KR102534215B1 - Lithium Secondary Battery Comprising Si Anode - Google Patents

Abstract

The present invention provides a lithium secondary battery including a positive electrode, a negative electrode, and a separator interposed therebetween, wherein the negative electrode includes a Si-based positive electrode active material and the initial efficiency of the positive electrode is lower than that of the negative electrode.

Description

Lithium secondary battery including a Si-based negative electrode {Lithium Secondary Battery Comprising Si Anode}

The present invention relates to a lithium secondary battery including a Si-based negative electrode.

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 have been commercialized and widely used.

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), which is a kind of passivation film on the surface of the negative electrode. This SEI can stabilize the structure of the anode by preventing the decomposition reaction of the electrolyte by suppressing the movement of electrons required for the reaction between the anode and the electrolyte, while it consumes lithium ions because it is an irreversible reaction. That is, lithium consumed by the formation of SEI 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. In the field of high capacity, attempts to use Si-based materials with high theoretical capacity as anode materials are increasing. .

On the other hand, as an effort to improve the performance of a lithium secondary battery, the development of a technology for accelerating the charging speed is required. In order to rapidly charge a lithium secondary battery, the lithium ion movement speed must be fast during the process of lithium ion insertion into the negative electrode. A battery design is being made to achieve high output by reducing internal resistance by forming and increasing conductivity.

However, it is difficult to realize a high capacity with an anode having such a thin film anode active material layer. In addition, it is difficult to implement rapid charging in a negative electrode having a high capacity.

Accordingly, the present invention is to solve the above problems, and one object of the present invention is to provide a lithium secondary battery capable of realizing both rapid charging and high capacity.

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-based negative electrode, and the initial efficiency of the positive electrode is lower than the initial efficiency of the negative electrode. A battery is provided.

According to the second aspect of the present invention, the negative electrode of the first aspect of the present invention may have a capacity of 1,200 mAh/g or more.

According to the third aspect of the present invention, the negative electrode in the first or second aspect of the present invention may have a capacity of 1,400 mAh/g to 4,000 mAh/g.

According to the fourth aspect of the present invention, the initial efficiency of the positive electrode in any one of the first to third aspects of the present invention may be 4 to 7% lower than the initial efficiency of the negative electrode.

According to the fifth aspect of the present invention, the initial efficiency of the negative electrode in any one of the first to fourth aspects of the present invention may be 90% or more.

According to the sixth aspect of the present invention, the negative electrode active material of the negative electrode in any one of the first to fifth aspects of the present invention may be made of Si, Si—C, or a mixture thereof.

According to the seventh aspect of the present invention, in any one of the first to sixth aspects of the present invention, the positive electrode may include lithium nickel-cobalt-manganese (NCM) oxide having a single crystal structure as a positive electrode active material.

According to an eighth aspect of the present invention, in any one of the first to seventh aspects of the present invention, the positive electrode includes LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _{1 -y} Co _y O as a positive electrode active material ₂ (O<y<1), LiCo _{1 - y} Mn _y O ₂ (O<y<1), LiNi _{1 - y} Mn _y O ₂ (O<y<1), LiMn _{2 - z} Ni _z O ₄ , LiMn _{2 -z} Co _z O4(O<z<2), LiCoPO ₄ And LiFePO ₄ It may further include one or a mixture of two or more selected from the group consisting of.

According to a ninth aspect of the present invention, in any one of the first to eighth aspects of the present invention, two or more types of conductive materials may be used in the negative electrode active material layer of the negative electrode.

According to a tenth aspect of the present invention, in any one of the first to ninth aspects of the present invention, the negative electrode may include a negative electrode active material layer having a thickness of 20 to 80 μm.

According to an eleventh aspect of the present invention, in any one of the first to tenth aspects of the present invention, the negative electrode may further include a carbon coating layer on the negative electrode active material particle.

The lithium secondary battery according to the present invention uses a Si-based active material as an anode active material, but simultaneously realizes high capacity and rapid charging by providing a negative electrode active material layer and lowering the initial efficiency of the positive electrode compared to the negative electrode.

In a preferred aspect of the present invention, when a Si-based active material having a negative electrode capacity of 1,200 mAh/g or more is used as the negative electrode active material in the lithium secondary battery according to the present invention, simultaneous realization of high capacity and rapid charging can be achieved more remarkably. .

