Negative active material comprising silicon composite, method for preparing the same, and lithium secondary battery comprising the same

Abstract

The present invention relates to a negative active material comprising a silicon composite material including: a core formed by coating a first crystalline carbon on the surfaces of needle-shaped silicon nanoparticles and plate-shaped silicon nanoparticles having different aspect ratios; and a shell surrounding the core and containing amorphous carbon, in which a second crystalline carbon is located on the entire surface or a part of the surface of the shell, a method for preparing the same, and a lithium secondary battery comprising the same to have excellent life characteristics, output characteristics, and safety.

Description

TECHNICAL FIELD

[0001] The present application relates to a high-capacity negative active material having high conductivity containing a silicon composite, a method for preparing the same, and a lithium secondary battery including the same and thus having excellent lifespan, output, and safety characteristics.

[0002] The national R&D project supporting the present application is described below.

[0003] Project-specific number 1425136568

[0004] Project serial number S2832482

[0005] Government agency name: Small and Medium Venture Business Administration, Republic of Korea

[0006] Project management professional organization name: Korea Small and Medium Enterprises Technology Information Promotion Association

[0007] Research institution name: (October) 2019 Business Growth Technology Development Business 'Innovation Start-up Project (Technology Innovation Enterprise)'

[0008] Research project name Development of carbon / Si nanocomposite and high-capacity (>1300 mAh / g) negative material for secondary battery

[0009] Executing agency name: Grapsil Co., Ltd.

[0010] Time 2019.12.01 ~ 2021.11.30 BACKGROUND

[0011] Lithium secondary batteries are widely used as power sources for mobile electronic devices including mobile phones, and as large devices such as electric vehicles increasingly use lithium secondary batteries, more applications of lithium secondary batteries are being sought.

[0012] Meanwhile, most of the lithium secondary batteries currently on the market use carbon-based materials as negative active materials. In particular, graphite exhibits highly reversible charge / discharge behavior due to the uniaxial orientation of the graphite layer, thereby having good lifespan characteristics. In addition, since graphite exhibits a potential very similar to that of lithium metal, a high-energy battery can be obtained when graphite is used in combination with a lithium metal positive electrode in a battery. However, despite these advantages, the low theoretical capacity (372 mAh / g) of graphite remains an obstacle to increasing the application field even in the current situation where high-capacity batteries are required.

[0013] Accordingly, attempts have been made to use metals such as Si, Sn, and Al having a relatively high capacity as a material to replace the existing carbon-based negative active material. However, these metals repeatedly undergo a large volume change during intercalation and deintercalation of lithium, resulting in pulverization and loss of conductive paths, thereby causing short cycle life and deterioration of overall battery performance.

[0014] To solve this problem, many efforts have been made. For example, various carbon materials are simply mixed with silicon (Si), fine powdered silicon is chemically fixed on the surface of carbon by a silane coupling agent, or amorphous carbon is deposited on the surface of silicon by a chemical vapor deposition (CVD) method or the like.

[0015] However, in the case of simply mixing the carbon material with silicon, the carbon is released from the silicon, and the silicon undergoes a large volume expansion and contraction during charging and discharging, which results in a decrease in electrical conductivity, causing a significant decrease in cycle life.

[0016] On the other hand, in the case of chemically fixing fine powdered silicon on the surface of carbon by CVD or using a coupling agent such as silane, the bonding duration provided by the silane coupling agent or CVD is not so long, and the cycle life decreases as the number of charging and discharging cycles increases. In addition, since it is difficult to obtain uniform physical and chemical bonding, there is a problem in that it is difficult to obtain a reliable negative active material.

[0017] Despite various attempts, there still remains a problem in that the electrode is damaged due to silicon expansion during discharging.

[0018] Accordingly, there is an increasing need for a high-capacity negative active material having excellent electrical conductivity and a lithium secondary battery having a long cycle life and good output characteristics by employing such a negative active material and being highly safe. SUMMARY

[0019] TECHNICAL PROBLEM

[0020] The present application aims to disclose a means for solving the problems of the prior art and the unsolved technical problems described above.

[0021] Specifically, an object of the present application is to provide a high-conductivity high-capacity negative active material including a silicon composite material.

[0022] Another object of the present application is to provide a method of manufacturing a high-conductivity high-capacity negative active material including a silicon composite material.

[0023] Still another object of the present application is to provide a lithium secondary battery employing the negative active material, thereby having a long cycle life, good output characteristics, and high safety.

[0024] TECHNICAL SOLUTION

[0025] The present application provides a negative active material including a silicon composite material including:

[0026] a core including needle-shaped silicon nanoparticles and plate-shaped silicon nanoparticles having different aspect ratios and coated with a first crystalline carbon; and

[0027] a shell surrounding the core and including amorphous carbon, wherein a second crystalline carbon is provided in a partial region or an entire region of a surface of the shell.

[0028] The aspect ratio of the needle-shaped silicon nanoparticles can be in a range of greater than 6 to 250, and the aspect ratio of the plate-shaped silicon nanoparticles can be in a range of 1 to 6.

[0029] The weight ratio of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles can be in a range of 30:70 to 70:30.

[0030] The content of the amorphous carbon can be in a range of 5 parts by weight to 30 parts by weight with respect to 100 parts by weight of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles.

[0031] The content of the first crystalline carbon can be 0.1 parts by weight to 80 parts by weight and the content of the second crystalline carbon can be 0.1 parts by weight to 80 parts by weight with respect to 100 parts by weight of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles.

[0032] The average particle diameter of the silicon composite material can be in a range of 5 μm to 50 μm.

[0033] The porosity of the silicon composite material can be in a range of 0.1% to 40%.

