Lithium ion secondary battery

Abstract

The invention relates to the technical field of batteries, in particular to a lithium ion secondary battery. Comprising a positive plate and electrolyte, the positive plate comprises a positive current collector and a positive active layer; the thermal weight loss curve of the positive electrode active layer has a first thermal weight loss interval between 150 DEG C and 360 DEG C, and a first thermal weight loss ratio m1 corresponding to the first thermal weight loss interval is 0.1%-1.5%; the mass content of ethylene carbonate in the electrolyte is c1, and c1 is more than or equal to 0 and less than or equal to 10%; the mass content of ethyl propionate in the electrolyte is c2, and c2 is more than or equal to 0 and less than or equal to 10%; the electrolyte comprises a nitrile additive and fluoroethyl acetate; the mass content of the nitrile additive in the electrolyte accounts for c3, and c3 is 0.1%-10%; the mass content of the fluoroethyl acetate in the electrolyte accounts for c4, and c4 is 20%-50%. The lithium ion secondary battery provided by the invention can give consideration to relatively high energy density, high-temperature cycling stability and rate capability.

Description

Technical Field

[0001] The present invention relates to the technical field of batteries, and in particular to a lithium-ion secondary battery. Background Art

[0002] Currently, lithium-ion batteries are widely used in 3C digital products due to their long cycle life and high energy density. However, with the rapid development of mobile electronic devices, the demand for the cycle life and energy density of lithium-ion batteries is increasing. Therefore, there is an urgent need to improve the cycle life and energy density of lithium-ion batteries. However, in actual production, it has been found that increasing the battery's energy density will lead to a decrease in the flexibility of the positive electrode, which seriously affects the battery's high-temperature cycling stability. Summary of the Invention

[0003] The present invention aims to overcome the aforementioned problems existing in the prior art and provides a lithium-ion secondary battery. The lithium-ion secondary battery (hereinafter referred to as the battery) of the present invention features synergistic improvements to the positive electrode sheet and electrolyte. This improves the flexibility of the positive electrode sheet while also regulating the electrolyte composition to achieve a compatible balance between the two, thereby enhancing the battery's energy density, high-temperature cycling stability, and rate performance.

[0004] The positive electrode usually uses polyvinylidene fluoride (PVDF) as a binder. PVDF has good chemical stability and strong bonding strength, and is widely used as a binder in the positive electrode. But at the same time, PVDF has strong rigidity and poor adaptability to materials with large volume changes (such as silicon-carbon materials), which makes the positive electrode sheet prone to cracking due to repeated expansion / contraction during long-term cycles. The positive electrode sheet using PVDF has poor flexibility and poor processing performance, and is prone to breakage during the process (especially the winding process). As a result, the positive electrode sheet cannot increase the rolling pressure, resulting in a low compaction density of the positive electrode sheet and a low energy density of the battery. If the rolling pressure increases, due to the poor flexibility of the positive electrode sheet, the positive electrode sheet may be damaged, causing the high-temperature cycle stability to deteriorate.

[0005] Based on the above reasons, extensive targeted research has revealed that specific additives can be added to the positive electrode active layer. These additives can, to a certain extent, alleviate the rigidity of PVDF, thereby improving the flexibility of the positive electrode sheet. Furthermore, these specific additives can absorb moisture from the air, providing lubrication and moisturizing properties. During the cold pressing process, these additives facilitate particle slippage in the positive electrode active material, reducing damage to the positive electrode current collector (e.g., aluminum foil), thereby increasing the compaction density of the positive electrode sheet. This specific additive is manifested by a weight loss peak in the thermal gravimetric loss curve of the positive electrode active layer within a specific weight loss range, specifically between 150°C and 360°C. If the temperature is below 150°C, the specific additive is unstable and easily decomposes in a higher temperature operating environment, resulting in its ineffectiveness and inability to improve the flexibility of the positive electrode sheet. Furthermore, its decomposition products can affect the normal operation of the battery and shorten its lifespan. If the temperature is above 360°C, the specific additive has a high degree of polymerization, a high molecular weight, or excessive crystallinity and a large number of functional groups, which reduces its effect on improving the flexibility of the positive electrode sheet.

[0006] Although the presence of a weight loss peak in the positive electrode active layer in a specific temperature range can improve the overall toughness of the positive electrode sheet, thereby improving the high-temperature cycle stability and energy density of the battery, it will lead to an increase in the internal resistance of the positive electrode sheet, affecting the rate performance of the battery. This is because the specific additive easily swells in ethylene carbonate (EC) and / or ethyl propionate (EP), increasing the gap between the positive electrode active material particles and increasing the contact internal resistance. Therefore, in order to match the positive electrode system of the present invention, the electrolyte needs to be improved simultaneously.

[0007] First, reducing the proportion of EC and EP content can effectively alleviate the swelling effect of the electrolyte on specific additives, thereby maintaining the internal resistance of the positive electrode sheet.

[0008] Second, add nitrile additives. The cyanide group in nitrile additives can strongly interact with the metal ions on the positive electrode surface, thereby forming a stable and uniform positive electrode interface film, effectively preventing direct contact between the electrolyte and the positive electrode, reducing the catalytic effect of metal ions on the electrolyte, and avoiding oxidative decomposition of the electrolyte. At the same time, it can enhance the stability of the positive electrode surface structure and reduce the risk of the positive electrode active material being easily decomposed and undergoing phase transformation after the battery charge cut-off voltage increases. This can effectively improve the battery's high-temperature cycling stability at high voltages.

[0009] Third, ethyl fluoroacetate (especially 2,2-difluoroethyl acetate (DFEA)) as a small molecule solvent has extremely high antioxidant properties and can exist stably for a long time at high voltage. Therefore, while the electrolyte of the present invention regulates the ratio of EC and EP contents, by regulating the ratio of ethyl fluoroacetate content, it is possible to further improve the cycle stability of the positive electrode sheet while ensuring a low internal resistance of the battery. When ethyl fluoroacetate is added to the electrolyte, the antioxidant stability of the electrolyte can be greatly improved, so that the electrolyte as a whole can be stably stored for a longer time at high voltage, and the battery can be stably cycled for a longer time at high voltage. When the mass content of ethyl fluoroacetate accounts for a small proportion (for example, less than 20%), due to the small amount added, its antioxidant ability cannot be exerted, and the high voltage performance of the battery cannot be improved; and when the mass content of ethyl fluoroacetate accounts for a large proportion (for example, greater than 50%), due to its high viscosity, the overall viscosity of the electrolyte increases, the conductivity decreases, and the battery rate performance decreases.

[0010] Based on this, the inventors of the present invention proposed the following solution:

[0011] The present invention provides a lithium-ion secondary battery, comprising a positive electrode sheet and an electrolyte; the positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer located on at least one side of the positive electrode current collector; a thermal gravimetric analysis curve of the positive electrode active layer has a first thermal gravimetric analysis interval, the first thermal gravimetric analysis interval is between 150° C. and 360° C., and a first thermal gravimetric analysis rate m1 corresponding to the first thermal gravimetric analysis interval is 0.1%-1.5%; wherein the thermal gravimetric analysis curve is obtained by testing using a thermogravimetric analyzer under a nitrogen atmosphere; the mass content of ethylene carbonate in the electrolyte is c1, 0≤c1≤10%; the mass content of ethyl propionate in the electrolyte is c2, 0≤c2≤10%; the electrolyte comprises a nitrile additive and ethyl fluoroacetate; the mass content of the nitrile additive in the electrolyte is c3, c3 is 0.1%-10%; the mass content of the ethyl fluoroacetate in the electrolyte is c4, c4 is 20%-50%.

[0012] Through the above technical solution, the present invention has at least the following advantages compared with the prior art: the lithium-ion secondary battery of the present invention can take into account higher energy density, high-temperature cycle stability and rate performance.

[0013] The endpoints of the ranges and any values ​​disclosed herein are not limited to the precise ranges or values, and these ranges or values ​​should be understood to include values ​​close to these ranges or values. For numerical ranges, the endpoints of each range, the endpoints of each range and individual point values, and the individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered to be specifically disclosed herein. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 Shown is a thermogravimetric curve of the positive electrode active layer and its corresponding thermogravimetric differential curve in an example of the present invention.

[0015] Figure 2 Shown is a schematic diagram of H1 and H2 in an example of the present invention.

[0016] Figure 3 FIG. 1 is a schematic diagram of the groove width in an embodiment of the present invention; wherein, Figure 3 (a)- Figure 3 (c) The two long sides of the groove are straight lines. Figure 3 In (d), the two long sides of the groove are curved.

[0017] Figure 4 The figure shows a schematic diagram of the groove spacing in an embodiment of the present invention; wherein, Figure 4 (a) is the case where two adjacent long sides are straight and parallel. Figure 4 (b) is the case where two adjacent long sides are straight lines and not parallel. Figure 4 (c) is the case where two adjacent long sides are curved lines.

[0018] Figure 5 FIG. 4 shows a volume particle size distribution curve of the positive electrode active material in an example of the present invention. DETAILED DESCRIPTION

[0019] The following is a detailed description of the specific embodiments of the present invention. It should be understood that the specific embodiments described herein are only used to illustrate and explain the present invention and are not intended to limit the present invention.

[0020] The present invention provides a lithium-ion secondary battery, comprising a positive electrode sheet and an electrolyte. The positive electrode sheet comprises a positive electrode current collector and a positive electrode active layer located on at least one side of the positive electrode current collector. The thermal weight loss curve of the positive electrode active layer has a first thermal weight loss interval, which is located between 150°C and 360°C (for example, 150°C, 160°C, 170°C, 180°C, 190°C, 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, 270°C, 280°C, 290°C, 300°C, 310°C, 320°C, 330°C, 340°C, 350°C or 360°C), and the first thermal weight loss rate m1 corresponding to the first thermal weight loss interval is 0.1%-1.5% (for example, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 1.1%, 1.2%, 1.3%, 1.4% or 1.5%). The thermogravimetric curve is obtained by using a thermogravimetric analyzer under a nitrogen atmosphere.

[0021] In one example, the first thermal weight loss range is between 160°C and 360°C.

[0022] In one example, m1 is 0.15%-1%.

