Negative electrode and lithium secondary battery

Abstract

The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery, the negative electrode comprising: a current collector; a first layer provided on the current collector and including a carbon-based active material; a second layer disposed on the first layer and including a silicon-based active material; and a third layer disposed on the second layer and including aluminum or an aluminum-containing compound, wherein a thickness of the third layer is greater than 0 and 1 / 20 or less with respect to a sum of thicknesses of the first layer and the second layer.

Description

Technical Field

[0001] This application claims priority to and the benefit of Korean Patent Application No. 10-2023-0118377 filed on September 6, 2023, in the Korean Intellectual Property Office, and Korean Patent Application No. 10-2024-0120050 filed on September 4, 2024, in the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a negative electrode and a lithium secondary battery, and a battery module and a battery pack including the same. Background Art

[0003] Recently, with the rapid popularization of electronic devices (such as mobile phones, laptops and electric vehicles) using batteries, the demand for small, lightweight and relatively high-capacity secondary batteries is rapidly increasing. In particular, as a driving power source for portable devices, lithium secondary batteries have attracted much attention due to their light weight and high energy density. Therefore, research and development work to improve the performance of lithium secondary batteries is being actively carried out.

[0004] Generally speaking, a lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, an electrolyte, and the like. Furthermore, the positive and negative electrodes may form active material layers, each containing a positive electrode active material and a negative electrode active material, on a current collector. Generally speaking, lithium-containing metal oxides such as LiCoO2 and LiMn2O4 are used as the positive electrode active material, while lithium-free carbon-based active materials or silicon-based active materials are used as the negative electrode active material.

[0005] Among negative electrode active materials, silicon-based active materials have higher capacities than carbon-based active materials and are attracting attention as high-capacity secondary battery materials. However, silicon-based active materials suffer from poor lifespan and expansion characteristics due to side reactions with the electrolyte. Therefore, there is a need to develop a technology for using silicon-based active materials while minimizing these drawbacks. Summary of the Invention

[0006] [Technical Issues]

[0007] The present invention relates to a negative electrode for a lithium secondary battery and a lithium secondary battery including the negative electrode, which can provide a high-capacity battery by using a silicon-based active material as a negative electrode active material while solving the aforementioned disadvantages of the silicon-based active material.

[0008] [Technical solution]

[0009] An exemplary embodiment of the present invention provides a negative electrode for a lithium secondary battery, the negative electrode for a lithium secondary battery comprising: a current collector; a first layer comprising a carbon-based active material disposed on the current collector; a second layer comprising a silicon-based active material disposed on the first layer; and a third layer comprising aluminum or an aluminum-containing compound disposed on the second layer, wherein the thickness of the third layer is greater than 0 and less than 1 / 20 of the sum of the thicknesses of the first layer and the second layer.

[0010] An exemplary embodiment of the present invention provides a lithium secondary battery including the negative electrode for the lithium secondary battery according to the exemplary embodiment, a separator, and a positive electrode, wherein the third layer of the negative electrode is disposed to face the separator.

[0011] An exemplary embodiment of the present invention provides a battery module including the lithium secondary battery according to the above exemplary embodiment.

[0012] An exemplary embodiment of the present invention provides a battery pack including the lithium secondary battery according to the above exemplary embodiment.

[0013] An exemplary embodiment of the present invention provides a battery pack including the battery module according to the above exemplary embodiment.

[0014] [Beneficial Effects]

[0015] A negative electrode for a lithium secondary battery according to an exemplary embodiment of the present invention includes a silicon-based active material. In this case, the silicon-based active material is included in the upper layer of a double-layer active material structure, and a layer containing aluminum or an aluminum-containing compound is also arranged on the surface facing the separator, thereby significantly improving cycle, life, and expansion characteristics. Specifically, the inorganic layer containing relatively hard aluminum can suppress volume changes in the active material layer containing the highly expanding silicon-based active material, thereby improving cycle, life, and expansion. DETAILED DESCRIPTION

[0016] Hereinafter, this specification will be described in more detail.

[0017] In the present specification, when it is mentioned that a part “comprises” a certain constituent element, unless otherwise specifically stated, it means that the part may further comprise other constituent elements, and does not exclude other constituent elements.

[0018] Throughout this specification, when a component is referred to as being "on" another component, the component may be in direct contact with the other component, or intervening components may also be present.

[0019] It should be understood that the terms or words used throughout the specification should not be interpreted as limited to their commonly used meanings or meanings in dictionaries, but should be interpreted as having meanings and concepts consistent with the technical spirit of the present invention based on the principle that the inventor can appropriately define the concepts of the words or terms to best explain the present invention.

[0020] As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0021] In this specification, a description simply referred to as a “negative electrode active material layer” without expressions such as first, second may apply to both the first layer and the second layer.

[0022] Hereinafter, preferred exemplary embodiments of the present invention will be described in detail. However, it should be understood that the exemplary embodiments of the present invention can be modified in various forms, and the scope of the present invention is not limited to the exemplary embodiments described below.

[0023] <Negative electrode>

[0024] According to an exemplary embodiment of the present invention, a negative electrode for a lithium secondary battery includes: a current collector; a first layer disposed on the current collector and containing a carbon-based active material; and a second layer disposed on the first layer and containing a silicon-based active material. The negative electrode for a lithium secondary battery further includes a third layer disposed on the second layer and containing aluminum or an aluminum-containing compound, wherein the thickness of the third layer is greater than 0 and less than 1 / 20 of the sum of the thicknesses of the first and second layers.

