Negative electrode material for lithium secondary battery and method of manufacturing same

Abstract

The negative electrode material for a lithium secondary battery of the present invention comprises a silicon-carbon composite including: silicon nanoparticles; crystalline carbon; carbon nanotubes; and first amorphous carbon, wherein the silicon nanoparticles, the crystalline carbon, and the carbon nanotubes are dispersed on the first amorphous carbon, and the minimum pore size inside the composite is more than 10 nm and 30 nm or less.

Description

Anode material for lithium secondary batteries and method for manufacturing the same

[0001] The present invention relates to a negative electrode material for a lithium secondary battery and a method for manufacturing the same.

[0002] Lithium secondary batteries typically consist of a positive electrode containing a positive electrode material, a negative electrode containing an anode material, a separator, and an electrolyte. Charging and discharging occur through the intercalation and deintercalation of lithium ions. Lithium secondary batteries boast high energy density, high electromotive force, and high capacity, making them widely used in various fields, including mobile devices, electric vehicles, and hybrid electric vehicles.

[0003] Metal oxides such as LiCoO2, LiMnO2, LiMn2O4 or LiCrO2 are used as cathode materials constituting the cathode of a lithium secondary battery, and metal materials such as metal lithium, carbon-based materials such as graphite or activated carbon and silicon oxide (SiOx) are used as anode materials constituting the anode.

[0004] Various types of carbon-based anode materials are used, including crystalline carbon-based materials such as natural graphite and artificial graphite, and amorphous carbon-based materials such as hard carbon and soft carbon. Among these, graphite-based anode materials, which have excellent reversibility and are advantageous for the lifespan characteristics of lithium secondary batteries, are the most widely used.

[0005] However, graphite-based anode materials have relatively low theoretical capacity values ​​(e.g., approximately 372 mAh / g for LiC6 anodes), which is still somewhat insufficient to satisfy the electrochemical characteristics required in the relevant market.

[0006] Therefore, many researchers are interested in group IV elements (Si, Ge, Sn) in the periodic table, and among them, silicon in particular has a very high theoretical capacity (Li 15 Si4 (Si: 3600 mAh / g) and low operating voltage (~0.1 V vs. Li / Li+) have attracted much attention as a very attractive material. However, during charge / discharge, silicon undergoes significant volume expansion and contraction due to its reaction with lithium, which can result in fine particle size loss of the silicon active material powder and poor electrical contact between the silicon active material powder and the current collector. Due to this phenomenon, lithium secondary batteries containing silicon have the problem that their capacity can rapidly decrease as the charge / discharge cycle progresses.

[0007] One aspect of the present invention is to provide a negative electrode material for a lithium secondary battery capable of securing high capacity characteristics and excellent lifespan characteristics, and a method for manufacturing the same.

[0008]

[0009] The objectives of the present invention are not limited to the above-described scope. Those skilled in the art will have no difficulty understanding the additional objectives of the present invention from the overall content of this specification.

[0010] According to one aspect of the present invention, a negative electrode material for a lithium secondary battery may include a silicon-carbon composite including silicon nanoparticles; crystalline carbon; carbon nanotubes; and first amorphous carbon, wherein the silicon nanoparticles, the crystalline carbon, and the carbon nanotubes may be dispersed on the first amorphous carbon, and the minimum pore size within the composite may be greater than 10 nm and less than or equal to 30 nm.

[0011] The above-described crystalline carbon may be at least one of artificial graphite, flake graphite, earth-like graphite, CNT, and graphene.

[0012] The above-described carbon nanotubes may be included in an amount of 0.10 wt% or more and 0.60 wt% or less based on the total weight of the above-described negative electrode material.

[0013] The specific surface area of ​​the carbon nanotubes described above is 500 m 2 / g or more than 800m 2 / g can be less.

[0014] The bulk density of the carbon nanotube described above may be 0.04 g / cc or more and 0.09 g / cc or less.

[0015] The first amorphous carbon described above may be a carbonized pitch having a fixed carbon ratio of 30% or more and a beta-resin value of 15% or more.

[0016]

[0017] *19 The negative electrode material for the lithium secondary battery described above may have the above-described silicon-carbon composite as a core and additionally include a second amorphous carbon layer covering the surface of the above-described silicon-carbon composite.

[0018] The above-mentioned negative electrode material for lithium secondary batteries has a specific surface area of ​​6 m 2 / g may be less.

