KR20210022237A - Secondary battery and preparation method thereof - Google Patents

Abstract

An embodiment of the present invention relates to a secondary battery comprising a silicon-based negative active material predoped with lithium, and a method for manufacturing the same, wherein the secondary battery not only realizes safety, but also significantly improves energy density, cycle characteristics, and rate characteristics. The present invention relates to the secondary battery comprising a positive electrode, a separator, and a negative electrode.

Description

Secondary battery and its manufacturing method {SECONDARY BATTERY AND PREPARATION METHOD THEREOF}

The present invention relates to a secondary battery including a silicon-based negative active material pre-doped with lithium and a method of manufacturing the same.

As electronic devices become smaller, lighter, thinner, and portable due to the recent development of the information and communication industry, there is an increasing demand for high energy density of batteries used as power sources for such electronic devices. A lithium secondary battery is a battery that can best meet these demands, and studies on the application of large-sized electronic devices such as automobiles and power storage systems as well as small batteries using the same are being actively conducted.

Lithium secondary batteries have _{mainly used lithium-containing transition metal oxides such as LiCoO 2} and LiMn ₂ O ₄ as positive electrodes, and carbon-based materials such as graphite as negative electrodes. There is a problem that adversely affects the safety of the lithium secondary battery by increasing the charging rate of an excessive active material, thinning the separator, or increasing the charging voltage by using a method of reducing the film thickness of the constituent material of.

Accordingly, various new materials capable of achieving high capacity and high energy density while realizing the reliability and safety of secondary batteries are being studied in various ways.

For example, an alloy-based negative electrode active material containing Si, Sn, and Li, and an oxide-based negative electrode active material are also attracting attention as high-capacity materials. However, among these, the alloy-based negative active material has difficulty in practical use due to the problem that the negative active material itself is finely divided in repeating charging and discharging. The oxide-based anode active material has an advantage of having less volume expansion than the alloy-based anode active material, but has a poor problem in terms of capacity of a secondary battery.

Meanwhile, a technology for pre-doping a secondary battery by supporting lithium ions in advance by supporting lithium ions on a negative electrode active material has been attracting attention. In this regard, for example, Japanese Laid-Open Patent Publication No. Hei 3-233860 discloses a secondary battery using a negative electrode in which an insoluble infusible gas containing a polyacene-based skeleton structure carrying lithium is bonded with a thermosetting resin. A method of implementing high voltage or high capacity is disclosed.

In addition, as a method of pre-doping the negative electrode, in International Patent Publication No. 1998/033227, an electrode layer is formed on a current collector including a through hole, and the lithium metal disposed in the battery is short-circuited with the negative electrode. A technology has been disclosed in which all negative electrodes are pre-doped through a through hole of this current collector.

Japanese Patent Publication No. 2009-076372 discloses a method of pre-doping lithium by attaching a lithium metal to a surface of a negative electrode and making it electrochemically contacted in a secondary battery. However, this method has problems such as remarkably deteriorating cycle characteristics or adversely affecting film-forming properties due to an exothermic reaction.

Japanese Patent Application Publication No. 6274253 discloses a method of pre-doping lithium into a silicon-based negative active material by mixing a silicon-based negative active material and a lithium metal having a thickness of 0.1 mm or more with a specific solvent.

In addition, Japanese Patent Laid-Open No. 10-294104 discloses a technique for producing a lithiated electrode by immersing an active material in a solution in which alkyl lithium is dissolved in an organic solvent, and reacting lithium with an electrode material. However, since this method requires a large amount of lithium, the process is complicated, such as removing lithium remaining on the active material after the reaction, and since it must be carried out in a solvent to conduct the reaction under mild conditions, the reaction rate is slow, It is difficult to confirm whether the reaction has been completed, which is difficult in mass production or practical use.

Japanese Patent Laid-Open Publication No. Hei 3-233860 International Patent Publication No. 1998/033227 Japanese Patent Publication No. 2009-076372 Japanese Registered Patent Publication No.6274253 Japanese Patent Publication No. Hei 10-294104

The present invention was devised to solve the problems of the prior art, and the technical problem to be solved by the present invention is to provide a secondary battery capable of improving energy density, cycle characteristics, capacity retention rate, and rate characteristics.

Another technical problem to be solved of the present invention is to provide a method of manufacturing the secondary battery.

In order to achieve the above object, an embodiment of the present invention is a secondary battery including a positive electrode, a separator, and a negative electrode, the positive electrode includes a metal oxide, a lithium metal is inserted between the positive electrode and the separator, and the negative electrode Silver (i) silicon fine particles, and (ii) a silicon-based negative active material containing a compound represented by SiOx (0.3≦x≦1.6), wherein the silicon-based negative active material is based on the initial irreversible capacity of the negative electrode. It provides a secondary battery, which is predoped with a significant amount of lithium.

In another embodiment, (1) a first process of increasing the initial efficiency of the negative electrode; And (2) a second process of manufacturing a secondary battery using the negative electrode obtained in the first process; including, and performing a process of pre-doping lithium in the first process and the second process, the secondary It provides a method of manufacturing a battery.

The secondary battery according to the above embodiment may realize safety, greatly improve energy density, and further improve cycle characteristics and rate characteristics.

1 is a graph showing capacity characteristics with respect to the number of cycles of Example 2 and Comparative Example 1. FIG.

The present invention is not limited to the contents disclosed below, and may be modified in various forms unless the gist of the invention is changed.

In the present specification, "including" means that other components may be further included unless otherwise specified.

In addition, all numbers and expressions representing amounts of components, reaction conditions, and the like described herein are to be understood as being modified by the term "about" in all cases unless otherwise specified.

[Secondary battery]

A secondary battery according to an embodiment of the present invention is a secondary battery including a positive electrode, a separator, and a negative electrode, wherein the positive electrode includes a metal oxide, a lithium metal is inserted between the positive electrode and the separator, and the negative electrode is (i ) Silicon fine particles, and (ii) a silicon oxide-based negative electrode active material including a compound represented by SiOx (0.3≦x≦1.6), wherein the silicon-based negative electrode active material corresponds to the initial irreversible capacity of the negative electrode. Positive lithium is predoped.

The secondary battery according to the embodiment of the present invention includes a silicon-based negative electrode active material pre-doped with lithium in an amount corresponding to the initial irreversible capacity of the negative electrode, thereby implementing the safety of the secondary battery and greatly improving the energy density. , Cycle characteristics and rate characteristics can be further improved. In addition, since lithium metal is inserted between the anode and the separator, when activated electrochemically, capacity characteristics as well as rate characteristics of the secondary battery can be further improved by a simple method.

In general, silicon-based negative active materials exhibit high capacity, but as the cycle progresses, the volume expansion ratio becomes more than 300%, which can lead to increased resistance and increased side reactions of the electrolyte.Solid electrolyte interface (SEI; Solid Electrolyte) such as electrode structure damage Interface) layer formation may intensify problems. In addition, the silicon oxide-based negative electrode active material has a lower volume expansion rate than that of the silicon-based negative electrode active material, but the _{initial irreversible capacity increases with Li 2} O due to oxygen in the negative electrode active material, thereby lowering the energy density and lowering the cycle characteristics and rate characteristics. There is a problem.

According to one embodiment of the present invention, in order to obtain a secondary battery having a high energy density, it is important to minimize the irreversible capacity of the silicon-based negative active material. Accordingly, the present invention includes a silicon-based negative electrode active material pre-doped with lithium, thereby lowering the initial 　 irreversible capacity of the negative electrode, preventing penetration of metal ions into the positive electrode to the SEI of the negative electrode surface, and improving high energy density. . The pre-doping compensates for irreversible capacity, or even when an active material that does not contain lithium is used in the positive electrode, lithium required for charging and discharging may be included in the negative electrode, thereby improving the performance of the secondary battery.

