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

Abstract

The present invention provides a negative electrode for a lithium secondary battery having excellent energy density, the negative electrode comprising a negative electrode current collector and a negative electrode mixture layer on at least one surface of the negative electrode current collector, the negative electrode mixture layer containing a carbon-based active material, the negative electrode for a lithium secondary battery has a crystal orientation index increase rate (ROI) according to formula 1 of 120% to 400%. [Formula 1] ROI = 100 (OI2-OI1) / (OI1) In Formula 1, ROI is a crystal orientation index (OI) increase rate (%), OI1 is a ratio of a peak intensity of a (004) plane to a peak intensity of a (110) plane as determined by X-ray diffraction analysis of a carbon-based active material, and OI2 is a ratio of a peak intensity of a (004) plane to a peak intensity of a (110) plane as determined by X-ray diffraction analysis of a negative electrode mixture layer.

Description

Lithium-ion secondary battery negative electrode and lithium-ion secondary battery including the negative electrode Technical Field

[0001] This invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery including the negative electrode. Background Technology

[0002] In recent years, extensive research has been conducted on electric vehicles (EVs), which are fossil fuel-powered vehicles such as gasoline and diesel vehicles, and are seen as alternatives to major sources of air pollution. These EVs primarily utilize lithium-ion batteries, which offer high discharge voltage and power stability. Consequently, the demand for high-performance lithium-ion battery anodes is continuously increasing. Summary of the Invention

[0003] (a) Technical problems to be solved

[0004] One aspect of the present invention is to improve the energy density of the negative electrode for lithium secondary batteries.

[0005] Another objective of this invention is to suppress the occurrence of springback in the negative electrode of lithium secondary batteries.

[0006] Another objective of the present invention is to provide a negative electrode for lithium secondary batteries with excellent lifespan characteristics.

[0007] Another objective of this invention is to reduce the manufacturing cost of negative electrodes for lithium secondary batteries.

[0008] (II) Technical Solution

[0009] A negative electrode for a lithium secondary battery according to one embodiment includes a negative electrode current collector and a negative electrode mixture layer on at least one side of the negative electrode current collector, the negative electrode mixture layer comprising a carbon-based active material, and the negative electrode for the lithium secondary battery exhibits a crystal orientation index increase rate (R) according to Formula 1 below. OI The percentage ranges from 120% to 400%.

[0010] [Formula 1]

[0011] R OI =100×(OI2-OI1) / (OI1)

[0012] In Equation 1, R OI OI is the increase rate (%) of the crystal orientation index (OI), and OI1 is the peak intensity (I) of the (004) plane based on X-ray diffraction (XRD) analysis of carbon-based active materials. 004 Peak intensity (I) of the (110) plane 110 The ratio of (I) 004 / I110 OI2 is the peak intensity (I) of the (004) plane based on X-ray diffraction (XRD) analysis of the negative electrode mixture layer. 004 Peak intensity (I) of the (110) plane 110 The ratio of (I) 004 / I 110 ).

[0013] In some specific embodiments, the increase rate (Rc) of the crystal orientation index of the negative electrode for the lithium secondary battery OI It can be between 190% and 270%.

[0014] In some specific implementations, the OI2 value of the negative electrode for the lithium secondary battery can be between 7 and 15.

[0015] In some specific embodiments, the carbon-based active material may comprise artificial graphite.

[0016] In some specific implementations, the artificial graphite may be in the form of single particles.

[0017] In some specific embodiments, the surface of the artificial graphite may include a carbon coating.

[0018] In some specific embodiments, the carbon-based active material may comprise artificial graphite and natural graphite.

[0019] In some specific embodiments, the weight of artificial graphite in the carbon-based active material may be greater than or equal to the weight of natural graphite.

[0020] In some specific embodiments, the surface of the natural graphite may include a carbon coating.

[0021] In some specific embodiments, the electrode density of the negative electrode mixture layer can be from 1.4 g / cm³ (cc) to 1.7 g / cm³.

[0022] A lithium secondary battery according to one embodiment includes a negative electrode for a lithium secondary battery as described in any of the above embodiments.

[0023] (III) Beneficial Effects

[0024] According to a specific embodiment of the present invention, a negative electrode for lithium secondary batteries with excellent energy density can be provided.

[0025] According to another specific embodiment of the present invention, the occurrence of rebound phenomenon in the negative electrode of lithium secondary battery can be suppressed.

[0026] According to another specific embodiment of the present invention, a negative electrode for lithium secondary batteries with excellent lifespan characteristics can be provided.

[0027] According to another specific embodiment of the present invention, economic efficiency can be ensured when manufacturing negative electrodes for lithium secondary batteries. Attached Figure Description

[0028] Figure 1 is a conceptual diagram showing the basal plane corresponding to the base plane and the edge plane formed by the aggregation of the edges of the aforementioned basal planes in a carbon-based active material with a parallel stacked structure.

[0029] Explanation of reference numerals in the attached figures:

[0030] 1: Carbon-based active substances

[0031] 2: Base surface

[0032] 3: Edge surface Detailed Implementation

[0033] In this specification, the “orientation” of the mixture layer and the active material refers to the peak intensity (I) of the (004) surface as measured by XRD. 004 Peak intensity (I) of the (110) plane 110 The ratio of peak intensity to peak intensity determines the "Orientation Index (OI)" value (Ic). 004 / I 110 The characteristics represented by the OI value are as follows: For example, the smaller the OI value of the mixture layer and the active material, the lower the orientation of the mixture layer and the active material, and the larger the OI value of the mixture layer and the active material, the higher the orientation of the mixture layer and the active material.

