Secondary battery and electronic device

Abstract

The present application provides a secondary battery and an electronic device. The secondary battery includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes a negative electrode current collector and a negative electrode material layer provided on at least one surface of the negative electrode current collector. The negative electrode material layer includes silicon-carbon particles, and the particle size of the silicon-carbon particles is D µm. When 5 ≤ D ≤ 13, the silicon-carbon particles are first silicon-carbon particles, and the circularity of the first silicon-carbon particles is R 1 , when 0.1 ≤ D ≤ 2, the silicon-carbon particles are second silicon-carbon particles, and the number of the second silicon-carbon particles is R 2 , the negative electrode material layer satisfies: R 1 / R 2 > 1; the electrolyte includes diethyl carbonate and ethyl propionate. Based on the mass of the electrolyte, the mass percentage content of diethyl carbonate is S 1 %, and the mass percentage content of ethyl propionate is S 2 %, 2.3 ≤ S 2 / S 1 ≤ 6. While achieving a relatively high energy density, the present application takes into account improving the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling.

Description

Technical Field

[0001] The present application belongs to the field of energy storage technology, and specifically relates to a secondary battery and an electronic device. Background Art

[0002] Secondary batteries can store and release energy during the charge and discharge process through electrochemical reactions, and can be recycled thousands of times. With their excellent energy efficiency and economy, secondary batteries are widely used in portable electronic devices, electric vehicles, energy storage systems and other fields. Among them, secondary batteries with high energy density have a relatively longer battery life, which is beneficial to improve the performance of application equipment. However, after long-term cyclic use, secondary batteries with high energy density are prone to capacity attenuation and diving, which is not conducive to the long-term and stable use of their battery life advantages, and are also prone to safety problems such as thermal runaway, creating safety hazards. Therefore, how to improve the capacity retention ability and safety performance of high energy density secondary batteries in the later stages of the cycle is a technical problem that needs to be solved urgently in this field. Summary of the invention

[0003] The purpose of the present application is to provide a secondary battery and an electronic device, which can achieve a higher energy density while improving the safety performance, discharge rate and capacity retention ability of the secondary battery in the later stage of the cycle.

[0004] In a first aspect, the present application provides a secondary battery. The secondary battery comprises a positive electrode, a negative electrode and an electrolyte, the negative electrode comprises a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, the negative electrode material layer comprises silicon carbon particles, and the particle size of the silicon carbon particles is D µm. The silicon carbon particles comprise first silicon carbon particles and second silicon carbon particles; when 5≤D≤13, the silicon carbon particles are first silicon carbon particles, and the circularity of the first silicon carbon particles is R 1 When 0.1≤D≤2, the silicon-carbon particle is the second silicon-carbon particle, and the roundness of the second silicon-carbon particle is R 2 , the negative electrode material layer satisfies: R 1 / R 2 >1; the electrolyte contains diethyl carbonate and ethyl propionate, and the mass percentage of diethyl carbonate based on the mass of the electrolyte is S 1 %, the mass percentage of ethyl propionate is S 2 %,2.3≤S 2 / S 1 ≤6. The secondary battery provided in the present application achieves a higher energy density while improving the safety performance, discharge rate and capacity retention of the secondary battery in the later stage of the cycle.

[0005] Without being limited to any theory, the inventors of the present application have found that although the tap density of the negative electrode sheet can be further increased by mixing silicon-carbon particles with large particle sizes and silicon-carbon particles with small particle sizes, thereby increasing the energy density of the secondary battery, the degrees of expansion and contraction of silicon-carbon particles with different particle sizes during the charge-discharge cycle are different. After multiple cycles, the negative electrode material is affected by uneven stress and accelerates pulverization. And compared with silicon-carbon particles with large particle sizes, silicon-carbon particles with small particle sizes are more likely to generate high stress points locally, and the risk and degree of pulverization are both higher, which also leads to uneven pulverization inside the negative electrode material layer. The above problems lead to an increase in the safety risk of the secondary battery in the later stage of the cycle, a decrease in the discharge rate, and an increase in the capacity loss rate. By regulating the roundness of the first silicon-carbon particles and the second silicon-carbon particles to satisfy the above relationship, the present application can achieve a high energy density while taking into account the improvement of the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of the cycle. And, by making the electrolyte contain both diethyl carbonate and ethyl propionate and adjusting the S 2 / S 1 value to satisfy: 2.3 ≤ S 2 / S 1 ≤ 6, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of the cycle can be further improved.

[0006] The inventor speculates that the first silicon-carbon particles have a particle size of 5 µm to 13 µm, which can act as a compressive skeleton in the negative electrode material layer. Making the first silicon-carbon particles have a higher roundness can buffer the stress caused by the volume change of the silicon-carbon particles to a greater extent, and reduce the probability of the second silicon-carbon particles being pulverized due to local high stress. In addition, when the first silicon-carbon particles with high roundness are stacked in the negative electrode material layer, larger pores will be formed. The second silicon-carbon particles have a particle size of 0.1 µm to 2 µm, which can be fully filled in the above pores to provide a conductive path for the negative electrode material. When the second silicon-carbon particles with lower roundness are filled in the above pores, more contact points and a larger contact area can be formed with the surrounding materials, providing a rich conductive network for the negative electrode material layer to make up for the problem of large pores and insufficient conductive paths in the negative electrode material layer when the first silicon-carbon particles with high roundness are stacked in the electrode. And, because the particle size difference between the two kinds of silicon-carbon particles is large, it is beneficial to have a large difference in the electrolytes for the two kinds of silicon-carbon particles to form a suitable solid electrolyte interface film (SEI film). For example, small-sized silicon-carbon particles have a higher specific surface area and higher reactivity with the electrolyte, so they consume more electrolyte and are more likely to form an SEI film with too large a thickness. Large-sized silicon-carbon particles have a lower specific surface area and are more likely to form an SEI film with too small a thickness. By adjusting the relationship between the mass percentage contents of diethyl carbonate and ethyl propionate, the requirements of the two kinds of silicon-carbon particles for the electrolyte components can be taken into account. In summary, by making the negative electrode and electrolyte of the secondary battery meet the above settings, it is possible to achieve a higher energy density while comprehensively improving the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling.

[0007] In some embodiments, 0.05 ≤ R 1 -R 2 ≤ 0.15. By adjusting the value of R 1 -R 2 within the above range, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be further improved.

[0008] In some embodiments, the first silicon-carbon particles contain silicon and carbon elements. Based on the sum of the masses of the silicon and carbon elements of the first silicon-carbon particles, the mass percentage of the silicon element is C 1 %, 35 ≤ C 1 ≤ 55; and / or, the second silicon-carbon particles contain silicon and carbon elements. Based on the sum of the masses of the silicon and carbon elements of the second silicon-carbon particles, the mass percentage of the silicon element is C 2 %, 45 ≤ C 2 ≤ 65. Adjusting C 1 and / or C 2When the value is within the above range, the expansion degree of the first silicon-carbon particles and / or the second silicon-carbon particles can be regulated, thereby improving the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle.

[0009] In some embodiments, 1.2 ≤ C 2 / C 1 ≤ 1.7. By adjusting the value of C 2 / C 1 within the above range, the expansion degrees of the first silicon-carbon particles and the second silicon-carbon particles can be coordinated, further improving the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle.

[0010] In some embodiments, the silicon-carbon particles include third silicon-carbon particles; when 2 < D < 5, the silicon-carbon particles are third silicon-carbon particles; the number of the first silicon-carbon particles is N 1 and the number of the second silicon-carbon particles is N 2 and the number of the third silicon-carbon particles is N 3 , N 3 / (N 1 +N 2 +N 3 ) < 0.2. By adjusting the value of N 3 / (N 1 +N 2 +N 3 ) within the above range, the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle can be improved.

[0011] In some embodiments, 2.8 ≤ S 2 / S 1 ≤ 4.5. By adjusting the value of S 2 / S 1 within the above range, the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle can be further improved.

[0012] In some embodiments, the electrolyte includes fluoroethylene carbonate and vinylene carbonate. Based on the mass of the electrolyte, the mass percentage of fluoroethylene carbonate is A 1 %, and the mass percentage of vinylene carbonate is A 2 %, 0.5 ≤ A 1 / (100 × A 2 ) ≤ 1.8. By making the electrolyte contain both fluoroethylene carbonate and vinylene carbonate and adjusting the value of A 1 / (100 × A 2 ) within the above range, the electrical conduction ability between the first silicon-carbon particles and the second silicon-carbon particles can be improved, thereby further improving the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle.

