Negative electrode and secondary battery including the negative electrode

Abstract

To provide a negative electrode comprising a silicon-based active material capable of improving initial efficiency and lifetime characteristics.SOLUTION: A negative electrode comprises a 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 having convexity of 0.8 or more as measured using a particle shape analyzer. The convexity is defined by a formula 1, Convexity (Cx)=Convex hull perimeter (Pc) / Actual perimeter (P). A secondary battery comprises the negative electrode.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] [CROSS REFERENCE TO RELATED APPLICATIONS] This application claims the benefit of priority based on Korean Patent Application No. 10-2019-0169171 filed on December 17, 2019, and all contents disclosed in the documents of said Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a negative electrode having excellent initial efficiency and life characteristics, and a secondary battery including the negative electrode. [Background technology]

[0003] With the rapid increase in the use of fossil fuels, there is an increasing demand for the use of alternative and clean energy. As part of this, the most actively researched field is the field of power generation and storage using electrochemical reactions.

[0004] At present, a representative example of such an electrochemical element that uses electrochemical energy is a secondary battery, and the range of its use is expanding. Recently, with the development of technology and increasing demand for portable devices such as portable computers, mobile phones, and cameras, the demand for secondary batteries as an energy source has increased rapidly.

[0005] In general, a secondary battery is composed of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode includes a negative electrode active material that inserts and extracts lithium ions from the positive electrode.

[0006] On the other hand, in order to increase the capacity of the battery, silicon-based active materials, such as SiO x (0≦x≦2) is mainly used. Conventionally used silicon-based active materials have rough particle surfaces and a small convexity value, which increases the specific surface area that can react with the electrolyte, causing excessive side reactions with the electrolyte, resulting in low initial efficiency and deteriorated life characteristics of the battery.

[0007] Therefore, a new type of anode capable of realizing a secondary battery having excellent initial efficiency and life characteristics is required. Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide a negative electrode containing a silicon-based active material capable of improving initial efficiency and life characteristics.

[0009] However, the problems to be solved by the present invention are not limited to those mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the following description. [Means for solving the problem]

[0010] According to one embodiment of the present invention, there is provided a negative electrode comprising a current collector and a negative electrode active material layer disposed on the current collector, the negative electrode active material layer including a conductive material, a negative electrode active material, and a binder, the negative electrode active material including a silicon-based active material having a convexity of 0.8 or more as measured by a particle shape analyzer, the convexity being defined by the following Equation 1:

[0011] [Formula 1] Convexity (C x ) = Convex hull perimeter (P c ) / Actual perimeter (P)

[0012] According to another embodiment of the present invention, there is provided a secondary battery including the negative electrode. Effect of the Invention

[0013] The negative electrode according to an embodiment of the present invention includes a silicon-based active material having a convexity of 0.8 or more, so that side reactions of the electrolyte on the surface of the silicon-based active material during operation of the battery are reduced, and damage to the battery due to excessive volume expansion of the silicon-based active material is suppressed, thereby improving the initial efficiency and life characteristics of the negative electrode and the battery. [Brief description of the drawings]

[0014] [Figure 1] FIG. 2 is a diagram showing the actual perimeter and convex hull perimeter of a measurement object. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] The present invention will now be described in further detail to aid in the understanding of the present invention.

[0016] The terms and words used in this specification should not be interpreted limited to their ordinary and dictionary meanings, but should be interpreted in a way that is consistent with the technical ideas of the present invention, based on the principle that an inventor may appropriately define the concepts of terms in order to best describe his or her invention.

[0017] The terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expression includes the plural expression unless otherwise clearly indicated in the context.

[0018] In this specification, terms such as "comprise", "comprise", or "have" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the possibility of the presence or addition of one or more other features, numbers, steps, components, or combinations thereof.

[0019] In this specification, D 50can be defined as the particle size that corresponds to 50% of the cumulative volume in the particle size distribution curve (graph curve of particle size distribution diagram). 50 can be measured by using, for example, a laser diffraction method. The laser diffraction method generally allows measurement of particle sizes from the submicron range to about several mm, and can provide results with high reproducibility and high resolution.

