%.

[0225] Capacity retention test after 500 cycles at 1C: Charge the battery at a constant current of 1C to 4.25V, then switch to constant voltage charging until the current drops to 0.05C and then cut off. Then, after leaving it for 10 minutes, discharge the battery at a constant current of 1C to 2.8V. Record the discharge capacity at this time as the initial discharge capacity. Repeat the charge-rest-discharge cycle and record the discharge capacity after 500 cycles. Capacity retention after 500 cycles at 1C = (discharge capacity after 500 cycles / initial discharge capacity) * 100%.

[0226] 70℃ storage gas production performance test: First, measure the battery volume as the volume before storage, then charge the battery to 4.25V at a constant current rate of 1C, then charge it at a constant voltage at 4.25V with a cut-off current of 0.05C, and then store the battery in a constant temperature box at 70℃ for 30 days. Then measure the volume as the volume after 30 days of storage. Volume growth rate = (volume after 30 days of storage / volume before storage - 1) * 100%.

[0227] Thermal stability test (150°C hot box): Fully charge the battery at 1C, clamp the battery cells with clamps, and place them in a heated explosion-proof box. Heat the box to 150°C and maintain this temperature for 60 minutes to observe whether the battery catches fire.

[0228] Needle penetration test: The battery is charged at a constant current and constant voltage of 0.2C to the charge cut-off voltage using a needle penetration tester. A 3mm diameter, rust-free steel needle is used to penetrate the center of the battery's largest surface at a speed of 2m / min for at least 1 hour using a Dongguan Bell battery needle penetration tester.

[0229] Flexibility test: The electrode is tested for flexibility using steel needles of different diameters. The electrode is cut into 2*10cm pieces, then wrapped around the steel needle and rolled once to observe whether the electrode has cracks. The naming method is: named after the smallest crack-free steel needle diameter. For example, if the electrode has no cracks after rolling on a 6mm steel needle, but cracks after rolling on a 5mm steel needle, the electrode flexibility is 6.

[0230] The above tests were all conducted at 25±5°C. The test results are shown in Table 1.

[0231] Table 1

[0232]

[0233] Referring to Table 1, it can be seen from the test results of Example 1 and Example 2 that because the cordierite powder particles are smaller, the gas storage channels become fewer and the volume change rate increases, but because the particles are small, the permeability to the lower layer active material is better and the adhesion is stronger, so it does not affect the subsequent safety experiments such as hot box and acupuncture. Compared with the test results of Example 3, it can be seen that because the cordierite powder particles become larger, the first effect and reversible capacity decrease, which may be related to the fact that the lithium inside the large particles cannot be released. The volume growth rate decreases significantly, because the gas enters the pores inside the cordierite. It can be seen from the test results of Example 8 that a decrease in cordierite content will cause a decrease in the first effect and reversible specific capacity and an increase in the volume growth rate, which proves that the addition of a cordierite safety layer will improve its capacity and gas production performance. It can be seen from the test results of Example 9 that the cordierite content increases, the binder content decreases, and the capacity retention rate after cycling decreases. This may be because the binder is reduced, the volume repeatedly expands and contracts, the cordierite powder falls off, the secondary particles begin to break, react with the fresh interface and electrolyte, lithium ions are lost, resulting in reduced capacity retention and reduced safety performance. The test results of Example 10 show that after the particle size of the positive active material increases, the long-term performance such as circulation and gas production decreases. This is related to the large number of secondary particles in the upper layer. However, due to the presence of the cordierite safety coating, the cycle does not drop suddenly, proving that its particles have not undergone large-scale crushing. Large particles also lead to a decrease in the peel strength of the electrode. As long as the failure occurs between the first and second layers of active materials, the safety layer does not fall off on a large scale, indicating that increasing the particles can make the safety coating penetrate better, but too large particles will cause poor bonding between the particles of the active material. The test results of Example 11 show that after the particle size of the positive active material decreases, the initial effect decreases, which is attributed to the increase in side reactions caused by the increase in specific surface area. After the particle size is reduced, the safety layer falls off during the peel strength test, indicating that the permeability of the safety layer decreases. The test results of Example 13 show that when the safety layer slurry does not contain a silicone resin adhesive, all performance properties deteriorate significantly. This is because the unbonded cordierite is very easy to fall off, and the bonding between the pole pieces is not tight, which makes lithium ion transmission difficult and the dynamics deteriorate. In addition, the flexibility of the pole piece without the silicone resin adhesive is above 6, and it is light-transmissive after folding the pole piece twice. The flexibility of the pole piece with the silicone resin adhesive is about 4.5, and there is no light transmission after folding the pole piece twice. This shows that the flexibility of the pole piece with the silicone resin adhesive is improved, and it will not easily break and pierce the diaphragm after being compressed.

