Silicon-carbon composite material, negative electrode sheet, secondary battery, and electronic device

Abstract

The application discloses a silicon-carbon composite material, a negative electrode sheet, a secondary battery and an electronic device. The silicon-carbon composite material comprises a porous carbon skeleton and a silicon material in the pores of the porous carbon skeleton. The porous carbon skeleton satisfies 1.5 < (c-a) / b < 5.0, wherein a represents a pore diameter corresponding to 10% of the cumulative pore volume in the total pore volume, b represents a pore diameter corresponding to 50% of the cumulative pore volume in the total pore volume, and c represents a pore diameter corresponding to 99% of the cumulative pore volume in the total pore volume. The technical scheme of the application can improve the cycle performance of the secondary battery and improve the high-temperature performance of the secondary battery while ensuring high energy density of the secondary battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a silicon-carbon composite material, a negative electrode sheet, a secondary battery and an electronic device. BACKGROUND

[0002] Silicon as a negative electrode material of lithium ion battery has a theoretical specific capacity of up to 4200 mAh / g, and is considered to be the most likely negative electrode material to replace traditional graphite. However, during the charging and discharging process of silicon particles, the insertion process of lithium ions will cause serious particle volume expansion due to phase change. The expansion of silicon is still unavoidable in the conventional sanding method of silicon-carbon. At present, the deposition of silicon-carbon material by silane method has become the mainstream direction of silicon material development, which provides an effective solution for the expansion problem of silicon by reserving holes. However, due to the complexity of the silane deposition process, many invalid holes will be formed in this reserved hole structure, which have low strength and are easily crushed and broken during the cold pressing process, thereby contacting the electrolyte and reacting, deteriorating the cycle and expansion performance of the battery. SUMMARY

[0003] Therefore, the present application provides a silicon-carbon composite material, a negative electrode sheet, a secondary battery and an electronic device. The technical scheme of the present application can improve the cycle performance and high temperature performance of the secondary battery while ensuring high energy density.

[0004] In a first aspect, the present application provides a silicon-carbon composite material, which comprises a porous carbon skeleton and a silicon material in the pores of the porous carbon skeleton. The silicon material can improve the energy density of the battery, and the porous carbon skeleton satisfies: 1.5 < (c-a) / b < 5.0 (in the present application, d = (c-a) / b), a represents the pore size corresponding to 10% of the cumulative pore volume based on the total pore volume, b represents the pore size corresponding to 50% of the cumulative pore volume based on the total pore volume, and c represents the pore size corresponding to 99% of the cumulative pore volume based on the total pore volume. The units of a, b and c are all nm. The ratio of (c-a) / b can represent the pore size distribution concentration degree of the porous carbon skeleton (i.e. the ratio of the proportion of large pores to the proportion of small pores and the proportion of intermediate distribution pores). The smaller the ratio, the closer the size of large pores and small pores, and the better the pore distribution concentration degree. By adjusting the ratio of (c-a) / b within the above range, the porous carbon skeleton has a high concentration degree of pore structure. After uniformly depositing silane in the deposition reactor, the overall structural strength of the material is better, which has good pressure resistance and is beneficial to improving the cycle performance and high temperature performance of the lithium ion battery. Preferably, 0.8 nm < a < 1.5 nm, 1.8 nm < b < 3.0 nm, and 5.0 nm < c < 10.0 nm.

[0005] In some embodiments, 2.0≤(c-a) / b≤2.9. In this way, the structural strength of the porous carbon framework is further improved, the pressure resistance of the silicon-carbon composite material is improved, and the cycle performance and high-temperature performance of the lithium ion battery are improved while the energy density is high. Specifically, in some examples, 2.0≤(c-a) / b≤2.7. In some examples, 2.0≤(c-a) / b≤2.5. In some examples, 2.0≤(c-a) / b≤2.4. In some examples, 2.0≤(c-a) / b≤2.3.

[0006] In some embodiments, 1.1 nm≤a≤1.4 nm. The value of a in the above range is more conducive to improving the pressure resistance of the silicon-carbon composite material in cooperation with b and c. Specifically, in some examples, 1.1 nm≤a≤1.3 nm. In some examples, 1.1 nm≤a≤1.2 nm.

[0007] In some embodiments, 2.2 nm≤b≤2.7 nm. The value of b in the above range is more conducive to improving the pressure resistance of the silicon-carbon composite material in cooperation with a and c. Specifically, in some examples, 2.3 nm≤b≤2.7 nm. In some examples, 2.4 nm≤b≤2.7 nm. In some examples, 2.5 nm≤b≤2.7 nm. In some examples, 2.6 nm≤b≤2.7 nm.

[0008] In some embodiments, 5.7 nm≤c≤6.5 nm. The value of c in the above range is more conducive to improving the structural strength of the material as a whole in cooperation with a and b. Specifically, in some examples, 5.9 nm≤c≤6.5 nm. In some examples, 6.1 nm≤c≤6.5 nm. In some examples, 6.3 nm≤c≤6.5 nm. In some examples, 6.4 nm≤c≤6.5 nm.

[0009] In some embodiments, the porous carbon framework comprises a phenolic resin-based porous carbon framework, i.e., formed by polymerization of phenolic monomers (purity greater than 99.9%) and has a curing degree greater than 97.0%, and contains ultramicropores, the volume fraction of the ultramicropores based on the total pore volume of the porous carbon framework E g / cc, satisfies: E > 0.55, e < 9.0, preferably, 0.75 ≤ E ≤ 0.85, e ≤ 4.0, and the pore size of the ultramicropores is less than 0.7 nm. By adjusting the volume fraction of the ultramicropores (<0.7 nm) in the above range, the application avoids the plugging phenomenon during silane deposition, thereby avoiding the formation of invalid pores inside the porous carbon framework, further improving the compressive strength of the silicon-carbon composite material, and obtaining a silicon-carbon material with good compression resistance. Under very high electrode compaction density (e.g., compaction density > 1.74 g / cc), the particles do not break, which is beneficial to the advantages of high volume energy density of silicon material, while the invalid pores are effectively reduced, which is beneficial to the reasonable matching of micropores and deposited silicon, and good semi-charge rebound and expansion performance can be obtained, thereby improving the cycle performance and high-temperature performance of the lithium ion battery.

