Positive pole piece, preparation method thereof and battery

Abstract

The invention relates to the technical field of batteries, in particular to a positive pole piece, a preparation method thereof and a battery. The positive pole piece comprises a current collector; the first positive electrode active material layer is arranged on the surface of at least one side of the current collector along the thickness direction, and the first positive electrode active material layer comprises a first positive electrode active material; the second positive electrode active material layer is arranged on the surface, far away from the current collector, of the first positive electrode active material layer, and the second positive electrode active material layer comprises a second positive electrode active material; the safety layer is arranged on the surface, far away from the first positive electrode active material layer, of the second positive electrode active material layer, and the safety layer comprises cordierite. According to the positive pole piece, the safety, the structural stability and the electrochemical performance of the battery can be improved.

Description

Technical Field

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

[0002] In order to endow lithium-ion batteries with high energy density, it is usually necessary for the positive electrode sheet to have a high tap density. Commonly used positive electrode active materials, such as ternary positive electrode materials, are secondary particles formed by the aggregation of primary particles. The binding force between the primary particles inside the secondary particles is not strong. Therefore, during the cold pressing process of the positive electrode sheet, the secondary particles are easily crushed under pressure. In particular, the positive electrode active material particles at the position where the surface of the positive electrode sheet contacts the cold pressing roller are very easy to break, ultimately leading to the deterioration of gas generation in the lithium-ion battery at high temperatures.

[0003] In related technologies, alumina ceramic coatings are often used as safety layers and coated on the surface of the active layer to solve the defect that secondary particles are easily crushed under pressure during the cold pressing process of the positive electrode sheet. However, conventional alumina ceramic coatings have problems such as powder shedding and demolding.

[0004] Therefore, it is necessary to take effective measures to improve the safety, structural stability, and electrochemical performance of the battery. 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 technologies to some extent. For this purpose, the present invention provides a positive electrode sheet, a preparation method thereof, and a battery, which can improve the safety, structural stability, and electrochemical performance of the battery.

[0006] To solve the above technical problems, the present application is implemented as follows: According to one aspect of the present application, an embodiment of the present application provides a positive electrode sheet, which includes: A current collector; A first positive electrode active material layer disposed on at least one surface of the current collector in the thickness direction, the first positive electrode active material layer including a first positive electrode active material; A second positive electrode active material layer disposed on the surface of the first positive electrode active material layer away from the current collector, the second positive electrode active material layer including a second positive electrode active material; and A safety layer disposed on the surface of the second positive electrode active material layer away from the first positive electrode active material layer, the safety layer including cordierite.

[0007] In some embodiments, the cordierite satisfies at least one of the following characteristics (1) to (4): (1) The cordierite has a porous structure, and the pores of the cordierite contain a lithium salt; Preferably, the mass content of the lithium salt in the cordierite is 30% - 40%; (2) The Dn90 of the cordierite is 50 nm - 500 nm; (3) The specific surface area of the cordierite is 1.5 m 2 / g - 3.5 m 2 / g; (4) The porosity of the cordierite is 65% - 70%.

[0008] In some of the embodiments, the safety layer further includes a third binder, a fourth binder, and a third conductive agent.

[0009] In some of the 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).

[0010] In some of the embodiments, the third binder includes at least one of polyvinylidene fluoride, styrene - butadiene rubber, nitrile rubber, or sodium polyacrylate.

[0011] In some of the embodiments, the fourth binder includes an organosilicon resin adhesive.

[0012] In some of the embodiments, the safety layer has a pore structure.

[0013] In some of the embodiments, the pore structure is honeycomb - shaped.

[0014] In some of the embodiments, the porosity is 50% - 60%.

[0015] In some of the embodiments, the thickness of the safety layer is 2 μm - 3 μm.

[0016] In some of the embodiments, both the first positive electrode active material and the second positive electrode active material include primary particles and secondary particles, and the content of primary particles in the first positive electrode active material is different from that in the second positive electrode active material, and the content of secondary particles in the first positive electrode active material is different from that in the second positive electrode active material.

[0017] In some of the embodiments, the content of primary particles in the first positive electrode active material is greater than that in the second positive electrode active material; and / or, the content of secondary particles in the first positive electrode active material is less than that in the second positive electrode active material.

[0018] In some of these embodiments, in the first positive electrode active material, the mass ratio of primary particles to secondary particles is (50 to 70):(30 to 50).

[0019] In some of these embodiments, in the second positive electrode active material, the mass ratio of primary particles to secondary particles is (10 to 30):(70 to 90).

[0020] In some of these embodiments, the first positive electrode active material includes LiNi x Co y M 1-x-y O2, where 0.7 ≤ x ≤ 0.9 and 0.1 ≤ y ≤ 0.3.

[0021] In some of these embodiments, M includes at least one of Mn or Al.

[0022] In some of these 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.

[0023] In some of these 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.

[0024] In some of these embodiments, the first positive electrode active material layer further includes a first conductive agent and a first binder.

[0025] In some of these embodiments, the mass ratio of the first positive electrode active material, the first conductive agent, and the first binder is (95 to 99):(1 to 3):(0.5 to 1.5).

[0026] In some of these embodiments, the thickness of the first positive electrode active material layer is 60 μm to 70 μm.

[0027] In some of these embodiments, the second positive electrode active material includes LiNi x Co y M 1-x-y O2, where 0.7 ≤ x ≤ 0.9 and 0.1 ≤ y ≤ 0.3.

[0028] In some of these embodiments, M includes at least one of Mn or Al.

[0029] In some of these 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.

[0030] In some of these 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.

[0031] In some of these embodiments, the second positive electrode active material layer further includes a second conductive agent and a second binder.

[0032] In some of these embodiments, the mass ratio of the second positive electrode active material, the second conductive agent, and the second binder is (95 to 99):(1 to 3):(0.5 to 1.5).

[0033] In some of these embodiments, the thickness of the second positive electrode active material layer is 60 μm to 70 μm.

[0034] According to another aspect of the present application, an embodiment of the present application provides a method for preparing a positive electrode sheet, including the following steps: Coating a first positive electrode slurry on at least one surface of a 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 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.

[0035] In some of these 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.

[0036] In some of these 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.

[0037] In some of these 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 a safety layer slurry.

