KR20190136382A - Lithium secondary battery - Google Patents

Abstract

The present invention relates to a lithium secondary battery which includes a positive electrode, a negative electrode, and an electrolyte. The negative electrode includes a mixed negative electrode active material including a silicon (Si)-based negative electrode active material and a carbon-based negative electrode active material. The carbon-based negative electrode active material satisfies equation 1 and equation 2, wherein the equation 1 is 1.4 <= D_P <= 1.6, and the equation 2 is D_p - D_T <= 0.6.

Description

Lithium secondary battery {LITHIUM SECONDARY BATTERY}

The present invention relates to a lithium secondary battery.

Recently, as demand for electronic devices such as mobile devices increases, technology development for mobile devices is expanding. The demand for lithium secondary batteries, such as lithium ion batteries, and lithium ion polymer batteries, has been greatly increased as a power source for driving such electronic devices. In addition, the growth of the electric vehicle market is accelerating as the regulations on automobile fuel economy and exhaust gas are tightened around the world.Along with this, medium and large secondary batteries such as secondary batteries for EVs and secondary batteries for energy storage devices (ESS) are accelerating. Demand for batteries is expected to skyrocket.

Meanwhile, as the negative electrode material of the secondary battery, a carbon-based negative electrode material having excellent cycle characteristics and a theoretical capacity of 372 mAh / g has been generally used. However, as secondary batteries, such as medium and large secondary batteries, are increasingly required, silicon (Si), germanium (Ge) tin (Sn), or antimony having a capacity of 500 mAh / g or more, which can replace the theoretical capacity of a carbon-based negative electrode material, is required. Inorganic negative electrode materials such as (Sb) have attracted attention. Among these inorganic negative electrode materials, the silicon negative electrode material exhibits a very large amount of lithium bonds.

However, the silicon-based negative electrode material may cause pulverization by causing a large volume change during insertion / desorption of lithium, that is, charging and discharging of a battery. As a result, a phenomenon in which the pulverized particles are aggregated may occur, and the negative electrode active material may be electrically detached from the current collector, which may lead to a loss of reversible capacity under a long cycle. For this reason, the silicon-based negative electrode material and the secondary battery including the same have a disadvantage in that they exhibit low cycle life characteristics in spite of their advantages due to their high charge capacity.

Accordingly, while a high-capacity is realized by suppressing the expansion of the silicon-based negative active material, it is necessary to develop a negative active material that can exhibit excellent cycle life characteristics.

One aspect of the present invention is to prevent the loss of the reversible capacity due to the charge and discharge of the silicon-based negative active material during the charge and discharge of the lithium secondary battery, a lithium secondary battery that can implement excellent life characteristics of the lithium secondary battery To provide.

An aspect of the present invention provides a lithium secondary battery including a positive electrode, a negative electrode, and an electrolyte, wherein the negative electrode includes a mixed negative electrode active material including a silicon (Si) -based negative electrode active material, and a carbon-based negative electrode active material, and the carbon-based The negative electrode active material provides a lithium secondary battery that satisfies Expressions 1 and 2 below.

[Equation 1]

1.4≤D _P ≤1.6

[Equation 2]

D _p -D _T ≤0.6

In Equations 1 and 2, D _P is a pellet density (g / cm ³ ) of the carbon-based negative active material, and D _T is a tap density (g / cm ³ ) of the carbon-based negative active material. to be.

The average particle diameter of the silicon-based negative electrode active material and the carbon-based negative electrode active material may satisfy the following Equation 3.

[Equation 3]

1.5≤A _c / A _si ≤4

In Formula 3, A _si is the D50 average particle diameter of the silicon-based negative electrode active material, A _c is the D50 average particle diameter of the carbon-based negative electrode active material.

The lithium secondary battery of one embodiment of the present invention may further satisfy Formula 4 below.

[Equation 4]

0.9≤D _T ≤1.1

In Equation 4, D _T is a tap density (g / cm ³ ) of the carbonaceous anode active material.

Equation 2 may satisfy the following Equation 5.

[Equation 5]

0.3≤D _p -D _T ≤0.6

In Equation 5, D _P is a pellet density (g / cm ³ ) of the carbon-based negative active material, and D _T is a tap density (g / cm ³ ) of the carbon-based negative active material.

