Positive pole piece, secondary battery and electronic device

Abstract

The invention provides a positive pole piece, a secondary battery and an electronic device, the positive pole piece comprises a positive pole current collector and a positive pole material layer arranged on at least one surface of the positive pole current collector, the positive pole material layer comprises a positive pole active material, a positive pole binder and an additive, the additive is at least one of a compound shown in a formula I and a compound shown in a formula II, based on the mass of the positive electrode material layer, the mass percentage content of the additive is W%, and W is greater than or equal to 0.01 and less than or equal to 2. According to the present invention, the positive electrode plate meets the characteristics, such that the positive electrode plate has good flexibility under the high compaction density condition, and the cycle performance and the safety performance of the secondary battery can be improved while the secondary battery has the high energy density.

Description

Technical Field

[0001] The present application relates to the field of electrochemical technology, and in particular to a positive electrode sheet, a secondary battery, and an electronic device. Background Art

[0002] With the development of the lithium-ion battery industry, the market requirements for the dynamic performance and energy density of lithium-ion batteries are becoming increasingly higher. The energy density of lithium-ion batteries can be improved by reducing the binder content in the material layer. However, reducing the binder content in the material layer will lead to poor slurry stability and reduced adhesion between the material layer and the current collector.

[0003] In addition, an effective way to increase the energy density of lithium-ion batteries is to increase the electrode sheet compaction density. By reducing the thickness of a single-layer electrode sheet, the thickness of the lithium-ion battery can be reduced and the volumetric energy density of the lithium-ion battery can be increased. However, as the electrode sheet compaction density increases, the active material particles squeeze and damage the current collector, which can cause the electrode sheet to fracture brittlely, leading to safety risks for the lithium-ion battery. Therefore, improving the electrode sheet brittle fracture problem under high compaction density is of great significance. Summary of the Invention

[0004] The purpose of this application is to provide a positive electrode sheet, a secondary battery, and an electronic device, which can ensure that the positive electrode sheet has good flexibility under high compaction density conditions, and the secondary battery has a high energy density while also improving the cycle performance and safety performance of the secondary battery. The specific technical solution is as follows:

[0005] The first aspect of the present application provides a positive electrode plate, comprising a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode material layer comprises a positive electrode active material, a positive electrode binder, and an additive, wherein the additive is at least one of a compound of formula I and a compound of formula II.

[0006]

[0007] wherein 9≤n≤20, R1 is selected from H, Li, Ca, Mg, Zn, Al, Fe, Ba or Co, and R2 is selected from C9 to C 20alkenylene. Based on the mass of the positive electrode material layer, the mass percentage of the additive is W%, 0.01≤W≤2, preferably, 0.02≤W≤0.5. The inventors have found that by adding additives to the positive electrode material layer, that is, the compound of formula I and / or the compound of formula II, the additives can be used as softeners. The softeners have grease-like lubricating properties. The softeners are evenly distributed between the positive electrode active material particles, which is beneficial to increase the slippage of the positive electrode active material particles during the rolling process of the electrode sheet, and improve the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles; at the same time, the softener, as a small molecule material, can be inserted between the molecular chains of the positive electrode binder, which can reduce the crystallinity of the positive electrode binder and improve the flexibility of the positive electrode sheet; and the softener and the positive electrode binder are more compatible, which can improve the distribution of the positive electrode binder and the positive electrode conductive agent to a certain extent, so that the friction between the positive electrode active material particles during rolling is reduced, and the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles is improved, and the brittle fracture of the positive electrode sheet is improved. By regulating the positive electrode material layer including the above-mentioned additives and the mass percentage of the additives within the scope of this application, the positive electrode plate can have good flexibility under high compaction density conditions, and the secondary battery can have a higher energy density while also improving the cycle performance and safety performance of the secondary battery.

[0008] In one embodiment of the present application, the compaction density of the positive electrode material layer is PD g / cm 3 , 3.0≤PD≤4.5, preferably, 3.8≤PD≤4.4. By regulating the compaction density of the positive electrode material layer within the above range, under the condition that the positive electrode material layer has a higher compaction density, the positive electrode active material particles are more densely packed, and the positive electrode conductive agent and the positive electrode active material are in closer contact. The addition of the softener can further increase the slippage of the positive electrode active material particles during the rolling process, further improve the damage to the positive electrode current collector caused by the squeezing of the positive electrode active material particles, and further improve the flexibility of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0009] In one embodiment of the present application, PD≥4.5529×W 0.0209 The compaction density of the positive electrode material layer and the mass percentage of the additive satisfy the above relationship. The compaction density of the positive electrode material layer matches the mass percentage of the additive within a certain range. The softener has a suitable addition amount. The softener can be more evenly distributed between the positive electrode active material particles in different stacking states, and can further increase the slippage of the positive electrode active material particles during the rolling process, further improve the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, and further improve the flexibility of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0010] In one embodiment of the present application, the coating weight of the positive electrode material layer is CW g / cm 2 , 0.018≤CW≤0.039. By regulating the coating weight of the positive electrode material layer within the above range, under the condition of a higher coating weight of the positive electrode material layer, the number of positive electrode active material particles within the same area is greater. The addition of the softener can further reduce the friction between the larger number of positive electrode active material particles during rolling, further improve the damage to the positive electrode current collector caused by the squeezing of the positive electrode active material particles, and further improve the brittle fracture resistance of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while achieving a higher energy density.

[0011] In one embodiment of the present application, the thickness of the positive electrode material layer is H1μm, 30≤H1≤110, preferably, 40≤H1≤80. By regulating the thickness of the positive electrode material layer within the above range, under the condition of a thicker positive electrode material layer, the number of positive electrode active material particles per unit volume is relatively large, and the addition of a softener can further reduce the friction between the large number of positive electrode active material particles during rolling, further improve the damage to the positive electrode current collector caused by the squeezing of the positive electrode active material particles, and further improve the brittle fracture of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0012] In one embodiment of the present application, the thickness of the positive electrode current collector is H2 μm, 7 ≤ H2 ≤ 20, preferably, 8 ≤ H2 ≤ 12. By regulating the thickness of the positive electrode current collector within the above range, the positive electrode current collector has high mechanical strength, which can effectively reduce damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, further improving the safety performance of the secondary battery.

[0013] In one embodiment of the present application, the compound of formula I includes at least one of undecanoic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, pearlic acid, stearic acid, lithium stearate, calcium stearate, magnesium stearate, zinc stearate, aluminum stearate, iron stearate, barium stearate, or cobalt stearate. The compound of formula I is selected because it has better grease lubrication characteristics and can be more evenly distributed between the positive electrode active material particles, further increasing the slippage of the positive electrode active material particles during rolling, further improving damage to the positive electrode current collector caused by squeezing of the positive electrode active material particles, and further improving the flexibility of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0014] In one embodiment of the present application, the compound of formula II includes at least one of myristoleic acid, oleic acid, or octadecenoic acid. The above-mentioned compound of formula II is selected because the compound of formula II has better grease lubrication characteristics. The compound of formula II can be more evenly distributed between the positive electrode active material particles, which can further increase the slippage of the positive electrode active material particles during the rolling process, further improve the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, and further improve the flexibility of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0015] In one embodiment of the present application, the melting point of the additive is Tm°C, 10≤Tm≤230°C, preferably, 65≤Tm≤190°C. By regulating the melting point of the additive within the above range, the additive has better dispersibility and lubricity, which is conducive to increasing the slippage of the positive electrode active material particles during the rolling process, and can further improve the dispersion performance of the positive electrode conductive agent and the positive electrode binder, further improve the flexibility of the positive electrode sheet, and further improve the cycle performance and safety performance of the secondary battery.

[0016] In one embodiment of the present application, the true density of the additive is pg / cm 3 , 0.7≤p≤1.3, preferably, 0.85≤p≤1.15. By regulating the true density of the additive within the above range, the additive has better thermal and mechanical stability. At the same time, the appropriate true density significantly improves the solubility and dispersibility of the additive, which can further improve the thermal stability and flexibility of the positive electrode sheet, and further improve the safety and cycle performance of the secondary battery.

