Positive electrode for lithium secondary battery and method for producing same

Abstract

The present invention relates to a positive electrode current collector and a positive electrode active material layer formed on at least one surface of the positive electrode current collector and containing a positive electrode active material, the positive electrode active material layer having a pore volume of 7.0×10 -3 cm 3 / g~8.0×10 -3 cm 3 The present invention relates to a positive electrode for a lithium secondary battery having a molecular weight of 1.0 to 1.0 g / g and a method for producing the same.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0004160 filed on January 11, 2022 and Korean Patent Application No. 10-2023-0003975 filed on January 11, 2023, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a positive electrode for a lithium secondary battery having excellent electrochemical properties and a method for producing the same. [Background technology]

[0003] Generally, a lithium secondary battery comprises a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode and the negative electrode contain active materials capable of intercalating and deintercalating lithium ions. Electrical energy is produced by oxidation and reduction reactions that occur during the intercalation and deintercalation of lithium ions at the positive electrode and the negative electrode.

[0004] The positive electrode of a lithium secondary battery is manufactured by applying a positive electrode active material to a current collector, rolling the current collector, and then drying the applied material. In recent years, there has been an increasing demand for lithium secondary batteries with a high energy density per unit volume so that they can be used in electric vehicles, etc. However, excessive rolling to increase the energy density can lead to problems such as particle cracking of the positive electrode active material and the formation of cracks inside the particles.

[0005] If particle breakage or cracks occur in the positive electrode active material, the contact area with the electrolyte increases, which increases the amount of gas generated due to side reactions with the electrolyte and accelerates the deterioration of the active material, which can reduce the battery's life characteristics.

[0006] Therefore, there is a need to develop a method for selecting a positive electrode having an optimal rolling condition that minimizes the occurrence of particle breakage and cracks and can exhibit excellent electrochemical properties when applied to a battery. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention provides a positive electrode that has little structural damage to particles even after rolling and has excellent electrochemical properties, and a method for producing the same. [Means for solving the problem]

[0008] The present invention provides a positive electrode current collector; and a positive electrode active material layer formed on at least one surface of the positive electrode current collector and containing a positive electrode active material, The positive electrode active material layer is Pore ​​volume is 7.0 x 10 -3 cm 3 / g~8.0×10 -3 cm 3 / g

[0009] The present invention also provides forming a positive electrode active material layer by coating at least one surface of a positive electrode current collector with a positive electrode slurry containing a positive electrode active material; and rolling the positive electrode current collector and the positive electrode active material layer, The pore volume of the positive electrode active material layer after rolling is 7.0 × 10 -3 cm 3 / g~8.0×10 -3 cm 3 The present invention provides a method for producing a positive electrode for a lithium secondary battery, in which the electrode has a specific surface area of ​​1000 μm / g. [Effects of the Invention]

[0010] The positive electrode according to one embodiment of the present invention includes a positive electrode active material with reduced particle cracking and internal cracking, and therefore can improve initial capacity, life characteristics, and gas generation rate when applied to a lithium secondary battery. [Brief explanation of the drawings]

[0011] [Figure 1] 1A and 1B are cross-sectional views of a positive electrode before and after rolling, observed with a scanning electron microscope. [Figure 2]1 shows the results of analyzing the positive electrodes manufactured in the examples and comparative examples of the present invention by the Barrett-Joyner-Halenda (BJH) method. [Figure 3] FIG. 1 is a graph showing the capacity retention rate and the resistance increase rate measured while charging and discharging batteries using positive electrodes manufactured in Examples and Comparative Examples of the present invention 600 times. [Figure 4] FIG. 10 is a graph showing the results of measuring the amount of gas generated after high-temperature storage for batteries using positive electrodes manufactured in Examples and Comparative Examples of the present invention. [Figure 5] FIG. 10 is a graph showing the results of measuring the rolling density depending on the mixing weight ratio of small particles to large particles in a cathode material having a bimodal particle size distribution. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present invention will now be described in more detail to aid in understanding the invention. In the present invention, "primary particle" refers to a particle unit composed of multiple determinants and having no apparent grain boundaries when observed under a scanning electron microscope at a magnification of 5,000 to 20,000 times. "Average particle size of primary particles" refers to the arithmetic mean value calculated after measuring the particle sizes of primary particles observed under a scanning electron microscope image. In the present invention, the "secondary particles" are particles formed by agglomeration of a plurality of primary particles.