[0025] According to the exemplary embodiment described above, the stacked structure of the first and second layers advantageously suppresses separation between the current collector and the active material due to the silicon-based active material exhibiting significant volume changes. By additionally stacking the third layer on the surface of the stacked structure containing the active material, the relatively hard aluminum-containing inorganic layer suppresses volume changes in the active material layer containing the highly expanding silicon-based active material, thereby improving battery life and expansion characteristics.

[0026] According to one exemplary embodiment, the first layer includes a carbon-based active material as the active material, and does not include a silicon-based active material. According to one example, the carbon-based active material may be graphite, and the graphite may be natural graphite, artificial graphite, or a mixture thereof. The carbon-based active material content of the first layer may be 80 to 100 parts by weight, for example, 90 to 100 parts by weight, or 95 to 100 parts by weight, based on 100 parts by weight of the active material in the first layer.

[0027] According to an exemplary embodiment, the silicon-based active material of the second layer includes at least one of silicon (Si), silicon oxide, metal-doped silicon oxide, silicon-carbon composite, and silicon-metal alloy. Based on 100 parts by weight of the active material in the second layer, the content of the silicon-based active material of the second layer may be 0.1 to 40 parts by weight, for example, 1 to 20 parts by weight or 5 to 12 parts by weight.

[0028] Based on 100 parts by weight of the active material in the second layer, when the content of the silicon-based active material of the second layer falls within the above range, the capacity of the negative electrode can be improved, and the electrode can be made thinner, resulting in improved battery life and swelling characteristics. Specifically, when the content of the silicon-based active material of the second layer is 0.1 part by weight or more, it is beneficial to improve the capacity of the negative electrode, and for achieving excellent capacity, the negative electrode can be formed relatively thinner, resulting in improved life and swelling characteristics. In addition, when the content of the silicon-based active material of the second layer is 40 parts by weight or less, the Si content is not too high and is within a suitable range, such that the swelling characteristics of Si are improved, resulting in improved life and swelling characteristics.

[0029] As needed, the second layer may further include a carbon-based active material. The carbon-based active material of the second layer may be natural graphite, artificial graphite, or a mixture thereof. Based on 100 parts by weight of the active material in the second layer, the content of the carbon-based active material of the second layer may be 60 to 99.9 parts by weight.

[0030] The silicon-based active material may be an active material containing SiO x (0 ≤ x < 2), and the active material may be silicon-based composite particles containing SiO x (0 < x < 2) and pores.

[0031] The active material containing SiO x (0 ≤ x < 2) may be silicon oxide particles containing SiO x (0 < x < 2) and pores.

[0032] The SiO x (0 < x < 2) corresponds to the matrix in the silicon oxide particles. The SiO x (0 < x < 2) may be in the form of containing Si and SiO2, and the Si may form a phase. That is, x corresponds to the number ratio of O to Si contained in the SiO x (0 < x < 2). When the silicon oxide particles contain SiO x (0 < x < 2), the discharge capacity of the secondary battery can be improved.

[0033] The silicon oxide particles may further contain at least one of a Mg compound and a Li compound. The Mg compound and the Li compound may correspond to dopants in the silicon oxide particles.

[0034] The Mg compound and / or the Li compound may be present inside the SiO x (0 < x < 2) and / or on the surface of the SiO x (0 < x < 2). The initial efficiency of the battery can be improved by the Mg compound and / or the Li compound.

[0035] The Mg compound may include at least one selected from the group consisting of Mg silicate, Mg silicide, and Mg oxide. The Mg silicate may include at least one of Mg2SiO4 and MgSiO3. The Mg silicide may include Mg2Si. The Mg oxide may include MgO.

[0036] In an exemplary embodiment of the present specification, based on 100% by weight of the total silicon oxide particles, the content of Mg element may be 0.1 to 20% by weight or 0.1 to 15% by weight. Specifically, the content of Mg element may be 0.5 to 10% by weight or 0.8 to 10% by weight. When the above range is satisfied, an appropriate content of Mg compound may be included in the silicon oxide particles, so that the volume change of the silicon oxide particles during battery charge and discharge can be easily suppressed, and the discharge capacity and initial efficiency of the battery can be improved.

[0037] The Li compound may include at least one selected from the group consisting of Li silicate, Li silicide, and Li oxide. The Li silicate may include at least one of Li2SiO3, Li4SiO4, and Li2Si2O5. The Li silicide may include Li7Si2. The Li oxide may include Li2O.

[0038] In an exemplary embodiment of the present invention, the Li compound may include a form of lithium silicate. The lithium silicate is represented by Li a Si b O c (2 ≤ a ≤ 4, 0 < b ≤ 2, 2 ≤ c ≤ 5), and can be divided into crystalline lithium silicate and amorphous lithium silicate. The crystalline lithium silicate may exist in the silicon oxide particles in the form of at least one lithium silicate selected from the group consisting of Li2SiO3, Li4SiO4, and Li2Si2O5, and the amorphous lithium silicate may exist in the form of Li a Si b O c (2 ≤ a ≤ 4, 0 < b ≤ 2, 2 ≤ c ≤ 5). However, it is not intended to be limited thereto.