[0017] In one embodiment of the present application, the thermogravimetric curve of the positive electrode sheet contains a first decomposition peak at 150°C to 220°C and / or a second decomposition peak at 250°C to 350°C. The thermogravimetric curve of the positive electrode sheet satisfies the above characteristics, indicating that the additives in the positive electrode material layer include acid additives and / or salt additives. The addition of these additives is beneficial for increasing the slippage of the positive electrode active material particles during rolling, improving damage to the positive electrode current collector caused by the squeezing of the positive electrode active material particles, and improving the flexibility of the positive electrode sheet. This enables the positive electrode sheet to have good flexibility even under high compaction density conditions, thereby achieving a higher energy density for the secondary battery and improving the cycle performance and safety performance of the secondary battery.

[0018] In one embodiment of the present application, the mass percentage of the positive electrode binder is X%, based on the mass of the positive electrode material layer, and 0.6≤X≤2.5. By regulating the mass percentage of the positive electrode binder within the above range, the positive electrode binder has an appropriate mass percentage, which can achieve a high bonding strength between the positive electrode material layer and the positive electrode current collector, further improving the cycle performance of the secondary battery.

[0019] In one embodiment of the present application, the positive electrode binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorine-containing acrylate resin, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polyethylene, polypropylene, ethylene-propylene-diene polymer, sulfonated ethylene-propylene-diene polymer, carboxymethyl cellulose, styrene-butadiene rubber, or fluororubber. The above-mentioned positive electrode binder can form a good dispersion effect with the above-mentioned additives, further improve the bonding performance between the positive electrode material layer and the positive electrode current collector, and further improve the cycle performance of the secondary battery.

[0020] In one embodiment of the present application, 7.5≤X / W≤120. By regulating the value of X / W within the above range and combining an appropriate amount of positive electrode binder with an appropriate amount of additive, the distribution of the positive electrode binder is further improved, thereby further reducing the friction between the positive electrode active material particles during rolling, further improving the damage to the positive electrode current collector caused by the squeezing of the positive electrode active material particles, and further improving the flexibility of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0021] In one embodiment of the present application, the bonding force between the positive electrode material layer and the positive electrode current collector is FN / m, where F ≥ 25. By regulating the bonding force between the positive electrode material layer and the positive electrode current collector within the above range, the bonding force between the positive electrode material layer and the positive electrode current collector is higher, further improving the cycle performance of the secondary battery.

[0022] In one embodiment of the present application, the particle size of the positive electrode active material satisfies the following conditions: 0.25 ≤ Dv10 / Dv50 ≤ 0.5, preferably, 0.33 ≤ Dv10 / Dv50 ≤ 0.45. The particle size of the positive electrode active material satisfies the above characteristics, the positive electrode active material has an appropriate gradation ratio, and the positive electrode active material has a blend of large and small particles. The positive electrode active material is more densely packed, and the positive electrode active material itself has a higher compaction density, which results in a higher compaction density of the positive electrode sheet, thereby providing a higher energy density for the secondary battery.

[0023] In one embodiment of the present application, the positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, or lithium iron phosphate. The selection of such positive electrode active materials allows the secondary battery to have both high energy density and good cycle performance and safety.

[0024] The second aspect of the present application provides a secondary battery, which includes the positive electrode sheet of any of the aforementioned embodiments. Therefore, the secondary battery provided by the present application has high energy density and good cycle performance and safety performance.

[0025] The third aspect of the present application provides an electronic device, which includes the secondary battery according to any of the aforementioned embodiments. Therefore, the electronic device provided by the present application has high energy density and good cycle performance and safety performance.

[0026] Beneficial effects of this application:

[0027] The present application provides a positive electrode sheet, a secondary battery, and an electronic device. The positive electrode sheet includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The positive electrode material layer includes a positive electrode active material, a positive electrode binder, and an additive. The additive is at least one of a compound of Formula I and a compound of Formula II. The mass percentage of the additive is W%, based on the mass of the positive electrode material layer, and 0.01≤W≤2. The positive electrode sheet meeting the above characteristics can ensure that the positive electrode sheet has good flexibility under high compaction density conditions, and the secondary battery has a high energy density while also improving the cycle performance and safety performance of the secondary battery.

[0028] Of course, it is not necessary to achieve all the advantages described above at the same time when implementing any product or method of the present application. DETAILED DESCRIPTION

[0029] The following will be combined with the embodiments of the present application to clearly and completely describe the technical solutions in this application. Obviously, the embodiments described are only part of the embodiments of the present application, rather than all the embodiments. All other embodiments obtained by those skilled in the art based on this application are within the scope of protection of this application.

[0030] It should be noted that, in the specific embodiments of the present application, lithium-ion batteries are used as an example of secondary batteries to explain the present application, but the secondary batteries of the present application are not limited to lithium-ion batteries.

[0031] The first aspect of the present application provides a positive electrode plate, comprising a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode material layer comprises a positive electrode active material, a positive electrode binder, and an additive, wherein the additive is at least one of a compound of formula I and a compound of formula II.

[0032]

[0033] wherein 9≤n≤20, R1 is selected from H, Li, Ca, Mg, Zn, Al, Fe, Ba or Co, and R2 is selected from C9 to C 20 alkenylene. Exemplarily, the value of n can be 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19 or 20. Based on the mass of the positive electrode material layer, the mass percentage of the additive is W%, 0.01≤W≤2, preferably, 0.02≤W≤0.5, more preferably, 0.02≤W≤0.3. Exemplarily, the value of W can be 0.01, 0.02, 0.03, 0.05, 0.07, 0.1, 0.3, 0.5, 0.7, 1, 1.1, 1.3, 1.5, 1.7, 2 or a range consisting of any two of the above values. The above-mentioned "positive electrode material layer provided on at least one surface of the positive electrode current collector" means that the positive electrode material layer can be provided on one surface of the positive electrode current collector along its own thickness direction, or on both surfaces of the positive electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of the positive electrode current collector or a partial area of the positive electrode current collector. This application has no special restrictions, as long as the purpose of this application can be achieved.

[0034] The inventors have found that by adding additives to the positive electrode material layer, namely the compound of formula I and / or the compound of formula II, the additives can be used as a softener. The softener has the lubricating properties of oils and fats. The softener is evenly distributed between the positive electrode active material particles, which is beneficial to increase the slippage of the positive electrode active material particles during the rolling of the electrode sheet, and improve the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles; at the same time, the softener, as a small molecule material, can be inserted between the molecular chains of the positive electrode binder, which can reduce the crystallinity of the positive electrode binder and improve the flexibility of the positive electrode sheet; and the softener and the positive electrode binder are more compatible, which can improve the distribution of the positive electrode binder and the positive electrode conductor to a certain extent, so that the friction between the positive electrode active material particles is reduced during rolling, improving the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, and improving the brittle fracture of the positive electrode sheet. When the weight percentage of the additive is too low, for example, less than 0.01%, the effect of increasing the slippage of the positive electrode active material particles during rolling is not significant, and the damage to the positive electrode current collector caused by the compression of the positive electrode active material particles cannot be effectively improved. When the weight percentage of the additive is too high, for example, greater than 2%, the positive electrode material layer will be more likely to detach from the surface of the positive electrode current collector, affecting the cycle performance of the secondary battery. Therefore, by regulating the positive electrode material layer to include the above-mentioned additives and the weight percentage of the additives within the scope of this application, the positive electrode sheet can have good flexibility under high compaction density conditions, and the secondary battery can have a higher energy density while also improving the cycle performance and safety performance of the secondary battery.

