Secondary battery

Abstract

The invention belongs to the technical field of secondary batteries, and particularly relates to a secondary battery which comprises a positive plate, a negative plate, a diaphragm and an electrolyte, the diaphragm is arranged between the positive plate and the negative plate, and the magnetization degree K of the negative plate, the viscosity eta of the electrolyte and the conductivity sigma of the electrolyte meet the relation that (38-20K) / (25 eta + 1.8) * 100 is larger than or equal to 2.3 and smaller than or equal to 7.2; 0.14 < = (0.5 * sigma + 1) / (35K + 3) < = 0.8; wherein K = OI1 / OI0, OI0 is the orientation degree of the negative plate before magnetization, and OI1 is the orientation degree of the negative plate after magnetization. Firstly, the negative plate is magnetized to change the graphite orientation, so that the lithium ion transmission path is reduced, the transmission and diffusion rate of lithium ions in the battery is improved, and the expansion of the battery in the thickness direction is reduced; and the physicochemical parameters of the electrolyte are adjusted to match the magnetized negative plate, so that the rate capability and the cycle expansion performance of the whole battery are improved to the greatest extent.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and particularly to a secondary battery. Background Art

[0002] At present, technical features such as high energy density and fast charging of lithium-ion batteries have been widely applied to digital products and power sources. With the continuous upgrading of technology, the energy density of lithium-ion batteries has reached its limit, and industrial silicon anode materials are increasingly applied to commercial lithium-ion batteries.

[0003] As the market's requirements for the capacity of lithium batteries are getting higher and higher, the silicon content of silicon-based battery cells is also gradually increasing. However, silicon is a semiconductor material with poor electron conduction ability. High-silicon systems have encountered great challenges in the field of fast charging and it is difficult to meet high-rate fast charging, or lithium deposition occurs at high rates, leading to cycle failure problems. During high-rate charging, the insertion speed of lithium ions into the negative electrode is slower than the extraction speed from the positive electrode, resulting in the precipitation of lithium ions on the surface of the negative electrode, forming lithium dendrites. This not only reduces the safety of the battery but also shortens the cycle life of the battery.

[0004] In view of this, there is an urgent need to develop a secondary battery that can effectively improve the rate performance and cycle expansion performance of lithium-ion batteries. Summary of the Invention

[0005] The purpose of the present invention is: aiming at the deficiencies of the prior art, to provide a secondary battery that can effectively improve the rate performance and cycle expansion performance of lithium-ion batteries.

[0006] To achieve the above purpose, the present invention adopts the following technical solutions:

[0007] A secondary battery includes a positive electrode sheet, a negative electrode sheet, a separator disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte. The magnetization degree K of the negative electrode sheet, the viscosity η of the electrolyte, and the conductivity σ of the electrolyte satisfy the relationships: 2.3 ≤ (38 - 20K) / (25η + 1.8) * 100 ≤ 7.2; 0.14 ≤ (0.5 * σ + 1) / (35K + 3) ≤ 0.8;

[0008] Wherein, K = OI1 / OI0, OI0 is the orientation degree before magnetization of the negative electrode sheet (here it is taken as 35), and OI1 is the orientation degree after magnetization of the negative electrode sheet.

[0009] Preferably, the K satisfies the relationship: 0 < K < 1, representing the range of K values involved in the electrode sheets of the entire experimental system.

[0010] Preferably, the K and OI1 satisfy the relationships: 0 < K < 0.25, 0 < OI1 < 8.75, that is, the (110) orientation of graphite on the negative electrode sheet is dominant.

[0011] Preferably, K and OI1 satisfy the relationship: 0.25 ≤ K < 0.5, 8.75 < OI1 < 17.5, that is, the proportion of the (110) orientation of graphite on the negative electrode sheet is still relatively large.

[0012] Preferably, K and OI1 satisfy the relationship: 0.5 ≤ K < 0.75, 17.5 < OI1 < 26.25, that is, the proportion of the (004) orientation of graphite on the negative electrode sheet increases.

[0013] Preferably, K and OI1 satisfy the relationship: 0.75 ≤ K < 1, 26.25 < OI1 < 35, that is, the (004) orientation of graphite on the negative electrode sheet is dominant.

[0014] Preferably, the viscosity η of the electrolyte is 20 - 40 mPa·s.

[0015] Preferably, the conductivity σ of the electrolyte is 6 - 8 mS / cm.

