Negative electrode active material, negative electrode comprising the negative electrode active material, and secondary battery comprising the negative electrode

Abstract

The present invention relates to: a negative electrode active material, the negative electrode active material comprising SiO x (0

Description

Technical Field

[0001] Cross - Reference to Related Applications

[0002] This application claims the benefit of priority of Korean Patent Application No. 10 - 2020 - 0117153, filed on September 11, 2020, the disclosure of which is incorporated herein by reference in its entirety. Technical Field

[0004] The present invention relates to a negative electrode active material, a negative electrode including the negative electrode active material, and a secondary battery including the negative electrode. Specifically, the negative electrode active material is characterized by including silicon - based composite particles containing SiO x (0 < x < 2) and MgSiO 3 phases, wherein the MgSiO 3 phase includes a first MgSiO 3 phase having an enstatite structure and a second MgSiO 3 phase having a clinoenstatite structure in a weight ratio of 1:1 to 1:5. Background Art

[0005] Due to the rapid increase in the use of fossil fuels, the demand for using alternative energy or clean energy has also increased. As part of this trend, power generation and power storage using electrochemical reactions are the most active research fields.

[0006] Currently, typical examples of electrochemical devices using electrochemical energy can be secondary batteries, and their fields of use have a tendency to expand. In recent years, with the increase in the technological development and demand for portable devices such as portable computers, mobile phones, and cameras, the demand for secondary batteries as an energy source has increased significantly. Among these secondary batteries, lithium secondary batteries with high energy density (i.e., high capacity) have been extensively studied and have been commercialized and widely used.

[0007] A secondary battery generally consists of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode includes a negative electrode active material in which lithium ions from the positive electrode are intercalated and deintercalated, and silicon - based particles with a high discharge capacity can be used as the negative electrode active material. However, in silicon - based particles such as SiO x (0 ≤ x < 2), the SiO 2 partially reacts with lithium ions from the positive electrode to form lithium silicate, and the lithium silicate acts as an irreversible phase, thereby reducing the initial efficiency of the battery. In addition, the silicon - based particles have an excessive volume change during charging and discharging and have side reactions with the electrolyte. Therefore, there is a problem of a decrease in battery life.

[0008] Traditionally, to solve this problem, a technique of intentionally doping a metal such as Mg into silicon-based particles to block reaction sites that may be formed in an irreversible manner, thereby improving the initial efficiency, has been used. However, although the initial efficiency can be improved, without considering the appropriate composition, there is a problem that it is difficult to improve the battery life characteristics only by doping the metal, and the unit weight capacity is significantly reduced due to the doping of the metal.

[0009] Therefore, there is an urgent need to develop a silicon-based active material that can improve the battery life characteristics while having an appropriate level of battery capacity and initial efficiency. Summary of the Invention

[0010] Technical Problem

[0011] One aspect of the present invention provides a negative electrode active material that can improve the battery life characteristics while having an appropriate level of battery capacity and initial efficiency; and a negative electrode and a secondary battery including the negative electrode active material.

[0012] Technical Solution

[0013] According to one aspect of the present invention, there is provided a negative electrode active material including silicon-based composite particles containing SiO x (0 < x < 2) and MgSiO 3 phases, wherein the MgSiO 3 phase includes a first MgSiO 3 phase having an enstatite structure and a second MgSiO 3 phase having a clinoenstatite structure in a weight ratio of 1:1 to 1:5.

[0014] According to another aspect of the present invention, there is provided a negative electrode including the negative electrode active material and a secondary battery including the negative electrode.

[0015] Advantageous Effects

[0016] The negative electrode active material according to an embodiment of the present invention includes a MgSiO 3 phase, and thus can improve the initial efficiency of the battery. In addition, the content of the first MgSiO 3 phase having an enstatite structure and the second MgSiO 3 phase having a clinoenstatite structure has a weight ratio of 1:1 to 1:5. Therefore, the second MgSiO 3The clinoenstatite phase can suppress the excessive volume expansion of the Si phase in the silicon-based composite particles during battery charging. Therefore, the structural stability of the negative electrode can be improved, and the life characteristics of the battery can be improved. In addition, since the resistance to lithium ion insertion is not large, the problems caused by the electrolyte side reaction can be minimized, and the discharge capacity can be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is used to confirm that the first MgSiO 3 Phase and the second MgSiO with clinoenstatite structure 3 XRD patterns of the phase weight ratios. DETAILED DESCRIPTION

[0018] The present invention will be described in more detail below to facilitate a clearer understanding of the present invention.

