Negative pole piece, secondary battery and electronic device

Abstract

The invention provides a negative pole piece, a secondary battery and an electronic device, the negative pole piece comprises a current collector and a negative electrode material layer arranged on the current collector, the negative electrode material layer comprises a negative electrode active material, the negative electrode active material comprises a composite graphite material and a silicon-based material, the composite graphite material comprises graphite and hard carbon on the surface of the graphite, and the silicon-based material comprises a silicon-based material. The ratio of the D peak to the G peak of the Raman spectrum of the composite graphite material is A, and A is more than or equal to 0.5 and less than or equal to 1; when the button cell assembled by the negative pole piece is charged to 2V, the lithium removal gram capacity is C, the gram capacity when the lithium removal potential P is larger than or equal to 0V and smaller than 0.1 V is C1, and C1 / C is larger than 0 and smaller than or equal to 30%. The negative pole piece provided by the invention can give consideration to the cycle performance and the volume energy density of the secondary battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of electrochemical energy storage, and in particular to a negative electrode sheet, a secondary battery using the negative electrode sheet, and an electronic device using the secondary battery. BACKGROUND

[0002] Lithium ion batteries have the advantages of high energy density, high working voltage, good safety, green environmental protection, etc., and have been widely used in consumer electronics such as mobile phones and notebook computers. Silicon-based materials are considered as an ideal choice for the next generation of lithium ion battery negative electrode materials due to their high theoretical specific capacity (about 4200 mAh / g). However, when silicon-based materials and graphite are used together as negative electrode materials, the graphite faces a significant structural stability problem during the charge and discharge cycle. In the related art, the amount of silicon in the silicon-based material of the negative electrode material is reduced to ensure the cycle performance, but this reduces the energy density of the lithium ion battery. SUMMARY

[0003] The present application provides a negative electrode sheet, a secondary battery and an electronic device.

[0004] The first aspect of the present application provides a negative electrode sheet, comprising a current collector and a negative electrode material layer arranged on the current collector, characterized in that the negative electrode material layer comprises a negative electrode active material, the negative electrode active material comprises a composite graphite material and a silicon-based material, the composite graphite material comprises graphite and hard carbon on the surface of the graphite, the ratio of the D peak to the G peak of the Raman spectrum of the composite graphite material is A, and 0.5≤A≤1; the lithium extraction capacity of a button cell assembled by the negative electrode sheet is C when charged to 2V, the capacity in the potential range of 0V≤P<0.1V is C1, and 0<C1 / C≤30%.

[0005] In the present application, the applicant finds that by introducing hard carbon on the surface of graphite and as a lithium storage transition layer, the hard carbon mainly stores lithium in micropores, and the lithium extraction potential of the hard carbon is below 0.1 V, so during the lithium extraction process of the negative electrode active material, the hard carbon will be extracted first, and after the lithium extraction of the graphite is completed, the hard carbon will be in a state of continuous deep lithium extraction at the transition potential interval when the silicon-based material has not been largely lithium extracted, which can improve the continuous deep lithium extraction of the graphite, and the hard carbon is stable and basically does not expand, so that the structural stability of the composite graphite material is better than that of the graphite, and the expansion of the silicon-based material during the cycle process can be relieved, thereby improving the structural stability of the graphite during the cycle process and the stable contact between the materials during the cycle process, and further improving the cycle performance of the secondary battery under the condition of good volume energy density of the secondary battery. In the present application, the Raman ratio A of the composite graphite material is controlled within the above range, the composite graphite material has a high Raman ratio A, the surface of the composite graphite material contains hard carbon, the appropriate hard carbon content improves the structural stability of the composite graphite material, and the expansion of the silicon-based material is relieved. Meanwhile, the applicant finds that C1 corresponds to the lithium extraction capacity of the hard carbon in the voltage interval of 0 to 0.1 V, and when C1 / C is kept within the above appropriate range, the contact stability between the materials is better under the condition of good compaction density of the negative electrode sheet, so that the secondary battery has good volume energy density, therefore, by controlling the relationship between the Raman ratio A and C1 / C of the composite graphite material, the secondary battery can have excellent cycle performance under the condition of high volume energy density.

[0006] Based on the first aspect, in some embodiments, the lithium extraction capacity of the button cell in the interval of 0.1 V≤P<0.25 V is C2, and 10%≤C2 / C≤60%. The contribution of the graphite to the lithium extraction capacity of the button cell assembled by the negative electrode sheet is appropriate, and the graphite has a higher compaction density than the silicon-based material and the hard carbon, which is also conducive to maintaining the negative electrode sheet to have a good compaction density, thereby improving the energy density of the secondary battery.

[0007] Based on the first aspect, in some embodiments, the lithium extraction capacity of the button cell in the interval of 0.25 V≤P≤2 V is C3, and 30%≤C3 / C≤60%. The content of silicon in the silicon-based material is appropriate, the negative electrode sheet has a higher capacity, and it is also conducive to shortening the time for the graphite to be in a state of continuous deep lithium extraction during the lithium extraction process of silicon, so that the composite graphite material has good stability, and it is also conducive to making the negative electrode sheet have a good compaction density, thereby further improving the cycle performance of the secondary battery under the condition of high energy density of the secondary battery.

[0008] In some embodiments of the first aspect, when 30%≤C3 / C≤50%, 0

[0009] In some embodiments of the first aspect, when 50%<C3 / C≤60%, 15%<C1 / C2≤30%, the graphite deep delithiation state can be improved, and the negative electrode sheet also has a good compaction density.

[0010] In some embodiments of the first aspect, the surface defect degree of the graphite is R, the ratio of the micropore volume of the hard carbon to the total pore volume is V, and 0.14≤R / V≤0.25. The surface defect degree of the graphite and the ratio of the micropore volume of the hard carbon to the total pore volume are combined and compared, so that the surface defect degree of the graphite and the ratio of the micropore volume of the hard carbon to the total pore volume are within the above range. The size of the micropore volume of the hard carbon to the total pore volume can improve the poor surface structure stability of the surface defect degree of the graphite during the cycle, thereby limiting the relationship between the two, which can further improve the cycle stability of the negative electrode sheet and the cycle performance of the secondary battery.

