Positive plate, preparation method thereof and battery

Abstract

The invention relates to the technical field of batteries, in particular to a positive plate, a preparation method thereof and a battery. The positive plate comprises a positive current collector, a polymer layer and a positive active material layer, the polymer layer is arranged on at least one surface of the positive current collector and comprises an expandable polymer, the expandable polymer is obtained by polymerizing a polymer matrix and a graft modifier, and the graft modifier comprises fluorine-containing olefin; the positive active material layer is arranged on the surface, away from the positive current collector, of the polymer layer; in the thickness direction of the positive plate, the projected area of the polymer layer is smaller than the projected areas of the positive active material layer and the positive current collector, and at least part of the positive active material layer is in direct contact with the positive current collector. The positive plate provided by the invention can effectively delay the occurrence of thermal runaway.

Description

Technical Field

[0001] The present application relates to the field of battery technology, and in particular to a positive electrode sheet, a preparation method thereof, and a battery. Background Art

[0002] Lithium iron phosphate has become one of the most promising cathode materials for lithium-ion batteries due to its high theoretical capacity, excellent electrochemical performance and thermodynamic stability, simple preparation method and low cost. However, with the high energy density of current batteries, it is difficult to further ensure safety during overcharging, and as the battery temperature rises due to overcharging, the battery is prone to thermal runaway. How to avoid such problems is a hot topic in current research. Summary of the invention

[0003] In view of this, the present invention provides a positive electrode sheet and a preparation method thereof and a battery. The positive electrode sheet can effectively delay the occurrence of thermal runaway.

[0004] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0005] In a first aspect, the present invention provides a positive electrode sheet, the positive electrode sheet comprising:

[0006] (a) positive electrode current collector;

[0007] (b) a polymer layer, the polymer layer being disposed on at least one surface of the positive electrode current collector;

[0008] The polymer layer includes an expandable polymer, which is obtained by polymerizing a polymer matrix and a graft modifier, and the graft modifier includes a fluorine-containing olefin;

[0009] (c) a positive electrode active material layer, the positive electrode active material layer being disposed on a surface of the polymer layer away from the positive electrode current collector;

[0010] In the thickness direction of the positive electrode sheet, the projected area of ​​the positive electrode current collector is recorded as S1, the projected area of ​​the polymer layer is recorded as S2, and the projected area of ​​the positive electrode active material layer is recorded as S3. S1, S2, and S3 satisfy the following relationship: S2<S3≤S1;

[0011] At least a portion of the positive electrode active material layer is in direct contact with the positive electrode current collector.

[0012] Preferably, the expansion temperature of the expandable polymer is 85-95°C.

[0013] Preferably, the thermal expansion rate of the expandable polymer is ≥ 20%.

[0014] Preferably, the melting point of the expandable polymer is 140-160°C.

[0015] In an embodiment of the present invention, the graft modifier includes at least one of hexafluoropropylene, hexafluoroethylene, and vinylidene fluoride.

[0016] Preferably, the resistivity of the polymer matrix is ​​greater than or equal to 10 MΩ / m.

[0017] In an embodiment of the present invention, the polymer matrix includes at least one of polyethylene, polyvinyl chloride, polypropylene, and polymethyl methacrylate.

[0018] Preferably, the mass ratio of the polymer matrix to the graft modifier is (10-30):(1-3).

[0019] In an embodiment of the present invention, the positive electrode active material layer includes a middle region and an edge region, the middle region is at least partially in direct contact with the polymer layer, and the edge region is in direct contact with the positive electrode current collector.

[0020] Preferably, the distance L1 between the edge of the polymer layer and the edge of the positive electrode active material layer is ≥3 mm.

[0021] Preferably, the ratio of S3 / S1 is 0.9-1.

[0022] Preferably, the ratio of S2 / S1 is 0.6 to 0.8.

[0023] Preferably, the thickness of the polymer layer is 0.5 to 1.2 μm.

[0024] In an embodiment of the present invention, the polymer layer comprises one or more sub-polymer layers.

[0025] Preferably, the number of the plurality of sub-polymer layers is ≥2.

[0026] In an embodiment of the present invention, the plurality of sub-polymer layers are evenly spaced and distributed.

[0027] Preferably, the distance L2 between two adjacent sub-polymer layers is ≥3 mm.

[0028] In an embodiment of the present invention, the polymer layer further comprises a thermally decomposed gas generating substance and / or an aqueous binder.

[0029] The thermal decomposition gas-generating substance includes at least one of sodium bicarbonate, lithium bicarbonate, ammonium bicarbonate, and lithium nitrate.

[0030] In an embodiment of the present invention, the aqueous binder includes at least one of polyacrylic acid PAA, polyethylene oxide PEO, and a propanol-based compound.

[0031] Preferably, the mass ratio of the expandable polymer to the thermally decomposed gas generating substance is (50-150):(1-10).

