Silicon-carbon mixture, method for preparing the same, and negative electrode active material and lithium secondary battery containing the same.

Abstract

The present invention relates to a silicon-carbon mixture, a method for preparing the same, and a negative electrode active material and a lithium secondary battery containing the same. The silicon-carbon mixture contains two or more composite materials, silicon particles, magnesium silicate, and carbon, and has a molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms that satisfies 0.06 to 0.90. Therefore, when the silicon-carbon mixture is applied to the negative electrode active material, the discharge capacity, initial efficiency, and capacity retention rate after cycling of the lithium secondary battery can be improved at the same time.

Description

Technical Field

[0001] The present invention relates to a silicon-carbon mixture, a method for preparing the same, a negative electrode active material, and a lithium secondary battery including the same.

Background Art

[0002] In recent years, with the development of the information and communication industry, as electronic devices become smaller, lighter, thinner, and more portable, the demand for higher energy density of batteries used as power sources for these electronic devices has been increasing. Lithium secondary batteries are the batteries that best meet this demand, and not only research on small batteries using them but also applications to large electronic devices such as automobiles and power storage systems have been actively carried out.

[0003] As a negative electrode active material of such a lithium secondary battery, a carbon material is widely used. In order to further improve the battery capacity, silicon-based negative electrode active materials have been studied. Since the theoretical capacity of silicon (4,199 mAh / g) is more than 10 times that of graphite (372 mAh / g), a significant improvement in battery capacity is expected. [[ID=??]]

[0004] The reaction scheme when lithium is intercalated into silicon is, for example, as follows: [Reaction Scheme 1]

Chemical formula

[0005] In the silicon-based negative electrode active material according to the above reaction scheme, a high-capacity alloy containing a maximum of 4.4 lithium atoms per silicon atom is formed. However, in most silicon-based negative electrode active materials, the intercalation of lithium causes a volume expansion of up to 300%, which destroys the negative electrode and makes it difficult to exhibit high cycle characteristics.

[0006] ​Furthermore, this volume change can cause cracks on the surface of the negative electrode active material, leading to the formation of ionic material inside the negative electrode active material, which can cause the negative electrode active material to electrically detach from the current collector. This electrical detachment phenomenon can significantly reduce the battery's capacity retention rate.

[0007] To solve this problem, Japanese Patent No. 4393610 discloses a negative electrode active material in which silicon and carbon are mechanically processed to form a composite material, and the surface of silicon particles is coated with a carbon layer using chemical vapor deposition (CVD).

[0008] Furthermore, Japanese Patent Publication No. 2016-502253 discloses a negative electrode active material that includes porous silicon-based particles and carbon particles, wherein the carbon particles include fine carbon particles and coarse carbon particles with different average particle sizes.

[0009] However, these prior art documents, although relating to negative electrode active materials containing silicon and carbon, have limitations in suppressing volume expansion and contraction during charging and discharging. Over long periods of thousands of cycles or more, there is a potential problem of significantly reduced capacity retention and decreased reliability. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent No. 4393610 [Patent Document 2] Japanese Patent Publication No. 2016-502253 [Patent Document 3] Korean Patent Application Publication No. 2018-0106485 Specification [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] The object of the present invention is to provide a silicon-carbon mixture that comprises two or more composite materials, including silicon particles, magnesium silicate, and carbon, and in which the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) is controlled within a specific range, thereby improving the performance of a lithium secondary battery when applied as a negative electrode active material.

[0012] Another object of the present invention is to provide a method for preparing the silicon-carbon mixture.

[0013] Another object of the present invention is to provide a negative electrode active material containing the silicon-carbon mixture and a lithium secondary battery containing the same. [Means for solving the problem]

[0014] To achieve the above objective, one embodiment of the present invention provides a silicon-carbon mixture comprising two or more composite materials, including silicon particles, magnesium silicate, and carbon, with a molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) of 0.06 and 0.90.

[0015] Another embodiment provides a method for preparing the silicon-carbon mixture, comprising a first step of obtaining a first composite material; a second step of obtaining a second composite material; and a third step of mixing the first composite material and the second composite material, wherein the first composite material comprises first silicon particles and first carbon, and the second composite material comprises second silicon particles, magnesium silicate and second carbon.

[0016] Another embodiment provides a negative electrode active material comprising the silicon-carbon mixture.

[0017] Furthermore, another embodiment provides a lithium secondary battery containing the negative electrode active material. [Effects of the Invention]

[0018] According to this embodiment, a silicon-carbon mixture is provided that comprises two or more composite materials, including silicon particles, magnesium silicate, and carbon, in which the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) is controlled within a specific range. Therefore, a lithium secondary battery with a well-balanced and excellent performance in terms of discharge capacity, initial efficiency, and capacity retention during cycling can be realized.

[0019] In particular, because the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) in the silicon-carbon mixture is controlled within a specific range, the carbon content and porosity can be easily controlled. As a result, excellent capacity retention can be maintained even after thousands of cycles over long periods, thereby further improving reliability.

[0020] In addition, the method of one embodiment has the advantage that mass production is possible by a continuous method with a minimum number of steps. [Brief explanation of the drawing]

[0021] The following drawings accompanying this specification illustrate preferred embodiments of the present invention and are useful in further understanding the technical idea of ​​the present invention together with the description of the invention. Accordingly, the present invention should not be construed as being limited only to what is depicted in the drawings.

[0022] [Figure 1] Figure 1 shows the results of analyzing the crystal structure of the silicon-carbon mixture prepared in Example 1 using an X-ray diffraction analyzer (X'Pert3, Malvern Panalytical). [Figure 2] Figure 2 shows the results of analyzing the crystal structure of the silicon-carbon mixture prepared in Example 7 using an X-ray diffraction analyzer (X'Pert3, Malvern Panalytical). [Figure 3] Figure 3 is an ion beam scanning electron microscope (FIB-SEM) image of the porous silicon composite material prepared in Preparation Example 2-1, observed at 10,000x magnification. [Figure 4]Figure 4 is an ion beam scanning electron microscope (FIB-SEM) image of the porous silicon composite material prepared in Preparation Example 2-1, observed at 30,000x magnification. [Figure 5] Figure 5 is an ion beam scanning electron microscope (FIB-SEM) image of the porous silicon composite material prepared in Preparation Example 2-1, observed at 100,000x magnification. [Figure 6] Figure 6 is a scanning electron microscope (FE-SEM) image of the first composite material prepared in Preparation Example 3-1, observed at 10,000x magnification. [Figure 7] Figure 7 is a bright-field image of the first composite material prepared in Preparation Example 3-1, taken using a transmission electron microscope (FE-TEM). [Figure 8] Figure 8 shows dark-field and bright-field images of the first composite material prepared in Preparation Example 3-1, taken using a transmission electron microscope (FE-TEM). [Modes for carrying out the invention]

[0023] The present invention is not limited to what is disclosed below. Rather, it can be modified in various ways, as long as the essence of the invention is not altered.

[0024] In this specification, if a part is referred to as "containing" an element, please understand that unless otherwise specified, that part may also contain other elements.

[0025] Furthermore, please understand that all numerical values ​​and expressions regarding the amounts of ingredients, reaction conditions, etc., used herein are modified by the term "approximately" unless otherwise specified. [Silicon-carbon mixture]

[0026] One embodiment of the silicon-carbon mixture comprises two or more composite materials, including silicon particles, magnesium silicate, and carbon, with a molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) of 0.06 and 0.90.

[0027] A silicon-carbon mixture according to one embodiment comprises two or more composite materials, including silicon particles, magnesium silicate, and carbon, and the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) is controlled within a specific range, thereby enabling the realization of a silicon-carbon mixture with desired structure and properties. When applied as a negative electrode active material for lithium secondary batteries, it can improve discharge capacity, initial efficiency, and capacity retention during cycling.

[0028] The molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon mixture is preferably 0.09 to 0.80, more preferably 0.09 to 0.60, even more preferably less than 0.10 to 0.60, and most preferably 0.1 to 0.50. By applying a silicon-carbon mixture satisfying the above molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon mixture to the negative electrode active material, the content of silicon particles as an active material can be increased, thereby simultaneously improving the capacity retention rate, discharge capacity, and initial efficiency of the lithium secondary battery. In particular, by controlling the pore structure and porosity while controlling the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the mixture within a specific range, it is possible to maintain an excellent capacity retention rate even after repeating thousands of cycles over a long period, providing the important advantage of improved reliability.

[0029] If the molar ratio of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon mixture (O / Si) is smaller than the above range, manufacturing becomes difficult due to process issues. If the molar ratio of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon mixture (O / Si) exceeds the above range, the desired effect may be reduced.

[0030] Furthermore, the molar ratio (Mg / Si) of magnesium (Mg) atoms to silicon (Si) atoms in the silicon-carbon mixture may be 0.009 to 0.55, 0.009 to 0.30, or 0.009 to 0.10. When the molar ratio (Mg / Si) of magnesium (Mg) atoms to silicon (Si) atoms in the silicon-carbon mixture satisfies the above range, the resistance during lithium desorption and intercalation is reduced, thereby further improving the discharge capacity, initial efficiency, and post-cycle capacity retention rate of the lithium secondary battery.

[0031] Furthermore, two or more composite materials may include a first composite material and a second composite material having different components, structures, and properties from each other.

[0032] Furthermore, according to various embodiments, the content of each component in the first and second composite materials and their mixing range can be adjusted to the optimal range necessary to improve the performance of the lithium secondary battery. In the manufacturing method, etching conditions and carbon coating conditions can be adjusted in various ways, thereby optimizing the physical properties of the silicon-carbon mixture, such as specific surface area, density, and porosity. Thus, the performance and reliability of the lithium secondary battery can be further improved.

[0033] The specific structure, composition, content, and physical properties of the first and second composite materials contained in the silicon-carbon mixture are as follows:

[0034] The weight ratio of the first composite material to the second composite material can be an important factor in achieving the desired performance of the lithium secondary battery.

[0035] For example, the weight ratio of the first composite material to the second composite material may be 90:10 to 5:95, 90:10 to 10:90, 90:10 to 20:80, 90:10 to 50:50, 90:10 to 60:40, 90:10 to 65:35, or 90:10 to 70:30. When the weight ratio of the first composite material to the second composite material satisfies the above range, it may be easier to achieve the desired effect, and in particular, it may be possible to provide a lithium secondary battery with well-balanced and excellent performance in terms of discharge capacity, initial efficiency, and capacity retention rate during cycling.

[0036] Furthermore, since the silicon-carbon mixture of one embodiment includes a first composite material and a second composite material uniformly mixed within the above range, a lithium secondary battery with the desired characteristics for its application can be realized by adjusting the physical properties of the final silicon-carbon mixture, such as the size of the silicon particles, porosity, magnesium content, specific surface area, thermal expansion coefficient, and porosity.

[0037] If the weight ratio of the first composite material to the second composite material falls outside the specified range, and the content of the first composite material is too low or the content of the second composite material is too high, it becomes difficult to achieve the desired value for the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon mixture. This can reduce the effect of mitigating the volume expansion of silicon particles due to charging and discharging. In such cases, the cycle characteristics and discharge capacity of the lithium secondary battery may decrease. Furthermore, the capacity retention rate after cycling of a lithium secondary battery made of a silicon-carbon mixture may decrease.

[0038] On the other hand, the silicon-carbon mixture contains magnesium silicate. Furthermore, the silicon-carbon mixture may contain magnesium silicate, which is included in the second composite material described later.

[0039] Furthermore, the carbon contained in the silicon-carbon mixture may include the first and second carbons described later.

[0040] On the other hand, the magnesium (Mg) content in the silicon-carbon mixture is 0.1% to 15% by weight, preferably 0.1% to 10% by weight, and more preferably 0.5% to 5% by weight, based on the total weight of the silicon-carbon mixture. If the magnesium (Mg) content in the silicon-carbon mixture is above the lower limit of the above range, the initial efficiency of the lithium secondary battery can be improved. If it is below the upper limit of this range, it may be advantageous in terms of the charge / discharge capacity, cycle characteristics, and handling stability of the lithium secondary battery.

[0041] The oxygen (O) content in the silicon-carbon mixture may be 1% to 25% by weight, preferably 1% to 21% by weight, more preferably 1% to 17% by weight, and even more preferably 3% to 17% by weight, based on the total weight of the silicon-carbon mixture.

[0042] If the oxygen (O) content in the silicon-carbon mixture is below the above range, the degree of expansion during charging of the lithium secondary battery increases, and the cycle characteristics deteriorate. Furthermore, if the oxygen (O) content in the silicon-carbon mixture exceeds the above range, when the silicon-carbon mixture is used as the negative electrode active material, irreversible reactions with lithium increase, leading to a decrease in initial charge-discharge efficiency, increased likelihood of detachment from the negative electrode current collector, and a potential deterioration in the charge-discharge cycle of the lithium secondary battery.

[0043] The silicon (Si) content in the silicon-carbon mixture may be 35% to 80% by weight, preferably 40% to 80% by weight, more preferably 45% to 80% by weight, and even more preferably 45% to 70% by weight, based on the total weight of the silicon-carbon mixture.

[0044] If the silicon (Si) content in the silicon-carbon mixture is below the above range, the amount of active material for lithium absorption and release decreases, which may reduce the charge and discharge capacity of the lithium secondary battery. On the other hand, if it exceeds the above range, the charge and discharge capacity of the lithium secondary battery may increase, but at the same time, the expansion and contraction of the electrodes during charging and discharging may become excessively large, further pulverizing the negative electrode active material powder, which may reduce the cycle characteristics of the lithium secondary battery.

[0045] The carbon (C) content in the silicon-carbon mixture may be 9% to 50% by weight, preferably 9% to 40% by weight, and more preferably 10% to 35% by weight, based on the total weight of the silicon-carbon mixture.

[0046] If the carbon (C) content in the silicon-carbon mixture is lower than the above range, there is a concern that the effect of improving conductivity will be small, and the electrode life characteristics of the lithium secondary battery may deteriorate. Also, if the carbon (C) content in the silicon-carbon mixture exceeds the above range, the discharge capacity of the lithium secondary battery may decrease, and the bulk density may decrease, which may reduce the discharge capacity per unit volume.

[0047] On the other hand, the silicon-carbon mixture may further contain silicon oxide (also called silicon oxide compounds). In that case, the capacity and life characteristics of the lithium secondary battery can be further improved, and volume expansion can be suppressed. In particular, the silicon oxide can be uniformly distributed with the silicon particles. In such cases, volume expansion can be reduced.

[0048] The silicon dioxide contained in the silicon-carbon mixture may include first silicon dioxide, second silicon dioxide, and combinations thereof as described below.

[0049] Furthermore, silicon dioxide is SiO x The compound may also be one represented by (0.4 ≤ x ≤ 2).

[0050] In other words, a silicon-carbon mixture is silicon oxide (SiO₂ xmay further contain (0.4 < x ≤ 2).

