Negative electrode active material, negative electrode containing the same, and secondary battery

Abstract

An objective is to provide a negative electrode active material for a secondary battery, a negative electrode, and a secondary battery. The negative electrode active material achieves both high initial efficiency and excellent discharge capacity and capacity retention. The negative electrode active material comprises: a first silicon oxide powder doped with at least one of an alkali metal and an alkaline earth metal; and an undoped second silicon oxide powder, wherein the second silicon oxide powder is amorphous.

Description

Technical Field

[0001] This application claims priority from Japanese Patent Application No. 2019-235038 filed with the Japan Patent Office on December 25, 2019, the disclosure of which is incorporated herein by reference. Embodiments of the present disclosure relate to a negative electrode active material, a negative electrode, and a secondary battery. Background Art

[0002] With the technological development and increasing demand for mobile devices, the demand for secondary batteries as energy sources has increased dramatically. Among secondary batteries, lithium-ion secondary batteries with high energy density and voltage, long cycle life and low self-discharge rate have been commercialized and widely used. Currently, many studies are underway to try to achieve higher capacity of lithium-ion secondary batteries.

[0003] The theoretical capacity density of silicon-based materials such as silicon alloys or silicon oxides is higher than that of carbon-based materials such as graphite that are commonly used nowadays, so there is great hope for using these silicon-based materials as negative electrode materials to improve the energy density of lithium-ion secondary batteries. x It showed a discharge capacity of over 1700 mAh / g during initial discharge, which is about 5 times that of graphite.

[0004] However, when using SiO x When using silicon-based materials as negative electrode active materials, the initial efficiency of the battery (i.e., the ratio of discharge capacity to charge capacity during the first charge-discharge cycle) is lower than when graphite is used. Regarding this issue, it is known that using silicon oxide powder pre-doped with Li or Mg as the negative electrode active material improves initial efficiency. However, pre-doping can sometimes result in a decrease in discharge capacity or deterioration in cycle characteristics.

[0005] Related Literature

[0006] [Patent Document]

[0007] Patent Document 1: Japanese Patent Publication No. 2013-114820

[0008] Patent Document 2: WO2015 / 059859

[0009] Patent Document 3: Japanese Patent Publication No. 2017-188319

[0010] Patent Document 4: Japanese Patent Publication No. 2012-33317 Summary of the Invention

[0011] Technical issues

[0012] The present disclosure aims to provide a negative electrode active material for a secondary battery, a negative electrode, and a secondary battery, the negative electrode active material being used to achieve high initial efficiency and improved discharge capacity and capacity retention.

[0013] Technical Solution

[0014] According to one embodiment of the present disclosure, there is provided a negative electrode active material for a secondary battery, the negative electrode active material including a first silicon oxide powder doped with at least one of an alkali metal and an alkaline earth metal and an undoped second silicon oxide powder, wherein the second silicon oxide powder is amorphous.

[0015] In the negative electrode active material according to the above embodiment, an average particle size of particles constituting the first silicon oxide powder may be larger than an average particle size of particles constituting the second silicon oxide powder.

[0016] In the negative electrode active material according to the embodiment, the average particle size of the particles constituting the first silicon oxide powder may be 3 μm to 15 μm inclusive. Furthermore, the average particle size of the particles constituting the second silicon oxide powder may be 0.5 μm to 2 μm inclusive.

[0017] In the negative electrode active material according to the above embodiment, a weight ratio of the second silicon oxide powder to the first silicon oxide powder may be equal to or greater than 0.2.

[0018] In the negative electrode active material according to the above embodiment, a weight ratio of the second silicon oxide powder to the first silicon oxide powder may be less than 10.

[0019] In the negative electrode active material according to the above embodiment, the first silicon oxide powder may include silicon crystallites having a crystallite size of 5 nm or more and 30 nm or less.

[0020] In the negative electrode active material according to the above embodiment, the first silicon oxide powder may include at least one of Li 2 SiO 3 , Li 2 Si 2 O 5 , Li 4 SiO 4 , and Mg 2 SiO 4 .

[0021] The negative electrode active material according to the above embodiment may further include a carbon material powder including at least one of natural graphite, artificial graphite, graphitized carbon fiber, and amorphous carbon.

[0022] According to another embodiment of the present disclosure, there is provided a negative electrode for a secondary battery, the negative electrode including a negative electrode active material layer formed on a negative electrode current collector, the negative electrode active material layer including the negative electrode active material according to the above embodiment.

[0023] According to another embodiment of the present disclosure, there is provided a secondary battery including the negative electrode according to the above embodiment. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 The graph plots the initial capacity, initial efficiency, and capacity retention as a function of the weight ratio of the first silicon oxide powder A to the second silicon oxide powder B in Example 1, Example 2, Comparative Example 1, and Comparative Example 2.

[0025] Figure 2 The graph plots the initial capacity, initial efficiency, and capacity retention as a function of the weight ratio of the first silicon oxide powder A to the second silicon oxide powder B in Example 3, Comparative Example 3, and Comparative Example 4. DETAILED DESCRIPTION

[0026] Hereinafter, embodiments of the present disclosure will be described. However, the present disclosure is not limited thereto.

[0027] In this specification, the "average particle size" refers to the particle size at 50% cumulative value in the particle size distribution measured by laser diffraction scattering, that is, the median diameter D 50 . In addition, in this specification, "silicon oxide powder" refers to a powder containing silicon oxide (which may contain any elements other than silicon and oxygen) in which the total content of silicon and oxygen is 80% by weight or more. In addition, in this specification, the symbol "to" is used to include both end values of the range indicated by the corresponding statement. For example, "1 to 2" means "1 or more and 2 or less."

[0028] In order to increase the capacity of lithium-ion secondary batteries, SiO x When silicon-based materials are used as negative electrode active materials, the initial efficiency tends to be lower than that when graphite is used. The reason is speculated as follows. x During the first cycle of charging, a reversible component that can be delithiated during the cycle, such as a Li-Si alloy, and an irreversible component that cannot be delithiated during the cycle, such as lithium silicate in a primary phase or a secondary phase or higher, may be formed. The lithium silicate suppresses the expansion of the silicon component in the negative electrode active material, but such irreversible components do not contribute to charging / discharging, resulting in a decrease in initial efficiency. Therefore, SiO x The initial efficiency of SiO is about 65% to 70%, which is very low compared with the initial efficiency of graphite (about 90% to 95%). x When used as negative electrode active materials, an imbalance occurs between the negative electrode active materials and the positive electrode active materials, resulting in a waste of positive electrode active materials and a reduction in energy density.

