Anode active material for secondary battery, method for preparing same, and lithium secondary battery comprising same

Abstract

One embodiment of the present invention provides an anode active material for a secondary battery, the anode active material comprising silicon-based composite particles in which a plurality of silicon particles, amorphous carbon, and lithium silicate are aggregated, wherein the silicon-based composite particles have a concentration ratio of lithium silicate at a surface portion to lithium silicate at a central portion of greater than 1, and the lithium silicate has a weight ratio (L1 / L2) of Li2Si2O5(L1) to Li2SiO3(L2) of 0.1 or more.

Description

Negative electrode active material for secondary batteries, method of manufacturing the same, and lithium secondary battery including the same

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

[0002] With the recent increase in demand for electronic devices, including mobile phones, technological development for these devices is expanding. Consequently, the demand for lithium-ion batteries, such as lithium batteries, lithium-ion batteries, and lithium-ion polymer batteries, is rising significantly as power sources for these electronic devices. Furthermore, driven by the global trend of tightening regulations on vehicle fuel efficiency and exhaust emissions, the growth of the electric vehicle (EV) market is accelerating. Along with this, demand for medium- and large-sized secondary batteries, such as those for EVs and Energy Storage Systems (ESS), is expected to surge.

[0003] Meanwhile, as the demand for higher capacity in secondary batteries, such as medium- and large-sized secondary batteries, has recently increased, silicon-based anode materials with excellent theoretical capacity are being researched as anode materials for secondary batteries. However, when an anode electrode is formed using silicon powder and then immersed in an electrolyte to perform an initial charge, the silicon active material particles within the anode active material powder react with the lithium ions in the electrolyte to form an SEI layer. In other words, the electrolyte decomposition reaction for the formation of the SEI layer causes the initial irreversible capacity of the secondary battery. This SEI layer is known to form as a thin material layer, passivate the surface of the active material electrode, and be advantageous for long-term cycle characteristics. However, because the difference between the initial charge capacity and the initial discharge capacity—that is, the initial irreversible capacity—becomes relatively large due to the formation of the SEI layer, this becomes a limitation in increasing the capacity of the secondary battery. Furthermore, if the SEI layer is excessively formed, the concentration of lithium ions in the electrolyte decreases, and gas generation is induced, which leads to problems that degrade cycle characteristics.

[0004] Prior art document 1 (Korean registered patent KR 2690050 B1) describes a negative electrode material comprising lithium silicate located on the surface of silicon nanoparticles and amorphous carbon surrounding it, which is intended to improve initial efficiency and capacity characteristics by forming silicon oxide, which can be formed on the silicon surface during the process of nano-sizing silicon, into the form of lithium silicate.

[0005] Prior art document 2 (Korean registered patent KR 1902071 B1) presents a technology that mixes silicon oxide powder and a lithium compound, performs a first heat treatment to form lithium silicate on the surface, and performs a second heat treatment to control the ratio of Li2SiO3 and Li4SiO4 and Li2Si2O5 in the lithium silicate phase.

[0006] However, in prior art 1 and 2, there is a problem of capacity reduction as excessive lithium silicate is synthesized due to the entire surface and interior of the manufactured assembled particles (negative active material) being lithium silicate.

[0007] Prior art document 3 (Korean registered patent KR 2318855 B1) presents a negative electrode material in which crystalline Li2SiO3 and Li2Si2O5 are formed on silicon compound particles. However, since Prior art document 3 uses SiOx particles (a mixture of metallic silicon particles and silicon dioxide particles), when using particles in the form of oxides such as SiOx, silicon and silicon oxide (SiO2) are mixed as individual particles, and oxygen is uniformly located within the active material. Therefore, it is not suitable because it is impossible to selectively convert only the irreversible phase silicon oxide into lithium silicate in addition to the reversible phase silicon.

[0008] Prior art document 4 (Korean Published Patent KR 2024-0000918 A) presents a cathode material in which lithium silicate is formed by mixing finely milled silicon powder, graphite, and an amorphous carbon precursor in a mixed solution, drying to spheroidize, mixing with lithium stearate, and then heat-treating. However, as the lithium silicate formed in Prior Art document 4 is analyzed as Li4SiO4 and Li2SiO3, there is a problem of capacity degradation due to increased synthesis of irreversible phases, and there is a problem that the initial efficiency and reversible capacity improvement are insufficient.

[0009] The objective of the present invention is to solve the problem of deterioration in lifespan characteristics caused by the occurrence of a decomposition reaction of the electrolyte in direct interfacial contact with the surface of the silicon-based negative electrode active material during the battery charging and discharging process.

[0010] Specifically, in the present invention, a silicon-based negative electrode active material can be coated with a carbon-based material having high electrical conductivity and lithium ion conductivity and lithium silicate to improve initial efficiency and resolve lifespan degradation.

[0011] In addition, the present invention can further improve lifespan characteristics by controlling the lithium silicate in a silicon-based negative electrode active material with a coating layer formed into a specific phase.

[0012] One embodiment of the present invention provides a negative electrode active material for a secondary battery comprising a plurality of silicon particles, amorphous carbon, and a silicon-based composite particles aggregated with lithium silicate, wherein the ratio of the lithium silicate concentration on the surface to the lithium silicate concentration in the center of the silicon-based composite particles is greater than 1, and the weight ratio (L1 / L2) of Li2Si2O5 (L1) to Li2SiO3 (L2) of the lithium silicate is 0.1 or greater.

[0013] The silicon-based composite particles may have an oxygen (O) content of 4.5 to 9.5 weight% and a silicon (Si) content of 60 to 80 weight% relative to the total weight of O, Si, and C elements in the center, and an oxygen (O) content of 10.5 to 15.5 weight% and a silicon (Si) content of 35 to 55 weight% relative to the total weight of O, Si, and C elements in the surface portion.