[0013] In some embodiments, 0.7 ≤ A 1 / (100 × A 2 ) ≤ 1.5. By adjusting the value of A 1 / (100 × A 2 ) within the above range, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be further improved.

[0014] In some embodiments, 0.90 ≤ R 1 ≤ 0.98. By adjusting the value of R 1 within the above range, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be further improved.

[0015] In some embodiments, 0.78 ≤ R 2 ≤ 0.92. By adjusting the value of R 2 within the above range, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be further improved.

[0016] In some embodiments, 5 ≤ S 1 ≤ 20. By adjusting the value of S 1 within the above range, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be further improved.

[0017] In some embodiments, 20 ≤ S 2 ≤ 45. By adjusting the value of S 2 within the above range, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be further improved.

[0018] In some embodiments, 11 ≤ A 1 ≤ 18. By adjusting the value of A 1 within the above range, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be further improved.

[0019] In some embodiments, 0.05 ≤ A 2 ≤ 0.5. By adjusting the value of A 2 within the above range, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be further improved.

[0020] In some embodiments, the electrolyte contains tetramethyltetravinylcyclotetrasiloxane. Based on the mass of the electrolyte, the mass percentage of tetramethyltetravinylcyclotetrasiloxane is 0.01% to 0.1%. By including tetramethyltetravinylcyclotetrasiloxane in the electrolyte and adjusting the mass percentage of tetramethyltetravinylcyclotetrasiloxane within the above range, the conductivity of the SEI film on the surface of the second silicon-carbon particles can be improved, and the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be improved.

[0021] In some embodiments, the electrolyte contains diethyl (cyanomethyl) phosphonate. Based on the mass of the electrolyte, the mass percentage of diethyl (cyanomethyl) phosphonate is 0.1% to 0.8%. By including diethyl (cyanomethyl) phosphonate in the electrolyte and adjusting the mass percentage of diethyl (cyanomethyl) phosphonate within the above range, the elasticity of the SEI film on the surface of the first silicon-carbon particles can be improved, and the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be improved.

[0022] In some embodiments, the elongation rate of the negative electrode current collector is δ%, and 3 ≤ δ ≤ 8. By adjusting the elongation rate of the negative electrode current collector within the above range, the possibility of the first silicon-carbon particles and the second silicon-carbon particles causing uneven stress on the negative electrode current collector and resulting in the deformation of the current collector can be reduced, and the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be improved.

[0023] In some embodiments, the average particle size of the first silicon-carbon particles is RA µm, and 1.3 ≤ RA / δ ≤ 3.6. By adjusting the value of RA / δ within the above range, the relationship between the stress exerted on the current collector when the first silicon-carbon particles expand and the expandable space provided by the elongation rate of the current collector to the negative electrode material layer can be balanced, and the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be improved.

[0024] In some embodiments, the silicon-carbon particles include a carbon carrier and silicon crystal grains at least partially distributed inside the carbon carrier. Through this setting, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be improved.

[0025] In a second aspect, the present application provides an electronic device, which includes the secondary battery provided in the first aspect of the present application. While achieving a higher energy density, the electronic device provided in the second aspect of the present application can further improve the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling.

[0026] Additional aspects and advantages of the embodiments of the present application will be described, shown, or explained in part in the subsequent description, or through the implementation of the embodiments of the present application. Detailed Embodiments

[0027] Embodiments of the present application will be described in detail below. Embodiments of the present application should not be construed as limiting the present application.

[0028] Unless otherwise expressly specified, the following terms used in the present application have the meanings set forth below.

[0029] In a first aspect, the present application provides a secondary battery. The secondary battery includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The negative electrode material layer includes silicon-carbon particles, and the particle size of the silicon-carbon particles is D µm. The silicon-carbon particles include first silicon-carbon particles and second silicon-carbon particles; when 5 ≤ D ≤ 13, the silicon-carbon particles are first silicon-carbon particles, and the roundness of the first silicon-carbon particles is R 1 , when 0.1 ≤ D ≤ 2, the silicon-carbon particles are second silicon-carbon particles, and the roundness of the second silicon-carbon particles is R 2 , and the negative electrode material layer satisfies: R 1 / R 2 > 1. The secondary battery provided by the present application achieves a relatively high energy density while taking into account improving the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling.

[0030] Negative electrode

[0031] The negative electrode includes a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector. The negative electrode material layer includes a negative electrode active material. In some embodiments, the rechargeable capacity of the negative electrode active material is greater than the discharge capacity of the positive electrode active material to prevent the unintentional precipitation of active materials (such as lithium metal) on the negative electrode during charging.

[0032] The negative electrode active material includes silicon-carbon particles. In some embodiments, the negative electrode material layer satisfies: R 1 / R 2 > 1. In some embodiments, 1.02 ≤ R 1 / R 2 ≤ 1.23. In some embodiments, 1.02 ≤ R 1 / R 2 ≤ 1.18. In some embodiments, 1.06 ≤ R 1 / R 2 ≤ 1.23. In some embodiments, 1.02 ≤ R 1 / R 2 ≤ 1.15. In some embodiments, 1.06 ≤ R 1 / R 2 ≤ 1.18. In some embodiments, 1.15 ≤ R 1 / R 2 ≤ 1.23. In some embodiments, 1.02 ≤ R 1 / R 2≤1.06. In some embodiments, 1.06 ≤ R 1 / R 2 ≤1.15. In some embodiments, 1.15 ≤ R 1 / R 2 ≤1.18. In some embodiments, 1.18 ≤ R 1 / R 2 ≤1.23. In some embodiments, R 1 / R 2 has a value within the range of 1.02, 1.03, 1.04, 1.05, 1.06, 1.07, 1.08, 1.09, 1.10, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.20, 1.21, 1.22, 1.23 or any value within the range formed by any two of them. By adjusting the value of R 1 / R 2 within the above range, the stacking relationship between the first silicon-carbon particles and the second silicon-carbon particles in the negative electrode material layer can be regulated, stress can be reduced, and more conductive paths can be provided, thereby further improving the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling.

[0033] In some embodiments, 0.05 ≤ R 1 -R 2 ≤0.15. In some embodiments, 0.05 ≤ R 1 -R 2 ≤0.13. In some embodiments, 0.13 ≤ R 1 -R 2 ≤0.15. In some embodiments, R 1 -R 2 has a value within the range of 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15 or any value within the range formed by any two of them. By adjusting the value of R 1 -R 2 within the above range, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be further improved.

[0034] In some embodiments, 0.90 ≤ R 1 ≤0.98. In some embodiments, 0.90 ≤ R 1 ≤0.96. In some embodiments, 0.92 ≤ R 1 ≤0.98. In some embodiments, 0.90 ≤ R 1 ≤0.94. In some embodiments, 0.92 ≤ R 1 ≤0.96. In some embodiments, 0.94 ≤ R1 ≤0.98. In some embodiments, 0.90 ≤ R 1 ≤0.92. In some embodiments, 0.92 ≤ R 1 ≤0.94. In some embodiments, 0.94 ≤ R 1 ≤0.96. In some embodiments, 0.96 ≤ R 1 ≤0.98. In some embodiments, R 1 is a value within the range composed of 0.90, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98 or any two of them. By adjusting the value of R 1 within the above range, the safety performance, discharge rate and capacity retention ability in the later stage of the secondary battery cycle can be further improved.

[0035] In some embodiments, 0.78 ≤ R 2 ≤0.92. In some embodiments, 0.78 ≤ R 2 ≤0.85. In some embodiments, 0.83 ≤ R 2 ≤0.92. In some embodiments, 0.78 ≤ R 2 ≤0.83. In some embodiments, 0.83 ≤ R 2 ≤0.85. In some embodiments, 0.85 ≤ R 2 ≤0.92. In some embodiments, R 2 is a value within the range composed of 0.78, 0.79, 0.80, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.89, 0.90, 0.91, 0.92 or any two of them. By adjusting the value of R 2 within the above range, the safety performance, discharge rate and capacity retention ability in the later stage of the secondary battery cycle can be further improved.

[0036] In some embodiments, the first silicon-carbon particles contain silicon element and carbon element. Based on the sum of the masses of silicon element and carbon element in the first silicon-carbon particles, the mass ratio of silicon element is C 1 %, 35 ≤ C 1 ≤55. In some embodiments, 35 ≤ C 1 ≤38. In some embodiments, 38 ≤ C 1 ≤55. In some embodiments, C 1 is a value within the range composed of 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55 or any two of them. Adjusting C 1When the value is within the above range, the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle can be further improved.