[0019] It should be understood that the terms or words used in the present disclosure and claims should not be interpreted as having the ordinary meaning or the meaning defined in the dictionary, but should be interpreted based on the principle that the inventor can appropriately define the concept of the terms in order to interpret the present invention in the best manner, consistent with the technical scope of the present invention.

[0020] The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to limit the present invention.As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0021] It should be understood that the terms "include", "comprises" or "has" when used in this specification indicate the presence of stated features, numbers, steps, elements or a combination thereof, but do not exclude the presence or addition of one or more other features, numbers, steps, elements or a combination thereof.

[0022] In this specification, the average particle size (D 50 The average particle size (D) can be defined as the particle size at which the cumulative volume is 50% in the particle size distribution curve. The average particle size (D) can be measured, for example, by using a laser diffraction method. 50 ). Laser diffraction can usually measure particle sizes from submicron to several millimeters, and can obtain results with high reproducibility and high resolution.

[0023] In this specification, the specific surface area is measured using a BET measuring instrument (BEL-SORP-MAX, Bell Co., Ltd., Japan) by degassing the object to be measured at 200°C for 8 hours and N2 2 Adsorption / desorption is measured.

[0024] <Negative Electrode Active Material>

[0025] The negative electrode active material according to one embodiment of the present invention contains silicon-based composite particles containing SiO x (0 < x < 2) and MgSiO 3 phases, and the MgSiO 3 phase may contain a first MgSiO 3 phase having an enstatite structure and a second MgSiO 3 phase having a clinoenstatite structure in a weight ratio of 1:1 to 1:5.

[0026] The negative electrode active material contains silicon-based composite particles.

[0027] The silicon-based composite particles contain the SiO x (0 < x < 2) and the MgSiO 3 phase.

[0028] The SiO x (0 < x < 2) corresponds to the matrix in the silicon-based composite particles. The SiO x (0 < x < 2) may be in a form containing Si and SiO 2 , and the Si may form a phase. That is, the above x corresponds to the ratio of the number of O to Si contained in SiO x (0 < x < 2). When the silicon-based composite particles contain SiO x (0 < x < 2), the discharge capacity of the secondary battery can be improved.

[0029] The MgSiO 3 phase may be present on the surface or inside of the SiO x (0 < x < 2) used as the matrix. The MgSiO 3 phase can improve the initial efficiency of the battery.

[0030] The content of the MgSiO 3 phase in the silicon-based composite particles may be 15 wt% to 60 wt%, particularly 15 wt% to 50 wt%, and more particularly 19 wt% to 22 wt%. When the above range is satisfied, the reactivity of the silicon-based composite particles with water is reduced, thereby improving the discharge capacity, initial efficiency, and life characteristics of the battery.

[0031] The MgSiO 3 phase may contain a first MgSiO 3 phase having an enstatite structure and a second MgSiO 3 phase having a clinoenstatite structure. The enstatite structure means that the MgSiO 3 crystal forms an orthorhombic structure, and the clinoenstatite structure means that MgSiO3 The crystals form a monoclinic structure.

[0032] The MgSiO 3 The phase may include a first MgSiO having an enstatite structure in a weight ratio of 1:1 to 1:5, specifically in a weight ratio of 1:2 to 1:3. 3 phase and a second MgSiO with a clinoenstatite structure 3 If the second MgSiO 3 If the amount of the second MgSiO phase (clinoenstatite structure) is too large and is not within the range of 1:1 to 1:5, the lithium insertion resistance increases excessively, thereby increasing the potential of the silicon-based composite particles and causing excessive electrolyte side reactions on the surface of the silicon-based composite particles. In addition, the substances generated by the electrolyte side reactions increase the resistance to lithium ion insertion, so there is a problem of continuous decline in the effect. In addition, if the second MgSiO 3 If the amount of the phase (clinoenstatite structure) is small and is not within the range of 1:1 to 1:5, the structural stability of the silicon-based composite particles is significantly reduced, thereby causing the structure of the silicon-based composite particles to collapse.