[0029] On the other hand, when SiO x When the silicon-based material is pre-doped with Li or Mg, the initial efficiency improves as the pre-doping amount increases, but a decrease in discharge capacity or deterioration in cycle characteristics may occur compared to when no pre-doping is performed. The reasons are presumably as follows. When pre-doping is performed, in some cases, in addition to not doping SiO x In addition to the irreversible components formed in the case of SiO2, in some cases, silicon compounds such as complex lithium silicate phases are formed, or excess lithium compounds are formed on the surface of the negative electrode active material particles. Therefore, the discharge capacity per unit weight may be much lower than that of undoped SiO2. x In addition, it can be speculated that during the doping period, SiO x The crystallinity of Si microcrystals in the battery increases, and with repeated charge / discharge, cracks are generated on the particle surface or inside the particle or the material expansion rate increases, leading to cycle degradation.

[0030] The inventors have discovered that by using a mixture of a first silicon oxide powder pre-doped with an alkali metal or alkaline earth metal and an undoped second silicon oxide powder as the negative electrode active material, both high initial efficiency and improved discharge capacity can be achieved. Furthermore, the inventors have discovered that by mixing the first and second silicon oxide powders, cycle characteristics are improved beyond expectations. Furthermore, the inventors have discovered that by adding a carbon material to the first and second silicon oxide powders, initial efficiency is improved beyond expectations.

[0031] [Non-aqueous electrolyte secondary battery]

[0032] Embodiments of the present disclosure relate to the nonaqueous electrolyte secondary battery. The nonaqueous electrolyte secondary battery according to this embodiment includes a negative electrode, a positive electrode, a separator disposed between the negative electrode and the positive electrode, and a nonaqueous electrolyte. Specific examples of the secondary battery may include lithium-ion secondary batteries having advantages such as high energy density, discharge voltage, and output stability.

[0033] Hereinafter, the lithium ion secondary battery is briefly described as an example, but the present disclosure is not limited to the lithium ion secondary battery and can be applied to various nonaqueous electrolyte secondary batteries.

[0034] The lithium-ion secondary battery according to an embodiment of the present disclosure includes a negative electrode, a positive electrode, a separator disposed between the negative electrode and the positive electrode, and a non-aqueous electrolyte. Furthermore, the lithium-ion secondary battery may optionally include: a battery case for accommodating an electrode assembly including the negative electrode, the positive electrode, and the separator; and a sealing member for sealing the battery case.

[0035] [negative electrode]

[0036] The negative electrode includes a negative electrode current collector and a negative electrode active material layer formed on one or both surfaces of the negative electrode current collector. The negative electrode active material layer may be formed on a portion or the entire surface of the negative electrode current collector.

[0037] (Negative electrode current collector)

[0038] The negative electrode current collector used in the negative electrode includes, but is not limited to, any type of negative electrode current collector that is conductive and does not cause chemical changes to the battery. For example, the negative electrode current collector may include: copper; stainless steel; aluminum; nickel; titanium; sintered carbon; copper or stainless steel with a surface treated with carbon, nickel, titanium, or silver; or aluminum-cadmium alloy.

[0039] The thickness of the negative electrode current collector may be 3 μm or more and 500 μm or less. The negative electrode current collector may have a fine texture on the surface to improve adhesion with the negative electrode active material. The negative electrode current collector may have various shapes such as a film, a sheet, a foil, a mesh, a porous body, a foam, and a non-woven fabric.

[0040] (Negative Electrode Active Material Layer)

[0041] The negative electrode active material layer can be formed by, for example, applying a negative electrode active material slurry prepared by dissolving or dispersing a mixture of a negative electrode active material, a binder, and a conductive agent in a solvent onto the negative electrode current collector, drying, and rolling; or by casting the negative electrode active material slurry onto a support and laminating a film separated from the support onto the negative electrode current collector. If necessary, the mixture may further contain a dispersant, a filler, or any other additives.

[0042] The negative electrode active material may be contained in an amount of 80 wt % or more and 99 wt % or less based on the total weight of the negative electrode active material layer.

[0043] (Negative electrode active material)

[0044] In the lithium ion secondary battery according to one embodiment, the negative electrode active material may include a first silicon oxide powder A doped with at least one of an alkali metal and an alkaline earth metal and an undoped second silicon oxide powder B. In addition, the negative electrode active material may further include carbon material powder.

[0045] The first silicon oxide powder A is the result of doping silicon oxide powder with at least one of an alkali metal and an alkaline earth metal. That is, the particles constituting the first silicon oxide powder A may contain at least one of the doped alkali metal element and the alkaline earth metal element. The first silicon oxide powder A may include, for example, silicon oxide SiO x(0 < x < 2), elemental silicon Si, elemental doped metal elements, metal silicates doped, any other silicon compounds or compounds of doped metals.

[0046] The silicon oxide powder used as a raw material before doping can be, for example, SiO x powder. SiO x can have a structure, for example, in which Si particles are dispersed in an amorphous silicon oxide matrix in a microcrystalline or amorphous form. The ratio x of oxygen to silicon is 0 < x < 2, preferably 0.5 ≤ x ≤ 1.6, more preferably 0.8 ≤ x ≤ 1.5. For example, the silicon oxide powder as a raw material can be SiO (x = 1). In addition, the silicon oxide powder as a raw material can be composed of SiO x with a specific x value, and can contain a mixture of different types of SiO x powders with different x values.

[0047] The silicon oxide powder used as a raw material can be an amorphous structure that does not have a crystalline phase except for the microcrystals of silicon dispersed in the structure. The dispersed microcrystals are so small that they do not appear as diffraction peaks in the X-ray diffraction (XRD) pattern, and the XRD pattern of the silicon oxide powder used as a raw material basically does not have diffraction peaks originating from the crystalline phase. In this specification, when no diffraction peaks are found in the XRD pattern, even a material containing microcrystals is referred to as "amorphous". On the other hand, the XRD pattern of the silicon oxide powder used as a raw material can have diffraction peaks originating from the dispersed microcrystals.

[0048] The metal elements used for doping include, but are not limited to, any alkali metal or alkaline earth metal. For example, at least one of lithium, sodium, potassium, magnesium, and calcium can be used to dope the silicon oxide powder used as a raw material, but it is not limited to this.

[0049] For example, when lithium is used for doping, the first silicon oxide powder A can contain particles of silicon or lithium silicate in the structure. For example, the first silicon oxide powder A can have a structure in which silicon or lithium silicate is dispersed in an amorphous silicon oxide matrix in a microcrystalline or amorphous form. Examples of the lithium silicate can include Li2SiO3, Li2Si2O5, and Li4SiO4, but are not limited to this. In addition to the above, there can be any other components, such as lithium-based materials, silicon-based materials, lithium-silicon compounds, etc.

[0050] Similarly, for example, when magnesium is used for doping, the first silicon oxide powder A can have a structure in which fine particles of silicon or magnesium silicate (MgSiO3 or Mg2SiO4) are dispersed in an amorphous silicon oxide matrix. Furthermore, when calcium is used for doping, the first silicon oxide powder A can have a structure in which fine particles of silicon or calcium silicate (CaSiO3 or Ca2SiO4) are dispersed in an amorphous silicon oxide matrix. The same is true for any other alkali metal or alkaline earth metal used for doping.