[0011] In some embodiments of the first aspect, the graphitization degree of the graphite is 92% to 96%. The graphite has a high graphitization degree, which is beneficial to improve the deintercalation speed of lithium ions, so that the negative electrode sheet has a high specific capacity and a high lithium ion diffusion coefficient, thereby improving the cycle performance and thermal stability of the secondary battery. In some embodiments, In some embodiments of the first aspect, the surface defect degree of the graphite is R, and 0.07≤R≤0.2. The graphite has good thermal stability, which is also beneficial to minimize the deterioration of the surface structure stability of the graphite during the cycle, and is also beneficial to improve the diffusion speed of lithium ions on the surface of the particles, reduce the interfacial diffusion impedance of lithium ions deintercalating from the graphite negative electrode material, and improve the thermal stability of the secondary battery.

[0012] In some embodiments of the first aspect, the particle size Dv50 of the graphite is 5 μm to 10 μm. The graphite has a suitable particle size, which can optimize the conductive network and is beneficial to improve the rate performance of the negative electrode sheet.

[0013] In some embodiments of the first aspect, the compaction density of the negative electrode sheet is T, and 1.05 g / cm3 ≤ T ≤ 1.35 g / cm 3 ; it is conducive to maintaining good electronic contact between particles in the negative electrode material layer, reducing the polarization of the secondary battery, and also conducive to reducing the side reactions of particle breakage, improving the energy density and thermal stability of the secondary battery.

[0014] Based on the first aspect, in some embodiments, the ratio of the micropore volume of the hard carbon to the total pore volume is V, and 50%≤V≤80%. The hard carbon is formed on the surface of the graphite, and the microporous structure can improve the surface structure stability of the graphite during cycling, thereby improving the cycle performance of the secondary battery.

[0015] Based on the first aspect, in some embodiments, when the mass content of the silicon-based material is 15% to 25% based on the mass of the negative electrode active material, 15% < C1 / C2≤30%; when the mass content of the silicon-based material is 15% to 25% in the negative electrode active material, the content of the silicon-based material is relatively high, and it has a greater impact on the structural stability of the graphite during cycling. The ratio of C1 / C2 can reflect the mass ratio of the hard carbon coating to the graphite, so when the content of the silicon-based material is relatively high, the ratio of C1 / C2 is increased to allow more hard carbon to be coated on the surface of the graphite, thereby further improving the structural stability of the graphite during cycling.

[0016] Based on the first aspect, in some embodiments, when the mass content of the silicon-based material is 10% to 15% based on the mass of the negative electrode active material, 0 < C1 / C2≤15%. When the mass content of the silicon-based material is 10% to 15% in the negative electrode active material, the content of the silicon-based material is relatively low, and the graphite structure is less affected by the lithium extraction of the silicon-based material. A small amount of hard carbon can be used to coat the graphite, so that the negative electrode sheet has good compaction density, thereby improving the energy density and cycle performance of the secondary battery.

[0017] Based on the first aspect, in some embodiments, the mass of the hard carbon is 5% to 35% of the mass of the graphite; on the basis of making the negative electrode sheet have good compaction density, the structural stability of the graphite during cycling can be improved, and the cycle performance of the secondary battery can be improved.

[0018] Based on the first aspect, in some embodiments, the compaction density of the powder of the composite graphite material is 1.4 g / cc to 1.8 g / cc; it is conducive to further improving the compaction density of the negative electrode sheet and improving the volume energy density of the secondary battery.

[0019] Based on the first aspect, in some embodiments, the specific surface area of the composite graphite material is less than 2 m 2The surface activity sites of the composite graphite material are improved, the composite graphite material has a suitable number of lithium ion deintercalation sites, and the decomposition of the SEI film is reduced, so as to improve the thermal stability of the secondary battery.

[0020] In some embodiments based on the first aspect, the mass ratio of the composite graphite material is 75% to 90% and the mass ratio of the silicon-based material is 10% to 25% based on the mass of the negative electrode active material. In the negative electrode active material, the mass ratios of the composite graphite material and the silicon-based material are within the above ranges, so that the negative electrode sheet has good cycle performance and energy density at a higher specific capacity.

[0021] The second aspect of the present application also provides a secondary battery, which comprises a positive electrode sheet, a separator and an electrolyte, the secondary battery comprising the negative electrode sheet as described above, and the separator is located between the positive electrode sheet and the negative electrode sheet. The cycle performance and the volume energy density of the secondary battery can be improved.

[0022] The third aspect of the present application also provides an electronic device comprising the secondary battery, which has excellent cycle performance and volume energy density, and is conducive to improving the service life of the electronic device. BRIEF DESCRIPTION OF DRAWINGS

[0023] The above and / or additional aspects and advantages of the present application will become apparent and more readily appreciated from the following description of the embodiments, taken in conjunction with the accompanying drawings, in which: Figure 1 Comparison of delithiation curves of button cells assembled by Comparative Example 1 and Example 8 of the present application. DETAILED DESCRIPTION

[0024] The technical solutions in the embodiments of the present application will be described in detail below. Obviously, the described embodiments are only some of the embodiments of the present application, not all. Unless otherwise defined, all technical and scientific terms used herein have the same meanings as understood by those skilled in the art to which the present application belongs. The terms used in the specification of the present application are only for the purpose of describing the specific embodiments and are not intended to limit the present application.

[0025] An embodiment of the present application provides a secondary battery, which comprises a shell, an electrode assembly and an electrolyte. The electrode assembly and the electrolyte are both located in the shell.

[0026] The shell can be a packaging bag obtained by packaging with a packaging film (such as an aluminum plastic film), such as a soft package battery. In other embodiments, the secondary battery can also be a steel shell battery, an aluminum shell battery, etc.

[0027] The electrode assembly includes a positive electrode tab, a negative electrode tab, and a separator membrane. The separator membrane is disposed between the positive electrode tab and the negative electrode tab. The electrode assembly can be a stacked structure formed by stacking the positive electrode tab, the separator membrane, and the negative electrode tab. In other embodiments, the electrode assembly can also be a wound structure formed by winding the positive electrode tab, the separator membrane, and the negative electrode tab after being stacked.

[0028] The negative electrode tab The negative electrode tab includes a negative current collector and a negative material layer on the surface of the negative current collector. The negative current collector includes a copper foil, an aluminum foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, or any combination thereof, and can also be any composite current collector disclosed in the prior art, such as but not limited to the current collector formed by combining the aforementioned conductive foils and polymer substrates. In the negative electrode tab, the negative material layer includes a negative active material.

[0029] The negative active material includes a composite graphite material and a silicon-based material. The composite graphite material includes graphite and hard carbon on the surface of the graphite, and the ratio of the D peak to the G peak in the Raman spectrum of the composite graphite material is A, 0.5≤A≤1. The lithium extraction capacity of the button cell assembled with the negative electrode tab is C when charged to 2V, the capacity in the potential range of 0V≤P<0.1V is C1, and 0<C1 / C≤30%.

