Negative electrode active material, method of preparing same, and rechargeable lithium battery

Abstract

Disclosed examples include a negative electrode active material, a method of preparing the negative electrode active material, and a rechargeable lithium battery. The negative electrode active material includes: a silicon-carbon composite including a secondary particle assembled with a plurality of nano-silicon primary particles and an amorphous carbon coating layer on a surface of the secondary particle; and a sodium element on the surface of the nano-silicon primary particles and on the amorphous carbon coating.

Description

[0001] This application claims the benefit of priority from Korean Patent Application No. 10-2024-0049633 filed on April 12, 2024, in the Korean Intellectual Property Office, the entire contents of which are incorporated herein by reference. Technical Field

[0002] Disclosed are a negative electrode active material, a method for preparing the negative electrode active material, and a rechargeable lithium battery. Background Art

[0003] As the use of electronic devices using batteries (such as mobile phones, notebook computers, electric vehicles, etc., for example) increases, the demand for small, lightweight, and relatively high-capacity rechargeable lithium batteries is rapidly increasing.

[0004] Such rechargeable lithium batteries use crystalline carbon such as graphite as a negative electrode active material, and the performance of the rechargeable lithium batteries can achieve close to theoretical energy density. However, significantly higher energy density is still required.

[0005] However, silicon-based active materials exhibit degraded lifecycle characteristics because silicon has lower initial efficiency than crystalline carbon and has large volume expansion during charge and discharge, which leads to electrode degradation, resulting in detachment of the electrode from the current collector, loss of the electrolyte, etc. Summary of the Invention

[0006] Disclosed examples include negative electrode active materials that reduce irreversible lithium capacity loss, improve charge and discharge efficiency, and reduce or suppress the generation of irreversible side reactants to improve battery life cycle characteristics.

[0007] In some example embodiments, the negative electrode active material includes: a silicon-carbon composite including secondary particles assembled with a plurality of nano-silicon primary particles and an amorphous carbon coating on surfaces of the secondary particles; and sodium element on the surfaces of the nano-silicon primary particles and the amorphous carbon coating.

[0008] In some example embodiments, a method of preparing a negative electrode active material includes: mixing silicon powder and a first sodium raw material in an organic solvent to prepare a mixed solution; drying the mixed solution; mixing the dried mixed solution and an amorphous carbon precursor, and performing a heat treatment; and immersing the dried mixed solution and the amorphous carbon precursor after the heat treatment in a solution including a second sodium raw material.

[0009] In some example embodiments, a rechargeable lithium battery includes a negative electrode including the negative electrode active material, a positive electrode, and an electrolyte.

[0010] By applying the negative electrode active material according to some example embodiments, irreversible lithium capacity loss may be reduced to improve charge / discharge efficiency, and generation of irreversible side reactants may be reduced or suppressed to improve battery life cycle characteristics. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figures 1 to 4 is a cross-sectional view schematically illustrating a rechargeable lithium battery according to some example embodiments.

[0012] Figure 5 and Figure 6 is a schematic cross-sectional view of a negative electrode active material according to some example embodiments. DETAILED DESCRIPTION

[0013] Hereinafter, example embodiments will be described in detail so that those skilled in the art can easily implement the example embodiments. However, the present disclosure may be implemented in many different forms and is not to be construed as limited to the example embodiments set forth herein.

[0014] The terms used herein are for describing example embodiments only and are not intended to limit the present disclosure. Unless the context clearly dictates otherwise, expressions in the singular include expressions in the plural.

[0015] As used herein, "combinations thereof" means mixtures, stacks, composites, copolymers, alloys, blends, reaction products, and the like, of the components.

[0016] Here, it should be understood that terms such as "include", "comprising" or "having" are intended to indicate the presence of the embodied features, quantities, steps, elements (components) or combinations thereof, but they do not exclude the possibility of the presence or addition of one or more other features, quantities, steps, elements (components) or combinations thereof.

[0017] In the drawings, the thickness of layers, films, panels, regions, etc. may be exaggerated for clarity, and like reference numerals denote like elements throughout the specification. It will be understood that when an element such as a layer, film, region, or substrate is referred to as being "on" another element, it can be directly on the other element or intervening elements may be present. In contrast, when an element is referred to as being "directly on" another element, there are no intervening elements present.

[0018] In addition, the "layer" here includes not only a shape formed on the entire surface when viewed from a plan view but also a shape formed on a part of the surface.

[0019] In addition, the average particle size can be measured by methods known to those skilled in the art, for example, it can be measured by a particle size analyzer, or it can be measured by a transmission electron microscope image or a scanning electron microscope image. Alternatively, the average particle size value can be obtained by performing data analysis using a dynamic light scattering method, counting the number of particles in each particle size range, and calculating the measured average particle size value therefrom. As used herein, when no definition is otherwise provided, the average particle size may mean the diameter (D50) of particles having a cumulative volume of 50% by volume in a particle size distribution. As used herein, when no definition is otherwise provided, the average particle size means the diameter (D50) of particles having a cumulative volume of 50% by volume in a particle size distribution obtained by randomly measuring the size (diameter or length of the major axis) of approximately 20 particles in an optical micrograph. 50 ).

[0020] Here, “or” is not to be interpreted as exclusive, for example, “A or B” is to be interpreted as including A, B, A+B, etc.

[0021] The term "metal" is interpreted as including common metals, transition metals, and metalloids (semimetals).

[0022] Soft carbon refers to a carbon material that can be graphitized and is easily graphitized by heat treatment at high temperatures (e.g., about 2800°C). Hard carbon refers to a carbon material that cannot be graphitized or is only slightly graphitized by heat treatment.