[0051] Based on the entire first silicon oxide powder A, the total doping amount of alkali metal or alkaline earth metal in the first silicon oxide powder A can be, for example, 0.1 wt % or more and 20 wt % or less, preferably 0.5 wt % or more and 15 wt % or less, and more preferably 1 wt % or more and 10 wt % or less.

[0052] The XRD pattern of the first silicon oxide powder A may have at least one diffraction peak originating from the microcrystals in the silicon oxide matrix. On the other hand, even when no diffraction peak is observed in the XRD pattern of the silicon oxide powder as a raw material, doping may cause crystallization or generate a new crystalline phase, thereby causing a diffraction peak to appear in the XRD pattern of the first silicon oxide powder A.

[0053] In the first silicon oxide powder A, the size of the silicon microcrystals in the silicon oxide matrix (hereinafter, the size of a single microcrystal is referred to as "crystallite size". In this specification, "crystallite size" refers to the D value calculated using the following Scherrer formula (1)) can be, for example, 5 nm or more and 30 nm or less, preferably 5 nm or more and 20 nm or less, and more preferably 5 nm or more and 10 nm or less. The crystallite size of the microcrystals can be calculated from the line width of the peak originating from each microcrystal on the XRD pattern of the first silicon oxide powder A using the following Scherrer formula (1) well known in the art.

[0054] D(nm)=Kλ / Bcosθ (1)

[0055] Here, D is the crystallite size of the microcrystal, B is the full width at half maximum (rad) of the target peak of the XRD pattern, θ is the diffraction angle of the XRD pattern, K=0.9, λ=0.154 nm (in the case of CuKα).

[0056] For example, in the case of silicon, a diffraction peak of the (111) plane is observed near 2θ=28.4°, and the crystallite size D of the silicon microcrystals in the first silicon oxide powder A can be estimated from the full width at half maximum of the (111) peak and the diffraction angle θ.

[0057] The average particle size of the particles of the first silicon oxide powder A used in the negative electrode active material may be, for example, 1 μm or more and 20 μm or less, preferably 3 μm or more and 15 μm or less, more preferably 4 μm or more and 10 μm or less.

[0058] Compared with the negative electrode active material containing only undoped silicon oxide powder, using the negative electrode active material containing the first silicon oxide powder A can improve the initial efficiency. It is speculated that in the undoped silicon oxide powder, irreversible components such as silicate phases that do not contribute to charge / discharge are generated during the first charge / discharge cycle. In contrast, the first silicon oxide powder A already contains silicate phases, thereby suppressing the decrease in the discharge capacity of the first cycle relative to the charge capacity of the first cycle to a certain extent.

[0059] The second silicon oxide powder B is a powder of undoped silicon oxide. Here, "undoped" means that it is not doped with metal elements and non-metal elements other than silicon and oxygen except for inevitable impurities. That is, the particles constituting the second silicon oxide powder B may contain no metal elements and non-metal elements other than silicon and oxygen except for inevitable impurities. For example, the second silicon oxide powder B is a powder of SiO x For example, SiO x may have a structure in which Si microparticles are dispersed in an amorphous silicon oxide matrix in the form of microcrystals or amorphous. The ratio x of oxygen to silicon is 0 < x < 2, preferably 0.5 ≤ x ≤ 1.6, more preferably 0.8 ≤ x ≤ 1.5. For example, the second silicon oxide powder B may be SiO (x = 1). In addition, the second silicon oxide powder B may be composed of SiO x having a specific x value, and may contain a mixture of at least two SiO x powders having different x values.

[0060] The second silicon oxide powder B may have an amorphous structure having no crystalline phase except for the microcrystals of silicon dispersed in the structure. For example, when the dispersed microcrystals are so small that they do not appear as diffraction peaks in the XRD pattern, the XRD pattern of the second silicon oxide powder B may substantially have no diffraction peaks derived from the crystalline phase.

[0061] For example, the average particle size of the particles of the second silicon oxide powder B used in the negative electrode active material may be smaller than the average particle size of the particles of the first silicon oxide powder A. The average particle size of the particles constituting the second silicon oxide powder B may be, for example, 0.1 μm or more and 5 μm or less, preferably 0.3 μm or more and 3 μm or less, more preferably 0.5 μm or more and 2 μm or less.

[0062] Compared with the negative electrode active material composed of silicon oxide powder doped with alkali metal or alkaline earth metal, the negative electrode active material containing the second silicon oxide powder B can improve the discharge capacity or cycle characteristics. It is speculated that the metal ions in the SiO contained in the second silicon oxide powder B x The diffusion of the particles is faster than that of the doped silicon oxide, but the SiO contained in the second silicon oxide powder B x is amorphous, and thus cracking or expansion / contraction caused by charge / discharge can be suppressed compared to doped silicon oxide having high crystallinity. However, this mechanism is merely an exemplary assumption and does not limit the present disclosure.

[0063] The weight ratio A:B of the first silicon oxide powder A and the second silicon oxide powder B in the negative electrode active material can be, for example, 1:9 to 9:1, preferably 3:7 to 9:1, more preferably 4:6 to 9:1, and most preferably 5:5 to 8:2. When the weight ratio is expressed as B / A, the weight ratio B / A can be, for example, 0.1 or more, preferably 0.2 or more, more preferably 0.25 or more. In addition, the weight ratio B / A can be, for example, less than 10, preferably less than 5, and more preferably less than 2. When the weight ratio is within the above range, high initial efficiency and improved discharge capacity and cycle characteristics can be achieved.

[0064] When the negative electrode active material includes the carbon material powder, the carbon material powder may include any type of carbon material commonly used in negative electrode active materials for non-aqueous electrolyte secondary batteries. For example, the carbon material powder may include at least one of the following: natural graphite, artificial graphite, graphitized carbon fiber and amorphous carbon, but is not limited thereto. A composite with any other element other than carbon may be used. On the other hand, the carbon material may include either low crystalline carbon or high crystalline carbon. The low crystalline carbon generally includes soft carbon and hard carbon, and the high crystalline carbon generally includes amorphous, plate-like, flaky, spherical or fibrous high temperature sintered carbon such as natural graphite or artificial graphite, floating graphite, pyrolytic carbon, mesophase pitch-type carbon fiber, mesophase carbon microbeads, mesophase pitch, as well as coal coke and petroleum coke.

[0065] The average particle size of the particles constituting the carbon material powder may be, for example, 1 μm or more and 50 μm or less, preferably 10 μm or more and 20 μm or less.

[0066] When the negative electrode active material contains the first silicon oxide powder A and the second silicon oxide powder B and also contains the carbon material powder, the weight ratio of the silicon material (i.e., the first silicon oxide powder A and the second silicon oxide powder B) and the carbon material in the negative electrode active material can be, for example, 1:99 to 50:50, preferably 5:95 to 30:80, and more preferably 8:92 to 20:80.