[0010] The preparation of the phenolic resin-based porous carbon can be, but is not limited to, using phenol and formaldehyde as monomers, adding a dispersing agent and a curing agent under the catalysis of one or more complex catalysts such as hydrochloric acid, ammonia, sodium hydroxide, and triethylamine.

[0011] In some embodiments, the porous carbon framework (phenolic resin-based porous carbon framework) is obtained by activating a phenolic resin-based porous carbon in an alkaline medium, and the activation conditions include: an activation temperature of 750-800°C, an activation time of 3-3.5 h, and an alkaline / carbon ratio of 3-3.5. In this way, by adjusting the activation temperature, activation time, and alkaline / carbon ratio in coordination, the value of (c-a) / b in the application can be controlled, and further, the volume fraction of the ultramicropores e% can be adjusted in a suitable range.

[0012] In some embodiments, the silicon-carbon composite material has a specific surface area (BET) of f m 2 / g after being compressed at a pressure of 295.33 MPa for 30 s, which satisfies: 2.0 < f < 5.0, and a compaction density of n g / cc after being compressed at a pressure of 295.33 MPa for 30 s, which satisfies: 0.95 g / cc < n < 1.10 g / cc. It can be seen that the silicon-carbon composite material of the application has a good compressive strength, which is beneficial to improving the cycle performance and high-temperature performance of the lithium ion battery.

[0013] In a second aspect, the present application provides a negative electrode sheet, which comprises a negative electrode current collector and a negative electrode material layer arranged on at least one surface of the negative electrode current collector, and the negative electrode material layer comprises a negative electrode material, and the negative electrode material comprises any one of the silicon-carbon composite materials of the first aspect.

[0014] In some embodiments, when the silicon-carbon composite material (35% to 55% of the mass percentage of silicon elements) is configured in the negative electrode material at an addition amount of less than 10 wt%, the compaction density of the negative electrode material layer is greater than 1.74 g / cc. It can be seen that the silicon-carbon composite material particles of the present application are not easy to break even at a very high electrode compaction density, which is beneficial to exert the advantages of high volume energy density of the silicon-carbon composite material, and is also beneficial to improve the cycle performance and high-temperature performance of the lithium ion battery.

[0015] In some embodiments, when the negative electrode sheet is cold-pressed, the compaction density corresponding to the breakage of the silicon-carbon composite material particles in the negative electrode sheet is m g / cc, and m is greater than 1.80. The cold-pressing conditions include: a cold-pressing pressure of 100 MPa to 500 MPa, a cold-pressing temperature of 20°C to 28°C, and a cold-pressing time of 10 to 60 h. By controlling the pore size distribution and the proportion of different pore sizes of the porous carbon precursor, the present application can effectively improve the pressure resistance of the silicon-carbon composite material, increase the compaction density of the silicon-carbon composite material, and make the cycle performance and high-temperature performance of the lithium ion battery better.

[0016] In a third aspect, the present application provides a preparation method of the above-mentioned silicon-carbon composite material, which comprises the following steps: 1) obtaining a carbonized porous carbon, activating the same by using an alkali medium at 750°C to 800°C, the activation time being 1.5 h to 2 h, and the alkali / carbon ratio being 3 to 3.5, to obtain a porous carbon framework; 2) placing the porous carbon framework in a deposition reactor, and performing first-stage deposition and second-stage deposition, wherein the first-stage deposition is performed by introducing a silane gas with a volume content of 1% to 100% at a temperature of 520°C to 600°C, and the other proportion being inert gas, and the deposition time being 2 h to 8 h; the second-stage deposition is performed by introducing a silane gas with a volume content of 1% to 100% at a temperature of 470°C to 520°C, and the other proportion being inert gas, and the deposition time being 15 h to 30 h; 3) after the completion of the deposition reaction, introducing pure inert gas for 60 min, and then introducing an acetylene gas with a concentration of 5% to 100% and depositing at 450-520°C for 1 h to 48 h.

[0017] In a fourth aspect, the present application provides a secondary battery, which comprises any one of the negative electrode sheets of the second aspect, or any one of the silicon-carbon composite materials of the first aspect, or comprises a silicon-carbon composite material prepared by the preparation method of the third aspect.

[0018] In a fifth aspect, the present application provides an electronic device comprising any one of the secondary batteries of the fourth aspect. DETAILED DESCRIPTION

[0019] In order to make the purposes, technical solutions and advantages of the present application clearer, the present application will be further described in detail below with reference to examples. It should be understood that the specific examples described herein are only intended to explain the present application and not to limit the present application.

[0020] The higher energy density and larger expansion rate of silicon-based materials are the main contradictions in promoting large-scale industrialization. In actual mass production and research, special structural design is performed on the silicon-based materials to provide space for the release and buffering of silicon expansion stress, which is the main scheme in the industry. Initially, micron silicon is adjusted to nanosilicon to avoid the pulverization problem of silicon particles due to stress differences at different positions of the particles during the charge and discharge cycle. Meanwhile, nanosilicon is mixed with graphite, pitch and other organic carbon sources to obtain a buffer space and better electrical contact. However, under this structure, the expansion of silicon will inevitably break the buffer layer, thereby causing the contact between silicon and electrolyte and the repeated formation of a solid-state electrolyte film, affecting the electrical performance of the secondary battery. To obtain a more stable structure, some researchers have deposited silane in a microporous structure of porous carbon to obtain space for silicon expansion by reserving the pore structure, and have obtained good electrical performance. However, the reserved pore structure is prone to cause particle breakage during cold pressing due to the complexity of the carbon activation and silane deposition process. The broken particles will repeatedly form a solid-state electrolyte film in the full battery, and the compaction density of the pole piece cannot be improved, which seriously affects the energy density of the secondary battery. To effectively solve this problem, the present application provides a silicon-carbon composite material, a negative pole piece, a secondary battery and an electronic device. By controlling the pore-forming process of carbon activation, on the one hand, a pore structure with high concentration is formed, so that after uniform deposition of silane, the overall structural strength of the material is better, and on the other hand, the volume of ultramicropores is specially controlled to avoid the phenomenon of pore blockage during silane deposition, thereby avoiding the formation of internal dead pores to reduce the compression resistance. Ultimately, the material has good pressure resistance, and the silicon-carbon material with good compression resistance can be obtained. When the pole piece has a very high compaction density, the particles will not be broken, which is conducive to the advantages of high volume energy density of the silicon-carbon composite material, and the dead pores are effectively reduced, so that the silicon-carbon composite material has good half-charge rebound and expansion performance.