According to one embodiment of the present invention, the negative electrode may include a silicon-based negative active material including silicon fine particles and a compound represented by SiOx (0.3≦x≦1.6). More specifically, the silicon-based negative active material includes silicon fine particles, and a silicon oxide-based compound represented by SiOx (0.3≦x≦1.6), and silicon dioxide, and a silicate containing magnesium, calcium, aluminum, or a combination thereof It may further include.

In addition, the silicon-based negative active material may be an active material including a matrix of oxides capable of occluding and releasing lithium, and the matrix may include silicon oxide, tin oxide, or a mixture thereof.

In the silicon-based negative active material, the content of silicon is 30% to 80% by weight, specifically 40% to 80% by weight, more specifically 40% to 70% by weight, particularly 40% by weight based on the total weight of the negative active material. % To 60% by weight. If the content of silicon is less than 30% by weight, the amount of negative active material acting upon the insertion and release of lithium is small, so the charge/discharge capacity of the secondary battery may be lowered. Conversely, when it exceeds 80% by weight, the secondary battery The charge/discharge capacity of may be large, but the expansion and contraction of the electrode during charging and discharging may become too large, and the cycle characteristics may be deteriorated.

The silicon fine particles may be crystalline or amorphous, and specifically, may be amorphous or a similar phase. When the silicon fine particles are amorphous or in a similar phase, the expansion or contraction of the lithium secondary battery during charging and discharging is small, and battery performance such as capacity characteristics may be further improved.

In addition, the silicon fine particles are uniformly dispersed in at least one selected from the group consisting of a compound represented by SiOx (0.3≦x≦1.6), silicon dioxide, and silicates including magnesium, calcium, aluminum, or a combination thereof. Or it may exist while surrounding them, and in this case, since expansion or contraction of silicon may be suppressed, the performance of the secondary battery may be improved. On the other hand, the silicon fine particles may have a structure dispersed in a silicon oxide compound or a silicate likewise when a carbon layer including a carbon film is formed.

In addition, when the silicon-based negative electrode active material contains the silicon fine particles, it is preferable because a lithium alloy is formed with a large specific surface area of the silicon fine particles to suppress destruction of the bulk. The silicon fine particles react with lithium upon charging _{to form Li 4.2} Si, and return to silicon upon discharge.

When the silicon fine particles are analyzed by X-ray diffraction (Cu-Kα) using copper as a cathode target, the half width of the diffraction peak of Si 220 centered around 2θ = 47.5° (FWHM, Full Width at Half Maximum) When calculated by the Sherrer equation on the basis of, it may have a crystallite size of 2 nm to 10 nm, specifically 2 nm to 9 nm, and more specifically 2 nm to 8 nm. When the crystallite size of the silicon fine particles exceeds 10 nm, the silicon-based negative active material may crack due to volume expansion or contraction occurring during charging and discharging, resulting in deterioration of cycle characteristics. In addition, when the crystallite size of the silicon fine particles is less than 2 nm, the charge/discharge capacity may be lowered, and the reactivity may be increased, so that physical properties may change during storage, which may cause fairness problems. When the crystallite size of the silicon fine particles is in the above range, there is hardly any region that does not contribute to charging and discharging, and a reduction in coulomb efficiency, that is, charging/discharging efficiency, indicating a ratio of charging capacity and discharging capacity can be suppressed.

When the silicon fine particles are further atomized and the amorphous or crystallite size is further atomized to about 2 to 6 nm, the density of the silicon-based negative electrode active material increases, so that the density of the silicon-based negative active material may be close to the theoretical density, and the pores may be greatly reduced. Accordingly, since the density of the matrix is improved and the strength is enhanced, cracking can be prevented, and thus the initial efficiency and cycle life characteristics of the secondary battery can be further improved.

In addition, the silicon-based negative active material includes a compound represented by SiOx (0.3≤x≤1.6), specifically SiOx (0.5≤x≤1.6), more specifically, SiOx (0.7≤x≤1.2). In addition, by including the silicon oxide compound, it is possible to improve capacity and reduce volume expansion when applied to a secondary battery. In addition, the silicon-based negative active material may further include silicon dioxide.

The silicon oxide compound is amorphous or when confirmed by energy dispersive X-ray spectroscopy (EDX) or a transmission electron microscope, fine particles of silicon (amorphous) of about several nm to tens of nm are contained in the silicon oxide compound. It can be a distributed structure.

The silicon oxide compound can be obtained by a method including cooling and depositing a silicon oxide gas generated by heating a mixture of silicon dioxide and sub-micron silicon powder or silicon fine particles of 50 nm or less.

The silicon oxide compound is amorphous and may cause an irreversible reaction with lithium ions upon discharge to form _{Li-Si-O or Si+Li 2 O.} Therefore, as the content of silicon dioxide in the silicon oxide compound increases, the initial irreversible reaction increases and the initial efficiency may decrease.

The silicon oxide compound may be included in an amount of 5 mol% to 20 mol% with respect to the entire negative active material. When the content of the silicon oxide compound is less than 5 mol%, the volume expansion and life characteristics of the secondary battery may be deteriorated, and when it exceeds 20 mol%, the initial irreversible reaction of the secondary battery may increase.

According to one embodiment of the present invention, the silicon-based negative active material may not contain silicon dioxide. The reason is that the formation of lithium silicate is suppressed when silicon is inserted, so that the initial efficiency can be improved.In the case of including silicon dioxide, when the silicon dioxide reacts with lithium, it causes an irreversible reaction to generate lithium silicate, thereby reducing the initial efficiency. Because it can be dropped.

In addition, the silicon-based negative active material may further include a silicate containing magnesium, calcium, aluminum, or a combination thereof. The magnesium, calcium, aluminum, or a combination thereof may be included in a doped form. Specifically, the silicon-based negative active material may include silicate containing magnesium, and magnesium may be included in a doped form.

According to an embodiment of the present invention, the silicate containing magnesium may include MgSiO ₃ crystal, Mg ₂ SiO ₄ crystal, or a mixture thereof.

In addition, according to an embodiment, the silicate containing magnesium includes a MgSiO ₃ crystal, and may further include a _{Mg 2} SiO _{4 crystal.}

In the case of the silicate containing magnesium, when magnesium is doped, for example, when SiO and Mg are reacted at 1:1, if the elements are uniformly distributed, only Si and MgO exist in terms of thermodynamics, but the element concentration distribution In the case of non-uniformity, not only Si and MgO, but also other materials such as unreacted SiO and metal Mg may be present.

According to an embodiment of the present invention, the silicon-based negative active material _{may contain Mg 2} SiO ₄ crystals and MgO, but may contain substantially a large amount of _{MgSiO 3} crystals to improve charge/discharge capacity and initial efficiency.

In the present specification, "substantially including a large amount" or "substantially including" means to include as a main component, or to mainly include.

More specifically, it is that substantially contains a lot of the MgSiO _3, the MgSiO ₃ determines the range of the means that much includes than the Mg ₂ SiO ₄ crystal and, for example, in X-ray diffraction 2θ = 22.3 ° to 23.3 ° IF/IE, which is the ratio of the intensity (IF) of the X-ray diffraction peak corresponding to the Mg ₂ SiO ₄ crystal appearing and the intensity (IE) of the X-ray diffraction peak corresponding _{to the MgSiO 3 crystal appearing in the range of 2θ = 30.5° to 31.5°} Means less than or equal to 1.

In the silicate, _{the content of magnesium relative to SiO X} may affect initial discharge characteristics or cycle characteristics during charge and discharge.

Silicon in the SiO _X is alloyed with a lithium atom to greatly improve initial discharge characteristics. On the other hand, when the MgSiO ₃ crystal contains substantially a large amount of magnesium silicate, the fineness of the silicon fine particles is maintained, so that the strength of the matrix may be further enhanced. In addition, the density can be improved to form a dense matrix. For this reason, an effect of improving the initial efficiency or improving the cycle during charging and discharging may be increased. However, _{if an excessive amount of Mg 2} SiO ₄ crystal is included, the size of the silicon fine particles increases, resulting in a decrease in the strength or density of the matrix, and the alloying of the silicon atom and the lithium atom decreases, resulting in the initial discharge of the secondary battery. It may cause deterioration of properties.