[0014] The above lithium silicate may have a weight ratio (L1 / L2) of Li2Si2O5 (L1) to Li2SiO3 (L2) of 0.4 to 20.

[0015] The above silicon-based composite particles have an average particle size (D50) of 10 to 25 μm and a BET specific surface area of ​​1.8 to 3.6 m² 2 It may contain spherical particles of / g.

[0016] The silicon-based composite particles are formed on a surface and include a coating layer containing the lithium silicate, and the coating layer may be in the form of a uniform film and formed with a thickness of 0.1 to 1 μm.

[0017] The above plurality of silicon particles may have a silicon oxide film formed on their surface by natural oxidation.

[0018] The above amorphous carbon may include at least one selected from hard carbon, soft carbon, graphene, and graphite sheets.

[0019] The above silicon-based composite particles may have a weight ratio of silicon (Si) to carbon (C) of 60:40 to 90:10.

[0020] Another embodiment of the present invention provides a method for manufacturing a negative electrode active material for a secondary battery, comprising: a spheroidizing step of mixing silicon powder and an amorphous carbon precursor in a solution and spheroidizing to form silicon-based assembled particles; a step of mixing the silicon-based assembled particles and a lithium precursor such that the lithium precursor is included in an amount of 5 to 20 parts by weight relative to the total weight of the silicon-based assembled particles and the lithium precursor; and a step of heat-treating the mixture of the silicon-based assembled particles and the lithium precursor.

[0021] The above spheroidization step spheroidizes the mixture using a mechano fusion method, so that the average particle size (D50) is 13 to 22 μm and the BET specific surface area is 1.1 to 2.1 m² 2 It may be possible to manufacture spherical silicon-based assembled particles such as / g.

[0022] The above mixing step may involve uniformly coating the lithium precursor onto the surface of the silicon-based assembled particles.

[0023] The above heat treatment step may be to produce silicon-based composite particles by heat treating at 700 to 900°C in an inert gas atmosphere for 1 to 20 hours.

[0024] Another embodiment of the present invention provides a negative electrode for a secondary battery comprising the negative electrode active material.

[0025] Another embodiment of the present invention provides a secondary battery comprising the cathode; the anode; and the electrolyte.

[0026] According to the present invention, by using silicon-based composite particles in which a plurality of silicon particles, amorphous carbon, and lithium silicate are aggregated, the electrical conductivity and lithium ion conductivity can be improved, thereby improving initial efficiency and lifespan characteristics.

[0027] In addition, it is characterized by increasing the ratio of lithium silicate formation on the surface portion relative to the center of the silicon-based composite particles. Accordingly, the electrolyte decomposition reaction caused by direct interfacial contact between the silicon-based anode material and the electrolyte can be suppressed, lifespan degradation can be improved, and capacity reduction caused by excessive lithium silicate synthesis can be prevented.

[0028] In addition, by controlling the ratio of Li2Si2O5 (L1) to Li2SiO3 (L2) in the synthesized lithium silicate phase to a specific weight range, a lifespan stabilization effect can be achieved through Li2Si2O5 capable of exhibiting reversible capacity. Furthermore, there is an effect of improving rate characteristics through the enhancement of the lithium diffusion rate.

[0029] Figures 1a and 1b are surface scanning electron microscope (SEM) images of silicon-based composite particles prepared in Comparative Example 1 and Example 1, respectively.

[0030] Figure 2a is a cross-sectional SEM-EDS (Scanning electron microscope-Energy dispersive spectroscopy) image of a silicon-based composite particle prepared in Comparative Example 1.

[0031] Figures 2b and 2c are cross-sectional SEM-EDS (Scanning electron microscope-Energy dispersive spectroscopy) images of silicon-based composite particles prepared in Example 1.

[0032] Figures 3a and 3b are cross-sectional Scanning electron microscope (SEM) images of silicon-based composite particles prepared in Comparative Example 1 and Example 2, respectively.

[0033] Figure 4 shows the X-ray Diffraction (XRD) analysis results of the cathode active materials prepared in Example 1, Example 2, and Comparative Example 1.

[0034] The advantages and features of the present invention and the methods for achieving them will become clear by referring to the embodiments described below in detail together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below but may be implemented in various different forms. These embodiments are provided merely to ensure that the disclosure of the present invention is complete and to fully inform those skilled in the art of the scope of the invention, and the present invention is defined only by the scope of the claims.

[0035] Unless otherwise defined, all terms used in this specification (including technical and scientific terms) may be used in a meaning that is commonly understood by those skilled in the art to which the present invention pertains. Throughout the specification, when a part is described as "comprising" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components.

[0036] In this specification, the singular form includes the plural form unless specifically stated otherwise in the text.

[0037] Additionally, when a part such as a layer, film, region, plate, etc. is described in this specification as being “on” or “on” another part, this includes not only cases where it is “immediately on” another part, but also cases where there is another part in between.

[0038] In this specification, "A to B" means "A or more and B or less" unless specifically defined otherwise. Additionally, "A and / or B" means at least one selected from the group consisting of A and B, unless specifically defined otherwise.

[0039]

[0040] One embodiment of the present invention provides a negative electrode active material for a secondary battery comprising silicon-based composite particles. The negative electrode active material comprises i) silicon-based composite particles in which a plurality of silicon particles, amorphous carbon, and lithium silicate are aggregated, wherein the ratio of the lithium silicate concentration on the surface to the lithium silicate concentration in the center of the silicon-based composite particles is greater than 1, and ii) the lithium silicate has a weight ratio (L1 / L2) of Li2Si2O5 (L1) to Li2SiO3 (L2) of 0.1 or more.