[0016] Preferably, the density ρ of the electrolyte is 1 - 1.3 g / cm 3 。

[0017] Preferably, the active material of the negative electrode sheet is at least one of natural graphite, artificial graphite, flake graphite, colloidal graphite, graphene, silicon, silicon carbide, silicon oxide, and silicon alloy.

[0018] Preferably, the silicon content in the active material is 15 - 50%.

[0019] Compared with the prior art, the beneficial effects of the present invention are as follows: By first magnetizing the negative electrode sheet to change the orientation arrangement of graphite particles, the present invention shortens the transmission path of lithium ions inside the negative electrode, significantly improves the transfer and diffusion rate of lithium ions in the battery cell, and effectively reduces the swelling tendency of the battery in the thickness direction. On this basis, the present invention further optimizes and adjusts the physical and chemical parameters such as the viscosity and conductivity of the electrolyte according to the characteristics of the magnetized negative electrode sheet, so that the electrolyte is fully adapted to the magnetically regulated negative electrode structure. Through the above technical solutions, the present invention can improve the high-rate performance of the battery and the structural stability during long-term cycling to a greater extent, thereby achieving a significant improvement in the overall performance. Detailed Embodiments

[0020] To make the technical solutions and advantages of the present invention clearer, the following will clearly and completely describe the technical solutions of the present invention in combination with specific embodiments. Obviously, the described embodiments are some, rather than all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0021] According to the first aspect of the present application, the present application provides a secondary battery, including a positive electrode sheet, a negative electrode sheet, a separator disposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte. The magnetization degree K of the negative electrode sheet, the viscosity η of the electrolyte, and the conductivity σ of the electrolyte satisfy the relationship: 2.3 ≤ (38 - 20K) / (25η + 1.8) * 100 ≤ 7.2; 0.14 ≤ (0.5*σ + 1) / (35K + 3) ≤ 0.8;

[0022] wherein, K = OI1 / OI0, OI0 is the degree of orientation before magnetization of the negative electrode sheet, and OI1 is the degree of orientation after magnetization of the negative electrode sheet.

[0023] It is known that when the viscosity of the electrolyte is too high, the ion transport impedance in the electrolyte will increase, and the transport rate will become slower, which will in turn affect the internal reaction rate of the battery. Moreover, when the viscosity increases to make (38 - 20K) / (25η + 1.8) * 100 less than the preset range, the ion transport impedance of the system will be very large, which will hinder the rate improvement ability brought by magnetization. In addition, the high-viscosity electrolyte will also increase the internal resistance of the battery, affect the energy efficiency and cycle life of the battery, and weaken the cycle benefits brought by magnetization; on the contrary, under the same conditions of electrode materials and lithium salt solution concentration, the lower the viscosity of the electrolyte, the faster the diffusion rate of lithium ions, but this will lead to a decrease in battery capacity. Too low viscosity of the electrolyte will cause the nanoparticles to be unstable in the battery and even precipitate. This unstable state will cause deep charge and discharge inside the battery, affecting the battery life. All of these will affect the cycle or rate benefits brought by magnetization itself. It has been verified that when (38 - 20K) / (25η + 1.8) * 100 is greater than the preset range, the cycle ability of the system itself will decay, which will weaken and even disappear the cycle benefits brought by magnetization. The influence of conductivity on the system is similar. When (0.5*σ + 1) / (35K + 3) is less than the preset range, due to the too low conductivity of the system, the migration speed of lithium ions in the electrolyte becomes slower, affecting the charge and discharge efficiency of the battery, and the low conductivity of the electrolyte will exacerbate the degradation of the electrode material, resulting in battery capacity loss and shortened cycle life. When (0.5*σ + 1) / (35K + 3) is greater than the preset range, due to the too high conductivity, exceeding the rate that the system can match after magnetization, it will instead limit the rate benefits. Therefore, when the following relationships are satisfied simultaneously: 2.3 ≤ (38 - 20K) / (25η + 1.8) * 100 ≤ 7.2; 0.14 ≤ (0.5*σ + 1) / (35K + 3) ≤ 0.8, the high-rate performance of the battery and the structural stability during long-term cycling can be improved, thus achieving a significant improvement in the overall performance.