When the silicon-based negative active material contains silicate containing magnesium, when preparing a negative active material composition using polyimide as a binder, a chemical reaction between the negative active material and the binder is suppressed compared to the case where lithium is doped instead of magnesium. I can. Accordingly, the safety of the negative electrode active material composition may be improved by using the silicon-based negative electrode active material, through which not only the safety of the negative electrode but also the cycle characteristics of the secondary battery may be improved. In addition, since silicate containing magnesium is difficult to react with lithium ions, it is considered that when applied to an electrode, the cycle characteristics can be improved by reducing the amount of expansion and contraction of the electrode when lithium ions are rapidly increased. In addition, it is thought that the strength of the matrix, which is a continuous phase surrounding the silicon fine particles, can be enhanced by the silicate.

When the silicon-based negative active material contains silicate containing magnesium, the doping amount of magnesium, that is, the content may be 3% to 15% by weight, specifically 4% to 12% by weight, based on the total weight of the negative electrode active material. . When the content of magnesium is 3% by weight or more, the initial efficiency of the secondary battery can be improved, and when it is 15% by weight or less, it may be advantageous in terms of cycle characteristics and handling safety of the secondary battery.

According to another embodiment of the present invention, in the case of a silicate in which the silicon-based negative active material includes calcium, aluminum, or a combination thereof, the doping amount of calcium or aluminum, that is, the content is 3% by weight based on the total weight of the negative active material. It may be to 15% by weight, specifically 4% to 12% by weight.

According to an embodiment of the present invention, the silicate containing magnesium, calcium or aluminum may be represented by the following formula (1):

[Formula 1]

M _x SiO _y

In Formula 1,

M is at least one metal selected from the group consisting of Mg, Ca and Al,

x is 0.5≤x≤2,

y is 2.5≤y≤4.

The silicate of Formula 1 is an oxide having a negative Gibbs free energy thermodynamically than a silicon oxide compound, and is amorphous, stable to lithium, and may play a role of suppressing the occurrence of an initial irreversible reaction.

According to an embodiment of the present invention, a carbon layer including a carbon film may be formed on the surface of the silicon-based negative active material, and the carbon layer may include carbon nanofibers, graphene, and graphene oxide. ) And reduced graphene oxide.

Specifically, the silicon-based negative active material, for example, silicon-based negative electrode active material including silicon fine particles and a compound represented by SiOx (0.3≦x≦1.6) has excellent conductivity on the surface of a carbon layer that is flexible in volume expansion, specifically carbon At least one selected from the group consisting of nanofibers, graphene, graphene oxide, and reduced graphene oxide may be directly grown. Accordingly, the volume expansion of the silicon-based negative active material can be suppressed, the pressing of the silicon-based negative active material can be reduced, and the formation of the SEI layer formed by the reaction of the silicon-based negative active material with the electrolyte due to the carbon layer. Can be reduced.

The carbon layer may be contained in an amount of 2% by weight to 10% by weight, specifically 4% by weight to 8% by weight, based on the total weight of the silicon-based negative active material. If the carbon content is less than 2% by weight, the conductivity of the negative electrode may decrease, and if it exceeds 10% by weight, the carbon content may be too large to reduce the capacity of the negative electrode. Can be difficult to achieve. The carbon layer forming method may include a method of chemical vapor deposition (CVD) in organic gas and/or vapor, or a method of introducing organic gas and/or vapor into the reactor during heat treatment.

_{In addition, the cumulative 50% average particle diameter (D 50} ) of the particle distribution of the silicon-based negative active material on which the carbon layer is formed is 0.5 µm to 15 µm, specifically 2 µm to 10 µm, more specifically 3 µm to 10 µm or 4 µm It may be in the range of 8 μm. If the D ₅₀ is too small, the bulk density may be too small, and the charge/discharge capacity per unit volume may decrease. On the contrary, if the D ₅₀ is too large, it becomes difficult to manufacture an electrode film of a secondary battery, which will be peeled off from the current collector. There is a concern. On the other hand, the D _{50 is a value measured as a weight average value D 50} (that is, a particle diameter or median diameter when the cumulative weight becomes 50%) in the particle size distribution measurement according to the laser light diffraction method.

According to one embodiment of the present invention, the average thickness of the carbon layer may be 10 nm to 200 nm, specifically 20 nm to 200 nm.

When the average thickness of the carbon layer is 10 nm or more, conductivity can be improved, and when the average thickness of the carbon layer is 200 nm or less, a decrease in capacity of the secondary battery can be suppressed. The average thickness of the carbon layer can be calculated, for example, by the following procedure.

First, a silicon-based negative electrode active material is observed at an arbitrary magnification by a transmission electron microscope (TEM). The magnification is preferably a degree that can be confirmed with the naked eye, for example. Next, at any 15 points, the thickness of the carbon layer is measured. In this case, it is preferable not to focus on a specific place as much as possible, and to set the measurement position widely and randomly. Finally, the average value of the thicknesses of the 15 carbon layers is calculated.

In addition, in the case of using the silicon-based negative active material according to an embodiment of the present invention, since the battery capacity per unit weight of the negative active material may be 1500 to 3000 mAh/g, the volume expansion is relatively small with high conductivity. This improved secondary battery can be provided. In addition, when used in combination with a carbon-based negative active material, Coulomb efficiency and cycle characteristics may be improved.

The positive electrode included in the secondary battery may include, for example, a metal oxide active material including lithium that can be released electrochemically, and specifically includes a spinel structure including lithium cobalt oxide, lithium manganate, or a mixture thereof. It may include an oxide having. More specifically, the metal oxide may include at least one selected from the group consisting of lithium composite cobalt oxide, lithium composite nickel oxide, lithium composite manganese oxide, and lithium composite titanium oxide.

Meanwhile, the separator has high ion permeability, mechanical strength, and durability against an electrolyte, a positive electrode active material and a negative electrode active material, and a porous material having no electrical conductivity may be used. The separator may include a conventional porous polymer film, for example, porous propylene. In addition, the separator is, for example, paper (cellulose), glass fiber, polyethylene or polystyrene, polyester, polytetrafluoroethylene, polyvinylidene fluoride, polyimide, polyphenylene sulfide, polyamide, polyamideimide, A porous cloth, a nonwoven fabric, or a porous body including at least one selected from the group consisting of polyethylene terephthalate, polybutylene terephthalate, and polyether ether ketone may be used. The thickness of the separation membrane can be appropriately set in consideration of the storage amount of the electrolyte solution, the strength of the separator, and the like, but the thickness of the separation membrane is preferably as thin as possible in order to decrease the direct current resistance of the power storage device or improve the energy density per volume. The thickness of the separator may be 10 μm to 30 μm, more specifically 15 μm to 25 μm. In addition, as a separator, a porous polymer film made of a polyolefin-based polymer may be used alone or by laminating them. Further, it is possible to use a thin film having high ion permeability and low insulating properties having mechanical strength. The separator may include a safety reinforced separator (SRS) in which a ceramic material is thinly coated on the surface of the separator.

According to one embodiment of the present invention, a lithium metal is inserted between the anode and the separator.

When the secondary battery according to an embodiment of the present invention satisfies Equation 1 below, the lithium metal has a thickness (TL) of 10 µm to 30 µm and a density (DA) of ^{0.3 g/cm 3} to 0.8 g/cm ^3. I can have it.

[Equation 1]

0.5 <{TAХDAХCA0(100-ICE)/100}/CLtХ(CLa/CLt)}ХTLХDL <TL <{TAХDAХCA0(100-ICE)/100}/CLtХ(CLa/CLt)ХDL

In Equation 1 above,

TA: The thickness of the cathode (㎛)

DA: The density of the cathode (g/cm ³ )

ICE: Initial efficiency (%)

CA0: The first charge capacity (mAh/g) of the negative electrode

CLt: Theoretical capacity of lithium (3600 mAh/g)

CLa: Actual capacity of lithium metal (mAh/g)

TL: Lithium metal thickness (㎛)

DL: Density of lithium metal (0.53g/cm ³ ).

