Negative electrode for lithium secondary battery and lithium secondary battery comprising the same

Abstract

According to one embodiment of the disclosed technology, a negative electrode active material for a secondary battery includes a negative electrode current collector, a first negative electrode active material layer located on at least one surface of the negative electrode current collector and including a graphite-based active material and a silicon-based active material doped with a metal element, and a second negative electrode active material layer located on the first negative electrode active material layer and including a porous structure. The porous structure includes carbon-based particles having pores and a silicon-containing coating layer formed inside the pores or on a surface of the carbon-based particles.

Description

[0001] Cross-reference and priority claims of related applications

[0002] This patent application claims priority and benefit to Korean Patent Application No. 10-2022-0169568, filed on December 7, 2022, with the Korean Intellectual Property Office (KIPO), the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] The technology disclosed herein relates to a negative electrode for a lithium secondary battery and a lithium secondary battery including the negative electrode. Background Technology

[0004] With the rapid development of electric vehicles and portable devices such as cameras, mobile phones, and laptops, the demand for rechargeable and dischargeable secondary batteries is increasing. Battery packs, including secondary batteries, are currently being developed for use as power sources in environmentally friendly vehicles such as electric vehicles.

[0005] Examples of rechargeable batteries include lithium-ion batteries, nickel-cadmium batteries, and nickel-metal hydride batteries. Lithium-ion batteries are widely used due to their advantages such as high operating voltage, high energy density per unit weight, high charge rate, and compact size.

[0006] A lithium secondary battery may include an electrode assembly comprising a positive electrode, a negative electrode, and a separator layer (separator), and an electrolyte immersing the electrode assembly. A lithium secondary battery may also include a housing having, for example, a pouch shape for containing the electrode assembly and the electrolyte. Summary of the Invention

[0007] The techniques disclosed herein can be implemented in certain embodiments to provide a negative electrode for lithium secondary batteries with improved capacity and lifetime characteristics.

[0008] In one aspect of the technology disclosed herein, a lithium secondary battery may include a negative electrode for the lithium secondary battery having improved capacity and lifespan characteristics.

[0009] A negative electrode for a lithium secondary battery includes a negative electrode current collector, a first negative electrode active material layer located on at least one surface of the negative electrode current collector and including a graphite-based active material and a silicon-based active material doped with a metal element, and a second negative electrode active material layer located on the first negative electrode active material layer and including a porous structure. The porous structure includes carbon-based particles having pores and a silicon-containing coating formed inside or on the surface of the pores of the carbon-based particles. In certain embodiments, the term "graphite-based active material" can be used to mean an active material including graphite. In certain embodiments, the term "silicon-based active material" can be used to mean an active material including silicon. In certain embodiments, the term "carbon-based particle" can be used to mean a particle including carbon.

[0010] In some embodiments, the content of the metal element doped in the silicon-based active material can be in the range of 7% to 17% by weight based on the total weight of the silicon-based active material.

[0011] In some embodiments, the metal element can include at least one selected from the group consisting of Mg, Li, Al, Ca, Fe, Ti, and V.

[0012] In some embodiments, the metal element can include Mg.

[0013] In some embodiments, a Mg1s spectrum of the surface of the silicon-based active material measured by X-ray photoelectron spectroscopy (XPS) can satisfy Equation 1.

[0014] [Equation 1]

[0015] P Mg / (P Mg + P MgO ) ≤ 0.6

[0016] In Equation 1, P Mg represents the area of a 1303 eV peak in the Mg1s spectrum, P MgO represents the area of a 1304.5 eV peak in the Mg1s spectrum.

[0017] In some embodiments, the content of the silicon-based active material can be in the range of 0.1% to 35% by weight based on the total weight of the first negative electrode active material layer.

[0018] In some embodiments, the silicon-based active material can include silicon-based active material particles and a carbon coating formed on the silicon-based active material particles.

[0019] In some embodiments, the graphite-based active material can include artificial graphite and natural graphite. The weight of the natural graphite included in the first negative electrode active material can be equal to or less than the weight of the artificial graphite included in the first negative electrode active material.

[0020] In some embodiments, the ratio of the weight of the natural graphite included in the first negative electrode active material to the weight of the artificial graphite included in the first negative electrode active material can be in the range of 0.025 to 1.

[0021] In some embodiments, the second negative electrode active material layer can further include artificial graphite and natural graphite. The weight of the natural graphite included in the second negative electrode active material can be equal to or less than the weight of the artificial graphite included in the second negative electrode active material.

[0022] In some embodiments, the ratio of the weight of the natural graphite included in the second negative electrode active material to the weight of the artificial graphite included in the second negative electrode active material can be in the range of 0.025 to 1.

[0023] In some embodiments, the content of the porous structure can be in the range of 0.1% by weight to 35% by weight, based on the total weight of the second negative electrode active material.

[0024] In some embodiments, the thickness of the second negative electrode active material layer can be 0.5% to 50% of the total thickness of the first negative electrode active material layer and the second negative electrode active material layer.

[0025] In some embodiments, the carbon-based particles included in the porous structure can include at least one selected from the group consisting of activated carbon, carbon nanotube, carbon nanowire, graphene, carbon fiber, carbon black, graphite, porous carbon, pyrolyzed cryogel, pyrolyzed xerogel, and pyrolyzed aerogel.

[0026] In some embodiments, the pores included in the carbon-based particles can have a pore diameter of 20 nm or less.

[0027] In some embodiments, the silicon included in the silicon-containing coating layer can have an amorphous structure or a crystallite size of 7 nm or less, as measured by X-ray diffraction (XRD) analysis.

[0028] In some embodiments, the crystallite size of the silicon included in the silicon-containing coating layer can be measured according to Equation 2.

[0029] [Equation 2]

[0030]

[0031] In Formula 2 above, L represents a crystallite size (nm), λ represents an X-ray wavelength (nm), β represents a full width at half maximum (radian) of a peak of a (111) plane of silicon included in the silicon-containing coating layer, and θ represents a diffraction angle (radian).

[0032] The lithium secondary battery includes the negative electrode for a lithium secondary battery according to the above-described embodiments, and a positive electrode opposite to the negative electrode.

[0033] In one embodiment of the disclosed technology, expansion of a silicon-based active material can be reduced to improve capacity retention of a lithium secondary battery during repeated charging and discharging.

[0034] In one embodiment of the disclosed technology, cracks can be prevented from being generated in a secondary battery during charging and discharging due to a difference in volume expansion ratio between carbon and silicon.

[0035] In one embodiment of the disclosed technology, a secondary battery having high capacity and high energy density can be realized while mitigating volume expansion of a silicon-based active material.

[0036] The negative electrode and the lithium secondary battery of the disclosed technology can be widely applied to green technology fields such as electric vehicles, battery charging stations, solar power generation, wind power generation using other batteries, etc. The negative electrode and the lithium secondary battery according to the disclosed technology can be used in eco-friendly electric vehicles and hybrid electric vehicles to prevent climate change by suppressing air pollution and greenhouse gas emissions, etc. BRIEF DESCRIPTION OF DRAWINGS

[0037] Figure 1 FIG. 1 is a schematic cross-sectional view illustrating a negative electrode for a lithium secondary battery based on some example embodiments.

[0038] Figure 2 FIG. 2 is a schematic plan view illustrating a secondary battery based on some example embodiments.

[0039] Figure 3 FIG. 3 is a schematic cross-sectional view illustrating a secondary battery based on some example embodiments. DETAILED DESCRIPTION

[0040] The chapter titles used herein are for the convenience of understanding only and do not limit the scope of the embodiments to the chapters described therein.

[0041] As the field of application of lithium secondary batteries continues to expand, lithium secondary batteries having high power and high capacity are in the spotlight. In addition, the negative electrode of a lithium secondary battery can include a negative electrode active material, for example, a high-capacity silicon material can be used together with carbon as a negative electrode active material to provide improved battery performance.

[0042] Due to the presence of both silicon and carbon in the negative active material, a difference between the volume expansion ratio of silicon and the volume expansion ratio of carbon can cause an undesirable effect, such as generation of cracks in the negative electrode, resulting in undesirable exposure of the material contained in the negative electrode to the electrolyte outside the negative electrode during repeated charging and discharging cycles.

[0043] To address these issues, the technology disclosed herein can be implemented in some embodiments to provide a negative electrode for a lithium secondary battery including a plurality of negative active material layers. Further, the technology disclosed herein can be implemented in some embodiments to provide a lithium secondary battery including a negative electrode. In some embodiments, the term "negative electrode" can be used to mean a negative electrode for a lithium secondary battery. In some embodiments, the term "secondary battery" can be used to mean a lithium secondary battery.

[0044] Reference will now be made in detail embodiments of the technology disclosed herein, examples of which are illustrated in the accompanying drawings. The following description is merely exemplary and is not intended to limit the disclosed technology to specific embodiments.

