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, a negative plate and electrolyte, the charging cut-off voltage of the lithium ion secondary battery is greater than or equal to 4.5 V; the positive plate comprises a positive active coating, the positive active coating comprises a positive active substance and a solid electrolyte, the positive active substance comprises lithium cobalt oxide containing an element Al, and the mass content c1 of the element Al in the positive active coating is 6800ppm-15000ppm; the solid electrolyte comprises at least one of lithium aluminum titanium phosphorus oxide, lithium lanthanum zirconium tantalum oxide and lithium lanthanum titanium oxide; the negative plate comprises a negative active coating, and the negative active coating comprises a silicon carbon material; the mass content c2 of the element Si in the negative electrode active coating is 1.5%-20%; the electrolyte comprises a carboxylic ester compound. The battery provided by the invention has high energy density, excellent cycle stability and low-temperature discharge performance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and specifically to a lithium-ion secondary battery. Background Technology

[0002] High capacity remains a constant demand from users for batteries. Improving battery capacity essentially involves increasing the specific capacity of the positive and negative electrode materials. This is achieved by doping the negative electrode with silicon and by raising the charging cutoff voltage to enhance the specific capacity of the positive electrode. However, at high voltages, the crystal structure of the positive electrode material collapses during charging and discharging, leading to decreased cycle stability. Furthermore, due to the poor conductivity of silicon itself, when silicon is doped into the negative electrode, especially when the silicon content is high (e.g., when the silicon content in the negative electrode active coating is greater than or equal to 5%), the battery's low-temperature discharge performance will significantly decrease.

[0003] Therefore, it is necessary to improve both the cycle stability and low-temperature discharge performance of high-energy-density batteries. Summary of the Invention

[0004] The purpose of this invention is to overcome the problems of poor cycle stability and low-temperature discharge performance in existing high-energy-density batteries, and to provide a lithium-ion secondary battery. The lithium-ion secondary battery of this invention (hereinafter referred to as the battery) uses silicon-carbon material as the negative electrode active material and lithium cobalt oxide as the positive electrode active material, and is equipped with a relatively high charging cut-off voltage (e.g., ≥4.5V) to achieve high energy density. Simultaneously, by controlling the Al content in lithium cobalt oxide, the crystal structure of lithium cobalt oxide is stabilized, thereby improving the cycle stability of the battery. Furthermore, addressing the problem of poor low-temperature discharge performance in batteries with high silicon doping (e.g., when the mass content of silicon in the negative electrode active coating is greater than or equal to 5%), the low-temperature discharge performance of the battery is improved by adding a solid electrolyte to the positive electrode and adding a specific solvent to the electrolyte.

[0005] This invention provides a lithium-ion secondary battery, comprising a positive electrode, a negative electrode, and an electrolyte; the charging cut-off voltage of the lithium-ion secondary battery is ≥4.5V; the positive electrode comprises a positive active coating, the positive active coating comprising a positive active material and a solid electrolyte, the positive active material comprising lithium cobalt oxide, the lithium cobalt oxide containing element Al, and the mass content (c1) of element Al in the positive active coating being 6800ppm-15000ppm; the solid electrolyte comprises at least one of lithium aluminum titanium phosphorus oxide, lithium lanthanum zirconium tantalum oxide, and lithium lanthanum titanium oxide; the negative electrode comprises a negative active coating, the negative active coating comprising a negative active material, the negative active material comprising silicon-carbon material; the mass content (c2) of element Si in the negative active coating being 1.5%-20%; the electrolyte comprises a solvent, the solvent comprising a carboxylic acid ester compound.

[0006] As the upper limit of battery charging voltage increases, the potential on the positive electrode side also increases, posing a greater challenge to the stability of the lithium cobalt oxide lattice structure. On the one hand, element Al can form Al-O bonds with element O in lithium cobalt oxide, effectively suppressing the extraction of lattice oxygen; on the other hand, Al... 3+ The octahedral structure of lithium cobalt oxide provides greater stability, making it more difficult for lithium cobalt oxide to transform into a monoclinic crystal system. This reduces the kinetics of monoclinic crystal formation and suppresses the phase transition of lithium cobalt oxide. Therefore, controlling the mass content of element Al in the positive electrode active coating can stabilize lithium cobalt oxide. However, it is not the case that the higher the Al content, the better. This is because an increase in Al content reduces the specific capacity of lithium cobalt oxide, thus affecting the overall energy density of the battery. Therefore, it is necessary to control the mass content of element Al in the positive electrode active coating. When a specific range is met, it can ensure that the positive electrode operates stably at high voltages (e.g., 4.5V and above) and that the positive electrode has sufficient capacity, without causing a loss in energy density due to excessive Al mass content.

[0007] Silicon-carbon materials exhibit poor electrical conductivity, and the ionic conductivity of the electrolyte decreases significantly with decreasing temperature. Silicon-doped systems, especially those with high silicon doping (e.g., silicon content greater than or equal to 5% in the negative electrode active coating), face the problem of failing to discharge in extreme low-temperature conditions (e.g., temperatures of -20°C and below). The ionic conductivity of solid-state electrolytes does not decrease significantly with decreasing temperature. Adding a solid-state electrolyte to the positive electrode allows ions to conduct along the solid-state electrolyte under extreme low-temperature conditions, thereby replacing part of the electrolyte's conduction path and ensuring normal battery discharge at low temperatures. Furthermore, since the discharge process involves lithium delithiation at the negative electrode and lithium insertion at the positive electrode, improving the battery's low-temperature discharge performance requires prioritizing the increase of lithium insertion speed on the positive electrode side; therefore, adding a solid-state electrolyte to the positive electrode is crucial.

[0008] The above methods can improve the cycle stability of the battery and enhance its low-temperature discharge performance to some extent, but the improvement in low-temperature discharge performance has not met expectations. Therefore, further improvements in the battery's low-temperature discharge performance are needed. Carboxylic acid esters have lower melting points and viscosities than carbonates, meaning they can more easily remain liquid at low temperatures, thus maintaining the battery's conductivity and discharge performance. Furthermore, carboxylic acid esters contribute to the formation of a stable, dense, and low-resistance SEI (Solid Electrolyte Interphase) film, which is crucial for the battery's low-temperature discharge performance. Therefore, adding carboxylic acid esters to the electrolyte can improve the electrolyte's electrochemical properties, optimize the solvation structure of lithium ions, and form a stable SEI layer, thereby significantly improving the battery's low-temperature discharge performance.

[0009] Through the above technical solution, the present invention has at least the following advantages compared with the prior art: the battery of the present invention can have high energy density, excellent cycle stability and low temperature discharge performance.

[0010] The endpoints and any values ​​of the ranges 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 endpoint values ​​of the various ranges, the endpoint values ​​of the various ranges and individual point values, and individual point values ​​can be combined with each other to obtain one or more new numerical ranges, which should be considered as specifically disclosed herein. Attached Figure Description

[0011] Figure 1 The diagram shown is a top view of the groove on the surface of the negative electrode sheet in an embodiment of the present invention. Figure 1 (a) The grooves are continuously arranged; Figure 1 (b) The groove is segmented.

[0012] Figure 2 The diagram shown is a schematic representation of the width of the groove in an embodiment of the present invention, wherein... Figure 2 (a)- Figure 2 In (c), the two long sides of the groove are straight lines. Figure 2 (d) has two long sides of the groove that are curved.

[0013] Figure 3 The diagram shown is a schematic representation of the groove spacing in an embodiment of the present invention, wherein... Figure 3 (a) represents the case where two adjacent long sides are straight lines and parallel. Figure 3 (b) represents the case where two adjacent long sides are straight lines and not parallel. Figure 3 (c) is the case where two adjacent long sides are curves.

[0014] Figure 4 The diagram shown is a structural schematic of the core in an example of the present invention.

[0015] Figure 5 The diagram shown is a schematic representation of the positive electrode sheet in an embodiment of the present invention, wherein... Figure 5 (a) is a top view. Figure 5 (b) is a cross-sectional view along the thickness direction.

[0016] Figure 6 The diagram shown is a top view of the first surface of the positive electrode in an embodiment of the present invention. Detailed Implementation

[0017] The following provides a detailed description of specific embodiments of the present invention. It should be understood that the specific embodiments described herein are for illustrative and explanatory purposes only and are not intended to limit the scope of the invention.

[0018] This invention provides a lithium-ion secondary battery, which may include a positive electrode, a negative electrode, and an electrolyte. The charging cut-off voltage of the lithium-ion secondary battery is ≥4.5V, for example, 4.5V or 4.53V. The term "charging cut-off voltage" has its conventional meaning in the art and generally refers to the maximum voltage value that the battery can safely reach during charging.

[0019] In this invention, the positive electrode sheet may include a positive electrode active coating. The positive electrode active coating may include a positive electrode active material and a solid electrolyte. The positive electrode active material may include lithium cobalt oxide. The lithium cobalt oxide contains element Al, and the mass content (c1) of element Al in the positive electrode active coating may be 6800ppm-15000ppm, for example, 6800ppm, 7000ppm, 8000ppm, 9000ppm, 10000ppm, 11000ppm, 12000ppm, 13000ppm, 14000ppm, or 15000ppm.

[0020] In one instance, c1 is 7100ppm-10000ppm.

[0021] In this invention, the mass content c1 of element Al in the positive electrode active coating can be tested by conventional methods in the art, such as using inductively coupled plasma-emission spectrometry (ICP-OES). The specific test method is as follows: discharge the battery to 0% SOC, disassemble and remove the positive electrode sheet, soak it in dimethyl carbonate (DMC) solvent for 12 hours; then rinse it with DMC solvent to remove the lithium salt attached to the positive electrode sheet, calcine it in a muffle furnace at 400°C for 3 hours, gently scrape the positive electrode active coating off the surface of the positive electrode current collector, and measure the mass content of element Al (in ppm, i.e., parts per million) by ICP-OES. The specific operation method is performed in accordance with GB / T 30902-2014.

