WO2017142261A1 - Negative electrode manufacturing method and negative electrode - Google Patents

Abstract

The present invention relates to a negative electrode manufacturing method and a negative electrode manufactured by the manufacturing method. Particularly, the present invention provides a negative electrode manufacturing method and a negative electrode manufactured by the method, which comprises: a first step of manufacturing a plurality of negative electrode samples comprising the same composition of active material layers and having different electrode densities; a second step of measuring a negative electrode expansion curve that follows a SOC at a first charging cycle with regard to each negative electrode sample; a third step of measuring, provided that x refers to an SOC value having "1" as the inclination of a tangent to the measured negative electrode expansion curve (but x<50), the difference between inclination values of tangents to the curve at (x-5) and (x+5); a fourth step of selecting an optimal electrode density such that the measured difference between inclination values satisfies the range of 0-0.5; and a fifth step of manufacturing a negative electrode in such a condition that the selected optimal electrode density is satisfied. According to the present invention, negative electrode samples having different electrode densities are manufactured, and, if the rate of change of the inclination value of a tangent to a negative electrode expansion curve that follows first charging of the negative samples satisfies a specific value in a range in which the expansion curve increases and in which the initial SOC is less than 50%, a secondary battery manufactured using the negative electrode that has the above electrode density can exhibit the most excellent life characteristics and initial efficiency in connection with the corresponding active material.

Description

Cathode Manufacturing Method and Cathode

Citation with Related Applications

This application claims the benefit of priority based on Korean Patent Application No. 2016-0017214 dated February 15, 2016 and 2017-0016846 dated February 7, 2017, and all contents disclosed in the documents of the Korean patent application are Included as part of the.

Technical Field

The present invention relates to a method for producing a negative electrode and a negative electrode produced by the above production method.

Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy or clean energy is increasing, and the most actively researched fields are power generation and storage using electrochemical reactions.

A representative example of an electrochemical device using such electrochemical energy is a secondary battery, and its use area is gradually increasing. Recently, as the development and demand for portable devices such as portable computers, portable telephones and cameras increases, the demand for secondary batteries as a source of energy is rapidly increasing. Many researches have been conducted on this long, low self-discharge rate lithium battery and are commercially available and widely used.

In general, a secondary battery is composed of a positive electrode, a negative electrode, and an electrolyte, and transfers energy while reciprocating both electrodes such that lithium ions from the positive electrode active material are inserted into a negative electrode active material such as carbon particles and are detached again during discharge by the first charge. Since it plays a role, it becomes possible to charge and discharge.

For example, a lithium secondary battery has a structure in which a lithium electrolyte is impregnated into an electrode assembly including a cathode including a lithium transition metal oxide as an electrode active material, a cathode including a carbon-based active material, and a porous separator. The positive electrode is prepared by coating a positive electrode mixture containing a lithium transition metal oxide on an aluminum foil, and the negative electrode is prepared by coating a negative electrode mixture including a carbon-based active material on a copper foil.

On the other hand, in order to achieve high capacity of the secondary battery, it is essential to implement a high-density electrode in order to utilize as many negative electrode active materials as possible in a unit volume while maintaining performances such as lifespan and resistance.

Conventionally, attempts have been made to implement the high density electrode by rolling as much as possible without considering the physical properties of the negative electrode active material, but this may cause peeling, cracking, etc. of the active material, causing increased resistance and reduced lifetime characteristics.

Accordingly, the development of a method for improving the performance of the battery through the implementation of the electrode density that can exhibit the optimum performance according to the type of the negative electrode active material is required.

The first technical problem to be solved of the present invention, by measuring the expansion curve of the plurality of negative electrode samples, by calculating the optimum electrode density according to the type of the negative electrode active material can be both excellent efficiency and life characteristics of the secondary battery It is to provide a method for producing a negative electrode.

The second technical problem to be solved of the present invention is to provide a negative electrode manufactured according to the manufacturing method of the negative electrode.

The third technical problem to be solved of the present invention is to provide a secondary battery, a battery module and a battery pack including the negative electrode.

In order to solve the above problems, the present invention includes a first step of preparing a plurality of negative electrode samples including the active material layer of the same composition, having a different electrode density; A second step of measuring a negative electrode expansion curve according to the SOC in the first charging cycle for each negative electrode sample; When the SOC value of the tangent slope with respect to the measured cathodic expansion curve is x (where x <50), the difference between the slope values of the tangent lines with respect to the curve at x-5 and x + 5 is measured. The third step; A fourth step of selecting an optimal electrode density such that the difference in the measured tilt values satisfies a range of 0 to 0.5; And a fifth step of manufacturing the negative electrode under conditions satisfying the selected optimal electrode density.

