Negative electrode active material, negative electrode, and secondary battery including the same

Abstract

A negative electrode active material is provided that recovers damage to a silicon crystal during a secondary particle formation process, improving conductivity and durability. One embodiment of the present invention is a negative electrode active material including second silicon particles in which first silicon particles are aggregated and a carbon layer on the second silicon particles, the negative electrode active material satisfying Formula 1 in a differential capacity plot for a half-cell including the negative electrode active material.

Description

Technical Field

[0001] The present invention relates to a negative electrode active material, and more specifically, to a negative electrode active material, a negative electrode, and a secondary battery including the same.

Background Art

[0002] With the rapid growth of the secondary battery market, the demand for high-capacity batteries has increased rapidly, and as a result, the demand for silicon-based negative electrode active material materials that can theoretically exhibit high capacity has increased rapidly. However, physical cracks due to volume expansion during charge and discharge and side reactions caused by them have occurred, and there has been a limit to expressing the performance only up to the theoretical capacity. In order to prevent such physical cracks due to volume expansion, silicon particles having a size of about 150 nm or less had to be used. Conventionally, silicon was synthesized in nanounits to produce single-crystalline silicon of 150 nm or less. However, there is a problem that mass production is difficult to synthesize silicon in nanounits, and in order to solve this, a technique of pulverizing silicon minerals to a level of 150 nm or less has been introduced.

[0003] However, when simply pulverizing the primary particle size to 150 nm or less and using it, there is a problem that the specific surface area becomes excessively large and it is difficult to adhere to the electrode. In order to solve this, a process of aggregating primary particles of 150 nm or less into secondary particles using a device such as a spray dryer was utilized. However, the silicon crystal was damaged during the process of pulverizing silicon into primary particles, and the crystal size decreased, which led to a capacity loss. Moreover, when secondary particle formation occurred, there was a problem that the electrical conductivity and physical durability were greatly reduced during the binding without a binder between the silicon primary particles.

Summary of the Invention

Problems to be Solved by the Invention

[0004] An object of the present invention is to provide a negative electrode active material capable of recovering a damaged silicon crystal during the process of forming secondary particles of silicon particles. Another object of the present invention is to provide a negative electrode active material with improved conductivity and durability.

[0005] Still another object of the present invention is to provide a negative electrode active material that satisfies a parameter related to the change amount of capacitance with respect to voltage (dQ / dV) and increases the initial capacitance and capacitance retention rate of a secondary battery. Still another object of the present invention is to provide a negative electrode including the negative electrode active material. Still another object of the present invention is to provide a secondary battery including the negative electrode.

[0006] The objects of the present invention are not limited to the objects mentioned above, and other objects and advantages of the present invention not mentioned can be understood from the following description and will be more clearly understood from the examples of the present invention. Also, it can be easily understood that the objects and advantages of the present invention can be realized by the means shown in the claims and combinations thereof.

Means for Solving the Problems

[0007] According to a first aspect of the present invention for achieving the above object, there is provided a negative electrode active material including second silicon particles in which first silicon particles are aggregated and a carbon layer on the second silicon particles, the negative electrode active material satisfying the following formula 1 in a differential capacity plot for a half-cell including the negative electrode active material. [Formula 1] R = (dQ 2 / dV 2 ) / (dQ 1 / dV 1 ) ≧ 1.2

[0008] In the above formula 1, dQ 1 / dV 1 is the maximum discharge peak of the first charge / discharge cycle at 0.3 to 0.6 V, dQ 2 / dV 2 is the maximum discharge peak of the second charge / discharge cycle at 0.3 to 0.6 V, and R is the ratio of the maximum discharge peaks.

[0009] According to the second aspect of the present invention, in the first aspect, in Formula 1, R may be 3.5 or less.

[0010] According to the third aspect of the present invention, in the first or second aspect, in Formula 1, R may be 1.5 to 2.8.

[0011] According to the fourth aspect of the present invention, in any one of the first to third aspects, the carbon layer can include any one selected from the group consisting of crystalline carbon, amorphous carbon, and combinations thereof.

[0012] According to the fifth aspect of the present invention, in any one of the first to fourth aspects, the carbon layer can include the crystalline carbon derived from pitch and the amorphous carbon derived from a carbon precursor.