[0032] Preferably, the mass ratio of the swellable polymer to the aqueous binder is (1-3):(1-3).

[0033] Preferably, the average particle size of the swellable polymer is 1.2-1.5 μm.

[0034] Preferably, the roughness of the surface of the polymer layer away from the positive electrode current collector is Ra≥0.5 mm.

[0035] Preferably, the roughness of the surface of the positive electrode current collector is Ra≥0.5 mm.

[0036] In a second aspect, the present invention provides a method for preparing the above-mentioned positive electrode sheet, comprising the following steps:

[0037] S1, mixing a polymer matrix, a grafting modifier and an organic solvent, performing a polymerization reaction, and drying and grinding to obtain a swellable polymer;

[0038] S2, mixing a swellable polymer and water to obtain a slurry, and coating the slurry on at least one surface of a positive electrode current collector to obtain a positive electrode current collector composited with a polymer layer;

[0039] S3, disposing a positive electrode active material layer on a surface of the polymer layer away from the positive electrode current collector to obtain a positive electrode sheet.

[0040] In an embodiment of the present invention, in step S1, the organic solvent includes at least one of N-methylpyrrolidone and acetone.

[0041] Preferably, the polymerization reaction temperature is 90-100° C., and the polymerization reaction time is 4-6 hours.

[0042] In an embodiment of the present invention, when the polymer layer includes a thermally decomposed gas-generating substance, in step S1, mixing the polymer matrix, the grafting modifier and the organic solvent comprises: mixing the polymer matrix, the grafting modifier, the thermally decomposed gas-generating substance and the organic solvent.

[0043] In an embodiment of the present invention, when the polymer layer includes an aqueous binder, in step S2, mixing the swellable polymer and water is as follows: mixing the swellable polymer, the aqueous binder and water.

[0044] In a third aspect, the present invention provides a battery, comprising the above-mentioned positive electrode sheet, and / or the positive electrode sheet prepared by the above-mentioned preparation method.

[0045] Compared with the prior art, the present invention has the following beneficial effects:

[0046] (1) The positive electrode sheet of the present invention includes a polymer layer, which is located between the positive electrode current collector and the positive electrode active material layer, and has an area smaller than that of the positive electrode current collector and the positive electrode active material layer. The polymer layer contains an expandable polymer, which is obtained by polymerizing a polymer matrix and a graft modifier, and the graft modifier includes a fluorine-containing olefin.

[0047] See also Figure 5 When the battery temperature is low (normal working state), although the polymer layer is located in the middle, the area of ​​the positive current collector and the positive active material layer is larger than that of the polymer layer, and part of the positive current collector and the positive active material layer can be in direct contact (direct contact part) to achieve electrical connection; as the battery temperature rises (overcharge), the expandable polymer in the polymer layer expands. On the one hand, it expands in the thickness direction, causing the positive active material layer above the polymer layer and the positive active material layer above the positive current collector to break and dislocate, increasing the resistance of the positive electrode, reducing the conductivity, and reducing the risk of thermal runaway; on the other hand, the expandable polymer expands in the transverse direction (perpendicular to the thickness direction of the electrode sheet), narrowing the conductive channel, making it impossible for lithium ions to pass through the polymer layer smoothly, further increasing the resistance of the positive electrode, and reducing the conductivity and the risk of thermal runaway.

[0048] The expandable polymer is a modified polymer, and the graft modifier in the modified polymer is a fluorinated olefin. The fluorinated olefin can further increase the expansion rate of the expandable polymer, thereby more effectively causing the positive electrode active material layer above the polymer layer and the positive electrode active material layer above the positive electrode current collector to break and dislocate, further narrowing the conductive path, making it impossible for lithium ions to pass through the polymer layer smoothly, further increasing the resistance, and delaying the occurrence of thermal runaway.

[0049] (2) The grafted modifier fluorinated olefin in the swellable polymer has good affinity with the fluorinated polymer (such as PVDF) of the positive electrode active material layer under normal working conditions, further increasing the bonding force between the polymer layer and the positive electrode active material layer.

[0050] (3) The melting point of the expandable polymer is 140-160°C. When the temperature further rises to the melting point, the expandable polymer melts, absorbs heat, and delays the occurrence of thermal runaway.

[0051] (4) The present invention preferably adds a thermal decomposition gas-generating substance to the polymer layer. Thermal decomposition gas-generating substances such as sodium bicarbonate, lithium bicarbonate, ammonium bicarbonate, and lithium nitrate generate inert gases such as carbon dioxide, nitrogen dioxide, and nitrogen when heated, which can increase the internal resistance. At the same time, the inert gas can play a partial flame retardant effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Figure 1This is a schematic diagram of the top view of the positive electrode current collector composited with a polymer layer in Example 1.

[0053] Figure 2 Schematic diagram of the structure of the positive electrode sheet of Example 1.

