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

Abstract

An aspect of the present invention relates to a negative electrode material for a lithium secondary battery, a lithium secondary battery including the negative electrode material and a method for manufacturing same, the negative electrode material including a silicon-carbon composite capable of ensuring high capacity characteristics and excellent lifespan characteristics and including silicon nanoparticles, crystalline carbon, and first amorphous carbon, wherein the silicon nanoparticles and the crystalline carbon are distributed in the first amorphous carbon, wherein the silicon nanoparticles have a Dmax particle size of less than 250 nm and a grain size of 16.5 nm or more and less than 18 nm.

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] 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.

[0009] 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, and first amorphous carbon, wherein the silicon nanoparticles and the crystalline carbon may be distributed within the first amorphous carbon, and the silicon nanoparticles may have a Dmax particle size of less than 250 nm and a crystal grain size of 16.5 nm or more and less than 18 nm. In addition, the first amorphous carbon described above may be a carbonized pitch having a fixed carbon ratio of 70% or more and a beta resin value of 25% or more.

[0010] The D50 particle size of the above-described silicon nanoparticles may be 115 nm or more and 150 nm or less.

[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 negative electrode material may additionally include a second amorphous carbon layer that has the silicon-carbon composite as a core and surrounds the surface of the silicon-carbon composite.

[0013] The above-described negative electrode material may have a D50 particle size of 10 μm or more and 15 μm or less.

[0014] The above-mentioned cathode material has a specific surface area of ​​6 m 2 / g may be less.

[0015] According to another aspect of the present invention, a method for manufacturing an anode material for a lithium secondary battery may include the steps of: obtaining silicon nanoparticles by pulverizing silicon raw material (poly-Si) through mechanical milling; adding crystalline carbon and pitch to the silicon nanoparticles and mixing them to obtain a mixture; pressurizing and molding the mixture to obtain a molded body; carbonizing the molded body; and crushing and classifying the molded body. In the mechanical milling, the slurry throughput per hour (flow rate) may be 1.5 kg / min or more and 2.3 kg / min or less. In addition, the pitch described above may have a fixed carbon ratio of 70% or more and a beta resin value of 25% or more.

[0016] The above-described crystalline carbon may be at least one of artificial graphite, flake graphite, earth-like graphite, CNT, and graphene.

[0017] The solvent in the mechanical milling described above may be ethanol or IPA.

[0018] The above-described mixing can be performed through a milling process via a contact medium with a powder.

[0019] When pressurizing as described above, 1 ton / cm 2A pressure of less than .

[0020] The above-described grinding may be a dry grinding process.

[0021] 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.

[0022] The negative electrode material of the present invention can secure high capacity characteristics and excellent life characteristics by controlling the particle size and crystal grain size of silicon nanoparticles contained in the negative electrode material.

[0023] In particular, the present invention can provide a method for manufacturing a negative electrode material for a lithium secondary battery that can facilitate mass production of the above-described negative electrode material.

[0024] 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.

[0025] 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.

[0026] Additionally, unless otherwise specifically provided in the specification of the present invention, the % unit means weight%.

[0027] 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.

[0028] 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.

[0029] In order to solve the above-described problem, the inventors of the present invention found that when the particle size and crystal grain size of silicon nanoparticles included in the negative electrode material are controlled to a certain level, the high-capacity characteristics of the negative electrode material can be maintained while the life characteristics of the negative electrode material can be improved.

[0030] In addition, as described above, the inventors of the present invention also found that, as a method for controlling the particle size and crystal grain size of silicon nanoparticles to a certain level, it is effective to control the hourly slurry processing amount (flow rate) of the slurry during mechanical milling of silicon raw materials.

[0031] In addition, the inventors realized that it is useful to set the fixed carbon ratio and beta resin content of the pitch within a specific range as another method for suppressing the expansion of the above-described silicon nanoparticles, and thus derived the present invention.