[0022] In this invention, the chemical formula of lithium cobalt oxide can be Li a Co b M 1 c O2, where 0.8 ≤ a ≤ 1.05, 0.85 ≤ b < 1, 0 <c≤0.15,M 1 The lithium cobalt oxide comprises at least one of Al, Mg, Ti, Y, La, Ga, Ge, Sn, Si, Zr, Ca, Sb, In, Ni, and Mn. The lithium cobalt oxide may include a first particle and a second particle. The average particle size of the first particle may be 0.3 μm-7 μm (e.g., 0.3 μm, 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, or 7 μm), and the average particle size of the second particle may be 7.5 μm-40 μm (e.g., 7.5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm, or 40 μm). Using first and second particles with different average particle sizes allows the difference in particle size distribution to directly affect the filling effect of the lithium cobalt oxide powder during the compression process, thereby affecting the compaction density and electronic conductivity of the lithium cobalt oxide, which is beneficial for improving the overall energy density and fast charging capability of the battery.

[0023] In this invention, the average particle size of the first particle and the average particle size of the second particle can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the positive electrode sheet is disassembled and removed. After soaking in DMC solvent for 12 hours, it is rinsed with DMC solvent to remove the lithium salt attached to the positive electrode sheet. After calcining in a muffle furnace at 400°C for 3 hours, the positive electrode active coating is gently scraped off from the surface of the positive electrode current collector. The particle size range of the first particle and the second particle can be obtained from the obtained curve.

[0024] In this invention, the solid electrolyte may include at least one of lithium aluminum titanium phosphorus oxide, lithium lanthanum zirconium tantalum oxide, and lithium lanthanum titanium oxide. The lithium aluminum titanium phosphorus oxide may be a superionic conductor type lithium aluminum titanium phosphorus oxide with the molecular formula Li.1+x Al x Ti 2-x (PO4)3, 0 < x ≤ 0.5. The lithium lanthanum zirconium tantalum oxide can be a garnet-type lithium lanthanum zirconium tantalum oxide with the molecular formula Li. 7-y La3Zr 2-y Ta y O 12 , 0≤y≤0.5. The lithium lanthanum titanium oxide can be a perovskite type lithium lanthanum titanium oxide with the molecular formula Li. 3z La 2 / 3-z TiO3, 0<z≤0.2. Solid electrolytes satisfying the above molecular formula have strong ion conductivity and high dielectric constant, and good affinity for electrolytes. When used in positive electrodes, they can improve the ion transport capacity of the positive electrode, improve battery kinetics, and thus enable the battery to have high energy density while having good low-temperature discharge performance and cycle stability.

[0025] In this invention, the negative electrode sheet may include a negative electrode active coating. The negative electrode active coating may include a negative electrode active material, which may include a silicon-carbon material. The silicon-carbon material refers to a composite material comprising elemental carbon and elemental silicon, such as a material formed by filling (including partially or completely filling) the pores of porous carbon with silicon and / or oxidized silicon.

[0026] In this invention, the mass content c2 of element Si in the negative electrode active coating can be 1.5%-20%, for example, 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19% or 20%.

[0027] In one instance, c2 is 4%-12%.

[0028] In this invention, the mass content c2 of element Si in the negative electrode active coating can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. After drying, the negative electrode sheet is subjected to high-temperature treatment at 400°C in an inert atmosphere for 2 hours (e.g., in a tube furnace under nitrogen or argon atmosphere). The negative electrode active coating can then be peeled off from the negative electrode current collector, and the negative electrode active coating is collected as a test sample. Using a thermogravimetric analyzer (e.g., a TGA 550 thermogravimetric analyzer), the test sample amount is 5mg-15mg. Under an air or oxygen atmosphere, the temperature is increased from room temperature (25°C) to 900°C at a heating rate of 10°C / min, and held at 900°C for 40 minutes, so that the non-silicon components in the negative electrode active coating volatilize while silicon is fully oxidized to silicon dioxide. The remaining substance is the ash content of the negative electrode active coating. The mass content of element Si in the negative electrode active coating can be calculated based on the mass of ash. The calculation formula is as follows: Mass content of element Si in negative electrode active coating = 7 × mass of ash / (15 × mass of test sample).

[0029] In this invention, the mass content of element Si in the silicon-carbon material can be 30%-80%, for example, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75% or 80%.

[0030] In this invention, the electrolyte may include a solvent. The solvent may include carboxylic acid ester compounds. The carboxylic acid ester compounds include, for example, at least one of the following solvents, whether fluorinated or unsubstituted: ethyl acetate, propyl acetate, n-butyl acetate, isobutyl acetate, n-amyl acetate, isoamyl acetate, methyl propionate (PP), ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, and n-ethyl butyrate.

[0031] In this invention, the mass content of the carboxylic acid ester compound in the electrolyte can be 10%-80%, for example, 10%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50%, 55%, 60%, 65%, 70%, 75% or 80%.

[0032] In one example, the carboxylic acid ester compound has a mass content of 20%-65% in the electrolyte.

[0033] In this invention, the mass content of the carboxylic acid ester compound in the electrolyte can be obtained by methods conventional in the art, such as discharging the battery to 0% SOC, extracting the electrolyte from the battery, and testing it by gas chromatography (GC).

[0034] In this invention, the solid electrolyte contains a characteristic element. The characteristic element includes at least one selected from Ti, Zr, La, and Ta. The ratio of the mass content c3 of the characteristic element in the positive electrode active coating to the mass content c4 of element Co in the positive electrode active coating can be 0.001-0.0055, for example, 0.001, 0.0015, 0.002, 0.0025, 0.003, 0.0035, 0.004, 0.0045, 0.005, or 0.0055.

[0035] In one example, the ratio of the mass content c3 of the characteristic element in the positive electrode active coating to the mass content c4 of element Co in the positive electrode active coating is 0.0015-0.0035.

[0036] By adjusting the mass ratio of characteristic elements to element Co in the positive electrode active coating, the battery can achieve a balance between low-temperature discharge performance and high-temperature stability. This is because the ratio indicates the mass ratio of solid electrolyte and lithium cobalt oxide in the positive electrode active coating. When the ratio is small (e.g., less than 0.001), the mass content of solid electrolyte in the positive electrode active coating is relatively low, and the improvement of low-temperature discharge performance by solid electrolyte is limited. Conversely, when the ratio is large (e.g., greater than 0.0055), the mass content of solid electrolyte in the positive electrode active coating is relatively high. Since solid electrolyte is unstable in high-temperature environments and prone to gas generation, a high content of solid electrolyte will adversely affect the high-temperature performance of the battery.

[0037] In this invention, the mass content c3 of the characteristic element in the positive electrode active coating can be 600ppm-3000ppm, for example, 600ppm, 700ppm, 800ppm, 900ppm, 1000ppm, 1100ppm, 1200ppm, 1300ppm, 1400ppm, 1500ppm, 1600ppm, 1700ppm, 1800ppm, 1900ppm, 2000ppm, 2500ppm or 3000ppm.

[0038] In one instance, c3 is 900ppm-2000ppm.

[0039] In this invention, the mass content c3 of the characteristic element in the positive electrode active coating can be obtained by conventional methods in the art, such as using ICP-OES. The specific test method is as follows: discharge the battery to 0% SOC, disassemble and remove the positive electrode sheet, soak it in DMC solvent for 12 hours; then rinse it with DMC solvent to remove the lithium salt attached to the positive electrode sheet, calcine it in a muffle furnace at 400°C for 3 hours, gently scrape the positive electrode active coating off the surface of the positive electrode current collector, and measure the mass content of the characteristic element (unit is ppm, i.e., parts per million) by ICP-OES. The specific operation method is performed in accordance with GB / T 30902-2014.

[0040] In this invention, "the mass content c3 of the characteristic element in the positive electrode active coating" refers to the mass content of at least one characteristic element in the solid electrolyte in the positive electrode active coating. For example, when the solid electrolyte is lithium lanthanum zirconium tantalum oxide, which contains three characteristic elements La, Zr, and Ta, then the mass content of at least one of element La, element Zr, or element Ta in the positive electrode active coating must satisfy the above-mentioned c3.

[0041] In this invention, the mass content of element Co in the positive electrode active coating, c4, can be 400,000 ppm to 750,000 ppm, for example, 400,000 ppm, 450,000 ppm, 500,000 ppm, 550,000 ppm, 600,000 ppm, 650,000 ppm, 700,000 ppm, or 750,000 ppm.

[0042] In one instance, c4 is 500,000 ppm to 650,000 ppm.

[0043] In this invention, the average particle size of the solid electrolyte can be 500 nm to 3 μm, for example, 500 nm, 600 nm, 700 nm, 800 nm, 900 nm, 1 μm, 1.1 μm, 1.2 μm, 1.3 μm, 1.4 μm, 1.5 μm, 1.6 μm, 1.7 μm, 1.8 μm, 1.9 μm, 2 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm or 3 μm.

[0044] In one example, the average particle size of the solid electrolyte is 1 μm-2 μm.

[0045] In this invention, the average particle size of the solid electrolyte can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the positive electrode is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the positive electrode. The positive electrode is then cut using an argon ion milling machine with a CP laser and observed using SEM at 5K magnification. At least 20 solid electrolyte particles are randomly selected, and the particle size of each solid electrolyte particle is measured, and the average value is taken. If the number of solid electrolyte particles at 5K magnification is less than 20, another microscopic image is taken until 20 solid electrolyte particles are measured.

[0046] In this invention, the electrolyte may further include vinylene carbonate (VC). VC is an unsaturated additive with good film-forming properties. During the battery formation stage, it can preferentially reduce and decompose at the negative electrode to participate in the formation of the SEI film (therefore, the VC content in the electrolyte obtained after battery manufacturing is significantly lower than the amount of VC added during battery preparation), playing a role in protecting the stability of the interface between the silicon-carbon material and the electrolyte. The SEI film derived from the reduction and decomposition of VC, due to its rich content of highly elastic polymers, can help suppress the volume expansion of the silicon-carbon material during cycling, improving cycle stability while ensuring efficient migration of lithium ions in the SEI film, thereby enhancing the low-temperature discharge performance of the battery.

[0047] In this invention, the mass content (c5) of vinylene carbonate in the electrolyte can be 0.01%-15%, for example, 0.01%, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or 15%.

