Negative electrode and secondary battery including the same

Abstract

The present invention relates to a negative electrode and a secondary battery comprising the negative electrode, wherein the negative electrode comprises: a current collector; and a negative electrode active material layer arranged on the current collector, wherein the negative electrode active material layer comprises: a conductive material; a negative electrode active material; and a binder, wherein the negative electrode active material comprises a silicon-based active material having an extension value of 0.05 to 0.35 and a circularity of 0.9 to 0.98 measured by a particle analyzer, wherein the extension value is defined by the following formula 1: [Formula 1] Extension value = 1-aspect ratio

Description

Technical Field

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims priority to and the benefit of Korean Patent Application No. 10-2019-0169170, filed on December 17, 2019, the disclosure of which is incorporated herein 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] As the use of fossil fuels has dramatically increased, there is an increasing demand for the use of alternative energy or clean energy, and the field of power generation and storage using electrochemical reactions has been most actively studied to meet this increasing demand.

[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 is rapidly increasing.

[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 enables lithium ions released from the positive electrode to be intercalated and deintercalated.

[0008] On the other hand, in order to increase the battery capacity, silicon-based active materials such as SiO x (0≤x≤2) has been widely used as a negative electrode active material. Conventionally used silicon-based active materials have high elongation values ​​and low circularity. Therefore, during battery operation, silicon-based active materials are easily damaged (cracked, etc.), so that side reactions with the electrolyte occur excessively, resulting in low initial efficiency and deteriorated life characteristics of the battery.

[0009] Therefore, a new type of negative electrode capable of realizing a secondary battery excellent in initial efficiency and life characteristics is required. Summary of the invention

[0010] [Technical issues]

[0011] The present invention is directed to providing a negative electrode including a silicon-based active material capable of improving initial efficiency and life 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 from the following description.

[0013] [Technical solution]

[0014] One aspect of the present invention provides a negative electrode, comprising a current collector and a negative electrode active material layer disposed 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 comprises a silicon-based active material having an extension value of 0.05 to 0.35 and a circularity of 0.9 to 0.98 measured using a particle shape analyzer, and the extension value is defined by the following formula 1.

[0015] [Formula 1]

[0016] Stretch value = 1-aspect ratio

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

[0018] [Beneficial Effects]

[0019] According to the present invention, since the negative electrode contains a negative electrode active material layer containing a silicon-based active material having an elongation value of 0.05 to 0.35 and a circularity of 0.9 to 0.98, the silicon-based active material can be prevented from being damaged during a rolling process in the manufacture of the negative electrode, and battery damage caused by excessive volume expansion of the silicon-based active material during operation of the battery can also be suppressed. Therefore, the initial efficiency and life characteristics of the negative electrode and the battery can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 The circularity and elongation values ​​for various shapes are shown. DETAILED DESCRIPTION

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

[0022] The terms and words provided herein should not be construed as limited to the commonly used meanings or meanings in dictionaries, and based on the principle that the inventors are able to appropriately define the concepts of the terms to describe the present invention in the best manner, these terms and words should be construed as meanings and concepts consistent with the technical scope of the present invention.

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

[0024] It should be understood that when used herein, the terms "comprises", "including", "includes", "containing", "has" and / or "having" indicate the presence of stated features, numbers, steps, elements and / or combinations thereof, but do not exclude the presence or addition of one or more other features, numbers, steps, elements and / or combinations thereof.

[0025] In the present invention, D 50 It can be defined as the particle size corresponding to 50% cumulative volume in the particle size distribution curve (curve on the particle size distribution graph). 50 It can be measured, for example, using laser diffraction. Laser diffraction is generally capable of measuring particle sizes from submicron levels to a few millimeters, and can produce results with high reproducibility and high resolution.

[0026] <Negative electrode>

[0027] According to one embodiment of the present invention, a negative electrode includes 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 an extension value of 0.05 to 0.35 and a circularity of 0.9 to 0.98 measured using a particle shape analyzer, and the extension value is defined by the following formula 1.

[0028] [Formula 1]

[0029] Stretch value = 1-aspect ratio

[0030] In this application, an "aspect ratio" is less than 1. The term "aspect ratio" in this application generally means:

[0031] In elliptical or other quasi-round particles, the ratio of the length of the minor axis to the length of the major axis, or

[0032] In a rectangular particle, the ratio of the length of the shorter side to the length of the longer side.