[0051] Specifically, silicon oxide (SiO x , (0.4 < x ≤ 2) can be used in an amount of 0.1 wt% to 5 wt% based on the total weight of the silicon-carbon mixture.

[0052] When the content of silicon oxide is outside the above range, silicon oxide may react with lithium to form strong alkali compounds such as Li4SiO4 and Li2O. Silicon oxide is preferably amorphous. When silicon oxide is amorphous, even if volume changes of silicon particles occur during intercalation and desorption of lithium, pulverization can be prevented or alleviated, and side reactions between silicon particles and the electrolyte can be prevented or reduced.

[0053] On the other hand, the silicon particles contained in the silicon-carbon mixture can include first silicon particles and second silicon particles, as described later.

[0054] Furthermore, the silicon-carbon mixture can include silicon aggregates in which silicon particles are bonded to each other. The silicon aggregates are as follows.

[0055] When the silicon particles (for example, first silicon particles and second silicon particles) in the silicon-carbon mixture are subjected to X-ray diffraction analysis (converted from the X-ray diffraction analysis results), the average value of the crystallite size of the silicon particles can be 1 nm to 15 nm, preferably 1 nm to 10 nm, more preferably 2 nm to 8 nm, and even more preferably 3 nm to 6 nm. In such cases, the performance of the lithium secondary battery such as discharge capacity, initial efficiency, or cycle life characteristics can be further improved.

[0056] The silicon-carbon mixture can have an average particle size (D 50 ) of 2 μm to 15 μm, preferably 2 μm to 10 μm, more preferably 2 μm to 6 μm.

[0057] In addition, the silicon-carbon mixture has a minimum particle size D of 0.3 or less.min and a maximum particle size D of 8 μm to 30 μm max can have. When the D min and D max of the silicon-carbon mixture respectively satisfy the above ranges, the specific surface area of the silicon-carbon mixture decreases, thereby further improving the initial efficiency and cycle characteristics of the lithium secondary battery.

[0058] On the other hand, the silicon-carbon mixture has a density of 1.8 g / cm 3 to 2.5 g / cm 3 , preferably 2.0 g / cm 3 to 2.5 g / cm 3 , more preferably 2.0 g / cm 3 to 2.4 g / cmC 3 can have. When the density of the silicon-carbon mixture satisfies the above range, it can be more advantageous for realizing the desired effect.

[0059] The silicon-carbon mixture can have a specific surface area (Brunauer-Emmett-Teller method; BET) of 2 m 2 / g to 50 m 2 / g, preferably 2 m 2 / g to 30 m 2 / g, more preferably 2 m 2 / g to 8 m 2 / g. When the specific surface area of the silicon-carbon mixture is less than the above range, the charge-discharge characteristics of the lithium secondary battery may deteriorate. When it exceeds the above range, since the contact area between the silicon-carbon mixture and the electrolyte increases, the decomposition reaction of the electrolyte is promoted, which may cause side reactions. When the specific surface area of the silicon-carbon mixture satisfies the above range, the doping and de-doping of lithium are actively carried out, and the irreversible capacity decreases, so excellent capacity characteristics and high rate characteristics can be achieved.

[0060] The silicon-carbon mixture can contain pores.

[0061] Furthermore, the silicon-carbon mixture may contain almost no pores.

[0062] Furthermore, silicon-carbon mixtures may not contain pores.

[0063] If the silicon-carbon mixture contains pores, the diameters of the pores may be the same or different.

[0064] Pores may include internal pores.

[0065] Internal pores include inner pores (also called closed pores) and open pores. Because at least a portion of the pore wall can be open to form an open structure, pores may or may not be connected to other pores.

[0066] Pores may include inner pores. Inner pores are independent pores that are not connected to other pores because their entire walls are closed, forming a closed structure.

[0067] Pores may include open pores.

[0068] According to one embodiment, the silicon-carbon mixture has a porosity of 0.1% to 30%, preferably 0.1% to 20%, and more preferably 0.1% to 10%. The term "pore" is sometimes used interchangeably with "void." Porosity may refer to internal porosity.

[0069] When the porosity of the silicon-carbon mixture meets the above range, it is possible to create a buffering effect against volume expansion while maintaining sufficient mechanical strength.

[0070] If the porosity of the silicon-carbon mixture, i.e., the internal porosity of the silicon-carbon mixture, is lower than the above range, it may be difficult to suppress the volume expansion of the negative electrode active material during charging and discharging. If it exceeds the above range, the mechanical strength may decrease due to the presence of numerous pores in the negative electrode active material, raising concerns that the negative electrode active material may collapse during the manufacturing process of lithium secondary batteries, for example, during the mixing of the negative electrode active material composition (slurry) or during the rolling process after coating.

[0071] If the internal porosity of the silicon-carbon mixture meets the above range, then when applied as the negative electrode active material for lithium secondary batteries, it is possible to obtain a volume expansion buffer effect while maintaining sufficient mechanical strength. Therefore, the volume expansion problem caused by the use of silicon particles can be minimized, high capacity can be achieved, and life characteristics can be improved.

[0072] The internal pores include open pores on the surface of the silicon-carbon mixture and internal pores.

[0073] In this specification, porosity can be defined as shown in Equation 1 below. Porosity may sometimes refer to internal porosity.

[0074] [Equation 1] Porosity (%) = Pore volume per unit mass / (Specific volume + Pore volume per unit mass)

[0075] The method of measuring porosity is not particularly limited. Based on one embodiment of the present invention, for example, it may be measured using an adsorbent gas such as nitrogen with a BELSORP (BET instrument) manufactured by BEL JAPAN, or it may be measured by mercury permeation (Hg porosimeter).

[0076] However, in this invention, it may be difficult to measure accurate values ​​using adsorption gases such as nitrogen or mercury permeation methods. This may be because the surfaces of each composite material are sealed by the carbon coating of the first composite material and / or the second composite material, preventing nitrogen gas from reaching the inner pores. In such cases, the internal porosity can be obtained by summing the value obtained by adsorption gas (BET) or mercury permeation method with the internal porosity given by equation 2 below.

[0077] [Equation 2] Internal porosity (%) = {1 - (measured density / calculated density)} × 100

[0078] Furthermore, in equation 2, the measured density can be measured using helium.

[0079] In silicon-carbon mixtures, if the inner pores are closed and nitrogen gas or mercury does not reach them, the porosity measured by the nitrogen adsorption method is considered to be close to the open porosity. Various examples of silicon-carbon mixtures

[0080] Silicon-carbon mixtures can have a variety of structures.

[0081] Specifically, the silicon-carbon mixture can include silicon-carbon mixture (1), silicon-carbon mixture (2), and combinations thereof, depending on the porosity.

[0082] Specifically, silicon-carbon mixture (1) may be a silicon-carbon mixture containing a small amount of pores, and silicon-carbon mixture (2) may be a porous mixture.

[0083] In one embodiment of the silicon-carbon mixture, silicon-carbon mixture (1) and silicon-carbon mixture (2) with controlled porosity can be obtained by adjusting the carbon coating method and the amount of carbon coating. The carbon coating can be formed by chemical thermal decomposition vapor deposition (CVD).

[0084] Furthermore, the porosity of the silicon-carbon mixture can be controlled in the silicon-carbon mixture preparation process described later by adjusting the etching conditions of the first composite material, such as the etching solution (etchant), its concentration, and etching time; and / or the carbon coating conditions and amount of carbon coating for the first and second composite materials, such as the type of carbon source, coating temperature, and coating time. The technical importance lies in the fact that the porosity of the silicon-carbon mixture can be optimized in this way according to the characteristics required for lithium secondary batteries. Silicon-carbon mixture (1)

[0085] The silicon-carbon mixture (1) of one embodiment may contain a small amount of pores.

[0086] Specifically, the silicon-carbon mixture (1) has an internal porosity of 0.1% to 10%, preferably 0.1% to 8%, and more preferably 0.1% to 5%. The method for measuring the internal porosity is as described above.

[0087] If the internal porosity of the silicon-carbon mixture (1) satisfies the above range, then when applied as the negative electrode active material for a lithium secondary battery, it is possible to obtain a volume expansion buffering effect while maintaining sufficient mechanical strength. Therefore, the volume expansion problem caused by the use of silicon particles can be minimized, high capacity can be achieved, and life characteristics can be improved.

[0088] The internal porosity of the silicon-carbon mixture (1) can be controlled by adjusting the etching conditions, carbon coating conditions, and carbon coating amount of the first and second composite materials, respectively.

[0089] For example, the internal porosity of the silicon composite material obtained by etching the first composite material before carbon coating (step 1-1 or 1-2 in the preparation process described later) may be 10% to 50%, preferably 15% to 50%, and more preferably 20% to 50%. However, the internal porosity of the first composite material obtained by forming the first carbon inside the silicon composite material using a chemical pyrolysis vapor deposition method after etching (step 1-3 in the preparation method described later) and forming the first carbon layer on the surface of the silicon composite material can be adjusted by adjusting the carbon coating conditions and amount of carbon coating so that the silicon-carbon mixture (1) has an internal porosity within the above range.

[0090] In other words, in the first method for preparing a composite material, the large number of internal pores present in the silicon composite material formed after etching may be filled with carbon by carbon coating using a chemical pyrolysis vapor deposition method, and the amount of carbon filling the internal pores can be adjusted by controlling the carbon coating conditions and the amount of carbon coating so that the silicon-carbon mixture (1) has an internal porosity that satisfies the above range.

[0091] On the other hand, the silicon-carbon mixture (1) may have an internal porosity of 0.1% to 6%, preferably 0.1% to 5%, and more preferably 0.1% to 4%.

[0092] If the internal porosity of the silicon-carbon mixture (1) satisfies the above range, then when applied as the negative electrode active material for a lithium secondary battery, it is possible to obtain a volume expansion buffering effect while maintaining sufficient mechanical strength. Therefore, the volume expansion problem caused by the use of silicon particles can be minimized, high capacity can be achieved, and life characteristics can be improved.

[0093] If the internal porosity and / or inner porosity of the silicon-carbon mixture (1) each meet the above ranges, then it is possible to maintain excellent capacity retention even after repeating thousands of cycles over a long period, which offers the significant advantage of improved reliability. Silicon-carbon mixture (2)

[0094] The silicon-carbon mixture (2) of one embodiment may be porous because it contains a large number of pores.

[0095] Specifically, the silicon-carbon mixture (2) may have an internal porosity of 4% to 20%, preferably 6% to 20%, and more preferably 8% to 20%.

[0096] If the internal porosity of the silicon-carbon mixture (2) satisfies the above range, the volume expansion of silicon particles that may occur during the charging and discharging of the lithium secondary battery can be mitigated and suppressed through the pores.

[0097] If the internal porosity of the silicon-carbon mixture (2) exceeds the above range, the mechanical strength decreases due to the numerous pores present in the negative electrode active material, and the negative electrode active material may collapse during the manufacturing process of lithium secondary batteries, for example, during the mixing of the negative electrode active material composition (slurry) or during the rolling process after coating.

[0098] On the other hand, the silicon-carbon mixture (2) has an internal porosity of 2% to 10%, preferably more than 4% and up to 10%, and more preferably 6% to 10%.

[0099] If the internal porosity of the silicon-carbon mixture (2) satisfies the above range, then when applied as the negative electrode active material for a lithium secondary battery, it is possible to obtain a volume expansion buffering effect while maintaining sufficient mechanical strength. Therefore, the volume expansion problem caused by the use of silicon particles can be minimized, high capacity can be achieved, and life characteristics can be improved.

[0100] Because the silicon-carbon mixture (2) has a porous structure, the electrolyte can easily penetrate the silicon-carbon mixture (2), thereby improving the charge and discharge characteristics of the lithium secondary battery.

[0101] The first and second composite materials contained in the silicon-carbon mixture will be described in detail below. <First Composite Material>

[0102] In one embodiment of the silicon-carbon mixture, the first composite material comprises first silicon particles and first carbon.

[0103] Specifically, the first composite material may contain first silicon particles. Furthermore, the first composite material may contain first silicon aggregates in which the first silicon particles are bonded together.

[0104] Specifically, the first composite material may include a first silicon aggregate having a three-dimensional (3D) structure in which two or more first silicon particles are bonded to each other. Since the first silicon particles are filled with lithium, if the first composite material does not contain the first silicon particles, the capacity of the lithium secondary battery may decrease. In particular, by using a first silicon aggregate in which the first silicon particles are bonded to each other, excellent mechanical properties such as strength can be obtained, a negative electrode active material composition with good dispersibility can be prepared, and a negative electrode active material composition with excellent processability can be applied to a current collector.

[0105] According to one embodiment, the first composite material may include a silicon composite material comprising first silicon particles and first carbon.

[0106] Furthermore, the first silicon aggregates can be uniformly distributed within the first composite material, specifically within the silicon composite material, and the first silicon particles may include silicon particles that are not bonded to each other. The first silicon particles and / or the first silicon aggregates can be uniformly distributed within the first composite material, specifically within the silicon composite material. In this case, excellent electrochemical properties such as charging and discharging can be achieved.

[0107] The first composite material may include first silicon particles, specifically, first silicon aggregates in which the first silicon particles are bonded (or linked) to one another. The first carbon layer may be formed on the surface of the first silicon particles and / or the first silicon aggregates. The first carbon may be formed between the first silicon particles because it is uniformly dispersed and / or distributed. Furthermore, the first carbon layer may be formed on the surface of the silicon composite material.

[0108] For example, the first composite material may include a first carbon layer on the surface of first silicon particles and / or first silicon aggregates, and a first carbon layer on the surface of a silicon composite material containing first silicon particles and / or first silicon aggregates.

[0109] In this specification, "bonded (or interconnected)" means a state in which silicon particles in the first composite material are bonded (fused) with at least one other silicon particle and connected to one another, or a structure in which silicon particles are continuously connected (silicon aggregate).

[0110] The interior of the first composite material can be more strongly bonded by the first carbon and the first carbon layer. As a result, the pulverization of the silicon-carbon mixture containing the first composite material due to volume changes during charging and discharging of the lithium secondary battery can be minimized.

[0111] The first composite material is a composite material in which first silicon aggregates, formed by the bonding of multiple first silicon particles to each other, are uniformly distributed within a composite material having a single, massive structure such as a polyhedral or spherical shape. It may also be a single composite material in which first carbon and a first carbon layer containing first carbon surround part or all of the surface of one or more first silicon particles or the surface of secondary first silicon particles (first silicon aggregates) formed by the aggregation of two or more first silicon particles.

[0112] In the first composite material, the interior of the silicon composite material obtained after etching and before carbon coating may have a structure similar to a three-dimensional pomegranate structure (hereinafter referred to as the pomegranate structure). In the pomegranate structure, walls may be formed inside the core when nano-sized first silicon particles are bonded to each other. Furthermore, internal pores surrounded by these walls may be formed. These internal pores can be distributed very uniformly inside the silicon composite material. These internal pores can include inner pores and open pores.

[0113] Subsequently, a large amount of the first carbon can be filled into the pores by carbon coating.