[0013] In the present invention, "D 50 " means the particle size at 50% of the volume cumulative particle size distribution of the lithium composite transition metal oxide powder or the positive electrode active material powder, and when the lithium composite transition metal oxide is a secondary particle, the D 50 The above D 50can be measured using the laser diffraction method. For example, after dispersing a lithium composite transition metal oxide powder or a positive electrode active material powder in a dispersion medium, the dispersion is introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac MT 3000) and irradiated with ultrasonic waves of about 28 kHz at an output of 60 W. After obtaining a graph of the volume cumulative particle size distribution, the particle size corresponding to 50% of the volume cumulative amount can be measured.

[0014] In the present invention, the "BET specific surface area" and "pore volume" are calculated from a nitrogen adsorption isotherm obtained in a liquid nitrogen atmosphere at 77 K using a BELSORP-MAX (MicrotracBEL corp.). The specific surface area is calculated by the BET (Brunauer-Emmett-Teller) multipoint method, and the pore volume is calculated using a BJH (Barrett-Joyner-Halenda) plot for pores with diameters of 2 nm to 185 nm.

[0015] positive electrode First, the positive electrode for a lithium secondary battery according to the present invention will be described. To achieve high energy density in lithium secondary batteries and ensure high electrical conductivity through contact between the current collector and the active material, it is advantageous to manufacture electrodes with high rolling density. However, as can be seen in Figure 1, which shows the surface of an electrode before rolling (A) and after rolling (B) observed with a scanning electron microscope (SEM), depending on the rolling conditions, not only is particle cracking likely to occur, causing the particle shape of the active material to be distorted, but cracks may also occur within the particles.

[0016] Specifically, excessively high rolling density can lead to breakage of the primary particles of the active material, resulting in disconnection of the conductive path, or an increase in the reaction area with the electrolyte, resulting in intensified gas generation. Therefore, the electrochemical performance of the electrode and gas generation due to side reactions are significantly affected by the rolling characteristics.

[0017] Therefore, the present inventors have devised a new parameter to enable selection of a positive electrode containing an active material in a good particle state from a rolled positive electrode. Specifically ... -3 cm 3 / g~8.0×10 -3 cm 3 / g, it was confirmed that the optimum positive electrode active material particles can be contained, which contributes to improving the performance of the battery.

[0018] Specifically, the pore volume of the positive electrode active material layer is 7.0 × 10 -3 cm 3 / g or more, preferably 7.1 × 10 -3 cm 3 / g or more, the contact between particles is sufficient, which can reduce the resistance and contribute to increasing the cell capacity. -3 cm 3 / g or less, preferably 7.6 × 10 -3 cm 3 When the average particle size is less than 1 / g, the degree of particle cracking of the active material inside the electrode is small, so that the phenomenon of gas generation inside the cell being intensified can be prevented.

[0019] Meanwhile, in one embodiment of the present invention, the BET specific surface area of ​​the positive electrode active material layer is 1.30 m 2 / g excess 1.50m 2 / g, preferably less than 1.32m 2 / g~1.48m 2 / g, more preferably 1.33m 2 / g~1.45m 2 In this case, the particle cracking of the active material inside the electrode is not excessive and the contact between the particles is sufficient, so that the resistance characteristics can be improved.

[0020] In one embodiment of the present invention, the positive electrode current collector is not particularly limited as long as it does not induce a chemical change in the battery and has conductivity. For example, aluminum; stainless steel; nickel; titanium; baked carbon; or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like may be used.

[0021] Meanwhile, the positive electrode active material is not particularly limited as long as it is a compound that allows reversible intercalation and deintercalation of lithium, and may include, for example, at least one selected from the group consisting of lithium cobalt-based oxides, lithium nickel-based oxides, lithium nickel cobalt-based oxides, lithium manganese-based oxides, lithium nickel manganese-based oxides, lithium phosphate-based oxides, and lithium composite transition metal oxides such as lithium nickel cobalt manganese-based oxides.

[0022] Specifically, the positive electrode active material may be a lithium cobalt-based oxide such as LiCoO2 (LCO); a lithium nickel-based oxide such as LiNiO2 (LNO); 1-y4 Co y4 Lithium nickel cobalt oxides such as LiO2 (0≦y4<1); Lithium manganese oxides such as LiMnO2 (LMO), LiMnO3, LiMn2O3, and Li2MnO3; Li 1+y1 Mn 2-y1 O4(0≦y1≦0.33), LiNi y2 Mn 2-y2 O4(0≦y2≦2), LiNi y3 Mn 2-y3 O2 (0.01≦y3≦0.1), and lithium nickel manganese-based oxides such as Li2NiMn3O8; lithium phosphate-based oxides such as LiFePO4 and LiCoPO4; and lithium nickel cobalt manganese-based oxides represented by the following chemical formula 1.

