Lithium secondary battery

Abstract

This invention relates to a lithium secondary battery comprising: a positive electrode, a negative electrode, and an electrolyte. The positive electrode comprises a positive electrode active material layer, which includes a single-particle positive electrode active material, a positive electrode conductive material, a positive electrode binder, and a positive electrode dispersant. The negative electrode comprises a first negative electrode active material layer and a second negative electrode active material layer formed on the first negative electrode active material layer. The first and second negative electrode active materials each independently comprise natural graphite, artificial graphite, or a combination thereof. The content ratio of the first negative electrode binder to the second negative electrode binder is 1.5 to 3.0. The porosity difference between the positive and negative electrodes is 4.2% to 9.8%, and the FBR value of the positive electrode, related to the respective amounts and true density values ​​of the binder and dispersant and the average particle size of the positive electrode active material, is 30 to 180.

Description

[0001] Cross-reference to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2023-0191346, filed on December 26, 2023, with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0003] This invention relates to lithium secondary batteries. Background Technology

[0004] Due to the rapid growth in fossil fuel use, the demand for alternative or clean energy is increasing, and as part of this trend, the most active research area is in power generation and energy storage using electrochemical reactions.

[0005] Currently, a typical example of an electrochemical device using this type of electrochemical energy is the secondary battery, and its application areas are expanding. In recent years, with the technological development and increasing demand for portable devices such as portable computers, mobile phones, and cameras, the demand for secondary batteries as energy sources has increased significantly, and among these secondary batteries, lithium secondary batteries have attracted much attention due to their high operating voltage and very high energy density.

[0006] Lithium-ion rechargeable batteries are typically manufactured as follows: an electrode assembly is provided by placing a separator between a positive electrode containing a positive electrode active material made of a lithium-containing transition metal oxide and a negative electrode containing a negative electrode active material capable of storing lithium ions; the electrode assembly is then housed in a battery case; a non-aqueous electrolyte is injected therein as a medium for lithium ion migration; and the battery case is then sealed. The non-aqueous electrolyte is typically composed of a lithium salt and an organic solvent capable of dissolving the lithium salt.

[0007] Recently, with the increasing demand for secondary batteries with high energy density (such as batteries for electric vehicles), the development of high-voltage secondary batteries driven at high voltage is actively underway.

[0008] Meanwhile, the demand for lithium-ion rechargeable batteries with excellent fast-charging performance has increased in order to shorten charging time (which is the biggest obstacle to the commercialization of electric vehicles). However, the fast-charging batteries developed to date have not shown satisfactory performance in terms of lifespan and energy density.

[0009] Therefore, there is a need to develop lithium secondary batteries that can improve the fast-charging performance of batteries and minimize the degradation of life properties or energy density. Summary of the Invention

[0010] Technical issues

[0011] One aspect of the present invention provides a lithium secondary battery comprising: a positive electrode having a positive electrode active material with an average particle size, a positive electrode binder and a positive electrode dispersant content, and a positive electrode binder and a positive electrode dispersant true density satisfying specific conditions; and a negative electrode having a negative electrode active material composition and a negative electrode binder distribution adjusted, wherein the porosity difference between the positive and negative electrodes is adjusted to a specific range, thereby improving fast charging performance, lifetime properties, and energy density properties.

[0012] Technical solution

[0013] [1] The present invention provides a lithium secondary battery comprising: a positive electrode, a negative electrode and an electrolyte, wherein the positive electrode comprises a positive electrode active material layer, the positive electrode active material layer comprises a single-particle positive electrode active material, a positive electrode conductive material, a positive electrode binder and a positive electrode dispersant, wherein the negative electrode comprises a first negative electrode active material layer and a second negative electrode active material layer, the first negative electrode active material layer being formed on a negative electrode current collector and comprising a first negative electrode active material, a first negative electrode conductive material and a first negative electrode binder, the second negative electrode active material layer being formed on the first negative electrode active material layer and comprising a second negative electrode active material, a second negative electrode conductive material and a second negative electrode binder, wherein the ratio of the content of the first negative electrode binder to the content of the second negative electrode binder is 1.5 to 3.0, and the porosity difference between the positive electrode and the negative electrode is 4.2% to 9.8%, and the FBR value of the positive electrode as defined in Equation 1 below is 30 to 180.

[0014] [Equation 1]

[0015] FBR=[(Rb×TDb)+(Rd×TDd)]×A 2

[0016] In Equation 1 above, Rb is a dimensionless percentage of the weight of the positive electrode binder relative to the total weight of the positive electrode active material layer, Rd is a dimensionless percentage of the weight of the positive electrode dispersant relative to the total weight of the positive electrode active material layer, TDb is a dimensionless true density (g / cc) of the positive electrode binder, TDd is a dimensionless true density (g / cc) of the positive electrode dispersant, and A is the average particle size (D) of the single-particle positive electrode active material. 50 The dimensionless number (μm).

[0017] [2] In the above [1], the present invention provides a lithium secondary battery, wherein the single-particle positive electrode active material comprises a lithium nickel oxide with a Ni content of less than 70 mol% based on the total moles of metals other than lithium.

[0018] [3] In [1] or [2] above, the present invention provides a lithium secondary battery, wherein the single-particle positive electrode active material comprises a lithium nickel oxide represented by the following [Formula 1].

[0019] [Formula 1]

[0020] Li 1+x [Ni a Co b Mn c M 1 d O2

[0021] In the above [Equation 1], M 1 Includes one or more elements selected from the group consisting of Ti, Mg, Al, Zr, Y, Ba, Ca, Sr, W, Ta, Nb, and Mo, and -0.1 ≤ x ≤ 0.1, 0.5 ≤ a ≤ 0.7, 0 <b<0.5,0<c<0.5,0≤d≤0.2。

[0022] [4] In at least one of [1] to [3] above, the present invention provides a lithium secondary battery, wherein the average particle size (D) of the single-particle positive electrode active material is... 50 The thickness ranges from 3.5 μm to 7.8 μm.

[0023] [5] In at least one of [1] to [4] above, the present invention provides a lithium secondary battery, wherein the single-particle positive electrode active material comprises 1 to 30 nodules.

[0024] [6] In at least one of [1] to [5] above, the present invention provides a lithium secondary battery, wherein the average particle size (D) of the nodules is... mean The thickness ranges from 0.8 μm to 4.0 μm.

[0025] [7] In at least one of [1] to [6] above, the present invention provides a lithium secondary battery, wherein the positive electrode active material layer comprises 0.5% to 2% by weight of the positive electrode binder.

[0026] [8] In at least one of [1] to [7] above, the present invention provides a lithium secondary battery, wherein the porosity of the positive electrode is 20% to 25%.

[0027] [9] In at least one of [1] to [8] above, the present invention provides a lithium secondary battery, wherein the first negative electrode active material and the second negative electrode active material are each independently composed of natural graphite, artificial graphite or a combination thereof.

[0028]

[10] In at least one of [1] to [9] above, the present invention provides a lithium secondary battery, wherein the first negative electrode active material comprises more than 50% by weight of natural graphite, and the second negative electrode active material comprises less than 50% by weight of natural graphite.

[0029]

[11] In at least one of [1] to

[10] above, the present invention provides a lithium secondary battery, wherein the ratio of the content of the first negative electrode binder to the content of the second negative electrode binder is 1.6 to 2.4.