[0020] <Negative electrode> According to an embodiment of the present invention, there is provided a negative electrode comprising a current collector and a negative electrode active material layer disposed on the current collector. The negative electrode active material comprises a silicon-based active material having a convexity of 0.8 or more as measured by a particle shape analyzer, and the convexity is defined by the following Equation 1:

[0021] [Formula 1] Convexity (C x ) = Convex hull perimeter (P c ) / Actual perimeter (P)

[0022] The convex hull perimeter may refer to the length of a rubber band when the measurement target is surrounded by the rubber band. For example, referring to FIG. 1, the black line in FIG. 1(a) indicates the actual perimeter (P), and the black line in FIG. 1(b) indicates the convex hull perimeter (P c ) In this way, the convexity is a variable related to the surface roughness and specific surface area, and is different from the sphericity, which is related to the macroscopic shape of the whole particle. For example, even if a particle is macroscopically spherical, if the surface of the sphere is excessively rough, the convexity may be less than 0.8.

[0023] The current collector is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. For example, the current collector may be copper, stainless steel, aluminum, nickel, titanium, baked carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. Specifically, transition metals that easily adsorb carbon, such as copper and nickel, can be used as the current collector. The thickness of the current collector may be 6 μm to 20 μm, but the thickness of the current collector is not limited thereto.

[0024] The negative electrode active material layer may be disposed on at least one surface of the current collector, specifically, on one or both surfaces of the current collector.

[0025] The negative electrode active material layer may include a conductive material, a negative electrode active material, and a binder.

[0026] The conductive material is not particularly limited as long as it is conductive without inducing a chemical change in the battery, and examples of the conductive material that can be used 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 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.

[0027] The negative electrode active material may include a silicon-based active material having a convexity of 0.8 or more, specifically 0.9 or more, as measured by a particle size analyzer. The upper limit of the convexity may be 1.0. The convexity may be defined by the following formula 1:

[0028] [Formula 1] Convexity (C x) = Convex hull perimeter (P c ) / Actual perimeter (P)

[0029] The convexity was measured, for example, by measuring the convexity of a 1 mm silicon-based active material sample using a Morphologi 4 manufactured by Malvern Instruments as a particle shape analyzer. 3 may be obtained by dispersing the silicon-based active material particles under conditions of 4 bar and 10 ms, and then capturing and analyzing the silicon-based active material particles in a 2-dimensional image, and the convexity may be 50% of the cumulative volume of 10,000 silicon active material particles.

[0030] When the convexity of the silicon-based active material satisfies 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. When the convexity of the silicon-based active material is less than 0.8, the specific surface area that can react with the electrolyte increases, and side reactions with the electrolyte occur excessively, resulting in low initial efficiency and deteriorated life characteristics of the battery. In addition, there is a problem that the binding characteristics of the electrode are reduced, resulting in low adhesive strength.

[0031] The silicon-based active material may be prepared by subjecting a material containing Si to a common pulverization or classification method known in the art, such as ball milling, sieving, etc. Specifically, for example, the silicon-based active material may be prepared by pulverizing a material containing Si with a ball mill pulverizer. The material containing Si may be a reactant obtained by heat-treating Si powder, silicon oxide powder, metal powder, etc. in a reduced pressure atmosphere and reacting in a gas phase, and then cooling the reactant.

[0032] Meanwhile, the convexity of the silicon-based active material can be adjusted by pulverizing the Si-containing material in a ball mill pulverizer having a ball size of 5 mm to 15 mm for 9 to 20 hours. Specifically, the convexity of the silicon-based active material can be adjusted by pulverizing the Si-containing material in a ball mill pulverizer having a ball size of 10 mm for 9 to 15 hours.

