KR20200022729A - Negative electrode active material, negative electrode and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a negative electrode active material which comprises: a silicon-based core; a first coating layer positioned on the silicon-based core; and a second coating layer positioned on the first coating layer, wherein the second coating layer includes one or more elements of N and S. According to the present invention, the negative electrode active material can be useful in the manufacture of lithium secondary batteries comprising the same.

Description

Negative active material, negative electrode and lithium secondary battery comprising the same {NEGATIVE ELECTRODE ACTIVE MATERIAL, NEGATIVE ELECTRODE AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to a negative electrode active material, a negative electrode and a lithium secondary battery including the same, and more particularly, to a negative electrode active material including a silicon core, a first coating layer and a second coating layer and a negative electrode and a lithium secondary battery comprising the same.

As the development and demand for mobile devices increases, the demand for secondary batteries as energy sources is rapidly increasing. Among them, lithium secondary batteries exhibiting high energy density and operating potential, long cycle life, and low self-discharge rate. Batteries have been commercialized and widely used.

A lithium secondary battery generally includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator, and an electrolyte, and is a secondary battery in which charge and discharge are performed by intercalation-decalation of lithium ions. Lithium secondary batteries have high energy density, high electromotive force, and high capacity, and thus have been applied to various fields.

On the other hand, metal oxides such as LiCoO ₂ , LiMnO ₂ , LiMn ₂ O _4, or LiCrO ₂ are used as positive electrode active materials constituting the positive electrode of a lithium secondary battery, and metal lithium and graphite are used as negative electrode active materials constituting the negative electrode. Carbon based materials such as graphite or activated carbon, or materials such as silicon oxide (SiO _x ) are used. Among the negative active materials, metal lithium was initially used, but as the charge and discharge cycles progress, lithium atoms grow on the surface of the metal lithium to damage the separator and damage the battery. Recently, carbon-based materials are mainly used. However, the carbon-based material has a disadvantage in that the capacity is small because the theoretical capacity is only about 400 mAh / g.

Accordingly, various studies have been conducted to replace the carbonaceous material using silicon (Si) having a high theoretical capacity (4,200 mAh / g) as a negative electrode active material. The reaction equation for the case of lithium in silicon is as follows:

Scheme 1

22Li + 5Si = Li ₂₂ Si ₅

 However, most silicon anode materials have a disadvantage that the silicon volume expands by up to 300% due to lithium insertion, which causes the cathode to be destroyed and thus does not exhibit high cycle characteristics. Furthermore, in the case of silicon, volume expansion occurs due to the lithium insertion as the cycle continues, pulverization, contact losses with conductive agents and current collectors, and unstable solids. May exhibit fading mechanisms such as solid-electrolyte-interphase (SEI) formation.

Thus, in order to solve this problem, structure-controlled silicon, such as the formation of complexes with nanowires, nanotubes, nanoparticles, porous structures and carbon-based materials Research using nanostructures has been reported. For example, although the silicon nano-structure coated with carbon has been studied, the lithium secondary battery using this as a negative electrode active material has a disadvantage in that the capacity of the negative electrode active material is not maintained as the charge and discharge cycle is repeated. In addition, when the carbon is coated on the silicon-based particles by a method such as CVD, the specific surface area is increased, so that the side reaction with the electrolyte is increased, thereby deteriorating cycle characteristics.

Therefore, there is still a need for development of a silicon-based negative active material and a negative electrode and a lithium secondary battery including the same, which can solve the problems caused by the conventional silicon.

Korean Patent Publication 2017-0078203 A

The problem to be solved by the present invention, while reducing the volume expansion caused by the insertion of lithium ions can further improve the performance of the lithium secondary battery, side reaction with the electrolyte according to the large specific surface area of the silicon-based negative active material with a conventional carbon coating layer It is to provide a negative electrode active material that can solve the problem.

Another object of the present invention is to provide a negative electrode for a lithium secondary battery including the negative electrode active material.

Another object of the present invention is to provide a lithium secondary battery comprising the negative electrode for a lithium secondary battery.

In order to solve the above problems, the present invention is a silicon-based core; A first coating layer on the silicon core; And a second coating layer positioned on the first coating layer, wherein the second coating layer provides a negative electrode active material including at least one element of N and S.

In addition, the present invention provides a negative electrode for a lithium secondary battery including the negative electrode active material in order to solve the other problem.

