Electrode for secondary battery and lithium secondary battery comprising same

Abstract

The present invention relates to an electrode for a secondary battery and a lithium secondary battery comprising the same, the electrode for the secondary battery according to the present invention comprising: an electrode current collector; and an electrode active material layer provided on one surface of the electrode current collector and including an electrode active material and a graphene conductive material. The number of layers, defined by the formula 1, of the graphene conductive material is 2-15. [Formula 1] Number of Layers = (Lc / D) + 1 In Formula 1, Lc is the grain size (f) calculated from the X-ray diffraction (XRD) analysis result of the graphene conductive material, and D is the d-pitch (f) calculated from the X-ray diffraction analysis result of the graphene conductive material.

Description

Technical Field

[0001] The present invention provides an electrode for a secondary battery and a lithium secondary battery including the electrode. Background Technology

[0002] Rechargeable batteries are batteries that can be repeatedly charged and discharged. With the development of the information communication and display industries, rechargeable batteries are widely used as power sources for portable electronic communication devices such as portable cameras, mobile phones, and laptops. In addition, in recent years, battery packs including rechargeable batteries have been developed for use as power sources in environmentally friendly vehicles such as hybrid electric vehicles.

[0003] Lithium-ion batteries include, for example, lithium-ion batteries, nickel-cadmium batteries, and nickel-metal hydride batteries. Among them, lithium-ion batteries have high operating voltage and energy density per unit weight, and are advantageous for charging speed and lightweight design, so they are being actively developed and applied.

[0004] For example, a lithium secondary battery may include an electrode assembly and an electrolyte impregnating the electrode assembly, the electrode assembly including a positive electrode, a negative electrode, and a separator (separation membrane). The lithium secondary battery may further include the electrode assembly and an outer packaging material containing the electrolyte, for example, a pouch-type outer packaging material.

[0005] Lithium-ion secondary batteries preferably possess low internal resistance and high rate capability, thereby exhibiting improved fast charge and discharge characteristics. Therefore, there is a need to develop an electrode for lithium-ion secondary batteries that can achieve both low resistance and high rate capability. Summary of the Invention

[0006] (a) Technical problems to be solved

[0007] One technical problem of the present invention is to provide an electrode for a secondary battery with improved electrochemical and physical properties.

[0008] One technical problem of the present invention is to provide a lithium secondary battery including the electrodes for the secondary battery.

[0009] (II) Technical Solution

[0010] The electrode for a secondary battery according to the present invention comprises: an electrode current collector; and an electrode active material layer disposed on one side of the electrode current collector, the electrode active material layer comprising an electrode active material and a graphene conductive material. The number of layers of the graphene conductive material as defined by Formula 1 is 2 to 15.

[0011] [Formula 1]

[0012] Number of layers = (L) c / D)+1

[0013] In Equation 1, L cThe grain size (Å) is calculated based on the X-ray diffraction (XRD) analysis results of the graphene conductive material, and D is the spacing (Å) calculated based on the X-ray diffraction analysis results of the graphene conductive material.

[0014] According to an exemplary implementation, the number of layers can be 3 to 12.

[0015] According to an exemplary implementation, in Formula 1, D can be from 2.5 Å to 3.5 Å.

[0016] According to an exemplary implementation, in Formula 1, L c It can range from 7.5 Å to 50 Å.

[0017] According to an exemplary embodiment, the ratio of the maximum peak intensity of the D band to the maximum peak intensity of the G band, calculated based on the Raman spectral analysis results of the graphene conductive material, is (I... D / I G () can be greater than 0 and less than 1.

[0018] According to an exemplary embodiment, the graphene conductive material has a conductivity of 1 g / cm³. 3 The resistivity of the powder can be below 0.005 Ω·cm.

[0019] According to an exemplary embodiment, the BET specific surface area of ​​the graphene conductive material can be 100 m². 2 / g to 1000m 2 / g.

[0020] According to an exemplary embodiment, the content of the graphene conductive material can be from 0.01% to 1% by weight of the total weight of the electrode active material layer.

[0021] According to an exemplary embodiment, the content of the graphene conductive material can be from 0.1% to 0.5% by weight of the total weight of the electrode active material layer.

[0022] According to an exemplary embodiment, the electrode active material may comprise lithium metal oxide.

[0023] According to an exemplary embodiment, the electrode active material may comprise a graphite-based active material or a silicon-based active material.

[0024] The lithium secondary battery according to the present invention includes a positive electrode and a negative electrode disposed opposite to the positive electrode. The positive electrode or the negative electrode is an electrode for the secondary battery.

[0025] (III) Beneficial Effects

[0026] The electrode for a secondary battery according to an exemplary embodiment of the present invention can have improved electrode internal resistance. Therefore, the fast charge / discharge performance of a battery including said electrode can be improved.

[0027] The electrode for a secondary battery according to an exemplary embodiment of the present invention can have improved rate performance. Therefore, a battery with improved lifespan characteristics can be realized.

[0028] The electrode for secondary batteries and the lithium secondary battery including the electrode of the present invention can be widely used in green technology fields such as electric vehicles, battery charging stations, and other battery-powered solar and wind power generation. Furthermore, the electrode for secondary batteries and the lithium secondary battery including the electrode of the present invention can be used in eco-friendly electric vehicles and hybrid vehicles, which prevent climate change by suppressing air pollution and greenhouse gas emissions. Attached Figure Description

[0029] Figure 1 and Figure 2 The images are scanning electron microscope (SEM) images of the graphene conductive materials prepared in Example 1 and Example 2, respectively.