[0045] Figure 1 FIG. 1 is a schematic cross-sectional view illustrating a negative electrode for a lithium secondary battery based on some example embodiments.

[0046] Referring to Figure 1 The negative electrode 100 can include a negative current collector 110, a first negative active material layer 120, and a second negative active material layer 130.

[0047] For example, the negative current collector 110 can include a copper foil, a nickel foil, a stainless steel foil, a titanium foil, a nickel foam, a copper foam, a polymer substrate coated with a conductive metal, or the like. The copper foil, the nickel foil, the stainless steel foil, the titanium foil, the nickel foam, the copper foam, and the polymer substrate coated with a conductive metal can be used alone or in combination of two or more thereof. For example, the thickness of the negative current collector 110 can be 10 µm to 50 µm.

[0048] The first negative active material layer 120 including a silicon-based active material and a graphite-based active material is formed on at least one surface of the negative current collector 110. In one embodiment, the first negative active material layer 120 can be in direct contact with the negative current collector 110.

[0049] The silicon-based active material is doped with a metal element. Accordingly, the expansion of the silicon-based active material can be reduced, and the capacity retention rate of the lithium secondary battery can be improved during repeated charging and discharging.

[0050] In some embodiments, the content of the doped metal element in the silicon-based active material can be in the range of 7 to 17% by weight, and in some embodiments, can be in the range of 10 to 17% by weight, based on the total weight of the silicon-based active material. Within this range, both the high capacity characteristics of silicon can be maintained, and the cycle characteristics can be improved due to the doping.

[0051] In some embodiments, the metal element can include at least one selected from the group consisting of Mg, Li, Al, Ca, Fe, Ti, and V.

[0052] In one embodiment, the metal element can include magnesium. For example, magnesium can be added to the silicon-based active material as a doping element, thereby reducing the volume expansion of the silicon-based active material. Accordingly, the life characteristics of the lithium secondary battery can be improved.

[0053] For example, magnesium can be doped by calcining a mixture of the silicon-based active material and a magnesium compound at a temperature of 900 to 1000°C.

[0054] For example, the silicon-based active material can be formed by mixing and depositing raw materials of the silicon-based active material, such as Si, SiO2, etc.

[0055] The mixing can be performed in a mixing device such as a tumbler mixer under an inert atmosphere.

[0056] The calcination can be performed within the above temperature range so as to sufficiently perform metal doping and prevent the silicon crystal size from excessively increasing, thereby preventing the capacity and life characteristics of the negative electrode from deteriorating.

[0057] For example, doping magnesium or other metals can be performed by mixing and firing the silicon-based active material and a compound of the metal element after the silicon-based active material is prepared as described above (two-step process).

[0058] For example, doping magnesium or other metals can be performed by mixing and firing raw materials of the silicon-based active material, such as Si, SiO2, etc., and raw materials of the metal element (one-step process).

[0059] For example, any doping method can be used without particular limitation.

[0060] In some embodiments, the silicon-based active material doped with magnesium or other metal elements can be washed with a cleaning solvent. For example, the cleaning solvent can include water, an organic solvent such as ethanol, methanol, acetone, hexane, etc., and / or an acidic solvent such as acetic acid, citric acid, hydrochloric acid, nitric acid, sulfuric acid, etc. The organic solvent, the acidic solvent, and water can be used alone, or a combination of two or more of the organic solvent, the acidic solvent, and water can be used.

[0061] In some embodiments, the magnesium compound for doping can include at least one selected from the group consisting of magnesium (Mg), magnesium hydroxide (Mg(OH)2), magnesium carbonate (MgCO3), and magnesium oxide (MgO).

[0062] In some embodiments, a Mg1s spectrum of the surface of the silicon-based active material measured by X-ray photoelectron spectroscopy (XPS) can satisfy the following Equation 1.

[0063] [Equation 1]

[0064] P Mg / (P Mg + PMgO) ≤ 0.6

[0065] In Equation 1, P Mg represents an area of a 1303 eV peak in the Mg1s spectrum, P MgO represents an area of a 1304.5 eV peak in the Mg1s spectrum.

[0066] For example, P Mg may be an area of a peak (1303 eV) representing a magnesium element, and P MgO may be an area of a peak (1304.5 eV) representing a combination of a magnesium element and an oxygen element. For example, P Mg / (P Mg + P MgO ) value in Equation 1 can represent a ratio of metallic magnesium present in the silicon-based active material surface to metallic magnesium in magnesium oxide and magnesium hydroxide.

[0067] In the case where the XPS spectrum satisfies Equation 1, a side reaction caused by conversion of magnesium remaining on the surface of the silicon-based active material into magnesium hydroxide can be inhibited. Accordingly, degradation of the life characteristics of the silicon-based active material can be prevented.

[0068] In some embodiments, the silicon-based active material can include silicon-based active material particles.

[0069] For example, the silicon-based active material particles can include at least one selected from the group consisting of Si, SiOx(0 < x < 2), Si-Q alloy (Q is an element selected from the group consisting of alkali metal, alkaline earth metal, group 13 element, group 14 element, group 15 element, group 16 element, transition metal, rare earth element, and combinations thereof, but excluding Si), and silicon-carbon composite. For example, the silicon-based active material particles can include a mixture of at least one selected from the above group and SiO2. In some embodiments, the silicon-based active material particles can be Si or SiOx(0 < x < 2), and in one embodiment can be SiOx(0 < x < 2).

[0070] In some embodiments, the silicon-based active material can further include a carbon coating layer formed on the silicon-based active material particles. Thus, the silicon-based active material particles can be prevented from contacting moisture in the air and / or water in the negative electrode slurry. Accordingly, a decrease in the discharge capacity of the secondary battery can be suppressed.

[0071] For example, the content of the carbon coating layer can be in the range of 2 to 12% by weight, and in some embodiments, can be in the range of 4 to 12% by weight, based on the total weight of the silicon-based active material. Within this range, the lifespan characteristics can be improved while maintaining the capacity characteristics of the silicon-based active material.

[0072] For example, the carbon coating layer can include at least one selected from the group consisting of amorphous carbon, carbon nanotubes, carbon nanofibers, graphite, graphene, graphene oxide, and reduced graphene oxide.

[0073] In some embodiments, the content of the silicon-based active material can be in the range of 0.1 to 35% by weight, and in some embodiments, can be in the range of 6 to 30% by weight, based on the total weight of the first negative electrode active material layer. Within this range, a sudden increase in the volume expansion ratio relative to the increase in the energy density of the secondary battery can be prevented. Accordingly, the lifespan characteristics of the lithium secondary battery can be improved during repeated rapid charging and discharging processes.

[0074] In some embodiments, the graphite-based active material can include both artificial graphite and natural graphite.

[0075] In the case where only artificial graphite is used as the negative electrode active material, the power characteristics can be improved, but the expansion of the silicon-based active material can not be sufficiently suppressed. Accordingly, the lifespan characteristics of the secondary battery can be decreased, and the adhesion strength to the negative electrode current collector can also be decreased, thereby causing the negative electrode active material layer to be peeled off during the charging and discharging processes.

[0076] When only natural graphite is used as the negative electrode active material, the adhesion to the negative electrode current collector can be improved, but the electrical resistance can increase during rapid charging and discharging processes, thereby decreasing the power characteristics.

[0077] In some embodiments of the technology disclosed herein, the first negative electrode active material layer 120 can include a silicon-based active material, artificial graphite, and natural graphite. Accordingly, the adhesion strength between the negative electrode current collector 110 and the first negative electrode active material layer 120 and the power characteristics of the secondary battery can both be improved.

[0078] In some embodiments, the weight of the natural graphite included in the first negative electrode active material layer 120 can be less than or equal to the weight of the artificial graphite included in the first negative electrode active material layer 120. Accordingly, the resistance during rapid charging can be reduced, the power characteristics can be improved, and the high-rate charge / discharge characteristics of the secondary battery can be improved.

[0079] In some embodiments, the ratio of the weight of the natural graphite included in the first negative electrode active material layer 120 to the weight of the artificial graphite included in the first negative electrode active material layer 120 can be in the range of 0.025 to 1. In this case, the adhesion strength at the interface between the negative electrode current collector 110 and the first negative electrode active material layer 120 can be maintained while sufficiently improving the high-rate charge and discharge characteristics. Accordingly, the resistance at the interface can be reduced while maintaining the mechanical stability of the negative electrode 100, so that the power characteristics can be improved.

[0080] For example, the first negative electrode active material composition including the silicon-based active material and the graphite-based active material can be coated on the negative electrode current collector 110, and then dried and calendered to form the first negative electrode active material layer 120.

