Negative electrode active material for lithium secondary battery and method for producing same

Abstract

The present invention relates to a negative electrode active material for a lithium secondary battery and a method for manufacturing the same, the negative electrode active material being obtained by selectively filling pores having a diameter of not more than a specific size that cannot accommodate a volume change of silicon. And a first carbon coating layer, a silicon coating layer, and a second carbon coating layer are coated in pores having a diameter greater than or equal to a specific size capable of accommodating a volume change of silicon, thereby achieving excellent charge capacity and conductivity.

Description

Technical Field

[0001] The present invention relates to a negative electrode active material for a lithium secondary battery, which achieves excellent charge capacity and conductivity by selectively filling pores having a diameter of less than a specific size that cannot accommodate volume changes of silicon, and coating pores having a diameter of more than a specific size that can accommodate volume changes of silicon with a first carbon coating layer, a silicon coating layer, and a second carbon coating layer. Background Art

[0002] Recently, as the demand for electric vehicles as a solution to environmental problems has increased dramatically, the demand for high-energy-density lithium secondary batteries, which are closely related to their performance improvement, has also grown rapidly. Lithium secondary batteries operate through the interaction between various components. Among them, graphite is mainly used as the negative electrode active material. However, the actual capacity of graphite (360mAh / g) is almost close to its theoretical capacity (372mAh / g, LiC6). Therefore, in order to improve the energy density of the battery, it is necessary to introduce a negative electrode material with a higher theoretical capacity than graphite.

[0003] When silicon alloys with lithium, its volume expands by more than three times due to changes in its crystal structure. However, during the dealloying process, the expanded volume contracts. This repeated volume expansion and contraction during battery cycling can lead to the pulverization of active materials and their shedding from the electrodes. Furthermore, silicon's lower conductivity than graphite makes its use in batteries difficult on its own.

[0004] Although various solutions have been proposed to address the volume expansion problem of silicon anode materials, these solutions still suffer from high volume expansion rates and low conductivity, making them difficult to apply to commercial batteries. In addition, these solutions are also accompanied by cost issues.

[0005] It should be noted that the above background technology is only provided to enhance the understanding of the background of the present invention and should not be regarded as an admission that it belongs to the prior art known to those skilled in the art. Summary of the Invention

[0006] The present invention aims to provide a negative electrode active material for a lithium secondary battery and a method for manufacturing the same, wherein the material achieves excellent charge capacity and conductivity by selectively filling pores having a diameter below a specific size that cannot accommodate the volume change of silicon, and coating pores having a diameter above a specific size that can accommodate the volume change of silicon with a first carbon coating layer, a silicon coating layer, and a second carbon coating layer.

[0007] The technical problems of the present invention are not limited to the above-mentioned technical problems, and those skilled in the art can clearly understand other technical problems not mentioned from the following description.

[0008] A negative electrode active material for a lithium secondary battery based on one embodiment of the present invention for achieving the above-mentioned purpose includes: porous particles containing graphite and containing first pores and second pores with a diameter larger than the first pores; a first carbon coating filling the interior of the first pores of the porous particles and coating the inner surface of the second pores; and a silicon coating coating the inner surface of the first carbon coating of the second pores.

[0009] For example, the feature may be further comprising a second carbon coating layer coated on the inner surface of the silicon coating layer of the second hole.

[0010] For example, the characteristic may be that the diameter of the first pore is 30 nm or less, but does not include 0 nm.

[0011] For example, the porous particles may be characterized in that the BET specific surface area of ​​the porous particles after being coated with the first carbon coating layer is 25% to 75% of the BET specific surface area of ​​the porous particles before being coated with the first carbon coating layer.

[0012] For example, the porous particles may be characterized in that the pore ratio in the state coated with the first carbon coating is 50% to 80% of the pore ratio before coating with the first carbon coating, where the pore ratio is defined as the value obtained by dividing the volume of the first pores by the volume of the second pores.

[0013] For example, the characteristic may be that the volume of the first pores is 4% by volume or less based on 100% by volume of the total pore volume (the sum of the volumes of the first pores and the second pores) of the porous particles coated with the first carbon coating layer.

[0014] For example, the characteristic may be the average particle size (D 50 ) is 7 to 18 μm.

[0015] For example, the porous particles may be characterized by a total pore volume including the first pores and the second pores of 0.01 to 0.05 cm 3 / g.

[0016] For example, the first carbon coating layer may have a weight of 3 to 10 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0017] For example, the characteristic may be that the weight of the silicon coating layer is 7.5 wt % to 13.5 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0018] For example, the second carbon coating layer may have a weight of 2 to 8 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0019] The negative electrode active material for a lithium secondary battery according to one embodiment of the present invention may be characterized in that graphite contains carbon particles and contains accommodating pores, the inner surface of the accommodating pores is coated with a first carbon coating, and the inner surface of the first carbon coating is coated with a silicon coating.

[0020] For example, the characteristic may be that the diameter of the receiving pore is greater than 30 nm.

[0021] For example, the characteristic may be that the diameter of the carbon particles is 30 nm or less.

[0022] A method for manufacturing a negative electrode active material for a lithium secondary battery based on an embodiment of the present invention for achieving the above-mentioned purpose is characterized in that it includes the following steps: filling a first carbon coating inside the first pore of a porous particle containing graphite and containing a first pore and a second pore with a diameter larger than the first pore, and coating the first carbon coating on the inner surface of the second pore; and coating a silicon coating on the inner surface of the first carbon coating in the second pore.

