KR20050090218A - Negative active material for lithium secondary battery, method of preparing same, and lithium secondary battery comprising same - Google Patents

Abstract

The present invention provides a silicon oxide (SiO _x ; x is 1.5 or less) is a core material doped with an element selected from the group consisting of B, P, Li, Ge, Al, V or a mixture thereof; And a silicon oxide-carbon composite including a carbon material present on the surface of the core material. The negative electrode active material of the present invention can improve the life characteristics and high rate charge and discharge characteristics of the lithium secondary battery excellently.

Description

A negative electrode active material for a lithium secondary battery, a method of manufacturing the same, and a lithium secondary battery including the same TECHNICAL FIELD

[Industrial use]

The present invention relates to a negative electrode active material for a lithium secondary battery, a method for manufacturing the same, and a lithium secondary battery including the same. More specifically, a negative electrode active material for a lithium secondary battery excellent in lifespan characteristics and high rate charge-discharge characteristics, a method for producing the same, and the same It relates to a lithium secondary battery.

[Prior art]

Recently, in order to cope with small weight and high performance of portable electronic devices, high capacity of lithium secondary batteries has emerged as an urgent problem. However, graphite, which is one of the negative electrode active materials of a lithium secondary battery, has a theoretical capacity of 372 mAh / g, but in order to obtain a higher capacity negative electrode active material, development of a new material that can replace amorphous carbon materials or carbon materials. It is necessary to proceed.

Silicon and its compound have been examined conventionally as a novel material which can replace graphite. Silicon and its compounds are known to form an alloy with lithium and exhibit a larger capacitance than graphite.

Therefore, recently, as a negative electrode material of a lithium secondary battery, (1) a material in which a silicon compound powder is simply mixed with graphite, (2) a fine powder silicon compound or the like is chemically fixed on the graphite surface by using a silane coupling agent or the like, (3) A material in which a graphite carbon material and a metal material such as Si is bonded or coated with an amorphous carbon material has been proposed.

However, since the graphite and the silicon compound are not necessarily in close contact with each other in the above-mentioned material (1) in which the powder of the silicon compound is simply mixed, the silicon compound is separated from the graphite when the graphite expands or contracts as the charge and discharge cycle proceeds. Since the silicon compound itself has low electron conductivity, there is a problem that the silicon compound is not sufficiently used as a negative electrode active material, and the cycle characteristics of the lithium secondary battery are lowered.

In addition, the above-mentioned material (2) chemically fixing finely powdered silicon compound on the graphite surface by using a silane coupling agent or the like is maintained in a state in which the silicon compound is in close contact with the graphite during the initial charge and discharge cycle. It functions as a negative electrode active material, but as the charge and discharge cycle progresses, the silicon compound itself expands as the alloy with lithium forms, thereby destroying the bond by the silane coupling agent, thereby releasing the silicon compound from the graphite, and the silicon compound as the negative electrode. There is a problem that the cycle characteristics of the lithium secondary battery may be lowered because it cannot be sufficiently used as an active material. In addition, the silane coupling treatment may not be performed homogeneously at the time of manufacturing the negative electrode material, and there is a problem that a negative electrode material of stable quality cannot be easily produced.

In addition, (3) the material which combines or coats the graphite-based carbon material and metal materials such as Si with an amorphous carbon material is (2) chemically fixing finely divided silicon compounds or the like on the graphite surface using a silane coupling agent or the like. There is the same problem as the above problem of the material. That is, as the charge and discharge cycle proceeds, the bond by the amorphous carbon material is broken by the expansion of the metal material itself due to the alloy formation with lithium, and the metal material is released from the graphite carbon material, so that the metal material is sufficiently used as the negative electrode active material. There was a problem that it was not used and the cycle characteristics were lowered.

The present invention is to solve the above problems, to provide a negative electrode active material for a lithium secondary battery excellent in life characteristics and high rate charge-discharge characteristics and a method of manufacturing the same.

The present invention also provides a lithium secondary battery comprising the negative electrode active material.