[0034] To provide a high-performance lithium-ion secondary battery, the negative electrode of a lithium-ion secondary battery according to one specific embodiment may include artificial graphite made from coke as the negative electrode active material. The artificial graphite has numerous channels through which lithium ions can pass, thereby improving the high-power performance and fast-charging performance of the lithium-ion secondary battery.

[0035] Specifically, referring to Figure 1, the orientation of the carbon-based active material such as artificial graphite will be described in detail below. Figure 1 is a conceptual diagram showing the base planes corresponding to the base planes and the edge planes formed by the aggregation of the edges of the aforementioned base planes in a carbon-based active material having a parallel stacked structure.

[0036] The carbon-based active material 1, such as artificial graphite, typically comprises a carbon layer consisting of six hexagonal rings connected in a planar manner, with the carbon layers stacked parallel to each other (see Figure 1). In the carbon-based active material 1, the basal surface 2 corresponds to the base surface (base plane) in the carbon layer with a parallel stacked structure, and the edge surface 3 refers to the surface formed by the aggregation of the edges of the aforementioned basal surfaces.

[0037] During the charging and discharging process of a lithium-ion secondary battery, the intercalation and deintercalation of lithium ions stored and released in the carbon-based active material 1 are mainly achieved through the edge surfaces 3. Therefore, as the number of these edge surfaces 3 increases, the intercalation and deintercalation of lithium ions during charging becomes easier, and the fast charging characteristics can also be improved.

[0038] Relatedly, the crystal orientation index (OI) value determined by X-ray diffraction (XRD) analysis of the carbon-based active material 1 refers to the ratio of the peak intensity of the (004) plane to the peak intensity of the (110) plane. The smaller the OI value of the carbon-based active material 1, the more edge faces 3 can have a structure with a relatively greater number of edge faces 3 than base faces 2. It is believed that this is because the smaller the crystal orientation index (OI) value, the greater the disorder of the crystal arrangement, thereby increasing the number of edge faces 3 that lithium ions can enter and exit. Therefore, the more low-orientation carbon-based active material with a smaller crystal orientation index (OI) value is contained in the negative electrode, the easier it is for lithium ions to enter and exit through many edge faces 3, thus resulting in excellent power performance and fast charging performance.

[0039] However, the lower the orientation, the higher the hardness of the carbon-based active material, making it difficult to roll low-oriented carbon-based active materials to a high density. Therefore, when the negative electrode contains low-oriented carbon-based active materials, it may be difficult to increase the rolling density of the negative electrode, thus potentially compromising its energy density. Furthermore, low-oriented carbon-based active materials may suffer particle damage during rolling, potentially leading to additional side reactions or contributing to reduced battery lifespan.

[0040] Relatedly, the artificial graphite is a low-orientation active material with low "orientation," making it potentially difficult to roll a negative electrode containing the artificial graphite to a high density. In particular, the so-called "springback" phenomenon—where the active material in the negative electrode expands again after rolling—may be exacerbated in negative electrodes containing artificial graphite. This "springback" phenomenon can cause the actual electrode thickness in the lithium-ion battery design to differ from the intended target thickness, potentially making it difficult to ensure the electrode's energy density.

[0041] Furthermore, the artificial graphite is subjected to significant stress during the calendering process of the negative electrode containing it, which may lead to particle damage. Therefore, this could potentially reduce the performance of the lithium-ion secondary battery including the negative electrode.

[0042] According to a specific embodiment of the present invention, a negative electrode for lithium secondary batteries is provided that can alleviate the above-mentioned problems. The specific embodiment of the present invention will be described in detail below.

[0043] Negative electrode for lithium secondary batteries

[0044] A negative electrode for a lithium secondary battery according to one embodiment includes a negative electrode current collector and a negative electrode mixture layer on at least one side of the negative electrode current collector, the negative electrode mixture layer comprising a carbon-based active material, and the negative electrode for the lithium secondary battery exhibits a crystal orientation index increase rate (R) according to Formula 1 below. OI The percentage ranges from 120% to 400%.

[0045] [Formula 1]

[0046] R OI =100×(OI2-OI1) / (OI1)

[0047] In Equation 1, R OI OI is the increase rate (%) of the crystal orientation index (OI), and OI1 is the peak intensity (I) of the (004) plane based on X-ray diffraction (XRD) analysis of carbon-based active materials. 004 Peak intensity (I) of the (110) plane 110 The ratio of (I) 004 / I 110 OI2 is the peak intensity (I) of the (004) plane based on X-ray diffraction (XRD) analysis of the negative electrode mixture layer. 004 Peak intensity (I) of the (110) plane 110 The ratio of (I) 004 / I 110 ).

[0048] The composition of the negative electrode current collector is not particularly limited. Exemplarily, the negative electrode current collector may be a plate or foil formed from one or more of indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), and their alloys. The thickness of the negative electrode current collector is not particularly limited. Exemplarily, the thickness of the negative electrode current collector may be from 0.1 μm to 50 μm.