[0034] In addition, the present application provides a method of preparing a negative active material, the method including:

[0035] (a) coating surfaces of needle-shaped silicon nanoparticles and surfaces of plate-shaped silicon nanoparticles with a first crystalline carbon to prepare a core, the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles having different aspect ratios;

[0036] (b) adding a precursor of amorphous carbon to the prepared core, and then heat-treating the prepared core, thereby forming a shell made of the amorphous carbon and surrounding the core; and

[0037] (c) adding a second crystalline carbon after the formation of the shell, and heat-treating, thereby obtaining a silicon composite material in which an entire surface or a partial surface of the shell has the second crystalline carbon.

[0038] A silicon composite material characterized by including a silicon composite material including needle-shaped silicon nanoparticles having an aspect ratio in a range of greater than 6 to 250 and plate-shaped silicon nanoparticles having an aspect ratio in a range of 1 to 6.

[0039] The weight ratio of the acicular silicon nanoparticles and the plate-like silicon nanoparticles can be in the range of 30:70 to 70:30.

[0040] The content of the amorphous carbon precursor can be in the range of 5 parts by weight to 30 parts by weight, with respect to 100 parts by weight of the acicular silicon nanoparticles and the plate-like silicon nanoparticles.

[0041] The content of the first crystalline carbon can be 0.1 parts by weight to 80 parts by weight, and the content of the second crystalline carbon can be 0.1 parts by weight to 80 parts by weight, with respect to 100 parts by weight of the acicular silicon nanoparticles and the plate-like silicon nanoparticles.

[0042] The average particle diameter of the silicon composite can be in the range of 5 μm to 50 μm.

[0043] The porosity of the silicon composite can be in the range of 0.1% to 40%.

[0044] The present application also provides a lithium secondary battery including the negative electrode active material.

[0045] Advantageous effects

[0046] Since the negative electrode active material according to the present application has a core made of acicular silicon nanoparticles and plate-like silicon nanoparticles having different aspect ratios, pores are minimized, and thus the capacity per unit volume is maximized. Accordingly, the lithium secondary battery employing the negative electrode active material has improved lifespan characteristics. In addition, since fine pores serve as a moving path for lithium ions, the lithium secondary battery exhibits good initial efficiency and output characteristics.

[0047] According to the present application, the negative electrode active material has a core-shell structure in which the silicon particle surface of the core is coated with the first crystalline carbon, and the shell is made of amorphous carbon and has the second crystalline carbon in a partial region or the entire region of its surface. Accordingly, although the contraction and expansion of the silicon particles repeatedly occur due to the intercalation and deintercalation of lithium ions during the charging and discharging of the battery, the volume expansion and damage of the silicon particles can be inhibited by the buffering effect. Accordingly, the negative electrode active material has good durability, thereby inhibiting the damage of the electrode. This can result in the improvement of the safety of the lithium secondary battery. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 is a schematic view showing a cross section of a negative electrode active material including a silicon composite according to one embodiment of the present application;

[0049] Figure 2a is an SEM image of acicular silicon nanoparticles, plate-like silicon nanoparticles, and their mixed state of Example 1;

[0050] Figure 2bis an SEM image of the silicon composite prepared in Example 1;

[0051] Figure 3a 、 Figure 3b and Figure 3c shows the pore distribution of the powder during the pore measurement of Experimental Example 2, the pore distribution during the mercury injection, and the pore distribution during the mercury discharge;

[0052] Figure 4 shows the capacity measurement results according to Experimental Example 3.

[0053] Figure 5 shows the conductivity measurement results according to Experimental Example 5-1; and

[0054] Figure 6 shows the capacity retention rate (CCR) measurement results according to Experimental Example 5-2. DETAILED DESCRIPTION

[0055] Negative electrode active material

[0056] Figure 1 is a schematic view schematically showing a cross-section of a negative active material including a silicon composite 100 according to one embodiment of the present application; however, the present application is not limited thereto.

[0057] Referring to Figure 1 , the present application provides a negative active material including a core and a shell. The core includes needle-shaped silicon nanoparticles 111 and plate-shaped silicon nanoparticles 112 having different aspect ratios, and is coated on its surface with first crystalline carbon. The shell surrounds the core and is made of amorphous carbon 130. The surface of the shell is entirely or partially covered with second crystalline carbon 140.

[0058] Silicon has a theoretical capacity of about 3,600 mA / g, which is higher than that of conventional carbon-based negative active materials, but has a problem in that silicon exhibits a large volume change during intercalation and deintercalation of lithium ions, resulting in pulverization of silicon and loss of conductive paths, which leads to deterioration of life characteristics.

[0059] The negative active material according to the present application includes a core in which a plurality of secondary particles are disposed. Each secondary particle includes a primary particle having a needle shape or a plate shape and a first crystalline carbon 120 coating layer. The aspect ratios of the needle-shaped silicon nanoparticles 111 and the plate-shaped silicon nanoparticles 112 are different. Accordingly, pores are minimized, so that the capacity per unit volume is maximized, so that the life characteristics of a lithium secondary battery are improved. In addition, since fine pores serve as a moving path for lithium ions, the lithium secondary battery exhibits good initial efficiency and output characteristics.

[0060] In the present application, the "needle shape" or "plate shape" can be approximately needle-shaped or approximately plate-shaped, and does not have to be completely needle-shaped or completely plate-shaped. The surface of the needle-shaped or plate-shaped silicon nanoparticles 111 and 112 can not be flat. The needle-shaped silicon nanoparticles 111 and the plate-shaped silicon nanoparticles 112 can be defined with an aspect ratio. The "aspect ratio" refers to a value obtained by dividing the length dimension of the silicon nanoparticles by the short dimension.