[0035] In one embodiment of the present application, the compaction density of the positive electrode material layer is PD g / cm 3 , 3.0≤PD≤4.5, preferably, 3.8≤PD≤4.4. Exemplarily, the value of PD can be 3.0, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4.0, 4.1, 4.2, 4.3, 4.4, 4.5 or a range consisting of any two of the above values. By regulating the compaction density of the positive electrode material layer within the above range, under the condition that the positive electrode material layer has a higher compaction density, the positive electrode active material particles are stacked more closely, and the positive electrode conductive agent and the positive electrode active material are in closer contact. The addition of a softener can further increase the slippage of the positive electrode active material particles during rolling, further improve the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, and further improve the flexibility of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0036] In one embodiment of the present application, PD≥4.5529×W 0.0209The compaction density of the positive electrode material layer and the mass percentage of the additive satisfy the above relationship. The compaction density of the positive electrode material layer matches the mass percentage of the additive within a certain range. The softener has a suitable addition amount. The softener can be more evenly distributed between the positive electrode active material particles in different stacking states, and can further increase the slippage of the positive electrode active material particles during the rolling process, further improve the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, and further improve the flexibility of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0037] In one embodiment of the present application, the coating weight of the positive electrode material layer is CW g / cm 2 , 0.018≤CW≤0.039. Exemplarily, the value of CW can be 0.018, 0.019, 0.020, 0.023, 0.025, 0.027, 0.029, 0.030, 0.033, 0.035, 0.037, 0.039 or a range consisting of any two of the above values. By regulating the coating weight of the positive electrode material layer within the above range, under the condition that the positive electrode material layer has a higher coating weight, the number of positive electrode active material particles in the same area is larger, and the addition of the softener can further reduce the friction between the larger number of positive electrode active material particles during rolling, further improve the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, further improve the brittle fracture of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0038] In one embodiment of the present application, the thickness of the positive electrode material layer is H1μm, 30≤H1≤110, preferably, 40≤H1≤80. Exemplarily, the value of H1 can be 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, 100, 105, 110 or a range consisting of any two of the above values. The thickness of the above-mentioned positive electrode material layer refers to the thickness of the single-sided positive electrode material layer after cold pressing. By regulating the thickness of the positive electrode material layer within the above range, under the condition that the positive electrode material layer is thicker, the number of positive electrode active material particles per unit volume is larger, and the addition of a softener can further reduce the friction between the larger number of positive electrode active material particles during rolling, further improve the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, and further improve the brittle fracture of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0039] In one embodiment of the present application, the thickness of the positive electrode current collector is H2μm, 7≤H2≤20, preferably, 8≤H2≤12. For example, the value of H2 can be 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, or a range consisting of any two of the above values. By regulating the thickness of the positive electrode current collector within the above range, the positive electrode current collector has higher mechanical strength, can effectively reduce the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, and further improve the safety performance of the secondary battery.

[0040] In one embodiment of the present application, the compound of formula I includes at least one of undecanoic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, pearlic acid, stearic acid, lithium stearate, calcium stearate, magnesium stearate, zinc stearate, aluminum stearate, iron stearate, barium stearate or cobalt stearate. Among them, stearic acid is also octadecanoic acid; palmitic acid is also hexadecanoic acid. The above-mentioned compound of formula I is selected. The compound of formula I has better grease lubrication characteristics. The compound of formula I can be more evenly distributed between the positive electrode active material particles, which can further increase the slippage of the positive electrode active material particles during the rolling process, further improve the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, and further improve the flexibility of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0041] In one embodiment of the present application, the compound of formula II includes at least one of myristoleic acid, oleic acid, or octadecenoic acid. The above-mentioned compound of formula II is selected because the compound of formula II has better grease lubrication characteristics. The compound of formula II can be more evenly distributed between the positive electrode active material particles, which can further increase the slippage of the positive electrode active material particles during the rolling process, further improve the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, and further improve the flexibility of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0042] In one embodiment of the present application, the melting point of the additive is Tm℃, 10≤Tm≤230, preferably, 65≤Tm≤190. Exemplarily, the value of Tm can be 10, 20, 30, 40, 50, 60, 65, 70, 90, 100, 110, 130, 150, 170, 190, 200, 210, 230 or a range consisting of any two of the above values. By regulating the melting point of the additive within the above range, the additive has better dispersibility and lubricity, which is beneficial to increase the slippage of the positive electrode active material particles during rolling, and can further improve the dispersion performance of the positive electrode conductor and the positive electrode binder, further improve the flexibility of the positive electrode sheet, and further improve the cycle performance and safety performance of the secondary battery.

[0043] In one embodiment of the present application, the true density of the additive is pg / cm 3 , 0.7≤p≤1.3, preferably, 0.85≤p≤1.15. Exemplarily, the value of p can be 0.7, 0.73, 0.75, 0.77, 0.8, 0.83, 0.85, 0.87, 0.9, 0.93, 0.95, 0.97, 1.0, 1.05, 1.1, 1.15, 1.2, 1.25, 1.3 or a range consisting of any two of the above values. By regulating the true density of the additive within the above range, the additive has better thermal stability and mechanical stability. At the same time, the appropriate true density greatly improves the solubility and dispersibility of the additive, which can further improve the thermal stability and flexibility of the positive electrode sheet, and further improve the safety performance and cycle performance of the secondary battery.

[0044] In one embodiment of the present application, there is a first decomposition peak at 150°C to 220°C or a second decomposition peak at 250°C to 350°C in the thermogravimetric curve of the positive electrode sheet. The first decomposition peak at 150°C to 220°C is the thermal decomposition peak of the acid additive, and the second decomposition peak at 250°C to 350°C is the thermal decomposition peak of the salt additive. The thermogravimetric curve of the positive electrode sheet meets the above characteristics, indicating that the additives in the positive electrode material layer include acid additives or salt additives. The addition of the above additives is beneficial to increase the slippage of the positive electrode active material particles during rolling, improve the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, and improve the flexibility of the positive electrode sheet. It can enable the positive electrode sheet to have good flexibility under high compaction density conditions, and the secondary battery can have a higher energy density while also improving the cycle performance and safety performance of the secondary battery.

[0045] In one embodiment of the present application, the thermogravimetric curve of the positive electrode sheet contains a first decomposition peak between 150°C and 220°C and a second decomposition peak between 250°C and 350°C. The thermogravimetric curve of the positive electrode sheet satisfies the above characteristics, indicating that the additives in the positive electrode material layer include acid additives and salt additives. The addition of these two types of additives is beneficial for increasing the slippage of the positive electrode active material particles during rolling, improving damage to the positive electrode current collector caused by the squeezing of the positive electrode active material particles, and improving the flexibility of the positive electrode sheet. This enables the positive electrode sheet to have good flexibility even under high compaction density conditions, thereby achieving a higher energy density for the secondary battery and improving the cycle performance and safety performance of the secondary battery.

[0046] In one embodiment of the present application, the thermogravimetric curve of the positive electrode sheet contains a first decomposition peak at 150°C to 220°C, a second decomposition peak at 250°C to 350°C, and a third decomposition peak at 350°C to 490°C. The third decomposition peak at 350°C to 490°C is the thermal decomposition peak of the positive electrode binder polyvinylidene fluoride. The thermogravimetric curve of the positive electrode sheet meets the above characteristics, indicating that the additives in the positive electrode material layer include acid additives and salt additives, and the positive electrode binder in the positive electrode material layer includes polyvinylidene fluoride. The combination of the additives and the positive electrode binder is conducive to further improving the distribution of the positive electrode binder, thereby further reducing the friction between the positive electrode active material particles during rolling, further improving the damage to the positive electrode current collector caused by the squeezing of the positive electrode active material particles, and further improving the flexibility of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density.

[0047] In one embodiment of the present application, based on the mass of the positive electrode material layer, the mass percentage of the positive electrode binder is X%, 0.6≤X≤2.5. For example, the value of X can be 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 2.1, 2.2, 2.3, 2.4, 2.5 or a range consisting of any two of the above values. By regulating the mass percentage of the positive electrode binder within the above range, the positive electrode binder has a suitable mass percentage, which can provide a higher bonding force between the positive electrode material layer and the positive electrode current collector, further improving the cycle performance of the secondary battery.

[0048] In one embodiment of the present application, the positive electrode binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorine-containing acrylate resin, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polyethylene, polypropylene, ethylene-propylene-diene polymer, sulfonated ethylene-propylene-diene polymer, carboxymethyl cellulose, styrene-butadiene rubber, or fluororubber. The above-mentioned positive electrode binder can form a good dispersion effect with the above-mentioned additives, further improve the bonding performance between the positive electrode material layer and the positive electrode current collector, and further improve the cycle performance of the secondary battery.

[0049] In one embodiment of the present application, 7.5≤X / W≤120. Exemplarily, the value of X / W can be 7.5, 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120 or a range consisting of any two of the above values. By regulating the value of X / W within the above range, a suitable content of positive electrode binder is matched with a suitable content of additives, which is conducive to further improving the distribution of the positive electrode binder, so that the friction between the positive electrode active material particles during rolling is further reduced, further improving the damage to the positive electrode current collector caused by the extrusion of the positive electrode active material particles, and further improving the flexibility of the positive electrode sheet, thereby further improving the cycle performance and safety performance of the secondary battery while having a higher energy density of the secondary battery.