[0032] From this perspective, one embodiment of the present invention may include a silicon-carbon composite comprising silicon nanoparticles, crystalline carbon, and a first amorphous carbon, wherein the silicon nanoparticles and the crystalline carbon may be distributed within the first amorphous carbon. Each component is described in detail below.

[0033] A silicon-carbon composite according to an example of the present invention may include silicon nanoparticles.

[0034] 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 115 nm for the purpose of securing high-capacity characteristics of the negative electrode material.

[0035] According to one example of the present invention, the Dmax particle size of the silicon nanoparticles may be less than 250 nm. In this case, the Dmax particle size may refer to the largest particle size in the particle size distribution. That is, the present invention can prevent expansion of the silicon nanoparticles by controlling the Dmax particle size while simultaneously controlling the D50 particle size of the silicon nanoparticles, thereby improving the life characteristics of the negative electrode material. As another example, the Dmax particle size may be less than 245 nm.

[0036] According to a non-limiting example of the present invention, the crystal grain size of the silicon nanoparticles may be less than 18 nm. If the crystal grain size is 18 nm or more, the silicon nanoparticles may expand due to repeated charge and discharge. As another example, the crystal grain size may be 17.5 nm or less or 17.3 nm or less. However, if the crystal grain size is excessively small, capturing by pitch may become difficult as the differentials such as D1 and D10 increase. As a result, the specific surface area may increase, and therefore, in a non-limiting example of the present invention, the lower limit of the crystal grain size may be set to 16.5 nm. In this case, the size of the crystal grains can generally be measured using XRD, and can be calculated by utilizing the Sherrer Equation below for the (111) plane showing the highest peak of the silicon nanoparticles.

[0037]

[0038] (Dp: Average Crystallite size, β: FWHM, θ: Bragg angle, λ: X-ray wavelength, K: shape factor)

[0039] According to one embodiment of the present invention, the negative electrode material for a lithium secondary battery can secure high capacity characteristics of the negative electrode material by containing 50 wt% or more of the silicon nanoparticles. More specifically, the content of the silicon nanoparticles may be 55 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 65 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 60 wt% or less.

[0040] 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.

[0041] As an example, the crystalline carbon may be at least one of artificial graphite, flake graphite, earth graphite, CNT, and graphene.

[0042] For the above-described purpose, one example of the present invention may include 15 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 20 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 25 wt% or less based on the total weight of the negative electrode material.

[0043] Meanwhile, a silicon-carbon composite according to an example of the present invention may include a first amorphous carbon, and the above-described silicon nanoparticles and crystalline carbon may be distributed within the first amorphous carbon. Accordingly, the first amorphous carbon may control the expansion of the silicon nanoparticles while also strengthening the contact path between the silicon nanoparticles and the conductive material.

[0044] In addition, the first amorphous carbon according to one embodiment of the present invention may be carbonized pitch having a fixed carbon ratio of 70% or more. In this case, the pitch may include coal-based pitch and petroleum-based pitch.

[0045] As the fixed carbon value of the above pitch increases, a conductive path with silicon nanoparticles having low self-conductivity can be created, thereby inducing an increase in 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 an increase in the 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 70%, and in another example, the lower limit of the fixed carbon ratio can be 73% or 75%.

[0046] 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 25% or more.

[0047] Specifically, the β-resin value refers to the value obtained by subtracting the quinoline capacity (benzeneinsoluble) from the benzene-insoluble. This β-resin value is proportional to the cohesion. According to one 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, one 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 26% or more, and as another example, it may be 28% or more.

[0048] Meanwhile, according to another example of the present invention, as described above, a silicon-carbon composite including silicon nanoparticles, crystalline carbon, 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.

[0049] The second amorphous carbon layer may refer to a carbon-based material carbonized coating layer, 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.

[0050] 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.

[0051] According to an example of the present invention, a negative electrode material for a lithium secondary battery may have a D50 particle size of 10.0 μm or more and 15.0 μm or less.

