Negative electrode for secondary battery and secondary battery comprising same

Abstract

The invention discloses a negative electrode for a secondary battery and a secondary battery including the same. The present invention provides a negative electrode for a secondary battery satisfying the following relational expression 1. [relational expression 1] (V1-V2) / V1100 > = 45% where V1 is the volume fraction (vol%) of the silicon-based active material particles having a gradation value of 30,000 or more when XRM measurement is performed on the negative electrode in a fully discharged state after chemical conversion, and V2 is the volume fraction (vol%) of the silicon-based active material particles having a gradation value of 30,000 or more when XRM measurement is performed on the negative electrode in a fully charged state after 500 cycles of charge and discharge. The negative electrode for a secondary battery according to the present invention can effectively suppress an electrical short circuit occurring between silicon-based active material particles due to volume expansion of the silicon-based active material particles occurring during repeated charge and discharge, thereby improving the ratio of the silicon-based active material capable of maintaining an active state during repeated charge and discharge cycles.

Description

[0001] This application is a divisional application of Chinese patent application No. 202211660417.1, filed with the Chinese Patent Office on December 23, 2022, entitled “Negative electrode for secondary battery and secondary battery including the same”. Technical Field

[0002] This invention relates to a negative electrode for a secondary battery and a secondary battery including the same. Background Technology

[0003] Recently, with the issue of global warming and the need to address it, the demand for environmentally friendly technologies is surging. In particular, with the growing demand for electric vehicles and ESS (energy storage systems), the demand for lithium-ion batteries, which are receiving much attention as energy storage devices, is also experiencing explosive growth.

[0004] Therefore, research is underway to improve the energy density of lithium-ion rechargeable batteries. Existing commercial lithium-ion rechargeable batteries generally use graphite-based active materials such as natural graphite and artificial graphite as anode materials. However, due to the low theoretical capacity of graphite (372 mAh / g), these batteries suffer from low energy density. Therefore, research is underway to develop new anode materials that can replace existing graphite-based anode materials to improve battery energy density.

[0005] As a solution to this problem, silicon-based anode materials with high theoretical capacity (3580 mAh / g) have attracted attention. However, silicon-based anode materials suffer from a decline in battery life due to significant volume expansion (~400%) during repeated charge-discharge cycles. Therefore, as a method to address the issue of large volume expansion in silicon-based anode materials, SiO2, which has a lower volume expansion rate than Si, has been developed. x Anode materials, but due to the formation of an irreversible phase in the early stages of battery operation, they suffer from low initial coulombic efficiency (ICE).

[0006] Prior technology literature

[0007] [Patent Documents]

[0008] (Patent Document 1) Korean Patent Publication No. 10-1996-0041439 (Publication Date: December 19, 1996) Summary of the Invention

[0009] Technical issues

[0010] An object of the present invention is to provide a negative electrode for a secondary battery that improves initial efficiency and capacity, and improves life characteristics by suppressing the transformation into an inactive state due to the volume expansion of silicon-based active material particles that occurs during repeated charge and discharge processes.

[0011] Technical Solution

[0012] As a technical solution to solve the above problems, according to an embodiment of the present invention, a negative electrode for a secondary battery satisfying the following relational expression 1 is provided.

[0013] [Relational Expression 1]

[0014] (V1 - V2) / V1 100 ≥ 45%

[0015] In the relational expression 1, V1 is the volume fraction (vol%) of silicon-based active material particles with a gray value of 30,000 or more when performing XRM measurement on the negative electrode in a fully discharged state after formation, and V2 is the volume fraction (vol%) of silicon-based active material particles with a gray value of 30,000 or more when performing XRM measurement on the negative electrode in a fully charged state after 500 charge-discharge cycles.

[0016] Moreover, for the negative electrode for a secondary battery according to an embodiment of the present invention, the silicon-based active material may include one or a combination of Si, SiOx (0 < x ≤ 2), Si-containing alloys, and Si / C composites.

[0017] Moreover, for the negative electrode for a secondary battery according to an embodiment of the present invention, the negative electrode may further include a carbon-based active material.

[0018] Moreover, for the negative electrode for a secondary battery according to an embodiment of the present invention, the carbon-based active material may include crystalline carbon.