[0030] Figure 3 and Figure 4 The images are transmission electron microscope (TEM) images of the graphene conductive materials prepared in Example 1 and Example 2, respectively.

[0031] Figures 5 to 9 The images shown are SEM images of the positive electrode active material layers of Example 1, Comparative Example 1, Comparative Example 3, Comparative Example 6, and Comparative Example 7, respectively. Detailed Implementation

[0032] This invention provides an electrode for a secondary battery comprising a conductive material having improved conductivity. Furthermore, this invention provides a lithium secondary battery including the aforementioned electrode.

[0033] The present invention will now be described in detail. However, this is merely an exemplary embodiment, and the present invention is not limited to the specific embodiments described herein.

[0034] The electrode for the secondary battery according to the present invention can be a positive electrode or a negative electrode.

[0035] The electrode for a secondary battery according to the present invention includes: an electrode current collector; and an electrode active material layer disposed on one side of the electrode current collector. The electrode active material layer may be disposed on one side of the electrode current collector, or it may be disposed on both sides.

[0036] When the electrode for the secondary battery is a positive electrode, the electrode current collector can be a positive electrode current collector. The positive electrode current collector can contain stainless steel, nickel, aluminum, titanium, or alloys thereof. The positive electrode current collector can also contain aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. The thickness of the positive electrode current collector is not limited thereto, and can be, for example, from 10 μm to 50 μm.

[0037] When the electrode for the secondary battery is a negative electrode, the electrode current collector can be a negative electrode current collector. Non-limiting examples of negative electrode current collectors include copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, and polymer substrates coated with conductive metals. The thickness of the negative electrode current collector is not limited to these, and can be, for example, from 10 μm to 50 μm.

[0038] The electrode active material layer contains electrode active materials. The electrode active materials can be positive electrode active materials or negative electrode active materials.

[0039] The positive electrode active material may contain lithium metal oxide. The lithium metal oxide may further contain at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

[0040] In some embodiments, the positive electrode active material or the lithium metal oxide may include a layered structure or a crystal structure represented by the following chemical formula 1.

[0041] [Chemical Formula 1]

[0042] Li x Ni a M b O 2+z

[0043] In chemical formula 1, the values ​​can be 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b≤0.4, and -0.5≤z≤0.1. As mentioned above, M can contain Co, Mn, and / or Al.

[0044] The chemical structure represented by Formula 1 indicates the bonding relationships contained in the layered or crystalline structure of the positive electrode active material, and does not exclude other additional elements. For example, M may contain Co and / or Mn, and Co and / or Mn may be provided together with Ni as the main active element of the positive electrode active material. Formula 1 is provided to represent the bonding relationships of the main active elements, and it should be understood that Formula 1 includes the introduction and substitution of additional elements.

[0045] In one embodiment, in addition to the primary active element, auxiliary elements may be further included to enhance the chemical stability of the positive electrode active material or the layered / crystal structure. These auxiliary elements may be incorporated into the layered / crystal structure to form a bond, and this should be understood to also include the chemical structures represented by Formula 1.

[0046] The auxiliary element may include at least one of, for example, Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, or Zr. The auxiliary element may function as an auxiliary active element, together with Co or Mn, to contribute to the capacity / power activity of the positive electrode active material; for example, Al.

[0047] For example, the positive electrode active material or the lithium metal oxide may include a layered structure or a crystal structure represented by the following chemical formula 1-1.

[0048] [Chemical Formula 1-1]

[0049] Li x Ni a M1 b1 M2 b2 O 2+z

[0050] In chemical formula 1-1, M1 may contain Co, Mn, and / or Al. M2 may contain the aforementioned auxiliary elements. In chemical formula 1-1, the following conditions may be met: 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b1+b2≤0.4, -0.5≤z≤0.1.

[0051] The positive electrode active material may further include coating elements or doping elements. For example, elements that are substantially the same as or similar to the auxiliary elements described above can be used as coating elements or doping elements. For example, one or more combinations of the elements described above can be used as coating elements or doping elements.

[0052] The coating element or doping element may exist on the surface of lithium metal oxide particles, or may penetrate through the surface of lithium-nickel metal composite oxide particles and be included in the bonding structure represented by chemical formula 1 or chemical formula 1-1.

[0053] The positive electrode active material may contain nickel-cobalt-manganese (NCM)-based lithium oxide. In this case, NCM-based lithium oxide with increased nickel content can be used.

[0054] Ni can be provided as a transition metal related to the power and capacity of a lithium secondary battery. Thus, as described above, by using a high-content (high nickel (High-Ni)) composition in the positive electrode active material, a high-capacity positive electrode and a high-capacity lithium secondary battery can be provided.

[0055] However, as the Ni content increases, the long-term storage stability and life stability of the positive electrode or secondary battery may be relatively reduced, and side reactions with the electrolyte may also increase. However, according to an exemplary embodiment, conductivity can be maintained by including Co, and life stability and capacity retention characteristics can be improved by Mn.