[0019] According to another aspect of the present invention, a method for manufacturing a negative electrode material for a lithium secondary battery may include the steps of preparing silicon nanoparticles; adding crystalline carbon, carbon nanotubes, and pitch to the silicon nanoparticles and then mixing them to obtain a mixture; press-molding the mixture to obtain a molded body; carbonizing the molded body; and crushing and classifying the molded body. The press-molding may be performed at a pressure of 0.40 ton / cm. 2 More than 0.90 ton / cm 2 It can be done with pressure below.

[0020] The above-described crystalline carbon may be at least one of artificial graphite, flake graphite, earth-like graphite, CNT, and graphene.

[0021] The above-mentioned pitch may have a fixed carbon ratio of 30% or more and a beta-resin value of 15% or more.

[0022] The above-described mixing can be performed through a milling process via a contact medium with a powder.

[0023] The above-described grinding may be a dry grinding process.

[0024] The method for manufacturing the above-described negative electrode material for a lithium secondary battery may additionally include a step of coating carbon on the surface of the manufactured negative electrode material.

[0025] The present invention can provide a cathode material that can suppress expansion of silicon nanoparticles and drastically reduce the specific surface area by forming a solid, high-density carbon layer.

[0026] As a result, the negative electrode material obtained from the negative electrode material of the present invention can secure high capacity characteristics and excellent life characteristics.

[0027] Figure 1 shows FIB-SEM photographs of cross-sections of each comparative example and example according to uniaxial compression molding conditions.

[0028] Figure 2 is a graph showing the particle strength of each comparative example and example.

[0029] Figure 3 is a graph showing the results of changes in specific surface area and pore distribution of each comparative example and example.

[0030] Hereinafter, preferred embodiments of the present invention will be described. However, the embodiments of the present invention may be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below.

[0031] In this specification, the term "including" is used to indicate that other components may be included rather than excluding other components unless specifically stated to the contrary.

[0032] Additionally, unless otherwise specifically provided in the specification of the present invention, the % unit means weight%.

[0033] Although not essential, it should be noted that the technical solutions according to each aspect of the present invention can also be usefully applied to other aspects of the present invention. Furthermore, the compositions and various useful parameters according to each aspect of the present invention can be appropriately combined with other aspects to achieve beneficial effects.

[0034] Since silicon undergoes significant volume expansion and contraction due to a reaction with lithium during charge and discharge, lithium secondary batteries containing silicon have the problem that their capacity may rapidly decrease as charge and discharge cycles progress.

[0035] To solve the above-described problems, the inventors of the present invention found that when a solid, high-density carbon layer is formed, not only can the contact path between silicon nanoparticles and a conductive material be strengthened, but also the expansion of the silicon nanoparticles can be controlled.

[0036] In addition, the inventors of the present invention have found that, as described above, controlling the size of the minimum pores within the silicon-carbon composite is effective as a method for forming a strong, high-density carbon layer.

[0037] In addition, the inventors of the present invention discovered that when carbon nanotubes are included in the negative electrode material, the high capacity characteristics of the negative electrode material can be maintained while improving the life characteristics of the negative electrode material, and thus derived the present invention.

[0038] From this perspective, a negative electrode material for a lithium secondary battery according to one embodiment of the present invention comprises a silicon-carbon composite including silicon nanoparticles; crystalline carbon; carbon nanotubes; and first amorphous carbon, wherein the silicon nanoparticles, the crystalline carbon, and the carbon nanotubes can be dispersed on the first amorphous carbon. Each component is described in detail below.

[0039] A silicon-carbon composite according to an example of the present invention may include silicon nanoparticles.

[0040] As a non-limiting example, the D50 particle size of the silicon nanoparticles may be less than 150 nm. In this case, D50 in the present specification may mean a particle size corresponding to 50% of the volume accumulation of the particle size distribution. Accordingly, reversible charge-discharge can be performed without cracking by suppressing the expansion of the silicon nanoparticles in the negative electrode material according to an example of the present invention. On the other hand, if the D50 particle size of the silicon nanoparticles is excessively small, the number of silicon nanoparticles included in the negative electrode material may increase excessively, thereby increasing the specific surface area, making it difficult to capture by the first amorphous carbon described below. In addition, in a non-limiting example of the present invention, the lower limit of the D50 particle size of the silicon nanoparticles may be 100 nm for the purpose of securing high-capacity characteristics of the negative electrode material.

