KR102571151B1 - Anode For Lithium Secondary Battery, Method Of Making The Same and Lithium Secondary Battery comprising the Same - Google Patents

Abstract

The present invention includes a positive electrode, a negative electrode, and a separator interposed therebetween, wherein the negative electrode includes a negative electrode active material layer having a web structure including a Si-containing negative electrode active material, a fibrous conductive material, and a binder, and the positive electrode And the capacity ratio (N / P ratio) of the negative electrode provides a lithium secondary battery of 1.5 to 3.0. In the lithium secondary battery according to the present invention, since the Si-based negative electrode is configured in a web structure using a fibrous conductive material, the charge/discharge characteristics can be improved by continuously maintaining the conductive network even when Si volume expansion occurs.

Description

Anode For Lithium Secondary Battery, Method Of Making The Same And Lithium Secondary Battery Comprising The Same}

The present invention relates to a negative electrode for a lithium secondary battery, a manufacturing method thereof, and a lithium secondary battery including the same.

As technology development and demand for mobile devices increase, demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries having high energy density and voltage and long cycle life are commercialized and widely used by using a mechanism in which lithium ions are intercalated into the crystal structure of a cathode material or an anode material.

A lithium secondary battery includes a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode to separate them, and an electrolyte that communicates electrochemically with the positive electrode and the negative electrode.

These lithium secondary batteries are typically manufactured by using a lithium-intercalated compound such as LiCoO ₂ or LiMn ₂ O ₄ for the cathode and a non-lithium-intercalated material such as carbon-based or Si-based for the anode, During charging, lithium ions inserted into the positive electrode move to the negative electrode through the electrolyte, and during discharging, lithium ions move from the negative electrode to the positive electrode. During the charging reaction, lithium moving from the positive electrode to the negative electrode reacts with the electrolyte to form a solid electrolyte interface (SEI) layer, which is a kind of passivation film, on the surface of the negative electrode. This SEI layer reduces direct contact between the anode and the electrolyte, preventing additional electrolyte decomposition reactions after the formation of the SEI layer, thereby stabilizing the structure of the anode and minimizing the consumption of the electrolyte solution. This results in the consumption of electrolyte additives and lithium ions at the beginning of activation or cycle. In addition, there is an area in which lithium can be irreversibly consumed depending on the structure of the active material itself, and in this area, lithium ions are not released again after charging, so lithium ions are consumed. That is, lithium consumed by reaction in the negative electrode during the initial activation process does not return to the positive electrode in the subsequent discharging process, reducing the capacity of the battery, and this phenomenon is called irreversible capacity.

As an anode material, carbon-based materials such as graphite are excellent in stability and reversibility, but have limitations in terms of capacity, and Si-based materials with high theoretical capacity are used as anode materials in the field of high capacity. However, the crystal structure of the Si-based material changes according to the intercalation and release of lithium ions during charging and discharging, resulting in rapid volume expansion. Si-based materials accompanied by volume expansion have high initial irreversible capacity, so lithium depletion is severe, resulting in low initial efficiency, as well as poor lifespan characteristics due to additional SEI reactions on the surface of newly exposed active materials as they undergo repeated charging and discharging. .

For example, referring to FIG. 1 showing a charging profile of a lithium secondary battery including a Si-based negative electrode, during charging, lithium ions are intercalated into the Si-based material and Li ₁₂ Si ₇ , Li ₂ Si, Li ₂₁ Si ₈ , Li Although Li _x Si compounds such as ₁₅ Si ₄ and Li ₂₂ Si ₅ are formed, only Li ₂ Si or Li ₂₁ Si ₈ shows reversible charge/discharge behavior, and the others do not.

Therefore, when charging is performed beyond the region of Li ₂ Si or Li ₂₁ Si ₈ showing reversible charge and discharge behavior in the Si-based negative electrode, the thickness change rate of the negative electrode increases due to volume expansion, resulting in deterioration of the conductive network in the negative electrode. From this, an electrical short circuit occurs, resulting in a problem in charge/discharge characteristics. When this phenomenon is accumulated, eventually cracks occur on the surface of the negative electrode active material, and there is a problem in that consumption of the electrolyte solution rapidly increases.