[0052] 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 controlling the characteristics of the silicon nanoparticles included in the negative electrode material. 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.

[0053] 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.

[0054] A method for manufacturing an anode material for a lithium secondary battery according to an example of the present invention may include the steps of: obtaining silicon nanoparticles by crushing silicon raw material (poly-Si) through mechanical milling; adding crystalline carbon and pitch to the silicon nanoparticles and mixing them to obtain a mixture; pressurizing and molding the mixture to obtain a molded body; and crushing and classifying the molded body. Each step will be described in detail below.

[0055] First, one embodiment of the present invention can obtain silicon nanoparticles by crushing silicon raw material (poly-Si) through mechanical milling.

[0056] As an example, the mechanical milling can be performed using stabilized zirconia. Furthermore, to improve nano-processing efficiency, suppress the generation of fine particles, and prevent oxidation of silicon particles, 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 are available in various sizes and can be utilized in process design, and are therefore not limited thereto.

[0057] 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.

[0058] 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.

[0059] In particular, according to a non-limiting example of the present invention, the hourly slurry throughput (flow rate) in the mechanical milling may be 1.5 kg / min or more and 2.3 kg / min or less. This is to secure the particle size characteristics of the target silicon nanoparticles by controlling the hourly slurry throughput at an appropriate level in the mechanical milling step described above. More specifically, as the hourly slurry throughput increases during the mechanical milling, the particle size of the silicon nanoparticles may tend to decrease. That is, in one example of the present invention, by setting the hourly slurry throughput to 1.5 kg / min or more and 2.3 kg / min or less, the D50 of the silicon nanoparticles can be 115 nm or more and less than 150 nm, and the Dmax particle size can be less than 250 nm. As another example, the hourly slurry throughput may be 1.6 kg / min to 2.1 kg / min, and as another example, 1.7 kg / min to 2.0 kg / min.

[0060] Next, in one embodiment of the present invention, crystalline carbon and pitch are added to the silicon nanoparticles obtained as described above, and then mixed to obtain a mixture.

[0061] The above crystalline carbon may be at least one of artificial graphite, flake graphite, earth graphite, CNT, and graphene, and the pitch may have a fixed carbon ratio of 70% or more and a beta resin value of 25% or more. Since this has been described above with respect to the negative electrode material of the present invention, a detailed description thereof will be omitted.

[0062] The mixing of the above silicon nanoparticles, the above crystalline carbon and the above pitch may be done simultaneously, but it is also possible that at least two of the three are mixed first and then the remaining one is mixed.

[0063] If, according to one embodiment of the present invention, the silicon nanoparticles and the crystalline carbon are mixed first and then the pitch is added, the mixture of the silicon nanoparticles and the crystalline carbon 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] In addition, as a non-limiting example, 1 ton / cm is used in the above pressurized molding. 2 A pressure of less than 1 ton / cm may be applied. If 2 When the pressure above is applied, the volatile matter of the internal pitch may not evaporate during subsequent carbonization, and when the pressure increases, the particle strength may increase, which may cause the side effect of increasing the generation of fine particles during the final particle crushing. Therefore, the present invention is to set the pressure during pressurization to 1 ton / cm as described above. 2 can be less than 0.9 ton / cm2. As another example, the pressure is 0.9 ton / cm2.2 Less than or equal to 0.8 ton / cm 2 It may be less than.

[0068] The present invention not only evaporates volatile components existing within the existing precursor by carbonizing the precursor at 800°C or higher, but also solidifies 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.

[0069] 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.

[0070] 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.

[0071] The above carbon coating step can be performed by adding a carbon-based material and performing a 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 considering that these can be appropriately designed by a person skilled in the art according to the purpose, they are not described in the present invention.

[0072] Hereinafter, the negative electrode of the lithium secondary battery of the present invention will be described in detail.

[0073] A negative electrode for a lithium ion secondary battery according to one embodiment of the present invention includes a negative electrode current collector and a negative electrode active material layer.

