Positive electrode for lithium secondary battery and lithium secondary battery comprising same

Abstract

A positive electrode for a lithium secondary battery according to an embodiment of the present invention may comprise: a positive electrode current collector; a first positive electrode active material layer formed on the positive electrode current collector and including a first positive electrode active material and a first positive electrode conductive material; and a second positive electrode active material layer formed on the first positive electrode active material layer and including a second positive electrode active material having an average particle diameter larger than that of the first positive electrode active material and a second positive electrode conductive material, a content of the first positive electrode conductive material in a total weight of the first positive electrode active material layer may be greater than a content of the second positive electrode conductive material in a total weight of the second positive electrode active material layer.

Description

Technical Field

[0001] The invention relates to a positive electrode for a lithium secondary battery and a lithium secondary battery comprising the positive electrode. Background Art

[0002] Secondary batteries are batteries that can be repeatedly charged and discharged, and are widely used as power sources for portable electronic devices such as mobile phones and notebook computers. In addition, battery packs including secondary batteries are being developed and used as power sources for environmentally friendly vehicles such as electric cars.

[0003] Lithium secondary batteries have high operating voltage and energy density per unit weight, and are advantageous in terms of charging speed and lightness, and therefore are being actively developed and applied.

[0004] For example, a lithium secondary battery may include an electrode assembly including a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and an electrolyte impregnating the electrode assembly.

[0005] For example, the positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector. For example, the positive electrode active material layer may include lithium metal oxide particles as a positive electrode active material.

[0006] For example, the lithium metal oxide particles may have a secondary particle form in which a plurality of primary particles are aggregated. In order to ensure high capacity of lithium secondary batteries, high nickel-based lithium metal oxide particles (eg, NCM 811 particles) with increased nickel content are being used as positive electrode active materials.

[0007] However, during repeated charge and discharge of the lithium secondary battery, cracks may be generated in the high nickel-based lithium metal oxide particles as lithium is inserted and removed. Therefore, the side reaction between the lithium metal oxide particles and the electrolyte may cause gas generation and reduce the life characteristics of the lithium secondary battery. In a high temperature environment, the generation of the above gas and the reduction of the life characteristics may be further aggravated. Summary of the invention

[0008] 1. Technical issues to be resolved

[0009] A technical problem of the present invention is to provide a positive electrode for a lithium secondary battery having improved lifespan characteristics.

[0010] A technical problem of the present invention is to provide a lithium secondary battery including a positive electrode for a lithium secondary battery having improved lifespan characteristics.

[0011] (II) Technical solution

[0012] According to an exemplary embodiment, a positive electrode for a lithium secondary battery may include: a positive electrode collector; a first positive electrode active material layer, the first positive electrode active material layer is formed on the positive electrode collector and includes a first positive electrode active material and a first positive electrode conductive material; and a second positive electrode active material layer, the second positive electrode active material layer is formed on the first positive electrode active material layer and includes a second positive electrode active material and a second positive electrode conductive material, the average particle size of the second positive electrode active material is greater than the average particle size of the first positive electrode active material, wherein the content of the first positive electrode conductive material in the total weight of the first positive electrode active material layer may be greater than the content of the second positive electrode conductive material in the total weight of the second positive electrode active material layer.

[0013] In some embodiments, the average particle size (D50) of the first positive electrode active material may be 2 μm to 10 μm, and the average particle size (D50) of the second positive electrode active material may be 5 μm to 20 μm.

[0014] In some embodiments, a ratio of an average particle size (D50) of the second positive active material to an average particle size (D50) of the first positive active material may be 2 to 5.

[0015] In some embodiments, the content of the first positive electrode conductive material may be 0.5 wt % to 3.0 wt % of the total weight of the first positive electrode active material layer, and the content of the second positive electrode conductive material may be 0.5 wt % to 2.0 wt % of the total weight of the second positive electrode active material layer.

[0016] In some embodiments, the first positive electrode conductive material may include two or more substances.

[0017] In some embodiments, the second positive electrode conductive material may be composed of a single substance.

[0018] In some embodiments, the first positive electrode active material layer may further include a first positive electrode binder, the second positive electrode active material layer may further include a second positive electrode binder, and the content of the first positive electrode binder in the total weight of the first positive electrode active material layer may be greater than the content of the second positive electrode binder in the total weight of the second positive electrode active material layer.

[0019] In some embodiments, the first positive electrode binder may contain 0.5 wt % to 4.0 wt % of the total weight of the first positive electrode active material layer, and the second positive electrode binder may contain 0.5 wt % to 2.5 wt % of the total weight of the second positive electrode active material layer.

[0020] In some embodiments, the positive electrode for the lithium secondary battery may further include a third positive electrode active material layer, which is formed on the second positive electrode active material layer and contains a third positive electrode active material and a third positive electrode conductive material, and the average particle size of the third positive electrode active material is greater than the average particle size of the second positive electrode active material.

[0021] In some embodiments, the content of the second positive electrode conductive material in the total weight of the second positive electrode active material layer may be greater than the content of the third positive electrode conductive material in the total weight of the third positive electrode active material layer.

[0022] A lithium secondary battery according to an exemplary embodiment may include: the above-described positive electrode for a lithium secondary battery; and a negative electrode disposed opposite to the positive electrode.

[0023] (III) Beneficial effects

[0024] According to an exemplary embodiment of the present invention, the degree of degradation of the positive electrode active material contained in the positive electrode for a lithium secondary battery can be made uniform as a whole, thereby suppressing side reactions, improving life characteristics, and suppressing cracks in the active material.

[0025] In some embodiments, a lithium secondary battery may include the positive electrode for a lithium secondary battery. Therefore, stable power characteristics and improved life characteristics may be simultaneously achieved.

