Negative pole piece, preparation method of silicon-carbon composite material, secondary battery and electronic device

Abstract

The invention provides a negative electrode piece, a preparation method of a silicon-carbon composite material, a secondary battery and an electronic device, the negative electrode piece comprises a negative electrode current collector and a negative electrode material layer arranged on at least one surface of the negative electrode current collector, the negative electrode material layer comprises the silicon-carbon composite material, the silicon-carbon composite material comprises a matrix and a carbon layer located on the surface of the matrix, and the carbon layer is arranged on the surface of the matrix. The matrix comprises a carbon material and a silicon material, the carbon material has a gap, and the silicon material is distributed in the gap; the negative electrode material layer and the potassium hydroxide aqueous solution are mixed to obtain a mixed system, the mixed system is subjected to heat preservation for 6 h at the temperature of T1 DEG C, then alkali solubility testing is carried out, the measured alkali solubility of the negative electrode material layer is alpha, alpha is smaller than 10%, the concentration of the potassium hydroxide aqueous solution is 1.78 mol / L, and the mass ratio of potassium hydroxide to the negative electrode material layer in the mixed system is 4: 1. When the negative pole piece provided by the invention is applied to the secondary battery, the high-temperature storage performance and the specific capacity of the secondary battery can be improved.

Description

Technical Field

[0001] The present application relates to the field of electrochemical energy storage, and particularly to a negative electrode sheet, a method for preparing a silicon-carbon composite material in the negative electrode sheet, a secondary battery using the negative electrode sheet, and an electronic device using the secondary battery. Background Art

[0002] Lithium-ion batteries have been widely used in the fields of electric vehicles, consumer electronics, and large-scale energy storage due to their ultra-high energy density, long cycle life, and low self-discharge characteristics. However, the negative electrodes of traditional commercial lithium-ion batteries are basically limited to graphite materials, and their low capacity (LiC6, 372 mAh g -1 ) has greatly hindered the improvement of energy density. In recent years, silicon-based materials have gradually entered the industrialization track of lithium-ion battery negative electrodes with their high specific capacity (Li 15 Si4, 3579 mAh g -1 ), rich resource reserves, and reasonable lithiation platform (<0.4V vs. Li / Li + ), and are considered to be the most promising negative electrode materials for the next generation.

[0003] Despite the above advantages, silicon-based materials will undergo severe volume changes during the lithiation / delithiation process, resulting in poor cycle performance, and then the electrode powder will fail, hindering the full play of the electrochemical performance of silicon-based materials. To address this problem, researchers have found that silicon-based materials can be nano-sized, combined with other substances to form composite materials, and the battery matching system can be changed to relieve structural strain, shorten the lithium-ion diffusion distance, and improve the effective electrochemical contact of active substances.

[0004] Carbon materials are considered to be the best candidates for composite with silicon due to their excellent electrical conductivity and tough strength. However, there are still problems with the formation of silicon-carbon materials by compounding silicon-based materials and carbon. For example, silicon-carbon materials are prone to generate gas during the preparation of anode slurries, which affects the specific capacity and also reduces the surface stability of silicon-carbon composite materials. Summary of the Invention

[0005] The present application provides a negative electrode sheet, a method for preparing a silicon-carbon composite material in the negative electrode sheet, a secondary battery, and an electronic device.

[0006] The first aspect of the present application provides a negative electrode plate, which 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 silicon-carbon composite material, which includes a matrix and a carbon layer located on the surface of the matrix. The matrix includes a carbon material and a silicon material. The carbon material has voids, and the silicon material is distributed in the voids. The negative electrode material layer and an aqueous potassium hydroxide solution are mixed to obtain a mixed system. The mixed system is kept warm at T1 for 6 hours and then subjected to an alkali solubility test. Among them, 53°C ≥ T1 ≥ 47°C, the alkali solubility of the negative electrode material layer is measured as α, α < 10%, the concentration of the aqueous potassium hydroxide solution is 1.78 mol / L, and the mass ratio of potassium hydroxide to the negative electrode material layer in the mixed system is 4:1.

[0007] In the negative electrode plate of the present application, the silicon-carbon composite material in the above-mentioned negative electrode material layer can react with an alkali solution. The size of the alkali solubility can reflect the reaction energy barrier between the silicon material and water in the negative electrode material layer. The interfacial stability of the silicon-carbon composite material affects the height of the reaction energy barrier between silicon and water. In the present application, in a lower alkali solubility range, the silicon-carbon composite material in the negative electrode material layer has good interfacial stability, so as to reduce the gas generation during the reaction of the silicon-carbon composite material and water in the preparation process of the negative electrode slurry of the negative electrode plate. When the negative electrode plate is applied to a secondary battery, during the cycling process of the secondary battery, it can reduce the erosion of the electrolyte on the silicon-carbon composite material, reduce the side reactions between the silicon-carbon composite material and the electrolyte, and improve the high-temperature storage performance and specific capacity of the secondary battery.

[0008] Based on the first aspect, in some possible implementation manners, 0.2% < α < 10%. By controlling 0.2% < α < 10%, while the silicon-carbon composite material in the negative electrode material layer has strong lithium intercalation ability, it has good interfacial stability, further reduces the gas generation during the reaction of the silicon-carbon composite material and water, and improves the high-temperature storage performance and specific capacity of the secondary battery.

[0009] Based on the first aspect, in some possible implementation manners, at T2, the negative electrode material layer and water are mixed in a mass ratio of 1:8 to obtain a dispersion system. Among them, 27°C ≥ T2 ≥ 23°C. After the dispersion system is sealed and left standing for 48 hours, the gas generation of the dispersion system is tested. Based on the total volume of the gas generation, the volume fraction of H2 is less than 2%. By controlling the volume fraction of H2 to be less than 2%, it is beneficial to reduce the side reactions between the silicon-carbon composite material in the negative electrode material layer and the electrolyte in the secondary battery, thereby further improving the high-temperature storage performance and specific capacity of the secondary battery.

[0010] Based on the first aspect, in some possible embodiments, the average particle size of the silicon-carbon composite material is 5.5 μm to 9.5 μm. An appropriate average particle size is conducive to reducing gas generation during the preparation of the negative electrode slurry, reducing the contact between the silicon material and the electrolyte, enabling the secondary battery to have good high-temperature storage performance, and also conducive to improving the kinetic performance of the secondary battery.

[0011] Based on the first aspect, in some possible embodiments, the average circularity of the silicon-carbon composite material is 0.7 to 0.85. An appropriate circularity is beneficial to adjusting the contact degree between the silicon material in the silicon-carbon composite material and the electrolyte, further leveraging the characteristic that the silicon-carbon composite material with an alkali solubility of less than <10% has a relatively high reaction energy barrier with water, reducing the occurrence of side reactions, and being conducive to improving the high-temperature storage performance and specific capacity of the secondary battery when the negative electrode material layer is applied to the secondary battery.

[0012] Based on the first aspect, in some possible embodiments, the silicon material includes silicon grains, and the average diameter of the silicon grains is 1.5 nm to 4.5 nm, which can further adjust the reaction energy barrier between the silicon-carbon composite material and water, reduce gas generation during the preparation of the negative electrode slurry of the silicon-carbon composite material, reduce swelling, and also be conducive to reducing the reaction with the electrolyte, thereby enabling the secondary battery to have good high-temperature storage performance.