[0038] In some of these embodiments, the solvent includes at least one of water, ethanol, or N-methylpyrrolidone.

[0039] In some of these embodiments, the first positive electrode paste 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 paste 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 paste is coated on the second positive electrode active material layer and dried to obtain the positive electrode plate.

[0040] In some of these embodiments, the preparation method of the cordierite includes: Mix an aluminum source, a magnesium source, a silicon source and an organic solvent to obtain a sol, then add a lithium salt and mix, and after adjusting the pH value, obtain a gel; Let the gel stand, and after drying, calcining and grinding, cordierite is obtained.

[0041] In some of these embodiments, the molar ratio of the aluminum source, the magnesium source and the silicon source is (2 - 6):(1 - 3):(2 - 8).

[0042] In some of these embodiments, the volume ratio of the silicon source to the organic solvent is 1:(3 - 5).

[0043] In some of these embodiments, based on the total mass of the sol, the lithium salt accounts for 30% - 40% of the total mass of the sol.

[0044] In some of these embodiments, the lithium salt includes at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)amide, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium bis(oxalato)borate, lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium phosphate or lithium difluorophosphate.

[0045] In some of these embodiments, the pH value is adjusted to 5 - 7.

[0046] In some of these embodiments, the standing time of the gel is 6h - 10h.

[0047] In some of these embodiments, the calcination temperature is 500°C - 700°C, and the calcination time is 1h - 3h.

[0048] According to another aspect of the present application, embodiments of the present application provide a battery, including a positive electrode plate, where the positive electrode plate includes the aforementioned positive electrode plate, or includes a positive electrode plate prepared according to the aforementioned preparation method.

[0049] Implementing the technical solutions of the present invention has at least the following beneficial effects: In the embodiments of the present application, a safety layer is provided in the positive electrode tab of the present application. Utilizing the electronic insulation property of cordierite, when particles pierce the separator, it can prevent the positive and negative electrodes from contacting and short-circuiting, replacing the uneven distribution and powder falling of ceramic powder on the separator; the inert metal inorganic salt on the surface of cordierite can replace the CEI film to improve the first efficiency and reversible discharge capacity; relying on the high mechanical strength and porous structure of cordierite itself, it can inhibit the harmful structural evolution caused by the volume change during the lithium deintercalation process of the positive electrode, store part of the gas generated after the secondary particles are broken, and reduce the impact of 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-like pores, inhibits the breakage of secondary particles, increases the gas storage space, and improves the gas generation problem caused by the easy breakage of secondary particles.

[0050] In some preferred embodiments of the present application, the proportion of secondary particles of the active material in the active material layer (the second active material layer) close to the electrolyte is relatively high. Since the kinetic performance of secondary particles is better, it can improve the fast charging ability of the battery; the proportion of primary particles in the active material layer (the first active material layer) close to the current collector is high, which can provide better thermodynamic performance for the electrode tab and improve the compressive resistance and cycling performance of the electrode tab.

[0051] Additional aspects and advantages of the present application will be given in part in the following description, will become apparent in part from the following description, or will be understood through the practice of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1 The figure shows a schematic diagram of the structure of a positive electrode tab provided in Embodiment 1 of the present invention.

[0053] Figure 2 It is a scanning electron microscope image of the safety layer of a positive electrode tab provided in Embodiment 1 of the present invention.

[0054] Figure 3 It is a scanning electron microscope image of the safety layer of a positive electrode tab provided in Embodiment 5 of the present invention.

[0055] 1 - Current collector; 2 - First positive electrode active material layer; 3 - Second positive electrode active material layer; 4 - Safety layer. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0056] The following further elaborates the present application in combination with specific embodiments. It should be understood that these embodiments of the present application are only used to illustrate the present application and not to limit the scope of the present application.

[0057] The endpoints and any values within the ranges disclosed herein are not limited to the exact ranges or values. These ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, between the endpoint values of each range, between the endpoint value of a range and a single point value, and between single point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein.

[0058] If not otherwise specified, all embodiments and alternative embodiments of the present application can be combined with each other to form new technical solutions.

[0059] If not otherwise specified, all technical features and alternative technical features of the present application can be combined with each other to form new technical solutions.

[0060] If not otherwise specified, all steps of the present application can be carried out in sequence or randomly, preferably in sequence. For example, the method includes steps (a) and (b), which means that the method can include steps (a) and (b) carried out in sequence, or can also include steps (b) and (a) carried out in sequence. For example, it is mentioned that the method may further include step (c), which means that step (c) can be added to the method in any order. For example, the method can include steps (a), (b) and (c), or can also include steps (a), (c) and (b), or can also include steps (c), (a) and (b), etc.

[0061] If not otherwise specified, the "including" and "comprising" mentioned in the present application mean open-ended or can also be closed-ended. For example, the "including" and "comprising" can mean that other components not listed can also be included or comprised, or can also only include or comprise the listed components.

[0062] [Positive electrode tab] In the related art, alumina ceramic coatings are often used as safety layers and are coated on the surface of the active layer to solve the defect that secondary particles are easily crushed under pressure during the cold pressing process of the positive electrode tab. However, conventional alumina ceramic coatings have problems such as powder shedding and demolding. Thereby affecting the service life and safety performance of the battery.

[0063] In view of this, an embodiment of the present application provides a positive electrode tab, which includes: Current collector 1; The first positive electrode active material layer 2 is disposed on at least one surface 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; The second positive electrode active material layer 3 is disposed on the 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 The safety layer 4 is disposed on the 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.

[0064] The provided positive electrode sheet has a multi-layer structure, which includes 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 that are sequentially stacked. That is, the first positive electrode active material layer 2 is disposed on at least one surface of the current collector 1 in the thickness direction, 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. Among them, the first positive electrode active material layer 2 can be a high-content layer of primary particles of the active material, the second positive electrode active material layer 3 can be a high-content layer of secondary particles of the active material, and the safety layer 4 can be a functional layer.

[0065] The above "the first positive electrode active material layer 2 is disposed on at least one surface of the current collector 1 in the thickness direction" means that the first positive electrode active material layer 2 can be disposed on one surface of the current collector 1 in its own thickness direction, or can be disposed on two surfaces of the current collector 1 in its own thickness direction. Here, the "surface" can be the entire area of the current collector 1 or a partial area of the current collector 1. For example, in this embodiment, the surface can be the entire area of the current collector 1. The present application has no particular limitation on this, as long as the purpose of the present application can be achieved.