[0033] The current collector is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, aluminum or stainless steel whose surface is treated with carbon, nickel, titanium, silver, etc., etc. can be used as the current collector. Specifically, a transition metal such as copper or nickel that satisfactorily adsorbs carbon can be used as the current collector. The current collector may have a thickness of 6 μm to 20 μm, but the thickness of the current collector is not limited thereto.

[0034] 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, may be disposed on one surface or both surfaces of the current collector.

[0035] The negative electrode active material layer may include a conductive material, a negative electrode active material, and a binder.

[0036] The conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity. For example, as the conductive material, graphite such as natural graphite, artificial graphite, etc.; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, etc.; conductive fibers such as carbon fibers, metal fibers, etc.; conductive tubes such as carbon nanotubes, etc.; metal powders such as fluorocarbon powders, aluminum powders, nickel powders, etc.; conductive whiskers such as zinc oxide, potassium titanate, etc.; conductive metal oxides such as titanium oxide, etc.; or conductive materials such as polyphenylene derivatives, etc. can be used.

[0037] The negative electrode active material may include a silicon-based active material having an extension value of 0.05 to 0.35, specifically 0.15 to 0.35, 0.20 to 0.28, or 0.20 to 0.25, more specifically 0.20 to 0.23 measured using a particle shape analyzer. In this case, the extension value is defined by the following Formula 1.

[0038] [Formula 1]

[0039] Stretch value = 1-aspect ratio

[0040] When the elongation value of the silicon-based active material is less than 0.05, problems such as cracking of the active material may occur during the rolling process in the manufacture of the negative electrode, and when the elongation value exceeds 0.35, the battery may be cracked and broken due to volume expansion during charging and discharging of the battery.

[0041] The elongation value can be obtained, for example, by dispersing a silicon-based active material sample (1 mm 2 ) at 4 bar and 10 ms using a particle shape analyzer (Morphologi 4, available from Malvern Panalytical Ltd.). 3 ), then capturing the silicon-based active material particles as a two-dimensional image, and analyzing the image. In addition, the extension value may be an extension value corresponding to 50% of the cumulative volume of 10,000 silicon-based active material particles.

[0042] The negative electrode active material may include a silicon-based active material having a circularity of 0.9 to 0.98, specifically 0.92 to 0.98, more specifically 0.94 to 0.97, and even more specifically 0.95 to 0.97. When the circularity of the silicon-based active material is less than 0.9, the battery may be ruptured and broken due to volume expansion during charge and discharge of the battery, and when the circularity exceeds 0.98, problems such as rupture of the active material composed of a mixture with graphite may occur during a rolling process in the manufacture of the negative electrode.

[0043] The circularity can be obtained, for example, by dispersing a silicon-based active material sample (1 mm 2 ) at 4 bar and 10 ms using a particle shape analyzer (Morphologi 4, available from Malvern Panalytical Ltd.). 3 ), then capturing the silicon-based active material particles as a two-dimensional image, and analyzing the image. In addition, the circularity may be a circularity corresponding to 50% of the cumulative volume of 10,000 silicon-based active material particles.

[0044] The circularity is a value obtained by dividing the circumference of a circle having an equivalent surface area (A) determined in a two-dimensional image obtained by capturing silicon-based active material particles by the actual circumference (P) and squaring the result. Therefore, when the radius of the circle is r, then A=πr 2 , and the circumference is 2πr. Therefore, the circularity can be defined by the following formula 2.

[0045] [Formula 2]

[0046]

[0047] When the silicon-based active material has an elongation value of 0.05 to 0.35 and a circularity of 0.9 to 0.98, the active material is hardly deformed during the rolling process in the manufacture of the negative electrode, and problems such as cracking and breaking hardly occur due to the uniform volume expansion of the active material during charging and discharging of the battery, thereby being able to improve the initial efficiency and life characteristics of the negative electrode and the battery.

[0048] The silicon-based active material may have an extension value of 0.15 to 0.35 and a circularity of 0.92 to 0.98. Specifically, the silicon-based active material may have an extension value of 0.2 to 0.28 and a circularity of 0.92 to 0.98. When the extension value and circularity of the silicon-based active material meet the above ranges, the initial efficiency and life characteristics of the battery can be further improved.