[0039] In an exemplary embodiment of the present specification, the content of Li element may be 0.1 to 20 wt% or 0.1 to 15 wt% based on 100 wt% of the total silicon oxide particles. Specifically, the content of Li element may be 0.5 to 10 wt%, and more specifically 0.5 to 10 wt%. When the above range is met, the silicon oxide particles can contain an appropriate content of Li compound, thereby easily suppressing the volume change of the negative electrode active material during battery charge and discharge, and improving the discharge capacity and initial efficiency of the battery.

[0040] The content of Mg element or Li element can be confirmed by ICP analysis. For ICP analysis, a predetermined amount (about 0.01 g) of negative electrode active material is accurately weighed, transferred to a platinum crucible, and completely decomposed on a hot plate by adding nitric acid, hydrofluoric acid and sulfuric acid thereto. Then, using an inductively coupled plasma atomic emission spectrometer (ICP-AES, Perkin-Elmer 7300), the intensity of a standard solution prepared using a standard solution (5 mg / kg) is measured at the inherent wavelength of the Mg element or Li element to obtain a reference calibration curve. Subsequently, the pretreated sample solution and the blank sample are introduced into the spectrometer, and the actual intensity is calculated by measuring the intensity of each, and the concentration of each component is calculated based on the obtained calibration curve. Then, the concentration of each component is converted so that the sum of the calculated concentrations of each component is equal to the theoretical value, and the content of the Mg element or Li element in the prepared silicon oxide particles can be analyzed.

[0041] In an exemplary embodiment of the present invention, the silicon oxide particles may contain additional metal atoms. The metal atoms may be present in the silicon oxide particles in the form of at least one of metal atoms, metal silicates, metal silicides, and metal oxides. The metal atoms may include at least one selected from the group consisting of Mg, Li, Al, and Ca. This may improve the initial efficiency of the negative electrode active material.

[0042] In one exemplary embodiment of the present invention, a carbon layer is provided on at least a portion of the surface of the silicon oxide particles. In this case, the carbon layer may be coated on at least a portion of the surface, i.e., may be coated on a portion of the particle surface, or may be coated on the entire surface of the particle. The carbon layer imparts conductivity to the negative electrode active material, thereby improving the initial efficiency, lifespan, and battery capacity characteristics of the secondary battery.

[0043] In an exemplary embodiment of the present invention, the carbon layer includes amorphous carbon. Alternatively, the carbon layer may include crystalline carbon.

[0044] The crystalline carbon may further improve the conductivity of the negative electrode active material. The crystalline carbon may include at least one selected from the group consisting of fullerene, carbon nanotube, and graphene.

[0045] The amorphous carbon can appropriately maintain the strength of the carbon layer, thereby suppressing the expansion of the silicon-based particles. The amorphous carbon can be a carbide of at least one substance selected from the group consisting of tar, pitch, and other organic materials, or can be a carbon-based material formed using hydrocarbons as a chemical vapor deposition source.

[0046] The carbide of the other organic material may be carbide of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose, ketohexose, or a carbide of an organic material selected from a combination thereof.

[0047] The hydrocarbon may be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or a substituted or unsubstituted aromatic hydrocarbon. The aliphatic or alicyclic hydrocarbon in the substituted or unsubstituted aliphatic or alicyclic hydrocarbon may be methane, ethane, ethylene, acetylene, propane, butane, butene, pentane, isobutane, hexane, etc. The aromatic hydrocarbon in the substituted or unsubstituted aromatic hydrocarbon may be benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, benzofuran, pyridine, anthracene, phenanthrene, etc.

[0048] In an exemplary embodiment of the present invention, the carbon layer may be an amorphous carbon layer.

[0049] In an exemplary embodiment of the present invention, the carbon layer may contain 0.1 to 50 parts by weight, 0.1 to 30 parts by weight, or 0.1 to 20 parts by weight, based on 100 parts by weight of the total silicon-based particles. More specifically, the carbon layer may contain 0.5 to 15 parts by weight, 1 to 10 parts by weight, or 1 to 5 parts by weight. When these ranges are met, the capacity and efficiency of the negative electrode active material can be prevented from decreasing.

[0050] In an exemplary embodiment of the present invention, the thickness of the carbon layer may be 1 to 500 nm, and specifically 5 to 300 nm. When the above range is met, the conductivity of the negative electrode active material is improved, the volume change of the negative electrode active material is easily suppressed, and the side reaction between the electrolyte and the negative electrode active material is suppressed, resulting in improved initial efficiency and / or life of the battery.

[0051] Specifically, the carbon layer may be formed by chemical vapor deposition (CVD) using at least one hydrocarbon gas selected from the group consisting of methane, ethane, and acetylene.

[0052] In the present invention, the crystallinity of the carbon layer can be confirmed by calculating the D / G band ratio using Raman spectroscopy. Specifically, measurements can be performed using a Renishaw 2000 Raman microscope system and 532 nm laser excitation. To avoid laser thermal effects, a 100× optical lens is used at low laser power density and a 30-second exposure time. To reduce positional deviations, a total of 25 points are measured in a 5 μm × 5 μm area and fitted using a Lorentzian function. The average values of the D and G bands are then calculated.

[0053] The silicon-based active material may be a silicon-carbon composite, which may be a Si / C-based active material.

[0054] In this specification, the silicon-carbon composite is a composite of Si and C, and is distinguished from silicon carbide expressed as SiC.

[0055] The silicon-carbon composite may be a composite of silicon, graphite, etc., and may have a structure in which a core formed by combining silicon and graphite, etc., is surrounded by graphene, amorphous carbon, etc. In the silicon-carbon composite, silicon may be nanosilicon.

