KR101225882B1 - Anode for secondary battery - Google Patents

Abstract

The present invention (a) Si-C composite (Si-C composite) formed by mechanical alloying silicon and the first carbonaceous material; And a negative electrode active material including a second carbonaceous material; And (b) relates to a negative electrode comprising a polyvinyl alcohol and a secondary battery comprising the same.

According to the present invention, by using a polyvinyl alcohol and a negative electrode active material including a Si-C composite material and a second carbonaceous material together, the volume change of the Si-C composite material can be minimized or suppressed during charging and discharging of a battery. In addition, the bonding force or adhesion between the negative electrode active materials or the current collector may be improved, thereby improving the capacity of the battery and the life characteristics of the battery.

Electrode active material, binder, secondary battery

Description

Anode for Secondary Battery {ANODE FOR SECONDARY BATTERY}

The present invention relates to a negative electrode for a secondary battery that uses polyvinyl alcohol in combination with a negative electrode active material containing a Si-C composite material and a carbonaceous material.

Recently, interest in energy storage technology is increasing. As the field of application extends to the energy of mobile phones, camcorders and notebook PCs, and even electric vehicles, efforts for research and development of electrochemical devices are becoming more concrete. The electrochemical device is one of the most remarkable fields in this respect, and development of a rechargeable secondary battery has become a focus of attention. Recently, in developing such a battery, research and development on the design of a new electrode and a battery have been conducted to improve capacity density and specific energy.

Among the currently applied secondary batteries, the lithium secondary battery developed in the early 1990s has advantages such as higher operating voltage and much higher energy density than conventional batteries such as N-MH, Ni-Cd and sulfuric acid-lead batteries using an aqueous electrolyte solution .

In general, a lithium secondary battery is prepared by using a material capable of intercalation and disintercalation of lithium ions as a cathode and an anode, and filling an organic electrolyte or a polymer electrolyte between a cathode and an anode, and lithium ions are a cathode and an anode. Electrical energy is generated by oxidation and reduction reaction when inserted and detached at

Currently, carbonaceous materials are mainly used as a negative electrode active material constituting the negative electrode of a lithium secondary battery. In the case of graphite, the theoretical capacity is about 372 mAh / g, and the actual capacity of commercially available graphite is realized up to about 350 to 360 mAh / g. However, the capacity of the carbonaceous material such as graphite does not meet the lithium secondary battery requiring a high capacity of the negative electrode active material.

In order to satisfy such a demand, there is an example of using a Si, Sn, etc., which is a metal that exhibits a higher charge / discharge capacity than the carbonaceous material and which is capable of being electrochemically alloyed with lithium, as a negative electrode active material. Among them, Si having a theoretical capacity of about 4200 mAh / g has received the most attention. However, in the case of a negative electrode including Si as a negative electrode active material, there is a problem in that the volume is greatly changed during charging and discharging of the battery, so that an interface is opened between all the negative electrode active materials, thereby degrading conductivity and significantly reducing cycle life. In addition, there was a tendency that the volume expansion occurred unevenly in the vertical direction of the electrode, and the electrode was warped and the volume expansion accelerated intensively.

Si is alloyed with other metals inert to control this volume change, but it is insufficient to control the volume expansion of Si, which still leads to electrode peeling, non-uniform electrode surface generation, lower electrode electrical conductivity, and distortion of the electrode. It was.

The inventors of the present invention found that when a conventional binder such as polyvinylidene fluoride (PVdF) is used in a silicon-graphite composite in which silicon and a carbonaceous material are mechanically alloyed, Due to the volume change of the composite of graphite, the structure of the negative electrode active material was collapsed and the electrode was deteriorated, and the negative electrode active material was peeled off from the current collector.

Accordingly, the present invention is a silicon-graphite produced by charging and discharging a battery by mixing a carbonaceous material in the silicon-graphite composite material as a negative electrode active material and simultaneously using polyvinyl alcohol having excellent adhesion. It is intended to prevent the negative electrode active material from peeling off from the current collector while minimizing or suppressing the volume change of the composite material.

The present invention (a) Si-C composite (Si-C composite) formed by mechanical alloying silicon and the first carbonaceous material; And a negative electrode active material including a second carbonaceous material; and (b) a polyvinyl alcohol.

In addition, the present invention is the negative electrode; anode; Separator; And it provides a secondary battery comprising an electrolyte.

