A secondary battery

Abstract

This invention discloses a secondary battery, belonging to the field of battery technology. By simultaneously controlling the tortuosity of the positive electrode sheet, the mass change rate of the separator after immersion in the mixed solution, and the ratio of the total amount of B and W elements in the positive electrode sheet to the particle size of the positive electrode material, the bonding force between the separator and the electrode sheet is improved, the interfacial impedance between them is reduced, and the pores of the separator can be fully utilized. The electrolyte has a high degree of wetting, which improves the lithium transport kinetics and stability, improves the coulombic efficiency of the battery, and ultimately achieves better cycle performance and rate performance.

Description

Technical Field

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

[0002] Existing conventional ternary or lithium iron phosphate secondary batteries, after fabrication (usually after formation), suffer from poor stability, high lithium-ion transport impedance, and low transport efficiency if the adhesion between the separator and the electrode is low. This results in low initial efficiency and makes it difficult to achieve breakthroughs in cycle stability. Furthermore, the battery's discharge performance at high rates is also difficult to meet requirements. However, if the adhesion between the separator and the electrode is designed to be too high, it may lead to the risk of blockage of the separator's transport pores and reduce the wettability of the electrolyte, which is also detrimental to the electrochemical performance of the secondary battery. 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 simultaneously controlling the tortuosity of the positive electrode, the mass change rate of the separator after immersion in the mixed solution, and the ratio of the total amount of B and W elements in the positive electrode to the particle size of the positive electrode material, the bonding force between the separator and the electrode is improved, the interfacial impedance between them is reduced, and the pores of the separator can be fully utilized. The electrolyte has a high degree of wetting, which improves the lithium transport kinetics and stability, improves the coulombic efficiency of the battery, and ultimately achieves better cycle performance and rate performance.

[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 electrode active material layer, the positive electrode active material layer includes a positive electrode material, the positive electrode material includes a nickel-cobalt-manganese-based material, and the positive electrode active material layer further contains at least one of element B and element W.

[0005] The secondary battery satisfies (b×a×10000) / c=0.1~1400μm / ppm;

[0006] a is the tortuosity of the positive electrode sheet, b is the mass change rate of the diaphragm after soaking in the mixed solution for 4 hours, c = c1 / c2, where c1 is the total mass content of B and W elements in the positive electrode active material layer in ppm, and c2 is the average particle size of the nickel-cobalt-manganese-based material in μm.

[0007] The beneficial effects of this application are as follows:

[0008] This application provides a secondary battery that improves the bonding force between the separator and the electrode by simultaneously controlling the tortuosity of the positive electrode sheet, the mass change rate of the separator after immersion in the mixed solution, and the ratio of the total amount of B and W elements in the positive electrode sheet to the particle size of the positive electrode material. This reduces the interfacial impedance between the two, while ensuring that the pores of the separator can be fully utilized and that the electrolyte has a high degree of wetting. This improves the lithium transport kinetics and stability, enhances the coulombic efficiency of the battery, and ultimately achieves superior cycle performance and rate performance. Detailed Implementation

[0009] 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.

[0010] 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.

[0011] 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.

[0012] The present application is further illustrated below with specific embodiments:

[0013] A secondary battery includes a positive electrode sheet and a separator. The positive electrode sheet includes a positive electrode active material layer, the positive electrode active material layer includes a positive electrode material, the positive electrode material includes a nickel-cobalt-manganese-based material, and the positive electrode active material layer further contains at least one of element B and element W.

[0014] The secondary battery satisfies: (b×a×10000) / c=0.1~1400μm / ppm;

[0015] a is the tortuosity of the positive electrode sheet, b is the mass change rate of the diaphragm after soaking in the mixed solution for 4 hours, c = c1 / c2, where c1 is the total mass content of B and W elements in the positive electrode active material layer in ppm, and c2 is the average particle size of the nickel-cobalt-manganese-based material in μm.

[0016] To improve the bonding strength between the separator and the positive electrode in a secondary battery, as well as the lithium-ion transport effect between them, while considering the battery's cycle performance and high-rate discharge performance, this application proposes a scheme that simultaneously regulates the tortuosity of the positive electrode, the mass change rate of the separator after immersion in a mixed solution, and the ratio of the total W and B content in the positive electrode active material layer to the average particle size of the nickel-cobalt-manganese-based material. This coordination of these three factors enhances the reliable connection and stability between the positive electrode and the separator, reduces the transport impedance at their interface, and decreases the lithium-ion transport path during lithium insertion / extraction cycling. Furthermore, the separator does not reduce transport channels due to swelling and pore blockage, thus improving both the lithium-ion transport rate and stability. Simultaneously, regulating the B and W content in the positive electrode active material layer and the ratio of the nickel-cobalt-manganese-based material particles ensures uniform distribution of B and W within the positive electrode material particles, improving lithium-ion extraction... The uniformity of the rate of lithium-ion insertion / extraction back into the positive electrode is crucial for improving the battery's high-rate discharge capability. If the particle size of the nickel-cobalt-manganese (NiCoMn)-based material increases or decreases, the content of boron (B) and hydroxyl (W) needs to be simultaneously controlled to avoid uneven element distribution leading to a lower consistency in the lithium-ion insertion / extraction rate. Furthermore, if the ratio of B to W is too small, in addition to uneven B and W distribution, the larger particle size of the positive electrode material will also result in an excessively long path for lithium-ion insertion / extraction, reducing lithium-ion conductivity. Conversely, if the ratio is too large, excessive B and W content or excessively small NiCoMn particle size will increase side reactions during lithium-ion insertion / extraction in contact with the electrolyte, reducing the coulombic efficiency of lithium-ion transport. Therefore, by controlling this ratio, the coulombic efficiency of lithium-ion transport in the secondary battery can be effectively improved, and the swelling effect of the separator can be further optimized, improving the initial efficiency of the secondary battery and ultimately achieving better cycle stability and high-rate discharge performance.

[0017] In some embodiments, the secondary battery satisfies the following range: (b×a×10000) / c = 0.1μm / ppm, 0.12μm / ppm, 0.15μm / ppm, 0.17μm / ppm, 0.19μm / ppm, 0.5μm / ppm, 0.7μm / ppm, 0.9μm / ppm, 0.94μm / ppm, 1μm / ppm, 1.2μm / ppm, 1.5μm / ppm, 2μm / ppm, 5μm / ppm, 10μm / ppm, 20μm / ppm, 50μm / ppm, 100μm / ppm, 150μm / ppm, 180μm / ppm, 200μm / ppm, 300μm / ppm, 500μm / ppm, 800μm / ppm, 1000μm / ppm, 1200μm / ppm, 1400μm / ppm, or any two of these values.

[0018] More preferably, the secondary battery satisfies: (b×a×10000) / c=0.5~302μm / ppm.

[0019] In this application, adjusting the rate of mass change of the separator after wetting in the secondary battery can improve the connection and bonding between the separator and the electrode, and reduce the interfacial transport impedance between them. However, it is also necessary to consider the retention rate of the electrolyte by the separator and the uniformity of electrolyte distribution. This is to avoid excessive electrolyte leakage from the separator due to the separator's limited binding capacity, which would reduce the electrolyte content in the separator, decrease the number of ion transport channels, and reduce ion transport efficiency. Alternatively, it could lead to uneven electrolyte distribution inside the separator, such as "local enrichment" or "local depletion," which could trigger lithium ion exchange. Uneven transport paths can easily lead to the deactivation of positive or negative electrode materials, jeopardizing the cycle stability of the corresponding secondary battery. Simultaneously, to improve the lithium-ion transport performance and the connection stability between the separator and the electrode, the tortuosity of the positive electrode also needs to be controlled. Tortuosity reflects the curvature and complexity of the path ions take in the porous structure of the electrode. Higher tortuosity means a longer actual distance for lithium ions to travel from the electrolyte to the active sites inside the positive electrode material particles, resulting in a lower effective diffusion coefficient. During high-rate discharge, a large current needs to flow through the battery instantaneously, requiring electrons to rapidly flow from the negative electrode. To reach the positive electrode, lithium ions need to rapidly escape through the electrolyte and separator to eventually embed into the positive electrode material. At this point, the effective diffusion coefficient is low, making it difficult for lithium ions to reach the reaction sites inside the positive electrode particles in a timely manner. This leads to a sharp increase in electrode polarization voltage, preventing effective capacity release and severely limiting the high-rate performance of the secondary battery. By controlling the tortuosity, shortening the lithium ion transport path, and increasing the effective diffusion coefficient, the kinetic performance of the electrode can be improved, concentration polarization suppressed, and the rate performance of the secondary battery enhanced. Furthermore, by controlling the balance between the separator and the electrode, as described above... The ratio of B / W elements and the particle size of nickel-cobalt-manganese-based materials on the positive electrode can reflect the relationship between the uniformity of the distribution of the two elements in the nickel-cobalt-manganese-based material particles and the length of the ion transport path. Further adjustment of this ratio can improve the effective intercalation and deintercalation of active lithium, avoid a large loss of active lithium, and improve the coulombic efficiency of the secondary battery. When the adjustment meets the preferred range of (b×a×10000) / c, the secondary battery can take into account better high-rate discharge performance, and the interface stability between the separator and the electrode is good, the degree of side reaction between the electrode and the electrolyte is low, and the cycle stability of the secondary battery is better.