[0049] On the other hand, circularity (a parameter that provides information about the contour and surface roughness of a particle) and extension value (a parameter that provides information about the relationship between the length and width of a particle and is not affected by the surface roughness of the particle) are different parameters, and if the circularity is closer to 1.0, the extension value is not necessarily closer to 0. In addition, even when the circularity is the same, the extension value may be different. For example, referring to the circularity and extension value diagrams showing various shapes Figure 1, it can be seen that even when the circularity increases from 0.47 to 0.52, the extension value is not close to 0, but increases from 0.24 to 0.79. In addition, it can be seen that even when the circularity is the same, such as 0.47, there are different extension values, such as 0.24 and 0.82.

[0050] The silicon-based active material can be prepared by a typical pulverization or size classification method known in the art, for example, by processing the Si-containing material using a ball mill, a sieve, etc. 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, etc. under a reduced pressure atmosphere to react the reactants in the gas phase and cool the reaction product. When the Si-containing material is pulverized using a ball mill instead of a jet mill, etc., the elongation value and circularity of the prepared silicon-based active material can fall within the range according to the present invention.

[0051] On the other hand, the extension value and circularity of the silicon-based active material can be adjusted by pulverizing the Si-containing material for 10 to 18 hours using a ball mill with a ball size of 5 to 15 mm. Specifically, the extension value and circularity of the silicon-based active material can be adjusted by pulverizing the Si-containing material for 13 to 17 hours using a ball mill with a ball size of 10 mm.

[0052] The silicon-based active material may have a 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 a porosity measurement analyzer (BELSORP-mini II, purchased from BelJapan Inc.) using a nitrogen adsorption flow method using a BET 6-point method.

[0053] The silicon-based active material may have an average particle size (D ) of 0.01 μm to 30 μm, specifically 0.05 μm to 20 μm, more specifically 0.1 μm to 10 μm. 50 ). When the average particle size of the silicon-based active material satisfies the above range, the electrode density is prevented from being reduced, thereby enabling an appropriate unit volume capacity to be achieved, and the electrode forming slurry can be applied with a uniform thickness.

[0054] The silicon-based active material may be contained in the negative electrode active material layer at 0.1 wt % to 80 wt %, 5 wt % to 80 wt %, 0.1 wt % to 30 wt %, 1 wt % to 25 wt %, or 2 wt % to 20 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.

[0055] The silicon-based active material may include SiO x (0≤x≤2). When the silicon-based active material comprises SiO x (0≤x≤2), the battery capacity can be increased. More specifically, the silicon-based active material can be SiO or SiO 2 The SiO may be SiO having crystallinity. In this case, excessive volume expansion of the silicon-based active material during charge and discharge of the battery is controlled, thereby improving the life characteristics of the battery. 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 SiO x The lithium silicate formation reaction of (0≤x≤2) is controlled, so that the degradation of the initial efficiency can be prevented and the initial capacity in the electrode design can be maximized.

[0056] The silicon-based active material may also contain a metal silicate phase. The metal silicate may be prepared by doping SiO with a metal. x (0≤x≤2), and the metal silicate phase means that the metal silicate exists in the silicon-based active material in the form of a domain. The metal silicate may be, for example, Mg 2 SiO 4 or MgSiO 3 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, Mg 2 Si, and the metal oxide may be, for example, MgO.

[0057] 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 lithium (Li), magnesium (Mg), calcium (Ca), aluminum (Al), sodium (Na), and titanium (Ti), and preferably Li and / or Mg. In this case, since the metal element of the metal silicate is more strongly bonded to oxygen, lithium silicate is prevented from being formed from lithium supplied from the positive electrode during initial charging, thereby preventing degradation of initial efficiency.

[0058] The metal may be contained 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 %, even more specifically 4 wt % to 15 wt %. When the content of the metal is within the above range, the silicon-based active material may have a high capacity and may also more effectively improve the initial efficiency of the silicon-based active material. The content of the metal may be determined by inductively coupled plasma (ICP) analysis.

[0059] The metal may be reacted 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 containing one or more selected from metal oxides and metal silicates.

[0060] The metal silicate may include one or more metal silicates selected from the group consisting of doping metals Li, Mg, Ca, Al, Na and Ti. Specifically, the metal silicate includes one or more metal silicates selected from Li and Mg, and more specifically includes magnesium silicate (Mg).

[0061] The metal oxide may include one or more metal oxides selected from the doping metals Li, Mg, Ca, Al, Na and Ti. Specifically, the metal oxide includes one or more metal oxides selected from Li and Mg, and more specifically includes magnesium oxide (Mg).