[0054] Figure 3 This is a schematic diagram of the top view of the positive electrode current collector composited with a polymer layer in Example 3.

[0055] Figure 4 Schematic diagram of the structure of the positive electrode sheet of Example 3.

[0056] Figure 5 It is a schematic diagram of preventing thermal runaway from occurring in the positive electrode sheet of the present invention.

[0057] The reference numerals are as follows:

[0058] 1: positive electrode current collector;

[0059] 2: polymer layer;

[0060] 3: Positive electrode active material layer. DETAILED DESCRIPTION

[0061] The present invention discloses a positive electrode sheet and a method for preparing the same and a battery. Those skilled in the art can refer to the contents of this article and appropriately improve the process parameters to achieve the same. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are deemed to be included in the present invention. The methods and applications of the present invention have been described through preferred embodiments, and relevant personnel can obviously modify or appropriately change and combine the methods and applications described herein without departing from the content, spirit and scope of the present invention to implement and apply the technology of the present invention.

[0062] In the description of the present invention, it should be noted that the terms "first", "second", etc. are only used for descriptive purposes, and do not indicate or imply relative importance.

[0063] In the description of the present invention, a list of items connected by the term "at least one of" or other similar terms may mean any combination of the listed items. For example, if items A, B are listed, the phrase "at least one of A, B" means only A; only B; or A and B. In another example, if items A, B, C are listed, the phrase "at least one of A, B, C" means only A; or only B; only C; A and B (excluding C); A and C (excluding B); B and C (excluding A); or all of A, B, and C. Item A may include a single element or multiple elements. Item B may include a single element or multiple elements. Item C may include a single element or multiple elements.

[0064] The endpoints and any values ​​of the ranges disclosed in this article are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoint values ​​of each range, the endpoint values ​​of each range or the individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed in this article.

[0065] If not otherwise specified, all embodiments and optional embodiments of the present application can be combined with each other to form a new technical solution.

[0066] Unless otherwise specified, all technical features and optional technical features of this application can be combined with each other to form a new technical solution.

[0067] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended or closed-ended. For example, the "include" and "comprising" may mean that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0068] Specifically, the present invention adopts the following technical solutions:

[0069] In a first aspect, the present invention provides a positive electrode sheet, the positive electrode sheet comprising:

[0070] (a) positive electrode current collector;

[0071] (b) a polymer layer, the polymer layer being disposed on at least one surface of the positive electrode current collector;

[0072] The polymer layer includes an expandable polymer, which is obtained by polymerizing a polymer matrix and a graft modifier, and the graft modifier includes a fluorine-containing olefin;

[0073] (c) a positive electrode active material layer, the positive electrode active material layer being disposed on a surface of the polymer layer away from the positive electrode current collector;

[0074] In the thickness direction of the positive electrode sheet, the projected area of ​​the positive electrode current collector is recorded as S1, the projected area of ​​the polymer layer is recorded as S2, and the projected area of ​​the positive electrode active material layer is recorded as S3. S1, S2, and S3 satisfy the following relationship: S2<S3≤S1;

[0075] At least a portion of the positive electrode active material layer is in direct contact with the positive electrode current collector.

[0076] In the present invention, the positive electrode sheet includes a polymer layer, which is located between the positive electrode current collector and the positive electrode active material layer, and has an area smaller than that of the positive electrode current collector and the positive electrode active material layer. The polymer layer includes a swellable polymer, which is obtained by polymerizing a polymer matrix and a graft modifier, and the graft modifier includes a fluorine-containing olefin. Figure 5 When the battery temperature is low (normal working state), although the polymer layer is located in the middle, the area of ​​the positive current collector and the positive active material layer is larger than that of the polymer layer, and part of the positive current collector and the positive active material layer can be in direct contact (direct contact part) to achieve electrical connection; as the battery temperature rises (overcharge), the expandable polymer in the polymer layer expands. On the one hand, it expands in the thickness direction, causing the positive active material layer above the polymer layer and the positive active material layer above the positive current collector to break and dislocate, increasing the resistance of the positive electrode, reducing the conductivity, and reducing the risk of thermal runaway; on the other hand, the expandable polymer expands in the transverse direction (perpendicular to the thickness direction of the electrode sheet), narrowing the conductive channel, making it impossible for lithium ions to pass through the polymer layer smoothly, further increasing the resistance of the positive electrode, and reducing the conductivity and the risk of thermal runaway. Moreover, the expandable polymer is a modified polymer, and the grafted modifier in the modified polymer is a fluorinated olefin, which can further increase the expansion rate of the expandable polymer, thereby more effectively causing the positive electrode active material layer above the polymer layer and the positive electrode active material layer above the positive electrode current collector to break and dislocate, further narrowing the conductive path, making it impossible for lithium ions to pass through the polymer layer smoothly, further increasing the resistance, and delaying the occurrence of thermal runaway.