In addition, the present invention provides a lithium secondary battery comprising the negative electrode for the lithium secondary battery, in order to solve the another problem.

While the negative electrode active material of the present invention can reduce the volume expansion of the silicon-based core due to the insertion of lithium ions, it can solve the problems associated with the increase in specific surface area generated when the carbon coating layer is formed on the silicon-based core. The performance of the secondary battery can be further improved. Therefore, the negative electrode active material of the present invention can be usefully used for the production of a lithium secondary battery including the same.

Hereinafter, the present invention will be described in more detail to aid in understanding the present invention.

The terms or words used in this specification and claims are not to be construed as limiting in their usual or dictionary meanings, and the inventors may appropriately define the concepts of terms in order to best describe their invention. It should be interpreted as meanings and concepts corresponding to the technical idea of the present invention based on the principle that the present invention.

The negative electrode active material of the present invention is a silicon-based core; A first coating layer on the silicon core; And a second coating layer positioned on the first coating layer, wherein the second coating layer includes one or more elements of N and S.

The silicon-based core may include at least one selected from the group consisting of Si, silicon oxide particles (SiO _x , 0 <x ≦ 2), and mixtures thereof, and the silicon oxide particles (SiO _x , 0 <x < 2) may be a composite composed of crystalline SiO ₂ and amorphous Si.

The silicon-based core may include at least one doped metal selected from the group consisting of Li, Mg, Ca, Al, and Ti. When the silicon-based core includes the doped metal, the doped metal may be a metal oxide, It may be included in the silicon-based core as a metal compound containing a metal silicate or both.

The silicon-based core may be a silicon-based particle composed of one lump, or alternatively, may be silicon secondary particles formed by agglomeration of silicon-based primary particles having a small particle size, or may include all of them.

The average particle diameter (D ₅₀ ) of the silicon core may be 0.01 μm to 30 μm, specifically 0.1 μm to 10 μm, and is 30 to 99 parts by weight, specifically 70 to 99 weight based on 100 parts by weight of the negative electrode active material. It may be wealth.

The first coating layer is formed on the silicon core, and specifically, may be formed on an outer surface of the silicon core.

The first coating layer may be formed on the silicon core to prevent or mitigate pulverization of the silicon core when the silicon core changes in volume due to the insertion and desorption of lithium. Side reactions of the electrolyte can be effectively prevented or alleviated. In addition, since the silicon-based core has low electrical conductivity, the first coating layer may allow the silicon-based core to have excellent conductivity, so that the negative electrode active material may easily react with lithium.

The first coating layer is amorphous carbon, natural graphite, artificial graphite, activated carbon, MCMB (MesoCarbon MicroBead), carbon fiber (carbon fiber), and coal tar pitch (petroleum pitch) and petroleum pitch (petroleum pitch) and organic materials ( organic material) may include at least one selected from the group consisting of carbon-based materials made by heat-treating raw materials, specifically, at least one selected from the group consisting of amorphous carbon, natural graphite, artificial graphite, and activated carbon. Can be.

The thickness of the first coating layer may be 5 nm to 100 nm, specifically 10 nm to 100 nm.

When the thickness of the first coating layer satisfies the above range, it is possible to appropriately prevent or alleviate pulverization of the silicon core in response to a volume change of the silicon core, and prevent side reaction with the electrolyte of the silicon core. It is possible to prevent the insertion and desorption of lithium by the first coating layer.

The first coating layer may be 0.1 parts by weight to 50 parts by weight based on 100 parts by weight of the negative electrode active material, and specifically 1 part by weight to 10 parts by weight. The content of the first coating layer is related to the thickness of the first coating layer. Accordingly, when the content of the first coating layer satisfies the above range, the silicon core may cause side reaction with the electrolyte solution while appropriately preventing or mitigating pulverization of the silicon core in response to the volume change of the silicon core. It can be prevented, and the ratio of the first coating layer of the negative electrode active material can be made to an appropriate degree so that the insertion and desorption of lithium can be prevented.

As described above, the first coating layer functions to prevent or mitigate pulverization of the silicon-based core in response to the volume change of the silicon-based core, while preventing the silicon-based core from causing side reactions with the electrolyte. . However, since the first coating layer located on the silicon-based core may itself have a large specific surface area in its formation, there is a possibility that the first coating layer causes side reactions with the electrolyte solution. Therefore, the negative electrode active material of the present invention includes a second coating layer located on the first coating layer to solve the problem caused by the large specific surface area of the first coating layer.