[0062] The metal compound containing one or more selected from the metal oxide and the metal silicate may be selected from Mg 2 SiO 4 MgSiO 3 Mg 2 One or more metal compounds selected from Si and MgO.

[0063] 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) 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 the excessive volume expansion of the silicon-based active material during battery charging and discharging, and may increase the conductivity of the active material to further reduce 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 further increased, and the conductivity of the silicon-based active material may be enhanced, so that charging and discharging occur uniformly, and thus the volume change during charging and discharging may be more effectively controlled.

[0064] The silicon-based active material further comprising the carbon coating on its surface may have a thickness of 0.5 m 2 / g to 15m 2 Specifically, the silicon-based active material further comprising the carbon coating on its surface may have a BET specific surface area of ​​1 m 2 / g to 13m 2 / g or 2m 2 / g to 10m 2 When the BET specific surface area of ​​the silicon-based active material further comprising the carbon coating on its surface is within the above range, a 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.

[0065] The silicon-based active material may include 0.1 wt % to 50 wt %, specifically 1 wt % to 25 wt %, more specifically 3 wt % to 15 wt % of the carbon coating layer on its surface relative 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 is enhanced, so that charging and discharging occur uniformly, and thus the volume change during charging and discharging can be more effectively controlled.

[0066] The carbon coating layer may have a thickness of 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 enhanced while maintaining a conductive path in the negative electrode active material.

[0067] 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 group consisting of artificial graphite, natural graphite, and graphitized mesophase carbon microbeads. Specifically, the carbon-based active material is preferably artificial graphite.

[0068] 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 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 improving the life of the negative electrode.

[0069] The adhesive may 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, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid and materials whose hydrogen is replaced by Li, Na, Ca, etc., and may also include various copolymers thereof.

[0070] The binder may be contained in the negative electrode active material layer at 30 wt % or less, specifically 0.1 wt % to 30 wt %. When the content of the binder satisfies the above range, the adhesive effect generated by using the binder can be exhibited, and the desired unit volume capacity of the negative electrode can be maintained.

[0071] <Secondary Battery>

[0072] 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.

[0073] Specifically, 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 above-mentioned negative electrode. Since the negative electrode has been described above, its detailed description will be omitted.

[0074] 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.

[0075] In the positive electrode, the positive electrode collector is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity. As the positive electrode collector, for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel treated with carbon, nickel, titanium, silver, etc. on the surface can be used. In addition, the positive electrode collector can generally have a thickness of 3μm to 500μm and have fine concavoconvexities formed on its surface to increase the adhesion of the positive electrode active material. In addition, the positive electrode collector can be used in various forms such as any of a film, a sheet, a foil, a net, a porous material, a foam, a non-woven fabric, etc.

[0076] 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 (LiCoO 2 ), lithium nickel oxide (LiNiO 2 ) or compounds obtained by substitution with one or more transition metals; lithium iron oxides, such as LiFe 3 O 4 ; Lithium manganese oxides, such as Li 1+c1 Mn 2-c1 O 4 (0≤c1≤0.33), LiMnO 3 、LiMn 2 O 3 、LiMnO 2 etc.; Lithium copper oxide (Li 2 CuO 2 ); Vanadium oxides, such as LiV 3 O 8 、V 2 O 5 , Cu 2 V 2 O 7 etc.; Ni-site lithium nickel oxide, composed of chemical formula LiNi 1-c2 M c2 O 2 (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); lithium manganese composite oxide, represented by the chemical formula LiMn 2-c3 M c3 O 2 (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 Li 2 Mn 3 MO 8 (wherein M is at least one selected from the group consisting of Fe, Co, Ni, Cu and Zn); LiMn2 O 4 , wherein some 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.

[0077] 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.

[0078] 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, artificial graphite, etc.; carbon materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, carbon fiber, etc.; metal powder or metal fiber containing copper, nickel, aluminum, silver, etc.; conductive whiskers, such as zinc oxide, potassium titanate, etc.; conductive metal oxides, such as titanium oxide, etc.; and conductive polymers, such as polyphenylene derivatives, etc., which can be used alone or in combination of two or more thereof.

[0079] In addition, the positive electrode binder is used to improve the cohesive force between the positive electrode active material particles and the adhesion between the positive electrode active material and the positive electrode 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, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (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.