[0077] Preferably, the expansion temperature of the expandable polymer is 85-95°C. Exemplarily, the expansion temperature of the expandable polymer is any value among 85°C, 87°C, 89°C, 90°C, 91°C, 93°C, 95°C or any value within the range of any two of the above values. When the battery temperature reaches this range, the expandable polymer in the polymer layer expands. On the one hand, it expands in the thickness direction, so that the positive electrode active material layer above the polymer layer and the positive electrode active material layer above the positive electrode current collector are broken and dislocated, the resistance of the positive electrode increases, the conductivity decreases, and the risk of thermal runaway is reduced; on the other hand, the expandable polymer expands in the lateral direction, narrowing the conductive channel, so that lithium ions cannot pass through the polymer layer smoothly, further increasing the resistance of the positive electrode, and reducing the conductivity and the risk of thermal runaway. When the expansion temperature of the expandable polymer is too low, it will affect the normal operation of the battery. When the expansion temperature of the expandable polymer is too high, it cannot respond to the thermal runaway problem during overcharging.

[0078] Preferably, the thermal expansion coefficient of the expandable polymer is ≥ 20%. Exemplarily, the thermal expansion coefficient of the expandable polymer is any value among 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, 150%, 200%, or any value within the range of any two of the above values. If the thermal expansion coefficient is too small, the tension in the thickness direction is too small, and the positive electrode active material layer above the polymer layer and the positive electrode active material layer above the positive electrode current collector cannot be broken or dislocated.

[0079] Preferably, the melting point of the expandable polymer is 140-160° C. Exemplarily, the melting point of the expandable polymer is any one of 140° C., 142° C., 144° C., 146° C., 148° C., 150° C., 152° C., 154° C., 156° C., 158° C., 160° C., or any value within a range of any two of the above values. When the temperature reaches the above melting point, the expandable polymer melts, absorbs heat, and prevents the occurrence of thermal runaway.

[0080] In an embodiment of the present invention, the graft modifier includes at least one of hexafluoropropylene, hexafluoroethylene, and vinylidene fluoride. The graft modifier fluorinated olefin can be grafted with the polymer matrix, and further promotes the bonding force and makes the expansion effect larger. Moreover, the fluorinated olefin and the fluorinated polymer (such as PVDF) of the positive electrode active material layer have good affinity under normal working conditions, further increasing the bonding force between the polymer layer and the positive electrode active material layer.

[0081] Preferably, the resistivity of the polymer matrix is ​​greater than or equal to 10 MΩ / m. Exemplarily, the resistivity of the polymer matrix is ​​any value among 10 MΩ / m, 20 MΩ / m, 30 MΩ / m, 40 MΩ / m, 50 MΩ / m, or any value within the range of any two of the above values. Within this resistance range, the polymer matrix can achieve insulation between the positive electrode current collector and the positive electrode active material layer.

[0082] In an embodiment of the present invention, the polymer matrix includes at least one of polyethylene, polyvinyl chloride, polypropylene, and polymethyl methacrylate.

[0083] Preferably, the mass ratio of the polymer matrix to the graft modifier is (10-30):(1-3). Exemplarily, the mass ratio of the polymer matrix to the graft modifier is any value among 10:3, 15:2, 10:1, 10:2, 20:1, 30:1 or any value within the range consisting of any two of the above values.

[0084] In an embodiment of the present invention, the positive electrode active material layer includes a middle region and an edge region, the middle region is at least partially in direct contact with the polymer layer, and the edge region is in direct contact with the positive electrode current collector. The polymer layer is in direct contact with the middle region of the positive electrode active material layer, so as to ensure that when the expandable polymer expands, the positive electrode sheet is subjected to tension at all locations in the thickness direction, and the overall tension is the largest, and the expandable polymer tends to cover the entire positive electrode sheet.

[0085] Preferably, the distance L1 between the edge of the polymer layer and the edge of the positive electrode active material layer is ≥3 mm. Exemplarily, L1 is any value among 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm or any value within the range of any two of the above values. When L1 is ≥3 mm, it can ensure that there is sufficient contact area between the positive electrode current collector and the positive electrode active material layer, so that the current between the positive electrode current collector and the positive electrode active material layer is conductive.

[0086] Preferably, the ratio of S3 / S1 is 0.9 to 1. Exemplarily, the ratio of S3 / S1 is any value among 0.9, 0.92, 0.94, 0.96, 0.98, 1, or any value within the range of any two of the above values. When the ratio of S3 / S1 is too small, the area of ​​the positive electrode active material layer is too small, and the energy density of the battery is small, which affects the performance of the battery.