[0033] In other words, when the first MgSiO 3 phase and a second MgSiO with a clinoenstatite structure 3 When the weight ratio of the phases satisfies a weight ratio of 1:1 to 1:5, the following effects are achieved. 3 The appropriate content of the phase, the second MgSiO 3 The clinoenstatite phase can suppress the excessive volume expansion of the Si phase in the silicon-based composite particles during battery charging. Therefore, the structural stability of the negative electrode can be improved, thereby improving the life characteristics of the battery. In addition, since the resistance to lithium ion insertion is not large, the problems caused by the electrolyte side reaction can be minimized, and the discharge capacity can be improved.

[0034] The first MgSiO with enstatite structure 3 phase and a second MgSiO with a clinoenstatite structure 3 The weight ratio of the phases can be confirmed by XRD Rietveld refinement method. Figure 1 The XRD measurement diagram of the silicon-based composite particles was drawn using a D4ENDEAVOR / X-ray diffraction device manufactured by Bruker ( Figure 1 Thereafter, the XRD reference pattern is compared with a plurality of XRD reference patterns ( Figure 1 The XRD reference pattern that is most similar to the measured XRD pattern is found (determined as Figure 1Thereafter, the XRD reference pattern can be identified as a first MgSiO 3 phase and a second MgSiO with a clinoenstatite structure 3 Here, the XRD reference pattern refers to the first MgSiO with enstatite structure. 3 Phase and the second MgSiO with clinoenstatite structure 3 The XRD pattern of the mixed state is the first MgSiO 3 phase and a second MgSiO with a clinoenstatite structure 3 The theoretical XRD patterns of each phase are derived from the appearance theory.

[0035] The silicon-based composite particles may not contain Mg 2 SiO 4 phase, or may contain 10 wt% or less, specifically 5 wt% or less of Mg 2 SiO 4 When the above range is satisfied, the capacity and efficiency of the battery can be improved.

[0036] The silicon-based composite particles may not contain the MgO phase, or may contain 1 wt % or less, specifically 0.5 wt % or less of the MgO phase. When the above range is satisfied, the side reaction of the silicon-carbon composite particles with moisture may be minimized, thereby improving the stability of the battery.

[0037] The silicon-based composite particles may not contain silicon crystals, or, although the silicon-based composite particles contain silicon crystals, the size of the silicon crystals may be less than 15 nm, specifically less than 10 nm. When the above range is met, the reduction in battery life due to volume expansion of silicon crystals can be minimized. The size of the crystals can be determined by the Scherrer formula.

[0038] The average particle size (D 50 ) may be 4 to 10 μm, specifically 5 to 6.5 μm, and more specifically 6.15 to 6.4 μm. When the above range is met, although the silicon-based composite particles undergo volume expansion, blocking of the conductive network in the negative electrode can be prevented, and battery life can be improved.

[0039] The specific surface area of ​​the silicon composite particles can be 1 m 2 / g to 15m 2 / g, specifically 4m 2 / g to 8m 2 / g, more specifically 6.45m 2 / g to 6.7m2 When the above range is satisfied, irreversible sites in the silicon-based composite particles can be minimized, thereby suppressing a decrease in battery efficiency.

[0040] The negative electrode active material may further include a carbon coating disposed on the surface of the silicon-based composite particle.

[0041] The carbon coating layer is formed on the silicon-based composite particles, thereby imparting conductivity to the silicon-based composite particles, and can improve initial efficiency, life characteristics, and capacity characteristics of a secondary battery including a negative electrode active material containing the silicon-based composite particles.

