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

Abstract

Disclosed are a negative electrode active material for a lithium secondary battery and a lithium secondary battery comprising the same. A negative electrode active material for a lithium secondary battery according to an embodiment of the present disclosure comprises composite particles, the composite particles comprising: carbon-based particles comprising pores; and a silicon-containing coating layer which is provided on the surface of the carbon-based particle, and in which the ratio of the number of oxygen atoms to the number of silicon atoms (O / Si atomic ratio) measured by X-ray photoelectron spectroscopy (XPS) analysis at a depth of 100 nm from the surface of the composite particle toward the center portion is 0.13 to 0.6. The negative electrode active material for a lithium secondary battery according to the present disclosure has improved lifespan characteristics.

Description

Technical Field

[0001] The disclosure of this application relates to a negative electrode active material for lithium secondary batteries, its preparation method, and a lithium secondary battery containing the same. Background Technology

[0002] Rechargeable batteries are batteries that can be repeatedly charged and discharged. With the development of the information communication and display industries, rechargeable batteries are widely used as power sources for portable electronic communication devices such as cameras, mobile phones, and laptops. Moreover, in recent years, battery packs incorporating rechargeable batteries have been developed for use as power sources for environmentally friendly vehicles such as electric vehicles.

[0003] Examples of secondary batteries include lithium batteries, nickel-cadmium batteries, and nickel-metal hydride batteries. Among them, lithium batteries have been actively developed and applied due to their high operating voltage and energy density per unit weight, as well as their advantages in charging speed and lightweight design.

[0004] Recently, with the expansion of applications for lithium-ion batteries, lithium-ion batteries with higher capacity and power are being developed. For example, high-capacity silicon and carbon composites can be used as negative electrode active materials.

[0005] However, the volume expansion rates of silicon-carbon composite anode active materials vary greatly. Therefore, repeated charging and discharging may cause cracks in the anode active material, which may then be exposed to the electrolyte. Summary of the Invention

[0006] Technical issues

[0007] According to one aspect of this disclosure, a negative electrode active material for lithium secondary batteries with improved lifespan characteristics can be provided.

[0008] According to another aspect of this disclosure, a lithium secondary battery with improved lifespan characteristics can be provided.

[0009] According to another aspect of this disclosure, a method for preparing a negative electrode active material for lithium secondary batteries with improved lifespan characteristics can be provided.

[0010] Technical solution

[0011] A negative electrode active material for lithium secondary batteries is provided, comprising composite particles, the composite particles comprising: carbon-based particles containing pores; and a silicon-containing coating disposed on the surface of the carbon-based particles, wherein the ratio of oxygen atoms to silicon atoms (O / Si atomic ratio) measured by X-ray photoelectron spectroscopy (XPS) at a depth of 100 nm from the surface of the composite particles toward the center is 0.13 to 0.6.

[0012] In some embodiments, the O / Si atomic ratio can be from 0.16 to 0.43.

[0013] In some embodiments, the size of the pores in the carbon-based particles can be from 0.1 nm to 10 nm.

[0014] In some embodiments, the size of the pores in the carbon-based particles can be from 1 nm to 5 nm.

[0015] In some embodiments, the composite particles may further comprise a carbon coating disposed on the silicon-containing coating.

[0016] In some embodiments, the carbon coating may comprise at least one selected from amorphous carbon and conductive polymers.

[0017] In some embodiments, the pores of the carbon-based particles may include a shape that is recessed from the outermost periphery of the carbon-based particles toward the interior of the carbon-based particles.

[0018] A lithium secondary battery is provided, comprising: a negative electrode containing the aforementioned negative electrode active material for lithium secondary batteries; and a positive electrode opposite to the negative electrode.

[0019] A method for preparing a negative electrode active material for lithium secondary batteries is provided, comprising: a step of preparing carbon-based particles containing pores; and a step of calcining the carbon-based particles and a silicon-containing gas three times at different temperatures to form composite particles containing a silicon-containing coating formed on the surface of the carbon-based particles, wherein the ratio of oxygen atoms to silicon atoms (O / Si atomic ratio) measured by X-ray photoelectron spectroscopy (XPS) at a depth of 100 nm from the surface of the composite particles toward the center is 0.13 to 0.6.

[0020] In some embodiments, the silicon-containing gas may include silane gas.

[0021] In some embodiments, the calcination may include a first calcination, a second calcination at a temperature higher than that of the first calcination, and a third calcination at a temperature higher than that of the second calcination.

[0022] In some embodiments, the first calcination, the second calcination, and the third calcination may be performed sequentially.

[0023] In some embodiments, the first calcination may be carried out at 400°C to 450°C.

[0024] In some embodiments, the second calcination can be carried out at 500°C to 550°C.

[0025] In some embodiments, the third calcination may be carried out at 600°C to 650°C.

[0026] Technical effect

[0027] According to one embodiment of this disclosure, battery swelling caused by side reactions of the negative electrode active material and electrolyte can be suppressed, and the life characteristics of the secondary battery can be improved.

[0028] According to one embodiment of the present disclosure, the chemical stability of the negative electrode active material can be improved, and the life characteristics of the secondary battery can be enhanced.

[0029] The negative electrode active material for lithium secondary batteries disclosed herein, and the lithium secondary battery containing the same, can be widely used in green technology fields such as electric vehicles, battery charging stations, and other battery-powered solar and wind power generation. The negative electrode active material for lithium secondary batteries disclosed herein, and the lithium secondary battery containing the same, can be used in eco-friendly electric vehicles, hybrid vehicles, and other applications that prevent climate change by suppressing air pollution and greenhouse gas emissions. Attached Figure Description

[0030] Figure 1 A schematic cross-sectional view is provided to illustrate composite particles according to an exemplary embodiment.

