Negative active material for lithium secondary battery, negative active material slurry containing the same, and lithium secondary battery

Abstract

According to an embodiment of the present invention, there is provided a negative electrode active material for a lithium secondary battery, which includes a silicon oxide having a carbon coating on its surface and doped with magnesium, wherein when measured by X-ray photoelectron spectroscopy (XPS), the ratio of the peak area at 1303 eV that appears in the Mg1s spectrum to the sum of the peak area at 1304.5 eV and the peak area at 1303 eV is 60% or less.

Description

Technical Field

[0001] The present invention relates to a negative electrode active material for a lithium secondary battery, a negative electrode active material slurry including the negative electrode active material, and a lithium secondary battery. More specifically, the present invention relates to a silicon oxide negative electrode active material, a negative electrode active material slurry including the silicon oxide negative electrode active material, and a lithium secondary battery. Background Art

[0002] A secondary battery is a battery that can be repeatedly charged and discharged. With the development of the information communication and display industries, secondary batteries are widely used as power sources for portable electronic communication devices such as portable cameras, mobile phones, and laptop computers. Recently, battery packs including secondary batteries have also been developed and applied to eco-friendly vehicles such as hybrid vehicles as their power sources.

[0003] Examples of secondary batteries may include lithium secondary batteries, nickel-cadmium batteries, nickel-metal hydride batteries, etc. In particular, when a lithium secondary battery is applied to a hybrid vehicle, it is advantageous in terms of charging speed and light weight due to its high operating voltage and energy density per unit weight.

[0004] Such a lithium secondary battery may include: an electrode assembly including a positive electrode, a negative electrode, and a separator; and an electrolyte solution immersing the electrode assembly. In addition, the lithium secondary battery may further include a housing in a pouch shape, for example, and the electrode assembly and the electrolyte solution are accommodated in the housing.

[0005] Recently, with the development of the electric vehicle industry, the development of lithium secondary batteries with high energy density has been carried out so that electric vehicles can travel a long distance on a single charge. Currently, the efficiency of silicon oxide used as a negative electrode active material for lithium secondary batteries applied in the art is low, which limits the increase in energy density. Therefore, various studies have been attempted to improve the energy density by doping silicon with metal magnesium.

[0006] For example, Korean Patent Registration No. 10-1783047 relates to a negative electrode material in which the surface of a powder containing at least one of silicon oxide and a silicon-silicon oxide composite is coated with carbon and then doped with magnesium. In this case, magnesium is exposed and remains on the surface of the negative electrode active material, which causes an increase in pH and shrinkage of a thickener during the preparation of the negative electrode active material slurry, making it difficult to manufacture a desired high-density electrode.

[0007] [Prior Art Documents]

[0008] [Patent Documents]

[0009] Korean Patent Registration Publication No. 10-1783047 Summary of the Invention

[0010] An object of the present invention is to provide a negative electrode active material for a lithium secondary battery having a high energy density and excellent electrode manufacturing stability.

[0011] In addition, another object of the present invention is to provide a negative electrode active material slurry including a negative electrode active material for a lithium secondary battery.

[0012] Furthermore, another object of the present invention is to provide a lithium secondary battery including a negative electrode active material for a lithium secondary battery.

[0013] To achieve the above object, according to one aspect of the present invention, there is provided a negative electrode active material for a lithium secondary battery, which includes silicon oxide, the silicon oxide includes a carbon coating formed on its surface, and is doped with magnesium, wherein when measured by X-ray photoelectron spectroscopy (XPS), the ratio of the peak area at 1303 eV appearing in the Mg1s spectrum to the sum of the peak areas at 1304.5 eV and 1303 eV is 60% or less.

[0014] In some embodiments, based on the total weight of the silicon oxide, the content of magnesium hydroxide remaining on the surface may be less than 0.05 wt%.

[0015] In some embodiments, based on the total weight of the silicon oxide, the content of magnesium hydroxide remaining on the surface may be 0 wt% or more and less than 0.05 wt%.

[0016] In some embodiments, based on the total weight of the silicon oxide, the content of doped magnesium may be 7 wt% to 17 wt%.

[0017] In some embodiments, based on the total weight of the silicon oxide, the carbon content in the coating may be 3 wt% to 12 wt%.

[0018] In some embodiments, the silicon oxide may be SiO x (0 < x < 2).

[0019] In some embodiments, the average particle size of the silicon crystals of the silicon oxide may be 20 nm or less.

[0020] In some embodiments, the average particle size of the silicon crystals of the silicon oxide may be 0.1 nm to 20 nm.