[0013] Based on the first aspect, in some possible embodiments, the negative electrode material layer includes silicon element and carbon element. Based on the mass of the negative electrode material layer, the mass ratio of the silicon element is 5% to 40%. This enables the silicon-carbon composite material to have a certain specific capacity and also minimizes the swelling performance of the silicon-carbon composite material as much as possible.

[0014] Based on the first aspect, in some possible embodiments, based on the mass of the negative electrode material layer, the mass ratio of the carbon element is 60% to 95%. When the mass ratio of the carbon element is within the above range, it is beneficial to improve the conductivity of the silicon-carbon composite material, thereby facilitating the improvement of the cycling performance of the silicon-carbon composite material.

[0015] Based on the first aspect, in some possible embodiments, the binder includes at least one of lithium polyacrylate, sodium carboxymethyl cellulose, methyl polyacrylate, styrene-butadiene rubber, or polyacrylic acid. Based on the mass of the negative electrode material layer, the mass ratio of the binder is 0.4% to 2.5%. This enables the negative electrode sheet to have a relatively high conductivity, can reduce the polarization of the secondary battery, promote the formation of the SEI film on the surface of the negative electrode sheet, make the surface potential distribution of the SEI film more uniform, and is conducive to improving the high-temperature storage performance of the secondary battery.

[0016] Based on the first aspect, in some possible implementation manners, the carbon layer contains Si-C bonds. The carbon layer containing Si-C bonds is beneficial to increasing the density of the carbon layer and reducing side reactions occurring at the contact between the silicon-carbon composite material and the electrolyte, thereby improving the high-temperature storage performance and specific capacity of the secondary battery.

[0017] The second aspect of the present application provides a method for preparing a silicon-carbon composite material in a negative electrode sheet, including introducing a silane mixed gas into a carbon material to deposit and form a silicon material in the carbon material to obtain a matrix; introducing a mixed gas into the matrix in an inert gas atmosphere at a temperature of 550°C to 700°C and reacting for 1 h to 6 h to obtain the silicon-carbon composite material.

[0018] In the above preparation method, a silicon-carbon composite material with good interfacial stability can be obtained to reduce side reactions occurring between the silicon in the silicon-carbon composite material and the electrolyte, which is beneficial to improving the high-temperature storage performance and specific capacity of the silicon-carbon composite material applied in a secondary battery.

[0019] Based on the second aspect, in some possible implementation manners, the silane mixed gas includes a silane gas and an inert gas, and the mixed gas includes one group of acetylene and propylene, acetylene and ethylene, acetylene and methane, acetylene and silane, silane and propylene, silane and ethylene. The above mixed gas can form a carbon layer with high density containing Si-C bonds on the surface of the matrix at high temperature to improve the interfacial stability of the silicon-carbon composite material.

[0020] The third aspect of the present application further provides a secondary battery, including a positive electrode sheet, the negative electrode sheet, an electrolyte, and a separator. The electrolyte includes fluoroethylene carbonate, and based on the mass of the electrolyte, the mass ratio of fluoroethylene carbonate is a, and 1.5% ≤ a ≤ 8.4%. The reduction potential of fluoroethylene carbonate is relatively high, and it will preferentially form a rigid inorganic component lithium fluoride. This inorganic component is distributed on the surface of the silicon-carbon composite material, which can block the erosion of the electrolyte on the silicon-carbon composite material. At the same time, the electrolyte containing fluoroethylene carbonate can further stabilize the silicon material, thereby improving the high-temperature storage performance of the secondary battery.

[0021] Based on the third aspect, in some possible implementation manners, the electrolyte includes difluoroethylene carbonate, and based on the mass of the electrolyte, the mass ratio of difluoroethylene carbonate is b, and 1.5% ≤ b ≤ 2.5%. In the electrolyte, difluoroethylene carbonate can form a rigid inorganic component lithium fluoride on the surface of the silicon-carbon composite material, further reducing the erosion of the silicon-carbon composite material and improving the high-temperature storage performance of the secondary battery.

[0022] Based on the third aspect, in some possible embodiments, the electrolyte includes 1,3 - propane sultone. Based on the mass of the electrolyte, the mass percentage of 1,3 - propane sultone is c, where 0.8% ≤ c ≤ 3.8%. 1,3 - propane sultone participates in the formation of the SEI film and forms a rigid inorganic component, lithium sulfate. Lithium sulfate is distributed on the surface of the silicon - carbon composite material, which can reduce the erosion of the silicon - carbon composite material. At the same time, the electrolyte containing 1,3 - propane sultone can further stabilize the silicon material and improve the high - temperature storage performance of the secondary battery.

[0023] Based on the third aspect, in some possible embodiments, the electrolyte includes vinylene sulfate. Based on the mass of the electrolyte, the mass percentage of vinylene sulfate is d, where 0.3% ≤ d ≤ 0.8%. In the secondary battery, vinylene sulfate forms a rigid inorganic component, lithium sulfate. Lithium sulfate is distributed on the surface of the silicon - carbon composite material, which can reduce the erosion of the silicon - carbon composite material, thereby improving the high - temperature storage performance of the secondary battery.

[0024] Based on the third aspect, in some possible embodiments, 0.1 ≤ a / α ≤ 1.26. By controlling 0.1 ≤ a / α ≤ 1.26, during the cycling process of the secondary battery, a solid - electrolyte interface film including an appropriate amount of lithium fluoride is formed on the surface of the composite silicon - carbon material to improve the stability of the negative electrode plate. Excessive lithium fluoride may cause insufficient elasticity of the SEI film and cannot further exert the stability advantage of the composite silicon - carbon material. By controlling the relationship between the alkali solubility of the negative electrode material layer and the mass percentage of fluoroethylene carbonate, the high - temperature storage performance and specific capacity of the secondary battery can be improved.

[0025] Based on the third aspect, in some possible embodiments, at a state of charge of 50% SOC, the thickness of the negative electrode plate is 72 μm to 85 μm. This is beneficial for the secondary battery to have both good high - temperature storage performance and energy density.

[0026] The fourth aspect of the present application provides an electronic device, including a secondary battery. The secondary battery supplies power to the load in the electronic device. The secondary battery including the negative electrode plate has excellent high - temperature storage performance and specific capacity, which is beneficial to improve the service life of the electronic device and its applicability in high - temperature environments. Specific Embodiments

[0027] The technical solutions in the embodiments of the present application will be clearly and detailedly described below. Obviously, the described embodiments are some, but not all, of the embodiments of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the technical field to which this application belongs. The terms used in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0028] In the related art, during the anode pulping process, it is easy to damage the interface of the silicon-carbon material during high-speed stirring. Silicon easily reacts with water to generate hydrogen, resulting in gas generation in the slurry, easy appearance of bubbles in the electrode sheet, and loss of specific capacity of the active material. Secondly, poor interface stability will also cause the electrolyte to gradually react with the internal silicon during the circulation process, resulting in the thickening of the SEI film and the reduction of the cycle life.

[0029] Si + 2H2O = SiO2 + 2H2↑

[0030] Therefore, how to enhance the interface stability of the silicon-carbon material needs further research.

[0031] In view of the above problems, a negative electrode sheet, a preparation method of a silicon-carbon composite material, a secondary battery, and an electronic device are provided in this application.

[0032] One embodiment of this application provides a secondary battery, which includes a housing, an electrode assembly, and an electrolyte. The electrode assembly and the electrolyte are both located inside the housing.

[0033] The housing can be a packaging bag obtained by encapsulating with a packaging film (such as an aluminum-plastic film). For example, the secondary battery is a soft-pack battery. In some other embodiments, the secondary battery can also be a steel-shell battery, an aluminum-shell battery, etc.

