Negative electrode and secondary battery comprising same

Abstract

The present invention provides a negative electrode and a secondary battery comprising the same, the negative electrode comprising a current collector and a negative electrode active material layer provided on the current collector, in which the negative electrode active material layer comprises a conductive material, a negative electrode active material and a binder, the negative electrode active material comprising a silicon-based active material having a convexity of 0.8 to 0.97, the convexity is measured using a particle shape analyzer, and the convexity is defined by the following Formula 1: [Formula 1] convexity (Cx) = convex hull circumference (Pc) / actual circumference (P). The battery has improved initial efficiency and lifetime characteristics.

Description

[0001] This patent application is a divisional application of Chinese Patent Application No. 202080073378.8, filed on December 17, 2020, entitled "Negative Electrode and Secondary Battery Comprising the Same". TECHNICAL FIELD

[0002] Cross Reference to Related Applications

[0003] This application claims priority to and the benefit of Korean Patent Application No. 10-2019-0169171, filed on December 17, 2019, the disclosure of which is incorporated herein in its entirety by reference. TECHNICAL FIELD

[0004] The present application relates to a negative electrode having excellent initial efficiency and excellent lifespan characteristics, and a secondary battery comprising the same. BACKGROUND

[0005] With the rapid increase in the use of fossil fuels, the demand for the use of alternative or clean energy is increasing, and in order to meet the growing demand, research has been most actively conducted on power generation and power storage using electrochemical reactions.

[0006] A representative example of an electrochemical device using electrochemical energy includes a secondary battery, and the field of application thereof is gradually expanding. Recently, with the technological development and increasing demand for portable devices such as portable computers, mobile phones, cameras, etc., the demand for secondary batteries as an energy source has rapidly increased.

[0007] A secondary battery is generally composed of a cathode, an anode, an electrolyte, and a separator. The anode comprises an anode active material that allows lithium ions released from the cathode to intercalate and deintercalate.

[0008] On the other hand, in order to increase the capacity of a battery, it has been common to use a silicon-based active material such as SiOx(0≤x≤2) as an anode active material. The silicon-based active material commonly used has a rough particle surface, thereby showing a low convex surface value. Therefore, the specific surface area of the silicon-based active material, which can react with an electrolyte, becomes large, thereby excessively reacting with the electrolyte, resulting in low initial efficiency of the battery and deterioration of lifespan characteristics.

[0009] Therefore, there is a need for a new anode that can achieve a secondary battery having excellent initial efficiency and lifespan characteristics. SUMMARY

[0010] Technical Problem

[0011] The present application aims to provide a negative electrode including a silicon-based active material capable of improving initial efficiency and life characteristics.

[0012] However, the object of the present application is not limited to the above-mentioned object, and other objects not described above will be clearly understood by those skilled in the art through the following description.

[0013] Technical Solution

[0014] One aspect of the present application provides a negative electrode including a current collector and a negative electrode active material layer disposed on the current collector, wherein the negative electrode active material layer includes a conductive material, a negative electrode active material, and a binder, the negative electrode active material includes a silicon-based active material having a convexity of 0.8 or more, the convexity is measured using a particle shape analyzer, and the convexity is defined by Equation 1 below.

[0015] [Equation 1]

[0016] Convexity (C x )= Convex hull perimeter (P c ) / Actual perimeter (P)

[0017] Another aspect of the present application provides a secondary battery including the above-mentioned negative electrode.

[0018] Advantages

[0019] Because the negative electrode according to one embodiment of the present application includes a silicon-based active material having a convexity of 0.8 or more, a side reaction with an electrolyte rarely occurs on the surface of the silicon-based active material during battery operation, thereby being able to inhibit a battery from being damaged due to excessive volume expansion of the silicon-based active material. Therefore, it is possible to improve initial efficiency and life characteristics of the negative electrode and the battery. BRIEF DESCRIPTION OF DRAWINGS

[0020] Figure 1 The actual perimeter and the convex hull perimeter of an object to be analyzed are shown. DETAILED DESCRIPTION

[0021] Hereinafter, the present application will be described in more detail to help understanding of the present application.

[0022] The terms and words provided herein should not be interpreted as limited to the commonly used meanings or meanings in dictionaries, but should be interpreted as having meanings and concepts consistent with the technical scope of the present application based on the principle that the inventor is able to properly define the concepts of the terms to describe the present application in the best way.

