Negative active materials containing core-shell complexes and their preparation methods

Abstract

The present invention provides a negative electrode active material for a lithium secondary battery, which is characterized by comprising a core-shell composite, and the core-shell composite comprises: a core, which comprises a silicon oxide (SiO x , 0 < x ≤ 2) and a graphite-based material containing a lithium-containing substance; and a shell, which is located on the core and comprises amorphous carbon, wherein the silicon oxide (SiO x , 0

Description

Technical Field

[0001] This invention relates to a negative electrode active material for lithium secondary batteries comprising a core-shell composite, a method for preparing the negative electrode active material, a negative electrode comprising the negative electrode active material, and a lithium secondary battery. Background Technology

[0002] With the increasing global warming problem in modern society, the demand for corresponding environmentally friendly technologies has increased dramatically. In particular, with the growing demand for electric vehicles and energy storage systems (ESS) technologies, the demand for lithium-ion batteries, as energy storage devices, has also surged. Therefore, research is underway to improve the energy density of lithium-ion batteries.

[0003] However, commercially available lithium-ion batteries typically use graphite active materials such as natural graphite and artificial graphite. Due to the low theoretical capacity of graphite (372 mAh / g), the energy density of the batteries is low. Therefore, research is underway to improve the energy density by developing new anode materials.

[0004] As a solution to this problem, Si-based materials with high theoretical capacity (3580 mAh / g) are emerging. However, these Si-based materials suffer from a drawback: large volume expansion (~400%) during repeated charge-discharge cycles leads to deterioration in battery life characteristics. Therefore, as a method to address the large volume expansion problem of Si materials, SiO2, which has a lower volume expansion rate compared to Si, has been developed. x Material. This SiO x The material exhibits excellent lifetime characteristics due to its low volume expansion rate, but its low initial efficiency (initial coulombic efficiency, ICE) caused by the formation of an irreversible phase in the early stage makes it difficult to apply to practical lithium secondary batteries. Summary of the Invention

[0005] Technical problems to be solved

[0006] The purpose of this invention is to provide a negative electrode active material that improves initial efficiency and capacity while enhancing lifetime characteristics.

[0007] Another object of the present invention is to provide a negative electrode active material that improves the stability and lifespan characteristics of the slurry by preventing the dissolution of lithium-containing substances remaining on the surface of lithium-pretreated silicon-based oxide particles, thereby suppressing the resulting increase in slurry pH and side reactions with the electrolyte during charge and discharge.

[0008] Technical solution

[0009] A specific embodiment of the present invention provides a negative electrode active material for a lithium secondary battery, which is characterized by comprising a core-shell composite, and the core-shell composite comprises: a core, which comprises a silicon oxide (SiO x , 0 < x ≤ 2) and a graphite-based material containing a lithium-containing substance; and a shell, which is located on the core and comprises amorphous carbon, wherein the silicon oxide (SiO x , 0 < x ≤ 2) contains at least one lithium silicate selected from Li2SiO3, Li2Si2O5 and Li4SiO4 in at least a part of the silicon oxide.

[0010] The lithium-containing substance can be at least one selected from LiOH, Li, LiH, Li2O and Li2CO3.

[0011] The negative electrode active material for a lithium secondary battery is characterized in that the position of the maximum peak according to Raman spectroscopy of the silicon oxide containing the lithium-containing substance can be 460 - 500 cm -1 .

[0012] The negative electrode active material for a lithium secondary battery is characterized in that the position of the maximum peak according to Raman spectroscopy of the silicon oxide containing the lithium-containing substance can be 500 - 530 cm -1 .

[0013] The negative electrode active material for a lithium secondary battery is characterized in that the core further comprises amorphous carbon.

[0014] The graphite-based material can be natural graphite, artificial graphite or a combination thereof.

[0015] Relative to 100 parts by weight of the composite, 5 - 50 parts by weight of the silicon oxide can be comprised.

[0016] Relative to 100 parts by weight of the composite, 30 - 80 parts by weight of the graphite-based material can be comprised.

[0017] The average thickness of the shell can be 0.1 - 100 nm.

[0018] Relative to 100 parts by weight of the negative electrode active material, more than 50 parts by weight of the composite can be comprised.

[0019] Another specific embodiment of the present invention provides a method for preparing a negative electrode active material for a lithium secondary battery, which is characterized by comprising the following steps: a) preparing a silicon oxide (SiO x, 0 < x ≤ 2); b) combining the lithium-containing silicon oxide, the graphite-based material, and the carbon precursor to prepare a core-shell composite precursor; and c) heat-treating the core-shell composite precursor to prepare a core-shell composite, wherein the combination in step b) includes a dry mixing step of applying shear stress and centrifugal force, and the carbon precursor is added at an intermediate time point during the dry mixing step, and the silicon oxide (SiO x , 0 < x ≤ 2) contains at least one lithium silicate selected from Li2SiO3, Li2Si2O5, and Li4SiO4 in at least a part of the silicon oxide particles.

[0020] The combination in step b) can be carried out by mechanochemical treatment.

[0021] Step c) can be carried out in an inert atmosphere.

[0022] In step a), a silicon compound and a lithium precursor can be mixed and heat-treated.

[0023] The lithium-containing substance can be at least one selected from LiOH, Li, LiH, Li2O, and Li2CO3.

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

[0025] Another specific embodiment of the present invention provides a lithium secondary battery, which includes: a negative electrode according to an embodiment of the present invention; a positive electrode; a separator located between the negative electrode and the positive electrode; and an electrolyte.

[0026] Advantageous Effects

[0027] The negative electrode active material for a lithium secondary battery of the present invention has the advantage of being able to solve the problems of initial efficiency and capacity deterioration.

[0028] In addition, by suppressing the expansion of silicon-based oxide particles during the charge and discharge process, the battery stability and life characteristics can be improved.

[0029] In addition, the dissolution of the lithium-containing substance remaining on the surface of the lithium-pretreated silicon-based oxide particles is prevented, thereby improving the stability of the slurry and the battery life characteristics. Description of the Drawings

[0030] Figure 1 (a) and (b) are cross-sectional schematic views of a core-shell composite of an embodiment of the present invention.

[0031] Description of the Reference Numerals

[0032] 10: Core-shell composite

[0033] 13: Nuclear

[0034] 15: Shell

[0035] 100: Silicon oxides containing lithium-containing substances

[0036] 200: Graphite-based materials

[0037] 300: Amorphous carbon (core)

[0038] 400: Amorphous carbon (shell) Detailed Implementation

[0039] The advantages and features of the present invention, as well as the methods of implementing them, can be clearly understood by referring to the accompanying drawings and detailed description of the embodiments. However, the present invention is not limited to the embodiments disclosed below and can be implemented in various ways, different from each other. These embodiments are provided to fully disclose the invention and to fully illustrate the scope of the invention to those skilled in the art, and the invention is defined only by the scope of the claims. The specific contents for implementing the invention are described in detail with reference to the accompanying drawings. Unrelated to the drawings, the same reference numerals refer to the same constituent elements, and "and / or" includes all combinations of the individual items mentioned and more than one of the mentioned items.

[0040] Unless otherwise defined, all terms used in this specification (including technical and scientific terms) may have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Throughout this specification, unless otherwise specifically stated to the contrary, the description of a part as "comprising" or "including" a constituent element means that other constituent elements may also be included, rather than excluding other constituent elements. Furthermore, unless otherwise specifically stated, the singular form also includes the plural form.