[0030] The above negative electrode tab is assembled into a button cell. According to the lithium extraction curve of the button cell with the negative active material, the applicant finds that the graphite in the composite graphite material starts to be extracted at about 0.1V and is basically extracted at no more than 0.25V. The silicon in the silicon-based material starts to be extracted at about 0.25V, is largely extracted at about 0.4V, and is completely extracted at 2V. Therefore, in the electrode tab mixed with graphite and silicon, the graphite is first extracted in the low potential range and the silicon is later extracted in the high potential range during the lithium extraction process. When the transition potential range is in the transition state between the completion of the lithium extraction of the graphite and the large amount of lithium extraction of the silicon, the graphite is in a state of continuous deep lithium extraction, which can cause the structure stability of the graphite to deteriorate during the cycle process, and further cause the cycle attenuation and cycle swelling of the electrochemical device to deteriorate.

[0031] For the above phenomenon, the applicant found that by introducing hard carbon on the surface of graphite as a lithium storage transition layer, the hard carbon mainly stores lithium in micropores, and the lithium extraction potential interval of the hard carbon is below 0.1 V, therefore, during the lithium extraction process of the negative electrode active material, the hard carbon will be extracted first, and after the lithium extraction of the graphite is completed, the hard carbon will be in a state of continuous deep lithium extraction at the transition potential interval when the silicon-based material has not been extracted much, which can improve or alleviate the continuous deep lithium extraction of the graphite, and the hard carbon is stable and basically does not expand, so that the structural stability of the composite graphite material is better than that of the graphite, and the expansion of the silicon-based material in the cycle process can be alleviated, thereby the structural stability of the graphite in the cycle process and the stable contact between the materials in the cycle process can be improved, and then the cycle performance of the secondary battery is improved under the condition of good volume energy density of the secondary battery. In the present application, the Raman ratio A of the composite graphite material is controlled within the above range, the composite graphite material has a high Raman ratio A, the surface of the composite graphite material contains hard carbon, the appropriate hard carbon content improves the structural stability of the composite graphite material, and the expansion of the silicon-based material is alleviated. Meanwhile, the applicant found that C1 corresponds to the lithium extraction capacity of the hard carbon in the voltage interval of 0 to 0.1 V, and when C1 / C is kept within the above appropriate range, the contact stability between the materials is better under the condition of good compaction density of the negative electrode sheet, so that the secondary battery has good volume energy density, therefore, by further controlling the relationship between the Raman ratio A of the composite graphite material and C1 / C, the secondary battery has excellent cycle performance under the condition of high volume energy density.

[0032] If the ratio of C1 / C is 0, it indicates that hard carbon is not introduced into the composite graphite material, and the hard carbon does not play a role in improving the composite graphite material, and the effect of improving the cycle performance of the secondary battery is not obvious. If the ratio of C1 / C is large, such as greater than 30%, the proportion of hard carbon in the composite graphite material is high, and the compaction density of the hard carbon is low, which will reduce the compaction density of the negative electrode sheet and the energy density of the secondary battery.

[0033] In some embodiments, the ratio A of the Raman spectrum of the composite graphite material can be 0.5, 0.6, 0.7, 0.8, 0.9, 1, or any value within the range formed by any two of the above values.

[0034] In some embodiments, the ratio of C1 / C can be 1%, 3%, 5%, 10%, 12%, 13%, 15%, 17%, 20%, 23%, 25%, 27%, 30%, or any value within the range formed by any two of the above values.

[0035] In some embodiments, the lithium extraction potential P of the button cell is in the range of 0.1 V≤P<0.25 V, and the gram capacity of the lithium extraction potential P is C2, 10%≤C2 / C≤60%. The applicant found that C2 can correspond to the lithium extraction capacity of the graphite in the voltage range of 0.1 V to 0.25 V. When the C2 / C ratio is in the above range, the contribution of the graphite to the lithium extraction gram capacity of the button cell assembled by the negative electrode sheet is appropriate, and the graphite has a higher compaction density than the silicon-based material and the hard carbon, which is also conducive to maintaining the negative electrode sheet with a good compaction density, thereby improving the energy density of the secondary battery. If the C2 / C ratio is small, such as less than 10%, the content of the graphite in the negative electrode active material is small, which reduces the compaction density of the negative electrode sheet and is not conducive to improving the energy density of the secondary battery; if the C2 / C ratio is large, such as greater than 60%, the content of the graphite in the negative electrode active material is large, and the capacity of the graphite is lower than that of silicon and hard carbon, which reduces the capacity of the negative electrode sheet and the energy density of the secondary battery. In some embodiments, the C2 / C ratio can be 10%, 15%, 20%, 25%, 27%, 30%, 35%, 37%, 40%, 45%, 47%, 50%, 55%, 60%, or any value within the range formed by any two of the above values.

[0036] In some embodiments, the lithium extraction potential P of the button cell is in the range of 0.25 V≤P≤2 V, and the gram capacity of the lithium extraction potential P is C3, 30%≤C3 / C≤60%. Wherein, C=C1+C2+C3. The applicant found that C3 can correspond to the lithium extraction capacity of the silicon in the voltage range of 0.25 V to 2 V. When the C3 / C ratio is in the above range, the content of silicon in the silicon-based material is appropriate, the negative electrode sheet has a high capacity, and it is also conducive to shortening the state that the graphite is continuously in deep lithium extraction during the lithium extraction of silicon, so that the composite graphite material has good stability, and it is also conducive to making the negative electrode sheet have a good compaction density, thereby improving the cycle performance of the secondary battery under the condition that the secondary battery has a high energy density. If the C3 / C ratio is small, such as less than 30%, the content of silicon is small, and the capacity of silicon is greater than that of graphite and hard carbon, which reduces the capacity of the negative electrode sheet; if the C3 / C ratio is large, the content of the silicon-based material in the negative electrode active material is large, which reduces the compaction density of the negative electrode sheet, and also exacerbates the state that the graphite is continuously in deep lithium extraction, reduces the structural stability of the composite graphite, and reduces the cycle performance of the secondary battery. In some embodiments, the C3 / C ratio can be 30%, 35%, 36%, 40%, 45%, 47%, 50%, 55%, 60%, or any value within the range formed by any two of the above values.