[0066] As an example, the current collector 1 has two surfaces opposite to each other in its own thickness direction, and the first positive electrode active material layer 2 is disposed on the two opposite surfaces of the current collector 1. Further, the 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, the safety layer 4 is formed on the surfaces of the second positive electrode active material layer 3 on both sides. It can be understood that in other embodiments, the first positive electrode active material layer 2 can also be stacked on any one of the two surfaces of the current collector 1.

[0067] In the present application, in the positive electrode sheet, the material of the current collector 1 is not particularly limited.

[0068] Therefore, a safety layer 4 is provided in the positive electrode sheet of the present application. By utilizing the electronic insulation property of cordierite, when particles pierce the separator, short circuit caused by the contact between the positive and negative electrodes can be prevented, replacing the uneven distribution and powder falling of ceramic powder on the separator; the inert metal inorganic salt on the surface of cordierite can replace the CEI film, improving the initial efficiency and reversible discharge capacity; relying on the high mechanical strength and porous structure of cordierite itself, the harmful structural evolution caused by the volume change during the lithium deintercalation and intercalation process of the positive electrode is inhibited, and part of the gas generated after the secondary particles are broken is stored, reducing the impact of gas on the long-term and safety performance of the battery cell. That is, the safety coating prepared by using cordierite has high mechanical strength and honeycomb-shaped pores, inhibits the breakage of secondary particles, increases the gas storage space, and improves the gas generation problem caused by the easy breakage of secondary particles.

[0069] In a preferred embodiment of the present application, the proportion of secondary particles of the active material in the active material layer (the second active material layer) close to the electrolyte is relatively high. Since the kinetic performance of the secondary particles is better, the fast charging ability of the battery can be improved; the proportion of primary particles in the active material layer (the first active material layer) close to the current collector 1 is high, which can provide better thermodynamic performance for the electrode sheet and improve the compressive resistance and cycle performance of the electrode sheet.

[0070] In some embodiments, cordierite has a porous structure, and the pores of cordierite contain lithium salts. The chemical formula of this cordierite is Mg2Al4Si5O 18 .

[0071] In some embodiments, the mass content of lithium salt in cordierite is 30% - 40%. As an example, the mass content of lithium salt in cordierite can be 30%, 35%, 40%, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here. If the lithium salt content is too high, the side reactions during the charge and discharge process of the battery will increase, and the safety coating will no longer be safe; if the lithium salt content is too low, the impedance of the battery will increase, the ionic conductivity will drop sharply, and the charge and discharge rate of the battery will be affected.

[0072] In some embodiments, the Dn90 of cordierite is 50nm - 500nm. As an example, the Dn90 of cordierite can be 50nm, 100nm, 150nm, 200nm, 250nm, 300nm, 350nm, 400nm, 450nm, 500nm, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here. If the Dn90 of cordierite is too small, the thermal expansion space inside the molecule will be reduced, thereby increasing the thermal expansion coefficient of cordierite and reducing the thermal stability of cordierite; if the Dn90 of cordierite is too large, the risk of powder falling from the safety coating will increase, which is not conducive to the conduction of lithium ions in the electrolyte and increases the risk of lithium deposition.

[0073] In some embodiments, the specific surface area of cordierite is 1.5m 2 / g~3.5 m 2 / g. As an example, the specific surface area of cordierite can be 1.5 m 2 / g, 2 m 2 / g, 2.5 m 2 / g, 3 m 2 / g, 3.5 m 2 / g, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here. If the specific surface area of cordierite is too large, it often represents an increase in porosity or a thinning of the wall thickness of cordierite, increasing the brittleness of the safety coating and the risk of piercing the diaphragm; if the specific surface area of cordierite is too small, the contact area with the electrolyte or lithium ions becomes smaller, making it difficult for lithium ions to be transported and deteriorating the kinetics.

[0074] In some embodiments, the porosity of cordierite is 65% - 70%. As an example, the porosity of cordierite can be 65%, 66%, 67%, 68%, 69%, 70%, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here. If the porosity of cordierite is too large, it will affect the mechanical strength of cordierite and it is difficult to inhibit the harmful structural evolution caused by the volume change during the lithium deintercalation process 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 broken, and the improvement of the long-term and safety performance of the battery cell is not obvious.

[0075] In some embodiments, the safety layer 4 further includes a third binder, a fourth binder, and a third conductive agent.

[0076] 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). As an 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, it can also be a certain ratio within the above range, and no specific limitation is made here.

[0077] 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, can be styrene-butadiene rubber, or can be a mixture of the two.

[0078] In some embodiments, the fourth binder includes an organosilicon resin adhesive. The safety layer 4 also contains a one-component organosilicon resin adhesive, which can fix the cordierite powder and inhibit the powder falling off of the safety coating. However, the amount of the organosilicon resin adhesive added should not be too much. First, its ionic conductivity is poor and the sealing effect is good. Adding too much will cause a sharp increase in impedance or even insulation; it will also increase the difficulty of the homogenization process, resulting in difficult stirring and coating.

[0079] In some embodiments, the safety layer 4 has a pore structure.

[0080] In some embodiments, the pore structure of the safety layer 4 is honeycomb-shaped. See the appended Figure 2 or the appended Figure 3 of the specification. This can provide stronger mechanical properties for the safety layer 4 and provide a larger gas storage space. It can inhibit the breakage of secondary particles, increase the gas storage space, and improve the gas generation problem caused by the easy breakage of secondary particles.

[0081] In some embodiments, the porosity of the safety layer 4 is 50% - 60%. As an example, the porosity of the safety layer 4 can be 50%, 55%, 60%, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here.

[0082] In some embodiments, the thickness of the safety layer 4 is 2μm - 3μm. As an example, the thickness of the safety layer 4 can be 2μm, 2.5μm, 3μm, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here.

[0083] In some embodiments, both the first positive electrode active material and the second positive electrode active material include primary particles and secondary particles, and 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.

[0084] 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 kinetic performance of secondary particles is better, the fast charging ability 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 sheet and improve the compressive resistance and cycling performance of the electrode sheet.

