Silicon carbon material and preparation method thereof, secondary battery and electronic device

Abstract

The invention provides a silicon-carbon material and a preparation method thereof, a secondary battery and an electronic device, the silicon-carbon material comprises a carbon skeleton and silicon particles, the carbon skeleton comprises amorphous carbon and carbon nanotubes, the cross section of the silicon-carbon material comprises a first test region and a second test region with the area of 1000 nm < 2 >, the first test region and the second test region are not overlapped, and the silicon particles are arranged in the first test region and the second test region. Based on the area of the first test region, the area proportion of the carbon nanotubes is S1%, based on the area of the second test region, the area proportion of the carbon nanotubes is S2%, and S1-S2 is less than or equal to 0.1. The silicon-carbon material provided by the invention can improve the cycle performance and expansion performance of the secondary battery.

Description

Technical Field

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

[0002] Lithium-ion batteries have the advantages of high energy density, high operating voltage, good safety, and green environmental protection. They have been widely used in consumer electronics such as mobile phones and laptops. Silicon-carbon materials are regarded as the ideal choice for the negative electrode materials of the next generation of lithium-ion batteries due to their high theoretical specific capacity (about 4200mAh / g). However, silicon-carbon materials face significant volume expansion problems during the charging and discharging process. This phenomenon mainly stems from the changes in the lattice structure caused by the insertion and extraction of lithium ions into silicon crystals. The repeated volume changes of the silicon lattice will cause the structural destruction of the silicon-carbon material, which will in turn cause the pulverization and shedding of the electrode material, and ultimately lead to the rapid attenuation of the capacity of the lithium-ion battery and the shortening of the cycle life. Summary of the invention

[0003] The present application provides a silicon-carbon material and a preparation method thereof, a secondary battery and an electronic device.

[0004] The present application provides a silicon-carbon material in a first aspect, comprising a carbon skeleton and silicon particles, wherein the carbon skeleton comprises amorphous carbon and carbon nanotubes, and a cross section of the silicon-carbon material comprises a silicon particle having an area of ​​1000 nm. 2 The first test area and the second test area do not overlap, the area of ​​the carbon nanotubes accounts for S1% based on the area of ​​the first test area, and the area of ​​the carbon nanotubes accounts for S2% based on the area of ​​the second test area, |S1-S2|≤0.1.

[0005] In the present application, adding carbon nanotubes to the carbon skeleton can enhance the mechanical structural stability of the carbon skeleton, inhibit the expansion of silicon during the charging and discharging process of the silicon-carbon material, and thus improve the expansion performance of the silicon-carbon material. The inventors found that when the value of |S1-S2| is within the above range, the carbon nanotubes are evenly distributed in the silicon-carbon material, reducing the possibility of regional agglomeration of carbon nanotubes in the carbon skeleton and sparseness in some areas, reducing the formation of macropores that are not conducive to lithium ion transmission, thereby reducing the generation of more useless SEI films, and improving the compressive strength of the silicon-carbon material, thereby improving the energy density and cycle performance of the silicon-carbon material. And when the value of |S1-S2| is within the above range, the expansion performance of the silicon-carbon material can be further improved.

[0006] Based on the first aspect, in some embodiments, 0.006≤|S1-S2|≤0.043, which is beneficial to further improve the cycle performance and expansion performance of the silicon-carbon material.

[0007] Based on the first aspect, in some embodiments, the diameter ratio of the carbon nanotubes is L, 1.2≤L≤2. It can effectively improve the electrical conductivity and mechanical properties of the silicon-carbon material, and at the same time, it is also beneficial to maintain the structural stability and cycle performance of the silicon-carbon material. And the diameter ratio of the carbon nanotubes is within the above range, which is beneficial to make the silicon-carbon material have a suitable specific surface area, improve the dispersion and uniformity of the carbon nanotubes, improve the mechanical properties and stability of the silicon-carbon material, and improve the overall electrical conductivity and electrochemical properties of the silicon-carbon material.

[0008] Based on the first aspect, in some embodiments, the average circularity of the silicon-carbon material is 0.81 to 0.97. When the silicon-carbon material is applied to the negative electrode sheet, the silicon-carbon material has good compressive strength, which can reduce the cracks or even breakage of the silicon-carbon material particles during the cold pressing process of preparing the negative electrode sheet, thereby facilitating the improvement of the cold pressing first efficiency and compaction density of the silicon-carbon material, and improving the volume energy density and cycle performance of the secondary battery.

[0009] Based on the first aspect, in some embodiments, the powder conductivity of the silicon-carbon material is 0.1S / cm to 10S / cm. The silicon-carbon material has good electrical conductivity, improves the charge and discharge efficiency of the secondary battery, reduces capacity decay, and improves the cycle life and rate performance of the secondary battery; and is also conducive to maintaining a suitable current during the charge and discharge process of the secondary battery, improving the thermal stability of the silicon-carbon material, and reducing the risk of thermal runaway or instability of the secondary battery.

[0010] Based on the first aspect, in some embodiments, the elastic modulus of the silicon-carbon material is 50 GPa to 200 GPa. This is beneficial for the silicon-carbon material to maintain good structural stability during the charge and discharge process, and to reduce the possibility of reducing the cycle stability of the silicon-carbon material due to cracks caused by volume changes in the silicon-carbon material particles during the charge and discharge process. The silicon-carbon material has a certain pressure resistance, reduces the damage of the silicon-carbon material during the cold pressing process, improves the mechanical strength of the silicon-carbon material, and improves the cycle performance and rate performance of the secondary battery.