[0050] In one embodiment of the present application, the bonding force between the positive electrode material layer and the positive electrode current collector is FN / m, where F ≥ 25. For example, the value of F can be 25, 27, 29, 30, 35, 37, 39, 40, 50, 60, 70, 80, 90, 100, 110, 120, or a range consisting of any two of the above values. By regulating the bonding force between the positive electrode material layer and the positive electrode current collector within the above range, the bonding force between the positive electrode material layer and the positive electrode current collector is higher, thereby further improving the cycle performance of the secondary battery.

[0051] In one embodiment of the present application, the particle size of the positive electrode active material satisfies: 0.25≤Dv10 / Dv50≤0.5, preferably, 0.33≤Dv10 / Dv50≤0.45. For example, the value of Dv10 / Dv50 can be 0.25, 0.27, 0.29, 0.3, 0.31, 0.33, 0.35, 0.37, 0.39, 0.4, 0.41, 0.43, 0.45, 0.47, 0.49, 0.5 or a range consisting of any two of the above values. The particle size of the positive electrode active material meets the above characteristics, the positive electrode active material has a suitable gradation ratio, large particles and small particles of positive electrode active materials are mixed, the positive electrode active materials are stacked more closely, the compaction density of the positive electrode active material itself is high, so that the compaction density of the positive electrode sheet is high, so that the secondary battery has a higher energy density. In the present application, 3.0 μm ≤ Dv10 ≤ 10.5 μm, and 6.8 μm ≤ Dv50 ≤ 19.6 μm.

[0052] In this application, Dv10 refers to the particle size at which 10% of the volume is accumulated, measured from the smallest particle size, in the volume-based particle size distribution of the material; Dv50 refers to the particle size at which 50% of the volume is accumulated, measured from the smallest particle size, in the volume-based particle size distribution of the material.

[0053] In one embodiment of the present application, the positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide or lithium iron phosphate. The above-mentioned lithium nickel cobalt manganese oxide may include LiNi 0.95 Co 0.03 Mn 0.02 O2(Ni95),LiNi 0.91 Co 0.03 Mn 0.06 O2(Ni91), LiNi 0.8 Co 0.1 Mn 0.1 O2(NCM811), LiNi 0.6 Co 0.2 Mn 0.2 O2(NCM622),LiNi 0.5 Co 0.2 Mn 0.3 O2(NCM523) or LiNi 1 / 3 Co 1 / 3 Mn 1 / 3 At least one of O2 (NCM111). By selecting the above positive electrode active materials, the secondary battery has high energy density and good cycle performance and safety performance.

[0054] The present application has no particular restrictions on the method for regulating the weight percentage of the additive, as long as the purpose of the present application can be achieved. For example, the weight percentage of the additive can be regulated by regulating the amount of the additive added.

[0055] In the present application, the compaction density of the positive electrode material layer can be controlled by methods known to those skilled in the art. For example, the compaction density of the positive electrode material layer can be controlled by controlling the cold pressing pressure during the cold pressing process. For example, when other conditions remain unchanged, increasing the cold pressing pressure increases the compaction density of the positive electrode material layer; decreasing the cold pressing pressure decreases the compaction density of the positive electrode material layer.

[0056] In the present application, the coating weight of the positive electrode material layer can be controlled by methods known to those skilled in the art. For example, when the positive electrode slurry is coated on the surface of the positive electrode current collector, the coating amount of the positive electrode slurry can be increased based on a certain solid content of the positive electrode slurry to increase the coating weight of the positive electrode material layer. This application is not particularly limited, as long as the purpose of this application can be achieved.

[0057] The present application does not particularly limit the method for regulating the thickness of the positive electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode material layer can be regulated by regulating the compaction density of the positive electrode material layer.

[0058] The present application does not particularly limit the method for adjusting the thickness of the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, commercially available positive electrode current collectors of different thicknesses can be selected.

[0059] The present application has no particular restrictions on the method for regulating the mass percentage of the positive electrode binder, as long as the purpose of the present application can be achieved. For example, the mass percentage of the positive electrode binder can be regulated by regulating the amount of the positive electrode binder added.

[0060] The present application does not particularly limit the method for regulating the value of X / W, as long as the purpose of the present application can be achieved. For example, the value of X / W can be regulated by regulating the values of X and W, and the regulation method is as described above.

[0061] The present application does not particularly limit the method for regulating the bonding force between the positive electrode material layer and the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, the bonding force between the positive electrode material layer and the positive electrode current collector can be regulated by regulating the amount of the positive electrode binder and / or additive added.

[0062] The present application does not particularly limit the method for regulating the particle size Dv10 and Dv50 of the positive electrode active material, as long as the purpose of the present application can be achieved. For example, the Dv10 and Dv50 of the positive electrode active material can be regulated by grinding the positive electrode active material. For example, the Dv10 of the positive electrode active material can be regulated by regulating the grinding time; the Dv50 of the positive electrode active material can be regulated by regulating the grinding time. For example, when other conditions remain unchanged, extending the grinding time will reduce the Dv10 of the positive electrode active material and reduce the Dv50 of the positive electrode active material; shortening the grinding time will increase the Dv10 of the positive electrode active material and increase the Dv50 of the positive electrode active material.

[0063] The present application does not particularly limit the method for regulating the Dv10 / Dv50 value, as long as the purpose of the present application can be achieved. For example, the Dv10 / Dv50 value can be regulated by regulating the respective values of Dv10 and Dv50, and the regulation method is as described above.

[0064] The present application has no particular limitation on the positive electrode current collector, as long as the purpose of the present application can be achieved. For example, it may include aluminum foil, aluminum alloy foil or a composite current collector (such as an aluminum-carbon composite current collector).

[0065] The positive electrode material layer of the present application also includes a positive electrode conductive agent. The present application has no particular restrictions on the positive electrode conductive agent, as long as the purpose of the present application can be achieved. For example, the positive electrode conductive agent may include but is not limited to at least one of conductive carbon black, carbon nanotubes (CNTs), carbon fibers, flake graphite, graphene, metal materials or conductive polymers. The above-mentioned conductive carbon black may include but is not limited to at least one of Super P, acetylene black or Ketjen black. The above-mentioned carbon nanotubes may include but is not limited to single-walled carbon nanotubes and / or multi-walled carbon nanotubes. The above-mentioned carbon fibers may include but are not limited to vapor-grown carbon fibers (VGCF) and / or nano-carbon fibers. The above-mentioned metal materials may include but are not limited to metal powder and / or metal fibers. Specifically, the metal may include but is not limited to at least one of copper, nickel, aluminum or silver. The above-mentioned conductive polymer may include but is not limited to at least one of polyphenylene derivatives, polyaniline, polythiophene, polyacetylene or polypyrrole. The present application has no particular restrictions on the mass percentage of the positive electrode active material and the positive electrode conductive agent in the positive electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved.

[0066] The second aspect of the present application provides a secondary battery, which includes the positive electrode sheet of any of the aforementioned embodiments. Therefore, the secondary battery provided by the present application has high energy density and good cycle performance and safety performance.

[0067] In the present application, the secondary battery also includes a negative electrode plate. The negative electrode plate includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. The above-mentioned "negative electrode material layer provided on at least one surface of the negative electrode current collector" means that the negative electrode material layer can be provided on one surface of the negative electrode current collector along its own thickness direction, or on two surfaces of the negative electrode current collector along its own thickness direction. It should be noted that the "surface" here can be the entire area of the negative electrode current collector or a partial area of the negative electrode current collector. This application is not particularly limited, as long as the purpose of this application can be achieved.

[0068] The present application has no particular limitation on the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, it can include copper foil, copper alloy foil, nickel foil, stainless steel foil, titanium foil, foam nickel, foam copper or a composite current collector.