[0049] According to the specific implementation scheme described above, in the process of manufacturing a negative electrode containing carbon-based active material, the performance of a lithium secondary battery can be improved by adjusting the rate of change of the "orientation" between the carbon-based active material and the negative electrode to a specific range. Specifically, in the manufacturing process of the negative electrode for the lithium secondary battery, the stress on the carbon-based active material is minimized, and the occurrence of the aforementioned "springback" phenomenon is suppressed, thereby the negative electrode for the lithium secondary battery can have excellent performance. Therefore, the increase rate of the crystal orientation index (R) of the negative electrode for the lithium secondary battery according to Formula 1 is... OI The value can be from 120% to 400%, specifically from 120% to 350%, and more specifically from 120% to 300%.

[0050] The crystal orientation index (OI) value can be determined by the peak intensity (I004) appearing on the (004) plane during X-ray diffraction (XRD) analysis of a specific object. 004 The peak intensity (I) appearing on the (110) surface 110 The ratio of (I) 004 / I 110 The peak intensity (I) appearing on the (004) surface is calculated using this method. 004 The peak intensity of the (004) surface, which appears at an angle of 2θ = 54.7 ± 0.2°, can be measured using CuKα XRD. The peak intensity of the (110) surface (I) 110 The peak intensity value can be the peak intensity value of the (110) surface appearing at an angle of 2θ = 77.5 ± 0.2° when measured by XRD using CuKα rays. The peak intensity value can refer to the height or integral area of ​​a specific peak. In some specific embodiments, the I... 004 / I 110 The value can be calculated by the ratio between the integral areas of each peak.

[0051] In some specific embodiments, the increase rate (Rc) of the crystal orientation index of the negative electrode for the lithium secondary battery OI The crystal orientation index increase rate (R) can be from 190% to 270%. For example, the increase rate of the crystal orientation index of the negative electrode for the lithium secondary battery (Rc) can be... OI The crystal orientation index (R) of the negative electrode for the lithium secondary battery can be above 195%, above 200%, above 210%, above 212%, above 213%, or above 220%, and can be below 265%, below 263%, below 260%, below 250%, below 240%, below 230%, or below 220%. OI Within the above range, the life performance of lithium secondary batteries can be improved while reducing the electrode expansion rate.

[0052] In some specific embodiments, the OI1 value of the negative electrode for the lithium secondary battery according to Formula 1 can be from 0.5 to 3.5. Specifically, the crystal orientation index (OI1) value of the carbon-based active material contained in the negative electrode for the lithium secondary battery can be from 0.5 to 3.5. Exemplarily, the crystal orientation index (OI1) value of the carbon-based active material can be 1 or more, 2.0 or more, 2.1 or more, 2.4 or more, 2.5 or more, 2.7 or more, 2.8 or more, or 3 or more, and can be less than 3.5, less than 3.2, less than 3, or less than 2.8. The crystal orientation index (OI1) value of the carbon-based active material can be a value obtained by measuring the crystal orientation index (OI) of the carbon-based active material collected after washing the negative electrode following disassembly of the lithium secondary battery.

[0053] In some specific embodiments, the OI2 value of the negative electrode for the lithium secondary battery according to Formula 1 can be from 7 to 15. Specifically, the crystal orientation index (OI2) value of the negative electrode mixture layer of the negative electrode for the lithium secondary battery can be from 7 to 15. Exemplarily, the crystal orientation index (OI2) value of the negative electrode mixture layer can be 7.1 or higher, 7.2 or higher, 9 or higher, 9.3 or higher, 10 or higher, 10.1 or higher, or 10.7 or higher, and can be 13 or lower, 11 or lower, 10.8 or lower, 10 or lower, or 9.2 or lower. The crystal orientation index (OI2) value of the negative electrode mixture layer can be a crystal orientation index (OI) value obtained by measuring the negative electrode mixture layer of the negative electrode after disassembling the lithium secondary battery.

[0054] In some specific embodiments, the average particle size (D50) of the carbon-based active material can be from 1 μm to 20 μm. Exemplarily, the average particle size (D50) of the carbon-based active material can be 5 μm or more, 8 μm or more, 10 μm or more, or 11 μm or more, and can be less than 15 μm or less than 12 μm.

[0055] The carbon-based active material is not particularly limited. Exemplarily, the carbon-based active material can be crystalline carbon, amorphous carbon, carbon composites, carbon fibers, or other carbon-based materials. Exemplarily, the crystalline carbon can be natural graphite, artificial graphite, graphitized coke, graphitized mesocarbon microbeads (MCMB), graphitized mesophase pitch-based carbon fiber (MPCF), or other graphite-based carbon. Exemplarily, the amorphous carbon can be hard carbon, soft carbon, coke, mesophase carbon microbeads (MCMB), or mesophase pitch-based carbon fiber (MPCF).

[0056] In some specific embodiments, the carbon-based active material may comprise artificial graphite. The artificial graphite may be made from coke. When the carbon-based active material comprises artificial graphite, cost competitiveness can be ensured while easily adjusting the "orientation" of the negative electrode mixture layer.

[0057] In some specific embodiments, the artificial graphite may be in single-particle form. When the negative electrode for the lithium secondary battery contains artificial graphite in single-particle form, the lifespan and power characteristics of the lithium secondary battery including the negative electrode can be further improved.

[0058] The single particle can be morphologically distinguished from secondary particles formed by the aggregation of primary particles. For example, the single particle and the secondary particle can be differentiated based on cross-sectional images of the particle measured by a scanning electron microscope (SEM).

[0059] In some specific implementations, the secondary particle can refer to a particle in which multiple primary particles aggregate but are substantially regarded or observed as a single particle. Exemplarily, the secondary particle can be composed of an aggregate of more than 10, 30, 50, or 100 to less than 1000 primary particles. Exemplarily, in the case of the secondary particle, the boundary of the primary particle can be observed in the SEM cross-sectional image.