[0023] In the present invention, the thermal gravimetric analysis of the positive electrode active layer, the position of the first thermal gravimetric analysis interval, and the first thermal gravimetric analysis rate can be obtained by thermal gravimetric analysis, specifically as follows: after discharging the battery to 0% SOC, the positive electrode sheet is disassembled and removed, and then soaked in dimethyl carbonate (DMC) solvent for 12 hours, and then rinsed with DMC to remove the lithium salt attached to the positive electrode sheet. After drying, the positive electrode active layer on the positive electrode sheet is peeled off from the positive electrode current collector with a metal scraper, and the positive electrode active layer is collected as a test sample. Using a thermogravimetric analyzer (such as a TGA550 thermogravimetric analyzer), the test sample amount is 5mg-15mg, and the temperature is increased from room temperature (25°C) to 1000°C at a heating rate of 1°C / min under a nitrogen atmosphere to obtain a thermal gravimetric analysis curve of the positive electrode active layer, and the thermal gravimetric analysis curve is further differentiated to obtain a thermal gravimetric differential curve. The position of the first weight loss interval can be obtained from the thermal weight loss differential curve (the position of the first weight loss interval can be obtained by the starting point and end point of the weight loss peak between 150℃ and 360℃ in the thermal weight loss differential curve). m1 = weight retention rate corresponding to the starting point of the weight loss peak - weight retention rate corresponding to the end point of the weight loss peak. Figure 1 The following figure shows the thermal gravimetric loss curve and its corresponding differential thermal gravimetric loss curve for the positive electrode active layer in one embodiment of the present invention. As can be seen from the figure, the first thermal gravimetric loss interval is between 245°C and 298°C, and m1 is 0.16%. The term "0% SOC" can refer to a battery discharged at 0.1C to 2.7V.

[0024] It can be understood that the first thermal weight loss interval being between 150°C and 360°C means that the first thermal weight loss interval can be located in the range formed by any two point values ​​between 150°C and 360°C, and includes 150°C and 360°C.

[0025] In the present invention, the mass content of ethylene carbonate in the electrolyte is c1, 0≤c1≤10%, for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1% or 0%. The mass content of ethyl propionate in the electrolyte is c2, 0≤c2≤10%, for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, 0.1% or 0%.

[0026] In one example, 0%≤c1≤5%.

[0027] In one example, 0%≤c2≤6%.

[0028] In one example, the electrolyte does not contain ethylene carbonate.

[0029] In one example, the electrolyte does not contain ethyl propionate.

[0030] In the present invention, the electrolyte comprises a nitrile additive and ethyl fluoroacetate. The mass content of the nitrile additive in the electrolyte is C3, and C3 is 0.1%-10%, for example, 0.1%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%. The mass content of the ethyl fluoroacetate in the electrolyte is C4, and C4 is 20%-20%, for example, 20%, 25%, 30%, 35%, 40%, 45%, or 50%.

[0031] In one example, 1%≤c3≤6%.

[0032] In the present invention, in the electrolyte, the mass content of ethylene carbonate accounts for c1, the mass content of ethyl propionate accounts for c2, the mass content of the nitrile additive accounts for c3 and the mass content of ethyl fluoroacetate accounts for c4 can be tested by conventional methods in the art, such as gas chromatography (GC).

[0033] In the present invention, the ethyl fluoroacetate refers to a substance in which a fluorine atom is substituted at any position of ethyl acetate.

[0034] In one example, the ethyl fluoroacetate includes 2,2-difluoroethyl acetate (DFEA) and / or ethyl 2,2-difluoroacetate.

[0035] In one example, the ethyl fluoroacetate includes DFEA.

[0036] In the present invention, the nitrile additive refers to an additive containing a cyano group (-CN), for example, at least one of glutaronitrile, adiponitrile (ADN), succinonitrile (SN), sebacononitrile, azelaic acid dinitrile, fumaronitrile, ethylene glycol bis(propionitrile) ether, glycerol trinitrile, 1,3,6-hexane trinitrile (HTCN) and 1,4-dicyano-2-butene.

[0037] In the present invention, the positive electrode active layer includes additives. The additives include polyvinyl butyral, polyurethane (PU) and its derivatives, polyamide, single-terminal polyether, double-terminal polyether, dioctyl phthalate, dibutyl phthalate, dimethyl phthalate, diethyl phthalate, di(butoxyethoxy)ethyl adipate, isopropyl titanate, n-butyl titanate, citrate, (2-ethyl)hexyl trimellitate, di(2-ethyl)hexyl phthalate, di(2-ethyl)hexyl sebacate, diethylene glycol dibenzoate, phthalic anhydride, dipropylene glycol dibenzoate, chlorosulfonated polyethylene, polyamideimide, pentaerythritol tetrabenzoate, trimethyl citrate, triethyl citrate, tributyl citrate, sodium ditridecyl sulfosuccinate, polysiloxane, fatty acid salt, quaternary ammonium salt, methyl silicone resin, phenyl silicone resin and vinyl silicone resin.

[0038] In the present invention, the polyurethane derivatives include, for example, at least one of perfluoropolyether-polyamide block copolymers (PFPE-PU), trifluoroacetic acid-terminated polyamides (TFA-PU), polyethylene oxide-urethane copolymers (PEO-PU), polycarbonate-polyurethanes, carboxylic acid-based waterborne polyurethanes, sulfonate-based waterborne polyurethanes, graphene / polyurethane composites, ceramic / polyurethane hybrids, castor oil-based polyurethanes, and lignin sulfonate-modified polyurethanes. The polyamides include, for example, at least one of nylon 6, nylon 66, poly(p-phenylene terephthalamide), poly(m-phenylene isophthalamide), poly(p-phenylene adipamide), poly(p-phenylene azelaamide), sulfurized polyamides, and fluorinated polyamides. Single-terminal polyethers refer to polymers in which only one end of the polyether molecular chain carries a reactive functional group (e.g., at least one of a hydroxyl, amino, or epoxy group) and the other end carries a blocked inactive group (e.g., a methyl and / or benzyl group). Double-end polyether refers to a polymer with reactive groups (such as at least one of hydroxyl, amino or epoxy groups) at both ends of the polyether molecular chain, and its flexible ether chain (such as polyethylene glycol and / or polypropylene glycol) is located between the reactive groups at both ends. The fatty acid salt, for example, includes at least one of aluminum stearate, magnesium stearate, aluminum isooctanoate and magnesium isooctanoate. The quaternary ammonium salt, for example, includes aliphatic quaternary ammonium salt and / or aromatic quaternary ammonium salt, wherein the aliphatic quaternary ammonium salt includes long-chain quaternary ammonium salt (referring to an alkyl chain length greater than or equal to 10, such as hexadecyltrimethylammonium bromide) and / or short-chain quaternary ammonium salt (referring to an alkyl chain length less than 10, such as tetramethylammonium chloride). The molecular structure of the methyl silicone resin is composed of a Si-O-Si inorganic main chain and a methyl side chain. The molecular structure of the phenyl silicone resin is composed of a Si-O-Si inorganic main chain and a phenyl side chain. The molecular structure of the vinyl silicone resin is composed of a Si-O-Si inorganic main chain and a vinyl side chain.

[0039] As mentioned above, the groups in specific additives can be sandwiched between PVDF molecular chains, shielding the polar CF bonds in PVDF, reducing the forces between PVDF molecular chains, and increasing the disorder of PVDF molecular chains. In particular, when combined with specific additives including ester groups with flexible segments, they can reduce hydrogen bonds and van der Waals forces between PVDF molecular chains, increase the flexibility and mobility of its molecular chains, optimize crystallinity, and at the same time exert a strong bonding effect, thereby improving the flexibility of the positive electrode sheet. This can not only improve the coating cracking and cold pressing embrittlement problems caused by polymer shrinkage during the coating and drying process of the positive electrode sheet, but also reduce the impact of winding or folding operations on the positive electrode sheet, improve the cracking of the electrode sheet or the breakage of the positive electrode active material particles, and reduce the side reactions of the surface and electrolyte caused by the breakage under cycling or high temperature conditions, thereby improving the cycle performance and high temperature performance of the battery. On this basis, the compaction density of the positive electrode sheet can also be improved, which is conducive to shortening the transmission path of the active material, reducing the internal resistance of the electrode sheet, and improving the dynamics of the battery. At the same time, specific additives can form an elastic network in the positive electrode active layer, which absorbs the volume change stress caused by lithium ion insertion / extraction through its own deformation, further improving the cycle life of the battery.

[0040] In one example, the additive has an ester group.

[0041] In one example, the additive includes the polyurethane and its derivatives.

[0042] In the present invention, the thermal gravimetric analysis curve of the positive electrode active layer further includes a second thermal gravimetric analysis interval. The second thermal gravimetric analysis interval is between 370°C and 600°C (for example, 370°C, 380°C, 390°C, 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C, or 600°C), and the second thermal gravimetric analysis interval corresponds to a second thermal gravimetric analysis rate m2 of 0.5% to 4% (for example, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, or 4%).

[0043] The second weight loss temperature range of the thermal weight loss curve of the positive electrode active layer reflects the molecular structure of PVDF, the strength of the CF chemical bond and the swelling state of PVDF in the electrolyte. By regulating the specific weight loss rate within the second weight loss temperature range, the flexibility of the positive electrode sheet can be further improved, thereby improving the energy density and high-temperature cycle stability of the battery.

[0044] In one embodiment, the second thermal weight loss range is between 430°C and 510°C.

[0045] In one embodiment, the second thermal weight loss range is between 440°C and 490°C.

[0046] In one example, m2 is 1%-2%.

[0047] In the present invention, the position of the second thermal weight loss interval and the second thermal weight loss rate can refer to the first thermal weight loss interval and the first thermal weight loss rate, which will not be repeated here.

[0048] In the present invention, the positive electrode active layer comprises polyvinylidene fluoride, and the crystallinity of the polyvinylidene fluoride is 40%-70%, for example, 40%, 50%, 60% or 70%.

[0049] In one embodiment, the polyvinylidene fluoride has a crystallinity of 50% to 60%.

[0050] The crystallinity of PVDF affects its bonding strength and flexibility. Specifically: the molecular chains in the crystalline region are closely arranged, forming strong intermolecular forces (such as dipole-dipole interactions and hydrogen bonds), which give PVDF higher strength and rigidity, ensuring a strong bond between the positive active layer and the positive current collector; while the molecular chains in the amorphous region are disordered and have a high degree of freedom, which can dissipate energy through local plastic deformation, inhibit crack propagation, and thus improve the flexibility of the positive electrode sheet. Therefore, an appropriate proportion of amorphous regions allows the positive electrode sheet to deform moderately when subjected to force, avoiding brittle fracture. Therefore, it is necessary to regulate the crystallinity of PVDF so that it can take into account both bonding strength and flexibility, thereby further improving the energy density and high-temperature cycle stability of the battery.

[0051] In the present invention, the crystallinity of polyvinylidene fluoride has the conventional meaning in the art, generally referring to the mass fraction of regularly ordered crystalline regions in the polyvinylidene fluoride. This can be calculated by analyzing the intensity ratio of the diffraction peak (i.e., the crystalline region) to the diffuse scattering (i.e., the amorphous region) using the X-ray diffraction characteristics of the crystal.

