Negative electrode and secondary battery including the same

Abstract

The present invention provides a negative electrode and a secondary battery including the negative electrode, wherein the negative electrode includes a current collector and a negative electrode active material layer provided on the current collector, wherein the negative electrode active material layer includes a conductive material, a negative electrode active material and a binder, and the negative electrode active material includes a silicon-based active material having a convexity of 0.8 or greater, the convexity being measured using a particle shape analyzer and being defined by the following formula 1: [Formula 1] Convexity (C x )=Convex hull perimeter(P c ) / actual circumference (P).

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] 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 hereby incorporated by reference in its entirety. Technical Field

[0004] The present invention relates to a negative electrode having excellent initial efficiency and excellent lifespan characteristics, and a secondary battery including the same. Background Art

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

[0006] Representative examples of electrochemical devices using electrochemical energy include secondary batteries, and their application fields are gradually expanding. Recently, with the technological development and increase in demand for portable devices such as portable computers, mobile phones, cameras, etc., the demand for secondary batteries as energy sources has increased dramatically.

[0007] Secondary batteries are generally composed of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode contains a negative electrode active material that allows lithium ions released from the positive electrode to be intercalated and deintercalated.

[0008] On the other hand, silicon-based active materials, such as SiOx (0 ≤ x ≤ 2), have been widely used as negative electrode active materials to increase battery capacity. Conventionally used silicon-based active materials have rough particle surfaces, resulting in low convex surface values. As a result, the specific surface area of ​​the silicon-based active material that can react with the electrolyte increases, leading to excessive side reactions with the electrolyte, resulting in low initial efficiency and deterioration of battery life characteristics.

[0009] Therefore, there is a need for a novel negative electrode that can realize a secondary battery excellent in initial efficiency and life characteristics. Summary of the Invention

[0010] Technical issues

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

[0012] However, the objects of the present invention are not limited to the above objects, 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 invention provides a negative electrode, which includes a collector and a negative electrode active material layer arranged on the collector, wherein the negative electrode active material layer includes a conductive material, a negative electrode active material and a binder, and the negative electrode active material includes a silicon-based active material with a convexity of 0.8 or more, the convexity is measured using a particle shape analyzer, and the convexity is defined by the following formula 1.

[0015] [Formula 1]

[0016] Convexity (C x ) = convex hull perimeter (P c ) / actual circumference (P)

[0017] Another aspect of the present invention provides a secondary battery including the negative electrode.

[0018] Beneficial effects

[0019] Because the negative electrode according to one embodiment of the present invention includes a silicon-based active material with a convexity of 0.8 or greater, side reactions with the electrolyte rarely occur on the surface of the silicon-based active material during battery operation, thereby suppressing battery damage caused by excessive volume expansion of the silicon-based active material. Consequently, the initial efficiency and lifespan characteristics of the negative electrode and battery can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

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

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

[0022] The terms and words provided herein should not be construed as limited to the commonly used meanings or dictionary meanings, but should be construed as having meanings and concepts consistent with the technical scope of the present invention on the basis of the principle that the inventor can appropriately define the concepts of the terms to describe the present invention in the best manner.

[0023] The terminology provided herein is for describing exemplary embodiments only and is not intended to be limiting of the present invention. Unless the context clearly indicates otherwise, the singular forms "a," "an," "the," and "said" include plural referents and are intended to include the plural forms as well.

[0024] It should be understood that the terms "comprises", "includes" and / or "having" used herein are used to clarify the existence of the stated features, integers, steps, components and / or their combinations, but do not preclude the existence or addition of at least one other feature, integer, step, component and / or its combination.

[0025] In the present invention, D 50 It can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve (curve on the particle size distribution graph). D can be measured using, for example, laser diffraction. 50 Laser diffraction methods generally allow the measurement of particle sizes in the range from submicrometers to several millimeters and can obtain results with high reproducibility and high resolution.

[0026] <Negative electrode>

[0027] According to one embodiment of the present invention, the negative electrode includes a current collector and a negative electrode active material layer arranged on the current collector, wherein the negative electrode active material layer includes a conductive material, a negative electrode active material and a binder, and the negative electrode active material includes a silicon-based active material having a convexity of 0.8 or more, and the convexity is measured using a particle shape analyzer, and the convexity is defined by the following formula 1.