Therefore, the present invention is to solve the above problems, and one object of the present invention is to manufacture a negative electrode for a lithium secondary battery capable of maintaining an excellent conductive network even during volume expansion when a Si-based negative electrode is applied, and the negative electrode for a lithium secondary battery It is to provide a method and a lithium secondary battery including the same.

In order to solve the above technical problem, according to a first aspect of the present invention, in a lithium secondary battery including a positive electrode, a negative electrode, and a separator interposed therebetween, the negative electrode is a Si-containing negative electrode active material, a fibrous conductive material And there is provided a negative electrode for a lithium secondary battery having a negative electrode active material layer of a web structure including a binder.

According to the second aspect of the present invention, the D50 average particle diameter of the Si-containing negative electrode active material in the first aspect may be in the range of 0.1 to 50 μm.

According to the third aspect of the present invention, the fibrous conductive material in the first or second aspect may have an average fiber length longer than the average diameter of the Si-based particles.

According to the fourth aspect of the present invention, the average fiber length of the fibrous conductive material in any one of the first to third aspects may be in the range of 3 to 50 μm.

According to the fifth aspect of the present invention, the average fiber length of the fibrous conductive material in any one of the first to fourth aspects may be in the range of 10 to 20 μm.

According to the sixth aspect of the present invention, the average fiber diameter of the fibrous conductive material in any one of the first to fifth aspects may be in the range of 100 to 500 nm.

According to the seventh aspect of the present invention, the average fiber diameter of the fibrous conductive material in any one of the first to sixth aspects may be in the range of 100 to 200 nm.

According to the eighth aspect of the present invention, in any one of the first to seventh aspects, the fibrous conductive material is carbon nanotubes, carbon nanofibers, a composite of carbon nanotubes and carbon black, and carbon nanofibers and carbon It may include one or a mixture of two or more selected from the group consisting of black composites.

According to the ninth aspect of the present invention, in any one of the first to eighth aspects, a weight ratio between the fibrous conductive material and the Si-containing negative electrode active material may be in the range of 1:2 to 1:7.

According to the tenth aspect of the present invention, in any one of the first to ninth aspects, a weight ratio between the fibrous conductive material and the Si-containing negative electrode active material may be in the range of 1:3 to 1:5.

According to an eleventh aspect of the present invention, in a lithium secondary battery including a positive electrode, a negative electrode, and a separator interposed therebetween, wherein the negative electrode is the negative electrode according to any one of the first to tenth aspects, the lithium secondary battery is provided do.

According to the twelfth aspect of the present invention, the capacity ratio (N / P ratio) of the positive electrode and the negative electrode in the eleventh aspect may be in the range of 1.5 to 3.0.

According to a thirteenth aspect of the present invention, forming a web structure by coating and drying a dispersion medium in which a fibrous conductive material and a binder polymer are dispersed on at least one surface of a current collector; and coating a negative electrode slurry in which a Si-containing negative electrode active material and a binder are dispersed in a dispersion medium on the conductive material web structure. is provided.

A negative electrode for a lithium secondary battery according to one aspect of the present invention is configured in a web form using a fibrous conductive material for a Si-based negative electrode, so that the conductive network can be continuously maintained even when volume expansion of the Si-based negative electrode active material occurs. In addition, a lithium secondary battery manufactured including the negative electrode for a lithium secondary battery may have improved charge/discharge characteristics.

The following drawings attached to this specification illustrate preferred embodiments of the present invention, and serve to further understand the technical idea of the present invention together with the contents of the above-described invention, so the present invention is limited to those described in the drawings. It should not be construed as limiting.
1 is a graph showing a charging profile of a lithium secondary battery including a Si-based negative electrode.
2 and 3 are SEM (scanning electron microscope) images showing the surface and cross-sectional structure of the fibrous conductive material used in Example 1 of the present invention.