[0037] In some embodiments, when 30%≤C3 / C≤50%, 0

[0038] In some embodiments, when 50%<C3 / C≤60%, 15%<C1 / C2≤30%. The applicant also found that when C3 / C is between 50% and 60%, the ratio of C1 / C2 is between 15% and 30%, which can improve the deep delithiation state of graphite, and also make the negative electrode sheet maintain good compaction density. In some embodiments, when 50%<C3 / C≤60%, the ratio of C1 / C2 can be 16%, 18%, 20%, 21%, 23%, 25%, 27%, 29%, 30%, or any value within the range formed by any two of the above values.

[0039] In some embodiments, the surface defect degree of the graphite is R, the ratio of the micropore volume of the hard carbon to the total pore volume is V, and 0.14≤R / V≤0.25. The surface defect degree of the graphite can reflect the stability of the graphite structure during the cycle of the negative electrode sheet. The applicant combines the surface defect degree of the graphite and the ratio of the micropore volume of the hard carbon to the total pore volume and compares the values of the two to make the surface defect degree of the graphite and the ratio of the micropore volume of the hard carbon to the total pore volume within the above range, and the size of the micropore volume of the hard carbon to the total pore volume can improve the phenomenon of poor surface structure stability of the surface defect degree of the graphite during the cycle, thereby limiting the relationship of the above two can further improve the cycle stability of the negative electrode sheet and improve the cycle performance of the secondary battery. When the surface defect degree of the graphite is low, the degree of reduction of the surface structure stability of the graphite during the cycle of the negative electrode sheet is small, and the use of hard carbon with a low micropore volume ratio to coat the graphite can improve the cycle stability of the negative electrode sheet. When the surface defect degree of the graphite is high, the degree of reduction of the surface structure stability of the graphite during the cycle is large, and the use of hard carbon with a high micropore volume ratio to coat the graphite to form a composite graphite material can further improve the cycle performance of the secondary battery.

[0040] In some embodiments, the ratio of R / V can be 0.14, 0.16, 0.18, 0.21, 0.23, 0.25, or any value within a range defined by any two of the above values.

[0041] In some embodiments, the graphitization degree of the graphite is 92% to 96%. The graphite has a high graphitization degree, which is conducive to improving the deintercalation speed of lithium ions, so that the negative electrode sheet has a high specific capacity and a high lithium ion diffusion coefficient, thereby improving the cycle performance and thermal stability of the secondary battery. In some embodiments, the graphitization degree of the graphite can be 92%, 93%, 94%, 95%, 96%, or any value within a range defined by any two of the above values.

[0042] In some embodiments, the surface defect degree of the graphite is R, 0.07≤R≤0.2. In the negative electrode active material, R is within the above range, so that the graphite has good thermal stability, and also helps to minimize the deterioration of the surface structure stability of the graphite during the cycle process, and also helps to improve the diffusion speed of lithium ions on the particle surface, reduce the interfacial diffusion impedance of lithium ions deintercalating from the graphite negative electrode material, and help to improve the thermal stability of the secondary battery. In some embodiments, R can be 0.07, 0.01, 0.012, 0.015, 0.018, 0.02, or any value within a range defined by any two of the above values.

[0043] In some embodiments, the particle size Dv50 of the graphite is 5 μm to 10 μm; the graphite has a suitable particle size, which can optimize the conductive network and help to improve the rate performance of the negative electrode sheet. In some embodiments, the particle size Dv50 can be 5 μm, 7 μm, 8 μm, 9 μm, 10 μm, or any value within a range defined by any two of the above values.

[0044] In some embodiments, the graphite includes at least one of artificial graphite or natural graphite. By selecting the above graphite, the graphite has high electronic conductivity, lithium ion transmission rate, and specific capacity, and the surface-coated graphite obtained after surface coating has higher specific capacity and higher rate performance, which can further improve the energy density and fast charging performance of the secondary battery.

[0045] In some embodiments, the powder compaction density of the composite graphite material is 1.4 g / cc to 1.8 g / cc; the specific surface area of the composite graphite material is less than 2 m 2 / g. The powder tap density of the composite graphite material is within the above range, which is conducive to further improving the tap density of the negative electrode sheet and improving the volume energy density of the secondary battery. The specific surface area is within the above range, which is conducive to improving the surface active sites of the composite graphite material, so that the composite graphite material has a suitable number of lithium ion extraction sites, and reduces the excessive formation of SEI film and the decomposition of the SEI film to improve the thermal stability of the secondary battery. In some embodiments, the powder tap density of the composite graphite material can be 1.4 g / cc, 1.5 g / cc, 1.6 g / cc, 1.7 g / cc, 1.8 g / cc, or any value within the range formed by any two of the above values. In some embodiments, the specific surface area of the composite graphite material can be 0.7 m 2 / g, 1 m 2 / g, 1.3 m 2 / g, 1.5 m 2 / g, 1.8 m 2 / g, 2 m 2 / g, or any value within the range formed by any two of the above values. In some embodiments, the specific surface area of the composite graphite material is 0.7 m 2 / g to 1.3 m 2 / g.

[0046] In some embodiments, the tap density of the negative electrode sheet is T, 1.05 g / cm 3 ≤ T ≤ 1.35 g / cm 3 , which is conducive to maintaining good electronic contact between particles in the negative electrode material layer and reducing the polarization of the secondary battery, and also conducive to reducing the side reactions of particle breakage and improving the energy density and thermal stability of the secondary battery. In some embodiments, T can be 1.05 g / cm 3 , 1.1 g / cm 3 , 1.15 g / cm 3 , 1.2 g / cm 3 , 1.25 g / cm 3 , 1.3 g / cm 3 , 1.35 g / cm 3 , or any value within the range formed by any two of the above values.

[0047] In some embodiments, the ratio of the micropore volume of the hard carbon to the total pore volume is V, 50% ≤ V ≤ 80%. Micropores refer to pore structures with a pore size less than 2 nm, and the ratio of the micropore volume to the total pore volume is within the above range. The hard carbon is formed on the surface of the graphite, and the micropore structure can improve the surface structure stability of the graphite during cycling, thereby improving the cycle performance of the secondary battery. In some embodiments, V can be 50%, 55%, 60%, 65%, 70%, 75%, 80%, or any value within the range formed by any two of the above values.