[0031] Figure 2 and Figure 3 The figures shown are a schematic top view and a cross-sectional view of a lithium secondary battery according to an exemplary embodiment. Detailed Implementation

[0032] Embodiments of this disclosure provide a negative electrode active material (hereinafter referred to as "negative electrode active material") for lithium secondary batteries comprising composite particles. Furthermore, a lithium secondary battery (hereinafter referred to as "secondary battery") comprising the negative electrode active material is provided.

[0033] The embodiments of this disclosure will be described in more detail below. However, this is merely exemplary, and this disclosure is not limited to the specific implementations described herein.

[0034] Figure 1 A schematic cross-sectional view illustrating composite particles in an exemplary embodiment.

[0035] For ease of explanation, Figure 1 The shape of the composite particles is schematically shown; however, the structure / shape of the composite particles disclosed herein is not limited to this. Figure 1As shown. For example, the cross-section of carbon-based particles can also vary randomly from a circle. Moreover, silicon-containing coatings can also be partially formed on the pores and surface of carbon-based particles, and can also be formed as multiple discontinuous islands or patterns.

[0036] Reference Figure 1 The composite particles 50 may include carbon-based particles 60 and a silicon (Si) coating 70. For example, the negative electrode active material may include multiple composite particles 50.

[0037] In embodiments of this disclosure, the carbon-based particles 60 may include pores 65. For example, the carbon-based particles 60 may be porous particles containing a plurality of pores.

[0038] In some embodiments, the carbon-based particles 60 may include activated carbon, carbon nanotubes, carbon nanowires, graphene, carbon fibers, carbon black, graphite, porous carbon, cryogel, xerogel, aerogel, etc. These can be used alone or in combination of two or more.

[0039] In some embodiments, the carbon-based particles 60 may include an amorphous structure or a crystalline structure.

[0040] According to one embodiment, the carbon-based particles 60 may include an amorphous structure. In this case, the durability of the negative electrode active material is increased, thereby suppressing cracking during charging and discharging or when subjected to external impacts. This improves the lifespan characteristics of the secondary battery.

[0041] A silicon-containing coating 70 can be formed on the surface of the carbon-based particles 60 containing pores 65. For example, the pores 65 can mitigate the volume expansion of the silicon contained in the silicon-containing coating 70. Thus, the relatively high capacity characteristics of silicon can be utilized, while preventing cracking caused by the difference between the volume expansion rate of carbon (e.g., less than about 150 vol%) and the volume expansion rate of silicon (e.g., more than about 400 vol%) during battery charging and discharging. Therefore, battery expansion caused by side reactions of the negative electrode active material and the electrolyte can be suppressed, and the life characteristics of the secondary battery can be improved.

[0042] The pore 65 of the carbon-based particle 60 may include a shape that is recessed from the outermost part of the carbon-based particle 60 into the interior of the carbon-based particle 60. For example, the pore 65 may include an open pore that opens toward the outside of the carbon-based particle 60.

[0043] The terms “surface of carbon-based particles” and / or “surface of carbon-based particles 60” as used in this specification may refer to the outer surface 62 of carbon-based particles 60, the inner surface 67 of pores 65, or the outer surface 62 of carbon-based particles 60 and the inner surface 67 of pores 65.

[0044] In some embodiments, the size of the pores 65 of the carbon-based particles 60 can be 0.1 nm to 10 nm, 0.5 nm to 8 nm, or 1 nm to 5 nm. Within these ranges, excessive silicon deposition can be prevented, thereby further suppressing cracking of the negative electrode active material during the charging and discharging of the secondary battery.

[0045] The size of the hole 65 refers to the diameter of the inlet of the hole 65 formed on the surface of the carbon-based particle 60.

[0046] For example, a silicon-containing coating 70 may be formed on at least a portion of the outer surface 62 of the carbon-based particles 60.

[0047] For example, a silicon-containing coating 70 may be formed on at least a portion of the inner surface 67 of the pores 65 of the carbon-based particles 60.

[0048] For example, a silicon-containing coating 70 may be formed on at least a portion of the outer surface 62 of the carbon-based particles 60 and at least a portion of the inner surface 67 of the pores 65.

[0049] In some embodiments, the silicon-containing coating 70 may comprise amorphous silicon. This can improve the stability of the composite particles and enhance the lifespan characteristics of the secondary battery.

[0050] According to one embodiment, the silicon-containing coating 70 may not contain crystalline silicon. This can further improve the stability of the composite particles and further enhance the lifespan characteristics of the secondary battery.

[0051] In some embodiments, the silicon-containing coating 70 may comprise silicon, and may also optionally comprise SiO2. x (0 < x < 4).

[0052] According to one embodiment, the composite particles 50 may not contain silicon carbide (SiC). In this case, the capacity thermal properties and lifetime characteristics of the negative electrode active material can be improved.

[0053] In embodiments of this disclosure, the ratio of oxygen atoms to silicon atoms (O / Si atom ratio) measured by X-ray photoelectron spectroscopy (XPS) at a depth of 100 nm from the surface of the composite particle 50 toward the center can be from 0.13 to 0.6.

[0054] In some embodiments, the ratio of oxygen atoms to silicon atoms (O / Si atom ratio) can be from 0.16 to 0.43. Within this range, the chemical stability of the composite particles 50 can be improved and its lifetime characteristics can be enhanced.

[0055] If the O / Si atomic ratio is less than 0.13, the chemical stability of the composite particles 50 will decrease, which may lead to the generation of gas in the negative electrode slurry and reduce the life characteristics of the secondary battery.

[0056] If the O / Si atomic ratio is greater than 0.6, the exposed area of ​​silicon increases, which may lead to an increase in the volume expansion rate of the negative electrode and a reduction in lifetime characteristics.

[0057] For example, the O / Si atomic ratio can represent the degree of silicon oxidation of the composite particles 50.

[0058] For example, the O / Si atomic ratio at a depth of more than 0 nm and less than 100 nm from the surface to the center of the composite particle 50 is subject to large fluctuations due to external environmental influences, which may reduce the reliability of judging the degree of silicon oxidation of the composite particle 50.

