A nano-homogeneous silicon-carbon negative electrode material and its preparation method and application

Abstract

The present invention belongs to the field of lithium-ion battery negative electrodes, and discloses a silicon-carbon negative electrode material with a nano-homogeneous structure, comprising a hard carbon matrix, a soft carbon coating layer on the surface of the hard carbon matrix, nano-silicon and a lithium aluminate film wrapped on the surface of the nano-silicon distributed inside the hard carbon matrix. The silicon-carbon negative electrode material has high structural density and good stability. The present invention also discloses a method for preparing the silicon-carbon negative electrode material: first, a dispersion of white carbon black and lithium aluminum hydride is prepared, hard carbon materials are added to mix, dried to obtain a solid, sintered, impregnated, roasted, crushed, and shaped. The method has low cost and good safety. The lithium aluminate formed by reduction is wrapped on the surface of the nano-silicon, and the uniformity is improved, which further inhibits the expansion of silicon. It can also be used as an artificial SEI film to prevent the reduction and decomposition of the electrolyte, reduce lithium consumption, and improve the coulombic efficiency during the charge and discharge cycle. The application of the silicon-carbon negative electrode material in lithium batteries is also disclosed, which has a significant improvement in electrical performance.

Description

Technical Field

[0001] The present invention belongs to the field of lithium-ion battery negative electrodes, and in particular relates to a silicon-carbon negative electrode material with a nano-homogeneous structure, a preparation method thereof, and an application thereof. Background Art

[0002] Benefiting from robust demand for lithium-ion batteries in downstream sectors such as electric vehicles, energy storage, and power tools, anode materials are expected to continue experiencing rapid growth. Across various application areas, there is a strong demand for high-energy-density lithium-ion batteries, while the energy density of graphite-based anode materials has already reached its theoretical limit. Silicon-based materials, due to their higher energy density, are becoming the mainstream focus of next-generation anode material research and development.

[0003] However, the current lithium battery market is still dominated by graphite-based materials, and silicon-based materials have yet to achieve widespread adoption. This is due to the large expansion, poor cycling performance, and relatively low initial coulombic efficiency of silicon-based anode materials. To address these issues, silicon-based materials are currently primarily used in combination with carbon and graphite materials. Therefore, silicon-carbon composite anode materials, which combine the high conductivity and stability of carbon materials with the high capacity of silicon materials, have become a popular research area for silicon-based materials.

[0004] The mainstream silicon-carbon negative electrodes in the industry mainly adopt two technical routes. One is to mix nano-silicon, graphite and other carbon materials prepared by sand milling to form secondary particles, and then coat them to obtain core-shell structure silicon-carbon materials; the other is to deposit silicon on a carbon matrix by a gas phase method to obtain a porous structure silicon-carbon material. Both technical routes can produce silicon-carbon materials with excellent performance, but they have limitations to a certain extent. The former sand milling method makes it difficult to refine the nano-silicon particle size to below 100nm and it is difficult to disperse it evenly with graphite; the latter technical route is costly, has poor safety, and is difficult to apply on a large scale. At the same time, the two routes do not deal with the repeated breakage and formation of the SEI film due to the volume expansion of nano-silicon during the cycle, so the cycle stability needs to be strengthened. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the deficiencies and defects mentioned in the above background technology and provide a silicon-carbon negative electrode material with a nano-homogeneous structure and a preparation method and application thereof.

[0006] In order to solve the above technical problems, the technical solution proposed by the present invention is:

[0007] A silicon-carbon negative electrode material with a nano-homogeneous structure comprises a hard carbon matrix, a soft carbon coating layer on the surface of the hard carbon matrix, nano-silicon and a lithium aluminate film wrapped on the surface of the nano-silicon uniformly distributed inside the hard carbon matrix.

[0008] In the above-mentioned nano-homogeneous silicon-carbon negative electrode material, preferably, the mass ratio of the hard carbon matrix, the soft carbon coating layer, the nano-silicon, and the lithium aluminate film is (15-20):(5-7):(35-38):(39-42).

[0009] Preferably, the hard carbon matrix is high molecular polymer hard carbon, which is obtained by sintering an organic polymer at 1200-1400° C.; and the soft carbon coating layer is pitch soft carbon.