[0042] The carbon coating layer may include at least one of amorphous carbon and crystalline carbon.

[0043] The crystalline carbon may further improve the conductivity of the silicon-based composite particles. The crystalline carbon may include at least one selected from the group consisting of fullerene, carbon nanotube, and graphene.

[0044] The amorphous carbon can suppress the expansion of the silicon-based composite particles by appropriately maintaining the strength of the carbon coating layer. The amorphous carbon can be at least one carbide selected from the group consisting of tar, pitch, and other organic materials, or can be a carbon-based material formed by using hydrocarbons as a chemical vapor deposition source.

[0045] The carbide of the other organic material may be a carbide of sucrose, glucose, galactose, fructose, lactose, mannose, ribose, aldohexose or ketohexose, and a carbide of an organic material selected from a combination thereof.

[0046] The hydrocarbon may be a substituted or unsubstituted aliphatic or alicyclic hydrocarbon, or a substituted or unsubstituted aromatic hydrocarbon. The substituted or unsubstituted aliphatic or alicyclic hydrocarbon may comprise methane, ethane, ethylene, acetylene, propane, butane, butylene, pentane, isobutane or hexane. The substituted or unsubstituted aromatic hydrocarbon may comprise benzene, toluene, xylene, styrene, ethylbenzene, diphenylmethane, naphthalene, phenol, cresol, nitrobenzene, chlorobenzene, indene, coumarone, pyridine, anthracene or phenanthrene.

[0047] The carbon coating layer may be contained in an amount of 3 to 5 parts by weight relative to 100 parts by weight of the silicon-based composite particles. When the above range is satisfied, the capacity and efficiency of the negative electrode active material may be prevented from decreasing.

[0048] The thickness of the carbon coating layer may be 10 μm to 100 μm, specifically 30 μm to 50 μm. When the above range is satisfied, the conductivity of the silicon-based composite particles is improved, thereby having an effect of improving the initial efficiency and life of the battery.

[0049] The method for preparing the negative electrode active material includes forming the silicon-based composite particles, and after forming the silicon-based composite particles, may further include forming a carbon coating layer as needed.

[0050] Forming the silicon-based composite particles may include: mixing Si, SiO 2 and Mg, and then react at a specific sublimation temperature, and then the mixture is cooled in a cooling zone having a specific cooling temperature, and subjected to a subsequent heat treatment at a specific subsequent heat treatment temperature, thereby forming Mg-SiO system particles.

[0051] Si and SiO 2 The mixing ratio (weight ratio) of the mixture of Si and SiO is preferably 97:3 to 85:15. 2 The molar ratio of MgSiO is preferably 1:0.8 to 1:1.2. In this case, the growth of silicon grains can be suppressed, and MgSiO 3 Mutually.

[0052] The sublimation temperature may be 1,400° C. to 1,600° C., specifically, 1,430° C. to 1,500° C. When the above range is satisfied, the silicon-based composite particles may be suppressed from having a porous structure by an appropriate sublimation rate.

[0053] The cooling temperature may be 600° C. to 900° C. When the above range is met, the silicon-based composite particles may be prevented from having a porous structure by an appropriate cooling rate. The cooling may be performed in a vacuum state.

[0054] The additional heat treatment temperature may be 700° C. to 1,000° C., specifically 750° C. to 850° C. When the above range is met, MgO generated during the preparation is converted into MgSiO. 3 , thereby suppressing the side reaction between the silicon-based composite particles and water, and adjusting the first MgSiO 3 phase and a second MgSiO with a clinoenstatite structure 3 The additional heating treatment can be carried out in an inert atmosphere such as Ar.

[0055] In the present invention, in order to make the first MgSiO 3 phase and a second MgSiO with a clinoenstatite structure 3 The weight ratio of the phases is 1:1 to 1:5. It is important to control the sublimation temperature, cooling temperature, additional heating temperature and Si / SiO 2 / Mg mixing amount to achieve the best combination.