[0085] In some embodiments, in the first positive electrode active material, the mass ratio of primary particles to secondary particles is (50 - 70):(30 - 50). As an example, in the first positive electrode active material, the mass ratio of primary particles to secondary particles can be 50:50, 40:60, 30:70, etc. Of course, it can also be a certain ratio within the above range, and no specific limitation is made here.

[0086] In some embodiments, in the second positive electrode active material, the mass ratio of primary particles to secondary particles is (10 - 30):(70 - 90). As an example, in the second positive electrode active material, the mass ratio of primary particles to secondary particles can be 10:90, 20:80, 30:70, etc. Of course, it can also be a certain ratio within the above range, and no specific limitation is made here.

[0087] 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 and 0.1 ≤ y ≤ 0.3.

[0088] In some embodiments, M includes but is not limited to at least one of Mn or Al. As an example, M can be Mn or can be Al.

[0089] In some embodiments, the Dn50 of the primary particles in the first positive electrode active material is 1 μm to 5 μm, and the Dn50 of the secondary particles is 10 μm to 20 μm. Preferably, 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. As an example, the Dn50 of the primary particles in the first positive electrode active material can be 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, etc. Of course, it can also be a certain point 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 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here. The particle sizes of the primary particles and secondary particles in the first positive electrode active material are relatively small, which can prevent stress concentration caused by larger particles, prevent them from detaching from the current collector 1 and causing electrochemical disconnection, and result in loss of active substances.

[0090] In some embodiments, the first positive electrode active material layer 2 further includes a first conductive agent and a first binder.

[0091] 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 a certain ratio within the above range, and no specific limitation is made here.

[0092] 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. Of course, it can also be a certain point value within the above range, and no specific limitation is made here.

[0093] 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 and 0.1 ≤ y ≤ 0.3.

[0094] In some embodiments, M includes, but is not limited to, at least one of Mn or Al. As an example, M can be Mn or can be Al.

[0095] In some embodiments, the Dn50 of the primary particles in the second cathode active material is 4 μm to 10 μm, and the Dn50 of the secondary particles is 10 μm to 20 μm. Preferably, 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. As an example, the Dn50 of the primary particles in the second cathode active material can be 4 μm, 6 μm, 8 μm, 10 μm, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here. The Dn50 of the primary particles in the second cathode active material can be 4 μm, 6 μm, 8 μm, 10 μm, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here. The Dn50 of the secondary particles in the second cathode active material can be 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here. The particle sizes of the primary and secondary particles in the second cathode active material are relatively large, which facilitates the penetration of the cordierite slurry, makes the adhesion of the safety coating stronger, and prevents shedding.

[0096] In some embodiments, the second cathode active material layer 3 further includes a second conductive agent and a second binder.

[0097] In some embodiments, the mass ratio of the second cathode active material, the second conductive agent, and the second binder is (95 to 99):(1 to 3):(0.5 to 1.5). As an example, the mass ratio of the second cathode active material, the second conductive agent, and the second binder can be 95:1:0.5, 97:2:1, 99:3:1.5, etc. Of course, it can also be a certain ratio within the above range, and no specific limitation is made here.

[0098] In some embodiments, the thickness of the second cathode active material layer 3 is 60 μm to 70 μm. As an example, the thickness of the second cathode active material layer 3 can be 60 μm, 65 μm, 70 μm, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here.

[0099] 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 nanotube, or graphene. Among them, the above-mentioned conductive carbon black includes acetylene black, Ketjen black, etc. The above-mentioned conductive carbon fiber includes vapor-grown carbon fiber.

[0100] It should also be noted that the first binder and the second binder each independently include, but are not limited to, at least one of polyvinylidene fluoride, sodium carboxymethyl cellulose, styrene-butadiene rubber, sodium alginate, or polyacrylic acid.

[0101] It can be understood that the mass ratios of the first cathode active material, the first conductive agent, and the first binder, the mass ratios of the second cathode 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 cathode electrode sheet. By controlling the proportions of the substances in the first cathode active material layer 2, the second cathode active material layer 3, and the safety layer 4 within the above ranges, the functions between the various materials are fully exerted, effectively improving the cycle performance, fast charging performance, and structural stability of the cathode electrode sheet.

[0102] It can also be understood that the surface thickness of the coating will affect the electrical performance of the battery (such as energy density, cycle performance, and rate performance), the manufacturing cost, and the safety performance. If the thicknesses of the first cathode active material layer 2, the second cathode active material layer 3, and the safety layer 4 are too large, the electron transport distance will increase, the electron resistance will increase, the rate performance will decrease, which will have an adverse effect on the electrical performance of the battery, and will further increase the difficulty of battery thermal management. However, if the thicknesses of the first cathode active material layer 2, the second cathode active material layer 3, and the safety layer 4 are too low, the improvement of the structural stability of the cathode electrode sheet will not be obvious, which will have an adverse effect on the safety and long-term cycle stability of the battery.

[0103] 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.

[0104] Thus, based on the above solution, in the cathode electrode sheet of the present application, the proportion of the active material secondary particles in the active material layer (the second active material layer) close to the electrolyte is relatively high. Since the kinetic performance of the secondary particles is better, the fast charging ability of the battery can be improved; the proportion of the primary particles in the active material layer (the first active material layer) close to the current collector 1 is high, which can provide better thermodynamic performance for the electrode sheet and improve the compressive resistance and cycle performance of the electrode sheet. In addition, the particle sizes of the primary particles and secondary particles in the first cathode active material are relatively small, which can prevent stress concentration caused by relatively large particles, prevent them from detaching from the current collector 1 and causing electrochemical disconnection, and result in the loss of active materials. The particle sizes of the primary particles and secondary particles in the second cathode active material are relatively large, which facilitates the penetration of the cordierite slurry, makes the adhesion of the safety coating stronger, and prevents shedding.