[0023] The terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to be limiting of the application. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0024] It is to be understood that the terms "comprising", "including", and / or "having" when used herein, refer to the presence of stated features, integers, steps, components and / or groups of features, integers, steps, components but do not preclude the presence or addition of one or more other features, integers, steps, components and / or groups thereof.

[0025] In the present application, D 50 may be defined as a particle diameter corresponding to a cumulative volume of 50% in a particle size distribution curve (a curve on a particle size distribution graph). D 50 may be measured using, for example, a laser diffraction method. The laser diffraction method generally allows measurement of particle diameters ranging from a sub-micron level to several millimeters, and results having high reproducibility and high resolution can be obtained.

[0026] <Negative electrode>

[0027] The negative electrode according to one embodiment of the present application includes a current collector and a negative electrode active material layer provided on the current collector, wherein the negative electrode active material layer includes an electrically conductive material, a negative electrode active material, and a binder, the negative electrode active material includes a silicon-based active material having a convexity of 0.8 or more, the convexity is measured using a particle shape analyzer, and the convexity is defined by Formula 1 below.

[0028] [Formula 1]

[0029] Convexity (C x ) = Convex perimeter (P c ) / Actual perimeter (P)

[0030] The convex perimeter can refer to the length of a rubber band when the object to be analyzed is surrounded by the rubber band. For example, referring to Figure 1 , Figure 1 the black line in a represents the actual perimeter (P), Figure 1 the black line in b represents the convex perimeter (P c ). Thus, the convexity is a variable related to surface roughness and specific surface area, and is different from sphericity related to the macroscopic shape of the entire particle. For example, even a particle that is macroscopically spherical can have a convexity of less than 0.8 when the surface of the spherical particle is very rough.

[0031] The current collector is not particularly limited as long as it does not cause a chemical change in the battery and has electrical conductivity. For example, as the current collector, it is possible to use: copper, stainless steel, aluminum, nickel, titanium, calcined carbon; aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, or the like; and the like. Specifically, as the current collector, it is possible to use a transition metal such as copper or nickel that adsorbs carbon well. The current collector can have a thickness of 6 μm to 20 μm, but the thickness of the current collector is not limited thereto.

[0032] The negative electrode active material layer is provided on the current collector. The negative electrode active material layer can be provided on at least one face of the current collector, specifically, on one face or both faces of the current collector.

[0033] The negative electrode active material layer can contain an electrically conductive material, a negative electrode active material, and a binder.

[0034] The electrically conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has electrical conductivity. As the electrically conductive material, it is possible to use, for example: graphite such as natural graphite, artificial graphite, and the like; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal-cracking black, and the like; electrically conductive fibers such as carbon fibers, metal fibers, and the like; electrically conductive tubes such as carbon nanotubes and the like; fluorocarbons; metal powders such as aluminum powder, nickel powder, and the like; electrically conductive whiskers such as zinc oxide, potassium titanate, and the like; electrically conductive metal oxides such as titanium oxide and the like; or electrically conductive materials such as polyphenylene derivatives and the like.

[0035] The negative electrode active material can contain a silicon-based active material having a convexity of 0.8 or greater, specifically, 0.9 or greater, which is measured using a particle shape analyzer. The upper limit of the convexity can be 1.0. Further, the convexity is defined by the following Formula 1.

[0036] [Formula 1]

[0037] Convexity (C x )= Convex perimeter (P c ) / Actual perimeter (P)

[0038] The convexity can be obtained, for example, by dispersing a silicon-based active material sample (1 mm 3 ) under conditions of 4 bar and 10 ms, then capturing the silicon-based active material particles as a two-dimensional image, and analyzing the image using a particle shape analyzer (Morphologi 4, commercially available from Malvern Panalytical Ltd.). In addition, the convexity can be a convexity corresponding to 50% of the cumulative volume of 10,000 silicon-based active material particles.

[0039] When the convexity of the silicon-based active material satisfies the above range, a side reaction between the silicon-based active material and the electrolyte solution can be prevented, and thus it is possible to improve the initial efficiency and the lifespan characteristics of the battery. When the convexity of the silicon-based active material is less than 0.8, the specific surface area that can react with the electrolyte solution increases, and thus a side reaction excessively occurs with the electrolyte solution, resulting in low initial efficiency and deterioration of the lifespan characteristics of the battery. In addition, the binding characteristics of the electrode can be deteriorated, and thus the adhesion can be reduced.