[0041] In this specification, when describing a layer, film, region, plate, or other part as being "on" or "above" another part, this includes not only the case where it is "directly" on the other part, but also the case where there are other parts in between.

[0042] In this specification, the average particle size may refer to D50, which is the particle diameter from the smallest particle size to 50% of the cumulative volume when the particle size distribution is measured by laser scattering. D50 can be sampled according to KS A ISO 13320-1 standard and the particle size distribution measured using a Malvern Mastersizer 3000. Specifically, ethanol can be used as a solvent, and if necessary, an ultrasonic disperser can be used for dispersion followed by volume density measurement.

[0043] The present invention provides a negative electrode active material for a lithium secondary battery, characterized in that the negative electrode active material contains a core-shell composite, and the core-shell composite includes: a core containing a silicon oxide (SiO x , 0 < x ≤ 2) containing a lithium-containing substance and a graphite-based material; and a shell located on the core and containing amorphous carbon, wherein the silicon oxide (SiO x , 0 < x ≤ 2) contains at least one lithium silicate selected from Li2SiO3, Li2Si2O5, and Li4SiO4 in at least a part of the silicon oxide.

[0044] The core contains a silicon oxide (SiO x , 0 < x ≤ 2) containing a lithium-containing substance and a graphite-based material.

[0045] The silicon oxide contains a lithium-containing substance. As an example, the lithium-containing substance may include at least one selected from LiOH, Li, LiH, Li2O, and Li2CO3. Such a lithium-containing substance remains on the surface of the silicon compound pretreated with lithium, is the residual lithium remaining unreacted during the lithium pretreatment, and is a substance that will inevitably be generated during the lithium pretreatment of the silicon-based compound.

[0046] As described above, the lithium-containing substance may exist on the surface of the silicon oxide, may dissolve out and be located inside the core during the preparation of the core-shell composite, and may also exist on the surface of the silicon oxide and / or the graphite-based material.

[0047] When the lithium-containing substance dissolves in a solvent (e.g., water) during the process of preparing a negative electrode paste containing the negative electrode active material, it may cause the pH of the negative electrode paste to rise. The rise in pH causes the chains of the polymer binder, which is an essential component of the paste, to contract. Therefore, problems such as a decrease in the viscosity of the paste resulting in a decrease in the adhesion between the current collector and the negative electrode active material layer and oxidation of the Si component of the silicon-based active material to generate gas may occur. Such results will not only reduce the stability of the negative electrode active material but also lead to a decrease in capacity.

[0048] In the present invention, the core-shell composite is prepared under specific conditions, thereby effectively suppressing the dissolution of the lithium-containing substance (residual lithium), and thus the above problems can be solved. In addition, the silicon oxide pretreated with lithium is included inside the core, thereby improving the problem of low initial efficiency, and as a result, the capacity and life characteristics of the battery can be improved.

[0049] The position of the maximum peak of the silicon oxide according to Raman spectroscopy is 500 - 530 cm -1 , for example, it can be more than 500 cm -1 and less than 530 cm -1 , 505 cm-1 above and less than 530 cm -1 or 510 cm -1 above and less than 530 cm -1 .

[0050] The position of the maximum peak of the silicon oxide according to the Raman spectrum is preferably 460 - 500 cm -1 , for example, preferably more than 460 cm -1 and less than 500 cm -1 , more than 460 cm -1 and 490 cm -1 or less, or more than 460 cm -1 and 480 cm -1 or less.

[0051] As the position of the maximum peak of the silicon oxide according to the Raman spectrum increases within the range of 500 - 530 cm -1 , compared with amorphous Si (hereinafter referred to as a - Si), the growth of crystalline Si (hereinafter referred to as c - Si) in the silicon oxide is promoted, so the ratio of c - Si / a - Si increases. In order to minimize the volume change caused by the insertion and extraction of lithium ions due to the high content of the c - Si, the position of the maximum peak can be 460 - 500 cm -1 , preferably 460 - 480 cm -1 . Within the above range, by increasing the content of a - Si, that is, increasing the content of Si in the amorphous state, the volume expansion occurring during charge and discharge can be suppressed, and further, the deterioration of the negative electrode active material can be suppressed, thereby improving the battery characteristics.

[0052] On the other hand, the crystallization means that the shape of a single Si located inside the particle has crystallinity, and the amorphous means that the shape of a single Si located inside the particle is amorphous, or refers to fine particles with a size that is difficult to measure by the Scherrer equation in XRD analysis.

[0053] In addition, the silicon oxide (SiO x , 0 < x ≤ 2) contains at least one lithium silicate selected from Li2SiO3, Li2Si2O5, and Li4SiO4 in at least a part of the silicon oxide.

[0054] When the Li2SiO3 phase is formed, less Si is consumed compared to lithium silicate such as Li2Si2O5, thereby improving capacity characteristics and reducing the severe volume change of Si during battery cycle life, which is beneficial for improving lifespan characteristics. On the other hand, the Li4SiO4 phase has high reactivity with water, making it difficult to adjust the physical properties of the slurry during electrode manufacturing, and is therefore not preferred.

[0055] Relative to 100 parts by weight of silicon oxide, the silicon oxide may contain 10-100 parts by weight of lithium silicate, preferably 30-90 parts by weight of lithium silicate, and more preferably 50-90 parts by weight of lithium silicate. Within the above content range, the formation of the initial irreversible phase of silicon oxide that occurs during initial charge and discharge can be suppressed, thereby increasing initial efficiency and capacity.

[0056] On the other hand, relative to 100 parts by weight of silicon oxide, the silicon oxide may contain 25 parts by weight or less of Li4SiO4, preferably 10 parts by weight or less of Li4SiO4, more preferably 5 parts by weight or less of Li4SiO4, and even more preferably less than 1 part by weight of Li4SiO4. The Li4SiO4 phase has irreversible properties to Li ions and is susceptible to moisture, therefore it is not preferred as an active material for a negative electrode using a water-based binder. Within the range of the Li4SiO4 phase, the water resistance of the negative electrode slurry can be improved.

[0057] At this time, the silicon oxide can have an average particle size of 2-30 μm, preferably 5-10 μm.

[0058] The graphite-based material is used for lithium ion insertion / extraction and to improve conductivity. The graphite-based material may be composed entirely or partially of graphite, or it may be a product of various chemical, thermal, oxidation, or physical treatments in the liquid, gas, or solid phases. Specific examples include amorphous, plate-like, flake-like, spherical, or fibrous natural graphite and / or synthetic graphite, but in terms of low resistivity and economy, the graphite-based material may be natural graphite.

[0059] The average particle size of the graphite-based material can be 1-50 μm, preferably 3-15 μm. Within this range, the difference in expansion rate between the graphite-based material and silicon oxide within the core can be reduced, thus suppressing interfacial separation. The core may also contain amorphous carbon. Examples of amorphous carbon include soft carbon, hard carbon, mesophase pitch carbides, and calcined coke. The amorphous carbon acts as a buffer to reduce the expansion of the interface between the graphite-based material and silicon oxide particles within the core, thus reducing the problem of reduced capacity and lifetime characteristics caused by volume expansion.