[0026] The positive electrode active material for lithium secondary batteries of the present invention and the lithium secondary batteries containing the positive electrode active material can be widely used in electric vehicles, battery charging stations, other green technology fields such as solar power generation and wind power generation using batteries. The positive electrode active material for lithium secondary batteries of the present invention and the lithium secondary batteries containing the positive electrode active material can be used in eco-friendly electric vehicles and hybrid vehicles, etc., to prevent climate change by suppressing air pollution and greenhouse gas emissions. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 and Figure 2 is a schematic cross-sectional view showing a positive electrode for a lithium secondary battery according to an exemplary embodiment.

[0028] Figure 3 and Figure 4 1 and 2 are a schematic plan view and a schematic cross-sectional view, respectively, showing a lithium secondary battery according to an exemplary embodiment. DETAILED DESCRIPTION

[0029] According to an exemplary embodiment of the present invention, a positive electrode for a lithium secondary battery having a multilayer structure may be provided. In addition, a lithium secondary battery including the positive electrode may be provided.

[0030] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings, but these are merely exemplary embodiments, and the present invention is not limited to the specific embodiments described exemplarily.

[0031] Figure 1 and Figure 2 is a schematic cross-sectional view showing a positive electrode for a lithium secondary battery according to an exemplary embodiment.

[0032] Reference Figure 1 The positive electrode 100 for a lithium secondary battery may include a positive electrode collector 105 , a first positive electrode active material layer 112 formed on the positive electrode collector 105 , and a second positive electrode active material layer 114 formed on the first positive electrode active material layer 112 .

[0033] In an exemplary embodiment, the first positive active material layer 112 may include a first positive active material and a first positive conductive material, and the second positive active material layer 114 may include a second positive active material and a second positive conductive material.

[0034] The positive electrode current collector 105 may include stainless steel, nickel, aluminum, titanium, copper, or alloys thereof. The positive electrode current collector 105 may also include aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. The thickness of the positive electrode current collector is not limited thereto, but for example, the thickness of the positive electrode current collector may be 10 μm to 50 μm.

[0035] The first positive electrode active material and the second positive electrode active material may include a compound that can reversibly intercalate and deintercalate lithium ions.

[0036] According to an exemplary embodiment, the first positive electrode active material and the second positive electrode active material may each independently include a lithium-nickel metal oxide. The lithium-nickel metal oxide may further include at least one of cobalt (Co), manganese (Mn) and aluminum (Al).

[0037] In some embodiments, the first positive active material and the second positive active material or the lithium-nickel metal oxide may include a layered structure or a crystal structure represented by the following Chemical Formula 1.

[0038] [Chemical formula 1]

[0039] Li x Ni a M b O 2+z

[0040] In Chemical Formula 1, 0.9≤x≤1.5, 0.6≤a≤0.99, 0.01≤b≤0.4, and -0.5≤z≤0.1 may be satisfied. As described above, M may include Co, Mn, and / or Al.

[0041] The chemical structure represented by Chemical Formula 1 represents the bonding relationship contained in the layered structure or crystal structure of the positive active material, and does not exclude other additional elements. For example, M may include Co and / or Mn, and Co and / or Mn may be provided together with Ni as the main active element of the positive active material. Chemical Formula 1 is provided to represent the bonding relationship of the main active elements, and it should be understood that Chemical Formula 1 is a formula including the introduction and substitution of additional elements.

[0042] In one embodiment, in addition to the main active element, an auxiliary element for enhancing the chemical stability of the positive electrode active material or the layered structure / crystal structure may be further included. The auxiliary element may be mixed together into the layered structure / crystal structure to form a bond, and it should be understood that this case is also included in the chemical structure represented by Chemical Formula 1.

[0043] The auxiliary element may include, for example, at least one of Na, Mg, Ca, Y, Ti, Tl, 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 that contributes to the capacity / power activity of the first positive electrode active material and the second positive electrode active material together with Co or Mn, such as Al.

[0044] For example, the first positive active material and the second positive active material or the lithium-nickel metal oxide may include a layered structure or a crystal structure represented by the following Chemical Formula 1-1.

[0045] [Chemical formula 1-1]

[0046] Li x Ni a M1 b1 M2 b2 O 2+z

[0047] In Chemical Formula 1-1, M1 may include Co, Mn and / or Al. M2 may include the auxiliary elements described above. In Chemical Formula 1-1, 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b1+b2≤0.4, and -0.5≤z≤0.1 may be selected.

[0048] The first positive electrode active material and the second positive electrode active material may each independently further contain a coating element or a doping element. For example, an element substantially the same as or similar to the auxiliary element may be used as a coating element or a doping element. For example, one of the above elements or a combination of two or more of the above elements may be used as a coating element or a doping element.

[0049] The coating element or the doping element may be present on the surface of the lithium-nickel metal oxide particle or permeate through the surface of the lithium-nickel metal oxide particle and be included in the bonding structure represented by Chemical Formula 1 or Chemical Formula 1-1.

[0050] The first positive electrode active material and the second positive electrode active material may each independently include a nickel-cobalt-manganese (NCM)-based lithium oxide. In this case, an NCM-based lithium oxide with an increased nickel content may be used.

[0051] Ni can be provided as a transition metal related to the power and capacity of a lithium secondary battery. Therefore, by using a high-content (high-nickel (High-Ni)) composition in the positive electrode active material as described above, a high-capacity positive electrode and a high-capacity lithium secondary battery can be provided.

[0052] 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 the side reaction with the electrolyte may also increase. However, according to an exemplary embodiment, the conductivity can be maintained by including Co, while the life stability and capacity retention characteristics can be improved by Mn.

[0053] In the NCM-based lithium oxide, the content of Ni (e.g., the molar fraction of nickel in the total moles of nickel, cobalt and manganese) may be greater than 0.6, greater than 0.7 or greater than 0.8. In some embodiments, the content of Ni may 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.