[0037] In some embodiments, the second silicon-carbon particles contain silicon and carbon elements. Based on the total mass of silicon and carbon elements in the second silicon-carbon particles, the mass percentage of silicon element is C 2 %, 45 ≤ C 2 ≤ 65. In some embodiments, 45 ≤ C 2 ≤ 63. In some embodiments, 48 ≤ C 2 ≤ 65. In some embodiments, 45 ≤ C 2 ≤ 48. In some embodiments, 48 ≤ C 2 ≤ 63. In some embodiments, 63 ≤ C 2 ≤ 65. In some embodiments, the value of C 2 is 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65 or a value within the range composed of any two of them. By adjusting the value of C 2 within the above range, the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle can be further improved.

[0038] In some embodiments, 1.2 ≤ C 2 / C 1 ≤ 1.7. In some embodiments, the value of C 2 / C 1 is 1.2, 1.3, 1.4, 1.5, 1.6, 1.7 or a value within the range composed of any two of them. By adjusting the value of C 2 / C 1 within the above range, the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle can be further improved.

[0039] In some embodiments, the silicon-carbon particles contain third silicon-carbon particles; when 2 < D < 5, the silicon-carbon particles are third silicon-carbon particles; the number of the first silicon-carbon particles is N 1 , the number of the second silicon-carbon particles is N 2 , the number of the third silicon-carbon particles is N 3 , N 3 / (N 1 +N 2 +N 3 ) < 0.2. 0.01 ≤ N 3 / (N 1 +N 2 +N 3 ) ≤ 0.19. In some embodiments, N 3 / (N1 +N 2 +N 3 ) has values of 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, or values within the range formed by any two of them. Adjust N 3 / (N 1 +N 2 +N 3 ) having a value within the above range can improve the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling.

[0040] In some embodiments, the silicon-carbon particles comprise a carbon carrier and silicon crystallites at least partially distributed inside the carbon carrier. By this arrangement, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be improved.

[0041] In some embodiments, the surface of the silicon-carbon particles may separately or simultaneously include functional materials such as polymers, conductive agents, and solid electrolytes to further improve the performance of the secondary battery.

[0042] In some embodiments, the surface of the silicon-carbon particles may include a carbon layer, and the thickness of the carbon layer is preferably 10 to 100 nanometers. The carbon layer may be provided on most of the surface of the silicon-carbon particles, and more preferably on the entire surface of the silicon-carbon particles. By providing a carbon layer on the surface of the silicon-carbon particles, it is beneficial to reduce the gas generation amount of the silicon-carbon particles during the preparation of the secondary battery, improve the flatness of the negative electrode sheet, and thereby improve the performance of the secondary battery.

[0043] Optionally, the negative electrode material layer may further include a crystalline carbon material and / or an amorphous carbon material. The crystalline carbon material may be natural graphite, artificial graphite, etc., and the amorphous carbon may be soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, or calcined coke, etc.

[0044] In some embodiments, the negative electrode material layer includes a conductive agent; the type of the conductive agent is not limited, and any known conductive material can be used. Examples of the conductive agent may include, but are not limited to, carbon blacks such as acetylene black and Super-P; amorphous carbon materials such as needle coke; carbon nanotubes; graphene, etc. The above conductive agents can be used alone or in any combination.

[0045] The present application places no particular limitation on the negative electrode current collector, as long as the object of the present application can be achieved. For example, the negative electrode current collector may include copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, or a polymer substrate coated with a conductive metal, etc. Among them, the conductive metal includes but is not limited to copper, nickel, or titanium, and the material of the polymer substrate includes but is not limited to at least one of polyethylene, polypropylene, ethylene-propylene copolymer, polyethylene terephthalate, polyethylene naphthalate, or poly(p-phenylene terephthalamide). In the present application, there is no particular limitation on the thickness of the negative electrode current collector and the negative electrode material layer, as long as the object of the present application can be achieved. For example, the thickness of the negative electrode current collector is 4 μm to 12 μm, and the thickness of the single-sided negative electrode material layer is 30 μm to 160 μm. In the present application, the negative electrode material layer may be provided on one surface in the thickness direction of the negative electrode current collector, or may be provided on both surfaces in the thickness direction of the negative electrode current collector. It should be noted that the "surface" here may be the entire area of the negative electrode current collector or a partial area of the negative electrode current collector. The present application has no particular limitation, as long as the object of the present application can be achieved.

[0046] In some embodiments, the elongation of the negative electrode current collector is δ%, 3 ≤ δ ≤ 8. In some embodiments, 3 ≤ δ ≤ 6. In some embodiments, 4 ≤ δ ≤ 8. In some embodiments, 3 ≤ δ ≤ 4. In some embodiments, 4 ≤ δ ≤ 6. In some embodiments, 6 ≤ δ ≤ 8. In some embodiments, the elongation of the negative electrode current collector is 3, 4, 5, 6, 7, 8, or a value within the range composed of any two of them. By adjusting the elongation of the negative electrode current collector within the above range, the possibility of the first silicon-carbon particles and the second silicon-carbon particles causing uneven stress on the negative electrode current collector and resulting in the deformation of the current collector can be reduced, and the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle can be improved.

[0047] In some embodiments, the average particle size of the first silicon-carbon particles is RA µm, and 1.3 ≤ RA / δ ≤ 3.6. In some embodiments, 1.3 ≤ RA / δ ≤ 2.9. In some embodiments, 2.2 ≤ RA / δ ≤ 3.6. In some embodiments, 1.3 ≤ RA / δ ≤ 2.2. In some embodiments, 2.2 ≤ RA / δ ≤ 2.9. In some embodiments, 2.9 ≤ RA / δ ≤ 3.6. In some embodiments, the value of RA / δ is 1.3, 1.4, 1.6, 1.7, 1.9, 2.0, 2.1, 2.2, 2.3, 2.5, 2.6, 2.7, 2.9, 2.9, 3.1, 3.2, 3.3, 3.4, 3.6 or a value within the range formed by any two of them. By adjusting the value of RA / δ within the above range, the relationship between the stress exerted on the current collector when the first silicon-carbon particles expand and the expandable space provided by the elongation rate of the current collector to the negative electrode material layer can be balanced, improving the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle.

[0048] In some embodiments, 7.9 ≤ RA ≤ 10.7. In some embodiments, 7.9 ≤ RA ≤ 8.6. In some embodiments, 8.6 ≤ RA ≤ 10.7. In some embodiments, the value of RA is 7.9, 8.1, 8.3, 8.4, 8.6, 8.7, 8.9, 9.0, 9.2, 9.3, 9.4, 9.6, 9.7, 9.8, 10.0, 10.1, 10.3, 10.5, 10.7 or a value within the range formed by any two of them. Controlling the value of RA to meet the above range can further improve the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle.

[0049] The negative electrode material layer may further include a negative electrode binder. The negative electrode binder can improve the binding between the negative electrode active material particles and the binding between the negative electrode active material and the current collector. There is no particular limitation on the type of the negative electrode binder, as long as it is a material that is stable to the electrolyte or the solvent used in electrode manufacturing. In some embodiments, the negative electrode binder includes a resin binder. Examples of the resin binder include, but are not limited to, fluororesin, polyacrylonitrile (PAN), polyimide resin, acrylic resin, polyolefin resin, etc. When preparing the negative electrode binder slurry using an aqueous solvent, the negative electrode binder includes, but is not limited to, hydroxyethyl carboxymethyl cellulose (HECMC) or its salt, carboxymethyl cellulose (CMC) or its salt, styrene-butadiene rubber (SBR), polyacrylic acid (PAA) or its salt, polyacrylate, polyvinyl alcohol, etc.

[0050] Exemplarily, the negative electrode can be prepared by the following method: Coating a negative electrode binder slurry containing a negative electrode binder, silicon-carbon composite particles, a conductive agent, etc. on the negative electrode current collector, drying, and then calendering to form a negative electrode material layer on both sides of the negative electrode current collector, whereby the negative electrode can be obtained.

[0051] There is no particular limitation on the tap density of the negative electrode sheet in this application, as long as the object of this application can be achieved. For example, the tap density of the negative electrode sheet can be 1.0 g / cm 3 to 1.85 g / cm 3 . There is no particular limitation on the cold pressing pressure of the negative electrode sheet in this application, as long as the object of this application can be achieved. For example, the cold pressing pressure of the negative electrode sheet can be 3 tons to 30 tons.

[0052] Electrolyte

[0053] The electrolyte used in the secondary battery of this application includes an electrolyte and a solvent for dissolving the electrolyte.