In addition, the present invention is a negative electrode prepared according to the manufacturing method of the negative electrode, the negative electrode is graphite composed of secondary particles of low graphite degree as an active material, the electrode density of the negative electrode is 1.3 g / cc to 1.5 g / cc Provide a cathode.

In addition, the present invention is a negative electrode prepared according to the manufacturing method of the negative electrode, the negative electrode is graphite composed of secondary particles of high graphite degree as an active material, the electrode density of the negative electrode is 1.4 g / cc to 1.7 g / cc Provide a cathode.

Furthermore, the present invention provides a secondary battery including the negative electrode, the positive electrode, a separator interposed between the negative electrode and the positive electrode, and an electrolyte, and a battery module and a battery pack including the same as a unit cell.

According to the present invention, after preparing a plurality of negative electrode samples having an active material layer of the same composition at different electrode densities, the negative electrode expansion curve according to the first charge cycle of each negative electrode sample is measured, and using the negative electrode expansion curve By calculating the optimum electrode density of the composition to produce a negative electrode, it is possible to manufacture a secondary battery having excellent life characteristics and initial efficiency.

1 is a graph showing the expansion curve of the negative electrode samples prepared by Examples 1 to 7 and Comparative Examples 1 to 3 of the present application.

2 is a graph showing the capacity characteristics according to the cycle of each of the negative electrode samples prepared by Example 2 and Comparative Examples 1, 2 of the present application.

3 is a graph showing the capacity characteristics according to the cycle of each of the negative electrode samples prepared by Examples 4, 6 and Comparative Example 3 of the present application.

Hereinafter, the present invention will be described in more detail to aid in understanding the present invention.

The terms or words used in this specification and claims are not to be construed as being limited to their ordinary or dictionary meanings, and the inventors may appropriately define the concept of terms in order to best describe their invention. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that the present invention.

The terminology used herein is for the purpose of describing exemplary embodiments only and is not intended to be limiting of the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

As used herein, the terms "comprise", "comprise" or "have" are intended to indicate that there is a feature, number, step, component, or combination thereof, that is, one or more other features, It should be understood that it does not exclude in advance the possibility of the presence or addition of numbers, steps, components, or combinations thereof.

Specifically, the method of manufacturing a negative electrode according to the present invention includes a first step of preparing a plurality of negative electrode samples including active material layers having the same composition, and having different electrode densities; A second step of measuring a negative electrode expansion curve according to the SOC in the first charging cycle for each negative electrode sample; When the SOC value of the tangent slope with respect to the measured cathodic expansion curve is x (where x <50), the difference between the slope values of the tangent lines with respect to the curve at x-5 and x + 5 is measured. The third step; A fourth step of selecting an optimal electrode density such that the difference in the measured tilt values satisfies a range of 0 to 0.5; And a fifth step of manufacturing the negative electrode under conditions satisfying the selected optimal electrode density.

Hereinafter, a method of manufacturing a negative electrode according to the present invention will be described in detail for each step.

First, a plurality of negative electrode samples including active material layers having the same composition but having different electrode densities are prepared (first step). Specifically, the preparation of the negative electrode may be prepared by applying a negative electrode slurry made by mixing a negative electrode active material, a conductive material and a binder mixed in an organic solvent on a negative electrode current collector, followed by drying and rolling. In this case, the electrode density may be adjusted in the drying and rolling steps after applying the negative electrode slurry.

Usually, the expansion curve of the negative electrode may vary according to the type of active material in the negative electrode, and in particular, may vary according to the powder compression density of the active material. By using the method of the present application, the optimum electrode density can be calculated according to the type of the active material, and the negative electrode having excellent life characteristics and initial efficiency characteristics can be manufactured using the same.

Here, the electrode density means the amount of the negative electrode copolymer coated in the same volume.

The powder compact density refers to the amount of the negative electrode active material coated in the same volume when 3 g of the negative electrode active material is compressed to 1000 kg under a powder compression density measurement condition of 10 mm. Further, the negative electrode active material having a powder pressing density of 1.75 g / cc does not refer to the negative electrode active material obtained by performing the compression treatment, but has a physical property value of a press density of 1.75 g / cc when the negative electrode active material is subjected to the compression treatment test. It means the negative electrode active material which has a.