In addition, the thickness of the lithium metal may be specifically 15 ㎛ to 25 ㎛. When considering the pre-doping rate, a lithium metal having a large surface area is preferable, and when considering the handling property, productivity, and pre-doping process atmosphere of the lithium metal, a lithium metal having a small surface area may be preferable. If the thickness of the lithium metal satisfies the above range, it may be good in terms of suppression of residual lithium metal, pre-doping rate, and productivity.

[Method of manufacturing secondary battery]

A method of manufacturing a secondary battery according to an embodiment of the present invention includes: (1) a first process of increasing the initial efficiency of the negative electrode; And (2) a second process of manufacturing a secondary battery using the negative electrode obtained in the first process; and a process of pre-doping lithium in the first process and the second process may be performed.

The first process is a process of increasing the initial efficiency of the negative electrode, and the initial efficiency of the negative electrode may be increased by pre-doping the negative electrode with lithium before manufacturing the secondary battery.

First, a silicon-based negative active material composition including a silicon-based negative active material may be coated on a negative electrode current collector, and then rolled and dried to prepare a negative electrode, which may be carried out by a conventional method.

Specifically, the negative electrode is prepared by applying a silicon-based negative active material composition in which the silicon-based negative active material, a conductive material, and a binder are mixed on a negative electrode current collector, followed by drying. Can be manufactured. For example, a silicon-based negative active material composition (slurry) is prepared by mixing and stirring a binder, a solvent, and, if necessary, a conductive material and a dispersant in the silicon-based negative active material according to an embodiment of the present invention, and then applied to the current collector. Then, it can be rolled and dried to prepare a negative electrode.

As the negative active material, a silicon-based negative active material including a compound represented by (i) silicon fine particles, and (ii) SiOx (0.3≦x≦1.6) may be used.

In the method of manufacturing a secondary battery according to an embodiment of the present invention, the silicon-based negative active material comprises: preparing a silicon-silicon oxide raw material powder mixture by mixing silicon powder and silicon dioxide powder; Heating and depositing the raw material powder mixture to obtain a silicon oxide composite; And pulverizing and classifying the silicon oxide composite to obtain a silicon-based negative active material.

Specifically, in the method of manufacturing the silicon-based negative active material, the first step may include preparing a silicon-silicon oxide raw material powder mixture by mixing silicon powder and silicon dioxide powder.

When mixing the silicon powder and the silicon dioxide powder, the mixing molar ratio of the silicon dioxide powder to the silicon powder, that is, the molar ratio of the silicon dioxide powder / silicon powder is more than 0.9 to less than 1.1, specifically 1.01 or more to 1.08 or less.

In the method of manufacturing the silicon-based negative active material, the second step may include heating and depositing the raw material powder mixture to obtain a silicon oxide composite.

The heating and deposition of the raw material powder mixture during the second step may be performed at 800°C to 1600°C, specifically 1200°C to 1500°C. When the heating and deposition temperature is less than 800° C., the reaction is difficult to proceed and productivity may be lowered, and when it exceeds 1600° C., the raw material powder mixture may melt and reactivity may be lowered.

In the method of manufacturing the silicon-based negative active material, the third step may include pulverizing and classifying the silicon oxide composite to obtain a silicon-based negative active material.

The pulverization may be performed by pulverizing and classifying the negative active material to have an average particle diameter (D ₅₀ ) of 0.5 µm to 15 µm, specifically 2 µm to 10 µm, and more specifically 4 µm to 8 µm. The pulverization may be performed using a commonly used pulverizer or sieve.

In addition, according to one embodiment of the present invention, between the second step and the third step, the step of cooling the silicon oxide composite may be further included.

The cooling may be performed at room temperature while injecting an inert gas. The inert gas may be at least one selected from carbon dioxide gas, argon (Ar), water vapor (H ₂ O), helium (He), nitrogen (N ₂ ), and hydrogen (H).

Meanwhile, according to an embodiment of the present invention, after the pulverization and classification in the third step, forming a carbon layer including a carbon film on the surface of the silicon-based negative active material by using a chemical thermal decomposition deposition method is further performed. Can include.

The formation of the carbon layer may include a compound represented by the following formula (2); A compound represented by the following formula (3); A compound represented by the following formula (4); And by introducing at least one carbon source gas selected from hydrocarbon gases containing at least one selected from acetylene, benzene, toluene and xylene, and reacting in a gaseous state at 600°C to 1200°C, a carbon film on the surface of the silicon-based negative active material It is possible to form a carbon layer containing.

[Formula 2]

C _n H _(2n+2-A) [OH] _A

In Chemical Formula 2,

n is an integer from 1 to 20,

A is 0 or 1,

[Formula 3]

C _m H _2m

In Chemical Formula 3,

m is an integer from 2 to 6,

[Formula 4]

C _x H _y O _z

In Chemical Formula 4,

x is an integer from 1 to 20 ,

y is an integer from 0 to 20,

z is 1 or 2.

The compound represented by Formula 2 is, for example, methane (CH ₄ ), ethane (C ₂ H ₆ ), propane (C ₃ H ₈ ), methanol (CH ₃ OH), ethanol (C ₂ H ₅ OH) And at least one selected from propanol (C ₃ H ₈ O), and the compound represented by Formula 3 is from methylene (CH ₂ ), ethylene (C ₂ H ₄ ) and propylene (C ₃ H ₆ ). It may include at least one selected. Meanwhile, the compound represented by Chemical Formula 4 is an oxygen-containing gas, and may include, for example, at least one selected from _{carbon monoxide (CO) and carbon dioxide (CO 2 ).}

In addition, when the carbon layer is formed, an inert gas containing at least one selected from carbon dioxide gas, argon (Ar), water vapor (H ₂ O), helium (He), nitrogen (N ₂ ), and hydrogen (H) is injected. I can. According to one embodiment, when water vapor is included in the formation of the carbon layer, the silicon-based negative active material may exhibit higher electrical conductivity. Specifically, when water vapor is included in the formation of the carbon layer, a highly crystalline carbon film may be formed on the surface of the silicon-based negative electrode active material by reaction with the gas mixture, and even when a smaller amount of carbon is coated, high It can represent electrical conductivity. The content of water vapor in the gas mixture is not particularly limited, and for example, 0.01% to 10% by volume may be used based on 100% by volume of the total carbon source gas.

According to one embodiment, when forming the carbon layer _{, a mixed gas of CH 4} and CO ₂ , or a mixed gas of CH ₄ , CO ₂ and H ₂ O may be used as the carbon source gas. In the case of using a mixture gas of _{CH 4} and CO _{2 , CH 4} : CO ₂ may be in a molar ratio of about 1:0.20 to 0.50, specifically in a molar ratio of about 1:0.25 to 0.45, and more specifically in a molar ratio of 1:0.30 to 0.40. When using a mixture of _{CH 4} , CO ₂ and H _{2 O, CH 4} :CO ₂ :H ₂ O is about 1:0.20 to 0.50:0.01 to 1.45 molar ratio, specifically about 1: 0.25 to 0.45: 0.10 to 1.35 The molar ratio may be, more specifically, about 1:0.30 to 0.40:0.50 to 1.0 molar ratio.

According to another embodiment, when forming the carbon layer, C0, CO ₂ or a combination thereof may be used as the carbon source gas.

According to another embodiment, when the carbon layer is formed, CH _{4 may be included as a carbon source gas, and N 2} may be used as an inert gas. The mixing ratio of the mixed gas of CH ₄ and N ₂ may be about 1:0.20 to 0.50 molar ratio, specifically about 1:0.25 to 0.45 molar ratio, more specifically about 1:0.30 to 0.40 molar ratio.

According to one embodiment, the inert gas may not be included.

In addition, during the reaction, the heat treatment temperature and the composition of the gas mixture may be selected in consideration of the amount of the desired carbon film.