[0060] In some specific implementations, the single particle may refer to a monolith rather than an aggregate. For example, in the case of a single particle, unlike secondary particles, the boundaries of the primary particle may not be observable in a SEM cross-sectional image. Additionally, the surface of the single particle may be covered with fine particles (for example, particles having a volume less than 1 / 100th of the volume of the single particle), a form not excluded from the concept of a single particle.

[0061] In some specific implementations, the individual particles may be in contact with each other. For example, the individual particles may exist in the form of 2 to 10, 2 to 5, or 2 to 3 individual particles in contact with each other.

[0062] In some specific embodiments, the surface of the artificial graphite may include a carbon coating. Exemplarily, the surface of the artificial graphite may include an amorphous carbon coating. The amorphous carbon coating can be formed by heat-treating a mixture of artificial graphite and a carbon precursor. The carbon precursor is not particularly limited. Exemplarily, the carbon precursor may be at least one selected from polymer resins such as polyvinyl alcohol, polyacrylonitrile, and polyamide; and asphalts such as coal-based pitch, petroleum-based pitch, and mesophase pitch. Exemplarily, the heat treatment temperature may be from 1000°C to 1800°C. In this case, the hardness of the artificial graphite can be increased, thereby reducing the electrode expansion rate of the negative electrode containing the artificial graphite.

[0063] In some specific embodiments, the carbon coating content can be from 1% to 10% by weight, based on the total weight of the artificial graphite. For example, the carbon coating content can be more than 1% by weight or more than 1.5% by weight, and can be less than 5% by weight, less than 3% by weight, or less than 1.5% by weight. When the carbon coating content is less than 1% by weight, it may be difficult to obtain a surface coating effect on the artificial graphite. On the other hand, when the carbon coating content exceeds 10% by weight, the hardness of the artificial graphite may become too high, making it difficult to ensure the energy density of the negative electrode containing the artificial graphite.

[0064] In some specific embodiments, the carbon-based active material may comprise artificial graphite and natural graphite. The detailed description of the artificial graphite is repeated above and therefore omitted. The natural graphite is a highly oriented carbon-based active material with relatively higher "orientation" compared to artificial graphite, and its hardness can be lower than that of artificial graphite. Therefore, the negative electrode containing the natural graphite is easily rolled to high density, thereby achieving high energy density. Furthermore, when the negative electrode contains natural graphite, the aforementioned "springback" phenomenon can be mitigated.

[0065] In some specific embodiments, the weight of artificial graphite in the carbon-based active material may be greater than or equal to the weight of natural graphite. As mentioned above, natural graphite is a highly oriented carbon-based active material, which, while helping to improve the energy density of the negative electrode, may lead to over-packing during the calendering process of a negative electrode containing natural graphite. Therefore, in a negative electrode containing natural graphite as the negative electrode active material, the surface pressure stress between the active materials may increase during the operation of the lithium secondary battery, potentially leading to delamination of the active material and a deterioration in the lifespan performance of the lithium secondary battery.

[0066] Therefore, in the negative electrode for a lithium secondary battery containing both artificial graphite and natural graphite as the carbon-based active material, when the weight of the artificial graphite is adjusted to be greater than or equal to the weight of the natural graphite, the aforementioned problems can be suppressed within a range that does not hinder the effects of adding natural graphite.

[0067] In some specific embodiments, the weight ratio of artificial graphite to natural graphite in the carbon-based active material can be from 51:49 to 99:1. For example, the weight ratio of artificial graphite to natural graphite in the carbon-based active material can be from 55:45 to 95:5 or from 60:40 to 80:20.

[0068] In some specific embodiments, the sphericity of the natural graphite can be between 0.85 and 0.99. When the natural graphite, after undergoing a sphericity process, has a sphericity within the above range, the energy density of the negative electrode containing the natural graphite and the power of the lithium secondary battery can be improved. The sphericity can be defined as the ratio of the shortest diameter (minor axis) to the longest diameter (major axis) among any diameters passing through the center of the particle. For example, the closer the sphericity is to 1, the closer the particle shape is to a sphere. The sphericity can be measured using a particle shape analyzer.

[0069] In some specific embodiments, the surface of the natural graphite may include a carbon coating. Exemplarily, the surface of the natural graphite may include an amorphous carbon coating. The amorphous carbon coating can be formed by heat-treating a mixture of natural graphite and a carbon precursor. The carbon precursor is not particularly limited. Exemplarily, the carbon precursor may be at least one selected from polymer resins such as polyvinyl alcohol resin, polyacrylonitrile resin, and polyamide resin; and asphalts such as coal-based pitch, petroleum-based pitch, and mesophase pitch. Exemplarily, the heat treatment temperature may be between 1000°C and 1800°C. In this case, the hardness of the natural graphite can be increased, thereby reducing the electrode expansion rate of the negative electrode containing the natural graphite.

[0070] In some specific embodiments, the carbon coating content can be from 1% to 10% by weight, based on the total weight of the natural graphite. For example, the carbon coating content can be more than 1% by weight or more than 1.5% by weight, and can be less than 5% by weight, less than 3% by weight, or less than 1.5% by weight. When the carbon coating content is less than 1% by weight, it may be difficult to obtain the desired surface coating effect on the natural graphite. On the other hand, when the carbon coating content exceeds 10% by weight, the hardness of the natural graphite may become too high, making it difficult to ensure the energy density of the negative electrode containing the natural graphite.