[0030]

[12] In at least one of [1] to

[11] above, the present invention provides a lithium secondary battery, wherein the first negative electrode active material layer comprises 1.5% to 3.0% by weight of the first negative electrode binder.

[0031]

[13] In at least one of [1] to

[12] above, the present invention provides a lithium secondary battery, wherein the QBR value of the negative electrode as defined by the following Equation 2 is 1.1 to 1.28.

[0032] [Equation 2]

[0033] QBR=Bs / Bf

[0034] Bs is the average adhesive content in the surface region of the negative electrode active material layer from the outermost surface of the second negative electrode active material layer to within 15% of the total thickness of the negative electrode active material layer, and Bf is the average adhesive content in the bottom region of the negative electrode active material layer from the interface between the first negative electrode active material layer and the negative electrode current collector to within 15% of the total thickness of the negative electrode active material layer.

[0035]

[14] In at least one of [1] to

[13] above, the present invention provides a lithium secondary battery, wherein the porosity of the negative electrode is 25% to 32%.

[0036]

[15] In at least one of [1] to

[14] above, the present invention provides a lithium secondary battery, wherein the difference in porosity between the positive electrode and the negative electrode is 6% to 8%.

[0037] Beneficial effects

[0038] This invention comprises a positive electrode having a positive electrode whose average particle size, content of positive electrode binder and positive electrode dispersant, and true density of positive electrode binder and positive electrode dispersant meet specific conditions; and a negative electrode comprising a first negative electrode active material layer (lower layer) with different compositions of negative electrode active material and distribution of negative electrode binder, and a second negative electrode active material layer (upper layer) formed on the first negative electrode active material layer, wherein the porosity difference between the positive and negative electrodes meets a specific range. By including a positive electrode that meets specific conditions, the initial capacity, initial resistance, and lifetime properties of the lithium secondary battery are improved. Simultaneously, by reducing the proportion of natural graphite in the upper layer of the negative electrode active material layer and increasing the proportion of artificial graphite, the initial resistance and fast charging performance are improved. Furthermore, by increasing the proportion of natural graphite in the lower layer of the negative electrode active material layer, adjusting the proportion of negative electrode binder, and appropriately adjusting the porosity difference between the positive and negative electrodes, a lithium secondary battery with excellent fast charging performance and excellent electrochemical performance (e.g., lifetime properties) can be achieved. Detailed Implementation

[0039] The invention will be described in more detail below.

[0040] It will be understood that the terms or words used in this specification and claims should not be construed as having the meanings defined in commonly used dictionaries, but should be interpreted as having meanings and concepts consistent with the technical concept of the invention, based on the principle that the inventors can appropriately define the concepts of the terms to best interpret the invention.

[0041] The terminology used herein is for describing embodiments and is not intended to limit the invention. In this specification, singular forms include plural forms unless the context clearly indicates otherwise.

[0042] In this specification, when a section is referred to as including a certain element, it means that the section may also include another element rather than excluding another element, unless otherwise stated.

[0043] In this invention, "single-particle type positive electrode active material" refers to a positive electrode active material particle composed of 30 or fewer subparticles. The subparticle unit that makes up the above-mentioned single-particle type positive electrode active material is called a "nodule". Single-particle type positive electrode active material includes single particles composed of a single nodule and particle-like particles that are composites of 30 or fewer nodules.

[0044] "Nodule" refers to the sub-particle unit that makes up single particles and quasi-single particles. Nodules can be single crystals without grain boundaries, or polycrystalline materials that do not appear to have grain boundaries when observed with a scanning electron microscope (SEM) at a magnification range of 5,000 to 20,000 times.

[0045] In this invention, "secondary particles" refer to particles formed by the aggregation of more than 30 subparticles. Each subparticle unit that makes up the above-mentioned secondary particles is called a "primary particle" to distinguish it from the subparticles that constitute the single-particle positive electrode active material particles.

[0046] The term "particle" as used in this invention may include any one or all of the following: single particle, quasi-single particle, primary particle, nodule, and secondary particle.

[0047] In this specification, the "average particle size (D)" of a nodule or primary particle is used. mean ")" refers to the arithmetic mean of the values ​​obtained after measuring the particle size of observed nodules or primary particles in SEM images obtained by scanning electron microscopy.

[0048] In this specification, "average particle size (D)" 50 "Average particle size" refers to the particle size corresponding to 50% of the cumulative volume of the volumetric particle size distribution of the powder being tested, and can be measured using laser diffraction. For example, the average particle size (D...) 50 The particle size distribution can be measured by dispersing the powder to be tested in a dispersion medium, then introducing the mixture into a commercially available laser diffraction particle size measurement device (e.g., Microtrac MT 3000), irradiating it with ultrasound at an output of 60 W at approximately 28 kHz, thereby obtaining a volumetric cumulative particle size distribution map, and then obtaining the particle size corresponding to 50% of the cumulative volume.

[0049] In this specification, "porosity (%)" can be calculated as 1 - (electrode density / electrode true density).

[0050] In this specification, "true density" can be measured using a gas specific gravity bottle. A gas specific gravity bottle is a device that allows the measurement of sample density by placing a known weight of the sample into a sample chamber, then injecting helium or nitrogen gas into it, and determining the volume occupied by the sample excluding pores. Specifically, after measuring the volume of the sample based on the pressure change between the sample chamber containing the sample and a reference chamber of known volume, the density value of the sample is calculated using the ideal gas equation (PV=nRT).

[0051] In this specification, "QBR" can be calculated using SEM images obtained through scanning electron microscopy analysis.

[0052] The lithium secondary battery of the present invention includes at least one of the following disclosed configurations, and may include any combination of technically possible configurations.

[0053] As a result of repeated research to develop lithium secondary batteries with excellent fast-charging performance and minimized degradation of lifetime properties, the inventors have discovered that a lithium secondary battery comprises: a multilayer structure of a negative electrode including a negative electrode current collector, a first negative electrode active material layer formed on the negative electrode current collector, and a second negative electrode active material layer formed on the first negative electrode active material layer, and also includes the content of a positive electrode binder and a positive electrode dispersant, as well as the average particle size (D) of the positive electrode active material. 50 The invention relates to a positive electrode that meets specific conditions, wherein the difference in porosity between the positive and negative electrodes meets a specific range. Under these conditions, the lithium secondary battery can have excellent fast charging performance and can suppress the degradation of cell performance during long-term cycling.

[0054] The invention will be described in detail below.

[0055] Lithium secondary batteries

[0056] The lithium secondary battery of the present invention comprises: a positive electrode, a negative electrode, and an electrolyte. The positive electrode comprises a positive electrode active material layer, which comprises a single-particle positive electrode active material, a positive electrode conductive material, a positive electrode binder, and a positive electrode dispersant. The negative electrode comprises a first negative electrode active material layer and a second negative electrode active material layer. The first negative electrode active material layer is formed on a negative electrode current collector and comprises a first negative electrode active material, a first negative electrode conductive material, and a first negative electrode binder. The second negative electrode active material layer is formed on the first negative electrode active material layer and comprises a second negative electrode active material, a second negative electrode conductive material, and a second negative electrode binder. The first and second negative electrode active materials each independently comprise natural graphite, artificial graphite, or a combination thereof. The ratio of the content of the first negative electrode binder to the content of the second negative electrode binder is 1.5 to 3.0. The porosity difference between the positive and negative electrodes is 4.2% to 9.8%. The FBR value of the positive electrode as defined in Equation 1 below is 30 to 180.