The present invention provides a silicon oxide (SiO _x ; x is 1.5 or less) is a core material doped with an element selected from the group consisting of B, P, Li, Ge, Al, V or a mixture thereof; And a silicon oxide-carbon composite including a carbon material present on the surface of the core material.

The invention also relates to a doping element selected from the group consisting of SiO ₂ , Si and B-containing compounds, P-containing compounds, Li-containing compounds, Ge-containing compounds, Al-containing compounds, V-containing compounds and mixtures thereof -Mixing the containing compound; Heat treating the mixture to prepare a core material of silicon oxide (SiO _x ; x is 1.5 or less) doped with B, P, Li, Ge, Al, V or a mixture thereof; And it provides a method for producing a negative electrode active material for a lithium secondary battery comprising the step of coating the core material with a carbon material to produce a silicon oxide-carbon composite.

The present invention also includes a negative electrode comprising the negative electrode active material; A positive electrode including a positive electrode active material capable of reversible intercalation / deintercalation of lithium; And it provides a lithium secondary battery comprising an electrolyte solution.

Hereinafter, the present invention will be described in detail.

The negative electrode active material of the present invention is a core material doped with an element selected from the group consisting of silicon oxide (SiO _x ; x is 1.5 or less) B, P, Li, Ge, Al, V or a mixture thereof and the core material Silicon oxide-carbon composite containing a carbon material present on the surface of. 1 is a cross-sectional view schematically showing the silicon oxide-carbon composite of the present invention. As can be seen in Figure 1, the silicon oxide-carbon composite of the present invention is in the amorphous silicon oxide (11) doped with an element selected from the group consisting of B, P, Li, Ge, Al, V or a mixture thereof. It has a structure in which the carbon material 21 is coated on the surface of the core material 10 in which the nano-sized silicon crystalline (Si nano-cryatalline) 12 is dispersed.

SiO _x constituting the core material has a high irreversible capacity, short lifetime, and poor high rate charge and discharge efficiency. This is thought to be due to the low structural stability during charging and discharging and the low diffusion rate of Li atoms. In the present invention, doping B, P, Li, Ge, Al, V or a mixture thereof to SiO _x increases the degree of amorphousness of SiO _x and improves the diffusion rate of Li atoms. The amorphous degree of at least 50% of the negative electrode active material of the present invention, when measuring the diffusion rate of Li according to GITT (Galvanic Intermitant time technique) how to 10 ^-8 cm ² / sec or more, preferably 10 ^-8 to 10 ^{- It} has a diffusion rate of ⁶ cm ² / sec. The amorphous degree is defined as in Equation 1 below.

[Calculation 1]

% Amorphous = = ((major XRD peak intensity after quench) / (major XRD peak intensity after quench)) * 100

The _x of the SiO _x is preferably adjusted to 1.5 or less, more preferably 0.5 to 1.5. If x exceeds 1.5, the relative amount of Si, the electrochemical reaction site, may be small, leading to a reduction in the overall energy density.

The doping amount of the doping element B, P, Li, Ge, Al, V or a mixture thereof is preferably 50% by weight or less, more preferably 10 to 30% by weight based on the total weight of the core material. When the doping amount exceeds 50% by weight, there is a problem in that energy density and irreversible capacity are increased.

As the carbon material coating the core material, crystalline carbon or amorphous carbon may be used. Examples of the crystalline carbon include plate-like, spherical or fibrous natural graphite or artificial graphite. The carbon material may be coated with 10 to 90% by weight, preferably 30 to 70% by weight, based on the total weight of the silicon oxide-carbon material composite. If the content of the carbon material is less than 10% by weight, the role as a support for the volume expansion of the active material that may occur during charging and discharging of the core material is significantly reduced, which may adversely affect the life of the electrode, 90% by weight When exceeded, the discharge capacity of the active material itself is reduced due to the reduction in the composition ratio of the core material, and thus it is impossible to pursue a high energy density compared with the conventional graphite material negative active material.

Examples of the amorphous carbon include digraphitizable carbon (soft carbon, low temperature calcined carbon) or nongraphitizable carbon (hard carbon).

The soft carbon may be obtained by heat treating coal-based pitch, petroleum-based pitch, tar, and low molecular weight heavy oil at about 1000 ° C.