[0041] According to one embodiment of the present invention, a negative electrode material for a lithium secondary battery can secure high capacity characteristics of the negative electrode material by containing 45 wt% or more of the silicon nanoparticles. More specifically, the content of the silicon nanoparticles may be 50 wt% or more. On the other hand, if the silicon nanoparticles are excessively included in the negative electrode material, the silicon nanoparticles and the crystalline carbon described below cannot be completely surrounded by the carbon coating layer, either individually or as a single body, and in this case, the structure of the negative electrode material, which is a silicon-carbon composite, may result in collapse. For this reason, one embodiment of the present invention may contain 60 wt% or less of the silicon nanoparticles based on the total weight of the negative electrode material for a lithium secondary battery. More specifically, the content of the silicon nanoparticles may be 55 wt% or less.

[0042] A silicon-carbon composite according to an example of the present invention may include, in addition to the silicon nanoparticles described above, crystalline carbon. Since the conductivity of the anode material can be improved through the crystalline carbon, the anode material according to an example of the present invention can increase cycling efficiency during repeated charge-discharge cycles by including the crystalline carbon.

[0043] As an example, the crystalline carbon may be at least one of artificial graphite, flake graphite, earthy graphite, expanded graphite, and graphene.

[0044] For the above-described purpose, one example of the present invention may include 10 wt% or more of the crystalline carbon based on the total weight of the negative electrode material for a lithium secondary battery. More specifically, the crystalline carbon may be included in an amount of 15 wt% or more based on the total weight of the negative electrode material. However, if the amount becomes excessive, capturing by the first amorphous carbon may become difficult, and therefore, one embodiment of the present invention may set the weight ratio of the crystalline carbon to 20 wt% or less based on the total weight of the negative electrode material.

[0045] A silicon-carbon composite according to one embodiment of the present invention may include carbon nanotubes. Such carbon nanotubes have excellent electrical and mechanical properties, which can help improve the life characteristics of the anode material. To achieve the above-described purpose, a non-limiting example of the present invention may include the carbon nanotubes in an amount of 0.10 wt% or more based on the total weight of the anode material. In another example, the carbon nanotubes may be included in an amount of 0.20 wt% or more, and in another example, the carbon nanotubes may be included in an amount of 0.30 wt% or more. However, if the carbon nanotubes are included in an excessive amount in the anode material, the carbon nanotubes that are not distributed in the first amorphous carbon described below may undergo a self-reaction during the cycle, which may negatively affect the life of the anode material. In consideration of this, an example of the present invention may set the upper limit of the content of the carbon nanotubes to 0.60%. As another example, the carbon nanotubes may be included in an amount of 0.50 wt% or less.

[0046] In addition, the specific surface area of ​​the above carbon nanotube is 500 m 2 / g or more than 800m 2 / g or less. That is, one example of the present invention is a Si-C particle having a specific surface area of ​​500 m to enhance conductivity and improve structural stability of the particle. 2 / g or more carbon nanotubes can be included in the negative electrode material. On the other hand, if the specific surface area of ​​the carbon nanotubes becomes excessively large, an excessive pitch is required to capture the carbon nanotubes, which may result in a deterioration in the performance of the negative electrode material. In addition, the excess carbon nanotubes that are not captured may increase the specific surface area of ​​the Si-C particles themselves, which may cause side reactions with the electrolyte, resulting in a deterioration in performance. Therefore, one example of the present invention is to set the upper limit of the specific surface area to 800 m 2 / g can be used. As another example, the specific surface area is 600 m 2 / g to 700m 2 / g or 650m 2 / g to 680m 2 / g may be.

[0047] For example, the bulk density of the carbon nanotubes may be 0.04 g / cc or more and 0.09 g / cc or less. That is, in one example of the present invention, carbon nanotubes having a bulk density of 0.04 g / cc or more may be included in the negative electrode material in order to enhance conductivity within the Si-C particles and improve the structural stability of the particles. On the other hand, if the bulk density of the carbon nanotubes is excessively large, an excessive amount of pitch may be required to capture the carbon nanotubes, which may result in a deterioration in the performance of the negative electrode material. In addition, if the bulk density is excessively high, the weight of the carbon nanotubes to be dispersed increases, and the resulting decrease in the silicon content may make it difficult to achieve the desired high capacity. Therefore, in one example of the present invention, the upper limit of the bulk density may be 0.09 g / cc. As another example, the bulk density may be from 0.05 g / cc to 0.08 g / cc or from 0.06 g / cc to 0.07 g / cc.

[0048] Meanwhile, a silicon-carbon composite according to an example of the present invention may include a first amorphous carbon, and the silicon nanoparticles, the crystalline carbon, and the carbon nanotubes may be dispersed on the first amorphous carbon. Accordingly, the first amorphous carbon may control the expansion of the silicon nanoparticles while reinforcing the contact path between the silicon nanoparticles, the crystalline carbon, and the carbon nanotubes.

