Positive electrode sheet and lithium ion battery

Abstract

The application relates to the technical field of lithium ion batteries, in particular to a positive plate and a lithium ion battery. The positive plate comprises a current collector, a protective layer and an active material layer. The protective layer is arranged on at least one side surface of the current collector, and the active material layer is arranged on the side surface of the protective layer away from the current collector. The protective layer comprises an organic polymer with a glass transition temperature lower than 25 DEG C. The positive plate has high toughness and elasticity, short-circuit between the aluminum foil and the graphite is avoided, and damage of the battery caused by the external environment is reduced. The lithium ion battery has high energy density and safety performance.

Description

Technical Field

[0001] This application relates to the field of lithium-ion battery technology, specifically to positive electrode sheets and lithium-ion batteries. Background Technology

[0002] Lithium-ion batteries are widely used in portable electronic devices and power batteries due to their high energy density. However, in daily life, lithium-ion batteries frequently catch fire and explode under impact, compression, or other stressors, seriously affecting people's lives and property safety and the user experience. Therefore, improving the safety performance of lithium-ion batteries is urgently needed. Among the four short-circuit modes in lithium-ion batteries, the most dangerous is the short circuit between the positive electrode current collector and the negative electrode active material layer. Therefore, the most effective way to improve the safety performance of lithium-ion batteries is to avoid short circuits between the aluminum foil and graphite.

[0003] Currently, the most common method to improve the safety performance of lithium-ion batteries is to introduce a safety coating on the surface of the positive electrode current collector or the surface of the negative electrode active material layer. Among these methods, coating the positive electrode current collector surface with a low binder content (<10%) shows good improvement in resistance to needle penetration and unilateral extrusion, but its effect on heavy impact is weak and results in significant energy density loss. Increasing the binder content (such as polyvinylidene fluoride, acrylate, polyacrylonitrile, etc.) in the positive electrode safety coating has some improvement effect on heavy impact, but the improvement is limited, and excessive use of polyvinylidene fluoride-based binders can lead to appearance problems in the top layer of lithium cobalt oxide coating.

[0004] In addition, using a safety coating with a high content of polyvinylidene fluoride or resin-based binders can make the positive electrode sheet brittle, reduce compaction during rolling, and lower the energy density of the battery. Furthermore, the addition of inorganic fillers with larger particle sizes to the safety coating can increase the thickness of the coating, resulting in a loss of energy density. Moreover, the protection effect on the aluminum foil is limited when the lithium-ion battery is subjected to significant impact or damage. Summary of the Invention

[0005] In view of this, the present invention provides a positive electrode sheet and a lithium-ion battery. This positive electrode sheet has high toughness and elasticity, preventing short circuits between the aluminum foil and graphite, reducing external damage to the battery cell, and enabling the lithium-ion battery to have high energy density and safety performance.

[0006] To achieve the above-mentioned objectives, the present invention provides the following technical solution:

[0007] The present invention provides a positive electrode sheet, which includes a current collector, a protective layer, and an active material layer. The protective layer is disposed on at least one side surface of the current collector, and the active material layer is disposed on the side surface of the protective layer away from the current collector. The protective layer includes an organic polymer with a glass transition temperature of less than 25°C.

[0008] To improve the safety performance of lithium-ion batteries while minimizing energy loss, this invention provides a high-toughness positive electrode sheet and a lithium-ion battery incorporating the positive electrode sheet. On one hand, by adding an organic polymer to the protective layer of the positive electrode sheet, both the positive electrode sheet and the protective layer possess high toughness. The high-toughness positive electrode sheet exhibits high elongation at break. When the lithium-ion battery is subjected to mechanical abuse, the greater deformation of the high-toughness protective layer can better encapsulate aluminum burrs, preventing short circuits between the aluminum foil and graphite. Figure 2 On the other hand, due to the high toughness and elasticity of the positive electrode, when the lithium-ion battery is subjected to impact from a heavy object, the high toughness and elasticity of the positive electrode can absorb part of the impact energy, reducing external damage to the cell. In addition, the protective coating of this positive electrode is relatively thin, which can reduce the energy density loss of the lithium-ion battery compared with conventional safety coatings. Therefore, the positive electrode with high toughness protective layer of the present invention and the lithium-ion battery containing it have high energy density and safety performance.