[0023] In one embodiment of the present invention, the positive electrode active material may include a lithium composite transition metal oxide represented by the following Chemical Formula 1:

[0024] [Chemical Formula 1] Li 1+x (Ni a Co b Mn c M d )O2

[0025] In Chemical Formula 1 above, M is any one or more selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo, x, a, b, c, and d are respectively -0.2 ≤ x ≤ 0.2, 0.50 ≤ a < 1, 0 < b ≤ 0.25, 0 < c ≤ 0.25, 0 ≤ d ≤ 0.1, and a + b + c + d = 1.

[0026] In addition, the positive electrode active material may contain a lithium composite transition metal oxide containing 70 mol% or more of nickel based on the total mol of the transition metal. Specifically, in Chemical Formula 1 above, it may contain a lithium composite transition metal oxide in which a, b, c, and d are respectively 0.70 ≤ a < 1, 0 < b ≤ 0.20, 0 < c ≤ 0.20, and 0 ≤ d ≤ 0.05. More specifically, it may contain a lithium composite transition metal oxide in which a, b, c, and d are respectively 0.80 ≤ a < 1, 0 < b ≤ 0.15, 0 < c ≤ 0.15, and 0 ≤ d ≤ 0.03.

[0027] In one embodiment of the present invention, the positive electrode active material may be one or more selected from Li(Ni 0.83 Co 0.05 Mn 0.10 Al 0.02 )O2 and Li(Ni 0.86 Co 0.05 Mn 0.07 Al« 0.02 )O2.

[0028] In addition, the positive electrode active material may contain small particle positive electrode active materials and large particle positive electrode active materials in which D 50 are different from each other. In this case, the rolling characteristics of the electrode can be improved.​ At this time, the D of the small particle positive electrode active material 50 may be 2 μm to 5 μm, and D of the large particle positive electrode active material 50 The thickness may be 8 μm to 12 μm.

[0029] In one embodiment of the present invention, the weight ratio of the small particles to the large particles (i.e., weight of small particle positive electrode active material / weight of large particle positive electrode active material) may be 1.0 to 2.0, preferably 1.2 to 1.8, and more preferably 1.5. Mixing at such a weight ratio is advantageous for achieving high rolling density under various pressure conditions.

[0030] In one embodiment of the present invention, the positive electrode active material layer may have a porosity of 20% to 25%, where the porosity is a value calculated by the following formula 1:

[0031] [Formula 1] Porosity of the positive electrode active material layer (%) = ((true density of the positive electrode active material - electrode density) / true density of the positive electrode active material) × 100 The electrode density in the above formula 1 is a value calculated by the following formula 2.

[0032] [Formula 2] Electrode density = (weight of positive electrode - weight of positive electrode current collector) / (a ​​× b × c) In the above formula 2, a, b, and c are the width, length, and height, respectively, measured after separating the positive electrode current collector from the positive electrode.

[0033] In one embodiment of the present invention, the thickness of the positive electrode active material layer may vary depending on the amount of positive electrode slurry applied, but may be, for example, 50 μm to 54 μm, preferably 51 μm to 53 μm, and the total thickness of the positive electrode including the thickness of the positive electrode current collector may be 60 μm to 100 μm. This has the advantage of improving energy density and being advantageous for controlling the drying process. The thickness of the positive electrode is measured in a state where the positive electrode is stretched to its maximum extent and flattened.

[0034] Positive electrode manufacturing method Next, a method for producing the positive electrode for a lithium secondary battery will be described. A method for manufacturing a positive electrode for a lithium secondary battery according to the present invention includes the steps of: coating at least one surface of a positive electrode current collector with a positive electrode slurry containing a positive electrode active material to form a positive electrode active material layer; and rolling the positive electrode current collector and the positive electrode active material layer, wherein the pore volume of the positive electrode active material layer after the rolling is 7.0×10 -3 cm 3 / g~8.0×10 -3 cm 3 For each component of the production method, the above-mentioned explanations regarding each component of the positive electrode can be cited.