[0059] According to embodiments of this disclosure, the O / Si atomic ratio can be measured at a depth of 100 nm from the surface of the composite particle 50 toward its center. This improves the reliability of the evaluation of the degree of silicon oxidation of the composite particle 50.

[0060] For example, the O / Si atomic ratio can be measured based on multiple peaks obtained by deconvolution of the XPS Si2p spectrum of the composite particles 50.

[0061] The deconvolution can be described as an analytical method that fits the Si2p spectrum obtained through XPS analysis into multiple peaks.

[0062] For example, depth profiling can be performed using sputtering from the surface of the composite particle 50 along the central direction to a depth of 100 nm, and XPS Si2p spectra can be obtained through XPS analysis. For the XPS Si2p spectra, a baseline can be set using the Shirley method in the range of 96 eV to 110 eV, and peak fitting, i.e., deconvolution, can be performed, for example, using a Gaussian-Lorentzian mixing function. By deconvolving the XPS Si2p spectra, Si can be obtained. 0 Si 1+ Si 2+ Si 3+ and Si 4+ Each has its corresponding 2p3 / 2 peak and 2p1 / 2 peak (a total of 10 peaks).

[0063] The relative peak area of ​​the obtained peak can be expressed in terms of Si. 0 Si 1+ Si2+ Si 3+ and Si 4+ The relative proportions of Si atoms in each oxidation state, and the sum of all peak areas, can be a value proportional to the total number of Si atoms, i.e., "total number of Si atoms × proportionality constant (C)". Si atoms in each oxidation state can be bonded to 0, 0.5, 1, 1.5, and 2 oxygen atoms, with Si:O ratios of 1:0, 1:0.5, 1:1, 1:1.5, and 1:2, respectively. By multiplying the peak area corresponding to each oxidation state of Si atoms by the aforementioned number of bonds, such as the Si-O bond number, a value proportional to the total number of oxygen atoms can be calculated, i.e., "total number of Si atoms × proportionality constant (C)". Dividing the calculated value proportional to the total number of oxygen atoms by the value proportional to the total number of Si atoms results in the proportionality constants canceling each other out, thus allowing the calculation of the O / Si atom ratio.

[0064] In some embodiments, the composite particles 50 may further include a carbon coating (not shown) formed on the silicon-containing coating 70. This prevents the silicon in the negative electrode active material from contacting water. Consequently, from the preparation of the negative electrode active material until the formation of the negative electrode, the reduction in the discharge capacity and capacity efficiency of the secondary battery can be suppressed.

[0065] In some embodiments, a carbon coating may also be formed on the portion of the carbon-based particle 60 on which the silicon-containing coating 70 is not formed. For example, the carbon coating may completely cover both the carbon-based particle 60 and the silicon-containing coating 70. This improves the mechanical and chemical stability of the negative electrode active material.

[0066] In one embodiment, the carbon coating may comprise at least one of carbon and a conductive polymer. For example, the carbon may include amorphous carbon. For example, the conductive polymer may include polyacetylene, polyaniline, polypyrrole, polythiophene, etc.

[0067] Hereinafter, a method for preparing the above-described negative electrode active material for lithium secondary batteries according to exemplary embodiments is provided.

[0068] In embodiments of this disclosure, carbon-based particles 60 comprising pores 65 may be prepared.

[0069] In some embodiments, primary carbon-based particles and additives may be mixed, heat-treated, and cleaned to form carbon-based particles 60.

[0070] In one embodiment, the primary carbon-based particles may include at least one selected from glucose, sucrose, cellulose, petroleum-based pitch, coal-based pitch, biomass, and resol oligomer.

[0071] In some embodiments, the additive may be provided as a chemical etchant or a hard template.

[0072] In one embodiment, the chemical etchant may include alkaline and / or acidic agents such as potassium hydroxide (KOH), potassium acetate, potassium carbonate (K₂CO₃), sodium hydroxide (NaOH), sodium carbonate (Na₂CO₃), ammonia (NH₄OH), and sulfuric acid (H₂SO₄). These may be used alone or in combination of two or more. For example, the reaction between the primary carbon-based particles and the additive may be carried out by a chemical activation method.

[0073] In one embodiment, the rigid template may include silica, polystyrene, etc. These can be used alone or in combination of two or more. For example, the rigid template may be provided as an additive for forming pores. For example, the size of the pores 65 may be adjusted according to the particle size of the rigid template.

[0074] In one embodiment, the heat treatment can be performed at 600°C to 900°C. Within this range, the size and / or volume of the hole 65 can be appropriately controlled.

[0075] In one embodiment, the cleaning can be performed by adding an acidic or alkaline solution to the mixture. For example, the acidic solution may include hydrochloric acid (HCl) solution, sulfuric acid (H₂SO₄) solution, etc. The alkaline solution may include sodium hydroxide (NaOH) solution, etc.

[0076] In an exemplary embodiment, carbon-based particles 60 and silicon-containing gas can be calcined together three times at different temperatures to form composite particles 50 including a silicon-containing coating 70 formed on the surface of carbon-based particles 60.

[0077] In some embodiments, the silicon-containing gas may include silane gas and inert gas. For example, the inert gas may include nitrogen (N2), argon (Ar), etc.

[0078] In some embodiments, the calcination may include a first calcination, a second calcination at a temperature higher than that of the first calcination, and a third calcination at a temperature higher than that of the second calcination.

[0079] For example, the first calcination, the second calcination, and the third calcination can be performed sequentially.

[0080] In some embodiments, the first calcination can be carried out at 400°C to 450°C. Within this range, the deposition rate of the silicon-containing coating is increased, and the O / Si atomic ratio can be controlled within an appropriate range.

[0081] In some embodiments, the second calcination can be carried out at a temperature of 500°C to 550°C. Within this range, the deposition rate of the silicon-containing coating is increased, while the O / Si atomic ratio can be controlled within an appropriate range.