[0114] Specifically, the first composite material is uniformly formed with first silicon particles surrounded by a large amount of first carbon and a first carbon layer containing the first carbon, which is sufficient to ensure high conductivity through conductive pathways and has the advantage of reducing the amount of carbon-based negative electrode active material added when manufacturing the negative electrode. Furthermore, the specific surface area of ​​the first composite material is appropriately adjusted to further improve the performance of the lithium secondary battery.

[0115] On the other hand, the first silicon particles include crystalline particles, and the crystallite size in X-ray diffraction analysis (calculated from the X-ray diffraction analysis results) may be 1 nm to 15 nm, preferably 1 nm to 10 nm.

[0116] If the crystallite size of the first silicon particle is less than the above range, it becomes difficult to form micropores inside the first composite material, making it impossible to suppress the decrease in Coulomb efficiency, which represents the ratio of charging capacity to discharging capacity, and the specific surface area becomes too large, making it impossible to prevent oxidation problems when handled in the atmosphere. Furthermore, if the crystallite size exceeds the above range, the volume expansion of silicon particles that may occur during charging and discharging cannot be sufficiently suppressed by the micropores, making it impossible to suppress the decrease in Coulomb efficiency, which represents the ratio of charging capacity to discharging capacity, due to repeated charging and discharging.

[0117] Specifically, when the first composite material of one embodiment is subjected to X-ray diffraction (Cu-Kα) analysis using copper as a cathode target, and calculated using the Scherrer equation based on the full width at half maximum (FWHM) of the Si(220) diffraction peak around 2θ = 47.5°, the first silicon particles may preferably have a crystallite size of 1 nm to 7.5 nm, more preferably 1 nm to 6 nm.

[0118] As the crystallite size of the first silicon particle is reduced within the above range, a denser composite material can be obtained, improving the strength of the matrix. Therefore, in such cases, the performance of lithium secondary batteries, such as discharge capacity, initial efficiency, or cycle life characteristics, can be further improved.

[0119] Furthermore, the first composite material may further contain amorphous silicon or silicon in a similar phase. The first silicon particles have high initial efficiency and battery capacity, but involve very complex crystalline changes through electrochemical absorption, storage, and release of lithium atoms.

[0120] On the other hand, the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the first composite material may be 0.005 to 0.50, preferably 0.01 to 0.40, and more preferably 0.01 to 0.30. Since the first composite material has a molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms that satisfies the above range, it may be more advantageous for satisfying the desired molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in a silicon-carbon mixture. When applied to lithium secondary batteries, it can improve discharge capacity and initial efficiency, along with excellent capacity retention.

[0121] In the first composite material, the first carbon may be present on the surface of the first silicon particles. Furthermore, the first carbon may be present on and / or within the first silicon aggregates contained in the first composite material.

[0122] Furthermore, the first carbon can function as a matrix, and the first silicon particles and / or first silicon aggregates can be dispersed in the first carbon matrix along with the pores.

[0123] Specifically, the first composite material may have a sea-island structure in which first silicon particles or closed pores (v) form islands and first carbon forms seas. The pores include open pores and closed pores. Closed pores may include pores whose interiors are not coated with first carbon.

[0124] In the first composite material, the first carbon may be present on the surface of the silicon composite particles containing the first silicon particles or inside the open pores.

[0125] The state in which the first silicon particle or the first carbon is uniformly dispersed can be confirmed by dark-field or bright-field imaging using a transmission electron microscope (TEM).

[0126] Furthermore, the first carbon exists on the surface of the first silicon particles or the first silicon aggregate. The first carbon can function as a matrix, and the first silicon particles and pores can be dispersed within the first carbon matrix.

[0127] Furthermore, the first composite material includes a silicon composite material and a first carbon layer on its surface. The first silicon particles may be present in the silicon composite material, and the first carbon may be included in at least a part of the interior of the first carbon layer and the silicon composite material.

[0128] Furthermore, according to one embodiment, since the thickness or the amount of carbon of the first carbon layer can be controlled, not only can the deterioration of the life characteristics be prevented, but an appropriate conductivity can be realized, thereby realizing a high-capacity negative electrode active material.

[0129] In addition, the first composite material may further include first silicon oxide (SiOx, 0.1 < x ≦ 2, also referred to as the first silicon oxide compound).

[0130] The first silicon oxide may be formed on the surface of the first silicon particles and / or silicon aggregates.

[0131] Furthermore, the first silicon oxide may be uniformly distributed and present together with the first silicon particles.

[0132] Furthermore, the first silicon oxide may be formed on the surface of the first silicon particles and may be uniformly distributed together with the first silicon particles or the first silicon aggregates.

[0133] The first silicon oxide may be a general term for amorphous silicon oxide (oxygen-containing silicon compound) obtained by cooling and depositing silicon monoxide gas generated by oxidation of metallic silicon, reduction of silicon dioxide, or heating of a mixture of silicon dioxide and metallic silicon.

[0134] For the first silicon oxide, SiO x in which, x is 0.4 ≦ x ≦ 2, preferably 0.6 ≦ x ≦ 1.6, more preferably 0.6 ≦ x ≦ 1.2.

[0135] For the formula SiO xIn this case, if the value of x is less than the above range, the expansion and contraction during charging and discharging of the lithium secondary battery become large, and the life characteristics may deteriorate. Further, if x exceeds the above range, there may be a problem that the initial efficiency of the lithium secondary battery decreases because the inactive oxide increases.

[0136] Furthermore, the first carbon may be present on the surface of the first silicon oxide (SiO x , 0.1 < x ≤ 2).

[0137] On the other hand, generally, in a negative electrode active material containing silicon, as the ratio of oxygen decreases, the ratio of silicon particles as the active material increases, so the discharge capacity and discharge efficiency improve. However, there may occur a problem that cracks can be formed in the first composite material or the silicon-carbon mixture caused by the volume expansion due to the charge and discharge of the silicon particles. As a result, the cycle characteristics of the lithium secondary battery may deteriorate. To solve this problem, silicon dioxide can be contained in the first composite material as an inactive material or an inactive phase.

[0138] Specifically, since silicon dioxide is an inactive material or an inactive phase, it has an effect of alleviating the volume expansion caused by the charge and discharge of silicon particles. Silicon dioxide is a component that forms the matrix of the first composite material, and has an effect of increasing the mechanical strength of the matrix of the first composite material to make it more robust and suppressing the generation of cracks even during volume expansion.

[0139] Furthermore, since the content of silicon dioxide is optimized, it is possible to suppress a decrease in the initial capacity and initial efficiency of the lithium secondary battery and improve the cycle characteristics by increasing the strength of the matrix.

[0140] Specifically, according to one embodiment, the first composite material may contain silicon dioxide in an amount of 0.3% to 6% by weight, preferably 0.5% to 6% by weight, more preferably 0.5% to 5% by weight, even more preferably 1% to 4% by weight, and most preferably 1% to 3% by weight, based on the total weight of the first composite material. If the silicon dioxide content is less than the above range, the effect of improving the cycle characteristics of the lithium secondary battery may be insufficient. If it exceeds the above range, it becomes difficult to achieve the desired value for the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the first composite material, which may reduce the cycle characteristics and discharge capacity of the lithium secondary battery.

[0141] The oxygen (O) content in the first composite material is 0.1% by weight to 16% by weight, preferably 0.1% by weight to 12% by weight, more preferably 0.1% by weight to 8% by weight, and most preferably 0.1% by weight to 6% by weight, based on the total weight of the first composite material.

[0142] If the oxygen (O) content in the first composite material is less than the above range, the degree of expansion during charging of the lithium secondary battery will increase, and the cycle characteristics will deteriorate. If the oxygen (O) content in the first composite material exceeds the above range, there is a concern that irreversible reactions with lithium will increase, the initial charge-discharge efficiency will decrease, it may become easier to detach from the negative electrode current collector, and the charge-discharge cycle of the lithium secondary battery will deteriorate.

[0143] On the other hand, in the first composite material, the silicon (Si) content may be 35% to 80% by weight, preferably 40% to 80% by weight, and more preferably 50% to 80% by weight, based on the total weight of the first composite material. If the silicon (Si) content in the first composite material is less than the above range, the amount of active material for lithium absorption and release will decrease, which may reduce the charge and discharge capacity of the lithium secondary battery. On the other hand, if it exceeds the above range, the charge and discharge capacity of the lithium secondary battery may increase, but the expansion and contraction of the electrodes during charging and discharging may become excessively large, which may further pulverize the negative electrode active material powder and reduce the cycle characteristics.

[0144] Furthermore, the first composite material includes a silicon composite material and a first carbon layer provided on its surface. The first silicon particles may be present in the silicon composite material, and the first carbon may be contained within the first carbon layer and the silicon composite material.

[0145] The carbon (C) content in the first composite material may be 10% to 50% by weight, based on the total weight of the first composite material. Specifically, the carbon (C) content may be 15% to 45% by weight, or 15% to 40% by weight, based on the total weight of the first composite material.

[0146] If the carbon (C) content in the first composite material is less than the above range, a sufficient improvement in conductivity cannot be expected, and the electrode life of the lithium secondary battery may decrease. Furthermore, if the carbon (C) content in the first composite material exceeds the above range, the discharge capacity of the lithium secondary battery may decrease, and the charge / discharge capacity per unit volume may decrease.

[0147] Furthermore, carbon (C) can be incorporated into the silicon composite material. The carbon (C) content within the silicon composite material may be 10% to 45% by weight, preferably 10% to 40% by weight, and more preferably 15% to 38% by weight, based on the total weight of the first composite material. When carbon is contained in the silicon composite material within the above content range, sufficient conductivity can be obtained.

[0148] Furthermore, the carbon (C) content in the first composite material, particularly the carbon (C) content in the silicon composite material, may be affected by the porosity.

[0149] For example, the first composite material may include the first composite material (1), the first composite material (2), or a mixture thereof, depending on the porosity.

[0150] The first composite material (1) may contain a small amount of pores, while the first composite material (2) may contain a large amount of pores.

[0151] Specifically, in a first composite material (1) containing a small amount of pores, the carbon (C) content contained within the silicon composite material can be increased to 30% to 45% by weight based on the total weight of the first composite material. Furthermore, in a porous first composite material (2) containing a large amount of pores, the carbon (C) content contained within the silicon composite material can be reduced to 10% to less than 30% by weight based on the total weight of the first composite material.

[0152] In this way, the carbon (C) content is adjusted to an optimal range, allowing control of the porosity of the first composite material. Thus, a silicon-carbon mixture with controlled porosity can be obtained.

[0153] Furthermore, the porosity of the first composite material can be controlled to adjust the carbon (C) content in the first composite material to an optimal range.

[0154] The first carbon layer may have a thickness of 2 nm to 100 nm, preferably 10 nm to 80 nm, more preferably 10 nm to 60 nm, and even more preferably 10 nm to 40 nm.

[0155] If the thickness of the first carbon layer is greater than or equal to the lower limit mentioned above, improved conductivity can be achieved. If it is less than or equal to the upper limit mentioned above, a decrease in the capacity of the lithium secondary battery can be suppressed.

[0156] The average thickness of the first carbon layer can be measured, for example, by the following procedure.

[0157] First, the composite material, mixture, or negative electrode active material is observed using a transmission electron microscope (TEM) at a magnification of any choice. The magnification is preferably such that it can be observed with the naked eye. Next, the thickness of the carbon layer is measured at 15 arbitrary points. In this case, it is desirable to select the measurement locations as broadly and randomly as possible, rather than concentrating them in a specific area. Finally, the average value of the carbon layer thicknesses at those 15 points is calculated.

[0158] The first carbon layer may include at least one selected from the group consisting of amorphous carbon, crystalline carbon, graphene, reduced graphene oxide, carbon nanotubes, and carbon nanofibers. Specifically, it may include graphene. At least one selected from the group consisting of amorphous carbon, crystalline carbon, graphene, reduced graphene oxide, carbon nanotubes, and carbon nanofibers may be present not only as the first carbon layer on the surface of the silicon composite material, but also inside the silicon composite material, on the surface of the first silicon particles, and in the first carbon matrix.

[0159] When the first carbon layer contains amorphous carbon, the strength of the first carbon layer is appropriately maintained, thereby suppressing the expansion of the composite material.

[0160] Amorphous carbon may include at least one selected from the group consisting of soft carbon (low-temperature calcined carbon), hard carbon, pitch carbide, mesophase pitch carbide, and calcined coke.

[0161] The raw materials for pitch may be, for example, petroleum-based, coal-based, or mixtures thereof. For example, it may be coal tar pitch, petroleum pitch, organically synthesized pitch obtained by polycondensation of condensed polycyclic aromatic hydrocarbon compounds, or organically synthesized pitch obtained by polycondensation of heteroatom-containing condensed polycyclic aromatic hydrocarbon compounds. Specifically, the raw material for pitch may be coal tar pitch.

[0162] When using pitch to form the first carbon layer, the conductivity of the negative electrode active material can be improved through a simple heat treatment process, which involves heat-treating the pitch raw material to carbonize it.

[0163] Furthermore, if the first carbon layer contains crystalline carbon, the conductivity of the negative electrode active material can be improved. The crystalline carbon may be at least one selected from the group consisting of plate-like, spherical, and fibrous natural graphite and fibrous artificial graphite.

[0164] Furthermore, it is possible to form a first carbon layer of the same type as the first carbon layer within the silicon composite material.

[0165] On the other hand, the first carbon layer of the first composite material may include one or more or two or more carbon layers.

[0166] For example, the first carbon layer may be a single-layer structure or a multilayer structure of two or more layers. If the first carbon layer is formed of two or more carbon layers, even if a crack forms on the surface of one of the carbon layers, the carbon layers can remain electrically connected until the other carbon layers that are not cracked are completely detached.

[0167] Specifically, if the first carbon layer of the first composite material contains two or more layers, it may include a 1-1 carbon layer and a 1-2 carbon layer.

[0168] The types of carbon layers 1-1 and 1-2 are as described above. Here, the types of carbon layers 1-1 and 1-2 may be different from each other.

[0169] For example, the first carbon layer (1-1) may include at least one selected from the group consisting of amorphous carbon, crystalline carbon, graphene, carbon nanotubes, and carbon nanofibers. The second carbon layer (1-2) may include reduced graphene oxide (RGO). Reduced graphene oxide can be formed by partially or substantially completely reducing graphene oxide.

[0170] The first silicon composite material has an average particle size (D) of 2 μm to 15 μm, preferably 4 μm to 10 μm, and more preferably 4 μm to 8 μm. 50 ) may have an average particle size (D 50 ) refers to the average particle size (D 50 ), that is, in particle size distribution measurement by laser beam diffraction, the average particle size (D 50 ), that is, the value measured as the particle diameter or median diameter when the cumulative volume is set to 50%. Average particle size (D50 If ) is below the above range, there is a concern that when preparing the negative electrode active material composition (slurry) using it, the particles of the first composite material may aggregate and the dispersibility may decrease. On the other hand, D 50 If the above range is exceeded, the expansion of composite material particles due to lithium-ion charging becomes severe, and as charging and discharging are repeated, the bonding ability between composite material particles and the bonding ability between composite material particles and the current collector decreases, which may significantly reduce the lifespan characteristics. In addition, there is a concern that the activity may decrease due to the reduction in specific surface area.