The active material layer may include an active material, a conductive material, and / or a binder, and may include a graphite-based active material. Among the graphite-based active materials, particularly, natural graphite and artificial graphite have a staging phenomenon due to the insertion of lithium ions into the mesh, and thus the negative electrode expansion curve shows an S-shaped opening and a thick expanding opening. For example, the active material layer may include graphite composed of secondary particles having low graphitization degree or graphite having high graphite degree secondary particles as an active material.

Meanwhile, manufacturing a plurality of negative electrode samples having different electrode densities in the first step may be performed by, for example, a method of manufacturing a plurality of negative electrode samples for each electrode density by varying the pressure at the time of manufacturing the negative electrode sample. Can be.

Specifically, the negative electrode sample may be prepared at an electrode density of 1.3 g / cc to 1.8 g / cc. For example, the negative electrode sample may be coated with a negative electrode slurry prepared by mixing a negative electrode mixture including an active material, a conductive material, and a binder in an organic solvent on a negative electrode current collector, and then dried in a range of 200 kg / 5 cm to 2000 kg / 5 cm. By rolling at different pressures, a negative electrode sample having the above electrode density can be prepared. At this time, the powder compression density characteristics and the electrode density characteristics are in proportion. However, in the method of manufacturing the negative electrode by different electrode densities, the electrode density of the negative electrode and the type of the negative electrode active material are not limited as described above, and after selecting the negative electrode active material to be prepared as the negative electrode, By preparing a plurality of negative electrode samples, the optimum negative electrode density according to the type of the negative electrode active material can be calculated.

The conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery. Examples of the conductive material include graphite such as natural graphite and artificial graphite; Carbon blacks such as carbon black, acetylene black, ketjen black, channel black, farnes black, lamp black and thermal black; Conductive fibers such as carbon fibers and metal fibers; Conductive tubes such as carbon nanotubes; Metal powders such as fluorocarbon, aluminum and nickel powders; Conductive whiskers such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials, such as a polyphenylene derivative, etc. can be used, Specifically, acetylene black can be used.

The binder is polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (polyvinylidenefluoride), polyacrylonitrile, polymethylmethacrylate, polymethylmethacrylate, poly Vinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), liquor Various types of binder polymers can be used, such as fonned EPDM, styrene butadiene rubber (SBR), fluorine rubber, poly acrylic acid and polymers in which hydrogen thereof is replaced with Li, Na or Ca, or various copolymers. In particular, CMC and SBR can be used.

Next, for each of the negative electrode samples prepared in the first step, the negative electrode expansion curve according to the SOC in the first charging cycle is measured (second step).

Specifically, in order to measure the expansion curve of each of a plurality of negative electrode samples having different electrode densities prepared in the first step, a coin-type half secondary battery including the negative electrode sample may be manufactured. For example, a metal lithium foil is used as a positive electrode, a plurality of negative electrode samples prepared in the first step are used as a negative electrode, an electrode assembly is prepared through a separator between the positive electrode and the negative electrode, and an electrolyte solution is injected. To produce a coin-type half secondary battery.

As described above, a coin type half secondary battery was manufactured for each of the plurality of negative electrode samples prepared in the first step, and then charged to measure the expansion curve of the negative electrode showing the change in thickness of the negative electrode according to SOC during charging. can do.

In this case, the thickness change of the SOC and the cathode may be measured by a real time thickness measurement method through a spring type real time displacement measuring device.

Next, when the SOC value of the tangent slope with respect to the measured cathodic expansion curve is x (where x <50), the difference of the slope of the tangent line with respect to the curve at x-5 and x + 5 is different. Measure (step 3).

Among the cathode expansion curves measured in the second step, the optimum electrode density can be found through the section in which the SOC value is less than 50%. For example, when analyzing a section in which the SOC value exceeds 50%, it is difficult to analyze the characteristics according to the slope because the change of the slope is not sudden but appears constant.

For example, when the slope of the tangent to the measured cathodic expansion curve is less than 50% SOC of 1 and the SOC is 25%, the slope of the expansion curve at 20% of the SOC is 20 g (20 ), The slope of the expansion curve at 30% SOC can be represented by g (30), the difference in the slope value of the curve can be represented by g (20)-g (30).

Subsequently, an optimal electrode density is selected such that the difference in the measured slope values satisfies the range of 0 to 0.5 (step 4).