[0023] When the term "about" or "substantially" is used in conjunction with a numerical value in this specification, it is intended that the relevant numerical value include a tolerance of ±10% around the stated numerical value. The expression "up to" includes the amount from zero to the stated upper limit and all values ​​therebetween. When a range is specified, the range includes all values ​​therebetween, such as increments of 0.1%.

[0024] Negative electrode active material In some example embodiments, the negative electrode active material includes: a silicon-carbon composite including secondary particles in which a plurality of nano-silicon primary particles are assembled and an amorphous carbon coating on surfaces of the secondary particles; and a sodium element on the surfaces of the nano-silicon primary particles and the amorphous carbon coating.

[0025] A potential challenge with silicon-based negative electrode active materials is that rapid volume changes typically occur during charge and discharge. Therefore, in order to buffer the volume expansion of silicon, common methods include processing silicon into nanoparticles, mixing silicon with amorphous carbon materials, or coating the surface of silicon with amorphous carbon materials. However, when silicon is converted into nanoparticles, a possible disadvantage is that the oxidation of the surface of the silicon nanoparticles may become severe, and the initial irreversible lithium capacity loss may increase accordingly. In addition, when silicon and amorphous carbon materials are mixed or coated, a possible challenge is that initial irreversible lithium capacity loss occurs due to defects in the amorphous carbon itself.

[0026] Some exemplary embodiments include a method for pre-sodiumizing a silicon-carbon composite, comprising a mixture of silicon nanoparticles and an amorphous carbon material, to reduce or suppress irreversible lithium loss, thereby improving charge / discharge efficiency and lifecycle characteristics. The surface of silicon nanoparticles is easily oxidized to form silicon oxide, which reacts with lithium during charging to form lithium silicate. This irreversible reaction, which prevents the release of lithium, reduces the reversible capacity of lithium. Some exemplary embodiments can pre-introduce sodium onto the surface of the silicon nanoparticles to form, for example, sodium silicate, thereby reducing irreversible lithium capacity loss and effectively reducing or suppressing the irreversible reaction between the silicon nanoparticles and lithium. Furthermore, when lithium is adsorbed onto defects in the amorphous carbon during charging, forming lithiated carbon, etc., the lithium ions are not released again, resulting in irreversible lithium capacity loss. However, in some exemplary embodiments, since sodium is pre-introduced into the amorphous carbon to form, for example, sodium carbide, the irreversible reaction between the amorphous carbon and lithium can be effectively reduced or suppressed, further reducing irreversible lithium capacity loss. Therefore, in the negative electrode active material according to some example embodiments, the capacity can be significantly increased by silicon while effectively buffering the volume change of silicon, and the irreversible capacity loss can be significantly reduced by silicon and amorphous carbon, thereby improving the charge / discharge efficiency. In addition, the life cycle characteristics of the battery can be improved by reducing or suppressing the generation of irreversible byproducts. In addition, the methods according to some example embodiments include a method for reducing irreversible capacity loss using sodium, which is cheaper than lithium, which is economically and commercially advantageous.

[0027] Silicon-carbon composite Figure 5 The left-hand image of FIG is a schematic diagram showing a cross section of a silicon-carbon composite according to some example embodiments. Figure 5, the silicon-carbon composite 1 includes secondary particles 4 assembled with a plurality of nano-silicon primary particles 3 and an amorphous carbon coating 5 on the surface of the secondary particles. The secondary particles 4 are a type of core particles, and the amorphous carbon coating 5 can be or include a shell surrounding the secondary particles. Here, the interior of the secondary particles 4, that is, between the nano-silicon primary particles 3, can be filled with amorphous carbon. In other words, the nano-silicon primary particles 3 can be coated with amorphous carbon. For example, the secondary particles 4 can be embedded in an amorphous carbon matrix, and the nano-silicon primary particles 3 can be dispersed in the amorphous carbon matrix.

[0028] The average particle size of the nano-silicon primary particles 3 (D 50 ) can be at about 10nm to about 600nm, for example, and about 10nm to about 500nm, about 10nm to about 400nm, about 10nm to about 300nm or about 10nm to about 200nm scope.When nano silicon primary particle 3 has the average grain diameter in the arbitrary scope in the above scope, can reduce or suppress the excessive volume expansion during charging and discharging, and can reduce or prevent the disconnection of the conductive path that causes owing to particle pulverization.The average grain diameter can be measured by using for example particle analyzer.

[0029] The shape of the nano-silicon primary particles 3 may not be particularly limited, and may be, for example, spherical, elliptical, sheet (plate)-shaped, flake-shaped, shapeless, or fiber-shaped.

[0030] Nano silicon primary particles 3 may include silicon, alloys of silicon and other metals, and partially oxidized silicon (SiO x , 0≤x<2), and is different from SiO2. When SiO2 particles are used instead of nano-silicon primary particles 3, irreversible capacity loss may be excessively high. Because SiO2 typically has a particle size of several microns or larger, achieving substantially uniform pre-sodiumization in the negative electrode active material may be challenging, and the effect of sodium introduction may be small. Furthermore, when SiO2 particles are pulverized to nanometer size, the Si content may decrease due to additional oxidation reactions, resulting in reduced capacity. In addition, SiO2 particles are highly reactive with the sodium raw material, so sodiumization reactions may occur excessively, forming excessive amounts of sodium silicate, which may reduce capacity.

[0031] The amorphous carbon may include, for example, at least one of soft carbon or hard carbon, a mesophase pitch carbonization product, coke, and combinations thereof, and may include, for example, hard carbon. The hard carbon may have a lattice spacing of approximately 0.385 nm corresponding to a diffraction peak of the (002) crystal plane, and thus may be suitable for reversible insertion and desorption of sodium ions.