[0171] On the other hand, since the surface of the silicon composite material particles or the interior of the silicon composite material and the pores within it can be covered with a carbon coating, the specific surface area of ​​the first composite material can change significantly.

[0172] The first composite material is 2m 2 / g~50m 2 / g, preferably 2m 2 / g~40m 2 / g, more comfortably, 2m 2 / g~20m 2 It may have a specific surface area (Brunauer-Emmett-Teller method; BET) of 1 / g. If the specific surface area of ​​the first composite material is below the above range, the charge-discharge characteristics of the lithium secondary battery may deteriorate. If it exceeds the above range, it becomes difficult to prepare a negative electrode active material composition (slurry) suitable for application to the negative electrode current collector of a lithium secondary battery, the contact area with the electrolyte increases, the decomposition reaction of the electrolyte is accelerated, or side reactions of the lithium secondary battery may occur.

[0173] The first composite material has a concentration of 1.8 g / cm³. 3 ~2.5g / cm 3 Preferably 2.0 g / cm³ 3 ~2.5g / cm 3 , more preferably 2.0 g / cm³ 3 ~2.4g / cm 3 More preferably 2.0 g / cm³ 3 ~2.35 g / cm³ 3It has a density of . The density can vary depending on the amount of coverage of the first carbon layer. The amount of carbon is constant, but the higher the density within the above range, the fewer pores there are in the first composite material. For this reason, when a silicon-carbon mixture containing the first composite material is used as the negative electrode active material, the conductivity is improved, the strength of the matrix is ​​strengthened, and thereby the initial efficiency and cycle life characteristics are improved. In that case, the density may refer to the true density. The measurement method is as described above.

[0174] If the density of the first composite material is within the above range, the volume expansion of the negative electrode active material during charging and discharging can be reduced, thereby minimizing cracking of the negative electrode active material. As a result, a decrease in cycle characteristics can be prevented, and the impregnation of the electrolyte can be improved, further improving the initial charge and discharge capacity of the lithium secondary battery.

[0175] On the other hand, the first composite material may contain pores.

[0176] In the first composite material, by adjusting the carbon coating method and the amount of carbon coating, a first composite material (1) and a first composite material (2) having different porosities can be obtained. The carbon coating can be formed by chemical thermal deposition (CVD) method.

[0177] Specifically, in the method for preparing the first composite material described later, the porosity of the first composite materials (1) and (2) can be controlled by adjusting etching conditions, such as the etching solution (etchant), its concentration, etching time, etc., and / or carbon coating conditions, such as the amount of carbon coating, such as the type of carbon source, coating temperature, coating time, etc. First composite material (1)

[0178] The first composite material (1) of one embodiment may contain a small amount of pores.

[0179] The definition and measurement method of pores are as described above.

[0180] Specifically, the first composite material (1) may have an internal porosity of 0.1% to 10%, preferably 0.1% to 8%, and more preferably 0.1% to 5%. The method for measuring the internal porosity is as described above.

[0181] When the internal porosity of the first composite material (1) satisfies the above range, a volume expansion buffering effect can be obtained while maintaining sufficient mechanical strength when applied as the negative electrode active material of a lithium secondary battery. Therefore, the volume expansion problem caused by the use of silicon particles can be minimized, high capacity can be achieved, and life characteristics can be improved.

[0182] If the internal porosity of the first composite material (1) is less than the above range, it may be difficult to suppress the volume expansion of the negative electrode active material during charging and discharging.

[0183] On the other hand, the first composite material (1) may have an internal porosity of 0.1% to 8%, preferably 0.1% to 5%, and more preferably 0.1% to 4%.

[0184] If the internal porosity of the first composite material (1) satisfies the above range, then when applied as the negative electrode active material of a lithium secondary battery, a volume expansion buffering effect can be obtained while maintaining sufficient mechanical strength. Therefore, the volume expansion problem caused by the use of silicon particles can be minimized, high capacity can be achieved, and life characteristics can be improved.

[0185] If the internal porosity and / or inner porosity of the first composite material (1) each meet the above ranges, then even after repeating thousands of cycles over a long period, it is possible to maintain excellent capacity retention, which is a significant advantage in terms of improved reliability. First composite material (2)

[0186] The first composite material (2) of one embodiment may be porous because it contains a large number of pores.

[0187] Specifically, the first composite material (2) may have an internal porosity of more than 10% and 40% or less, preferably more than 10% and 30% or less, and more preferably more than 10% and 25% or less.

[0188] If the internal porosity of the first composite material (2) satisfies the above range, the volume expansion of silicon particles that may occur during the charging and discharging of the lithium secondary battery can be mitigated and suppressed through the pores.

[0189] If the internal porosity of the first composite material (2) exceeds the above range, the presence of numerous pores in the negative electrode active material reduces its mechanical strength, potentially causing the negative electrode active material to collapse during the manufacturing process of lithium secondary batteries, for example, during the mixing of the negative electrode active material composition (slurry) or the rolling process after coating.

[0190] Since the first composite material (2) has a porous structure, the electrolyte can easily penetrate the first composite material (2), thereby improving the charge and discharge characteristics of the lithium secondary battery.

[0191] On the other hand, the first composite material (2) may have an internal porosity of 5% to 35% or less, preferably more than 5% and 25% or less, and more preferably more than 5% and 20% or less.

[0192] If the internal porosity of the first composite material (2) satisfies the above range, then when applied as the negative electrode active material of a lithium secondary battery, a volume expansion buffering effect can be obtained while maintaining sufficient mechanical strength. Therefore, the volume expansion problem caused by the use of silicon particles can be minimized, high capacity can be achieved, and life characteristics can be improved. <Second type of composite material>

[0193] In one embodiment of the silicon-carbon mixture, the second composite material may contain second silicon particles, magnesium silicate, and second carbon.

[0194] According to one embodiment, since a first composite material and a second composite material are used, the problem of reduced lifespan characteristics, which is an issue with the second composite material, can be solved while further improving the capacity characteristics (initial capacity) and initial efficiency, which are excellent properties of the second composite material. In particular, since the silicon-carbon mixture is included in the second composite material, the initial efficiency improvement effect of magnesium silicate can be maximized, thereby improving the performance of the lithium secondary battery.

[0195] Specifically, the second composite material may include a silicon composite oxide and a second carbon layer on its surface, the second silicon particles and magnesium silicate may be present in the silicon composite oxide, and the second carbon may be included in at least one selected from the group consisting of the second carbon layer, the surface of the second silicon particles, and the surface of the second magnesium silicate.

[0196] Furthermore, the second silicon particles and magnesium silicate can be uniformly distributed within the silicon composite oxide. In that case, excellent electrochemical properties can be exhibited.

[0197] The second silicon particle may include crystalline particles, amorphous particles, particles having phases similar to these, or a combination thereof.

[0198] Specifically, the second silicon particle may include crystalline particles, and the crystallite size in X-ray diffraction analysis may be between 3 nm and 15 nm (calculated from the X-ray diffraction analysis results).

[0199] For example, if the second composite material is subjected to X-ray diffraction (Cu-Kα) analysis using copper as a cathode target, and calculated using the Scherrer equation based on the full width at half maximum (FWHM) of the Si(220) diffraction peak around 2θ = 47.5°, the second silicon particles may preferably have a crystallite size of 4 nm to 15 nm, more preferably 4 nm to 12 nm, even more preferably 4 nm to 10 nm, and most preferably 4 nm to 8 nm.

[0200] Furthermore, if the crystallite size of the second silicon particle is less than the above range, the initial efficiency, discharge capacity, and capacity retention rate may decrease sharply. If the crystallite size of the second silicon particle satisfies the above range, the Coulomb efficiency, which represents the ratio of charging capacity to discharging capacity, i.e., the charge-discharge efficiency, can be improved.

[0201] The second silicon particle may be an amorphous particle, a crystalline particle with a crystallite size of 4 nm to 15 nm, or a mixture thereof.

[0202] While it may be advantageous for the second silicon particle to be nearly 100% amorphous, obtaining a completely amorphous second silicon particle in the process is difficult. Therefore, the second silicon particle may exist as a mixture of amorphous and crystalline particles.

[0203] Furthermore, if the second silicon particle is a mixture of amorphous and crystalline particles, the proportion of amorphous particles may be 50% or more.

[0204] If the second silicon particles include amorphous particles, crystalline particles with crystallite sizes within the above range, or a mixture thereof, cracks during the initial charge and discharge of the lithium secondary battery can be suppressed.

[0205] If several cracks form during the initial charge and discharge of a lithium-ion battery, these cracks may gradually widen as the charge and discharge cycle is repeated, potentially leading to performance problems with the lithium-ion battery.

[0206] If the second silicon particle contains crystalline particles and the crystallite size of the second silicon particle is below the above range, the charge / discharge capacity of the lithium secondary battery may decrease, and the reactivity may increase during storage, potentially altering the material's properties and causing problems in the process.

[0207] If the second silicon particle contains crystalline particles and the crystallite size of the second silicon particle is greater than or equal to the lower limit, the excellent charge-discharge characteristics of the lithium secondary battery can be maintained. If the crystallite size of the second silicon particle is less than or equal to the upper limit, the decrease in the Coulomb efficiency, which represents the ratio of the charge capacity to the discharge capacity of the lithium secondary battery, can be suppressed.

[0208] Furthermore, by further crushing the second silicon particles to an amorphous state or with crystallite sizes of 4 nm to 7 nm, the density can be increased, the strength strengthened, and crack formation can be prevented. Therefore, the initial efficiency and cycle life characteristics of lithium secondary batteries can be further improved.

[0209] The second composite material is a composite material in which second silicon aggregates, formed by the bonding of multiple second silicon particles to each other, are uniformly distributed within the composite material having a single-block structure, for example, polyhedral, spherical, or similarly shaped. It may also be a single composite material in which a second carbon layer containing second carbon surrounds part or all of the surface of one or more second silicon particles, or the surface of secondary second silicon particles (second silicon aggregates) formed by the aggregation of two or more second silicon particles.

[0210] Furthermore, the silicon (Si) content in the second composite material may be 30% to 60% by weight, 30% to 55% by weight, 35% to 55% by weight, or 35% to 50% by weight, based on the total weight of the second composite material.

[0211] If the silicon (Si) content in the second composite material meets the above range, the charge / discharge capacity and cycle characteristics of the lithium secondary battery can be further improved.

[0212] If the silicon (Si) content is below the above range, the charge and discharge capacity of the lithium secondary battery may decrease. If the silicon (Si) content exceeds the above range, the charge and discharge capacity of the lithium secondary battery may increase, but on the other hand, excessive contraction and expansion of the electrodes during charging and discharging may occur, further pulverizing the negative electrode active material powder and potentially degrading the cycle characteristics of the lithium secondary battery.

[0213] On the other hand, according to one embodiment, the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the second composite material is 0.8 to 1.2, preferably 0.85 to 1.2, and more preferably 0.9 to 1.2. When a second composite material having a molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms within the above range is applied as a negative electrode active material, it is possible to improve discharge capacity and initial efficiency while maintaining excellent capacity retention.

[0214] According to one embodiment, if the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the second composite material is outside the above range, the capacity characteristics of the secondary battery may decrease.

[0215] On the other hand, the second composite material contains magnesium silicate.

[0216] Because the second composite material contains magnesium silicate, it exhibits excellent slurry stability when manufacturing lithium secondary batteries, and can improve capacity retention, charge / discharge characteristics, and cycle characteristics when applied to lithium secondary batteries.

[0217] Magnesium silicate hardly reacts with lithium ions during the charging and discharging of lithium secondary batteries, thus reducing the expansion and contraction of the electrodes when lithium ions are absorbed into them, thereby improving the cycle characteristics of lithium secondary batteries. Furthermore, the strength of the matrix, which is the continuous phase surrounding silicon, can be enhanced by magnesium silicate.

[0218] Magnesium silicate is represented by the following formula A: [Formula A] [ka]

[0219] In equation A, x is 0.5 ≤ x ≤ 2 and y is 2.5 ≤ y ≤ 4.

[0220] Magnesium silicate may contain MgSiO3, Mg2SiO4, or mixtures thereof.

[0221] Specifically, magnesium silicate may include at least one selected from MgSiO3 crystals (enstatite) and Mg2SiO4 crystals (forsterite).

[0222] In addition, according to one embodiment, the magnesium silicate may contain MgSiO3 crystals and may further contain Mg2SiO4 crystals.

[0223] When magnesium silicate contains a mixture of MgSiO3 and Mg2SiO4 crystals, the ratio of MgSiO3 to Mg2SiO4 crystals may vary depending on the amount of magnesium used in the raw material process.

[0224] Furthermore, magnesium silicate can contain a substantial amount of MgSiO3 crystals to improve Coulomb efficiency, charge / discharge capacity, initial efficiency, and capacity retention.

[0225] In this specification, the expression "substantially contains a certain component" may mean that the component is the main component, or that the product mainly contains that component.

[0226] Specifically, according to one embodiment, the magnesium silicate contains MgSiO3 crystals and further contains Mg2SiO4 crystals. In such a case, in X-ray diffraction analysis, the ratio of the intensity of the X-ray diffraction peak corresponding to the Mg2SiO4 crystal appearing in the range of 2θ = 22.3° to 23.3° (IF) to the intensity of the X-ray diffraction peak corresponding to the MgSiO3 crystal appearing in the range of 2θ = 30.5° to 31.5° (IE) may be greater than 0 and less than or equal to 1, specifically 0.1 to 1.

[0227] By using a substantial amount of MgSiO3 crystals in magnesium silicate, the improvement in the number of cycles during charging and discharging can be enhanced.

[0228] When magnesium silicate contains both MgSiO3 and Mg2SiO4 crystals, the initial efficiency can be improved. If more Mg2SiO4 crystals are used than MgSiO3 crystals, the degree of alloying between silicon and lithium atoms decreases, which may result in a decrease in initial discharge characteristics.

[0229] According to one embodiment, if the second composite material contains both MgSiO3 crystals and Mg2SiO4 crystals, the initial efficiency can be further improved.

[0230] If the second composite material contains MgSiO3 crystals, then the MgSiO3 crystals (for example, with a density of 2.7 g / cm³) 3 ) is a Mg2SiO4 crystal (for example, with a density of 3.2 g / cm³ 3 Compared to (for example, its density is 2.33 g / cm³), silicon (for example, its density is 2.33 g / cm³) 3 The volume change based on the volume change of (is) is small. Therefore, the cycle characteristics of lithium secondary batteries can be further improved. Furthermore, MgSiO3 crystals and Mg2SiO4 crystals can act as diluents or inert materials in the negative electrode active material. In addition, when MgSiO3 crystals are formed, pulverization caused by the contraction and expansion of silicon is suppressed, thereby improving the initial efficiency.