In another preferred embodiment of the present invention, the lithium secondary battery according to the present invention uses a Si-based active material having a negative electrode capacity of 1,200 mAh / g or more as a negative electrode active material and includes lithium nickel-cobalt-manganese oxide having a single crystal structure as a positive electrode active material In this case, simultaneous realization of high capacity and rapid charging can be achieved more remarkably.

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 shows charge and discharge potential profiles of lithium secondary batteries according to Example 2 and Comparative 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.

A lithium secondary battery according to an embodiment of the present invention includes a positive electrode, a negative electrode, and a separator interposed therebetween, and the positive electrode and the negative electrode are designed to simultaneously satisfy high capacity and rapid charging.

In one embodiment of the present invention, the negative electrode may have a capacity of 1,200 mAh/g or more, specifically 1,400 mAh/g to 4,000 mAh/g. When the negative electrode has a capacity of less than 1,200 mAh/g, it may be difficult to realize a high-capacity negative electrode.

In addition, in one embodiment of the present invention, the negative electrode may have a negative electrode active material layer having a thickness of 20 to 80 μm, specifically 30 to 70 μm. When the negative electrode active material layer has a thickness within the above range, a high-capacity negative electrode may be more easily realized. In conventional lithium secondary batteries for rapid charging, rapid charging is achieved by reducing the diffusion distance of lithium by forming the thickness of the negative electrode active material layer to a thin film level. However, in this case, since the loading amount of the negative electrode active material layer is too low, there is a limit to realizing a high-capacity battery.

Therefore, in a preferred embodiment of the present invention, the minimum thickness of the negative electrode active material layer is designed to be 20 μm or more to secure a high capacity of 1,200 mAh / g or more, while minimizing the moving distance of lithium ions during rapid charging. The upper limit of the thickness may be limited to 80 μm.

The negative electrode active material layer is formed by preparing a slurry by mixing a negative electrode active material capable of imparting a high capacity as a negative electrode active material, a binder, a conductive material, and a dispersion medium, coating the slurry on at least one surface of the current collector, and then drying and rolling. can

An anode active material usable in the present invention may include a Si-based compound having an initial efficiency of 88% or more or 90% or more. Preferably, the negative electrode active material usable in the present invention may consist only of a Si-based compound having an initial efficiency of 88% or more. Non-limiting examples of the Si-based compound include Si, Si-C, or mixtures thereof, but are not limited thereto.

The Si-based compound having an initial efficiency of 88% or more may have an average D50 particle diameter of 0.8 to 10 µm, or a D50 average particle diameter of 1 to 8 µm, or a D50 average particle diameter of 2 to 7 µm. The Si-based compound having an average particle diameter of D50 within the above numerical range is advantageous in terms of forming a conductive network even when the conductive material is used in the same amount as compared to a Si-based compound having an average particle diameter larger or smaller than this.

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 passing through 50% of the cumulative weight 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 average particle diameter of the particles can generally be measured using X-ray diffraction (XRD) analysis or electron microscopy (SEM, TEM).

The Si-based compound having an initial efficiency of 88% or more may be applied alone to the active material constituting the negative electrode active material layer. When a graphite-based negative electrode active material is applied, it is difficult to develop battery capacity, and it is difficult to set a reversible range due to mismatch in charge and discharge potential.

In addition to the negative electrode active material, other Si-containing materials, such as SiO, SiO ₂ or mixtures thereof, within a trace amount that does not affect the desired effect in the present invention, for example, within a small amount that does not lower the initial efficiency of the negative electrode; Sn-based materials such as Sn, SnO, and SnO2; carbon-based materials such as artificial graphite, natural graphite, amorphous hard carbon, and low crystalline soft carbon; metal composite oxides such as lithium titanium oxide; Alternatively, a mixture of two or more of these may be used as the negative electrode active material.

Such negative electrode active materials may be used in an amount of 50 to 80% by weight based on the total weight of the negative electrode slurry based on the solid content when preparing the negative electrode.

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

The conductive material is a carbon-based conductive material having conductivity without causing chemical change in the battery. For example, it may be one or a mixture of two or more selected from the group consisting of carbon black, acetylene black, ketjen black, channel black, farnes black, lamp black, thermal black, carbon nanofibers, and carbon nanotubes. Considering the characteristics of the Si-based negative electrode, it is preferable to use a mixture of two or more materials having different shapes and particle sizes as the conductive material. As a non-limiting example, it can be used in the form of a mixture of at least one selected from carbon black, acetylene black, ketjen black, channel black, farnes black, lamp black and thermal black, and at least one selected from carbon nanofibers and carbon nanotubes. there is.