[0049] In addition, the first amorphous carbon according to one embodiment of the present invention may be a carbonized pitch having a fixed carbon ratio of 30% or more. In this case, the pitch may include coal-based pitch and petroleum-based pitch.

[0050] As the fixed carbon value of the above pitch increases, a conductive path with silicon nanoparticles having low intrinsic conductivity can be created, leading to increased capacity and efficiency. In addition, when the fixed carbon value satisfies the above range, the internal pores of the negative electrode material of the present embodiment can be reduced. Accordingly, side reactions with the electrolyte can also be reduced, thereby contributing to increased initial efficiency of the battery. To this end, in one example of the present invention, the lower limit of the fixed carbon ratio of the first amorphous carbon can be set to 30%, and in another example, the lower limit of the fixed carbon ratio can be 40% or 50%.

[0051] In addition, the first amorphous carbon according to one embodiment of the present invention may be a carbonized pitch having a β-resin value of 15% or more.

[0052] Specifically, the β-resin value refers to the value obtained by subtracting the quinoline capacity (benzeneinsoluble) from the benzene-insoluble value. This β-resin value is proportional to the cohesion. According to an embodiment of the present invention, when coal-based pitch or petroleum-based pitch having a β-resin value satisfying the above range is used, the porous structure of the silicon-carbon composite can be more stably maintained. Accordingly, an embodiment of the present invention can implement a lithium secondary battery having excellent life characteristics and electrode expansion characteristics. As another example, the β-resin value of the pitch may be 20% or more.

[0053] In addition, the silicon-carbon composite according to an example of the present invention may have a minimum size of pores within the composite exceeding 10 nm. At this time, the minimum size of the pores can be obtained using a nitrogen adsorption method, and may refer to the size of the pores initially measured in DFT porosity analysis. The anode material for a lithium secondary battery according to an example of the present invention can provide a dense carbon layer without micropores by making the minimum size of the pores within the composite described above equal to or greater than a certain level. Thereby, the expansion of silicon nanoparticles can be controlled, and as a result, the anode material according to an embodiment of the present invention can exhibit high-capacity characteristics and excellent cycle-life characteristics.

[0054] On the other hand, if the minimum size of the pores becomes excessively large, a deterioration in the lifespan, etc. may occur due to a side reaction between the exposed nano-silicon and the electrolyte. Therefore, the minimum size of the pores inside the composite according to a non-limiting example of the present invention may be 30 nm or less. In another example, the minimum size of the pores inside the composite may be 12 nm to 25 nm, and in another example, 15 nm to 20 nm.

[0055] Meanwhile, according to another example of the present invention, as described above, a silicon-carbon composite including silicon nanoparticles, crystalline carbon, carbon nanotubes, and first amorphous carbon may be used as a core, and a second amorphous carbon layer covering the surface of the silicon-carbon composite may be additionally included.

[0056] The second amorphous carbon layer may refer to a carbonized coating layer of a carbon-based material, and examples of the carbon-based material may include petroleum pitch or coal tar, PAA, and PVA having a softening point of less than 250°C. This second amorphous carbon layer may serve to minimize side reactions with the electrolyte by ultimately carbon-coating silicon nanoparticles exposed on the surface during a pulverization process such as a jet mill of Si-C particles.

[0057] Hereinafter, a negative electrode material for a lithium secondary battery according to one embodiment of the present invention including all or part of the above-described configurations is described.

[0058] According to an example of the present invention, a negative electrode material for a lithium secondary battery may have a D50 particle size of 8.0 μm or more and 15.0 μm or less.

[0059] In addition, as described above, the negative electrode material according to an example of the present invention can exhibit a low specific surface area by providing a high-density carbon layer. More specifically, the specific surface area of ​​the negative electrode material according to an example of the present invention is 6 m 2 / g may be less than . As another example, the specific surface area is 5 m 2 / g or less or 4.5m 2 / g may be less.

[0060] Hereinafter, a method for manufacturing an anode material for a lithium secondary battery of the present invention will be described. However, the following method for manufacturing an anode material for a lithium secondary battery is merely an example, and it is not necessary for the anode material for a lithium secondary battery of the present invention to be manufactured by this manufacturing method. It should be noted that any manufacturing method that satisfies the claims of the present invention will not cause any problems in implementing and using each embodiment of the present invention.

[0061] A method for manufacturing a negative electrode material for a lithium secondary battery according to an example of the present invention may include the steps of preparing silicon nanoparticles; adding crystalline carbon, carbon nanotubes, and pitch to the silicon nanoparticles and mixing them to obtain a mixture; pressurizing and molding the mixture to obtain a molded article; and crushing and classifying the molded article. Each step is described in detail below.