Hereinafter, the terms or words used in this specification and claims should not be construed as being limited to ordinary or dictionary meanings, and the inventor appropriately defines the concept of terms in order to explain his/her invention in the best way. It should be interpreted as meaning and concept consistent with the technical idea of the present invention based on the principle that it can be done.

One embodiment of the present invention relates to a negative electrode for a lithium secondary battery including a positive electrode, a negative electrode, and a separator interposed therebetween.

The negative electrode includes a negative electrode active material layer in the form of a web including a Si-containing negative electrode active material, a fibrous conductive material, and a binder.

The negative electrode advantageously has a capacity of 1,200 mAh/g or more, specifically 1,400 mAh/g to 4,000 mAh/g, in order to have a high capacity.

The Si-containing negative electrode active material is a component that causes an electrochemical reaction by combining with lithium ions moving in the positive electrode during a charging reaction, and has a D50 average particle diameter of 0.1 to 50 μm or 0.2 to 30 μm or 0.8 to 10 μm or 1 to 8 μm or 2 to 7 μm.

In the present specification, 'D50 average particle diameter' refers to the representative diameter of particles having two or more types of particle diameters, which corresponds to 50% of the weight percentage in the particle size distribution curve, that is, particles in which 50% of the cumulative weight passes in the particle size distribution curve It refers to the diameter of and is understood as the same meaning as the diameter of the particle corresponding to the size of the sieve passing 50% of the total.

The D50 average particle diameter of the Si-based particles can generally be measured using X-ray diffraction (XRD) analysis or electron microscopy (SEM, TEM).

Examples of such a Si-containing negative electrode active material include Si, SiO, SiO ₂ , Si—C, or a mixture of two or more thereof. In one embodiment, Si may be used alone as an anode active material used in the anode active material layer.

In addition, the negative electrode, if necessary Sn, SnO, SnO ₂ Sn-based materials such as; carbon-based materials such as artificial graphite, natural graphite, amorphous hard carbon, and low crystalline soft carbon; And metal complex oxides such as lithium titanium oxide; may further include one or a mixture of two or more selected from the group consisting of.

The negative electrode is characterized in that it comprises a fibrous conductive material as a conductive material. Here, the fibrous form means a form in which the length is more than 10 times longer than the diameter. For example, the fibrous conductive material used in one embodiment of the present invention may have an average fiber diameter of 10 to 500 nm, specifically 20 to 300 nm. In addition, for example, the fibrous conductive material used in one embodiment of the present invention is 3 to 50 μm, specifically 5 to 30 μm, more specifically 5 to 20 μm, more specifically 10 to 20 μm, more specifically Preferably, it may have an average fiber length of 13 to 17 μm.

Here, the average fiber diameter is a value obtained by arithmetic average of the diameters of vertical planes in the longitudinal direction of the fibrous conductive material, and can be obtained, for example, from an SEM image at a magnification of 2,000 to 50,000 times taken by a field emission scanning electron microscope. . In addition, the average fiber length is an arithmetic average of the lengths of the fibrous conductive material, and can be obtained in the same way as the average fiber diameter.

In addition, it is advantageous in terms of maintaining the conductivity of the Si-containing active material that the fibrous conductive material has an average fiber length longer than the average diameter of the Si-based particles. For example, it is advantageous in forming a conductive material web structure that the fibrous conductive material used in one embodiment of the present invention has a length two or more times or three times longer than the particle diameter or D50 average particle diameter of the active material particles.