[0056] According to an exemplary embodiment, the silicon-carbon composite includes porous carbon-based particles and silicon present on the surface or in internal pores of the porous carbon-based particles.

[0057] According to an exemplary embodiment, the pore volume of the silicon-carbon composite measured by the BET method may be 0.005 to 0.03 cm 3 / g, and the pore size measured by the BET method may be 10 to 20 nm. The pore volume of the silicon-carbon composite measured by mercury penetration method (Hg porosimeter) may be 0.005 to 0.03 cm 3 / g.

[0058] According to one exemplary embodiment, the silicon-carbon composite may be manufactured by a method including forming silicon on the surface and in the internal pores of porous carbon-based particles.

[0059] The porous carbon-based particles can be produced using methods known in the art. For example, they can be obtained by carbonizing an organic material (e.g., a petroleum-based material or a polymer) or by chemically treating a naturally occurring material (e.g., palm bark) and then carbonizing it. As another example, the porous carbon-based particles can be obtained by a method comprising etching a carbon-based particle containing internal pores to enlarge the internal pores of the carbon-based particle.

[0060] The step of expanding the internal pores of the carbon-based particles may be performed in a nitrogen (N2) atmosphere, an oxygen (O2) atmosphere, or an air atmosphere. Specifically, the flow rate of oxygen (O2) or oxygen-containing air may be controlled to be 0.1 to 10 L / min.

[0061] The step of expanding the internal pores of the carbon-based particles may be performed at a temperature ranging from 400° C. to 1200° C. for 30 minutes to 4 hours.

[0062] The pore characteristics of the obtained porous carbon-based particles may vary depending on the conditions for enlarging the internal pores of the carbon-based particles.

[0063] The step of forming silicon can be performed using chemical vapor deposition. In this case, silicon nanoparticles are deposited on the surface of the carbon-based particles having enlarged internal pores and / or in the internal pores, thereby forming silicon in the form of a film, an island, or a combination thereof.

[0064] The silicon nanoparticles may be crystalline, quasi-crystalline, amorphous, or a combination thereof.

[0065] The silicon-based active material may be a silicon-metal alloy. For example, the silicon-metal alloy (Si alloy) may be an alloy of Si with one or more metals selected from the group consisting of Zn, Al, Mn, Ti, Fe, and Sn, and may include, but is not limited to, solid solutions, intermetallic compounds, and eutectic alloys of Si and these metals.

[0066] In this specification, the average particle size (D50) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. The average particle size (D50) can be measured using, for example, laser diffraction. Laser diffraction methods can generally measure particle sizes ranging from submicron to several millimeters, and can produce results with high reproducibility and high resolution.

[0067] The average particle size (D50) of the silicon-based active material may be 1 to 10 μm. When this range is met, the structural stability of the active material during charge and discharge can be ensured, and the problem of increased volume expansion / contraction as the particle size becomes too large can be prevented, as well as the problem of reduced initial efficiency when the particle size is too small can be prevented.

[0068] The first layer and the second layer may each additionally contain a binder and a conductive material, and may further contain a thickener and / or a conductive material as needed.

[0069] The first layer and the second layer can be formed by applying a negative electrode slurry containing the aforementioned active material, a binder, a thickener, and / or a conductive material to at least one surface of a current collector, followed by drying and roll-pressing. The first layer and the second layer can be formed sequentially on the current collector. The thickness of the first layer and the second layer can be the same, or the ratio of the thickness of the second layer to the thickness of the first layer can be 0.7 to 1.3.

[0070] The negative electrode slurry may further include additional negative electrode active material.

[0071] As the additional negative electrode active material, a compound capable of reversibly intercalating and deintercalating lithium can be used. Specific examples include: carbonaceous materials, such as artificial graphite, natural graphite, graphitized carbon fibers, and amorphous carbon; metal substances capable of alloying with lithium, such as Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys, or Al alloys; metal oxides capable of doping and dedoping lithium, such as SiO β (0<β<2), SnO2, vanadium oxide, lithium titanium oxide and lithium vanadium oxide; or a composite comprising a metal substance and a carbonaceous material, such as a Si-C composite or a Sn-C composite, any one of which or a mixture of two or more thereof can be used. In addition, a metallic lithium thin film can be used as the negative electrode active material. In addition, both low-crystalline carbon and high-crystalline carbon can be used as the carbon material. Representative examples of the low-crystalline carbon include soft carbon and hard carbon, and representative examples of the high-crystalline carbon include amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, floating graphite, pyrolytic carbon, mesophase pitch-based carbon fibers, mesocarbon microbeads, mesophase pitch, high-temperature fired carbon such as coke derived from petroleum or coal tar pitch.

[0072] According to an exemplary embodiment of the present invention, the negative electrode slurry may have a pH of 7 to 11 at 25°C. When the pH of the negative electrode slurry is within the above range, the rheological properties of the slurry are stabilized. On the other hand, when the pH of the negative electrode slurry is less than 7 or exceeds 11, carboxymethyl cellulose (CMC) used as a thickener decomposes, resulting in a decrease in the viscosity of the slurry and a decrease in the dispersion of the active material contained in the slurry.

[0073] The negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., can be used for the current collector. Specifically, transition metals that adsorb carbon well (such as copper and nickel) can be used for the current collector. The thickness of the current collector may be 6 to 20 μm. However, the thickness of the current collector is not limited thereto.

[0074] The adhesive may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, polyacrylic acid, and the above materials in which hydrogen is replaced by Li, Na, Ca, etc., and may also include various copolymers thereof.