[0048] In this invention, the mass content c5 of vinylene carbonate in the electrolyte can be obtained by conventional methods in the art, such as discharging the battery to 0% SOC, extracting the electrolyte from the battery, and obtaining it by GC testing.

[0049] In this invention, the mass content c3 of the characteristic element in the positive electrode active coating and the mass content c1 of element Al in the positive electrode active coating satisfy: 5 × 10⁻⁶. -6 ≤c3×c1≤2.5×10 -5 For example, 5×10 -6 6×10 -6 7×10 -6 8×10 -6 9×10 -6 1×10 -5 1.5×10 -52×10 -5 Or 2.5×10 -5 .

[0050] In one instance, 9.3 × 10 -6 ≤c3×c1≤1.5×10 -5 .

[0051] The mass content of element Al in the positive electrode active coating affects the specific capacity of the positive electrode active material; the low-temperature discharge capacity of the battery decreases with increasing Al mass content. Conversely, the content of characteristic elements directly affects the battery's low-temperature discharge performance; the battery's low-temperature discharge capacity increases to some extent with increasing characteristic element content. Therefore, when these two factors satisfy a specific relationship, the battery exhibits a high low-temperature discharge capacity without experiencing a decrease in high-temperature performance due to excessive solid electrolyte.

[0052] In this invention, the silicon-carbon material may include primary spherical particles. The average particle size of the primary spherical particles may be 1 μm-6 μm, for example, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm or 6 μm.

[0053] In one example, the average particle size of the primary spherical particles is 3 μm-5 μm.

[0054] In this invention, the average particle size of the primary spherical particles can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt adhering to the negative electrode sheet. The negative electrode sheet is then cut using an argon-ion milling machine with a CP laser and observed using SEM (using high-voltage mode (Back-scattered ElectronsBSE)). In this mode, the contrast of silicon-carbon materials is brighter (which can be used to distinguish graphite materials and conductive agents in the negative electrode active coating). Measurements are taken at 5K magnification, at least 20 primary spherical particles are randomly selected, and the particle size of each primary spherical particle is measured, and the average value is taken. If the number of primary spherical particles at 5K magnification is less than 20, another microscopic image is taken until 20 primary spherical particles are measured.

[0055] Conventional silicon-carbon materials are in block form with an average particle size of approximately 6μm-12μm. This relatively large average particle size results in poor conductivity. Furthermore, the high hardness of silicon-carbon materials leads to low compaction density, making it difficult to achieve a silicon content of over 5% in the negative electrode active coating, thus limiting the improvement in battery energy density. Using silicon-carbon materials with primary spherical particles of 1μm-6μm not only provides a higher silicon content—1.5%-20% in the negative electrode active coating, significantly improving battery energy density—but also enhances the overall conductivity of the negative electrode sheet due to its smaller particle size, thereby improving the battery's low-temperature discharge performance.

[0056] In this invention, the silicon-carbon material further includes secondary spherical particles formed by a plurality of the primary spherical particles. The term "a plurality of" refers to a number of primary spherical particles forming the secondary spherical particles that is greater than or equal to two.

[0057] Smaller primary spherical particles are beneficial for increasing battery energy density; however, due to their smaller size, they have a larger specific surface area, which increases the risk of side reactions with the electrolyte and leads to poorer stability. To reduce side reactions between silicon-carbon materials and the electrolyte, secondary spherical particles, formed from several primary spherical particles, are further added. These larger secondary spherical particles have a smaller specific surface area, thus reducing the risk of side reactions with the electrolyte and resulting in better stability.

[0058] In this invention, the average particle size of the secondary spherical particles can be 3μm-20μm, for example, 3μm, 4μm, 5μm, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm or 20μm.

[0059] In this invention, the average particle size of the secondary spherical particles can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC to remove the lithium salt adhering to the negative electrode sheet. The negative electrode sheet is then cut using an argon-ion milling machine with a CP laser, and observed using SEM (using high-voltage mode). Measurements are taken at 5K magnification, and at least 20 secondary spherical particles are randomly selected. The particle size of each secondary spherical particle is measured, and the average value is taken. If the number of secondary spherical particles at 5K magnification is less than 20, microscopic images are taken again until 20 secondary spherical particles are measured.

[0060] In this invention, in the negative electrode active coating, the number of primary spherical particles accounts for 0.1-0.9% of the total number of primary and secondary spherical particles, for example, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8 or 0.9%.

[0061] In one example, in the negative electrode active coating, the number of primary spherical particles accounts for 0.3-0.8% of the total number of primary and secondary spherical particles.

[0062] In this invention, the number of primary spherical particles and the number of secondary spherical particles in the negative electrode active coating can be tested 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 DMC solvent for 12 hours, it is rinsed with DMC solvent to remove the lithium salt attached to the negative electrode sheet. Along the thickness direction of the negative electrode sheet, the negative electrode sheet is cut with an argon ion milling machine using a CP laser. Then, it is observed using SEM (using high voltage mode) to obtain a mirror image of the cross-section of the negative electrode sheet along the thickness direction. At least 20 mirror images with different cross-sections are selected and the number of primary spherical particles and the number of secondary spherical particles in each mirror image are counted and the average value is taken.

[0063] In this invention, the negative electrode active material may further include graphite material. The graphite material includes, for example, artificial graphite and / or natural graphite. The graphite material comprises secondary particles. The secondary particles are formed from a plurality of primary particles. "A plurality" means that the number of primary particles forming the secondary particles is greater than or equal to 2.

[0064] In this invention, the average particle size of the secondary particles can be 6μm-20μm, for example, 6μm, 7μm, 8μm, 9μm, 10μm, 11μm, 12μm, 13μm, 14μm, 15μm, 16μm, 17μm, 18μm, 19μm or 20μm.

[0065] By combining specific graphite materials and controlling their structure and particle size, the graphite material is made to include secondary particles composed of primary particles, with an average particle size of 6μm-20μm, which further enhances the energy density of the battery. Compared to primary graphite materials, secondary particles of graphite with specific particle sizes can improve the energy density of the battery. Therefore, this graphite material has a high degree of compatibility with specific silicon-carbon materials, which is beneficial for improving the battery's energy density.

[0066] In this invention, the average particle size of the secondary particles can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the negative electrode sheet is disassembled, soaked in DMC solvent for 12 hours, rinsed with DMC solvent to remove lithium salts adhering to the negative electrode sheet, and then the negative electrode active coating is rinsed off the negative electrode current collector with deionized water. After ultrasonication and centrifugation to remove the filtrate, the sample is dried and dispersed in deionized water containing nonylphenol polyoxyethylene ether (wherein the mass content of nonylphenol polyoxyethylene ether is 0.02%-0.03%) to form a mixture. The mixture is ultrasonicated for 2 minutes and then tested using a Malvern particle size analyzer. The median particle size Dv50 obtained is the average particle size of the secondary particles. Due to the specific composition and particle size of the graphite and silicon carbide materials in this invention, the silicon carbide material has a relatively small impact on the average particle size of the graphite secondary particles. Therefore, the data obtained by the above testing method is the average particle size of the secondary particles.

[0067] In this invention, the lithium-ion secondary battery may further include a separator. The separator may include an organic coating comprising polymer particles containing at least one of cyano, isocyanate, isocyanate, and triazine groups. The polymer particles may include, for example, at least one of polyacrylonitrile, nitrile rubber, 1,3,5-triazine-2,4,6-triamine, cyanuric acid, melamine, and melamine trithiocyanate.

[0068] In one example, the organic coating faces the positive electrode.

[0069] When the organic coating is facing the positive electrode, the organic cyanide in the organic coating can diffuse to the surface of the positive electrode, which can stabilize the crystal structure of metal ions and positive electrode active materials, reduce the release of active oxygen, and help improve the stability of positive electrode active materials, thereby improving the cycle stability of the battery.

[0070] In this invention, the mass content of element N in the organic coating can be 10.5%-55%, for example, 10.5%, 15%, 20%, 25%, 30%, 35%, 40%, 45%, 50% or 55%.

[0071] In one example, the mass content of element N in the organic coating is 15%-35%.

[0072] In this invention, the mass content of element N in the organic coating can be obtained by conventional methods in the art. For example, after discharging the battery to 0% SOC, the separator is disassembled, soaked in DMC solvent for 12 hours, and then rinsed with DMC solvent to remove the lithium salt attached to the separator. The adhesive layer on the outer surface of the organic coating is gently removed with tape (this step is omitted if no adhesive layer is set on the outer surface of the organic coating). The organic coating is observed with SEM, and then combined with energy dispersive spectroscopy (EDS) at 30K magnification, the particles in the organic coating are scanned, at least 20 points are selected, the mass content of nitrogen element is tested, and the average value is taken.

[0073] In this invention, the diaphragm may further include a substrate layer and / or an adhesive layer. The substrate layer may, for example, comprise polyethylene. The adhesive layer may, for example, comprise polyvinylidene fluoride (PVDF) and / or polymethyl methacrylate (PMMA).

[0074] In one example, the diaphragm includes the substrate layer, the organic coating on one side surface of the substrate layer, and the adhesive layer on the other side surface of the substrate layer.

[0075] In one example, the diaphragm includes the substrate layer, the organic coating on one side surface of the substrate layer, and the adhesive layer on both outer surfaces of the diaphragm.

[0076] In this invention, the thickness h of the organic coating can be 0.5μm-4μm, for example, 0.5μm, 1μm, 2μm, 3μm or 4μm.

[0077] In one instance, h is 1μm-2μm.

[0078] In this invention, the outer surface of the negative electrode active coating may have a first recess. The negative electrode sheet includes a negative electrode current collector and the negative electrode active coating located on at least one side of the surface of the negative electrode current collector. The outer surface of the negative electrode active coating refers to the surface of the negative electrode active coating facing away from the negative electrode current collector.

[0079] A first recess is provided on the outer surface of the negative electrode active coating. This first recess can provide a certain release space for the volume expansion of the negative electrode active material, which can reduce the extension of the negative electrode current collector caused by the volume expansion of the negative electrode active material, and is conducive to further improving the cycle stability of the battery.