[0010] Based on a general inventive concept, the present invention also provides a method for preparing a silicon-carbon negative electrode material with a nano-homogeneous structure, comprising the following steps:

[0011] (1) dispersing white carbon black in a solvent, stirring, adding lithium aluminum hydride powder, and grinding to obtain a dispersion of white carbon black and lithium aluminum hydride;

[0012] (2) adding a hard carbon material to the dispersion of step (1), mixing the mixture uniformly, and drying the mixture to obtain a solid;

[0013] (3) The solid obtained in step (2) is placed in an atmosphere furnace for sintering, and then coated with a soft carbon material, roasted, crushed, and shaped to obtain the silicon-carbon negative electrode material with a nano-homogeneous structure.

[0014] The present invention reduces fine white carbon to form nano-silicon dispersed in a hard carbon matrix. At the same time, a high ionic conductivity film formed on the surface of the silicon particles can not only serve as a buffer medium for the volume expansion of silicon, but also serve as a stable artificial SEI to isolate the entry of electrolyte solvent molecules, thereby improving the electrochemical stability; at the same time, a soft carbon coating layer is coated on the surface of the hard carbon matrix, and the internal hard carbon has a large interlayer spacing, which is conducive to the rapid deintercalation of lithium ions, and has excellent rate performance and cycle performance. The soft carbon film formed on the surface is relatively denser and more complete, and can well fill the defective holes of the hard carbon, has good compatibility with the electrolyte, and reduces the irreversible lithium consumption during the first charge and discharge, so that the material has good cycle performance while ensuring the first coulomb efficiency; more importantly, unlike the conventional reduction route, this process route is controllable in operation, low in preparation cost, high in safety, and easy to achieve large-scale production.

[0015] In the above-mentioned preparation method, preferably, in step (1), the white carbon black includes any one or more of nano-silicon dioxide, fumed white carbon black, ultrafine silica gel and aerogel, and its primary particle size or porous diameter is below 100 nm; preferably below 50 nm. The white carbon black at this particle size has a large specific surface area, high activity, and is easy to undergo reduction reaction. At the same time, the size of the silicon crystal generated by the reaction can be controlled to be below 50 nm, which is smaller than the nano-silicon crystals on the market, has lower cost, and has advantages in electrochemical performance; the solvent is an alcohol, hydrocarbon or fatty acid solvent, preferably an oxygen-free hydrocarbon solvent, specifically including any one or more of octane, isopropanol, and dodecanol.

[0016] Preferably, in step (1), the molar ratio of white carbon black to lithium aluminum hydride is 1:(1-1.2); the grinding method is ball milling or sand milling, and the humidity condition during grinding is controlled below 10%.

[0017] Preferably, in step (2), the hard carbon material is an organic polymer, preferably one or more of phenolic resin, epoxy resin, and acrylic resin, and a corresponding curing agent can be added as needed; the weight ratio of the white carbon black to the hard carbon material is (1 to 1.5):1.

[0018] Preferably, in step (2), the drying temperature is 100-200°C, the drying time is 24-48 hours, and the drying equipment is a vacuum dryer, a double cone dryer, or a rake dryer. The drying process is a process of solvent volatilization, polymer cross-linking, and solidification.

[0019] Preferably, in step (3), the sintering temperature is 1200-1400°C, the heating rate is 1-20°C / min, the holding time is 4-10h, and the sintering atmosphere is N2, Ar or a reducing gas, preferably a reducing gas, wherein the reducing gas is CO or H2; the sintering process is a chemical reaction process in which the polymer is decomposed and carbonized, the white carbon black is reduced by lithium aluminum hydride, and the white carbon black is reduced by hydrogen; the specific reduction reaction equation is as follows: LiAlH4+SiO2=LiAlO2+Si+2H2↑, 2H2+SiO2=Si+2H2O;

[0020] The soft carbon material is impregnated with pitch, and the specific operation of coating the soft carbon material is as follows: the sintered material obtained after sintering is placed in a vacuum pressure tank, impregnated with pitch, heated at a temperature of 50 to 150° C., a pressure of 2 to 5 MPa, and a heating time of 2 to 6 hours; the impregnation is to fill the pores formed by the escape of volatiles during the sintering process, thereby improving the density of the material;

[0021] The calcination temperature is 700-1000° C., the holding time is 2-5 hours, and the calcination atmosphere is N 2 or Ar.