[0011] The second aspect of the present application provides a method for preparing a silicon-carbon material, comprising the following steps: S1. adding carbon nanotubes, a carbon precursor, a curing agent, and a surfactant to an organic solvent, stirring to obtain a mixed liquid, wherein the mass ratio of the carbon nanotubes to the surfactant is (1:1) to (1:400); S2. granulating the mixed liquid by spray drying to obtain mixed particles; S3. carbonizing and activating the mixed particles in sequence to obtain porous carbon; S4. using chemical vapor deposition to deposit silicon and carbon-coat the porous carbon to obtain a silicon-carbon material. In the preparation method of silicon-carbon material provided in the present application, carbon nanotubes, carbon precursors, curing agents, and surfactants are added to an organic solvent to mix the carbon nanotubes, carbon precursors, curing agents, and surfactants evenly, thereby improving the uniformity of distribution of carbon nanotubes and carbon precursors, and facilitating reduction of agglomeration of some carbon nanotubes in the subsequent preparation process. In the above preparation method, the mass ratio of carbon nanotubes and surfactants is controlled to regulate the uniformity of distribution of carbon nanotubes in the silicon-carbon material, thereby improving the cyclic performance and expansion performance of the silicon-carbon material.

[0012] Based on the second aspect, in some embodiments, before the mixed liquid is spray-dried, the mixed liquid is also subjected to a sand mill stirring treatment, the sand mill speed is 200r / min to 2000r / min, and the sand mill time is 0.1h to 6h, which is beneficial to further improve the distribution uniformity of carbon nanotubes in the silicon-carbon material particles, thereby improving the cycle performance and expansion performance of the silicon-carbon material.

[0013] The third aspect of the present application provides a secondary battery, comprising a negative electrode plate, a positive electrode plate and an electrolyte, wherein the negative electrode plate comprises the silicon-carbon material or the silicon-carbon material obtained by the preparation method. The secondary battery comprises the silicon-carbon material, which improves the cycle performance and expansion performance of the secondary battery.

[0014] Based on the third aspect, in some embodiments, the electrolyte includes ethyl propionate and propyl propionate, and based on the mass of the electrolyte, the mass proportion of ethyl propionate is A1, the mass proportion of propyl propionate is A2, and 2≤A2 / A1≤6. This is beneficial to further improve the cycle performance, rate performance and expansion performance of the secondary battery.

[0015] Based on the third aspect, in some embodiments, the electrolyte includes a lithium salt additive, the lithium salt additive is selected from at least one of lithium bis(fluorosulfonyl)imide or lithium bis(trifluoromethanesulfonyl)imide, and the mass proportion of the lithium salt additive is 0.01% to 1.5% based on the mass of the electrolyte, which is beneficial to further improve the cycle performance and expansion performance of the secondary battery.

[0016] The fourth aspect of the present application provides an electronic device, comprising the secondary battery. The secondary battery has excellent cycle performance and expansion performance, which is beneficial to improving the service life of the electronic device. DETAILED DESCRIPTION

[0017] The technical scheme in the embodiment of the present application is described clearly and in detail below. Obviously, the described embodiment is a part of the embodiment of the present application, rather than all the embodiments. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art of the present application. The terms used in the specification of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application.

[0018] In order to solve the poor rate performance and poor structural stability of silicon-carbon materials, researchers have proposed a variety of modification strategies, including nano-sizing, carbon coating, introduction of conductive additives, and design of porous structures. However, these methods still have certain limitations in practical applications. For example, nano-sizing may lead to an increase in the surface area of ​​the material, an increase in side reactions, and affect the cycle performance of the material. Therefore, the development of a silicon-carbon material that can both improve the cycle performance and enhance the structural stability remains the focus and difficulty of current research. Future research directions may include the development of new nanostructures, optimization of the conductivity and stability of the carbon matrix, and exploration of new synthesis methods to achieve efficient preparation and performance improvement of silicon-carbon materials.

[0019] An embodiment of the present application provides a secondary battery, which includes a housing, an electrode assembly and an electrolyte. The electrode assembly and the electrolyte are both located in the housing.

[0020] The outer shell can be a packaging bag encapsulated by an encapsulation film (such as an aluminum-plastic film), such as a soft-pack battery. In other embodiments, the secondary battery can also be a steel shell battery, an aluminum shell battery, etc.

[0021] The electrode assembly includes a positive electrode sheet, a negative electrode sheet and a separator, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet. The electrode assembly may be a laminated structure, which is formed by stacking the positive electrode sheet, the separator and the negative electrode sheet. In other embodiments, the electrode assembly may also be a wound structure, which is formed by stacking the positive electrode sheet, the separator and the negative electrode sheet and then winding them.

[0022] Negative electrode

[0023] The negative electrode plate includes a negative electrode current collector and a negative electrode active layer located on the surface of the negative electrode current collector. The negative electrode current collector includes: copper foil, aluminum foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate covered with a conductive metal or any combination thereof, and may 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 foil and the polymer substrate. In the negative electrode plate, the negative electrode active layer includes a silicon-carbon material.