[0060] The shell located on the core comprises amorphous carbon, specifically, the amorphous carbon can be the same material as the amorphous carbon contained in the core. In this case, the average thickness of the shell can be 0.1-100 nm, preferably 1-10 nm, and more preferably 2-5 nm. Within the above thickness range, the leaching of lithium-containing materials contained in the core into the slurry and electrolyte can be effectively prevented, thus improving lifetime characteristics.

[0061] Specifically, the shell can be formed such that the lithium-containing material and silicon oxide within the core do not leach to the surface of the core-shell composite. Preferably, it can be formed such that the lithium-containing material, silicon oxide, and graphite-based material within the core do not leach to the surface of the core-shell composite. More preferably, amorphous carbon is uniformly coated on the core to form a layer.

[0062] That is, the shell of the present invention comprises amorphous carbon and may not contain lithium-containing materials contained in the core, silicon oxide containing lithium-containing materials, graphite-based materials, or combinations thereof.

[0063] Figure 1 (a) and (b) are schematic cross-sectional views of a core-shell composite according to an embodiment of the present invention. Referring to the above... Figure 1 In (a) and (b), the core-shell composite 10 comprises a lithium-containing silicon oxide 100, a graphite-based material 200, and an amorphous carbon 300 within the core 13, and the shell 15 enclosing the core may have a structure containing amorphous carbon 400. Because the amorphous carbon is present both inside the core and on the shell, a conductive path is provided to the interior of the core, thereby exhibiting excellent conductivity while effectively preventing the dissolution of the lithium-containing material within the core.

[0064] The core-shell composite may contain 5-50 parts by weight of the silicon oxide, preferably 10-50 parts by weight, and more preferably 20-40 parts by weight, relative to 100 parts by weight of the silicon oxide.

[0065] Relative to 100 parts by weight of the core-shell composite, it may contain 30-80 parts by weight of the graphite-based material, preferably 40-80 parts by weight of the graphite-based material, and more preferably 50-80 parts by weight of the graphite-based material.

[0066] Relative to 100 parts by weight of the core-shell composite, it may contain 5-25 parts by weight of the amorphous carbon, preferably 5-15 parts by weight, and more preferably 7-12 parts by weight. Within the above range, a shell with uniformly distributed amorphous carbon can be formed, and it also penetrates into the core, thus increasing conductivity according to the supply of conductive pathways.

[0067] The lithium content in the core-shell composite can be 1.3% by weight or more, 1.3-5% by weight, 1.3-4% by weight, 2-5% by weight, 2.5-5% by weight, 2.5-4% by weight, 2.5-3.5% by weight, or 3-3.5% by weight. In this case, the lithium content can refer to the total weight of lithium elements present in the core-shell composite, specifically, the weight of residual lithium that does not dissolve into the slurry and the weight of lithium elements contained in lithium silicate. On the other hand, as the content of lithium-containing substances dissolved in the solvent during the preparation of the negative electrode slurry increases, i.e., the amount of lithium-containing substances dissolved, the residual lithium decreases. Therefore, under the same lithium silicate content conditions, a higher residual lithium content is more advantageous in suppressing the dissolution of lithium-containing substances, but in terms of suppressing the reduction in initial charge-discharge efficiency and capacity due to the use of silicon oxide, the lithium content in the core-shell composite meeting the above range is advantageous.

[0068] More specifically, in the core-shell composite, the ratio (A / B) of the lithium content (A, wt%) in the core-shell composite to the lithium silicate content (B, wt%) in the silicon-based oxide can be 0.020 or more, 0.020-0.050, and preferably 0.033-0.043. Within the above range, the residual lithium content in the core-shell composite can be maximized, thus preventing the pH increase of the negative electrode slurry caused by the dissolution of lithium-containing substances inside the core. Furthermore, when the pH of the slurry increases, the chains of the carboxymethyl cellulose polymer in the binder, an essential component of the slurry, undergo condensation, thereby reducing the viscosity of the slurry and decreasing the adhesion during electrode coating.

[0069] On the other hand, the lithium content in the core-shell composite can be measured by inductively coupled plasma atomic emission spectrometry (ICP), and the lithium silicate content can be measured by X-ray diffraction (XRD), but the present invention is not limited thereto.

[0070] With respect to 100 parts by weight of the negative electrode active material, 50 parts by weight or more, preferably 60 parts by weight or more or 70 parts by weight or more, more preferably 80 parts by weight or more or 90 parts by weight or more of the core-shell composite can be included. As an example, 100 parts by weight of the core-shell composite can be included. Conventionally, due to the volume expansion of the electrode using silicon oxide, excellent life characteristics cannot be achieved. Therefore, more than half of the graphite-based active material that can alleviate the shrinkage / expansion of the silicon oxide-based particles, etc., is mixed, or an excessive amount of amorphous carbon is used in the core-shell composite containing a silicon-based material. In the present invention, the generation of crystalline c-Si can be suppressed and the proportion of a-Si can be increased during the Li pretreatment of the silicon oxide particles. Therefore, a negative electrode active material using a high content of the core-shell composite can be provided, and the core-shell composite contains silicon oxide containing the Li2SiO3. Therefore, compared with the prior art, the initial efficiency and life characteristics can be improved while further increasing the discharge capacity.

[0071] The present invention also provides a method for preparing a negative electrode active material for a lithium secondary battery, characterized by comprising the following steps: a) preparing silicon oxide (SiO x , 0 < x ≤ 2) containing a lithium-containing substance; b) compounding the silicon oxide containing the lithium-containing substance, a graphite-based material, and a carbon precursor to prepare a core-shell composite precursor; and c) heat-treating the core-shell composite precursor to prepare a core-shell composite, wherein the compounding is carried out by dry mixing applying shear stress and centrifugal force, and the carbon precursor is added in the middle stage of the dry mixing, and the silicon oxide (SiO x , 0 < x ≤ 2) contains at least one lithium silicate selected from Li2SiO3, Li2Si2O5, and Li4SiO4 in at least a part of the silicon oxide particles.

[0072] The step a) is a step of preparing silicon oxide, which can be divided into: a first step of preparing a silicon compound; and a second step of subjecting the silicon compound to Li pretreatment to prepare silicon oxide containing a lithium-containing substance.

[0073] In the first step, the mixing ratio of Si powder and SiO2 powder is appropriately adjusted and mixed to form the molar ratio of Si and O of the prepared silicon compound particles (SiO x , 0 < x ≤ 2). Then, heat treatment can be carried out for 1 to 12 hours or 1 to 8 hours at a temperature below 900 °C, preferably at a temperature below 800 °C or 500 - 700 °C, more preferably at 500 - 650 °C in an inert atmosphere and under reduced pressure. Conventionally, in order to prepare silicon compound particles, heat treatment is carried out at a high temperature of 900 - 1600 °C, but SiO xWhen SiO material is heat-treated at temperatures above 800°C, c-Si seed crystals grow, while microcrystals grow significantly at around 900°C. Therefore, in this invention, to suppress the formation of c-Si seed crystals and the growth of c-Si to prepare amorphous or microcrystalline silicon compounds, heat treatment at 500-650°C can be performed. The prepared silicon compounds can be pulverized to prepare silicon compound particles.

[0074] The second step is to pretreat the prepared silicon compound particles with lithium. By mixing with a lithium precursor and heat-treating, a negative electrode active material containing at least one lithium silicate selected from Li2SiO3, Li2Si2O5 and Li4SiO4 in at least a portion of the silicon oxide can be prepared.