[0054] In some embodiments, the first positive electrode active material and the second positive electrode active material may also each independently 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 (e.g., LiFePO4).

[0055] In some embodiments, the first positive electrode active material and the second positive electrode active material may each independently include, for example, an Mn-rich based active material having a chemical structure or crystal structure represented by Chemical Formula 2, an Li rich layered oxide (LLO) / Over Lithiated Oxide (OLO) based active material, or a Co-less based active material.

[0056] [Chemical Formula 2]

[0057] p[Li2MnO3]·(1-p)[Li q JO2]

[0058] 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.

[0059] In an exemplary embodiment, the average particle diameter (D50) of the second positive electrode active material may be greater than the average particle diameter of the first positive electrode active material.

[0060] As used herein, the term "average particle diameter" or "D50" may refer to the particle diameter corresponding to a cumulative volume percentage of 50% in the particle size distribution obtained from the particle volume.

[0061] In some embodiments, the average particle diameter of the first positive electrode active material may be 2 μm to 10 μm, 2 μm to 8 μm, or 2 μm to 6 μm. Within the above ranges, the power characteristics of the positive electrode can be improved by the first positive electrode active material having a small average particle diameter.

[0062] In some embodiments, the average particle diameter of the second positive electrode active material may be 5 μm to 20 μm, 5 μm to 15 μm, 7 μm to 15 μm, or 9 μm to 15 μm. Within the above ranges, the life characteristics can be improved by the second positive electrode active material having a large average particle diameter.

[0063] For example, the ratio of the average particle diameter of the second positive electrode active material to the average particle diameter of the first positive electrode active material may be 2 to 5, 3 to 5, or 3 to 4. Within the above ranges, the deterioration of the positive electrode active material in the positive electrode may occur uniformly as a whole. Therefore, side reactions of the positive electrode active material can be suppressed, and cracks in the first positive electrode active material and the second positive electrode active material can be suppressed, thereby improving the life characteristics.

[0064] For example, the positive electrode active material may be mixed in a solvent to prepare a positive electrode slurry. The positive electrode slurry may be coated on the positive electrode current collector 105 and then dried and rolled to manufacture the positive electrode active material layer 110. The coating process may be performed by gravure coating, slot extrusion coating, multi-layer simultaneous die coating, stamping, doctor blade coating, dip coating, bar coating, casting, and the like, and is not limited thereto.

[0065] In one embodiment, a first positive electrode active material may be mixed in a solvent to prepare a first positive electrode slurry. A second positive electrode active material may be mixed in a solvent to prepare a second positive electrode slurry. The first positive electrode slurry may be coated on the positive electrode collector 105, dried and rolled to manufacture a first positive electrode active material layer 112. The second positive electrode slurry may be coated on the first positive electrode active material layer 112, dried and rolled to manufacture a second positive electrode active material layer 114. Thus, the first positive electrode active material layer 112 and the second positive electrode active material layer 114 may be manufactured in sequence.

[0066] In one embodiment, a first positive electrode active material may be mixed in a solvent to prepare a first positive electrode slurry. A second positive electrode active material may be mixed in a solvent to prepare a second positive electrode slurry. The first positive electrode slurry and the second positive electrode slurry may be coated on the positive electrode collector 105 and then dried and rolled to manufacture a first positive electrode active material layer 112 and a second positive electrode active material layer 114. Therefore, the first positive electrode active material layer 112 and the second positive electrode active material layer 114 may be manufactured at the same time.

[0067] The positive electrode active material layer 110 may further include a conductive material, and may optionally further include a binder, a thickener, etc. For example, the first positive electrode active material layer 112 may further include a first positive electrode conductive material, and may optionally further include a first positive electrode binder, a thickener, etc. For example, the second positive electrode active material layer 114 may further include a second positive electrode conductive material, and may optionally further include a second positive electrode binder, a thickener, etc.

[0068] Non-limiting examples of the solvent used to manufacture the positive active material layer 110 include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, and the like.

[0069] The first positive electrode conductive material and the second positive electrode conductive material may be added to enhance conductivity and / or mobility of lithium ions or electrons. For example, the first positive electrode conductive material and the second positive electrode conductive material may each independently include carbon-based conductive materials such as graphite, carbon black, acetylene black, Ketjen black, graphene, carbon nanotubes, vapor-grown carbon fiber (VGCF), carbon fiber, and / or metal-based conductive materials including perovskite materials such as tin, tin oxide, titanium oxide, LaSrCoO3, LaSrMnO3, etc., but are not limited thereto.

[0070] In an exemplary embodiment, the content of the first positive electrode conductive material in the total weight of the first positive electrode active material layer 112 may be greater than the content of the second positive electrode conductive material in the total weight of the second positive electrode active material layer 114. The conductivity of the first positive electrode active material layer 112 disposed on the positive electrode collector 105 may be higher than the conductivity of the second positive electrode active material layer 114 disposed at the outermost portion of the positive electrode 100. Therefore, the power characteristics of the first positive electrode active material layer 112 protected by the second positive electrode active material layer 114 may be maintained or improved, while the lifespan characteristics may be improved.

[0071] In some embodiments, the content of the first positive electrode conductive material may be 0.5 wt % to 3.0 wt %, 0.5 wt % to 2.5 wt %, or 1.0 wt % to 2.5 wt % of the total weight of the first positive electrode active material layer 112. Within the above range, the first positive electrode active material having a small particle size is sufficiently included in the first positive electrode active material layer 112, thereby improving conductivity.

[0072] In one embodiment, the content of the first positive electrode conductive material may be 1.5 wt % to 2.5 wt %, 1.5 wt % to 2.3 wt % or 1.8 wt % to 2.3 wt % of the total weight of the first positive electrode active material layer 112. Within the above range, the conductivity of the positive electrode 100 may be further improved.

