Positive plate and battery

Abstract

The invention provides a positive plate and a battery, the positive plate comprises a current collector and a positive active layer, the positive active layer comprises a positive active material and a non-fluorine adhesive, the non-fluorine adhesive comprises a first adhesive, the relative crystallinity of the first adhesive meets the following conditions: 10% < = I1 / I2 < = 30%, I1 is the peak area of the diffraction peak of the XRD pattern of the first adhesive when 2 theta is 16-22 degrees, I2 is the peak area of the diffraction peak of the XRD pattern of the first adhesive when 2 theta is 16-22 degrees, and I3 is the peak area of the diffraction peak of the XRD pattern of the first adhesive when 2 theta is 16-22 degrees. I2 is the peak area of all diffraction peaks in the XRD spectrum of the first adhesive; the swelling ratio of the first adhesive is 10-30%. The battery comprising the positive plate has high cycle performance and high energy density.

Description

Technical Field

[0001] The invention relates to a positive electrode sheet, in particular to a positive electrode sheet and a battery, belonging to the field of lithium ion batteries. Background Art

[0002] In recent years, with the rapid development of new energy vehicles, energy storage and other markets, lithium-ion battery research has attracted more and more attention, and the demand for lithium-ion batteries with long cycle life, safe use and good rate characteristics is becoming increasingly urgent.

[0003] Lithium-ion batteries are usually composed of positive electrodes, negative electrodes, separators and electrolytes. However, existing lithium-ion batteries will have serious capacity decay problems after long-term cycles, which may be caused by the positive active layer on the positive electrode falling off after long-term cycles. In addition, the energy density of the battery is also difficult to meet people's increasingly high requirements. In order to meet the requirements of higher and higher energy density, increasing the compaction density of the positive electrode is one of the means to achieve high energy density. However, the increase in compaction density will cause the positive electrode to be hard and brittle after rolling, resulting in abnormal winding process and excessive width of the next battery cell, which will lead to a decrease in energy density.

[0004] Based on the above shortcomings, it is urgent to develop a positive electrode sheet that has both high adhesion and high compaction density, so that the battery can exhibit high cycle performance and high energy density. Summary of the invention

[0005] The present invention provides a positive electrode sheet and a battery. The positive electrode sheet has high adhesion and high compaction density, so that the battery can present high cycle performance and high energy density.

[0006] The present invention provides a positive electrode sheet, wherein the positive electrode sheet comprises a current collector and a positive electrode active layer, the positive electrode active layer comprises a positive electrode active material and a non-fluorine binder, the non-fluorine binder comprises a first binder, and the relative crystallinity of the first binder satisfies: 10%≤I 1 / I 2 ≤30%, wherein the I 1 is the peak area of ​​the diffraction peak at 2θ of 16° to 22° in the XRD spectrum of the first adhesive, wherein I 2 It is the peak area of ​​all diffraction peaks in the XRD spectrum of the first adhesive; the swelling rate of the first adhesive is 10-30%.

[0007] The positive electrode sheet as described above, wherein, based on the mass of the positive electrode active layer, the mass percentage of the non-fluorine adhesive is A, the peeling force between the current collector and the positive electrode active layer is B, A satisfies: 0.8%≤A<2.0%, and B is not less than 25N / m.

[0008] The positive electrode sheet as described above, wherein, based on the mass of the positive electrode active layer, the mass percentage content of the first binder is m, the particle size of the positive electrode active material is D50, and m and D50 satisfy: m = -0.08 * D50 + 2.24, where m satisfies: 0.80 to 1.20%, and D50 satisfies: 8 μm ≤ D50 ≤ 20 μm.

[0009] The positive electrode sheet as described above, wherein the non-fluorine binder further includes a second binder, and the mass percentage content of the second binder in the positive electrode active layer is 0.10 to 0.50%.

[0010] The positive electrode sheet as described above, wherein in the infrared wave number range of 1730 to 1749 cm -1 there is a first characteristic absorption peak of the first binder, and based on the total characteristic peak area of the first binder, the peak area ratio of the first characteristic absorption peak is 5.0 to 20.0%;

[0011] and / or, in the infrared wave number range of 2150 to 2450 cm -1 there is a second characteristic absorption peak of the first binder, and based on the total characteristic peak area of the first binder, the peak area ratio of the second characteristic absorption peak is 20.0 to 35.0%;

[0012] and / or, the dissolution rate of the first binder is not higher than 2.0%;

[0013] and / or, the first binder has no redox peak in the voltage range of 3 to 5V, and the order of magnitude range of the current value of the CV test curve is 10 -4 ~10 -3 .

[0014] The positive electrode sheet as described above, wherein the first binder includes at least one of polyacrylic acid-based binders, polyacrylonitrile-based binders, and polyimide-based binders.

[0015] The positive electrode sheet as described above, wherein the second binder has no redox peak in the voltage range of 3 to 5V, and the order of magnitude range of the current value of the CV test curve is 10 -5 ~10 -4 ;

[0016] and / or, the second binder includes at least one of fluorosulfonates, fluorosulfonylimides, and sulfate esters.

[0017] The positive electrode sheet as described above, wherein the fluorosulfonate includes at least one of N-butyl-N-methylpyrrole trifluoromethanesulfonate and 1-ethyl-3-methylimidazole trifluoromethanesulfonate;

[0018] And / or, the fluorosulfonylimide salt includes at least one of N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide, N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, and N-propyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide;

[0019] And / or, the sulfuric acid ester compound includes at least one of dimethyl sulfate, diethyl sulfate, vinylene sulfite, and propylene sulfite.

[0020] The positive electrode sheet as described above, wherein the areal density of the positive electrode active layer is 18 to 28 mg / cm 2 ;

[0021] And / or, the tap density of the positive electrode active layer is 4.20 to 4.40 g / cc;

[0022] And / or, the thermal weight loss rate of the positive electrode sheet at 200 °C is not higher than 3.0%.

[0023] The present invention also provides a battery including the positive electrode sheet as described above.

[0024] The positive electrode sheet of the present invention includes a current collector and a positive electrode active layer. The positive electrode active layer includes a positive electrode active material and a non-fluorine binder. The non-fluorine binder includes a first binder. The relative crystallinity of the first binder satisfies: 10% ≤ I 1 / I 2 ≤ 30%, where I 1 is the peak area of the diffraction peak of the XRD pattern of the first binder at 16° to 22°, and I 2 is the peak area of all diffraction peaks in the XRD pattern of the first binder; the swelling rate of the first binder is 10 to 30%. When the positive electrode sheet including the first binder is applied to a battery, the electrolyte has less damage to the structure of the binder, and can still maintain a high adhesive force in the later stage of cycling, maintaining the stability of the structure of the positive electrode sheet, so that the battery has high cycling performance.

[0025] The battery provided by the present invention is prepared based on the above positive electrode sheet and has high cycling performance and high energy density. Description of the Drawings

[0026] Figure 1 It is the cyclic voltammogram of the first binder in Example 1 of the present invention;

[0027] Figure 2 It is the infrared absorption spectrum of the first binder and PVDF in Example 1 of the present invention;

[0028] Figure 3 It is the cyclic voltammogram of the second binder in Example 1 of the present invention;

[0029] Figure 4 TG diagram of the first binder in Example 1 of the present invention;

[0030] Figure 5 Flexibility test diagram of the positive electrode sheet including Comparative Example 1 and the positive electrode sheet of Example 1;

[0031] Figure 6 Four-probe resistance test diagram of the positive electrode sheet of Comparative Example 1 and the positive electrode sheet of Example 1;

[0032] Figure 7 Two-probe resistance test diagram of the positive electrode sheet of Comparative Example 1 and the positive electrode sheet of Example 1;

[0033] Figure 8 Peeling strength diagram of the positive electrode sheet of Comparative Example 1 and the positive electrode sheet of Example 1. Detailed implementation manners

[0034] To make the objectives, technical solutions and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the protection scope of the present invention.