[0075] The conductive material is not particularly limited as long as it has conductivity without causing chemical changes in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbons, aluminum powders, and nickel powders; conductive whiskers such as zinc oxide, potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives, etc.

[0076] The thickener may be carboxymethyl cellulose (CMC), but is not limited thereto, and thickeners used in the art may be appropriately employed.

[0077] In an exemplary embodiment of the present invention, the content of all negative electrode active materials contained in the negative electrode slurry may be 60 to 99 parts by weight, and specifically 70 to 98 parts by weight, based on 100 parts by weight of the total solid content of the negative electrode slurry.

[0078] In an exemplary embodiment of the present invention, the binder may be included in an amount of 0.5 to 30 parts by weight, and specifically 1 to 20 parts by weight, based on 100 parts by weight of the total solid content of the negative electrode slurry.

[0079] In an exemplary embodiment of the present invention, the conductive material may be included in an amount of 0.01 to 25 parts by weight, and specifically 0.05 to 20 parts by weight, based on 100 parts by weight of the total solid content of the negative electrode slurry.

[0080] In an exemplary embodiment of the present invention, the thickener may be included in an amount of 0.5 to 25 parts by weight, specifically 0.5 to 20 parts by weight, and more specifically 1 to 20 parts by weight, based on 100 parts by weight of the total solid content of the negative electrode slurry.

[0081] The negative electrode slurry according to an exemplary embodiment of the present invention may further include a solvent for forming the negative electrode slurry. Specifically, from the perspective of facilitating the dispersion of components, the solvent for forming the negative electrode slurry may include at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropyl alcohol, and may specifically include distilled water.

[0082] In an exemplary embodiment of the present invention, the solid content of the negative electrode slurry may be 20 to 75 parts by weight, and specifically 30 to 70 parts by weight, based on 100 parts by weight of the total negative electrode slurry.

[0083] In the present invention, the third layer contains aluminum or an aluminum-containing compound, and its thickness is greater than zero and less than 1 / 20 of the sum of the thicknesses of the first and second layers. The thickness of the third layer refers to the thickness of the third layer in the portion where the third layer is present. In other words, the third layer may completely cover one surface of the second layer, or may only cover a portion of one surface of the second layer. In this case, the thickness of the third layer refers to the thickness of the region where the third layer covers the second layer. The thickness and thickness ratio of each layer can be obtained by measuring the thickness of an electrode cross-section using SEM. The thickness can be measured at any of the following points: before immersion in the electrolyte, after immersion, or after battery cycling. The effects of the present invention are achieved when the thickness ratio calculated from the thickness measured at any point in time falls within the range of the present invention. In the case of the above materials, the third layer containing relatively hard aluminum has the advantage of improving cycle life and expansion characteristics by suppressing volume change in the active material layer containing a highly expanding silicon-based active material. Furthermore, when the thickness falls within the above range, it has the advantage of suppressing volume expansion without reducing negative electrode conductivity.

[0084] According to an exemplary embodiment, the third layer is configured to partially or completely cover the second layer. When the third layer partially covers the second layer, it may cover more than 30%, more than 50%, more than 70%, or more than 90% of the second layer.

[0085] According to an exemplary embodiment, the third layer may further include a binder in addition to aluminum or an aluminum-containing compound. In this case, the binder may be present in an amount of 1 to 20 parts by weight, for example, 5 to 15 parts by weight, per 100 parts by weight of the third layer.

[0086] According to an exemplary embodiment, the third layer can be manufactured by the following method. First, a dispersion is prepared by mixing and dispersing at least one of aluminum and an aluminum-containing compound with a solvent and a polymer binder. Then, the dispersion is applied and coated on the second layer of the stacked structure of the first and second layers. Here, the stacked structure of the first and second layers can be in a state where calendering has been completed. A drying process can be additionally performed as needed. The dispersion of aluminum or an aluminum-containing compound according to an exemplary embodiment of the present invention may further include a solvent for dispersion formation. Specifically, from the aspect of being conducive to the dispersion of the components, the solvent for dispersion formation may include at least one selected from the group consisting of distilled water, ethanol, methanol and isopropanol, and may specifically include distilled water.

[0087] The polymer binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, polyacrylic acid, and the above materials in which hydrogen is replaced by Li, Na, Ca, etc., and may also include various copolymers thereof.

[0088] According to an exemplary embodiment, the aluminum-containing compound of the third layer includes at least one of aluminum oxide and Al(OH) 3 .

[0089] According to an exemplary embodiment, the content of the aluminum or the aluminum-containing compound is 90 to 100 parts by weight, for example, 90 to 95 parts by weight, based on 100 parts by weight of the third layer composition.

[0090] When the above range is met, the battery life and expansion characteristics can be improved. Specifically, when the content of aluminum or an aluminum-containing compound in the third layer falls within the above range, the content of the polymer binder in the third layer composition does not increase to more than 10 parts by weight. This prevents the polymer binder from acting as a resistance layer and thereby reducing conductivity, thereby improving battery life characteristics.

[0091] <Secondary Battery>

[0092] An exemplary embodiment of the present invention provides a lithium secondary battery comprising the negative electrode according to the exemplary embodiment described above; a separator; and a positive electrode, wherein the inorganic layer of the negative electrode is disposed facing the separator. In this case, the third layer of the negative electrode is disposed facing the separator. When the third layer contacts the separator, the bonding strength with the separator is enhanced, thereby further facilitating battery manufacturing.