[0024] The silicon-carbon material includes a carbon skeleton and silicon particles. The carbon skeleton includes amorphous carbon and carbon nanotubes. The cross section of the silicon-carbon material includes an area of ​​1000nm. 2 The first test area and area is 1000nm 2 The first test area and the second test area do not overlap, the area of ​​the carbon nanotubes accounts for S1% based on the area of ​​the first test area, and the area of ​​the carbon nanotubes accounts for S2% based on the area of ​​the second test area, |S1-S2|≤0.1.

[0025] In the silicon-carbon material of the present application, the value of |S1-S2| can reflect the uniformity of distribution of carbon nanotubes in the silicon-carbon material. When the value of |S1-S2| is closer to 0, it indicates that the carbon nanotubes are more uniformly distributed in the particles of the silicon-carbon material. The greater the difference between the value of |S1-S2| and 0, the worse the uniformity of distribution of carbon nanotubes in the particles of the silicon-carbon material. Adding carbon nanotubes to the carbon skeleton can enhance the mechanical structural stability of the carbon skeleton, inhibit the expansion of silicon during the charging and discharging process of the silicon-carbon material, thereby improving the expansion performance of the silicon-carbon material. The inventors found that when the value of |S1-S2| is within the above range, the carbon nanotubes are evenly distributed in the silicon-carbon material, reducing the possibility of regional agglomeration of carbon nanotubes in the carbon skeleton and the phenomenon of sparseness in some areas, reducing the formation of macropores, thereby reducing the generation of more useless SEI films, and improving the compressive strength of the silicon-carbon material, thereby improving the energy density and cycle performance of the silicon-carbon material. And when the value of |S1-S2| is within the above range, the expansion performance of the silicon-carbon material can be further improved.

[0026] If the value of |S1-S2| is large, such as greater than 0.1, the distribution of carbon nanotubes in the silicon-carbon material is relatively uneven, and the carbon nanotubes agglomerate, resulting in the formation of large pores in the silicon-carbon material, which reduces the compressive strength of the silicon-carbon material and increases the generation of more useless SEI films, thereby reducing the cycle performance of the secondary battery.

[0027] In some embodiments, the value of |S1-S2| can be 0.001, 0.002, 0.003, 0.006, 0.007, 0.008, 0.009, 0.01, 0.011, 0.012, 0.015, 0.017, 0.02, 0.025, 0.028, 0.031, 0.04, 0.043, 0.05, 0.06, 0.077, 0.08, 0.09, 0.1, or any value within the range formed by any two of the above values.

[0028] In some embodiments, 0.006≤|S1-S2|≤0.043, which is beneficial to further improve the cycle performance and expansion performance of the silicon-carbon material.

[0029] In some embodiments, the diameter ratio of the carbon nanotubes is L, 1.2≤L≤2. The diameter ratio of the carbon nanotubes may refer to the ratio of the outer diameter to the inner diameter of the carbon nanotubes. The diameter ratio of the carbon nanotubes within the above range can effectively improve the electrical conductivity and mechanical properties of the silicon-carbon material, while also helping to maintain the structural stability and cycle performance of the silicon-carbon material. And the diameter ratio of the carbon nanotubes within the above range is conducive to making the silicon-carbon material have a suitable specific surface area, improving the dispersibility and uniformity of the carbon nanotubes, improving the mechanical properties and stability of the silicon-carbon material, and improving the overall electrical conductivity and electrochemical properties of the silicon-carbon material. In some embodiments, the diameter ratio L of the carbon nanotubes may be 1.2, 1.3, 1.5, 1.6, 1.8, 2 or any value within the range formed by any two of the above values.

[0030] In some embodiments, the average circularity of the silicon-carbon material is 0.81 to 0.97. When the circularity of the silicon-carbon material is within the above-mentioned suitable range, the silicon-carbon material has good compressive strength, which can reduce the cracks or even breakage of the silicon-carbon material particles during the cold pressing process of preparing the negative electrode sheet, thereby facilitating the improvement of the cold pressing initial efficiency and compaction density of the silicon-carbon material, and improving the volume energy density and cycle performance of the secondary battery. In some embodiments, the average circularity of the silicon-carbon material can be 0.81, 0.83, 0.85, 0.87, 0.9, 0.93, 0.95, 0.97 or any value within the range formed by any two of the above values.

[0031] In some embodiments, the powder conductivity of the silicon-carbon material is 0.1S / cm to 10S / cm. The powder conductivity of the silicon-carbon material is within the above-mentioned suitable range, so that the silicon-carbon material has good electrical conductivity, improves the charge and discharge efficiency of the secondary battery, reduces capacity decay, and improves the cycle life and rate performance of the secondary battery; and it is also beneficial to maintain a suitable current during the charge and discharge process of the secondary battery, improve the thermal stability of the silicon-carbon material, and reduce the risk of thermal runaway or instability of the secondary battery. In some embodiments, the powder conductivity of the silicon-carbon material is 0.1S / cm, 0.5S / cm, 1S / cm, 2S / cm, 3S / cm, 4S / cm, 5S / cm, 6S / cm, 7S / cm, 8S / cm, 9S / cm, 10S / cm or any value within the range composed of any two of the above values.