[0013] According to the sixth aspect of the present invention, in any one of the first to fifth aspects, based on the total weight of the negative electrode active material, the carbon content may be 5 to 80% by weight.

[0014] According to the seventh aspect of the present invention, in any one of the first to sixth aspects, based on the total weight of the negative electrode active material, the carbon content may be 35 to 60% by weight.

[0015] According to the eighth aspect of the present invention, in any one of the first to seventh aspects, the crystalline size of the first silicon particles may be 15 to 30 nm.

[0016] According to the ninth aspect of the present invention, in any one of the first to eighth aspects, the crystalline size of the first silicon particles may be 17 to 20 nm.

[0017] According to the tenth aspect of the present invention, in any one of the first to ninth aspects, the thickness of the carbon layer may be 20 to 80 nm.

[0018] According to the 11th aspect of the present invention, in any one of the 1st to 10th aspects, the thickness of the carbon layer may be 40 to 80 nm.

[0019] According to the 12th aspect of the present invention, a negative electrode including a negative electrode active material according to any one of the 1st to 11th aspects can be provided.

[0020] According to the 13th aspect of the present invention, a secondary battery including the negative electrode according to the 12th aspect can be provided.

Effects of the Invention

[0021] According to one aspect of the present invention, it is possible to provide a negative electrode active material with improved conductivity and physical durability while repairing silicon crystals damaged in the process of secondary particle formation of the first silicon particles (primary particles). By increasing the initial capacity and capacity retention rate of the secondary battery using such a negative electrode active material, the performance of the secondary battery can be significantly improved. Together with the effects described above, the specific effects of the present invention will be described together while explaining the embodiments for carrying out the following invention.

Embodiments for Carrying Out the Invention

[0022] Hereinafter, each configuration of the present invention will be described in more detail so that a person having ordinary knowledge in the technical field to which the present invention pertains can easily implement it. However, this is merely an example, and the scope of the rights of the present invention is not limited by the following content. In this specification, singular expressions include plural expressions unless the context clearly indicates otherwise.

[0023] In this specification, a numerical range indicated using the term "~" indicates a numerical range including the values described before and after the term as the lower limit value and the upper limit value, respectively. When a plurality of numerical values are disclosed as the upper limit and the lower limit of an arbitrary numerical range, the numerical range disclosed in this specification can be understood as an arbitrary numerical range having any one of the plurality of lower limit values and any one of the plurality of upper limit values as the lower limit value and the upper limit value, respectively.

[0024] In this specification, the "differential capacity plot" is a graph showing the relationship between charge and potential. Specifically, after measuring the change in charge that varies with the potential adjustment of the battery, it is a graph showing the change in the measured charge as a derivative with respect to potential. Generally, although the electrochemical characteristics of a secondary battery change during the charging and discharging processes, these characteristics can be confirmed using the differential capacity plot.

[0025] One embodiment of the present invention provides a negative electrode active material including second silicon particles in which first silicon particles are aggregated and a carbon layer on the second silicon particles, and satisfying the following formula (1) in a differential capacity plot for a half-cell including the negative electrode active material. [Formula (1)] R = (dQ 2 / dV 2 ) / (dQ 1 / dV 1 ) ≥ 1.2

[0026] In Formula (1), dQ 1 / dV 1 is the maximum discharge peak of the first charge / discharge cycle at 0.3 to 0.6 V, dQ 2 / dV 2 is the maximum discharge peak of the second charge / discharge cycle at 0.3 to 0.6 V, and R is the ratio of the maximum discharge peaks. According to one embodiment of the present invention, by providing a negative electrode active material that satisfies the parameters related to Formula (1), while repairing the damaged silicon crystal during the process of secondary particle formation of the first silicon particles (primary particles), a negative electrode active material with improved conductivity and physical durability can be provided. By increasing the initial capacity and capacity retention rate of the secondary battery using such a negative electrode active material, the performance of the secondary battery can be significantly enhanced. Hereinafter, the configuration of the present invention will be described in more detail.