According to the present invention, by using a polyvinyl alcohol and a negative electrode active material including a Si-C composite material and a second carbonaceous material together, the volume change of the Si-C composite material can be minimized or suppressed during charging and discharging of a battery. In addition, the bonding strength or adhesion between the negative electrode active materials or the current collector may be improved, thereby improving the capacity of the battery and the life characteristics of the battery.

Hereinafter, the present invention will be described in detail.

According to an embodiment of the present invention, a cathode active material including a Si-C composite and a second carbonaceous material formed by mechanical alloying of silicon and a first carbonaceous material, and a cathode active material, It is characterized by using a combination of polyvinyl alcohol which can impart the bonding force or the adhesive force between the negative electrode active material and the current collector as a binder (竝 用).

The Si-C composite is formed by mechanically alloying silicon (Si) and a carbonaceous material (hereinafter, referred to as a 'first carbonaceous material'), and in addition to a high charge / discharge capacity, the first carbonaceous material. It may also have a high electrical conductivity.

However, in the case of the negative electrode including the Si-C composite material, the silicon and lithium ions in the Si-C composite material repeatedly react during charge and discharge of the battery, thereby causing the volume of the Si-C composite material to expand or contract repeatedly. Due to the repeated volume change, the stress in the negative electrode accumulates and the negative electrode collapses or cracks, which may cause deterioration of the electrode, resulting in a decrease in capacity of the battery.

In addition, when the Si-C composite material is used in combination with polyvinyledene fluoride (PVdF), styrene-butadiene rubber, and the like, which are widely used as binders in the art, Conventional binders do not provide a strong bonding force or adhesion sufficient to withstand the volume change of the Si-C composite material generated during charging and discharging of the battery between the negative electrode active material or between the negative electrode active material and the current collector. For this reason, due to the volume change of the negative electrode active material generated at the time of charge / discharge of the battery, the negative electrode active material is separated from the current collector and peeled off, thereby causing a decrease in capacity of the battery and a deterioration of life characteristics. However, when an excessive amount of binder is used to increase the bonding strength between the negative electrode active materials to prevent the volume change of the negative electrode active materials, not only the resistance in the electrode may be increased but also the ratio of the electrode active material in the electrode may be relatively decreased, thereby reducing the battery capacity. have.

Particularly, in the case of polyvinylidene fluoride (PVdF), since the negative electrode active material is covered in a form in which fibers are tightly woven, it is a factor that lowers the battery performance inherent in the negative electrode active material such as capacity and efficiency. In addition, polyvinylidene fluoride (PVdF) lacks flexibility and has a large specific surface area like natural graphite, and when used in combination with a negative electrode active material having a large volume change due to charging and discharging of a battery, the negative electrode active material and a current collector, etc. The bonds can break and tend to degrade life characteristics. In addition, the polyvinylidene fluoride (PVdF) has a tendency to absorb and expand the carbonate-based electrolyte, so that the output capacity is greatly reduced as the cycle progresses.

Accordingly, in the present invention, a negative active material containing a carbonaceous material (hereinafter referred to as a 'second carbonaceous material') and a polyvinyl alcohol are used in combination with the Si-C composite material, thereby providing Si- Even if the C composite reacts with lithium ions, the volume change of the Si-C composite may be minimized or suppressed by the second carbonaceous material, thereby preventing the battery capacity from being lowered. In addition, the polyvinyl alcohol can be provided with a strong bonding force or adhesive force between the negative electrode active material or between the negative electrode active material and the current collector, it is possible to prevent the negative electrode active material from peeling, detached from the current collector, thereby reducing the capacity of the battery and The life characteristic can be prevented from deteriorating. In addition, as the cycle progresses, even if the Si-C composite degenerates, the lifetime of the negative electrode may be maintained at a constant rate due to the second carbonaceous material, thereby maintaining a high capacity of the battery.

The negative electrode of the present invention may include polyvinyl alcohol as a binder. In general, the polyvinyl alcohol may be formed by hydrolysis of polyvinyl acetate, which is a polyvinyl alcohol precursor produced through polymerization of vinyl acetate (CH ₃ COOCHCH ₂ ), as shown in Scheme 1 below. At this time, the degree of hydrolysis is referred to as safflower (see Equation 1).

(In Equation 1, l is the number of units of vinyl alcohol formed by hydrolysis of polyvinylacetate, m is the number of units of vinyl acetate not hydrolyzed).

