Secondary battery

Abstract

The invention discloses a secondary battery, which belongs to the field of batteries, and is characterized in that the structural stability and chemical stability of a positive electrode active material lithium nickel cobalt manganese oxide in a lithium deintercalation process are improved by regulating and controlling the content of free lithium in a positive electrode active material layer of a positive electrode plate and the microstress of the positive electrode active material layer; meanwhile, the air permeability of the diaphragm is synchronously controlled to guarantee the transmission efficiency of lithium ions, so that the impedance of the secondary battery in the fast charging process is relatively low, the DCR is controlled at a relatively small level, meanwhile, the cycle stability is excellent, and the fast charging performance is improved and the cycle life is prolonged.

Description

Technical Field

[0001] This application relates to the field of battery technology, specifically to a secondary battery. Background Technology

[0002] Lithium nickel cobalt manganese oxide (NiCoMnO) is a ternary cathode material in lithium-ion secondary batteries with high energy density. However, this material is prone to reacting with carbon dioxide and water in the air, which consumes the surface-active lithium in the material and increases the difference in bulk ion concentration between the material surface and the interior. Internal lithium ions migrate to the surface, which not only increases the surface impedance but also damages the material structure. Ultimately, it affects the normal insertion and extraction of lithium ions, resulting in increased impedance during charging and discharging, decreased fast-charging performance, and affected cycle life. As a result, the overall performance of the battery cannot be balanced. Summary of the Invention

[0003] The purpose of this application is to overcome the shortcomings of the existing technology and provide a secondary battery. By controlling the free lithium content and micro-stress in the positive electrode active material layer of the positive electrode sheet, the structural and chemical stability of the positive electrode active material lithium nickel cobalt manganese oxide during the lithium intercalation and deintercalation process is improved. At the same time, the permeability of the separator is controlled to ensure the lithium ion transport efficiency. This results in a secondary battery with low impedance and low DCR during fast charging, as well as excellent cycle stability, thus improving fast charging performance and extending cycle life.

[0004] To achieve the above objectives, in a first aspect of this application, this application provides a secondary battery, including a positive electrode sheet and a separator, wherein the positive electrode sheet includes a positive active material layer, the positive active material includes a positive active material, and the positive active material includes a layered transition metal oxide. The secondary battery satisfies: a×b×c=0.06~5.3; Where a% is the mass percentage of free lithium in the positive electrode active material layer; b is the micro-stress of the positive electrode active material layer, in GPa; and c is the air permeability of the separator, in s / 100mL.

[0005] The beneficial effects of this application are as follows: This application provides a secondary battery that improves the structural and chemical stability of lithium nickel cobalt manganese oxide (LiCO) during lithium deintercalation by regulating the free lithium content and micro-stress in the positive electrode active material layer of the positive electrode sheet. At the same time, the permeability of the separator is controlled to ensure the lithium-ion transport efficiency, resulting in a secondary battery with low impedance and low DCR during fast charging, as well as excellent cycle stability, thus improving fast charging performance and extending cycle life. Detailed Implementation

[0006] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments in this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0007] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0008] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0009] The present application is further illustrated below with specific embodiments: A secondary battery includes a positive electrode sheet and a separator, wherein the positive electrode sheet includes a positive active material layer, the positive active material includes a positive active material, and the positive active material includes a layered transition metal oxide. The secondary battery satisfies: a×b×c=0.06~5.3; Where a% is the mass percentage of free lithium in the positive electrode active material layer; b is the micro-stress of the positive electrode active material layer, in GPa; and c is the air permeability of the separator, in s / 100mL.

[0010] Layered transition metal oxides, such as lithium nickel cobalt manganese oxide (LCM), offer high energy density when used as cathode materials. However, their chemical stability is low, and after being fabricated into cathode sheets, they readily react with carbon dioxide and water in the air. This leads to the deactivation of active lithium on the material surface and causes a concentration difference between the internal and external lithium phases of the material. Internal lithium migrates to the surface, ultimately increasing the surface impedance of the material and reducing the battery's fast-charging performance. It also causes structural damage to the material, weakening the battery's cycle life. Therefore, in this application, after using layered transition metal oxides as the cathode active material, the free lithium content (a%) of the cathode active material layer in the secondary battery is controlled. The higher the free lithium content, the more side reactions occur with the electrolyte, making it easier to form an unstable solid electrolyte interphase (CEI) film. Furthermore, it induces the continuous transfer of lithium ions from inside the particles to the particle surface. Surface lithium enrichment occupies delithiation sites, reducing vacancies inside the particles, hindering subsequent lithium ion insertion / extraction, and exacerbating polarization, resulting in a decrease in the fast-charging performance of the secondary battery. However, the free lithium content (a%) of the positive electrode active material layer is not necessarily better the lower it is. If the free lithium content is too low, it cannot compensate for the consumption of active lithium caused by the dynamic damage and regeneration of the SEI film during cycling, resulting in a decrease in the cycle life of the secondary battery. At the same time, when controlling the free lithium, the micro-stress of the positive electrode active material layer in the secondary battery described in this application also needs to be controlled synchronously. On the one hand, if the micro-stress of the positive electrode active material layer is too low, the layered transition metal oxide will lack sufficient compressive stress to offset the contraction stress when lithium is extracted, which can easily cause particle cracks, leading to electrolyte intrusion and dissolution of the transition metal, resulting in loss of positive electrode active material and structural damage and collapse, thus reducing the cycle performance of the secondary battery. On the other hand, if the micro-stress is too high, the number of microcracks inside the particles will increase, which can easily damage the conductive network and ion transport channels, hinder the diffusion of lithium ions, reduce kinetics, and reduce the fast charging performance of the secondary battery. In addition to the regulation of the positive electrode, the permeability of the separator also needs to be adjusted in order to balance the DCR performance and cycle life of the secondary battery. This improves the ion transport capacity of the separator and reduces side reactions between the positive and negative electrode interfaces and the electrolyte. Ultimately, based on the synergistic regulation of these three factors, the secondary battery can achieve excellent comprehensive electrochemical performance. It not only has excellent kinetic performance and low DCR under fast charging conditions, but also has good cycle stability and can achieve a long cycle life even under high temperature environments.

[0011] In some implementations, a×b×c = 0.06~5.3.

[0012] More preferably, the range of a×b×c is one or any two of the following: 0.06, 0.08, 0.1, 0.12, 0.15, 0.18, 0.2, 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.3.

[0013] More preferably, a×b×c=0.18~2.