[0009] Preferably, the glass transition temperature of the organic polymer is below 25°C. Exemplary values ​​are any one of -150°C, -140°C, -130°C, -120°C, -110°C, -100°C, -90°C, -80°C, -70°C, -60°C, -50°C, -40°C, -30°C, -20°C, -10°C, 0°C, 10°C, 20°C, and 24°C, or any value within a range of any two values.

[0010] Preferably, the glass transition temperature of the organic polymer is below 15°C.

[0011] More preferably, the glass transition temperature of the organic polymer is below 0°C.

[0012] More preferably, the glass transition temperature of the organic polymer is below -20°C.

[0013] In the specific embodiments provided by the present invention, the glass transition temperature of the organic polymer is -120℃ to -50℃.

[0014] Preferably, the organic polymer includes rubber materials.

[0015] Preferably, the organic polymers include, but are not limited to, at least one of styrene-butadiene rubber, nitrile rubber, styrene-isoprene-styrene block copolymer, silicone rubber, cis-butadiene rubber, thermoplastic styrene-butadiene elastomer, natural rubber, and butyl rubber. The common characteristic of these polymeric materials is that they are in a highly elastic state at room temperature (i.e., their glass transition temperature is below room temperature). This ensures that the positive electrode protective layer of the lithium-ion battery has good toughness and elasticity within the normal operating temperature range, thus better protecting the aluminum foil and preventing the most dangerous short-circuit mode (short circuit between the aluminum foil and the negative electrode), thereby improving the safety performance of the lithium-ion battery.

[0016] More preferably, the organic polymer is styrene-butadiene rubber.

[0017] In embodiments of the present invention, the particle size of the organic polymer particles can be in the nanometer, micrometer, or millimeter range. Preferably, the particle size of the organic polymer is less than 200 μm.

[0018] Preferably, the particle size of the organic polymer is less than 20 μm.

[0019] Preferably, the protective layer further includes a conductive agent, or the protective layer further includes a conductive agent and an adhesive.

[0020] Preferably, the content of each component in the protective layer is as follows:

[0021] Organic polymers 50wt%~99wt%

[0022] Conductive agent 1wt%~20wt%

[0023] Adhesive 0wt%~40wt%.

[0024] Preferably, the content of each component in the protective layer is as follows:

[0025] Organic polymers 60wt%~95wt%

[0026] Conductive agent 1wt%~20wt%

[0027] Adhesive 1wt%~20wt%.

[0028] More preferably, the content of each component in the protective layer is:

[0029] Organic polymers 80wt%~95wt%

[0030] Conductive agent 1wt%~10wt%

[0031] Adhesive 1wt% to 10wt%.

[0032] In embodiments of the present invention, the conductive agent includes, but is not limited to, at least one of conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, conductive metal particles, Ketjen black, graphene, carbon fiber, and amorphous carbon.

[0033] In embodiments of the present invention, the conductive metal particles include, but are not limited to, at least one of aluminum powder, copper powder, silver powder, and zinc powder.

[0034] In embodiments of the present invention, the adhesive includes, but is not limited to, at least one of polyvinylidene fluoride (PVDF), acrylic acid-modified polyvinylidene fluoride, carboxylic acid-modified polyvinylidene fluoride, polymethyl methacrylate (PMMA), polyacrylic acid copolymer, polyacrylic acid, and polyimide (PI).

[0035] More preferably, the adhesive is a polyacrylic acid copolymer or polyacrylic acid.

[0036] Preferably, the thickness of the protective layer is 0.5 to 10 μm. Exemplary examples include any value from 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, and 10 μm, or any value between any two values.

[0037] Preferably, the thickness of the protective layer is 1–5 μm.

[0038] More preferably, the thickness of the protective layer is 2–4 μm.

[0039] Preferably, the active material in the active material layer includes at least one of lithium cobalt oxide, lithium iron phosphate, lithium nickel cobalt manganese, lithium nickel cobalt aluminum, lithium manganese oxide, and lithium-rich manganese-based lithium.

[0040] Preferably, the active material layer also includes a conductive agent and / or an adhesive.