[0035] The present invention provides a positive electrode sheet, which includes a current collector and a positive electrode active layer. The positive electrode active layer includes a positive electrode active material and a non-fluorine binder. The non-fluorine binder includes a first binder, and the relative crystallinity of the first binder satisfies: 10% ≤ I 1 / I 2 ≤ 30%, where I 1 is the peak area of the diffraction peak of the XRD pattern of the first binder at 2θ of 16° - 22°, and I 2 is the peak area of all diffraction peaks in the XRD pattern of the first binder; the swelling rate of the first binder is 10 - 30%.

[0036] The positive electrode sheet of the present invention includes a current collector. The present invention does not limit the specific selection of the current collector, which can be selected according to actual needs, such as aluminum foil, etc.

[0037] The positive electrode sheet of the present invention includes a positive electrode active layer, and the positive electrode active layer may include a positive electrode active material, a conductive agent, a binder, etc. The positive electrode active material may be selected from one or more of lithium cobaltate, lithium nickelate, lithium manganate, lithium manganese oxide, nickel cobalt manganese ternary material, nickel cobalt aluminum ternary material, lithium iron phosphate, lithium nickel manganate, etc.; the conductive agent may be selected from one or more of conductive carbon black, Super-C, acetylene black, Ketjen black, carbon nanofibers. The binder is the non-fluorine binder described in the present invention. The present invention does not limit the specific selection of each component in the positive electrode active layer, and can be selected according to actual needs.

[0038] The positive electrode active layer of the present invention includes a non-fluorine binder, and the non-fluorine binder refers to a binder that does not contain per- and polyfluoroalkyl substances (PFAS). Since the non-fluorine binder does not contain PFAS and is environmentally friendly, the positive electrode sheet can be widely used, which is conducive to the wide application of subsequent products such as batteries.

[0039] Specifically, the first binder of the present invention is subjected to XRD testing to obtain the XRD pattern of the first binder. According to the XRD pattern of the first binder, the relative crystallinity of the first binder can be determined. For example, the relative crystallinity of the first binder can be 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28% or 30%, etc. The higher the crystallinity of the first binder, the stronger the interaction between the molecular chains of the first binder, making the first binder exhibit a higher adhesion strength, which can enhance the binding force between the positive electrode active material, the conductive agent and the current collector, and reduce the shedding of the positive electrode active material caused by volume change during the cycle, thereby improving the cycle performance of the battery. If the crystallinity is too high, it will lead to an increase in the brittleness of the material, and the stress generated by the expansion of the positive electrode active material during charge and discharge is likely to cause microcracks, resulting in the peeling of the positive electrode active material from the current collector. Therefore, only when the crystallinity of the first binder is within the above range can the cycle performance of the battery be improved.

[0040] In a specific embodiment, the XRD pattern of the first binder includes diffraction peaks at 2θ of 16° to 22°. For example, the XRD pattern of the first binder includes diffraction peaks at 2θ of 16°, 16.5°, 17°, 17.5°, 18°, 18.5°, 19°, 19.5°, 20°, 20.5°, 21°, 21.5° or 22°. The 2θ position in the XRD pattern of the first binder can reflect the crystallinity of the first binder. Only when the 2θ position in the XRD pattern of the first binder is within the above range, the crystallinity of the first binder can be within the above range.

[0041] In a specific embodiment, the conditions for testing the first adhesive of the present invention by XRD can be as follows: The first adhesive is placed in an oven at 85 °C and dried for 24 h until all the solvent has evaporated to obtain a film of the first adhesive. Using conventional scanning, X-ray diffraction is performed on the film of the first adhesive in the range of 10 °C - 90 °C, and its XRD pattern is analyzed to obtain the relative crystallinity of the first adhesive.

[0042] In a specific embodiment, the swelling ratio of the first binder is 10 - 30%, for example, 10%, 12%, 14%, 16%, 18%, 20%, 22%, 24%, 26%, 28% or 30%, etc. The cross-linking density of the polymer structure in the first binder is large, and the resistance of small solvent molecules in the electrolyte to penetrate into the polymer is large, resulting in low swelling of the first binder. Therefore, the dissolution of the electrode sheet prepared with the first adhesive of the present invention is very low, which means that during the cycling process, the structure of the first adhesive is stable, the electrolyte has less damage to the structure of the first adhesive, and a high adhesive force can still be maintained in the later stage of cycling, maintaining the stability of the structure of the positive electrode sheet and improving the capacity retention rate, making the battery have high cycling performance. If the swelling ratio is too high, small solvent molecules in the electrolyte are likely to penetrate into the first adhesive, that is, the electrolyte has a large destructive effect on the structure of the first adhesive, resulting in the inability of the first adhesive to maintain a high adhesive force in the later stage of cycling, reducing the binding force between the positive active material, the conductive agent and the current collector, and increasing the shedding of the positive active material caused by volume change during cycling, thereby reducing the cycling performance of the battery. If the swelling ratio is too low, the spacing of the polymer chains in the first adhesive cannot be effectively expanded, resulting in the diffusion path of lithium ions in the first adhesive being blocked, thereby reducing the cycling performance. Therefore, only when the swelling ratio of the first adhesive is within the above range can the cycling performance of the battery be improved.

[0043] It can be understood that in the present invention, the "swelling ratio of the first binder" can be obtained by making the first binder into a film of 30 mm * 10 mm * 1 mm, soaking it in an electrolyte of EC:PC:DEC:PP = 15:10:10:60 at 80 °C for 72 h, and then testing. The average value of multiple measurement results can be used as the swelling ratio of the first adhesive.

[0044] In a specific embodiment, based on the mass of the positive active layer, the mass percentage content of the non-fluorine binder is A, and the peel force between the current collector and the positive active layer is B. A satisfies: 0.8% ≤ A < 2.0%. For example, A can be 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%;

[0045] B is not less than 25 N / m. For example, B can be 25 N / m, 26 N / m, 27 N / m, 28 N / m, 29 N / m, 30 N / m, 31 N / m, 32 N / m, 33 N / m, 34 N / m, 35 N / m, 36 N / m, 37 N / m, 38 N / m, 39 N / m or 40 N / m, etc.

[0046] The increase in the amount of non-fluorine adhesive can improve its wettability and distribution between the current collector and the active layer, forming a tighter mechanical interlock or chemical bonding, thereby increasing the peel force. When the amount of non-fluorine adhesive reaches a certain level, the interfacial bonding is already close to saturation (such as forming a continuous film), and continuing to increase the amount of non-fluorine adhesive has limited improvement on the actual effective bonding.

[0047] The present invention does not limit the method for achieving the peel force between the current collector and the positive electrode active layer. For example, different positive electrode active materials, conductive agents, and binders can be used to prepare the positive electrode active layer so that the peel force between the current collector and the positive electrode active layer is within the above range.

[0048] Specifically, the test method for the peel force between the current collector and the positive electrode active layer includes the following steps:

[0049] (1) Sample preparation: Cut the positive electrode sheet to be tested into specimens of a fixed size; (2) Glue application: Bond one side of the specimen to be tested with double-sided tape and press it firmly with a roller to make the double-sided tape fit perfectly with the electrode sheet; Stick the other side of the double-sided tape to the surface of the stainless steel substrate and bend one end of the specimen by 180 degrees; (3) Testing: Fix one end of the stainless steel substrate to the lower clamp of the tensile machine, fix the bent end of the specimen to the upper clamp, and stretch it at a fixed speed until the specimen is completely peeled off. The force during the stretching process is the peel force between the current collector and the positive electrode active layer.

[0050] In a specific embodiment, based on the mass of the positive electrode active layer, the mass percentage content of the first binder is m, and the particle size of the positive electrode active material is D50. m and D50 satisfy: m = -0.08 * D50 + 2.24; where m satisfies: 0.8 to 1.2%, for example, 0.8%, 0.9%, 1%, 1.1% or 1.2%, etc.; D50 satisfies: 8μm ≤ D50 ≤ 20μm. For example, D50 is 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm or 20μm. In order to achieve the same adhesive force, the smaller the particle size of the positive electrode active material, the more the amount of the first binder used. However, too much of the first binder will result in high impedance, poor rate performance and poor cycle performance of the battery; while too little of the first binder cannot achieve a high adhesive force, resulting in poor cycle performance of the battery. Therefore, when the mass percentage content m of the first binder in the positive electrode active layer and D50 of the positive electrode active material in the positive electrode active layer satisfy the above relationship, it not only has a high adhesive force, but also has low impedance, excellent rate performance and cycle performance of the battery.