[0014] In this application, by synergistically regulating a, b, and c, and further optimizing them within the above-mentioned range, not only can the stability of the layered transition metal oxide be effectively improved during cycling, but also the transport efficiency of lithium ions between the positive and negative electrodes and the separator can be guaranteed, thereby achieving better fast charging performance and longer cycle life.

[0015] In some implementations, a% = 0.2% to 0.65%.

[0016] More preferably, a% is a range of one or any two of the following: 0.2%, 0.22%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, and 0.65%.

[0017] More preferably, a% = 0.25%~0.55%.

[0018] In this application, as described above, the free lithium on the positive electrode active material layer mainly comes from the side reactions and self-decomposition between the layered transition metal oxide and the electrolyte. The higher the content of free lithium, the more side reactions or self-decomposition there are between the layered transition metal oxide and the electrolyte, and it will induce lithium ions inside the particles to be transferred to the surface, resulting in surface lithium enrichment. At this time, these lithium will occupy delithiation sites, leading to a reduction in vacancies inside the particles, hindering subsequent lithium ion intercalation and deintercalation, and exacerbating the polarization effect. However, if the content is too low, the active lithium consumed by the SEI film during dynamic damage / repair in the subsequent cycle process cannot be replenished in time, resulting in a decrease in the cycle performance of the battery. It is necessary to control the stress of the active material layer and the permeability of the separator to coordinate. When the battery meets the product limit of the above three factors, and the free lithium content of the positive electrode active material layer is preferably within the above range, it can not only effectively control the impedance of the electrode and improve the dynamic performance of the electrode, but also ensure that the lithium concentration difference inside and outside the material is small, and the stability during lithium intercalation and deintercalation is improved. Ultimately, it can achieve lower impedance, good fast charging performance, and further improve cycle stability.

[0019] It should be noted that, in the present application, the mass percentage of free lithium in the positive electrode active material layer can be controlled by the doping elements and their amounts in the positive electrode active material, but it is not limited to this. Those skilled in the art can also do so in other ways.

[0020] It should be noted that, in the technical solution of this application, the term 'a' can be confirmed by, but is not limited to, the following methods: The secondary battery is discharged at a rate of 0.33C to the lower voltage limit of 2.5V. The positive electrode is then disassembled, soaked in dimethyl carbonate (DMC) for 4 hours, and air-dried naturally. At 25°C, 5g of the positive electrode is cut and soaked in 50g of deionized water for 10 minutes. The soaking solution is then subjected to ICP testing: the testing instrument is ICP-thermoscientific iCAP PRO, with a radio frequency power of 1150W, and the testing method refers to the standard method of "Chemical Analysis Method for Lithium Nickel Cobalt Manganese Oxide" (YS / T1006.2-2014). After determining the content of Li element M1 and Ni element M2 in the soaking solution, 'a' is confirmed by calculating a = M1 - M2.

[0021] In some implementations, b = 0.005~0.03.

[0022] More preferably, b is a range of one or any two of the following: 0.005, 0.006, 0.008, 0.01, 0.012, 0.015, 0.02, 0.025, and 0.03.

[0023] More preferably, b = 0.008~0.02.

[0024] When the micro-stress of the positive electrode active material layer changes, the stability of its layered transition metal oxide particles during lithium intercalation / deintercalation also changes. Within a larger range, the anisotropic stress of the particles is high, and the material is prone to cracking due to thermodynamic instability during cycling. The increased number of microcracks in the particles leads to damage to the conductive network and ion transport channels, hindering lithium-ion diffusion, and reducing kinetic performance and fast-charging performance. Within a smaller range, the particles lack sufficient compressive stress to offset the contraction stress during lithium deintercalation, making them prone to particle cracks. In this case, electrolyte can invade the particle interior, easily triggering transition metal dissolution, which in turn leads to material loss and structural collapse, affecting cycle performance. It is necessary to coordinate the regulation of the free lithium content in the positive electrode and the permeability of the separator to ensure the battery's kinetic performance and cycle stability. When the battery meets the product of these three conditions, and is further preferably within the above-mentioned range, it is possible to maintain both the battery's kinetic performance and cycle stability at a higher level.

[0025] It should be noted that, in this application, the micro-stress of the positive electrode active material layer can be controlled by the calcination temperature, calcination time and heating rate in the sintering process of the positive electrode active material, but it is not limited to this. Those skilled in the art can also do so in other ways.

[0026] It should be noted that the b described in this application can be confirmed by, but is not limited to, the following methods: the secondary battery is discharged to the lower limit of 2.5V at a rate of 0.33C, and then the positive electrode is disassembled. The electrode is soaked in DMC for 4 hours, air-dried in a natural environment, and then cut into test samples with the positive electrode active material layer facing upwards. The sample is then placed in an X-ray powder diffractometer (XRD) for testing. The diffraction source is Cu-Kα line, the working voltage is set to 40kV, the working current to 40mA, the scanning speed to 1° / min, and the scanning range to 10°-90°. The obtained XRD curve is processed by smoothing, filtering, and background subtraction. The half-peak width β and diffraction angle θ of the diffraction peak corresponding to each crystal plane diffraction peak are calculated. Then, a linear fit is performed with 4sinθ as the abscissa and β×COSθ as the ordinate. The slope of the fitted line is the micro-stress b.

[0027] In some implementations, c = 50~300.

[0028] More preferably, c is a range of one or any two of the following: 50, 60, 80, 100, 150, 180, 200, 250, and 300.

[0029] More preferably, c = 80~200.

[0030] In addition to the positive electrode end, this application also simultaneously regulates the separator. When the separator's permeability (c) increases, side reactions at the electrolyte-positive / negative electrode interface are reduced, crosstalk of byproducts between the positive and negative electrodes is decreased, and cycle performance is improved. However, this also hinders electrolyte wetting of the separator, reduces lithium-ion transport channels, and decreases battery fast-charging performance. Therefore, it is necessary to simultaneously regulate the free lithium content of the positive electrode and the micro-stress of the positive electrode active material layer. When the product of these three factors meets the requirements of this application, and the separator's permeability is further optimized within the aforementioned range, high lithium-ion transport efficiency and low positive / negative electrode crosstalk can be effectively balanced, achieving better cycle stability and fast-charging performance.

[0031] It should be noted that the air permeability of the diaphragm described in this application can be controlled by the thickness of the diaphragm base film and the coating, but it is not limited to this. Those skilled in the art can also do so in other ways.