[0054] In some embodiments, the electrolyte contains diethyl carbonate and ethyl propionate. Based on the mass of the electrolyte, the mass percentage of diethyl carbonate is S 1 %, and the mass percentage of ethyl propionate is S 2 %, 2.3 ≤ S 2 / S 1 ≤ 6. In some embodiments, 2.3 ≤ S 2 / S 1 ≤ 4.5. In some embodiments, 2.8 ≤ S 2 / S 1 ≤ 6. In some embodiments, 2.3 ≤ S 2 / S 1 ≤ 4. In some preferred embodiments, 2.8 ≤ S 2 / S 1 ≤ 4.5. In some embodiments, 4 ≤ S 2 / S 1 ≤ 6. In some embodiments, 2.3 ≤ S 2 / S 1 ≤ 2.8. In some embodiments, 2.8 ≤ S 2 / S 1 ≤ 4. In some embodiments, 4 ≤ S 2 / S 1 ≤ 4.5. In some embodiments, 4.5 ≤ S 2 / S 1 ≤ 6. In some embodiments, the value of S 2 / S 1 is a value within the range composed of 2.3, 2.5, 2.6, 2.9, 3, 3.2, 3.4, 3.6, 3.8, 3.9, 4.1, 4.3, 4.6, 4.7, 4.9, 5.1, 5.4, 5.4, 5.7, 6 or any two of them. By making the electrolyte contain both diethyl carbonate and ethyl propionate and adjusting S 2 / S 1When the value is within the above range, the film formation thickness of the solid electrolyte interface film (SEI film) on the surface of the first silicon-carbon particle and the surface of the second silicon-carbon particle can be coordinated, further improving the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle.

[0055] In some embodiments, 5 ≤ S 1 ≤ 20. In some embodiments, 5 ≤ S 1 ≤ 15. In some embodiments, 6 ≤ S 1 ≤ 20. In some embodiments, 5 ≤ S 1 ≤ 10. In some embodiments, 6 ≤ S 1 ≤ 15. In some embodiments, 10 ≤ S 1 ≤ 20. In some embodiments, 5 ≤ S 1 ≤ 6. In some embodiments, 6 ≤ S 1 ≤ 10. In some embodiments, 10 ≤ S 1 ≤ 15. In some embodiments, 15 ≤ S 1 ≤ 20. In some embodiments, S 1 is a value within the range composed of 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 or any two of them. By adjusting the value of S 1 within the above range, the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle can be further improved.

[0056] In some embodiments, 20 ≤ S 2 ≤ 45. In some embodiments, 20 ≤ S 2 ≤ 42. In some embodiments, 35 ≤ S 2 ≤ 45. In some embodiments, 20 ≤ S 2 ≤ 40. In some embodiments, 35 ≤ S 2 ≤ 42. In some embodiments, 40 ≤ S 2 ≤ 45. In some embodiments, 20 ≤ S 2 ≤ 35. In some embodiments, 35 ≤ S 2 ≤ 40. In some embodiments, 40 ≤ S 2 ≤ 42. In some embodiments, 42 ≤ S 2 ≤ 45. In some embodiments, S 2 is a value within the range composed of 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45 or any two of them. By adjusting S 2When the value is within the above range, the safety performance, discharge rate and capacity retention ability in the later stage of the secondary battery cycle can be further improved.

[0057] In some embodiments, the electrolyte contains fluoroethylene carbonate and vinylene carbonate. Based on the mass of the electrolyte, the mass percentage of fluoroethylene carbonate is A 1 %, and the mass percentage of vinylene carbonate is A 2 %, 0.5 ≤ A 1 / (100 × A 2 ) ≤ 1.8. In some embodiments, 0.5 ≤ A 1 / (100 × A 2 ) ≤ 1.5. In some embodiments, 0.7 ≤ A 1 / (100 × A 2 ) ≤ 1.8. In some embodiments, 0.5 ≤ A 1 / (100 × A 2 ) ≤ 1.2. In some preferred embodiments, 0.7 ≤ A 1 / (100 × A 2 ) ≤ 1.5. In some embodiments, 1.2 ≤ A 1 / (100 × A 2 ) ≤ 1.8. In some embodiments, 0.5 ≤ A 1 / (100 × A 2 ) ≤ 0.7. In some embodiments, 0.7 ≤ A 1 / (100 × A 2 ) ≤ 1.2. In some embodiments, 1.2 ≤ A 1 / (100 × A 2 ) ≤ 1.5. In some embodiments, 1.5 ≤ A 1 / (100 × A 2 ) ≤ 1.8. In some embodiments, the value of A 1 / (100 × A 2 ) is a value within the range composed of 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 or any two of them. By making the electrolyte contain both fluoroethylene carbonate and vinylene carbonate and adjusting the value of A 1 / (100 × A 2 ) within the above range, the safety performance, discharge rate and capacity retention ability in the later stage of the secondary battery cycle can be further improved.

[0058] In some embodiments, 11 ≤ A 1 ≤ 18. In some embodiments, 11 ≤ A 1 ≤ 15. In some embodiments, 12 ≤ A1 ≤18. In some embodiments, 11 ≤ A 1 ≤14. In some embodiments, 12 ≤ A 1 ≤15. In some embodiments, 14 ≤ A 1 ≤18. In some embodiments, 11 ≤ A 1 ≤12. In some embodiments, 12 ≤ A 1 ≤14. In some embodiments, 14 ≤ A 1 ≤15. In some embodiments, 15 ≤ A 1 ≤18. In some embodiments, A 1 has a value of 11, 12, 13, 14, 15, 16, 17, 18 or a value within the range formed by any two of them. By adjusting the value of A 1 within the above range, the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle can be further improved.

[0059] In some embodiments, 0.05 ≤ A 2 ≤0.5. In some embodiments, 0.05 ≤ A 2 ≤0.3. In some embodiments, 0.1 ≤ A 2 ≤0.5. In some embodiments, 0.05 ≤ A 2 ≤0.2. In some embodiments, 0.1 ≤ A 2 ≤0.3. In some embodiments, 0.2 ≤ A 2 ≤0.5. In some embodiments, 0.05 ≤ A 2 ≤0.1. In some embodiments, 0.1 ≤ A 2 ≤0.2. In some embodiments, 0.2 ≤ A 2 ≤0.3. In some embodiments, 0.3 ≤ A 2 ≤0.5. In some embodiments, A 2 has a value of 0.05, 0.1, 0.2, 0.3, 0.4, 0.5 or a value within the range formed by any two of them. By adjusting the value of A 2 within the above range, the safety performance, discharge rate, and capacity retention ability in the later stage of the secondary battery cycle can be further improved.

[0060] In some embodiments, the electrolyte contains tetramethyltetravinylcyclotetrasiloxane. Based on the mass of the electrolyte, the mass percentage content of tetramethyltetravinylcyclotetrasiloxane is 0.01% to 0.1%. Exemplarily, the mass percentage content of tetramethyltetravinylcyclotetrasiloxane is 0.01%, 0.02%, 0.03%, 0.04%, 0.05%, 0.06%, 0.07%, 0.08%, 0.09%, 0.1%, or a value within the range composed of any two of them. Making the electrolyte include tetramethyltetravinylcyclotetrasiloxane and adjusting the mass percentage content of tetramethyltetravinylcyclotetrasiloxane within the above range can improve the conductivity of the SEI film on the surface of the second silicon-carbon particles, and improve the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling.

[0061] In some embodiments, the electrolyte contains diethyl (cyanomethyl) phosphonate. Based on the mass of the electrolyte, the mass percentage content of diethyl (cyanomethyl) phosphonate is 0.1% to 0.8%. Exemplarily, the mass percentage content of diethyl (cyanomethyl) phosphonate is 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, or a value within the range composed of any two of them. Making the electrolyte contain diethyl (cyanomethyl) phosphonate and adjusting the mass percentage content of diethyl (cyanomethyl) phosphonate within the above range can improve the elasticity of the SEI film on the surface of the first silicon-carbon particles, and improve the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling.

[0062] The present application has no particular limitation on the types of other components of the electrolyte, as long as the object of the present application can be achieved. For example, it may include but is not limited to at least one of dimethyl carbonate, ethylene carbonate, propylene carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl ethyl carbonate, butylene carbonate, ethylene ethylene carbonate, methyl formate, methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, propyl propionate, γ-butyrolactone, decanolide, valerolactone, or caprolactone, ethylene glycol dimethyl ether, dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1-ethoxy-1-methoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, dimethyl sulfoxide, 1,2-dioxolane, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, N-methyl-2-pyrrolidone, dimethylformamide, acetonitrile, trimethyl phosphate, triethyl phosphate, or trioctyl phosphate.