[0056] The content of Ni in the NCM-based lithium oxide (for example, the mole fraction of Ni in the total moles of nickel, cobalt, and manganese) can be 0.6 or more, 0.7 or more, or 0.8 or more. In some embodiments, the content of Ni can be 0.8 to 0.95, 0.82 to 0.95, 0.83 to 0.95, 0.84 to 0.95, 0.85 to 0.95, or 0.88 to 0.95.

[0057] In some embodiments, the positive electrode active material may further include a lithium cobalt oxide-based active material, a lithium manganese oxide-based active material, a lithium nickel oxide-based active material, or a lithium iron phosphate (LFP)-based active material (for example, LiFePO4).

[0058] In some embodiments, the positive electrode active material may include, for example, a manganese-rich (Mn-rich)-based active material, a lithium-rich layered oxide (LLO) / over-lithiated oxide (OLO)-based active material, or a cobalt-less (Co-less)-based active material having a chemical structure or crystal structure represented by Chemical Formula 2.

[0059] [Chemical Formula 2]

[0060] p[Li2MnO3]·(1-p)[Li q JO2]

[0061] In Chemical Formula 2, 0 < p < 1, 0.9 ≤ q ≤ 1.2, and J may include at least one element selected from Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, and B.

[0062] The negative electrode active material may include a graphite-based active material or a silicon-based active material. The negative electrode active material may use a material that allows lithium ions to be inserted and extracted. For example, the negative electrode active material may use carbon-based materials such as crystalline carbon, amorphous carbon, carbon composites, carbon fibers, etc.; lithium metal; lithium alloys; silicon (Si)-containing substances or tin (Sn)-containing substances, etc.

[0063] Examples of the amorphous carbon may include hard carbon, soft carbon, coke, mesocarbon microbead (MCMB), mesophase pitch-based carbon fiber (MPCF), etc.

[0064] Examples of the crystalline carbon may include graphite-based carbons such as natural graphite, artificial graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc.

[0065] The lithium metal may include pure lithium metal or lithium metal formed with a protective layer for suppressing dendrite growth, etc.

[0066] As the elements included in the lithium alloy, aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium, etc. may be mentioned.

[0067] The silicon-containing substance may provide further enhanced capacity characteristics. The silicon-containing substance may include Si, SiO x (0 < x < 2), metal-doped SiO x (0 < x < 2), silicon-carbon composites, etc. The metal may include lithium and / or magnesium, and the metal-doped SiO x (0 < x < 2) may include metal silicate.

[0068] The content of the electrode active material in the total weight of the electrode active material layer may be, for example, 80% by weight or more. In some embodiments, the content of the electrode active material in the total weight of the electrode active material layer may be, for example, 90% by weight or more or 95% by weight or more.

[0069] The electrode active material layer contains a graphene conductive material. The graphene conductive material has a shape in which multiple sheets are stacked, and it is disposed between the electrode active material particles, thereby forming a conductive network with high conductivity.

[0070] According to an exemplary embodiment, the number of layers of the graphene conductive material defined by the following formula 1 is 2 to 15. According to some embodiments, the number of layers of the graphene conductive material defined by the following formula 1 is 3 to 12 or 4 to 10.

[0071] [Formula 1]

[0072] Number of layers = (L) c / D)+1

[0073] In Equation 1, L c The grain size (Å) is calculated based on the X-ray diffraction (XRD) analysis results of the graphene conductive material, and D is the spacing (Å) calculated based on the X-ray diffraction analysis results of the graphene conductive material.

[0074] In this specification, "number of layers" can refer to the average number of overlapping layers in the thickness direction of a two-dimensional plate-like structure formed by the bonding of carbon atoms in a carbon-based material.

[0075] When the number of layers is less than 2, the solid content of the graphene dispersion may be reduced, or there may be time-related stability problems caused by re-aggregation.

[0076] When the number of layers exceeds 15, it may excessively increase the internal resistance of the electrodes, which may reduce the rate performance of the battery.

[0077] The number of layers can be obtained based on the XRD analysis results of the graphene conductive material.

[0078] The XRD analysis can be performed using CuK α (1.5406 Å) rays as the target and with an XRD instrument (PANalytical's Empyrean or X'Pert). For example, XRD analysis can be performed under the following conditions.

[0079] - 2θ = 10° to 70°

[0080] - Scanning speed = 0.01-0.02° / second

[0081] - Step size = 0.027-0.012° / step

[0082] Based on the XRD analysis results, the grain size can be calculated using the following Equation 2, based on the full width at half maximum (FWHM) of the peaks observed in the range of 2θ from 23° to 28° or from 25.2° to 26.3°.

[0083] [Equation 2]

[0084] L c =0.89 λ / (βcosθ)

[0085] In Equation 2, L cλ is the grain size in Å, λ is the wavelength of the target line, β is the full width at half maximum (FWHM) of the peak converted to radians after correction according to the basic width value of the device (e.g., 0.01 to 0.1), and θ is the peak position 2θ converted to radians.

[0086] According to an exemplary implementation, in Formula 1, L c It can be from 7.5 Å to 50 Å. According to some embodiments, in Equation 1, L... c It can be from 10 Å to 30 Å.

[0087] Within the aforementioned range, the graphene conductive material can have grains of appropriate size, thereby improving conductivity.

[0088] Based on the XRD analysis results, the d-interval can be calculated using the following formula 3, based on the peak position values ​​observed in the range of 2θ from 23° to 28°.