[0082] In some embodiments, the third calcination can be performed at 600°C to 650°C. Within this range, the deposition rate of the silicon-containing coating is increased, while the O / Si atomic ratio can be controlled within an appropriate range.

[0083] The O / Si atomic ratio of the composite particles 50 formed by the above method can be from 0.13 to 0.6.

[0084] Figure 2 and Figure 3 These are schematic top and cross-sectional views, respectively, illustrating a lithium secondary battery according to an exemplary embodiment. For example, Figure 3 For along Figure 2 A cross-sectional view cut along line II′ in the thickness direction.

[0085] The lithium secondary battery may include: a negative electrode 130 containing the above-mentioned negative electrode active material; and a positive electrode 100 opposite to the negative electrode 130.

[0086] The positive electrode 100 may include: a positive electrode current collector 105; and a positive electrode active material layer 110 formed on at least one side of the positive electrode current collector 105.

[0087] The positive electrode current collector 105 may comprise stainless steel, nickel, aluminum, titanium, or alloys thereof. The positive electrode current collector 105 may also comprise aluminum or stainless steel surface-treated with carbon, nickel, titanium, or silver. For example, the thickness of the positive electrode current collector 105 may be from 10 μm to 50 μm.

[0088] The positive electrode active material layer 110 may contain a positive electrode active material. The positive electrode active material may contain a compound capable of reversibly inserting and deintercalating lithium ions.

[0089] According to an exemplary embodiment, the positive electrode active material may comprise a lithium nickel metal oxide. The lithium nickel metal oxide may further comprise at least one of cobalt (Co), manganese (Mn), and aluminum (Al).

[0090] In some embodiments, the positive electrode active material or the lithium nickel metal oxide may contain a layered structure or a crystal structure represented by the following chemical formula 1.

[0091] [Chemical Formula 1]

[0092] Li x Ni a M b O 2+z

[0093] In chemical formula 1, the values ​​can be 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b≤0.4, and -0.5≤z≤0.1. As mentioned above, M can include Co, Mn, and / or Al.

[0094] The chemical structure represented by Formula 1 indicates the bonding relationships contained in the layered or crystalline structure of the cathode active material, and does not exclude other additional elements. For example, M may include Co and / or Mn, and Co and / or Mn may be provided together with Ni as the main active element of the cathode active material. Formula 1 is provided to represent the bonding relationships of the main active element and should be understood as including the introduction and substitution of additional elements.

[0095] In one embodiment, auxiliary elements may also be included, which are added to the main active element to enhance the chemical stability of the positive electrode active material or the layered / crystal structure. The auxiliary elements may be mixed into the layered / crystal structure to form bonds; in this case, they should also be understood as being included within the chemical structure range represented by Formula 1.

[0096] For example, the auxiliary element may include at least one selected from Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Cu, Ag, Zn, B, Al, Ga, C, Si, Sn, Sr, Ba, Ra, P, and Zr. For example, the auxiliary element may act as an auxiliary active element, together with Co or Mn, to contribute to the capacity / power activity of the positive electrode active material; for example, Al.

[0097] For example, the positive electrode active material or the lithium nickel metal oxide may contain a layered structure or a crystal structure represented by the following chemical formula 1-1.

[0098] [Chemical Formula 1-1]

[0099] Li x Ni a M1 b1 M2 b2 O 2+z

[0100] In chemical formula 1-1, M1 may include Co, Mn and / or Al. M2 may include the aforementioned auxiliary elements. In chemical formula 1-1, the elements may be 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b1+b2≤0.4, and -0.5≤z≤0.1.

[0101] The positive electrode active material may also include coating elements or doping elements. For example, elements that are substantially the same as or similar to the auxiliary elements described above can be used as coating elements or doping elements. For example, two or more of the above elements may be used alone or in combination as coating elements or doping elements.

[0102] The coating element or doping element may exist on the surface of the lithium nickel metal oxide particles, or may be included in the bonding structure represented by the chemical formula 1 or chemical formula 1-1 through surface penetration of the lithium nickel metal composite oxide particles.

[0103] The positive electrode active material may include nickel-cobalt-manganese (NCM)-based lithium oxide. In this case, NCM-based lithium oxide with increased nickel content can be used.

[0104] Ni can serve as a transition metal related to the power and capacity of lithium secondary batteries. Therefore, as described above, by employing a high-content (High-Ni) composition in the positive electrode active material, a high-capacity positive electrode and a high-capacity lithium secondary battery can be provided.

[0105] However, with increasing Ni content, the long-term storage stability and lifetime stability of the cathode or secondary battery may relatively decrease, and side reactions with the electrolyte may also increase. However, according to exemplary embodiments, conductivity can be maintained by including Co, while lifetime stability and capacity retention characteristics can be improved by including Mn.

[0106] The Ni content in the NCM-based lithium oxide (e.g., the mole fraction of nickel in the total moles of nickel, cobalt, and manganese) can be 0.5 or more, 0.6 or more, 0.7 or more, or 0.8 or more. In some embodiments, the Ni content can be 0.8 to 0.95, 0.82 to 0.95, 0.83 to 0.95, 0.84 to 0.95, 0.85 to 0.95, or 0.88 to 0.95.

[0107] In some embodiments, the positive electrode active material may also include lithium cobalt oxide-based active material, lithium manganese oxide-based active material, lithium nickel oxide-based active material, or lithium iron phosphate (LFP)-based active material (e.g., LiFePO4).

[0108] In some embodiments, for example, the positive electrode active material may comprise lithium-rich layered oxide (LLO) / overLithiated oxide (OLO) based active materials, manganese-rich based active materials, cobalt-less based active materials, etc., having a chemical structure or crystal structure represented by Formula 2. These can be used alone or in combination of two or more.

[0109] [Chemical Formula 2]

[0110] p[Li₂MnO₃]·(1-p)[LiqJO₂]

[0111] In chemical formula 2, 0 < p < 1, 0.9 ≤ q ≤ 1.2, and J may include at least one element selected from Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg and B.

