A positive electrode sheet, a method for manufacturing the same, and a battery

Abstract

The application 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 polymerization of a polymer matrix and a grafting modifier. The grafting modifier comprises a fluorine-containing olefin. The positive active material layer is arranged on the surface of the polymer layer away from the positive current collector. In the thickness direction of the positive plate, the projection area of the polymer layer is smaller than the projection area of the positive active material layer and the positive current collector, and the positive active material layer is at least partially in direct contact with the positive current collector. The positive plate can effectively delay the occurrence of thermal runaway.

Description

TECHNICAL FIELD

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

[0002] Due to the high theoretical capacity, excellent electrochemical performance and thermodynamic stability of lithium iron phosphate material, and the simple preparation method and low cost, it has become one of the most potential cathode materials for lithium ion batteries. However, with the high energy density of the current battery, it is difficult to further ensure the safety during overcharging, and the battery is prone to thermal runaway phenomenon due to the temperature rise caused by overcharging. How to avoid such problems is a hot research topic at present. SUMMARY

[0003] Therefore, the present application provides a positive electrode sheet, 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 application purposes, the present application provides the following technical solutions:

[0005] In a first aspect, the present application provides a positive electrode sheet, which comprises:

[0006] (a) a positive current collector;

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

[0008] The polymer layer comprises an expandable polymer, the expandable polymer being obtained by polymerization of a polymer matrix and a grafting modifier, the grafting modifier comprising a fluorine-containing olefin;

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

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

[0011] The positive active material layer is at least partially in direct contact with the positive current collector.

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

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

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

[0015] In an embodiment of the present application, the grafting modifier comprises at least one of hexafluoropropylene, hexafluoroethylene, and vinylidene fluoride.

[0016] Preferably, the polymer matrix has a resistivity of 10 MΩ / m or more.

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

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

[0019] In an embodiment of the present application, the positive active material layer comprises an intermediate region and an edge region, the intermediate region is at least partially in direct contact with the polymer layer, and the edge region is in direct contact with the positive current collector.

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

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

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

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

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

[0025] Preferably, the number of the sub-polymer layers is 2 or more.

[0026] In an embodiment of the present application, the sub-polymer layers are uniformly spaced.

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

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

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

[0030] In an embodiment of the present application, the aqueous binder comprises at least one of polyacrylic acid (PAA), polyethylene oxide (PEO), and a propyl alcohol-based compound.

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

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

[0033] As preferred, the average particle size of the expandable polymer is 1.2-1.5 μm.

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

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

[0036] In a second aspect, the present application provides a preparation method of the above positive sheet, comprising the following steps:

[0037] S1, mixing the polymer matrix, the grafting modifier and the organic solvent to perform a polymerization reaction, drying and grinding to obtain the expandable polymer;

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

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

[0040] In the embodiments of the present application, in step S1, the organic solvent comprises at least one of N-methyl pyrrolidone and acetone.

[0041] As preferred, the temperature of the polymerization reaction is 90-100℃, and the time of the polymerization reaction is 4-6 h.

[0042] In the embodiments of the present application, when the polymer layer comprises the thermal decomposition gas generating substance, in step S1, the mixing of the polymer matrix, the grafting modifier and the organic solvent is: mixing the polymer matrix, the grafting modifier, the thermal decomposition gas generating substance and the organic solvent.

[0043] In the embodiments of the present application, when the polymer layer comprises the aqueous binder, in step S2, the mixing of the expandable polymer and water is: mixing the expandable polymer, the aqueous binder and water.

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

[0045] Compared with the prior art, the present application has the beneficial effects that:

[0046] (1) The positive electrode sheet of the present application comprises 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, and the polymer layer contains an expandable polymer, which is obtained by polymerization of a polymer matrix and a grafting modifier, and the grafting modifier comprises a fluorine-containing olefin.