The second coating layer is formed on the first coating layer, specifically, may be formed on the other surface of the surface contacting the silicon-based core of the first coating layer, that is, on the outer surface of the first coating layer.

The second coating layer has a smoother surface than the first coating layer, and is formed on the first coating layer to reduce side reaction with the electrolyte due to the large specific surface area of the first coating layer, thereby improving cycle characteristics of the negative electrode active material. Can be. In addition, the second coating layer helps to maintain a physical bond between the silicon core and the first coating layer, and the first coating caused by the volume change of the silicon core when the volume of the silicon core changes due to the insertion and desorption of lithium. It is possible to accommodate the volume change of the silicon-based core while suppressing the detachment of the coating layer. In particular, since the second coating layer includes at least one element of N and S, the N and S may further improve the conductivity of the negative electrode active material.

The second coating layer may be made by carbonizing a polymer containing at least one element of N and S in the molecule.

The polymer containing at least one element of N and S in the molecule is not particularly limited as long as it corresponds thereto, for example, polyacrylonitrile, polyvinylpyrrolidone, polyethyleneimine, polypyrrole, polyaniline, urethane, polythiophene It may be at least one selected from the group consisting of polyurea and polythiophene, and may be used in combination with a suitable solvent.

The second coating layer may be formed by carbonizing a single layer film of a polymer, or a multilayer film in which two or more coating films are stacked.

The thickness of the second coating layer may be 5 nm to 100 nm, specifically 10 nm to 100 nm.

When the thickness of the second coating layer satisfies the above range, it is possible to appropriately reduce the influence of the specific surface area of the first coating layer, assist in the physical bonding of the silicon-based core and the first coating layer, the insertion of lithium and While the volume change of the silicon-based core due to desorption can be suitably accommodated, insertion and desorption of lithium can be prevented from interfering with the second coating layer.

The second coating layer may be 0.1 parts by weight to 50 parts by weight based on 100 parts by weight of the negative electrode active material, and specifically 1 part by weight to 10 parts by weight. The content of the second coating layer is related to the thickness of the second coating layer. Therefore, when the content of the second coating layer satisfies the above range, it is possible to exert an effect according to the thickness limitation of the second coating layer as described above.

The negative active material may have an average particle diameter (D ₅₀ ) of 0.05 to 40 ㎛, specifically, may have an average particle diameter of 0.05 to 20 ㎛, more specifically 1 to 10 ㎛.

When the average particle diameter of the negative electrode active material is 0.05 μm or more, the density of the electrode may be prevented from being lowered to have an appropriate volume / volume, and when the average particle size is 40 μm or less, the slurry for forming the electrode may be appropriately uniformly formed. Can be coated.

In the present invention, the average particle diameter (D ₅₀ ) of the silicon-based core and the silicon oxide-carbon-polymer composite may be defined as the particle size at 50% of the particle size distribution. The average particle diameter is not particularly limited, but may be measured using, for example, a laser diffraction method or a scanning electron microscope (SEM) photograph. In general, the laser diffraction method can measure a particle diameter of about several mm from the submicron region, and a result having high reproducibility and high resolution can be obtained.

The negative electrode active material according to an example of the present invention may be manufactured by, for example, a manufacturing method described below.

An anode active material according to an example of the present invention comprises the steps of (1) forming a first coating layer on a silicon-based core; And (2) it may be prepared by a manufacturing method comprising the step of forming a second coating layer on the first coating layer.

(1) forming the first coating layer on the silicon-based core may be achieved by forming a composite by growing carbon on the silicon-based core, for example, amorphous carbon, natural graphite, artificial graphite, activated carbon, Mesocarbon MicroBead (MCMB), carbon fiber, and carbon-based material made by heat treatment of coal tar pitch, petroleum pitch and organic material as raw materials At least one selected may be formed by coating using chemical vapor deposition (CVD), coating using pitch, solvent evaporation, coprecipitation, precipitation, sol-gel or sputtering, and specifically organic The carbon-based material formed by heat-treating an organic material as a raw material may be formed by depositing by chemical vapor deposition (CVD).