[0093] According to an exemplary embodiment of the present application, the positive electrode may include a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector and including the positive electrode active material.

[0094] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity and does not cause chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, fired carbon, aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. can be used. In addition, the positive electrode current collector can generally have a thickness of 3 to 500 μm, and the surface of the current collector can be formed with fine concave and convex to enhance 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, nets, porous bodies, foams, and non-woven fabrics.

[0095] The positive electrode active material layer may further include a positive electrode conductive material and a positive electrode binder in addition to the positive electrode active material.

[0096] In this case, the positive electrode conductive material is used to impart conductivity to the electrode and can be used without particular limitation, as long as the positive electrode conductive material has electronic conductivity and does not cause chemical changes in the battery to be constructed. Specific examples may include: graphite, 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 black, and carbon fiber; 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; or conductive polymers, such as polyphenylene derivatives, and the like, and any one of these or a mixture of two or more thereof may be used.

[0097] In addition, the role of the positive electrode binder is to improve the bonding between the 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), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one thereof or a mixture of two or more thereof may be used.

[0098] The content of the positive electrode binder may be 0.1 parts by weight or more and 50 parts by weight or less, for example, preferably 0.3 parts by weight or more and 35 parts by weight or less, and more preferably 0.5 parts by weight or more and 20 parts by weight or less, based on 100 parts by weight of the positive electrode active material layer.

[0099] The conductive material contained in the positive electrode active material layer is used to impart conductivity to the electrode and can be used without particular limitation, as long as the conductive material has electronic conductivity and does not cause chemical changes in the battery. Specific examples include: graphite, such as natural graphite and artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; 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; or conductive polymers, such as polyphenylene derivatives, and the like. Any one of these or a mixture of two or more thereof can be used.

[0100] Specifically, in one exemplary embodiment, the conductive material may include one or more of single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs). The content of the conductive material may be 0.1 parts by weight or more and 2 parts by weight or less, for example, preferably 0.3 parts by weight or more and 1.5 parts by weight or less, and more preferably 0.5 parts by weight or more and 1.2 parts by weight or less, based on 100 parts by weight of the positive electrode active material layer composition.

[0101] The secondary battery according to an exemplary embodiment of the present invention may include the negative electrode, the positive electrode, and a separator interposed between the positive electrode and the negative electrode, and may further include an electrolyte.

[0102] The diaphragm serves to separate the negative electrode from the positive electrode and to provide a path for the movement of lithium ions, wherein any diaphragm can be used as the diaphragm without particular limitation, as long as it is commonly used in secondary batteries, and in particular, it is preferred to use a diaphragm having high moisture retention and low resistance to the movement of electrolyte ions. Specifically, a porous polymer film can be used, for example, a porous polymer film manufactured by a polyolefin-based polymer (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer), or a stacked structure of more than two layers thereof. In addition, a common porous non-woven fabric can be used, for example, a non-woven fabric formed by high melting point glass fiber, polyethylene terephthalate fiber, etc. In addition, a coated diaphragm comprising a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and a diaphragm with a single layer or multilayer structure can be selectively used.

[0103] According to an exemplary embodiment of the present invention, the diaphragm may be a safety reinforced diaphragm (SRS).

[0104] Examples of the electrolyte may include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, molten inorganic electrolytes, etc., which may be used to manufacture lithium secondary batteries, but are not limited thereto.

[0105] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0106] As the non-aqueous organic solvent, for example, an aprotic organic solvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate or ethyl propionate can be used.

[0107] Especially, in carbonate-based organic solvents, ethylene carbonate and propylene carbonate as cyclic carbonate are high viscosity organic solvents, and because dielectric constant is high so as to dissociate lithium salt well, therefore can preferably be used.When described cyclic carbonate is mixed with low viscosity, the linear carbonate of low dielectric constant (for example dimethyl carbonate or diethyl carbonate) when using in appropriate ratio, can prepare the electrolyte with high conductivity, so such combination use can be more preferred.

[0108] As the metal salt, a lithium salt can be used. The lithium salt is a material that is easily soluble in the non-aqueous electrolyte. For example, a lithium salt selected from the group consisting of F - 、Cl - , I - 、NO3 - 、N(CN)2 - 、BF4 - 、ClO4 - PF6 - 、(CF3)2PF4 - 、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - CF3SO3 - CF3CF2SO3 - 、(CF3SO2)2N - 、(FSO2)2N - CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - 、CH3CO2 - 、SCN - and (CF3CF2SO2)2N - One or more kinds of the group consisting of are used as anions of the lithium salt.

[0109] In order to improve the life characteristics of the battery, inhibit the decline in battery capacity and improve the discharge capacity of the battery, in addition to containing the above-mentioned electrolyte components, the electrolyte may also contain one or more additives, for example: halogenated alkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme dimethyl ether, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol or aluminum chloride.

[0110] According to another exemplary embodiment of the present invention, there are provided a battery module including the lithium secondary battery as a unit battery, and a battery pack including the same.

[0111] According to another exemplary embodiment of the present invention, there is provided a battery pack including the lithium secondary battery.

[0112] According to another exemplary embodiment of the present invention, there is provided a battery pack including the battery module.

[0113] Since the battery module and the battery pack include the lithium secondary battery having high capacity, high rate capability, and high cycle characteristics, the battery module and the battery pack can be used as a power source for medium and large-sized equipment selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid vehicles, and power storage systems.