[0056] <Negative electrode>

[0057] The negative electrode according to another embodiment of the present invention may include a negative electrode active material, wherein the negative electrode active material is the same as the negative electrode active material of the above embodiment. Specifically, the negative electrode may include a negative electrode current collector and a negative electrode active material layer arranged on the negative electrode current collector. The negative electrode active material layer may include the negative electrode active material. In addition, the negative electrode active material layer may further include a binder and / or a conductive agent.

[0058] The negative electrode current collector is not particularly limited as long as it has conductivity without causing adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel whose surface is treated with one of carbon, nickel, titanium, and silver, etc. can be used. Specifically, transition metals such as copper and nickel that adsorb carbon well can be used as the current collector. The thickness of the current collector can be 6 μm to 20 μm, but the thickness of the current collector is not limited thereto.

[0059] The binder 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, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and materials in which hydrogen is replaced by lithium (Li), sodium (Na) or calcium (Ca), or may include various copolymers thereof.

[0060] The conductive agent is not particularly limited as long as it has conductivity without causing adverse chemical changes in the battery, and conductive materials such as: 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 or metal fibers; conductive tubes, such as carbon nanotubes; fluorocarbon powder; metal powder, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives.

[0061] <Secondary Battery>

[0062] A secondary battery according to another embodiment of the present invention may include the negative electrode of the above embodiment. Specifically, the secondary battery may include a negative electrode, a positive electrode, a separator arranged between the negative electrode and the positive electrode, and an electrolyte, wherein the negative electrode is the same as the above negative electrode. Since the negative electrode has been described above, its detailed description will be omitted.

[0063] The positive electrode may include a positive electrode collector and a positive electrode active material layer formed on the positive electrode collector and including a positive electrode active material.

[0064] In the positive electrode, the positive electrode current collector is not particularly limited as long as it has conductivity without causing adverse chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface is treated with one of carbon, nickel, titanium, and silver, etc. can be used. In addition, the positive electrode current collector can generally have a thickness of 3 μm to 500 μm, and can have a surface with fine concavoconvexities to improve adhesion to the positive electrode active material. For example, the positive electrode current collector can be used in various forms such as films, sheets, foils, nets, porous bodies, foams, and non-woven fabrics.

[0065] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may include a layered compound such as lithium cobalt oxide (LiCoO 2 ) or lithium nickel oxide (LiNiO 2 ), or compounds substituted by 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 and LiMnO 2 ; Lithium copper oxide (Li 2 CuO 2 ); Vanadium oxides, such as LiV 3 O 8 、V 2 O 5 and Cu 2 V 2 O 7 ; By chemical formula LiNi 1-c2 M c2 O 2 (wherein M is at least one selected from the group consisting of cobalt (Co), manganese (Mn), aluminum (Al), copper (Cu), iron (Fe), magnesium (Mg), boron (B) and gallium (Ga), and c2 satisfies 0.01≤c2≤0.3) represented by a nickel (Ni) site type lithium nickel oxide; a lithium nickel oxide having a chemical formula of LiMn 2-c3 M c3 O 2 (wherein M is at least one selected from the group consisting of Co, Ni, Fe, chromium (Cr), zinc (Zn) and tantalum (Ta), and c3 satisfies 0.01≤c3≤0.1) or Li2 Mn 3 MO 8 (wherein M is at least one selected from the group consisting of Fe, Co, Ni, Cu and Zn); and LiMn 2 O 4 , wherein a portion of Li is replaced by alkaline earth metal ions, but the positive electrode active material is not limited thereto. The positive electrode may be Li metal.

[0066] The positive electrode active material layer may further include a positive electrode conductor and a positive electrode binder in addition to the positive electrode active material.

[0067] In this case, the positive electrode conductive agent is used to provide conductivity to the electrode, wherein any conductive agent can be used without particular limitation as long as it has electronic conductivity and does not cause adverse chemical changes in the battery. Specific examples of the conductive agent may be graphite, such as natural graphite or artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; powder or fiber of metals such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one of them or a mixture of two or more thereof may be used.

[0068] In addition, the positive electrode binder is used to improve the bonding 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 can be polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber and various copolymers thereof, and any one thereof or a mixture of two or more thereof can be used.