[0019] Moreover, for the negative electrode for a secondary battery according to an embodiment of the present invention, the negative electrode may include an acrylic-based binder.

[0020] Moreover, for the negative electrode for a secondary battery according to an embodiment of the present invention, the negative electrode includes a carbon nanotube conductive material, and the conductive material may be included in an amount less than 5% by weight based on the total weight of the negative electrode active material layer.

[0021] Moreover, for the negative electrode for a secondary battery according to an embodiment of the present invention, the conductive material may include single-walled carbon nanotubes (SWCNT).

[0022] Moreover, as another technical solution to solve the above problems, according to another embodiment of the present invention, a secondary battery including the aforementioned negative electrode is provided.

[0023] [[ID=

[0024] The negative electrode for secondary batteries according to the present invention can improve initial efficiency and capacity.

[0025] Furthermore, according to the negative electrode for a secondary battery of the present invention, by controlling the following relationship 1 to be satisfied, the ratio of silicon-based active materials that can effectively suppress electrical short circuits between silicon-based active material particles caused by volume expansion during repeated charge and discharge processes, and thus maintain an activated state under repeated charge and discharge cycles, can be increased. This improves the stability and lifespan characteristics of the secondary battery.

[0026] [Relation 1]

[0027] (V1-V2) / V1 100 ≥ 45%

[0028] In Equation 1, V1 is the volume fraction (vol%) of silicon-based active material particles with a gray value of 30,000 or higher when XRM measurement is performed on the negative electrode in the fully discharged state after formation, and V2 is the volume fraction (vol%) of silicon-based active material particles with a gray value of 30,000 or higher when XRM measurement is performed on the negative electrode in the fully charged state after 500 charge-discharge cycles. Attached Figure Description

[0029] Figure 1a This is a schematic diagram showing the X-ray microscopy (XRM) analysis results of the negative electrode of Example 1 after 500 cycles in a fully charged state.

[0030] Figure 1b This is a schematic diagram showing the X-ray microscopy (XRM) analysis results of the negative electrode of Example 6 after 500 cycles in a fully charged state. Detailed Implementation

[0031] The advantages, features, and implementation methods of the present invention will become clear from the following detailed description of the embodiments in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but can be implemented in many different forms. The embodiments provided here are intended to make the disclosure of the present invention complete and to fully inform those skilled in the art of the scope of the invention, which is defined only by the scope of the claims. The following detailed description of the specific contents used to implement the present invention is provided with reference to the accompanying drawings. Unless otherwise specified in the drawings, the same reference numerals refer to the same constituent elements, and "and / or" includes each mentioned item and all combinations thereof.

[0032] Unless otherwise defined, all terms used in this specification (including technical and scientific terms) are to be used in a meaning consistent with that understood by one of ordinary skill in the art to which this invention pertains. Throughout this specification, when a part "comprises" a constituent element, it means, unless specifically stated otherwise, that other constituent elements may be included, rather than excluded. Furthermore, unless specifically stated otherwise, the singular form also includes the plural form.

[0033] In this specification, when describing layers, films, regions, plates, etc., as being located "above" or "on top of" another part, this not only means being "directly above" another part, but also includes situations where there are other parts in between.

[0034] This implementation example provides a negative electrode for a secondary battery that satisfies the following relation 1.

[0035] [Relation 1]

[0036] (V1-V2) / V1 100 ≥ 45%

[0037] V1 is the volume fraction (vol%) of silicon-based active material particles with a grayscale value of 30,000 or higher when XRM measurement is performed on the negative electrode in a fully discharged state after formation, and V2 is the volume fraction (vol%) of silicon-based active material particles with a grayscale value of 30,000 or higher when XRM measurement is performed on the negative electrode in a fully charged state after 500 charge-discharge cycles.

[0038] The volume fraction is calculated based on the overall volume of the negative electrode active material particles and pores derived from X-ray microscopy (XRM) image analysis of the negative electrode. The XRM image analysis is performed non-destructively on the negative electrode using 3D XRM analysis, employing a Zeiss Versa 520XRM instrument. The XRM image analysis conditions are voxel 350nm and power 5W (60 kV).