[0061] For example, the aspect ratio of the needle-shaped silicon nanoparticles 111 can be in the range of greater than 6 to 250, and the aspect ratio of the plate-shaped silicon nanoparticles 112 can be in the range of 1 to less than 6. When the aspect ratio exceeds the above range, the intended pore reduction effect by the arrangement of the particles cannot be achieved, the bonding between the silicon nanoparticles can be weakened, and the cycle characteristics can be deteriorated, which is not desirable.

[0062] Specifically, in the present application, the length of the needle-shaped silicon nanoparticles 111 of the primary particles is about 190 nm to 250 nm, and the width is about 1 nm to 30 nm, and the length of the plate-shaped silicon nanoparticles of the primary particles is about 150 nm to 240 nm, and the width is about 40 nm to 100 nm. More specifically, the length of the needle-shaped silicon nanoparticles 111 is about 190 nm to 220 nm, and the width is about 1 nm to 30 nm, and the length of the plate-shaped silicon nanoparticles is about 180 nm to 220 nm, and the width is about 40 nm to 100 nm. When the above range is satisfied, the pores between the secondary particles can be minimized, resulting in an increase in capacity and an improvement in initial efficiency.

[0063] The weight ratio of the needle-shaped silicon nanoparticles 111 and the plate-shaped silicon nanoparticles 112 can be in the range of 30:70 to 70:30. When the amount of the needle-shaped silicon nanoparticles 111 is too much, there is a risk of serious undifferentiation during the charging and discharging of the battery. When the amount of the plate-shaped silicon nanoparticles 112 is too much, there is a risk of deterioration in economic feasibility. In addition, if the above range is not satisfied, the intended effect of minimizing the pores by arranging silicon particles having different aspect ratios cannot be achieved, and thus is not desirable. Specifically, the weight ratio of the needle-shaped silicon nanoparticles 111 to the plate-shaped silicon nanoparticles 112 can be in the range of 40:60 to 60:40.

[0064] The negative active material according to the present application has a core-shell structure. The core includes silicon nanoparticles having different aspect ratios and coated with a first crystalline carbon 120, and the shell is made of amorphous carbon 130 and is entirely or partially covered with a second crystalline carbon 140. Therefore, even if the silicon particles repeatedly shrink and expand due to the intercalation and deintercalation of lithium ions during the charging and discharging of the battery, the volume expansion and damage of the silicon particles can be inhibited due to the buffering effect. This can result in an improvement in the durability of the silicon particles, and thus the electrode damage can be minimized. Furthermore, since the contact between the silicon particles and the electrolyte is inhibited, the safety of the lithium secondary battery can be improved.

[0065] According to the present application, since part of the surface area or the entire surface area of each of the needle-shaped silicon nanoparticles 111 and the plate-shaped silicon nanoparticles 112 having different aspect ratios is coated with the first crystalline carbon 120 to form a conductive path, the electrical conductivity is improved.

[0066] The content of the first crystalline carbon 120 can be 0.1 parts by weight to 80 parts by weight with respect to 100 parts by weight of the needle-shaped silicon nanoparticles 111 and the plate-shaped silicon nanoparticles 112. When the content of the first crystalline carbon 120 is too low, it is difficult to achieve the intended effect of improving the electrical conductivity. When the content of the first crystalline carbon 120 is too high, the lithium ion conductivity decreases, resulting in deterioration of the output characteristics. The content of the first crystalline carbon 120 can be preferably 1 part by weight to 30 parts by weight, more preferably 1 part by weight to 3 parts by weight, with respect to 100 parts by weight of the needle-shaped silicon nanoparticles 111 and the plate-shaped silicon nanoparticles 112.

[0067] As the first crystalline carbon 120, any carbon material known in the art can be used without particular limitation. For example, one or more carbons selected from the group consisting of natural graphite, synthetic graphite, expanded graphite, graphene, and fullerene soot, but not limited thereto. Specifically, it can be graphene that is a single layer of graphite.

[0068] The amorphous carbon 130 can form a covering film on the entire surface of the secondary particle constituting the core. In some cases, the amorphous carbon 130 can be located between the primary particles. The amorphous carbon 130 can impart sufficient strength to maintain the shape of the core-shell structure, thereby suppressing swelling of the core.

[0069] The content of the amorphous carbon 130 can be in the range of 5 parts by weight to 30 parts by weight with respect to 100 parts by weight of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles. When the content of the amorphous carbon 130 is lower than the above range, the core cannot be sufficiently surrounded by the amorphous carbon. In this case, the risk of the electrolyte and the negative active material reacting with each other increases. This can result in deterioration of the cycle characteristics. When the content of the amorphous carbon 130 is too high, it is undesirable because the dispersion stability decreases.

[0070] As the first amorphous carbon 130, any amorphous carbon known in the art can be used without limitation. For example, any one material selected from the group consisting of soft carbon, hard carbon, pitch carbonization, mesophase pitch carbonization, and baked coke, or a combination of two or more materials can be used. In particular, pitch carbonization can be used. More specifically, pitch carbonization derived from petroleum-based pitch can be used. The petroleum-based pitch can be obtained by distilling crude oil and removing impurity components from the remaining high-boiling residue.

[0071] The second crystalline carbon 140 can form a covering film on the entire region or a partial region of the shell made of the amorphous carbon 130.

[0072] The content of the second crystalline carbon can be in the range of 0.1 parts by weight to 80 parts by weight with respect to 100 parts by weight of the needle-shaped silicon nanoparticles 111 and the plate-shaped silicon nanoparticles 112. When the content of the second crystalline carbon is lower than the above range, the shell cannot be sufficiently covered with the second crystalline carbon. In this case, the risk of the electrolyte and the negative active material reacting with each other can increase. This can cause the cycle characteristics to deteriorate. On the other hand, if the content is too high, the cycle characteristics and the reaction processability can deteriorate, and thus are not desirable. The content of the second crystalline carbon can be preferably 0.1 parts by weight to 30 parts by weight, more preferably 0.1 parts by weight to 3 parts by weight, with respect to 100 parts by weight of the needle-shaped silicon nanoparticles 111 and the plate-shaped silicon nanoparticles.