[0112] For example, a positive electrode slurry can be prepared by mixing the positive electrode active material in a solvent. The positive electrode active material layer 110 can be prepared by coating the positive electrode slurry onto at least one side of the positive electrode current collector 105, followed by drying and calendering. The coating may include gravure coating, slot die coating, multilayer simultaneous die coating, embossing, doctor blade coating, dip coating, bar coating, casting, and other methods. The positive electrode active material layer 110 may also contain a binder and optionally include conductive materials, thickeners, etc.

[0113] As the solvent, N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc. can be used.

[0114] The adhesive may include polyvinylidene fluoride (PVDF), poly(vinylidene fluoride-co-hexafluoropropylene) copolymer, polyacrylonitrile, polymethyl methacrylate, nitrile rubber (NBR), polybutadiene rubber (BR), styrene-butadiene rubber (SBR), etc. These can be used alone or in combination of two or more.

[0115] In one embodiment, a PVDF-based binder can be used as the positive electrode binder. In this case, the amount of binder used to form the positive electrode active material layer 110 can be reduced, and the amount of positive electrode active material can be relatively increased. This improves the power and capacity characteristics of the secondary battery.

[0116] The conductive material can be added to enhance the conductivity and / or the mobility of lithium ions or electrons in the positive electrode active material layer 110. For example, the conductive material may include carbon-based conductive materials such as graphite, carbon black, acetylene black, Ketjen black, graphene, carbon nanotubes (CNTs), vapor-grown carbon fiber (VGCF), and carbon fibers, and / or metallic conductive materials including perovskite materials such as tin, tin oxide, titanium oxide, LaSrCoO3, and LaSrMnO3. These can be used alone or in combination of two or more.

[0117] The positive electrode slurry may also contain thickeners and / or dispersants. In one embodiment, the positive electrode slurry may contain thickeners such as carboxymethyl cellulose (CMC).

[0118] The negative electrode 130 may include a negative electrode current collector 125 and a negative electrode active material layer 120 formed on at least one side of the negative electrode current collector 125.

[0119] For example, the negative electrode current collector 125 may include copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with conductive metal, etc. These can be used alone or in combination of two or more. For example, the thickness of the negative electrode current collector 125 can be from 10 μm to 50 μm.

[0120] The negative electrode active material layer 120 may contain a negative electrode active material comprising the aforementioned composite particles 50. For example, the negative electrode active material may comprise a plurality of composite particles 50.

[0121] In some embodiments, the negative electrode active material may comprise a plurality of composite particles 50 and a graphite-based active material. For example, the graphite-based active material may comprise artificial graphite and / or natural graphite.

[0122] In the total weight of the negative electrode active material (e.g., the total weight of multiple composite particles 50 and graphite-based active material), the content of composite particles 50 can be 1% or more by weight, 3% or more by weight, 5% or more by weight, 10% or more by weight, 15% or more by weight, 20% or more by weight, 25% or more by weight, 30% or more by weight, 35% or more by weight, 40% or more by weight, or 45% or more by weight.

[0123] In the total weight of the negative electrode active material, the weight of the composite particles can be less than 99% by weight, less than 97% by weight, less than 95% by weight, less than 90% by weight, less than 85% by weight, less than 80% by weight, less than 75% by weight, less than 70% by weight, less than 65% by weight, less than 60% by weight, less than 55% by weight, or less than 50% by weight.

[0124] In one embodiment, the negative electrode active material may substantially consist of composite particles 50 and the graphite-based active material.

[0125] A negative electrode slurry can be prepared by mixing the negative electrode active material in a solvent. The negative electrode slurry can be coated / deposited onto the negative electrode current collector 125, followed by drying and calendering to prepare the negative electrode active material layer 120. The coating may include gravure coating, slot die coating, multi-layer simultaneous die coating, embossing, doctor blade coating, dip coating, bar coating, casting, and other methods. The negative electrode active material layer 120 may also contain a binder and may optionally contain conductive materials, thickeners, etc.

[0126] The solvents contained in the negative electrode slurry may include water, pure water, deionized water, distilled water, ethanol, isopropanol, methanol, acetone, n-propanol, tert-butanol, etc. These can be used alone or in combination of two or more.

[0127] The aforementioned substances used in preparing the positive electrode 100 can be used as the binder, conductive material, and thickener.

[0128] In some embodiments, styrene-butadiene-rubber (SBR) based adhesives, carboxymethyl cellulose (CMC), polyacrylic acid based adhesives, and poly(3,4-ethylenedioxythiophene) (PEDOT) adhesives can be used as negative electrode binders. These can be used alone or in combination of two or more.

[0129] In an exemplary embodiment, a separator 140 may be inserted between the positive electrode 100 and the negative electrode 130. The separator 140 may be configured to prevent short circuits between the positive electrode 100 and the negative electrode 130 and to allow ion flow. For example, the thickness of the separator may be from 10 μm to 20 μm.

[0130] For example, diaphragm 140 may include a porous polymer membrane or a porous nonwoven fabric.

[0131] The porous polymer membrane may include polyolefin-based polymers, such as ethylene polymers, propylene polymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers. These can be used alone or in combination of two or more.

[0132] The porous nonwoven fabric may include high melting point glass fiber, polyethylene terephthalate fiber, etc.

[0133] The diaphragm 140 may also include a ceramic-based material. For example, inorganic particles may be coated on or dispersed within the polymer membrane to improve heat resistance.

[0134] The diaphragm 140 may have a single-layer or multi-layer structure including the polymer membrane and / or nonwoven fabric described above.

[0135] According to an exemplary embodiment, an electrode cell is defined by a positive electrode 100, a negative electrode 130, and a separator 140. An electrode assembly 150, for example, in the form of a jelly roll, can be formed by stacking multiple said electrode cells. For example, the electrode assembly 150 can be formed by winding, stacking, z-folding, or stack-folding of the separator 140.