[0063] In some embodiments, there is no particular limitation on the electrolyte, and a substance known as an electrolyte can be arbitrarily used. The weight of the electrolyte is not particularly limited as long as the effects of the present application are not impaired.

[0064] Positive electrode

[0065] The positive electrode includes a positive electrode current collector and a positive electrode material layer provided on the surface of the positive electrode current collector.

[0066] The positive electrode material layer contains a positive electrode active material, and the positive electrode material layer may be one layer or multiple layers. Each layer of the multiple positive electrode active materials may contain the same or different positive electrode active materials. The positive electrode active material is any material capable of reversibly inserting and extracting alkali metal ions.

[0067] The positive electrode active material includes a lithium transition metal oxide containing nickel and other transition metals. In the lithium transition metal oxide containing nickel and other transition metals, the amount of nickel may be 60 mol% or more, for example, 75 mol% or more, for example, 80 mol% or more, for example, 85 mol% or more, or for example, 90 mol% or more, relative to the total molar amount of the transition metals.

[0068] For example, the lithium transition metal oxide may be a compound represented by the following formula α:

[0069] Formula α: Li a Ni x Co y M z O 2-b A b ,

[0070] Wherein in formula α, 0.9 ≤ a ≤ 1.2, 0 ≤ b ≤ 0.2, 0.6 ≤ x < 1, 0 < y ≤ 0.3, 0 < z ≤ 0.3, and x + y + z = 1, M is at least one selected from manganese (Mn), vanadium (V), magnesium (Mg), gallium (Ga), silicon (Si), tungsten (W), molybdenum (Mo), iron (Fe), chromium (Cr), copper (Cu), zinc (Zn), titanium (Ti), aluminum (Al), or boron (B), and A is F, S, Cl, Br, or a combination thereof. For example, the above subscripts may be 0.7 ≤ x < 1, 0 < y ≤ 0.3, and 0 < z ≤ 0.3; 0.8 ≤ x < 1, 0 < y ≤ 0.3, and 0 < z ≤ 0.3; 0.8 ≤ x < 1, 0 < y ≤ 0.2, and 0 < z ≤ 0.2; 0.83 ≤ x < 0.97, 0 < y ≤ 0.15, and 0 < z ≤ 0.15; or 0.85 ≤ x < 0.95, 0 < y ≤ 0.1, and 0 < z ≤ 0.1.

[0071] For example, the lithium transition metal oxide may be at least one compound represented by the following formula β or formula γ:

[0072] Formula β: LiNi x Co y Mn z O 2 ,

[0073] Among them, in formula β, 0.6 ≤ x ≤ 0.95, 0 < y ≤ 0.2, and 0 < z ≤ 0.1. For example, 0.7 ≤ x ≤ 0.95, 0 < y ≤ 0.3, and 0 < z ≤ 0.3.

[0074] Formula γ: LiNi x Co y Al z O 2 ,

[0075] Among them, in formula γ, 0.6 ≤ x ≤ 0.95, 0 < y ≤ 0.2, and 0 < z ≤ 0.1. For example, 0.7 ≤ x ≤ 0.95, 0 < y ≤ 0.3, and 0 < z ≤ 0.3. For example, 0.8 ≤ x ≤ 0.95, 0 < y ≤ 0.3, and 0 < z ≤ 0.3. For example, 0.82 ≤ x ≤ 0.95, 0 < y ≤ 0.15, and 0 < z ≤ 0.15. For example, 0.85 ≤ x ≤ 0.95, 0 < y ≤ 0.1, and 0 < z ≤ 0.1.

[0076] For example, the lithium transition metal oxide can be LiNi 0.6 Co 0.2 Mn 0.2 O 2 , LiNi 0.88 Co 0.08 Mn 0.04 O 2 , LiNi 0.8 Co 0.15 Mn 0.05 O 2 , LiNi 0.8 Co 0.1 Mn 0.1 O 2 , LiNi 0.88 Co 0.1 Mn 0.02 O 2 , LiNi 0.8 Co 0.15 Al 0.05 O 2 or LiNi 0.88 Co 0.1 Al 0.02 O 2 .

[0077] According to another embodiment, the positive electrode active material includes at least one active material selected from the group consisting of: Li-Ni-Co-Al (NCA), Li-Ni-Co-Mn (NCM), lithium cobalt oxide (LiCoO 2 ), lithium manganese oxide (LiMnO 2 ), lithium nickel oxide (LiNiO 2) and lithium iron phosphate (LiFePO 4 ).

[0078] In some embodiments, the positive electrode material layer includes a positive electrode conductive material; the type of the positive electrode conductive material is not limited, and any known conductive material can be used. Examples of the positive electrode conductive material may include, but are not limited to, carbon blacks such as acetylene black and Super-P; amorphous carbons such as needle coke; materials such as carbon nanotubes; graphene, etc. The above positive electrode conductive materials can be used alone or in any combination.

[0079] The type of the positive electrode binder used in the manufacture of the positive electrode material layer is not particularly limited. In the case of the coating method, any material that can be dissolved or dispersed in the liquid medium used during electrode manufacturing is acceptable. Examples of the positive electrode binder may include, but are not limited to, one or more of the following: resin-based polymers such as polyethylene, polypropylene, polyethylene terephthalate, polymethyl methacrylate, polyimide, aromatic polyamide, cellulose, and nitrocellulose; rubber-like polymers such as styrene-butadiene rubber (SBR), nitrile rubber (NBR), fluororubber, isoprene rubber, polybutene rubber, and ethylene-propylene rubber; thermoplastic elastomer-like polymers such as styrene-butadiene-styrene block copolymer or its hydride, ethylene-propylene-diene terpolymer (EPDM), styrene-ethylene-butadiene-ethylene copolymer, and styrene-isoprene-styrene block copolymer or its hydride; soft resin-like polymers such as syndiotactic-1,2-polybutadiene, polyvinyl acetate, ethylene-vinyl acetate copolymer, and propylene-α-olefin copolymer; fluorine-based polymers such as polyvinylidene fluoride (PVDF), polytetrafluoroethylene, fluorinated polyvinylidene fluoride, and polytetrafluoroethylene-ethylene copolymer; polymer compositions having ionic conductivity of alkali metal ions (especially lithium ions), etc. The above positive electrode binders can be used alone or in any combination.

[0080] The type of the solvent used to form the positive electrode slurry is not limited, as long as it is a solvent that can dissolve or disperse the positive electrode active material, conductive material, positive electrode binder, and thickener used as required. Examples of the solvent used to form the positive electrode slurry may include either an aqueous solvent or an organic solvent. Examples of the aqueous medium may include, but are not limited to, a mixed medium of alcohol and water or water, etc. Examples of the organic medium may include, but are not limited to, aliphatic hydrocarbons such as hexane; aromatic hydrocarbons such as benzene, toluene, xylene, and methylnaphthalene; heterocyclic compounds such as quinoline and pyridine; ketones such as acetone, methyl ethyl ketone, and cyclohexanone; esters such as methyl acetate and methyl acrylate; amines such as diethylenetriamine and N,N-dimethylaminopropylamine; ethers such as diethyl ether, propylene oxide, and tetrahydrofuran (THF); amides such as N-methylpyrrolidone (NMP), dimethylformamide, and dimethylacetamide; aprotic polar solvents such as hexamethylphosphoramide and dimethyl sulfoxide, etc.

[0081] Thickeners are usually used to adjust the viscosity of the slurry. In the case of using an aqueous medium, thickeners and styrene-butadiene rubber (SBR) emulsions can be used for slurrying. The types of thickeners are not particularly limited, and examples thereof may include, but are not limited to, carboxymethyl cellulose, methyl cellulose, hydroxymethyl cellulose, ethyl cellulose, polyvinyl alcohol, oxidized starch, phosphorylated starch, casein, and their salts, etc. The above thickeners can be used alone or in any combination.

[0082] The type of the positive electrode current collector is not particularly limited, and it can be any known material suitable for use as a positive electrode current collector. Examples of the positive electrode current collector may include, but are not limited to, metal materials such as aluminum, stainless steel, nickel plating, titanium, tantalum, etc.; materials such as carbon cloth and carbon paper. In some embodiments, the positive electrode current collector is a metal material. In some embodiments, the positive electrode current collector is aluminum.