[0023] For example, the feature may be that after the step of applying the silicon coating, the step of applying a second carbon coating on the inner surface of the silicon coating is further included.

[0024] For example, it may be characterized in that, in the step of applying the first carbon coating layer, coating is performed by a sol-gel method using at least one substance selected from the group consisting of pitch, PVP, citric acid, and a mixture thereof.

[0025] For example, it may be characterized in that, in the step of applying the silicon coating layer, the coating is performed by a chemical vapor deposition method using a substance selected from the group consisting of silane-based gases including SiH 4 (g).

[0026] For example, the feature may be that, in the step of applying the second carbon coating layer, the coating is performed by chemical vapor deposition using at least one substance selected from the group consisting of hydrocarbon gases C2H4, CH4 and mixtures thereof.

[0027] The electrode for a lithium secondary battery of the present invention for achieving the above-mentioned object may include the negative electrode active material for a lithium secondary battery of the present invention.

[0028] The lithium secondary battery of the present invention for achieving the above-mentioned object may include the negative electrode active material for a lithium secondary battery of the present invention.

[0029] The present invention can provide a negative electrode active material for a lithium secondary battery and a method for manufacturing the same, wherein the material achieves excellent charge capacity and conductivity by selectively filling pores having a diameter below a specific size that cannot accommodate the volume change of silicon, and coating a first carbon coating, a silicon coating, and a second carbon coating in pores having a diameter above a specific size that can accommodate the volume change of silicon.

[0030] Effects obtainable by the present invention are not limited to the effects described above, and those skilled in the art can clearly understand other effects not described from the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 This figure shows a negative electrode active material for a lithium secondary battery according to one embodiment of the present invention.

[0032] Figure 2 This figure shows a method for producing a negative electrode active material for a lithium secondary battery according to one embodiment of the present invention.

[0033] Figure 3 Graphs showing the cumulative pore volume (Cumulative pore volume) based on pore diameter in Examples 1 and 2 and Comparative Examples 1 and 2.

[0034] Figure 4 Graphs showing incremental pore volume based on pore diameter in Examples 1 and 2 and Comparative Examples 1 and 2.

[0035] Figure 5 It will Figure 4 The graph is an enlarged view of a region where the pore diameter is 30 nm or less.

[0036] Figure 6 This is a SEM cross-sectional photograph of the electrode produced in Comparative Example 1.

[0037] Figure 7 This is a SEM cross-sectional photograph of the electrode produced in Example 1.

[0038] Figure 8 This is a graph comparing the electrochemical performance of Examples 1 to 2 and Comparative Example 1.

[0039] Figure 9 This is a graph comparing the discharge capacity of Examples 1 and 2 and Comparative Example 1 based on the number of charge and discharge cycles.

[0040] Figure 10 This is a graph comparing CE (Coulombic Efficiency) based on the number of charge and discharge cycles of Examples 1 and 2 and Comparative Example 1.

[0041] Description of Reference Numerals

[0042] 100: Porous particles

[0043] 110: First hole

[0044] 130: Second hole

[0045] 200: First carbon coating

[0046] 300: Silicon coating

[0047] 400: Second carbon coating. DETAILED DESCRIPTION

[0048] The present invention is susceptible to various modifications and embodiments. A specific embodiment will now be described in detail with reference to the accompanying drawings. However, these descriptions are not intended to limit the present invention to these specific embodiments. It should be understood that all variations, equivalents, and alternatives of the present invention, as long as they fall within the conceptual and technical scope of the present invention, are deemed to be included within the scope of protection of the present invention.

[0049] The terms used in this specification are for the purpose of describing specific embodiments only and are not intended to limit the scope of the present invention. Unless the context clearly indicates otherwise, expressions in the singular should generally be understood to include plural forms.

[0050] In this specification, terms such as "comprise", "contain" or "have" should be understood to indicate the presence of the features, values, steps, constituent elements or their combinations described in the specification, but do not exclude the presence of one or more other features, values, steps, constituent elements or their combinations.

[0051] In this specification, when a range is described as "X to Y", unless otherwise explicitly limited, it should be understood that it also includes all values ​​between X and Y. For example, if a range is described as 1 to 10, it includes not only 1 and 10, but also all integers and decimals therebetween.

[0052] All terms used in this application (including technical and scientific terms) should be understood to have the meanings commonly understood by those skilled in the art. Terms that are consistent with the content of commonly used dictionary definitions should be interpreted as having the same meaning in the context of the relevant technology and should not be idealized or overly formalized unless the meaning is clearly indicated in this application.

[0053] In this specification, specific surface area can be measured using the BET (Brunauer-Emmett-Teller) method. For example, it can be measured using a pore distribution analyzer (e.g., the Belsorp-II mini model from Bell Japan) using the BET 6-point method using nitrogen adsorption flow. In addition to the aforementioned equipment, other equipment commonly used in the art can also be appropriately used. Here, specific surface area refers to the surface area per unit mass or unit volume.

[0054] In this specification, the pore diameter can be measured by nitrogen adsorption according to the BJH (Barrett-Joyer-Halenda) calculation formula. Specifically, the measurement can be performed using a BELSORP-miniⅡ device manufactured by BEL (Japan). In addition to the aforementioned equipment, other equipment commonly used in the art can also be appropriately used.

[0055] The thickness can be measured by photographing the surface or cross-section of the negative electrode active material using a microscope such as a scanning electron microscope. However, this is merely an example, and measurement can be performed using various other methods.