[0047] Referring to Figure 5 When the battery temperature is low (normal working state), the polymer layer is located in the middle, but the area of the positive electrode current collector and the positive electrode active material layer is larger than that of the polymer layer, so part of the positive electrode current collector and the positive electrode active material layer can be directly contacted (directly contacted part), realizing electrical connection; as the battery temperature rises (overcharge), the expandable polymer in the polymer layer expands, on the one hand, 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, and the conductivity decreases, 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), making the conductive channel narrow, so that lithium ions cannot pass through the polymer layer smoothly, further increasing the resistance of the positive electrode, reducing the conductivity and the risk of thermal runaway.

[0048] The expandable polymer is a modified polymer, and the grafting modifier in the modified polymer is a fluorine-containing olefin, which can further increase the expansion rate of the expandable polymer, so as to more effectively make the positive electrode active material layer above the polymer layer and the positive electrode active material layer above the positive electrode current collector broken and dislocated, further narrow the conductive channel, so that lithium ions cannot pass through the polymer layer smoothly, further increase the resistance, and delay the occurrence of thermal runaway.

[0049] (2) The grafting modifier fluorine-containing olefin in the expandable polymer and the fluorine-containing polymer (such as PVDF) in the positive electrode active material layer have good affinity in the normal working state, further increasing the adhesion between the polymer layer and the positive electrode active material layer.

[0050] (3) The melting point of the expandable polymer is 140-160℃, 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 application preferably adds a thermal decomposition gas generating substance in the polymer layer, such as sodium bicarbonate, lithium bicarbonate, ammonium bicarbonate, lithium nitrate, which generates carbon dioxide, nitrogen dioxide, nitrogen and other inert gases when heated, which can increase the internal resistance, and the inert gas can also have a part of the flame retardant effect. BRIEF DESCRIPTION OF DRAWINGS

[0052] Figure 1A schematic diagram of a top view of the positive current collector to which the polymer layer is applied in Example 1.

[0053] Figure 2 A schematic diagram of the structure of the positive electrode sheet in Example 1.

[0054] Figure 3 A schematic diagram of a top view of the positive current collector to which the polymer layer is applied in Example 3.

[0055] Figure 4 A schematic diagram of the structure of the positive electrode sheet in Example 3.

[0056] Figure 5 A schematic diagram of the positive electrode sheet of the present application for blocking the occurrence of thermal runaway.

[0057] Reference signs are as follows:

[0058] 1: positive current collector;

[0059] 2: polymer layer;

[0060] 3: positive active material layer. DETAILED DESCRIPTION

[0061] The present application discloses a positive electrode sheet, a preparation method thereof and a battery. Those skilled in the art can refer to the content herein and appropriately improve the process parameters for implementation. It is particularly pointed out that all similar substitutions and changes are obvious to those skilled in the art, and they are considered to be included in the present application. The method and application of the present application have been described by the preferred embodiments, and the 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 application, to realize and apply the present application technology.

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

[0063] In the description of the present application, the list of items connected by the term "at least one of" or other similar terms means 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 can include a single element or multiple elements. Item B can include a single element or multiple elements. Item C can include a single element or multiple elements.

[0064] The endpoints of the ranges and any values disclosed herein are not limited to the precise values recited as exactly that endpoint. Any ranges of values, whether small or large, are intended to include values approximating the recited endpoint values, unless expressly indicated otherwise. For values having a range, the endpoints of the ranges are included within the range, and the range includes any intervening values between the endpoints, unless expressly indicated otherwise.

[0065] If there is no specific indication, all the embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0066] If there is no specific indication, all the technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0067] If there is no specific indication, the "includes" and "contains" mentioned in the present application represent open type, and can also be closed type. For example, the "includes" and "contains" can represent that other components not listed can also be included or contained, or only the listed components can be included or contained.