[0062] First, one embodiment of the present invention can prepare silicon nanoparticles. According to a non-limiting example of the present invention, such silicon nanoparticles can be obtained by grinding silicon raw material (poly-Si) through mechanical milling.

[0063] Furthermore, as an example, the mechanical milling may be performed using stabilized zirconia. Furthermore, to improve nano-processing efficiency, suppress the generation of fine particles, and prevent silicon particles from oxidizing, the size of the zirconia beads may be less than twice the D99 of the silicon raw material being introduced. However, the zirconia beads used may be of various sizes and can be utilized in process design, and are therefore not limited thereto.

[0064] Other detailed conditions for mechanical milling are not specifically limited, as they can be sufficiently designed by a skilled technician according to the purpose. However, as an example, the BPR (Ball Per Ratio) of the silicon raw material to the stabilized zirconia balls may be set to 5:1, and the rotation speed of the rotor inside the grinder may be maintained at 2,500 rpm.

[0065] And, according to one example of the present invention, the solvent in the mechanical milling may be an organic solvent such as ethanol or IPA to prevent oxidation of the silicon raw material.

[0066] The D50 of the silicon nanoparticles obtained by the above-described grinding may be 100 nm or more and less than 150 nm. Since this has been described above with respect to the negative electrode material, a detailed description thereof will be omitted.

[0067] Next, in one embodiment of the present invention, a mixture can be obtained by adding crystalline carbon, carbon nanotubes, and pitch to the silicon nanoparticles obtained as described above and then mixing them.

[0068] The above crystalline carbon may be at least one of artificial graphite, flake graphite, earthy graphite, expanded graphite, and graphene, and the pitch may have a fixed carbon ratio of 30% or more and a beta-resin value of 15% or more.

[0069] The mixing of the above silicon nanoparticles, the above crystalline carbon, the above carbon nanotubes and the above pitch may be done simultaneously, but it is also possible that at least two of the four are mixed first and then the remainder is mixed.

[0070] If, according to one embodiment of the present invention, the silicon nanoparticles, the crystalline carbon, and the carbon nanotubes are mixed first, and then the pitch is added, the mixture of the silicon nanoparticles, the crystalline carbon, and the carbon nanotubes can be spray-dried to remove the solvent, and the pitch can be added to the spray-dried body obtained thereby, and then mixed to obtain a negative electrode precursor.

[0071] Additionally, as a non-limiting example, the center particle size (D50) of the crystalline carbon may be smaller than the center particle size of the spray dryer so that the crystalline carbon can be completely captured within the spray dryer, and the center particle size (D50) may be 5 μm to 10 μm.

[0072] Additionally, the above mixing can be performed through a milling process via contact with the powder, and non-limiting examples thereof include mechanofusion or a ball mill. When performing a milling process via contact with the powder, the independent flow of silicon nanoparticles can be reduced when mixed with a solvent.

[0073] Next, the method for manufacturing a negative electrode material for a lithium secondary battery according to an example of the present invention may include a step of pressure-molding the mixture to obtain a molded body.

[0074] In addition, as a non-limiting example, 0.40 ton / cm during the pressurized forming 2 More than 0.90 ton / cm 2 The pressure below can be applied. That is, one example of the present invention can provide a high-density carbon layer. As a result, one example of the present invention can not only eliminate micropores within the negative electrode material, but also increase the surface coverage of the silicon nanoparticles as the internal carbon layer sufficiently wraps the silicon nanoparticles. As a result, an negative electrode material with excellent electrochemical performance can be provided. In another example, the pressure is 0.60 ton / cm. 2 It may be ideal. On the other hand, 0.90 ton / cm 2 When excessive pressure is applied, as the strength of the cathode material increases, the generation of fine particles may increase during subsequent crushing. Therefore, the present invention is to set the pressure during pressurization to 0.90 ton / cm as described above. 2 It can be as follows. As another example, the pressure above is 0.80 ton / cm 2 Less than or equal to 0.70 ton / cm 2 It may be less than.

[0075] The present invention can not only evaporate volatile components existing within the existing precursor by carbonizing the molded body at 800°C or higher, but also solidify the pitch to secure the internal structure of the negative electrode material. In addition, if the temperature during carbonization becomes too high, silicon carbide and silicon nitride without electrochemical performance may be generated, which may reduce capacity and efficiency. Therefore, the carbonization may be performed at a temperature range of 1000°C or lower. In particular, the carbonization may be performed in an inert atmosphere to suppress oxidation of silicon particles.