[0040] The silicon-based active material can be prepared by processing the Si-containing material through a typical pulverization or size classification method known in the art, for example, by using a ball mill, a sieve, or the like. Specifically, the silicon-based active material can be prepared by pulverizing the Si-containing material using a ball mill. The Si-containing material can be obtained by heat-treating Si powder, silicon oxide powder, metal powder, or the like under a reduced pressure atmosphere to allow reactants in a gas phase to react, and cooling the reaction product.

[0041] On the other hand, the convexity of the silicon-based active material can be adjusted by pulverizing the Si-containing material using a ball mill having a ball size range of 5 mm to 15 mm for 9 hours to 20 hours. Specifically, the convexity of the silicon-based active material can be adjusted by pulverizing the Si-containing material using a ball mill having a ball size of 10 mm for 9 hours to 15 hours.

[0042] The silicon-based active material can have a BET specific surface area of 1 m 2 / g to 60 m 2 / g. When the BET specific surface area of the silicon-based active material is within the above range, a side reaction between the silicon-based active material and the electrolyte solution can be prevented, and thus it is possible to improve the initial efficiency and the lifespan characteristics of the battery. The specific surface area of the silicon-based active material can be measured by a Brunauer-Emmett-Teller (BET) method. For example, the specific surface area can be measured by using a nitrogen adsorption flow method using a BET 6-point method using a porosimetry analyzer (BELSORP-mini II, commercially available from Bel Japan Inc.).

[0043] The silicon-based active material can have an average particle diameter (D 50 ) of 0.01 μm to 30 μm, specifically 0.05 μm to 20 μm, and more specifically 0.1 μm to 10 μm. When the average particle diameter of the silicon-based active material satisfies the above range, a decrease in the density of the electrode is prevented, and thus it is possible to achieve an appropriate capacity per unit volume, and to coat a slurry for forming an electrode at a uniform thickness.

[0044] The content of the silicon-based active material in the negative active material layer can be 5 to 80% by weight or 9 to 80% by weight. When the content of the silicon-based active material is within the above range, the initial efficiency and the life characteristics of the negative electrode and the battery can be improved.

[0045] The silicon-based active material can include SiO x (0≤x≤2). When the silicon-based active material includes SiO x (0≤x≤2), the battery capacity can be improved. More specifically, the silicon-based active material can be SiO or SiO2. The SiO can be SiO having crystallinity. In this case, excessive volume expansion of the silicon-based active material during charging and discharging of the battery is controlled, and thus the life characteristics of the battery can be improved. The SiO x (0≤x≤2) can be particles having an average particle diameter (D 50 ) of 1 μm to 15 μm. When the average particle diameter (D x ) of the SiO 50 (0≤x≤2) particles is within the above range, the side reaction between the SiO x (0≤x≤2) and the electrolyte is inhibited, and the formation of lithium silicate from the SiO x (0≤x≤2) is controlled, thereby being able to prevent a decrease in the initial efficiency and to maximize the initial capacity in the electrode design.

[0046] The silicon-based active material can further include a metal silicate phase. The metal silicate can be formed by doping SiO x (0≤x≤2) with a metal, and the metal silicate phase refers to the metal silicate existing in the silicon-based active material in the form of a domain. The metal silicate can be, for example, Mg2SiO4 or MgSiO3. In addition, the silicon-based active material can further include a metal silicide and a metal oxide. In this case, the metal silicide can be, for example, Mg2Si, and the metal oxide can be, for example, MgO.

[0047] The metal can be an element selected from the group consisting of alkali metals, alkaline earth metals, transition metals, Group 13 elements, Group 14 elements, rare earth elements, and combinations thereof. Specifically, the metal can be one or more selected from the group consisting of lithium (Li), magnesium (Mg), calcium (Ca), aluminum (Al), sodium (Na), and titanium (Ti), preferably Li and / or Mg. In this case, since the bonding between the metal element of the metal silicate and oxygen is stronger, the formation of lithium silicate from lithium supplied from the positive electrode during initial charging is prevented, and thus a decrease in the initial efficiency can be prevented.

[0048] The content of the metal in the silicon-based active material can be 0.1 to 30% by weight, specifically 1 to 25% by weight, more specifically 3 to 20% by weight, and still more specifically 4 to 15% by weight. When the content of the metal is within the above range, the silicon-based active material can have a high capacity, and can also more effectively increase the initial efficiency of the silicon-based active material. The content of the metal can be determined by inductively coupled plasma (ICP) analysis.