[0073] In some embodiments, the content of the second positive electrode conductive material may be 0.5 wt % to 2.0 wt %, 0.5 wt % to 1.5 wt %, or 0.8 wt % to 1.5 wt % of the total weight of the second positive electrode active material layer 114. Within the above range, the second positive electrode active material having a large particle size is sufficiently included in the second positive electrode active material layer 114, so that cracks can be prevented and life characteristics can be improved.

[0074] In one embodiment, the content of the second positive electrode conductive material may be 1.0 wt % to 1.5 wt % or 1.3 wt % to 1.5 wt % of the total weight of the second positive electrode active material layer 114. Within the above range, the life characteristics of the positive electrode 100 may be further improved.

[0075] In some embodiments, the first positive electrode conductive material may include more types of conductive materials than the second positive electrode conductive material. For example, the first positive electrode conductive material and the second positive electrode conductive material may each include a mixture of two or more conductive materials. For example, the number of types of conductive materials mixed in the first positive electrode conductive material may be greater than the number of types of conductive materials mixed in the second positive electrode conductive material. Therefore, the conductivity of the first positive active material layer 112 may be higher than the conductivity of the second positive active material layer 114.

[0076] According to some embodiments, the first positive electrode conductive material may include two or more substances. For example, the first positive electrode conductive material may include two or more substances, and the second positive electrode conductive material may consist of a single substance.

[0077] In some embodiments, the first positive active material layer 112 may further include a first positive binder, and the second positive active material layer 114 may further include a second positive binder.

[0078] The first positive electrode binder and the second positive electrode binder may each independently include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (poly(vinylidene fluoride-co-hexafluoropropylene) copolymer, PVDF-co-HFP), polyacrylonitrile (polyacrylonitrile), polymethyl methacrylate (polymethylmethacrylate), acrylonitrile butadiene rubber (acrylonitrile butadiene rubber, NBR), polybutadiene rubber (polybutadiene rubber, BR), styrene-butadiene rubber (styrene-butadiene rubber, SBR), etc.

[0079] In one embodiment, as the first positive electrode binder and the second positive electrode binder, a PVDF-based binder may be used. Therefore, the amount of the first positive electrode binder and the second positive electrode binder respectively contained in the first positive electrode active material layer 112 and the second positive electrode active material layer 114 may be reduced. In addition, the amount of the first positive electrode active material and the second positive electrode active material respectively contained in the first positive electrode active material layer 112 and the second positive electrode active material layer 114 may be increased, and the power characteristics and capacity characteristics of the secondary battery may be improved.

[0080] In some embodiments, the content of the first positive electrode binder in the total weight of the first positive electrode active material layer 112 may be greater than the content of the second positive electrode binder in the total weight of the second positive electrode active material layer 114. Therefore, the first positive electrode binder having a relatively high content may be protected by the second positive electrode active material layer 114, thereby improving the mechanical stability of the positive electrode.

[0081] In one embodiment, the content of the first positive electrode binder may be 0.5 wt % to 4.0 wt %, 1.0 wt % to 4.0 wt %, 1.5 wt % to 4.0 wt %, 2.0 wt % to 4.0 wt %, or 2.5 wt % to 4.0 wt % of the total weight of the first positive electrode active material layer 112. Within the above range, the first positive electrode active material having a small particle size is sufficiently included in the first positive electrode active material layer 112, so that conductivity can be improved.

[0082] In one embodiment, the content of the second positive electrode binder may be 0.5 wt % to 2.5 wt %, 1.0 wt % to 2.5 wt %, 1.5 wt % to 2.5 wt %, 1.5 wt % to 2.2 wt %, or 1.8 wt % to 2.2 wt % relative to the total weight of the second positive electrode active material layer 114. Within the above range, the second positive electrode active material having a large particle size is sufficiently included in the second positive electrode active material layer 114, so that cracks can be prevented and life characteristics can be improved.

[0083] In one embodiment, the positive active material layer 110 may further include a thickener and / or a dispersant, etc. As one embodiment, the positive active material layer 110 may include a thickener such as carboxymethylcellulose (CMC).

[0084] Reference Figure 2 The positive electrode 100 may further include a third positive active material layer 116 formed on the second positive active material layer 114 and including a third positive active material and a third positive conductive material.

[0085] In some embodiments, the average particle size of the third positive electrode active material may be greater than the average particle size of the second positive electrode active material. For example, the average particle size of the third positive electrode active material may be greater than the average particle size of the second positive electrode active material, and the average particle size of the second positive electrode active material may be greater than the average particle size of the first positive electrode active material.

[0086] Therefore, the degree of degradation of the positive electrode active material (e.g., the first positive electrode active material, the second positive electrode active material, and the third positive electrode active material) contained in the positive electrode can be more uniform as a whole. Therefore, the side reaction between the positive electrode active material and the electrolyte solvent can be suppressed, and the generation of cracks in the positive electrode active material can be suppressed, thereby further improving the life characteristics.

[0087] In some embodiments, the content of the second positive electrode conductive material relative to the total weight of the second positive electrode active material layer 114 may be greater than the content of the third positive electrode conductive material relative to the total weight of the third positive electrode active material layer 116. In this case, the conductivity of the second positive electrode active material layer 114 may be higher than the conductivity of the third positive electrode active material layer 116 disposed at the outermost portion of the positive electrode 100. Therefore, the power characteristics of the second positive electrode active material layer 114 protected by the third positive electrode active material layer 116 may be maintained or improved, while the life characteristics may be improved.

[0088] In some embodiments, the second positive electrode conductive material may include more types of conductive materials than the third positive electrode conductive material. For example, the second positive electrode conductive material and the third positive electrode conductive material may use two or more conductive materials as a mixture.