[0032] In some embodiments, the elastic modulus of the silicon-carbon material is 50 GPa to 200 GPa. The elastic modulus of the silicon-carbon material is maintained within the above-mentioned suitable range, which is conducive to the silicon-carbon material to maintain good structural stability during the charge and discharge process, and reduces the possibility of reducing the cycle stability of the silicon-carbon material due to cracks caused by volume changes in the silicon-carbon material particles during the charge and discharge process. The silicon-carbon material has a certain pressure resistance, reduces the damage of the silicon-carbon material during the cold pressing process, improves the mechanical strength of the silicon-carbon material, and improves the cycle performance and rate performance of the secondary battery. In some embodiments, the elastic modulus of the silicon-carbon material can be 50 GPa, 60 GPa, 70 GPa, 80 GPa, 90 GPa, 95 GPa, 100 GPa, 110 GPa, 120 GPa, 130 GPa, 140 GPa, 150 GPa, 160 GPa, 170 GPa, 180 GPa, 190 GPa, 200 GPa or any value within the range composed of any two of the above values.

[0033] In some embodiments, the electrolyte includes ethyl propionate and propyl propionate. Based on the mass of the electrolyte, the mass proportion of ethyl propionate is A1, the mass proportion of propyl propionate is A2, and 2≤A2 / A1≤6. The inclusion of ethyl propionate and propyl propionate in the electrolyte is beneficial to improving the cycle performance, rate performance and expansion performance of the secondary battery; when A2 / A1 satisfies the above relationship, it is beneficial to further improve the cycle performance, rate performance and expansion performance of the secondary battery. In some embodiments, the ratio of A2 / A1 can be 2, 3, 4, 5, 6 or any value within the range of any two of the above values. Preferably, 3≤A2 / A1≤5.

[0034] In some embodiments, the electrolyte includes a lithium salt additive, the lithium salt additive includes at least one of lithium bis(fluorosulfonyl)imide (LiFSI) or lithium bis(trifluoromethanesulfonyl)imide (LiTFSI), and the mass proportion of the lithium salt additive is 0.01% to 1.5% based on the mass of the electrolyte. It is beneficial to improve the cycle performance, rate performance and expansion performance of the secondary battery; when the lithium salt additive meets the above range, it is beneficial to further improve the cycle performance and expansion performance of the secondary battery. In some embodiments, the mass proportion of the lithium salt additive can be 0.01%, 0.03%, 0.05%, 0.1%, 0.2%, 0.3%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5% or any value within the range of any two of the above values. Preferably, based on the mass of the electrolyte, the mass proportion of the lithium salt additive is 0.01% to 0.8%.

[0035] In some embodiments, the negative electrode active layer further comprises a binder and a conductive agent. In some embodiments, the binder comprises, but is not limited to: polyvinyl alcohol, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic (ester) styrene-butadiene rubber, epoxy resin or nylon, etc.

[0036] 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 material is selected from carbon black, acetylene black, Ketjen black, carbon fiber, or any combination thereof. In some embodiments, the metal-based material is selected from metal powder, metal fiber, copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer is a polyphenylene derivative.

[0037] The present application provides a method for preparing a silicon-carbon material, comprising the following steps:

[0038] S1. Add carbon nanotubes, carbon precursor, curing agent and surfactant into an organic solvent and stir to obtain a mixed solution.

[0039] In the step, the carbon precursor may be a linear resin, the curing agent may be urotropine, the organic solvent may be ethanol, or an alcohol such as methanol. The surfactant includes at least one of polyvinyl pyrrolidone (PVP), sodium carboxymethyl cellulose (CMC) or cetyl trimethyl ammonium bromide (CTAB).

[0040] The mass ratio of carbon nanotubes, carbon precursor and curing agent can be in the range of 1: (350-900): 100. The mass ratio of carbon nanotubes and surfactant is (1: 1) to (1: 400). In the mixed solution, the mass ratio of carbon nanotubes and surfactant affects the uniformity of dispersion of carbon nanotubes in silicon-carbon materials. In some embodiments, the mass ratio of carbon nanotubes and surfactant is (1: 10) to (1: 400). Preferably, the mass ratio of carbon nanotubes and surfactant is 1: 100.

[0041] The mixed liquid is also subjected to sand milling and stirring treatment, the sand milling speed is 200r / min to 2000r / min, and the sand milling time is 0.1h to 6h. During the above sand milling speed and sand milling time, the carbon nanotubes, carbon precursors, curing agents and surfactants in the mixed liquid are further fully mixed and uniform. In some embodiments, the sand milling speed can be 200r / min, 300r / min, 500r / min, 700r / min, 1000r / min, 1200r / min, 1500r / min, 1700r / min, 2000r / min or any value within the range formed by any two of the above values; the sand milling time can be 0.1h, 0.5h, 1h, 2h, 3h, 5h, 6h or any value within the range formed by any two of the above values.

[0042] S2. Granulate the mixed solution by spray drying to obtain mixed particles.

[0043] During the spray drying process, the air inlet temperature is 200°C to 260°C, the air outlet temperature is 80°C to 100°C, the atomization pressure is 0.3MPa, and the feed rate is 0.03L / min to 0.1L / min. During the spray drying process, the feed rate can adjust the average particle size of the mixed particles. When the feed rate is large, the average particle size of the mixed particles can be increased. In this step, mixed particles with an average particle size of 5μm to 10μm can be obtained. In some embodiments, the air inlet temperature can be 200°C, 210°C, 220°C, 230°C, 240°C, 250°C, 260°C, or any value within the range of any two of the above values; the air outlet temperature can be 80°C, 90°C, 100°C, or any value within the range of any two of the above values; the feed rate can be 0.03L / min, 0.05L / min, 0.08L / min, 0.1L / min, or any value within the range of any two of the above values.