[0080] In this invention, the first recess may include a recessed hole or a groove. The first recess can be obtained using laser drilling or wire drilling techniques. When the first recess is a groove, the groove can be continuously provided or segmented. Figure 1 The diagram shown is a top view of the groove on the surface of the negative electrode sheet in an embodiment of the present invention. Figure 1(a) The grooves are continuously arranged; Figure 1 (b) The grooves are segmented. As can be seen from the figure, the surface of the negative electrode (i.e., the surface of the negative electrode active coating) has several grooves. Figure 1 In (a), the groove is continuously arranged in the width direction of the negative electrode sheet. Figure 1 In (b), the groove is segmented along the width direction of the negative electrode sheet. It is understood that... Figure 1 The example only shows the case where the groove is set along the width direction of the negative electrode sheet; the groove can also be set along the length direction of the negative electrode sheet.

[0081] In this invention, the depth of the first recess can be 5μm-40μm, for example, 5μm, 10μm, 15μm, 20μm, 25μm, 30μm, 35μm, or 40μm. The depth of the first recess has a conventional meaning in the art, referring to the vertical distance from the lowest point within the first recess to the surface of the negative electrode sheet. The depth of the first recess can be measured using conventional methods in the art, such as using a 3D profilometer to measure the depth of all or at least 20 first recesses on the surface of the negative electrode active coating and taking the average value.

[0082] In one example, the depth of the first recess is 15μm-30μm.

[0083] In this invention, the width of the first recess can be 40μm-200μm, for example, 40μm, 50μm, 60μm, 70μm, 80μm, 90μm, 100μm, 150μm or 200μm.

[0084] In one example, the width of the first recess is 60μm-100μm.

[0085] When the first recess is a concave hole, the width of the first recess refers to the diameter of the concave hole. The diameter of the concave hole has a conventional meaning in the art. When the shape of the orthographic projection of the concave hole on the surface of the negative electrode sheet is a "regular circle", the diameter of the concave hole is the diameter of the regular circle; when the shape of the orthographic projection of the concave hole on the surface of the negative electrode sheet is a non-"regular circle" (e.g., ellipse or irregular curved polygon), the diameter of the concave hole is the diameter of an equivalent circle with the same area as the non-"regular circle". The diameter of the concave hole can be tested by conventional means in the art, for example, by using a 3D profilometer to test the diameter of at least 20 concave holes on the surface of the negative electrode active coating and taking the average value.

[0086] When the first recess is a groove, the width of the first recess refers to the width of the groove. The width of the groove has a conventional meaning in the art. The orthographic projection of the groove onto the surface of the negative electrode includes two long sides, and the width of the groove refers to the average distance from one long side to the other along the length or width direction of the negative electrode. For example... Figure 2 The diagram shown is a schematic representation of the width of the groove in an embodiment of the present invention, wherein... Figure 2 (a)- Figure 2 In (c), the two long sides of the groove are straight lines. Figure 2 In (d), the two long sides of the groove are curved. Figure 2 (a) and Figure 2 In (b), the two long sides are arranged parallel to each other. Therefore, in the width direction of the negative electrode sheet, the perpendicular distance from any point on one long side to the other long side is equal. In this case, the width of the groove is the perpendicular distance L1 from any point on one long side to the other long side in the length or width direction of the negative electrode sheet. Figure 2 In (c), the two long sides of the groove are straight lines, but not parallel. Therefore, in the width direction, 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 taken as the average value. That is, on one long side, based on the length of that side, 50 points are selected at equal intervals (i.e., the distance between each point is equal, so that the calculation result is more accurate), and the width L1 corresponding to each point is measured. The average value is then taken to obtain the width of the groove. Figure 2 In (d), the two long sides are curves. Therefore, in the width direction, 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 taken as the average value, that is, 50 points are randomly selected on one long side (because...). Figure 2 In (d), the two longer sides are curves, and there is no... Figure 2 (c) Given the relationship between the two long sides, 50 points can be randomly selected for measurement. The width L1 corresponding to each point is measured, and the average value is taken to obtain the width of the groove. The width of the groove can be tested by conventional methods in the art, such as using a 3D profilometer to test the width of all grooves or at least 5 grooves on the surface of the negative electrode active coating and take the average value.

[0087] In this invention, the spacing of the first recess can be 0.5mm-5mm, for example, 0.5mm, 0.6mm, 0.7mm, 0.8mm, 0.9mm, 1mm, 1.5mm, 2mm, 2.5mm, 3mm, 3.5mm, 4mm, 4.5mm or 5mm.

[0088] In one example, the spacing of the first recess is 0.8mm-1.5mm.

[0089] When the first recess is a hole, the spacing of the first recess refers to the spacing of the holes. The spacing of the holes has a conventional meaning in the art. It refers to the shortest distance between the edges of two adjacent holes on the surface of the negative electrode. The spacing of the holes can be tested by conventional methods in the art, such as by SEM, selecting all or at least 10 groups of adjacent holes in the electron microscope field, measuring the spacing, and taking the average value.

[0090] When the first recess is a groove, the spacing between the first recesses refers to the spacing between the grooves. It is understood that when there is only one groove on the surface of the negative electrode, there is no spacing between the grooves. The spacing between the grooves has a conventional meaning in the art, referring to the average distance between the two adjacent long sides of two adjacent grooves in the length or width direction of the negative electrode. Figure 3 The diagram shown is a schematic representation of the groove spacing in an embodiment of the present invention, wherein... Figure 3 (a) represents the case where two adjacent long sides are straight lines and parallel. Figure 3 (b) represents the case where two adjacent long sides are straight lines and not parallel. Figure 3 (c) represents the case where two adjacent long sides are curves. Figure 3 In (a), the two adjacent long sides are straight lines and parallel to each other. Therefore, in the width direction, the distance from any point on one long side to the other long side is equal. At this time, the spacing of the grooves is L2, the distance from any point on one long side to the other long side in the width direction. Figure 3 In (b), the two adjacent long sides are straight lines, but not parallel. Therefore, in the width direction, 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 that side, 50 points are selected at equal intervals (i.e., the distance between each point is equal, so that the calculation results are more accurate), and the width L2 corresponding to each point is measured. The average value is then taken to obtain the spacing. Figure 3 In (c), two adjacent long sides are curves. Therefore, in the width direction, 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 taken as the average value, that is, 50 points are randomly selected on one long side (because...). Figure 3 (c) has two long sides that are curves and do not exist. Figure 3 (b) Given the relationship between the two long sides, 50 points can be randomly selected for measurement. The width L2 corresponding to each point is measured, and the average value is taken to obtain the spacing. The spacing of the grooves can be tested by conventional means in the art, such as using a 3D profilometer to test the spacing of all grooves or at least 5 grooves on the surface of the negative electrode active coating, and taking the average value.

[0091] In this invention, the positive electrode sheet may include a positive current collector and a positive active coating located on at least one surface of the positive current collector. The length of the positive active coating on the first surface of the positive current collector is greater than the length of the positive active coating on the second surface of the positive current collector. It is understood that during coating, the positive electrode sheet has a single-sided coating area and a double-sided coating area. The single-sided coating area is where the positive current collector has a positive active coating on only one surface; while the double-sided coating area is where the positive current collector has a positive active coating on both surfaces. This results in the positive electrode sheet having unequal lengths of the positive active coating on both surfaces. In this invention, the side of the positive active coating on the surface of the positive current collector with a relatively longer length is defined as the first surface, and the side with a relatively shorter length is defined as the second surface. The area where the projections of the positive active coating on the first surface and the positive active coating on the second surface overlap in the thickness direction of the positive electrode sheet is the double-sided coating area, and the area where the projections do not overlap is the single-sided coating area. Due to the special structure of wound batteries, the first surface usually faces the winding center of the core, while the second surface faces away from the winding center of the core.

[0092] In this invention, the surface of the positive electrode active coating located on the first surface has a second recess, and the surface of the positive electrode active coating located on the second surface has a convex portion. An embossing process can be performed on the surface of the positive electrode sheet to obtain a structure with a recess on one side and a convex portion on the other.

[0093] The electrolyte storage locations inside the battery are distributed in the gaps between the core and the aluminum-plastic film, as well as in the gaps between the electrode layers. Interlayer storage mainly relies on the electrode pores and capillary effect for slow wetting, which requires more time and is more difficult than wetting the gaps between the core and the aluminum-plastic film. Therefore, the electrolyte storage capacity of the inner electrode layers is significantly less than that of the outer electrode layers. Using an embossing process on the positive electrode, creating a concave side and a convex side, provides more storage space for electrolyte, reduces ion transport distance, decreases polarization between the positive and negative electrodes, and improves the overall charging speed of the battery.

[0094] Furthermore, the inventors of this invention conducted a stress analysis on the positive electrode active coating facing and away from the winding center in the core, and found that when a concave portion is provided on the first surface facing the winding center and a convex portion is provided on the second surface away from the winding center, it not only benefits the structural stability of the positive electrode itself, but also provides a better buffer space for the volume expansion of the negative electrode, thereby improving the cycle life of the battery. Figure 4The following is a schematic structural diagram of a core in an embodiment of the present invention. As can be seen from the figure, the core includes a negative electrode sheet 1 and a positive electrode sheet 2. The surface of the positive electrode active coating on the first surface has a second recess, and the surface of the positive electrode active coating on the second surface has a protrusion. The first surface faces the winding center of the core, and the second surface faces away from the winding center of the core.

[0095] In the present invention, the shape of the orthographic projection of the second recess and the protrusion on the surface of the positive electrode sheet is not limited, and it can be a circular, elliptical, linear (including straight or wavy lines), polygonal or other shapes.

[0096] In the present invention, the positive electrode sheet includes a positive electrode tab welding area, a paste coating area, and an empty foil area. The empty foil area refers to the area on the positive electrode current collector where the positive electrode active coating is not coated except for the positive electrode tab welding area. The paste coating area includes a double-sided coating area and a single-sided coating area. As Figure 5 The following is a schematic structural diagram of a positive electrode sheet in an embodiment of the present invention, where Figure 5 (a) is a top view, Figure 5 (b) is a cross-sectional view along the thickness direction. As can be seen from the figure, the positive electrode sheet includes a positive electrode tab welding area 3, a paste coating area 4, and an empty foil area 5, where the paste coating area 4 includes a double-sided coating area 10 and a single-sided coating area 20.