[0041] Preferably, the mass percentage of each component in the positive electrode active material layer is: 80wt% to 99.8wt% of positive electrode active material, 0.1wt% to 10wt% of conductive agent, and 0.1wt% to 10wt% of binder.

[0042] Preferably, the mass percentage of each component in the positive electrode active material layer is: 90wt% to 99.6wt% of positive electrode active material, 0.2wt% to 5wt% of conductive agent, and 0.2wt% to 5wt% of binder.

[0043] Preferably, the protective layer is disposed on both sides of the current collector. This arrangement ensures that, in the event of damage to the electrode, the protective layer can effectively encapsulate the aluminum foil current collector, preventing aluminum burrs from being exposed and thus avoiding fire and explosion, resulting in high safety.

[0044] In this embodiment of the invention, the current collector of the positive electrode is aluminum foil.

[0045] The present invention also provides a method for preparing the above-mentioned positive electrode, comprising the following steps:

[0046] (1) Mix the protective layer material and solvent to obtain a protective layer slurry, coat the protective layer slurry onto the current collector, and dry it to obtain a current collector coated with a protective layer; the protective layer material includes an organic polymer.

[0047] (2) Mix the active material layer material and solvent to obtain an active material layer slurry. Coat the active material layer slurry onto the protective layer of the current collector, and obtain a positive electrode sheet by drying and rolling.

[0048] In the embodiment of the present invention, the protective layer material further comprises a conductive agent, or a conductive agent and a binder.

[0049] In the embodiment of the present invention, the solvent comprises water and / or an organic solvent.

[0050] In the embodiment of the present invention, the organic solvent includes, but is not limited to, at least one of N-methylpyrrolidone (NMP), xylene, toluene, dimethylformamide, acetone, and methyl ethyl ketone.

[0051] The present invention also provides a lithium ion battery comprising the above positive electrode plate.

[0052] In the embodiment of the present invention, the lithium ion battery further comprises a negative electrode plate, a separator and an electrolyte.

[0053] In the embodiment provided by the present invention, the negative electrode plate comprises a negative current collector and a negative active material layer coated on one or both surfaces of the negative current collector, and the negative active material layer comprises a negative active material, a conductive agent and a binder.

[0054] Preferably, the mass percentage of each component in the negative active material layer is: 80wt% ~ 99.8wt% of the negative active material, 0.1wt% ~ 10wt% of the conductive agent, and 0.1wt% ~ 10wt% of the binder.

[0055] Preferably, the mass percentage of each component in the negative active material layer is: 90wt% ~ 99.6wt% of the negative active material, 0.2wt% ~ 5wt% of the conductive agent, and 0.2wt% ~ 5wt% of the binder.

[0056] Preferably, the negative active material comprises a carbon-based negative electrode material.

[0057] Preferably, the carbon-based negative electrode material comprises at least one of artificial graphite, natural graphite, mesocarbon microbeads, hard carbon and soft carbon.

[0058] Preferably, the negative active material may further comprise a silicon-based negative electrode material.

[0059] Preferably, the silicon-based negative electrode material is selected from at least one of nano-silicon, silicon-oxygen negative electrode material (SiOx, 0<x<2) or silicon-carbon negative electrode material.

[0060] Preferably, in the negative active material, the mass ratio of the carbon-based negative electrode material to the silicon-based negative electrode material is 10:0 to 1:19.

[0061] The present invention also provides a method for preparing the above lithium ion battery, comprising the following steps: winding the positive electrode plate, the separator, and the negative electrode plate to obtain a wound core, and subjecting the wound core to packaging, baking, injecting, formation, second sealing, sorting and OCV to obtain the lithium ion battery.

[0062] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0063] (1) The positive electrode current collector of the present invention is coated with a high-toughness protective layer (polymer safety coating). When the lithium-ion battery is subjected to mechanical abuse such as heavy object impact, needle puncture and unilateral extrusion, on the one hand, the large deformation of the safety coating can better wrap the aluminum burrs and avoid short circuit between aluminum foil and graphite; on the other hand, the high-toughness and elasticity of the coating can absorb some impact energy when subjected to a large impact, especially heavy object impact, thereby reducing the degree of damage to the lithium-ion battery and reducing the probability of short circuit.