[0032] The thickness of the amorphous carbon coating 5 on the surface of the secondary particle 4 may be in the range of about 2 nm to about 800 nm, for example, about 5 nm to about 600 nm, about 10 nm to about 400 nm, or about 20 nm to about 200 nm. The thickness of the amorphous carbon coating 5 can be measured, for example, by scanning electron microscopy (SEM) or transmission electron microscopy (TEM) images of a cross-section of the silicon-carbon composite.

[0033] The silicon-carbon composite 1 may have an average particle size (D 50 ), for example, an average particle size (D) in the range of about 1 μm to about 30 μm, about 2 μm to about 25 μm, about 3 μm to about 20 μm, or about 5 μm to about 15 μm. 50 When the silicon-carbon composite 1 has an average particle size within any of the above ranges, energy density can be increased, and lithium ions can easily diffuse into the negative electrode active material composite, reducing battery resistance and improving rate capability. Furthermore, because excessive increases in the specific surface area of ​​the negative electrode active material can be reduced or suppressed, side reactions with the electrolyte can also be reduced. The average particle size can be measured, for example, using a particle size analyzer.

[0034] In the silicon-carbon composite, based on 100 wt % of the total silicon and amorphous carbon, silicon may be included in an amount within a range of about 50 wt % to about 90 wt %, for example, about 60 wt % to about 80 wt %, and amorphous carbon may be included in an amount within a range of about 10 wt % to about 50 wt % or about 20 wt % to about 40 wt %. When the contents of silicon and amorphous carbon satisfy any of the above ranges, a negative electrode active material that achieves high capacity and effectively reduces or suppresses volume expansion can be obtained.

[0035] In an example, the silicon-carbon composite 1 according to some example embodiments may further include crystalline carbon 6 . Figure 6 The left-hand side illustration of shows a cross section of the silicon-carbon composite 1 further comprising crystalline carbon 6. Figure 6 The silicon-carbon composite 1 may, for example, include a core comprising nano-silicon primary particles 3 and crystalline carbon 6, and an amorphous carbon coating 5 on the surface of the core. The crystalline carbon 6 may be located inside the secondary particles 4, so that the silicon primary particles 3 and the crystalline carbon 6 may be dispersed in the amorphous carbon matrix. Similarly, the amorphous carbon may be filled between the nano-silicon primary particles 3 or between the crystalline carbon 6.

[0036] The crystalline carbon 6 may be or include natural graphite or artificial graphite, and is spherical, elliptical, flake-like, flaky, amorphous, or fibrous.

[0037] The crystalline carbon 6 may be included in an amount ranging from about 1 wt % to about 20 wt %, for example, from about 3 wt % to about 17 wt % or from about 5 wt % to about 15 wt %, based on 100 wt % of the total silicon, amorphous carbon, and crystalline carbon. When the crystalline carbon 6 is included in any of the above content ranges, electrical conductivity may be enhanced, thereby improving rate capability.

[0038] Based on the total 100 wt% of silicon, amorphous carbon, and crystalline carbon, the silicon-carbon composite 1 may include 30 wt% to 89 wt% of silicon, 10 wt% to 59 wt% of amorphous carbon, and 1 wt% to 20 wt% of crystalline carbon. For example, based on the total 100 wt% of silicon, amorphous carbon, and crystalline carbon, the silicon-carbon composite 1 may include 40 wt% to 87 wt% of silicon, 10 wt% to 49 wt% of amorphous carbon, and 3 wt% to 17 wt% of crystalline carbon. Alternatively, based on the total 100 wt% of silicon, amorphous carbon, and crystalline carbon, the silicon-carbon composite 1 may include 50 wt% to 75 wt% of silicon, 20 wt% to 45 wt% of amorphous carbon, and 5 wt% to 15 wt% of crystalline carbon. Alternatively, based on the total 100 wt% of silicon, amorphous carbon, and crystalline carbon, the silicon-carbon composite 1 may include 60 wt% to 70 wt% of silicon, 25 wt% to 35 wt% of amorphous carbon, and 5 wt% to 15 wt% of crystalline carbon.

[0039] sodium Figure 5 and Figure 6 The right figure is a schematic diagram of an enlarged cross-section of a negative electrode active material according to some example embodiments. In the negative electrode active material according to some example embodiments, sodium is present in the nano-silicon primary particles 3 and on the amorphous carbon coating 5. Sodium may be present on the surface of the nano-silicon primary particles 3 or within the nano-silicon primary particles 3, for example, on the surface of the nano-silicon primary particles 3. In addition, sodium may be substantially uniformly or locally distributed in the amorphous carbon coating 5.

[0040] As an example, the negative electrode active material may include at least one of the aforementioned silicon-carbon composite, sodium silicate, and sodium carbide.

[0041] For example, the negative electrode active material may include sodium silicate 7 on the surface of the nano-silicon primary particles 3 and sodium carbide 9 on the amorphous carbon coating 5. Sodium silicate and sodium carbide do not react with lithium and therefore do not cause side reactions, and effectively hinder or prevent lithium ions from irreversibly reacting with silicon (or silicon oxide) or with amorphous carbon during charging, thereby improving the charge / discharge efficiency and life cycle characteristics of the battery.

[0042] Sodium silicate 7 may be represented by the chemical formula (Na2O)·n(SiO2) and may include at least one of Na2SiO3, Na4SiO4, Na2Si2O5, Na2Si4O9, and combinations thereof. The presence of sodium silicate in the negative electrode active material may be determined by X-ray diffraction analysis (XRD).

[0043] Sodium carbide 9 can be represented by the chemical formula Na x C represents sodium ions, and sodium ions may be adsorbed or bound within the amorphous carbon. Amorphous carbon coating 5 present on the outermost layer of the negative electrode active material particles according to some example embodiments may include sodium carbide. Therefore, the sodium element may be substantially uniformly distributed on the surface of the negative electrode active material, as can be confirmed by scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) on the surface of the negative electrode active material.