[0051] D50 (50% volume average particle size) refers to the diameter of the particle at the 50% position of the volume when the particles in the positive electrode active material are arranged in ascending order of particle size.

[0052] The particle size of the positive electrode active material can be measured by a laser particle size analysis method. Specifically, a certain amount of positive electrode active material particles are dispersed in deionized water containing a dispersant (such as nonylphenol polyoxyethylene ether, content 0.02 - 0.03 wt%) to form a mixture. The mixture is ultrasonicated for 2 minutes and then put into a Malvern particle size analyzer for testing to obtain the average particle size.

[0053] In a specific embodiment, the non-fluorine binder further includes a second binder, and the mass percentage content of the second binder in the positive electrode active layer is 0.1-0.5%, such as 0.1%, 0.2%, 0.3%, 0.4% or 0.5%, etc. When the mass percentage contents of the first binder and the second binder are respectively within the above ranges, the first binder and the second binder can fully cooperate with each other, which can improve the adhesiveness of the positive electrode sheet, so that the peel strength between the positive electrode current collector and the positive electrode active layer is not less than 25 N / m, such as 25 N / m, 26 N / m, 28 N / m, 30 N / m, 32 N / m, 35 N / m or 37 N / m, etc.; and the cohesive strength of the positive electrode active layer is not less than 30 N / m, such as 30 N / m, 31 N / m, 32 N / m, 33 N / m, 34 N / m, 35 N / m, 36 N / m, 37 N / m, 38 N / m, 39 N / m, 40 N / m, etc.; the higher cohesive strength and peel strength can reduce the probability of the positive electrode active layer falling off during the cycling process, avoid problems such as explosion and fire caused by the short circuit between the cathode and the aluminum foil, and at the same time can reduce the repeated shrinkage and expansion of the positive electrode active material during the cycling process, avoid problems such as powder falling off from the positive electrode active layer and even the structure collapse of the positive electrode active material, thereby improving the cycling performance of the battery.

[0054] Among them, the cohesive strength of the positive electrode active layer refers to the attraction between the components inside the positive electrode active layer. Exemplarily, the cohesive strength of the positive electrode active layer can be measured by the powder pressing method, including the following steps: (1) Sample preparation: Take the positive electrode sheet and scrape off the positive electrode active layer powder on the surface of the positive electrode current collector with a blade; (2) Pressing: Put the positive electrode active layer powder into a tablet press to make a sample; (3) Strength test: Use a tensile machine to test the breaking strength and displacement of the sample; (4) Calculation: Determine the cohesive strength of the positive electrode active layer according to the breaking strength and displacement of different samples.

[0055] In a specific embodiment, in the range of infrared wave numbers 1730-1749 cm -1 there is a first characteristic absorption peak for the first binder. Based on the total characteristic peak area of the first binder, the peak area ratio of the first characteristic absorption peak is 5.0-20.0%. When the first binder is in the range of infrared wave numbers 1730-1749 cm -1When a characteristic absorption peak exists within the range, it indicates that the first adhesive contains carboxyl groups (-COOH). Carboxyl groups can form strong hydrogen bond interactions with active substances (current collectors, cathode active materials) with hydroxyl groups on the surface, which can enhance the adhesion between the cathode active layer and the current collector, prevent the cathode active layer from peeling off during cycling, and thus improve the cycling performance of the battery. However, if the peak area ratio of carboxyl groups is too low (less than 20%), it may lead to insufficient adhesion between the cathode active layer and the current collector, causing the cathode active layer to peel off during cycling, thereby reducing the cycling performance of the battery; if the peak area ratio of carboxyl groups is too high (exceeding 35%), it may lead to an increase in the rigidity of the molecular chains in the cathode active layer, reducing the flexibility of the cathode active layer, making the cathode active layer unable to adapt to the volume changes during the charge and discharge process of the electrode, thereby triggering the peeling off of the cathode active layer and reducing the cycling performance of the battery.

[0056] In a specific embodiment, within the range of infrared wave numbers 2150 - 2450 cm -1 , the first binder has a second characteristic absorption peak. Based on the total characteristic peak area of the positive electrode sheet, the peak area ratio of the second characteristic absorption peak is 20.0 - 35.0%. When the first adhesive has a characteristic absorption peak within the range of infrared wave numbers 2150 - 2450 cm -1 , it indicates that the first adhesive contains cyano groups (-CN). Cyano groups have high polarity and can form strong hydrogen bond interactions with active substances (current collectors, cathode active materials) with hydroxyl groups on the surface, which can enhance the adhesion between the cathode active layer and the current collector, prevent the cathode active layer from peeling off during cycling, and thus improve the cycling performance of the battery. However, if the peak area ratio of cyano groups is too low (less than 20%), it may lead to insufficient adhesion between the cathode active layer and the current collector, causing the cathode active layer to peel off during cycling, thereby reducing the cycling performance of the battery; if the peak area ratio of cyano groups is too high (exceeding 35%), it may lead to an increase in the rigidity of the molecular chains in the cathode active layer, reducing the flexibility of the cathode active layer, making the cathode active layer unable to adapt to the volume changes during the charge and discharge process of the electrode, thereby triggering the peeling off of the cathode active layer and reducing the cycling performance of the battery.

[0057] In a specific embodiment, based on the peak area of the total characteristic absorption peak of the first adhesive, the ratio of the peak area of the second characteristic absorption peak to the peak area of the first characteristic absorption peak is R, and R satisfies: 2.0 ≤ R ≤ 3.4. If the R value is too low, that is, there are too many carboxyl groups, it may cause electrolyte decomposition or side reactions, affecting the cycle performance of the battery; if the R value is too high, that is, there are too many cyano groups, it may lead to an increase in the rigidity of the molecular chain in the positive electrode active layer, reducing the flexibility of the positive electrode active layer, making the positive electrode active layer unable to adapt to the volume change during the charge and discharge process of the electrode, thereby causing the shedding of the positive electrode active layer and reducing the cycle performance of the battery. When R is within the above range, the carboxyl group and the cyano group can form a strong hydrogen bond with the active substances (current collector, positive electrode active substance) on the surface hydroxyl group; it can improve the adhesion between the positive electrode active layer and the current collector, avoid the shedding of the positive electrode active layer during the cycle, and thus improve the cycle performance of the battery.

[0058] Specifically, the present invention performs an infrared absorption spectrum test on the first binder to obtain the infrared absorption spectrum of the first binder.

[0059] In a specific embodiment, the dissolution rate of the first adhesive is not higher than 2.0%, for example, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9% or 2.0%, etc. Since the contact between the short branched chains inside the polymer structure of the first adhesive and the electrolyte decreases, the dissolved part of the similar phase solubility structure is smaller. Therefore, the dissolution of the electrode sheet prepared by the first adhesive of the present invention is very low, which means that during the cycle, the structure of the first adhesive is stable, the electrolyte has less damage to the structure of the first adhesive, and a high adhesive force can still be maintained in the later stage of the cycle, maintaining the stability of the positive electrode sheet structure and improving the capacity retention rate, making the battery have high cycle performance.

[0060] It can be understood that in the present invention, the "dissolution rate of the first adhesive" can be obtained by making the first adhesive into a 30mm * 10mm * 1mm adhesive film, soaking it in an electrolyte of EC:PC:DEC:PP = 15:10:10:60, and testing it after soaking at 80 °C for 72 h. The average value of multiple measurement results can be used as the dissolution rate of the first adhesive.

[0061] In a specific embodiment, the glass transition temperature of the first adhesive is -30 to 60 °C, such as -30 °C, -20 °C, -10 °C, 0 °C, 10 °C, 20 °C, 30 °C, 40 °C, 50 °C or 60 °C, etc. The glass transition temperature is directly related to the flexibility of the polymer chain. The greater the flexibility of the molecular chain, the lower the glass transition temperature; the greater the rigidity of the molecular chain, the higher the glass transition temperature. When the glass transition temperature of the first adhesive is within the above range, the toughness of the first adhesive is better, which can improve the toughness of the positive electrode sheet and the energy density of the battery.