[0056] The sol-gel method or sol-gel process refers to a process in which a sol is formed from a selected precursor, which is a stable colloidal particle (a state in which tiny particles are dispersed in a solvent). This sol is then transformed into a gel (a state in which solid particles are partially filled with a solvent) through a gelation process, thereby producing an inorganic material. Alternatively, the obtained colloidal particles are precipitated to prepare a desired form such as a single crystal, fiber, film, or powder of a single size. Specifically, it includes: a precursor solution preparation step in which a metal organic compound or inorganic salt is dissolved in a solvent to prepare a precursor solution; a coagulation and sol formation step in which the particles are combined to form a sol; a gel formation step in which the coagulation is continued in the sol state to transform it into a gel; a drying step in which the gel is dried to remove the solvent and obtain a porous solid; and a heat treatment step in which the dried gel is heat-treated to transform it into a nanomaterial such as a ceramic, glass, or metal oxide having the desired crystal structure and physical properties. It should be noted that the above steps are merely exemplary descriptions, and other steps may be added or some steps may be omitted in a specific process.

[0057] Chemical vapor deposition (CVD) is a method of transporting molecules or ions to the surface of an object through a fluid such as gas, thereby forming a solid thin film layer on the surface.

[0058] The present invention relates to a negative electrode active material for a lithium secondary battery, which achieves excellent charge capacity and conductivity by selectively filling pores having a diameter of less than a specific size that cannot accommodate volume changes of silicon, and coating pores having a diameter of more than a specific size that can accommodate volume changes of silicon with a first carbon coating layer, a silicon coating layer, and a second carbon coating layer.

[0059] Figure 1 Schematic diagram of the negative electrode active material for lithium secondary battery in one embodiment of the present invention. Figure 1 , the negative electrode active material for lithium secondary battery of the present invention is described.

[0060] A negative electrode active material for a lithium secondary battery according to one embodiment of the present invention may include: a porous particle 100, which contains graphite and includes a first pore 110 and a second pore 130 having a diameter larger than that of the first pore; a first carbon coating layer 200, which fills the interior of the first pore 110 of the porous particle 100 and fills the interior, and is coated on the inner surface of the second pore 130; a silicon coating layer 300, which is coated on the first carbon coating layer 200 in the second pore 130; and a second carbon coating layer 400, which is coated on the silicon coating layer 300.

[0061] The negative electrode active material for a lithium secondary battery of the present invention may include porous particles 100 containing graphite.

[0062] Furthermore, the first pores 110 included in the porous particle 100 are filled with the first carbon coating layer, the first carbon coating layer 200 is coated on the inner surface of the second pore 130 , and the silicon coating layer 300 is formed on the inner surface of the first carbon coating layer 200 .

[0063] On the other hand, the inner surface of the silicon coating layer 300 of the second hole 130 may be coated with a second carbon coating layer 400 having high conductivity.

[0064] At this time, the porous particle 100 may have pores of various diameters inside and outside, including the first pores 110 and the second pores 130 .

[0065] At this time, the graphite contained in the porous particles 100 may be spherical or amorphous, or may be natural graphite that has not been subjected to additional processing.

[0066] In addition, the average particle size (D 50 ) can be 7 to 18 μm.

[0067] The second pores 130 may have a diameter greater than a certain size capable of accommodating volume expansion of the silicon negative electrode material during charge and discharge, and the diameter of the first pores 110 may be smaller than that of the second pores 130 .

[0068] For example, the diameter of the first hole 110 may be less than 30 nm (but not including 0 nm), and the diameter of the second hole 130 may be greater than 30 nm.

[0069] Before applying the first carbon coating layer, the sum of the unit weight volume ratios of the first pores and the second pores of the porous particles may be 0.01 to 0.05 cm 3 / g.

[0070] In addition, the first carbon coating layer 200 may fill the interior of the first pore to completely fill the interior, and coat the inner surface of the second pore.

[0071] The first carbon coating layer 200 may be filled into the interior of the first pores by a sol-gel method and coated on the inner surface of the second pores.

[0072] Furthermore, the first carbon coating layer 200 may fill the interior of the first pores with at least one substance selected from the group consisting of pitch, PVP, citric acid, and a mixture thereof, and coat the inner surface of the second pores.

[0073] The weight of the first carbon coating layer may be 1 to 10 weight %, preferably 3 to 10 weight %, based on 100 weight % of the total weight of the negative electrode active material.

[0074] This is because, as confirmed in the Examples described below, below this range, pores in the porous particles having diameters below a specific size that cannot accommodate volume changes of silicon cannot be effectively filled. Conversely, above this range, pores in the porous particles having diameters above a specific size that can accommodate volume changes of silicon can also be filled.

[0075] In addition, the silicon coating layer 300 may perform the role of a negative electrode material that stores and releases lithium through an alloy reaction when the lithium secondary battery is charged and discharged.

[0076] The silicon coating layer 300 may be coated on the inner surface of the first carbon coating layer 200 of the second hole 130 .

[0077] The silicon coating layer 300 may be coated by chemical vapor deposition (CVD), and may be coated using a gas containing a substance selected from a silane-based gas group including SiH 4 (g).

[0078] The weight of the silicon coating layer 300 may be 7.5 to 13.5 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0079] This is because, below this range, the silicon coating layer 300 may not be sufficiently formed. On the other hand, above this range, the silicon coating layer 300 may be excessively formed, resulting in the second pores 130 being unable to accommodate volume changes during charge and discharge, thereby possibly causing damage and shedding of the negative electrode active material for the lithium secondary battery of the present invention.