(2) The forming of the second coating layer on the first coating layer may be performed by coating a polymer material constituting the second coating layer on the first coating layer and then carbonizing it.

The polymer material is a polymer containing at least one element of N and S in a molecule, and may be polyacrylonitrile, polyvinylpyrrolidone, polyethyleneimine, polypyrrole, polyaniline, urethane, polythiophene, polyurea and polythiophene. At least one selected from the group consisting of, which is coated by chemical vapor deposition (CVD), solvent evaporation method, coprecipitation method, precipitation method, sol-gel method or sputtering method and carbonized by heat treatment to the second coating layer Can be formed.

The carbonization in step (2) may be performed for 2 to 6 hours at a temperature of, for example, 600 ° C to 800 ° C.

The negative electrode active material according to an example of the present invention may be used alone as a negative electrode active material, or may be mixed with a material capable of alloying with carbon and / or lithium and used as a negative electrode active material. The material capable of alloying with lithium is 1 selected from the group consisting of Si, SiO _x , Sn, SnO _x , Ge, GeO _x , Pb, PbO _x , Ag, Mg, Zn, ZnO _x , Ga, In, Sb and Bi. Or more species.

The present invention also provides a negative electrode for a lithium secondary battery including the negative electrode active material, and provides a lithium secondary battery including the negative electrode.

The lithium secondary battery may include a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode.

The negative electrode may be prepared by a conventional method known in the art, for example, by mixing and stirring the negative electrode active material and additives such as a binder and a conductive material to prepare a negative electrode active material slurry, and then apply it to the negative electrode current collector and dry After compression can be prepared.

The binder may be used to bind the negative electrode active material particles to maintain the molded body, and is not particularly limited as long as it is a conventional binder used when preparing a slurry for the negative electrode active material, and for example, polyvinyl alcohol, carboxymethyl cellulose, and hydroxy, which are non-aqueous binders. Propylene cellulose, diacetylene cellulose, polyvinyl chloride, polyvinylpyrrolidone, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyethylene or polypropylene, and the like. Any one or a mixture of two or more selected from the group consisting of ronitrile-butadiene rubber, styrene-butadiene rubber and acrylic rubber can be used. Aqueous binders are economical and environmentally friendly compared to non-aqueous binders, are harmless to the health of workers, and have a superior binding effect than non-aqueous binders. Specifically styrene-butadiene rubber may be used.

The binder may be included in less than 10% by weight of the total weight of the slurry for the negative electrode active material, specifically, may be included in 0.1% by weight to 10% by weight. If the content of the binder is less than 0.1% by weight, the effect of using the binder is insignificant and undesirable. If the content of the binder is more than 10% by weight, the capacity per volume may decrease due to the decrease in the relative content of the active material due to the increase in the content of the binder. not.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; Carbon blacks such as acetylene black, Ketjen black, channel black, furnace black, lamp black and summer black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Or conductive materials such as polyphenylene derivatives. The conductive material may be used in an amount of 1% by weight to 9% by weight based on the total weight of the slurry for the negative electrode active material.

The negative electrode current collector used for the negative electrode according to an embodiment of the present invention may have a thickness of 3 ㎛ to 500 ㎛. The negative electrode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and is not particularly limited to the surface of copper, gold, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel. Surface-treated with carbon, nickel, titanium, silver and the like, aluminum-cadmium alloy and the like can be used. In addition, it is possible to enhance the bonding strength of the negative electrode active material by forming fine irregularities on the surface, it can be used in various forms such as film, sheet, foil, net, porous body, foam, non-woven fabric.

The positive electrode can be prepared by conventional methods known in the art. For example, a slurry may be prepared by mixing and stirring a solvent, a binder, a conductive material, and a dispersant in a cathode active material, if necessary, and then applying (coating) to a current collector of a metal material, compressing, and drying the same to prepare a cathode. have.

The current collector of the metal material is a highly conductive metal, and is a metal to which the slurry of the positive electrode active material can easily adhere, and is particularly limited as long as it has high conductivity without causing chemical change in the battery in the voltage range of the battery. For example, the surface-treated with stainless steel, aluminum, nickel, titanium, calcined carbon, or carbon, nickel, titanium, silver, or the like on the surface of aluminum or stainless steel may be used. In addition, fine unevenness may be formed on the surface of the current collector to increase the adhesion of the positive electrode active material. The current collector may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven fabric, and may have a thickness of 3 to 500 μm.