[0136] The electrode assembly 150 can be housed together with the electrolyte in a housing 160 to define a lithium secondary battery. According to an exemplary embodiment, a non-aqueous electrolyte can be used as the electrolyte.

[0137] The non-aqueous electrolyte contains a lithium salt as the electrolyte and an organic solvent; for example, the lithium salt may be represented as Li. + X - The lithium salt anion (X - ) can be exemplified by F - Cl - ,Br - I - NO3 - N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 -, (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - etc.

[0138] For example, propylene carbonate (PC), ethylene carbonate (EC), butylene carbonate, diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, vinylene carbonate, methyl acetate (MA), ethyl acetate (EA), n-propylacetate (n-PA), 1,1-dimethylethyl acetate (DMEA), methyl propionate (MP), ethyl propionate (EP), ethyl fluoroacetate (FEA), ethyl difluoroacetate (DFEA), ethyl trifluoroacetate (TFEA), dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), and diethylene glycol dimethyl ether can be used as the organic solvent. These include ether (DEGDME), dimethoxyethane, tetrahydrofuran (THF), 2-methyltetrahydrofuran, ethyl alcohol, isopropyl alcohol, dimethyl sulfoxide, acetonitrile, sulfolane, γ-butyrolactone, and propylene sulfite, among others. These can be used alone or in combination of two or more.

[0139] The non-aqueous electrolyte may also contain additives. These additives may include, for example, cyclic carbonate compounds, fluorinated carbonate compounds, sulfonyl lactone compounds, cyclic sulfate compounds, cyclic sulfite compounds, phosphate compounds, and borate compounds. These may be used alone or in combination of two or more.

[0140] The cyclic carbonate compounds may include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), etc.

[0141] The fluorine-substituted cyclic carbonate compounds may include fluoroethylene carbonate (FEC), etc.

[0142] The sulfonyl compounds may include 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, etc.

[0143] The cyclic sulfate compounds may include 1,2-ethylene sulfate, 1,2-propylene sulfate, etc.

[0144] The cyclic sulfite compounds may include ethylene sulfite, butene sulfite, etc.

[0145] The phosphate compounds may include lithium difluorobis-oxalatophosphate, lithium difluorophosphate, etc.

[0146] The borate compounds may include lithium bis(oxalate) borate, etc.

[0147] In some embodiments, a solid electrolyte can be used instead of the non-aqueous electrolyte described above. In this case, the lithium secondary battery can be fabricated as an all-solid-state battery. Furthermore, a solid electrolyte layer can be disposed between the positive electrode 100 and the negative electrode 130 instead of the separator 140 described above.

[0148] The solid electrolyte may include a sulfide-based electrolyte. As a non-limiting example, the sulfide-based electrolyte may include Li₂S-P₂S₅, Li₂S-P₂S₅-LiCl, Li₂S-P₂S₅-LiBr, Li₂S-P₂S₅-LiCl-LiBr, Li₂S-P₂S₅-Li₂O, Li₂S-P₂S₅-Li₂O-LiI, Li₂S-SiS₂, Li₂S-SiS₂-LiI, Li₂S-SiS₂-LiBr, Li₂S-SiS₂-LiCl, Li₂S-SiS₂-B₂S₃-LiI, Li₂S-SiS₂-P₂S₅-LiI, Li₂S-B₂S₃, and Li₂S-P₂S₅-Z. m S n(m and n are positive numbers, Z is Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (p and q are positive numbers, M is P, Si, Ge, B, Al, Ga, or In), Li7-xPS6-xCl x (0≤x≤2), Li7-xPS6-xBr x (0≤x≤2), Li7-xPS6-xI x (0≤x≤2), etc. These can be used individually or in combination of two or more.

[0149] In one embodiment, the solid electrolyte may also include oxide-based amorphous solid electrolytes such as Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3, and Li2O-B2O3-ZnO.

[0150] like Figure 3 As shown, the tabs (positive tab and negative tab) can protrude from the positive current collector 105 and the negative current collector 125 included in the respective electrode cells and extend to one side of the housing 160. The tabs can be fused to said side of the housing 160 to form electrode leads (positive lead 107 and negative lead 127) extending or exposed to the outside of the housing 160.

[0151] For example, the lithium secondary battery can be made into a cylindrical, prismatic, pouch, or coin shape for use in containers.

[0152] The embodiments of this disclosure will be further illustrated below with specific experimental examples. The embodiments and comparative examples included in the experimental examples are for illustrative purposes only and do not limit the scope of the appended claims. Various changes and modifications can be made to the embodiments within the scope and technical concept of this disclosure, which will be obvious to those skilled in the art, and such variations and modifications are naturally within the scope of the appended claims.

[0153] Example 1

[0154] (1) Preparation of composite particles

[0155] 1) Preparation of carbon-based particles

[0156] Asphalt and potassium acetate were dry-mixed at a weight ratio of 5:5 and heat-treated at 800°C for two hours. The heat-treated mixture was then washed with excess 0.5M HCl aqueous solution to prepare porous carbon-based particles. The specific volume of the pores in the prepared carbon-based particles reached 0.6 cm³. 3 / g of carbon-based particles.

[0157] 2) Formation of a silicon-containing coating

[0158] A silicon-containing gas, comprising silane and nitrogen, is injected into the CVD coater at a flow rate of 50 mL / min to 100 mL / min. The silane gas content is 70% by volume relative to the total volume of the silicon-containing gas.

[0159] The CVD coater is heated to 400°C at a heating rate of 5°C / min to 20°C / min and held at 400°C for 30 minutes for the first calcination.

[0160] After the first calcination, the CVD coater is heated to 550°C at a heating rate of 5°C / min to 20°C / min and held at 550°C for 3 hours for the second calcination.

[0161] After the second calcination, the CVD coater is heated to 600°C at a heating rate of 5°C / min to 20°C / min and held at 600°C for 30 minutes for a third calcination, thereby preparing composite particles containing a silicon-containing coating.