[0080] On the other hand, the second carbon coating layer 400 may be coated on the inner surface of the silicon coating layer 300 .

[0081] The second carbon coating layer may be an amorphous carbon coating layer having high electrical conductivity, thereby being able to improve the electrical conductivity of the negative electrode active material.

[0082] The second carbon coating layer 400 may be coated by chemical vapor deposition (CVD) using a gas containing at least one selected from the group consisting of hydrocarbon gases including C 2 H 4 , CH 4 , and mixtures thereof.

[0083] The weight of the second carbon coating layer may be 2 to 8 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0084] This is because, if the thickness is below this range, the second carbon coating layer 400 may not be sufficiently formed, resulting in a weak conductivity improvement effect. If the thickness is above this range, the second carbon coating layer 400 may be excessively formed, thereby hindering the entry and exit of metal ions including lithium ions into and out of the silicon coating layer 300, thereby causing a decrease in electrochemical performance.

[0085] The BET specific surface area of ​​the porous particle 100 in the negative electrode active material for a lithium secondary battery according to the present invention, when coated with the first carbon coating layer 200 , is 25% to 75% of the BET specific surface area before coating with the first carbon coating layer 200 .

[0086] The porous particle 100 coated with the first carbon coating layer 200 has a pore ratio of 50% to 80% of the pore ratio before coating with the first carbon coating layer 200 . The pore ratio is defined as the volume of the first pores divided by the volume of the second pores.

[0087] The volume of the first pores may be 4% by volume or less based on 100% by volume of the total pore volume (the sum of the volumes of the first pores and the second pores) of the porous particles coated with the first carbon coating layer 200 .

[0088] The total pore volume of the porous particle 100 including the first pores 110 and the second pores 130 before coating the first carbon coating layer 200 may be 0.01 to 0.05 cm 3 / g.

[0089] On the other hand, the negative electrode active material for a lithium secondary battery according to one embodiment of the present invention will be described from another perspective.

[0090] According to one embodiment of the present invention, the negative electrode active material for a lithium secondary battery includes graphite, which includes carbon particles with a diameter below a specific size and a receiving pore with a diameter above a certain diameter, the inner surface of the receiving pore is coated with a first carbon coating; the inner surface of the first carbon coating is coated with a silicon coating; the inner surface of the silicon coating of the receiving pore is coated with a second carbon coating.

[0091] On the other hand, graphite can correspond to the above-mentioned porous particles 100, carbon particles can correspond to the first carbon coating filled in the above-mentioned first holes 110, the receiving holes can correspond to the above-mentioned second holes 130, and the silicon coating and the second carbon coating can correspond to the above-mentioned silicon coating 300 and the second carbon coating 400, respectively.

[0092] At this time, the graphite may be spherical or amorphous, or may be natural graphite that has not been processed in an additional process.50 ) can be 7 to 18 μm.

[0093] The carbon particles may be formed by coating a first carbon coating layer in pores having a diameter smaller than a diameter of receiving pores included in graphite.

[0094] The carbon particles may have a diameter smaller than the receiving pores, for example, the diameter of the carbon particles may be less than 30 nm.

[0095] The carbon particles may be formed by a sol-gel method, and may be formed using at least one substance selected from the group consisting of pitch, PVP, citric acid, and mixtures thereof.

[0096] The accommodating pore may have a diameter greater than or equal to a specific size capable of accommodating volume changes of silicon present in the graphite during charge and discharge.

[0097] For example, the diameter of the receiving hole may be greater than 30 nm.

[0098] On the other hand, the inner surface of the receiving hole may be coated with a first carbon coating.

[0099] The first carbon coating layer may be coated by a sol-gel method, and may be coated using at least one substance selected from the group consisting of pitch, PVP, citric acid, and mixtures thereof.

[0100] The weight of the first carbon coating layer may be 1 to 10 weight %, preferably 3 to 10 weight %, based on 100 weight % of the total weight of the negative electrode active material.

[0101] On the other hand, a silicon coating layer may be coated on the inner surface of the first carbon coating layer containing the pores, and the silicon coating layer functions as a negative electrode material that stores and releases lithium through an alloy reaction during charge and discharge.

[0102] The silicon coating may be applied by chemical vapor deposition (CVD), and may be applied using a gas selected from the group consisting of silane gases including SiH 4 (g).

[0103] The weight of the silicon coating layer 300 may be 7.5 to 13.5 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0104] On the other hand, a second carbon coating layer may be coated on the inner surface of the silicon coating layer containing the pores.

[0105] The second carbon coating layer may be an amorphous carbon coating layer having high electrical conductivity, thereby being able to improve the electrical conductivity of the negative electrode active material.

[0106] The second carbon coating layer may be applied by chemical vapor deposition (CVD), and may be applied using a gas containing at least one selected from the group consisting of hydrocarbon gases such as C2H4, CH4, and mixtures thereof.

[0107] The weight of the second carbon coating layer may be 2 to 8 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0108] Figure 2 This is a flow chart of a method for producing a negative electrode active material for a lithium secondary battery according to one embodiment of the present invention. The method for producing a negative electrode active material for a lithium secondary battery according to the present invention will be described based on this figure.