[0044] Based on the total 100 wt % of the silicon-carbon composite and the sodium element, the amount of the sodium element may be in the range of about 0.1 wt % to about 20 wt %, for example, about 0.5 wt % to about 15 wt % or about 1 wt % to about 10 wt %. In addition, based on the total 100 at % of the silicon-carbon composite and the sodium element, the sodium element may be included in an amount in the range of about 1 at % to about 25 at %, for example, about 3 at % to about 20 at % or about 5 at % to about 15 at %. When the sodium content is within any of the above-mentioned contents or ranges, irreversible lithium capacity loss can be effectively reduced, thereby improving charge / discharge efficiency and life cycle characteristics. The sodium amount can be measured, for example, by atomic absorption spectroscopy (AAS), X-ray photoelectron spectroscopy (XPS), or SEM-EDS quantitative analysis.

[0045] Method for preparing negative electrode active material In some example embodiments, a method for preparing a negative electrode active material includes: (i) mixing silicon powder and a first sodium raw material in an organic solvent to prepare a mixed solution; (ii) drying the mixed solution; (iii) mixing the dried product with an amorphous carbon precursor and heat-treating the mixture; and (iv) immersing the heat-treated product in a solution including a second sodium raw material. This method can prepare the aforementioned pre-sodiumized silicon-carbon composite negative electrode active material.

[0046] Step (i) may be or include a process for preparing a silicon dispersion. For example, step (i) may include (i-1) first preparing a silicon dispersion by adding silicon powder to an organic solvent and mixing, and then (i-2) adding the first sodium raw material to the dispersion. The mixing process may be or include a milling process using a bead mill or a ball mill, and the silicon particles may be reduced to nanometer size through the mixing process. The silicon in the prepared mixed solution may have a size ranging from several nanometers to several hundred nanometers.

[0047] The organic solvent may be or include an alcohol solvent that is easily volatilized without oxidizing the silicon powder, and may include, for example, at least one of methanol, ethanol, isopropyl alcohol, butanol, propylene glycol, and combinations thereof.

[0048] The average particle size of the added silicon powder (D 50 ) may be on the micrometer or nanometer scale without particular limitation, but may be in the range of, for example, about 10 nm to about 200 μm.

[0049] The first sodium raw material is a material for inducing pre-sodiumization of silicon and may include, for example, at least one of NaOH, Na 2 CO 3 , and a combination thereof.

[0050] In step (i), the silicon powder and the first sodium raw material may be mixed to have a sodium content of about 1 wt% to about 21 wt% based on 100 wt% of the total amount of silicon and sodium. Thus, an appropriate amount of sodium may be introduced to effectively reduce irreversible capacity loss and improve electrochemical performance of the battery.

[0051] In step (i), when preparing a mixed solution by mixing silicon powder and a first sodium raw material in an organic solvent, crystalline carbon may be added. The description of the crystalline carbon may be the same as above. Based on 100 wt% of the total amount of silicon powder and crystalline carbon, the content of the crystalline carbon added above may be in the range of about 3 wt% to about 25 wt%. This can improve the conductivity of the negative electrode and improve the overall electrochemical characteristics including rate capability.

[0052] In step (ii), the mixed solution can be dried to form secondary particles assembled with nano-silicon primary particles, wherein the secondary particles can include the first sodium raw material. In other words, the dried product can be secondary particles assembled with nano-silicon primary particles. In step (i), when crystalline carbon is added, the nano-silicon primary particles are assembled with the crystalline carbon to form secondary particles.

[0053] Step (ii) may be or include, for example, a spray drying method, which allows the production of secondary particles under mild conditions. The produced secondary particles may have a uniform particle size and few defects. Drying may be performed, for example, at a temperature ranging from about 50°C to about 150°C or from about 100°C to about 140°C.

[0054] In step (iii), about 60 wt % to about 90 wt % of the dried product and about 10 wt % to about 40 wt % of the amorphous carbon precursor may be mixed. For example, about 60 wt % to about 80 wt % of the dried product and about 20 wt % to about 40 wt % of the amorphous carbon precursor may be mixed. Here, because an excessive amount of amorphous carbon may not be included in the final negative electrode active material, the capacity increase effect of silicon and the desired or improved initial efficiency may be achieved.

[0055] The amorphous carbon precursor may include, for example, at least one of coal-based pitch, mesophase pitch, petroleum-based pitch, mesocarbon pitch, coal-based oil, heavy petroleum, phenolic resin, furan resin, and polyimide resin.

[0056] In step (iii), the amorphous carbon precursor is converted into amorphous carbon by heat treatment, and the amorphous carbon can surround the surface of the secondary particles to form an amorphous carbon coating and also fill the interior of the secondary particles. In addition, during the heat treatment process, silicon oxide (e.g., silicon oxide on the surface of the nano-silicon primary particles) can react with the first sodium raw material to form sodium silicate. Silicon oxide is naturally formed by the reaction of silicon with oxygen in the air, but can be removed by reaction with sodium, which can reduce or inhibit the irreversible reaction of silicon oxide with lithium and reduce irreversible capacity loss.

[0057] The heat treatment may be performed under an inert atmosphere at a temperature range of about 700° C. to about 1000° C. for about 1 hour to about 10 hours. When the heat treatment is performed under such conditions, amorphous carbon may be effectively generated, and while oxidation or carbonization of silicon may be reduced or suppressed, pre-sodiumization of silicon may be performed.

[0058] Step (iv) may be or include a process of pre-sodiumizing the amorphous carbon coating to form a sodium carbide. For example, after preparing a sodium solution by adding the second sodium raw material to an organic solvent, the heat-treated product may be immersed in the sodium solution for a desired or predetermined time and then removed therefrom.

