KR20050087147A - Negative active material for lithium secondary battery, method of preparing same, and lithium secondary battery comprising same - Google Patents

Abstract

The present invention relates to a silicon oxide (SiO _x ; x is 1 or less) core material doped with an element selected from the group consisting of Mg, Ca 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 the 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 relates to a silicon oxide (SiO _x ; x is 1 or less) core material doped with an element selected from the group consisting of Mg, Ca 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 comprises the steps of mixing a doping element-containing compound selected from the group consisting of SiO ₂ , Si and Mg-containing compounds, Ca-containing compounds and mixtures thereof; Heat treating the mixture to prepare a silicon oxide (SiO _x ; x is 1 or less) core material doped with Mg, Ca or a mixture thereof; Quenching the heat-treated silicon oxide core material; 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 comprises a core material doped with an element selected from the group consisting of silicon oxide (SiO _x ; x is 1 or less), Mg, Ca or a mixture thereof and a carbon material present on the surface of the core material. It includes a silicon oxide-carbon composite. The silicon oxide doped with the Mg, Ca or a mixture thereof is represented by the following Chemical Formula 1.

[Formula 1]

(MO) y (SiOx)

(Wherein, 2.2≤y≤5.5, 0 <x≤1, and M is Mg, Ca or a mixture thereof.)

SiO _x constituting the core material has a high irreversible capacity, short lifespan, and poor high rate charge / 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. The present invention, Mg, Ca, or by doping the mixture thereof to the SiO _x to increase the amorphous degree of SiO _x, inhibit the growth of the Si metal agglomerates which can be formed during the reduction of SiO _x, thereby improving the diffusion rate of Li atoms .

Amorphization degree of the negative electrode active material of the present invention is 70% or more, preferably 70 to 99%. In addition, the negative electrode active material is 10 ^-8 cm ² / sec or more, preferably 10 ^-8 to 10 ^-6 cm ² / sec when measuring the diffusion rate of Li according to the Galvanic Intermitant time technique (GITT) method Has The degree of amorphousness is defined as in Equation 1 below.

[Calculation 1]

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

The range of _x in the SiO x is preferably adjusted to less than 1, more preferably of 0.5 to 1. If the range of x exceeds 1, the portion of the irreversible capacity when reacting with lithium increases, which may lower initial efficiency.

The doping amount of the doping element Mg, Ca or a mixture thereof is 50 parts by weight or less, preferably 20 to 50 parts by weight based on 100 parts by weight of silicon oxide. When the doping amount exceeds 50 parts by weight, there is a problem in that energy density and irreversible capacity are increased.

In the silicon oxide-carbon composite of the present invention, crystalline carbon or amorphous carbon may be used as the carbon material coating the core material. The crystalline carbon may be a natural graphite or artificial graphite in the form of a plate, sphere or fiber, and the content of the carbon material may be included in an amount of 5 to 50% by weight based on the total weight of the silicon oxide-carbon composite. It may be included in 5 to 30% by weight.

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 a phenol resin, naphthalene resin, polyvinyl alcohol resin, urethane resin, poly The mid resin, furan resin, cellulose resin, epoxy resin, polystyrene resin and the like can be obtained by heat treatment 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, to prepare a core material, an Mg-containing compound, a Ca-containing compound, or a mixture thereof is added to a mixture of SiO ₂ and Si, followed by heat treatment together. SiO ₂ and Si are preferably mixed in a molar ratio of 1: 1 to 1: 3.

Examples of the Mg-containing compound include MgO and the like, and examples of the Ca-containing compound include CaO and the like. These compounds are preferably added in an amount of 20 to 50 parts by weight based on 100 parts by weight of a mixture of SiO ₂ and Si.

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 SiOx due to the reduction of the thermal diffusion phenomenon, it is difficult to form a uniform SiOx doped with Mg, Ca or a mixture thereof. Moreover, when it exceeds 1000 degreeC, since there exists a possibility of decomposition reaction of Si, it is unpreferable. The heat treatment atmosphere is preferably an inert atmosphere or a vacuum atmosphere. By such a heat treatment process, Mg, Ca, or both of these elements may be doped into silicon oxide to improve amorphousness of silicon oxide and diffusion rate of Li.

After the heat treatment step is subjected to a quenching process to glass. 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 is a negative electrode active material according to the present invention can be prepared as a negative electrode by applying a negative electrode mixture prepared by mixing with a binder to a current collector, such as copper, may be prepared as a negative electrode by mixing a conductive material as needed.

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

1 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. 1 may be in a cylindrical shape, but in addition, various shapes such as a cylindrical shape, a square shape, a coin shape, a sheet shape, and the like.

The positive electrode is provided with a positive electrode mixture composed of a positive electrode active material, a conductive agent 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 one preferred embodiment of the present invention and the present invention is not limited to the following examples.

Example 1

MgO doped by 17% by weight of a mixture of 40 parts by weight of MgO was added to 100 parts by weight of a mixture of SiO ₂ and Si in a 1: 1 molar ratio, and then quenched at a speed of 10 ⁷ K / sec. SiO _x (x = 1) core material was prepared. A negative electrode active material was prepared by coating an amorphous carbon material at 30% by weight using a chemical vapor deposition (CVD) coating process on the surface of the doped SiO _x core material.