[0089] [Formula 3]

[0090] D=λ / 2(sinθ)

[0091] In Equation 3, D is the d-space, and λ and θ are the same as those described in Equation 2.

[0092] According to an exemplary embodiment, in Formula 1, D can be from 2.5 Å to 3.5 Å. According to some embodiments, in Formula 1, D can be from 3 Å to 3.4 Å.

[0093] Within the aforementioned range, the graphene conductive material can have an appropriate inter-plane spacing, thereby improving conductivity.

[0094] According to an exemplary embodiment, the ratio of the maximum peak intensity of the D band to the maximum peak intensity of the G band, calculated based on the Raman spectral analysis results of the graphene conductive material, is (I... D / I G () can be greater than 0 and less than 1.

[0095] The Raman spectroscopy analysis can obtain the Raman spectrum of the graphene conductive material and calculate the Raman I from it. D / I G The value is used for analysis. Raman spectroscopy is not particularly limited, but it can be performed using the following methods.

[0096] In calculating the Raman I D / I GDuring the process, the Raman spectra of graphene conductive materials can be obtained using Raman spectroscopy analysis equipment equipped with a continuous wave laser (manufacturer: HÜBNER Photonics, model name: Cobolt 04-01 Series Samba™, output wavelength: 532.1±0.3nm) and a charge-coupled device (CCD) camera (manufacturer: Renishaw, model name: inVia™ confocal Raman microscope)).

[0097] In obtaining the Raman spectrum of the graphene conductive material, a laser source can be focused onto the graphene conductive material using an objective lens (manufacturer: Leica, model name: N PLAN EPI 50x / 0.75NA). The laser intensity reaching the sample can be from about 0.1mW to about 5mW.

[0098] Raman I D / I G The value is the intensity of the D band (I) D ) and the intensity of G-band (I) G The proportion of (e.g., in the Raman spectrum obtained from graphene conductive materials, at 1540 cm⁻¹) -1 Up to 1620cm -1 The peak observed in the region can be a G-band peak, at 1300 cm⁻¹. -1 Up to 1420cm -1 The peaks observed in the region can be peaks of the D band.

[0099] According to some implementation schemes, the maximum peak intensity (I) of the D band D ) and the maximum peak intensity of the G-band (I G The proportion of (I) D / I G The value can be 0.1 to 0.8 or 0.5 to 0.7.

[0100] Within the aforementioned range, the graphene conductive material can better fill the gaps between the electrode active material particles, reducing the volume of gaps that might contribute to the internal resistance of the electrode. Therefore, the internal resistance of the electrode can be reduced.

[0101] According to an exemplary embodiment, the graphene conductive material has a conductivity of 1 g / cm³. 3 The resistivity of the powder can be below 0.005 Ω·cm. According to some embodiments, the graphene conductive material has a resistivity of 1 g / cm. 3 The resistivity of the powder can be greater than 0 Ω·cm and less than 0.004 Ω·cm.

[0102] Within the aforementioned range, the conductivity of the electrode active material layer can be further improved.

[0103] For example, the powder resistance can be measured by filling the graphene conductive material into an insulating mold, applying different pressures, and then using four probes to measure the current and voltage on the surfaces of each sample with different densities, thereby measuring the powder resistance in Ω·cm units.

[0104] A fitted line can be derived with the sample density as the X-axis and the sample volume resistivity as the Y-axis, and X can be calculated as 1 g / cm³. 3 The resistance of the powder at that time.

[0105] According to an exemplary embodiment, the BET specific surface area of ​​the graphene conductive material can be 100 m². 2 / g to 1000m 2 / g. According to some embodiments, the BET specific surface area of ​​the graphene conductive material can be 300m². 2 / g to 600m 2 / g.

[0106] Within the aforementioned range, sufficient contact sites can be formed between the electrode active material particles and / or graphene conductive material particles in the electrode active material layer. Therefore, a highly conductive network can be formed, and the rate performance of the battery can be improved.

[0107] For example, the BET specific surface area can be measured using an ASAP2420 (Micromeritics) instrument, with nitrogen as the adsorbate gas and helium as the carrier gas, via a continuous flow method using 5 predefined points for BET relative pressure. After heating the graphene conductive material, the pressure is reduced to below 10 μmHg to remove internal moisture and gases. Then, it is cooled to liquid nitrogen temperature (77.300 K), adsorbing a mixture of nitrogen and helium, and then heated to room temperature (e.g., approximately 25°C) to desorb the adsorbed nitrogen. The amount of adsorbed and desorbed nitrogen is detected using a thermal conductivity detector, allowing the calculation of the sample's specific surface area.

[0108] According to an exemplary embodiment, the content of the graphene conductive material can be from 0.01 wt% to 1 wt% of the total weight of the electrode active material layer. According to some embodiments, the content of the graphene conductive material can be from 0.05 wt% to 0.7 wt%, 0.2 wt% to 0.5 wt%, or 0.1 wt% to 0.5 wt% of the total weight of the electrode active material layer.

[0109] Within the aforementioned range, the internal resistance of the electrodes can be further reduced, and the rate performance of the battery can be further improved.

[0110] In addition to the electrode active material and conductive material, the electrode active material layer may further contain other additives.