[0067] When the negative electrode active material contains the carbon material powder, the carbon material tends to exhibit better initial efficiency and cycle characteristics than the silicon material, thereby achieving improved initial efficiency and cycle characteristics compared to the negative electrode active material composed of the first silicon oxide powder A and the second silicon oxide powder B.

[0068] On the other hand, the negative electrode active material may further contain any other material in addition to the first silicon oxide powder A, the second silicon oxide powder B, and the carbon material powder.

[0069] (Adhesive)

[0070] The binder is added to promote bonding between the active material and the conductive agent or between the active material and the current collector. Examples of the binder may include at least one of polyvinylidene fluoride (PVdF), polyvinyl alcohol (PVA), polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), polyacrylate, acrylamide, polyimide, fluororubber, and copolymers thereof, but are not limited thereto.

[0071] The content of the binder may be 0.1% by weight or more and 30% by weight or less based on the total weight of the negative electrode active material layer. The content of the binder may preferably be 0.5% by weight or more and 20% by weight or less, more preferably 1% by weight or more and 10% by weight or less. When the content of the binder satisfies the above range, degradation of the battery capacity characteristics can be prevented and sufficient bonding strength can be imparted to the electrode.

[0072] (Conductive agent)

[0073] The conductive agent includes, but is not limited to, any type of conductive material that does not cause chemical changes. Examples of the conductive agent may include at least one of the following: carbon materials such as artificial graphite, natural graphite, carbon nanotubes, graphene, carbon black, acetylene black, Ketjen black, Denka black, thermal black, channel black, furnace black, lamp black, carbon fiber; metal powders or metal fibers of aluminum, tin, bismuth, silicon, antimony, nickel, copper, titanium, vanadium, chromium, manganese, iron, cobalt, zinc, molybdenum, tungsten, silver, gold, lanthanum, ruthenium, platinum, or iridium; conductive whiskers of zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyaniline, polythiophene, polyacetylene, polypyrrole, or polyphenylene derivatives, but are not limited thereto.

[0074] The amount of the conductive agent may be 0.1% by weight or more and 30% by weight or less based on the total weight of the negative electrode active material layer. The amount of the conductive agent may preferably be 0.5% by weight or more and 15% by weight or less, and more preferably 0.5% by weight or more and 10% by weight or less. When the amount of the conductive agent falls within the above range, sufficient conductivity can be imparted, and since the amount of the negative electrode active material is not reduced, the battery capacity can be ensured.

[0075] (Thickener)

[0076] The negative electrode active material slurry may further include a thickener. Specifically, the thickener may be a cellulose compound such as carboxymethyl cellulose (CMC). For example, based on the total weight of the negative electrode active material layer, the content of the thickener may be greater than 0.5% by mass and less than 10% by mass.

[0077] (Solvent)

[0078] The solvent used for the negative electrode active material slurry includes, but is not limited to, any type of solvent commonly used to manufacture the negative electrode. Examples of the solvent may include at least one of the following: N-methyl-2-pyrrolidone (NMP), dimethyl sulfoxide (DMSO), isopropyl alcohol, acetone, and water, but are not limited thereto.

[0079] [Method for Manufacturing Negative Electrode]

[0080] According to one embodiment, a method for manufacturing a negative electrode for a lithium ion secondary battery may include: (1) obtaining a negative electrode active material; (2) obtaining a negative electrode active material slurry from the negative electrode active material; and (3) obtaining a negative electrode from the negative electrode active material slurry.

[0081] (1) Steps for obtaining negative electrode active material

[0082] The first silicon oxide powder A can be obtained, for example, by thermal doping. More specifically, for example, by mixing a silicon oxide powder as a raw material and a doping metal source powder and performing high-temperature sintering in an inert atmosphere such as an argon atmosphere or a nitrogen atmosphere, the first silicon oxide powder A can be obtained. If necessary, the obtained first silicon oxide powder A can be ground by a bead mill to adjust the particle size.

[0083] For example, commercially available SiO x (0 < x < 2) powder can be used as the silicon oxide powder as the raw material. Here, the ratio x of oxygen to silicon is 0 < x < 2, preferably 0.5 ≤ x ≤ 1.6, and more preferably 0.8 ≤ x ≤ 1.5. Examples of the doping metal source can include: in the case of doping lithium, containing metallic lithium (Li) or lithium hydride (LiH); in the case of doping magnesium, containing magnesium hydride (MgH2); and in the case of doping calcium, containing calcium hydride (CaH2), but are not limited thereto. The sintering temperature is, for example, 650 °C or higher and 850 °C or lower.

[0084] For example, commercially available SiO x (0 < x < 2) can be used as the second silicon oxide powder B. The SiO x powder used can be the same as or different from the SiO x powder used as the raw material of the first silicon oxide powder A. If necessary, the second silicon oxide powder B can be ground by a bead mill to adjust the particle size.

[0085] The first silicon oxide powder A and the second silicon oxide powder B can be mixed with any other material such as a carbon material as needed to obtain the negative electrode active material.

[0086] (2) Step of obtaining a negative electrode active material paste from the negative electrode active material

[0087] A solvent is added to the negative electrode active material obtained in the above step (1). In this case, the conductive agent, the binder, and the thickener can be added as needed. By dissolving or dispersing the negative electrode active material, the conductive agent, the binder, and the thickener in the solvent, the negative electrode active material paste can be obtained.

[0088] (3) Step of obtaining a negative electrode from the negative electrode active material paste

[0089] The negative electrode active material paste can be coated on the negative electrode current collector, dried, and roll-pressed to manufacture a negative electrode having a negative electrode active material layer on the negative electrode current collector.

[0090] Alternatively, for example, the negative electrode may be manufactured by casting the negative electrode active material slurry on a support and laminating the film separated from the support on the negative electrode collector. In addition, the negative electrode active material layer may be formed on the negative electrode collector by any other method.

[0091] [positive electrode]

[0092] In the lithium ion secondary battery according to one embodiment, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on one or both surfaces of the positive electrode current collector. The positive electrode active material layer may be formed on a portion or the entire surface of the positive electrode current collector.

[0093] (Positive electrode current collector)

[0094] The positive electrode current collector used in the positive electrode includes, but is not limited to, any type of positive electrode current collector that is conductive and does not cause chemical changes to the battery. For example, the positive electrode current collector may include: stainless steel; aluminum; nickel; titanium; sintered carbon; aluminum or stainless steel with a surface treated with carbon, nickel, titanium, or silver.

[0095] The thickness of the positive electrode current collector may be 3 μm or more and 500 μm or less. The positive electrode current collector may have a fine texture on the surface to improve adhesion with the positive electrode active material. The positive electrode current collector may have various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam, and a non-woven fabric.

[0096] (Positive Electrode Active Material Layer)

[0097] The positive electrode active material layer can be formed by, for example, applying a positive electrode active material slurry comprising a mixture of a positive electrode active material, a conductive agent, and a binder dissolved or dispersed in a solvent onto the positive electrode current collector, drying, and rolling. If necessary, the mixture may further contain a dispersant, a filler, or any other additives.