[0041] In the present invention, i) by compounding (sphericalizing) a carbon-based material with silicon particles and coating the surface of the compound particles with lithium silicate, direct contact between silicon and the electrolyte is physically prevented to suppress the electrolyte decomposition reaction, and the initial efficiency and lifespan characteristics can be improved through the effect of enhancing electrical conductivity and lithium ion conductivity due to the composition of the compound particles. In addition, by increasing the ratio of lithium silicate formation on the surface of the silicon-based compound particles relative to the center, excessive synthesis of the irreversible phase can be prevented, thereby preventing capacity degradation. ii) By controlling the lithium silicate such that the weight ratio (L1 / L2) of Li2Si2O5 (L1) to Li2SiO3 (L2) is 0.1 or higher, lifespan characteristics can be improved through Li2Si2O5 capable of exhibiting reversible capacity. In addition, the lithium ion diffusion rate is enhanced, thereby improving rate characteristics.

[0042] As in one embodiment, the silicon-based composite particle may have an oxygen (O) content of 4.5 to 9.5 weight% relative to the total weight of O, Si, and C elements in the center, and a silicon (Si) content of 60 to 80 weight%. Additionally, the silicon-based composite particle may have an oxygen (O) content of 10.5 to 15.5 weight% relative to the total weight of O, Si, and C elements in the surface portion, and a silicon (Si) content of 35 to 55 weight%.

[0043] In the present invention, by increasing the ratio of lithium silicate formation in the surface portion relative to the center of silicon-based composite particles, the oxygen content in the surface portion relative to the center can be controlled to be high and the silicon content to be low. Accordingly, direct contact between silicon and the electrolyte is physically prevented to suppress the electrolyte decomposition reaction, while simultaneously preventing excessive synthesis of the irreversible phase and reducing capacity degradation.

[0044] Specifically, the oxygen (O) content in the center of the silicon-based composite particle may be 4.5 wt% or more, 5 wt% or more, 6 wt% or more, or 6.5 wt% or more, and 9.5 wt% or less, 9 wt% or less, or 8 wt% or less, and the silicon (Si) content may be 60 wt% or more, 65 wt% or more, and 80 wt% or less, or 75 wt% or less.

[0045] If the oxygen content becomes excessively high and the silicon content becomes low due to pre-lithiation reactions in the core of silicon-based composite particles, the synthesis of irreversible phases such as lithium silicate and silicon oxide increases, leading to a problem of capacity degradation. Conversely, if the oxygen content becomes excessively low and the silicon content becomes high, it causes significant volume changes during lithium insertion / extraction, which can reduce reversible capacity and degrade lifespan characteristics.

[0046] Specifically, the oxygen (O) content on the surface of the silicon-based composite particle may be 10.5 wt% or more, 11 wt% or more, 12 wt% or more, or 13 wt% or more, and 15.5 wt% or less, 15 wt% or less, or 14 wt% or less, and the silicon (Si) content may be 35 wt% or more, 40 wt% or more, or 45 wt% or more, and 55 wt% or less, or 50 wt% or less.

[0047] If the oxygen content becomes excessively high and the silicon content becomes low due to pre-lithiation reactions on the surface of silicon-based composite particles, the amount of Li2SiO3 synthesized in the lithium silicate increases, making it difficult to control the weight ratio (L1 / L2) of Li2SiO5 (L1) to Li2SiO3 (L2) to 0.1 or higher. Consequently, the synthesis of the irreversible phase increases in the lithium silicate, leading to a problem of capacity degradation. Conversely, if the oxygen content becomes excessively low and the silicon content becomes high on the surface, direct contact between the silicon and the electrolyte cannot be sufficiently suppressed, leading to a problem of lifespan degradation due to electrolyte decomposition reactions.

[0048] Meanwhile, the above surface portion may refer to an area within 2 µm depth from the outermost surface in the central direction from the surface of the silicon-based composite particle, an area within 1.5 µm depth from the outermost surface, and preferably an area within 1 µm depth from the outermost surface. The above center portion may refer to an area excluding the surface portion, and specifically, may refer to an area within 1 µm depth to 5 µm depth, an area within 1.5 µm depth to 5 µm depth, or an area within 2 µm depth to 5 µm depth in the central direction from the surface of the silicon-based composite particle.

[0049] As in one embodiment, the lithium silicate has a weight ratio (L1 / L2) of Li2Si2O5 (L1) to Li2SiO3 (L2) of 0.1 or more, and specifically, it may be 0.4 to 20, 0.5 to 20, 1 to 20, 2 to 20, 3 to 20, 5 to 20, or 10 to 20.

[0050] The above Li2Si2O5 can form Si and Li2SiO3 at the negative electrode during charging / discharging of a secondary battery as shown in Reaction Scheme 1 below, thereby exhibiting reversible capacity and improving initial efficiency.

[0051] [Reaction Equation 1]

[0052] 3Li2Si2O5+ 4Li → Si + 5Li2SiO3(lithiation)

[0053] The above Li2SiO3 has a reversible capacity of 200 mAh / g or less, a lifespan stabilization effect, and an effect of improving rate characteristics through improved lithium diffusion rate.

[0054] If the weight ratio L1 / L2 of Li2Si2O5 (L1) to Li2SiO3 (L2) exceeds the upper limit, the total content of lithium silicate decreases, making it difficult to exhibit the effect of suppressing side reactions in the electrolyte through the formation of a coating layer. Conversely, if the weight ratio L1 / L2 is below the lower limit, the synthesis of irreversible phases such as lithium silicate and silicon oxide increases, leading to a problem of capacity reduction, and the amount of Li2Si2O5 synthesized decreases, which may degrade the initial efficiency and reversible capacity characteristics.

[0055] As in one embodiment, the silicon-based composite particles have an average particle size (D50) of 10 to 25 μm and a BET specific surface area of ​​1.8 to 3.6 m² 2 It may contain spherical particles of / g.