[0064] (2) The safety coating can meet the mechanical safety performance of lithium-ion batteries by applying a thin layer. Compared with other positive electrode safety coatings containing inorganic fillers, this technology can reduce the loss of mass energy density and volume energy density of lithium-ion batteries.

[0065] (3) When this safety positive electrode is used in lithium-ion batteries, the pass rate of the lithium-ion battery for needle penetration, unilateral extrusion and heavy object impact is greater than 90%. Attached Figure Description

[0066] Figure 1 Schematic diagrams of the positive electrode structure in Examples 1-9;

[0067] Figure 2 A schematic diagram of the positive electrode current collector and protective layer after damage. The end of the protective layer that is deformed is the location where the electrode is damaged. As can be seen from this schematic diagram, when the electrode is damaged, the protective layer can well wrap the aluminum foil and prevent the aluminum burrs from being exposed.

[0068] The attached figures are labeled as follows:

[0069] 1. Positive current collector, 2. Protective layer, 3. Positive active material layer. Detailed Implementation

[0070] This invention discloses a positive electrode and a lithium-ion battery. Those skilled in the art can refer to the content of this document and appropriately modify the process parameters to achieve the desired result. It should be particularly noted that all similar substitutions and modifications are obvious to those skilled in the art and are considered to be included in this invention. The methods and applications of this invention have been described through preferred embodiments. Those skilled in the art can obviously make modifications or appropriate alterations and combinations to the methods and applications described herein without departing from the content, spirit, and scope of this invention to realize and apply the technology of this invention.

[0071] Terminology Explanation:

[0072] Glass transition temperature (Tg) is the temperature at which a material transitions from a glassy state to a rubbery state. The glass transition is an inherent property of amorphous polymers, a macroscopic manifestation of the change in the form of polymer motion, and it directly affects the material's performance in use and processing. The glass transition temperature (Tg) is the lowest temperature at which molecular chain segments can move; its value is directly related to the flexibility of the molecular chains. The greater the flexibility of the molecular chains, the lower the glass transition temperature; conversely, the greater the rigidity of the molecular chains, the higher the glass transition temperature.

[0073] The specific types of organic polymers, their English abbreviations, and their glass transition temperatures (Tg) are as follows:

[0074]

[0075] The reagents, instruments, and materials used in this invention can all be obtained through commercial channels.

[0076] The present invention will be further illustrated below with reference to the embodiments:

[0077] Example 1:

[0078] (1) Battery structure

[0079] The battery in this embodiment uses a wound cell, which includes a positive electrode, a separator, and a negative electrode in sequence.

[0080] 1) Positive electrode plate

[0081] See Figure 1 The positive electrode includes a positive current collector 1, and a protective layer 2 and a positive active material layer 3 are respectively disposed on both sides of the positive current collector.

[0082] 2) Negative electrode plate

[0083] The negative electrode sheet includes a negative current collector and negative active material layers disposed on both sides thereof.

[0084] 3) Separating membrane

[0085] The separator is a 5μm thick polymer membrane made of PP material.

[0086] (2) Preparation method

[0087] Step 1: Prepare the positive electrode protective layer slurry. Coat the slurry onto aluminum foil using gravure or skim coating. After drying, a positive electrode current collector with a high-toughness coating is obtained.

[0088] The high-toughness positive electrode protective layer provided in this embodiment is prepared as follows: the organic polymer styrene-butadiene rubber (SBR) is mixed evenly with conductive agent carbon black and carbon nanotubes, and then a certain amount of adhesive polyacrylic acid copolymer liquid and deionized water are added and stirred evenly to obtain the positive electrode protective layer slurry.

[0089] In the aforementioned high-toughness cathode protective layer, the mass fraction of SBR is 88%, the mass fraction of polyacrylic acid copolymer is 6%, and the mass fraction of conductive agent is 6% (of which carbon black is 3% and carbon nanotubes are 3%). The thickness of the high-toughness cathode protective layer is 3 μm.