[0083] In order to reduce the electron contact resistance between the positive electrode current collector and the positive electrode material layer, the surface of the positive electrode current collector may include a conductive aid or a conductive coating. Examples of the conductive aid may include, but are not limited to, carbon and noble metals such as gold, platinum, silver, etc. Examples of the conductive coating may include a mixture layer containing inorganic oxides, conductive agents, and binders.

[0084] The positive electrode can be fabricated by forming a positive electrode material layer containing a positive electrode active material and a binder on the current collector. The manufacture of a positive electrode using a positive electrode active material can be carried out by a conventional method, that is, dry-mixing the positive electrode active material, the binder, and, if necessary, a conductive material, a thickener, etc., making it into a sheet, and pressing the obtained sheet onto the positive electrode current collector; or dissolving or dispersing these materials in a liquid medium to make a slurry, coating the slurry onto the positive electrode current collector and drying it, thereby forming a positive electrode material layer on the current collector, and thus a positive electrode can be obtained.

[0085] Separator

[0086] In order to prevent short circuit, a separator is usually provided between the positive electrode and the negative electrode. In this case, the electrolyte of the present application usually penetrates into the separator for use.

[0087] There is no particular limitation on the material and shape of the separator, as long as the effects of the present application are not significantly impaired. The separator can be a resin, glass fiber, inorganic substance, etc. formed of a material stable to the electrolyte of the present application. In some embodiments, the separator includes a porous sheet or a non-woven fabric-like substance with excellent liquid retention properties, etc. Examples of the material of the resin or glass fiber separator may include, but are not limited to, polyolefin, aromatic polyamide, polytetrafluoroethylene, polyethersulfone, etc. In some embodiments, the polyolefin is polyethylene or polypropylene. In some embodiments, the polyolefin is polypropylene. The above materials of the separator can be used alone or in any combination.

[0088] The separator membrane may also be a material formed by laminating the above materials. Examples thereof include, but are not limited to, a three-layer separator membrane laminated in the order of polypropylene, polyethylene, and polypropylene, etc.

[0089] Examples of the inorganic material may include, but are not limited to, oxides such as alumina and silica, nitrides such as aluminum nitride and silicon nitride, and sulfates (e.g., barium sulfate, calcium sulfate, etc.). The form of the inorganic material may include, but is not limited to, granular or fibrous.

[0090] The morphology of the separator membrane may be in the form of a thin film. Examples thereof include, but are not limited to, non-woven fabric, woven fabric, microporous membrane, etc. In the form of a thin film, the pore diameter of the separator membrane is 0.01 μm to 1 μm, and the thickness is 5 μm to 50 μm. In addition to the above independent thin film-like separator membrane, the following separator membrane may also be used: a separator membrane formed by forming a composite porous layer containing the above inorganic particles on the surface of the positive electrode and / or negative electrode by using a resin-based adhesive. For example, a separator membrane formed by using a fluororesin as an adhesive to form a porous layer on both sides of a positive electrode with 90% of the alumina particles having a particle size less than 1 μm.

[0091] The thickness of the separator membrane is arbitrary. In some embodiments, the thickness of the separator membrane is greater than 1 μm, greater than 5 μm, or greater than 8 μm. In some embodiments, the thickness of the separator membrane is less than 50 μm, less than 40 μm, or less than 30 μm. When the thickness of the separator membrane is within the above range, insulation and mechanical strength can be ensured, and the rate performance and energy density of the secondary battery can be ensured.

[0092] This application further provides an electronic device, which includes the secondary battery according to this application.

[0093] The use of the secondary battery of this application is not particularly limited, and it can be used in any electronic device known in the prior art. In some embodiments, the secondary battery of this application can be used in, but not limited to, laptop computers, pen input computers, mobile computers, e-book players, mobile phones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, liquid crystal TVs, cleaners, portable CD players, mini discs, transceivers, electronic notebooks, calculators, memory cards, portable recorders, radios, backup power supplies, motors, automobiles, motorcycles, motorized bicycles, bicycles, lighting fixtures, toys, game consoles, clocks, power tools, flashlights, cameras, large household storage batteries, and lithium-ion capacitors, etc.

[0094] Examples

[0095] Hereinafter, taking a lithium-ion battery as an example, examples and comparative examples are given to more specifically illustrate the implementation manners of the secondary battery of the present application. Those skilled in the art will understand that the preparation methods described in the present application are only examples, and any other suitable preparation methods are within the scope of the present application. Various tests and evaluations are carried out according to the following methods. In addition, unless otherwise specified, "parts" and "%" are based on mass.

[0096] Test method

[0097] Safety performance in the later stage of cycling (thermal shock test)

[0098] (1) At 25°C ± 1°C, the lithium-ion battery is charged at a constant current of 1.0C to 4.45V, then charged at a constant voltage of 4.45V until the current drops to 0.05C, and then the lithium-ion battery is discharged at a constant current of 1.0C to 3.0V and then stopped. This is taken as one charge-discharge cycle. The lithium-ion battery is subjected to 200 charge-discharge cycles.

[0099] (2) Under the condition of 25°C ± 1°C, the lithium-ion battery is charged at a constant current of 0.5C to 4.45V, then charged at a constant voltage of 4.45V until the current is 0.025C, and then the lithium-ion battery is vertically placed in a box and heated at a rate of 5°C / min. Heat up to a specific temperature T (T = 100°C) and hold for 100 minutes. If the lithium-ion battery does not catch fire or explode within 100 minutes, it is considered to pass the test.

[0100] When the lithium-ion battery passes the test, repeat steps (1) and (2). When repeating step (2), increase the specific temperature T by 5°C, and keep other operations unchanged. Do this until the lithium-ion battery fails the test, and record the specific temperature T at the last time when the lithium-ion battery passes the test as the maximum thermal shock temperature.

[0101] For example, "110°C" means that the lithium-ion battery has passed the test at a specific temperature T = 110°C and has not passed the test at a specific temperature T = 115°C.

[0102] Discharge rate in the later stage of cycling

[0103] (1) At 25°C ± 1°C, the lithium-ion battery is charged at a constant current of 1.0C to 4.45V, then charged at a constant voltage of 4.45V until the current drops to 0.05C, and then the lithium-ion battery is discharged at a constant current of 1.0C to 3.0V and then stopped. This is taken as one charge-discharge cycle. The lithium-ion battery is subjected to 200 charge-discharge cycles.

[0104] (2) At 25°C ± 1°C, charge at a constant current of 0.5C to 4.45V, after constant voltage charging to 0.05C, let it stand for 5 minutes, discharge at a constant current of 0.2C to 3.0V, and record the 0.2C discharge capacity.

[0105] (3) At 25°C ± 1°C, charge at a constant current of 0.5C to 4.45V, then charge at a constant voltage until the current drops to 0.05C, and let it stand for 5 minutes. Then discharge at a constant current of 2.0C to 3.0V, and record the 2.0C discharge capacity.

[0106] Discharge rate in the later stage of the cycle = 2.0C discharge capacity / 0.2C discharge capacity × 100%.

[0107] Capacity loss rate in the later stage of the cycle

[0108] (1) At 25°C ± 1°C, charge the lithium-ion battery at a constant current of 0.5C to 4.45V, then charge at a constant voltage of 4.45V until the current drops to 0.025C, let it stand for 5 minutes, and then discharge at a constant current of 0.5C to 3.0V, and record the discharge amount Q of the lithium-ion battery. 1 ;

[0109] (2) At 25°C ± 1°C, charge the lithium-ion battery at a constant current of 1.0C to 4.45V, then charge at a constant voltage of 4.45V until the current drops to 0.05C, and then discharge the lithium-ion battery at a constant current of 1.0C to 3.0V and stop. This is taken as a standard charge-discharge cycle. After performing 100 standard charge-discharge cycles on the lithium-ion battery, charge at a constant current of 0.5C to 4.45V, then charge at a constant voltage of 4.45V until the current drops to 0.025C, let it stand for 5 minutes, and then discharge at a constant current of 0.5C to 3.0V, and record the discharge amount Q of the lithium-ion battery. 2 ;

[0110] (3) At 25°C ± 1°C, after performing 100 standard charge-discharge cycles on the lithium-ion battery, charge at a constant current of 0.5C to 4.45V, then charge at a constant voltage of 4.45V until the current drops to 0.025C, let it stand for 5 minutes, and then discharge at a constant current of 0.5C to 3.0V, and record the discharge amount Q of the lithium-ion battery. 3 .

[0111] Capacity loss rate in the later stage of the cycle = (Q 3 - Q 2 ) / (Q 2 - Q 1 ) × 100%.