[0052] In the present invention, the peel strength between the positive electrode active layer and the positive electrode current collector is 10N / m-50N / m, for example, 10N / m, 15N / m, 20N / m, 25N / m, 30N / m, 35N / m, 40N / m, 45N / m or 50N / m.

[0053] In one example, the peel strength between the positive electrode active layer and the positive electrode current collector is 15 N / m-25 N / m.

[0054] In the present invention, the peel strength between the positive electrode active layer and the positive electrode current collector can be tested by conventional testing methods in the field, for example, discharging the battery to 0% SOC, disassembling the battery, soaking it in DMC solvent for 12 hours, and then rinsing it with DMC to remove the lithium salt attached to the positive electrode sheet. After drying, use a 3M double-sided tape with a length of more than 60 mm and a width of 24 mm to adhere to the positive electrode sheet; then use a roller with a diameter of 100 mm to roll the positive electrode sheet with the double-sided tape back and forth once to ensure that the double-sided tape is evenly adhered; then tear off a section from one end for fixing the fixture, and use a WD-D3 electronic universal testing machine to test the peel strength.

[0055] In the present invention, the positive electrode active layer may further include a positive electrode conductive agent and a positive electrode binder. The positive electrode conductive agent may include, for example, at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, carbon nanotubes, and carbon fibers. The positive electrode binder may include, for example, at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, PAN, polyurethane, polymethacrylate, polyacrylate, and polyethylene oxide.

[0056] In the present invention, based on the total mass of the positive electrode active layer, the content of the positive electrode active material is 80%-99.8% (for example, 80%, 82%, 84%, 86%, 88%, 90%, 92%, 94%, 96%, 98%, 99% or 99.8%), the content of the positive electrode conductor is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%), and the content of the positive electrode binder is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%).

[0057] In the present invention, the lithium-ion secondary battery further includes a negative electrode sheet, comprising a negative electrode current collector and a negative electrode active layer located on at least one surface of the negative electrode current collector. The negative electrode active layer includes a first active layer and a second active layer disposed along the thickness of the negative electrode sheet, with the first active layer located between the negative electrode current collector and the second active layer. The first active layer comprises a first graphite material, and the second active layer comprises a silicon-carbon material.

[0058] The improved positive electrode sheet of the present invention has significantly improved flexibility and better processing characteristics. Therefore, a negative electrode sheet with good flexibility is required to match it during the manufacturing process (such as winding). The negative electrode active layer is divided into zones, with silicon-carbon materials arranged in the area away from the negative current collector and graphite materials arranged in the area close to the negative current collector. The graphite material has a high modulus but is relatively brittle. Placing it in the area close to the negative current collector can provide structural stability; while the silicon-carbon material arranged in the area away from the negative current collector compensates for the brittleness of the graphite material through its plastic deformation ability, which is beneficial to improving the flexibility of the negative electrode sheet. In addition, the silicon-carbon material has a higher porosity than the graphite material and can absorb some of the strain caused by volume changes; the graphite material has a higher density, which ensures the integrity of the conductive network. This pore gradient distribution allows the negative electrode sheet to adapt to deformation through pore compression / rebound when bent, thereby improving the flexibility of the negative electrode sheet. Therefore, the negative electrode active layer is coated in different areas, and the first graphite material is arranged in the area close to the negative electrode current collector, and the silicon-carbon material is arranged in the area away from the negative electrode current collector, so that the flexibility of the negative electrode sheet can be maintained while maintaining a high gram capacity. The improved negative electrode sheet can be used in combination with a positive electrode sheet with good flexibility, which not only has better processing performance, but also can further improve the energy density and high-temperature cycle stability of the battery. In addition, from a dynamic point of view, the silicon-carbon material is arranged in the area away from the negative electrode current collector, so that the silicon-carbon material is closer to the positive electrode sheet, the electrolyte infiltration of the silicon-carbon material is more sufficient, and the distance for lithium ion migration is very short. Therefore, the number of lithium ions that can be quickly embedded in lithium is greatly increased, thereby improving the kinetic performance of the negative electrode.

[0059] In the present invention, the dimension of the first active layer in the thickness direction of the negative electrode sheet is H1, and the dimension of the second active layer in the thickness direction of the negative electrode sheet is H2, and H2 / H1 ≥ 0.4, for example, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1, 1.2, 1.3, or 1.4. By adjusting H2 / H1, the flexibility of the negative electrode sheet can be further improved, thereby making it more compatible with the positive electrode sheet, which is conducive to further improving the energy density and high-temperature cycling stability of the battery.

[0060] In one example, 0.5≤H2 / H1≤0.95.

[0061] In the present invention, the size H1 of the first active layer in the thickness direction and the size H2 of the second active layer in the thickness direction can be obtained by testing conventional methods in the art. For example, after the battery is discharged to 0% SOC, the negative electrode sheet is disassembled and taken out, and then soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt attached to the negative electrode sheet. The negative electrode sheet is cut using an argon ion milling instrument CP, and then observed using a scanning electron microscope (SEM) (using backscatter imaging mode). In this mode, the contrast of the silicon-carbon material is brighter (which can be used to distinguish the graphite material and the conductive agent in the negative electrode active layer). ), select at least 10 electron microscope images, select the silicon-carbon material with the edge closest to the surface of the negative electrode current collector in each electron microscope image, select the point closest to the surface of the negative electrode current collector on the edge of the silicon-carbon material, and use this point as the starting point to draw a vertical line perpendicular to the negative electrode current collector, recorded as the first vertical line, and use the size of the first vertical line as the dimension H1 of the first active layer in the thickness direction, and use this point as the starting point to draw a vertical line perpendicular to the surface of the second active layer away from the negative electrode current collector, recorded as the second vertical line, and use the size of the second vertical line as the dimension H2 of the second active layer in the thickness direction. Take the average of the measured values ​​in each microscope image to obtain H1 and H2. Figure 2 Shown is a schematic diagram of H1 and H2 in an embodiment of the present invention, where H1 and H2 are marked.

[0062] In one example, the silicon-carbon material includes a porous carbon matrix and silicon material located in pores of the porous carbon matrix.

[0063] In one embodiment, the second active layer further comprises a second graphite material. The first graphite material and the second graphite material each independently comprise at least one of artificial graphite, natural graphite, and mesophase carbon microbead graphite.

[0064] In the present invention, the sphericity of the silicon-carbon material is 0.8-1, for example, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 0.99 or 1. The sphericity of the silicon-carbon material can be tested by conventional methods in the art. For example, discharge the battery to 0% SOC, disassemble and remove the negative electrode sheet, polish the cross-section of the negative electrode sheet using an argon ion mill, and observe it in the backscatter imaging mode of the SEM equipment. Find a silicon-carbon material with a continuous and smooth contour, connect any two points on the edge of the particle to form a straight line segment inside the particle, select the longest straight line segment within the particle, and record its length as Z1. Take the midpoint of the longest straight line segment, draw a straight line through it, and form a straight line segment with both ends at the edge of the particle. Select the shortest straight line segment, and record its length as Z2. The sphericity of the particle is then Z2 / Z1. Select at least 10 silicon-carbon material particles, measure the sphericity, and take the average value.

[0065] Silicon-carbon materials with high sphericity (for example, 0.8-1) have no selectivity in the direction of expansion and can expand uniformly in all directions. Therefore, they can reduce problems such as thickness increase, electrode interface instability, and electrode stress accumulation caused by silicon in the negative electrode active material.

[0066] In one example, the sphericity of the silicon-carbon material is 0.9-1.

[0067] In the present invention, the average particle size of the silicon-carbon material is 2.5μm-26μm, for example, 2.5μm, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm, 20μm, 21μm, 22μm, 23μm, 24μm, 25μm or 26μm. The average particle size of the silicon-carbon material can be measured using conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled and removed. After soaking in a DMC solvent for 12 hours, the negative electrode sheet is rinsed with DMC solvent to remove the lithium salt attached to the negative electrode sheet. The negative electrode sheet is cut using an argon ion milling instrument (CP) and observed in a SEM device using backscatter imaging mode. The particle size of at least 20 silicon-carbon material particles is measured at a magnification of 5K, and the average value is taken. If the number of particles at a magnification of 5K is less than 20, additional mirror images are taken until 20 particles are measured. If the particle in the mirror image is a regular circle, the particle size of the particle is the diameter of the regular circle. If the particle in the mirror image is not a "regular circle", any two points on the edge of the particle are connected to form a straight line segment inside the particle. The longest straight line segment inside the particle is selected as the particle size.

[0068] In one embodiment, the average particle size of the silicon-carbon material is 8 μm-18 μm.

[0069] In the present invention, the mass content of silicon in the silicon-carbon material is 20%-60%, for example, 20%, 30%, 40%, 50% or 60%. The mass content of silicon in the silicon-carbon material can be tested by conventional methods in the art, such as discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, using an argon ion milling instrument CP to cut the negative electrode sheet, and using the backscattering imaging mode in the SEM equipment to observe the silicon-carbon material and magnify it as much as possible; using an energy dispersive spectrometer (EDS) to scan the cross section of the silicon-carbon material particles, the surface scanning area should not be less than 50% of the particle cross section, and the scanning range should be completely within the particle cross section, and the mass content of silicon is calculated. At least 10 particles are selected for measurement and the average value is taken.

[0070] In the present invention, the mass content of silicon in the negative electrode active layer is 3%-30%, for example, 3%, 5%, 10%, 15%, 20%, 25% or 30%. The mass content of silicon in the negative electrode active layer can be tested by conventional methods in the art, for example, after discharging the battery to 0% SOC, disassembling and removing the negative electrode sheet, soaking it in DMC solvent for 12 hours, then rinsing it with DMC solvent to remove the lithium salt attached to the negative electrode sheet, soaking the negative electrode active layer from the negative electrode current collector with deionized water, and then drying the detached negative electrode active layer, and collecting the negative electrode active layer as a test sample. Using a thermogravimetric analyzer (e.g., a TGA 550 thermogravimetric analyzer), test a sample size of 5mg-15mg. In an air or oxygen atmosphere, heat the sample from room temperature (25°C) to 900°C at a rate of 10°C / min and maintain the temperature at 900°C for 40 minutes. This allows the non-silicon components in the negative electrode active layer to volatilize while the silicon is fully oxidized to silicon dioxide. The remaining material is the ash content of the negative electrode active layer. The mass content of silicon in the negative electrode active layer can be calculated based on the mass of the ash. The calculation formula is as follows: Mass content of silicon in the negative electrode active layer = 7 × mass of ash / (15 × mass of the test sample).