[0059] The second sodium raw material is or includes a material for pre-sodiumizing amorphous carbon, such as sodium biphenyl. Sodium biphenyl can be easily prepared at room temperature and is configured to effectively sodiumize amorphous carbon. For example, when equivalent amounts of biphenyl and sodium are added to a non-polar organic solvent (such as dimethyl ether (DME)), sodium biphenyl rapidly forms at room temperature. The conjugated biphenyl ring, with its strong electron affinity, can accept electrons from sodium and rapidly form a biphenyl radical anion. This sodium biphenyl can readily induce the sodiumization of amorphous carbon at room temperature.

[0060] The resulting product after heat treatment can be immersed in a solution including the second sodium raw material under mild conditions (such as room temperature and normal pressure) for a relatively short time (eg, about 0.5 minutes to about 10 minutes).

[0061] The method of preparing a negative electrode active material may further include a process of washing with an organic solvent after immersing the resultant after the heat treatment in a solution including a second sodium raw material and then taking it out.

[0062] negative electrode In some example embodiments, a negative electrode including the aforementioned negative electrode active material is provided. The negative electrode may include a current collector and a negative electrode active material layer on the current collector, and the negative electrode active material layer may include the aforementioned negative electrode active material. The negative electrode active material layer may further optionally include other types of negative electrode active materials, and may also include a binder, a conductive material, or a combination thereof.

[0063] For example, the negative electrode according to some example embodiments may include the aforementioned negative electrode active material as a first negative electrode active material, and may include a second negative electrode active material containing crystalline carbon. Here, based on a total of 100 wt% of the first negative electrode active material and the second negative electrode active material, the first negative electrode active material may be included in an amount ranging from about 1 wt% to about 60 wt%, about 1 wt% to about 50 wt%, about 3 wt% to about 30 wt%, or about 3 wt% to about 10 wt%, and may be appropriately mixed according to the desired capacity.

[0064] adhesive The binder is configured to attach the negative electrode active material particles to each other and also facilitates attachment of the negative electrode active material to the current collector. The binder may be or include a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof.

[0065] The non-aqueous binder may be or include at least one of polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, and combinations thereof.

[0066] The aqueous binder may be or include at least one of styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinyl pyrrolidone, polyepichlorohydrin, polyphosphazene, poly(meth)acrylonitrile, ethylene propylene diene copolymer, polyvinyl pyridine, chlorosulfonated polyethylene, latex, polyester resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0067] When an aqueous binder is included as the negative electrode binder, a cellulose compound that can impart viscosity may be further included. The cellulose compound may be a mixture of two or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be or include at least one of Na, K, and Li.

[0068] The dry binder can be or include a polymeric material that can be fibrous, and can be or include, for example, at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, and combinations thereof.

[0069] Conductive materials The conductive material is configured to impart conductivity to the electrode, and any electrically conductive material can be a conductive material unless the electrically conductive material causes chemical changes in the battery. Examples of the conductive material may include: carbon-based materials such as at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fibers, carbon nanofibers, and carbon nanotubes; metal-based materials including at least one of copper, nickel, aluminum, and silver in the form of metal powder or metal fibers; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0070] Based on 100 wt % of the negative electrode active material layer, the amount of the negative electrode active material may be in the range of about 95 wt % to about 99.5 wt %, and based on 100 wt % of the negative electrode active material layer, the content of the binder may be in the range of about 0.5 wt % to about 5 wt %. For example, the negative electrode active material layer may include about 90 wt % to about 99 wt % of the negative electrode active material, about 0.5 wt % to about 5 wt % of the binder, and about 0.5 wt % to about 5 wt % of the conductive material.

[0071] current collector The negative electrode current collector may include, for example, at least one 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 alloys thereof, and may be in the form of foil, sheet, or foam. The negative electrode current collector may have a thickness, for example, in the range of about 1 μm to about 20 μm, about 5 μm to about 15 μm, or about 7 μm to about 10 μm.

[0072] Rechargeable lithium battery Some exemplary embodiments include a rechargeable lithium battery comprising the aforementioned negative electrode, positive electrode, and electrolyte. As an example, a rechargeable lithium battery may include a positive electrode, a negative electrode, a separator between the positive and negative electrodes, and an electrolyte. Some exemplary embodiments include an all-solid-state rechargeable battery comprising a positive electrode, a negative electrode, and a solid electrolyte layer between the positive and negative electrodes. Below, for convenience, the construction of a lithium-ion battery using an electrolyte is explained in detail.

[0073] Rechargeable lithium batteries can be classified into cylindrical, prismatic, pouch, coin, etc. according to their shapes. Figures 1 to 4is a schematic diagram illustrating a rechargeable lithium battery according to some example embodiments, wherein Figure 1 It is a cylindrical battery. Figure 2 It is a prismatic battery. Figure 3 and Figure 4 It is a pouch-shaped battery. Figures 1 to 4 , the rechargeable lithium battery 100 includes an electrode assembly 40 having a separator 30 interposed between a positive electrode 10 and a negative electrode 20, and a case 50 accommodating the electrode assembly 40. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte (not shown). Figure 1 As shown in FIG, the rechargeable lithium battery 100 may include a sealing member 60 that seals the housing 50. Figure 2 In the embodiment, the rechargeable lithium battery 100 may include a positive electrode lead tab 11, a positive electrode terminal 12, a negative electrode lead tab 21, and a negative electrode terminal 22. Figure 3 and Figure 4 As shown in FIG, the rechargeable lithium battery 100 includes Figure 4 The electrode tab 70 shown in Figure 3 The positive electrode tab 71 and the negative electrode tab 72 shown in FIG. 7 , the tabs 70 / 71 / 72 form an electrical path for inducing current formed in the electrode assembly 40 to the outside of the battery 100 .

[0074] positive electrode A positive electrode for a rechargeable lithium battery may include a current collector and a positive electrode active material layer formed on the current collector. The positive electrode active material layer includes a positive electrode active material and may further include a binder and / or a conductive material.