For example, when the difference between the measured tilt values is greater than 0.5, the electrode is overcompressed, and thus there is a problem that the initial efficiency is low and the swelling is severe due to poor life characteristics.

Therefore, when the difference in the slope value of the curve is in the range of 0 to 0.5, it has an optimal electrode density corresponding to a specific negative electrode active material, and thus has a lifespan characteristic due to problems such as peeling or cracking of the active material that appeared when the electrode density is large. Problems such as a decrease in resistance and an increase in resistance can be prevented, and conversely, problems such as a decrease in capacity of the battery, which can occur when the electrode density is low, can be prevented.

Finally, a cathode is manufactured under the conditions satisfying the selected optimal electrode density (step 5).

When the optimal electrode density selected as described above is satisfied, a negative electrode having excellent life characteristics, efficiency, and capacity characteristics may all be manufactured, and a secondary battery having excellent life characteristics and efficiency may be manufactured including the same. The secondary battery may include a separator and an electrolyte interposed between the negative electrode, the positive electrode, the negative electrode and the positive electrode.

In this case, the negative electrode may be manufactured in the same manner as the manufacturing process of the negative electrode sample in the first step. Specifically, the negative electrode mixture including the active material, the conductive material, and the binder may be mixed with an organic solvent to prepare a negative electrode slurry, and then the negative electrode slurry may be applied onto a current collector, dried, and rolled.

Specifically, the negative electrode may include graphite composed of secondary particles having a low graphitization degree or graphite composed of secondary particles having a high graphitization degree as an active material.

When the negative electrode active material is graphite composed of secondary particles having a low graphitization degree, an optimum electrode density of the negative electrode may be 1.3 g / cc to 1.5 g / cc. When the electrode density of a negative electrode including graphite composed of secondary particles having low graphite degree as an active material is less than 1.3 g / cc, the lifetime characteristics and initial efficiency of the battery including the electrode are low, and thus the electrode density is not suitable. If is more than 1.5 g / cc, the electrode may be over-compressed to increase the resistance or to decrease the life characteristics.

When the negative electrode active material is graphite composed of secondary particles having a high graphitization degree, the optimum electrode density of the negative electrode may be 1.4 g / cc to 1.7 g / cc. When the electrode density of the negative electrode including graphite composed of secondary particles having high graphite degree as an active material is less than 1.4 g / cc, the lifespan characteristics and initial efficiency of the battery including the electrode are low, and thus the electrode density is not suitable. If is greater than 1.7 g / cc, the electrode may be over-compressed to increase the resistance or decrease the life characteristics.

In addition, the conductive material and the binder may be the same as described above, specifically, the conductive material may include acetylene black, the binder may include CMC and SBR.

The active material, the conductive material and the binder may be included in a weight ratio of 95 to 95.5: 0.5 to 1.5: 3.5 to 4, and specifically, may be included in a weight ratio of 95.3: 1: 3.7.

In addition, the positive electrode may be prepared by applying a slurry prepared by mixing a positive electrode mixture including an active material, a conductive material, and a binder to an organic solvent on a current collector, followed by drying and rolling.

The positive electrode active material is, in addition to lithium nickel manganese composite oxide (LNMO), for example, layered compounds such as lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ); Lithium transition metal composite oxide substituted with a transition metal such as Li (Ni _a Co _b Mn _c ) O ₂ (0 <a <1, 0 <b <1, 0 <c <1, a + b + c = 1) ; Lithium manganese oxides such as LiMnO ₃ , LiMn ₂ O ₄ , LiMnO ₂ ; Lithium copper oxide (Li ₂ CuO ₂ ); Vanadium oxides such as LiV ₃ O ₈ , V ₂ O ₅ , Cu ₂ V ₂ O _7, and the like; LiFe ₃ O ₄ ; LiFePO ₄ , Lithium phosphate such as LiCoPO ₄ , LiFe _x Mn _{1 - x} PO ₄ ; Ni-site type lithium nickel oxide represented by the formula LiNi _{1 - x} MxO ₂ , wherein M is Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x is 0.01 to 0.3; Formula LiMn _{2 - x} MxO ₂ , wherein M is Co, Ni, Fe, Cr, Zn or Ta, x is 0.01 to 0.1, or Li ₂ Mn ₃ MO ₈ , where M is Fe, Co, Ni, Lithium manganese composite oxides represented by Cu or Zn) may be used together, but are not limited thereto.