[0080] The separator is used to separate the negative electrode and the positive electrode, and provides a channel for lithium ions to migrate through, and any separator commonly used as a separator in a secondary battery can be used without particular limitation. In particular, it is preferred to exhibit low resistance to electrolyte ion migration and have excellent electrolyte impregnation ability. Specifically, a porous polymer film can be used, for example, a porous polymer film made of polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, ethylene / methacrylate copolymers, etc., or a stacked structure having two or more layers thereof. Alternatively, a typical porous non-woven fabric can be used, such as a non-woven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, etc. Alternatively, a coated separator containing a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and optionally, the separator can be used in a monolayer or multilayer structure.

[0081] 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 may be used to manufacture a lithium secondary battery, but the present invention is not limited thereto.

[0082] Specifically, the electrolyte may include a nonaqueous organic solvent and a metal salt.

[0083] 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, 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.

[0084] In the carbonate organic solvent, it is particularly preferred to use ethylene carbonate and propylene carbonate as cyclic carbonates, because they are high viscosity organic solvents and have high dielectric constants, thus satisfactorily dissociating lithium salts. When using a mixture formed by mixing such cyclic carbonates with low viscosity low dielectric constant linear carbonates such as dimethyl carbonate and diethyl carbonate in an appropriate ratio, an electrolyte with high conductivity can be prepared. Therefore, it is more preferred to use the mixture.

[0085] As the metal salt, a lithium salt can be used, and the lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte. For example, as the anion of the lithium salt, one or more selected from the group consisting of: F - , Cl - ,I - 、NO 3 - 、N(CN) 2 - , BF 4 - , ClO 4 - PF 6 - ,(CF 3 ) 2 PF 4 - ,(CF 3 ) 3 PF 3 - ,(CF3 ) 4 PF 2 - 、(CF 3 ) 5 PF - 、(CF 3 ) 6 P - 、CF 3 SO 3 - 、CF 3 CF 2 SO 3 - 、(CF 3 SO 2 ) 2 N - 、(FSO 2 ) 2 N - 、CF 3 CF 2 (CF 3 ) 2 CO - 、(CF 3 SO 2 ) 2 CH - 、(SF 5 ) 3 C - 、(CF 3 SO 2 ) 3 C - 、CF 3 (CF 2 ) 7 SO 3 - 、CF 3 CO 2 - 、CH 3 CO 2 - 、SCN - 和(CF 3 CF 2 SO 2 ) 2 N - 。

[0086] In addition to the above electrolyte components, the electrolyte may also contain one or more additives selected from the following for the purpose of improving the life characteristics of the battery, inhibiting the reduction of battery capacity, improving the discharge capacity of the battery, etc.: halogenated alkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, 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.

[0087] According to another embodiment of the present invention, a battery module including the above-mentioned secondary battery as a unit cell and a battery pack including the battery module are provided. Since the battery module and the battery pack include secondary batteries having high capacity, high rate characteristics and cycle characteristics, they can be used as a power source for medium and large devices selected from the group consisting of electric vehicles, hybrid electric vehicles and plug-in hybrid electric vehicles, and power storage systems.

[0088] Modes for carrying out the invention

[0089] 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 various changes and modifications may be made within the scope and technical spirit of the present invention, so that the present invention encompasses all such changes and modifications, provided that they are within the scope of the appended claims.

[0090] Preparation example: Preparation of silicon-based active materials

[0091] Preparation Example 1

[0092] The Si powder and silicon oxide (SiO 2 ) powder and magnesium (Mg) were heat treated at 1400°C and 700°C respectively under reduced pressure atmosphere, so that silicon oxide vapor and magnesium vapor derived from the Si and the silicon oxide were simultaneously generated, thereby reacting the reactants in the gas phase. The reaction product was cooled to induce precipitation, and then pulverized by a ball mill using 10 mm balls for 14 hours. Then, size classification was performed to collect particles with an average particle size (D 50 ) and contains MgSiO 3 and Mg 2 SiO 4 Silicon active materials.

[0093] The collected silicon-based active material was introduced 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 having a carbon coating layer having a carbon content of 5 wt % formed on the surface thereof.

[0094] As a result of determining the Mg content of the prepared silicon-based active material through inductively coupled plasma (ICP) analysis, it was found that the Mg content was 8 wt % relative to the total weight of the silicon-based active material.