Examples of the fibrous conductive material include one or a mixture of two or more selected from the group consisting of carbon nanotubes, carbon nanofibers, composites of carbon nanotubes and carbon black, and composites of carbon nanofibers and carbon black. When these fibrous conductive materials are included in an amount of 3 to 30% by weight, specifically 5 to 25% by weight based on the total content of the negative electrode active material layer, conductivity is maintained even if the volume expansion of the active material occurs during charging and discharging of the battery. It is advantageous to be able to In addition, when the weight ratio of the fiber-type conductive material and the Si-containing negative electrode active material is 1:2 to 1:7, specifically 1:3 to 1:5, a conductive path between the fiber-type conductive materials is secured and the fiber-type conductive material It is advantageous in terms of incorporating the Si-containing negative electrode active material by securing an empty space.

The fibrous conductive material may be included in the negative electrode active material layer to impart conductivity, and Si-based particles, which are active materials, may be positioned between the fibrous conductive materials to form a web structure. Such a web structure induces structural stabilization of the negative electrode active material layer, so that the conductive network can be continuously maintained even when the volume expansion of the Si-based particles occurs.

Therefore, the Si-based negative electrode using the fibrous conductive material as described above can prevent deterioration of the conductive network, thereby suppressing electrical short circuit and improving charge/discharge characteristics.

In the lithium secondary battery according to an embodiment of the present invention, the capacity ratio (N/P ratio) of the positive electrode and the negative electrode, specifically, the capacity ratio of the positive electrode and the negative electrode to the opposing area (the areal capacity ratio of negative to positive electrode) is 1.5 to 1.5. By designing in the range of 3.0, specifically 1.8 to 2.5, the balance between the positive electrode and the Si-based negative electrode can be adjusted. At this time, the N / P ratio is designed by adjusting the ratio of the active material specific capacity and the active material coating amount of the positive electrode / negative electrode facing each other in the battery configuration.

In the present specification, the capacity ratio between the Si-based negative electrode and the positive electrode, that is, N / P ratio, is understood as a value representing the ratio of the capacity per unit area of the negative electrode and the positive electrode (opposite N / P), (capacity per unit area of the negative electrode) / ( capacity per unit area of the anode).

In general, in a lithium secondary battery, the potential of the negative electrode is lowered to near 0V during charging in which lithium ions of the positive electrode are inserted into the negative electrode, and during discharging, the potential of the negative electrode decreases to about 1.5V as the lithium ions of the negative electrode move back to the positive electrode. go up Considering such charging and discharging, it is recommended to set the N/P ratio in the range of approximately 1 to 1.4 so that the potential of the negative electrode does not fall below OV in order to prevent precipitation of lithium metal on the negative electrode opposite to lithium ions released from the positive electrode. It is common.

However, while applying a Si-based negative electrode to a lithium secondary battery, if the N / P ratio is set in the usual range of 1 to 1.4, the compound of Li _x Si generated during the charging process in which lithium ions are inserted into the Si-based material ( For example, among Li ₁₂ Si ₇ , Li ₂ Si, Li ₂₁ Si ₈ , Li ₁₅ Si ₄ and Li ₂₂ Si ₅ ), except for Li ₂ Si or Li ₂₁ Si ₈ , the remaining region accompanies rapid volume expansion, resulting in reversible charging and discharging. The behavior is not seen (see Fig. 1).

For this reason, in the case of a Si-based negative electrode, a region in which volume expansion increases during the intercalation and desorption of lithium during charging and discharging of Si-based particles is excluded, and a region showing reversible charge and discharge behavior due to small volume expansion, such as Li ₂ in FIG. 1 In order to limitly utilize the region of Si or Li ₂₁ Si ₈ , it may be necessary to adjust the N/P ratio to 3 or more.

However, as the lithium secondary battery according to an embodiment of the present invention uses a web-structured Si-based negative electrode using a fibrous conductive material, it is possible to continuously maintain a conductive network even when volume expansion of Si occurs, resulting in N/ Energy density can be increased even if the P ratio is designed in the range of 1.5 to 3.0.