The conductive material may be included in the range of 15 to 40% by weight based on 100% by weight of the solids constituting the negative electrode active material layer. When the conductive material is used in an amount within the above numerical range, a sufficient conductive network may be formed and a desired battery capacity may be exhibited.

Meanwhile, water or an organic solvent such as N-methyl-2-pyrrolidone (NMP) may be used as a dispersion medium for dispersing the negative electrode active material, the binder, and the conductive material to obtain a negative electrode slurry. In the case of using an aqueous composition using water as a dispersion medium, it may be used in an amount such that the solid content concentration in the negative electrode slurry is 20 to 55% by weight, preferably 30 to 50% by weight.

The current collector is not particularly limited as long as it does not cause chemical change in the battery and has conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, carbon, nickel on the surface of copper or stainless steel , titanium, one surface-treated with silver, etc., an aluminum-cadmium alloy, etc. 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 for preparing the negative electrode is not particularly limited as long as it is a method commonly used in the art. For example, a coating method using a slot die may be used, and other than that, a Mayer bar coating method, a gravure coating method, a dip coating method, a spray coating method, and the like may be used.

In one embodiment of the present invention, the positive electrode includes lithium nickel-cobalt-manganese (NCM) oxide as a positive electrode active material.

The lithium nickel-cobalt-manganese (NCM) oxide is Li _{1 + x} (Ni _a Co _b Mn _C ) O ₂ (0.97≤x ≤1.06, 0<a<1, 0<b<1, 0<c<1 , a+b+c=1) Ni-Co-Mn ternary positive electrode active material, Li _1+x (Ni _a Co _b Mn _C )O ₄ (0.97≤x ≤1.06, 0<a<2, 0 <b<2, 0<c<2, a+b+c=2) Ni-Co-Mn three-component cathode active material, or a mixture thereof, wherein the nickel-containing oxide has high capacity and the manganese-containing oxide has thermal conductivity. It is a composite that combines the advantages of stability and good electrochemical properties of cobalt-containing oxides. The lithium nickel-cobalt-manganese (NCM) oxide may be doped with a small amount of a metal element to the extent that meets the purpose of the present invention. For example, Li _{1 + x} (Ni _a Co _b Mn _C ) O ₂ (0.97≤x ≤1.06, 0<a<1, 0<b<1, 0<c<1, a+b+c=1) or Li _1+x (Ni _a Co _b Mn _C )O ₄ (0.97≤x ≤1.06, 0<a<2, 0<b<2, 0<c<2, a+b+c=2) Each of the lithium nickel-cobalt-manganese (NCM) oxides represented by the Ni-Co-Mn ternary positive electrode active material is independently, but is not limited to, Na, K, Mg, Ca, Sr, Ni, Co, Ti, Al, It may be doped with one or two or more metals selected from the group consisting of Si, Sn, Mn, Cr, Fe, V, and Zr. The content of the metal doped into the lithium nickel-cobalt-manganese (NCM) may be within a range that does not significantly increase anode resistance. For example, the doping metal in lithium nickel-cobalt-manganese (NCM) oxide may be in the range of 10 to 1500 ppm, 50 to 1000 ppm, or 100 to 500 ppm based on each element, but is not limited thereto. As a non-limiting example, lithium nickel-cobalt-manganese (NCM) oxide generally uses 2 to 3 types of elements as doping metals, and the doping metals may be present in hundreds of ppm based on each element.

In general, a lithium cobalt oxide-based positive electrode in a lithium secondary battery has an initial efficiency value of (discharge capacity in one cycle / charge capacity in one cycle) × 100 higher than that of the negative electrode due to the intrinsic charge and discharge characteristics of the material.

However, considering the operating range of the negative electrode, especially the Si-based negative electrode, it is advantageous to control the initial efficiency of the positive electrode lower than that of the negative electrode for rapid charging of the secondary battery. Here, rapid charging means that the time required to reach SOC (state of charge) 70% when charging at a certain C-rate, for example, 2.4C or higher, specifically 3C or higher, is within 30 minutes, preferably reaching SOC 80%. This means that it takes less than 20 minutes.