[0028] [Formula 1]

[0029] Convexity (C x )=Convex hull perimeter(P c ) / actual circumference (P)

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

[0031] There are no particular limitations on the current collector, as long as it does not cause chemical changes in the battery and is conductive. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, can be used as the current collector. Specifically, transition metals that adsorb carbon well, such as copper or nickel, can be used. The current collector can have a thickness of 6 to 20 μm, but the thickness is not limited thereto.

[0032] The negative electrode active material layer is disposed on the current collector. The negative electrode active material layer may be disposed on at least one surface of the current collector, specifically, on one surface or both surfaces of the current collector.

[0033] The negative electrode active material layer may include a conductive material, a negative electrode active material, and a binder.

[0034] There are no particular limitations on the conductive material, as long as it does not cause chemical changes in the battery and is conductive. Examples of the conductive material include graphite, such as natural graphite and artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers, such as carbon fibers and metal fibers; conductive tubes, such as carbon nanotubes; fluorocarbons; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; and conductive materials, such as polyphenylene derivatives.

[0035] The negative electrode active material may include a silicon-based active material having a convexity of 0.8 or greater, specifically 0.9 or greater, as measured using a particle shape analyzer. The upper limit of the convexity may be 1.0. Furthermore, the convexity is defined by the following formula 1.

[0036] [Formula 1]

[0037] Convexity (C x )=Convex hull perimeter(P c ) / actual circumference (P)

[0038] The convexity can be obtained, for example, by dispersing a sample of silicon-based active material (1 mm 3 ), then the silicon-based active material particles are captured as a two-dimensional image, and the image is analyzed using a particle shape analyzer (Morphologi 4 commercially available from Malvern Panalytical Ltd.). Alternatively, the convexity may 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 falls within the above range, side reactions between the silicon-based active material and the electrolyte can be prevented, thereby improving the initial efficiency and battery lifespan. When the convexity of the silicon-based active material is less than 0.8, the specific surface area available for reaction with the electrolyte increases, leading to excessive side reactions with the electrolyte, resulting in low initial efficiency and degraded battery lifespan. Furthermore, the bonding properties of the electrodes may deteriorate, reducing adhesion.

[0040] Silicon-based active materials can be prepared by treating the Si-containing material using typical pulverization or size classification methods known in the art, such as using a ball mill, a sieve, or the like. Specifically, the Si-containing material can be pulverized 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 reduced pressure to react the reactants in the gas phase, and then 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 for 9 to 20 hours using a ball mill with a ball size ranging from 5 mm to 15 mm. Specifically, the convexity of the silicon-based active material can be adjusted by pulverizing the Si-containing material for 9 to 15 hours using a ball mill with a ball size of 10 mm.

[0042] Silicon active materials can have 1m 2 / g to 60m 2 / g of BET specific surface area. When the BET specific surface area of ​​the silicon-based active material is within the above range, the side reaction between the silicon-based active material and the electrolyte can be prevented, thereby improving the initial efficiency and life characteristics of the battery. The specific surface area of ​​the silicon-based active material can be measured by the Brunauer-Emmett-Teller (BET) method. For example, the specific surface area can be measured by the BET 6-point method using a nitrogen adsorption flow method using a porosity analyzer (BELSORP-mini II commercially available from BelJapan Inc.).

[0043] The average particle size of the silicon active material (D 50 ) can be 0.01 μm to 30 μm, specifically 0.05 μm to 20 μm, more specifically 0.1 μm to 10 μm. When the average particle size of the silicon-based active material satisfies the above range, the reduction of the electrode density is prevented, thereby achieving an appropriate capacity per unit volume, and the slurry for forming the electrode can be applied with a uniform thickness.