[0081] For example, the first negative electrode active material composition can be prepared by mixing the silicon-based active material and the graphite-based active material with a negative electrode binder, a conductive material, and / or a dispersant in a solvent.

[0082] The solvent can include an aqueous solvent such as water, an aqueous hydrochloric acid solution, or an aqueous sodium hydroxide solution, or a non-aqueous solvent such as N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, oxirane, tetrahydrofuran, etc.

[0083] For example, the negative electrode binder can include a polymeric material such as styrene butadiene rubber (SBR). In one embodiment, a thickening agent such as carboxymethyl cellulose (CMC) can also be used with the negative electrode binder.

[0084] The conductive material can be added to facilitate electron movement between active material particles. For example, the conductive material can include a carbon-based conductive material such as graphite, carbon black, graphene, and carbon nanotubes, and / or a metal-based conductive material such as tin, tin oxide, titanium oxide, a perovskite material including, for example, LaSrCoO3, LaSrMnO3, etc.

[0085] In some example embodiments, the silicon-based active material and the graphite-based active material can be used together, thereby improving the lifespan characteristics while achieving the high-capacity characteristics of silicon.

[0086] In some example embodiments, a second negative electrode active material layer 130 including a porous structure can be formed on the first negative electrode active material layer 120.

[0087] The porous structure can include carbon-based particles including pores and a silicon-containing coating layer formed inside the pores of the carbon-based particles and / or on the surface of the carbon-based particles.

[0088] For example, the negative electrode active material can be formed to include both silicon and carbon-based particles. In this case, carbon can partially alleviate the volume expansion of silicon. However, a difference between the volume expansion rate of silicon (e.g., about 400% or more) and the volume expansion rate of carbon (e.g., about 150% or less) can cause cracks in the negative electrode active material during charging and discharging of the secondary battery. Accordingly, the negative electrode active material can be exposed to the electrolyte during repeated charging and discharging, causing side reactions such as gas generation, and the life characteristics of the secondary battery can deteriorate.

[0089] In some embodiments of the technology disclosed herein, the carbon-based particles included in the porous structure can include pores. For example, the carbon-based particles can be porous particles including a plurality of pores.

[0090] In some embodiments, the silicon-containing coating layer can be formed inside the pores and / or on the surface of the carbon-based particles. Accordingly, cracks due to the difference in the volume expansion rate of carbon and silicon can be prevented during charging and discharging of the secondary battery.

[0091] For example, the second negative electrode active material layer 130 can be disposed as the outermost layer of the negative electrode 100. Accordingly, side reactions between the electrolyte and silicon particles located at the outermost periphery of the negative electrode 100 can be suppressed, thereby improving the life characteristics of the secondary battery.

[0092] If the second negative electrode active material layer 130 including the porous structure is not disposed as the outermost layer of the negative electrode 100, side reactions between the silicon-based active material in the first negative electrode active material layer 120 and the electrolyte can increase. Accordingly, the life characteristics of the secondary battery can deteriorate.

[0093] For example, the second negative electrode active material layer 130 including the porous structure including the silicon-containing coating layer can improve the capacity and energy density of the secondary battery. For example, even if the content of the silicon-based active material included in the first negative electrode active material layer 120 is reduced, the porous structure included in the second negative electrode active material layer 130 can improve the capacity of the secondary battery. Accordingly, while reducing the volume expansion of the silicon-based active material in the first negative electrode active material layer 120, the capacity and energy density of the secondary battery can be improved.

[0094] In some embodiments, the carbon-based particles can have a pore diameter of 20 nm or less. In some embodiments, the carbon-based particles can have a pore diameter of less than 10 nm. Within this range, over-deposition of silicon in the pores can be prevented. Thus, defects caused by the difference in volume expansion rates between carbon and silicon can be further mitigated during the charging and discharging of the secondary battery. In some embodiments, the carbon-based particles can have a pore diameter in the range of 0.1 nm to 20 nm, or 0.1 nm to 10 nm.

[0095] In some embodiments, the term "pore diameter" can refer to the diameter of the pore inlet formed on the surface of the carbon-based particles.

[0096] For example, the carbon-based particles described above can include activated carbon, carbon nanotubes, carbon nanowires, graphene, carbon fibers, carbon black, graphite, porous carbon, pyrolytic cold gel, pyrolytic dry gel, pyrolytic aerogel, etc. The activated carbon, carbon nanotubes, carbon nanowires, graphene, carbon fibers, carbon black, graphite, porous carbon, pyrolytic cold gel, pyrolytic dry gel, and pyrolytic aerogel can be used as carbon-based particles alone, or can be used as carbon-based particles in combination of two or more of the activated carbon, carbon nanotubes, carbon nanowires, graphene, carbon fibers, carbon black, graphite, porous carbon, pyrolytic cold gel, pyrolytic dry gel, and pyrolytic aerogel.

[0097] In some embodiments, the carbon-based particles described above can have an amorphous structure or a crystalline structure. For example, the carbon-based particles can have an amorphous structure. In this case, the durability of the porous structure can be improved, and cracks generated by charging and discharging or external impact can be inhibited. Thus, the life characteristics of the secondary battery can be improved.

[0098] In some example embodiments, the porous structure can include a silicon-containing coating layer formed inside the pores of the carbon-based particles or on the surface of the carbon-based particles. While taking advantage of the high capacity characteristics of silicon in the silicon-containing coating layer, the difference in the ratio of volume expansion between carbon and silicon can be mitigated. Thus, micro-cracks and exposure to the electrolyte caused by repeated charging and discharging of the secondary battery can be reduced, and the life characteristics of the secondary battery can be improved while maintaining the power characteristics.

[0099] For example, the silicon-containing coating layer can refer to a layer of silicon particles formed on at least a portion of the surface and / or inside the pores of the carbon-based particles.

[0100] In some example embodiments, the silicon-containing coating layer described above can have an amorphous structure, or can contain silicon having a crystallite size of 7 nm or less as measured by X-ray diffraction (XRD) analysis. In some embodiments, the crystallite size can be 4 nm or less.

[0101] Within the above range, the mechanical stability of the negative active material can be improved in a press process for manufacturing a secondary battery or in a repeated charging and discharging process. Accordingly, the life characteristics and capacity retention rate of the secondary battery can be improved.

[0102] In some embodiments, the term "amorphous structure" means that the shape of the single silicon included in the silicon-containing coating layer is amorphous, or means that the particle is too small to be substantially measured by the Scherrer equation represented by the following Equation 2 in X-ray diffraction (XRD) analysis.

[0103] [Equation 2]

[0104]

[0105] In the above Equation 2, L represents a crystallite size (nm), λ represents an X-ray wavelength (nm), β represents a full width at half maximum (radian) of a corresponding peak, and θ represents a diffraction angle (radian). In some example embodiments, the full width at half maximum in the XRD analysis for measuring the crystallite size can be measured from a peak of a (111) plane of silicon included in the silicon-containing coating layer.

[0106] In some embodiments, the full width at half maximum (FWHM) corrected by the instrument-derived value can be used as β in the above Equation 2. In one embodiment, Si can be used as a reference material to reflect the instrument-derived value. In this case, the instrument-derived FWHM can be expressed as a function of 2θ by fitting a FWHM profile over the entire 2θ range of Si. Thereafter, the instrument-derived FWHM value at the corresponding 2θ obtained from the function can be corrected to be used as β.

[0107] In some embodiments, the silicon-containing coating layer can further include at least one of SiOx (0 < x < 2) and silicon carbide (SiC).

[0108] In some embodiments, silicon carbide can not be formed inside the pores of the carbon-based particles or on the surface of the carbon-based particles. For example, the silicon-containing coating layer can not include silicon carbide. For example, the silicon-containing coating layer can include only silicon and / or silicon oxide. Accordingly, the capacity characteristics of the secondary battery can be improved.

[0109] For example, the formation of silicon carbide can be suppressed by adjusting the temperature at which silicon deposition is performed and the time at which silicon deposition is performed.

[0110] In some embodiments, the second negative active material layer 130 can further include artificial graphite and natural graphite.

[0111] For example, the weight of the natural graphite included in the second negative electrode active material layer 130 can be equal to or less than the weight of the artificial graphite included in the second negative electrode active material layer 130. In this case, the resistance at the time of rapid charging can be reduced, and the power characteristics can be improved. Accordingly, the high-rate charge / discharge characteristics of the secondary battery can be improved.

[0112] In some embodiments, the ratio of the weight of the natural graphite included in the second negative electrode active material layer 130 to the weight of the artificial graphite included in the second negative electrode active material layer 130 can be in the range of 0.025 to 1. In this case, the adhesion strength at the interface between the first negative electrode active material layer 120 and the second negative electrode active material layer 130 can be maintained while sufficiently improving the high-rate charge / discharge characteristics. Accordingly, the resistance at the interface can be reduced while maintaining the mechanical stability of the negative electrode 100, thereby improving the power characteristics.