[0071] In some specific embodiments, the content of the carbon-based active material contained in the negative electrode mixture layer can be 70% to 99% by weight. Exemplarily, the content of the carbon-based active material contained in the negative electrode mixture layer can be 90% to 95% by weight.

[0072] In some specific embodiments, the negative electrode mixture layer can further contain a negative electrode active material other than the carbon-based active material. Exemplarily, the negative electrode mixture layer can further contain at least one selected from lithium metal, lithium alloy, silicon-containing material, and tin-containing material as the negative electrode active material.

[0073] Exemplarily, the elements contained in the lithium alloy can be aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, or indium.

[0074] The silicon-containing material is not particularly limited as long as it contains silicon, and can be an active material that can be alloyed with lithium (Li). Exemplarily, the silicon-containing material can be one or more selected from silicon (Si), silicon oxide (SiOx, 0 < x < 2), metal-doped silicon oxide (SiOx, 0 < x < 2), carbon-coated silicon oxide (SiOx, 0 < x < 2), silicon-carbon composite (Si-C), and silicon alloy.

[0075] In some specific embodiments, the negative electrode mixture layer can further contain an adhesive. The adhesive is not particularly limited. Exemplarily, the adhesive can be selected from rubber-based adhesives such as styrene-butadiene rubber (SBR), fluorine-based rubber, ethylene-propylene rubber, butadiene rubber, isoprene rubber, and silyl rubber; cellulose-based adhesives such as carboxymethyl cellulose (CMC), hydroxypropyl methyl cellulose, methyl cellulose, or their alkali metal salts; and combinations thereof. Exemplarily, the content of the adhesive contained in the negative electrode mixture layer can be 0.1% to 10% by weight.

[0076] In some specific embodiments, the negative electrode mixture layer can further contain a conductive material. The conductive material is not particularly limited. Exemplarily, the conductive material can be selected from graphites such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking carbon black, carbon fiber, and carbon nanotube (CNT); metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive polymers such as polyphenylene derivatives; and combinations thereof. Exemplarily, the content of the conductive material contained in the negative electrode mixture layer can be 0.1% to 10% by weight.

[0077] In some specific embodiments, the electrode density of the negative electrode mixture layer can be from 1.4 g / cm³ to 1.7 g / cm³. Exemplarily, the electrode density of the negative electrode mixture layer can be 1.5 g / cm³ or more, or 1.6 g / cm³ or more, and can be less than 1.7 g / cm³. When the electrode density of the negative electrode mixture layer is within the above range, a negative electrode for lithium-ion batteries with excellent energy density and lifetime performance can be provided.

[0078] Manufacturing method of negative electrode for lithium secondary batteries

[0079] A method for manufacturing a negative electrode for a lithium secondary battery according to one embodiment includes the step of forming a negative electrode mixture layer on at least one side of a negative electrode current collector, said negative electrode mixture layer comprising a carbon-based active material, wherein the negative electrode for the lithium secondary battery exhibits a crystal orientation index increase rate (R) according to Formula 1. OI The percentage is 120% to 400%. Detailed descriptions of the negative electrode current collector, negative electrode mixture layer, carbon-based active material, Formula 1, etc., are repeated above and therefore omitted.

[0080] In some specific embodiments, the negative electrode mixture layer can be formed by coating a negative electrode slurry containing the aforementioned carbon-based active material onto at least one side of the negative electrode current collector and then drying it. The coating method of the negative electrode slurry is not particularly limited. Exemplarily, the negative electrode slurry can be coated onto the surface of the negative electrode current collector by methods such as rod coating, casting, or spraying. The drying temperature of the negative electrode slurry is not particularly limited. Exemplarily, the drying of the negative electrode slurry can be carried out at a temperature between 90°C and 120°C.

[0081] In some specific embodiments, the negative electrode slurry may further comprise at least one of a conductive material and a binder. Detailed descriptions of the conductive material, binder, etc., are repeated above and therefore omitted.

[0082] In some specific embodiments, the negative electrode slurry may further contain a solvent. The solvent is not particularly limited. Exemplarily, the solvent may be dimethyl sulfoxide (DMSO), isopropyl alcohol, N-methylpyrrolidone (NMP), acetone, water, etc. The amount of solvent used is not particularly limited, as long as it dissolves or disperses the components while considering the coating thickness, manufacturing yield, etc., and gives the slurry a viscosity that exhibits excellent thickness uniformity when coated on the current collector.

[0083] In some specific embodiments, the method for manufacturing the negative electrode for a lithium secondary battery may further include a step of calendering a negative electrode current collector and a negative electrode mixture layer formed on at least one side of the negative electrode current collector. Exemplarily, the calendering may be performed such that the electrode density of the negative electrode mixture layer is between 1.4 g / cm³ and 1.7 g / cm³. A detailed description of the electrode density is repeated above and therefore omitted.

[0084] Lithium secondary batteries

[0085] A lithium secondary battery according to one embodiment includes a negative electrode for a lithium secondary battery as described in any of the above embodiments. Exemplarily, the lithium secondary battery may include a single cell, the single cell including the aforementioned negative electrode, positive electrode, and separator. The separator may be disposed between the positive and negative electrodes.

[0086] The positive electrode may include a positive electrode current collector and a positive electrode mixture layer on at least one side of the positive electrode current collector.