[0062] It can be understood that in the present invention, the DSC curve of the first adhesive can be obtained by DSC test, and the "glass transition temperature of the first adhesive" can be analyzed from the DSC curve. Among them, the conditions for DSC test are: placing the first adhesive in an oven at 85 °C and drying it for 24 h until all the solvent volatilizes to obtain the adhesive film of the first adhesive. The DSC test conditions are: (1) cooling from room temperature to -60 °C at a cooling rate of 10 °C / min; (2) maintaining at -60 °C for 20 min; (3) heating to 200 °C at a heating rate of 5 °C / min.

[0063] In a specific embodiment, the first binder has no redox peak in the voltage range of 3 to 5 V, and the order of magnitude range of the current value of the CV test curve is 10 -4 ~10 -3 , that is, the first adhesive can work stably within the above voltage range. The first binder has excellent high-voltage resistance, which can prevent the first binder from decomposing and causing the positive active layer to shed powder or the battery to have side reactions when the battery works at high voltage, thereby improving the cycle performance of the battery.

[0064] Specifically, the CV test includes the following steps: (1) Preparation of button cell: Coating the first adhesive on the surface of the positive current collector (such as aluminum foil); drying at 85 °C and punching into small round pieces with a diameter of 12 mm; arranging the lithium sheet, the current collector coated with the adhesive and the stainless steel in sequence to make a button cell, adding a spring gasket on one side of the stainless steel to ensure full contact, and dropping the electrolyte to complete the preparation of the button cell; (2) Testing: The test voltage range is 3 V to 5 V; the scanning frequency is 0.2 mV / s; scanning 3 cycles to obtain the CV curve graph.

[0065] In a specific embodiment, the first binder includes at least one of polyacrylic acid binders, polyacrylonitrile binders, and polyimide binders. When the first binder selects the above compounds, the positive electrode sheet can have both high adhesiveness and high tap density, thereby improving the cycle performance and energy density of the battery. When the first binder is a polyacrylic acid binder and / or a polyacrylonitrile binder, based on the total amount of functional groups of carboxyl, cyano, and ester groups in the first binder, the functional group ratio of carboxyl, cyano, and ester groups in the first binder satisfies 50%: 30% - 40%: 10% - 20%.

[0066] In a specific embodiment, the second binder exists in a liquid form within the range of 80 - 200°C. The second binder does not volatilize at high temperatures and exists in a liquid form within the range of 80 - 200°C, having high stability, which can improve the thermal stability of the positive electrode sheet, avoid side reactions of the positive electrode sheet at high temperatures, and thus can improve the cycle performance of the battery.

[0067] In a specific embodiment, the viscosity of the second binder is 0.5 - 5 mPa·s, for example, 0.5 mPa·s, 1 mPa·s, 1.5 mPa·s, 2 mPa·s, 2.5 mPa·s, 3 mPa·s, 3.5 mPa·s, 4 mPa·s, 4.5 mPa·s, or 5 mPa·s, etc. When the viscosity of the second binder is within the above range, the second binder has a low viscosity and similar properties to the electrolyte. When the battery including the positive electrode sheet is formed, the second binder can dissolve from the positive electrode sheet into the electrolyte, thereby improving the conductivity of the electrolyte, further improving the ionic conductivity of the battery, reducing the impedance of the battery, and thus being able to improve the cycle performance of the battery to a greater extent.

[0068] The viscosity of the second binder can be measured by the rotational viscometer method. Specifically, by measuring the torque required for the rotating disk to rotate in the second binder, the viscosity of the second binder is calculated.

[0069] In a specific embodiment, the relative dielectric constant of the second binder is 2 - 100, for example, 2, 5, 10, 20, 30, 40, 50, 60, 70, 80, 90, or 100. When the relative dielectric constant of the second binder is within the above range, more ion channels can be provided to promote the migration of lithium ions, the internal resistance of the battery can be reduced, and thus the charge-discharge performance of the battery can be improved.

[0070] It can be understood that in the present invention, the relative dielectric constant of the second binder can be obtained by capacitance method testing. The specific testing method is: filling the second binder into a capacitor, then by measuring the capacitance value of the capacitor, and combining the geometric parameters of the capacitor and the dielectric constant in air, the relative dielectric constant of the second binder is calculated.

[0071] In a specific embodiment, the ionic conductivity of the second adhesive is 10 -4 ~10 -3 S / cm. The ionic conductivity is positively correlated with the relative dielectric constant. The larger the dielectric constant, the higher the ionic conductivity. When the ionic conductivity of the second adhesive is within the above range, more ionic channels can be provided to promote the migration of lithium ions, reduce the internal resistance of the battery, and thus improve the charge-discharge performance of the battery.

[0072] It can be understood that in the present invention, the ionic conductivity of the second adhesive can be obtained through ionic conductivity testing. The specific testing method is as follows: After applying a certain external electric field to the second adhesive, under the action of the electric field, ions will migrate to form a current. By measuring the magnitude of the current and the applied electric field strength, the ion mobility and conductivity can be calculated. The formula is: ionic conductivity = ion concentration * ion mobility. It should be noted that the second adhesive of the present invention is a solvent type itself, so the ionic conductivity of the second adhesive can be directly tested without preparing a solution additionally.

[0073] In a specific embodiment, the second binder has no redox peak in the voltage range of 3 - 5V, and the order of magnitude range of the current value of the CV test curve is 10 -5 ~10 -4 , that is, the second adhesive works stably within the above voltage range. The second binder has excellent high-voltage resistance, which can ensure the stability of the positive electrode sheet under high voltage, avoid problems such as powder falling and structural collapse of the positive electrode sheet, and thus improve the cycle performance of the battery.

[0074] In a specific embodiment, the dissolution rate w of the positive electrode sheet including the second adhesive satisfies: 0.30% < w < 0.60%, for example, w is 0.30%, 0.35%, 0.40%, 0.45%, 0.50%, 0.55% or 0.60%, etc. When the dissolution rate of the positive electrode sheet containing the second adhesive is within the above range, it indicates that the electrolyte has little damage to the structure of the positive electrode sheet, and the structure of the positive electrode sheet is more stable in the later stage of long-term cycling.

[0075] The "dissolution rate of the positive electrode sheet including the second adhesive" described in the present invention can be understood as when the positive electrode sheet includes the second adhesive, the dissolution rate of the positive electrode sheet is w.

[0076] In a specific embodiment, the second binder includes at least one of fluorosulfonates, fluorosulfonylimides, and sulfate esters.

[0077] Preferably, the fluorosulfonate includes at least one of N-butyl-N-methylpyrrole trifluoromethanesulfonate and 1-ethyl-3-methylimidazole trifluoromethanesulfonate, the fluorosulfonimide salt includes at least one of N-butyl-N-methylpyrrolidine bis(fluorosulfonyl)imide salt, N-butyl-N-methylpyrrolidine bis(trifluoromethanesulfonyl)imide salt, and N-propyl-N-methylpyrrolidine bis(trifluoromethanesulfonyl)imide salt, and the sulfate ester compound includes at least one of dimethyl sulfate, diethyl sulfate, ethylene sulfite, and propylene sulfite. When the second binder selects the above compounds, the second binder and the first binder can act synergistically, enabling the positive electrode sheet to have both high adhesiveness and high tap density, thereby improving the cycle performance and energy density of the battery.