[0113] In some embodiments, the content of the porous structure can be in the range of 0.1% by weight to 35% by weight. In some embodiments, the content of the porous structure can be in the range of 6% by weight to 30% by weight, based on the total weight of the second negative electrode active material layer 130. Within the above range, the volume expansion rate of silicon can be effectively suppressed by the pores of the carbon-based particles, and the increase in the volume expansion rate can be further suppressed compared to the increase in the energy density of the secondary battery. Accordingly, the life characteristics of the lithium secondary battery can be further improved during repeated rapid charging and discharging.

[0114] In some embodiments, the thickness of the second negative electrode active material layer 130 can be 0.5% to 50% of the total thickness of the first negative electrode active material layer 120 and the second negative electrode active material layer 130. Within this range, a negative electrode 100 having high capacity and improved life can be achieved while sufficiently including the porous structure. In addition, it is also possible to prevent a decrease in energy density due to excessive increase in the thickness of the negative electrode 100. Accordingly, a secondary battery having high capacity and high energy density can be achieved.

[0115] For example, the second negative electrode active material composition including the porous structure can be coated on the first negative electrode active material layer 120, and then dried and calendered to form the above-described second negative electrode active material layer 130.

[0116] For example, the second negative electrode active material composition can be prepared by mixing the porous structure with a negative electrode binder, a conductive material, and / or a dispersant in a solvent. In one embodiment, the second negative electrode active material composition can be prepared by mixing the artificial graphite and the natural graphite with the porous structure, the negative electrode binder, the conductive material, and / or the dispersant.

[0117] For example, the solvent, the negative electrode binder, the conductive material, and the dispersant can include the same types of compounds as the above-described compounds.

[0118] Figure 2 and Figure 3 are schematic top views and schematic cross-sectional views showing a secondary battery based on example embodiments. For example, Figure 3 is a cross-sectional view taken along line I-I' in Figure 2 in a thickness direction.

[0119] Referring to Figure 2 and Figure 3 , the lithium secondary battery can include an electrode assembly 180 including the above-described negative electrode 100 and a positive electrode 150 opposite the negative electrode 100. The electrode assembly 180 can be accommodated in a case 190 and impregnated with an electrolyte.

[0120] The positive electrode 150 can include a positive electrode active material layer 170 formed by coating a mixture including a positive electrode active material on at least one surface of a positive electrode current collector 160.

[0121] The positive electrode current collector 160 can include stainless steel, nickel, aluminum, titanium, or an alloy thereof. The positive electrode current collector 160 can include aluminum or stainless steel that is surface-treated with carbon, nickel, titanium, or silver. For example, the thickness of the positive electrode current collector 160 can be 10 µm to 50 µm.

[0122] The positive electrode active material can include a compound capable of reversibly intercalating and deintercalating lithium ions.

[0123] In some example embodiments, the positive electrode active material can include a lithium nickel metal oxide. For example, the lithium nickel metal oxide includes nickel (Ni) and can further include at least one of cobalt (Co) and manganese (Mn).

[0124] For example, the lithium nickel metal oxide can include a layered structure or a crystal structure represented by Chemical Formula 1 below.

[0125] [Chemical Formula 1]

[0126] Li x Ni 1-y M y O 2+z

[0127] In Chemical Formula 1, 0.9 ≤ x ≤ 1.2, 0 ≤ y ≤ 0.7, -0.1 ≤ z ≤ 0.1, and M can include at least one element selected from Na, Mg, Ca, Y, Ti, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Co, Fe, Cu, Ag, Zn, B, Al, Ga, Sn, and Zr.

[0128] In some embodiments, the molar ratio or concentration of Ni in Chemical Formula 1 (1-y) can be greater than or equal to 0.8, and in some embodiments, can exceed 0.8.

[0129] The chemical structure represented by Chemical Formula 1 indicates a bonding relationship included in a layered structure or a crystal structure of a positive active material, and does not mean that other additional elements are excluded. For example, M includes Co and / or Mn, and Co and / or Mn can be included together with Ni as a main active element of the positive active material. Chemical Formula 1 is provided to express a bonding relationship of a main active element, and should be understood as a chemical formula including introduction and substitution of additional elements.

[0130] In one embodiment, in addition to the main active element, an auxiliary element for enhancing the chemical stability of the positive active material or the layered structure / crystal structure can be further added. The auxiliary element can be added to the layered structure / crystal structure to form a bond, and it should be understood that this case is also included in the chemical structure represented by Chemical Formula 1.

[0131] The auxiliary element can include, for example, at least one of 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. The auxiliary element can be included as an auxiliary active element, such as Al, which can increase the capacity / power activity of the positive active material together with Co or Mn.

[0132] For example, the positive active material or lithium nickel metal oxide can have a layered structure or a crystal structure represented by the following Chemical Formula 1-1.

[0133] [Chemical Formula 1-1]

[0134] Li x Ni a M1 b1 M2 b2 O 2+z

[0135] In Chemical Formula 1-1, M1 can include Co and / or Mn. M2 can include the above-described auxiliary element. In Chemical Formula 1-1, 0.9≤x≤1.2, 0.6≤a≤0.99, 0.01≤b1+b2≤0.4, and -0.5≤z≤0.1.

[0136] The positive active material can further include a coating element or a doping element. For example, an element substantially the same as or similar to the above-described auxiliary element can be used as the coating element or the doping element. For example, one of the above-described elements or a combination of two or more of the above-described elements can be used as the coating element or the doping element.

[0137] The coating element or the doping element can exist on the surface of the lithium nickel metal oxide particles, and can penetrate the surface of the lithium nickel metal oxide particles and be included in the bonding structure represented by Chemical Formula 1 or Chemical Formula 1-1.

[0138] The positive active material can include a nickel cobalt manganese (NCM)-based lithium oxide. In this case, an NCM-based lithium oxide having an increased nickel content can be used.

[0139] Nickel can serve as a transition metal related to the power and capacity of a lithium secondary battery. Accordingly, as described above, the positive active material can employ a high-Ni composition, thereby achieving a high-capacity positive electrode and a high-capacity lithium secondary battery.

[0140] However, as the Ni content increases, the long-term storage stability and the life stability of the positive electrode or the secondary battery can be relatively deteriorated, and the side reaction with the electrolyte can also increase. However, in some example embodiments, Mn can improve the life stability and the capacity retention rate, while the electrical conductivity can be maintained by the addition of Co.

[0141] The nickel content (e.g., the mole fraction of nickel in the total moles of nickel, cobalt, and manganese) in the NCM-based lithium oxide can be 0.6 or more, 0.7 or more, or 0.8 or more. In some embodiments, the nickel 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.

[0142] In some embodiments, the positive active material can include a lithium cobalt oxide-based active material, a lithium manganese oxide-based active material, a lithium nickel oxide-based active material, or a lithium iron phosphate (LFP) active material (e.g., LiFePO4).

[0143] In some embodiments, the positive active material can include a lithium-rich layered oxide (LLO) / over-lithiated oxide (OLO)-based active material, a manganese-rich-based active material, a low cobalt-based active material, etc. having a chemical structure or a crystal structure represented by, for example, Chemical Formula 2. The lithium-rich layered oxide (LLO) / over-lithiated oxide (OLO)-based active material, the manganese-rich-based active material, and the low cobalt-based active material can be used alone as the positive active material, and a combination of two or more of the lithium-rich layered oxide (LLO) / over-lithiated oxide (OLO)-based active material, the manganese-rich-based active material, and the low cobalt-based active material can be used as the positive active material.

[0144] [Chemical Formula 2]

[0145] p[Li2MnO3]·(1-p)[Li q JO2]

[0146] In Chemical Formula 2, 0 < p < 1, 0.9 ≤ q ≤ 1.2, and J includes at least one element selected from Mn, Ni, Co, Fe, Cr, V, Cu, Zn, Ti, Al, Mg, and B.

[0147] The mixture can be prepared by mixing and stirring the positive active material with a positive binder, a conductive material, and / or a dispersant in a solvent. The mixture can be coated on the positive current collector 160, and then dried and calendered to form the positive active material layer 150.

[0148] The solvent can include N-methyl-2-pyrrolidone (NMP), dimethylformamide, dimethylacetamide, N,N-dimethylaminopropylamine, oxirane, tetrahydrofuran, etc.

[0149] For example, the positive binder can include an organic-based binder such as polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyacrylonitrile, polymethyl methacrylate, etc., or an aqueous binder such as styrene butadiene rubber (SBR), and the positive binder can be used with a thickening agent such as carboxymethyl cellulose (CMC).

[0150] For example, a PVDF-based binder can be used as the positive binder. In this case, the amount of the binder used to form the positive active material layer can be reduced, and the amount of the positive active material can be relatively increased. Accordingly, the capacity and the power of the lithium secondary battery can be further improved.