[0056] Specifically, the silicon-based composite particles may have an average particle size (D50) of 10–25 µm, 12–25 µm, 14–25 µm, 15–23 µm, or 15–20 µm, and a BET specific surface area of ​​1.8–3.6 m² 2 / g, 2.0~3.4 m 2 / g or 2.2~3.4 m 2 / g. When the average particle size and specific surface area are controlled through spheroidization (secondary particle formation) within the above design range, the exposure of the silicon natural oxide film is reduced, allowing a relatively small amount of lithium precursor to be used to form a uniform lithium silicate coating layer on the surface of the secondary particles.

[0057] As in one embodiment, the silicon-based composite particle may include a coating layer comprising lithium silicate formed on its surface, wherein the coating layer may be in the form of a uniform film and may be formed with a thickness of 0.1 to 1 μm. Specifically, the coating layer may be formed in the form of a film that uniformly surrounds the surface portion of the silicon-based composite particle, and the thickness of the coating layer may be 0.1 to 1 μm, 0.3 to 1 μm, or 0.5 to 1 μm.

[0058] In addition, as the coating layer is formed in the form of a uniform film, direct contact between silicon and the electrolyte is physically prevented, thereby suppressing the electrolyte decomposition reaction while simultaneously preventing excessive synthesis of the irreversible phase and reducing capacity degradation.

[0059] Meanwhile, the above coating layer may refer to the surface portion of the silicon-based composite particle.

[0060] As in one embodiment, the plurality of silicon particles may be used having a silicon oxide film formed on their surface by natural oxidation.

[0061] Specifically, the silicon particles may have an oxygen (O) content of 1 to 15 wt%, 3 to 15 wt%, or 5 to 10 wt%, and may be used in which the oxygen is distributed on the surface of the silicon particles in the form of a silicon oxide (SiO2) film. The silicon particles are particles consisting solely of Si, making it easy to convert the oxide film (SiO2) that can naturally form on the surface into a silicate form. On the other hand, when using particles in the form of oxides such as SiOx, the silicon and silicon oxide (SiO2) are mixed as individual particles, and oxygen is uniformly located within the active material. Therefore, it is not suitable because it is impossible to selectively convert only the irreversible silicon oxide, in addition to the reversible silicon, into lithium silicate.

[0062] In addition, the silicon particles may exist in a crystalline or amorphous form, for example, and may be spherical, disc-shaped, needle-shaped, plate-shaped, sheet-shaped, fragment-shaped, or amorphous particles, and the average particle size (D50) may be 2 to 800 nm, 10 to 500 nm, or 100 to 500 nm.

[0063] Spherical silicon-based secondary particles (composite particles) can be manufactured in which multiple silicon particles are surrounded by amorphous carbon and uniformly dispersed by a carbon spheroidization process, and since the manufactured composite particles are spherical, a uniform lithium silicate coating layer can be manufactured on the surface of the secondary particles without being affected by the shape of the silicon primary particles.

[0064] As in one embodiment, the amorphous carbon may comprise at least one selected from hard carbon, soft carbon, graphene, and graphite sheets, and may be, for example, a coal-based pitch pyrolysis product or a petroleum-based pitch pyrolysis product.

[0065] The above hard carbon is an amorphous carbonaceous material obtained by thermally decomposing various organic materials such as phenolic resin or furan resin, and the soft carbon is an amorphous carbonaceous material obtained by carbonizing coke, needle coke, coal tar pitch, petroleum pitch, or heavy oil. The above pitch is a general term for a black carbonaceous solid residue obtained when distilling tar obtained by the dry distillation of coal, wood, or other organic materials, and refers to a type of bitumen, and coke is a lumpy carbonaceous material produced by the high-temperature dry distillation of coal, wood, or other organic materials.

[0066] As in one embodiment, the silicon-based composite particles may have a weight ratio of silicon (Si) to carbon (C) of 60:40 to 90:10, for example, 60:40 to 80:20 or 60:40 to 70:30. A plurality of silicon particles may be uniformly dispersed and arranged by being surrounded by amorphous carbon through a spheroidization process.

[0067] Another embodiment of the present invention provides a method for manufacturing a negative electrode active material for a secondary battery.

[0068] The above manufacturing method comprises: a spheroidizing step of mixing silicon powder and an amorphous carbon precursor in a solution and spheroidizing them to form silicon-based assembled particles; a step of mixing the silicon-based assembled particles and a lithium precursor such that the lithium precursor is included in an amount of 5 to 20 parts by weight relative to the total weight of the silicon-based assembled particles and the lithium precursor; and a step of heat-treating the mixture of the silicon-based assembled particles and the lithium precursor.

[0069] As in one embodiment, in the spheroidization step, silicon powder and an amorphous carbon precursor are mixed in a mixing solvent and spheroidized to produce silicon-based assembled particles (secondary particles).

[0070] The above mixing can be performed by mixing the silicon powder and the amorphous carbon precursor in a weight ratio of 60:40 to 90:10, for example, 60:40 to 80:20 or 60:40 to 70:30 in a mixed solution using a solvent. By mixing, the silicon powder can be surrounded by the amorphous carbon precursor and uniformly dispersed.

[0071] The above spheroidization can be performed by a coating method in which an amorphous carbon precursor and silicon powder are dissolved in an organic solvent using a mechano fusion device, followed by stirring and filtration.

[0072] The aggregated particles formed through a meccanofusion device (meccanofusion method) have an average particle size (D50) of 13 to 22 μm and a BET specific surface area of ​​1.1 to 2.1 m² 2 It can be manufactured in a spherical shape such as / g. If the average particle size of the aggregated particles increases to 30㎛ or more, the lithium silicate coating layer may be thin or formed locally, which may be disadvantageous in suppressing side reactions in the electrolyte.