[0073] As the second crystalline carbon 140, any known carbon material can be used without particular limitation. For example, one or more carbons selected from the group consisting of natural graphite, synthetic graphite, expanded graphite, graphene, and fullerene soot, but is not limited thereto. Specifically, it can be graphene that is a single layer of graphite.

[0074] The average particle diameter of the silicon composite of the present application is not particularly limited, and can be in the range of 5 μm to 50 μm, for example. When the average particle diameter of the silicon composite is too small, i.e., less than the above range, the reactivity between the electrolyte and the negative active material can be high, thereby reducing the cycle characteristics. When the average particle diameter of the silicon composite is too large, the dispersion stability can decrease and the negative electrode surface can become rough. Specifically, the average particle diameter can be in the range of 5 μm to 20 μm.

[0075] In the present application, the porosity of the silicon composite can be in the range of 0.1% to 40%. Here, the porosity refers to a value expressed as "(volume of pores per unit mass) / (specific volume + volume of pores per unit mass)", and can be measured by a mercury porosimetry test or a Bruanuer-Emmett-Teller (BET) test. That is, since the core as a secondary particle is made of primary particles of silicon nanoparticles having different aspect ratios, it is possible to minimize the pores present in the secondary particle, thereby maximizing the capacity per unit volume. Further, since fine pores can serve as a moving path for lithium ions, it is possible to improve the initial efficiency. Therefore, when the porosity exceeds the above range, the effects of increasing the capacity and the initial efficiency cannot be obtained at the same time. Specifically, the porosity can be in the range of 0.1% to 4%.

[0076] Method for producing a negative electrode active material.

[0077] The present invention provides a method of preparing a negative active material, the method comprising:

[0078] (a) preparing a core by coating surfaces of acicular silicon nanoparticles and surfaces of plate-like silicon nanoparticles, which have different aspect ratios, with first crystalline carbon;

[0079] (b) performing heat treatment on the prepared core after adding a precursor of amorphous carbon to the prepared core, thereby forming a shell made of amorphous carbon and surrounding the core; and

[0080] (c) adding second crystalline carbon after the formation of the shell and the heat treatment, thereby obtaining a silicon composite in which the surfaces of the shell have the second crystalline carbon in whole or in part.

[0081] In the core, a plurality of secondary particles are disposed, wherein the secondary particles are formed by coating primary particles including acicular silicon nanoparticles and plate-like silicon nanoparticles, which have different aspect ratios, with first crystalline carbon.

[0082] The negative active material according to the present invention includes a core in which secondary particles are disposed. The secondary particles are obtained by coating primary particles including acicular silicon nanoparticles and plate-like silicon nanoparticles, which have different aspect ratios, with first crystalline carbon 120. Accordingly, the negative active material has minimized pores and maximized capacity per unit volume, thereby improving the lifespan characteristics of a lithium secondary battery. In addition, since fine pores are used as a moving passage for lithium ions, the lithium secondary battery exhibits good initial efficiency.

[0083] In step (a), the acicular silicon nanoparticles and the plate-like silicon nanoparticles can be obtained through a grinding process.

[0084] The aspect ratio of the acicular silicon nanoparticles can be in the range of greater than 6 to 250, and the aspect ratio of the plate-like silicon nanoparticles can be in the range of 1 to less than 6. When the aspect ratio is not in the above range, the pore-reducing effect desired by the present invention cannot be achieved through the arrangement of the particles, and the bonds between the silicon nanoparticles will weaken, and the cycle characteristics can deteriorate, which is undesirable.

[0085] Specifically, in the present invention, the length of the acicular silicon nanoparticles 111 of the primary particles is about 190 nm to 250 nm, the width is about 1 nm to 30 nm, and the length of the plate-like silicon nanoparticles of the primary particles is about 150 nm to 240 nm, the width is about 40 nm to 100 nm. More specifically, the length of the acicular silicon nanoparticles is about 190 nm to 220 nm, the width is about 1 nm to 30 nm, and the length of the plate-like silicon nanoparticles is about 180 nm to 220 nm, the width is about 40 nm to 100 nm. When the above range is satisfied, the pores between the secondary particles can be minimized, such that the capacity increases and the initial efficiency improves.

[0086] The weight ratio of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles can be in the range of 30:70 to 70:30. When the amount of the needle-shaped silicon nanoparticles is too much, there is a serious risk of undifferentiation during charging and discharging of the battery cell. When the amount of the plate-shaped silicon nanoparticles is too much, there is a risk of economic feasibility degradation. In addition, if the above range is not satisfied, the intended effect of minimizing the pores by arranging silicon particles having different aspect ratios cannot be achieved, and thus is not desirable. Specifically, the polycrystalline silicon particles and the single-crystal silicon particles 112 can be mixed in a weight ratio in the range of 40:60 to 60:40 based on weight.

[0087] The negative electrode active material according to the present application has a core-shell structure. The core includes silicon nanoparticles having different aspect ratios and being coated with first crystalline carbon, and the shell is made of amorphous carbon and has all or part of second crystalline carbon. Thus, even in the process of charging and discharging of the battery, due to the intercalation and deintercalation of lithium ions, shrinkage and expansion of the silicon particles repeatedly occur, but due to the buffering effect, the volume expansion and damage of the silicon particles can also be inhibited. This can lead to an improvement in the durability of the silicon particles, thereby minimizing the damage to the electrode. Furthermore, since the contact between the silicon particles and the electrolyte is inhibited, the safety of the lithium secondary battery can be improved.