[0109] The method for manufacturing a negative electrode active material for a lithium secondary battery of the present invention includes the following steps: filling a first carbon coating layer in first pores of porous particles containing graphite and including first pores and second pores with a diameter larger than the first pores, and coating the first carbon coating layer on the inner surface of the second pores (S210).

[0110] At this time, the first carbon coating layer may be coated in the first pores and the second pores of the porous particles including graphite by a sol-gel method.

[0111] In this case, the solvent used in the sol-gel method may be an organic solvent selected from the group consisting of organic solvents including THF (tetrahydrofuran).

[0112] Porous particles and pitch as a precursor of the first carbon coating layer may be added to the organic solvent.

[0113] On the other hand, the substance used to form the first carbon coating layer may be at least one substance selected from the group consisting of asphalt, PVP, citric acid, and mixtures thereof.

[0114] For example, the residual carbon rate of the added asphalt is 60% and the softening point can be 260°C.

[0115] At this time, the addition amount of pitch as a precursor of the first carbon coating layer can be set to 1 to 10 wt %, preferably 3 to 10 wt %, based on 100 wt % of the total weight of the negative electrode active material.

[0116] In particular, the diameter of the pores filled with the first carbon coating layer can be controlled by adjusting the weight of the first carbon coating layer relative to the negative electrode active material.

[0117] For example, when the weight of the first carbon coating layer relative to the negative electrode active material increases, the maximum diameter of the pores filled by the first carbon coating layer can increase; when the weight of the first carbon coating layer relative to the negative electrode active material decreases, the maximum diameter of the pores filled by the first carbon coating layer can decrease.

[0118] Specifically, based on 100 weight % of the total weight of the negative electrode active material, when the weight of the first carbon coating layer is more than 10 weight %, pores with a diameter greater than 30 nm can be filled by the first carbon coating layer; when the weight of the first carbon coating layer is less than 3 weight %, pores with a diameter less than 30 nm can be filled by the first carbon coating layer.

[0119] As described above, the pretreatment step may be performed after the pores of the porous particles are coated with at least one substance selected from the group consisting of asphalt, PVP, citric acid, and a mixture thereof.

[0120] The pretreatment step can be performed in an argon inert atmosphere.

[0121] The pretreatment process may include a softening heat treatment process and a carbonization heat treatment process.

[0122] Specifically, first, a softening heat treatment step may be performed by heating the precursor pitch at the softening point (ie, 260° C.) for 2 hours.

[0123] Through this process, the asphalt can be softened and evenly distributed inside and outside the porous particles, thereby better filling the first pores.

[0124] Next, a carbonization heat treatment step may be performed by heating at 920° C. (ie, the combustion temperature of the precursor) for 2 hours. This step can increase the crystallinity of the pitch distributed inside and outside the porous particles and remove impurities.

[0125] Through this step, first pores having a diameter below a specific size that cannot accommodate volume changes of silicon can be selectively filled, and a first carbon coating can be formed on the inner surfaces of second pores having a diameter above a specific size that can accommodate volume changes of silicon.

[0126] Next, a step of coating the inner surface of the first carbon coating layer in the second hole with a silicon coating layer may be performed ( S220 ).

[0127] In the first carbon coating coating step, the first pores may have already been filled with the first carbon coating. Therefore, the silicon coating can be selectively formed only on the first carbon coating formed in the second pores.

[0128] The silicon coating may be coated on the first carbon coating in the second hole by chemical vapor deposition (CVD).

[0129] At this time, the gas used for coating the silicon coating layer may be a gas containing a substance selected from the group of silane-based gases including SiH 4 (g).

[0130] Specifically, in order to apply the silicon coating, the silane-based gas may be introduced at a decomposition temperature of 475° C. at a flow rate of 100 sccm for 62.2 minutes.

[0131] At this time, in order to reduce the aggregation of the powder in the chemical vapor deposition process, the powder may be rotated while being heat-treated in a rotary furnace that rotates once per minute.

[0132] The silicon coating layer prepared by this process can be formed into a quasi-crystalline state. Based on 100 wt % of the total weight of the negative electrode active material, the weight of the silicon coating layer can be 7.5 to 13.5 wt %.

[0133] Next, a step of coating a second carbon coating layer on the silicon coating layer of the second hole may be performed ( S230 ).

[0134] An amorphous carbon coating having high conductivity may be coated on the silicon coating formed in the second pores of the porous particles.

[0135] At this time, a second carbon coating layer may be coated on the silicon coating layer in the second hole by chemical vapor deposition.

[0136] First, the temperature can be raised in an argon atmosphere to a temperature at which hydrocarbon gas can decompose.

[0137] Specifically, the temperature can be above 930°C, and then hydrocarbon gas is injected at a flow rate of 500sccm for 10 minutes to deposit the second carbon coating on the silicon coating. After replacing it with an argon atmosphere, it is cooled so that the second carbon coating can be coated on the silicon coating in the second hole.

[0138] At this time, the hydrocarbon gas used for coating the second carbon coating layer may be a gas containing at least one substance selected from the hydrocarbon gas substance group consisting of C 2 H 4 , CH 4 and a mixture thereof.

[0139] The second carbon coating layer formed through this process has high conductivity and can be an amorphous carbon coating layer.

[0140] In addition, the weight of the second carbon coating layer may be 2 to 8 wt % based on 100 wt % of the total weight of the negative electrode active material.

[0141] This method can produce a negative electrode active material for lithium secondary batteries, which can not only ensure the high charging capacity of the silicon negative electrode material, but also accommodate the drastic volume changes of the silicon negative electrode material during charging and discharging.