[0231] Furthermore, since magnesium silicate hardly reacts with lithium ions, incorporating it into electrodes can reduce the expansion and contraction of the electrodes when lithium ions are absorbed into them, thereby improving the cycle characteristics of lithium secondary batteries.

[0232] Furthermore, the strength of the matrix, which is a continuous phase surrounding silicon, can be enhanced by magnesium silicate.

[0233] Because the second composite material contains magnesium silicate, it can suppress the chemical reaction between the negative electrode active material and the binder during the manufacturing of lithium secondary batteries, improve the stability of the slurry, and enhance both the stability and cycle characteristics of the negative electrode.

[0234] Furthermore, when the magnesium silicate contains both MgSiO3 crystals and Mg2SiO4 crystals, it is preferable that the MgSiO3 and Mg2SiO4 crystals are uniformly distributed. It is also preferable that the size of their crystallites is 10 nm or less.

[0235] When MgSiO3 and Mg2SiO4 crystals are uniformly distributed, the second silicon particles and the constituent elements of the MgSiO3 and Mg2SiO4 crystals diffuse with each other, and the phase interface is bonded; that is, each phase is bonded at the atomic level. Therefore, the volume change when lithium ions are intercalated and released may be small, and cracks may not easily form in the negative electrode active material even after repeated charging and discharging. As a result, there will likely be little decrease in capacity even with a large number of cycles.

[0236] On the other hand, the magnesium (Mg) content in the second composite material may be 2% to 20% by weight, preferably 4% to 15% by weight, and most preferably 4% to 12% by weight, based on the total weight of the second composite material.

[0237] If the content of magnesium (Mg) in the second composite material is equal to or higher than the lower limit within the above range, the initial efficiency of the lithium secondary battery can be improved. If it is equal to or lower than the upper limit within the above range, it may be advantageous in terms of the improvement effects of charge-discharge capacity, cycle characteristics, and handling stability.

[0238] Also, as the content of magnesium (Mg) increases, the content of Mg2SiO4 crystals can increase. As a result, the crystal size of magnesium silicate becomes larger, the initial efficiency rises excessively, and the cycle characteristics relatively deteriorate. Therefore, it may be important to control the content of magnesium (Mg) within an optimal range.

[0239] Magnesium silicate can be formed by being uniformly distributed together with the second silicon particles.

[0240] On the other hand, the second composite material can contain second silicon oxide (also referred to as a second silicon oxide compound).

[0241] The second silicon oxide may contain the same or similar compounds as the first silicon oxide.

[0242] Furthermore, the second silicon oxide may be, for example, a silicon-based oxide represented by the formula SiO x (0.5 ≦ x ≦ 1.5). Specifically, the second silicon oxide may be SiOx (0.8 ≦ x ≦ 1.2), and more specifically, SiO x (0.9 < x ≦ 1.1). In the formula SiO x if the value of x is less than the above range, expansion and contraction during charge and discharge of the lithium secondary battery become large, and the life characteristics may deteriorate. Also, if x exceeds the above range, there may be a problem that the initial efficiency of the lithium secondary battery decreases due to an increase in inert oxides.

[0243] The second silicon oxide can be obtained by a method including mixing and heating silicon powder and silicon dioxide powder, and cooling and depositing the silicon oxide gas generated thereby.

[0244] The second silicon dioxide can be used in an amount of 0.1% to 45% by weight, preferably 0.5% to 20% by weight, based on the total weight of the second composite material.

[0245] If the content of the second silicon dioxide is below the above range, the volume expansion characteristics and lifespan characteristics of the lithium secondary battery may decrease. If it exceeds the above range, the initial irreversible reaction of the lithium secondary battery may increase.

[0246] According to one embodiment, the silicon composite oxide of the second composite material may contain a second silicon oxide, and the second silicon particles, magnesium silicate, and second silicon oxide may be present in a uniform mixture. Furthermore, the second silicon oxide may be present on the surface of the second silicon particles.

[0247] When the second composite material includes second silicon particles, second silicon oxide, and magnesium silicate, the volume change during the intercalation and release of lithium ions is reduced, preventing crack formation in the negative electrode active material during repeated charging and discharging. In such cases, a rapid decrease in capacity with respect to the number of cycles of the lithium secondary battery can be prevented, thereby improving the cycle characteristics of the lithium secondary battery.

[0248] Furthermore, the second composite material contains second silicon particles, second silicon oxide, and magnesium silicate, and because the second silicon particles are linked and bonded to each other, lithium ions are easily released during the discharge of the lithium secondary battery. Therefore, the balance between the charging and discharging of lithium ions is good, and the initial efficiency (charge-discharge efficiency) can be increased.

[0249] The second silicon dioxide can be used in an amount of 40% to 50% by weight based on the total weight of the second composite material.

[0250] If the content of the second silicon oxide is less than the above range, the volume expansion characteristics and life characteristics of the lithium secondary battery may deteriorate. If it exceeds the above range, the initial irreversible reaction of the lithium secondary battery may increase.

[0251] The content of oxygen (O) in the second composite material may be 20% to 40% by weight, preferably 25% to 40% by weight, more preferably 25% to 35% by weight based on the total weight of the second composite material. If the content of oxygen (O) in the second composite material is less than the above range, the degree of expansion during charging of the lithium secondary battery becomes large and the cycle characteristics deteriorate. Also, if the content of oxygen (O) in the second composite material exceeds the above range, the charge-discharge cycle of the lithium secondary battery may deteriorate.

[0252] On the other hand, according to one embodiment, the second composite material may include a second carbon layer on the surface of the silicon composite oxide, and the second carbon (C) may be included in the second carbon layer.

[0253] Since the second composite material contains the second carbon, excellent conductivity can be provided even after the electrode expands during charge and discharge, and the performance of the lithium secondary battery can be further improved.

[0254] Furthermore, according to one embodiment, the second composite material may include a silicon composite oxide and a second carbon layer on its surface, and the second silicon particles and magnesium silicate may be present in the silicon composite oxide. Also, the second carbon may be included in at least one selected from the group consisting of the second carbon layer, the surface of the second silicon particles, and the surface of the second magnesium silicate. Furthermore, the second carbon may be present on the surface of the second silicon oxide.

[0255] Specifically, the second carbon may be included in the second carbon layer. Furthermore, the second carbon may be included by forming a second carbon layer on at least a part of one or more surfaces selected from the group consisting of the second silicon particles, the second magnesium silicate, and the second silicon oxide.

[0256] More specifically, the second carbon may be included in a second carbon layer formed on the surface of the silicon oxide. Further, the second carbon can be incorporated into a second carbon layer formed on at least a part of the surface of the second silicon particles, the surface of the second silicon oxide, and the surface of the second magnesium silicate.

[0257] Since the second carbon is included in the second carbon layer, the mechanical properties can be enhanced, excellent conductivity can be ensured even when the electrode expands during charge and discharge, side reactions with the electrolyte are suppressed, and thus the charge-discharge capacity, initial charge-discharge efficiency, and capacity retention rate of the lithium secondary battery can be improved.

[0258] Also, since the thickness and carbon amount of the second carbon layer can be controlled, not only can the degradation of the life characteristics be prevented, but an appropriate conductivity can be obtained, thereby enabling the realization of a high-capacity negative electrode active material.

[0259] Furthermore, the content of carbon (C) in the second carbon composite material may be relatively less than the content of carbon (C) in the first composite material.

[0260] The content of carbon (C) in the second composite material may be 3 wt% to 15 wt%, preferably 3 wt% to 10 wt%, more preferably 3 wt% to 8 wt%, and most preferably 5 wt% to 8 wt% based on the total weight of the second composite material.

[0261] If the content of carbon (C) in the second composite material is less than the above range, the thickness of the second carbon layer is too thin, so the desired effects of conductivity and suppression of volume expansion may be insufficient, irreversible reactions may occur, and the problem of a significant decrease in discharge capacity may arise. Also, if the content of carbon (C) in the second composite material exceeds the above range, the thickness of the second carbon layer is too thick, making it difficult for lithium ions to detach. In such a case, the initial efficiency and discharge capacity of the lithium secondary battery may decrease.

[0262] The thickness of the second carbon layer may be 10 nm to 150 nm, preferably 20 nm to 100 nm, and more preferably 20 nm to 80 nm. When the thickness of the second carbon layer is within the above range, it is possible to effectively prevent or reduce the pulverization of the second composite material, effectively prevent side reactions between the second silicon particles and the electrolyte, and thereby further improve the performance of the lithium secondary battery.

[0263] If the thickness of the second carbon layer is greater than or equal to the lower limit within the above range, improved conductivity can be achieved. If it is less than or equal to the upper limit within this range, the decrease in the capacity of the lithium secondary battery can be suppressed.

[0264] The method for measuring the average thickness of the second carbon layer is as described above.

[0265] The second carbon layer may include at least one selected from the group consisting of amorphous carbon, crystalline carbon, graphene, reduced graphene oxide, carbon nanotubes, and carbon nanofibers. Specifically, the second carbon layer may include at least one selected from the group consisting of amorphous carbon and crystalline carbon. The types of the second carbon layer are as described for the first carbon layer.

[0266] The second carbon layer may include one or more carbon layers.

[0267] If the second composite material includes one or more layers of second carbon layers, each carbon layer can be formed thinly and uniformly along the entire outline of the carbon layer while maintaining the outline of the carbon layer. Part or most of the second carbon layer can surround the entire surface of the silicon composite oxide. In such cases, if the second carbon layer is formed thinly and uniformly on the surface of the silicon composite oxide, the electrical contact between particles contained in the second composite material or negative electrode active material can be improved.

[0268] Specifically, if the second carbon layer of the second composite material contains two or more layers, it may include a 2-1 carbon layer and a 2-2 carbon layer.

[0269] The types of carbon layers 2-1 and 2-2 are as described above. Here, the types of carbon layers 2-1 and 2-2 may be different from each other.

[0270] The second composite material has an average particle size (D) of 2 μm to 15 μm, preferably 3 μm to 12 μm, more preferably 4 μm to 10 μm, even more preferably 4 μm to 7 μm, and most preferably 4 μm to 6 μm. 50 ) may have.

[0271] Average particle diameter (D 50 If ) is below the above range, when preparing a negative electrode slurry (negative electrode active material composition) using it, the particles of the second composite material may aggregate and the dispersibility may decrease. On the other hand, D 50 If the above range is exceeded, the expansion of composite material particles due to lithium-ion charging becomes severe, and as charging and discharging are repeated, the bonding ability between composite material particles and the bonding ability between composite material particles and the current collector decreases, so the life characteristics deteriorate significantly, and the activity may decrease due to the decrease in specific surface area.

[0272] Furthermore, according to one embodiment, the second composite material may have a composite material structure in which second silicon particles, second silicon oxide, and magnesium silicate are simultaneously contained and these components are uniformly dispersed. In addition, the individual components and their particle sizes contained in the second composite material can be adjusted to a specific range. If the sizes of the second silicon particles, second silicon oxide, and magnesium silicate are too large, there may be a problem in that the second composite material cannot fully perform the desired functions.

[0273] On the other hand, the second composite material is 3m 2 / g~20m 2 / g, preferably 3m 2 / g~15m 2 / g, comfortably, 3m 2 / g~10m 2It may have a specific surface area (Brunauer-Emmett-Teller method; BET) of 1 / g. If the specific surface area of ​​the second composite material is below the above range, the output characteristics of the lithium secondary battery may decrease. If it exceeds the above range, it becomes difficult to prepare a negative electrode active material slurry suitable for application to the negative electrode current collector of a lithium secondary battery, the contact area with the electrolyte increases, the decomposition reaction of the electrolyte is accelerated, or side reactions of the lithium secondary battery may occur.

[0274] The second composite material has a concentration of 1.8 g / cm³. 3 ~2.5g / cm 3 Preferably 2.0 g / cm³ 3 ~2.5g / cm 3 , more preferably 2.0 g / cm³ 3 ~2.4g / cm 3 It may have a density of [value missing]. The density may vary depending on the amount of coverage of the second carbon layer. In that case, the density may refer to the true density. The measurement method is as described above.

[0275] If the density of the second composite material meets the above range, the impregnation of the electrolyte is improved, thereby further improving the initial charge / discharge capacity and life characteristics of the lithium secondary battery.

[0276] On the other hand, according to one embodiment, the second composite material may include a silicate containing other metals in addition to magnesium. The other metal may be at least one selected from the group consisting of alkali metals, alkaline earth metals, group 13 to group 16 elements, transition metals, rare earth elements, and combinations thereof. Specific examples include Li, Ca, Sr, Ba, Y, Ti, Zr, Hf, V, Nb, Cr, Mo, W, Fe, Pb, Ru, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, and Se.

[0277] Lithium-ion secondary batteries using a second composite material as the negative electrode can further improve their discharge capacity, initial efficiency, and capacity retention rate.

[0278] According to one embodiment, silicon-carbon mixtures, specifically, silicon-carbon mixtures (1) and (2) having desired physical properties can be obtained as the first composite material and the second composite material, respectively, by appropriately selecting and adjusting at least one of the first composite materials (1) and (2).

[0279] For example, the silicon-carbon mixture can include the first composite material (1) and the second composite material.

[0280] Alternatively, the silicon-carbon mixture can include the first composite material (2) and the second composite material.

[0281] Alternatively, the silicon-carbon mixture can include the first composite material (1), the first composite material (2), and the second composite material. [Method for Preparing Silicon-Carbon Mixture]

[0282] A method for preparing a silicon-carbon mixture according to one embodiment includes a first step of obtaining a first composite material; a second step of obtaining a second composite material; and a third step of mixing the first composite material and the second composite material, where the first composite material includes first silicon particles and first carbon, and the second composite material includes second silicon particles, magnesium silicate, and second carbon.

[0283] According to this method for preparing a silicon-carbon mixture, etching conditions and carbon coating conditions can be adjusted in various ways, thereby adjusting and optimizing physical properties such as the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms, specific surface area, density, and porosity in the silicon-carbon mixture accordingly. Therefore, the performance of lithium secondary batteries can be further improved, and the reliability can be further improved.

[0284] Furthermore, this preparation method has the advantage that mass production is possible by a continuous method with a minimum number of steps. The first step of obtaining the first composite material

[0285] First, the method for preparing a silicon-carbon mixture includes a first step of obtaining a first composite material comprising first silicon particles and first carbon.

[0286] The first step of obtaining the first composite material may include: step 1-1 etching a first silicon-based raw material powder using an etching solution containing a fluorine (F) atom-containing compound; step 1-2 filtering and drying the product obtained by etching to obtain a silicon composite material; and step 1-3 forming a first carbon layer inside the silicon composite material and forming a first carbon layer on the surface of the silicon composite material using a chemical pyrolysis vapor deposition method to obtain the first composite material.

[0287] Step 1-1 may include etching the first silicon-based raw material powder using an etching solution containing a fluorine (F) atom-containing compound.