[0049] The metal can be in the form of a metal silicate or a metal oxide by reacting with the silicon-based particles, specifically SiO x (0≤x≤2) particles. Accordingly, the silicon-based active material according to one embodiment of the present application can include SiO x (0≤x≤2) and a phase including a metal compound selected from one or more of a metal oxide and a metal silicate.

[0050] The metal silicate can include a metal silicate selected from one or more of the following doped metals: Li, Mg, Ca, Al, Na, and Ti. Specifically, the metal silicate includes a metal silicate selected from one or more of Li and Mg, and more specifically includes magnesium silicate (Mg).

[0051] The metal oxide can include a metal oxide selected from one or more of the following doped metals: Li, Mg, Ca, Al, Na, and Ti. Specifically, the metal oxide includes a metal oxide selected from one or more of Li and Mg, and more specifically includes magnesium oxide (Mg).

[0052] The metal compound including one or more of a metal oxide and a metal silicate can be one or more of the following metal compounds: Mg2SiO4, MgSiO3, Mg2Si, and MgO.

[0053] The silicon-based active material can also include a carbon coating layer on the surface thereof. Specifically, the silicon-based active material can also include a carbon coating layer on the surface of SiO x (0≤x≤2). The carbon coating layer can cover at least a portion of the surface of the silicon-based active material. The carbon coating layer can more effectively control the excessive volume expansion of the silicon-based active material during charging and discharging of the battery, and can increase the electrical conductivity of the active material, thereby further reducing the resistance of the negative electrode. In addition, when the carbon coating layer is included, the surface hardness of the silicon-based active material can also be increased, and the electrical conductivity of the silicon-based active material is enhanced, so that charging and discharging occur uniformly, whereby the volume change during charging and discharging can be more effectively controlled.

[0054] The silicon-based active material further including the carbon coating layer on the surface thereof can have a specific surface area of 0.5 m2 / g to 15 m 2 / g. Specifically, the silicon-based active material can have a BET specific surface area of 1 m 2 / g to 13 m 2 / g. When the BET specific surface area of the silicon-based active material including a carbon coating layer on the surface thereof is within the above range, a side reaction between the silicon-based active material and the electrolyte solution can be prevented, thereby enabling improvement in initial efficiency and life characteristics of the battery.

[0055] The content of the carbon coating layer on the surface of the silicon-based active material can be 0.1% to 50% by weight, specifically 1% to 25% by weight, and more specifically 3% to 15% by weight, with respect to the total weight of the silicon-based active material. When the content of the carbon coating layer satisfies the above range, the conductivity of the silicon-based active material increases, and thus charging and discharging occur uniformly, thereby enabling more effective control of volume change during charging and discharging.

[0056] The thickness of the carbon coating layer can be 1 nm to 200 nm, specifically 5 nm to 100 nm. When the thickness of the carbon coating layer satisfies the above range, the conductivity of the negative electrode can be improved while maintaining the conductive path in the negative active material.

[0057] The negative active material can further include a carbon-based negative active material. The carbon-based negative active material can include at least one selected from the group consisting of artificial graphite, natural graphite, and graphitized mesocarbon microbeads. Specifically, the carbon-based active material is preferably artificial graphite.

[0058] When the negative active material includes the silicon-based active material and the carbon-based negative active material, the silicon-based active material and the carbon-based negative active material can be included in a weight ratio of 3:97 to 20:80, 5:95 to 20:80, or 10:90 to 20:80. When the weight ratio of the silicon-based active material and the carbon-based negative active material satisfies the above range, the battery capacity can be improved, and the volume change of the negative active material that can occur during charging and discharging of the negative electrode can be suppressed, thereby prolonging the life of the negative electrode.

[0059] The binder can include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, polyacrylic acid, and a material in which the hydrogen thereof is replaced with Li, Na, Ca, or the like, and further includes various copolymers thereof.

[0060] In the negative electrode active material layer, the content of the binder can be 30% by weight or less, specifically 0.1% to 30% by weight. When the content of the binder satisfies the above range, the adhesion effect resulting from the use of the binder can be exerted, and the capacity per unit volume of the negative electrode desired can be maintained.

[0061] <Secondary battery>

[0062] The secondary battery according to another embodiment of the present application includes a negative electrode, and the negative electrode is the same as the above-described negative electrode.

[0063] Specifically, the secondary battery can include: a negative electrode; a positive electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte, and the negative electrode is the same as the above-described negative electrode. Since the negative electrode has been described above, detailed description thereof will be omitted.