[0057] [Equation 1]

[0058] FBR=[(Rb×TDb)+(Rd×TDd)]×A 2

[0059] In Equation 1 above, Rb is the weight percentage of the positive electrode binder relative to the total weight of the positive electrode active material layer, Rd is the weight percentage of the positive electrode dispersant relative to the total weight of the positive electrode active material layer, TDb is the true density (g / cc) of the positive electrode binder, TDd is the true density (g / cc) of the positive electrode dispersant, and A is the average particle size (D) of the positive electrode active material. 50 ,μm).

[0060] According to the inventors' research, a lithium secondary battery comprises a multilayer structure of a positive electrode having an FBR value satisfying a specific range, a first negative electrode active material layer formed on a negative electrode current collector, and a second negative electrode active material layer formed on the first negative electrode active material layer. In this case, it has been shown to have excellent cell resistance and fast charging performance, and to suppress the degradation of cell performance during long-term cycling.

[0061] Meanwhile, the porosity difference between the positive and negative electrodes is between 4.2% and 9.8%. Specifically, the porosity difference between the positive and negative electrodes can be above 4.2%, above 4.5%, above 4.8%, above 5%, above 5.2%, above 5.5%, above 5.8%, above 6%, above 6.2%, above 6.5%, above 6.8%, above 7%, above 7.2%, above 7.5%, above 7.8%, above 8%, below 9.8%, below 9.5%, below 9.2%, below 9%, below 8.8%, below 8.5%, below 8.2%, below 8%, below 7.8%, below 7.5%, below 7.2%, below 7%, below 6.8%, below 6.5%, below 6.2%, or below 6%. For example, the porosity difference between the positive and negative electrodes can be 4.2% to 9.8%, 4.5% to 9.5%, 5% to 9%, 5.5% to 8.5%, or 6% to 8%. If the porosity difference between the positive and negative electrodes meets the above ranges, the battery volume is optimized according to the electrode thickness, thereby improving the energy density. Furthermore, when lithium ions migrate from the positive electrode to the negative electrode, the diffusion resistance of lithium ions in the positive electrode is reduced, and lithium ions are more easily inserted into the negative electrode, thereby improving fast charging properties. It also reduces the battery resistance and improves the structural stability of the negative and positive electrodes, thus improving the adhesion of the negative electrode and the lifetime properties even more effectively.

[0062] According to the present invention, the effects of shortening fast charging time, improving capacity properties, reducing initial resistance and improving lifetime properties can only be achieved when the ratio of the content of the first negative electrode adhesive to the content of the second negative electrode adhesive is 1.5 to 3.0, the difference in porosity between the positive and negative electrodes is 4.2% to 9.8%, and the FBR value defined in Equation 1 above is 30 to 180.

[0063] The components of the lithium secondary battery of the present invention will be described in more detail below.

[0064] positive electrode

[0065] The lithium secondary battery of the present invention may include: a positive electrode comprising a positive electrode active material layer and a positive electrode current collector. Specifically, the positive electrode comprises a positive electrode active material layer, which includes a positive electrode active material, a positive electrode conductive material, a positive electrode binder, and a positive electrode dispersant. The positive electrode active material comprises single-particle particles, wherein the FBR defined by Formula 1 can be 30 to 180, preferably 35 to 180, and more preferably 40 to 175.

[0066] [Equation 1]

[0067] FBR=[(Rb×TDb)+(Rd×TDd)]×A 2

[0068] In Equation 1 above, Rb is the weight percentage of the positive electrode binder relative to the total weight of the positive electrode active material layer, Rd is the weight percentage of the positive electrode dispersant relative to the total weight of the positive electrode active material layer, TDb is the true density (g / cc) of the positive electrode binder, TDd is the true density (g / cc) of the positive electrode dispersant, and A is the average particle size (D) of the positive electrode active material. 50 ,μm).

[0069] If the FBR meets the scope of this invention, the initial capacity, initial resistance, and lifespan properties of the lithium secondary battery can be improved. Specifically, if the FBR is less than 30, the energy density and cell stability may decrease, and if the FBR is greater than 180, the cell resistance increases, which may lead to problems of degraded output performance and degraded fast charging performance.

[0070] Meanwhile, there are no particular restrictions on the positive electrode current collector, as long as it is conductive and will not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, or silver can be used. Furthermore, the thickness of the positive electrode current collector can typically range from 3 μm to 500 μm, and fine irregularities can be formed on its surface to improve the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0071] The positive electrode active material of the present invention may comprise a lithium nickel oxide with a Ni content of 70 mol% or less based on the total molar number of metals other than lithium. Specifically, the Ni content of the positive electrode active material may be 50 mol% to 70 mol%, preferably 55 mol% to 70 mol%, more preferably 60 mol% to 70 mol%. As described above, if a positive electrode active material with a relatively low Ni content is used, the contact area with the electrolyte solution is small, thereby achieving high energy density through stable driving at a high voltage of 4.35 V or higher, and side reactions with the electrolyte solution are suppressed, thereby reducing gas generation, and cell performance degradation caused by the agglomeration of conductive materials can be suppressed, thereby improving fast charging performance.

[0072] More specifically, the positive electrode active material may contain lithium nickel oxide as represented by [Formula 1].

[0073] [Formula 1]

[0074] Li 1+x [Ni a Co b Mn c M 1 d O2

[0075] In the above [Equation 1], M 1 It may include one or more doping elements selected from the group consisting of Ti, Mg, Al, Zr, Y, Ba, Ca, Sr, W, Ta, Nb, and Mo, and preferably, it may be one or more doping elements selected from the group consisting of Al, W, Y, Ba, Ca, Ti, Mg, and Nb. 1 The element may or may not be selectively included in lithium nickel oxides, and if included, then M 1 Elements can promote particle growth or improve structural stability during the calcination process of the cathode.

[0076] Meanwhile, 1 + x represents the molar ratio of lithium in lithium-nickel oxides, where -0.1 ≤ x ≤ 0.1, 0 ≤ x ≤ 0.1, or 0 ≤ x ≤ 0.07. If x satisfies the above range, a stable layered crystal structure can be formed.

[0077] 'a' represents the molar ratio of nickel to all metals other than lithium in lithium-nickel oxides, where 0.5 ≤ a ≤ 0.7, 0.55 ≤ a ≤ 0.7, or 0.55 ≤ a ≤ 0.65. If 'a' meets the above range, structural stability can be ensured during charge and discharge to suppress cathode degradation, and high energy density can be achieved.

[0078] b is the molar ratio of cobalt among all metals other than lithium in the lithium nickel-based oxide, where 0 < b < 0.5, 0.03 ≤ b ≤ 0.4, 0.05 ≤ b ≤ 0.3, or 0.08 ≤ b ≤ 0.15. If b satisfies the above range, good resistance properties and output properties can be achieved, and the molar ratio of manganese can be relatively increased, thereby improving the structural stability.