[0022] Preferably, in step (3), the pulverization method is mechanical grinding or air flow grinding, and the median particle size of the material after pulverization is 6 to 15 μm, preferably 6 to 10 μm; the shaping is performed by spheroidization in a pulverizing device with a shaping function, and the median particle size of the material after shaping is 5 to 9 μm.

[0023] Based on a general inventive concept, the present invention also provides an application of a nano-homogeneous silicon-carbon negative electrode material in a lithium battery.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] 1. The silicon-carbon negative electrode material of the present invention is formed by converting fine white carbon into nano-silicon under the action of a strong reducing agent and uniformly dispersing it in a carbon matrix to form a nano-homogeneous structure. The fine-particle nano-silicon has a more stable structure to maintain the charge and discharge cycle; the lithium aluminate high ion conductivity film formed on the surface of the silicon particles can not only serve as a buffer medium for the volume expansion of silicon, but also serve as a stable artificial SEI to isolate the electrolyte solvent molecules from entering, thereby improving the electrochemical stability; at the same time, the hard carbon structure of the hard carbon matrix formed by the high molecular polymer not only has good cycle stability, but also provides a good buffer for the expansion caused by silicon embedded in lithium; the soft carbon coating layer is coated on the surface of the hard carbon matrix, and the soft carbon film is relatively denser and more complete, which can well fill the defective holes of the hard carbon, has good compatibility with the electrolyte, and reduces the irreversible lithium consumption during the first charge and discharge; in general, the silicon-carbon negative electrode material of the present invention has high structural density and good stability, and has a significant improvement in electrical performance after being applied to lithium batteries.

[0026] 2. Unlike the conventional magnesium thermal reduction method, the technical route of the preparation method of the present invention does not require acid washing to remove excess impurities, has low preparation cost and good safety. The lithium aluminate formed by the reduction method is wrapped on the surface of nano-silicon, which improves the uniformity and further inhibits the expansion of silicon. More importantly, lithium aluminate is a high ionic conductor at room temperature and can be used as an artificial SEI film to prevent the reduction and decomposition of the electrolyte, reduce lithium consumption, and improve the coulombic efficiency during the charge and discharge cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0028] Figure 1 Schematic diagram of the structure of the silicon-carbon negative electrode material of the present invention;

[0029] Figure 2This is a metallographic electron microscope image of the material of the present invention;

[0030] Figure 3 It is the XRD test result of the material of the present invention. DETAILED DESCRIPTION

[0031] To facilitate understanding of the present invention, the present invention will be described in more comprehensive and detailed form below in conjunction with the accompanying drawings and preferred embodiments. However, the protection scope of the present invention is not limited to the following specific embodiments.

[0032] Unless otherwise defined, all technical terms used hereinafter have the same meanings as those generally understood by those skilled in the art. The technical terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0033] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods.

[0034] Example 1:

[0035] A nano-homogeneous silicon-carbon negative electrode material of the present invention includes a hard carbon matrix (polymer hard carbon), a soft carbon coating layer (asphalt soft carbon) wrapped on the surface of the hard carbon matrix, nano-silicon and a lithium aluminate film wrapped on the surface of the nano-silicon distributed inside the hard carbon matrix, and the mass ratio of the hard carbon matrix, the soft carbon coating layer, the nano-silicon and the lithium aluminate film is 19:5:35:41.