[0095] For example, by using a particle shape analyzer (Morphologi 4, available from Malvern Panalytical Ltd.) to disperse a silicon-based active material sample (1 mm 3 ), then capturing the silicon-based active material particles as a two-dimensional image, and analyzing the image, the extension value and circularity corresponding to 50% of the cumulative volume of 10,000 silicon-based active material particles were 0.25 and 0.94, respectively.

[0096] The silicon-based active material including a carbon coating layer formed thereon has a 4m 2 / g to 7m 2 / g BET specific surface area.

[0097] Preparation Examples 2 to 4

[0098] A silicon-based active material including a carbon coating layer formed thereon was prepared in the same manner as in Preparation Example 1, except that the pulverization was performed by a ball mill for 16 hours, 6 hours, and 20 hours, respectively.

[0099] As can be seen by using a particle shape analyzer (Morphologi 4, available from Malvern Panalytical Ltd.) to disperse each of the silicon-based active material samples (1 mm 3 ), then capturing the silicon-based active material particles as a two-dimensional image, and analyzing the image, for Preparation Example 2, the extension value and circularity corresponding to 50% of the cumulative volume of 10,000 silicon-based active material particles are 0.20 and 0.97; for Preparation Example 3, the extension value and circularity corresponding to 50% of the cumulative volume of 10,000 silicon-based active material particles are 0.5 and 0.85; and for Preparation Example 4, the extension value and circularity corresponding to 50% of the cumulative volume of 10,000 silicon-based active material particles are 0.02 and 0.99.

[0100] All of the silicon-based active materials prepared in Preparation Examples 2 to 4 including the carbon coating layers formed thereon had 4m 2 / g to 7m 2 / g BET specific surface area.

[0101] Preparation Example 5

[0102] A silicon-based active material including a carbon coating layer formed thereon was prepared in the same manner as in Preparation Example 1, except that the pulverization was performed by a jet mill instead of a ball mill.

[0103] The silicon-based active material sample (1 mm 2 ) prepared in Preparation Example 5 was dispersed under the conditions of 4 bar and 10 ms by using a particle shape analyzer (Morphologi 4, available from Malvern Panalytical Ltd.). 3 ), then capturing the silicon-based active material particles as a two-dimensional image, and analyzing the image, the extension value and circularity corresponding to 50% of the cumulative volume of 10,000 silicon-based active material particles were 0.39 and 0.93, respectively.

[0104] The silicon-based active material prepared in Preparation Example 5 and including a carbon coating layer formed thereon has a 4m 2 / g to 7m 2 / g BET specific surface area.

[0105] Preparation Example 6

[0106] A silicon-based active material including a carbon coating layer formed thereon was prepared in the same manner as in Preparation Example 1, except that pulverization was performed by a ball mill using 12 mm balls for 3 hours.

[0107] As can be seen by using a particle shape analyzer (Morphologi 4, available from Malvern Panalytical Ltd.) to disperse the silicon-based active material sample (1 mm2) prepared in Preparation Example 6 under the conditions of 4 bar and 10 ms. 3 ), then capturing the silicon-based active material particles as a two-dimensional image and analyzing the image, the extension value and circularity corresponding to 50% of the cumulative volume of 10,000 silicon-based active material particles were 0.25 and 0.88, respectively.

[0108] The silicon-based active material prepared in Preparation Example 6 and including a carbon coating layer formed thereon has a 4m 2 / g to 7m 2 / g BET specific surface area.

[0109] Examples and Comparative Examples

[0110] Example 1

[0111] The silicon-based active material prepared in Preparation Example 1 as a negative electrode active material, carbon black as a conductive material, and polyacrylic acid (PAA) as a binder were mixed in a weight ratio of 80:10:10 in water (H2O) as a solvent. 2 O) to prepare a uniform negative electrode slurry. The negative electrode slurry is applied to one surface of a copper current collector, dried and roll-pressed, and then the resultant is punched out in a constant size to manufacture a negative electrode.

[0112] Li metal was used as the counter electrode, a polyolefin separator was inserted between the negative electrode and the Li metal, and then 1 M LiPF 6 The coin-type half-cell of Example 1 was manufactured by dissolving an electrolyte prepared in a solvent containing ethylene carbonate (EC) and diethyl carbonate (DEC) mixed in a volume ratio of 30:70.

[0113] Example 2

[0114] 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 prepared in Preparation Example 2 was used as a negative electrode active material.