[0089] For example, the number of types of the mixed conductive materials of the second positive electrode conductive material may be greater than the number of types of the mixed conductive materials of the third positive electrode conductive material. Therefore, the conductivity of the second positive active material layer 114 may be higher than that of the third positive active material layer 116.

[0090] Figure 3 and Figure 4 1 and 2 are a schematic plan view and a schematic cross-sectional view, respectively, showing a lithium secondary battery according to an exemplary embodiment. Figure 4 It is along Figure 3 A cross-sectional view taken along line I-I'.

[0091] Reference Figure 3 and Figure 4 The lithium secondary battery may include the positive electrode 100 , the negative electrode 120 , and the separator 140 .

[0092] The positive electrode 100 may include the above-mentioned positive electrode current collector 105, a first positive electrode active material layer 112 formed on the positive electrode current collector 105, and a second positive electrode active material layer 114 formed on the first positive electrode active material layer 112.

[0093] The negative electrode 120 may include a negative electrode current collector 125 and a negative electrode active material layer 130 provided on at least one surface of the negative electrode current collector 125.

[0094] As non-limiting examples of the negative electrode current collector 125, a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a foam nickel, a foam copper, a polymer substrate coated with a conductive metal, etc. may be mentioned. The thickness of the negative electrode current collector 125 is not limited thereto, but for example, the thickness of the negative electrode current collector 125 may be 10 μm to 50 μm.

[0095] The negative electrode active material layer 130 may contain a negative electrode active material. As the negative electrode active material, a material that allows lithium ions to be inserted and extracted may be used. For example, as the negative electrode active material, a carbon-based material such as crystalline carbon, amorphous carbon, a carbon composite, or carbon fiber; lithium metal; a lithium alloy; a silicon (Si)-containing substance or a tin (Sn)-containing substance, etc. may be used.

[0096] As examples of the amorphous carbon, hard carbon, coke, mesocarbon microbead (MCMB), mesophase pitch-based carbon fiber (MPCF), etc. may be mentioned.

[0097] As examples of the crystalline carbon, graphite-based carbon such as natural graphite, artificial graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc. may be mentioned.

[0098] The lithium metal may include pure lithium metal or lithium metal formed with a protective layer for suppressing dendrite growth, etc. In one embodiment, a layer containing lithium metal deposited or coated on the negative electrode current collector may be used as the negative electrode active material layer 130. In one embodiment, a lithium thin film layer may be used as the negative electrode active material layer.

[0099] As elements included in the lithium alloy, aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium, etc. may be mentioned.

[0100] The silicon-containing substance may provide further increased capacity characteristics. The silicon-containing substance may include Si, SiO x (0 < x < 2), SiO doped with a metal x (0 < x < 2), a silicon-carbon composite, etc. The metal may include lithium and / or magnesium, and SiO doped with a metal x(0 < x < 2) may contain metal silicate.

[0101] For example, the negative electrode active material may be mixed in a solvent to prepare a negative electrode paste. The negative electrode paste may be coated / deposited on the negative electrode current collector 125 and then dried and calendered to form the negative electrode active material layer 130. The coating process may include gravure coating, slot die coating, multi-layer simultaneous die coating, imprinting, knife coating, dip coating, rod coating, casting, etc.

[0102] The negative electrode active material layer 130 may further contain a binder, and may optionally further contain a conductive material, a thickening agent, etc.

[0103] In some embodiments, the negative electrode 120 may further include a negative electrode active material layer 130 in the form of lithium metal formed by a deposition / coating process.

[0104] Non-limiting examples of solvents for the negative electrode active material layer include water, pure water, deionized water, distilled water, ethanol, isopropyl alcohol, methanol, acetone, n-propanol, tert-butanol, etc.

[0105] As the binder, conductive material, and thickening agent, the above substances that can be used in manufacturing the positive electrode 100 can be used.

[0106] In some embodiments, the negative electrode binder may use a styrene-butadiene rubber (SBR)-based binder, carboxymethyl cellulose (CMC), a polyacrylic acid-based binder, a poly(3,4-ethylenedioxythiophene) (PEDOT)-based binder, etc.

[0107] A separator 140 may be disposed between the positive electrode 100 and the negative electrode 120. The separator 140 may be arranged to prevent a short circuit between the positive electrode 100 and the negative electrode 120 and allow the flow of ions. For example, the thickness of the separator may be 10 μm to 20 μm.

[0108] For example, the separator 140 may include a porous polymer membrane or a porous non-woven fabric. The porous polymer membrane may include polyolefin-based polymers such as ethylene polymers, propylene polymers, ethylene / butene copolymers, ethylene / hexene copolymers, ethylene / methacrylate copolymers, etc.

[0109] The porous nonwoven fabric may include high melting point glass fiber, polyethylene terephthalate fiber, and the like.

[0110] The separator may also include a ceramic-based material. For example, inorganic particles are coated on or dispersed in the polymer film, thereby improving heat resistance.

[0111] The separator may have a single-layer or multi-layer structure including the above-mentioned polymer film and / or nonwoven fabric.

[0112] According to an exemplary embodiment, the positive electrode 100, the negative electrode 120 and the separator 140 may be repeatedly disposed to form an electrode assembly 150. In some embodiments, the electrode assembly 150 may be a winding type, a stack type, a z-folding type, or a stack-folding type.

[0113] The electrode assembly 150 is accommodated in the case 160 together with an electrolyte, so that a lithium secondary battery may be defined. According to an exemplary embodiment, the electrolyte may use a non-aqueous electrolyte.

[0114] The non-aqueous electrolyte may include a lithium salt as an electrolyte and an organic solvent. The lithium salt may be, for example, Li + X - Represents, and as the anion of the lithium salt (X - ), we can exemplify 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 - 、(CF3CF2SO2)2N - wait.

