KR102750799B1 - Binder for secondary battery, anode for secondary battery comprising the same and secondary battery comprising the same - Google Patents

Abstract

A binder for a secondary battery according to exemplary embodiments includes a copolymer having a repeating unit represented by chemical formula 1 and a repeating unit represented by chemical formula 2. By including a binder for a secondary battery including a copolymer including two types of repeating units having different structures, the discharge characteristics of an electrode and the life characteristics of the electrode can be evenly improved.

Description

Binder for secondary battery, anode for secondary battery and secondary battery comprising same {BINDER FOR SECONDARY BATTERY, ANODE FOR SECONDARY BATTERY COMPRISING THE SAME AND SECONDARY BATTERY COMPRISING THE SAME}

The present invention relates to a binder for a secondary battery, a negative electrode for a secondary battery including the same, and a secondary battery. More specifically, the present invention relates to a binder for a secondary battery including a polymer compound, a negative electrode for a secondary battery including the same, and a secondary battery.

Secondary batteries are batteries that can be repeatedly charged and discharged, and have been widely applied to portable electronic communication devices such as camcorders, mobile phones, and notebook PCs along with the development of the information and communication and display industries. Examples of secondary batteries include lithium secondary batteries, nickel-cadmium batteries, and nickel-hydrogen batteries, and among these, lithium secondary batteries have been actively developed and applied because they have high operating voltage and energy density per unit weight, and are advantageous in terms of charging speed and weight reduction.

A lithium secondary battery may include, for example, an electrode assembly including a positive electrode, a negative electrode, and a separator, and an electrolyte impregnating the electrode assembly. The lithium secondary battery may further include an outer packaging material, for example in the form of a pouch, that accommodates the electrode assembly and the electrolyte.

For example, the negative electrode may use carbon-based or silicon-based active material particles as the negative electrode active material. When charge/discharge is repeated, the active material particles may experience side reactions due to contact with the electrolyte, and mechanical and chemical damage such as cracks in the particles may occur.

For example, commonly known negative electrode binders, such as styrene-butadiene polymers and styrene-acrylate polymers, have poor high-temperature stability and adhesive strength, and may cause side reactions with the electrolyte.

For example, Korean Patent No. 10-1320381 discloses a secondary battery having CMC and SBR satisfying certain property conditions as a cathode aqueous binder, but fails to overcome the limitations mentioned above.

Korean Patent Registration No. 10-1320381

One object of the present invention is to provide a binder for a secondary battery having excellent stability.

One object of the present invention is to provide a binder for a secondary battery that can contribute to improving the discharge characteristics of the secondary battery.

One object of the present invention is to provide a negative electrode for a secondary battery having improved stability and discharge characteristics.

One object of the present invention is to provide a secondary battery having improved stability and discharge characteristics.

A binder for a secondary battery according to exemplary embodiments includes a copolymer having a repeating unit represented by the following chemical formula 1 and a repeating unit represented by the following chemical formula 2.

[Chemical Formula 1]

In chemical formula 1, R ₁ and R ₂ are each hydrogen, a C1 to C5 alkyl group, or a C1 to C5 carboxylic acid group or carboxylic acid salt.

[Chemical formula 2]

In chemical formula 2, X is either O, NH, or CH ₂ .

In a binder for a secondary battery according to some exemplary embodiments, R ₁ of the chemical formula 1 may be a C1 to C5 alkyl group, and R ₂ of the chemical formula 1 may be a C1 to C5 carboxylic acid group or carboxylic acid salt.

In a binder for a secondary battery according to some exemplary embodiments, X in the chemical formula 2 is O, and the copolymer may be a polyetheretherketone-based compound.

In some exemplary embodiments of a binder for a secondary battery, the ratio of the number of repeating units represented by the chemical formula 2 to the number of repeating units represented by the chemical formula 1 may be 0.5 to 1.

In a binder for a secondary battery according to some exemplary embodiments, the copolymer may be represented by the following chemical formula 3.

[Chemical Formula 3]

In chemical formula 3, R ₁ is a C1 to C5 alkyl group, R ₂ is a C1 to C5 carboxylic acid group or carboxylic acid salt, and n can be an integer from 1 to 1000.

In some exemplary embodiments of the binder for a secondary battery, the molecular weight of the copolymer may be 1,000 to 700,000 g/mol.