[0288] The first silicon-based raw material powder may include a powder containing silicon that can react with lithium, for example, a powder containing at least one of silicon, silicon oxide, and silicon dioxide.

[0289] Specifically, the first silicon-based raw material powder may include, for example, at least two selected from silicon, silicon oxide, and silicon dioxide. Specifically, the first silicon-based raw material powder may have the formula SiO x It may contain lower silicon dioxide powder represented by (0.9 ≤ x < 1.2).

[0290] The first silicon-based raw material powder is amorphous or crystalline SiO2 prepared by a gas-phase method. x (The size of the silicon crystals may be approximately 2 nm to 3 nm.) The average particle size of the first silicon-based raw material powder as the median diameter may be approximately 0.5 μm to 30 μm, preferably approximately 0.5 μm to 25 μm, and more preferably approximately 0.5 μm to 10 μm.

[0291] If the average particle size of the first silicon-based raw material powder is below the above range, the volume density may become too low, and the charge / discharge capacity per unit volume may decrease. If the average particle size of the first silicon-based raw material powder exceeds the above range, it may become difficult to manufacture the electrode, or it may detach from the current collector. The average particle size is measured by the particle size distribution measurement method using laser diffraction, D 50 (That is, the particle size when the cumulative weight is 50%).

[0292] Furthermore, this method may further include forming a carbon layer on the surface of the first silicon-based raw material powder using a chemical pyrolysis vapor deposition method. Therefore, the first silicon-based raw material powder on which the carbon layer has been formed can be used as the silicon-based raw material powder.

[0293] Specifically, as soon as a carbon layer is formed on the surface of the first silicon-based raw material powder containing silicon particles, the etching process of step 1-1 can be carried out. In this case, there is the advantage that uniform etching is possible and a high yield can be obtained.

[0294] The step of forming the carbon layer can be carried out in the same manner as or by the same method as the step of forming the first carbon layer in steps 1-3 of the method for preparing the first composite material described later.

[0295] The etching process can include dry etching and wet etching.

[0296] When dry etching is used, selective etching may be possible.

[0297] The silicon dioxide in the first silicon-based raw material powder forms pores by dissolving and eluting through the etching process.

[0298] Furthermore, pores can be considered to be formed, for example, by the following reaction schemes 2 and 3. [Reaction Scheme 2] [ka] [Reaction Scheme 3] [ka]

[0299] Through a reaction mechanism like the one described in the reaction scheme above, pores (voids) can be formed in which silicon dioxide is dissolved and removed in the forms of SiF4 and H2SiF6.

[0300] Furthermore, depending on the degree of etching, silicon dioxide, such as silicon dioxide, contained in the first composite material may be removed, and pores may be formed therein.

[0301] The degree of pore formation can vary depending on the degree of etching.

[0302] Furthermore, the O / Si ratio during etching and the specific surface area of ​​the silicon composite material can change significantly. Additionally, the specific surface area and density of the silicon composite material with formed pores can change considerably before and after carbon coating.

[0303] Etching can be used to obtain silicon composite powder in which multiple pores are formed on the surface of the silicon composite particles, or on the surface and inside them.

[0304] Here, etching refers to a method of treating the first silicon-based raw material powder with an etching solution containing a fluorine (F) atom-containing compound.

[0305] Etching solutions containing fluorine atoms that are commonly used can be used without restriction as etching solutions containing fluorine (F) atom-containing compounds, as long as their effectiveness is not impaired.

[0306] Specifically, the fluorine (F) atom-containing compound may include at least one selected from the group consisting of HF, NH4F, and HF2. The use of a fluorine (F) atom-containing compound allows for a faster etching process.

[0307] The etching solution may further contain one or more acids selected from the group consisting of organic acids, sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, and chromic acid.

[0308] As etching conditions, the stirring temperature (processing temperature) may be, for example, 10°C to 90°C, preferably 10°C to 80°C, and more preferably 20°C to 70°C.

[0309] The silicon composite material obtained by etching may contain the first silicon particle.

[0310] This etching process yields a silicon composite material in which multiple pores are formed on the surface, inside, or both of the silicon composite particles. Here, the silicon composite material may have a three-dimensional (3D) structure in which two or more first silicon particles are bonded to each other.

[0311] Furthermore, etching has the characteristic that the average particle size of the silicon composite material hardly changes.

[0312] In other words, the average particle size of the first silicon-based raw material powder before etching is approximately the same as the average particle size of the silicon composite material obtained by etching. The difference (change) between the average particle size of the first silicon-based raw material powder and the average particle size of the silicon composite material can be within approximately 5%.

[0313] Furthermore, etching can reduce the number of oxygen atoms present on the surface of silicon composite materials. In other words, etching can significantly reduce the oxygen fraction on the surface of silicon composite materials, thereby reducing surface resistance. In such cases, the electrochemical properties of lithium secondary batteries, particularly their lifespan, can be further improved.

[0314] Furthermore, because a large amount of silicon dioxide is removed by selective etching, the first silicon particles can contain silicon (Si) at a much higher fraction than oxygen (O) on their surface. In other words, the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms present in the silicon composite material can be significantly reduced. In such a case, a lithium secondary battery with excellent capacity retention, improved discharge capacity, and increased initial efficiency can be obtained.

[0315] Furthermore, pores and voids may form in the areas where silicon dioxide has been removed. As a result, the specific surface area of ​​the silicon composite material may increase compared to the specific surface area of ​​the first silicon-based raw material powder before etching.

[0316] According to one embodiment, physical properties such as elemental content and specific surface area may change before and after the etching process. That is, the first silicon-based raw material powder before the etching process and the silicon composite material after the etching process may have different physical properties such as elemental content and specific surface area.

[0317] Here, the silicon composite material after the etching process and before carbon coating, and the second silicon composite material, have a porous structure unless otherwise specified.

[0318] The internal porosity of the silicon composite material obtained by the etching process is 10% to 50%, preferably 15% to 50%, and more preferably 20% to 50%.

[0319] If the internal porosity of the silicon composite material meets the above range, it may be more advantageous in obtaining the desired effect.

[0320] , In the first method for preparing a silicon composite material, the first and second steps may include filtering and drying the product obtained by etching to obtain the silicon composite material. The filtering and drying steps can be carried out using commonly used methods.

[0321] In the first method for preparing a composite material, steps 1-3 may include forming a first carbon layer inside a silicon composite material using a chemical pyrolysis vapor deposition method, and forming a first carbon layer on the surface of the silicon composite material to prepare the first composite material.

[0322] The first carbon layer may include at least one selected from the group consisting of amorphous carbon, crystalline carbon, graphene, reduced graphene oxide, carbon nanotubes, and carbon nanofibers.

[0323] The process of forming a first carbon layer inside the silicon composite material and forming a first carbon layer on the surface of the silicon composite material can be carried out by injecting at least one carbon source gas selected from the compounds represented by the following formulas 1 to 3, and reacting the silicon composite material obtained in steps 1 and 2 in a gaseous state at 400°C to 1200°C.

[0324] [Formula 1] [ka] (In Equation 1, N is an integer between 1 and 20, and A is either 0 or 1.) [Formula 2] [ka] (In Equation 2, N is an integer between 2 and 6, and B is an integer between 0 and 2.) [Formula 3] [ka] (In equation 3, x is an integer between 1 and 20, y is an integer between 0 and 25, and z is an integer between 0 and 5.)

[0325] The compound represented by Formula 1 may be at least one selected from the group consisting of methane, ethane, propane, butane, methanol, ethanol, propanol, propanediol, and butanediol. The compound represented by Formula 2 may be at least one selected from the group consisting of ethylene, acetylene, propylene, butylene, butadiene, and cyclopentene. The compound represented by Formula 3 may be at least one selected from the group consisting of benzene, toluene, xylene, ethylbenzene, naphthalene, anthracene, and dibutylhydroxytoluene (BHT).

[0326] The carbon source gas may further include at least one inert gas selected from hydrogen, nitrogen, helium, and argon.

[0327] The reaction can be carried out, for example, at 400°C to 1,200°C, more specifically at 500°C to 1,100°C, or more specifically at 600°C to 1,000°C.

[0328] The reaction time (or heat treatment time) can be appropriately adjusted depending on the heat treatment temperature, the pressure during heat treatment, the composition of the gas mixture, and the desired amount of carbon coating. For example, the reaction time may be 10 minutes to 100 hours, more specifically 30 minutes to 90 hours, or more specifically 50 minutes to 40 hours, but is not limited to these ranges.

[0329] According to the first composite material preparation method of one embodiment, a thin, uniform first carbon layer can be formed on the surface of the silicon composite material even at relatively low temperatures by a gas-phase reaction of a carbon source gas. Furthermore, peeling reactions do not substantially occur in the first carbon layer formed in this manner.

[0330] Furthermore, since the first carbon layer is uniformly formed across the entire surface of the silicon composite material by a gas-phase reaction using chemical pyrolysis vapor deposition, a highly crystalline carbon film (carbon layer) can be formed.

[0331] Furthermore, since carbon can be coated (embedded) into the multiple pores formed by etching in step 1-1 and / or step 1-2 by a gas-phase reaction using chemical pyrolysis vapor deposition, the porosity of the first composite material and the silicon-carbon mixture can be controlled, and the desired effect can be obtained.

[0332] Therefore, when a silicon-carbon mixture containing the first composite material is used as the negative electrode active material, the conductivity of the negative electrode active material can be improved without changing its structure.

[0333] According to one embodiment, when a reaction gas containing a carbon source gas and an inert gas is supplied to a silicon composite material, the reaction gas can penetrate into the pores of the silicon composite material, thereby allowing sufficient first carbon to be formed (coated or embedded) inside the silicon composite material, and a first carbon layer to be formed on the surface of the silicon composite material. As a result, the conductivity of the negative electrode active material can be improved.

[0334] The specific surface area of ​​the first composite material may decrease with respect to the amount of carbon coating.

[0335] Furthermore, by adjusting the etching conditions and / or carbon coating conditions, the porosity of the first composite material, and furthermore, the porosity of the silicon-carbon mixture, can be controlled as desired. That is, since carbon penetrates the silicon composite material and fills the pores, and a carbon layer is formed on the surface of the silicon composite material, a first composite material with controlled porosity, specifically, a first composite material (1) and a first composite material (2), can be obtained. By this method, the porosity of the silicon-carbon mixture can be optimized according to the characteristics required for lithium secondary batteries.

[0336] The structure of the graphene-containing material may be in the form of layers, nanosheets, or a mixture of several flakes.

[0337] When a first carbon layer containing graphene is uniformly formed across the entire surface of the silicon composite material, the graphene-containing material, which has improved conductivity and flexibility against volume expansion, can be directly grown on the surface of the first silicon particles, thereby suppressing volume expansion. Furthermore, the coating of the first carbon layer reduces the possibility of silicon coming into direct contact with the electrolyte, thereby suppressing the formation of a solid electrolyte interface (SEI) layer.

[0338] Alternatively, after the numerous pores formed after etching are coated with carbon, the first carbon may be inserted into the surface and internal pores of the first composite material.

[0339] The porosity of the first composite material after carbon coating is as described above.

[0340] Furthermore, according to one embodiment, after steps 1-3 (in which steps 1-3 a first carbon is formed inside the silicon composite material and a first carbon layer is formed on the surface of the silicon composite material), one or more of the first composite materials can bond to each other to form an aggregate. The first composite material may be further crushed and classified so that the average particle size of the first composite material is 2 μm to 15 μm. Classification can be performed to adjust the particle size distribution of the first composite material, and dry classification, wet classification, or sieving classification can be used. In dry classification, the steps of dispersion, separation, recovery (separation of solid and gas), and discharge are performed sequentially or simultaneously using an airflow. At this time, in order to prevent a decrease in classification efficiency due to interference between particles, particle shape, airflow turbulence, velocity distribution, static electricity, etc., pretreatment (adjustment of moisture, dispersibility, humidity, etc.) can be performed before classification to adjust the moisture or oxygen concentration in the airflow used. Furthermore, by performing the pulverization and classification of the first composite material in a single step, the desired particle size distribution can be obtained. After pulverization, it is effective to separate the coarse powder portion from the granular portion using a classifier or sieve. Second step to obtain a second composite material

[0341] A method for preparing a silicon-carbon mixture according to one embodiment includes a second step of obtaining a second composite material comprising a second silicon particle, magnesium silicate, and a second carbon.

[0342] The process for obtaining the second composite material includes: step 2-1, which involves obtaining a silicon composite oxide using a second silicon-based raw material and a magnesium-based raw material; step 2-2, which involves crushing and / or classifying the silicon composite oxide to obtain silicon composite oxide powder; and step 2-3, which involves forming a second carbon layer on the surface of the silicon composite oxide powder using a chemical pyrolysis vapor deposition method to obtain the second composite material.

[0343] The second method for preparing composite materials has the advantage of enabling mass production through a continuous process with minimal steps.

[0344] Specifically, in the second method for preparing a composite material, step 2-1 may include obtaining a silicon composite oxide using a silicon-based raw material and a magnesium-based raw material. Step 2-1 can be carried out, for example, using the method described in Korean Published Patent Publication No. 2018-0106485.

[0345] Furthermore, the second silicon-based raw material may be the same as or similar to the first silicon-based raw material.

[0346] Silicon composite oxides may contain magnesium silicate.

[0347] Specifically, a silicon-based composite oxide can be obtained by heating a second silicon-based raw material and a magnesium-based raw material at different temperatures to evaporate them, and then cooling the resulting vapor-deposited material.

[0348] The second silicon-based raw material powder may contain at least one selected from silicon, silicon oxide, and silicon dioxide.

[0349] Magnesium-based raw materials may contain metallic magnesium.

[0350] On the other hand, the average particle size of the second silicon-based raw material is not particularly limited. For example, the average particle size of the silicon powder may be 5 μm to 50 μm, 10 μm to 40 μm, or 15 μm to 30 μm, and the average particle size of the silicon dioxide powder may be 5 nm to 50 nm, 10 nm to 40 nm, or 15 nm to 30 nm.

[0351] When a second silicon-based raw material having an average particle size within the above range is used, the deposition and evaporation of silicon dioxide become uniform, thereby enabling the production of fine second silicon particles.

[0352] The second silicon-based raw material and the magnesium-based raw material are placed in separate crucibles in a vacuum reactor, heated to evaporate at different temperatures, and then cooled by deposition to obtain a silicon composite oxide.

[0353] The heating and evaporation of the second silicon-based raw material can be carried out at a pressure of 0.0001 Torr to 2 Torr, at a temperature of 900°C to 1,800°C, preferably 1,000°C to 1,600°C, and more preferably 1,200°C to 1,600°C. If the temperature is lower than the above range, the reaction may not proceed smoothly, which may reduce productivity. If the temperature exceeds the above range, the reactivity may decrease.

[0354] The magnesium-based raw materials can be heated and evaporated at a pressure of 0.0001 Torr to 2 Torr, at temperatures of 500°C to 1,100°C, 600°C to 1,000°C, or 650°C to 900°C.