[0111] For example, the electrode active material layer may further include a dispersant for dispersing the graphene conductive material. The dispersant can be used to uniformly disperse graphene conductive materials with strong aggregation. For example, the content of the dispersant may be from 1 part by weight to 30 parts by weight relative to 100 parts by weight of the graphene conductive material.

[0112] For example, when the electrode active material layer is a positive electrode active material layer, it may further include a positive electrode binder, a thickener, etc.

[0113] The positive electrode adhesive may include polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) copolymer, polyacrylonitrile, polymethyl methacrylate, nitrile rubber (NBR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), etc. In one embodiment, a PVDF-based adhesive may be used as the positive electrode adhesive.

[0114] The thickener may include carboxymethyl cellulose (CMC), etc.

[0115] For example, when the electrode active material layer is a negative electrode active material layer, it may further include a negative electrode binder, the thickener, etc.

[0116] The negative electrode adhesive may include styrene-butadiene rubber (SBR) based adhesives, carboxymethyl cellulose (CMC), polyacrylic acid based adhesives, poly(3,4-ethylenedioxythiophene) (PEDOT) based adhesives, etc.

[0117] According to an exemplary embodiment, the electrode can be manufactured by forming an electrode active material layer on the surface of an electrode current collector. The electrode active material layer can be formed from an electrode slurry comprising an electrode active material and a conductive material. The electrode slurry can be coated onto the surface of the electrode current collector, and then dried and calendered to form the electrode active material layer.

[0118] The coating method is not particularly limited and can be carried out by gravure coating, slot extrusion coating, multi-layer simultaneous die coating, embossing, doctor blade coating, dip coating, bar coating, casting, etc.

[0119] The electrode slurry may contain the aforementioned dispersants, binders, thickeners, etc., and may also contain solvents.

[0120] For example, when the electrode slurry is a positive electrode slurry, the solvent may include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc.

[0121] For example, when the electrode slurry is a negative electrode slurry, the solvent may include water, pure water, deionized water, distilled water, ethanol, isopropanol, methanol, acetone, n-propanol, tert-butanol, etc.

[0122] The lithium secondary battery according to the present invention includes a positive electrode and a negative electrode disposed opposite to the positive electrode. The positive electrode or the negative electrode is an electrode for the secondary battery.

[0123] According to some embodiments, both the positive electrode and the negative electrode can be electrodes for the secondary battery. According to one embodiment, the positive electrode may include a positive electrode active material layer containing a first graphene conductive material, and the negative electrode may include a negative electrode active material layer containing a second graphene conductive material. The number of layers of the first graphene conductive material and the second graphene conductive material, as defined by Formula 1, can each independently be 2 to 15.

[0124] A membrane can be disposed between the positive and negative electrodes. The membrane can be disposed to prevent short circuits between the positive and negative electrodes and to allow ion flow. According to an embodiment, the thickness of the membrane can be from 10 μm to 20 μm, but the invention is not limited thereto.

[0125] For example, the diaphragm may comprise a porous polymer membrane or a porous nonwoven fabric. The porous polymer membrane may comprise polyolefin-based polymers such as ethylene polymers, propylene polymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers. The porous nonwoven fabric may comprise high-melting-point glass fibers, polyethylene terephthalate fibers, etc. The diaphragm may also comprise ceramic-based materials. For example, inorganic particles may be coated on or dispersed within the polymer membrane to improve heat resistance.

[0126] The diaphragm may have a single-layer or multi-layer structure comprising the aforementioned polymer membrane and / or nonwoven fabric.

[0127] According to exemplary embodiments, an electrode assembly can be formed by repeatedly setting up a positive electrode, a negative electrode, and a separator. In some embodiments, the electrode assembly can be of the winding type, stacking type, z-folding type, or stack-folding type.

[0128] The electrode assembly can be housed together with the electrolyte in a casing, thereby defining a lithium secondary battery. According to an exemplary embodiment, the electrolyte can be a non-aqueous electrolyte.

[0129] The non-aqueous electrolyte comprises a lithium salt as the electrolyte and an organic solvent, the lithium salt being, for example, Li... + X - This indicates that the anion (X) of the lithium salt is... - ), can be exemplified by F - Cl - ,Br - I - NO3 - N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3 - (CF3SO2)2N- 、(FSO2)2N - 、CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - 、CF3(CF2)7SO3 - 、CF3CO2 - 、CH3CO2 - 、SCN - and (CF3CF2SO2)2N - etc.

[0130] The organic solvent may comprise an organic compound that has sufficient solubility for the lithium salt and additives and is non-reactive within the battery. The organic solvent may include at least one of, for example, carbonate-based solvents, ester-based solvents, ether-based solvents, ketone-based solvents, alcohol-based solvents, and aprotic solvents. The organic solvent may be, for example, propylene carbonate (PC), ethylene carbonate (EC), butene carbonate, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, vinylene carbonate, methyl acetate (MA), ethyl acetate (EA), n-propylacetate (n-PA), 1,1-dimethylethyl acetate (DMEA), methyl propionate (MP), ethyl propionate (EP), ethyl fluoroacetate (FEA), ethyl difluoroacetate (DFEA), ethyl trifluoroacetate (TFEA), dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), and diethylene glycol dimethyl ether. These include ether (DEGDME), tetrahydrofuran (THF), 2-methyltetrahydrofuran, ethyl alcohol, isopropyl alcohol, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, sulfolane, γ-butyrolactone, and propylene sulfite, etc. These can be used alone or in combination of two or more.