[0074] The negative current collector may be a conductive plate including, but not limited to, copper, stainless steel, aluminum, nickel, titanium, carbon or a combination thereof.

[0075] The negative active material layer may include a negative active material, a binder, and optionally a conductive material.

[0076] The negative electrode active material may be the negative electrode material for the lithium ion secondary battery described above, and the specific description is as described above. In addition to the negative electrode material for the lithium ion secondary battery described above, the negative electrode active material may further include a lithium-containing metal oxide. The negative electrode active material may be included in an amount of 75 wt% or more based on the total content of the negative electrode active material layer, and may be included in an amount of, for example, 75 to 99 wt%, 80 to 99 wt%, or 85 to 99 wt%.

[0077] The binder is a component for increasing the bonding between the negative electrode active material and the conductive material and the adhesion to the negative electrode current collector, and may include, but is not limited to, polyvinylidene fluoride (PVDF), polyvinyl alcohol (PVA), carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, fluoroelastomer, copolymers thereof, or combinations thereof. The binder may be included in an amount of 30 wt% or less based on the total content of the negative electrode active material layer, and may be included in an amount of, for example, 0.1 to 30 wt%, 1 to 20 wt%, or 3 to 15 wt%.

[0078] The conductive material may be a carbon material, such as, but not limited to, graphite, carbon black, fluorocarbon, conductive fiber, conductive metal oxide, or a combination thereof. The conductive material may be included in an amount of 20 wt% or less based on the total content of the negative electrode active material layer, and may be included in an amount of, for example, 0.1 to 20 wt%, 1 to 15 wt%, or 1 to 10 wt%.

[0079] A lithium ion secondary battery according to one embodiment includes a positive electrode, a negative electrode, a separator, and an electrolyte.

[0080] The positive electrode includes a positive electrode current collector and a positive electrode active material layer.

[0081] The cathode current collector may be a conductive plate including, but not limited to, stainless steel, aluminum, nickel, titanium, carbon, or a combination thereof.

[0082] The cathode active material layer includes a cathode active material, a binder, and optionally a conductive material.

[0083] The cathode active material is a compound capable of reversible intercalation and deintercalation of lithium, and may include, for example, a lithium metal oxide containing one or more metals such as nickel (Ni), cobalt (Co), manganese (Mn), and / or aluminum (Al). The lithium metal oxide may be, for example, lithium nickel oxide, lithium manganese oxide, lithium cobalt oxide, lithium nickel cobalt oxide, lithium nickel manganese oxide, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, or a combination thereof. The cathode active material may be included in an amount of 75 wt% or more based on the total content of the cathode active material layer, for example, 75 to 99 wt%, 80 to 99 wt%, or 85 to 99 wt%.

[0084] The binder is a component for increasing the bonding between the positive active material and the conductive material and the adhesion to the positive current collector, and may include, but is not limited to, polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluororubber, copolymers thereof, or combinations thereof. The binder may be included in an amount of 30 wt% or less based on the total content of the positive active material layer, and may be included in an amount of, for example, 0.1 to 30 wt%, 1 to 20 wt%, or 3 to 15 wt%.

[0085] The conductive material may be a carbon material, such as, but not limited to, graphite, carbon black, fluorocarbon, conductive fiber, conductive metal oxide, or a combination thereof. The conductive material may be included in an amount of 20 wt% or less based on the total content of the positive electrode active material layer, for example, 0.1 to 20 wt%, 1 to 15 wt%, or 1 to 10 wt%.

[0086] The separator may be positioned between the anode and cathode and may be a porous polymer film.

[0087] The electrolyte may be, for example, an organic electrolyte or a polymer electrolyte in which a lithium salt is dissolved in a non-aqueous solvent. The non-aqueous solvent may be propylene carbonate, ethylene carbonate, butylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyltetrahydrofuran, gamma-butyrolactone, dioxolane, 4-methyldioxolane, N,N-dimethylformamide, dimethylacetamide, dimethylsulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, dimethyl carbonate, methylethyl carbonate, diethyl carbonate, methylpropyl carbonate, methylisopropyl carbonate, ethylbutyl carbonate, dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate, diethylene glycol, dimethyl ether, or a combination thereof.