[0048] In some embodiments, when the mass content of the silicon-based material is 15% to 25% based on the mass of the negative electrode active material, 15% < C1 / C2≤ 30%. When the mass content of the silicon-based material is 15% to 25% in the negative electrode active material, the content of the silicon-based material is relatively high, and the structure stability of the graphite in the cycle process is greatly affected. The ratio of C1 / C2 can reflect the mass ratio of the hard carbon coating amount to the graphite, and therefore, when the content of the silicon-based material is relatively high, the ratio of C1 / C2 is increased to make more hard carbon coated on the surface of the graphite, thereby further improving the structure stability of the graphite in the cycle process. In some embodiments, when the mass content of the silicon-based material is 15% to 25%, the ratio of C1 / C2 can be 16%, 18%, 20%, 21%, 23%, 25%, 27%, 29%, 30%, or any value within the range formed by any two of the above values.

[0049] In some embodiments, when the mass content of the silicon-based material is 10% to 15% based on the mass of the negative electrode active material, 0 < C1 / C2≤ 15%. When the mass content of the silicon-based material is 10% to 15% in the negative electrode active material, the content of the silicon-based material is relatively low, and the structure of the graphite is less affected by the silicon-based material delithiation. A small amount of hard carbon can be used to coat the graphite, so that the negative electrode sheet has good compaction density, thereby improving the energy density and cycle performance of the secondary battery. In some embodiments, when the mass content of the silicon-based material is 10% to 15%, the ratio of C1 / C2 can be 1%, 2%, 3%, 5%, 7%, 10%, 12%, 13%, 15%, or any value within the range formed by any two of the above values.

[0050] In some embodiments, the mass of the hard carbon is 5% to 35% of the mass of the graphite. The mass ratio of the hard carbon within the above range can improve the structure stability of the graphite in the cycle process and improve the cycle performance of the secondary battery on the basis of making the negative electrode sheet have good compaction density. In some embodiments, the mass of the hard carbon is 5%, 10%, 12%, 15%, 20%, 25%, 30%, 33%, 35% of the mass of the graphite, or any value within the range formed by any two of the above values.

[0051] In some embodiments, based on the mass of the negative electrode active material, the mass ratio of the composite graphite material is 75% to 90%, and the mass ratio of the silicon-based material is 10% to 25%. In the negative electrode active material, the mass ratios of the composite graphite material and the silicon-based material are within the above ranges, so that the negative electrode sheet has good cycle performance and energy density at a relatively high specific capacity.

[0052] The capacity density E of the negative electrode sheet is C x T, 650 mAh / cm 3≤ E ≤ 800 mAh / cm 3 The electrode capacity density E represents the de-lithium capacity of the active material per unit volume, and the greater the value of E, the higher the volume energy density of the secondary battery composed of the electrode. By controlling the composite graphite material and the ratio range of C1, C2, and C3 as above, the electrode capacity density can be controlled to be in the range of 650 mAh / cm 3 to 800 mAh / cm 3

[0053] In the present application, the silicon-based material can be a silicon-carbon material, which takes porous carbon as the core and uses a chemical vapor deposition device. The porous carbon material is used as the precursor, and a silicon-containing gas is used as the silicon source. Through chemical vapor deposition, elemental nanosilicon is adsorbed and deposited in the pores or on the surface of the porous carbon, and a carbon source gas is then introduced into the pores, and a deposition reaction occurs, resulting in carbon deposition in part of the pores and on the outer surface of the product to form a shell, thereby obtaining the silicon-carbon material. The silicon-containing gas can include, but is not limited to, at least one of monosilane, disilane, trisilane, tetrasilane, chlorosilane, dichlorosilane, trichlorosilane, or tetrachlorosilane. The carbon source gas can include, but is not limited to, at least one of methane, acetylene, ethylene, ethane, propyne, propylene, propane, butyne, butene, or butane; and the porous carbon material can include, but is not limited to, at least one of activated carbon, biomass-derived carbon, resin-based carbon, expanded graphite, carbon molecular sieve, carbon nanofiber, or carbon nanotube.

[0054] In some embodiments, the negative electrode material layer further includes a binder and a conductive agent. In some embodiments, the binder includes, but is not limited to, polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymer, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, butadiene rubber, acrylated butadiene rubber, epoxy resin, or nylon, etc.

[0055] In some embodiments, the conductive agent includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, and mixtures thereof. In some embodiments, the carbon-based materials are selected from carbon black, acetylene black, ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based materials are selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0056] Separator film The material and shape of the separator film used in the secondary battery of the present application are not particularly limited and can be any of the techniques disclosed in the prior art. In some embodiments, the separator film includes a polymer or inorganic material formed of a material stable to the electrolyte of the present application, etc.

[0057] ​The separation film can include a base layer and a surface treatment layer. The base layer is a nonwoven fabric, a film, or a composite film having a porous structure, and the material of the base layer is selected from at least one of polyethylene, polypropylene, polyethylene terephthalate, and polyimide. Specifically, a polypropylene porous film, a polyethylene porous film, a polypropylene nonwoven fabric, a polyethylene nonwoven fabric, or a polypropylene-polyethylene-polypropylene porous composite film can be used.

[0058] The surface treatment layer is provided on at least one surface of the base layer, and can be a polymer layer or an inorganic layer, or a layer formed by mixing a polymer and an inorganic substance. The inorganic layer includes inorganic particles and a binder, and the inorganic particles are selected from at least one of alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinylalkoxide, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer includes a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinylalkoxide, polyvinylidene fluoride, and poly(vinylidene fluoride-hexafluoropropylene).

[0059] Electrolyte According to some embodiments of the present application, the electrolyte includes an organic solvent, a lithium salt, and optionally an additive. The organic solvent in the electrolyte of the present application can be any organic solvent known in the art that can be used as a solvent for an electrolyte. The electrolyte used in the electrolyte of the present application is not limited and can be any electrolyte known in the art. The additive of the electrolyte of the present application can be any additive known in the art that can be used as an additive for an electrolyte. In some embodiments, the organic solvent includes, but is not limited to, ethylene carbonate (EC), propylene carbonate (PC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), propylene carbonate, or ethyl propionate. In some embodiments, the organic solvent includes an ether-based solvent, such as including at least one of 1,3-dioxolane (DOL) and dimethoxyethane (DME). In some embodiments, the lithium salt includes at least one of an organic lithium salt or an inorganic lithium salt. In some embodiments, the lithium salt includes, but is not limited to, lithium hexafluorophosphate (LiPF6), lithium tetrafluoroborate (LiBF4), lithium difluorophosphate (LiPO2F2), lithium bis-trifluoromethanesulfonimide LiN(CF3SO2)2 (LiTFSI), lithium bis(fluorosulfonyl)imide Li(N(SO2F)2) (LiFSI), lithium bis(oxalato)borate LiB(C2O4)2 (LiBOB), or lithium difluoro(oxalato)borate LiBF2(C2O4) (LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0060] Positive electrode tab The positive electrode tab includes a positive current collector and a positive active layer disposed on the positive current collector. The positive current collector can use an aluminum foil or a nickel foil, or the like, and can be any composite current collector disclosed in the art, such as, but not limited to, a current collector formed by combining the aforementioned conductive foil and a polymer substrate. The positive active layer contains a positive active material, which includes a compound that reversibly intercalates and deintercalates lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive active material can include a lithium transition metal composite oxide. The lithium transition metal composite oxide contains lithium and at least one element selected from cobalt, manganese, and nickel. In some embodiments, the positive active material can include, but is not limited to, at least one of lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminate, lithium iron phosphate, lithium vanadium phosphate, lithium cobalt phosphate, lithium manganese phosphate, lithium manganese iron phosphate, lithium iron silicate, lithium vanadium silicate, lithium cobalt silicate, lithium manganese silicate, spinel-type lithium manganate, spinel-type lithium nickel manganate, and lithium titanate.