[0088] In step (a), since part of the area or the entire area of the surface of each of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles having different aspect ratios is coated with the first crystalline carbon to form a conductive path, the electrical conductivity is improved.

[0089] Any coating method known in the art can be used without particular limitation. For example, a dry coating method such as a deposition method, a chemical vapor deposition (CVD) method, or the like, or a liquid coating method such as dipping, spraying, or the like, can be used.

[0090] The content of the first crystalline carbon 120 can be 0.1 parts by weight to 80 parts by weight with respect to 100 parts by weight of the needle-shaped silicon nanoparticles 111 and the plate-shaped silicon nanoparticles 112. When the content of the first crystalline carbon 120 is too low, it is difficult to achieve the intended effect of improving the electrical conductivity. When the content of the first crystalline carbon 120 is too high, the lithium ion conductivity is reduced, leading to deterioration of the output characteristics. The content of the first crystalline carbon 120 can be preferably 1 part by weight to 30 parts by weight, more preferably 1 part by weight to 3 parts by weight, with respect to 100 parts by weight of the needle-shaped silicon nanoparticles 111 and the plate-shaped silicon nanoparticles 112.

[0091] As the first crystalline carbon, any carbon material known in the art can be used without particular limitation. For example, one or more carbons selected from the group consisting of natural graphite, synthetic graphite, expanded graphite, graphene, and fullerene soot, but is not limited thereto. Specifically, it can be graphene that is a single layer of graphite.

[0092] In step (a), the heat treatment for the first crystalline carbon coating can be performed at 100°C to 400°C for 1 hour to 3 hours in an inert atmosphere. When coating is performed at a temperature lower than the lower limit of the above range, it can be impossible to sufficiently perform coating, and thus is undesirable. On the other hand, when coating is performed at a temperature higher than the upper limit of the above range, process efficiency can be reduced.

[0093] Step (b) is a heat treatment process performed after adding a precursor of amorphous carbon. By performing step (b), an amorphous carbon shell is formed around the secondary particles.

[0094] Any coating method known in the art can be used without particular limitation. For example, a dry coating method such as a deposition method, a chemical vapor deposition (CVD) method, or the like, or a liquid coating method such as dipping, spraying, or the like, can be used.

[0095] The amorphous carbon can impart sufficient strength to maintain the shape of the core-shell structure, thereby suppressing swelling of the core. The precursor of amorphous carbon can be one or more materials selected from the group consisting of coal-based pitch, mesophase pitch, petroleum-based pitch, coal-based oil, petroleum-based heavy oil, organic synthetic pitch, phenol resin, furan resin, and polyimide resin.

[0096] The content of amorphous carbon can be in the range of 5 parts by weight to 30 parts by weight with respect to 100 parts by weight of the acicular silicon nanoparticles and the platy silicon nanoparticles. When the content of amorphous carbon is lower than the above range, the core can not be sufficiently covered with amorphous carbon. In this case, the risk of the electrolyte and the negative active material reacting with each other increases. This can result in deterioration of the cycle characteristics. When the content of amorphous carbon is too much, dispersion stability can be reduced, and thus is undesirable.

[0097] As the first amorphous carbon, any amorphous carbon known in the art can be used without particular limitation. For example, any one material or a combination of two or more materials selected from the group consisting of soft carbon, hard carbon, pitch carbide, mesophase pitch carbide, and baked coke can be used. Specifically, pitch carbide can be used. More specifically, pitch carbide derived from petroleum-based pitch can be used. Petroleum-based pitch can be obtained by fractionating crude oil and removing impurity components from the remaining high-boiling-point residue.

[0098] In step (b), a precursor of amorphous carbon is added, and heat treatment is performed in an inert atmosphere at a temperature in the range of 600°C to 1300°C to remove impurities, whereby a shell made of amorphous carbon can be formed to surround the core. For example, when petroleum-based pitch is used as the precursor of amorphous carbon, a carbide of pitch can be formed into amorphous carbon by heat treatment.

[0099] In step (c), after the formation of the core-shell structure, second crystalline carbon is added and heat treatment is performed so that a silicon composite in which the second crystalline carbon is disposed on the entire region or a partial region of the surface of the shell can be prepared.

[0100] The second crystalline carbon can form an electrically conductive channel, thereby further improving the electrical conductivity.

[0101] The content of the second crystalline carbon can be in the range of 0.1 parts by weight to 80 parts by weight with respect to 100 parts by weight of the acicular silicon nanoparticles and the platy silicon nanoparticles. When the content of the second crystalline carbon is lower than the above range, the shell can not be sufficiently covered with the second crystalline carbon. In this case, the risk that the electrolyte and the negative active material can react with each other increases. This can result in deterioration of the cycle characteristics. On the other hand, the content of the second crystalline carbon is too high because the cycle characteristics and the reaction processability can be deteriorated, which is not desirable. The content of the second crystalline carbon can be preferably 0.1 parts by weight to 30 parts by weight, more preferably 0.1 parts by weight to 3 parts by weight, with respect to 100 parts by weight of the acicular silicon nanoparticles and the platy silicon nanoparticles.

[0102] As the second crystalline carbon, any carbon material known in the art can be used without particular limitation. For example, one or more carbons selected from the group consisting of natural graphite, synthetic graphite, expanded graphite, graphene, and fullerene soot, but is not limited thereto. Specifically, it can be graphene which is a single layer of graphite.