[0044] S3. Carbonizing and activating the mixed particles in sequence to obtain porous carbon.

[0045] In this step, the mixed particles go through a carbonization stage and an activation stage in sequence. Carbonization stage: In an inert gas (such as nitrogen or argon), the carbonization temperature is 600°C to 1500°C, and the carbonization time is 2h to 6h. Activation stage: In a mixed gas atmosphere of water and carbon dioxide, the activation temperature is 700°C to 1000°C, and the activation time is 2h to 20h. In this step, the mixed particles are carbonized and activated as described above, thereby facilitating the acquisition of porous carbon with a more stable structure. In some embodiments, the carbonization temperature can be 600°C, 800°C, 900°C, 1000°C, 1200°C, 1400°C, 1500°C, or any value within the range formed by any two of the above values; the carbonization time can be 2h, 3h, 4h, 5h, 6h, or any value within the range formed by any two of the above values; the activation temperature can be 700°C, 800°C, 900°C, 1000°C, or any value within the range formed by any two of the above values; the activation time can be 2h, 5h, 10h, 15h, 20h, or any value within the range formed by any two of the above values.

[0046] S4. Using chemical vapor deposition, silicon is deposited on the porous carbon and carbon is coated to obtain the silicon-carbon material.

[0047] Chemical vapor deposition is used to deposit silicon-containing gas on porous carbon containing carbon nanotubes in a silane atmosphere, the deposition temperature is 400°C to 600°C, and the deposition time is 2h to 20h. Then, the porous carbon deposited with silicon particles is carbon-coated, and the carbon coating temperature is 500°C to 900°C in a carbon-containing gas such as acetylene atmosphere, and the carbon coating time is 0.1h to 6h to further stabilize the structure of the porous carbon and obtain a silicon-carbon material. In some embodiments, the deposition temperature can be 400°C, 500°C, 600°C, or any value within the range formed by any two of the above values; the deposition time can be 2h, 5h, 10h, 15h, 20h, or any value within the range formed by any two of the above values. The temperature of carbon coating can be 500°C, 600°C, 700°C, 800°C, 900°C or any value within the range formed by any two of the above values; the time of carbon coating can be 0.1h, 0.5h, 1h, 2h, 3h, 4h, 5h, 6h or any value within the range formed by any two of the above values.

[0048] In the preparation method of silicon-carbon material provided in the present application, carbon nanotubes, carbon precursors, curing agents and surfactants are added to an organic solvent to mix the carbon nanotubes, carbon precursors, curing agents and surfactants evenly through solid-liquid mixing, thereby improving the uniformity of distribution of carbon nanotubes and carbon precursors, and helping to reduce the agglomeration of some carbon nanotubes in the subsequent preparation process; in the above-mentioned preparation method, the mass ratio of carbon nanotubes and surfactants is controlled to regulate the uniformity of distribution of carbon nanotubes in the silicon-carbon material, so that |S1-S2| in the first test area and the second test area of ​​the carbon nanotubes meets the specific range, thereby further improving the cycle performance and expansion performance of the silicon-carbon material.

[0049] Isolation film

[0050] The material and shape of the separator used in the secondary battery of the present application are not particularly limited, and can be any technology disclosed in the prior art. In some embodiments, the separator includes a polymer or inorganic substance formed of a material that is stable to the electrolyte of the present application.

[0051] For example, the isolation film may include a substrate layer and a surface treatment layer. The substrate layer is a non-woven fabric, a film or a composite film having a porous structure, and the material of the substrate 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 may be selected.

[0052] A surface treatment layer is provided on at least one surface of the substrate layer, and the surface treatment layer can be a polymer layer or an inorganic layer, or a layer formed by a mixed 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 aluminum oxide, silicon oxide, magnesium oxide, 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, a copolymer of vinylidene fluoride-hexafluoropropylene, polyamide, polyacrylonitrile, polyacrylate, polyacrylic acid, polyacrylic acid salt, polyvinylpyrrolidone, polyethylene alkoxy, 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, polyacrylic acid salt, polyvinylpyrrolidone, polyethylene alkoxy, polyvinylidene fluoride and poly (vinylidene fluoride-hexafluoropropylene).

[0053] Electrolyte

[0054] 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 may be any organic solvent known in the prior art that can be used as a solvent for the electrolyte. There is no restriction on the electrolyte used in the electrolyte of the present application, which may be any electrolyte known in the prior art. The additive of the electrolyte according to the present application may be any additive known in the prior art that can be used as an electrolyte additive. 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, including at least one of 1,3-dioxolane (DOL) and ethylene glycol dimethyl ether (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(oxalatoborate) LiB(C2O4)2 (LiBOB), or lithium di(oxalatoborate) LiBF2(C2O4) (LiDFOB). In some embodiments, the additive includes at least one of fluoroethylene carbonate and adiponitrile.

[0055] Positive electrode

[0056] The positive electrode plate includes a positive current collector and a positive active layer disposed on the positive current collector. The positive current collector may be aluminum foil or nickel foil, etc., or may 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 foil and the polymer substrate. The positive active layer contains a positive active material, and the positive active material includes a compound that reversibly embeds and deintercalates 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 lithium cobaltate, lithium nickel manganese cobaltate, lithium nickel manganese aluminum, 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 lithium manganese oxide, spinel lithium nickel manganese oxide and lithium titanate.