[0071] In the present invention, the outer surface of the negative electrode active layer has a plurality of grooves. The depth of the grooves is 5 μm to 50 μm, for example, 5 μm, 10 μm, 20 μm, 30 μm, 40 μm, or 50 μm. The width of the grooves is 20 μm to 150 μm, for example, 20 μm, 30 μm, 40 μm, 50 μm, 60 μm, 70 μm, 80 μm, 90 μm, 100 μm, 110 μm, 120 μm, 130 μm, 140 μm, or 150 μm. The spacing of the grooves is 500 μm to 2000 μm, for example, 500 μm, 600 μm, 700 μm, 800 μm, 900 μm, 1000 μm, 1100 μm, 1200 μm, 1300 μm, 1400 μm, 1500 μm, 1600 μm, 1700 μm, 1800 μm, 1900 μm or 2000 μm. The "several" refers to the number of grooves on the outer surface of the negative electrode active layer being greater than or equal to 2. The "outer surface" refers to the surface of the negative electrode active layer away from the negative electrode current collector.

[0072] Providing grooves on the outer surface of the negative electrode active layer can further provide buffer space for the volume expansion of the silicon-carbon material, thereby reducing the increase in the thickness of the negative electrode sheet and helping to improve the cycle stability of the battery.

[0073] In the present invention, the depth of the groove has the conventional meaning in the art, and refers to the vertical distance from the lowest point in the groove to the outer surface of the negative electrode active layer. This depth can be determined by conventional testing methods in the art. For example, the depths of all grooves or at least five grooves on the outer surface of the negative electrode active layer can be measured using a 3D profilometer or SEM, and the average value can be calculated.

[0074] In the present invention, the projection of the groove on the negative electrode active layer includes two long sides, and the width of the groove refers to the average distance from one long side to the other long side in the length direction or width direction of the negative electrode sheet. Figure 3 FIG. 1 is a schematic diagram of the groove width in an embodiment of the present invention; wherein, Figure 3 (a)- Figure 3 (c) The two long sides of the groove are straight lines. Figure 3 The two long sides of the groove in (d) are curved. It should be noted that Figure 3 Only one groove is schematically drawn in the figure, which does not mean that there is only one groove on the surface of the negative electrode sheet of the present invention. Figure 3 (a) and Figure 3 In (b), the two long sides are arranged in parallel. Therefore, in the width direction of the negative electrode sheet, the distance from any point on one long side to the other long side is equal. In this case, the width of the groove is ( Figure 3In the width direction), the distance d from any point on one long side to the other long side; Figure 3 In (c), the two long sides of the groove are straight lines, but they are not parallel. Therefore, the distance from any point on one long side to the other long side is not equal. In this case, the width of the groove can be averaged. That is, on one long side, based on the length of the side, 50 points are selected at equal distances (that is, the distance between each point is equal, so the selection of points can make the calculation result more accurate), and the width d corresponding to each point is measured. The average value is used to obtain the width of the groove. Figure 3 In (d), the two long sides are curved. Therefore, the distance from any point on one long side to the other long side is not equal. In this case, the width of the groove can also be averaged, that is, 50 points are randomly selected on one long side (due to Figure 3 In (d), the two long sides are curved and do not exist. Figure 3 (c) The relationship between the two long sides, therefore, 50 points can be randomly selected for measurement), and the width d corresponding to each point is measured, and the average value is taken to obtain the groove width. The groove width can be measured by conventional testing methods in the art, for example, by measuring the width of all grooves or at least 5 grooves on the outer surface of the negative electrode active layer using a 3D profilometer or SEM, and taking the average value.

[0075] In the present invention, the spacing of the grooves refers to the average distance between the two adjacent long sides of two adjacent grooves in the length direction or width direction of the negative electrode sheet. Figure 4 The figure shows a schematic diagram of the groove spacing in an embodiment of the present invention; wherein, Figure 4 (a) is the case where two adjacent long sides are straight and parallel. Figure 4 (b) is the case where two adjacent long sides are straight lines and not parallel. Figure 4 (c) is the case where two adjacent long sides are curved. Figure 4 In (a), the two adjacent long sides are straight and parallel. Therefore, in the width direction, the distance from any point on one long side to the other long side is equal. In this case, the spacing of the grooves is the distance d1 from any point on one long side to the other long side in the width direction. Figure 4 In (b), the two adjacent long sides are straight lines, but they are not parallel. Therefore, the distance from any point on one long side to the other long side is not equal. In this case, the spacing of the grooves can be averaged. That is, on one long side, based on the length of the side, 50 points are selected at equal distances (that is, the distance between each point is equal, so the selection of points can make the calculation result more accurate), and the width d1 corresponding to each point is measured. The average value is used to obtain the spacing of the grooves. Figure 4In (c), the two adjacent long sides are curved. Therefore, the distance from any point on one long side to the other long side is not equal. In this case, the spacing of the grooves can also be averaged, that is, 50 points are randomly selected on one long side (due to Figure 4 In (c), the two long sides are curved and do not exist. Figure 4 (b) The relationship between the two long sides, therefore, 50 points can be randomly selected for measurement) and the width d1 corresponding to each point is measured. The average value is used to obtain the groove spacing. The groove spacing can be measured using conventional testing methods in the art. For example, using a 3D profilometer or SEM, the spacing of all grooves or at least five groups of adjacent grooves on the outer surface of the negative electrode active layer is measured and the average value is calculated.

[0076] In the present invention, the negative electrode active layer may further include a negative electrode conductive agent, a negative electrode binder, and a thickener. The negative electrode conductive agent may include, for example, at least one of conductive carbon black, acetylene black, Ketjen black, conductive graphite, carbon nanotubes, and carbon fibers. The negative electrode binder may include, for example, at least one of polyacrylic acid, polyvinylidene fluoride, polytetrafluoroethylene, styrene-butadiene rubber, polyacrylonitrile (PAN), polyurethane, polymethacrylate, polyacrylate, and polyethylene oxide. The thickener may include, for example, at least one of carboxymethyl cellulose (CMC), sodium carboxymethyl cellulose, and lithium carboxymethyl cellulose.

[0077] In the present invention, the first active layer includes a first negative electrode active material, the negative electrode conductor, the negative electrode binder and the thickener. The first negative electrode active material includes the first graphite material. Based on the total mass of the first active layer, the content of the first negative electrode active material is 70%-99.7% (for example, 70%, 73%, 76%, 79%, 82%, 85%, 88%, 91%, 94%, 97%, 98.5% or 99.7%), and the content of the negative electrode conductor is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%), the content of the negative electrode binder is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%), and the content of the thickener is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%).

[0078] In the present invention, the second active layer includes a second negative electrode active material, the negative electrode conductor, the negative electrode binder and the thickener. The second negative electrode active material includes the second graphite material and the silicon-carbon material. Based on the total mass of the second active layer, the content of the second negative electrode active material is 70%-99.7% (for example, 70%, 73%, 76%, 79%, 82%, 85%, 88%, 91%, 94%, 97%, 98.5% or 99.7%), and the content of the negative electrode conductor is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%), the content of the negative electrode binder is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%), and the content of the thickener is 0.1%-10% (for example, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5% or 0.1%).

[0079] In the present invention, the positive electrode active layer further includes a positive electrode active material. The volume particle size distribution curve of the positive electrode active material has a first peak and a second peak; the abscissa corresponding to the peak value of the first peak is P1, and the abscissa corresponding to the peak value of the second peak is P2. P1 is 2 μm to 6 μm, for example, 2 μm, 3 μm, 4 μm, 5 μm, or 6 μm. P2 is 10 μm to 25 μm, for example, 10 μm, 15 μm, 20 μm, or 25 μm.

[0080] When the volume particle size distribution curve of the positive electrode active material has a first peak and a second peak, it indicates that the positive electrode active material has particles of different sizes with different average particle sizes (i.e., the positive electrode active material includes first particles and second particles, wherein the average particle size of the first particles is P1, the average particle size of the second particles is P2, and the average particle size of the first particles is smaller than the average particle size of the second particles). Such a combination can increase the surface density of the positive electrode active layer. Among them, particles with large particle size (second particles) can provide more capacity, and particles with small particle size (first particles) fill the gaps formed by particles with large particle size, which can not only play the role of transmitting electrons, but also increase the specific surface area and increase the active sites of the reaction, thereby improving the kinetic performance of the positive electrode sheet.

[0081] In one example, P1 is 3 μm-5 μm.

[0082] In one example, P2 is 15 μm-23 μm.

[0083] In the present invention, the volume particle size distribution curve of the positive electrode active material can be obtained by testing conventional methods in the field, for example, after discharging the battery to 0% SOC, disassembling and removing the positive electrode sheet, soaking it in DMC solvent for 12 hours, and then rinsing it with DMC solvent to remove the lithium salt attached to the positive electrode sheet, using air atmosphere in a muffle furnace, heating from room temperature to 400°C at 5°C / min, keeping warm for 3 hours, and then cooling to room temperature, taking out the positive electrode sheet, brushing the powder on the surface of the positive electrode sheet from the positive electrode current collector with a brush, and then testing the volume particle size distribution with a laser particle size analyzer. Figure 5 The figure shows the volume particle size distribution curve of the positive electrode active material in an example of the present invention. It can be seen from the figure that the volume particle size distribution curve of the positive electrode active material has a first peak and a second peak; the peak value P1 of the first peak is 4.034μm, and the peak value P2 of the second peak is 18.664μm.

[0084] In the present invention, the charging cut-off voltage of the lithium-ion secondary battery is ≥4.5V.

[0085] In one embodiment, the positive electrode active material includes lithium cobalt oxide. The lithium cobalt oxide includes titanium, and the mass content of titanium in the lithium cobalt oxide is 200 ppm-3000 ppm, for example, 200 ppm, 300 ppm, 400 ppm, 500 ppm, 600 ppm, 700 ppm, 800 ppm, 900 ppm, 1000 ppm, 1100 ppm, 1200 ppm, 1300 ppm, 1400 ppm, 1500 ppm, 1600 ppm, 1700 ppm, 1800 ppm, 1900 ppm, 2000 ppm, 2100 ppm, 2200 ppm, 2300 ppm, 2400 ppm, 2500 ppm, 2600 ppm, 2700 ppm, 2800 ppm, 2900 ppm, or 3000 ppm.

[0086] In one embodiment, the mass content of titanium in lithium cobalt oxide is 800 ppm-2000 ppm.