[0088] Lithium salts contain Li as a cation. + Contains F as an anion - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , AlO4 - , AlCl4 - , PF6 - , SbF6 - , AsF6 - , BF2C2O4 - , BC4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , C4F9SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH- , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - , (CF3CF2SO2)2N - or a combination thereof.

[0089] These lithium-ion secondary batteries can be applied to various electronic devices requiring energy supply, such as electric vehicles, hybrid electric vehicles, power storage devices, or electronic products.

[0090] 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.

[0091] (Example 1)

[0092] (1) Manufacturing of cathode material

[0093] Mechanical milling was performed on silicon raw materials having particle sizes shown in Table 1 below. The hourly slurry throughput during the mechanical milling is shown in Table 2 below. The mechanical milling was performed for 12 hours. The particle size distribution of the silicon nanoparticles thus obtained is shown in Table 2 below. The prepared silicon nanoparticle slurry was added in an amount of 53 wt% based on the weight of the final negative electrode material, and flaky graphite particles having a center particle size of 8 μm were added in an amount of 18 wt% and dispersed using a high-speed mixer. Afterwards, a silicon-graphite spray-dried precursor having a center particle size of approximately 20 μm was synthesized and mixed with pitch powder. In order to form a carbon support for the powder, the powder was charged into a cylindrical mold with a radius of 5 cm, and uniaxial pressing was performed at a pressure of approximately 50 tons. 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.

[0094] Afterwards, the crystal grain size of the silicon nanoparticles was measured, and the measured values ​​are shown in Table 3 below.

[0095] At this time, the particle size of the silicon raw material, the particle size of the silicon nanoparticles, and the particle size of the final cathode material were measured using a particle size measuring device from Beckmann Coulter.

[0096] In addition, the half width of silicon nanoparticles was measured using XRD, and more specifically, the half width of the X-ray diffraction angle (2theta) using CuKα rays on the (111) plane was measured using the XRD. Then, the crystal grain size of the silicon nanoparticles was derived by the Sherrer equation below using the half width on the (111) plane of the silicon nanoparticles obtained as described above.

[0097]

[0098] (Dp: Average Crystallite size, β: FWHM, θ: Bragg angle, λ: X-ray wavelength, K: shape factor)

[0099] Classification Raw material particle size (㎛) Crystalline grain size (nm) Electrochemical performance Discharge capacity (mAh / g) Initial efficiency (%) Lifespan (%) Expansion rate (%) D10.1939.03, 21785.724.1284.7 D101.07 D501.56 D907.93 D9910.44

[0100] Slurry flow rate (kg / min) Particle size (㎛) D1 D10 D50 D90 Dmax Span Comparative example 11.3400.0460.0760.1240.1900.4540.915 Comparative example 22.3800.0460.0750.1140.1720.2380.845 Comparative example 31.1040.0460.0800.1330.24 30.7461.225 Invention Example 12.0840.0460.0760.1190.1790.2430.866 Comparative Example 41.2010.0460.0790.1280.2130.6701.045 Invention Example 21.7850.0460.0760.1220.1840.2490.882

[0101] (2) Electrochemical evaluation of single electrode

[0102] Based on the total weight of the negative electrode, 75 wt% of the negative electrode material manufactured as described above, 1 wt% of the conductive material, and 24 wt% of the PAA binder (Mw 240,000) were mixed in a triple-distilled water solvent to prepare a negative electrode active material slurry. The negative electrode active material slurry was loaded onto a copper (Cu) current collector at a loading amount of 4 mg / cm. 2 After applying the coating, it was dried and pressed in a roll press to have a rolling density of 1.00 g / cc. Afterwards, the negative electrode was manufactured by vacuum drying in a vacuum oven.