When the N/P ratio in the above range is satisfied, the charging potential of the Si-based negative electrode may be maintained up to 0.08V, and may exhibit, for example, a charging potential of 0.08V to 0.45V. This generally means that charging progresses less than when the charging potential is lowered to near 0V. As a result, the balance between the positive and negative electrodes is controlled, and the volume expansion of the Si-based negative electrode is reduced, which is more advantageous in maintaining excellent charge/discharge characteristics and a conductive network.

For example, even if the lithium secondary battery according to an embodiment of the present invention is overcharged to an SOC of 200% under intended charging and discharging conditions, short circuit does not occur, which means that an excellent conductive network of the Si-based negative electrode is maintained.

If necessary, the Si-based negative electrode used in one embodiment of the present invention may be prelithiated to supplement irreversible capacity. Pre-lithiation may be performed by a conventional method in the art, for example, by attaching lithium metal to an anode active material or an anode layer including the anode active material by deposition or powder coating. As such, the pre-lithiated Si-based negative electrode secures a high initial efficiency of 88% or more and can substantially exhibit a high capacity of 3000 mAh/g or more.

In one embodiment of the present invention, the negative electrode may be prepared by coating at least one surface of a current collector with a negative electrode slurry obtained by dispersing a Si-containing negative electrode active material, a fibrous conductive material, and a binder in a dispersion medium, followed by drying and rolling. there is.

In one embodiment of the present invention, the negative electrode is coated with a dispersion medium in which a fibrous conductive material and a binder polymer are dispersed on at least one surface of a current collector and dried to form a web structure, and then the Si-containing negative electrode active material and the binder are dispersed in the dispersion medium. After coating the negative electrode slurry on the conductive material web structure, it may be prepared through drying and rolling. The dispersion medium in which the fibrous conductive material and the binder polymer are dispersed may further include a dispersant.

The binder is a component that assists in bonding between the conductive material and the active material or the current collector, and is typically included in an amount of 0.1 to 20% by weight based on the total weight of the negative electrode slurry composition. Examples of such binders include polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HEP), polyvinylidenefluoride (PVDF), polyacrylonitrile, polymethyl methacrylate ( polymethylmethacrylate), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, styrene butadiene rubber (SBR), etc. can be heard The carboxymethylcellulose (CMC) may also be used as a thickener for adjusting the viscosity of the slurry.

Meanwhile, water or an organic solvent such as NMP (N-methyl-2-pyrrolidone) may be used as a dispersion medium to obtain a negative electrode slurry by dispersing the negative electrode active material, the binder, and the conductive material, and the solid content concentration in the negative electrode slurry may be 50 to 95% by weight, preferably 70 to 90% by weight.

The current collector used for the negative electrode is not particularly limited as long as it does not cause chemical change in the battery and has conductivity. For example, the surface of copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel. For example, a surface treated with carbon, nickel, titanium, silver, or the like, an aluminum-cadmium alloy, or the like may be used. The thickness of the current collector is not particularly limited, but may have a commonly applied thickness of 3 to 500 μm.

In addition, in the present invention, the coating method of the negative electrode slurry when preparing the negative electrode is not particularly limited as long as it is a method commonly used in the field. For example, a coating method using an impregnation or slot die may be used, and besides this, a Mayer bar coating method, a gravure coating method, a dip coating method, a spray coating method, and the like may be used.

In the lithium secondary battery according to an embodiment of the present invention, the positive electrode is prepared by mixing a positive electrode active material, a conductive material, a binder, and a solvent to prepare a slurry, coating the slurry on at least one surface of a current collector, and then drying and rolling can do.

Active materials used in the cathode include LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ , LiCoPO ₄ , LiFePO ₄ and LiNi _1-xyz Co _x M1 _y M2 _z O ₂ (M1 and M2 are independently Al, Ni, Co, It is any one selected from the group consisting of Fe, Mn, V, Cr, Ti, W, Ta, Mg, and Mo, and x, y, and z are atomic fractions of oxide composition elements independently of each other, and each satisfies a value of 1 to 0.5. and 0<x+y+z<1), and may include any one active material particle selected from the group consisting of or a mixture of two or more of them.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery. For example, carbon black, acetylene black, ketjen black, channel black, farnes black, lamp black, thermal black, etc. carbon 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, and the combined amount of the conductive material used in the positive electrode active material layer and the conductive material used in the negative electrode active material layer is 100 parts by weight of the sum of the solid contents of each of the positive electrode active material layer and the negative electrode active material layer. It is 30 parts by weight or less as a standard, and since as many active materials as possible can be used, it is advantageous to secure battery capacity.