The polyvinyl alcohol prepared through such a reaction is repeatedly formed with a hydroxy group capable of hydrogen bonding, it is possible to hydrogen bond with the surface of the current collector, thereby forming a physical cross-linking can be improved adhesion. Therefore, when the polyvinyl alcohol is used as the binder in the present invention, the binding force between the negative electrode active materials is enhanced due to the polyvinyl alcohol, so that the volume change of the negative electrode active material can be alleviated or suppressed. In addition, since the polyvinyl alcohol provides strong adhesion between the negative electrode active material and the surface of the current collector, even if the formation of the electrode changes due to the volume change of the negative electrode active material generated during charging and discharging of the battery, the negative electrode active material is peeled off from the current collector. Can be suppressed.

The degree of polymerization of the polyvinyl alcohol may be about 2000 to 6000, preferably about 4000 to 5000. If the degree of polymerization of polyvinyl alcohol is too low, polyvinyl alcohol is dissolved in a carbonate-based electrolyte, and as the cycle progresses, the resistance of the electrolyte is increased, leading to a decrease in efficiency of the battery. The alcohol may increase the electrical resistance in the cathode and may increase the viscosity of the electrode mixture, making it difficult to handle during manufacture.

The saponification degree of the polyvinyl alcohol is about 98% or more, preferably about 99.5% or more. If polyvinyl alcohol has low saponification, the number of hydroxyl groups capable of exerting adhesion between the current collector such as a metal foil and the negative electrode active material is reduced, and the polyvinyl alcohol can be dissolved in the carbonate-based electrolyte. As this progresses, the adhesive force between the negative electrode active material and the current collector decreases, so that the negative electrode active material may be separated from the current collector.

In addition, the polyvinyl alcohol may be included in the negative electrode about 1 to 10 parts by weight, preferably about 2 to 5 parts by weight based on 100 parts by weight of the negative electrode mixture. When the content of the polyvinyl alcohol is too small, the bonding strength between the negative electrode active materials or the adhesion between the negative electrode active material and the current collector is low, so that the polyvinyl alcohol, which is electrically insulating, cannot withstand the volume change of the negative electrode active material generated during charging and discharging of the battery. Too much can increase the resistance in the cathode and relatively reduce the amount of cathode active material, thereby reducing the capacity of the electrode. Here, the negative electrode mixture means that a negative electrode active material and a binder and optionally a conductive agent, a filler, and the like are mixed.

The negative electrode of the present invention may include a Si-C composite material and a second carbonaceous material formed by mechanical alloying of silicon and the first carbonaceous material as a negative electrode active material in addition to the polyvinyl alcohol.

The Si-C composite may include silicon and a first carbonaceous material dispersed therebetween, or may include silicon and a first carbonaceous material formed on part or all of the surface of the silicon. Can be. In this case, even though silicon has low electrical conductivity, high conductivity may be imparted by the first carbonaceous material, so that the Si-C composite material included in the negative electrode active material of the present invention may have high electrical conductivity.

The first carbonaceous material is not particularly limited as long as the volume is expanded by intercalation of lithium during charging and discharging of the battery, and the volume can be retracted to its original state by deintercalation of lithium during discharge. Non-limiting examples thereof include amorphous carbon made by heat treatment using coal tar pitch, petroleum pitch, various organic materials, and natural graphite, artificial graphite having a high degree of graphitization. Crystalline carbon such as graphite, soft carbon, hard carbon, carbon black, MCMB (MesoCarbon MicroBead), carbon fiber and the like. Of these, graphite-based materials such as artificial graphite or natural graphite are preferred.

The silicon and the first carbonaceous material may form a Si-C composite at a ratio of 40 to 80 parts by weight: 20 to 60 parts by weight. If the silicon content is less than 40 parts by weight, the mechanically alloyed graphite content is increased, the initial efficiency is lowered, the specific surface area may be increased, and if the silicon content is more than 80 parts by weight, the volume expansion of the Si-C composite Due to this, the life characteristics may be lowered. By controlling the mixing ratio of silicon and the first carbonaceous material at this specific ratio, the Si-C composite can have a high initial discharge capacity of about 400 mAh / g or more. Therefore, even when the Si-C composite material is included in the negative electrode active material in a content of about 2 to 10 parts by weight, the secondary battery including the negative electrode active material may have an initial efficiency of about 90% or more (see FIG. 1). .