[0115] The organic solvent may include an organic compound that has sufficient solubility for the lithium salt and additives and is not reactive in the battery. The organic solvent may include, for example, a carbonate-based solvent, an ester-based solvent, an ether-based solvent, a ketone-based solvent, an alcohol-based solvent, and / or an aprotic solvent.

[0116] The organic solvent may be propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate (butylene 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-propyl acetate (n-PA), 1,1-dimethylethyl acetate (DMEA), methyl propionate (MP), ethyl propionate (EP), fluoroethyl acetate (FEA), difluoroethyl acetate (DFEA), trifluoroethyl acetate (TFEA), dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether ( ... ether, DEGDME), dimethoxyethane, tetrahydrofuran (THF), 2-methyltetrahydrofuran, ethyl alcohol, isopropyl alcohol, dimethyl sulfoxide, acetonitrile, diethoxyethane, cyclopentane, γ-butyrolactone and propylene sulfite, etc. These can be used alone or in combination of two or more.

[0117] The non-aqueous electrolyte may further include an additive, such as a cyclic carbonate-based compound, a fluorine-substituted carbonate-based compound, a sultone-based compound, a cyclic sulfate-based compound, a cyclic sulfite-based compound, a phosphate-based compound, a borate-based compound, and the like.

[0118] The cyclic carbonate-based compound may include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and the like.

[0119] The fluorine-substituted carbonate-based compound may include fluoroethylene carbonate (FEC) and the like.

[0120] The sultone-based compound may include 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, and the like.

[0121] The cyclic sulfate-based compound may include 1,2-ethylene sulfate, 1,2-propylene sulfate, and the like.

[0122] The cyclic sulfite-based compound may include ethylene sulfite, butylene sulfite, and the like.

[0123] The phosphate-based compound may include lithium difluoro bis-oxalato phosphate, lithium difluoro phosphate, and the like.

[0124] The borate-based compound may include lithium bis(oxalato) borate and the like.

[0125] like Figure 4 As shown, the tabs (positive tab and negative tab) may protrude from the positive current collector 105 and the negative current collector 125, respectively, and extend to one side of the case 160. The tabs may be fused to the one side of the case 160 and connected to the electrode leads (positive lead 107 and negative lead 127) extending to the outside of the case 160 or exposed to the outside of the case 160.

[0126] For example, a pouch-type case, a prismatic case, a cylindrical case, a coin-shaped case, etc. may be used.

[0127] The embodiments of the present invention are further described below with reference to specific experimental examples. The embodiments and comparative examples included in the experimental examples are only used to illustrate the present invention and are not used to limit the claims. Various modifications and variations can be made to the embodiments within the scope of the present invention and the technical concept, which is obvious to those skilled in the art, and it is natural that such modifications and variations belong to the scope of the claims.

[0128] Examples and Comparative Examples

[0129] Example 1

[0130] (1) Manufacturing of positive electrode

[0131] LiNi as the first positive electrode active material 0.8 Co 0.1 Mn 0.1 O2 (average particle size of 4 μm), carbon black as a first positive electrode conductive material, and polyvinylidene fluoride (PVDF) as a first positive electrode binder are mixed in a weight ratio of 94.5:1.5:4 to prepare a first positive electrode active material layer slurry. An aluminum foil having a protrusion (positive electrode tab) on one side is prepared. The slurry is applied to the area of ​​the aluminum foil other than the protrusion, and then dried and rolled to form a first positive electrode active material layer.

[0132] Then, LiNi as the second positive electrode active material 0.8 Co 0.1 Mn 0.1 O2 (with an average particle size of 15 μm), carbon black as a second positive electrode conductive material, and polyvinylidene fluoride (PVDF) as a second positive electrode binder are mixed in a weight ratio of 97:1:2 to prepare a second positive electrode active material layer slurry, and then the second positive electrode active material layer slurry is coated on the first positive electrode active material layer and dried and rolled to form a second positive electrode active material layer.

[0133] (2) Manufacturing of negative electrode

[0134] A negative electrode slurry was prepared by dispersing a negative electrode active material prepared by mixing artificial graphite and natural graphite at a weight ratio of 7:3, styrene-butadiene rubber, and carboxymethyl cellulose at a weight ratio of 97:1:2 in distilled water. A copper foil having a protrusion (negative electrode tab) on one side was prepared. The negative electrode slurry was applied to the area of ​​the copper foil except the protrusion, dried, and rolled to manufacture a negative electrode.

[0135] (3) Manufacturing of lithium secondary batteries

[0136] A polyethylene separator (thickness of 20 μm) was placed between the positive electrode and the negative electrode to form an electrode assembly. A positive electrode lead and a negative electrode lead were connected to the positive electrode tab and the negative electrode tab, respectively, by welding.

[0137] After preparing a 1M LiPF6 solution (30:70 v / v EC / EMC mixed solvent), 1 wt% of fluoroethylene carbonate (FEC), 0.3 wt% of vinylethylene carbonate (VEC), 1 wt% of lithium difluorophosphate (LiPO2F2), 0.5 wt% of 1,3-propane sultone (PS) and 0.5 wt% of 1-propylene-1,3-sultone (PRS) were added to a total of 100 wt% of the electrolyte to prepare the electrolyte.

[0138] The electrode assembly is housed in a soft pack (casing) such that a portion of the positive electrode lead and the negative electrode lead are exposed to the outside, and three surfaces other than the electrolyte injection portion surface are sealed.

[0139] The electrolyte is injected into the soft package, and the electrolyte injection portion is sealed, thereby manufacturing a lithium secondary battery.

[0140] Example 2

[0141] A lithium secondary battery was manufactured by the same method as in Example 1, except that the weight ratio of the first positive electrode active material: the first positive electrode conductive material: the first positive electrode binder was changed to 94:2:4.