[0151] The conductive material can include a compound substantially the same as the type of compound used to form the first positive active material layer 120.

[0152] The negative electrode 100 can be formed as described above.

[0153] A separator 140 can be disposed between the negative electrode 150 and the positive electrode 100. The separator 140 can include a porous polymer film formed of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, an ethylene / methacrylate copolymer, or the like. The separator 140 can include a non-woven fabric formed of glass fibers having a high melting point, polyethylene terephthalate fibers, or the like.

[0154] In some embodiments, the area (e.g., the contact area with the separator 140) and / or the volume of the negative electrode 100 can be greater than the area and / or the volume of the positive electrode 150. Accordingly, the lithium ions generated from the positive electrode 150 can be facilitated to transfer to the negative electrode 100 without being, for example, precipitated. Accordingly, it can be easier to achieve the effects of improving the capacity and the output according to the positive electrode active material described above.

[0155] In some example embodiments, an electrode cell can be formed of the positive electrode 150, the negative electrode 100, and the separator 140, and a plurality of electrode cells can be stacked to form an electrode assembly 180 having, for example, a jelly-roll shape. For example, the electrode assembly 180 can be formed by winding, stacking, or z-folding of the separator 140.

[0156] The electrode assembly 150 can be loaded into a case 190 together with an electrolyte to form a lithium secondary battery. In some example embodiments, a non-aqueous electrolyte can be used as the electrolyte.

[0157] The non-aqueous electrolyte can include a lithium salt and an organic solvent. The lithium salt can be represented by Li + X - . The anion of the lithium salt X - may include, for example, 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 - , (CF3CF2SO2)2N - , etc.

[0158] The organic solvent can include, 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-propyl acetate (n-PA), 1,1-dimethyl ethyl acetate (DMEA), methyl propionate (MP), ethyl propionate (EP), fluoroethyl acetate (FEA), difluoroethyl acetate (DFEA), trifluoroethyl acetate (TFEA), dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether (DEGDME), dimethoxyethane, tetrahydrofuran (THF), 2-methyltetrahydrofuran, ethanol, isopropanol, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, sulfolane, gamma-butyrolactone, propylene sulfite, etc. These substances can be used alone or in combination.

[0159] The nonaqueous electrolyte solution can further include an additive. The additive can include, for example, a cyclic carbonate-based compound, a fluorine-substituted carbonate-based compound, a sultone-based compound, a cyclic sulfate-based compound, a cyclic sulfite-based compound, a phosphate-based compound, a borate-based compound, etc. These compounds can be used alone or in combination.

[0160] The cyclic carbonate-based compound can include vinylene carbonate (VC), vinylethylene carbonate (VEC), etc.

[0161] The fluorine-substituted cyclic carbonate-based compound can include fluoroethylene carbonate (FEC), etc.

[0162] The sultone-based compound can include 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, etc.

[0163] The cyclic sulfate-based compound can include 1,2-ethylene sulfate, 1,2-propylene sulfate, etc.

[0164] The cyclic sulfite-based compound can include ethylene sulfite, butylene sulfite, etc.

[0165] The phosphate-based compound can include lithium difluorophosphate, lithium difluorophosphate, etc.

[0166] The borate-based compound can include lithium borate, etc.

[0167] In some embodiments, a solid electrolyte can be used instead of the above-described non-aqueous electrolyte. In this case, the lithium secondary battery can be manufactured in the form of a full solid-state battery. In addition, a solid electrolyte layer can be further disposed between the cathode 150 and the anode 100, instead of the separator 140 described above.

[0168] The solid electrolyte can include a sulfide-based electrolyte. Non-limiting examples of the sulfide-based electrolyte can 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, Li2S-P2S5-Z m S n (m and n are positive numbers, and Z represents Ge, Zn, or Ga), Li2S-GeS2, Li2S-SiS2-Li3PO4, Li2S-SiS2-Li p MO q (p and q are positive numbers, M represents P,

[0169] Si, Ge, B, Al, Ga, or In), Li 7-x PS 6-x Cl x (0≤x≤2), Li 7-x PS 6-x Br x (0≤x≤2),

[0170] Li 7-x PS 6-x I x (0≤x≤2), etc. These substances can be used alone or in combination.

[0171] In one embodiment, the solid electrolyte can include, for example, an oxide-based amorphous solid electrolyte such as Li2O-B2O3-P2O5, Li2O-SiO2, Li2O-B2O3, Li2O-B2O3-ZnO, etc.

[0172] As shown in FIG. 1, the lithium secondary battery 100 can include an electrode assembly 120, a case 190, and an electrode terminal 150. Figure 3 As shown in FIG. 1, the lithium secondary battery 100 can include an electrode assembly 120, a case 190, and an electrode terminal 150.

[0173] The lithium secondary battery can be manufactured, for example, in a cylindrical shape (using a can), a square shape, a pouch shape, or a coin shape.

[0174] The experimental examples discussed below illustrate some embodiments of the technology disclosed herein.

[0175] Manufacture of negative electrode and secondary battery

[0176] Example 1

[0177] (1) Formation of first negative electrode active material layer

[0178] 1) Preparation of silicon-based active material

[0179] A mixture of silicon (Si), silicon dioxide (SiO2), and magnesium (Mg) was added to a reaction furnace and fired at a temperature of 600°C for 5 hours under a vacuum atmosphere of 10 Pa.

[0180] The fired mixture was cooled, and a precipitate was taken out, pulverized, and classified using a ball mill, thereby obtaining a plurality of Mg-doped SiO particles. The amount of magnesium was adjusted to 12% by weight based on the total weight of the silicon-based active material.

[0181] The amounts of silicon and silicon dioxide in the raw material were adjusted so that the ratio of the content of silicon element in the total weight of the silicon-based active material particles to the content of magnesium element in the total weight of the silicon-based active material particles (Si / Mg) was in the range of 2 to 12.

[0182] The obtained plurality of Mg-doped SiO particles were put into a CVD coater, and a mixed gas of methane gas and argon gas was injected at a flow rate of 50 mL / minute to 100 mL / minute. The temperature of the CVD coater was increased to about 400°C to 800°C at a temperature increase rate of 5°C / minute to 20°C / minute, and maintained for about 60 minutes to 360 minutes, to form a carbon coating layer.

[0183] SiO particles having a carbon coating and doped with magnesium were used as the silicon-based active material.

[0184] 2) Preparation of first negative electrode active material

[0185] An anode active material including artificial graphite (D50: 20 μm) and the prepared silicon-based active material (weight ratio of 8:2) at 96.4% by weight, 0.1% by weight of SWCNT conductive material, and 3.5% by weight of CMC / SBR (binder, weight ratio of 1.5 / 2.0) were mixed in water to prepare a first anode active material composition in the form of a slurry.

[0186] The first anode active material composition was coated, dried, and calendered on one surface of a copper current collector (copper foil having a thickness of 8 μm) to form a first anode active material layer.

[0187] (2) Formation of second negative electrode active material layer

[0188] 1) Preparation of porous structure

[0189] Formation of carbon-based particle

[0190] i) Synthesis of resol oligomer: phenol and formaldehyde were mixed at a molar ratio of 1:2, and then 1.5% by weight of triethylamine was added thereto, followed by a reaction at 85°C for 4 hours and 160 rpm (stirring).

[0191] ii) Suspension stabilization of resol oligomer: 1 g of PVA was dispersed in a water dispersion medium, and then added to the resol oligomer.

[0192] iii) Curing of resol oligomer: 3 g of HMTA was added as a curing agent, and reacted at 98°C for 12 hours at 400 rpm (stirring).

[0193] iv) Obtaining of carbon substance: the cured resol oligomer was fractionated using a sieve, and then washed with H2O.

[0194] v) Removal of unreacted monomers and oligomers from the washed resol oligomer using ethanol, and then drying.

[0195] vi) Carbonization and activation: the dried resol oligomer was calcined at 900°C for 1 hour under a nitrogen atmosphere. During the calcination, carbon dioxide gas was introduced at a flow rate of 1 L / min, and carbonization was performed at 900°C.

[0196] Formation of silicon-containing coating layer

[0197] The silane gas is injected into the CVD coater at a flow rate of 50 mL / min to 100 mL / min, the temperature is raised to above 400°C and below 600°C at a temperature increase rate of 5°C / min to 20°C / min, and then maintained for about 120 minutes to 240 minutes to obtain a porous structure in which a silicon-containing coating layer is formed inside and on the surface of the pores of the carbon-based particle.

[0198] 2) Preparation of second negative electrode active material layer

[0199] A second negative active material composition in the form of a slurry is prepared by mixing 90.4 wt% of a graphite-based active material (in which the weight ratio of artificial graphite and natural graphite is 5:5), 6.0 wt% of the prepared porous structure, 0.1 wt% of a SWCNT conductive material, and 3.5 wt% of CMC / SBR (binder, weight ratio 1.5 / 2.0) in water.