[0112] Particle size and particle number (D µm, N 1 , N 2 , N 3 , R 1 , R 2 )

[0113] Disassemble the battery under test to obtain the negative electrode, cut the negative electrode into a size of 1 cm × 1 cm, and then use argon plasma polishing technology to polish the cut surface of the negative electrode to prepare a negative electrode cross-section sample.

[0114] Observe the cross-section of the negative electrode in the backscattered mode of a scanning electron microscope, and randomly select a test area of 40 µm × 80 µm in the cross-section of the negative electrode. Statistically analyze the cross-sectional area S of the first silicon-carbon particles in the test area. According to the formula S = π × R 2 calculate the particle size D = 2R of the selected silicon-carbon particles. Silicon-carbon particles with a particle size D greater than or equal to 5 µm and less than or equal to 13 µm are the first silicon-carbon particles, silicon-carbon particles with a particle size D greater than or equal to 0.1 µm and less than or equal to 2 µm are the second silicon-carbon particles, and silicon-carbon particles with a particle size D greater than 2 µm and less than 5 µm are the third silicon-carbon particles. The number of the first silicon-carbon particles is N 1 , the number of the second silicon-carbon particles is N 2 , and the number of the third silicon-carbon particles is N 3 . Statistically analyze the area S' of the circumscribed circle of the cross-section of the first silicon-carbon particles. Circularity = S / S'. Randomly select three first silicon-carbon particles in the test area to measure the circularity and calculate the arithmetic mean to obtain the circularity R 1 of the first silicon-carbon particles. The test method for the circularity R 2 of the second silicon-carbon particles is the same.

[0115] Test for the mass percentage of silicon element (C 1 % and C 2 %)

[0116] Cut the negative electrode into a size of 1 cm × 1 cm, and then use argon plasma polishing technology to polish the cut surface of the negative electrode to obtain a cross-section sample of the negative electrode.

[0117] Observe the cross-section of the negative electrode in the backscattered mode of a scanning electron microscope, and randomly select three first silicon-carbon particles and three second silicon-carbon particles in the cross-section sample of the negative electrode. Respectively test the weight percentage contents of silicon element and carbon element of the selected particles by energy-dispersive X-ray analysis, and calculate the mass percentage of silicon element of each particle. Calculate the arithmetic mean of the mass percentages of silicon element of the obtained first silicon-carbon particles or second silicon-carbon particles respectively, and the mass percentages of silicon element can be obtained as C 1 % and C 2 %.

[0118] Test for the elongation rate (δ) of the negative electrode current collector

[0119] Disassemble the battery under test to obtain the negative electrode, and remove the negative electrode material layer to obtain the negative electrode current collector. Cut the negative electrode current collector into a test sample with a length × width of 200 mm × 15 mm, and mark the original gauge length L 0 on the test sample. At a test temperature of 25 °C ± 1 °C, use a universal material testing machine to stretch the negative electrode current collector along the length direction at a stretching rate of 5 mm / min. The elongation at break of the test sample is L 1mm, the elongation rate δ of the negative electrode current collector = L 1 / L 0 × 100.

[0120] Example 1-1

[0121] <Preparation of Negative Electrode>

[0122] (1)Preparation of silicon-carbon particles:

[0123] Put the first carbon skeleton material with an average particle size of 9.2 µm and a roundness of 0.96 into a rotary furnace. After heating to 480 °C in an argon atmosphere and holding for one hour, then switch the atmosphere to a mixed gas of argon and silane (the volume ratio of silane to argon is 2:8), and carry out chemical vapor deposition at a deposition temperature of 480 °C for 6.5 hours to obtain the first silicon-carbon particles.

[0124] Put the second carbon skeleton material with an average particle size of 1.1 µm and a roundness of 0.78 into a rotary furnace. After heating to 480 °C in an argon atmosphere and holding for one hour, then switch the atmosphere to a mixed gas of argon and silane (the volume ratio of silane to argon is 2:8), and carry out chemical vapor deposition at a deposition temperature of 530 °C for 4 hours to obtain the second silicon-carbon particles.

[0125] Set carbon layers on the surfaces of the first silicon-carbon particles and the second silicon-carbon particles by chemical vapor deposition. By controlling the time of chemical vapor deposition, the thickness of the carbon layer can be controlled. The average thickness of the carbon layer on the surface of the first silicon-carbon material is 33 nm, and the average thickness of the carbon layer on the surface of the second silicon-carbon particles is 12 nm.

[0126] (2)Preparation of negative electrode sheet:

[0127] Mix the first silicon-carbon particles with a carbon layer on the surface, the second silicon-carbon particles with a carbon layer on the surface, and artificial graphite evenly as the negative electrode active material in a mass ratio of 9.5:0.5:90. Mix the negative electrode active material, styrene-butadiene rubber, polyacrylic acid, carbon nanotubes, and carboxymethyl cellulose in a mass ratio of 95.8:2.4:0.5:0.5:0.8, then add deionized water as a solvent and stir evenly to prepare a negative electrode slurry with a solid content of 45 wt%. Coat the negative electrode slurry evenly on one surface of the negative electrode current collector copper foil, dry it to obtain a negative electrode sheet with a single-sided coated negative electrode material layer. Repeat the above steps on the other surface of the negative electrode current collector copper foil, that is, obtain a negative electrode sheet with a double-sided coated negative electrode material layer. After cold pressing, slicing, slitting, and drying, obtain the negative electrode sheet.

[0128] <Preparation of Positive Electrode>

[0129] Lithium cobalt oxide, conductive carbon black, and polyvinylidene fluoride (PVDF) were mixed at a mass ratio of 95:2:3, and N-methylpyrrolidone (NMP) was added. The mixture was stirred evenly under a vacuum mixer to obtain a positive electrode slurry with a solid content of 70 wt%. The positive electrode slurry was evenly coated on one surface of a positive electrode current collector aluminum foil with a thickness of 9 μm, and then dried to obtain a positive electrode sheet with a single-sided coated positive electrode material layer. The above steps were repeated on the other surface of the positive electrode current collector aluminum foil to obtain a positive electrode sheet with a double-sided coated positive electrode material layer. After cold pressing, slicing, and slitting, and then drying, a positive electrode sheet was obtained.

[0130] <Preparation of electrolyte>

[0131] In a dry argon atmosphere glove box, lithium hexafluorophosphate (LiPF 6 6) was dissolved in a mixed solution containing propylene carbonate, ethylene carbonate, ethyl propionate, diethyl carbonate, fluoroethylene carbonate, and vinylene carbonate. Based on the mass of the electrolyte, the mass percentage content of LiPF 6 6 was 12.5%, the mass percentage content of diethyl carbonate was 10%, the mass percentage content of ethyl propionate was 40%, the mass percentage content of fluoroethylene carbonate was 12%, and the mass percentage content of vinylene carbonate was 0.1%. The balance was propylene carbonate and ethylene carbonate, and the ratio of the mass percentage content of propylene carbonate to ethylene carbonate was 3:1.

[0132] <Separator>

[0133] A porous polyethylene film with a thickness of 15 μm was used as the separator.

[0134] <Preparation of lithium-ion battery>

[0135] The positive electrode and negative electrode prepared as above were each connected to a tab. They were laminated via the separator to obtain a laminate. Then, the laminate and the electrolyte were housed together in an aluminum laminated casing. The opening of the casing was heat-sealed, and through steps such as formation and degassing, a lithium-ion battery was manufactured.

[0136] Examples 1-2 to 1-15, Comparative Example 1

[0137] Except for adjusting the parameters according to Table 1, the rest was the same as Example 1-1. By controlling the roundness of the porous carbon skeleton, the roundness R 1 of the first silicon-carbon particle and the roundness R 2 of the second silicon-carbon particle were as shown in Table 1. The temperature of chemical vapor deposition during the preparation of the silicon-carbon particles was adjusted so that the silicon element ratio C 1 of the first silicon-carbon particle and the silicon element ratio C 2 of the second silicon-carbon particle were as shown in Table 1.

[0138] Table 1

[0139]

[0140] As can be seen from Table 1, the negative electrode material layer of the secondary battery regulated in this application includes first silicon-carbon particles and second silicon-carbon particles. By controlling the roundness of the first silicon-carbon particles and the second silicon-carbon particles to satisfy R 1 / R 2 > 1, it is possible to achieve a relatively high energy density while taking into account the improvement of the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling. In particular, on this basis, when the roundness of both also satisfies 0.05 ≤ R 1 -R 2 ≤ 0.15, the secondary battery can exhibit stronger safety performance, discharge rate, and capacity retention ability in the later stage of cycling.