[0097] In one embodiment, the second recess and the protrusion are located in the paste coating area.

[0098] In one embodiment, the second recess and the protrusion are located in the double-sided coating area.

[0099] In one embodiment, the second recess and the protrusion are located in the paste coating area and in the double-sided coating area.

[0100] In the present invention, the distance from the second recess to the edge of the positive electrode tab welding area is w1, 0 mm < w1 ≤ 10 mm, for example, 0.1 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm or 10 mm.

[0101] In the present invention, the distance from the second recess to the edge of the first side of the paste coating area is w2, 2 mm ≤ w2 ≤ 40 mm, for example, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm, 25 mm, 30 mm, 35 mm or 40 mm. The first side is the side where the positive electrode tab welding area is provided.

[0102] In the present invention, the distance from the second recess to the edge of the second side of the pasting area is w3, where 2 mm ≤ w3 ≤ 25 mm, for example, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm, 20 mm or 25 mm. The second side is the side opposite to the side where the positive tab welding area is provided.

[0103] In the present invention, the distance from the second recess to the edge of the third side of the pasting area is w4, where 0 mm < w4 ≤ 20 mm, for example, 0.1 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm or 20 mm. The third side is the side of the pasting area close to the first end of winding.

[0104] In the present invention, the distance from the second recess to the boundary line between the double-sided coating area and the single-sided coating area is w5, where 0 mm < w5 ≤ 20 mm, for example, 0.1 mm, 0.5 mm, 1 mm, 2 mm, 3 mm, 4 mm, 5 mm, 6 mm, 7 mm, 8 mm, 9 mm, 10 mm, 11 mm, 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, 18 mm, 19 mm or 20 mm.

[0105] By controlling w1, w2, w3, w4 and w5, it is possible to ensure that the positive electrode sheet does not shed powder, which is beneficial to improving the cycle life of the battery.

[0106] As Figure 6 shown in the top view schematic diagram of the first surface of the positive electrode sheet in an example of the present invention, it can be seen from the figure that the first surface has a plurality of second recesses 6. The distance from the second recess 6 to the edge of the positive tab welding area 3 is w1, the distance from the second recess 6 to the edge of the first side of the pasting area 4 is w2, the distance from the second recess 6 to the edge of the second side of the pasting area 4 is w3, the distance from the second recess 6 to the edge of the third side of the pasting area 4 is w4, and the distance from the second recess 6 to the boundary line between the double-sided coating area 10 and the single-sided coating area 20 is w5. <00XXX319>In the present invention, the depth of the second recess can be 3 μm - 40 μm, for example, 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm or 40 μm. The height of the convex portion can be 3 μm - 40 μm, for example, 3 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm, 35 μm or 40 μm.

[0108] In one example, the depth of the second recess is 10 μm-30 μm. The height of the convex portion is 10 μm-30 μm.

[0109] In this invention, the depth of the second recess and the height of the convex portion have conventional meanings in the art. The depth of the second recess refers to the vertical distance from the lowest point within the second recess to the surface of the positive electrode sheet. The height of the convex portion refers to the vertical distance from the highest point on the convex portion to the surface of the positive electrode sheet. The depth of the second recess and the height of the convex portion can be obtained by conventional methods in the art, such as using a 3D profilometer to select at least 20 second recesses or 20 convex portions on the surface of the positive electrode sheet, measuring the depth of each second recess, and taking the average value to obtain the depth of the second recess; measuring the height of each convex portion, and taking the average value to obtain the height of the convex portion.

[0110] In this invention, the width of the second recess can be 0.2mm-8mm, for example, 0.2mm, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm or 8mm. The width of the protrusion can be 0.2mm-8mm, for example, 0.2mm, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm or 8mm.

[0111] In one example, the width of the second recess is 1mm-3mm. The width of the convex portion is 1mm-3mm.

[0112] In this invention, when the projection of the second recess onto the thickness direction of the positive electrode sheet is a regular circle, the width of the second recess is the diameter of the regular circle; when the projection of the second recess onto the thickness direction of the positive electrode sheet is not a regular circle, the width of the second recess is the equivalent diameter of a circle with the same area as the irregular circle. Similarly, when the projection of the convex portion onto the thickness direction of the positive electrode sheet is a regular circle, the width of the convex portion is the diameter of the regular circle; when the projection of the convex portion onto the thickness direction of the positive electrode sheet is not a regular circle, the width of the convex portion is the equivalent diameter of a circle with the same area as the irregular circle. The widths of the second recess and the convex portion can be measured using conventional methods in the art, such as a 3D profilometer.

[0113] In this invention, the spacing of the second recess can be 0.5mm-8mm, for example, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm or 8mm. The spacing of the protrusions can be 0.5mm-8mm, for example, 0.5mm, 1mm, 2mm, 3mm, 4mm, 5mm, 6mm, 7mm or 8mm.

[0114] In one example, the spacing between the second recesses is 1mm-3mm. The spacing between the protrusions is 1mm-3mm.

[0115] In this invention, the spacing between the second recesses refers to the shortest distance between the edges of the orthographic projections of two adjacent second recesses onto the surface of the positive electrode sheet. Similarly, the spacing between the convexes refers to the shortest distance between the edges of the orthographic projections of two adjacent convexes onto the surface of the positive electrode sheet. The spacing between the second recesses and the spacing between the convexes can be tested using conventional methods in the art, such as using a 3D profilometer to select at least 10 sets of adjacent second recesses or convexes on the surface of the positive electrode sheet, measuring the spacing, and taking the average value.

[0116] In this invention, the positive electrode active coating may further include a positive electrode conductive agent and a positive electrode binder, and the negative electrode active coating may further include a negative electrode conductive agent and a negative electrode binder. The positive electrode conductive agent and the negative electrode conductive agent can be conductive agents conventionally used in the art. The positive electrode binder and the negative electrode binder can be binders conventionally used in the art.

[0117] It should be noted that the numerical designations such as "first" and "second" in this invention are only used to distinguish different substances or methods of use, and do not represent a difference in order.

[0118] The present invention will be described in detail below through embodiments. The embodiments described herein are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0119] In the following examples, unless otherwise specified, all materials used are commercially available analytical grade.

[0120] The following examples illustrate the lithium-ion secondary battery of the present invention.

[0121] Example 1 The battery is prepared according to the following method: (1) Preparation of positive electrode sheet Lithium cobalt oxide (M) 1 Including Al), solid electrolyte (lithium aluminum titanium phosphorus oxide, chemical formula Li 1.3 Al0.3 Ti 1.7 (PO4)3 (average particle size 1.5 μm), positive electrode conductive agent (conductive carbon black), and positive electrode binder (polyvinylidene fluoride) are mixed in a mass ratio of 96.039:0.961:1:2, and N-methylpyrrolidone (NMP) is added. The mixture is stirred evenly to prepare a positive electrode slurry. The positive electrode slurry is coated on the first and second surfaces of an aluminum foil (the coating length of the positive electrode slurry on the first surface of the aluminum foil is greater than the coating length on the second surface), baked, and rolled to obtain a thickness of 100 μm. The positive electrode sheet is m; a positive electrode tab welding area of ​​a fixed size is set on the paste area of ​​the positive electrode sheet (the size of the positive electrode tab welding area in the width direction of the positive electrode sheet is 20mm), and nickel tabs are laser welded to the positive electrode tab welding area; then, after passing through a roller, in the double-sided coating area (and avoiding the positive electrode tab welding area), embossing is performed from the first surface to the second surface to obtain a second concave part (first surface) and a convex part (second surface); the shape of the orthographic projection of the second concave part and the convex part on the surface of the positive electrode sheet is circular; The mass content of element Al in the positive electrode active coating is 8273 ppm (c1), the mass content of characteristic element (Ti) in the positive electrode active coating is 1578 ppm (c3), and the mass content of element Co in the positive electrode active coating is 580215 ppm (c4). The ratio of c3 to c4 is 0.0027, and the ratio of c3 to c1 is 1.31 × 10⁻⁶. -5 The width of the second recess is 2mm, the depth is 20μm, and the spacing is 2mm; w1 is 7mm, w2 is 27mm, w3 is 15mm, w4 is 7mm, and w5 is 7mm.

[0122] (2) Preparation of negative electrode sheet Artificial graphite (secondary particles), silicon-carbon material (the number of primary spherical particles accounts for 0.55 of the total number of primary and secondary spherical particles, the average particle size of the primary spherical particles is 4.1 μm, and the mass content of element Si in the silicon-carbon material is 40%), negative electrode conductive agent (carbon nanotubes), negative electrode dispersant (lithium carboxymethyl cellulose), and negative electrode binder (polyacrylic acid) are mixed in a mass ratio of 77:20:0.4:0.1:2.5, and deionized water is added to prepare a negative electrode slurry. The above negative electrode slurry is coated on both sides of carbon-coated copper foil, baked, and rolled to obtain a negative electrode sheet with a thickness of 110 μm. A first concave part (groove) is created on the surface of the negative electrode sheet using a laser. The mass content of element Si in the negative electrode active coating is 8% (c2), and the groove has a width of 79.6 μm, a depth of 20.2 μm, and a spacing of 1.3 mm.

[0123] (3) Preparation of electrolyte In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), ethylene carbonate, propylene carbonate, and diethyl carbonate were mixed in a weight ratio of 1:3:6 to obtain an organic solvent; then, 45% of a carboxylic acid ester compound (PP) based on the total mass of the electrolyte was added to form a mixed solvent; 15% of fluoroethylene carbonate, 7.5% of vitamin C (C5 = 7.5%), and 12.5% ​​of lithium hexafluorophosphate based on the total mass of the electrolyte were dissolved in the above mixed solvent to obtain the electrolyte.

[0124] (4) Preparation of diaphragm 1,3,5-triazine-2,4,6-triamine was ground and then mixed with styrene-butadiene rubber and lithium polyacrylate at a mass ratio of 63:30:7. NMP was added to obtain an organic coating slurry. The organic coating slurry was coated onto one side of a polyethylene film and dried (forming an organic coating with a thickness h of 1.3 μm). A PMMA adhesive layer was coated onto the other side of the polyethylene film, and a PVDF+PMMA adhesive layer (PVDF to PMMA mass ratio of 7:3) was coated onto the outer surface of the organic coating to obtain a separator. The mass content of element N in the organic coating was 28%.