[0162] The deposition time can be adjusted as described above to make the target silicon content reach 50% by weight relative to the total weight of the composite particles.

[0163] (2) Manufacturing the negative electrode

[0164] A negative electrode slurry is obtained by mixing 95.5% by weight of a negative electrode active material composed of 15% by weight of the composite particles and 80.5% by weight of artificial graphite, 1% by weight of carbon nanotubes (CNTs) as a conductive material, 2% by weight of styrene-butadiene rubber (SBR) as a binder, and 1.5% by weight of carboxymethyl cellulose (CMC) as a thickener.

[0165] The negative electrode slurry is coated onto a copper substrate, dried, and rolled to manufacture the negative electrode.

[0166] (3) Manufacturing lithium secondary batteries

[0167] Manufacturing a lithium secondary battery, the lithium secondary battery including the negative electrode and using lithium metal as the counter electrode (positive electrode).

[0168] Specifically, a separator (polyethylene, 20 μm thick) is inserted between the negative electrode and the lithium metal (1 mm thick) to form a CR2016 (20 mm in diameter, 1.6 mm thick) lithium coin half-cell.

[0169] The lithium metal / separator / anode assembly is placed in a coin cell plate, electrolyte is injected, and then the cap is placed on and clamped. The electrolyte used is a 1M LiPF6 solution prepared using a mixed solvent of EC / EMC (3:7 volume ratio), with 2.0% (v / v) of fluoroethylene carbonate (FEC) added relative to the total electrolyte volume. After clamping the lithium coin cell, it is allowed to stand for 3 to 24 hours, followed by three charge-discharge cycles at 0.1C (charging conditions: CC-CV 0.1C 0.01V 0.01C cut-off; discharging conditions: CC 0.1C 1.5V cut-off)).

[0170] Examples 2 to 10 and Comparative Examples 1 and 2

[0171] Except for changing the temperatures of the first, second, and third calcinations as shown in Table 1, composite particles, anodes, and lithium secondary batteries were prepared using the same method as in Example 1.

[0172] Example 11

[0173] Except for adding an equal amount of silica dispersion with an average particle size of 5 nm to replace potassium acetate, and using 0.5 M NaOH aqueous solution to replace HCl aqueous solution for cleaning, composite particles, negative electrode and lithium secondary battery were prepared according to the same method as in Example 1.

[0174] Example 12

[0175] Except for adding an equal amount of silica dispersion with an average particle size of 10 nm to replace potassium acetate, and using 0.5 M NaOH aqueous solution to replace HCl aqueous solution for cleaning, composite particles, negative electrode and lithium secondary battery were prepared according to the same method as in Example 1.

[0176] Example 13

[0177] Except for adding an equal amount of silica dispersion with an average particle size of 12 nm to replace potassium acetate, and using 0.5 M NaOH aqueous solution to replace HCl aqueous solution for cleaning, composite particles, negative electrode and lithium secondary battery were prepared according to the same method as in Example 1.

[0178] Comparative Example 3

[0179] Except for replacing the first, second, and third calcinations by heating the CVD coater to 450°C at a heating rate of 5°C / min to 20°C / min and holding it at 450°C for 4 hours, composite particles, anodes, and lithium secondary batteries were prepared according to the same method as in Example 1.

[0180] Comparative Examples 4 to 6

[0181] Except for changing the calcination temperature as shown in Table 1, composite particles, negative electrodes, and lithium secondary batteries were prepared using the same method as in Comparative Example 3.

[0182] Experimental Example

[0183] (1) XPS analysis - measurement of O / Si atomic ratio

[0184] The composite particles prepared according to the above examples and comparative examples were attached to carbon tape to prepare samples, and the samples were placed in an XPS device (ESCALAB 250Xi, Thermo Scientific).

[0185] XPS analysis was performed under the following conditions to obtain the XPS Si2p spectrum (hereinafter, surface spectrum) of the composite particle surface.

[0186] The composite particles were etched under the following etching conditions for the same sample, and XPS analysis was performed under the same XPS analysis conditions to obtain the XPS Si2p spectrum (hereinafter, 100nm spectrum) at a depth of 100nm from the surface to the center of the composite particles.

[0187] [XPS Analysis Conditions]

[0188] i) X-ray type: Al kα monochromatic source

[0189] ii) Source Energy (hv): 1486.68 eV

[0190] iii) Beam size: 500μm

[0191] iv) Analyzer: CAE (Constant Energy Analyzer) Mode

[0192] v) Number of scans: 50

[0193] vi) Pass energy: 20eV

[0194] vii) Dwell Time: 100ms

[0195] [Etching Conditions]

[0196] i) Type: Argon ion gun

[0197] ii) Ion energy: 4000 eV

[0198] iii) Raster size: 3mm

[0199] iv) Etch cycle time: 280 seconds

[0200] In the above etching, the etching depth was calculated based on the Ta2O5 data, and the etching rate was 0.3538 nm / s.

[0201] For the acquired XPS Si2p spectra, a baseline was set using the Shirley method in the range of 96 eV to 110 eV. Then, peak fitting (deconvolution) was performed using a Gaussian-Lorentzian mixture function. This separated the Si... 0 Si 1+ Si 2+ Si 3+ and Si 4+ Each has its corresponding 2p3 / 2 peak and 2p1 / 2 peak.

[0202] For the surface spectrum, the areas of the fitted Si 2p peaks are summed to calculate a value proportional to the total number of Si atoms, i.e., "total number of Si atoms × proportionality constant (C)". Since Si atoms in each oxidation state can bond with 0, 0.5, 1, 1.5, and 2 oxygen atoms, the peak area corresponding to each oxidation state of Si atoms is multiplied by the aforementioned number of bonds, i.e., the Si-O bond number, to calculate a value proportional to the total number of oxygen atoms, i.e., "total number of Si atoms × proportionality constant (C)".

[0203] Divide the calculated value proportional to the total number of oxygen atoms by the value proportional to the total number of Si atoms (the same proportionality constant cancels each other out) to calculate the O / Si atom ratio (surface).