[0098] The content of the positive electrode active material may be 80 wt % or more and 99 wt % or less based on the total weight of the positive electrode active material layer.

[0099] (Positive electrode active material)

[0100] The positive electrode active material may include a compound capable of reversibly intercalating and deintercalating lithium. Specific examples may include, for example, lithium metal composite oxides containing at least one of the following metals and lithium: cobalt, manganese, nickel, copper, vanadium, and aluminum. More specifically, the lithium metal composite oxide may include: lithium manganese-based oxides (such as LiMnO2, LiMnO3, LiMn2O3, LiMn2O4); lithium cobalt-based oxides (such as LiCoO2); lithium nickel-based oxides (such as LiNiO2); lithium copper-based oxides (such as Li2CuO2); lithium vanadium-based oxides (such as LiV3O8); lithium nickel manganese-based oxides (such as LiNi 1-z Mn z O2(0<z<1), LiMn 2- z Ni z O4(0<z<2)); lithium nickel cobalt-based oxides (such as LiNi 1-y Co y O2(0<y<1)); lithium manganese cobalt-based oxides (such as LiCo 1- z Mn z O2(0<z<1), LiMn 2-y Co y O4(0<y<2)); lithium nickel manganese cobalt-based oxides (such as Li(Ni x Co y Mn z )O2(0<x<1, 0<y<1, 0<z<1, x + y + z = 1), Li(Ni x Co y Mn z )O4(0<x<2, 0<y<2, 0<z<2, x + y + z = 2)); lithium nickel cobalt metal (M) oxides (such as Li(Ni x Co y Mn z M w )O2 (M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, 0<x<1, 0<y<1, 0<z<1, 0<w<1, x + y + z + w = 1)); compounds in which the transition metal element in the above compounds is partially replaced by at least one other metal element. The positive electrode active material layer may include at least one of them. However, the positive electrode active material layer is not limited thereto.

[0101] In particular, in terms of improving the capacity characteristics and stability of the battery, LiCoO2, LiMnO2, LiMn2O4, LiNiO2, lithium nickel manganese cobalt oxide (such as Li(Ni 1 / 3 Mn 1 / 3 Co 1 / 3)O2、Li(Ni 0.6 Mn 0.2 Co 0.2 )O2、Li(Ni 0.4 Mn 0.3 Co 0.3 )O2、Li(Ni 0.5 Mn 0.3 Co 0.2 )O2、Li(Ni 0.7 Mn 0.15 Co 0.15 )O2、Li(Ni 0.8 Mn 0.1 Co 0.1 )O2), lithium nickel cobalt aluminum oxide (such as Li(Ni 0.8 Co 0.15 Al 0.05 )O2) is expected.

[0102] (adhesive and conductive agent)

[0103] The types and amounts of the binder and the conductive agent used in the positive electrode active material slurry may be the same as those described above for the negative electrode.

[0104] (Solvent)

[0105] The solvent used in the positive electrode active material slurry includes, but is not limited to, any type of solvent commonly used to manufacture the positive electrode. Examples of the solvent may include at least one of the following: an amine solvent such as N,N-dimethylaminopropylamine, diethylenetriamine, N,N-dimethylformamide (DMF), an ether solvent such as tetrahydrofuran, a ketone solvent such as methyl ethyl ketone, an ester solvent such as methyl acetate, an amide solvent such as dimethylacetamide, 1-methyl-2-pyrrolidone (NMP), or dimethyl sulfoxide (DMSO), but are not limited thereto.

[0106] Considering the coating thickness or yield of the slurry, the amount of the solvent used should be large enough to dissolve or disperse the positive active material, the conductive material and the binder, and should be viscous enough to ensure high thickness uniformity when coated on the positive current collector.

[0107] [Method for Manufacturing Positive Electrode]

[0108] According to one embodiment, the method for manufacturing a positive electrode for a lithium ion secondary battery may include: a step of dissolving or dispersing the positive electrode active material and optionally the binder, the conductive agent, and the thickener in the solvent to obtain the positive electrode active material slurry; and a step of obtaining the positive electrode by coating the positive electrode active material slurry on the positive electrode collector in the same manner as the method for manufacturing a negative electrode to form the positive electrode active material layer on the positive electrode collector.

[0109] [Diaphragm]

[0110] In the lithium ion secondary battery according to one embodiment, the separator separates the negative electrode and the positive electrode to provide a mobile channel for lithium ions, and may include but is not limited to any type of separator commonly used as a lithium ion secondary battery separator. In particular, the separator preferably has low resistance to the ion movement of the electrolyte and high wettability to the electrolyte. For example, the separator may include: a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, an ethylene / methacrylate copolymer; or a stacked structure of two or more layers thereof. In addition, commonly used porous non-woven fabrics, such as non-woven fabrics made of high melting point glass fibers or polyethylene terephthalate fibers, may be used. In addition, in order to ensure heat resistance or mechanical strength, the separator may be coated with a ceramic or polymer material.

[0111] [Non-aqueous electrolyte]

[0112] In the nonaqueous electrolyte secondary battery according to one embodiment, the nonaqueous electrolyte may include an organic liquid electrolyte and an inorganic liquid electrolyte used to manufacture a secondary battery, but is not limited thereto.

[0113] The nonaqueous electrolyte may include an organic solvent and a lithium salt, and may further include an additive if necessary. Hereinafter, the liquid electrolyte is referred to as an "electrolyte."

[0114] The organic solvent includes, but is not limited to, any type of organic solvent used as a medium for enabling the movement of ions participating in the electrochemical reaction of the battery. Examples of the organic solvent may include at least one of the following: ester solvents such as methyl acetate, ethyl acetate, γ-butyrolactone, ε-caprolactone; ether solvents such as dibutyl ether, tetrahydrofuran; ketone solvents such as cyclohexanone; aromatic hydrocarbon solvents such as benzene and fluorobenzene; carbonate solvents such as dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), propylene carbonate (PC); alcohol solvents such as ethanol and isopropanol; nitrile solvents such as R-CN (wherein R is a C2 to C20 hydrocarbon group in a linear, branched or cyclic structure, and may contain a double bond aromatic ring or an ether bond); amide solvents such as dimethylformamide; dioxolane solvents such as 1,3-dioxolane; or sulfolane solvents, but are not limited thereto. In particular, the carbonate solvent is desirable, and more desirable is a mixture of a cyclic carbonate (e.g., ethylene carbonate or propylene carbonate) having high ionic conductivity and a high dielectric constant to improve the charge / discharge performance of the battery and a low-viscosity linear carbonate compound (e.g., ethyl methyl carbonate, dimethyl carbonate, diethyl carbonate). In this case, a mixture of a cyclic carbonate and a linear carbonate in a volume ratio of about 1:1 to 1:9 can provide excellent electrolyte performance.