The positive electrode active material may be, for example, lithium cobalt oxide (LiCoO ₂ ); Lithium nickel oxide (LiNiO ₂ ); Li [Ni _a Co _b Mn _c M ¹ _d ] O ₂ (wherein M ¹ is any one selected from the group consisting of Al, Ga, and In or two or more elements thereof, and 0.3 ≦ a <1.0, 0 ≤ b ≤ 0.5, 0 ≤ c ≤ 0.5, 0 ≤ d ≤ 0.1, a + b + c + d = 1); Li (Li _e M ² _{fe-f '} M ³ _f' ) O _2-g A _g (wherein 0≤e≤0.2, 0.6≤f≤1, 0≤f'≤0.2, 0≤g≤0.2 , M ² includes Mn and at least one selected from the group consisting of Ni, Co, Fe, Cr, V, Cu, Zn and Ti, M ³ is 1 selected from the group consisting of Al, Mg and B At least one species, and A is at least one species selected from the group consisting of P, F, S and N), or a compound substituted with one or more transition metals; Lithium manganese oxides such as Li _{1 + h} Mn _2-h O ₄ (wherein 0 ≦ _h ≦ 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 , Cu ₂ V ₂ O _7, and the like; Formula ^{_{LiNi 1 - i M 4 i O}} 2 Ni site type lithium nickel oxides represented by (wherein, M = ^4, and Co, Mn, Al, Cu, Fe, Mg, B or Ga, 0.01≤i≤0.3); Formula LiMn _{2 - j} M ⁵ _j O ₂ (wherein M ⁵ = Co, Ni, Fe, Cr, Zn or Ta, 0.01 ≦ j ≦ 0.1) or Li ₂ Mn ₃ M ⁶ O ₈ (wherein M ⁶ = lithium manganese composite oxide represented by Fe, Co, Ni, Cu or Zn); LiMn ₂ O _{4 in} which a part of Li in the formula is substituted with alkaline earth metal ions; Disulfide compounds; LiFe ₃ O ₄ , Fe ₂ (MoO ₄ ) ₃ , and the like, but are not limited thereto.

The solvent for forming the positive electrode includes an organic solvent such as NMP (N-methyl pyrrolidone), DMF (dimethyl formamide), acetone, dimethyl acetamide or water, and these solvents are used alone or in combination of two or more. Can be used by mixing. The amount of the solvent used is sufficient to dissolve and disperse the positive electrode active material, the binder, and the conductive material in consideration of the coating thickness of the slurry and the production yield.

The binder may be polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, Polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), Sulfonated EPDM, styrene butadiene rubber (SBR), fluorine rubber, poly acrylic acid and polymers in which hydrogen thereof is replaced with Li, Na or Ca, or the like, or Various kinds of binder polymers such as various copolymers can be used.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; Carbon blacks such as acetylene black, Ketjen black, channel black, farnes black, lamp black and thermal black; Conductive fibers such as carbon fibers and metal fibers; Conductive tubes such as carbon nanotubes; Metal powders such as fluorocarbon, aluminum and nickel powders; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used. The conductive material may be used in an amount of 1 wt% to 20 wt% with respect to the total weight of the positive electrode slurry.

The dispersant may be an aqueous dispersant or an organic dispersant such as N-methyl-2-pyrrolidone.

In addition, as the separator, conventional porous polymer films conventionally used as separators, such as polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene-butene copolymer, ethylene-hexene copolymer and ethylene-methacrylate copolymer, etc. The porous polymer film prepared by using a single or lamination thereof may be used, or alternatively, a conventional porous nonwoven fabric, such as a high melting point glass fiber, polyethylene terephthalate fiber, etc. may be used, but is not limited thereto.

A lithium salt which can be included as an electrolyte used in the present invention can be used without limitation, those which are commonly used in a lithium secondary battery electrolyte, for example the lithium salt of the anion is ^{F -, Cl -, Br -} , 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 may be any one selected from the group consisting of _{^{-, CH 3 CO 2 -,}} SCN - , and _{(CF 3 CF 2 SO 2)} 2 N.

Examples of the electrolyte used in the present invention include an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, a molten inorganic electrolyte, and the like, which can be used in manufacturing a lithium secondary battery. no.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be cylindrical, square, pouch type or coin type using a can.