[0200] (3) Manufacture of half-cell

[0201] An electrolyte is injected into an electrode assembly including the above-prepared negative electrode, a counter electrode (lithium metal), and a PE separator between the negative electrode and the counter electrode to assemble a coin cell (CR2032). The assembled coin cell is left to stand at room temperature for 3 hours to 24 hours to prepare a half cell. As the electrolyte, a 1.0 M LiPF6 solution in a mixed organic solvent (EC: FEC: EMC / DEC = 2:1:2:5 vol%) is used.

[0202] (4) Manufacture of lithium secondary battery

[0203] Li[Ni 0.88 Co 0.1 Mn 0.02 ]O2 as a positive active material, carbon black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder are mixed at a weight ratio of 96.5:2:1.5 to prepare a slurry. The slurry is uniformly coated on an aluminum foil having a thickness of 12 μm, and then vacuum-dried to prepare a positive electrode for a secondary battery.

[0204] The positive electrode and the negative electrode prepared as described above are respectively cut (notched) to a predetermined size, and stacked together with a separator (polyethylene, thickness: 13 μm) interposed between the positive electrode and the negative electrode to form an electrode core. The respective tab portions of the positive electrode and the negative electrode are welded. The welded positive electrode / separator / negative electrode assembly is inserted into a soft pack, and three sides of the soft pack except for an electrolyte injection side are sealed. The sealed portion also includes the tab portions.

[0205] An electrolyte is injected through the electrolyte injection side, and then the electrolyte injection side is also sealed. Subsequently, the above structure is immersed for more than 12 hours.

[0206] In preparing the electrolyte, a 1M LiPF6solution was prepared using an EC / EMC mixed solvent (volume ratio of 25 / 75), and 8 wt% of fluoroethylene carbonate (FEC), 0.5 wt% of 1,3-propensultone (PRS), and 1.0 wt% of 1,3-propanesultone (PS) were added to the solution.

[0207] After that, hot calendering pre-charge was performed for 60 minutes at a current corresponding to 0.5C. After being stabilized for 12 hours or more, degassing was performed, and then aging was performed for greater than 24 hours and formation charge / discharge was performed (charge condition CC-CV 0.25C 4.2V 0.05C cutoff, discharge condition CC 0.25C 2.5V cutoff).

[0208] After that, standard charge and discharge were performed (charge condition CC-CV 0.33C 4.2V 0.05C cutoff, discharge condition CC 0.33C 2.5V cutoff).

[0209] Example 2

[0210] A negative electrode and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the amount of magnesium added corresponded to 5 wt% based on the total weight of the silicon-based active material.

[0211] Example 3

[0212] A negative electrode and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the amount of magnesium added corresponded to 18 wt% based on the total weight of the silicon-based active material.

[0213] Example 4

[0214] A negative electrode and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the amount of magnesium added corresponded to 15 wt% based on the total weight of the silicon-based active material.

[0215] Comparative Example 1

[0216] A negative electrode and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the second negative electrode active material layer was not formed.

[0217] Comparative Example 2

[0218] A negative electrode and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the same amount of a silicon-based active material (the same as the silicon-based active material contained in the first negative electrode active material layer) was added to the second negative electrode active material layer instead of the porous structure.

[0219] Comparative Example 3

[0220] A negative electrode and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the same amount of silicon oxide (SiOx, 0 < x < 2, D50: 5 μm) (not doped with magnesium) was added to the first negative electrode active material instead of the silicon-based active material in Example 1.

[0221] Comparative Example 4

[0222] A first negative electrode active material composition was formed by mixing 96.4 wt% of a graphite-based active material (in which artificial graphite and natural graphite were mixed at a weight ratio of 5:5), 0.1 wt% of a SWCNT conductive material, and 3.5 wt% of CMC / SBR (binder, weight ratio of 1.5 / 2.0) in water.

[0223] A second negative electrode active material composition was formed by mixing 96.4 wt% of a graphite-based active material (in which artificial graphite and natural graphite were mixed at a weight ratio of 5:5), 0.1 wt% of a SWCNT conductive material, and 3.5 wt% of CMC / SBR (binder, weight ratio of 1.5 / 2.0) in water.

[0224] A negative electrode and a lithium secondary battery were manufactured in the same manner as in Example 1, except for the above details.

[0225] Comparative Example 5

[0226] A negative electrode and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the same amount of silicon oxide (SiOx, 0 < x < 2, D50: 5 μm) (not doped with magnesium) was added to the first negative electrode active material instead of the silicon-based active material in Example 1.

[0227] Comparative Example 6

[0228] A negative electrode and a lithium secondary battery were manufactured in the same manner as in Comparative Example 5, except that the same amount of silicon oxide (SiOx, 0 < x < 2, D50: 5 μm) (not doped with magnesium) was added to the second negative electrode active material instead of the porous structure.

[0229] [Example 1]

[0230] (1) Peak area ratio in Mg Is spectrum on surface of silicon-based active material

[0231] The peak area of 1303 eV and the peak area of 1304.5 eV appearing in the Mg1s spectrum were measured by XPS measurement of the first negative electrode active material layer prepared in Example 1 to Example 4 and Comparative Example 1 to Comparative Example 6.

[0232] As shown in Table 1, the peak area was substituted into Equation 1.

[0233] (2) Evaluation of volume expansion rate and peeling of negative electrode

[0234] After charging (CC / CV 0.1C 0.01V (vs. Li) 0.01C cutoff) of the half cells prepared in Example 1 to Example 4 and Comparative Example 1 to Comparative Example 6 at room temperature (25℃), the coin cells were disassembled.

[0235] The thickness of the uncharged negative electrode (SOC 0, t1) and the thickness of the charged negative electrode (SOC 100, t2) were measured, and the expansion rate of the negative electrode was calculated by the following equation. The calculated expansion rate is shown in Table 2 below.

[0236] [Equation]

[0237] Expansion rate (%) = (t2 - t1) / (t1 - current collector thickness) x 100

[0238] In the above equation, the current collector thickness refers to the thickness of the negative electrode current collector used when manufacturing the secondary battery negative electrode.

[0239] Then, the charged negative electrode was left at room temperature (25℃) for 10 minutes without an additional washing process, and the state of the adhesion surface between the negative electrode current collector and the first negative electrode active material was visually observed to evaluate detachment, as follows:

[0240] O: detachment was detected.

[0241] X: no detachment was detected.

[0242] (3) Evaluation of power characteristics

[0243] The lithium secondary batteries prepared in Example 1 to Example 4 and Comparative Example 1 to Comparative Example 6 were charged (CC / CV 0.1C 0.01V (vs. Li) 0.01C cutoff) and discharged (CC 0.1C 1.5V (vs. Li)) at room temperature. The DCIR (mΩ) and the power (W / kg) during the charging and discharging processes were measured, and are shown in Table 2 below.

[0244] The lithium secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 to 4 were charged (CC / CV 0.3C 4.2V 0.05C cutoff) and discharged (CC 0.3C 2.5V cutoff) twice at room temperature (25°C). Thereafter, each battery was discharged (CC 0.3C) from a charged state (CC / CV 0.3C 4.2V 0.05C cutoff) to SOC 50 points, and the 10-second DCIR (mΩ) and the power at SOC 50 points (W / kg) were measured, as shown in Table 2 below.

[0245] (4) Evaluation of rapid charging life characteristics

[0246] The lithium secondary batteries prepared in Examples 1 to 4 and Comparative Examples 1 to 6 were charged to a DOD 72 state at a C rate of 3.25C / 3.0C / 2.75C / 2.5C / 2.25C / 2.0C / 1.75C / 1.5C / 1.25C / 1.0C / 0.7C / 0.5C in 25 minutes according to a step charging method, and were discharged at 1 / 3C. The rapid charge evaluation was performed in a case where the charge and discharge cycles were repeated as one cycle. After repeating the cycles 100 / 200 / 300 / 400 / 500 times with an interval of 10 minutes between the charge and discharge cycles, the rapid charge capacity retention rate was measured, as shown in Table 3 below.

[0247] [Table 1]

[0248]

[0249]

[0250] [Table 2]

[0251]

[0252] [Table 3]

[0253]

[0254] Referring to Tables 1 to 3, in Examples 1 to 4 including the first negative electrode active material layer containing the silicon-based active material doped with magnesium and the second negative electrode active material layer containing the porous structure, the resistance was reduced, and the power characteristics and the capacity retention rate were improved, as compared with the case of Comparative Examples 1 to 6.