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

[0045] Silicon-based active materials may 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. SiO can be SiO with crystallinity. In this case, the excessive volume expansion of the silicon-based active material during battery charging and discharging is controlled, thereby improving the life characteristics of the battery. SiO x (0≤x≤2) can be the average particle size (D 50 ) are particles of 1 μm to 15 μm. x (0≤x≤2) Average particle size (D 50 ) is within the above range, SiO x The side reaction with the electrolyte is suppressed, and the SiO x (0≤x≤2) The formation of lithium silicate is controlled, thereby preventing a decrease in initial efficiency and maximizing the initial capacity in electrode design.

[0046] Silicon active materials can also include metal silicate phases. Metal silicates can be formed by doping SiO with metals. x (0≤x≤2), and the metal silicate phase refers to the presence of metal silicate in the form of domains in the silicon-based active material. The metal silicate may be, for example, Mg2SiO4 or MgSiO3. In addition, the silicon-based active material may also include metal silicide and metal oxide. In this case, the metal silicide may be, for example, Mg2Si, and the metal oxide may be, for example, MgO.

[0047] The metal may 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 may 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, because the metal element of the metal silicate has a stronger bond with oxygen, the lithium supplied from the positive electrode during initial charging is prevented from forming lithium silicate, thereby preventing a decrease in initial efficiency.

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

[0049] The metal can react with the silicon-based particles, specifically SiO x(0≤x≤2) particles react to form metal silicates or metal oxides. Therefore, the silicon-based active material according to one embodiment of the present invention may include SiO x (0≤x≤2) and a metal compound phase comprising one or more selected from metal oxides and metal silicates.

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

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

[0052] The metal compound including one or more selected from metal oxides and metal silicates may be one or more metal compounds selected from the group consisting of Mg2SiO4, MgSiO3, Mg2Si, and MgO.

[0053] The silicon-based active material may further include a carbon coating on its surface. Specifically, the silicon-based active material may further include SiO x (0≤x≤2) on the surface. The carbon coating may cover at least a portion of the surface of the silicon-based active material. The carbon coating may more effectively control excessive volume expansion of the silicon-based active material during battery charging and discharging, and may increase the conductivity of the active material, thereby further reducing the resistance of the negative electrode. In addition, when the carbon coating is included, the surface hardness of the silicon-based active material may be increased, and the conductivity of the silicon-based active material may be enhanced, thereby making charging and discharging uniform, thereby more effectively controlling volume changes during charging and discharging.

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

[0055] The carbon coating layer on the surface of the silicon-based active material may be present in an amount of 0.1 to 50 wt %, specifically 1 to 25 wt %, and more specifically 3 to 15 wt %, relative to the total weight of the silicon-based active material. When the carbon coating layer content satisfies the above range, the conductivity of the silicon-based active material increases, thereby uniformly charging and discharging, and thus more effectively controlling the volume change during charging and discharging.

[0056] The thickness of the carbon coating layer may 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 electrode active material.

[0057] The negative electrode active material may also include a carbon-based negative electrode active material. The carbon-based negative electrode active material may include at least one selected from the following: artificial graphite, natural graphite, and graphitized mesophase carbon microbeads. Specifically, the carbon-based active material is preferably artificial graphite.

[0058] When the negative electrode active material includes a silicon-based active material and a carbon-based negative electrode active material, the silicon-based active material and the carbon-based negative electrode active material may 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 to the carbon-based negative electrode active material satisfies the above range, the battery capacity can be improved, and the volume change of the negative electrode active material that may occur during the charge and discharge of the negative electrode can be suppressed, thereby extending the life of the negative electrode.

[0059] The adhesive may include at least one selected from the following: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene propylene diene monomer rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, polyacrylic acid and materials whose hydrogen is replaced by Li, Na, Ca, etc., and also include various copolymers thereof.

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

[0061] <Secondary Battery>

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

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

[0064] The positive electrode may 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, there is no particular limitation on the positive electrode current collector as long as it does not cause chemical changes in the battery and has conductivity. As the positive electrode current collector, for example, stainless steel, aluminum, nickel, titanium, calcined carbon; or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, etc. can be used. In addition, the positive electrode current collector can generally have a thickness of 3μm to 500μm, and can have fine concavoconvexities 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 of a variety of forms such as a film, a sheet, a foil, a mesh, a porous material, a foam, a non-woven fabric, etc.