[0103] In step (c), the heat treatment can be performed in an inert atmosphere at a temperature in the range of 100°C to 400°C for 1 hour to 3 hours. When coating is performed at a too low temperature lower than the lower limit of the above range, the structural stability of the prepared silicon composite coating can be deteriorated, which is not desirable. On the other hand, when coating is performed at a too high temperature, the process efficiency can be reduced.

[0104] The average particle diameter of the silicon composite of the present application is not particularly limited, but can be in the range of, for example, 5 μm to 50 μm. When the average particle diameter of the silicon composite is too small, i.e., less than the above range, the reactivity between the electrolyte and the negative active material can be high, thereby reducing the cycle characteristics. When the average particle diameter of the silicon composite is too large, the dispersion stability can be reduced and the negative electrode surface can become rough. Specifically, the average particle diameter can be in the range of 5 μm to 20 μm.

[0105] In the present application, the porosity of the silicon composite can be in the range of 0.1% to 40%. Here, the porosity refers to a value expressed as "(volume of pores per unit mass) / (specific volume + volume of pores per unit mass)", and can be measured by a mercury porosimetry test or a Bruanuer-Emmett-Teller (BET) test. That is, since the core of the secondary particle is made of the primary particles of silicon nanoparticles having different aspect ratios, it is possible to minimize the pores present in the secondary particle, thereby maximizing the capacity per unit volume. Further, since the fine pores can serve as a moving path for lithium ions, it is possible to improve the initial efficiency. Therefore, when the porosity exceeds the above range, the effects of increasing the capacity and the initial efficiency cannot be obtained at the same time. Specifically, the porosity can be in the range of 0.1% to 4%.

[0106] Lithium secondary battery

[0107] The present application provides a lithium secondary battery including the above-described negative active material.

[0108] The lithium secondary battery can include a positive electrode including a positive active material, a negative electrode including the above-described negative active material, and an electrolyte.

[0109] The positive electrode is formed by coating a positive electrode preparation mixture including a positive active material onto a current collector, and if necessary, the positive electrode preparation mixture can further include a binder and a conductive material.

[0110] The positive active material is, for example, a lithium metal oxide (0 0.8-x Co 0.2 Al x O2, LiCo x Mn y O2, LiNi x Co y O2, LiNi x Mn y O2, LiNi x Co y Mn z O2, LiCoO2, LiNiO2, LiMnO2, LiFePO4, LiCoPO4, LiMnPO4, or Li4Ti5O 12 Alternatively, the positive active material can be a chalcogenide such as Cu2Mo6S8, FeS, CoS, or MiS. Further alternatively, the positive active material can be any one selected from oxides, sulfides, and halides of scandium, ruthenium, titanium, vanadium, molybdenum, chromium, manganese, iron, cobalt, nickel, copper, zinc, etc. More specifically, LiNi 0.8 Co 0.1 Mn 0.1O2, TiS2, ZrS2, RuO2, Co3O4, Mo6S8, and V2O5, or the like can be used as the positive electrode active material, but the positive electrode active material is not limited thereto.

[0111] The shape of the positive electrode active material is not particularly limited. That is, the positive electrode active material can be in a granular form. For example, the shape of the positive electrode active material can be spherical, ellipsoidal, or cuboid. The average particle diameter of the positive electrode active material can be in the range of 1 μm to 50 μm, but is not limited thereto. The average particle diameter of the positive electrode active material can be obtained by measuring the particle diameter of the active material with a scanning electron microscope (SEM) and calculating the average thereof.

[0112] The binder is not particularly limited. A fluorine-containing binder such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) can be used, but the binder is not limited thereto.

[0113] The content of the binder is not particularly limited, as long as the binder can bind the positive electrode active material. The content can be in the range of 0% to 10% by weight, relative to the total weight of the positive electrode.

[0114] The conductive material is not particularly limited, as long as it can improve the conductivity of the positive electrode. For example, nickel powder, cobalt oxide, titanium oxide, carbon, or the like can be used. Specifically, the carbon can be any one or a combination of two or more selected from the group consisting of ketjen black, acetylene black, furnace black, graphite, carbon fiber, and fullerene.

[0115] The content of the conductive material can be determined in consideration of other battery conditions such as the type of the conductive material. For example, the content can be in the range of 1% to 10% by weight, relative to the total weight of the positive electrode.

[0116] When the positive electrode preparation mixture is coated on the current collector, the thickness of the layer made of the positive electrode preparation mixture including the positive electrode active material, the binder, and the conductive material can be, for example, in the range of 0.1 μm to 1000 μm.

[0117] In some cases, in the present application, the positive electrode preparation mixture can include 0.1% to 60% by weight, particularly 10% to 50% by weight, of the solid electrolyte, relative to the total weight of the positive electrode preparation mixture.

[0118] The thickness of the layer of the positive electrode preparation mixture can be, for example, in the range of 0.1 μm to 1000 μm.

[0119] The material of the positive electrode current collector is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, or baked carbon can be used. Alternatively, the base member of the positive electrode current collector is made of stainless steel, aluminum, nickel, titanium, or baked carbon, and the surface of the base member can be treated with carbon, nickel, titanium, silver, or the like. In addition, the positive electrode current collector can take any form such as a film, a sheet, a foil, a mesh, a porous body, a foam, or a nonwoven body, and the surface of the positive electrode current collector can have fine irregularities.

[0120] The negative electrode can be formed by coating the negative electrode preparation mixture containing the negative electrode active material according to the present application on a negative electrode current collector. If necessary, the negative electrode preparation mixture can further include the same binder and conductive material as described above.