The hard carbon may be obtained by heat treatment of a phenol resin, naphthalene resin, polyvinyl alcohol resin, urethane resin, polyimide resin, furan resin, cellulose resin, epoxy resin, polystyrene resin and the like at about 1000 ° C. In addition, the mesoface pitch, raw cokes, and carbon raw materials obtained by heat-treating petroleum, coal-based carbon raw materials, or resin-based carbons at 300 to 600 ° C., are heat-treated at 600 to 1500 ° C. after infusible treatment or without infusible treatment. It is also possible to use amorphous carbon such as one mesophase pitch carbide, calcined coke, or the like.

Hereinafter, a manufacturing process of a silicon oxide-carbon material composite which is a negative electrode material of the present invention will be described. First, a B-containing compound, a P-containing compound, a Li-containing compound, a Ge-containing compound, an Al-containing compound, a V-containing compound, or a mixture thereof is added to a mixture of SiO ₂ and Si to prepare a core material. Heat treatment together. SiO ₂ and Si are preferably mixed in a weight ratio of 3: 1 to 1: 1.

The doping element-containing compound is preferably a glass network former. Examples of the B-containing compound include B ₂ O ₃ , B ₂ O, and the like, and examples of the P-containing compound include P ₂ O ₅ , P ₂ O ₃ , and the like, and Li-containing compounds include Li ₂ O, Li ₂ CO ₃ , LiOH, and the like, Ge-containing compounds include GeO ₂ , Al-containing compounds include Al ₂ O ₃ , and V-containing compounds such as V ₂ O ₅ . The glassy precursor is mixed with a mixture of SiO ₂ and Si, preferably added in an amount of up to 50% by weight, more preferably in an amount of 10 to 30% by weight, based on the total weight of the mixture.

It is preferable that it is 600-1000 degreeC, and, as for the said heat processing temperature, it is more preferable that it is 800-1000 degreeC. If the heat treatment temperature is less than 600 ℃, it is difficult to form a uniform amorphous SiO _x due to the reduction of the thermal diffusion phenomenon, it becomes difficult to form a uniform SiO _x doped with the doping element. In addition, when the heat treatment temperature exceeds 1000 ° C., there is a possibility that decomposition reaction of Si and other components occurs, and an unwanted second phase is formed. In addition, the heat treatment atmosphere is preferably an inert atmosphere or a vacuum atmosphere. By the heat treatment process, B, P, Li, Ge, Al, V, or a mixture thereof may be doped into silicon oxide to improve the amorphousness of silicon oxide and the diffusion rate of Li.

After the heat treatment step is subjected to a quenching step. The method of the quenching process is not limited to a specific method, but may be water-cooled or melt-spinning. The melt spinning method refers to a method of rapidly solidifying the dissolved melt by spraying it on a Cu-roll which rotates at a high speed with a surface temperature below normal temperature by using a gas of a specific pressure and having a surface temperature below normal temperature. The cooling rate during the quench is preferably 10 ² to 10 ⁷ K / sec.

The SiO _x core material including the doping element is manufactured by the heat treatment process and the quenching process. The core material is coated with a carbon material to prepare a silicon oxide-carbon composite anode material of the present invention.

The carbon material may be crystalline carbon or amorphous carbon, and in the case of crystalline carbon, the crystalline carbon may be coated on the core material by performing a coating process after mixing the core material and the crystalline carbon in a solid or liquid state.

In the case of mixing in a solid phase, a coating process may be mainly performed by a mechanical mixing method. As an example of the mechanical mixing method, a kneading method and a shear stress may occur during mixing. Or a mechanical mixing method in which the wing structure is changed, or a mechanochemical method for inducing fusion between particle surfaces by mechanically applying shear force between particles.

In the case of mixing in a liquid phase, as in the case of mixing in a solid phase, mechanical mixing, spray drying, spray pyrolysis, or freeze drying may be performed. In the case of liquid mixing, water, an organic solvent, or a mixture thereof may be used as the solvent, and as the organic solvent, ethanol, isopropyl alcohol, toluene, benzene, hexane, tetrahydrofuran, or the like may be used.