[0036] The preparation method of the silicon-carbon negative electrode material comprises the following steps:

[0037] (1) Weigh 60 g of fumed silica (primary particle size below 100 nm) in 300 g of octane, stir ultrasonically for 30 min, add 38 g of lithium aluminum hydride powder, and grind in a sand mill for 1 h. During grinding, the humidity is controlled below 10% to obtain a silica / lithium aluminum hydride dispersion;

[0038] (2) Add 50 g of phenolic resin and 10 g of NL curing agent to the dispersion and stir for 2 h; place the mixture in a double cone dryer and dry at 150° C. for 36 h to obtain a dry solid;

[0039] (3) The dried solid was sintered at 1200°C for 4 h in an Ar atmosphere with a heating rate of 20°C / min; the sintered material was placed in a vacuum pressure tank and impregnated in asphalt at a pressure of 3 MPa for 4 h, then taken out and calcined at 800°C for 2 h in a N2 atmosphere; finally, the calcined material was mechanically ground (the median particle size of the material after crushing was 6-15 μm) and spheroidized (the median particle size of the material after shaping was 5-9 μm) in turn to obtain a silicon-carbon negative electrode material.

[0040] The structure of the silicon-carbon negative electrode material obtained in this embodiment is shown in FIG. Figure 1 and metallographic electron microscope Figure 2 As shown, the nano-composite particles of nano-silicon crystals and lithium aluminate (lithium aluminate film wrapped on the surface of nano-silicon crystals) generated by the reaction are uniformly dispersed in the carbon matrix, and the outer layer is wrapped by a layer of soft carbon. This dense structure not only prevents the electrolyte from contacting with the internal silicon to produce side reactions, but also inhibits the volume expansion of silicon in the material during charging and discharging from many aspects. At the same time, it has good ionic conductivity and excellent electrochemical stability.

[0041] Figure 3 This is the XRD result of the material of this embodiment. It can be seen from the figure that the material is a coexistence of silicon and lithium aluminate phases, and lithium aluminate plays an important role in improving the performance of the material.

[0042] Example 2:

[0043] A nano-homogeneous silicon-carbon negative electrode material of the present invention includes a hard carbon matrix (polymer hard carbon), a soft carbon coating layer (asphalt soft carbon) wrapped on the surface of the hard carbon matrix, nano-silicon and a lithium aluminate film wrapped on the surface of the nano-silicon distributed inside the hard carbon matrix, and the mass ratio of the hard carbon matrix, the soft carbon coating layer, the nano-silicon and the lithium aluminate film is 20:5:35:40.

[0044] The preparation method of the silicon-carbon negative electrode material comprises the following steps:

[0045] (1) Weigh 100 g of nano-silica (primary particle size less than 100 nm) in 500 g of isopropyl alcohol, stir ultrasonically for 30 min, add 60 g of lithium aluminum hydride powder, and grind in a sand mill for 1.5 h. During grinding, the humidity is controlled below 10% to obtain a silica / lithium aluminum hydride dispersion.

[0046] (2) adding 100 g of acrylic resin and stirring for 3 h; placing the mixture into a rake dryer and drying at 100 ° C for 36 h to obtain a dry solid;

[0047] (3) The dried solid was sintered at 1300°C for 2 h in a CO atmosphere with a heating rate of 20°C / min; the sintered material was placed in a vacuum pressure tank and impregnated in asphalt at a pressure of 5 MPa for 2 h, then taken out and calcined at 1000°C for 4 h in a N2 atmosphere; finally, the calcined material was mechanically ground (the median particle size of the material after crushing was 6-15 μm) and spheroidized (the median particle size of the material after shaping was 5-9 μm) in turn to obtain a silicon-carbon negative electrode material.

[0048] Example 3:

[0049] A nano-homogeneous silicon-carbon negative electrode material of the present invention includes a hard carbon matrix (polymer hard carbon), a soft carbon coating layer (asphalt soft carbon) wrapped on the surface of the hard carbon matrix, nano-silicon and a lithium aluminate film wrapped on the surface of the nano-silicon distributed inside the hard carbon matrix, and the mass ratio of the hard carbon matrix, the soft carbon coating layer, the nano-silicon and the lithium aluminate film is 16:7:36:42.

[0050] The preparation method of the silicon-carbon negative electrode material comprises the following steps:

[0051] (1) Weigh 120 g of fumed silica (primary particle size below 100 nm) in 600 g of dodecanol, stir ultrasonically for 30 min, add 150 g of lithium aluminum hydride powder, and grind in a sand mill for 2 h. During grinding, the humidity is controlled below 10% to obtain a silica / lithium aluminum hydride dispersion.