[0075] Specifically, in the mixing process, the silicon compound and the lithium precursor can be mixed such that the Li / Si molar ratio is 0.3-1.0, preferably 0.3-0.8, and more preferably 0.4-0.8. Within the above mixing range, the optimal rate capability of Li₂SiO₃ and Li₂Si₂O₅ can be obtained, and the electrochemical performance of the battery can be greatly improved by suppressing the formation of c-Si and Li₄SiO₄.

[0076] The lithium precursor may be selected from at least one of LiOH, Li, LiH, Li2O and Li2CO3, as long as it is a compound that can be decomposed during heat treatment, without particular limitation.

[0077] Next, the mixture can be heat-treated in an inactive atmosphere at a temperature of 500-700°C for 1-12 hours. When heat treatment is performed at temperatures above 700°C, uneven reactions occur or Si crystal growth is accelerated, thus c-Si growth is inevitable. When the raw material is prepared at temperatures below 700°C, crystal growth is suppressed, thereby producing amorphous or microcrystalline silicon oxide particles. Furthermore, when the heat treatment is performed at a low temperature below 500°C, lithium pretreatment is not sufficiently carried out. On the other hand, in lithium pretreatment by electrochemical or redox methods, Li4SiO4, which is lithium silicate, is easily formed. However, according to the present invention, due to heat treatment, lithium silicate with different desired compositions can be synthesized with high purity. In this case, the inactive gas constituting the inactive atmosphere can be selected from Ne, Ar, Kr, and N2, etc., and Ar or N2 is preferred, but not limited to these.

[0078] The silicon oxide prepared in step a) may contain at least one lithium-containing substance selected from LiOH, Li, LiH, Li2O and Li2CO3.

[0079] Step b) is the preparation of a core-shell composite precursor, which can be achieved by combining the lithium-containing silicon oxide, graphite-based material, and carbon precursor prepared in step a) through a solid-phase reaction without the use of solvents. Specifically, the composite can be carried out by dry mixing under shear stress and centrifugal force, preferably by mechanochemical treatment.

[0080] The mechanochemical process described above is a method for preparing composite particles by applying compressive and shear forces to two or more different particles, causing them to adhere to each other. This can be achieved using the Mechanofusion powder composite system from Hosokawa Micron. Preferably, the circumferential speed difference between the roller and the internal components is 10-50 m / s, the distance between them is 1-100 mm, and the processing time is 30-120 minutes. By performing mechanochemical treatment under the above conditions, after step c), the density of amorphous carbon on the surface of the silicon oxide particles is increased, resulting in a smooth coating (shell), thus effectively suppressing the dissolution of lithium-containing substances (residual lithium). On the other hand, simple mixing methods such as ball milling or spraying are irregular mixing methods, making it difficult to ensure a uniform and smooth coating.

[0081] In step b), a carbon precursor can be added during the dry mixing process, specifically during an intermediate stage of the mechanochemical process. This intermediate stage can refer to a time point of 10-90% of the cumulative mechanochemical treatment time relative to 100%, preferably 30-90%, and more preferably 40-90%. Further mechanochemical treatment following the addition of the carbon precursor within the above range yields a core-shell composite structure with a graphite-based material attached to a lithium-containing silicon oxide substrate, and a structure in which the carbon-based precursor is uniformly distributed both inside the core and on the shell. Therefore, capacity degradation can be suppressed by mitigating the expansion of the interface between the graphite-based material and silicon oxide particles within the core, and the shell effectively prevents the lithium-containing material within the core from dissolving into the slurry and electrolyte, thus improving lifetime characteristics.

[0082] Step c) is the preparation of the core-shell composite, which can be obtained by heat-treating the core-shell composite precursor. The heat treatment process can be carried out at a temperature of 400-800°C, but more preferably at 550-700°C. Within the above heat treatment temperature range, not only can the carbon precursor be sufficiently carbonized to improve conductivity, but the amorphous Si content in the silicon oxide is also increased to suppress the degradation of the negative electrode active material, thereby improving the characteristics of the battery containing the core-shell composite. In this case, the heat treatment can be carried out in an inactive atmosphere, specifically selected from Ne, Ar, Kr, and N2, etc., preferably using Ar or N2, but not limited to these.

[0083] The present invention also provides a negative electrode for a lithium secondary battery comprising a negative electrode active material according to a specific embodiment of the present invention. Specifically, the negative electrode includes: a current collector; and a negative electrode active material layer located on the current collector and containing the negative electrode active material and a water-based binder.

[0084] The current collector can be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof, but is not limited thereto.

[0085] The negative electrode active material layer can contain a negative electrode active material and a water-based binder, and optionally can also contain a conductive material.

[0086] The negative electrode active material contains a core-shell composite, and the core-shell composite contains: a core containing a silicon oxide (SiO x , 0 < x ≤ 2) containing a lithium-containing substance and a graphite-based substance; and a shell located on the core and containing amorphous carbon, and optionally can also contain a substance capable of reversibly intercalating / deintercalating lithium ions, lithium metal, an alloy of lithium metal, a substance capable of doping and dedoping in lithium, or a transition metal oxide. At this time, the silicon oxide (SiO x , 0 < x ≤ 2) contains at least one lithium silicate selected from Li2SiO3, Li2Si2O5, and Li4SiO4 in at least a part of the silicon oxide.

[0087] As the substance capable of reversibly intercalating / deintercalating the lithium ions, for example, a carbon substance can be cited, that is, a carbon-based negative electrode active material commonly used in lithium secondary batteries can be cited. As a representative example of the carbon-based negative electrode active material, crystalline carbon, amorphous carbon, or they can be used together. As an example of the crystalline carbon, graphite such as amorphous, plate-shaped, flaky, spherical, or fibrous natural graphite or artificial graphite can be cited, and as an example of the amorphous carbon, soft carbon, hard carbon, mesophase pitch carbide, calcined coke, etc. can be cited.

[0088] The alloy of lithium metal can use an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn.

[0089] The substance capable of doping and dedoping in lithium can cite a silicon-based substance, for example, Si, SiO x(0 < x < 2), Si-Q alloy (where Q is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Si-carbon composite, Sn, SnO2, Sn-R alloy (where R is an element selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof, and is not Sn), Sn-carbon composite, etc., and at least one of them can also be mixed with SiO2 for use. The elements Q and R can be selected from Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0090] The transition metal oxide can be lithium titanium oxide.

[0091] The water-based binder serves to bond the negative electrode active material particles well to each other and bond the negative electrode active material well to the current collector. Examples of the water-based binder include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, various copolymers thereof, etc. Specifically, the binder can include a binder composed of carboxymethyl cellulose (CMC), styrene-butadiene rubber (SBR), and mixtures thereof.

[0092] The conductive material is used to impart conductivity to the electrode, and any conductive material can be used as long as it does not cause chemical changes in the constructed battery. As examples of the conductive material, a conductive material containing carbon-based substances such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber; metal-based substances such as metal powders or metal fibers of copper, nickel, aluminum, silver; conductive polymers such as polyphenylene derivatives; or mixtures thereof can be used.

[0093] Relative to the total weight of the negative electrode active material layer, the contents of the binder and the conductive material in the negative electrode active material layer can be 1-10% by weight, preferably 1-5% by weight, but are not limited thereto.

[0094] The present invention also provides a lithium secondary battery, the lithium secondary battery comprising: the negative electrode; the positive electrode; a separator located between the negative electrode and the positive electrode; and an electrolyte.

