Negative electrode for secondary battery and secondary battery comprising the same

Abstract

Disclosed are 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 relation 1. [Relation 1] (V1-V2) / V1100 ≥ 45%, in the relation 1, V1 is a volume fraction (vol%) of silicon-based active material particles having a gray scale value of 30,000 or more when XRM measurement is performed on a negative electrode in a full discharge state after formation, and V2 is a volume fraction (vol%) of silicon-based active material particles having a gray scale value of 30,000 or more when XRM measurement is performed on a negative electrode in a full charge state after 500 cycles of charge and discharge. According to the negative electrode for a secondary battery of the present invention, electrical short-circuiting between silicon-based active material particles caused by volume expansion of the silicon-based active material particles occurring during repeated charge and discharge can be effectively suppressed, so that the ratio of silicon-based active material particles that can maintain an activated state during repeated charge and discharge cycles can be increased.

Description

TECHNICAL FIELD

[0001] The present application relates to a negative electrode for a secondary battery and a secondary battery including the same. BACKGROUND

[0002] Recently, as the global warming problem and its response, the demand for environmental protection technology is increasing. In particular, as the demand for technology for electric vehicles and ESS (Energy Storage System) is increasing, the demand for lithium secondary batteries, which are attracting attention as energy storage devices, is also increasing explosively.

[0003] Therefore, research is being conducted to improve the energy density of lithium secondary batteries. Existing commercial lithium secondary batteries generally use graphite-based active materials such as natural graphite, artificial graphite, etc. as negative electrode materials, but have a problem of low energy density of the battery due to the low theoretical capacity of graphite (372 mAh / g). Therefore, research is being conducted to develop new negative electrode materials that can replace existing graphite-based negative electrode materials to improve the energy density of the battery.

[0004] As a solution to this, silicon-based negative electrode materials having a high theoretical capacity (3580 mAh / g) are attracting attention as a solution. However, since the silicon-based negative electrode material undergoes a large volume expansion (~400%) during repeated charge and discharge, it has the disadvantage that the life characteristics of the battery are degraded. Therefore, as a method of solving the problem of large volume expansion of the silicon-based negative electrode material, SiO x negative electrode materials, but have a problem of low initial efficiency (ICE; Initial coulombic efficiency) due to the formation of an irreversible phase at the initial stage of battery operation.

[0005] PRIOR ART DOCUMENT

[0006] PATENT DOCUMENT

[0007] (Patent Document 1) Korean Patent Laid-Open Publication No. 10-1996-0041439 (Publication Date: December 19, 1996) SUMMARY

[0008] TECHNICAL PROBLEM

[0009] The present application aims to provide a negative electrode for a secondary battery that improves the initial efficiency and capacity while improving the life characteristics by suppressing the conversion to an inactive state due to the volume expansion of silicon-based active material particles occurring during repeated charge and discharge.

[0010] TECHNICAL SOLUTION

[0011] As a technical solution to the above problems, according to one implementation example of the present application, there is provided a negative electrode for a secondary battery satisfying the following relationship 1.

[0012] [Relationship 1]

[0013] (V1-V2) / V1 100 ≥ 45%

[0014] In the relationship 1, V1 is a volume fraction (vol%) of silicon-based active material particles having a gray scale value of 30,000 or more when XRM measurement is performed on the negative electrode in a full discharge state after formation, and V2 is a volume fraction (vol%) of silicon-based active material particles having a gray scale value of 30,000 or more when XRM measurement is performed on the negative electrode in a full charge state after 500 cycles of charge and discharge.

[0015] Further, according to the negative electrode for a secondary battery of one implementation example of the present application, the silicon-based active material can include one or a combination of Si, SiOx (0

[0016] Further, according to the negative electrode for a secondary battery of one implementation example of the present application, the negative electrode can further include a carbon-based active material.

[0017] Further, according to the negative electrode for a secondary battery of one implementation example of the present application, the carbon-based active material can include crystalline carbon.

[0018] Further, according to the negative electrode for a secondary battery of one implementation example of the present application, the negative electrode can include an acrylic binder.