Meanwhile, the binder, the solvent, and the current collector are as described in the negative electrode.

In the lithium secondary battery according to an embodiment of the present invention, the separator is a conventional porous polymer film used as a conventional separator, for example, ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and Porous polymer films made of polyolefin-based polymers such as ethylene/methacrylate copolymers may be used alone or in a laminated manner. 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 a surface of the separator. In addition, conventional porous non-woven fabrics, for example, non-woven fabrics made of high melting point glass fibers, polyethylene terephthalate fibers, etc. may be used, but are not limited thereto.

In the lithium secondary battery according to an embodiment of the present invention, an electrode assembly is formed by interposing a separator between a positive electrode and a negative electrode, the electrode assembly is placed in, for example, a pouch, a cylindrical battery case, or a prismatic battery case, and then the electrolyte is When injected, a lithium secondary battery can be completed. Alternatively, a lithium secondary battery may be completed by laminating the electrode assemblies, impregnating them with an electrolyte, and sealing the resulting product in a battery case.

The electrolyte solution includes a lithium salt as an electrolyte and an organic solvent for dissolving the lithium salt.

The lithium salt may be used without limitation as long as it is commonly used in an electrolyte solution for a secondary battery. For example, as an anion of the lithium salt, F ^- , Cl ^- , 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 ^- , At least one selected from the group consisting of SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- may be used.

Any organic solvent included in the electrolyte may be used without limitation as long as it is commonly used, and representative examples include propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate, dipropyl carbonate, and dimethyl sulfoxide. At least one selected from the group consisting of side, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran may be used.

A lithium secondary battery according to an embodiment of the present invention may be a stack type, a winding type, a stack and folding type, or a cable type.

The lithium secondary battery of the present invention can be used not only as a battery cell used as a power source for a small device, but also can be preferably used as a unit battery in a medium or large battery module including a plurality of battery cells. Preferable examples of the medium-to-large-sized device include an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system. In particular, it is useful for hybrid electric vehicles and batteries for renewable energy storage, which are areas where high power is required. can be used

Hereinafter, examples will be described in detail to aid understanding of the present invention. However, the embodiments according to the present invention can be modified in many different forms, and the scope of the present invention should not be construed as being limited to the following examples. Embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example One:

<Manufacture of cathode>

Carbon nanotubes (average fiber diameter: 150nm, average fiber length: 15㎛, product name: VGCF, manufacturer: Showa-Denko) as a fibrous conductive material, polyacrylic acid (PAA) (weight average molecular weight 110,000, Toyo Chem) as a dispersing agent and binder ) was dispersed in water at a weight ratio of 10:1 to prepare a conductive material slurry. The conductive material slurry was coated on one surface of a copper foil having a thickness of 10 μm and dried to prepare a web structure having conductivity. At this time, the coating amount of the fiber-type conductive material was used in an amount such that the weight ratio of the fiber-type conductive material and the negative electrode active material to be used later was 1:3.

As can be seen in the surface and cross-sectional scanning electron microscope (SEM) images of FIGS. 2 and 3, the conductive web structure has a structure in which fibrous conductive materials are entangled.

Subsequently, Si particles (Wacker) having a D50 average particle diameter of 5 μm as an anode active material and polyacrylic acid (PAA) (weight average molecular weight 250,000, Sigma-Aldrich) as a dispersant and binder were dispersed in water at a weight ratio of 10:1 to form a negative electrode An active material slurry was prepared. An anode was prepared by impregnating the anode active material slurry on a previously prepared conductive web structure, drying and rolling to form a cathode active material layer. The prepared negative electrode was loaded to a capacity of 6.0 mAh/cm ² .