[0095] The negative electrode is as described above.

[0096] The positive electrode includes: a current collector; and a positive electrode active material layer, which is formed by coating the current collector with a positive electrode slurry containing the positive electrode active material.

[0097] The current collector can be the negative electrode current collector or a material known in the art, but the present invention is not limited thereto.

[0098] The positive electrode active material layer comprises a positive electrode active material, and optionally may also comprise a binder and a conductive material. The positive electrode active material may be any positive electrode active material known in the art, for example, preferably a composite oxide of a metal selected from cobalt, manganese, nickel, and combinations thereof, and lithium, but the invention is not limited thereto.

[0099] The adhesive and conductive material can be the same as the negative electrode adhesive and negative electrode conductive material, or can be any substances known in the art, but the invention is not limited thereto.

[0100] For example, the separator can be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or combinations thereof, and can be in the form of nonwoven or woven fabric. For example, polyolefin-based polymer separators such as polyethylene and polypropylene can be mainly used in lithium secondary batteries, and to ensure heat resistance or mechanical strength, separators coated with ceramic components or compositions containing polymer substances can also be used. Single-layer or multi-layer structures can be selectively used, and separators known in the art can also be used, but the invention is not limited thereto.

[0101] The electrolyte contains an organic solvent and a lithium salt.

[0102] The organic solvent acts as a medium that facilitates the movement of ions participating in the electrochemical reaction of the battery. For example, carbonate-based, ester-based, ether-based, ketone-based, alcohol-based, or aprotic solvents can be used. The organic solvent can be used alone or in combination of two or more. When using two or more solvents, the mixing ratio can be adjusted appropriately according to the desired battery performance. Alternatively, organic solvents known in the art can also be used, but the present invention is not limited thereto.

[0103] The lithium salt is dissolved in an organic solvent and used in the battery as a source of lithium ions to enable the basic operation of the lithium secondary battery, and is a substance that promotes the movement of lithium ions between the positive and negative electrodes. Examples of such lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiN(SO3C2F5)2, LiN(CF3SO2)2, LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, and LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2 (where x and y are natural numbers), LiCl, LiI, LiB(C2O4)2 or combinations thereof, but the invention is not limited thereto.

[0104] The concentration of the lithium salt can be used in the range of 0.1-2.0 M. When the concentration of the lithium salt is within this range, the electrolyte has suitable conductivity and viscosity, thus exhibiting excellent electrolyte performance, and lithium ions can move effectively.

[0105] Furthermore, as needed, to improve charge / discharge characteristics, flame retardant properties, etc., the electrolyte may further contain pyridine, triethyl phosphate, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, hexaphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolides, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, aluminum trichloride, etc. In some cases, to impart non-flammability, halogen-containing solvents such as carbon tetrachloride and ethylene trifluoride may also be included. To improve high-temperature storage characteristics, fluoroethylene carbonate (FEC), propenesultone (PRS), fluoropropylene carbonate (FPC), etc., may also be included.

[0106] In the method for manufacturing a lithium secondary battery according to the present invention for achieving the objectives described above, the battery can be manufactured by sequentially stacking a negative electrode, a separator, and a positive electrode to form an electrode assembly, placing the manufactured electrode assembly into a cylindrical or angular battery casing, and then injecting an electrolyte. Alternatively, the battery can be manufactured by immersing the stacked electrode assembly in an electrolyte and then sealing the resulting product in a battery casing.

[0107] The battery casing used in this invention can be any type of battery casing commonly used in the art, and is not limited by the shape of the casing depending on the purpose of the battery. For example, it can be cylindrical, angular, pouch-shaped, or coin-shaped, etc.

[0108] The lithium secondary battery of the present invention can be used not only as a battery cell for powering small devices, but also preferably as a unit battery in medium to large-sized battery modules comprising multiple battery cells. Preferred examples of such medium to large-sized devices include electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems, but are not limited thereto.

[0109] Hereinafter, preferred embodiments and comparative examples of the present invention are described. However, the following embodiments are merely preferred implementations of the present invention, and the present invention is not limited to the following embodiments.

[0110] Example

[0111] (Example 1)

[0112] Step 1: Preparation of silicon compounds

[0113] A raw material mixed with metallic silicon and silicon dioxide was introduced into a reactor, vaporized at 600°C and a vacuum atmosphere of 10 Pa for 5 hours, and deposited onto an adsorption plate. After sufficient cooling, the deposit was removed and pulverized using a ball mill. The resulting silicon compound particles were SiO₂. x The value of x is 1.0. Next, by adjusting the particle size of the silicon compound particles in stages, SiO particles with an average particle size (D50) of 8 μm are obtained.

[0114] Step 2: Preparation of silicon oxide containing lithium-containing substances

[0115] A mixed powder is formed by mixing the prepared SiO particles and LiOH powder to achieve a Li / Si molar ratio of 0.75. The mixed powder and zirconia balls (1-20 times the volume of the mixed powder) are placed in a sealed container and shaken and mixed for 30 minutes. Afterward, the mixed powder is filtered through a 25-500 μm sieve and then placed in an alumina crucible.

[0116] The alumina crucible was heat-treated at 550°C for 8 hours under a nitrogen atmosphere. The heat-treated powder was then recovered and pulverized in a mortar to prepare a silicon oxide containing lithium. At this point, the silicon oxide had an average particle size of 6.7 μm.

[0117] Step 3: Preparation of the core-shell complex

[0118] 30 wt% of the prepared silicon oxide (D50: 6.7 μm) and 60 wt% of flake-like natural graphite particles (D50: 12.5 μm) were mechanically dry-mixed, and then subjected to mechanochemical treatment for 30 minutes using a mechanofusion machine (Hosokawa Micron Co., Ltd., AMS). During this process, the drum rotation speed was 20 m / s, and the distance between the drum and the internal components was 10 mm. Then, 10 wt% of coal-based pitch binder was added, and the process was further subjected to mechanochemical treatment for 30 minutes.

[0119] Subsequently, the temperature was increased to 600°C at a rate of 3°C / min in a high-purity argon atmosphere, and then heat-treated at 600°C for 4 hours to prepare the core-shell composite.

[0120] Step 4: Manufacturing the negative electrode

[0121] A slurry was prepared by mixing 95 wt% of the prepared core-shell composite, 1.5 wt% of carboxymethyl cellulose, 2 wt% of styrene-butadiene rubber, and 1.5 wt% of the conductive material Super C in distilled water. The slurry was coated onto a Cu foil current collector, and a negative electrode was manufactured using conventional processes of drying and pressing.

[0122] Step 5: Manufacturing the half-cell

[0123] The CR2016 coin cell battery is manufactured by using a fabricated negative electrode and lithium metal as a counter electrode, inserting a PE separator between the negative and counter electrodes, and then injecting an electrolyte. The assembled coin cell battery is left to stand at room temperature for 3-24 hours to produce a half-cell. At this time, the electrolyte is prepared by mixing 1.0 M LiPF6 as the lithium salt and 2 vol% FEC electrolyte additive in an organic solvent (EC:EMC = 3:7 vol%).