The conductive material and the binder may be the same as or different from that used for the negative electrode active material.

In addition, the electrolyte may include a non-aqueous organic solvent and a metal salt.

Examples of the non-aqueous organic solvent include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, and gamma Butyl lactone, 1,2-dimethoxy ethane, tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxorone, formamide, dimethylformamide, dioxolon , Acetonitrile, nitromethane, methyl formate, methyl acetate, phosphate triester, trimethoxy methane, dioxorone derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbo Aprotic organic solvents such as nate derivatives, tetrahydrofuran derivatives, ethers, methyl pyroionate and ethyl propionate can be used.

The metal salt may be a lithium salt, the lithium salt is a material that is good to dissolve in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF _{3 SO 3, LiCF 3 CO 2,} LiAsF 6, LiSbF 6, LiAlCl 4, CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, 4-phenyl Lithium borate, imide and the like can be used.

The separator is a conventional porous polymer film conventionally used as a separator, for example, polyolefin-based, such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer The porous polymer film made of a polymer may be used alone or by laminating them, or a conventional porous nonwoven fabric, for example, a non-woven fabric made of high melting point glass fiber, polyethylene terephthalate fiber, or the like may be used. no.

Further, according to another embodiment of the present invention, a battery module including the secondary battery as a unit cell and a battery pack including the same are provided. Since the battery module and the battery pack include the secondary battery having excellent capacity, efficiency characteristics and lifespan characteristics, a medium-large device selected from the group consisting of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, and a power storage system It can be used as a power source.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily practice the present invention. As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention.

Example  One

Preparation of Cathode Sample

Graphite composed of secondary particles of low graphitization degree having a powder compression density of 1.75 g / cc was prepared. 95.3% by weight of graphite composed of secondary particles of low graphitization degree, N-methyl-2P as a solvent of a negative electrode mixture containing 1% by weight of acetylene black series carbon particles as a conductive material and 3.7% by weight of CMC and SBR as a binder. A negative electrode slurry was prepared by adding to Rollidone (NMP). The negative electrode slurry was applied to a copper thin film, which is a negative electrode current collector having a thickness of 10 μm, and dried to prepare a negative electrode sample, followed by roll pressing.

At this time, the loading amount of the negative electrode was 250 mg / 25 cm ² and roll press to 200 kg / 5 cm so that the electrode density of the negative electrode is 1.3 g / cc.

[Manufacture of Secondary Battery]

Using the negative electrode sample prepared above, a spring was installed on the top plate to prepare a secondary battery capable of real-time thickness measurement according to the expansion of the negative electrode. A real-time displacement measuring device was installed on the top plate to measure the thickness change according to charge and discharge.

In this case, an LCO-based cathode active material of 1.8 cm ² was used as an anode, and an electrode assembly was prepared through a polyethylene separator between the cathode and the anode. A non-aqueous electrolyte was prepared by adding 1 M LiPF ₆ to a nonaqueous electrolyte solvent in which ethylene carbonate and diethyl carbonate were mixed at a volume ratio of 1: 2. Was prepared.

Example  2

Except for rolling press at 500 kg / 5 cm to obtain an electrode density of 1.4 g / cc of a negative electrode sample prepared using graphite composed of low graphitization secondary particles having a powder compaction density of 1.75 g / cc as an active material. In the same manner as in Example 1, a negative electrode and a secondary battery including the same were prepared.

Example  3

Except for rolling press at 800 kg / 5 cm to obtain an electrode density of 1.5 g / cc of the negative electrode sample prepared by using graphite composed of low graphitization secondary particles having a powder compression density of 1.75 g / cc as an active material. In the same manner as in Example 1, a negative electrode and a secondary battery including the same were prepared.

Example  4

A negative electrode sample was prepared using graphite composed of high graphitization secondary particles having a powder compaction density of 1.95 g / cc as an active material, wherein the negative electrode was rolled at 200 kg / 5 cm to obtain an electrode density of 1.4 g / cc. Except for pressing, a negative electrode and a secondary battery including the same were manufactured in the same manner as in Example 1.

Example  5

A negative electrode sample was prepared using graphite composed of high graphitized secondary particles having a powder compaction density of 1.95 g / cc as an active material, wherein the negative electrode was roll-pressed at 400 kg / 5 cm to obtain an electrode density of 1.5 g / cc. A negative electrode and a secondary battery including the same were manufactured in the same manner as in Example 1, except that.