[0057] The positive electrode active layer also includes a binder to bond the positive electrode active material particles to facilitate the formation of a film layer, and at the same time, it can also improve the bonding force between the positive electrode active layer and the positive 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, ethylene oxide-containing polymers, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene butadiene rubber, acrylic (ester) styrene butadiene rubber, epoxy resin or nylon, etc. At least one of the above.

[0058] The positive electrode active layer may also include a conductive material, including but 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 may include but is 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 material may include but is not limited to metal powder or metal fiber, such as copper, nickel, aluminum, or silver. In some embodiments, the conductive polymer may be a polyphenylene derivative.

[0059] The above-mentioned secondary battery is applied to an electronic device to power the load in the electronic device. Moreover, the silicon-carbon material in the above-mentioned secondary battery has excellent cycle performance and expansion performance, which is conducive to improving the service life of the electronic device. Among them, the electronic device may include but is not limited to a laptop computer, a pen-input computer, a mobile computer, an e-book player, a portable phone, a portable fax machine, a portable copier, a portable printer, a head-mounted stereo headset, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini-disc, a transceiver, an electronic notepad, a calculator, a memory card, a portable recorder, a radio, a backup power supply, a motor, a car, a motorcycle, a power-assisted bicycle, a bicycle, a lighting fixture, a toy, a game console, a clock, an electric tool, a flashlight, a camera, a large household battery and a lithium-ion capacitor, etc.

[0060] The present application is described below by 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.

[0061] Example 1-1

[0062] Preparation of silicon carbon materials:

[0063] 1 gram of carbon nanotubes, 750 grams of linear resin, 100 grams of hexamethylenetetramine (HMT), and 380 grams of polyvinyl pyrrolidone (PVP) were dissolved in 5000 milliliters of ethanol solution, the sand milling speed was 1000 r / min, and the sand milling stirring time was 3 hours to form a uniform mixed solution containing carbon nanotubes. Subsequently, the mixed solution was granulated by spray drying, and the inlet air temperature during the spray drying process was 200°C, the outlet air temperature was 80°C, the atomization pressure was 0.3 MPa, and the feed rate was 0.03 L / min to obtain mixed particles containing carbon nanotubes with a diameter of about 10 μm, and the mixed particles were vacuum dried at 80°C for 24 hours to remove the organic solvent.

[0064] Then, the dried mixed particles are carbonized and activated in sequence. In the carbonization stage, the carbonization temperature of the mixed particles is 600°C and the carbonization time is 2 hours under an argon atmosphere. In the activation stage, the activation temperature is 900°C and the activation time is 4 hours under a mixed gas atmosphere of water and carbon dioxide to obtain porous carbon containing carbon nanotubes.

[0065] Finally, by chemical vapor deposition, silicon-containing gas (such as silane, SiH4) is deposited on the porous carbon containing carbon nanotubes in silane gas. The deposition temperature is 550°C and the deposition time is 6 hours. Then, carbon-containing gas (such as acetylene, C2H2) is coated in acetylene atmosphere at 600°C and the carbon coating time is 0.5 hours to obtain silicon-carbon particle material.

[0066] Preparation of lithium-ion batteries:

[0067] Preparation of positive electrode: Lithium cobalt oxide, conductive carbon black and polyvinylidene fluoride (PVDF) are fully stirred and mixed in an N-methylpyrrolidone solvent system at a weight ratio of 95%:2.5%:2.5% to prepare positive electrode slurry. The prepared positive electrode slurry is coated on the positive electrode current collector aluminum foil, dried, cold pressed, cut, and the pole ears are welded to obtain positive electrode sheets.

[0068] Preparation of negative electrode: Graphite, silicon-carbon material, conductive agent carbon nanotubes, and binder polymethyl acrylate are mixed according to a solid mass ratio of 88.4%: 9.8%: 0.1%: 1.7%, kneaded and dispersed at a solid content of about 38% to 50wt%, to form a negative electrode slurry, and the prepared negative electrode slurry is coated on the negative electrode collector copper foil, dried, cold pressed, cut, and welded to obtain the negative electrode.

[0069] Preparation of electrolyte: In a dry argon environment, LiPF6 was added to a mixture of 1,3-propane sultone (1,3-PS), propyl propionate (PP), ethyl propionate (EP), fluoroethylene carbonate (FEC), propylene carbonate (PC), ethylene carbonate (EC) and LiTFSI, wherein, based on the mass of the electrolyte, the mass proportion of LiPF6 was 12.5%, the mass proportion of 1,3-PS was 3%, the mass proportion of PP was 45%, the mass proportion of EP was 15%, the mass proportion of FEC was 10%, the mass proportion of LiTFSI was 0.5%, and the remainder was PC and EC. The mass ratio of PC to EC was 1:1.

[0070] Preparation of isolation membrane: PE composite film is used as isolation membrane.

[0071] Preparation of lithium-ion battery: stack the positive electrode, separator and negative electrode in order, so that the separator is between the positive electrode and the negative electrode to play a role of isolation. Wind to get the electrode assembly. After welding the pole ear, put the electrode assembly in an aluminum-plastic film packaging bag, dry it at 80°C, inject the electrolyte, and go through vacuum packaging, static, formation, shaping, and capacity testing to get a soft-pack lithium-ion battery.