When forming the carbon layer, the heat treatment temperature may be 600°C to 1200°C, specifically 700°C to 1100°C, and more specifically 700°C to 1000°C.

In addition, the pressure during the heat treatment may be controlled by adjusting the amount of the gas mixture to be introduced. For example, the pressure during heat treatment may be 1 atm or more, for example, 1 atm to 5 atm, but is not limited thereto.

The heat treatment time is not limited, but can be appropriately adjusted according to the heat treatment temperature, the pressure during the heat treatment, the composition of the gas mixture, and the amount of the desired carbon coating. For example, the heat treatment time may be 10 minutes to 100 hours, specifically 30 minutes to 90 hours, more specifically 50 minutes to 40 hours. According to an embodiment, as the heat treatment time increases, the thickness of the formed carbon film may increase, and when the thickness is adjusted to an appropriate thickness, the electrical properties of the silicon-based negative active material may be improved.

According to an embodiment of the present invention, the carbon film can be formed uniformly and thinly over the entire surface of the negative electrode active material, and the carbon film forms substantially a lot of carbon nanofibers, graphene, graphene oxide and reduced graphene oxide. It is desirable.

The formation of the carbon layer of the silicon-based negative active material may be performed through a gas phase reaction of a carbon source gas to obtain a carbon layer including a uniform carbon film on the surface of the silicon-based negative active material even at a relatively low temperature. The silicon-based negative active material thus produced does not readily undergo a desorption reaction of the carbon film. In addition, since a carbon film having high crystallinity can be formed by intervening the gas phase reaction, the electrical conductivity of the negative electrode active material can be improved without changing the structure when it is used as a negative electrode active material.

The carbon layer may include at least one selected from the group consisting of carbon nanofibers, graphene, graphene oxide, and reduced graphene oxide.

The structure of the carbon layer may be a layer, a nano sheet type, or a structure in which several flakes are mixed.

The film may refer to a form of a film in which graphene is continuously and uniformly formed on a surface of a silicon-based negative active material.

In addition, the nanosheet may refer to a case in which a carbon nanofiber or a graphene-containing material is formed in an irregular state on the surface of the silicon-based negative active material.

In addition, the flake may mean a case where a part of the nanosheet or film is damaged or deformed.

After forming the carbon layer, it may further include the step of pulverizing and classifying.

_{The average particle diameter (D 50} ) of the negative active material in which the carbon layer is formed may be formed by grinding as described above. In addition, classification may be performed to arrange particle size distribution after pulverization, which may be used for dry classification, wet classification, or filtration. The dry classification is mainly performed in order or simultaneously with the processes of dispersion, separation (separation of fine particles and defective particles), collection (separation of solids and gases), and discharge using an air stream. The moisture or oxygen concentration of the air stream used can be adjusted by performing pre-treatment (adjusting moisture, dispersibility, humidity, etc.) before classification so as not to reduce the classification efficiency due to confusion, velocity distribution, or static electricity. . In addition, it can be pulverized and classified at a time to obtain a desired particle size distribution.

When the silicon-based negative electrode active material having the average particle diameter is used by the pulverization and classification treatment, initial efficiency or cycle characteristics may be improved by about 10% to 20% compared to before classification. The silicon-based negative active material after the pulverization and classification has a Dmax of about 10 μm or less, and in this case, the specific surface area of the silicon-based negative active material may decrease, and thus lithium supplemented in the SEI layer may decrease.

In addition, the specific surface area of the silicon-based negative active material may be 2 ㎡/g to 20 ㎡/g. When the specific surface area of the silicon-based negative active material is less than 2 ㎡/g, the rate characteristics of the secondary battery may be deteriorated. When it exceeds 20 ㎡/g, the contact area with the electrolyte increases, resulting in a decomposition reaction of the electrolyte solution. It may be accelerated or a side reaction of the battery may occur. The specific surface area of the silicon-based negative active material may be specifically 3 ㎡/g to 15 ㎡/g, more specifically 3 ㎡/g to 10 ㎡/g. The specific surface area can be measured by the BET method by nitrogen adsorption, for example, a specific surface area measuring device generally used in the art (MOUNTECH's Macsorb HM (model 1210) or MicrotracBEL's Belsorp-mini±, etc.) Can be used.

The specific gravity of the silicon-based negative active material may be 1.8 g/cm 3 to 3.2 g/cm 3, specifically 2.5 g/cm 3 to 3.2 g/cm 3. As the specific gravity increases, since the number of pores in the silicon-based negative active material decreases, the conductivity may be improved, the strength of the matrix may be enhanced, and thus initial efficiency and cycle life characteristics may be improved.

In the present specification, specific gravity may be expressed in the same sense as true specific gravity, density, or true density. For the measurement of the specific gravity, for example, measurement of the specific gravity by a dry density meter, an AccuPeak II1340 manufactured by Shimadzu Corporation can be used as the dry density meter. The purge gas to be used can be measured after repeated purging 200 times in a sample holder set at a temperature of 23°C using helium gas.

According to an embodiment of the present invention, the silicon-based negative active material including the carbon layer directly grows a carbon layer having excellent conductivity and flexible in volume expansion on the surface of the negative electrode active material, in particular, a carbon nanofiber or a graphene-containing material, Volume expansion may be suppressed, and contraction due to compression of the silicon-based negative active material may be reduced. In addition, since it is possible to control the direct reaction of silicon contained in the silicon-based negative active material with the electrolyte by carbon nanofibers or graphene, it is possible to reduce the generation of the SEI layer of the electrode. In this way, when a silicon-based negative active material including a carbon layer is used, structural collapse due to volume expansion of the silicon-based negative active material can be suppressed without a binder when preparing the silicon-based negative active material composition, and electrical conductivity by minimizing the increase in resistance And it can be usefully used to manufacture an electrode and a lithium secondary battery having excellent capacity characteristics.

In addition, according to an embodiment of the present invention, the negative active material may further include a carbon-based negative electrode material.

Specifically, the negative electrode active material may be used by mixing the silicon-based negative electrode active material and the carbon-based negative electrode material. In this case, it is possible to reduce the electrical resistance of the negative electrode active material and to alleviate the expansion stress accompanying charging. The carbon-based negative electrode material is, for example, natural graphite, artificial graphite, soft carbon, hard carbon, mesocarbon, carbon fiber, carbon nanotubes, pyrolysis carbons, coke, glass carbon fiber, organic It may include at least one selected from the group consisting of a high molecular compound fired body and carbon black.

The content of the carbon-based negative electrode material, for example, a graphite-based negative electrode material, is 30% to 95% by weight, specifically 30% to 90% by weight, more specifically 50% to 80% by weight based on the total weight of the negative active material. Can be In particular, when the silicon-based negative active material contains silicon fine particles, since the silicon fine particles do not cause volume expansion, a secondary battery having excellent cycle characteristics can be obtained when used in combination with the carbon-based negative electrode material.

The binder is not particularly limited, for example, polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidene fluoride, polytetrafluoroethylene, etc. Resin; rubber-based materials such as fluorine rubber and styrene butylene rubber (SBR); polyolefins such as polyethylene and polypropylene; acrylic resin; And one or more selected from the group consisting of polyimide-based resins. However, since the silicon-based negative active material has a large volume change during charging and discharging, a material having excellent adhesion may be more preferable. It may include a polyimide-based resin including mid, polyamide-imide, or a mixture thereof. In addition, when manufacturing the negative electrode, the blending amount with the binder may be used in consideration of the type, particle diameter, shape, and composition amount of the negative electrode active material of the present invention. Good cycle characteristics can be obtained.

The conductive material is not particularly limited as long as it has conductivity without causing a chemical change in the battery, and examples thereof include graphite such as natural graphite or artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, Parnes black, lamp black, and thermal black; Conductive fibers such as carbon fibers and metal fibers; Metal powders such as fluorocarbon, aluminum, and nickel powder; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives may be used.