[0069] The negative electrode material layer of the present application includes a negative electrode active material. The present application has no particular limitation on the negative electrode active material, as long as the purpose of the present application can be achieved. For example, the negative electrode active material may include natural graphite, artificial graphite, mesophase microcarbon beads (MCMB), hard carbon, soft carbon, silicon, silicon-carbon composite, SiO x (0.5<x<1.6), Li-Sn alloy, Li-Sn-O alloy, Sn, SnO, SnO2, spinel structure lithium titanate Li4Ti5O 12 , Li-Al alloy or metallic lithium. The negative electrode material layer of the present application also includes a negative electrode binder and a negative electrode conductive agent. The present application has no special restrictions on the negative electrode binder and the negative electrode conductive agent in the negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the negative electrode binder may include but is not limited to at least one of the above-mentioned positive electrode binders, and the negative electrode conductive agent may include but is not limited to at least one of the above-mentioned positive electrode conductive agents. The present application has no special restrictions on the mass ratio of the negative electrode active material, the negative electrode binder and the negative electrode conductive agent in the negative electrode material layer. Those skilled in the art can choose according to actual needs, as long as the purpose of the present application can be achieved.

[0070] The present application does not particularly limit the thickness of the negative electrode current collector, as long as the purpose of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 20 μm. The present application does not particularly limit the thickness of the negative electrode material layer, as long as the purpose of the present application can be achieved. For example, the thickness of the single-sided negative electrode material layer is 30 μm to 250 μm.

[0071] Optionally, the negative electrode plate may further include a conductive layer, which is located between the negative electrode current collector and the negative electrode material layer. The present application does not particularly limit the composition of the conductive layer, and it can be a conductive layer commonly used in the art. The conductive layer includes a conductive agent and a binder. The present application does not particularly limit the conductive agent and binder in the conductive layer, and it can be at least one of the above-mentioned negative electrode conductive agent and the above-mentioned negative electrode binder. The present application does not particularly limit the mass ratio of the conductive agent and binder in the conductive layer, and those skilled in the art can select according to actual needs, as long as the purpose of the present application can be achieved.

[0072] In the present application, the secondary battery also includes an electrolyte. The electrolyte includes a lithium salt and a non-aqueous solvent. The lithium salt may include various lithium salts commonly used in the art, such as at least one of LiPF6, LiBF4, LiAsF6, LiClO4, LiB(C6H5)4, LiCH3SO3, LiCF3SO3, LiN(SO2CF3)2, LiC(SO2CF3)3, Li2SiF6, lithium bis(oxalatoborate) (LiBOB) or lithium difluoroborate. The present application has no particular restrictions on the non-aqueous solvent, as long as the purpose of the present application can be achieved. For example, it may include but is not limited to at least one of a carbonate compound, a carboxylate compound, an ether compound or other organic solvent. The above-mentioned carbonate compound may include but is not limited to at least one of a chain carbonate compound, a cyclic carbonate compound or a fluorocarbonate compound. The above-mentioned chain carbonate compound may include but is not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC) or ethyl methyl carbonate (EMC). The cyclic carbonate compound may include but is not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) or vinyl ethylene carbonate (VEC). The fluorinated carbonate compound may include but is not limited to at least one of fluorinated ethylene carbonate (FEC), 1,2-difluoroethylene carbonate, 1,1-difluoroethylene carbonate, 1,1,2-trifluoroethylene carbonate, 1,1,2,2-tetrafluoroethylene carbonate, 1-fluoro-2-methylethylene carbonate, 1-fluoro-1-methylethylene carbonate, 1,2-difluoro-1-methylethylene carbonate, 1,1,2-trifluoro-2-methylethylene carbonate or trifluoromethylethylene carbonate. The carboxylate compound may include but is not limited to at least one of methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolactone, valerolactone or caprolactone. The above-mentioned ether compound may include, but is not limited to, at least one of dibutyl ether, tetraglyme, diglyme, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, or tetrahydrofuran. The above-mentioned other organic solvents may include, but are not limited to, at least one of dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate. The present application does not particularly limit the weight percentage of the lithium salt and the non-aqueous solvent, as long as the purpose of the present application can be achieved.

[0073] In the present application, the secondary battery also includes a diaphragm. The diaphragm is used to separate the positive electrode plate and the negative electrode plate, prevent internal short circuit of the secondary battery, allow electrolyte ions to pass freely, and do not affect the electrochemical charge and discharge process. The present application has no special restrictions on the diaphragm, as long as the purpose of the present application can be achieved. For example, the material of the diaphragm may include but is not limited to polyethylene (PE), polypropylene (PP)-based polyolefins (PO), polyesters (for example, polyethylene terephthalate (PET) film), cellulose, polyimide (PI), polyamide (PA), spandex or aramid; the type of diaphragm may include at least one of a woven membrane, a non-woven membrane, a microporous membrane, a composite membrane, a rolled membrane or a spun membrane.

[0074] In the present application, the diaphragm may include a base film and a surface treatment layer. The base film may be a non-woven fabric or a composite film having a porous structure, and the material of the base film may include at least one of polyethylene, polypropylene, polyethylene terephthalate or polyimide. Optionally, a polypropylene porous film, a polyethylene porous film, a polypropylene non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film may be used. Optionally, a surface treatment layer is provided on at least one surface of the base film, and the surface treatment layer may be a polymer layer or an inorganic layer, or a layer formed by a mixed polymer and an inorganic substance. For example, the inorganic layer includes inorganic particles and a binder for the diaphragm. The present application does not particularly limit the above-mentioned inorganic particles, and for example, it may include at least one of aluminum oxide, silicon oxide, magnesium oxide, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide or barium sulfate. The present application does not particularly limit the above-mentioned separator binder, for example, it can be at least one of the above-mentioned positive electrode binders. The polymer layer includes a polymer, and the polymer material includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyvinyl pyrrolidone, polyvinyl ether, polyvinylidene fluoride, or poly(vinylidene fluoride-hexafluoropropylene).

[0075] The secondary battery of this application also includes a packaging bag for containing the positive electrode sheet, separator, negative electrode sheet, and electrolyte, as well as other components of the secondary battery known in the art. This application does not limit these other components. This application does not specifically limit the packaging bag and can be any packaging bag known in the art, as long as it can achieve the purpose of this application.

[0076] The present application does not particularly limit the type of secondary battery, which may include any device that generates an electrochemical reaction. In the present application, secondary batteries may include but are not limited to: lithium metal secondary batteries, lithium ion secondary batteries (lithium ion batteries), lithium polymer secondary batteries or lithium ion polymer secondary batteries, etc.

[0077] The preparation process of the secondary battery of the present application is well known to those skilled in the art, and is not particularly limited in the present application. For example, it may include but is not limited to the following steps: stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and winding, folding and other operations as needed to obtain an electrode assembly with a wound structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery; or stacking the positive electrode sheet, the separator and the negative electrode sheet in order, and then fixing the four corners of the entire stacked structure with tape to obtain an electrode assembly with a stacked structure, placing the electrode assembly in a packaging bag, injecting the electrolyte into the packaging bag and sealing it to obtain a secondary battery. In addition, overcurrent protection elements, guide plates, etc. may also be placed in the packaging bag as needed to prevent the pressure inside the secondary battery from rising and overcharging and discharging. Among them, the packaging bag is a packaging bag known in the art, and the present application does not limit this.

[0078] The third aspect of the present application provides an electronic device, which includes the secondary battery according to any of the aforementioned embodiments. Therefore, the electronic device provided by the present application has high energy density and good cycle performance and safety performance.

[0079] The present application does not particularly limit the type of electronic device, and the electronic device may be any electronic device known in the art. In some embodiments, the electronic device may include, but is not limited to, a laptop computer, a pen-type computer, a mobile computer, an electronic book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a headset, a video recorder, an LCD television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, a power tool, a flashlight, a camera, a large household battery, and a lithium-ion capacitor.