[0020] In some embodiments, the tortuosity a of the positive electrode sheet is 1 to 5.5.

[0021] In some implementations, a is a range of one or both of the following: 1, 1.2, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5.

[0022] More preferably, a = 1.5~3.5.

[0023] The tortuosity of the positive electrode sheet affects its wettability in the secondary battery, as well as the electrochemical environment of the positive electrode material during charging and discharging, lithium-ion transport efficiency, especially at high rates. Higher tortuosity lengthens the lithium-ion transport path, increasing the actual distance lithium ions travel from the electrolyte to the active sites inside the positive electrode material, resulting in a lower effective diffusion coefficient, lower kinetic performance, and a higher polarization voltage, hindering the full realization of rate performance. However, if the tortuosity of the electrode sheet is too low, it can lead to aggregation after immersion in the electrolyte, increasing the degree of local side reactions and affecting the cycle stability and performance of the secondary battery. Therefore, it is necessary to coordinate the control of the separator swelling ratio and the ratio of W / B elements to the particle size of the positive electrode material. When the control meets the limits defined in this application, and the tortuosity of the positive electrode sheet is preferably within the aforementioned range, it can not only effectively improve the lithium-ion transport rate but also enhance the stability of the electrode sheet after immersion in the electrolyte, thus improving both the rate performance and cycle performance of the secondary battery.

[0024] It should be noted that the tortuosity of the positive electrode sheet described in this application can be confirmed by, but is not limited to, the following methods:

[0025] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V for venting treatment. The positive electrode was then disassembled and immersed in dimethyl carbonate (DMC) solution at 25°C for 4 hours. After drying, the electrode was examined using a dual-beam electron microscope (FIB, model: SCIOS 2). HiVacFIB is used to slice the positive electrode region. The cutting voltage is 30kV, the cutting beam current is 7nA, and the cutting range is 60μm×65μm×30μm. The imaging voltage is 5kV and the imaging beam current is 0.8nA. The number of sections is determined by the z-axis cutting range and the interlayer spacing. For example, when the cutting sample is 30μm thick and the interlayer spacing is 60nm, the number of sections is about 500. The slices are reconstructed in three dimensions, and then the pores on the sections are segmented and extracted. The centroid of the connecting pores in each section is calculated by software. The tortuosity of adjacent two surfaces is calculated by the ratio of the distance between the centroids to the straight distance between the sections. Finally, the tortuosity of the entire positive electrode is calculated. The tortuosity of the positive electrode is a=∑d(i) / H, d=Cn-Cn-1, where d represents the path of the pore between two adjacent surfaces and H represents the cross-sectional distance of the entire slice.

[0026] It should be noted that the tortuosity of the positive electrode sheet described in this application can be adjusted by, but is not limited to, conventional adjustment methods. Specifically, it can be adjusted by a combination of factors such as the compaction density of the positive electrode sheet and the particle size of the positive electrode active material. Other conventional methods can also be used by those skilled in the art, and no specific limitations are made here.

[0027] In some implementations, b = 0.05~0.2.

[0028] In some implementations, b is a range of one or both of the following: 0.05, 0.06, 0.08, 0.1, 0.12, 0.15, 0.18, and 0.2.

[0029] More preferably, b = 0.1~0.175.

[0030] b refers to the rate of mass change of the separator before immersion in the mixed solution and after 4 hours of immersion. This rate needs to be controlled synchronously with the tortuosity of the positive electrode. On the one hand, increasing b results in higher binding force of the separator to the electrolyte, and actually better adhesion between the separator and the positive electrode, improving connectivity and reducing the contact resistance between the electrode and the separator, which is beneficial for lithium-ion conduction and thus reduces the battery's internal resistance. However, if b is increased further, the separator expands after absorbing the electrolyte, causing some pores in the separator to become blocked, and electrolyte is easily lost, leading to a decrease in the electrolyte content in the separator. This, in turn, reduces the number of lithium-ion transport channels and decreases transport efficiency. Increasing the internal resistance of the secondary battery reduces its discharge performance at high rates. However, if b is too low, it will also reduce the bonding effect between the separator and the positive electrode, increase the interface impedance, and easily lead to uneven distribution of electrolyte in the separator, which is also not conducive to lithium ion transport. When b is synchronously controlled with the tortuosity of the electrode and the ratio c to the range defined in this application, and b is further preferably within the above range, the separator can fully achieve electrolyte wetting and will not have too much separator pore blockage when it is strongly connected with the electrode. The lithium ion transport channels are sufficient, and the high-rate discharge performance of the secondary battery is better.

[0031] It should be noted that 'b' in this application refers to the mass change rate of the diaphragm after soaking in a mixed solution at 25°C for 4 hours, which can be confirmed in the following ways, but is not limited to:

[0032] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V for venting treatment. Then the separator was disassembled, soaked in dimethyl carbonate for 2 hours, and then vacuum dried at 50°C for 4 hours. The resulting separator was cut into 10mm×10mm test samples and weighed, and the mass m0 was recorded. Then the test samples were placed in a mixed solution (a mixture of ethylene carbonate, dimethyl carbonate and diethyl carbonate, with a mass ratio of 2:2:1) at 25°C for 4 hours. The free electrolyte was wiped off, and the mass was recorded as m1. Then b = (m1-m0) / m0.

[0033] In some implementations, c = 7~6875ppm / μm.

[0034] In some embodiments, c = 7ppm / μm, 8ppm / μm, 10ppm / μm, 15ppm / μm, 20ppm / μm, 25ppm / μm, 50ppm / μm, 70ppm / μm, 100ppm / μm, 200ppm / μm, 500ppm / μm, 1000ppm / μm, 1500ppm / μm, 2000ppm / μm, 2200ppm / μm, 2500ppm / μm, 3000ppm / μm, 3200ppm / μm, 3400ppm / μm, 4000ppm / μm, 4500ppm / μm, 5000ppm / μm, 5500ppm / μm, 6000ppm / μm, 6500ppm / μm, 6875ppm / μm is a range of one or any two of these values.

[0035] More preferably, the c = 20~4000ppm / μm.

[0036] If the ratio of boron (B) and wheat (W) content to cathode material particle size is too small, the particle diameter will be large, and the low content of B and W will lead to poor dispersion uniformity of these two elements, resulting in weak lithium-ion transport capability. Reducing this ratio can improve this situation, thereby enhancing the high-rate discharge capability of the secondary battery. However, if the ratio is too large, the high content of W and B will cause excessive W to forcibly expand the crystal structure of the cathode material particles, resulting in distortion and reducing coulombic efficiency and capacity. Excessive B will block ion channels, and the imbalance of surface conditioning energy of B, combined with the grain-refining effect of W, will further exacerbate the problem. Excessive heat can cause particle agglomeration, and the combined effect of these factors can lead to very low reversible capacity and very high internal resistance, resulting in extremely poor rate performance and affecting high-rate discharge capability. In addition, complex inert impurity phases (such as borostungstates) may form between the two elements and Li and O elements, further consuming active material, increasing impedance, and deteriorating rate capability. When the ratio of the two elements is preferably within the above range, the distribution uniformity of B and W elements can be significantly improved, and it can be ensured that the two elements do not excessively affect the crystal structure of the material particles, maintaining low internal resistance and high chemical stability, ultimately resulting in better rate performance of the secondary battery.