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

A mixture of SiO ₂ and Si in a molar ratio of 1: 1 was heat-treated under reduced pressure at 900 ° C., and then quenched at a rate of 10 ⁷ K / sec to prepare SiO _x (x = 1) core material. The anode active material was prepared by coating an amorphous carbon material at 30% by weight using a chemical vapor deposition (CVD) coating process on the surface of the prepared SiO _x core material. .

[Create test cell for charge / discharge test]

The negative electrode active material of Example 1 and Comparative Examples 1 and 2 and polyvinylidene fluoride were mixed in N-methylpyrrolidone in a ratio of 90:10 to prepare a negative electrode slurry. This slurry was applied to a copper current collector having a thickness of 18 µm by a doctor blade method, dried at 100 ° C. for 24 hours in a vacuum atmosphere, and volatilized N-methylpyrrolidone. In this manner, a negative electrode active material layer having a thickness of 120 μm was laminated on the copper current collector, and then a hole was cut in a circular shape having a diameter of 13 mm to form a negative electrode.

A separator made of a porous polypropylene film is inserted between the working electrode and the counter electrode by using a metal lithium foil cut into a circular shape having the same diameter as the working electrode as a counter electrode, and propylene carbonate (PC) and diethyl carbonate ( Coin-type cells were prepared using a solution in which LiPF ₆ was dissolved at a concentration of 1 (mol / L) in a mixed solvent of DEC) and ethylene carbonate (EC) (PC: DEC: EC = 1: 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 in Example 1 and Comparative Examples 1 and 2, 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 Alloy composition Carbon coating, Mg doped SiO _x (x = 1) Carbon coating, Si Carbon coated SiO _x (x = 1) Discharge Capacity (mAh / g) 700 1200 850 Initial Efficiency (%) 88 90 78 Electrode life (% at 100th cycle) > 90 <40 <70 Amorphous Degree (%) 80 0 50

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 can increase the amorphousness of SiO _x and improve the diffusion rate of Li atoms by doping Mg, Ca, or a mixture thereof to SiO _x , thereby increasing the lifetime of the lithium secondary battery. The characteristics and high rate charge / discharge characteristics can be excellently improved.

1 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 (21)

A core material doped with an element selected from the group consisting of Mg, Ca, or a mixture thereof in silicon oxide (SiO _x ; x is 1 or less); 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 70% or more, and the diffusion rate of Li according to a galvanic intermitant time technique (GITT) method is 10 ⁻⁸ cm ² / sec or more. The negative electrode active material of claim 1, wherein the degree of amorphousness of the negative electrode active material is 70 to 99%, and the diffusion rate of Li according to the Galvanic Intermitant time technique (GITT) method is 10 ⁻⁸ to 10 ⁻⁶ cm ² / sec. Active material. The negative active material of claim 1, wherein _x of SiO x is 0.5 to 1. 3. The negative active material of claim 1, wherein the doping amount of Mg, Ca, or a mixture thereof is 50 parts by weight or less based on 100 parts by weight of silicon oxide. The negative active material of claim 5, wherein the doping amount of Mg, Ca, or a mixture thereof is 20 to 50 parts by weight based on 100 parts by weight of silicon oxide. The negative active material of claim 1, wherein the carbon material is crystalline carbon or amorphous carbon. Mixing doping element-containing compounds selected from the group consisting of SiO ₂ , Si and Mg-containing compounds, Ca-containing compounds and mixtures thereof; Heat treating the mixture to prepare a silicon oxide (SiO _x ; x is 1 or less) core material doped with Mg, Ca or a mixture thereof; Quenching the heat-treated silicon oxide 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 8, wherein the SiO ₂ and Si are mixed in a molar ratio of 1: 1 to 1: 3. The method of claim 8, wherein the doping element-containing compound is a glass-forming precursor. The method of claim 8, wherein the Mg-containing compound is MgO and the Ca-containing compound is CaO. The method of claim 8, wherein the doping element-containing compound is added in an amount of 20 to 50 parts by weight based on 100 parts by weight of the mixture of SiO ₂ and Si. The method of claim 8, wherein the heat treatment temperature is 600 to 1000 ° C. 10. The method of claim 13, wherein the heat treatment temperature is 800 to 1000 ° C. 15. 10. The method of claim 9, wherein the degree of amorphousness of the negative electrode active material is 70% or more, and the diffusion rate of Li according to the Galvanic Intermitant time technique (GITT) method is 10 ^-8 cm ² / sec or more. 16. The method of claim 15, wherein the negative electrode active material is amorphous to 70 to 99%, the diffusion rate of Li according to the Galvanic Intermitant time technique (GITT) method is 10 ^-8 to 10 ^-6 cm ² / sec negative electrode for a lithium secondary battery Method for producing an active material. The method of claim 8, wherein _x of SiO _x is 0.5 to 1. The method of claim 8, wherein the doping amount of Mg, Ca, or a mixture thereof is 50 parts by weight or less based on 100 parts by weight of silicon oxide. The method of claim 18, wherein the doping amount of Mg, Ca or a mixture thereof is 20 to 50 parts by weight based on 100 parts by weight of silicon oxide. The method of claim 8, 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 7; 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.