[0021] In some embodiments, the carbon in the coating may include amorphous carbon, carbon nanofibers, carbon nanotubes, graphite, graphene, graphene oxide, reduced graphene oxide, or a mixture thereof.

[0022] In addition, according to another aspect of the present invention, there is provided a negative electrode active material slurry for a lithium secondary battery including a negative electrode active material.

[0023] In some embodiments, the pH of the slurry may be less than 10.

[0024] In some embodiments, the pH of the slurry may be 7 or more and less than 10.

[0025] In some embodiments, the viscosity of the slurry may be 4000 cPs or more.

[0026] In some embodiments, the viscosity of the slurry may be from 4000 cPs to 9000 cPs.

[0027] In addition, according to another aspect of the present invention, there is provided a lithium secondary battery including: a positive electrode; and a negative electrode including a negative electrode active material for a lithium secondary battery.

[0028] According to the above exemplary embodiments, the negative electrode active material for a lithium secondary battery includes silicon oxide doped with magnesium, and the silicon oxide includes a carbon coating on its surface, wherein when measured by X-ray photoelectron spectroscopy (XPS), the ratio of the peak area at 1303 eV that appears in the Mg1s spectrum to the sum of the peak areas at 1304.5 eV and 1303 eV is 60% or less, so that stability during the preparation of the negative electrode active material slurry can be ensured.

[0029] According to some exemplary embodiments, the negative electrode active material slurry may have a pH maintained in a low range of 10 or less to prevent a decrease in the slurry viscosity, and by ensuring a viscosity of 4000 centipoise (cps) or more, it can contribute to ensuring the stability of the slurry.

[0030] According to some exemplary embodiments, the negative electrode active material slurry having excellent stability can provide excellent uniformity during the manufacture of the negative electrode.

[0031] According to some exemplary embodiments, the negative electrode active material for a secondary battery can realize a lithium secondary battery having high energy density characteristics and excellent electrochemical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] The above and other objects, features and other advantages of the present invention will be more clearly understood from the following detailed description in conjunction with the accompanying drawings, in which:

[0033] Figure 1 For showing photographs of slurries including negative electrode active materials according to Examples and Comparative Examples;

[0034] Figure 2 Photographs of coated electrodes according to the embodiments and comparative examples are shown. Detailed Description

[0035] According to an embodiment of the present invention, there is provided a negative electrode active material for a lithium secondary battery including silicon oxide doped with magnesium and having a carbon coating on its surface, wherein when measured by X-ray photoelectron spectroscopy (XPS), the ratio of the peak area at 1303 eV to the sum of the peak areas at 1304.5 eV and 1303 eV in the Mg1s spectrum is 60% or less.

[0036] Furthermore, according to an embodiment of the present invention, there is provided a lithium secondary battery including a positive electrode; and a negative electrode including a negative electrode active material for a lithium secondary battery.

[0037] Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, these embodiments are merely examples, and the present invention is not limited to the specific embodiments described as examples.

[0038] <Negative Electrode Active Material for Lithium Secondary Battery>

[0039] The negative electrode active material for a lithium secondary battery (hereinafter, may be abbreviated as the negative electrode active material) according to an embodiment of the present invention may include silicon oxide doped with magnesium. In addition, the surface of the silicon oxide may include a carbon coating, and magnesium may remain on the surface of the silicon oxide during the doping process. The magnesium remaining on the surface may be, for example, magnesium hydroxide Mg(OH)2. The content of magnesium hydroxide can be measured by the peak area appearing in the spectrum according to X-ray photoelectron spectroscopy (XPS) or by acid titration.

[0040] According to an embodiment of the present invention, the content of magnesium hydroxide remaining on the surface of the silicon oxide measured by X-ray photoelectron spectroscopy (XPS) may be provided as the ratio of magnesium hydroxide to the sum of magnesium oxide and magnesium hydroxide. In addition, the content of magnesium hydroxide can be confirmed by acid titration.

[0041] That is, the present invention aims to provide the ratio of magnesium hydroxide remaining on the surface of the silicon-based negative electrode active material measured according to an embodiment of the present invention, which is a technical indicator capable of confirming the electrode stability and electrochemical performance of the secondary battery.

[0042] According to an embodiment of the present invention, the negative electrode active material for a lithium secondary battery may include silicon oxide doped with magnesium, and the surface of the silicon oxide may include a carbon coating, wherein when measured by X-ray photoelectron spectroscopy (XPS), the ratio of the peak area at 1303 eV that appears in the Mg1s spectrum to the sum of the peak areas at 1304.5 eV and 1303 eV may be 60% or less.