[0032] It should be noted that the method described in this application, c, can be confirmed by, but is not limited to, the following: The secondary battery is discharged, then the battery is disassembled and the separator is removed. It is then soaked in DMC for 10 minutes, allowed to air dry naturally, and subsequently tested using an air permeability tester at 25°C under a pressure of 1.21 kPa, with 100 mL of air passing through an area of ​​6.45 cm². 2 The time required for the diaphragm to be formed is expressed in seconds, which is the value of c.

[0033] More preferably, the layered transition metal oxide includes lithium nickel cobalt manganese oxide, wherein the XPS peak area ratio of divalent nickel ions to trivalent nickel ions in lithium nickel cobalt manganese oxide is 0.5~0.8.

[0034] In the layered phase, trivalent nickel is the dominant valence state, forming a stable layered structure with oxygen. Lithium ions can be reversibly inserted and extracted between the transition metal layers. When trivalent nickel is reduced to divalent nickel, the larger ionic radius exacerbates lattice distortion, triggering a transformation of the layered phase to the rock salt phase. When the XPS peak area ratio of divalent nickel to trivalent nickel in the lithium nickel cobalt manganese oxide active material in the positive electrode is preferably within the above range, the layered structure phase in lithium nickel cobalt manganese oxide is dominant, while the rock salt phase accounts for a smaller proportion. The material has higher stability during lithium insertion and extraction, which is more conducive to improving the cycle stability of the secondary battery.

[0035] It should be noted that the XPS peak area ratio of divalent nickel ions to trivalent nickel ions in the lithium nickel cobalt manganese oxide described in this application can be confirmed by, but is not limited to, the following method: Discharge the secondary battery to the lower limit voltage of 2.5V at 0.33C, remove the empty battery, disassemble the positive electrode, soak the positive electrode in DMC for 4 hours, allow it to air dry, and then use XPS testing (model - Thermo Scientifi K-Alpha, Al Kα X-ray monochromator, X-ray monochromator selectable 400 µm analysis area, analyzer mode: CAE, power: 50.0 eV, analysis energy step: 0.1 eV, number of energy analysis points: 301) for surface inspection and Ar etching to test Ni element. The test results are then subjected to XPS fitting analysis to obtain Ni... 2+ and Ni 3+ Characteristic peak area A Ni 2+ (Coordinates 853.7 eV) and A Ni 3+ (Coordinate 856.1 eV), then the XPS peak area ratio of divalent nickel ions to trivalent nickel ions in lithium nickel cobalt manganese oxide is ANi. 2+ / ANi 3+ .

[0036] In some embodiments, the molar percentage of nickel in the transition metal elements of the positive electrode active material is 70-95%.

[0037] In some embodiments, the molar percentage of nickel in the transition metal element of the positive electrode active material is one or any two of the following values: 70%, 72%, 75%, 78%, 80%, 82%, 85%, 88%, 90%, 92%, and 95%.

[0038] The nickel element in the transition metal elements of the positive electrode active material mainly comes from lithium nickel cobalt manganese oxide. The higher the nickel content, the higher the energy density of the material, but it is also more likely to react with air and increase the surface resistance of the material, ultimately reducing the fast charging performance of the secondary battery. When the nickel content in the active material layer is preferably within the above range, the fast charging performance and cycle performance of the secondary battery can be balanced to achieve better comprehensive electrochemical performance.

[0039] It should be noted that the molar percentage of nickel in the transition metal elements of the positive electrode active material described in this application can be confirmed by, but is not limited to, the following methods: The secondary battery is discharged, disassembled to obtain the positive electrode sheet, the positive electrode sheet is soaked in DMC for 240 min, naturally dried, and then the positive electrode active material layer is scraped off. The resulting powder is calcined at 400℃ for 3 h and ground to obtain the test powder; the powder is dispersed in 20 mL of water, then 10 mL of nitric acid is added and mixed evenly before heating. After the powder dissolves, the material is diluted with water to 100 mL to obtain the test solution; the test solution is subjected to ICP testing. Before the test, a 1000 mg / L standard solution needs to be diluted with deionized water to different concentrations (0, 1 mg / 100 mL, 2 ... (mg / 100mL, 3mg / 100mL), and selected the element detection spectrum wavelength, set the experimental conditions: gas flow rate 0.5L / min, power 1150W, selected nickel (231.60nm), cobalt (238.89nm), and manganese (257.61nm) test wavelengths. The ICP test software can read the mass content of nickel, cobalt and manganese in the sample by self-analysis function, and convert it into the molar percentage of nickel in the total molar of nickel, cobalt and manganese.

[0040] In some embodiments, the positive electrode active material layer also contains dopant elements.

[0041] More preferably, the doping element includes at least one selected from Zr, Y, Sr, Al, W, Nb, B, La, Mo, V, Ti, and F.

[0042] More preferably, the total mass percentage of doped elements in the positive electrode active material layer is 300~10000ppm.

[0043] By doping with the above-mentioned doping elements in optimal amounts, the crystal structure stability of lithium nickel cobalt manganese oxide in the positive electrode active material layer can be effectively improved, the structural deterioration of lithium nickel cobalt manganese oxide caused by stress or chemical side reactions during lithium deintercalation can be reduced, and the overall cycle stability of the secondary battery can be improved, while ensuring the high kinetic performance of the material.

[0044] It should be noted that the content of doped elements in the positive electrode active material layer described in this application can be confirmed by, but is not limited to, the following methods: The secondary battery is discharged, and the positive electrode sheet is obtained by disassembly. The positive electrode sheet is soaked in DMC for 240 minutes, air-dried naturally, and then the positive electrode active material layer is scraped off. The resulting powder is calcined at 400℃ for 3 hours and ground to obtain the powder to be tested. The powder is dispersed in 20 mL of water, and then 10 mL of aqua regia is added and mixed evenly. After heating, the powder is dissolved, and the material is diluted with water to 100 mL to obtain the test solution. The test solution is then subjected to ICP testing. Before the test, a 1000 mg / L standard solution needs to be diluted with deionized water to different concentrations (0, 1 mg / 100 mL, 2 ... (mg / 100mL, 3mg / 100mL), and select the element detection spectrum wavelength, set the experimental conditions: gas flow rate 0.5L / min, power 1150W, select the element test wavelengths of Zr, Y, Sr, Al, W, Nb, B, La, Mo, V, Ti, and F. The content of doped elements in the sample can be read by the self-analysis function of the ICP test software, which is the content of doped elements in the positive electrode active material layer.

[0045] In some embodiments, the cumulative volume distribution particle size D of the positive electrode active material v50 The value is 2~10μm.