In the case of coating with amorphous carbon, a method of carbonizing the carbon precursor by coating with an amorphous carbon precursor and performing heat treatment may be used. The coating method may be used both dry and wet mixing. Also, a deposition method such as chemical vapor deposition (CVD) using a gas containing carbon such as methane, ethane, propane, or the like may also be used. As the amorphous carbon precursor, resins such as phenol resin, naphthalene resin, polyvinyl alcohol resin, urethane resin, polyimide resin, furan resin, cellulose resin, epoxy resin, polystyrene resin, coal pitch, petroleum pitch, tar (tar) ) Or low molecular weight heavy oil.

 The lithium secondary battery of the present invention includes a negative electrode made of the negative electrode active material. The negative electrode may be prepared as a negative electrode by applying a negative electrode mixture prepared by mixing the negative electrode active material according to the present invention with a binder to a current collector, such as copper, may be prepared as a negative electrode by adding a conductive material as necessary.

Examples of the conductive material include nickel powder, cobalt oxide, titanium oxide, carbon, and the like, and examples of carbon used as the conductive material include ketjen black, acetylene black, furnace black, graphite, carbon fiber, and fullerene.

2 shows a lithium secondary battery 1 which is an embodiment of the present invention. The lithium secondary battery 1 includes a negative electrode 2, an electrode 3, a separator 4 disposed between the negative electrode 2 and the positive electrode 3, the negative electrode 2, the positive electrode 3, and the separator 4. ), The battery container 5, and the sealing member 6 for sealing the battery container 5 are constituted as main parts. The shape of the lithium secondary battery illustrated in FIG. 2 may be in the form of a cylinder, in addition to various shapes such as a cylinder, a square, a coin, or a sheet.

The positive electrode is provided with a positive electrode mixture composed of a positive electrode active material, a conductive material and a binder. The positive electrode active material is a compound capable of reversibly intercalating / deintercalating lithium, such as LiMn ₂ O ₄ , LiCoO ₂ , LiNiO ₂ , LiFeO ₂ , V ₂ O ₅ , TiS, MoS, and the like. As the separator, an olefin porous film such as polyethylene or polypropylene can be used.

As electrolyte, propylene carbonate, ethylene carbonate, butylene carbonate, benzonitrile, acetonitrile, tetrahydrofuran, 2-methyl tetrahydrofuran, γ-butyrolactone, dioxolane, 4-methyldioxolane, N, N-dimethyl Formamide, dimethylacetoamide, dimethyl sulfoxide, dioxane, 1,2-dimethoxyethane, sulfolane, dichloroethane, chlorobenzene, nitrobenzene, dimethyl carbonate, methylethyl carbonate, diethyl carbonate, methylpropyl carbonate, methyl LiPF ₆ to an aprotic solvent such as isopropyl carbonate, ethylbutyl carbonate, dipropyl carbonate, diisopropyl carbonate, dibutyl carbonate, diethylene glycol, dimethyl ether, or a mixed solvent of two or more of these solvents. , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , Li (CF ₃ SO ₂ ) ₂ N, LiC ₄ F ₉ SO ₃ , LiSbF ₆ , LiAlO ₄ , LiAlCl ₄ , LiN (C _x F _{2x + 1} SO ₂ ) (C _y F _{2y + 1} SO ₂ ) (where x and y are natural waters), a mixture of one or two or more electrolytes consisting of lithium salts such as LiCl and LiI can be used. have.

In addition, a polymer solid electrolyte may be used instead of the electrolyte solution. In this case, it is preferable to use a polymer having high ion conductivity with respect to lithium ions, and polyethylene oxide, polypropylene oxide, polyethyleneimine and the like can be used. It is also possible to use a gel in which the solvent and the solute are added to the polymer of.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention, and the present invention is not limited to the following examples.