[0131] The non-aqueous electrolyte may further contain additives. These additives may include, for example, cyclic carbonate compounds, fluorinated carbonate compounds, sulcolone compounds, cyclic sulfate compounds, cyclic sulfite compounds, phosphate compounds, and borate compounds.

[0132] The cyclic carbonate-based compound may include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), etc.

[0133] The fluorinated carbonate compounds may include fluoroethylene carbonate (FEC), etc.

[0134] The sulfonyl compounds may include 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, etc.

[0135] The cyclic sulfate-based compounds may include 1,2-ethylene sulfate, 1,2-propylene sulfate, etc.

[0136] The cyclic sulfite-based compound may include ethylene sulfite, butylene sulfite, etc.

[0137] The phosphate-based compounds may include lithium difluorobis-oxalato phosphate, lithium difluorophosphate, etc.

[0138] The borate-based compounds may include lithium bis(oxalate) borate, etc.

[0139] In some implementations, a solid electrolyte can be used instead of the aforementioned separator and non-aqueous electrolyte. In this case, the lithium secondary battery can be manufactured as an all-solid-state battery.

[0140] The solid electrolyte may include a sulfide-based electrolyte. As a non-limiting example, the sulfide-based electrolyte may include Li₂S-P₂S₅, Li₂S-P₂S₅-LiCl, Li₂S-P₂S₅-LiBr, Li₂S-P₂S₅-LiCl-LiBr, Li₂S-P₂S₅-Li₂O, Li₂S-P₂S₅-Li₂O-LiI, Li₂S-SiS₂, Li₂S-SiS₂-LiI, Li₂S-SiS₂-LiBr, Li₂S-SiS₂-LiCl, Li₂S-SiS₂-B₂S₃-LiI, Li₂S-SiS₂-P₂S₅-LiI, Li₂S-B₂S₃, and Li₂S-P₂S₅-Z. m S n (m and n are positive numbers, Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (p and q are positive numbers, M is P, Si, Ge, B, Al, Ga, or In), Li7-xPS6-xCl x (0≤x≤2), Li7-xPS6-xBr x (0≤x≤2), Li7-xPS6-xI x (0≤x≤2), etc. These can be used individually or in combination of two or more.

[0141] In one embodiment, the solid electrolyte may further include oxide-based amorphous solid electrolytes such as Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3, and Li2O-B2O3-ZnO.

[0142] For example, tabs (positive and negative tabs) may protrude from the positive and negative current collectors and extend to one side of the housing, respectively. The tabs may be fused to said side of the housing to connect to electrode leads (positive and negative leads) extending to or exposed outside the housing.

[0143] For example, pouch-shaped shells, prismatic shells, cylindrical shells, coin-shaped shells, etc. can be used.

[0144] The embodiments of the present invention will be further described below with reference to specific experimental examples. The embodiments and comparative examples included in the experimental examples are only for illustrating the present invention and are not intended to limit the scope of the claims. Various changes and modifications can be made to the embodiments within the scope of the present invention and its technical concept, which is obvious to those skilled in the art, and such variations and modifications naturally fall within the scope of the claims.

[0145] Preparation Example

[0146] Graphite was added to a concentrated sulfuric acid aqueous solution and reacted in a cooling bath in the presence of a KMnO4 catalyst for pre-oxidation. Then, water was added and further oxidized in a heating bath to prepare graphene oxide. The graphene oxide was then dispersed in a dilute acid aqueous solution to remove impurities for purification.

[0147] Graphene oxide was obtained by filtration and low-temperature drying, and then reduced at high temperature under a protective gas atmosphere to prepare a graphene conductive material with the following physical properties.

[0148] Figure 1 and Figure 2 The images are scanning electron microscope (SEM) images of the graphene conductive materials prepared in Example 1 and Example 2, respectively. Figure 3 and Figure 4 The images are transmission electron microscope (TEM) images of the graphene conductive materials prepared in Example 1 and Example 2, respectively.

[0149] The results of X-ray diffraction (XRD) analysis of the graphene conductive materials of Preparation Example 1 and Preparation Example 2, namely the full width at half maximum (FWHM) of the (002) peak and the grain size calculated therefrom, and the results of X-ray photoelectron spectroscopy (XPS) analysis, namely the ratio of C 1s and O 1s atoms, are shown in Table 1 below.

[0150] [Table 1]

[0151]

[0152] Referring to Table 1, it can be confirmed that the graphene conductive materials of Preparation Example 1 and Preparation Example 2 have low oxygen content.

[0153] Example 1

[0154] LiNi will be used as the positive electrode active material 0.8 Co 0.1 Mn 0.1 O2, graphene conductive material of Example 1, polyvinylpyrrolidone (PVP) as a conductive material dispersant, and polyvinylidene fluoride (PVDF, Solvay, S514) as a binder are mixed and dispersed in N-methylpyrrolidone at a weight ratio of 98.175:0.5:0.125:1.2 to prepare a positive electrode slurry (solid concentration: 70%).

[0155] The positive electrode slurry was coated onto one side of an aluminum foil (12 μm thick) using a doctor blade method, dried at 120°C for 1 hour, and then calendered to form a 110 μm thick electrode with a loading of 18.0 g / cm³. 2The positive electrode active material layer is used to fabricate the positive electrode.