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

[0069] In a first aspect, the present application provides a positive electrode sheet, which comprises:

[0070] (a) a positive electrode current collector;

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

[0072] The polymer layer comprises an expandable polymer, the expandable polymer being obtained by polymerization of a polymer matrix and a grafting modifier, the grafting modifier comprising a fluorine-containing olefin;

[0073] (c) a positive electrode active material layer, the positive electrode active material layer being arranged 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 denoted as S1, the projected area of the polymer layer is denoted as S2, and the projected area of the positive electrode active material layer is denoted as S3, S1, S2 and S3 satisfy the following relationship: S2 < S3 ≤ S1;

[0075] The positive electrode active material layer is at least partially in direct contact with the positive electrode current collector.

[0076] In the present application, the positive electrode sheet comprises a polymer layer, the polymer layer is located between the positive electrode current collector and the positive electrode active material layer, and the area of the polymer layer is smaller than the area of the positive electrode current collector and the positive electrode active material layer, the polymer layer comprises an expandable polymer, the expandable polymer is obtained by polymerization of a polymer matrix and a grafting modifier, and the grafting modifier comprises a fluorine-containing olefin. See 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 electrode current collector and the positive electrode active material layer is larger than the area of the polymer layer, and part of the positive electrode current collector and the positive electrode active material layer can be directly contacted (directly contacted part), so as to realize electrical connection; with the increase of the battery temperature (overcharge), the expandable polymer in the polymer layer expands, on the one hand, 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 transverse direction (perpendicular to the thickness direction of the electrode sheet), so that the conductive channel is narrowed, the lithium ions cannot pass through the polymer layer smoothly, the resistance of the positive electrode is further increased, the conductivity is reduced, and the risk of thermal runaway is reduced. Moreover, the expandable polymer is a modified polymer, the grafting modifier in the modified polymer is a fluorine-containing olefin, and the fluorine-containing olefin can further increase the expansion rate of the expandable polymer, so as to more effectively make the positive electrode active material layer above the polymer layer and the positive electrode active material layer above the positive electrode current collector broken and dislocated, further narrow the conductive channel, make the lithium ions unable to pass through the polymer layer smoothly, further increase the resistance, and delay the occurrence of thermal runaway.

[0077] As a preferred, the expansion temperature of the expandable polymer is 85-95℃. As an example, the expansion temperature of the expandable polymer is any one of 85℃, 87℃, 89℃, 90℃, 91℃, 93℃, 95℃ or any value within the range composed 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, 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 transverse direction, so that the conductive channel is narrowed, the lithium ions cannot pass through the polymer layer smoothly, the resistance of the positive electrode is further increased, the conductivity is reduced, and the risk of thermal runaway is reduced. When the expansion temperature of the expandable polymer is too low, it will affect the normal work of the battery, and when the expansion temperature of the expandable polymer is too high, it cannot play a role in response to the problem of thermal runaway in overcharge.

[0078] As preferred, the thermal expansion rate of the expandable polymer is greater than or equal to 20%. Exemplarily, the thermal expansion rate of the expandable polymer is any one of 20%, 22%, 24%, 26%, 28%, 30%, 32%, 34%, 36%, 38%, 40%, 50%, 60%, 70%, 80%, 90%, 100%, 150%, 200% or any range between any two of the above values. If the thermal expansion rate 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] As preferred, 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 range between any two of the above values. When the temperature reaches the above melting point, the expandable polymer melts, absorbs heat, and prevents thermal runaway.

[0080] In the embodiments of the present application, the grafting modifier includes at least one of hexafluoropropylene, hexafluoroethylene, and vinylidene fluoride. The fluorine-containing olefin of the grafting modifier can be grafted with the polymer matrix, further improve the adhesion, and make the expansion larger. Moreover, the fluorine-containing olefin and the fluorine-containing polymer (such as PVDF) of the positive electrode active material layer have good affinity in the normal working state, further increasing the adhesion between the polymer layer and the positive electrode active material layer.