[0069] The separator separates the negative electrode from the positive electrode and provides a path for lithium ion movement, wherein any separator can be used as the separator without particular limitation, as long as it is commonly used in secondary batteries, in particular, a separator having high moisture retention capacity for electrolytes and low resistance to the transfer of electrolyte ions can be used. Specifically, a porous polymer film can be used, for example, a porous polymer film prepared by polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers and ethylene / methacrylate copolymers, or a layered structure of two or more layers thereof. In addition, a typical porous nonwoven fabric can be used, for example, a nonwoven fabric formed by high melting point glass fibers, polyethylene terephthalate fibers, etc. In addition, a coating separator comprising a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and can be optionally used in the form of a monolayer or multilayer structure.

[0070] The electrolyte may include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel-type polymer electrolyte, a solid inorganic electrolyte, or a molten inorganic electrolyte that can be used to manufacture a lithium secondary battery, but is not limited thereto.

[0071] Specifically, the electrolyte may include a nonaqueous organic solvent and a metal salt.

[0072] 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, ethers, methyl propionate and ethyl propionate can be used.

[0073] Especially, in carbonate-based organic solvents, because ethylene carbonate and propylene carbonate as cyclic carbonate are high viscosity organic solvents and dissociate lithium salts well due to high dielectric constant, cyclic carbonate can be preferably used. Because the above-mentioned cyclic carbonate can be prepared with electrolytes with high conductivity when mixed and used with low viscosity, low dielectric constant linear carbonates such as dimethyl carbonate and diethyl carbonate in an appropriate ratio, cyclic carbonate can be more preferably used.

[0074] As the metal salt, a lithium salt can be used, and the lithium salt is a material that is easily soluble in a non-aqueous electrolyte, wherein, for example, as an anion of the lithium salt, at least one 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 - 、(CF 3 ) 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 - and (CF 3 CF 2 SO 2 ) 2 N - .

[0075] In addition to the above electrolyte components, in order to improve the life characteristics of the battery, prevent the battery capacity from decreasing and improve the discharge capacity of the battery, the electrolyte may further contain at least one additive, for example, a halogenated alkylene carbonate compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme dimethyl ether, hexamethylphosphoric triamide, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted Oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol or aluminum trichloride.

[0076] According to another embodiment of the present invention, a battery module including the 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 capability and high cycle characteristics, the battery pack module and the battery pack can be used as a power source for medium and large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and power storage systems.

[0077] Preferred embodiments will be provided below to better understand the present invention. It is clear to those skilled in the art that these embodiments are provided only to illustrate the present invention, and various modifications and changes may be made within the scope of the present invention and technical spirit, and such modifications and changes are clearly included in the appended claims.

[0078] Examples and Comparative Examples

[0079] Example 1: Preparation of negative electrode active material

[0080] In the reactor, Si and SiO 2 The obtained powder (94 g) was mixed with Mg (6 g) in a 1:1 molar ratio, and the resulting mixture was heated at a sublimation temperature of 1,400° C. under vacuum. Then, the vaporized Si and SiO 2The mixed gas of Mg and Mg reacts in a cooling zone in a vacuum state with a cooling temperature of 800°C to cool to a solid phase. Then, heat treatment is performed at a temperature of 800°C (additional heating treatment temperature) in an inert atmosphere to prepare Mg-SiO-based particles. Then, the Mg-SiO-based particles are crushed by using a ball mill to prepare silicon-based composite particles with a size of 6 μm. Then, while passing Ar gas to maintain an inert atmosphere, the silicon-based composite particles are placed in a hot zone of a CVD device, and methane is blown into the hot zone at 900°C using Ar as a carrier gas, and the mixture is heated to 100°C at 300°C. -1 The reaction was allowed to proceed for 20 minutes to prepare a negative electrode active material having a carbon coating formed on the surface of the silicon-based composite particles.

[0081] Example 2: Preparation of negative electrode active material

[0082] A negative electrode active material was prepared in the same manner as in Example 1, except that the sublimation temperature was 1,450°C.