The lithium secondary battery according to the present invention may not only be used in a battery cell used as a power source for a small device, but also may be used as a unit battery in a medium-large battery module including a plurality of battery cells.

Preferred examples of the medium-to-large device include, but are not limited to, electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

Example

Hereinafter, the present invention will be described in more detail with reference to Examples and Experimental Examples, but the present invention is not limited to these Examples and Experimental Examples. Embodiments according to the present invention can be modified in many different forms, the scope of the invention should not be construed as limited to the embodiments described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example 1

Silicon monoxide (SiO) having an average particle diameter (D ₅₀ ) of about 5 μm was introduced into a tube-shaped tubular furnace and subjected to CVD under a mixed gas of argon (Ar) and methane (CH ₄ ) to about 2.5 weights of carbon. Silicon-based particles having a carbon coating layer of% were prepared.

A polyurea polymer was dissolved in an alcohol solvent to prepare a polymer solution, and the silicon-based particles having a carbon coating layer formed thereon were immersed in the polymer solution and sufficiently stirred using a stirrer. The filter-filtered solution was filtered and dried to obtain a silicon-based particle-carbon coating layer-N-containing polymer composite. Thereafter, the dried silicon-based particle-carbon coating layer-N-containing polymer composite was heat-treated at 800 ° C. for 3 hours to perform a carbonization reaction. Finally, a negative electrode active material in the form of a silicon-based particle-carbon coating layer-N-containing polymer carbonized coating layer was prepared. The carbon content of the prepared particles was about 5% by weight.

Example 2

Except for using a polythiophene polymer in place of the polyurea polymer, a negative electrode active material in the form of a silicon-based particle-carbon coating layer-S containing a polymer carbide coating layer in the same manner as in Example 1.

Comparative Example 1

Silicon monoxide (SiO) having an average particle diameter (D ₅₀ ) of about 5 μm was introduced into a tube-shaped tubular furnace, and subjected to CVD under a mixed gas of argon (Ar) and methane (CH ₄ ) to have a carbon content of about 5 weight. Silicon-based particles having a carbon coating layer of% were prepared.

Comparative Example 2

A polymer solution was prepared by dissolving a polyurea polymer in an alcohol solvent, and silicon monoxide (SiO) particles having an average particle diameter (D ₅₀ ) of about 5 μm were immersed in the polymer solution and sufficiently stirred using a stirrer. The filter-filtered solution was filtered and dried to obtain a silicone-based particle-N-containing polymer composite.

Thereafter, the dried silicon-based particle-N-containing polymer composite was heat-treated at 800 ° C. for 3 hours to perform a carbonization reaction, and finally, particles in the form of silicon-based particle-N-containing polymer carbonized coating layer were prepared. The carbon content of the prepared particles was about 5% by weight.

Comparative Example 3

Except for using a polythiophene polymer in place of the polyurea polymer, particles in the form of a silicon-based particle-S-containing polymer carbide coating layer was prepared in the same manner as in Comparative Example 2.

<Production of Cathode and Lithium Secondary Battery>

Examples 1A and 2A

As the negative electrode active material, the silicon composites prepared in Examples 1 and 2 were each made of carbon black as a conductive material, polyacrylic acid (PAA) as a binder, and water (H ₂ O) as a solvent in a weight ratio of 80:10:10. Mixing together produced a uniform negative electrode slurry. The prepared negative electrode slurry was coated on one surface of a copper current collector, dried and rolled, and then punched to a predetermined size to prepare a negative electrode.

Li metal was used as a counter electrode, and a polyolefin separator was interposed between the negative electrode and the Li metal, and then 1M in a solvent in which ethylene carbonate (EC) and diethyl carbonate (EMC) were mixed at a volume ratio of 30:70. A coin-type half cell was prepared by injecting an electrolyte in which LiPF ₆ was dissolved.

Comparative Examples 1A, 2A and 3A

A negative electrode and a coin-type half cell were manufactured in the same manner as in Example 1A, except that the silicon composites of Comparative Examples 1, 2, and 3 were used as the negative electrode active materials, respectively.

Experimental Example 1: Evaluation of Capacity Retention Rate and Electrode Thickness Change Rate

Charge and discharge were performed on the batteries prepared in Examples 1A and 2A, Comparative Examples 1A, 2A, and 3A, respectively, to evaluate the capacity retention rate and the electrode thickness change rate, which are described in Table 1 below.