[0121] The material of the negative electrode current collector is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, or baked carbon can be used. Alternatively, the base member of the negative electrode current collector is made of copper or stainless steel, and carbon, nickel, titanium, silver, or the like can be formed on the copper or stainless steel base member. In addition, similarly to the positive electrode current collector, the negative electrode current collector can take any form such as a film, a sheet, a foil, a mesh, a porous body, a foam, and a nonwoven body, and the surface of such a negative electrode current collector can have fine irregularities.

[0122] The electrolytic solution is composed of an organic solvent and an electrolyte material.

[0123] If the organic solvent is common, it is not particularly limited. For example, one or more selected from propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, and tetrahydrofuran can be used.

[0124] As long as a lithium salt that can be contained as the electrolytic solution is common, there is no limitation. For example, as the anion of the lithium salt, one or more selected from F - , Cl - , I - , NO 3- , N(CN) 2- , BF 4- , ClO 4- , PF 6- , (CF3)2PF 4- , (CF3)3PF 3- , (CF3)4PF 2- , (CF3)5PF - , (CF3)6P- , CF3SO2 3- , CF3CF2SO2 3- , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO 3- , CF3CO 2- , CH3CO 2- , SCN - , and (CF3CF2SO2)2N - one or more anions.

[0125] The battery structure is wound or folded by disposing a separator between the positive electrode and the negative electrode, and then placed in a cylindrical battery case or a corner battery case, to form a battery structure. Next, an electrolyte is injected into the battery case to complete the secondary battery. Alternatively, the lithium secondary battery is completed by stacking the battery structure in a form of a double cell structure, impregnating the double cell structure with an electrolyte, and sealing the obtained result in a soft pack.

[0126] Although it will be described with reference to the following examples, the following examples are to explain the present application, and the scope of the present application is not limited thereto.

[0127] [Example 1]

[0128] The needle-shaped silicon nanoparticles (200 nm long, 1 nm to 30 nm wide) and the plate-shaped silicon nanoparticles (200 nm long, 40 nm to 100 nm wide) were mixed at a weight ratio of 2:2. 1.5 parts by weight of graphene was added per 100 parts by weight of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles, and heat-treated at 250°C for 2 hours in an inert atmosphere to form a core.

[0129] Next, 20 parts by weight of pitch was added per 100 parts by weight of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles, and heat-treated at 935°C for 3 hours in an inert atmosphere to form a shell surrounding the core and made of amorphous carbon, which is a carbonization product of the pitch.

[0130] Next, 0.5 parts by weight of graphene was added per 100 parts by weight of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles, and heat-treated at 250°C for 2 hours in an inert atmosphere to produce a silicon composite including a graphene film formed on the entire surface of the core-shell structure.

[0131] [Comparative Example 1]

[0132] A silicon composite was prepared in the same manner as in Example 1, except that only needle-shaped silicon nanoparticles having a length of 200 nm and a width of 1 nm to 30 nm were used as the silicon particles.

[0133] (Comparative Example 2)

[0134] A silicon composite was prepared in the same manner as in Example 1, except that only plate-shaped silicon nanoparticles having a length of 200 nm and a width of 40 nm to 100 nm were used as the silicon particles.

[0135] (Comparative Example 3)

[0136] A silicon composite was prepared in the same manner as in Example 1, except that the core-shell structure was not covered with the graphene film.

[0137] <Experimental Example 1>

[0138] In Example 1, SEM images of the needle-shaped silicon nanoparticles, the plate-shaped silicon nanoparticles, and the mixed state of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles were obtained. The SEM images are shown in Figure 2a .

[0139] In addition, the SEM images of the silicon composite prepared in Example 1 are shown in Figure 2b . The average particle diameter of the silicon composite was 8 μm.

[0140] <Experimental Example 2>

[0141] The porosity of each of the silicon composites prepared in Example 1, Comparative Example 1, and Comparative Example 2 was measured, and the results are shown in Table 1 and Figure 1 and Figure 3a , Figure 3b and Figure 3c . The porosity was measured as follows: a rod was inserted into a cylindrical container having a hole in the center, and the loading amount of the powder was in the range of 0.20 g to 0.21 g. Then, the container was loaded into a cylindrical container loading unit in a low power mode, and mercury was injected into the cylindrical container. The cylindrical container filled with mercury was loaded into a loading part of a micromeritics instrument. After loading, an argon gas atmosphere was generated, and the external pressure was continuously increased so that the mercury was injected into the pores of the powder. During the injection of the mercury, the porosity was measured. Then, the pressure was gradually decreased to discharge the mercury. However, the measurement results did not show the pore distribution.

[0142] [Table 1]

[0143]

[0144] In Figures 3a to 3cThe x-axis in each of the graphs indicates the pore diameter of the powder, and the axis indicates the volume (mL) of the corresponding pore diameter. The red line indicates the pore measurement at the time of mercury injection, and the green line indicates the pore measurement at the time of discharge of mercury by reducing the internal pressure of the cylindrical container. Specifically, referring to Table 1, it can be seen that the silicon composite of Example 1 has the lowest porosity and the highest filling rate. That is, the capacity can be improved.

[0145] <Experimental Example 3>

[0146] Each of several negative electrode preparation mixtures was prepared by mixing one of the silicon composites prepared according to Example 1, Comparative Example 1, and Comparative Example 2, graphite, a conductive material, CMC, and SBR in a weight ratio of 16.2:75.8:5:1.5:1.5. A negative electrode plate was made from each of the respective negative electrode preparation mixtures, and cut into a circular electrode to be used as a negative electrode. A positive electrode having the same size and shape as the negative electrode was made of a lithium metal thin film. Here, a lithium coin half-cell was manufactured under the following conditions, and the capacity and initial efficiency of each half-cell were measured. The results are shown in Table 2 and Figure 4 .