[0103] Next, a lithium secondary battery was manufactured to evaluate its electrochemical properties.

[0104] Specifically, a half coin cell of the 2032 coin cell type was manufactured according to a conventional manufacturing method using an electrolyte in which 1 mol of LiPF6 solution was dissolved in a mixed solvent having a volume ratio of 3:7 of ethylene carbonate (EC): ethyl methyl carbonate (EMC) and a negative electrode manufactured by the above method, a counter electrode of lithium metal (Li-metal), and ethylene carbonate (EC): ethyl methyl carbonate (EMC), respectively. 1.5 wt% of a VC additive was added to the electrolyte.

[0105] The discharge capacity and initial efficiency of the above battery were measured and are shown in Table 3 below.

[0106] The current during charge and discharge was measured at 0.1 C in the initial cycle. Furthermore, a current of 0.5 C was applied during charge and discharge based on the first 1 C capacity, and the 50-cycle life was measured. The charge cut-off current was set to 0.005 C.

[0107] (3) Electrochemical evaluation of mixed electrodes

[0108] The manufactured negative electrode material and commercially available spherical natural graphite were mixed at 21 wt% and 79 wt%, respectively, to obtain a negative electrode active material, and the negative electrode capacity was designed to be 620 mAh / g, and a half-cell of the 2032 coin cell type was manufactured. The composition of the negative electrode was composed of the above negative electrode active material: conductive material: CMC: SBR = 96.1:1:1.7:1.2 in weight ratio based on the total weight, and the rolling density was maintained at 1.3 to 1.4 g / cc.

[0109] The electrolyte for the life test was EC: EMC = 3: 7 (1.0 M LiPF6)+VC1.5 wt%+FEC 7 wt%. The charge-discharge protocol was evaluated in the same manner as the electrochemical evaluation of the negative electrode alone described above, and the expansion rate of the mixed electrode was measured by fully charging to 0.005 V (0.005 C cut-off) after the 50th cycle using the half cell of the 2032 coin cell type, disassembling the half cell, and measuring the thickness of the charged electrode.

[0110] Distinction crystal grain size (nm)Single electrodeMixed electrodeDischarge capacity (mAh / g)Initial efficiency (%)Discharge capacity (mAh / g)Initial Efficiency (%), Lifespan (%), Expansion Rate (%), Specific Surface Area (㎡ / g), Comparative Example 1 18.05 1,56 4 84.95 9 188.175.168.14.01, Comparative Example 2 16.46 1,51 2 85.45 89 88.47 8.161.86.87, Comparative Example 3 18.84 1,62 3 83.96 17 88.461.48 8.73.54, Invention Example 1 17.16 1,65 5 86.16 2 5 9 0.58 4.45 7.84.72, Comparative Example 4 18.3 11,62 4 84.46 15 89.171.172.43.61, Invention Example 2 17.24 1,66 4 86.26 2 2 9 0.18 0.159.54.41

[0111] As shown in Tables 2 and 3 above, Comparative Examples 1, 3, and 4 had excessively large Dmax particle sizes due to slurry processing capacities of less than 1.5 kg / min per hour. As a result, the electrodes of Comparative Examples 1, 3, and 4 had poor life characteristics and expansion rate characteristics.

[0112] In Comparative Example 2, as the slurry throughput per hour exceeded 2.3 kg / min, the number of silicon nanoparticles increased excessively, thereby reducing the pitch capturing effect. As a result, the specific surface area was large, and the electrochemical performance was also poor compared to Examples 1 and 2.

[0113] On the other hand, looking at Tables 2 and 3 above, it can be confirmed that in the case of Invention Examples 1 and 2, which satisfy both the characteristics and manufacturing conditions of the silicon nanoparticles proposed in the present invention, the electrochemical performance was excellent. More specifically, Invention Examples 1 and 2 exhibited a lifespan characteristic of 80% or more, an expansion ratio of 60% or less, and a specific surface area of ​​6 m2 / g.