[0105] Moreover, a safety layer 4 is also provided on the positive electrode plate of the present application. Utilizing the electronic insulation property of cordierite, it can prevent short circuit caused by the contact between the positive and negative electrodes when particles pierce the separator, replacing the uneven distribution and powder falling of ceramic powder on the separator; the inert metal inorganic salt on the surface of cordierite can replace the CEI film to improve the initial efficiency and reversible discharge capacity; relying on the high mechanical strength and porous structure of cordierite itself, it can inhibit the harmful structural evolution caused by volume change during the lithium deintercalation process of the positive electrode, store part of the gas generated after the secondary particles are broken, and reduce the impact of gas on the long-term and safety performance of the battery cell. That is to say, the safety coating prepared by using cordierite has high mechanical strength and honeycomb-like pores, which can inhibit the breakage of secondary particles, increase the gas storage space, and improve the gas generation problem caused by the easy breakage of secondary particles.

[0106] [Preparation of Positive Electrode Plate] According to another aspect of the present application, an embodiment of the present application provides a method for preparing a positive electrode plate, including the following steps: Coating a first positive electrode slurry on at least one surface of the current collector 1 to obtain a first positive electrode active material layer 2; 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; Coating a safety layer 4 slurry on the surface of the second positive electrode active material layer 3 to obtain a safety layer 4; Wherein, 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.

[0107] It should be understood that all the features and advantages described above for the "positive electrode plate" also apply to the "method for preparing a positive electrode plate", and will not be elaborated here one by one.

[0108] In some embodiments, the preparation of the first positive electrode slurry includes: mixing the first positive electrode active material, the first conductive agent, and the first binder evenly in a solvent to obtain a first active material slurry.

[0109] In some embodiments, the preparation of the second positive electrode slurry includes: mixing the second positive electrode active material, the second conductive agent, and the second binder evenly in a solvent to obtain a second active material slurry.

[0110] In some embodiments, the preparation of the safety layer 4 slurry includes: mixing cordierite, a third binder, a fourth binder, and a third conductive agent evenly in a solvent to obtain a safety layer 4 slurry.

[0111] 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.

[0112] In some embodiments, the first positive electrode paste is coated on the surface of the current collector 1. After drying, a first positive electrode active material layer 2 is formed on the surface of the current collector 1. The second positive electrode paste is coated on the first positive electrode active material layer 2. After drying, a second positive electrode active material layer 3 is formed on the surface of the first positive electrode active material layer 2. The safety layer 4 paste is coated on the second positive electrode active material layer 3 and dried to obtain a positive electrode sheet.

[0113] [Preparation of Cordierite] In some embodiments, the method for preparing cordierite includes: Mixing an aluminum source, a magnesium source, a silicon source and an organic solvent to obtain a sol, then adding a lithium salt and mixing, and adjusting the pH value to obtain a gel; Letting the gel stand, drying, calcining and grinding to obtain cordierite.

[0114] 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 a certain ratio within the above range, and no specific limitation is made here.

[0115] 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. Of course, it can also be a certain ratio within the above range, and no specific limitation is made here.

[0116] In some embodiments, the silicon source includes but is not limited to at least one of tetraethyl orthosilicate, tetramethyl orthosilicate or alkoxysilane.

[0117] In some embodiments, the magnesium source includes but is not limited to at least one of magnesium nitrate, magnesium carbonate or magnesium oxide.

[0118] In some embodiments, the aluminum source includes but is not limited to at least one of aluminum nitrate, aluminum chloride or aluminum hydroxide.

[0119] In some embodiments, based on the total mass of the sol, the lithium salt accounts for 30% - 40% of the total mass of the sol. As an example, based on the total mass of the sol, the lithium salt can be 30%, 35%, 40%, etc. of the total mass of the sol. Of course, it can also be a certain point value within the above range, and no specific limitation is made here.

[0120] In some embodiments, the lithium salt includes but is not limited to at least one of lithium bis(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)amide, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium bis(oxalato)borate, lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium phosphate or lithium difluorophosphate. As an example, the lithium salt can be lithium bis(trifluoromethanesulfonyl)imide, can be lithium bis(fluorosulfonyl)amide, or a mixture of the two.

[0121] In some embodiments, after adding the lithium salt, sufficient stirring can reduce the residual lithium content.

[0122] 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. Of course, it can also be a certain point value within the above range, and no specific limitation is made here. The pH value has a great influence on the particle size and porosity of cordierite powder. In a neutral environment, it is easier to produce small-particle and densified cordierite powder, but the powder is not the smaller the better. Excessively small particles may reduce the thermal expansion space within the molecule, thereby increasing the thermal expansion coefficient of cordierite and reducing the thermal stability of cordierite.

[0123] In some embodiments, the standing time of the gel is 6h - 10h. As an example, the standing time of the gel can be 6h, 8h, 10h, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here.

[0124] In some embodiments, the calcination temperature is 500°C - 700°C, and the calcination time is 1h - 3h. As an example, the calcination temperature can be 500°C, 600°C, 700°C, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here. The calcination time can be 1h, 2h, 3h, etc. Of course, it can also be a certain point value within the above range, and no specific limitation is made here.

[0125] In some embodiments, the grinding method is not limited, as long as the particle size of cordierite reaches the specified range. As an example, grinding can be carried out by ball milling.

[0126] In some embodiments, the preparation of the silicone resin adhesive includes: copolymerizing 80 parts of dimethylsiloxane capped with vinyl diisopropoxysilyl, 20 parts of bis(p-glycidylphenyl) propane, and 70 parts of styrene at 160°C for 6h to obtain a polymer mixture, and then mixing 80 parts of the obtained polymer mixture with 2 parts of a silane coupling agent (γ-aminopropyltriethoxysilane), 60 parts of butyl acrylate, and 0.02 parts of a catalyst (dimethoxydibutyltin), and then heating and curing to obtain the silicone resin adhesive.

[0127] Thus, based on the above solution, in the positive electrode tab of the present application, the proportion of the secondary particles of the active material in the active material layer (the second active material layer) close to the electrolyte is relatively high. Since the kinetic performance of the secondary particles is better, the fast charging ability of the battery can be improved; the proportion of the primary particles in the active material layer (the first active material layer) close to the current collector 1 is high, which can provide better thermodynamic performance for the tab and improve the compressive resistance and cycling performance of the tab. In addition, the particle sizes of the primary particles and secondary particles in the first positive electrode active material are relatively small, which can prevent stress concentration caused by relatively large particles and prevent electrochemical disconnection due to their detachment from the current collector 1, resulting in loss of active material. The particle sizes of the primary particles and 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 detachment.