[0142] Example 3

[0143] A lithium secondary battery was manufactured by the same method as in Example 1, except that the weight ratio of the first positive electrode active material: the first positive electrode conductive material (carbon black): the first positive electrode conductive material (carbon nanotubes (CNTs)): the first positive electrode binder was changed to 93.5:1.5:1:4.

[0144] Example 4

[0145] LiNi as the first positive electrode active material 0.8 Co 0.1 Mn 0.1O2 (average particle size of 4 μm), carbon black as the first positive electrode conductive material, and polyvinylidene fluoride (PVDF) as the first positive electrode binder are mixed in a weight ratio of 94.5:1.5:4 to prepare a first positive electrode active material layer slurry. An aluminum foil having a protrusion (positive electrode tab) on one side is prepared. The slurry is applied to the area of ​​the aluminum foil other than the protrusion and dried and rolled to form a first positive electrode active material layer.

[0146] LiNi as the second positive electrode active material 0.8 Co 0.1 Mn 0.1 O2 (average particle size of 6 μm), carbon black as a second positive electrode conductive material, and polyvinylidene fluoride (PVDF) as a second positive electrode binder are mixed in a weight ratio of 95.75:1.25:3 to prepare a second positive electrode active material layer slurry. The second positive electrode active material layer slurry is coated on the first positive electrode active material layer and dried and rolled to form a second positive electrode active material layer.

[0147] LiNi as the third positive electrode active material 0.8 Co 0.1 Mn 0.1 O2 (average particle size of 15 μm), carbon black as a third positive electrode conductive material, and polyvinylidene fluoride (PVDF) as a third positive electrode binder are mixed in a weight ratio of 97:1:2 to prepare a third positive electrode active material layer slurry. The third positive electrode active material layer slurry is coated on the second positive electrode active material layer and dried and rolled to form a third positive electrode active material layer.

[0148] Thereafter, a negative electrode and a lithium secondary battery were manufactured by the same method as in Example 1.

[0149] Comparative Example 1

[0150] The LiNi with large particle size (average particle size of 15 μm) 0.8 Co 0.1 Mn 0.1 O2 and LiNi with small particle size (average particle size of 4 μm) 0.8 Co 0.1 Mn 0.1 O2 was mixed at a ratio of 1:1 to prepare a positive electrode active material.

[0151] The positive electrode active material, carbon black as a positive electrode conductive material, and polyvinylidene fluoride (PVDF) as a positive electrode binder are mixed in a weight ratio of 95.75:1.25:3 to prepare a positive electrode slurry. An aluminum foil having a protrusion (positive electrode tab) on one side is prepared. The slurry is applied to the area of ​​the aluminum foil except the protrusion and dried and rolled to form a positive electrode.

[0152] A lithium secondary battery was manufactured by the same method as in Example 1 except for the above contents.

[0153] Comparative Example 2

[0154] The LiNi with large particle size (average particle size of 15 μm) 0.8 Co 0.1 Mn 0.1 O2, LiNi with medium particle size (average particle size of 6 μm) 0.8 Co 0.1 Mn 0.1 O2 and LiNi with small particle size (average particle size of 4 μm) 0.8 Co 0.1 Mn 0.1 O2 was mixed at a ratio of 1:1:1 to prepare a positive electrode active material.

[0155] The positive electrode active material, carbon black as a positive electrode conductive material, and polyvinylidene fluoride (PVDF) as a positive electrode binder are mixed in a weight ratio of 95.75:1.25:3 to prepare a positive electrode slurry. An aluminum foil having a protrusion (positive electrode tab) on one side is prepared. The slurry is applied to the area of ​​the aluminum foil except the protrusion and dried and rolled to form a positive electrode.

[0156] A lithium secondary battery was manufactured by the same method as in Example 1 except for the above contents.

[0157] Comparative Example 3

[0158] A lithium secondary battery was manufactured by the same method as in Example 1, except that the average particle size of the first positive electrode active material was set to 15 μm, the average particle size of the second positive electrode active material was set to 4 μm, the weight ratio of the first positive electrode active material: the first positive electrode conductive material: the first positive electrode binder was changed to 97:1:2, and the weight ratio of the second positive electrode active material: the second positive electrode conductive material: the second positive electrode binder was changed to 94.5:1.5:4.

[0159] Comparative Example 4

[0160] A lithium secondary battery was manufactured by the same method as in Example 1, except that the weight ratio of the first positive electrode active material: the first positive electrode conductive material: the first positive electrode binder was changed to 95:1:4, and the weight ratio of the second positive electrode active material: the second positive electrode conductive material: the second positive electrode binder was changed to 96.5:1.5:2.

[0161] Experimental example

[0162] (1) Evaluation of initial charge capacity, initial discharge capacity, and initial capacity efficiency

[0163] The lithium secondary batteries manufactured according to the above-mentioned embodiments and comparative examples were charged in a chamber at 25° C. (CC-CV 0.1C 4.3V 0.005C cut-off (CUT-OFF)) and then the battery capacity (initial charge capacity) was measured. The battery was discharged (CC 0.1C 3.0V cut-off) and then the battery capacity (initial discharge capacity) was measured.

[0164] The initial capacity efficiency was evaluated by converting the value obtained by dividing the measured initial discharge capacity by the measured initial charge capacity into a percentage (%).

[0165] (2) Evaluation of internal resistance (DC-IR)

[0166] The lithium secondary battery manufactured according to the above embodiment and comparative example was charged at 0.3C CC / CV (4.2V 0.05C cut-off) at 25°C, and then discharged at 0.3C CC to SOC 50%. At SOC 50%, the C-rate was changed to 0.2C, 0.5C, 1.0C, 1.5C, 2.0C, 2.5C and 3.0C, and the discharge and supplementary charge were performed for 10 seconds respectively. When performing the discharge and supplementary charge, the end point of the voltage was set as the linear equation, and its slope was taken as DC-IR.