[0087] The composition of the positive electrode current collector is not particularly limited. Exemplarily, the positive electrode current collector may be a sheet or foil formed from one or more of indium (In), copper (Cu), magnesium (Mg), stainless steel, titanium (Ti), iron (Fe), cobalt (Co), nickel (Ni), zinc (Zn), aluminum (Al), germanium (Ge), lithium (Li), and their alloys. The thickness of the positive electrode current collector is not particularly limited. Exemplarily, the thickness of the positive electrode current collector may be from 0.1 μm to 50 μm.

[0088] The positive electrode mixture layer may contain a positive electrode active material. The positive electrode active material is not particularly limited, and may contain compounds that enable reversible insertion and extraction of lithium ions. Exemplarily, the positive electrode active material may contain a lithium-nickel metal oxide. The lithium-nickel metal oxide may further contain at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

[0089] In some specific embodiments, the positive electrode active material or the lithium-nickel metal oxide may contain a layered structure or a crystal structure represented by the following chemical formula 1.

[0090] [Chemical Formula 1]

[0091] Li x Ni a M b O 2+z

[0092] In the chemical formula 1, the values ​​can be 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b≤0.4, and -0.5≤z≤0.1. As mentioned above, M can contain Co, Mn, and / or Al.

[0093] The chemical structure represented by Formula 1 indicates the bonding relationships contained in the layered or crystalline structure of the positive electrode active material, and does not exclude other additional elements. For example, M may contain Co and / or Mn, and Co and / or Mn may be provided together with Ni as the main active element of the positive electrode active material. Formula 1 is provided to represent the bonding relationships of the main active elements, and it should be understood that Formula 1 includes the introduction and substitution of additional elements.

[0094] In some specific embodiments, in addition to the main active element, auxiliary elements may be further included to enhance the chemical stability of the positive electrode active material or the layered / crystal structure. These auxiliary elements may be incorporated into the layered / crystal structure and form bonds; this should be understood to also include the chemical structures represented by Formula 1.

[0095] For example, the auxiliary element may include at least one selected from Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, or Zr. The auxiliary element may function as an auxiliary active element, together with Co or Mn, to contribute to the capacity / power activity of the positive electrode active material; for example, Al.

[0096] For example, the positive electrode active material or the lithium-nickel metal oxide may contain a layered structure or a crystal structure represented by the following chemical formula 1-1.

[0097] [Chemical Formula 1-1]

[0098] Li x Ni a M1 b1 M2 b2 O 2+z

[0099] In chemical formula 1-1, M1 may contain Co, Mn, and / or Al. M2 may contain the aforementioned auxiliary elements. In chemical formula 1-1, the following conditions may be met: 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b1+b2≤0.4, -0.5≤z≤0.1.

[0100] The positive electrode active material may further include a coating element or a doping element. For example, an element that is substantially the same as or similar to the above-described auxiliary element may be used as the coating element or the doping element. Exemplarily, one or a combination of two or more of the above elements may be used as the coating element or the doping element.

[0101] The coating element or the doping element may be present on the surface of the lithium-nickel metal oxide particles or penetrate through the surface of the lithium-nickel metal oxide particles and be included in the bonding structure represented by Chemical Formula 1 or Chemical Formula 1-1.

[0102] The positive electrode active material may include a nickel-cobalt-manganese (NCM)-based lithium oxide. In this case, an NCM-based lithium oxide with an increased nickel content may be used.

[0103] The content of Ni in the NCM-based lithium oxide (e.g., the mole fraction of Ni in the total moles of nickel, cobalt, and manganese) may be 0.6 or more, 0.7 or more, or 0.8 or more. In some embodiments, the content of Ni may be 0.8 to 0.95, 0.82 to 0.95, 0.83 to 0.95, 0.84 to 0.95, 0.85 to 0.95, or 0.88 to 0.95.

[0104] In some specific embodiments, the positive electrode active material may further include a lithium cobalt oxide-based active material, a lithium manganese oxide-based active material, a lithium nickel oxide-based active material, or a lithium iron phosphate (LFP)-based active material (e.g., LiFePO4).

[0105] In some specific embodiments, the positive electrode active material may include a manganese-rich (Mn-rich)-based active material, a lithium-rich layered oxide (LLO) / over-lithiated oxide (OLO)-based active material, or a low-cobalt (Co-less)-based active material having a chemical structure or crystal structure represented by Chemical Formula 2.

[0106] [Chemical Formula 2]

[0107] p[Li2MnO3]·(1-p)[Li q JO2]

[0108] In Chemical Formula 2, 0 < p < 1, 0.9 ≤ q ≤ 1.2, and J may include at least one element selected from Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, and B.

[0109] The positive electrode mixture layer may further comprise an adhesive. The adhesive is not particularly limited. Exemplarily, the adhesive may include one or more of the following: styrene-butadiene rubber (SBR), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), PVDF / hexafluoropropylene copolymer, polyacrylonitrile, and polymethyl methacrylate (PMMA).

[0110] The positive electrode mixture layer may further comprise a conductive material. The conductive material is not particularly limited. Exemplarily, the conductive material may include graphite such as natural or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, carbon fiber, and carbon nanotubes (CNTs); 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; or conductive polymers such as polyphenylene derivatives, etc., one or more of the following.

[0111] The diaphragm is not particularly limited. Exemplarily, the diaphragm may include a porous polymer membrane prepared from polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer. Furthermore, the diaphragm may also include a nonwoven fabric formed from high-melting-point glass fibers, polyethylene terephthalate fibers, etc.