Example 1

20 parts by weight of B ₂ O ₃ was added to 100 parts by weight of a mixture of SiO ₂ and Si in a molar ratio of 1: 1, and the mixture was heat-treated under reduced pressure at 800 ° C., and then quenched at a rate of 10 ³ K / sec. 6.2 wt% doped SiO _x (x = 1.48) was prepared. The doped SiO _x was used as a core material, and an amorphous carbon material was coated at 30 wt% using a chemical vapor deposition (CVD) coating process on the surface thereof to prepare a SiO _x composite anode active material coated with carbon.

Comparative Example 1

A carbon-coated Si composite anode active material was prepared by coating an amorphous carbon material at 30 wt% using a chemical vapor deposition (CVD) coating process on a surface of a Si powder having a particle size of 5 μm.

Comparative Example 2

SiO ₂ and Si were mixed at a molar ratio of 1: 1 by heat treatment at 800 ° C. under reduced pressure, and then rapidly cooled at a rate of 10 ³ K / sec. To prepare SiO _x (x = 1). By using the prepared SiO _x as a core material, After coating 30% by weight of the amorphous carbon material on the surface by using a chemical vapor deposition (CVD) coating process, a carbon-coated SiO _x composite anode active material was prepared.

 Comparative Example 3

20 parts by weight of B ₂ O ₃ was added to 100 parts by weight of the mixture of SiO ₂ and Si in a molar ratio of 3: 1, and the mixture was heat-treated under reduced pressure at 800 ° C., followed by quenching at a speed of 10 ³ K / sec. 5.2 wt% doped SiO _x (x = 1.95) was prepared. The doped SiO _x was used as a core material, and an amorphous carbon material was coated at 30 wt% using a chemical vapor deposition (CVD) coating process on the surface thereof to prepare a SiO _x composite anode active material coated with carbon .

[Create test cell for charge / discharge test]

Each negative electrode slurry was prepared by mixing the negative active materials of Example 1 and Comparative Examples 1 to 3 and polyvinylidene fluoride in N-methylpyrrolidone in a ratio of 90:10. This slurry was applied to a copper current collector having a thickness of 10 μm by a doctor blade method, dried at 100 ° C. for 1 hour in a vacuum atmosphere to volatilize N-methylpyrrolidone. Thus, the negative electrode active material layer of 50 micrometers in thickness was laminated | stacked on the copper collector, and it punched out circularly [diameter 16mm], and made it as a negative electrode.

A separator made of a porous polypropylene film is inserted between the working electrode and the counter electrode with a metal lithium foil cut into a circular electrode having the same diameter as the working electrode as the counter electrode, and diethyl carbonate (DEC) and ethylene carbonate ( A coin-type cell was created using what dissolved LiPF ₆ so that it might become 1 (mol / L) in the mixed solvent of EC) (DEC: EC = 1: 1).

Then, charging and discharging experiments were carried out with a charging and discharging current density of 0.2C, a charging end voltage of 0 V (Li / Li ⁺ ), and a discharging end voltage of 2.0 V (Li / Li ⁺ ). Table 1 shows the alloy composition amorphousness of the negative electrode active materials prepared according to Example 1 and Comparative Examples 1 to 3, and the discharge capacity, initial efficiency, and electrode lifetime measurement of the battery manufactured using the same.

TABLE 1

Example 1 Comparative Example 1 Comparative Example 2 Comparative Example 3 Alloy composition Carbon coating, B doped SiO _x (x = 1.48) Carbon coating, Si Carbon coated SiO _x (x = 1) Carbon coating, B doped SiO _x (x = 1.95) Discharge Capacity (mAh / g) 700 1200 850 420 Initial Efficiency (%) 88 90 78 70 Electrode life (% at 100th cycle) > 90 <40 <70 To 75 Amorphous Degree (%) 80 0 50 70

The technical scope of the present invention is not limited to the above embodiments, and various changes can be made without departing from the spirit of the present invention.

As described in detail above, the anode active material for a lithium secondary battery of the present invention increases the amorphousness of SiO _x and the diffusion rate of Li atoms by doping B, P, Li, Ge, Al, V, or a mixture thereof to SiO _x . It is possible to excellently improve the life characteristics and high rate charge and discharge characteristics of the lithium secondary battery.

1 is a cross-sectional view schematically showing the silicon oxide-carbon composite of the present invention.