[0046] Specifically, the cumulative volume distribution particle size D of the positive electrode active material v50 The range is one or any two of the following: 2μm, 2.5μm, 4μm, 5μm, 6μm, 8μm, 9μm, and 10μm.

[0047] In the positive electrode active material layer, the effect of lithium intercalation / deintercalation varies depending on the particle size of the positive electrode active material. Within a smaller range, the particles have a larger specific surface area and a larger wetting area when in contact with the electrolyte, resulting in a higher probability of side reactions. As the particle size gradually increases, the probability of side reactions between the particles and the electrolyte decreases, while the lithium ion transport path gradually increases. When the average particle size of the positive electrode active material is preferably within the above range, it can balance chemical stability and lithium ion transport rate, achieving better kinetic performance and lithium intercalation / deintercalation stability.

[0048] It should be noted that the cumulative volume distribution particle size D of the positive electrode active material described in this application... v50The following methods can be used for confirmation, but are not limited to: discharging the secondary battery to 2.5V at 0.33C, disassembling the secondary battery to obtain the positive electrode, immersing it in dimethyl carbonate (DMC) solution at room temperature for 4 hours, removing the positive electrode and drying it in a vacuum environment, scraping off the active material powder from the electrode surface with a ceramic knife; using a laser particle size distribution measuring instrument (Mastersizer 3000), measuring the particle size distribution according to the particle size distribution laser diffraction method (specific steps refer to GB / T19077-2016), the particle size corresponding to the cumulative particle size distribution percentage reaching 50% is D. v50 .

[0049] In some embodiments, the layered transition metal oxide includes lithium nickel cobalt manganese oxide, wherein the chemical formula of lithium nickel cobalt manganese oxide is LiNi. a Co b Mn c O2, where a is greater than 0 and less than 1; b is greater than 0 and less than 1; c is greater than 0 and less than 1; a + b + c = 1.

[0050] In some embodiments, the lithium nickel cobalt manganese oxide can be a commercially available product or can be obtained by a self-made method. Specifically, the lithium nickel cobalt manganese oxide can be obtained by the following preparation method: Nickel, cobalt, and manganese sources are mixed in a solvent, a precipitant is added to precipitate the reaction, the mixture is allowed to stand, filtered, washed, and dried. The resulting mixed precursor is then mixed with a lithium source and oxides of doped elements, ground, calcined, and sieved to obtain the lithium nickel cobalt manganese oxide.

[0051] In some embodiments, the nickel source includes at least one of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate; In some embodiments, the cobalt source used includes at least one of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate, and cobalt acetate; In some embodiments, the manganese source includes at least one of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate; In some embodiments, the lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium nitrate, lithium oxalate, lithium dihydrogen phosphate, lithium citrate, and lithium acetate. In some embodiments, the molar ratio of lithium atoms in the lithium source to the total atoms of nickel, cobalt, and manganese in the mixed precursor is 1.02 to 1.08.

[0052] As mentioned above, in addition to adjusting the type and content of doping elements, the free lithium content of the positive electrode sheet described in this application can also be controlled by adjusting the molar ratio of lithium to transition metal elements during the preparation of the positive electrode active material. The larger the molar ratio, the larger the free lithium content of the positive electrode sheet. However, it is not limited to this. Those skilled in the art can also control it in other ways.

[0053] In some embodiments, the precipitant includes at least one of sodium hydroxide, sodium carbonate, oxalic acid, and ammonia.

[0054] In some embodiments, the solvent includes water.

[0055] In some embodiments, the precipitation reaction takes 0.5 to 1.5 hours.

[0056] In some embodiments, the calcination includes a primary calcination and a secondary calcination, and the calcination is carried out in an air atmosphere.

[0057] In some embodiments, the heating rate during the first calcination is 4~8℃ / min, the calcination temperature is 450~550℃, and the time is 4~8h.

[0058] In some embodiments, the heating rate during the secondary calcination is 4~8℃ / min, the calcination temperature is 750~1000℃, and the time is 12~18h.

[0059] In some embodiments, the oxide of the doping element includes at least one of zirconium oxide, yttrium oxide, niobium oxide, and aluminum oxide, and the amount of the oxide of the doping element added is 500 to 2000 ppm based on the mixture of the precursor and the lithium source.

[0060] In some embodiments, the preparation method of the lithium nickel cobalt manganese oxide can also be carried out as follows: a nickel source, a cobalt source and a manganese source are mixed in a solvent, a precipitant is added to precipitate the reaction, the mixture is allowed to stand, filtered, washed and dried, and the resulting mixed precursor is mixed with a lithium source and an oxide of the dopant element, ground and sieved, and the resulting calcined powder is mixed with a coating agent and sintered to obtain the lithium nickel cobalt manganese oxide.

[0061] In some embodiments, the oxide of the doping element includes at least one of zirconium oxide, strontium oxide, and tungsten oxide.

[0062] In some embodiments, the coating agent includes at least one of cobalt oxide and aluminum oxide, and the coating agent is 500 to 10,000 ppm based on a mixture of calcined powder and coating agent.

[0063] In some embodiments, the heating rate during sintering is 4~8℃ / min, the temperature is 500~750℃, and the time is 4~8h.

[0064] In some embodiments, the positive electrode active material layer in the positive electrode sheet includes a positive electrode active material, a binder, and a conductive agent. The positive electrode active material includes lithium nickel cobalt manganese oxide, and the mass percentage of the positive electrode material in the positive electrode material layer is 92-99%.

[0065] In some embodiments, the binder is used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Any binder can be used without particular limitation, as long as it has suitable adhesion and does not significantly cause adverse chemical changes in the battery. For example, the binder includes fluorinated polyolefin binders, including but not limited to polyvinylidene fluoride (PVDF), PVDF copolymers, or their modified derivatives (e.g., modified with carboxylic acids, acrylic acid, acrylonitrile, etc.).

[0066] Specifically, the adhesive is selected from polytetrafluoroethylene or polyvinylidene fluoride.

[0067] In some embodiments, the mass percentage of the binder in the positive electrode active material layer is 1% to 4.0%, such as 1%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, or any range formed by any two of the above values.

[0068] In some embodiments, the conductive agent is used to provide conductivity. Any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. Exemplary examples of conductive agents in the positive electrode active material layer include, but are not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes, wherein carbon fibers are, for example, carbon nanofibers; and carbon black is, for example, SP (Super P), acetylene black, Ketjen black, etc.