[0204] For the 100 nm spectrum, the values ​​proportional to the total Si atoms and the total oxygen atoms were calculated using the same method as described above. The calculated value proportional to the total oxygen atoms was divided by the value proportional to the total Si atoms (the same proportionality constants cancel each other out) to calculate the O / Si atom ratio (100 nm).

[0205] (2) Measuring the pore size of carbon-based particles

[0206] The pore size of the carbon-based particles prepared according to the above examples and comparative examples was measured using a surface area analyzer (ASAP-2420) from Micromeritics.

[0207] Specifically, the pore size distribution curves were derived from the nitrogen gas sorption isotherm of the samples obtained from the examples and comparative examples using the BJH (Barrett-Joyner-Halenda) method. The position of the maximum peak in the pore size distribution curve was measured and thus used as the pore size of the carbon-based particles.

[0208] (3) Battery volume expansion rate

[0209] After fabricating three identical lithium coin-type half-cells, they were charged at 0.1C and 0.01V. The thickness of the negative electrode before and after charging was measured using a micrometer, and the average of the three values ​​was calculated. This average value was evaluated as the battery's volume expansion rate.

[0210] (4) Evaluate capacity retention rate (300 cycles)

[0211] The lithium secondary batteries of the examples and comparative examples were subjected to charge-discharge cycle tests at room temperature (25°C) under the following conditions. Charging was performed in CC-CV mode, charging to 0.01V at a current of 0.5C and then cutting off at 0.01C. Discharging was performed in CC mode, discharging to 3.0V at a current of 0.1C and then cutting off. After repeating this charge-discharge process 300 times, the capacity retention was evaluated by multiplying the discharge capacity of the 300th cycle by the discharge capacity of the first cycle by 100.

[0212] The measurement and evaluation results are shown in Tables 1 and 2.

[0213] In Table 2, "-" indicates that no significant XPS analysis results were detected.

[0214] Table 1

[0215]

[0216] Table 2

[0217]

[0218] Referring to Tables 1 and 2, in the embodiments where the O / Si atomic ratio of the composite particles at a depth of 100 nm from the surface of the composite particles to the center is 0.13 to 0.6, the volume expansion rate is reduced and the capacity retention rate is improved compared with the comparative example.

[0219] In Examples 4 and 5, where the first calcination temperature exceeds 400°C to 450°C, the capacity retention rate is relatively lower compared to other examples.

[0220] In Examples 7 and 8, where the second calcination temperature exceeds 500°C to 550°C, the capacity retention rate is relatively lower compared to other examples.

[0221] In Examples 9 and 10, where the third calcination temperature exceeds 600°C to 650°C, the volume expansion rate relatively increases and the capacity retention rate relatively decreases compared to other examples.

[0222] In Example 13, where the pore size of the carbon-based particles exceeds 10 nm, the capacity retention rate is relatively lower compared to other examples.

[0223] Compared with the O / Si atomic ratio (surface), the O / Si atomic ratio (100nm) is less affected by various external factors, thus ensuring the reliability of the silicon oxidation degree evaluation index of composite particles.

Claims

1. A negative electrode active material for lithium secondary batteries, wherein, The composite particles comprise: Carbon-based particles, including pores; and A silicon-containing coating is applied to the surface of the carbon-based particles. The ratio of oxygen atoms to silicon atoms, measured by X-ray photoelectron spectroscopy at a depth of 100 nm from the surface to the center of the composite particle, i.e., the O / Si atom ratio, is 0.13 to 0.

6.

2. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The O / Si atomic ratio is between 0.16 and 0.

43.

3. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The size of the pores in the carbon-based particles is from 0.1 nm to 10 nm.

4. The negative electrode active material for lithium secondary batteries according to claim 3, wherein, The pore size of the carbon-based particles is from 1 nm to 5 nm.

5. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The composite particles also include a carbon coating disposed on the silicon-containing coating.

6. The negative electrode active material for lithium secondary batteries according to claim 5, wherein, The carbon coating comprises at least one selected from amorphous carbon and conductive polymers.

7. The negative electrode active material for lithium secondary batteries according to claim 1, wherein, The pores of the carbon-based particles include a shape that is recessed from the outermost part of the carbon-based particles into the interior of the carbon-based particles.

8. A lithium secondary battery, wherein, include: The negative electrode comprises the negative electrode active material for lithium secondary batteries as described in claim 1; as well as The positive electrode is opposite to the negative electrode.

9. A method for preparing a negative electrode active material for lithium secondary batteries, wherein, include: Steps for preparing porous carbon-based particles; as well as The step of calcining the carbon-based particles and silicon-containing gas three times at different temperatures to form composite particles including a silicon-containing coating formed on the surface of the carbon-based particles. The ratio of oxygen atoms to silicon atoms, measured by X-ray photoelectron spectroscopy at a depth of 100 nm from the surface to the center of the composite particle, i.e., the O / Si atom ratio, is 0.13 to 0.

6.

10. The method for preparing the negative electrode active material for lithium secondary batteries according to claim 9, wherein, The silicon-containing gas includes silane gas.

11. The method for preparing the negative electrode active material for lithium secondary batteries according to claim 9, wherein, The calcination includes a first calcination, a second calcination at a temperature higher than that of the first calcination, and a third calcination at a temperature higher than that of the second calcination.

12. The method for preparing the negative electrode active material for lithium secondary batteries according to claim 11, wherein, The first calcination, the second calcination, and the third calcination are carried out in sequence.

13. The method for preparing the negative electrode active material for lithium secondary batteries according to claim 11, wherein, The first calcination is carried out at 400°C to 450°C.

14. The method for preparing the negative electrode active material for lithium secondary batteries according to claim 11, wherein, The second calcination is carried out at 500°C to 550°C.

15. The method for preparing the negative electrode active material for lithium secondary batteries according to claim 11, wherein, The third calcination is carried out at 600°C to 650°C.

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