The Si-C composite is preferably one having a particle size as small as about 20 μm or less. Preferably the particle diameter may be about 5 to 20 ㎛. If the particle diameter of the Si-C composite material exceeds 20 μm, the volume change of the negative electrode active material is large because the volume change of the negative electrode active material is not effective because the volume expansion of the Si-C composite material by the second carbonaceous material is not effective during charging and discharging of the battery. May crack or collapse. In addition, the Si-C composite can be concentrated on one side without being uniformly dispersed between the second carbonaceous materials, causing it to expand unevenly during charging and discharging so as to crack or collapse more quickly.

In addition, the Si-C composite material has a specific surface area of 100 m 2 / g or less, preferably 3 to 100 m 2 / g. If the specific surface area exceeds 100 m 2 / g, the amount of electrolyte decomposed when the initial solid electrolyte interface (SEI) film is formed may increase. Due to the decomposition of the electrolyte may reduce the life of the battery.

Such Si-C composites may be uniformly dispersed between carbonaceous materials (hereinafter referred to as 'second carbonaceous materials') or the Si-C composites may be surrounded by second carbonaceous materials. In particular, it is preferred that the Si-C composites are present separately between the interfaces of the second carbonaceous materials and are evenly spread over the electrodes. In the negative electrode including the negative electrode active material of the present invention, the life of the negative electrode may be maintained at a constant rate due to the second carbonaceous material even when the Si-C composite materials are degraded due to the charge and discharge of the battery. In addition, since the second carbonaceous material can minimize or suppress the volume change of the Si-C composite material generated during charging and discharging of the battery, it is possible to prevent a decrease in capacity of the battery due to deterioration of the electrode.

As the second carbonaceous material, any material capable of intercalation or deintercalation of lithium ions may be used without particular limitation, and may be the same as the first carbonaceous material used in the aforementioned Si-C composite material. . For example, crystalline carbon such as natural graphite and artificial graphite having a high degree of graphitization, and the graphite-based materials include MesoCarbon MicroBead (MCMB), Carbon fiber, Natural graphite, Soft carbon, hard carbon and carbon black. In particular, as the surface treated graphite, it is appropriate that the specific surface area is about 10 m 2 / g or less, preferably about 5 to 2 m 2 / g or an average particle diameter of about 5 to 40 m.

The Si-C composite material and the second carbonaceous material may be mixed in a ratio of 2 to 10 parts by weight: 90 to 98 parts by weight, preferably 2 to 5 parts by weight: 95 to 98 parts by weight. (See Figure 2). If the content of the Si-C composite material is less than 2 parts by weight, the reversible capacity is small and meaningless as a high capacity negative electrode active material, and if it is more than 10 parts by weight, the Si-C composite material may be concentrated in the vertical direction of the electrode. During charging and discharging, it may cause a problem of uneven thickness expansion.

The negative electrode active material of the present invention comprises the steps of: mechanically alloying a mixture of silicon and a first carbonaceous material in a Mechano Fusion device to produce a Si-C composite; And it may be prepared by a method comprising the step of mixing the Si-C composite with a second carbonaceous material.

In the present invention, when manufacturing the Si-C composite material, if the silicon and the first carbonaceous material is mixed non-uniformly, the gap between the negative electrode active materials during the charging and discharging of the battery may be widened, thereby lowering the electrical conductivity, thereby The life of the battery may be reduced. In addition, when the silicon is concentrated in the vertical direction of the electrode, the negative electrode active material in the vertical direction of the electrode may cause volume expansion, such that the electrode is deformed, such that the electrode may be deteriorated, thereby lowering the capacity of the battery.

Accordingly, in the present invention, the mixing ratio of silicon and the first carbonaceous material can be easily controlled, and the Si-C composite material is formed by mechanical alloying so that the silicon can be uniformly mixed with the first carbonaceous material. By manufacturing, it is possible to prevent the silicon and the first carbonaceous material from being mixed unevenly or the silicon is concentrated in the vertical direction of the electrode. At this time, mechanical alloying (Mechanical Alloying) is to make an alloy of a uniform composition by applying a mechanical force.

The mechanical alloy may be made in a Mechano fusion apparatus using beads, such as beads that do not react with silicon and the first carbonaceous material, that is, chemically inert beads. Non-limiting examples of the Mechano fusion apparatus include a high energy ball mill, a planetary mill, a stirred ball mill, a vibration mill, and the like.