[0087] Preferably, the ratio of S2 / S1 is 0.6 to 0.8. Exemplarily, the ratio of S2 / S1 is any of 0.6, 0.62, 0.64, 0.66, 0.68, 0.7, 0.72, 0.74, 0.76, 0.78, 0.8 or any of the ranges of any two of the above values. When the ratio of S2 / S1 is too small, the area of ​​the polymer layer is too small, the expansion effect is small, and the positive active material layer above the polymer layer and the positive active material layer above the positive current collector cannot be broken or dislocated, and the occurrence of thermal runaway cannot be effectively prevented; when the ratio of S2 / S1 is too large, the area of ​​the polymer layer is too large, the area of ​​direct contact between the positive active material layer and the positive current collector is too small, and the conductivity of the battery under normal working conditions is affected.

[0088] Preferably, the thickness of the polymer layer is 0.5 to 1.2 μm. Exemplarily, the thickness of the polymer layer is any value among 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, or any value within the range of any two of the above values. If the thickness of the polymer layer is too small, the expandable polymer is not sufficient to cause the positive electrode active material layer above the polymer layer and the positive electrode active material layer above the positive electrode current collector to break or dislocate; if the thickness of the polymer layer is too large, the transmission path between the positive electrode current collector and the positive electrode active material layer is too long, which is not conducive to conductivity under normal conditions.

[0089] In an embodiment of the present invention, the polymer layer comprises one or more sub-polymer layers.

[0090] Preferably, the number of the plurality of sub-polymer layers is ≥ 2. Exemplarily, the number of the sub-polymer layers is any value among 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 or any value within the range consisting of any two of the above values.

[0091] In an embodiment of the present invention, multiple sub-polymer layers are evenly spaced and distributed. The advantages of evenly spaced distribution of multiple sub-polymer layers are: on the one hand, the transmission path of electrons between the positive electrode active material layer and the positive electrode current collector is shorter (in the case of only one entire polymer layer, the polymer layer is in the middle area, and the electrons need to reach the edge first and then transmit between layers), and the conductivity under normal conditions is better; on the other hand, the expandable polymer is more evenly dispersed, and when it expands at high temperature (overcharge), it can expand evenly, so that all parts of the positive electrode sheet can be affected by the expandable polymer.

[0092] Preferably, the spacing L2 between two adjacent sub-polymer layers is ≥3 mm. Exemplarily, L2 is any value among 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm or any value within the range of any two of the above values. When L2 ≥ 3 mm, it can be ensured that under normal conditions, there are sufficient channels between the sub-polymer layers for electron transmission between the positive electrode current collector and the positive electrode active material layer.

[0093] In an embodiment of the present invention, the polymer layer further comprises a thermally decomposed gas generating substance and / or an aqueous binder.

[0094] In an embodiment of the present invention, the thermal decomposition gas-generating substance includes at least one of sodium bicarbonate, lithium bicarbonate, ammonium bicarbonate, and lithium nitrate. The thermal decomposition gas-generating substance generates inert gases such as carbon dioxide, nitrogen dioxide, or nitrogen when heated. These inert gases can increase the internal resistance on the one hand, and can also play a part of the flame retardant effect on the other hand.

[0095] In an embodiment of the present invention, the aqueous binder includes at least one of polyacrylic acid PAA, polyethylene oxide PEO, and a propanol-based compound.

[0096] Preferably, the mass ratio of the expandable polymer to the thermal decomposition gas generating substance is (50-150):(1-10). Exemplarily, the mass ratio of the expandable polymer to the thermal decomposition gas generating substance is any value among 150:1, 110:5, 100:1, 50:1, 5:1 or any value within the range consisting of any two of the above values.

[0097] Preferably, the mass ratio of the swellable polymer to the aqueous binder is (1-3):(1-3). Exemplarily, the mass ratio of the swellable polymer to the aqueous binder is any value among 1:1, 1:2, 1:3, 2:1, 3:1 or any value within the range consisting of any two of the above values.

[0098] Preferably, the average particle size of the expandable polymer is 1.2 to 1.5 μm. Exemplarily, the average particle size of the expandable polymer is any value among 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, or any value within the range of any two of the above values. The average particle size of the expandable polymer is within this range, which facilitates the formation of a rough polymer layer surface. The rough polymer layer surface can form a riveting effect with the positive electrode active material layer, so that the polymer layer and the positive electrode active material layer have a certain degree of adhesion, preventing the positive electrode active material layer from falling off and peeling off.

[0099] Preferably, the roughness Ra of the surface of the polymer layer away from the positive electrode current collector is ≥ 0.5 mm. Exemplarily, the Ra of the surface of the polymer layer is any value among 0.5 mm, 0.7 mm, 0.9 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, or any value within the range of any two of the above values. The rough surface of the polymer layer can form a riveting effect with the positive electrode active material layer, so that the polymer layer and the positive electrode active material layer have a certain degree of adhesion, and prevent the positive electrode active material layer from falling off and peeling off.