[0072] Example 1-2 to Example 1-13

[0073] The difference between Examples 1-2 to 1-13 and Example 1-1 is that the mass ratio of carbon nanotubes and surfactants and the preparation condition parameters in the preparation of silicon-carbon materials are changed, and the remaining steps are the same as Example 1-1. The specific preparation parameters can be referred to Tables 1 and 2.

[0074] Example 2-1 to Example 2-9

[0075] The difference between Examples 2-1 to 2-9 and Example 1-1 is that the diameter ratio of the carbon nanotubes or the average circularity of the silicon-carbon material is changed, and the remaining steps are the same as Example 1-1. The specific preparation parameters can be referred to Table 2.

[0076] Example 3-1 to Example 3-13

[0077] The difference between Examples 3-1 to 3-13 and Example 2-3 is that the components of the electrolyte in the lithium-ion battery and the mass ratio of the lithium salt additive are adjusted. Except for adjusting the parameters of the relevant electrolyte components according to Table 3, the rest is the same as Example 2-3.

[0078] Comparative Example 1

[0079] The difference between Comparative Example 1 and Example 1-1 is that the mass ratio of carbon nanotubes and surfactants and the preparation condition parameters in the preparation of silicon-carbon material are changed, and the other conditions are the same as Example 1-1. For specific preparation parameters, please refer to Table 1 and Table 2.

[0080] The silicon-carbon materials prepared in various embodiments and comparative examples and the assembled lithium-ion batteries were tested.

[0081] Lithium-ion battery related performance test

[0082] (1) Cyclic performance test

[0083] Place the lithium-ion battery in a 25°C constant temperature box, charge it to 4.4V at a constant current of 1.5C, charge it to 0.05C at a constant voltage of 4.4V, and then discharge it to 3.0V at a constant current of 1.0C. This is the first charge and discharge cycle. Carry out the charge and discharge cycle test according to the above steps, and make a ratio of the discharge capacity of each step to the initial capacity to obtain the discharge capacity retention rate of each step, and record the number of cycles of the lithium-ion battery at 25°C until the discharge capacity retention rate is 90%. The cycle performance of the lithium-ion battery is characterized by the number of cycles at 25°C until the discharge capacity retention rate is 90%. The higher the number of cycles of the lithium-ion battery at 25°C until the discharge capacity retention rate is 90%, the better the cycle performance of the lithium-ion battery.

[0084] (2) Expansion performance test

[0085] At 25°C, a flat thickness gauge (with a load of 600g) is used to test the thickness M of the lithium-ion battery at the second half charge (50% SOC). When the above charge and discharge process is cycled to 500 cycles, the lithium-ion battery is in a fully charged state (100% SOC). The thickness N of the lithium-ion battery at this time is tested again with a flat thickness gauge (with a load of 600g). The expansion rate of the lithium-ion battery after 500 cycles at 25°C is L=(NM) / M×100%. The thickness of the tested lithium-ion battery is the cold measurement thickness, that is, the lithium-ion battery is taken back and placed in a normal temperature test room for testing.

[0086] Related test methods for silicon carbon materials:

[0087] (1) Test method for silicon-carbon material |S1-S2|

[0088] The silicon-carbon material particles containing carbon nanotubes were made into thin sheets with a thickness of about 80 nanometers by ion thinning. The prepared samples were placed on the sample stage of the TEM, and the distribution of carbon nanotubes inside the particles was observed and images were collected in high-resolution TEM mode. The collected images were processed using image analysis software, and cross-sections with an area of ​​1000nm were randomly selected. 2In the first test area and the second test area, since the carbon nanotubes have obvious TEM lattice stripes, the carbon skeleton and the silicon particles are all amorphous except the area containing the carbon nanotubes. Therefore, by measuring the area of ​​the area with obvious lattice stripes in the first test area and the second test area, the distribution area proportions S1 and S2 of the carbon nanotubes in the two test areas can be obtained, and the value of |S1-S2| can be calculated.

[0089] (2) Testing method for carbon nanotube diameter ratio:

[0090] First, the carbon nanotubes are dispersed in an appropriate solvent to ensure uniform dispersion and avoid agglomeration. Then, the structure of the carbon nanotubes is observed using a transmission electron microscope (TEM) and high-resolution images are taken. Next, the outer diameter and inner diameter of the carbon nanotubes are measured using image processing software, and the diameter ratio (outer diameter / inner diameter) is calculated. Finally, the diameter ratios of more than 10 carbon nanotubes are statistically analyzed to obtain an average value.

[0091] (3) Test method for average circularity of silicon-carbon materials:

[0092] The two-dimensional image of the particles is obtained through a microscope, and the perimeter, area and other parameters of the particles are measured using image analysis software. Then, according to the formula, sphericity = (π × (area) (1 / 2) ) / (circumference) to calculate the circularity of a single particle; finally, the circularity of more than 20 particles is statistically analyzed to obtain an average value to obtain the average circularity of the silicon-carbon material.

[0093] (4) Testing method for elastic modulus of silicon-carbon materials:

[0094] Using Shimadzu single particle crusher (FLAT50), 10 silicon-carbon material particles were randomly selected for crushing force test, with minimum / maximum pressure of 0.02 / 10mN. The elastic modulus of each silicon-carbon material particle was calculated based on the deformation and approximate contact surface corresponding to the maximum pressure, and using Hooke's law and pressure formula, and the elastic modulus of the silicon-carbon material was obtained by calculating the arithmetic mean.