[0069] In some embodiments, the mass percentage of the conductive agent in the positive electrode active material layer is 1.0% to 4.0%, such as 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.5%, 3%, 3.5%, 4%, or any range formed by any two of the above values.

[0070] In some embodiments, the positive electrode sheet can be prepared by, but is not limited to, the following methods: The positive electrode active material, conductive agent, and binder are mixed in a solvent and then stirred to prepare a slurry. The slurry is coated onto a current collector in one or two layers, dried, rolled, and cut to obtain the positive electrode sheet.

[0071] In some embodiments, the solvent includes N-methylpyrrolidone (NMP).

[0072] In some embodiments, the compaction density of the positive electrode sheet is 3.4~3.6 g / cm³. 3 .

[0073] It should be noted that the compaction density of the positive electrode sheet described in this application can be confirmed by, but is not limited to, the following methods: The pretreated positive electrode sheet is punched into circular pieces of a fixed area using a punching machine. The area is denoted as S0, and the unit is mm. 2 To ensure testing accuracy, select a flat location in the middle of the electrode sheet and take three or more circular pieces as parallel samples. Weigh each of the three circular pieces using an electronic balance, and record the mass of each circular piece as M1 (in grams). Measure the thickness of the active material layer (after removing the current collector) in each of the three circular pieces using a micrometer, and record it as H (in mm). Take the average value. Finally, add an appropriate amount of deionized water to each of the three circular pieces, gently wipe off the coating with lint-free paper to expose the current collector, and let them stand (dry) at room temperature for 10 minutes. After the current collector is dry, weigh each of the three current collectors and record it as M0. Take the average value and calculate the coating compaction density A using the following formula: A = (M1 - M0) / (H * S0).

[0074] In some embodiments, the diaphragm includes a basement membrane.

[0075] More preferably, the base film includes at least one of a polypropylene base film and a polyethylene base film.

[0076] In some embodiments, the thickness of the base membrane in the diaphragm accounts for 50-80%.

[0077] The type of base membrane described in this application is not specifically limited, as long as it can achieve the basic support strength required for secondary battery separators, it is acceptable.

[0078] As a supporting structure, if the thickness of the base film is too small, it will reduce the difficulty of material transport between the positive and negative electrodes, increase the shuttle frequency of by-products, and make it easier to cause crosstalk between the positive and negative electrodes. If it is too large, it will increase the transport path of lithium ions, reduce the transport efficiency, and reduce the kinetic performance. Therefore, when the thickness ratio is set as described above, the overall performance of the secondary battery is better.

[0079] In some embodiments, the diaphragm is further provided with a coating, the coating being disposed on at least one side of the diaphragm, the coating comprising an organic coating containing at least one of aramid and polyvinylidene fluoride.

[0080] More preferably, the organic coating comprises an aramid coating, wherein the thickness of the aramid coating on one side of the diaphragm is 1~4μm.

[0081] When an aramid coating is preferably applied to the separator, the polar functional groups in the aramid and the polar solvent of the electrolyte have strong intermolecular forces, which can effectively improve the wettability of the electrolyte in the separator, improve the lithium-ion transport efficiency, and reduce the DCR of the secondary battery.

[0082] In some embodiments, the thickness of the diaphragm is 12~40 μm.

[0083] In some embodiments, the total thickness of the coating on the diaphragm is 2 to 8 μm.

[0084] In some embodiments, the diaphragm can be a commercially available product, or it can be prepared by, but is not limited to, the following methods: constructing a base membrane by melt extrusion, applying a coating to the base membrane and performing heat treatment and stretching to obtain the diaphragm; or it can be prepared by the following methods: mixing base membrane preparation raw materials, then adding a pore-forming agent, melt extruding and casting, then performing longitudinal and transverse stretching, extracting and separating the pore-forming agent, shaping, and cutting to obtain the diaphragm.

[0085] In some embodiments, the secondary battery further includes an electrolyte.

[0086] More preferably, the pH of the electrolyte is 4.5 to 6.5.

[0087] In this application, when the preferred acidic electrolyte is selected, it can react with alkaline free lithium after contacting the positive electrode active material layer. At the same time, the lithium fluoride generated by the reaction will not be deposited on the surface of the positive electrode active material layer and block the lithium ion transport channels. This improves the cycle stability of the secondary battery and ensures good kinetic performance, resulting in a low DCR of the battery.

[0088] It should be noted that the pH of the electrolyte can be confirmed by, but is not limited to, the following methods: The secondary battery was discharged at a rate of 0.33C to the lower limit of 2.5V for venting treatment. Then, the battery was disassembled and the electrolyte collected in a glove box (H2O≤0.1ppm, O2≤0.1ppm). There are three methods for collecting the electrolyte: 1) After removing the battery cover, if there is free electrolyte, collect it with a pipette into a 5mL sample tube and seal it with adhesive tape to prevent leakage; 2) If there is no free electrolyte, use a hydraulic press to continuously pressurize until free electrolyte appears, collect it into a sample tube, and seal it; 3) Add an appropriate amount of dichloromethane extractant to the battery and record the dichloromethane content. After adding dichloromethane, place the battery in an aluminum-plastic bag and seal it with a heat sealer. Transfer it to an ultrasonic vibrator and vibrate for 12 hours to allow the electrolyte and dichloromethane in the electrodes to mix thoroughly. Then, use a pipette to draw the mixture of dichloromethane and electrolyte into a 5mL sample tube and seal the sample tube with adhesive tape. The test method for the obtained electrolyte was carried out in accordance with the free acid content detection method in GB / T 19282-2014, and the pH of the electrolyte was calculated.

[0089] In some embodiments, the electrolyte comprises a lithium salt and a solvent.

[0090] In some embodiments, the solvent includes at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents.

[0091] Exemplary examples include, but are not limited to, at least one of propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), and ethyl methyl carbonate (EMC); carboxylic acid ester solvents include, but are not limited to, at least one of ethyl acetate, methyl formate, and 1,4-butyrolactone; ether solvents include, at least one of dimethyltetrahydrofuran, tetrahydrofuran, and 1,2-dimethoxyethane; sulfone solvents include, at least one of methyl sulfone and dimethyl sulfoxide; nitrile solvents include, at least one of propionitrile, butyronitrile, 1-(2-cyanoethyl)pyrrole, and 1,3,6-hexanetrionitrile; and phosphate ester solvents include, at least one of trimethyl triphosphate and triethyl phosphate.

[0092] More preferably, the solvent includes EC, DMC, and EMC, with a volume ratio of (1~3):(1~3):(1~4).