[0073] Specifically, the average particle size (D50) of the aggregated particles is 10–25 µm, 12–25 µm, 14–25 µm, 15–23 µm, or 15–20 µm, and the BET specific surface area is 1.1–2.1 m² 2 / g, 1.3~1.9 m 2 / g or 1.5~1.7 m 2 It can be manufactured to be / g.

[0074] In addition, the assembled particles are formed such that a plurality of silicon particles are surrounded by an amorphous carbon precursor inside, and may exist in the form of an encapsulated capsule or shell, or as a continuously covering layer.

[0075] Meanwhile, devices for performing meccanofusion include, for example, high energy ball mill devices, planetary mill devices, stirred ball mill devices, and vibrating mill devices.

[0076] The above silicon powder may be a particle powder consisting solely of Si, on which a silicon oxide film formed by natural oxidation is formed on the surface. Specifically, the silicon particles may have an oxygen (O) content of 1 to 15 weight%, 3 to 15 weight%, or 5 to 10 weight%, and the oxygen may be distributed on the surface of the silicon particles in the form of a silicon oxide (SiO2) film.

[0077] In addition, the silicon powder may exist in a crystalline or amorphous form, for example, and may be spherical, disc-shaped, needle-shaped, plate-shaped, sheet-shaped, fragment-shaped, or amorphous particles, and the average particle size (D50) may be 2 to 800 nm, 10 to 500 nm, or 100 to 500 nm.

[0078] The above amorphous carbon precursor may include at least one selected from various organic materials such as phenolic resin or furan resin that form hard carbon upon pyrolysis (carbonization); coke, needle coke, coal tar pitch, petroleum pitch, or heavy oil that form soft carbon upon pyrolysis (carbonization); a graphene precursor; and a graphite sheet precursor.

[0079] As in one embodiment, the mixing step may be performed by mixing the silicon-based assembled particles and the lithium precursor such that the lithium precursor is included in an amount of 5 to 20 parts by weight relative to the total weight of the silicon-based assembled particles and the lithium precursor, thereby uniformly coating the lithium precursor on the surface of the silicon-based assembled particles.

[0080] Specifically, the lithium precursor may be mixed in such a way that it is included in an amount of 5 to 20 weight%, 5 to 15 weight%, and 5 to 10 weight% based on the total weight of the assembled particles and the lithium precursor. The chemical reaction in which each lithium precursor and silicon oxide react to form a lithium silicate phase is as follows, but is not necessarily limited thereto.

[0081] (1) 2LiOH + SiO2→ Li2SiO3+ H2O

[0082] (2) 2LiOH + 2SiO2→ Li2Si2O5+ H2O

[0083] (3) Li2CO3+ SiO2→ Li2SiO3+ CO2

[0084] (4) Li2CO3+ 2SiO2→ Li2SiO3+ CO2

[0085] According to the above chemical reaction, since the lithium precursor and silicon oxide react in molar ratios of 1:1, 1:2, and 2:1 to form lithium silicate, the phase can be controlled by varying the input ratio during the mixing step, but it is not necessarily limited to this.

[0086] The above lithium precursor may include lithium hydride, lithium hydroxide, lithium fluoride, lithium hexafluorophosphate, lithium oxide, lithium carbonate, lithium particles, or a combination thereof, and may include at least one selected from the group consisting of, for example, LiOH, Li, and Li2CO3, and preferably may be LiOH or Li2CO3.

[0087] As in one embodiment, the heat treatment step involves heat-treating a mixture of the silicon-based assembled particles and the lithium precursor to produce silicon-based composite particles.

[0088] In the above heat treatment step, the lithium precursor diffuses onto the surface of the silicon-based assembled particles, which can induce an irreversible reaction between the lithium precursor and the silicon particles, for example, a pre-lithiation reaction may occur. Accordingly, lithium silicates including Li2Si2O5 (lithium disilicate) and Li2SiO3 (lithium metasilicate) can be formed.

[0089] The above heat treatment can be performed by heat treating at 700 to 900°C in an inert gas atmosphere for 1 to 20 hours, and specifically, can be performed at 700 to 900°C or 800 to 900°C in a nitrogen atmosphere for 1 to 20 hours, 4 to 15 hours, or 8 to 12 hours.

[0090]

[0091] Another embodiment of the present invention provides a negative electrode for a secondary battery and a secondary battery comprising the negative electrode active material.

[0092] The above-mentioned cathode comprises a cathode current collector and a cathode active material layer located on the cathode current collector, and the cathode active material according to one aspect of the present invention is present in the cathode active material layer.

[0093] The negative electrode current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, etc. may be used. In addition, the negative electrode current collector can typically have a thickness of 3 to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesion of the negative electrode active material. Such negative electrode current collectors may be provided in various forms such as films, sheets, foils, nets, porous bodies, foams, nonwoven bodies, etc.

[0094] The negative electrode active material layer may be a layer comprising a conductive material and a binder together with the negative electrode active material described above.

[0095] The above conductive material is used to impart conductivity to the electrode, and can be used without special limitations as long as it is conductive without causing chemical changes to the negative electrode active material. Non-limiting examples of conductive materials include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fibers; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskies such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives. The conductive material may typically be included in an amount of 1% to 30% by weight based on the total weight of the negative electrode active material layer.

[0096] The above binder is a material that serves to improve adhesion between negative electrode active material particles and adhesion between the negative electrode active material and the current collector. Non-limiting examples of binders include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof. The binder may typically be included in an amount of 1% to 30% by weight based on the total weight of the negative electrode active material layer.

[0097] A cathode according to one embodiment of the present invention may be manufactured according to a conventional method for manufacturing a cathode for a secondary battery, except for using the cathode active material described above. For example, a cathode may be manufactured by applying a slurry for forming a cathode active material layer, comprising a cathode active material and optionally a binder and a conductive material, onto a cathode current collector, and then drying and rolling. According to another example, a cathode may be manufactured by casting a slurry for forming a cathode active material layer onto a separate support, and then laminating a film obtained by peeling off the cathode active material layer from the support onto a cathode current collector.