[0043] According to one embodiment, based on the total weight of the silicon oxide, the content of magnesium hydroxide remaining on the surface may be less than 0.05 wt%, preferably 0 wt%. More specifically, 1 g of silicon oxide is added to 99 g of water, stirred for 3 minutes, and titrated with 0.1 M HCl while confirming the content of residual magnesium at room temperature, which can be calculated by the following Reaction Formula 1.

[0044] [Reaction Formula 1]

[0045] Equivalent point 1: Mg(OH)2 (aqueous solution) + 2HCl (aqueous solution) → MgCl2 (aqueous solution) + 2H2O (liquid)

[0046] Equivalent point 2: MgCO3 (aqueous solution) + 2HCl (aqueous solution) → MgCl2 (aqueous solution) + 2H2O (liquid) + CO2

[0047] In addition, the contents of MgCO3 and Mg(OH)2 can be calculated by measuring the content of 0.1 M HCl titrated from equivalent point 1 to equivalent point 2 according to the following formula.

[0048] (1) Mg(OH)2 content: ((titration volume (equivalent point 1) × HCl molar concentration × Mg(OH)2 molecular weight) / (sample weight))1 / 2

[0049] (2) MgCO3 content: ((titration volume (equivalent point 2 - equivalent point 1) × HCl molar concentration × MgCO3 molecular weight) / (sample weight))1 / 2

[0050] In some embodiments, when the value according to formula (1) is less than 0.05 wt%, no gas is generated during the preparation of the negative electrode active material slurry, thereby solving the problem of changes in the physical properties of the slurry.

[0051] During the preparation of the negative electrode active material slurry, Mg(OH)2 in the surface residual magnesium meets water, increasing the pH of the slurry. When the pH of the slurry increases, carboxymethyl cellulose (CMC), which is a thickening agent, shrinks, and the viscosity of the slurry decreases. Eventually, this may cause problems in manufacturing the electrode.

[0052] In addition, as shown in Reaction Scheme 1 below, hydroxide ions (OH - ) generated in the slurry react with silicon (Si) to produce hydrogen gas.

[0053] [Reaction Scheme 1]

[0054] Si (solid) + 2OH - (aqueous solution) + 2H2O (liquid) → Si(OH)4 (aqueous solution) + H2 (g) → SiO2(OH)2 2- (aqueous solution) + 2H2 (gas)

[0055] When hydrogen gas is generated in the slurry to form bubbles, it is difficult to uniformly coat the current collector with the slurry. This may have an adverse effect on the electrochemical performance of the secondary battery. That is, due to the influence of hydroxide ions in the slurry, silicon as the active material is consumed, thereby reducing the discharge capacity. In this case, it may lead to a result contrary to the purpose of using silicon oxide as the active material to increase the energy density.

[0056] Therefore, according to an exemplary embodiment of the present invention, when the negative electrode active material for a lithium secondary battery includes silicon oxide doped with magnesium and the surface of the silicon oxide includes a carbon coating, where when measured by X-ray photoelectron spectroscopy (XPS), the ratio of the peak area at 1303 eV that appears in the Mg1s spectrum to the sum of the peak areas at 1304.5 eV and 1303 eV is 60% or less, the above problems can be solved.

[0057] In addition, according to an embodiment of the present invention, when the content of magnesium hydroxide remaining on the surface of the silicon oxide satisfies the range of less than 0.05% by weight based on the total weight of the silicon oxide, such problems can be solved.

[0058] In some embodiments, when the organic material is subjected to heat treatment in a non-oxidizing atmosphere, a carbon coating can be formed by thermal decomposition of the organic material.

[0059] In some embodiments, a coating can be formed by heat-treating the organic material on the surface of the silicon oxide by chemical vapor deposition (CVD) at 800 °C to 1000 °C in a gas and / or vapor atmosphere. At this time, when the process temperature is lower than 800 °C, carbon crystallization on the surface of the silicon oxide may not be sufficiently carried out. In addition, when the process temperature exceeds 1000 °C, the particle diameter of the silicon crystals of the silicon oxide increases to more than 20 nm, and the life characteristics of the secondary battery may thus be reduced.

[0060] In some embodiments, the organic substance can be a single hydrocarbon or a mixture of hydrocarbons such as methane, ethane, ethylene, acetylene, propane, butane, butene, pentane, isobutane, n-hexane, cyclohexane, etc.; monocyclic to tricyclic aromatic hydrocarbons such as benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumaron, pyridine, anthracene, phenanthrene, or mixtures thereof, etc.

[0061] In addition, for example, light gas oil, creosote oil, anthracene oil obtained during tar distillation, and tar produced by naphtha cracking can be used alone or in the form of a mixture thereof.