[0027] 1. Negative Electrode Active Material and Method for Manufacturing the Same The negative electrode active material according to the present invention includes second silicon particles in which first silicon particles are aggregated. The second silicon particles may be those generated by secondary particle formation of the first silicon particles. When an electrode is manufactured using the first silicon particles (primary particles) alone without going through the secondary particle formation process, during charging and discharging of the secondary battery, the surface of silicon may be physically cracked during the process of lithium insertion / desorption into silicon, the specific surface area of silicon may become excessively high, the binding property with the electrode may decrease, the crystal size may become small, and a problem may occur that the capacity of the battery cannot be sufficiently exhibited. According to an embodiment of the present invention, by using the second silicon particles that have been secondary particleized, the physical cracking phenomenon on the silicon surface can be effectively prevented, and the decrease in the binding property with the electrode can be prevented.

[0028] Specifically, the average particle size (D50) of the first silicon particles may be 100 to 200 nm, and more specifically may be 100 to 150 nm. The average particle size (D50) of the second silicon particles may be 3 to 30 μm, and specifically may be 3 to 10 μm. When the average particle size of the second silicon particles satisfies the numerical range, the binding property and durability with the electrode can be improved.

[0029] The negative electrode active material according to the present invention includes a carbon layer on the second silicon particles. Specifically, the carbon layer can include any one selected from the group consisting of crystalline carbon, amorphous carbon, and combinations thereof, and more specifically, can include a combination of crystalline carbon and amorphous carbon. Specifically, the carbon layer can include crystalline carbon derived from pitch and amorphous carbon derived from a carbon precursor. By the carbon layer including crystalline carbon and amorphous carbon, the conductivity and durability of the negative electrode active material are improved, and the capacity and maintenance rate of the battery can be significantly improved.

[0030] The carbon source for generating the crystalline carbon may be, for example, pitch such as coal tar pitch or petroleum pitch, saccharides, aromatic carbon materials such as lignin or cellulose, polyacrylonitrile, etc. Specifically, when pitch is used as the carbon source for generating the crystalline carbon, there is an advantage that the carbon content remaining after heat treatment can be adjusted up to 80% depending on the softening point.

[0031] The carbon precursor for generating the amorphous carbon may be, for example, a hydrocarbon in gas form, and specifically, it may be acetylene, toluene, methane, etc. By chemical vapor deposition (CVD), the carbon precursor can be deposited as an amorphous carbon layer on the second silicon particles. Since the carbon precursor is continuously supplied in gas form during the process of being deposited as a carbon layer containing amorphous carbon, a carbon layer without voids can be formed. Thereby, it can suppress the generation of an excessive solid electrolyte interphase (SEI) layer during the charge and discharge process, and at the same time play the role of a protective film that can protect against the volume expansion of silicon.

[0032] The half-cell containing the negative electrode active material according to the present invention can satisfy the following formula 1 in a differential capacity plot. [Formula 1] R=(dQ 2 / dV 2 ) / (dQ 1 / dV 1 )≧1.2

[0033] dQ 1 / dV 1 is the maximum discharge peak of the first charge-discharge cycle at 0.3 - 0.6V, and dQ 2 / dV 2is the maximum discharge peak of the second charge-discharge cycle at 0.3 to 0.6 V. According to an embodiment of the present invention, dQ corresponding to two variables 1 / dV 1 with respect to dQ 2 / dV 2 When the ratio of satisfies the above numerical range, it is possible to provide a negative electrode active material that effectively recovers the crystals damaged by the heat treatment temperature and improves conductivity and durability. The negative electrode active material can be heat-treated simultaneously with or after forming the second silicon particles by secondary particle formation of the first silicon particles. The negative electrode active material can be heat-treated simultaneously with or after depositing the carbon layer on the second silicon particles. At least one of the second silicon particles and the carbon layer may be formed by a heat treatment step at 600 to 1200 °C or lower, specifically 700 to 1000 °C, and more specifically 700 to 900 °C. If the temperature of the heat treatment step is less than the above numerical range, the damaged crystals may not be fully recovered, and if it exceeds the numerical range, a problem may occur in that the first silicon particles melt and cannot maintain the shape of the primary particles. Specifically, secondary particles produced by dispersing the pulverized first silicon particles in a solvent; or secondary particles produced by dispersing the pulverized first silicon particles and pitch in a solvent; and / or secondary particles with a carbon layer deposited by introducing a carbon precursor in gas form by chemical vapor deposition are put into a firing furnace to perform the heat treatment step, whereby a negative electrode active material can be produced. An embodiment of the present invention can improve the durability of the electrode by recovering the damaged silicon crystals by deriving the optimal range of dQ 1 / dV 1 with respect to dQ 2 / dV 2 by performing a heat treatment step.