[0098] According to another aspect of the present invention, an electrochemical device comprising the aforementioned cathode is provided. Herein, the electrochemical device may specifically be a battery, a capacitor, etc., and more specifically, a lithium secondary battery.

[0099] A secondary battery comprises a negative electrode, a positive electrode positioned opposite the negative electrode, a separator optionally interposed between the negative electrode and the positive electrode, and an electrolyte. Additionally, a lithium secondary battery may include a battery container (case) housing an electrode assembly comprising a positive electrode, a negative electrode, and a separator, and a sealing member for sealing the battery container.

[0100] At this time, depending on the shape of the battery container (case), lithium secondary batteries can be classified into can-type lithium secondary batteries in which the electrode assembly is embedded in a metal can and pouch-type lithium secondary batteries in which the electrode assembly is embedded in a pouch made of a sheet such as aluminum laminate.

[0101]

[0102] The present invention will be described in detail below through examples, but these are intended to explain the invention in more detail and the scope of the present invention is not limited by the following examples.

[0103] Examples

[0104] (Examples 1 to 4 and Comparative Example 1)

[0105] 1) Conceptualization stage

[0106] A slurry was formed by using dimethyl sulfoxide as a solvent and adding an amorphous carbon precursor (petroleum-based pitch) at the raw material input ratios shown in Table 1 to Si particle powder (D50: 400 nm) with a naturally formed oxide film. This slurry was then spheroidized using a planetary mill at 5,000 rpm for 20 minutes. The manufactured silicon-based aggregated particles had an average particle size (D50) of 15–20 µm and a BET specific surface area of ​​1.0–2.0 m². 2 It was a spherical particle of / g.

[0107] 2) Mixing and heat treatment steps

[0108] A silicon-based composite particle negative electrode active material was prepared by mixing the manufactured silicon-based assembled particles and the lithium precursor LiOH in the raw material mixing ratio shown in Table 1 below, and then heat-treating the mixture in a calcination furnace at 900°C under a nitrogen atmosphere for 2 hours.

[0109] 3) Manufacturing steps for the cathode and lithium secondary battery

[0110] A negative electrode slurry was prepared by dispersing 30g of 96wt% of manufactured silicon negative electrode active material, 1wt% of carbon black, 1.5wt% of SBR binder, and 1.5wt% of carboxymethyl cellulose in distilled water. The prepared negative electrode slurry was uniformly coated onto a Cu thin film and vacuum dried at 135°C to fabricate a negative electrode for a lithium secondary battery.

[0111] LiNi 0.8 Co 0.1 Mn 0.1 A cathode slurry was prepared by mixing 94 wt% O2 (NCM811, EcoPro BM), 3 wt% carbon black, and 3 wt% PVdF binder in N-methyl-2-pyrrolidone. The prepared cathode slurry was uniformly coated onto an Al thin film, vacuum dried at 135°C, and then rolled to produce a cathode for a lithium secondary battery.

[0112] A lithium secondary battery was manufactured by placing the manufactured positive and negative electrodes and a porous polyethylene membrane (Celgard 2300, thickness: 25 μm) separator into a battery container and using an electrolyte containing LiPF6 at a concentration of 1.15 M in a solvent mixed with EC:EMC in a volume ratio of 3:7.

[0113] Spheroidization Step Raw Material Input Ratio (wt%) Mixing Step Raw Material Mixing Ratio (wt%) Heat Treatment Step Conditions Si Organic Solvent Pitch Granulated Particles LiOH Comparative Example 15 2 13 35 -- N2, 900℃, 2h Example 19 0 10 Example 29 28 Example 39 37 Example 49 55

[0114]

[0115] Experimental Example

[0116] * XRD (X-ray diffraction) analysis

[0117] XRD analysis of the cathode active material (Cu-Kα ray, 2θ: 10°–80°, scan rate: 0.01° / step) was performed using a Bruker D8 ADVANCE A25, with measurements taken by applying a current of 30 mA at a voltage of 45 kV. The obtained results confirmed the positions of all peaks corresponding to each crystal phase.

[0118] Method for measuring the weight ratio of lithium silicate

[0119] The Li2Si2O5 / Li2SiO3 (weight ratio) was confirmed by calculating the ratio of the intensity of the Li2Si2O5 peak (2 theta=24.9°) to the intensity of the Li2SiO3 peak (2 theta=18.8°) using the X-ray Diffraction (XRD) analysis results of the cathode active material.

[0120] * Cell performance evaluation

[0121] For a sodium secondary battery, the initial charge capacity, initial discharge capacity, and initial reversible efficiency were measured through charge-discharge experiments using an electrochemical analyzer (Toyo, Toscat-3100) at 25°C, a voltage range of 2.2V to 4.1V, and a discharge rate of 0.1C.

[0122] In addition, for the same sodium secondary battery, 1 to 50 charge / discharge cycles were performed under conditions of 0.5C / 0.5C within a driving voltage range of 2.2V to 4.1V at 25℃, and the ratio of the discharge capacity at the 30th cycle to the discharge capacity at the 1st cycle (cycle capacity retention rate) was measured.

[0123]

[0124] [Experimental Example 1] Particle Size Analysis and BET Specific Surface Area Analysis via SEM Images of Particle Surfaces

[0125] Figures 1a and 1b are surface scanning electron microscope (SEM) images of silicon-based composite particles prepared in Comparative Example 1 and Example 2, respectively.

[0126] As shown in Figures 1a and 1b, it can be confirmed that spherical silicon-based composite particles were manufactured, and compared to Comparative Example 1, it can be confirmed that a lithium silicate coating layer was uniformly formed on the surface of the composite particles manufactured in Example 1.