[0093] When the aforementioned low-viscosity components are selected as solvents and their volume ratios are optimized within the above range, the fluidity and wettability of the electrolyte can be effectively improved, the lithium-ion transport efficiency can be enhanced, and ultimately the kinetic performance of the battery can be improved, while reducing DCR.

[0094] In some embodiments, the lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium dioxalate borate, lithium difluorooxalate borate, lithium trifluoromethanesulfonate, lithium difluoromethanesulfonylimide, lithium ditrifluoromethanesulfonylimide, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

[0095] More preferably, the lithium salt comprises lithium hexafluorophosphate; More preferably, the lithium salt has a mass content of 8-15% in the electrolyte.

[0096] When the electrolyte has the preferred lithium salt content, not only can the ion conductivity of the electrolyte be guaranteed, but also the problem of poor wettability and obstructed lithium ion transport caused by excessively high electrolyte viscosity can be avoided, and the DCR of the secondary battery can be kept at a low level.

[0097] In some embodiments, the electrolyte further includes additives, including but not limited to at least one of fluoroethylene carbonate (FEC), vinylene carbonate (VC), and chloroethylene carbonate (CEC).

[0098] More preferably, the mass content of the additive in the electrolyte is 1-5%.

[0099] When using a lithium nickel cobalt manganese oxide system for the positive electrode, selecting an electrolyte containing the aforementioned additives can reduce the probability of side reactions between the electrolyte and the electrode, effectively balancing the cycle performance and fast charging performance of the secondary battery.

[0100] In some embodiments, the porosity of the negative electrode sheet is 20-40%.

[0101] Besides the positive electrode and the separator, the porosity of the negative electrode also affects the electrochemical performance of the battery. When the porosity of the negative electrode is preferably within the above range, it can not only avoid the excessive contact area between the negative electrode and the electrolyte due to excessive porosity, which would lead to an increase in side reactions, but also avoid the reduction of lithium ion insertion / extraction sites due to excessive porosity, which would hinder lithium ion transport and increase impedance. Ultimately, this balances the battery's fast charging performance and cycle stability.

[0102] It should be noted that the porosity of the negative electrode sheet described in this application can be confirmed by, but is not limited to, the following method: The negative electrode sheet is cut into a circular piece with a diameter D = 12 mm, and the thickness of the electrode sheet and the current collector is measured using a thickness gauge and recorded as h1 and h2 respectively. The porosity is then determined using V1 = πR. 2Calculate the volume V1 of the negative electrode active material layer (h1-h2), then weigh the electrode and record the mass as m1. Next, immerse the electrode completely in a sealed container of hexadecane for 1 hour (the volume of hexadecane in the sealed container is not critical, but the amount must be sufficient to completely submerge the electrode). Remove the electrode and dry it with filter paper until a constant weight is achieved (generally after 1 hour). Weigh the electrode and record the weight as m2. Calculate the porosity of the negative electrode using the formula porosity% = (m2-m1 / ρ) / V1 × 100%, where ρ is the density of hexadecane, 0.7734 g / cm³. 3 .

[0103] In some embodiments, the negative electrode sheet includes a negative electrode active material layer, which includes a negative electrode active material, including at least one of natural graphite, artificial graphite, needle coke, mesophase carbon microspheres, hard carbon, soft carbon, elemental silicon, silicon suboxide, silicon-carbon composite material, and lithium titanate.

[0104] In some embodiments, the cumulative volume distribution particle size D of the negative electrode active material v50 The value is 8~20μm.

[0105] The negative electrode active material layer may also contain conductive agents and / or binders.

[0106] The conductive agent in the negative electrode active material layer is used to provide conductivity. Any conductive agent can be used without particular limitation, as long as it has suitable electronic conductivity and does not significantly cause adverse chemical changes in the battery. For example, the conductive agent includes, but is not limited to, at least one of carbon nanotubes, carbon black, graphite, carbon fibers, activated carbon, mesoporous carbon, and fullerenes, wherein carbon fibers are, for example, carbon nanofibers; and carbon black is, for example, SP, acetylene black, Ketjen black, etc.

[0107] In some embodiments, the mass percentage of the conductive agent in the negative electrode active material layer is 0.4% to 2%, such as 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, or any range formed by any two of the above values.

[0108] The binder in the negative electrode active material layer is used to improve the adhesion between negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector. Any binder can be used without particular limitation, as long as it has suitable adhesion and does not significantly cause adverse chemical changes in the battery. For example, the binder includes, but is not limited to, at least one of carboxymethyl cellulose (CMC), styrene-butadiene rubber, polyvinylidene fluoride, polytetrafluoroethylene, polyvinyl butyral, and aqueous acrylic resin.

[0109] In some embodiments, the mass percentage of the binder in the negative electrode active material layer is 1.0% to 4.5%, such as 1.0%, 1.2%, 1.5%, 1.8%, 2.0%, 2.2%, 2.5%, 2.7%, 3.0%, 3.2%, 3.5%, 3.8%, 4.0%, 4.2%, 4.5%, or any range formed by two of the above values. In some embodiments, the negative electrode active material layer in the negative electrode sheet includes a negative electrode material, a binder, and a conductive agent, and the mass percentage of the negative electrode active material in the negative electrode active material is 70% to 99%.

[0110] In some embodiments, the compaction density of the negative electrode sheet is 1.4~1.6 g / m³. 3 The surface density is 90~120 g / m³ 2 .

[0111] In some embodiments, the secondary battery is a wound cell structure, and the secondary battery satisfies: a×b×c=1.5~5.3.

[0112] When the secondary battery adopts a wound cell structure, the positive and negative electrode plates in the cell have a higher contact density. In order to ensure the lithium ion transmission efficiency, the relationship between the free lithium of the positive electrode plate, the micro-stress of the positive electrode active material layer, and the air permeability of the separator is preferably controlled to a larger value range as mentioned above. The secondary battery can achieve better fast charging performance.

[0113] More preferably, the battery cell is a cylindrical battery cell, and the diameter of the cylinder is ≥20cm and the height is ≥70cm; In some embodiments, the secondary battery includes a cell and a casing; More preferably, the housing comprises an aluminum-plastic film or a metal shell; More preferably, the metal shell may be, but is not limited to, at least one of aluminum or its alloy shell, steel shell, titanium or its alloy shell.