Therefore, in the present invention, among NCM-based positive electrode active materials, NCM-based positive electrode active materials obtained under conditions in which crystal growth of particles is induced through over-firing by controlling the firing temperature to a high level have low initial efficiency and fast CV arrival time. The initial efficiency of the positive electrode may be higher than the initial efficiency of the negative electrode during charging and discharging of the secondary battery. In one embodiment of the present invention, the cell can be designed such that the initial efficiency of the positive electrode is higher than the initial efficiency of the negative electrode, but the initial efficiency of the positive electrode is 90% or less. In another embodiment of the present invention, the cell can be designed such that the initial efficiency of the positive electrode is higher than the initial efficiency of the negative electrode, but the initial efficiency of the Si-based negative electrode is greater than 88% or greater than 90%. In another embodiment of the present invention, the initial efficiency of the negative electrode minus the initial efficiency of the positive electrode is 0.1 to 12% or 1 to 12% or 0.1 to 10% or 1 to 10% or 1 to 7%. to 6 to 7%. When the positive electrode and the negative electrode satisfy this initial efficiency range, the use period of the negative electrode is moved to a more stable region, and rapid charging and high capacity can be achieved at the same time. In addition, by rapidly bringing the CV arrival point through this, the charging speed at the end of charging, in which lithium precipitation is concerned, is automatically lowered, thereby suppressing the generation of by-products in the charging process.

In the present specification, the term 'efficiency' means the amount of discharge capacity compared to the charge capacity. Discharge capacity is understood to mean a value of (discharge capacity of n cycles / charge capacity of n cycles) x 100, measured three times based on a 0.1C current condition.

The charge/discharge measurement is performed at room temperature, that is, at 25°C.

In addition, it is advantageous for the potential to rise rapidly in order to rapidly charge the lithium secondary battery. That is, when the positive electrode resistance is designed to be high enough, the burden on the negative electrode can be reduced by controlling the overflow of lithium ions to the negative electrode. In the case of rapid charging, it is advantageous to have a fast diffusion rate of lithium ions, but if the diffusion rate of lithium ions is excessive, a problem of lithium precipitation in the negative electrode may occur.

Therefore, in the secondary battery according to an embodiment of the present invention, the CV arrival point during 3C charging in the first charging cycle was controlled to be 75% or less of the total SOC and maintained at 60% or more of the total SOC after 25 cycles.

Referring to the term 'CV arrival point' used in the present specification, a charging process generally proceeds in two steps: constant current (CC) and constant voltage (CV). When charging is carried out with the set current, the potential gradually rises, and when it finally reaches the set upper limit voltage, charging continues while maintaining the voltage and reducing the current. The 'CV arrival time' means the time point at which CC charging ends, that is, the time point at which first-stage charging ends. Since this CV arrival point corresponds to high-rate charging, it is important in the art that the CV arrival point region is appropriately controlled. In the present specification, when the CV arrival time is 75% or less of the total SOC, it means that 75% or less of the total SOC is charged by the first-stage CC charging and the remaining 25% or more of the SOC is charged by the second-stage CV charging.

In order to obtain the resistance of the cathode in the above range, it is preferable that the lithium nickel-cobalt-manganese (NCM) oxide used as the cathode active material is in a single crystal form. This is because lithium nickel-cobalt-manganese (NCM) oxide, which is in the form of a single crystal, has the characteristic of advancing the CV arrival point to 75% or less, reducing the burden imposed on the late stage of charging and preventing lithium precipitation. A non-limiting example of the single-crystal form of lithium nickel-cobalt-manganese (NCM) oxide refers to primary particles formed by a solid phase method, but excludes lithium nickel-cobalt-manganese (NCM) oxide formed as secondary particles. It is not.

The single-crystal lithium nickel-cobalt-manganese (NCM) oxide is preferably included in an amount of 30 to 90 wt% or 40 to 85 wt% based on 100 wt% of the cathode active material. When the single-crystal form of lithium nickel-cobalt-manganese (NCM) oxide is used in the cathode active material within the above content range, a cathode that achieves the cathode efficiency and resistance level desired in the present invention and secures electrode rolling processability can be obtained. there is.