[0034] The electrode assembly includes a positive electrode sheet, a negative electrode sheet, and a separator. The separator is disposed between the positive electrode sheet and the negative electrode sheet. The electrode assembly can be a stacked structure, which is formed by alternately laminating the positive electrode sheet, the separator, and the negative electrode sheet. In some other embodiments, the electrode assembly can also be a wound structure, which is formed by laminating and winding the positive electrode sheet, the separator, and the negative electrode sheet.

[0035] Negative electrode sheet

[0036] The negative electrode sheet 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 current collector can use at least one of copper foil, nickel foil, stainless steel foil, titanium foil, composite current collector, or carbon-based current collector, etc.

[0037] One embodiment of this application provides a negative electrode sheet. The negative electrode material layer includes a silicon-carbon composite material. The silicon-carbon composite material includes a matrix and a carbon layer on the surface of the matrix. The matrix includes a carbon material and a silicon material. The carbon material has voids, and the silicon material is distributed in the voids. The negative electrode material layer and an aqueous potassium hydroxide solution are mixed to obtain a mixed system. The mixed system is kept warm at T1 for 6 hours and then subjected to an alkali solubility test. Among them, 53°C ≥ T1 ≥ 47°C. The measured alkali solubility of the negative electrode material layer is α, α < 10%, the concentration of the aqueous potassium hydroxide solution is 1.78 mol / L, and the mass ratio of potassium hydroxide to the negative electrode material layer in the mixed system is 4:1.

[0038] In the negative electrode sheet of the present application, the silicon-carbon composite material in the above-mentioned negative electrode material layer can react with an alkali solution (the chemical reaction formula is as shown in Formula (1)). The magnitude of the alkali solubility can reflect the reaction energy barrier between the silicon material and water in the negative electrode material layer. The interfacial stability of the silicon-carbon composite material affects the height of the reaction energy barrier between the silicon material and water. In the present application, a dense carbon layer is formed on the surface of the matrix in the silicon-carbon composite material, improving the integrity of the carbon layer coating the matrix, capable of enhancing the corrosion resistance of the silicon-carbon composite material to the alkali, reducing the reaction between the silicon-carbon composite material and water, and combined with the lower alkali solubility range of the negative electrode material layer, enabling the silicon-carbon composite material in the negative electrode material layer to have good interfacial stability, so as to further reduce the gas generation during the reaction between the silicon-carbon composite material and water in the preparation process of the negative electrode slurry of the negative electrode sheet, and when the negative electrode sheet is applied to a secondary battery, further reducing the erosion of the electrolyte on the silicon-carbon composite material during the cycling process of the secondary battery, reducing the side reactions between the silicon-carbon composite material and the electrolyte, and improving the high-temperature storage performance and specific capacity of the secondary battery.

[0039] Chemical reaction of silicon with an alkali solution:

[0040] Si + H2O + 2KOH = K2SiO3 + 2H2↑ (1)

[0041] If under the above conditions, the alkali solubility of the negative electrode material layer is large, the reaction energy barrier between the silicon-carbon composite material and water in the negative electrode material layer is low, and the interfacial stability of the silicon-carbon composite material is poor, which is not conducive to reducing the gas generation during the reaction between the silicon-carbon composite material and water in the preparation process of the negative electrode slurry, and is not conducive to reducing the side reactions between the silicon-carbon composite material and the electrolyte, thus not conducive to improving the high-temperature storage performance and specific capacity of the negative electrode sheet applied to a secondary battery. In some embodiments, the alkali solubility α of the negative electrode material layer can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 9.5% or any value within the range composed of any two of the above values.

[0042] In some embodiments, 0.2% < α < 10%. Within this range, while enabling the silicon-carbon composite material in the negative electrode material layer to have strong lithium insertion ability, it has good interfacial stability, further reducing the gas generation during the reaction between the silicon-carbon composite material and water, and improving the high-temperature storage performance and specific capacity of the secondary battery.

[0043] In some embodiments, at T2, the negative electrode material layer and water are mixed at a mass ratio of 1:8 to obtain a dispersion system, where 27°C ≥ T2 ≥ 23°C. After the dispersion system is sealed and left standing for 48 h, when testing the gas generated by the dispersion system, based on the total volume of the generated gas, the volume fraction of H2 is less than 2%. The volume fraction of H2 is within a suitable range, which enables the negative electrode material layer to have better interfacial stability, reduces gas generation during the preparation of the negative electrode paste, and reduces side reactions between the silicon-carbon composite material and the electrolyte, thereby facilitating further improvement of the high-temperature storage performance and specific capacity of the secondary battery. In some embodiments, the volume fraction of H2 can be 0.1%, 0.2%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 1.8%, 1.9% or any value within the range formed by any two of the above values. Preferably, the volume fraction of H2 in the gas generated by the dispersion system is 0.3% to 2%.

[0044] In some embodiments, the average particle size of the silicon-carbon composite material is 5.5 μm to 9.5 μm. Within the above suitable range of average particle size, it is beneficial to reduce gas generation during the preparation of the negative electrode paste. When the negative electrode material layer is applied to a secondary battery, it reduces the contact between the silicon material and the electrolyte and reduces the occurrence of side reactions, enabling the secondary battery to have good high-temperature storage performance. Also, within the above suitable range of average particle size, it is beneficial to improve the kinetic performance of the secondary battery. In some embodiments, the average particle size of the silicon-carbon composite material can be 5.5 μm, 5.8 μm, 6 μm, 6.2 μm, 6.4 μm, 6.8 μm, 7 μm, 7.5 μm, 8 μm, 8.5 μm, 9 μm, 9.5 μm or any value within the range formed by any two of the above values.

[0045] In some embodiments, the carbon layer contains Si-C bonds. The carbon layer containing Si-C bonds is beneficial to increase the density of the carbon layer, reduce side reactions occurring due to the contact between the silicon-carbon composite material and the electrolyte, thereby improving the high-temperature storage performance and specific capacity of the secondary battery.

[0046] In some embodiments, the negative electrode material layer includes silicon and carbon elements. Based on the mass of the negative electrode material layer, the mass fraction of the silicon element is 5% to 40%. The mass fraction of the silicon element is within the above range to enable the silicon-carbon composite material to have a certain specific capacity, inhibit the expansion of the silicon-carbon composite material, and improve the high-temperature storage performance of the secondary battery. In some embodiments, based on the mass of the negative electrode material layer, the mass fraction of the silicon element can be 5%, 7%, 8%, 10%, 12%, 15%, 20%, 25%, 30%, 35%, 40% or any value within the range formed by any two of the above values.

[0047] In some embodiments, based on the mass of the negative electrode material layer, the mass fraction of carbon element is 60% to 95%. When the mass fraction of carbon element is within the above range, it is beneficial to improve the electrical conductivity of the silicon-carbon composite material, thereby facilitating the improvement of the cycling performance of the silicon-carbon composite material. In some embodiments, based on the mass of the negative electrode material layer, the mass fraction of carbon element can be 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95% or any value within the range composed of any two of the above values.

[0048] In some embodiments, the average circularity of the silicon-carbon composite material is 0.7 to 0.85. When the average circularity of the silicon-carbon composite material is within the above range, it is beneficial to make the silicon-carbon composite material have a suitable specific surface area, reduce the contact between silicon in the silicon-carbon composite material and the electrolyte, further utilize the characteristic that the silicon-carbon composite material with an alkali solubility of less than <10% has a relatively high reaction energy barrier with water, reduce the occurrence of side reactions, and when the negative electrode material layer is applied to a secondary battery, it is beneficial to improve the high-temperature storage performance and specific capacity of the secondary battery. In some embodiments, the average circularity of the silicon-carbon composite material can be 0.7, 0.75, 0.78, 0.8, 0.74, 0.77, 0.8, 0.84, 0.85 or any value within the range composed of any two of the above values.