[0037] It should be noted that, in the scheme described in this application, c1 can be confirmed in, but is not limited to, the following ways:

[0038] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V for venting treatment. Then, the positive electrode was disassembled and soaked in dimethyl carbonate (DMC) solution at 25°C for 60 minutes. After drying, the positive electrode powder was scraped off, weighed, dispersed in 20 mL of water, and then 10 mL of nitric acid was added. After mixing evenly, the mixture was heated to dissolve the positive electrode powder. The mixture was then diluted with water to 100 mL to obtain the test solution. The test solution was subjected to ICP testing. Before the test, a standard solution must be prepared. The linear correlation coefficient of the standard concentration must be above 0.999 to be used as a normal standard. The 1000 mg / L standard solution was diluted with deionized water to different concentrations (0, 1 mg / 100 mL, 2 mg / 100 mL, 3 mg / 100 mL), and the element detection wavelength was selected. Experimental conditions were set as follows: an ICP inductively coupled plasma atomic emission spectrometer was used, with a gas flow rate of 0.5 L / min and a power of 1150 W. Wavelengths for elements B and W were selected. The element content in the sample was determined using the self-analysis function of the ICP testing software, expressed in ppm. The standard solution refers to a solution containing the element to be analyzed; standard solutions are commercially available.

[0039] In some implementations, c1 = 100~8000ppm.

[0040] In some embodiments, the mass content of element B in the positive electrode active material layer is 100~1500 ppm.

[0041] In some embodiments, the W element content in the positive electrode active material layer is 5~5375 ppm by mass.

[0042] By optimizing the content ratio of the two elements within the above range, the uniformity of their distribution can be further improved, resulting in better lithium-ion insertion / extraction efficiency. At the same time, the chemical stability of the cathode material will not be excessively reduced, the degree of side reactions will be smaller, and the cycle stability and rate performance of the secondary battery will be better.

[0043] It should be noted that, in the scheme described in this application, c2 can be confirmed in, but is not limited to, the following ways:

[0044] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V for venting treatment. Then, the positive electrode was disassembled and soaked in dimethyl carbonate (DMC) solution at 25°C for 4 hours. After drying, the positive electrode powder was scraped off and fixed onto the SEM test sample holder with conductive adhesive. The sample was then polished with CP argon ion and coated with a conductive layer. Subsequently, three locations were randomly selected under the SEM at 3kx magnification for imaging. The powder particles in the obtained areas were pre-positioned and identified by EDS elemental analysis, and the particle size was measured using the diagonal scribe method with Mearsure Nano software. 200 particles were measured, and the average value was calculated, which is the c2, in μm.

[0045] In some implementations, c2 = 0.8~20 μm.

[0046] More preferably, c2 = 1.2~12μm.

[0047] When the average particle size of the nickel-cobalt-manganese-based material is preferably within the above-mentioned range, the length of the ion transport path and the uniformity of the distribution of B and W elements can be further balanced, taking into account the faster transport rate of lithium ions during the insertion and extraction process and the lower degree of side reactions. This results in better kinetic performance and cycle stability of the secondary battery, as well as better rate performance and cycle performance.

[0048] In some embodiments, the ratio of the mass content of element B to the mass content of element W in the positive electrode active material layer is 0.01 to 40.

[0049] When the ratio of B to W elements in the positive electrode active material layer is preferably within the above range, it can not only effectively improve the stability of the crystal structure of the positive electrode material and optimize the interface performance, suppress lattice distortion and microcracks, but also optimize the lithium ion insertion / extraction efficiency to a certain extent, especially the insertion / extraction efficiency at high rates, thereby further improving the cycle performance and rate performance of the secondary battery.

[0050] In some embodiments, the positive electrode active material layer further contains at least one of the following elements: Al, Zr, Sr, Mg, Ti, Nb, Y, Mo, Ta, and Sb.

[0051] In addition to elements B and W, the inventors discovered that when these elements are further introduced into the positive electrode active material layer, the reversible insertion / extraction efficiency of lithium ions is higher, the effective diffusion coefficient is larger, and it is more conducive to improving the rate performance of the corresponding secondary battery.

[0052] In some embodiments, the positive electrode active material layer also contains Nb and / or Sb elements.

[0053] More preferably, the mass content of Nb element in the positive electrode active material layer is 1~2000ppm;

[0054] More preferably, the mass content of Sb element in the positive electrode active material layer is 3~2000ppm.

[0055] Among the aforementioned elements, the doping of Nb and Sb elements can introduce lattice defects or adjust the electronic state density, reduce the charge transfer impedance during lithium-ion transport, and improve electronic conductivity. When Nb and Sb elements are introduced into the positive electrode active material layer and their content is further optimized and controlled within the aforementioned range, the diffusion channels of lithium ions during transport can be effectively broadened, the lithium-ion migration resistance can be reduced, and the effective diffusion coefficient of lithium ions can be improved. This allows for smoother rapid insertion and extraction of lithium ions in secondary batteries at high rates, avoiding a sharp decline in capacity.

[0056] In some embodiments, the diaphragm includes a base membrane and a coating disposed on at least one side of the base membrane, wherein the thickness of the coating on one side of the base membrane is in the ratio of the thickness of the base membrane to the thickness of the base membrane, which is 0.05 to 1.5.

[0057] It should be noted that the coating on the diaphragm described in this application can be disposed on one side of the base membrane or on both sides simultaneously.

[0058] In some embodiments, the thickness of the base film is 5 to 15 μm.

[0059] In some embodiments, the thickness of the coating on one side of the substrate is 1~4 μm.

[0060] More preferably, the coating comprises an organic substance, which includes at least one of PVDF (polyvinylidene fluoride), PAA (polyacrylic acid), SBR (styrene-butadiene rubber), and CMC (carboxymethyl cellulose).

[0061] After the separator comes into contact with the electrolyte, the thickness and type of the organic material coating will affect not only the degree of mass change of the separator after immersion in the electrolyte and the number and width of lithium-ion transport channels, but also the magnitude of the interfacial impedance and interfacial stability between the separator and the positive electrode due to its chemical properties. When the thickness ratio of the coating and / or the organic material selection is preferably within the above range, the separator can achieve a moderate swelling effect, and the separator has high interfacial stability and low impedance, which is more conducive to the rapid and stable transport of lithium ions, and the rate performance and cycle performance of the secondary battery are better.

[0062] In addition, the coating may also include inorganic substances, including at least one of alumina, silicon dioxide, and boehmite.

[0063] Based on actual needs, those skilled in the art can further add inorganic substances such as ceramic components like alumina and silicon dioxide after introducing organic coating components, thereby further improving the mechanical and interfacial stability of the separator and optimizing the performance of the secondary battery. No special limitations are imposed on this.

[0064] More preferably, the organic matter in the coating has a mass percentage content of 5-10 wt%.

[0065] More preferably, the diaphragm further includes an adhesive layer, the thickness of which is 1~3μm on one side of the base membrane.

[0066] It should be noted that when the diaphragm described in this application contains a coating, the adhesive layer is disposed on the surface of the coating away from the base membrane; when the diaphragm described in this application does not contain a coating, the adhesive layer is disposed on the surface of the base membrane.

[0067] It should be noted that the coating and / or adhesive layer can be applied to one side of the base film or to both sides of the base film. The coating and adhesive layer are not bonded together. When the base film has a coating on one or both sides, the adhesive layer may not be applied. There is no limitation on this.

[0068] More preferably, the adhesive layer comprises PVDF and PVA, with a mass ratio of (90:10) to (99:1).

[0069] In some embodiments, the puncture strength of the diaphragm is 100~700gF.

[0070] In addition to swelling ratio, optimizing the strength performance of the separator can also improve the electrochemical performance of the secondary battery: when the puncture strength of the separator is further optimized within the above range, the separator can not only maintain excellent lithium-ion transport efficiency, but also, when the separator swells, it contacts the positive and negative electrodes, and its stress resistance to the active particles on the electrodes is higher and it is less likely to be damaged. At the same time, the stability of the separator is better when lithium ions pass through the separator quickly, and the cycle performance and rate performance of the secondary battery can be further improved.

[0071] It should be noted that the b mentioned in this application can be controlled by conventional control methods during the preparation of the diaphragm. Specifically, for example, it can be adjusted by the mass ratio of organic matter in the coating, the type and ratio of adhesive in the adhesive layer, and the thickness of the adhesive layer. However, those skilled in the art are not limited to this. Based on the actual situation, other methods can also be used for control, and no specific limitation is made.

[0072] In some embodiments, the nickel-cobalt-manganese-based material includes lithium nickel cobalt-manganese oxide particles.

[0073] In some embodiments, the lithium nickel cobalt manganese oxide particles have the chemical formula Li. x Ni o Co p Mn q O2, where o is greater than 0 and less than 1, p is greater than 0 and less than 1, q is greater than 0 and less than 1, o+p+q = 1, x = 0.9~1.1.

[0074] In some embodiments, the lithium nickel cobalt manganese oxide particles also contain metal elements, including B and W, and at least one of Al, Zr, Sr, Mg, Ti, Nb, Y, Mo, Ta, and Sb.