[0115] Example 3

[0116] A negative electrode active material including the silicon-based active material prepared in Preparation Example 1 and natural graphite mixed in a weight ratio of 1:9, carbon black as a conductive material, and carboxymethyl cellulose and styrene-butadiene rubber (SBR) as a binder in a weight ratio of 95.4:1:1.1:2.5 were mixed in water (H2O) as a solvent. 2 O) to prepare a uniform negative electrode slurry. The negative electrode slurry is applied to one surface of a copper current collector, dried and roll-pressed, and then the resultant is punched out in a constant size to manufacture a negative electrode.

[0117] The positive electrode active material includes Li[Ni 0.6 Co 0.2 Mn 0.2 ]O 2 The positive electrode was used as the counter electrode, a polyolefin separator was inserted between the negative electrode and the positive electrode, and then 1M LiPF 6 The bi-cell type lithium secondary battery of Example 3 was manufactured by dissolving the prepared electrolyte in a solvent containing ethylene carbonate and diethyl carbonate mixed in a volume ratio of 30:70.

[0118] Example 4

[0119] A dual-cell type lithium secondary battery of 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 prepared in Preparation Example 2 and natural graphite mixed in a weight ratio of 1:9 was used.

[0120] Comparative Example 1

[0121] 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 prepared in Preparation Example 3 was used as a negative electrode active material.

[0122] Comparative Example 2

[0123] 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 prepared in Preparation Example 4 was used as a negative electrode active material.

[0124] Comparative Example 3

[0125] A coin-type half cell of Comparative Example 3 was manufactured in the same manner as in Example 1, except that the silicon-based active material prepared in Preparation Example 5 was used as a negative electrode active material.

[0126] Comparative Example 4

[0127] A coin-type half cell of Comparative Example 4 was manufactured in the same manner as in Example 1, except that the silicon-based active material prepared in Preparation Example 6 was used as a negative electrode active material.

[0128] Comparative Example 5

[0129] A dual-cell type lithium secondary battery of Comparative Example 5 was manufactured in the same manner as in Example 3 except that a negative electrode active material including the silicon-based active material prepared in Preparation Example 3 and natural graphite mixed in a weight ratio of 1:9 was used.

[0130] Comparative Example 6

[0131] A dual-cell type lithium secondary battery of Comparative Example 6 was manufactured in the same manner as in Example 3 except that a negative electrode active material including the silicon-based active material prepared in Preparation Example 4 and natural graphite mixed in a weight ratio of 1:9 was used.

[0132] Comparative Example 7

[0133] A dual-cell type lithium secondary battery of Comparative Example 7 was manufactured in the same manner as in Example 3 except that a negative electrode active material including the silicon-based active material prepared in Preparation Example 5 and natural graphite mixed in a weight ratio of 1:9 was used.

[0134] Comparative Example 8

[0135] A dual-cell type lithium secondary battery of Comparative Example 8 was manufactured in the same manner as in Example 3 except that a negative electrode active material including the silicon-based active material prepared in Preparation Example 6 and natural graphite mixed in a weight ratio of 1:9 was used.

[0136] Experimental example

[0137] Experimental Example 1: Evaluation of discharge capacity and initial efficiency

[0138] Each coin-type half-cell of Examples 1 and 2 and Comparative Examples 1 to 4 was charged to 5 mV at a constant current (CC) of 0.1 C at 25° C., and then the first charge was performed at a constant voltage (CV) to a charge current (cut-off current) of 0.005 C. After the battery was left for 20 minutes and then discharged to 1.5 V at a constant current (CC) of 0.1 C, the discharge capacity (mAh / g) and the initial efficiency (%) were evaluated, and the results are shown in Table 1 below. The initial efficiency (%) was calculated by the following formula 3.

[0139] [Formula 3]

[0140] Initial efficiency (%) = (discharge capacity after first discharge / initial charge capacity) × 100

[0141] [Table 1]

[0142] Discharge capacity (mAh / g) Initial efficiency (%) Example 1 1414 82.1 Example 2 1420 82.3 Comparative Example 1 1367 79.2 Comparative Example 2 1412 82.0 Comparative Example 3 1389 80.1 Comparative Example 4 1353 79.0

[0143] Referring to Table 1, it can be seen that the batteries of Examples 1 and 2 containing silicon-based active materials having elongation values ​​of 0.05 to 0.35 and circularities of 0.9 to 0.98 exhibit excellent initial efficiency and excellent discharge capacity due to prevention of side reactions with the electrolyte, compared with the batteries of Comparative Examples 1 to 4 containing silicon-based active materials whose elongation values ​​and / or circularities do not satisfy the ranges according to the present invention.