[0125] (5) Battery fabrication The positive electrode sheet prepared in step (1), the separator prepared in step (4), and the negative electrode sheet prepared in step (2) are wound together to obtain a core (wherein the organic coating faces the positive electrode sheet); the battery is obtained through encapsulation, baking, liquid injection, formation, secondary sealing, sorting, and OCV.

[0126] Example 2 The battery is prepared according to the following method: (1) Preparation of positive electrode sheet Lithium cobalt oxide (M) 1 Including Al), solid electrolyte (lithium aluminum titanium phosphorus oxide, chemical formula Li 1.3 Al 0.3 Ti 1.7(PO4)3 (average particle size 1.2 μm), positive electrode conductive agent (conductive carbon black), and positive electrode binder (polyvinylidene fluoride) are mixed in a mass ratio of 96.042:0.958:1:2. N-methylpyrrolidone (NMP) is added, and the mixture is stirred evenly to prepare a positive electrode slurry. The above positive electrode slurry is coated on the first and second surfaces of an aluminum foil (the coating length of the positive electrode slurry on the first surface of the aluminum foil is greater than the coating length on the second surface), baked, and rolled to obtain a thickness of 100 μm. The positive electrode sheet is m; a positive electrode tab welding area of ​​a fixed size is set on the paste area of ​​the positive electrode sheet (the size of the positive electrode tab welding area in the width direction of the positive electrode sheet is 15mm), and nickel tabs are laser welded to the positive electrode tab welding area; then, after passing through a roller, in the double-sided coating area (and avoiding the positive electrode tab welding area), embossing is performed from the first surface to the second surface to obtain a second concave part (first surface) and a convex part (second surface); the shape of the orthographic projection of the second concave part and the convex part on the surface of the positive electrode sheet is circular; The mass content of element Al in the positive electrode active coating is 7126 ppm (c1), the mass content of characteristic element (Ti) in the positive electrode active coating is 1991 ppm (c3), and the mass content of element Co in the positive electrode active coating is 564282 ppm (c4). The ratio of c3 to c4 is 0.0035, and the ratio of c3 to c1 is 1.42 × 10⁻⁶. -5 The width of the second recess is 1mm, the depth is 10μm, and the spacing is 1mm; w1 is 5mm, w2 is 20mm, w3 is 10mm, w4 is 5mm, and w5 is 5mm.

[0127] (2) Preparation of negative electrode sheet Artificial graphite (secondary particles), silicon-carbon material (the number of primary spherical particles accounts for 0.32 of the total number of primary and secondary spherical particles, the average particle size of the primary spherical particles is 3μm, and the mass content of element Si in the silicon-carbon material is 40%), negative electrode conductive agent (carbon nanotubes), negative electrode dispersant (lithium carboxymethyl cellulose), and negative electrode binder (polyacrylic acid) are mixed in a mass ratio of 77:20:0.4:0.1:2.5, and deionized water is added to prepare a negative electrode slurry. The above negative electrode slurry is coated on both sides of carbon-coated copper foil, baked, and rolled to obtain a negative electrode sheet with a thickness of 110μm. A first concave part (groove) is created on the surface of the negative electrode sheet using a laser. The mass content (c2) of element Si in the negative electrode active coating is 8%, and the groove has a width of 60.1 μm, a depth of 15.4 μm, and a spacing of 0.8 mm.

[0128] (3) Preparation of electrolyte In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), ethylene carbonate, propylene carbonate, and diethyl carbonate are mixed in a weight ratio of 1:3:6 to obtain an organic solvent; then, 20% of a carboxylic acid ester compound (PP) based on the total mass of the electrolyte is added to form a mixed solvent; 15% of fluoroethylene carbonate, 10% of VC (C5 is 10%), and 12.5% ​​of lithium hexafluorophosphate based on the total mass of the electrolyte are dissolved in the above mixed solvent to obtain the electrolyte.

[0129] (4) Preparation of diaphragm 1,3,5-triazine-2,4,6-triamine was ground and then mixed with styrene-butadiene rubber and lithium polyacrylate at a mass ratio of 40:50:10. NMP was added to obtain an organic coating slurry. The organic coating slurry was coated onto one side of a polyethylene film and dried (forming an organic coating with a thickness h of 1 μm). A PMMA adhesive layer was coated onto the other side of the polyethylene film, and a PVDF+PMMA adhesive layer (PVDF to PMMA mass ratio of 7:3) was coated onto the outer surface of the organic coating to obtain a separator. The mass content of element N in the organic coating was 15%.

[0130] (5) Battery fabrication The positive electrode sheet prepared in step (1), the separator prepared in step (4), and the negative electrode sheet prepared in step (2) are wound together to obtain a core (wherein the organic coating faces the positive electrode sheet); the battery is obtained through encapsulation, baking, liquid injection, formation, secondary sealing, sorting, and OCV.

[0131] Example 3 The battery is prepared according to the following method: (1) Preparation of positive electrode sheet Lithium cobalt oxide (M) 1 Including Al), solid electrolyte (lithium aluminum titanium phosphorus oxide, chemical formula Li 1.3 Al 0.3 Ti 1.7(PO4)3 (average particle size 1.7 μm), positive electrode conductive agent (conductive carbon black), and positive electrode binder (polyvinylidene fluoride) are mixed in a mass ratio of 96.033:0.967:1:2. N-methylpyrrolidone (NMP) is added, and the mixture is stirred evenly to prepare a positive electrode slurry. The above positive electrode slurry is coated on the first and second surfaces of an aluminum foil (the coating length of the positive electrode slurry on the first surface of the aluminum foil is greater than the coating length on the second surface), baked, and rolled to obtain a thickness of 100 μm. The positive electrode sheet is m; a positive electrode tab welding area of ​​a fixed size is set on the paste area of ​​the positive electrode sheet (the size of the positive electrode tab welding area in the width direction of the positive electrode sheet is 25mm), and the nickel electrode tab is laser welded to the positive electrode tab welding area; then, after passing through a roller, in the double-sided coating area (and avoiding the positive electrode tab welding area), embossing is performed from the first surface to the second surface to obtain a second concave part (first surface) and a convex part (second surface); the shape of the orthographic projection of the second concave part and the convex part on the surface of the positive electrode sheet is circular; The mass content of element Al in the positive electrode active coating is c1, which is 9986 ppm; the mass content of characteristic element (Ti) in the positive electrode active coating is c3, which is 936 ppm; and the mass content of element Co in the positive electrode active coating is c4, which is 618542 ppm. The ratio of c3 to c4 is 0.0015, and the ratio of c3 to c1 is 9.35 × 10⁻⁶. -6 The width of the second recess is 3mm, the depth is 30μm, and the spacing is 3mm; w1 is 9mm, w2 is 34mm, w3 is 20mm, w4 is 10mm, and w5 is 10mm.

[0132] (2) Preparation of negative electrode sheet Artificial graphite (secondary particles), silicon-carbon material (the number of primary spherical particles accounts for 0.79 of the total number of primary and secondary spherical particles, the average particle size of the primary spherical particles is 4.9 μm, and the mass content of element Si in the silicon-carbon material is 40%), negative electrode conductive agent (carbon nanotubes), negative electrode dispersant (lithium carboxymethyl cellulose), and negative electrode binder (polyacrylic acid) are mixed in a mass ratio of 77:20:0.4:0.1:2.5, and deionized water is added to prepare a negative electrode slurry. The above negative electrode slurry is coated on both sides of carbon-coated copper foil, baked, and rolled to obtain a negative electrode sheet with a thickness of 110 μm. A first concave part (groove) is created on the surface of the negative electrode sheet using a laser. The mass content of element Si in the negative electrode active coating is 8% (c2), and the groove has a width of 99.2 μm, a depth of 29.5 μm, and a spacing of 1.5 mm.

[0133] (3) Preparation of electrolyte In a glove box (H2O < 0.01 ppm, O2 < 0.01 ppm, Ar atmosphere), ethylene carbonate, propylene carbonate, and diethyl carbonate are mixed in a weight ratio of 1:3:6 to obtain an organic solvent; then, 65% of a carboxylic acid ester compound (PP) based on the total mass of the electrolyte is added to form a mixed solvent; 15% of fluoroethylene carbonate, 5% of VC (C5 is 5%), and 12.5% ​​of lithium hexafluorophosphate based on the total mass of the electrolyte are dissolved in the above mixed solvent to obtain the electrolyte.

[0134] (4) Preparation of diaphragm 1,3,5-triazine-2,4,6-triamine was ground and then mixed with styrene-butadiene rubber and lithium polyacrylate at a mass ratio of 80:15:5. NMP was added to obtain an organic coating slurry. The organic coating slurry was coated onto one side of a polyethylene film and dried (forming an organic coating with a thickness h of 2 μm). A PMMA adhesive layer was coated onto the other side of the polyethylene film, and a PVDF+PMMA adhesive layer (PVDF to PMMA mass ratio of 7:3) was coated onto the outer surface of the organic coating to obtain a separator. The mass content of element N in the organic coating was 35%.

[0135] (5) Battery fabrication The positive electrode sheet prepared in step (1), the separator prepared in step (4), and the negative electrode sheet prepared in step (2) are wound together to obtain a core (wherein the organic coating faces the positive electrode sheet); the battery is obtained through encapsulation, baking, liquid injection, formation, secondary sealing, sorting, and OCV.

[0136] Example 4 group This set of examples is used to verify the impact of changing the "mass content c1 of element Al in the positive electrode active coating".

[0137] This set of embodiments is based on Embodiment 1, except that c1 is changed, as follows: Example 4a, c1 is 6852 ppm; wherein, c3 × c1 is 1.08 × 10 -5 ; Example 4b, c1 is 14874 ppm; wherein, c3 × c1 is 2.35 × 10 -5 .

[0138] Example 5 group This set of examples is used to verify the impact of changing the "solid electrolyte type".