[0078] In a specific embodiment, the areal density of the positive electrode active layer is 18-28 mg / cm 2 , and the tap density of the positive electrode active layer is 4.20-4.40 g / cc. For example, the areal density of the positive electrode active layer can be 18 mg / cm 2 , 18.5 mg / cm 2 , 19 mg / cm 2 , 19.5 mg / cm 2 , 20 mg / cm 2 , 20.5 mg / cm 2 , 21 mg / cm 2 , 21.5 mg / cm 2 , 22 mg / cm 2 , 22.5 mg / cm 2 , 23 mg / cm 2 , 23.5 mg / cm 2 , 24 mg / cm 2 , 24.5 mg / cm 2 , 25 mg / cm 2 , 25.5 mg / cm 2 , 26 mg / cm 2 , 26.5 mg / cm 2 , 27 mg / cm 2 , 27.5 mg / cm 2 or 28 mg / cm 2 etc., preferably 20-25 mg / cm 2The tap density of the positive electrode active layer can be 4.20 g / cc, 4.21 g / cc, 4.22 g / cc, 4.23 g / cc, 4.24 g / cc, 4.25 g / cc, 4.26 g / cc, 4.27 g / cc, 4.28 g / cc, 4.29 g / cc, 4.3 g / cc, 4.31 g / cc, 4.32 g / cc, 4.33 g / cc, 4.34 g / cc, 4.35 g / cc, 4.36 g / cc, 4.37 g / cc, 4.38 g / cc, 4.39 g / cc or 4.4 g / cc, etc., preferably 4.25 to 4.35 g / cc. When the areal density and tap density of the positive electrode active layer meet the above ranges, the thickness of the battery can be reduced, and the energy density of the battery can be increased. Moreover, when the tap density of the positive electrode sheet is within this range, problems such as brittleness, powder shedding, and abnormal winding process will not occur during the rolling of the positive electrode sheet, thereby maintaining the high energy density of the battery.

[0079] In the present invention, the areal density of the positive electrode active layer is the coating areal density of the positive electrode active paste. The tap density of the positive electrode active layer is the parameter of rolling after the positive electrode active paste is coated on the current collector and dried.

[0080] In a specific embodiment, a thermogravimetric test is performed on the positive electrode sheet. The thermogravimetric loss rate of the positive electrode sheet at 200 °C is not higher than 3.0%, such as 1%, 1.2%, 1.4%, 1.6%, 1.8%, 2%, 2.2%, 2.4%, 2.6%, 2.8% or 3%, etc. When the thermogravimetric loss rate of the positive electrode sheet is within the above range, it indicates that the positive electrode active layer has excellent thermal stability. Even if the battery including the positive electrode sheet operates in a cycle at a high temperature, the positive electrode active layer can still perform its function, and the occurrence of side reactions caused by the decomposition of the components in the positive electrode active layer can be avoided, thereby ensuring the high cycle performance of the battery.

[0081] The present invention does not limit the method for achieving the thermogravimetric loss rate of the positive electrode sheet. For example, the contents of the positive electrode active material, conductive agent, and binder in the positive electrode active layer can be adjusted to make the thermogravimetric loss rate of the positive electrode sheet within the above range.

[0082] Since only the first binder in the positive electrode sheet affects the thermogravimetric loss rate of the positive electrode sheet, the thermogravimetric loss rate of the first binder can be used in the present invention to indirectly evaluate the thermogravimetric loss rate of the positive electrode sheet.

[0083] Specifically, the thermogravimetric loss rate of the first binder can be tested by a thermogravimetric analyzer.

[0084] In a specific embodiment, the positive electrode active layer is formed by coating a positive electrode slurry on a current collector and drying it. The viscosity of the positive electrode slurry is 2000 - 7000 mPa·s, such as 2000 mPa·s, 4500 mPa·s, 5000 mPa·s, 5500 mPa·s, 6000 mPa·s, 6500 mPa·s or 7000 mPa·s, and the solid content of the positive electrode slurry is 73 - 76%, such as 73%, 73.5%, 74%, 74.5%, 75%, 75.5% or 76%, etc. When the viscosity and solid content of the positive electrode slurry are within the above ranges respectively, it is beneficial to improve the leveling property of the coating, so as to ensure the uniform coating thickness of the positive electrode sheet, avoid the uneven thickness of the positive electrode active layer, shorten the migration path of lithium ions, and thus improve the cycle performance of the battery.

[0085] It should be noted that the positive electrode sheet of the present invention is the positive electrode sheet after formation.

[0086] In a specific embodiment, before the battery including the positive electrode sheet is formed, the second binder exists in the positive electrode active layer. After the battery including the positive electrode sheet is formed, the second binder is partially dissolved into the electrolyte. Before the battery including the positive electrode sheet is formed, the second binder is physically embedded into the polymer chain segments of the first binder, reducing the secondary forces (hydrogen bonds, van der Waals forces) between / within the molecular chains of the first binder, so as to improve the brittleness of the positive electrode sheet, increase the toughness of the positive electrode sheet, and increase the energy density of the battery. After the battery including the positive electrode sheet is formed, the second binder is dissolved into the electrolyte, and the position occupied by the second binder in the positive electrode active layer will turn into pores in the positive electrode active layer, increasing the porosity of the positive electrode active layer, so as to increase the infiltration rate and liquid retention capacity of the electrolyte to the positive electrode active layer, and can also increase the conductivity of the electrolyte, thus improving the battery cycle performance. However, if the amount of the first binder is too small, an insufficient bonding network cannot be formed, resulting in a decrease in the peeling force between the positive electrode current collector and the positive electrode active layer, and the shedding of the positive electrode active layer during cycling. At this time, even if the second binder is embedded into the chain segments of the first binder before formation, the problems caused by the insufficient amount of the first binder cannot be compensated. After the second binder is dissolved and forms pores after formation, if the amount of the second binder is too large, the porosity of the positive electrode active layer will be too high. The too high porosity may damage the conductive network, increase the electron transfer impedance, and at the same time, excessive infiltration of the electrolyte may lead to an aggravation of side reactions (such as an increase in the contact area between the positive electrode material and the electrolyte, accelerating the dissolution of transition metals). Therefore, only when the mass percentage contents of the first binder and the second binder are within the above ranges respectively, the first binder and the second binder can fully cooperate, improve the adhesion of the positive electrode sheet, reduce the probability of the positive electrode active layer shedding during cycling, and thus improve the battery cycle performance.

[0087] The present invention also provides a battery including the positive electrode sheet as described above. The battery is prepared based on the above positive electrode sheet and has excellent cycle performance and energy density.

[0088] In a specific embodiment, the battery further includes a negative electrode sheet, a separator, and an electrolyte. Specifically:

[0089] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer. The negative electrode active material layer includes a negative electrode active material, a conductive agent, and a binder. Among them, the negative electrode current collector is generally a copper foil, and the negative electrode active material is selected from one or more of carbonaceous materials, silicon-carbon materials, alloy materials, and sodium-containing metal composite oxides.

[0090] The present invention does not limit the selection of the conductive agent and the binder in the negative electrode active layer. For example, the conductive agent and the binder can be selected from conventional materials in the art and can be specifically set according to needs. The present invention does not limit the preparation methods of the positive electrode sheet and the negative electrode sheet, and can be selected from conventional technical means in the art or other preparation processes, and can be specifically selected according to requirements.

[0091] The separator is a separator well-known in the art that can be used in the battery and is stable to the electrolyte used, and can include one or more of polyolefin, aromatic polyamide, polytetrafluoroethylene, and polyethersulfone, and can be specifically set according to needs.

[0092] The electrolyte is an electrolyte well-known in the art that can be used in the battery and makes the electrochemical performance of the battery excellent, and includes a lithium salt and an organic solvent, and can be specifically set according to needs.

[0093] Hereinafter, the present invention will be further described in detail through specific examples.

[0094] Example 1

[0095] The preparation process of the battery provided in this example includes the following steps:

[0096] 1. Preparation of the positive electrode sheet: According to the mass ratio of lithium cobaltate, conductive agent, and non-fluorine binder of 97.6:1.44:0.96 respectively, where the non-fluorine binder includes a first binder, the conductive agent includes conductive carbon black (SP) and carbon nanotubes (CNT), and the mass ratio of SP and CNT is 1:1, and the D50 of the positive electrode active material (lithium cobaltate) is 16 μm, prepare the positive electrode slurry according to the following batching process: The first binder (carboxyl: cyano: ester group = 50%: 34%: 16%, relative crystallinity is 20%, swelling rate is 24%, dissolution rate is 1.06%, glass transition temperature is 48 °C, and its XRD pattern includes a diffraction peak at 2θ of 19.4 °, and the cyclic voltammogram is shown in Figure 1) It is added to N-methylpyrrolidone (NMP), stirred at 1600 rpm for 30 min to fully disperse the first binder in NMP; then SP and CNT are added, stirred at 1600 rpm for 20 min, and then the vacuum is started, and stirring continues for 2 h to ensure uniform dispersion of the conductive agent; stirred at 1600 rpm for 30 min; finally, lithium cobaltate is added and stirred at 1600 rpm for 20 min. The vacuum needs to be closed at this stage, and the vacuum is started again after 20 min. After stirring for 4 h, the batching is completed.