Example  6

A negative electrode sample was prepared using graphite composed of high graphitization secondary particles having a powder compaction density of 1.95 g / cc as an active material, wherein the negative electrode was rolled at 700 kg / 5 cm to have an electrode density of 1.6 g / cc. Except for pressing, a negative electrode and a secondary battery including the same were manufactured in the same manner as in Example 1.

Example  7

A negative electrode sample was prepared using graphite composed of high graphitization secondary particles having a powder compression density of 1.95 g / cc as an active material, wherein the negative electrode was rolled at 900 kg / 5 cm so that the electrode density of the negative electrode sample was 1.7 g / cc. Except for pressing, a negative electrode and a secondary battery including the same were manufactured in the same manner as in Example 1.

Comparative example  One

Except for rolling press at 1000 kg / 5 cm so that the electrode density of the negative electrode sample prepared using the graphite composed of low graphitization secondary particles having a powder compression density of 1.75 g / cc as an active material is 1.6 g / cc. In the same manner as in Example 1, a negative electrode and a secondary battery including the same were prepared.

Comparative example  2

Except for rolling press at 2000 kg / 5 cm so that the electrode density of the negative electrode sample prepared using the graphite composed of low graphitization secondary particles having a powder compaction density of 1.75 g / cc is 1.8 g / cc as an active material. In the same manner as in Example 1, a negative electrode and a secondary battery including the same were prepared.

Comparative example  3

A negative electrode sample was prepared using graphite composed of high graphitization secondary particles having a powder compression density of 1.95 g / cc as an active material, wherein the negative electrode was rolled at 1200 kg / 5 cm so that the electrode density of the negative electrode sample was 1.8 g / cc. Except for pressing, a negative electrode and a secondary battery including the same were manufactured in the same manner as in Example 1.

Experimental Example  1: Measurement of Optimal Electrode Density by Cathode Expansion Curve

After the first cycle of the coin-type half secondary batteries prepared in Examples 1 to 7 and Comparative Examples 1 to 3, respectively, under the conditions of charging CC / CV, 0.2 C, 5 mv, and 0.005 C cut, , The cathodic expansion curve for fullness of the first cycle is shown in FIG. 1. For example, as in Example 2, when the electrode density of the cathode is 1.4 g / cc, the SOC value is 25% when the slope is 1 when the SOC value is less than 50%, which is g (25) When, the difference in the slope value of the tangent to the curve at g (20) and g (30) was measured as 0.1.

In Table 1 below, in the expansion curves of the negative electrodes of the coin-type half secondary batteries prepared in Examples 1 to 7 and Comparative Examples 1 to 3, respectively, the SOC value when the slope of the tangent line is less than 50% of SOC is x. When the difference between the slope of the tangent to the curve at x-5 and x + 5, respectively.

Type of cathode	SOC (x) when the tangent slope is less than 50% SOC	Difference of Slope Value of Tangent to Curve at x-5 and x + 5
Example 1	25	0.1
Example 2	25	0.1
Example 3	21	0.5
Comparative Example 1	12	0.9
Comparative Example 2	12	1.2
Example 4	24	0.2
Example 5	24	0.2
Example 6	24	0.2
Example 7	19	0.5
Comparative Example 3	15	1.1

As a result of observing through Table 1, when using graphite composed of secondary particles of low graphite degree as the active material as in Examples 1 to 3 and Comparative Examples 1 and 2, the curves for x-5 and x + 5 It can be seen that Examples 1 to 3, in which the difference in the tangent slope value is within 0.5, are the optimum electrode densities for the case where the graphite composed of the low graphitization secondary particles is used as the active material. In addition, the difference in the slope value of the tangent to the curves at x-5 and x + 5 represents 0.5 in Examples 3 and above 0.5 in Comparative Examples 1 and 2, whereby the active material exceeds Example 3 It can be expected that the overpressure of the cathode will increase the resistance or decrease the life characteristics at the electrode density.

In addition, when using graphite composed of secondary particles of high graphite degree as in Examples 4 to 6, and Comparative Example 3, Example 4, the difference in the slope value of the curve at x-5 and x + 5 is within 0.5 It can be seen that the value of 6 to 6 can be an optimum electrode density for the case of using graphite composed of the secondary particles of the high graphitization degree as the active material. In addition, the difference in the slope value of the tangent to the curves at x-5 and x + 5 represents 0.5 in Example 7, and exceeds 0.5 in Comparative Example 3, whereby the active material exceeds the electrode of Example 7. It can be expected that the overcompression of the cathode in density will increase the resistance or decrease the lifetime characteristics.