[0142] Another embodiment of the present invention can provide a negative electrode including the negative electrode active material for a lithium secondary battery of the present invention.

[0143] Furthermore, another embodiment of the present invention provides a lithium secondary battery including: a negative electrode including the negative electrode active material for a lithium secondary battery of the present invention; a positive electrode including a positive electrode active material; and an electrolyte.

[0144] Specifically, the lithium secondary battery of the present invention can be manufactured by injecting the nonaqueous electrolyte of the present invention into an electrode structure consisting of a positive electrode, a negative electrode, and a separator interposed between the positive and negative electrodes. In this case, the positive electrode, separator, and electrolyte that constitute the electrode structure can all be made of materials commonly used in lithium secondary battery manufacturing. The following describes each component.

[0145] <Positive electrode>

[0146] The positive electrode active material may be formed using a material capable of inserting and extracting lithium ions or inducing a conversion reaction.

[0147] The positive electrode material may be obtained by mixing a positive electrode active material, a conductive agent, and a binder.

[0148] The positive electrode material can be coated on a positive electrode current collector to form the positive electrode. The positive electrode current collector can be a conductor. The positive electrode material can be coated on the positive electrode current collector by compression molding or by using an organic solvent to form a paste, which is then coated on the current collector and pressed to solidify.

[0149] <Electrolyte>

[0150] The electrolyte may contain lithium. Alternatively, a fluorine-containing electrolyte may be used. Furthermore, the electrolyte may be dissolved in an organic solvent and used as a non-aqueous electrolyte. Alternatively, a solid electrolyte may be used. Furthermore, the solid electrolyte may also function as a separator, described later, in which case a separator may not be required.

[0151] <Spacer>

[0152] A separator can be placed between the positive and negative electrodes. This separator can be made of a porous film, non-woven fabric, or woven fabric. The thinner the separator, while maintaining mechanical strength, the better. This is because a thinner separator can increase the volumetric energy density of the battery and reduce internal impedance.

[0153] <Method for Manufacturing Lithium Secondary Battery>

[0154] A lithium secondary battery can be manufactured by stacking a positive electrode, a separator, and a negative electrode in sequence to form an electrode group, which is then wound and placed in a battery case as needed. The electrode group is then impregnated with a non-aqueous electrolyte. Alternatively, a positive electrode, a solid electrolyte, and a negative electrode are stacked to form an electrode group, which is then wound and placed in a battery case as needed to manufacture a secondary battery.

[0155] The following preferred embodiments are provided to help understand the present invention. However, it should be noted that the following embodiments are only used to help understand the present invention, and the present invention is not limited to the following embodiments.

[0156] Examples and Comparative Examples

[0157] Example 1

[0158] After coating 50 g of natural spherical graphite with a first carbon coating by a sol-gel method, silane (SiH4) was added at a flow rate of 100 sccm in an argon atmosphere chamber at 475°C for 62.2 minutes to form a silicon coating, thereby preparing a negative electrode active material. At this time, based on the total weight of the negative electrode active material as 100 weight %, The weight of the first carbon coating is 3 weight% .

[0159] Example 2

[0160] Based on 100% by weight of the total weight of the negative electrode active material, The weight of the first carbon coating layer is 10 wt % , except that, the preparation was carried out under the same conditions as in Example 1.

[0161] Comparative Example 1

[0162] Not on natural spherical graphite Apply the first carbon coating (0 wt%) , except that, the preparation was carried out under the same conditions as in Example 1.

[0163] Comparative Example 2

[0164] Based on 100% by weight of the total weight of the negative electrode active material, The weight of the first carbon coating is 1 wt% , except that, the preparation was carried out under the same conditions as in Example 1.

[0165] Experimental example

[0166] (1) Electrode preparation

[0167] The negative electrode active material of the present invention, the conductive agent and the binder are mixed in a ratio of 96:1:3 to prepare a slurry.

[0168] At this time, super-P was used as the conductive agent, and a mixture of styrene butadiene rubber (SBR) and sodium carboxymethyl cellulose (CMC) at a weight ratio of 1.5:1.5 was used as the binder.

[0169] The slurry was evenly coated on copper foil, dried in an oven at 80°C for 1 hour, rolled, and dried in a vacuum oven at 120°C for an additional 6 hours and 30 minutes to prepare a negative electrode sheet.

[0170] (2) Half-cell production

[0171] The prepared negative electrode sheet and lithium foil were set as counter electrodes.

[0172] A porous polyethylene film was used as a separator.

[0173] A CR2032 button-type half-cell was prepared using a liquid electrolyte obtained by dissolving 1.3M LiPF6, 10 wt% fluoro-ethylene carbonate (FEC), 0.2 wt% lithium tetrafluoroborate (LiBF4), 0.5 wt% vinylene carbonate (VC), and 1 wt% propane sultone (PS) additive in a solvent obtained by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 3:5:2.

[0174] (3) Pore analysis (BET, BJH)

[0175] The BET specific surface area (BET Surface Area), the volume of pores with a diameter of 30 nm or less (first pores), and the volume of pores with a diameter of more than 30 nm (second pores) of Examples 1 to 2 and Comparative Examples 1 to 2 were analyzed by BET and BJH analysis.

[0176] Figure 3 is a graph showing the cumulative pore volume (Cumulative pore volume) based on pore diameter in Examples 1 to 2 and Comparative Examples 1 to 2, Figure 4 is a graph showing the incremental pore volume based on the pore diameter in Examples 1 to 2 and Comparative Examples 1 to 2, Figure 5 It will Figure 4 The graph is an enlarged view of a region where the pore diameter is 30 nm or less.