[0114] Invention Mode

[0115] <Examples and Comparative Examples>

[0116] Example 1

[0117] Prepare a first layer forming composition, based on 100 parts by weight of the first layer forming composition, comprising 96.3 parts by weight of graphite, 2.55 parts by weight of SBR (styrene-butadiene rubber) as a binder, 1 part by weight of CMC (carboxymethyl cellulose), and further comprising a CNT pre-dispersion comprising 0.1 parts by weight of a dispersant and 0.05 parts by weight of single-walled CNTs. In this case, the graphite is prepared by a jet milling method to a size of D50 = 17 μm. The first layer forming composition is applied to a copper foil having a thickness of 15 μm so that the thickness is 50 μm in a dry state.

[0118] Prepare a second layer-forming composition, based on 100 parts by weight of the second layer-forming composition, comprising graphite (90 parts by weight based on 100 parts by weight of the negative electrode active material) and a silicon oxide active material coated with carbon by methane CVD (10 parts by weight based on 100 parts by weight of the negative electrode active material) 97.7 parts by weight, 1.15 parts by weight of SBR (styrene-butadiene rubber) as a binder, 1 part by weight of CMC (carboxymethyl cellulose), and further comprising a CNT pre-dispersion comprising 0.1 parts by weight of a dispersant and 0.05 parts by weight of single-walled CNTs. In this case, the carbon-coated silicon oxide active material is prepared to a size of D50 = 6 μm and the graphite is prepared to a size of D50 = 17 μm by a jet milling method. The second layer-forming composition is applied to the first layer so that the thickness in the dry state is 100 μm.

[0119] The negative electrode coated with the first and second layers was rolled into an electrode with a thickness of 60 μm. Next, alumina (Al2O3) as an inorganic material, SBR as a binder, and CMC were mixed in a ratio of 90:6:4 and dissolved in water. The inorganic particles were then ball-milled and dispersed for more than 12 hours to prepare an aluminum compound dispersion. This dispersion was applied to the second layer of the negative electrode coated with the first and second layers, resulting in a negative electrode with a combined thickness of 62 μm for the first to third layers in the dry state.

[0120] Example 2

[0121] A negative electrode was prepared in the same manner as in Example 1, except that a Mg-doped carbon-coated silicon oxide active material was used instead of the carbon-coated silicon oxide active material.

[0122] Example 3

[0123] A negative electrode was prepared in the same manner as in Example 1, except that a Li-doped carbon-coated silicon oxide active material was used instead of the carbon-coated silicon oxide active material.

[0124] Example 4

[0125] A negative electrode was prepared in the same manner as in Example 1, except that a silicon-carbon composite (Si / C) was used instead of the carbon-coated silicon oxide active material.

[0126] Example 5

[0127] A negative electrode was prepared in the same manner as in Example 1, except that a Si alloy (Si6Cu3Sn1) was used instead of the carbon-coated silicon oxide active material.

[0128] Example 6

[0129] A negative electrode was prepared in the same manner as in Example 1, except that the sum of the thicknesses of the first to third layers after applying the third layer was 61 μm.

[0130] Example 7

[0131] The negative electrode was prepared in the same manner as in Example 1, except that 100 parts by weight of the negative electrode active material based on the second layer included graphite (50 parts by weight based on 100 parts by weight of the negative electrode active material) and a silicon oxide active material coated with carbon by methane CVD (50 parts by weight based on 100 parts by weight of the negative electrode active material).

[0132] Example 8

[0133] A negative electrode was prepared in the same manner as in Example 1, except that the ratio of alumina (Al2O3) as an inorganic material, SBR as a binder, and CMC was 50:30:20 based on 100 parts by weight of the third layer composition.

[0134] Comparative Example 1

[0135] A negative electrode was prepared in the same manner as in Example 1, except that the third layer was not applied.

[0136] Comparative Example 2

[0137] A negative electrode was prepared in the same manner as in Example 2, except that the third layer was not applied.

[0138] Comparative Example 3

[0139] A negative electrode was prepared in the same manner as in Example 3, except that the third layer was not applied.

[0140] Comparative Example 4

[0141] A negative electrode was prepared in the same manner as in Example 4, except that the third layer was not applied.

[0142] Comparative Example 5

[0143] A negative electrode was prepared in the same manner as in Example 5, except that the third layer was not applied.

[0144] Comparative Example 6

[0145] A negative electrode was prepared in the same manner as in Example 1, except that the sum of the thicknesses of the first to third layers after applying the third layer was 65 μm.

[0146] Comparative Example 7

[0147] A negative electrode was prepared in the same manner as in Example 2, except that the sum of the thicknesses of the first to third layers after applying the third layer was 65 μm.

[0148] Comparative Example 8

[0149] A negative electrode was prepared in the same manner as in Example 3, except that the sum of the thicknesses of the first to third layers after applying the third layer was 65 μm.

[0150] Comparative Example 9

[0151] A negative electrode was prepared in the same manner as in Example 4, except that the sum of the thicknesses of the first to third layers after applying the third layer was 65 μm.

[0152] Comparative Example 10

[0153] A negative electrode was prepared in the same manner as in Example 5, except that the sum of the thicknesses of the first to third layers after applying the third layer was 65 μm.

[0154] Comparative Example 11

[0155] A negative electrode was prepared in the same manner as in Example 1, except that the sum of the thicknesses of the first to third layers after applying the third layer was 64.6 μm.