[0024] When K is smaller, the degree of magnetization of the graphite in the negative electrode sheet is higher, and the proportion of the (004) orientation of the graphite is smaller. Graphite has a layered structure. Lithium ions need to be inserted into the gaps between the graphite layers from the edges of the layered graphite, diffuse through the SEI film, and then reach the reaction sites to combine with electrons. The tortuous route of lithium deintercalation and intercalation is likely to cause lithium deposition. After the negative electrode sheet is magnetized, the arrangement direction of the graphite is changed, which is more conducive to the intercalation and deintercalation of lithium ions, improves the rate performance, and alleviates the swelling problem in the thickness direction. According to the magnetization degree K of the negative electrode sheet of the battery cell, the viscosity and conductivity of the electrolyte are adjusted. Specifically, the viscosity and conductivity of the electrolyte are changed by adjusting the electrolyte composition, including but not limited to the solvent system, various additives, and lithium salts. Through these measures, the effects of the electrolyte and magnetization can be better exerted to achieve a synergistic effect.

[0025] Orientation degree = I(004) / I(110), where (004) corresponds to the crystal plane perpendicular to the Z-axis, (110) corresponds to the crystal plane perpendicular to the X and Y axes, I(004) represents the X-ray diffraction peak intensity of the (004) crystal plane, I(110) represents the X-ray diffraction peak intensity of the (110) crystal plane, and I(004) / I(110) represents the ratio of the (004) crystal plane to the (110) crystal plane in the diffraction direction. It can be calculated from XRD data, and its value is affected by crystal structure, graphitization degree, particle size, surface area, etc. Its value will directly affect the electrolyte infiltration of the negative electrode, the surface impedance, the high-rate charge and discharge performance, and also directly affect the swelling of the negative electrode during cycling.

[0026] In some embodiments, K satisfies the relationship: 0 < K < 1, for example, it can be 0.1, 0.2, 0.4, 0.6, 0.7, 0.8, 0.9, or 0.95.

[0027] Among them, when the value of K is small and the proportion of the (004) orientation of the graphite after rolling is small, the tortuosity τ of the electrode sheet decreases. At this time, the system is more conducive to the intercalation and deintercalation of lithium ions, improving the rate performance and electrolyte infiltration. When 0.5 < K < 1, the degree of magnetization of the graphite in the electrode sheet is weak, and the proportion of the (004) orientation of the graphite after rolling is still small; when 0 < K < 0.5, the degree of magnetization of the graphite in the electrode sheet is strong, and the proportion of the (004) orientation of the graphite after rolling is large.

[0028] In some embodiments, K and OI1 satisfy the relationship: 0 < K < 0.25, for example, it can be 0.1, 0.15, 0.2, or 0.24; 0 < OI1 < 8.75, for example, it can be 0.1, 1, 2, 6, 8, or 8.74. At this time, the degree of magnetization of the negative electrode is strong.

[0029] In some embodiments, K and OI1 satisfy the relationship: 0.25 ≤ K < 0.5, for example, it can be 0.25, 0.3, 0.35, 0.4, 0.45, or 0.49, 8.75 < OI1 < 17.5, for example, it can be 8.76, 9, 10, 15, 16, or 17.4. At this time, the degree of magnetization of the negative electrode is relatively strong.

[0030] In some embodiments, K and OI1 satisfy the relationship: 0.5 ≤ K < 0.75, for example, it can be 0.5, 0.55, 0.6, 0.65, 0.7, or 0.74, 17.5 < OI1 < 26.25, for example, it can be 17.6, 18, 20, 24, 26, or 26.24. At this time, the degree of magnetization of the negative electrode is relatively poor.

[0031] In some embodiments, K and OI1 satisfy the relationship: 0.75 ≤ K < 1, for example, it can be 0.75, 0.8, 0.85, 0.9, 0.95, or 0.99, 26.25 < OI1 < 35, for example, it can be 26.3, 28, 30, 32, 34, or 34.9. At this time, the degree of magnetization of the negative electrode is very poor.

[0032] In some embodiments, the viscosity η of the electrolyte is 20 - 40 mPa·s, for example, it can be 20 mPa·s, 25 mPa·s, 30 mPa·s, 35 mPa·s, or 40 mPa·s.

[0033] In some embodiments, the conductivity σ of the electrolyte is 6 - 8 mS / cm, for example, it can be 6 mS / cm, 6.5 mS / cm, 7 mS / cm, 7.5 mS / cm, or 8 mS / cm.

[0034] In some embodiments, the density ρ of the electrolyte is 1 - 1.3 g / cm 3 , for example, it can be 1 g / cm 3 , 1.1 g / cm 3 , 1.2 g / cm 3 or 1.3 g / cm 3 .