[0061] The positive active layer can further include a binder to bind the positive active material particles to facilitate formation of a film layer and to improve the adhesion between the positive active layer and the positive current collector. In some embodiments, the binder can include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylated styrene butadiene rubber, epoxy resin, or nylon.

[0062] The positive active layer can further include a conductive material, which can include, but is not limited to, a carbon-based material, a metal-based material, a conductive polymer, or any combination thereof. In some embodiments, the carbon-based material can include, but is not limited to, at least one of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material can include, but is not limited to, metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer can be a polyphenylene derivative.

[0063] The secondary battery described above can be used in an electronic device to supply power to a load in the electronic device. Moreover, the negative electrode sheet in the secondary battery has excellent cycle performance and energy density, which is beneficial to improve the service life of the electronic device. The electronic device can include, but is not limited to, a notebook computer, a pen-input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile, a portable copying machine, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable audio player, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, a power tool, a flashlight, a camera, a household large storage battery, and a lithium ion capacitor.

[0064] The present application is described below by way of specific examples and comparative examples. Those skilled in the art will understand that the preparation methods described in the present application are merely examples, and any other suitable preparation method is within the scope of the present application.

[0065] Example 1 Preparation of the negative active material: (1) Composite graphite material The hard carbon precursor phenolic resin (30% residual carbon content after carbonization) is dissolved in tetrahydrofuran to obtain a hard carbon precursor solution; the hard carbon precursor solution and the artificial graphite matrix material are added into a reaction kettle, coated at 85°C for 2h, and then dried at 150°C for 4h to remove the solvent, to obtain a preliminarily coated graphite material; the preliminarily coated graphite material is added into a box furnace, carbonized at 900°C under a nitrogen atmosphere for 3h, to obtain a carbonized graphite material, wherein the mass of the hard carbon accounts for 15% of the mass of the graphite in the carbonized graphite material; the carbonized graphite material is added into a rotary furnace, and surface carbon modification is performed in a methane atmosphere at 800°C for 1h, to obtain the composite graphite material.

[0066] (2) Silicon-based material The porous carbon (D50 of 7 μm, pore volume of 0.9 cm 3 / g, specific surface area of 1800 m 2 / g) is subjected to a drying treatment, and the treated porous carbon material is placed in a chemical vapor deposition furnace, heated to 420°C under an argon protective atmosphere, and subjected to a reaction under a micro-positive pressure gas phase pressure for 9h using 10% silane as a silicon source and 8L / min argon as an inert gas at a total gas flow rate of 18L / min, so that elemental nanosilicon is adsorbed and deposited in the pores of the porous carbon.

[0067] In the same vapor deposition furnace, heating is continued to 550°C under an argon protective atmosphere, and a reaction is continued for 5.6h under a micro-positive pressure gas phase pressure using a carbon source gas mixture containing argon at a ratio of 50% at a total gas flow rate of 5L / min, so that the outer surface of the porous carbon in which elemental nanosilicon is deposited is carbon-deposited and forms a carbon coating layer; the carbon source gas is acetylene, propyne, and other alkyne hydrocarbons, ethylene, propylene, and other olefin hydrocarbons, methane, hexane, and other alkane hydrocarbons, and mixtures thereof.

[0068] The obtained deposition sample is ground and sieved through a 400-mesh sieve to obtain a silicon-based material.

[0069] Preparation of negative electrode sheet The negative electrode active material, sodium carboxymethyl cellulose (CMC), polyacrylic acid (PAA), and conductive agent (carbon nanotube) are uniformly dispersed in a proper amount of deionized water at a mass ratio of 92:1.5:5:1.5 to obtain a slurry. A copper foil that has been subjected to a conductive coating treatment in advance is used as a current collector, and the slurry is uniformly coated on the current collector. After coating, drying and cold pressing are performed to obtain a negative electrode sheet.

[0070] Preparation of positive electrode sheet The positive electrode sheet takes lithium cobaltate as the active material, which is mixed with conductive agent acetylene black and binder polyvinylidene fluoride (abbreviated as PVDF) in a weight ratio of 96.3:2.2:1.5 in an appropriate amount of N-methyl pyrrolidone (abbreviated as NMP) solvent, and is fully stirred to form a uniform positive electrode slurry. The slurry is coated on the current collector Al foil, and after drying and cold pressing, the positive electrode sheet is obtained. After the tab is welded, it is ready for use.

[0071] Preparation of electrolyte In a dry argon atmosphere glove box, ethylene carbonate (EC), propylene carbonate (PC), methyl ethyl carbonate (EMC), and diethyl carbonate (DEC) are mixed in a mass ratio of EC: EMC: DEC = 1:3:3:3. Then, fluoroethylene carbonate and 1,3-propane sultone are added. After dissolution and thorough stirring, lithium salt LiPF6 is added, and the mixture is uniformly mixed to obtain the electrolyte. The mass percentage of LiPF6 is 12.5%, the mass percentage of fluoroethylene carbonate is 2%, the mass percentage of 1,3-propane sultone is 2%, and the rest is EC, PC, EMC, and DEC. The mass percentages of the substances are calculated based on the mass of the electrolyte.

[0072] Preparation of separator film The separator film is a polypropylene film with a porous structure (porosity of 35%).