[0128] Moreover, the positive electrode tab of the present application is further provided with a safety layer 4. By utilizing the electronic insulation property of cordierite, when particles pierce the separator, it can prevent short circuit caused by contact between the positive and negative electrodes, replacing the uneven distribution and powder dropping of ceramic powder on the separator; the inert metal inorganic salt on the surface of 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 cordierite itself, it can inhibit the harmful structural evolution caused by volume change during the lithium deintercalation process of the positive electrode, store part of the gas generated after the secondary particles are broken, and reduce the impact of gas on the long-term and safety performance of the battery cell. That is to say, the safety coating prepared from cordierite has high mechanical strength and honeycomb-shaped pores, inhibits the breakage of secondary particles, increases the gas storage space, and improves the gas generation problem caused by the easy breakage of secondary particles.

[0129] A preparation method of cordierite is also additionally provided. The cordierite prepared in the present application has a smaller particle size and a larger specific surface area compared with the cordierite prepared by the traditional solid-phase sintering method. When it is used to prepare the safety coating, it is not easy to pierce the separator during hot pressing, thereby improving the safety performance of the battery. The cordierite of the present application has excellent chemical stability, can prevent the attack of HF acid on the positive electrode material, reduce the consumption of Li on the CEI film during the first charge, and improve the first efficiency.

[0130] According to another aspect of the present application, an embodiment of the present application provides a battery, including a positive electrode tab, and the positive electrode tab includes the aforementioned positive electrode tab, or includes the positive electrode tab prepared according to the aforementioned preparation method.

[0131] Since the battery includes the positive electrode tab provided by the embodiment of the present application, it has relatively excellent safety, structural stability, and electrochemical performance.

[0132] 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 prismatic (aluminum case, steel case, etc.) battery, a soft-pack battery, or a cylindrical battery, etc., without specific limitation. The battery has relatively excellent safety and structural stability.

[0133] 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.

[0134] In this embodiment, for the negative electrode sheet, materials and structures such as the negative electrode current collector and the conductive agent and binder 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.

[0135] 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.

[0136] 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.

[0137] Since the battery provided by the embodiment of the present invention adopts all the technical solutions of the above embodiments, it has at least all the beneficial effects brought by the technical solutions of the above embodiments, which will not be elaborated herein one by one.

[0138] 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 techniques or conditions noted in the implementation manners, the techniques or conditions described in the literature in the art or the product specifications are followed. For reagents, materials, or instruments without the manufacturer noted, they are all conventional products that can be obtained through commercial purchase.

[0139] Example 1 S1: Mix LiNi 0.8 Co 0.1 Mn 0.1 O2 cathode 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 cathode slurry; 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%.

[0140] S2: Mix LiNi 0.8 Co 0.1 Mn0.1 The O2 positive electrode 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 positive electrode slurry; Among them, the particle sizes of the primary particles and the secondary particles are 4 μm < Dn50 < 8 μm and 13 μm < Dn50 < 16 μm respectively, the discharge viscosity ≤ 3000 mPa·s, and the solid content is 70%.

[0141] 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 make 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; Among them, the particle size of the cordierite powder is in the range of 50 nm ≤ Dn90 ≤ 100 nm, the specific surface area is in 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; The mass ratio of cordierite powder, PVDF, silicone resin adhesive and acetylene black is 75:15:0.6:3; 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; Add (Al(NO3)3) and Mg(NO3)2 to anhydrous ethanol and let it stand for a period of time, then heat and 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 pH of the solution 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 °C, then remove the organic matter and water at 200 °C. Finally, calcine it at 600 °C for 2 h, and put the obtained powder into a ball mill for sufficient grinding to obtain nano-scale cordierite powder.

[0142] S4. Use a double-layer die to uniformly coat the first positive electrode slurry and the second positive electrode slurry on a 13-μm-thick aluminum foil, and after drying treatment, form a first active material layer and a second active material layer; continue to coat the safety layer slurry at a coating speed of 5 m / min, and after coating, dry it in a 5-section oven, and 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 roll press for pressure treatment to obtain a positive electrode plate with a compaction density of 3.4 g / cm 3 of the positive electrode plate, and the structure of the positive electrode plate is shown in the attached Figure 1 .

[0143] Example 2 The difference between Example 2 and Example 1 is that the particle size Dn90 of the cordierite powder for preparing the safety layer is less than 50 nm.

[0144] Example 3 The difference between Example 3 and Example 1 is that the particle size Dn90 of the cordierite powder for preparing the safety layer is less than or equal to 300 nm.

[0145] Example 4 The difference between Example 4 and Example 1 is that the particle size Dn90 of the cordierite powder for preparing the safety layer is 120 nm ≤ Dn90 ≤ 150 nm.

[0146] Example 5 The difference between Example 5 and Example 1 is that the porosity of the cordierite powder for preparing the safety layer is 65%, and the porosity of the safety layer is 50%.

[0147] Example 6 The difference between Example 6 and Example 1 is that the proportion of the secondary particle material in the first positive electrode active material is 50 wt%; the proportion of the secondary particle material in the second positive electrode active material is 90 wt%.

[0148] Example 7 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.

[0149] Example 8 The difference between Example 8 and Example 1 is that the cordierite powder, PVDF, silicone resin adhesive, and acetylene black are used to prepare the safety layer in a ratio of 60:18:0.5:1.5.

[0150] Example 9 The difference between Example 9 and Example 1 is that the cordierite powder, PVDF, silicone resin adhesive, and acetylene black are used to prepare the safety layer in a ratio of 95:4:0.5:1.5.

[0151] Example 10 The difference between Example 10 and Example 1 is 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; 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.

[0152] Example 11 The difference between Example 11 and Example 1 is 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; 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.

[0153] Example 12 The difference between Example 12 and Example 1 is 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; 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.

[0154] Example 13 The difference between Example 13 and Example 1 is that the safety layer slurry does not contain silicone resin adhesive.

[0155] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that mullite ceramic (3Al2O3·2SiO2) is selected to replace cordierite.

[0156] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that spinel ceramic (MgO·Al2O3) is selected to replace cordierite.

[0157] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that there is no safety layer.

[0158] Performance Test 1. Battery Preparation Separator: A composite membrane (substrate (polypropylene) 9μm + double-sided coated 3μm) is selected.