[0033] The silicon-based active material has a BET specific surface area of ​​1 m 2 / g to 60m 2 / g. When the BET specific surface area of ​​the silicon-based active material 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. The specific surface area of ​​the silicon-based active material can be measured by the BET (Brunauer-Emmett-Teller) method. For example, it can be measured by the BET 6-point method using a porosimetry analyzer (Belsorp-II mini, Bell Japan Inc.) and a nitrogen gas adsorption and flow method.

[0034] The silicon-based active material has an average particle size (D 50 ), specifically, the average particle size may be 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 electrode can have an appropriate capacity per volume by preventing the density from decreasing, and the slurry for forming the electrode can be coated to a uniform thickness.

[0035] The silicon-based active material may be included in the negative electrode active material layer in an amount of 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.

[0036] The silicon-based active material is SiO x (0≦x≦2). The silicon-based active material may include SiO x When the silicon-based active material contains (0≦x≦2), the capacity of the battery can be increased. More specifically, the silicon-based active material may be SiO or SiO2. The SiO may be crystalline SiO. In this case, excessive volume expansion of the silicon-based active material during charging and discharging of the battery can be controlled, and the life characteristics of the battery can be improved. The SiO x (0≦x≦2) is the average particle size (D 50) may be 1 μm to 15 μm. x (0≦x≦2) Average particle size (D 50 ) satisfies the above range, x The side reaction between the SiO and the electrolyte is suppressed. x Since the formation reaction of lithium silicate (0≦x≦2) is controlled, the decrease in initial efficiency is prevented, and the realization of the initial capacity in the electrode design can be maximized.

[0037] The silicon-based active material may further include a metal silicate phase. The metal silicate is SiO x (0≦x≦2), and the metal silicate phase means that the metal silicate exists in the form of a domain in the silicon-based active material. The metal silicate may be, for example, Mg2SiO4 or MgSiO3. The silicon-based active material may further include a metal silicide and a metal oxide, and the metal silicide may be, for example, Mg2Si and the metal oxide may be, for example, MgO.

[0038] 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 at least one selected from lithium (Li), magnesium (Mg), calcium (Ca), aluminum (Al), sodium (Na), and titanium (Ti), and is preferably lithium and / or magnesium. In this case, the bonding strength between the metal element of the metal-silicate and oxygen is superior, so that lithium silicate is prevented from being formed by lithium supplied from the positive electrode during initial charging, and thus a decrease in initial efficiency can be prevented.

[0039] The metal may be included in the silicon-based active material at 0.1 wt% to 30 wt%, specifically 1 wt% to 25 wt%, more specifically 3 wt% to 20 wt%, and even more specifically 4 wt% to 15 wt%. When the metal content is within the above range, the silicon-based active material has a high capacity and the initial efficiency of the silicon-based active material can be more effectively increased. The metal content can be confirmed by inductively coupled plasma (ICP) analysis.

[0040] The metal is a silicon-based particle contained in the silicon-based active material, specifically, SiO x (0≦x≦2) The silicon-based active material according to one embodiment of the present invention may react with the particles to form a metal silicate or a metal oxide. x (0≦x≦2); and a metal compound phase including at least one selected from metal oxides and metal silicates.

[0041] The metal silicate may include one or more metal silicates selected from the doping metals Li, Mg, Ca, Al, Na, and Ti, specifically, may include one or more metal silicates selected from Li and Mg, and more specifically, may include a silicate of Mg.

[0042] The metal oxide may include one or more metal oxides selected from the doping metals Li, Mg, Ca, Al, Na, and Ti, specifically, one or more metal oxides selected from Li and Mg, and more specifically, Mg oxide.

[0043] The metal compound containing at least one selected from the group consisting of metal oxides and metal silicates may be at least one metal compound selected from the group consisting of Mg2SiO4, MgSiO3, Mg2Si, and MgO.