[0139] This set of embodiments is based on Embodiment 1, except that the solid electrolyte is changed, as follows: Example 5a: Lithium aluminum titanium phosphorus oxide was replaced with lithium lanthanum zirconium tantalum oxide, with the chemical formula Li 6.5 La3Zr1.5 Ta 0.5 O 12 In addition, in order to control the mass content c3 of the characteristic element to be close to that of Example 1, the ratio of the positive electrode slurry was adjusted: the mass ratio of lithium cobalt oxide, solid electrolyte, positive electrode conductive agent and positive electrode binder was 96.047:0.953:1:2, and the mass content c3 of the characteristic element (Zr) was 1676. Example 5b, where lithium aluminum titanium phosphorus oxide was replaced with lithium lanthanum titanium oxide, chemical formula Li 30.3 La 0.567 TiO3, as above, with the positive electrode slurry ratio adjusted: the mass ratio of lithium cobalt oxide, solid electrolyte, positive electrode conductive agent and positive electrode binder is 96.032:0.968:1:2, and the mass content of characteristic element (La) c3 is 1628.

[0140] Example 6 group This set of examples is used to verify the impact of changing the "mass content c2 of element Si in the negative electrode active coating".

[0141] This set of embodiments follows the same procedure as Embodiment 1, except that c2 is controlled by changing the mass content of graphite and silicon carbide materials in the negative electrode slurry, as detailed below: Example 6a: Artificial graphite, silicon carbide material, negative electrode conductive agent, negative electrode dispersant, and negative electrode binder are mixed in a mass ratio of 87:10:0.4:0.1:2.5, wherein c2 is 4%; Example 6b: Artificial graphite, silicon carbide material, negative electrode conductive agent, negative electrode dispersant, and negative electrode binder are mixed in a mass ratio of 67:30:0.4:0.1:2.5, wherein c2 is 12%; Example 6c: Artificial graphite, silicon carbide material, negative electrode conductive agent, negative electrode dispersant, and negative electrode binder are mixed in a mass ratio of 93:4:0.4:0.1:2.5, wherein c2 is 1.6%; In Example 6d, artificial graphite, silicon carbide material, negative electrode conductive agent, negative electrode dispersant and negative electrode binder were mixed in a mass ratio of 52:45:0.4:0.1:2.5, wherein c2 was 18%.

[0142] Example 7 group This set of examples is used to verify the impact of changing the "mass content c3 of characteristic elements in the positive electrode active coating".

[0143] This set of embodiments follows the same procedure as Embodiment 1, except that c3 is controlled by changing the content of solid electrolyte in the positive electrode slurry, as detailed below: Example 7a, the positive electrode slurry formulation: the mass ratio of lithium cobalt oxide, solid electrolyte, positive electrode conductive agent, and positive electrode binder is 96.031:0.969:1:2, c3 is 605 ppm; c3 / c4 is 0.001, and c3×c1 is 5.01×10 -6 ; Example 7b, positive electrode slurry formulation: the mass ratio of lithium cobalt oxide, solid electrolyte, positive electrode conductive agent, and positive electrode binder is 96.05:0.95:1:2, c3 is 2967 ppm; c3 / c4 is 0.0051, and c3×c1 is 2.45×10 -5 .

[0144] Example 8 group This set of examples is used to verify the effect of changing "the product of the mass content of the characteristic element in the positive electrode active coating c3 and the mass content of element Al in the positive electrode active coating c1 c3×c1".

[0145] This set of embodiments refers to Embodiments 2 and 3, the difference being that c3×c1 is adjusted by changing c1, as detailed below: Example 8a was performed in accordance with Example 2, except that c1 was 9875 ppm and c3 × c1 was 1.97 × 10⁻⁶. -5 ; Example 8b was performed in accordance with Example 3, except that c1 was 7233 ppm and c3 × c1 was 6.77 × 10⁻⁶. -6 .

[0146] Example 9 This embodiment is used to verify the effect of whether the second concave portion is located on the surface of the positive active coating of the first surface and whether the convex portion is located on the surface of the positive active coating of the second surface.

[0147] The process is carried out with reference to Example 1, except that the embossing process is performed from the second surface to the first surface, that is, the surface of the positive electrode active coating on the second surface has a second concave portion, and the surface of the positive electrode active coating on the first surface has a convex portion.

[0148] Example 10 This embodiment is used to verify the effect of "not setting a second concave and convex part on the surface of the positive electrode active coating".

[0149] The procedure was carried out in accordance with Example 1, except that no embossing was performed.

[0150] Example 11 This embodiment is used to verify the impact of whether the single-sided coating area has a second recess.

[0151] The procedure was carried out in accordance with Example 1, except that embossing was performed on both the single-sided and double-sided coated areas (avoiding the positive electrode tab welding area). Example 12 group This set of examples is used to verify the impact of changes to "w1, w2, w3, w4 and w5".

[0152] This set of embodiments is based on Embodiment 1, except that w1, w2, w3, w4, and w5 are changed, as follows: Example 12a, w1 is 0.5mm, w2 is 2mm, w3 is 2mm, w4 is 0.5mm, w5 is 0.5mm; Example 12b: w1 is 10mm, w2 is 40mm, w3 is 25mm, w4 is 20mm, and w5 is 20mm.

[0153] Example 13 group This set of examples is used to verify the impact of changes in the "average particle size of primary spherical particles".

[0154] This set of embodiments is based on Embodiment 1, except that the control is achieved through changes, as detailed below: In Example 13a, the average particle size of the primary spherical particles was 1.1 μm; Example 13b: The average particle size of the primary spherical particles is 6 μm.

[0155] Example 14 group This set of examples is used to verify the impact of changing the "value of the number of primary spherical particles as a percentage of the total number of primary and secondary spherical particles".

[0156] This set of embodiments is based on Embodiment 1, except that the proportion of spherical particles is changed, as follows: In Example 14a, all silicon-carbon materials are primary spherical particles, meaning that the number of primary spherical particles accounts for 1% of the total number of primary and secondary spherical particles. In Example 14b, the number of primary spherical particles accounts for 0.11 of the total number of primary and secondary spherical particles.

[0157] Example 15 The procedure was carried out in accordance with Example 1, except that the artificial graphite was primary particles with an average particle size of 6.1 μm.

[0158] Example 16 This embodiment is used to verify the impact of changing the "type of the first recess".

[0159] The procedure was carried out in accordance with Example 1, except that a laser was used to create recesses on the surface of the negative electrode. The recesses had a width of 85.2 μm, a depth of 20.1 μm, and a spacing of 0.5 mm.

[0160] Example 17 This embodiment is used to verify the effect of changing the "mass content of carboxylic acid ester compounds in the electrolyte".

[0161] The procedure was carried out in accordance with Example 1, except that the PP content was changed to 10%.

[0162] Example 18 group This set of examples is used to verify the effect of changing the "mass content of vinylene carbonate in the electrolyte c5".

[0163] This set of examples follows the same procedure as Example 1, except that C5 is adjusted by changing the amount of vinylene carbonate added to the electrolyte, as detailed below: Example 18a: No vinylene carbonate was added to the electrolyte, i.e., C5 was 0; Example 18b, c5 is 0.05%; Example 18c, c5 is 15%.

[0164] Example 19 This embodiment is used to verify the impact of changes to the "organic coating".

[0165] The procedure was carried out in accordance with Example 1, except that the organic coating was replaced with a boehmite ceramic coating of the same thickness.

[0166] Example 20 This embodiment is used to verify the effect of "whether the organic coating is directly facing the positive electrode".

[0167] The procedure was carried out in accordance with Example 1, except that the organic coating was placed directly opposite the negative electrode.

[0168] Example 21 group This set of examples is used to verify the impact of changing the "mass content of element N in organic coatings".

[0169] This set of embodiments is based on Embodiment 1, except that the mass content of element N in the organic coating is controlled by changing the substances in the organic coating or the composition of each substance, as follows: Example 21a, in which 1,3,5-triazine-2,4,6-triamine was replaced with the same mass of polyacrylonitrile, wherein the mass content of element N in the organic coating was 10.5%; Example 21b: The mass ratio of 1,3,5-triazine-2,4,6-triamine, styrene-butadiene rubber, and lithium polyacrylate is 94:5:1, wherein the mass content of element N in the organic coating is 55%.

[0170] Example 22 group This set of examples is used to verify the effect of changing the thickness h of the organic coating.

[0171] This set of embodiments is based on Embodiment 1, except that h is changed, as follows: Example 22a, h is 0.5 μm; Example 22b, h is 4μm.

[0172] All of the above embodiments satisfy the following: the average particle size of the first lithium cobalt oxide particle is 0.3μm-7μm, and the average particle size of the second lithium cobalt oxide particle is 7.5μm-40μm.

[0173] Except for Example 14a, all the above embodiments satisfy the requirement that the average particle size of the secondary spherical particles of silicon-carbon material is 3μm-20μm. Except for Example 15, all the above embodiments satisfy the requirement that the average particle size of the secondary particles of graphite material is 6μm-20μm.

[0174] Comparative Example 1 The process was carried out in accordance with Example 1, except that no solid electrolyte was added to the positive electrode sheet. Instead, lithium cobalt oxide, positive electrode conductive agent and positive electrode binder were mixed in a mass ratio of 97:1:2, and NMP was added to prepare a positive electrode slurry.

[0175] Comparative Example 2 This comparative group is used to verify the effect of changing the "mass content c1 of element Al in the positive electrode active coating".

[0176] This comparison group follows the same proportions as in Example 1, except that c1 is changed, as follows: Comparative Example 2a, c1 was 6354 ppm; Comparative Examples 2b and 2c1 were 17895 ppm.

[0177] Comparative Example 3 This comparative example is used to verify the effect of changing the "mass content c2 of element Si in the negative electrode active coating".

[0178] The experiment was carried out in accordance with Example 1, except that c2 was controlled by changing the mass content of graphite and silicon carbide materials in the negative electrode slurry. Specifically, artificial graphite, silicon carbide materials, negative electrode conductive agent, negative electrode dispersant and negative electrode binder were mixed in a mass ratio of 37:60:0.4:0.1:2.5, wherein c2 was 24%.