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

The negative electrode current collector is not particularly limited, and examples thereof include copper foil, stainless steel foil, or titanium foil. In addition, a surface embossed or a porous current collector, for example, an expanded metal, a mesh, or a punched metal may be used.

The initial efficiency of the negative electrode may be increased by using the negative electrode thus prepared, and this may be achieved by electrochemically activating a cell by preparing a cell by interposing a separator between the negative electrode and the lithium metal. By the above method, lithium is pre-doped on the negative electrode, thereby increasing the initial efficiency of the negative electrode.

According to one embodiment of the present invention, the lithium metal may be a lithium metal having the above-described thickness and/or density.

Meanwhile, as the separator, the above-described separator may be used. In addition, it may include a conventional porous polymer film used as a conventional separator, for example, porous propylene. In addition, the separator is a porous polymer film made of a polyolefin-based polymer such as ethylene homopolymer, propylene homopolymer, ethylene/butene copolymer, ethylene/hexene copolymer, and ethylene/methacrylate copolymer, etc. I can. In addition, an insulating thin film having high ion permeability and mechanical strength may be used. The separator may include a safety reinforced separator (SRS) in which a ceramic material is thinly coated on the surface of the separator. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of a high melting point glass fiber, polyethylene terephthalate fiber, or the like may be used, but is not limited thereto. The separator may have a thickness of 10 µm to 30 µm, specifically 15 µm to 25 µm.

In the method of manufacturing the secondary battery, the process of increasing the initial efficiency of the negative electrode may be performed by electrochemically activating a cell in which a separator is interposed between the negative electrode and the lithium metal, and predoping lithium to the negative electrode.

In the present specification, pre-doping means doping lithium by occluding and supporting lithium in the negative electrode active material before normal charging and discharging between the positive electrode and the negative electrode.

In order to obtain a secondary battery having a high energy density, which is the object of the present invention, it is important to minimize the irreversible capacity of the negative electrode active material. Theoretically, since it is impossible to reduce more than a certain level, high energy density can be improved by pre-doping lithium on the silicon-based negative active material. The pre-doping compensates for irreversible capacity, or even when an active material that does not contain lithium is used in the positive electrode, lithium required for charging and discharging may be included in the negative electrode, thereby improving the performance of the secondary battery.

As a pre-doping method according to an embodiment of the present invention, a method of doping lithium into an anode active material by electrochemically activating a lithium metal as a counter electrode in an electrolyte solution used for a secondary battery may be mentioned. The pre-doping may include doping an irreversible capacity of lithium by performing absorption and desorption of lithium while the negative electrode is in electrochemical contact with the lithium metal. Specifically, lithium metal is used as a negative electrode and a counter electrode, and a cell is manufactured by interposing a separator between the negative electrode and the lithium metal, and then electrochemically activated in the electrolyte to pre-dope lithium for the first irreversible capacity. I can. The electrolyte may include a non-aqueous electrolyte, a polymer electrolyte, or a polymer gel electrolyte.

In addition, the electrochemical activation may be performed by charging and discharging the cell, and although not particularly limited, for example, after the voltage reaches 0.005 V with a constant current of 0.1 C, until the current becomes 0.005 C with a constant voltage. It can be charged and discharged with a constant current of 0.1 C until the voltage reaches 1.5 V.

As described above, by interposing a separator between the negative electrode and the lithium metal in the electrolyte, by electrochemically pre-doping lithium without directly contacting the lithium metal to the negative electrode, the lithium metal is formed by directly attaching or depositing the lithium metal to the negative electrode. It is possible to solve the problem of adversely affecting the separator due to acicular crystals (dendrite). This can ensure the safety of the secondary battery.

In addition, the pre-doping method according to another embodiment of the present invention may be a method of removing excess lithium (irreversible capacity remains in the negative electrode) by the method of de-doping lithium after the doping. The pre-doping may include electrochemically doping lithium on the negative electrode, leaving an initial irreversible amount of lithium and releasing (de-doping) all other amounts of lithium.

Specifically, lithium metal is used as a negative electrode and a counter electrode, and a cell is manufactured by interposing a separator between the negative electrode and the lithium metal, and then electrochemically activated in an electrolytic solution, for example, a non-aqueous electrolytic solution, for the first irreversible capacity. After pre-doping of lithium in minutes, lithium can be released (de-doped) in total, leaving the initial irreversible capacity portion. In the case of pre-doping the non-reversible capacity of lithium by the pre-doping method, particularly de-doping, the initial efficiency can be achieved close to 100%. As a result, the cathode/anode capacity ratio (N/P ratio) can approach 1.0, and consequently, the capacity of the secondary battery itself can be improved. In addition, although a large exothermic reaction may be accompanied during pre-doping of lithium, the speed can be controlled by controlling the current during doping with lithium, which can be a great advantage in terms of safety.

Accordingly, according to the present invention, by electrochemically doping or doping with lithium to the negative electrode active material by the pre-doping method, or by de-doping again after doping, an amount corresponding to the irreversible capacity of lithium may be pre-doped on the negative electrode active material. . Accordingly, the negative electrode can reduce the volume expansion of the electrode accompanying the insertion and release of lithium. In addition, since the rate of doping is made possible by controlling the current, safety of doping including large heat generation can be secured. In particular, when doping is performed in a large amount, since the temperature can be controlled, the method may be advantageous for mass production.

The 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, such as the lithium salt, 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 ^- , CH ₃ CO ₂ ^- , SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^-It may be any one selected from the group consisting of.

In the electrolytic solution used in the present invention, as the organic solvent contained in the electrolytic solution, those commonly used in the electrolytic solution for lithium secondary batteries may be used without limitation, typically propylene carbonate (PC), ethylene carbonate, EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, dimethyl sulfur oxide, acetonitrile, dimethoxyethane, diethoxy Any one selected from the group consisting of ethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, methyl acetate, methyl formate, and tetrahydrofuran, or a mixture of two or more of them may be typically used. . The concentration of the electrolyte solution is not particularly limited, but may be, for example, 0.5 to 2 mol/L. It is preferable to use the electrolyte solution having a moisture content of 100 ppm or less. Meanwhile, the term "non-aqueous electrolyte" used in the present specification not only refers to a concept including a non-aqueous electrolyte and an organic electrolyte, but may also mean a concept including a gel-like and solid electrolyte.

According to an exemplary embodiment of the present invention, a secondary battery including a silicon-based negative active material pre-doped with lithium may significantly improve energy density and further improve cycle characteristics and rate characteristics.

In this way, a secondary battery can be manufactured using the negative electrode with improved initial efficiency.

Specifically, as described above, after electrochemical activation to increase the initial efficiency of the negative electrode, the cell is separated to prepare a secondary battery using a lithium-pre-doped negative electrode, a positive electrode including a positive electrode active material, and a separator. can do.

When manufacturing the secondary battery, the positive electrode active material included in the positive electrode is not particularly limited as long as it is a material capable of electrochemically storing and releasing lithium. For example, it may include a metal oxide active material including lithium that can be released electrochemically, and specifically, may include an oxide having a spinel structure including lithium cobalt oxide, lithium manganate, or a mixture thereof. More specifically, the positive electrode active material may include at least one selected from the group consisting of lithium composite cobalt oxide, lithium composite nickel oxide, lithium composite manganese oxide, and lithium composite titanium oxide, and the composite oxide contains at least one different metal element. An added active material or the like can be used. In addition, one or more metal oxides selected from the group consisting of manganese, vanadium and iron as not containing electrochemically releasable lithium, disulfide-based compounds, polyacene-based compounds, and one or more selected from the group consisting of activated carbon Etc. can be used.

In addition, as the separator, a commonly used separator may be used, and a separator used in a process of increasing the initial efficiency of the negative electrode may be used.

The pre-doping of lithium in the second process may be performed by electrochemically activating lithium metal in an electrolyte solution, for example, a non-aqueous electrolyte solution, in which lithium metal is inserted between the anode and the separator. The electrolyte may be an electrolyte used in a process of increasing the initial efficiency of the negative electrode.