In the lithium secondary battery of one embodiment of the present invention, with respect to 100% by weight of the mixed negative electrode active material, it may include 3 to 50% by weight of the silicon-based negative active material, and 50 to 97% by weight of the carbon-based negative electrode active material.

The negative electrode of the lithium secondary battery of one embodiment of the present invention may have a volume expansion ratio of less than 15%.

In the negative electrode of the lithium secondary battery of one embodiment of the present invention, the discharge capacity per unit weight of the mixed negative electrode active material may be 400 to 700 mAh / g.

In the negative electrode of the lithium secondary battery of one embodiment of the present invention, the silicon-based negative active material may be Si, SiO _x (0 <x <2), or silicon-carbon composites.

In the negative electrode of the lithium secondary battery of one embodiment of the present invention, the carbon-based negative electrode active material may be artificial graphite or natural graphite.

In the lithium secondary battery of one embodiment of the present invention, since the expansion of the silicon-based negative active material is suppressed during charge and discharge of the lithium secondary battery, loss of reversible capacity due to charge and discharge can be prevented. Thus, excellent lifespan characteristics of the lithium secondary battery may be realized.

Unless otherwise defined, all terms used in the present specification (including technical and scientific terms) may be used as meanings that can be commonly understood by those skilled in the art. When any part of the specification is to "include" any component, this means that it may further include other components, except to exclude other components unless specifically stated otherwise. In addition, singular forms also include the plural unless specifically stated otherwise in the text.

An aspect of the present invention includes a positive electrode, a negative electrode, and an electrolyte, and the negative electrode includes a mixed negative electrode active material including a silicon (Si) -based negative electrode active material and a carbon-based negative electrode active material, and the carbon-based negative electrode active material is as follows. The lithium secondary battery which satisfy | fills Formula 1 and Formula 2 is provided.

[Equation 1]

1.4≤D _P ≤1.6

[Equation 2]

D _p -D _T ≤0.6

In Equations 1 and 2, D _P is a pellet density (g / cm ³ ) of the carbon-based negative active material, and D _T is a tap density (g / cm ³ ) of the carbon-based negative active material. to be.

Throughout this specification, "pellet density" of the negative electrode active material is 1g of a negative electrode active material into a cylindrical pelletizer having a diameter of 13 mm, and then pressurized for 10 seconds with 3 metric tons to the pelletizer, followed by pelletizer Measure the height of, means the density of the pellet calculated from the height difference with the initial empty pelletizer. In this case, for example, a pressure applied to the pelletizer may be used as a manual type pressure (Carver, 3853-0).

Throughout this specification, "tap density" of the negative electrode active material is filled with 10 g of the negative electrode active material in a 25 ml measuring cylinder, the measuring cylinder is fixed, and tapping and rotation are performed 3,000 times simultaneously. After that, it means the density value measured from the volume after falling. In this case, the fixing of the measuring cylinder may be performed using, for example, a tap device (Quantachrome, Autotap).

The negative electrode of the lithium secondary battery of one embodiment of the present invention includes a mixed negative electrode active material of a silicon-based negative electrode active material and a carbon-based negative electrode active material, and the mixed carbon-based negative electrode active material satisfies Equations 1 and 2 above.

By including the carbon-based negative electrode active material that satisfies Formula 1 and Formula 2 together with the silicon-based negative electrode active material, compared to the case of including a carbon-based negative electrode active material that does not satisfy any one or both of Formula 1 and Formula 2, The volume expansion of the negative electrode is greatly reduced, and the life characteristics of the lithium secondary battery can be greatly improved.

This is also confirmed as a comparison between Examples and Comparative Examples of the present invention described below, when the carbon-based negative electrode active material that satisfies both Formula 1 and Formula 2 is not satisfied at least any one of Formula 1 and Formula 2 Compared with this, the volume expansion of the negative electrode was greatly reduced, and the life characteristics of the lithium secondary battery were greatly improved.

Specifically, in the embodiment of the present invention, when charging and discharging after battery assembly, the volume expansion rate of the negative electrode after charging was very low to less than 15% of the discharge rate, and the capacity retention rate after repeating 300 charge / discharge cycles was 90% or more. Very good life characteristics.

In the negative electrode of the lithium secondary battery of one embodiment of the present invention, the average particle diameter of the silicon-based negative electrode active material and the carbon-based negative electrode active material may satisfy the following Equation 3.