Experimental Example  2: Evaluation of Battery Characteristics Through Final Cathode Thickness and Initial Efficiency

The thickness of the negative electrode of the coin-type half secondary battery (CHC) was measured before and after the first cycle was carried out in Examples 1 to 7, and Comparative Examples 1 to 3, respectively.

In addition, after full charge of the first cycle, the discharge was advanced to CC, 0.2 C, 1.0 V, and then the initial efficiency of the battery was measured and the results are shown in Table 2 below.

Type of cathode sample	Initial thickness of the cathode (μm)	Final thickness of the cathode (μm)	CHC Initial Efficiency (%)	Discharge Capacity (mAh)
Example 1	72	98.0	93.8	5.67
Example 2	72	98.0	93.8	5.68
Example 3	72	97.5	93.2	5.59
Comparative Example 1	72	94.0	92.0	5.41
Comparative Example 2	72	94.0	91.5	5.33
Example 4	72	96.0	94.1	5.69
Example 5	72	96.0	94.0	5.68
Example 6	72	94.5	94.0	5.68
Example 7	72	94.0	93.5	5.61
Comparative Example 3	72	93.0	92.2	5.39

As shown in Table 2, the initial efficiency characteristics of Examples 1 to 3 and Comparative Examples 1 and 2 using graphite composed of secondary particles of low graphitization degree as the active material were the best in Examples 1 and 2, and , Comparative Examples 2 and 3 can be seen that the equivalent level by 0.5% p difference. Further, even when judged by the thickness of the negative electrode, the expansion characteristics of the negative electrode were the lowest in Examples 1 and 2, and the same in Comparative Examples 1 and 2, and the electrode density point at which the final thickness and initial efficiency characteristics rapidly changed in the negative electrode. It can be seen that this is similar.

In general, in the secondary battery, the swelling characteristic of the negative electrode is determined based on the full layer thickness, and when the swelling is small, the performance of the secondary battery is determined to be excellent. It tends to decrease. This means that while the charging capacity is the same, the discharge capacity is lowered and the initial efficiency is lowered. As the rolling density increases, the absolute amount of the active material participating in the overall battery reaction decreases, thereby lowering the swelling characteristics. Accordingly, the initial efficiency of Examples 1 to 7 is excellent compared to Comparative Examples 1 to 3 where the swelling characteristics are relatively high.

Through this, it can be seen that the graphite composed of the low graphitization secondary particles as an active material, and the negative electrode having excellent initial efficiency and expansion characteristics, is the electrode density between Example 1 and Example 3, which is the implementation As an example, the difference in the slope value according to the SOC of the expansion curve is 0.5 or less, it can be seen that the results supporting the confirmation that the optimum electrode density between the electrode density between Example 1 and Example 3.

In addition, when examining the initial efficiency characteristics of Examples 5 to 7 and Comparative Example 3 using graphite composed of the secondary particles of the high graphitization degree, Examples 5 to 7 are excellent at a similar level, Comparative Example 3 It can be seen that the characteristics rapidly deteriorate at.

Through this, it can be seen that the graphite composed of the secondary particles of the high graphitization degree as an active material, and the negative electrode excellent in both initial efficiency and expansion characteristics, is the electrode density between Example 4 and Example 7, which is the embodiment As shown, the difference in the slope value according to the SOC of the expansion curve is 0.5 or less, it can be seen that the results supporting the confirmation that the optimum electrode density between the electrode density between Example 4 and Example 7.

Experimental Example  3: evaluation of battery life characteristics

The life characteristics of the secondary batteries prepared in Examples 2, 4, 6, and Comparative Examples 1 to 3, respectively, were evaluated.

Specifically, a secondary battery including graphite composed of secondary particles of low graphite degree according to Example 2 and Comparative Examples 1 and 2 as a negative electrode active material, and high graphite degree according to Examples 4, 6, and Comparative Example 3 Capacity characteristics according to cycles were compared with respect to each of the secondary batteries including graphite composed of the secondary particles as the negative electrode active material, and the results are shown in FIGS. 2 and 3, respectively.