[0090] Step 2: Prepare the positive electrode active layer slurry. Add conductive carbon black and carbon nanotubes to the PVDF adhesive and stir until homogeneous. Then add lithium cobalt oxide and stir until homogeneous again to prepare the positive electrode active material slurry. Coat the positive electrode active material slurry onto the surface of the positive electrode protective layer. After baking and rolling, obtain the positive electrode sheet. The mass fraction of lithium cobalt oxide in the positive electrode active material layer is 97.6%, the mass fraction of PVDF is 1.05%, and the mass fraction of conductive carbon black and carbon nanotubes is 1.35% (where the mass ratio of carbon black to carbon nanotubes is 1:1).

[0091] Step 3: Prepare the negative electrode active layer slurry. Mix 97.3% graphite, 0.5% conductive carbon black, 1.3% binder and 0.9% dispersant evenly, then add an appropriate amount of deionized water and disperse evenly to prepare the negative electrode slurry. Coat the negative electrode slurry onto carbon-coated copper foil, and obtain the negative electrode sheet after baking and rolling.

[0092] Step 4: After the positive and negative electrode sheets are slit, made into sheets, and wound with the separator to obtain the core, the core is then packaged, baked, injected with electrolyte, formed, resealed, sorted, and OCV to obtain the lithium-ion battery.

[0093] The electrolyte is a commercially available conventional electrolyte, and the lithium salt in it is LiFP6.

[0094] Example 2:

[0095] In this embodiment, the polymer material used in the positive electrode protective layer is thermoplastic styrene-butadiene elastomer (SBS), the adhesive is polyvinylidene fluoride (PVDF), and the solvent is N-methylpyrrolidone (NMP). Other aspects are the same as in Example 1.

[0096] Example 3:

[0097] In this embodiment, the polymer material used in the positive electrode protective layer is styrene-isoprene-styrene block copolymer (SIS), the adhesive is polyvinylidene fluoride (PVDF), and the solvent is a mixed solution of NMP and xylene. Other aspects are the same as in Example 1.

[0098] Example 4:

[0099] In this embodiment, the polymer material used in the positive electrode protective layer is silicone rubber (SIR), the adhesive is polyvinylidene fluoride (PVDF), and the solvent is a mixed solution of NMP and xylene. Other aspects are the same as in Example 1.

[0100] Example 5:

[0101] In this embodiment, the SBR content in the positive electrode protective layer is 93%, the conductive agent content is 1%, the polyacrylic acid copolymer adhesive content is 6%, and the rest is the same as in Example 1.

[0102] Example 6:

[0103] In this embodiment, the SBR content in the positive electrode protective layer is 79%, the conductive agent content is 15%, the polyacrylic acid copolymer adhesive content is 6%, and the rest is the same as in Example 1.

[0104] Example 7:

[0105] In this embodiment, the SBR content in the positive electrode protective layer is 94%, the conductive agent content is 6%, the polyacrylic acid copolymer adhesive content is 0%, and other aspects are the same as in Example 1.

[0106] Example 8:

[0107] In this embodiment, the SBR content in the positive electrode protective layer is 54%, the conductive agent content is 6%, the polyacrylic acid copolymer adhesive content is 40%, and the other contents are the same as in Example 1.

[0108] Example 9:

[0109] In this embodiment, the thickness of the positive electrode protective layer is 1 μm, and the rest is the same as in Embodiment 1.

[0110] Comparative Example 1:

[0111] This comparative example has no positive electrode protective layer, but everything else is the same as in Example 1.

[0112] Comparative Example 2:

[0113] In this comparative example, the positive electrode protective layer (safety coating) is a lithium iron phosphate coating. The mass fraction of lithium iron phosphate in the protective layer is 94%, the mass fraction of PVDF binder is 5%, the mass fraction of conductive agent is 1%, the solvent used is NMP, and the thickness of the protective layer is 3μm.

[0114] Except for the positive electrode protective layer, which is different from that in Example 1, this comparative example is the same as Example 1.

[0115] Comparative Example 3:

[0116] In this comparative example, the thickness of the protective layer is 5 μm, and other parameters are the same as in comparative example 2.

[0117] The composition of the positive electrode protective layer in each embodiment and comparative example is shown in Table 1.

[0118] Table 1. Composition of the positive electrode protective layer in each embodiment and comparative example.

[0119]

[0120] Battery performance test

[0121] The lithium-ion batteries of the examples and comparative examples were subjected to performance tests such as needle penetration, heavy object impact, and unilateral extrusion.