[0075] Positive electrode active material The positive electrode active material may include a compound capable of intercalating and deintercalating lithium (lithiated intercalation compound). Specifically, at least one composite oxide of lithium and a metal including at least one of cobalt, manganese, nickel, and a combination thereof may be used.

[0076] The composite oxide may be or include a lithium transition metal composite oxide, and examples thereof may include at least one of lithium nickel-based oxides, lithium cobalt-based oxides, lithium manganese-based oxides, lithium iron phosphate-based compounds, lithium nickel manganese-based oxides not containing cobalt, and combinations thereof.

[0077] As an example, a compound represented by any one of the following chemical formulae can be used. a A 1-b X b O 2-c D c (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05); Li a Mn2-b X b About 4-c D c (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.05);Li a Ni 1-b-c Co b X c About 2-α D α (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0<α<2);Li a Ni 1-b- c Mn b X c About 2-α D α (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0<α<2);Li a Ni b Co c L 1 d G e O2(0.90≤a≤1.8,0≤b≤0.9,0≤c≤0.5,0≤d≤0.5,0≤e≤0.1);Li a NiG b O2(0.90≤a≤1.8,0.001≤b≤0.1);Li a CoG b O2(0.90≤a≤1.8,0.001≤b≤0.1);Li a Mn 1-b G b O2(0.90≤a≤1.8,0.001≤b≤0.1);Li a Mn2G b O4(0.90≤a≤1.8,0.001≤b≤0.1);Li a Mn 1-g G g PO4 (0.90≤a≤1.8, 0≤g≤0.5);Li (3-f) Fe2(PO4)3(0≤f≤2);Li a FePO4 (0.90≤a≤1.8).

[0078] In the above chemical formula, A is or includes at least one of Ni, Co, Mn, and combinations thereof; X is or includes at least one of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements, and combinations thereof; D is or includes at least one of O, F, S, P, and combinations thereof; G is or includes at least one of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, and combinations thereof; and L 1 It is or includes at least one of Mn, Al and combinations thereof.

[0079] As an example, the positive electrode active material may be or include a high-nickel positive electrode active material having a nickel content of greater than or equal to about 80 mol%, greater than or equal to about 85 mol%, greater than or equal to about 90 mol%, greater than or equal to about 91 mol%, or greater than or equal to about 94 mol% and less than or equal to about 99 mol%, based on 100 mol% of metals other than lithium in the lithium transition metal composite oxide. The high-nickel positive electrode active material can achieve high capacity and can be applied to high-capacity, high-density rechargeable lithium batteries.

[0080] adhesive The binder is configured to attach the positive electrode active material particles to each other and to attach the positive electrode active material to the current collector. The binder may include at least one of polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, a polymer containing ethylene oxide, polyvinyl pyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, nylon, etc., but is not limited thereto.

[0081] Conductive materials The conductive material is configured to impart conductivity to the electrode and can include any electrically conductive material as the conductive material, unless the electrically conductive material causes chemical changes in the battery. Examples of the conductive material can include: carbon-based materials such as at least one of natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fibers, carbon nanofibers, and carbon nanotubes; metal-based materials including at least one of copper, nickel, aluminum, and silver in the form of metal powder or metal fibers; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0082] The amount of the positive electrode active material may be in the range of about 90 wt % to about 99 wt % based on 100 wt % of the positive electrode active material layer, and the amount of each of the binder and the conductive material may be in the range of about 0.5 wt % to about 5 wt % based on 100 wt % of the positive electrode active material layer.

[0083] The current collector may include Al, but is not limited thereto.

[0084] electrolyte An electrolyte for a rechargeable lithium battery may include a non-aqueous organic solvent and a lithium salt.

[0085] The non-aqueous organic solvent is or constitutes a medium for transporting ions participating in the electrochemical reaction of the battery.

[0086] The non-aqueous organic solvent may include at least one of a carbonate solvent, an ester solvent, an ether solvent, a ketone solvent, an alcohol solvent, an aprotic solvent, and combinations thereof.

[0087] Carbonate solvents may include at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), ethylmethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). Ester solvents may include at least one of methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, decanoic acid lactone, mevalonolactone, valerolactone, and caprolactone. Ether solvents may include at least one of dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and tetrahydrofuran. Ketone solvents may include cyclohexanone, and the like. The alcohol solvent may include at least one of ethanol and isopropanol, etc., and the aprotic solvent may include at least one of nitrile (such as R-CN (wherein R may include a C2 to C20 linear, branched or cyclic hydrocarbon group, and may include a double bond, an aromatic ring or an ether bond)), amide (such as dimethylformamide), dioxolane (such as 1,3-dioxolane, 1,4-dioxolane, etc.), cyclopentane, etc.

[0088] The non-aqueous organic solvent may be used alone or in combination of two or more solvents.

[0089] In an example, in the case of using a carbonate-based solvent, a mixture of a cyclic carbonate and a chain carbonate may be used, and the cyclic carbonate and the chain carbonate may be mixed in a volume ratio of about 1:1 to about 1:9.

[0090] Lithium salts are dissolved in organic solvents to supply lithium ions to the battery, aiding the operation of rechargeable lithium batteries and improving the transport of lithium ions between the positive electrode and the negative electrode. Examples of lithium salts may include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide, LiFSI), LiC4F9SO3, LiN(C F 2x+1 SO2)(C y F 2y+2 SO2), wherein x and y are integers in the range of 1 to 20, one or more of lithium trifluoromethanesulfonate, lithium tetrafluoroethanesulfonate, lithium difluorobis(oxalato)phosphate (LiDFBOP), and lithium bis(oxalato)borate (LiBOB).