[0087] The peak values ​​of the first and second peaks correspond to the average particle sizes of the first and second particles, respectively. Since PVDF reduces the compaction density of the positive electrode active layer, the problem of low compaction density can be further improved by combining large and small particles to fill the gaps between large particles. Furthermore, the addition of titanium to lithium cobalt oxide improves the adhesion of the lithium cobalt oxide particle surface, enhancing the bonding performance of the positive electrode sheet and thus improving the battery's cycling stability.

[0088] In the present invention, the mass content ratio of titanium element in lithium cobalt oxide can be tested by conventional methods in the field, such as using an inductively coupled plasma-optical emission spectrometer (ICP-OES). The specific testing method is as follows: discharge the battery to 0% SOC, disassemble and remove the positive electrode sheet, and soak it in DMC solvent for 12 hours; then rinse with DMC solvent to remove the lithium salt attached to the positive electrode sheet, calcined in a muffle furnace at 400°C for 3 hours, gently scraped the positive electrode active layer from the surface of the positive electrode current collector, and measured the mass content ratio of titanium element in lithium cobalt oxide by ICP-OES (in ppm, i.e., one part per million). The specific operation method is carried out in accordance with GB / T30902-2014.

[0089] In the present invention, the surface density of the positive electrode active layer is P (unit: mg / cm 2 ), the compaction density of the positive electrode active layer is Q (unit: g / cm 3 ), P, Q and m1 respectively satisfy: P / m1 is 1000-18000 (for example, 1000, 2000, 3000, 4000, 5000, 6000, 7000, 8000, 9000, 10000, 11000, 12000, 13000, 14000, 15000, 16000, 17000 or 18000), Q / m1 is 200-3000 (for example, 200, 300, 400, 500, 600, 700, 800, 900, 1000, 2000 or 3000).

[0090] In one embodiment, P / m1 is 1000-12500.

[0091] In one embodiment, P / m1 is 1300-10000.

[0092] In one embodiment, Q / m1 is 330-2600.

[0093] In one embodiment, Q / m1 is 400-2400.

[0094] The first thermal weight loss rate m1 corresponding to the first thermal weight loss interval can reflect the mass content ratio of the additive in the positive electrode active layer to a certain extent. The surface density and compaction of the positive electrode active layer are positively correlated with the mass content ratio of the additive. The greater the surface density of the positive electrode active layer, the denser the stacking of the active materials in the positive electrode active layer, and the poorer the flexibility of the positive electrode sheet. At this time, more additives are needed to improve the flexibility of the positive electrode sheet. Similarly, the greater the compaction density of the positive electrode active layer, the denser the stacking of the active materials in the positive electrode active layer, and the poorer the flexibility of the positive electrode sheet. At this time, more additives are needed to improve the flexibility of the positive electrode sheet.

[0095] In the present invention, P is 8 mg / cm 2-28mg / cm 2 , for example 8 mg / cm 2 , 9mg / cm 2 、10mg / cm 2 、11mg / cm 2 , 12mg / cm 2 、13mg / cm 2 , 14mg / cm 2 、15mg / cm 2 、16mg / cm 2 、17mg / cm 2 、18mg / cm 2 、19mg / cm 2 , 20mg / cm 2 , 21mg / cm 2 , 22mg / cm 2 , 23mg / cm 2 , 24mg / cm 2 , 25mg / cm 2 , 26mg / cm 2 , 27mg / cm 2 or 28 mg / cm 2 . Q is 3g / cm 3 -4.6g / cm 3 , for example 3g / cm 3 , 3.1g / cm 3 、3.2g / cm 3 , 3.3g / cm 3 、3.4g / cm 3 , 3.5g / cm 3 、3.6g / cm 3 、3.7g / cm 3 、3.8g / cm 3 、3.9g / cm 3 , 4g / cm 3 , 4.1g / cm 3 , 4.2g / cm 3 , 4.3g / cm 3 , 4.4g / cm 3 , 4.5g / cm 3 or 4.6g / cm 3 .

[0096] In one example, P is 10 mg / cm 2 -20mg / cm 2 .

[0097] In one embodiment, Q is 3.3 g / cm 3 -4.2g / cm3 .

[0098] In the present invention, the surface density P and the compacted density Q of the positive electrode active layer can be obtained by conventional methods in the art, for example, the battery is discharged to 0% SOC, the positive electrode sheet is disassembled and taken out, and then immersed in DMC solvent for 12 hours; then the DMC solvent is used to rinse to remove the lithium salt attached to the positive electrode sheet, and a 15.4025 cm 2 Weigh the disc. Subtract the weight of a positive electrode current collector with the same area as the disc from the original disc weight. Divide the difference by 2 to obtain the weight of the single-sided positive electrode active layer. Divide the weight of the single-sided positive electrode active layer by 15.4025 to obtain the areal density. Divide the obtained areal density by the thickness of the single-sided positive electrode active layer to obtain the compacted density. The thickness of the single-sided positive electrode active layer is measured by SEM cross-section observation. The thickness is averaged at at least five locations and the thickness is measured.

[0099] In the present invention, the roughness of the positive electrode current collector is 0.2 μm-0.9 μm, for example, 0.2 μm, 0.3 μm, 0.4 μm, 0.5 μm, 0.6 μm, 0.7 μm, 0.8 μm or 0.9 μm.

[0100] In one example, the roughness of the positive electrode current collector is 0.3 μm-0.6 μm.

[0101] The roughness of the positive electrode current collector affects the mechanical engagement and surface contact area between the current collector and the positive electrode active material. A roughness within an appropriate range can enhance the adhesion of the positive electrode active layer, but excessive roughness can cause burrs or increase the internal resistance of the positive electrode sheet.

[0102] In the present invention, the roughness of the positive electrode current collector can be obtained by conventional methods in the art, such as using a surface roughness tester, specifically as follows: the battery is discharged to 0% SOC, the positive electrode sheet is disassembled and removed, and soaked in DMC solvent for 12 hours; then rinsed with DMC solvent to remove the lithium salt attached to the positive electrode sheet, and the single-sided empty positive electrode current collector or double-sided empty positive electrode current collector area on the positive electrode sheet is taken for testing.

[0103] In the present invention, the electrolyte further comprises lithium bis(trifluoromethylsulfonyl)imide (LiTFSI) and lithium hexafluorophosphate (LiFP6). The mass content of lithium bis(trifluoromethylsulfonyl)imide in the electrolyte is c5, and c5 is 3%-10%, for example, 3%, 4%, 5%, 6%, 7%, 8%, 9% or 10%. The mass content of lithium hexafluorophosphate in the electrolyte is c6, and c6 is 8%-20%, for example, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19% or 20%.

[0104] In one example, c5 is 4%-7%.

[0105] In one example, c6 is 10%-16%.

[0106] Since the mass content ratio of EC is regulated in the electrolyte system of the present invention, the solubility of LiFP6 is reduced, the solvated lithium ions are reduced, and the ionic conductivity of the electrolyte is reduced. Therefore, it is necessary to introduce a lithium salt with high solubility in a variety of organic solvents. LiTFSI not only has the characteristics of high solubility in a variety of organic solvents, but also has high ionic conductivity. Its chemical structure makes it easy to dissociate into Li+ and TFSI- in organic solvents. TFSI- is large in volume and has dispersed charges, which weakens the Coulomb force with Li+ and improves ion mobility. The decomposition temperature is higher than 300°C, and the high temperature resistance is better than LiFP6. It is suitable for battery systems in high temperature environments and has a wide electrochemical window (charging cut-off voltage is greater than or equal to 4.5V); and TFSI- can form a dense passivation layer on the aluminum surface to prevent the oxidation reaction between the electrolyte and aluminum, thereby protecting the positive electrode current collector (such as aluminum foil) from oxidation in a high voltage system. LiTFSI-containing electrolytes can maintain the stability of the positive electrode current collector (such as aluminum foil) when the battery's charge cutoff voltage is 4.5V or above. LiTFSI maintains a high degree of dissociation at low temperatures, reducing the electrolyte viscosity and ensuring efficient lithium ion transfer, allowing the battery to continue operating below -20°C.

[0107] In the present invention, the mass content ratio c5 of lithium bis(trifluoromethylsulfonyl)imide in the electrolyte and the mass content ratio c6 of lithium hexafluorophosphate in the electrolyte can be measured by conventional methods in the art, such as ion chromatography (IC).

[0108] It should be noted that the numerical expressions such as "first" and "second" in the present invention are only used to distinguish different substances or usage methods, and do not represent a difference in order.

[0109] The present invention will be described in detail below through examples. The examples described in the present invention are only some examples of the present invention, not all examples. All other examples obtained by persons of ordinary skill in the art based on the examples of the present invention without creative work are within the scope of protection of the present invention.

[0110] In the following examples, unless otherwise specified, all materials used were commercially available analytical grade.

[0111] The following examples are used to illustrate the lithium-ion secondary battery of the present invention.

[0112] Example 1

[0113] Prepare the battery as follows:

[0114] (1) Preparation of negative electrode sheet

[0115] Artificial graphite, conductive carbon black, lithium carboxymethyl cellulose and styrene butadiene rubber are mixed in a mass ratio of 97:0.5:1.2:1.3, and deionized water is added to prepare a first negative electrode slurry; artificial graphite, silicon-carbon material, carbon nanotubes, lithium carboxymethyl cellulose, styrene butadiene rubber and polyacrylic acid are mixed in a mass ratio of 55:42:0.5:0.5:0.8:1.2, and deionized water is added to prepare a second negative electrode slurry; the first negative electrode slurry is coated on both sides of a copper foil and dried; the second negative electrode slurry is then coated on both sides of the dried copper foil coated with the first negative electrode slurry, and after baking, roller pressing, die cutting and cold pressing, grooves are etched on the outer surface of the negative electrode active layer using laser processing technology to obtain a negative electrode sheet;

[0116] The dimension H1 of the first active layer in the thickness direction of the negative electrode sheet is 20.3 μm, the dimension H2 of the second active layer in the thickness direction of the negative electrode sheet is 15.1 μm, and H2 / H1 is 0.744.

[0117] The silicon-carbon material includes a porous carbon matrix and a silicon material located in the pores of the porous carbon matrix; the silicon-carbon material has a sphericity of 1 and an average particle size of 12.3 μm;

[0118] The grooves have a depth of 24.5 μm, a width of 80 μm, and a pitch of 1320 μm.

[0119] (2) Preparation of positive electrode sheet

[0120] Lithium cobalt oxide (containing titanium), an additive (polyurethane), a positive electrode conductive agent (a mixture of conductive carbon black and carbon nanotubes in a mass ratio of 2:1), and polyvinylidene fluoride (55% crystallinity) are mixed in a mass ratio of 97.84:0.16:1.5:0.5, N-methylpyrrolidone (NMP) is added, and the mixture is stirred evenly to prepare a positive electrode slurry; the positive electrode slurry is coated on both sides of a positive electrode current collector (aluminum foil with a roughness of 0.5 μm), and the positive electrode sheet is obtained after drying, roll pressing, and slitting.