[0097] The temperature of the entire batching process is maintained at 15 - 35 °C, and the viscosity of the slurry is controlled at about 6000 mPa·s. The positive electrode active slurry is coated on the aluminum foil by spraying: the coating speed is controlled at 20 m / min, and the coating areal density is 20 mg / cm 2 , and the wet film is dried in three different ovens: the temperature of the first stage is controlled at 100 °C, the temperature of the second stage is controlled at 130 °C, and the temperature of the third stage is controlled at 110 °C. Then, it is roll-pressed and slit to obtain the positive electrode sheet, and the compaction density during roll-pressing is 4.25 g / cc.

[0098] 2. Preparation of the negative electrode sheet: The negative electrode active material graphite (specific capacity of 355 mAh / g, Dv50 of 12 μm), conductive agent carbon black, thickening agent lithium carboxymethyl cellulose (CMC-Li), and binder styrene-butadiene rubber are mixed according to a weight ratio of 97.2%:0.5%:1.0%:1.3%, and deionized water is added to obtain the negative electrode slurry under the action of a vacuum mixer; the negative electrode slurry is uniformly coated on the negative electrode current collector copper foil, and the coating areal density is 12 mg / cm 2 , dried at 85 °C and then cold-pressed, trimmed, sliced, and slit. After slitting, it is dried at 85 °C for 4 h under vacuum conditions, and the tab is welded to obtain the negative electrode sheet.

[0099] 3. Preparation of the electrolyte: In a glove box filled with argon (moisture < 10 ppm, oxygen content < 1 ppm), the solvents EC:PC:DEC:PP = 15:10:10:65 are mixed evenly, and lithium hexafluorophosphate that has been fully dried is quickly added to the mixed solvent, and after fully mixing evenly, the electrolyte is obtained. Among them, the mass percentage content of lithium hexafluorophosphate in the electrolyte is 10%.

[0100] 4. The positive electrode sheet, negative electrode sheet, and macroporous oil-based separator (thickness of 9 μm, where the thicknesses of the substrate, ceramic, and PVDF adhesive layer are 5 μm, 2 μm, and 2 μm in sequence) prepared according to the above process are wound into an electric core. After drying the electric core for 24 h, the electrolyte is injected, soaked at 45 °C for 2 h, and then left standing at room temperature for 24 h; then, formation treatment is carried out at 1.2 MPa and 83 °C, and then aging treatment is carried out to obtain the battery.

[0101] The infrared absorption spectra of the first binder and PVDF of the present invention are shown in Figure 2 . From Figure 2 , it can be seen that PVDF has typical characteristic peaks: 1177.35 cm -1 is the stretching vibration absorption peak of CF2, 1401 cm -1 is the deformation vibration absorption peak of CH2 connected to CF2 in PVDF. However, the above-mentioned same characteristic peaks are not found in the first binder of the present invention, and the absorption characteristic peaks of the main functional groups of the first binder of the present invention include: -COOH (1730 cm -1 ), -CN (2238 cm -1 ), butadiene (964 cm -1 ). Based on the area of the total characteristic absorption peak of the first binder, at 1730 cm -1 , the peak area ratio of the absorption peak is 12%, at 2238 cm ～1 , the peak area ratio of the absorption peak is 32%, and the peak intensity ratio R of the absorption peak at 2238 cm -～1 and the absorption peak at 1730 cm -～1 is 2.67.

[0102] Example 2

[0103] The preparation process of the battery provided in this example is substantially the same as that in Example 1. The difference is that the carboxyl group: cyanide group: ester group in the first binder is 50%: 38%: 12%, the relative crystallinity of the first binder is 10%, and the swelling rate is 18%.

[0104] Example 3

[0105] The preparation process of the battery provided in this example is substantially the same as that in Example 1. The difference is that the carboxyl group: cyanide group: ester group in the first binder is 50%: 32%: 18%, the relative crystallinity of the first binder is 30%, and the swelling rate is 28%.

[0106] Example 4

[0107] The preparation process of the battery provided in this example is substantially the same as that in Example 1. The difference is that the carboxyl group: cyanide group: ester group in the first binder is 40%: 50%: 10%, the relative crystallinity of the first binder is 8%, and the swelling rate is 10%. During the preparation of the positive electrode sheet, the compaction density during rolling is 4.23 g / cc.

[0108] Example 5

[0109] The preparation process of the battery provided in this embodiment is substantially the same as that in Embodiment 1. The difference is that in the first binder, carboxyl group: cyano group: ester group = 50%: 45%: 5%, the relative crystallinity of the first binder is 5%, and the swelling ratio is 4%. During the preparation of the positive electrode sheet, the compaction density during roll pressing is 4.20 g / cc.

[0110] Example 6

[0111] The preparation process of the battery provided in this embodiment is substantially the same as that in Embodiment 1. The difference is that in the first binder, carboxyl group: cyano group: ester group = 50%: 25%: 25%, the relative crystallinity of the first binder is 38%, and the swelling ratio is 54%.

[0112] Example 7

[0113] The preparation process of the battery provided in this embodiment is substantially the same as that in Embodiment 1. The difference is that the mass percentage content of the first binder in the positive electrode active layer is 0.9%, and the D50 of the positive electrode active material is 19 μm. During the preparation of the positive electrode sheet, the compaction density during roll pressing is 4.28 g / cc.

[0114] Example 8

[0115] The preparation process of the battery provided in this embodiment is substantially the same as that in Embodiment 1. The difference is that the mass percentage content of the first binder in the positive electrode active layer is 1.1%, and the D50 of the positive electrode active material is 12 μm. During the preparation of the positive electrode sheet, the compaction density during roll pressing is 4.20 g / cc.

[0116] Example 9

[0117] The preparation process of the battery provided in this embodiment is substantially the same as that in Embodiment 1. The difference is that the mass percentage content of the first binder in the positive electrode active layer is 0.8%, and the D50 of the positive electrode active material is 18 μm. During the preparation of the positive electrode sheet, the compaction density during roll pressing is 4.28 g / cc.

[0118] Example 10

[0119] The preparation process of the battery provided in this embodiment is substantially the same as that in Embodiment 1. The difference is that the non-fluorine binder further includes a second binder N-butyl-N-methylpyrrolidinium bis(fluorosulfonyl)imide (CAS No. 1057745-51-3, ionic conductivity is 9.8*10 -3 , relative dielectric constant is 10, and the cyclic voltammogram is shown in Figure 3 ), the mass percentage content of the first binder in the positive electrode active layer is 0.86%, the mass percentage content of the second binder in the positive electrode active layer is 0.1%, and the D50 of the positive electrode active material is 16 μm. During the preparation of the positive electrode sheet, the compaction density during roll pressing is 4.40 g / cc.

[0120] Example 11

[0121] The preparation process of the battery provided in this example is substantially the same as that in Example 10. The difference is that the mass percentage content of the first binder in the positive active layer is 0.9%, the mass percentage content of the second binder in the positive active layer is 0.2%, and the D50 of the positive active material is 15.5 μm. During the preparation of the positive electrode sheet, the compaction density during roll pressing is 4.40 g / cc.

[0122] Example 12

[0123] The preparation process of the battery provided in this example is substantially the same as that in Example 10. The difference is that the mass percentage content of the first binder in the positive active layer is 1.2%, the mass percentage content of the second binder in the positive active layer is 0.5%, and the D50 of the positive active material is 13 μm. During the preparation of the positive electrode sheet, the compaction density during roll pressing is 4.35 g / cc.