[0114] In this application, there are no special restrictions on the specific type of metal casing used for the secondary battery. For example, the metal casing can be an aluminum casing, an aluminum alloy casing, a titanium casing, or a titanium alloy casing. The metal casing can also be a steel casing, specifically a stainless steel casing, a nickel-plated steel casing, a carbon steel casing, etc., depending on the actual needs.

[0115] The present invention is further illustrated below with specific embodiments, which should not be construed as limiting the scope of protection claimed by the present invention: Example 1 A secondary battery, the preparation method comprising the following steps: (1) Preparation of lithium nickel cobalt manganese oxide: Nickel sulfate solution, cobalt sulfate solution and manganese sulfate solution were mixed in stoichiometric ratio, and then sodium hydroxide precipitant was added. The pH was adjusted to be within the range of 12.1 and the temperature was 55℃ for 1h. After filtration, washing and drying, the precursor obtained was mixed with lithium carbonate in a molar ratio of lithium atoms to the total atoms of nickel, cobalt and manganese in the precursor of 1.05:1. Then aluminum oxide (Al mass percentage in the positive electrode active material layer is 5000ppm) was added and mixed. The mixture was ground and calcined once at 5℃ / min to 500℃ for 6h in air atmosphere, and then calcined again at 5℃ / min to 850℃ for 15h. The mixture was then sieved to obtain lithium nickel cobalt manganese oxide particles. (2) Preparation of the positive electrode sheet: Lithium nickel cobalt manganese oxide particles are used as the positive electrode material. Then, the positive electrode material, conductive agent acetylene black, and binder polyvinylidene fluoride are dispersed in N-methylpyrrolidone at a mass ratio of 98:1:1. The mixture is vacuum stirred to prepare a slurry, which is then coated on both sides of the current collector aluminum foil. After drying, cold pressing, slitting, and rolling, the positive electrode sheet is obtained. The areal density of the positive electrode sheet is 350 g / m³. 2 The compacted density is 3.6 g / cm³. 3 ; (2) Preparation of negative electrode sheet: After graphitizing needle-shaped coke at 3000℃ for 8 hours, it was crushed into artificial graphite with a particle size Dv50 of 10μm. Conductive agent SP, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber were dispersed in water at a mass ratio of 96.4:1:1.2:1.4. The mixture was vacuum stirred to prepare a slurry, which was then coated onto the current collector copper foil. After coating, drying, cold pressing, slitting, and rolling, the negative electrode sheet was obtained. The areal density of the negative electrode sheet was 185g / m³. 2 The compacted density is 1.55 g / cm³. 3 ; (3) Preparation of electrolyte: EC, EMC and DEC are mixed in a volume ratio of 1:1:1 as solvent. Then, based on the total mass of the electrolyte, lithium hexafluorophosphate and additive 2,2,2-trifluoroethyl methyl carbonate are added to prepare an electrolyte with a lithium hexafluorophosphate concentration of 12wt%. (4) Preparation of the diaphragm: Aramid (poly(m-phenylene isophthalamide)) (8% of the total amount added by mass), alumina (86%) with a particle size of 0.54 μm, acrylate polymer emulsion (4%) and CMC (1.5%) were added to solvent DMAC (dimethylacetamide) and dispersed at high speed for 3.5 h (dispersion speed 2500 rpm). Polyvinylpyrrolidone dispersant (0.5%) was added and dispersed at low speed for another 1.5 h (1000 rpm) to obtain aramid coating liquid with a viscosity of 200 cps (25℃) and a solid content of 30%.

[0116] Aramid coating solution was coated on both sides of an 8μm thick PE base film at a coating speed of 20 m / min and a drying temperature of 90℃ to obtain an aramid diaphragm. The thickness of the aramid coating on one side was 2.2μm. (5) The positive electrode, separator and negative electrode are wound and assembled into a cell in sequence. The cell is placed in the outer packaging shell, dried and injected with electrolyte. After vacuum sealing, standing, formation and volume adjustment, the lithium-ion secondary battery is obtained. The parameters of the battery during preparation are shown in Table 1 and the parameters of the finished product are shown in Table 2.

[0117] Examples 2-22, Comparative Examples 1-4 A secondary battery differs from Example 1 only in the parameters of the manufacturing process and the finished product parameters.

[0118] Table 1 Continued from Table 1 Table 2 Continued from Table 2 Continued from Table 2 The lithium-ion batteries obtained in each embodiment and comparative example were tested as follows: (1) Loop test: (I) The secondary batteries obtained in each embodiment and comparative example were pre-charged at 25°C with a constant current of 1C to 4.25V, and then charged with a constant voltage to the cutoff current of 0.05C. (II) Let stand for 10 minutes; (III) Discharge to 2.5V at a 1C rate; (IV) Let stand for 10 minutes; The above four steps constitute one cycle. Using the discharge capacity of the first cycle as the initial capacity, repeat the above four steps until the battery capacity is ≤ 80% of the initial capacity, and record the number of cycles. (2) DCR test: The secondary battery was charged at 25°C with a constant current and constant voltage of 0.33C to 4.25V, with a cutoff current of 0.05C. After resting for 10 minutes, it was discharged at a constant current of 0.33C to 2.5V. This cycle was repeated 3 times, and the discharge capacity C1 was recorded. After resting for 5 minutes, it was charged at a constant current and constant voltage of 0.33C to 4.25V. After discharging to 50% of the discharge capacity of the third cycle, it was discharged at a 1C rate for 18 seconds with a sampling interval of 0.1 seconds. The battery voltage V1 and current I1 before the discharge stopped, and the voltage V2 after the discharge voltage stabilized were recorded. Then, the DCR of the battery is calculated as |V2-V1| / I1.

[0119] The test results are shown in Table 3.

[0120] Table 3 As can be seen from Table 3: This application's technical solution uses layered transition metal oxides as the positive electrode active material. Simultaneously, it regulates the content of free lithium in the positive electrode sheet and the micro-stress of the positive electrode active material layer. This effectively suppresses side reactions between the positive electrode sheet and the electrolyte, reducing polarization effects. Furthermore, it does not excessively affect the dynamic damage of the SEI film during cycling or the consumption of active lithium during regeneration. During cycling, the positive electrode active material layer can effectively counteract the stress generated during lithium insertion / extraction, resulting in high particle integrity and high ion diffusion efficiency. Additionally, it further synergistically regulates the permeability of the separator, enhancing its ion transport capacity and reducing the degree of interfacial side reactions. Ultimately, the a×b×c ratio is controlled within the range of 0.06~5.3, enabling the secondary battery to achieve both superior fast-charging and cycling performance. During testing, the maximum DCR does not exceed 50mΩ, while the stability cycle count can reach over 300 cycles.