[0100] Preferably, the roughness Ra of the positive electrode current collector surface is ≥ 0.5 mm. Exemplarily, the Ra of the positive electrode current collector surface is any value among 0.5 mm, 0.7 mm, 0.9 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, or any value within the range of any two of the above values. The rough positive electrode current collector surface can better combine with the polymer layer and the positive electrode active material layer to ensure adhesion and prevent powdering and peeling.

[0101] In a second aspect, the present invention provides a method for preparing the above-mentioned positive electrode sheet, comprising the following steps:

[0102] S1, mixing a polymer matrix, a grafting modifier and an organic solvent, performing a polymerization reaction, and drying and grinding to obtain a swellable polymer;

[0103] S2, mixing a swellable polymer and water to obtain a slurry, and coating the slurry on at least one surface of a positive electrode current collector to obtain a positive electrode current collector composited with a polymer layer;

[0104] S3, disposing a positive electrode active material layer on a surface of the polymer layer away from the positive electrode current collector to obtain a positive electrode sheet.

[0105] In an embodiment of the present invention, in step S1, the organic solvent includes at least one of N-methylpyrrolidone (NMP) and acetone.

[0106] Preferably, the polymerization temperature is 90-100°C, and the polymerization time is 4-6h. Exemplarily, the polymerization temperature is any one of 90°C, 92°C, 94°C, 96°C, 98°C, 100°C, or any value in the range of any two of the above values, and the polymerization time is any one of 4h, 4.5h, 5h, 5.5h, 6h, or any value in the range of any two of the above values.

[0107] In an embodiment of the present invention, when the polymer layer includes a thermally decomposed gas-generating substance, in step S1, mixing the polymer matrix, the grafting modifier and the organic solvent comprises: mixing the polymer matrix, the grafting modifier, the thermally decomposed gas-generating substance and the organic solvent.

[0108] In an embodiment of the present invention, when the polymer layer includes an aqueous binder, in step S2, mixing the swellable polymer and water is as follows: mixing the swellable polymer, the aqueous binder and water.

[0109] In a third aspect, the present invention provides a battery, comprising the above-mentioned positive electrode sheet, and / or the positive electrode sheet prepared by the above-mentioned preparation method.

[0110] In the embodiment of the present invention, the battery structure includes but is not limited to button batteries, soft-pack batteries, cylindrical batteries, etc.

[0111] The present application has no particular restrictions on the negative electrode sheet, separator, and electrolyte in the battery, and those skilled in the art can select them according to actual needs as long as the purpose of the present application can be achieved.

[0112] The reagents, instruments or materials used in the present invention can be obtained through commercial channels.

[0113] The present invention will be further described below in conjunction with embodiments:

[0114] Embodiment 1:

[0115] 1. Preparation of positive electrode:

[0116] (1) Preparation of expandable polymer:

[0117] Mix 100 g of polyethylene (polymer matrix), 10 g of hexafluoropropylene (grafting modifier), and 5 g of sodium bicarbonate (thermal decomposition gas-producing substance), add 30 g of N-methylpyrrolidone (NMP), heat and stir to mix evenly, then heat at 95°C for 5 hours, dry and grind to obtain an expandable polymer with an average particle size of 1.3 μm, which includes thermal decomposition gas-producing substances.

[0118] (2) Preparation of polymer layer:

[0119] The swellable polymer is dissolved in water, and the aqueous binder polyacrylic acid (PAA) is added for high-speed dispersion. The mass ratio of the swellable polymer to PAA is 1:1. A slurry is prepared and coated on the positive electrode current collector (the positive electrode current collector has a length of 250 mm, a width of 40 mm, and a roughness of 0.5 mm) with a coating thickness of 1 μm. Three sub-polymer layers with a length of 79.3 mm are formed in the length direction of the pole piece, and the interval between each sub-polymer layer in the length direction is 3 mm. Two sub-polymer layers with a width of 15.5 mm are formed in the width direction, and the interval between each sub-polymer layer in the width direction is 3 mm. The polymer layer is in the middle of the current collector layer, forming a polymer layer with a surface roughness of 0.8 mm. The schematic diagram of the structure of the positive electrode current collector composited with a polymer layer is shown as follows: Figure 1 shown.

[0120] (3) Preparation of positive electrode active material layer:

[0121] Lithium iron phosphate, carbon nanotubes (CNT), conductive carbon black (SP), and polytetrafluoroethylene (PVDF) were mixed in a mass ratio of 96.5:1:0.5:2, added to the solvent NMP to make a slurry, and coated on the polymer layer. The length of the positive electrode active material layer was 250 mm, the width was 40 mm, and the coating thickness was 192 μm, to obtain a positive electrode sheet. The schematic diagram of the positive electrode sheet structure is shown in FIG. Figure 2 shown.

[0122] 2. Diaphragm:

[0123] The diaphragm is made of polyethylene.