[0021] Silicon-carbon composite material

[0022] The first aspect of the embodiments of the present application provides a silicon-carbon composite material, which comprises a porous carbon framework and a silicon material in the pores of the porous carbon framework, wherein the porous carbon framework satisfies 1.5 < (c-a) / b < 5.0, wherein a represents a pore size corresponding to 10% of the cumulative pore volume in the total pore volume, b represents a pore size corresponding to 50% of the cumulative pore volume in the total pore volume, and c represents a pore size corresponding to 99% of the cumulative pore volume in the total pore volume, and the units of a, b and c are nm. By adjusting the ratio of (c-a) / b in the above range, the porous carbon framework has a higher concentrated pore structure. After the silicon is uniformly deposited in the deposition reactor, the overall structural strength of the material is better, the pressure resistance is better, and the cycle performance and high-temperature performance of the lithium ion battery are improved. For example, the ratio of (c-a) / b is 1.5, 1.6, 1.8, 2.0, 2.1, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3.0, 3.3, 3.5, 4.0, 4.2, 4.5, 4.9, 4.99 or a range formed by any two of the above values.

[0023] In some embodiments, 0.8nm < a < 1.5nm. The value of a in the above range is more conducive to improving the pressure resistance of the silicon-carbon composite material in cooperation with b and c. For example, the value of a is in the range of 0.81nm, 0.85nm, 0.9nm, 1.0nm, 1.1nm, 1.2nm, 1.3nm, 1.4nm, 1.41nm, 1.43nm, 1.45nm, 1.49nm or a range formed by any two of the above values.

[0024] In some embodiments, 1.8nm < b < 3.0nm. The value of b in the above range is more conducive to improving the pressure resistance of the silicon-carbon composite material in cooperation with a and c. For example, the value of b is in the range of 1.81nm, 1.9nm, 2.0nm, 2.2nm, 2.3nm, 2.5nm, 2.6nm, 2.7nm, 2.8nm, 2.9nm, 2.99nm or a range formed by any two of the above values.

[0025] In some embodiments, 5.0nm < c < 10.0nm. The value of c in the above range is more conducive to improving the overall structural strength of the material in cooperation with a and b. For example, the value of c is in the range of 5.1nm, 5.3nm, 5.5nm, 5.7nm, 6.0nm, 6.2nm, 6.3nm, 6.5nm, 7.0nm, 8.5nm, 9.0nm, 9.5nm, 9.9nm or a range formed by any two of the above values.

[0026] In some embodiments, the porous carbon framework comprises a phenolic resin-based porous carbon framework, the porous carbon framework contains ultramicropores, the volume fraction of the ultramicropores is e%, based on the total pore volume E g / cc of the porous carbon framework, and the following conditions are met: E > 0.55, e < 9.0, preferably, 0.75 ≤ E ≤ 0.85, e ≤ 4.0, wherein the pore size of the ultramicropores is less than 0.7 nm. The volume fraction of the ultramicropores (<0.7 nm) is controlled in the above range to avoid the pore blocking phenomenon during silane deposition, thereby avoiding the formation of invalid pores inside the porous carbon framework, further improving the compressive strength of the silicon-carbon composite material, and obtaining a silicon-carbon material with better compressive capacity, which can further improve the cycle performance and high-temperature performance of the lithium ion battery. For example, the total pore volume E of the porous carbon framework is 0.6 g / cc, 0.65 g / cc, 0.7 g / cc, 0.75 g / cc, 0.78 g / cc, 0.80 g / cc, 0.82 g / cc, 0.83 g / cc, 0.85 g / cc, 0.9 g / cc, or a range defined by any two of the above values. For example, the volume fraction of the ultramicropores is 0.01%, 0.1%, 0.5%, 0.8%, 1.0%, 1.5%, 2.0%, 2.5%, 3.0%, 3.5%, 4.0%, 6.5%, 7.0%, 8.5%, 8.9%, or a range defined by any two of the above values. For example, the pore size of the ultramicropores is 0.01 nm, 0.01 nm, 0.1 nm, 0.3 nm, 0.4 nm, 0.5 nm, 0.6 nm, 0.65 nm, 0.69 nm, or a range defined by any two of the above values.

[0027] In some embodiments, the phenolic resin-based porous carbon framework is obtained by activating a phenolic resin-based porous carbon in an alkaline medium, and the activation conditions include: an activation temperature of 750-800°C, an activation time of 3-3.5 h, and an alkaline / carbon ratio of 3-3.5. For example, the activation temperature is 750°C, 760°C, 770°C, 780°C, 790°C, 800°C, or a range defined by any two of the above values. For example, the activation time is 3 h, 3.1 h, 3.2 h, 3.3 h, 3.4 h, 3.5 h, or a range defined by any two of the above values. For example, the alkaline / carbon ratio is 3, 3.1, 3.2, 3.3, 3.4, 3.5, or a range defined by any two of the above values.

[0028] In some embodiments, the silicon-carbon composite material has a BET of f m 2 / g after being compressed at a pressure of 295.33 MPa for 30 s, and the following condition is met: 2.0 < f < 5.0. It can be seen that the silicon-carbon composite material of the present application has a better compressive strength, which is beneficial to improving the cycle performance and high-temperature performance of the lithium ion battery. For example, the BET of the silicon-carbon composite material after compression is 2.05 m2 / g, 2.1m 2 / g, 2.3m 2 / g, 2.8m 2 / g, 3.0m 2 / g, 3.5m 2 / g, 4.0m 2 / g, 4.5m 2 / g, 4.8m 2 / g, 4.9m 2 / g, 4.95m 2 / g or a range consisting of any two of the above values.