[0039] In Equation 1, V1 is derived from XRM measurement of the negative electrode in the fully discharged state after formation, i.e., SOC 0%. V1 represents the volume fraction (vol%) of silicon-based active material in the unreacted state after formation. The total amount of silicon-based active material that can participate in the battery reaction after formation can be quantitatively evaluated based on V1.

[0040] On the one hand, the formation process is the step of forming a solid electrolyte interface (SEI) film on the surface of the negative electrode through initial charge and discharge. This is not particularly limited as long as it is a method generally used in this technical field. After formation, a negative electrode at 0% SOC can be obtained through a complete discharge process. As an example, the discharge process is performed at a discharge rate of 0.33C and a cut-off condition of 2.5V, but it is not limited to this.

[0041] In Equation 1, V2 is derived by XRM measurement of the negative electrode after 500 charge-discharge cycles at a fully charged state (SOC 100%). V2 represents the volume fraction (vol%) of unreacted silicon-based active material particles that cannot participate in the battery reaction and are in an electrical short-circuit state after 500 cycles. V2 can be used to quantitatively evaluate the total amount of inactive silicon-based active material that cannot participate in the battery reaction due to repeated charge-discharge cycles. The charge-discharge conditions for these cycles are as follows: charging can be performed at 0.33C and 4.2V cut-off, and discharging can be performed at 0.5C and 2.5V cut-off. The conditions for full charging after 500 cycles are not specifically limited; as a non-limiting example, charging at 0.33C under 4.2V cut-off conditions is acceptable.

[0042] According to the present invention, the relationship between V1, which can quantitatively evaluate the total amount of silicon-based active materials that can participate in the reaction, and V2, which can quantitatively evaluate the total amount of inactive silicon-based active materials that cannot participate in the battery reaction due to repeated charge-discharge cycles, is Equation 1, thereby increasing the ratio of silicon-based active materials that can remain in an active state even during repeated charge-discharge cycles.

[0043] Specifically, (V1-V2) can quantitatively evaluate the total amount of silicon-based active material in an activated state that can participate in the battery reaction even after repeated charge-discharge cycles. According to the present invention, by satisfying Equation 1, which represents the ratio of the total amount of silicon-based active material in an activated state (V1-V2) relative to the total amount of silicon-based active material (V1), the ratio of silicon-based active material in an activated state that can be maintained during repeated charge-discharge cycles can be increased. Therefore, the battery life characteristics can be significantly improved.

[0044] The active state can be defined as a state in which an electrical short circuit does not occur between silicon active material particles during repeated charging and discharging processes due to the volume expansion of the silicon active material particles, and an electrochemical reaction can continue to occur while maintaining the state of the electrical network.

[0045] From the perspective of further improving the life characteristics of the battery, the value of (V1 - V2) / V1 in Formula 1 of 100 may be 50% or more, or 55% or more, or 60% or more.

[0046] The silicon-based active material may include one or a combination of Si, SiOx (0 < x ≤ 2), Si-containing alloy, and Si / C composite.

[0047] The Si-containing alloy is not particularly limited, but may be expressed as, for example, Si-Q alloy. The Q is an element selected from the group consisting of 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, excluding Si. The element Q may be, for example, an element selected from the group consisting of 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.

[0048] The negative electrode according to an embodiment of the present invention includes a current collector and a negative electrode active material layer located on the current collector and containing a negative electrode active material and a binder.

[0049] The negative electrode active material may further include a carbon-based active material. Specifically, among the carbon-based active materials, any material that can reversibly intercalate / deintercalate lithium ions can be used. Specific examples may include one or a mixture of crystalline carbon and amorphous carbon. As non-limiting examples, the amorphous carbon may be soft carbon (low-temperature sintered carbon), hard carbon, coke, mesocarbon microbead (MCMB) sintered at 1500°C or lower, and mesophase pitch-based carbon fiber (MPCF), or a combination thereof. As non-limiting examples, the crystalline carbon may be natural graphite, artificial graphite, expanded graphite, etc., graphite, carbon black, graphene, fullerene soot, carbon nanotubes, carbon nanowires, carbon fibers, and micro / meso / macro porous carbon, or a combination thereof. The carbon-based active material may be used in a spherical, plate-like, fibrous, tubular, or powder form.