[0019] Further, according to the negative electrode for a secondary battery of one implementation example of the present application, the negative electrode includes a carbon nanotube conductive material, and the conductive material can be contained in an amount of less than 5% by weight with respect to the total weight of the negative electrode active material layer.

[0020] Further, according to the negative electrode for a secondary battery of one implementation example of the present application, the conductive material can include single-walled carbon nanotubes (SWCNTs).

[0021] Further, as another technical solution to the above problems, according to another implementation example of the present application, there is provided a secondary battery including the aforementioned negative electrode.

[0022] Technical Effects

[0023] The negative electrode for a secondary battery according to the present application can improve initial efficiency and capacity.

[0024] Further, the negative electrode for a secondary battery according to the present application can improve the stability and the life characteristics of the secondary battery by controlling the ratio of the silicon-based active material that can effectively suppress the electrical short between the silicon-based active material particles caused by the volume expansion of the silicon-based active material particles during repeated charge and discharge and can maintain the activated state even in the case of repeated charge and discharge cycles, by satisfying the following relation 1.

[0025] [Relation 1]

[0026] (V1-V2) / V1 100 ≥ 45%

[0027] In the relation 1, V1 is the volume fraction (vol%) of the silicon-based active material particles having a gray scale value of 30,000 or more when the negative electrode in the full discharge state after formation is measured by XRM, and V2 is the volume fraction (vol%) of the silicon-based active material particles having a gray scale value of 30,000 or more when the negative electrode in the full charge state after 500 cycles is measured by XRM. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1a is a schematic diagram showing the results of X-ray microscopy (XRM) analysis of the negative electrode of Example 1 in the full charge state after 500 cycles;

[0029] Figure 1b is a schematic diagram showing the results of X-ray microscopy (XRM) analysis of the negative electrode of Example 6 in the full charge state after 500 cycles. DETAILED DESCRIPTION

[0030] The advantages, features and implementation methods of the present application will be apparent from the following embodiments described in detail in conjunction with the accompanying drawings. However, the present application is not limited to the following disclosed embodiments, but can be implemented in various forms, but the provision of the present embodiment aims to make the disclosure of the present application complete, and to completely inform the person skilled in the art of the scope of the present application, which is defined only by the scope of the claims. The specific content for implementing the present application is described in detail below. Irrespective of the accompanying drawings, the same reference signs refer to the same constituent elements, and "and / or" includes each of the mentioned items and all combinations of one or more.

[0031] Unless otherwise defined, all terms (including technical and scientific terms) used in the present specification are used in a meaning that is the same as commonly understood by one of ordinary skill in the art to which the present application belongs. In the present specification, when a part "comprises" a certain constituent element, unless otherwise specified, it means that another constituent element can be further included, rather than excluding another constituent element. Also, unless otherwise specified, the singular form includes the plural form.

[0032] In the present specification, when it is described that a part is "on" or "above" another part, it means not only the case where it is "directly on" another part, but also the case where there is another part therebetween.

[0033] The present embodiment provides a negative electrode for a secondary battery satisfying the following relation 1.

[0034] [Relation 1]

[0035] (V1-V2) / V1 100 ≥ 45%

[0036] The V1 is a volume fraction (vol%) of silicon-based active material particles having a grayscale value of 30,000 or more when XRM measurement is performed on the negative electrode in a full discharge state after formation, and the V2 is a volume fraction (vol%) of silicon-based active material particles having a grayscale value of 30,000 or more when XRM measurement is performed on the negative electrode in a full charge state after 500 cycles of charge and discharge.

[0037] The volume fraction is calculated based on the total 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 a non-destructive analysis of the negative electrode by a 3D XRM analysis method, and in this case, an X-ray microscope uses a versa 520 XRM device of Zeiss. The XRM image analysis conditions are voxel 350 nm, power 5 W (60 kV).

[0038] In the relation 1, the V1 is derived from XRM measurement of the negative electrode in a full discharge state, i.e., SOC 0% after formation, and the V1 represents a volume fraction (vol%) of silicon-based active material in an unreacted state after formation, and the total amount of silicon-based active material capable of participating in a battery reaction after formation can be quantitatively evaluated based on the V1.