[0044] The silicon-based active material may further include a carbon coating layer on the surface. x The silicon-based active material may further include a carbon coating layer on the surface (0≦x≦2). The carbon coating layer may cover at least a portion of the surface of the silicon-based active material. The carbon coating layer may more effectively control excessive expansion of the volume of the silicon-based active material during charging / discharging of the battery, and may increase the conductivity of the active material to further reduce the resistance of the negative electrode. In addition, when the carbon coating layer is included, the surface hardness of the silicon-based active material may be further increased, and the electrical conductivity of the silicon-based active material may be improved to enable uniform charging / discharging, thereby more effectively controlling volume change during charging / discharging.

[0045] The BET specific surface area of ​​the silicon-based active material further including a carbon coating layer on the surface is 0.5 m 2 / g to 15m 2 Specifically, the BET specific surface area of ​​the silicon-based active material may be 1 m 2 / g to 13m 2 When the BET specific surface area of ​​the silicon-based active material further including the carbon coating layer on the 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.

[0046] The silicon-based active material may include a carbon coating layer on the surface of the silicon-based active material in an amount of 0.1 wt % to 50 wt %, specifically 1 wt % to 25 wt %, more specifically 3 wt % to 15 wt %, based on the total weight of the silicon-based active material. When the carbon coating layer satisfies the above range, the electrical conductivity of the silicon-based active material is improved, enabling uniform charging / discharging, and volume change during charging / discharging can be more effectively controlled.

[0047] The carbon coating layer may have a thickness of 1 nm to 200 nm, specifically, 5 nm to 100 nm. When the thickness is within this range, the electrical conductivity of the negative electrode can be improved while maintaining the conductive path in the negative electrode active material.

[0048] The negative electrode active material may further include a carbon-based negative electrode active material. The carbon-based negative electrode active material may 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.

[0049] When the negative electrode active material includes both 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 and the carbon-based negative electrode active material satisfies the above range, the capacity of the battery is improved, and a volume change of the negative electrode active material that may occur during charging / discharging of the negative electrode is suppressed, thereby improving the life of the negative electrode.

[0050] The binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which hydrogen is substituted with Li, Na, Ca, or the like, or may include various copolymers thereof.

[0051] The binder may be included in the negative electrode active material layer in an amount of 30 wt% or less, specifically, 0.1 wt% to 30 wt%. When the binder content is within this range, the adhesive effect due to the use of the binder can be exhibited and the intended capacity per volume of the negative electrode can be maintained.

[0052] <Secondary battery> A secondary battery according to another embodiment of the present invention may include a negative electrode, which may be the same as the negative electrode of the above-described embodiment.

[0053] In particular, the secondary battery may include the negative electrode, a positive electrode, a separator interposed between the positive electrode and the negative electrode, and an electrolyte, and the negative electrode is the same as the negative electrode described above. The negative electrode has been described above, so a detailed description thereof will be omitted.

[0054] 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.

[0055] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity without inducing a chemical change in the battery, and may be, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. The positive electrode current collector may generally have a thickness of 3 μm to 500 μm, and fine irregularities may be formed on the collector surface to increase the adhesive strength of the positive electrode active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0056] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; a lithium iron oxide such as LiFe3O4; 1+c1 Mn2-c1 O4 (0≦c1≦0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 Ni-site type lithium nickel oxide represented by the chemical formula LiMn 2-c3 Examples of the lithium manganese composite oxide include, but are not limited to, LiMn2O4 in which part of the Li in the chemical formula is replaced with an alkaline earth metal ion, and Li3O2 (wherein 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 (wherein M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn). The positive electrode may be Li metal.

[0057] The positive electrode active material layer may contain a positive electrode conductive material and a positive electrode binder in addition to the positive electrode active material described above.

[0058] In this case, the positive electrode conductive material is used to impart conductivity to the electrode, and can be used without any particular limitation as long as it has electronic conductivity without causing chemical changes in the battery that is constructed.Specific examples include graphite such as natural graphite and artificial graphite; carbon-based materials such as acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fibers; metal powders or metal fibers such as copper, nickel, aluminum, and silver; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like, and one or more of these may be used alone or in mixture.