[0064] The positive electrode can include: a positive electrode current collector; and a positive electrode active material layer formed on the positive electrode current collector and including a positive electrode active material.

[0065] In the positive electrode, the positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has electrical conductivity. As the positive electrode current collector, for example, stainless steel, aluminum, nickel, titanium, calcined carbon; or aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, or the like can be used. In addition, the positive electrode current collector can generally have a thickness of 3 μm to 500 μm, and can have fine irregularities formed on its surface to increase the adhesion of the positive electrode active material. In addition, the positive electrode current collector can be used in any one of a variety of forms such as a film, a sheet, a foil, a mesh, a porous material, a foam, a nonwoven fabric, or the like.

[0066] The positive electrode active material can be a generally used positive electrode active material. Specific examples of the positive electrode active material include: a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or the like or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; lithium manganese oxide such as LiMnO4 (0≤c1≤0.33), LiMnO3, LiMn2O3, LiMnO2, or the like; lithium copper oxide (Li2CuO2); vanadium oxide such as LiV3O8, V2O5, Cu2V2O7, or the like; Ni-site type lithium nickel oxide represented by the chemical formula LiNi 1+c1 Mn 2-c1 O4 (0≤c1≤0.33), LiMnO3, LiMn2O3, LiMnO2, or the like; lithium copper oxide (Li2CuO2); vanadium oxide such as LiV3O8, V2O5, Cu2V2O7, or the like; Ni-site type lithium nickel oxide represented by the chemical formula LiNi 1-c2 M c2 O2 (here, M is at least one selected from Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01≤c2≤0.3); lithium manganese oxide represented by the chemical formula LiMn 2-c3 M c3O2 (here, M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 0.01 ≤ c3 ≤ 0.1) or Li2Mn3MO8 (here, M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn) represents a lithium-manganese composite oxide; LiMn2O4 in which part of Li ions in the chemical formula are replaced with alkaline earth metal ions, or the like, but the present application is not limited thereto. The positive electrode can be a Li metal.

[0067] In addition to the above-described positive electrode active material, the positive electrode active material layer can further include a positive electrode conductive material and a positive electrode binder.

[0068] In this case, the positive electrode conductive material serves to impart conductivity to the electrode, and any conductive material that does not cause a chemical change in the battery and has electron conductivity can be used without particular limitation. Specific examples of the conductive material include graphite such as natural graphite, artificial graphite, and the like; carbon-based materials such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal crack black, carbon fiber, and the like; metal powders or metal fibers containing copper, nickel, aluminum, silver, and the like; conductive whiskers such as zinc oxide, potassium titanate, and the like; conductive metal oxides such as titanium oxide, and the like; and conductive polymers such as polyphenylene derivatives, and the like, which can be used alone or in combination of two or more thereof.

[0069] In addition, the positive electrode binder serves to improve adhesion between positive electrode active material particles and adhesion between the positive electrode active material and the positive electrode current collector. Specific examples of the binder include polyvinylidene fluoride (PVDF), a vinylidene-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, and various copolymers thereof, which can be used alone or in combination of two or more thereof.

[0070] The separator is used to separate the negative electrode from the positive electrode and to provide a passage for lithium ions to move, and any separator commonly used as a separator in a secondary battery can be used without particular limitation. In particular, a separator that is low in resistance to movement of electrolyte ions and has excellent electrolyte impregnation ability is preferable. Specifically, a porous polymer film, such as a porous polymer film made 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; or having a stacked structure of two or more layers thereof can be used. Alternatively, a typical porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fibers, polyethylene terephthalate fibers, or the like can be used. Alternatively, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymeric material can be used, and optionally, a separator of a single layer or a multilayer structure can be used.

[0071] Examples of the electrolyte include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, an inorganic solid electrolyte, a molten-type inorganic electrolyte, and the like, which are useful for manufacturing a lithium secondary battery, but the present application is not limited thereto.

[0072] Specifically, the electrolyte can contain a nonaqueous organic solvent and a metal salt.

[0073] As the nonaqueous organic solvent, for example, an aprotic organic solvent such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxy methane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, ethyl propionate, and the like can be used.

[0074] In particular, among carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are preferably used because they are high-viscosity organic solvents and have a high dielectric constant, thereby dissociating lithium salt well. When a mixture formed by mixing these cyclic carbonates with linear carbonates such as dimethyl carbonate and diethyl carbonate, which are low-viscosity, low-dielectric-constant, in an appropriate ratio is used, an electrolyte having a high electric conductivity can be prepared. Thus, the mixture is more preferably used.