[0062] In some embodiments, based on the total weight of the silicon oxide, the carbon content of the carbon coating can be 3 to 12 wt%. When the carbon coating is provided within the above range, appropriate conductivity can be imparted to the negative electrode active material, and since no gas is generated during the preparation of the negative electrode active material slurry, an electrode with excellent uniformity can be manufactured.

[0063] The carbon of the coating can include amorphous carbon, carbon nanofibers, carbon nanotubes, natural graphite, artificial graphite, graphene, graphene oxide, reduced graphene oxide, or mixtures thereof.

[0064] After coating with carbon, the silicon oxide is doped with magnesium. The magnesium doping can be provided by mixing the carbon-coated silicon oxide with a magnesium compound and then heating the mixture at a doping temperature of 900 °C to 1000 °C.

[0065] In this document, the mixing is not particularly limited, but a mixing device such as a tumbler mixer can be used, and the mixing can be carried out in an inert atmosphere. After mixing, doping is carried out at 900 °C to 1000 °C, so that magnesium doping can be fully carried out. When the temperature during doping is lower than 900 °C, the magnesium silicate phase generated by magnesium as the doping element cannot be stably formed, so that the increase in the initial efficiency may not be significant. When the temperature exceeds 1000 °C, there is a problem that the increase in the silicon crystal is greater than required, resulting in a decrease in the life characteristics.

[0066] In some embodiments, after doping with magnesium, the obtained silicon oxide can be washed with a washing solvent. The washing solvent can be, for example, water, organic solvents such as ethanol, methanol, acetone, hexane, etc., acids such as acetic acid, citric acid, hydrochloric acid, nitric acid, sulfuric acid, etc., or mixtures thereof.

[0067] In some exemplary embodiments, the magnesium compound for doping can be magnesium (Mg), magnesium hydroxide (Mg(OH)2), magnesium carbonate (MgCO3), or mixtures thereof.

[0068] In some embodiments, based on the total weight of the silicon oxide, when the magnesium doping content is 7 to 17% by weight, no gas is generated during the preparation of the negative electrode active material slurry, and when the current collector is coated with the silicon oxide, it can be uniformly coated on the current collector to ensure the stability of the electrode.

[0069] That is, according to the present invention, by adjusting the amount of carbon contained in the silicon oxide coating, the heat treatment temperature during magnesium doping, the magnesium doping amount, the Mg(OH)2 ratio, etc., an effect of not generating gas during the preparation of the negative electrode active material slurry can be provided, so that it can be used as a technical index for confirming the electrode stability and electrochemical performance of the secondary battery.

[0070] In some embodiments, the silicon oxide may be SiO x (0 < x < 2). Generally, there is a problem that as the value of x decreases, the battery capacity increases and the battery life decreases, while as the value of x increases, the battery capacity decreases and the energy density of the electrode decreases. Therefore, in the present invention, when x is within the above range, the energy density can be ensured while achieving the desired capacity of the battery.

[0071] In some embodiments, the silicon crystal particle size of the silicon oxide may be 20 nm or less, and preferably 0.1 nm to 20 nm. If the silicon crystal particle size of the silicon oxide is 20 nm or less, when the doped metal reacts with silicon during charging, the expansion of the active material can be suppressed, thereby improving the life characteristics of the battery. In addition, after measuring the silicon-based negative electrode active material by XRD (X-ray diffraction, Cu negative electrode source), the silicon crystal particle size can be calculated based on the full width at half maximum (FWHM) of the silicon peak measured at 2θ of 28.5°, 47.5°, and 56.0°.

[0072] <Negative electrode active material slurry for lithium secondary battery>

[0073] According to an embodiment of the present invention, the negative electrode active material slurry can be prepared by mixing a mixture of a negative electrode active material and a binder with a conductive material, a thickener, etc. in a solvent and stirring.

[0074] For example, examples of the conductive material may include carbon-based conductive materials such as carbon black, graphene, carbon nanotubes, etc., and / or metal-based conductive materials such as tin, tin oxide, titanium oxide, or perovskite minerals such as LaSrCoO3 or LaSrMnO3. Examples of the thickener may include, for example, carboxymethyl cellulose (CMC).

[0075] In some embodiments, based on the total weight of the negative electrode active material slurry, the content of the negative electrode active material may be about 90 wt% to 98 wt%, the content of the binder mixture may be about 1 wt% to 5 wt%, the content of the conductive material may be about 0.5 wt% to 5 wt%, and the content of the thickener may be about 0.5 wt% to 5 wt%.