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

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

[0083] As preferred, the mass ratio of the polymer matrix to the grafting modifier is (10-30):(1-3). Exemplarily, the mass ratio of the polymer matrix to the grafting modifier is any one of 10:3, 15:2, 10:1, 10:2, 20:1, 30:1 or any range between any two of the above values.

[0084] In the embodiments of the present application, the positive active material layer comprises an intermediate region and an edge region, the intermediate region is at least partially in direct contact with the polymer layer, and the edge region is in direct contact with the positive current collector. The polymer layer is in direct contact with the intermediate region of the positive active material layer, so that when the expandable polymer expands, the positive sheet is subjected to tension in the thickness direction everywhere, and the overall tension is the largest, and the expandable polymer tends to cover the entire positive sheet.

[0085] Preferably, the distance L1 between the edge of the polymer layer and the edge of the positive active material layer is ≥ 3 mm. For example, L1 is any one of 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, or any value within the range formed by any two of the above values. When L1 ≥ 3 mm, the positive current collector and the positive active material layer have sufficient contact area, so that the current between the positive current collector and the positive active material layer is conducted.

[0086] Preferably, the ratio of S3 / S1 is 0.9-1. For example, the ratio of S3 / S1 is any one of 0.9, 0.92, 0.94, 0.96, 0.98, 1, or any value within the range formed by any two of the above values. If the ratio of S3 / S1 is too small, the area of the positive 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 0.6-0.8. For example, the ratio of S2 / S1 is any one 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 value within the range formed by any two of the above values. If the ratio of S2 / S1 is too small, the area of the polymer layer is too small, and the expansion effect is small, which cannot cause the positive active material layer above the polymer layer and the positive active material layer above the positive current collector to be broken and dislocated, and cannot effectively prevent the occurrence of thermal runaway; if the ratio of S2 / S1 is too large, the area of the polymer layer is too large, and the area of the direct contact between the positive active material layer and the positive current collector is too small, which affects the conductivity of the battery under normal working conditions.

[0088] As preferred, the thickness of the polymer layer is 0.5-1.2 μm. Exemplarily, the thickness of the polymer layer is any one of 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 one of the range values formed by any two of the above values. If the thickness of the polymer layer is too small, the expandable polymer is insufficient to cause the positive active material layer above the polymer layer to break and dislocate from the positive active material layer above the positive current collector; if the thickness of the polymer layer is too large, the transmission path of the positive current collector and the positive active material layer is too long, which is not conducive to the conduction in normal state.

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

[0090] As preferred, the number of the plurality of sub-polymer layers is ≥2. Exemplarily, the number of the sub-polymer layers is any one of 2, 4, 6, 8, 10, 12, 14, 16, 18, 20 or any one of the range values formed by any two of the above values.

[0091] In an embodiment of the present application, the plurality of sub-polymer layers are uniformly spaced. The plurality of sub-polymer layers are uniformly spaced have the advantages that, on the one hand, the transmission path of the electrons between the positive active material layer and the positive current collector is shorter (in the case of only one entire polymer layer, the polymer layer is in the middle region, and the electrons need to first reach the edge and then be transmitted between layers), and the conduction performance in normal state is better; on the other hand, the expandable polymer is uniformly dispersed, and can uniformly expand when expanding at high temperature (overcharging), so that the positive plate can be affected by the expandable polymer everywhere.

[0092] As preferred, the distance L2 between the two adjacent sub-polymer layers is ≥3 mm. Exemplarily, L2 is any one of 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm or any one of the range values formed by any two of the above values. When L2 is ≥3 mm, it can be ensured that there is sufficient channel between the sub-polymer layers for the transmission of electrons between the positive current collector and the positive active material layer in normal state.

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

[0094] In an embodiment of the present application, the thermal decomposition gas generating substance comprises 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, which can on the one hand increase the internal resistance and on the other hand have a part of the flame retardant effect.

[0095] In the embodiments of the present application, the aqueous binder comprises at least one of polyacrylic acid (PAA), polyethylene oxide (PEO), and a propanol-based compound.