[0076] After carbonization, the molded body can be pulverized and classified according to the method for manufacturing a negative electrode material for a lithium secondary battery according to one embodiment of the present invention. Through this, a negative electrode material of a desired size can be obtained. As a non-limiting example, the pulverization can be performed by dry pulverization, and the dry pulverization can be performed using a jet mill or a pin mill.

[0077] As described above, after obtaining the negative electrode material by crushing and classifying, one example of the present invention may additionally include a step of coating the surface of the negative electrode material with carbon.

[0078] The above carbon coating step can be performed by adding a carbon-based material and performing heat treatment, and the carbon-based material can be at least one of petroleum pitch, coal tar, PAA, and PVA having a softening point of less than 250°C. In addition, but not limited thereto, the coating can be performed using a twisted blade mixer. At this time, process variables such as time and rotation speed, but since these are matters that can be appropriately designed by a person skilled in the art according to the purpose, they are not described in the present invention. Hereinafter, a method for manufacturing a negative electrode of a lithium secondary battery of the present invention will be described in detail.

[0079] The above negative electrode can be manufactured by mixing a negative electrode material manufactured according to one embodiment of the present invention, a binder, and optionally a conductive material to manufacture a composition for forming a negative electrode material layer, and then applying the composition to a negative electrode current collector.

[0080] The negative electrode current collector may be, for example, copper foil, nickel foil, stainless steel, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, or a combination thereof.

[0081] Examples of the binder include, but are not limited to, polyacrylic acid, polyvinyl alcohol, carboxymethyl cellulose / styrene-butadiene rubber, hydroxypropylene cellulose, diacetylene cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, or polypropylene. The binder may be mixed in an amount of 1 wt% to 30 wt% based on the total amount of the composition for forming the negative electrode material layer.

[0082] The conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the battery, and specifically, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, or summer black; conductive fibers such as carbon fiber or metal fiber; metal powders such as fluorinated carbon, aluminum, or nickel powder; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives can be used. The conductive material can be mixed in an amount of 0.1 wt% to 30 wt% based on the total amount of the composition for forming the negative electrode material layer.

[0083] Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are intended only to illustrate and further illustrate the present invention and are not intended to limit the scope of the invention. This is because the scope of the invention is determined by the matters set forth in the patent claims and matters reasonably inferred therefrom.

[0084] (Example 1)

[0085] (1) Manufacturing of cathode material

[0086] First, mechanical milling was performed on the silicon raw material for 12 hours. The D50 particle size of the silicon nanoparticles obtained as a result was 115 nm. The manufactured silicon nanoparticle slurry was added in an amount of 53 wt% based on the weight of the final negative electrode material, and flake graphite and carbon nanotubes with a center particle size of 8 μm were added in amounts of 18 wt% and 0.50 wt%, respectively, and dispersed using a high-speed mixer. The physical properties of the added carbon nanotubes are shown in Table 1 below. Afterwards, a spray-dried precursor with a center particle size of approximately 20 μm was synthesized and mixed with pitch powder. The powder was loaded into a mold of a specific size to form a carbon support for the powder, and uniaxial pressing was performed at a pressure shown in Table 2 below. The fixed carbon ratio of the pitch powder was 77.71%, and the beta resin value was 28.1%. The block obtained in the pressurized process was heat-treated at 950 degrees in an inert atmosphere to prevent oxidation of silicon nanoparticles, and then ground to a size ranging from 10 μm to 15 μm based on D50 using a JET mill. The same amount of coal tar in the range of 5 wt% to 10 wt% relative to the amount of silicon nanoparticles added was stirred for about 30 minutes using a twisted blade mixer, and then heat-treated in an inert atmosphere below 1000 degrees, and then sieved through a #635mesh (20 μm) sieve to obtain the final negative electrode material.

[0087] FIB-SEM images of cross-sections of each example are shown in Fig. 1 below. In addition, particle strength was measured and the results are shown in a graph in Fig. 2 below. In addition, the changes in specific surface area and pore distribution results of each example are shown in Fig. 3 below.

[0088] Here, the specific surface area refers to the BET specific surface area, and the specific surface area change and pore distribution results in Fig. 3 were measured using a 3Flex analyzer from Micromeritics. In addition, the particle strength was measured using a MCT-W500 (Micro particle Compression Test) from SHIMADZU.