In addition to the lithium nickel-cobalt-manganese (NCM) oxide as described above, the positive electrode can be LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _1-y as long as the positive electrode satisfies the initial efficiency of 90% or less. Co _y O ₂ (O<y<1), LiCo _{1 - y} Mn _y O ₂ (O<y<1), LiNi _{1 - y} Mn _y O ₂ (O<y<1), LiMn _{2 - z} Ni _z O ₄ , LiMn _{2 -z} Co _z O4(O<z<2), LiCoPO ₄ And LiFePO ₄ It may further include one or a mixture of two or more selected from the group consisting of.

The conductive material used for the cathode is not particularly limited as long as it has conductivity without causing chemical change to the battery, and examples thereof include carbon black, acetylene black, ketjen black, channel black, farnes black, lamp black, and thermal paste. carbon black such as 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; One or a mixture of two or more selected from the group consisting of conductive materials such as polyphenylene derivatives may be used. The conductive material may be added in an amount of 0.1 to 20% by weight based on the total weight of the positive electrode slurry composition.

The positive electrode may be prepared by preparing a slurry by mixing a positive electrode active material, a conductive material, a binder, and a dispersion medium, and then coating the slurry on at least one surface of the current collector. The binder and solvent may be used the same as those used in preparing the negative electrode. can

The lithium secondary battery according to one embodiment of the present invention is configured with the positive electrode and the negative electrode as described above, so that high capacity can be implemented and rapid charging that reaches SOC 80% within 20 minutes when charging at 2.4C or higher is possible.

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 A porous polymer film made of polyolefin-based polymer, which is one or a mixture of two or more selected from the group consisting of 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, and 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 ^- , One or a mixture of two or more 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. One or a mixture of two or more 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>

Si particles (Wacker Co., Ltd.) with a D50 average particle diameter of 5 ㎛ as a negative electrode active material, graphite-based conductive material (KS6L, Timcal) and carbon black (Super C, Imerys Graphite & Carbon Co.) as a conductive material, styrene-butadiene rubber (Styrene-butadiene rubber) as a binder SBR) (BM-L302, Zeon Co.) and carboxymethyl cellulose (CMC) (BG-L02, GL Chem Co.) as a thickener were dispersed in water at a weight ratio of 70:20:5:3.5:1.5 to obtain an anode slurry. .

The negative electrode slurry was coated on one side of a 10 μm-thick copper foil, dried and rolled to prepare a negative electrode (capacity: 2380 mAh/g) having a negative electrode active material layer having a thickness of 32 μm.

<Manufacture of positive electrode>

Li(Ni _0.5 Co _0.2 Mn _0.3 )O ₂ (S740, manufacturer: Ronbay, initial efficiency: 84%) in the form of single crystal primary particles obtained through an over-firing step as a cathode active material, and carbon black (Super C as a conductive material) , Imerys Graphite & Carbon Co.) and polyvinylidene fluoride (PVDF) as a binder were dispersed in N-methyl-2-pyrrolidone (NMP) at a weight ratio of 97:1.5:1.5 to prepare a cathode slurry. The positive electrode slurry was coated on one side of aluminum foil having a thickness of 15 μm, dried and rolled to prepare a positive electrode having an 80 μm thick positive electrode active material layer formed thereon.

<Manufacture of lithium secondary battery>

A polyolefin-based separator (SRS, LG Chem Co., Ltd.) having a thickness of 12 μm was interposed between the cathode and anode prepared above, and then ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed at a volume ratio of 30:70. An electrolyte solution in which 1.2M LiPF ₆ was dissolved in a solvent was injected to prepare a lithium secondary battery having an initial efficiency of 84% for the positive electrode and an initial efficiency of 91% for the negative electrode.

Example 2:

Li(Ni _0.5 Co _0.2 Mn _0.3 )O ₂ (S740, manufacturer: Ronbay Co., initial efficiency: 84%) and LiCoO ₂ ( A lithium secondary battery was prepared in the same manner as in Example 1, except that LC10S, manufacturer: L&F new material, initial efficiency: 97%) was used in a 1:1 mixing ratio (wt/wt).