[0234] From the test data of Comparative Example 1, it can be seen that mullite has poorer high-temperature gas production and hot box performance than cordierite. This is because mullite has a high thermal expansion coefficient and a large volume expansion rate at high temperatures, which cannot effectively inhibit the expansion of the silicon negative electrode, causing the high-temperature performance to deteriorate. From the test data of Comparative Example 2, it can be seen that spinel has poorer peel strength and safety performance than cordierite. This is because spinel has a very high hardness, which results in the safety coating formed by spinel being more brittle under the same formula. After the needle puncture test, the safety coating cannot be effectively extended, causing the positive and negative electrodes to short-circuit, resulting in differences in safety performance.

[0235] Parts of the present invention that are not described in detail are well known to those skilled in the art.

[0236] The basic principles of the present invention have been described above in conjunction with specific embodiments. However, it should be noted that the advantages, strengths, and effects mentioned in the present invention are merely illustrative and non-limiting, and should not be construed as necessarily possessed by each embodiment of the present invention. Furthermore, the specific details disclosed above are provided for illustrative purposes and to facilitate understanding, and are not intended to be limiting. These details do not necessarily limit the present invention to being implemented using these specific details.

[0237] It should be noted that the terms "and / or" or " / " used herein are merely a description of an association relationship between associated objects, indicating that three relationships may exist. For example, A and / or B may represent three situations: A exists alone, A and B exist at the same time, and B exists alone. The singular forms "a," "the," and "the" used in the embodiments of the present invention and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0238] In the detailed description and claims, a list of items connected by the terms "at least one of," "at least one of," "at least one of," or other similar terms may mean any combination of the listed items. For example, if items A and B are listed, the phrase "at least one of A and B" means only A; only B; or A and B. In another example, if items A, B, and C are listed, the phrase "at least one of A, B, and C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may include a single element or multiple elements. Item B may include a single element or multiple elements. Item C may include a single element or multiple elements.

[0239] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. However, these modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present invention.

Claims

1. A positive electrode plate, characterized in that: The positive electrode plate comprises: current collector; a first positive electrode active material layer, disposed on at least one surface of the current collector in a thickness direction, the first positive electrode active material layer comprising a first positive electrode active material; a second positive electrode active material layer, disposed on a surface of the first positive electrode active material layer away from the current collector, the second positive electrode active material layer comprising a second positive electrode active material; and a safety layer, disposed on a surface of the second positive electrode active material layer away from the first positive electrode active material layer, the safety layer comprising cordierite; The cordierite has a porous structure, and the pores of the cordierite contain lithium salts; The mass content of lithium salt in the cordierite is 30% to 40%; The Dn90 of the cordierite is 50nm~500nm; The specific surface area of the cordierite is 1.5 m 2 / g~3.5 m 2 / g; The porosity of the cordierite is 65% to 70%.

2. The positive electrode sheet according to claim 1, characterized in that: The security layer satisfies at least one of the following characteristics (1) to (3): (1) The safety layer further includes a third adhesive, a fourth adhesive and a third conductive agent; The mass ratio of the cordierite, the third binder, the fourth binder and the third conductive agent is (70-80): (0.3-1): (10-20): (1-5); The third binder includes at least one of polyvinylidene fluoride, styrene-butadiene rubber, nitrile rubber, or sodium polyacrylate; The fourth adhesive includes a silicone resin adhesive; (2) The safety layer has a porous structure; The pore structure is honeycomb-shaped; The porosity is 50% to 60%; (3) The thickness of the safety layer is 2 μm to 3 μm.

3. The positive electrode sheet according to claim 1, characterized in that: The first positive electrode active material and the second positive electrode active material both include primary particles and secondary particles, the content of primary particles in the first positive electrode active material is different from the content of primary particles in the second positive electrode active material, and the content of secondary particles in the first positive electrode active material is different from the content of secondary particles in the second positive electrode active material; The primary particle content of the first positive electrode active material is greater than the primary particle content of the second positive electrode active material; and / or the secondary particle content of the first positive electrode active material is less than the secondary particle content of the second positive electrode active material; In the first positive electrode active material, the mass ratio of primary particles to secondary particles is (50-70): (30-50); In the second positive electrode active material, the mass ratio of primary particles to secondary particles is (10-30): (70-90).