[0034] According to still another embodiment of the present invention, in Formula 1, R may be 3.5 or less. Specifically, in Formula 1, R may be 1.5 to 2.8. When the range of R in Formula 1 satisfies the numerical range, the durability of the negative electrode active material, the capacity of the battery, and the retention rate can be significantly improved.

[0035] The carbon content according to the present invention may be 5 to 80% by weight, specifically 35 to 60% by weight, and more specifically 50 to 60% by weight based on the total weight of the negative electrode active material. The carbon content can be measured using an ordinary elemental analyzer in the art. When the carbon content satisfies the numerical range, the capacity or retention rate of the battery can be improved.

[0036] The crystalline size of the first silicon particles according to the present invention may be 15 to 30 nm, specifically 17 to 20 nm. The crystalline size of the first silicon particles can be calculated by XRD (X-ray diffraction) analysis and the Scherrer-Wilson formula. When the crystalline size of the first silicon particles satisfies the numerical range, the capacity and capacity retention rate of the battery can be further increased.

[0037] The thickness of the carbon layer of the negative electrode active material according to the present invention may be 1 to 110 nm, specifically 10 to 100 nm, more specifically 20 to 80 nm, and even more specifically 40 to 80 nm. The thickness of the carbon layer can be measured by observing a TEM (Transmission Electron Microscope) photograph of the manufactured negative electrode active material. When the thickness of the carbon layer satisfies the numerical range, the capacity and capacity retention rate of the battery can be further increased.

[0038] The BET specific surface area of the negative electrode active material according to the present invention may be 10 to 200 m 2 / g, specifically 10 to 150 m 2 / g, and more specifically 10 to 80 m 2It may also be / g. The specific surface area of the negative electrode active material may be the result of satisfying the average particle size range of the negative electrode active material, measured by the BET (Brunauer-Emmett-Teller) analysis method.

[0039] The total pore volume contained in the negative electrode active material according to the present invention is 0.05 to 1.0 cm 3 / g may also be acceptable, specifically 0.05 to 0.5 cm 3 / g may also be acceptable, and more specifically 0.05 to 0.2 cm 3 / g may also be acceptable. The total pore volume can be measured by the BJH (Barrett-Joyner-Halenda) analysis method. Still another embodiment of the present invention can provide a method for manufacturing a negative electrode active material.

[0040] The method for manufacturing a negative electrode active material according to an embodiment of the present invention may include: (S1) a step of manufacturing a dispersion liquid containing first silicon particles; and (S2) a step of spraying the dispersion liquid to manufacture second silicon particles. Optionally, the dispersion liquid may further contain pitch. The content of the pitch may be more than 0 and 50 parts by weight or less based on 100 parts by weight of the first silicon particles, and specifically may be 20 to 40 parts by weight. If the content of the pitch is less than the numerical range, a carbon layer containing sufficient crystalline carbon may not be formed, and if it exceeds the numerical range, the voids contained in the negative electrode active material may become excessively numerous.

[0041] The method for manufacturing a negative electrode active material according to still another embodiment of the present invention may further include: (S3) a step of putting the second silicon particles into a firing furnace and performing heat treatment at 600 to 1200 °C or lower under a carbon precursor gas. By the step (S3), a carbon layer can be realized in a state where crystalline carbon and amorphous carbon are mixed, thereby simultaneously improving conductivity and durability. As a result, the capacity and retention rate of the battery can be significantly improved. The thickness and carbon content of the carbon layer can be appropriately adjusted according to the content of the pitch or the content of the carbon precursor (gas).