[0091] diaphragm Depending on the type of rechargeable lithium battery, a separator may be present between the positive electrode and the negative electrode. Examples of suitable separator materials include at least one of polyethylene, polypropylene, polyvinylidene fluoride, and multilayers thereof, such as a polyethylene / polypropylene double-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, and a polypropylene / polyethylene / polypropylene three-layer separator.

[0092] The separator may include a porous substrate and a coating layer including an organic material, an inorganic material, or a combination thereof on one or both surfaces of the porous substrate.

[0093] The porous substrate may be or include at least one polymer film formed from or including any one of polyolefins (such as polyethylene and polypropylene), polyesters (such as polyethylene terephthalate and polybutylene terephthalate), polyacetals, polyamides, polyimides, polycarbonates, polyetherketones, polyaryletherketones, polyetherimides, polyamideimides, polybenzimidazoles, polyethersulfones, polyphenylene oxides, cyclic olefin copolymers, polyphenylene sulfide, polyethylene naphthalate, fiberglass, Teflon (tetrafluoroethylene) and polytetrafluoroethylene, or copolymers or mixtures of two or more thereof.

[0094] The organic material may include a polyvinylidene fluoride-based polymer or a (meth)acrylic-based polymer.

[0095] The inorganic material may include inorganic particles, and the inorganic particles include at least one of Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite and combinations thereof, but are not limited thereto.

[0096] An organic material and an inorganic material may be mixed in one coating layer, or a coating layer including an organic material and a coating layer including an inorganic material may be stacked.

[0097] Examples and comparative examples of the present disclosure are described below. The following examples are merely examples of the present disclosure, but the present disclosure is not limited to the following examples.

[0098] Example 1 1. Preparation of negative electrode active material An ethanol solvent and silicon powder were mixed in a weight ratio of 9:1 using a bead mill to prepare a nano-silicon dispersion. Graphite was added to the nano-silicon dispersion so that the weight ratio between silicon and graphite was 9:1. NaOH was added thereto to have about 1.4 wt% of Na based on a total of 100 wt% of silicon, graphite and Na to prepare a mixed solution. The mixed solution was spray-dried at 120°C using a spray dryer. The dried product obtained was mixed with mesophase pitch in a weight ratio of 70:30, and the mixture was heat-treated at 1000°C for 4 hours under an N2 atmosphere.

[0099] A 0.5 M sodium biphenyl solution was prepared by mixing equal amounts of biphenyl and sodium in a dimethyl ether (DME) solvent at room temperature. The heat-treated product was immersed in the solution for 1 minute, taken out, and washed with a DME solvent to prepare a negative electrode active material.

[0100] The prepared negative electrode active material is in the form of secondary particles (cores) including nano-silicon primary particles and graphite, and a mesophase pitch carbonization product coating formed on the surface of the secondary particles, wherein amorphous carbon (e.g., mesophase pitch carbonization product) is even filled between the nano-silicon primary particles. The content of each component is as follows: about 63wt% silicon content, about 7wt% graphite content, and 30wt% amorphous carbon content, wherein the sodium content is about 1wt% based on 100wt% of the negative electrode active material. According to a particle size analyzer, the negative electrode active material has an average particle size (D 50 ).

[0101] When examined by X-ray diffraction analysis (XRD), the dried product obtained by spray drying showed a peak corresponding to Na2SiO3 in the range of 30° to 70°, indicating that sodium silicate was formed on the surfaces of the nano-silicon primary particles.

[0102] The prepared negative electrode active material, when examined by SEM-EDS analysis, showed that the sodium element was substantially uniformly dispersed on the surface of the negative electrode active material particles, indicating that sodium carbide was formed in the amorphous carbon coating.

[0103] 2. Manufacturing of rechargeable lithium battery cells 97.5 wt% of the prepared negative electrode active material, 1 wt% of carboxymethyl cellulose, and 1.5 wt% of styrene butadiene rubber were mixed in an aqueous solvent to prepare a negative electrode active material layer slurry. The negative electrode active material layer slurry was coated on a Cu foil current collector, dried, and pressed to form a negative electrode active material layer, thereby manufacturing a negative electrode.

[0104] A polytetrafluoroethylene separator was inserted between the negative electrode and the lithium metal counter electrode, and an electrolyte was injected to make a half-cell. The electrolyte was prepared by dissolving 1M LiPF6 in a mixed solvent of ethylene carbonate and dimethyl carbonate in a volume ratio of 3:7.

[0105] Example 2 A negative electrode active material and a half-cell were manufactured in substantially the same manner as in Example 1, except that a mixed solution was prepared by adding NaOH to have about 7.5 wt % of Na based on the total 100 wt % of silicon, graphite, and Na, so that the sodium content was changed to about 5 wt % based on 100 wt % of the final negative electrode active material.

[0106] Example 3 A negative electrode active material and a half-cell were manufactured in substantially the same manner as in Example 1, except that a mixed solution was prepared by adding NaOH to have about 15.9 wt % of Na based on a total of 100 wt % of silicon, graphite, and Na, so that the sodium content was changed to about 10 wt % based on 100 wt % of the final negative electrode active material.

[0107] Example 4 A negative electrode active material and a half-cell were manufactured in substantially the same manner as in Example 1, except that graphite was not mixed in the nano-silicon dispersion. The negative electrode active material of Example 4 contained 70 wt% silicon and 30 wt% amorphous carbon based on 100 wt% of silicon and amorphous carbon, and 1 wt% sodium based on 100 wt% of the negative electrode active material.

[0108] Comparative Example 1 A negative electrode active material and a half-cell were prepared in substantially the same manner as in Example 1, except that the mixed solution was prepared without adding NaOH and the heat-treated product was not immersed in a sodium biphenyl solution, i.e., sodium treatment was not performed to prepare the silicon-carbon composite. The negative electrode active material of Comparative Example 1 contained 63 wt% silicon, 7 wt% graphite, and 30 wt% amorphous carbon, based on a total of 100 wt% of silicon, graphite, and amorphous carbon, and did not contain sodium.