[0141] Particularly, by separately regulating the mass ratio of silicon element in the first silicon-carbon particles and the second silicon-carbon particles to satisfy 35 ≤ C 1 ≤ 55 and / or 45 ≤ C 2 ≤ 65, it is possible to improve the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling. Preferably, when adjusting the mass ratio of silicon element to satisfy 1.2 ≤ C 2 / C 1 ≤ 1.7, it is possible to further improve the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling.

[0142] Examples 2-1 to 2-17

[0143] Except for adjusting the parameters according to Table 2, the rest are the same as in Examples 1-10. When the parameters shown in Table 2 increase compared to Examples 1-10, the contents of propylene carbonate and ethylene carbonate are correspondingly reduced; when the parameters shown in Table 2 decrease compared to Examples 1-10, the contents of propylene carbonate and ethylene carbonate are correspondingly increased. The ratio of the mass percentages of propylene carbonate and ethylene carbonate remains unchanged. "\\" indicates that the electrolyte does not contain this substance.

[0144] Table 2

[0145]

[0146] As can be seen from Table 2, by controlling the electrolyte to contain diethyl carbonate and ethyl propionate, when regulating the mass percentages of diethyl carbonate and ethyl propionate in the electrolyte to satisfy 2.3 ≤ S 2 / S 1 ≤ 6, it is possible to improve the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling. In particular, when 2.8 ≤ S 2 / S 1When ≤ 4.5, it can further improve the safety performance, discharge rate and capacity retention ability of the secondary battery in the later stage of cycling. Among them, by comparing Examples 1-10 and Examples 2-6, it can be seen that when controlling the mass content of ethyl propionate in the electrolyte to be the same, by regulating the mass content of diethyl carbonate so that its relationship with the mass content of ethyl propionate conforms to the above range, the safety performance, discharge rate and capacity retention ability of the secondary battery in the later stage of cycling have been significantly improved. This proves that it is not only by adjusting the mass content of propyl propionate that the above performance of the secondary battery can be improved, but it is necessary to regulate the content relationship between the two to satisfy 2.3 ≤ S 2 / S 1 ≤ 6, through the combined action of propyl propionate and diethyl carbonate, the safety performance, discharge rate and capacity retention ability of the secondary battery in the later stage of cycling are improved.

[0147] In particular, when controlling the electrolyte to contain fluoroethylene carbonate and vinylene carbonate, when regulating the mass percentage content of fluoroethylene carbonate and vinylene carbonate in the electrolyte to satisfy 0.5 ≤ A 1 / (100 × A 2 ) ≤ 1.8, the safety performance, discharge rate and capacity retention ability of the secondary battery in the later stage of cycling can be further improved. In particular, when controlling to satisfy the relationship of 0.7 ≤ A 1 / (100 × A 2 ) ≤ 1.5, the secondary battery can exhibit more excellent safety performance, discharge rate and capacity retention ability in the later stage of cycling.

[0148] In particular, when regulating the electrolyte to contain tetramethyltetravinylcyclotetrasiloxane and controlling its mass percentage content in the electrolyte to be 0.01% - 0.1%, the safety performance, discharge rate and capacity retention ability of the secondary battery in the later stage of cycling can be further improved. In particular, when regulating the electrolyte to contain diethyl (cyanomethyl) phosphonate and controlling its mass percentage content in the electrolyte to be 0.1% - 0.8%, the safety performance, discharge rate and capacity retention ability of the secondary battery in the later stage of cycling can be further improved.

[0149] Examples 3-1 to Examples 3-6

[0150] Except for adjusting the parameters according to Table 3, the rest are the same as in Examples 1-2. By regulating the average particle size of the carbon skeleton material, RA is as shown in Table 3. By adjusting the rolling speed used in the preparation process of the negative current collector, the elongation δ of the negative current collector is as shown in Table 3.

[0151] Table 3

[0152]

[0153] As can be seen from Table 3, when the elongation rate δ% of the negative current collector is regulated to satisfy 3 ≤ δ ≤ 8, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be further improved. In particular, when the elongation rate δ% of the negative current collector and the average particle size RA µm of the first silicon-carbon particles satisfy the relationship: 1.3 ≤ RA / δ ≤ 3.6, the safety performance, discharge rate, and capacity retention ability of the secondary battery in the later stage of cycling can be further improved.

[0154] The above are only the preferred embodiments of the present application and are not intended to limit the present application. Any modifications, equivalent replacements, and improvements made within the principles of the present application shall be included within the protection scope of the present application.

Claims

1. A secondary battery comprising a positive electrode, a negative electrode and an electrolyte, wherein: The negative electrode comprises a negative electrode current collector and a negative electrode material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode material layer comprises silicon carbon particles, and the particle size of the silicon carbon particles is D μm; when observing the cross section of the negative electrode in the backscattered mode of a scanning electron microscope, the cross-sectional area of ​​the silicon carbon particles is S, according to S=π×R 2 Calculate R, the particle size of the silicon-carbon particles D=2×R; The silicon-carbon particles include a carbon carrier and silicon crystal grains at least partially distributed inside the carbon carrier; The silicon-carbon particles include first silicon-carbon particles and second silicon-carbon particles; when 5≤D≤13, the silicon-carbon particles are first silicon-carbon particles, and the circularity of the first silicon-carbon particles is R1; when 0.1≤D≤2, the silicon-carbon particles are second silicon-carbon particles, and the circularity of the second silicon-carbon particles is R2; R1 / R2>1;0.78≤R2≤0.92; The electrolyte comprises diethyl carbonate and ethyl propionate, and based on the mass of the electrolyte, the mass percentage of the diethyl carbonate is S1%, the mass percentage of the ethyl propionate is S2%, and 2.3≤S2 / S1≤6; The electrolyte includes fluoroethylene carbonate and vinylene carbonate. Based on the mass of the electrolyte, the mass percentage of the fluoroethylene carbonate is A1%, the mass percentage of the vinylene carbonate is A2%, 0.5≤A1 / (100×A2)≤1.8; 0.05≤A2≤0.

5. 2 . The secondary battery according to claim 1 , wherein 0.05≤R1-R2≤0.

15.

3. The secondary battery according to claim 1, which satisfies at least one of the following: (1) The first silicon-carbon particles contain silicon and carbon, and based on the sum of the mass of the silicon and carbon elements in the first silicon-carbon particles, the mass proportion of the silicon element is C1%, 35≤C1≤55; and / or, (2) The second silicon-carbon particles contain silicon and carbon. Based on the sum of the mass of the silicon and carbon elements in the second silicon-carbon particles, the mass proportion of the silicon is C2%, and 45≤C2≤65. The secondary battery according to claim 3 , wherein 1.2≤C2 / C1≤1.

7.

5. The secondary battery according to claim 1, wherein the silicon-carbon particles include third silicon-carbon particles; when 2<D<5, the silicon-carbon particles are third silicon-carbon particles; the number of the first silicon-carbon particles is N1, the number of the second silicon-carbon particles is N2, the number of the third silicon-carbon particles is N3, and N3 / (N1+N2+N3)<0.

2. The secondary battery according to claim 1 , wherein 2.8≤S2 / S1≤4.

5. 7 . The secondary battery according to claim 1 , wherein 0.7≤A1 / (100×A2)≤1.

5. 8 . The secondary battery according to claim 1 , wherein 0.90≤R1≤0.

98.

9. The secondary battery according to claim 1 or 6, which satisfies at least one of the following: （1）5≤S1≤20； （2）20≤S2≤45。 10 . The secondary battery according to claim 1 , wherein 11≤A1≤18.

11. The secondary battery according to any one of claims 1 to 7, wherein the electrolyte satisfies at least one of the following conditions: (1) The electrolyte contains tetramethyltetravinylcyclotetrasiloxane, and the mass percentage of the tetramethyltetravinylcyclotetrasiloxane is 0.01% to 0.1% based on the mass of the electrolyte; (2) The electrolyte contains diethyl (cyanomethyl)phosphonate, and the mass percentage of the diethyl (cyanomethyl)phosphonate is 0.1% to 0.8% based on the mass of the electrolyte. 12 . The secondary battery according to claim 1 , wherein the elongation of the negative electrode current collector is δ%, and 3≤δ≤8.

13. The secondary battery according to claim 12, wherein the average particle size of the first silicon-carbon particles is RA μm, and 1.3≤RA / δ≤3.

6. 14 . An electronic device comprising the secondary battery according to claim 1 .

Images