[0083] Example 3: Preparation of negative electrode active material

[0084] A negative electrode active material was prepared in the same manner as in Example 1, except that the sublimation temperature was 1,350°C.

[0085] Example 4: Preparation of negative electrode active material

[0086] A negative electrode active material was prepared in the same manner as in Example 1, except that the additional heat treatment temperature was 1,000°C.

[0087] Comparative Example 1: Preparation of negative electrode active material

[0088] A negative electrode active material was prepared in the same manner as in Example 1, except that the sublimation temperature was 1,480°C.

[0089] Comparative Example 2: Preparation of negative electrode active material

[0090] A negative electrode active material was prepared in the same manner as in Example 1, except that the sublimation temperature was 1,320°C.

[0091] Comparative Example 3: Preparation of negative electrode active material

[0092] A negative electrode active material was prepared in the same manner as in Example 1, except that the sublimation temperature was 1,500°C.

[0093] Comparative Example 4: Preparation of negative electrode active material

[0094] A negative electrode active material was prepared in the same manner as in Example 1, except that the sublimation temperature was 1,280°C.

[0095] Comparative Example 5: Preparation of negative electrode active material

[0096] A negative electrode active material was prepared in the same manner as in Example 1, except that no additional heating treatment was performed.

[0097] Comparative Example 6: Preparation of negative electrode active material

[0098] A negative electrode active material was prepared in the same manner as in Example 1, except that the additional heat treatment temperature was 600°C.

[0099] Comparative Example 7: Preparation of negative electrode active material

[0100] A negative electrode active material was prepared in the same manner as in Example 1, except that the additional heat treatment temperature was 1,100°C.

[0101] [Table 1]

[0102]

[0103] MgSiO 3 Content, Mg 2 SiO 4 The content of Mg and the content of MgO refer to the MgSiO in the silicon-based composite particles. 3 Mg 2 SiO 4 The contents (wt%) of Ag and MgO can be confirmed by an XRD Rietveld refinement method using a D4 ENDEAVOR / X-ray diffraction apparatus manufactured by Bruker Corporation.

[0104] The first MgSiO with enstatite structure can be confirmed by XRD Rietvelt refinement method using D4 ENDEAVOR / X-ray diffraction equipment manufactured by Bruker Corporation. 3 The content of the second MgSiO phase with a clinoenstatite structure 3 The content of the first MgSiO phase and the enstatite structure 3 Phase and the second MgSiO with clinoenstatite structure 3 Phase weight ratio.

[0105] The average particle size (D 50 ).

[0106] The specific surface area of ​​the silicon-based composite particles was confirmed using a BET analyzer.

[0107] Experimental Example 1: Evaluation of discharge capacity, initial efficiency, and life (capacity retention) characteristics A negative electrode and a battery were prepared using each negative electrode active material of the examples and comparative examples.

[0108] The negative electrode active material, carbon black as a conductive agent, and carboxymethyl cellulose (CMC) and styrene-butadiene rubber (SBR) as a binder were mixed in a weight ratio of 95.8:1:1.7:1.5 to prepare a mixture. Then, distilled water (7.8 g) was added to the mixture (5 g) and stirred to prepare a negative electrode slurry. The negative electrode slurry was coated on a 20 μm thick copper (Cu) metal film as a negative electrode collector and then dried. In this case, the temperature of the circulating air was 60 ° C. Subsequently, the negative electrode collector was roll-pressed and dried in a vacuum oven at 130 ° C for 12 hours to prepare a negative electrode.

[0109] The prepared negative electrode was cut into pieces with an area of ​​1.7671 cm 2 A lithium (Li) metal film is prepared as a positive electrode. A separator formed of porous polyethylene is inserted between the positive electrode and the negative electrode, and then an electrolyte is injected therein to prepare a lithium coin half-cell. The electrolyte is prepared by dissolving 0.5 wt % of vinylene carbonate in a mixed solution of ethyl methyl carbonate (EMC) and ethylene carbonate (EC) in a mixed volume ratio of 7:3, and dissolving 1 M concentration of LiPF 6 And get.