[0079] c is the molar ratio of manganese among all metals other than lithium in the lithium nickel-based oxide, where 0 < c < 0.5, 0.1 ≤ c ≤ 0.45, 0.15 ≤ c ≤ 0.4, 0.2 ≤ c ≤ 0.35, or 0.25 ≤ c ≤ 0.35. If c satisfies the above range, the positive electrode active material can have excellent structural stability.

[0080] d is the molar ratio of the M 1 element among all metals other than lithium in the lithium nickel-based oxide, where 0 ≤ d ≤ 0.2, 0 ≤ d ≤ 0.1, 0 ≤ d ≤ 0.05, or d = 0. The M 1 element is not necessarily included as a doping element, but if included in an appropriate amount, the M 1 element can be used to promote particle growth during the calcination process or improve the stability of the crystal structure.

[0081] Meanwhile, the average particle size (D 50 ) of the single-particle type positive electrode active material can be 3.0 μm to 7.9 μm, preferably 3.5 μm to 7.8 μm, and more preferably 3.7 μm to 7.6 μm. If the average particle size of the positive electrode active material is too small, the processability during the manufacture of the positive electrode may be reduced, and the wettability of the electrolyte solution may be reduced, thereby increasing the electrochemical properties. And if the average particle size is too large, the resistance may increase, and the output performance may deteriorate.

[0082] Meanwhile, preferably, the single-particle type positive electrode active material contains 1 to 30, preferably 1 to 25, and more preferably 1 to 15 nodules in the particles. This is because if the number of nodules constituting the positive electrode active material particles is greater than 30, particle breakage may increase during the electrode manufacturing process, and due to the volume expansion / contraction of the nodules during charge / discharge, internal cracking may increase, so the life properties may deteriorate.

[0083] Meanwhile, the average particle size (D mean ) of the nodules can be 0.8 μm to 4.0 μm, preferably 0.8 μm to 3.0 μm, and more preferably 1.0 μm to 3.0 μm. If the average particle size of the nodules satisfies the above range, particle breakage can be minimized during the electrode manufacturing process, and an increase in resistance can be more effectively suppressed.

[0084] In addition to single-particle positive electrode active materials, the positive electrode active material layer may also include positive electrode conductive materials, positive electrode binders, and positive electrode dispersants.

[0085] At this point, the positive electrode conductive material is used to impart conductivity to the electrode, and any conductive material can be used without particular restrictions, as long as it has electronic conductivity and will not cause chemical changes in the battery to be constructed. Specific examples may include graphite, such as natural or artificial graphite; carbon materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermally cracked carbon black, carbon fibers, and carbon nanotubes; metal powders or metal fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one or a mixture of two or more of them may be used. Based on the total weight of the positive electrode active material layer, the content of the conductive material can be from 0.4 wt% to 10 wt%, preferably from 0.4 wt% to 7 wt%, more preferably from 0.4 wt% to 5 wt%. If the content of the conductive material meets the above range, excellent positive electrode conductivity and capacity can be achieved.

[0086] Simultaneously, the positive electrode binder is used to improve the adhesion between positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples may include polyvinylidene fluoride (PVdF), vinylidene fluoride-hexafluoropropylene copolymer (PVdF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one or a mixture of two or more of them may be used.

[0087] Based on the total weight of the positive electrode active material layer, the content of the positive electrode binder can be from 0.5% to 2% by weight, preferably from 1% to 2% by weight, and more preferably from 1.5% to 2% by weight. If the content of the positive electrode binder meets the above range, the adhesion between the current collector and the positive electrode active material layer is high, thus achieving excellent capacity and lifetime properties even after long-term cycling.

[0088] Meanwhile, the positive electrode dispersant is used to improve the dispersibility of lithium nickel oxides, conductive materials, etc., such as hydrogenated nitrile butadiene rubber (H-NBR), but the positive electrode dispersant is not limited to these. Based on the total weight of the positive electrode active material layer, the content of the dispersant can be less than 2% by weight, preferably from 0.1% to 2% by weight, more preferably from 0.1% to 0.5% by weight. If the content of the dispersant is too low, the effect of improving dispersibility will not be significant; if it is too high, the battery performance may be adversely affected.

[0089] Meanwhile, the porosity of the positive electrode can be 20% to 25%, 20.5% to 25%, 21% to 25%, or 21% to 23%. If the porosity of the positive electrode meets the above range, the positive electrode is thinner, so when applied to a battery, the battery volume is reduced, thereby improving the energy density.

[0090] Meanwhile, the positive electrode can be manufactured according to conventional manufacturing methods for positive electrodes. For example, the positive electrode can be manufactured by mixing the positive electrode active material, the positive electrode binder, and / or the positive electrode conductive material in a solvent to prepare a positive electrode slurry, coating the positive electrode slurry onto the positive electrode current collector, and then drying and rolling it, or casting the positive electrode slurry onto a separate support, and then laminating the film layer peeled off from the support onto the positive electrode current collector.

[0091] At this point, solvents commonly used in the art can be used as the solvent for the positive electrode slurry. For example, dimethyl sulfoxide (DMSO), isopropanol, N-methylpyrrolidone (NMP), acetone, water, etc., can be used alone, or a mixture of two or more of them can be used. The amount of solvent used is sufficient to dissolve or disperse the positive electrode active material, the positive electrode conductive material, and the positive electrode binder, taking into account the coating thickness and preparation yield of the slurry, and is subsequently sufficient to give the slurry a viscosity that exhibits excellent thickness uniformity when coated to manufacture the positive electrode.

[0092] negative electrode

[0093] The lithium secondary battery of the present invention may comprise: a negative electrode comprising a negative electrode active material, a negative electrode conductive material, and a negative electrode binder. Specifically, the negative electrode comprises: a first negative electrode active material layer formed on a negative electrode current collector and comprising a first negative electrode active material, a first negative electrode conductive material, and a first negative electrode binder; and a second negative electrode active material layer formed on the first negative electrode active material layer and comprising a second negative electrode active material, a second negative electrode conductive material, and a second negative electrode binder, wherein the first negative electrode active material and the second negative electrode active material may each independently comprise natural graphite, artificial graphite, or a combination thereof.

[0094] There are no particular limitations on the negative electrode current collector, as long as it has high conductivity and will not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., and aluminum-cadmium alloys can be used. Furthermore, the thickness of the negative electrode current collector can typically range from 3 μm to 500 μm, and similar to the positive electrode current collector, fine irregularities can be formed on its surface to improve the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0095] The first and second negative electrode active materials can each independently contain natural graphite, artificial graphite, or a combination thereof.

[0096] At this point, the first negative electrode active material may contain more than 50% by weight and less than 50% by weight and less than 100% by weight, preferably 55% by weight to 100% by weight, of natural graphite, and the second negative electrode active material may contain less than 50% by weight, less than 50% by weight and less than 45% by weight, of natural graphite. If the above ranges are met, the proportion of natural graphite in the first negative electrode active material layer is high, and the proportion of artificial graphite in the second negative electrode active material layer is high, thereby reducing cell resistance and improving initial capacity performance to improve energy density.

[0097] Meanwhile, the first negative electrode conductive material and the second negative electrode conductive material can be the same as those in the positive electrode.

[0098] In addition, the first negative electrode adhesive and the second negative electrode adhesive are used to improve the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material and the negative electrode current collector, and specific examples can be the same as those in the positive electrode described above.