[0075] More preferably, o ≥ 0.5.

[0076] When the nickel content in lithium nickel cobalt manganese oxide particles increases, the tendency for dissolved nickel ions in the cathode material to migrate to the surface membrane and interior increases. At this time, the reduction and deposition of these nickel ions may cause blockage of the membrane micropores, increasing the resistance to ion transport. By limiting the total amount of B and W elements in the cathode material, the tortuosity of the cathode sheet, and the mass change rate of the membrane after immersion in the mixed solution, the impact of the increased nickel content can be effectively controlled, the probability of membrane micropore blockage can be reduced, and the rate performance of the secondary battery can be optimized.

[0077] In some embodiments, the lithium nickel cobalt manganese oxide particles include a core and a coating layer, the coating layer containing elements B and / or W.

[0078] In some embodiments, the lithium nickel cobalt manganese oxide particles contain boron and / or w elements in a region from a depth of 1 / 3R to 2 / 3R from the surface, where R is the diameter of the lithium nickel cobalt manganese oxide particles.

[0079] As mentioned above, the introduction of boron (B) and wattage (W) elements into cathode materials can effectively improve their electrochemical performance. B mainly optimizes the radial arrangement of the cathode material, thereby shortening the lithium-ion transport path and improving kinetic performance, thus optimizing the rate performance of the material. W can refine the grain size of the cathode material to a certain extent, which can also shorten the lithium-ion transport path. When these two elements are set in the coating layer or on the outer surface of the particles, not only can the rate performance of the secondary battery be improved, but also the interface protection of the cathode material particles can be achieved to a certain extent, improving the stability of the cathode material during the lithium-ion insertion / extraction process.

[0080] In some embodiments, the cathode material contains rod-shaped particles.

[0081] In some embodiments, the lithium nickel cobalt manganese oxide particles satisfy: e=1~1.8, where e is the average ratio of e1 to e2, and e1 is the length of the lithium nickel cobalt manganese oxide particle in the long axis direction, and e2 is the length of the lithium nickel cobalt manganese oxide particle at the middle position in the long axis direction.

[0082] After the tortuosity of the positive electrode sheet is adjusted, the lithium nickel cobalt manganese oxide particles on the positive electrode sheet are further configured with an aspect ratio greater than 1. This shortens the lithium-ion transport path based on the short axis orientation of the particles, further optimizing the particle packing effect and the porosity and utilization rate of the positive electrode sheet. This further enhances the lithium-ion transport kinetics of the positive electrode sheet and, together with the tortuosity adjustment, improves the lithium insertion / extraction stability of the electrode sheet, simultaneously improving the rate performance and cycle performance of the secondary battery. However, the aspect ratio of the lithium nickel cobalt manganese oxide particles cannot be too high, otherwise... The difference between the diffusion rate of lithium ions along the short axis and the diffusion rate along the long axis is too large, especially under high-rate conditions. The reaction is incomplete in the long axis region of the particles, and the lithium ion insertion and extraction are uneven. This not only prevents the full utilization of capacity, but also causes polarization effects and even stress accumulation, reducing the stability of the particle structure and weakening the cycle performance and rate performance of the secondary battery. When the aspect ratio of the particles is further optimized to the range of 1 to 1.8, the lithium nickel cobalt manganese oxide particles are not only more efficient in insertion and extraction, but also have better uniformity, which is more conducive to improving the cycle performance and rate performance of the secondary battery.

[0083] It should be noted that the long axis direction of the lithium nickel cobalt manganese oxide particle refers to the longest length that the plane can reach when the particle is taken in a two-dimensional plane along the height direction. The direction along this long axis is the long axis direction, and the middle position of the long axis direction is the position of 1 / 2 length of the particle in the long axis direction.

[0084] It should be noted that the term 'e' can be confirmed in, but is not limited to, the following ways:

[0085] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V for venting treatment. Then, the positive electrode was disassembled and soaked in dimethyl carbonate (DMC) solution at 25°C for 4 hours. After drying, the powder on the surface of the current collector was scraped off and used as a sample. The sample was fixed with conductive adhesive and polished with CP argon ion, coated with a conductive film, placed on the sample stage and observed under a scanning electron microscope (SEM). The magnification was adjusted to 3kx and three areas were selected for photography. The images were opened in Nano Measure software, the scale was selected, and the length and unit of the scale were set to be consistent with the scale in the lower left corner of the SEM. Then, a lithium nickel cobalt manganese oxide particle was identified and located by EDS elemental analysis. The short axis length e2 corresponding to the middle position of the particle was measured, and the long axis length e1 was measured. The ratio e1 / e2 was calculated. The ratio data of 100 particles were collected and the average value was calculated to obtain e.

[0086] In some embodiments, the lithium nickel cobalt manganese oxide particles comprise single-crystal particles, and the nickel cobalt manganese-based material satisfies: e=1~1.5.

[0087] In some embodiments, the lithium nickel cobalt manganese oxide particles comprise polycrystalline particles, and the nickel cobalt manganese-based material satisfies: e=1~1.8.

[0088] Polycrystalline particles are composed of multiple primary particles, so lithium-ion diffusion efficiency is higher regardless of whether the long axis or short axis is large. In contrast, single-crystal particles have larger sizes, and the greater the change in e, the greater the variation in the uniformity of lithium-ion insertion / extraction. When using the above-mentioned optimized ratio of the long axis to the short axis of the particles in different particle settings, the lithium-ion insertion / extraction rate, utilization rate and uniformity of nickel cobalt manganese oxide particles can be further improved, while also improving the rate performance and cycle performance of secondary batteries.

[0089] In some embodiments, the lithium nickel cobalt manganese oxide particles comprise polycrystalline particles;

[0090] More preferably, the polycrystalline particles include primary particles, and the average particle size of the primary particles is 50~500nm.

[0091] When polycrystalline particles are set in lithium nickel cobalt manganese oxide particles, these particles contain more than or equal to primary particles. Therefore, the transmission path of the positive electrode sheet during lithium ion transport can be further shortened, thereby improving the lithium ion transport rate and making the high-rate discharge performance of the secondary battery better.

[0092] In some embodiments, the lithium nickel cobalt manganese oxide particles further include single-crystal particles with an average particle size of 800~5000 nm.

[0093] In some embodiments, the single crystal particle comprises a single particle, or an agglomeration of no more than two particles.

[0094] In some embodiments, the lithium nickel cobalt manganese oxide particles can be commercially available products, or they can be prepared by the following methods:

[0095] 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, ground, and calcined once. The resulting material is then further heated and calcined a second time, and then calcined a third time at a adjusted temperature. After crushing, the lithium nickel cobalt manganese oxide particles are obtained.

[0096] In some embodiments, the nickel source includes at least one of nickel sulfate, nickel nitrate, nickel chloride, nickel oxalate, and nickel acetate;

[0097] In some embodiments, the cobalt source used includes at least one of cobalt sulfate, cobalt nitrate, cobalt chloride, cobalt oxalate, and cobalt acetate;

[0098] In some embodiments, the manganese source includes at least one of manganese sulfate, manganese nitrate, manganese chloride, manganese oxalate, and manganese acetate;

[0099] 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.

[0100] In some embodiments, the precipitant includes at least one of sodium hydroxide, sodium carbonate, oxalic acid, and ammonia.

[0101] In some embodiments, the solvent includes water.

[0102] In some embodiments, the precipitation reaction takes 0.5 to 1.5 hours.

[0103] In some embodiments, the heating rate during the first calcination is 1~10℃ / min, the calcination temperature is 450~550℃, and the time is 2~6h.

[0104] In some embodiments, the heating rate during the secondary calcination is 1~10℃ / min, the calcination temperature is 750~950℃, and the time is 5~15min.

[0105] In some embodiments, the temperature of the three calcinations is 780~820℃ and the time is 8~15h.

[0106] In some embodiments, the grinding can be achieved by ball milling, with a grinding time of 1.5h to 2.5h and a rotation speed of 400 to 500 r / min.

[0107] In some embodiments, the crushing is carried out using an air jet mill, wherein the air jet mill has an induced draft frequency of 20~50Hz.

[0108] In some embodiments, the positive electrode active material layer contains 92-99% by mass of the positive electrode material.

[0109] In some embodiments, the positive electrode active material layer further includes a binder and a conductive agent.

[0110] In some embodiments, the binder is used to improve the adhesion between cathode material particles and the adhesion between the cathode material and the cathode current collector. Any binder can be used without particular limitation, as long as it has suitable adhesive properties 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.).

[0111] Specifically, the adhesive is selected from polytetrafluoroethylene or polyvinylidene fluoride.