[0029] In some embodiments, the silicon-carbon composite material has a compaction density of n g / cc after being compressed at a pressure of 295.33 MPa for 30 s, and satisfies: 0.95 < n < 1.10. It can be seen that the silicon-carbon composite material of the present application has a better pressure resistance, which is beneficial to improving the cycle performance and high-temperature performance of the lithium ion battery. Exemplarily, the compaction density after compression is 0.96 g / cc, 0.98 g / cc, 1.0 g / cc, 1.05 g / cc, 1.08 g / cc, 1.10 g / cc or a range consisting of any two of the above values.

[0030] In some embodiments, the negative electrode sheet has a compaction density of m g / cc when the silicon-carbon composite material particles in the negative electrode sheet are broken after cold pressing, and m > 1.80. The cold pressing conditions include: a cold pressing pressure of 100 MPa to 500 MPa, a cold pressing temperature of 20℃ to 28℃, and a cold pressing time of 10 to 60 h. By controlling the pore size distribution and the proportion of different pore sizes of the porous carbon precursor, the present application can effectively improve the pressure resistance of the silicon-carbon composite material, increase the compaction density of the silicon-carbon composite material, and make the cycle performance and high-temperature performance of the lithium ion battery better.

[0031] Method for producing silicon-carbon composite material

[0032] (1) Obtain the carbonized porous carbon, activate it at 750℃ to 800℃ using an alkali medium, the activation time is 1.5h to 2h, the alkali-carbon ratio is 3 to 3.5, and obtain the porous carbon skeleton;

[0033] (2) Place the porous carbon skeleton in a deposition reactor, and perform first-stage deposition and second-stage deposition, the first-stage deposition is to pass in 1% to 100% volume content of silane gas at a temperature of 520℃ to 600℃, the other proportion is inert gas, and the deposition time is 2h to 8h;

[0034] The second stage deposition is carried out at a temperature of 470-520°C, with a silane gas with a volume content of 1-100%, and other inert gas, and a deposition time of 15-30h.

[0035] (3) After the deposition reaction is completed, pure inert gas is introduced for 60min, and then acetylene gas with a concentration of 5-100% is introduced, and deposition is carried out at 450-520°C for 1-48h.

[0036] Negative electrode sheet

[0037] The second aspect of the embodiment of the present application provides a negative electrode tab, which comprises a negative electrode current collector and a negative electrode material layer arranged on at least one surface of the negative electrode current collector, wherein the negative electrode material layer comprises a negative electrode material, the negative electrode material comprises any one of the silicon-carbon composite materials of the first aspect and a carbon material, the carbon material includes but is not limited to at least one of natural graphite, artificial graphite, mesophase carbon microbeads, hard carbon or soft carbon, the mass ratio of the silicon-carbon composite material and the carbon material is about 9:1, and when the silicon-carbon composite material is configured in the negative electrode material at an addition amount of less than 10wt%, the compaction density of the negative electrode material layer is >1.74g / cc.

[0038] The thickness of the negative electrode material layer is not particularly limited in the present application, as long as the object of the present application can be achieved, for example, the thickness of the negative electrode active layer is 30 pm to 120 pm. The negative electrode current collector is not particularly limited in the present application, as long as the object of the present application can be achieved, for example, it can include a copper foil, a copper alloy foil, a nickel foil, a stainless steel foil, a titanium foil, a foamed nickel, a foamed copper, or a composite current collector (for example, a composite current collector in which a metal layer is provided on the surface of a polymer layer), and the like. The thickness of the negative electrode current collector is not particularly limited in the present application, as long as the object of the present application can be achieved, for example, the thickness of the negative electrode current collector is 5 pm to 12 pm. The negative electrode material layer can further include a binder and a thickening agent, and the kind of the binder and the thickening agent is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the binder can include, but is not limited to, at least one of polyvinyl alcohol, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, styrene butadiene rubber, or acrylated styrene butadiene rubber; and the thickening agent can include, but is not limited to, at least one of sodium carboxymethyl cellulose or lithium carboxymethyl cellulose. In addition, the negative electrode material layer can further include a conductive agent, and the kind of the conductive agent is not particularly limited in the present application, as long as the object of the present application can be achieved. For example, the conductive agent can include, but is not limited to, at least one of conductive carbon black, carbon nanotubes (CNTs), carbon fibers, Ketjen black, graphene, a metal material, or a conductive polymer. The mass ratio of the negative electrode material, the conductive agent, the binder, and the thickening agent in the negative electrode material layer is not particularly limited in the present application, and a person skilled in the art can select according to the actual needs, as long as the object of the present application can be achieved. Optionally, the negative electrode sheet can further include a conductive layer, and the conductive layer is located between the negative electrode current collector and the negative electrode active layer. The composition of the conductive layer is not particularly limited in the present application, and it can be a commonly used conductive layer in the art. For example, the conductive layer includes a conductive agent and a binder. The conductive agent and the binder in the conductive layer are not particularly limited in the present application, and for example, they can be at least one of the conductive agent and the binder in the above-mentioned negative electrode active layer.

[0039] Secondary battery

[0040] The third aspect of the embodiments of the present application provides a secondary battery, which includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte, the separator is arranged between the positive electrode sheet and the negative electrode sheet, and the negative electrode sheet is any one of the negative electrode sheets according to the second aspect.

[0041] According to some embodiments of the present application, the secondary battery of the present application includes, but is not limited to, a lithium ion battery or a sodium ion battery. In some embodiments, the secondary battery includes a lithium ion battery.