[0076] In an exemplary embodiment, the pH of the negative electrode active material slurry may be less than 10, and preferably, its pH is above 7 and less than 10. When the pH of the negative electrode active material slurry is 10 or above, carboxymethyl cellulose (CMC) used as a thickener shrinks to reduce the viscosity of the slurry, which may cause difficulties in electrode manufacturing. On the other hand, when the pH of the negative electrode active material slurry is less than 7, there is a problem of reduced slurry viscosity. In particular, when the pH is below 5, the reduction of the slurry viscosity is further deepened, and when the pH is 2 to less than or equal to 3, there may be a problem of precipitation. Therefore, when the pH range is less than 10, preferably the pH is above 7 and less than 10, a slurry viscosity favorable for electrode preparation can be provided. The viscosity of the slurry may be 4000 centipoises (cPs) or more, preferably 4000 cPs to 9000 cPs. If the viscosity exceeds the above range, it may be difficult to uniformly disperse the components in the slurry, and there may be problems leading to deterioration of the electrode quality.

[0077] <Lithium secondary battery>

[0078] According to an embodiment of the present invention, a lithium secondary battery may include an electrode assembly, which includes: a positive electrode, a negative electrode including a negative electrode active material; and a separation membrane interposed between the positive electrode and the negative electrode. The electrode assembly may be accommodated in a housing together with an electrolyte in which the electrode assembly is immersed.

[0079] The positive electrode may include a positive electrode active material layer formed by coating a positive electrode active material on a positive electrode current collector. The positive electrode active material may include a compound capable of reversibly inserting and extracting lithium ions.

[0080] In an exemplary embodiment, the positive electrode active material may include a lithium transition metal oxide. For example, the lithium transition metal oxide contains nickel (Ni), and may further contain at least one of cobalt (Co) and manganese (Mn).

[0081] For example, the lithium transition metal oxide may be represented by the following Formula 1.

[0082] [Formula 1]

[0083] Li 1+a Ni 1-(x+y) Co x M yO2

[0084] In Formula 1, a, x, and y can be in the ranges of -0.05 ≤ a ≤ 0.15, 0.01 ≤ x ≤ 0.3, and 0.01 ≤ y ≤ 0.3, and M can be at least one element selected from Mn, Mg, Sr, Ba, B, Al, Si, Ti, Zr, and W.

[0085] The positive electrode active material paste can be prepared by mixing the positive electrode active material with a binder, a conductive material, and / or a dispersant in a solvent and then stirring them. The paste can be coated on the positive electrode current collector, and then dried and pressed to fabricate the positive electrode.

[0086] The positive electrode current collector can include, for example, stainless steel, nickel, aluminum, titanium, copper, or their alloys, and preferably includes aluminum or an aluminum alloy.

[0087] The binder can be selected from, for example, organic binders such as poly(vinylidene fluoride - hexafluoropropylene) copolymer (PVDF - co - HFP), poly(vinylidene fluoride) (PVDF), polyacrylonitrile, polymethyl methacrylate, etc., or aqueous binders such as styrene - butadiene rubber (SBR), and can be used together with a thickener such as carboxymethyl cellulose (CMC).

[0088] For example, a PVDF - based binder can be used as the binder for forming the positive electrode. In this case, the amount of the binder used for forming the positive electrode active material layer can be reduced, and the amount of the positive electrode active material can be relatively increased, thereby improving the output and capacity of the secondary battery.

[0089] A conductive material can be included to facilitate electron transfer between the active material particles. For example, the conductive material can include carbon - based conductive materials such as graphite, carbon black, graphene, or carbon nanotubes, and / or metal - based conductive materials such as tin, tin oxide, titanium oxide, or perovskite minerals such as LaSrCoO3 or LaSrMnO3.

[0090] The negative electrode can include a negative electrode current collector and a negative electrode active material layer formed by coating the negative electrode current collector with the composition for the negative electrode containing the above - mentioned negative electrode active material.

[0091] In addition, the composition for the negative electrode can include an acrylic polymer binder and styrene - butadiene rubber (SBR) as a binder mixture.

[0092] For example, the composition for the negative electrode can be prepared in the form of a negative electrode active material paste by mixing the negative electrode active material and the binder mixture with a conductive material, a thickener, etc. in a solvent and then stirring. The paste can be coated on at least one surface of the negative electrode current collector, and then dried and pressed to fabricate the negative electrode.

[0093] The separator can be interposed between the positive electrode and the negative electrode. The separator can include, for example, a porous polymer membrane made of a polyolefin polymer such as a homopolymer of ethylene, a homopolymer of propylene, an ethylene / butene copolymer, an ethylene / hexene copolymer, an ethylene / methacrylate copolymer, etc. In addition, the separator can include a nonwoven fabric made of glass fibers with a high melting point, polyethylene terephthalate fibers, etc.