[0121] As can be seen from the various embodiments, by synergistically regulating the free lithium content (a%) of the positive electrode sheet, the micro-stress (b) of the positive electrode active material layer, and the permeability (c) of the separator, and further optimizing the values ​​within the range of a×b×c=0.18~2, not only can the stability of the layered transition metal oxide during cycling be effectively improved, but the transport efficiency of lithium ions between the positive and negative electrodes and the separator can also be guaranteed, thereby achieving better fast charging performance and longer cycle life.

[0122] Furthermore, a higher content of free lithium on the positive electrode indicates more side reactions or self-decomposition between the layered transition metal oxide and the electrolyte. This induces lithium ions inside the particles to transfer to the surface, resulting in surface lithium enrichment, occupying delithiation sites, reducing vacancies inside the particles, hindering subsequent lithium ion intercalation / deintercalation, and exacerbating the polarization effect. However, if the content is too low, the active lithium consumed by the SEI film during dynamic damage / repair in subsequent cycles cannot be replenished in time, leading to a decrease in battery cycle performance. When simultaneously controlling the a×b×c range, further optimizing a% to be in the range of 0.25~0.55% can not only effectively control the impedance of the electrode and improve its kinetic performance, but also ensure a small lithium concentration difference inside and outside the material, improving stability during lithium intercalation / deintercalation. Ultimately, this achieves lower impedance, good fast-charging performance, and further improved cycle stability.

[0123] Meanwhile, when the micro-stress 'b' of the positive electrode active material layer changes, the stability of its layered transition metal oxide particles during lithium intercalation / deintercalation also changes. Within a larger range, the anisotropic stress of the particles is high, and the material is prone to cracking due to thermodynamic instability during cycling. The number of microcracks in the particles increases, leading to damage to the conductive network and ion transport channels, hindering lithium-ion diffusion, and reducing kinetic performance and fast-charging performance. Within a smaller range, the particles lack sufficient compressive stress to offset the contraction stress during lithium deintercalation, making them prone to particle cracks. In this case, the electrolyte can invade the particles, easily causing the dissolution of transition metals, which in turn leads to material loss and structural collapse, affecting cycle performance. Therefore, when 'b' is preferably in the range of 0.008~0.02 GPa, both the kinetic performance and cycle stability of the battery can be maintained at a higher level.

[0124] Finally, in addition to the positive electrode end, the separator is synchronously regulated to be optimally 80~200s / 100mL, which can effectively balance high lithium-ion transport efficiency and low positive and negative electrode crosstalk, achieving better cycle stability and fast charging performance.

Claims

1. A secondary battery, characterized in that, It includes a positive electrode sheet and a separator, wherein the positive electrode sheet includes a positive electrode active material layer, the positive electrode active material includes a positive electrode active material, and the positive electrode active material includes a layered transition metal oxide; The secondary battery satisfies: a×b×c=0.06~5.3; Where a% is the mass percentage of free lithium in the positive electrode active material layer; b is the micro-stress of the positive electrode active material layer, in GPa; and c is the air permeability of the separator, in s / 100mL.

2. The secondary battery as described in claim 1, characterized in that, The a×b×c = 0.18~2.

3. The secondary battery as described in claim 1, characterized in that, a% = 0.2%~0.65%, and / or b = 0.005~0.03%, and / or c = 50~300.

4. The secondary battery as described in claim 1, characterized in that, The layered transition metal oxide includes lithium nickel cobalt manganese oxide, wherein the XPS peak area ratio of divalent nickel ions to trivalent nickel ions in the lithium nickel cobalt manganese oxide is 0.5~0.

8.

5. The secondary battery as described in claim 1, characterized in that, The molar percentage of nickel in the transition metal elements of the positive electrode active material is 70-95%.

6. The secondary battery as described in claim 1, characterized in that, The positive electrode active material layer also contains doping elements; the doping elements include at least one of Zr, Y, Sr, Al, W, Nb, B, La, Mo, V, Ti, and F; the total mass percentage of the doping elements in the positive electrode active material layer is 300~10000ppm.

7. The secondary battery as described in claim 1, characterized in that, The cumulative volume distribution particle size D of the positive electrode active material v50 The value is 2~10μm.

8. The secondary battery as described in claim 1, characterized in that, The diaphragm includes a base membrane; the base membrane includes at least one of a polypropylene base membrane and a polyethylene base membrane; the thickness of the base membrane in the diaphragm accounts for 50-80%.

9. The secondary battery as described in claim 1, characterized in that, The diaphragm is further provided with a coating, the coating including an organic coating containing at least one of aramid and polyvinylidene fluoride.

10. The secondary battery as described in claim 9, characterized in that, The organic coating includes an aramid coating, the thickness of which is 1~4μm on one side of the diaphragm.

11. The secondary battery as described in claim 1, characterized in that, The secondary battery also includes an electrolyte; the pH of the electrolyte is 4.5 to 6.

5.

12. The secondary battery as described in claim 11, characterized in that, The electrolyte comprises lithium salt and solvent.

13. The secondary battery as described in claim 12, characterized in that, The solvent includes at least one of carbonate solvents, carboxylic acid ester solvents, ether solvents, sulfone solvents, nitrile solvents, and phosphate ester solvents.

14. The secondary battery as described in claim 13, characterized in that, The solvents include EC, DMC, and EMC.

15. The secondary battery as described in claim 12, characterized in that, The lithium salt includes at least one of lithium hexafluorophosphate, lithium hexafluoroarsenate, lithium perchlorate, lithium tetrafluoroborate, lithium dioxalate borate, lithium difluorooxalate borate, lithium trifluoromethanesulfonate, lithium difluoromethanesulfonylimide, lithium ditrifluoromethanesulfonylimide, lithium difluorodioxalate phosphate, and lithium tetrafluorooxalate phosphate.

16. The secondary battery as described in claim 15, characterized in that, More preferably, the lithium salt comprises lithium hexafluorophosphate; the mass content of the lithium salt in the electrolyte is 8-15%.

17. The secondary battery as described in claim 11, characterized in that, The electrolyte also includes additives, including at least one of fluoroethylene carbonate, vinylene carbonate, and chloroethylene carbonate.

18. The secondary battery as described in claim 12, characterized in that, The mass content of the additive in the electrolyte is 1-5%.

19. The secondary battery as described in claim 1, characterized in that, The secondary battery also includes a negative electrode sheet, the porosity of which is 20-40%.

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