Charge each cell at 25 ° C with a constant current (CC) of 0.1 C until 5 mV, then charge with a constant voltage (CV) until the charging current reaches 0.005 C (cut-off current). The filling was performed. Thereafter, it was left for 20 minutes, and then discharged until it became 1.5 V with a constant current (CC) of 0.1 C to perform the first discharge. Since the charge and discharge was repeated at 0.5 C until 40 cycles to evaluate the capacity retention rate. After completion of the cycle experiment, 41 cycles were terminated in a charged state, the cell was disassembled to measure the thickness of the negative electrode, and then, based on the negative electrode thickness immediately after manufacture, the increase in the negative electrode thickness after 41 cycles was calculated as a% value. This is represented by the change rate of the electrode thickness.

Capacity retention rate (%) % Change in electrode thickness Example 1A 55.9 48.2 Example 2A 55.3 47.8 Comparative Example 1A 53.7 50.5 Comparative Example 2A 50.2 52.6 Comparative Example 3A 49.7 53.1

When Examples 1A and 2A were compared with Comparative Example 1A, the negative electrode using the negative electrode active material including the first coating layer and the second coating layer including N or S had a small thickness change rate and an excellent capacity retention rate. On the other hand, in the case of the particles of Comparative Example 1A, since the specific surface area of the carbon coating layer on the surface is large, the area that can be contacted with the liquid electrolyte is large, thereby increasing the side reactions and increasing the resistance. Judging by the heat.

Compared to Examples 1A and 2A and Comparative Examples 2A and 3A, the negative electrode using the negative electrode active material including the first coating layer and the second coating layer containing N or S has a small thickness change rate and excellent capacity retention rate. It was. On the other hand, in the case of the negative electrode including the particles of Comparative Examples 2A and 3A, the increase in the thickness of the silicon-based particles (silicon monoxide) could not be effectively suppressed, so the performance is judged to be inferior. In addition, in the case of the negative electrode active material of Examples 1A and 2A in addition, since the first coating layer is included, compared with the particles of Comparative Examples 2A and 3A that do not include it, the electrical conductivity is high, the battery resistance is small, and thus the battery It is believed that the performance has improved.

Claims (12)

Silicon-based cores;
A first coating layer on the silicon core; And
A second coating layer on the first coating layer,
The second coating layer, the negative electrode active material containing at least one element of N and S.
The method of claim 1,
The silicon-based core comprises at least one selected from the group consisting of Si, silicon oxide particles (SiO _x , 0 <x≤2) and mixtures thereof, negative electrode active material.
The method of claim 2,
The silicon oxide particles (SiO _x , 0 <x <2) is a composite consisting of crystalline SiO ₂ and amorphous Si, the negative electrode active material.
The method of claim 1,
The silicon-based core is at least one doped metal selected from the group consisting of Li, Mg, Ca, Al, Ti and Na, the negative electrode active material.
The method of claim 1,
The average particle diameter (D ₅₀ ) of the silicon core is 0.01 μm to 30 μm, the negative electrode active material.
The method of claim 1,
The first coating layer is amorphous carbon, natural graphite, artificial graphite, activated carbon, MCMB (MesoCarbon MicroBead), carbon fiber (carbon fiber), and coal tar pitch (petroleum pitch) and petroleum pitch (petroleum pitch) and organic materials ( An anode active material comprising at least one selected from the group consisting of carbon-based materials made by heat-treating an organic material as a raw material.
The method of claim 1,
The first coating layer has a thickness of 5 nm to 100 nm, the negative electrode active material.
The method of claim 1,
The second coating layer is made by carbonizing a polymer containing at least one element of N and S in the molecule, the negative electrode active material.
The method of claim 8,
The polymer comprising at least one element of N and S in the molecule is selected from the group consisting of polyacrylonitrile, polyvinylpyrrolidone, polyethyleneimine, polypyrrole, polyaniline, urethane, polythiophene, polyurea and polythiophene. At least one selected, the negative electrode active material.
The method of claim 1,
The second coating layer has a thickness of 5 nm to 100 nm, the negative electrode active material.
A negative electrode for a lithium secondary battery comprising the negative electrode active material according to claim 1.
A lithium secondary battery comprising the negative electrode according to claim 11.