[0039] In one aspect, the formation is a process of forming a solid electrolyte interface (SEI) film on the surface of the negative electrode by initial charge and discharge, and is not particularly limited as long as it is a method generally used in the technical field. After the formation, the negative electrode in a state of SOC 0% can be obtained by a complete discharge process. As an example, the discharge process is performed at a discharge rate of 0.33 C under a 2.5 V cut-off condition, but is not limited thereto.

[0040] In the relationship 1, V2 is derived by XRM measurement on the negative electrode in a full charge state, i.e., SOC 100%, after 500 cycles of charge and discharge, and V2 represents the volume fraction (vol%) of the silicon-based active material particles in a non-reactive state that cannot participate in the battery reaction and that have electrical short circuits after 500 cycles. The total amount of non-active silicon-based active materials that cannot participate in the battery reaction due to repeated charge and discharge cycles can be quantitatively evaluated based on V2. Here, the charge and discharge conditions when the charge and discharge cycles are performed are as follows: the charge can be performed at 0.33 C under a 4.2 V cut-off condition, and the discharge can be performed at 0.5 C under a 2.5 V cut-off condition. The condition for full charging after the 500 cycles is not particularly limited, and as a non-limiting example, it can be performed at a 0.33 C charge rate under a 4.2 V cut-off condition.

[0041] According to the present application, the relationship between V1, which quantitatively evaluates the total amount of silicon-based active materials that can participate in the reaction, and V2, which quantitatively evaluates the total amount of non-active silicon-based active materials that cannot participate in the battery reaction due to repeated charge and discharge cycles, i.e., relationship 1, is satisfied, thereby enabling an increase in the ratio of silicon-based active materials that can maintain an active state even after repeated charge and discharge cycles.

[0042] Specifically, (V1-V2) quantitatively evaluates the total amount of silicon-based active materials in an active state that can participate in the battery reaction even after repeated charge and discharge cycles. According to the present application, by satisfying relationship 1, which represents the ratio of the total amount of silicon-based active materials in an active state (V1-V2) to the total amount of silicon-based active materials (V1), it is possible to increase the ratio of silicon-based active materials that can maintain an active state even after repeated charge and discharge cycles. Therefore, it is possible to significantly improve the life characteristics of the battery.

[0043] Here, the active state can mean a state in which electrical short circuits between silicon-based active material particles do not occur as the volume of the silicon-based active material particles expands during repeated charge and discharge processes, and can mean a state in which the electrical network is maintained in a good state and the electrochemical reaction can continue to occur.

[0044] From the viewpoint of further improving the life characteristics of the battery, the value of (V1-V2) / V1 of the relational expression 1 The value of 100 can be 50% or more, or 55% or more, or 60% or more.

[0045] The silicon-based active material can include one or a combination of Si, SiOx (0 < x ≤ 2), a Si-containing alloy, and a Si / C composite.

[0046] The Si-containing alloy is not particularly limited, but can be represented by Si-Q alloy, for example. 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, except for Si. The element Q can be 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, for example.

[0047] The negative electrode according to one embodiment of the present application includes a current collector and a negative electrode active material layer containing a negative electrode active material and a binder on the current collector.

[0048] The negative electrode active material can further include a carbon-based active material. Specifically, any carbon-based active material that can reversibly intercalate / deintercalate lithium ions can be used. As specific examples, one or a mixture of crystalline carbon and amorphous carbon can be included. As non-limiting examples, the amorphous carbon can be one or a combination of soft carbon (low-temperature sintered carbon), hard carbon, coke, mesocarbon microbead (MCMB) sintered at 1500°C or less, and mesophase pitch-based carbon fiber (MPCF). As non-limiting examples, the crystalline carbon can be one or a combination of natural graphite, artificial graphite, graphite such as exfoliated graphite, carbon black, graphene, fullerene soot, carbon nanotube, carbon nanowire, carbon fiber, and micro / meso / macro porous carbon. The carbon-based active material can be used in a spherical, plate-like, fibrous, tubular, or powder form.