For example, the mixture is prepared by mixing silicon and the first carbonaceous material in a ratio of 40 to 80 parts by weight: 20 to 60 parts by weight. The mixture is then placed together with a ball into a Mechano fusion device, such as a high energy ball mill, and then subjected to mechanical alloying at room temperature for about 30 to 50 minutes at a rotational speed of 500 to 3000 rpm. To perform. As a result, the high energy of the ball-milling causes the mixture of silicon and the first carbonaceous material to be pulverized and uniformly mixed and simultaneously bonded between the silicon and the first carbonaceous material so that the silicon and the first carbonaceous material are dispersed with each other or Si-C composites with a first carbonaceous material formed on part or all of the surface may be formed.

In this case, the mixture and the ball of the silicon and the first carbonaceous material may be a ratio of 2 to 50 parts by weight: 50 to 98 parts by weight. Beyond this range it may not be possible to transfer the compressive stress to the mixture, and more balls than necessary may be used to reduce yield.

The ball may be a stainless ball or a zirconia ball having a particle diameter of 0.1 to 10 mm.

Thereafter, the prepared Si-C composite material is blended with the second carbonaceous material at a ratio of 2 to 10 parts by weight: 90 to 98 parts by weight to prepare a final negative electrode active material.

The mixing of the Si-C composite material and the second carbonaceous material may be performed by chemical mixing or mechanical mixing, which is known in the art, among which the mixing ratio of the Si-C composite material and the second carbonaceous material may be easily adjusted. It is preferable to carry out by mechanical mixing.

Specifically, the mechanical mixing may use the Mechano fusion device used in the mechanical alloy applied in the production of the Si-C composite material as the mechanical mixing device. However, process variables such as rotation speed, ball-to-powder weight ratio, ball size, mixing time, mixing temperature and atmosphere of the apparatus used may differ from those of mechanical alloys. In addition, alcohols such as ethanol and higher fatty acids such as stearic acid may be added as processing control agents to obtain good mixing effects. The process control agent may be added at about 2.0 parts by weight or less, preferably 0.5 parts by weight or less, based on 100 parts by weight of the mixture. When the process control agent is added, the mixing time can be reduced.

The negative electrode in the present invention can be prepared by conventional methods known in the art. For example, a slurry is prepared by mixing and stirring the polyvinyl alcohol and solvent DMSO of the present invention as a binder, a conductive agent and a dispersant as necessary in a negative electrode active material of the present invention, and then applying it to a current collector of a metal material and compressing it. After drying, the electrode may be manufactured.

Non-limiting examples of the solvent include dimethyl sulfoxide (DMSO), N-methyl pyrrolidon (NMP), dimethyl formamide (DMF) and the like. Of these, dimethyl sulfoxide (DMSO) is preferably used as a solvent.

In addition, as the conductive agent, carbon black may be generally used, and as a product currently available as a conductive agent, acetylene black series (such as Chevron Chemical Company or Gulf Oil Company). Ketjen Black EC series (from Armak Company), Vulcan XC-72 (manufactured by Cabot Company, etc.), and Super P (manufactured by MMM), and the like, are not limited thereto.

These conducting agents may slightly improve the battery's cycle life when used, but may also degrade the battery's capacity. Therefore, the conductive agent is suitably used in an amount of about 1 to 30 parts by weight, preferably about 1 to 2 parts by weight with respect to the negative electrode active material.

The current collector of the metal material is a highly conductive metal, and a metal to which the slurry of the negative electrode active material can easily adhere can be used as long as it is not reactive in the voltage range of the battery. Representative examples include meshes, foils, and the like, which are made of aluminum, copper, gold, nickel or an aluminum alloy or a combination thereof. Among these, copper foil of about 8 micrometers or more is suitable.

The negative electrode active material of the present invention can be used in all devices that undergo an electrochemical reaction. For example, there are all kinds of primary cells, secondary batteries, fuel cells, solar cells or capacitors, and the like, of which secondary batteries are preferred.

The secondary battery of the present invention can be produced by a conventional method known in the art, including a negative electrode prepared by using the negative electrode active material and the binder of the present invention in combination. For example, a porous separator may be inserted between the positive electrode and the negative electrode to add an electrolyte solution. Secondary batteries include lithium ion secondary batteries, lithium polymer secondary batteries or lithium ion polymer secondary batteries.