[0050] According to one embodiment of the invention, the negative electrode can use a mixture of silicon-based active material and carbon-based active material in a weight ratio of 1 to 100:99 to 0. Therefore, high capacity characteristics can be achieved, and excessive volume expansion of the silicon-based active material during repeated charge and discharge can be buffered. More specifically, the mixture can be used in weight ratios of 5 to 100:95 to 0, or 1 to 50:99 to 50, or 5 to 50:95 to 50, or 1 to 30:99 to 70, or 5 to 30:95 to 70, or 1 to 20:99 to 80, or 5 to 20:95 to 80.

[0051] The adhesive is not particularly limited, but an acrylic adhesive is preferred. The acrylic adhesive may include, for example, one or more of the group consisting of polyacrylic acid (PAA), polyacrylamide, and polyvinyl alcohol coacrylic acid. Therefore, it can function as a slurry for the active materials within the negative electrode, enabling the active materials to adhere to each other and to adhere to the current collector. Furthermore, its excellent adhesive strength allows for good electrical contact between the active materials even during volume changes in the silicon-based active materials within the negative electrode.

[0052] In order to efficiently achieve the above-mentioned effects, the content of the acrylic adhesive may be, for example, 0.1 to 30% by weight or 1 to 10% by weight of the total weight of the negative electrode active material layer.

[0053] The conductive material may include, for example, multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs). MWCNTs consist of 10 or more walls, resulting in a larger diameter and more surface defects. Compared to single-walled carbon nanotubes, which are composed of an average of 1-3 walls, MWCNTs have lower conductivity and therefore require a higher content. Here, the content of the conductive material may be less than 10% by weight, less than 5% by weight, or 0.5 to 1% by weight of the total weight of the negative electrode active material layer. Single-walled carbon nanotubes have high conductivity, so even when the content is reduced to one-tenth of that of typical conductive materials, the conductivity between silicon-based active material particles within the negative electrode can be stably maintained. Specifically, by including single-walled carbon nanotube conductive material, the electrical network between the negative electrode active material particles can be maintained even when the volume of silicon-based active material particles expands during repeated charge-discharge processes, thereby improving lifetime characteristics. Here, the content of the conductive material may be less than 5% by weight, less than 1% by weight, or 0.05 to 0.5% by weight of the total weight of the negative electrode active material layer.

[0054] According to this embodiment, the negative electrode can be manufactured by coating a current collector with a negative electrode slurry containing the aforementioned negative electrode active material, binder, conductive material and solvent, and then drying it.

[0055] More specifically, the negative electrode slurry is preferably prepared by dispersing the conductive material in the binder solution and then mixing it with the aforementioned negative electrode active material. This ensures the phase stability of the final negative electrode slurry while uniformly dispersing the conductive material within it, thereby efficiently exhibiting the aforementioned effects of the conductive material.

[0056] On the one hand, the adhesive solution refers to a mixture in which the adhesive is not dissolved but exists in particulate form in the solvent, and thickeners or the like can be added as needed for use. Here, the content of solid components in the adhesive solution other than the solvent can be 0.5 to 50% by weight, or 10 to 40% by weight.

[0057] The solvent may be at least one selected from the group consisting of water, pure water, deionized water, ethanol, isopropanol, methanol, acetone, n-propanol and t-butanol, but is not limited thereto.

[0058] The thickener may be a cellulose compound, specifically a mixture of at least one of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or their alkali metal salts. The alkali metal may be Na, K, or Li.

[0059] The current collector may be a material selected from, but not limited to, copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with conductive metal, and combinations thereof.

[0060] The coating can be applied using any coating method known to be commonly used for coating liquids to form thin films. For example, spraying, dip coating, spin coating, concave coating, grooved coating, blade coating, roller coating, inkjet printing, elastic printing, screen printing, electrodynamic printing, micro-contact printing, embossing, reverse offset printing, bar coating, gravure offset printing, etc., but are not limited to these.

[0061] The drying process can be carried out at a temperature of 80 to 130°C, preferably 100 to 130°C, for 10 to 50 minutes, or more preferably 15 to 30 minutes.

[0062] Next, a negative electrode can be manufactured by rolling the dried negative electrode to an appropriate density to form a layer of negative electrode active material on the current collector.