[0121] The thermal weight loss curve of the positive electrode active layer has a first thermal weight loss interval and a second thermal weight loss interval. The first thermal weight loss interval is between 245°C and 298°C, and m1 is 0.16%. The second thermal weight loss interval is between 451°C and 483°C, and m2 is 0.5%.

[0122] The peel strength between the positive electrode active layer and the positive electrode current collector is 15.2N / m;

[0123] The volume particle size distribution curve of lithium cobalt oxide has a first peak and a second peak. The peak value P1 of the first peak is 4.034 μm, and the peak value P2 of the second peak is 18.664 μm. The mass content of titanium in lithium cobalt oxide is 1547 ppm.

[0124] The surface density P of the positive electrode active layer is 15.67 mg / cm 2 , compacted density Q is 3.82g / cm 3 ;

[0125] P / m1 is 9793.8 and Q / m1 is 2387.5.

[0126] (3) Preparation of electrolyte

[0127] In a glove box (H2O <0.01ppm, O2 <0.01ppm, Ar atmosphere), EP and DFEA were mixed uniformly, and then PC and DEC were added in a mass ratio of 1:2; lithium hexafluorophosphate, LiTFSI, fluoroethylene carbonate, and a nitrile additive (HTCN, ADN, and SN were mixed in a mass ratio of 3:3:4) were added and mixed uniformly to obtain an electrolyte;

[0128] Among them, based on the total mass of the electrolyte, the content of EP accounts for c2 3%, the content of DFEA accounts for c4 35%, the content of lithium hexafluorophosphate accounts for c6 16%, the content of LiTFSI accounts for c5 4%, and the content of nitrile additives accounts for c3 3%.

[0129] (4) Preparation of batteries

[0130] The negative electrode sheet prepared in step (1), the separator (a polyethylene film with a thickness of 5 μm, a boehmite ceramic layer with a thickness of 2 μm on one side of the polyethylene film, a polyvinylidene fluoride and polymethyl methacrylate mixed adhesive layer with a thickness of 1 μm on the outer surface of the boehmite ceramic layer, and a polymethyl methacrylate adhesive layer with a thickness of 1 μm on the other side of the polyethylene film) and the positive electrode sheet prepared in step (2) are wound to obtain a winding core; and a battery is obtained through packaging, baking, liquid injection, formation, secondary sealing, sorting and OCV.

[0131] Example 2

[0132] Prepare the battery as follows:

[0133] (1) Preparation of negative electrode sheet

[0134] Artificial graphite, conductive carbon black, lithium carboxymethyl cellulose and styrene-butadiene rubber are mixed in a mass ratio of 97:0.5:1.2:1.3, and deionized water is added to prepare a first negative electrode slurry; artificial graphite, silicon-carbon material, carbon nanotubes, lithium carboxymethyl cellulose, styrene-butadiene rubber and polyacrylic acid are mixed in a mass ratio of 55:42:0.5:0.8:0.5:1.2, and deionized water is added to prepare a second negative electrode slurry; the first negative electrode slurry is coated on both sides of a copper foil and dried; the second negative electrode slurry is then coated on both sides of the dried copper foil coated with the first negative electrode slurry, and after baking, roller pressing, die cutting and cold pressing, grooves are etched on the outer surface of the negative electrode active layer using laser processing technology to obtain a negative electrode sheet;

[0135] The dimension H1 of the first active layer in the thickness direction of the negative electrode sheet is 21.5 μm, the dimension H2 of the second active layer in the thickness direction of the negative electrode sheet is 20.3 μm, and H2 / H1 is 0.944.

[0136] The silicon-carbon material includes a porous carbon matrix and silicon material located in the pores of the porous carbon matrix; the sphericity of the silicon-carbon material is 0.95 and the average particle size is 16.2 μm;

[0137] The grooves have a depth of 5.3 μm, a width of 20.6 μm, and a pitch of 523 μm.

[0138] (2) Preparation of positive electrode sheet

[0139] Lithium cobalt oxide (containing titanium), an additive (polycarbonate-polyurethane), a positive electrode conductive agent (a mixture of conductive carbon black and carbon nanotubes in a mass ratio of 2:1), and polyvinylidene fluoride (50% crystallinity) are mixed in a mass ratio of 96.7:0.8:1.5:1, N-methylpyrrolidone (NMP) is added, and the mixture is stirred evenly to prepare a positive electrode slurry; the positive electrode slurry is coated on both sides of a positive electrode current collector (aluminum foil with a roughness of 0.3 μm), and the positive electrode sheet is obtained after drying, rolling, and slitting.

[0140] The thermal weight loss curve of the positive electrode active layer has a first thermal weight loss interval and a second thermal weight loss interval. The first thermal weight loss interval is between 300°C and 360°C, and m1 is 0.8%. The second thermal weight loss interval is between 440°C and 485°C, and m2 is 1%.

[0141] The peel strength between the positive electrode active layer and the positive electrode current collector is 19.3N / m;

[0142] The volume particle size distribution curve of lithium cobalt oxide has a first peak and a second peak, the peak value P1 of the first peak is 3.027 μm, and the peak value P2 of the second peak is 15.219 μm; the mass content of titanium in lithium cobalt oxide is 823 ppm;

[0143] The surface density P of the positive electrode active layer is 10.67 mg / cm 2 , compacted density Q is 3.36g / cm 3 ;

[0144] P / m1 is 1333.8 and Q / m1 is 420.

[0145] (3) Preparation of electrolyte

[0146] In a glove box (H2O <0.01ppm, O2 <0.01ppm, Ar atmosphere), EC, EP, and DFEA were mixed uniformly, and then PC and DEC were added in a mass ratio of 1:2; lithium hexafluorophosphate, LiTFSI, fluoroethylene carbonate, and a nitrile additive (HTCN, ADN, and SN were mixed in a mass ratio of 3:3:4) were added and mixed uniformly to obtain an electrolyte;

[0147] Among them, based on the total mass of the electrolyte, the content of EC accounts for c1 2.5%, the content of EP accounts for c2 6%, the content of DFEA accounts for c4 20%, the content of lithium hexafluorophosphate accounts for c6 13%, the content of LiTFSI accounts for c5 5.5%, and the content of nitrile additives accounts for c3 1%.

[0148] (4) Preparation of batteries

[0149] The negative electrode sheet prepared in step (1), the separator (a polyethylene film with a thickness of 5 μm, a boehmite ceramic layer with a thickness of 2 μm on one side of the polyethylene film, a polyvinylidene fluoride and polymethyl methacrylate mixed adhesive layer with a thickness of 1 μm on the outer surface of the boehmite ceramic layer, and a polymethyl methacrylate adhesive layer with a thickness of 1 μm on the other side of the polyethylene film) and the positive electrode sheet prepared in step (2) are wound to obtain a winding core; and a battery is obtained through packaging, baking, liquid injection, formation, secondary sealing, sorting and OCV.

[0150] Example 3

[0151] Prepare the battery as follows:

[0152] (1) Preparation of negative electrode sheet

[0153] Artificial graphite, conductive carbon black, lithium carboxymethyl cellulose and styrene-butadiene rubber are mixed in a mass ratio of 97:0.5:1.2:1.3, and deionized water is added to prepare a first negative electrode slurry; artificial graphite, silicon-carbon material, carbon nanotubes, lithium carboxymethyl cellulose, styrene-butadiene rubber and polyacrylic acid are mixed in a mass ratio of 55:42:0.5:0.8:0.5:1.2, and deionized water is added to prepare a second negative electrode slurry; the first negative electrode slurry is coated on both sides of a copper foil and dried; the second negative electrode slurry is then coated on both sides of the dried copper foil coated with the first negative electrode slurry, and after baking, roller pressing, die cutting and cold pressing, grooves are etched on the outer surface of the negative electrode active layer using laser processing technology to obtain a negative electrode sheet;

[0154] The dimension H1 of the first active layer in the thickness direction of the negative electrode sheet is 25.8 μm, the dimension H2 of the second active layer in the thickness direction of the negative electrode sheet is 13.7 μm, and H2 / H1 is 0.531.

[0155] The silicon-carbon material includes a porous carbon matrix and silicon material located in the pores of the porous carbon matrix; the sphericity of the silicon-carbon material is 0.9 and the average particle size is 8.1 μm;

[0156] The grooves have a depth of 49.5 μm, a width of 147.6 μm, and a pitch of 1895 μm.

[0157] (2) Preparation of positive electrode sheet

[0158] Lithium cobalt oxide (containing titanium), an additive (polyvinyl alcohol carboxybutyral), a positive electrode conductive agent (a mixture of conductive carbon black and carbon nanotubes in a mass ratio of 2:1), and polyvinylidene fluoride (crystallinity of 60%) are mixed in a mass ratio of 95.5:1:1.5:2, N-methylpyrrolidone (NMP) is added, and the mixture is stirred evenly to prepare a positive electrode slurry; the positive electrode slurry is coated on both sides of a positive electrode current collector (aluminum foil with a roughness of 0.6 μm), and the positive electrode sheet is obtained after drying, roll pressing, and slitting.

[0159] The thermal weight loss curve of the positive electrode active layer has a first thermal weight loss interval and a second thermal weight loss interval. The first thermal weight loss interval is between 160°C and 240°C, and m1 is 1%. The second thermal weight loss interval is between 448°C and 480°C, and m2 is 2%.

[0160] The peel strength between the positive electrode active layer and the positive electrode current collector is 23.7N / m;

[0161] The volume particle size distribution curve of lithium cobalt oxide has a first peak and a second peak. The peak value P1 of the first peak is 4.986 μm, and the peak value P2 of the second peak is 22.583 μm. The mass content of titanium in lithium cobalt oxide is 1964 ppm.

[0162] The surface density P of the positive electrode active layer is 19.23 mg / cm 2 , compacted density Q is 4.08g / cm 3 ;

[0163] P / m1 is 1923 and Q / m1 is 408.

[0164] (3) Preparation of electrolyte

[0165] In a glove box (H2O <0.01ppm, O2 <0.01ppm, Ar atmosphere), EC and DFEA were mixed uniformly, and then PC and DEC were added in a mass ratio of 1:2; lithium hexafluorophosphate, LiTFSI, fluoroethylene carbonate, and a nitrile additive (HTCN, ADN, and SN were mixed in a mass ratio of 3:3:4) were added and mixed uniformly to obtain an electrolyte;

[0166] Among them, based on the total mass of the electrolyte, the content of EC accounts for c1 5%, the content of DFEA accounts for c4 50%, the content of lithium hexafluorophosphate accounts for c6 10%, the content of LiTFSI accounts for c5 7%, and the content of nitrile additives accounts for c3 6%.