[0042] According to still another embodiment of the present invention, the negative electrode active material may further contain one or more other negative electrode active materials in addition to the silicon-carbon composite negative electrode active material, such as carbon-based negative electrode active materials, silicon-based negative electrode active materials, tin-based negative electrode active materials, etc. The other negative electrode active materials may be, for example, graphite such as natural graphite and artificial graphite, hard carbon, soft carbon, silicon, silicon alloy, silicon oxide, tin, tin alloy, tin oxide or silicon-tin alloy. The other negative electrode active materials can be mixed with the second silicon particles to form the negative electrode active material. In this case, the content of the silicon-carbon composite negative electrode active material according to the present invention may be, for example, 0.5 to 30% by weight based on the weight of the solid content of the total negative electrode active material including the silicon-carbon composite negative electrode active material and other negative electrode active materials, and specifically may be 5 to 15% by weight.

[0043] 2. Negative electrode Other embodiments of the present invention can provide a negative electrode slurry and a negative electrode containing the negative electrode active material.

[0044] The content of the negative electrode active material according to one embodiment of the present invention may be 60 to 80% by weight based on the weight of the total solid content of the negative electrode slurry. When the negative electrode active material according to other embodiments of the present invention further contains other negative electrode active materials, the content of the negative electrode active material may be 90 to 99% by weight based on the weight of the total solid content of the negative electrode slurry, and specifically may be 95 to 99% by weight. When the content of the negative electrode active material satisfies the above numerical range, a large-capacity or high-output battery can be realized by effectively storing or releasing lithium ions emitted from the positive electrode. The negative electrode slurry according to the present invention may further contain a conductive material, a binder and a solvent.

[0045] The conductive material is not particularly limited as long as it has conductivity without inducing chemical changes in the battery. Examples of the conductive material include carbon blacks such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; carbon nanotubes; fluorocarbons; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. One or more of these can be used. The content of the conductive material may be, for example, 0.01 to 20% by weight based on the total solid content weight of the negative electrode slurry.

[0046] The binder is a component that assists in binding the conductive material, the negative electrode active material, or the current collector, and may usually be contained in an amount of 0.1 to 20% by weight based on the total solid content weight of the negative electrode slurry. The binder may be, for example, a mixture of one or more selected from the group consisting of polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HEP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose, starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene - butadiene rubber, nitrile - butadiene rubber, and lithium - substituted polyacrylate (Li - PAA).

[0047] The solvent can contain, for example, water or an organic solvent such as NMP (N - methyl - 2 - pyrrolidone). The negative electrode slurry according to the present invention can further contain additives such as a dispersant, if necessary. The negative electrode according to the present invention can be manufactured through a drying and rolling process after the negative electrode slurry is coated on at least one surface of the current collector.

[0048] The current collector is not particularly limited as long as it has conductivity without inducing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, those with a surface treatment of carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. can be used. The thickness of the current collector is not particularly limited, but it may be usually 3 to 500 μm which is applicable.

[0049] 3. Secondary battery A further embodiment of the present invention can provide a secondary battery including the negative electrode. Repeated explanations of the above-mentioned parts will be briefly described or omitted.

[0050] The secondary battery according to the present invention further includes a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolytic solution provided for ion movement between the negative electrode and the positive electrode.

[0051] The positive electrode can be manufactured by mixing a positive electrode active material, a conductive material, a binder, and a solvent to produce a positive electrode slurry, and then directly coating this on a current collector, or casting it on another support and laminating the positive electrode active material film peeled from the support onto a metal current collector.

[0052] The positive electrode active material is, for example, LiCoO 2 , LiNiO 2 , LiMn 2 O 4 , LiCoPO 4 , LiFePO 4 , Li[NiCoMn]O 2 and LiNi 1-x-y-z Co x M1 y M2 z O 2It may also be one or a mixture of two or more selected from the group consisting of. Said M1 and M2 are each independently any one selected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg and Mo, and x, y and z are each independently the atomic fraction of the oxide composition element, and can satisfy 0≦x<0.5, 0≦y<0.5, 0≦z<0.5, 0<x+y+z<1.

[0053] The separator can be a normal porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as a polyethylene homopolymer, a polypropylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, etc., and can be used alone or laminated. Also, a thin insulating film having high ion permeability and mechanical strength can be used. The separator may include a safety reinforced separator (SRS) with a ceramic substance thinly coated on the surface. In addition, as the separator, a normal porous non-woven fabric, for example, a non-woven fabric made of high melting point glass fibers, polyethylene terephthalate fibers, etc. can be used, but it is not limited thereto.