[0159] Electrolyte: In a glove box filled with inert gas, ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate were mixed at a mass ratio of 1:1:1 to obtain an organic solvent. Then, lithium salt LiPF6 was dissolved in the organic solvent, and the concentration of the lithium salt was 1.2 mol / L to obtain the electrolyte.

[0160] 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 evenly 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 min, and then made into the negative electrode sheet of the lithium-ion battery after cold pressing and slitting. The compaction density was 1.65 g / cm 3 .

[0161] The positive electrode sheets, negative electrode sheets, and separators prepared in each example and comparative example were wound by a winding machine to obtain a core with a wound structure wrapped by the positive electrode. It was encapsulated with an aluminum-plastic film, baked for 48 h in a vacuum state to remove moisture, then the electrolyte was injected, and the battery was subjected to conventional formation and grading to obtain a prismatic soft-pack lithium-ion battery.

[0162] 2. Testing Peel strength test: The electrode sheet was fixed on the clamp of a tensile machine. One end of the tape was adhered to the electrode sheet, and the other end was folded 180° and then fixed on the lower clamp of the tensile machine. It was pulled apart at a test rate of 50 mm / min, and the peel strength (N / m) was measured by the force required to continuously peel the tape from the electrode sheet.

[0163] Initial efficiency test: During formation, it was charged with a current of 0.1C until 85% SOC, and the charging capacity at this time was recorded as C1; after standing for 5 minutes, during grading, it was continued to be charged with a small current of 0.05C until 4.25V, and the charging capacity at this time was recorded as C2; after standing for 5 minutes, it was discharged at a constant current of 1C until 2.8V, and after standing for 5 minutes, it was discharged at a constant current of 0.2C until 2.8V. The total discharge capacity of the two steps was recorded as C3. The charge and discharge capacities obtained by the battery were collected respectively, and the initial efficiency = ((C1 + C2) / C3) * 100%.

[0164] Capacity retention rate test after 500 cycles at 1C: The battery was charged at a constant current of 1C to 4.25V and then switched to constant voltage charging until the current dropped to 0.05C and then stopped. Then, after standing for 10 minutes, the battery was discharged at a constant current of 1C until 2.8V, and the discharge capacity at this time was recorded as the initial discharge capacity. The cycle of charging - standing - discharging was repeated, and the discharge capacity after 500 cycles was recorded. The capacity retention rate after 500 cycles at 1C = (discharge capacity after 500 cycles / initial discharge capacity) * 100%.

[0165] Gas generation performance test at 70°C storage: First, measure the volume of the battery as the volume before storage. Then, charge the battery at a constant current of 1C to 4.25V, and then charge it at a constant voltage of 4.25V with a cut-off current of 0.05C. Then, place the battery in an incubator at 70°C for 30 days of storage. Then, measure the volume as the volume after 30 days of storage. The volume growth rate = (volume after 30 days of storage / volume before storage - 1) * 100%.

[0166] Thermal stability test (150°C hot box): Charge the battery to full capacity with a current of 1C. Then, clamp the battery cell with a clamping plate and place it in an explosion-proof box with a heating function. Heat the temperature in the explosion-proof box to 150°C and maintain it for 60 minutes. Observe whether the battery catches fire.

[0167] Pinprick detection test: Use a pinprick testing machine to conduct a pinprick test. Charge at a constant current and constant voltage of 0.2C until the charging cut-off voltage. Use the Dongguan Bell battery pinprick testing machine to pierce the center position of the largest surface of the battery with a stainless steel needle with a diameter of 3mm at a speed of 2m / min and keep it for more than 1 hour.

[0168] Flexibility test: Use steel needles with different diameters to conduct a flexibility test on the electrode sheet. Cut the electrode sheet into an electrode sheet of 2 * 10 cm, and then wrap the cut electrode sheet around the steel needle and roll it for one week. Observe whether there are cracks on the electrode sheet. The naming method is: name it with the smallest diameter of the steel needle without cracks. For example, if there are no cracks on the electrode sheet after rolling on a 6mm steel needle, but there are cracks on a 5mm steel needle, then the flexibility of this electrode sheet is 6.

[0169] The above tests are all carried out at 25 ± 5°C. The test results are shown in Table 1.

[0170] Table 1 Referring to Table 1, from the test results of Example 1 and Example 2, it can be seen that since the cordierite powder particles are smaller, the gas storage channels are fewer and the volume change rate increases. However, due to the small particle size, the permeability to the lower active material is better and the adhesion is stronger. Therefore, it does not affect the subsequent safety experiments such as the hot box and needle punching. Comparing with the test results of Example 3, it can be seen that since the cordierite powder particles become larger, the initial efficiency and reversible capacity decrease, which may be related to the fact that lithium inside the large particles cannot be released. The volume growth rate decreases significantly because the gas enters the pores inside the cordierite. From the test results of Example 8, it can be seen that the decrease in the content of cordierite will cause the decrease of the initial efficiency and reversible specific capacity and the increase of the volume growth rate, which proves that the addition of the cordierite safety layer will improve its capacity and gas generation performance. From the test results of Example 9, it can be seen that when the content of cordierite increases and the content of the binder decreases, the capacity retention rate after cycling decreases. This may be because with the decrease of the binder, the cordierite powder falls off after repeated volume expansion and contraction, the secondary particles begin to break, react with the fresh interface and the electrolyte, and lithium ions are lost, resulting in the decrease of capacity retention and the decline of safety performance. From the test results of Example 10, it can be seen that after the particle size of the positive active material increases, the long-term performance such as cycling and gas generation decreases, which is related to the relatively large number of upper secondary particles. However, due to the presence of the cordierite safety coating, the cycling does not drop suddenly, indicating that the particles do not undergo large-scale fragmentation. The large particles also cause the peel strength of the electrode sheet to decrease, and its failure mainly occurs between the first layer and the second layer of active materials. The safety layer does not fall off on a large scale, indicating that the increase in particle size can make the safety coating penetrate better, but too large particles will cause poor particle adhesion between the active materials. From the test results of Example 11, it can be seen that after the particle size of the positive active material decreases, the initial efficiency decreases, which is attributed to the increase in the specific surface area resulting in more side reactions. After the particle size decreases, the safety layer falls off during the peel strength test, indicating that the permeability of the safety layer decreases. From the test results of Example 13, it can be seen that when the safety layer slurry does not contain silicone resin adhesive, all performances deteriorate significantly because the unbonded cordierite is extremely easy to fall off, the electrode sheets are not tightly bonded, resulting in difficult lithium ion transmission and deteriorated kinetics. In addition, the flexibility of the electrode sheet without silicone resin adhesive is above 6, and the electrode sheet is translucent after folding back and forth twice. The flexibility of the electrode sheet with silicone resin adhesive is about 4.5, and there is no light transmission phenomenon after folding the electrode sheet back and forth twice, which indicates that the flexibility of the electrode sheet with silicone resin adhesive is improved and it will not easily break and pierce the diaphragm after being pressed.