[0059] The positive electrode binder plays a role in improving the adhesion between the positive electrode active material particles and the adhesive strength between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these may be used alone or in combination.

[0060] The separator is a membrane that separates the negative electrode and the positive electrode to provide a passage for lithium ions to move. Any separator that is generally used in secondary batteries can be used without any particular limitation. In particular, a separator that has low resistance to ion movement of the electrolyte and has excellent humidification ability of the electrolyte solution is preferable. Specifically, a porous polymer film, for example, 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, and an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of a high-melting point glass fiber, a polyethylene terephthalate fiber, or the like, may be used. In addition, a coated separator containing a ceramic component or a polymeric substance to ensure heat resistance or mechanical strength may be used, and may be selectively used in a single layer or multilayer structure.

[0061] Examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries.

[0062] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0063] Examples of the non-aqueous organic solvent that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate.

[0064] In particular, among the carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, may be preferably used since they are high-viscosity organic solvents with high dielectric constants and good dissociation of lithium salts. When such a cyclic carbonate is mixed with a linear carbonate having a low viscosity and low dielectric constant, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte having high electrical conductivity can be produced, and thus these carbonates may be more preferably used.

[0065] The metal salt may be a lithium salt, and the lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte solution. For example, the anion of the lithium salt may be 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 - One or more selected from the group consisting of may be used.

[0066] In addition to the components of the electrolyte, the electrolyte may further include one or more additives such as a haloalkylene carbonate compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing a decrease in the capacity of the battery, and improving the discharge capacity of the battery.

[0067] According to another embodiment of the present invention, there is provided a battery module including the secondary battery as a unit cell, and a battery pack including the same. The battery module and the battery pack include the secondary battery having high capacity, high rate characteristics and cycle characteristics, and therefore may be used as a power source for a medium- to large-sized device selected from the group consisting of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system.

[0068] Below, preferred embodiments are presented to aid in understanding the present invention. However, the above embodiments are merely illustrative of the present description, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope and technical ideas of the present description. Naturally, such changes and modifications fall within the scope of the claims.

[0069] Manufacturing example: Manufacturing of silicon-based active materials Manufacturing Example 1 A homogeneous mixture of Si powder and silicon oxide (SiO2) powder in a 1:1 molar ratio was heat-treated with magnesium (Mg) at 1400°C and 700°C, respectively, in a reduced pressure atmosphere, and silicon oxide vapor and magnesium vapor were generated simultaneously from the Si and silicon oxide, causing a gas phase reaction. The reactants were cooled and precipitated, and then pulverized in a ball mill using 10 mm balls for 10 hours. The average particle size (D 50 ) was 5 μm, and a silicon-based active material containing MgSiO3 and Mg2SiO4 was recovered.

[0070] The recovered silicon-based active material was placed in a tube-shaped furnace and subjected to CVD treatment under a mixed gas of argon and methane to produce a silicon-based active material having a carbon coating layer with a carbon content of 5 wt % formed on the surface.

[0071] In order to confirm the Mg content of the prepared silicon-based active material, an inductively coupled plasma (ICP) analysis was performed, and the analysis result showed that the magnesium content was 8 wt % based on the total weight of the silicon-based active material.

[0072] Using a particle size analyzer (Morphologi 4, manufactured by Malvern Instruments), 1 mm of silicon-based active material samples were 3 After dispersing under conditions of 4 bar and 10 ms, the silicon-based active material particles were captured and analyzed in a 2D image. The convexity corresponding to 50% of the cumulative volume of 10,000 silicon active material particles was 0.89.

[0073] Manufacturing Examples 2 to 4 Silicon-based active materials were prepared in the same manner as in Preparation Example 1, except that the materials were ground in a ball mill for 14 hours, 8 hours, and 6 hours, respectively.