[0073] Preparation of lithium ion battery: The positive electrode sheet, the separator film, and the negative electrode sheet after welding the tab are stacked in order, with the separator film between the positive electrode and the negative electrode to act as a separator. The electrode assembly is obtained by winding. The electrode assembly is placed in an aluminum plastic film packaging bag, dried at 80°C, and then injected with electrolyte. After vacuum packaging, standing, formation, shaping, and capacity testing processes, the soft package lithium ion battery is obtained.

[0074] Preparation of button cell: The cold-pressed negative electrode sheet is taken as the negative electrode of the button cell, and assembled with a lithium sheet, a separator film, an electrolyte, a steel sheet, a foamed nickel, and a button cell shell to obtain a button cell. The button cell is statically placed for 6h before testing.

[0075] Examples 2 to 15 Examples 2 to 15 differ from Example 1 in that the mass ratio of the composite graphite material and the silicon-based material is changed, as well as the mass content of the hard carbon in the preparation of the composite graphite material. The specific preparation parameters can be referred to Tables 1 and 2.

[0076] Comparative Examples 1 to 3 Comparative Examples 1 to 3 and Example 1 differ in that the mass ratio of the composite graphite material and the silicon-based material and the mass content of the hard carbon in the composite graphite material are changed, and the rest of the conditions are the same as in Example 1. The specific preparation parameters can be referred to Tables 1 and 2.

[0077] The negative electrode active materials prepared in each of the examples and comparative examples and the assembled lithium ion batteries were tested.

[0078] Lithium ion battery related performance tests (1) Test of discharge capacity in different potential intervals The button cell was placed on a blue instrument tester for testing. The test procedure was as follows: discharged to 5 mV at 0.05 C, rested for 5 min, discharged to 5 mV at 0.05 mA, discharged to 5 mV at 0.01 mA, charged to 2.0 V at 0.1 C to obtain the charge capacity, and finally divided by the active material weight to obtain the gram capacity C of the sample. The lithium extraction curve of the material was obtained by plotting the charge voltage and the gram capacity. The gram capacity corresponding to the voltage P in the interval 0 V≤P<0.1 V was denoted as C1, the gram capacity corresponding to the voltage P in the interval 0.1 V≤P<0.25 V was denoted as C2, and the gram capacity corresponding to the voltage P in the interval 0.25 V≤P≤2 V was denoted as C3.

[0079] (2) Test of the compaction density and capacity density of the negative electrode sheet The negative electrode sheet was taken, the thickness of the negative electrode sheet was measured by a micrometer and denoted as H1, the thickness of the negative electrode current collector was denoted as H2, a small disc with a fixed area on the negative electrode sheet was punched by a mold, the area was denoted as S, the weight of the small disc was denoted as M1, the weight of the negative electrode current collector with the same area was denoted as M2, the compaction density T of the negative electrode sheet was (M2-M1) / (H1-H2) / S, and the capacity density E of the negative electrode sheet was C×T, wherein C was the gram capacity obtained in the above-mentioned “test of discharge capacity in different potential intervals”.

[0080] (3) Cycle performance test The soft package lithium ion battery was placed in a 25℃ constant temperature box, rested for 30 min, charged to 4.5 V at a constant current of 1.5 C, charged to 0.025 C at a constant voltage of 4.5 V, rested for another 30 min, discharged to 3.0 V at a constant current of 0.5 C, and rested for another 30 min. At this time, it was the first charge and discharge cycle. The charging and discharging cycle test was carried out according to the above-mentioned steps, and the cycle was 800 times. The capacity retention rate of the battery was the ratio of the discharge capacity at the 800th cycle to the discharge capacity at the 3rd cycle.

[0081] Related test methods of the negative electrode sheet: (1) Raman spectrum test of the composite graphite material Select a region of 100 μm x 100 μm of the solid powder of the composite graphite material, scan the particles in the region using a laser microscopic confocal Raman spectrometer (Raman, HR Evolution, HORIBA Scientific Instrument Co., Ltd.), the laser wavelength of the Raman spectrometer can be in the range of 532 nm to 785 nm, obtain the D peak and G peak of all particles in the area, use LabSpec software to process the data to obtain the peak intensity of the D peak and G peak of each particle, respectively recorded as Id and Ig, and the average value of the ratio of Id and Ig of all particles in the region is obtained, and the corresponding A is obtained.

[0082] (2) Surface defect degree test of graphite Select a region of 100 μm x 100 μm of the solid powder of the composite graphite material, scan the particles in the region using a laser microscopic confocal Raman spectrometer (Raman, HR Evolution, HORIBA Scientific Instrument Co., Ltd.), the laser wavelength of the Raman spectrometer can be in the range of 532 nm to 785 nm, obtain the D peak and G peak of all particles in the area, use LabSpec software to process the data to obtain the peak intensity of the D peak and G peak of each particle, respectively recorded as Id1 and Ig1, and the average value of the ratio of Id1 and Ig1 of all particles in the region is obtained, and the surface defect degree of the graphite is obtained.

[0083] (3) Graphitization degree test of graphite Take 0.5g~1g of graphite powder, load it into a tablet press, and press it into a flat sheet under a pressure of 10-20 MPa, then put it into an X-ray diffractometer mold for testing, the target material is CuKα(λ=0.15418nm), the voltage and current are 40KV / 40mA, the scanning angle range is 20° to 80°, the scanning step is 0.02°, and the time for each step is 2° / min, and the silicon standard sample is used to calibrate the instrument angle, according to the obtained X-ray diffraction pattern, take the 002 peak diffraction angle θ, calculate the 002 crystal plane spacing d through the Bragg equation d=λ / 2sin(θ), and then calculate the graphitization degree through the graphitization degree calculation formula g=(0.344-d) / (0.344-0.3354) x 100%.

[0084] (4) Particle size Dv50 test of graphite In a 50ml clean beaker, 0.02g of graphite powder sample in each example and comparative example was added, 20ml of deionized water was added, and surfactant was added dropwise to disperse the powder sample in water completely, ultrasonic cleaning for 5min in a 120W ultrasonic cleaner, MasterSizer 2000 equipment was used to test the graphite from 0° to 135° detection angle, according to the particle size distribution laser diffraction method GB / T19077-2016, the particle size distribution was measured, and the DV50 value of the graphite was obtained.

[0085] (5) Test of ratio of micropore volume to total pore volume of hard carbon A gas adsorption instrument was used for testing, 1-5g of hard carbon sample was weighed, activated at 250℃ for 6h to remove surface impurities, then high-purity nitrogen was introduced, and the hard carbon sample was adsorbed with nitrogen at liquid nitrogen temperature until saturation, and the obtained adsorption and desorption curve was calculated by DFT model to calculate the total pore volume P and micropore volume P0 of the hard carbon material, and the ratio of micropore volume to total pore volume V=P0 / P of the hard carbon.