[0095] (5) Test method for electrical conductivity of silicon-carbon material powder:

[0096] The conductivity of silicon-carbon material powder was tested using a conductivity tester (instrument model: Suzhou Jingge Electronics ST-2255A). Take 5g of silicon-carbon material powder sample, use an electronic press to press the sample, press to 5000kg±2kg, maintain for 20s, and prepare the test sample. Place the test sample between the electrodes of the conductivity tester, and obtain the resistance R (in Ω) through the voltage U and current I at both ends. The height of the test sample is h (in cm), and the area of ​​the test sample is S = 3.14cm 2The powder conductivity of the silicon-carbon material is calculated according to the formula powder conductivity δ = h / (S×R) and the unit is S / cm.

[0097] Table 1

[0098]

[0099]

[0100] Table 2

[0101]

[0102]

[0103] In combination with Table 1 and Table 2, compared with the comparative example, in Examples 1-1 to 1-13, in the process of preparing the negative electrode active material, the mass ratio of carbon nanotubes and surfactants in the preparation of the silicon-carbon material and the preparation condition parameters are changed. When the area ratio of the carbon nanotubes in the first test area and the second test area meets a specific range, the cycle performance and expansion performance of the silicon-carbon material can be improved.

[0104] In Table 2, during the preparation of the negative electrode active material, the diameter ratio of the carbon nanotubes or the average circularity of the silicon-carbon material is changed. When the diameter ratio of the carbon nanotubes or the average circularity of the silicon-carbon material is within a specific range, the cycle performance and expansion performance of the silicon-carbon material can be further improved.

[0105] Table 3

[0106]

[0107]

[0108] Combined with Table 3, in lithium-ion batteries, changing the components of the electrolyte and the mass ratio of the lithium salt additive, when the electrolyte contains ethyl propionate and propyl propionate and is within an appropriate mass ratio range, and the mass ratio of the lithium salt additive is maintained within a specific range, the cycle performance and expansion performance of the silicon-carbon material can be further improved.

[0109] The above disclosure is only a preferred implementation mode of the present application, and certainly cannot be used to limit the present application. Therefore, equivalent changes made according to the present application are still within the scope covered by the present application.

Claims

1. A silicon-carbon material, characterized in that: The invention comprises a carbon skeleton and silicon particles, wherein the carbon skeleton comprises amorphous carbon and carbon nanotubes, and the cross section of the silicon-carbon material comprises an area of ​​1000 nm. 2 A first test area and a second test area, the first test area and the second test area do not overlap, based on the area of ​​the first test area, the area of ​​the carbon nanotubes accounts for S1%, based on the area of ​​the second test area, the area of ​​the carbon nanotubes accounts for S2%, |S1-S2|≤0.

1.

2. The silicon-carbon material according to claim 1, characterized in that 0.006≤|S1-S2|≤0.

043.

3. The silicon-carbon material according to claim 1, characterized in that: The diameter ratio of the carbon nanotubes is L, 1.2≤L≤2.

4. The silicon-carbon material according to any one of claims 1 to 3, characterized in that The silicon-carbon material satisfies at least one of the following conditions: (1) The average circularity of the silicon-carbon material is 0.81 to 0.97; (2) The powder conductivity of the silicon-carbon material is 0.1 S / cm to 10 S / cm; (3) The elastic modulus of the silicon-carbon material is 50 GPa to 200 GPa.

5. A method for preparing a silicon-carbon material according to any one of claims 1 to 4, characterized in that: The following steps are involved: S1. adding carbon nanotubes, a carbon precursor, a curing agent, and a surfactant to an organic solvent and stirring to obtain a mixed solution, wherein the mass ratio of the carbon nanotubes to the surfactant is (1:1) to (1:400); S2. The mixed solution is granulated by spray drying to obtain mixed particles; S3. The mixed particles are sequentially carbonized and activated to obtain porous carbon; S4. Using chemical vapor deposition, silicon is deposited and carbon is coated on the porous carbon to obtain the silicon-carbon material.

6. The preparation method according to claim 5, characterized in that: Before the spray drying treatment, the mixed liquid is further subjected to a sand mill stirring treatment, the sand mill speed is 200 r / min to 2000 r / min, and the sand mill time is 0.1 h to 6 h.

7. A secondary battery comprising a negative electrode sheet, a positive electrode sheet and an electrolyte, characterized in that: The negative electrode plate comprises the silicon-carbon material according to any one of claims 1 to 4 or the silicon-carbon material obtained by the preparation method according to claim 5 or 6.

8. The secondary battery according to claim 7, wherein: The electrolyte includes ethyl propionate and propyl propionate. Based on the mass of the electrolyte, the mass proportion of ethyl propionate is A1, the mass proportion of propyl propionate is A2, and 2≤A2 / A1≤6.

9. The secondary battery according to claim 7 or 8, characterized in that: The electrolyte includes a lithium salt additive, wherein the lithium salt additive includes at least one of lithium bis(fluorosulfonyl)imide or lithium bis(trifluoromethanesulfonyl)imide. Based on the mass of the electrolyte, the mass proportion of the lithium salt additive is 0.01% to 1.5%.

10. An electronic device, characterized in that: The invention comprises the secondary battery according to any one of claims 7 to 9.

Images