[Equation 3]

1.5≤A _c / A _si ≤4

In Formula 3, A _si is the D50 average particle diameter of the silicon-based negative electrode active material, A _c is the D50 average particle diameter of the carbon-based negative electrode active material.

The "D50 average particle diameter" means the value measured as the volume average value D50 (that is, the particle diameter when the cumulative volume reaches 50%) in the particle size distribution measurement by the laser light diffraction method, and the "average particle diameter" herein. This means the D50 average particle diameter unless otherwise specified.

In the range where the average particle diameter of the silicon-based negative electrode active material and the carbon-based negative electrode active material satisfies Equation 3, in the mixed negative electrode active material of the silicon-based negative electrode active material and the carbon-based negative electrode active material, sufficient voids are secured between the carbon-based negative electrode active material, thereby causing volume expansion of the silicon-based negative electrode active material. As this can be effectively suppressed, volume expansion of the negative electrode can be suppressed, and the life characteristics of the lithium secondary battery can be improved.

The upper limit of Formula 3 may be more specifically 3.8.

Thus, when satisfying the formula 3 together with the formula 1 and formula 2, the low volumetric expansion rate of the negative electrode and excellent life characteristics of the lithium secondary battery can be implemented better.

The negative electrode of the lithium secondary battery of one embodiment of the present invention can further satisfy the following formula (4).

[Equation 4]

0.9≤D _T ≤1.1

In Equation 4, D _T is the tap density of the carbon-based negative electrode active material, which is the same as the previous definition, and when more satisfies Equation 4, it was confirmed that the low volumetric expansion rate of the negative electrode and excellent life characteristics of the lithium secondary battery.

In the negative electrode of the lithium secondary battery of one embodiment of the present invention, Equation 2 may satisfy the following Equation 5 as a more specific range.

[Equation 5]

0.3≤D _p -D _T ≤0.6

In Equation 5, D _P and D _T are the pellet density and the tap density of the carbon-based negative electrode active material, respectively, as defined above. When Equation 5 is satisfied, the low volumetric expansion rate of the negative electrode and the excellent life characteristics of the lithium secondary battery were confirmed. The lower limit of Formula 5 may be more specifically 0.32, and the upper limit may be more specifically 0.58.

In the negative electrode of the lithium secondary battery of one embodiment of the present invention, the mixed negative electrode active material may include 3 to 50% by weight of the silicon-based negative active material, and 50 to 97% by weight of the carbon-based negative electrode active material with respect to 100% by weight of the mixed negative electrode active material. have. More specifically, it may include 5 to 30% by weight of the silicon-based negative active material, and 70 to 95% by weight of the carbon-based negative electrode active material based on 100% by weight of the mixed negative electrode active material. By including a mixed negative electrode active material of the silicon-based negative electrode active material and the carbon-based negative electrode active material in the above range can be implemented a low volume expansion rate of the negative electrode and excellent life characteristics of the lithium secondary battery. However, the present invention is not necessarily limited thereto.

The negative electrode of the lithium secondary battery of one embodiment of the present invention may have a volume expansion ratio of less than 15% during charge and discharge.

In this case, the volume expansion rate is the same as using the above-described negative electrode as the negative electrode, and using a Li metal as the positive electrode in each of the six coin cells (coin cell) in the same manner, the battery voltage is 0.01V (vs. Apply a constant current until Li) and when the battery voltage reaches 0.01V, charge it with a constant voltage until the current reaches 0.01C rate, and then disassemble three coin cells to measure the thickness of the electrode in SOC100 state. The three coin cells were discharged at a constant current of 0.1C rate until the voltage reached 1.5V (vs. Li), and then disassembled to determine the average value of the change rate calculated by measuring the thickness of the electrode in SOC0 state. have.

Specifically, a low volume expansion ratio of 14% or less, more specifically 1% or more and 14% or less, 5% or more and 14% or less, 10% or more and 14% or less, 10.5% or more and 13.8% or less may be implemented.

In the lithium secondary battery negative electrode of one embodiment of the present invention, the discharge capacity per unit weight of the mixed negative electrode active material may be 400 to 700 mAh / g, more specifically may be 500 to 700 mAh / g. As the negative electrode of the lithium secondary battery of one embodiment of the present invention includes a mixed negative electrode active material as described above, low volume expansion rate and excellent discharge capacity characteristics may be simultaneously implemented.