[0080] Example

[0081] The following examples and comparative examples are provided to more specifically illustrate the embodiments of the present invention. Various tests and evaluations were performed according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0082] Test methods and equipment:

[0083] Types of additives and their mass percentage test:

[0084] (1) disassembling the lithium-ion battery, taking out the positive electrode sheet, soaking the positive electrode sheet in N-methylpyrrolidone (NMP), centrifuging and drying the upper layer solution to obtain a mixture of a positive electrode binder, a positive electrode conductive agent and an additive;

[0085] (2) taking the mixture obtained in step (1), performing thermogravimetric analysis (TG, instrument model: synchronous thermal analyzer STA449F3), the test temperature: 25°C to 600°C, and calculating the weight loss; wherein, the weight percentage of the decomposition substance between 25°C and 400°C is the mass percentage of the additive;

[0086] (3) By performing infrared testing on the mixture obtained in step (1), the infrared spectrum of the additive shows a peak at 1710 cm -1 to 1725cm -1 , 2500cm -1 to 3300cm -1 The infrared characteristic peak of the above mixture is located at 1710cm -1 to 1725cm -1 , 2500cm -1 to 3300cm -1 If the infrared characteristic peak of is detected, the positive electrode material layer contains additives;

[0087] (4) Performing a DSC melting point test on the mixture obtained in step (1), if there is a characteristic melting peak only between 60°C and 80°C, it indicates that the additive in the positive electrode material layer is stearic acid; if there is a characteristic melting peak only between 100°C and 120°C and between 200°C and 230°C, it indicates that the additive in the positive electrode material layer is stearate; if there are characteristic melting peaks between 60°C and 80°C and between 100°C and 120°C and between 200°C and 230°C, it indicates that the additive in the positive electrode material layer is stearate and stearic acid.

[0088] Compaction density test of positive electrode material layer:

[0089] Disassemble the lithium-ion battery, remove the positive electrode sheet, clean the positive electrode sheet with DMC, and dry the positive electrode sheet. Select the area on the positive electrode sheet including the double-sided positive electrode material layer, and cut 10 pieces with an area of 1540.25mm using a cutting machine. 2 Weigh the small discs and take the average value M. Then measure the thickness of the discs with a caliper and take the average value H3. Then wipe off the positive electrode material layer from 10 discs, weigh them, and take the average value m. Then measure the thickness of the positive electrode current collector with a caliper and take the average value H2. The compacted density of the positive electrode material layer, PD, = (Mm) / [1540.25 × (H3 - H2)].

[0090] Melting point test:

[0091] The melting point of the additive was tested by differential scanning calorimetry (DSC). An aluminum crucible was used for the test. The weight of the additive sample was 10 mg. The test temperature range was from room temperature to 400°C. The heating rate was 10°C / min.

[0092] Thermogravimetric testing:

[0093] Disassemble the lithium-ion battery, remove the positive electrode sheet, clean the positive electrode sheet with DMC, and dry the positive electrode sheet. Use a knife to scrape off the positive electrode material layer powder, weigh 2mg of the powder sample, and perform thermogravimetric analysis. Place the powder sample in a small crucible of a synchronous thermal analyzer (model STA449F3). Turn on the synchronous thermal analyzer and open N2 to purge the synchronous thermal analyzer at a rate of 20mL / min. Set the program temperature range from 25℃ to 500℃, and the heating rate at 5℃ / min until the program is completed. Output the thermogravimetric curve.

[0094] Adhesion test:

[0095] (1) Disassemble the lithium-ion battery, remove the positive electrode, clean the positive electrode with DMC, and then dry the positive electrode;

[0096] (2) Take the dried positive electrode sheet and cut a sample with a width of 30 mm and a length of 100 mm using a blade;

[0097] (3) Apply special double-sided tape to the steel plate with a width of 20 mm and a length of 90 mm;

[0098] (4) Stick the sample cut in step (2) on the double-sided tape with the test surface facing downward;

[0099] (5) Insert a paper tape with a width equal to the width of the specimen and a length of 180 mm under the specimen and secure it with wrinkle glue;

[0100] (6) Turn on the power of the Sansi tensile testing machine, the indicator light will light up, and adjust the position of the limit block;

[0101] (7) Fix the sample prepared in step (5) on the test bench, set the speed to 10 mm / min, the test range to 0 mm to 40 mm, and pull the paper tape in a 90° direction until the test is completed to obtain the bonding force between the positive electrode material layer and the positive electrode current collector.

[0102] Particle size test:

[0103] (1) Turn on the equipment: first turn on the equipment sampling system, turn on the optical system and computer, and preheat the equipment for 30 minutes;

[0104] (2) Cleaning the injection system: Fill the injector with water and repeat the cleaning three times to ensure that the injector is clean;

[0105] (3) Enter the "Manual Measurement" interface and set parameters such as material name, refractive index, material type, test time, and number of tests in sequence;

[0106] (4) Click "Start" to measure the light and background light;

[0107] (5) Disperse 0.2 g of the positive electrode active material in 10 mL of aqueous solution and test using a laser diffraction / scattering particle size distribution analyzer (Model Master Sizer 3000). Add the above sample to the sample cell. The shading increases with the amount of sample added. When the shading increases to 8% to 12%, stop adding the sample. Wait until the shading remains stable (no fluctuation in the value for 30 seconds), click "Start" to test the particle size and obtain the Dv10 and Dv50 values of the positive electrode active material.

[0108] (6) Test three parallel samples separately and calculate the average values of Dv10 and Dv50.

[0109] Positive electrode brittle fracture test:

[0110] At 25°C and 40% RH, the positive electrode sheets in the examples and comparative examples were 3 The positive electrode sheet is cold pressed with a compaction density of 100 mm, and then cut into 20 mm × 100 mm positive electrode sheets, which are folded in half. A 2 kg roller is used to roll the folded positive electrode sheet once. After that, the positive electrode sheet is spread out and observed against the light, and the number of light-transmitting points is recorded as Q. The fewer the number of light-transmitting points, the lower the degree of brittle fracture of the positive electrode sheet.

[0111] Specific surface area test of positive electrode particle crushing:

[0112] (1) Take the cold-pressed positive electrode sheets from the examples and comparative examples and cut them into 10 cm × 10 cm sizes. Prepare 20 positive electrode sheet samples for each group.

[0113] (2) Performing high-temperature calcination treatment: placing the positive electrode sample prepared in step (1) in a suspended state in a muffle furnace, setting the temperature to 600°C ± 5°C, heating rate to 5°C / min, and treatment time to 3 hours;

[0114] (3) After the high-temperature calcination treatment, cool it down to room temperature, remove the positive electrode sample, use a brush to brush off the positive electrode material layer powder, and collect the positive electrode material layer powder for later use;

[0115] (4) The specific surface area test was conducted in accordance with the national standard “Determination of Specific Surface Area of Solid Substances by Gas Adsorption BET Method” (GB / T 19587-2017). The test steps are as follows:

[0116] a. Take 20 g of the positive electrode material layer powder, degas in a vacuum at 200 ° C for 2 h, and weigh the actual positive electrode material layer powder mass;

[0117] b. Place the cathode material layer powder into the filling rod and then load it into the analysis station;

[0118] c. Open the test software to conduct an operation test;

[0119] d. Output the operation report and confirm the specific surface area data;

[0120] e. Repeat steps (a) to (d) three times, add parallel sample tests, and take the average value as the specific surface area of the broken positive electrode particles.

[0121] Among them, the larger the specific surface area of the broken positive electrode sheet particles, the more broken positive electrode active material particles are, and the poorer the flexibility of the positive electrode sheet; the smaller the specific surface area of the broken positive electrode sheet particles, the fewer broken positive electrode active material particles are, and the better the flexibility of the positive electrode sheet.

[0122] Cyclic performance test:

[0123] The lithium-ion battery in the embodiment or comparative example is placed in an environment of 25°C and charged at a constant current of 0.5C to a set voltage (the voltage setting value is 4.5V when the positive electrode active material is lithium cobalt oxide, the voltage setting value is 4.2V when the positive electrode active material is lithium nickel cobalt manganese oxide, and the voltage setting value is 3.6V when the positive electrode active material is lithium iron phosphate). Then, the battery is charged at a constant voltage to a cutoff current of 0.05C, allowed to stand for 5 minutes, and discharged at a constant current of 0.5C to a set voltage (the voltage setting value is 3.0V when the positive electrode active material is lithium cobalt oxide, the voltage setting value is 2.8V when the positive electrode active material is lithium nickel cobalt manganese oxide, and the voltage setting value is 2.5V when the positive electrode active material is lithium iron phosphate). After standing for 5 minutes, the discharge capacity of the first cycle is recorded. Then, 800 cycles of charge and discharge are performed using the same steps, and the discharge capacity of the 800th cycle is recorded.

[0124] 800-cycle capacity retention rate of lithium-ion battery (%) = (discharge capacity at the 800th cycle / discharge capacity at the first cycle) × 100%.