[0054] The electrolyte can include a lithium salt as an electrolyte and an organic solvent for dissolving the same. The lithium salt can be used without limitation as long as it is usually used in an electrolyte for a secondary battery.

[0055] As the organic solvent contained in the electrolyte, those commonly used in electrolytes for lithium secondary batteries can be used without limitation. As the organic solvent, for example, any one selected from the group consisting of propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran, or a mixture of two or more of these can be used.

Examples

[0056] Hereinafter, the examples of the present invention will be described in detail so that those having ordinary knowledge in the technical field to which the present invention pertains can easily implement them. However, this is merely an example, and the scope of the rights of the present invention is not limited by the following content.

[0057] [Production Example 1: Production of negative electrode active material] <Examples 1 to 4: When the source of the carbon layer is pitch> 1.1 Production of secondary particles The milled first silicon particles (D50: 120 nm) and pitch powder were dispersed in ethanol, and then the dispersion was injected by a spray dryer to produce second silicon particles (secondary particles). The content of the milled first silicon particles was adjusted to 3% by weight based on the total weight of the dispersion. Using the nozzle-type spray dryer, pressure (100 torr) was applied with an inert gas (Ar) so that the droplets could be uniformly injected, and the outlet temperature of the spray dryer was set to 85°C to produce second silicon particles (secondary particles) with an average size (D50) of 5 μm.

[0058] 1.2 Heat treatment process The second silicon particles (secondary particles) were put into a firing furnace, and a heat treatment process was performed under the heat treatment temperature conditions shown in Table 1 below to produce a negative electrode active material with a final carbon layer laminated thereon.

[0059] <Examples 5 to 9: When the source of the carbon layer is a-C> After milling the first silicon particles (D50: 120 nm) and dispersing them in ethanol, the dispersion was sprayed by a spray dryer to produce second silicon particles (secondary particles). The content of the milled first silicon particles was adjusted to 3% by weight based on the total weight of the dispersion. Using the nozzle-type spray dryer, pressure (100 torr) was applied with an inert gas (Ar) so that the droplets could be sprayed uniformly, the outlet temperature of the spray dryer was set to 85°C, and second silicon particles (secondary particles) with an average size (D50) of 5 μm were produced. Thereafter, acetylene gas was additionally introduced in the heat treatment step by chemical vapor deposition to produce a final negative electrode active material.

[0060] <Examples 10 to 21: When the source of the carbon layer is pitch + a-C> A negative electrode active material was produced in the same manner as in Examples 1 to 4, but acetylene gas was additionally introduced in the heat treatment step by chemical vapor deposition to produce a final negative electrode active material.

[0061] <Comparative Example 1: Negative electrode active material that has not undergone a heat treatment process> A negative electrode active material was produced in the same manner as in Examples 5 to 9, but the heat treatment step and the chemical vapor deposition step were omitted, and the crystal size of the first silicon particles was adjusted to the crystal size shown in Table 1 below.

[0062] <Comparative Example 2: When the temperature of the heat treatment step is 400°C> A negative electrode active material was produced in the same manner as in Comparative Example 1, but the crystal size of the first silicon particles was adjusted to the crystal size shown in Table 1 below, and the second silicon particles were put into a firing furnace and a heat treatment step was performed at 400°C.

[0063] <Comparative Example 3: When the temperature of the heat treatment process is 600°C> The negative electrode active material was produced in the same manner as in Comparative Example 1, but the crystal size of the first silicon particles was adjusted to the crystal size shown in Table 1 below. The second silicon particles were put into a firing furnace, and a heat treatment process was performed at 600°C.

[0064] [Table 1]

[0065] [Production Example 2: Production of a half-cell] Manufacture of the negative electrode Graphite:negative electrode active material:CMC (carboxy methyl cellulose):SBR (styrene-butadiene rubber) produced in Production Example 1 were mixed at a weight ratio of 87:10:1.5:1.5 to produce a negative electrode slurry dispersed in water (total solid content: 50% by weight). Thereafter, the negative electrode slurry was coated on one surface of a copper foil (thickness: 18 μm) as a current collector, and then dried at 90°C for 180 minutes to produce a negative electrode with a total thickness of 90 μm.