According to some exemplary embodiments, a binder for a secondary battery may further include one or more of styrene butadiene rubber (SBR), polyvinyl alcohol, poly acrylic acid (PAA), carboxymethyl cellulose (CMC), hydroxypropyl cellulose, and diacetyl cellulose.

In some exemplary embodiments of the binder for a secondary battery, the copolymer may be included in an amount of 30 wt% or more based on the total weight of the binder for a secondary battery.

In addition, the negative electrode for a secondary battery according to the exemplary embodiment includes a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector and including a binder and a negative electrode active material for each secondary battery according to the exemplary embodiment.

In the negative electrode for a secondary battery according to an exemplary embodiment, the secondary battery binder may be included in an amount of 0.001 to 2 wt% based on the total weight of the negative electrode active material layer.

Additionally, a secondary battery according to an exemplary embodiment may include a cathode comprising lithium metal oxide; and a cathode disposed opposite the cathode and according to an exemplary embodiment.

According to exemplary embodiments, a binder for a secondary battery including a copolymer including two types of repeating units having different structures can be included, so that discharge characteristics of an electrode such as initial discharge retention rate and discharge capacity can be improved.

In addition, by including the secondary battery binder as described above, volume expansion of the active material can be suppressed, delamination of the electrode active material can be delayed, and life characteristics of the secondary battery electrode, such as discharge capacity retention rate, can be improved.

In addition, by including a secondary battery binder as described above, the structure of the electrode active material can be stabilized and the mechanical stability of the electrode can be improved.

FIG. 1 is a schematic plan view showing a secondary battery according to exemplary embodiments.
FIG. 2 is a schematic cross-sectional view showing a secondary battery according to exemplary embodiments.

Exemplary embodiments of the present invention provide a binder for a secondary battery comprising a copolymer having a repeating unit represented by the following chemical formula 1 and a repeating unit represented by the following chemical formula 2.

[Chemical Formula 1]

In chemical formula 1, R ₁ and R ₂ are each hydrogen, a C1 to C5 alkyl group, or a C1 to C5 carboxylic acid group or carboxylic acid salt.

[Chemical formula 2]

In chemical formula 2, X is either O, NH, or CH ₂ .

Unless otherwise defined herein, “copolymerization” may mean a block copolymer, a random copolymer, a graft copolymer or an alternating copolymer, and “copolymer” may mean a block copolymer, a random copolymer, a graft copolymer or an alternating copolymer.

In a binder for a secondary battery according to some exemplary embodiments, R ₁ of the chemical formula 1 may be a C1 to C5 alkyl group, and R ₂ of the chemical formula 1 may be a C1 to C5 carboxylic acid group or carboxylic acid salt.

In the above chemical formula 1, an alkyl group of C1 to C5 may be substituted to induce additional steric hindrance, and the alkyl group of C1 to C5 may contribute to uniform dispersion of the binder in the slurry for the electrode.

In addition, a C1 to C5 carboxylic acid group or a carboxylic acid salt may be substituted in the chemical formula 1 to increase polar interaction, and the adhesiveness of the binder according to an exemplary embodiment may be improved.

For example, hydrogen bonding between binders may be increased by the carboxylic acid group substituted in Chemical Formula 1, and an attractive force may be applied between an ether group and a carboxylic acid group included in the binder according to an exemplary embodiment. In addition, a dimeric interaction may be established between two or more carboxylic acid groups included in the binder according to an exemplary embodiment.

In a binder for a secondary battery according to some exemplary embodiments, X in the chemical formula 2 is O, and the copolymer may be a polyetheretherketone-based compound.

In addition, the polyetheretherketone compound may include one or more carboxylic acid groups or carboxylic acid salts. The interaction between polyetheretherketone compounds may be increased by the carboxylic acid groups or carboxylic acid salts included in the polyetheretherketone compound.

In addition, the polar interaction by the carboxylic acid group or carboxylic acid salt and the nonpolar interaction by the aromatic ring can occur evenly, thereby increasing the interaction between the mixture included in the slurry composition, and the dispersibility and adhesiveness of the binder according to some exemplary embodiments can be improved simultaneously.

In some exemplary embodiments of a binder for a secondary battery, the ratio of the number of repeating units represented by the chemical formula 2 to the number of repeating units represented by the chemical formula 1 may be 0.5 to 1, and more preferably 0.6 to 0.9.