[0049] In some embodiments, the binder includes at least one of lithium polyacrylate, sodium carboxymethyl cellulose, methyl polyacrylate, styrene-butadiene rubber or polyacrylic acid. Based on the mass of the negative electrode material layer, the mass fraction of the binder is 0.4% to 2.5%. By adjusting the type and content of the binder within the above range, the negative electrode sheet can have a high electrical conductivity, which can reduce the polarization of the secondary battery, promote the formation of the SEI film on the surface of the negative electrode sheet, make the surface potential distribution of the SEI film more uniform, and is beneficial to improving the high-temperature storage performance of the secondary battery; selecting the above binder type and content range can make the binding force of the negative electrode sheet higher, and the materials in the negative electrode material layer are not easy to fall off, reducing the risk of disconnection of the electron path, which is beneficial to improving the stability of the negative electrode sheet. At the same time, the binder also covers part of the silicon-carbon composite material, which can block the erosion of the electrolyte on the silicon-carbon composite material, thereby being beneficial to further improving the high-temperature storage performance of the secondary battery. In some embodiments, the mass fraction of the binder can be 0.4%, 0.5%, 0.8%, 1%, 1.4%, 1.8%, 2%, 2.5% or any value within the range composed of any two of the above values.

[0050] In some embodiments, the silicon material comprises silicon crystallites with an average diameter of 1.5 nm to 4.5 nm. With the average diameter of the silicon crystallites within a suitable range, the reaction energy barrier between the silicon-carbon composite material and water can be further adjusted, gas generation during the preparation of the negative electrode paste of the silicon-carbon composite material can be reduced, swelling can be decreased, and the reaction with the electrolyte can also be reduced, thereby facilitating the secondary battery to have good high-temperature storage performance. In some embodiments, the average diameter of the silicon crystallites in the silicon-carbon composite material can be 1.5 nm, 1.8 nm, 2 nm, 2.5 nm, 2.8 nm, 3 nm, 3.5 nm, 3.8 nm, 4 nm, 4.5 nm or any value within the range formed by any two of the above values.

[0051] In some embodiments, at a state of charge (SOC) of 50%, the thickness of the negative electrode sheet is 72 μm to 85 μm. With the thickness of the negative electrode sheet within the above range, it is beneficial for the secondary battery to have good high-temperature storage performance and energy density simultaneously. Under the above conditions, if the thickness of the negative electrode sheet is relatively large, the degree of side reactions occurring in the negative electrode material layer may increase, reducing the high-temperature storage performance of the secondary battery; if the thickness of the negative electrode sheet is relatively small, the energy density of the secondary battery will be reduced. In some embodiments, at a state of charge (SOC) of 50%, the thickness of the negative electrode sheet can be 72 μm, 75 μm, 78 μm, 80 μm, 82 μm, 85 μm or any value within the range formed by any two of the above values.

[0052] This application also provides a method for preparing a negative electrode sheet, comprising:

[0053] S1. Pass a silane mixed gas into the carbon material to deposit a silicon material in the carbon material, obtaining a substrate.

[0054] In this step, under an inert gas, the carbon material is heated from room temperature to 400°C to 480°C at a rate of 5°C / min to 20°C / min and then held at a constant temperature. Then, the inert gas atmosphere is switched to the silane mixed gas, and deposition is carried out in the silane mixed gas for 2 h to 30 h. The silane mixed gas contains 10% to 50% by mass of silane gas and 50% to 90% by mass of inert gas.

[0055] In some embodiments, the heating temperature of the carbon material can be 400°C, 410°C, 420°C, 430°C, 440°C, 450°C, 460°C, 470°C, 480°C or any value within the range formed by any two of the above values.

[0056] In some embodiments, the heating rate of the carbon material is 5°C / min, 8°C / min, 10°C / min, 13°C / min, 15°C / min, 18°C / min, 20°C / min or any value within the range formed by any two of the above values.

[0057] In some embodiments, the deposition time of the carbon material in the silane mixed gas can be 2h, 3h, 5h, 10h, 15h, 20h, 25h, 30h, or any value within the range formed by any two of the above values.

[0058] In some embodiments, based on the mass of the silane mixed gas, the mass ratio of the silane gas can be 10%, 20%, 30%, 40%, 50%, or any value within the range formed by any two of the above values. In some embodiments, the silane gas may include, but is not limited to, at least one of silane, disilane, trisilane, tetrasilane, chlorosilane, dichlorosilane, trichlorosilane, or tetrachlorosilane.

[0059] Among them, the inert gas includes at least one of nitrogen and argon.

[0060] In this step, silicon is deposited in the pores or on the surface of the porous carbon material by chemical vapor deposition to obtain a substrate.

[0061] S2. In an inert gas atmosphere at a temperature of 550 °C to 700 °C, a mixed gas is introduced into the substrate and reacted for 1h to 6h to obtain a silicon-carbon composite material.

[0062] At the above temperature, a silicon-carbon composite material with good interfacial stability can be obtained to reduce the side reactions between the silicon in the silicon-carbon composite material and the electrolyte, which is beneficial to improving the high-temperature storage performance and specific capacity of the silicon-carbon composite material applied in secondary batteries.

[0063] In some embodiments, in this step, the reaction temperature for heating the substrate can be 550 °C, 570 °C, 600 °C, 650 °C, 680 °C, 700 °C, or any value within the range formed by any two of the above values. In this step, the reaction time for heating the substrate can be 1h, 2h, 3h, 4h, 5h, 6h, or any value within the range formed by any two of the above values.

[0064] The mixed gas includes one group among the mixtures of acetylene and propylene, acetylene and ethylene, acetylene and methane, acetylene and silane, silane and propylene, and silane and ethylene. In this application, by selecting mixed gases with different components and simultaneously controlling the volumes of the two gases, a dense and continuous carbon layer can be obtained. Taking the selection of acetylene and propylene as an example, at the above high temperature, acetylene cracks into linear molecular small polycyclic aromatic hydrocarbons, while ethylene, propylene, etc. crack into linear macromolecular polycyclic aromatic hydrocarbons. The linear macromolecular polycyclic aromatic hydrocarbons formed by cracking are relatively large and have a low diffusion coefficient. During the process of deposition on the substrate, they will first agglomerate to form large particles attached to the surface of the substrate, which provides good conditions for forming large defects on the surface of the silicon-carbon composite material. However, the above large particles are not easily filled in the gaps and holes, and the linear macromolecular polycyclic aromatic hydrocarbons will also form a crust on the surface of the substrate earlier, blocking the further deposition of cracked carbon particles on the defects, and a carbon layer with a lower density will be formed. The linear small polycyclic aromatic hydrocarbons cracked from acetylene have a high diffusion coefficient, are easy to fill in small pore defects, and can also enter the interior of the substrate through the defects and be filled. However, due to the small size of the linear molecular small polycyclic aromatic hydrocarbons, it is not easy to fill larger defects, so it is not easy to form a continuous carbon layer. Therefore, in this application, two different carbon source gases are combined, so as to take into account the advantages of both gases to form an ideal coating layer and obtain a continuous and dense carbon layer containing Si-C.