[0066] Manufacture of the half-cell The produced negative electrode, a counter electrode (Li metal), and a separator made of polyethylene interposed between the negative electrode and the counter electrode were placed in a battery case. Then, an electrolytic solution (1M LiPF 6 dissolved) in which FEC (fluoroethylene carbonate) / DMC (dimethyl carbonate) was mixed at a volume ratio of 3:7 was injected. A half-cell into which the electrolytic solution was injected was produced.

[0067] [Experimental Example 2: Performance evaluation of a half-cell] For the half-cell according to Production Example 2, the performance was evaluated by the following measurement method using a WBCS3000 measuring device manufactured by WonATech.

[0068] 1) R (Ratio of Discharge Peak) For the stable charge-discharge evaluation of the half-cell according to Production Example 2, evaluation was performed with a current level of 0.1 C. After drawing a dQ / dV curve with respect to voltage, with the change amount of capacitance with respect to voltage (dQ / dV) on the y-axis and voltage on the x-axis using the charge amount and voltage value during charge and discharge, the ratio value of the discharge peak with respect to dQ / dV near 0.5 V was shown in Table 2 below.

[0069] 2) 1-cycle Capacity After the initial stabilization step, when evaluating the half-cell according to Production Example 2 at a current level of 0.5 C, the initial capacity was measured and shown in Table 2 below.

[0070] 3) Retention Rate Evaluation (100-cycle Retention Rate) After the initial stabilization step, for the half-cell according to Production Example 2 at a current level of 0.5 C, after performing 100 charge-discharges, the ratio (%) of discharge capacity / initial capacity was measured and shown in Table 2 below.

[0071]

Table 2

[0072] In Table 2 above, when comparing Examples 1 to 4, it can be confirmed that as the carbon content increases, the initial capacity and retention rate tend to increase, and R = (dQ 2 / dV 2 ) / (dQ 1 / dV 1 ) increases significantly. It can be inferred that this is due to the interaction between the pitch and silicon crystals during the crystallization process of the pitch.

[0073] In Table 2, when comparing Examples 5 to 9, similar to Examples 1 to 4, it can be confirmed that as the carbon content increases, the initial capacity and the retention rate tend to increase. However, it can be confirmed that the thickness of the carbon layer of the outer shell compared to the same carbon amount as the pitch tends to be large. In the case of pitch, in the crystallization step, a part cannot be crystallized and disappears, generating voids (pores). However, in the process of depositing the amorphous carbon layer, acetylene gas is continuously supplied, so it can be confirmed that there is no factor inducing voids.

[0074] In Table 2, when comparing Examples 10 to 15, to some extent, it can be confirmed that as the crystal size of the first silicon particles increases, R tends to increase. However, when the crystal size of the first silicon particles becomes excessively large, a-SiLi δ is excessively generated, and it can be confirmed that the capacity and the retention rate tend to decrease accordingly. Therefore, it can be confirmed that the capacity of the battery is high when the crystal size of the first silicon particles (primary particles) is 15 to 30 nm, specifically 17 to 20 nm.

[0075] In Table 2, when comparing Examples 16 to 21 in which all pitch and acetylene gas are used as carbon sources (source), it can be confirmed that the thickness of the carbon layer compared to the carbon content decreases significantly. Also, when the thickness of the carbon layer in Example 17 is at about 60 nm level, it shows the best capacity and retention rate. When the carbon layer is laminated excessively beyond this, the amount of silicon that can accept lithium decreases, and it can be confirmed that it rather tends to decrease in terms of capacity and retention rate. Furthermore, when the thickness of the carbon layer is laminated from more than 80 nm to 110 nm, it can be confirmed that the capacity and the retention rate decrease compared to the case where the amorphous carbon layer is laminated alone. When manufacturing a composite in which the carbon layer is laminated with two carbon sources, the thickness of the carbon layer is preferably 20 to 80 nm, and it can be confirmed that it is most preferable at 40 to 80 nm.

[0076] In Table 2, when comparing Comparative Examples 1 to 3, as a result of adjusting the crystal size of the first silicon particles (primary particles) by heat treatment without adding a carbon layer, it can be confirmed that the capacity and retention rate are improved compared to Comparative Example 1 without heat treatment, and it can be confirmed that the increased crystal size is reflected in R.

[0077] Referring to the comparative examples and examples in Table 2, it can be confirmed that in order to recover the crystal size of the pulverized silicon particles and exhibit the original capacity, R is 1.2 or more.