<Manufacture of positive electrode>

Li(Ni _0.5 Co _0.3 Mn _0.2 )O ₂ as a cathode active material, carbon black (Super C, Imerys Graphite & Carbon) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder in a weight ratio of 97:1.5:1.5 A cathode slurry was prepared by dispersing in N-methyl-2-pyrrolidone (NMP). The positive electrode slurry was coated on one surface of aluminum foil having a thickness of 15 μm, dried and rolled to prepare a positive electrode. The prepared positive electrode was loaded to a capacity of 2.9 mAh/cm ² .

<Manufacture of lithium secondary battery>

After interposing a polyolefin-based separator (SRS, LG Chem Co., Ltd.) having a thickness of 12 μm between the Si-based negative electrode and the positive electrode prepared above, ethylene carbonate (EC) and ethylmethyl carbonate (EMC) were mixed at a volume ratio of 30:70 An electrolyte solution in which 1.2M LiPF ₆ was dissolved was injected into the mixed solvent to prepare a lithium secondary battery having an N/P ratio of 2.1 between the positive electrode and the negative electrode, which was obtained by dividing the negative electrode capacity by the positive electrode capacity.

Example 2:

The process of Example 1 was repeated, but the loading amount was increased during manufacture of the cathode to prepare a lithium secondary battery in which the capacity ratio (N/P ratio) of the cathode and anode was adjusted to 1.8.

Example 3:

A lithium secondary battery having a capacity ratio (N/P ratio) of the positive electrode and the negative electrode adjusted to 2.3 was prepared by performing prelithiation in which lithium metal was deposited on the negative electrode prepared in Example 1.

comparative example One:

The same process was performed as in Example 1, except that carbon black (Super C65, spherical particles with a D50 of about 200 nm on a single particle basis, Imerys Graphite & Carbon Co.) was used as the conductive material, and the capacity ratio of the anode and cathode (N/ A lithium secondary battery having a P ratio of 2.1 was prepared.

Experimental example One: charge and discharge Experiment

Initial (one-time) charge and discharge was performed on the lithium secondary batteries of Examples and Comparative Examples using an electrochemical charger and discharger. At this time, all charging and discharging conditions were carried out at 0.1C. Thereafter, charging and discharging continued to check the capacity retention rate. At this time, charging was performed in a constant current/constant voltage (CC/CV) method until a current of 0.7C reached 4.3V and a current of 1/20C, and discharge was performed at 0.5C. It was carried out in a constant current (CC) method up to a voltage of 3.2V. Charging and discharging was performed a total of 300 times.

During the charge/discharge process as described above, the charge termination potential, initial efficiency, and capacity retention rate of the negative electrode included in each battery were measured, and the results are shown in Table 1.

Experimental example 2: Measurement of resistance increase rate

After measuring the AC resistance of the lithium secondary batteries of Examples and Comparative Examples at the initial full charge and at the time of full charge at 300 cycles, the resistance increase rate was confirmed by comparing the measured values, and the results are shown in Table 1. At this time, the lower the resistance increase rate means the better the conductive network.

Example 1 Example 2 Example 3 Comparative Example 1 Cathode Conductive Material (Product Name) Fiber type conductive material (VGCF) fiber type conductive material
(VGCF) fiber type conductive material
(VGCF) Spherical Conductive Material Carbon Black
(Super C65) N/P ratio 2.1 1.8 2.3 2.1 Charge end potential (V) 0.11 0.09 0.12 0.12 Initial efficiency (%) ^{1 )} 90.6 90.4 96.8 90.1 Capacity retention rate (%) ^{2 )} 88.9 86.1 90.6 21.6 resistance increase rate ^{3 )} 31.3 35.9 28.1 229.8 1) Initial efficiency (%) = (discharge capacity in one cycle / charge capacity in one cycle) × 100
2) Capacity retention rate (%) = (Discharge capacity after 300 cycles / Discharge capacity after 1 cycle)Х100
3) Resistance increase rate (%) = [(resistance when fully charged after 300 cycles - resistance when fully charged once)/ resistance when fully charged once] Х100