Example 3:

A lithium secondary battery was manufactured in the same manner as in Example 1, except that the surface of the negative electrode active material was treated with a carbon coating layer to form a carbon coating layer to a thickness of 5 nm during the preparation of the negative electrode.

comparative example One:

Li(Ni _0.5 Co _0.3 Mn _0.2 )O ₂ (N502, manufacturer: LG Chem Co., Ltd., initial efficiency: 88%) and LiCoO ₂ (LC10S, L&F new material, initial efficiency: 97%) were used in a 1:1 mixing ratio (wt/wt), and a lithium secondary battery was prepared by performing the same procedure as in Example 1. did

comparative example 2:

In the manufacture of the cathode, Li(Ni _0.6 Co _0.2 Mn _0.2 )O ₂ (34LG, manufacturer: Nichia, initial efficiency: 92%) and LiCoO ₂ (LC10S, L&F new material, initial efficiency) in the form of polycrystalline secondary particles were used as cathode active materials. : 97%) was used in a 1:1 mixing ratio (wt/wt), and a lithium secondary battery was prepared in the same manner as in Example 1.

Experimental example One: lithium secondary battery performance evaluation

For the manufactured lithium secondary battery, the initial (one-time) charge/discharge was performed under CC/CV conditions at 3.0 C for charging and under CC conditions for discharging at 1.0 C, and then the initial efficiency of the positive and negative electrodes (1 cycle of It was calculated as a value of discharge capacity / charge capacity of one cycle) × 100.

In addition, while charging and discharging under the above conditions, the potential of the battery was simultaneously measured.

Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Initial efficiency of anode (%) 84 90 84 93 95 Initial efficiency of cathode (%) 91 91 90 91 91 Initial CV reached SOC 72 69 72 69 78 CV reached SOC after 25 cycles 71 68 71 33 26 Capacity retention rate after 25 cycles (%) 98 99 98 98 84

As can be seen in Table 1, Examples 1 to 3 maintain fast charging performance by stably controlling the initial CV arrival time and after 25 cycles, while Comparative Example 1 satisfied the initial CV arrival time, but 25 It was confirmed that the rapid deterioration of the rapid charging performance after the cycle was confirmed, and in Comparative Example 2, both the initial and after 25 cycles, the CV control range was out of range, and it was confirmed that it was not appropriate in terms of not only rapid charging but also capacity retention rate.

In addition, from FIG. 1, it can be confirmed in more detail the CV change during charging and discharging according to Example 2 and Comparative Example 1, and even in the case of Comparative Example 1, the capacity decrease is somewhat gradual, but the CV arrival point after 25 cycles is rapidly It can be seen that the fast charging performance is not maintained.

Claims (11)

In a lithium secondary battery including a positive electrode, a negative electrode, and a separator interposed therebetween,
The cathode is a Si-based cathode,
The negative electrode has a capacity of 1,200 mAh / g or more,
The negative electrode includes a negative electrode active material layer having a thickness of 30 to 70 μm,
Including two or more kinds of carbon-based conductive materials in the negative electrode active material layer of the negative electrode,
The initial efficiency of the positive electrode is lower than the initial efficiency of the negative electrode,
The positive electrode includes lithium nickel-cobalt-manganese (NCM) oxide having a single crystal structure as a positive electrode active material,
In the first cycle of charging the lithium secondary battery, the point at which the current voltage (CV), which is the upper limit voltage set during 3C charging, is reached is 75% or less of the total SOC and at the same time maintained at 60% or more of the total SOC after 25 cycles, the lithium secondary battery .
delete According to claim 1,
A lithium secondary battery wherein the negative electrode has a capacity of 1,400 mAh/g to 4,000 mAh/g.
According to claim 1,
A lithium secondary battery in which the initial efficiency of the positive electrode is 1 to 7% lower than the initial efficiency of the negative electrode.
According to claim 1,
The initial efficiency of the negative electrode is a lithium secondary battery of 90% or more.
According to claim 1,
A lithium secondary battery in which the anode active material of the anode is made of Si, Si-C, or a mixture thereof.
delete According to claim 1,
The positive electrode is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNi _1-y Co _y O ₂ (O<y<1), LiCo _1-y Mn _y O ₂ (O<y<1) as a positive electrode active material. ), LiNi _1-y Mn _y O ₂ (O<y<1), LiMn _2-z Ni _z O ₄ , LiMn _2-z Co _z O4 (O<z<2), LiCoPO ₄ and LiFePO ₄ A lithium secondary battery further comprising one or a mixture of two or more selected from
delete delete According to claim 1,
A lithium secondary battery further comprising a carbon coating layer on the negative electrode active material layer of the negative electrode.

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