Here, the discharge capacity means the initial discharge capacity, and a constant current is applied until the battery voltage reaches 0.01 V (vs. Li) at a current of 0.1 C rate at 25 ° C., and the current reaches 0.01 C rate when the battery voltage reaches 0.01 V. After charging by applying a constant voltage until it reaches to 1.5V (vs. Li) it may mean the capacity measured by discharging at a constant current of 0.1C rate until the voltage reaches 1.5V (vs. Li).

The lithium secondary battery of one embodiment of the present invention may have a high discharge capacity and have a small volume expansion ratio of the negative electrode, thereby improving lifetime characteristics of the lithium secondary battery, thereby enabling high-capacity long term implementation.

In the negative electrode of the lithium secondary battery of one embodiment of the present invention, the silicon-based negative active material may be Si, SiO _x (0 <x <2) or silicon-carbon composites, but the present invention is not limited thereto. no.

In the negative electrode of the lithium secondary battery of one embodiment of the present invention, the carbon-based negative electrode active material may be, for example, a crystalline carbon-based material, and specifically, may be natural graphite or artificial graphite. More specifically, it may be artificial graphite, and since artificial graphite can manufacture a relatively high density electrode due to high particle strength, the capacity density per volume of the negative electrode may be increased, and the life characteristics of the battery may contribute to improvement. Can be. However, the present invention is not necessarily limited thereto.

In the negative electrode of the lithium secondary battery of one embodiment of the present invention, the average particle diameter of the carbon-based negative electrode active material is not particularly limited, but is 500 nm or more and 100 μm or less, specifically 1 μm or more and 40 μm or less, more specifically 5 μm or more. It may be 30 μm or less.

In the negative electrode of the lithium secondary battery of one embodiment of the present invention, the average particle diameter of the silicon-based negative electrode active material is not particularly limited, but is 500 nm or more and 100 μm or less, specifically 1 μm or more and 40 μm or less, more specifically 3 μm or more. 20 μm or less.

The negative electrode of the lithium secondary battery of one embodiment of the present invention includes a current collector and a negative electrode active material layer formed on the current collector, and the negative electrode active material layer may include the mixed negative electrode active material of one embodiment of the present invention described above.

In the negative electrode of the lithium secondary battery of one embodiment of the present invention, after mixing and stirring a solvent, a negative electrode binder, and a conductive material in the mixed negative electrode active material as necessary, to prepare a slurry, it is applied to a current collector, compressed and dried It can be produced by forming a negative electrode active material layer on the current collector. Since the description of the mixed negative electrode active material is as described above, it will be omitted.

Hereinafter, the current collector, the negative electrode binder, and the conductive material will be described in more detail. However, the present invention is not limited thereto.

The negative electrode binder adheres the negative electrode active material particles to each other well, and also serves to adhere the negative electrode active material to the current collector well. As the binder, a water-insoluble binder, a water-soluble binder or a combination thereof can be used.

The water-insoluble binder includes polyvinyl chloride, carboxylated polyvinylchloride, polyvinyl fluoride, polymers including ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride , Polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

Examples of the water-soluble binder include styrene-butadiene rubber, acrylated styrene-butadiene rubber, polyvinyl alcohol, sodium polyacrylate, propylene and olefin copolymers having 2 to 8 carbon atoms, and (meth) acrylic acid and (meth) acrylic acid alkyl esters. Copolymers, or a combination thereof.

When using the water-soluble binder as the negative electrode binder, it may further include a cellulose-based compound that can impart viscosity. As this cellulose type compound, carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, these alkali metal salts, etc. can be used in mixture of 1 or more types. Na, K or Li may be used as the alkali metal.

 The conductive material is used to impart conductivity to the electrode, and any battery can be used as long as it is an electronic conductive material without causing chemical changes in the battery. For example, natural graphite, artificial graphite, carbon black, acetylene black, and ketjen. Carbon-based materials such as black, carbon fiber and carbon nanotubes; Metal materials such as metal powder or metal fibers such as copper, nickel, aluminum and silver; Conductive polymers such as polyphenylene derivatives; Or a conductive material containing a mixture thereof.

In addition, the current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with conductive metal, and combinations thereof. Can be.