[0124] Example 13

[0125] The preparation process of the battery provided in this example is substantially the same as that in Example 10. The difference is that the mass percentage content of the first binder in the positive active layer is 1.3%, the mass percentage content of the second binder in the positive active layer is 0.4%, and the D50 of the positive active material is 11.75 μm. During the preparation of the positive electrode sheet, the compaction density during roll pressing is 4.30 g / cc.

[0126] Example 14

[0127] The preparation process of the battery provided in this example is substantially the same as that in Example 10. The difference is that the mass percentage content of the first binder in the positive active layer is 1.3%, the mass percentage content of the second binder in the positive active layer is 0.6%, and the D50 of the positive active material is 10.5 μm. During the preparation of the positive electrode sheet, the compaction density during roll pressing is 4.30 g / cc.

[0128] Example 15

[0129] The preparation process of the battery provided in this example is substantially the same as that in Example 10. The difference is that the mass percentage content of the first binder in the positive active layer is 1.4%, the mass percentage content of the second binder in the positive active layer is 0.6%, and the D50 of the positive active material is 8 μm. During the preparation of the positive electrode sheet, the compaction density during roll pressing is 4.32 g / cc.

[0130] Example 16

[0131] The preparation process of the battery provided in this embodiment is substantially the same as that in Embodiment 10. The difference is that the mass percentage content of the first binder in the positive electrode active layer is 0.64%, the mass percentage content of the second binder in the positive electrode active layer is 0.06%, and the D50 of the positive electrode active material is 20 μm. During the preparation of the positive electrode sheet, the compaction density during rolling is 4.30 g / cc.

[0132] Comparative Example 1

[0133] The preparation process of the battery provided in this embodiment is substantially the same as that in Embodiment 1. The difference is that according to the mass ratio of lithium cobaltate, conductive agent (carbon black, carbon nanotube or graphene), and PVDF (H900) of 97.6:1.44:0.96 respectively, the following batching process is used to prepare the positive electrode slurry: First, polyvinylidene fluoride (PVDF) is fully dispersed in N-methylpyrrolidone (NMP), the self-rotation is set to 800 rpm - 1000 rpm, and stirred for 20 - 30 min; then the conductive agent is added, the self-rotation is set to 1000 rpm - 2000 rpm, and the stirring time is 2 h; subsequently, lithium cobaltate is added, the self-rotation is set to 2000 rpm - 3000 rpm, and the stirring time is 2 h to ensure that the slurry is evenly dispersed. The temperature of the entire batching process is maintained at 15 - 35 °C, and the viscosity of the slurry is controlled at 2000 - 7000 mPa·s. The above slurry is sieved and then coated on the positive electrode current collector, and the coating areal density is 20 mg / cm 2 , dried at 110 - 120 °C, and after rolling and slitting, a positive electrode sheet is obtained, and the compaction density of rolling is 4.20 g / cc.

[0134] Test Example 1

[0135] 1.1. The thermal weight loss rate of the positive electrode sheet prepared in Embodiment 1 is tested. Since the thermal weight loss rate of the positive electrode sheet can be represented by the thermal weight loss rate of the first binder, therefore, testing the thermal weight loss rate of the positive electrode sheet is equivalent to testing the thermal weight loss rate of the first binder. Figure 4 is the TG diagram of the first binder in Embodiment 1. From Figure 4 it can be seen that the first binder of the present invention has almost no weight loss rate at 100 °C, and the weight loss rate at 200 °C is 2.1%. This process is mainly the removal of external moisture, and its main decomposition temperature occurs at 200 °C - 600 °C.

[0136] 1.2. Perform a dissolution rate test on the positive electrode sheet containing the second binder (i.e., the positive electrode sheet prepared in Example 11): Take the positive electrode sheets before and after injecting the liquid (electrolyte) respectively, cut them into small round sheets with a diameter of 40 mm. The mass of the positive electrode sheet before injecting the liquid is denoted as m1, and the positive electrode sheet after injecting the liquid is placed in a vacuum oven and dried completely at 80 °C - 90 °C, and the mass after drying is denoted as m2. The dissolution rate w of the positive electrode sheet is: (m1 - m2) / m1. Through the above test, the dissolution rate of the positive electrode sheet in Example 11 of the present invention is measured to be 0.4%.

[0137] 1.3. Perform a flexibility test on the positive electrode sheet. The flexibility test method includes the following steps: Wrap the positive electrode sheet around a 1.5-mm winding needle, align the part around the winding needle with force, and slowly rotate the winding needle to observe the crack situation on the surface of the electrode sheet. The flexibility test diagrams of the positive electrode sheets including PVDF-based binders and the positive electrode sheet of Example 1 are shown in Figure 5 . It can be seen from Figure 5 that under the same rolling conditions, the surface of the positive electrode sheet prepared with PVDF has relatively serious cracks and slight powder shedding problems, while the surface of the positive electrode sheet of Example 1 has slight cracks.

[0138] 1.4. Perform a four-probe resistance test and a two-probe resistance test on the positive electrode sheet. The four-probe resistance test includes the following steps: Arrange four metal probes in a straight line and apply a certain pressure on the surface of the positive electrode sheet. Pass a current between the first and fourth probes, then a potential difference is generated between the second and third probes, and finally the overall resistance of the positive electrode sheet is obtained; the two-probe resistance test includes the following steps: Place the test electrodes on the upper and lower sides of the positive electrode sheet respectively, apply a certain current, and detect the voltage on the upper and lower sides of the positive electrode sheet, and finally obtain the overall resistance of the positive electrode sheet. The four-probe resistance test of the positive electrode sheet of Comparative Example 1 and the positive electrode sheet of Example 1 is shown in Figure 6 , and the two-probe resistance test of the positive electrode sheet of Comparative Example 1 and the positive electrode sheet of Example 1 is shown in Figure 7 . It can be seen from Figure 6 and Figure 7 that the resistance of the positive electrode sheet of Example 1 is lower, which is beneficial to improving the cycle performance of the battery.

[0139] 1.5. Cut the positive electrode sheet into small round sheets with a diameter of 40 mm for swelling test. At 80 °C, immerse the positive electrode sheet in the electrolyte for 48 h, and then test the swelling rate and dissolution rate of the positive electrode sheet. The swelling rate and dissolution rate of the positive electrode sheet of Comparative Example 1 and the positive electrode sheet of Example 1 are shown in Table 1. It can be seen from Table 1 that the dissolution of the positive electrode sheet of Example 1 is very low, which means that during the cycling process, the electrolyte has less damage to the binder structure, and a higher adhesive force can still be maintained in the later stage of cycling, maintaining the stability of the positive electrode sheet structure and improving the capacity retention rate.

[0140] Table 1

[0141]

[0142]

[0143] 1.6. Perform 180° peel strength tests on the positive electrode sheets before and after formation. The peel strengths of the positive electrode sheets in Comparative Example 1 and Example 1 are shown in Figure 8 , and it can be seen from Figure 8 that compared with the positive electrode sheet of Comparative Example 1, the peel force of the positive electrode sheet of Example 1 can be increased by 2 times.

[0144] 1.7. Cohesion test: (1) Sample preparation: Take the positive electrode sheet and scrape off the positive electrode active layer powder on the surface of the positive electrode current collector with a blade. The mass of the scraped positive electrode active layer powder is greater than 15 g; (2) Tablet pressing: Put the positive electrode active layer powder into a tablet press and press it into a disc-shaped sample with a pressure of 15 MPa. The diameter of the disc-shaped sample is 5 cm; (3) Strength test: The upper pressing head of the high-speed tensile testing machine presses down the disc-shaped sample at a speed of 50 mm / min until the disc-shaped sample is damaged, and record the displacement and strength during the process; (4) Calculation: Determine the cohesion of the positive electrode active layer according to the failure strength and displacement of different samples. The results are shown in Table 2.

[0145] 1.8. Peel force test between the current collector and the positive electrode active layer: (1) Sample preparation: Cut the positive electrode sheet to be tested into a sample of 24 mm * 100 mm; (2) Glue sticking: Bond one side of the sample to be tested with double-sided tape and press it firmly with a roller to make the double-sided tape fit completely with the electrode sheet; Stick the other side of the double-sided tape on the surface of the stainless steel substrate and bend one end of the sample by 180 degrees; (3) Test: Fix one end of the stainless steel substrate on the lower clamp of the tensile testing machine, fix the bent end of the sample on the upper clamp, and stretch it at a speed of 50 mm / min until the sample is completely peeled off. The acting force during the stretching process is the peel force between the current collector and the positive electrode active layer. The results are shown in Table 2.