[0035] In one embodiment of the present invention, the BET specific surface area of ​​the positive electrode active material layer after rolling is 1.30 m 2 / g excess 1.50m 2 / g, preferably less than 1.32m 2 / g~1.48m 2 / g, more preferably 1.33m 2 / g~1.45m 2 / g.

[0036] In one embodiment of the present invention, the coating of the positive electrode slurry is carried out by applying the positive electrode slurry to at least one surface of the positive electrode current collector in an amount of 350 mg / 25 cm. 2 ~480mg / 25cm 2 Loading amount, preferably 380mg / 25cm 2 ~460mg / 25cm 2 It may also be coated with

[0037] In one embodiment of the present invention, the thickness of the positive electrode active material layer after rolling may be 66% to 72%, preferably 68% to 72%, of the thickness of the positive electrode active material layer before rolling. In this case, the BET specific surface area and pore volume of the positive electrode active material can be adjusted within the ranges of the present invention while achieving an appropriate rolling density.

[0038] Meanwhile, the positive electrode slurry may further include a binder and / or a conductive material in addition to the positive electrode active material, and may be prepared by dissolving them in a solvent.

[0039] The positive electrode active material may be included in an amount of 80 wt% to 99 wt%, specifically 90 wt% to 99 wt%, based on the total weight of the solid content in the positive electrode slurry. In this case, if the content of the positive electrode active material is 80 wt% or less, the energy density may be reduced, resulting in a decrease in capacity.

[0040] The binder is a component that aids in binding the active material and conductive material, etc., and in binding to the current collector, and may typically be added in an amount of 1 wt% to 30 wt% based on the total weight of the solids in the positive electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

[0041] The conductive material is a substance that does not induce chemical changes in the battery and provides conductivity, and may be added in an amount of 0.5 wt % to 20 wt % based on the total weight of the solid content in the positive electrode slurry.

[0042] The conductive material may be selected from conductive materials such as carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, carbon nanotubes, and graphite; conductive fibers such as carbon fibers and metal fibers; conductive powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives.

[0043] The solvent for the positive electrode slurry may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount that provides a preferred viscosity when containing the positive electrode active material, binder, conductive material, etc. For example, the solvent may be included so that the solids concentration in the positive electrode slurry containing the positive electrode active material, binder, and conductive material is 40 wt % to 90 wt %, preferably 60 wt % to 80 wt %.

[0044] In one embodiment of the present invention, the rolling step may be performed by cutting the positive electrode current collector coated with the positive electrode slurry into a size of 10 cm to 12 cm in width and 3 cm to 4 cm in length, placing the cut piece between two rolling rolls, and compressing the cut piece by adjusting the gap between the rolling rolls.

[0045] Lithium secondary battery Next, the lithium secondary battery according to the present invention will be described. The lithium secondary battery of the present invention includes the above-described positive electrode of the present invention, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and may be manufactured by a conventional method for manufacturing a lithium secondary battery, except for using the positive electrode of the present invention.

[0046] Meanwhile, the lithium secondary battery may optionally further include a battery container that houses the electrode assembly of the positive electrode, the negative electrode, and the separator, and a sealing member that seals the battery container.

[0047] In the lithium secondary battery, the negative electrode includes a negative electrode current collector and a negative electrode active material layer located on the negative electrode current collector. The negative electrode current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy may be used. The negative electrode current collector may typically have a thickness of 3 to 500 μm. As with the positive electrode current collector, the surface of the current collector may be formed with fine irregularities to strengthen the binding force of the negative electrode active material. The negative electrode current collector may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0048] The negative electrode active material layer includes a negative electrode active material, and optionally a binder and a conductive material. The negative electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium. Specific examples include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber, and amorphous carbon; metallic compounds capable of alloying with lithium such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, and Al alloys; and SiO β (0<β<2), metal oxides that can be doped and dedoped with lithium, such as SnO2, vanadium oxide, and lithium vanadium oxide; or composites containing the metallic compounds and carbonaceous materials, such as Si-C composites or Sn-C composites, and any one or a mixture of two or more of these may be used.

[0049] In addition, a metallic lithium thin film may be used as the negative electrode active material. In addition, either low-crystalline carbon or high-crystalline carbon may be used as the carbon material. Typical low-crystalline carbons include soft carbon and hard carbon, while typical high-crystalline carbons include amorphous, plate-like, flake-like, spherical, or fibrous natural or artificial graphite, Kish graphite, pyrolytic carbon, mesophase pitch-based carbon fiber, mesocarbon microbeads, mesophase pitches, and high-temperature-calcined carbons such as petroleum or coal tar pitch-derived cokes.