The positive electrode may include a current collector and a positive electrode active material layer positioned on the current collector. Al or Cu may be used as the current collector, but is not limited thereto.

As the cathode active material, a compound (lithiated intercalation compound) capable of reversible intercalation and deintercalation of lithium may be used. Specifically, as the lithium metal oxide, for example, one or more of a complex oxide of metal and lithium selected from cobalt, manganese, nickel, and combinations thereof known in the art may be used, and is not particularly limited to a specific composition. Does not.

The cathode active material layer may further include a cathode binder and a conductive material.

The binder adheres the positive electrode active material particles to each other well, and also serves to adhere the positive electrode active material to the current collector well, and representative examples thereof include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, and polyvinyl. Chloride, carboxylated polyvinylchloride, polyvinylfluoride, polymer comprising ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene- Butadiene rubber, acrylic styrene-butadiene rubber, epoxy resin, nylon and the like can be used, but is not limited thereto.

The conductive material is used to impart conductivity to the electrode, and any battery can be used as long as it is an electronic conductive material without causing chemical changes in the battery. For example, natural graphite, artificial graphite, carbon black, acetylene black, and ketjen. Metal powders such as black, carbon fiber, carbon nanotubes, copper, nickel, aluminum, silver, and metal fibers may be used, and conductive materials such as polyphenylene derivatives may be used alone or in combination of one or more thereof. However, the present invention is not limited thereto.

The lithium secondary battery may be a nonaqueous electrolyte secondary battery, wherein the nonaqueous electrolyte may include a nonaqueous organic solvent and a lithium salt.

The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the cell can move.

The non-aqueous organic solvent and the lithium salt may employ a material commonly used in the lithium secondary battery art, and are not limited to specific materials.

In addition, as mentioned above, a separator may exist between the anode and the cathode. As the separator, polyethylene, polypropylene, polyvinylidene fluoride or two or more multilayer films thereof may be used, and polyethylene / polypropylene two-layer separator, polyethylene / polypropylene / polyethylene three-layer separator, polypropylene / polyethylene / poly Of course, a mixed multilayer membrane such as a propylene three-layer separator can be used, and the like is not particularly limited.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred examples of the present invention and the present invention is not limited to the following examples.

[Assessment Methods]

Initial discharge capacity

At 25 ° C., a constant current was applied until the battery voltage reached 0.01 V (vs. Li) at a current of 0.1 C rate, and charged with a constant voltage until the current reached 0.01 C rate when the battery voltage reached 0.01 V. At the time of discharge, the battery was discharged at a constant current of 0.1 C rate until the voltage reached 1.5 V (vs. Li).

-Volume expansion rate

Six coin cells were fabricated in the same manner using Li metal as the anode, and a constant current was applied until the battery voltage reached 0.01V (vs. Li) at a current of 0.1C rate. When the voltage reaches 0.01V, the battery is charged by applying a constant voltage until the current reaches 0.01C rate. After discharging the three coin cells, the thickness of the electrode in SOC100 state is measured with a micrometer, and the three coin cells have a voltage of 1.5. It is the average value of the rate of change calculated by measuring the thickness of the electrode in the SOC0 state after discharging at a constant current of 0.1 C rate until V (vs. Li).

Life characteristics (capacity retention)

All cells are manufactured using the same anode and have a capacity of about 20 Ah. Then, 1C charge / 1C discharge c-rate is used to evaluate the life (35 ℃) in a chamber maintained at a constant temperature set within the range of DOD90. It was.

 [Examples and Comparative Examples]

As shown in Table 1, a negative electrode active material was prepared by mixing SiO negative active material and artificial graphite, and a binder was prepared by mixing 2.5: 1.5 of styrene-butadiene rubber (SBR): carboxymethyl cellulose (CMC) as an aqueous binder. As a conductive material, a carbon nanotube (CNT) dispersion was prepared.

The prepared negative electrode active material: binder: conductive material was mixed in a weight ratio of 95: 4: 1, and then dispersed in water to prepare a negative electrode slurry. The negative electrode slurry was coated on an 8 μm thick copper thin film, dried in an 80 ° C. oven for about 2 hours, and then rolled to a density of the negative electrode active material layer of 1.6 g / cc, and further dried in a 110 ° C. vacuum oven for 12 hours. A negative electrode for a secondary battery was prepared.