[0063] This embodiment also provides a secondary battery including the negative electrode. The secondary battery may include a negative electrode; a positive electrode; a separator and an electrolyte between the negative electrode and the positive electrode.

[0064] The negative electrode is the same as described above.

[0065] The positive electrode may include a current collector and a layer of positive electrode active material formed by coating the current collector with a positive electrode slurry containing positive electrode active material.

[0066] The current collector can be the aforementioned negative electrode current collector, or any material known in the art, but the present invention is not limited thereto.

[0067] The positive electrode active material layer includes a positive electrode active material, and optionally may also include a binder and a conductive material. It is permissible to use a positive electrode active material known in the art, such as a composite oxide of a metal selected from cobalt, manganese, nickel, or combinations thereof, and lithium, which is preferred, but the invention is not limited thereto.

[0068] The adhesive may be used without particular limitation as long as it is commonly used in this technical field. For example, it may include organic adhesives such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, and polymethyl methacrylate, or water-based adhesives such as styrene-butadiene rubber (SBR), and may be used with thickeners such as carboxymethyl cellulose (CMC).

[0069] The conductive material is used to impart conductivity to the electrodes, and any electronically conductive material that will not cause chemical changes within the battery can be used. Examples of conductive materials include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, and carbon fiber; metallic materials such as metal powders or fibers of copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; or conductive materials of mixtures thereof.

[0070] The diaphragm is not particularly limited as long as it is a diaphragm known in the art. For example, it can be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene or combinations thereof, and can be in non-woven or woven form, and can be used selectively in single-layer or multi-layer structure.

[0071] The electrolyte comprises a non-aqueous organic solvent and an electrolytic salt. The non-aqueous organic solvent may be ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (EMC), 1,2-dimethoxyethane (DME), γ-butyrolactone (BL), tetrahydrofuran (THF), 1,3-dioxolane (DOL), diethyl ether (DEE), methyl formate (MF), methyl propionate (MP), sulfolane (S), dimethyl sulfoxide (DMSO), acetonitrile (AN), or mixtures thereof, but is not limited thereto. The electrolytic salt is dissolved in the non-aqueous organic solvent and serves as a source of electrolytic metal ions in the battery, enabling the secondary battery to operate essentially and promoting the movement of electrolytic metal ions between the positive and negative electrodes. In a non-limiting example, when the electrolytic metal is lithium, the electrolytic salt may be LiPF6, LiBF4, LiTFSI, LiSbF6, LiAsF6, LiClO4, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, LiSbF6, LiAlO4, LiAlCl4, or LiN(C x F 2x+1 SO2)(C y F 2y+ The electrolyte salt may contain, but is not limited to, 1SO2 (where x and y are natural numbers), LiCl, LiI, or mixtures thereof. Furthermore, the electrolyte salt may use known substances at concentrations appropriate to the purpose, and may also include, as needed, known solvents or additives for improving charge / discharge characteristics, flame retardant properties, etc.

[0072] The present invention will now be described in detail with reference to the embodiments, but this is only for the purpose of illustrating the present invention in more detail, and the scope of the present invention is not limited to the following embodiments.

[0073] Example

[0074] (Example 1)

[0075] Step 1: Preparation of negative electrode slurry

[0076] Simultaneously, 83% by weight of artificial graphite and SiO were mixed in. x A negative electrode slurry with a solid content of 43% by weight was prepared by using 13% by weight of acrylic acid (PAA) binder, 3% by weight of MWCNT (C-nano Co., Ltd.) and 1% by weight of MWCNT (C-nano Co., Ltd.).

[0077] Step 2: Manufacturing the negative electrode

[0078] The negative electrode slurry prepared in step 1 was coated onto a copper current collector (8 μm thick copper foil) using a grooved coating process. The negative electrode active material layer was then dried in a drying oven heated to 120°C for 30 minutes to complete the drying process. Here, the thickness of the negative electrode active material layer was set to 50 μm.