[0125] Hot box test:

[0126] (1) Take 10 lithium-ion batteries from each group of the examples or comparative examples, and at a temperature of 20±5°C, charge the lithium-ion batteries at a constant current (CC) of 1C until the voltage reaches a set value (the voltage setting value is 4.5V when the positive electrode active material is lithium cobalt oxide, the voltage setting value is 4.2V when the positive electrode active material is lithium nickel cobalt manganese oxide, and the voltage setting value is 3.6V when the positive electrode active material is lithium iron phosphate), and then charge them at a constant voltage (CV) until the cutoff current reaches 0.05C; let them stand for 5 minutes;

[0127] (2) Inspect the appearance of the lithium-ion battery and take photos. Then attach a temperature sensor to the center of the lithium-ion battery and place 10 lithium-ion battery samples vertically in the box. Heat the temperature to 135±2℃ at a rate of 5±2℃ and maintain for 60 minutes. Observe the appearance of the lithium-ion battery and take photos. If the lithium-ion battery does not catch fire or explode, it is considered to have passed the hot box test.

[0128] The hot box test results reflect the safety performance of lithium-ion batteries. The more lithium-ion batteries that pass the hot box test, the better the safety performance of the lithium-ion batteries.

[0129] Example 1-1

[0130] <Preparation of positive electrode sheet>

[0131] The positive electrode active material, lithium cobalt oxide (LiCoO2), the positive electrode conductive agent, acetylene black, the positive electrode binder, polyvinylidene fluoride (PVDF), and the additive, stearic acid, were mixed in a weight ratio of 97.52:1.3:1.1:0.08, and N-methylpyrrolidone (NMP) was added as a solvent. The mixture was stirred and mixed to obtain a positive electrode slurry having a solid content of 70 wt %. The positive electrode slurry was evenly coated on one surface of a 12 μm thick positive electrode current collector aluminum foil and dried at 120° C. for 1 hour to obtain a positive electrode sheet coated with a positive electrode material layer on one side. The coating weight CW of the positive electrode material layer was 0.025 g / cm 2 Repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode sheet coated with a positive electrode material layer on both sides. Dry under vacuum conditions at 120°C for 1 hour, then cold press, cut and slit to obtain a positive electrode sheet with a specification of 74mm×867mm. The compacted density PD of the positive electrode material layer is 4.25g / cm 3 The thickness H1 of the single-sided positive electrode material layer after cold pressing is 58.8μm.

[0132] <Preparation of negative electrode sheet>

[0133] The negative electrode active material artificial graphite, the negative electrode binder sodium carboxymethyl cellulose (CMC-Na), and the negative electrode binder styrene-butadiene rubber (SBR) were mixed in a weight ratio of 95:2:3, and deionized water was added as a solvent. The mixture was stirred and mixed to obtain a negative electrode slurry, wherein the solid content of the negative electrode slurry was 75wt%. The negative electrode slurry was evenly coated on one surface of a negative electrode current collector copper foil with a thickness of 12μm, and dried at 120°C to obtain a negative electrode sheet with a negative electrode material layer coated on one side with a thickness of 120μm. The above steps were repeated on the other surface of the copper foil to obtain a negative electrode sheet with a negative electrode material layer coated on both sides. After drying under vacuum conditions at 120°C for 1h, the negative electrode sheet with a specification of 78mm×875mm was obtained after cold pressing, cutting, and slitting. The compaction density of the negative electrode material layer is 1.75g / cm 3 .

[0134] <Preparation of Electrolyte>

[0135] In an argon atmosphere glove box with a water content of less than 10 ppm, ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a weight ratio of 1:1:1 to create a base solvent. Lithium hexafluorophosphate (LiPF6) was then added and mixed thoroughly to create an electrolyte. The lithium salt content was 12.5% by weight of the electrolyte, with the remainder being the base solvent.

[0136] <Preparation of Separator>

[0137] A polyethylene (PE) film with a thickness of 15 μm is used.

[0138] <Preparation of lithium-ion batteries>

[0139] The prepared positive electrode sheet, separator, negative electrode sheet, and separator are stacked in order, with the separator positioned between the positive and negative electrode sheets to provide isolation. The electrodes are then wound to form an electrode assembly. After welding the tabs, the electrode assembly is placed in an aluminum-plastic film packaging bag, dried, and then filled with electrolyte. The lithium-ion battery is then vacuum packaged, allowed to stand, formed, degassed, and trimmed.

[0140] Example 1-2 to Example 1-28

[0141] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0142] Example 2-1 to Example 2-4

[0143] Except for adjusting the coating amount of the positive electrode slurry so that the coating weight of the positive electrode material layer is as shown in Table 2, the rest is the same as Example 1-1.

[0144] Example 2-5 to Example 2-10

[0145] The same procedures as in Example 1-1 were used except that the cold pressing pressure during the cold pressing process was adjusted to achieve the compaction density of the positive electrode material layer as shown in Table 2 and the relevant preparation parameters were adjusted according to Table 2. When the weight percentage of the additives changes, the weight percentage of the positive electrode active material also changes, while the weight percentages of the positive electrode conductive agent and the positive electrode binder remain unchanged.

[0146] Example 2-11 to Example 2-13

[0147] Except for adjusting the relevant preparation parameters according to Table 2, the rest is the same as Example 1-1.

[0148] Comparative Examples 1 to 5

[0149] Except for adjusting the relevant preparation parameters according to Table 1, the rest is the same as Example 1-1.

[0150] The preparation parameters and electrical performance parameters of each embodiment and comparative example are shown in Tables 1 and 2.

[0151]

[0152]

[0153]

[0154]

[0155] From Examples 1-1 to 1-28 and Comparative Examples 1 to 5, it can be seen that when the positive electrode material layer includes the above-mentioned additives and the mass percentage of the additives is within the range of this application, the number of light-transmitting points on the positive electrode sheet is small, the specific surface area of the broken positive electrode sheet particles is small, the lithium-ion battery has a high cycle capacity retention rate after 800 cycles, and a large number of hot box test passes, indicating that the positive electrode sheet has good flexibility, and the lithium-ion battery has a high energy density while having good cycle performance and safety performance. In Comparative Example 1, the positive electrode material layer does not include the above-mentioned additives, and the mass percentage of the additives in Comparative Examples 2 to 5 is not within the range of this application. The number of light-transmitting points on the positive electrode sheet is large, the specific surface area of the broken positive electrode sheet particles is large, the lithium-ion battery has a low cycle capacity retention rate after 800 cycles, and a small number of hot box test passes, indicating that the flexibility of the positive electrode sheet is poor, and the cycle performance and safety performance of the lithium-ion battery are poor.

[0156] The type of positive electrode binder generally affects the cycling performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-19, and 1-20, by adjusting the type of positive electrode binder within the scope of this application, the number of light-transmitting points on the positive electrode sheet is reduced, the specific surface area of the broken positive electrode sheet particles is reduced, the lithium-ion battery has a high capacity retention rate after 800 cycles, and a high number of hot box test passes, indicating that the positive electrode sheet has good flexibility, and the lithium-ion battery has high energy density while also having good cycling performance and safety performance.

[0157] The mass percentage of the positive electrode binder generally affects the cycling performance of lithium-ion batteries. As can be seen from Examples 1-1, 1-21, and 1-22, by adjusting the mass percentage of the positive electrode binder within the scope of this application, the number of light-transmitting points on the positive electrode sheet is reduced, the specific surface area of the broken positive electrode sheet particles is reduced, the lithium-ion battery has a high capacity retention rate after 800 cycles, and a high number of cells pass the hot box test, indicating that the positive electrode sheet has good flexibility, and the lithium-ion battery has high energy density while also having good cycling performance and safety performance.

[0158] The value of X / W usually affects the flexibility of the positive electrode sheet, the cycle performance and safety performance of the lithium-ion battery. From Examples 1-1, 1-8, 1-9, 1-12 to 1-18, and 1-21 to 1-23, it can be seen that by adjusting the value of X / W within the scope of this application, the number of light-transmitting points on the positive electrode sheet is small, the specific surface area of the broken positive electrode sheet particles is small, the lithium-ion battery has a high capacity retention rate after 800 cycles, and a large number of hot box test passes, indicating that the positive electrode sheet has good flexibility, and the lithium-ion battery has a high energy density while having good cycle performance and safety performance.