[0179] Comparative Example 4 This comparative example is used to verify the effect of "not adding carboxylic acid ester compounds to the electrolyte".

[0180] The procedure was carried out in accordance with Example 1, except that no PP was added to the electrolyte.

[0181] Test case (1) Volumetric energy density test The volumetric energy density of the batteries prepared in the examples and comparative examples was tested, and the specific testing methods are as follows: The battery was charged to 4.5V at a current of 0.2C, then charged at a constant voltage until the current dropped to 0.02C. It was then discharged at a current of 0.2C to 3.0V, and the energy discharged was recorded as E. The thickness, width, and length of the battery were measured, and their product was calculated to obtain the battery volume, recorded as V. The formula for calculating the volumetric energy density is VED = E / V, and the results are recorded in Table 1.

[0182] (2) Low-temperature discharge test The batteries prepared in the examples and comparative examples were subjected to low-temperature discharge tests. The specific test methods are as follows: At room temperature (25℃), the battery was charged at 0.2C with constant current and constant voltage to 4.5V, then stopped at 0.02C and allowed to stand for 5 minutes before being discharged at 0.2C to 3.0V. The discharge capacity C1 was recorded. The battery was then charged at room temperature again with constant current and constant voltage to 4.5V, then stopped at 0.02C to fully charge. The fully charged battery was placed in a -20℃ constant temperature chamber and allowed to stand for 2 hours before being discharged at 0.2C to 3.0V. The discharge capacity C2 was recorded. The ratio of C2 to C1 is the low-temperature discharge capacity retention rate at -20℃ and 0.2C. The results are recorded in Table 1.

[0183] (3) 45℃ high temperature cycling test The batteries prepared in the examples and comparative examples were subjected to a high-temperature cycling test at 45°C. The specific test method is as follows: The battery was left to stand in a 45℃ constant temperature room for 2 hours, then charged at a constant current of 3C to 4.2V, then charged at a constant current and constant voltage of 2C to 4.5V, and finally stopped at 0.05C and left to stand for 10 minutes. After that, it was discharged at 0.7C to 3.0V. This cycle was repeated 500 times. The discharge capacity of the battery when it was fully charged on the 500th cycle was measured as C1, and the discharge capacity of the battery when it was fully charged on the first cycle was measured as C0. C1 / C0 is the capacity retention rate after 500 cycles. The results are recorded in Table 1.

[0184] (4) High-temperature storage test The batteries prepared in the examples and comparative examples were subjected to high-temperature storage tests. The specific test methods are as follows: The battery was kept at a constant current and voltage of 0.2C to 4.5V at room temperature (25℃) and cut off at 0.02C. The thickness h1 under full charge was measured using a thickness tester with a capacity of 600ppg. Then the battery was placed at 85℃±2℃ for 6 hours. The thickness h2 was measured immediately after the battery was removed. (h2-h1) / h1 is the thickness expansion rate. The results are recorded in Table 1.

[0185] Table 1 As can be seen from Table 1, compared with the comparative example, the battery of the present invention can combine high energy density, excellent cycle stability, low-temperature discharge performance and high-temperature storage performance.

[0186] The preferred embodiments of the present invention have been described in detail above; however, the present invention is not limited thereto. Within the scope of the inventive concept, various simple modifications can be made to the technical solutions of the present invention, including combinations of various technical features in any other suitable manner. These simple modifications and combinations should also be considered as the content disclosed in the present invention and are all within the protection scope of the present invention.

Claims

1. A lithium-ion secondary battery, characterized by comprising: The lithium ion secondary battery comprises a positive electrode sheet, a negative electrode sheet and a separator; the charging cut-off voltage of the lithium ion secondary battery is greater than or equal to 4.5 V; The positive electrode sheet comprises a positive electrode active coating, the positive electrode active coating comprises a positive electrode active material and a solid-state electrolyte, the positive electrode active material comprises lithium cobaltate, the lithium cobaltate contains element Al, the mass content c1 of element Al in the positive electrode active coating is 6800 ppm-15000 ppm; the solid-state electrolyte comprises at least one of lithium aluminum titanium phosphorus oxide, lithium lanthanum zirconium tantalum oxide and lithium lanthanum titanium oxide; The negative electrode sheet comprises a negative electrode active coating, the negative electrode active coating comprises a negative electrode active material, the negative electrode active material comprises a silicon-carbon material; the mass content c2 of element Si in the negative electrode active coating is 1.5%-20%; The separator comprises an organic coating, the organic coating contains polymer particles.

2. The lithium-ion secondary battery according to claim 1, wherein The mass content c1 of element Al in the positive electrode active coating is 7100 ppm-10000 ppm; And / or, the mass content c2 of element Si in the negative electrode active coating is 4%-12%; And / or, the lithium ion secondary battery further comprises an electrolyte, the electrolyte comprises a solvent, the solvent comprises a carboxylic acid ester compound; Preferably, the mass content of the carboxylic acid ester compound in the electrolyte is 10%-80%; preferably 20%-65%; Preferably, the carboxylic acid ester compound comprises at least one of the following solvents which are substituted or unsubstituted by fluorine: ethyl acetate, propyl acetate, n-butyl acetate, isobutyl acetate, n-pentyl acetate, isopentyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate and n-ethyl butyrate.

3. The lithium-ion secondary battery according to claim 1 or 2, wherein The solid-state electrolyte contains characteristic elements, the characteristic elements comprise at least one of Ti, Zr, La and Ta; Preferably, the ratio of the mass content c3 of the characteristic elements in the positive electrode active coating to the mass content c4 of element Co in the positive electrode active coating is 0.001-0.0055; more preferably 0.0015-0.0035; Preferably, the mass content c3 of the characteristic elements in the positive electrode active coating is 600 ppm-3000 ppm; more preferably 900 ppm-2000 ppm; Preferably, the mass content c4 of element Co in the positive electrode active coating is 400000 ppm-750000 ppm; more preferably 500000 ppm-650000 ppm; Preferably, the average particle size of the solid-state electrolyte is 500 nm-3 μm; more preferably 1 μm-2 μm.

4. The lithium-ion secondary battery according to claim 1 or 2, wherein The electrolyte further comprises vinylene carbonate; Preferably, the mass content c5 of vinylene carbonate in the electrolyte is 0.01%-15%.

5. The lithium-ion secondary battery according to claim 1 or 2, wherein The solid-state electrolyte contains characteristic elements, the characteristic elements comprise at least one of Ti, Zr, La and Ta; The mass content c3 of the characteristic element in the positive electrode active coating satisfies: 5x10 -6 ≤ c3 < 2.5x10 -5 ; Preferably, 9.3 x 10 -6 ≤ c3 x cl ≤ 1.5 x 10 -5 .

6. The lithium-ion secondary battery according to claim 1 or 2, wherein The polymer particles contain at least one of cyano, isocyano, isocyanate and triazine groups; Preferably, the polymer particles contain element N, and the mass content of element N in the organic coating is 10.5%-55%; more preferably, 15%-35%; Preferably, the organic coating faces the positive electrode sheet; Preferably, the polymer particles include at least one of polyacrylonitrile, nitrile rubber, 1,3,5-triazine-2,4,6-triamine, cyanuric acid, melamine, and melamine trimer thiocyanate; Preferably, the thickness h of the organic coating is 0.5 μm-4 μm.

7. The lithium-ion secondary battery according to claim 1 or 2, wherein The silicon-carbon material includes primary spherical particles; And / or, the mass content of element Si in the silicon-carbon material is 30%-80%; And / or, the negative electrode active material further includes a graphite material.

8. The lithium-ion secondary battery according to claim 1 or 2, wherein The outer surface of the negative electrode active coating has a first recess; Preferably, the depth of the first recess is 5 μm-40 μm; Preferably, the width of the first recess is 40 μm-200 μm; Preferably, the pitch of the first recess is 0.5 mm-5 mm.

9. The lithium-ion secondary battery according to claim 1 or 2, wherein The positive electrode sheet includes a positive electrode current collector and the positive electrode active coating located on at least one side surface of the positive electrode current collector; the length of the positive electrode active coating located on a first surface of the positive electrode current collector is greater than the length of the positive electrode active coating located on a second surface of the positive electrode current collector; the area where the projection of the positive electrode active coating located on the first surface and the projection of the positive electrode active coating located on the second surface overlap in the thickness direction of the positive electrode sheet is a double-sided coating area, and the area where the projections do not overlap is a single-sided coating area; The surface of the positive electrode active coating located on the first surface has a second recess, and the surface of the positive electrode active coating located on the second surface has a protrusion; Preferably, the second recess and the protrusion are located in the double-sided coating area; Preferably, the positive electrode sheet includes a positive electrode tab welding area, a coating area, and a blank foil area, and the second recess and the protrusion are located in the coating area; Preferably, the depth of the second recess is 3 μm-40 μm, the width of the second recess is 0.2 mm-8 mm, and the pitch of the second recess is 0.5 mm-8 mm; Preferably, the height of the protrusion is 3 μm-40 μm, the width of the protrusion is 0.2 mm-8 mm, and the pitch of the protrusion is 0.5 mm-8 mm.

10. The lithium-ion secondary battery according to claim 9, wherein The positive electrode sheet includes a positive electrode tab welding area, a coating area, and a blank foil area, and the coating area includes the double-sided coating area and the single-sided coating area; The distance from the second recess to the edge of the positive electrode tab welding area is w1, 0 mm < w1 ≤ 10 mm; And / or, the distance from the second recess to the edge of the first side of the coating area is w2, 2 mm ≤ w2 ≤ 40 mm, and the first side is the side where the positive electrode tab welding area is arranged; And / or, the distance from the second recess to the edge of the second side of the coating area is w3, 2 mm ≤ w3 ≤ 25 mm, and the second side is the side opposite to the side where the positive electrode tab welding area is arranged; And / or, the distance from the second recess to the third edge of the coating area is w4, 0mm < w4≤ 20mm, and the third edge is an edge of the coating area close to the winding starting end; And / or, the distance from the second recess to the intersection line of the double-sided coating area and the single-sided coating area is w5, 0mm < w5≤ 20mm.

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