In the prepared cathode, a lithium foil, a porous polyethylene film separator as a counter electrode, and electrolyte solution are ethylene carbonate (EC) containing 1M LiPF ₆ in / ethyl methyl carbonate (EMC) / diethyl carbonate (DEC) / Using a solution in which fluoroethylene carbonate (FEC) was mixed at a volume ratio of 2/2/5/1, the CR2016 coin-type half cell was prepared according to a commonly known manufacturing process.

In addition, in order to fabricate a cell having a capacity of about 20 Ah, LiNi _0.8 Co _0.1 Mn _0.1 O ₂ (average particle diameter (D ₅₀ ) = 12 μm) was used, and Denka Black and flake-shaped black were used as the conductive material. A positive electrode slurry was prepared by using a KS6 as a linking conductive material and PVDF as a binder, and having a mass ratio of 96.5: 1: 1: 1.5, respectively, and coating, drying, and pressing a 12 μm thick aluminum substrate to prepare a positive electrode. .

The positive electrode plate and the negative electrode plate prepared above are notched and stacked in appropriate sizes, respectively, and a cell is formed between the positive electrode plate and the negative electrode plate through a separator (polyethylene, 13 μm thick), and the tab portion of the positive electrode and the negative electrode Each tab part was welded. The welded anode / separator / cathode combination was placed in a pouch and sealed on three surfaces except the electrolyte injection surface. In this case, the tabbed portion is included in the sealing portion. The lithium secondary battery was manufactured by injecting the electrolyte into the remaining portion, sealing the remaining side, and impregnating for more than 24 hours.

Electrolyte was prepared by preparing a 1M LiPF ₆ solution with a mixed solvent of EC / EMC / DEC (volume ratio 25:45:30), then 7wt% fluoroethylene carbonate (FEC), 0.5wt 1,3-propenesultone (PRS) %, 0.5 wt% of lithium bis (oxalato) borate (LiBOB) and 0.5 wt% of ethylene sulfate (ESA) were used.

division Artificial graphite SiO - Tap Density (D _T )
g / cm ³ Pellet  Density (D _P )
g / cm ³ D _P -D _T Average particle size (A _c )
μm Amount
g Average particle size (A _si )
μm Amount
g A _c / A _si Example  One 0.93 1.43 0.5 15 4 6 0.75 2.5 Example  2 1.03 1.55 0.52 12 4 6 0.75 2 Example  3 1.01 1.59 0.58 23 4 6 0.75 3.8 Example  4 1.1 1.42 0.32 9 4 6 0.75 1.5 Comparative example  One 1.02 1.37 0.35 15 4 6 0.75 2.5 Comparative example  2 1.1 1.65 0.55 26 4 6 0.75 4.3 Comparative example  3 0.9 1.52 0.62 15 4 6 0.75 2.5 Comparative example  4 0.93 1.69 0.76 8 4 6 0.75 1.3

In Table 1, the pellet density (D _P ) is 1g of a negative electrode active material in a cylindrical pelletizer having a diameter of 13 mm, and then the pelletizer was changed to 3 metric tons using manual type pressure (Carver, 3853-0). After applying pressure for a second, the height of the pelletizer is measured and the density of the pellet calculated from the height difference from the initial empty pelletizer.

In Table 1, the tap density (D _T ) is obtained by filling 10 g of the negative electrode active material in a 25 ml measuring cylinder, and then fixing the measuring cylinder to a tap device (Quantachrome, Autotap), and simultaneously tapping and rotating. It is the density measured from the volume after falling after having performed 3000 times.

Table 2 shows the initial discharge capacity of the coin-type half cell including the cathodes prepared in Examples 1 to 4 and Comparative Examples 1 to 4, and the charge / discharge 300 times of the volume expansion rate (SOC100 / SOC0) and the 20Ah-class full cell, respectively. Dose retention rate after repeated performance is shown.

division Discharge capacity Volume expansion rate Capacity maintenance rate mAh / g % , SOC100 / SOC0 %, @ 300CY Example 1 502 11.7 92.5% Example 2 503 13.8 91.6% Example 3 500 12.5 90.7% Example 4 505 10.5 93.4% Comparative Example 1 495 15 87.0% Comparative Example 2 498 20 78.0% Comparative Example 3 493 18 80.0% Comparative Example 4 501 22 75.0%

In Table 2, for Examples 1 to 4 where the pellet density of artificial graphite is 1.4 to 1.6 and the difference between the pellet density and the tap density is 0.6 or less, compared with Comparative Examples 1 to 4 which do not satisfy at least one of the two. It can be seen that the volume expansion rate of the prepared negative electrode was small and the capacity retention rate was very excellent.