[0355] When the heating and evaporation of the second silicon-based raw material and the magnesium-based raw material meet the above range, fine second silicon particles and fine magnesium silicate particles can be produced, thereby producing the desired SiO x Silicon oxide (silicon oxide compounds) having a composition of (0.4 ≤ x ≤ 2) can be obtained.

[0356] On the other hand, vapor deposition can be carried out at 300°C to 800°C, preferably 400°C to 700°C.

[0357] Cooling can be performed by rapidly cooling to room temperature using water cooling. Alternatively, it can be performed at room temperature while injecting an inert gas. The inert gas may be at least one selected from carbon dioxide, argon (Ar), helium (He), nitrogen (N2), and hydrogen (H2).

[0358] The second silicon-based raw material and the magnesium-based raw material are heated and evaporated, then deposited onto a substrate in the reactor, and a silicon composite oxide is synthesized by a uniform gas-phase reaction of the particles. Therefore, it is possible to prevent the rapid growth of the second silicon due to exothermic reactions that occur when magnesium is locally added in excess, as in solid-phase reactions.

[0359] On the other hand, the method may further include heat treatment of silicon composite oxides.

[0360] The heat treatment can be carried out at 700°C to 950°C. Specifically, the heat treatment can be carried out at 800°C to 950°C, preferably 900°C to 930°C, for 1 to 10 hours or 1 to 5 hours.

[0361] Step 2-2 of one embodiment may further include grinding and / or classifying the silicon composite oxide obtained in step 2-1 to have an average particle size of 3 μm to 15 μm.

[0362] More specifically, the silicon composite oxide obtained in step 2-1 may be pulverized, and the pulverization is performed to reduce the average particle size (D) of the heat-treated silicon composite oxide. 50 The procedure may be carried out so that the size is between 3 μm and 10 μm, specifically between 3 μm and 8 μm.

[0363] For grinding, grinding equipment known in the art can be used. For example, grinding can be carried out using at least one selected from the group consisting of jet mills, ball mills, agitated media mills, roll mills, hammer mills, pin mills, disc mills, colloidal mills, and atomizer mills.

[0364] Specifically, grinding can be performed using a ball mill, which grinds the material to be ground by moving a grinding medium such as balls or beads and utilizing the impact force, friction force, or compressive force supplied by the kinetic energy; a mixed medium mill; or a roll mill, which grinds the material by utilizing the compressive force of rollers. Furthermore, a jet mill may be used, which grinds the material by colliding it with interior materials or by colliding the material to be ground with each other at high speed and utilizing the impact force supplied by the collision. In addition, hammer mills, pin mills, and disc mills can also be used. Moreover, a colloid mill or atomizer mill that utilizes shear force may be used as a high-pressure wet opposing impact type disperser.

[0365] Furthermore, classification may be performed using at least one selected from dry classification, wet classification, and sieving classification.

[0366] According to one embodiment, a dry classifier equipped with a cyclone can be used together with a jet mill.

[0367] In a jet mill, the processes of dispersion, separation (separation of fine and coarse particles), recovery (separation of solids and gases), and discharge can be carried out sequentially using airflow. In such cases, the classification efficiency must not be impaired by interference between particles, particle shape, airflow turbulence, velocity distribution, and the effects of static electricity.

[0368] In other words, the airflow can be pre-treated before classification (to adjust moisture content, dispersibility, humidity, etc.) to adjust the concentration of moisture and oxygen.

[0369] Furthermore, by performing grinding and classification in a single step, the desired particle size distribution can be obtained.

[0370] In the method for preparing the second composite material, the second and third steps may include forming a second carbon layer on the surface of the silicon composite oxide powder using a chemical pyrolysis vapor deposition method to obtain the second composite material.

[0371] The second carbon layer may be formed not only on the surface of the silicon composite oxide powder, but also on the surface of at least one particle selected from the group consisting of second silicon particles, magnesium silicate (particles), and second silicon oxide (particles) that constitute the silicon composite oxide powder.

[0372] The second carbon layer may include at least one selected from the group consisting of amorphous carbon, crystalline carbon, graphene, reduced graphene oxide, carbon nanotubes, and carbon nanofibers.

[0373] The process of forming the second carbon layer can be carried out in the same manner as the first to third steps.

[0374] Furthermore, according to one embodiment, after the second and third steps (after the formation of the second carbon layer in the second and third steps), the second composite material may be further crushed and classified so that the second composite material has a desired average particle size. The crushing and classification steps are as described above. Third step

[0375] The process of preparing a silicon-carbon mixture includes a third step of mixing a first composite material with a second composite material.

[0376] The mixed weight ratio of the first composite material to the second composite material may be 90:10 to 5:95, 90:10 to 10:90, 90:10 to 20:80, 90:10 to 60:40, 90:10 to 65:35, or 90:10 to 70:30. When the mixed weight ratio of the first composite material to the second composite material satisfies the above range, the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon mixture can be optimized. As a result, a silicon-carbon mixture with desired properties can be obtained.

[0377] If the mixed weight ratio of the first composite material and the second composite material is outside the above range, it becomes difficult to obtain the desired effect, and in particular, the cycle characteristics of the lithium secondary battery may deteriorate.

[0378] The mixing can be carried out by at least one method selected from dry mixing and wet mixing.

[0379] According to one embodiment, mixing can be performed, for example, using a co-rotating mixer, to prepare a powder containing mixed phase particles. The rotation speed during mixing may be 1,000 to 3,000 rpm, for example, about 2,000 rpm, and the mixing time may be 2 to 20 minutes, preferably 3 to 10 minutes, more preferably 5 to 8 minutes, for example, about 6 minutes. When the mixing speed and mixing time are within the above ranges, the first composite material and the second composite material can be mixed efficiently and uniformly.

[0380] On the other hand, according to one embodiment, the silicon-carbon mixture can be prepared by a preparation method that includes the steps of: preparing a silicon composite material mixture by mixing the silicon composite material obtained in step 1-2 with the silicon composite oxide powder obtained in step 2-2; and forming a carbon layer on the silicon composite material mixture.

[0381] In the silicon composite material mixture, the mixing weight ratio of the silicon composite material obtained in step 1-2 and the silicon composite oxide powder obtained in step 2-2 may be 90:10-5:95, 90:10-10:90, 90:10-20:80, 90:10-60:40, 90:10-65:35, or 90:10-70:30. When the mixing weight ratio of the silicon composite material mixture satisfies the above range, the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon mixture can be optimized. As a result, a silicon-carbon mixture with desired properties can be obtained.

[0382] Furthermore, the step of forming a carbon layer on the silicon composite material mixture can be carried out in the same or similar manner as in steps 1-3. This method may further include a step of grinding the powder obtained after the formation of the carbon layer (grinding step). The grinding step is as described above. negative electrode active material

[0383] The negative electrode active material in one embodiment may include the silicon-carbon mixture described above.

[0384] Furthermore, the above-mentioned anode active material may further include a carbon-based anode active material, specifically a graphite-based anode active material.

[0385] When the negative electrode active material is used as a mixture of a silicon-carbon mixture and a carbon-based negative electrode active material, such as a graphite-based negative electrode active material, the electrical resistance of the negative electrode active material can be reduced, and the expansion stress associated with charging can be simultaneously relieved. The carbon-based negative electrode material may include, for example, at least one selected from the group consisting of natural graphite, synthetic graphite, soft carbon, hard carbon, mesocarbon, carbon fiber, carbon nanotubes, pyrolysis carbon, coke, glass carbon fiber, sintered organic polymer compounds, and carbon black.

[0386] The content of the carbon-based anode material may be 5% to 70% by weight, preferably 10% to 60% by weight, and more preferably 10% to 50% by weight, based on the total weight of the anode active material.

[0387] Furthermore, when silicon particles with a crystallite size of 20 nm or less, for example 15 nm or less, are mixed with graphite-based materials, which generally exhibit small volume expansion, the silicon particles alone do not cause significant volume expansion. Therefore, lithium secondary batteries with excellent cycle characteristics can be obtained. Lithium-ion battery

[0388] According to one embodiment, a negative electrode containing a negative electrode active material and a lithium secondary battery containing the same are provided.

[0389] A lithium secondary battery may include a positive electrode, a negative electrode, a separator inserted between the positive and negative electrodes, and a non-aqueous liquid electrolyte in which a lithium salt is dissolved. The negative electrode active material may include a silicon-carbon mixture.

[0390] The negative electrode may consist solely of a negative electrode mixture, or it may consist of a negative electrode current collector and a negative electrode mixture layer (negative electrode active material layer) supported thereon. Similarly, the positive electrode may consist solely of a positive electrode mixture, or it may consist of a positive electrode current collector and a positive electrode mixture layer (positive electrode active material layer) supported thereon. Furthermore, the negative electrode mixture and the positive electrode mixture may each further contain a conductive agent and a binder.

[0391] Materials known in the art can be used as the materials constituting the negative electrode current collector and the positive electrode current collector. Materials known in the art can be used as the binder and conductive material added to the negative electrode and positive electrode.

[0392] When the negative electrode consists of a current collector and an active material layer supported thereon, the negative electrode can be manufactured by coating the surface of the current collector with a negative electrode active material composition containing a silicon-carbon mixture and drying it.

[0393] Furthermore, lithium secondary batteries include a non-aqueous liquid electrolyte containing a non-aqueous solvent and a lithium salt dissolved in that non-aqueous solvent. Any solvent commonly used in the field can be used as the non-aqueous solvent. Specifically, aprotic organic solvents can be used. Examples of aprotic organic solvents include cyclic carbonates such as ethylene carbonate, propylene carbonate, and butylene carbonate; cyclic carboxylic acid esters such as furanone; linear carbonates such as diethyl carbonate, ethyl methyl carbonate, and dimethyl carbonate; linear ethers such as 1,2-methoxyethane, 1,2-ethoxyethane, and ethoxymethoxyethane; and cyclic ethers such as tetrahydrofuran and 2-methyltetrahydrofuran. These can be used individually or in combination of two or more.

[0394] Lithium secondary batteries may include non-aqueous lithium secondary batteries.

[0395] In one embodiment, a negative electrode active material and lithium secondary battery using a silicon-carbon mixture can improve not only the initial charge-discharge efficiency and capacity retention rate but also the charge-discharge capacity. [Examples]

[0396] The present invention will be described in detail below with reference to examples. The following examples are merely illustrative of the present invention, and the scope of the present invention is not limited thereto. <Preparation of the first composite material> Preparation Example 3-1

[0397] (1) Step 1-1: Silicon oxide (oxygen-containing silicon compound) (Preparation Example 1-1) having the physical properties shown in Table 1 below, prepared by vapor deposition using silicon powder and silicon dioxide powder, is silicon oxide (SiO2). x (x=0.9) 50g of powder was dispersed in water. While stirring at a speed of 300 rpm, 50 mL of a 40 wt% HF aqueous solution was added as an etching solution, and the silicon-based raw material (first silicon-based raw material) powder was etched for 1 hour. (2) Step 1-2: The product obtained by the above etching was filtered and dried at 150°C for 2 hours to prepare a dried composite material. Next, in order to control the particle size of the composite material, it was ground using a mortar and pestle to an average particle size of 4.5 μm, and a porous silicon composite material (Preparation Example 2-1) having the content and physical properties of each component shown in Table 2 below was prepared.

[0398] (3) Steps 1-3: 10 g of porous silicon composite material was placed in a tubular electric furnace, and argon (Ar) and methane gas were flowed through it at a rate of 1 liter / min each. After maintaining the temperature at 900°C for 1 hour, the material was cooled to room temperature to form a first carbon layer inside the silicon composite material and a first carbon layer on the surface of the silicon composite material, thereby preparing a first silicon-carbon composite material having the content and physical properties of each component shown in Table 3 below.

[0399] (4) Steps 1-4: In order to control the particle size of the first silicon-carbon composite material, it was crushed by mechanical means and classified so that the average particle size was 5.0 μm, thereby preparing the first composite material having the content and physical properties of each component shown in Table 3 below. Preparation Examples 3-2 to 3-7

[0400] Using porous silicon composite materials (Preparation Examples 2-2 to 2-5) having the component content and physical properties shown in Table 2 below, first composite materials (Preparation Examples 3-2 to 3-7) having the component content and physical properties shown in Table 3 below were prepared in the same manner as Preparation Example 3-1, except that the component content and physical properties were adjusted. Preparation Examples 3-8 to 3-10

[0401] Using porous silicon composite materials (Preparation Examples 2-2 to 2-5) having the content and physical properties of each component shown in Table 2 below, the content and physical properties of each component were adjusted. Except for the following, 10 g of the porous silicon composite material was placed in a tubular electric furnace, argon (Ar) gas and ethylene gas were flowed through it at a rate of 0.5 liters / minute each, and the temperature was maintained at 750°C for 2 hours before being cooled to room temperature, first composite materials (Preparation Examples 3-8 to 3-10) having the content and physical properties of each component shown in Table 3 below were prepared in the same manner as Preparation Example 3-1, in which a first carbon layer was formed inside the silicon composite material and a first carbon layer on the surface of the silicon composite material. <Preparation of the second composite material> Preparation Example 4-1

[0402] (1) Step 2-1: 8 kg of silicon powder with an average particle size of 20 μm and 16 kg of silicon dioxide powder with an average particle size of 20 nm were added to 50 kg of water, stirred for 12 hours to mix uniformly, and then dried at 200°C for 24 hours to obtain a raw material powder mixture.

[0403] The raw material powder mixture and 2 kg of metallic magnesium were placed in crucible A and crucible B, respectively, in a vacuum reactor. After reducing the pressure to 0.1 Torr, the temperature of crucible A was raised to 1,500°C and the temperature of crucible B to 900°C, and the reaction was carried out for 5 hours.

[0404] The silicon composite oxide deposits deposited onto the substrate inside the reactor by a reaction in the high-temperature gas phase were rapidly cooled to room temperature using a water-cooled substrate.

[0405] (2) Step 2-2: The silicon composite oxide mass was crushed by mechanical means to control the particle size and obtain silicon composite oxide powder with an average particle size of 6 μm.

[0406] (3) Step 2-3: 50 g of silicon composite oxide powder was placed in a tubular electric furnace, and argon (Ar) and methane gas were flowed through it at a rate of 1 liter / min each. This was maintained at 900°C for 1 hour to prepare a second composite material (Preparation Example 4-1) having the content and physical properties of each component shown in Table 4 below. Preparation Example 4-2

[0407] A second composite material (Preparation Example 4-2) was prepared in the same manner as Preparation Example 4-1, except that the content of each component and the physical properties of the composite material were adjusted as shown in Table 4 below. Example 1 Preparation of silicon-carbon mixture

[0408] The first composite material from Preparation Example 3-1 and the second composite material from Preparation Example 4-1 were mixed in a weight ratio of 90:10. Mixing was performed using a co-rotating mixer (THINKY MIXER) to prepare a powder containing mixed phase particles composed of the first and second composite materials. Mixing was carried out at a rotation speed of approximately 2,000 rpm for approximately 6 minutes. The physical properties of the silicon-carbon mixture thus prepared are shown in Tables 5 and 7 below. Manufacturing of lithium secondary batteries

[0409] A negative electrode and a battery (coin cell) containing this silicon-carbon mixture as the negative electrode active material were fabricated.