[0115] The lithium salt may include, but is not limited to, any type of compound capable of providing the lithium ions used in the lithium ion secondary battery. Examples of the lithium salt may include at least one of the following: LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, and LiB(C2O4)2, but are not limited thereto. For example, the lithium salt may be included in the electrolyte at a concentration of 0.1 mol / L or more and 2 mol / L or less. When the concentration of the lithium salt is within the above range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance, thereby achieving efficient movement of lithium ions.

[0116] If necessary, in order to improve the life characteristics of the battery, prevent the reduction of the battery capacity and improve the discharge capacity of the battery, additives can be used. Examples of the additives may include at least one of the following: halogenated alkylene carbonate compounds such as fluoroethylene carbonate (FEC) or difluoroethylene carbonate (DFEC), pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted Oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol and aluminum chloride, but not limited thereto. For example, the content of the additive may be 0.1 wt % or more and 15 wt % or less based on the total weight of the electrolyte.

[0117] In particular, fluoroethylene carbonate and difluoroethylene carbonate can act as film formers to form a film in the electrode-electrolyte interface. For example, when at least one of fluoroethylene carbonate and difluoroethylene carbonate is included, a good SEI layer can be formed during the alloying of the silicon-based material and lithium in the negative electrode using the negative electrode active material including the silicon-based material, thereby achieving stable charge / discharge. Based on the total weight of the electrolyte, the content of the film former can be, for example, 0.1% by weight or more and 15% by weight or less, preferably 0.5% by weight or more and 10% by weight or less, more preferably 1% by weight or more and 7% by weight or less. The film former may include at least one of fluoroethylene carbonate and difluoroethylene carbonate.

[0118] [Method for Manufacturing Non-Aqueous Electrolyte Secondary Battery]

[0119] The non-aqueous electrolyte secondary battery according to one embodiment can be manufactured by placing the separator and the electrolyte between the negative electrode manufactured as described above and the positive electrode manufactured as described above. More specifically, the separator is placed between the negative electrode and the positive electrode to form an electrode assembly, the electrode assembly is placed in a battery case such as a cylindrical battery case or a prismatic battery case, and the electrolyte is injected to manufacture the non-aqueous electrolyte secondary battery. Alternatively, the non-aqueous electrolyte secondary battery can be manufactured by placing the obtained product obtained by stacking the electrode assemblies and wetting them in the electrolyte into the battery case, and then sealing it.

[0120] The battery case may be a commonly used battery case in the art. The battery case may be in a cylindrical shape such as a can, a prismatic shape, a bag shape, or a coin shape.

[0121] The lithium-ion secondary battery according to one embodiment can be used as a power source for small devices and as a unit battery for medium- to large-sized battery modules including battery cells. Preferred examples of the medium- to large-sized devices include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0122] Example

[0123] Hereinafter, examples and comparative examples will be described, but the present disclosure is not limited thereto. In addition, the mechanism described below is merely an exemplary speculation to help understand the present disclosure and is not intended to limit the present disclosure.

[0124] [Example 1]

[0125] (Manufacturing of negative electrode)

[0126] Amorphous SiO powder was doped with lithium by thermal doping to prepare silicon oxide powder (first silicon oxide powder A). The average particle size of the particles constituting the first silicon oxide powder A was 7.0 μm. Based on the total weight of the first silicon oxide powder A, the lithium content measured by inductively coupled plasma spectroscopy was 6 wt %. As a result of X-ray diffraction (XRD) measurement of the first silicon oxide powder A, a diffraction peak of the (111) plane of silicon (Si) was observed near 2θ=28.4°. The crystallite size of the silicon microcrystals calculated from the (111) peak using the Scherrer formula was about 9 nm. In addition, peaks derived from Li2SiO3 and Li2Si2O5 were observed on the XRD pattern.

[0127] Amorphous SiO powder (Sigma-Aldrich) was ground using a bead mill to prepare an undoped silicon oxide powder (second silicon oxide powder B). The particles constituting the second silicon oxide powder B had an average particle size of 1.6 μm, a lower limit of the particle size distribution of 0.3 μm, and an upper limit of the particle size distribution of 6.0 μm. An XRD pattern of the second silicon oxide powder B was measured, revealing no diffraction peak indicating a crystalline phase.

[0128] The first silicon oxide powder A and the second silicon oxide powder B were mixed in a weight ratio of 5:5 to obtain a negative electrode active material powder. To 85 parts by weight of the negative electrode active material powder, 5 parts by weight of carbon black as a conductive agent and 10 parts by weight of polyacrylate as a binder were added. Pure water was added as a solvent and mixed to obtain a negative electrode active material slurry. The negative electrode active material slurry was coated on copper foil, dried in a vacuum, and then pressed to a predetermined density to obtain a negative electrode.

[0129] (Battery Manufacturing)

[0130] A coin cell (half cell) was produced using metallic lithium as a counter electrode (ie, a positive electrode) of the obtained negative electrode.

[0131] [Example 2]

[0132] A coin cell was manufactured in the same manner as in Example 1, except that the weight ratio of the first silicon oxide powder A to the second silicon oxide powder B was 8:2.

[0133] [Example 3]

[0134] In the same manner as in Example 1, the first silicon oxide powder A and the second silicon oxide powder B are mixed in a weight ratio of 5:5 to obtain a mixed powder. Natural graphite is added to the mixed powder so that the weight ratio of the mixed powder to the natural graphite is 1:9 and mixed to obtain a negative electrode active material powder (i.e., the weight ratio of the first silicon oxide powder A, the second silicon oxide powder B and the natural graphite is 0.5:0.5:9). 1.0 parts by weight of carbon black as a conductive agent, 1.5 parts by weight of styrene-butadiene rubber (SBR) as a binder, and 1.5 parts by weight of carboxymethyl cellulose (CMC) as a thickener are added to 96 parts by weight of the negative electrode active material powder, and pure water as a solvent is added and mixed to obtain a negative electrode active material slurry. The negative electrode active material slurry is coated on a copper foil and dried in a vacuum, and then pressed to a predetermined density to obtain a negative electrode. Metallic lithium is used as the counter electrode (i.e., the positive electrode) of the negative electrode to manufacture a coin cell.

[0135] [Example 4]

[0136] A coin cell was manufactured in the same manner as in Example 1, except that the average particle size of the first silicon oxide powder A was adjusted to 4.2 μm.

[0137] [Example 5]

[0138] A coin cell was manufactured in the same manner as in Example 1, except that the average particle size of the second silicon oxide powder B was adjusted to 0.8 μm.

[0139] [Comparative Example 1]

[0140] A coin cell was manufactured in the same manner as in Example 1, except that only the first silicon oxide powder A was used instead of the second silicon oxide powder B (ie, the weight ratio of the first silicon oxide powder A to the second silicon oxide powder B was 10:0).