[0099] Meanwhile, the ratio of the content of the first negative electrode adhesive to the content of the second negative electrode adhesive can be 1.5 to 3.0, preferably 1.5 to 2.8, and more preferably 1.6 to 2.4.

[0100] At this time, the first negative electrode active material layer may contain 1.5% to 3.0% by weight, preferably 1.5% to 2.8% by weight, more preferably 1.6% to 2.4% by weight of the first negative electrode binder.

[0101] If the above range is met, the proportion of the first negative electrode adhesive contained in the first negative electrode active material layer in contact with the negative electrode current collector increases, thereby improving the adhesion between the negative electrode current collector and the first negative electrode active material layer, and suppressing the structural disintegration of the negative electrode active material by the adhesive, thereby significantly improving the lifetime properties.

[0102] Meanwhile, the Quantitative Binder Ratio (QBR) value defined in Equation 2 below for the negative electrode of the present invention can be 1.1 to 1.28, preferably 1.1 to 1.26, and more preferably 1.1 to 1.24.

[0103] [Equation 2]

[0104] QBR=Bs / Bf

[0105] Bs represents the average adhesive content in the surface region of the negative electrode active material layer from the outermost surface of the second negative electrode active material layer to within 15% of the total thickness of the negative electrode active material layer, and Bf represents the average adhesive content in the bottom region of the negative electrode active material layer from the interface between the first negative electrode active material layer and the negative electrode current collector to within 15% of the total thickness of the negative electrode active material layer.

[0106] At this point, the QBR value can be calculated as follows.

[0107] The cross-section of the negative electrode was generated using argon ion milling. Subsequently, the composition of the negative electrode active material layer in the generated negative electrode cross-section was mapped using an energy-dispersive X-ray spectroscopy (EDS) detector on a scanning electron microscope (SEM).

[0108] From the EDS mapping results, the line profile is extracted along the thickness direction of the negative electrode active material layer. From the extracted line profile results, the average value of the binder content Bs in the surface region of the negative electrode active material layer and the average value of the binder content Bf in the bottom region of the negative electrode active material layer are extracted, and the QBR value is calculated.

[0109] The QBR value represents the uniformity of adhesive distribution in the thickness direction within the entire negative electrode active material layer, expressed as the ratio of the amount of adhesive contained in the surface area to the amount of adhesive contained in the bottom area.

[0110] If the QBR value meets the above range, then when a negative electrode binder with the same proportion is introduced, the negative electrode binder is properly distributed in the negative electrode, thereby having the effect of improving lifetime properties, because even after long-term cycling during the evaluation of lifetime properties, side reactions are suppressed due to uniform charge and discharge.

[0111] Meanwhile, the porosity of the negative electrode can be 25% to 32%, 25% to 31%, 25.5% to 31%, 27% to 30.5%, or 28% to 30%. If the porosity of the negative electrode meets the above range, it promotes lithium migration, resulting in excellent cell resistance properties. Furthermore, the porosity buffers the volume expansion caused by the continuous degradation of the negative electrode active material, thereby improving expansion properties and ultimately improving lifetime properties.

[0112] As described above, in the lithium secondary battery of the present invention, the negative electrode active material layer may have a multilayer structure comprising two or more layers including a first negative electrode active material layer and a second negative electrode active material layer.

[0113] As described above, if the negative electrode active material layer has a multilayer structure consisting of two or more layers, then each layer may contain different types and / or contents of negative electrode active materials, binders, and / or conductive materials.

[0114] For example, the weight ratio of natural graphite to the total weight of the negative electrode active material in the first negative electrode active material layer (lower layer) can be higher than that in the second negative electrode active material layer (upper layer). Furthermore, the weight ratio of artificial graphite to the total weight of the negative electrode active material in the second negative electrode active material layer can be higher than that in the first negative electrode active material layer.

[0115] Alternatively, the weight ratio of the first negative electrode adhesive to the total weight of the first negative electrode active material layer (lower layer) may be higher than the weight ratio of the second negative electrode adhesive to the total weight of the second negative electrode active material layer (upper layer).

[0116] By forming a multi-layered negative electrode active material layer as described above, and by changing the composition of each layer, the battery's performance properties, such as fast charging performance and lifespan, can be further improved.

[0117] Meanwhile, the negative electrode can be manufactured according to conventional manufacturing methods for negative electrodes. For example, the negative electrode can be manufactured by mixing the negative electrode active material, negative electrode binder and / or negative electrode conductive material in a solvent to prepare a negative electrode slurry, coating the negative electrode slurry onto the negative electrode current collector, and then drying and rolling it, or by casting the negative electrode slurry onto a separate support, and then laminating the film layer peeled off from the support onto the negative electrode current collector.

[0118] Meanwhile, as a solvent for the negative electrode slurry, a solvent commonly used in the art can be used, and its specific content is the same as that of the solvent for the positive electrode slurry.

[0119] diaphragm

[0120] If necessary, the lithium secondary battery of the present invention may include a separator between the positive and negative electrodes. The separator serves to separate the negative and positive electrodes and provide a migration path for lithium ions. Any separator can be used without particular limitation, as long as it is a separator commonly used in lithium secondary batteries. In particular, a separator with excellent moisture retention capacity in the electrolyte solution and low resistance to ion migration in the electrolyte is preferred. Specifically, porous polymer membranes can be used, for example, porous polymer membranes made from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures having two or more of these layers. Alternatively, conventional porous nonwoven fabrics can be used, for example, nonwoven fabrics made from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Furthermore, coated separators containing ceramic components or polymer materials can be used to ensure heat resistance or mechanical strength, and can be selectively used in single-layer or multi-layer structures.

[0121] electrolytes

[0122] In the lithium secondary battery of the present invention, the electrolyte may contain an organic solvent and a lithium salt.

[0123] As organic solvents, any organic solvent can be used without particular restrictions, as long as it can serve as a medium through which ions participating in the electrochemical reactions of the battery can migrate. Specifically, as organic solvents, ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, and ε-caprolactone can be used; ether solvents such as dibutyl ether and tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), and propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitriles such as R-CN (where R is a straight-chain, branched, or cyclic C2 to C20 hydrocarbon group, and may contain double-bonded aromatic rings or ether bonds); amides such as dimethylformamide; dioxolane, such as 1,3-dioxolane; or sulfolane. Preferably, carbonate solvents are preferred, and more preferably, a mixture of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant and low viscosity linear carbonate compounds (e.g., ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate, etc.) that can improve the charge / discharge performance of the battery is preferred.

[0124] As a lithium salt, any compound can be used without particular restrictions, as long as it can provide the lithium ions used in lithium secondary batteries. Specifically, the anion of the lithium salt can be selected from F... - Cl - ,Br- I - NO3 - N(CN)2 - BF4 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - At least one of the constituent groups, and as a lithium salt, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, etc., can be used. The lithium salt can be used in a concentration range of 0.1 M to 4.0 M, preferably 0.5 M to 3.0 M, more preferably 1.0 M to 2.0 M. If the concentration of the lithium salt is within the above range, the electrolyte has suitable conductivity and viscosity, and therefore can exhibit excellent electrolyte performance, and lithium ions can migrate efficiently.