[0112] In some specific embodiments, the lithium secondary battery can be manufactured by housing the aforementioned single cell in a soft pack that serves as the battery casing and then injecting electrolyte.

[0113] The electrolyte may contain an organic solvent and a lithium salt. The organic solvent acts as a medium for the migration of ions participating in the electrochemical reactions of the battery. For example, the organic solvent may be used alone or in combination with two or more of the following solvents: carbonate-based solvents, ester-based solvents, ether-based solvents, ketone-based solvents, alcohol-based solvents, or aprotic solvents. The mixing ratio when using two or more solvents can be appropriately adjusted according to the desired battery performance.

[0114] The lithium salt is dissolved in an organic solvent and acts as a lithium-ion supply source within the battery, enabling the basic operation of the lithium secondary battery and promoting the migration of lithium ions between the positive and negative electrodes. The lithium salt can be a known substance used at a suitable concentration. The electrolyte may further contain known solvents as needed to improve charge-discharge characteristics, flame-retardant properties, etc., and may contain known additives.

[0115] In some specific embodiments, the single-cell battery may include a solid electrolyte instead of a separator between the positive and negative electrodes. The solid electrolyte is not particularly limited; exemplaryly, it may be an oxide-based solid electrolyte, a sulfide-based solid electrolyte, or a polymer-based solid electrolyte.

[0116] Examples and Comparative Examples

[0117] 1) Preparation of negative electrode active material

[0118] As shown in Table 1 below, carbon-based active materials were prepared as negative electrode active materials for the examples and comparative examples. Specifically, as artificial graphite, seven types of artificial graphite (A to E, A', and A") with different average particle sizes (D50) and / or particle forms were prepared. Among them, A' is a carbon-based active material with an amorphous carbon coating of 1.0 wt% based on the total weight of the artificial graphite formed on the surface of an artificial graphite (A), and A" is a carbon-based active material with an amorphous carbon coating of 1.5 wt% based on the total weight of the artificial graphite formed on the surface of an artificial graphite (A). In addition, as natural graphite, natural graphite (G) spheroidized by a spheroidizing process and containing an amorphous carbon coating of 5 wt% on its surface was prepared.

[0119] The carbon-based active materials described above were mixed in the combinations shown in Table 2 below to prepare carbon-based active materials, which were then used as negative electrode active materials in the examples and comparative examples. The crystal orientation index (OI) values ​​(OI1) of the carbon-based active materials in the examples and comparative examples were then measured and are shown in Table 2 below.

[0120] Specifically, the crystal orientation index (OI) is determined by measuring the peak intensity (I004) of the (004) surface using X-ray diffraction (XRD) analysis. 004 Peak intensity (I) of the (110) plane and the (110) plane 110 Then, through the peak intensity (I) of the (004) surface. 004 ) and the peak intensity (I) of the (110) surface 110 The ratio of (I) 004 / I 110 To calculate. At this time, the I... 004 / I 110 The values ​​are calculated as the ratios between the integrated areas of each peak. Furthermore, the peak intensity values ​​are measured using CuKα rays as the target and an XRD apparatus (Panalytical's Empyrean). The measurement conditions are: 2θ = 10° to 80°, scan rate (° / sec) = 3, and step size set to 0.025° / step.

[0121] 2) Manufacturing of negative electrodes for lithium secondary batteries

[0122] The negative electrode active material, binder (carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR)) and conductive material (amorphous carbon) prepared as described above were mixed with a solvent to prepare a negative electrode slurry. At this time, based on the solids content, the contents of the negative electrode active material, binder, and conductive material were set to 94 wt%, 3 wt%, and 3 wt%, respectively. Then, the above negative electrode slurry was subjected to a solvent at 10 mg / cm³. 2 Up to 14 mg / cm 2 The loading amount is coated on one side of the copper foil (Cu-foil) that serves as the negative electrode current collector, and then dried at 90°C to 120°C, thereby forming a negative electrode mixture layer on one side of the negative electrode current collector.

[0123] Subsequently, as shown in Table 2 below, according to the Examples and Comparative Examples, a rolling process was applied to change the electrode density, thereby manufacturing the negative electrodes for lithium secondary batteries of the Examples and Comparative Examples. The crystal orientation index (OI2) values ​​of the negative electrode mixture layer included in the negative electrodes of the above Examples and Comparative Examples were measured and are shown in Table 2 below. Specifically, XRD analysis was performed on the surface of the above negative electrode mixture layer, and the crystal orientation index (OI) values ​​were measured by the same method as described above.

[0124] 3) Manufacturing of lithium secondary batteries

[0125] A lithium transition metal composite oxide, Li[Ni], is coated onto an aluminum foil (Al-foil) serving as the positive electrode current collector. 0.8 Co 0.1 Mn 0.1 A positive electrode slurry containing O2 is dried to produce a positive electrode for a lithium secondary battery. A polyolefin separator is then placed between the positive and negative electrodes to obtain a secondary battery cell. The obtained secondary battery cell is placed in a pouch for a secondary battery, and an electrolyte is injected into the pouch. The electrolyte is a solution of 1 M LiPF6 dissolved in a solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC). The pouch is then sealed to produce a pouch-type lithium secondary battery. The obtained pouch-type lithium secondary batteries are used as secondary battery samples in the examples and comparative examples.