[0050] The conductive material is used to impart conductivity to the electrode. Any material that exhibits electronic conductivity and does not cause chemical changes in the resulting battery can be used without particular limitations. Specific examples include graphite, such as natural graphite or artificial graphite; carbon black, such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; carbon-based materials, such as carbon fibers and carbon nanotubes; metal powders or fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive polymers, such as polyphenylene derivatives. These materials may be used alone or in combination. The conductive material may typically be present in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0051] The binder improves adhesion between negative electrode active material particles and between the negative electrode active material and the negative electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer rubber (EPDM rubber), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. One or more of these may be used alone or in combination. The binder may be included in an amount of 1 to 30 wt %, preferably 1 to 20 wt %, and more preferably 1 to 10 wt %, based on the total weight of the negative electrode active material layer.

[0052] For example, the negative electrode active material layer may be manufactured by applying a negative electrode slurry containing a negative electrode active material, and optionally a binder and a conductive material, onto a negative electrode current collector and drying the applied slurry, or by casting the negative electrode slurry onto a separate support, peeling the resulting film from the support, and laminating the resulting film onto the negative electrode current collector.

[0053] The solvent for the negative electrode slurry may include water or an organic solvent such as NMP or alcohol, and may be used in an amount that provides a preferred viscosity when containing the negative electrode active material, binder, conductive material, etc. For example, the solvent may be included so that the solids concentration in the slurry containing the negative electrode active material, binder, and conductive material is 30 wt % to 80 wt %, preferably 40 wt % to 70 wt %.

[0054] Meanwhile, in the lithium secondary battery, the separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator typically used in lithium secondary batteries can be used without particular limitations. It is particularly preferred that the separator exhibits low resistance to ion migration and excellent electrolyte humidification. Specifically, a porous polymer film, such as a porous polymer film made from a polyolefin polymer, such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material may be used, and may be selectively used as a single-layer or multi-layer structure.

[0055] Furthermore, examples of the electrolyte used in the present invention include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the production of lithium secondary batteries, but are not limited to these.

[0056] Specifically, the electrolyte may include an organic solvent and a lithium salt. The organic solvent may be any solvent capable of acting as a medium through which ions involved in the electrochemical reaction of the battery can move. Specifically, examples of the organic solvent include ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; dimethylcarbonate (DMC), diethylcarbonate (DEC), ethylmethylcarbonate (EMC), ethylene carbonate (EC), and propylene carbonate. Examples of solvents that can be used include carbonate-based solvents such as ethylene carbonate (PC), alcohol-based solvents such as ethyl alcohol and isopropyl alcohol, nitriles such as R-CN (R is a C2-C20 hydrocarbon group having a linear, branched, or cyclic structure, and may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane, and sulfolanes. Among these, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and a high dielectric constant, which can improve the charge / discharge performance of batteries, and low-viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, or diethyl carbonate) are more preferred.

[0057] The lithium salt may be any compound capable of providing lithium ions used in lithium secondary batteries without particular limitation. Specifically, the lithium salt may be LiPF, LiClO, LiAsF, LiBF, LiSbF, LiAlO, LiAlCl, LiCF, SO, LiCF, SO, LiN(C, F, SO), LiN(C, F, SO), LiN(CF, SO), LiCl, LiI, or LiB(C, O) . The lithium salt is used at a concentration of 0.1 to 5.0 M, preferably 0.1 to 3.0 M. When the lithium salt concentration is within the above range, the electrolyte has suitable conductivity and viscosity, exhibiting excellent electrolyte performance and allowing lithium ions to migrate effectively.

[0058] In addition to the electrolyte components, the electrolyte may further contain additives to improve battery life characteristics, suppress battery capacity loss, and improve battery discharge capacity. Examples of additives include, but are not limited to, haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, and aluminum trichloride, which may be used alone or in combination. The additives may be present in an amount of 0.1 to 10 wt %, preferably 0.1 to 5 wt %, based on the total weight of the electrolyte.

[0059] As described above, the lithium secondary battery including the positive electrode according to the present invention stably exhibits excellent discharge capacity, output characteristics, and capacity retention rate, and is therefore useful in the fields of portable devices such as mobile phones, notebook computers, and digital cameras, and electric vehicles such as hybrid electric vehicles (HEVs).