[0124] 3. Preparation of negative electrode sheet:

[0125] Graphite, SP, carboxymethyl cellulose (CMC), and styrene-butadiene rubber (SBR) were mixed in a mass ratio of 95.5:1.5:1.2:1.8, added into solvent water to prepare slurry, and coated onto a negative electrode current collector to obtain a negative electrode sheet.

[0126] 4. Electrolyte:

[0127] The lithium salt of the electrolyte is 1 mol / L LiPF6, and the solvent is a mixture of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1.

[0128] 5. Battery assembly:

[0129] The positive electrode sheet, separator, negative electrode sheet and electrolyte are assembled into a lithium-ion battery.

[0130] Embodiment 2:

[0131] The difference between this embodiment and embodiment 1 is that the raw polymer matrix, grafting modifier and thermal decomposition gas generating substance for preparing the expandable polymer are different, and the particle size of the expandable polymer is different. The specific differences are as follows:

[0132] 1. The polymer matrix is ​​polyvinyl chloride;

[0133] 2. The grafting modifier is hexafluoroethylene;

[0134] 3. The gas-producing substance produced by thermal decomposition is lithium bicarbonate;

[0135] 4. The average particle size of the expandable polymer is 1.2 μm.

[0136] Embodiment 3:

[0137] The difference between this embodiment and embodiment 1 is that there are no multiple sub-polymer layers, and the total area is equal to the integrated polymer layer. Figure 3 The schematic diagram of the positive electrode structure is shown in Figure 4 shown.

[0138] Embodiment 4:

[0139] The present embodiment is different from the embodiment 1 in that the particle size of the expandable polymer is 0.5 μm, and the roughness of the sub-polymer layer is 0.4 mm (no riveting effect is formed).

[0140] Comparative Example 1:

[0141] The difference between this comparative example and Example 1 is that the positive electrode active material layer is directly coated on the surface of the current collector without a polymer layer.

[0142] Comparative Example 2:

[0143] The difference between this comparative example and Example 1 is that the area of ​​the polymer layer (expandable) is equal to the positive electrode active material layer (the positive electrode active material layer is directly and completely insulated from the current collector and cannot conduct electricity, so the battery cannot operate normally).

[0144] Comparative Example 3:

[0145] The difference between this comparative example and Example 1 is that the expandable polymer is replaced by aluminum oxide (aluminum oxide is a non-expandable insulating material).

[0146] Comparative Example 4:

[0147] The difference between this comparative example and Example 1 is that the expandable polymer is not graft-modified with hexafluoropropylene (grafting modifier).

[0148] Performance Test:

[0149] Testing methods include:

[0150] (1) Test method for expansion temperature of expandable polymer:

[0151] The temperature was raised from room temperature at a rate of 5°C / min and the temperature was recorded when expansion began.

[0152] (2) Expansion rate test method of expandable polymer:

[0153] Expansion ratio = (volume after expansion - volume before expansion) / volume before expansion.

[0154] (3) Test method for electrical conductivity:

[0155] The conductivity was tested using a film resistor.

[0156] (4) Overcharge performance test method:

[0157] Charge the 1C battery, observe the temperature change, and record the temperature change until the battery catches fire and fails. If the battery does not catch fire or explode during the entire overcharge performance test, the test is judged to have passed (OK); otherwise, the test is judged to have failed (NG).

[0158] (5) Thermal runaway temperature test method:

[0159] Heat the battery and record the temperature when the battery swells and smokes.

[0160] (6) Peel strength test method:

[0161] The peeling strength between the positive electrode active material layer and the polymer layer was tested using a universal tensile testing machine in a 180-degree peeling mode.

[0162] Table 1

[0163]

[0164] From the above results, it can be seen that the expandable polymers of Examples 1-4 begin to expand at 95° C., with an expansion rate of at least 20%, have excellent conductivity under normal conditions, good overcharge performance, and can effectively prevent the occurrence of thermal runaway.

[0165] In Comparative Example 1, since no polymer layer is provided on the surface of the current collector, the overcharge performance is poor and the occurrence of thermal runaway cannot be prevented.

[0166] In Comparative Example 2, since the area of ​​the polymer layer is equal to that of the positive electrode active material layer, the positive electrode active material layer and the current collector are directly and completely insulated and cannot conduct electricity, and the battery cannot operate normally.

[0167] In Comparative Example 3, since the expandable polymer is replaced by alumina, which is a non-expandable insulating material, when the temperature rises due to overcharging, the positive electrode active material layer above the polymer layer and the positive electrode active material layer above the positive electrode current collector cannot be broken or dislocated, and thermal runaway cannot be prevented.

[0168] In Comparative Example 4, since the expandable polymer has not been grafted with hexafluoropropylene, the thermal expansion coefficient of the unmodified polymer is small, and the expansion coefficient is small when the overcharge temperature rises. The positive electrode active material layer above the polymer layer and the positive electrode active material layer above the positive electrode current collector cannot be broken or dislocated, and thermal runaway cannot be prevented.