An increase in the number of repeating units represented by the above chemical formula 1 can contribute to an increase in polar interactions. In addition, an increase in the number of repeating units represented by the above chemical formula 2 can contribute to an increase in nonpolar interactions. Therefore, by satisfying the ratio of the numbers of repeating units as described above, the expression of polar interactions and nonpolar interactions according to the exemplary embodiments can be pursued in a balanced manner.

As a result, the adhesiveness of the binder according to the exemplary embodiment can be further improved, and the detachment of the active material for the electrode interacting with the binder according to the exemplary embodiment can be further suppressed. Furthermore, the life characteristics of the electrode for a secondary battery including the binder according to the exemplary embodiment can be further improved.

In a binder for a secondary battery according to some exemplary embodiments, the copolymer may be represented by the following chemical formula 3.

[Chemical Formula 3]

In chemical formula 3, R ₁ is a C1 to C5 alkyl group, R ₂ is a C1 to C5 carboxylic acid group or a carboxylic acid salt, and n can be an integer from 1 to 1000. The carboxylic acid salt can be an alkali salt, and can be a lithium salt, a sodium salt, or a potassium salt.

Both the bisphenol moiety and the benzophenone moiety included in the copolymer represented by the above chemical formula 3 have a symmetrical structure, and the aromatic rings included in the bisphenol moiety and the benzophenone moiety each exhibit nonpolarity.

Therefore, by including the copolymer represented by the above chemical formula 3 in a binder for a secondary battery, it is possible to pursue a balanced balance of polar interactions and non-polar interactions, and the uniformity of mixing of the slurry for the electrode can be increased.

In the binder for secondary batteries according to exemplary embodiments, the molecular weight of the copolymer may be 1,000 to 700,000 g/mol. By satisfying the molecular weight range described above, it is possible to pursue a balanced mixability and adhesiveness of the binder for secondary batteries according to exemplary embodiments.

According to some exemplary embodiments, a binder for a secondary battery may further include one or more of styrene butadiene rubber (SBR), polyvinyl alcohol, poly acrylic acid (PAA), carboxymethyl cellulose (CMC), hydroxypropyl cellulose, and diacetyl cellulose. By further including one or more of the above-described compounds, the mixing property of the binder is improved, and the discharge characteristics of the electrode, such as the discharge capacity and the initial charge/discharge efficiency, can be further improved.

In some exemplary embodiments of the binder for a secondary battery, the copolymer may be included in an amount of 30 wt% or more based on the total weight of the binder for a secondary battery. By satisfying the numerical range described above, the adhesiveness of the binder may be further improved, and the life characteristics of the electrode, such as the discharge capacity retention rate, may be further improved.

In addition, the electrode including the binder according to the exemplary embodiment may be a negative electrode. The negative electrode for a secondary battery according to the exemplary embodiment may include a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector and including each of the binder for a secondary battery according to the exemplary embodiment and the negative electrode active material.

Additionally, the negative electrode current collector may include gold, stainless steel, nickel, aluminum, titanium, copper or an alloy thereof, and preferably may include copper or a copper alloy.

In exemplary embodiments, a carbon-based active material and/or a silicon-based active material may be provided as a material providing negative electrode activity.

For example, the silicon-based active material may include one or more of silicon metal, crystalline silicon oxide, or amorphous silicon oxide. Additionally, the silicon-based active material may be derived from a siloxane.

For example, the graphite-based active material may include at least one of a crystalline carbon-based material or an amorphous carbon-based material. Additionally, the carbon-based active material may be derived from artificial graphite and/or natural graphite.

In some embodiments, the carbon-based active material may include artificial graphite. Artificial graphite may have a lower capacity than natural graphite, but may have relatively high chemical and thermal stability.

In some embodiments, the carbon-based active material may include an amorphous carbon-based material. Examples of the amorphous carbon-based material include, but are not limited to, a carbon-based compound derived from one or a mixture of two or more of the following: glucose, fructose, galactose, maltose, lactose, sucrose, phenolic resins, naphthalene resins, polyvinyl alcohol resins, urethane resins, polyimide resins, furan resins, cellulose resins, epoxy resins, polystyrene resins, resorcinol-based resins, phloroglucinol-based resins, coal pitch, petroleum pitch, tar, and low-molecular-weight heavy oils.

In the negative electrode for a secondary battery according to an exemplary embodiment, the secondary battery binder may be included in an amount of 0.001 to 2 wt% based on the total weight of the negative electrode active material layer. By satisfying the above-described numerical range, it is possible to pursue a balanced improvement in the electrical characteristics of the negative electrode and an improvement in the physical and chemical stability.