[0079] Step 3: Manufacturing the positive electrode

[0080] Will Li[Ni 0.88 Co 0.1 Mn 0.02 A positive electrode slurry was prepared by mixing O2 as the positive electrode active material, carbon black as the conductive material, and polyvinylidene fluoride (PVdF) as the binder in a weight ratio of 96.5:2:1.5. The positive electrode slurry was uniformly coated onto a 12 μm thick aluminum foil and then vacuum dried to manufacture a positive electrode for secondary batteries.

[0081] Step 4: Manufacturing the secondary battery

[0082] The positive and negative electrodes are notched to a predetermined size and stacked. A separator (polyethylene, 13 μm thick) is placed between the positive and negative electrodes to form an electrode cell. The tabs of the positive and negative electrodes are then welded onto each other. The welded positive / separator / negative electrode assembly is placed in a bag, and the three sides except for the electrolyte filling section are sealed. The portion with the electrode tabs is included in the sealed section.

[0083] After injecting electrolyte into the remaining surfaces other than the sealing part, the remaining surfaces were sealed and then immersed for more than 12 hours.

[0084] An electrolyte was prepared by dissolving 1M LiPF6 in a mixed solvent of EC / EMC / DEC (25 / 45 / 30; volume ratio), followed by the addition of 1wt% vinylene carbonate (VC) and 3wt% fluoroethylene carbonate (FEC).

[0085] (Example 2)

[0086] In addition to using 83.4% by weight of artificial graphite and SiO2, x Except for the negative electrode, which was manufactured using a negative electrode slurry containing 13% by weight, 2% by weight, SBR binder, 1.5% by weight, CMC thickener, and 0.1% by weight, SWCNT (Ocsial) was used to manufacture the negative electrode, the negative electrode, positive electrode, and secondary battery were manufactured in the same manner as in Example 1.

[0087] (Example 3)

[0088] In addition to using 83.9% by weight of artificial graphite and SiO2,x Except for the negative electrode, which was manufactured using a negative electrode slurry containing 13% by weight of acrylic (PAA) binder, 3% by weight of polyacrylic acid (PAA) binder, and 0.1% by weight of SWCNT (Ocsial Co., Ltd.), the negative electrode, positive electrode, and secondary battery were manufactured in the same manner as in Example 1.

[0089] (Comparative Example 1)

[0090] Except for the negative electrode slurry containing 2% by weight of SBR binder and 1% by weight of CMC thickener instead of 3% by weight of polyacrylic acid (PAA) binder, the negative electrode, positive electrode and secondary battery were manufactured in the same manner as in Example 1.

[0091] [Evaluation Example 1] Evaluation of lifespan characteristics based on the type of adhesive and conductive agent

[0092] The secondary batteries manufactured in Examples 1-3 and Comparative Example 1 were charged at 0.25C and 4.2V, discharged at 0.25C and 2.5V, and then charged at 0.25C to 3.5V. After a formation process, they were fully discharged at 0.33C and 2.5V cut-off. After formation, XRM analysis was performed on the negative electrode (state 1) in the fully discharged state, and the volume fraction (V1, vol%) of silicon-based active material particles with a gray value of 30,000 or higher was derived. The results are shown in Table 1 below.

[0093] Subsequently, 500 charge-discharge cycles were performed under the following conditions: charging condition: 0.33C, 4.2V cut-off; discharging condition: 0.5C, 2.5V cut-off. After measuring the capacity retention (%), a full charge was performed under the 0.33C, 4.2V cut-off condition. After the 500 charge-discharge cycles, XRM analysis was performed on the fully charged negative electrode (state 2), and the volume fraction (V2, vol%) of silicon-based active material particles with a grayscale value of 30,000 or higher was derived. The results are shown in Table 1 below.

[0094] The volume fractions V1 and V2 were calculated using the grayscale values ​​of X-ray microscopy (XRM) images. The XRM microscope used was a Zeiss Versa 520 XRM system. The XRM analysis conditions were as follows:

[0095] - Voxel: 350nm

[0096] - Power: 60kV, 5W

[0097] Table 1

[0098]

[0099] In Table 1, State 1 represents the negative electrode in the fully discharged state (SOC 0%) after formation, and State 2 represents the negative electrode in the fully charged state (SOC 100%) after 500 charge-discharge cycles.