[0089] Specific surface area (m 2 / g)645bulk density (g / cc)0.057

[0090] Comparative Example 1 Comparative Example 2 Comparative Example 3 Invention Comparative Example 4 Pressure (ton / cm) 2 )00.130.380.640.96

[0091] (2) Electrochemical evaluation

[0092] A half-cell of the 2032 coin cell type was manufactured by mixing the manufactured negative electrode material and commercially available spherical natural graphite at 15 wt% and 85 wt%, respectively, and designing the negative electrode capacity to be 550 mAh / g as the target value. The composition of the negative electrode was composed of active material: conductive material: CMC: SBR = 96.1:1:1.7:1.2, and the loading amount was 7 mg / cm 2, and the rolling density was maintained at 1.35 g / cc. The electrolyte for the life test was EC: EMC = 3: 7 (1.0 M LiPF6)+VC1.5 wt%+FEC 7 wt%. The discharge capacity and initial efficiency of the battery were measured and shown in Table 3 below. At this time, the current during charge and discharge was measured at 0.1 C in the initial cycle. In addition, based on the first 1 C capacity, a current of 0.5 C was applied during charge and discharge to measure the life of 50 cycles. At this time, the charge cut-off current was set to 0.005 C, and the discharge cut-off voltage was set to 1.0 V.

[0093] The expansion ratio of the mixed electrode was measured by disassembling the half cell and measuring the thickness of the charged electrode after the 50th cycle was completed using the half cell of the 2032 coin cell type, fully charging it to 0.005 V (0.005 C cut-off).

[0094] Specific surface area of ​​the mixed electrode (m) 2 / g)Discharge capacity (mAh / g)Initial efficiency (%)Lifespan (%)Expansion rate (%)Comparative example 112.7848184.147.2128.4Comparative example 29.3254089.778.574.2Comparative example 37.0755190.279.567.0Invention example 4.0355490.287.156.7Comparative example 45.5754489.881.461.1

[0095] As described above, Fig. 1 is an FIB-SEM photograph showing cross-sectional photographs of each example according to the uniaxial compression molding conditions. Looking at Figs. 1a to 1e, it can be confirmed that in the case of Comparative Example 1, where uniaxial compression molding was not applied, a dense carbon layer was not formed inside the material, and it can be seen that the pores inside the material tend to decrease as the compression pressure increases. In the case of Comparative Example 4, although it has an overall dense structure, it can be confirmed that cracks were formed inside the particles during crushing.

[0096] Figure 2 is a graph showing the particle strength of each example. This shows that the higher the pressure applied during press molding, the higher the particle strength. Meanwhile, in Comparative Example 1, which did not use uniaxial compression molding, no particle strength was measured.

[0097] The surface area measurement results in Table 3 show that the surface area tends to decrease as the uniaxial compression molding pressure increases. This trend is attributed to the formation of a dense first amorphous carbon layer, which is in good agreement with the observation results in Fig. 1. On the other hand, in the case of Comparative Example 4, the surface area was 5.57 m even though the pressure during press molding was the highest. 2 / g increased, which is believed to be due to the surface area increasing as the cracks within the particles described above were exposed to the outside.

[0098] In addition, looking at the pore distribution results of Fig. 3, it can be seen that micro-pores were observed in the 2 to 3 nm range for Comparative Examples 1 and 2. In addition, as the pressure increased, the pore size decreased, but in the case of Comparative Example 4, it can be confirmed that pores were observed around 6 to 7 nm, deviating from this trend. This is presumed to be due to the formation of internal cracks during particle crushing as the particle strength increased, as described above, and the formation of pores as a result.

[0099] In summary, in Comparative Examples 1 to 3, the pressure applied during pressurization fell short of the range proposed by the present invention, resulting in the remaining micropores and, as a result, the formation of a high-density carbon layer was not possible. Consequently, as shown in Table 3, the service life and expansion characteristics were poor.

[0100] In the case of Comparative Example 4, since the pressure during pressurization exceeded the range proposed by the present invention, cracks occurred within the particles during crushing, and accordingly, the specific surface area increased, making it impossible to secure excellent electrochemical performance.

[0101] On the other hand, in the case of the invention example where an appropriate level of pressure was applied during pressurization and the initially measured pore size exceeded 10 nm as a result of pore distribution analysis, the pitch and silicon nanoparticles, carbon nanotubes, and crystalline carbon could be bonded as much as possible under a certain pressure before carbonization, and as a result, a dense carbon layer was secured during carbonization, so that excellent capacity characteristics and life characteristics could be secured.

[0102]

[0103] (Example 2)

[0104] Except for the difference in the content of carbon nanotubes added to the silicon nanoparticles, a negative electrode material was manufactured in the same order and under the same conditions as the invention example in (Example 1). An electrode was manufactured using the negative electrode material, and the electrochemical performance was evaluated, which is shown in Table 4 below. The method for evaluating the electrochemical performance is the same as in (Example 1).