[0127] (Example 1-1 and Comparative Example 2-1)

[0128] A silicon-based composite particle negative electrode active material, a negative electrode, and a lithium secondary battery were manufactured by proceeding in the same manner as in Example 1, except that spherical particles were manufactured with the average particle size (D50) and BET specific surface area listed in Table 2 below in the above 1) spheroidization step.

[0129] (Comparative Example 2-2)

[0130] A silicon-based composite particle negative electrode active material, a negative electrode, and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the above 1) spheroidization step was not performed, and 2) Si particle (D50:400nm) powder was used instead of silicon-based assembled particles in the mixing and heat treatment steps.

[0131] Comparative Example 1 Example 1 Example 2 Example 1-1 Comparative Example 2-1 Comparative Example 2-2 Spherical aggregated particles D50 (㎛) 18.0 18.0 18.0 5.0 38.0 -BET(m 2 / g)1.6 1.6 1.6 2 3.4 0.8 - Heat-treated composite particles D50 (㎛)18.0 19.2 18.4 5.5 38.1 0.4 3BET(m 2 / g)1.6 2.4 3.2 25 148 Li2Si2O5 / Li2SiO3 (weight ratio) Phase not formed 0.1 0.5 0.5 0.1 Less than 0.6

[0132] Referring to Table 2, as in Example 2-1, as the D50 decreases and the BET specific surface area increases, more oxide film is exposed compared to Example 1, allowing more lithium silicate (Li2Si2O5) to be formed with the same amount of Li precursor.

[0133] On the other hand, when primary particles that have not undergone aggregation are pretreated as in Comparative Example 2-2, it is not easy to lithium pretreat the silicon natural oxide film of each silicon powder (primary particle) with the same amount of Li precursor, making it practically difficult to form a uniform coating layer.

[0134] In addition, as in Comparative Example 2-1, if the particle size of the spherical composite particles (secondary particles) increases excessively, there is no change in the amount of lithium silicate formed, but the thickness of the coating layer decreases, making it difficult to form a uniform coating layer, and the proportion of Li2SiO3 lithium silicate may increase because the content of the Li precursor increases relatively compared to the exposed oxide film (SiO2).

[0135] Accordingly, the particle size of the aggregated particles is preferably 5 to 20 or 10 to 20 μm, and the corresponding specific surface area is 1.5 to 25 or 2.0 to 5 m 2 / g is desirable.

[0136]

[0137] [Experimental Example 2] Analysis of Si, O, and C concentration distributions in the center and surface of composite particles via particle cross-sectional SEM-EDS images

[0138] Figures 2a and 3a are cross-sectional SEM-EDS (Scanning electron microscope-Energy dispersive spectroscopy) images of silicon-based composite particles prepared in Comparative Example 1 and cross-sectional SEM (Scanning electron microscope) images of silicon-based composite particles prepared in Comparative Example 1, respectively, and Figures 2b, 2c, and 3b are cross-sectional SEM-EDS (Scanning electron microscope-Energy dispersive spectroscopy) images of silicon-based composite particles prepared in Example 2 and cross-sectional SEM (Scanning electron microscope) images of silicon-based composite particles prepared in Example 2.

[0139] The average content (wt%) of Si, C, and O was measured through SEM-EDS analysis for Spot 1 (area within 1 μm from the outermost surface), Spot 2 (area from 1 to 2 μm from the outermost surface), and Spot 3 (area from 2 to 3 μm from the outermost surface), respectively, as indicated in Figures 3a and 3b, and the measurement results are summarized in Table 3 below.

[0140] Comparative Example 1 Example 2 Spot 1 (Surface) 2 (Surface to Center) 3 (Center) 1 (Surface) 2 (Surface to Center) 3 (Center) Si (wt%) 66.07 7.24 76.8 24 7.30 72.40 70.25 C (wt%) 26.88 17.11 16.93 39.43 21.04 21.85 O (wt%) 7.05 5.65 6.26 13.28 6.56 7.90

[0141] As shown in FIGS. 2a to 2c and Table 3, compared to Comparative Example 1, in which a lithium silicate reaction was not performed, it was confirmed that in Example 2, a lithium silicate coating was performed on the surface of the silicon-based composite particle (Spot 1), as the oxygen concentration was high and the silicon concentration was relatively low. Accordingly, it is expected that the initial efficiency and lifespan characteristics can be improved by physically preventing direct contact between silicon and the electrolyte to suppress the electrolyte decomposition reaction, and by enhancing electrical conductivity and lithium ion conductivity through the composite particle composition.

[0142] In addition, no significant difference in silicon concentration (oxygen concentration) was observed in the center (Spot 2, Spot 3) of the composite particles between Comparative Example 1 and Example 2. Accordingly, it is expected that the silicon-based composite particles of the present invention can prevent capacity degradation by increasing the ratio of lithium silicate formation in the surface portion relative to the center, thereby preventing excessive synthesis of the irreversible phase.

[0143]

[0144] [Experimental Example 3]: Evaluation of Lithium Silicate Phase Control by Changes in Silicon Raw Material and Li Precursor Contents

[0145] Si raw material surface O content (wt%) Li source content (wt%) Li2Si2O5 / Li2SiO3 (weight ratio) Comparative Example 14.0 - No phase formed Example 14.0 100.1 Example 24.0 80.5 Example 34.0 73.0 Example 44.0 519.0 Comparative Example 3-14.0 327.0 Comparative Example 3-24.0 131.4 Comparative Example 4-10 (Oxide film removed) No phase formed Comparative Example 4-216 (SiOx used) Less than 50.1

[0146] Referring to Table 4, as the amount of Li precursor mixed decreases, the weight ratio of Li2Si2O5 / Li2SiO3 of the lithium silicate increases, but as in Comparative Example 4-1, the lithium silicate phase is not formed when the natural oxide film is removed. Conversely, it was confirmed that when the content of SiO2 inside the negative electrode active material is high, such as with SiOx powder, the internal SiO2 and the Li source do not react sufficiently, resulting in a low weight ratio of Li2Si2O5 / Li2SiO3.