[0065] Preferably, the mixed gas includes acetylene and silane, silane and propylene, and silane and ethylene. By selecting the mixed gas of the above components to obtain a continuous and dense carbon layer containing Si-C, the reaction energy barrier between the silicon material and the electrolyte can be further increased, and the silicon-carbon composite material can be stabilized.

[0066] In this application, the composition of the carbon layer can be adjusted by adjusting the reaction temperature, reaction time of heating the substrate in the coating stage, and the composition of the mixed gas, changing the continuity and density of the carbon layer, and changing the integrity of the carbon layer coating on the substrate, so as to adjust the stability of the surface of the silicon-carbon composite material, and further affect the alkali solubility of the negative electrode material layer.

[0067] S3. Mix the silicon-carbon composite material, graphite, binder, and conductive agent to make a negative electrode active paste, coat the negative electrode active paste on the negative electrode current collector, and dry to obtain a negative electrode sheet.

[0068] The negative electrode active paste is dried to obtain the negative electrode material layer. The binder in the negative electrode material layer is used to bond the negative electrode active material particles to facilitate the formation of the film layer, and at the same time can also improve the bonding force between the negative electrode material layer and the negative electrode current collector. In some embodiments, the binder may include but is not limited to polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin or nylon, etc.

[0069] The conductive agent in the negative electrode material layer, the conductive agent includes but is not limited to carbon-based materials, metal-based materials, conductive polymers or any combination thereof. In some embodiments, the carbon-based materials may include but are not limited to natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber or any combination thereof. In some embodiments, the metal-based materials may include but are not limited to metal powders or metal fibers, such as copper, nickel, aluminum or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0070] The negative electrode active material contains graphite, and the graphite includes at least one of natural graphite or artificial graphite. Since graphite has a certain flexibility, its combination with the silicon-carbon composite material can relieve the overall volume expansion of the negative electrode material layer. At the same time, graphite and the silicon-carbon composite material as the negative electrode active material can also make full use of the advantages of both the silicon-carbon composite material and graphite to achieve better electrochemical performance.

[0071] Separator

[0072] There is no particular limitation on the material and shape of the separator used in the secondary battery of the present application, and it can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or an inorganic substance formed of a material stable to the electrolyte of the present application.

[0073] For example, the separator may include a base material layer and a surface treatment layer. The base material layer is a non-woven fabric, a film or a composite film having a porous structure, and the material of the base material 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 non-woven fabric, a polyethylene non-woven fabric or a polypropylene-polyethylene-polypropylene porous composite film can be selected.

[0074] A surface treatment layer is provided on at least one surface of the substrate layer. The surface treatment layer can be a polymer layer, an inorganic layer, or a layer formed by mixing polymers and inorganic substances. The inorganic layer includes inorganic particles and a binder. The inorganic particles are selected from at least one of alumina, silica, magnesia, titanium oxide, hafnium dioxide, tin oxide, cerium dioxide, nickel oxide, zinc oxide, calcium oxide, zirconium oxide, yttrium oxide, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, and barium sulfate. The binder is selected from at least one of polyvinylidene fluoride, copolymer of vinylidene fluoride and hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxide, polymethyl methacrylate, polytetrafluoroethylene, and polyhexafluoropropylene. The polymer layer contains a polymer, and the material of the polymer is selected from at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate salt, polyvinylpyrrolidone, polyethylene alkoxide, polyvinylidene fluoride, and poly(vinylidene fluoride - hexafluoropropylene).

[0075] Electrolyte

[0076] According to some embodiments of the present application, the electrolyte includes an organic solvent, a lithium salt, and an optional additive. The organic solvent in the electrolyte of the present application can be any organic solvent known in the prior art that can be used as a solvent for the electrolyte. There is no limitation on the electrolyte used in the electrolyte according to the present application, and it can be any electrolyte known in the prior art. The additive of the electrolyte according to the present application can be any additive known in the prior art that can be used as an electrolyte additive.

[0077] In some embodiments, the electrolyte includes fluoroethylene carbonate. Based on the mass of the electrolyte, the mass ratio of fluoroethylene carbonate is a, and 1.5% ≤ a ≤ 8.4%. In the electrolyte, fluoroethylene carbonate has a relatively high reduction potential and will be reduced prior to the solvent (such as carbonate compounds) to form a rigid inorganic component, lithium fluoride. This inorganic component is distributed on the surface of the silicon - carbon composite material and can further block the erosion of the electrolyte on the silicon - carbon composite material by combining with the continuous carbon layer in the silicon - carbon composite material, thereby improving the high - temperature storage performance of the secondary battery. Within the above mass range, an appropriate content of the inorganic component will be formed, further improving the high - temperature storage performance of the secondary battery. In some embodiments, in the electrolyte, a can be 1.5%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 8.4% or any value within the range composed of any two of the above values.

[0078] In some embodiments, the electrolyte includes vinylene difluoride carbonate. Based on the mass of the electrolyte, the mass percentage of vinylene difluoride carbonate is b, where 1.5% ≤ b ≤ 2.5%. In the electrolyte, vinylene difluoride carbonate can form a rigid inorganic component, lithium fluoride, and participate in the formation of the SEI film. The inorganic component can further reduce the erosion of the silicon-carbon composite material and improve the high-temperature storage performance of the secondary battery. In some embodiments, in the electrolyte, b is 1.5%, 1.8%, 2%, 2.3%, 2.5% or any value within the range formed by any two of the above values.

[0079] In some embodiments, the electrolyte includes 1,3 - propane sultone. Based on the mass of the electrolyte, the mass percentage of 1,3 - propane sultone is c, where 0.8% ≤ c ≤ 3.8%. In the electrolyte, 1,3 - propane sultone participates in the formation of the SEI film and has a relatively high reduction potential, making it easy to form a rigid inorganic component, lithium sulfate. The lithium sulfate is distributed on the surface of the silicon-carbon composite material, which can reduce the erosion of the silicon-carbon composite material and improve the high-temperature storage performance of the secondary battery. In some embodiments, in the electrolyte, c can be 0.8%, 1%, 1.2%, 1.5%, 1.8%, 2%, 2.3%, 2.5%, 3%, 3.5%, 3.8% or any value within the range formed by any two of the above values.

[0080] In some embodiments, the electrolyte includes ethylene sulfate. Based on the mass of the electrolyte, the mass percentage of ethylene sulfate is d, where 0.3% ≤ d ≤ 0.8%. In the secondary battery, ethylene sulfate will form a rigid inorganic component, lithium sulfate. The lithium sulfate is distributed on the surface of the silicon-carbon composite material, which can reduce the erosion of the silicon-carbon composite material and improve the high-temperature storage performance of the secondary battery. In some embodiments, in the electrolyte, d can be 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8% or any value within the range formed by any two of the above values.

[0081] In some embodiments, the ratio a / α of the mass percentage of vinylene carbonate fluoride in the electrolyte to the alkali solubility of the negative electrode material layer satisfies: 0.1 ≤ a / α ≤ 1.26. The mass percentage of vinylene carbonate fluoride in the electrolyte and the alkali solubility of the negative electrode material layer satisfy the above relationship to form a solid electrolyte interface film including an appropriate amount of lithium fluoride on the surface of the composite silicon-carbon material during the cycling process of the secondary battery, improving the stability of the negative electrode sheet. By controlling the relationship between the alkali solubility of the negative electrode material layer and the mass percentage of vinylene carbonate fluoride, the high-temperature storage performance and specific capacity of the secondary battery can be improved. In some embodiments, the a / α ratio can be 0.1, 0.15, 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.26 or any value within the range formed by any two of the above values.