[0124] Evaluation example

[0125] Evaluation Example 1: Evaluation based on the characteristics of coal-based pitch binder at the time of addition

[0126] (Examples 1 and 2 and Comparative Examples 1 and 2)

[0127] (Example 2)

[0128] Except for step 3 of Example 1, in which 30 wt% of the prepared silicon oxide (D50: 6.7 μm) and 60 wt% of flake-like primary natural graphite particles (D50: 12.5 μm) are mechanically dry-mixed, and then mechanically chemically treated for 50 minutes using a mechanical fusion machine (Hosokawa Micron Co., AMS) at a drum rotation speed of 20 m / s and a distance of 10 mm from the internal components, and then 10 wt% of coal-based pitch binder is added and further mechanically chemically treated for 10 minutes, the process is carried out in the same manner as in Example 1.

[0129] (Comparative Example 1)

[0130] Except for step 3 of Example 1, in which 30 wt% of the prepared silicon oxide (D50: 6.7 μm) and 10 wt% of coal-based pitch binder are mechanically dry-mixed, and then mechanically chemically treated for 30 minutes using a mechanical fusion machine (Hosokawa Micron Co., AMS) at a drum rotation speed of 20 m / s and a distance of 10 mm from the internal components, and then 60 wt% of flake-shaped primary natural graphite particles (D50: 12.5 μm) are added and further mechanically chemically treated for 30 minutes, the process is carried out in the same manner as in Example 1.

[0131] (Comparative Example 2)

[0132] Except for step 3 of Example 1, in which 30 wt% of the prepared silicon oxide (D50: 6.7 μm), 60 wt% of flake-like primary natural graphite particles (D50: 12.5 μm) and 10 wt% of coal-based pitch binder are mechanically dry-mixed and then subjected to mechanochemical treatment for 60 minutes using a mechanical fusion machine (Hosokawa Micron Co., AMS) at a drum rotation speed of 20 m / s and a distance of 10 mm from the internal components, the process is carried out by the same method as in Example 1.

[0133] (Evaluation Method)

[0134] *Evaluation of interfacial resistance

[0135] The interface resistance of the negative electrode of the electrodes manufactured in Example 1, Example 2, Comparative Example 1 and Comparative Example 2 was measured using an XF057 probe unit from HIOKI Corporation of Japan under conditions of 10mA current and 0.5V voltage range.

[0136] The measurement results are shown in Table 1 below.

[0137] * pH measurement of slurries containing core-shell complexes

[0138] The pH of the slurries prepared in Example 1, Example 2, Comparative Example 1 and Comparative Example 2 was measured, and the results are shown in Table 1 below.

[0139] *Evaluation of the interfacial adhesion between the negative electrode active material layer and the current collector

[0140] The negative electrodes manufactured in Examples 1, 2, Comparative Example 1, and 2 were cut into pieces with a transverse length of 18 mm and a longitudinal length of 150 mm. A 18 mm wide adhesive tape was then applied to the foil layer of the negative electrode, and a roller with a 2 kg load was used to ensure thorough adhesion. The active material layer of the negative electrode was adhered to one side of a tensile testing machine using double-sided adhesive tape. The tape adhered to the foil was then secured to the other side of the tensile testing machine, and the adhesive force was measured. The measurement results are shown in Table 1 below.

[0141] [Table 1]

[0142]

[0143] As can be seen from Table 1, in Examples 1 and 2, where a coal-based pitch binder was added during the intermediate stage of the mechanochemical process of silicon oxide and natural graphite particles, a low slurry pH was observed, and both the electrical resistance and adhesive strength were superior to the comparative example. This is because the uniform coating of the coal-based pitch binder on the core and shell of the core-shell composite effectively inhibits the dissolution of the lithium-containing material into the slurry, resulting not only in a low slurry pH but also in low electrical resistance due to the sufficient formation of conductive pathways within the silicon oxide core caused by the coal-based pitch binder. Furthermore, it can be determined that while the viscosity of the slurry decreases and the adhesive strength decreases when coating the electrode due to the condensation of the carboxymethyl cellulose polymer chains in the binder as the slurry pH increases, the low pH of the slurry results in excellent adhesive strength.

[0144] On the other hand, it can be determined that in the case of Comparative Example 1, where coal-based pitch binder was added at the initial time point before the start of the mechanochemical process, the core-shell composite was uniformly coated with coal-based pitch binder, and the pH and adhesive strength of the slurry showed excellent results. However, the insufficient penetration into the core and the lack of conductive pathways led to an increase in resistance.

[0145] Furthermore, it can be determined that in Comparative Example 2, where silicon oxide, natural graphite particles, and coal-based pitch binder were mixed in a single step and then subjected to mechanochemical treatment, the coal-based pitch binder and silicon oxide achieved uniform bonding within the core of the core-shell composite, resulting in low resistance. However, the coating was not sufficiently applied to the shell. The lithium-containing silicon oxide directly contacted water in the water-based slurry, causing the lithium-containing substances to dissolve and raising the pH of the slurry. It can be determined that the increase in the slurry's pH reduced its viscosity, significantly decreasing the adhesion force during electrode coating, thus resulting in a significant decrease in adhesion strength.

[0146] Evaluation Example 2: Evaluation based on the characteristics of composite processes

[0147] (Comparative Examples 2 to 5)

[0148] (Comparative Example 3)

[0149] Except for step 3 of Example 1, in which 30 wt% of the prepared silicon oxide (D50: 6.7 μm) and 70 wt% of flake-like primary natural graphite particles (D50: 12.5 μm) are mechanically dry-mixed and then subjected to mechanochemical treatment for 60 minutes using a mechanical fusion machine (Hosokawa Micron Corporation, AMS) at a drum rotation speed of 20 m / s and a distance of 10 mm from the internal components, the process is carried out in the same manner as in Example 1.

[0150] (Comparative Example 4)

[0151] Except for step 3 of Example 1, in which 30 wt% of the prepared silicon oxide (D50: 6.7 μm), 60 wt% of flake-like primary natural graphite particles (D50: 12.5 μm) and 10 wt% of coal-based pitch binder are simply mechanically mixed, then heated to 600 °C at a rate of 3 °C / min in a high-purity argon atmosphere and heat-treated at 600 °C for 4 hours, the process is carried out by the same method as in Example 1.

[0152] (Comparative Example 5)

[0153] Except for step 3 of Example 1, in which 30 wt% of the prepared silicon oxide (D50: 6.7 μm) and 70 wt% of flake-like primary natural graphite particles (D50: 12.5 μm) are simply mechanically mixed, then heated to 600 °C at a rate of 3 °C / min in a high-purity argon atmosphere and heat-treated at 600 °C for 4 hours, the process is carried out by the same method as in Example 1.

[0154] (Evaluation Method)

[0155] * pH measurement of negative electrode slurry containing core-shell complex

[0156] The pH of the slurries prepared in Comparative Examples 2 to 5 was measured, and the results are shown in Table 2 below.

[0157] *Evaluation of interfacial resistance

[0158] For the half-cells manufactured in Comparative Examples 2 to 5, the interface resistance of the negative electrode was measured using the same method as in Evaluation Example 1, and the results are shown in Table 2 below.

[0159] [Table 2]

[0160]

[0161] As can be seen from Table 2, compared with Comparative Examples 2 and 3, which were coal-based pitch binders at the same time points, Comparative Examples 4 and 5, which involved simple mechanical mixing via ball milling followed by heat treatment without mechanochemical treatment, exhibited higher slurry pH. This can be attributed to the fact that the ball-milled silicon oxide and natural graphite particles were not fully combined, potentially remaining in a simple mixed state. The silicon oxide was directly exposed to the outside rather than within the core, resulting in the dissolution of a large amount of lithium-containing substances during the slurry preparation process, thus increasing the slurry pH.