[0169] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principle of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode sheet comprises: (a) positive electrode current collector; (b) a polymer layer, the polymer layer being disposed on at least one surface of the positive electrode current collector; The polymer layer includes an expandable polymer, which is obtained by polymerizing a polymer matrix and a graft modifier, and the graft modifier includes a fluorine-containing olefin; (c) a positive electrode active material layer, the positive electrode active material layer being disposed on a surface of the polymer layer away from the positive electrode current collector; In the thickness direction of the positive electrode sheet, the projected area of ​​the positive electrode current collector is recorded as S1, the projected area of ​​the polymer layer is recorded as S2, and the projected area of ​​the positive electrode active material layer is recorded as S3, and S1, S2, and S3 satisfy the following relationship: S2<S3≤S1; At least a portion of the positive electrode active material layer is in direct contact with the positive electrode current collector.

2. The positive electrode sheet according to claim 1, characterized in that: The expansion temperature of the expandable polymer is 85 to 95°C; and / or, the thermal expansion rate of the expandable polymer is ≥ 20%; and / or, the melting point of the expandable polymer is 140-160° C.; And / or, the graft modifier includes at least one of hexafluoropropylene, hexafluoroethylene, and vinylidene fluoride; And / or, the resistivity of the polymer matrix is ​​greater than or equal to 10 MΩ / m; And / or, the polymer matrix includes at least one of polyethylene, polyvinyl chloride, polypropylene, and polymethyl methacrylate; And / or, the mass ratio of the polymer matrix to the graft modifier is (10-30):(1-3).

3. The positive electrode sheet according to claim 1, characterized in that: The positive electrode active material layer comprises a middle region and an edge region, wherein at least a portion of the middle region is in direct contact with the polymer layer, and the edge region is in direct contact with the positive electrode current collector; and / or, a distance L1 between an edge of the polymer layer and an edge of the positive electrode active material layer ≥ 3 mm; and / or, the ratio of S3 / S1 is 0.9 to 1; and / or, the ratio of S2 / S1 is 0.6 to 0.8; And / or, the polymer layer has a thickness of 0.5-1.2 μm.

4. The positive electrode sheet according to claim 1, characterized in that: The polymer layer comprises one or more sub-polymer layers; The number of the plurality of sub-polymer layers is ≥ 2; and / or, the plurality of sub-polymer layers are evenly spaced and distributed; And / or, the distance L2 between two adjacent sub-polymer layers is ≥3 mm.

5. The positive electrode sheet according to claim 1, characterized in that: The polymer layer further comprises a thermally decomposed gas generating substance and / or an aqueous binder; The thermal decomposition gas-generating substance includes at least one of sodium bicarbonate, lithium bicarbonate, ammonium bicarbonate, and lithium nitrate; And / or, the aqueous binder includes at least one of polyacrylic acid PAA, polyethylene oxide PEO, and a propanol-based compound; and / or, the mass ratio of the expandable polymer to the thermally decomposed gas-generating substance is (50-150):(1-10); And / or, the mass ratio of the swellable polymer to the aqueous binder is (1-3):(1-3).

6. The positive electrode sheet according to claim 1, characterized in that: The average particle size of the expandable polymer is 1.2 to 1.5 μm; And / or, the roughness of the surface of the polymer layer away from the positive electrode current collector is Ra≥0.5 mm; And / or, the roughness of the surface of the positive electrode current collector is Ra≥0.5 mm.

7. A method for preparing the positive electrode sheet according to any one of claims 1 to 6, characterized in that: The steps include: S1, mixing a polymer matrix, a grafting modifier and an organic solvent, performing a polymerization reaction, and drying and grinding to obtain a swellable polymer; S2, mixing the swellable polymer and water to obtain a slurry, and coating the slurry on at least one surface of the positive electrode current collector to obtain a positive electrode current collector composited with a polymer layer; S3, disposing a positive electrode active material layer on the surface of the polymer layer away from the positive electrode current collector to obtain a positive electrode sheet.

8. The preparation method according to claim 7, characterized in that: In the step S1, the organic solvent includes at least one of N-methylpyrrolidone and acetone; And / or, the polymerization reaction temperature is 90-100° C., and the polymerization reaction time is 4-6 hours.

9. The preparation method according to claim 7, characterized in that: When the polymer layer includes a thermally decomposed gas-generating substance, in step S1, the polymer matrix, the graft modifier and the organic solvent are mixed as follows: the polymer matrix, the graft modifier, the thermally decomposed gas-generating substance and the organic solvent are mixed; And / or, when the polymer layer includes an aqueous binder, in step S2, mixing the swellable polymer with water comprises: mixing the swellable polymer, the aqueous binder and water.

10. A battery, characterized in that: The battery comprises the positive electrode sheet according to any one of claims 1 to 6, and / or the positive electrode sheet prepared by the preparation method according to any one of claims 7 to 9.

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