[0094] In some embodiments, the area and / or volume of the negative electrode (e.g., the contact area with the separator) can be larger than the area and / or volume of the positive electrode. Thus, for example, lithium ions generated from the positive electrode can move smoothly to the negative electrode without precipitating in the middle. Therefore, by using the above negative electrode active material, the effects of improving capacity and output can be more easily achieved.

[0095] According to an exemplary embodiment, an electrode cell is defined by a positive electrode, a negative electrode, and a separator, and a plurality of electrode units can be stacked to form an electrode assembly such as a jelly roll type. For example, the electrode assembly can be formed by winding, laminating, folding, etc. the separator.

[0096] The electrode assembly can be housed in a casing together with an electrolytic solution to define a lithium secondary battery. According to an exemplary embodiment, a non-aqueous electrolytic solution can be used as the electrolytic solution.

[0097] The non-aqueous electrolytic solution includes a lithium salt of an electrolyte and an organic solvent, and the lithium salt is represented by, for example, Li + X - and, as the anion (X - ) of the lithium salt, examples include: 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.

[0098] As the organic solvent, for example, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, γ-butyrolactone, propylene sulfite, tetrahydrofuran, etc. can be used. These compounds can be used alone or in combination of two or more of them.

[0099] The electrode tabs (the positive electrode tab and the negative electrode tab) can protrude from the positive electrode current collector and the negative electrode current collector belonging to each electrode unit, respectively, and can extend to one side of the outer case. The electrode tabs can be fused to one side of the outer case to form the electrode leads (the positive electrode lead and the negative electrode lead) extending or exposed to the outside of the outer case.

[0100] The lithium secondary battery can be manufactured, for example, in a cylindrical shape (using a can), a square shape, a pouch type, or a coin shape.

[0101] Hereinafter, specific experimental examples are presented for easy understanding of the present invention. However, the following examples are given only for illustrating the present invention, and those skilled in the art will clearly understand that various changes and modifications can be made within the scope and spirit of the present invention. Such changes and modifications are appropriately included in the appended claims.

[0102] Preparation Example 1: Preparation of Silicon-Based Anode Active Material

[0103] In order to form a coating on the surface of silicon oxide (SiO, Sigma-Aldrich Co.), ethylene was subjected to CVD (heat treatment) at 900 °C to form silicon oxide with a carbon-coated surface. The silicon oxide was mixed with magnesium hydroxide (Mg(OH)2), and then heated to incorporate magnesium. Then, the magnesium-doped silicon oxide was washed in distilled water for 10 minutes to prepare the negative electrode active material for the lithium secondary battery.

[0104] The carbon coating amount of the prepared silicon-based anode active material was measured using a CS analyzer (CS844) from LECO Corporation, and the magnesium doping content was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES, model Optima 8300) from Perkin-Elmer Corporation.

[0105] In addition, as described in Table 1 below, anode active materials of Examples 1 to 6 and Comparative Examples 1 to 4 were prepared by changing the conditions of the carbon coating amount, magnesium doping amount, heat treatment temperature during doping, and Mg(OH)2 ratio.

[0106] [Table 1]

[0107]

[0108] Preparation Example 2: Preparation of Anode Active Material Slurry

[0109] An anode active material slurry was prepared by mixing the anode active material prepared according to the conditions in Table 1, graphite and carbon nanotubes (CNT) as conductive agents, and carboxymethyl cellulose (CMC) and SBR binder as binder mixtures in a ratio of 84.0:13.0:0.3:1.2:1.5 parts by weight.

[0110] Experimental Example 1

[0111] (1) X-ray photoelectron spectroscopy (XPS)

[0112] X-ray photoelectron spectroscopy analysis was performed on the silicon-based anode active material prepared according to the conditions in Table 1 above. The X-ray photoelectron spectroscopy analysis was carried out using an Al kα X-ray beam with a photon energy of 1486.68 eV and a beam size of 650 μm. The analysis was performed using the CAE mode.

[0113] The method for calculating the XPS-Mg1s scanning area is as follows. After measuring the XPS of the active material, the Mg1s spectrum was deconvoluted to calculate the peak area at 1304.5 eV as the area of MgO and the peak area at 1303 eV as the area of Mg(OH)2. Thereafter, the Mg(OH)2 ratio was calculated as follows, and the results are described in Table 2 below.

[0114] (1) XPS-Mg1s scanning area ratio

[0115] When measured by X-ray photoelectron spectroscopy (XPS), the ratio of the peak area at 1303 eV to the sum of the peak areas at 1304.5 eV and 1303 eV that appear in the Mg1s spectrum was calculated.