[0255] In Examples 1 to 4, the power characteristics and the life characteristics were improved, as compared with Comparative Examples 1 to 4 and Comparative Example 6 in which the silicon-based active material was directly exposed to the electrolyte.

[0256] In Example 2, the doping amount of magnesium is less than 7 wt% based on the total weight of the silicon-based active material, and the cycle performance is relatively reduced.

[0257] In Example 3, the doping amount of magnesium exceeds 17 wt% based on the total weight of the silicon-based active material, and the resistance is relatively increased.

[0258] In Example 4 in which the peak area ratio according to Formula 1 exceeds 0.6, the power characteristics are relatively reduced.

[0259] [Example 2] Evaluation of charging properties by rate and rapid charging life characteristics according to weight ratio of natural graphite and artificial graphite (1) Example 5 and

[0260] (2) Evaluation method Example 6

[0261] A negative electrode and a lithium secondary battery were prepared in the same manner as in Example 1, except that the weight ratio of the total weight of the natural graphite to the total weight of the artificial graphite in the first negative electrode active material layer and in the second negative electrode active material layer was adjusted as shown in Table 4.

[0262] 1) Evaluation of rapid charging life characteristics

[0263] 2) Evaluation of charging properties by rate

[0264] The evaluation method of the rapid charge life characteristics of the lithium secondary batteries according to Example 1, Example 5, and Example 6 after 100 / 200 / 300 cycles was the same as in (4) of Evaluation Example 1, and the results are shown in Table 4.

[0265] [Example 3] Evaluation of battery performance according to thickness ratio of first negative electrode active material layer and second negative electrode active material layer

[0266] The lithium secondary batteries manufactured according to Example 1, Example 5, and Example 6 were charged (first charge) (CC / CV 0.2C 4.2V 0.05C cutoff) and discharged (CC 0.2C 2.5V cutoff), and then subjected to a second charge (CC / CV xC 4.2V 0.05C cutoff).

[0267] In the second charge, x was 0.2C, 0.333C, 0.5C, 1.0C, 1.5C, 2.0C, 2.5C, and 3.0C, respectively, and the second charge was performed in a thermostatically maintained room (25°C).

[0268] The constant current section charge capacity (%) of each charge rate with respect to the initial 0.2C constant current charge capacity was measured, and the results are shown in Table 5 below.

[0269] [Table 4]

[0270]

[0271] [Table 5]

[0272]

[0273] Referring to Table 4 and Table 5, in Example 5 and Example 6 in which the amount of natural graphite is greater than the amount of artificial graphite in the first negative active material layer or the second negative active material layer, the high-rate chargeability is relatively reduced.

[0274] (1) Example 7 to (2) Evaluation method

[0275] 1) Evaluation of energy density Example 10

[0276] A negative electrode and a lithium secondary battery were manufactured according to the same method as in Example 1, except that the thickness ratio of the second negative active material layer based on the total thickness of the first negative active material layer and the second negative active material layer was modified, as shown in Table 6 below.

[0277] 2) Evaluation of rapid charging life characteristics

[0278] [Example 4] Evaluation of battery performance according to content of silicon-based active material

[0279] The lithium secondary batteries manufactured in Example 1 and Example 7 to Example 10 were charged (CC / CV 0.3C 4.2V 0.05C cut-off) and discharged (CC 0.3C 2.5V cut-off) to obtain the discharge capacity (Ah) and the energy (Wh).

[0280] The volume-energy density was calculated by measuring the volume of each battery in the 4.2V charged state, and the results are shown in Table 6 below.

[0281] (1) Example 11 to

[0282] The rapid charge life characteristics of the secondary batteries according to Example 1 and Example 7 to Example 10 after 100 cycles were evaluated according to the same method as in (4) of Evaluation Example 1, and the results are shown in Table 6 below.

[0283] [Table 6]

[0284]

[0285] Referring to Table 6, in Example 1, Example 7, and Example 8 in which the thickness ratio of the second negative active material layer based on the total thickness of the negative active material layer is in the range of 0.5% to 50%, the side reaction with the electrolyte is suppressed, the life characteristics are improved, and the energy density is also improved.

[0286] In Example 9 in which the thickness ratio of the second negative electrode active material layer was less than 0.5%, the capacity retention was relatively low.

[0287] In Example 10 in which the thickness ratio of the second negative electrode active material layer exceeded 50%, the energy density was relatively low.

[0288] (2) Evaluation method

[0289] 1) Evaluation of volume expansion rate Example 15

[0290] A negative electrode, a half-cell, and a lithium secondary battery were prepared in the same manner as in Example 1, except that the content of the silicon-based active material in the first negative electrode active material layer was modified as shown in Table 7 based on the total weight of the first negative electrode active material layer.

[0291] 2) Evaluation of energy density

[0292] 3) Evaluation of rapid charging life characteristics

[0293] The negative electrode volume expansion rate of each negative electrode prepared in Example 1 and Examples 11 to 15 was measured in the same manner as in (2) of Evaluation Example 1, and the results are shown in Table 7.

[0294] 4) Evaluation of normal charging life characteristics

[0295] The energy density of the lithium secondary battery prepared in Example 1 and Examples 11 to 15 was measured in the same manner as in 1) of (2) of Evaluation Example 3, and the results are shown in Table 7.

[0296] [Example 5] Measurement of pore diameter and crystallite size of porous structure

[0297] The rapid charging life characteristics after 100 cycles of the lithium secondary battery according to Example 1 and Examples 11 to 15 were evaluated in the same manner as in (4) of Evaluation Example 1, and the results are shown in Table 7.

[0298] (1) Example 16 and

[0299] The normal charge life characteristics of the lithium secondary batteries prepared in Example 1 and Examples 11 to 15 were evaluated in the range of DOD 94 (SOC 2-96) at 35°C in a room. Each battery was charged at 0.3C to a voltage corresponding to SOC 96 under constant current / constant voltage (CC / CV) conditions, and then cut off at 0.05C. Then, the battery was discharged at 0.3C to a voltage corresponding to SOC 2 under constant current (CC) conditions, and the discharge capacity was measured. The above process was repeated for 500 cycles, and the discharge capacity retention rate was evaluated as the normal charge life characteristics. The results are shown in Table 7 below.

[0300] [Table 7]

[0301]

[0302] Referring to Table 7, in Example 1 and Examples 11 to 13, the volume expansion rate, the energy density, and the capacity retention were all improved. As the content of the silicon-based active material increased, both the volume expansion rate and the energy density were improved, while the capacity retention was decreased.

[0303] In Example 14, the content of the silicon-based active material exceeded 35 wt%, the volume expansion rate was relatively increased, and the capacity retention rate was relatively decreased.

[0304] In Example 15, the content of the silicon-based active material was less than 0.1 wt%, and the energy density was relatively decreased.

[0305] (2) Evaluation method

[0306] 1) Measurement of pore diameter of carbon-based particle Example 17

[0307] A negative electrode, a half-cell, and a lithium secondary battery were prepared by the same method as in Example 1, except that, in preparing the porous structure, the temperature and the stirring time when synthesizing the phenol-formaldehyde resin oligomer in the first stage and the firing temperature when carbonizing and activating were changed, thereby obtaining a porous structure having a pore diameter and a crystallite size shown in Table 9.

[0308] 2) Measurement of amorphous degree and crystallite size of silicon particle contained in silicon-containing coating layer.

[0309] 3) Measurement of volume expansion rate with respect to capacity

[0310] The pore size of the carbon-based particles prepared according to Example 1, Example 16, and Example 17 was measured using a Micromeritics surface area analyzer (ASAP-2420). Specifically, the position of the maximum peak in the Barrett-Joyner-Halenda (BJH) pore size distribution curve obtained by measuring the nitrogen adsorption isotherm of the sample according to Example 1, Example 16, and Example 17 was determined to determine the pore size of the carbon-based particles.

[0311] 4) Evaluation of rapid charging life characteristics

[0312] The crystallite size of the porous structure prepared according to Example 1, Example 16, and Example 17 was analyzed using XRD and calculated according to Equation 1, as described above.

[0313] If the particle size of silicon is too small to be practically measured by XRD analysis, the silicon in the silicon-containing coating layer was determined to be amorphous silicon.

[0314] The specific XRD analysis equipment / conditions are described in Table 8 below.

[0315] [Table 8]

[0316]

[0317]

[0318] [Example 6] Battery performance according to content of porous structure

[0319] The half-cells prepared according to Example 1, Example 16, and Example 17, respectively, were charged at room temperature (25℃) (CC / CV 0.1C 0.01V (vs. Li) 0.01C cutoff). The percentage increase in the volume of the negative electrode after charging with respect to the initial volume of the negative electrode was calculated, and then divided by the charging capacity, to evaluate the volume expansion rate.