[0125] To improve battery life, suppress capacity reduction, and improve discharge capacity, in addition to the electrolyte components mentioned above, the electrolyte may also contain additives. As additives, various additives used in the art can be used, such as fluoroethylene carbonate (FEC), vinylene carbonate (VC), vinyl ethylene carbonate (VEC), ethylene sulfate (ESa), lithium difluorophosphate (LiPO2F2), lithium bis(oxalato)borate (LiBOB), lithium tetrafluoroborate (LiBF4), lithium difluorooxalatoborate (LiDFOB), lithium difluorobis(oxalato)phosphate (LiDFBP), and lithium tetrafluorooxalato)phosphate (LiTF). Examples of additives include OP, lithium methyl sulfate (LiMS), lithium ethyl sulfate (LiES), propane sulpholol (PS), propene sulpholol (PRS), succinic acid nitrile (SN), adiponitrile (AND), 1,3,6-hexanetrionitrile (HTCN), 1,4-dicyano-2-butene (DCB), fluorobenzene (FB), ethyl di(prop-2-yl-1-yl) phosphate (EDP), 5-methyl-5-alkynoxycarbonyl-1,3-dioxolane-2-one (MPOD), etc., but additives are not limited to these. The content of additives can be from 0.1% to 10% by weight, preferably from 0.1% to 5% by weight, based on the total weight of the electrolyte.

[0126] The lithium secondary battery of the present invention, as described above, can be effectively used in portable devices such as mobile phones, laptops, and digital cameras, as well as in electric vehicles such as hybrid electric vehicles (HEVs).

[0127] The invention will be described in more detail below with reference to embodiments. However, the following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention.

[0128] Example 1

[0129] <The Manufacturing of the Positive Electrode>

[0130] A positive electrode slurry containing positive electrode active material, positive electrode conductive material, positive electrode binder, and positive electrode dispersant in a weight ratio of 97:1.2:1.44:0.36 was prepared. At this point, the average particle size D... 50 Single-particle Li[Ni] with a particle size of 3.7 μm 0.60 Co 0.10 Mn 0.30 O2 is used as the positive electrode active material, and carbon nanotubes (CNTs) are used as the positive electrode conductive material. In addition, polyvinylidene fluoride (PVdF) with a true density of 1.77 g / cc is used as the positive electrode binder, and hydrogenated nitrile butadiene rubber (H-NBR) with a true density of 1.3 g / cc is used as the positive electrode dispersant.

[0131] Simultaneously, the positive electrode conductive material, positive electrode binder, and positive electrode dispersant are added to the positive electrode slurry solvent (N-methylpyrrolidone) in the form of a pre-dispersion dispersed in a dispersion liquid. Subsequently, the positive electrode slurry is coated onto an aluminum current collector, dried, and then rolled to manufacture the positive electrode. The porosity of the positive electrode is 22%.

[0132] <Manufacturing the Negative Electrode>

[0133] The first negative electrode active material (natural graphite), the first negative electrode conductive material (super C), the thickener (carboxymethyl cellulose (CMC)) and the first negative electrode binder (styrene-butadiene rubber (SBR)) were mixed in water at a weight ratio of 96.9:0.5:1.0:1.6 to prepare the first negative electrode slurry.

[0134] The second negative electrode active material (artificial graphite), the second negative electrode conductive material (super C), the thickener (carboxymethyl cellulose (CMC)), and the second negative electrode binder (styrene-butadiene rubber (SBR)) are mixed in water at a weight ratio of 97.4:0.5:1.1:1.0 to prepare the second negative electrode slurry.

[0135] A first negative electrode slurry and a second negative electrode slurry are simultaneously coated onto a negative electrode current collector (copper metal film), wherein the first negative electrode slurry is disposed on the negative electrode current collector, and the second negative electrode slurry is disposed on the first negative electrode slurry. Subsequently, the negative electrode current collector coated with the first and second negative electrode slurries is dried and then rolled to manufacture the negative electrode. The porosity of the negative electrode is 30%.

[0136] <Manufacturing of Lithium Secondary Batteries>

[0137] A separator is placed between the positive and negative electrodes manufactured as described above to create a dual-cell structure with separator / negative electrode / separator / positive electrode / separator / negative electrode / separator. The dual-cell structure is then wound together with the separator to prepare an electrode assembly, which is then placed in a battery case. 1 M of LiPF6 is dissolved in a solvent containing ethylene carbonate, ethyl methyl carbonate, and diethyl carbonate in a 1:1:1 ratio to obtain an electrolyte solution. The electrolyte solution is then injected into the battery case to manufacture a lithium secondary battery.

[0138] Example 2

[0139] <Manufacturing the Negative Electrode>

[0140] The first negative electrode active material (artificial graphite: natural graphite = 25:75), the first negative electrode conductive material (superC), the thickener (carboxymethyl cellulose (CMC)) and the first negative electrode binder (styrene-butadiene rubber (SBR)) were mixed in water at a weight ratio of 96.1:0.5:1.0:2.4 to prepare the first negative electrode slurry.

[0141] The second negative electrode active material (artificial graphite: natural graphite = 75:25), the second negative electrode conductive material (super C), the thickener (carboxymethyl cellulose (CMC)), and the second negative electrode binder (styrene-butadiene rubber (SBR)) are mixed in water at a weight ratio of 97.4:0.5:1.1:1.0 to prepare the second negative electrode slurry.

[0142] A first negative electrode slurry and a second negative electrode slurry are simultaneously coated onto a negative electrode current collector (copper metal film), wherein the first negative electrode slurry is disposed on the negative electrode current collector, and the second negative electrode slurry is disposed on the first negative electrode slurry. Subsequently, the negative electrode current collector coated with the first and second negative electrode slurries is dried and then rolled to manufacture the negative electrode. The porosity of the obtained negative electrode is 29%.

[0143] The positive electrode and the lithium secondary battery were manufactured in the same manner as in Example 1, except that the negative electrode described above was used.

[0144] Example 3

[0145] The positive electrode, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 2, except that the average particle size D was adjusted. 50 Single-particle Li[Ni] with a particle size of 7.6 μm 0.60 Co 0.10 Mn 0.30 O2 was used as the positive electrode active material. The porosity of the obtained positive electrode was 22%.

[0146] Example 4

[0147] <The Manufacturing of the Positive Electrode>

[0148] The positive electrode was manufactured in the same manner as in Example 1, except that a positive electrode slurry was prepared with a weight ratio of positive electrode active material, positive electrode conductive material, positive electrode binder, and positive electrode dispersant of 97.24:1.2:1.35:0.21. The porosity of the obtained positive electrode was 22%.

[0149] The negative electrode and the lithium secondary battery were manufactured in the same manner as in Example 2, except that the above-described positive electrode was used.

[0150] Example 5

[0151] <The Manufacturing of the Positive Electrode>

[0152] The positive electrode was manufactured in the same manner as in Example 1, except that a positive electrode with a porosity of 22% was manufactured.

[0153] <Manufacturing the Negative Electrode>

[0154] The negative electrode was manufactured in the same manner as in Example 1, except that a negative electrode with a porosity of 28% was manufactured.

[0155] <Manufacturing of Lithium Secondary Batteries>

[0156] The lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive and negative electrodes manufactured above were used.