[0105] Classification CNT content (weight%) Specific surface area (m 2 / g)Discharge capacity (mAh / g)Initial efficiency (%)Lifespan (%)Expansion rate (%)Example 2-10.00%3.8855990.482.460.4Example 2-20.10%3.8755890.282.960.1Example 2-30.20%3.9455890.184.759.2Example 2-40.50%4.0355490.287.156.7Example 2-50.70%4.2255189.485.157.1Example 2-61.00%4.9854889.184.157.4

[0106] Looking at Examples 2-1 to 2-4 of Table 4 above, it can be confirmed that as the content of carbon nanotubes increases, it is helpful in securing excellent life characteristics. However, when the carbon nanotubes are included in an amount exceeding 0.60%, it can also be confirmed that the carbon nanotubes are not distributed in the first amorphous carbon phase, resulting in an increase in the specific surface area. In addition, in the case of Examples 2-5 and 2-6, as described above, it was confirmed that the carbon nanotubes that were not distributed in the first amorphous carbon phase caused their own reaction, which had a negative effect on the capacity, efficiency, and life characteristics.

[0107] Therefore, it can be seen through the above-described experiment that in order to manufacture a cathode material with excellent performance, it is necessary to add carbon nanotubes at an appropriate level, and in order for the carbon nanotubes to be distributed within the first amorphous carbon, it can be seen that it is preferable to add them at 0.60 wt% or less within the cathode material.

Claims

1. Silicon nanoparticles; crystalline carbon; carbon nanotubes; and Comprising a silicon-carbon composite comprising first amorphous carbon, The silicon nanoparticles, the crystalline carbon and the carbon nanotubes are dispersed on the first amorphous carbon, A negative electrode material for a lithium secondary battery, wherein the minimum pore size inside the above complex is greater than 10 nm and less than or equal to 30 nm.

2. In paragraph 1, The above crystalline carbon is an anode material for a lithium secondary battery, wherein at least one of artificial graphite, flaky graphite, earth-like graphite, CNT, and graphene.

3. In paragraph 1, A negative electrode material for a lithium secondary battery, wherein the carbon nanotubes are contained in an amount of 0.10 wt% or more and 0.60 wt% or less based on the total weight of the negative electrode material.

4. In paragraph 1, The specific surface area of ​​the above carbon nanotube is 500 m 2 / g over 800m 2 / g or less negative electrode material for lithium secondary batteries.

5. In paragraph 1, A negative electrode material for a lithium secondary battery, wherein the bulk density of the carbon nanotube is 0.04 g / cc or more and 0.09 g / cc or less.

6. In paragraph 1, The above first amorphous carbon is a negative electrode material for a lithium secondary battery, wherein the pitch is carbonized and has a fixed carbon ratio of 30% or more and a beta resin value of 15% or more.

7. In paragraph 1, With the above silicon-carbon composite as the core, A negative electrode material for a lithium secondary battery, further comprising a second amorphous carbon layer covering the surface of the silicon-carbon composite.

8. In clauses 1 to 7, Specific surface area is 6m 2 / g or less for lithium secondary battery negative electrode material.

9. Step of preparing silicon nanoparticles; A step of adding crystalline carbon, carbon nanotubes and pitch to the above silicon nanoparticles and then mixing them to obtain a mixture; A step of pressure-molding the above mixture to obtain a molded body; a step of carbonizing the above-mentioned molded body; and A step of crushing and classifying the above-mentioned molded body; The above pressurization is 0.40 ton / cm 2 Above 0.90ton / cm 2 A method for manufacturing a negative electrode material for a lithium secondary battery, which is performed under a pressure of less than 100 psi.

10. In paragraph 9, A method for manufacturing an anode material for a lithium secondary battery, wherein the above crystalline carbon is at least one of artificial graphite, flaky graphite, earth-like graphite, CNT, and graphene.

11. In paragraph 9, The above pitch is a method for manufacturing a negative electrode material for a lithium secondary battery having a fixed carbon ratio of 30% or more and a beta resin value of 15% or more.

12. In paragraph 9, The above mixing is a method for manufacturing a negative electrode material for a lithium secondary battery, which is performed through a milling process via contact mediation with powder.

13. In paragraph 9, The above grinding is a method for manufacturing a negative electrode material for a lithium secondary battery, which is a dry grinding process.

14. In paragraph 9, A method for manufacturing an anode material for a lithium secondary battery, further comprising a step of coating the surface of the manufactured anode material with carbon.

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