[0146] Test Example 2

[0147] 2.1. Cycle performance test: Charge the batteries prepared in the examples and comparative examples at a rate of 1.0C and discharge at a rate of 0.5C for 1000 cycles at 25°C; At the same time, divide the battery capacity at the 1000th cycle by the battery capacity at the 1st cycle to obtain the cycle capacity retention rate. The results are shown in Table 2.

[0148] 1.0C rate charging, 0.5C rate discharging for 1000 cycles; At the same time, divide the battery capacity at the 1000th cycle by the battery capacity at the 1st cycle to obtain the cycle capacity retention rate. The results are shown in Table 2.

[0149] 2.2. High-temperature cycle performance test: Charge the batteries prepared in the examples and comparative examples at a rate of 1.0C and discharge at a rate of 0.5C for 800 cycles at 45°C; At the same time, divide the battery capacity at the 800th cycle by the battery capacity at the 1st cycle to obtain the cycle capacity retention rate. The results are shown in Table 2.

[0150] 2.3. Energy density: The batteries prepared in the examples and comparative examples were left standing in a constant temperature chamber at 25 ± 1°C for 30 min, charged at a constant current of 1.0C to 4.3V, charged at a constant voltage of 4.3V until the charging current was less than 0.05C, left standing for 10 min, the length (L) and width (W) of the battery cell were measured with a caliper, and the thickness (H) of the battery cell was measured with a thickness gauge. Discharged at a constant current of 0.5C to 3.0V, left standing for 10 min. The above tests were a complete cycle process, and the cycle was repeated 3 times. The energy density (ED) was calculated using the discharge capacity (R3) of the third cycle; the volume of the battery cell was V = L * W * H; the average discharge voltage of the third cycle was V3; the mass energy density ED (Wh / L) = R3 * V3 / V. The results are shown in Table 2.

[0151] 2.4. Electrochemical performance test: In an environment of 25°C ± 5°C, the batteries prepared in the above examples and comparative examples were discharged at 0.2C to the lower limit voltage; left standing for 10 min; adjusted the SOC: charged at 0.7C constant current to 50% SOC of the declared capacity; left standing for 10 min; tested the full charge state voltage and internal resistance at 25°C ± 5°C; then carried out an AC impedance test, and the parameters were as follows: the highest frequency was 50 kHz to 5 kHz, the starting frequency was the same as the highest frequency value, the lowest frequency was about 100 mHz, and scanned from high frequency to low frequency. The results are shown in Table 2.

[0152] Table 2

[0153]

[0154]

[0155] As can be seen from Table 2, by comparing Examples 1 - 16 with Comparative Example 1, it can be known that using the non-fluorine binder of the present invention can improve the cycle performance of the battery and maintain the high energy density of the battery.

[0156] By comparing Examples 1 - 6, it can be known that when the relative crystallinity and swelling rate of the first binder are within the foregoing ranges, there is a high peeling force between the positive electrode current collector and the positive electrode active layer, and the positive electrode active layer has a high cohesive force, reducing the shedding of the positive electrode active material caused by volume change during the cycle, thereby improving the capacity retention rate and energy density of the battery.

[0157] By comparing Examples 1, 7 - 12, it can be known that when the mass percentage m of the first binder in the positive electrode active layer and the particle size D50 of the positive electrode active material satisfy the foregoing relational formula, the battery has low impedance, excellent cycle performance and energy density.

[0158] According to the comparison of Examples 1, 9 - 16, when the dosage of the non - fluorine binder is 0.8 - 2.0%, the mass percentage content of the first binder in the positive active layer is 0.8 - 1.2%, and the mass percentage content of the second binder in the positive active layer is 0.1 - 0.5%, the first binder and the second binder can fully cooperate, enabling the battery to have both high cycle performance and high energy density.

[0159] Finally, it should be noted that: the above - mentioned embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A positive electrode sheet, characterized in that: The positive electrode sheet includes a current collector and a positive electrode active layer, the positive electrode active layer includes a positive electrode active material and a non-fluorine binder, the non-fluorine binder includes a first binder, and the relative crystallinity of the first binder satisfies: 10%≤I1 / I2≤30%, wherein I1 is the peak area of ​​the diffraction peak of the XRD spectrum of the first binder at 2θ of 16° to 22°, and I2 is the peak area of ​​all diffraction peaks in the XRD spectrum of the first binder; The swelling rate of the first adhesive is 10-30%.

2. The positive electrode sheet according to claim 1, characterized in that: Based on the mass of the positive electrode active layer, the mass percentage of the non-fluorine adhesive is A, the peeling force between the current collector and the positive electrode active layer is B, A satisfies: 0.8%≤A<2.0%, and B is not less than 25N / m.

3. The positive electrode sheet according to claim 1, characterized in that: Based on the mass of the positive electrode active layer, the mass percentage of the first binder is m, the particle size of the positive electrode active material is D50, and m and D50 satisfy: m=-0.08*D50+2.24, wherein m satisfies: 0.80~1.20%, and D50 satisfies: 8μm≤D50≤20μm.

4. The positive electrode sheet according to claim 3, characterized in that: The non-fluorine binder further comprises a second binder, and the mass percentage of the second binder in the positive electrode active layer is 0.10-0.50%.

5. The positive electrode sheet according to any one of claims 1 to 4, characterized in that: In infrared wave number 1730~1749cm -1 Within the range of, the first adhesive has a first characteristic absorption peak, and based on the peak area of ​​the total characteristic absorption peak of the first adhesive, the peak area of ​​the first characteristic absorption peak accounts for 5.0-20.0%; and / or, in the infrared wave number 2150~2450cm -1 Within the range of, the first adhesive has a second characteristic absorption peak, and based on the peak area of ​​the total characteristic absorption peak of the first adhesive, the peak area of ​​the second characteristic absorption peak accounts for 20.0-35.0%; and / or, the dissolution rate of the first binder is not higher than 2.0%; And / or, the first binder has no redox peak in the voltage range of 3 to 5 V, and the current value of the CV test curve has an order of magnitude range of 10 -4 ~10 -3 .

6. The positive electrode sheet according to claim 5, characterized in that: The first adhesive includes at least one of a polyacrylic acid adhesive, a polyacrylonitrile adhesive, and a polyimide adhesive.

7. The positive electrode sheet according to claim 4, characterized in that: The second binder has no redox peak in the voltage range of 3 to 5 V, and the current value of the CV test curve has an order of magnitude range of 10 -5 ~10 -4 ; And / or, the second binder includes at least one of fluorine-containing sulfonates, fluorine-containing sulfonimide salts and sulfate compounds.

8. The positive electrode sheet according to claim 7, characterized in that: The fluorine-containing sulfonate includes at least one of N-butyl-N-methylpyrrole trifluoromethanesulfonate and 1-ethyl-3-methylimidazolium trifluoromethanesulfonate; and / or, the fluorine-containing sulfonyl imide salt comprises at least one of N-butyl-N-methylpyrrolidine bis(fluorosulfonyl)imide salt, N-butyl-N-methylpyrrolidine bis(trifluoromethanesulfonyl)imide salt and N-propyl-N-methylpyrrolidine bis(trifluoromethanesulfonyl)imide salt; And / or, the sulfate ester compound includes at least one of dimethyl sulfite, diethyl sulfite, ethylene sulfite and propylene sulfite.

9. The positive electrode sheet according to any one of claims 1 to 8, characterized in that: The surface density of the positive electrode active layer is 18 to 28 mg / cm 2 ; and / or, the compaction density of the positive electrode active layer is 4.20 to 4.40 g / cc; And / or, the thermal weight loss rate of the positive electrode sheet at 200° C. is not higher than 3.0%.

10. A battery, characterized in that: The battery comprises the positive electrode sheet according to any one of claims 1 to 9.

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