[0157] Comparative Example 1

[0158] <Manufacturing the Negative Electrode>

[0159] The first negative electrode active material (artificial graphite: natural graphite = 50:50), the first negative electrode conductive material (superC), the thickener (carboxymethyl cellulose (CMC)) and the first negative electrode binder (styrene-butadiene rubber (SBR)) were mixed in water at a weight ratio of 95.2:0.5:1.0:3.3 to prepare the first negative electrode slurry.

[0160] The second negative electrode active material (artificial graphite: natural graphite = 50:50), the second negative electrode conductive material (super C), the thickener (carboxymethyl cellulose (CMC)), and the second negative electrode binder (styrene-butadiene rubber (SBR)) are mixed in water at a weight ratio of 97.4:0.5:1.1:1.0 to prepare the second negative electrode slurry.

[0161] A first negative electrode slurry and a second negative electrode slurry are simultaneously coated onto a negative electrode current collector (copper metal film), wherein the first negative electrode slurry is disposed on the negative electrode current collector, and the second negative electrode slurry is disposed on the first negative electrode slurry. Subsequently, the negative electrode current collector coated with the first and second negative electrode slurries is dried and then rolled to manufacture the negative electrode. The porosity of the obtained negative electrode is 27%.

[0162] The positive electrode, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 1, except that the aforementioned negative electrode was used.

[0163] Comparative Example 2

[0164] The positive electrode, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 2, except that the first negative electrode active material (artificial graphite:natural graphite = 25:75), the first negative electrode conductive material (super C), the thickener (carboxymethyl cellulose (CMC)), and the first negative electrode binder (styrene-butadiene rubber (SBR)) were mixed in water at a weight ratio of 93.5:0.5:1.0:5.0 to prepare the first negative electrode slurry. The porosity of the obtained negative electrode was 26%.

[0165] Comparative Example 3

[0166] The positive electrode, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 2, except that the first negative electrode active material (artificial graphite:natural graphite = 25:75), the first negative electrode conductive material (super C), the thickener (carboxymethyl cellulose (CMC)), and the first negative electrode binder (styrene-butadiene rubber (SBR)) were mixed in water at a weight ratio of 97.5:0.5:1.0:1.0 to prepare the first negative electrode slurry. The porosity of the obtained negative electrode was 29%.

[0167] Comparative Example 4

[0168] The positive electrode, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 2, except that the average particle size D was adjusted. 50 Single-particle Li[Ni] with a particle size of 8.0 μm 0.60 Co 0.10 Mn 0.30 O2 was used as the positive electrode active material. The porosity of the obtained positive electrode was 22%.

[0169] Comparative Example 5

[0170] <Manufacturing the Negative Electrode>

[0171] A negative electrode slurry was prepared by mixing the negative electrode active material (artificial graphite: natural graphite = 50:50), the negative electrode conductive material (super C), the thickener (carboxymethyl cellulose (CMC)), and the first negative electrode binder (styrene-butadiene rubber (SBR)) in water at a weight ratio of 96.8:0.5:1.0:1.7.

[0172] The negative electrode slurry is coated onto the negative electrode current collector (copper metal film), dried, and rolled to manufacture the negative electrode. The porosity of the resulting negative electrode is 29%.

[0173] The positive electrode, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 2, except that the aforementioned negative electrode was used.

[0174] Comparative Example 6

[0175] The positive electrode, negative electrode, and lithium secondary battery were manufactured in the same manner as in Example 2, except that the average particle size D was adjusted. 50 Single-particle Li[Ni] with a particle size of 2.2 μm 0.60 Co 0.10 Mn 0.30 O2 was used as the positive electrode active material. The porosity of the obtained positive electrode was 22%.

[0176] Comparative Example 7

[0177] <The Manufacturing of the Positive Electrode>

[0178] The positive electrode was manufactured in the same manner as in Example 1, except that a positive electrode with a porosity of 22% was manufactured.

[0179] <Manufacturing the Negative Electrode>

[0180] The negative electrode was manufactured in the same manner as in Example 1, except that a negative electrode with a porosity of 32% was manufactured.

[0181] <Manufacturing of Lithium Secondary Batteries>

[0182] The lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive and negative electrodes manufactured above were used.

[0183] Comparative Example 8

[0184] <The Manufacturing of the Positive Electrode>

[0185] The positive electrode was manufactured in the same manner as in Example 1, except that a positive electrode with a porosity of 22% was manufactured.

[0186] <Manufacturing the Negative Electrode>

[0187] The negative electrode was manufactured in the same manner as in Example 1, except that a negative electrode with a porosity of 26% was manufactured.

[0188] <Manufacturing of Lithium Secondary Batteries>

[0189] The lithium secondary battery was manufactured in the same manner as in Example 1, except that the positive and negative electrodes manufactured above were used.

[0190] Experimental Example 1: Evaluation of FBR value, negative electrode binder, and porosity

[0191] (1) Measurement of FBR value

[0192] For the lithium secondary batteries manufactured in Examples 1 to 5 and Comparative Examples 1 to 8, the FBR value defined by Equation 1 below was measured. The results are shown in Table 1 below.

[0193] [Equation 1]

[0194] FBR=[(Rb×TDb)+(Rd×TDd)]×A 2

[0195] Rb is the dimensionless percentage (by weight) of the positive electrode binder, Rd is the dimensionless percentage (by weight) of the positive electrode dispersant, TDb is the dimensionless true density (g / cc) of the positive electrode binder, TDd is the dimensionless true density (g / cc) of the positive electrode dispersant, and A is the average particle size (D) of the positive electrode active material. 50 The dimensionless number (μm).

[0196] (2) Evaluation of negative electrode binder and porosity

[0197] The ratio of the first negative electrode binder content to the second negative electrode binder content, the total binder content in the negative electrode, and the difference in porosity between the positive and negative electrodes of the lithium secondary batteries manufactured in Examples 1 to 5 and Comparative Examples 1 to 8, are shown in Table 1 below.

[0198] [Table 1]

[0199] Experimental Example 2. Evaluation of Adhesive Migration

[0200] For the negative electrodes manufactured in Examples 1 to 5 and Comparative Examples 1 to 8, the QBR value defined in Equation 2 below was measured. The results are shown in Table 2 below.

[0201] [Equation 2]

[0202] QBR=Bs / Bf

[0203] Bs represents the average adhesive content in the surface region of the negative electrode active material layer from the outermost surface of the second negative electrode active material layer to within 15% of the total thickness of the negative electrode active material layer, and Bf represents the average adhesive content in the bottom region of the negative electrode active material layer from the interface between the first negative electrode active material layer and the negative electrode current collector to within 15% of the total thickness of the negative electrode active material layer.

[0204] At this point, the binder content is measured using the following procedure: A cross-section of the negative electrode is generated using argon ion milling. Subsequently, the composition of the negative electrode active material layer in the generated negative electrode cross-section is EDS-mapped using an energy-dispersive X-ray spectroscopy (EDS) detector on a scanning electron microscope (SEM). From the EDS-mapped results, a line profile is extracted along the thickness direction of the negative electrode active material layer, and from the extracted line profile, the average binder content Bs of the surface region of the negative electrode active material layer and the average binder content Bf of the bottom region of the negative electrode active material layer are extracted.