[0122] 1. Impact test

[0123] At room temperature, the battery is discharged to 3.0V at 0.5C, then charged to 4.48V at 0.2C, with a constant voltage charging cutoff current of 0.02C. This cycle of discharging to 3.0V at 0.5C is repeated five times. Next, the battery is charged at a constant current rate of 0.7C to 4.48V, with a constant voltage cutoff current of 0.02C. Finally, the fully charged battery cell is placed on a flat surface. A steel column with a diameter of 15.8±0.2mm is placed in the center of the cell, with its longitudinal axis parallel to the plane. A weight of 9.8±0.1Kg is dropped freely from a height of 610±25mm onto the steel column above the center of the cell. The cell passes if it does not catch fire or explode. The battery must undergo a heavy impact test within 48 hours of being fully charged.

[0124] 2. Single-sided extrusion test

[0125] At room temperature, the battery cell is charged and discharged 5 times at a rate of 0.7C / 1C (cutoff current of 0.02C). Then, the cell is placed on an 8mm thick platform for a compression test. The pressing speed is 150mm / s, the total travel distance between the pressing block and the bottom of the platform is 300mm, the pressing stroke is 294mm, the distance between the pressing block and the fixed platform is 0.3±0.1mm, and the distance between the pressing block and the bottom of the platform is <0.1mm. The compression time is 1 second. Both sides of the cell are tested separately. A cell that does not ignite or explode is considered to have passed the test. The battery cell must be fully charged and tested within 48 hours.

[0126] 3. Needle prick test

[0127] At room temperature, the battery cell is discharged at 1C to 3.0V, then charged at a constant current of 0.7C to 4.48V with a cutoff current of 0.02C. It is then discharged at 1C to 3.0V, and this cycle is repeated 5 times. Finally, it is charged at a constant current of 0.7C to 4.48V with a cutoff current of 0.02C. Within 48 hours after the test, a 2.5mm diameter steel needle is used to vertically pass through the left, center, and right positions of the lithium-ion battery at a speed of 30mm / s. If the battery does not catch fire or explode, it is considered to have passed the test.

[0128] 4. Energy density test

[0129] The lithium-ion battery was charged at a constant current rate of 0.2C to 4.48V, and then charged at a constant voltage rate to 0.025C to complete the full charge of the lithium-ion battery. Next, it was discharged at a constant current rate of 0.2C until the lithium-ion battery voltage dropped to 3.0V. The total capacity discharged during the discharge process was recorded as C, and the actual lithium-ion battery volume V was calculated.

[0130] ED = Discharge Capacity C * Voltage Plateau / Cell Volume V

[0131] The platform voltage of the 4.48V system is typically 3.89V.

[0132] ED loss rate = ED difference / ED of battery without safety coating * 100%

[0133] 5. Coating appearance

[0134] If microcracks or water ripples appear on the surface of the dried positive electrode, it is considered an abnormal appearance; if no microcracks or water ripples appear on the dried positive electrode, it is considered to have no abnormal appearance.

[0135] 6. Toughness after rolling

[0136] If the rolled positive electrode sheet is bent twice and the sheet is opaque, it is considered to have acceptable toughness.

[0137] The test results are as follows:

[0138] Table 2 shows the safety performance test results for each embodiment and comparative example.

[0139]

[0140] As shown in Table 2, the protective layer (high-toughness safety primer) of this invention significantly improves the protection against heavy impacts compared to conventional lithium iron phosphate primers (Examples 1-4 vs. Comparative Examples 2 & 3). Compared to conventional lithium iron phosphate primers, the high-toughness safety primer achieves or even surpasses the safety improvement effect of lithium iron phosphate primers on lithium-ion batteries with less energy density loss. Therefore, in addition to improving the safety performance of lithium-ion batteries, the high-toughness safety primer can also reduce the energy density loss of lithium-ion batteries.

[0141] Examples 1, 5, and 6 are experiments on the gradient of conductive agent content in the positive electrode protective layer. It can be seen that as the conductive agent content decreases, the safety performance of the lithium-ion battery is improved. However, the conductive agent content should not be too low, otherwise it will affect the cycle stability of the battery.