[0082] 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 solvent, for example, includes 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(trifluoromethanesulfonyl)imide 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 vinylene carbonate fluoride and adiponitrile.

[0083] Positive electrode sheet

[0084] The positive electrode plate includes a positive current collector and a positive active layer provided on the positive current collector. Aluminum foil, nickel foil, etc. can be used as the positive current collector. The positive active layer contains a positive active material, and the positive active material includes a compound that can reversibly intercalate and deintercalate lithium ions (i.e., a lithiated intercalation compound). In some embodiments, the positive active material may 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 may 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.

[0085] The positive active layer further contains a binder for bonding the positive active material particles to facilitate the formation of a film layer and at the same time improve the bonding force between the positive active layer and the positive current collector. In some embodiments, the binder may include, but is not limited to, at least one of polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene 1,1-difluoroethylene, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, or nylon.

[0086] The positive active layer may further contain a conductive material, and the conductive material includes, but is not limited to, carbon-based materials, metal-based materials, conductive polymers, or any combination thereof. In some embodiments, the carbon-based materials may include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0087] The above secondary battery is applied to an electronic device to supply power to a load in the electronic device. Moreover, the above secondary battery including a negative electrode plate has excellent high-temperature storage performance and specific capacity, which is beneficial to improving the service life of the electronic device and the applicability in a high-temperature environment. Among them, the electronic device may include, but is not limited to, laptop computers, pen-input computers, mobile computers, e-book players, portable telephones, portable fax machines, portable copiers, portable printers, head-mounted stereo headphones, video recorders, liquid crystal televisions, hand-held 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.

[0088] The present application will be described below through specific examples and comparative examples. Those skilled in the art should 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.

[0089] Example 1-1

[0090] (1) Preparation of silicon-carbon composite material

[0091] S1) 100 g of carbon material (i.e., porous carbon material carrier, the pore volume of non-local density functional theory (NLDFT) is 0.9 cm 3 / g) is heated to 450 °C at a rate of 10 °C / min in an argon atmosphere, and then the argon atmosphere is switched to a silane mixed gas (by mass percentage, 20% silane and 80% argon), and deposited at 450 °C for 10 h to form a matrix.

[0092] S2) Pure argon is introduced into the matrix, the silane mixed gas is stopped from being introduced and evacuated, and then the matrix is heated to 550 °C in a pure argon atmosphere, and then a mixed gas of acetylene and silane (by volume percentage, 50% acetylene and 50% silane in the mixed gas) is introduced for surface coating for 2 hours to obtain a silicon-carbon composite material. The average particle size of the silicon-carbon composite material obtained in this example is 8.2 μm, and the average roundness of the silicon-carbon composite material is 0.7. The average size of the silicon grains in the silicon-carbon composite material is 3.3 nm.

[0093] (2) Preparation of negative electrode sheet

[0094] A mixture of artificial graphite and the above-mentioned silicon-carbon composite material with a weight ratio of 90:10 is used as the negative electrode active material. The negative electrode active material, polymethyl acrylate and sodium carboxymethyl cellulose (CMC) are fully stirred and mixed in an appropriate amount of deionized water according to a weight ratio of 97:2:1 to form a uniform negative electrode slurry, and the solid content of the negative electrode slurry is 40 wt%. This slurry is coated on a negative electrode current collector copper foil, dried at 85 °C, and then dried under vacuum at 120 °C for 12 hours after cold pressing, slicing, and slitting to obtain a single-sided coated negative electrode sheet.

[0095] Preparation of lithium-ion coin half-cell

[0096] A mixed solution of ethylene carbonate / dimethyl carbonate (EC / DMC, volume ratio 1:1) dissolved in 1 mol / L LiPF6 and containing 5 vol.% fluoroethylene carbonate (FEC) is used as the electrolyte, and a 7-μm polypropylene / polyethylene composite separator is used. Lithium sheets and negative electrode sheets are used as counter electrodes respectively, and coin half-cells are assembled in a glove box.

[0097] Preparation of lithium-ion full cell

[0098] Repeat the above steps on the other surface of the copper foil in the above single-sided coated negative electrode tab to obtain a negative electrode tab with a double-sided coated negative electrode material layer. Then, through cold pressing, slitting, and cutting, dry it under vacuum conditions at 120 °C for 12 h to obtain a negative electrode tab with a specification of 78 mm × 875 mm for use. Among them, the compaction density of the negative electrode material layer after cold pressing is 1.7 g / cm 3 。

[0099] Preparation of the positive electrode tab: Mix the positive electrode active material lithium cobalt oxide (LiCoO₂), conductive carbon black Super P, and positive electrode binder polyvinylidene fluoride (PVDF) according to a mass ratio of 97:1.4:1.6, add N-methylpyrrolidone (NMP) as a solvent, and formulate it into a positive electrode slurry with a solid content of 72 wt%. After vacuum stirring evenly, obtain the positive electrode slurry. Coat the positive electrode slurry evenly on one surface of a positive electrode current collector aluminum foil with a thickness of 12 μm, and dry it at 85 °C to obtain a positive electrode tab with a single-sided coated positive electrode material layer. When coating, the coating weight of the positive electrode material layer is 19 mg / cm 2 。Then repeat the above steps on the other surface of the aluminum foil to obtain a positive electrode tab with a double-sided coated positive electrode material layer. Then, through cold pressing, slitting, and cutting, dry it under vacuum conditions at 85 °C for 4 h to obtain a positive electrode tab with a specification of 74 mm × 867 mm for use. Among them, the compaction density of the positive electrode material layer after cold pressing is 4.2 g / cm 3 。

[0100] Preparation of the electrolyte: In a glove box with an argon atmosphere where the water content is less than 10 ppm, mix ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) evenly to obtain a basic solvent. Then, add lithium salt lithium hexafluorophosphate (LiPF₆) to the above basic solvent and mix evenly to obtain the electrolyte. Among them, based on the mass of the electrolyte, the mass percentage content of the lithium salt is 12.5%, the mass percentage content of fluoroethylene carbonate is 5%, and the balance is the basic solvent. The mass percentage content ratio of EC, EMC, and DEC is 1:1:1.

[0101] Preparation of the separator: Use a porous polyethylene (PE) film with a thickness of 7 μm as the separator.

[0102] Preparation of the lithium-ion battery: Stack the positive electrode tab, separator, negative electrode tab, and separator in sequence, so that the separator is in the middle of the positive electrode tab and the negative electrode tab to play an isolation role, and wind it to obtain an electrode assembly. After welding the electrode tabs, place the electrode assembly in an aluminum-plastic film packaging bag, dry it at 80 °C, inject the electrolyte, and obtain a soft-pack lithium-ion battery through processes such as vacuum packaging, standing, formation, shaping, and capacity testing.

[0103] Examples 1-2 to Examples 1-5

[0104] Examples 1-2 to 1-5 are different from Example 1-1 in that the reaction temperature, reaction time of the substrate, or the volume ratio of the mixed gas in step S2) is adjusted. For specific preparation parameters, please refer to Table 1, and the rest is the same as in Example 1-1.

[0105] Examples 1-6 to 1-8

[0106] Examples 1-6 to 1-8 are different from Example 1-1 in that the reaction temperature of the substrate in step S2) is adjusted. For specific preparation parameters, please refer to Tables 2 and 3, and the rest is the same as in Example 1-1.