[0162] Furthermore, it can be determined that, compared with Comparative Examples 2 and 4, Comparative Examples 3 and 5, which did not use coal-based pitch binder, did not have any effect on preventing the leaching of lithium-containing substances based on the shell containing coal-based pitch binder, and therefore exhibited a high slurry pH. In particular, in the case of Comparative Example 5, due to simple mechanical mixing, the silicon oxide and natural graphite particles were not sufficiently combined, and the silicon oxide was directly exposed to the slurry, thus exhibiting a high slurry pH compared with Comparative Example 3.

[0163] Furthermore, it can be confirmed that Comparative Examples 4 and 5 both exhibited higher resistance values ​​compared to Comparative Examples 2 and 3. This can be attributed to the weak composite formation of silicon oxide and natural graphite particles resulting from the simple mechanical mixing via ball milling, leading to insufficient conductive paths within the silicon oxide and thus high resistance. In particular, it can be determined that in Comparative Examples 3 and 5, which did not use a coal-based pitch binder, no conductive paths were formed that penetrated into the core of the coal-based pitch binder, resulting in even higher resistance compared to Comparative Examples 2 and 4.

[0164] Evaluation Example 3: Evaluation based on the characteristics of heat treatment temperature

[0165] (Examples 1 and Examples 3 through 7)

[0166] (Examples 3 to 7)

[0167] Except that in step 3 of embodiment 1, the heat treatment temperature is set to the heat treatment temperature shown in Table 3 below instead of 600°C, the same method as in embodiment 1 is used.

[0168] (Evaluation Method)

[0169] Raman spectroscopy analysis

[0170] Raman spectroscopy analysis was performed on the negative electrode active materials prepared in Examples 1 and 3 to 7. Specifically, an Invia confocal Raman microscope from Renishaw (UK) was used with a laser wavelength of 532 nm and a lens magnification of 50x, in static mode at 67-1800 cm⁻¹. -1 The particle surface was measured 8 times within the range and the average value was applied.

[0171] On the other hand, in the Raman spectroscopy analysis method, 515±15cm -1 The region above can be defined as the region of crystalline Si (c-Si), and the region below it can be defined as the region derived from amorphous silicon oxide (SiO2). x The region contains a mixture of amorphous Si (a-Si) and amorphous Si derived from lithium silicate. The evaluation results are recorded in Table 3 below.

[0172] *Analysis of Si crystallite size in silicon compounds based on X-ray diffraction analysis

[0173] X-ray diffraction analysis was performed on the composites prepared in Examples 1 and 3 through 7, specifically using an Empyrean XRD diffractometer from PANalytical, with a current of 40 mA applied at a voltage of 45 kV. Specifically, the full width at half maximum (FWHM) of the diffraction peaks based on the Si(111) crystal plane (2θ = 28.4 ± 0.3°) was obtained using Cu-Kα line X-ray diffraction. The Si crystallite size was analyzed using the Scherrer equation, and the results are shown in Table 3 below.

[0174] Scherrer equation: τ=(Kλ) / (βcosθ)

[0175] K: Dimensionless shape factor, 0.9

[0176] λ: X-ray wavelength, 0.1540598 nm

[0177] β: Full width at half maximum

[0178] θ: Bragg angle

[0179] *Evaluation of lifespan characteristics

[0180] At room temperature (25°C), the half-cells manufactured in Examples 1 and 3 to 7 were charged with a constant current at a rate of 0.1C until the voltage reached 0.01V (vs. Li). Then, while maintaining the voltage at 0.01V in constant voltage mode, the charge was cut off at a rate of 0.01C, thus charging at a constant voltage. The cells were then discharged with a constant current at a rate of 0.1C until the voltage reached 1.5V (vs. Li). This charge and discharge cycle was considered as one cycle. Another cycle of charge and discharge was performed in the same manner. Then, the applied current during charge and discharge was changed to 0.5C and 50 cycles were performed, with a 10-minute rest period between cycles. The lifetime characteristics were measured as the capacity retention rate (%) relative to the discharge capacity of one cycle, and the results are shown in Table 3 below.

[0181] [Table 3]

[0182]

[0183] As can be seen from Table 3, when the heat treatment temperature is below 700℃, the maximum peak position of the Raman spectrum is 461-471 cm⁻¹. -1 Therefore, the formation of the Si crystalline phase is not obvious. On the other hand, it can be confirmed that when the heat treatment temperature is 800-1000℃, the maximum peak position of the Raman spectrum is 500 cm⁻¹. -1 Therefore, this promotes the formation of the Si crystalline phase.

[0184] Furthermore, it can be confirmed that the amorphous properties of Si within silicon oxide are well preserved when the heat treatment temperature is below 700°C, and it can also be confirmed that the size of the formed Si crystallites increases with increasing temperature when the heat treatment temperature exceeds 700°C.

[0185] It can be determined that, under the heat treatment temperature of 600℃ in Example 1, the amorphous properties of Si within the silicon oxide are well maintained while the carbonization of the coal-based pitch binder is fully achieved. Furthermore, the carbonized coal-based pitch binder is uniformly coated on both the core and shell of the core-shell composite, thus exhibiting a low slurry pH and high capacity retention. This is because the well-maintained amorphous properties of Si suppress the volume expansion of the composite caused by the growth of c-Si (crystalline Si) within the silicon oxide, effectively inhibiting the degradation of the negative electrode active material. Additionally, the carbonized coal-based pitch binder acts as a buffer to reduce the volume expansion at the interface between the silicon oxide and natural graphite particles within the core, thereby suppressing capacity reduction. Moreover, the uniformly formed shell inhibits the dissolution of lithium-containing substances into the slurry, effectively suppressing the rise in pH.

[0186] It can be determined that, in the case of Example 3, as the heat treatment temperature decreases to 500°C, the carbonization of the coal-based pitch binder is not fully achieved, the conductivity of the core-shell composite decreases, and thus the conductive path is reduced and the capacity retention is low during charge and discharge.

[0187] It can be determined that, in Example 4, with the heat treatment temperature increased to 700°C, the coal-based pitch binder was fully carbonized, and a high capacity retention rate was observed as the conductivity of the core-shell composite increased. However, as the heat treatment temperature increased, the Si phase within the silicon oxide crystallized, reducing the structural stability of the negative electrode active material containing the silicon oxide, thus resulting in a lower capacity retention rate compared to Example 1.

[0188] It can be determined that, in the cases of Examples 5 to 7, as the heat treatment temperature was increased to 800°C, 900°C and 1000°C respectively, the crystallization of the Si phase became more severe. Therefore, as the temperature increased, the capacity retention rate based on the volume expansion of the composite gradually decreased.

[0189] Evaluation Example 4: Measurement of Lithium Content and Lithium Silicate Content in Core-Shell Composites

[0190] (Examples 1, 8 to 10, and Comparative Example 2)

[0191] (Examples 8 to 10)

[0192] Except that in step 2 of Example 1, the Li / Si molar ratio and the weight % of the coal-based pitch binder are set to the molar ratios shown in Table 4 below, the same method as in Example 1 is used.