[0075] As the metal salt, a lithium salt can be used, and the lithium salt is a substance that is easily dissolved in a nonaqueous electrolyte solution. For example, as the anion of the lithium salt, one or more selected from the group consisting of F - , Cl - , 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 - .

[0076] In order to improve the life characteristics of the battery, suppress the decrease in the capacity of the battery, improve the discharge capacity of the battery, and the like, the electrolyte can contain, in addition to the above-mentioned electrolyte components, one or more additives selected from the group consisting of halogenated alkylene carbonate compounds such as difluoroethylene carbonate and the like, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, (glycol) dimethyl ether, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinonimine dyes, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, and the like.

[0077] According to still another embodiment of the present application, there is provided a battery module containing the above-mentioned secondary battery as a unit cell and a battery pack containing the battery module. Since the battery module and the battery pack contain a secondary battery having high capacity, high rate characteristics, and cycle characteristics, they can be used as a power source for a large-sized device selected from the group consisting of an electric vehicle, a hybrid electric vehicle, and a plug-in hybrid electric vehicle; and a system for storing electric power.

[0078] Examples

[0079] In the following, exemplary embodiments of the present application will be described to facilitate the understanding of the present application. However, it will be apparent to those skilled in the art that the exemplary embodiments presented herein are intended to exemplify the present application and that various changes and variations can be made within the scope and technical spirit of the present application, so that the present application encompasses all such changes and variations if they are within the scope of the appended claims.

[0080] Preparation Example: Preparation of Silicon-Based Active Material

[0081] Preparation Example 1

[0082] A powder containing Si powder and silica (SiO2) powder uniformly mixed at a molar ratio of 1:1 was heat-treated at 1400°C and 700°C, respectively, under a reduced pressure atmosphere, to simultaneously generate silica vapor and magnesium vapor from Si and silica, and to cause the reactants in the gas phase to react. The reaction product was cooled to induce precipitation, and then pulverized for 10 hours by using a ball mill with 10 mm balls. Then, size classification was performed to collect a silicon-based active material having an average particle diameter (D 50 ) of 5 μm and containing MgSiO3 and Mg2SiO4.

[0083] The collected silicon-based active material was put into a tube furnace and subjected to chemical vapor deposition (CVD) under a mixed gas of argon and methane to produce a silicon-based active material containing a carbon coating layer having a carbon content of 5% by weight formed on the surface thereof.

[0084] As a result of performing inductively coupled plasma (ICP) analysis to determine the Mg content of the produced silicon-based active material, it was found that the content of Mg with respect to the total weight of the silicon-based active material was 8% by weight.

[0085] The convexity corresponding to 50% of the cumulative volume of 10000 silicon-based active material particles was 0.89, which was obtained by dispersing a silicon-based active material sample (1 mm 3 ) under conditions of 4 bar and 10 ms, then capturing the silicon-based active material particles as a two-dimensional image, and analyzing the image using a particle shape analyzer (Morphologi 4 commercially available from Malvern Panalytical Ltd.).

[0086] Preparation Examples 2 to 4

[0087] A silicon-based active material was produced in the same manner as in Production Example 1, except that the pulverization by the ball mill was performed for 14 hours, 8 hours, and 6 hours, respectively.

[0088] The convexities corresponding to 50% of the cumulative volume of 10,000 silicon-based active material particles were 0.97, 0.75, and 0.69, respectively, which were obtained by dispersing the silicon-based active material samples prepared in Preparation Examples 2 to 4 (1 mm 3 ) under the conditions of 4 bar and 10 ms, then capturing the silicon-based active material particles as two-dimensional images, and analyzing the images using a particle shape analyzer (Morphologi 4 commercially available from Malvern Panalytical Ltd.).

[0089] Examples and Comparative Examples

[0090] Example 1

[0091] A silicon-based active material having a convexity of 0.89 prepared in Preparation Example 1 as a negative active material, carbon black as a conductive material, and polyacrylic acid (PAA) as a binder were mixed in water (H2O) as a solvent at a weight ratio of 80:10:10 to prepare a uniform negative electrode slurry. The negative electrode slurry was coated on one face of a copper current collector, dried and calendered, and then the resultant was punched into a constant size to manufacture a negative electrode.

[0092] A coin-type half cell of Example 1 was manufactured using Li metal as a counter electrode, inserting a polyolefin separator between the negative electrode and the Li metal, and then injecting an electrolyte prepared by dissolving 1M LiPF6 in a solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) mixed at a volume ratio of 30:70.