[0060] Therefore, according to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same. The battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, such as power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0061] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following detailed description of the preferred embodiments of the present invention will be made in order to enable those skilled in the art to easily carry out the present invention. [Example]

[0062] [Examples and Comparative Examples: Production of Positive Electrode] Reference Experiment Example 1. Prior to manufacturing the positive electrode, in order to confirm the optimum mixing ratio of small particles and large particles, the rolling density value according to the mixing weight ratio of the bimodal positive electrode material was confirmed by the following method.

[0063] Specifically, D 50 Li(Ni 0.83 Co 0.05 Mn 0.10 Al 0.02 )O2 and D 50 Li(Ni 0.86 Co 0.5 Mn 0.7 Al 0.2 )O2(D 50 = 10 μm) were mixed in weight ratios of 7:3, 6:4, 5:5, 4:6, and 3:7, respectively, to produce cathode materials with bimodal particle size distribution.

[0064] The rolling density of each cathode material was measured using an HPRM-1000. Specifically, 5 g of each cathode material prepared at each weight ratio was placed in a cylindrical mold, and the mold containing each cathode material was pressurized at 2 tons. The height of the pressed mold was then measured using a Vernier caliper to determine the rolling density. The rolling density was obtained in the same manner when the pressure applied to the mold was changed to 5 tons and 9 tons, and the results are shown in Figure 5.

[0065] From the results of FIG. 5, it can be seen that the rolling density is highest when the mixing weight ratio of small particles to large particles is 6:4 under all pressure conditions of 2 tons, 5 tons, and 9 tons. Therefore, in the following examples, cathode materials were manufactured using this weight ratio.

[0066] Comparative Example 1 D 50 Li(Ni 0.83 Co 0.05 Mn 0.10 Al 0.02 )O2 and D 50 Li(Ni 0.86 Co 0.5 Mn 0.7 Al 0.2 )O2(D 50 =10 μm) were mixed in a weight ratio of 6:4 to prepare a bimodal cathode material.

[0067] The cathode material, conductive material (carbon black), and binder (polyvinylidene fluoride, PVdF) were mixed in a weight ratio of 97.5:1.0:1.5 in N-methyl-2-pyrrolidone (NMP) solvent to prepare a cathode slurry (solid content: 76 wt%). The true density of the cathode active material was 4.475 g / cc.

[0068] The positive electrode slurry was applied to one side of an aluminum current collector and dried at 130°C. The loading amount was 450g / 25cm 2An electrode including an active material layer of this material was manufactured. The thickness of the active material layer was 75 μm. After cutting into an 11 cm × 3.5 cm size, the electrode was placed between two rolling mills and the gap between the rolling mills was adjusted at 25°C to manufacture a positive electrode having an active material layer thickness of 70 μm and a porosity of 42.5%, respectively.

[0069] Comparative Example 2 In Comparative Example 1, a positive electrode was prepared in the same manner as in Example 1, except that the active material layer was rolled to a thickness of 55 μm and a porosity of 26.8%, respectively.

[0070] Comparative Example 3. In Comparative Example 1, a positive electrode was manufactured in the same manner as in Example 1, except that the active material layer was rolled to have a thickness of 49 μm and a porosity of 17.8%, respectively.

[0071] Example 1 In Comparative Example 1, a positive electrode was prepared in the same manner as in Example 1, except that the active material layer was rolled to a thickness of 53 μm and a porosity of 24.0%, respectively.

[0072] Example 2. In Comparative Example 1, a positive electrode was prepared in the same manner as in Example 1, except that the active material layer was rolled to have a thickness of 51 μm and a porosity of 21.1%.

[0073] [Experimental Example 1: Measurement of BET specific surface area and pore volume] The positive electrodes of Comparative Examples 1 to 3 and Examples 1 and 2 prepared above were each cut into a 1.96 cm x 1.96 cm sample, with the total weight of each positive electrode being 3.5 g or more. These samples were then placed in a BELSORP-MAX (MicrotracBEL Corp.). The BET (Brunauer-Emmett-Teller) specific surface area of ​​the positive electrode active material layer, after subtracting the weight of the current collector, and the pore volume were determined using a BJH (Barrett-Joyner-Halenda) plot. The BJH plot, obtained for pores with diameters of 2 nm to 185 nm, is shown in Figure 2, and the pore volume values ​​calculated from this plot are listed in Table 1 below.