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

[0067] The positive electrode active material layer may further include a positive electrode conductive material and a positive electrode binder in addition to the positive electrode active material described above.

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

[0069] In addition, the positive electrode binder is used to improve the adhesion between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode current collector. Specific examples of the binder include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene propylene diene monomer rubber (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber and various copolymers thereof, and the above substances 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 provide a channel for the movement of lithium ions, and any separator commonly used as a separator in a secondary battery can be used without particular limitation. In particular, a separator having low resistance to the movement of electrolyte ions and excellent electrolyte impregnation ability is preferred. Specifically, it is possible to use: a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, an ethylene / methacrylate copolymer, etc.; or a stacked structure having two or more layers thereof. Alternatively, a typical porous non-woven fabric can be used, for example, a non-woven fabric made of a high melting point glass fiber, polyethylene terephthalate fiber, etc. Alternatively, a coated separator comprising a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and optionally, a separator having a single-layer or multi-layer structure can be used.

[0071] Examples of the electrolyte include organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, inorganic solid electrolytes, molten inorganic electrolytes, etc., which can be used to manufacture lithium secondary batteries, but the present invention is not limited thereto.

[0072] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0073] As the non-aqueous 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, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, ethyl propionate, etc. can be used.

[0074] Especially, in carbonate organic solvents, preferably use ethylene carbonate and propylene carbonate as cyclic carbonate, because they are high viscosity organic solvents and have high dielectric constant, thus well dissociate lithium salt.When using by these cyclic carbonates and low viscosity, low dielectric constant straight chain carbonate such as dimethyl carbonate and diethyl carbonate mixed in appropriate proportion and formed mixture, can prepare the electrolyte with high conductivity.Therefore, more preferably use described mixture.

[0075] As the metal salt, lithium salt can be used, and lithium salt is a substance that is easily soluble in non-aqueous electrolyte. For example, as the anion of the lithium salt, one or more selected from the following can be used: 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, inhibit the decline of the battery capacity, improve the discharge capacity of the battery, etc., in addition to the above-mentioned electrolyte components, the electrolyte may also contain one or more additives selected from the following: halogenated alkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, (condensed) glycol dimethyl ether, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted Oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum chloride, etc.

[0077] According to another embodiment of the present invention, a battery module including the aforementioned secondary battery as a unit cell and a battery pack including the battery module are provided. Because the battery module and the battery pack include secondary batteries with high capacity, high rate characteristics, and high cycle characteristics, they can be used as power sources for medium- to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, and plug-in hybrid electric vehicles; and systems for storing electricity.

[0078] Example

[0079] Hereinafter, exemplary embodiments of the present invention will be described to facilitate understanding of the present invention. However, it will be apparent to those skilled in the art that the exemplary embodiments set forth herein are intended to illustrate the present invention and that various changes and modifications can be made within the scope and technical spirit of the present invention, so that the present invention encompasses all such changes and modifications if they are within the scope of the appended claims.

[0080] Preparation example: Preparation of silicon active materials

[0081] Preparation Example 1

[0082] The powder containing Si powder and silicon oxide (SiO2) powder uniformly mixed in a molar ratio of 1:1 was heat-treated with magnesium (Mg) at 1400°C and 700°C respectively under a reduced pressure atmosphere, thereby simultaneously generating silicon oxide vapor and magnesium vapor from Si and silicon oxide, and reacting the reactants in the gas phase. The reaction product was cooled to induce precipitation and then pulverized for 10 hours by a ball mill using 10mm balls. Then, size classification was performed to collect the average particle size (D 50 ) is 5 μm and contains silicon-based active materials such as 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 prepare a silicon-based active material including a carbon coating layer having a carbon content of 5 wt % formed on the surface thereof.

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

[0085] The convexity corresponding to 50% of the cumulative volume of 10,000 silicon-based active material particles is 0.89, which is obtained by dispersing the silicon-based active material sample (1 mm 3 ), the silicon-based active material particles were then captured as a two-dimensional image, and the image was analyzed using a particle shape analyzer (Morphologi 4 commercially available from Malvern Panalytical Ltd.).