2 is a perspective view showing an example of a lithium secondary battery according to an embodiment of the present invention.

<Description of the symbols for the main parts of the drawings>

1: lithium secondary battery 2: negative electrode

3: anode 4: separator

5: battery container 6: sealing member

Claims (20)

A core material doped with silicon oxide (SiO _x ; x is 1.5 or less), an element selected from the group consisting of B, P, Li, Ge, Al, V, or a mixture thereof; And Carbon material present on the surface of the core material A negative electrode active material for a lithium secondary battery made of a silicon oxide-carbon composite comprising a. The negative electrode active material of claim 1, wherein the degree of amorphousness of the negative electrode active material is 50% or more, and the diffusion rate of Li according to a galvanic intermitant time technique (GITT) method is 10 ⁻⁸ to 10 ⁻⁶ cm ² / sec. . The negative active material of claim 1, wherein _x of SiO x is 0.5 to 1.5. The negative active material of claim 1, wherein the doping amount of B, P, Li, Ge, Al, V, or a mixture thereof is 50 wt% or less based on the total core material. The negative active material of claim 4, wherein the doping amount of B, P, Li, Ge, Al, V, or a mixture thereof is 10 to 30 wt% based on the total core material. The negative active material of claim 1, wherein the carbon material is crystalline carbon or amorphous carbon. A doping element-containing compound selected from the group consisting of SiO ₂ , Si and B-containing compounds, P-containing compounds, Li-containing compounds, Ge-containing compounds, Al-containing compounds, V-containing compounds and mixtures thereof Mixing; Heat treating the mixture to prepare a core material of silicon oxide (SiO _x ; x is 1.5 or less) doped with B, P, Li, Ge, Al, V or a mixture thereof; Quenching the heat-treated core material; And The method of manufacturing a negative active material for a lithium secondary battery comprising coating the core material with a carbon material to produce a silicon oxide-carbon composite. The method of claim 7, wherein the SiO ₂ and Si are mixed in a weight ratio of 3: 1 to 1: 1. The method of claim 7, wherein the doping element-containing compound is a glassy oxide. The method of claim 7, wherein the B-containing compound is selected from the group consisting of B ₂ O ₃ and B ₂ O, P-containing compound is selected from the group consisting of P ₂ O ₅ and P ₂ O ₃ , Li- The containing compound is selected from the group consisting of Li ₂ O, Li ₂ CO ₃ , and LiOH, the Ge-containing compound is GeO ₂ , the Al-containing compound is Al ₂ O ₃ , the V-containing compound is V ₂ O ₅ The manufacturing method of the negative electrode active material for secondary batteries. The method of claim 7, wherein the doping element-containing compound is added in an amount of 50 wt% or less based on the total weight of the mixture of SiO ₂ , Si, and the doping element-containing compound. The method of claim 7, wherein the doping element-containing compound is added in an amount of 10 to 30 wt% based on the total weight of the mixture of SiO ₂ , Si, and the doping element-containing compound. The method of claim 7, wherein the heat treatment temperature is 600 to 1000 ° C. 9. The method of claim 13, wherein the heat treatment temperature is 800 to 1000 ° C. 15. The negative active material of claim 7, wherein the degree of amorphousness of the negative electrode active material is 50% or more, and the diffusion rate of Li according to the galvanic intermitant time technique (GITT) method is 10 ⁻⁸ to 10 ⁻⁶ cm ² / sec or more. Method of preparation. The method of claim 7, wherein _x of SiO _x is 0.5 to 1.5. The method of claim 7, wherein the doping amount of B, P, Li, Ge, Al, V, or a mixture thereof is 50 wt% or less with respect to the total core material. The method of claim 17, wherein the doping amount of B, P, Li, Ge, Al, V or a mixture thereof is 10 to 30% by weight based on the total core material. The method of claim 7, wherein the carbon material is crystalline carbon or amorphous carbon. A negative electrode comprising the negative electrode active material according to any one of claims 1 to 6; A positive electrode including a positive electrode active material capable of reversible intercalation / deintercalation of lithium; And a lithium secondary battery comprising an electrolyte solution.