[0193] (Evaluation Method)

[0194] *Inductively Coupled Plasma Emission Spectroscopy of Core-Shell Complexes Spectrometer (ICP) analysis

[0195] The half-cells manufactured in Examples 1, 8 to 10 and Comparative Example 2 were disassembled, washed and dried, and subjected to ICP analysis of the core-shell composite. The lithium content (wt%) is shown in Table 4 below.

[0196] * Measurement of lithium silicate content

[0197] The core-shell composites prepared in Examples 1, 8 to 10, and Comparative Example 2 were subjected to X-ray diffraction (XRD) analysis. XRD analysis was performed using an Empyrean XRD diffractometer from PANalytical, with measurements taken at a voltage of 45 kV and a current of 40 mA. Analysis of each phase was compared using JCPDS card numbers 98-002-9287 (Si), 98-002-8192 (Li₂SiO₃), 98-028-0481 (Li₂Si₂O₅), and 98-003-5169 (Li₄SiO₄). The results confirmed peaks at 22.2 ± 0.3° for Li₄SiO₄ (110), 24.9 ± 0.3° for Li₂Si₂O₅ (111), and 26.9 ± 0.3° for Li₂SiO₃ (111).

[0198] For each lithium silicate, the area of ​​each peak was calculated using the Rietveld method for quantitative analysis. The results are summarized in Table 4 below.

[0199] [Table 4]

[0200]

[0201] (In Table 4, lithium content is % by weight relative to the total amount of the core-shell composite, and lithium silicate content is % by weight relative to the total amount of silicon oxide in the core-shell composite.)

[0202] As can be seen from Table 4, under the same core-shell composite preparation conditions (Examples 1, 8, and 9), the lithium content (A) in the core-shell composite tends to be proportional to the Li / Si molar ratio during Li pretreatment. In the case of Example 9, although the Li / Si molar ratio increased by 33% compared to Example 1, the increase in the lithium silicate content (B) in the core-shell composite was not significant. It can be determined that this is because when the Li / Si molar ratio is 1, more Li4SiO4 phase is formed compared to Li2SiO3 or Li2Si2O5, thus reducing the lithium silicate content.

[0203] Furthermore, in the case of Example 10, when preparing the core-shell composite, as the content of the coal-based pitch binder decreased, the lithium content in the composite was lower than in Example 1, despite having the same Li / Si molar ratio. It can be determined that this is because the content of amorphous carbon in the formed composite decreased, thus reducing the shell's effectiveness in inhibiting the dissolution of lithium-containing substances.

[0204] In Example 11, the content of coal-based pitch binder was increased during the preparation of the core-shell composite, but the lithium content was similar to that of Example 1, which had the same Li / Si molar ratio. It can be determined that this is because when the content of amorphous carbon contained in the composite shell exceeds a certain amount (8% by weight), the effect of inhibiting the dissolution of lithium-containing substances does not increase further.

[0205] In Comparative Examples 2 and 3, the addition of coal-based pitch binder at the beginning of the process or the absence of coal-based pitch binder resulted in greater leaching of lithium-containing substances, exhibiting a low lithium content despite having the same Li / Si molar ratio as Example 1. In particular, in Comparative Example 3, since no coal-based pitch binder was included, no shell was formed, resulting in a very low lithium content in the composite.

[0206] On the other hand, based on the results of Examples 1 and 8 to 9, it is known that the preferred range of lithium content in the composite is 1.3% by weight or more, preferably 1.3-5% by weight, and more preferably 1.3-4% by weight.

[0207] The embodiments of the present invention have been described above, but the present invention is not limited to the described embodiments. It can be manufactured in various different ways, and those skilled in the art will understand that it can be implemented in other specific ways without changing the technical concept or essential technical features of the present invention. Therefore, it should be understood that the above embodiments are exemplary and not limiting in all respects.

Claims

1. A negative electrode active material for lithium secondary batteries, characterized in that, The core-shell complex comprises: a core comprising a silicon oxide containing a lithium-containing material and a graphite-based material, wherein the silicon oxide is represented by SiO x where 0 < x ≤ 2; and A shell, located on the core, and comprising amorphous carbon. The silicon oxide comprises, in at least a portion thereof, lithium silicate, which is a combination of Li2SiO3, Li2Si2O5 and Li4SiO4. The silicon oxide contains less than 1 part by weight of Li4SiO4 relative to 100 parts by weight of the silicon oxide. In the core-shell composite, the ratio of the weight percentage of lithium A in the core-shell composite to the weight percentage of lithium silicate B in the silicon oxide, A / B, is 0.020 or higher. The shell has an average thickness of 1-5 nm, and The Raman spectrum of the lithium-containing silicon oxide showed a maximum peak position of 461-471 cm⁻¹. -1 Furthermore, in the lithium-containing silicon oxide, the content of amorphous Si is higher than the content of crystalline Si.

2. The negative electrode active material according to claim 1, characterized in that, The lithium-containing substance is selected from at least one of LiOH, Li, LiH, Li2O and Li2CO3.

3. The negative electrode active material according to claim 1, characterized in that, The core also contains amorphous carbon.

4. The negative electrode active material according to claim 1, characterized in that, The graphite-based material is natural graphite, artificial graphite, or a combination thereof.

5. The negative electrode active material according to claim 1, characterized in that, The silicon oxide comprises 5-50 parts by weight relative to 100 parts by weight of the core-shell composite.

6. The negative electrode active material according to claim 1, characterized in that, The core-shell composite contains 30-80 parts by weight of the graphite-based material relative to 100 parts by weight.

7. The negative electrode active material according to claim 1, characterized in that, The core-shell complex comprises 50 or more parts by weight relative to 100 parts by weight of the negative electrode active material.

8. A method for preparing the negative electrode active material according to claim 1, characterized in that, Includes the following steps: a) Prepare silicon oxide containing lithium-containing substances; b) A core-shell composite precursor is prepared by combining silicon oxide, graphite-based material and carbon precursor containing the lithium-containing material; as well as c) The core-shell composite precursor is heat-treated at a temperature of 550-700°C to prepare the core-shell composite. Step b) of the compounding process includes a dry mixing step that applies shear stress and centrifugal force, during which the carbon precursor is added at an intermediate time point. The silicon oxide contains, in at least a portion thereof, lithium silicate in the form of a combination of Li₂SiO₃, Li₂Si₂O₅ and Li₄SiO₄. Of which, relative to 100 parts by weight of silicon oxide, the silicon oxide contains less than 1 part by weight of Li4SiO4, and The intermediate time point refers to the time point between 10% and 90% of the cumulative time of 100% dry mixing.

9. The method for preparing the negative electrode active material according to claim 8, characterized in that, The composite process in step b) is carried out through mechanochemical treatment.

10. The method for preparing the negative electrode active material according to claim 8, characterized in that, Step c) is carried out in a non-reactive atmosphere.

11. The method for preparing the negative electrode active material according to claim 8, characterized in that, In step a), the silicon compound and lithium precursor are mixed and then subjected to heat treatment.

12. The method for preparing the negative electrode active material according to claim 8, characterized in that, The lithium-containing substance is selected from at least one of LiOH, Li, LiH, Li2O and Li2CO3.

13. A negative electrode for a lithium secondary battery, comprising the negative electrode active material according to any one of claims 1 to 7.

14. A lithium secondary battery, comprising: The negative electrode as described in claim 13; the positive electrode; and the membrane located between the negative electrode and the positive electrode; And electrolyte.

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