[0035] In some embodiments, the active material of the negative electrode sheet is at least one of natural graphite, artificial graphite, flake graphite, colloidal graphite, graphene, silicon, silicon carbide, silicon oxide, and silicon alloy;

[0036] and / or, the silicon content in the active material is 15 - 50%, for example, it can be 15%, 20%, 30%, 40%, or 50%.

[0037] Among them, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer provided on at least one surface of the positive electrode current collector surface. The positive electrode active material layer includes a positive electrode active material. The positive electrode active material may be one or more combinations of compounds represented by, but not limited to, chemical formulas such as Li a Ni x Co y M z O 2-b N b (where 0.95 ≤ a ≤ 1.2, x > 0, y ≥ 0, z ≥ 0, and x + y + z = 1, 0 ≤ b ≤ 1, M is selected from one or more combinations of Mn and Al, and N is selected from one or more combinations of F, P, and S). The positive electrode active material may also be one or more combinations of, but not limited to, LiCoO2, LiNiO2, LiVO2, LiCrO2, LiMn2O4, LiCoMnO4, Li2NiMn3O8, LiNi 0.5 Mn 1.5 O4, LiCoPO4, LiMnPO4, LiFePO4, LiNiPO4, LiCoFSO4, CuS2, FeS2, MoS2, NiS, TiS2, etc. The positive electrode active material may also be subjected to modification treatment. The method of modifying the positive electrode active material should be known to those skilled in the art. For example, methods such as coating and doping can be used to modify the positive electrode active material. The materials used for the modification treatment may be one or more combinations of, but not limited to, Al, B, P, Zr, Si, Ti, Ge, Sn, Mg, Ce, W, etc. The positive electrode current collector is usually a structure or part that collects current. The positive electrode current collector may be various materials in the art suitable for use as a positive electrode current collector of a lithium-ion battery. For example, the positive electrode current collector may be, but not limited to, a metal foil, and more specifically, it may be, but not limited to, an aluminum foil, etc.

[0038] The separator may be various materials in the art suitable for use as a separator of a secondary battery. For example, it may be one or more combinations of, but not limited to, polyethylene, polypropylene, polyvinylidene fluoride, aramid, polyethylene terephthalate, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, polyester, and natural fibers, etc.

[0039] The secondary battery further includes an electrolyte, which includes an organic solvent, an electrolyte lithium salt, and an additive. Among them, the electrolyte lithium salt can be LiPF6 and / or LiBOB used in high-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, and LiPF6 used in low-temperature electrolytes; it can also be at least one of LiBF4, LiBOB, LiPF6, and LiTFSI used in overcharge-proof electrolytes; it can also be at least one of LiClO4, LiAsF6, LiCF3SO3, and LiN(CF3SO2)2. The organic solvent can be a cyclic carbonate, including PC and EC; it can also be a chain carbonate, including DFC, DMC, or EMC; it can also be a carboxylic acid ester, including MF, MA, EA, MP, etc. The additive includes at least one of, but is not limited to, a film-forming additive, a conductive additive, a flame-retardant additive, an overcharge-proof additive, an additive for controlling the content of H2O and HF in the electrolyte, an additive for improving low-temperature performance, and a multifunctional additive.

[0040] In some embodiments, when the viscosity η of the electrolyte is 35 mPa·s, the conductivity σ is 6.51 mS / cm, and the density ρ is 1.23 g / cm 3 ³, the amount of EC and PC can be appropriately increased, and the amount of PP and EP can be reduced, which can increase the density, viscosity, and conductivity of the electrolyte. Conversely, the density, viscosity, and conductivity of the electrolyte can be decreased.

[0041] The present invention will be further described in detail below in conjunction with specific embodiments, but the embodiments of the present invention are not limited thereto.

[0042] Example 1

[0043] Preparation of the positive electrode sheet

[0044] The positive electrode active material LiCoO2, the conductive agent acetylene black, the conductive carbon nanotubes, and the binder polyvinylidene fluoride (PVDF) are fully dispersed in an N-methylpyrrolidone (NMP) solvent system according to a weight ratio of 97.6:0.5:0.6:1.3, and mixed evenly to form a positive electrode paste. Then, it is coated on an aluminum foil, dried, cold-pressed, and cut into strips to make a positive electrode sheet.

[0045] Preparation of the negative electrode sheet

[0046] Mix the negative electrode active material (including graphite and silicon-carbon material), conductive agent (a mixture of SP and CNT with a mass ratio of 0.45:0.05), and binder (a mixture of SBR and PAA-Li with a mass ratio of 0.5:1.8) in a weight ratio of 97.7:1.1:1.2 to prepare a slurry of the negative electrode active material, and evenly coat it on both sides of the negative electrode current collector. When coating, the graphite on the wet film needs to be magnetized by the magnetic force (about 6000 gs) of a magnetic bar at the same time. Then dry, cold press, and cut into strips to obtain the negative electrode plate.