[0320] (1) Example 18 to

[0321] The rapid charging life characteristics of the secondary batteries according to Example 1, Example 16, and Example 17 after 100 cycles were evaluated in the same manner as in (4) of Example 1,

[0322] The evaluation results are shown in Table 9 below.

[0323] [Table 9]

[0324]

[0325] Referring to Table 9, in Example 1, amorphous silicon was deposited on the carbon-based particles including pores having a size of 20 nm or less, the volume expansion rate was reduced, and the capacity retention rate was improved.

[0326] In Example 16 in which the pore diameter exceeds 20 nm, the side reaction of the silicon particles with the electrolyte solution increases.

[0327] Accordingly, the volume expansion rate increases and the capacity retention rate relatively decreases compared to Example 1.

[0328] In Example 17 in which the crystallite size exceeds 7 nm, the mechanical stability of the porous structure during the calendering process or the repeated charging and discharging process relatively decreases. Accordingly, the capacity retention rate relatively decreases compared to Example 1.

[0329] (2) Evaluation method

[0330] 1) Evaluation of volume expansion rate Example 22

[0331] The negative electrode, half-cell, and lithium secondary battery were prepared in the same manner as in Example 1, except that the content of the porous structure based on the total weight of the second negative electrode active material layer was modified as shown in Table 10.

[0332] 2) Evaluation of energy density

[0333] 3) Evaluation of rapid charging life characteristics

[0334] The negative electrode volume expansion rate was measured for each of the negative electrodes prepared in Example 1 and Examples 18 to 22 in the same manner as in (2) of Evaluation Example 1.

[0335] ​

[0336] The energy density of the lithium secondary batteries prepared in Example 1 and Examples 18 to 22 was measured in the same manner as in 1) of (2) of Evaluation Example 3.

[0337] ​

[0338] The rapid charging life characteristics of the secondary batteries prepared according to Example 1 and Examples 18 to 22 after 100 cycles were evaluated in the same manner as in (4) of Evaluation Example 1.

[0339] 4) Evaluation of normal charging life characteristics

[0340] The rapid charging life characteristics of the secondary batteries according to Example 1 and Examples 18 to 22 after 100 cycles were evaluated in the same manner as in 4) of (2) of Example 4.

[0341] The evaluation results are shown in Table 10.

[0342] [Table 10]

[0343]

[0344] Referring to Table 10, in Example 1 and Examples 18 to 20, the volume expansion rate, the energy density, and the capacity retention rate are all improved. As the content of the porous structure increases, the volume expansion rate and the energy density increase, and the capacity retention rate decreases.

[0345] In Example 21 in which the content of the porous structure exceeds 35 wt%, the volume expansion rate relatively increases, and the capacity retention rate relatively decreases.

[0346] In Example 22 in which the content of the porous structure is less than 0.1 wt%, the energy density relatively decreases.

[0347] In some example embodiments of the technology disclosed herein, by adjusting the composition and materials of the first negative electrode active material layer and the second negative electrode active material layer according to the above conditions, the mechanical stability, the power, and the life characteristics of the lithium secondary battery can be improved.

[0348] The technology disclosed herein can be implemented in a rechargeable secondary battery that is widely used in battery-powered devices or systems, including, for example, digital cameras, mobile phones, notebook computers, hybrid electric vehicles, electric vehicles, uninterruptible power supplies, battery energy storage power stations, and other battery energy storage including batteries for solar panels, wind turbines, and other green technology power generation equipment. Specifically, the technology disclosed herein can be implemented in certain embodiments to provide improved electrochemical devices, such as batteries used in various power sources and power supply devices, to mitigate climate change associated with the use of power sources and power supply devices. Lithium secondary batteries based on the technology disclosed herein can be used to address various adverse effects such as air pollution and greenhouse gas emissions, to power electric vehicles (EVs) to replace cars using fossil fuel engines, and to provide battery-based energy storage systems (ESS) to store renewable energy such as solar and wind energy.

[0349] Only specific implementation examples of certain embodiments are described herein. Variations, improvements, and enhancements can be made to the disclosed embodiments and other embodiments in accordance with the disclosure of the present document.

Claims

1. A negative electrode for a lithium secondary battery, comprising: a negative electrode current collector; a first negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the first negative electrode active material layer including a graphite-based active material and a silicon-based active material doped with a metal element; and a second negative electrode active material layer disposed on the first negative electrode active material layer, the second negative electrode active material layer including a porous structure, artificial graphite, and natural graphite, wherein the porous structure includes a carbon-based particle including a pore and a silicon-containing coating layer disposed in the pore or on a surface of the carbon-based particle, wherein the negative electrode current collector, the first negative electrode active material layer, and the second negative electrode active material layer are sequentially stacked.

2. The negative electrode for a lithium secondary battery according to claim 1, wherein The content of the metal element doped in the silicon-based active material is in a range of 7% by weight to 17% by weight, based on the total weight of the silicon-based active material. 3.The negative electrode for a lithium secondary battery according to claim 1, wherein the metal element includes at least one of Mg, Li, Al, Ca, Fe, Ti, or V. 4.The negative electrode for a lithium secondary battery according to claim 1, wherein the metal element includes Mg, wherein a Mg1s spectrum of a surface of the silicon-based active material measured by X-ray photoelectron spectroscopy (XPS) satisfies Formula 1: [Formula 1] P Mg / (P Mg +P MgO )≤0.6 wherein In Formula 1, P Mg represents the area of the 1303 eV peak in the Mg Is spectrum, P MgO represents the area of the 1304.5 eV peak in the Mg Is spectrum. 5.The negative electrode for a lithium secondary battery according to claim 1, wherein the content of the silicon-based active material is in a range of 0.1% by weight to 35% by weight, based on the total weight of the first negative electrode active material layer. 6.The negative electrode for a lithium secondary battery according to claim 1, wherein the silicon-based active material includes a silicon-based active material particle and a carbon coating layer formed on the silicon-based active material particle. 7.The negative electrode for a lithium secondary battery according to claim 1, wherein the graphite-based active material includes artificial graphite and natural graphite, and the weight of the natural graphite included in the first negative electrode active material layer is equal to or less than the weight of the artificial graphite included in the first negative electrode active material layer. 8.The negative electrode for a lithium secondary battery according to claim 7, wherein the ratio of the weight of the natural graphite included in the first negative electrode active material layer to the weight of the artificial graphite included in the first negative electrode active material layer is in a range of 0.025 to 1. 9.The negative electrode for a lithium secondary battery according to claim 1, wherein the weight of the natural graphite included in the second negative electrode active material layer is equal to or less than the weight of the artificial graphite included in the second negative electrode active material layer. 10.The negative electrode for a lithium secondary battery according to claim 9, wherein the ratio of the weight of the natural graphite included in the second negative electrode active material layer to the weight of the artificial graphite included in the second negative electrode active material layer is in a range of 0.025 to 1. 11.The negative electrode for a lithium secondary battery according to claim 1, wherein a content of the porous structure is in a range of 0.1% to 35% by weight, based on a total weight of the second negative electrode active material layer. 12.The negative electrode for a lithium secondary battery according to claim 1, wherein a thickness of the second negative electrode active material layer is 0.5% to 50% of a total thickness of the first negative electrode active material layer and the second negative electrode active material layer. 13.The negative electrode for a lithium secondary battery according to claim 1, wherein the carbon-based particle included in the porous structure includes at least one selected from the group consisting of activated carbon, carbon nanotube, carbon nanowire, graphene, carbon fiber, carbon black, graphite, porous carbon, pyrolyzed xerogel, pyrolyzed xerogel, and pyrolyzed aerogel. 14.The negative electrode for a lithium secondary battery according to claim 1, wherein a pore diameter of the pore included in the carbon-based particle is 20 nm or less. 15.The negative electrode for a lithium secondary battery according to claim 1, wherein the silicon included in the silicon-containing coating has an amorphous structure or a crystallite size of 7 nm or less measured by X-ray diffraction analysis, wherein the crystallite size of the silicon included in the silicon-containing coating is measured according to Formula 2: [Formula 2] L = λ / β × tan θ wherein, In the above Formula 2, L denotes a crystallite size in nm, λ denotes an X-ray wavelength in nm, β denotes a full width at half maximum of a peak of a (111) plane of the silicon included in the silicon-containing coating in rad, and θ denotes a diffraction angle in rad. 16.A lithium secondary battery comprising: a negative electrode; and a positive electrode disposed opposite to the negative electrode, wherein the negative electrode comprises: a negative electrode current collector; a first negative electrode active material layer disposed on at least one surface of the negative electrode current collector, the first negative electrode active material layer including a graphite-based active material and a silicon-based active material doped with a metal element; and a second negative electrode active material layer disposed on the first negative electrode active material layer and including a porous structure including a carbon-based particle of a pore and a silicon-containing coating, wherein the silicon-containing coating is disposed in or on the pore of the carbon-based particle.

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