4. The positive electrode sheet according to claim 3, characterized in that: The first positive electrode active material layer satisfies at least one of the following characteristics (1) to (4): (1) The first positive electrode active material includes LiNi x Co y M 1-x-y O2, where 0.7≤x≤0.9, 0.1≤y≤0.3; The M comprises at least one of Mn or Al; (2) The Dn50 of the primary particles is 1 μm to 5 μm, and the Dn50 of the secondary particles is 10 μm to 20 μm; (3) The first positive electrode active material layer further includes a first conductive agent and a first binder; The mass ratio of the first positive electrode active material, the first conductive agent and the first binder is (95-99): (1-3): (0.5-1.5); (4) The thickness of the first positive electrode active material layer is 60 μm to 70 μm.

5. The positive electrode sheet according to claim 3, characterized in that: The second positive electrode active material layer satisfies at least one of the following characteristics (1) to (4): (1) The second positive electrode active material includes LiNi x Co y M 1-x-y O2, where 0.7≤x≤0.9, 0.1≤y≤0.3; The M comprises at least one of Mn or Al; (2) The Dn50 of the primary particles is 4 μm to 10 μm, and the Dn50 of the secondary particles is 10 μm to 20 μm; (3) The second positive electrode active material layer further includes a second conductive agent and a second binder; The mass ratio of the second positive electrode active material, the second conductive agent and the second binder is (95-99): (1-3): (0.5-1.5); (4) The thickness of the second positive electrode active material layer is 60 μm to 70 μm.

6. A method for preparing a positive electrode sheet, characterized in that: The following steps are involved: Coating a first positive electrode slurry on at least one side of the current collector to obtain a first positive electrode active material layer; coating a second positive electrode slurry on the surface of the first positive electrode active material layer to obtain a second positive electrode active material layer; coating a safety layer slurry on the surface of the second positive electrode active material layer to obtain a safety layer; Wherein, the first positive electrode active material layer comprises a first positive electrode active material; the second positive electrode active material layer comprises a second positive electrode active material; and the safety layer comprises cordierite; The preparation method of the cordierite comprises: An aluminum source, a magnesium source, a silicon source and an organic solvent are mixed to obtain a sol, and then a lithium salt is added and mixed, and the pH value is adjusted to obtain a gel; The gel is allowed to stand, dried, calcined, and ground to obtain cordierite; The molar ratio of the aluminum source, magnesium source and silicon source is (2-6): (1-3): (2-8); The volume ratio of the silicon source to the organic solvent is 1:(3-5); Based on the total mass of the sol, the lithium salt accounts for 30% to 40% of the total mass of the sol; The lithium salt includes at least one of lithium bis(trifluorosulfonyl)imide, lithium bis(fluorosulfonyl)amide, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium bis(oxalatoborate), lithium bis(oxalatoborate), lithium tetrafluoroborate, lithium phosphate or lithium difluorophosphate; The pH value is adjusted to 5-7; The gel is allowed to stand for 6 to 10 hours. The calcination temperature is 500° C. to 700° C., and the calcination time is 1 hour to 3 hours.

7. The method for preparing a positive electrode sheet according to claim 6, characterized in that: The preparation of the first positive electrode slurry includes: uniformly mixing a first positive electrode active material, a first conductive agent and a first binder in a solvent to obtain a first active material slurry; The preparation of the second positive electrode slurry includes: uniformly mixing a second positive electrode active material, a second conductive agent, and a second binder in a solvent to obtain a second active material slurry; The preparation of the safety layer slurry comprises: uniformly mixing cordierite, a third binder, a fourth binder and a third conductive agent in a solvent to obtain the safety layer slurry; The solvent includes at least one of water, ethanol or N-methylpyrrolidone; The first positive electrode slurry is coated on the surface of the current collector, and after drying, a first positive electrode active material layer is formed on the surface of the current collector; the second positive electrode slurry is coated on the first positive electrode active material layer, and after drying, a second positive electrode active material layer is formed on the surface of the first positive electrode active material layer; the safety layer slurry is coated on the second positive electrode active material layer, and dried to obtain the positive electrode sheet.

8. A battery comprising a positive electrode plate, characterized in that: The positive electrode sheet includes the positive electrode sheet according to any one of claims 1 to 5, or includes the positive electrode sheet prepared by the preparation method according to any one of claims 6 to 7.

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