[0156] Artificial graphite, natural graphite, and SiOx (0 < x < 2) as the negative electrode active material, multi-walled carbon nanotubes (MWCNT, JEIO, 10B) as the conductive material, PVP as the dispersion material, styrene-butadiene rubber (SBR, Hansol, A200) as the binder, and carboxymethyl cellulose (CMC, Nippon Paper Industries, MAC800LC) as the thickener are mixed and dispersed in water at a weight ratio of 47.525:47.525:2.0:0.35:0.1:1.3:1.2 to prepare a negative electrode slurry (solid content concentration: 50%).

[0157] The negative electrode slurry is coated on one side of a copper foil (thickness: 8 μm) using a doctor blade method, dried at 120 °C for 1 hour, and calendered to form a negative electrode active material layer with a thickness of 131 μm and a loading of 10.2 g / cm 2 to fabricate the negative electrode.

[0158] The above positive electrode and negative electrode are respectively punched into circles with dimensions of 14Φ and 16Φ. In a coin cell casing, a separator (polyethylene, thickness: 13 μm) is stacked successively between the positive electrode and the negative electrode. After injecting the electrolyte, the coin cell casing is clamped, and then it is immersed for more than 1 hour.

[0159] The electrolyte used is an electrolyte in which 1.1 M LiPF6 is dissolved in a mixed solvent of EC / EMC (25 / 75; volume ratio), and then 8 wt% of fluoroethylene carbonate (FEC), 0.5 wt% of 1,3-propylene sultone (PRS), and 1.0 wt% of 1,3-propane sultone (PS) are added.

[0160] After that, it is stabilized for more than 12 hours, and three charge-discharge cycles of formation are carried out (charging conditions: CC-CV 0.1C 4.2V 0.05C cut-off (CUT-OFF), discharging conditions: CC 0.1C 2.5V cut-off).

[0161] Examples and Comparative Examples

[0162] The positive electrode and the battery are fabricated by the same method as in Example 1, except that the composition of the positive electrode active material layer is adjusted according to Table 2 below.

[0163] [Table 2]

[0164]

[0165] G-1: Graphene of Preparation Example 1

[0166] G-2: Graphene of Preparation Example 2

[0167] C-1: Carbon black (Cabot, XC72)

[0168] C-2: Graphite-based conductive material (Imerray, KS6L)

[0169] SWCNT: Single-walled carbon nanotubes (SWCNT) (Ocsial, TUBALL)

[0170] Measurement example

[0171] The physical properties of the conductive materials used in manufacturing the positive electrode in the examples and comparative examples were measured using the following methods and are shown in Table 3 below.

[0172] (1) Measurement of BET specific surface area

[0173] Using an ASAP2420 (McMerrittik) instrument, nitrogen was used as the adsorbate gas and helium as the carrier gas. Specific surface area was measured using a continuous flow method with BET relative pressure at 5 predefined points. 2g of conductive material was measured and heated at 150°C for 8 hours to reduce the pressure to below 10 μmHg, removing internal moisture and gases. Next, it was cooled to liquid nitrogen temperature (77.300K) to adsorb a mixture of nitrogen and helium. Then, it was heated to room temperature to desorb the adsorbed nitrogen. The amount of adsorbed and desorbed nitrogen was detected using a thermal conductivity detector to calculate the sample's specific surface area.

[0174] (2) Measurement of powder resistance

[0175] 0.2 g of conductive material was filled into a 1 mm insulating mold, and pressures of 4 kN, 8 kN, 12 kN, 16 kN, or 20 kN were applied. Four probes were then used to measure the surface current and voltage of each sample with different densities, thus measuring the powder resistivity in Ω·cm. A fitted line was derived with the sample density as the X-axis and the sample volume resistivity as the Y-axis, and X was calculated to be 1 g / cm³. 3 The resistance of the powder at that time.

[0176] (3) Raman spectroscopy analysis

[0177] Raman spectra of conductive materials were obtained using a Raman spectroscopy analysis apparatus (manufacturer: Renishaw, model name: inVia™ confocal Raman microscope) equipped with a continuous wave laser (manufacturer: HÜBNER Photonics, model name: Cobolt 04-01 Series Samba™, output wavelength: 532.1±0.3nm) and a charge-coupled device (CCD) camera.

[0178] In obtaining the Raman spectrum, a laser source was focused onto the conductive material using an objective lens (manufacturer: Leica, model name: N PLAN EPI 50x / 0.75NA), with the laser intensity reaching the sample ranging from approximately 0.1 mW to approximately 5 mW.

[0179] Calculate the Raman spectrum at 1300 cm⁻¹ -1 Up to 1420cm -1 The peak intensity of the D band observed in the region is similar to that at 1540 cm⁻¹ -1 Up to 1620cm -1 The proportion of peak intensity of the G band observed in the region (I) D / I G ).

[0180] (4) XRD analysis

[0181] Using CuK α (1.5406 Å) rays as the target, XRD analysis was performed using XRD equipment (Panaco's Empyrean or X'Pert) under the following conditions. Furthermore, based on the XRD analysis results, the spacing d and L were calculated using Equations 1 to 3 above. c And the number of floors.