[0167] (4) Preparation of batteries

[0168] The negative electrode sheet prepared in step (1), the separator (a polyethylene film with a thickness of 5 μm, a boehmite ceramic layer with a thickness of 2 μm on one side of the polyethylene film, a polyvinylidene fluoride and polymethyl methacrylate mixed adhesive layer with a thickness of 1 μm on the outer surface of the boehmite ceramic layer, and a polymethyl methacrylate adhesive layer with a thickness of 1 μm on the other side of the polyethylene film) and the positive electrode sheet prepared in step (2) are wound to obtain a winding core; and a battery is obtained through packaging, baking, liquid injection, formation, secondary sealing, sorting and OCV.

[0169] Example 4 Group

[0170] This group of examples is used to verify the impact of changes in "additives".

[0171] This group of examples is carried out with reference to Example 1, except that the additives are changed as follows:

[0172] In Example 4a, the additive was replaced with dioctyl phthalate of the same mass, and the first thermal weight loss interval was between 330° C. and 360° C.;

[0173] In Example 4b, the additive was replaced with polyamide-imide of the same mass, and the first thermal weight loss range was between 278° C. and 311° C.

[0174] Example 5 group

[0175] This set of examples is used to verify the impact of changes in the “first thermal weight loss rate m1”.

[0176] This group of examples is carried out with reference to Example 1, except that m1 is changed by adjusting the content ratio of the additives, as follows:

[0177] Example 5a, m1 is 0.1%;

[0178] In Example 5b, m1 is 1.5%.

[0179] Example 6

[0180] Used to verify the impact of changes in "the mass content ratio c1 of ethylene carbonate in the electrolyte".

[0181] The same procedure was carried out with reference to Example 1, except that c1 was changed, specifically, c1 was 10%.

[0182] Example 7

[0183] Used to verify the impact of changes in "the mass content ratio c2 of ethyl propionate in the electrolyte".

[0184] The same procedure was carried out with reference to Example 1, except that c2 was changed, specifically, c2 was 10%.

[0185] Example 8 Group

[0186] This set of examples is used to verify the impact of changes in "the mass content proportion c3 of nitrile additives in the electrolyte".

[0187] This group of examples is carried out with reference to Example 1, except that c3 is changed, specifically as follows:

[0188] Example 8a, c3 is 0.1%;

[0189] Example 8b, c3 is 10%.

[0190] Example 9

[0191] Used to verify the impact of changes in "ethyl fluoroacetate".

[0192] The reaction was carried out in accordance with Example 1, except that DFEA was replaced with ethyl 2,2-difluoroacetate of the same mass.

[0193] Example 10 Group

[0194] This set of examples is used to verify the impact of the change in the “second thermal weight loss interval position”.

[0195] This group of examples is carried out with reference to Example 1, except that the position of the second thermal weight loss interval is changed by adjusting the crystallinity of polyvinylidene fluoride, as follows:

[0196] In Example 10a, the crystallinity of the polyvinylidene fluoride is 40%, and the second thermal weight loss range is between 436° C. and 474° C.; wherein, the peel strength between the positive electrode active layer and the positive electrode current collector is 13.1 N / m;

[0197] In Example 10b, the crystallinity of the polyvinylidene fluoride is 70%, and the second thermal weight loss range is between 460° C. and 502° C.; wherein, the peel strength between the positive electrode active layer and the positive electrode current collector is 16.8 N / m.

[0198] Example 11 Group

[0199] This set of examples is used to verify the impact of changes in the "second thermal weight loss rate m2".

[0200] This group of examples is carried out with reference to Example 1, except that m2 is changed by adjusting the content ratio of polyvinylidene fluoride, as follows:

[0201] In Example 11a, m2 is 0.5%; wherein, the peel strength between the positive electrode active layer and the positive electrode current collector is 10.4 N / m;

[0202] In Example 11b, m2 is 4%; wherein, the peel strength between the positive electrode active layer and the positive electrode current collector is 45.5 N / m.

[0203] Example 12 Group

[0204] This set of embodiments is used to verify the impact of changes in “the peak value P1 of the first peak and the peak value P2 of the second peak”.

[0205] This group of examples is carried out with reference to Example 1, except that P1 and P2 are changed as follows:

[0206] Example 12a, P1 is 2.132 μm, and the peak value P2 of the second peak is 10.21 μm;

[0207] In Example 12b, P1 is 5.954 μm, and the peak value P2 of the second peak is 24.056 μm.

[0208] Example 13 Group

[0209] This group of examples is used to verify the impact of changes in the "mass content ratio of titanium in lithium cobalt oxide".

[0210] This group of examples was carried out with reference to Example 1, except that the mass content ratio of titanium element in lithium cobalt oxide was changed, as follows:

[0211] In Example 13a, the mass content of titanium in lithium cobalt oxide is 217 ppm;

[0212] In Example 13b, the mass content of titanium in lithium cobalt oxide is 2984 ppm.

[0213] Example 14 Group

[0214] This group of examples is used to verify the impact of changes in the "area density P of the positive electrode active layer".

[0215] This group of examples is carried out with reference to Example 1, except that P is changed as follows:

[0216] Example 14a, P is 8.36 mg / cm 2 ; Among them, P / m1 is 5225;

[0217] Example 14b, P is 27.62 mg / cm 2 ; Among them, P / m1 is 17262.5.

[0218] Example 15 group

[0219] This group of examples is used to verify the impact of changes in the "compaction density Q of the positive electrode active layer".

[0220] This group of examples is carried out with reference to Example 1, except that Q is changed, as follows:

[0221] Example 15a, Q is 3.02 g / cm 3 ; Among them, Q / m1 is 1887.5;

[0222] Example 15b, Q is 4.33 g / cm 3 ; Among them, Q / m1 is 2706.25.

[0223] Example 16

[0224] This set of examples is used to verify the impact of changes in "P / m1".

[0225] This group of examples were carried out with reference to Examples 1 and 3, except that the surface density P of the positive electrode active layer was adjusted to change P / m1, as follows:

[0226] Example 16a was carried out in accordance with Example 1, except that P was 19.25 mg / cm 2 ;P / m1 is 12031.3;

[0227] Example 16b was carried out in accordance with Example 3, except that P was 10.69 mg / cm 2 ;P / m1 is 1069.

[0228] Example 17 Group

[0229] This set of examples is used to verify the impact of changes in “Q / m1”.

[0230] This group of examples were carried out with reference to Examples 1 and 3, except that the compaction density Q of the positive electrode active layer was adjusted to change Q / m1, as follows:

[0231] Example 17a was carried out in the same manner as in Example 1, except that Q was 4.06 g / cm 3 ;Q / m1 is 2537.5;

[0232] Example 17b was carried out according to Example 3, except that Q was 3.35 g / cm 3 ;Q / m1 is 335.

[0233] Example 18 Group

[0234] This group of examples is used to verify the impact of changes in the “roughness of the positive electrode current collector”.

[0235] This group of examples is carried out with reference to Example 1, except that the roughness of the positive electrode current collector is changed, as follows:

[0236] In Example 18a, the roughness of the positive electrode current collector is 0.2 μm;

[0237] In Example 18b, the roughness of the positive electrode current collector is 0.9 μm.

[0238] Example 19

[0239] Used to verify the impact of changes in the "negative electrode active layer partition setting".

[0240] The process is carried out in accordance with Example 1, except that the negative electrode active layer is not partitioned in step (1). Specifically:

[0241] (1) Preparation of negative electrode sheet

[0242] Artificial graphite, silicon-carbon material, carbon nanotubes, lithium carboxymethyl cellulose, styrene-butadiene rubber and polyacrylic acid are mixed in a mass ratio of 55:42:0.5:0.8:0.5:1.2, and deionized water is added to prepare a negative electrode slurry; the negative electrode slurry is coated on both sides of the copper foil, and after baking, roller pressing, die cutting and cold pressing, grooves are etched on the outer surface of the negative electrode active layer using laser processing technology to obtain a negative electrode sheet.

[0243] Example 20

[0244] This is used to verify the impact of changes in the positions of the first active layer and the second active layer.

[0245] The process was carried out in accordance with Example 1, except that the positions of the first active layer and the second active layer were changed. Specifically, the second negative electrode slurry was first applied to both sides of the negative electrode current collector and dried; then the first negative electrode slurry was applied to both sides of the negative electrode current collector coated with the second negative electrode slurry after drying.

[0246] Example 21 Group

[0247] This set of examples is used to verify the impact of the change of "H2 / H1".

[0248] This group of examples was carried out with reference to Example 1, except that H2 / H1 was changed by regulating H1 and H2, as follows:

[0249] In Example 21a, H1 is 25.1 μm, H2 is 10.6 μm, and H2 / H1 is 0.422; wherein, the average particle size of the silicon-carbon material is 8.3 μm;

[0250] Example 21b, H1 is 17.7 μm, H2 is 17.8 μm, and H2 / H1 is 1.006;

[0251] In Example 21c, H1 is 15.4 μm, H2 is 20.2 μm, and H2 / H1 is 1.312.

[0252] Example 22 Group

[0253] This set of examples is used to verify the effects of changes in "the mass content of LiTFSI in the electrolyte accounts for c5 and the mass content of lithium hexafluorophosphate in the electrolyte accounts for c6".

[0254] This group of examples is carried out with reference to Example 1, except that c5 and c6 are changed as follows:

[0255] Example 22a, c5 is 3%, c6 is 20%;

[0256] In Example 22b, c5 is 10% and c6 is 8%.

[0257] Comparative Example 1

[0258] The process is carried out in accordance with Example 1, except that the positive electrode sheet does not include any additives, and the thermogravimetric curve of the positive electrode active layer does not have the first thermogravimetric interval.

[0259] Comparative Example 2

[0260] The same procedure was carried out as in Example 1, except that m1 was 2%.

[0261] Comparative Example 3

[0262] The process is carried out in accordance with Example 1, except that the mass content of ethylene carbonate in the electrolyte is 12%.

[0263] Comparative Example 4

[0264] The process was carried out in accordance with Example 1, except that the mass content of ethyl propionate in the electrolyte (C2) was 12%.

[0265] Comparative Example 5

[0266] The process was carried out with reference to Example 1, except that the mass content of the nitrile additive in the electrolyte was changed to c3, specifically: c3 was 12%.