Specifically, the lithium secondary battery having a battery capacity of 30mAh manufactured in Examples 2, 4, 6 and Comparative Examples 1 to 3 was charged at 25 ° C. until 1V constant current was 4.4V, and then charged at 4.4V constant voltage. The charging was terminated when the charging current reached 1.5 mA. Thereafter, it was left for 10 minutes, and then discharged until it became 3V at 0.5C constant current. The charge-discharge behavior was used as one cycle, and the cycle was repeated 200 times, and the capacity according to the cycles according to the present example and the comparative example was measured.

As shown in FIG. 2, in a secondary battery including graphite composed of secondary particles having low graphitization degree as a negative electrode active material, the secondary battery prepared in Example 2 was manufactured in Comparative Examples 1 and 2 while the cycle was repeated 200 times. Compared to the secondary battery, the cycle characteristics were found to be much better. This, as shown in Experimental Example 1, in the case of Example 2 in the negative electrode expansion curve according to SOC in the first charging cycle, when the SOC value when the slope of the tangent is less than 50% SOC 1, x, This is because the difference in the slope values of the curves at x-5 and x + 5 satisfies a value of 0.5 or less, and thus has an optimum electrode density.

On the other hand, Figure 3 is a graph showing the capacity change during the cycle is repeated 200 times in the secondary battery comprising graphite composed of secondary particles of high graphite degree as a negative electrode active material, as shown in Figure 3, Example 4 And the secondary battery prepared in 6 exhibited excellent cycle characteristics compared to the secondary battery prepared in Comparative Example 3. As shown in Experimental Example 1, in Examples 4 and 6, in the cathode expansion curve according to SOC in the first charging cycle, the SOC value when the tangential slope is less than 50% of SOC is x. This is because the difference in the slope values of the curves at x-5 and x + 5 satisfies a value of 0.5 or less, respectively, to have an optimum electrode density.

Although the preferred embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and improvements of those skilled in the art using the basic concepts of the present invention defined in the following claims are also provided. It belongs to the scope of the invention.

Claims (12)

1. A first step of preparing a plurality of negative electrode samples including active material layers having the same composition and having different electrode densities;

   A second step of measuring a negative electrode expansion curve according to the SOC in the first charging cycle for each negative electrode sample;

   When the SOC value of the tangent slope with respect to the measured cathodic expansion curve is x (where x <50), the difference between the slope values of the tangent lines with respect to the curve at x-5 and x + 5 is measured. The third step;

   A fourth step of selecting an optimal electrode density such that the difference in the measured tilt values satisfies a range of 0 to 0.5; And

   The manufacturing method of the negative electrode comprising a fifth step of manufacturing a negative electrode under the conditions satisfying the selected optimal electrode density.
2. The method according to claim 1,

   The active material layer is a method for producing a negative electrode comprising a graphite consisting of secondary particles of low graphite degree or a high graphite degree as an active material.
3. The method according to claim 1,

   The active material layer is a method for producing a negative electrode comprising an active material, a conductive material, and a binder in a weight ratio of 95 to 95.5: 0.5 to 1.5: 3.5 to 4.
4. The method according to claim 1,

   In the first step, the negative electrode sample is prepared with an electrode density of 1.3 g / cc to 1.8 g / cc respectively.
5. The method according to claim 1,

   The fifth step is performed by applying a slurry made by mixing the negative electrode mixture including the active material, the conductive material and the binder in an organic solvent on a current collector, followed by drying and rolling,

   The rolling is performed to satisfy the optimum electrode density selected in the fourth step.
6. As a negative electrode produced by the method of claim 1,

   The negative electrode includes a graphite composed of secondary particles of low graphite degree as an active material, the negative electrode has an electrode density of 1.3 g / cc to 1.5 g / cc.
7. As a negative electrode produced by the method of claim 1,

   The negative electrode comprises a graphite composed of secondary particles of high graphite degree as an active material, the negative electrode having an electrode density of 1.4 g / cc to 1.7 g / cc.
8. The method according to claim 6 or 7,

   The weight ratio of the active material, the conductive material and the binder is 95 to 95.5: 0.5 to 1.5: 3.5 to the negative electrode, characterized in that.
9. A secondary battery comprising a negative electrode of claim 6 or 7, a separator interposed between the positive electrode, the negative electrode and the positive electrode, and an electrolyte solution.
10. A battery module comprising the secondary battery of claim 9 as a unit cell.
11. A battery pack comprising the battery module of claim 10 and used as a power source for medium and large devices.
12. The method according to claim 11,

    The medium-to-large device is a battery pack, characterized in that selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and power storage systems.

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