[0177] With reference to the above-mentioned figures, the experimental results are organized into the following Table 1.

[0178] [Table 1]

[0179]

[0180] The BET surface area shows the highest value in Comparative Example 1 in which the first carbon coating layer is not formed, and in Comparative Example 2 and Examples 1 and 2, the BET surface area tends to decrease gradually with increasing the content of the first carbon coating layer.

[0181] This confirmed that the first carbon coating layer filled the pores, and as the content of the first carbon coating layer increased, more pores were filled, thereby reducing the BET specific surface area of ​​the porous particles.

[0182] In addition, the volume of pores with a diameter of less than 30 nm showed the highest value in Comparative Example 1 in which the first carbon coating layer was not formed, while in Comparative Example 2 and Examples 1 and 2, there was a tendency that the volume of pores with a diameter of less than 30 nm gradually decreased as the weight of the first carbon coating layer increased.

[0183] On the other hand, for the volume of pores with a diameter greater than 30 nm, Comparative Example 1 in which the first carbon coating layer was not formed and Comparative Example 2 in which the first carbon coating layer content was 1 wt% showed similar values; while from Example 1 to Example 2, there was a tendency that the volume of pores with a diameter greater than 30 nm gradually decreased with the increase in the content of the first carbon coating layer.

[0184] This confirms that the pore-filling effect of the porous particles improves as the weight of the first carbon coating layer increases. Furthermore, as the weight of the first carbon coating layer increases, not only does the volume of pores with diameters of 30 nm or less decrease, but the volume of pores with diameters greater than 30 nm also decreases. This confirms that increasing the weight of the first carbon coating layer can also fill pores with diameters greater than 30 nm.

[0185] In addition, for the BJH adsorption pore volume, Comparative Example 1 in which the first carbon coating layer was not formed and Comparative Example 2 in which the first carbon coating layer had a content of 1 wt% showed similar values; while from Example 1 to Example 2, there was a tendency that the BJH adsorption pore volume gradually decreased with the increase in the weight of the first carbon coating layer.

[0186] (4) SEM cross-sectional photo analysis

[0187] Electrodes were prepared using the negative electrode active materials of Example 1 and Comparative Example 1 using an ion polisher (Gatan 697Ilion II), and then charged and discharged. Cross-sectional samples of the electrodes were prepared, and SEM cross-sectional photographs were taken for analysis.

[0188] Figure 6 This is a SEM cross-sectional photograph of the electrode produced using Comparative Example 1. Figure 7This is a SEM cross-sectional photograph of the electrode produced in Example 1.

[0189] exist Figure 6 In the figure, large black gaps can be seen between the gray active materials, i.e., electrical short circuits between the active materials.

[0190] The reason for the above results may be that during charge and discharge, the silicon coating is formed in pores with a diameter below a certain size that cannot accommodate the volume change of silicon, so that during charge and discharge, the volume change of silicon causes an electrical short circuit between active materials.

[0191] Figure 7 In the figure, black gaps between the gray active materials can be observed compared to Figure 3 1 is smaller than that of Comparative Example 1, which indicates that the degree of electrical short circuit between active materials is reduced.

[0192] The reason for the above results may be that during charging and discharging, the silicon coating is formed in pores with a diameter larger than a specific size that can accommodate the volume change of silicon, so that during charging and discharging, the volume change of the silicon coating will not cause an electrical short circuit between the active materials.

[0193] (5) Evaluation of electrochemical characteristics

[0194] The batteries of Example 1 and Comparative Example 1 were electrochemically analyzed under the following conditions.

[0195] Cut-off voltage (V): 0.005-1.5V

[0196] Formation C rate (C): 0.1C for lithiation, 0.1C for delithiation

[0197] Cycle C rate (C): 0.5C lithiation, 0.5C delithiation

[0198] At this time, ICE (Initial Coulombic Efficiency) is calculated by the following mathematical formula 1. A higher ICE value indicates better initial reversibility.

[0199] The capacity retention rate was calculated using the following mathematical formula 2. A higher capacity retention rate indicates better lifespan characteristics.

[0200] [Mathematical formula 1]

[0201] ICE = [initial discharge capacity / initial charge capacity]

[0202] [Mathematical formula 2]

[0203] Capacity retention rate = [initial charge capacity / charge capacity after 50 charge-discharge cycles]

[0204] Figure 8 is a graph comparing the electrochemical performance of the batteries of Examples 1 to 2 and Comparative Example 1. Figure 9 This is a graph comparing the discharge capacity of the batteries of Examples 1 to 2 and Comparative Example 1 based on the number of charge and discharge cycles. Figure 10 This is a graph comparing the ICE (Initial Coulombic Efficiency) based on the number of charge and discharge cycles of the batteries of Examples 1 and 2 and Comparative Example 1. The results are shown in Table 2.

[0205] [Table 2]

[0206]

[0207] In terms of charge capacity and discharge capacity, Comparative Example 1 had the highest value, Example 1 had the second highest value, and Example 2 had the lowest value.

[0208] In terms of ICE, Example 2 had the highest value, Example 1 had the second highest value, and Comparative Example 1 had the lowest value.

[0209] In terms of capacity retention, it can be confirmed that the negative electrode active material of Example 1 still has the highest capacity retention after 50 charge and discharge cycles.