[0112] In some embodiments, the mass percentage of the binder in the positive electrode active material layer is 0.5% to 4.0%, such as 0.5%, 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.

[0113] 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.

[0114] In some embodiments, the mass percentage of the conductive agent in the positive electrode active material layer is 0.5% to 4.0%, such as 0.5%, 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 two of the above values. In some embodiments, the positive electrode sheet can be prepared by, but is not limited to, the following methods:

[0115] The positive electrode 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.

[0116] In some embodiments, the solvent includes N-methylpyrrolidone (NMP).

[0117] In some embodiments, the compaction density of the positive electrode sheet is 3.2~3.85 g / cm³. 3 .

[0118] 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:

[0119] The pretreated positive electrode sheet is punched into circular sheets of a fixed area using a punching machine. The area is denoted as S0, and the unit is m. 2 To ensure testing accuracy, select a flat location in the middle of the electrode sheet and take three or more discs as parallel samples. Weigh each of the three discs using an electronic balance and record the mass of each disc as M1 (g). Measure the thickness of the active material layer (after removing the current collector) in each of the three discs using a micrometer and record it as H (m). Take the average value. Finally, add an appropriate amount of deionized water to each of the three discs and gently wipe off the coating with lint-free paper to expose the current collector. 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 compaction density of the positive electrode sheet using the following formula: (M1-M0) / (H×S0).

[0120] In some embodiments, the positive electrode active material layer of the positive electrode sheet is further provided with a plurality of recesses, the depth of which is 0.5~20μm, the pore diameter of which is 2~40μm, and the pore density of which is 6~500pt / m³. 2 .

[0121] More preferably, the concave hole is disposed on the side of the positive electrode active material layer away from the current collector.

[0122] After adjusting the tortuosity of the positive electrode sheet, further setting concave holes on the positive electrode active material layer can improve the surface roughness of the positive electrode active material layer, further improving the connection effect between the positive electrode sheet and the separator after contact, resulting in lower interfacial impedance and better interfacial stability. At the same time, it can also improve the wetting effect of the electrolyte on the positive electrode sheet of the separator, improve ion transport efficiency, and improve the lithium insertion / extraction transport effect in the secondary battery.

[0123] In some embodiments, the adhesion force between the positive electrode active material layer and the current collector in the positive electrode sheet is 3~20 N / m.

[0124] The adhesion force of the positive electrode sheet is mainly determined by the bonding strength between the positive electrode active material layer and the current collector. Optimizing this bonding strength so that the adhesion force of the positive electrode sheet is preferably within the above range can improve the connectivity between the positive electrode active material layer and the current collector, ensure stable electron transport between the positive electrode active material layer and the current collector, and have a high tolerance for high-speed ion / electron transport at high rates. This ensures that the coulombic effect of the secondary battery is maintained at a better level, and ultimately achieves better rate performance.

[0125] It should be noted that the adhesion between the positive electrode active material layer and the current collector in the positive electrode sheet described in this application can be confirmed by, but is not limited to, the following methods:

[0126] Discharge the secondary battery at 0.33C to the lower limit voltage of 2.5V, remove the empty battery, disassemble the positive electrode, soak the electrode in dimethyl carbonate solution for 4 hours; air dry; take a standard steel plate (50mm×125mm) as the rigid test base plate, and wipe the surface of the steel plate clean with lint-free paper dipped in alcohol. Attach one side of a 50mm×125mm piece of 3M double-sided tape to the steel plate, ensuring a smooth and wrinkle-free adhesion. Cut the positive electrode sheet into 50mm×125mm test samples. Attach the test sample to the other side of the double-sided tape, then apply another layer of double-sided tape to the other surface of the electrode sheet, ensuring a smooth and wrinkle-free contact during the adhesion process. After pressing with a pressure roller, use a universal testing machine's tensile clamps to hold the steel plate at one end and the 3M tape at the other. Set the tensile testing machine's stroke to 100mm and perform a tensile test at a speed of 300mm / min. Record the tensile test results in the software graph when the curve flattens out and the displacement is greater than 80mm, then stop the machine. The average tensile force value of the flattened portion of the curve is the adhesion force between the positive electrode active material layer and the current collector in the positive electrode sheet.

[0127] In some embodiments, the secondary battery further includes a negative electrode.

[0128] In some embodiments, the tortuosity of the negative electrode sheet is 1 to 5.

[0129] After adjusting the tortuosity of the positive electrode, the tortuosity of the negative electrode can be further adapted, which can shorten the ion transport path on the entire electrode, thereby further improving the transport rate of lithium ions in the secondary battery. At the same time, matching the tortuosity of the two electrodes can also further improve the stability of the electrodes when lithium ions are transported back and forth at high rates.

[0130] It should be noted that the tortuosity of the negative electrode sheet described in this application can be confirmed by the method for confirming the tortuosity of the positive electrode sheet described above, but it is not limited to this method, and will not be elaborated further.

[0131] In some embodiments, the negative electrode sheet includes a negative electrode active material layer with a porosity of 20-50%.

[0132] It should be noted that the porosity of the negative electrode active material layer described in this application can be confirmed by, but is not limited to, the following methods:

[0133] The negative electrode sheet was cut into a circular piece with a diameter of D=12mm. Simultaneously, a thickness gauge was used to measure the thickness of the electrode sheet and the current collector, recording the results as h1 and h2 respectively. This was then calculated using V1=πR. 2 Calculate 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 active material layer using the formula porosity% = [(m2-m1) / ρ] / V1 × 100%, where ρ is the density of hexadecane, 0.7734 g / cm³. 3 .

[0134] In some embodiments, the negative electrode sheet includes a negative electrode active material layer, the negative electrode active material layer includes a negative electrode material, and the negative electrode material has a particle size D corresponding to a volume cumulative distribution of 50%. v50 The value is 5~25μm.

[0135] It should be noted that the particle size D of the negative electrode material described in this application v50 Confirmation can be made through, but is not limited to, the following methods:

[0136] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V for venting treatment. The negative electrode was then disassembled and immersed in dimethyl carbonate solution at 25°C for 4 hours. After drying, the negative electrode active material layer powder was scraped off. The powder was then directly tested using a laser particle size analyzer (Malvin 3000) according to GB / T19077-2016. The particle size D corresponding to a cumulative volume distribution of 50% of the negative electrode material is defined as the particle size of the negative electrode material. v50 .

[0137] In some embodiments, the negative electrode material includes at least one of natural graphite, artificial graphite, mesophase carbon microspheres, hard carbon, soft carbon, elemental silicon, silicon suboxide, silicon-carbon composite material, and lithium titanate.

[0138] The negative electrode active material layer may also contain conductive agents and / or binders.

[0139] 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.

[0140] 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.

[0141] The binder in the negative electrode active material layer is used to improve the adhesion between negative electrode material particles and the adhesion between the negative electrode material and the negative electrode current collector. Any binder can be used without particular limitation, as long as it has suitable adhesive properties 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.

[0142] In some embodiments, the mass percentage of the binder in the negative electrode active material layer is 0.6% to 4.5%, such as 0.6%, 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 any two of the above values.

[0143] In some embodiments, the mass percentage of the negative electrode material in the negative electrode active material layer is 93.5-99%.

[0144] In some embodiments, the secondary battery further includes an electrolyte.

[0145] In some embodiments, the electrolyte has an ionic conductivity of 5~25 mS / cm at 25°C.

[0146] More preferably, the electrolyte has an ionic conductivity of 8~20 mS / cm at 25°C.

[0147] It should be noted that the ionic conductivity of the electrolyte described in this application can be confirmed in the following ways:

[0148] The secondary battery was discharged at 0.33C to the lower limit voltage of 2.5V for venting. The battery was then disassembled and the electrolyte collected in a glove box (H2O ≤ 0.1ppm, O2 ≤ 0.1ppm). There are three methods for collecting the electrolyte: After removing the battery cover: ① If there is free electrolyte, collect it into a 5mL sample tube using a pipette and seal it with sealing tape to prevent leakage. ② If there is no free electrolyte, a hydraulic press can be used to continuously pressurize until free electrolyte appears. Collect the electrolyte 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 in the electrode to mix thoroughly with the dichloromethane. Then, use a pipette to draw the mixture of dichloromethane and electrolyte into a 5mL sample tube and seal the sample tube with sealing tape. After collecting the electrolyte, use a Mettler conductivity meter (model: S230) to measure the electrolyte content. The test was conducted at 25℃. Before testing, calibration was performed as follows: 10 mL of a conductivity standard solution with a conductivity of 12.88 mS / cm was placed in a centrifuge tube and incubated in a 25℃ water bath for 30 minutes. The temperature of the standard solution was measured using a temperature electrode; if the temperature was 25.0 ± 0.2℃, calibration began. If the conductivity of the standard solution was 12.88 ± 0.15 mS / cm, the calibration was successful. Approximately 100 mL of electrolyte sample was then taken and sealed. The sample was placed in a constant temperature water bath at 25.0 ± 0.2℃ for 30 minutes. Finally, the test was performed, and the stable reading was taken as the ionic conductivity of the electrolyte.