As can be seen in Table 1, the lithium secondary batteries of Examples 1 to 3 include a negative electrode having a negative electrode active material layer having a web structure using a fibrous conductive material, so that the N/P ratio is in the range of 1.5 to 3.0. When applied as , it was confirmed that the challenge network was excellent. In addition, the charge termination potential of the negative electrode is maintained at 0.09 V or more, and charging is performed only up to a region showing reversible charge and discharge behavior of the Si-based negative electrode, thereby improving charge and discharge characteristics.

In contrast, in Comparative Example 1 using carbon black, a common conductive material, the capacity retention rate was lowered and the resistance increase rate was increased due to the deterioration of the conductive network when the same N/P ratio was applied. The results of Comparative Example 1 indicate that the battery characteristics were maintained up to 200 cycles and then completely degraded thereafter.

Claims (13)

In the negative electrode for a lithium secondary battery comprising a positive electrode, a negative electrode and a separator interposed therebetween,
The negative electrode includes a negative electrode active material layer having a web structure including a Si-containing negative electrode active material, a fibrous conductive material, and a binder,
The fibrous conductive material has an average fiber diameter of 20 to 500 nm and an average fiber length of 10 to 50 μm,
The fibrous conductive material is a negative electrode for a lithium secondary battery included in an amount of 3 to 30% by weight based on the total content of the negative electrode active material layer.
According to claim 1,
A negative electrode for a lithium secondary battery comprising a D50 average particle diameter of the Si-containing negative electrode active material in the range of 0.1 to 50 μm.
According to claim 1,
The fibrous conductive material is a negative electrode for a lithium secondary battery having an average fiber length longer than the average diameter of the Si-containing negative electrode active material.
According to claim 1,
The average fiber length of the fibrous conductive material is a negative electrode for a lithium secondary battery in the range of 13 to 50 μm.
According to claim 1,
The average fiber length of the fibrous conductive material is a negative electrode for a lithium secondary battery in the range of 13 to 17 μm.
According to claim 1,
The average fiber diameter of the fibrous conductive material is a negative electrode for a lithium secondary battery in the range of 100 to 500 nm.
According to claim 1,
The average fiber diameter of the fibrous conductive material is a negative electrode for a lithium secondary battery in the range of 100 to 200 nm.
According to claim 1,
The fibrous conductive material is a negative electrode for a lithium secondary battery including one or a mixture of two or more selected from the group consisting of carbon nanotubes, carbon nanofibers, composites of carbon nanotubes and carbon black, and composites of carbon nanofibers and carbon black. .
According to claim 1,
A negative electrode for a lithium secondary battery in which the weight ratio of the fibrous conductive material and the Si-containing negative electrode active material is in the range of 1:2 to 1:7.
According to claim 1,
A negative electrode for a lithium secondary battery in which the weight ratio of the fibrous conductive material and the Si-containing negative electrode active material is in the range of 1:3 to 1:5.
In a lithium secondary battery including a positive electrode, a negative electrode, and a separator interposed therebetween,
A lithium secondary battery in which the negative electrode is the negative electrode according to claim 1.
According to claim 11,
A lithium secondary battery having a capacity ratio (N/P ratio) of the positive electrode and the negative electrode in the range of 1.5 to 3.0.
Forming a web structure by coating and drying a dispersion medium in which a fibrous conductive material and a binder polymer are dispersed on at least one surface of a current collector; and
Coating a negative electrode slurry in which a Si-containing negative electrode active material and a binder are dispersed in a dispersion medium on the web structure; comprising
A method for manufacturing the negative electrode for a lithium secondary battery according to claim 1.