[0100] The results in Table 1 confirm that ((V1-V2) / V1) When the percentage of silicon-based active material (SAP) is 45% or higher, meaning that in Examples 1 to 3 satisfying Equation 1, the capacity retention rate is 80% or higher compared to Comparative Example 1, exhibiting superior lifespan characteristics. The results show that when the negative electrode satisfies Equation 1, the proportion of silicon-based active material that remains active during repeated charge-discharge cycles is relatively high, thus resulting in excellent battery lifespan characteristics.

[0101] Furthermore, in Examples 2-3 using SWCNT conductive material, it can be confirmed that compared to Example 1 using MWCNT conductive material, the ((V1-V2) / V1) The 100) value is higher, and it exhibits better capacity retention. In SWCNTs, the CNTs consist of 1-3 walls, resulting in a very small diameter and very high conductivity. Therefore, even during 500 cycles, SiO2... x The volume expansion of the particles can also effectively preserve the SiO₂. x The electrical contact between particles makes it easier to ensure excellent lifetime characteristics compared to MWCNTs. Conversely, it is judged that MWCNTs are composed of more than 10 walls, resulting in a large diameter and many surface defects. Therefore, even if the content is increased 10 times compared to SWCNTs, the above-mentioned effect cannot be achieved, and its lifetime characteristics are more inferior.

[0102] Furthermore, in Example 3 using an acrylic adhesive, it was confirmed that it exhibited superior lifetime characteristics compared to Example 2 using an SBR / CMC adhesive. This result is attributed to the acrylic adhesive causing SiO2 to form during repeated charge-discharge processes. x When the particle volume changes, it exhibits stronger adhesion compared to SBR / CMC adhesives, thus maintaining SiO2 more stably. x Electrical contact between particles.

[0103] [Evaluation Example 2] Evaluation of the lifetime characteristics based on the preparation method of the negative electrode slurry

[0104] (Example 4)

[0105] In addition to dispersing MWCNTs (C-nano) in an acrylic (PAA) binder solution, 83% by weight of artificial graphite and SiO2 were added.x Except for the preparation of the negative electrode slurry (13% by weight), the negative electrode, positive electrode, and secondary battery were manufactured in the same manner as in Example 1. The negative electrode slurry prepared here contained 1% by weight of MWCNT conductive material and 3% by weight of polyacrylic acid (PAA) binder.

[0106] (Example 5)

[0107] In addition to dispersing 0.1 wt% SWCNT (Ocsial) in the SBR / CMC binder solution, 83.4 wt% artificial graphite and SiO2 were added. x Except for the preparation of the negative electrode slurry (13% by weight), the negative electrode, positive electrode, and secondary battery were manufactured in the same manner as in Example 1. The negative electrode slurry prepared here contained 0.1% by weight of SWCNT conductive material and 3.5% by weight of SBR / CMC binder.

[0108] (Example 6)

[0109] Except for the preparation of a negative electrode slurry by linearly dispersing SWCNT (Ocsial) in an acrylic (PAA) binder solution and then adding 83.9% by weight of artificial graphite and 13% by weight of SiOx, a negative electrode, a positive electrode, and a secondary battery were manufactured in the same manner as in Example 1. The negative electrode slurry prepared here contained 0.1% by weight of SWCNT conductive material and 3% by weight of acrylic (PAA) binder.

[0110] (Example 7)

[0111] In addition to dispersing MWCNT (C-nano) in the SBR / CMC binder solution line, 82.5% by weight of artificial graphite and SiO2 were added. x Except for the preparation of the negative electrode slurry (13% by weight), the negative electrode, positive electrode, and secondary battery were manufactured in the same manner as in Example 1. The negative electrode slurry prepared here contained 1% by weight of MWCNT conductive material and 3.5% by weight of SBR / CMC binder.

[0112] The secondary batteries manufactured in Examples 4-7 were subjected to formation and charge-discharge experiments using the same method as in Evaluation Example 1. XRM analysis was performed on the negative electrodes in the fully discharged state (State 1) after formation and in the fully charged state (State 2) after 500 charge-discharge cycles. The volume fraction (vol%) of silicon-based active material particles with a grayscale value of 30,000 or higher was derived, and the capacity retention was measured and is shown in Table 2 below. The methods for measuring the volume fraction and capacity retention were the same as in Evaluation Example 1.