[0149] In some embodiments, the electrolyte includes additives, solvents, and lithium salts.

[0150] 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.

[0151] 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 (EA), 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.

[0152] More preferably, the solvent includes at least one of DMC, DEC, EMC, EC, EA, and PC.

[0153] More preferably, the solvent includes EC.

[0154] When the above-mentioned solvent is used as the electrolyte component, it can reduce the degree of swelling of the separator after adsorbing the electrolyte based on the strong polarity of EC and its low compatibility with the separator, resulting in better rate performance and cycle performance of the secondary battery.

[0155] More preferably, the solvent has a mass percentage content of 75-95% in the electrolyte.

[0156] When the parameters of the positive electrode and the separator meet the specified requirements, further adaptation using the above-mentioned preferred electrolyte solvent formulation can greatly improve the rate of lithium-ion transport in the secondary battery, thereby improving the kinetic performance of the secondary battery.

[0157] 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.

[0158] In some embodiments, the additives include, but are not limited to, at least one of vinylene carbonate (VC), fluoroethylene carbonate (FEC), vinyl sulfate (DTD), propylene sulfite (PS), and methylene disulfonate (MMDS).

[0159] When the above-mentioned electrolyte additives with moderate viscosity and high ion conductivity are selected for compounding, the lithium ion conductivity of the electrolyte is higher, which is more conducive to improving the transmission rate of lithium ions between the positive electrode, separator and negative electrode, and the rate performance of the secondary battery is better.

[0160] More preferably, the additive has a mass percentage content of 0.1-10% in the electrolyte.

[0161] In some embodiments, the secondary battery includes a cell structure comprising a negative electrode, a positive electrode, and a separator, the cell structure operating by the movement of lithium ions between the positive and negative electrodes.

[0162] In some embodiments, the secondary battery has a wound cell structure.

[0163] It should be noted that the winding of the battery cell structure refers to the battery cell made by winding the continuous positive electrode, negative electrode and separator in the battery cell structure, with the separator located between adjacent positive and negative electrode sheets.

[0164] When the secondary battery adopts a wound cell structure, the positive and negative electrode plates in the cell have a higher contact tightness, resulting in a higher connection between the positive electrode plate and the separator. In order to improve the lithium-ion transmission efficiency, the relationship between the tortuosity of the positive electrode plate, the B / W element content and the particle size ratio of the material, and the mass change rate of the separator after immersion is optimally controlled, and the secondary battery can achieve better rate performance.

[0165] In some embodiments, the battery cell includes a cylindrical battery cell or a prismatic battery cell.

[0166] It should be noted that the cylindrical cell refers to an electrode assembly in which the negative electrode, positive electrode, and separator are wound into a cylindrical shape, while the square cell refers to an electrode assembly in which the negative electrode, positive electrode, and separator are wound or stacked into a roughly rectangular shape.

[0167] More preferably, the battery cell is a cylindrical battery cell, and a = 1.5~4.

[0168] When a cylindrical cell structure is used, the stacking density of the electrode sheets is higher. Further optimization of the tortuosity of the positive electrode sheet within the above range results in a faster lithium-ion insertion / extraction rate and better structural stability of the electrode sheet during insertion / extraction, which is beneficial to the performance improvement of the battery.

[0169] More preferably, the secondary battery includes a casing, a cell, and tabs. The casing is provided with a terminal post, one of which serves as the positive output terminal and the other as the negative output terminal. The tabs include a positive tab and a negative tab, one of which is electrically connected to the terminal post and the other is electrically connected to the casing, and is led out from both ends of the cell along the axis. The secondary battery satisfies: (b×a×10000) / c=0.6~900.

[0170] When assembling battery cells, multiple cells need to be connected in series and parallel via a busbar. The busbar usually needs to be on the same side to connect multiple cells in series and parallel. The final output terminals of the positive and negative electrodes are on the same side. Therefore, when the positive and negative electrodes are output from both ends, the current transmission of one polarity needs to go from one end to the other. The current path becomes longer, and the high-rate discharge performance of the battery becomes lower. Therefore, further adjustment of the positive electrode and separator is required to ensure that the battery's rate performance is at a better level.

[0171] In some embodiments, the secondary battery includes a casing, a cell, and tabs. The casing has a terminal post, one of which serves as the positive output terminal and the other as the negative output terminal. The tabs include a positive tab and a negative tab, one of which is electrically connected to the terminal post and the other is electrically connected to the casing, and both are led out from the same end along the axial direction of the cell. The secondary battery satisfies: (b×a×10000) / c=0.6~543.

[0172] More preferably, the electrode tab comprises a metal and / or an alloy thereof, specifically including but not limited to at least one of copper, copper alloy, aluminum, aluminum alloy, nickel, and nickel alloy.

[0173] In some embodiments, the secondary battery is a stacked cell structure, wherein adjacent positive and / or negative electrode layers within the stacked cell structure are discontinuous, and the cell structure is stacked by means of layering or Z-shaped folding.

[0174] In some embodiments, the secondary battery is a stacked cell structure, and the secondary battery satisfies: (b×a×10000) / c=0.3~1200.

[0175] When the secondary battery adopts a stacked structure, the connection between the electrode and the separator is lower due to the gap between the electrode. The relationship needs to be adjusted within the above range to ensure that the lithium-ion transport rate is at a high level and the transport stability is high, thus ensuring the rate performance and cycle performance of the secondary battery.

[0176] In some embodiments, the secondary battery satisfies: b×D=2.5~80, where D is the DCR obtained by discharging the secondary battery at a 4C rate for 60s, and the unit is mΩ.

[0177] The DC resistance (DCR) of a secondary battery is mainly composed of the ohmic resistance, interfacial charge transfer resistance, and diffusion resistance within the battery. At high rates, a higher DCR results in poorer rate performance. However, simply adjusting the ohmic resistance cannot completely improve rate performance. By synergistically regulating the DCR of the secondary battery at high rates and the rate of mass change of the separator after immersion in the mixed solution, the interfacial contact state and interfacial stability between the separator and the electrode can be effectively reduced, thereby lowering the interfacial charge transfer resistance and diffusion resistance in the secondary battery, improving the utilization efficiency of ion transport channels, and thus effectively improving the rate and efficiency of ion transport, ultimately optimizing the rate performance of the secondary battery.

[0178] In some implementations, b×D = one or any two of the following values: 2.5, 5, 8, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80.

[0179] In some implementations, D = 40~500mΩ.

[0180] More preferably, D = 50~450mΩ.

[0181] 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:

[0182] Example 1

[0183] A secondary battery, the preparation method comprising the following steps:

[0184] (1) Preparation of lithium nickel cobalt manganese oxide particles: 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 within the range of 12.1, and the reaction was carried out at 55℃ for 1 h. After filtration, washing, and drying, the resulting precursor was mixed with lithium carbonate at a molar ratio of lithium atoms to the total atoms of nickel, cobalt, and manganese in the precursor of 1.05:1. Then, boron oxide and tungsten oxide, which are dopants, were added. The mixture was ball-milled at 435 r / min for 2 h, and calcined once at 500℃ for 5 h under an oxygen atmosphere by heating at 5℃ / min. The resulting material was then calcined a second time at 925℃ for 5 min by heating at 5℃ / min, and finally calcined a third time at 805℃ for 10 h. The mixture was then crushed by air jet milling at 40 Hz to obtain lithium nickel cobalt manganese oxide particles (LiNi) containing doped B and W elements. 0.94 Co 0.04 Mn 0.02 O2;

[0185] (2) Preparation of positive electrode sheet: Lithium nickel cobalt manganese oxide particles were used as positive electrode material. Then, the positive electrode material, conductive agent acetylene black, and binder polyvinylidene fluoride were dispersed in N-methylpyrrolidone at a mass ratio of 98:1:1. The slurry was prepared by vacuum stirring and then coated onto the current collector aluminum foil. After coating, drying, cold pressing, slitting and rolling were performed. Then, concave holes (depth 5μm, pore diameter 10μm, pore density 200pt / m) were set by mechanical punching. 2 The positive electrode sheet is thus obtained; the areal density of the positive electrode sheet is 300 g / m³. 2 The compacted density is 3.5 g / cm³. 3 ;

[0186] (2) Preparation of negative electrode sheet: The negative electrode material artificial graphite, conductive agent acetylene black, thickener sodium carboxymethyl cellulose, and binder styrene-butadiene rubber are dispersed in water at a mass ratio of 96.4:1:1.2:1.4. The mixture is then vacuum stirred to prepare a slurry, which is then coated onto the current collector copper foil. After coating, drying, cold pressing, slitting, and rolling, the negative electrode sheet is obtained. The areal density of the negative electrode sheet is 180 g / m³. 2 The compacted density is 1.55 g / cm³. 3 The average particle size of the negative electrode material on the negative electrode sheet is 15.2 μm.