The pre-doping amount of lithium may vary depending on the amount of lithium of the positive electrode, positive electrode efficiency, and negative electrode characteristics (charge/discharge potential, change in internal resistance accompanying charge/discharge).

The shape and size of the secondary battery of the present invention are not particularly limited, but may include a cylindrical shape, a square shape, a pouch type, a film type, or a coin type, depending on each use.

The secondary battery manufactured according to the above embodiment includes a silicon-based negative electrode active material pre-doped with lithium, and is manufactured by inserting a lithium metal between the positive electrode and the separator, thereby realizing the safety of the secondary battery and greatly improving the energy density. , Cycle characteristics and rate characteristics can be further improved.

[Example]

The above will be described in more detail by the following examples. However, the following examples are for illustrative purposes only, and the scope of the examples is not limited thereto.

Example 1

Using a silicon-based negative electrode active material (made by DAEJOO) with a carbon layer having the following properties, Super-P (Super-P, TIMCAL) as a conductive material, and polyacrylic acid (PAA) as a binder, Sigma-Aldrich (Sigma-Aldrich). -Aldrich)) was mixed in a weight ratio of 80:10:10 to prepare a silicon-based negative active material composition.

SiOx: x = 1.0

Carbon content: 5% by weight

Average particle diameter of silicon-based negative active material (D ₅₀ ): 7.0 ㎛

The silicon-based negative active material composition was applied and dried on a copper foil having a thickness of 18 µm, 41 μm, with a density of 1.3 g/cm 3 The cathode was produced. This negative electrode was punched out to a diameter of 14 mmφ, and dried under vacuum at 80° C. for 12 hours to obtain a negative electrode. When the prepared negative electrode was wound and unfolded with a 4 mmφ axis, the negative electrode active material was peeled off from the current collector and did not desorb, so it was confirmed that it had sufficient strength for electrochemical evaluation.

Thereafter, in the glove box, the cathode and the thickness are 20 μm A cell was prepared by interposing a porous polypropylene sheet (30 μm) as a separator between lithium metals. _{Put the cell in a reaction tank, in a lithium non-aqueous electrolyte containing 1M LiPF 6} in a mixed solvent of 1:1 by volume of ethylene carbonate (EC) and diethylene carbonate (DEC), for a current of 1.5 mA and a lithium metal potential After reaching the constant current until 1 mV, 1 mV, a constant voltage of 1 mV was applied to the lithium potential for a predetermined time. Thereafter, de-doping (release of lithium) was performed with a current of 0.6 mA and a constant current of 2.0 V with respect to the lithium metal potential, and pre-doping of lithium on the negative electrode was performed to increase the initial efficiency of the negative electrode. The initial efficiency measured in this way was 76.1%.

The cell was separated to prepare a secondary battery using a lithium-pre-doped negative electrode.

Specifically, LiCoO ₂ is used as a positive electrode active material, Super-P (TIMCAL) as a conductive material, and polyacrylic acid (PAA, Poly Acrylic acid, Sigma-Aldrich) as a binder 80:10:10 The mixture was mixed so as to have a weight ratio to obtain a positive electrode active material composition. The positive electrode active material composition was coated on an aluminum current collector and punched with 16 mmφ to prepare a positive electrode. A 15 µm-thick lithium metal-attached separator was stacked on the positive electrode, and at this time, the lithium metal was disposed toward the positive electrode. A porous polypropylene sheet (25 μm) was used as the separator. After preparing a cell by stacking the lithium-pre-doped negative electrode on the separator, a secondary battery was manufactured using a non-aqueous electrolyte. When manufacturing the secondary battery, the same non-aqueous electrolyte was used as the non-aqueous electrolyte used in the process of increasing the initial efficiency of the negative electrode.

Example 2

A silicon-based negative electrode active material (manufactured by DAEJOO) having a carbon layer having the following physical properties, and a polyimide resin (manufactured by Ube Xinshan: U-varnish A, NMP (N-methyl-2-pyrrolidone) solution, solid content 18.1 weight as a binder) %) was used. The silicon-based negative active material and the binder were mixed at a weight ratio of 90:10 (based on the solid content), and the negative active material composition obtained by adjusting the viscosity with NMP was carried out in the same manner as in Example 1, and the thickness was 37 µm and the density was A negative electrode having 1.03 g/cm 3 was prepared. The negative electrode was punched with 16 mmφ, dried under vacuum at 200° C. for 10 hours and used, and a lithium non-aqueous system containing _{1M LiPF 6 in a mixed solvent of ethylene carbonate (EC) and methylethylene carbonate as an electrolyte in a volume ratio of 3:7} A silicon-based negative electrode and a secondary battery pre-doped with lithium were prepared in the same manner as in Example 1, except that the electrolyte was used:

SiOx: x = 1.0

Carbon content: 5.1% by weight

_{Average particle diameter (D 50} ) of the silicon-based negative active material: 5.3 µm.

Example 3

A negative electrode (thickness of 37) was carried out in the same manner as in Example 1, except that the weight ratio of the silicon-based negative electrode active material (manufactured by Shin-Etsu Chemical Co., Ltd.: KSC801) and the binder was used in the same manner as in Example 1, except that a weight ratio of 85:15 (based on solid content) was used. Μm, density 1.00 g/cm 3) was prepared:

SiOx：x = 1. 0

SiOx carbon complex amount: 5% by mass based on SiOx

SiOx carbon composite particle diameter (D50): 7. 0μm

The negative electrode was punched with a diameter of 17 mmφ, dried under vacuum at 80° C. for 10 hours, and then a negative electrode, a separator, and a cell including a lithium metal having a diameter of 20 μm were prepared. _{A lithium non-aqueous electrolyte solution containing 1M LiPF 6} was injected into a mixed solvent of ethylene carbonate and methyl ethyl carbonate as an electrolytic solution in a volume ratio of 3:7, and left to stand for 60 hours to pre-dope lithium on the negative electrode.

On the other hand, LiNi _0.8 Co _0.2 O ₂ as a positive electrode active material, acetylene black as a conductive material, and polyvinylidene fluoride (PVdF, Solvay) as a binder in a 92:4:4 weight ratio and NMP (N-methyl pyrrolidone) By mixing in a solvent, a positive electrode active material composition (slurry) was prepared. The positive electrode active material composition was coated on an aluminum foil having a thickness of 20 μm, and then dried and rolled to obtain a positive electrode. The initial charge/discharge characteristics of the positive electrode had a capacity of 175 mAh/g at 4.3 V-2.7 V.

A secondary battery was obtained in the same manner as in Example 1 using a silicon-based negative electrode (dried under vacuum at 200° C. for 10 hours) with a thickness of 40 μm prepared in the same manner as described above.

<Characteristic evaluation of secondary battery using silicon-based negative electrode>

In the lithium-pre-doped negative electrode of Example 3, the thickness of the pre-doped negative electrode was 60 μm. Therefore, by confirming that the thickness corresponding to the lithium metal (20 μm) increased, it was confirmed that a gap due to the disappearance of the lithium metal did not occur.

The secondary battery of Example 3 was charged with a current of 3 mA until it reached 4.3 V, and then charged for 8 hours by applying a constant voltage of 4.3 V. Subsequently, discharge was performed with a constant current of 3 mA until it became 2.0 V (the negative electrode voltage corresponds to about 0.7 V with respect to the lithium potential). The charging capacity of the secondary battery was 18.0 mAh, the discharge capacity was 14.9 mAh, and the average discharge voltage was 3.60 V. In addition, the discharge capacity up to 3.0 V was 14.0 mAh.

Comparative Example 1

In Example 1, a negative electrode and a secondary battery were manufactured in the same manner as in Example 1, except that lithium was not pre-doped on the negative electrode and the lithium metal was not in contact with the positive electrode.

Comparative Example 2

A negative electrode and a secondary battery were obtained in the same manner as in Example 3, except that the silicon-based negative electrode having a thickness of 40 μm was not pre-doped with lithium, and the lithium metal was not contacted with the positive electrode.