[0126] 4) Performance Evaluation

[0127] (1) Capacity retention

[0128] For the above-mentioned secondary battery samples, the battery was charged at a rate of 1C for 17 minutes within the range of SOC 2% to 96% and discharged at 1C as one cycle. This cycle was repeated 300 times at 25°C. The discharge capacity retention rate relative to the initial discharge capacity was then measured as a percentage (%), and the results are shown in Table 2 below.

[0129] (2) Electrode expansion rate

[0130] First, for the negative electrodes of the embodiments and comparative examples, the electrode thickness (T1) of the negative electrode before charging (SOC 0%) was measured. Furthermore, after charging the secondary battery samples including the negative electrodes of the embodiments and comparative examples described above (CC-CV; 1.0C, 4.2V, 0.1C cut-off), the secondary batteries were disassembled, and the electrode thickness (T2) of the negative electrode after charging (SOC 100%) was measured. Then, the electrode expansion rate of the charged negative electrode was calculated according to Equation 2 below, and the results are shown in Table 2 below.

[0131] [Equation 2]

[0132] Electrode expansion rate (%) = 100 × (T2 - T1) / (T1)

[0133] In Equation 2, T1 is the electrode thickness at SOC 0%, and T2 is the electrode thickness at SOC 100%.

[0134] [Table 1]

[0135]

[0136] [Table 2]

[0137]

[0138] Referring to Tables 1 and 2 above, it can be confirmed that the increase rate (R) of the crystal orientation index (OI) is... OI When the capacity retention rate is less than 120% (Comparative Examples 1, 2, and 4) or exceeds 400% (Comparative Examples 3 and 5), the capacity retention rate of the lithium secondary battery is relatively low. In particular, it can be confirmed that the capacity retention rate increases with the crystal orientation index (OI) increase rate (R0). OI In Comparative Examples 1 and 2, where the electrode expansion rate was less than 100%, the electrode expansion rate was also relatively high.

[0139] It was determined that this was because, in Comparative Examples 1 and 2, a significant "springback" phenomenon occurred in the negative electrode after rolling, resulting in a decrease in the crystal orientation index increase rate. Furthermore, in Comparative Examples 3 and 5, the carbon-based active material contained in the negative electrode was damaged by extreme stress during the rolling process, causing an excessively large increase rate in the crystal orientation index of the negative electrode, thereby reducing the performance of the lithium secondary battery.

[0140] On the other hand, it can be confirmed that the increase rate (R) of the crystal orientation index (OI) is... OIWhen the crystal orientation index increase rate (R0.05) is 120% to 400% (Examples 1 to 6), the capacity retention rate of the lithium secondary battery is maintained at a relatively high level. In particular, it can be confirmed that the capacity retention rate is maintained at the aforementioned crystal orientation index increase rate (R0.05). OI With electrode expansion rates ranging from 190% to 270% (Examples 1, 2, 5, and 6), a low level of electrode expansion rate was ensured, and the capacity retention rate of the lithium secondary battery was maintained at a very high level.

[0141] Considering the results described above, it can be determined that when the crystal orientation index according to Equation 1 increases by a rate (R) OI When the negative electrode for lithium secondary batteries is manufactured in a manner ranging from 120% to 400%, lithium secondary batteries with excellent performance can be provided.

Claims

1. A negative electrode for a lithium secondary battery, the negative electrode comprising a negative electrode current collector and a negative electrode mixture layer on at least one side of the negative electrode current collector, the negative electrode mixture layer comprising a carbon-based active material, the negative electrode having a crystal orientation index increase rate R according to Formula 1 below. OI For 120% to 400%, [Equation 1]R OI =100×(OI2-OI1) / (OI1) In Equation 1, R OI OI represents the increase rate of the crystal orientation index OI, expressed as a percentage, and OI1 represents the peak intensity I of the (004) plane based on X-ray diffraction (XRD) analysis of the carbon-based active material. 004 Peak intensity I of (110) plane 110 The ratio of I 004 / I 110 OI2 is the peak intensity I of the (004) plane based on X-ray diffraction (XRD) analysis of the negative electrode mixture layer. 004 Peak intensity I of (110) plane 110 The ratio of I 004 / I 110 .

2. The negative electrode for a lithium secondary battery according to claim 1, wherein, The increase rate R of the crystal orientation index of the negative electrode for the lithium secondary battery OI It ranges from 190% to 270%.

3. The negative electrode for a lithium secondary battery according to claim 1, wherein, The negative electrode for the lithium secondary battery has an OI2 value of 7 to 15.

4. The negative electrode for a lithium secondary battery according to claim 1, wherein, The carbon-based active material includes artificial graphite.

5. The negative electrode for a lithium secondary battery according to claim 4, wherein, The artificial graphite has a single-particle form.

6. The negative electrode for a lithium secondary battery according to claim 4, wherein, The surface of the artificial graphite contains a carbon coating.

7. The negative electrode for a lithium secondary battery according to claim 1, wherein, The carbon-based active material includes artificial graphite and natural graphite.

8. The negative electrode for a lithium secondary battery according to claim 7, wherein, The weight of the artificial graphite in the carbon-based active material is greater than or equal to the weight of the natural graphite.

9. The negative electrode for a lithium secondary battery according to claim 7, wherein, The surface of the natural graphite contains a carbon coating.

10. The negative electrode for a lithium secondary battery according to claim 1, wherein, The electrode density of the negative electrode mixture layer is from 1.4 g / cm³ to 1.7 g / cm³.

11. A lithium secondary battery comprising a negative electrode for a lithium secondary battery as described in any one of claims 1 to 10.

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