[0142] Examples 1, 7, and 8 are experiments on the gradient of binder content in the positive electrode protective layer. It can be seen that as the binder content decreases, the safety performance of the lithium-ion battery deteriorates. The adhesion of the bottom coating to the aluminum foil is weak. The protective layer without binder has a weaker protective effect on the aluminum foil compared to the coating with binder, which will reduce the safety performance of the lithium-ion battery. However, excessive binder content will affect the toughness and cycle performance of the positive electrode sheet of the lithium-ion battery.

[0143] The thickness of the protective layer plays a crucial role in the safety performance of lithium-ion batteries. In Example 9, when the thickness of the high-toughness positive electrode protective layer was reduced from 3μm to 1μm, the lithium-ion battery could not pass various safety tests.

[0144] Furthermore, lithium-ion batteries using water-soluble SBR rubber for the positive electrode protective layer have better appearance and safety performance compared to SBS rubber, which is easily soluble in NMP (Examples 1 and 2). This is mainly because the bottom layer of the water-based protective layer electrode does not dissolve or collapse when the active material coating is applied, thus avoiding appearance problems. In addition, the adhesion between the water-based polyacrylic acid adhesive and the aluminum foil is better than that between the oil-based PVDF and the aluminum foil, which is beneficial for protecting the aluminum foil and thus improving safety performance.

[0145] The main reason for the presence of microcracks in the positive electrode sheet in Example 2 is speculated to be that when the substances in the safety coating (polymer or binder) encounter solvents that easily dissolve them during the coating of the active material, some of the substances in the safety coating will dissolve. This causes structural collapse in some areas of the bottom layer, resulting in uneven thickness distribution after the top layer is coated, and microcracks or water ripples appear in some areas. In this example, the SBS in the safety coating will slightly dissolve when it encounters NMP during the coating of the active material layer, thus causing an appearance abnormality. However, this appearance abnormality will not affect the safety performance of the battery.

[0146] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A positive electrode sheet characterized by comprising: The positive electrode includes a current collector, a protective layer, and an active material layer. The protective layer is disposed on at least one side of the current collector, and the active material layer is disposed on the side of the protective layer away from the current collector. The protective layer includes an organic polymer with a glass transition temperature of -120℃ to -55℃. The active material in the active material layer includes lithium cobalt oxide; The current collector of the positive electrode is aluminum foil; The protective layer may also include a conductive agent, or a conductive agent and an adhesive; The content of each component in the protective layer is as follows: Organic polymers 50 wt%~99 wt% Conductive agent 1 wt%~20 wt%, Adhesive 0 wt%~40 wt% The thickness of the protective layer is 0.5~3 μm.

2. The positive electrode sheet according to claim 1, characterized by The organic polymer includes rubber materials.

3. The positive electrode sheet according to claim 2, characterized by The organic polymer includes at least one of styrene-butadiene rubber, nitrile rubber, styrene-isoprene-styrene block copolymer, silicone rubber, cis-butadiene rubber, natural rubber, and butyl rubber.

4. The positive electrode sheet according to claim 2, characterized in that, The organic polymer includes thermoplastic styrene-butadiene elastomer.

5. The positive electrode sheet according to claim 1, characterized in that, The particle size of the organic polymer is less than 200 μm.

6. The positive electrode sheet according to claim 1, characterized in that, The conductive agent includes at least one of conductive carbon black, single-walled carbon nanotubes, multi-walled carbon nanotubes, conductive metal particles, graphene, and carbon fiber.

7. The positive electrode sheet according to claim 1, characterized in that, The adhesive includes at least one of polyvinylidene fluoride, acrylic acid-modified polyvinylidene fluoride, carboxylic acid-modified polyvinylidene fluoride, polymethyl methacrylate, polyacrylic acid, polyacrylic acid copolymer, and polyimide.

8. The positive electrode sheet according to any one of claims 1-7, characterized in that, The active material in the active material layer also includes at least one of lithium iron phosphate, lithium nickel cobalt manganese, lithium nickel cobalt aluminum, lithium manganese oxide, and lithium-rich manganese-based lithium.

9. A lithium-ion battery, characterized in that, The lithium-ion battery includes the positive electrode sheet according to any one of claims 1-8.

Images