[0116] (2) Confirmation of gas generation

[0117] The gas generation of the negative electrode active material paste prepared according to Preparation Example 2 was confirmed by visually observing the bubbles formed in the paste.

[0118] (3) Measurement of pH

[0119] After adding 1 g of silicon oxide as the negative electrode active material to 99 g of water and stirring the mixture for 3 minutes, the pH was measured at room temperature using a pH meter.

[0120] (4) Measurement of viscosity

[0121] The viscosity of the prepared paste was measured using a Brookfield viscometer (rotor type: CZ-52, torque 60 - 67%, 25.0 °C).

[0122] [Table 2]

[0123]

[0124] (In Table 2, X indicates no gas generation, and O indicates gas generation.)

[0125] In the cases of Examples 1 to 6, when controlling the magnesium doping amount, the amount of carbon in the coating, the heat treatment temperature during doping, and the Mg(OH)₂ ratio is less than 60%, no gas generation can be visually confirmed because no bubbles are formed during the manufacture of the negative electrode active material paste.

[0126] Furthermore, an increase in pH during the preparation of the negative electrode active material paste can be prevented, which means that a decrease in viscosity due to the shrinkage of the thickener can be prevented, thus ensuring stability during the electrode preparation process.

[0127] On the other hand, it can be confirmed that in the case of Comparative Example 1, since the negative electrode active material paste was not doped with magnesium, no gas was generated, and gas was generated when the ratio of Mg(OH)₂ exceeded 60%.

[0128] Experimental Example 2

[0129] The residual magnesium content of the negative electrode active material paste prepared in Preparation Example 2 was confirmed according to the following Reaction Formula 1. The presence or absence of gas generation, pH, and viscosity are the same as those shown in Table 2.

[0130] [Reaction Formula 1]

[0131] Equivalence point 1: Mg(OH)₂ (aqueous solution) + 2HCl (aqueous solution) → MgCl₂ (aqueous solution) + 2H₂O (l)

[0132] Equivalence Point 2: MgCO3(aqueous solution) + 2HCl(aqueous solution) → MgCl2(aqueous solution) + 2H2O(liquid) + CO2

[0133] In addition, the contents of MgCO3 and Mg(OH)2 can be calculated by measuring the amount of 0.1 M HCl titrated from Equivalence Point 1 to Equivalence Point 2 according to the following formula.

[0134] (1) Content of Mg(OH)2: ((Titration volume (Equivalence Point 1) × Molar concentration of HCl × Molecular weight of Mg(OH)2) / (Sample weight))1 / 2

[0135] (2) Content of MgCO3: ((Titration volume (Equivalence Point 2 - Equivalence Point 1) × Molar concentration of HCl × Molecular weight of MgCO3) / (Sample weight))1 / 2

[0136] [Table 3]

[0137]

[0138] (In Table 3, X indicates no gas generation, and O indicates gas generation.)

[0139] As proposed in Examples 1 to 6, when adjusting the magnesium doping amount, the amount of carbon in the coating, and the heat treatment temperature during doping and calculating according to the above Reaction Formula 1, if Mg(OH)2 satisfies the range of less than 0.05 wt%, it can be confirmed that no gas is generated because no bubbles are formed during the preparation of the negative electrode active material slurry.

[0140] In addition, an increase in pH during the preparation of the negative electrode active material slurry can be prevented, thereby ensuring the stability during the electrode preparation process.

[0141] On the other hand, it can be confirmed that in Comparative Example 1, since the negative electrode active material slurry is not doped with magnesium, no gas is generated, while in Comparative Examples 2, 3, and 4, since the negative electrode active material slurry contains more than 0.05 wt% of Mg(OH)2, gas is generated.

[0142] As a result, according to the XPS-Mg1s scanning area ratio or the content of magnesium residues remaining on the surface of the silicon-based negative electrode active material, measured according to Reaction Formula 1, the stability of the electrode can be confirmed.

[0143] Experimental Example 3

[0144] Figure 1 Pictures of the negative electrode active material slurries (Example 1 and Comparative Example 2) prepared according to Preparation Example 2 are shown. In the case of Example 1, it can be confirmed that no gas is generated, while in the case of Comparative Example 2, it can be visually confirmed that bubbles are formed, thus confirming that gas is generated.