[0147]

[0148] [Experimental Example 4]: Cell Performance Evaluation by Change in Lithium Silicate Composition

[0149] Li2Si2O5 / Li2SiO3 (Weight Ratio) Discharge Capacity (mAh / g) ICE (%) 30 cycle Lifetime (%) Comparative Example 1 Phase Not Formed 480 92.47 0.5 Example 1 0.14 85 92.47 6.1 Example 2 0.54 86 92.28 0.9 Example 3 3.04 87 92.28 4.4 Example 4 19.04 86 92.38 7.6 Comparative Example 3 12.704 819 0.17 0.3 Comparative Example 3 23.144 848 9.57 0.0

[0150] As shown in Table 5, it was confirmed that the discharge capacity, initial efficiency (ICE), and lifespan characteristics of the example were improved by forming a lithium silicate coating layer on the surface of the silicon-based composite particles and controlling the Li2Si2O5 / Li2SiO3 weight ratio of the lithium silicate to 0.1 or higher.

[0151] In addition, it was confirmed that the above-mentioned effect is further improved as the ratio of Li2Si2O5 increases within the preferred phase control range of lithium silicate (Li2Si2O5 / Li2SiO3 = 0.5~20 weight ratio).

[0152] On the other hand, when the weight ratio of Li2Si2O5 / Li2SiO3 deviates from the desirable Li2Si2O5 / Li2SiO3, the total content of lithium silicate decreases, so the effect of suppressing side reactions in the electrolyte by forming a coating layer is not sufficiently expressed (Comparative Examples 3-1, 3-2), or the synthesis of the irreversible phase increases, leading to a decrease in capacity, and it was confirmed that the initial efficiency and reversible capacity characteristics deteriorate due to a decrease in the amount of Li2Si2O5 synthesized (Comparative Example 1, Example 1).

Claims

1. Silicon-based composite particles comprising a plurality of silicon particles, amorphous carbon, and lithium silicate aggregated therein, The above silicon-based composite particle has a ratio of the lithium silicate concentration on the surface to the lithium silicate concentration in the center of greater than 1, and The above lithium silicate is a negative electrode active material for a secondary battery, wherein the weight ratio (L1 / L2) of Li2Si2O5 (L1) to Li2SiO3 (L2) is 0.1 or greater.

2. In Paragraph 1, The silicon-based composite particles have an oxygen (O) content of 4.5 to 9.5 weight% relative to the total weight of O, Si, and C elements in the center, and a silicon (Si) content of 60 to 80 weight%, and A negative electrode active material for a secondary battery, wherein the oxygen (O) content relative to the total weight of O, Si, and C elements on the surface portion is 10.5 to 15.5 weight% and the silicon (Si) content is 35 to 55 weight%.

3. In Paragraph 1, The above lithium silicate is a negative electrode active material for a secondary battery, wherein the weight ratio (L1 / L2) of Li2Si2O5 (L1) to Li2SiO3 (L2) is 0.4 to 20.

4. In Paragraph 1, The above silicon-based composite particles have an average particle size (D50) of 10 to 25 μm and a BET specific surface area of ​​1.8 to 3.6 m² 2 A negative electrode active material for a secondary battery containing spherical particles of / g.

5. In Paragraph 1, The above silicon-based composite particles are formed on a surface and include a coating layer comprising the above lithium silicate, The above coating layer is in the form of a uniform film and is formed with a thickness of 0.1 to 1 μm, a negative electrode active material for a secondary battery.

6. In Paragraph 1, The above plurality of silicon particles are negative electrode active materials for secondary batteries, wherein a silicon oxide film formed on the surface by natural oxidation.

7. In Paragraph 1, The above amorphous carbon comprises at least one selected from hard carbon, soft carbon, graphene, and graphite sheets, a negative electrode active material for a secondary battery.

8. In Paragraph 1, The above silicon-based composite particles are a negative electrode active material for a secondary battery, having a weight ratio of silicon (Si) to carbon (C) of 60:40 to 90:

10.

9. A spheroidization step of mixing silicon powder and an amorphous carbon precursor in a solution and spheroidizing them to form silicon-based assembled particles; A step of mixing the silicon-based assembled particles and the lithium precursor, wherein the lithium precursor is included in an amount of 5 to 20 parts by weight relative to the total weight of the silicon-based assembled particles and the lithium precursor; and A method for manufacturing a negative electrode active material for a secondary battery, comprising the step of heat-treating a mixture of the silicon-based assembled particles and the lithium precursor.

10. In Paragraph 9, The above spheroidization step spheroidizes the mixture using a mechano fusion method, so that the average particle size (D50) is 13 to 22 μm and the BET specific surface area is 1.1 to 2.1 m² 2 A method for manufacturing a negative electrode active material for a secondary battery, wherein spherical silicon-based assembled particles are manufactured to be of the size of / g.

11. In Paragraph 9, A method for manufacturing a negative electrode active material for a secondary battery, wherein the above mixing step involves uniformly coating the lithium precursor onto the surface of silicon-based assembled particles.

12. In Paragraph 9, A method for manufacturing a negative electrode active material for a secondary battery, wherein the above heat treatment step involves heat treating at 700 to 900°C in an inert gas atmosphere for 1 to 20 hours to produce silicon-based composite particles.

13. A negative electrode for a secondary battery comprising a negative electrode active material according to paragraph 1.

14. A secondary battery comprising a negative electrode according to paragraph 13; a positive electrode; and an electrolyte.

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