[0187] (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, lithium difluorosulfonyl imide and the additive fluoroethylene carbonate are added to prepare an electrolyte with a concentration of 1 mol / L of lithium hexafluorophosphate and lithium difluorosulfonyl imide (molar ratio of the two is 1:1) and a mass content of 1 wt% of fluoroethylene carbonate.

[0188] (4) Preparation of the diaphragm: Polypropylene, 0.1 wt% antioxidant, and 0.1 wt% slip agent were added to a twin-screw extruder and melt-blended at 180~200℃. The screw speed was 250 r / min. The mixture was extruded through a T-die to form a 150 μm thick green film. The film was then cooled and shaped by a 45℃ cooling roller. The green film was then introduced into a longitudinal stretching machine and uniaxially stretched at 90℃ with a stretch ratio of 4 (stretch rate controlled at 80 mm / s). Subsequently, it was introduced into a transverse stretching machine and transversely stretched at 150℃ with a stretch ratio of 6 and a stretch rate of 70 mm / s. The film was then heated at 180℃. After setting for 10 seconds and cooling to room temperature, a coating is applied to one side of the diaphragm. After drying and fixing, an adhesive layer is then applied on top of the coating. The adhesive layer comprises PVDF and PAA. The coating comprises alumina particles with an average particle size of 5 μm and organic substances PVDF and PAA. The mass ratio of PVDF to PAA is 90:10. The mass percentage of the organic substances in the coating is 8.2%. The mass ratio of PVDF to PAA in the adhesive layer is 94:6. The thickness of the coating is 2 μm, and the thickness of the adhesive layer is 2.4 μm.

[0189] (5) The positive electrode, separator (coated side facing the positive electrode), and negative electrode are stacked, wound and assembled into a cell in sequence. The cell is placed in the outer packaging shell, dried and then injected with electrolyte. After vacuum sealing, standing, formation and volume adjustment, the secondary battery is obtained. The parameters of the secondary battery are shown in Tables 2 and 3.

[0190] Examples 2-37, Comparative Examples 1-6

[0191] A secondary battery differs from Example 1 only in its manufacturing process, as shown in Table 1. The parameters of the secondary battery are shown in Tables 2 and 3.

[0192] The difference between Embodiments 31-34 and Embodiment 1 is that the battery cells are assembled into cylindrical structure battery cells. The cylindrical structure of Embodiments 31-33 has a diameter of 45mm and a height of 90mm, while that of Embodiment 34 has a diameter of 25mm and a height of 60mm.

[0193] Examples 35-37 and Comparative Examples 5-6 are stacked cell structures.

[0194] Table 1

[0195]

[0196] Continued from Table 1

[0197]

[0198] Continued from Table 1

[0199]

[0200] Continued from Table 1

[0201]

[0202] Continued from Table 1

[0203]

[0204] Table 2

[0205]

[0206] Continued from Table 2

[0207]

[0208] Continued from Table 2

[0209]

[0210] Continued from Table 2

[0211]

[0212] Continued from Table 2

[0213]

[0214] Table 3

[0215]

[0216] Continued from Table 3

[0217]

[0218] The lithium-ion batteries obtained in each embodiment and comparative example were tested as follows:

[0219] (1) High-rate discharge test:

[0220] The secondary batteries obtained in each embodiment and comparative example were charged at 0.33C to the upper limit voltage of 4.25V, then charged at constant voltage to the cutoff current of 0.05C, and then discharged at 0.33C to the lower limit voltage of 2.5V. One cycle was completed, and the charge and discharge cycle was repeated for 3 cycles. The discharge capacity of the third cycle was taken as the fixed capacity A0 of the battery. After the fixed capacity was completed, the battery was charged at a constant current and constant voltage rate of 6C to 4.25V with a cutoff current of 0.05C. The discharge capacity A1 was recorded. The high-rate discharge capacity retention rate was recorded as 100% × A1 / A0.

[0221] (2) Cyclic performance test:

[0222] 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 Q0 of the 3rd cycle was recorded. Then, the 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; and discharged at a constant current of 0.33C to 2.5V. This was considered one cycle, and the cycle was repeated 300 times. The discharge capacity Q1 of the 300th cycle was recorded, and the cycle capacity retention rate was recorded as 100% × Q1 / Q0.

[0223] The test results are shown in Table 4.

[0224] Table 4

[0225]

[0226] As can be seen from Table 4:

[0227] (1) The secondary battery described in this application synchronously regulates the tortuosity of the positive electrode sheet, the mass change rate of the separator after immersion in the electrolyte, and the ratio of the total content of W and B elements in the positive electrode active material layer to the average particle size of the positive electrode material, thereby coordinating the relationship between the three, thereby improving the reliable connection and connection stability between the positive electrode sheet and the separator, reducing the transmission impedance between the two interfaces, and at the same time, during the lithium insertion / extraction cycle, the lithium ion transmission path is reduced, and the separator will not reduce the transmission channel due to swelling and pore blockage, thus improving the lithium ion transmission rate and transmission stability; through the comparative value c for synergistic regulation, the coulombic efficiency of lithium ion transmission in the secondary battery can be effectively improved, and the swelling effect of the separator can be further optimized, the first efficiency of the secondary battery is improved, and finally, better cycle stability and high-rate discharge performance are achieved. The capacity retention rate of the secondary battery can reach more than 70% after high-rate testing, and the capacity retention rate can reach more than 68% after 300 long-cycle tests.

[0228] (2) As mentioned above, adjusting the thickness change rate of the separator after wetting in the secondary battery can improve the connection and bonding between the separator and the electrode, and reduce the interfacial transmission impedance between them. However, it is also necessary to take into account the uniformity of the electrolyte retention rate of the separator. At the same time, in order to improve the lithium-ion transmission performance of the secondary battery and the connection stability between the separator and the electrode, the tortuosity of the positive electrode also needs to be controlled synchronously. The tortuosity reflects the curvature and complexity of the path taken by ions in the porous structure of the electrode, which is related to the effective diffusion coefficient of lithium ions. When discharging at a high rate, a large current needs to pass through the battery instantly, electrons need to flow quickly from the negative electrode to the positive electrode, and lithium ions need to quickly escape through the electrolyte and the separator, and finally embed into the positive electrode material. At this time, the effective diffusion coefficient is low, and lithium ions have difficulty reaching the reaction sites inside the positive electrode particles in time, and the capacity cannot be effectively released, thus seriously limiting the high rate of discharge of the secondary battery. In addition, after adjusting the balance between the separator and the electrode, the ratio c can reflect the relationship between the uniformity of the distribution of B and W elements in the positive electrode material particles and the length of the ion transport path. Further adjustment of this ratio can improve the effective intercalation and deintercalation of active lithium and improve the coulombic efficiency of the secondary battery. When the adjustment satisfies the above (b×a×10000) / c, which is preferably 0.5~302, the secondary battery can take into account better high-rate discharge performance, and the interface stability between the separator and the electrode is good, the degree of side reaction between the electrode and the electrolyte is low, and the cycle stability of the secondary battery is better. At the same time, if the tortuosity of the positive electrode is preferably 1.5~3.5, or the electrolyte immersion mass change rate of the separator is preferably 0.1~0.175, or the ratio c is preferably 20~4000ppm / μm, the electrochemical performance of the secondary battery can also be further improved.

[0229] (3) In addition, the DCR of a secondary battery is mainly composed of the internal ohmic resistance, the interfacial charge transfer resistance and the diffusion resistance inside the battery. The larger the DCR at high rates, the worse the rate performance of the secondary battery. However, simply adjusting the internal ohmic resistance cannot completely improve the rate performance. When the DCR of the secondary battery at high rates and the mass change rate of the separator after soaking in the mixed solution are synergistically controlled, so that b×D=2.5~80, the cross-sectional contact state and cross-sectional stability of the separator and the electrode can be effectively reduced, the interfacial charge transfer resistance and diffusion resistance in the secondary battery can be reduced, and the utilization efficiency of the ion transport channel can be improved, thereby effectively improving the rate and efficiency of ion transport and ultimately optimizing the rate performance of the secondary battery.