[0086] (6) Test of powder compaction density of composite graphite material A Sansi Zongheng UTM7305 type battery powder compaction density instrument was used to test the compaction density of the composite graphite material under a pressure of 5 tons.

[0087] (7) Test of specific surface area of composite graphite material The solid powder of the composite graphite material was vacuum degassed at 100℃ for 12h in a sample tube, the adsorption amount of nitrogen by the material under different pressures was tested by a physical adsorption analyzer (model ASAP1460), the adsorption and desorption isotherms were drawn, the shape of the composite graphite material pores was determined according to the shape of the hysteresis loop, the pore size distribution curve was fitted using the DFT model, and the specific surface area of the material was calculated. The test was carried out according to the national standard GB / T 19587-2017 "Gas adsorption BET method for measuring specific surface area of solid materials".

[0088] Table 1 Table 2 According to Table 1 and Table 2, compared with Comparative Examples 1-3, it can be seen from Examples 1-7 in Table 1 that when the Raman value A of the composite graphite material in the negative electrode sheet satisfies 0.5≤A≤1 and when the capacity ratio C1 / C corresponding to the potential 0V≤P<0.1V of the negative electrode sheet satisfies the range 0<C1 / C≤30%, the negative electrode sheet has a high capacity density and also has excellent cycle performance.

[0089] And in the range of 0.1V≤P<0.25V, the capacity ratio C2 / C satisfies the range 10%≤C2 / C≤60%, or / and in the range of 0.25V≤P≤2V, the capacity ratio C3 / C satisfies the range 30%≤C3 / C≤60%, the negative electrode sheet has a higher capacity density and an excellent cycle capacity retention rate.

[0090] Table 3 In combination with the above Table 3 and Figure 1 On the basis of Example 2, in Examples 8 to 11, the mass ratio of the silicon-based material and the composite graphite material and the coating amount of the hard carbon material are changed, when the range of C3 / C is 30%≤C3 / C≤50%, C1 / C2 satisfies the range 0%<C1 / C2≤15%, or when the mass ratio of the silicon-based material is 10% to 15%, 0<C1 / C2≤15%, the negative electrode sheet has a higher capacity density and a cycle capacity retention rate.

[0091] Table 4 In combination with Table 4, on the basis of Example 1, in Examples 12 to 15, the mass ratio of the silicon-based material and the composite graphite material and the coating amount of the hard carbon material are changed, when the range of C3 / C is 50%<C3 / C≤60%, C1 / C2 satisfies the range 15%<C1 / C2≤30%, or when the mass content of the silicon-based material is 15% to 25%, 15%<C1 / C2≤30%, the negative electrode sheet has a higher capacity density and a cycle capacity retention rate.

[0092] The above disclosure is only the preferred embodiment of the present application, of course, cannot be limited by this, therefore, the equivalent changes made by the present application, still belongs to the scope covered by the present application.

Claims

1. A negative electrode sheet comprising a current collector and a negative electrode material layer provided on the current collector, characterized by, The negative material layer comprises a negative active material, the negative active material comprises a composite graphite material and a silicon-based material, the composite graphite material comprises graphite and hard carbon on the surface of the graphite, a ratio of a D peak to a G peak in a Raman spectrum of the composite graphite material is A, 0.5≤A≤1; a lithium extraction capacity of a button cell comprising the negative electrode sheet is C when charged to 2V, a lithium extraction capacity in a range of 0V≤P<0.1V is C1, C=C1+C2+C3, 0 2. The negative electrode sheet according to claim 1, wherein The lithium extraction capacity of the button cell in a range of 0.1V≤P<0.25V is C2, 10%≤C2 / C≤60%.

3. The negative electrode sheet according to claim 1, wherein The lithium extraction capacity of the button cell in a range of 0.25V≤P≤2V is C3, 30%≤C3 / C≤60%.

4. The negative electrode sheet according to claim 1, wherein The lithium extraction capacities of the button cell in ranges of 0.1V≤P<0.25V and 0.25V≤P≤2V are C2 and C3 respectively, C=C1+C2+C3, the negative electrode sheet satisfies at least one of the following conditions: (1) when 30%≤C3 / C≤50%, 0 The negative electrode sheet satisfies at least one of the following conditions:

5. The negative electrode sheet according to any one of claims 1 to 4, wherein (1) a surface defect degree of the graphite is R, a ratio of a micropore volume of the hard carbon to a total pore volume is V, 0.14≤R / V≤0.25; (2) a graphitization degree of the graphite is 92% to 96%; (3) a surface defect degree of the graphite is R, 0.07≤R≤0.2; (4) a particle size Dv50 of the graphite is 5μm to 10μm; (6) a ratio of a micropore volume of the hard carbon to a total pore volume is V, 50%≤V≤80%. (5) the compaction density of the negative electrode plate is T, 1.05 g / cm 3 ≤ T ≤ 1.35 g / cm 3 ; The lithium extraction capacity of the button cell in a range of 0.1V≤P<0.25V is C2, the negative electrode sheet satisfies at least one of the following conditions:

6. The negative electrode sheet according to any one of claims 1 to 4, wherein (1) when a mass content of the silicon-based material is 15% to 25% based on a mass of the negative active material, 15%≤C1 / C2≤30%; (2) when a mass content of the silicon-based material is 10% to 15% based on a mass of the negative active material, 0 The negative electrode sheet satisfies at least one of the following conditions:

7. The negative electrode sheet according to any one of claims 1 to 4, wherein (1) a mass of the hard carbon is 5% to 35% of a mass of the graphite; (2) a powder compaction density of the composite graphite material is 1.4g / cc to 1.8g / cc; A mass ratio of the composite graphite material is 75% to 90% and a mass ratio of the silicon-based material is 10% to 25% based on a mass of the negative active material. (3) the specific surface area of the composite graphite material is less than 2 m 2 / g.

8. The negative electrode sheet according to any one of claims 1 to 4, wherein The secondary battery further comprises the negative electrode sheet of any one of claims 1 to 8, and the separator film is located between the positive electrode sheet and the negative electrode sheet.

9. A secondary battery comprising a positive electrode sheet, a separator, and an electrolyte, characterized by The secondary battery comprises the secondary battery of claim 9.

10. An electronic device, comprising: ​

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