In addition, a secondary battery according to an exemplary embodiment may include a cathode including lithium metal oxide; and a negative electrode according to an exemplary embodiment, which is disposed opposite the cathode. The secondary battery and the positive electrode according to an exemplary embodiment are described based on the attached drawings.

Hereinafter, embodiments of the present invention will be described in detail with reference to the attached drawings. However, these are merely exemplary and the present invention is not limited to the specific embodiments described as exemplary.

FIGS. 1 and 2 are schematic plan views and cross-sectional views, respectively, showing a secondary battery according to exemplary embodiments. For example, FIG. 2 is a cross-sectional view taken along the line I-I' shown in FIG. 1 in the thickness direction of the lithium secondary battery.

Referring to FIGS. 1 and 2, the secondary battery may be provided as a lithium secondary battery. According to exemplary embodiments, the secondary battery may include an electrode assembly (150) and a case (160) that accommodates the electrode assembly (150). The electrode assembly (150) may include a positive electrode (100), a negative electrode (130), and a separator (140).

The positive electrode (100) may include a positive electrode current collector (105) and a positive electrode active material layer (110) formed on at least one surface of the positive electrode current collector (105). According to exemplary embodiments, the positive electrode active material layer (110) may be formed on both surfaces (e.g., the upper surface and the lower surface) of the positive electrode current collector (105). For example, the positive electrode active material layer (110) may be coated on each of the upper surface and the lower surface of the positive electrode current collector (105), and may be directly coated on the surface of the positive electrode current collector (105).

The cathode current collector (105) may include, for example, stainless steel, nickel, aluminum, titanium, copper or an alloy thereof, and preferably may include aluminum or an aluminum alloy.

The cathode active material layer (110) includes a lithium metal oxide as a cathode active material, and according to exemplary embodiments, may include a lithium (Li)-nickel (Ni)-based oxide.

In some embodiments, the lithium metal oxide included in the positive electrode active material layer (110) may be represented by the following chemical formula 4.

[Chemical Formula 4]

Li _1+a Ni _1-(x+y) Co _x M _y O ₂

In the above chemical formula 4, -0.05≤α≤0.15, 0.01≤x≤0.2, 0≤y≤0.2, and M may be one or more elements selected from the group consisting of Mg, Sr, Ba, B, Al, Si, Mn, Ti, Zr, and W. In one embodiment, 0.01≤x≤0.20, 0.01≤y≤0.15.

Preferably, in chemical formula 4, M may be manganese (Mn). In this case, a nickel-cobalt-manganese (NCM) lithium oxide may be used as the positive electrode active material.

For example, nickel (Ni) may be provided as a metal associated with the capacity of a lithium secondary battery. As the nickel content increases, the capacity of the lithium secondary battery may be improved, but if the nickel content increases excessively, the lifespan may be reduced and may be disadvantageous in terms of mechanical and electrical stability. For example, cobalt (Co) may be a metal associated with the conductivity or resistance and output of a lithium secondary battery. In one embodiment, M includes manganese (Mn), and Mn may be provided as a metal associated with the mechanical and electrical stability of a lithium secondary battery.

Through the interaction of nickel, cobalt and manganese described above, the capacity, output, low resistance and life stability can be improved together from the positive electrode active material layer (110).

For example, a slurry can be prepared by mixing and stirring a positive electrode active material with a positive electrode binder, a conductive agent, and/or a dispersing agent in a solvent. After coating the slurry on a positive electrode current collector (105), it can be compressed and dried to form a positive electrode active material layer (110).

The above-mentioned positive electrode binder may include, for example, an organic binder such as vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, or an aqueous binder such as styrene-butadiene rubber (SBR), and may be used together with a thickener such as carboxymethyl cellulose (CMC).

For example, a PVDF series binder can be used as a positive electrode binder. In this case, the amount of the positive electrode binder for forming the positive electrode active material layer (110) can be reduced and the amount of the positive electrode active material or lithium metal oxide particles can be relatively increased, thereby improving the output and capacity of the secondary battery.

The conductive material may be included to promote electron transfer between active material particles. For example, the conductive material may include a carbon-based conductive material such as graphite, carbon black, graphene, carbon nanotubes, and/or a metal-based conductive material including perovskite materials such as tin, tin oxide, titanium oxide, LaSrCoO ₃ , and LaSrMnO ₃ .