[0107] Examples 1-9 to 1-17

[0108] Examples 1-9 to 1-17 are different from Example 1-1 in that the reaction time of the substrate, the components of the mixed gas introduced into the substrate, or the pore volume of the carbon material, and the proportion of the silicon-carbon composite material in the negative electrode active material in step S2) are adjusted. For specific preparation parameters, please refer to Tables 2, 3, and 4, and the rest is the same as in Example 1-1.

[0109] Examples 1-18 to 1-19

[0110] Examples 1-18 to 1-19 are different from Example 1-1 in that the components of the mixed gas introduced into the substrate and the content of the binder in step S2) are adjusted. For specific preparation parameters, please refer to Tables 2 and 5, and the rest is the same as in Example 1-1.

[0111] Comparative Examples 1 to 2

[0112] Comparative Examples 1 to 2 are different from Example 1-1 in that the reaction temperature introduced into the substrate, or the components and volume ratio of the mixed gas in step S2) are adjusted. For specific preparation parameters, please refer to Table 1, and the rest is the same as in Example 1-1. In Comparative Example 2, oxygen and nitrogen are used to replace acetylene and silane.

[0113] Examples 2-1 to 2-15

[0114] Examples 2-1 to 2-15 are different from Example 1-1 in that the components of the electrolyte in the lithium-ion battery and the alkali solubility of the negative electrode material layer are adjusted. Except for adjusting the parameters of the relevant electrolyte components according to Table 6, the rest is the same as in Example 1-1.

[0115] Testing Method

[0116] (1) Testing and calculation of alkali solubility

[0117] Prepare a potassium hydroxide aqueous solution with a concentration of 1.78 mol / L using a 500 mL volumetric flask, 50 g of potassium hydroxide, and an appropriate amount of deionized water. Add 5 g of the negative electrode material layer powder and 200 mL of the pre-prepared potassium hydroxide solution into an aluminum-plastic bag, and seal the aluminum-plastic bag. Place the aluminum-plastic bag in a constant temperature water bath with a set temperature of 50 °C and keep it warm for 6 h. Record the volume change by the water displacement method during the reaction process.

[0118] Alkaline solubility calculation formula:

[0119] Among them, the total volume of the aluminum-plastic bag and water initially immersed in the water bath is V1, the total volume of the aluminum-plastic bag and water after 6 h in the water bath is V2, △V is the change in the system volume, and △V = V2 - V1. P is the ambient pressure (101.325 kPa). R is the molar gas constant, which is 8.314 J(mol K)-1. T is the water bath temperature, 323 K. m is the mass of the negative electrode material layer; f Si is the mass percentage of Si in the negative electrode material layer; Msi is the molar mass of Si.

[0120] (2) Powder gas production test

[0121] At 25 °C, mix 0.5 g of the negative electrode material layer powder and 4 g of deionized water and place them in a 10 mL sealed bottle. After standing at room temperature (25 °C) for 48 h, perform chemical gas chromatography analysis (Aglient 7890A, GC). The gas production components and ratios can be obtained, and thus the volume percentage of H2 can be obtained.

[0122] (3) Element mass percentage test

[0123] Determine the mass percentage of each element in the negative electrode material layer by means of alkali etching and high-temperature oxidation tests. Specifically, add 5 g of the negative electrode material layer and 50 g of sodium hydroxide into a crucible, heat to 400 °C and keep it warm for 1 h, then cool to below 100 °C, put the reactants into 80 °C deionized water, wash, filter, and dry them multiple times with 1 M hydrochloric acid solution and deionized water, and weigh the mass of the remaining carbon filter residue product as m1. The mass percentage of carbon element is m1 divided by the total mass of the negative electrode material layer, 5 g. Collect the filtrate, and then perform a constant volume ICP element test to obtain the mass percentage of Si.

[0124] (4) Test for the average particle size of the silicon-carbon composite material

[0125] After FIB cutting the negative electrode plate, use a scanning electron microscope to select the silicon-carbon composite material at 500 times magnification by area scan EDS Mapping. Randomly select the particle sizes of 12 silicon-carbon composite materials and calculate their average value to obtain the average particle size.

[0126] (5) Test of the average circularity of the silicon-carbon composite material

[0127] The cross-section of the particles is analyzed through particle image analysis software, so as to obtain the surface area equivalent diameter and volume equivalent diameter of the silicon-carbon composite material, and the average circularity of the particles can be calculated. The average circularity is defined as the ratio of the surface area equivalent diameter to the volume equivalent diameter of the particles.

[0128] (6) Test of the average diameter of silicon grains in the silicon-carbon composite material

[0129] Using the D8 Advance equipment, with a Cu target as the target material, test in the angular range from 2θ = 10° to 80° at a voltage of 60 kV. After obtaining the XRD pattern of the silicon-carbon composite material by using Highscore for diffraction pattern analysis, apply the Debye-Scherrer formula at 2θ = 28.4°: D = Kλ / βcosθ, and the average diameter of silicon grains in the silicon-carbon composite material can be calculated. Among them, K is the Scherrer constant, D is the average diameter of silicon grains, B is the full width at half maximum of the diffraction peak of the measured sample, θ is the Bragg diffraction angle, and γ is the X-ray wavelength.

[0130] (7) Test of the thickness of the negative electrode plate

[0131] At 50% SOC, disassemble the lithium-ion battery in a drying room and take out the negative electrode plate. Measure the thickness of the negative electrode plate (including the negative electrode current collector) with a micrometer, and take the average value of 12 measurement points as the thickness of the negative electrode plate.

[0132] Battery performance test

[0133] (1) Specific capacity

[0134] First, discharge the coin-type half-cell at a small current of 0.05C / 50μA / 10μA in three stages to 0.005V, and record the first discharge capacity of the coin-type half-cell; then charge it at a constant current of 0.1C to 0.8V, and record the first charge capacity of the coin-type half-cell. The first reversible specific capacity of the negative electrode active material from 0.005V to 0.8V = the first charge capacity of the coin-type half-cell / the mass of the negative electrode active material.

[0135] (2) High-temperature storage performance

[0136] At 25°C, discharge the lithium-ion full-cell at a current density of 0.2C to 3.0V. Then charge it at a constant current of 0.2C to 4.53V, and then charge it at a constant voltage until the current density is 0.05C. At this time, the SOC of the lithium-ion full-cell is 100%, and the initial thickness is T0. Then place the lithium-ion full-cell in a test chamber, raise the temperature to 80°C, keep it for 6 hours, and measure the thickness of the lithium-ion full-cell as T1 after cooling to 25°C.

[0137] The thickness expansion rate after high-temperature storage is: (T1 - T0) / T0.

[0138] Among them, the preparation conditions and test results of each example and comparative example are recorded in Tables 1 to 6.

[0139] Table 1

[0140]

[0141] In Table 1 above, compared with Comparative Examples 1 to 2, the alkali solubility of the negative electrode material layer in Examples 1-1 to 1-5 satisfies a specific range, which improves the high-temperature storage performance and specific capacity of the secondary battery. A smaller alkali solubility of the negative electrode material layer in Examples 1-1 to 1-5 indicates a high reaction energy barrier between the negative electrode material layer and water and strong interface stability, so that the gas generation during the preparation of the negative electrode paste of the negative electrode sheet can be further reduced, and during the cycling process of the secondary battery, the erosion of the silicon-carbon composite material by the electrolyte can be further reduced, the side reactions between the silicon-carbon composite material and the electrolyte can be reduced, and the high-temperature storage performance and specific capacity of the secondary battery can be improved.