[0110] The prepared battery was charged and discharged, and the discharge capacity, initial efficiency and capacity retention rate were evaluated. The results are shown in Table 2 below.

[0111] The 1st and 2nd cycles were charged and discharged at 0.1 C, and the 3rd to 49th cycles were charged and discharged at 0.5 C. The 300th cycle was terminated in the charged state (the state in which lithium is located in the negative electrode), the battery was disassembled to measure the thickness, and then the electrode thickness change rate was calculated.

[0112] Charging conditions: CC (constant current) / CV (constant voltage) (5mV / 0.005C current cutoff value)

[0113] Discharge conditions: CC (constant current) condition 1.5V

[0114] The discharge capacity (mAh / g) and the initial efficiency (%) were obtained from the results during the first charge and discharge. Specifically, the initial efficiency (%) was obtained by the following calculation.

[0115] Initial efficiency (%) = (discharge capacity after the first discharge / first charge capacity) × 100 The capacity retention rate and the electrode thickness change rate were calculated as follows.

[0116] Capacity retention rate (%) = (300th discharge capacity / 1st discharge capacity) × 100 Electrode thickness change rate (%) = (final negative electrode thickness change / initial negative electrode thickness) × 100 [Table 2]

[0117] Battery Discharge capacity (mAh / g) Initial efficiency (%) Capacity retention rate (%) Example 1 1460 82.6 92 Example 2 1450 82.4 91 Example 3 1430 82.4 91 Example 4 1455 82.5 88 Comparative Example 1 1400 81.9 85 Comparative Example 2 1380 81.7 86 Comparative Example 3 1360 81.2 84 Comparative Example 4 1350 81.3 83 Comparative Example 5 1280 78.2 80 Comparative Example 6 1300 78.6 81 Comparative Example 7 1456 82.1 70

Claims

1. A negative electrode active material for a secondary battery, the negative electrode active material comprising silicon-based composite particles containing SiO x and MgSiO 3 phases, where 0 < x < 2, wherein the MgSiO 3 phase comprises a first MgSiO 3 phase having a protoenstatite structure and a second MgSiO 3 phase having a clinoenstatite structure in a weight ratio of 1:1 to 1:5, The first MgSiO 3 phase and the second MgSiO having a clinoenstatite structure 3 The weight ratios of the phases were confirmed by XRD Rietveld refinement method.

2. The negative electrode active material according to claim 1, wherein the MgSiO 3 The content of the phase in the silicon-based composite particles is 15 wt % to 60 wt %.

3. The negative electrode active material according to claim 1, wherein the silicon-based composite particles do not contain Mg 2 SiO 4 phase, or contains 10 wt% or less of the Mg 2 SiO 4 Mutually.

4. The negative electrode active material according to claim 1, wherein the average particle size D of the silicon-based composite particles is 50 4μm to 10μm, The average particle size D 50 It is measured by laser diffraction.

5. The negative electrode active material according to claim 1, wherein the specific surface area of ​​the silicon-based composite particles is 1 m 2 / g to 15m 2 / g, The specific surface area is measured by using a BET measuring instrument by degassing the object to be measured at 200° C. for 8 hours and N 2 Adsorption / desorption is measured. 6 . The negative electrode active material according to claim 1 , further comprising a carbon coating layer disposed on a surface of the silicon-based composite particle. 7 . The negative electrode active material according to claim 6 , wherein the content of the carbon coating layer is 3 to 5 parts by weight relative to 100 parts by weight of the silicon-based composite particles.

8. The negative electrode active material according to claim 1, wherein The silicon-based composite particles do not contain silicon crystals, or The silicon-based composite particles include silicon crystal grains, but the size of the silicon crystal grains is less than 15 nm. The size of the silicon grains is determined by the Scherrer equation. 9 . A negative electrode for a secondary battery, comprising the negative electrode active material for a secondary battery according to claim 1 . 10 . A secondary battery comprising the negative electrode for a secondary battery according to claim 9 .

Images