In some embodiments, the electrode density of the positive electrode (100) may be 3.0 to 3.9 g/cc, preferably 3.2 to 3.8 g/cc.

The negative electrode (130) may include a negative electrode current collector (125) and a negative electrode active material layer (120) formed on at least one surface of the negative electrode current collector (125). According to exemplary embodiments, the negative electrode active material layer (120) may be formed on both surfaces (e.g., the upper surface and the lower surface) of the negative electrode current collector (125). The negative electrode active material layer (120) may be coated on the upper surface and the lower surface of the negative electrode current collector (125), respectively. For example, the negative electrode active material layer (120) may be in direct contact with the surface of the negative electrode current collector (125).

The negative electrode current collector (125) and the negative electrode active material layer (120) according to the exemplary embodiment are as described above.

For example, a negative electrode slurry may be prepared by mixing and stirring a negative electrode active material with a binder, a conductive agent, and/or a dispersing agent for a secondary battery according to an exemplary embodiment in a solvent. After the negative electrode slurry is applied (coated) on a negative electrode current collector (125), it may be compressed (rolled) and dried to form a negative electrode active material layer (120).

Materials substantially identical to or similar to those used to form the anode (100) as the above-mentioned challenge material may be used.

In exemplary embodiments, the packing density of the negative active material layer (120) may be 1.4 to 1.9 g/cc.

In some embodiments, the area (e.g., the contact area with the separator (140)) and/or volume of the negative electrode (130) may be larger than that of the positive electrode (100). Accordingly, lithium ions generated from the positive electrode (100) may smoothly move to the negative electrode (130) without being precipitated in the middle, for example, thereby further improving the output and capacity characteristics.

A separator (140) may be interposed between the positive electrode (100) and the negative electrode (130). The separator (140) may include a porous polymer film manufactured from a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, an ethylene/methacrylate copolymer, etc. The separator may also include a nonwoven fabric formed from high-melting-point glass fibers, polyethylene terephthalate fibers, etc.

The separator (140) extends in the second direction between the positive electrode (100) and the negative electrode (130), and can be folded and rolled along the thickness direction of the lithium secondary battery. Accordingly, a plurality of positive electrodes (100) and negative electrodes (130) can be stacked in the thickness direction through the separator (140).

According to exemplary embodiments, an electrode cell is defined by an anode (100), a cathode (130), and a separator (140), and a plurality of electrode cells may be laminated to form an electrode assembly (150) in the form of, for example, a jelly roll. For example, the electrode assembly (150) may be formed by winding, lamination, folding, or the like of the separator (140).

The electrode assembly (150) is accommodated in a case (160), and an electrolyte can be injected into the case (160) together. The case (160) may include, for example, a pouch, a can, or the like.

According to exemplary embodiments, a non-aqueous electrolyte may be used as the electrolyte.

The non-aqueous electrolyte contains a lithium salt as an electrolyte and an organic solvent, wherein the lithium salt is expressed as, for example, Li ⁺ X ^{- ,} and the anion (X ^- ) of the lithium salt is F ^- , Cl ^- , Br ^- , 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 ^- , SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- can be exemplified.

Examples of the organic solvent that can be used include propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinylene carbonate, sulfolane, gamma-butyrolactone, propylene sulfite, and tetrahydrofuran. These may be used alone or in combination of two or more.

As illustrated in FIG. 2, electrode tabs (positive electrode tabs and negative electrode tabs) may protrude from the positive electrode collector (105) and negative electrode collector (125) belonging to each electrode cell and extend to one side of the outer case (160). The electrode tabs may be fused together with the one side of the outer case (160) and connected to electrode leads (positive electrode lead (107) and negative electrode lead (127)) that extend or are exposed to the outside of the outer case (160).

In FIG. 1, the positive lead (107) and the negative lead (127) are shown as being formed on the same side of the lithium secondary battery or external case (160), but they may be formed on opposite sides.

For example, the positive lead (107) may be formed on one side of the outer case (160), and the negative lead (127) may be formed on the other side of the outer case (160).

Lithium secondary batteries can be manufactured in a cylindrical, square, pouch or coin shape, for example, using a can.

Hereinafter, experimental examples including specific examples and comparative examples are presented to help understand the present invention, but these are only illustrative of the present invention and do not limit the scope of the appended claims. It will be apparent to those skilled in the art that various changes and modifications to the examples are possible within the scope and technical idea of the present invention, and it is natural that such changes and modifications fall within the scope of the appended claims.