[0113] In addition, the XRM analysis results of the negative electrodes of Examples 1 and 6 after 500 cycles in the fully charged state (state 2) are shown in the figures below. Figure 1a , Figure 1b .

[0114] Table 2

[0115]

[0116] As shown in Table 2, in the cases of Examples 1-3, which confirmed that the negative electrode slurry was prepared by simultaneously adding the negative electrode active material, binder, and conductive material, the ratio ((V1-V2) / V1) was lower than that in Examples 4-6, which used negative electrode slurries with the same composition. 100) value and capacity retention rate. It is judged that in the cases of Examples 4 to 6, the active substance is added after mixing the binder and conductive material to uniformly disperse the conductive material, thereby achieving uniform dispersion of the conductive material, thus ensuring excellent conductivity and exhibiting superior performance compared to Examples 1 to 3.

[0117] Furthermore, in Examples 4-6, it can be confirmed that ((V1-V2) / V1) varies depending on the type of adhesive and conductive material. The changes in capacity retention (100%) and capacity retention rate were similar to those in Evaluation Example 1. It was determined that in Example 6, which used SWCNT conductive material and acrylic adhesive, the electrical contact between active material particles was maintained stably and well even with the volume expansion of silicon particles due to prolonged charging and discharging, resulting in the highest volume fraction change and thus the best lifetime characteristics.

[0118] For Example 7, it was determined that although a linearly dispersed conductive material was used, the volume expansion of silicon particles due to long-term charging and discharging could not maintain good electrical contact between active material particles, resulting in relatively low volume fraction change and lifetime characteristics.

[0119] Additionally, see Figure 1a and Figure 1b It can be confirmed that the SiO2 in Example 6, after 500 cycles and in a fully charged state, is... x The volume expansion of the particles was significantly reduced compared to Example 1.

[0120] The embodiments of the present invention have been described above, but the present invention is not limited to the described embodiments and can be manufactured in various different forms. It is understood that those skilled in the art to which this invention pertains can implement it in other specific forms without changing the essential features of the present invention. Therefore, it should be understood that the embodiments described above are exemplary in all respects and not limiting.

Claims

1. A negative electrode for a secondary battery, comprising a silicon-based active material, a conductive material, and a binder; When performing XRM measurements on a fully charged negative electrode after 500 charge-discharge cycles using a Zeiss Versa 520 XRM device, the volume fraction V2 of silicon-based active material particles with a grayscale value of 30000 or higher is 2.7 vol.% or higher and below 5.3 vol.%.

2. The negative electrode for a secondary battery according to claim 1, wherein: The silicon-based active material includes Si and SiO. x One or a combination of Si-containing alloys and Si / C composites, 0 <x≤2。 3. The negative electrode for a secondary battery according to claim 1, wherein: The conductive material includes carbon nanotubes.

4. The negative electrode for a secondary battery according to claim 3, wherein: The conductive material includes multi-walled carbon nanotubes or single-walled carbon nanotubes.

5. The negative electrode for a secondary battery according to claim 3, wherein: Based on the total weight of the negative electrode active material layer, the content of the conductive material is less than 5% by weight.

6. The negative electrode for a secondary battery according to claim 1, wherein, The negative electrode of the secondary battery satisfies the following relationship 1: [Relation 1] (V1-V2) / V1 100 ≥ 45% In the aforementioned Equation 1, V1 is the volume fraction of silicon-based active material particles with a gray value of 30,000 or higher when XRM measurement is performed on the negative electrode in a fully discharged state after formation, and V2 is the volume fraction of silicon-based active material particles with a gray value of 30,000 or higher when XRM measurement is performed on the negative electrode in a fully charged state after 500 charge-discharge cycles. The unit of the volume fraction is vol.

7. The negative electrode for a secondary battery according to claim 1, wherein: The negative electrode contains carbon-based active materials.

8. The negative electrode for a secondary battery according to claim 7, wherein: The carbon-based active material includes crystalline carbon.

9. The negative electrode for a secondary battery according to claim 1, wherein: The negative electrode contains an acrylic adhesive.

10. A secondary battery comprising a negative electrode for a secondary battery according to any one of claims 1 to 9.

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