[0171] 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 temperature, 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 makes the safety coating formed by spinel more brittle under the same formula. After the puncture test, the safety coating cannot be effectively extended, causing the positive and negative electrodes to short-circuit, resulting in differences in safety performance.

[0172] Parts of the present invention that are not described in detail are well known to those skilled in the art.

[0173] The basic principle of the present invention is described above in conjunction with specific embodiments. However, it should be pointed out that the advantages, strengths, effects, etc. mentioned in the present invention are only examples and not limitations, and it cannot be considered that these advantages, strengths, effects, etc. must be possessed by each embodiment of the present invention. In addition, the specific details disclosed above are only for the purpose of illustration and facilitation of understanding, rather than limitation, and the above details do not limit the present invention to being implemented by adopting the above specific details.

[0174] It should be noted that the term "and / or" or " / " used in this document is only a description of the association relationship of associated objects, indicating that three relationships may exist. For example, A and / or B can represent: A exists alone, A and B exist at the same time, and B exists alone. The singular forms of "a", "the" and "the" used in the embodiments of the present invention and the appended claims are also intended to include plural forms, unless the context clearly indicates other meanings.

[0175] 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, B are listed, the phrase "at least one of A, B" means only A; only B; or A and B. In another example, if items A, B, C are listed, the phrase "at least one of A, B, 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.

[0176] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features. These modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode sheet, characterized in that, The positive electrode sheet includes: A current collector; A first positive electrode active material layer disposed on at least one surface of the current collector in the thickness direction, the first positive electrode active material layer including a first positive electrode active material; A second positive electrode active material layer disposed on the surface of the first positive electrode active material layer away from the current collector, the second positive electrode active material layer including a second positive electrode active material; and A safety layer disposed on the surface of the second positive electrode active material layer away from the first positive electrode active material layer, the safety layer including cordierite.

2. The positive electrode sheet according to claim 1, characterized in that, The cordierite satisfies at least one of the following characteristics (1) to (4): (1) The cordierite has a porous structure, and the pores of the cordierite contain a lithium salt; The mass content of the lithium salt in the cordierite is 30% to 40%; (2) The Dn90 of the cordierite is 50 nm to 500 nm; (3) The specific surface area of the cordierite is 1.5 m 2 / g to 3.5 m 2 / g; (4) The porosity of the cordierite is 65% to 70%.

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

4. The positive electrode sheet according to claim 1, characterized in that, Both the first positive electrode active material and the second positive electrode active material include primary particles and secondary particles, and 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 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; 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; In the first positive electrode active material, the mass ratio of primary particles to secondary particles is (50 to 70): (30 to 50); In the second positive electrode active material, the mass ratio of primary particles to secondary particles is (10 to 30): (70 to 90).

5. The positive electrode sheet according to any one of claims 4, 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 and 0.1 ≤ y ≤ 0.3; The M includes 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 to 99): (1 to 3): (0.5 to 1.5); The thickness of the first positive electrode active material layer is 60 μm to 70 μm.

6. The positive electrode sheet according to any one of claims 4, 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 and 0.1 ≤ y ≤ 0.3; The M includes 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 to 99): (1 to 3): (0.5 to 1.5); (4) The thickness of the second positive electrode active material layer is 60 μm to 70 μm.

7. A method for preparing a positive electrode plate, characterized in that, It includes the following steps: Coat a first positive electrode paste on at least one surface of the current collector to obtain a first positive electrode active material layer; Coat a second positive electrode paste on the surface of the first positive electrode active material layer to obtain a second positive electrode active material layer; Coat a safety layer paste on the surface of the second positive electrode active material layer to obtain a safety layer; Among them, 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; the safety layer includes cordierite.

8. The method for preparing the positive electrode sheet according to claim 7, wherein, The preparation of the first positive electrode paste 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 paste; The preparation of the second positive electrode paste 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 paste; The preparation of the safety layer paste includes: uniformly mixing cordierite, a third binder, a fourth binder, and a third conductive agent in a solvent to obtain a safety layer paste; The solvent includes at least one of water, ethanol, or N-methylpyrrolidone; Coat the first positive electrode paste on the surface of the current collector, and after drying, form a first positive electrode active material layer on the surface of the current collector; coat the second positive electrode paste on the first positive electrode active material layer, and after drying, form a second positive electrode active material layer on the surface of the first positive electrode active material layer; coat the safety layer paste on the second positive electrode active material layer, and dry to obtain the positive electrode sheet.

9. The method for preparing a positive electrode sheet according to claim 7, wherein The preparation method of the cordierite includes: Mix an aluminum source, a magnesium source, a silicon source, and an organic solvent to obtain a sol, then add a lithium salt and mix, and after adjusting the pH value, obtain a gel; Let the gel stand, and after drying, calcining, and grinding, obtain cordierite; The molar ratio of the aluminum source, the magnesium source, and the silicon source is (2 to 6): (1 to 3): (2 to 8); The volume ratio of the silicon source to the organic solvent is 1: (3 to 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(trifluoromethanesulfonyl)imide, lithium bis(fluorosulfonyl)amide, lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium bis(oxalato)borate, lithium bis(oxalato)borate, lithium tetrafluoroborate, lithium phosphate, or lithium difluorophosphate; Adjust the pH value to 5 to 7; The standing time of the gel is 6 h to 10 h; The calcination temperature is 500°C to 700°C, and the calcination time is 1 h to 3 h.

10. 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 6, or includes the positive electrode sheet prepared by the preparation method according to any one of claims 7 to 9.

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