[0042] Other

[0043] The positive electrode tab includes a positive electrode current collector and a positive electrode material layer disposed on at least one surface of the positive electrode current collector. The positive electrode current collector is not particularly limited as long as the purpose of the present application can be achieved, and for example, can include an aluminum foil, an aluminum alloy foil, or a composite current collector (for example, a composite current collector in which a metal layer is disposed on the surface of a polymer layer), etc. The thickness of the positive electrode current collector is not particularly limited as long as the purpose of the present application can be achieved. For example, the thickness of the positive electrode current collector is 5 μm to 13 μm. The positive electrode material layer includes a positive electrode active material, and the positive electrode active material is not particularly limited as long as the purpose of the present application can be achieved. For example, the positive electrode active material can include, but is not limited to, at least one of nickel-cobalt-manganese lithium acid (for example, commonly known as NCM811, NCM622, NCM523, NCM111), nickel-cobalt-aluminum lithium acid, lithium iron phosphate, lithium-rich manganese-based material, lithium cobalt acid, lithium manganate, or lithium manganese iron phosphate, and in addition, the positive electrode material layer can further include a conductive agent and a binder, and the type of the conductive agent and the binder is not particularly limited as long as the purpose of the present application can be achieved. For example, the conductive agent can include, but is not limited to, at least one of conductive carbon black, carbon nanotubes (CNTs), carbon fibers, Ketjen black, graphene, a metal material, or a conductive polymer. The above-mentioned metal material can include, but is not limited to, metal powder and / or metal fibers, and specifically, the metal can include, but is not limited to, at least one of copper, nickel, aluminum, or silver. The above-mentioned conductive polymer can include, but is not limited to, at least one of polyphenylene derivative, polyaniline, polythiophene, polyacetylene, or polypyrrole. The binder can include, but is not limited to, at least one of polyacrylic acid, polyacrylic acid salt, acrylate polymer, polyvinyl alcohol, polyvinylidene fluoride, polytetrafluoroethylene, or vinylidene-hexafluoropropylene copolymer. The mass ratio of the positive electrode active material, the conductive agent, and the binder in the positive electrode material layer is not particularly limited, and a person skilled in the art can select according to the actual needs as long as the purpose of the present application can be achieved.

[0044] The separator film can include a base material layer and a surface treatment layer. The base material layer can be a nonwoven fabric, a film, or a composite film having a porous structure, and a material of the base material layer can include at least one of polyethylene, polypropylene, polyethylene terephthalate, or polyimide. Optionally, at least one surface of the base material layer is provided with a surface treatment layer, and the surface treatment layer can be an adhesive layer or a heat-resistant layer. For example, the adhesive layer includes an adhesive, and a material of the adhesive includes at least one of polyamide, polyacrylonitrile, acrylate polymer, polyacrylic acid, polyacrylate, polyvinylpyrrolidone, polyvinyl ether, or polyvinylidene fluoride or vinylidene-hexafluoropropylene copolymer. The heat-resistant layer includes inorganic particles and an adhesive, and the inorganic particles are not particularly limited, and for example, can include at least one of alumina, silica, magnesia, titania, hafnia, tin oxide, ceria, nickel oxide, zinc oxide, calcium oxide, zirconia, yttria, silicon carbide, boehmite, aluminum hydroxide, magnesium hydroxide, calcium hydroxide, or barium sulfate. The adhesive is not particularly limited, and for example, can be at least one of the adhesives in the above-described adhesive layer.

[0045] The electrolyte solution includes an organic solvent, an electrolyte lithium salt, and an additive. The present application does not particularly limit the kind thereof, and it can be selected according to the actual demand.

[0046] Illustratively, the above-described organic solvent includes one or more, preferably two or more, of ethylene carbonate (EC), propylene carbonate (PC), methyl ethylene carbonate (EMC), diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), butylene carbonate (BC), fluoroethylene carbonate (FEC), methyl formate (MF), methyl acetate (MA), ethyl acetate (EA), propyl acetate (PA), methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), methyl butyrate (MB), ethyl butyrate (EB), 1,4-butyrolactone (GBL), sulfolane (SF), dimethyl sulfone (MSM), methyl ethyl sulfone (EMS), or diethyl sulfone (ESE).

[0047] Illustratively, the above-described electrolyte lithium salt includes one or more of LiPF6 (lithium hexafluorophosphate), LiBF4 (lithium tetrafluoroborate), LiClO4 (lithium perchlorate), LiAsF6 (lithium hexafluoroarsenate), LiFSI (lithium bisfluorosulfonylimide), LiTFSI (lithium bis-trifluoromethanesulfonylimide), LiTFS (lithium trifluoromethanesulfonate), LiDFOB (lithium difluoro(oxalato)borate), LiBOB (lithium bis(oxalato)borate), LiPO2F2 (lithium difluorophosphate), LiDFOP (lithium difluoro(dioxalato)phosphate), or LiTFOP (lithium tetrafluoro(oxalato)phosphate).

[0048] The electrolyte solution can optionally further include other additives, which can be any additive that can be used for lithium ion secondary batteries, and the present application is not particularly limited, and can be selected as needed. As an example, the additive can be one or more of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), fluoroethylene carbonate (FEC), 1,3-propane sultone (PS), succinonitrile (SN), adiponitrile (ADN), 1,3-propene sultone (PST), tris(trimethylsilyl)phosphate (TMSP), or tris(trimethylsilyl)borate (TMSB).

[0049] The secondary battery can be prepared according to conventional methods in the art. Illustratively, the positive electrode sheet, the separator, and the negative electrode sheet described above are stacked in order with the separator between the positive electrode sheet and the negative electrode sheet to function as a separator, to obtain an electrode assembly, which can also be obtained by winding. The electrode assembly is placed in a packaging case, electrolyte solution is injected, and the case is sealed to obtain a secondary battery.

[0050] The structure of the lithium battery is not particularly limited, and can be a coin-type battery, a cylindrical battery, a square battery, or a soft pack battery, etc. having a single layer or multiple layers of a separator.

[0051] The use of the lithium ion battery of the present application is not particularly limited, and it can be used for any electronic device known in the art. In some embodiments, the lithium ion battery of the present application can be used for, but is not limited to, a notebook computer, a pen input computer, a mobile computer, an electronic book player, a portable telephone, a portable facsimile machine, a portable copying machine, a portable printer, a headphone, a video recorder, a liquid crystal television, a portable cleaner, a portable CD player, a mini disc, a transceiver, an electronic organizer, a calculator, a memory card, a portable recorder, a radio, a backup power supply, an electric motor, an automobile, a motorcycle, a power-assisted bicycle, a bicycle, a lighting appliance, a toy, a game machine, a clock, an electric tool, a flash, a camera, a household large storage battery, and a lithium ion capacitor, etc.

[0052] The embodiments of the present application will be described more specifically below with reference to lithium ion batteries as examples, in conjunction with specific examples and comparative examples. The listed parts, percentages, and ratios are all based on weight unless otherwise stated. The reagents, materials, and instruments used are commercially available unless otherwise specified.