[0096] As preferred, the mass ratio of the expandable polymer to the thermal decomposition gas generating substance is (50-150):(1-10). For example, the mass ratio of the expandable polymer to the thermal decomposition gas generating substance is any one of 150:1, 110:5, 100:1, 50:1, or any value within the range formed by any two of the above values.

[0097] As preferred, the mass ratio of the expandable polymer to the aqueous binder is (1-3):(1-3). For example, the mass ratio of the expandable polymer to the aqueous binder is any one of 1:1, 1:2, 1:3, 2:1, 3:1, or any value within the range formed by any two of the above values.

[0098] As preferred, the average particle size of the expandable polymer is 1.2-1.5 μm. For example, the average particle size of the expandable polymer is any one of 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, or any value within the range formed by any two of the above values. When the average particle size of the expandable polymer is within this range, a rough polymer layer surface can be formed. The rough polymer layer surface can form a riveting effect with the positive active material layer, so that the polymer layer and the positive active material layer have a certain adhesion, preventing the positive active material layer from peeling off.

[0099] As preferred, the roughness Ra of the surface of the polymer layer away from the positive current collector is ≥0.5 mm. For example, the Ra of the surface of the polymer layer is any one of 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 formed by any two of the above values. The rough polymer layer surface can form a riveting effect with the positive active material layer, so that the polymer layer and the positive active material layer have a certain adhesion, preventing the positive active material layer from peeling off.

[0100] As preferred, the roughness Ra of the surface of the positive current collector is ≥0.5 mm. For example, the Ra of the surface of the positive current collector is any one of 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 formed by any two of the above values. The rough positive current collector surface can better combine with the polymer layer and the positive active material layer, ensuring adhesion and preventing peeling off.

[0101] In a second aspect, the present application provides a method for preparing the positive electrode sheet as described above, comprising the following steps:

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

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

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

[0105] In the embodiment of the present application, in step S1, the organic solvent comprises at least one of N-methyl pyrrolidone (NMP) and acetone.

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

[0107] In the embodiment of the present application, when the polymer layer comprises a thermal decomposition gas generating substance, in step S1, the polymer matrix, the grafting modifier and the organic solvent are mixed as follows: the polymer matrix, the grafting modifier, the thermal decomposition gas generating substance and the organic solvent are mixed.

[0108] In the embodiment of the present application, when the polymer layer comprises a water-based binder, in step S2, the expandable polymer and water are mixed as follows: the expandable polymer, the water-based binder and water are mixed.

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

[0110] In the embodiment of the present application, the battery structure includes but is not limited to a button cell, a soft pack battery, a cylindrical battery, etc.

[0111] The negative electrode sheet, the separator and the electrolyte in the battery of the present application are not particularly limited, and can be selected by those skilled in the art 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 application can be obtained through commercial channels.

[0113] The present application will be further described below in conjunction with examples:

[0114] Example 1:

[0115] 1. Preparation of positive electrode sheet:

[0116] (1) Preparation of expandable polymer:

[0117] Polyethylene (polymer matrix) 100 g, hexafluoropropylene (grafting modifier) 10 g, sodium bicarbonate (thermal decomposition gas generating substance) 5 g were mixed, N-methyl pyrrolidone (NMP) 30 g was added, heated and stirred to mix uniformly, then heated, the heating temperature was 95°C, the time was 5 h, after drying and grinding, an expandable polymer with an average particle size of 1.3 μm was obtained, which included a thermal decomposition gas generating substance.