[0049] The negative electrode according to one embodiment of the present application can use the silicon-based active material and the carbon-based active material in a weight ratio of 1 to 100: 99 to 0. Thus, high capacity characteristics can be achieved, and excessive volume expansion of the silicon-based active material occurring during repeated charge and discharge can be buffered. More specifically, the silicon-based active material and the carbon-based active material can be used in a weight ratio 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.

[0050] The binder is not particularly limited, and an acrylic binder can be preferred. The acrylic binder can include one or more selected from the group consisting of polyacrylic acid (PAA), polyacrylamide, and polyvinyl alcohol co-acrylic acid, for example. Thus, the binder can function as a slurry of the active material, adhesion between the active materials, and adhesion to the current collector, and can maintain electrical contact between the active materials even when volume change of the silicon-based active material occurs in the negative electrode due to excellent adhesion.

[0051] The content of the acrylic binder can be 0.1 to 30% by weight, or 1 to 10% by weight, of the total weight of the negative electrode active material layer, for example, in terms of efficient realization of the above effects.

[0052] The conductive material can include multi-walled carbon nanotubes (MWCNTs) or single-walled carbon nanotubes (SWCNTs), for example. MWCNTs are composed of 10 or more walls, and thus have a large diameter and many surface defects, and thus have lower conductivity than SWCNTs composed of an average of 1 to 3 walls. Thus, a higher content of the conductive material than SWCNTs is required. The content of the conductive material can be 10% by weight or less, or 5% by weight or less, or 0.5 to 1% by weight, of the total weight of the negative electrode active material layer. SWCNTs have high conductivity, and thus can stably maintain conductivity between silicon-based active material particles in the negative electrode even when the content of the conductive material is reduced to one-tenth of that of general conductive materials. Specifically, by including SWCNTs as the conductive material, electrical networks between the negative electrode active material particles can be maintained even when volume expansion of the silicon-based active material particles occurs during repeated charge and discharge, and thus life characteristics can be improved. The content of the conductive material can be 5% by weight or less, or 1% by weight or less, or 0.05 to 0.5% by weight, of the total weight of the negative electrode active material layer.

[0053] The negative electrode according to the present embodiment can be manufactured by coating a negative electrode slurry including the above-described negative electrode active material, binder, conductive material, and solvent on a current collector and drying the same.

[0054] More specifically, for the negative electrode slurry, it is preferable to prepare by mixing the aforementioned negative electrode active material after dispersing the conductive material in the binder solution line. Thereby, it is possible to ensure the phase stability of the finally prepared negative electrode slurry while uniformly dispersing the conductive material in the negative electrode slurry, and further it is possible to efficiently exhibit the aforementioned effects of the conductive material.

[0055] On the other hand, the binder solution represents a mixture in which the binder is not dissolved but exists in the solvent in a granular form, and a thickening agent or the like can be additionally mixed as needed. Here, the content of the solid component in the binder solution other than the solvent can be 0.5 to 50% by weight, or 10 to 40% by weight.

[0056] The solvent can be at least one selected from the group consisting of water, pure water, deionized water, ethanol, isopropyl alcohol, methanol, acetone, n-propanol, and t-butanol, but is not limited thereto.

[0057] The thickening agent can use a cellulose compound, and specifically at least one of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or an alkali metal salt thereof, or the like can be mixedly used. As the alkali metal, Na, K, or Li can be used.

[0058] The current collector can use a material selected from the group consisting of a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, and a combination thereof, but is not limited thereto.

[0059] The coating can use any coating method commonly known for coating a liquid to form a thin film, without limitation. For example, spray coating, dip coating, spin coating, gravure coating, slot die coating, blade coating, roll coating, inkjet printing, flexographic printing, screen printing, electrohydrodynamic printing, microcontact printing, stamping, reverse offset printing, bar coating, and gravure offset printing can be used, but are not limited thereto.