[0053] Example 1-1

[0054] (I) Preparation of a lithium ion battery

[0055] <Preparation of a negative electrode sheet>

[0056] Method for preparing a silicon-carbon composite material

[0057] 1) Obtain the carbonized phenolic resin-based porous carbon (obtained by polymerization of phenol and formaldehyde, carbonization temperature 700°C), activate it with sodium hydroxide at an activation temperature of 760°C, activation time 1.7h, alkali / carbon ratio 3.2, to obtain a porous carbon skeleton (total pore volume 0.85g / cc, micropore volume ratio 6.0%;

[0058] 2) Place the porous carbon skeleton in a deposition reactor, carry out first-stage deposition and second-stage deposition, the first-stage deposition is carried out at a temperature of 580°C, with a volume content of 50% silane gas, the rest being inert gas, deposition time 5h;

[0059] The second-stage deposition is carried out at a temperature of 510°C, with a volume content of 50% silane gas, the rest being inert gas, deposition time 24h;

[0060] 3) After the deposition reaction is completed, pass pure inert gas for 60min, then pass 50% concentration acetylene gas, deposit at 600°C for 10h, to obtain the silicon-carbon composite material.

[0061] The preparation parameters of the silicon-carbon composite materials of Example 1-2 to Example 1-12 and Comparative Example 1 can be seen in Table 1-a, and the others are the same as those of Example 1-1. By adjusting the parameters in the preparation process, especially the activation temperature, activation time and alkali / carbon ratio, porous carbon skeletons with different parameters can be prepared. The parameters of the porous carbon skeletons in Example 1-1 to Example 1-12 and Comparative Example 1 can be seen in Table 1-b.

[0062] Mix 88.4% artificial graphite, 9.8% silicon-carbon composite material, 0.1% conductive agent carbon nanotube (CNT) and 1.7% binder PAA in solid mass ratio, add deionized water as solvent and stir uniformly, to prepare a negative electrode slurry with a solid content of 45wt%, uniformly coat the negative electrode slurry on one surface of a negative electrode current collector copper foil with a thickness of 6μm, dry the copper foil at 85°C for 4 hours, to obtain a negative electrode sheet with a single-sided coated negative electrode mixture layer. After cold pressing (cold pressing weight 20 tons), cutting and slitting, dry the negative electrode sheet at 120°C under vacuum conditions for 12 hours, to obtain a negative electrode sheet with a specification of 76.6mm x 875mm.

[0063] <Preparation of a positive electrode sheet>

[0064] The positive active material lithium cobaltate (LiCoO2), positive conductive agent acetylene black, and positive binder polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 95:2.5:2.5, N-methyl pyrrolidone (NMP) is added as a solvent and stirred uniformly to prepare a positive slurry with a solid content of 75 wt%. The positive slurry is uniformly coated on one surface of a positive current collector aluminum foil with a thickness of 9 μm, dried at 85°C to obtain a positive electrode sheet with a single-sided coated positive electrode mixture layer. After cold pressing (cold pressing weight of 20 tons), cutting, and slitting, the positive electrode sheet is dried at 85°C under vacuum for 4 hours to obtain a positive electrode sheet with a size of 74 mm x 867 mm.

[0065] <Preparation of electrolyte>

[0066] In a dry argon environment, LiPF6 is added to a solvent prepared by mixing propylene carbonate (PC), ethylene carbonate (EC), and diethyl carbonate (DEC) in a weight ratio of 1:1:1, and stirred uniformly. Then, fluoroethylene carbonate (FEC) is added and stirred uniformly to obtain an electrolyte. The mass percentage of LiPF6 is 12.5%, and the mass percentage of FEC is 4.5%.

[0067] <Separator>

[0068] A porous polyethylene film (provided by Celgard) with a thickness of 12 μm is used as a separator.

[0069] <Preparation of lithium ion battery>

[0070] The positive electrode and the negative electrode prepared above are connected with tabs, respectively, and stacked via a separator to obtain an electrode assembly. Then, the electrode assembly is accommodated in an aluminum plastic film packaging bag together with an electrolyte. The opening of the packaging bag is heat sealed, and a lithium ion battery is prepared through formation, capacity, and other steps. The test results are shown in Table 1-b.

[0071] (II) Test method

[0072] (1) The compaction density n g / cc is tested after being pressed at a pressure of 295.33 MPa for 30 s.

[0073] The test reference standard: GB / T 24533-2009 "lithium ion battery graphite negative material", the operation principle is that a certain mass of silicon-carbon composite material is weighed and added to the special mold for compaction (the mold diameter is known), and a metal disc is placed on the upper and lower hollows in the mold. The silicon-carbon composite material is placed between the metal discs, a metal cylinder is placed on the top, the mold is placed on the compaction density instrument, the required pressure is set, and the thickness of the silicon-carbon composite material under the corresponding pressure can be read on the device. The compaction density is calculated by the density formula p = m / V. The device model used in this test is Sansi Zongheng UTM7305, the pressurization and pressure maintaining time is 30s, the pressure relief and pressure maintaining time is 10s, and the compaction density is calculated according to the following formula: n = m / V = m / (S x H), wherein m represents the mass of the silicon-carbon composite material tested, S represents the surface area of the mold, and H is the height of the silicon-carbon composite material after pressure relief.

[0074] (2) After being pressed for 30s under a pressure of 295.33Mpa, the specific surface area f m 2 / g

[0075] The test reference standard: GB / T 24533-2009 "lithium ion battery graphite negative material", the operation principle is that a certain mass of silicon-carbon composite material is weighed and added to the special mold for compaction (the mold diameter is known), and a metal disc is placed on the upper and lower hollows in the mold. The silicon-carbon composite material is placed between the metal discs, a metal cylinder is placed on the top, the mold is placed on the compaction density instrument, the required pressure is set; the device model used in this test is Sansi Zongheng UTM7305, the pressurization and pressure maintaining time is 30s, the pressure relief and pressure maintaining time is 10s; the silicon-carbon composite material after pressure is taken out, and the specific surface area of the silicon-carbon composite material after pressure is tested by using the specific surface area tester.