[0210] In addition, it can be confirmed that the negative electrode active material of Comparative Example 1 has the lowest capacity retention rate after 50 charge and discharge cycles.

[0211] This may be because, in Example 1 and Example 2, the first carbon coating is filled in the first pores with a diameter of less than 30 nm that cannot accommodate the volume change of silicon, while the first carbon coating and the silicon coating are formed in the second pores with a diameter greater than 30 nm that can accommodate the volume change of silicon, which can effectively accommodate the volume change of silicon caused by charging and discharging, thereby preventing the falling off and damage of the negative electrode active material.

[0212] In addition, in the case of Comparative Example 1, since the first carbon coating layer was not formed, the volume change of silicon generated during charge and discharge could not be accommodated, resulting in the fall-off and destruction of the negative electrode active material.

[0213] This confirms that the presence of the first carbon coating layer has a positive effect on the life characteristics of the battery.

[0214] The content of the above detailed description should not be understood as limiting, but should be regarded as exemplary content. The scope of the present invention should be determined by reasonable interpretation of the appended claims, and all modifications within the equivalent scope of the present invention should be included in the scope of the present invention.

Claims

1. A negative electrode active material for a lithium secondary battery, comprising: a porous particle comprising graphite and comprising first pores and second pores having a larger diameter than the first pores; a first carbon coating layer filling the interior of the first pores of the porous particle and coating the inner surface of the second pores; as well as A silicon coating layer is coated on the inner surface of the first carbon coating layer in the second pore.

2. The negative electrode active material for lithium secondary batteries according to claim 1, characterized in that Also includes: A second carbon coating layer is coated on the inner surface of the silicon coating layer in the second hole.

3. The negative electrode active material for lithium secondary batteries according to claim 1, characterized in that The diameter of the first pore is 30 nm or less, excluding 0 nm.

4. The negative electrode active material for lithium secondary batteries according to claim 1, characterized in that The BET specific surface area of ​​the porous particles in a state of being coated with the first carbon coating layer is 25% to 75% of the BET specific surface area before being coated with the first carbon coating layer.

5. The negative electrode active material for lithium secondary batteries according to claim 1, characterized in that The porous particles coated with the first carbon coating have a pore ratio of 50% to 80% of the pore ratio before the first carbon coating, the pore ratio being defined as the volume of the first pores divided by the volume of the second pores.

6. The negative electrode active material for lithium secondary batteries according to claim 1, characterized in that The volume of the first pores is 4% by volume or less based on 100% by volume of the total pore volume (the sum of the volumes of the first pores and the second pores) of the porous particles coated with the first carbon coating layer.

7. The negative electrode active material for lithium secondary batteries according to claim 1, characterized in that The weight of the first carbon coating layer is 3 wt % to 10 wt % based on 100 wt % of the total weight of the negative electrode active material.

8. The negative electrode active material for lithium secondary batteries according to claim 1, characterized in that The weight of the silicon coating layer is 7.5 wt % to 13.5 wt % based on 100 wt % of the total weight of the negative electrode active material.

9. The negative electrode active material for lithium secondary batteries according to claim 2, characterized in that: The weight of the second carbon coating layer is 2 wt % to 8 wt % based on 100 wt % of the total weight of the negative electrode active material.

10. A negative electrode active material for a lithium secondary battery, characterized in that: Graphite contains carbon particles and contains pores, The inner surface of the receiving hole is coated with a first carbon coating, and A silicon coating is coated on the inner surface of the first carbon coating.

11. The negative electrode active material for lithium secondary batteries according to claim 10, characterized in that The diameter of the receiving pore is greater than 30 nm.

12. The negative electrode active material for lithium secondary batteries according to claim 10, characterized in that: The diameter of the carbon particles is 30 nm or less.

13. The negative electrode active material for lithium secondary batteries according to claim 10, characterized in that: A second carbon coating is coated on the inner surface of the silicon coating.

14. A method for producing a negative electrode active material for a lithium secondary battery, characterized in that: The steps include: Filling first pores of porous particles containing graphite and including first pores and second pores having a diameter larger than the first pores with a first carbon coating layer, and coating inner surfaces of the second pores with the first carbon coating layer; as well as A silicon coating is coated on the inner surface of the first carbon coating in the second hole.

15. The method for producing a negative electrode active material for a lithium secondary battery according to claim 14, wherein: After the step of applying the silicon coating layer, the method further includes the step of applying a second carbon coating layer on the inner surface of the silicon coating layer.

16. The method for producing a negative electrode active material for a lithium secondary battery according to claim 14, wherein: In the step of applying the first carbon coating layer, coating is performed by a sol-gel method using at least one substance selected from the group consisting of pitch, PVP, citric acid, and a mixture thereof.

17. The method for producing a negative electrode active material for a lithium secondary battery according to claim 14, wherein: In the step of applying the silicon coating layer, the coating is performed by a chemical vapor deposition method using a substance selected from the group consisting of silane-based gases including SiH 4 .

18. The method for producing a negative electrode active material for a lithium secondary battery according to claim 15, wherein: In the step of coating the second carbon coating layer, coating is performed by chemical vapor deposition using at least one substance selected from the group consisting of hydrocarbon gases C2H4, CH4 and mixtures thereof. 19 . An electrode for a lithium secondary battery, comprising the negative electrode active material for a lithium secondary battery according to claim 1 . 20 . A lithium secondary battery comprising the negative electrode active material for a lithium secondary battery according to claim 1 .

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