The positive electrode active material that can be used for the positive electrode of the secondary battery according to the present invention includes a lithium-containing transition metal oxide, and specifically, LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _a Co _b Mn _c ) O ₂ (0 <a <1, 0 <b <1, a + b + c = 1), LiNi _1-Y Co _Y O ₂ , LiCo _1-Y Mn _Y O ₂ , LiNi _1-Y Mn _Y O ₂ (where , 0 ≦ Y <1), Li (Ni _a Co _b Mn _c ) O ₄ (0 <a <2, 0 <b <2, a + b + c = 2), LiMn _2-Z Ni _Z O ₄ , LiMn _2-Z Co _Z O ₄ , where 0 <Z <2, LiCoPO ₄ , LiFePO ₄ and mixtures thereof can be used.

The positive electrode to be applied to the secondary battery of the present invention is not particularly limited, and according to a conventional method known in the art, the positive electrode active material may be prepared in a form bound to the positive electrode current collector.

The electrolyte may include a nonaqueous solvent and an electrolyte salt.

The nonaqueous solvent is not particularly limited as long as it is normally used as a nonaqueous solvent for nonaqueous electrolyte, and cyclic carbonate, linear carbonate, lactone, ether, ester or ketone can be used.

Examples of the cyclic carbonate include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BE), fluoroethylene carbonate (FEC), and the like. Examples of the linear carbonate include diethyl carbonate (DEC) and dimethyl. Carbonate (DMC), dipropyl carbonate (DPC), ethylmethyl carbonate (EMC) and methylpropyl carbonate (MPC). Examples of the lactone include gamma butyrolactone (GBL), and examples of the ether include dibutyl ether, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, and the like. There is this. In addition, examples of the ester include n-methyl acetate, n-ethyl acetate, methyl propionate, methyl pivalate, and the like, and the ketone includes polymethylvinyl ketone. These nonaqueous solvents can be used individually or in mixture of 2 or more types.

The electrolyte salt is not particularly limited as long as it is usually used as an electrolyte salt for nonaqueous electrolyte. Non-limiting examples of the electrolyte salt is A ⁺ B ^- A salt of the structure, such as, A ⁺ is Li ^+, Na ^+, and comprising an alkali metal cation or an ion composed of a combination thereof, such as K ^+, B ^- is PF _{6 ^{-, BF 4 -, Cl -}} , Br -, I -, ClO 4 -, AsF 6 -, CH 3 CO 2 -, CF 3 SO 3 -, N (CF 3 SO 2) 2 -, C (CF 2 SO ₂ ) salts containing ions consisting of anions such as ₂ ^- or a combination thereof. In particular, lithium salts are preferred. These electrolyte salts can be used individually or in mixture of 2 or more types.

The secondary battery of the present invention may include a separator. There is no restriction | limiting in particular in the separator which can be used, It is preferable to use a porous separator, A non-limiting example is a polypropylene type, a polyethylene type, or a polyolefin type porous separator etc.

The secondary battery of the present invention is not limited in appearance, but may be cylindrical, square, pouch or coin using a can.

Hereinafter, the present invention will be described in more detail with reference to examples and comparative examples. However, the following examples serve to illustrate the present invention, and the scope of the present invention is not limited thereto.

Example  One

Silicon having a particle size of 3 μm and natural graphite having a particle size of 50 to 100 μm ('first carbonaceous material') were mixed in a weight ratio of 70:30. Thereafter, the mixture was introduced into a ball-milling machine (ZOZ-mill, Germany) and milled at a rotational speed of 500 to 3000 rpm to prepare Si-C composite particles. The prepared Si-C composite particles were mixed with artificial graphite ('second carbonaceous material') in a weight ratio of 3:97.

As a negative electrode active material, a mixture of the Si-C composite particles prepared above and artificial graphite and polyvinyl alcohol (polyvinylachol (PVA)) as a binder were mixed at a weight ratio of 98: 2, and these were mixed with N-methyl-2-pyrrolidone. (N-methyl-2-pyrrolidone: NMP) was added to prepare a negative electrode slurry. The prepared slurry was coated on a copper (Cu) current collector to prepare a negative electrode.

LiCoO ₂ as a positive electrode active material, super-p as a conductive agent and polyvinylidene fluoride (PVdF) as a binder were mixed in a weight ratio of 94: 3: 3, and these were solvents N-methyl-2-pyrrolidone (N-methyl-2-pyrrolidone: NMP) was added to prepare a positive electrode slurry, and then coated on an aluminum current collector to prepare a positive electrode.

Fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) were mixed in a weight ratio of 30:70. Thereafter, 1 M of LiPF ₆ was added to the nonaqueous electrolyte solvent to prepare a nonaqueous electrolyte.

A polyethylene separator was interposed between the positive electrode and the negative electrode prepared as described above. The secondary battery was manufactured by injecting the nonaqueous electrolyte solution prepared above.

Comparative Example 1

A negative electrode and a secondary battery were manufactured in the same manner as in Example 1, except that a mixture of Si-C composite particles and artificial graphite as a negative electrode active material was mixed with polyvinylidene fluoride as a binder in a weight ratio of 93: 7.

Comparative Example 2

Blending silicon and artificial graphite having a particle diameter of about 3 μm at a weight ratio of 2:98 to prepare a mixture, and mixing the mixture with a negative electrode active material as a binder with polyvinylidene fluoride at a weight ratio of 93: 7 A battery and a secondary battery were manufactured in the same manner as in Example 1 except for the above.

Comparative Example 3

A negative electrode and a secondary battery were manufactured in the same manner as in Example 1, except that silicon and artificial graphite having a particle diameter of about 3 μm were blended at a weight ratio of 2:98 as a negative electrode active material to prepare a mixture.

Experimental Example 1

(1) In order to evaluate the performance of the secondary battery using the negative electrode prepared in Example 1 prepared according to the present invention, after preparing a half secondary battery as described below, this half secondary battery was charged and discharged.

Metal lithium foil was used as an electrode (electrode active material = Si-C composite + second carbonaceous material, binder = polyvinyl alcohol) prepared in Example 1 and a counter electrode thereof. A polyethylene-based separator was interposed between the electrodes. Thereafter, a non-aqueous electrolyte prepared by adding 1 M LiPF ₆ to a non-aqueous electrolyte solvent containing fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a weight ratio of 30:70 was injected between the electrodes. A coin type half secondary battery was prepared.

As can be seen in Figure 1, the initial efficiency of the negative electrode according to the present invention was about 90% or more. As a result, it was confirmed that the negative electrode of the present invention has high initial efficiency.

(2) In order to evaluate the performance of the secondary battery according to the present invention including the Si-C composite material, the thickness change of the battery with respect to the content change of the Si-C composite material was measured at 300 cycles. The experimental results are shown in FIG.

As a result, when the content of Si-C composite material is less than 10% by weight it can be seen that the thickness increase rate of the battery is about 20%. In particular, when the content of the Si-C composite material is 3% by weight or less, it can be seen that the volume expansion of the battery was suppressed to about 10% or less. As a result, the Si-C composite material and the second carbonaceous material have a weight ratio of 2 to 10:90 to 98, preferably 2 to 5:95 to 98, more preferably in terms of high charge and discharge capacity and a change in volume of the battery. Was found to be suitable for mixing at a weight ratio of 2 to 3: 97 to 98.

(3) In order to confirm the performance characteristics of the secondary battery using polyvinyl alcohol as a binder according to the present invention, the secondary batteries prepared in Example 1 and Comparative Examples 1 to 3 were charged and discharged. Experimental results are shown in FIGS. 3 and 4.

As can be seen in Figures 3 and 4, the negative electrode of Example 1 in which polyvinyl alcohol (PVA) was used as the binder has an initial efficiency of about 92% and then a capacity retention rate of R _150/1 (%) at 150 cycles. It was about 90%.

On the other hand, the negative electrode of Comparative Example 1, in which polyvinylidene fluoride (PVdF) was used as the binder, had an initial efficiency of about 88% and a capacity retention rate of R _150/1 (%) at about 20% at 150 cycles (Fig. See 3). In addition, as can be seen in Figure 4, the negative electrode of Comparative Example 3 using a mixture prepared by blending silicon and graphite as a negative electrode active material was about 70% capacity retention rate R _150/1 (%) at 150 cycles. In addition, the negative electrode of Comparative Example 2, in which the mixture prepared by blending silicon and graphite as a negative electrode active material and polyvinylidene fluoride as a binder, had a capacity retention rate of R _150/1 (%) of about 10% at 150 cycles. .