In addition, when the average particle diameter of the artificial graphite to the average particle diameter of SiO satisfies the range of 1.5 to 4, it can be seen that the volume expansion rate of the negative electrode and the capacity retention rate after 300 cycles are better.

Specifically, when comparing the comparative example 1 and the comparative example 2 in which the difference between the pellet density and the tap density of the artificial graphite is 0.6 or less and the pellet density is outside the range of 1.4 to 1.6, the artificial graphite with respect to the average particle diameter of SiO In Comparative Example 2, which does not satisfy the average particle diameter of 1.5 to 4, it can be seen that the volume expansion ratio of the negative electrode is higher than that of Comparative Example 1 satisfying the above range, and the capacity retention ratio is inferior.

In addition, although the pellet density of artificial graphite is in the range of 1.4 to 1.6, but the difference in pellet density and tap density is greater than 0.6 and the difference in pellet density and tap density is 0.6 or less, the pellet density of artificial graphite is 1.4 to 1.6 Comparing Comparative Example 2 outside the range of Comparative Example 3 and Comparative Example 2, neither of the difference between the pellet density and the tap density and the range of the pellet density, in Comparative Example 3 with respect to the average particle diameter of SiO Since the average particle diameter of the artificial graphite satisfies 1.5 to 4, the volume expansion rate and capacity retention rate of the negative electrode were better than those of Comparative Example 2, which was not.

In addition, in the case of Comparative Example 4 in which the difference between the pellet density and the tap density of the artificial graphite and the pellet density were outside the scope of the present invention, and the average particle diameter of the artificial graphite relative to the average particle diameter of SiO was also 1.5 to 4, the volume expansion rate of the negative electrode Was the largest and the capacity maintenance rate was inferior.

Claims (9)

Lithium secondary battery comprising a positive electrode, a negative electrode, and an electrolyte,
The negative electrode includes a mixed negative electrode active material including a silicon (Si) -based negative electrode active material, and a carbon-based negative electrode active material,
The carbon-based negative electrode active material satisfies the following formula 1 and formula 2.
[Equation 1]
1.4≤D _P ≤1.6
[Equation 2]
D _p -D _T ≤0.6
(In the above formula 1 and formula 2, D _P is the pellet density of the carbon-based negative electrode active material ^{(pellet density, g / cm 3} ), D T is the tap density of the carbon-based negative electrode active material (tap density, g / cm ³ )to be.) In claim 1,
An average particle diameter of the silicon-based negative electrode active material and the carbon-based negative electrode active material satisfies Equation 3 below.
[Equation 3]
1.5≤A _c / A _si ≤4
(In Formula 3, A _si is the D50 average particle diameter of the silicon-based negative electrode active material, A _c is the D50 average particle diameter of the carbon-based negative electrode active material.) In claim 1,
The lithium secondary battery which satisfies following formula 4 further.
[Equation 4]
0.9≤D _T ≤1.1
(In Formula 4, D _T is a tap density (g / cm ³ ) of the carbon-based negative electrode active material.) In claim 1,
A lithium secondary battery satisfying the following formula 5.
[Equation 5]
0.3≤D _p -D _T ≤0.6
In Equation 5, D _P is a pellet density (g / cm ³ ) of the carbon-based negative active material, and D _T is a tap density (g / cm ³ ) of the carbon-based negative active material. ) In claim 1,
A lithium secondary battery comprising 3 to 50% by weight of the silicon-based negative active material and 50 to 97% by weight of the carbon-based negative electrode active material with respect to 100% by weight of the mixed negative electrode active material. In claim 1,
A lithium secondary battery having a volume expansion ratio of the negative electrode less than 15%. In claim 1,
A lithium secondary battery having a discharge capacity per unit weight of the mixed negative electrode active material is 400 to 700mAh / g. In claim 1,
The silicon-based negative active material is Si, SiO _x (0 <x <2), or a lithium secondary battery (Si / C composites). In claim 1,
The carbon-based negative electrode active material is artificial graphite or natural graphite lithium secondary battery.