Manufacturing example

Manufacturing example 1

Prepare by mixing 1 equivalent of a bisphenol compound (4,4-bis(4-hydroxyphenyl)pentanoic acid), 0.5 equivalents of a benzophenone compound (bis(4-fluorophenyl)methanone), and a pH adjuster (potassium carbonate) in DMF solvent. Stirring was continued while heating so that DMF was refluxed until the benzophenone compound was no longer confirmed on TLC.

After confirming that no benzophenone compounds were detected on TLC, the mixture was dissolved in water and the pH of the aqueous solution was adjusted to 2 using sulfuric acid. When the solid precipitated, the upper layer of solvent was removed. The remaining solid was dried with hot air to obtain a solid binder.

The weight average molecular weight (Mw) of the binder measured by GPC was 5,800. The specific reaction formula can be depicted as shown in the following reaction formula 1.

[Reaction Formula 1]

Manufacturing examples 2 to 5

Manufactured in the same manner as in Manufacturing Example 1, but the equivalent (eq) of the benzophenone compound was adjusted as shown in Table 1 below. The weight average molecular weight (M _w ) of the binder obtained as a result is as shown in Table 1 below.

Example 1 2 3 4 5 equivalent weight 0.5 0.6 0.7 0.8 0.9 Weight average molecular weight (Mw) 5,800 7,600 21,000 48,000 135,000

Manufacturing example 6

A mixed binder was prepared by mixing the binder of Manufacturing Example 1, styrene-butadiene rubber binder particles (SBR), and carboxymethyl cellulose (CMC) in a weight ratio of 1:1:1.

Manufacturing example 7

A mixed binder was prepared by mixing styrene-butadiene rubber binder (SBR) and carboxymethyl cellulose (CMC) at a weight ratio of 1:1.

Examples and Comparative Examples

A slurry having a total weight of 100 parts was prepared by mixing 98 parts by weight of artificial graphite and 2 parts by weight of the binder of the manufacturing example, respectively. The cathodes of Examples 1 to 6 each contained the binder of Manufacturing Examples 1 to 6, and the cathode of Comparative Example 1 contained the binder of Manufacturing Example 7.

The above slurry was applied to copper foil, dried, and roll pressed to manufacture negative electrodes of examples and comparative examples having the same loading level and packing density. The loading level of the negative electrodes according to each example and comparative example was 13 mg/cm ² , and the packing density was 1.65 g/cc.

Experimental example

In order to evaluate the characteristics of the battery, a lithium secondary battery was manufactured as follows. A coin-shaped battery was manufactured by compressing the negative electrode, lithium counter electrode, and 20㎛ thick polyethylene separator Celgard 2500 according to each of the above-described examples and comparative examples, and injecting an electrolyte inside.

As a solvent of the above electrolyte, a solvent in which ethylene carbonate (EC) and diethyl carbonate (DEC) were mixed in a volume ratio of 30 to 70 (EC:DEC=30v%:70v%) was used, and 1.3 M LiPF ₆ was added as an electrolyte to the solvent. In addition, 5 parts by weight of fluoroethylene carbonate (FEC) was used as an additive with respect to 100 parts by weight of the total electrolyte.

1. Evaluation of the adhesiveness of the cathode

The adhesiveness of the negative electrodes according to each example and comparative example was evaluated by attaching the negative active material layer formed on the copper foil to the tape using a universal testing machine and then peeling the negative electrode at a 180° peel and a speed of 50 mm/min. The evaluation results are shown in Table 2 below.

2. Crack evaluation of the cathode

After folding each cathode once back and forth according to the examples and comparative examples, the occurrence of cracks and the presence of pinholes were observed. The specific evaluation criteria are as follows.

<Evaluation criteria>

◎: No cracks occurred and no active material fell off during slitting.

○: No cracks occurred

△: A pinhole has occurred

X: Crack occurs

Example Comparative example 1 2 3 4 5 6 1 Adhesive strength
(gf/cm) 22 27 25 31 29 26 15 Crack ○ ◎ ◎ ◎ ◎ ◎ △

3. Evaluation of charge and discharge characteristics of secondary batteries

For the secondary batteries including each of the negative electrodes of the above examples and comparative examples, one charge and discharge cycle was performed at 0.05 C to measure the charge capacity, discharge capacity, and initial efficiency, respectively. The evaluation results are shown in Table 3 below.