[0109] Evaluation Example 1: Initial charge / discharge efficiency The rechargeable lithium battery cells of Examples 1 to 4 and Comparative Example 1 were charged to 4.45 V at a constant current of 0.2 C and charged to 0.05 C at a constant voltage, and then discharged to 3.0 V at 0.2 C at 25° C. for initial charge and discharge, and then the ratio of the initial discharge capacity to the initial charge capacity was calculated and shown as efficiency in Table 1.

[0110] Table 1:

[0111] Referring to Table 1, the battery cells of Examples 1 to 4 exhibited high initial charge / discharge efficiency compared to the battery cell of Comparative Example 1. The example of applying the silicon-carbon composite prepared by performing pre-sodiumization as the negative electrode active material is understood to effectively reduce or suppress irreversible lithium loss and thus exhibit improved charge / discharge efficiency.

[0112] While the present disclosure has been described in connection with what are presently considered to be practical example embodiments, it is to be understood that the disclosure is not limited to the disclosed example embodiments, but is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

[0113] Description of Reference Numerals: 100: Rechargeable lithium battery 10: Positive electrode 11: Positive electrode lead lug 12: Positive electrode terminal 20: Negative electrode 21: Negative electrode lead lug 22: Negative electrode terminal 30: Diaphragm 40: Electrode assembly 50: Shell 60: Sealing component 70: Electrode terminal 71: Positive electrode terminal 72: Negative electrode terminal.

Claims

1. A negative electrode active material, comprising: A silicon-carbon composite comprising secondary particles assembled with a plurality of nano-silicon primary particles and an amorphous carbon coating on the surface of the secondary particles; as well as Sodium element is present in the nano-silicon primary particles and in the amorphous carbon coating.

2. The negative electrode active material according to claim 1, wherein The sodium element is included in an amount of 0.1 wt % to 20 wt % based on 100 wt % of the total of the silicon-carbon composite and the sodium element.

3. The negative electrode active material according to claim 1, wherein The sodium element is included in an amount of 1 at % to 25 at % based on 100 at % of the total of the silicon-carbon composite and the sodium element.

4. The negative electrode active material according to claim 1, wherein The sodium element is substantially uniformly distributed on the surface of the negative electrode active material.

5. The negative electrode active material according to claim 1, further comprising: Sodium silicate, on the surface of the nano-silicon primary particles; as well as Sodium carbide, in the amorphous carbon coating.

6. The negative electrode active material according to claim 1, wherein The amorphous carbon is filled between the nano-silicon primary particles.

7. The negative electrode active material according to claim 1, wherein In the silicon-carbon composite, the negative electrode active material includes 50 wt % to 90 wt % of silicon and 10 wt % to 50 wt % of amorphous carbon based on a total of 100 wt % of silicon and amorphous carbon.

8. The negative electrode active material according to claim 1, wherein: The thickness of the amorphous carbon coating on the surface of the secondary particle is in the range of 2 nm to 800 nm, The average particle size D of the nano-silicon primary particles 50 In the range of 10nm to 600nm, and The average particle size D of the silicon-carbon composite 50 In the range of 1μm to 30μm.

9. The negative electrode active material according to claim 1, wherein The silicon-carbon composite further includes crystalline carbon.

10. The negative electrode active material according to claim 9, wherein The silicon-carbon composite includes a core including the nano-silicon primary particle and the crystalline carbon, and the amorphous carbon coating on a surface of the core.

11. The negative electrode active material according to claim 9, wherein The crystalline carbon is included in an amount of about 1 wt % to about 20 wt % based on the total 100 wt % of silicon, amorphous carbon, and crystalline carbon.

12. A method for preparing a negative electrode active material, the method comprising the following steps: mixing silicon powder and a first sodium raw material in an organic solvent to prepare a mixed solution; drying the mixed solution; mixing the dried product of the mixed solution and an amorphous carbon precursor, and performing a heat treatment to form a mixture; and The heat-treated mixture is immersed in a solution including a second sodium raw material.

13. The method according to claim 12, wherein: The organic solvent includes an alcohol solvent, The average particle size D of the silicon powder 50 In the range of 10nm to 200μm, and The first sodium raw material includes at least one of NaOH and Na2CO3.

14. The method according to claim 12, wherein: The step of mixing the silicon powder and the first sodium raw material comprises: preparing a silicon dispersion by adding the silicon powder to the organic solvent and mixing the silicon powder and the organic solvent; and The first sodium raw material is added to the silicon dispersion.

15. The method of claim 12, wherein: When the mixed solution is prepared by mixing the silicon powder and the first sodium raw material in the organic solvent, crystalline carbon is added, and The amount of the crystalline carbon added is in the range of 3 wt % to 25 wt % based on the total 100 wt % of the silicon powder and the crystalline carbon.

16. The method according to claim 12, wherein: The step of drying the mixed solution includes spray drying the mixed solution at a temperature ranging from 50°C to 150°C.

17. The method according to claim 12, wherein: The step of mixing the dried product of the dried mixed solution with the amorphous carbon precursor includes mixing 60 wt % to 90 wt % of the dried product and 10 wt % to 40 wt % of the amorphous carbon precursor.

18. The method according to claim 12, wherein: The heat treatment is performed at a temperature ranging from 700°C to 1000°C.

19. The method of claim 12, wherein: The second sodium raw material includes sodium biphenyl, and The heat-treated mixture is immersed in the solution including the second sodium raw material for 0.5 to 10 minutes.

20. A rechargeable lithium battery, comprising: A negative electrode comprising a negative electrode active material according to any one of claims 1 to 11; positive electrode; as well as electrolytes.

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