[0093] Example 2

[0094] A coin-type half cell of Example 2 was manufactured in the same manner as in Example 1, except that a silicon-based active material having a convexity of 0.97 prepared in Preparation Example 2 was used as a negative active material.

[0095] Example 3

[0096] A negative active material containing a silicon-based active material having a convexity of 0.89 prepared in Preparation Example 1 and natural graphite mixed at a weight ratio of 1:9 as a negative active material, carbon black as a conductive material, and carboxymethyl cellulose and styrene butadiene rubber (SBR) as binders were mixed in water (H2O) as a solvent at a weight ratio of 95.4:1:1.1:2.5 to prepare a uniform negative electrode slurry. The negative electrode slurry was coated on one face of a copper current collector, dried and calendered, and then the resultant was punched into a constant size to manufacture a negative electrode.

[0097] A coin-type half cell of Example 3 was manufactured using Li metal as a counter electrode, inserting a polyolefin separator between the negative electrode and the Li metal, and then injecting an electrolyte prepared by dissolving 1M LiPF6 in a solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) mixed at a volume ratio of 30:70. 0.6 CO 0.2Mn 0.2 ]O2 as a positive electrode active material as a positive electrode, a polyolefin separator was inserted between the negative electrode and the positive electrode, and then an electrolyte prepared by dissolving 1 M LiPF6 in a solvent containing ethylene carbonate and diethyl carbonate mixed at a volume ratio of 30:70 was injected, thereby manufacturing a coin-type half cell of Example 1.

[0098] Example 4

[0099] A coin-type half cell of Comparative Example 1 was manufactured in the same manner as in Example 1, except that the silicon-based active material having a convexity of 0.75 prepared in Preparation Example 3 was used as the negative electrode active material.

[0100] Comparative Example 1

[0101] A coin-type half cell of Comparative Example 1 was manufactured in the same manner as in Example 1, except that the silicon-based active material having a convexity of 0.75 prepared in Preparation Example 3 was used as the negative electrode active material.

[0102] Comparative Example 2

[0103] A coin-type half cell of Comparative Example 2 was manufactured in the same manner as in Example 1, except that the silicon-based active material having a convexity of 0.69 prepared in Preparation Example 4 was used as the negative electrode active material.

[0104] Comparative Example 3

[0105] A coin-type half cell of Comparative Example 1 was manufactured in the same manner as in Example 1, except that the silicon-based active material having a convexity of 0.75 prepared in Preparation Example 3 was used as the negative electrode active material.

[0106] Comparative Example 4

[0107] A coin-type half cell of Comparative Example 2 was manufactured in the same manner as in Example 1, except that the silicon-based active material having a convexity of 0.69 prepared in Preparation Example 4 was used as the negative electrode active material.

[0108] Experimental Examples

[0109] Experimental Example 1: Evaluation of Discharge Capacity and Initial Efficiency

[0110] The various coin-type half cells of Examples 1 and 2 and Comparative Examples 1 and 2 were charged at a constant current (CC) of 0.1 C at 25°C until 5 mV, and then subjected to a first charge by charging at a constant voltage (CV) until a charge current of 0.005 C (cutoff current). After the battery was held for 20 minutes, and then discharged at a constant current (CC) of 0.1 C until 1.5 V, the discharge capacity (mAh / g) and initial efficiency (%) were evaluated, and the results thereof are shown in Table 1 below. The initial efficiency (%) was calculated by Formula 2 below.

[0111] [Formula 2]

[0112] Initial efficiency (%) = (discharge capacity after first discharge / initial charge capacity) x 100

[0113]

[0114] Referring to Table 1, it can be seen that the batteries of Examples 1 and 2, which contain a silicon-based active material having a convexity of 0.8 or more, exhibit more excellent initial efficiency and more excellent discharge capacity due to the prevention of side reactions with the electrolyte, compared to the batteries of Comparative Examples 1 and 2, which contain a silicon-based active material having a convexity of less than 0.8.

[0115] Experimental Example 2: Evaluation of Capacity Retention Rate and Electrode Thickness Increase Rate

[0116] The various coin-type half cells of Examples 1 and 2 and Comparative Examples 1 and 2 were charged at a constant current (CC) of 0.1 C at 25°C until 5 mV, and then subjected to a first charge by charging at a constant voltage (CV) until a charge current of 0.005 C (cutoff current). After the battery was held for 20 minutes, and then discharged at a constant current (CC) of 0.1 C until 1.5 V, the discharge capacity (mAh / g) and initial efficiency (%) were evaluated, and the results thereof are shown in Table 1 below. The initial efficiency (%) was calculated by Formula 2 below.