[0142] Table 2

[0143]

[0144] Table 3

[0145]

[0146]

[0147] In Tables 2 and 3, in Examples 1-6 to 1-8, the volume fraction of H2 is within a specific range, which improves the high-temperature storage performance and specific capacity of the secondary battery. A volume fraction of H2 within a suitable range is conducive to making the negative electrode material layer have better interface stability, reducing gas generation during the preparation of the negative electrode paste, and reducing side reactions between the silicon-carbon composite material and the electrolyte in the negative electrode material layer of the secondary battery, thereby facilitating further improvement of the high-temperature storage performance and specific capacity of the secondary battery.

[0148] Combining Tables 2 and 3, in Examples 1-9 to 1-10, the average particle size of the silicon-carbon composite material in the negative electrode material layer is adjusted. When the average particle size of the silicon-carbon composite material is within a specific range, it improves the high-temperature storage performance and specific capacity of the secondary battery.

[0149] Table 4

[0150]

[0151] Combined with Table 2 and Table 4, in Examples 1-11 to 1-13, when the silicon element and carbon element in the negative electrode material layer are within an appropriate range, it improves the high-temperature storage performance of the secondary battery. In Examples 1-14 to 1-15, when the average diameter of the silicon grains is between 1.5 nm and 4.5 nm, the corresponding secondary battery has good high-temperature storage performance.

[0152] Table 5

[0153]

[0154] Combined with Table 2 and Table 5, in Examples 1-16 to 1-17, by adjusting the average circularity of the carbon material to change the average circularity of the silicon-carbon composite material, when the average circularity of the silicon-carbon composite material is within an appropriate range, it improves the high-temperature storage performance and specific capacity of the secondary battery. The average circularity within the appropriate range enables the silicon-carbon composite material to have a suitable specific surface area, reduces the contact between the silicon in the silicon-carbon composite material and the electrolyte, reduces the occurrence of side reactions, and is conducive to improving the high-temperature storage performance and specific capacity of the secondary battery.

[0155] In Examples 1-18 to 1-19, changing the mass ratio of the binder in the negative electrode material layer within an appropriate range, the corresponding secondary battery has good high-temperature storage performance and specific capacity.

[0156] Table 6

[0157]

[0158] In Table 6 above, based on Example 1-1, Examples 2-1 to 2-3 adjusted the mass ratio of fluoroethylene carbonate, which improved the high-temperature storage performance and specific capacity of the secondary battery. And when a / α satisfies the appropriate range, the high-temperature storage performance and specific capacity of the corresponding secondary battery.

[0159] In Examples 2-4 to 2-6 and Examples 2-10 to 2-12, when in the secondary battery, the mass ratio of difluoroethylene carbonate and the mass ratio of vinylene sulfate in the electrolyte satisfy the appropriate range, the high-temperature storage performance and specific capacity of the corresponding secondary battery.

[0160] In Examples 2-7 to 2-9 and Examples 2-13 to 2-15, when the electrolyte contains two or three of fluoroethylene carbonate, difluoroethylene carbonate, vinylene sulfate, and 1,3-propane sultone, it further improves the high-temperature storage performance and specific capacity of the secondary battery.

[0161] The above disclosure is only the preferred embodiment of the present application. Of course, the present application cannot be limited thereby. Therefore, equivalent changes made according to the present application still fall within the scope covered by the present application.

Claims

1. A negative electrode sheet, wherein: The negative electrode plate 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 a silicon-carbon composite material, wherein the silicon-carbon composite material comprises a substrate and a carbon layer located on the surface of the substrate, wherein the substrate comprises a carbon material and a silicon material, wherein the carbon material has gaps, and the silicon material is distributed in the gaps; The negative electrode material layer and the potassium hydroxide aqueous solution are mixed to obtain a mixed system, and the mixed system is kept warm for 6 hours at T1 and then subjected to an alkali solubility test, wherein 47°C≤T1≤53°C, the alkali solubility of the negative electrode material layer is measured to be α, α<10%, the concentration of the potassium hydroxide aqueous solution is 1.78 mol / L, and the mass ratio of potassium hydroxide to the negative electrode material layer in the mixed system is 4:

1.

2. The negative electrode sheet according to claim 1, wherein: 0.2％＜α＜10％。 3. The negative electrode sheet according to claim 1, wherein: At T2, the negative electrode material layer and water are mixed in a mass ratio of 1:8 to obtain a dispersion system, wherein 23°C≤T2≤27°C, and the dispersion system is sealed and allowed to stand for 48 hours. The gas production of the dispersion system is tested, and based on the total volume of the gas produced, the volume proportion of H2 is less than 2%.

4. The negative electrode sheet according to claim 1, wherein: The silicon-carbon composite material satisfies at least one of the following: (1) The average particle size of the silicon-carbon composite material is 5.5 μm to 9.5 μm; (2) The average circularity of the silicon-carbon composite material is 0.7 to 0.85; (3) The silicon material comprises silicon grains, and the average diameter of the silicon grains is 1.5 nm to 4.5 nm.

5. The negative electrode sheet according to any one of claims 1 to 4, wherein: The negative electrode material layer includes silicon and carbon, and the negative electrode material layer satisfies at least one of the following conditions: (1) The negative electrode material layer further includes a binder, and the binder includes at least one of lithium polyacrylate, sodium carboxymethyl cellulose, styrene-butadiene rubber, polymethyl acrylate or polyacrylic acid, and the mass proportion of the binder is 0.4% to 2.5% based on the mass of the negative electrode material layer; (2) The carbon layer contains Si—C bonds.

6. A method for preparing the silicon-carbon composite material in the negative electrode sheet according to any one of claims 1 to 5, wherein: include: S1) introducing a silane mixed gas into the carbon material to deposit a silicon material in the carbon material to obtain a matrix; S2) In an inert gas atmosphere at a temperature of 550° C. to 700° C., introducing a mixed gas into the matrix and reacting for 1 hour to 6 hours to obtain the silicon-carbon composite material.

7. The preparation method according to claim 6, wherein The silane mixed gas includes silane gas and inert gas; the mixed gas includes a group selected from acetylene and propylene, acetylene and ethylene, acetylene and methane, acetylene and silane, silane and propylene, and silane and ethylene.

8. A secondary battery comprising a positive electrode plate, an electrolyte and a separator, wherein: Also included is a negative electrode sheet according to any one of claims 1 to 5, and a negative electrode sheet comprising a silicon-carbon composite material prepared by the preparation method according to claim 6 or 7, wherein the electrolyte satisfies at least one of the following conditions: (1) The electrolyte comprises fluoroethylene carbonate, and the mass proportion of the fluoroethylene carbonate is a, 1.5%≤a≤8.4%, based on the mass of the electrolyte; (2) The electrolyte comprises bisfluoroethylene carbonate, and the mass proportion of the bisfluoroethylene carbonate is b based on the mass of the electrolyte, 1.5%≤b≤2.5%; (3) The electrolyte includes 1,3-propane sultone, and the mass proportion of the 1,3-propane sultone is c, 0.8%≤c≤3.8% based on the mass of the electrolyte; (4) The electrolyte includes vinyl sulfate, and the mass proportion of the vinyl sulfate based on the mass of the electrolyte is d, 0.3%≤d≤0.8%.

9. The secondary battery according to claim 8, wherein: 0.1≤a / α≤1.26。 10. An electronic device, wherein: Comprising the secondary battery as claimed in claim 8 or 9.

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