[0060] The drying can be performed at a temperature of 80 to 130°C, preferably 100 to 130°C, for 10 to 50 minutes, preferably 15 to 30 minutes.

[0061] Next, a negative electrode in which a negative electrode active material layer is formed on a current collector can be manufactured by rolling the dried negative electrode to an appropriate density.

[0062] The present implementation example also provides a secondary battery including the negative electrode. The secondary battery can include a negative electrode; a positive electrode; a separator and an electrolyte interposed between the negative electrode and the positive electrode.

[0063] The negative electrode is the same as the aforementioned.

[0064] The positive electrode can include a current collector and a positive electrode active material layer formed by coating a positive electrode slurry containing a positive electrode active material on the current collector.

[0065] The current collector can use the negative electrode current collector described above, and can use a material known in the art, but the present application is not limited thereto.

[0066] The positive electrode active material layer includes a positive electrode active material, and can optionally include a binder and a conductive material. The positive electrode active material can use a 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 present application is not limited thereto.

[0067] The binder is not particularly limited as long as it is generally used in the art. For example, it can include an organic binder such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, or the like, or a water-based binder such as styrene butadiene rubber (SBR), and can be used with a thickening agent such as carboxymethyl cellulose (CMC).

[0068] The conductive material is used to impart electrical conductivity to the electrode, and can be arbitrarily used as long as it is an electronic conductive material that does not cause chemical changes in the battery. As examples of the conductive material, a conductive material containing a carbon-based substance such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, or the like; a metal-based substance such as a metal powder or a metal fiber of copper, nickel, aluminum, silver, or the like; a conductive polymer such as a polyphenylene derivative; or a mixture thereof can be used.

[0069] The separator is not particularly limited as long as it is a separator known in the art. For example, it can be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or a combination thereof, and can be in the form of a non-woven fabric or a woven fabric, and can optionally be used in a single layer or a multi-layer structure.

[0070] The electrolyte includes a non-aqueous organic solvent and an electrolytic salt. The non-aqueous organic solvent can be ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethylmethyl 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 a mixture thereof, but is not limited thereto. The electrolytic salt is a substance that is dissolved in the non-aqueous organic solvent, enables the secondary battery to substantially operate as a supply source of electrolytic metal ions in the battery, and facilitates movement of the electrolytic metal ions between the cathode and the anode. In the case of lithium as the electrolytic metal, the electrolytic salt can be LiPF6, LiBF4, LiTFSI, LiSbF6, LiAsF6, LiClO4, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, LiSbF6, LiAlO4, LiAlCl4, LiN(C x F 2x+1 SO2)(C y F 2y+ 1SO2)(but, x, y are natural numbers), LiCl, LiI, or a mixture thereof, but is not limited thereto. Also, the electrolytic salt can use a known substance at a concentration matching the purpose, and can further include a known solvent or an additive for improving charge-discharge characteristics, flame retardant characteristics, etc. as needed.

[0071] Hereinafter, the present application will be described in detail through examples, but this is for more detailed description of the present application, and the scope of the present application is not limited to the following examples.

[0072] Example

[0073] (Example 1)

[0074] Step 1: Preparation of negative electrode slurry

[0075] Meanwhile, artificial graphite 83 wt%, SiO x 13 wt%, polyacrylic acid (PAA) binder 3 wt%, and MWCNT (C-nano) 1 wt% were mixed to prepare a negative electrode slurry having a solid content of 43 wt%.

[0076] Step 2: Manufacture of negative electrode

[0077] The negative electrode slurry prepared in the step 1 was coated on a copper current collector (copper foil of 8 μm thickness) using slot die coating. Thereafter, the negative electrode active material layer was completed by drying for 30 minutes in a drying oven heated with a hot air at 120°C. At this time, the thickness of the negative electrode active material layer was made to be 50 μm.

[0078] Step 3: Manufacture of positive electrode

[0079] Li[Ni 0.88 Co 0.1 Mn 0.02 ]O2 was prepared as a positive electrode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder, mixed in a weight ratio of 96.5:2:1.5. The positive electrode slurry was uniformly coated on an aluminum foil of 12 μm thickness, and vacuum drying was performed to manufacture a positive electrode for a secondary battery.