[0078] [Production Example 3: Production of full cell] Manufacture of the positive electrode NCM(Li[NiCoMn]O 2 ): A mixture in which carbon black: nitrile-butadiene rubber (NBR) is mixed at a weight ratio of 96:2:2 was added to a solvent (N-methylpyrrolidone), and then a positive electrode slurry with a total solid content of 75% by weight was produced. After coating the positive electrode slurry on one side of an aluminum foil (thickness: 20 μm), it was dried at 120 °C for 120 minutes to produce a positive electrode with a total thickness of 70 μm.

[0079] Manufacture of the full cell A battery was produced in the same manner as in Production Example 2, but instead of the counter electrode (Li metal), a full cell was produced using the produced positive electrode. Here, the ratio of the capacity per unit area of the negative electrode to the positive electrode (N / P ratio) was adjusted to 1.1, and the positive electrode was punched out at 14 pi and the negative electrode was punched out at 16 pi in a 2032 coin cell.

[0080] [Experimental Example 3: Performance evaluation of full cell] For the full cell according to Production Example 3, with a charging voltage of 4.2 V and a discharging voltage of 2.8 V, a retention rate evaluation was carried out at a current level of 0.5 C using the same measurement method as in Experimental Example 2.

[0081]

Table 3

[0082] Referring to Table 3 above, it can be confirmed that by applying heat treatment to the second silicon particles with a carbon layer laminated thereon, the capacity and retention rate are significantly improved in a full cell, and it can be confirmed that the performance of the full cell of Example 17 with a carbon layer laminated with a mixture of crystalline carbon and amorphous carbon using both pitch and acetylene gas as two carbon sources is the best. Thereby, in order to ensure the conductivity and durability in a full cell, it can be confirmed that when using silicon particles with a carbon layer in which amorphous carbon and crystalline carbon are mixed, the capacity and retention rate of the cell are further improved.

[0083] As described above, the preferred embodiments of the present invention have been described in detail. However, the scope of the rights of the present invention is not limited thereto, and various modifications and improvements by those skilled in the art using the basic concept of the present invention defined in the following claims also belong to the scope of the rights of the present invention.

Claims

1. A negative electrode active material comprising second silicon particles in which first silicon particles are aggregated and a carbon layer on the second silicon particles, wherein the differential capacity plot for a half-cell containing the negative electrode active material satisfies the following formula (1): Negative electrode active material: [[Formula 1]] In Formula (1), R = (dQ ２ / dV ２ ) / (dQ １ / dV １ ) ≥ 1.2 R is the ratio of the maximum discharge peak. dQ １ / dV １ is the maximum discharge peak of the first charge / discharge cycle at 0.3 to 0.6 V, dQ ２ / dV ２ is the maximum discharge peak of the second charge / discharge cycle at 0.3 to 0.6 V,

2. In Formula (1), R is 3.5 or less, The negative electrode active material according to Claim 1.

3. In Formula (1), R is 1.5 to 2.8, The negative electrode active material according to Claim 2.

4. The carbon layer contains any one selected from the group consisting of crystalline carbon, amorphous carbon, and combinations thereof, The negative electrode active material according to Claim 1.

5. The carbon layer contains the crystalline carbon derived from pitch and the amorphous carbon derived from a carbon precursor, The negative electrode active material according to Claim 4.

6. Based on the total weight of the negative electrode active material, the carbon content is 5 to 80% by weight, The negative electrode active material according to Claim 1.

7. Based on the total weight of the negative electrode active material, the carbon content is 35 to 60% by weight, The negative electrode active material according to Claim 6.

8. The crystal size of the first silicon particles is 15 to 30 nm, The negative electrode active material according to Claim 1.

9. The crystal size of the first silicon particles is 17 to 20 nm, The negative electrode active material according to Claim 8.

10. The thickness of the carbon layer is 20 to 80 nm, The negative electrode active material according to Claim 1.

11. The thickness of the carbon layer is 40 to 80 nm, The negative electrode active material according to Claim 10.

12. A negative electrode containing the negative electrode active material according to any one of Claims 1 to 11.

13. A secondary battery containing the negative electrode according to Claim 12. ​ ​ ​ ​

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