[0205] Experimental Example 3. Evaluation of Negative Electrode Adhesion

[0206] The negative electrodes manufactured in Examples 1 to 5 and Comparative Examples 1 to 8 were cut to a width of 20 mm and a length of 15 cm, and attached to a glass slide using double-sided adhesive tape. The negative electrodes were then pressed under constant pressure. Specifically, a 90-degree peel test was performed to confirm the adhesion of the negative electrodes, measured in gf / 20 mm. The results are shown in Table 2 below.

[0207] [Table 2]

[0208] Referring to Table 2 above, compared with the lithium secondary batteries manufactured in Comparative Examples 1 to 8, the lithium secondary batteries manufactured in Examples 1 to 5 each meet the QBR values ​​of 1.1 to 1.28. Therefore, it can be confirmed that the adhesive is uniformly distributed in the thickness direction throughout the entire negative electrode active material layer, and the negative electrode has excellent adhesion.

[0209] Experiment Example 4. Evaluation of Fast Charging Performance

[0210] The lithium secondary batteries manufactured in Examples 1 to 5 and Comparative Examples 1 to 8 were charged using a step-charging method to measure the time (in minutes) required to charge the battery from 8% SOC to 80%. Step-charging was performed by charging at a constant charging rate (C-rate) and then reducing the charging rate sequentially when a certain voltage (4.35 V) was reached. In this invention, charging was performed sequentially at a C-rate of 0.25 C → 0.5 C → 0.75 C → 1 C → 1.25 C → 1.5 C → 1.75 C → 2.25 C → 2.5 C → 2.75 C → 3 C → 3.25 C. The measurement results are shown in Table 3 below.

[0211] Experimental Example 5. Evaluation of Initial Capacitance and Resistive Properties

[0212] At 25°C, the lithium secondary batteries manufactured in Examples 1 to 5 and Comparative Examples 1 to 8 were charged to 4.35 V at 0.1 C and then discharged to 2.5 V at 0.1 C to measure the initial capacity. The resistance was then measured by the voltage change when a current of 2.5 C was applied for 10 seconds. The measurement results are shown in Table 3 below.

[0213] Experimental Example 6. Evaluation of Lifetime Properties

[0214] At 25°C, the lithium secondary batteries manufactured in Examples 1 to 5 and Comparative Examples 1 to 8 were charged to 4.35 V at 0.33 C and then discharged to 2.5 V at 0.33 C. This entire process was defined as one cycle, and 200 charge-discharge cycles were performed to measure capacity retention and thus evaluate lifetime properties. The measurement results are shown in Table 3 below.

[0215] [Table 3]

[0216] Referring to Table 3 above, compared with the lithium secondary batteries manufactured in Comparative Examples 1 to 8, it can be confirmed that the lithium secondary batteries manufactured in Examples 1 to 5 simultaneously possess excellent fast charging properties, initial capacity, initial resistance, and capacity retention.

Claims

1. A lithium secondary battery, comprising: The positive electrode comprises a positive electrode active material layer, the positive electrode active material layer comprising a single-particle positive electrode active material, a positive electrode conductive material, a positive electrode binder, and a positive electrode dispersant; a negative electrode; and an electrolyte. in, The negative electrode comprises: a first negative electrode active material layer formed on a negative electrode current collector and comprising a first negative electrode active material, a first negative electrode conductive material, and a first negative electrode binder; and a second negative electrode active material layer formed on the first negative electrode active material layer and comprising a second negative electrode active material, a second negative electrode conductive material, and a second negative electrode binder. in, The first negative electrode active material and the second negative electrode active material each independently comprise natural graphite, artificial graphite, or a combination thereof, and The ratio of the content of the first negative electrode adhesive to the content of the second negative electrode adhesive is 1.5 to 3.

0. and The porosity difference between the positive and negative electrodes is 4.2% to 9.8%, and The FBR value of the positive electrode, as defined by Equation 1 below, is between 30 and 180. [Equation 1] FBR=[(Rb×TDb)+(Rd×TDd)]×A 2 In equation 1 above, Rb is a dimensionless number representing a percentage by weight of the positive electrode adhesive relative to the total weight of the positive electrode active material layer. Rd is a dimensionless number representing a weight percentage of the positive electrode dispersant relative to the total weight of the positive electrode active material layer. TDb is the dimensionless number of the true density of the positive electrode adhesive in g / cc. TDd is the dimensionless number of the true density of the positive electrode dispersant in g / cc, and A is the average particle size of the positive electrode active material in μm. 50 . dimensionless number.

2. The lithium secondary battery as described in claim 1, wherein, The single-particle positive electrode active material comprises lithium nickel oxide with a Ni content of less than 70 mol% based on the total molar number of metals other than lithium.

3. The lithium secondary battery as described in claim 1, wherein, The single-particle type positive electrode active material comprises lithium nickel oxide represented by the following formula 1. [Formula 1] Li 1+x [Ni a Co b Mr c M 1 d ]O2 In Equation 1 above, M 1 Includes one or more doping elements selected from the group consisting of Ti, Mg, Al, Zr, Y, Ba, Ca, Sr, W, Ta, Nb, and Mo, and -0.1 ≤ x ≤ 0.1, 0.5 ≤ a ≤ 0.7, 0 <b<0.5,0<c<0.5,0≤d≤0.2。 4. The lithium secondary battery as described in claim 1, wherein, The average particle size D of the single-particle positive electrode active material 50 The thickness ranges from 3.5 μm to 7.8 μm.

5. The lithium secondary battery as described in claim 1, wherein, The single-particle positive electrode active material contains 1 to 30 nodules.

6. The lithium secondary battery as described in claim 5, wherein, The average particle size (D) of the nodules mean The thickness ranges from 0.8 μm to 4.0 μm.

7. The lithium secondary battery as described in claim 1, wherein, The positive electrode active material layer comprises 0.5% to 2% by weight of the positive electrode binder.

8. The lithium secondary battery as described in claim 1, wherein, The porosity of the positive electrode is 20% to 25%.

9. The lithium secondary battery as described in claim 1, wherein, The first negative electrode active material and the second negative electrode active material are each independently composed of natural graphite, artificial graphite, or a combination thereof.

10. The lithium secondary battery as described in claim 1, wherein, The first negative electrode active material contains more than 50% by weight of natural graphite, and The second negative electrode active material contains less than 50% by weight of natural graphite.

11. The lithium secondary battery as described in claim 1, wherein, The ratio of the content of the first negative electrode adhesive to the content of the second negative electrode adhesive is 1.6 to 2.

4.

12. The lithium secondary battery as described in claim 1, wherein, The first negative electrode active material layer contains 1.5% to 3.0% by weight of the first negative electrode binder.

13. The lithium secondary battery as described in claim 1, wherein, The QBR value of the negative electrode, as defined by Equation 2 below, is between 1.1 and 1.

28. [Equation 2] QBR=Bs / Bf Bs is the average amount of binder content in the surface region of the negative electrode active material layer, from the outermost surface of the second negative electrode active material layer to within 15% of the total thickness of the negative electrode active material layer. Bf is the average amount of binder content in the bottom region of the negative electrode active material layer, from the interface between the first negative electrode active material layer and the negative electrode current collector to 15% of the total thickness of the negative electrode active material layer.

14. The lithium secondary battery as described in claim 1, wherein, The porosity of the negative electrode is 25% to 32%.

15. The lithium secondary battery as described in claim 1, wherein, The porosity difference between the positive electrode and the negative electrode is 6% to 8%.

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