[0144] Experimental Example 2: Evaluation of capacity retention and electrode thickness increase rate

[0145] Each of the dual-cell type lithium secondary batteries of Examples 3 and 4 and Comparative Examples 5 to 8 was charged and discharged. Then, the capacity retention ratio and the electrode thickness increase ratio were evaluated, and the results thereof are shown in Table 2 below.

[0146] Specifically, each dual-monomer lithium secondary battery of Examples 3 and 4 and Comparative Examples 5 to 8 was charged to 4.25V at a constant current (CC) of 1C at 25°C, and then the first charge was performed at a constant voltage (CV) until a charging current (cut-off current) of 0.05C. After the battery was left for 20 minutes, the battery was discharged to 2.5V at a constant current (CC) of 1C. This cycle was repeated 50 times, and then 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 increase rate were calculated by the following formulas 4 and 5, respectively.

[0147] [Formula 4]

[0148] Capacity retention rate (%) = (discharge capacity after 50th cycle / discharge capacity at 1st cycle) × 100

[0149] [Formula 5]

[0150] Electrode thickness increase rate (%) = (negative electrode thickness increase after 51st charge / initial negative electrode thickness) × 100

[0151] [Table 2]

[0152] Capacity retention rate (%) Electrode thickness increase rate (%) Example 3 92.7 51.8 Example 4 93.5 50.2 Comparative Example 5 85.4 65.9 Comparative Example 6 86.9 62.3 Comparative Example 7 87.3 64.5 Comparative Example 8 85.1 66.3

[0153] Referring to Table 2, it can be seen that the secondary batteries of Examples 3 and 4 containing silicon-based active materials having an extension value of 0.05 to 0.35 and a circularity of 0.9 to 0.98 exhibit excellent capacity retention and low electrode thickness increase rate, compared to the secondary batteries of Comparative Examples 5 to 8 containing silicon-based active materials whose extension value and / or circularity do not meet the range according to the present invention. Therefore, in the case of a secondary battery containing a silicon-based active material having an extension value of 0.05 to 0.35 and a circularity of 0.9 to 0.98, it can be seen that the active material is hardly broken and crushed due to volume expansion during rolling and charging / discharging, and the occurrence of side reactions with the electrolyte on the surface of the active material is less.

Claims

1. A negative electrode, comprising: 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 comprises a silicon-based active material having an elongation value of 0.15 to 0.28 and a circularity of 0.9 to 0.97 measured using a particle shape analyzer, The content of the silicon-based active material in the negative electrode active material layer is 0.1 wt % to 80 wt %, and The extension value is defined by the following formula 1, [Formula 1] Stretch value = 1-aspect ratio, The aspect ratio is the ratio of the length of the minor axis to the length of the major axis in elliptical or other quasi-circular particles, or the ratio of the length of the short side to the length of the long side in rectangular particles, and The circularity is defined by the following formula 2: [Formula 2] Wherein, in Formula 2, A is the equivalent surface area determined in the two-dimensional image obtained by capturing the silicon-based active material particles, and p is the actual perimeter of the silicon-based active material particles. 2 . The negative electrode according to claim 1 , wherein the silicon-based active material has an elongation value of 0.20 to 0.25 and a circularity of 0.92 to 0.

97.

3. The negative electrode according to claim 1, wherein the silicon-based active material comprises SiO x , where 0≤x≤2. 4 . The negative electrode according to claim 1 , wherein the silicon-based active material further comprises a metal silicate phase formed by doping with a metal. 5 . The negative electrode according to claim 4 , wherein the metal is one or more selected from lithium, magnesium, calcium, aluminum, sodium and titanium. 6 . The negative electrode according to claim 4 , wherein the metal is contained in the silicon-based active material at 0.1 wt % to 30 wt %.

7. The negative electrode according to claim 1, wherein the silicon-based active material has 1 m 2 / g to 60m 2 / g BET specific surface area. 8 . The negative electrode according to claim 1 , wherein the silicon-based active material further comprises a carbon coating on a surface thereof. 9 . A secondary battery comprising the negative electrode according to claim 1 .

Images