[0147] Silicon composite (1300 mAh / g): 16.02 wt%, Graphite (360 mAh / g): 75.8 wt%, Conductive material (330 mAh / g): 5 wt%, CMC: 1.5 wt%, SBR (solid 40 wt%): 1.5 wt% (solution basis: 3.75)

[0148] Loading level (L / L) = 5.39 g / cm 2 , Pressed density (E / D) = 1.54 grams per cubic centimeter (g / cc)

[0149] Electrolyte: EC:DEC = 1:1 + 1M LiPF6 in 5 wt% FEC

[0150] Charge: CC / CV, 0.1C / 0.01V, 0.01C cut-off

[0151] Discharge: CC, 0.1C / 1.5V cut-off

[0152] [Table 2]

[0153] Initial efficiency (%) Comparative example 2 (plate-shaped) 84.7 Comparative example 1 (needle-shaped) 86.5 Example 1 (plate-shaped and needle-shaped) 89.3

[0154] Referring Figure 4 to Table 2, it can be seen that the capacity and initial efficiency of the battery using the silicon composite of Example 1 are the highest.

[0155] <Experimental Example 4-1>

[0156] The silicon composite prepared in Example 1 and Comparative Example 3, respectively, was coated on a copper electrode plate of 5 cm x 5 cm, and the conductivity of each silicon composite was measured.

[0157] [Table 3]

[0158]

[0159] Referring to Table 3 above, the silicon composite of Example 1 exhibited high conductivity.

[0160] [Experimental Example 5-1]

[0161] Under the conditions of Experimental Example 3, lithium coin half-batteries were manufactured using the silicon composites of Example 1 and Comparative Example 3, respectively, and the capacity and initial efficiency were measured. The measurement results are shown in Table 3. Figure 5 and Table 3.

[0162] [Table 4]

[0163]

[0164] Referring to Figure 5 and Table 4, it can be seen that the capacity and initial efficiency of the battery using the silicon composite of Example 1 were the highest.

[0165] [Experimental Example 5-2]

[0166] Under the conditions of Experimental Example 3, lithium coin half-batteries were manufactured using the silicon composite of Example 1, and the capacity retention rate (CRR) according to the number of cycles was measured. The measurement results are shown in Table 4. Figure 6 and Table 4.

[0167] [Table 5]

[0168]

[0169] Referring to Table 5, it can be seen that the capacity and capacity retention rate (CCR) of the battery using the silicon composite of Example 1 were the highest.

Claims

1. A negative active material comprising a silicon composite material including a core and a shell, the core including needle-shaped silicon nanoparticles and plate-shaped silicon nanoparticles having different aspect ratios and coated with a first crystalline carbon, the shell surrounding the core and containing amorphous carbon, wherein a second crystalline carbon is provided in an entire region or a partial region of a surface of the shell, wherein the aspect ratio of the needle-shaped silicon nanoparticles is in a range of greater than 6 to 250, and the aspect ratio of the plate-shaped silicon nanoparticles is in a range of 1 to 6, wherein the length of the needle-shaped silicon nanoparticles is 190 nm to 250 nm, the width is 1 nm to 30 nm, and the length of the plate-shaped silicon nanoparticles is 150 nm to 240 nm, the width is 40 nm to 100 nm, wherein the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles are mixed in a weight ratio of 30:70 to 70:

30.

2. The negative electrode active material according to claim 1, wherein, The content of the amorphous carbon is in a range of 5 parts by weight to 30 parts by weight per 100 parts by weight of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles.

3. The negative electrode active material according to claim 1, wherein, The content of the first crystalline carbon is 0.1 parts by weight to 80 parts by weight, and the content of the second crystalline carbon is 0.1 parts by weight to 80 parts by weight per 100 parts by weight of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles.

4. The negative electrode active material according to claim 1, wherein, The average particle diameter of the silicon composite material is 5 μm to 50 μm.

5. The negative electrode active material according to claim 1, wherein, The porosity of the silicon composite material is 0.1% to 40%.

6. A method for producing a negative electrode active material, the method comprising the steps of: (a) coating surfaces of needle-shaped silicon nanoparticles and surfaces of plate-shaped silicon nanoparticles with a first crystalline carbon to prepare a core, the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles having different aspect ratios; (b) adding a precursor of amorphous carbon to the prepared core and performing heat treatment to obtain a shell made of amorphous carbon and configured to surround the core; and (c) adding a second crystalline carbon after the formation of the shell and performing heat treatment to prepare a silicon composite material in which an entire surface or a partial surface of the shell has the second crystalline carbon, wherein the aspect ratio of the needle-shaped silicon nanoparticles is in a range of greater than 6 to 250, and the aspect ratio of the plate-shaped silicon nanoparticles is in a range of 1 to 6, wherein the length of the needle-shaped silicon nanoparticles is 190 nm to 250 nm, the width is 1 nm to 30 nm, and the length of the plate-shaped silicon nanoparticles is 150 nm to 240 nm, the width is 40 nm to 100 nm, wherein the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles are mixed in a weight ratio of 30:70 to 70:

30.

7. The method of claim 6, wherein, The content of the amorphous carbon is in a range of 5 parts by weight to 30 parts by weight per 100 parts by weight of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles.

8. The method of claim 6, wherein, The content of the first crystalline carbon is 0.1 parts by weight to 80 parts by weight, and the content of the second crystalline carbon is 0.1 parts by weight to 80 parts by weight per 100 parts by weight of the needle-shaped silicon nanoparticles and the plate-shaped silicon nanoparticles.

9. The method of claim 6, wherein, The average particle diameter of the silicon composite material is 5 μm to 50 μm.

10. The method of claim 6, wherein, The porosity of the silicon composite material is 0.1% to 40%.

11. A lithium secondary battery comprising the negative electrode active material according to claim 1.

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