[0047] Among them, the magnetization degree K of the negative electrode plate is 0.75, and the orientation degree of the negative electrode plate is 28.

[0048] Preparation of the separator

[0049] Coat the surface of PE with ceramics, PVDF, etc. as the separator membrane.

[0050] Preparation of the electrolyte

[0051] Mix ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), propyl propionate (PP), and ethyl propionate (EP) according to the mass fractions of 5.3, 12.8, 10.3, 10.4, and 12.9 to obtain an organic solvent. Then dissolve the fully dried lithium salt LiPF6 in the above organic solvent at a concentration of 1 mol / L, and finally add additives to prepare the electrolyte. Adjust the viscosity and conductivity of the electrolyte by controlling the mass of each substance in the electrolyte.

[0052] Among them, the viscosity η of the electrolyte is 30 mPa·s, the conductivity σ is 8 mS / cm, and the density ρ is 1.23 g / cm 3 .

[0053] Preparation of the battery

[0054] Make the above positive electrode plate, separator membrane, and negative electrode plate into a bare battery cell in the form of winding or stacking, then encapsulate and bake, inject the electrolyte, and finally obtain the required finished battery.

[0055] Among them, the preparation methods of Examples 2-13 and Comparative Examples 1-5 are the same as that of Example 1, except that the specific parameters of the magnetization degree K, orientation degree OI1 of the negative electrode plate, viscosity η of the electrolyte, conductivity σ of the electrolyte, and density ρ of the electrolyte are shown in Table 1 below. The viscosity, conductivity, and density of the electrolyte are achieved by adjusting the mass fractions of each component in the organic solvent.

[0056] Table 1

[0057]

[0058]

[0059] Performance Test

[0060] 1. Rate Performance Test: Under the high-silicon system and standard charging conditions, the capacity ratios M1 = C1 / C0 * 100% and M2 = C2 / C0 * 100% at discharge rates of 1C, 1.5C respectively compared to 0.2C are taken.

[0061] Charge at a constant current of 0.5C and constant voltage up to 4.53V, with a cut-off current of 0.02C; rest for 0.5 hours; discharge at a constant current of 0.2C down to 3.0V and read the capacity C0;

[0062] Charge at a constant current of 0.5C and constant voltage up to 4.53V, with a cut-off current of 0.02C; rest for 0.5 hours; discharge at a constant current of 1.0C down to 3.0V and read the capacity C1;

[0063] Charge at a constant current of 0.5C and constant voltage up to 4.53V, with a cut-off current of 0.02C; rest for 0.5 hours; discharge at a constant current of 1.5C down to 3.0V and read the capacity C2.

[0064] 2. Cycle Expansion Test: Under environments of 25°C and 45°C, the cycle tests are carried out respectively according to the following methods. Charging regime: 3.2C CC to 4.3V, 2.6C CC to 4.35V, CV to 1.8C, 1.8C CC to 4.4V, CV to 1.5C, 1.5C CC to 4.53V, CV to 1.2C, 1.2C CC to 4.58V, CV to 0.22C; Discharging regime: 0.7C DC to 3.0V. After 500 cycles, the battery thickness is tested by using 600g PPG. Cycle expansion rate = fully charged thickness at the 500th cycle / initial half-charged thickness * 100%.

[0065] The lithium-ion batteries prepared in the examples and comparative examples are respectively subjected to performance tests, and the test results are shown in Table 2.

[0066] Table 2

[0067]

[0068]

[0069] From the comparison of the experimental data of Examples 1-20 and Comparative Examples 1-5 in Table 2, it can be seen that by magnetizing the negative electrode sheet in this application, the rate performance and cycle performance are improved compared with the non-magnetized group. Secondly, under different magnetization degrees, especially at a high magnetization degree (such as when K = 0.1), by adjusting the electrolyte, the rate performance and cycle expansion performance of the battery are significantly improved. Therefore, a certain relationship can be established among the magnetization degree K value of the negative electrode sheet of the present invention, the viscosity, conductivity, etc. of the electrolyte, which can be used to improve the rate performance and cycle performance of lithium-ion batteries and solve the problem of battery failure caused by large expansion of materials.