[0074] [Table 1]

[0075] [Experimental Example 2: Evaluation of battery performance] (1) Initial capacity evaluation An electrode assembly was fabricated by interposing a porous polyethylene separator between each of the positive electrodes and lithium metal negative electrodes fabricated in Examples 1 and 2 and Comparative Examples 1 to 3. The assembly was then placed inside a battery case, and an electrolyte solution was injected into the case to fabricate a coin half-cell lithium secondary battery. The electrolyte solution was a 1M LiPF6 solution dissolved in a mixed organic solvent of ethylene carbonate, dimethyl carbonate, and diethyl carbonate in a volume ratio of 3:4:3. After activation, the fabricated coin half-cells were each charged / discharged once at room temperature (25°C) at a charge / discharge rate of 0.1C / 0.1C. The initial capacity after one charge / discharge cycle is shown in Table 2 below.

[0076] (2) Evaluation of life characteristics Lithium secondary batteries using the positive electrodes of Examples 1 and 2 and Comparative Examples 2 and 3 were fabricated in the same manner as in the coin half-cell, except that the negative electrode prepared as follows was used instead of the lithium metal negative electrode.

[0077] The negative electrode mixture slurry was prepared by mixing graphite (a mixture of artificial graphite and natural graphite in a weight ratio of 8:2) as the negative electrode active material, styrene-butadiene rubber (SBR) as the binder, and carbon black as the conductive material in a weight ratio of 97.6:0.8:1.6, and then adding it to water as the solvent. The negative electrode mixture slurry was applied to a 10 μm-thick copper (Cu) thin film as the negative electrode current collector, dried, and then roll pressed to prepare the negative electrode.

[0078] The fabricated lithium secondary batteries were each subjected to an activation process at 25°C and a 0.1C rate, after which gas was removed from the batteries by a degassing process. The degassed lithium secondary batteries were subjected to constant current / constant voltage (CC / CV) charging at 25°C and a 0.33C rate up to 4.2V, followed by a 0.05C cut-off charge, and then constant current (CC) discharging at 0.33C rate down to 2.5V.

[0079] One cycle was defined as one charge / discharge cycle, and the discharge capacity in the initial state (one cycle) was measured using a PNE-0506 charger / discharger (manufacturer: PNE solution Co., Ltd., 5 V, 6 A). The results are shown in Table 2 below as the initial capacity.

[0080] Thereafter, the same charge / discharge cycle was repeated 600 times, and the discharge capacity retention rate was measured. The results are shown in FIG. 3. The retention rate after 600 charge / discharge cycles was calculated based on the initial capacity, and is shown in Table 2 below as the capacity retention rate.

[0081] In addition, the resistance increase rate relative to the initial resistance was measured by calculating the DC-iR based on the voltage drop that occurred when a discharge pulse of 1 C was applied for 10 seconds after charging to 50% SOC at 25°C. The results are shown in FIG. 3. The resistance increase rate after 600 charge / discharge cycles relative to the initial resistance was calculated and listed as the resistance increase rate in Table 2 below.

[0082] (3) Measurement of volume change Lithium secondary batteries using the positive electrodes of Examples 1 and 2 and Comparative Examples 2 and 3 were fabricated in the same manner as described in the evaluation section of life characteristics. An activation process was performed at 25°C and a 0.2C rate, and then gas inside the battery was removed by a degassing process. Then, at room temperature (25°C), the batteries were charged at a 0.33C rate up to 4.2V under constant current / constant voltage conditions, and charged to a 0.05C cutoff, followed by discharging to 2.5V at 0.33C.

[0083] Then, the battery was charged under the same conditions as above until it reached a full SOC of 100%. The volume of the fully charged battery was measured at room temperature using the buoyancy method, and this was taken as the initial volume.

[0084] After completing the volume measurement, the batteries were stored at 60°C for 16 weeks, and the amount of gas generated was measured based on the volume change in the following manner. The results are shown in Figure 4. After each volume measurement, the batteries were transferred to a charger / discharger at room temperature (25°C) and charged at a 0.33C rate up to 4.2V under constant current / constant voltage conditions, followed by a 0.05C cut-off charge, and then fully charged to 100% SOC. The volume of the fully charged batteries was measured at room temperature using the buoyancy method and then stored at 60°C again. The difference between the volume measured after 16 weeks of storage and the initial volume was calculated and reported as the amount of gas generated in Table 2 below.