[0052] (2) adding 100 g of thermosetting phenolic acid resin and stirring for 3 h; placing the above mixture into a double cone dryer and drying at 120° C. for 48 h to obtain a dry solid;

[0053] (3) The dried solid was sintered at 1000°C for 5 h in a H2 atmosphere with a heating rate of 20°C / min; the sintered material was placed in a vacuum pressure tank and impregnated in asphalt at a pressure of 4 MPa for 2 h, then taken out and calcined at 1000°C for 2 h in an Ar atmosphere; finally, the calcined material was subjected to mechanical grinding (the median particle size of the material after crushing was 6-15 μm) and spheroidization (the median particle size of the material after shaping was 5-9 μm) in turn to obtain a silicon-carbon negative electrode material.

[0054] Comparative Example 1:

[0055] A method for preparing a silicon-carbon negative electrode material comprises the following steps:

[0056] (1) Weigh 60 g of nano-silicon powder and 90 g of flake graphite in 500 g of ethanol, ultrasonically disperse for 30 min, add 15 g of phenolic resin, and mix for 2 h to obtain a silicon / graphite slurry; dry the slurry at 100° C. under vacuum to obtain a silicon / graphite powder;

[0057] (2) The dry powder is molded into a mold to obtain a silicon / graphite blank; the blank is placed in a sintering furnace, kept at 700°C for 2 hours under an argon atmosphere, and naturally cooled to room temperature before being taken out to obtain a silicon / graphite sintered blank; the silicon / graphite sintered blank is crushed and shaped to obtain a silicon-carbon negative electrode material.

[0058] Comparative Example 2:

[0059] A method for preparing a silicon-carbon negative electrode material comprises the following steps:

[0060] (1) 60 g of nano-silica, 42 g of graphite powder, and 10 g of polyvinyl pyrrolidone were weighed and dissolved in 300 g of ethanol, and the mixture was ball-milled to obtain a homogenous mixture, which was then dried at 100° C. to obtain a silica / carbon composite material;

[0061] (2) The composite material is crushed and mixed with magnesium powder to undergo a magnesium thermal reduction reaction at 800°C for 4 hours; the product is collected and acid-washed and water-washed 3-5 times, and then dried to obtain a silicon-carbon negative electrode material.

[0062] Comparative Example 3:

[0063] A method for preparing a silicon-carbon negative electrode material comprises the following steps:

[0064] (1) 80 g of silica, 45 g of lithium aluminum hydride, and 70 g of resin were weighed and mixed for 30 min to obtain a silica / lithium aluminum hydride / resin mixture; the mixture was sintered at 1100 °C for 6 h in an Ar atmosphere;

[0065] (2) The sintered material was immersed in impregnation asphalt at a pressure of 3 MPa for 2 h, and then taken out and calcined at 1000 ° C for 3 h in a N2 atmosphere; finally, the calcined material was mechanically ground and spheroidized in turn to obtain a silicon-carbon negative electrode material.

[0066] Comparative Example 4:

[0067] A method for preparing a silicon-carbon negative electrode material comprises the following steps:

[0068] The phenolic resin and curing agent in step (2) of Example 1 were replaced with asphalt, and the other steps remained unchanged to obtain a silicon-carbon negative electrode material.

[0069] Comparative Example 5:

[0070] A method for preparing a silicon-carbon negative electrode material comprises the following steps:

[0071] The impregnating asphalt in step (3) of Example 1 was replaced with phenolic resin, and the remaining steps remained unchanged to obtain a silicon-carbon negative electrode material.

[0072] The silicon-carbon negative electrode materials prepared in the above examples and comparative examples were mixed with conductive carbon black and LA133 binder at a mass ratio of 7:2:1 to form a slurry. The slurry was then coated onto copper foil, vacuum-dried, and roller-pressed to form the negative electrode. A ternary electrode sheet was used as the positive electrode. CR2430 button-type batteries were assembled in the following order: positive electrode shell, positive electrode, electrolyte, separator, electrolyte, negative electrode, nickel foam, and negative electrode shell. After 24 hours of rest, the batteries were charged and discharged at a constant current of 0.2C, with the voltage limited to 0-1.5V.