[0070] Among them, the rate and cycle expansion of Example 16 are the best, which is the optimal example, that is, the smaller the K value, the viscosity and conductivity of the electrolyte satisfy 2.3 ≤ (38 - 20K) / (25η + 1.8) * 100 ≤ 7.2; 0.14 ≤ (0.5 * σ + 1) / (35K + 3) ≤ 0.8. In this range, as the electrolyte viscosity decreases and the conductivity increases, in the system with a stronger magnetization degree of the electrode sheet, the greater the benefits of the rate and cycle expansion performance obtained. That is, due to the change of graphite orientation after magnetization of the anode sheet in the magnetized system, the ion transport rate is greatly accelerated, resulting in a greater demand for the charge transport ability of the electrolyte. Therefore, the higher the magnetization degree, the more the electrolyte is needed to reduce the viscosity and enhance the conductivity for matching.

[0071] From the comparison of the experimental data of Comparative Example 5, it can be seen that when the negative electrode sheet is not magnetized, that is, when the orientation degree of the negative electrode sheet is greater than the preset range, due to the large silicon content in the active material, the rate and cycle performance of the system itself are poor, and the expansion rate is large. For the magnetized system, when (38 - 20K) / (25η + 1.8) * 100 is less than the preset range, due to the too high viscosity of the electrolyte, the ion transport is blocked and slowed down, and the rate of the system can no longer be improved by the graphite orientation of the magnetized electrode sheet; when (38 - 20K) / (25η + 1.8) * 100 is greater than the preset range, due to the too low viscosity of the electrolyte, the rate and cycle performance of the system deteriorate, and this is mainly caused by capacity attenuation at this time, and magnetization cannot provide enough support to improve this situation; when (0.5 * σ + 1) / (35K + 3) is too large or too small, the rate and cycle performance of the system itself will deteriorate, and the performance of the system itself determines the upper limit of the magnetization benefit.

[0072] According to the disclosure and teaching of the above specification, those skilled in the art to which the present invention pertains can also make changes and modifications to the above embodiments. Therefore, the present invention is not limited to the above specific embodiments, and any obvious improvements, substitutions or variations made by those skilled in the art on the basis of the present invention all belong to the protection scope of the present invention. In addition, although some specific terms are used in this specification, these terms are only for convenience of description and do not constitute any limitation to the present invention.

Claims

1. A secondary battery, comprising a positive electrode sheet, a negative electrode sheet, a separator interposed between the positive electrode sheet and the negative electrode sheet, and an electrolyte, characterized in that, The magnetization degree K of the negative electrode sheet, the viscosity η of the electrolyte, and the conductivity σ of the electrolyte satisfy the relationships: 2.3 ≤ (38 - 20K) / (25η + 1.8) * 100 ≤ 7.2; 0.14 ≤ (0.5 * σ + 1) / (35K + 3) ≤ 0.8; Wherein, K = OI1 / OI0, OI0 is the degree of orientation of the negative electrode sheet before magnetization, and OI1 is the degree of orientation of the negative electrode sheet after magnetization.

2. The secondary battery according to claim 1, characterized in that, The K satisfies the relationship: 0 < K < 1.

3. A secondary battery according to claim 2, characterized in that, The K and OI1 satisfy the relationships: 0 < K < 0.25, 0 < OI1 < 8.

75.

4. A secondary battery according to claim 2, characterized in that, The K and OI1 satisfy the relationships: 0.25 ≤ K < 0.5, 8.75 < OI1 < 17.

5.

5. A secondary battery according to claim 2, characterized in that, The K and OI1 satisfy the relationships: 0.5 ≤ K < 0.75, 17.5 < OI1 < 26.

25.

6. The secondary battery according to claim 2, wherein The K and OI1 satisfy the relationships: 0.75 ≤ K < 1, 26.25 < OI1 < 35.

7. The secondary battery according to claim 1, characterized in that, The viscosity η of the electrolyte is 20 - 40 mPa·s.

8. A secondary battery according to claim 1, characterized in that, The conductivity σ of the electrolyte is 6 - 8 mS / cm.

9. A secondary battery according to claim 1, characterized in that, The density ρ of the electrolyte is 1 - 1.3 g / cm 3 .

10. A secondary battery according to claim 1, characterized in that, The active material of the negative electrode sheet is at least one of natural graphite, artificial graphite, flake graphite, colloidal graphite, graphene, silicon, silicon carbide, silicon oxide, and silicon alloy; And / or, the silicon content in the active material is 15 - 50%.

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