[0085] [Table 2]

[0086] From the results in Table 2, it can be seen that the batteries manufactured using the positive electrodes of Examples 1 and 2, in which the positive electrode active material layer satisfies the pore volume range of the present invention, exhibited consistently excellent initial capacity, capacity retention rate, resistance increase rate, and gas generation amount.

[0087] Specifically, the pore volume of the positive electrode active material layer is 7.0 × 10 -3 cm 3In Comparative Examples 1 and 2, in which the positive electrode was used with a pore volume of less than 8.0 × 10 -3 cm 3 In the case of Comparative Example 3 using a positive electrode exceeding / g, it can be seen that the amount of gas generated is significantly increased.

Claims

1. a positive electrode current collector; and a positive electrode active material layer formed on at least one surface of the positive electrode current collector and containing a positive electrode active material, The positive electrode active material layer has a pore volume of 7.0×10 －３ cm ３ / g to 8.0 x 10 －３ cm ３ / g, The positive electrode active material includes a lithium composite transition metal oxide represented by the following Chemical Formula 1: The pore volume is calculated based on pores having a diameter of 2 nm to 185 nm. Positive electrode for lithium secondary battery: [Chemical formula 1] Li 1+x (Nia Co b Mn c M d ) O 2 In the above Chemical Formula 1, M is at least one selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; x, a, b, c, and d are −0.2≦x≦0.2, 0.50≦a<1, 0<b≦0.25, 0<c≦0.25, 0≦d≦0.1, and a+b+c+d=1, respectively.

2. The positive electrode active material layer has a BET specific surface area of ​​1.30 m ２ / g excess 1.50m ２ 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the SiO2 content is less than 1 / g.

3. The positive electrode active material is D ５０ 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode particles are different from each other in small particle positive electrode active material and large particle positive electrode active material.

4. D of the small particle positive electrode active material ５０ The positive electrode for a lithium secondary battery according to claim 3, wherein the thickness is 2 μm to 5 μm.

5. D of the large particle positive electrode active material ５０ The positive electrode for a lithium secondary battery according to claim 3, wherein the thickness is 8 μm to 12 μm.

6. 4. The positive electrode for a lithium secondary battery according to claim 3, wherein the weight ratio of the small particle positive electrode active material to the large particle positive electrode active material is 1.0 to 2.

0.

7. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode active material comprises a lithium composite transition metal oxide containing 70 mol % or more of nickel relative to the total moles of transition metals.

8. 2. The positive electrode for a lithium secondary battery according to claim 1, wherein the positive electrode active material layer has a porosity of 20% to 25%.

9. forming a positive electrode active material layer by coating at least one surface of a positive electrode current collector with a positive electrode slurry containing a positive electrode active material; and rolling the positive electrode current collector and the positive electrode active material layer, The pore volume of the positive electrode active material layer after rolling is 7.0 × 10 －３ cm ３ / g to 8.0 x 10 －３ cm ３ / g, The positive electrode active material includes a lithium composite transition metal oxide represented by the following Chemical Formula 1: The pore volume is calculated based on pores having a diameter of 2 nm to 185 nm. [Chemical formula 1] Li 1+x (Nia Co b Mn c M d ) O 2 In the above Chemical Formula 1, M is at least one selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; x, a, b, c, and d are −0.2≦x≦0.2, 0.50≦a<1, 0<b≦0.25, 0<c≦0.25, 0≦d≦0.1, and a+b+c+d=1, respectively.

10. The BET specific surface area of ​​the positive electrode active material layer after rolling is 1.30 m ２ / g excess 1.50m ２ The method for producing a positive electrode for a lithium secondary battery according to claim 9 , wherein the SiO 2 content is less than 1 / g.

11. The positive electrode slurry was coated on at least one surface of the positive electrode current collector in an amount of 350 mg / 25 cm. ２ ~480mg / 25cm ２ The method for producing a positive electrode for a lithium secondary battery according to claim 9, wherein the coating is carried out with a loading amount of

12. 10. The method for producing a positive electrode for a lithium secondary battery according to claim 9, wherein the thickness of the positive electrode active material layer after rolling is 66% to 72% of the thickness of the positive electrode active material layer before rolling.

13. A lithium secondary battery comprising: the positive electrode for a lithium secondary battery according to claim 1; a negative electrode; a separator interposed between the positive electrode and the negative electrode; and an electrolyte.

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