[0117] Specifically, the various coin-type half cells of Examples 1 and 2 and Comparative Examples 1 and 2 were charged at a constant current (CC) of 0.1 C at 25°C until 5 mV, and then subjected to a first charge by charging at a constant voltage (CV) until a charge current of 0.005 C (cutoff current). After the battery was held for 20 minutes, and then discharged at a constant current (CC) of 0.1 C until 1.5 V, the discharge capacity (mAh / g) and initial efficiency (%) were evaluated, and the results thereof are shown in Table 1 below. The initial efficiency (%) was calculated by Formula 2 below.

[0118] [Formula 3]

[0119] Capacity retention rate (%) = (discharge capacity after 45th cycle / discharge capacity of 1st cycle) x 100

[0120] [Formula 4]

[0121] Electrode thickness change rate (%) = (final negative electrode thickness change amount / initial negative electrode thickness) x 100

[0122] Experimental Example 3: Evaluation of Electrode Adhesion

[0123] Before the batteries in Examples 3 and 4 and Comparative Examples 3 and 4 were manufactured, the adhesion of the manufactured negative electrodes was measured, and the results thereof are shown in Table 2 below.

[0124] Specifically, a double-sided tape was pasted on a glass slide, and each of the negative electrodes of Examples 3 and 4 and Comparative Examples 3 and 4, which were punched to a size of 20 mm x 180 mm, was placed thereon, and then calendered by using a 2 kg roller back and forth (one round trip) 10 times to be adhered thereto. Then, a universal testing machine (UTM; TA Instruments) was used to measure the force applied when the electrode was pulled at a speed of 20 mm / minute so as to be peeled from the glass slide. In this case, the measurement angle between the glass slide and the negative electrode was 90°.

[0125]

[0126] Referring to Table 2, it can be seen that the secondary batteries of Examples 3 and 4, which contain the silicon-based active material having a convexity of 0.8 or more, exhibit a more excellent capacity retention rate, a lower electrode thickness change rate, and a more excellent electrode adhesion force, compared to the secondary batteries of Comparative Examples 3 and 4, which contain the silicon-based active material having a convexity of less than 0.8. It can thus be seen that because the secondary batteries containing the silicon-based active material having a convexity of 0.8 or more have a relatively small specific surface area, side reactions with the electrolyte on the surface of the active material can occur less frequently, and the binder can be properly positioned with the same binder content, thereby effectively maintaining the shape of the electrode during charging and discharging. It can thus be seen that the secondary batteries containing the silicon-based active material having a convexity of 0.8 or more exhibit a low electrode thickness change rate and a high capacity retention rate during charging and discharging.

Claims

1. A negative electrode, said negative electrode comprising: Current collector; and A negative electrode active material layer disposed on the current collector. The negative electrode active material layer comprises a conductive material, a negative electrode active material, and a binder. The negative electrode active material comprises a silicon-based active material with a convexity of 0.8 to 0.97, the convexity being measured using a particle shape analyzer. The convexity is defined by the following equation 1. [Formula 1] Convexity = convex hull perimeter / actual perimeter.

2. The negative electrode according to claim 1, wherein the silicon-based active material has a convexity of 0.89 to 0.

97.

3. The negative electrode according to claim 1, wherein the silicon-based active material has a 1 μm... 2 / g to 60 m 2 / g BET specific surface area.

4. The negative electrode according to claim 1, wherein the silicon-based active material comprises SiO₂. x (0≤x≤2).

5. The negative electrode according to claim 1, wherein the silicon-based active material further comprises a metal silicate phase formed by doping with metal.

6. The negative electrode according to claim 5, wherein the metal is selected from one or more of the following: lithium, magnesium, calcium, aluminum, sodium and titanium.

7. The negative electrode according to claim 5, wherein the metal content in the silicon-based active material is from 0.1% to 30% by weight.

8. The negative electrode according to claim 1, wherein the content of the silicon-based active material in the negative electrode active material layer is from 5% to 80% by weight.

9. The negative electrode according to claim 1, wherein the silicon-based active material further comprises a carbon coating on its surface.

10. A secondary battery comprising a negative electrode according to any one of claims 1 to 9.

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