[0114] (Example 2)

[0115] Comparative Examples 5 and 6 were manufactured according to the same procedure as Inventive Example 1 in Table 3, except that pitches with different fixed carbon ratios and caking characteristics (β-resin values) were used. The fixed carbon ratios and β-resin values ​​of the pitches used are shown in Table 4 below. Using these, single electrodes and mixed electrodes were manufactured, and their electrochemical performances were measured. The values ​​measured for Comparative Examples 5 and 6 are shown in Table 5 below, along with the values ​​measured for Inventive Example 1.

[0116] Fixed carbon ratio (%)β-resin value (%)Pitch A77.7128.05Pitch B71.0115.17Pitch C57.1525.11

[0117] Grain size range (nm) Grain size Actual value (nm) Si-C alone 21% Si-C+ 79% Natural graphite mixed Discharge capacity (mAh / g) Initial efficiency (%) Discharge capacity (mAh / g) Initial efficiency (%) Lifespan (%) Expansion rate (%) Specific surface area (㎡ / g) Invention example 1 Pitch A 17±0.5 17.161,65 586.162 59 0.58 4.45 7.8 4.72 Comparative example 5 Pitch B 17.3 71,55 584.15 7487.77 0.47 0.47 77 Comparative example 6 Pitch C 17.42 1,588 84.75 8188.17 6.86 5.46.89

[0118] As in the case of Comparative Example 5, when the beta resin value of the pitch was low, it was difficult to form a high-density carbon matrix, and thus the life characteristics and expansion rate characteristics were poor.

[0119] Additionally, in the case of Comparative Example 6, the fixed carbon ratio of the pitch was low, so the amount of carbon converted to pitch was low. As a result, Comparative Example 6 was unable to secure excellent electrochemical performance.

Claims

1. A silicon-carbon composite comprising silicon nanoparticles, crystalline carbon, and first amorphous carbon, The above silicon nanoparticles and the above crystalline carbon are distributed within the first amorphous carbon, The above silicon nanoparticles are a negative electrode material for a lithium secondary battery having a Dmax particle size of less than 250 nm and a crystal grain size of 16.5 nm or more and less than 18 nm.

2. In paragraph 1, A negative electrode material for a lithium secondary battery, wherein the D50 particle size of the above silicon nanoparticles is 115 nm or more and 150 nm or less.

3. 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.

4. 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 70% or more and a beta resin value of 25% or more.

5. 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.

6. In paragraph 1, A negative electrode material for a lithium secondary battery having a D50 particle size of 10 μm or more and 15 μm or less.

7. In at least one of paragraphs 1 to 6, Specific surface area is 6m 2 / g or less for lithium secondary battery negative electrode material.

8. A step of obtaining silicon nanoparticles by crushing silicon raw material (poly-Si) through mechanical milling; A step of adding crystalline carbon 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; In the above mechanical milling, the slurry processing amount per hour (flow rate) is 1.5 kg / min or more and 2.3 kg / min or less, The above pitch is a method for manufacturing an anode material for a lithium secondary battery, wherein the fixed carbon ratio is 70% or more and the beta resin value is 25% or more.

9. In paragraph 8, 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.

10. In paragraph 8, A method for manufacturing an anode material for a lithium secondary battery, wherein the solvent in the mechanical milling is ethanol or IPA.

11. In paragraph 8, 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.

12. In paragraph 8, During the above pressurized forming, 1 ton / cm 2 A method for manufacturing a negative electrode material for a lithium secondary battery by applying a pressure of less than 100 psi.

13. In paragraph 8, 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 8, 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.

15. A negative electrode comprising a negative electrode material for a lithium ion secondary battery according to at least one of claims 1 to 7.

16. A lithium ion secondary battery comprising a positive electrode, a negative electrode according to Article 15, and an electrolyte.

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