[0141] [Comparative Example 2]

[0142] A coin cell was manufactured in the same manner as in Example 1, except that the first silicon oxide powder A was not used and only the second silicon oxide powder B was used (ie, the weight ratio of the first silicon oxide powder A to the second silicon oxide powder B was 0:10).

[0143] [Comparative Example 3]

[0144] A coin battery was manufactured in the same manner as in Example 3, except that the second silicon oxide powder B was not used and only the first silicon oxide powder A was mixed with natural graphite (i.e., the weight ratio of the first silicon oxide powder A, the second silicon oxide powder B and the natural graphite was 1:0:9).

[0145] [Comparative Example 4]

[0146] A coin battery was manufactured in the same manner as in Example 3, except that the first silicon oxide powder A was not used and only the second silicon oxide powder B was mixed with natural graphite (i.e., the weight ratio of the first silicon oxide powder A, the second silicon oxide powder B and the natural graphite was 0:1:9).

[0147] [Comparative Example 5]

[0148] A coin cell was manufactured in the same manner as in Example 1, except that the average particle size of the first silicon oxide powder A was adjusted to 18 μm.

[0149] [Comparative Example 6]

[0150] A coin cell was manufactured in the same manner as in Example 1, except that the average particle size of the second silicon oxide powder B was adjusted to 5 μm.

[0151] [Evaluation Example 1: Initial Charge / Discharge Characteristics]

[0152] The coin cells manufactured by each Example and each Comparative Example were charged / discharged at a constant current of 0.2 C and a cutoff voltage of 1.5 V. The “initial capacity” is a value obtained by dividing the discharge capacity during the initial charge / discharge period by the weight (g) of the negative electrode active material powder used in each Example and each Comparative Example, and is defined as follows:

[0153] [Formula 1]

[0154]

[0155] In addition, the charge / discharge efficiency during the initial charge / discharge period (hereinafter referred to as “initial efficiency”) is defined as follows:

[0156] [Formula 2]

[0157]

[0158] [Evaluation Example 2: Capacity Retention Rate]

[0159] After the initial charge / discharge in Evaluation Example 1, the coin cells manufactured in each Example and each Comparative Example were charged / discharged again under the same conditions, and then the charge / discharge was repeated for another 48 cycles at a constant current of 0.5 C. That is, the charge / discharge was repeated for a total of 50 cycles, including the first and second charge / discharge cycles. The capacity retention rate during repeated charge / discharge was defined as follows:

[0160] [Formula 3]

[0161]

[0162] The initial capacity, initial efficiency, and capacity retention calculated for each coin cell manufactured from each Example and each Comparative Example are as follows. The table also shows the weight ratio of the first silicon oxide powder A to the second silicon oxide powder B ("A:B" and "B / A"), as well as the weight ratio of the silicon oxide powder to natural graphite ("Silicon Oxide:Graphite").

[0163] [Table 1]

[0164]

[0165] First, Examples 1 and 2, which do not use natural graphite, and Comparative Examples 1 and 2 were compared. Comparative Example 1, which used only the first silicon oxide powder A, had the lowest initial capacity (1355 mAh / g), and the initial capacity increased as the ratio of the second silicon oxide powder B to the first silicon oxide powder A increased. On the other hand, Comparative Example 2, which used only the second silicon oxide powder B, had the lowest initial efficiency (75.4%), and the initial efficiency increased as the ratio of the second silicon oxide powder B to the first silicon oxide powder A decreased. Furthermore, similarly to the initial capacity, Comparative Example 1, which used only the first silicon oxide powder A, had the lowest capacity retention rate (71.3%), and the capacity retention rate increased as the ratio of the second silicon oxide powder B to the first silicon oxide powder A increased.

[0166] Comparative Example 1, which used the first silicon oxide powder A alone, had high initial efficiency but low initial capacity and capacity retention. Furthermore, Comparative Example 2, which used the second silicon oxide powder B alone, had high initial capacity and capacity retention, but low initial efficiency. Therefore, Comparative Examples 1 and 2, which used either the first silicon oxide powder A or the second silicon oxide powder B, found it difficult to simultaneously achieve high initial efficiency and excellent discharge capacity and capacity retention.

[0167] In Examples 1 and 2 including the first silicon oxide powder A and the second silicon oxide powder B, none of the initial capacity, initial efficiency, and capacity retention ratio was too bad, thereby achieving high initial efficiency and excellent discharge capacity and capacity retention ratio in a balanced manner.

[0168] Figure 1 The initial capacity (○), initial efficiency (△), and capacity retention (□) as a function of the weight ratio (A:B) of the first silicon oxide powder A to the second silicon oxide powder B in Examples 1 and 2 and Comparative Examples 1 and 2 not using natural graphite are plotted. Figure 1As shown, the graphs of the initial capacity (○) and initial efficiency (△) as a function of A:B are almost linear. That is, the initial capacity increases almost linearly as A:B changes from 10:0 to 0:10, while the initial efficiency decreases almost linearly as A:B changes from 10:0 to 0:10. On the other hand, the graph of the capacity retention rate as a function of A:B (□) does not show a simple proportional relationship, but rather an upward-bending curve. That is, compared with Comparative Example 1 in which only the first silicon oxide powder A is used as the negative electrode active material, the capacity retention rate is significantly improved even when the first silicon oxide powder A is mixed with a small amount of the second silicon oxide powder B. For example, although the first silicon oxide powder A, which may cause cycle degradation, occupies half of the negative electrode active material, Example 1 with a weight ratio of A:B of 5:5 shows a capacity retention rate of 96%, which is much higher than the capacity retention rate (71%) of Comparative Example 1 in which only the first silicon oxide powder A is used as the negative electrode active material. The results show that when the first silicon oxide powder A and the second silicon oxide powder B are mixed, the capacity retention rate is improved beyond expectation due to a synergistic effect.

[0169] The mechanism of the synergistic effect is explained, for example, as follows. However, the following description is merely an exemplary speculation to help understand the present disclosure and is not intended to limit the present disclosure.

[0170] In various embodiments, the first silicon oxide powder A and the second silicon oxide powder B are mixed. Compared to the lithium-doped first silicon oxide powder A, the undoped second silicon oxide powder B facilitates rapid lithium intercalation and alloying due to its high lithium intercalation capacity. Presumably, this alloying reduces the resistance of the second silicon oxide powder B, allowing lithium to diffuse smoothly from the second silicon oxide powder B to the adjacent first silicon oxide powder A. Consequently, the surface resistance of the first silicon oxide powder A is significantly reduced, and the charge / discharge process proceeds smoothly, contributing to improved lifespan characteristics.

[0171] Furthermore, when the average particle size of the particles comprising the second silicon oxide powder B is smaller than the average particle size of the particles comprising the first silicon oxide powder A, the smaller second silicon oxide powder B can easily enter the spaces between the larger first silicon oxide powder A. Consequently, the overall density of the negative electrode active material, and thus the charge / discharge capacity per unit weight, can be increased. Furthermore, it is speculated that this increases the substantial contact area between the first silicon oxide powder A and the second silicon oxide powder B, and that lithium diffusion, i.e., the charge / discharge of the battery, becomes smoother, thereby improving battery life characteristics.