[0073] The electrochemical test results of the negative electrode materials prepared in various embodiments and comparative examples are shown in Table 1.

[0074] Table 1: Electrochemical test results of various materials

[0075]

[0076] As shown in the table above, compared with the negative electrode materials prepared by different methods in comparative examples 1-3, the negative electrode materials prepared in the embodiment have obvious advantages in all aspects of performance, indicating that the structural design of this method has a great improvement on the first efficiency and stability of the material; at the same time, it can be seen from comparative examples 4-5 that the hard carbon matrix has better cycle performance for the material than the soft carbon matrix, and the soft carbon coating has higher first efficiency for the material than the hard carbon coating, so the material performance prepared by combining the hard carbon matrix and the soft carbon coating is the best.

Claims

1. A method for preparing a nano-homogeneous silicon-carbon negative electrode material, characterized in that: The invention comprises a hard carbon matrix, a soft carbon coating layer is provided on the surface of the hard carbon matrix, nano-silicon and a lithium aluminate film wrapped on the surface of the nano-silicon are distributed inside the hard carbon matrix, and the preparation method comprises the following steps: (1) dispersing white carbon black in a solvent, stirring, adding lithium aluminum hydride powder, and grinding to obtain a dispersion of white carbon black and lithium aluminum hydride; (2) adding a hard carbon material to the dispersion in step (1), mixing the mixture evenly, and drying the mixture to obtain a solid; (3) The solid obtained in step (2) is placed in an atmosphere furnace for sintering, and then coated with a soft carbon material, roasted, crushed, and shaped to obtain the silicon-carbon negative electrode material with a nano-homogeneous structure.

2. The preparation method according to claim 1, characterized in that The mass ratio of the hard carbon matrix, the soft carbon coating layer, the nano-silicon and the lithium aluminate film is (15-20): (5-7): (35-38): (39-42).

3. The preparation method according to claim 1 or 2, characterized in that The hard carbon matrix is high molecular polymer hard carbon, which is obtained by sintering an organic polymer at 1200-1400° C.; the soft carbon coating layer is pitch soft carbon.

4. The preparation method according to claim 1, characterized in that In step (1), the white carbon black includes any one or more of nano-silica, fumed white carbon black, ultrafine silica gel and aerogel, and its primary particle size or porous diameter is below 100 nm; the solvent includes any one or more of octane, isopropyl alcohol and dodecanol.

5. The preparation method according to claim 1, characterized in that In step (1), the molar ratio of white carbon black to lithium aluminum hydride is 1:(1-1.2); the grinding method is ball milling or sand milling, and the humidity condition during grinding is controlled below 10%.

6. The preparation method according to claim 1, characterized in that In step (2), the hard carbon material is an organic polymer, including any one or more of phenolic resin, epoxy resin, and acrylic resin; and the weight ratio of the white carbon black to the hard carbon material is (1-1.5):

1.

7. The preparation method according to claim 1, characterized in that In step (2), the drying temperature is 100-200° C., and the drying time is 24-48 hours; the drying equipment is a vacuum dryer, a double-cone dryer, or a rake dryer.

8. The preparation method according to any one of claims 1 to 7, characterized in that In step (3), the sintering temperature is 1200-1400°C, the heating rate is 1-20°C / min, the holding time is 4-10h, and the sintering atmosphere is N2, Ar, CO or H2; The soft carbon material is impregnated with pitch, and the specific operation of coating the soft carbon material is as follows: the sintered material obtained after sintering is put into a vacuum pressure tank, impregnated with pitch, heated at a temperature of 50-150° C., a pressure of 2-5 MPa, and a time of 2-6 hours; The calcination temperature is 700-1000°C, the holding time is 2-5h, and the calcination atmosphere is N2 or Ar; The pulverization method is mechanical grinding or air flow grinding, and the median particle size of the material after pulverization is 6-15 μm; The shaping is carried out in a pulverizing device with a shaping function to achieve spheroidization, and the median particle size of the material after shaping is 5-9 μm.

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