The secondary battery prepared using the negative electrode was charged to 4.3 V with a current of 1 mA, and then charged for 20 hours by applying a constant voltage of 4.3 V. Subsequently, discharge was performed with a constant current of 2 mA until it reached 3.0 V (the negative electrode voltage corresponds to about 0.7 V with respect to the lithium potential). The charging capacity of the secondary battery was 18.2 mAh, the discharge capacity was 10.6 mAh, and the average discharge voltage was 3.57 V.

Comparative Example 3

The same silicon-based negative active material as in Example 1 and a carbon-based negative electrode material of natural graphite (average particle size: 11 µm) were mixed in a weight ratio of 50:50, and were not pre-doped with lithium, and a thickness of 30 µm. A negative electrode and a secondary premise were prepared in the same manner as in Comparative Example 1, except that a sub-cathode was used. The charging capacity of the secondary battery was 17.8 mAh, the discharge capacity was 11.0 mAh, and the average discharge voltage was 3.59 V.

<Evaluation of capacity characteristics and initial efficiency of secondary battery>

After aging the lithium secondary battery obtained in Examples and Comparative Examples for 12 hours, using a charge/discharge test equipment (WonA Tech) with a constant current of 0.1 C until it reaches 4. 2 V with a constant current of 0.5 mA/cm 2 Charging was performed. A constant current of 5 mA/cm 2 was used and discharge was performed until the cell voltage reached 2.5 V. This charge/discharge test was repeated 50 times to determine the capacity retention rate.

The discharge capacity and the capacity retention rate after the cycle were calculated as follows, and the results are shown in FIGS. 1 and 1.

Initial discharge capacity: discharge capacity at 1 ^st cycle

Capacity retention: 100X ( ^{discharge capacity at 50 th} cycle) / (discharge capacity at ^{1 st cycle)}

Remark Initial efficiency
(%) Capacity retention rate
(%) Example 1 88 98 Example 2 89 99 Example 3 89 98 Comparative Example 1 62 95 Comparative Example 2 58 97 Comparative Example 3 62 96

As can be seen from Table 1, the secondary batteries of Examples 1 to 3 using the negative electrode pre-doped with lithium in the negative electrode active material had an initial efficiency of 88% to 89%, and a capacity retention rate of 98% to after 50 cycles. It was 99%.

In contrast, the initial efficiencies of the secondary batteries of Comparative Examples 1 to 3 using a negative electrode not pre-doped with lithium in the negative electrode active material and not contacting the positive electrode with lithium metal were 58% to 62%, compared to the secondary battery of Example. It decreased by more than about 20%. In addition, it was confirmed that the secondary batteries of Comparative Examples 1 to 3 also reduced capacity retention compared to the secondary batteries of Examples.

Meanwhile, FIG. 1 shows the capacity characteristics of the secondary batteries of Example 2 and Comparative Example 1 with respect to the number of cycles. As can be seen from FIG. 1, it can be seen that the secondary battery of Example 2 was improved by about 20% or more even when the charging/discharging test was repeated 50 times compared to the secondary battery of Comparative Example 1.

Claims (21)

As a secondary battery comprising a positive electrode, a separator and a negative electrode,
The anode includes a metal oxide,
Lithium metal is inserted between the anode and the separator,
The negative electrode includes (i) silicon fine particles, and (ii) a silicon-based negative active material including a compound represented by SiOx (0.3≦x≦1.6),
In this case, the silicon-based negative active material is pre-doped with lithium in an amount corresponding to the initial irreversible capacity of the negative electrode.
The method of claim 1,
The negative electrode active material is an active material including an oxide matrix capable of storing and releasing lithium.
The method of claim 2,
The secondary battery, wherein the matrix comprises silicon oxide, tin oxide, or a mixture thereof.
The method of claim 1,
When the secondary battery satisfies Equation 1 below, the lithium metal has a thickness of 10 μm to 30 μm and ^{a density of 0.3 g/cm 3} to 0.8 g/cm ³ , a secondary battery:
[Equation 1]
0.5 <{TAХDAХCA0(100-ICE)/100}/CLtХ(CLa/CLt)}ХTLХDL <TL <{TAХDAХCA0(100-ICE)/100}/CLtХ(CLa/CLt)ХDL
In Equation 1 above,
TA: The thickness of the cathode (㎛)
DA: The density of the cathode (g/cm ³ )
ICE: Initial efficiency (%)
CA0: The first charge capacity (mAh/g) of the negative electrode
CLt: Theoretical capacity of lithium (3600 mAh/g)
CLa: Actual capacity of lithium metal (mAh/g)
TL: Lithium metal thickness (㎛)
DL: Density of lithium metal (0. 53 g/cm ³ ).
The method of claim 1,
The silicon fine particles have a crystallite size of 2 nm to 10 nm, secondary battery.
The method of claim 1,
A secondary battery, wherein a carbon layer including a carbon film is formed on the surface of the negative active material.
The method of claim 6,
The secondary battery, wherein the carbon layer comprises at least one selected from the group consisting of carbon nanofibers, graphene, graphene oxide, and reduced graphene oxide.
The method of claim 6,
The secondary battery, wherein the carbon content of the carbon layer is 2% by weight to 10% by weight based on the total weight of the negative active material.
The method of claim 6,
_{A secondary battery having a cumulative 50% average particle diameter (D 50} ) of 0.5 µm to 15 µm of the particle distribution of the negative active material on which the carbon layer is formed.
The method of claim 1,
The negative active material has a specific gravity of 1.8 g/cm 3 to 3.2 g/cm 3 and a specific surface area (Brunauer-Emmett-Teller; BET) of 2 m ² /g to 20 m ² /g.
(1) a first step of increasing the initial efficiency of the cathode; And
(2) a second process of manufacturing a secondary battery using the negative electrode obtained in the first process;
Including,
The method of manufacturing a secondary battery according to claim 1, wherein a process of pre-doping lithium is performed in the first process and the second process.
The method of claim 11,
The method of manufacturing a secondary battery, wherein the first process is performed by electrochemically activating a cell in which a separator is interposed between a negative electrode and a lithium metal.
The method of claim 12,
The separator comprises a porous propylene, and the thickness is 10 ㎛ to 30 ㎛, a method of manufacturing a secondary battery.
The method of claim 11,
The negative electrode includes a negative electrode active material, a binder, and a current collector,
The negative active material is a method of manufacturing a secondary battery comprising a silicon oxide-based compound including silicon fine particles and a compound represented by SiOx (0.3≦x≦1.6).
The method of claim 14,
The method of manufacturing a secondary battery, wherein the binder comprises at least one selected from the group consisting of a fluorine-based resin, a rubber-based material, a polyolefin, an acrylic resin, and a polyimide-based resin.
The method of claim 14,
The negative active material,
Preparing a silicon-silicon oxide raw material powder mixture by mixing silicon powder and silicon dioxide powder;
Heating and depositing the raw material powder mixture to obtain a silicon oxide composite; And
Pulverizing and classifying the silicon oxide composite to obtain a negative active material;
That is manufactured by a process comprising a, a method of manufacturing a secondary battery.
The method of claim 16,
The heating is made in the range of 800 to 1600 ℃, the method of manufacturing a secondary battery.
The method of claim 16,
The pulverization is performed so that the average particle diameter of the silicon oxide composite is 0.5 µm to 15 µm.
The method of claim 16,
After the pulverization and classification, the method of manufacturing a secondary battery further comprising the step of forming a carbon layer including a carbon film on the surface of the negative electrode active material by using a chemical pyrolytic deposition method.
The method of claim 19,
After forming the carbon layer, the method of manufacturing a secondary battery further comprising the step of pulverizing and classifying.
The method of claim 11,
The pre-doping is performed in an electrolyte solution,
The method of manufacturing a secondary battery, wherein the electrolyte solution comprises a non-aqueous electrolyte, a polymer electrolyte, or a polymer gel electrolyte.

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