[0080] Step 4: Manufacture of secondary battery

[0081] The positive electrode and the negative electrode were notched to a predetermined size and stacked, and a separator (polyethylene, 13 μm thick) was disposed between the positive electrode and the negative electrode to form an electrode core, and then the tab portions of the positive electrode and the negative electrode were welded, respectively. The welded positive electrode / separator / negative electrode assembly was put into a pouch, and three sides other than the electrolyte injection side were sealed. At this time, the portion having the electrode tab was included in the sealed portion.

[0082] After injecting the electrolyte through the remaining side other than the sealed portion, the remaining side was sealed, and then immersed for 12 hours or more.

[0083] An electrolyte in which 1M LiPF6 was dissolved in a mixed solvent of EC / EMC / DEC (25 / 45 / 30; volume ratio), and 1 wt% of vinylene carbonate (VC) and 3 wt% of fluoroethylene carbonate (FEC) were further added, was used.

[0084] (Example 2)

[0085] Except that a negative electrode slurry in which artificial graphite 83.4 wt%, SiO x 13 wt%, SBR binder 2 wt%, CMC thickener 1.5 wt%, and SWCNT (Ocsial) 0.1 wt% were mixed at the same time was used to manufacture a negative electrode, the negative electrode, the positive electrode, and the secondary battery were manufactured in the same manner as in Example 1.

[0086] (Example 3)

[0087] Except that artificial graphite 83.9 wt%, SiOx 13 wt% of polyacrylic acid (PAA) binder 3 wt% and 0.1 wt% of SWCNT (Ocsial) were used to manufacture the negative electrode slurry. The negative electrode, the positive electrode and the secondary battery were manufactured in the same manner as in Example 1, except that the negative electrode was manufactured.

[0088] (Comparative Example 1)

[0089] The negative electrode, the positive electrode and the secondary battery were manufactured in the same manner as in Example 1, except that 2 wt% of SBR binder and 1 wt% of CMC thickener were used instead of 3 wt% of polyacrylic acid (PAA) binder to manufacture the negative electrode slurry.

[0090] [Example 1] Evaluation of life characteristics according to types of binder and conductive agent

[0091] After performing the formation process under the conditions of charging at 0.25C 4.2V and discharging at 0.25C 2.5V to 3.5V, the full discharge was performed at 0.33C under the 2.5V cut-off condition, and the XRM analysis was performed on the negative electrode in the full discharge state (State 1) after the formation. The volume fraction (V1, vol%) of the silicon-based active material particles having a grayscale value of 30,000 or more was derived. The results are shown in Table 1 below.

[0092] After performing the charge-discharge 500 cycles under the conditions of charging at 0.33C 4.2V cut-off and discharging at 0.5C 2.5V cut-off, the capacity retention rate (%) was measured, and the full charge was performed at 0.33C 4.2V cut-off. The XRM analysis was performed on the negative electrode in the full charge state (State 2) after the charge-discharge 500 cycles, and the volume fraction (V2, vol%) of the silicon-based active material particles having a grayscale value of 30,000 or more was derived. The results are shown in Table 1 below.

[0093] The V1 and V2 volume fractions were calculated using the grayscale value of the X-ray microscopy (XRM) image. The X-ray microscopy used a Versa 520 XRM device from Zeiss. The XRM analysis conditions were as follows:

[0094] - Voxel: 350 nm

[0095] - Power: 60 kV, 5 W

[0096]

Table 1

[0097]

[0098] In Table 1, State 1 indicates a negative electrode in a full discharge state (SOC 0%) after formation, and State 2 indicates a negative electrode in a full charge state (SOC 100%) after 500 cycles of charge and discharge.

[0099] From the results of Table 1, it can be confirmed that ((V1-V2) / V1 In the case of Examples 1 to 3 in which 100) is 45% or more, i.e., satisfying the relationship 1, the capacity retention rate is 80% or more, compared to Comparative Example 1, and has a relatively excellent life characteristic. From the results, in the case of the negative electrode satisfying the relationship 1, the ratio of the silicon-based active material that can maintain the activated state during repeated charge and discharge cycles is high, and thus has an excellent life characteristic of the battery.