As a result, according to the present invention, in the case of the secondary battery in which the Si-C composite material and the second carbonaceous material were used as the negative electrode active material and polyvinyl alcohol was used as the binder, the lifespan characteristics of the battery were improved.

(4) In order to check whether the secondary battery according to the present invention to inhibit the volume expansion during charging and discharging, the thickness of the battery prepared in Example 1 and Comparative Example 1 after charging and discharging after 150 cycles was measured.

Table 1 shows the negative electrode thickness change of Example 1 and Comparative Example 1 according to the charge and discharge after 150 cycles.

Initial cell thickness
(Mm) Cell thickness at 150 cycles
(Mm) Electrode Expansion Rate (%)
(△ t / t ₁ ) Example 1 4.5 4.9 8.8 Comparative Example 1 4.5 5.6 24

Referring to Table 1, in Example 1, the thickness change of the electrode was observed about 8.8% (4.5 mm → 4.9 mm), but in Comparative Example 1, the thickness of the electrode was about 24% (4.5 mm → 5.6 mm). A change in thickness was observed, indicating that the negative electrode of the present invention had a volume expansion inhibiting effect. Thus, in the case of the electrode according to the present invention, it was confirmed that the volume change of the Si-C composite material can be minimized or suppressed during charge and discharge.

Experimental Example 2

In the case of using polyvinyl alcohol as a binder according to the present invention, in order to confirm that the negative electrode active material is prevented from peeling from the current collector, the negative electrode active material was observed for 30 hours after joining the electrolyte solution.

As a result, as shown in FIG. 5, the negative electrode active material of Example 1 in which polyvinyl alcohol was used as the binder did not peel off from the surface, whereas the negative electrode active material of Comparative Example 1 in which polyvinylidene fluoride was used as the binder had a surface. Peeled off.

As a result, when the negative electrode active material of the Si-C composite material and the second carbonaceous material was used in combination with polyvinyl alcohol as a binder, it was confirmed that the negative electrode active material could be prevented from being removed from the current collector by the polyvinyl alcohol.

1 is a curve showing a state of charge and discharge of a half cell using the negative electrode of Example 1.

Figure 2 is a curve showing the increase rate of the battery thickness with respect to the content change of the Si-C composite (Si-C composite).

3 is a curve showing the life characteristics of the negative electrode prepared in Example 1 and Comparative Example 1.

Figure 4 is a curve showing the life characteristics of the negative electrode prepared in Example 1 and Comparative Examples 2 and 3.

5 is a photograph showing whether the negative electrode active material of Example 1 and Comparative Example 1 peeled from the current collector.

Claims (13)

(a) a Si-C composite formed by mechanical alloying of silicon and a first carbonaceous material; And a negative electrode active material including a second carbonaceous material; and (b) A negative electrode comprising polyvinyl alcohol, wherein the Si-C composite material is dispersed between the second carbonaceous material. delete The negative electrode of claim 1, wherein the Si-C composite material and the second carbonaceous material are included in a ratio of Si-C composite material: second carbonaceous material = 2 to 10 parts by weight: 90 to 98 parts by weight. The negative electrode of claim 1, wherein the Si-C composite material comprises silicon (Si): carbon (C) = 40 to 80 parts by weight: 20 to 60 parts by weight. The cathode of claim 1, wherein the Si-C composite material has a particle diameter of 20 μm or less. The negative electrode of claim 1, wherein the first carbonaceous material is selected from the group consisting of natural graphite, artificial graphite, MesoCarbon MicroBead (MCMB), carbon fiber, and carbon black. The negative electrode of claim 1, wherein the second carbonaceous material is selected from the group consisting of natural graphite, artificial graphite, MesoCarbon MicroBead (MCMB), carbon fiber, and carbon black. The negative electrode according to claim 1, wherein the second carbonaceous material is graphite having a specific surface area of 10 m 2 / g or less. The negative electrode of claim 1, wherein the second carbonaceous material is graphite having a particle diameter of 5 to 40 μm. The negative electrode of claim 1, wherein the polyvinyl alcohol has a degree of polymerization of 2000 to 6000. The negative electrode of claim 1, wherein the polyvinyl alcohol has a saponification degree of 98% or more. The negative electrode of claim 1, wherein the polyvinyl alcohol is included in an amount of 1 to 10 parts by weight based on 100 parts by weight of the negative electrode mixture. The negative electrode of claim 1; anode; Separator; And a secondary battery comprising an electrolyte solution.

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