[0172] Subsequently, Example 3, Comparative Examples 3, and Comparative Examples 4, all of which used natural graphite, were compared. Similar to the case without natural graphite, Comparative Example 3, which used only the first silicon oxide powder A, had the lowest initial capacity and capacity retention (464 mAh / g, 92.7%). These initial capacity and capacity retention increased as the ratio of the second silicon oxide powder B to the first silicon oxide powder A increased. On the other hand, Comparative Example 4, which used only the second silicon oxide powder B, had the lowest initial efficiency (86.9%), and the initial efficiency increased as the ratio of the second silicon oxide powder B to the first silicon oxide powder A decreased.

[0173] Figure 2 For Figure 1 In the same way, the graphs of initial capacity (○), initial efficiency (△), and capacity retention rate (□) as a function of the weight ratio (A:B) of the first silicon oxide powder A and the second silicon oxide powder B in Example 3 and Comparative Examples 3 and 4 using natural graphite are plotted. In the same way as when natural graphite is not used, the graph of initial capacity (○) as a function of A:B is almost linear. That is, as A:B changes from 10:0 to 0:10, the initial capacity increases almost linearly. On the other hand, in the same way as when natural graphite is not used, the graph of capacity retention rate (□) as a function of A:B does not show a simple proportional relationship, but an upward-bending curve. That is, by adding the second silicon oxide powder B, the capacity retention rate is significantly improved compared to Comparative Example 3 using only the first silicon oxide powder A as the negative electrode active material. Therefore, in the case of using natural graphite, when the first silicon oxide powder A and the second silicon oxide powder B are mixed, the capacity retention rate is also improved beyond expectations.

[0174] Furthermore, in contrast to the case where natural graphite is not used, the graph of initial efficiency (Δ) as a function of the A:B ratio does not exhibit a simple proportional relationship, but rather an upwardly curved curve. That is, despite the addition of the second silicon oxide powder B to the first silicon oxide powder A, the initial efficiency does not decrease linearly, but rather decreases more gradually as a function of the A:B ratio. In other words, by adding the first silicon oxide powder A to the second silicon oxide powder B, the initial efficiency is significantly improved compared to Comparative Example 4, which used only the second silicon oxide powder B as the negative electrode active material. These results demonstrate that mixing the first silicon oxide powder A and the second silicon oxide powder B in the presence of the carbon material produces a synergistic effect, resulting in an initial efficiency exceeding expectations.

[0175] The mechanism of the synergistic effect in the presence of the carbon material is explained as follows. However, the following description is merely an exemplary speculation to help understand the present disclosure and is not intended to limit the present disclosure.

[0176] The reason for the high initial efficiency achieved by adding the carbon material is speculated to be that when a carbon material such as graphite, whose volume change due to charge / discharge is smaller than that of silicon oxide, is mixed in, the volume change of the electrode during the first cycle of charging, when the change due to charging is the largest, is smaller than when silicon oxide is used alone as the negative electrode active material. Furthermore, the improved capacity retention is attributed to the deformability and high electron conductivity of the carbon material. Specifically, silicon oxide is hard and does not deform when the electrode is pressed. Therefore, when the negative electrode active material is composed of silicon oxide, even if the first silicon oxide powder A and the second silicon oxide powder B of different particle sizes are mixed, voids that could disrupt the conductive path are easily formed in some areas of the electrode. It is speculated that when graphite, which is soft and easily deformed by pressing, is mixed in, particularly natural graphite, such voids are significantly reduced. Furthermore, it is speculated that because natural graphite, which has high electron conductivity, is in close contact with silicon oxide, which has relatively low electron conductivity, natural graphite helps improve the intercalation and deintercalation of lithium ions in silicon oxide. Therefore, it is speculated that the addition of the carbon material improves the first cycle charging and lifespan characteristics, namely, capacity retention.

[0177] On the other hand, Examples 1 and 2 and Comparative Examples 1 and 2, which do not use natural graphite, differ from Example 3 and Comparative Examples 3 and 4, which use natural graphite, in the types of binder and thickener used. However, the selection of an appropriate binder and thickener is determined solely by whether or not natural graphite is used. This difference does not significantly affect the initial capacity, initial efficiency, or capacity retention of the battery.

[0178] Also in the case of Examples 4 and 5, compared with Example 1, the particle size of each of A and B was reduced, and the initial efficiency was slightly reduced, but in the same manner as in Example 1, it was found that the initial capacity and life (capacity retention rate) were good. On the other hand, as in Comparative Example 5, when the particle size of A became larger, electrode damage due to expansion occurred from the first cycle of charge / discharge, and thus even if B was added, a conductive path between the particles could not be obtained and the life characteristics were significantly deteriorated. In addition, as in Comparative Example 6, when the particle size of B became larger, it was difficult to uniformly fill the gaps between the particles, and due to the expansion of B itself, the degradation of the electrode was more serious than in Example 1, resulting in deterioration of the life.

Claims

1. A negative electrode active material for a lithium ion secondary battery, comprising: a first silicon oxide powder doped with at least one of an alkali metal and an alkaline earth metal; and undoped second silicon oxide powder, wherein the second silicon oxide powder is amorphous, and The average particle size of the particles constituting the first silicon oxide powder is greater than the average particle size of the particles constituting the second silicon oxide powder, the average particle size of the particles constituting the first silicon oxide powder is 3 μm or more and 15 μm or less, and the average particle size of the particles constituting the second silicon oxide powder is 0.5 μm or more and 2 μm or less. 2 . The negative electrode active material according to claim 1 , wherein a weight ratio of the second silicon oxide powder to the first silicon oxide powder is equal to or greater than 0.

2. 3 . The negative electrode active material according to claim 1 , wherein a weight ratio of the second silicon oxide powder to the first silicon oxide powder is less than 10. 4 . The negative electrode active material according to claim 1 , wherein the first silicon oxide powder comprises silicon fine crystals having a crystallite size of 5 nm or more and 30 nm or less. 5 . The negative electrode active material according to claim 1 , wherein the first silicon oxide powder comprises at least one of the following: Li 2 SiO 3 , Li 2 Si 2 O 5 , Li 4 SiO 4 , and Mg 2 SiO 4 . 6 . The negative electrode active material according to claim 1 , further comprising a carbon material powder comprising at least one of natural graphite, artificial graphite, graphitized carbon fiber, and amorphous carbon. 7 . A negative electrode for a lithium ion secondary battery, comprising a negative electrode active material layer formed on a negative electrode current collector, the negative electrode active material layer comprising the negative electrode active material according to claim 1 . 8 . A lithium ion secondary battery comprising the negative electrode according to claim 7 .

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