[0100] In addition, in the case of Examples 2 to 3 using the SWCNT conductive material, it can be confirmed that, compared to Example 1 using the MWCNT conductive material, the ((V1-V2) / V1 100) value exhibits a higher value, and has a more excellent capacity retention rate. In the SWCNT, the CNT is composed of 1 to 3 walls, and thus has a very small diameter and a very high conductivity, and thus even if the SiO x particles expand in volume during 500 cycles, the electrical contact between the SiO x particles can be well maintained, and thus it is easy to secure an excellent life characteristic compared to the MWCNT. In contrast, it is judged that the MWCNT is composed of 10 or more walls, and thus has a large diameter and a large number of surface defects, and thus even if the content is increased 10 times compared to the SWCNT, the above effect cannot be achieved, and the life characteristic is more disadvantageous.

[0101] In addition, in the case of Example 3 using the acrylic adhesive, it can be confirmed that an excellent life characteristic is exhibited compared to Example 2 using the SBR / CMC adhesive. It is judged that the results are because the acrylic adhesive has a strong adhesion force when the SiO x particles change in volume due to repeated charge and discharge, and thus more stably maintains the electrical contact between the SiO x particles, compared to the SBR / CMC adhesive.

[0102] [Example 2] Evaluation of life characteristics according to preparation method of negative electrode slurry

[0103] (Example 4)

[0104] After dispersing MWCNT (C-nano) in a polyacrylic acid (PAA) adhesive solution, 83 wt% of artificial graphite and 17 wt% of SiOx 13 weight% of the negative electrode slurry, the negative electrode, the positive electrode and the secondary battery were manufactured in the same manner as in Example 1. The content of the SWCNT conductive material in the negative electrode slurry thus prepared was 0.1 weight%, and the content of the SBR / CMC binder was 3.5 weight%.

[0105] (Example 5)

[0106] Except for dispersing MWCNT (C-nano) 0.1 weight% in the SBR / CMC binder solution line, artificial graphite 82.5 weight% and SiO x 13 weight% of the negative electrode slurry, the negative electrode, the positive electrode and the secondary battery were manufactured in the same manner as in Example 1. The content of the SWCNT conductive material in the negative electrode slurry thus prepared was 0.1 weight%, and the content of the SBR / CMC binder was 3.5 weight%.

[0107] (Example 6)

[0108] Except for dispersing SWCNT (Ocsial) 0.1 weight% in the polyacrylic acid (PAA) binder solution line, artificial graphite 83.9 weight%, SiOx 13 weight% of the negative electrode slurry, the negative electrode, the positive electrode and the secondary battery were manufactured in the same manner as in Example 1. The content of the SWCNT conductive material in the negative electrode slurry thus prepared was 0.1 weight%, and the content of the polyacrylic acid (PAA) binder was 3 weight%.

[0109] (Example 7)

[0110] Except for dispersing MWCNT (C-nano) 0.1 weight% in the SBR / CMC binder solution line, artificial graphite 82.5 weight% and SiO x 13 weight% of the negative electrode slurry, the negative electrode, the positive electrode and the secondary battery were manufactured in the same manner as in Example 1. The content of the SWCNT conductive material in the negative electrode slurry thus prepared was 0.1 weight%, and the content of the SBR / CMC binder was 3.5 weight%.

[0111] The secondary batteries manufactured through Examples 4 to 7 were subjected to formation and charge-discharge experiments in the same manner as in Evaluation Example 1, and XRM analysis was performed on the negative electrode in the fully discharged state (State 1) after the formation and in the fully charged state (State 2) after 500 cycles of charge-discharge, and the volume fraction (vol%) of the silicon-based active material particles having a gray scale value of 30,000 or more was derived, and the capacity retention rate was measured and shown in Table 2 below. Here, the volume fraction and the capacity retention rate measurement method were the same as in Evaluation Example 1.