[0267] Comparative Example 6

[0268] The method was carried out in accordance with Example 1, except that the mass content of ethyl fluoroacetate in the electrolyte was changed to c4, as follows:

[0269] Comparative Example 6a, c4 is 15%;

[0270] Comparative Example 6b, c4 is 55%.

[0271] Test Case

[0272] (1) Positive electrode flexibility test

[0273] The batteries prepared in the examples and comparative examples were subjected to a positive electrode sheet flexibility test. The specific test method is as follows:

[0274] The battery was discharged to 3V, dissected, and the positive electrode was removed. The positive electrode was soaked in DMC solvent for 12 hours, and after drying, the positive electrode was cut into strips 15 mm wide and 80 mm long. One end of the strip was placed on the table with about 2 mm of one end, and the other end was suspended in the air and drooped naturally with about 6 mm of the other end. The vertical distance from the lowest point of the drooping end to the plane was measured. If it was greater than or equal to 4 mm, it was excellent; if it was greater than or equal to 3 mm and less than 4 mm, it was good; if it was greater than or equal to 2 mm and less than 3 mm, it was medium; and if it was less than 2 mm, it was poor. The results are recorded in Table 1.

[0275] (2) Energy density test

[0276] The batteries prepared in the examples and comparative examples were subjected to energy density tests. The specific test methods are as follows:

[0277] Charge at 0.2C to an upper voltage limit of 4.5V (cut off at 0.02C), and discharge at 0.2C to a lower voltage limit of 3V, and repeat this three times; record the third discharge energy as Q; then use a 2.5D microscope to test the width L and height W of the battery, and use a PPG thickness tester to test the fully charged thickness T of the battery; the energy density is Q / (L×W×T), in units of Wh / L. The results are recorded in Table 1.

[0278] (3) 45℃ high temperature cycle test

[0279] The battery was placed at 45°C for 1 hour, charged at 0.5C to a cutoff voltage of 4.5V, charged at constant voltage to a cutoff current of 0.05C, and discharged at 0.5C to 3V. The above charge and discharge steps were repeated 200 times; the 200th discharge capacity was divided by the maximum value of the 1st to 3rd discharge capacities, which was recorded as the capacity retention rate; the battery was fully charged before the high-temperature cycle test, and its thickness was measured and recorded as T1. After 200 cycles, the battery was fully charged and its thickness was measured and recorded as T2. The thickness expansion rate = (T2-T1) / T1; the above test results are recorded in Table 1.

[0280] (4) Rate test

[0281] The batteries prepared in the examples and comparative examples were subjected to rate tests. The specific test methods are as follows:

[0282] At 25°C, discharge the battery at 0.2C to 3V, then charge it at 0.7C constant current to a cutoff voltage of 4.5V, record the constant current charge capacity R1, continue constant voltage charging to a cutoff current of 0.05C, record the constant voltage charge capacity R2, constant current charge ratio = R1 / (R1+R2);

[0283] At 25°C, charge the battery to 4.5V at 0.5C, record the time t1, then charge it to 0.05C at a constant voltage, record the time t2. The constant voltage charging time ratio is t2 / (t1+t2).

[0284] The above results are recorded in Table 1.

[0285] Table 1

[0286]

[0287]

[0288]

[0289] It can be seen from Table 1 that the lithium ion secondary battery of the present invention can achieve higher energy density, high temperature cycle performance and rate performance compared with the comparative example.

[0290] The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited thereto. Within the technical concept of the present invention, various simple variations of the technical solution of the present invention may be made, including combining the various technical features in any other appropriate manner. These simple variations and combinations should also be regarded as disclosed in the present invention and fall within the scope of protection of the present invention.

Claims

1. A lithium-ion secondary battery, characterized in that: Including positive electrode and electrolyte; The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer located on at least one side of the positive electrode current collector; the thermal gravimetric loss curve of the positive electrode active layer has a first thermal gravimetric loss range, the first thermal gravimetric loss range is between 150° C. and 360° C., and the first thermal gravimetric loss rate m1 corresponding to the first thermal gravimetric loss range is 0.1% to 1.5%; wherein the thermal gravimetric loss curve is obtained by testing using a thermogravimetric analyzer under a nitrogen atmosphere; The mass content of ethylene carbonate in the electrolyte is c1, 0≤c1≤10%; the mass content of ethyl propionate in the electrolyte is c2, 0≤c2≤10%; The electrolyte includes a nitrile additive and ethyl fluoroacetate; The mass content of the nitrile additive in the electrolyte is C3, and C3 is 0.1%-10%; The mass content of the ethyl fluoroacetate in the electrolyte is C4, and C4 is 20%-50%.

2. The lithium ion secondary battery according to claim 1, wherein The first thermal weight loss range is between 160°C and 360°C; and / or, m1 is 0.15%-1%; and / or, 0%≤c1≤5%; preferably, the electrolyte does not contain ethylene carbonate; and / or, 0%≤c2≤6%; preferably, the electrolyte does not contain ethyl propionate; and / or, 1% ≤ c3 ≤ 6%; And / or, the compaction density of the positive electrode active layer is 3g / cm 3 -4.6g / cm 3 ; and / or, the ethyl fluoroacetate comprises 2,2-difluoroethyl acetate; And / or, the nitrile additive includes at least one of glutaronitrile, adiponitrile, succinonitrile, sebacononitrile, azelaic acid dinitrile, fumaronitrile, ethylene glycol bis(propionitrile) ether, glycerol trinitrile, 1,3,6-hexane trinitrile and 1,4-dicyano-2-butene.

3. The lithium ion secondary battery according to claim 1 or 2, wherein The positive electrode active layer includes additives, and the additives include at least one of polyvinyl butyral, polyurethane and its derivatives, polyamide, single-terminal polyether, double-terminal polyether, dioctyl phthalate, dibutyl phthalate, dimethyl phthalate, diethyl phthalate, di(butoxyethoxy)ethyl adipate, isopropyl titanate, n-butyl titanate, citrate, (2-ethyl)hexyl trimellitate, di(2-ethyl)hexyl phthalate, di(2-ethyl)hexyl sebacate, diethylene glycol dibenzoate, phthalic anhydride, dipropylene glycol dibenzoate, chlorosulfonated polyethylene, polyamideimide, pentaerythritol tetrabenzoate, trimethyl citrate, triethyl citrate, tributyl citrate, sodium ditridecyl sulfosuccinate, polysiloxane, fatty acid salt, quaternary ammonium salt, methyl silicone resin, phenyl silicone resin and vinyl silicone resin; Preferably, the additive has an ester group; More preferably, the additive includes the polyurethane and its derivatives.

4. The lithium ion secondary battery according to claim 1 or 2, wherein The thermal weight loss curve of the positive electrode active layer has a second thermal weight loss range, the second thermal weight loss range is between 370°C and 600°C, and the second thermal weight loss rate m2 corresponding to the second thermal weight loss range is 0.5%-4%; preferably, the second thermal weight loss range is between 430°C and 510°C; more preferably, between 440°C and 490°C; preferably, m2 is 1%-2%; And / or, the positive electrode active layer comprises polyvinylidene fluoride; preferably, the crystallinity of the polyvinylidene fluoride is 40%-70%; And / or, the peel strength between the positive electrode active layer and the positive electrode current collector is 10 N / m-50 N / m; preferably 15 N / m-25 N / m.

5. The lithium ion secondary battery according to claim 1 or 2, wherein The lithium-ion secondary battery further includes a negative electrode sheet, the negative electrode sheet including a negative electrode current collector and a negative electrode active layer located on at least one side of the negative electrode current collector; the negative electrode active layer includes a first active layer and a second active layer arranged along the thickness direction of the negative electrode sheet, the first active layer being located between the negative electrode current collector and the second active layer; the first active layer includes a first graphite material, and the second active layer includes a silicon-carbon material; Preferably, the dimension of the first active layer in the thickness direction of the negative electrode sheet is H1, the dimension of the second active layer in the thickness direction of the negative electrode sheet is H2, H2 / H1≥0.4; more preferably, 0.5≤H2 / H1≤0.95; Preferably, the silicon-carbon material comprises a porous carbon matrix and silicon material located in the pores inside the porous carbon matrix; Preferably, the sphericity of the silicon-carbon material is 0.8-1; more preferably 0.9-1; Preferably, the average particle size of the silicon-carbon material is 2.5 μm-26 μm; more preferably 8 μm-18 μm; Preferably, the mass content of silicon in the silicon-carbon material is 20%-60%; Preferably, the mass content of silicon in the negative electrode active layer is 3%-30%.

6. The lithium ion secondary battery according to claim 5, wherein The outer surface of the negative electrode active layer has a plurality of grooves; Preferably, the depth of the groove is 5 μm-50 μm, the width of the groove is 20 μm-150 μm, and the spacing between the grooves is 500 μm-2000 μm.

7. The lithium ion secondary battery according to claim 1 or 2, wherein The positive electrode active layer includes a positive electrode active material, and the volume particle size distribution curve of the positive electrode active material has a first peak and a second peak; the abscissa corresponding to the peak value of the first peak is P1, and the abscissa corresponding to the peak value of the second peak is P2; Preferably, P1 is 2 μm-6 μm; P2 is 10 μm-25 μm; Preferably, the charging cut-off voltage of the lithium-ion secondary battery is ≥4.5V; Preferably, the positive electrode active material comprises lithium cobalt oxide; More preferably, the lithium cobalt oxide includes titanium, and the mass content of titanium in the lithium cobalt oxide is 200ppm-3000ppm.

8. The lithium ion secondary battery according to claim 1 or 2, wherein The surface density of the positive electrode active layer is P, and the compacted density of the positive electrode active layer is Q, P, and the unit is mg / cm 2 , Q, unit is g / cm 3 and m1 respectively meet the following requirements: P / m1 is 1000-18000, Q / m1 is 200-3000; Preferably, P / m1 is 1300-10000; Preferably, Q / m1 is 400-2400; Preferably, P is 8 mg / cm 2 -28mg / cm 2 .

9. The lithium ion secondary battery according to claim 1 or 2, wherein The roughness of the positive electrode current collector is 0.2 μm-0.9 μm; Preferably it is 0.3 μm to 0.6 μm.

10. The lithium ion secondary battery according to claim 1 or 2, wherein The electrolyte also includes lithium bis(trifluoromethylsulfonyl)imide and lithium hexafluorophosphate; Preferably, the mass content of lithium bis(trifluoromethylsulfonyl)imide in the electrolyte is c5, and c5 is 3%-10%; Preferably, the mass content of lithium hexafluorophosphate in the electrolyte is C6, and C6 is 8%-20%.

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