[0156] Experimental Example 1

[0157] The negative electrode was dried in a vacuum oven at 130° C. for 12 hours.

[0158] The prepared negative electrode was used and cut into 1.7671 cm 2 A circular lithium (Li) metal film was used as the positive electrode. A porous polyethylene separator was interposed between the positive and negative electrodes, and an electrolyte solution was injected, wherein 0.5 parts by weight of vinylene carbonate was dissolved in a mixed solution in which ethyl methyl carbonate (EMC) and ethylene carbonate (EC) were mixed at a volume ratio of 7:3, and LiPF6 was dissolved to a concentration of 1M, thereby preparing a lithium coin half-cell.

[0159] The prepared battery was charged and discharged to evaluate the capacity retention rate and the electrode expansion rate, and the results are shown in Table 1 below.

[0160] For the 1st and 2nd cycles, charge and discharge were performed at 0.1 C, and from the 3rd to 49th cycles, charge and discharge were performed at 0.5 C. At the 50th cycle, charge and discharge were terminated in a charged state (lithium was contained in the negative electrode).

[0161] Charging conditions: CC (constant current) / CV (constant voltage) (5mV / 0.005C current cutoff)

[0162] Discharge conditions: CC (constant current) condition 1.5V

[0163] Based on the results of one charge / discharge cycle, the capacity retention ratio was calculated as follows.

[0164] Capacity retention rate (%) = (49th discharge capacity / 1st discharge capacity) × 100%

[0165] The electrode expansion ratio is calculated as follows.

[0166] Electrode expansion rate (%) = (electrode thickness after 50 cycles / initial electrode thickness) × 100%

[0167]

[0168] As shown in Table 1 above, it can be confirmed that the batteries prepared in Examples 1 to 6, in which the thickness of the third layer is greater than 0 and less than 1 / 20 of the sum of the thicknesses of the first and second layers, have significantly higher capacity retention and significantly lower electrode expansion than Comparative Examples 1 to 5 in which no third layer is formed. In addition, in Comparative Examples 6 to 11 in which the third layer is formed thicker than in the above-described examples, the electrode expansion rate is reduced to some extent; however, despite the increase in the thickness of the third layer, the electrode expansion rate is relatively greater than in the examples in which the third layer is formed thinner. In addition, Comparative Examples 6 to 11 exhibit poor capacity retention due to the thick thickness of the third layer.

[0169] The battery prepared in Example 7, in which the content of silicon-based active material in the second layer is 50 parts by weight based on 100 parts by weight of active material in the second layer, has a higher capacity retention rate than that of Comparative Example 1, but has a lower capacity retention rate than that of Example 1, in which the content of silicon-based active material in the second layer is 0.4 to 40 parts by weight based on 100 parts by weight of active material in the second layer.

[0170] In addition, the battery prepared in Example 8, in which the content of the aluminum-containing compound is 50 parts by weight based on 100 parts by weight of the third layer composition, has a higher capacity retention rate than that of Comparative Example 1, but has a lower capacity retention rate than that of Example 1, in which the content of the aluminum-containing compound is 90 parts by weight based on 100 parts by weight of the third layer composition.

Claims

1. A negative electrode for a lithium secondary battery, comprising: Current collector; a first layer comprising a carbon-based active material disposed on the current collector; a second layer disposed on the first layer and comprising a silicon-based active material; and a third layer disposed on the second layer and comprising aluminum or an aluminum-containing compound, The thickness of the third layer is greater than 0 and less than 1 / 20 of the sum of the thicknesses of the first layer and the second layer. 2 . The negative electrode for a lithium secondary battery according to claim 1 , wherein the carbon-based active material in the first layer is graphite. 3 . The negative electrode for a lithium secondary battery according to claim 1 , wherein a content of the carbon-based active material in the first layer is 80 to 100 parts by weight based on 100 parts by weight of the active material in the first layer. 4 . The negative electrode for a lithium secondary battery according to claim 1 , wherein the silicon-based active material in the second layer comprises at least one of silicon (Si), silicon oxide, metal-doped silicon oxide, silicon-carbon composite, and silicon-metal alloy. 5 . The negative electrode for a lithium secondary battery according to claim 1 , wherein a content of the silicon-based active material in the second layer is 0.1 to 40 parts by weight based on 100 parts by weight of the active material in the second layer. 6 . The negative electrode for a lithium secondary battery according to claim 1 , wherein the aluminum-containing compound in the third layer comprises at least one of aluminum oxide and Al(OH) 3 . 7 . The negative electrode for a lithium secondary battery according to claim 1 , wherein the content of the aluminum or the aluminum-containing compound in the third layer is 90 to 100 parts by weight based on 100 parts by weight of the composition of the third layer. 8 . The negative electrode for a lithium secondary battery according to claim 1 , wherein the third layer is provided to partially or completely cover the second layer. 9 . The negative electrode for a lithium secondary battery according to claim 1 , wherein the first layer and the second layer each further contain a binder and a conductive material.

10. A lithium secondary battery, comprising: The negative electrode according to any one of claims 1 to 9; diaphragm; and positive electrode, The third layer of the negative electrode is disposed to face the separator. The lithium secondary battery according to claim 10 , wherein the separator is a safety reinforced separator (SRS). 12 . A battery module comprising the lithium secondary battery according to claim 11 . 13 . A battery pack comprising the lithium secondary battery according to claim 11 . 14 . A battery pack comprising the battery module according to claim 12 .

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