[0118] (2) Preparation of polymer layer:

[0119] The expandable polymer was dissolved in water, and an aqueous binder polyacrylic acid (PAA) was dispersed at high speed, the mass ratio of the expandable polymer to PAA was 1:1, a slurry was prepared, and was coated on the positive current collector (the length of the positive current collector was 250 mm, the width was 40 mm, and the roughness was 0.5 mm), the coating thickness was 1 μm. Three sub-polymer layers with a length of 79.3 mm were formed in the length direction of the electrode sheet, the interval of each sub-polymer layer in the length direction was 3 mm, two sub-polymer layers with a width of 15.5 mm were formed in the width direction, the interval of each sub-polymer layer in the width direction was 3 mm, the polymer layer was in the middle of the current collector layer, and a polymer layer with a surface roughness of 0.8 mm was formed. The structure diagram of the positive current collector with the polymer layer is shown in Figure 1 .

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

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

[0122] 2. Separator:

[0123] The separator was a polyethylene separator.

[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, and were added to the solvent water to prepare a slurry, which was coated on the negative current collector to obtain a negative electrode sheet.

[0126] 4. Electrolyte:

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

[0128] 5. Assembly of the battery:

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

[0130] Example 2:

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

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

[0133] 2. The grafting modifier is hexafluoroethylene;

[0134] 3. The thermal decomposition gas generating substance is lithium bicarbonate;

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

[0136] Example 3:

[0137] The difference between this example and Example 1 is that there is no multiple sub-polymer layer, but a one-piece polymer layer with equal total area. The structure of the positive electrode current collector with the polymer layer is shown in Figure 3 . The structure of the positive electrode sheet is shown in Figure 4 .

[0138] Example 4:

[0139] The difference between this example and Example 1 is that the particle size of the expandable polymer is 0.5 μm, and the roughness of the sub-polymer layer is 0.4 mm (without forming a riveting effect).

[0140] Comparative Example 1:

[0141] The difference between this comparative example and Example 1 is that the current collector surface is directly coated with a positive electrode active material layer 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 that of the positive electrode active material layer (the positive electrode active material layer is directly and completely insulated from the current collector, cannot conduct electricity, and 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 alumina (alumina is an insulating material that is not expandable).

[0146] Comparative Example 4:

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

[0148] Performance test:

[0149] Test methods include:

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

[0151] The temperature is recorded when the expansion begins at a rate of 5°C / min from room temperature.

[0152] (2) Test method for expansion rate of expandable polymer:

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

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

[0155] The electrical conductivity is tested by film resistance meter.

[0156] (4) Test method for overcharge performance:

[0157] 1C battery charging, observing temperature changes, recording temperature changes until the battery ignites and fails. If the battery does not catch fire or explode during the entire overcharge performance test, it is determined to pass the test (OK); otherwise, it is determined to fail the test (NG).

[0158] (5) Test method for thermal runaway temperature:

[0159] The battery is heated, and the temperature at which the battery swells and smokes is recorded.

[0160] (6) Test method for peel strength:

[0161] The peel strength between the positive active material layer and the polymer layer is tested by a universal testing machine in 180-degree peel 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%, superior electrical conductivity in normal state, good overcharge performance, and can effectively prevent the occurrence of thermal runaway.

[0165] Comparative Example 1, since the current collector surface is not provided with a polymer layer, the overcharge performance is poor, and the occurrence of thermal runaway cannot be prevented.

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

[0167] Comparative Example 3, since the expandable polymer is replaced by alumina, which is an insulating material that cannot expand, when the temperature rises due to overcharge, the positive active material layer above the polymer layer cannot be broken and misaligned with the positive active material layer above the positive current collector, and the occurrence of thermal runaway cannot be prevented.

[0168] Comparative Example 4, since the expandable polymer is not modified by grafting with hexafluoropropylene, the unmodified polymer has a small thermal expansion rate, and when the temperature rises due to overcharge, the expansion coefficient is small, and the positive active material layer above the polymer layer cannot be broken and misaligned with the positive active material layer above the positive current collector, and thermal runaway cannot be prevented.

[0169] The above is only a preferred embodiment of the present application, and it should be noted that for those skilled in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, and these improvements and refinements should also be considered within the scope of protection of the present application.