[0167] (3) Measurement of capacity retention (lifetime characteristics) during repeated charge and discharge

[0168] The lithium secondary batteries according to the examples and comparative examples were charged (CC / CV 1.0C 4.2V 0.05C cutoff) and discharged (CC 1.0C 2.5V cutoff) 500 times in a chamber at 45° C. The capacity retention was evaluated by dividing the 500th discharge capacity by the 1st discharge capacity.

[0169] The evaluation results are shown in Table 1 below.

[0170] [Table 1]

[0171] category Initial charge capacity (mAh / g) Initial discharge capacity (mAh / g) Initial capacity efficiency (%) Internal resistance (DC-IR) (mΩ) Capacity retention rate (500 cycles, %) Example 1 229.8 204.5 89.0 1.103 88.2 Example 2 229.2 206.3 90.0 1.074 89.3 Example 3 226.8 204.8 90.3 1.043 89.2 Example 4 229.8 203.6 88.6 1.002 89.1 Comparative Example 1 229.8 198.1 86.2 1.190 87.2 Comparative Example 2 229.8 199.9 87.0 1.160 88.3 Comparative Example 3 229.8 201.3 87.6 1.219 86.3 Comparative Example 4 229.8 198.8 86.5 1.213 86.1

[0172] Referring to Table 1, in the embodiment where a positive active material having a small particle size is used in the positive active material layer close to the positive current collector based on the positive current collector and the content of the conductive material is increased, the initial capacity efficiency is 88.5% or more and the capacity retention rate exceeds 88%.

[0173] In Example 3 using two conductive materials as the conductive material of the first positive electrode active material layer, the initial capacity efficiency and the capacity retention ratio increased.

[0174] In Example 4 in which the positive electrode active material layer included the first positive electrode active material layer, the second positive electrode active material layer, and the third positive electrode active material layer, the capacity retention ratio increased.

[0175] In Comparative Example 1 in which two cathode active materials having different particle sizes were used but the two cathode active materials were mixed to form a single cathode active material layer, the initial capacity efficiency and the capacity retention ratio were reduced.

[0176] In Comparative Example 2, in which three positive electrode active materials having different particle sizes were used but the three positive electrode active materials were mixed to form a single positive electrode active material layer, the initial capacity efficiency was reduced.

[0177] In Comparative Example 3, in which a plurality of positive electrode active material layers were used but the particle size of the positive electrode active material in the positive electrode active material layer close to the positive electrode current collector was larger and the content of the conductive material was lower, the initial capacity efficiency and the capacity retention ratio decreased.

[0178] In Comparative Example 4, in which a plurality of positive electrode active material layers were used but the content of the conductive material in the positive electrode active material layer close to the positive electrode current collector was lower, the initial capacity efficiency and the capacity retention ratio decreased.

Claims

1. A positive electrode for a lithium secondary battery, comprising: Positive electrode current collector; a first positive electrode active material layer, the first positive electrode active material layer being formed on the positive electrode current collector and comprising a first positive electrode active material and a first positive electrode conductive material; as well as a second positive electrode active material layer, the second positive electrode active material layer is formed on the first positive electrode active material layer and comprises a second positive electrode active material and a second positive electrode conductive material, the average particle size of the second positive electrode active material is larger than the average particle size of the first positive electrode active material, The content of the first positive electrode conductive material in the total weight of the first positive electrode active material layer is greater than the content of the second positive electrode conductive material in the total weight of the second positive electrode active material layer.

2. The positive electrode for a lithium secondary battery according to claim 1, wherein The average particle size of the first positive electrode active material is 2 μm to 10 μm, and the average particle size of the second positive electrode active material is 5 μm to 20 μm.

3. The positive electrode for a lithium secondary battery according to claim 1, wherein The ratio of the average particle size of the second positive electrode active material to the average particle size of the first positive electrode active material is 2 to 5.

4. The positive electrode for a lithium secondary battery according to claim 1, wherein The content of the first positive electrode conductive material is 0.5 wt % to 3.0 wt % of the total weight of the first positive electrode active material layer, and the content of the second positive electrode conductive material is 0.5 wt % to 2.0 wt % of the total weight of the second positive electrode active material layer.

5. The positive electrode for a lithium secondary battery according to claim 1, wherein The first positive electrode conductive material includes two or more substances.

6. The positive electrode for a lithium secondary battery according to claim 5, wherein The second positive electrode conductive material is composed of a single substance.

7. The positive electrode for a lithium secondary battery according to claim 1, wherein The first positive electrode active material layer further includes a first positive electrode binder, and the second positive electrode active material layer further includes a second positive electrode binder, and the content of the first positive electrode binder in the total weight of the first positive electrode active material layer is greater than the content of the second positive electrode binder in the total weight of the second positive electrode active material layer.

8. The positive electrode for a lithium secondary battery according to claim 7, wherein The content of the first positive electrode binder is 0.5 wt % to 4.0 wt % of the total weight of the first positive electrode active material layer, and the content of the second positive electrode binder is 0.5 wt % to 2.5 wt % of the total weight of the second positive electrode active material layer.

9. The positive electrode for a lithium secondary battery according to claim 1, wherein The positive electrode for lithium secondary battery further includes a third positive electrode active material layer, which is formed on the second positive electrode active material layer and contains a third positive electrode active material and a third positive electrode conductive material. The average particle size of the third positive electrode active material is greater than that of the second positive electrode active material.

10. The positive electrode for a lithium secondary battery according to claim 9, wherein The content of the second positive electrode conductive material in the total weight of the second positive electrode active material layer is greater than the content of the third positive electrode conductive material in the total weight of the third positive electrode active material layer.

11. A lithium secondary battery comprising: The positive electrode for a lithium secondary battery according to claim 1; as well as A negative electrode is arranged opposite to the positive electrode.

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