[0159] The particle size Dv10 / Dv50 of the positive electrode active material generally affects the energy density of lithium-ion batteries. As can be seen from Examples 1-1, 1-24, and 1-27, by adjusting the particle size Dv10 / Dv50 of the positive electrode active material within the scope of this application, the number of light-transmitting points on the positive electrode sheet is reduced, the specific surface area of the broken positive electrode sheet particles is reduced, the lithium-ion battery has a high capacity retention rate after 800 cycles, and a high number of hot box test passes, indicating that the positive electrode sheet has good flexibility, and the lithium-ion battery has high energy density while also having good cycling performance and safety performance.

[0160] It can be seen from Examples 1-1 and 1-28 that by regulating the type of positive electrode active material within the scope of this application, the number of light-transmitting points on the positive electrode sheet is small, the specific surface area of the broken positive electrode sheet particles is small, the lithium-ion battery has a high cycle capacity retention rate of 800 cycles, and a large number of hot box test passes, indicating that the positive electrode sheet has good flexibility, and the lithium-ion battery has a high energy density while having good cycle performance and safety performance.

[0161] Table 2

[0162]

[0163] The coating weight of the positive electrode material layer generally affects the flexibility of the positive electrode sheet, the cycling performance, and the safety performance of the lithium-ion battery. As can be seen from Examples 1-1, 2-1, and 2-4, by adjusting the coating weight of the positive electrode material layer within the scope of this application, the number of light-transmitting points on the positive electrode sheet is reduced, the specific surface area of the broken positive electrode sheet particles is reduced, the lithium-ion battery has a high capacity retention rate after 800 cycles, and a high number of hot box test passes, indicating that the positive electrode sheet has good flexibility, and the lithium-ion battery has a high energy density while also having good cycling performance and safety performance.

[0164] The compaction density of the positive electrode material layer usually affects the flexibility of the positive electrode sheet, the cycle performance and safety performance of the lithium-ion battery. It can be seen from Example 1-1, Example 2-5 to Example 2-10 that by regulating the compaction density of the positive electrode material layer within the scope of this application, the number of light-transmitting points on the positive electrode sheet is small, the specific surface area of the broken positive electrode sheet particles is small, the lithium-ion battery has a high cycle capacity retention rate of 800 cycles, and a large number of hot box test passes, indicating that the positive electrode sheet has good flexibility, and the lithium-ion battery has a high energy density while having good cycle performance and safety performance. In Example 1-1, Example 2-5 to Example 2-10, the compaction density of the positive electrode material layer of Example 2-5 is small, and the thickness of the positive electrode material layer is thick, which will affect the energy density of the lithium-ion battery.

[0165] The thickness of the positive electrode material layer generally affects the flexibility of the positive electrode sheet, the cycling performance, and the safety performance of the lithium-ion battery. As can be seen from Examples 1-1, 2-1, and 2-10, by adjusting the thickness of the positive electrode material layer within the scope of this application, the number of light-transmitting points on the positive electrode sheet is reduced, the specific surface area of the broken positive electrode sheet particles is reduced, the lithium-ion battery has a high capacity retention rate after 800 cycles, and a high number of hot box test passes, indicating that the positive electrode sheet has good flexibility, and the lithium-ion battery has a high energy density while also having good cycling performance and safety performance.

[0166] The thickness of the positive electrode current collector usually affects the safety performance of lithium-ion batteries. From Examples 1-1, 2-11 to 2-13, it can be seen that by regulating the thickness of the positive electrode current collector within the scope of this application, the number of light-transmitting points on the positive electrode sheet is small, the specific surface area of the broken positive electrode sheet particles is small, the lithium-ion battery has a high capacity retention rate after 800 cycles, and a large number of hot box test passes, indicating that the positive electrode sheet has good flexibility, and the lithium-ion battery has a high energy density while having good cycle performance and safety performance. Among Examples 1-1, 2-11 to 2-13, the thickness of the positive electrode current collector in Example 2-13 is relatively thick, which will affect the energy density of the lithium-ion battery.

[0167] It should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, or article comprising a list of elements includes not only those elements but also other elements not explicitly listed, or elements inherent to such process, method, or article.

[0168] Each embodiment in this specification is described in a related manner. The same or similar parts between the embodiments can be referred to each other. Each embodiment focuses on the differences from other embodiments.

[0169] The above description is only a preferred embodiment of the present application and is not intended to limit the present application. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A positive electrode plate, comprising a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode material layer comprises a positive electrode active material, a positive electrode binder, and an additive, wherein the additive is at least one of a compound of formula I and a compound of formula II. in, 9≤n≤20, R1 is selected from H, Li, Ca, Mg, Zn, Al, Fe, Ba or Co, R2 is selected from C9 to C 20 alkenylene; Based on the mass of the positive electrode material layer, the mass percentage of the additive is W%, and 0.01≤W≤2.

2. The positive electrode sheet according to claim 1, wherein: The compaction density of the positive electrode material layer is PD g / cm 3 , 3.0≤PD≤4.

5.

3. The positive electrode sheet according to claim 2, wherein: PD≥4.5529×W 0.0209 。 4. The positive electrode sheet according to claim 1, wherein: The coating weight of the positive electrode material layer is CW g / cm 2 , 0.018≤CW≤0.

039.

5. The positive electrode sheet according to claim 1, wherein: The thickness of the positive electrode material layer is H1 μm, 30≤H1≤110.

6. The positive electrode sheet according to claim 1, wherein: The thickness of the positive electrode current collector is H2 μm, 7≤H2≤20.

7. The positive electrode sheet according to claim 1, wherein: The compound of formula I includes at least one of undecanoic acid, lauric acid, tridecanoic acid, myristic acid, pentadecanoic acid, palmitic acid, pearlic acid, stearic acid, lithium stearate, calcium stearate, magnesium stearate, zinc stearate, aluminum stearate, iron stearate, barium stearate or cobalt stearate.

8. The positive electrode sheet according to claim 1, wherein: The compound of formula II includes at least one of myristoleic acid, oleic acid or octadecenoic acid.

9. The positive electrode sheet according to claim 1, wherein: The melting point of the additive is Tm°C, 10≤Tm≤230.

10. The positive electrode sheet according to claim 1, wherein: The true density of the additive is pg / cm 3 , 0.7≤p≤1.

3.

11. The positive electrode sheet according to claim 1, wherein: The thermogravimetric curve of the positive electrode sheet has a first decomposition peak at 150° C. to 220° C. and / or a second decomposition peak at 250° C. to 350° C.

12. The positive electrode sheet according to claim 1, wherein: Based on the mass of the positive electrode material layer, the mass percentage of the positive electrode binder is X%, and 0.6≤X≤2.

5.

13. The positive electrode sheet according to claim 12, wherein: The positive electrode binder includes at least one of polyvinylidene fluoride, polytetrafluoroethylene, vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, tetrafluoroethylene-hexafluoropropylene copolymer, fluorine-containing acrylate resin, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinyl alcohol, polyacrylonitrile, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polyethylene, polypropylene, ethylene-propylene-diene polymer, sulfonated ethylene-propylene-diene polymer, carboxymethyl cellulose, styrene-butadiene rubber or fluororubber.

14. The positive electrode sheet according to claim 12, wherein: 7.5≤X / W≤120.

15. The positive electrode sheet according to claim 1, wherein: The bonding force between the positive electrode material layer and the positive electrode current collector is FN / m, and F≥25.

16. The positive electrode sheet according to claim 1, wherein: The particle size of the positive electrode active material satisfies: 0.25≤Dv10 / Dv50≤0.

5.

17. The positive electrode sheet according to claim 1, which satisfies at least one of the following characteristics: (1)0.02≤W≤0.5； (2) The compaction density of the positive electrode material layer is PD g / cm 3 , 3.8≤PD≤4.4; (3) The thickness of the positive electrode material layer is H1 μm, 40≤H1≤80; (4) The thickness of the positive electrode current collector is H2 μm, 8≤H2≤12; (5) The melting point of the additive is Tm°C, 65≤Tm≤190; (6) The true density of the additive is pg / cm 3 , 0.85≤p≤1.15; (7) The particle size of the positive electrode active material satisfies the following: 0.33≤Dv10 / Dv50≤0.45; (8) The positive electrode active material includes at least one of lithium cobalt oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide or lithium iron phosphate. 18 . A secondary battery comprising the positive electrode sheet according to claim 1 .

19. An electronic device comprising the secondary battery according to claim 18.

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