[0086] Preparation Examples 2 to 4

[0087] Silicon-based active materials were prepared in the same manner as in Preparation Example 1, except that the pulverization was performed by a ball mill 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, and the convexities were obtained by dispersing the silicon-based active material samples (1 mm 2 ) prepared in Preparation Examples 2 to 4 under the conditions of 4 bar and 10 ms. 3 ), the silicon-based active material particles were then captured as a two-dimensional image, and the image was analyzed using a particle shape analyzer (Morphologi 4 commercially available from Malvern Panalytical Ltd.).

[0089] Examples and Comparative Examples

[0090] Example 1

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

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

[0093] Example 2

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

[0095] Example 3

[0096] A negative electrode active material comprising the silicon-based active material with a convexity of 0.89 and natural graphite prepared in Preparation Example 1, mixed at a weight ratio of 1:9, carbon black as a conductive material, and carboxymethyl cellulose and styrene-butadiene rubber (SBR) as a binder at a weight ratio of 95.4:1:1.1:2.5 were mixed in water (HO) as a solvent to prepare a uniform negative electrode slurry. The negative electrode slurry was applied to one surface of a copper current collector, dried, and rolled, and the resultant was then punched into a constant size to produce a negative electrode.

[0097] will contain Li[Ni 0.6 CO 0.2 Mn 0.2 ]O2 as a positive electrode active material is used as a counter electrode, a polyolefin separator is inserted between the negative electrode and the positive electrode, and then an electrolyte prepared by dissolving 1M LiPF6 in a solvent containing ethylene carbonate and diethyl carbonate mixed in a volume ratio of 30:70 is injected to manufacture a dual-cell type lithium secondary battery of Example 3.

[0098] Example 4

[0099] A bicell type lithium secondary battery of Example 4 was manufactured in the same manner as Example 3 except that a negative electrode active material including the silicon-based active material having a convexity of 0.97 prepared in Preparation Example 2 and natural graphite mixed at a weight ratio of 1:9 was used.

[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 bicell type lithium secondary battery of Comparative Example 3 was manufactured in the same manner as Example 3 except that a negative electrode active material including the silicon-based active material having a convexity of 0.75 prepared in Preparation Example 3 and natural graphite mixed at a weight ratio of 1:9 was used.

[0106] Comparative Example 4

[0107] A bicell type lithium secondary battery of Comparative Example 4 was manufactured in the same manner as in Example 3 except that a negative electrode active material including the silicon-based active material having a convexity of 0.69 prepared in Preparation Example 4 and natural graphite mixed at a weight ratio of 1:9 was used.

[0108] Experimental example

[0109] Experimental Example 1: Evaluation of discharge capacity and initial efficiency

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

[0111] [Formula 2]

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

[0113] [Table 1]

[0114] Discharge capacity (mAh / g) Initial efficiency (%) Example 1 1403 81.8 Example 2 1414 82.1 Comparative Example 1 1385 80.9 Comparative Example 2 1367 79.2

[0115] Referring to Table 1, it can be seen that compared with the batteries of Comparative Examples 1 and 2 containing silicon-based active materials with a convexity of less than 0.8, the batteries of Examples 1 and 2 containing silicon-based active materials with a convexity of 0.8 or more exhibit better initial efficiency and better discharge capacity due to the prevention of side reactions with the electrolyte.

[0116] Experimental Example 2: Evaluation of Capacity Retention and Electrode Thickness Increase

[0117] Charge and discharge were performed on each of the dual-cell type lithium secondary batteries of Examples 3 and 4 and Comparative Examples 3 and 4. Then, the capacity retention rate and the electrode thickness increase rate were evaluated, and the results are shown in Table 2 below.

[0118] Specifically, the various dual-battery lithium secondary batteries of Examples 3 and 4 and Comparative Examples 3 and 4 were charged at a constant current (CC) of 1C to 4.25V at 25°C, and then charged for the first time, and the first charging was performed as follows: charging at a constant voltage (CV) to a charging current (cut-off current) of 0.05C. After the battery was kept for 20 minutes, the battery was discharged at a constant current (CC) of 1C to 2.5V. The cycle was repeated 50 times, and the capacity retention rate was evaluated, and after the 51st charge, the thickness of the electrode was measured, and the thickness increase rate was evaluated. The capacity retention rate and the electrode thickness change rate were calculated by the following formulas 3 and 4, respectively.