[0074] Using a particle size analyzer (Morphologi 4, manufactured by Malvern Instruments), the silicon-based active material samples produced in Examples 2 to 4 were analyzed by 1 mm 3 After dispersing under conditions of 4 bar and 10 ms, the silicon-based active material particles were captured and analyzed in a 2D image. The convexity corresponding to 50% of the cumulative volume of 10,000 silicon active material particles was 0.97, 0.75, and 0.69, respectively.

[0075] Examples and Comparative Examples Example 1 A uniform anode slurry was prepared by mixing the silicon-based active material with a convexity of 0.89 prepared in Preparation Example 1 as the anode active material, carbon black as the conductive material, and polyacrylic acid (PAA) as the binder in a weight ratio of 80:10:10 with water (H2O) as the solvent. The anode slurry was coated on one side of a copper current collector, dried, rolled, and punched to a certain size to prepare an anode.

[0076] A Li metal was used as a counter electrode, a polyolefin separator was interposed between the negative electrode and the Li metal, and an electrolyte in which 1M LiPF6 was dissolved in a solvent in which ethylene carbonate (EC) and diethyl carbonate (EMC) were mixed in a volume ratio of 30:70 was injected to prepare a coin-type half cell of Example 1.

[0077] Example 2 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 manufactured in Preparation Example 2 was used as the negative active material.

[0078] Example 3 The negative active material was a mixture of the silicon-based active material with a convexity of 0.89 prepared in Preparation Example 1 and natural graphite in a weight ratio of 1:9, carbon black as a conductive material, carboxymethyl cellulose and styrene butadiene rubber (SBR) as a binder in a weight ratio of 95.4:1:1.1:2.5, and water (H2O) as a solvent to prepare a uniform negative electrode slurry. The negative electrode slurry was coated on one side of a copper current collector, dried and rolled, and then punched to a certain size to prepare a negative electrode.

[0079] As the counter electrode, Li[Ni 0.6 Co 0.2 Mn 0.2 A bi-cell type lithium secondary battery of Example 3 was manufactured by using a positive electrode containing ]O2, interposing a polyolefin separator between the negative electrode and the positive electrode, and injecting an electrolyte in which 1M LiPF6 was dissolved in a solvent in which ethylene carbonate and diethyl carbonate were mixed in a volume ratio of 30:70.

[0080] Example 4 A bi-cell type lithium secondary battery of Example 4 was prepared in the same manner as in Example 3, except that a negative active material prepared by mixing the silicon-based active material having a convexity of 0.97 prepared in Preparation Example 2 and natural graphite in a weight ratio of 1:9 was used.

[0081] Comparative Example 1 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 active material.

[0082] Comparative Example 2 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 active material.

[0083] Comparative Example 3 A bi-cell type lithium secondary battery of Comparative Example 3 was prepared in the same manner as in Example 3, except that a negative active material prepared in Preparation Example 3, in which the silicon-based active material having a convexity of 0.75 and natural graphite were mixed in a weight ratio of 1:9, was used as the negative active material.

[0084] Comparative Example 4 A bi-cell type lithium secondary battery of Comparative Example 4 was prepared in the same manner as in Example 3, except that a negative active material prepared in Preparation Example 4, which was a mixture of the silicon-based active material having a convexity of 0.69 and natural graphite in a weight ratio of 1:9, was used.

[0085] Experimental Example Experimental Example 1: Evaluation of discharge capacity and initial efficiency The coin-type half cells of Examples 1 and 2 and Comparative Examples 1 and 2 were charged at 25° C. at a constant current (CC) of 0.1 C until the voltage dropped to 5 mV, and then charged at a constant voltage (CV) until the charging current reached 0.005 C (cut-off current), for the first time. After leaving the cells for 20 minutes, the cells were discharged at a constant current (CC) of 0.1 C until the voltage dropped to 1.5 V, and 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.

[0086] [Formula 2] Initial efficiency (%) = (discharge capacity after one discharge / one charge capacity) x 100

[0087] [Table 1]

[0088] Referring to Table 1, it can be seen that the batteries of Examples 1 and 2 including the silicon-based active material with a convexity of 0.8 or more have excellent initial efficiency and discharge capacity as compared to the batteries of Comparative Examples 1 and 2 including the silicon-based active material with a convexity of less than 0.8, as side reactions with the electrolyte are prevented.