Claims

1. A positive electrode sheet characterized by comprising: The positive electrode sheet comprises: (a) a positive electrode current collector; (b) a polymer layer, which is arranged on at least one surface of the positive electrode current collector; The polymer layer comprises an expandable polymer, which is obtained by polymerization of a polymer matrix and a grafting modifier, comprising the following steps: mixing the polymer matrix, the grafting modifier and an organic solvent, performing a polymerization reaction, drying and grinding to obtain the expandable polymer; the temperature of the polymerization reaction is 90-100°C, and the time of the polymerization reaction is 4-6h; The polymer matrix comprises at least one of polyethylene, polyvinyl chloride, polypropylene and polymethyl methacrylate; the grafting modifier comprises at least one of hexafluoropropylene, hexafluoroethylene and vinylidene fluoride; the average particle size of the expandable polymer is 1.2-1.5μm; the roughness Ra of the surface of the polymer layer away from the positive electrode current collector is greater than or equal to 0.5mm; (c) a positive electrode active material layer, which is arranged on the 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 denoted as S1, the projected area of the polymer layer is denoted as S2, and the projected area of the positive electrode active material layer is denoted as S3, and the S1, S2 and S3 satisfy the following relationship: S2<S3≤S1; the ratio of S2 / S1 is 0.6-0.8; The positive electrode active material layer is at least partially in direct contact with the positive electrode current collector.

2. The positive electrode sheet according to claim 1, characterized by The expansion temperature of the expandable polymer is 85-95°C; And / or, the thermal expansion rate of the expandable polymer is greater than or equal to 20%; And / or, the melting point of the expandable polymer is 140-160°C; And / or, the resistivity of the polymer matrix is greater than or equal to 10MΩ / m; And / or, the mass ratio of the polymer matrix to the grafting modifier is (10-30):(1-3).

3. The positive electrode sheet according to claim 1, characterized by The positive electrode active material layer comprises 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; And / or, the distance L1 between the edge of the polymer layer and the edge of the positive electrode active material layer is greater than or equal to 3mm; And / or, the ratio of S3 / S1 is 0.9-1; And / or, the thickness of the polymer layer is 0.5-1.2μm.

4. The positive electrode sheet according to claim 1, characterized by The polymer layer comprises one or more sub-polymer layers; The number of the plurality of sub-polymer layers is greater than or equal to 2; And / or, the plurality of sub-polymer layers are uniformly spaced; And / or, the spacing L2 between two adjacent sub-polymer layers is greater than or equal to 3mm.

5. The positive electrode sheet according to claim 1, characterized by The polymer layer further comprises a thermal decomposition gas-producing substance and / or a water-based binder; The thermal decomposition gas-producing substance comprises at least one of sodium bicarbonate, lithium bicarbonate, ammonium bicarbonate and lithium nitrate; And / or, the water-based binder comprises at least one of polyacrylic acid PAA, polyethylene oxide PEO and propanol-based compounds; And / or, the mass ratio of the expandable polymer to the thermal decomposition gas-producing 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 by The roughness Ra of the surface of the positive electrode current collector is greater than or equal to 0.5 mm.

7. A method for producing the positive electrode sheet according to any one of claims 1 to 6, characterized by, The method comprises the following steps: S1, mixing a polymer matrix, a grafting modifier and an organic solvent to perform a polymerization reaction, 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 with a polymer layer; S3, arranging 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 comprises at least one of N-methyl pyrrolidone and acetone.

9. The preparation method according to claim 7, characterized in that, When the polymer layer comprises a thermal decomposition gas generating substance, in the step S1, the polymer matrix, the grafting modifier and the organic solvent are mixed as follows: the polymer matrix, the grafting modifier, the thermal decomposition gas generating substance and the organic solvent are mixed. When the polymer layer comprises an aqueous binder, in the step S2, the swellable polymer and water are mixed as follows: the swellable polymer, the aqueous binder and water are mixed.

10. A battery, characterized by 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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