[0119] [Formula 3]

[0120] Capacity retention (%) = (discharge capacity after 45 cycles / discharge capacity at 1st cycle) × 100

[0121] [Formula 4]

[0122] Electrode thickness change rate (%) = (final negative electrode thickness change / initial negative electrode thickness) × 100 Experimental Example 3: Evaluation of electrode adhesion

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

[0124] Specifically, double-sided tape was attached to a glass slide, and each negative electrode punched into a size of 20 mm × 180 mm of Examples 3 and 4 and Comparative Examples 3 and 4 was placed thereon, and then rolled by using a 2 kg roller to roll back and forth (one round trip) 10 times to adhere to it. 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 / min to peel off from the glass slide. In this case, the measurement angle between the glass slide and the negative electrode was 90°.

[0125] [Table 2]

[0126] Capacity retention rate (%) Electrode thickness change rate (%) Electrode adhesion (gf / 20mm) Example 3 90.3 53.2 31 Example 4 92.7 51.8 35 Comparative Example 3 87.6 60.4 23 Comparative Example 4 85.4 65.9 18

[0127] Referring to Table 2, it can be seen that compared with the secondary batteries of Comparative Examples 3 and 4 containing silicon-based active materials with a convexity of less than 0.8, the secondary batteries of Examples 3 and 4 containing silicon-based active materials with a convexity of 0.8 or more exhibited better capacity retention, lower electrode thickness change rate, and better electrode adhesion. Therefore, it can be seen that because the secondary battery containing silicon-based active materials with a convexity of 0.8 or more has a relatively small specific surface area, side reactions with the electrolyte on the surface of the active material can be less likely to occur, and the binder can be properly positioned with the same binder content, thereby effectively maintaining the shape of the electrode during charge and discharge. Therefore, it can be seen that the secondary battery containing silicon-based active materials with a convexity of 0.8 or more exhibits a low electrode thickness change rate and a high capacity retention rate during charge and discharge.

Claims

1. A method for preventing a side reaction between a silicon-based active material and an electrolyte by adjusting the convexity of the silicon-based active material, wherein the silicon-based active material is contained in a negative electrode, The negative electrode comprises: current collector; and A negative electrode active material layer is provided on the current collector, wherein the negative electrode active material layer comprises a conductive material, a negative electrode active material and a binder, The negative electrode active material includes the silicon-based active material, and the silicon-based active material has a convexity of 0.8 or greater, and the convexity is measured using a particle shape analyzer. The silicon-based active material includes SiO x , where 0≤x≤2, and The convexity is defined by the following equation 1: [Formula 1] Convexity = convex hull perimeter / actual perimeter, The convex hull perimeter refers to the length of the rubber band when the object to be analyzed is surrounded by the rubber band. The convexity is obtained by: dispersing 1 mm at 4 bar and 10 ms 3 The silicon-based active material sample is then captured as a two-dimensional image of the silicon-based active material particles and the image is analyzed using a particle shape analyzer. The convexity is a convexity corresponding to 50% of the cumulative volume of 10,000 silicon-based active material particles. The method according to claim 1 , wherein the silicon-based active material has a convexity of 0.9 or greater.

3. The method according to claim 1, wherein the silicon-based active material has 1m 2 / g to 60m 2 / g of BET specific surface area. The method according to claim 1 , wherein the silicon-based active material further comprises a metal silicate phase formed by doping with a metal.

5. The method according to claim 4, wherein the metal is one or more selected from the group consisting of lithium, magnesium, calcium, aluminum, sodium and titanium. The method according to claim 4 , wherein the content of the metal in the silicon-based active material is 0.1 wt % to 30 wt %. 7 . The method according to claim 1 , wherein the silicon-based active material is present in an amount of 5 wt % to 80 wt % in the negative electrode active material layer. The method according to claim 1 , wherein the silicon-based active material further comprises a carbon coating on a surface thereof. 9 . The method according to claim 1 , wherein the negative electrode is included in a secondary battery.

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