4. Evaluation of life characteristics of secondary batteries

For the secondary batteries including each of the negative electrodes of the above examples and comparative examples, charge and discharge were performed under the conditions of 0.1C↔0.1C (one charge and discharge) between 0.01 V and 2 V to evaluate the life characteristics.

The life characteristics were evaluated based on the discharge capacity retention rate. The discharge capacity retention rate (30 ^th /1 ^st ) is an indicator that expresses the discharge capacity of 30 cycles as a % ratio compared to the initial discharge capacity. The evaluation results are as shown in Table 3 below.

Example Comparative example 1 2 3 4 5 6 1 Charging capacity
(mAh/g) 3455 3488 3476 3465 3459 3436 3473 Discharge capacity (mAh/g) 3083 3132 3130 3129 3099 3102 3091 I.C.E.(Initial charge/discharge efficiency, %) 89.2 89.8 90.0 90.2 89.6 90.3 89.0 Discharge capacity retention rate (%) 83.4 87.2 85.4 86.1 88.0 84.9 81.5

Referring to Tables 1 to 3 above, it can be confirmed that the cathode according to the exemplary embodiment has excellent stability against external force and excellent charge/discharge characteristics and life characteristics.

Referring to Table 3, the discharge capacity of each cathode according to Examples 2 to 6 was found to exceed approximately 3100 mAh/g.

Referring to Table 3, it can be confirmed that the initial charge/discharge efficiency of each negative electrode according to the embodiment exceeds 89%, and the initial charge/discharge efficiency of the negative electrode according to Examples 3, 4, and 6 exceeds 90%. In addition, the maintenance rate of the discharge capacity of the negative electrode according to Examples 2 to 5 exceeds 85%, and the maintenance rate of the discharge capacity of the negative electrode according to Example 5 exceeds 88%.

When the evaluation results of each negative electrode according to the exemplary embodiment are summarized, it was evaluated that the initial charge/discharge efficiency and discharge capacity retention rate of each negative electrode were improved simultaneously, and the charge/discharge characteristics and life characteristics of the negative electrode were improved.

100: Anode 105: Anode current collector
110: Cathode active material layer 120: Cathode active material layer
125: Negative current collector 130: Negative electrode
140: Separator 150: Electrode assembly
160: Case

Claims (11)

A binder for a secondary battery, comprising a copolymer having a repeating unit represented by the following chemical formula 1 and a repeating unit represented by the following chemical formula 2:
[Chemical Formula 1]

(In chemical formula 1, R ₁ is a C1 to C5 alkyl group, and R ₂ is a C2 to C5 carboxylic acid group or carboxylic acid salt.)
[Chemical formula 2]

(In chemical formula 2, X is either O, NH, or CH ₂ )
delete A binder for a secondary battery according to claim 1, wherein X in the chemical formula 2 is O, and the copolymer is a polyetheretherketone compound. A binder for a secondary battery according to claim 1, wherein the ratio of the number of repeating units represented by the chemical formula 2 to the number of repeating units represented by the chemical formula 1 is 0.5 to 1. In claim 1, the copolymer is a binder for a secondary battery represented by the following chemical formula 3:
[Chemical Formula 3]

(In chemical formula 3, R ₁ is a C1 to C5 alkyl group, R ₂ is a C2 to C5 carboxylic acid group or carboxylic acid salt, and n is an integer from 1 to 1000.) A binder for a secondary battery according to claim 5, wherein the molecular weight of the copolymer is 1,000 to 700,000 g/mol. A binder for a secondary battery according to claim 1, further comprising at least one of styrene butadiene rubber (SBR), polyvinyl alcohol, poly acrylic acid (PAA), carboxymethyl cellulose (CMC), hydroxypropyl cellulose, and diacetyl cellulose. A binder for a secondary battery according to claim 7, wherein the copolymer is contained in an amount of 30 wt% or more based on the total weight of the binder for a secondary battery. A negative electrode for a secondary battery, comprising a negative electrode current collector and a negative electrode active material layer formed on the negative electrode current collector and including a binder for a secondary battery and a negative electrode active material according to claim 1. A secondary battery negative electrode according to claim 9, wherein the secondary battery binder is included in an amount of 0.001 to 2 wt% based on the total weight of the negative electrode active material layer. An anode comprising a lithium metal oxide; and
A secondary battery comprising a cathode according to claim 9 and arranged opposite the positive electrode.