[0089] Experimental Example 2: Evaluation of Capacity Retention Rate and Electrode Thickness Increase Rate The bi-cell type lithium secondary batteries of Examples 3 and 4 and Comparative Examples 3 and 4 were charged / discharged, and the capacity retention rate and the electrode thickness increase rate were evaluated. The results are shown in Table 2 below.

[0090] Specifically, the bi-cell type lithium secondary batteries of Examples 3 and 4 and Comparative Examples 3 and 4 were charged at 25° C. at a constant current (CC) of 1C until the voltage reached 4.25V, and then charged at a constant voltage (CV) until the charge current reached 0.05C (cut-off current), and the first charge was performed. After that, the batteries were left for 20 minutes, and then discharged at a constant current (CC) of 1C until the voltage reached 2.5V. This was repeated up to 50 cycles to evaluate the capacity retention rate, and the 51st charge was performed to measure the thickness of the electrode, and then the thickness increase rate was evaluated. The capacity retention rate and the electrode thickness change rate were derived from the following formulas 3 and 4.

[0091] [Formula 3] Capacity retention rate (%) = (45 discharge capacity / 1 discharge capacity) × 100

[0092] [Formula 4] Electrode thickness change rate (%) = (final negative electrode thickness change / initial negative electrode thickness) x 100

[0093] Experimental Example 3: Evaluation of electrode adhesion Before fabricating the batteries in Examples 3 and 4 and Comparative Examples 3 and 4, the adhesive strength of the prepared negative electrodes was measured, and the results are shown in Table 2 below.

[0094] Specifically, double-sided tape was attached to a slide glass, and the negative electrodes of Examples 3 and 4 and Comparative Examples 3 and 4, which were punched out to 20 mm x 180 mm, were placed on the tape, and then the negative electrodes were adhered by rolling them back and forth 10 times with a 2 kg roller. The force at which the negative electrodes peeled off from the slide glass was measured by pulling the negative electrodes at 20 mm / min using a UTM (manufactured by TA Co., Ltd.) device. The measurement angle between the slide glass and the negative electrodes was 90°.

[0095] [Table 2]

[0096] Referring to Table 2, it can be seen that the secondary batteries of Examples 3 and 4 including the silicon-based active material with a convexity of 0.8 or more have excellent capacity retention, small electrode thickness change rate, and excellent electrode adhesion, compared to the secondary batteries of Comparative Examples 3 and 4 including the silicon-based active material with a convexity of less than 0.8. Accordingly, it can be seen that the secondary battery including the silicon-based active material with a convexity of 0.8 or more has a relatively small specific surface area, which can reduce the side reaction of the electrolyte on the active material surface, and the binder can be appropriately positioned with the same binder content, so that the electrode shape can be efficiently maintained during charging / discharging. Based on this, it can be seen that the secondary battery including the silicon-based active material with a convexity of 0.8 or more has a small change in electrode thickness during charging / discharging and a high capacity retention rate.

Claims

1. a current collector and a negative electrode active material layer disposed on the current collector, 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 to 0.97 as measured by a particle size analyzer, the convexity being 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 silicon-based active material has a BET specific surface area of ​​1 m ２ / g to 60m ２ The negative electrode according to claim 1 , wherein the anode has a Cr content of 1.0 / g.

4. The silicon-based active material is SiO ｘ 10. The negative electrode of claim 1 , comprising: (0≦x≦2).

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

6. The negative electrode according to claim 5 , wherein the metal is at least one selected from the group consisting of lithium, magnesium, calcium, aluminum, sodium, and titanium.

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

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

9. The negative electrode of claim 1 , wherein the silicon-based active material further comprises a carbon coating layer on a surface thereof.

10. A secondary battery comprising the negative electrode according to claim 1 .

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