[0145] In addition, Figure 2 Photographs of the coated electrodes are shown, in which the copper current collectors are coated with the negative electrode active material slurry prepared according to Preparation Example 2 (Example 1 and Comparative Example 2). It can be seen that the electrode coated with the slurry of Example 1 in which no bubbles are formed has a uniformly coated surface, while it can be seen that the electrode coated with the slurry of Comparative Example 2 in which bubbles are formed has a non-uniformly coated surface due to the bubbles.

[0146] Experimental Example 4

[0147] A coated electrode was prepared using the negative electrode active material slurry prepared according to Preparation Example 2, and then a lithium (Li) coin half-cell was prepared, and the electrochemical properties such as charge capacity, discharge capacity, and initial efficiency were measured.

[0148] A lithium coin half-cell was formed by inserting a separator (polyethylene, thickness 20 μm) between the prepared negative electrode coated electrode and lithium metal (thickness 2 mm). The combination of lithium metal / separator / negative electrode was placed in a coin cell plate, electrolyte was injected, and then the lid was covered and clamped. The electrode used herein was prepared by dissolving 1 M LiPF6 solution in a mixed solvent of EC / FEC / EMC / DEC (20 / 10 / 20 / 50; volume ratio).

[0149] After the prepared lithium coin half-cell was impregnated for 12 hours or longer, the battery was charged (CC-CV 0.1C 0.01V 0.01C cut-off) in a chamber at 25 °C, and then the battery capacity (charge capacity) was measured. After discharging it (CC 0.1C 1.5V cut-off), the battery capacity (discharge capacity) was measured again. The initial capacity efficiency of each lithium secondary battery was calculated by dividing the measured initial discharge capacity by the measured initial charge capacity and then multiplying by 100. However, the electrodes of the comparative examples did not have a uniform coating surface, so the most uniform part was selected for electrochemical evaluation. The results are shown in Table 4 below.

[0150] [Table 4]

[0151]

[0152] It was found that in the case of Example 1, compared with Comparative Example 1, the charge capacity and discharge capacity decreased, but the initial efficiency increased depending on the purpose of magnesium doping. In addition, as described above, even when preparing the negative electrode active material slurry, no gas is generated, and the uniformity of the coated electrode can be ensured.

[0153] On the other hand, it was found that in the case of Comparative Example 2, due to magnesium doping being carried out as in Example 1, the initial efficiency increased similarly to Example 1, but both the charge capacity and the discharge capacity decreased. Therefore, it is expected that as silicon (Si) in the silicon oxide is ionized, it can no longer act as an active material and can no longer play the role of being able to receive lithium, such that the charge capacity will decrease, and similarly, the discharge capacity will also decrease together. Therefore, when the battery capacity decreases, it is no longer possible to realize the advantages of the silicon-based negative electrode active material, namely high capacity and high energy density.

Claims

1. A negative electrode active material for a lithium secondary battery, comprising: a silicon oxide, the silicon oxide including a carbon coating formed on its surface and doped with magnesium, wherein the negative electrode active material contains magnesium hydroxide on its surface, wherein when measured by X-ray photoelectron spectroscopy (XPS), the ratio of the peak area at 1303 eV that appears in the Mg1s spectrum to the sum of the peak area at 1304.5 eV and the peak area at 1303 eV is 60% or less, based on the total weight of the silicon oxide, the content of magnesium hydroxide remaining on the surface is less than 0.05% by weight, the carbon content of the carbon coating is 3% to 12% by weight, based on the total weight of the silicon oxide, the content of the doped magnesium is 7% to 17% by weight, and the magnesium doping is achieved by mixing the silicon oxide coated with a carbon coating with magnesium hydroxide and then heating at a temperature of 900 - 1000 °C.

2. The negative electrode active material for a lithium secondary battery according to claim 1, wherein the silicon oxide is SiO x , and the range of x is 0 < x < 2.

3. The negative electrode active material for a lithium secondary battery according to claim 1, wherein the average particle size of the silicon crystal of the silicon oxide is 20 nm or less.

4. The negative electrode active material for a lithium secondary battery according to claim 1, wherein the carbon of the coating includes amorphous carbon, carbon nanofibers, carbon nanotubes, graphite, graphene, graphene oxide, reduced graphene oxide, or a mixture thereof.

5. A negative electrode active material slurry for a lithium secondary battery, comprising the negative electrode active material according to claim 1.

6. The negative electrode active material slurry for a lithium secondary battery according to claim 5, wherein the pH of the slurry is less than 10.

7. The negative electrode active material slurry for a lithium secondary battery according to claim 5, wherein the viscosity of the slurry is 4000 cPs or more.

8. A lithium secondary battery, comprising: a positive electrode; a negative electrode containing the negative electrode active material for a lithium secondary battery according to claim 1; and a separator interposed between the positive electrode and the negative electrode.

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