[0410] A negative electrode active material composition with a solid content of 45% was prepared by mixing Super-P as a conductive material and polyacrylic acid with water in a weight ratio of 80:10:10.

[0411] This negative electrode active material composition was coated onto a copper foil with a thickness of 18 μm and dried to produce an electrode with a thickness of 70 μm. This copper foil coated with the electrode was punched out into a circle with a diameter of 14 mm to produce a negative electrode plate for a coin cell.

[0412] On the other hand, a 0.3 mm thick metallic lithium foil was used as the positive electrode plate.

[0413] A porous polyethylene sheet with a thickness of 25 μm was used as a separator. As the electrolyte, a liquid electrolyte was used, prepared by dissolving LiPF6 at a concentration of 1 M in a mixed solvent of ethylene carbonate (EC) and diethylene carbonate (DEC) in a 1:1 volume ratio. Using the above components, a coin cell with a thickness of 3.2 mm and a diameter of 20 mm was fabricated. Examples 2-12

[0414] Except for changing the types of the first and second composite materials, their mixed weight ratios, and the O / Si molar ratio as shown in Table 5 below, silicon-carbon mixtures and lithium secondary batteries were prepared in the same manner as in Example 1. Comparative Example 1

[0415] As shown in Table 6 below, a lithium secondary battery was fabricated in the same manner as in Example 1, except that the first composite material obtained in Preparation Example 3-1 was used alone without the second composite material. Comparative Example 2

[0416] As shown in Table 6 below, a lithium secondary battery was fabricated in the same manner as in Example 1, except that the second composite material obtained in Preparation Example 4-1 was used alone without the first composite material. Comparative Example 3

[0417] As shown in Table 6 below, silicon-carbon mixtures and lithium secondary batteries were prepared in the same manner as in Example 1, except that the types of the first and second composite materials, their mixed weight ratios, and the O / Si molar ratio were changed. <Test Example 1> X-ray diffraction analysis Measurement of the size of Si crystals

[0418] The size of the Si crystals in the first and second silicon particles in the silicon-carbon mixture was determined by the Scherrer equation 3 below, based on the full width at half maximum (FWHM) of the peak corresponding to Si(220) around 2θ = 47.5° in X-ray diffraction analysis. [Equation 3] [ka]

[0419] In equation 3, K is 0.9, λ is 0.154 nm, B is the full width at half maximum (FWHM), and θ is the peak position (angle). Crystal structure of silicon-carbon mixture

[0420] The crystal structure of the silicon-carbon mixture prepared in the examples was analyzed using an X-ray diffraction analyzer (X'Pert3, Malvern Panalytical). Specifically, the applied voltage was 40kV and the applied current was 40mA. The 2θ range was 10° to 80°, and measurements were performed by scanning at 0.05° intervals.

[0421] Analysis revealed that the silicon-carbon mixtures in the examples each had peaks corresponding to Si crystals, indicating that they contained crystalline Si, and peaks for MgSiO3 and SiO2 were also observed.

[0422] Specifically, as can be seen from the X-ray diffraction pattern in Figure 1, the silicon-carbon mixture of Example 1 has peaks corresponding to Si crystals at diffraction angles (2θ) around 28.6°, 47.5°, 56.2°, 69.3°, and 76.6°, indicating the presence of crystalline Si. In addition, diffraction angles (2θ) of MgSiO3 were observed at 30.4° and 35.6°. SiO2 was observed at 21.3°.

[0423] As can be seen from the X-ray diffraction pattern in Figure 2, the silicon-carbon mixture of Example 7 has peaks corresponding to Si crystals at diffraction angles (2θ) around 28.5°, 47.3°, 56.2°, 69.2°, and 76.7°, indicating the presence of crystalline Si. Furthermore, diffraction angles (2θ) of MgSiO3 were observed at 31.0° and 35.4°. SiO2 was observed at 21.3°. <Test Example 2> Electron Microscopy Analysis

[0424] The porous silicon composite material prepared in Preparation Example 2-1 was observed at magnifications of 10,000x, 30,000x, and 100,000x using an ion beam scanning electron microscope (FIB-SEM, Focused Ion Beam System / Quanta 3D FEG). The results are shown in Figures 3 to 5.

[0425] Furthermore, the first composite material prepared in Preparation Example 3-1 was observed at a magnification of 10,000x using a scanning electron microscope (FE-SEM, S-4700; Hitachi). The results are shown in Figure 6.

[0426] In Figure 6, the bright white particles represent the first silicon (Si) particles (crystalline), while the dark areas represent carbon (C).

[0427] Furthermore, the first composite material prepared in Preparation Example 3-1 was observed using a transmission electron microscope (FE-TEM, TECNAI G2 F20). The results are shown in Figures 7 and 8.

[0428] Figure 7 shows the FE-TEM image of the first composite material prepared in Preparation Example 3-1. The bright-field image results are shown. Figure 8 shows the dark-field and bright-field image results.

[0429] As can be seen from Figures 7 and 8, a carbon coating layer with a thickness of approximately 27 ± 5 nm was formed on the surface of the first composite material. <Test Example 3> Analysis of elemental content and density of composite material

[0430] The magnesium (Mg), oxygen (O), carbon (C), and silicon (Si) content in the first composite material, the second composite material, and the silicon-carbon mixture was analyzed.

[0431] The magnesium (Mg) content was analyzed by inductively coupled plasma (ICP) analysis. The oxygen (O) and carbon (C) content were measured using elemental analyzers. The silicon (Si) content was calculated based on the oxygen (O) and magnesium (Mg) content.

[0432] Meanwhile, the components contained in the silicon-carbon mixture were identified using FIB-SEM EDS (S-4700; Hitachi, QUANTA 3D FEG; FEI, EDS system; EDAX). As a result, Mg, C, O, and Si components were observed in the silicon-carbon mixture.

[0433] On the other hand, the densities of the first composite particle, the second composite particle, and the silicon-carbon mixture were measured using a Shimadzu Acupick II1340 after purging 200 times with helium gas in a sample holder set to a temperature of 23°C. <Test Example 4> Measurement of the average particle size of composite materials and composite material particles

[0434] The average particle size (D) of the first and second composite material particles and the silicon-carbon mixture particles. 50 ) is the average diameter D in particle size distribution measurement by laser beam diffraction. 50 That is, it was measured as the particle diameter or median diameter when the cumulative volume reached 50%. <Test Example 5> Measurement of Capacity, Initial Efficiency, and Capacity Retention Rate of a Lithium-ion Battery

[0435] The monocells (lithium secondary batteries) prepared in the examples and comparative examples were charged with a constant current of 0.1C until the voltage reached 0.005V, and then the current was increased to 0.005C at a constant voltage. Next, they were discharged with a constant current of 0.1C until the voltage reached 2.0V, and the charging capacity (mAh / g), discharging capacity (mAh / g), and initial efficiency (%) were measured. The results are shown in Tables 5 and 6 below. [Equation 4] Initial efficiency (%) = Discharge capacity / Charge capacity × 100

[0436] Furthermore, the monocells (lithium secondary batteries) prepared in the examples and comparative examples were charged with a constant current of 0.1C until the voltage reached 4.25V, and then the current reached 0.05C at a constant voltage. Next, they were discharged with a constant current of 0.1C until the voltage reached 2.5V. 1,000 cycles were performed under these conditions, and the capacity retention rate after 1,000 cycles was measured according to equation 5 below. [Equation 5] Capacity retention rate after 1,000 cycles (%) = Discharge capacity after 1,000 cycles / Discharge capacity in the first cycle × 100

[0437] The elemental content and physical properties of the first and second composite materials prepared in the preparation example are summarized in Tables 3 and 4 below, while the elemental content and physical properties of the mixtures or composite materials prepared in the examples and comparative examples, as well as the characteristics of lithium secondary batteries using them, are summarized in Tables 5 to 7 below.

[0438] [Table 1]

[0439] [Table 2]

[0440] [Table 3]

[0441] [Table 4]

[0442] [Table 5]

[0443] [Table 6]

[0444] [Table 7]

[0445] As can be seen from Tables 5 to 7 above, in the lithium secondary batteries of Examples 1 to 12, which used a silicon-carbon mixture containing two or more composite materials, a first composite material and a second composite material, silicon particles, magnesium silicate, and carbon, and in which the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) satisfies a specific range, the discharge capacity, initial efficiency, and capacity retention rate were all improved compared to the lithium secondary batteries of Comparative Examples 1 to 3 when evaluated comprehensively.

[0446] Specifically, the lithium secondary batteries of Examples 1 to 12, which used a silicon-carbon mixture containing a first composite material and a second composite material with a molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) of 0.09 to 0.56, almost always exhibited a discharge capacity of approximately 1,400 mAh / g or higher, an initial efficiency of approximately 83% or higher, and a capacity retention rate of approximately 85% or higher after 50 cycles. Furthermore, the lithium secondary batteries of Examples 10 to 12 exhibited a capacity retention rate of approximately 66% or higher after 1,000 cycles, demonstrating overall superior performance and improved reliability of the lithium secondary batteries.

[0447] In contrast, Comparative Example 1, a lithium secondary battery in which a silicon-carbon mixture containing only the first composite material was used as the negative electrode active material, and the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) was 0.01, showed a capacity retention rate of 78.4% and a capacity retention rate of 52% after 1000 cycles, which was a significant decrease compared to the lithium secondary battery of the example.

[0448] Furthermore, in Comparative Example 2, a lithium secondary battery using a silicon-carbon mixture containing only the second composite material as the negative electrode active material, with a molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) of 0.98, the discharge capacity was approximately 1,354 mAh / g, the initial efficiency was 81%, and the capacity retention rate after 1,000 cycles was 57%, showing a significant decrease compared to the lithium secondary battery of the example.

[0449] On the other hand, in Comparative Example 3, a lithium secondary battery using a silicon-carbon mixture containing the first and second composite materials, although the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) was 0.91, the discharge capacity was 1,368 mAh / g, the initial efficiency was 82.1%, and the capacity retention rate after 1,000 cycles was approximately 54%, showing a significant decrease compared to the lithium secondary battery of the example.

[0450] On the other hand, if the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) was less than 0.06, the fabrication itself was impossible.

[0451] Furthermore, according to one embodiment, the carbon content and porosity can be easily controlled, and as a result, while satisfying the above molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the silicon-carbon mixture, excellent capacity retention can be maintained even after repeating thousands of cycles over a long period of time, thereby further improving reliability.

Claims

1. A silicon-carbon mixture comprising two or more composite materials, comprising silicon particles, magnesium silicate, and carbon, wherein the molar ratio of oxygen (O) atoms to silicon (Si) atoms (O / Si) is 0.06 to 0.90, The two or more composite materials include a first composite material and a second composite material. The first composite material comprises first silicon particles and first carbon, The second composite material comprises second silicon particles, magnesium silicate, and second carbon. A silicon-carbon mixture in which the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the first composite material is 0.005 to 0.50, and the molar ratio (O / Si) of oxygen (O) atoms to silicon (Si) atoms in the second composite material is 0.8 to 1.

2.

2. The silicon-carbon mixture according to claim 1, wherein the weight ratio of the first composite material to the second composite material is 90:10 to 5:

95.

3. The silicon-carbon mixture contains silicon aggregates in which silicon particles are bonded to each other, and the magnesium silicate is MgSiO ３ Mg ２ SiO ４ A silicon-carbon mixture according to claim 1, comprising, or a mixture thereof.

4. In the silicon-carbon mixture, based on the total weight of the silicon-carbon mixture, The oxygen (O) content is 1% to 25% by weight. The silicon (Si) content is 35% to 80% by weight. The magnesium (Mg) content is 0.1% to 15% by weight. The carbon (C) content is 9% to 50% by weight. The silicon-carbon mixture according to claim 1, wherein the molar ratio (Mg / Si) of magnesium (Mg) atoms to silicon (Si) atoms in the silicon-carbon mixture is 0.009 to 0.

55.

5. The silicon-carbon mixture is silicon oxide (SiO ｘ The silicon-carbon mixture according to claim 1, further comprising 0.4 < x ≤ 2.

6. The first composite material comprises a silicon composite material and a first carbon layer on its surface, the first silicon particles are present in the silicon composite material, and the first carbon is contained in the first carbon layer and inside the silicon composite material. The second composite material comprises a silicon composite oxide and a second carbon layer on its surface, the second silicon particles and the second magnesium silicate are present in the silicon composite oxide, and the second carbon is included in at least one selected from the group consisting of the second carbon layer, the surface of the second silicon particles, and the surface of the second magnesium silicate. The silicon-carbon mixture according to claim 1.

7. The oxygen (O) content in the first composite material is 0.1% by weight to 16% by weight, based on the total weight of the first composite material. The silicon-carbon mixture according to claim 1, wherein the oxygen (O) content in the second composite material is 20% by weight to 40% by weight based on the total weight of the second composite material.

8. The carbon (C) content in the first composite material is 10% by weight to 50% by weight based on the total weight of the first composite material. The silicon-carbon mixture according to claim 1, wherein the carbon (C) content in the second composite material is 3% by weight to 15% by weight based on the total weight of the second composite material.

9. A first step to obtain a first composite material; A second step to obtain a second composite material; and A third step of mixing the first composite material and the second composite material. A method for preparing a silicon-carbon mixture according to claim 1, comprising: A method for preparing a composite material, wherein the first composite material comprises first silicon particles and first carbon, and the second composite material comprises second silicon particles, magnesium silicate, and second carbon.

10. The first step of obtaining the first composite material is Step 1-1 involves etching a first silicon-based raw material using an etching solution containing a fluorine (F) atom-containing compound; Steps 1 and 2 to obtain a silicon composite material by filtering and drying the product obtained by the etching; and Steps 1-3: Using a chemical pyrolysis vapor deposition method, a first carbon layer is formed inside the silicon composite material, and a first carbon layer is formed on the surface of the silicon composite material to obtain the first composite material. A method for preparing a silicon-carbon mixture according to claim 9, comprising:

11. The second step of preparing the second composite material is: Step 2-1 for obtaining a silicon composite oxide using a second silicon-based raw material and a magnesium-based raw material; Step 2-2 involves grinding and / or classifying the silicon composite oxide to obtain silicon composite oxide powder; and Steps 2-3 involve forming a second carbon layer on the surface of the silicon composite oxide powder using a chemical pyrolysis vapor deposition method to obtain a second composite material. A method for preparing a silicon-carbon mixture according to claim 9, comprising:

12. A negative electrode active material comprising the silicon-carbon mixture described in claim 1.

13. A lithium secondary battery comprising the negative electrode active material according to claim 12.

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