Claims

1. A secondary battery, characterized in that, The device includes a positive electrode sheet and a separator. The positive electrode sheet includes a positive electrode active material layer, which includes a positive electrode material. The positive electrode material includes a nickel-cobalt-manganese-based material, and the positive electrode active material layer also contains boron (B) and wattage (W). The mass content of boron in the positive electrode active material layer is 100-1500 ppm, and the mass content of wattage (W) in the positive electrode active material layer is 5-5375 ppm. The secondary battery satisfies: (b×a×10000) / c=0.5~302μm / ppm; The value of 'a' is the tortuosity of the positive electrode sheet, and the value of 'a' is 1 to 5.

5. The b is the mass change rate of the diaphragm after soaking in the mixed solution for 4 hours, and the b = 0.05~0.2; the mixed solution is a mixture of ethylene carbonate, dimethyl carbonate and diethyl carbonate, and the mass ratio of the three is 2:2:1; The c = c1 / c2, where c1 is the total mass content of B and W elements in the positive electrode active material layer, in ppm, and c1 = 100~8000ppm; c2 is the average particle size of the nickel-cobalt-manganese-based material, in μm, and c2 = 0.8~20μm, and c = 7~6875 ppm / μm.

2. The secondary battery as described in claim 1, characterized in that, The a is 1.5 to 3.5, and / or the b is 0.1 to 0.175, and / or the c is 20 to 4000 ppm / μm.

3. The secondary battery as described in claim 1, characterized in that, The ratio of the mass content of element B to the mass content of element W in the positive electrode active material layer is 0.01 to 40.

4. The secondary battery as described in claim 1, characterized in that, The positive electrode active material layer also contains at least one of the following elements: Al, Zr, Sr, Mg, Ti, Nb, Y, Mo, Ta, and Sb.

5. The secondary battery as described in claim 4, characterized in that, The positive electrode active material layer also contains Nb and / or Sb elements.

6. The secondary battery as described in claim 5, characterized in that, The mass content of Nb in the positive electrode active material layer is 1~2000ppm, and / or the mass content of Sb in the positive electrode active material layer is 3~2000ppm.

7. The secondary battery as described in claim 1, characterized in that, The diaphragm includes a base membrane and a coating disposed on at least one side of the base membrane, wherein the thickness of the coating on one side of the base membrane is in the ratio of the thickness of the base membrane to the thickness of the base membrane, which is 0.05 to 1.

5.

8. The secondary battery as described in claim 7, characterized in that, The thickness of the base film is 5~15μm, and / or the thickness of the coating on one side of the base film is 1~4μm.

9. The secondary battery as described in claim 7, characterized in that, The diaphragm also includes an adhesive layer, the thickness of which is 1~3μm on one side of the base membrane.

10. The secondary battery as described in claim 7, characterized in that, The coating comprises an organic substance, which includes at least one of PVDF, PAA, SBR, and CMC.

11. The secondary battery as described in claim 10, characterized in that, The coating also includes inorganic substances, including at least one of alumina, silicon dioxide, and boehmite.

12. The secondary battery as described in claim 1, characterized in that, The puncture strength of the diaphragm is 100~700gF.

13. The secondary battery as described in claim 1, characterized in that, The nickel-cobalt-manganese-based material includes lithium nickel cobalt-manganese oxide particles; the chemical formula of the lithium nickel cobalt-manganese oxide particles is Li. x Ni o Co p Mn q O2, where o is greater than 0 and less than 1, p is greater than 0 and less than 1, q is greater than 0 and less than 1, o+p+q=1, and x=0.9~1.

1.

14. The secondary battery as described in claim 13, characterized in that, The lithium nickel cobalt manganese oxide particles also contain metal elements, including B and W, and at least one of Al, Zr, Sr, Mg, Ti, Nb, Y, Mo, Ta, and Sb.

15. The secondary battery as described in claim 13, characterized in that, The value of o is ≥ 0.

5.

16. The secondary battery as described in claim 13, characterized in that, The lithium nickel cobalt manganese oxide particles include a core and a coating layer, wherein the coating layer contains boron and / or w.

17. The secondary battery as described in claim 13, characterized in that, The lithium nickel cobalt manganese oxide particles contain B and / or W elements in the region from a depth of 1 / 3R to a depth of 2 / 3R from the surface, where R is the diameter of the lithium nickel cobalt manganese oxide particles.

18. The secondary battery as described in claim 13, characterized in that, The lithium nickel cobalt manganese oxide particles satisfy the following: e = 1~1.8, where e is the average ratio of e1 to e2, e1 is the length of the lithium nickel cobalt manganese oxide particle in the long axis direction, and e2 is the length of the lithium nickel cobalt manganese oxide particle at the middle position in the long axis direction.

19. The secondary battery as described in claim 18, characterized in that, The lithium nickel cobalt manganese oxide particles include single-crystal particles, wherein the lithium nickel cobalt manganese oxide particles satisfy e=1~1.5, and / or, the lithium nickel cobalt manganese oxide particles include polycrystalline particles, wherein the lithium nickel cobalt manganese oxide particles satisfy e=1~1.

8.

20. The secondary battery as described in claim 1, characterized in that, The nickel-cobalt-manganese-based material includes lithium nickel cobalt-manganese oxide particles, which include polycrystalline particles; the polycrystalline particles include primary particles, and the average particle size of the primary particles is 50~500nm.

21. The secondary battery as described in claim 1, characterized in that, The nickel-cobalt-manganese-based material includes lithium nickel cobalt-manganese oxide particles, which further include single-crystal particles with an average particle size of 800~5000nm.

22. The secondary battery as described in claim 1, characterized in that, The positive electrode active material layer of the positive electrode plate is further provided with a plurality of recessed holes, the depth of the recessed holes is 0.5-20μm, the aperture of the recessed holes is 2-40μm, and the hole density of the recessed holes is 6-500pt / m 2 .

23. The secondary battery as described in claim 1, characterized in that, The adhesion force between the positive electrode active material layer and the current collector in the positive electrode sheet is 3~20N / m.

24. The secondary battery as described in claim 1, characterized in that, The secondary battery also includes a negative electrode sheet, the tortuosity of which is 1 to 5.

25. The secondary battery as described in claim 14, characterized in that, The secondary battery further includes a negative electrode tab including a negative electrode active material layer having a porosity of 20 to 50%, and / or the negative electrode tab includes a negative electrode active material layer including a negative electrode material having a cumulative volume distribution reaching 50% at a particle diameter D v50 of 5 to 25 μm.

26. The secondary battery as described in claim 1, characterized in that, The secondary battery also includes an electrolyte, which has an ionic conductivity of 5~25 mS / cm at 25°C.

27. The secondary battery as described in claim 26, characterized in that, The electrolyte includes additives, solvents, and lithium salts.

28. The secondary battery as described in claim 27, characterized in that, The solvent includes at least one of DMC, DEC, EMC, EC, EA, and PC, and the solvent has a mass percentage content of 75-95% in the electrolyte.

29. The secondary battery as described in claim 28, characterized in that, The solvent includes EC.

30. The secondary battery as described in claim 29, characterized in that, The additives include at least one of VC, FEC, DTD, PS, and MMDS.

31. The secondary battery as described in claim 1, characterized in that, The secondary battery has a wound cell structure.

32. The secondary battery as described in claim 31, characterized in that, The battery cell is a cylindrical battery cell, and a = 1.5~4.

33. The secondary battery as described in claim 32, characterized in that, The secondary battery includes a casing, a cell, and tabs. The casing has a terminal post, one of which serves as the positive output terminal and the other as the negative output terminal. The tabs include a positive tab and a negative tab, one of which is electrically connected to the terminal post and the other is electrically connected to the casing, and is led out from both ends of the cell along the axis. The secondary battery satisfies: (b×a×10000) / c=0.6~900.

34. The secondary battery as described in claim 1, characterized in that, The secondary battery has a stacked cell structure and satisfies the following condition: (b×a×10000) / c=0.3~1200.

35. The secondary battery as described in claim 1, characterized in that, The secondary battery satisfies the following condition: b×D=2.5~80, where D is the DCR obtained by discharging the secondary battery at a 4C rate for 60s, and the unit is mΩ.

36. The secondary battery as described in claim 35, characterized in that, The value of D is 40~500mΩ.

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