[0076] (3) Test of the compaction density of the electrode sheet

[0077] A certain pressure is applied to the negative electrode sheet, and then the thickness of the negative electrode sheet is measured by using a micrometer. At the same time, the negative electrode sheet is cut into small discs with a size of 1540.25mm 2 , the mass of the small disc is weighed, multiple small discs can be taken for weighting and then the value is taken, and the mass of the single-sided negative material layer is obtained by deducting the mass of the copper foil divided by 2. According to the formula p = m0 / V, the compaction density of the electrode sheet under this pressure can be calculated, wherein m0 is the mass of the small disc with a size of 1540.25mm 2 , V is the mass of the small disc with a size of 1540.25mm 2 , and the thickness of the single-sided negative material layer obtained by deducting the thickness of the current collector from the small disc with a size of 1540.25mm 2 . After the electrode sheet is cold-pressed, the compaction density corresponding to the rupture of the silicon-carbon composite material particles in the negative electrode sheet is: consistent with the above-mentioned electrode sheet compaction density test method.

[0078] (4) Test of pore volume and pore distribution of porous carbon

[0079] The pore structure of the sample was tested by using a physical adsorption instrument (model: ipore 620). 0.15 g of the silicon-carbon composite material was taken in a sample tube, degassed at 200°C for 6 h, and then the adsorption amount of argon by the silicon-carbon composite material at different pressures was tested to draw the isothermal adsorption curve of the sample. The surface area, pore volume, and pore size distribution of the silicon-carbon composite material were calculated by using BET and NLDFT fitting. The total pore volume E was g / cc, and the pore volume ratio of ultramicropores e was %. In the pore volume and pore size distribution graph calculated by NLDFT fitting, the pore diameters corresponding to 10%, 50%, and 99% of the cumulative pore volume of the total pore volume were a, b, and c, respectively.

[0080] (5) Test of the compaction density m g / cc corresponding to the case where the silicon-carbon composite material particles in the pole piece are broken

[0081] The compaction density test method is consistent with the above-mentioned pole piece compaction density test method, except that the CP test is used to trace the case where the silicon-carbon composite material is broken in the pole piece. Specifically, the ZEISS SEM (Sigma-02-33) (0.1-30KV) equipment is used in the back scattering mode at a magnification of 1000K. If a crack with a longest diameter greater than 100 nm (the longest distance between two points on the edge of the crack is the longest diameter) is observed in the silicon-carbon composite material particles, it is judged that the silicon-carbon composite material particles are broken. At this time, the compaction density of the pole piece corresponding to the case where the particles in the pole piece are broken is the minimum anode pole piece compaction density corresponding to the case where the particles are broken.

[0082] (6) Cycle performance test

[0083] The test temperature was 25°C. The lithium ion battery was charged at 0.7C to 4.4V, charged at constant voltage to 0.025C, and discharged at 0.5C to 3.0V after standing for 5 minutes. The capacity obtained by this step is the initial capacity. The cycle test was performed at 0.7C charge / 0.5C discharge. The capacity decay curve was obtained by taking the ratio of the capacity of each step to the initial capacity. The number of cycles at which the capacity retention rate is 90% at 25°C is recorded as the room temperature cycle performance of the lithium ion battery. The thickness M of the lithium ion battery at the second half-charge (50% SOC) was tested by using a flat plate thickness gauge (load of 600g). After the lithium ion battery was cycled according to the above charge and discharge process to 500 cycles, the lithium ion battery was in a fully charged state (100% SOC). The thickness N of the lithium ion battery was tested again by using a flat plate thickness gauge (600g load). The expansion rate L of the lithium ion battery after 500 cycles at 25°C was (N-M) / M x 100%. The thickness of the lithium ion battery was tested as the cold thickness, i.e., the lithium ion battery was taken back and tested in a normal temperature test room.

[0084] Test temperature is 45℃, charge to 4.4V at 0.7C, constant voltage to 0.025C, rest for 5min, then discharge to 3.0V at 0.5C. The capacity obtained in this step is initial capacity, cycle test is carried out at 0.7C charge / 0.5C discharge, the capacity decay curve is obtained by comparing the capacity of each step with the initial capacity. The cycle number at which the capacity retention rate is 80% at 45℃ is recorded as the high temperature cycle performance of the lithium ion battery. The thickness P of the lithium ion battery at the second half charge (50% SOC) is tested by a flat plate thickness gauge (load is 600g), and when the lithium ion battery is cycled according to the above charge and discharge process to 500 cycles, the lithium ion battery is in a full charge state (100% SOC), and the thickness O of the lithium ion battery is tested again by a flat plate thickness gauge (600g load). The expansion rate Q of the lithium ion battery cycled at 45℃ to 500 cycles is (O-P) / P x 100%, wherein the tested thickness is cold thickness, that is, the lithium ion battery is taken back and tested in a normal temperature test room.

[0085] (7) Test method for expansion rate of lithium ion battery stored at 70℃ for 24h

[0086] Test pre-CAP-1 process: 1) test temperature is 25℃, 2) Rest 5min, 3) 0.2C D to 3V, 4) Rest 5min, 5) 0.2C C to 4.53V, CV to 0.02C---hot measurement-THK (700g PPG), at this time the thickness of the lithium ion battery is X, 6) Rest 5min, 7) adjust the test temperature to 70℃, store at this temperature for 24h------hot measurement-THK (700g PPG), at this time the thickness of the lithium ion battery is Y. The expansion rate of the lithium ion battery stored at 70℃ for 24h is represented as Z=(Y-X) / X x 100%.

[0087] (8) Test method for thermal shock test

[0088] Pre-treatment-1 CAP process: 1) test temperature is 20℃ (±5℃), 2) Rest 5min, 3) 0.7C C to 4.53V, CV to 0.05C, 4) Rest 5min. Test process: ① check the appearance before and after the test and take pictures, ② temperature sensing line position, ③ place the sample vertically in the box and heat to 135±2℃ at a temperature rising speed of 5±2℃ and keep for 60min, ④ measurement frequency: voltage and resistance measurement uses 1KHz specification, pre-treatment, post-treatment measurement, ⑤ judgment standard: no fire, no explosion. The temperature at which the lithium ion battery appears fire and explosion phenomenon is the lowest temperature of thermal shock test failure.

[0089] The preparation parameters of the lithium ion batteries of Example 1-2 to Example 1-12 and Comparative Example 1 are the same as those of Example 1-1 except for the differences listed in Table 1-a. The test results of each example and comparative example are shown in Table 1-b.