[0182] - 2θ = 10° to 70°

[0183] - Scanning speed = 0.01-0.02° / second

[0184] - Step size = 0.027-0.012° / step

[0185] [Table 3]

[0186]

[0187] Experimental Example

[0188] The physical properties of the cathode and battery were evaluated using the following methods, and the results are shown in Table 4 below.

[0189] (1) Measurement of electrode internal resistance

[0190] The positive electrode slurry was coated onto one side of a PET film (100 μm thick) using a doctor blade method and dried at 120°C for 30 minutes to produce an electrode sample.

[0191] Furthermore, the thickness of the electrode sample was input into the resistance measuring instrument (Mitsubishi, LORESTA-GPMCP-T610), and four probes (MCP-TP03P) were brought into contact with the sample surface to measure the internal resistance of the electrode (Ω·cm). The average value was used after eight measurements.

[0192] When the internal resistance of the electrode is above 100,000 Ω·cm, it is marked as unmeasurable.

[0193] (2) Evaluation of battery rate performance

[0194] The discharge rate characteristics of the batteries in the examples and comparative examples were evaluated (discharge conditions: CC 0.1C, 1.5C 2.5V cutoff; charging conditions: CC-CV 0.3C 4.2V 0.05C cutoff).

[0195] (3) Analysis of electrode surface images

[0196] For the positive electrode of Example 1, a scanning electron microscope (SEM) image of the surface of the positive electrode active material layer was measured.

[0197] Figures 5 to 9 The images shown are SEM images of the positive electrode active material layers of Example 1, Comparative Example 1, Comparative Example 3, Comparative Example 6, and Comparative Example 7, respectively.

[0198] [Table 4]

[0199]

[0200] Referring to Table 4, in the embodiments containing a positive electrode of graphene conductive material with 2 to 15 layers, the electrode internal resistance is reduced. Therefore, the rate performance of the battery in these embodiments is improved.

[0201] On the other hand, in Comparative Examples 1 and 2, which are positive electrodes made of graphene conductive materials containing more than 15 layers and with a slightly low BET specific surface area, the number of graphene particles per unit weight is reduced, and the contact area between the active material particles and the conductive material is reduced. Therefore, it is not conducive to forming a conductive path between the active material and the conductive material, resulting in a deterioration in the physical properties of the electrode's internal resistance.

[0202] Furthermore, the internal resistance of the electrodes in Comparative Examples 3 to 7, which are positive electrodes that do not contain graphene conductive materials, is much higher than that in the embodiments, and the rate performance of the batteries is also reduced.

[0203] Reference Figure 5 In Example 1, the conductive material on the surface of the positive electrode active material layer is uniformly mixed with and in contact with the positive electrode active material particles.

[0204] Reference Figures 6 to 9 In the comparative example, the conductive material on the surface of the positive electrode active material layer is locally distributed in certain areas, the connection between the positive electrode active material particles and the conductive material is poor, and there are aggregates of conductive material in the voids of the positive electrode active material layer, which may hinder the movement of lithium ions. Therefore, the conductivity of the positive electrode active material layer is reduced.

[0205] The above description is merely an example of applying the principles of this invention, and other configurations may be further included without departing from the scope of this invention.

Claims

1. An electrode for a secondary battery, comprising: Electrode current collector; as well as An electrode active material layer is disposed on one side of the electrode current collector and comprises an electrode active material and a graphene conductive material. Wherein, the number of layers of the graphene conductive material, as defined by Formula 1, is 2 to 15. [Formula 1] Number of layers = (L) c / D)+1 In Equation 1, L c The grain size, in Å, is calculated based on the X-ray diffraction (XRD) analysis results of the graphene conductive material, and D is the d-interval, in Å, calculated based on the X-ray diffraction analysis results of the graphene conductive material.

2. The electrode for a secondary battery according to claim 1, wherein, The number of layers is 3 to 12.

3. The electrode for a secondary battery according to claim 1, wherein, In Equation 1, D is 2.5 Å to 3.5 Å.

4. The electrode for a secondary battery according to claim 1, wherein, In Equation 1, L c The range is 7.5 Å to 50 Å.

5. The electrode for a secondary battery according to claim 1, wherein, The ratio I of the maximum peak intensity of the D band to the maximum peak intensity of the G band, calculated based on the Raman spectral analysis results of the graphene conductive material. D / I G It is greater than 0 and less than 1.

6. The electrode for a secondary battery according to claim 1, wherein, The graphene conductive material has a strength of 1 g / cm³. 3 The resistivity of the powder is below 0.005 Ω·cm.

7. The electrode for a secondary battery according to claim 1, wherein, The BET specific surface area of ​​the graphene conductive material is 100 m². 2 / g to 1000m 2 / g.

8. The electrode for a secondary battery according to claim 1, wherein, The content of the graphene conductive material is 0.01% to 1% by weight of the total weight of the electrode active material layer.

9. The electrode for a secondary battery according to claim 1, wherein, The content of the graphene conductive material is 0.1% to 0.5% of the total weight of the electrode active material layer.

10. The electrode for a secondary battery according to claim 1, wherein, The electrode active material contains lithium metal oxide.

11. The electrode for a secondary battery according to claim 1, wherein, The electrode active material includes graphite-based active material or silicon-based active material.

12. A lithium secondary battery comprising a positive electrode and a negative electrode disposed opposite to the positive electrode, wherein the positive electrode or the negative electrode is an electrode for a secondary battery according to claim 1.

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