[0112] In addition, the results of XRM analysis of the negative electrode of Example 1 and Example 6 after 500 cycles at the full charge state (state 2) are shown in Figure 1a , Figure 1b .

[0113]

Table 2

[0114]

[0115] As shown in Table 2, it can be confirmed that in the case of Examples 1 to 3 in which the negative electrode slurry is prepared by simultaneously adding the negative electrode active material, the binder, and the conductive material, the (V1-V2) / V1 100) value and the capacity retention rate are lower than in the case of Examples 4 to 6 in which the negative electrode slurry having the same composition is used. It is judged that in the case of Examples 4 to 6, the active material is first added in a state in which the binder and the conductive material are mixed to uniformly disperse the conductive material, thereby achieving uniform dispersion of the conductive material, and thus excellent conductivity can be stably ensured, and thus excellent performance is exhibited compared to Examples 1 to 3.

[0116] In addition, in Examples 4 to 6, it can be confirmed that the change tendency of the (V1-V2) / V1 100) and the capacity retention rate according to the type of the binder and the conductive material is the same as in Evaluation Example 1. It is judged that in the case of Example 6 in which the SWCNT conductive material and the acrylic binder are used, the volume expansion of the silicon particles due to long-time charge and discharge also stably and favorably maintains the electrical contact between the active material particles, and thus the volume fraction change is the highest, and thus the life characteristics are also the best.

[0117] In the case of Example 7, it is judged that although the wire dispersion conductive material is used, the electrical contact between the active material particles cannot be favorably maintained when the volume of the silicon particles expands due to long-time charge and discharge, and thus relatively low volume fraction change and life characteristics are exhibited.

[0118] In addition, referring to Figure 1a and Figure 1b , it can be confirmed that the volume expansion of the SiO x particles of the negative electrode of Example 6 at the full charge state after 500 cycles is significantly reduced compared to Example 1.

[0119] The above describes the embodiments of the present application, but the present application is not limited to the described embodiments, and can be manufactured in different various forms, and it can be understood that a person of ordinary skill in the art to which the present application pertains can implement in other specific forms without changing the essential characteristics of the present application. Therefore, it should be understood that the above-described embodiments are exemplary in all aspects, and are not limiting.

Claims

1. A negative electrode for a secondary battery, wherein: the negative electrode for a secondary battery satisfies the following relation 1: [Relation 1] (V1-V2) / V1 100 ≥ 45% In the relation 1, VI is a volume fraction (vol%) of silicon-based active material particles having a gray scale value of 30000 or more when XRM measurement is performed on the negative electrode in a full discharge state, i.e., SOC 0%, after formation, and V2 is a volume fraction (vol%) of silicon-based active material particles having a gray scale value of 30000 or more when XRM measurement is performed on the negative electrode in a full charge state, i.e., SOC 100%, after charge and discharge of 500 cycles under a charge condition of 0.33C and 4.2V cut-off and a discharge condition of 0.5C and 2.5V cut-off, the XRM measurement being measured by a Zeiss versa 520 XRM device.

2. The negative electrode for a secondary battery according to claim 1, wherein: the silicon-based active material includes one or a combination of Si, SiOx, Si-containing alloy, and Si / C composite, 0 < x < 2.

3. The negative electrode for a secondary battery according to claim 1, wherein: the negative electrode further includes a carbon-based active material.

4. The negative electrode for a secondary battery according to claim 3, wherein: the carbon-based active material includes crystalline carbon.

5. The negative electrode for a secondary battery according to claim 1, wherein: the negative electrode includes an acrylic-based binder.

6. The negative electrode for a secondary battery according to claim 1, wherein: the negative electrode includes a carbon nanotube conductive material, the conductive material being contained at less than 5% by weight with respect to the total weight of the negative electrode active material layer.

7. The negative electrode for a secondary battery according to claim 6, wherein: the conductive material includes single-walled carbon nanotubes (SWCNTs).

8. A secondary battery including the negative electrode according to any one of claims 1 to 7.

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