[0090] Table 1-a

[0091]

[0092] Table 1-b

[0093]

[0094]

[0095] Note: d = (c-a) / b represents the concentration of the pore size distribution of the porous carbon.

[0096] In combination with Table 1-a and Table 1-b, the pore structure of the porous carbon framework in the examples has a higher concentration, the overall structural strength of the material is better, and the lithium ion battery has better pressure resistance, cycle performance and high temperature performance. The pore size distribution concentration of the porous carbon framework in the comparative example is not suitable, and the cycle performance and high temperature performance of the lithium ion battery in the comparative example are significantly lower than those of the lithium ion battery in the examples.

[0097] In particular, when the porous carbon framework is further adjusted to satisfy 0.8 nm < a < 1.5 nm and 1.8 nm < b < 3.0 nm, the cycle performance and high temperature performance of the lithium ion battery are more optimal.

[0098] In particular, when the silicon-carbon composite material satisfies 2.0 < f < 5.0, 0.95 < n < 1.10 or m > 1.80, it indicates that the silicon-carbon composite material has better pressure resistance, and the lithium ion battery has better cycle performance and high temperature performance.

[0099] Examples 2-1 to 2-5 are based on the further adjustment of the activation parameters of Example 1-2 to change the proportion of the ultra-micropore pore volume e%. The specific operation can be referred to the conventional technical means, and the test results are shown in Table 2.

[0100] Table 2

[0101]

[0102] In combination with Table 2, when the proportion of the ultra-micropore pore volume e% < 9.0%, it is beneficial to further improve the pressure resistance of the silicon-carbon composite material, and the cycle performance and high temperature performance of the lithium ion battery are optimal.

[0103] The above merely provides preferred embodiments of the application, and is not used to limit the application. Any modification, equivalent replacement, and improvement made within the spirit and principle of the application shall fall within the protection scope of the application.

Claims

1. A silicon-carbon composite material, characterized in that, The silicon-carbon composite material includes a porous carbon skeleton and silicon material located within the pores of the porous carbon skeleton. The porous carbon framework satisfies: 1.5 < (ca) / b < 5.0; Where a represents the pore diameter corresponding to 10% of the total pore volume, b represents the pore diameter corresponding to 50% of the total pore volume, and c represents the pore diameter corresponding to 99% of the total pore volume. The units of a, b, and c are all nm.

2. The silicon-carbon composite material according to claim 1, characterized in that, The porous carbon framework satisfies at least one of the following conditions: (1) 0.8nm < a < 1.5nm; (2) 1.8nm < b < 3.0nm; (3) 5.0nm < c < 10.0nm.

3. The silicon-carbon composite material according to claim 2, characterized in that, The porous carbon framework satisfies at least one of the following conditions: (1) 2.0 ≤ (ca) / b ≤ 2.9; (2) 1.1nm ≤ a ≤ 1.4nm; (3) 2.2nm≤b≤2.7nm; (4) 5.7nm≤c≤6.5nm.

4. The silicon-carbon composite material according to any one of claims 1 to 3, characterized in that, The porous carbon framework includes a phenolic resin-based porous carbon framework, which also contains ultramicropores. Based on the total pore volume E g / cc of the porous carbon framework, the pore volume ratio of the ultramicropores is e%, satisfying: E ​​> 0.55, e < 9.0; The pore size of the ultramicropore is less than 0.7 nm.

5. The silicon-carbon composite material according to claim 4, characterized in that, 0.75≤E≤0.85, e≤4.

0.

6. The silicon-carbon composite material according to claim 4, characterized in that, The phenolic resin-based porous carbon framework is obtained by activating phenolic resin-based porous carbon in an alkaline medium. The activation conditions include: activation temperature of 750℃~800℃, activation time of 3h~3.5h, and alkali-to-carbon ratio of 3~3.

5.

7. The silicon-carbon composite material according to any one of claims 1 to 3, characterized in that, The silicon-carbon composite material satisfies at least one of the following conditions: (1) After being subjected to a pressure of 295.33 MPa for 30 seconds, the specific surface area of ​​the silicon-carbon composite material is fm. 2 / g, satisfying: 2.0 < f < 5.0; (2) After being subjected to a pressure of 295.33 MPa for 30 seconds, the compaction density of the silicon-carbon composite material is ng / cc, which satisfies: 0.95 < n < 1.10; (3) The silicon material includes silicon-carbon material and / or silicon-oxygen material.

8. A negative electrode sheet, characterized in that, 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 material layer includes a negative electrode material, which includes a silicon-carbon composite material as described in any one of claims 1 to 7.

9. The negative electrode sheet according to claim 8, characterized in that, The silicon-carbon composite material is added to the negative electrode material at an addition amount of less than 10 wt%, and the compaction density of the negative electrode material layer is >1.74 g / cc.

10. A method for preparing a silicon-carbon composite material, characterized in that, Includes the following steps: (1) Obtain the carbonized porous carbon and activate it with an alkaline medium at 750℃~800℃ for 1.5 h~2 h with an alkali-to-carbon ratio of 3~3.5 to obtain a porous carbon framework; (2) The porous carbon skeleton is placed in a deposition reactor for first-stage deposition and second-stage deposition. The first-stage deposition is carried out at a temperature of 520℃~600℃, with silane gas of 1%~100% by volume and inert gas of the other proportion, for a deposition time of 2 h~8 h. The second stage of deposition involves introducing silane gas with a volume content of 1% to 100% at a temperature of 470℃ to 520℃, with the remainder being inert gas, for a deposition time of 15 h to 30 h. (3) After the deposition reaction is completed, pure inert gas is introduced for 60 min, and then acetylene gas with a concentration of 5%~100% is introduced. Deposition is carried out at 550~600℃ for 10~15 h.

11. A secondary battery, characterized in that, The secondary battery comprises the silicon-carbon composite material according to any one of claims 1 to 7; or... The secondary battery comprises the negative electrode sheet as described in any one of claims 8 to 9; or... The secondary battery comprises a silicon-carbon composite material, which is prepared by the preparation method according to claim 10.

12. An electronic device, characterized in that, The electronic device includes the secondary battery as described in claim 11.

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